Pillay J., Ozoemena K.I., Tshikhudo R.T., Moutloali R.M.
Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa; Advanced Materials Division, Mintek, 200 Hans Strijdom Drive, Randburg 2125, South Africa; Energy and Processes Unit, Materials Science and Manufacturing, Council for Scientific and Industrial Research (CSIR), Pretoria 0001, South Africa
Pillay, J., Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa, Advanced Materials Division, Mintek, 200 Hans Strijdom Drive, Randburg 2125, South Africa; Ozoemena, K.I., Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa, Energy and Processes Unit, Materials Science and Manufacturing, Council for Scientific and Industrial Research (CSIR), Pretoria 0001, South Africa; Tshikhudo, R.T., Advanced Materials Division, Mintek, 200 Hans Strijdom Drive, Randburg 2125, South Africa; Moutloali, R.M., Advanced Materials Division, Mintek, 200 Hans Strijdom Drive, Randburg 2125, South Africa
Surface electrochemistry of novel monolayer-protected gold nanoparticles (MPCAuNPs) is described. Protecting ligands, (1-sulfanylundec-11-yl) tetraethylene glycol (PEG-OH) and (1-sulfanylundec-11-yl)poly(ethylene glycol)ic acid (PEG-COOH), of three different percent ratios (PEG-COOH:PEG-OH), 1:99 (MPCAuNP-COOH1%), 50:50 (MPCAuNP-COOH50%), and 99:1 (MPCAuNP-COOH99%), were studied. The electron transfer rate constants (ket/s-1) in organic medium decreased as the concentration of the surface-exposed -COOH group in the protecting monolayer ligand is increased: MPCAuNP-COOH1% (∼5 s-1) > MPCAuNP-COOH50% (∼4 s-1) > MPCAuNP-COOH 99% (∼0.5 s-1). In aqueous medium, the trend is reversed. The surface pKa was estimated as ∼8.2 for the MPCAuNP-COOH1%, while both MPCAuNP-COOH50% and MPCAuNP-COOH99% showed two pKa values of about 5.0 and ∼8.0. These results have been interpreted in terms of the quasi-solidity and quasi-liquidity of the terminal -OH and -COOH head groups, respectively. MPCAuNP-COOH99% excellently suppressed the voltammetric response of the ascorbic acid but enhanced the electrocatalytic detection of epinephrine compared to the other MPCAuNPs studied. This study reveals important factors that should be considered when designing electrode devices that employ monolayer-protected gold nanoparticles and possibly for some other redox-active metal nanoparticles. © 2010 American Chemical Society.
Aqueous medium; Ascorbic acids; Concentration of; COOH group; Electrocatalytic; Electron-transfer rate constants; Gold Nanoparticles; Head groups; Heterogeneous electron transfer; Monolayer protected clusters; Monolayer-protected gold nanoparticles; Organic medium; Redox-active metals; Stabilizing ligands; Surface electrochemistry; Surface pK; Tetraethylene glycols; Voltammetric detection; Voltammetric response; Drug products; Electron transitions; Ethylene; Ethylene glycol; Gold; Gold compounds; Ketones; Ligands; Metals; Monolayers; Nanoparticles; Organic acids; Polyethylene oxides; Rate constants; Polyethylene glycols