Surface modification of colloidal gold by chemisorption of alkanethiols in the presence of a nonionic surfactant

Surface modification of colloidal gold with 11-mercaptoundecanoic acid or 16-mercaptohexadecanoic acid was performed in the absence or in the presence of the nonionic surfactant polyoxyethylene (20) sorbitan monolaurate (Tween 20). The stability of the colloidal systems was assessed with optical absorption spectroscopy. The surface-modified nanoparticles were stable only within a narrow range of intermediate pH values when chemisorption of alkanethiols was performed in the absence of Tween 20. This was explained in terms of partial ionization of the surface carboxylic groups and charge neutralization at high pH values by counterions present in the buffer solutions. Formation of a physisorbed monolayer of Tween 20 onto the nanoparticles prior to chemisorption of alkanethiols resulted in surface-modified colloidal gold that was stable over a broader range of pH values. Parallel experiments demonstrated that self-assembled monolayers could form on flat substrates in the presence of Tween 20. Therefore, possible incorporation of alkanethiols within micelles or the presence of a physisorbed layer of Tween 20 on gold surfaces did not prevent their chemisorption. The chemisorption of alkanethiols on gold surfaces with a physisorbed layer of Tween 20 is slow and may be due to a decreased sticking coefficient of the alkanethiols on gold with a physisorbed layer of surfactant. Nanoparticles whose surface was modified in the presence of Tween 20 do not appear to undergo irreversible aggregation. They can be frozen or dried and resuspended again with mild sonication.