Terahertz Spectroscopic Molecular Sensor for Rapid and Highly Specific Quantitative Analytical Gas Sensing

Quantitative analytical gas sampling is of great importance in a range of environmental, safety, and scientific applications. In this article, we present the design, operation, and performance of a recently developed tabletop terahertz (THz) spectroscopic molecular sensor capable of rapid (minutes) and sensitive detection of polar gaseous analytes with near "absolute" specificity. A novel double-coil absorption cell design and an array of room-temperature sorbent-based preconcentration modules facilitate quantitative THz detection of light polar volatile compounds, which often challenge the capabilities of established gas sensing techniques. Acetone, ethanol, methanol, acetaldehyde, formaldehyde, and isoprene are detected at low parts-per-billion to high parts-per-trillion levels. This work evaluates performance-limiting factors for THz spectroscopy-based chemical identification: (1) spectral signal to noise and (2) preconcentrator efficiency.

Biomarkers and Detection Platforms for Human Health and Performance Monitoring: A Review

Human health and performance monitoring (HHPM) is imperative to provide information necessary for protecting, sustaining, evaluating, and improving personnel in various occupational sectors, such as industry, academy, sports, recreation, and military. While various commercially wearable sensors are on the market with their capability of "quantitative assessments" on human health, physical, and psychological states, their sensing is mostly based on physical traits, and thus lacks precision in HHPM. Minimally or noninvasive biomarkers detectable from the human body, such as body fluid (e.g., sweat, tear, urine, and interstitial fluid), exhaled breath, and skin surface, can provide abundant additional information to the HHPM. Detecting these biomarkers with novel or existing sensor technologies is emerging as critical human monitoring research. This review provides a broad perspective on the state of the art biosensor technologies for HHPM, including the list of biomarkers and their physiochemical/physical characteristics, fundamental sensing principles, and high-performance sensing transducers. Further, this paper expands to the additional scope on the key technical challenges in applying the current HHPM system to the real field.

COVID-19 Seroprevalence and Active Infection in an Asymptomatic Population

In response to the COVID-19 pandemic, immediate and scalable testing solutions are needed to direct return to full capacity planning in the general public and across the Department of Defense (DoD). To fully understand the extent to which a population has been affected by COVID-19, active monitoring approaches require an estimation of overall seroprevalence in addition to accurate, affordable, and rapid tests to detect current SARS-CoV-2 infection. In this study, researchers in the Air Force Research Laboratory's 711th Human Performance Wing, Airman Systems Directorate evaluated the performance of various testing methods for the detection of SARS-CoV-2 antibodies and viral RNA in asymptomatic adults working at Wright-Patterson Air Force Base and the surrounding area during the period of 23 July 2020-23 Oct 2020. Altogether, there was a seroprevalance of 3.09% and an active infection rate of 0.5% (determined via the testing of saliva samples) amongst individuals tested, both of which were comparable to local and national averages at the time. This work also presents technical and non-technical assessments of various testing strategies as compared to the gold standard approaches (e.g., lateral flow assays vs. ELISA and RT-LAMP vs. RT-PCR) in order to explore orthogonal supply chains and fieldability. Exploration and validation of multiple testing strategies will allow the DoD and other workforces to make informed responses to COVID-19 and future pandemics.

Science and Technology Solutions for Scalable SARS-CoV-2 Testing to Inform Return to Full Capacity Strategy in United States Air Force Workforce Personnel

SARS-CoV-2 has highlighted the requirement for a drastic change in pandemic response. While cases continue to rise, there is an urgent need to deploy sensitive and rapid testing in order to identify potential outbreaks before there is an opportunity for further community spread. Currently, reverse transcription quantitative polymerase chain reaction (RT-qPCR) is considered the gold standard for diagnosing an active infection, using a nasopharyngeal swab; however, it can take days after symptoms develop to properly identify and trace the infection. While many civilian jobs can be performed remotely, the Department of Defense (DOD) is by nature a very fluid organization which requires in-person interaction and a physical presence to maintain effectiveness. In this commentary, we examine several current and emergent technologies and their ability to identify both active and previous SARS-CoV-2 infection, possibly in those without symptoms. Further, we will explore an ongoing study at the Air Force Research Laboratory, utilizing Reverse Transcription Loop-mediated isothermal amplification (RT-LAMP), next-generation sequencing, and the presence of SARS-CoV-2 antibodies through Lateral Flow Immunoassays. The ability to identify SARS-CoV-2 through volatile organic compound biomarker identification will also be explored. By exploring and validating multiple testing strategies, and contributing to Operation Warp Speed, the DOD is postured to respond to SARS-CoV-2, and future pandemics.

