daniel review 2004

Gold Nanoparticles: Assembly, Supramolecular Chemistry,Quantum-Size-Related Properties, and Applications toward Biology, Catalysis,

and Nanotechnology

Marie-Christine Daniel and Didier Astruc*

Molecular Nanosciences and Catalysis Group, LCOO, UMR CNRS No. 5802, Universite Bordeaux I, 33405 Talence Cedex, France

Received August 6, 2003

Contents

1. Historic Introduction 2932. General Background: Quantum Size Effect and

Single-Electron Transitions294

3. Synthesis and Assembly 2963.1. Citrate Reduction 2963.2. The BrustSchiffrin Method: Two-Phase

Synthesis and Stabilization by Thiols296

3.3. Other Sulfur Ligands 2973.4. Other Ligands 298

3.4.1. Phosphine, Phosphine Oxide, Amine, andCarboxylate Ligands

298

3.4.2. Isocyanide 2983.4.3. Acetone 2983.4.4. Iodine 298

3.5. Microemulsion, Reversed Micelles,Surfactants, Membranes, and Polyelectrolytes

298

3.6. Seeding Growth 2983.7. Physical Methods: Photochemistry (UV,

Near-IR), Sonochemistry, Radiolysis, andThermolysis

298

3.8. Solubilization in Fluorous and Aqueous Media 2993.9. Characterization Techniques 300

3.10. Bimetallic Nanoparticles 3033.11. Polymers 3043.12. Dendrimers 3073.13. Surfaces, Films, Silica, and Other AuNP

Materials308

4. Physical Properties 3124.1. The Surface Plasmon Band (SPB) 3124.2. Fluorescence 3144.3. Electrochemistry 3154.4. Electronic Properties Using Other Physical

Methods315

5. Chemical, Supramolecular, and RecognitionProperties

317

5.1. Reactions of Thiolate-Stabilized AuNPs 3175.2. Supramolecular Chemistry 3185.3. Molecular Recognition 319

5.3.1. Redox Recognition Using FunctionalizedAuNPs as Exoreceptors

319

5.3.2. Miscellaneous Recognition and Sensors 3206. Biology 321

6.1. DNAAuNPs Assemblies and Sensors 3216.2. AuNP-Enhanced Immuno-Sensing 323

6.3. AuNP Sugar Sensors 3236.4. Other AuNP Bioconjugates: Peptides, Lipids,

Enzymes, Drugs, and Viruses324

6.5. AuNP Biosynthesis 3257. Catalysis 325

7.1. Catalysis of CO Oxidation 3257.2. Electrochemical Redox Catalysis of CO and

CH3OH Oxidation and O2 Reduction326

7.3. Catalysis of Hydrogenation of UnsaturatedSubstrates

326

7.4. Catalysis by Functional Thiolate-StabilizedAuNPs

326

7.5. Other Types of Catalysis 3278. Nonlinear Optics (NLO) 3279. Miscellaneous Applications 328

10. Conclusion and Perspectives 32911. Acknowledgment 32912. Abbreviations 32913. References 330

1. Historic IntroductionAlthough gold is the subject of one of the most

ancient themes of investigation in science, its renais-sance now leads to an exponentially increasingnumber of publications, especially in the context ofemerging nanoscience and nanotechnology with nano-particles and self-assembled monolayers (SAMs). Wewill limit the present review to gold nanoparticles(AuNPs), also called gold colloids. AuNPs are themost stable metal nanoparticles, and they presentfascinating aspects such as their assembly of multipletypes involving materials science, the behavior of theindividual particles, size-related electronic, magneticand optical properties (quantum size effect), and theirapplications to catalysis and biology. Their promisesare in these fields as well as in the bottom-upapproach of nanotechnology, and they will be keymaterials and building block in the 21st century.

Whereas the extraction of gold started in the 5thmillennium B.C. near Varna (Bulgaria) and reached10 tons per year in Egypt around 1200-1300 B.C.when the marvelous statue of Touthankamon wasconstructed, it is probable that soluble gold ap-peared around the 5th or 4th century B.C. in Egyptand China. In antiquity, materials were used in anecological sense for both aesthetic and curativepurposes. Colloidal gold was used to make ruby glass

293Chem. Rev. 2004, 104, 293346

10.1021/cr030698+ CCC: $48.50 2004 American Chemical SocietyPublished on Web 12/20/2003

and for coloring ceramics, and these applications arestill continuing now. Perhaps the most famous ex-ample is the Lycurgus Cup that was manufacturedin the 5th to 4th century B.C. It is ruby red intransmitted light and green in reflected light, due tothe presence of gold colloids. The reputation of solublegold until the Middle Ages was to disclose fabulouscurative powers for various diseases, such as heartand venereal problems, dysentery, epilepsy, andtumors, and for diagnosis of syphilis. This is welldetailed in what is considered as the first book oncolloidal gold, published by the philosopher andmedical doctor Francisci Antonii in 1618.1 This bookincludes considerable information on the formationof colloidal gold sols and their medical uses, includingsuccessful practical cases. In 1676, the Germanchemist Johann Kunckels published another book,2awhose chapter 7 concerned drinkable gold thatcontains metallic gold in a neutral, slightly pinksolution that exert curative properties for severaldiseases. He concluded, well before Michael Faraday

(vide infra), that gold must be present in such adegree of communition that it is not visible to thehuman eye. A colorant in glasses, Purple of Cas-sius, is a colloid resulting from the heterocoagulationof gold particles and tin dioxide, and it was popularin the 17th century.2b A complete treatise on colloidalgold was published in 1718 by Hans HeinrichHelcher.3 In this treatise, this philosopher and doctorstated that the use of boiled starch in its drinkablegold preparation noticeably enhanced its stability.These ideas were common in the 18th century, asindicated in a French dictionary, dated 1769,4 underthe heading or potable, where it was said thatdrinkable gold contained gold in its elementary formbut under extreme sub-division suspended in aliquid. In 1794, Mrs. Fuhlame reported in a book5that she had dyed silk with colloidal gold. In 1818,Jeremias Benjamin Richters suggested an explana-tion for the differences in color shown by variouspreparation of drinkable gold:6 pink or purple solu-tions contain gold in the finest degree of subdivision,whereas yellow solutions are found when the fineparticles have aggregated.

In 1857, Faraday reported the formation of deep-red solutions of colloidal gold by reduction of anaqueous solution of chloroaurate (AuCl4-) usingphosphorus in CS2 (a two-phase system) in a well-known work. He investigated the optical propertiesof thin films prepared from dried colloidal solutionsand observed reversible color changes of the filmsupon mechanical compression (from bluish-purple togreen upon pressurizing).7 The term colloid (fromthe French, colle) was coined shortly thereafter byGraham, in 1861.8 Although the major use of goldcolloids in medicine in the Middle Ages was perhapsfor the diagnosis of syphilis, a method which re-mained in use until the 20th century, the test is notcompletely reliable.9-11

In the 20th century, various methods for thepreparation of gold colloids were reported andreviewed.11-17 In the past decade, gold colloids havebeen the subject of a considerably increased numberof books and reviews,15-44 especially after the break-throughs reported by Schmid17,19,21 and Brust et al.22,27The subject is now so intensively investigated, dueto fundamental and applied aspects relevant to thequantum size effect, that a majority of the referencesreported in the present review article have appearedin the 21st century. Readers interested in nano-particles in general can consult the excellent bookscited in refs 15, 24, 28, 33, and 43. The book byHayat, published in 1989,14 essentially deals withbiological aspects and imaging of AuNPs.

2. General Background: Quantum Size Effect andSingle-Electron Transitions

Physicists predicted that nanoparticles in thediameter range 1-10 nm (intermediate between thesize of small molecules and that of bulk metal) woulddisplay electronic structures, reflecting the electronicband structure of the nanoparticles, owing to quan-tum-mechanical rules.29 The resulting physical prop-erties are neither those of bulk metal nor those ofmolecular compounds, but they strongly depend on

Marie-Christine Daniel was born in Vannes, France. She graduated fromthe University of Rennes (France). She is now finishing her Ph.D. onexoreceptors at the Bordeaux 1 University in the research group ofProfessor Didier Astruc. Her doctoral research is concerned with therecognition of anions of biological interest using functionnalized goldnanoparticles and redox-active metallodendrimers.

Didier Astruc is Professor of Chemistry at the University Bordeaux I andhas been a Senior Member of the Institut Universitaire de France since1995. He studied in Rennes (thesis with R. Dabard), and then did hispostdoctoral research at MIT with R. R. Schrock. He is the author ofElectron Transfer and Radical Processes in Transition-Metal Chemistry(VCH, 1995, prefaced by Henry Taube) and Chimie Organometallique(EDP Science, 2000; Spanish version in 2003). His research interestsare in organometallic chemistry at the interface with nanosciences,including sensing, catalysis, and molecular electronics.

294 Chemical Reviews, 2004, Vol. 104, No. 1 Daniel and Astruc

the particle size, interparticle distance, nature of theprotecting organic shell, and shape of the nano-particles.27 The few last metallic electrons are usedfor tunneling processes between neighboring par-ticles, an effect that can be detected by impedancemeasurements that distinguish intra- and intermo-lecular processes. The quantum size effect is involvedwhen the de Broglie wavelength of the valenceelectrons is of the same order as the size of theparticle itself. Then, the particles behave electroni-cally as zero-dimensional quantum dots (or quantumboxes) relevant to quantum-mechanical rules. Freelymobile electrons are trapped in such metal boxes andshow a characteristic collective oscillation frequencyof the plasma resonance, giving rise to the so-calledplasmon resonance band (PRB) observed near 530nm in the 5-20-nm-diameter range. In nanoparticles,there is a gap between the valence band and theconduction band, unlike in bulk metals. The size-induced metal-insulator transition, described in1988, is observed if the metal particle is small enough(about 20 nm) that size-dependent quantization ef-fects occur. Then, standing electron waves withdiscrete energy levels are formed. Single-electrontransitions occur between a tip and a nanoparticle,causing the observation of so-called Coulomb block-ades if the electrostatic energy, Eel ) e2/2C, is largerthan the thermal energy, ET ) kT. The capacitanceC becomes smaller with smaller particles. This meansthat single-electron transitions can be observed at agiven temperature only if C is very small, i.e., fornanoparticles since they are small enough (C < 10-18F). Large variations of electrical and optical proper-ties are observed when the energy level spacingexceeds the temperature, and this flexibility is ofgreat practical interest for applications (transistors,switches, electrometers, oscillators, biosensors, cataly-sis).32-38 For instance, single-electron tunneling re-lated to the electrical resistance of a single rod-shaped molecule provided a value of 18 ( 12 M forself-assembled monolayers on gold (1,1,1) substrateused to tether AuNPs deposited from a clusterbeam.32 The transition from metal-like capacitivecharging to redox-like charging was observed withalkanethiolate-gold nanoparticles of low dispersityin an electrochemical setup for Coulomb staircaseexperiments.39,40 Indeed, it was initially indicatedthat these AuNPs could accommodate 10 redoxstates.39a In a subsequent paper published in 2003,it was shown that lower temperatures enhance theresolution of quantized double-layer charging peaksin differential pulse voltammetry (DPV) observations.This led to the resolution of 13 peaks in CH2Cl2 at263 K for Au140 particles.39b At the same time,however, a publication by Quinns group revealedremarkably well-resolved DPV of analogous Au147particles, showing 15 evenly spaced peaks at roomtemperature (295 K) corresponding to 15 oxidationstates (Figure 1). It was also anticipated that, thenumber of observable charge states being limited bythe size of the available potential window, additionalpeaks should be observed in controlled atmosphereand reduced temperature conditions.40 Thus, AuNPsbehave as other delocalized redox molecules, disclos-

ing redox cascades that are well known in inorganicand organometallic electrochemistry for other transi-tion metal clusters and bi-sandwich complexes.

