ECOTOXICITY OF AG-NANOPARTICLES ON TWO MICROALGAE, CHLORELLA VULGARIS AND DUNALIELLA TERTIOLECTA

Assay-Dependent Phytotoxicity of Nanoparticles to PlantsD I M I T R I O S S T A M P O U L I S,*,†S A I O N K.S I N H A,‡A N D J A S O N C.W H I T E*,§Department of Biology and Environmental Science and Department of Physics,University of New Haven,300Boston Post Road,West Haven,Connecticut06516,and Department of Analytical Chemistry,The Connecticut Agricultural Experiment Station,123Huntington Street,New Haven, Connecticut06504Received June11,2009.Revised manuscript received November9,2009.Accepted November10,2009.The effects offive nanomaterials(multiwalled carbon nanotubes[MWCNTs],Ag,Cu,ZnO,Si)and their corresponding bulk counterparts on seed germination,root elongation,and biomass of Cucurbita pepo(zucchini)were investigated.The plants were grown in hydroponic solutions amended with nanoparticles or bulk material suspensions at1000mg/L.Seed germination was unaffected by any of the treatments,but Cu nanoparticles reduced emerging root length by77%and64% relative to unamended controls and seeds exposed to bulk Cu powder,respectively.During a15-day hydroponic trial,the biomass of plants exposed to MWCNTs and Ag nanoparticles was reduced by60%and75%,respectively,as compared to control plants and corresponding bulk carbon and Ag powder solutions.Although bulk Cu powder reduced biomass by 69%,Cu nanoparticle exposure resulted in90%reduction relative to control plants.Both Ag and Cu ion controls(1-1000mg/L) andsupernatantfromcentrifugednanoparticlesolutions(1000mg/ L)indicate that half the observed phytotoxicity is from the elementalnanoparticlesthemselves.Thebiomassandtranspiration volume of zucchini exposed to Ag nanoparticles or bulk powder at0-1000mg/mL for17days was measured.Exposure to Ag nanoparticles at500and100mg/L resulted in57%and 41%decreases in plant biomass and transpiration,respectively, as compared to controls or to plants exposed to bulk Ag.On average,zucchini shoots exposed to Ag nanoparticles contained 4.7greater Ag concentration than did the plants from the corresponding bulk solutions.Thesefindings demonstrate that standard phytotoxicity tests such as germination and root elongation may not be sensitive enough or appropriate when evaluating nanoparticle toxicity to terrestrial plant species.IntroductionIn2005,total global investment in nanotechnologies ex-ceeded$4billion,and the estimated annual value for nanotechnology-related products is expected to reach$1 trillion by2015(1).The manufacture and use of particulate material in the size range of a few nanometers(nm)is the driving force behind this growth(2).The extremely small size,structure,and surface characteristics of nanoparticles result in unique physicochemical properties not observed with larger or bulk particles of the same material.For example, materials with dimensions less than5nm exhibit unique electronic states,magnetic/optical properties,and catalytic reactivities that differ from corresponding atomic and bulk scale counterparts(3).In addition,insoluble substances can exhibit drastically enhanced solubility when the particle size is less than100nm.Nanoparticles also have a greater surface area than larger particles with equivalent mass,yielding a greater proportion of atoms on the surface relative to the interior of the structure and resulting in higher surface reactivity(4).Surfactants and other additives can modify the surface active properties of nanomaterials and may prevent particle aggregation(5).Such effects can be particularly important for hydrophobic particles such as carbon nano-tubes and fullerenes.The U.S.EPA organizes engineered nanomaterials into four categories,including(1)carbon-based materials in tubes, spheres,or ellipsoids;(2)metal-based materials such as Au, Ag,but also including metal oxides and quantum dots;(3) dendrimers or nanosized polymers;and(4)composites integrating nanoparticles with other bulk scale materials. Hundreds of nanotechnology-based products are currently in the marketplace,with common applications being found in electronics,optics,food packaging,textiles,medical devices,cosmetics,water treatment technology,fuel cells, catalysts,biosensors,and components for environmental remediation(6-8).For example,nanoscale zerovalent iron can be used to detoxify halogenated molecules such as polychlorinated biphenyls or to reduce nitrates in ground-water(9).The antimicrobial properties of Ag nanoparticles have found use in a range of products,including textiles, bandages,airfilters,and vacuum cleaners.Carbon-based nanomaterials are integrated into plastics,catalysts,battery/ fuel cell electrodes,water purification systems,orthopedic implants,adhesives/composites,and electronics(10).Nanoparticulate matter can be produced by naturally occurring processes such as volcanic activity,fire,and erosion;as such,organisms have long been exposed to and have evolved with these materials.However,the current magnitude of exposure and the unique nature of engineered particulates warrant caution.Manufactured nanoparticles can enter the environment unintentionally through atmo-spheric emissions,domestic wastewater,agriculture,and accidental release during manufacture/transport;or through intentional releases such as during remediation efforts(11). The interaction and impact of nanomaterials,with their unique physical and chemical properties,on living systems has only recently been explored(12).Obviously,some direct data on human response to nanoparticle exposure has been acquired(13).A range of species have been investigated in nanotoxicology studies,including bacteria(14,15),algae(16), invertebrates such as nematodes and crustaceans(17,18), and vertebrates such asfish and rats(19,20).However,this literature is far from complete and is plagued by shortcom-ings,with many studies failing to directly compare bulk and nanoparticle toxicity for a given material.In terms of ecotoxicity,there has been significantly greater focus on aquatic rather than terrestrial species,and very little work has focused on terrestrial plants.Some studies have reported the toxic effects of nanoparticles on the germination and/or root growth of some plant species(21,22).A recent study*Corresponding author e-mail:Jason.White@;phone:203-974-8523;fax:203-974-8502.†Department of Biology and Environmental Science,Universityof New Haven.‡Department of Physics,University of New Haven.§The Connecticut Agricultural Experiment Station.Environ.Sci.Technol.43,9473–947910.1021/es901695c 2009American Chemical Society VOL.43,NO.24,2009/ENVIRONMENTAL SCIENCE&TECHNOLOGY99473 Published on Web11/19/2009(23)showed that select nanoparticles can be absorbed,translocated,and accumulated within tissues of pumpkinplants.The current study focuses on comparing the effects offive types of commonly used nanoparticles(multiwalledcarbon nanotubes[MWCNTs],Ag,Cu,Si,and Zn oxide)totheir corresponding bulk material counterparts on germina-tion,root elongation,and biomass of the agricultural plantCucurbita pepo(zucchini).In this preliminary nanotoxicologystudy,initial concentrations of1000mg/L were chosen toensure observation of relevant phytotoxic responses.Inaddition,the effect of nanoparticle or bulk Ag concentration(0-1000mg/L)on zucchini biomass,transpiration,and Agcontent was determined in a dose-response study.Assessingthe impacts of nanoparticles on agricultural plants willprovide insight into the risk of ecological exposure to thesematerials,as well as to the potential for human exposurethrough food chain contamination.Experimental SectionChemicals.The nanoparticles and corresponding bulkmaterials used were acquired commercially and used aspurchased.The particles were MWCNTs,Ag,Cu,Si,and Znoxide;information on these materials can be found inSupporting Information Table1.The MWCNTs were pro-duced through a high-yield catalytic process based onchemical vapor deposition(CVD)that results in a high degreeof purity(>99%),low concentration of residual catalyst,andabsence of amorphous carbon.Their number of walls rangedfrom3to15.A10.1-cm Si wafer was pulverized with a mortarand pestle.Hoagland Solution and sodium dodecyl sulfate(SDS)were purchased from MP Biomedicals(Solon,OH).Nanoparticle or bulk material solutions were prepared at1000mg/L in25%Hoagland solution with0%or0.2%SDS.Solutions were sonicated for20min and manually agitatedprior to use.As this study was a straightforward comparisonof phytotoxicity from exposure to equivalent nanoparticleand bulk particle treatments,no further information onparticle aggregation/dissolution was obtained.Seeds.Zucchini seeds(Cucurbita pepo cv Costata Ro-manesco)were purchased from Johnny’s Selected Seeds(Albion,ME).Seeds were soaked in10%sodium hypochloritefor10min and were then rinsed thoroughly with reverseosmosis(RO)water.The seeds were air-dried for several daysand were then stored at room temperature.Germination and Root Elongation Assays.The effect ofnanoparticles or corresponding bulk materials on seedgermination and root elongation was determined.Theseassays were conducted with R.O.water amended with0%or0.2%SDS to promote particle dispersion.For the elongationassay,seeds were pregerminated and those with a radical ofapproximately0.5mm were selected.Three individuallymarked seeds(germination)or emerging seeds(elongation)were placed in Petri dishes(35mm×10mm);each dish wasamended with3mL of nanoparticle or bulk material solution(1000mg/L)with or without0.2%SDS.There werefivereplicate dishes for each treatment.The dishes were closedand placed on an orbital shaker operating at50rpm and28°C for5days(elongation)or12days(germination).The solution was replenished as needed with the appropriatenanoparticle or bulk particle containing solution so as toavoid exposure concentration dilution.Hydroponic Biomass Assays.A batch hydroponic experi-ment was conducted to determine the effect of nanoparticlesand corresponding bulk materials on the biomass of zucchiniseedlings.Seeds were added to moist germination paper and4-day-old seedlings were subsequently added to8mL ambervials containing7.5mL of25%Hoagland solution.Theseedlings were placed in a growth room maintained at25°Cwith a12h photoperiod of200µmol m-2s-1photosynthetic active radiation(PAR)for14days.Seedlings were thentransferred to40-mL amber vials containing39mL of solutionamended with either the nanoparticles or corresponding bulkmaterials at1000mg/L.There were six replicate plants pertreatment.The plants were then returned to the growth room,and the biomass was monitored during a subsequent15-dayexposure period.The solution was replenished as neededwith the appropriate nanoparticle or bulk particle containingsolution so as to avoid exposure concentration dilution.For particles showing significant differences in planttoxicity between bulk and nanomaterials(Ag and Cu),experiments were run to differentiate soluble ion phytotox-icity from that of the elemental nanoparticles.Seven-day-old seedlings were transferred to40-mL amber vials con-taining39mL of solution amended with1.0,10,100,or1000mg/L AgNO3or Cu(NO3)2·5H20in25%Hoagland solution.Additional KNO3controls were included to address potentialenhanced growth from added nitrate.Also,solutions of Agand Cu nanoparticles(1000mg/L in25%Hoagland solution)were added to50mL Teflon centrifuge tubes and were spunat2000rpm for5min.The elemental nanoparticles settled,and the ion containing supernatants were collected for useas a growth medium(referred to as“Cu or Ag NP super-natant”).The seedlings were incubated as above,and biomasswas measured daily for15days.There werefive seedlingsper treatment.Ag Dose-Uptake Assay.On the basis of the large differencein phytotoxicity observed between bulk and nanoparticulateAg in the hydroponic biomass assay,the effect of Agnanoparticles or bulk powder at0,1.0,10,50,100,500,and1000mg/L on zucchini biomass and transpiration volumewas determined.Seeds were added to moist germinationpaper and4-day-old seedlings were subsequently added to40mL amber vials containing39mL of25%HoaglandSolution amended with bulk or nanoparticle Ag at0-1000mg/L.There were six replicate plants per treatment.Theplants were placed in a greenhouse at22-28°C with ambientlight.The biomass and transpiration volume(determinedby mass change of solution)were measured during the17day exposure period.The solution was replenished as neededwith the appropriate nanoparticle or bulk particle containingsolution so as to avoid exposure concentration dilution.At17days,stems were severed with a razor blade at least4cmabove the level of solution so as to acquire tissues never indirect contact with nanoparticle or bulk Ag.The shoots wereoven-dried at100°C for72h and digested on a hot blockwith concentrated HNO3for1h at115°C.The digested planttissues were analyzed by inductively coupled plasma massspectroscopy(ICP-MS)for Ag content.Results and DiscussionGermination Assay.The effect of1000mg/L nanoparticlesor corresponding bulk material on the germination ofzucchini(C.pepo)seeds was determined(Table1).In thefirst assay,solutions were amended with the surfactantsodium dodecyl sulfate(0.2%)to facilitate particle dissolution.The percent germination in the RO water and RO water+SDS(0.2%)were90%and60%,respectively;these values aresignificantly different(Student t test;p<0.05).Similarly,thetime to66%germination(2of3seeds for a given replicatedish)was3.0and5.5days for the RO water and RO water +SDS(0.2%),respectively(significantly different at p<0.05). Because of the reduction in germination rate in the presenceof the surfactant,the RO water+SDS was used as the controlagainst which the nanoparticles and their corresponding bulkmaterials were statistically evaluated.In general,no furtherreductions in germination were observed in the varioustreatments.The exception was silicon nanoparticles,wheregermination was completely inhibited;although these valuesare statistically different from the unamended control,there94749ENVIRONMENTAL SCIENCE&TECHNOLOGY/VOL.43,NO.24,2009is no difference from the silicon bulk material(40%germi-nation).The average time to66%germination across the nanoparticle and corresponding bulk materials was6.1and 7.5days.These values are not significantly different from each other or from the control,although this analysis excludes the Si nanoparticles which had0%germination.In trial two,which lacked the confounding effect of the surfactant,none of the investigated nanoparticles or cor-responding bulk materials at1000mg/L had a statistically significant impact on seed germination.The average percent germination across all treatments was82%((9.0%).The time to66%germination was not different across treatments and averaged3.4days((0.68days).Our observation that,in the absence of SDS,nanoparticles had little impact on seed germination agrees with Lin and Xing(22),who evaluated the impact offive nanoparticles on the germination of six plant species.Although corresponding bulk materials were not evaluated in that study,the authors noted only two instances of reduced germination out of30treatments. Cucumber was the most closely related plant to zucchini (same family)in the Lin and Xing(22)study;even at concentrations twice that of our work,none of thefive nanoparticles resulted in phytotoxicity.Conversely,Zheng et al.