The Use of Polyurethane Composites with Sensing Polymers as New Coating Materials for Surface Acoustic Wave-Based Chemical Sensors—Part III: Ultrasonic Analyses and Optical Microscopy Characterization of the Coating Results

The chemical sensitization of surface acoustic wave (SAW) sensors plays a key role for this technology. The analysis of the resulting nanometric sensing layer is crucial for the development of new sensing materials as well as for the quality control of SAW sensors systems for commercial applications. In the previous works, the resulting coating layers using new coating materials based on polyurethane-polymer composites were evaluated considering the ultrasonic analysis, the adhesion, and the sensor responses. In this work, the characterization of the coating process, Bright Field Microscopy (BFM) and Dark Field Microscopy (DFM) were used to evaluate the quality of the material distribution and homogeneity of the obtained sensing layers. The sensing materials analyzed were the four polymers used in the previous works and their respective new composites with polyurethane (PU). The combination of BFM and DFM allows the characterization of the resulting material distribution obtained by the coating process, providing inferences about the interaction of each coating material with the surface of the SAW sensor element as well as about the correlation between the results of the ultrasonic parameters, the real material distribution and the homogeneity of the obtained coating layer of each coating material.