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Analytica Chimica Acta 598 (2007) 181–192

Review

Synthesis and electrochemical applications of gold nanoparticles

Shaojun Guo a,b, Erkang Wang a,b,∗a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, Jilin, Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China

Received 31 May 2007; received in revised form 12 July 2007; accepted 13 July 2007Available online 31 July 2007

bstract

This review covers recent advances in synthesis and electrochemical applications of gold nanoparticles (AuNPs). Described approaches includehe synthesis of AuNPs via designing and choosing new protecting ligands; and applications in electrochemistry of AuNPs including AuNPs-basedioelectrochemical sensors, such as direct electrochemistry of redox-proteins, genosensors and immunosensors, and AuNPs as enhancing platformor electrocatalysis and electrochemical sensors.

2007 Elsevier B.V. All rights reserved.

eywords: Gold nanoparticle; Biosensor; Electrocatalysis; Electrochemical sensor; Nanomaterial

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1812. Synthesis of gold nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

2.1. Biomolecule protected gold nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1822.2. Green agent protected gold nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1832.3. Polymer protected gold nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1832.4. Dendrimer protected gold nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

3. Electrochemical applications of gold nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1843.1. Gold nanoparticles-based electrochemical sensor and bioelectrochemical sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

3.1.1. Direct electrochemistry of redox-protein on gold nanoparticles and third-generation electrochemical biosensors . . . . 1853.1.2. Gold nanoparticles for genosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1863.1.3. Gold nanoparticles for immunosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

3.2. Gold nanoparticles as enhancing platform for electrocatalysis and electrochemical sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1894. Summary, conclusions, and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

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ao

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

Nanosized particles of noble metals, especially gold nanopar-icles (AuNPs), have received great interests due to theirttractive electronic, optical, and thermal properties as well

∗ Corresponding author. Tel.: +86 431 85262003; fax: +86 431 85687911.E-mail address: ekwang@ciac.jl.cn (E. Wang).

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003-2670/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2007.07.054

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s catalytic properties and potential applications in the fieldsf physics, chemistry, biology, medicine, and material sciencend their different interdisciplinary fields [1], and therefore, theynthesis and characterization of AuNPs have attracted consid-rable attention from a fundamental and practical point of view.

fter the breakthroughs reported by Schmid [2–4] and Brust

t al. [5,6], a variety of methods have been developed to pre-are AuNPs, and many reviews [7–9] are now available. Fornstance, the report about synthesis, assembly, properties and

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pplication of AuNPs has been reviewed in “Gold Nanoparticles:ssembly, Supramolecular Chemistry, Quantum-Size-Relatedroperties, and Applications toward Biology, Catalysis, andanotechnology” [9]. As is known, the preparation of AuNPsenerally involves the chemical reduction of gold salt in aque-us, organic phase or two phases. However, the high surfacenergy of AuNPs makes them extremely reactive, and mostystems undergo aggregation without protection or passivationf their surfaces. Thus, special precautions have to be takeno avoid their aggregation or precipitation. Typically, AuNPsre prepared by chemical reduction of the corresponding tran-ition metal salts in the presence of a stabilizer which bindso their surface to impart high stability and rich linking chem-stry and provide the desired charge and solubility properties.ome of the commonly used methods for surface passivation

nclude protection by self-assembled monolayers, the most pop-lar being citrate [10] and thiol-functionalized organics [11];ncapsulation in the H2O pools of reverse microemulsions [12];nd dispersion in polymeric matrixes [13]. Although the syn-hesis of AuNPs makes great progress, how to control the size,

onodisperse, morphology, and surface chemistry of AuNPs istill a great challenge. Recently, designing novel protectors foruNPs have been the focus of intense research because surface

hemistry of AuNPs will play a key role in future applicationelds such as nanosensor, biosensor, catalysis, nanodevice andanoelectrochemistry. Thus, in the synthesis part, we will focusn recent advances on how to choose novel protecting agents forhe synthesis of AuNPs.

For electroanalytical chemist, more attention has been paido AuNPs because of their good biological compatibility, excel-ent conducting capability and high surface-to-volume ratio.he introduction of AuNPs onto the electrochemical interfacesas infused new vigor into electrochemistry [9,14–20]. Ret-ospecting the history of electroanalytical chemistry, we havexperienced the times of developing new electrochemical tech-iques and different electrode modified strategies for enhancingnalytical selectivity and sensitivity [21]. The development ofanomaterial will offer new opportunities in the developmentf electroanalytical chemistry. Recently, AuNPs modified elec-rode surfaces, generating functional electrochemical sensingnterfaces, have been reported in great quantity. Thus, in the partf electrochemical application, we will summary some recentrogress on AuNPs-based electrochemical sensing and electro-atalytic systems.

However, due to the explosion of publications in this field, weo not claim that this review includes all of the published workbout the synthesis and electrochemical applications of AuNPs.e apologize to the authors of much excellent work that due to

he large activity in this field, we have unintentionally left out.

. Synthesis of gold nanoparticles

.1. Biomolecule protected gold nanoparticles

With the recent development of biotechnology, bioconjugatesusing some biomolecules to bind the AuNPs) have receivedonsiderable research interests because the obtained hybrid bio-

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ca Acta 598 (2007) 181–192

aterials own unique properties provided by the AuNPs. Thetabilization and functionalization of AuNPs with biomolecu-ar recognition motif have provided flexibility for a variety ofpplications, including bioassay, bioimaging and biosensor. As aesult, the ability to synthesize aqueous-stabilized nanoparticlesf controlled size and shape that can be easily functionalizedith biomolecules (peptides, enzymes, antibodies, DNA) isighly desirable.

A number of methods have been reported to synthesize theiomolecule protected AuNPs in aqueous media [22–25]. Fornstance, peptide protected AuNPs have been reported by sev-ral groups. Naik and co-workers [22] demonstrated the usef a simple one-pot process for synthesizing water-stabilized,onodisperse AuNPs that were coated with biomolecular

ecognition multifunctional peptides on their surfaces. The mul-ifunctional peptides not only reduced the choloraurate but alsooated the surface of the AuNPs, resulting in the stabiliza-ion of the nanoparticles in aqueous solution. Higashi et al.23] described the preparation of polypeptide (poly(�-benzyl--glutamate)) monolayer-covered AuNPs using the modifiedrust–Schiffern method. In addition, thiolated peptides based on

he R-aminoisobutyric acid (Aib) unit have been well employedo the synthesis of AuNPs [24].

In addition to peptide stabilized AuNPs, some lipids or syn-hetic lipids such as phospholipid [26–28], didodecyldimethy-ammonium bromide (DDAB) [29,30], cetyltrimethyl ammo-ium bromide (CTAB) [31], tetraoctammonium (TOA) [32,33],ould also be used as capping agents for the synthesis ofuNPs. Our group has employed DDAB [29,30], CTAB [31]

nd TOA [32] as protective agents to the synthesis of AuNPsith high stability. For instance, DDAB lipid bilayer-protecteduNPs, which were stable and hydrophilic, were synthesizedy in situ reduction of HAuCl4 with NaBH4 in an aqueousedium in the presence of DDAB [29]. In addition, phospho-

ipid stabilized AuNPs have been developed by several groups.or instance, Urban and co-worker [26] provided a simple

wo-step approach of modification of 1 nm diameter AuNPssing an aqueous solution of (1, 2-dipalmitoyl-sn-glycero-3-hosphothio-ethanol) phospholipid (PL). Morphology and sizeontrol of AuNPs by phospholipids has been reported by Zhund co-worker [28] via a seed growth method. These lipids pro-ected AuNPs will find potential applications in biomoleculeeparation, researching biomolecule interaction, and biosensor.

Recently, there has been great and increasing interest inooking at biological systems for inspiration and using microor-anisms as workers in the living factory for the production of newunctional nanomaterials [33–35]. It was found that microor-anism could be used as capping agents for the fabrication ofuNPs. Ahmad et al. [33] reported extracellular biosynthesis ofonodisperse AuNPs by a novel extremophilic actinomycete,hermomonospora sp. Our group [34] has also attempted tobtain AuNPs assisted by Escherichia coli DH5a. Further-ore, viruses can be also exploited as a constrained template

o direct nanoparticle synthesis [36,37]. As templates, virusesffer confined cages, high symmetry, robust functional pro-ein capsids, unique structural architectures, repeating motifs,nd are amenable to molecular biology manipulations, all of

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hich will be suitable for the synthesis of AuNPs. In fact, somether small biomolecules [38–40] can also be used to direct theynthesis of AuNPs. A new reagent (dopamine hydrochloride)as been reported to achieve controlled fabrication of AuNPs39]. Another interesting chiral molecule protected AuNPsecently have been reported using N-isobutyryl-l-cysteine or-isobutyryl-d-cysteine as stabilized agent [40].