The pioneering work by Schmid and co-workers onwell-defined phosphine-stabilized gold clusters showedthe properties of quantum-dot particles for the firsttime.30 The number of atoms in these gold clustersis based on the dense packing of atoms taken asspheres, each atom being surrounded by 12 nearestneighbors. Thus, the smallest cluster contains 13atoms, and the following layers contain 10n2 + 2atoms, n being the layer number. For instance, thesecond layer contains 42 atoms, which leads to a totalof 55 atoms for a gold cluster, and the compound[Au55(PPh3)12Cl6] has been well characterized bySchmids group. Recently, spectroscopic data haverevealed discrete energy level spacings of 170 meVthat can be attributed to the Au55 core.30c Largerclusters containing, respectively, 147, 309, 561, 923,1415, or 2057 (n ) 3-8) atoms have been isolated.30,31Discrete organogold clusters are also well known withsmall numbers of atoms and various geometries, andthey will not be reviewed here.41,42 Large ones forma fuzzy frontier between clusters and colloids (AuNPs),the latter being defined by some dispersity material-ized by a histogram determined using transmissionelectron microscopy (TEM) data.

Despite the considerable variety of contributions,we will focus first on synthesis, stabilization, andvarious types of assemblies, and then on physicalproperties and on chemical, supramolecular, andsensor properties, and finally on applications tobiochemistry, catalysis, and nonlinear optical proper-ties before concluding on the perspectives of AuNPsin nanosciences and nanotechnology. Many publica-tions involve two or even sometimes several of thesetopics. Thus our classification is arbitrary, but thereader will often better understand the spirit of eachpaper from its title given in the reference section.

Figure 1. Differential pulse voltammetry (DPV) responsesfor AuNP solutions measured at a Pt microelectrode;(upper) as-prepared 177 M hexanethiol-capped Au147showing 15 high-resolution quantized double-layer charg-ing (QDL) peaks and (lower) 170 M hexanethiol-cappedAu38 showing a HOMO-LUMO gap. It can be seen thatthe as-prepared solution contains a residual fraction of Au38that smears out the charging response in E regions whereQDL peaks overlap. The electrode potential scanned nega-tive to positive. Reprinted with permission from ref 40(Quinns group). Copyright 2003 American Chemical So-ciety.

Gold Nanoparticles Chemical Reviews, 2004, Vol. 104, No. 1 295

3. Synthesis and Assembly

3.1. Citrate ReductionAmong the conventional methods of synthesis of

AuNPs by reduction of gold(III) derivatives, the mostpopular one for a long time has been that usingcitrate reduction of HAuCl4 in water, which wasintroduced by Turkevitch in 1951.12 It leads to AuNPsof ca. 20 nm. In an early effort, reported in 1973 byFrens,13 to obtain AuNPs of prechosen size (between16 and 147 nm) via their controlled formation, amethod was proposed where the ratio between thereducing/stabilizing agents (the trisodium citrate-to-gold ratio) was varied. This method is very often usedeven now when a rather loose shell of ligands isrequired around the gold core in order to prepare aprecursor to valuable AuNP-based materials. Re-cently, a practical preparation of sodium 3-mercap-topropionate-stabilized AuNPs was reported in whichsimultaneous addition of citrate salt and an am-phiphile surfactant was adopted; the size could becontrolled by varying the stabilizer/gold ratio (Figure2).44

3.2. The BrustSchiffrin Method: Two-PhaseSynthesis and Stabilization by Thiols

Schmids cluster [Au55(PPh3)12Cl6], reported in 1981,long remained unique with its narrow dispersity (1.4( 0.4 nm) for the study of a quantum-dot nanoma-terial, despite its delicate synthesis.45 The stabiliza-tion of AuNPs with alkanethiols was first reportedin 1993 by Mulvaney and Giersig, who showed thepossibility of using thiols of different chain lengthsand their analysis.46a The Brust-Schiffrin method forAuNP synthesis, published in 1994, has had aconsiderable impact on the overall field in less thana decade, because it allowed the facile synthesis ofthermally stable and air-stable AuNPs of reduceddispersity and controlled size for the first time(ranging in diameter between 1.5 and 5.2 nm).Indeed, these AuNPs can be repeatedly isolated and

redissolved in common organic solvents withoutirreversible aggregation or decomposition, and theycan be easily handled and functionalized just asstable organic and molecular compounds. The tech-nique of synthesis is inspired by Faradays two-phasesystem7 and uses the thiol ligands that strongly bindgold due to the soft character of both Au and S.47AuCl4- is transferred to toluene using tetraoctylam-monium bromide as the phase-transfer reagent andreduced by NaBH4 in the presence of dodecanethiol(Figure 3).47a The organic phase changes color fromorange to deep brown within a few seconds uponaddition of NaBH4:

The TEM photographs showed that the diameterswere in the range 1-3 nm, with a maximum in theparticle size distribution at 2.0-2.5 nm, with apreponderance of cuboctahedral and icosahedral struc-tures. Larger thiol/gold mole ratios give smalleraverage core sizes, and fast reductant addition andcooled solutions produced smaller, more monodis-perse particles. A higher abundance of small coresizes (e2 nm) is obtained by quenching the reactionimmediately following reduction or by using stericallybulky ligands.48-50 Brust et al. extended this synthe-sis to p-mercaptophenol-stabilized AuNPs in a single-phase system,47b which opened an avenue to thesynthesis of AuNPs stabilized by a variety of func-tional thiol ligands.47,48 Subsequently, many publica-tions appeared describing the use of the Brust-Schiffrin procedure for the synthesis of other stableAuNPs, also sometimes called monolayer-protectedclusters (MPCs), of this kind that contained func-tional thiols.49-53 The proportion thiol:AuCl4- usedin the synthesis controls the size of the AuNPs (forinstance, a 1:6 ratio leads to the maximum averagecore diameter of 5.2 nm, i.e., ca. 2951 Au atoms andca. 371 thiolate ligands; core diameter dispersity of (10%). Murray et al. reported and studied theplace exchange of a controlled proportion of thiolligands by various functional thiols52 (Figures 4 and5) and the subsequent reactions of these functionalAuNPs.50,52 Schiffrin reported the purification ofdodecanethiol-stabilized AuNPs from tetraoctylam-

Figure 2. Preparation procedure of anionic mercapto-ligand-stabilized AuNPs in water. Reprinted with permis-sion from ref 44 (Kunitakes group). Copyright 1999Elsevier.

Figure 3. Formation of AuNPs coated with organic shellsby reduction of AuIII compounds in the presence of thiols.Reprinted with permission from ref 73 (Crookss group).Copyright 2001 Royal Society of Chemistry.

AuCl4-(aq) + N(C8H17)4

+(C6H5Me) f

N(C8H17)4+AuCl4

-(C6H5Me)

mAuCl4-(C6H5Me) +

nC12H25SH(C6H5Me) + 3m e- f

4m Cl-(aq) + [Aum(C12H25SH)n](C6H5Me)


monium impurities by Soxhlet extraction.54 Theinfluence of nonionic surfactant polyoxoethylene(20)sorbitan monolaurate (Tween 20) on surface modifi-cation of AuNPs was studied with mercaptoalkanoicacids.55 Digestive ripening, i.e., heating a colloidalsuspension near the boiling point in the presence ofalkanethiols (for instance, 138 C for 2 min, followedby 5 h at 110 C), significantly reduced the averageparticle size and polydispersity in a convenient andefficient way. This technique also led to the formationof 2D and 3D superlattices,56,57 a subject of intenseinvestigation (see also section 3.13 on materials).58-63For instance, AuNPs obtained using acid-facilitatedtransfer are free of tetraalkylammonium impurity,are remarkably monodisperse, and form crystallinesuperstructures.63a The truncated icosahedron struc-ture is formed in growth conditions in which theequilibrium shape is achieved.63b Molecular dynamicssimulations showed that AuNPs with 1157 Au atoms

attained an icosahedral structure upon freezing.63cA single-toluene phase method was also reportedwhereby the ammonium salt-stabilized AuNPs weresynthesized, followed by an exchange reaction withdodecanethiol.58 Superhydride64a and hexadecyl-aniline64b (inter alia) have been used as alternativereagents to NaBH4 for the reduction of gold(III) inthe synthesis of thiol-stabilized AuNPs. Shape sepa-ration of suspended AuNPs by size-exclusion chro-matography was monitored by examining the 3Dchromatograms obtained by employing a diode-arraydetection system.65

3.3. Other Sulfur Ligands

Other sulfur-containing ligands,67-70 such as xan-thates66 and disulfides,67-69 di-70a and trithiols,70b andresorcinarene tetrathiols,70d have been used to sta-bilize AuNPs. Disulfides are not as good stabilizingagents as thiols,67-70 which is eventually useful forcatalysis.70 Similarly, thioethers do not bind AuNPsstrongly,71 but the use of polythioethers by Rhein-houts group astutely circumvented this problem.72aTetradentate thiethers have also been used to revers-ibly form AuNP assemblies.72b On the other hand,oxidation of thiol-stabilized AuNPs by iodine pro-vokes their decomposition to gold iodide with forma-tion of disulfides, which led Crooks to form polycy-clodextrin hollow spheres by templating AuNPs.73

Figure 4. General scheme for the ligand-exchange reac-tion between alkanethiol-AuNPs of the Brust type andvarious functionalized thiols.

