On the Adsorption of Dimethyl Methylphosphonate on Self-Assembled Alkanethiolate Monolayers:  Influence of Humidity

Abstract

Humidity influences the adsorption of dimethyl methylphosphonate (DMMP) on three different organic model interfaces to quite different extents. The model interfaces, characterized by the following tail groups (−OH, −CH3, −COOH), were prepared by solution self-assembly of ω-substituted alkanethiols on gold, and the coadsorption of water and DMMP was followed by a surface acoustic wave sensor and infrared reflection absorption spectroscopy. The surfaces show both quantitative and qualitative differences concerning the interaction with DMMP. Humidity is found to influence the hydrogen bonds formed between DMMP and the tail groups, increasing the strength of interaction. However, the changes in surface coverage of DMMP, as measured by infrared absorption, are not fully registered by the surface acoustic wave sensor, because of a concomitant desorption of water. Indeed there is even a loss in sensor signal due to the presence of humidity in the case of the −COOH-terminated surface, where the strongest interaction with DMMP is found. After compensation for the loss of water, the coverage of DMMP increases with the relative humidity on the OH surface and decreases with the relative humidity on the COOH surface. The consequences of the results are discussed in terms of their impact on the design of sensing interfaces based on hydrogen bonding.

Adsorption on Carbon Nanotubes Studied Using Polarization-Modulated Infrared Reflection−Absorption Spectroscopy

“…The adsorption of DMMP on nonmetals, primarily on oxides, [71][72][73][74][75][76] has been studied extensively due to its importance as a simulant for toxic nerve agents and pesticides. The interaction of DMMP with activated carbon 77 and with alkanethiolate self-assembled monolayers (SAMs) on Au 78 has also been investigated. Activated carbon is found 77 to catalyze the decomposition of DMMP at elevated temperatures, and the SAM results 78 show that functional groups able to form H-bonds to the PdO group significantly increase the heat of adsorption of DMMP.…”

“…The interaction of DMMP with activated carbon 77 and with alkanethiolate self-assembled monolayers (SAMs) on Au 78 has also been investigated. Activated carbon is found 77 to catalyze the decomposition of DMMP at elevated temperatures, and the SAM results 78 show that functional groups able to form H-bonds to the PdO group significantly increase the heat of adsorption of DMMP. Detailed vibrational mode assignments for the IR and Raman spectra of liquid DMMP have been given by Moravie et al 79 Figure 5 shows IR data for DMMP adsorbed on SWNTs and in the vapor phase.…”

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“…Bertilsson et al 16 has conducted a detailed investigation of the adsorption of DMMP on three SAM coatings with respective tail groups of-OH, -CH 3 , and -COOH prepared by the self-assembly of alkanethiols ͑SHCH 2 ͒ m X ͑X = -OH, -CH 3 , -COOH͒ on gold. The -COOH-terminated surface ͓obtained with SH͑CH 2 ͒ 15 COOH coating͔ yielded the highest response from DMMP, while the -CH 3 -terminated surface ͓obtained with SH͑CH 2 ͒ 15 CH 3 coating͔ yielded the lowest response.…”

“…The -COOH-terminated surface ͓obtained with SH͑CH 2 ͒ 15 COOH coating͔ yielded the highest response from DMMP, while the -CH 3 -terminated surface ͓obtained with SH͑CH 2 ͒ 15 CH 3 coating͔ yielded the lowest response. 16 The apparatus used in the present study is shown in Fig. 2.…”

“…Regardless of the flow rate, 0.25 ng/ min is expected to be delivered to the flow cell. In order to compare with the data of Bertilsson et al, 16 a cantilever on a separate chip was coated with SH͑CH 2 ͒ 11 CH 3 . The response of that cantilever to a 1 min pulse of DMMP at a flow rate of 25 SCCM is also shown in Fig.…”