Xunchen Liu

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.

Application of quantum cascade lasers for infrared spectroscopy of jet-cooled molecules and complexes

The combination of infrared laser spectroscopy with a molecular jet expansion provides a powerful technique to investigate medium sized organic molecules and clusters. The coupling of quantum cascade lasers (QCLs) with two slit jet infrared spectrometers, namely an off-axis cavity enhanced absorption (CEA) spectrometer and a rapid scan spectrometer with an astigmatic multi-pass cell assembly, are described. Two types of QCLs, specifically a continuous wave (cw) liquid nitrogen cooled distributed-feedback QCL at 5.7 μm, and a cw room temperature mode-hop-free external cavity QCL centered at 6.1 μm, were employed as the light sources. A pair of 1 inch highly reflective cavity ring-down mirrors (R = 99.98% at 5.2 μm) separated by 55 cm or a pair of 1.5 inch astigmatic mirrors separated by 20 cm, served as the optical cavities. To automate and to synchronize the timing of the CEA or rapid scan experiments with a pulsed slit jet molecular expansion, two LabVIEW computer programs were developed. For the CEA experiments, one of the cavity mirrors was mounted on a piezoelectric actuator with 1 inch clear aperture to maximize the effective mirror size. The effects of mirror size and laser sweep rate were evaluated. A minimum detection sensitivity of 1.8×10-8 cm-1 was achieved. Jet-cooled molecules were generated using a homemade pulsed slit jet nozzle assembly. A jet-cooled infrared spectrum of methyl lactate was recorded to demonstrate the performance of the CEA spectrometer. Preliminary results obtained with the room temperature QCL coupled to the rapid scan spectrometer are also presented.

Molecular Self-Recognition: Rotational Spectra of the Dimeric 2-Fluoroethanol Conformers

Fluoroalcohols show competitive formation of intra- and intermolecular hydrogen bonds, a property that may be crucial for the protein-altering process in a fluoroalcohol/water solution. In this study, we examine the intra- and intermolecular interactions of 2-fluoroethanol (FE) in its dimeric conformers by using rotational spectroscopy and ab initio calculations. Three pairs of homo- and heterochiral dimeric FE conformers are predicted to be local minima at the MP2/6-311++G(d,p) level of theory. They are solely made of the slightly distorted most stable G+g-/G-g+ FE monomer units. Jet-cooled rotational spectra of four out of the six predicted dimeric conformers were observed and unambiguously assigned for the first time. All four observed dimeric conformers have compact geometries in which the fluoromethyl group of the acceptor tilts towards the donor and ensures a large contact area. Experimentally, the insertion of the O-H group of one FE subunit into the intramolecular O-H...F bond of the other was found to lead to a higher stabilisation than the pure association through an intermolecular O-H...O-H link. The hetero- and homochiral combinations were observed to be preferred in the inserted and the associated dimeric conformers, respectively. The experimental rotational constants and the stability ordering are compared with the ab initio calculations at the MP2 level with the 6-311++G(d,p) and aug-cc-pVTZ basis sets. The effects of fluorination and the competing inter- and intramolecular hydrogen bonds on the stability of the dimeric FE conformers are discussed.

Standoff detection of explosive residues on unknown surfaces

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 meters.

High-resolution infrared spectrum of jet-cooled methyl acetate in the C=O stretching region: Internal rotations of two inequivalent methyl tops

The jet-cooled high resolution infrared (IR) spectrum of methyl acetate (MA), CH(3)-C(=O)-O-CH(3), in the C=O fundamental band region was recorded by using a rapid scan IR laser spectrometer equipped with an astigmatic multipass cell. No high resolution IR analyses of the ro-vibrational transitions between the ground and non-torsionally excited vibrational states have hitherto been reported for molecules with two inequivalent methyl rotors. Because of the two chemically different methyl tops in MA, i.e., the acetyl -CH(3) and methoxy -CH(3), each rotational energy level is split into more than two torsional sublevels by internal rotations of these methyl groups. We were able to assign ro-vibrational transitions of four torsional species by using the ground state combination differences calculated from the molecular constants of the vibrational ground state recently determined by a global fit of the microwave and millimeter wave lines [M. Tudorie, I. Kleiner, J. T. Hougen, S. Melandri, L. W. Sutikdja, and W. Stahl, J. Mol. Spectrosc. 269, 211 (2011)]. The assigned lines were successfully fitted using the BELGI-Cs-IR program to an overall standard deviation which is comparable to the measurement accuracy. This study is also of interest in understanding the role of methyl rotors in the intramolecular vibrational-energy redistribution processes in mid-size organic molecules.

Point and standoff detection of trace explosives using quantum cascade lasers

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.