Figure 5. Ligand substitution reactions (CH2Cl2, 2 d, room temperature) for the syntheses of the AuNPs containingmixed dodecanethiol and (amidoferrocenyl) alkanethiol-type ligands with variation of the chain length (C11 vs C6) andring structure of the ferrocenyl motif (Cp, Cp*, C5H4COMe). Reprinted with permission from ref 140 (Astrucs group).Copyright 2002 American Chemical Society.


3.4. Other Ligands

3.4.1. Phosphine, Phosphine Oxide, Amine, andCarboxylate Ligands

The Brust biphasic method of synthesis was ap-plied to PPh3 in order to improve the synthesis ofSchmids cluster [Au55(PPh3)12Cl6], using HAuCl43H2O and N(C8H15)4Br in a water-toluene mixtureto which PPh3 and then NaBH4 were added. It wasestimated that the cluster synthesized in this wayhad the formula [Au101(PPh3)21Cl5] and contained 3.7mass percent of [Au(PPh3)Cl] as an impurity.74aThermolysis of [AuI(C13H27COO)(PPh3)] at 180 Cunder N2 yielded monodispersed AuNPs capped bymyristate and a small amount of PPh3 ligands; theAuNP diameter increased with reaction time, from12 nm for 1 h to 28 nm for 10 h, and with increasingtemperature (42 nm for 5 h at 200 C).74b Variousother gold complexes, in particular gold(I) aminecomplexes, have been used as precursors for thesynthesis of amine-stabilized AuNPs.75,76a,b Reductionof AuIVCl4 by NaBH4 in a mixture of tri-n-octylphos-phine oxide (TOPO) and octadecylamine (1:0.57molar ratio) at 190 C resulted in the controlledgrowth of spherical AuNPs (8.59 ( 1.09 nm diameter)that are stable for months in toluene and weremanipulated into crystals and 2D arrays (Figure 6).76cCapping aqueous AuNPs with the amino acid lysinestabilizes the AuNPs in solution electrostatically andrenders them air-stable and water-dispersible, afinding that is promising toward biologically relevantresearch.76d Efficient synthesis of stable AuNPs byreaction of AuCl4- ions with the alkalothermophilicactinomycete Thermomonospora sp. has beendescribed.76e

3.4.2. Isocyanide

Aryl isocyanide thin films have attracted someattention, due to their potential application as mo-

lecular wires,76f and 1,4-diisocyanide-AuNP formslarge aggregate superstructures that have beenexamined by IR and Raman spectroscopy, showingbonding to the AuNP core via the carbon lonepair.76g,h

3.4.3. AcetonePure Au0NPs, obtained by replacement of citrate

by acetone, were shown to be stable against attackby BH4- or HCl.76i

3.4.4. IodineIodine adsorption was shown to displace citrate

ions from AuNPs, leading to superstructures that arealso formed upon addition of KI.76j

3.5. Microemulsion, Reversed Micelles,Surfactants, Membranes, and Polyelectrolytes

The use of microemulsions,77 copolymer micelles,78reversed micelles,77 surfactant, membranes, and otheramphiphiles is a significant research field for thesynthesis38 of stabilized AuNPs in the presence or inthe absence of thiol ligands.77-95 The synthesesinvolve a two-phase system with a surfactant thatcauses the formation of the microemulsion or themicelle maintaining a favorable microenvironment,together with the extraction of metal ions from theaqueous phase to the organic phase. This is anadvantage over the conventional two-phase system.This dual role of the surfactant and the interactionbetween the thiol and the AuNP surface control thegrowth and stabilization of the AuNP or nanocrystal.The narrow size distribution allows the ordering ofthe particles into a 2D hexagonal close-packed array.AuNP sizes of the order of 4 nm diameter have beenfound.79 Polyelectrolytes have also been extensivelyused for the synthesis of AuNPs (Figure 7).94,98-102The polyelectrolyte coating of carboxylic acid-deriva-tized AuNPs with diameters less than 10 nm hasbeen achieved by electrostatic self-assembly of op-positely charged polyelectrolytes.102b

3.6. Seeding GrowthThe seeding-growth procedure is another popular

technique that has been used for a century. Recentstudies have successfully led to control of the sizedistribution (typically 10-15%) in the range 5-40nm, whereas the sizes can be manipulated by varyingthe ratio of seed to metal salt (Figure 8).103-105 Thestep-by-step particle enlargement is more effectivethan a one-step seeding method to avoid secondarynucleation.87a Gold nanorods have been convenientlyfabricated using the seeding-growth method.87b

3.7. Physical Methods: Photochemistry (UV,Near-IR), Sonochemistry, Radiolysis, andThermolysis

UV irradiation is another parameter that canimprove the quality of the AuNPs,86,104,105 includingwhen it is used in synergy with micelles86 or seeds.104Near-IR laser irradiation provokes an enormous sizegrowth of thiol-stabilized AuNPs.106 The presence ofan ultrasonic field (200 kHz) allowed the control ofthe rate of AuCl4- reduction in an aqueous solutioncontaining only a small amount of 2-propanol and the

Figure 6. 2D lattice of octadecylamine/TOPO-cappedAuNPs spontaneously formed when the latter are depositedon a copper grid; bar ) 20 nm. (Inset) Scanning electronmicroscopy (SEM) image of a cubic colloidal crystal pre-pared from octadecylamine/TOPO-capped AuNPs (190 C);bar ) 80 m. Reprinted with permission from ref 76c(OBriens group). Copyright 2000 The Royal Society ofChemistry.


sizes of the formed AuNPs by using parameters suchas the temperature of the solution, the intensity ofthe ultrasound, and the positioning of the reac-tor.107,108 Sonochemistry was also used for the syn-thesis of AuNPs within the pores of silica111-113 andfor the synthesis of Au/Pd bimetallic particles.114Radiolysis has been used to control the AuNP size115aor to synthesize them in the presence of specificradicals,115b and the mechanism of AuNP formationupon -irradiation has been carefully examined (Fig-ure 9).116

AuNPs have been fabricated via decomposition of[AuCl(PPh3)] upon reduction in a monolayer at thegas/liquid interface.117 The thermolysis of [C14H29-Me3N][Au(SC12H25)2] at 180 C for 5 h under N2

produced alkyl-groups-passivated AuNPs of 26 nm.118aThermolysis of crude preparations of Brusts AuNPswithout removing the phase-transfer reagent, tet-raoctylammonium bromide, to 150-25 C led to anincrease of the particle sizes to 3.4-9.7 nm, and thissize evolution was discussed on the basis of athermodynamic model. The heat-treated AuNPsformed 2D superlattices with hexagonal packing. Theconformation of the alkanethiol is all-trans, and theseligands interpenetrate each other (Figure 10).118bLaser photolysis has been used to form AuNPs inblock copolymer micelles.119 Laser ablation is anothertechnique of AuNP synthesis that has been usedunder various conditions whereby size control can beinduced by the laser.120-122 The evolution of thiol-stabilized AuNPs has been induced by and observedupon heating.123-126,135 Structural changes of spheri-cal aggregates composed of mercaptoacetate-stabi-lized AuNPs suspended in water were monitored bymaintaining the spheroid suspension at a constanttemperature, ranging from 65 to 91 C, for 2-12 h.The spheroid diameter was reduced to almost 70%of the original size, due to an irreversible coagula-tive transition resulting from fusion among thenanocolloids in spheroids.126b Morphology changes ofAuNPs were also shown during sintering.126c Sput-tering AuNPs by single ions and clusters was shownto eject AuNPs.178

3.8. Solubilization in Fluorous and AqueousMedia

AuNPs stabilized by perfluorodecanethiol or1H,1H,2H,2H-perfluorooctathiol, with an average

Figure 7. Schematic diagram illustrating the layer-by-layer polymer deposition process applied to AuNPs. Re-printed with permission from ref 98 (Carusos group).Copyright 2001 American Chemical Society.

Figure 8. TEM image of larger gold particles preparedfrom seed: (a) 5.5 ( 0.6, (b) 8.0 ( 0.8, (c) 17 ( 2.5, and (d)37 ( 5 nm after separation of rods. The 5.5 ( 0.6-nmAuNPs were extracted into toluene after thiol capping forTEM in order to remove excess surfactant. The otherparticles were separated from excess surfactant by cen-trifugation. Reprinted with permission from ref 89 (Mur-phys group). Copyright 2001 American Chemical Society.


diameter of 2.4-2.6 nm, were prepared by reductionof HAuCl4 by NaBH4 (dropwise addition) in ethanoland were soluble only in fluorocarbon media.127

Special emphasis has been placed on the synthesisof stable water-soluble thiol-stabilized AuNPs128-134using thiols containing poly(ethylene oxide)chains128-130a (Figure 11) or carboxylate modification(Figure 12).130b,134 Poly(N-vinyl-2-pyrrolidone) (PVP)is the polymer of choice for the stabilization in waterof AuNPs prepared by reduction of HAuCl4 (see thesection on polymers).

3.9. Characterization TechniquesThe most common characterization technique is

high-resolution transmission electron microscopy(HRTEM), which gives a photograph of the gold coreof the AuNPs,46 but the core dimensions can also bedetermined using scanning tunneling microscopy(STM), atomic force microscopy (AFM), small-angleX-ray scattering (SAXS),50,137a laser desorption-ionization mass spectrometry (LDI-MS),137b-139 andX-ray diffraction.136 A detailed high-resolution studyof the AuNP shape using HRTEM, reported by Brustet al., revealed that the truncated cuboctahedronpredominated, and that decahedra, dodecahedra andicosahedra were also present in the same preparationof alkanethiol-stabilized AuNPs.47 The histogram

providing the size distribution of these cores givescrucial information on the dispersity of the samplethat is usually obtained from TEM pictures.47 Themean diameter, d, of the cores allows determinationof the mean number of gold atoms, NAu, in thecores:47 NAu ) 4(d/2)3/vAu. For instance, with d )2.06 nm, NAu ) 269.140 From these data, the ele-mental analysis, giving the Au/S ratio, allows calcu-lation of the average number of S ligands. Thisnumber can also be deduced from X-ray photoelectronspectroscopy (XPS) or thermogravimetric analysis(TGA).50

Figure 9. Taping-mode AFM images of the species formedin a solution irradiated with rays (1.5 kGy) and thendeposited on highly ordered pyrolytic graphite (HOPG) 30days after the irradiation and dried under a mild N2 streamfor visualization. The solution contains 10-3 molL-1 AuIIIand poly(vinyl alcohol) but no alcohol. (A) Height-mode (zrange 80.0 nm) image and (B) phase (z range 42.2) image,run simultaneously on the same area of the sample. (C, D)Close-up images showing the same area as in (A) and (B),respectively. Reprinted from ref 116 (Bellonis group) bypermission of The Royal Society of Chemistry (RSC) onbehalf of the Centre National de la Recherche Scientifique(CNRS). Copyright 1998.

