Chemical derivatization of self-assembled 3-mercaptopropionic and 16-mercaptohexadecanoic acids at platinum surfaces with 3-aminopropyltrimethoxysilane: a spectroscopic and electrochemical study

Platinum surfaces have been chemically modified and derivatized with mercapto and amine compounds. Self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (HSC2H4COOH, 3-MPA) and 16-mercaptohexadecanoic acid (HSC15H30CO2H, 16-MHDA) were formed at Pt surfaces using 1 mM solutions of the modifiers in ethanol. The surface modification with 3-MPA and 16-MHDA was followed by a chemical derivatization with 3-aminopropyltrimethoxysilane (H2NC3H6Si(OCH3)3, 3-APS). The derivatization of the self-assembled monolayers (SAMs) formed at Pt surfaces were done using a 2 mM 3-APS in dimethylformamide (DMF), solution. The SAM and derivatized Pt surfaces were characterized by FT-IR and X-ray photoelectron spectroscopy (XPS). The FT-IR study presented the disappearance of an absorption band at 1700 cm−1 corresponding to νCO, from the COOH groups at the 3-MPA/Pt and 16-MHDA/Pt surfaces, and the appearance of an absorption band at 1565 cm−1, corresponding to the amide II band, after derivatization with 3-APS. FT-IR absorption bands of other chemical bonds were also detected. In the XPS spectra, binding energy peaks for the Pt substrate, as well as for the C, O, S, N, and Si elements were present in the SAMs and derivatized surfaces, as well. The final surfaces formed by the derivatization of the SAM/Pt surfaces were characterized by cyclic voltammetry using a 2.5 mM K3Fe(CN)6 in 0.1 M KCl solution. This redox system was chosen to study the defect site induced by electrochemical desorption and its effect on the charge transfer processes. In summary, the 3-APS derivatization of 16-MHDA/Pt [amide(16-MHDA)/Pt] resulted in the surface being less accessible to mass transfer effects compared with the derivatized 3-MPA/Pt surface [amide(3-MPA)/Pt].