(24)evaluated the effects of nanoparticulate(5nm)and bulk TiO2on spinach seed germination;however,in their study,seeds were presoaked in nanoparticle or bulk TiO2 solution for48h prior to planting in perlite growth media. Under these conditions,the authors noted that while nanoparticle exposure significantly increased germination at their lower concentrations(250-4000mg/L),higher levels of exposure(6000-8000mg/L)had the opposite effect.Thefinding that seed germination was reduced in the presence of SDS clearly indicates surfactant-mediated phy-totoxicity.The literature on the ability of surfactants to emulsify cell membranes and other lipid-containing cellular constituents is mature,and the precise mechanisms are well characterized(25-27).Clearly,the use of surfactants to facilitate nanoparticle dispersion is useful but must be employed cautiously and with the inclusion of appropriate controls.In the current study,when comparing within individual particle treatments,SDS reduced germination in the following instances:MWCNT,ZnO powder,ZnO(5nm) nanoparticles,Si power,Si nanoparticles,Cu powder,Cu nanoparticles,and Ag nanoparticles.Clearly,any discussion of mechanisms for reduced germination is confounded by the30%reduction caused by SDS alone.However,one could speculate on potential additive effects in certain treatments where reductions in germination are greater than30%.For example,the reduction in germination in the SDS-MWCNT, -Si nanoparticle,and-Ag nanoparticle treatments were47%, 80%,and54%,respectively;however,further investigation is necessary to statistically evaluate these trends.Root Elongation Assay.The effect of1000mg/L nano-particles or corresponding bulk material on the root elonga-tion of pregerminated zucchini(C.pepo)seeds over5days was determined in the presence or absence of0.2%SDS.For the purposes of data analysis,an individual nanoparticle treatment was statistically evaluated only against the cor-responding bulk material and the control.In thefirst trial (surfactant),the root elongation in the RO water and RO water+SDS(0.2%)were13and3.4mm,respectively;these values are significantly different(Mann-Whitney Rank sum test;p<0.001).Since the surfactant reduced root growth,the RO water+SDS was used as the control against which the nanoparticles and bulk materials were statistically evaluated. No further reductions in root growth were observed in the various treatments.On day3,both bulk and nanoparticle Ag treatments had significantly greater root growth than the controls but that difference disappeared by the following day(data not shown).In the absence of SDS,the average root length of untreated control seeds after5days reached24mm.All treatments except Cu had no significant impact on root elongation;the average lengths for nanoparticles and corresponding bulk materials at day5were23and27mm,respectively.Cu nanoparticles resulted in a significantly decreased root length after24h;at day5,the average root lengths of the unamended controls,Cu powder,and Cu nanoparticles were24,16,and 5.7mm,respectively(one-way ANOVA followed by Student-Newman-Keuls multiple comparison test)(Supporting Information Figure1).Thesefindings agree with those of Yang and Watts(21),who determined the impact of alumina nanoparticles on the root elongation of corn,cucumber, cabbage,and carrot.Although20and200mg/L alumina nanoparticles did not impact the root growth of the plants, 2000mg/L resulted in a13%reduction in root length across all four species.However,the authors did not compare these findings to the impact of bulk alumina.Similarly,Lin and Xing(22)investigated the impact offive nanoparticles on the root elongation of radish,rape,ryegrass,lettuce,corn, and cucumber.Again,the authors did not include the corresponding bulk counterparts,but MWCNT,Al2O3,and Al nanoparticles did not impact root elongation at2000mg/ L.Conversely,ZnO nanoparticles at2000mg/L dramaticallyTABLE1.Percent Germination of Zucchini Seeds Exposed to Nanoparticles and Corresponding Bulk Materials aTrial One(SDS)RO water bROwater+SDS AC c MWCNT cZnOpowderZnO5nmZnO10nmSipowderSi100nmCupowderCu50nmAgpowderAg100nm90a60A d60Aa e40Aa67Aa86Aa67Aa40ABa0Ba40Aa53Aa67Aa33Aa (0.05)(0.07)(0.17)(0.11)(0.13)(0.07)(0.08)(0.11)(NA)(0.07)(0.08)(0.15)(0.15)Trial Two(No SDS)RO waterROwater+SDS AC MWCNTZnOpowderZnO5nmZnO10nmSipowderSi10nmCupowderCu50nmAgpowderAg100nm87Aa NA67Aa87Ab80Ab*67Ab*87Aa93Ab80Ab73Ab*80Aa93Aa87Ab(0.08)(0.21)(0.08)(0.08)(0.13)(0.11)(0.07)(0.08)(0.13)(0.13)(0.07)(0.08)a Trial one included the surfactant sodium dodecyl sulfate(SDS,0.2%)in all treatments to facilitate particle dispersion; trial2did not use the surfactant.Values represent%germination after4days.Values in parentheses represent standard errors.b For trial one,all treatments except RO water included SDS.c AC)activated carbon,MWCNT)multiwalled carbon nanotubes.d Within a row,values followed by different capital letters are significantly different by one-way ANOVA followed by a Dunns multiple comparison test involving three treatments(trial1,RO water+SDS,a nanomaterial,and the corresponding bulk material;trial2,RO water,a nanomaterial,and the corresponding bulk material).e Within a column,values followed by different lowercase letters are significantly different(Student t test,p<0.05;*indicates difference at p<0.10).VOL.43,NO.24,2009/ENVIRONMENTAL SCIENCE&TECHNOLOGY99475reduced root growth for all five species.In our study,no statistical differences were observed between the control,ZnO powder,and ZnO nanoparticles.This difference may be due to the higher particle concentration used by Lin and Xing,the different plant species used,and also the high replicate variability within our ZnO treatments.Can ˜as et al.(28)exposed six crop species to carbon nanotubes and also reported inherently high variability among replicates of a given treatment.Again,the authors of this study did not include a corresponding non-nanoparticle carbon control,but they did note that impact of nanotube exposure on root growth was species-specific and that functionalization of the material generally reduced toxicity.Given the above discussion on the impact of anionic surfactants on seed germination,it is not surprising that SDS similarly reduced root elongation.However,when comparing an individual treatment in the presence and absence of SDS,the surfactant reduced root elongation in the following instances:Ag powder,Ag nanoparticles,Si powder,Si nanoparticles,and Cu powder.Given the 73%reduction in root length caused by the surfactant alone,the mechanism for reduced germination among the various treatments may have little to do with particle size.Hydroponic Biomass Assay.The effect of 1000mg/L nanoparticles or corresponding bulk material on the biomass of zucchini (C.pepo )seedlings grown under batch hydroponic conditions was determined.The results fall into three general categories.First,the impact of nanoparticles on plant growth is not significantly different from the corresponding bulk material (Supporting Information Figure 2).The biomass of plants exposed to Si nanoparticles or bulk Si powder were not significantly different from the control plants.Conversely,all ZnO treatments reduced zucchini biomass by 78-90%relative to the controls (significant at p <0.01),but there were no differences between the nanoparticles and bulk material.These findings are in partial agreement with Lin and Xing (30),where ryegrass was exposed for 12days to ZnO nanoparticlesat0-1000mg/L.Theauthorsobservedsignificant reductions in shoot mass at solution concentrations as low as 0.05mg/L and 50%reduction at 1000mg/L.Although no corresponding bulk ZnO was investigated,the authors did include ZnSO 4·7H 2O as a source of Zn 2+and determined that the phytotoxicity observed with the ZnO nanoparticles could not be fully explained by dissolution to the free ion.Second,as shown in Figures 1and 2A,the bulk material has no significant impact on plant biomass relative to thecontrols,but the corresponding nanoparticle does reduce plant growth.MWCNT and Ag nanoparticle exposure resulted in 38%and 69%reductions,respectively,in zucchini biomass relative to the control and corresponding bulk materials.These results disagree with those of Zheng et al.(24),who showed that,at concentrations of 250-4000mg/L,exposure to nanoparticle TiO 2for 48h prior to planting in clean media significantly increased seedling growth whereas the corre-sponding bulk material had little impact.However,the alternative exposure method and given that at higher concentrations (8000mg/L)both bulk and nanoparticle TiO 2reduce plant growth make comparison with the current study problematic.Exposure to AgNO 3at 10mg/L most closely approximates plant growth upon exposure to Ag nanopar-ticles.Exposure to the supernatant of a 1000mg/L Ag nanoparticle solution results in significantly greater plant growth than that observed with the noncentrifuged nano-particle solution,clearly indicating that increased Ag ion dissolution from the nanoparticles only partially explains the observedphytotoxicity.FIGURE 1.Effect of activated carbon powder and MWCNT on zucchini biomass.Curves displaying different numbers are significantly different (one-way ANOVA on slopes of lines regressed through replicate data followed by Student -Newman -Keuls multiple comparison test).Error bars represent standard errors of themean.FIGURE 2.Effect of Ag powder or Ag nanoparticles (A)and Cu powder or Cu nanoparticles (B)on zucchini biomass under hydroponic conditions.Dissolved ion controls were included,as were controls consisting of the supernatant of centrifuged nanoparticle solutions.Within a graph,curves displaying different letters are significantly different (one-way ANOVA on slopes of lines regressed through replicate data followed by Student -Newman -Keuls multiple comparison test).Error bars represent standard errors of the mean.94769ENVIRONMENTAL SCIENCE &TECHNOLOGY /VOL.43,NO.24,2009Third,as shown in Figure2B,Cu powder significantly reduced plant biomass relative to the controls,but exposure to Cu nanoparticles resulted in even greater toxicity.Copper powder and nanoparticles reduced zucchini biomass by69% and90%,respectively,relative to untreated control plants. Thesefindings are in partial agreement with those of Lee et al.(31),who exposed bean and wheat seedlings to Cu nanoparticles(0-1000mg/L)for48h.Although no corre-sponding bulk Cu was included,which is obviously significant given our data,the authors did note40%reduction in the biomass of both species at200mg/L and80%reduction at 1000mg/L.Plant growth in the Cu nanoparticle solution most closely tracks with exposure to10mg/L Cu(NO3)2·5H20. Similar to Ag,exposure to the supernatant1000mg/L Cu nanoparticle solution results in significantly greater plant growth than that observed with the noncentrifuged nano-particle solution,suggesting direct phytotoxic effects from the elemental nanoparticles.Ag Dose-Uptake Assay.The biomass and transpiration volume of zucchini exposed to bulk or nanoparticle Ag at 0-1000mg/L was determined.Bulk Ag at all concentrations had no impact on zucchini biomass or transpiration volume; control plants and plants exposed to bulk Ag(all concentra-tions)had an average mass of4.4g(wet)((0.95)and transpired an average of70mL((14)of solution over17 days.Exposure to Ag nanoparticles at1000and500mg/L reduced zucchini biomass by71%and57%respectively,as compared to unamended controls and corresponding bulk Ag treatments(significantly different at p<0.05)(Supporting Information Figure3).Exposure at100mg/L or lower did not significantly impact zucchini biomass.The transpiration volume of plants exposed to Ag nanoparticles at100,500, and1000mg/L were reduced by41%,78%,and79%, respectively(significantly different from control and corre-sponding bulk material at p<0.05)(Figure3).Exposure to Ag at concentrations of50mg/L or lower did not impact zucchini transpiration volume.The Ag content of shoots from nanoparticle or bulk Ag solutions is shown in Figure4.The Ag content of plants grown in unamended Hoagland’s solution was22µg/kg.At all levels except1mg/L,the Ag concentration in plants exposed to nanoparticles was significantly greater than those exposed to bulk Ag powder(Student t test,p<0.05).On average,the nanoparticle solutions yielded shoots with4.7-fold greater Ag content than present in plants grown in the corresponding levels of bulk element.Information on the uptake of nanoparticles by plants,as well as on the potential mechanisms of phytotoxicity,remains largely unknown.Clearly nanoparticle chemical structure, size,shape,and surface area significantly impact their behavior and fate in the environment,as well as interactions with biota.Metals may release ions that result in toxicity; differences between nanoparticle and corresponding bulk material phytotoxicity could be due to greater ion release from the nanomaterials.In this study,increased release of Cu and Ag ions from the nanopowders appears to have been partially responsible for the observed toxicity(Figures2-4). This is further supported by the greater shoot Ag levels in plants exposed to the nanoparticles as compared to the Ag powder.However,exposure to the elemental Ag nanoparticles seems to result in at least half of the observed phytotoxicity. Ratte(31)notes that the toxicity of Ag to plant species varies, with sensitive cultivars being impacted at aqueous concen-trations as low as75mg/L.Conversely,in sludges spiked with photographic Ag waste at120mg/kg,no impact on the growth of several agricultural species was noted.In the caseof bulk and potentially nanoparticle Ag,toxicity is clearly related to availability of the ion.Similarly,El-Ghamery et al.(32)reported that Zn2+phytotoxicity includes decreased or stunted growth;in the current study,such symptoms were observed in plants exposed to bulk and nanoparticle ZnO.However,Lin and Xing(22)reported that ZnO nanoparticle-induced toxicity resulted not only from the dissolution of ions into the nutrient solution but also something specific to the mere presence of nanoparticles.Although in a later study,the same authors(29)reported little or noZn FIGURE3.Impact of bulk or nanoparticle(<100nm)Ag on the transpiration volume of zucchini grown hydroponically.Ag was at0,100,500,or1000mg/L.Curves followed by different letters are significantly different(data was log-transformed and a one-way ANOVA was performed on the slopes of lines regressed through replicate data followed by Student-Newman-Keuls multiple comparison test).Error bars represent standard errors of themean.FIGURE4.Shoot Ag content of plants not directly exposed to nanoparticle or bulk Ag solutions.At a given Ag solution concentration,bars with different letters are significantly different(Student t test).VOL.43,NO.24,2009/ENVIRONMENTAL SCIENCE&TECHNOLOGY99477。

5月CATTI英语笔译三级综合能力试题CATTI英语笔译三级综合能力比较考验英语基本功，大家可以看中日报双语新闻，因为它涵盖了许多热词，紧跟时事热点。

通过它，你会感觉词汇量上升了一个等级。

下面给大家带来CATTI英语笔译三级综合能力试题，盼望对你们有所关心。