Figure 10. UV-vis spectra (A) and TEM images and sizedistributions (B) of (a) [AuCl4]- before reduction; dode-canethiol-AuNPs (b) as prepared and after heat treatmentat (c) 150, (d) 190, and (e) 230 C; and (f) octadecanethiol-AuNPs heat-treated at 250 C. Reprinted with permissionfrom ref 188b (Miyakes group). Copyright 2003 AmericanChemical Society.


The oxidation state of the gold atoms of the corehas been examined by Brust et al in their seminalarticle using X-ray photoelectron spectra that showedthe binding energies of the doublet for Au 4f7/2 (83.8eV) and Au 4f5/2 (87.5 eV) characteristic of Au0. Noband was found for AuI at 84.9 eV, although one-thirdof the gold atoms are located at the surface andbonded to thiols for 2.0-2.5 nm sized particle cores.On this basis, Brust et al. suggested that the gold-thiol bond does not have the character of goldsulfide.46b This matter of a thiol vs thiolate bond tothe gold core atoms, however, has been debated. Forinstance, it was suggested that a high coverage of

gold cores by thiolate ligands139 was due to largeligand/Au binding ratios on core edges and vertexes(Figure 13), in accord with theoretical calculations.140Moreover, it was reported that thermolysis of thethiolate-stabilized AuNPs produces only the corre-sponding disulfide. The absence of thiol by thermaldesorption mass spectrometry was considered to beevidence that the chemisorbed ligand consisted of analkanethiolate (not thiol) fragment. This would meanthat H2 is produced during the reductive synthesisfrom thiols, but this formation has never beendetected. Theoretical calculations suggested the for-mation of disulfides when the number of thiol mol-ecules around a AuNP was enough to saturate theflat planes, whereas thiolate behavior was observedwhen the sulfur atoms were not enough.139a Thisfinding was reported139 to corroborate the observationof a S-S distance of 2.32 from grazing incidenceX-ray in self-assembly of n-alkanethiols on a (1,1,1)gold crystal surface.139b Brust et al. recently provided1H NMR evidence for intact thiols adsorbed onAuNPs. They showed that the loss of hydrogen couldbe prevented to some extent as long as there is noeasy reaction path for hydrogen removal.140

X-ray diffraction also demonstrated the strikingtendency of thiolates-AuNPs to spontaneously formhighly ordered superlattices141-144 with periodicityextending to three dimensions up to several tens ofmicrometers.143 These superlattices were obtainedupon slow evaporation of the organic solvent or evenwater on a suitable surface.144b Such self-organizedsuperlattices of AuNPs on highly ordered pyrolyticgraphite (HOPG) are also observable by STM (Figure14).27 Monodispersity is a very important criterionfor the formation of ordered superlattices.145a Whenthe gas phase at the gas-suspension interface of asynthetic medium leading to sodium mercaptosucci-nate-AuNPs contained nonpolar organic molecules,spherical AuNPs formed. On the other hand, whenthe gas phase contained polar organic vapors suchas MeCN or CHCl3, irregular-shaped AuNPsformed.145b

Langmuir-Blodgett (LB) films of Schmids Au55cluster were characterized by STM, Brewter anglemicroscopy (BAM), and scanning force microscopy(SFM). These techniques showed that the clusterformed monolayers, as indicated by the surfacepressure-area (-A) isotherms and the area-time(A-t) isobars between 20 and 30 C and 15-30 nN/mof surface pressure, and the cluster size could beestimated from the -A isotherms as 2.17 nm (calcd2.1 nm).19 The Au55 cluster has also been studied byMossbauer spectroscopy, extended X-ray absorptionfine structure (EXAFS), electron spectroscopy forchemical analysis (ESCA), and conductivity measure-ments. These techniques show that the Au55 particlesbehave like a system with a few last metallicelectrons that are used for tunneling between neigh-boring clusters. This metallizing situation wasobserved by applying an alternating current in the10-kHz range as for impedance measurements.146,147Scanning tunneling spectroscopy (STS) had beenused to observe Coulomb blockade in metal nano-particles. The tunneling current is induced by an

Figure 11. Schematic representation of a AuNP protectedby a monolayer of monohydroxy (1-mercaptoundec-11-yl)tetraethylene glycol. The hydrophobic C11 chain confersextreme stability to the cluster, while the hydrophilictetraethylene glycol unit ensures solubility in water.Reprinted with permission from ref 129 (Brusts group).Copyright 2002 The Royal Society of Chemistry.

Figure 12. Possible combinations of H2O molecules withmercaptosuccinic acid (MSA)-capped AuNPs: one H2Omolecule connected with two carbonyl groups in either (a)one MSA molecule, (b) adjoining MSA molecules on oneAuNP, making a successive hydrogen-bonding network, or(c) MSA molecules from different AuNPs, the water actingas glue to join two neighboring particles. Reprinted withpermission from ref 130b (Chens group). Copyright 1999American Chemical Society.


applied voltage and leads to the charging of a metalparticle with at least one single electron.148-150 Elec-trostatic trapping (ET) is a technique used to inves-tigate isolated nanosize metal particles. It is basedon moving a polarized particle in an electric field tothe point of strongest field, which is the positionbetween two electrodes (dipped in a solution of theparticles) at a distance comparable to the particlediameter.27,148,149

The UV-vis and IR spectra provide an identifica-tion of the ligand that is also confirmed by NMRspectroscopy, except that the ligand atoms close tothe core give broad signals. This latter phenomenonis due to (i) spin-spin relaxational (T2) broadening(main factor), (ii) variations among the gold-sulfurbonding sites around the particle, and (iii) a gradientin the packing density of the thiolate ligands fromthe core region to the ligand terminus at theperiphery.151-153 The NMR spectra are very informa-tive, as for all molecular compounds, for the part ofthe ligand remote from the core. The latter can alsobe more fully analyzed, if desired, after oxidativedecomplexation using iodide.

IR spectroscopy shows that, as in SAMs,154 thethiolate ligands of AuNPs are essentially in all-transzigzag conformations, with 5-25% of gauche defectsat both inner and terminal locations.50,152 IR andNMR spectroscopies allow, together with differentialscanning calorimetry (DSC),152-155 the detection oforder-disorder transitions in AuNPs in the solid

state. The temperature of the transition increaseswith the chain length, and FTIR shows the increasingamount of gauche defects. Variable-temperaturedeuterium NMR in the solid state shows that thedisorder, materialized by the increased proportion ofgauche bonds, propagates from the chain terminustoward the middle of the chain, but not further tothe ligand atom, and causes chain melting.152 Calo-rimetric measurements led to the determination ofthe formation enthalpy of AuNPs in a water/sodiumbis(2-ethylhexyl) sulfosuccinate/n-heptane micro-emulsion. The results indicated that the energeticstates and the dimensions of the AuNPs were influ-enced by the radii and concentrations of the reversedmicelles.80

Capillary zone electrophoresis in acetate buffershowed that the mobility of AuNPs with a given corediameter decreased with decreasing ionic strength.At the highest ionic strength investigated (6 mmol/L), a good linear dependence of the mobility on thereciprocal of the core radius allowed the character-ization of the size of the AuNPs.156

The AFM images of AuNPs operating in the contactmode in air at room temperature showed an attrac-tive interaction among the particles, leading to theformation of aggregates and a mean size that is afunction of the size of the reverse micelle used forthe synthesis. This was taken into account in termsof the formation of an adsorbed layer of surfactantmolecules at the particle surface.81

Figure 13. Stable configurations of AuNPs covered with n-alkanethiol molecules. The sequence shows n ) 4 butanethiol,n ) 6 hexanethiol, n ) 8 octanethiol, n ) 10 decanethiol, n ) 12 dodecanethiol, n ) 14 butanedecanethiol, and n ) 16hexanedecanethiol (n ) number of C atoms). Reprinted with permission from ref 139 (Jose-Yacamans group). Copyright1998 Kluwer.


The use of incoherent light experiments, performedin the vicinity of the surface plasmon resonancefrequency, allowed measurement of the phase relax-ation time and nonlinear susceptibility of AuNPs of5-40 nm.157

Surface-enhanced Raman scattering (SERS)70b andXPS made it possible to analyze the chemisorptiveproperties of tetrathiol ligands and indicated thatsurface passivation was an important factor in thedispersibility of AuNPs in nonpolar solvents.70c

EXAFS allowed investigation of the size-dependentdistance contraction in thiol-stabilized AuNPs, andthe short metal-ligand bond found suggested arather strong surface interaction.158

High-resolution time-of-flight mass spectroscopyanalysis of alkanethiol-stabilized AuNPs allowed theassignment of the number of gold and sulfur atoms,although alkyl chains were not evident. Pure goldcluster ions of various sizes could be generated fromthe AuNPs in a two-laser experiment.159

Small-angle X-ray scattering, STM, and AFM wereconsistent with a small, monodisperse (2.4 nm diam-eter) gold core.160a The formation by physical vapordeposition and growth of AuNPs was studied by STM

on TiO2 (1,1,0) surfaces.160b The surface of AuNPs wasanalyzed using a phase reconstruction technique inTEM, extended to simultaneous correction of spheri-cal aberration and two-fold astigmatism.160c Thevacancy formation energy of AuNPs has been shownto decrease with decreasing particle size.160d

3.10. Bimetallic NanoparticlesBimetallic nanoparticles20,37 containing gold as one

of the elements have been synthesized in a varietyof ways. Bimetallic AuNPs have been reported withAg (Figure 15),161-168 Pd (Figure 16),97,161,166,167 Pt,161,167TiO2,99 Fe,169-171 Zn,168 Cu,165,168 ZrO2,172 CdS,173,174Fe2O3,175 and Eu.176 Although bimetallic nano-particles have been known for a long time, Schmidsgroup were the first to report the synthesis of core-shell bimetallic nanoparticles, the core-shell struc-ture being demonstrated using HRTEM and energy-disperse X-ray (EDX) microanalysis. AuNPs of 18 nmdiameter were covered with a Pd or Pt shell whenan aqueous solution of these AuNPs was added to asolution of H2PtCl6 or H2PdCl4 and H3NOHCl. Theoriginal color of the AuNPs then changed to brown-black. Addition of p-H2NC6H4SO3Na stabilized thegenerated particles in the same manner as P(m-C6H4-SO3Na)3 stabilized the AuNPs. The colloids showeda metallic luster and were of uniform 35 nm diam-eter. For instance, Au/Pt particles had an averagegold content of 15% atom % located at the coresurrounded by Pt crystals of about 5 nm that werepregrown before being added to the Au surface.161Stabilization of the bimetallic particles could beachieved using the Brust procedure in the presenceof thiols. Such stable bimetallic particles were syn-thesized with group 10 (Pd, Pt) and group 11 (Cu,Ag, Au) metals, all containing Au as one of the two

Figure 14. Nanostruture preparation from AuNPs: STMimage of self-assembled superlattice of 3.5-nm gold par-ticles on a HOPG substrate. The particles are stabilizedby hexanethiol. TEM micrograph of an AB2 superlattice ofAuNPs having a bimodal size distribution (4.5 and 7.8 nm).The AuNPs are stabilized by decanethiol. Reprinted withpermission from ref 27 (Brusts group). Copyright 2002Elsevier.

