Two-Step Functionalization of Neutral and Positively Charged Thiols onto Citrate-Stabilized Au Nanoparticles

Abstract

We propose a two-step approach to functionalize gold nanoparticles prepared by reducing tetrachloroauric acid by trisodium citrate in water. The chloride and citrate physisorbed on the gold nanoparticles are first displaced by thioctic acid (TA), which is then exchanged by thiols containing the desired functionality during the second step. TA bears a carboxylate group and disulfide; at high pH, the negative charge of the former stabilizes gold nanoparticles and the disulfide develops two S−Au bonds, retarding the desorption kinetics upon further functionalization. The slow kinetics of TA desorption is crucial to establishing sufficient steric stabilization for the gold core while losing electrostatic stabilization. Successful attachment of carboxylate, crown ether, cyclodextrin, pyridine, and amino functionalities is verified by surface sensitive techniques. In most cases, ca. 80% of the TA molecules are displaced. The nanoparticles are stable under solution pH where the surface group is ionized. Although at certain solution acidities, the gold sols have difficulty carrying charges and appear flocculated, the nanoparticles become dispersive after being centrifuged and redissolved in suitable buffer solutions, which indicates the successful protection of the gold cores by steric stabilization.

Surface modification, functionalization and bioconjugation of colloidal inorganic nanoparticles

“…in the presence of salts. The citrate layer can be replaced by ligands binding stronger to the particle surface; popular examples include sulphonated phosphines or mercaptocarboxylic acids, and common examples being mercaptoacetic acid (MAA) mercatoacetic acid to mercaptoacetic acid, mercaptopropionic acid (MPA) or mercaptoundecanoic acid (MUA) (Lin et al 2004a). Modifying the nanoparticles with phosphines already allows for achieving highly concentrated particle solutions; the particles can be precipitated by salt-induced aggregation and redissolved again as single particles in low-salt buffers.…”

“…Naturally, ligand molecules that are strongly bound to the nanoparticle surface or more tightly to each other will be less subject to get off the particles' surface, as, for instance, shown for different peptide sequences (Levy et al 2004;Fabris et al 2006). In analogy to the concept of critical micelle concentration, such ligand molecules bind dynamically to the particle surface and are thus subject to mass action and may be washed off by continued purification of the nanoparticles (Lin et al 2004a). …”

Incorporation of the fluoride induced SiO bond cleavage and functionalized gold nanoparticle aggregation into one colorimetric probe for highly specific and sensitive detection of fluoride

“…Then, into 200 mL of the obtained citrate-AuNPs (12 nM, pH 11.0) was added 1.0 mL of LA ethanol solution (0.1 M) and stirred overnight (12 h) to prepare the LA capped AuNPs (LA-AuNPs) [47]. To purify the LA-AuNPs, the solution was centrifuged for 20 min at 15700 g and decantated by supernatants, and the resulting AuNPs were resuspended in water.…”

“…Since direct modification of the citrate-AuNPs with MPD induced irreversible aggregation, a two-step ligand exchange approach was adopted to prepare the MPD-AuNPs [47], i.e., the chloride and citrate physically adsorbed on AuNPs were at first displaced by LA, which was further partly replaced by MPD. The residual of negatively charged LA on the prepared MPD-AuNPs provided strong electrostatic repulsion to keep the MPD-AuNPs stability.…”

Controlled Assembly of Gold Nanostructures on a Solid Substrate via Imidazole Directed Hydrogen Bonding for High Performance Surface Enhance Raman Scattering Sensing of Hypochlorous Acid

“…36 Then, an ethanolic solution of LA (1.0 mL, 0.1 mol L −1 ) was added into the prepared citrate-AuNPs (200 mL, 10 nmol L −1 , pH 9.0) and stirred overnight (12 h) to prepare the LA-capped AuNPs (LA-AuNPs). 37 To purify the LA-AuNPs, we centrifuged the solution for 20 min at 15700 g and the supernatant was decanted. The NPs were then resuspended in water.…”