Chemical sensors based on micro/nanoelectromechanical systems (M/NEMS) offer many advantages. However, obtaining chemical selectivity in M/NEMS sensors using chemoselective interfaces has been a longstanding challenge. Despite their many advantages, M/NEMS devices relying on chemoselective interfaces do not have sufficient selectivity. Therefore, highly sensitive and selective detection and quantification of chemical molecules using real-time, miniature sensor platforms still remains as a crucial challenge. Incorporating photothermal/photoacoustic spectroscopic techniques with M/NEMS using quantum cascade lasers can provide the chemical selectivity without sacrificing the sensitivity of the miniaturized sensing system. Point sensing is defined as sensing that requires collection and delivery of the target molecules to the sensor for detection and analysis. For example, photothermal cantilever deflection spectroscopy, which combines the high thermomechanical sensitivity of a bimetallic microcantilever with high selectivity of the mid infrared (IR) spectroscopy, is capable of obtaining molecular signatures of extremely small quantities of adsorbed explosive molecules (tens of picogram). On the other hand, standoff sensing is defined as sensing where the sensor and the operator are at distance from the target samples. Therefore, the standoff sensing is a non-contact method of obtaining molecular signatures without sample collection and processing. The distance of detection depends on the power of IR source, the sensitivity of a detector, and the efficiency of the collecting optics. By employing broadly tunable, high power quantum cascade lasers and a boxcar averager, molecular recognition of trace explosive compounds (1 μg/cm2 of RDX) on a stainless steel surface has been achieved at a distance of five meters.
The highly sensitive nanoporous cantilever beam without immobilized receptors was combined with highly selective mid-infrared (IR)
spectroscopy for molecular recognition of analytes using characteristic molecular vibrations. Unlike conventional IR spectroscopy, in
addition, the detection sensitivity and resolution are drastically enhanced by combining high power tunable quantum cascade laser
with a nanoporous cantilever having large surface area, low modulus, and nanowell structures. Further, analytes can be easily loaded
on the porous microcantilever without receptor due to nanowells. In addition, orthogonal signals, variations in the mass and IR
spectrum, provide more reliable and quantitative results including physical as well as chemical information of samples. We have used
this technique to rapidly identify single and double stranded DNA.