Self-Assembled Nanoreactors-Chem.Rev.-2005-Vol.105

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Self-Assembled NanoreactorsDennis M.Vriezema,Marta Comellas Aragone`s,Johannes A.A.W.Elemans,Jeroen J.L.M.Cornelissen,Alan E.Rowan,and Roeland J.M.Nolte*Department of Organic Chemistry,Institute for Molecules and Materials,Radboud University Nijmegen,Toernooiveld1,6525ED Nijmegen,The NetherlandsReceived September9,2004Contents1.Introduction14452.Molecular Nanoreactors14462.1.Capsules and Boxes14462.1.1.Covalent Systems14462.1.2.Noncovalent Systems14502.2.Micelle-Based Systems14562.3.Vesicle-Based Systems14592.3.1.Reactions in or at the Vesicle Membrane14592.3.2.Reactions in the Inner Compartment ofVesicles14633.Macromolecular Nanoreactors14653.1.Polymersomes as Nanoreactors14653.2.Polymer Micelles as Nanoreactors14703.3.Unimolecular Nanoreactors14733.4.Other Polymeric Systems as Nanoreactors14734.Biomacromolecular Nanoreactors14764.1.Protein Cages14764.1.1.Ferritin14774.1.2.Other Protein Cages14784.2.Viruses14784.2.1.Rod-Shaped Viruses14784.2.2.Cage-Structured Viruses14825.Conclusions and Outlook14856.Acknowledgment14857.References1485 1.IntroductionIn nature,chemical conversions take place in a confined environment and are closely geared to each other such that the product of one reaction is the substrate or catalyst of the subsequent one.Such a coupling of reactions in time and space(i.e.,perform-ing cascade reactions)is of growing interest to chemists as it is expected to increase the efficiency of chemical conversions on scales ranging from the laboratory bench to industrial plants.Nature realizes this coupling and the control and fine-tuning of the products,among others,by using well-defined reac-tion environments,which vary from nanometer-sized and relatively simple systems,such as enzymes,to micrometer-sized and extremely complex assemblies,such as cells.Industry currently uses whole cells as complete factories to produce on a large scale a variety of products.For a synthetic chemist,the coupling of reactions in time and space begins with the design and construction of a confined reaction environment within which the conversion(s)will take place,that is,a reactor.Toward this goal,the cell acts as the primary source of inspiration.Initial attempts to create such reactors involved fully syn-thetic low molecular weight receptors capable of encapsulating reagents;however,in most cases,the construction of covalent reactors implies complicated multistep synthesis,which is a severe drawback for their application on a larger scale.The development of synthetically more accessible systems demanded a more natural approach,that is,the self-assembly of small molecular components,for example,phos-pholipids,into discrete,capsule-containing architec-tures,and vesicles.Although the construction of an artificial cell is the ultimate fantasy,more simple systems such as micelles,vesicles,and other as-semblies of molecules may already partly solve the problem.In these simple reactors not only the reac-tion pathways can be influenced,but in many cases also the size and morphology of the products(e.g., crystals).The aim of this review is to give an overview of the wide range of nanoreactors that have been con-structed from synthetic and biological building blocks using both covalent and noncovalent approaches.The primary focus is on self-assembled systems,varying in size from a few nanometers to tens of micrometers. Not included is the vast area of inorganic compounds, such as zeolites and sol-gels.The self-assembled nanoreactors that are discussed can be considered as the simplest mimics of a cell and contain a cavity in which a chemical reaction can take place.The review is divided into several sections that cover the devel-opment of tailor-made nanoreactors,starting from small organic molecular containers expanding to large compartment-containing assemblies.The con-struction of capsules from low molecular weight compounds by means of covalent synthesis and self-assembly by highly directive and pre-designed inter-actions,such as hydrogen bonding and metal-ligand coordination,is presented in section2.1.In subse-quent sections(2.2and2.3),nanocapsules based on micellar and vesicular assemblies that are built up from low molecular weight molecules will be dis-*To whom correspondence should be addressed.Fax:+3124 3652929.E-mail:r.nolte@science.ru.nl.1445Chem.Rev.2005,105,1445−148910.1021/cr0300688CCC:$53.50©2005American Chemical SocietyPublished on Web03/01/2005cussed,whereas section 3covers the construction of nanoreactors from macromolecular building blocks.Finally,in section 4we will give an overview of the most recent developments in the use of viruses as nanocontainers and -reactors.We have focused in our review on developments in the past decade,covering the fields of organic and macromolecular chemistry as well as the emerging area of biomacromolecules (e.g.,viruses).All important articles in these fields are highlighted,and where appropriate the founda-tion papers from relevant earlier work are included.2.Molecular Nanoreactors2.1.Capsules and Boxes2.1.1.Covalent SystemsThe most efficient catalysts known are nature’s enzymes,whose tremendously high efficiency and selectivity have always been the drive for chemists to create synthetic catalytic systems that approach a similar superior activity and selectivity.In this respect,the general architecture of an enzyme has been a key of inspiration for the development of artificial supramolecular catalytic systems.Many of the systems of this type rely on principles reported already 60years ago by Pauling in his fundamental theories of enzyme catalysis with respect to transition states and molecular recognition 1and are therefore commonly referred to as artificial enzymes.2The most important feature of an enzyme is that the activated complex is stabilized to a larger extent than the enzyme -substrate complex itself,by the presence of additional binding interactions in the transition state of the reaction.In other words,enzymes can be considered complementary in structure to the transi-tion state of the reaction they catalyze.The enzyme’s binding properties,coupled with catalytic function-alities strategically placed with the active site,de-crease the activation energy for the reaction.3,4To be an efficient enzyme mimic,the designed molecule therefore needs to possess a binding cavity or site that is able to selectively recognize and bind a desired substrate,which in the next step has to be converted at a catalytic center in its direct proximity.Finally,and in the design of many artificial enzymes this has proven to be a major bottleneck,the catalyst should be able to release the converted substrate and have the ability to be regenerated;that is,turnover has to occur.Using nature’s enzymes as a blueprint,many approaches to synthesize low molecular weight cata-lysts,which contain a substrate binding site that recognizes substrates next to an active site,have been described in the literature.5The first,simple ex-amples of enzyme mimics were crown ethers and cryptands with catalytically active functionalities attached in the correct position near the reacting group of a complexed substrate.6-9From the early 1970s on,cyclodextrins (CDs),naturally abundant cavity molecules,have been extensively used as binding sites in supramolecular catalysts.10-13A myriad of functionalized cyclodextrins have shown to be selective catalysts for ester hydrolysis.14-17ADennis M.Vriezema (second from left)completed his Chemistry degree at the University of Nijmegen in 1999and became a researcher in a joint project of this university with the Dutch Research Institute TNO,working on a project to develop a percutaneous glucose monitor.In 2000,he joined the group of Professor R.J.M.Nolte to carry out a Ph.D.project on the aggregation behavior of isocyanide polymers and the development of polymeric nanoreactors.He currently is a postdoctoral fellow in the group of Professor Nolte and is in the process of setting up a spin-off company called Encapson.Marta Comellas Aragone `s (far left)studied Chemistry at the University of Barcelona.After graduating in 2002,she carried out her Masters research at the University of Nijmegen,investigating the assembly of polymer/protein biohybrids.Currently,she is a Ph.D.student in the group of Professor R.J.M.Nolte,working on the exploration of viral capsids as nanoreactors.Hans A.A.W.Elemans (standing,right)completed his Ph.D.in Physical Organic Chemistry in 2001at the University of Nijmegen with Professor R.J.M.Nolte,after which he commenced a postdoctoral position investigating scanning probe techniques in conjunction with the Physics Department and Professor Nolte.Since 2003,he occupies a bridging position between the Solid State Physics and Organic Chemistry groups,where he is conducting research into the self-organization and function of organic molecules at solid −liquid interfaces.For this work,he was awarded a Veni Innovative Research Grant in 2004.His main scientific interests are at the interface of Chemistry and Physics,in particular in the areas of catalysis and self-assembled architectures.Jeroen J.L.M.Cornelissen (third from left)studied Chemistry at the University of Nijmegen and received his Ph.D.(cum laude)in 2001on the hierarchical transfer of stereochemical information in synthetic polymers with Professor R.J.M.Nolte.For this work,he was awarded the 2002IUPAC Prize for Young Chemists and the 2002SNS Bank Prize.From 2001to 2002,he worked as a postdoctoral fellow at the IBM Almaden Research Center with ler.Thereafter,he returned to the University of Nijmegen with a Veni Innovative Research Grant to work on structurally defined polymers,biohybrid macromolecules,and protein cages as reactors and supramolecular building blocks.Alan E.Rowan (seated,left)completed his Ph.D.study at the University of Liverpool in the field of Physical Organic Chemistry in 1990.He then departed to New Zealand for a postdoctoral stage at the University of Otago.In 1993,he returned to Europe to take up a TMR Fellowship at the University of Nijmegen in the field of Supramolecular Chemistry in the group of Professor R.J.M.Nolte.Since then,he became Lecturer,Senior Lecturer,and recently Professor of Molecules and Materials at the University of Nijmegen.His passion is for fine chemistry and good single malts.Roeland J.M.Nolte (seated,right)received his Ph.D.in Physical Organic Chemistry from the University of Utrecht (1973),where he stayed and became Assistant Professor and then Associate Professor.In 1981,he was a visiting scientist at UCLA in the group of Professor Donald J.Cram.In 1987,he moved to the University of Nijmegen and became a Full Professor of Organic Chemistry,and since 1994he has also been a part-time Professor of Supramolecular Chemistry at the Eindhoven University of Technology.In 2003,he was awarded the first Royal Netherlands Academy of Arts and Science Chair in Chemistry.His research interests span a broad range of topics at the interfaces of Supramolecular Chemistry,Macromolecular Chemistry,and Biomimetic Chemistry.In his work,he focuses on the design of catalysts and (macro)molecular materials.He and his group have published ca.450scientific papers.1446Chemical Reviews,2005,Vol.105,No.4Vriezema et al.representative example is Breslow’s mimic of the enzyme ribonuclease A,14,18which works according to principles of acid -base catalysis (Figure 1a).In this mimic,two imidazole rings are connected to the primary side of a -CD,and the resulting cavity molecule 1binds and selectively hydrolyzes cyclic phosphor diester 2.Apart from a 120-fold rate enhancement as compared to the uncatalyzed solu-tion reaction using NaOH as a base,a greater than 99%selectivity for one of the two possible products (i.e.,3a )was observed.Following a similar approach,Diederich covalently attached a thiazolium and a flavin moiety to a cyclophane cavity,creating a synthetic pyruvate oxidase mimic 4(Figure 1b).19,20The system is capable of catalyzing,on a truly preparative scale,the conversion of complexed naph-ththalene-2-carbaldehyde 5into the methyl ester 6.During the reaction,the flavin residue is reduced,and for turnover to occur it is reoxidized by regenera-tion in an electrochemical cell.Many other examples are known in which a transi-tion metal catalyst is covalently attached to oneorFigure 1.Examples of artificial enzymes:(a)Breslow’s ribonuclease A mimic which catalyzes the hydrolysis of cyclic phosphodiesters,(b)Diederich’s pyruvate oxidase mimic which catalyzes the oxidation of naphththalene-2-carbaldehyde,and (c)Breslow’s manganese porphyrin tetra-cyclodextrin receptor which regio-and stereoselectively hydroxylizes steroid derivatives.Self-Assembled Nanoreactors Chemical Reviews,2005,Vol.105,No.41447more substrate binding cavities.For example,iron porphyrins connected to one21,22or more21-24CDs have been applied as oxidation catalysts with varying efficiencies and selectivities of substrates that are bound in the CD cavity.Different types of cavity-containing catalysts are metal-ligand systems con-nected to cyclocholates,25,26cyclophanes,27-29 calixarenes,30clips,31-34and other concave binding sites.35-38An artificial enzyme system which perfectly illustrates the possibility to achieve high selectivity is the manganese porphyrin7,which possesses four covalently linked peripheral -CDs(Figure1c).39-43 In the presence of iodosyl benzene,this tetramer is capable of selectively epoxidizing or hydroxylating stilbene derivatives.Its superior selectivity is best illustrated by the fact that it can regio-and stereo-selectively oxidize the unactivated6-CH2position in the B-ring of steroid derivative8.The selectivity relies on a precise positioning of this methylene group directly over the porphyrin metal center,caused bythe encapsulation of the two tert-butylphenyl groups of the substrate by two trans-positioned CD cavities, as was concluded from molecular modeling calcula-tions.As a result of concomitant hydrolysis of the ester groups during the reaction,product9was obtained in a yield of40%.The majority of the examples discussed above refer to supramolecular catalysts which convert a single bound substrate.In recent years,however,new receptors