title: Strong resistance of citrate anions on metal nanoparticles to desorption under thiol functionalization.
abstract: Thiols are widely utilized to functionalize metal nanoparticles, including ubiquitous citrate-stabilized gold nanoparticles (AuNPs), for fundamental studies and biomedical applications. For more than two decades, citrate-to-thiol ligand exchange has been used to introduce functionality to AuNPs in the 5-100 nm size regime. Contrary to conventional assumptions about the completion of ligand exchange processes and formation of a uniform self-assembled monolayer (SAM) on the NP surface, coadsorption of thiols with preadsorbed citrates as a mixed layer on AuNPs is demonstrated. Hydrogen bonding between carboxyl moieties primarily is attributed to the strong adsorption of citrate, leading to the formation of a stabilized network that is challenging to displace. In these studies, adsorbed citrates, probed by Fourier transform infrared and X-ray photoelectron spectroscopy (XPS) analyses, remain on the surface following thiol addition to the AuNPs, whereas acetoacetate anions are desorbed. XPS quantitative analysis indicates that the surface density of alkyl and aryl thiolates for AuNPs with an average diameter of ∼40 nm is 50-65% of the value of a close-packed SAM on Au(111). We present a detailed citrate/thiolate coadsorption model that describes this final mixed surface composition. Intermolecular interactions between weakly coordinated oxyanions, such as polyprotic carboxylic acids, can lead to enhanced stability of the metal-ligand interactions, and this needs to be considered in the surface modification of metal nanoparticles by thiols or other anchor groups.
authors.affiliation: Department of Chemistry, University of Utah , 1400 East 315 South, RM 2020, Salt Lake City, Utah 84112, United States .
authors.forename: Jong-Won
authors.initials: JW
authors.lastname: Park
authors.forename: Jennifer S
authors.initials: JS
authors.lastname: Shumaker-Parry
mesh_terms: D000327:Adsorption; D019343:Citric Acid / Q000737:chemistry*; D006046:Gold / Q000737:chemistry; D006860:Hydrogen Bonding; D008024:Ligands; D053768:Metal Nanoparticles / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010316:Particle Size; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D008024:Ligands; D013438:Sulfhydryl Compounds; D019343:Citric Acid; D006046:Gold
keywords: XPS quantitative analysis; citrate displacement; gold nanoparticles; ligand exchange; self-assembled monolayer; surface coverage