[University home]

Centre for Atmospheric Science

Development of QCM Sensors

Quartz crystals have the unusual property of piezoelectric resonance. When any molecules stick to the surface of the QCM the resonant frequency will change and hence the mass can be detected, but the molecules can't be identified.  The aim of the work is to develop artificial recognition materials to coat onto the surface of a QCM to produce a selective detector. Among the most promising examples of artificially generated recognition materials are the molecularly imprinted polymers (MIPs). MIPs are highly cross-linked polymers, which are inherently stable and capable of high selectivities, approaching those of their natural counterparts, indeed, the selectivity and binding affinities of MIPs are comparable to nature's antibody-antigen interactions.

Selective detectors have been developed for a range of analytes ranging from anabolic steroids to PAHs (polycyclic aromatic hydrocarbons), to terpenes, to atmospheric aerosols  and amino acids. More recently in collaboration with Nottingham Trent University to develop Acoustic wave sensors to assess MHC-peptide interactions. (http://www.ntu.ac.uk/research/groups_centres/sat/81254.html)

Other recent QCM work involved testing different types of coating for the detection of ozone and the polymer polybutadiene was found to be most sensitive. The sensitivity was improved by operating the crystal at higher harmonics. More details can be found here: http://pubs.rsc.org/en/content/articlelanding/2011/an/c1an15106a

Ozone Sensors

Ozone is a critically important tropospheric trace chemical. It plays a central role in the atmospheric chemistry of many species on local to global scales (e.g. NOx, VOCs, halogenated organics, aerosol). Knowledge of surface deposition processes for ozone are therefore important in regional and global chemical modelling, especially in regional assessments of oxidant behaviour. Ozone also has the greatest irritant effect in the human respiratory tract of all of the commonly occurring gas phase pollutants. Epidemiological studies have shown that ozone pollution episodes in cities are related to incidence of human respiratory disorder. Apart from the implications of ozone for human health, it is known to be phytotoxic, causing damage to crops and sensitive natural ecosystems. The main aim of our research is to develop sensors based on acoustic wave technology for the selective detection of ozone for the use in epidemiological and atmospheric flux studies (link to surface exchange).

SAW Delay Line
Fig 1. SAW Delay Line.

A surface acoustic wave is a mechanical "ripple" travelling along the surface of a solid. On a piezoelectric substrate the wave can be generated by fabricating a set of metal interdigital transducers (IDTs) and applying an rf voltage across them. The spacing of the fingers of the IDT determines the wavelength and this together with the substrate orientation, which defines the propagation speed, determines the frequency of operation. A SAW device using a delay line configuration provides a free surface area between the IDT’s that can be used as a sensor (Fig. 1). As the wave is surface localised, any rigid mass deposited on the propagation path changes the speed of propagation and decreases the resonant frequency of the device; the path can have a specific binding recognition layer added to create a sensor. Using a polybutadiene coating with have developed a sensor with a limit of detection of 3 ppb ozone whilst taking measurements every 10 seconds was obtained when using a 5 MHz device.