THE POLYMER-COATED SAW SENSOR AS A GRAVIMETRIC SENSOR

The absorption and desorption of chlorobenzene, o-dichlorobenzene, and chloroform in poly[n-butyl methacrylate] (PBMA) was studied in polyme r-coated 104 MHz surface acoustic wave (SAW) sensors, and in freestand ing polymer films by thermogravimetric analysis (TGA). The sorption pr ocesses were analyzed by a Fickian simulation and best-fit partition, and diffusion coefficients were derived. Good correlations were found between simulated and observed data. Partition coefficients derived fr om SAW response were independent of coating thickness and were found t o be about two to three times bigger than those derived from the gravi metric response, In contrast,, the diffusion coefficients increased li nearly with csa ting thickness in the range 70-560 kHz. For the thicke st polymer coating, SAW-derived diffusion coefficients were comparable with TGA-related diffusion coefficients. This study reconfirms the fi nding for other polymers that the response of SAW chemosensors is high er than that anticipated from a mass change. only. The viscoelastic ef fect is again more pronounced than the gravimetric effect, Moreover, t he similarity of diffusion coefficients obtained at higher polymer thi cknesses suggests that the rate of change of the SAW viscoelastic comp onent is similar to that of the gravimetric element. It is fair to ass ume that both processes originate from the same event: the absorption of the analyte in the polymer, In this view the polymer-coated SAW sen sor may be regarded as an enhanced gravimetric sensor.