5月CATTI英语笔译三级综合能力试题Section 1: Vocabulary and Grammar(25 points)This section consists of 3 parts. Read the directions for each part before answering the questions. The time for thissection is 25 minutes.Part 1 Vocabulary SelectionIn this part, there are 20 incomplete sentences. Below each sentence, there are 4 choices respectively marked by letters A, B, C and D. Choose the word or phrase which best completes each sentence. There is only ONE right answer. Then blacken the corresponding letter as requiredon yourMachine-scoring ANSWER SHEET. 1.Grover Cleveland was the first president __________ in the White House.A.got marriedB. to get marriedC. has got marriedD. was married2.If cauliflowers are not __________ from extreme temperatures, the heads getdiscolored.A.protectedB. shelterC. shadeD. saved3.The gas __________ from the tank is dangerous.A.given offB. giving outC. giving awayD. given up4.When it started to snow, we turned round and __________ the hotel.A.got byB. searched forC. made forD. cleared up5.Since writing home to their parents for money, they had lived _________ hope.A.inB. forC. onD. through6.Rice is the __________ food of most Southeast Asians.monB. generalC. stapleD. popular7.William Byrd was the owner of the largest library in colonial __________.A.periodB. timeC. timesD. periods8.Exobiology is the study of life __________ other planets.A.inB. atC. onD. to9.The Declaration of Independence, __________ the Constitution of the UnitedStates, was drawn up with the help of Benjamin Franklin.A.andB. alsoC. as well asD. so too10.It was from the Lowell Laboratory that the ninth __________ , Pluto, wassighted in 1930.A.planetB. constellationC. stardomD. satellite11. The rodent, __________ the mouse, rat, guinea pig, and porcupine, are mammals with incisor-like teeth in both jaws.A.made upB. includingC. consistingD. constitute12.___________ into oceans and rivers is a serious form of pollution. A.Pouring sewage B. Emptying litter C. Throwing garbage D. Dumping sewage13. Products which are made from dirts and are __________ high temperatures are known as ceramics.A.tempered inB. subjected toC. exposed toD. baked in14.A pigment called melanin protects the ________ layers of skin from sun rays.A.underB. belowC. underlyingD. underneath15.Oranges are a __________source of vitamin C.A.wellB. betterC. goodD. very16. Even after having their grandchildren live with them for ten years, the couple felt that __________ children these days was the most difficult of all familymatters. A. rising B. raising C. caring D. taking care17. The most important __________ of the farmers in Iraq is dates, ofwhich Iraq is the worlds leading exporter.A.economic cropB. cash cropC. money cropD. staple18. More has been learned about the Moon than any other of the Earth’s neighbors in space because of the Apollo program, which enabled men to walk on the Moon andbring back hundreds of pounds of __________.A.rocksB. rockC. stoneD. stones19. __________ the variety that the average family has in beaf, fish, poultry, and vegetarian recipes, they findmost meals unexciting.A.In spiteB. InspiteC. Despite ofD. Despite20. The speaker __________ have criticized the paraprofessionals, knowing full well that they were seated in the audience.A.should not toB. must notC. ought not toD. may notPart 2 Vocabulary ReplacementThis part consists of 15 sentences in which one word or phrase is underlined. Below each sentence, there are 4 choices respectivelymarked by letters A, B, C and D. Choose the word or phrase that can replace the underlined part without causing any grammatical error or changing the basic meaning of the sentence. There is only ONE right answer. Blacken the corresponding letter as required on your Machine-scoring ANSWER SHEET.21. Iceland has the oldest parliament, which goes as far back to 930A.D. when Althing, the legislative organization, was established.A.officeB. adobeC. assemblyD. building22. The only problem with the debate last week was that the beginning sounded more like a personal attack than a dispassionate, intellectual arguing.A.discussionB. argumentC. talkD. speech23. Susan Jones was at the bus stop well on time to take the 7:01 bus, but she had to miss her breakfast to do it.A.catch up withB. catchC. run up toD. be catching24.Since her father could not drive her to the airport, she requested her uncle todrive her instead. A. take B. bring C. dispatch D. deliver 25. A famous collection of Persian, Indian, and Arabian folktales, the Arabian Nights was supposedly told by the legendary queen Scheherazade to her husband everynight for 1,001 days.A.imaginary B imagery C. fabled D. legend26.What may be the oldest fossil footprint yet found was discovered in June 1968by William J. Meister, a non-professional fossil collector.A.a part-timeB. a spare-timeC. an untrainedD. an amateur27.Most of us think of sharks as dangerous, owing to lack of information ratherthan fear.A.due toB. becauseC. asD.for28.Double Eagle II, the first trans-Atlantic balloon, was greeted by avid crowdsin France.A.eagerB. surgingC. appreciativeD. vigorous29. The discovery of the connection between aspirin and Reyessyndrome,a rare and deadly ailment, is a recent example of the caution with which drugs must be used, even for medical purposes.A.diseaseB. sickC. illD. illness30. My parents moved out of their old home sometime last year after they had celebrated their 50th year there.A.anniversaryB. years oldC. ageD. wedding31. The library she worked in lent books, magazines, audio-cassettes and maps to its customers, who could keep them for four weeks.A.borrowersB. lendersC. patronsD. clients32.A common question that people ask a story writer is whether or not he hasexperienced what he has written about.A.fictionB. scienceC. imaginaryD. literary33.At the World Literacy Center, an organization that works to help people read,thehelpers work hard, enabling them to successfully reach their goals.A.assistantsB. volunteersC. part-timersD. amateurs34.The officers made it clear that they were letting her go only because that shewas old and not because she was above suspicion.A.for reasonB. due toC. because ofD. on the grounds35. The book, which is a useful guide for today’s young people, deals with many questions and problems that face them at school and at home as well as in society.A.are facedB. confrontC. in oppositionD. meetPart 3 Error CorrectionThis part consists of 15 sentences in which there is an underlined part that indicates a grammatical error. Below each sentence, thereare 4 choices respectively marked by letters A, B, C and D. Choose the word orphrase that can replace the underlined part so that the error is corrected. There is only ONE right answer. Blacken the corresponding letter as required on your Machine-scoring ANSWER SHEET.36.All don’t have a free ticket must pay the admission fee.A.Everyone who doesn’t have a free ticketB.No one who doesn’t have a free ticketC.No one who has free ticketsD.Anyone who has free tickets37.When I last saw them, the police had chased the robbers down Columbus Street.A.were chasingB. was chasingC. chasedD. were on a chase38. Erosion that is a slow process, but it constantly changes the features on the surface of the earth.A.which isB. althoughC. beingD. is39. When an organism is completely encapsulated and preserved, it becomesa fossil, therefore turning into evidence of things thatonce lived.A.therebyB. as a result ofC. soD.in the end40.The pictures of the Loch Ness Monster show a remarkable resemblance to aplesiosaur, a large water reptile of the Mesozoic era presuming extinct formore than 70 million years.A.supposedB. presumablyC. presumptuousD. is presumed41. In our own galaxy, the Milky Way, there are perhaps 200 billion stars,a small part of them probably have planets onwhich life is feasible.A.a small fraction in whichB.a small fraction of whichC.a small fraction whichD.which a fraction of 42.“But you’ll be able to come, won’t you?”“Yes, I think such.”A.thatB. itC. soD.this43.The professor is quite difficult pleased.A.to pleaseB. to be pleasedC. for pleasingD. pleasing44.Because everyone knows, facts speak louder than words.A.SinceB. ThatC. ItD.As45.The trapeze artist who ran away with the clown broke up the lion tamer’s heart.A.broke awayB. broke downC. brokeD. broken down46.His heavy drinking and fond of gambling makes him a poor role model.A.and fact that he gamblesB.and that he gamblesC.and he gambles whichD.and gambling47.Depression that inflicts people who believe their lives lack content when therush of the busy week stops referred to by a prominent psychiatrist as Sunday Neurosis.A.has been referred to by a prominent psychiatristB.has been referred to as by a prominent psychiatristC.a prominent psychiatrist has referred to itD.it has been referred to by a prominent psychiatrist48.Just as there are occupations that require college degrees also there areoccupations for which technical training is necessary.A.so to there areB. so too there areC. so there areD. so too are there49.Most of the older civilizations which flourished during the fifth century B.C. are died out.A.they have died outB. has died outC. have died outD. they had died out 50.The student asked her professor if he would have gone on the space ship he did know earlier.A. if he knewB. if heknowsC. he had knownD. had he knownSection 2: Reading Comprehension(55 points)In this section you will find after each of the passages a number of questions or unfinished statements about the passage, each with 4 (A, B, C and D) choices to complete the statement. You must choose the one which you think fits best. Then blacken the corresponding letter as required on your Machine-scoring ANSWER SHEET. The time for this section is 75 minute. Questions 51-56 are based on the following passage.Awardedthe Nobel Prize for physics in 1918, German physicist Max Planck is bestremembered as the originator of the quantum theory. His work helped usher in anew era in theoretical physics and revolutionized the scientific community’sunderstanding of atomic and subatomic processes.Planck introduced an idea that led to the quantum theory, which became the foundation of twentieth century physics. In December 1900, Planck worked out an equation that described the distribution of radiation accurately over the range of low to highfrequencies. He had developed a theory which depended on a model of matter that seemed very strange at the time. The model required the emission of electromagnetic radiation in small chunks or particles. These particles were later calledquantums. The energy associated with each quantum is measured by multiplying the frequency of the radiation, v, by a universal constant, h. Thus, energy, or E, equals hv. The constant, h, is known as Planck’s constant. It is now recognized as one of the fundamental constants of the world.Planck announced his findings in 1900, but it was years before the full consequences of his revolutionary quantum theory were recognized. Throughout his life, Planck made significant contributions to optics, thermodynamics and statistical mechanics, physical chemistry, among other fields.51.In which of the following fields did Max Planck not make a significantcontribution?A.Optics.B. Thermodynamics.C. Statistical mechanics.D. Biology.52.The word “revolutionary” as used in line 16 means .A.radicalB. extremistC. momentousD. militaristic53.It can be inferred from the passage that Planck’s work led to the developmentof which of the following?A.The rocket.B. The atomic bomb.C. The internal combustion engine.D. The computer.54.The particles of electromagnetic radiation given off by matter are known as .A.quantumsB. atomsC. electronsD. valences55.The implication in this passage is that .A.only a German physicist could discover such a theoryB.quantum theory, which led to the development of twentieth century physics, is basically a mathematical formulaC.Planck’s constant was not discernible before 1900D.radiation was hard to study56.“An idea” as used in line 5, refers to .A.a model of matterB.emission of electromagnetic radiationC.quantumsD.the equation that described the distribution of radiation accurately over therange of low to high frequenciesQuestions 57-62are based on the following passage.There has been much speculation about the origin of baseball. In 1907 a special commission decided that themodern game was invented by Abner Doubleday in 1839. One hundred years later the National Baseball Museum was opened to honor Doubleday. Historians, however, disagree about the origin of baseball. Some say that baseball comes from bat-and-ball games of ancient times. It is a matter ofrecord that in the 1700s English boys played a game they called “baseball”.Americans have played a kind of baseball since about 1800. At first the American game had different rules and differentnames in various parts of the country —“town ball”, “rounders”, or “one oldcat”. Youngsters today still play some of these simplified forms of thegame. Baseball did not receive a standard set of rules until 1845, when Alexander Cartwright organized the Knickerbocker Baseball Club of New York City. The rulesCartwright set up for his nine-player team were widely adopted by other clubs and formed the basis of modern baseball. The game was played on a “diamond” infieldwith the bases 90 feet apart. The first team to score 21 runs was declared the winner. By 1858 the National Association of BaseballPlayers was formed with 25 amateur teams.The Cincinnati Red Stockings began to pay players in 1869.57.Which of the following is true about the origins of baseball?A.Historians agree that baseball was invented by Abner Doubleday.B.Baseball, as played in the early 19th century, differed very little fromtoday’s game.C.As early as the 1700s, English boys played a game called “baseball.”D.The first standard set of baseball rules was established at the turn ofthe century.58.What was the first professional baseball team called?A.New York Knickerbockers.B. MilwaukeeBraves.C. Cincinnati Red Stockings.D. Brooklyn Dodgers.59.Who first gave baseball a standard set of rules?A.Abner Doubleday.B. AlexanderCartwright.C. Albert Spalding.D. Babe Ruth.60.Which of the following was not a predecessor of baseball?A.Rounders.B. Town ball.C. Cricket.D. One old cat.61.The tone of the passage is .A.persuasiveB. informativeC. biasedD. argumentative62. The passage implies that until 1869, baseball was played for all of the following reasonsexcept .A.exerciseB. leisureC. profitD. socializingQuestions 63-68are based on the following passage.Theblue of the sea is caused by the scattering of sunlight by tiny particlessuspended in the water. Blue light, being of short wavelength, is scattered more efficiently than light of longer wavelengths. Althoughwaters of the open ocean are commonly some shade of blue, green water iscommonly seen near coasts, especially in tropical or subtropical regions. Thisiscaused by yellow pigments being mixed with blue water. Phytoplankton are onesource of the yellow pigment. Other microscopic plants may color the waterbrown or brownish-red. Near the shore, silt or sediment in suspension can give water a brownish hue. Outflow of large rivers can often be observed many miles offshore by the coloration of suspendedsoil particles.Marine phytoplankton (Greek for “plant wanderers”) are microscopic single-celled plants that include diatoms, dinoflagellates, coccolithophorids, green algae, and blue-green algae, among others. The growth of these organisms, whichphotosynthesize light, depends on a delicate balance of nutrient enrichment via vertical mixing, which is often limited by the availability of nitrogen and light. Diatoms are one-celled plants with patterned glass coverings. Each glass, or silicon dioxide box, is ornamented with species-specific designs, pits, and perforations making them popular with microscopists and, morerecently, electron scanning microscopists. 63.Green water near coastlines is almost always caused by .A.sand colorB.red pigments in coastal watersC.blue pigmentD.reflected light and yellow pigment from plant life64.Phytoplankton are the source of which color pigment?A.Red.B. Green.C. Yellow.D. Blue.65.What can give waters a brownish hue near the shore?A.Sediment.B. Phytoplankton.C. Blue pigment.D. Diatoms.66.Which of the following is not a type of phytoplankton?A.Green algae.B. Diatoms.C. Blue-green algae.D. Amoeba.67.The growth of phytoplankton is often limited by the availability of .A.oxygenB. hydrogenC. nitrogenD. carbon dioxide68.The main idea of this passage is that .A.light causes sea colorB.sea coloration is varied because of a combination of length of light waves and microscopic plant life and siltC.microscopic plant life causes sea colorD.water composition causes sea colorQuestions 69-75are based on the following passage.The United States government publishes guidelines for appropriate nutrient intakes. These are known as the RecommendedDietary Allowances (RDAs) and are updated regularly based on new research in nutrition. RDAs are suggested amounts of calories, protein, and some minerals and vitamins for an adequate diet. For other dietary substances, specific goals must await further research. However, forthe U.S. population as a whole, increasing starch and fiber in one’s diet andreducing calories (primarily from fats, sugar, and alcohol) is sensible. These suggestions are especially appropriate forpeople who have other factors for chronic diseases due to family history of obesity, premature heart disease, diabetes, high blood pressure, and high blood cholesterol, or for those who usetobacco.Snacks can furnish about one-fourth of the calorie requirements among teenagers. Those snacks should also provide much of the day’s allowances for protein, minerals, and vitamins. Sandwiches, fruit, and milk make good snacksfor active teenagers. Food from the food pyramid may be part of any meal. A grilled cheese sandwich or a bowl of whole-grain cereal is just as nutritious in the morning as it is at noon.In addition, a good breakfast consists of any foods that supply aboutone-fourth of the necessary nutrients for the day.69. The passage directly states that most of the U.S. population should increase their intake of .A.proteinB. fatsC. starch and fiberD. sandwiches70.A good breakfast should supply about what percentage of the necessary nutrientsfor the day?A.One-half.B. One-third.C. One-fourth.D. Less than one-fourth.71.The passage implies which of the following?A.The time of day when food is consumed affects its nutritive value.B.Different foods can be combined to increase total nutrition value.C.It can be detrimental to your health to eat breakfast foods later in the day.D.When food is eaten has no bearing on its nutritive effects.72.Why are RDAs regularly updated?A.New discoveries in the science of nutrition are constantly being made.B.Americans’ diets are constantly changing.C.As people age, their nutritional needs change.D.Very little is currently known about nutrition. 