have been developed that are designed in such a way that they can simultaneously complex two or even more substrates,and in addition promote a reaction between them.In many cases,the binding site is modified in such a way that a reaction chamber is created which encapsulates guest molecules,con-fines them,and effectively shields them from their environment,generally the solvent.As a result,the transition state of more than one simultaneously captured substrate in a bimolecular reaction can be stabilized.The appropriate catalysis of bimolecular reactions is,however,not trivial,because apart from binding the two substrates it is also necessary to orient them in the correct position for the reaction to occur effectively.44On the other hand,in such a way a careful design of the host might allow for a geometrical restriction of the bound substrates and a concomitantly enforced stereo-or regioselectivity in product formation.A frequently occurring example is the acceleration of cycloaddition reactions under encapsulation conditions.45When both partners of the cycloaddition reaction are oriented in the correct geometry within the host,the latter acts as the template for a transition state analogue,on which also the development of catalytically active imprinted polymers,46catalytic antibodies,47and the recently developed selection of a catalyst from a dynamic combinatorial library(vide infra)are based.A serious limitation of the use of templating hosts,however, is that they are in many cases not truly catalytic. Because their design generally relies on binding the reactants in a geometry that resembles that of the eventual products,the latter almost inevitably bind to the host with a similar strength or even stronger, thereby preventing effective turnover by product in-hibition.Approaches to address this problem include a careful design of the binding sites of the capsule, in such a way that they bind the products weaker than the reactants,or by aiming for reaction products that are too small or too large to fit within the host. In particular,the latter approach has proven to be extremely versatile in cases where the hosting cap-sule itself can be reversibly assembled and dis-assembled(see section2.1.2).Initially,CDs were utilized as catalysts of Diels-Alder reactions in water,48,49in one case revealing Michaelis-Menten kinetics with saturation occurring at high substrate concentrations,50whereas inhibition of the reaction occurred in the presence of a hydro-phobic dienophile.In a related study,significant changes in diastereoselectivity of the reaction could be achieved in the presence of CD catalysts.51An-other family of macrocycles that have been applied as nanoreactors more recently are cucurbituril de-rivatives.52,53The internal cavity of the hexameric glycoluril derivative cucurbit[6]uril10is sufficiently large to accommodate aliphatic and aromatic five-membered rings,and consequently it was investi-gated as a nanoreactor for1,3-dipolar cycloaddition reactions.54The uncatalyzed reaction between prop-argylammonium11and azidoethylammonium12 yields a mixture of two regioisomeric diazole adducts 13a and13b,which are formed in equal amounts by a standard concerted process(Scheme1).In the presence of a catalytic amount of10,the rate of this reaction was accelerated55000×.In addition,the 1,4-disubstituted product was exclusively formed. These results are explained by assuming the forma-tion of a transient ternary complex between cucur-bituril and the both reactants.The reactants are bound simultaneously by electrostatic interactions between the charged ammonium groups and the urea oxygen atoms lining the portals of10,with the acetylene and azide substituents extending into the core of the host.The reactive groups are perfectly aligned with respect to each other,and a rapid reaction occurs.Not unexpectedly,the release of the pseudorotaxane product,which has a higher affinity for the host than each of the both reactants,is the Scheme1.1,3-Dipolar Cycloaddition between Propargylammonium12and Azidoethylammonium 11Catalyzed by Cucurbit[6]uril101448Chemical Reviews,2005,Vol.105,No.4Vriezema et al.rate-limiting step.In more recent work,Tuncel and Steinke have shown that this cucurbituril-catalyzed Diels-Alder reaction can also be applied in the synthesis of polyrotaxanes by using diazides and diacetylenes as the substrates.55,56The octameric analogue cucurbit[8]uril,which has a cavity with a size comparable to that ofγ-CD,can accommodate two aromatic guest molecules.The cavity was used as a nanoreactor in the[2+2]-photodimerization of(E)-diammoniumstilbene in aque-ous solution.57Apart from a large rate acceleration as compared to a similar reaction catalyzed byγ-CD (0.5vs72h forγ-CD),a considerable stereoselectivity was observed resulting in a ratio between the syn-and anti-adducts of95:5.In addition,no isomeriza-tion,which is usually observed for stilbene deriva-tives,was observed during the reaction.The observed results are explained by assuming a stabilization of both guest molecules in a favored geometry in which the olefinic groups are oriented in a parallel fashion within the substrate binding cavity.Sanders and co-workers were among the first to synthesize more sophisticated hosts for the use as nanoreactors,viz.,cyclic zinc porphyrin hosts,which possess convergent binding sites that can specifically complex two reaction partners in close proximity by providing them with ligands that coordinate to the porphyrin zinc ions.In the presence of trimer14a, the Diels-Alder reaction between pyridine-function-alized maleimide15and furan derivative16ap-peared to occur with a200-fold increase in initial rate as compared to when the reaction was carried out inthe absence of14a(Scheme2).58In addition,when using14a,the exo-adduct17was the only product formed.These results can only be explained by assuming a simultaneous binding of15 and16within14a,which positions the starting materials in the correct geometry for producing the exo-product exclusively.Acceleration of the reaction occurs simply by virtue of the binding of the reactants by14a:its favorable enthalpic gain compensates for the highly unfavorable entropy of activation.Further evidence for the occurrence of the reaction within the cavity of14a was provided by the fact that in the presence of tripyridyltriazine,a guest that is bound extremely strong within14a,the Diels-Alder reac-tion was completely inhibited.As expected,the major drawback that prevents14a from acting as a true catalyst is the occurrence of product inhibition, because17is complexed by14far more favorably than the starting materials.The formation of the endo-adduct is disfavored because of a poor fit of its transition state within the cavity of14a.However, upon contraction of the cavity,the stereochemistry of the reaction could be completely reversed.59Host 14b,which contains two ethyne instead of butadiyne bridges,incorporates two different Zn-Zn distances, viz.,15Å,as in the case of14a,and12Å.The latter distance is more complementary to binding of the endo-adduct18,which is formed exclusively upon reaction of15and16.Similar rate accelerations using cyclic metalloporphyrin trimers were observed in the case of hetero Diels-Alder reactions between pyridyl dienes and nitrosopyridines to yield oxa-zines.60-62Host14a was turned into a true catalyst when it was used in a catalytic acyl transfer reac-tion.63Upon hosting4-(hydroxymethyl)pyridine and N-acetylimidazole,the substrates are perfectly ori-ented to undergo a transacylation reaction.As com-pared to the uncatalyzed reaction,14a increased the initial rate of the reaction16-fold.In contrast to the case of the Diels-Alder reactions,however,the products are now coordinated to14a by only a single zinc-ligand interaction,resulting in a binding affin-ity of the product which is comparable to those of the substrates,allowing turnover to occur.In the above examples,molecular capsules are utilized to induce cycloaddition reactions between two simultaneously complexed substrates.In the reverse approach,encapsulation has also proven to be an extremely powerful tool to prevent reactions by the trapping of reactive intermediates.64The inclusion of reactive compounds in a confined space has thus become a means to control their reactivity toward compounds other than themselves.For example, Cram’s hemicarcerand-type containers can incarcer-ate o-benzyne or cyclobutadiene and stabilize them by protecting them for dimerization.65-67Recently, Warmuth et al.reported the generation and stabili-zation of cycloheptatetraene inside the cavity of a hemicarcerand,68,69and reactive cations have been stabilized by trapping them inside a tetrahedral gallium-ligand cluster.70Similarly,gases such as NO2 have been chemically“fixated”and utilized as stable nitrosation reagents by encapsulating them within calixarenes.71-73Scheme2.Controlled Direction of a Diels-Alder Reaction by Means of the Geometrical Constraints of Cyclic PorphyrinHostsSelf-Assembled Nanoreactors Chemical Reviews,2005,Vol.105,No.414492.1.2.Noncovalent SystemsCovalently built capsules have proven their value as catalysts of a wide range of chemical reactions,and in several cases impressive efficiencies and selectivities have been achieved (see the examples described in section 2.1.1).However,as the scale and complexity of the target products of their reactions increase,designing and preparing capsule-shaped catalysts in a covalent fashion rapidly becomes very difficult,involving costly multistep syntheses that often produce only milligram yields of material.19,32,59Moreover,the resulting catalysts are not so versatile because they are often highly specific to only a limited number of reactions.54,58,63-69It is for this reason that in the past decade a shift has been observed in the approach to construct these catalysts,from tradi-tional covalent chemistry to multicomponent self-assembly of relatively small complementary building blocks.Self-assembled capsules are receptors with enclosed cavities that are formed through reversible,noncovalent interactions between two or more comple-mentary subunits.Typically,such capsules self-assemble through hydrogen-bonding and metal -ligand interactions.74,75Hydrogen-bonding interactions are highly directional and specific,whereas metal -ligand bonds are generally much stronger,yielding more robust self-assembled structures.In this sec-tion,self-assembled capsules that can function as nanoreactors will be discussed.The group of Rebek has become one of the leading groups in the exploration of self-assembly of comple-mentary concave receptors into a myriad of hydrogen-bonded capsular species.76-79The first capsule,the so-called “tennis-ball”19-19(Figure 2a),consists oftwo bis-glycoluril units which self-assemble into a closed cavity.The self-assembly is governed by the specific curvature encoded within the building block,and the concomitant formation of eight hydrogen bonds be-tween the self-complementary C d O and N -H func-tions of the two subunits.80The inner volume of the capsule (∼50Å3)is rather small and only allows for the encapsulation of small molecules such as meth-ane and noble gases.To complex larger guests,the spacers between the glycoluril moieties were varied,generating a number of dimeric and higher order capsules with inner compartments of varying sizes.By comparing the binding properties of these cap-sules,it turned out that the encapsulation process was largely determined by the size and shape of the guest.An extensive study toward the understanding of the factors that govern the binding of guests within Rebek’s capsules revealed the “55%occupance”rule,which implies that in the absence of specific inter-molecular interactions between the guest and the capsule the binding is most efficient if only 55%of the available inner space is filled.81A typical example of a larger capsule generated by the assembly of extended subunits is the “molecular softball”20-20,in which the glycoluril moieties are connected across seven fused rings (Figure 2b).The central ring contains an ethylene bridge function that encodes the extra curvature that is required for the specific formation of a hydrogen-bonded dimer,re-sulting in a capsule which is now held together by a seam of 12hydrogen bonds.The complexation of guest molecules within the softball is,interestingly,an entropy-driven process,which was concluded from the fact that the association constants increase with temperature.This was unexpected,because most host -guest complexation processes in organic solu-tion are entropically unfavorable and enthalpy-driven.Upon binding of a guest,for example,ada-mantane or ferrocene,in 20-20,two molecules of solvent that are present in the cavity are released,resulting in a favorable increase in entropy.NMR studies also revealed that the softball was capable of encapsulating two aromatic solvent molecules,82,83and this propensity stimulated the researchers to utilize the “softball”as a microreactor for bimolecular reactions.An example is depicted in Figure 3a,viz.,the Diels -Alder reaction between p -benzoquinone 21and cyclohexadiene 22.84,85In p -xylene-d 10solution,the rate of this reaction is 200times faster in the presence of the 20-20capsule than in its absence.This was explained by assuming that both starting materials are complexed within the capsule and,because of their close proximity,the rate of reaction is enhanced.Due to the restricted space inside the capsule,the endo -isomer 23is favored as the exclu-sive product.In contrast,if 1,4-naphthoquinone,a molecule that does not fit in the capsule,was used as the dienophile,the reaction was not accelerated.Addition of a nonreactive guest that effectively competes for encapsulation with the reactants also was found to prevent rate acceleration.A major deficit of the reaction,as expected,is that effective turnover is prevented due to productinhibi-Figure 2.Glycoluril-based capsules which are self-as-sembled by hydrogen-bonding interactions:(a)a “tennis ball”and (b)a “softball”.In the computer modeled struc-tures,some of the substituents and hydrogen bonds have been omitted for clarity.(Reproduced with permission from ref 74.Copyright 2002Wiley.)1450Chemical Reviews,2005,Vol.105,No.4Vriezema et al.。

