Synthesis of Au Nanorods through Prereduction with Curcumin: Preferential Enhancement of Au Nanorod Formation Prepared from CTAB-Capped over Citrate-Capped Au Seeds
Rasha N.Moussawi and Digambara Patra*
Department of Chemistry,American University of Beirut,Beirut11072020,Lebanon
*Supporting Information
formation when the ascorbic acid and citrate capped seed
encourages nanorod formation for only CTAB capped seed
solution.A mechanism has been suggested for this
process by bringing new insight in the mechanism
may trigger better design of nanomaterials.
1.INTRODUCTION
One of the aims of materials science is de?ning and understanding the mechanisms that eventually dictate crystal shape.Without a?rm understanding of these formation processes,oriented production of crystals with the desired shape and crystalline structure will not be ful?lled.In particular, gold has attracted intense research because of its fascinating optical,electronic,and chemical properties as well as biocompatibility.1,2These properties nominate gold as an ideal candidate for promising applications in nanoelectronics, biomedicine,sensing,and catalysis.3,4
Various wet chemical techniques have been established to produce gold nanoparticles with various shapes such as rods,5?7 wires,8,9plates,10,11prisms,12,13cubes,14polyhedra,15,16and branched particles.17,18Gold nanorods have received tremen-dous attention in recent years due to their exciting potential applications in chemical sensing,biological imaging,drug delivery,and phototherapeutics.A“nanorod”is de?ned to be a nanoscale particle with an aspect ratio(length/width ratio) that is between1and～20?25;higher aspect ratio materials are termed as“nanowires”(with diameter<100nm).19Gold nanorods can be obtained via many methods,from lithographic deposition of thin gold?lms on a substrate followed by various chopping procedures20to electrochemical deposition in hard nanotube templates21,22to photochemical reactions in solution.Among the various ways,the seed-mediated growth approach introduced by Murphy and co-workers6is the state of art method for synthesis of gold nanorods.However,here choice of the reducing agent determines to a great extent the rate of formation of the nanorods and the?nal shape and anisotropy.Ascorbic acid(AA)is the secondary mild reducing agent usually used because it is too weak to reduce the additional gold salt in the growth step from Au3+to Au0alone. This allows for the growth to occur over a long time(minutes to hours),which aids in anisotropic growth.The structure-directing agent used is cetyltrimethylammonium bromide (CTAB).6Research on the seed mediated method has studied the e?ects of many parameters including the gold seed, concentration of reactants,temperature,duration of growth, additives,etc.23?27The choice of the capping agent used for Au seed is also an important factor that a?ects the yield of nanorods since it a?ects the crystal structure of the seed itself. Another approach is to use a silver-assisted method.5Low aspect ratio nanorods(～1?6,up to ca.90nm in length) starting from1.5nm CTAB capped Au seeds28,29can be synthesized with high yield(～90%)after two rounds of
Received:May8,2015
Revised:July31,2015
puri ?cation by centrifugation.19Nonetheless,the overall yield relative to the initial gold ions is less than 15%.5
Prominent work has been done as demonstrated above on gold nanorods;however,these studies illustrate the lack of fundamental understanding of how the kinetic,thermodynamic,or other factors in ?uence the underlying mechanism in these and other interesting systems.30?33Thus,considerable research is needed to explore the mechanisms governing morphology and geometry control over particle growth,because they are still not well understood.The present work could serve the purpose of gaining a better understanding on how di ?erent parameters are a ?ecting the formation of gold nanorods.Little research is done on the nature of the secondary reducing agent.In this work,we replaced the conventional (secondary)reducing agent such as ascorbic acid with curcumin.Recently we found curcumin to be a good candidate to reduce silver ions to synthesize Ag nanoparticles.34?36Importance of curcumin has been greatly realized in probe chemistry.37?39In addition to the reducing ability of curcumin,its safe pro ?le (at doses as high as 8g/day in clinical trials)40makes it attractive to use this molecule in the synthesis of nanorods as a reducing agent.We systematically changed some reaction variables,such as pH,concentrations of reactants,addition of silver,and the capping of the gold seed to prepare gold nanorods using curcumin as shown in Scheme 1.These modi ?cations have been carried out to explore any distinctive features arising and to help gain a better understanding of the reaction mechanism during nanorod synthesis.