73.In this passage RDAs refers to .A.types of vitaminsB. types of proteinC. types of mineralsD. amounts of energy, protein, vitamins, andminerals74.One implication in this passage is that .A.all RDAs have been establishedB.not all RDAs have been established yetC.it’s not important to know RDAsD.RDAs are necessary only for sick people75.The reduction of calories in the diet is particularly good for people whosuffer from .A.obesityB.premature heart disease and diabetesC.high blood pressure and cholesterol levelsD.all of the aboveQuestions 76-81are based on the following passage.The most popular organic gem is the pearl. A pearl is the response of a marine mollusk to the presence of an irritating impurity accidentally introduced into its body; a cultured pearl is the result of the intentional insertion of a mother-of-pearl bead into a live mollusk. Whether introduced accidentally or intentionally, the pearl-making process is the same: the mollusk coats the irritant with a substance called nacre. Nacre is composed chiefly of calcium carbonate. Because very few natural pearls are now on the market, most pearls used in fine jewelry are cultured. These include “Biwa”pearls and most other freshwater pearls. Cultured pearls are not easily distinguished from natural pearlsexcept by an expert.76. Which of the following people could tell the difference between a cultured pearl and an organic pearl?A.Scuba diver.B. Fisherman.C. Jeweler.D. Clerk. 77.What is the chief component of nacre?A.Sand.B. Bead.C. Calcium carbonate.D. Biwa.78.The difference between a pearl and a cultured pearl is the nature of the .A.colorB. introduction of the irritatingimpurityC. coating materialD. irritating impurity79.Nacre is a substance that is .A.mechanically manufacturedB.the result of laboratory testinganically secreted by the molluskD.present in the chemical composition of freshwater pounds80.The main idea in this passage is that .A. most marketable pearls are cultured because nature does not produce enough of its own to satisfy the marketB.cultured pearls are of a higher quality than natural pearlsC.there are two major methods of pearl-makingD.a natural “drought” of pearl production is taking place81.Cultured pearl is formed by .A.insertion of a pearl into a live molluskB.an oyster into which a piece of grit has been placedC.putting in a live molluskD.placing a bead into cultureQuestions 82-87are based on the following passage.Stress is with us all the time. It comes from mental or emotional activity as well as physical activity. It is uniqueand personal to each of us. So personal, in fact, that what may be relaxing toone person may be stressful to another. For example, if you’re a busy executivewho likes to keep occupied all of the time, “taking it easy” at the beach on a beautiful day may be extremely frustrating, nonproductive, and upsetting. You may be emotionally distressed from “doing nothing.” Too much emotional stress can cause physical illnesses such as high blood pressure, ulcers, or even heart disease. Physical stress from work or exercise is not likely to cause such ailments. The truth is that physical exercise can help you to relax andto better handle your mental or emotional stress.82.Which of the following people would find “taking it easy” stressful?A.Construction workers.B. Businessexecutives.C. Farm workers.D. Truck drivers.83.Which of the following would be a determinant as to what people find stressful?A.Personality.B. Education.C. Marital status.D. Shoe size.84.This article, published by the Department of Health and Human Services,probably came from the .A.Federal Bureau of InvestigationB.Alcohol, Drug Abuse, and Mental Health Administrationcation Administrationmunicable Diseases Administration85.A source of stress not specifically mentioned in this passage is .cational activityB. physicalactivityC. mental activityD. emotional activity86.Physical problems caused by emotional stress can appear as all of the followingexcept .A.ulcersB. pregnancyC. heart diseaseD. high blood pressure87.One method mentioned to help handle stress is .A.physical exerciseB. tranquilizersC. drugsD. taking it easyQuestions 88-92are based on the following passage.With the sudden onset of severe psychotic symptoms, the individual is said to be experiencing acute schizophrenia (精神分裂症). “Psychotic”means out of touch with reality, or unable to separate real from unreal experiences. Somepeople have only one such psychotic episode. Others have many episodes during alifetime but lead relatively normal lives during interim periods. Theindividual with chronic (continuous or recurring) schizophrenia often does not fully recover normal functioning and typically requires long-term treatment, generallyincluding medication, to control the symptoms. These symptoms may include hallucinations(幻觉), incoherence, delusions, lackof judgment, deterioration of the abilities to reason and feel emotion, and alack of interaction between the patient and his environment. The hallucinationsmay be a visual, auditory, or tactile. Some chronic schizophrenic patients maynever be able to function without assistance of one sort or another.88.Which of the following is not a symptom of schizophrenia?A.Hallucinations.B. Delusions.C. Incoherence.D. Vertigo.89.It can be inferred from the passage that a person experiencing acuteschizophrenia most likely .A.cannot live without medicationB.cannot go on livingC.can hold a full-time jobD.cannot distinguish real from unreal90.According to this passage, thinking that one can fly might be an example of .A.medicine overdoseB.being out of touch with realityC.recovering normal functioningD.symptom control91.The passage suggests that the beginning of severe psychotic symptoms of acute schizophrenia may be any of the following except .A.debilitatingB.sudden occurrenceC.occurring after a long period of normalcyD.drug-induced92.The passage implies that normal life may be possible for the chronicschizophrenic with the help of .A.medicinesB. neurotic episodesC. psychotic episodesD. timeQuestions 93-100are based on the following passage.Aspirinis one of the safest and most effective drugs invented by man. The most popularmedicine in the world today, it is an effective pain reliever. Its bad effects are relatively mild. It is also cheap.Formillions of people suffering from arthritis, it is the only thing that works.Aspirin, in short, is truly the 20th-century wonder drug. It isalso the second largest suicide drug and is the leading cause of poisoning among children. It has side effects that, although relatively mild, are largely unrecognized among users.Although aspirin was first sold by a German company in 1899, it has been around much longer than that. Hippocrates, in ancient Greece, understood the medical value of tree barks and leaves which today are known to contain a chemical found in aspirin. During the19th century, there was a great deal of experimentation in Europe with this chemical, and it led to the introduction of aspirin. By 1915, aspirin tablets were available in the United States.Asmall quantity of aspirin relieves pain and inflammation. It also reduces feverby affecting some of the body’s reactions. Aspirin is very irritating to thestomach lining. The best way is to chew the tablets before swallowing them withwater, but few people can stand the bitter taste. Some people suggest crushingthe tablets in milk or orange juice.93.Which of the following statements is not true?A.Aspirin is good to arthritis sufferers.B.Aspirin may be used as suicide drug.C.Aspirin is dangerous to small children.D.Aspirin has unrecognizable side effects.94.The second paragraph points out that __________.A.aspirin is always safeB. aspirin can bedangerousC. aspirin has been long usedD. aspirin is not truly effective95.Aspirin was invented in .A.the 20th centuryB. the 19th centuryC. ancient GreeceD. ancient Germany96.The third paragraph describes the _________ of aspirin.esB. valueC. effectsD. history。

小学上册英语第2单元真题试卷[有答案]英语试题一、综合题(本题有50小题，每小题1分，共100分.每小题不选、错误，均不给分)1 The chemical symbol for neptunium is _______.2 I saw a ______ at the zoo. (lion)3 My brother is __________ (富有想象力).4 My father is a . (我爸爸是个。

)5 I enjoy playing ______ (拼图) with my family on rainy days.6 I can explore different cultures with my ________ (玩具).7 The __________ (人权) must be respected worldwide.8 What do we call the act of enhancing communication skills?A. DevelopmentB. ImprovementC. TrainingD. All of the Above答案：D9 Acidic solutions have a pH less than ______.10 She is ________ to music.11 The ______ (鲸鱼) is a gentle giant of the ocean.12 A _______ measures the amount of energy consumed.13 I have a new _______ (手机).14 The library is _______ (很安静).15 The _____ (猴子) is known for its playful behavior.16 What do we call the loud sound made by thunder?A. ClapB. BoomC. CrashD. Bang答案:B17 How many colors are there in a standard box of crayons?a. 12b. 24c. 48d. 64答案:B18 What is the term for the change of state from liquid to gas?A. FreezingB. MeltingC. CondensationD. Evaporation答案: D19 The ______ (蟋蟀) sings at night.20 What is the name of the famous mouse created by Walt Disney?A. Donald DuckB. Mickey MouseC. Goofy答案:B21 A rabbit's ears are very ______ (灵敏).22 environmental education program) informs citizens. The ____23 Nitrogen makes up a large part of the ______.24 I saw a rabbit hopping in the ______.25 The baby is _____ (sleeping).26 A ____ is a small bird that builds nests in trees.27 I have a toy _______ that rolls and spins everywhere.28 ts have a ______ (独特的) appearance. Some pla29 My dad often takes me to _______ (地方). 我们一起 _______ (动词).30 Our teacher taught us about the ________ (大陆).31 A chemical _______ shows how many atoms of each element are in a molecule.32 n bring heavy ______ (降水). Studying33 I have a ______ in my pocket.34 The atomic number tells you the number of ______.35 The first man to reach the North Pole was _______. (皮尔·阿蒙森)36 Which of the following is a type of tree?A. OakB. FlowerC. GrassD. Weed37 Star formation occurs in regions of space called molecular _______.38 What is the largest planet in our solar system?A. EarthC. JupiterD. Saturn39 __________ (植物) use water and sunlight for photosynthesis.40 A reaction that produces gas is called a ______ reaction.41 The __________ (历史的反思) provokes thought.42 Many _______ are grown for their beauty.43 What is the term for a period of ten years?A. CenturyB. DecadeC. MillenniumD. Era44 The country of Italy is shaped like a ________ (靴子).45 The _______ of a wave can be described as its height.46 What is the name of the famous band from Liverpool?A. The Rolling StonesB. The WhoC. The BeatlesD. Pink Floyd答案: C. The Beatles47 I love to decorate my _________ (玩具房间) with colorful items.48 What is the name of the story about a girl and her adventures in Wonderland?A. Alice in WonderlandB. Peter PanC. The Wizard of OzD. Little Red Riding Hood49 She is ___ her lunch. (packing)50 What do we call the study of the Earth's structure and substance?A. BiologyB. ChemistryC. GeologyD. Physics答案: C51 What do you call a place where people go to vote?A. Polling StationB. CourtC. SchoolD. Office答案: A52 A __________ is a reaction that produces new substances.53 I like to _______ (听音乐) in my room.54 The _____ (butterfly/bird) is colorful.55 The horse runs very ______.56 What do we call the light emitted by the sun?A. Solar RadiationB. Stellar LightC. Cosmic LightD. Sunlight57 What do we call the place where we watch movies?A. TheaterC. RestaurantD. School答案:A58 What do you call a baby dolphin?A. CalfB. PupC. KitD. Fawn答案:A59 Canal opened in the year ________. The Suez60 Near-Earth objects (NEOs) are asteroids and comets that can come close to _______.61 What do you call it when water falls from the sky?A. RainB. SnowC. HailD. Sleet答案:A62 A wave can carry energy through a medium without transporting ______.63 A _____ (emulsion) is a mixture of two liquids that normally do not mix.64 The _____ (花瓣) are often colorful and bright.65 The first female Prime Minister of the UK was _______ Thatcher.66 __________ are used in pharmaceuticals for their healing properties.67 What is the opposite of hard?A. SoftC. ShinyD. Bright68 The capital of Laos is _______.69 What is the name of the famous scientist known for his work on plate tectonics?A. Alfred WegenerB. Harry HessC. Charles LyellD. James Hutton答案: A70 A rabbit has large _______ to help it hear all the sounds around it.71 ts are _____ (edible) and can be eaten. Some pla72 The rabbit enjoys eating _________. (新鲜的蔬菜)73 We have _____ (春天) flowers blooming.74 The _______ (Pilgrims) traveled on the Mayflower to America in 1620.75 What do you call a small, flying insect that is often a pest?A. MothB. FlyC. SpiderD. Beetle答案:B76 The bear is very _______ (熊非常_______).77 The _______ (Moscow Kremlin) is a fortified complex in Russia.78 What is the term for a baby sheep?A. CalfC. LambD. Kid答案:C79 A molecule with a high affinity for electrons is called ______.80 Baking soda is used as a ______ agent.81 The superhero is very ___ (brave).82 What do you call a group of stars?A. GalaxyB. Solar SystemC. UniverseD. Nebula答案: A83 My grandma has a lot of _______ (名词). 她的生活经历很 _______ (形容词).84 The chemical symbol for hydrogen is _______.85 What is the term for a baby kangaroo?A. JoeyB. PupC. CalfD. Kit答案:A86 I want to ________ (improve) my skills.87 What is the opposite of beautiful?A. UglyB. PrettyC. AttractiveD. Lovely答案:A88 The chemical reaction that occurs in batteries is called ______.89 What is the name of the famous car race held every year in France?A. Monaco Grand PrixB. Le MansC. Daytona 500D. Indianapolis 500答案：B90 The process of ______ is influenced by wind and water.91 What is the name of the ocean on the west coast of the USA?A. Atlantic OceanB. Indian OceanC. Pacific OceanD. Arctic Ocean答案:C92 Which animal is known for its slow movement and shell?A. TurtleB. SlothC. SnailD. Armadillo答案:A93 What instrument has keys and is played with fingers?A. GuitarB. ViolinC. PianoD. Drum94 The process of producing energy in muscle cells is called ______.95 A ____(carbon footprint) measures environmental impact.96 His favorite movie is a ________.97 The ________ has a long neck.98 The _____ (植物群落) consists of various plants in an area.99 The toucan's beak is large and colorful, aiding in attracting ________________ (配偶). 100 What do we call a scientist who studies insects?A. EntomologistB. BiologistC. ChemistD. Ecologist答案: A。

