A low-cost, ultra sensitive hand-held system for explosive detection using piezo-resistive micro-cantilevers

This paper presents a hand-held explosive detector system based on piezo-resistive cantilever sensors. A novel signal conditioning scheme comprising current excitation method with modified half bridge configuration, is implemented to measure as low as 20 ppb variations in the sensor resistance. The proposed circuit is insensitive to thermoelectric and stray noise effects as well as to temperature changes with minimum SNR value of 20 dB which is better than previously reported values. The hand held system comprises micro-cantilever sensor, analog signal conditioning module, sigma-delta ADC, digital modules along with digital output displayed on the LCD. The micro-cantilevers feature a special coating like 6-Marcaptonicotonic acid (6-MNA) or 4-mercaptobenzoic acid (4-MBA) on one side of the cantilever. The selective reaction takes place with TNT and RDX molecules (the target analyte molecules) on the functionalized cantilever surface with special coating. This reaction changes the resistance of the piezo-resistive sensor. On-chip implementation of one of the techniques is presented while some other methods are realized using discrete components. The objective is to explore different techniques for converting cantilever deflection into a measurable quantity and challenges ahead of converting explosive detector system into a complete SoC. These challenges are also discussed in the paper.