2.MATERIALS AND METHODS
2.1.Materials.Gold(III)chloride hydrate (99.999%trace metal basis)was obtained from Aldrich.Cetyltrimethylammo-nium bromide (CTAB)(Acros Organics),ascorbic acid (Fluka),and curcumin (Sigma)were used as received.Trisodium citrate dehydrate and silver nitrate were obtained from Sigma-Aldrich.Sodium borohydride was obtained from Acros Organics.DMSO was obtained from Merck,and absolute ethanol was obtained from Sigma-Aldrich.All these chemicals
were used directly without further puri ?cation.Stock solutions were prepared in deionized water unless otherwise indicated.2.2.Synthesis of Gold Nanorods from Citrate-Capped Gold Seeds (Procedure A).In a typical experiment,2.5mL of a 0.01M H[AuCl 4]·3H 2O solution and 3.6445g of CTAB were replenished in a 100mL volumetric glass ?ask.For completely dissolving the CTAB powder,it was necessary to keep this solution at elevated temperature (33°C)for at least 2h.This solution,which contained 0.1M CTAB and 2.5×10?4M H[AuCl 4]·3H 2O,was used as growth solution .During the dissolution of the CTAB in the above growth solution,a seed solution containing small gold particles was prepared as follows:a volume of 0.5mL of 0.01M H[AuCl 4]·3H 2O solution (5×10?6mol,volumetric glass ?ask,not cooled)was added to 18.4mL of ice-cooled deionized water in a wide-necked PE-?ask (PE =polyethylene).Thereafter,0.5mL of freshly prepared 0.01M sodium citrate solution (5×10?6mol,solution in a volumetric glass ?ask,not cooled)was added.Then immediately 0.6mL of freshly prepared 0.1M NaBH 4solution (in a glass vessel)cooled in an ice-bath was added,and the resulting solution was stirred for 30s.The solution turned red (after 24h,particles of ～8nm formed).For the following ?nal reaction steps,cooling was not required:80μL of a freshly prepared 0.1M curcumin solution (8.0×10?6mol)was added without stirring to 14.4mL of the growth solution (age between 2and 24h,containing 3.6×10?6mol of gold).The solution was mixed by shaking,whereupon the solution became brighter yellow.After addition of 16μL of seed solution (age between 2and 24h,containing 4.0×10?9mol of gold),the solution was mixed by shaking and became brownish.Then the test tubes were placed in a water bath at 33°C overnight and then stored in a refrigerator (temperature ≈6°C)to precipitate a major fraction of CTAB.Samples were withdrawn from the bottom precipitate and centrifuged at low speeds to separate the rods from the spheres.However,this method was not very e ?cient in separation.To investigate the e ?ect of pH,the same procedure was done except the growth solution containing the curcumin was adjusted to pH 9.2?9.6.
Scheme 1.Illustration of Au Nanorod Formation by Using Curcumin as Secondary Reducing Agent through Seed Mediated
Method
2.3.Synthesis of Gold Nanorods from CTAB Capped Gold Seeds(Procedure B).The gold nanoparticle seeds were prepared following a literature procedure.41Brie?y,250μL of a 10.0mM aqueous HAuCl4solution was added to a plastic
conical centrifuge tube.To this,7.50mL of a100mM aqueous CTAB solution was added.The mixture of these reagents changed the solution color from bright yellow to deep orange. To this,600μL of a freshly prepared,ice-cold10.0mM NaBH4 solution was added,which immediately changed the solution color to pale brown.The reaction was then left undisturbed for ～3.0h to ensure complete Au3+reduction;however,the seeds were used within2?4h after synthesis.After seed generation, Au nanorods were prepared in a25.0mL scale.For this,23.75 mL of100mM CTAB was mixed with1.0mL of10.0mM HAuCl4in a conical centrifuge tube.To this,150μL of10.0 mM AgNO3and160μL of100mM curcumin(in ethanol) were added.All of these solutions were freshly prepared and added to the reaction in the listed order,and the system was mixed by inversion after the addition of each solution.Finally, 105μL of the Au seed solution was added followed by gentle mixing,after which the reaction was left undisturbed for at least 3.0h at25.0°C before use.Washings were done with1?2mM CTAB as they preserve the nanorods.When changing the initial concentrations of reactants one at a time,we slightly changed the way we prepared CTAB capped gold seeds for the sake of comparison with literature.42This was done as follows: Typically,a10mL solution of Au seeds was prepared by the reduction of HAuCl4·3H2O(2.5×10?4M)by ice-cold NaBH4 (6.0×10?4M)in the presence of CTAB(7.5×10?2M).The NaBH4solution was added at a time to the solution containing CTAB and HAuCl4,and the reaction mixture was then shaken (or magnetically stirred)for2min allowing the escape of the gas formed during the reaction.These seeds were4nm or smaller in diameter.
