Size dependent electrocatalytic activity of gold nanoparticles immobilized onto three dimensional sol–gel network

The electrocatalytic activity of different sizes (2.6, 12.6, 20, 40 and 60 nm) of gold nanoparticles (AuNPs) immobilized onto (3-mercaptopropyl)-trimethoxysilane sol–gel modified Au electrodes (Au-MPTS) were examined by taking biologically important compounds such as ascorbic acid (AA), uric acid (UA) and 3,4-dihydroxyphenylacetic acid (DOPAC). The oxidation current of these compounds decreases and the oxidation potentials of them were shifted to more positive potential when the size of the immobilized AuNPs increases. Since AuNPs with smaller size provides higher surface area when compared to AuNPs with larger size, higher oxidation currents were observed for these compounds at smaller size AuNPs modified electrodes. AFM images show that AuNPs with 20 nm size were densely packed onto MPTS sol–gel film and generated continuous array whereas AuNPs with 60 nm size were less densely packed. The diffuse reflectance spectra (DRS) of 2.6 nm and 60 nm AuNPs immobilized onto MPTS sol–gel film show the absorption maximum around 520 nm and 540 nm, respectively indicating that the immobilized AuNPs were not aggregated in the sol–gel network. The AuNPs immobilized Au-MPTS electrodes successfully resolve the oxidation peaks of AA and UA in 0.2 M phosphate buffer solution (pH 7.2) whereas bare Au electrode fails to resolve the oxidation peaks of them. Interestingly the peak separation between AA and UA was identical (180 mV) irrespective of the size of AuNPs though the oxidation potentials of them were shifted to more positive potentials. Further, the detection limit of 200 nM DOPAC could be achieved at 2.6 nm AuNPs immobilized electrode whereas 500 nM was achieved at 60 nm AuNPs.