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

Abstract

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.

Methane sensing at room temperature using photothermal cantilever deflection spectroscopy

“…Unlike the detectors used in conventional IR absorption spectroscopy, which require cooled mid-IR detectors, the bimetallic cantilever is uncooled and has temperature sensitivity in 10 mK range. Also, unlike conventional photon detection based sensing, the detection of optical absorption using thermal effects offers a higher signal-to-noise ratio, which increases with incident photon intensity [11]. Therefore, by using recent advances in high brightness mid infrared light sources such as quantum cascade lasers (QCL), it is possible to detect extremely small amounts of materials [11].…”

“…The spectrum in the mid infrared (IR) region, however, provides a "molecular finger print" due to the uniqueness of molecular vibrations in the mid infrared range that is free from overtones. Though highly selective, IR spectroscopy is not very sensitive in the mid-IR region [10,11].…”

“…This technique thus enables obtaining the molecular vibrational signature of trace amounts of adsorbed molecules on the cantilever surface [11][12][13][14]. In this technique, the target molecules are first adsorbed on the bimetallic cantilever.…”

“…Also, resonance frequency measurements conducted on the cantilevers during methane adsorption confirm the femtogram level mass loading due to methane adsorption. Unlike chemoselective interface-based microsensors in which operation life is shortened by degradation of the coating, the PCDS technique offers a robust, selective and sensitive single microstructure without a requisite chemoselective interface [11].…”

Nanomechanical identification of liquid reagents in a microfluidic channel

“…Second, recent advances in nanomechanical calorimetric infrared (IR) spectroscopy of adsorbed materials using a bi-material cantilever offer high selectivity detection without relying on extrinsic labels or specific receptors. 16 However, this technique has been limited to solid samples placed on the cantilever and could not be used for liquid analytes.…”