.2. Green agent protected gold nanoparticles

Over the past decade, there has been an increasing emphasisn the topic of “green” chemistry because of the minimizeddverse environmental effect. For achieving this, utilizationf nontoxic chemicals, environmentally benign solvents, andenewable materials are some of the key issues that meritmportant consideration in a green synthetic strategy. With thencreasing interest in the minimization or total elimination ofaste and the implementation of sustainable processes through

he adoption of 12 fundamental principles of green chemistry41], the development of green chemistry approaches for therowth of advanced AuNPs is greatly desirable. In this part, weill discuss some green chemistry approaches to the synthesisf the AuNPs.

Room-temperature ionic liquids are attracting considerablenterest in many fields of chemistry and industry, due to theirotential as a “green” recyclable alternative to the traditionalrganic solvents [42]. They are known to have the potentialo enhance certain properties of metal nanoparticles, and alsoeen used as stabilized agents to prepare inorganic nanoparti-les [43–45]. Recently, the synthesis of AuNPs in ionic liquidsr modified by them has been the focus of intensive research46–48]. Naka and co-workers [46] reported the synthesis andunctions of AuNPs modified with ionic liquids based on themidazolium cation. Hydrophilic and hydrophobic properties ofhe as-prepared AuNPs could easily be tuned by anion exchangef the ionic liquid moiety. Bond and co-workers [47] reportedhe fabrication of AuNPs in a distillable ionic liquid, which arelso easily purified, recovered and separated to their constituentarts by low temperature distillation.

Recently, polysaccharides, e.g. chitosan [49–53] and sucrose54], etc., have been employed as green agents or protectinggents to synthesize “green” AuNPs. For instance, chitosan haseen used as a protecting agent by Esumi et al. [49] in the prepa-ation of AuNPs. Later, Yang and co-worker [50] reported then situ synthesis of AuNPs with controllable size via heatinghe mixture of HAuCl4 and chitosan using different molecu-ar weight chitosan as reducing/stabilizing agent. Furthermore,hitosan has been adopted as a stabilizer and reducing agent toynthesize AuNPs in solid surface or thin films. For instance,ao and co-workers [51] presented the chitosan-mediated syn-

hesis of AuNPs on poly(dimethylsiloxane) (PDMS) surfacesnder mild conditions. In addition to the synthesis of AuNPssing chitosan as the ligand, other polysaccarides could also be

mployed to the preparation of AuNPs. For instance, Qi et al.54] reported the synthesis of AuNPs using sucrose, which isnother kind of sugar. The above prepared AuNPs are expectedo be biocompatible, which will be found potential applica-

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ca Acta 598 (2007) 181–192 183

ions in several fields such as biosensor, bioelectrochemistry,nd bioassay.

.3. Polymer protected gold nanoparticles

The polymer-assisted synthesis of metal nanoparticles haseceived considerable attention because of the small concen-rations of homopolymers and block copolymers capable oftabilizing nanoparticles effectively by steric stabilization andherefore their providing good stability than that protected byhe well-known other ligands such as sodium citrate.

As is known, AuNPs are useful in a broad range ofpplications, but practical limitations are apparent whenonodispersity is required, for example, in electrochemical

uantized capacitance charging, single-electron transistor, andiosensor. It is difficult for the common protecting agents tobtain monodisperse AuNPs. However, polymers acting asrotecting agents provide good opportunities for monodisperseuNPs. Recently, some polymers with high steric effectsave been employed as stabilizing agents for the synthesis ofonodisperse AuNPs. For instance, Brust et al. [55] reported

ne-step method which led to near-monodisperse AuNPs inhe 1–4 nm size range in the presence of a water-soluble alkylhioether end-functionalized poly(methacrylic acid) stabilizer.he particle size could be controlled precisely by the molar

atio of Au to capping ligand, and the particles were readilybtainable in both aqueous and nonaqueous solutions. Fig. 1hows the typical TEM images of the AuNPs of four differentizes. All images confirm very narrow size distributions whichre unprecedented in the sub-5 nm size range. In addition to thebove novel polymer ligand, some other designing functionalolymers such as polyampholyte [56], hyperbranched polymer57–59], linear polymers [60], �-conjugated polymer [61],mphiphilic polymer [62–64] and thermoresponsive andH-responsive polymers [65,66] have been designed or used asood stabilizing agents for obtaining the AuNPs. For instance,hang’s group [66] reported the synthesis of three kinds of col-

oidal AuNPs such as discrete AuNPs, gold@polymer core-shellanoparticles, and AuNPs clusters mediated with a thermore-ponsive and pH-responsive coordination triblock copolymerf poly(ethylene glycol)-b-poly(4-vinylpyridine)-b-poly(N-sopropylacrylamide) (PEG110-b-P4VP35-b-PNIPAM22). Thes-prepared colloidal AuNPs were stable and thermoresponsive,hich might have very important applications such as acting as

esponsive catalysts.Recently, employing polymers containing NH2 group [67,68]

s in situ reductant and stabilizing agents has received con-iderable interests because of its simplicity and speedinessor the synthesis of AuNPs. The advantage of this methodas to combine the reduction and stabilization process in one

tep with only one component. As a result, the production ofanoscaled gold particles was reported by several groups, usingoly(ethylenemine) as reducing and stabilizing agent. Our group

67] reported that polyelectrolyte-protected AuNPs have beenacilely obtained by heating poly(ethylenemine)/HAuCl4 aque-us solution without the additional step of introducing othereducing agents. It is noted that some OH group-containing poly-

184 S. Guo, E. Wang / Analytica Chimi

Fig. 1. TEM images of the AuNPs of four different sizes prepared by controllingtMS

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er [69] and poly(diallyl dimethylammonium) chloride [70]ere also good capping and reductive agents for the synthesis ofuNPs. The above one-pot method for the synthesis of AuNPsill greatly reduce the cost of materials for future technical

pplications.

.4. Dendrimer protected gold nanoparticles

Dendrimers have received considerable attention becausehey can provide a dimensional functionality which is differentrom that of conventional linear polymers. Using dendrimerss protecting agents for the synthesis of AuNPs will supplyew opportunity for nanochemist due to the following advan-ages [71]: (1) the dendrimer templates themselves are of fairlyniform composition and structure, and therefore they yieldell-defined nanoparticle replicas; (2) the nanoparticles are sta-ilized by encapsulation within the dendrimer, and thereforehey do not agglomerate; (3) the dendrimer branches can besed as selective gates to control access of small moleculessubstrates) to the encapsulated nanoparticles; (4) the AuNPsrotected by dendrimer own very small size, typical 1–5 nm,

hich will find application in catalysis due to their high activity

nd surface-to-volume ratio.Based on the above advantage of dendrimer, a great deal of

esearch work has been reported to the synthesis of AuNPs via

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ca Acta 598 (2007) 181–192

esigning different dendrimers. Early example included goldolloids coated with poly(amidoamine) (PAMAM) dendrimersontaining terminal amines [72]. Later, Esumi et al. studiedhe formation of AuNPs within PAMAM dendrimers carryingmine [73], sugar [74], methyl ester, and alkyl groups on theireriphery. However, relatively large particles (2.1–12.8 nm) typ-cally having broad size distributions were formed both in thenterior and on the dendrimer exterior. Thus, the synthesis ofonodisperse AuNPs using other novel dendrimers as ligands is

ntensely required. Crooks and co-workers [75] reported a highlyonodisperse, 1–2 nm diameter AuNPs using OH-containing

oly(amidoamine) (PAMAM) dendrimers as templates. Moreecently, Loading of HAuCl4 in poly(amidoamine) dendrimersaving poly(ethylene glycol) (PEG) grafts at all chain ends andubsequent reduction with NaBH4 yielded PEG-modified den-rimers encapsulating monodisperse AuNPs of 2 nm diameter76]. The AuNPs held in the dendrimers were stable in aqueousolutions and dissolved readily, even after freeze-drying, whichs necessary for the subsequent applications.