Microreactor Microfluidic Systems with Human Microsomes and Hepatocytes for use

Biomedical Microdevices7:2,117–125,2005C 2005Springer Science+Business Media,Inc.Manufactured in TheNetherlands.Microreactor Microﬂuidic Systems with Human Microsomes and Hepatocytes for use in Metabolite StudiesJeanna C.Zguris,1Laura J.Itle,2Daniel Hayes,3andMichael V.Pishko4,∗1Department of Chemical Engineering,2Department of Chemical Engineering and The Huck Institute for theLife Sciences,3Department of Material Science,and4Departments of Chemical Engineering,Chemistry,and MaterialsScience&Engineering,The Pennsylvania State University,UniversityPark,PA16802-4420,USAE-mail:mpishko@Abstract.In the area of drug discovery,high-speed synthesis has increased the number of drug candidates produced.These poten-tial drugs need to be evaluated for their adsorption,distribution, metabolism,elimination,and toxicology(ADMET)properties as early in the drug development stage as possible.Previously,a poten-tial drug’s ADMET properties have been found out by using mono-layer cell cultures and live animals.These methods can be costly, time-intensive,and impractical for screening the large amount of potential drugs created by combinatorial chemistry.A quick,small, inexpensive,and highly parallel device would be desirable to deter-mine a drug candidate’s properties(i.e.,metabolism of the drug). Here we fabricate a microﬂuidic device entrapping human micro-somes within poly(ethylene)glycol hydrogels thereby generating an in situ microreactor to assess a drug candidate’s metabolic proper-ties that can be coupled to analysis equipment.We show that mi-crosomes can be entrapped without the loss of enzymatic activity during photopolymerization.Additionally,a microreactor utilizing hepatocytes was also created for comparison with the microsome microreactor.Key Words.microsomes,hydrogels,microreactor,cytochrome P450IntroductionAdsorption,distribution,metabolism,elimination,and toxicology(ADMET)studies are a key area of drug development in which potential drugs are evaluated in an attempt to determine adverse in vivo reactions prior to human trials.Despite rigorous testing,approximately 75,000–135,000deaths due to adverse drug reactions oc-cur annually in the United States,making it the sixth lead-ing cause of death(Hodgson,2001).In addition to limit-ing the number of deaths due to adverse drug reactions, a distinct economic advantage exists for pharmaceutical companies that determine ADMET properties early in the development process(Myers and Baker,2001).An esti-mated90%of drugs are dropped from development in the late clinical stage;eliminating any number of these dead-end drug candidates earlier in the process would allow for the reinvestment of research moneys into more productive avenues(Contag,2002).We have devised a microreactor with microsomes en-trapped in a polymer matrix of poly(ethylene)glycol that can be used to determine the metabolic products of potential drug candidates.Derived from the ultrasonic homogenization of human liver,microsomes retain the metabolic properties of the liver,the organ largely re-sponsible for drug metabolism(Pearce,et al.,1996). Microsomes contain a high percentage of cytochrome P450,group of hemeproteins(Khan,2003)that com-pose an enzyme superfamily subdivided by enzyme struc-ture(Chauret et al.,1999).The cytochrome P450family plays a major role in the oxidative metabolism of xeno-biotics,including toxins,exogenous chemicals,and drugs (Pearce et al.,1996;Chauret et al.,1999;V ondracek et al., 2001).Cytochrome P450metabolism of potential drug can-didates is becoming a routine aspect of high through-put drug screening.For example,the inhibition of Cy-tochrome P450activity can indicate an adverse drug-drug interaction(Crespi et al.,1997).In vitro drug metabolism and cytochrome P450activity related to drug exposure is measured in a variety of biological systems with in-creasing orders of structure and organization.These sys-tems include microsomes,supersomes,hepatocytes,and liver slices(Brandon et al.,2003).The production of drug metabolites can vary depending on the system used,with microsomes and supersomes producing Phase I metabo-lites due to Cytochrome P450,while hepatocytes and liver slices can produce Phase I(hydrolysis,oxidation, and reduction)and Phase II(conjugation)metabolites in in vitro environments(Brandon,2003;DeGraaf et al.,2003).Despite incomplete metabolism of some drug candidates,microsomes are an inexpensive,easy to use *Corresponding author.117118Zguris et al.alternative for initial testing of a drug’s metabolism.In this case human microsomes are used,which will pro-vide us with information regarding the metabolites that will be produced in humans(Sivapathasundaram et al., 2003).The immobilization of microsomes in a three-dimensional matrix provides several advantages over co-valent linkage or physical adsorption including increased storage stability,the ability to separate products from the incubation step,and the ability to reuse and recycle the microsomes for later tests(Sakai-Kato et al.,2002).It has been shown by Sakai-Kato and co-workers that micro-somes can be entrapped within a sol-gel matrix(Sakai-Kato et al.,2002).Microsomes can also be entrapped in a poly(ethylene)glycol matrix,which has several advan-tages over sol-gel technology.These advantages include biocompatibility of the matrix,non-toxic crosslinking so-lutions,faster polymerization time,and ease of patterning (Honiger et al.,1995;Cruise et al.,1998;Alcantar et al., 2000;Elisseeff et al.,2000;Revzin et al.,2001;Burdick and Anseth,2002;Koh et al.,2002,2003;Liu,2002;Liu et al.,2002;Revzin,2003).Previously,we have encap-sulated and patterned enzymes and whole cells for use in sensing applications(Revzin et al.,2001;Koh et al., 2002,2003).However,microsomes harness the advan-tages of both enzyme and whole cell encapsulation.Mi-crosomes contain multiple enzymes,leading to a variety of products,but without needing a tightly regulated external environment.Here we describe the fabrication of a poly(ethylene) glycol microreactor containing microsomes in microﬂu-idic channels for easy sample delivery.Because it is con-tained within a microﬂuidic device,this microreactor has the ability to be coupled with a variety of downstream de-tection devices for drug metabolites.However,this method relies on the ability of drug candidates to be transferred through the hydrogel matrix and for drug products to be returned to the process stream.To this end,we charac-terized the mass transfer by calculating the mesh size of PEG hydrogels.Additionally,we compare the activ-ity of encapsulated microsomes with microsomes in so-lution,hepatocytes in solution,and encapsulated hepa-tocytes by utilizing aﬂuorescent substrate,EFC and its ﬂuorescent product HFC(Roberts et al.,1995;Kent et al., 2004).Experimental SectionMicrosomesGLP human liver microsomes(CellzDirect,Austin TX) were used.For imaging,the microsome membrane was stained with3,3 -dioctadecyloxacarbocyanine perchlorate (DiOC)(Molecular Probes,Eugene,OR)dissolved in dimethyl sulfoxide(DMSO)(VWR International,West Chester,PA).Cell cultureSV-40transformed murine hepatocytes were obtained from American Type Culture Collection(Manassas, V A).Cells were incubated at37◦C in5%CO2and 95%humidiﬁed air in Dulbecco’s Modiﬁed Eagle’s Medium(DMEM,Sigma,St.Louis,MO)with4.5g/L glucose,200nM dexamethasone,4%fetal bovine serum(FBS),and1%antibiotic/antimycotic solution (Sigma,St.Louis,MO).Hepatocytes were grown to conﬂuence in75cm2polystyrene tissue cultureﬂasks and conﬂuent cells were subcultured every2to3days by trypsinization with0.25%(w/v)trypsin and0.13%(w/v) EDTA.Forﬂuorescent whole cell imaging,cells were stained with Cell Tracker Orange CMTMR(5-(and-6)-(((4-chloromethyl)benzyl)amino)tetramethylrhodamine) (Molecular Probes,Eugene,OR).Preparation of PEG hydrogel spheresHydrogel spheres were prepared by following a pre-viously described procedure(Russell et al.,1999).A poly(ethylene)glycol diacrylate(PEG-DA,Sigma,St. Louis,MO)precursor solution was extruded through a21-gauge needle into a graduated cylinder of mineral oil.The precursor droplets were photopolymerized with a365nm, 300mW/cm2light source(EFOS Ultracure100ss Plus, UV spot lamp,Mississauga,Ontario).The spheres were collected and washed repeatedly with water.For mass transfer experiments,spheres containing 100nmﬂuorescently particles(Molecular Probes,Eu-gene,OR)and spheres that did not contain any particles with the composition of700µL of PEG-DA,300µL of phosphate buffer saline solution(0.1M,pH7.3)and 10µL of Darocur1173.For spheres that were created mi-crosome activity measurements,a precursor solution with 700µL of PEG-DA,200µL of phosphate buffered saline solution(0.1M,pH7.4),100µL of glucose-6-phosphate dehydrogenase at0.07KU/mL,20µL of microsomes at 20mg protein/mL and10µL of2-Hydroxy-2-methyl-1-phenyl-1-propanone(Darocur1173,Ciba,Tarrytown, NY)was prepared.To entrap hepatocytes in PEG spheres, a cell suspension of1.0×106to1.5×106cells/mL in90%phosphate buffered saline,10%PEG-DA(MW 575),and1%2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur1173,Ciba,Tarrytown,NY)photoinitiator was used.Microreactor Microﬂuidic Systems with Human Microsomes and Hepatocytes119Cytochrome P450activity in microsomes and mammalian cellsFor the determination of cytochrome P450in microsomes, suspension or entrapped microsomes were used.For sus-pension studies,solutions were made with3000µL of phosphate buffer saline solution(0.1M,pH7.4),100µL of glucose-6-phosphate dehydrogenase at0.07KU/mL, 20µL of microsomes at20mg protein/mL,100µL of glucose-6-phosphate(0.57M,Sigma,Sigma,St.Louis, MO),and100µL of nicotinamide adenine dinucleotide phosphate oxidized sodium salt(NADPH)(VWR Inter-national,West Chester,PA).The NADPH,glucose-6-phosphate and glucose-6-phosphate dehydrogenase were added for the NADPH regenerating system that is needed with the use of microsomes.Microsome containing spheres were generated as previously described and sus-pended in2mL PBS with100µL of glucose-6-phosphate (0.57M,Sigma,Sigma,St.Louis,MO)and100µL of NADPH.7-ethoxy-4-triﬂuoromethylcoumarin(1g/L, EFC)(Molecular Probes,Eugene,OR)were added to microsomes or sphere suspensions in varied amounts. Fluorescent emission spectra of the EFC or the prod-uct of the reaction,7-hydroxy-4-triﬂuoromethyl coumarin (HFC)(DeLuca et al.,1988;Buters et al.,1993)were ob-tained using aﬂuorescence spectrometer(QM-1,Photon Technology International).For the determination of cytochrome P450activity in mammalian hepatocytes,cell suspensions or entrapped hepatocytes were used.For suspension studies,adherent cultures were detached via trypsinization,pelleted,and re-suspended at a density of1.0×106to1.5×106cells/mL in sterile0.1M phosphate buffer saline solution(PBS),pH 7.4.Cell containing spheres were generated as previously described and suspended in PBS.10µL of1g/L7-ethoxy-4-triﬂuoromethylcoumarin(EFC)(Molecular Probes,Eu-gene,OR)were added to cell or sphere suspensions.Flu-orescent emission spectra of the EFC or the HFC were obtained using aﬂuorescence spectrometer(QM-1,Pho-ton Technology International).Preparation of PEG hydrogel microstructuresThe hydrogel microstructures were made using a simi-lar method as previously reported(Revzin et al.,2001).In short,the hydrogel arrays were patterned photolithograph-ically with poly(ethylene)glycol diacrylate(PEG-DA) (MW575,Sigma,Milwaukee,WI)on glass substrates.To prepare the substrates for the arrays,an oxidized surface was created by an acid wash(6N sulfuric acid,Aldrich Chemical Co.,Milwaukee,WI)followed by a base wash (0.1M sodium hydroxide,Sigma,St.Louis,MO)of the glass followed by treatment with3-(trichlorosilyl)propyl methacrylate(Sigma,St.Louis,MO)to form a self as-sembled monolayer(SAM)with pendant methacrylate groups.A solution containing PEG-DA(MW575,Sigma, St.Louis,MO),0.1M phosphate buffered saline, and a photoinitiator,2-Hydroxy-2-methyl-1-phenyl-1-propanone(Darocur1173,Ciba,Tarrytown,NY)was prepared.For microsome containing arrays,the con-centration of components was700µL of PEG-DA, 200µL of phosphate buffer saline solution(0.1M, pH7.3),100µL of glucose-6-phosphate dehydrogenase at0.07KU/mL,20µL of microsomes at20mg pro-tein/mL that were previously exposed to DIOC and10µL of2-Hydroxy-2-methyl-1-phenyl-1-propanone.To gen-erate cell-containing hydrogel arrays,a cell suspension of1.0×106to1.5×106cells/mL in PBS was ing the cell suspension,a10%PEG-DA solution with1%photoinitiator,2-Hydroxy-2-methyl-1-phenyl-1-propanone(Darocur1173,Ciba,Tarrytown,NY),was prepared.The microsome or cell-containing precursor solution was subsequently placed onto the methacrylated substrate. The precursor solution was then covered with a photomask and exposed to ultraviolet light at365nm,300mW/cm2 (EFOS Ultracure100ss Plus,UV spot lamp,Mississauga, Ontario)region for1–2seconds.Areas exposed to light cross-linked via a free radical mechanism and the result-ing hydrogel network isﬁxed in place while the remaining unreacted monomer is washed away with water.Arrays were imaged on an Axiovert Zeiss200Mﬂuorescent mi-croscope with a mercury light source(Zeiss,Thornwood, NY).Preparation of microﬂuidic channelsThe masters had negative patterns that were prepared us-ing photolithography with SU-850negative photoresist (Microlithography Chemical Corp,Newton MA).The patterns were made on glass slides and silica wafers. The pattern was placed on the middle of the substrate utilizing a chrome sodalime photomask made by Ad-vanced Reproductions(Andover,MA).To create the poly(dimethylsiloxane)(PDMS)channels for the genera-tion of microreactors within microﬂuidic channels,mas-ters were placed in petri dishes and covered with a10:1 mixture of PDMS prepolymer to curing agent(Dow Corn-ing Sylgard184,Midland,MI).The resulting degassed mixture was poured into the Teﬂon mold with the master and cured at60◦C for at least2hours.After curing,the PDMS replica was removed from the master and the in-let and outlet ports were placed by piercing the replica with a blunted syringe needle through the backside of the network.PDMS channels and glass coverslips were treated with a115V high frequency generator model120Zguris et al.BD-10AS(Electro-Technic