Figure 15. Schematic illustrating the proposed inter-actions of thiocyanate ion-coated 2.8 ( 0.8- and 11.6 ( 0.8-nm-diameter AuNPs with 37.8 ( 9-nm-diameter ethylene-diaminetetraacetic acid (EDTA)-covered AgNPs. Reprintedwith permission from ref 164 (Fendlers group). Copyright2002 The Royal Society of Chemistry.


metals, and were characterized using TEM, 1H NMRline broadening, XPS, elemental analysis, and TGA.TEM showed that Pd/Au cores are small (1.7 nm) andrelatively monodisperse (average 20% dispersity),while Ag/Au cores are larger (3.2 nm), and otherbimetallic particles are of intermediate size. The moleratios of metals both in and on the surface of thebimetallic cores differed significantly from the metal:salt ratio used in the bimetallic particle synthesis.162Partially segregated alloys indeed form easily, andmore noble metals prefer the nonsurface (core) loca-tion.20 Metal galvanic exchange reactions are yetanother quite facile way to synthesize stable bimetal-lic particles. This procedure relies on reactions ofalkanethiolate-metal particles or other metal par-ticles (Ag, Pd, Cu) with the complexes [AuISCH2-(C6H4)CMe3] and [PdII{S(CH2)11Me}2].163,166 Au core-Ag shell and Au core-Pt shell nanoparticles havebeen formed using photochemically reduced phos-photungstate Keggin ions.168b Specific properties andfunctions of bimetallic nanoparticles will be discussedin the appropriate sections devoted to catalytic,electronic, and optical properties.

3.11. PolymersSince the report in Helchers treatise in 1718,3

indicating that starch stabilizes water-soluble goldparticles, it has been known that such materials,recognized two centuries later as polymers, favor theisolation of AuNPs.14,179 With the considerably im-

proved recent understanding of the parameters lead-ing to the stabilization of AuNPs and of their quan-tum-size-related interest, there has been a revivalof activity in the field of polymer-stabilizedAuNPs.33,177,180-181 The most commonly used polymersfor the stabilization of AuNPs are PVP and poly-(ethylene glycol).11b,14

Although there are a variety of ways to achievenanoparticle-polymer composites,182,183 two differentapproaches dominate. The first one consists of thein situ synthesis of the nanoparticles in the polymermatrix either by reduction of the metal salts dissolvedin that matrix184 or by evaporation of the metals onthe heated polymer surface.185 The second one, lessfrequently used, involves polymerization of the ma-trix around the nanoparticles.186 Recently, however,blending of premade AuNPs into a presynthesizedpolystyrene polymer (synthesized by anionic polym-erization) bound to a thiol group was also reported.187Whereas the physical process involving mechanicalcrushing or pulverization of bulk metals and arcdischarge yielded large nanoparticles with a wide sizedistribution, nanoparticles prepared by reduction ofmetal salts are small, with a narrow size distribution.This reduction processes most often use a reagentsuch as NaBH4188 which is added in situ, or thereductant can also be the solvent, such as an alco-hol.189,190 For instance, HAuCl44H2O gives stableAuNPs upon refluxing in methanol/water in thepresence of PVP, even if NaOH is added subsequentlyto the preparation of the AuNPs.191 In poly(acryl-amide), AuCl4- cannot be reduced by alcohol, but itcan be reduced by NaBH4.192 Other reductants aregenerated involving radiolysis, photolysis,193 or elec-trochemistry.194 The polymer-nanoparticle compositecan be generated from solution (the classic mode) orcan involve the immobilization by a solid polymersuch as poly(acrylic acid), poly(vinyl alcohol), or PVPfrequently used. Reduction of metal ions in thepresence of the polymer is most often chosen becausethe complexation of the metal cations by the ligandatoms of the polymer is crucial before reduction. Inparticular, it dramatically limits the particle size.195

The most important role of the stabilizing polymeris to protect the nanoparticles from coagulation.Toshima has expressed this function quantitativelyby the gold number, i.e. the number of milligramsof protective polymer that just prevents 10 mL of ared gold sol from changing color to violet uponaddition of 1 mL of 10% aqueous NaCl. The goldnumber is smaller for protective polymers that arebetter stabilizers.189 Core-shell PVP-stabilized Au/Pd196 and Au/Pt197,198 nanoparticles were prepared byYonezawa and Toshima by simultaneous alcoholreduction of the two corresponding metal salts andcharacterized by EXAFS. The relative order of reduc-tion in alcohol/water is seemingly controlled by therelative redox potentials, HAuCl4 being reduced morerapidly than Pd(OH)2 and PtCl62-. The AuNPs formfirst, and then the Pd or Pt shell forms around theAuNPs to produce the core-shell bimetallic particles.In fact, the Pd0 formed reduces AuCl4- to Au0 andthus acts as a mediator or redox catalyst for thereduction of AuCl4-, as long as any AuCl4- is left in

Figure 16. Cartoon diagram of core metal galvanicexchange reactions. MPC, monolayer-protected cluster;MPAC, monolayer-protected alloy cluster; SC12, S(CH2)11-CH3. Reprinted with permission from ref 166a (Murraysgroup). Copyright 2002 American Chemical Society.


the solution.199,200 Attempts to synthesize Pd-core/Au-shell bimetallic particles led instead to a remarkablecluster-in-cluster structure because of this redoxpriority (Figure 16).201

Many ordered polymer-AuNPs are known. Forinstance, AuNPs in PVP were prepared by hydrazinereduction of incorporated HAuCl4. The color of thesolution of HAuCl4-loaded block copolymer changedfrom yellow to purple, and then to bluish uponaddition of a large excess of anhydrous hydrazine.The reduction can be stopped by addition of HCl,which protonates hydrazine in order to avoid coagu-lation of the AuNPs.202,203 These phenomena werealso obtained with (styrene-block-ethylene oxide).204,205

The use of a diaminotriazine-functionalized diblockcopolymer led to size-controlled synthesis of AuNPaggregates in solution and in thin films with thyminefunctionality.206 AuNPs were generated in polymericmicelles composed of amphiphilic block copoly-mers,207,208 and amphilic star-block copolymers were

an ideal choice to serve as a confined reactionvessel.209 The formation of AuNPs was also controlledby using poly(methylphosphazene), whose lone pairsstabilized the AuNPs.210 Functionalized polymershave also been used as stabilizers. Poly(ethyleneglycol)-based polymer was used to fabricate an AuNPsensor that reversibly binds lectin for recognition andbioassay.211 The so-called grafting from techniquehas been used to construct highly dense polymerbrushes. For instance, several methods,212-214 includ-ing the efficient living radical polymerization (LRP),have indeed been applied to the synthesis of AuNPscoated with such a high-density polymer brush.AuNP-based nanoscale architectures could be fore-casted using this simple technique (Figure 17).214

Polymer hollow spheres have been synthesizedwith movable AuNPs at their interiors.215 AuNPs canserve as templates for the synthesis of conductivecapsules216 (Figure 18) and for the oligomerizationof L-cysteine in aqueous solution (Figure 19).217

Figure 17. Schematic representation for the synthesis of polymer-coated AuNPs by surface-initiated living-radicalpolymerization (LRP). Reprinted with permission from ref 214 (Fukudas group). Copyright 2002 American Chemical Society.

Figure 18. SEM image of (a) 50-m ceramic hollow spheres (CHSs), (b)-50 m gold-seeded CHSs, (c) 50-m gold hollowspheres (GHSs) obtained by calcination and dissolution of gold-seeded CHSs, (d) a 100-m CHS, (e) a 100-m gold-seededCHS, and (f) a 100-m broken GHS. The arrow in (c) indicates a broken particle, which proves that it is hollow. From (f),it can be seen that the inside of GHS is empty. Reprinted with permission from ref 216b (Fendlers group). Copyright 2002Elsevier.


Nanosized domains of block copolymers can be usedas nanoreactors to synthesize AuNPs by expansionof the nanosized domains and period of block copoly-mers, such as polystyrene-block-poly(4-vinylpyridine)(PS-PVP) diblock copolymers.218 Self-assemblies ofAuNPs/polymer multilayer films have been formedusing surface functionalization.219,220 AuNPs of aver-age size between 1 and 50 nm have also beenstabilized by many water-soluble polymers, and someof then have been shown to be stable after 9 monthsin air. The most stable ones were obtained withpolymers possessing hydrophobic backbones and sidegroups, allowing good interactions with the AuCl4-ion. The preparations were carried out using eitherUV irradiation or KBH4 to reduce HAuCl4 in thepresence of a mass ratio of polymer:gold 25:1.221

Linear polymers having cyano or mercapto groupsstabilize AuNPs of 1.5-3 nm diameter and narrowsize distributions.222 AuNPs of Brust type with somethiol chain termini bearing exo-norbornene unitswere polymerized using ring-opening metathesispolymerization (ROMP) to produce a block copolymershell.223,224 Small AuNPs (5 nm diameter) stabilizedwith sodium citrate225 were attached to the surfaceof silica nanoparticles protected by polymer layers toprovide contrast in the final TEM image, a strategyalso used to obtain TEM contrast for many types ofmolecular97 and biological materials.226 Solution be-havior, i.e., transformation in the morphology fromsmall spherical AuNPs to large anisotropic objects,was observed by decreasing the concentration ofpolystyrene-block-poly(2-vinylpyridine) micelles be-low the critical micelle concentration (Figure 20).85

Networks of AuNPs prepared in water were ob-served by TEM upon adding poly(acrylic acid) toAuNPs stabilized by thiolated poly(ethylene oxide)chains of high molecular weight (necessary to stabi-lize AuNPs in water). Moreover, thin and linearthermally robust arrangements were formed whenchondroitin sulfate c sodium salt (a polysaccharidecarrying sulfuric acid groups and carboxylic acidgroups) was added (Figure 21).227 AuNPs of about 20nm size were formed upon reduction of AuCl3 bypolyaniline in N-methylpyrrolidinone.228 An amine-functionalized polymer was used to simultaneouslyassemble carboxylic-acid-functionalized AuNPs andsilica naoparticles into extended agregates.229 Sucha strategy also led to spherical silica templates(Figure 22).226 Macroporous Au spheres with a di-ameter 9 m have been formed by employing porousorganic bead templates and preformed AuNPs.230AuNPs were stabilized by the lone nitrogen pair onthe backbone of polymethylphosphazene, [Me(Ph)-PN]n, and varying the ratio of [Me(Ph)PN]n to HAuCl4

Figure 19. AuNPs as templates for the synthesis of hollowpolymer capsules. Reprinted with permission from ref 216c(Feldheims group). Copyright 1999 American ChemicalSociety.