生物专业外语笔试题目及答案一、选择题（每题2分，共20分）1. The term "gene" was first introduced by which scientist?A. Charles DarwinB. Gregor MendelC. James WatsonD. Francis Crick答案：B2. Which of the following is not a function of DNA?A. Store genetic informationB. Control cell divisionC. Direct protein synthesisD. Provide energy答案：D3. What is the basic unit of a protein?A. CarbohydrateB. LipidC. Amino acidD. Nucleotide答案：C4. The process of DNA replication occurs during which phase of the cell cycle?A. G1 phaseB. S phaseC. G2 phaseD. M phase答案：B5. Which of the following is a type of genetic mutation?A. TranscriptionB. TranslationC. TransversionD. Translocation答案：C二、填空题（每空2分，共20分）6. The central dogma of molecular biology states that genetic information flows from DNA to RNA and then to _______.答案：protein7. In eukaryotic cells, the process of protein synthesis takes place in the _______.答案：cytoplasm8. The term "genome" refers to all the genetic material of an _______.答案：organism9. The process by which a fertilized egg develops into afully formed individual is known as _______.答案：development10. The study of the relationships among various species is known as _______.答案：taxonomy三、简答题（每题10分，共40分）11. Briefly describe the structure of a typical eukaryotic cell.答案：A typical eukaryotic cell has a nucleus that contains the genetic material, a cell membrane that encloses the cell, cytoplasm where organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus perform various functions, and other structures like lysosomes and a cytoskeleton.12. Explain the concept of natural selection and its importance in evolution.答案：Natural selection is the process by which individuals with traits that are better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring. It is a key mechanism of evolution, leading to adaptation and the diversity of life forms.13. What are the main differences between prokaryotic and eukaryotic cells?答案：Prokaryotic cells lack a nucleus and membrane-bound organelles, whereas eukaryotic cells have a defined nucleus and various membrane-bound organelles such as mitochondria and the endoplasmic reticulum. Prokaryotes are generally smaller and simpler in structure compared to eukaryotes.14. Describe the process of photosynthesis and its significance for life on Earth.答案：Photosynthesis is the process by which green plants and some other organisms convert light energy into chemical energy stored in glucose or other organic molecules. It is significant for life on Earth as it provides oxygen and is the primary source of energy for most food chains.四、论述题（每题20分，共20分）15. Discuss the impact of genetic engineering on modern agriculture and medicine.答案：Genetic engineering has revolutionized agriculture by enabling the development of crops with improved resistance to pests and diseases, better tolerance to environmental stresses, and enhanced nutritional content. In medicine, it has facilitated the production of recombinant proteins and vaccines, the development of gene therapies, and the advancement of personalized medicine based on genetic profiles.结束语：本试题旨在考察学生对生物专业外语知识的掌握程度以及应用能力，希望同学们能够通过本试题加深对生物学基本概念和原理的理解，并在实际应用中不断进步。

环糊精2药物复合纳米粒子的制备及其控制释放研究进展3周应学,范晓东,任　杰,田　威(西北工业大学理学院应用化学系,西安710129)摘要 从直接法、小分子键合、星形、树枝状和超支化环糊精大分子胶束及水凝胶、超分子组装7方面论述了环糊精2药物纳米复合体的制备,认为扩散控制、溶胀控制和化学控制是环糊精2药物纳米复合体主要的释放机理。

结合释放机理,指出具有超分子结构的复合体系可望成为智能靶向释放领域的主导。

关键词 环糊精　纳米粒子　药控释放　两亲性　包合R esearch Advance in Preparation of N anoparticles B ased on Cyclodextrins andTheir Applications in Controlled Drug R elease B ehaviorsZHOU Y ingxue ,FAN Xiaodong ,R EN Jie ,TIAN Wei(Department of Applied Chemistry ,School of Science ,Northwestern Polytechnical University ,Xi ’an 710129)Abstract The preparation of CDs 2drug nanoparticle is summarized f rom aspects including non 2covalent inclu 2sion ,covalent conjugate ,micelles inclusion of CDs polymer and supramolecular assembly in nano size.Star 2shape ,dendritic and hyperbranched 2CD polymers and CD hydrogel 2drug inclusions are described in details.Mechanism of drug controlled release is elucidated in diff usion ,target controlled and swelling controlled bination to drug release mechanisms ,CDs 2drug complexes integrated supramolecular structure is desired to guide in smart target con 2trolled release.K ey w ords cyclodextrins ,nanoparticles ,drug controlled release ,amphiphilicity ,inclusion　3国家自然科学基金(基金号20674060)　周应学:男,1974年生,博士生,讲师,从事功能高分子材料的研究　E 2mail :yxzhou2001@ 范晓东:通讯作者,博导,研究方向为生物医用高分子材料和有机硅0　引言环糊精(Cyclodextrins ,CDs )是一类由α2D 2吡喃葡萄糖单元通过1,42糖苷键首尾相连形成的六、七或八环寡糖,其结构呈“锥筒”状,中间是直径为0.7～1.0nm 的空洞。

Applied Surface Science 277 (2013) 105–110Contents lists available at SciVerse ScienceDirectApplied SurfaceSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u scElectrodeposited Ag nanoparticles on TiO 2nanorods for enhanced UV visible light photoreduction CO 2to CH 4Dan Kong,Jeannie Ziang Yie Tan,Fei Yang,Jieliang Zeng,Xiwen Zhang ∗State Key Laboratory of Silicon Materials,Department of Materials Science and Engineering,ZheJiang University,Hangzhou 310027,PR Chinaa r t i c l ei n f oArticle history:Received 26February 2013Received in revised form 31March 2013Accepted 3April 2013Available online 17 April 2013Keywords:Ag nanoparticle Electrodeposition CO 2photoreduction TiO 2nanorodsa b s t r a c tWe employed the double-potentiostatic methodology to electrodeposit Ag nanoparticles on oriented single-crystalline rutile TiO 2nanorods synthesized by hydrothermal method.The synthesized composites were used as the photocatalyst to reduce CO 2to CH 4under UV irradiation,and tested by SEM,XRD,TEM,XPS,UV–vis and photoluminescence.Deposition with Ag nanoparticles was observed to enhance the pho-tocatalytic activity (≈1.5–2.64␮mol (g catal h)−1)up to 5times with respect to undecorated TiO 2nanorods (≈0.5␮mol (g catal h)−1).The increase in the CH 4yield was correlated with the surface morphology and structure of TiO 2nanorods.© 2013 Elsevier B.V. All rights reserved.1.IntroductionAs is known to everyone,carbon dioxide (CO 2)is one of the major greenhouse gases in recent years because the emissions have sharply increased from fossil fuel combustion over the past few centuries [1].CO 2photoreduction is one promising strategy to switch the rising emissions to renewable energy products such as CO,methane (CH 4),methanol (CH 3OH),formaldehyde (HCOOH)[2].Many researchers have reported that titanium dioxide (TiO 2),TiO 2nanocomposites [3]and other metal oxide catalysts can con-vert CO 2[1,4]to methane or methanol in the presence of water.TiO 2is almost a perfect photocatalyst due to its excellent chemical sta-bility,easy operation,low cost and high catalytic activity.However,the relatively high energy bandgap (3.2eV)limits TiO 2only operat-ing under UV irradiation with a wavelength shorter than 387.5nm [4].An effective resolution to improve the photocatalytic activity is modifying TiO 2by doping with metal impurities,such as noble and transition metals [5–8].For instance,Ag has successfully decorated TiO 2for increasing yields of several photocatalytic reactions [9–14].The advantages of Ag doped or deposited on TiO 2are (i)to modify the surface morphologies and structures of TiO 2,(ii)to improve the electron–hole separation by performing as electron traps [15],(iii)to increase the surface electron activity by localized surface plasma Resonance [16,17].∗Corresponding author.E-mail address:zhangxw@ (X.Zhang).Koˇc i et al.[12]prepared Ag-enriched TiO 2powders by sol–gel process controlled in the reverse micellar environ-ment.The catalysts were tested in CO 2photocatalytic reduction under UV irradiation,where the highest yields of methane was 0.38␮mol g catal −1after 24h of UV irradiation.The Ag-doped sol–gel TiO 2powder possessed higher photocatalytic activity than pure TiO 2.Xingtian Yin et al.[17]probed that Ag–TiO 2nanocomposites were prepared at low temperature in polyethylene glycol solution.TiO 2powders with different Ag contents were employed to decom-pose methyl orange under UV.The rapid separation of photoexcited charge carriers and the surface Plasmon effect of Ag nanoparticles in the visible region contributed to a better photocatalytic activity of samples.However,the sol–gel method to preparing Ag–TiO 2nanocom-posites is not so successful,as the particle size and the spatial distribution is not homogenous compared to the electrochem-ical deposition [18–21].Until now,several methods have been employed such as double potential pulses [20],potential step [22],or cyclic voltammetry scan [23].Depositing Ag nanoparticles onto substrates by double-potentiostatic method is rarely recorded.The double-potentiostatic method can realize a quick nucleation and slow growth under the high nucleation potential and low precur-sor concentration,maintaining a suitable growth rate and obtaining uniform and dense metallic nanoparticles [24,25].In our report,Ag nanoparticles were deposited on TiO 2nanorods by double-potentiostatic methods under different nucleation potentials.Hydrothermal method was used to grow TiO 2nanorods on transparent conductive fluorine-doped tin oxide (FTO)sub-strates.The prepared catalysts were evaluated by photoreduction0169-4332/$–see front matter © 2013 Elsevier B.V. All rights reserved./10.1016/j.apsusc.2013.04.010106 D.Kong et al./Applied Surface Science277 (2013) 105–110CO2under UV irradiation.The results showed that TiO2decorated with Ag nanoparticles improved photocatalytic activity,and Ag deposited TiO2under−1.0V nucleation potential had better perfor-mance for the better size variation and uniform spatial distribution compared to other nucleation potentials.2.Experimental2.1.Preparation of photocatalystsTiO2nanorods were prepared by the hydrothermal method on FTO substrates following the report[26].In the beginning,FTO sub-strates(F:SnO2,Tec15,10 m,Hartford Glass Company)were ultrasonically cleaned for60min in a mixture of deionized water, acetone and2-propanol with volume ratios of1:1:1.Tecdeionized water and concentrated hydrochloric acid(36.5–38%by weight) were mixed(ratio1:1)to reach a total volume of480mL,stirred at ambient conditions for15min,and then stirred for15min after the addition of8mL of titanium butoxide(97%Aldrich).The pre-cursor solution immersed FTO substrates,placed at angle against the wall of the Teflon-liner with the conduction side facing down. TiO2nanorods were growing on FTO at150◦C for20h in an elec-tric oven.After that,the FTO substrates were removed,rinsed completely with deionized water overnight and dried in ambi-ent conditions.0.1M KNO3,0.2mM sodium citrate(C6H5Na3O7) and0.05mM AgNO3made up of aqueous electrolyte.The double-potentiostatic method was used on an electrochemical workstation (ES550,Gaoss Union Technology Co.,Ltd.,Wuhan,China).Based on linear sweep voltammogram of TiO2nanorodsfilm from0.2V to −1.5V at−0.05V/s,the nucleation potential was chosen to deposit Ag atoms ranging from−1.4V to−0.8V for100s and the growth potential was−0.2V for2400s at room temperature(28±1◦C)in a standard three-electrode system.FTO coated with TiO2nanorods was used as working electrode,Pt plate as counter electrode and saturated calomel electrode(SCE)as reference electrode.2.2.Characterization of photocatalystsScanning electron microscopy(SEM,Hitachi S4800)was employed to characterize the surface morphologies of TiO2 nanorods and silver nanoparticles.The crystal structure of the as-prepared samples was examined by X-ray diffraction(XRD)in a X’Pert PRO diffractometer using Cu K␣radiation( =1.5406˚A) from20◦to80◦at a scanning speed of2◦/min.The elemental composition of as-prepared samples was analyzed by X-ray photo-electron spectroscopy(XPS)in a VG ESCALAB Mark II instrument using Mg-K␣excitation source.The amount of spectra,recorded at normal emission with pass energies of0.8eV at300W,was collected from the area under the curve of Ag nanoparticles on TiO2film according to the Handbook of X-ray Photoelectron Spec-troscopy(Physical Electronics Division,Eden Prairie,Minnesota, USA,1979).Microstructure was characterized by transmission electron microscopy(TEM)image on a Philips CM200TEM with an acceleration voltage of160kV.The absorption spectra were recorded with a TU-1901UV–vis spectrophotometer by using bare FTO coated glass as the reference.The photoluminescence spectra were carried out on a FLS920fluorescence spectrometer(Edinburg Instruments Ltd.)using a325nm UV xenon lamp as the excitation source.2.3.Photocatalytic reduction experimentsPhotoreduction of CO2was conducted in a quartz reactor with the as-prepared TiO2film placed at the center of the container bottom.The ultraviolet light irradiation system consisted four 8W UVA lamps with a wavelength of365nm(average intensity:3.25mW cm−2,measured by UVX radiometer,UVP)and located in two groups on opposite sides of the container.The details of the photoreduction process and analytical methods were described in our previous report[27].Ultra-pure gaseous CO2(Air Products, 99.995%)wasflowed through deionized water into the reactor for 30min beforeflowing into the reactor for30min to degas the air from the reactor were automatically analyzed by chromatography (GC/FID,Thermo-Fisher,Trace GC)once an hour during the whole 8h reaction time.Methane was the main organic product from the reactor and the reactor and carbon monoxide was occasionally measured within the detection limits of our method(∼200ppb). Therefore,the direct measure of activity toward CO2photoreduc-tion is referenced to the methane yield.The results were compared with our previous work[27].3.Results and discussionFig.1A(a)shows the linear sweep voltammogram of the elec-trolyte from0.2V to−1.5V at the scanning speed of−0.05V/s. The scanning peak at about−0.6V referred to the electrochemi-cal reduction of Ag nanoparticles[28].The related potential–time and current density–time curves were shown in Fig.1A(b)and (c)respectively.Double-potential methods were implemented to study influence of the nucleation potential for Ag nanoparticles. Such short nucleation periods(100s)and long growth periods (2400s)ensured quick nucleation and slow particle growth pro-cess[29].As the nucleation potential was more negative than −0.8V,the current densities were higher in Fig.1A(c).When the nucleation period prolonged with higher nucleation poten-tial,the Ag precursor concentration was reduced at higher rate. Then the low precursor concentration restrained the growth of the nuclei[21,30].Thus,nucleation potential and deposition time was the key to control the size and diameter of Ag nanoparti-cles if the precursor solution were determined.In other words, the nucleation and growth process of Ag nanoparticles could be realized the controllability of size and diameter by the double-potential.This phenomenon could be further confirmed with SEM.Fig.1B shows the SEM image of the as-prepared TiO2nanorods after deposition with Ag nanoparticles.The top view showed that the nanorods were tetragonal and the lengths being approximately 2␮m.The cross-sectional views(b–e)exhibited a uniform distri-bution of the metal on the surface of the nanorods and the size and diameter of nanoparticles diminishing with increasingly nega-tive nucleation potential.The number of nanoparticles increased as more positions were invoked by increasing the nucleation poten-tial.When the nucleation potential reached−0.8V,agglomeration (van der Waals interactions)or aggregation(chemical bands)of Ag nanoparticles at the top part of the nanorods was observed as shown in Fig.2b.In addition,the high nucleation potential had also destroyed the morphology of TiO2nanorods in Fig.2b.High-resolution XPS spectra of Ag(3d)were displayed in Fig.1C. The peaks observed at363.3and369.4eV referred to Ag3d3/2and 3d5/2electronic states of metallic silver respectively.The6.0eV difference between the binding energy of the peaks was also the characteristic of metallic Ag3d states[6,31].There was no peak for oxidized silver corresponding to Ag2O or AgO observed in the full XPS spectra of all the samples.Thus,XPS data together with SEM images,suggested that Ag nanoparticles were deposited on the nanorods.When the nucleation potential increased from−1.2V to−0.8V,XPS intensity was also increasing,indicating that differ-ent content of Ag nanoparticles were formed on the surfaces.There was no XPS spectra of the Ag component deposited under nuclea-tion potential of−1.4V,which was because that Ag nanoparticles were too small to be observed by XPS.D.Kong et al./Applied Surface Science277 (2013) 105–110107Fig.1.(A)(a)Linear sweep voltammogram of TiO2nanorods from0.2V to−1.5V at−0.05V/s,(b)potential–time curves(−0.8V,−1.0V,−1.2V,−1.4V for100s respectively and−0.2V for2400s)and(c)the corresponding current density–time curves of the double-potentiostatic electrodeposition process,(B)SEM images of top view(a)and cross-sectional view(b)of Ag nanoparticles deposited on TiO2nanorods at nucleation potentials of−0.8V,−1.0V(c),−1.2V(d)and−1.4V(e);(C)XPS spectra of TiO2 nanorods after Ag nanoparticles deposition under the different nucleation potentials.Fig.2.(A)XRD patterns of the TiO2nanorods before(a)and after(b–d)Ag electrodeposition under different nucleation potentials as shown in the legend and(B)TEM pattern and(C)HRTEM pattern of TiO2nanorods after Ag electrodeposition under the nucleation potention of−1.0V.108 D.Kong et al./Applied Surface Science 277 (2013) 105–110Fig.3.