Products,Inc,Chicago,IL) to irreversibly seal the channel to the glass.To create microreactors within the microﬂuidic net-work,polymer precursor solution previously described containing either microsomes or hepatocytes were pumped into the microﬂuidic channels.A photomask was positioned over the microﬂuidic channel,and the polymer precursor solution was exposed to ultraviolet light as de-scribed above.Unreacted polymer precursor was rinsed from the channel using water.Arrays within microﬂuidic networks were imaged using an Axiovert Zeiss200Mﬂu-orescent microscope with a mercury light source(Zeiss, Thornwood,NY).Results and DiscussionFabrication of microsome containing hydrogel arrays The process of making hydrogel microstructures with mi-crosomes has been developed.The microsomes are bought with a20mg/mL protein,which includes a large amount of tissue debris.The large debris can be seen in Fig-ure1(a).The large tissue debris could block the microﬂu-idic channel and alter the mass transfer of the analyte to the microsomes in the hydrogel.Depending on the chan-nel width it could be critical that the larger tissue debris was removed.This was accomplished by centrifuging the solution,removing and keeping theﬂuid and throwing out the pellet.After thisﬁltering technique most of the large debris is removed.This can be shown by Figure1(b), there is sufﬁciently less debris after thisﬁltering technique.Theﬁltered or unﬁltered solution of microsomes can be entrapped in hydrogel microstructures.We have shown that the microsomes in the unﬁltered state can be en-trapped with a polymer matrix and stay intact.This is shown in Figure2,where the lipid membranes are stained with DiOC remainingﬂuorescently active.The hydrogels contain a7:3ratio of PEG:PBS precursor solution.Mass transfer to microsomes in PEG hydrogels Microsomes must be permanentlyﬁxed within the hy-drogel or free microsomes could disrupt any analysis of products.PEG hydrogels containingﬂuorescently tagged nanoparticles the same size as microsomes were moni-tored for2.5hours to insure that no particles leached into the surroundingﬂuid.In Figure3,theﬂuorescent inten-sity of the hydrogels is shown as a function of time.No substantial decrease inﬂuorescent intensity was shown, indicating that no particles leached from the hydrogels.To substantiate this result and to verify that drug candidates could reach microsomes entrapped in a hydrogel matrix, the mesh size of the gels was calculated.Additionally,the mesh size of the gel can be used to ﬁnd the effective diffusivity(Peppas et al.,2000)of the potential drug;the effective diffusivity would allow the incubation time of the microsomes with the drug candi-date to be calculated.Mesh size was estimated by using the Peppas-Merrill equation.Though the Peppas-Merrill equation corrected the Flory-Rehner model(Peppas et al., 2000)for solvated systems,it still contains some limita-tions(Wu et al.,2004).A key limitation precludes the cal-culation of mesh size for polymer networks formed with dilute precursor solutions;in this case,the mesh size can-not be calculated mathematically for hydrogels entrapping hepatocytes.The determination of the mesh size has been deter-mined for other polymer systems(Mellott et al.,2001; Russell et al.,2001;Kim and Peppas,2002;Berger et al., 2003;Wang et al.,2003)by using the Peppas-Merrill equation(Peppas and Merrill,1976)(equation(1)).In the equation,v denotes the volume of the polymer af-ter crosslinking in either the relaxed state(subscript r) and in the swollen state(subscript s).V is the molar vol-ume of the medium,M N is the molecular weight of lin-ear polymer chains polymerized in the same conditions without crosslinks,andχis the Flory-Huggins interaction parameter.1c=2N−(v/V1)ln(1−v2,s)+v2,s+χ1v22,sv2,s(v2,s/v2,r)1/3−12(v2,s/v2,r)(1) N is the number of links between crosslinks,which is calculated by equation(2).In this equation,M r is the average molecular weight of the repeating unit.n=2M cM r(2)Equation(3)is used to determine r,the root mean squared end to end distance in the freely jointed state.In this equa-tion,l is the carbon-carbon bond length(1.54angstroms) and n is obtained from equation(2).(r2)1/2=l√n(3) Equation(4)is used to calculate the root mean square end to end distance of the polymer chain in the unperturbed state,where C n is the Flory characteristic ratio or rigidity factor of polymer.r o1/2=C n(r)1/2(4)The above value can be used to calculate the mesh size (ξ)in equation(5).ξ=υ−1/32,sr2o1/2(5)Microreactor Microﬂuidic Systems with Human Microsomes and Hepatocytes121Fig.1.Microsomes with lipids stained with DiOC(a)unﬁltered microsome mixture originally purchased from CellzDirect at10×(b) Filtered microsome mixture at10×(c)Filtered microsome mixture at 40×magniﬁcation.Fig.2.Microsomes entrapped in PEG hydrogel microstructures(2.5×magniﬁcation).To calculate mesh size,hydrogel spheres were fabri-cated with and without nanoparticles.They are investi-gated in the relaxed,hydrated,and dry states.The av-erage mesh size was found to be11.4±1.6Angstroms (standard deviation,n=22)for a precursor solution of 700µL of PEG,300µL of water,10µL of Darocur 1173,and nanoparticles.The average mesh size of hy-drogels without nanoparticles was10.0±2.5(standard deviation,n=71)for a precursor solution of700µL of PEG,300µL of water,and10µL of Darocur1173. There is no signiﬁcant difference between these two val-ues,indicating that100nm microsomes will notleach Fig.3.Leaching of the100nmﬂuorescently tagged particles from a hydrogel over time.122Zguris etal.Fig.4.Investigation of activity of microsomes of the average values between the wavelengths of 468.5±2.5nm (A)product emission of microsomes in solution over time and (B)product emission of microsomes in PEG sphere microstructures over time.from the gel and that the inclusion of nanoparticles in the hydrogels does not signiﬁcantly alter hydrogel mesh size.Cytochrome P450activityThe cytochrome P450activity of microsomes in solu-tion was compared to the activity of the microsomes entrapped in the hydrogel microstructures,to ensure that encapsulation did not disrupt enzymatic activity.As shown in Figures 4(A)and (B),microsomes en-trapped in PEG maintained cytochrome P450activity de-spite undergoing the gelation process.The EFC emission peak increased over time in both microsomes in solution and in hydrogel spheres;no such increase was notice-able in a control sample containing only PEG spheres (Figure 4(C)).Similar experiments were conducted to determine the cytochrome P450activity of hepatocytes both in solution and in spheres (Figures 5(A)and (B)).These graphs show that for the same activity a larger amount ofcellsFig.5.Cytochrome P450activity of hepatocytes in free solution (A)and in PEG hydrogels (B)expressed in counts/second.or total protein are needed.This is due to the concen-trated nature of cytochrome P450present in microsomes and the lack of competitive pathways (Brandon,Raap et al.,2003).This indicates that the microreactor can be tai-lored to the needs of the user whether they want phase I metabolites (microsomes)or a combination of Phase I &II (hepatocytes).Fabrication of microreactors within PDMS microchannelsTo illustrate the feasibility of placing microsome or hep-atocyte microreactors within microﬂuidic channels for easy sample delivery,we generated arrays microsome and hepatocyte microreactors in microﬂuidic networks.Poly-mer precursor solutions containing either microsomes or hepatocytes were placed in the microﬂuidic networks.A chrome mask was positioned over the reactor chamber to generate patterns of hydrogels within the microﬂuidic channel.Arrays of microsomes are shown in Figure 6,while hepatocytes are shown in Figure 7.Microreactor Microﬂuidic Systems with Human Microsomes and Hepatocytes123(A)Fig.6.Images of microsomes in microﬂuidic channels.(A)Schematic drawing of hydrogel structures within in the microﬂuidic chamber.(B)2.5×Fluorescent image of the channel with hydrogel structures with microsomes stained with DIOC lipid stain.(C)20×Fluorescent image of one hydrogel structure in the channel with microsomes stained with DIOClipid.(A)(C)Fig.7.Array of hydrogels containing hepatocytes:(A)schematic drawing,(B)ﬂuorescent,and (C)an enlarged image of one array element.124Zguris et al.ConclusionWe have demonstrated the ability to entrap microsomes in a poly(ethylene)glycol hydrogel microstructures without loss of activity during the gelation process.The activ-ity of cytochrome P450in microsomes in PEG structures was compared with the activity of hepatocytes in simi-lar microstructures.It was shown that fewer microsomes are needed than hepatocytes to generate the same level of activity in this system.Hydrogel arrays containing mi-crosomes and hepatocytes were created in microﬂuidic channels for easy sample delivery.Additionally,these de-vices can be coupled to mass spectrometers to create a device that requires little of the operator.Previously it has been shown that microﬂuidic devices have been cou-pled to MALDI-TOF MS(Brivio et al.,2002;Gustafsson et al.,2004)and electrospray ionization-mass spectrome-try(Kim and Knapp,2001;Chiou et al.,2002;Benetton et al.,2003;LeGac et al.,2004),which are both used to detect metabolite products currently. AcknowledgmentWe thank NASA(contract BIOTECH-01-0023-0131)for their kind support.ReferencesN.A.Alcantar,E.S.Aydil et al.,“Polyethylene glycol-coated biocom-patible surfaces,”J Biomed Mater Res51(3),343–351(2000).S.Benetton,J.Kameoka et al.,“Chip-based P450drug metabolism cou-pled to electrospray ionization-mass spectrometry detection,”Analyt-ical Chemistry:A-G.(2003).J.Berger,M.Reist et al.,“Structure and interactions in covalently and ionically crosslinked chitosan hydrogels for biomedical applications,”European Journal of Pharmaceutics and Biopharmaceutics(57),19–34(2003).E.F.A.Brandon,C.D.Raap et al.,“An update on in vitro test methods in human hepatic drug biotransformation research:Pros and cons.”Toxicology and Applied Pharmacology(189),233–246(2003).E.F.A.Brandon,C.D.Raap,I.B.Meijerman,H.Jos,and Jan H.M. Schellens,“An update on in vitro test methods in human hepatic drug biotransformation research:Pros and cons,”Toxicology and Applied Pharmacology189,233–246(2003).M.Brivio,R.H.Fokkens et al.,“Integrated microﬂuidic systems enabling (Bio)chemical reactions with on-line MALDI-TOF mass spectrome-try,”Anal Chem74(16),3972–3976(2002).J.A.Burdick and K.S.Anseth,“Photoencapsulation of osteoblasts in injectable RGD-modiﬁed PEG hydrogels for bone tissue engineering,”Biomaterials23(22),4315–4323(2002).J.T.Buters,C.D.Schiller et al.,“A highly sensitive tool for the assay of cyctochrome P450enzyme activity in rat,dog and man,”Biochemical Pharmacology46(9),1577–1584(1993).N.Chauret,N.Tremblay et al.,“Description of a96-well plate assay to measure cytochrome p4503a inhibition in human liver microsomesusing a selectiveﬂuorescent probe,”Analytical Biochemistry276, 215–226(1999).C.-H.Chiou,G.-B.Lee et al.,“Micro devices integrated with microchan-nels and electrospray nozzles using PDMS casting techniques,”Sen-sors and Actuators B86,280–286(2002).P.R.Contag,“Whole-animal cellular and molecular imaging to acceler-ate drug development,”Drug Discovery Today7(10),555–562(2002).C.L.Crespi,ler et al.,“Microtiter plate assays for inhibition of human,drug-metabolizing cyctochromes P450,”Analytical Biochem-istry248,188–190(1997).G.M.Cruise,D.S.Scharp et al.,“Characterization of permeability and network structure of interfacially photopolymerized poly(ethylene glycol)diacrylate hydrogels,”Biomaterials19(14),1287–1294 (1998).I.A.M.DeGraaf,C.E.Van Meijeren et al.,“Comparison of in vitro prepa-rations for semi-quantitative prediction of in vivo drug metabolism,”Drug Metabolism and Disposition30(10),1129–1136(2003).J.G.DeLuca,G.R.Dysart et al.,“A direct,highly sensitive assay for cytochrome P-450catalyzed O-Deethylation using a novel coumarin analog,”Biochemical Pharmacology37(9),1731–1739(1988).J.Elisseeff,W.McIntosh et al.,“Photoencapsulation of chondrocytes in poly(ethylene oxide)-based semi-interpenetrating networks,”J Biomed Mater Res51(2),164–171(2000).M.Gustafsson,D.Hirschberg et al.,“Integrated sample prepartation and MALDI mass spectrometry on a microﬂuidic compact disk,”Anal Chem76,345–350(2004).J.Hodgson,“ADMET—Turning chemicals into drugs,”Nature Biotech-nology19,722–726(2001).J.Honiger,P.Balladur et al.,“Permeability and biocompatibility of a new hydrogel used for encapsulation of hepatocytes,”Biomaterials 16(10),753–759(1995).U.M.Kent,L.Pascual et al.,“Mechanistic studies with N-benzyl-1-aminobenzotriazole-inactivated CYP2B1:Differential effects on the metabolism of7-ethoxy-4-(triﬂuoromethyl)coumarin,testosterone and benzphetamine,”Archives of Biochemistry and Biophysics423, 277–287(2004).K.K.Khan,“Homotropic versus heterotopic cooperativity of cytochrome P450eryF:A substrate oxidation and spectral titration study,”Drug Metabolism and Disposition31(4),356–359(2003).B.Kim and N.A.Peppas,“Synthesis and characterization of pH-sensitive glycopolymer for oral drug delivery systems,”J.Biomater.Sci.Poly-mer Edn13(11),1271–1281(2002).J.-S.Kim and D.R.Knapp,“Microfabricated PDMS multichannel emit-ter for electrospray ionization mass spectrometry,”J Am Soc Mass Spectrom12,463–469(2001).W.G.Koh,L.J.Itle et al.,“Molding of hydrogel microstructures to cre-ate multiphenotype cell microarrays,”Anal Chem75(21),5783–5789 (2003).W.G.Koh, A.Revzin et al.,“Poly(ethylene glycol)hydrogel mi-crostructures encapsulating living cells,”Langmuir18(7),2459–2462 (2002).S.LeGac,J.Carlier et al.,“Monoliths for microﬂuidic devices in pro-teomics,”Journal of Chromatography B808,3–14(2004).V.Liu and S.N.Bhatia,“Three-dimensional photopatterning of hydro-gels containing living cells,”Biomedical Microdevices4(4),257–266 (2002).V.A.Liu,W.E.Jastromb et al.,“Engineering protein and cell adhesivity using PEO-terminated triblock polymers,”J Biomed Mater Res60(1), 126–134(2002).M.B.,Mellott,K.Searcy et al.,“Release of protein from highly cross-linked hydrogels of poly(ethylene glycol)diacrylate fabricated by UV polymerization,”Biomaterials22,929–941(2001).S.Myers and A.Baker”Drug discovery—an operating model for a new era,”Nature Biotechnology19,727–730(2001).。