2.4.Characterization.Scanning electron microscopy (SEM)analysis was carried out using Tescan,Vega3LMU with Oxford EDX detector(Inca XmaW20)SEM.The sample was deposited on a carbon?lm for SEM analysis.The absorption spectra were recorded at room temperature using a
JASCO V-570UV?vis?NIR spectrophotometer.Similarly, UV?visible di?use re?ectance spectra were measured using a JASCO V-570UV?vis?NIR spectrophotometer in the range 200to800nm.The steady-state photoluminescence spectra (excitation and emission)were recorded at room temperature using Jobin-Yvon-Horiba Fluorolog III?uorometer and the FluorEssence program where the excitation and emission slits width were5nm.The source of excitation was a100W xenon lamp,and the used detector was R-928operating at a voltage of 950V.
3.RESULTS AND DISCUSSION
3.1.Gold Nanorods Prepared from Citrate Capped Au Seeds.The interesting optical properties of metals in general stem from the excitation of the free electrons on their surface, which results in surface plasmon resonance(SPR).43?50Factors such as the particle’s size,shape,and particle-to-particle interaction dictate the surface plasmon modes of metal nanoparticles.51?62Thus,UV?vis absorption spectrum has been a fast,easy,and reliable tool for characterization of Au and Ag nanoparticles.Figure1a shows the UV?vis absorption spectrum of citrate capped Au seed,which is described in experimental section.Gold nanorods were synthesized from these citrate capped Au using curcumin as the secondary reducing agent as speci?ed in procedure A.This procedure was adopted because it has rendered the seed mediated synthesis procedure simpler,decreased the susceptibility to impurities, and improved the reproducibility of the product distribution. UV?vis absorption spectrum is also sensitive to the shape and aggregation states of the materials and will shift or change the position of the plasmon bands.Anisotropic(nonspherical shapes constituting nanocubes,nanoprisms,nanorods,and nanowires)particles allow for a greater degree of tunability of the SPR while keeping almost the same volume in comparison to spherical particles that show a limited tunability accom-panied by a signi?cant change in volume;thus,preparing anisotropic gold particles has been a great interest.Gold nanorods give rise to two absorption bands.One of them is caused by transverse oscillations of the electrons(transverse absorption band),located in the visible wavelength region,and can interfere with the absorption of spherical gold particles. The other is due to the longitudinal oscillations of the electrons (longitudinal absorption band),which shifts from the visible toward the near-infrared region with increasing aspect ratio. The longitudinal absorption band of such particles in UV?vis?NIR absorption spectra falls in the absorption region of water (ca.1300to above2500nm)and is therefore not accessible in the aqueous solutions?nally resulting from the
nanorod Figure1.(a)UV?vis absorption spectrum of citrate capped Au seed and(b)comparison of UV?vis absorption spectra of Au NRs prepared from citrate capped Au seeds reduced with curcumin(Cur)and ascorbic acid(AA)(procedure A).
synthesis.When AA was used to prepare Au nanorods,UV?vis absorption spectrum showed peaks in the region of500?700 nm to be“attributed to the more or less spherical particles, triangles,and the transversal plasmon absorption of the nanorods and that around1955nm to the longitudinal absorption of the nanorods”(a technique based on embedding the particles in poly(vinyl alcohol)(PVAL)was used to detect bands beyond1000nm63).In our case the nanorods reduced with AA displayed two plasmon bands at552nm for the transverse surface plasmon and759nm for the longitudinal surface plasmon in the visible region of the spectrum as shown in Figure1b.The longitudinal band for nanorods reduced with curcumin did not show clearly and the transverse band was lower in intensity than that of the AA reduced-Au NRs due to probably loss of material during decanting.It also shows an additional peak at～404nm attributed to curcumin absorption. The longitudinal band might be located beyond1000nm.