(A)UV–vis spectra of TiO 2nanorods before and after Ag deposition at differ-ent nucleation potentials,(B)Fluorescence spectra of pure and Ag–TiO 2nanorods under different nucleation potentials.In Fig.2A XRD patterns showed TiO 2nanorods were rutiles with growth axis in the (101)and (002)directions.The nanorods before and after electrodeposition showed the similar XRD curves.The absence of Ag component on TiO 2nanorods showed that Ag did not enter the TiO 2lattice [32].Ag-deposited TiO 2nanorods at nucleation potential of −1.0V were further investigated by TEM as shown in Fig.2B and C.It clearly displayed that Ag nanoparticles amorphously covered TiO 2nanorods with a diameter of 200nm in Fig.2B.Fig.2C shows the high resolution TEM image of the Ag-TiO 2nanocomposites.The spacing between two adjacent lattice fringes were 0.35nm and 0.24nm,corresponding to the (101)plane of TiO 2and the (111)plane of Ag,respectively.The distinguished interface further confirmed the XRD results.The formation of chem-ical bond between TiO 2and Ag nanoparticles was verified by the continuity of lattice fringes between them.Fig.3A shows the UV–vis spectroscopy of the Ag-deposited TiO 2nanorods at the different nucleation potentials.All the samples had higher absorbance intensities than the unmodified TiO 2nanorod films in the range from 300to 400nm.It was observed a red shift and a broadening peak width with increase of nucleation poten-tial between −1.4V and −1.0V.According to the SEM images,the nanorods-deposited Ag nanoparticles at the nucleation potential of −1.0V exhibited a shorter distance between particles and a larger coverage area of particles without agglomeration,suggesting a better-proportioned metal dispersion than other samples.With the condition of unaggregation,the increase of the deposition amount of Ag on TiO 2nanorods with the increasing nucleation potentials had increased the localized surface Plasmon resonance intensity of Ag nanoparticles [6,33,34,16].Thus,the increase of nucleation potentials leaded to the slight red-shift of absorption edge,which contributed to enhancing photoactivity under visible light [35].Fig.4.(A)Sums of methane yield of prepared catalysts at Ag electrodeposition nucleation potentials of −1.0V and −1.2V under UV irradiation (365nm);(B)pho-tocatalytic process:(a)absorbing electron activity and (b)localized surface plasma resonance of Ag nanoparticles effecting carriers transfer process under irradiation.Fig.3B shows the photoluminescence spectra of pure and Ag-deposited TiO 2nanorods.The fluorescence peaks of Ag–TiO 2were the same as pure TiO 2nanorods in the region of 400–450nm.But the sample after Ag electrodeposition at the nucleation poten-tial of −0.8V had another peak in the region of 350–400nm.This was because that aggregating Ag nanoparticles improved the recombination of excited electron–hole pairs [36].The lower inten-sity of Ag–TiO 2nanorods revealed the decrease in recombining electron–hole pairs on metal-loaded TiO 2nanorod surfaces.The positively charged plasmas of Ag nanoparticles attracted electrons in the conduction band of TiO 2and then increased the capability of electron–hole separation [37].However,if the Ag nanoparticles were adjacent to each other,the separation pairs would have been recombining together very soon even under lower energy [38,39].As the nucleation potential of Ag deposition decreased,the cor-responding intensity of the fluorescence decreased.This suggested that the size and the directional distribution of Ag nanoparticles sig-nificantly influenced the rate of e −/h +separation in semiconductor,as well as re-dox reactions.The photocatalytic activity of Ag–TiO 2with the deposited nucleation potentials of −1.2V and −1.0V was compared to that of pure TiO 2nannorods.Fig.4A exhibited the photocat-alytic results in terms of the methane yield versus reaction time.Both Ag-deposited nanorods exhibited much higher methane total outputs (≈1.5–2.64␮mol (g catal h)−1)than the pure TiO 2(≈0.5␮mol (g catal h)−1),suggesting the metal-decorated TiO 2nanorods could highly enhance photocatalytic activity.The uni-form distribution of Ag nanoparticles on the nanorods improved the separation of photogenerated electrons,as well as providing more electron traps than pure TiO 2nanorods.In addition,the AgD.Kong et al./Applied Surface Science277 (2013) 105–110109nanoparticles as intermediates were more convenient to carry out photocatalytic reductions at superior rates.Both of them were responsible for the excellent photocatalytic performance of Ag–TiO2nanorods.The photocatalytic process of Ag–TiO2nanocomposites,as shown in Fig.4B,was put forward to discuss the above phenomena in detail.In Fig.4B(a)the electron transferred from the excited TiO2nanorods to Ag nanoparticles under UV light,due to the lower Fermi level of Ag(E f=0.4V)than that of TiO2.Thus,more electrons assembled in the Fermi level of the metal and the whole Fermi level of Ag–TiO2was nearer to the conduction band of the TiO2, which brought better reductive power of nanocomposites[40]. The improved electron–hole separation and the enhanced reduc-tive power contributed to the photocatalytic activity of Ag–TiO2 nanocomposites under UV irradiation.In addition,due to the local-ized surface plasma resonance of Ag nanoparticles,incident light induced an electricfield in metal nanoparticles,and those nega-tive charge plasmas and positive charge plasmas were separated, as shown in Fig.4B(b).Then the negative charge plasmas near the valence band reacted with holes and positive charge plasmas near the conduction band reacted with electrons[27].The electrons transferring through the Ag nanoparticles reacted with absorbed oxidants to produce oxygen radicals(O2•−),which reduced recom-bination probability in TiO2[37].Together,Ag-deposited TiO2 nanorods exhibit much better photocatalytic performance than the pure TiO2nanorods in our report.However,as the nucleation potential increased from−1.0V to−0.8V,the Ag nanopartilces aggregated together,which infringed the photocatalytic process. The possible explanation was that the aggregation improved the probability of holes captured by Ag nanoparticles and recombina-tion of electron–hole pairs,thus the poorer photoreduction activity [40].4.ConclusionThis work demonstrated Ag nanoparticles successfully deposited on TiO2nanorods by electrochemical method,could highly improve the photocatalytic activity.Under the different nucleation potentials the different size and distribution of Ag nanoparticles grew up on TiO2nanorods.Photoreduction activity measurements were evaluated by photoreduction of CO2under UV irradiation.The results showed that Ag–TiO2nanocomposites exhibited higher photocatalytic activity of conversion CO2to CH4 with rates up to 2.64␮mol(g catal h)−1than that of pure TiO2, possibly due to their specific structures and morphology assisting in the separation of photogenerated electrons and holes.This report might provide new insights into the design and fabrication of advanced photocatalytic materials with complex hierarchical architectures and enhanced photocatalytic activity. 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水溶性银纳米颗粒的制备及抗菌性能孙磊1,*刘爱心1黄红莹2陶小军1赵彦保1张治军1(1河南大学特种功能材料教育部重点实验室,河南开封475004;2河南大学医学院,河南开封475004)摘要:采用液相还原法,以单宁酸为还原剂,聚乙烯吡咯烷酮(PVP)为修饰剂制备出了水溶性的表面修饰Ag纳米颗粒.通过X 射线粉末衍射仪(XRD)、透射电子显微镜(TEM)、紫外-可见吸收分光光度计(UV-Vis)、傅里叶变换红外(FTIR)光谱仪等对所得样品的形貌和结构进行了表征.采用肉汤稀释法测试了样品的抗菌性能,考察了样品在水相中的分散稳定性,提出了PVP 修饰Ag 纳米颗粒的形成机理.结果表明所制备的样品具有Ag 的面心立方晶体结构,平均粒径为15-17nm.样品在水相中能长时间稳定分散;对埃希氏大肠杆菌(E.coli )、金黄色葡萄球菌(S.aureus )具有明显的抗菌作用.操作简便、条件温和的制备方法易于在工业规模上放大;试剂无毒,使得所制备的PVP 修饰Ag 纳米颗粒作为抗菌剂具有良好的应用前景.关键词:银纳米颗粒;水溶性;表面修饰;绿色无毒;抗菌性中图分类号:O648Preparation and Antibacterial Properties of Water-SolubleAg NanoparticlesSUN Lei 1,*LIU Ai-Xin 1HUANG Hong-Ying 2TAO Xiao-Jun 1ZHAO Yan-Bao 1ZHANG Zhi-Jun 1(1Key Laboratory for Special Functional Materials of Ministry of Education,Henan University,Kaifeng 475004,Henan Province,P .R.China ;2Medical College of Henan University,Kaifeng 475004,Henan Province,P .R.China )Abstract:Water-soluble surface modified silver nanoparticles were synthesized by liquid phase reduction with tannic acid as the reductant and polyvinyl pyrrolidone (PVP)as the surface modification agent.The structure and morphology of the as-synthesized powders were investigated by X-ray powder diffraction (XRD),transmission electron microscopy (TEM),ultraviolet-visible (UV-Vis)absorption spectroscopy,and Fourier-transform infrared (FTIR)spectrometry.The antibacterial activity of the water-soluble Ag nanoparticles against Escherichia coli (E.coli )and Staphylococcus aureus (S.aureus )was investigated by broth dilution.The stable dispersion duration of the as-synthesized Ag nanoparticles in water was also determined.A mechanism for PVP modified Ag nanoparticle formation is proposed.The results show that the as-synthesized PVP modified Ag nanoparticles have a face-centered cubic crystalline structure.The average diameter of the Ag nanoparticles ranges from 15to 17nm.The as-synthesized powders have good solubility in water over a long period of time.PVP modified Ag nanoparticles exhibit good antibacterial properties against E.coli and S.aureus .This simple and mild preparation method can be easily increased to an industrial scale process and,therefore,PVP modified Ag nanoparticles are potentially a new type of antibacterial.Key Words:Ag nanoparticles;Water-soluble;Surface modification;“Green ”and nontoxic;Antibacterial property[Article]物理化学学报(Wuli Huaxue Xuebao )Acta Phys.⁃Chim.Sin .2011,27(3),722-728MarchReceived:August 24,2010;Revised:November 1,2010;Published on Web:January 12,2011.∗Corresponding author.Email:sunlei@;Tel:+86-378-3881358.The project was supported by the National Natural Science Foundation of China (50701016)and Natural Science Foundation of Education Department of Henan Province,China (2007150008,2008B150003).国家自然科学基金(50701016)和河南省教育厅自然科学基金(2007150008,2008B150003)资助项目ⒸEditorial office of Acta Physico ⁃Chimica Sinica722No.3孙磊等1引言银纳米材料由于兼具本身固有的物理化学性质和纳米尺度的特殊效应,使其在电学、1,2光学、3-5催化、6,7摩擦学、8生物材料9,10等诸多领域都具有十分广泛的应用.近年来,随着数次大规模传染性疾病的爆发和细菌对传统抗菌剂耐药性的增强,开发新型抗菌剂和抗菌材料已成为科研工作者所关注的一个热点.纳米材料因具有大的比表面积和独特的物理化学性质,在抗菌性能方面的应用前景引起了人们的广泛兴趣.具有抗菌性能的无机纳米材料主要包括Cu、Ag、Au、ZnO、TiO2和藻酸盐等,相关研究已表明其中纳米Ag对于细菌、病毒和真核微生物具有最好的杀灭效果.11-15银离子、金属银或银纳米颗粒作为抗菌药物可用于烧伤治疗、牙科材料、不锈钢涂层、织物、水处理、防晒液等,它们具有对人体细胞的低毒性、高的稳定性和低挥发性等优点.16Leaper17对用于伤口愈合的含银纳米颗粒敷料进行了综述报道,总结了Ag纳米颗粒的抗菌作用机理、人体毒性、临床应用等研究结果.Kumar等18报道了在植物油中制备的银纳米颗粒作为涂料添加剂的抗菌性,其对革兰氏阳性菌和阴性菌均有杀灭作用.尽管银纳米颗粒具有优异的抗菌性,但由于其本身的非水溶性,使其作为抗菌材料在生物、医药方面的应用受到了一些限制;因而有必要对其进行表面改性,使其具有亲水性而便于实际应用.目前,水溶性银纳米材料的制备已有不少报道,19-25但这些研究中的多数并不以生物医用材料的应用为目的,在制备过程中通常没有考虑所用试剂的毒性和合成方法的经济性,往往实验条件苛刻,操作过程繁琐且试剂毒性大,不利于水溶性Ag纳米颗粒作为抗菌材料的应用和制备方法在工业化规模上的放大.因而以经济简便、易于实现工业化生产的方法制备出“绿色”无毒且具有良好抗菌性能的水溶性Ag纳米颗粒已成极为重要的一项课题.本文采用液相化学还原结合原位表面修饰的方法,以单宁酸为还原剂,聚乙烯吡咯烷酮(PVP)为修饰剂,制备了具有良好水溶性的Ag纳米颗粒,解决了产物的分离、提纯和水相中的再分散问题,方法操作简便,反应条件温和,易于工业化生产.制备过程所用还原剂单宁酸、修饰剂PVP均为食品医药领域常用的添加剂,与人体生物相容性好.这使得所制备的Ag纳米颗粒作为活性成分,在抗菌消毒药水等方面具有良好的应用前景.2实验部分2.1试剂与仪器硝酸银(AgNO3)购于国药集团化学试剂有限公司;单宁酸购于郑州市德众化学试剂厂;PVP(K30, M W=40000)为进口分装,德国BASF公司生产;丙酮购于天津市精细化工有限公司;氨水(25%-28%)购于洛阳市化学试剂厂,以上试剂均为分析纯,使用前均未经纯化处理.实验用水为蒸馏水.X射线粉末衍射分析(XRD)采用荷兰Philips公司制造的X′Pert Pro型粉末衍射仪(Cu靶Kα)测定;样品形貌及选区电子衍射(SAED)采用日本电子株式会社(JEOL Ltd)制造的JEM-100CX型透射电子显微镜(TEM)进行观察;采用英国UNICAM公司制造的Unicam UV540型紫外-可见(UV-Vis)吸收分光光度计测定样品的表面等离子体共振(SPR)吸收特性;采用美国Nicolet公司制造的A V ATAR360型傅里叶变换红外(FTIR)光谱仪测定样品的表面化学键合状态.2.2PVP修饰Ag纳米颗粒的制备为了得到粒径小而且分布均匀、水溶性好的PVP修饰Ag纳米颗粒,考察了不同反应物浓度、化学计量比等实验条件对产物的影响,制备了一系列样品.得到最佳实验条件下的制备过程如下:称取PVP0.1359g溶于45mL蒸馏水,移入250mL三颈烧瓶中.室温电磁搅拌下,取50mL2.4mmol·L-1单宁酸溶液加入烧瓶中,随后加入5mL0.5mol·L-1氨水,此时溶液为橙黄色透明液,测得pH约为9.取20mL120mmol·L-1AgNO3溶液,用恒压滴液漏斗将之缓慢逐滴加到上述三颈烧瓶中,溶液中很快出现许多黑色细小颗粒,且溶液由橙黄色透明液变为红黑色悬浮液.AgNO3溶液滴加完毕后,继续搅拌1 h,旋转蒸发浓缩样品溶液,然后加入适量丙酮使沉淀析出、抽滤,用丙酮洗涤数次,真空干燥24h.得到深红棕色粉末样品即为所要制备的PVP修饰Ag纳米颗粒.出于应用目的,采用与上述制备过程同样的反应物计量比和实验条件，在1000mL烧瓶中将反应物和溶剂的量均扩大为4倍进行了实验室的初步放大实验(反应液体积由120mL扩大为480mL).2.3PVP修饰Ag纳米颗粒的抗菌性能测试通过检测最小抑菌浓度(MIC)和最小杀菌浓度(MBC)评价所制备PVP修饰Ag纳米颗粒的抗菌性Vol.27 Acta Phys.⁃Chim.Sin.2011能.实验用菌株大肠杆菌(E.coli)、金黄色葡萄球菌(S.aureus)均购于中国普通微生物菌种保藏管理中心(CGMCC),细菌培养采用牛肉膏蛋白胨培养基和营养琼脂培养基,使用前灭菌保存.2.3.1菌悬液的制备26取4°C保存的菌种接种于牛肉膏蛋白胨培养基,放入37°C孵箱中培养18h;然后取该增菌培养物,再接种于牛肉膏蛋白胨培养基中,仍置于37°C 孵箱中培养6h,即制成含有一定菌落浓度的培养物.将一定量培养物加到无菌试管中,用无菌生理盐水调配成含菌量约为107colony-forming units(cfu)/ mL的菌悬液,菌落计数采用显微镜直接计数法.2.3.2MIC值测试采用不连续分步对倍稀释法配制样品溶液测定MIC值.27,28具体方法如下:配制1000.0μg·mL-1PVP 修饰Ag纳米颗水溶液10mL,以细菌过滤器过滤除菌备用.排列具透气胶塞的无菌试管,一列10管.量取2.0mL Ag纳米颗粒水溶液加到第1支试管中,接着加入2.0mL灭菌的牛肉膏蛋白胨培养基,混合均匀,配制成Ag纳米颗粒浓度为500.0μg·mL-1的样品.从第1支试管中取出2.0mL溶液,滴加到第2支试管中,接着加入2.0mL灭菌牛肉膏蛋白胨培养基,混合均匀后配成浓度为250.0μg·mL-1的样品.依此类推,浓度依次减半一直稀释到第9管,并从第9支试管中吸取混合后的溶液2.0mL弃去.第10支试管为仅含2.0mL灭菌牛肉膏蛋白胨培养基的无样品对照管.得到浓度分别为500.0、250.0、125.0、62.5、31.3、15.6、7.8、3.9、2.0、0μg·mL-1的梯度样品.采用微量加样器向各试管均加入制备好的菌悬液0.2μL,摇匀后放入孵箱中37°C培养24h.取出观察细菌生长情况.先观察对照管中细菌生长情况,在对照管中细菌呈混浊状生长;然后观察含有不同浓度Ag纳米颗粒的各试管中溶液的混浊度,溶液开始出现澄清的最低浓度确定为样品的MIC值.本实验重复三次,结果取平均值或重复值.2.3.3MBC值测试从样品浓度高于MIC值(包括MIC浓度)的试管中各吸取100μL分别滴到灭菌的琼脂平板上,涂布均匀,在孵箱中37°C培养24h.肉眼观察实验结果,菌落数小于5个或无菌落生长的最低样品浓度确定为MBC值.29实验重复三次,结果取平均值或重复值.3结果与讨论3.1PVP修饰Ag纳米颗粒的XRD分析图1(a,b)为在250mL和1000mL烧瓶中制备出的PVP修饰Ag纳米颗粒的XRD图谱.从图1可以看出2个样品在衍射角2θ为38.2°、44.3°、64.5°、77.5°和81.6°的位置均出现了多重衍射峰,这些衍射峰位分别归属于面心立方(fcc)晶型银的(111)、(200)、(220)、(311)和(222)晶面的衍射(JCPDS, No.87-0720),说明所制备样品为单质银.除了图1(b)在2θ约23°附近出现的修饰层PVP的非晶态衍射峰外,样品的XRD图谱中没有出现杂质的衍射峰,说明所合成的样品纯度较高.从图1还可看到,样品XRD衍射峰宽化较为明显,利用谢乐公式对(111)晶面的衍射数据进行计算可知,图1(a)样品的晶粒度约为6.3nm,图1(b)样品的约为7.4nm.除了晶粒度稍有增大外,图1(b)的相对衍射强度比图1(a)的略为减弱,这说明放大实验的样品结晶度稍差,推测其原因,可能是由于在1000mL烧瓶中制备样品,反应液体积比在250mL中的大,虽然反应物浓度、反应物化学计量比等实验条件均相同,但反应过程受到的搅拌力和传热传质速率会有一定区别,因而导致2个样品晶粒度和晶体缺陷程度略有不同.3.2PVP修饰Ag纳米颗粒的形貌分析图2为在250mL和1000mL烧瓶中制备出的PVP修饰Ag纳米颗粒的TEM照片,SAED图谱和用scion image软件对TEM照片进行粒径统计分析得到的粒径分布柱状图.30从TEM图中可以看出所合成的银纳米微粒形貌均为球形、分散较好,无明显聚集现象,粒径大小比较均一.由粒径分布图可以明显看出,在250mL和1000mL烧瓶中合成出的样品图1不同容积烧瓶中制备出的PVP修饰Ag纳米颗粒的XRD图谱Fig.1XRD patterns of PVP modified Ag nanoparticles synthesized in flasks with different volumes(a)250mL;(b)1000mL724No.3孙磊等:水溶性银纳米颗粒的制备及抗菌性能平均粒径分别约为15和17nm,此结果与XRD估算的值有所不同,原因是XRD给出的是样品的晶粒度,而TEM图片给出的是颗粒的表观粒径.TEM观察到的一个纳米颗粒由2-3个银微晶所构成的.31由TEM图中插入的对应样品的SAED图可以看出,衍射斑点呈明显的环状,进一步证实了纳米颗粒由多晶构成.利用透射电镜电子衍射公式可计算出靠近中心斑点的三个电子衍射环分别归属于(fcc)晶型Ag的(111)、(200)、(220)晶面衍射.由TEM和粒径分布图分析可知，放大实验所制备出的样品颗粒平均粒径稍有增大,且粒径大小分布更宽.其原因可归结为反应液体积放大后,影响到了搅拌受力和传质传热的速率,纳米颗粒的成核生长过程受到影响.从而导致了粒径大小和分布的不同.3.3PVP修饰Ag纳米颗粒的UV-Vis分析将所得到的粉末样品重新溶于蒸馏水后进行UV-Vis测试,图3为在250mL和1000mL烧瓶中制备出的PVP修饰Ag纳米颗粒的UV-Vis图谱.从图3可以看出,样品分别在408和413nm附近出现了Ag 纳米颗粒的SPR特征吸收峰.与在250mL烧瓶中合成出的Ag纳米颗粒相比,放大实验后样品(曲线b)的吸收峰红移了约5nm,表明颗粒的平均粒径有所增大;吸收峰宽化也更为明显,表明其粒径分布更宽,这均与TEM分析的结果相一致.3.4PVP修饰Ag纳米颗粒的FT-IR分析图4所示为在250、1000mL烧瓶中制备出的图2不同容积烧瓶中制备出的PVP修饰Ag纳米颗粒的TEM图(a,b)、SAED(插图)和粒径分布图(a′,b′)Fig.2TEM images(a,b),SAED patterns(inset)and particles sizes distributions(a′,b′)of PVP modified Ag nanoparticlessynthesized in flasks with different volumes(a,a′)250mL,(b,b′)1000mL;SAED:selected area electron diffraction3不同容积烧瓶中制备出的PVP修饰Ag纳米颗粒的UV-Vis图谱Fig.3UV-Vis spectra of PVP modified Ag nanoparticlessynthesized in flasks with different volumes(a)250mL;(b)1000mL725Vol.27 Acta Phys.⁃Chim.Sin.2011PVP修饰Ag纳米颗粒和修饰剂PVP的红外光谱图.在图4(c)中,3428cm-1处的吸收峰为―OH的反对称及对称伸缩振动峰,它来自于PVP聚合链端的―OH;2950cm-1处的吸收峰对应于亚甲基中C―H 的伸缩振动;1663cm-1处的吸收峰归属于C＝O的伸缩振动;1440cm-1处的吸收峰为C―H键的面内摇摆振动峰;1290cm-1处的吸收峰对应于C―N键的反对称及对称伸缩振动;656cm-1处的吸收峰为C―H键的面外摇摆振动峰.以上各吸收峰都可归属于修饰剂PVP的不同基团化学键的振动吸收.图4(a,b)吸收峰位及透过率大致相同,表明在不同容积烧瓶中制备出的PVP修饰Ag纳米颗粒表面化学键合性质基本一致.而从它们与修饰剂PVP红外图谱的对比可以看出,表面修饰Ag纳米颗粒在1440 cm-1处出现PVP中C―H键的面内摇摆振动峰,在1290cm-1处出现PVP中C―N键的反对称及对称伸缩振动峰,这说明Ag纳米颗粒表面确实存在PVP 修饰层.而C＝O的伸缩振动峰,则从PVP的1663 cm-1向短波数方向移到了1645cm-1.C＝O振动的减弱,是因为PVP中的O原子提供电子对填充纳米颗粒表面Ag原子的空轨道,换言之,PVP通过化学作用修饰到了Ag纳米颗粒表面.32从图4还可看出, (a)、(b)在2950和656cm-1处C―H键的振动峰与(c)相比消失了.这是由于反应物PVP与AgNO3的质量比只有1:3,在反应过程中也不能完全修饰到Ag纳米颗粒表面,即样品中修饰剂的含量较低,而且由于所制备PVP修饰Ag纳米颗粒的粉末颜色为深红棕色,与KBr粉末压片制成薄膜后,其对红外入射光的吸收仍较PVP样品的强,一定程度上掩盖了修饰剂PVP振动峰的信息,从而导致样品中某些振动峰强度减弱以致消失.而图4(a,b)中3416cm-1附近振动峰强度减弱与图4(c)相比并不明显的原因是―OH 除了来自PVP以外,也有可能是由样品中吸附水所引起的.3.5PVP修饰Ag纳米颗粒的抗菌性能虽然银纳米材料抗菌剂的具体作用机理目前仍不很清楚,但Ag纳米颗粒与体相材料相比具有更优异抗菌性能的原因普遍被认为来自纳米材料具有大的比表面积这一特性.15表1所示分别为250和1000 mL烧瓶中合成的PVP修饰Ag纳米颗粒对E.coli和S.aureus这两类菌种的MIC及MBC值.由表1可见,所制备的PVP修饰Ag纳米颗粒最小抑菌浓度分别只有3.9和7.8μg·mL-1,与商品抗菌剂的值(约800μg·mL-1)相比均要低得多,33这表明PVP修饰Ag纳米颗粒在很低浓度下就表现出抑菌性能.最低杀菌浓度也比文献报道的水溶性Ag纳米颗粒的MBC值(120μg·mL-1)小,34表明其杀菌性能较好.S. aureus和E.coli分属于革兰氏阳性菌和革兰氏阴性菌,所制备的PVP修饰Ag纳米颗粒对它们均有较好的抑菌和杀菌性能,这说明样品具有一定的广谱抗菌性.