Enhanced sensitivity of SAW gas sensor coated molecularly imprinted polymer incorporating high frequency stability oscillator

This paper presents a 300 MHz surface acoustic wave (SAW) gas sensor coated molecularly imprinted polymer incorporating a novel SAW oscillator with high frequency stability. A SAW delay line on ST-X quartz substrate with low insertion loss (less than 12 dB) and single mode selection capability was fabricated as the oscillator element. Electrode width control single phase unidirectional transducer (EWC/SPUDT) configuration and comb transducer were used to structure the SAW device to minimize the insertion loss and accomplish the single mode selection, respectively. Prior to fabrication, coupling of modes (COM) simulation was performed to predict device performance. The measured frequency response S12 showed a good agreement with simulated results. The effect of oscillator circuit system temperature shift upon frequency stability of the fabricated oscillator was studied in detail. Typical short-term frequency stability (1 h) of ∼0.06 ppm (∼20 Hz) for the fabricated 300 MHz SAW oscillator was observed in a laboratory environment with temperature control. The oscillator was successfully applied to gas sensor coated 10 nm molecularly imprinted polymer (MIP) as sensor material for dimethyl-methyl-phosphonate (DMMP), and the MIP was prepared using o-phenylenediamine (o-PD) as a functional monomer, Sarin acid as a molecular template and co-polymerized using cyclic voltammetry (CV). The sensitivity for DMMP concentrations detection in a range of 1–100 mg/m3 was evaluated as ∼96 Hz/mg/m3, and the threshold detection limit was up to 0.5 mg/m3.