Achievements and Challenges for Real-Time Sensing of Analytes in Sweat within Wearable Platforms

Physiological sensors in a wearable form have rapidly emerged on the market due to technological breakthroughs and have become nearly ubiquitous with the Apple Watch, FitBit, and other wearable devices. While these wearables mostly monitor simple biometric signatures, new devices that can report on the human readiness level through sensing molecular biomarkers are critical to optimizing the human factor in both commercial sectors and the Department of Defense. The military is particularly interested in real-time, wearable, minimally invasive monitoring of fatigue and human performance to improve the readiness and performance of the war fighter. However, very few devices have ventured into the realm of reporting directly on biomarkers of interest. Primarily this is because of the difficulties of sampling biological fluids in real-time and providing accurate readouts using highly selective and sensitive sensors. When additional restrictions to only use sweat, an excretory fluid, are enforced to minimize invasiveness, the demands on sensors becomes even greater due to the dilution of the biomarkers of interest, as well as variability in salinity, pH, and other physicochemical variables which directly impact the read-out of real-time biosensors. This Account will provide a synopsis not only on exemplary demonstrations and technological achievements toward implementation of real-time, wearable sweat sensors but also on defining problems that still remain toward implementation in wearable devices that can detect molecular biomarkers for real world applications. First, the authors describe the composition of minimally invasive biofluids and then identify what biomarkers are of interest as biophysical indicators. This Account then reviews demonstrated techniques for extracting biofluids from the site of generation and transport to the sensor developed by the authors. Included in this discussion is a detailed description on biosensing recognition elements and transducers developed by the authors to enable generation of selective electrochemical sensing platforms. The authors also discuss ongoing efforts to identify biorecognition elements and the chemistries necessary to enable high affinity, selective biorecognition elements. Finally, this Account presents the requirements for wearable, real-time sensors to be (1) highly stable, (2) portable, (3) reagentless, (4) continuous, and (5) responsive in real-time, before delving into specific methodologies to sense classes of biomarkers that have been explored by academia, government laboratories, and industry. Each platform has its areas of greatest utility, but also come with corresponding weaknesses: (1) ion selective electrodes are robust and have been demonstrated in wearables but are limited to detection of ions, (2) enzymatic sensors enable indirect detection of metabolites and have been demonstrated in wearables, but the compounds that can be detected are limited to a subset of small molecules and the sensors are sensitive to flow, (3) impedance-based sensors can detect a wide range of compounds but require further research and development for deployment in wearables. In conclusion, while substantial progress has been made toward wearable molecular biosensors, substantial barriers remain and need to be solved to enable deployment of minimally invasive, wearable biomarker monitoring devices that can accurately report on psychophysiological status.

Chemically Enhanced Polymer-Coated Carbon Nanotube Electronic Gas Sensor for Isopropyl Alcohol Detection

Breathing-air quality within commercial airline cabins has come under increased scrutiny because of the identification of volatile organic compounds (VOCs) from the engine bleed air used to provide oxygen to cabins. Ideally, a sensor would be placed within the bleed air pipe itself, enabling detection before it permeated through and contaminated the entire cabin. Current gas-phase sensors suffer from issues with selectivity, do not have the appropriate form factor, or are too complex for commercial deployment. Here, we chose isopropyl alcohol (IPA), a main component of de-icer spray used in the aerospace community, as a target analyte: IPA exposure has been hypothesized to be a key component of aerotoxic syndrome in pre, during, and postflight. IPAs proposed mechanism of action is that of an anesthetic and central nervous system depressant. In this work, we describe IPA sensor development by showing (1) the integration of a polymer as an IPA capture matrix, (2) the adoption of a redox chemical additives as an IPA oxidizer, and (3) the application of carbon nanotubes as an electronic sensing conduit. We demonstrate the ability to not only detect IPA at 100-10 000 ppm in unfiltered, laboratory air but also discriminate among IPA, isoprene, and acetone, especially in comparison to a typical photoionization detector. Overall, we show an electronic device that operates at room temperature and responds preferentially to IPA, where the increase in the resistance corresponds directly to the concentration of IPA. Ultimately, this study opens up the pathway to selective electronic sensors that can enable real-time monitoring in a variety of environments for the force health prevention and protection, and the potential through future work to enable low parts-per-million and possibly high parts-per-billion selective detection of gas-phase VOCs of interest.

