Sensitivity Enhancement in Plasma Polymer Films for Surface Acoustic Wave Based Sensor Applications

Ivan Avramov,Ekatherina Radeva,Teodor Grakov,Yuliyan Lazarov,Lazar Vergov

doi:10.3390/coatings11101193

Ivan Avramov, Ekatherina Radeva + Show 3 more

Open Access

https://doi.org/10.3390/coatings11101193

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Abstract

Plasma polymer films (PPF), widely used as sensing layers in surface acoustic wave (SAW) based gas and liquid phase sensors, have a major drawback: high concentrations of the sensed analytes easily drive these films into saturation, where accurate measurements are no longer possible. This work suggests a solution to this problem by modifying the PPF with the sensed chemical compound to improve the overall sorption properties and sensor dynamic range. Thin polymer films were synthesized from hexamethyldisiloxane (HMDSO) and triethylsilane (TES) monomers in a plasma-enhanced chemical vapor deposition (PECVD) process using a RF plasma reactor. We used these Si-containing compounds because they are known for their excellent sensing properties. In this work, the layers were deposited onto the active surface of high-Q 438 MHz Rayleigh SAW two-port resonators, used as mass sensitive sensor elements. We call these devices quartz surface microbalances (QSM). In a second step, ammonia plasma modification was applied to the HMDSO and TES films, in order to achieve a higher sensitivity to NH3. The sensors were probed at different NH3 gas concentrations in a computer controlled gas probing setup. A comparison with unmodified films revealed a 74% to 85% improvement in both the sensitivity and sorption ability of the HMDSO sensing layers, and of about 8% for the TES films.

Highlights

Plasma polymer (PP) films (PPFs) have established themselves as reliable solid and semisolid sensing layers in surface acoustic wave (SAW)-based mass sensitive elements in gas and liquid phase chemical and biological sensor systems [1,2]
The sensor sensitivity to the measured analyte and its dynamic range are significantly increased, and a certain amount of selectivity is achieved. We demonstrate this principle with two NH3 sensing Plasma polymer films (PPF) of different origin: hexamethildisiloxane (HMDSO) and triethylsilane (TES), deposited on Rayleigh SAW two-port resonators used as sensor elements
We have demonstrated a significant increase in the probing sensitivity of plasma polymer sensing films, as used in quartz surface microbalance (QSM)-based SAW gas sensors

Summary

Introduction

Plasma polymer (PP) films (PPFs) have established themselves as reliable solid and semisolid sensing layers in surface acoustic wave (SAW)-based mass sensitive elements in gas and liquid phase chemical and biological sensor systems [1,2]. They feature excellent physical sorption characteristics; are obtained using radio frequency (RF) plasma reactors, in a well-controlled and reproducible deposition process; can polymerize, even at room temperature; and are very stable over time and with environmental changes [11,12] This makes them appropriate for operation as sensing films in highly reactive chemical environments. In applications where accurate measurements at high concentrations of the probing compound are required, the probing analyte drives the sensing PPFs into saturation, whereby the sensor signal becomes nonlinear and correct concentration measurements are no longer possible [10,13,14] Such applications include dosing anesthetic gases in surgical operations, refrigerating equipment, and monitoring chemical processes in industrial equipment, etc

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