In SEM,spherical particles of30?60nm were detected alongside fewer triangular particles for curcumin mediated Au particle synthesis,which is similar to results obtained on seed-mediated growth of gold nanorods using ascorbic acid.63The aspect ratio distribution of nanorods was broad as seen in Figure2a?e;high to low aspect ratio nanorods with varying lengths(100?800nm)and widths(30?40nm)were obtained. The yield was relatively low.When AA was used,nanorods of aspect ratios around6?10were obtained.63In addition signi?cant quantities of spherical(of typically30to60nm) and,to a lesser extent,triangular particles also arose.A particularly interesting thing here to notice in Figure2d(closer look is given in Figure2e),besides the apparent layer around the nanorods(which is the CTAB bilayer formed around)was the ghostly nanorod form so to speak.We could not relate this shadowy template but to the CTAB micelle formed before any gold becomes adsorbed on it.This could be a hint to the mechanism of formation of the gold nanorods.
In fact,there are several mechanisms proposed in the literature.Garg and co-workers constructed a structural model based on the HR-TEM results and proposed a mechanism for the seeding growth of[110]oriented,single crystalline Au nanorods.19The CTAB capped Au seeds are typically quasi-spherical particles(～2nm)enclosed by low-index[111]and
[100]facets;with the[111]facets possessing the lowest surface energy(hence,the most stable facet)among the three types of facets.The[110]facets have the highest surface energy,and thus it is not favored that these facets be exposed;the[100] facets come in between.One of the major roles of Br ions in the nanorod growth process is their etching interaction with the Au seeds,which leads to small single crystalline,polyhedral particles at the early stage of the growth process.Twinned particles may be dissolved due to Br etching,and this etching process occurs much more easily on smaller seeds because larger particles are more di?cult to etch by Br to form the right type of polyhedral seeds for rod growth.Since the Au[111]and Au[100]facets are more stable than[110],it is expected that the single crystalline seeds prefer to elongate along the[110] direction to maximize[111]facets.So,?110?elongation of the single crystalline seeds results in[111]and[100]side facets.As the side facets are passivated by Br,the radial(or side)growth is retarded,and further elongation along the[110]direction is facilitated.The preferred growth of[110]nanorods,as opposed to[100]or[111]nanorods,was energetically favorable according to the theoretical calculations by Barnard and Curtiss.64It is thought that CTA+mainly provides steric protection of the formed nanorods,but its templating role could not be dismissed.On the other hand,Murphy et al.have proposed a di?erent mechanism that falls in line with the above stated one since it suggests that CTAB would bind preferentially to the Au[100]and[110]seed facets and then the thermodynamically favorable intermolecular interactions of the16-carbon cetyl chains take action to promote surface adhesion.This preferential binding regime agrees with the size of the quaternary ammonium headgroup relative to the larger binding sites available on the Au[100]and Au[110]faces of the crystalline rods than at the[111]face.25,28
However,our result stands opposite to the above mechanisms for nanorod growth,and the cylindrical CTAB micelle-templated based growth mechanism seems to be operative rather than the preferential binding of CTAB molecules onto certain facets of developing NRs adopted when AA is used.It could be that these two mechanisms exist and one of them would take place under certain conditions.It is suggested that the presence of curcumin could have favored the cylindrical CTAB micelle growth(seen in SEM image)in some way.In fact,it has been shown that introducing certain aromatic compounds could modify(improve)the
micellization Figure2.(a?e)SEM images of gold nanorods prepared from citrate capped Au seeds obtained from di?erent regions(procedure A).
behavior of CTAB surfactant.65This suggestion is consistent with the proposed mechanism by Liz-Marzan and co-workers that considered the CTAB micellar structure promoted the deposition of the metal at the tips of gold seed particles that are also surrounded by CTAB.66The mode of action proposed is that AuCl4?ions displace Br?ions and then tightly bind to CTA+micelles.Upon addition of curcumin,AuCl4?is reduced to AuCl2?at the micelle surface,and the rate of growth of the di?erent nanorod facets would be determined by the approach of the micelle,and thus gold species,toward the facets of the seed that is also covered with CTAB.Liz-Marzan et al. supported this mechanism by performing calculations for the surface potential of metal ellipsoids in1mM NaCl,which showed that this potential decays more rapidly at the nanorod tip than at the sides,and thus the micelle can more readily approach the tips of the rods than the sides and allow deposition of gold.19It is crucial,however,to have initially a twinning plane or stacking fault in the seed to cause an asymmetric electric?eld.28,66
3.1.1.E?ect of pH.The slow reduction of gold is crucial for nanorod formation,and mild reducing agents are used for this purpose.As a con?rmation,we increased the reducing ability of curcumin by adjusting the pH to its p K a value(～9.3?9.6).As expected,no nanorods were observed,and much smaller spherical particles(see Supporting Information,Figure S1a) appeared as further con?rmed by the blue shift in the UV?vis spectrum(band at505nm,see Supporting Information,Figure S1b).
