Chemical Manipulation by X-rays of Functionalized Thiolate Self-Assembled Monolayers on Au

• Published in: Langmuir Vol. 24; no. 24; pp. 13969 - 13976
• Main Authors: Iqbal, P, Critchley, K, Attwood, D, Tunnicliffe, D, Evans, S. D, Preece, J. A
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 16.12.2008
• Subjects: Chemistry; Colloidal state and disperse state; Electrons; Exact sciences and technology; General and physical chemistry; Gold - chemistry; Interfaces: Adsorption, Reactions, Films, Forces; Molecular Structure; Photochemical Processes; Spectrophotometry; Sulfhydryl Compounds - chemistry; Surface physical chemistry; X-Rays; Nitrogen oxide; Photoelectron; Self assembly; Monolayer; Thiolate; X ray; Conversion; X ray irradiation; Chemical reduction; Substrate; Ester; Chemical modification; Amine; Carbon content; Acids; Hexane; Photoelectron spectrometry
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la802244a

The chemical modification caused by prolonged exposure to X-rays on a series of para-substituted phenyl moieties (−NO2, −CN, −CHO, −COOH, −CO2Me, and −CO2 tBu) at the surface of thiolate-Au self-assembled monolayers (SAMs) has been investigated by X-ray photoelectron spectroscopy (XPS). Furthermore, the influence that the phenyl group has on the chemical modification induced by the X-ray irradiation on the SAMs was investigated by comparing the XPS results obtained from irradiation on a NO2-aromatic-terminated SAM (6-(4-nitro-phenoxy)-hexane-1-thiolate (NPHT)) and NO2-aliphatic-terminated SAM (thioacetic acid S-(12-nitrododecyl) ester (TNDDE)). The NPHT and TNDDE SAMs have been shown to behave differently to X-ray exposure. The irradiation of the NPHT SAM led to the reduction of the nitro (−NO2) moiety to the amine (−NH2) moiety, as shown by the decrease in the intensity of the N 1s photoelectron peak for −NO2 (406 eV) in the XPS spectra with the concomitant increase in the N 1s photoelectron peak for −NH2 (399 eV). On the TNDDE SAM, XPS showed the −NO2 photoelectron peak again decreasing with prolonged X-ray irradiation whereas no peak was observed at 399 eV; therefore, the −NO2 moieties are selectively cleaved. No change was observed on the other functionalized monolayers apart from the −CO2 tBu-functionalized monolayer, where after 100 min of X-ray irradiation approximately 11% of the carbon content was lost. The S 2p and O 1s spectra remained unchanged during the irradiation suggesting the conversion of the −CO2 tBu to the −COOH moiety, although the conversion was not complete because the tertiary butyl moiety contributes 25% to the total carbon content of the SAM. Also, there was no evidence of the molecules desorbing from the substrate for any of the SAMs studied during the X-ray irradiation as shown by no change in the S 2p and C 1s XPS spectra taken during the X-ray irradiation.