Techniques for Probing the Processes by Which Microwaves Interact with Chemical and Biological Systems

Abstract

Microwave heating is a relatively mature field and is theoretically well understood. However, recently there has been debate as to whether microwaves can interact with chemical and biological systems by means other than heating alone. There is some theoretical justification for such interactions but experimental evidence is often unreliable due to poor or non-existent measurement of heating and/or poor control experiments. Therefore improved techniques for probing these systems are required. One of the reasons why microwave-assisted chemistry is poorly understood is that there is little available dielectric property data even for common solvents. A simple method for the measurement of the dielectric spectra of liquids was verified and used to probe a room temperature ionic liquid and a chemical mixture used in a stage of a microwave-heated industrial process. The temperature and frequency dependence of the dielectric properties explained the observed rapid microwave heating of the ionic liquid and the relative failure of the process as a result of changing the irradiation conditions in order to scale it up. Temperature measurement during microwave-assisted chemistry, whilst crucial to the elucidation of non-thermal effects, is problematic. A method of component specific or spatiallyresolved thermometry during microwave heating of solid-phase organic synthesis (SPOS) suspensions has been developed. Measurements of the temperature-dependent lifetime of a fluorophore covalently attached to SPOS resin beads yield temperature values accurate to within around 0.5°C. Selective microwave heating of the resin was not observed, even for a system artificially designed to have significant dielectric inhomogeneity. Techniques for the in situ and in vitro probing of model biological systems offer significant improvements over previous methods used to determine the possibility of microwaves effecting living things by non-thermal means. Thermally induced changes in the structures of a lipid and a globular protein were followed by small angel neutron scattering and circular dichroism respectively during microwave exposure. No evidence for non-thermal effects was obtained.