In addition to PAMAM dendrimers, other dendrimersecently reported, such as poly(propyleneimine) (PPI) den-rimers [77], oligothia and dendron [78–82], have been extendeds novel ligands for the synthesis of AuNPs. A dendron is aegment of dendrimer that possesses a focal point onto whichhe branching units of a dendritic architecture are attached. Ifhe focal moiety is capable of metal complexation, the specific

etal-dendron interactions can be utilized to control reactionst this site. Thus, several research groups have reported the syn-hesis of gold AuNPs using dendron. Zheng and co-workers [80]emonstrated that dendrons which are focally modified withmetal-coordinating functionality could be utilized as stabi-

izing media for the controlled growth of nanocrystals. With-pyridone-based dendrons, the obtained AuNPs were stableor 6 months both in solution and in the solid state. Frechet-ype dendrons possessing a single thiol group at the focal pointave been reported by Kim et al. [82] to obtain small AuNPsith narrow size distribution and remarkably high stability. In

act, similar to the polymer-assisted in situ synthesis of AuNPs,mino-containing dendrimers [77] could also be used as sta-ilizing and reducing agents for the synthesis of AuNPs. Ourroup [77] demonstrated that dendrimer protected AuNPs hadeen facilely obtained by heating an aqueous solution containinghird-generation poly(propyleneimine) dendrimers and HAuCl4ithout the additional step of introducing other reducing agents.ue to their small size (1–5 nm) and particular optical, electric

nd thermal properties, the above dendrimer protected AuNPshould be a strong enticement to workers in the fields of catal-sis, biotechnology, medicine, electronics, photonics, physics,aterials science, etc.

. Electrochemical applications of gold nanoparticles

The unique physical and chemical properties of nanos-

ructured materials provide excellent prospects for interfacingiological recognition events with electronic signal transduc-ion and for designing a new generation of bioelectronic devicesith novel functions [83]. Especially, AuNPs represent excel-

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ent biocompatibility and display unique structural, electronic,agnetic, optical and catalytic properties which have made themvery attractive material for biosensor, chemisensor and elec-

rocatalyst. In recent years, many research articles using AuNPsor electrochemical applications such as bioassays, biosensor,hemical sensor and electrocatalysis have been published. Therere several reviews available which partly deal with the usef AuNPs for amperometric or voltammetric electrochemicalanobiosensors [83–87]. However, to the best of our knowledge,here is no review reporting the electrochemical applications ofuNPs. The aim of this part is mainly to review recent impor-

ant achievements about AuNPs in the field of electrochemicalspects.

.1. Gold nanoparticles-based electrochemical sensor andioelectrochemical sensor

.1.1. Direct electrochemistry of redox-protein on goldanoparticles and third-generation electrochemicaliosensors

The direct electron transfer (DET) from redox-protein to thelectrode surface is a very important subject in bioelectrochem-stry, which might help us understand the mechanism of manyioelectrochemical reactions and construct the biochemical sen-ors. Therefore, many scientists have devoted their efforts toealize the direct electrochemistry of proteins. An extremelymportant challenge in the direct electrochemistry of proteins

s the establishment of satisfactory electrical communicationetween the active site of the enzyme and the electrode sur-ace [88]. However, the redox center of most oxidoreductasess electrically insulated by a protein shell. Because of this shell,

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ca Acta 598 (2007) 181–192 185

he protein cannot be oxidized or reduced at an electrode atny potential. In order to achieve this task, mediator (discrete,lectroactive intermediaries between electrodes and solutionouples) have been utilized. More recently, it is interesting tond that the DET of some redox-proteins can also take placeith the help of nanoparticles without need of additional medi-

tors. Modification of electrode surfaces with the AuNPs willrovide a microenvironment similar to that of the redox-proteinsn native systems and gives the protein molecules more free-om in orientation, thereby reducing the insulating effect ofhe protein shell for the DET through the conducting tunnelsf AuNPs. In 1996, Natan and co-workers [89] have reported aeversible electrochemistry of horse heart cytochrome c at SnO2lectrodes modified with 12 nm-diameter AuNPs. Since then, areat deal of literatures have been reported to complete the DETf redox-proteins using AuNPs as promoter.

When nanoparticle/protein conjugates are assembled onhe electrode via simple self-assembly technology, the third-eneration nanoparticle-based biosensors can be facilelyabricated. Dong’s group [90] has developed a novel methodo construct a third-generation horseradish peroxidase biosen-or by self-assembling AuNPs into three-dimensional sol–geletwork. Fig. 2 shows the stepwise preparation process ofhe biosensor. First, a clean gold electrode was modified withhree-dimensional matrix by treatment with hydrolyzed (3-

ercaptopropyl)-trimethoxysilane (MPS), then AuNPs werenfiltrated into the matrix by forming Au-S covalent linkage.

inally horseradish peroxidase was introduced into the electrodeurface by electrostatic attraction between negatively chargeduNPs and positively charged horseradish peroxidase. The as-roduced biosensor could be fabricated reproducibly, exhibiting

printed with permission from Ref. [90], S. Dong, Anal. Chem. 74 (2002) 2217,

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ast amperometric responses (2.5 s) to H2O2, high sensitivitynd long-term stability. The detection limit of the biosensorould attain 2.0 �M, and the linear range was between 5.0 �Mnd 10.0 mM. Similarly, our group [91] has also completedirect electrochemistry of cytochrome c on a novel electro-hemical interface constructed by self-assembling AuNPs ontothree-dimensional silica gel network. In addition, some thi-

ls with specific functions could also be assembled on thelectrode surface. Thus, AuNPs could be immobilized on theelf-assembly monolayer surface and complete the DET ofome redox-proteins. For instance, Gu et al have reported theET of hemoglobin on the citrated-capped AuNPs assembledn a cysteamine modified gold substrate [92]. Furthermore,hey investigated the electrocatalytic activity of nanoparti-le/hemoglobin electrode towards H2O2 reduction. As a result,stable nanoparticle biosensor was constructed. In addition,

he DET of glucose oxidase and horseradish peroxidase wasell demonstrated by Pingarron and co-workers [93] andhen [94] on AuNPs immobilized cysteamine modified goldlectrode.

The AuNPs modified carbon paste electrodes have providedgood microenvironment for completing the DET of differ-

nt redox-proteins [95–99]. For instance, Ju and co-worker [95]eported that the DET between immobilized myoglobin andolloidal gold modified carbon paste electrode was completed.he myoglobin immobilized on the colloidal AuNPs displayedpair of redox peaks in 0.1 M pH 7.0 PBS with a formal

otential of about −0.108 V (versus NHE). Furthermore, thereparation of a xanthine oxidase biosensor, based on a carbonaste electrode modified with electrodeposited AuNPs, for themperometric determination of hypoxanthine was reported byingarron group [96]. Our group synthesized a kind of goldanoparticle protected by a synthetic lipid (DDAB). With theelp of these AuNPs, hemoglobin could exhibit a DET reactionn DDAB protected AuNPs modified glassy carbon electrode29]. In addition, the AuNPs modified ITO and screen-printedhodium–graphite electrodes could be also developed to com-lete the DET of some redox-protein such as myoglobin [100]nd cytochrome P450scc [101].

Recently, layer-by-layer (LBL) assembly technique based onlectrostatic interaction [102–105] could also be used to tai-or the electrochemical interface for completing the DET ofome redox-proteins and constructing novel electrochemicaliosensors. For instance, Hoshi et al. [102] prepared multi-ayer membranes by the LBL deposition of glucose oxidase anduNPs on sensor substrates, such as a Pt electrode and a quartzlass plate, to prepare glucose sensors. Sun et al. [103] reportedfeasible approach to construct multilayer films of glucose oxi-ase/AuNPs on the Au electrode surface using a cysteamine ascovalent attachment cross-linker. The biosensor constructedith six bilayers of GOD/AuNPs showed a wide linear response

o glucose in the range of 10 �M–0.013 M, with a fast responseess than 4 s, high sensitivity of 5.72 �A mM−1 cm−2, as well

s good stability and long-term life.

It is well-known that the polymer–nanoparticles compositesossess the interesting electrical, optical and magnetic proper-ies superior to those of the parent polymer and nanoparticles.

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ca Acta 598 (2007) 181–192

he nanocomposite composed of AuNPs and biopolymer suchs chitosan and carboxymethyl chitosan was also employeds excellent matrix for completing the DET of some redox-rotein and fabricating novel biosensor [106–109]. For instance,hen’s group [106] demonstrated a novel biocomposite madef chitosan hydrogel, glucose oxidase, and AuNPs by a directnd facile electrochemical deposition method under enzyme-riendly conditions for glucose biosensor. The biocompositerovided a shelter for the enzyme to retain its bioactivity atonsiderably extreme conditions, and the decorated AuNPsn the biocomposite offered excellent affinity to enzyme. Theiosensor exhibited a rapid response (within 7 s) and a linear cal-bration range from 5.0 �M to 2.4 mM with a detection limit of.7 �M for the detection of glucose. Later, Zhu and co-workers107] reported the DET of horseradish peroxidase based oniocompatible carboxymethyl chitosan–AuNPs nanocompos-te. A novel biosensor for H2O2 was constructed based on thebove nanocomposite. The biosensor exhibited a fast ampero-etric response (5 s), a good wide linear range of concentrations

rom 5.0 × 10−6 to 1.4 × 10−3 M, and a low detection limit of.01 × 10−7 M. Furthermore, Indium tin oxide (ITO) electrode108] could also be used to fabricate a novel disposable biosensorased on enzyme immobilized on Au-chitosan nanocompositeombined with flow injection analysis for the rapid determina-ion of H2O2.

