Interactions between self-assembled monolayers and an organophosphonate Detailed study using surface acoustic wave-based mass analysis, polarization modulation-FTIR spectroscopy and ellipsometry

Abstract

Self-assembled monolayers (SAMs) having surfaces terminated in –CH₃, –OH, –CO₂H, and –(CO₂^-)₂Cu²⁺ (MUA–Cu²⁺) have been prepared and examined as potential chemically sensitive interfaces. Mass measurements made using surface acoustic wave (SAW) devices indicate that these surfaces display different degrees of selectivity and sensitivity to a range ofanalytes. The response of the MUA–Cu²⁺ SAM to the nerve-agent simulant diisopropyl methylphosphonate (DIMP) is particularly intriguing. Exposure of this surface to 50%-of-saturation DIMP yields a surface concentration equivalent to ca. 16 DIMP monolayers. Such a high surface concentration, in equilibrium with a much lower-than-saturation vapour pressure, has not previously been observed. Newly developed analytical tools have made it possible to measure the IR spectrum of the chemically receptive surface during analyte dosing. Coupled with insitu SAW–ellipsometry measurements, which permit simultaneous measurement of mass and thickness with ng and Åresolution, respectively, it has been possible to develop a model for the surface chemistry leading to the unusual behaviour of this system. The results indicate that DIMP interacts strongly with surface-confined Cu²⁺via the phosphoryl oxygen to yield a DIMP/Cu²⁺ adduct that nucleates growth of a semi-ordered, solid-like phase having substantially lower vapour pressure than the liquid.