Reverse Size-Dependent Electrooxidation Of Gold Nanoparticles Coated With Alkanethiol Self-Assembled Monolayers

The oxidation potential of weakly stabilized, citrate-coated Au and other spherical metal nanoparticles (NPs) shifts negative by an amount proportional to 1/radius due to a size-dependent negative shift in the thermodynamic standard potential, which is related to the increase in surface-area-to-volume ratio (SA/V) of the NPs with decreasing size. Strongly bound stabilizers, such as alkanethiolates, provide stability by preventing aggregation in solution but also strongly influence the oxidation potential of the metal NPs. In this paper, cyclic voltammetry (CV) shows that the oxidative stripping of Au by Br- is hindered significantly for 4.1 +/- 0.7, 15.1 +/- 1.3, and 50.3 +/- 1.7 nm diameter Au NPs when coated with butanethiolate (C4S), decanethiolate (C10S), and hexadecanethiolate (C16S) ligands. The resistance to oxidation increases with increasing chain length, consistent with the numerous studies on planar two-dimensional (2D) Au surfaces. When sizes are compared, the 4.1 nm alkanethiolate-coated Au NPs show greater resistance to oxidative stripping compared to the 15.1 and 50.3 nm diameter Au NPs coated with the same thiolates. Chronocoulometry (CC) experiments show that 15.1 and 50.3 nm diameter thiolate-coated Au NPs oxidize to a much greater extent in acidic Br- within 1000 s at 1.0 V vs Ag/AgCl compared to 4.1 nm Au NPs. Almost 4 times more Au dissolves from C4S-coated 50.3 nm Au NPs as compared to C4S-coated 4.1 nm Au NPs, despite the much higher SA/V for the 4.1 nm Au NPs. The stability of the Au NPs against oxidation therefore has a unique reverse size dependence for thiolate-coated Au NPs, where the extent of oxidation increases with increasing size, which is opposite from the trend observed for weakly stabilized or bare Au NPs.