.1.2. Gold nanoparticles for genosensorsThe development of electrical DNA hybridization biosen-

ors has attracted considerable research efforts [110,111]. SuchNA sensing applications require high sensitivity through

mplified transduction of the oligonucleotide interaction. Elec-rochemical devices offer elegant routes for interfacing, at the

olecular level, the DNA recognition and signal transductionlements, and are uniquely qualified for meeting the low-cost,ow-volume, and power requirements of decentralized DNAiagnostics [83,111,112]. The AuNPs modified electrochemi-al sensing interfaces offer elegant ways for interfacing DNAecognition events with electrochemical signal transduction, andor amplifying the resulting electrical response. AuNPs-basedmplification schemes reported have led to improved sensitivityf bioelectronic assays by several orders of magnitude. Thus,uNPs-based electrochemical device will provide new oppor-

unity for gene diagnostics in the future.Merkoci and co-workers reviewed [85] recent important

chievements on the electrochemical sensing of DNA usinguNPs. In that review, the author discussed recent some novel

trategies for genosensors based on AuNPs. Fig. 3 depicted achematic of the most important strategies used to integrateuNPs in DNA detection systems. These strategies consist of:

A) the electrochemical detection of AuNPs label by detectinghe gold ions released after acidic dissolving; (B) direct detectionf AuNPs anchored onto the surface of a conventional genosen-or (based on stripping voltammetry); (C) silver enhancement

sing conductometric technique; (D) enhancement of AuNPsnchored to conventional genosensor surface by using silver orold; (E) AuNPs as carriers of other AuNPs; (F) using AuNPss carriers for other electroactive labels.

S. Guo, E. Wang / Analytica Chimica Acta 598 (2007) 181–192 187

Fig. 3. Schematic procedure of the different strategies used for the integration of AuNPs into DNA sensing systems: (A) previous dissolving of AuNPs by usingH NPs ae of othp (2007

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Br/Br2 mixture followed by Au(III) ions detection, (B) direct detection of Aunhancement with silver or gold followed by detection, (E) AuNPs as carriersermission from Ref. [85], A. Merkoci, 19 (2007) 743. Copyright Wiley-VCH

In 2001 both Wang’s group [113] and that of Limoges [114]eported on the use of colloidal gold tags for electronic detec-ion of DNA hybridization. This protocol relied on capturinghe AuNPs to the hybridized target, followed by highly sensi-ive anodic stripping electrochemical measurement of the metalracer. This approach could attain a detection limit in the picomo-ar range. In addition, the electrochemical genosensors based onuNPs labels could be amplified by the catalytic electrodeposi-

ion of silver and its subsequent stripping. A better detectionimit was reported when a silver enhancement method wasmployed, based on the precipitation of silver on AuNPs tagsnd its dissolution (in HNO3) and subsequent electrochemicalotentiometric stripping detection [115,116]. This method waseported to obtain a detection limit in the femtomolar range.

Because the HBr/Br2 solution is highly toxic and thereforeethods based on direct electrochemical detection of AuNPs

ags, which replace the chemical oxidation agent, have beenlso reported recently [117–119]. For instance, Merkoci ando-workers [117] reported a novel AuNPs-based protocol foretection of DNA hybridization based on a magnetically triggedirect electrochemical detection of gold quantum dot tracers.t relied on binding target DNA with Au67 quantum dot in

ratio 1:1, followed by a genomagnetic hybridization assayetween Au67-DNA and complementary probe DNA markedaramagnetic beads. Differential pulse voltammetry was usedor a direct voltammetric detection of resulting Au67 quantumot-target DNA/complementary DNA-paramagnetic bead con-ugate on magnetic graphite-epoxy composite electrode. This

ethod could attain a low detection limit in the nanomolar range.

Enhancements by precipitation of silver or gold onto the

uNPs labels have been reported so as to achieve amplifiedignals and lower detection limits [84,120–125]. For instance,ang’s group [120] demonstrated an electrochemical detection

a

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nchored onto the surface of the genosensor, (C) conductometric detection, (D)er AuNPs, (F) AuNPs as carriers of other electroactive labels. Reprinted with).

ethod for analyzing sequence-specific DNA using AuNPsarked DNA probes and subsequent signal amplification step

y silver enhancement. The assay relied on the electrostaticdsorption of target oligonucleotides onto the sensing surfacef the glassy carbon electrode (GCE) and its hybridization tohe AuNPs-labeled oligonucleotides DNA probe. After silvereposition onto AuNPs, binding events between probe and tar-et were monitored by the differential pulse voltammetry signalf the large number of silver atoms anchored on the hybrids at thelectrode surface. A detection limit of 50 pM of complementaryligonucleotides was obtained based on this novel approach. Inddition to silver enhanced technology, Rochelet-Dequaire etl. [124] developed a new efficient protocol for the sensitiveuantification of a 35 base-pair human cytomegalovirus nucleiccid target (tDNA). In this assay, the hybridization of the targetdsorbed on the bottom of microwells with an oligonucleotideodified AuNPs detection probe (pDNA-Au) was monitored

y the anodic stripping detection of the chemically oxidizedold label at a screen-printed microband electrode (SPMBE).hanks to the combination of the sensitive AuIII determinationt a SPMBE with the large amount of AuIII released from eachDNA-Au, the picomolar detection limits of tDNA could bechieved. Further enhancement of the hybridization signal basedn the autocatalytic reductive deposition of ionic gold (AuIII) onhe surface of the AuNPs labels anchored on the hybrids was firstnvisaged by incubating the commonly used mixture of AuIII andydroxylamine. This strategy, which led to an efficient increasef the hybridization response, allowed detection of tDNA con-entrations as low as 600 aM (i.e., 104 lower than that without

mplification).

Another signal amplification strategy is to attach electroactiveerrocenylhexanethiol molecules [126–128] or electrogeneratedhemiluminescence (ECL) indicator [129] to the AuNPs labels.

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88 S. Guo, E. Wang / Analytica C

hou’s group [126] reported that AuNPs/streptavidin conju-ates covered with 6-ferrocenylhexanethiol were attached ontobiotinylated DNA detection probe of a sandwich DNA com-lex. Due to the elasticity of the DNA strands, the ferroceneaps on AuNPs/streptavidin conjugates were positioned in closeroximity to the underlying electrode modified with a mixedNA capture probe/hexanethiol self-assembled monolayer and

ould undergo reversible electron-transfer reactions. A detectionevel, down to 2.0 pM for oligodeoxynucleotide samples coulde obtained. In addition, a novel sensitive ECL method for theetection DNA hybridization based on AuNPs carrying multi-le ECL probes was developed by Zhang and co-workers [129].

detection limit of 5.0 × 10−12 mol L−1 for target DNA waschieved.

Obviously the above DNA ultrasensitive electrochemicaletection by using AuNPs will play an important role on theevelopment of specific and sensitive assays for clinical diagno-is, bioassay, detection of pathogenic microorganisms in foodsnd the environment.

.1.3. Gold nanoparticles for immunosensorsImmunosensors are important analytical tools based on the

etection of the binding event between antibody and antigen.he recent development of immunoassay techniques focused

n most cases on decreasing analysis times, improving assayensitivity, simplification and automation of the assay proce-ures, low-volume analysis. Among types of immunosensors,lectrochemical immunosensors are attractive tools and haveeceived considerable attention because they are easy and eco-omical to mass production, they are robust, and they achievexcellent detection limits with small analyte volumes. Fur-hermore, the availability of a variety of new materials withnique properties at nanoscale dimension, such as AuNPs, hasttracted widespread attention in their utilization for the bioas-ay, especially for electrochemical detection. Recently, severalovel strategies have been proposed to develop electrochemicalmmunosensors with high sensitivity using AuNPs.

