Chemical Interface Damping as an Indicator for Hexadecyltrimethylammonium Bromide Replacement by Short-Chain Thiols on Gold Nanorods

Abstract

The binding of thiols on hexadecyltrimethylammonium bromide (CTAB)-capped gold nanorods is investigated both at the ensemble and individual particle level using two similar sized but oppositely charged small molecules, namely, cysteamine and mercaptopropionic acid (MPA). Changes in the width and position of the localized surface plasmon resonance of individual gold nanorods are used to elucidate differences between the accumulation of these two short-chain thiols at the particles' surface. It is shown that the interplay between the charged nature of the thiol molecules and the concentration of CTAB in the bulk phase determines to which extent the thiols bind to the particles. On the basis of the changes in the resonance widths, the binding of the negatively charged MPA is reduced compared to that of the positively charged cysteamine, especially at higher CTAB bulk concentrations. This is interpreted as the result of the interaction between the small molecule thiols and CTAB: while cysteamine is effectively replacing CTAB, the interaction of readily bound MPA and CTAB from the bulk results in a self-limiting process.