3.1.2.E?ect of Seed Concentration.Previous results using ascorbic acid did not provide a de?nite trend for increasing the seed concentration.63Typically,increasing the seed concen-tration resulted in a decrease in AR of NRs.However,the results of some reports were contrary to expectations where AR increased upon increasing the amount of seed.In our case,we varied the seed concentrations as given in Table1(1s?3s);
when we decreased the seed concentration from2.76×10?7M (in sample1s)to2×10?7M(in sample2s),there was an increase in the AR and yield of NRs;however,further decreasing to1×10?7M reversed the outcome seen in Figure 3a?c.There is no clear trend of NR formation with the seed concentration at least in our case.The UV?vis spectra of the samples is in agreement with what is seen in SEM images.The thinnest NRs observed(see Supporting Information,Figure S2)have the transverse PB blue-shifted,while the thicker NRs of the other samples lie at higher wavelengths.
3.1.3.E?ect of Curcumin Concentration.Curcumin concentration was varied as given in Table1(1cur?3cur). Sample1cur shows high AR gold NRs and spherical NPs(20?30nm)(see Supporting Information,Figure S3a).Upon doubling the curcumin concentration in sample2cur(see Supporting Information,Figure S3b),we noticed a tremendous decrease in AR in accordance with what is seen in literature.29 Further increasing curcumin by5-fold(sample3cur,see Supporting Information,Figure S3c)resulted in larger spherical particles(50?70nm).In the UV?vis absorption spectra(see Figure4a),absorption peaks in the region of500?700nm correspond to the more or less spherical particles,triangles,and the transversal plasmon absorption of the nanorods.So shoulders or broad bands seen in the spectra are basically attributed to the minority shapes present in the sample. Speci?cally,shoulders at656and795nm for samples2cur and 3cur,respectively,are attributed to the triangular shapes observed more clearly in the sample containing the highest curcumin concentration(Figure4a).The band at537nm for 1cur red-shifted to549and568nm,respectively,for higher curcumin concentration,indicating spherical size increase.As the concentration of curcumin increases,there is a faster supply of gold to the seeds resulting in shorter rods and larger spheres and triangles(morphological change).Similar observations have been reported for ascorbic acid.29The photoluminescence spectra of1cur,2cur,and3cur at excitation wavelength425 nm are shown in Figure4b.All of them gave an emission maximum around550nm and another band around670nm. The maximum around550nm is due to curcumin conjugated gold nanoparticles that includes nanorods,triangles,and other shapes,whereas the emission at around670nm is due to gold particles/nanorods(triangles,etc.).The?uorescence intensity at around550nm provided interesting information;as the curcumin concentration increased to2-fold(for2cur)and subsequently10-fold(for3cur),the?uorescence intensity similarly enhanced roughly in the same order.This indicates that the amount of curcumin conjugated to gold particles depends on the curcumin concentration.As more curcumin is present in the solution,the attachment of curcumin per gold nanoparticles increases where it does not necessarily transfer into gold nanorods solely.In other words increase in curcumin concentration per gold discourages formation of gold nanorods.
3.1.
