A gas sensing system for indoor air quality control and polluted environmental monitoring

The objective of this work is to develop a miniature electronic nose (E-nose) to detect the poisonous organic vapors in the indoor. A novel bio-chemical surface-acoustic-wave array with multiplexed oscillator and readout electronics was developed and tested. The sensor array was fabricated on a 128 degrees YX LiNbO3 sensing substrate. On the surface of this substrate, an interdigital transducer (IDT) was made with a Cr/Au film as a metallic structure. The center frequency of the sensing chips was designed at 99.8 MHz. Seven polymers, Poly-4-vinylphenol (P4VP), Poly-vinylacetate (PVAc), poly-N-vinylpyrrolidone (PNVP), Polyethyleneglycol (PEG), Polystyrene (PS), polystyrene-co-maleic anhydride (PSMA), and Polysulfone (PSu), were coated on the sensing area of Au surface between the center of the IDTs. The 2 x 2 non-continuous SAW array could be controlled by a multiplexing technique and sensor signals were obtained by readout electronics with an 89C51 microprocessor. Five different organic vapors were adopted as samples to test the developed system. The frequency shifts (Delta f) were measured and the stable state was about 1 kHz. Two-way hierarchical clustering analysis was used to analyze the measured data. It showed that based on the analysis method, gases and polymers with similar chemical properties were grouped into the same family. The results suggest that the designed miniature e-nose and the applied analysis method are promising for practical applications of gas detection and recognition.