For example, a novel and sensitive electrochemicalmmunoassay for immunoglobulin G (IgG) has been devel-ped by Limoges and co-workers [130] using a colloidal goldabel via anodic stripping voltammetry technology. A low detec-ion limit (concentration as low as 3 × 10−12 M) could bebtained, which was competitive with colorimetric enzymeinked immuno-sorbent assay or with immunoassays based onuorescent europium chelate labels. Furthermore, Shen’s group131] reported a novel electrochemical immunoassay based onhe precipitation of silver on colloidal gold labels. After metalilver dissolution in an acidic solution, the signal was indirectlyetermined by anodic stripping voltammetry at a glassy carbonlectrode. A detection limit as low as 1 ng mL−1 human IgG waschieved. The enhancement in sensitivity for an electrochemi-al immunoassay by the autocatalytic deposition of Au3+ ontouNPs has been studied by Huang’s group [132]. By coupling

he autocatalytic deposition with square-wave stripping voltam-etry, the rabbit immunoglobulin G analyte could be determined

uantitatively. A very low detection limit, 0.25 pg mL−1 (1.6 fM)as obtained, which is three orders of magnitude lower than that

ualI

ca Acta 598 (2007) 181–192

btained by a conventional immunoassay using the same AuNPsabels.

Novel enzyme-labeled electrochemical immunosensors wereell developed by several groups [133–135]. For instance,

u’s group [133] reported that a highly hydrophilic and con-uctive colloidal AuNPs/titania sol–gel composite membraneould be employed as electrochemical sensing interface fororseradish peroxidase-labeled electrochemical immunosen-or. Later, a novel electrochemical immunosensor for humanhorionic gonadotrophin (hCG) was developed by the sameroup [134] via the immobilization of hCG on AuNPsoped three-dimensional (3D) sol–gel matrix. The 3D orga-ized composite structure was prepared by assembling AuNPsnto a hydrolyzed (3-mercaptopropyl)-trimethoxysilane sol–gel

atrix, which showed good biocompatibility. After the inter-acial competitive immunoreaction, the formed HRP-labeledmmunoconjugate showed good enzymatic activity for the oxi-ation of o-phenylenediamine by H2O2. The immunosensorhowed good precision, high sensitivity, acceptable stability andeproducibility.

Label-free electrochemical immunosensors using AuNPss enhancing sensing component have been the focus ofntense research due to their simplicity, speedy analysisnd high sensitivity. The technique is mainly based on theetection of a change in physical properties as a result ofntibody–antigen complex formation. The direct determina-ion of immunospecies by detecting the change of impedance136–138] caused by immunoreactions has been demonstrated.

simple and sensitive label-free electrochemical immunoas-ay electrode for detection of carcinoembryonic antigen (CEA)as been developed by Yao’s group [136]. CEA antibodyCEAAb) was covalently attached on glutathione (GSH)onolayer-modified AuNPs and the resulting CEAAb-AuNPs

ioconjugates were immobilized on Au electrode by electro-opolymerization with o-aminophenol (OAP). Electrochemicalmpedance spectroscopy studies demonstrated that the for-

ation of CEA antibody–antigen complexes increased thelectron-transfer resistance of [Fe(CN)6]3−/4− redox pair athe poly-OAP/CEAAb-AuNPs/Au electrode. The immunosen-or could detect the CEA with a detection limit of 0.1 ng mL−1

nd a linear range of 0.5–20 ng mL−1.DNA-free ultrasensitive electrochemical immunosensors

ave received considerable interests because of their advantagencluding simplify, rapidness and high sensitivity. Yang’s group139] developed an ultrasensitive and simple electrochemicalethod for the fabrication of a sandwich-type heterogeneous

lectrochemical immunosensor. Fig. 4 shows a typical fabri-ation procedure of DNA-free electrochemical immunosensor.n IgG layer was formed on an ITO electrode via a stepwise

ssembly process (Fig. 4a). First, partially ferrocenyltetheredendrimer (Fc-D) was immobilized to the ITO electrode byovalent bonding between dendrimer amines and carboxyliccids of a phosphonate self-assembled monolayer. Some of the

nreacted amines of Fc-D were modified with biotin groups tollow the specific binding of streptavidin. Afterward, biotiny-ated antibodies were immobilized to the streptavidin-modifiedTO electrode. An IgG-nanocatalyst conjugate was prepared via

S. Guo, E. Wang / Analytica Chimica Acta 598 (2007) 181–192 189

F er. (b)a c. 128

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ig. 4. (a) Schematic representation of the preparation of an immunosensing layntigen. Reprinted with permission from Ref. [139], H. Yang, J. Am. Chem. So

irect adsorption of IgG on 10 nm AuNPs. Mouse IgG or prostatepecific antigen was chosen as a target protein (Fig. 4b). ThegG-nanocatalyst conjugate and the immunosensing layer sand-iched the target protein. Signal amplification was achieved by

atalytic reduction of p-nitrophenol (NP) to p-aminophenol (AP)sing gold nanocatalyst labels and the chemical reduction of p-uinone imine (QI) by NaBH4. This novel DNA-free methodould attain a very low detection limit (1 fg mL−1).

.2. Gold nanoparticles as enhancing platform forlectrocatalysis and electrochemical sensor

As is known, nanometer-sized AuNPs exhibiting excel-ent catalytic activity have received considerable attention dueo their relative high surface area-to-volume ratio, and theirnterface-dominated properties, which significantly differ fromheir bulk counterparts. Thus, interest in the catalytic proper-ies of AuNPs has increased rapidly. In particular, AuNPs haveeen studied extensively for the design and fabrication of elec-rocatalysts and using as an enhancing component of catalyticctivity or selectivity. The large surface-to-volume ratios andctive sites of AuNPs constitute part of the driving force ineveloping nanosized electrocatalysts. Various methodologiesave been used for the tailoring of AuNPs on electrode surfacesor electrocatalytic applications, which include the anchoring bylectrostatic interaction, covalent linkage, and electrochemicaleposition, etc. Thus, AuNPs modified electrochemical interfaceehaving as nanoelectrode ensembles have been widely used asnhancing catalytic interface for the development of electro-hemical sensors. In principle, the electroanalytical detectionimit at a nanoelectrode ensemble can be much lower than thatt an analogous macrosized electrode because the ratio betweenhe faradaic and capacitive currents is higher [140]. Severalroups [19,20,141–143] have been interested in the development

f novel 2-D or 3-D AuNPs modified nanoelectrode ensemblesor enhancing electrochemical responses.

For example, AuNPs have been employed as electrochemi-al enhancing materials for ECL sensors with high sensitivity.

s

As

Schematic view of electrochemical detection of mouse IgG or prostate specific(2006) 16022, Copyright American Chemical Society (2006).

ur group [144] has devised a simple method for the effec-ive immobilization of Ru(bpy)3

2+ on an electrode surface usinguNPs as enhancing materials. The whole preparation pro-

ess is shown in Fig. 5. First, the electrostatic interactionsetween citrate-capped AuNPs and Ru(bpy)3Cl2 in aqueousedium were used to fabricate the Ru(bpy)3

2+-AuNPs aggre-ates (Ru-AuNPs), and then the Au-S interactions betweens-formed Ru-AuNPs and sulfhydryl groups were used to effec-ively immobilize the Ru-AuNPs on a sulfhydryl-derivated ITOlectrode surface. The as-prepared ITO electrode showed excel-ent stability, and the ECL active species (Ru(bpy)3

2+) containedxhibited excellent ECL behaviors. Later, Dong’s group [145]eveloped an attractive alcohol dehydrogenase biosensor basedn the above strategy. This biosensor displayed wide linearange, good stability and high sensitivity with the detection limitf 3.33 × 10−6 M. The ECL of lucigenin on a AuNPs modi-ed gold electrode in neutral and alkaline solutions was studiedy Cui’s group [146] under conventional cyclic voltammetryonditions. The AuNPs self-assembled gold electrode exhibitedxcellent ECL property for the lucigenin ECL system.

In fact, AuNPs could also be employed as enhancingaterials for electrochemical investigation of cell [147] and

lectrocatalyzing some small biomolecules such as glucose148–151], norepinephrine [152], dopamine [153], catechol154], epinephrine [155] and ascorbic acid [156], etc. Fornstance, Raj and co-worker [150] reported a nonenzymaticlectrochemical method for the detection of glucose by usinguNPs self-assembled on a 3D silicate network obtained bysing sol–gel processes. The nanosized Au particles have beenelf-assembled on the thiol tail groups of the silicate network andnlarged by hydroxylamine. The AuNPs efficiently catalyzed thexidation of glucose at less-positive potential (0.16 V) in phos-hate buffer solution (pH 9.2) in the absence of any enzymesr redox mediators. This novel nonenzymatic glucose sensor

howed excellent sensitivity with a detection limit of 50 nM.

