title: IR spectroscopic studies of adsorption of dithiol-containing ligands on CdS nanocrystal films in aqueous solutions.
abstract: The adsorption of the ligands alpha-lipoic acid, dihydrolipoic acid, and dithiothreitol to films of deposited CdS nanoparticles was studied in situ by ATR-IR spectroscopy. For alpha-lipoic acid and dihydrolipoic acid, the spectra of the adsorbed species closely resemble those of the respective solution species. However, for dithiothreitol, the spectrum of the adsorbed species is significantly different from that of the solution species and is attributed to an interruption of intermolecular hydrogen bonding upon adsorption to the CdS. The S-H stretching absorption of the dihydrolipoic acid solution species at pH=8.6 is observed at 2542 cm(-1). The corresponding absorptions for dithiothreitol occur at 2578 and 2528 cm(-1) and are attributed to monomers and dimers. Adsorption of dihydrolipoic acid and dithiothreitol is found to occur via both thiol functional groups and an additional interaction between the carboxylate and the CdS surface. The adsorption of alpha-lipoic acid to CdS in the presence of light proceeds with photo-oxidation of the CdS surface and reductive cleavage of the disulfide bond of alpha-lipoic acid to produce some adsorbed dihydrolipoic acid and thiosulfate. The adsorption of alpha-lipoic acid to CdS in the absence of visible light shows no photo-oxidation and suggests that adsorption occurs via retention of the disulfide bond. The adsorption isotherm data for dihydrolipoic acid and dithiothreitol gave good fits to the Langmuir isotherm, with adsorption constants higher than those for monothiol-containing ligands on CdS. The Langmuir adsorption constant for n-octanoic acid on CdS indicates that the additional interaction between the carboxylate group of dihydrolipoic acid and the CdS is weak in comparison with the dithiol interaction with CdS.
authors.affiliation: Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand.
authors.forename: Aidan G
authors.initials: AG
authors.lastname: Young
authors.forename: David P
authors.initials: DP
authors.lastname: Green
authors.forename: A James
authors.initials: AJ
authors.lastname: McQuillan
mesh_terms: D000327:Adsorption; D019187:Cadmium Compounds / Q000737:chemistry*; D006860:Hydrogen Bonding; D008024:Ligands; D012996:Solutions; D013055:Spectrophotometry, Infrared / Q000379:methods*; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D019187:Cadmium Compounds; D008024:Ligands; D012996:Solutions; D013438:Sulfhydryl Compounds; D013440:Sulfides; C034939:cadmium sulfide; D014867:Water