4.E?ect of Silver.It is noted that adding AgNO3would increase the nanorod yield but would at the same time decrease the aspect ratio of the rods(aspect ratio achievable is typically <6versus～25without AgNO3).28However,there is a critical Ag+concentration,above which the aspect ratio of the nanorods decreases again.5In fact,silver also a?ects the growth mechanism,the crystal structure of nanorods,and the results of varying di?erent parameters.Because the addition of silver into the growth solution is thought to alter the chemistry at the interface of the growing particle and the growth solution,it is convenient to di?erentiate seed-mediated approaches per-formed in the absence and in the presence of silver.Silver was added at di?erent concentrations as provided in Table2in an attempt to understand its e?ect.We expected a rise in the yield of nanorods;however,this addition caused the total disappearance of rods at high Ag concentrations(2A?4A)(see Figure5a?d).At low Ag concentration(1A,see Figure5d), few nanorods appeared,much less than the silverless procedure (compare with1s,see Figure3a).Although the silver concentrations used are similar and even lower than those used in the literature,it seems that the critical Ag+
Table1.Concentrations of Reactants in Growth Solutions Di?ering in Seed and Curcumin Concentration
sample [CTAB]
(mM)
[Au]seed(10?7
M)
[Au3+]
(mM)
[curcumin]
(mM)
1s100 2.760.250.533 2s10020.250.533 3s10010.250.533 1cur10020.250.533 2cur10020.25 1.067 3cur10020.25 5.330 11s160.1250.23
22s160.250.23
33s16 2.500.23
11cur160.1250.26
22cur160.1250.23
concentration when curcumin is used is much below the concentrations used,or it might be due to an interaction of curcumin with silver.No trace of silver was detected in EDX (see Figure 6),which goes along with the assumption that silver may be directly interacting with curcumin instead of assisting in
gold nanorod formation.Since this interaction a ?ects the nanorod growth,it is expected to result in smaller spherical particles as seen in SEM images and in the blue shift of the surface plasmon band (SPB).In the UV ?vis absorption spectrum (see Figure 7a),all the samples exhibited a band around 540nm,and only 1A showed LSPB at around 983nm since the rods are shorter than those in the silverless procedure.The bands seen at around 405nm is due to curcumin left after washing.Except a change in ?uorescence intensity,
the
Figure 3.SEM images of samples (a)1s ,(b)2s ,and (c)3s .Refer Table 1for experimental
conditions.
Figure 4.UV ?vis absorption spectra (a)and photoluminescence (emission)spectra (b)of samples 1cur ,2cur ,and 3cur .For photoluminescence measurement,the excitation wavelength used was 425nm.Refer to Table 1for experimental conditions.
Table 2.Concentrations of Reactants in Growth Solutions Di ?ering in Silver Content
sample [CTAB](mM)[Au]seed (10?7M)[Au 3+](mM)[AgNO 3](10?5M)[curcumin](mM)1A 10020.250.160.532A 100 2.760.25 1.40.533A 10020.25 1.60.534A
100
2.76
0.25
2.7
0.53
Figure 5.SEM images of (a)2A ,(b)4A ,(c)3A ,and (d)1A .Refer to Table 1for experimental
conditions.
Figure 6.EDX of gold nanoparticles made from citrate capped gold nanoparticles in the presence of silver.
?uorescence spectra shown in Figure 7b did not show any di ?erence in the spectral shape at low (with Au nanorod formation,1A )and high (without Au nanorod formation,3A )concentration of silver (10-fold increase in concentration).The band at around 670nm was not clear due to low intensity in this region.The increase in ?uorescence intensity suggests that reduction in formation of nanorods increases the possibility of smaller nanosphere formation that increases the overall surface area,thus re ?ecting higher curcumin conjugation with gold.3.2.Gold Nanorods Prepared from CTAB Capped Au Seeds.Gold nanorods were synthesized from CTAB capped Au seeds using curcumin as the reducing agent as indicated in procedure B.Interestingly,much higher aspect ratio nanorods were obtained when curcumin was used as the secondary reducing agent compared with ascorbic acid.41The average length,width,and aspect ratio of the nanorods were determined by counting the nanostructures in several images.Nanorods of ～30nm by 410nm were obtained (～30%yield)(see Figure 8).The average length,width,and AR of the nanorods are 410nm (spanning from 118to 848nm),30nm (in the range 17?44nm),and 15(from 3to 32),respectively.Spherical particles of ～60nm made up the majority of the nanostructures.The gold NRs reduced with curcumin displayed a transverse PB around 550nm that probably is also attributed to the spherical particles (see Figure 9).Having obtained a higher AR,the longitudinal band would appear in the IR region.To understand the growth mechanism,morphology from the initial seed particles to nanorod formation was monitored.It