In addition to enhancing detection of small biomolecules,uNPs derivated electrodes were also used to detect some toxic

ubstances [157–159]. AuNPs modified carbon screen-printed,

190 S. Guo, E. Wang / Analytica Chimica Acta 598 (2007) 181–192

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ig. 5. Scheme illustrating (A) the formation of Ru-AuNPs in aqueous mediumB) the immobilization of Ru-AuNPs on a sulfhydryl-derivated ITO electrode s166, Copyright American Chemical Society (2005).

lassy carbon and basal plane pyrolytic graphite electrodes haveeen reported to detect Sb (III) [157] and As (III) [158,159] withigh sensitivity. The electrolytic oxidation of nitric oxide andydrazine was also developed by several groups [160–163]. It isound that the AuNPs modified electrode exhibited high catalyticctivity for NO and hydrazine. For instance, Raj and co-worker163] reported an ultrasensitive electrochemical detection ofydrazine using AuNPs self-assembled on a sol–gel-derived 3Dilicate network, followed by seed-mediated growth of gold.his nanostructured platform was highly sensitive toward thelectrochemical oxidation of hydrazine. A very large decreasen the overpotential (∼800 mV) and significant enhancement inhe peak currents with respect to the bulk Au electrode werebserved without using any redox mediator. The nanostruc-ured platform showed excellent sensitivity with an experimentaletection limit of 200 pM.

Studies on the electrocatalytic oxidation of methanolnd the reduction of oxygen using precious metal cata-ysts have received considerable attention mainly becausef their energy-related applications such as fuel cell tech-ology. AuNPs can be employed as a good electrocatalystue to its high surface-to-volume ratio. Several groups haveonstructed attractive electrocatalytic interfaces via simplyropping, self-assembly and layer-by-layer techniques forioxygen reduction and oxidation of methanol [164–167].t is found that such AuNPs modified electrode exhib-ted good electrocatalytic performance. For example, our

roup [166] demonstrated that 2D and 3D AuNPs/[tetrakis(N-ethylpyridyl)porphyrinato]cobalt (CoTMPyP) nanostructuredaterials were prepared by “bottom-up” self-assembly

echnique. This novel AuNPs/CoTMPyP self-assembled nanos-

icdr

o electrostatic interactions between Ru(bpy)32+ and citrate-capped AuNPs and

. Reprinted with permission from Ref. [144], E. Wang, Anal. Chem. 77 (2005)

ructured material exhibited tunable electrocatalytic activity forioxygen reduction. The electrodeposited AuNPs modified Pt,lassy carbon and highly oriented pyrolytic graphite electrode168–170] could also be used as electrocatalyst for dioxygeneduction and oxidation of methanol. It must be noted that if theuNPs were combined with some supporting materials such

s carbon nanotube, the obtained hybrid nanomaterials showedigher electrocatalytic activity [171]. Thus, the application ofuNPs as a good electrocatalyst will be greatly extended.

. Summary, conclusions, and outlook

In this review, some recent advances have been addressedn the synthesis and electrochemical applications of AuNPs.uNPs have been facilely obtained via designing different lig-

nds. In addition, the introduction of the modish AuNPs intolectrochemistry implants their novel functions into electro-hemical sensing interfaces, resulting in many novel strategiesested and some highly sensitive systems fabricated. This rapidlyxtending interdisciplinary area has attracted great researchfforts from chemists, physicists, biologists and materials scien-ists. However, this area is still on the horizon from the viewpointf applied research, and the widespread practical application ofuNPs-based electroanalytical devices is not possible currently.

n order to fully exploit the potential application of AuNPs inlectrochemistry, more perfect nanoparticles with well-definedeometry, well-defined properties, and long-term stability in var-

ous environments have to be designed and synthesized. Forompleting this, the novel ligands for AuNPs must be quicklyevised. In addition, other attractive gold nanostructured mate-ials in the form of disk, plate, sponge, star, flake, urchin,

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S. Guo, E. Wang / Analytica C

risms, wires and rods have been the focus of intense researchnd been primarily synthesized via many strategies. Combinedhese AuNPs with particular morphologies and electrochemicalechnology, more novel and sensitive electrochemical sensingnterface will be constructed, which will result in the deeperevelopment of nanoelectrochemistry.

cknowledgment

This work was supported by the National Natural Scienceoundation of China (Nos. 20575064, 20427003) and the Chi-ese Academy of Sciences (No. KJCX2.YW.HO9)

eferences

[1] M.H. Rashid, R.R. Bhattacharjee, A. Kotal, T.K. Mandal, Langmuir 22(2006) 7141.

[2] G. Schmid, Large clusters and colloids, Chem. Rev. 92 (1992) 1709.[3] G. Schmid, L.F. Chi, Metal clusters and colloids, Adv. Mater. 10 (1998)

515.[4] G. Schmid, in: K.J. Klabunde (Ed.), Nanoscale Materials in Chemistry,

Wiley, New York, 2001.[5] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R.J. Whyman, J. Chem.

Soc. Chem. Commun. (1994) 801.[6] M. Brust, J. Fink, D. Bethell, D.J. Schiffrin, C.J. Kiely, J. Chem. Soc.

Chem. Commun. (1995) 1655.[7] D.L. Feldheim, A.F. Colby Jr. (Eds.), Metal Nanoparticles-Synthesis,

Characterization and Applications [M], Marcel Dekker, New York, 2002.[8] G. Schmid, M. Baumle, M. Geerkens, I. Heim, C. Osemann, T. Saw-

itowski, Chem. Soc. Rev. 28 (1999) 179.[9] M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293.

[10] G. Frens, Nature (Phys. Sci.) 241 (1973) 20.[11] A. Ullman, Chem. Rev. 96 (1996) 1533.[12] C. Petit, P. Lixon, M. Pileni, J. Phys. Chem. B 97 (1993) 12974.[13] K.S. Suslick, M. Fang, T. Hyeon, J. Am. Chem. Soc. 118 (1996) 11960.[14] D.L. Feldheim, C.D. Keating, Chem. Soc. Rev. 27 (1998) 1.[15] A.N. Shipway, E. Katz, I. Willner, Chem. Phys. Chem. 1 (2000) 18.[16] N.I. Kovtyukhova, T.E. Mallouk, Chem. Eur. J. 8 (2002) 4354.[17] P. Kohli, C.C. Harrell, Z. Cao, R. Gasparac, W. Tan, C.R. Martin, Science

305 (2004) 984.[18] W. Cheng, S. Dong, E. Wang, Chem. Mater. 15 (2003) 2495.[19] W. Cheng, S. Dong, E. Wang, Anal. Chem. 74 (2002) 3599.[20] W. Cheng, S. Dong, E. Wang, Langmuir 18 (2002) 9947.[21] R.W. Murray, in: A.J. Bard (Ed.), Electroanaytical Chemistry, vol. 13,

Marcel Dekker, New York, 1984, p. 191.[22] J.M. Slocik, M.O. Stone, R.R. Naik, Small 1 (2005) 1048.[23] N. Higashi, J. Kawahara, M. Niwa, J. Colloid Interface Sci. 288 (2005)

83.[24] L. Fabris, S. Antonello, L. Armelao, R.L. Donkers, F. Polo, C. Toniolo,

F. Maran, J. Am. Chem. Soc. 128 (2006) 326.[25] D. Aili, K. Enander, J. Rydberg, I. Lundstrom, L. Baltzer, B. Liedberg, J.

Am. Chem. Soc. 128 (2006) 2194.[26] P. He, M.W. Urban, Biomacromolecules 6 (2005) 1224.[27] H.F. Zhu, C. Tao, S.P. Zheng, J.B. Li, Colloids Surf. A: Physicochem.

Eng. Aspects 257 (2005) 411.[28] P. He, X. Zhu, Mater. Res. Bull. 42 (2007) 1310.[29] X. Han, W. Cheng, Z. Zhang, S. Dong, E. Wang, Biochim. Biophys. Acta

1556 (2002) 273.[30] L. Zhang, X. Sun, Y. Song, X. Jiang, S. Dong, E. Wang, Langmuir 22

(2006) 2838.

[31] W. Cheng, S. Dong, E. Wang, Langmuir 19 (2003) 9434.[32] W. Cheng, E. Wang, J. Phys. Chem. B 108 (2004) 24.[33] S.R. Isaacs, E.C. Cutler, J.S. Park, T.R. Lee, Y.S. Shon, Langmuir 21

(2005) 5689.[34] L. Du, H. Jiang, X. Liu, E. Wang, Electrochem. Commun. 9 (2007) 1165.

ca Acta 598 (2007) 181–192 191

[35] B. Nair, T. Pradeep, Cryst Growth Des. 2 (2002) 293.[36] L.N. Loo, R.H. Guenther, V.R. Basnayake, S.A. Lommel, S. Franzen, J.

Am. Chem. Soc. 128 (2006) 4502.[37] J.M. Slocik, R.R. Naik, M.O. Stone, D.W. Wright, J. Mater. Chem. 15

(2005) 749.[38] R.C. Mucic, J.J. Storhoff, C.A. Mirkin, R.L. Letsinger, J. Am. Chem. Soc.