chem.rev.2010

Charge Photogeneration in Organic Solar CellsTracey M.Clarke and James R.Durrant*Centre for Plastic Electronics,Department of Chemistry,Imperial College London,London,SW72AZ,United KingdomReceived August5,2009Contents1.Introduction Aanic Solar Cells A1.2.Charge Photogeneration D2.Theoretical Background F2.1.Properties of Excitons F2.2.Nonadiabatic Electron Transfer Theory F2.3.Onsager Theory H3.Charge Photogeneration in Organic BulkHeterojunction Solar CellsJ3.1.Charge Separation Interface J3.2.Evidence for the Presence of InterfacialCharge Transfer States in Organic Donor/Acceptor FilmsL3.3.Exciton and Charge Transfer-State BindingEnergiesN3.4.Photophysics of Charge Photogeneration inOrganic FilmsO3.4.1.Interfacial Energetics:Singlet and TripletExciton Energies versus CT-State EnergiesP3.4.2.Geminate versus BimolecularRecombination KineticsQ3.4.3.Ultrafast Spectroscopy of Charge TransferStates in Organic FilmsQ3.5.Functional Importance of Charge TransferStates for Charge PhotogenerationR3.6.Role of Excess Thermal Energy in DrivingCharge DissociationS3.7.Role of Electric Fields in Driving ChargeDissociationT3.8.Role of Nanomorphology in Charge Dissociation V3.9.Inﬂuence of Thermal Annealing on ChargePhotogenerationY3.10.Other Factors Inﬂuencing ChargePhotogeneration:Inﬂuence of Molecular/Interface StructureZ3.11.Summary of the Role of Charge-TransferStates in Inﬂuencing Charge PhotogenerationAB4.Implications for Organic Photovoltaic Materials andDevice DesignAB5.Acknowledgments AC6.References AC 1.Introductionanic Solar CellsIn recent years,organic solar cells utilizingπ-conjugated polymers have attracted widespread interest in both the academic and,increasingly,the commercial communities. These polymers are promising in terms of their electronic properties,low cost,versatility of functionalization,thinﬁlm ﬂexibility,and ease of processing.These factors indicate that organic solar cells,although currently producing relatively low power conversion efﬁciencies(∼5-7%),1–3compared to inorganic solar cells,have the potential to compete effectively with alternative solar cell technologies.However, in order for this to be feasible,the efﬁciencies of organic solar cells need further improvement.This is the focus of extensive studies worldwide.The backbone of aπ-conjugated polymer is comprised of a linear series of overlapping p z orbitals that have formed via sp2hybridization,thereby creating a conjugated chain of delocalized electron density.It is the interaction of theseπelectrons that dictates the electronic charac-teristics of the polymer.The energy levels become closely spaced as the delocalization length increases,resulting in a‘band’structure somewhat similar to that observed in inorganic solid-state semiconductors.In contrast to the latter,however,the primary photoexcitations in conjugated polymers are bound electron-hole pairs(excitons)rather than free charge carriers;this is largely due to their low dielectric constant and the presence of signiﬁcant electron-lattice interactions and electron correlation ef-fects.4In the absence of a mechanism to dissociate the excitons into free charge carriers,the exciton will undergo radiative and nonradiative decay,with a typical exciton lifetime in the range from100ps to1ns.Achieving efﬁcient charge photogeneration has long been recognized as a vital challenge for molecular-based solar cells.For example,theﬁrst organic solar cells were simple single-layer devices based on the pristine polymer and two electrodes of different work function.These devices,based on a Schottky diode structure,resulted in poor photocurrent efﬁciency.5–7Relatively efﬁcient pho-tocurrent generation in an organic device wasﬁrst reported by Tang in1986,8employing a vacuum-deposited CuPc/ perylene derivative donor/acceptor bilayer device.The dif-fering electron afﬁnities(and/or ionization potentials)be-tween these two materials created an energy offset at their interface,thereby driving exciton dissociation.However,the efﬁciency of such bilayer devices is limited by the require-ment of exciton diffusion to the donor/acceptor interface, typically requiringﬁlm thicknesses less than the optical absorption anic materials usually exhibit exciton diffusion lengths of∼10nm and optical absorption depths of100nm,although we note signiﬁcant progress is now being made with organic materials with exciton diffusion lengths comparable to or exceeding their optical absorption depth.9–12The observation of ultrafast photoinduced electron transfer13,14from a conjugated polymer to C60and the*To whom correspondence should be addressed.E-mail:j.durrant@.Chem.Rev.XXXX,xxx,000–000A10.1021/cr900271s XXXX American Chemical Societyconsequent enhancement in charge photogeneration yield opened up the potential to employ solution-processable polymers in the next generation of solar cells.A key breakthrough in the utilization of such materials was the introduction of the bulk heterojunction15to overcome the primary issue in bilayer devices:the short exciton diffusion length limiting the active layer thickness(this is generally a more severe limitation for polymers than small molecule materials).Bulk heterojunctions involve a bicontinuous interpenetrating network of the electron donor(polymer)and acceptor materials(fullerene).This enhances the donor/ acceptor interfacial area available for exciton dissociation and thus reduces the distance the exciton needs to travel before reaching an interface.Charge photogeneration is therefore increased in this case.The overall function of such bulk heterojunction solar cells is illustrated in Figure1and reviewed elsewhere in this issue.16The process of charge photogeneration in a polymer: fullerene blend can be summarized in a series of sequential steps as follows.Absorption of a photon,usually by the donor,generates the exciton.The exciton subsequently diffuses until it reaches the polymer:fullerene interface,where it may be quenched by electron transfer from the polymer to the fullerene(see section 3.1for a more detailed description).However,this electron transfer does not neces-sarily directly generate free(dissociated)charge carriers. Despite being located on different materials,the electron and hole pair are still expected to exhibit signiﬁcant Coulomb attraction(of the range0.1-0.5eV),potentially resulting in the formation of Coulombically bound interfacial elec-tron-hole pairs.The overall process of charge photogenera-tion requires dissociation of these initially generated bound electron-hole pairs into free charge carriers,which can then be transported to and collected by the device electrodes.The term‘charge photogeneration’used herein refers to the yield of fully dissociated(free)charges.A key challenge for the development of organic pho-tovoltaic devices is to develop a predictive understanding of the relationship between molecular structure and photovoltaic device performance.The molecular structures of the main polymers and acceptors discussed in this review are displayed in Figure 2.These molecular structures can inﬂuence device performance in many ways; for example,the HOMO/LUMO energy gap governs the optical band gap of the device(although the exciton binding energy also needs to be taken into account),while the charge carrier mobilities inﬂuence the charge collection efﬁciency.In this review,we focus on one speciﬁc aspect of the materials structure/device function relationship:the efﬁciency of charge photogeneration at the donor/acceptor interface.In particular,we concentrate on the behavior of the interfacial bound electron-hole pair states referred to above,which appear to be of great signiﬁcance in organic photovoltaic devices.These Coulombically bound electron-hole pair states are comprised of partially charge-separated states,where the hole is primarily localized on the donor HOMO orbital and the electron on the acceptor LUMO orbital but where the Coulomb attraction between the electron and hole remains signiﬁcantly greater than k B T.A widely ranging nomenclature is currently employed in the literature to refer to such states,including geminate pairs,boundTracey M.Clarke obtained her Ph.D.degree in2007at the University of Otago(New Zealand),studying computational chemistry and Raman spectroscopy of oligothiophenes.After this,she accepted a postdoctoral research associate position at Imperial College London,working with James Durrant on the spectroscopic characterization and analysis of polymer-based solar cells and their constituent materials,using techniques such as transient absorption spectroscopy.James R.Durrant is Professor of Photochemistry in the Department of Chemistry,Imperial College London,and Deputy Director of Imperial College’s Energy Futures Lab.Following undergraduate studies in Physics, his Ph.D.and postdoctoral studies focused on the primary processes of plant photosynthesis.He joined the Chemistry Department in1999,where he established an interdisciplinary research group focusing upon chemical approaches to solar energy conversion.His research is based around employing photochemical studies to elucidate design principles which enable technological development.His group is currently researching organic and dye-sensitized nanostructured solar cells as well as photo-electodes for solar fuel generation.He has published over170research papers and5patents and has recently been awarded the2009 Environment Prize by the Royal Society of Chemistry.Figure1.Schematic of the charge photogeneration and transport processes in a polymer:fullerene photovoltaic device.Reprinted with permission from ref17.Copyright2008Material Research Society.B Chemical Reviews,XXXX,Vol.xxx,No.xx Clarke andDurrantpolaron (or electron -hole or radical)pairs,charge-transfer excitons,and exciplexes.18–28In some cases the nomen-clature differs to reﬂect particular characteristics of these states.For example,the term ‘charge-transfer state’has been used to refer to such interfacial states that exhibit an observable transition dipole to the molecular ground state,as evidenced by the observation of weak charge-transfer optical absorption and (electro)luminescence bands.18,29–32In some cases,this luminescence can be relatively intense,appearing as a red-shifted and broadened photoluminescence (PL)19,25analogous to that observed for bound excited-state complexes (exciplexes)in solution.In other cases,different terminology is used to differentiate between thermally hot and thermally relaxed charge-transfer states.21For simplicity,throughout this review we will follow the most commonly used terminology and refer to all such states as ‘charge-transfer’(CT)states.This terminology indicates that such states are intermediate in terms of charge separation between excitons and fully dissociated charges.We note however that this ‘charge-transfer’terminology does not necessarily imply the observa-tion of radiative coupling to the electronic ground state.Several factors inﬂuence the overall process of photocur-rent generation in organic solar cells,as has been the subject of several recent reviews.7,33–40The photocurrent generation efﬁciency is directly related to the fraction of light absorbed by the blend ﬁlm,7which is a function of the absorption spectra (e.g.,optical band gap),the absorption strength (extinction coefﬁcient),and the absorbing layer thickness.Conjugated polymers typically have high extinction coef-ﬁcients,but their optical band gaps are often not well matched to the solar emission spectrum,thus limiting the fraction of solar radiation absorbed.In addition,the nano-morphology created upon blending the two materials together is crucial in determining the efﬁciency of exciton diffusion to the donor/acceptor interface as well as,potentially,the efﬁciency by which the interfacial charge-transfer states can dissociate into free charges.The energy levels of the donor and acceptor are also critical to ensure efﬁcient exciton quenching at the interface and to determine whether thisFigure 2.Structures of the polymers (and PCBM)discussed in this review.Charge Photogeneration in Organic Solar Cells Chemical Reviews,XXXX,Vol.xxx,No.xxCquenching is associated with electron or energy transfer processes.41Following donor/acceptor electron transfer,two recombination pathways may compete with photocurrent generation.Geminate recombination of the initially generated bound charges(or charge-transfer states)back to the ground state may compete with dissociation of these charges into free charge carriers.Alternatively,bimolecular recombina-tion,the recombination of the dissociated free charge carriers, competes with charge transport to the electrodes and therefore may inﬂuence photocurrent generation.1.2.Charge PhotogenerationThe primary process in almost all electronic(as opposed to thermal)solar energy conversion systems is the utilization of incident solar energy to generate separated electronic charges. These energetic charges can subsequently be used for electrical power generation(photovoltaics)or employed to drive redox chemistry(photosynthesis).Efﬁcient energy transduction re-quires separation of this photogenerated electron-hole pair into long-lived dissociated charges with a high quantum yield and minimal loss of free energy.A potential concern in this charge-separation process is that the electron and hole must overcome their mutual Coulomb attraction,VV)e24πεrε0r(1)where e is the charge of an electron,εr is the dielectric constant of the surrounding medium,ε0is the permittivity of vacuum,and r is the electron-hole separation distance. Gregg et al.42presented a comparison of inorganic and organic semiconductors on the basis of their respective Coulomb potentials.In conventional inorganic photovoltaic devices,such as those based on silicon p-n junctions, overcoming the Coulomb attraction is facile due to the highdielectric constant of silicon(εr≈12)and because the electronic states involved are already highly delocalized (corresponding to a larger average‘r’in eq1).Similarly, dye-sensitized photoelectrochemical solar cells exhibit a high dielectric constant for the electron acceptor nanoparticles (TiO2,εr≈80)in addition to a high ionic strength electrolyte. As a result,after the initial electron injection step from the molecular excited state,the Coulomb attraction of electrons and holes in dye-sensitized photoelectrochemical cells is effectively screened.43However,for solar cells based upon on molecular materials,overcoming this Coulomb attraction is signiﬁcantly more demanding due to both their smaller dielectric constant(εr≈2-4)and the more localized nature of the electronic states involved.As such,achieving efﬁcient charge photogeneration is a key challenge for solar energy conversion technologies based upon molecular materials. The most sophisticated examples of molecular-based solar energy conversion systems are undoubtedly the natural photosynthetic reaction centers.Despite the wide range of photosynthetic organisms,the primary functional components of their reaction centers are remarkably invariant.Charge photogeneration in such reaction centers is achieved by the use of a redox relay or cascade,as illustrated in Figure3. Light absorption by the reaction center results,after various energy transfer steps,in the formation of a short-lived (nanoseconds)molecular singlet excited state localized on a primary donor species.Charge separation is achieved by an energetically downhill electron transfer from the donor excited-state LUMO orbital to a neighboring molecular acceptor,achieving an initial separation of the electron and hole.Subsequent secondary electron(and hole)transfers produce further charge separation,resulting in the almost unity quantum yield generation of a long-lived(milliseconds to seconds)charge-separated state.These long-lived charge pairs are subsequently coupled to multielectron redox chemistry that ultimately results in the storage of the incident solar energy in chemical bonds(sugars).However,as is apparent from Figure3b,the photoinduced charge separation achieved in the natural photosynthetic reaction centers comes at a signiﬁcant energy cost.The long-lived charge-separated state generated in these reaction centers typically retains only one-half the free energy of the initial molecular excited state. This loss of free energy is required both to drive the sequence of electron transfer steps at sufﬁcient rates to compete with the unwanted charge recombination pathways back to the ground state and to prevent thermally driven electron transfer back to the initial short-lived excited state.It is thus apparent that even for these highly evolved photosystems,charge photogeneration comes at a signiﬁcant energy cost,which of course has an impact upon the overall energy conversion efﬁciency of biological photosynthesis.There have been numerous studies of photoinduced charge separation in molecular mimics of the natural photosystems based on molecular donor/acceptor redox relays.Extensive studies of these donor/acceptor systems in dilute solution have led to a detailed understanding of their structure/ function relationship in terms of nonadiabatic electron transfer theory45–48(see section2.2for summary).This has Figure3.