should be noted that formation of gold nanoparticles gave a UV ?visible absorption maximum at 512nm and during the preparation of seed solution the absorbance at 512nm increased continuously for 3h and then afterward it saturated.This enhancement is due to formation of more gold nanoparticles.The morphology of the initial seed solution is shown in Supporting Information ,Figure S4a.When the seed solution was used in the growth solution for gold nanorod preparation and the reaction was stopped after 2min to 1h,the nanoparticles could not be separated using centrifugation method adopted in this work.In this case,no nanorod was detected and the transverse PB was found at ～512nm,which was not di ?erent from seed solution.However,after 2h of reaction in growth solution,relatively large size spherical gold nanoparticles along with a few nonspherical structures such as sphero-cylindrical or triangular shaped particles were observed (see Supporting Information ,Figure S4b).This is in accordance with an earlier report that all seeds grow isotropically until a critical size for transition is reached.33Here,the transverse PB rapidly red-shifted to around 552nm.After 3h of reaction,this band shifted to 550nm and longer gold nanorods were observed along with spherical and triangular shape particles (see Supporting Information ,Figure S4b).The particles that grew isotropically in all directions gave large spherical shaped particles due to clusters with [111]facets.33Those particles that grew in one direction gave longer but thinner nanorods because curcumin facilitates cylindrical CTAB micelle growth.These cylindrical micelles support growth of the particles in one direction giving rod shaped particles as discussed earlier (see Supporting Information ,Figure S4c).Unlike case of hydrophilic ascorbic acid,curcumin is also immediately available to AuCl 4?ions at the micelle surface because of its hydrophobic nature,which facilitates reduction of AuCl 4?to AuCl 2?at the micelle surface containing seed particles.Thus,curcumin has a better control to form nanorods compared with ascorbic acid.However,after continuation of the reaction for 24h,there was little change in morphology.
3.2.1.E ?ect of Seed Concentration.In an attempt to increase the AR of nanorods obtained in sample 11s ,we increased the seed concentrations in 22s and 33s by 10and 100times,respectively (see Table 1(11s ?33s ));however,we could not notice any noteworthy changes in the SEM images (See Supporting Information ,Figure S5a ?c).Also no signi ?cant change was detected in the UV ?vis absorption spectra in terms of the SPB (see Supporting Information ,Figure S6).The photoluminescence spectra of 11s at excitation wavelength 425nm is shown in Figure 10a;as expected emission maximum around 550nm for curcumin conjugated gold nanoparticles and another band around 670nm due to gold nanorods or nanoparticles were obtained.To further con ?rm that the ?uorescence at longer wavelength band (above 670nm)is due to gold nanorods and nanospheres,excitation spectra of nanorods and nanoparticles were recorded at emission wavelength 610nm.As depicted in Figure 10b,the excitation spectrum showed two maximum,one at around 425nm corresponding to curcumin (coming from curcumin conjugated nanorods)and other at around 540nm conforming to the absorption of gold nanorods and nanospeheres (compare with Figure S6,Supporting Information ).
3.2.2.E ?ect of Curcumin Concentration.The e ?ect of curcumin concentration was studied as per Table 1(11cur ?22cur ).Sample 11cur (Figure 11a-b)seemed to
constitute
Figure 7.(a)UV ?vis absorption spectra of Au NPs from citrate capped Au seeds (procedure A)in the presence of di ?erent silver content.(b)Photoluminescence spectra of 1A and 3A at excitation wavelength 425nm.Refer to Table 2for experimental conditions.
almost all shapes from hexagons,pentagons (see inset of Figure 11a),and triangles to spheres (～25nm)and rods,which is translated in the shoulders seen for the sample ’s UV ?vis
spectrum in Figure 12at 620and 644nm.Spherical particles made up the bulk of the sample (band seen at 540nm).The SEM image suggests that hexagons originate from triangles that seem to be chopped at their vertices.Intermediate structures that look like rectangular parallelograms under SEM (notice the ends of the nanorods)were noticed and seem to originate from triangles.Upon decreasing the curcumin ’s concentration to half in sample 22cur (Figure 11b),we only noticed that triangles appear smaller by almost half their size in sample 11cur .The ?uorescence excitation and emission spectra further con ?rmed the presence of curcumin conjugation and formation of gold nanoparticles (see Supporting Information ,Figure S7and S8).3.2.3.E ?ect of Silver on Citrate versus CTAB Capped Gold Seeds Used in Nanorod Formation.It should be recalled that when silver was used in procedure A (for citrate capped Au seeds),no nanorods were obtained.However,when CTAB capped Au seeds were used in procedure B,gold nanorods were obtained that were even better than procedure A without silver.This suggests that curcumin may not be able to interact directly with the silver ion (thus not be able to reduce involvement of silver in nanorod formation)in the presence of CTAB-capped gold seeds.This can be explained based on the proposed mechanism by Nikoobakht and El-Sayed who considered
the
Figure 8.(a ?g)SEM images of gold nanorods prepared from CTAB capped Au seeds obtained from di ?erent regions (procedure
B).
