Detection of phenol in aqueous solution using an AT-cut quartz thickness shear mode sensor

The AT-cut quartz crystal, operating in the thickness shear mode, has been used to detect 4-aminophenol. The mechanism of response involves a specific reaction with o-cresol to produce a hydrophobic indophenol dye that precipitates out and adsorbs to the quartz surface. Adsorption specificity for the crystal surface is enhanced via the pretreatment of the surface with a hydrophobic poly(vinyl chloride) (PVC) layer. It is shown that there is a critical PVC loading that affords optimised speed of response and amplitude. At a high loading of 0.05% (w/v) of PVC (in tetrahydrofuran), the resulting desolvated polymer layer exhibits a defined porous structure. The pores act as nucleation sites for the oligomeric precipitating species. At lower PVC loadings, the membrane is non-uniform and inhomogeneous. However, the speed and amplitude of response to indophenol at such loadings are enhanced compared with that of a bare crystal or crystal with high PVC loadings.