Figure 20. (a) Reaction scheme for the synthesis of the PEO-GMA-DEAR triblock copolymers. (b) Schematic illustrationof the formation of three-layer onion-like micelles and shell cross-linked micelles from PEO-GMA-DEA triblock copolymers.PEO-GMA-DEA, poly[(ethylene oxide)-block-glycerol monomethacrylate-block-2-(diethylamino)ethyl methacrylate]. Re-printed with permission from ref 85 (Armess group). Copyright 2002 American Chemical Society.


prior to reduction allowed control of the AuNP size.231AuNPs (4-12 nm) were associated with thiol-func-tionalized polyoxometalates -[SiW10O36(RSi)O]4- (R) HSC3H6), where the R group played the role of bothstabilizing the AuNPs via the thiolate ligand andforming a covalent link to the polyanion through thetrimethoxysilane group.232 The preparation of poly-(N-isopropylacrylamide)-protected AuNPs has beencarried out in a homogeneous phase using variousmethods, and this polymer was found to be a betterpassivant than alkanethiols.233a

AuNPs were prepared in both aqueous and organicsystems by reducing HAuCl4 with o-anisidine in thepresence of 1:1 N-methyl-2-pyrrolidone/toluene.233bAuNPs of 6 nm diameter and narrow size distribu-tions were stabilized by -conjugated poly(dithiaful-vene) polymers, and the oxidized form of this polymerinduced a strong red shift of the absorption spectrumof the AuNPs to 550 nm (whereas the theory predicts510-515 nm for the plasmon band in water).234AuNPs with improved stability against long-termaggregation up to one month were prepared usingpoly(styrene)-block-poly(2-vinylpyridine) star-blockcopolymer.235

Water-soluble polymer-stabilized AuNPs were pre-pared from citrate-capped AuNPs by simple contactwith dilute aqueous solutions of hydrophilic nonionicpolymers based on the monomers N-[tris(hydroxy-methy)methyl]acrylamide and N-(isopropyl)acryl-

amide that were functionalized with disulfide an-choring groups. The resulting polymer-coated AuNPscould be stored in the dry state and redispersed inwater to yield sterically stabilized AuNP suspensions.The disulfide-bearing polymers exhibited only aslightly larger affinity for the gold surface than thosethat do not have the disulfide groups. The polymerlayers allowed the free diffusion of small solutes butefficiently minimized the nonspecific absorption oflarge molecules such as proteins, a promising prop-erty (Figure 23).236a AuNPs have been synthesizedin graft copolymer micelles,87 and the diffusion ofAuNPs in a polymer matrix has been analyzed.236bCore-shell AuNPs have been prepared by the layer-by-layer technique, utilizing polyelectrolyte multi-layers assembled onto polystyrene cores as thin filmsin which to infiltrate AuNPs, and hollow sphereswere obtained by removal of the templated polysty-rene cores.236c

3.12. DendrimersA variety of assemblies between PAMAM dendrim-

ers and AuNPs were reported in which the AuNPswere stabilized by the dendrimer that acted as botha polymer and a ligand. The AuNPs were stabilizedonly in the presence of excess PAMAM dendrimersand in solution, but PAMAM dendrimers function-alized with thiol termini could completely stabilizethe AuNPs.237 PAMAM dendrimers were also func-tionalized with hydrophobic groups for solubilizationof the AuNPs in organic solvents.238,368 Such den-drimer-AuNP assemblies were deposited as films onsurfaces and used as sensors.239-241 AuNPs weresynthesized from AuCl3 in DMF using PAMAMdendrimers that were modified with surface methylester groups.242 The use of PAMAM dendrimers forthe stabilization of AuNPs allowed control of the

Figure 21. TEM images of AuNPs covered with PEGSH2000, observed in the presence of chondroitin sulfate csodium salt (polysaccharide carrying sulfuric acid groupsand carboxylic acid groups, which are expected to interactwith the PEG chain). Reprinted with permission from ref228 (Ishiwataris group). Copyright 2002 The ChemicalSociety of Japan.

Figure 22. Schematic illustration for the synthesis ofAu@HCMS (hollow core/mesoporous shell) polymer andcarbon capsules. Reprinted with permission from ref 226(Hyeons group). Copyright 2002 American Chemical So-ciety.


interparticle distance.243 PAMAM-dendrimer-AuNPsassemblies were incorporated into SiO2 matrices.244The pH dependence of water-soluble PAMAM-den-drimer-stabilized AuNPs was examined,245 and suchassemblies were used for imaging in cells.246 Bimetal-lic Au-Pd nanoparticles were synthesized by Crookssgroup from PAMAM-dendrimer-Pd nanoparticlesassemblies for a catalytic purpose.247-249 Dendrimerscontaining a AuNP core were synthesized by usingthe Brust-Schiffrin technique with dendrons thatwere functionalized with thiols at the focal point(Figures 24 and 25).250-254 PAMAM-dendrimer-AuNP composites were used as chemiresistor sensors

for the detection of volatile organic compounds andstudied by specular neutron reflectometry.255a AuNPswere formed in the presence of stiff polyphenylenedendrimer templates with 16 thiomethyl groups onthe outside.255b

3.13. Surfaces, Films, Silica, and Other AuNPMaterials

AuNPs were deposited on surfaces for a numberof purposes, including physical studies256-275 (Figures26-28) and derivatization of self-assembled mono-layers.276-278 Privileged materials for deposition are

Figure 23. Stepwise grafting-to derivatization of AuNPs. (a) Fixation of the polymer with disulfide anchoring groups.(b) Activation of the polymer by unsymmetrical bifunctional linker groups. (c) Functionalization of the polymer by receptors.Step (b) is omitted when activated polymers are used. Reprinted with permission from ref 236a (Mangeneys group).Copyright 2002 American Chemical Society.

Figure 24. Syntheses of dendronized AuNPs using the thiol ligand substitution procedure. Reprinted with permissionfrom ref 251 (Astrucs group). Copyright 2003 American Chemical Society.


Si,279 various molecular silicon substrates,280 TiO2(Figure 29),281-285a BaTiO3,285b SrTiO3,285c Al2O3 (Fig-ure 30),286 ammonium salts,287 and various forms ofcarbon288-293 ([60] fullerene (Figure 31),288,289 nano-tubes,290,291 and diamond292). AuNPs were also de-posited together with [RuIItris(2,2-bipyridine)] to

form multistructures whose photocurrent responseswere recorded.294

Thin films of AuNPs were prepared,126,295-309 inparticular LB films310-319 (Figure 32) and monolay-ers320 at the liquid interface92,321 and from the gasphase.322a Typically, the Brust-Schiffrin method wasused to synthesize alkanethiol-stabilized AuNPs thatwere subsequently spread on the water surface.These totally hydrophobic films could sustain reason-able pressures, and the compressibility was high. Itappeared likely that multilayers were generatedabove 6-8 mN/m.317 Films of several tens of cm2 inwidth and several microns in thickness were formedby cross-linking AuNPs with alkanedithiols, followedby filtration onto nanoporous supports.322b A conduct-ing AuNP film is simply produced by rinsing apolystyrene plate with ethanol and water, followedby stirring it at room temperature in an aqueoussolution consisting of a thiol and a AuNP solution.323AuNPs were dispersed into Nylon-1,1 thin films, andthe resulting materials were studied during heattreatment.324-326 Phase transfer of AuNPs across a

Figure 25. Direct syntheses of dendronized AuNPs containing a nonaferrocenyl thiol dendron (about 180 ferrocenyl groups).Reprinted with permission from ref 251 (Astrucs group). Copyright 2003 American Chemical Society.

Figure 26. Schematic representation of the proceduresdeveloped for fabricating two-dimensional arrays of AuNPson a silicon substrate in combination with self-assemblyof an (aminopropyl)triethoxysilane monolayer, immobiliza-tion of nanoparticles, alkanethiol treatment, and solventevaporation technique. Reprinted from ref 258 (Lius group)by permission of the PCCP Owner Societies. Copyright2002.

Figure 27. Tapping-mode AFM images (2 m 2 m) ofthe morphologies of a 0.02-mL gold film deposited onfocused ion beam patterned surfaces for different ionfluences If and periodicities ldef: (a) ldef ) 300 nm, If )37 500 ions per point; (b) ldef ) 300 nm, If ) 3750 ions perpoint; (c) ldef ) 500 nm, If ) 37 500 ions per point. Reprintedwith permission from ref 262 (Bardottis group). Copyright2002 Elsevier.


water/oil interface was achieved by stoichiometricion-pair formation between carboxylate anions onparticle surfaces and tetraoctylammonium cationsand revealed by TEM, IR, UV-vis absorption, andEDX.327a Complete phase transfer of negativelycharged (-CO2- or -SO3-), surface-modified AuNPsfrom the aqueous phase to the organic phase wascarried out by hydrophobization using primary

amines.327b AuP nanowires that are between 100 and200 nm wide could be formed by ultrashort laserpulses.327c

There are many reports of the preparation, char-acterization, and study of AuNPs dispersed withinmesoporous silica, [email protected],125,328-360 The influ-ence of size, 328,329 the biofunctionalization,331 thecrystal growth,332 the organization in high order

Figure 28. Schematic configuration of the experimentalapparatus for measuring current-voltage characteristicsby an STM tip. (a) Conventional structure of single-electrontunneling (SET) devices in which a small AuNP is sepa-rated from a substrate electrode by an ultrathin tunnelinglayer. (b) Current structure of the SET devices in which asmall AuNP is separated from an HOPG ground plane bya surface-capped tunneling layer on the gold particle.Reprinted with permission from ref 272 (Huangs group).Copyright 1998 Elsevier.

Figure 29. Schematic illustration of sequential surfacesol-gel technique and alternate assembly. Reprinted withpermission from ref 284a (Kunitakes group). Copyright1999 American Chemical Society.

Figure 30. AuNP deposits on SiO2/Al2O3 nanoparticlesproduced by reduction of [AuCl4]- with freshly preparedsodium borohydride solution. Reprinted with permissionfrom ref 115b (Kamats group). Copyright 2000 AmericanChemical Society.

Figure 31. Fullerene-induced network of -cyclodextrin-capped AuNPs in aqueous solution. Reprinted with permis-sion from ref 288a (Kaifers group). Copyright 2001 Ameri-can Chemical Society.