(a)Schematic of the photosystem II reaction center of higher plants.Reprinted with permission from ref44.Copyright 2004Elsevier.(b)State diagram schematic of the energetics and kinetics of charge separation in this reaction center.D Chemical Reviews,XXXX,Vol.xxx,No.xx Clarke andDurrantin turn led to impressive advances in the molecular control of electron transfer dynamics in such systems.It has been shown,for example,that charge photogeneration in these systems is dependent upon the use of redox relays to increase the spatial separation of the charges and on control of the state energetics to minimize activation barriers to forward charge separation while ensuring large activation barriers for the undesired recombination pathways,leading to reports of remarkably efﬁcient,long-lived charge separation.45,46,48,49 The use of solidﬁlms allows,in principle,the formation of percolation pathways to achieve the electrical‘wiring’of charge photogeneration at the donor/acceptor interface to external device electrodes,as illustrated in Figure1.Typical examples of organic donor/acceptor heterojunction photo-voltaic devices are polymer:fullerene blend and small molecule bilayer solar cells.The mechanism of charge photogeneration in such devices is of particular interest. These organic materials typically do not exhibit the highly delocalized electronic states,weak electron-lattice interac-tions,and high dielectric constant present in silicon solar cells.Moreover,the interfaces are typically fabricated from one donor and one acceptor material and therefore do not exhibit the redox relay of distinct molecular donor and acceptors utilized in photosynthetic reaction centers to achieve efﬁcient charge photogeneration.The mechanism by which such devices can overcome the Coulomb attraction of the photogenerated electron-hole pair,and in particular achieve this with a high quantum and energy efﬁciency,is central to the development of such devices.The signiﬁcance of this charge photogeneration challenge for organic solar cells can be readily appreciated by simple consideration of the energetics of the donor/acceptor inter-face.It is generally agreed that the initial charge-separation step,electron transfer from the donor exciton state into the acceptor conduction band(or the reverse hole transfer from acceptor excited states),can be achieved by utilizing a suitable energy offset between the donor and acceptor LUMO levels,as illustrated in Figure4a.This requires that the energy offset is greater than the Coulomb binding energy of the exciton,E B exc(we discuss the details of these energetics in section3.3below),enabling the initial electron transfer step to be energetically downhill.50However,as the donor and acceptor molecules are physically adjacent at the charge-separation interface,this initial electron transfer step results in only a modest spatial separation of the electron and hole,typically of the same order of magnitude as the size of the molecules(or molecular orbitals)concerned:0.5-1nm.At this spatial separation,the electron and hole still exhibit a signiﬁcant Coulomb attraction.Employing a simple point charge approximation(eq1),the magnitude of this Coulomb attraction can be estimated to be in the range0.1-0.5eV, as illustrated in Figure4b(we will refer herein to this attraction as the charge-transfer-state binding energy E B CT). The magnitude of this Coulomb attraction is clearly sizable compared to the thermal energy(0.025eV)and,in principle, represents a large energetic barrier to charge photogeneration at donor/acceptor organic interfaces.There now exists several donor/acceptor organic hetero-junctions that can yield photovoltaic devices with near unity quantum yields of photocurrent generation and increasingly efﬁcient overall device efﬁciencies.Thus,it is clear that it is possible to achieve efﬁcient charge photogeneration at organic donor/acceptor interfaces,even when employing low dielectric constant materials and without the use of redox relays.There are,however,many more examples of organic donor/acceptor heterojunctions that achieve efﬁcient exciton quenching at the donor/acceptor interface but do not achieve efﬁcient photocurrent generation.This can often be attributed to inefﬁcient charge percolation or transport,i.e.,poor ‘wiring’of the interface to the device electrodes.However, in many other systems the charge collection process appears to be relatively efﬁcient;in such cases it appears likely that poor photocurrent generation is related to the efﬁciency of charge photogeneration at the donor/acceptor interface. Indeed,many reports have related subunity photocurrent quantum yields to charge photogeneration limitations.21,25,38,52,53 Moreover,the voltage dependence of charge photogeneration has been widely proposed as a key factor limiting the voltage output of such devices.27,32,54It is thus apparent that understanding the mechanism of charge photogeneration at donor/acceptor interfaces is not only of academic interest but also of technological importance for the development of efﬁcient organic photovoltaic devices.Figure4.(a)Energy level diagram of a donor/acceptor interface showing a simpliﬁed viewpoint of photoexcitation of an electron into the donor LUMO followed by electron transfer into the acceptor LUMO and migration of the separated charges away from the interface.(b)Illustration of the formation of interfacial electron-hole pairs or charge-transfer(CT)states.The energy of this state depends upon the Coulomb attraction of the electron and hole and therefore their spatial separation,as illustrated by the dotted curve.(b)Typical binding energies for the exciton and CT states(E B exc and E B CT, respectively).E B exc corresponds to the difference between the optical and electrochemical band gaps.Note that for simplicity,the energy of the exciton and CT states are shown relative to the polaron HOMO level.Figure4b adapted with permission from ref51. Copyright2008American Physical Society.Charge Photogeneration in Organic Solar Cells Chemical Reviews,XXXX,Vol.xxx,No.xxEOur understanding of the mechanism of charge photoge-neration in organic solar cells remains incomplete.However, signiﬁcant progress has been made over the past few years, both experimentally and theoretically,on this topic.In particular,progress has been made in understanding the roles of macroscopic electricﬁelds,interfacial energetics,and nanomorphology in determining the efﬁciency of this process.In this review,we discuss this progress,focusing in particular upon the role of interfacial charge-transfer states in inﬂuencing the efﬁciency of photocurrent generation in organic solar cells.The following section will summarize two signiﬁcant theories relevant to charge separation at such donor/acceptor interfaces:nonadiabatic electron transfer theory and the Onsager theory of charge dissociation.The third section will concentrate on the current understanding of charge photogeneration in polymer:fullerene solar cells from the viewpoint of the charge-transfer state:its formation, recombination,and dissociation.Theﬁnal section will discuss the implications for materials and photovoltaic device design.2.Theoretical Background2.1.Properties of ExcitonsPhotoexcitation of a conjugated polymer generates a singlet excited state in which the electron and hole are still inﬂuenced by a strong Coulomb attraction;this state is termed a singlet exciton.The Coulomb interaction causes a large binding energy for the exciton(E B exc)that is signiﬁcantly greater than the thermal energy k B T.Other contributors to the appreciable binding energy are electron-lattice and electron-electron interactions.4The exciton is required to overcome this binding energy in order to dissociate within its lifetime and achieve charge photogeneration;this typically occurs at the interface with an electron acceptor but can also occur through interactions with impurities and defect sites or under the inﬂuence of an applied electricﬁeld.The properties of excitons in polymer materials have been studied extensively using both experimental and theoretical meth-ods.4,55–61Some of these properties,such as the exciton binding energy and diffusion length,are important parameters for organic solar cells as they strongly inﬂuence the prob-ability of exciton dissociation and thus the efﬁciency of charge photogeneration.The exciton of a conjugated polymer is spatially more localized compared to excitations in inorganic semiconduc-tors.The creation of this localized electronic excited state in a conjugated polymer is accompanied by a localized relaxation of the surrounding molecular structure.4This occurs because of the strong electron-phonon coupling in these materials,and the term‘exciton’in this context therefore refers to both the electronic excitation and the local structural deformation it induces.In nondegenerate polymers such as polythiophene,this structural relaxation is character-ized by a reversal of the C-C bond length alternation to create a domain of semiquinoidal bond sequences localized over several monomer units.The magnitude of this bond length reversal is greatest at the center of the exciton and progressively diminishes along the chain until an aromatic bond structure is restored.This bond rearrangement can result in a relatively more rigid,planar geometry in the excited state compared to the ground state,as is evident from photoluminescence spectra as an enhancement in the vibra-tionalﬁne structure compared to the inhomogeneously broadened ground-state absorption spectra.62,63Note that both polymer polarons and triplet excitons also possess these localized structural deformations.64–67The spatial extent of the structural changes depends upon the nature of the excitation and the chemical structure.In PPVs,for example, photoluminescence data suggest that the singlet exciton extends over six monomer units,68whereas the triplet exciton of PPV has been reported to have a much more pronounced structural deformation localized over a shorter length:only one or two monomer units.58These structural relaxation effects(electron-lattice interactions)are part of the reason for the relatively high exciton binding energies in conjugated polymers compared to conventional inorganic semiconduc-tors.4One of the other reasons is the low dielectric constant, which prevents effective screening of charge and results in a large Coulombic interaction between the excited electron and hole,as discussed above.Efﬁcient device performance relies upon the photogener-ated exciton moving to a donor/acceptor interface so that exciton dissociation can occur.69Due to their electrical neutrality,the motion of excitons is not affected by electric ﬁelds,and thus,they diffuse through the blend randomly. This diffusion is typically described as a Fo¨rster-type incoherent energy transfer process,which can be either intramolecular or intermolecular and usually acts to lower the energy of the exciton.This downhill energy transfer can result in trapping of the exciton in the tail of the inhomo-geneously broadened density of states;the trap sites are also often associated with defects and aggregates.At this point, any further exciton migration will rely on thermalﬂuctua-tions.At the same time,if the initially generated exciton is vibrationally excited,the diffusion process allows time for intramolecular vibrational relaxation to take place(within 100fs).55Exciton dissociation can therefore occur from either the vibrationally excited‘hot’exciton(i.e.,the Franck-Condon state)70–72or the thermally equilibrated,geometrically relaxed,‘cold’exciton.55,73Another important parameter of polymer excitons is the diffusion length,the distance an exciton can migrate before relaxing back to the ground state.Dissociation of the exciton into charges must therefore occur within this distance. Clearly,this will limit the extent of phase segregation possible in a bulk heterojunction blend for efﬁcient device performance.In general,phase segregation on the order of the exciton diffusion length is desired.A typical method of estimating this parameter is to assess the photoluminescence quenching of polymer/acceptor bilayers as a function of layer thickness.Measurements of this type,56,59,74and also using photocurrent experiments,60have yielded exciton diffusion lengths of5-14nm for PPV derivatives and3-9nm for P3HT.75,762.2.Nonadiabatic Electron Transfer TheoryOnce the photogenerated exciton has diffused to a donor/ acceptor interface,it can be dissociated by an interfacial electron transfer reaction.In cases where the donor and acceptor phases are intimately mixed,such that excitons are generated directly at the donor/acceptor interface and thus avoiding the need for exciton diffusion,this electron transfer reaction can occur extremely rapidly,on the femtosecond time scale.14,77,78Marcus’s theory of semiclassical nonadia-batic electron transfer,ﬁrst developed in1956,79has been successfully applied to a number of chemical systems47,80 and also been extended to photoinduced charge transfer in conjugated polymer blends.81–84Marcus theory considers theF Chemical Reviews,XXXX,Vol.xxx,No.xx Clarke and Durrant。

