Electrocatalytic oxidation of glucose at gold–silver alloy, silver and gold nanoparticles in an alkaline solution

Gold, silver and gold–silver alloy nanoparticles capped with decanethiolate monolayer shells (DT-Au, DT-Ag and DT-Au/Ag) were synthesized, with core sizes 2.3 (±1.0), 3.3 (±1.0) and 2.0 (±1.0) nm, respectively. To activate the synthesized nanoparticles for the electrocatalytic oxidation of glucose, nanoparticles were treated at 300 °C for 2 h. Heat-treated nanoparticles surfaces were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and cyclic voltammetry (CV). The elimination of C–H alkyl chains and thiolates from DT capped Au and Au/Ag nanoparticles was evident post heat-treatment by TGA, FT-IR and XPS investigations. In DT-Ag nanoparticles, C–H chains from DT were eliminated by heat-treatment, though thiolate was still present on nanoparticle surfaces. However, the thiolate from DT was eventually removed by further oxidation and reduction cycle treatments in an alkaline solution. After heat-treatment at 300 °C for 2 h, the surface content ratio of Au and Ag changed from Au:Ag(84:16) to Au:Ag(73:27). This tendency to increase the surface content ratio of Ag after heat-treatment was also observed in other Au–Ag alloy nanoparticle content ratios. Results from cyclic voltammograms at Au/Ag nanoparticles modified PFC electrodes in H2SO4 and NaOH solutions indicated that the distribution of Au and Ag atoms of Au/Ag nanoparticles on nanocrystal surfaces is homogeneous. Electrocatalytic peaks for glucose oxidation in a 0.1 mol dm−3 NaOH solution were observed around −0.4 and 0.4 V (vs. Ag/AgCl) at heat-treated Au nanoparticle modified carbon electrode and around −0.4 and 0.6 V at heat-treated Au/Ag nanoparticle modified electrode, which correspond to the oxidation of glucose and further oxidation of gluconolactone generated by the first oxidation peak (−0.4 V), respectively. It is interesting to note that the catalytic current at Au/Ag nanoparticle modified electrodes was observed from ca. −0.75 V, which represents a negative potential shift of ca. 0.1 V compared to that at Au nanoparticle modified electrodes. This result indicates that Au–Ag alloy nanoparticles are effective catalysts for the electrocatalytic oxidation of glucose. At both Au and Au/Ag nanoparticles, aldose-type monosaccharides showed catalytic oxidation peaks in an alkaline solution, however ketose-type monosaccharides did not show any catalytic peaks in the potential region of −0.8 ∼ 0.8 V. After the controlled-potential electrolysis at a potential of −0.3 V, gluconolactone (or gluconate, a two-electron oxidation product) was only detected at a current efficiency of 100% at Au and Au/Ag nanoparticles modified carbon electrodes. In the case at 0.3 V, oxalate (an 18-electron oxidation product) and gluconolactone as the main product were detected at Au nanoparticle modified electrodes, and formate (a 12-electron oxidation product) in addition to oxalate and gluconolactone as the main products were detected at both Au/Ag and Ag nanoparticles modified electrodes. These results indicate that the catalytic selectivity at a potential of 0.3 V would be strongly governed by silver atoms containing Au/Ag nanoparticles surfaces.