Figure 32. Schematics of the formation of the defects inLB monolayers prepared from (a) a low particle concentra-tion and (b) a high AuNP concentration of LB spreadingsuspensions. Reprinted with permission from ref 312(Huangs group). Copyright 2001 American Institute ofPhysics.


(Figure 33),333 the influence of radiations on thenucleation,342 and the sol-gel approach354,359 havebeen examined in the preparations of Au@SiO2. Thetwo most common synthetic methods to form sol-gel matrices of AuNPs are the citrate route followedby stabilization by a (3-aminopropyl)trimethoxysilane(APTMS)-derived aminosilicate and the sol-gel pro-cessing in inverse micelles (Figure 34).354 For in-stance, 2- and 5-nm AuNPs have been inserted inmesoporous silica materials MCM-41 and MCM-48by the Somorjai group.352 Another simple and suc-cessful method involves tetramethoxysilane, partiallyhydrolyzed tetrakis(hydroxymethyl)phosphonium chlo-ride, and HAuCl4 in aqueous solution to produceAu@SiO2 containing up to 1% weight Au.342 Reduc-tion of HAuCl4 using H2 at 973 K for 1 h was alsosuccessfully used.344 The sonochemical approach ofHAuCl4 reduction leads to the deposition of AuNPson the surface of the silica spheres113 or within thepores of mesoporous silica.111,112 Some methods forthe engineering of the AuNP surface have beendescribed.355 The homogeneous incorporation of silica-coated AuNPs into a transparent silica gel without

any aggregation of particles has been reported.335There is growing interest for the assembly and studyof AuNPs on silanized glass plates,295,300,347,360,492,645-648

Figure 33. SEM images of plasmonic waveguide struc-tures fabricated by assembling the Au@SiO2 core-shellAuNP against templates (see insets) patterned in thin filmsof photoresist. These core-shell AuNPs had a core diam-eter of 50 nm and a shell thickness of 100 nm. Reprintedwith permission from ref 329a (Xias group). Copyright2002 American Chemical Society.

Figure 34. Schematic illustration of AuNPs in a silicatematrix. Reprinted with permission from ref 353 (Levsgroup). Copyright 1997 The Royal Society of Chemistry.

Figure 35. AuNP superlattices. (Top) Packing sequencesobserved in AuNP superlattices: (a) hexagonal close-packing ABAB, (b) cubic close-packing ABC ABC, and (c)anomalous packing in which AuNPs sit on two fold saddlepositions D. The packing becomes ADA or ADC. (Bottom)High-dispersion diffraction patterns from an FCC AuNPsuperlattice on the [111] zone axis. The reflections (220),corresponding to the second row of reflections, correspondto the void superlattice. Reprinted with permission fromref 366 (Jose-Yacamans group). Copyright 2000 Springer.


and highly stable AuNPs have been prepared insideHY zeolite supercages.361

AuNPs have been manipulated to form highlyordered 1D,65,362-364 2D,141,365 or 3D59,366,367 (Figure 35)nanonetworks and superstructures.60,368 Syntheticopals369,370 were fabricated using a layer-by-layerprocess.370 AuNPs have been formed in lipids follow-ing electrostatic entrapment of AuCl4-,371 and bio-molecular templating allowed site-specific organiza-tion of AuNPs.372 AuNPs have been used to decoratelatexes via conducting polymer templates (Figure36).373a The spin-coating method was shown to be farsuperior to the traditional immersion method for thefast fabrication of AuNPs attached to APTMS-modi-fied fused silica.373b

AuNP-containing materials, whose assembly ismentioned in this section, will be also discussed fortheir applications in the sensors and catalysis sec-tions.

4. Physical Properties

4.1. The Surface Plasmon Band (SPB)The deep-red color of AuNP sols in water and

glasses reflects the surface plasmon band (SPB; seealso section 2 for background), a broad absorptionband in the visible region around 520 nm. The SPBis due to the collective oscillations of the electron gasat the surface of nanoparticles (6s electrons of theconduction band for AuNPs) that is correlated withthe electromagnetic field of the incoming light, i.e.,the excitation of the coherent oscillation of theconduction band. The study of the SPB has remainedan area of very active research from both scientificand technological standpoints, especially when theparticles are embedded in ionic matrices andglasses.374,375 For instance, a driving force for thisinterest is the application to the photographic pro-cess.376 Thus, the SPB provides a considerable bodyof information on the development of the bandstructure in metals and has been the subject ofextensive study of optical spectroscopic properties ofAuNPs.375,377-379

The nature of the SPB was rationalized in a masterpublication authored by Mie in 1908.379 According toMie theory, the total cross section composed of theSP absorption and scattering is given as a summationover all electric and magnetic oscillations. The reso-nances denoted as surface plasmons were described

quantitatively by solving Maxwells equations forspherical particles with the appropriate boundaryconditions. Mie theory attributes the plasmon bandof spherical particles to the dipole oscillations of thefree electrons in the conduction band occupying theenergy states immediately above the Fermi energylevel.380 All the numerous subsequent and recentreports correlate the spectroscopic behavior of AuNPswith the Mie theory.24,375,380-383 The main character-istics of the SPB are (i) its position around 520 nm;(ii) its sharp decrease with decreasing core size forAuNPs with 1.4-3.2-nm core diameters due to theonset of quantum size effects that become importantfor particles with core sizes

or gold chloride.386gAnother influential parameter is the core charge.

Excess electronic charge causes shifts to higherenergy, whereas electron deficiency causes shifts tolower energy.386,387 A convenient theoretical expres-sion has been derived for the SPB position as afunction of the changes in free electron concentra-tion:

For instance, in the AuNPs of 5.2 nm diametercontaining 2951 Au atoms, the authors assume thatthere is one free electron per Au atom (e.g., Ninitial )2951 free electrons per AuNP); AuNPs charged to+0.82 V vs Ag leads to the removal of 19 electronsper AuNP, and thus Nfinal is 19 electrons less (Nfinal) 2932). This corresponds to a predicted red shift of1.7 nm, which is much smaller than that observedexperimentally (9 nm from 516 to 525 nm, whichwould theoretically correspond to the removal of 100electrons from the AuNP). The reason for this dif-ference is not clear, and tentative explanationsinvolve a much reduced number of free electrons andlarge shift amplification by the thiolate ligand shell.386

The SPB width was found to increase with decreas-ing size in the intrinsic size region (mean diametersmaller than 25 nm) and also to increase withincreasing size in the extrinsic region (mean diameterlarger than 25 nm). A small temperature effect wasalso found. It was proposed that the dominantelectronic dephasing mechanism involves electron-electron interactions rather than electron-phononcoupling.387a Femtosecond light scattering of AuNPsof 80 nm diameter showed, on the other hand, thatboth electron-phonon and phonon-phonon couplingprocesses occur in the individual AuNPs.387b In Au-SiO2 core-shell particles (Au@SiO2), varying the SiO2shell thickness and the refractive index of the solventallowed control over the optical properties of thedispersions, and the optical spectra were in goodagreement with Mie theory.356 The SPB position inAu@SiO2 (including films) was accurately predictedby the Maxwell-Garnett model, and it was concludedthat it is possible to synthesize composite materialswith optical properties that lie anywhere betweenthose of transparent glass and those of metallicgold.388-390

A near-field optical antenna effect was used tomeasure the line shape of the SPB in single AuNPs,and the results were found to be in agreement withMie theory; double-peak shapes caused by electro-magnetic coupling between close-lying particles wereobserved.391

From optical spectra of oriented AuNPs/polyethyl-ene, the SPB extinction maxima of AuNPs for inci-dent fields polarized parallel to the direction ofnanoparticle orientation (max - (0)) were found tobe red-shifted relative to the extinction maxima forperpendicular polarization (max - (90)).392 For rodlikeparticles, the extinction maximum for incident elec-tric fields polarized along the long axis occurs at alonger wavelength than the max for polarizationalong the particle radius.393 Selective suppression of

extinction of the SPB was observed.394,395 Picoseconddynamics of AuNPs was studied by laser excitationclose to the SPB at 66 nm, leading to the formationof hot non-Fermi electronic distribution within theAuNPs.396 Visible-laser-induced fusion and fragmen-tation could be induced by thionicotamide, and ag-gregation effects disappear following laser pulseexcitation.397a Softening of the coherently excitedbreathing mode on AuNPs was observed by time-resolved spectroscopy, showing that the period ofbreathing mode increases with pump laser power.397bThe coagulation (along with Oswald ripening) ofAuNPs dispersed in organic liquids was dramaticallyaccelerated by visible light, and the process wasshown to be wavelength dependent; UV irradiationcaused coalescence.398 Laser irradiation at the SPBof suspended AuNPs in 2-propanol causes coagulationor dispersion, depending on concentration, the pho-tochemical reaction being due to electron transferfrom a solvent molecule to the AuNP.399 AuNPs werepulverized into smaller AuNPs with a desired aver-age diameter and a narrow distribution by a suitableselection of laser irradiation.400 When a molecularlinker, 4-aminobenzenethiol, attached several AuNPstogether, the optical absorption differed, consistentwith plasmon-plasmon interactions between theAuNPs of the assembly (Figure 37).401

Applications of the sensitivity of the position of theSPB are known, especially in the fields of sensors andbiology. A shift of the SPB of AuNPs has beenmeasured upon adsorption of gelatin, and quantita-tive yield measurements of the adsorbed amountwere obtained.402 Rhodamine 6G was shown to pro-voke morphological changes and particle growth uponlaser irradiation of the SPB as a result of meltingand fusion of AuNPs, and the multiphoton processleading to the fusion process has been elucidatedusing picosecond laser flash photolysis (Figure 38).403Increasing the solution temperature from 10 to 40C thermally triggered the reversible hydrophilic-to-hydrophobic phase transition of the adsorbed elastin-

final/initial ) (Ninitial/Nfinal)1/2

Figure 37. (a) UV/vis solution spectra of 4-mercaptoben-zoic acid-capped AuNPs as a function of the addition ofNaOH(aq) and (b) the variation of the intensity of theplasmon absorbance, at 525 nm, as a function of the pH ofthe solution. Points i, iii, v, vii, ix, and xi in (b) wereobtained from curves i, iii, v, vii, ix, and xi in (a),respectively. Reprinted with permission from ref 93 (Evanssgroup). Copyright 1998 American Chemical Society.