化学反应工程英文课件Chapter 5

t NA0
XA 0 XA dXA dXA CA0 0 ( r )(1 X ) ( rA )V0 (1 A XA ) A A A
(5)
化学反应工程
In one form or another, Eqs.2 to 5 have all been encountered in Chapter 3. They are applicable to both isothermal and nonisothermal operations. For the latter the variation of rate with temperature, and the variation of temperature with conversion, must be known before solution is possible. Figure 5.2 is a graphical representation of two of these equations.
accumulati on of A, moles/time
dNA d [ N A0 (1 X A )] dX A N A0 dt dt dt
化学反应工程
By replacing these two terms in Eq.1, we obtain
( rA )V N A 0
rate of loss of reactant A or within reactor due to chemical reaction rate of accumudation of reactant A within the reactor (1)
In the treatment to follow it should be understood that the term V, called the reactor volume, really refers to the volume of fluid in the reactor. When this differs from the internal volume of reactor, then Vr ‘designates(指明,任命) the internal volume of reactor while V designates the volume of reacting fluid. For example, in solid catalyzed reactors with voidage (空隙率) we have V = Vr For homogeneous systems, however, we usually use the term V alone.

双面神Janus 粒子

DOI:10.1002/cphc.201402111Self-Propelled Janus Mesoporous Silica Nanomotors with Sub-100nm Diameters for Drug Encapsulation and DeliveryMingjun Xuan,[a]Jingxin Shao,[a]Xiankun Lin,[a]Luru Dai,*[b]and Qiang He*[a]1.IntroductionIt remains a challenge to deliver therapeutic and diagnostic agents to predefined locations of the body by current drug-de-livery systems.Thus,new-generation drug-delivery vehicles are required to integrate self-driven and navigation capabilities,so that they could achieve not only smart encapsulation and re-lease,but also precise guidance and control.In the realm of biology,autonomous molecular motors,such as kinesin,utilize chemical energy derived from the hydrolysis of adenosine tri-phosphate (ATP)to process motion on microtubules at the nanoscale.[1]Such directed motion plays a crucial role in active transport within a cell.Inspired by the motion of biomolecular motors,substantial efforts have been devoted to making versa-tile,catalytic artificial motors over the past few years.[2–7]In most cases,autonomous motion of artificial motors is achieved by employing colloidal particles with nonuniform surface prop-erties (i.e.Janus particles),and thus,their motion is greatly de-termined by the gradient of an external field or gas-bubble propulsion.[8–12]Janus particles are able to generate their own gradient in the surrounding medium and then convert avail-able energy into mechanical work (i.e.self-phoresis and bubble driven).For example,Janus silica microparticles containinga platinum component could generate oxygen bubbles due to the catalytic decomposition of hydrogen peroxide fuel,and thus,cause self-propulsion.[13,14]It has been demonstrated that these chemically powered synthetic motors are capable of the pick-up,transportation,and release of various cargo,including polymer particles,cancer cells,nucleic acids,and bacteria.[15–18]However,they have some inherent limitations,such as com-plex preparation technology,poor biocompatibility,biodegrad-ability,and large sizes.More specifically,on the basis of physiological parameters such as hepatic filtration,tissue extravasation/diffusion,and kidney excretion,[19]it is clear that particle size plays a key factor in the circulation and biodistribution of nanoparticles.Nanoparticles with diameters of less than 100nm had a higher potential to circulate in the blood for a long period of time and experienced reduced hepatic pared with the first chemical man-made motor,the dimensions of chemically powered synthetic motors have been gradually decreased from centimeters to several hundreds of nanometers.[10,20,21]However,a sub-100-nanometer autonomous motor is still a great challenge because of several technical barriers in the fabrication of synthetic nanomotors,motor–cargo integration,post-modification,and functionalization.In this work,we successfully constructed a well-defined Janus mesoporous silica nanoparticle (MSN)nanomotor with a diameter of less than 100nm through a vacuum metal sput-ter deposition method.Sub-100-nanometer MSNs that are uni-form in size,porous,and dispersible have potential application as “nanocarriers”in drug delivery.MSNs have exceptionally high surface areas (exceeding 1000m 2g À1)and are able to in-dependently modify pore size and surface chemistry,[22–28]which enables the loading of diverse cargo and cargo combi-nations at a level higher than that achieved withother[a]M.Xuan,J.Shao,Dr.X.Lin,Prof.Q.HeState Key Lab of Urban Water Resource and Environment Micro/Nanotechnology Research Centre Harbin Institute of Technology Harbin 150080(P .R.China)E-mail:qianghe@[b]Dr.L.DaiCAS Key Lab for Biomedical Effects of Nanomaterials and Nanosafety National Center for Nanoscience and Technology Beijing 100190(P .R.China)E-mail:dai@Supporting Information for this article is available on the WWW under /10.1002/cphc.201402111.CHEM PHYS CHEMARTICLEScommon drug-delivery carriers,such as liposomes or polymer conjugates.[17,29]In the present study,one hemisphere of Janus MSN nanomotors is uncovered and retains the original meso-porous structure,so that drug molecules can be encapsulated and released from this smart carrier.The other hemisphere is deposited on a thin platinum film with a thickness of about 7nm.The platinum layer serves as an engine to catalytically decompose hydrogen peroxide fuel to eject oxygen bubbles,which in turn give rise to a surface tension gradient in the sur-rounding medium,and thus,lead to self-propelled motion of the Janus MSNs.Additionally,due to the small size of the pre-pared Janus MSN nanomotors,a homemade high-resolution optical microscope was employed to record the motion of Janus MSN nanomotors.Intracellular localization and drug re-lease in vitro revealed that it took fluorescently labeled Janus MSN nanomotors a shorter time to enrich the inside of HeLa cells relative to normal MSNs,and subsequently,the anticancer drug doxorubicin hydrochloride (DOX)encapsulated in the Janus MSNs was slowly released with catalytic hydrolysis of lipid bilayers by cellular enzymes.2.Results and Discussion2.1.Synthesis and Characterization of Janus MSN NanomotorsSelf-propelled Janus MSN nanomotors with sub-100-nanome-ter diameters were prepared and used for drug delivery as il-lustrated in Scheme 1.Initially,spherical MSNs were synthe-sized according to a modified Stçber method.The average particle diameter was tuned to about 65nm and confirmed by transmission electron microscopy (TEM)and scanning electron microscopy (SEM;Figure 1a and Figure S1ab in the Supporting Information).The TEM image in Figure 1a clearly shows that as-synthesized MSNs have regular hexagonal-shaped ordered channels and the pore size is estimated to be about 3.4nm (Figure S2in the Supporting Information).Once the sub-100-nanometer MSNs were obtained,they were asymmetrically functionalized by following a similar method to that described elsewhere.[30–32]Briefly,MSNs were spread on a silicon slide to form a two-dimensional MSN monolayer (Figure S1a in theSupporting Information)and subsequently coated with an ad-hesive layer (Cr)and a catalytic layer (Pt)by using a sputter coater.After brief sonication of the silicon wafer in water,Janus MSN nanomotors were released and suspended in de-ionized water.TEM,SEM,and EDX elemental mapping results displayed the morphology of the MSNs coated on one side with metallic layers (Figure 1b)and the presence of complex platinum layers on the MSNs (Figure 1c and d).Also,the ele-mental content curve in Figure 1c confirms the intensity of the platinum layer deposited on the MSNs;the thickness was esti-mated to be approximately 7nm.2.2.Motion Analysis for Janus MSN NanomotorsAs illustrated in Scheme 1,the resulting Janus MSN nanomo-tors are self-propelled in a solution of hydrogen peroxide by oxygen bubbles generated from the catalytic decomposition of hydrogen peroxide fuel.Because the size of the as-prepared Janus MSN nanomotors is roughly 75nm,the resolution of normal optical microscopes cannot satisfy requirements.Thus,a homemade high-resolution optical microscope was used to track,in real time,the fast movement of Janus MSN nanomo-tors in aqueous solutions in fluorescence mode.In this case,a fluorescent-labeled component,rhodamine–poly(allylamine hydrochloride)(Rhd-PAH)was chosen to mark the nanomotors.Figure 2a displays an image taken from Video S1in the Sup-porting Information of the movement of Janus MSN nanomo-tors in a 10%solution of hydrogen peroxide.The trajectories show the autonomously directional movement coupled with a significant increase in speed relative to that of JanusMSNsScheme 1.Synthetic procedure for the preparation of Janus MSN nanomo-tors,as well as subsequent drug loading,lipid bilayer functionalization,transportation,and drugrelease.Figure 1.a)TEM image of original spherical MSNs with regular hexagonal-shaped ordered channels.b)SEM image of the MSN monolayer after sputter-ing with a metallic layer (Pt);the inset shows a TEM image of a Janus MSN.c)Energy-dispersive X-ray spectroscopy (EDX)elemental analysis of a Janus MSN with a Pt layer;the inset gives a scanning transmission electron micros-copy (STEM)image of a Janus MSN;the line indicates the direction profile.d)Pt elemental mapping on the Janus MSN.without hydrogen peroxide fuel (Video S2in the SupportingInformation).From Video S2in the Supporting Information,one can clearly see that the trajectory of Janus MSNs in the ab-sence of hydrogen peroxide followed the typical random walkof Brownian motion and did not show any directionality or in-crease in movement.The average speed of the catalyticallypowered Janus MSN nanomotors is in the order of 6m m s À1(80body length per second)and the motion is visibly non-Brownian.Due to the influence of the surroundings and possi-ble tumbling of the nanomotor itself,swimming trajectories are not straight lines in a long run without controlling the field (Figure 2a),but these trajectories are approximately straight lines in a short run (Figure S3in the Supporting Information).Although the catalytic reaction proceeds with the formation of oxygen bubble,tails similar to those of big Janus micromotors were not observed.[30,31]The velocity of the catalytically powered Janus MSN nano-motors can be readily modulated through a change in the con-centration of the hydrogen peroxide fuel.Figure 2b showsa histogram of the speed distribution of Janus MSN nanomo-tors in different fuel concentrations ranging from 0to 30%(v/v)at 248C.The average speed of Janus MSN nanomotors in-creases from 1.8m m s À1without H 2O 2to 20.2m m s À1with 30%H 2O 2(about 267body lengths per second),which is a signifi-cant increase.The inset image in Figure 2b shows that the speed of movement appears to in-crease with increasing tempera-ture.The speed of movement in 0.2%H 2O 2at 248C is approxi-mately 2.4m m s À1,which is close to that of Brownian motion (1.8m m s À1).The speed of move-ment of the Janus MSN nanomo-tors at 378C (physiological tem-perature),however,can reach 3.6m m s À1in 0.2%H 2O 2.This demonstrates the self-powered transportation ability of Janus MSN nanomotors for biomedical fields,such as directed drug transportation,because the con-centration of hydrogen peroxide fuel at 0.2%has less cytotoxicity in an in vitro environment.It is important to note that,with de-creasing fuel concentrations,the percentage of active nanomo-tors reduces accordingly.As shown in Figure S4in the Sup-porting Information,76%ofJanus MSN nanomotors remain active at 30%H 2O 2,but the per-centage decreases to 22and 28%in a 0.2%solution of H 2O 2at 24and at 378C,respectively.The observed speed–fuel con-centration relationship can also be expressed in terms of diffu-sion coefficient–fuel concentration,as shown in Figure 2c.Dif-fusion coefficients were determined experimentally by using trajectory diffusion methods and the two-dimensional diffusion coefficient (D )is defined as X 2=4Dt ,in which X is the displace-ment in time interval t .[33–35]In the absence of hydrogen perox-ide,the diffusion coefficient of Janus MSN nanomotors calcu-lated by trajectory analysis is 0.6 10À12m 2s À1and is independ-ent of the movement time.This value is notably different fromthose obtained for the same Janus MSNs in aqueous solutions of hydrogen peroxide.Hence,it becomes easy to distinguish self-powered movement from Brownian motion in Figure 2c.Assuming that the forces exerted by the Janus MSN nano-motors are equal to the hydrodynamic drag forces at a steady state,the driving force of a single spherical Janus MSN nano-motor is thus approximately proportional to its velocity atsmall Reynolds numbers and can be estimated by using the Stokes law [Eq.(1)]:F ¼6ph rv ð1Þin which F is the drag force,h is the viscosity of fluid,r is the radius of the sphere,and v is the relative velocity between the fluid and sphere.Equation (1)is only valid if theReynolds Figure 2.Autonomously directional movements of Janus MSN nanomotors.a)Video frame and the trajectories of Janus MSN nanomotors in a 10%solution of H 2O 2in fluorescence mode.Bright dots represent the Rhd-PAH-la-beled MSN nanomotors,and the lines indicate the trajectories of different Janus MSN nanomotors;scale bar =20m m.b)Histograms of the speed distribution of Janus MSN nanomotors in different H 2O 2concentrationsat 248C.The inset shows the motion speed in a 1%solution of H 2O 2at 24and 378C,respectively.c)The diffusioncoefficient of Janus MSN nanomotors in different H 2O 2concentrations or without H 2O 2fuel at 248C.d)Relation-ship of the drag force of Janus MSN nanomotors with different H 2O 2concentrations at 248C.number has a value of less than1.0.[31,35]The Reynolds number of Janus MSN nanomotors is in the order of10À12(as shown in Figure S5in the Supporting Information),and therefore,Equa-tion(1)is valid to calculate the drag force of a Janus MSN motor.Figure2d demonstrates that the drag force of a Janus MSN motor increases with increasing concentration of H2O2. When the concentration of hydrogen peroxide fuel is30%,the maximum speed is20.2m m sÀ1.As a result,the Stokes drag force is calculated to be about13.47 10À15N;this means that the Janus MSN nanomotor can indeed serve as both a powerful motor and a smart cargo because this MSN motor system is in-herently multifunctional,as demonstrated previously.[11]2.3.Intracellular Localization and Drug Release in VitroAs stated in the Introduction,MSNs have an exceptionally high surface area,which enables the loading of diverse cargoes.