Chemical modification of parylene C coatings for SAW biosensors

Surface acoustic wave (SAW) devices based on horizontally polarised surface shear waves (HPSSW) enable label-free, sensitive and cost-effective detection of biomolecules in real time. Binding reactions on the sensor surface are detected by determining changes in surface wave velocity caused mainly by the uptake of mass or viscosity changes in the sensing layer. Our SAW devices consist of a lithium tantalate substrate with gold transducers. They are coated with parylene C (poly(2-chloro-p-xylylene)) to get a chemically homogeneous surface. For biosensing applications capture molecules or ligands corresponding to the analyte are bound on the sensor surface, usually via an intermediate hydrogel layer, e.g., dextran. In our case, the challenge is to modify the parylene C layer as it is chemically largely inert. One possibility successfully applied already is using photolinkers, but this is restricted to compounds containing photoactive groups. Our new approach aims at a more versatile platform for covalent sensor coatings on parylene C. Here, parylene C is activated by plasma treatment and silanisation first, then aminodextran (AMD) is bound covalently. The shielding effect of this sensor modification against unspecific protein adsorption is determined. Sensors coated with parylene C and covalently bound AMD are successfully applied in an immunoassay.