Super-Absorbent Polymer Valves and Colorimetric Chemistries for Time-Sequenced Discrete Sampling and Chloride Analysis of Sweat via Skin-Mounted Soft Microfluidics

This paper introduces super absorbent polymer valves and colorimetric sensing reagents as enabling components of soft, skin-mounted microfluidic devices designed to capture, store, and chemically analyze sweat released from eccrine glands. The valving technology enables robust means for guiding the flow of sweat from an inlet location into a collection of isolated reservoirs, in a well-defined sequence. Analysis in these reservoirs involves a color responsive indicator of chloride concentration with a formulation tailored to offer stable operation with sensitivity optimized for the relevant physiological range. Evaluations on human subjects with comparisons against ex situ analysis illustrate the practical utility of these advances.

Cross-Reactive Plasmonic Aptasensors for Controlled Substance Identification

In this work, we developed an assay to determine if an arbitrary white powder is a controlled substance, given the plasmonic response of aptamer-gold nanoparticle conjugates (Apt-AuNPs). Toward this end, we designed Apt-AuNPs with specific a response to common controlled substances without cross reactivity to chemicals typically used as fillers in street formulations. Plasmonic sensor variation was shown to produce unique data fingerprints for each chemical analyzed, supporting the application of multivariate statistical techniques to annotate unknown samples by chemical similarity. Importantly, the assay takes less than fifteen minutes to run, and requires only a few micrograms of the material, making the proposed assay easily deployable in field operations.

Storage stability of exhaled breath on Tenax TA

Exhaled breath is coming to the forefront of non-invasive biomarker discovery efforts. Concentration of exhaled breath volatile organic compounds (VOCs) on thermal desorption (TD) tubes with subsequent analysis by gas chromatography-mass spectrometry (GC-MS) has dominated this field. As discovery experimentation increases in frequency, the need to evaluate the long-term storage stability of exhaled breath VOCs on thermal desorption adsorbent material is critical. To address this gap, exhaled breath was loaded on Tenax TA thermal desorption tubes and stored at various temperature conditions. 74 VOCs, 56 of which have been previously uncharacterized, were monitored using GC-MS over a period of 31 d. The results suggest that storage of exhaled breath at cold temperatures (4 °C) provides the most consistent retention of exhaled breath VOCs temporally. Samples were determined to be stable up to 14 d across storage conditions prior to gaining or losing 1-2 standard deviations in abundance. Through gene set enrichment analysis (GSEA), certain chemical classes were found to be positively (acids) or negatively (sulfur-containing) enriched temporally. By means of field sample collections, the effect of storage and shipping was found to be similar to those studies preformed in the laboratory at 4 °C. Collectively this study not only provides recommendations for proper storage conditions and storage length, but also illustrates the use of GSEA to exhaled breath based GC-MS data.

The identification of hypoxia biomarkers from exhaled breath under normobaric conditions

Pilots have reported experiencing in-flight hypoxic-like symptoms since the inception of high-altitude aviation. As a result, the need to monitor pilots, in-flight, for the onset of hypoxic conditions is of great interest to the aviation community. We propose that exhaled breath is an appropriate non-invasive medium for monitoring pilot hypoxic risk through volatile organic compound (VOC) analysis. To identify changes in the exhaled breath VOCs produced during periods of reduced O2 levels, volunteers were exposed to simulated flight profiles, i.e. sea level for 5 min, O2 levels found at elevated altitudes for 5 min or placebo and 5 min at 100% O2 recovery gas, using a modified flight mask interfaced with a reduced O2 breathing device. During the course of these test events, time series breath samples from the flight mask and pre/post bag samples were collected and analyzed by gas chromatography/mass spectrometry (GC/MS). Seven compounds (pentanal, 4-butyrolactone, 2-pentanone, 2-hexanone, 2-cyclopenten-1-one, 3-methylheptane and 2-heptanone) were found to significantly change in response to hypoxic conditions. Additionally, the isoprene, 2-methyl-1,3-butadiene, was found to increase following the overall exposure profile. This study establishes an experimental means for monitoring changes in VOCs in response to hypoxic conditions, a computational workflow for compound analysis via the Metabolite Differentiation and Discovery Lab and MatLab(©) software and identifies potential volatile organic compound biomarkers of hypoxia exposure.

