电化学 纳米金修饰电极检测VC和尿酸

Published:April 02,2011

/ac

Electrochemical Sensing Using Quantum-Sized Gold Nanoparticles

S.Senthil Kumar,Kyuju Kwak,and Dongil Lee*

Department of Chemistry,Yonsei University,Seoul 120-749,Korea

b

Supporting Information R

ecent 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 network

by 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 25SGE

Received: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 electron

transfer study has further revealed the correlation between the electronic conductivity of the Au 25film and the sensing

sensitivity.