The Use of Polyurethane Composites with Sensing Polymers as New Coating Materials for Surface Acoustic Wave-Based Chemical Sensors—Part I: Analysis of the Coating Results, Sensing Responses and Adhesion of the Coating Layers of Polyurethane–Polybutylmethacrylate Composites

Abstract

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.

The Use of Polyurethane Composites with Sensing Polymers as New Coating Materials for Surface Acoustic Wave-Based Chemical Sensors—Part II: Polyurethane Composites with Polylaurylmetacrylate, Polyisobutene, and Poly(chlorotrifluoroethylene-co-vinylidene Fluoride): Coating Results, Relative Sensor Responses and Adhesion Analysis

“…The quartz sensor elements are highly polished piezoelectric quartz (SiO 2 , 37.5 • cut orientation, 0.5 mm thickness) devices with gold electrodes lithographically deposited, with an optimized geometry [7], purchased from SCD Components, Dresden, Germany. The SAW sensor system was developed internally and is described in the first part of the work [44].…”

“…The procedure and the experimental setup for the measurements of the sensor responses are described in the first part of this work [44]. The sensor responses for each coating material were obtained by measuring the maximum frequency shift with the SAW sensor system for the same nine organic compounds and water as in the previous work.…”

“…The CAT test is a house-made test devised to investigate the stability of the polymeric coating layer in terms of its chemical resistance to perchloroethylene, a strong organic solvent, and, at the same time, to infer the adhesion of the polymeric layer to the sensor element surface. The method, described in the first part of this work [44], is performed by submitting the coated sensor elements to the limit condition of a complete immersion in a bath of perchloroethylene for 24 h. After this, the coated sensor is left at room temperature for twelve hours to complete the evaporation of the solvent before the measurements.…”

“…New developed sensing materials are composed of polyurethane and, at least, a second polymer that acts as the sensing polymer. In the first part of this work, we present the results of the methodology for the polymer composites formed by PBMA as the sensing polymer and polyurethane and their overall practical evaluation as sensing layers for their application in the SAW technology [44]. The results for the composites of PU with PBMA showed that the methodology generated reliable sensor coatings, with improved adhesion and chemical resistance regarding the test of immersion in an organic solvent (perchloroethylene), maintaining the sensitivity characteristics of the sensing polymer (PBMA) and fulfilling all the other requirements to be used in the SAW sensor device [44].…”

“…It was also inferred that, depending on the relative concentration of PBMA and PU in the composite, some excess of the sensing polymer (PBMA) remained as not nonbound in the PU-PBMA structure of the coating layer. According to the results, this excess should be removed from the composite coating layers during the CAT [44].…”