Rapid Photoligation of Gold Nanocolloids with Lipoic Acid-Based Ligands

Abstract

An effective and easy to implement ligand exchange strategy is paramount to the design of stable and multifunctional gold and other inorganic nanocolloids. This is also crucial for their use in biology and medicine. In this contribution, we demonstrate that photomediated ligand substitution of gold nanocrystals with a series of lipoic acid-modified ligands yields several advantages, including rapid phase transfer and great long-term colloidal stability. This strategy combines photochemical reduction of the dithiolane group with energetically favorable in situ ligand chemisorption, yielding rapid modification of the surfaces. It requires substantially smaller amounts of excess ligands compared to conventional incubation starting with the oxidized form of the ligands. Complete substitution of the ligands is confirmed by using 1H NMR and FT-IR spectroscopy. The colloidal properties of the resulting materials have been tested by using a combination of long-term stability in ion-rich media, sodium cyanide digestion, and dithiothreitol competition tests. They show that photoligation preserves the structure and photophysical properties of the various colloids. Mechanistic arguments have been discussed to explain the effectiveness of this ligation strategy. These findings prove the practical benefits of this approach for designing biocompatible gold colloids and bode well for using such materials in a variety of biological assays and photothermal therapy.