在1000mL烧瓶中制备出的PVP修饰Ag纳米颗粒,其MIC和MBC值普遍要比在250mL烧瓶中制备出的样品偏高,原因可能是放大实验后Ag纳米颗粒的粒径有所增大,比表面积减小,从而影响了其抗菌性能.3.6PVP修饰Ag纳米颗粒形成机理及分散稳定性PVP是一种水溶性非离子型表面活性剂,常用于制备纳米材料的修饰(包覆)剂.PVP作为修饰剂在还原AgNO3制备Ag纳米颗粒的过程中起到了重要的作用,主要体现在三个方面:首先,PVP中的N 和O原子提供电子对给Ag+的sp轨道,形成配合物;其次,在还原的过程中,由于PVP-Ag+的存在,使Ag+图4不同容积烧瓶中制备出的PVP修饰Ag纳米颗粒和修饰剂PVP的FTIR图谱Fig.4FTIR spectra of PVP modified Ag nanoparticlessynthesized in flasks with different volumes andmodification agent of PVP(a)250mL;(b)1000mL;(c)PVP1不同容积烧瓶中制备出的PVP修饰Ag纳米颗粒的抗菌性能Table1Antibacterial properties of PVP modified Agnanoparticles synthesized in flasks with different volumesStrainE.coli S.aureusMIC/(μg·mL-1)250mL3.93.91000mL7.87.8MBC/(μg·mL-1)250mL31.315.61000mL31.362.5 MIC:minimum inhibitory concentration;MBC:minimum bacterical concentration726No.3孙磊等:水溶性银纳米颗粒的制备及抗菌性能更容易被还原,Ag 纳米晶核更易形成;最后,由于PVP 以化学作用修饰到了新生成的Ag 纳米颗粒表面,PVP 的空间位阻作用限制了颗粒之间的团聚,起到了控制粒径大小的作用.以上PVP 修饰Ag 纳米颗粒形成机理的推测在TEM 、FTIR 分析中都得到了证实,图5是其示意图.Ag 纳米颗粒在水相中的分散稳定性是影响其抗菌性能的重要因素之一.图6(a)是不同实验条件下(具体条件如表2所示，图中数字对应表格中的样品编号)制备的PVP 修饰Ag 纳米颗粒粉末以0.5g ·L -1的浓度溶解于水的照片,可以看到这些样品的水溶液均澄清透明,分散性很好,由于颗粒粒径不同而呈现出的颜色则稍有差别.对于本文实验部分详细描述的最佳实验条件下制备出的样品(表2中1号样品),测得其在水中的最大溶解浓度约为2.0g ·L -1,由抗菌性能测试结果可知,这个浓度完全能满足作为抗菌剂配制溶液的需要.图6(b)是该样品以最大含量分散于水中的照片.由于水溶性的PVP 修饰到了Ag 纳米颗粒表面,且颗粒粒径均比较小,上述样品在水中都有很好的分散稳定性,放置12个月仍6PVP 修饰Ag 纳米颗粒在水相中分散稳定性的照片Fig.6Photos of PVP modified Ag nanoparticles dispersed stably in water(a)samples (1-6)synthesized in different conditions (shown in Table 2)with the same concentration of 0.5g ·L -1;(b)sample synthesized in the optimum condition with the largest concentration of 2.0g ·L -1表2制备PVP 修饰Ag 纳米颗粒的不同实验条件图5PVP 修饰Ag 纳米颗粒形成机理示意图Fig.5Schematic illustration of the formation mechanism of PVP modified Ag nanoparticles727Vol.27 Acta Phys.⁃Chim.Sin.2011无聚沉.4结论采用液相化学还原的方法,制备了PVP修饰Ag 纳米颗粒.所得颗粒平均粒径为15-17nm,粒径大小分布均匀.PVP通过化学作用修饰在Ag纳米颗粒表面,使得所制备的样品在水中具有良好的分散稳定性.抗菌性能测试表明PVP修饰Ag纳米颗粒对于埃希氏大肠杆菌和金黄色葡萄球菌都具有优异的抑菌杀菌效果.该方法操作过程简便、所用试剂无毒、反应条件温和,所制备的PVP修饰Ag纳米颗粒作为新型抗菌剂具有良好的应用前景.References(1)Endrino,J.L.;Horwat,D.;Gago,R.;Andersson,J.;Liu,Y.S.;Guo,J.;Anders,A.Solid State Sci.2009,11,1742.(2)Sun,J.;Zhang,J.;Liu,W.;Liu,S.;Sun,H.;Jiang,K.;Li,Q.;Guo,J.Nanotechnology2005,16,2412.(3)Cobley,C.M.;Rycenga,M.;Zhou,F.;Li,Z.Y.;Xia,Y.J.Phys.Chem.C2009,113,16975.(4)Dong,X.;Ji,X.;Wu,H.;Zhao,L.;Li,J.;Yang,W.J.Phys.Chem.C2009,113,6573.(5)Wani,I.A.;Khatoon,S.;Ganguly,A.;Ahmed,J.;Ganguli,A.K.;Ahmad,T.Mater.Res.Bull.2010,45,1033.(6)Yao,W.;Guo,Y.L.;Lu,G.Z.;Guo,Y.;Wang,Y.Q.;Zhang,Z.G.;He,D.N.Acta 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双金属纳米颗粒的英文文献2000字左右Dual-metallic nanoparticles have gained increasing attention in various fields due to their unique properties and wide range of applications. These nanoparticles, composed of two different metals, exhibit synergistic effects that enhance their catalytic, optical, and magnetic properties. In this review, we discuss the synthesis, properties, and applications of dual-metallic nanoparticles, focusing on their importance in nanotechnology.Synthesis methods for dual-metallic nanoparticles include chemical reduction, thermal decomposition, and galvanic replacement. These methods allow for control over the size, shape, composition, and structure of the nanoparticles, which ultimately influences their properties and applications. For example, alloying two metals can induce a shift in the surface plasmon resonance of the nanoparticles, leading to enhanced catalytic activity for various reactions.The properties of dual-metallic nanoparticles can be tailored by adjusting the ratio of the two metals, the nanoparticle size, and the synthesis conditions. For instance, bimetallic nanoparticles can exhibit improved stability, selectivity, and activity compared to their monometallic counterparts. The presence of two different metals also enables multifunctionality,allowing for applications in catalysis, sensing, imaging, and drug delivery.In catalysis, dual-metallic nanoparticles have shown great potential as efficient and selective catalysts for a wide range of reactions, including hydrogenation, oxidation, and coupling reactions. The synergistic effects between the two metals enhance the catalytic activity, while the unique structure of the nanoparticles provides a high surface area for catalysis. These properties make dual-metallic nanoparticles ideal candidates for sustainable and green chemistry applications.In addition to catalysis, dual-metallic nanoparticles have been used in other fields such as optoelectronics and photonics. The plasmonic properties of these nanoparticles can be tuned to absorb and scatter light in specific wavelengths, enabling applications in sensing, imaging, and photothermal therapy. Moreover, the magnetic properties of dual-metallic nanoparticles make them promising candidates for magnetic separation, drug delivery, and hyperthermia treatments.Overall, dual-metallic nanoparticles represent a versatile class of nanomaterials with significant potential for a wide range of applications. Their unique properties, tunable nature, and multifunctionality make them promising candidates for variousfields, including catalysis, sensing, imaging, and therapy. With further research and development, dual-metallic nanoparticles have the potential to revolutionize nanotechnology and contribute to the advancement of science and technology.。

/CRNucleation and Growth of Nanoparticles in the AtmosphereRenyi Zhang,*,†,‡,§Alexei Khalizov,†Lin Wang,‡Min Hu,§and Wen Xu††Department of Atmospheric Sciences and Department of Chemistry,Center for Atmospheric Chemistry and Environment,Texas A&M University,College Station,Texas77843,United States‡Department of Environmental Science&Engineering and Institute of Global Environment Change Research,Fudan University, Shanghai200433,China§State Key Laboratory of Environmental Simulation and Pollution Control,College of Environmental Sciences and Engineering, Peking University,Beijing,100871,ChinaCONTENTS1.Introduction B2.Overview of Vapor Nucleation D2.1.Nucleation Theories and ComputationalApproaches D2.1.1.Classical Nucleation Theory D2.1.2.Kinetic Theories F2.1.3.Molecular Dynamics and Monte CarloMethods F2.1.4.Density Functional Theory G2.1.5.Nucleation Theorem G2.2.Nucleation Experiments H2.2.1.Adiabatic Expansion Approaches H2.2.2.Diffusion Chamber H2.2.minar Flow Chamber I2.2.4.Turbulent Mixing Chamber I2.2.5.Continuous Generation of NucleatingVapors from Chemical Reaction sources Iparison between ExperimentalResults and Nucleation Theories I 3.Nucleation of Nanoparticles in the Atmosphere K3.1.Atmospheric Measurements K3.1.1.Concentrations and Size Distributions ofAtmospheric Nanoparticles L3.1.2.Chemical Composition of AtmosphericNanoparticles M3.1.3.Measurements of Charged and NeutralAtmospheric Clusters Pboratory Studies R3.2.1.Binary Nucleation of H2SO4ÀH2O R3.2.2.Ternary Nucleation of H2SO4ÀH2O Involv-ing Ammonia and Amines T3.2.3.Nucleation of H2SO4ÀH2O Assisted byOrganic Acids U3.2.4.Nucleation of Iodine Oxides W3.2.5.Ion-Induced Nucleation X3.2.6.Chemical Composition,Reactivity,andThermodynamics of Nucleating Clusters Y3.2.7.Other Species AA3.3.Theoretical and Computational Studies AA3.3.1.Quantum Chemical Calculations AA3.3.2.Molecular Dynamics and Monte CarloSimulations AD3.4.Parameterizations of AtmosphericNucleation AF 4.Growth of Nanoparticles in the Atmosphere AG4.1.Role of the Kelvin(Curvature)Effect in Growthof Nanoparticles AH4.2.Condensation AI4.2.1.Condensation of Sulfuric Acid AI4.2.2.Condensation of Low-VolatilityOrganics AI4.3.Heterogeneous Reactions AJ4.3.1.Ammonia AJ4.3.2.Amines AJ4.3.3.Aldehydes AL4.3.4.α-Dicarbonyls AM4.3.5.Alcohols AN4.3.6.Other Species AO5.Numerical Treatment of Ambient NanoparticleNucleation and Growth Rates AP5.1.Measured Nucleation and Growth Rates AP5.2.Condensation Sink of Low-Volatility Vapor APbined Growth Including Condensationand Intramodal/Extramodal Coagulation AP5.4.Derivation of Nucleation Rates fromAtmospheric Measurements AQ 6.Summary and Future Research Needs AR Author Information AS Biographies AS Acknowledgment AT Glossary of Acronyms AT References AT Received:May17,20111.INTRODUCTIONThis review intends to critically assess recent findings related to nucleation and growth of atmospheric nanoparticles,with an emphasis on the understanding of these processes at a funda-mental molecular level.Aerosols (small particles suspended in air)can be directly emitted into the atmosphere from primary sources or be formed in the atmosphere through nucleation of gas-phase species.Aerosol nucleation events produce a large fraction of atmospheric aerosols.New particle formation occurs in two distinct stages,1i.e.,nucleation to form a critical nucleus and subsequent growth of the critical nucleus to a larger size (>2À3nm)that competes with capture and removal of the freshly nucleated nanoparticles by coagulation with pre-existing aerosols.Nucleation is generally de fined as creation of molecular embryos or clusters prior to formation of a new phase during the transformation of vapor f liquid f solid.This process is char-acterized by a decrease in both enthalpy and entropy of the nucleating system (i.e.,ΔH <0and ΔS <0).Hence,although thermodynamically favorable according to the first law of thermo-dynamics,(i.e.,exothermic)nucleation is hindered in entropy according to the second law of thermodynamics.A free energy barrier,ΔG (ΔG =ΔH ÀT ΔS >0),is often involved and needs to be surmounted before transformation to the new phase becomes spontaneous.Another major limitation in the nucleation and growth of atmospheric nanoparticles lies in signi ficantly elevated equilibrium vapor pressures above small clusters and nanoparti-cles,also known as the Kelvin (curvature)e ffect,which considerably restricts growth of freshly nucleated nanoparticles.Formation of molecular clusters occurs through random collisions and rearrangements of atoms or molecules of the existing phase (Figure 1a).Growth of a cluster can be repre-sented as a reversible,stepwise kinetic process.After reaching a critical size (the critical cluster or nucleus),further growth of the cluster becomes spontaneous.At each step,formation and decomposition of a cluster can be described by fundamental kinetic rate theories.A cluster can form homogeneously within the original phase or heterogeneously on various irregularities,such as pre-existing small particles or ions,which assist in surmounting the free energy barrier associated with formation of an interfacebetween the small cluster of the new phase and the original phase (Figure 1b).The lifetime of clusters is extremely short,but since a very large number of clusters form and dissociate at any time,a few can reach the critical size and continue to grow sponta-neously to form larger particles.Atmospheric nucleation of aerosols from vapors 1,2is,in principle,analogous to that of freezing of liquids,3crystallization of supersaturated solutions,4and formation of vapor bubbles inside the bulk liquid;5all proceed by the same basic mechanism.The common feature of the nucleation process is that there exists a dividing surface 6,7at the critical nucleus that separates the properties of the original and new phases.From an energetic perspective,the free energy of cluster formation,ΔG ,increases with cluster size prior to but decreases after the critical nucleus,reaching a maximal value at the critical size,i =i *.Hence,the critical nucleus can be identi fied if the free energy surface leading to cluster growth is available 6ð∂ΔG =∂i Þi ¼i ü0:ð1:1ÞThe properties of the critical nucleus are central to nucleation theory.The rate at which nucleation occurs is related to the chemical makeup of the critical nucleus and the gaseous con-centrations of the nucleating species and is an important variable in simulations of aerosol formation in atmospheric models.1Nucleation from the vapor phase is homomolecular when a single type of a gas is involved in formation of a critical nucleus and heteromolecular when several types of gases are involved in formation of a critical nucleus.In the absence of existing heterogeneities,homomolecular nucleation requires an extre-mely high supersaturation.For instance,homogeneous nuclea-tion of pure water vapor requires a supersaturation of a few hundred percent.Since such a condition is hardly realized in the atmosphere,homomolecular nucleation of water vapor,leading to formation of cloud droplets,is always heterogeneous in nature,taking place on pre-existing water-soluble seeds,i.e.,cloud condensation nuclei (CCN).In fact,clouds would have never formed in the Earth ’s atmosphere in the absence of CCN.Homogeneous nucleation of atmospheric nanoparticles,the focus area of this review,is always heteromolecular,involving two (binary),three (ternary),orpossibly more mutually interactingFigure 1.Schematic representation of the transformation from the molecular complex through the critical nucleus to 2À3nm nanoparticle (top)and associated free energy variation (bottom).(Reprinted with permission from ref 1.Copyright 2010American Association for the Advancement of Science.)vapors (multicomponent).The abundance,volatility,and reac-tivity likely determine the potential of a chemical species as a nucleation precursor.Atmospheric aerosol formation is closely linked with the gas-phase chemistry because the abun-dances required for nucleation to occur are achieved through a gradual increase in the concentration of the nucleating vapors produced from photo-oxidation of atmospheric gases,such as sulfur dioxide and volatile organic compounds (VOCs),includ-ing many saturated,unsaturated,or aromatic hydrocarbons,SO 2þOH f O 2,H 2OH 2SO 4ð1:2ÞVOCs þOH f O 2oxidized organicsð1:3ÞThe most common nucleating species is sulfuric acid because of its low vapor pressure at typical atmospheric temperatures,which is further reduced in the presence of water due to the large mixing enthalpy of these two substances.8À10The pre-sence of gaseous H 2SO 4in concentrations exceeding 105molecules cm À3has been shown as a necessary condition to observe new particle formation in the atmosphere.11,12In addition to sulfuric acid,a number of other nucleating pre-cursors,including atmospheric ions,ammonia,amines,organic acids,and iodine oxides,have been proposed to be involved in formation of the critical nucleus under di fferent ambient environments.The size and chemical make up of atmospheric critical nuclei are not well-known presently,because of the lack of existing analytical methods to directly probe the critical nucleus.Indirect measurements and theoretical calculations suggest that the critical nucleus has a diameter on the order of 1nm and consists of a relatively small number of molecules held together by noncovalent van der Waals (vdW)interactions.Since the molecules of known nucleating vapors possess a signi ficant dipole moment and/or contain a hydrogen atom connected with an electronegative atom (nitrogen or oxygen),electrostatic,polarization,and hydrogen-bonding interactions have been recognized to play a signi ficant role in formation of the smallest clusters.As clusters grow,proton transfer from an acid moiety (e.g.,H 2SO 4)to a base moiety (e.g.,H 2O or NH 3)becomes possible because the resulting ion pair is stabilized by interactions with surrounding polar molecules (e.g.,H 2O)within the cluster.Formation of an ion pair can signi ficantly increase the nucleation rate by reducing the free energy of the critical nucleus.However,current understanding of the role of proton transfer and other possible chemical processes in the nucleation of atmospheric clusters is still inadequate.Aerosol nucleation events,which are re flected as episodes with very high concentrations (up to 104particle cm À3or higher)of nanoparticles generated in a short period of time,are frequently observed in the free troposphere and under remote,urban,forested,and marine environments of the lower troposphere.Thermodynamically stable larger clusters and small nanoparti-cles formed during a nucleation event need to grow quickly so that they are not scavenged by coagulation through collisions with existing larger particles.The surface of pre-existing particles also acts as a condensation sink for nucleating vapors,reducing their concentration and inhibiting nucleation.Whereas conden-sation of low-volatility