Figure 9.UV ?vis absorption spectrum of gold nanorods prepared from CTAB capped Au seeds (procedure B).
silver ions located between the head groups of the capping surfactant (CTAB)as Ag ?Br pairs.This would lead to the decrease of the charge density on the bromide ions and consequently the repulsion between neighboring head-groups on the gold surface and results in CTAB template elongation.5The growth solution contains gold and silver ions,and when curcumin is added only gold ions are reduced because silver ions and curcumin cannot come close to each other due to the favorable hydrophobic interaction between the alkyl chain of the CTAB molecule 38and curcumin and the unfavorable
interaction between silver ion and surfactant (the silver will be present near the Stern layer).Furthermore,Guyot-Sionnest and co-workers proposed that,during seed-mediated synthesis,under potential deposition (UPD)of silver preferentially occurs on the [110]gold facets compared with the [111]and [100]facets leading to nanorod growth in the [100]direction.67Because the silver monolayer will cover the [110]facet,although the silver may be oxidized and replaced by gold,other facets will grow faster because they are less covered with silver.Therefore di ?ering degrees of silver passivation on the [110]facets lead to varying ratios of growth on this facet and the nanorod ’s end facets.Because this is the case,the curcumin will reduce gold at the [111]and [100]faces,which are not covered by silver and exposed toward the cetyl chains of CTAB where curcumin itself is embedded;thus elongation occurs in the 110direction.
4.CONCLUSION
The study on gold nanorods produced through reduction with curcumin seemed to stress the micelle-templated growth of nanorods as revealed by the direct witness of micelle formation under SEM.At the same time,the rationale of curcumin being embedded into the alkyl chains of CTAB and its consequent reduction along the [110]direction complies with the mechanism of preferential binding of CTAB molecules onto certain facets of the gold NRs.The behavior of curcumin was a ?ected by the choice of the capping agent used for the gold seeds.In the presence of silver,CTAB-capped gold seeds gave higher AR NRs than when citrate-capped seeds were used.In fact,silver hindered the formation of NRs from citrate-capped seeds,which was attributed to an interaction between curcumin and silver.Such an interaction does not take place in case of CTAB-capped seeds since silver is found at the surface of seeds by forming Ag ?Br pairs and curcumin is located among the hydrophobic part of CTAB.The outcome of varying seed curcumin concentrations separately was similar to results observed with AA as well.The conjugation of curcumin into the di ?erent shapes of nanoparticles was con ?rmed by the ?uorescence signature of curcumin.Present results trigger new insight to gain more understanding of the formation process and the parameters a ?ecting it for a better design of
materials.
Figure 10.(a)Photoluminescence (emission)spectra of 11s at excitation wavelength 425nm and (b)excitation spectra of 11s at emission wavelength 610
nm.
Figure 11.SEM images of samples (a)11cur and (b)22cur .Refer to Table 1for experimental
conditions.
Figure 12.UV ?vis spectra of gold nanorods prepared from CTAB capped Au seeds di ?ering in the curcumin concentration.Refer to Table 1for experimental conditions.
ASSOCIATED CONTENT
*Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acs.jpcc.5b04447.
SEM images of gold nanorods from citrate capped Au seeds,1cur,2cur,3cur,11s,22s,33s,and seed solution,
after2h of reaction and after3h of reaction during
preparation of gold nanorods,UV?vis absorption spectra
of gold nanorods from citrate capped Au seed,1s,2s,3s,
11s,22s,and33s,and photoluminescence emission and excitation spectra of22cur(PDF)
■AUTHOR INFORMATION
Corresponding Author
*Tel:+9611350000,ext3985.Fax:+9611365217.E-mail: dp03@https://www.doczj.com/doc/248611137.html,.lb.
Notes
The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS
Financial support provided by Lebanese National Council of Scienti?c Research(NCSR)and American University of Beirut, Lebanon through URB,Kamal A.Shair Research Fund as well as Kamal A.Shair Central Research Science Laboratory(KAS
CRSL)facilities to carry out this work is greatly acknowledged.■REFERENCES
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