[22–28] A well-known anticancer drug,DOX,was chosen to examine the capability of drug encapsulation,transportation,and re-lease.DOX was loaded into mesoporous pore channels of Janus MSN motors by physical adsorption,[36,37]and then cov-ered with an egg phosphatidylcholine(PC)bilayer containing 1%1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[fola-te(polyethylene glycol)-2000](folic acid modified DSPE);the addition of the folic acid component is helpful for the adhe-sion of the used MSNs or Janus MSN motors on the surface of cancer cells.[38]The color change of a0.1mg mLÀ1solution of DOX before and after the addition of Janus MSNs(2mg)illus-trated in Figure S6in the Supporting Information shows that most of the DOX molecules were absorbed into the pore chan-nels of the Janus MSNs.The confocal laser scanning microsco-py(CLSM)image in Figure S7in the Supporting Information also shows the successful loading of DOX components in Janus MSN nanomotors.Interestingly,UV measurements of egg PC modified Janus MSN nanomotors with encapsulated DOX in a solution of phosphate-buffered saline(PBS)at pH7.4 at378C demonstrates that lipid bilayers could effectively pre-vent the release of water-soluble DOX molecules from the pores of the Janus MSNs.[38–40]After incubating egg PC modified,DOX-loaded Janus MSN nanomotors with HeLa cells in culture medium containing 0.2%H2O2for1h,they were washed three times with fresh culture medium and then continued to be cultivated in normalculture medium without H2O2fuel for different culture inter-vals(Figure3).Both longitudinal and horizontal profile images from three-dimensional CLSM images were employed to esti-mate the position of the Janus MSNs(Figure4).It can be seen that Janus MSN motors were indeed taken up and located in the cytoplasm of HeLa cells.Next,these Janus MSN-loaded cells were cultivated in normal culture medium without H2O2 fuel for0,1,2,and3h,respectively,as shown in Figure5.The red color in Figure5a comes from encapsulated DOX in the Janus MSNs.With increasing culture time,the red area gradual-ly became larger due to the release of encapsulated DOX into the cytoplasm(Figure5b).DOX release should be triggered by the catalytic hydrolysis of the egg PC bilayers coated on Janus MSN nanomotors by phospholipases inside the cells.[41–43]Sub-sequently,the whole nucleus of HeLa cells turns red and the fluorescence intensity of DOX-loaded Janus MSN motors de-creases accordingly(Figure5c).This is because the DOX mole-cules can recognize DNA chains,and thus,released DOX mole-cules easily enrich the nucleus of HeLa cells.In addition,DOX can inhibit DNA synthesis,and thus,result in striking and col-lapse of HeLa cells,as shown in Figure5d.[44]3.ConclusionsWe have demonstrated the first example of self-propelled sub-100-nanometer Janus MSN nanomotors through a simple metal deposition method.The speed of movement of these nanomotors could be tuned through changing the fuelcon-Figure3.CLSM overlay images of fluorescence image and a differential in-terference contrast(DIC)image of egg PC modified MSNs(a–c)and Janus MSN nanomotors(d–f)incubated with HeLa cells in normal culture medium containing0.2%H2O2for1(a,d),2(b,e),and3h(c,f).Scale bars=10mm.Figure4.CLSM images of DOX-loaded Janus MSN nanomotors inside HeLa cells incubated with normal medium containing0.2%H2O2:a)DOX-loaded Janus MSNs,b)HeLa cells,c)DIC image,d)CLSM horizontal and longitudinal profile images of the overlaid3D images from a)and b).Scale bars=10m m.centration and temperature.More interestingly,the Janus MSN nanomotor combined an autonomous nanomotor and a smart cargo into an integrated system,and thus,was more promis-ing for performing drug loading,targeted transportation,and controlled release in the vicinity of cells and tissues in an or-ganism.Although the peroxide fuel employed for self-propul-sion is still toxic to mammalian cellular functions,biocompat-ible fuels or alternative powered methods have to be devel-oped.However,such self-propelling drug-delivery systems could provide a novel concept to develop rapidly delivering drug carriers in the future.Experimental SectionMaterialsTetraethyl orthosilicate (TEOS)and hexadecyltrimethylammonium bromide (CTAB)were purchased from J&K.NaOH,acetone,metha-nol,H 2O 2were purchased from Beijing Chemical Works,P .R.China.DOX,rhodamine B isothiocyanate,and PAH (M w =70000)were pur-chased from Sigma–Aldrich.PAH was labeled with Rhd according to a procedure reported in the literature.[45]Egg PC and folic acid modified DSPE were purchased from Avanti Polar Lipids.Texas Red 1,2-dihexadecanoyl-sn -glycero-3-phophoethanolamine (DHPE)was purchased from Life Technologies Corporation.Dulbecco’s modi-fied eagle medium (DMEM),0.25%trypsin ethylenediaminetetra-acetic acid (EDTA)and cell membrane marker Alexa Fluor 488dye were purchased from Life Technologies Corporation.Mycoplasma-free fetal bovine serum was purchased from Tian Hang Biological Technologies Corporation,P.R.China.All chemicals were used as re-ceived;all solutions were freshly prepared.Deionized water (Milli-pore Milli-Q)with a resistivity of 18.2M W was used in all the ex-periments.CharacterizationThe metal layer of Janus MSNs was coated in a sputtering coating machine (K575;EMITECH).Electron microscopy images were col-lected by SEM (S-4300;Hitachi)at an operating voltage of 10keV and TEM (Tecnai G2F30;FEI)at an acceleration voltage of 300kV.A homemade high-resolution fluorescence microscope was used to record the movement of Janus MSN nanomotors.A UV/Vis spec-trophotometer (U-4100;Hitachi)was used to measure the loading and release of DOX.CLSM (TCS SP5;Leica)recorded the interaction between Janus MSN motors and HeLa cells.Preparation of Janus MSNsMSNs were prepared according to a base-catalyzed sol–gel method reported previously.[46]Briefly,CTAB (250mg)as the cation-ic surfactant and NaOH (70mg)were dissolved in deionized water (120mL)and then the mixture was heated to 808C.TEOS (1.25mL)was introduced dropwise to the surfactant solution and allowed to react for 2h at 808C.The dispersion was then washed thoroughly with methanol and dried under vacuum at room temperature.A drop of MSN solution in ethanol was spread on a hydrophilic sili-con slide and the substrate was kept at an angle of 98.After evap-oration of ethanol in air,a MSN monolayer on the silicon slide formed and was subsequently transferred into the sputter coater.A complex metallic layer containing Cr (3nm)and Pt (7nm)was deposited on the MSN monolayer.Finally,Janus MSNs were ob-tained after ultrasonic treatment of the MSN monolayer in pure water.Motion ExperimentsThe prepared Janus MSN nanomotors were first incubated in a 2mg mL À1solution of Rhd-PAH for 2h,so that,through electro-static interactions,Rhd-PAH assembled on the surface of the un-coated metallic hemisphere and adsorbed in the pore channel of the Janus MSN nanomotor.The nanomotors were then centrifugal-ly washed with fresh deionized water.A drop of Janus MSN nano-motors suspended in water was put into H 2O 2at different concen-trations at 24and 378C.A homemade high-resolution fluorescence microscope (Figure S10in the Supporting Information)was em-ployed to record the movement of Janus MSN nanomotors and MATLAB 2012software was used to analyze and track the motion of the nanomotors.Drug Loading and EncapsulationJanus MSN nanomotors (2mg)were dispersed into a 0.1mg mL À1solution of DOX (0.2mL)and left to stand for 3days.The DOX-loaded Janus MSN nanomotors were collected by centrifugation at 10000rpm for 10min and incubated with 1mg mL À1egg PC con-taining 1%folic acid modified DSPE for 2h for encapsulation (Fig-ure S5in the Supporting Information).Egg PC mixed with 1%folic acid modified DSPE through the extrusion method formed lipo-some vesicles to encapsulate the uncovered part of the Janus MSN nanomotors.Egg PC from lipid bilayers adsorbed on the naked MSN and protected DOX from extravasation.[36,39]In previous re-ports,lipid bilayers did not form membranes on the catalytic sur-face,[47]and no DOX was observed to release from the egg PC–DOX–Janus MSN nanomotors at 378C in PBS (pH 7.4),as monitored by using a UVspectrophotometer.Figure 5.DOX release from egg PC modified Janus MSN nanomotors inside HeLa cells:a)0,b)1,c)2,and d)3h.The images are overlays of fluorescence and DIC channels.Scale bars =10m m.Intracellular Localization and Drug Release in VitroA standard cell culture procedure was used for the in vitro experi-ments.Briefly,HeLa cells were first maintained in DMEM with10% fetal calf serum,1%penicillin,and streptomycin at378C in an at-mosphere of5%CO2for2days.Then,egg PC containing2% Texas Red labeled DHPE liposomes was used to label MSNs and Janus MSN nanomotors;the same amount(1mg mLÀ1)incubated with HeLa cells in a culture medium containing0.2%H2O2at378C. the intracellular localization was checked after1,2,and3h.Two groups of samples were checked by using CLSM(TCS SP5,Leica). In the experiment in which DOX was released from egg PC modi-fied Janus MSN nanomotors,egg PC–DOX–Janus MSN nanomotors and HeLa cells were incubated in a culture medium mixed with 0.2%H2O2and left to stand for1h,then washed with fresh culture medium3times.The process of drug release from the nanomotor (0,1,2,3h)was recorded by CLSM:with egg PC catalytic hydroly-sis by phospholipase in HeLa cells,DOX was released from the nanomotor.AcknowledgementsWe thank Prof.J.B.Li and Prof.H.Mçhwald for their helpful dis-cussions.This work was supported by the National Nature Sci-ence Foundation of China(91027045and21273053),the100-Talent Program of HIT,the New Century Excellent Talent Program (NCET110800),and State Key Lab of Urban Water Resource and Environment in HIT(no.ESK201302).Keywords:drug delivery·mesoporous materials·nanoparticles·self-propelled·vesicles[1]J.Howard,Nature1997,389,561.[2]Y.Mei,A.Solovev,S.Sanchez,O.Schmidt,Chem.Soc.Rev.2011,40,2109.[3]J.Wang,W.Gao,ACS Nano2012,6,5745.[4]S.Sengupta,M.E.Ibele,A.Sen,Angew.Chem.Int.Ed.2012,51,8434;Angew.Chem.2012,124,8560.[5]T.Mirkovic,N.S.Zacharia,G.D.Scholes,G.A.Ozin,ACS Nano2010,4,1782.[6]M.Pumera,Nanoscale2010,2,1643.[7]W.Wang,W.Duan,A.Sen,T.E.Mallouk,A2013,110,17744.[8]S.Sundararajan,mmert,A.W.Zudans,V.H.Crespi,A.Sen,NanoLett.2008,8,1271.[9]A.A.Solovev,S.Sanchez,M.Pumera,Y.F.Mei,O.G.Schmidt,Adv.Funct.Mater.2010,20,2430.[10]R.F.Ismagilov,A.Schwartz,N.Bowden,G.M.Whitesides,Angew.Chem.Int.Ed.2002,41,652;Angew.Chem.2002,114,674.[11]Y.J.Wu,Z.G.Wu,X.K.Lin,Q.He,J.B.Li,ACS Nano2012,6,10910.[12]W.Wang,T.-Y.Chiang,D.Velegol,T.E.Mallouk,J.Am.Chem.Soc.2013,135,10557.[13]O.S.Pak,W.Gao,J.Wang,uga,Soft Matter2011,7,8169–8181.[14]S.J.Ebbens,J.R.Howse,Soft Matter2010,6,726.[15]S.Campuzano,J.Orozco,D.Kagan,M.Guix,W.Gao,S.Sattayasamitsa-thit,J.C.Claussen,A.MerkoÅi,J.Wang,Nano Lett.2012,12,396. [16]J.Simmchen,A.Baeza,D.Ruiz,M.J.Esplandiu,M.Vallet-Regí,Small2012,8,2053.[17]W.Gao,S.Sattayasamitsathit,J.Orozco,J.Wang,J.Am.Chem.Soc.2011,133,11862.[18]S.Balasubramanian,D.Kagan,C.M.J.Hu,S.Campuzano,M.J.Lobo-CastaÇon,N.Lim, D.Y.Kang,M.Zimmerman,L.F.Zhang,J.Wang, Angew.Chem.2011,123,4247;Angew.Chem.Int.Ed.2011,50,4161. [19]P.Wunderbaldinger,L.Josephson,R.Weissleder,Bioconjugate Chem.2002,13,264.[20]T.R.Kline,W.F.Paxton,T.E.Mallouk,A.Sen,Angew.Chem.2005,117,754;Angew.Chem.Int.Ed.2005,44,744.[21]D.A.Wilson,R.J.M.Nolte,J.C.M.van Hest,Nat.Chem.2012,4,268.[22]Y.Wan,D.Y.Zhao,Chem.Rev.2007,107,2821.[23]D.Y.Zhao,J.L.Feng,Q.S.Huo,N.Melosh,G.H.Fredrickson, B.F.Chmelka,G.D.Stucky,Science1998,279,548–552.[24]G.S.Attard,J.G.Glyde,C.G.Gçltner,Nature1995,378,366.[25]F.Hoffmann,M.Cornelius,J.Morell,M.Frçba,Angew.Chem.2006,118,3290;Angew.Chem.Int.Ed.2006,45,3216.[26]M.Vallet-Regí,F.Balas,D.Arcos,Angew.Chem.2007,119,7692;Angew.Chem.Int.Ed.2007,46,7548.[27]C.BarbØ,J.Bartlett,L.Kong,K.Finnie,H.Q.Lin,rkin,S.Calleja,A.Bush,G.Calleja,Adv.Mater.2004,16,1959.[28]G.L.Li,Z.L.Zheng,H.Mçhwald,D.G.Shchukin,ACS Nano2013,7,2470.[29]D.Kagan,ocharoensuk,M.Zimmerman,C.Clawson,S.Balasubra-manian,D.Kang,D.Bishop,S.Sattayasamitsathit,L.F.Zhang,J.Wang, Small2010,6,2741.[30]L.Baraban,D.Makarov,R.Streubel,I.Mçnch,D.Grimm,S.Sanchez,O.G.Schmidt,ACS Nano2012,6,3383.[31]J.Gibbs,S.Kothari,D.Saintillan,Y.P.Zhao,Nano Lett.2011,11,2543.[32]H.R.Jiang,N.Yoshinaga,M.Sano,Phys.Rev.Lett.2010,105,268302.[33]A.Einstein,Ann.Phys.1905,322,549.[34]P.S.Lovely,F.W.Dahlquist,J.Theor.Biol.1975,50,477.[35]T.Mirkovic,N.S.Zacharia,G.D.Scholes,G.A.Ozin,Small2010,6,159.[36]S.Mornet,mbert,E.Duguet,A.Brisson,Nano Lett.2005,5,281.[37]M.Claesson,N.-J.Cho,C.W.Frank,M.Andersson,Langmuir2010,26,16630.[38]J.Liu,X.Jiang,C.Ashley,C.J.Brinker,J.Am.Chem.Soc.2009,131,7567.[39]C.L.Du,J.Zhao,J.Fei,Y.Cui,J.B.Li,Adv.Healthcare Mater.2013,2,1246.[40]Z.G.Wu,Y.J.Wu,W.P.He,X.K.Lin,J.M.Sun,Q.He,Angew.Chem.2013,125,7138;Angew.Chem.Int.Ed.2013,52,7000.[41]H.Mçhwald,Ann.Rev.Phys.Chem.1990,41,441.[42]U.P.Steinbrecher,S.Parthasarathy,D.S.Leake,J.L.Witztum,D.Stein-berg,A1984,81,3883.[43]J.B.Li,Z.J.Chen,X.L.Wang,G.Brezesinski,H.Mçhwald,Angew.Chem.2000,112,3187;Angew.Chem.Int.Ed.2000,39,3059.[44]X.H.Yang,T.L.Sladek,X.Liu, B.R.Butler, C.J.Froelich, A.D.Thor,Cancer Res.2001,61,348.[45]L.Duan,Q.He,K.W.Wang,X.H.Yan,Y.Cui,H.Mçhwald,J.B.Li,Angew.Chem.2007,119,7126;Angew.Chem.Int.Ed.2007,46,6996.[46]S.Huh,J.W.Wiench,J.C.Yoo,M.Pruski,V.S.Y.Lin,Chem.Mater.2003,15,4247.[47]B.Wang,L.Zhang,S.C.Bae,S.Granick,A2008,105,18171.Received:March10,2014Published online on&&&&,2014ARTICLESM.Xuan,J.Shao,X.Lin,L.Dai,*Q.He*&&–&&Self-Propelled Janus Mesoporous Silica Nanomotors with Sub-100nm Diameters for Drug Encapsulation andDeliveryDrugs in motion:Self-propelled Janus mesoporous silica nanomotors with di-ameters of sub-100nm are presented.This new active delivery system com-bines an autonomous nanomotor and a smart cargo into an integrated system and may be helpful in dealing with cur-rent nanoparticle drug-delivery system obstacles (see picture).。

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页数:8
Nanoscale Research Letters 2011, 674…

页数:8
J. AM. CHEM. SOC. 2003, 125, 10790-10791

页数:2
Ke-Qin Zhang, Journal of Chemical Physics, 2009

页数:8
Chem. Commun. 2008, 5203-5205

页数:3
Res Chem Intermed (2012) 38693–703

页数:11
Chem. Eur. J. 2008, 14, 7782–7790

页数:9
Macromol. Chem. Phys. 2005, 206, 1769-1777

页数:9
Cramer_et_al-2004-Journal_of_Polymer_Science_Part_A-_Polymer_Chemistry (1)

页数:6
Inorg. Chem., 2011, 50 (11), pp 5249–5257s

页数:12
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页数:16
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页数:2

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