Anion binding to surfactant aggregates: AuCl4- in cationic, anionic and zwitterionic micelles

Quenching of the fluorescence of pyrene by AuCl4- ion was investigated in aqueous micellar solutions of hexadecyltrimethylammonium chloride (CTAC) and 3-(N,N-dimethylmyristylammonio)propanesulfonate (SB3-14) in the presence of 0.010 mol L--(1) HCl. Because AuCl4- is excluded from anionic sodium dodecyl sulfate (SDS) micelles due to electrostatic repulsion effects, no quenching of pyrene fluorescence by AuCl4- was observed in micellar SDS. Absorption spectral shifts show that AuCl4- binds strongly to both CTAC and SB3-14 micelles and time-resolved fluorescence results indicate that the quenching of the fluorescence of micelle-solubilized pyrene by AuCl4- is dynamic in nature. Fits of the pyrene fluorescence decays with the Tachiya model for micelle quenching in the limit of slow exchange of quenchers between the aqueous phase and micelles provided estimates of the micelle aggregation numbers that are slightly smaller than those determined with other quenchers, and intrarnicellar quenching rates 3-4 times higher than those observed with known collisional quenchers. The strong interaction of AuCl4- with SB3-14 and CTAC led to the stabilization of gold nanoparticles, highlighting the use of zwitterionic surfactants for the synthesis of stable and biocompatible gold nanoparticles. (C) 2020 Elsevier B.V. All rights reserved.