Evaluation of Bio-VOC Sampler for Analysis of Volatile Organic Compounds in Exhaled Breath

Monitoring volatile organic compounds (VOCs) from exhaled breath has been used to determine exposures of humans to chemicals. Prior to analysis of VOCs, breath samples are often collected with canisters or bags and concentrated. The Bio-VOC breath sampler, a commercial sampling device, has been recently introduced to the market with growing use. The main advantage for this sampler is to collect the last portion of exhaled breath, which is more likely to represent the air deep in the lungs. However, information about the Bio-VOC sampler is somewhat limited. Therefore, we have thoroughly evaluated the sampler here. We determined the volume of the breath air collected in the sampler was approximately 88 mL. When sampling was repeated multiple times, with the succeeding exhalations applied to a single sorbent tube, we observed linear relationships between the normalized peak intensity and the number of repeated collections with the sampler in many of the breath VOCs detected. No moisture effect was observed on the Tenax sorbent tubes used. However, due to the limitation in the collection volume, the use of the Bio-VOC sampler is recommended only for detection of VOCs present at high concentrations unless repeated collections of breath samples on the sampler are conducted.

Changes in volatile compounds of human urine as it ages: their interaction with water

The urinary odors are commonly perceived as unpleasant. While numerous studies have identified the volatile organic compounds (VOCs) released from urine, the odorants responsible for the urine odor are not well characterized. Furthermore, anecdotal reports suggest that the odor of aged urine is different from that of fresh urine. However, no study has yet to investigate the specific VOCs released from aged urine. In this study, we analyzed and compared the VOCs released from fresh and aged urine samples, investigating the changes in the urinary VOCs as urine aged. We found an overall decrease in concentration of many urinary VOCs, and concluded this was due to the urine evaporating as it aged. On the contrary, some highly water-soluble compounds such as short and branched-chain organic acids and trimethylamine, increased. Their increased release is most likely due to the loss of water and the subsequent release of water-soluble VOCs as urine ages. We suggest that these VOCs may contribute to the odor of the aged urine.

Changes in volatile compounds of mouse urine as it ages: their interactions with water and urinary proteins

Mice release a variety of chemical signals, particularly through urine, which mediate social interactions and endocrine function. Studies have been conducted to investigate the stability of urinary chemosignals in mice. Neuroendocrine and behavioral responses of mice to urine samples of male and female conspecifics which have aged for different amounts of time have been examined, demonstrating that the quality and intensity of signaling molecules in urine change over time. In this study, we monitored changes in volatile organic compounds (VOCs) released from male and female mouse urine following aging the urine samples. Substantial amounts of some VOCs were lost during the aging process of urine, whereas other VOCs increased. Considerable portions of the VOCs which exhibited the increased release were shown to have previously been dissolved in water and subsequently released as the urine dried. We also demonstrated that some VOCs decreased slightly due to their binding with the major urinary proteins (MUPs) and identified MUP ligands whose headspace concentrations increased as the urine aged. Our results underscore the important role of MUPs and the hydration status in the release of VOCs in urine, which may largely account for the changes in the quality and intensity of urinary signals over time.

Metabolite differentiation and discovery lab (MeDDL): a new tool for biomarker discovery and mass spectral visualization

The goal of this work was to design and implement a prototype software tool for the visualization and analysis of small molecule metabolite GC-MS and LC-MS data for biomarker discovery. The key features of the Metabolite Differentiation and Discovery Lab (MeDDL) software platform include support for the manipulation of large data sets, tools to provide a multifaceted view of the individual experimental results, and a software architecture amenable to modification and addition of new algorithms and software components. The MeDDL tool, through its emphasis on visualization, provides unique opportunities by combining the following: easy use of both GC-MS and LC-MS data; use of both manufacturer specific data files as well as netCDF (network Common Data Form); preprocessing (peak registration and alignment in both time and mass); powerful visualization tools; and built in data analysis functionality.

Age-dependent partition coefficients for a mixture of volatile organic solvents in Sprague-Dawley rats and humans

The absorption, distribution, metabolism, and excretion of volatile organic compounds (VOCs) are critically determined by a few chemical-specific factors, notably their blood and tissue partition coefficients (PC) and metabolism. Age-specific values for PCs in rats have rarely been reported or utilized in pharmacokinetic modeling for predicting dosimetry in toxicity studies with rats progressing through their lifestages. A mixture of six VOCs (benzene, chloroform, methyl ethyl ketone, methylene chloride, trichloroethylene, and perchloroethylene) was used to determine blood:air and tissue:air PCs in rats at three different ages (postnatal d 10, 60 d, and 2 yr) and blood:air PCs in pediatric and adult human blood. No differences with age in human blood:air PCs for the six compounds were observed. Rat blood:air PCs increased with age varying with compound. Tissue:air PCs showed tissue-specific changes with age. Water-soluble methyl ethyl ketone showed no age-dependent differences. Partition coefficients, particularly the blood:air PC, are key determinants of the rodent and human blood concentrations; age-appropriate values improve the accuracy of pharmacokinetic model predictions of population variability and age-specific exposures.