Ross C. Thomas

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-)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 Aresolution, 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.

Thin film porous membranes for catalytic sensors

This paper reports on new and surprising experimental data for catalytic film gas sensing resistors coated with nanoporous sol-gel films to impart selectivity and durability to the sensor structure. This work is the result of attempts to build selectivity and reactivity to the surface of a sensor by modifying it with a series of sol-gel layers. The initial sol-gel SiO/sub 2/ layer applied to the sensor surprisingly showed enhanced O/sub 2/ interaction with H/sub 2/ and reduced susceptibility to poisons such as H/sub 2/S.

Self-assembling monolayers on SAW devices for selective chemical detection

The authors have utilized gold-coated, 98-MHz surface acoustic wave (SAW) devices on ST-cut quartz substrates to monitor in real time the self-assembly of organized alkanethiol monolayer films. The kinetics of assembly of these films indicate that the self-assembly process is dominated by the interaction between the thiol head group and the gold layer, rather than interactions between adjacent alkanethiol molecules. SAW devices modified with a self-assembled monolayer (SAM) of mercaptoundecanoic acid provide a response specific to the presence of organophosphonates (OPs) relative to water and a range of common volatile organic compounds (VOCs): the OP response is characteristic of increased mass loading of the device, while water and VOCs yield responses indicative of changes in the mechanical properties of the SAM film.<<ETX>>