120 (1998) 12674.[39] C. Gautier, T. Burgi, J. Am. Chem. Soc. 128 (2006) 11079.[40] A. Pal, Mater. Lett. 58 (2004) 529.[41] P. Raveendran, J. Fu, S.L. Wallen, J. Am. Chem. Soc. 125 (2003) 13940.[42] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000) 3772.[43] G.S. Fonseca, A.P. Umpierre, P.F.P. Fichtner, S.R. Teixeira, J. Dupont,

Chem. Eur. J. 9 (2003) 3263.[44] C.W. Scheeren, G. Machado, J. Dupont, P.F.P. Fichtner, S.R. Texeira,

Inorg. Chem. 42 (2003) 4738.[45] J. Dupont, G.S. Fonseca, A.P. Umpierre, P.F.P. Fichtner, S.R. Teixeira, J.

Am. Chem. Soc. 124 (2002) 4228.[46] H. Itoh, K. Naka, Y. Chujo, J. Am. Chem. Soc. 126 (2004) 3026.[47] A.I. Bhatt, A. Mechler, L.L. Martin, A.M. Bond, J. Mater. Chem. 17

(2007) 2241.[48] K.S. Kim, D. Demberelnyamba, H. Lee, Langmuir 20 (2004) 556.[49] K. Esumi, N. Takei, T. Yoshimura, Colloids Surf. B: Biointerfaces 32

(2003) 117.[50] H.Z. Huang, X.R. Yang, Biomacromolecules 5 (2004) 2340.[51] B. Wang, K. Chen, S. Jiang, F.O. Reincke, W.J. Tong, D.Y. Wang, C.Y.

Gao, Biomacromolecules 7 (2006) 1203.[52] D.S. dos Santos, J.P.J.G. Goulet, N.P.W. Pieczonka, O.N. Oliveira, J.R.F.

Aroca, Langmuir 20 (2004) 10273.[53] T. Miyama, Y. Yonezawa, Langmuir 20 (2004) 5918.[54] Z.M. Qi, H.S. Zhou, N.K. Matsuda, I. Honma, K. Shimada, A. Takatsu,

K. Kato, J. Phys. Chem. B 108 (2004) 7006.[55] I. Hussain, S. Graham, Z.X. Wang, B. Tan, D.C. Sherrington, S.P. Ran-

nard, A.I. Cooper, M. Brust, J. Am. Chem. Soc. 127 (2005) 16398.[56] C. Note, J. Koetz, L. Wattebled, A. Laschewsky, J. Colloid Interface Sci.

308 (2007) 162.[57] D.C. Wan, Q. Fu, J.L. Huang, J. Appl. Polym. Sci. 101 (2006) 509.[58] N. Perignon, J.D. Marty, A.F. Mingotaud, M. Dumont, I. Rico-Lattes, C.

Mingotaud, Macromolecules 40 (2007) 3034.[59] H.W. Duan, S.M. Nie, J. Am. Chem. Soc. 129 (2007) 2412.[60] T. Teranishi, I. Kiyokawa, M. Miyake, Adv. Mater. 10 (1998) 596.[61] J.V. Herrikhuyzen, R.A.J. Janssen, E.W. Meijer, S.C.J. Meskers, A.P.H.J.

Schenning, J. Am. Chem. Soc. 128 (2006) 686.[62] S.K. Jewrajka, U. Chatterjee, J. Polym. Sci. Pol. Chem. 44 (2006) 1841.[63] Y.J. Kang, T.A. Taton, Angew. Chem. Int. Ed. 44 (2005) 409.[64] E.R. Zubarev, J. Xu, A. Sayyad, J.D. Gibson, J. Am. Chem. Soc. 128

(2006) 4958.[65] J.H. Kim, T.R. Lee, Chem. Mater. 16 (2004) 3647.[66] P.W. Zheng, X.W. Jiang, X. Zhang, W.Q. Zhang, L.Q. Shi, Langmuir 22

(2006) 9393.[67] X. Sun, S. Dong, E. Wang, Polymer 45 (2004) 2181.[68] C. Note, S. Kosmella, J. Koetz, Colloid Surf. A 290 (2006) 150.[69] T. Ishii, H. Otsuka, K. Kataoka, Y. Nagasaki, Langmuir 20 (2004) 561.[70] H. Chen, Y. Wang, Y. Wang, S. Dong, E. Wang, Polymer 47 (2006) 763.[71] R.M. Crooks, M.Q. Zhao, L. Sun, V. Chechik, L.K. Yeung, Acc. Chem.

Res. 34 (2001) 181.[72] M.E. Garcia, L.A. Baker, R.M. Crooks, Anal. Chem. 71 (1999) 256.[73] K. Esumi, K. Torigoe, Prog. Colloid Polym. Sci. 117 (2001) 80.[74] K. Esumi, T. Hosoya, A. Suzuki, K. Torigoe, Langmuir 16 (2000) 2978.[75] Y.G. Kim, S.K. Oh, R.M. Crooks, Chem. Mater. 16 (2004) 167.[76] Y. Haba, C. Kojima, A. Harada, T. Ura, H. Horinaka, K. Kono, Langmuir

23 (2007) 5243.[77] X. Sun, X. Jiang, S. Dong, E. Wang, Macromol. Rapid Commun. 24

(2003) 1024.

[78] C.S. Love, I. Ashworth, C. Brennan, V. Chechik, D.K. Smith, J. Colloid

Interface Sci. 302 (2006) 178.[79] A.D. Aleo, R.M. Williams, F. Osswald, P. Edamana, U. Hahn, J. van

Heyst, F.D. Tichelaar, F. Vogtle, L.D. Cola, Adv. Funct. Mater. 14 (2004)1167.

1 himi

92 S. Guo, E. Wang / Analytica C

[80] R.Y. Wang, J. Yang, Z.P. Zheng, M.D. Carducci, J. Jiao, S. Seraphin,Angew. Chem. Int. Ed. 40 (2001) 549.

[81] S. Nakao, K. Torigoe, K. Kon-No, T. Yonezawa, J. Phys. Chem. B 106(2002) 12097.

[82] M.K. Kim, Y.M. Jeon, W.S. Jeon, H.J. Kim, S.G. Hong, C.G. Park, K.Kim, Chem. Commun. (2001) 667.

[83] J. Wang, Analyst 130 (2005) 421.[84] M. Pumera, S. Sanchez, I. Ichinose, J. Tang, Sens. Actuators B 123 (2007)

1195.[85] M.T. Castaneda, S. Alegret, A. Merkoci, Electroanalysis 19 (2007) 743.[86] E. Katz, I. Willner, Angew. Chem. Int. Ed. 43 (2004) 6042.[87] J. Wang, Anal. Chim. Acta 500 (2003) 247.[88] A.L. Guindilis, P. Atanasov, E. Wilkins, Electroanalysis 9 (1997) 661.[89] K.R. Brown, A.P. Fox, M.J. Natan, J. Am. Chem. Soc. 118 (1996)

1154.[90] J. Jia, B. Wang, A. Wu, G. Cheng, Z. Li, S. Dong, Anal. Chem. 74 (2002)

2217.[91] L. Wang, E. Wang, Electrochem. Commun. 6 (2004) 49.[92] H. Gu, A. Yu, H. Chen, J. Electroanal. Chem. 516 (2001) 119.[93] M.L. Mena, P. Yanez-Sedeno, J.M. Pingarron, Anal. Biochem. 336 (2005)

20.[94] Y. Xiao, H. Ju, H. Chen, Anal. Biochem. 278 (2000) 22.[95] S. Liu, H. Ju, Electroanalysis 15 (2003) 1488.[96] L. Aguı, J. Manso, P. Yanez-Sedeno, J.M. Pingarron, Sens. Actuators B

113 (2006) 272.[97] S. Liu, H. Ju, Anal. Biochem. 307 (2002) 110.[98] S. Liu, H. Ju, Biosens. Bioelectron. 19 (2003) 177.[99] H. Ju, S. Liu, B. Ge, F. Lisdat, F.W. Scheller, Electroanalysis 14 (2002)

141.[100] J. Zhang, M. Oyama, J. Electroanal. Chem. 577 (2005) 273.[101] V.V. Shumyantseva, S. Carrara, V. Bavastrello, D.J. Riley, T.V. Bulko,

K.G. Skryabin, A.I. Archakov, C. Nicolini, Biosens. Bioelectron. 21(2005) 217.

[102] T. Hoshi, N. Sagae, K. Daikuhara, K. Takahara, J.I. Anzai, Mater. Sci.Eng. C 27 (2007) 890.

[103] W. Yang, J. Wang, S. Zhao, Y. Sun, C. Sun, Electrochem. Commun. 8(2006) 665.

