Nanoscale imaging techniques that can be used to visualize and characterize local aggregations of the embedded nanoparticulates with sufficient resolution have attracted a great deal of interest. Ultrasonic scanning probe microscopy (SPM) and its derivatives are nondestructive techniques that can be used to elucidate subsurface nanoscale features and mechanical properties. Although many different ultrasonic methods have been used for subsurface imaging, the mechanisms and crucial parameters associated with the contrast formation in subsurface imaging are still unclear. Here, the impact of mechanical properties of the nanoparticulates/matrix, size of the nanoparticulates, buried depth of the nanoparticulates, and the ultrasonic excitation frequency on the developed ultrasonic SPM images have been investigated. To verify our theoretical model, experimental measurements of scanning near-field ultrasound holography (SNFUH) have been recreated in our theoretical analysis to reveal comparable variations in phase contrast measured in SNFUH while scanning over the nanoparticulates embedded in bacteria.
Multi-modal chemical sensors based on microelectromechanical systems (MEMS) have been developed with an electrical readout. Opto-calorimetric infrared (IR) spectroscopy, capable of obtaining molecular signatures of extremely small quantities of adsorbed explosive molecules, has been realized with a microthermometer/microheater device using a widely tunable quantum cascade laser. A microthermometer/microheater device responds to the heat generated by nonradiative decay process when the adsorbed explosive molecules are resonantly excited with IR light. Monitoring the variation in microthermometer signal as a function of illuminating IR wavelength corresponds to the conventional IR absorption spectrum of the adsorbed molecules. Moreover, the mass of the adsorbed molecules is determined by measuring the resonance frequency shift of the cantilever shape microthermometer for the quantitative opto-calorimetric IR spectroscopy. In addition, micro-differential thermal analysis, which can be used to differentiate exothermic or endothermic reaction of heated molecules, has been performed with the same device to provide additional orthogonal signal for trace explosive detection and sensor surface regeneration. In summary, we have designed, fabricated and tested microcantilever shape devices integrated with a microthermometer/microheater which can provide electrical responses used to acquire both opto-calorimetric IR spectra and microcalorimetric thermal responses. We have demonstrated the successful detection, differentiation, and quantification of trace amounts of explosive molecules and their mixtures (cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN)) using three orthogonal sensing signals which improve chemical selectivity.
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/cm<sup>2</sup> 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.
Standoff identification of explosive residues may offer early warnings to many hazards plaguing present and future
military operations. The greatest challenge is posed by the need for molecular recognition of trace explosive compounds
on real-world surfaces. Most techniques that offer eye-safe, long-range detection fail when unknown surfaces with no
prior knowledge of the surface spectral properties are interrogated. Inhomogeneity in the surface concentration and
optical absorption from background molecules can introduce significant reproducibility challenges for reliable detection
when surface residue concentrations are below tens of micrograms per square centimeter. Here we present a coupled
standoff technique that allows identification of explosive residues concentrations in the sub microgram per square
centimeter range on real-world surfaces. Our technique is a variation of standoff photoacoustic spectroscopy merged
with ultraviolet chemical photodecomposition for selective identification of explosives. We demonstrate the detection of
standard military grade explosives including RDX, PETN, and TNT along with a couple of common compounds such as
diesel and sugar. We obtain identification at several hundred nanograms per centimeter square at a distance of four