Published:April 02,2011/acElectrochemical Sensing Using Quantum-Sized Gold NanoparticlesS.Senthil Kumar,Kyuju Kwak,and Dongil Lee*Department of Chemistry,Yonsei University,Seoul 120-749,KoreabSupporting Information Recent advances in the synthesis of ultrasmall gold nanoparticles protected with organothiolate (SR)have opened the possibility to synthesize stable,atomically monodisperse gold nanoparticles.1À4Au 25(SR)18,Au 38(SR)24,and Au 144(SR)60are the examples of the quantum-sized gold nanoparticles that exhibit discrete electronic states and quantum con finement e ffects.5,6These nanoparticles have received considerable attention recently because of their unique size-dependent electrochemical,optical,and catalytic properties.1À9Much progress has been made toward understanding their structures and fundamental physical and chemical properties.For example,electrochemical and optical study of the Au 25nanoparticles has revealed that Au 25has the highest occupied molecular orbital (HOMO)Àlowest unoccupied molecular orbital (LUMO)gap of ca.1.33eV,representing the molecule-like property.5However,the technological application of such nanoparticles is still scarce.7À9It will be of great interest to utilize these functional materials in technolog-ical areas such as nanoelectronics,optoelectronics,and sensors since these nanoparticles could exhibit unique properties that di ffer sub-stantially from the corresponding atoms and bulk materials.Herein,we report the first utilization of the quantum-sized Au 25nanoparticles in electrocatalysis and electrochemical sensing.The sol Àgel technique has been used to immobilize gold nanoparticles to form a modi fied electrode.10À12Gold nanoparticles employed for electrochemical sensing thus far were,however,redox inactive nanoparticles with core diameters usually larger than 3nm and,thus,they were entrapped into the sol Àgel network along with redox mediators or redox enzymes.10À12The sol Àgel matrix provides stability to the redox mediator or the enzyme that interacts selectively with the target analyte,and the gold nanoparticles act as tiny con-ductors.In the present study,the unique electrochemical properties of Au 25nanoparticles o ffer particular virtues for the development of the modi fied electrode in which Au 25can serve as an electronic conductor as well as a redox mediator.Highly monodisperse,hexanethiolate-pro-tected Au 25nanoparticles (Au 25)were synthesized and characterized as [Au 25(SC 6H 13)18]À(see Supporting Information for experimental details).Au 25nanoparticles were entrapped into the sol Àgel networkby the hydrolysis of ethyltrimethoxy silane according to a literature procedure 13with slight modi fication.In a typical procedure,Au 25solution (10mg in 0.2mL of CH 2Cl 2)was mixed with 0.1mL of water containing 25%(v/v)glutaraldehyde and 0.2mL of ethyltri-methoxy silane,and the mixture was sonicated for 30min.The resulting homogeneous solution was subsequently stored at room temperature for 2h.10μL of this mixture was then dropcast on the surface of a glassy carbon electrode (GCE,3mm diameter)and allowed to dry overnight at room temperature to form the modi fied sol Àgel electrode (Au 25SGE).The Au 25SGE was then washed thoroughly with water and used as a working electrode.Scheme 1depicts the cartoon of Au 25SGE 14with the Au 25entrapped in the sol Àgel network.The square wave voltammogram (SWV)of Au 25in CH 2Cl 2shown in Figure 1A displays the redox characteristics of Au 25;three sets of well-de fined redox peaks with formal potentials at 0.62,0.31,and À1.33V vs Ag wire quasi-reference electrode (AgQRE)can be assigned to Au 251þ/0,Au 250/1Àand Au 251À/2Àredox couples,respectively.1Cyclic voltammogram (CV)of the Au 25SGE in 0.1M KCl (Figure 1B)also shows well-de fined and reversible redox peaks with formal potential at 0.34V vs Ag/AgCl corresponding to Au 250/1Àcouple.The redox peaks of Au 251þ/0couple are not well-resolved,and they appear as a small shoulder around 0.43V.The reason for this behavior is unclear at this time.It could re flect the fact that small peak spacings between Au 251þ/0and Au 250/1Àcouples are expected when the dielectric constant of the medium is higher.5It could also be due to the fact that limited charge-compensating counterions are available in the sol Àgel network for Au 251þ/0upon the first oxidation (Au 250/1À)reaction,as has been observed in the voltammogram of a Langmuir monolayer of similar particles.15The first oxidation (Au 250/1À)appears,however,to be very stable and reproducible;the peak potentials and peak currents of the Au 25SGEReceived:February 14,2011Accepted:April 2,2011ABSTRACT:This paper describes the electrocatalytic activity of quantum-sized thiolate protected Au 25nanoparticles and their use in electrochemical sensing.The Au 25film modi fied electrode exhibited excellent mediated electrocatalytic activity that was utilized for amperometric sensing of biologically relevant ana-lytes,namely,ascorbic acid and uric acid.The electron transfer dynamics in the Au 25film was examined as a function of Au 25concentration,which manifested the dual role of Au 25as an