Charles W. Van Neste

Trace Explosive Detection using Photothermal Deflection Spectroscopy

Satisfying the conditions of high sensitivity and high selectivity using portable sensors that are also reversible is a challenge. Miniature sensors such as microcantilevers offer high sensitivity but suffer from poor selectivity due to the lack of sufficiently selective receptors. Although many of the mass deployable spectroscopic techniques provide high selectivity, they do not have high sensitivity. Here, we show that this challenge can be overcome by combining photothermal spectroscopy on a bimaterial microcantilever with the mass induced change in the cantilever’s resonance frequency. Detection using adsorption-induced resonant frequency shift together with photothermal deflection spectroscopy shows extremely high selectivity with a subnanogram limit of detection for vapor phase adsorbed explosives, such as pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), and trinitrotoluene (TNT).

Molecular recognition using receptor-free nanomechanical infrared spectroscopy based on a quantum cascade laser

Speciation of complex mixtures of trace explosives presents a formidable challenge for sensors that rely on chemoselective interfaces due to the unspecific nature of weak intermolecular interactions. Nanomechanical infrared (IR) spectroscopy provides higher selectivity in molecular detection without using chemoselective interfaces by measuring the photothermal effect of adsorbed molecules on a thermally sensitive microcantilever. In addition, unlike conventional IR spectroscopy, the detection sensitivity is drastically enhanced by increasing the IR laser power, since the photothermal signal comes from the absorption of IR photons and nonradiative decay processes. By using a broadly tunable quantum cascade laser for the resonant excitation of molecules, we increased the detection sensitivity by one order of magnitude compared to the use of a conventional IR monochromator. Here, we demonstrate the successful speciation and quantification of picogram levels of ternary mixtures of similar explosives (trinitrotoluene (TNT), cyclotrimethylene trinitramine (RDX), and pentaerythritol tetranitrate (PETN)) using nanomechanical IR spectroscopy.

Photothermal Sensing of Chemical Vapors Using Microcantilevers

Although microfabricated cantilevers have been used for detecting a variety of chemicals with high sensitivity, their selectivity appears to be poor. This selectivity problem is directly related to the poor selectivity of chemical interfaces immobilized on cantilever surfaces. Here we discuss two cantilever-based techniques that can be used for obtaining orthogonal signals for multimodal operation for enhanced selectivity. The first technique is based on photothermal deflection spectroscopy where selectivity is achieved through a mechanical response due to optical absorption by the adsorbed molecules on a cantilever. In the second technique, the position of the resonance frequency peak of the cantilever is monitored for shifting due to mass loading. This technique allows the precise measurement of the mass of the surface-adsorbed molecules. These two methods are demonstrated for adsorbed explosives.