Electrochemical Size Discrimination of Gold Nanoparticles Attached to Glass/Indium-Tin-Oxide Electrodes by Oxidation in Bromide-Containing Electrolyte

Here we describe the electrochemical oxidation of an assembly of gold nanoparlicles (Au NPs) attached to glass/indium-tin-oxide (ITO) electrodes as a function of particle size. We synthesized Au NP arrays with NP diameters ranging from 8 to 250 nm by electrodeposition of Au from HAuCl4 in H2SO4 at potentials of -0.2 to 0.8 V versus Ag/AgCl using chronocoulometry to keep the amount of Au deposited constant. The average Au NP size increased with increasing deposition potential. The chemical reduction of HAuCl4 by NaBH4 in trisodium citrate solution led to 4 nm average diameter Au NPs, which we chemisorbed to the glass/ITO electrode. Linear sweep voltammograms (LSVs) obtained on the glass/ITO/Au NP (4 to 250 nm) electrodes (with a constant coverage of Au in terms of Au atoms per cm(2)) from 0.5 to 1.1 V in 0.01 M potassium bromide plus 0.1 M HClO4 showed a positive shift in oxidation potential from 734 +/- 1 mV to 913 +/- 19 mV with increasing Au NP diameter. The shift agrees qualitatively with that predicted by a shift in the redox potential based on a difference in free energy associated with a change in surface energy as a function of particle size. On the basis of the charge during Au deposition versus the charge during oxidation, the oxidation process produces a mixture of (AuBr4-)-Br-III (25%) and (AuBr2-)-Br-I (75%). A glass/ITO electrode coated with a mixture of 4 and 250 nm Au NPs revealed 2 oxidation peaks, consistent with the two Au NP size populations present on the surface.