electronic conductor as well as a redox mediator.The electrontransfer study has further revealed the correlation between the electronic conductivity of the Au 25film and the sensingsensitivity.were found to remain unaltered for 25continuous cycles (Figure 1B),re flecting the stable immobilization of the Au 25in the sol Àgel network.The electrocatalytic activity of the Au 25SGE toward the oxidation of biologically relevant analytes,namely,ascorbic acid (AA)and uric acid (UA),has been examined.Figure 2shows the CVs of Au 25SGE and bare GCE (inset to Figure 2)recorded in the absence and presence of the analytes in 0.1M KCl.As shown in the figures,there is a dramatic enhancement in the anodic peak current at the Au 25SGE (curves b Àf)upon the addition of analytes in 1μM increments,whereas only a slight increase in the anodic current was observed at the bare GCE even after the addition of 5μM of the analytes (curve h).Both AA and UA are known to undergo irreversible oxidation,16and thus,the en-hancement was observed only in the anodic current.Moreover,AA and UA were found to undergo oxidation,respectively,at 380and 405mV,signi ficantly lower potentials at the Au 25SGE than those at the bare GCE,as compared in Table 1.The oxidation potentials of AA and UA at the Au 25SGE are also considerably lower than those at the bare gold electrode,which were found to be 460and 574mV for AA and UA,respectively.The decrease in the potential for oxidation of these analytes to a potential closer to the oxidation potential of Au 25,accompanied with an en-hancement in anodic current,clearly demonstrates the mediated electrocatalytic activity 17of the immobilized Au 25according to the following reactions:Au 25Àf Au 250ð1ÞAu 250þanalyte ðreduced Þf Au 25Àþanalyte ðoxidized Þð2ÞThe Au 25nanoparticles in the sol Àgel network are first oxidized at the electrode (eq 1).In the presence of analyte,the oxidized Au 250electrocatalytically oxidizes the analyte while it is reduced to Au 25À(eq 2).The unique electronic structure of Au 25nanoparticles has been computed,18,19which reveals uneven charge distribution between the Au 13core and the Au 12shell.The electron-de ficient Au 12shell and low-coordinate surface gold atoms 9appear to be responsible for the observed electrocatalytic activity of the Au 25nanoparticles.The regeneration of Au 25Àwould result in an increase in the anodic current with increasing analyte concentra-tion.In addition,the oxidation of AA and UA at the Au 25SGE (Figure 2A,B)shows only one peak nearer to the oxidation peak of the mediator (Au 25),indicating that all the available AA and UA undergo mediated electrocatalytic oxidation at the Au 25SGE.As can be seen in the calibration graphs in Figure 2,the increases in the anodic peak currents were found to vary linearly with the concentration of the analyte added for both analytes.The amperometric sensing performance of Au 25SGE summar-ized in Table 1shows that the Au 25SGE can be employed for the amperometric determination of these analytes over a good linear range with low detection limit and high sensitivity.The obtained linear range,detection limit,and sensitivity are comparable or better than the recently reported electrochemical sensors for the determination of these analytes.16,20À22To the best of our knowledge,this is the first result demonstrating amperometric sensing based on a redox-active gold nanoparticle.As noted above,Au 25could play the dual role as an electronic conductor as well as a redox mediator.In order to gain further insights into the role of Au 25as a conductor,we examined the elec-tron transport in the Au 25SGE as a function of the Au 25concentra-tion (C ).24Stable voltammetric responses were observed for all the concentrations in the range from 3.94to 15.97mM.A representativeScheme 1.Mechanism Depicting the Mediated Electrocata-lytic Oxidation and Ensuing Electron Transport Across the EntrappedAu 25inAu 25SGEFigure 1.(A)SWV of Au 25in CH 2Cl 2containing 0.1M Bu 4NPF 6at Pt working electrode.(B)CV of the Au 25SGE for 25continuous cycles in 0.1M KCl at 20mVs À1.Figure 2.Voltammograms demonstrating the electrocatalytic oxidation of (A)ascorbic acid and (B)uric acid in 0.1M KCl at 20mVs À1;(a)CV of Au 25SGE in the absence of analyte,(b Àf)in the presence of 1,2,3,4,and 5μM of the analyte,(g)CV of bare GCE in the absence of analyte,and (h)in the presence of 5μM of analyte,and calibration graphs for the determination of (C)ascorbic acid and (D)uric acid from the voltammograms of Au 25SGE.voltammetric response of the Au25SGE(C=15.97mM)at varying scan rate is shown in Figure S4(Supporting Information).Both anodic and cathodic peak currents for all the modified electrodes made with different Au25concentrations were found to vary linearly with the square root of scan rate from2to100mVsÀ1(Figure S5,Support-ing Information),indicating the electron transport in thefilm is a diffusion-controlled process.The apparent diffusion coefficients (D APP)were calculated from the peak currents.25The diffusion pro-cess may involve one or both of physical diffusion(D PHYS)and electron hopping diffusion(D E)between