Probing the bilayer-monolayer switching of capping agents on Au nanorods and its interaction with guest molecules $$^{\S }$$ §

Abstract

The anisotropic features of Au nanorods make them an attractive nanoscale precursor for the design of higher order nanostructured materials. However, the mode of interaction of various molecular systems on Au nanorods is not well-understood. In the present study, we have employed isothermal titration calorimetry and surface-enhanced Raman scattering for understanding various types of interactions of functional molecules on the surface of gold nanorods. The binding of thiol-bearing analyte molecules is effective with the surface of gold nanorods in acetonitrile-rich solvents and found to be weak in an aqueous medium. The effective interaction of thiol-bearing analyte molecules on nanorods is facilitated by the breakdown of cetyltrimethylammonium bromide bilayer to a monolayer in organic-rich solvent systems, thereby resulting in appreciable signals in isothermal titration calorimetry and surface-enhanced Raman spectra. The electrostatic interaction of analyte molecule is mainly driven by the charge reversal on the surface of Au nanorods on switching the solvent from aqueous to organic medium. Thus, based on isothermal titration calorimetry and surface-enhanced Raman scattering investigations, it is established that the microheterogeneous environment around the Au nanorods plays a crucial role in driving the interaction of analyte molecules.

Quantitative Surface-Enhanced Spectroscopy

“…At the highest chloride concentrations, no analyte was detected at the interface because the surface was saturated by chloride. The same effect was observed for surfactant-stabilized nanoparticles, in which a surfactant bilayer prevented target molecules from diffusing near the metal interface for SERS detection (81,82). CTAB bilayers, for example, blocked the binding and SERS detection of 4-mercaptopyridine onto gold nanorods suspended in water, as the surfactant bilayer formed a steric barrier that inhibited ligand exchange and minimized SERS activity.…”

“…CTAB bilayers, for example, blocked the binding and SERS detection of 4-mercaptopyridine onto gold nanorods suspended in water, as the surfactant bilayer formed a steric barrier that inhibited ligand exchange and minimized SERS activity. When the CTAB bilayer was disrupted by the addition of acetonitrile, the molecule was readily detected, as it was able to bind to gold (82).…”