like polypeptide, a thermally responsive biopoly-mer.404 Formation of large aggregates caused areversible change in color of the AuNP suspensionfrom red to violet due to coupling to surface plasmonsin aggregated colloids. The SPB was used to studythe dispersibility of AuNPs in a variety of solvents(Figure 39).405 Phase transfer of dodecylamine-cappedAuNPs dispersed in an organic solvent into watercontaining the surfactant cetyltrimethylammoniumbromide (CTAB) was monitored by color changesinitiated upon shaking.406 Surface interaction ofAuNPs with functional organic molecules was probedusing the shifts of the SPB. For instance, a red shiftwas observed with an increase of the solvent dielec-tric constant with solvents that do not coordinate thegold core, but the SPB is unaffected in polar solventsthat do not bind to the core.407 AuNPs consisting ofa mixture of triangular/hexagonal and smaller, close-to-spherical particles display two SPBs at 540 and680 nm, respectively, as expected. UV-visible dataindicate preferential adsorption of the flat AuNPs onpolyelectrolyte films and the development of a newband at 650 nm as the number of bilayers in-creased.408a Compared to solid AuNPs of similar size,nanorings produced on soda-glass substrates usingcolloidal lithography exhibit a red-shifted localizedSPB that could be tuned over an extended wave-length range by varying the ratio of the ring thick-ness to its radius.408b

The magnetic circular dichroism (MCD) spectra ofAuNPs and Au9(PPh3)83+ encapsulated in opticallytransparent xerogels have been shown to be temper-ature dependent between 5 and 295 K only for theformer, and indicate that the MCD spectra of thelatter reflect an excited-state phenomenon with astrong intermixing of the excited spin-orbit states.382bThe hot electron dynamics of AuNPs was character-ized using femtosecond two-color pump-probe spec-troscopy in the SPB region.382c

4.2. FluorescenceFluorescence studies of AuNPs have been carried

out under various conditions,409-419 including femto-second emission410 and steady-state investigation of

the interaction between thiolate ligands and the goldcore.411 Capping fluorescent groups are pyrenyl,413polyoctylthiophenyl,415 fluorenyl,419 and otherprobes.414,416-418 Indeed, resonant energy transfer wasobserved in fluorescent ligand-capped AnNPs, thisphenomenon being of great interest in biophotonics420and materials science.421-423 Both radiative and non-radiative rates critically depend on the size and shapeof the AuNPs, the distance between the dye mol-ecules, the orientation of the dipole with respect tothe dye-nanoparticle axis, and the overlap of themolecules emission with the nanoparticles absorp-tion spectrum.418 Orders-of-magnitude higher ef-ficiencies were obtained with nanometer-dimensionmetal samples.424-426 The observed increase in thefluorescence yield reflected the suppression of the

Figure 38. Schematic diagram illustrating the possiblemorphological changes associated with laser irradiation ofthe AuNP-dye assembly. Au@SCN, AuNPs obtained byreduction of AuCl4- with SCN-. Reprinted with permissionfrom ref 402 (Kamats group). Copyright 2000 AmericanChemical Society.

Figure 39. Optical absorption spectra of AuNPs 8.3 nmin diameter dispersed in (a) water, (b) ethanol, and (c)chloroform. The dashed lines represent the values calcu-lated from the Mie equation. The solid lines represent theexperimental data. Reprinted with permission from ref 404(Huangs group). Copyright 2002 American Institute ofPhysics.


nonradiative decay upon binding to AuNPs.414 Visibleluminescence has been reported for water-solubleAuNPs, for which a hypothetical mechanism involv-ing 5 d10 f 6 (sp)1 interband transition has beensuggested.409 On the basis of the ability of discretephotoisomeric states of spiropyrans to exhibit distinctphysical properties, photoswitchable AuNP assem-blies of various amino acids were designed by an-choring spiropyrans to allow the release of the outersphere of amino acids on irradiation.427

4.3. ElectrochemistryThe electrochemistry of thiolate-stabilized AuNPs

showing the formation of core charge has beenindicated in section 2 (staircase blockade).39,40 More-over, the quantized capacitance charging of AuNPsself-assembled monolayers on electrode surfaces couldbe rectified by certain hydrophobic electrolyte ions,such as PF6-, in aqueous solution (Figure 40).39c Inthe presence of some p-nitrothiophenolate ligands,the peak spacings corresponding to the quantizedcapacitance charging were found to decrease slightlycompared to those obtained in the absence of p-nitrothiophenolate ligands, corresponding to a smalldecrease of the particle capacitance due to the pres-ence of more polar ligands.428a Magnetoelectrochem-istry of AuNP quantized capacitance charging showedthe influence of a magnetic field on the electrochem-istry of AuNPs, and in particular the effect of electronparity upon their charging states.428b Double-layercapacitance was obtained in aqueous media by dif-ferential pulse voltammetry.428c The voltammetry ofsmall AuNPs containing respectively 11429a and 38429bcore Au atoms has been reported.

Viologen thiols and dithiols have been synthesizedand connected to AuNPs.430,431 In particular, viologenthiols have been used as redox-active linkers in orderto study electron transfer between the linkedAuNPs.430a AuNPs have been shown to tune theelectrochemical properties of the electrode/solutioninterface using the Fe(CN)63-/4- redox system.432

The electrochemistry of functional AuNPs contain-ing thiolate ligands bearing ferrocenyl49b or amido-or silylferrocenyl140,250-252,433 and biferrocenyl groupshas been reported, and these ferrocenyl- or biferro-cenyl-derived AuNPs have been deposited on elec-

trodes by scanning around the potential regionaround the FeII/FeIII waves.251,252 Nishiharas grouphas modified alkanethiol-stabilized AuNPs by thiolligands bearing biferrocenyl434-437 or anthraquino-ne436,437 using the thiolate-exchange method. Thesecond oxidation process of the biferrocenyl units ofAuNPs-biferrocenylthiolate led to a uniform redox-active AuNP film on an electrode. AuNPs-an-thraquinonethiolate was found to be aggregated bytwo-electron reduction of anthraquinone groups. Thesefindings confirmed that AuNPs containing multipleredox molecules could assemble according to chargeaccumulation. The multielectron system seemed tobe indispensable for these deposition phenomena, asthey were not observed for AuNPs linked via thiolateligands with a single redox species such as ferro-cenyl.49b This property also allowed Yamada andNishihara to construct alternating multilayed struc-tures of palladium and gold nanoparticles connectedwith biferrocenyl groups (Figure 41).440 Ferrocenyl-dendronized AuNPs also adsorb very well, a usefulproperty for molecular recognition (vide infra).251,252C60-functionalized AuNPs, synthesized using theligand-exchange procedure, can form a derivatizedelectrode upon immersion of a gold electrode, and thismodified electrode showed two peaks correspondingto the reduction of C60 that were stable uponscanning.441a The charge injection energetics betweena solution redox couple and hexanethiol-AuNPs hasbeen probed by scanning electrochemical micros-copy.441b Altogether, electrochemistry has been usedin several ways to fabricate and depositAuNPs,251,252,436-441 inter alia on porous silicon.441c AAPTMS-supported AuNP electrode was constructed;therefore, the preparation procedure using 3-mer-captopropionic acid-bridged copper hexacyanoferratemultilayers on a planar macroelectrode was copiedto the as-prepared AuNP electrode.441d AuNPs formedfrom ferrocenylthiophenol (2.5 nm) were studied interalia by voltammetry, showing only 15% coverage withthe composition Au490Fc80.441e AuNP microelectrodeshave been prepared with uniform Pt group over-layers.442a

4.4. Electronic Properties Using Other PhysicalMethods

It was found that, for 2D superlattices consistingof large (5 nm) AuNPs, the electronic behavior wasdominated by the Coulomb blockade effect at lowtemperature, while the I-V response was ohmic at

Figure 40. Schematic chart of electron tunneling throughsurface-confined AuNP layers. Reprinted with permissionfrom ref 39c (Chens group). Copyright 2001 AmericanChemical Society.

Figure 41. Combination of electro-oxidative deposition ofPdNPs attached with biferrocene-terminated thiolates andthat of AuNPs with the same thiolates, to form a thinredox-active film with a layered hybrid structure. Reprintedwith permission from ref 440 (Nishiharas group). Copy-right 2002 The Royal Society of Chemistry.


room temperature.61 TiO2 nanoparticles exhibited ablue coloration due to stored electrons within theparticles when subjected to UV. A partial disappear-ance of the blue color was seen upon contact with

AuNPs as electrons were transferred from TiO2 toAuNPs (Figure 42).442b TiO2 films cast on conductingglass plates were modified by adsorbing AuNPs (5nm diameter) from a toluene solution, and selectiveformation of Au islands and larger particles on theTiO2 suface was observed by TEM and AFM (Figure43).459 A technique was described in which a AuNPwas connected to two Cr electrodes, and electrontransport in this sample could be fitted to orthodoxtheory of electron tunneling.443a Scanning electro-chemical microscopy was used to investigate thekinetics of electron-transfer reactions between methylviologen (MV2+) and protons, giving H2 and MV+,catalyzed by AuNPs supported on an insulatingsubstrate. This technique allows investigation ofAuNP size effects on the kinetics and study ofreactions at semiconductor nanoparticle surfaces inthe absence of a metallic conductor.443b An electronicconductivity study of composites made from 1,10-decanethiol and AuNPs using pressed pellets be-tween 1-m electrode gaps showed that the I-V curvewas sigmoidal (Figure 44).438 The rectifying effectsof electrolyte ions on interfacial electron transferwere investigated with AuNP monolayers anchoredby bifunctional chemical bridges containing viologengroups (Figure 45).439 X-ray absorption near-edgestructure (XANES) of 2-nm AuNPs capped withdendrimer and thiol revealed the gain of a 5d electronrelative to the bulk when capped with weakly inter-acting dendrimers and the loss of a 5d electron whencapped with strongly interacting thiol ligands.444a Thechemical states of the self-assembled AuNPs ab-sorbed onto the surface of a silane were studied byangle-resolved X-ray photoelectron spectroscopy, whichmade it possible to distinguish bound thiolate ligandsand unbound thiols444b and to investigate the size-dependent systematics of lattice contraction andcharge redistribution.444c Using a pump-probe tech-nique, the dynamics of the hot carriers in nanodotsinduced by femtosecond laser pulses showed that

Figure 42. Charge distribution between TiO2 and AuNPs,leading to equilibration with the C60/C60- redox couple (a)in the absence and (b) in the presence of metal nanopar-ticles. EF and EF refer to the Fermi levels of TiO2 beforeand after attaining equilibrium, respectively. Reprintedwith permission from ref 442b (Kamats group). Copyright2003 American Chemical Society.

Figure 43. Charge separation in a AuNP-modified nano-structured TiO2 electrode. OTE, optically transparentelectrode. Reprinted with permission from ref 459 (Kamatsgroup). Copyright 2000 American Chemical Society.

Figure 44. Schematic illustration of t

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