[104] T. Hoshi, N. Sagae, K. Daikuhara, K. Takahara, J.I. Anzai, Talanta 71(2007) 644.

[105] S.H. Chen, R. Yuan, Y.Q. Chai, L. Xu, N. Wang, X.N. Li, L.Y. Zhang,Electroanalysis 18 (2006) 471.

[106] X. Luo, J. Xu, Y. Du, H. Chen, Anal. Biochem. 334 (2004) 284.[107] Q. Xu, C. Mao, N.N. Liu, J.J. Zhu, J. Sheng, Biosens. Bioelectron. 22

(2006) 768.[108] J.H. Lin, W. Qu, S.S. Zhang, Anal. Biochem. 360 (2007) 288.[109] M.H. Xue, Q. Xu, M. Zhou, J.J. Zhu, Electrochem. Commun. 8 (2006)

1468.[110] J.J. Gooding, Electroanalysis 14 (2002) 1149.[111] E. Palecek, M. Fojta, Anal. Chem. 73 (2001) 75A.[112] J. Wang, Anal. Chim. Acta 469 (2002) 63.[113] J. Wang, D. Xu, A.N. Kawde, R. Polsky, Anal. Chem. 73 (2001) 5576.[114] L. Authier, C. Grossiord, P. Brossier, B. Limoges, Anal. Chem. 73 (2001)

4450.[115] J. Wang, R. Polsky, D. Xu, Langmuir 17 (2001) 5739.[116] J. Wang, D. Xu, R. Polsky, J. Am. Chem. Soc. 124 (2002) 4208.[117] M. Pumera, M.T. Castaneda, M.I. Pividori, R. Eritja, A. Merkoci, S.

Alegret, Langmuir 21 (2005) 9625.[118] M. Pumera, M. Aldavert, C. Miles, A. Merkoci, S. Alegret, Electrochim.

Acta 50 (2005) 3702.[119] M.T. Castaneda, A. Merkoci, M. Pumera, S. Alegret, Biosens. Bioelec-

tron. 22 (2007) 1961.

[120] H. Cai, Y. Wang, P. He, Y. Fang, Anal. Chim. Acta 469 (2002) 165.[121] H. Cai, C. Shang, H. Ming, Anal. Chim. Acta 523 (2004) 61.[122] T.M.H. Lee, L.L. Li, I.M. Hsing, Langmuir 19 (2003) 4338.[123] T.M.H. Lee, H. Cai, I.M. Hsing, Electroanalysis 16 (2004) 1628.[124] M. Rochelet-Dequaire, B. Limoges, P. Brossier, Analyst 131 (2006) 923.

ca Acta 598 (2007) 181–192

[125] M.J. Wang, C.Y. Sun, L.Y. Wang, X.H. Ji, Y.B. Bai, T.J. Li, J.H. Li, J.Pharm. Biomed. Anal. 33 (2003) 1117.

[126] J. Wang, J. Li, A.J. Baca, J. Hu, F. Zhou, W. Yan, D.-W. Pang, Anal.Chem. 75 (2003) 3941.

[127] A.J. Baca, F. Zhou, J. Wang, J. Hu, J. Li, J. Wang, Z.S. Chikneyan,Electroanalysis 16 (2004) 73.

[128] Y. Jin, W. Lu, J.Q. Hu, X. Yao, J.H. Li, Electrochem. Commun. 9 (2007)1086.

[129] H. Wang, C.X. Zhang, Y. Li, H.L. Qi, Anal. Chim. Acta 575 (2006) 205.[130] M. Dequaire, C. Degrand, B. Limoges, Anal. Chem. 72 (2000) 5521.[131] X. Chu, X. Fu, K. Chen, G.L. Shen, R.Q. Yu, Biosens. Bioelectron. 20

(2005) 1805.[132] K.T. Liao, H.J. Huang, Anal. Chim. Acta 538 (2005) 159.[133] J. Chen, J. Tang, F. Yan, H. Ju, Biomaterials 27 (2006) 2313.[134] J. Chen, F. Yan, F. Tan, H. Ju, Electroanalysis 18 (2006) 1696.[135] C. Milligan, A. Ghindilis, Electroanalysis 14 (2002) 415.[136] H. Tang, J.H. Chen, L.H. Nie, Y.F. Kuang, S.Z. Yao, Biosens. Bioelectron.

22 (2007) 1061.[137] S. Zhang, F. Huang, B.H. Liu, J.J. Ding, X. Xu, J.L. Kong, Talanta 71

(2007) 874.[138] H.Z. Huang, Z.G. Liu, X.R. Yang, Anal. Biochem. 356 (2006) 208.[139] J. Das, M.A. Aziz, H. Yang, J. Am. Chem. Soc. 128 (2006) 16022.[140] R.M. Penner, C.R. Martin, Anal. Chem. 59 (1987) 2625.[141] A. Escorcia, A.A. Dhirani, J. Electroanal. Chem. 601 (2007) 260.[142] R.H. Tian, J.F. Zhi, Electrochem. Commun. 9 (2007) 1120.[143] F. Wang, J. Wang, H. Chen, S. Dong, J. Electroanal. Chem. 600 (2007)

265.[144] X. Sun, Y. Du, S. Dong, E. Wang, Anal. Chem. 77 (2005) 8166.[145] L. Zhang, Z. Xu, X. Sun, S. Dong, Biosens. Bioelectron. 22 (2007) 1097.[146] H. Cui, Y. Dong, J. Electroanal. Chem. 595 (2006) 37.[147] L. Ding, C. Hao, Y. Xue, H. Ju, Biomacromolecules 8 (2007) 1341.[148] M. Tominaga, T. Shimazoe, M. Nagashima, I. Taniguchi, Electrochem.

Commun. 7 (2005) 189.[149] M. Comotti, C.D. Pina, E. Falletta, M. Rossi, Adv. Synth. Catal. 348

(2006) 313.[150] B.K. Jena, C.R. Raj, Chem. Eur. J. 12 (2006) 2702.[151] F. Kurniawan, V. Tsakova, V.M. Mirsky, Electroanalysis 18 (2006) 1937.[152] L.P. Lu, S.Q. Wang, X.Q. Lin, Anal. Chim. Acta 519 (2004) 161.[153] C.R. Raj, T. Okajima, T. Ohsaka, J. Electroanal. Chem. 543 (2003) 127.[154] L. Su, L.Q. Mao, Talanta 70 (2006) 68.[155] L. Wang, J. Bai, P. Huang, H. Wang, L. Zhang, Y. Zhao, Electrochem.

Commun. 8 (2006) 1035.[156] J. Wang, F. Wang, X. Zou, Z. Xu, S. Dong, Electrochem. Commun. 9

(2007) 343.[157] O.D. Renedo, M.J.A. Martınez, Anal. Chim. Acta 589 (2007) 255.[158] Y.S. Song, G. Muthuraman, Y.Z. Chen, C.C. Lin, J.M. Zen, Electroanal-

ysis 18 (2006) 1763.[159] X. Dai, R.G. Compton, Electroanalysis 17 (2005) 1325.[160] E.V. Milsom, J. Novak, M. Oyama, F. Marken, Electrochem. Commun.

9 (2007) 436.[161] A.M. Yu, Z.J. Liang, J.H. Cho, F. Caruso, Nano Lett. 3 (2003) 1203.[162] J.D. Zhang, M. Oyama, Anal. Chim. Acta 540 (2005) 299.[163] B.K. Jena, C.R. Raj, J. Phys. Chem. C 111 (2007) 6228.[164] J. Hernandez, J. Solla-Gullon, E. Herrero, A. Aldaz, J.M. Feliu, Elec-

trochim. Acta 52 (2006) 1662.[165] C.R. Raj, A.I. Abdelrahman, T. Ohsaka, J. Electroanal. Chem. 553 (2003)

107.[166] W. Cheng, S. Dong, E. Wang, J. Phys. Chem. B 108 (2004) 19146.[167] F.N. Crespilho, F.C. Nart, O.N. Oliveira Jr., C.M.A. Brett, Electrochim.

Acta 52 (2007) 4649.[168] M.S. El-Deab, T. Ohsaka, J. Electroanal. Chem. 553 (2003) 107.

[169] M.S. El-Deab, T. Sotomura, T. Ohsaka, Electrochim. Acta 52 (2006) 1792.[170] Y.R. Zhang, S. Asahina, S. Yoshihara, T. Shirakashi, Electrochim. Acta

48 (2003) 741.[171] N. Alexeyeva, T. Laaksonen, K. Kontturi, F. Mirkhalaf, D.J. Schiffrin, K.

Tammeveski, Electrochem. Commun. 8 (2006) 1475.