The sensing layers for surface acoustic wave-based (SAW) sensors are the main factor in defining the selectivity and reproducibility of the responses of the sensor systems. Among the materials used as sensing layers for SAW sensors, polymers present a wide range of advantages, from availability to a large choice of chemical-sensing environments. However, depending on the physical–chemical properties of the polymer, issues about the chemical and mechanical stability of the sensing layer have been reported that can compromise the application of sensor systems in the long-term. The sensor properties are defined basically by the properties of the coating material and the quality of the coating process. The strategy used to improve the properties of polymeric coating layers for SAW technology involved the use of polyurethane (PU) in combination with a second polymer that is responsible for the sensing properties of the resulting layer; this is obtained by a reproducible and robust coating procedure. In this first part of our research, we used polymer composites of different compositions of polybutylmetacrylate (PBMA) as the sensing polymer with polyurethane. The analysis of the coating (ultrasonic parameters), the relative sensor responses and the adhesion results for the PU–PBMA composites were determined. The ultrasonic analysis and the relative sensor responses showed very reproducible and precise results, indicating the reproducibility and robustness of the coating process. Accurate correlations between the results of the ultrasonic parameters due to the coating and the relative sensor responses for the organic analytes analyzed were obtained, showing a precise quantitative relationship between the results and the constitution of the composite coating materials. The composites show practically no significant sensor responses to water. The PU–PBMA composites substantially enhanced adhesion to the surface of the piezoelectric sensor element in comparison to the coating with pure PBMA, without loss of its sensing properties. Other PU–polymer composites will be presented in the future, as well as an analysis of the selectivity for the organic analytes for these types of coating materials.

This work presents the application of the methodology for the sensitization of surface acoustic wave-based sensors (SAW), developed in the first part of this work. The strategy of the method is the obtention of sensing layers with tailored chemical environments by taking advantage of the wide variety of chemical composition of the organic polymers, which have been used as sensing polymers, and combining them with polyurethane (PU) to form polymeric composites that show enhanced properties as sensing materials for the SAW sensor technology. In the first part of this work, the ultrasonic and adhesion characterization was correlated to the sensor responses of PU-polybutylmethacrylate (PBMA) composites of different relative concentrations of the sensing polymer (PBMA) and PU. The resulting coating layers obtained with the PU polymer composites improved the chemical and mechanical properties of the sensing layer without interfering with the quality of their sensor responses in comparison to those with the pristine polymer as the sensing material. In this second part of this work, three new polyurethane polymeric composites were analyzed. The new sensing materials were produced using polylaurylmetacrylate (PLMA), polyisobutene (PIB), and poly(chlorotrifluoroethylene-co-vinylidene fluoride) (PCTFE) as the sensing polymers combined with PU. The results of the new PU polymer composites showed consequently different properties depending on the type of sensing polymer used, reproducing, however, the previous features achieved with PU and polybutylmetacrylate (PBMA) composites, like the improvements in the adhesion and the resistance against an organic solvent and preserving, in each case, the sensor response characteristic of each sensing polymer used, as was also observed for the PU-PBMA polymeric composites. The results obtained with the new sensing materials validated the strategy and confirmed its generalization as a very suitable methodology for the sensitization of SAW sensors, strongly indicating the applicability and reliability of the method, which makes possible the choice of virtually any chemical environments for the sensitization of SAW sensor systems.

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.

Surface acoustic wave-based (SAW) sensors are of great interest due to their high sensibility and fast and stable responses. They can be obtained at an overall low cost and with an intuitive and easy-to-use method. The chemical sensitization of a piezoelectric transducer plays a key role in defining the properties of SAW sensors. In this study, we investigate the structural and adhesion properties of a new class of coating material based on polyurethane polymeric composites. We used dark-field microscopy (DFM) and scanning electron microscopy (SEM) to observe the microstructure of polyurethane composite coatings on piezoelectric sensor elements and to analyze the effects of the chemical resistance and adhesion test (CAT) on the coating layers obtained with the polyurethane polymeric composites. The results of the microscopy showed that all polyurethane composite coatings exhibited excellent uniformity and stability after chemical adherence testing (CAT). All of the observations were correlated with the results of the ultrasonic analysis, which demonstrated the role of polyurethane as a binder to form the stable structure of the composites and, at the same time, as an adhesion promoter, increasing the chemical resistance and the adherence of the coating layer to the complex surface of the piezoelectric sensor element.

The technology of surface acoustic wave-based (SAW) sensors greatly depends on the performance of the resulting sensing layer. To ensure real applications, the sensors must be produced with reproducibility, as well as the statistical consistency of analytical sensor response results must be assured. In this work, we investigated the reproducibility and the statistical performance of the coating procedure used in previous works for the development of new polymeric coating materials, and the statistics of the respective sensor responses were analyzed. The polylaurylmethacrilate (PLMA) is used as an example of polymeric coating material. Two series of sensors coated with distinct quantities of the polymer were produced and analyzed. The statistical analysis of the ultrasonic parameters of the sensor production presented very low variability for both series of sensors. The respective sensor responses, obtained with a set of analytes with distinct chemical functions, presented, in the same way, excellent reproducibility for both series of sensors. The very good reproducibility and statistical robustness of the sensor production data and of the respective sensor responses confirm the reliability of the methodology to produce sensors for the SAW technology.