C. W. Van Neste

Standoff photoacoustic spectroscopy

Here, we demonstrate a variation of photoacoustic spectroscopy that can be used for obtaining spectroscopic information of surface adsorbed chemicals in a standoff fashion. Pulsed light scattered from a target excites an acoustic resonator and the variation of the resonance amplitude as a function of illumination wavelength yields a representation of the absorption spectrum of the target. We report sensitive and selective detection of surface adsorbed compounds such as tributyl phosphate and residues of explosives such as trinitrotoluene at standoff distances ranging from 0.5–20m, with a detection limit on the order of 100ng∕cm2.

Standoff Spectroscopy of Surface Adsorbed Chemicals

Despite its immediate applications, selective detection of trace quantities of surface adsorbed chemicals, such as explosives, without physically collecting the sample molecules is a challenging task. Standoff spectroscopic techniques offer an ideal method of detecting chemicals without using a sample collection step. Though standoff spectroscopic techniques are capable of providing high selectivity, their demonstrated sensitivities are poor. Here we describe standoff detection of trace quantities of surface adsorbed chemicals using two quantum cascade lasers operated simultaneously, with tunable wavelength windows that match with absorption peaks of the analytes. This standoff method is a variation of photoacoustic spectroscopy, where scattered light from the sample surface is used for exciting acoustic resonance of the detector. We demonstrate a sensitivity of 100 ng/cm(2) and a standoff detection distance of 20 m for surface adsorbed analytes such as explosives and tributyl phosphate.

Photoacoustic spectroscopy of surface adsorbed molecules using a nanostructured coupled resonator array.

A rapid method of obtaining photoacoustic spectroscopic signals for trace amounts of surface adsorbed molecules using a nanostructured coupled resonator array is described. Explosive molecules adsorbed on a nanoporous anodic aluminum oxide cantilever, which has hexagonally ordered nanowells with diameters and well-to-well distances of 35 nm and 100 nm, respectively, are excited using pulsed infrared (IR) light with a frequency matching the common mode resonance frequency of the coupled resonator. The common mode resonance amplitudes of the coupled resonator as a function of illuminating IR wavelength present a photoacoustic IR absorption spectrum representing the chemical signatures of the adsorbed explosive molecules. In addition, the mass of the adsorbed molecules as an orthogonal signal for quantitative analysis is determined by measuring the variation of the localized, individual mode resonance frequency of a cantilever on the array. The limit of detection of the ternary mixture of explosive molecules (1:1:1 of trinitrotoluene (TNT), cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN)) is estimated to be ~ 100 ng cm(-2). These multi-modal signals enable us to perform quantitative and rapid chemical sensing and analysis in ambient conditions.

Parametric energy conversion of thermoacoustic vibrations

We demonstrate a parametric energy conversion method of thermoacoustic (TA) vibrations into electrical oscillations of a LC circuit. The inductance modulation necessary to excite the parametric oscillations is achieved by varying the air gap between two halves of a ferrite E-core coil. As a proof-of-concept, the parametric converter was attached to a Sondhauss tube that converts the heat into acoustic vibrations. The maximum total acoustic power output of this thermoacoustic engine was ∼5.3 mW. A flexible metallic membrane capping the Sondhauss tube connected to the moving half E-core served as a mechanical oscillator. The resonance frequency of the membrane was matched with the operating frequency (130 Hz) of the Sondhauss tube for resonant energy extraction. We have characterized the power output of the complete system as a function of electrical load. The maximum electrical power of 2.3 mW produced by the system corresponds to an acoustic-to-electric conversion efficiency of 44%.

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.

Wireless single contact power delivery

We expand on our recently developed single contact transmission method and apply it toward short-range wireless power delivery. The connection between a conductive surface and a receiver, which was directly connected in our initial system, can be made capacitive by extending the receiver a short distance off the surface. This allows the system to function in a purely wireless mode. We demonstrate the wireless transmission of power to a 25 W load at varying distances from an aluminum foil sheet. The transfer efficiency is given with respect to receding distance from the sheet with an average efficiency of 80% over a 3 cm separation for the experimental system.

The role of chloride ions in plasma-activated water treatment processes

The effect of chloride ions (Cl−) on the efficiency of hydroxyl-based (OH˙) water treatment processes, especially plasma treatment systems, remains controversial with conflicting reports of enhanced and deteriorating roles. In this study, we show that during the plasma treatment stage, the scavenging nature of Cl− towards OH˙ decreases the percentage of contaminant removal. On the other hand, the percentage of contaminant removal increases during the post-treatment phase due to the formation of singlet oxygen (1O2) from the reaction of hypochlorous acid (HOCl) and hydrogen peroxide (H2O2). Our results show that there exists an optimum Cl− concentration at which the removal percentage is at its maximum. We also investigated the effect of pH on the role of Cl−. We present possible solution characteristic-dependent reaction pathways and their effect on the treatment process.

Ozone alteration for background references using QCL-based mid infrared standoff spectroscopy

Mid-Infrared standoff spectroscopy using Quantum Cascade Lasers has been a focus of on-going research for many years. When attempting to detect trace analyte residues, the greatest challenge facing this technology is not in the lasers, but the difficulty in creating a spectroscopic background reference for an unknown surface. Such techniques as Differential Location Measurements fail when analyte concentrations are below 1 μg/cm2. To overcome this challenge of unknown surface backgrounds, we propose a technique to alter the IR absorption peaks of a target analyte by exposing the surface to a high intensity, alternating electric field in a standoff fashion. The high intensity electric field generates ozone radicals from the local air, oxidizing organic compounds on the surface. A spectrum of the surface before and after the ozone radicals is obtained. The ozone altered spectrum acts as the reference background and is compared against the un-altered spectrum, generating a differential signal used to identify the target analyte.

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.