INTERACTIONS BETWEEN SELF-ASSEMBLED MONOLAYERS AND AN ORGANOPHOSPHONATE - DETAILED STUDY USING SURFACE ACOUSTIC WAVE-BASED MASS ANALYSIS, POLARIZATION MODULATION FTIR SPECTROSCOPY AND ELLIPSOMETRY

Self-assembled monolayers (SAMs) having surfaces terminated in -CH3, - OH, -CO2H and -(CO2-)(2)Cu2+ (MUA-Cu2+) have been prepared and examine d as potential chemically sensitive interfaces. Mass measurements made using surface acoustic wave (SAW) devices indicate that these surface s display different degrees of selectivity and sensitivity to a range of analytes. The response of the MUA-Cu2+ SAIM to the nerve-agent simu lant diisopropyl methylphosphonate (DIMP) is particularly intriguing. Exposure of this surface to 50%-of-saturation DIMP yields a surface co ncentration 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 analytica l tools have made it possible to measure the IR spectrum of the chemic ally receptive surface during analyte dosing. Coupled with in situ SAW -ellipsometry measurements, which permit simultaneous measurement of m ass and thickness with ng and Angstrom resolution, respectively, it ha s been possible to develop a model for the surface chemistry leading t o the unusual behaviour of this system. The results indicate that DIMP interacts strongly with surface-confined Cu2+ via the phosphoryl oxyg en to yield a DIMP/Cu2+ adduct that nucleates growth of a semi-ordered , solid-like phase having substantially lower vapour pressure than the liquid.