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 nanoparticles (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 cm²) 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 Au^IIIBr₄⁻ (25%) and Au^IBr₂⁻ (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.