vapors and reversible partitioning of semivolatile vapors are commonly recognized as the major contributors to growth of aerosols,the role of heterogeneous chemical reactions between gas-phase chemical compounds andparticles is not well understood and is a subject of intensive research.13When reaching a size of about 50À100nm,aerosols become e fficient light scatterers and CCN.14Overall,during the atmospheric lifetime,the size of particles may vary over 5orders of magnitude,from a lower limit of about 1nm corresponding to stable molecular clusters to an upper limit of about 1mm for cloud droplets.Growth of nanoparticles driven by condensation,partitioning,heterogeneous chemical reactions,and coagulation is another focus area of this review.Atmospheric aerosols have profound impacts on the Earth Àatmosphere system,in fluencing the weather,climate,atmo-spheric chemistry and air quality,ecosystem,and public health.15Those particles cool the atmosphere by directly scattering a fraction of the incoming solar radiation back to space,an e ffect commonly referred to as direct climate forcing.By acting as CCN and ice nuclei (IN),aerosols play an important role in controlling cloud formation,development,and precipitation,impacting the albedo,frequency of occur-rence,and lifetime of clouds on local,regional,and global scales,16À21which is often referred to as indirect climate forcing.Presently,the aerosol direct and indirect e ffects represent the largest uncertainty in climate predictions.22Also,chemical reactions occurring on the surface or in the bulk of aerosols 23,24may alter the properties of aerosols and the gaseous composition of the atmosphere.For example,hetero-geneous reactions on particle surfaces convert inactive chlorine species into photochemically active forms in the middle atmo-sphere (between 20and 50km altitudes),leading to depletion of stratospheric ozone,25À35which acts as a UV shield.In the lower atmosphere (below 20km),particle-phase reactions can modulate formation of tropospheric ozone,36À41which is a key criteria air pollutant.On the regional and local scales,fine particulate matter (i.e.,aerosols smaller than 2.5μm or PM 2.5)represents a major contributor to air pollution.42Elevated concentrations of PM 2.5cause degradation in visibility,exacer-bate accumulation of pollutants in the planetary boundary layer (PBL),and adversely a ffect human health.43Increasing evi-dence has implicated aerosols not only in aggravation of existing health symptoms but also in the development of serious chronic diseases.44When inhaled,aerosols can amplify the adverse e ffect of gaseous pollutants,such as ozone,45and the smallest particles cause the most severe health impacts 46because they have higher probability than larger particles to deposit in the pulmonary region and penetrate into the bloodstream.47,48Several previous review articles have provided a detailed account of di fferent aspects of new particle formation in the atmosphere,including field measurements of atmospheric aero-sols and nucleation events,49À51coastal new particle formation,52the relation between laboratory,field,and modeling nucleation studies,53,54and the role of di fferent types of nucleation processes in the atmosphere.55Over the past few years,there has been substantial research progress in the area of atmospheric aerosol nucleation,including development of novel detection methods for atmospheric nanoparticles and clusters,as summarized in a recent review by Bzdek and Johnston.56Advances in analytical instruments have led to a number of laboratory and field studies that produced exciting yet often contradictory results regarding the compositions of the critical nucleus and the role of sulfuric acid and other species in the nucleation and growth of nanoparticles.57À61In the present review,we first provide the background information on theoretical and experimental ap-proaches toward investigation of homogeneous vapor nucleationand then introduce recent advances in nucleation and growth of atmospheric nanoparticles.Throughout this review,we strive to present the various nucleation aspects from a fundamental chemical prospective.Since there is a vast body of literature in the area of atmospheric aerosol nucleation,we do not attempt to be inclusive to cover all available publications on this subject.Instead,we choose in this review to focus on the studies that make the most important advances in this field.In section 2,we introduce the nucleation theories and illustrate how the predicted nucleation rates are related with results of laboratory experiments for a number of simple nucle-ating systems.Nucleation of atmospheric aerosols,including ambient measurements,laboratory experiments,and theoretical studies,is described in section 3.Results of laboratory experi-ments and ambient measurements of nanoparticle growth are presented in section 4,and section 5provides the numerical approaches developed to connect measured aerosol nucleation and growth rates.Section 6contains the concluding remarks and describes future research needs.A glossary of acronyms is provided at the end of the review.2.OVERVIEW OF VAPOR NUCLEATION2.1.Nucleation Theories and Computational ApproachesIn the absence of heterogeneities,formation of a new phase occurs through random fluctuations in the vapor density,generating clusters that can grow or decay by gaining or losing a monomer molecule.Growth of the cluster can be represented by a reversible,stepwise kinetic process in a single or multicomponent system:::C s f þA i À1,k þi À1i À1r s k ÀiC Ãs fþA i ,k þi i r s k Ài þ1C i þ1:::ð2:1Þwhere A i À1denotes a monomer species to be added to the clusterC i À1at the (i À1)th step and k i Àand k i +represent the cluster decomposition and association rate constants,respectively.A complete nucleation theory can be established to describe the evolution of the population of clusters,i.e.,the rates and mechanism by which these clusters grow and decay.As re flected by equation 1.1,the free energy of the nucleating system reaches a maximum (i.e.,the nucleation barrier)when the critical nucleus forms.In addition,a multicomponent system may exhibit multiple nucleation barriers,leading to further complication in the identi fication of the critical nucleus on the basis of the free energy surface of the cluster growth.Kinetically,at the critical nucleus,the rate to form the (i +1)th cluster is equal to that of decomposition of the critical nucleus to form the (i À1)th cluster,i.e.k Ài ½C Ãi ¼k þi ½A i ½C Ãið2:2Þwhere [A i ]and [C i ]are the number concentrations of the associating monomer and the cluster of size i ,respectively.Furthermore,since the molecular flux between adjacent clusters achieves the minimum at the critical nucleus (commonly referred to as a bottleneck 7),another practical approach to locate the critical nucleus is to variationally minimize the molecular flux as a function of the cluster site d F i =d i ¼0ð2:3Þwhere F i is the number of clusters growing from a size i to a size i +1per second.The rate of nucleation,J ,is de fined as the rate of growth of the critical nucleusJ ¼k i þ½C i Ãð2:4ÞThe association and decomposition rate constants can be calculated employing the kinetic rate theories,such as transition state theory (TST).62For each cluster,the association rate is related to the dissociation rate by detailed balance 63À70k þi À1k Ài ¼Q C Ãi Q C i À1Q A i À1exp D C ÃikT ð2:5Þwhere Q C i *is the partition function of the critical nucleus,Q C i À1andQ A i À1are the partition functions of the respective (i À1)th cluster and monomer,k is the Boltzmann constant,T is the temperature,and D C i *is the binding energy of the critical nucleus relative to monomer and (i À1)th cluster.The decomposition rate constant of each cluster can be calculated according to the following expression 63À70k Ài ¼kT h Q qC ÃiQ C Ãi exp ÀΔE kT ð2.6Þwhere Q C i *q is the partition function of the transition state,h is the Planck constant,and ΔE is the transition state energy relative to the critical nucleus.In the case that the association reaction proceeds without an activation barrier (a loose transition state),the location of the transition state can be determined variationally by minimizing the decomposition reaction rate constant using canonical variational transition state theory (CVTST).71The partition functions required for eqs 2.5and 2.6can be evaluated by treating the rotational and translational motion classically and treating vibrational modes quan-tum mechanically.Vibrational frequencies,moments of inertia,and reaction energies can be taken from quantum chemical calculations.72An example of such an approach is the dynamical nucleation theory (DNT)of Kathmann et al.,73À75which uses CVTST to locate transition states and calculate evaporation rate constants k i Àfor each stage of the nucleation process.Depending on the assumptions and approximations made,three major types of theoretical approaches have been estab-lished to characterize the nucleation process.Phenomenological theories,e.g.,classical nucleation theory,attempt to obtain the free energy of formation of the critical nucleus from macroscopic parameters,such as the surface tension and the bulk liquid density.Some kinetic theories derive the cluster distribution and hence the nucleation rate by calculating rate constants for association and decomposition of clusters,avoiding explicit evaluation of cluster formation energies from macroscopic parameters.Molecular-scale approaches,including molecular dynamics,Monte Carlo simula-tions,and density functional theory,apply first principles to calculate the cluster structure and free energy of cluster formation.2.1.1.Classical Nucleation Theory.The classical nuclea-tion theory (CNT)was formulated by Becker and D €o ring 76and Frenkel 77on the basis of the kinetic theory of nucleation established by the work of Volmer and Weber 78and Farkas.79CNT includes the thermodynamic and kinetic components by evaluating the free energy change of formation of a nascent phase cluster and calculating the nucleation rate.The phenomenologi-cal approach to CNT describes the nucleation process in terms of the change in Gibbs free energy of the system upon transfer of i molecules from the vapor phase to an i -mer cluster of radius r ΔG ¼ÀikT ln S þ4πr 2σð2.7Þwhere S =p A /p A S is the saturation ratio,p A is the vapor pressure of substance A in the gas phase,p A S is the vapor pressure ofsubstance A over a flat surface of the corresponding liquid,and σis the surface tension.Although the cluster may consist of only a few molecules,it is assumed to have sharp boundaries and the same physical and chemical properties as the bulk phase (capillarity approximation).For a spherical cluster,the number of molecules i can be explicitly related to its radius i =(4/3)πr 3/v l ,where v l is the volume of a single molecule in liquid.Equation 2.7is one of the forms of the Kelvin equation,which expresses the ele-vation in the saturation vapor pressure above a curved surface,such as the interface between a small liquid droplet and surrounding air.The free energy change of the cluster formation given by the right-hand side of eq 2.7consists of two terms.The first term represents the energy decrease upon the transition from vapor to liquid and may be either negative or positive,depending on the vapor saturation ratio.The second term,which is related to the excess of the free energy at the liquid/vapor interface,is always positive.When vapor is subsaturated (S <1),the free energy of cluster formation is always positive and condensation is prohib-ited (Figure 2).If the system is supersaturated (S >1),the free energy term is negative,favoring condensation of vapor mol-ecules and growth of the embryonic droplet.For very small particles,the increase in the free energy due to formation of the new surface area dominates over the free energy decrease from bulk phase formation,resulting in an energy barrier to nucleation.For droplets with size greater than the critical radius r *,the condensation term dominates,leading to a decrease in ΔG as shown in Figure 2.The free energy of cluster formation ΔG reaches a maximum at r *,and the location of the critical nucleus can be determined by di fferentiation of eq 2.7with respect to r r ü2σv l kT ln Sð2.8ÞThe corresponding number of molecules i *at the critical size and free energy barrier height ΔG *are given in the following relations i ü32πσ3v 2l 3ðkT ln S Þ3ð2.9ÞΔG ü4π3σr Ã2¼16π3σ3v 2l ðkT ln S Þ2ð2.10ÞThe critical nucleus at the top of the ΔG curve is in a metastableequilibrium with the vapor.If a single molecule is removed from the critical nucleus,the free energy decreases and the cluster decomposes.If a molecule is added to the critical nucleus,the freeenergy also decreases and the cluster continues to grow sponta-neously.The nucleation rate J can be de fined as the number of clusters that grow past the critical size per unit volume per unit timeJ ¼J 0exp ÀΔG ÃkTð2.11Þwhere J 0is the pre-exponential factor typically determined from gas Àkinetic considerations.The nucleation rate has a negative exponential dependence on the height of the free energy barrier.An increasing saturation ratio decreases the critical nucleus size and the height of the free energy barrier,resulting in a faster nucleation rate (eqs 2.10and 2.11).Classical nucleation theory can be applied to nucleation of multicomponent vapors.When several molecular species parti-cipate in nucleation,the chemical composition of the critical nucleus,which is usually di fferent from the vapor composition,becomes an additional degree of T of binary homogeneous nucleation is first introduced by Flood 80and further developed by Reiss.81The free energy change,ΔG*(i 1,i 2),associated with formation of a critical nucleus from binary vapor depends on the concentrations of molecules of both components,i 1and i 2.The critical nucleus is located at the saddlepoint on the ΔG*(i 1,i 2)surface and corresponds to the smallest cluster for which growth by addition of another mole-cule of vapor of either component is a spontaneous process.An alternative kinetic formulation of the classical nucleation theory can be obtained for cluster formation and dissociation corresponding to reaction 2.1d ½C i d t¼k þi À1½C i À1 ½A i À1 Àk Ài ½C i Àk þi ½C i ½A i þk Ài þ1½C i þ1ð2:12ÞIn the steady state,the concentrations of clusters of di fferent sizes are independent of time and the net rate,at which clusters C i become C i +1,is constant for all i .This simpli fication reduces the problem of calculating the nucleation rate to the derivation of the association and decomposition rate constants.82Whereas the association rate constant k i +can be calculated from first princi-ples,usually assuming that it is the gas Àkinetic collision rate,the decomposition rate k i Àrequires evaluation of the cluster stability,typically from the free energy change of cluster formation,based on the properties of bulk solutions.83For this reason,not only the resulting nucleation rate derived by the kinetic approach takes a general form given by eq 2.11but also it is exactly equivalent to the nucleation rate obtained within the framework of the phenomenological approach.The advantage of CNT lies in its simplicity.The CNT approach provides closed analytical expressions for the critical saturation and nucleation rate based on the free energy of critical nucleus formation derived from measurable bulk properties,readily available for many substances.Although CNT allows estimation of critical supersaturations reasonably well,it fails frequently,by many orders of magnitude,in reproducing mea-sured nucleation rates for a broad range of substances and experimental conditions.Speci fically,the nucleation rates are underestimated at low temperatures and overestimated at high temperatures,84and critical supersaturations are signi ficantly underestimated for strongly associated vapors,such as organic carboxylic acids.85One of the majorreasons for the poorFigure 2.Gibbs free energy change for formation of a droplet of radius rfrom unsaturated (S <1)and supersaturated (S >1)vapor;ΔG *corresponds to a critical nucleus of radius r *.。