Stepwise Monitoring of Ligand Exchange on Gold Nanorods: From Cetyltrimethylammonium Bromide to Thiol-Functionalized Biocompatible Phosphorylcholine Using Matrix-Free LDI-TOF Mass Spectrometry

Gold nanorods (AuNRs) possess anisotropic optical and electronic properties, primarily determined by their aspect ratio and surface ligands, which make them attractive for applications in sensing, catalysis, and nanomedicine. While these nanorods are typically stabilized using cetyltrimethylammonium bromide (CTAB) to ensure colloidal dispersion, the cytotoxicity and strong surface affinity of CTAB hinder further surface modification through ligand exchange. In this study, we employed matrix-free laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) to directly monitor the ligand exchange process on AuNRs. This technique enables the detection of intact CTAB, transient intermediates, and final thiol-bound ligands without requiring chemical derivatization. By correlating mass spectral data with ultraviolet-visible-near-infrared absorption and zeta potential measurements, we elucidate a stepwise ligand exchange mechanism in which CTAB is gradually displaced by a thiol-functionalized phosphorylcholine ligand, facilitated by electrostatic interaction with poly(styrene sulfonate). These findings highlight the utility of matrix-free LDI-TOF-MS as a powerful analytical tool for gaining mechanistic insights into ligand exchange reactions at the nanoscale, particularly in aqueous environments.