Au25cores(i.e.,electron self-exchange)in thefilm.The possible electrocatalytic reaction between the oxidized Au250and analyte and ensuing electron transport process across the Au25film is shown in Scheme1.To calculate the electron hopping rates from D E,we make the assumption that D E.D PHYS.This is entirely reasonable considering the Au25SGE structure (Scheme1)in which Au25nanoparticles are entrapped in the polymer network.Thus,the self-exchange rate constant(k EX)of the Au250/Àcouple in thefilm can be calculated from D E using26,27D APP¼D PHYSþD E%D E¼k EXδ2C=6ð3Þwhereδis the centerÀcenter Au25core separation.The calculated DE and k EX are plotted versus the Au25concentration in Figure3,and values are listed in Table S1(Supporting Information).The structurally well-defined Au25SGE opens the way to unravel the effect of the electronic conductivity of thefilm on electrochemical sensing.As can be seen in Figure3,both D E and k EX increase dramatically at lower Au25concentrations and gradually reach the maximum at concentrations higher than9.23mM.The consequence of this behavior is well reflected in the sensitivity results for ascorbic acid(S AA)and uric acid(S UA).Both S AA and S UA show more than a 25-fold increase as the concentration increases from3.94mM to 9.23mM by2.3-fold,whereas S AA and S UA increase almost linearly with concentration at concentrations higher than9.23mM.These results clearly indicate that the sensitivity is dominantly controlled by the electron transfer dynamics in thefilm at lower Au25concentra-tions.In addition,the fact that the sensitivity increases linearly with concentration at higher concentrations indicates that all Au25 nanoparticles in thefilm are electroactive and electronically well-connected with each other.Finally,the dependence of k EX on Au25 concentration provides new insights into the electron hopping mechanism in thefilm.When the Au25concentration is higher than 9.23mM,the k EX reaches the maximum rate which appears to be limited by the tunneling rate through the hexanethiolate ligands between the Au25cores.The maximum k EX of2.27Â108MÀ1sÀ1 (Table S1,Supporting Information)compares very well with those obtained from networkfilms of gold nanoparticles.28At lower concentrations(C<9.23mM),the electron hopping rate appears to be limited by the mass transport rate of Au25to form a precursor complex for electron transfer.26This interpretation is well supported by the structural data of Au25SGEs in Table S1(Supporting Infor-mation)in which the average Au25centerÀcenter distances29seem to be too long for the electron transfer to occur at that equilibrium distance.In summary,we have shown that redox-active Au25nanopar-ticles can be utilized to develop amperometric sensors based on their excellent electrocatalytic activity.The electron transfer dynamics study of the Au25modified electrode manifests that Au25nanopar-ticles play the dual role as an electronic conductor as well as a redox mediator.The electron transfer study further provides thefirst quantitative results,revealing the correlation between the electron transfer dynamics in the nanoparticlefilm and the sensing sensitivity. These studies are important to the frontier of fundamental science and also highly relevant to the potential technological applications of the quantum-sized gold nanoparticles.Another potential advantage of these sensors is that it may be possible to engineer the ligand shell of the nanoparticles to improve the selectivity.This will be pursued in the future work.’ASSOCIATED CONTENTb Supporting Information.Experimental details including synthesis and characterization of Au25and electron transport data.This material is available free of charge via the Internet at .’AUTHOR INFORMATIONCorresponding Author*Phone:(þ82)2-2123-5638.Fax:(þ82)2-364-7050.E-mail: dongil@yonsei.ac.kr.Homepage:http://chem.yonsei.ac.kr/∼nanomat/.’ACKNOWLEDGMENTThis research was supported by World Class University(R32-2008-000-10217-0),Priority Research Centers(2009-0093823), and Basic Science Research(2010-0009244)Programs throughTable1.Electrocatalytic Activity and Amperometric Sensing Performance of Au25SGEoxidation potentials(mV)aanalyte BareGCE(E1)Au25SGE(E2)E1-E2linear range(μM)LOD(μM)b S(μA/μM)c AA450380700.13À11.60.068 1.556 UA5104051050.13À7.50.071 1.489a Oxidation potentials are compared at the analyte concentration of5μM.b Limit of detection.c Sensitivity of determinationof the analytes.23Figure3.Dependence of electron diffusion coefficient(D E),self-exchange rate constant(k EX),and sensitivity of determination for ascorbic acid(S AA)and uric acid(S UA)on the concentration of Au25.the National Research Foundation of Korea(NRF)funded by the Ministry of Education,Science and Technology and Yonsei University Research Fund.’REFERENCES(1)Murray,R.W.Chem.Rev.2008,108,2688–2720.(2)Jin,R.Nanoscale2010,2,343–362.(3)Parker,J.F.;Fields-Zinna,C.A.;Murray,R.W.Acc.Chem.Res. 2010,43,1289–1296.(4)Chaki,N.K.;Negishi,Y.;Tsunoyama,H.;Shichibu,Y.;Tsukuda, T.J.Am.Chem.Soc.2008,130,8608–8610.(5)Lee,D.;Donkers,R.L.;Wang,G.;Harper,A.S.;Murray,R.W. 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