iHWG-MOX: A Hybrid Breath Analysis System via the Combination of Substrate-Integrated Hollow Waveguide Infrared Spectroscopy with Metal Oxide Gas Sensors

SARS CoV-2 infection screening via the exhaled breath fingerprint obtained by FTIR spectroscopic gas-phase analysis. A proof of concept

The COVID-19 pandemic remains a global challenge now with the long-COVID arising. Mitigation measures focused on case counting, assessment and determination of variants and their likely targets of infection and transmission, the pursuit of drug treatments, use and enhancement of masks, social distancing, vaccination, post-infection rehabilitation, and mass screening. The latter is of utmost importance given the current scenario of infections, reinfections, and long-term health effects. Research on screening platforms has been developed to provide more sensitive, specific, and reliable tests that are accessible to the entire population and can be used to assess the prognosis of the disease as well as the subsequent health follow-up of patients with sequelae of COVID-19. Therefore, the aim of the present study was the simulation of exhaled breath of COVID-19 patients by evaluation of three identified COVID-19 indicator breath biomarkers (acetone (ACE), acetaldehyde (ACH) and nitric oxide (NO)) by gas-phase infrared spectroscopy as a proof-of-concept principle for the detection of infected patients' exhaled breath fingerprint and subsequent follow-up. The specific fingerprints of each of the compounds and the overall fingerprint were obtained. The synthetic exhaled breath evaluation concept revealed a linearity of r = 0.99 for all compounds, and LODs of 6.42, 13.81, 9.22 ppm, and LOQs of 42.26, 52.57, 69.23 ppm for NO, ACE, and ACH, respectively. This study proves the fundamental feasibility of gas-phase infrared spectroscopy for fingerprinting lung damage biomarkers in exhaled breath of patients with COVID-19. This analysis would allow faster and cheaper screening and follow-up of infected individuals, which could improve mass screening in POC settings.

Pattern Recognition with Temperature Regulation: A Single YSZ-Based Mixed Potential Sensor Classifies Multiple Mixtures of Isoprene, n-Propanol, and Acetone

Gas sensors integrated with machine learning algorithms have aroused keen interest in pattern recognition, which ameliorates the drawback of poor selectivity on a sensor. Among various kinds of gas sensors, the yttria-stabilized zirconia (YSZ)-based mixed potential-type sensor possesses advantages of low cost, simple structure, high sensitivity, and superior stability. However, as the number of sensors increases, the increased power consumption and more complicated integration technology may impede their extensive application. Herein, we focus on the development of a single YSZ-based mixed potential sensor from sensing material to machine learning for effective detection and discrimination of unary, binary, and ternary gas mixtures. The sensor that is sensitive to isoprene, n-propanol, and acetone is manufactured with the MgSb2O6 sensing electrode prepared by a simple sol-gel method. Unique response patterns for specific gas mixtures could be generated with temperature regulation. We chose seven algorithm models to be separately trained for discrimination. In order to realize more accurate discrimination, we further discuss the selection of suitable feature parameters and its reasons. With temperature regulation coefficients which are easily available as feature input to model, a single sensor is verified to achieve elevated accuracy rates of 95 and 99% for the discrimination of seven gases (three unary gases, three binary gas mixtures, and one ternary gas mixture) and redefined six gas mixtures. This article provides a potential new approach via a mixed potential sensor instead of a sensor array that could provide a wide application prospect in the field of electronic nose and artificial olfaction.

Flexible terahertz gas sensing platform based on substrate-integrated hollow waveguides and an opto-electronic light source

We report on a flexible platform for molecular sensing in the terahertz range. Merging the well-established technologies of near-infrared electro-optic modulation and photomixing realizes a spectrally adaptable terahertz source, which is combined with a new generation of compact gas cells, so-called substrate-integrated hollow waveguides (iHWGs). iHWGs have been developed in the mid-infrared and provide flexibility in the design of the optical absorption path. Here, we demonstrate its suitability for the terahertz domain by presenting its low propagation losses and by measuring rotational transitions of nitrious oxide (N₂O). A fast frequency sideband modulation technique results in substantially reduced measurement times and increased accuracy compared to a standard wavelength tuning method.

Monitoring Ozone Using Portable Substrate-Integrated Hollow Waveguide-Based Absorbance Sensors in the Ultraviolet Range

Ozone is an oxidizing molecule used for disinfecting a wide variety of environments, such as in dental clinics, and has most recently been promoted as a sanitizing agent to prevent coronavirus transmission. The easy access to ozone-generating sources also enables their ubiquitous use. However, exposure to ozone may seriously affect human health by amplifying or inducing respiratory diseases and distress syndromes and has been associated with premature deaths from other diseases. In this scenario, miniaturized, low-cost, and portable optical sensors based on the absorption signature of ozone in the ultraviolet (UV) range of the electromagnetic spectrum are an innovative approach for providing real-time monitoring of gaseous ozone, ensuring the safety of indoor and workplace environments. In this paper, a miniaturized ozone sensor based on the absorption signature of ozone at deep-UV frequencies was developed by integration of so-called substrate-integrated hollow waveguides (iHWG) with a miniaturized ultraviolet lamp and a fiber-optic USB-connected spectrophotometer. The innovative concept of iHWGs facilitates unprecedented compact dimensions with a high degree of flexibility in the optical design of the actual photon absorption path. The proposed device rapidly responded to the presence of ozone (<1 min) and revealed a suitable linearity (r ² > 0.99) in the evaluated concentration range. The limit of detection was determined at 29.4 ppbv, which renders the device suitable for measurements in the threshold range of the main regulatory agencies. Given the adaptability and modularity of this platform, we anticipate the application of this innovative concept to be equally suitable for the in situ and real-time analysis of other relevant gases providing suitable UV absorption signatures.

From Light Pipes to Substrate-Integrated Hollow Waveguides for Gas Sensing: A Review

Absorption-based spectroscopy in the mid-infrared (MIR) spectral range (i.e., 2.5-25 μm) is an excellent choice for directly sensing trace gas analytes providing discriminatory molecular information due to inherently specific fundamental vibrational, rovibrational, and rotational transitions. Complimentarily, the miniaturization of optical components has aided the utility of optical sensing techniques in a wide variety of application scenarios that demand compact, portable, easy-to-use, and robust analytical platforms yet providing suitable accuracy, sensitivity, and selectivity. While MIR sensing technologies have clearly benefitted from the development of advanced on-chip light sources such as quantum cascade and interband cascade lasers and equally small MIR detectors, less attention has been paid to the development of modular/tailored waveguide technologies reproducibly and reliably interfacing photons with sample molecules in a compact format. In this context, the first generation of a new type of hollow waveguides gas cells-the so-called substrate-integrated hollow waveguides (iHWG)-with unprecedented compact dimensions published by the research team of Mizaikoff and collaborators has led to a paradigm change in optical transducer technology for gas sensors. Features of iHWGs included an adaptable (i.e., designable) well-defined optical path length via the integration of meandered hollow waveguide structures at virtually any desired dimension and geometry into an otherwise planar substrate, a high degree of robustness, compactness, and cost-effectiveness in fabrication. Moreover, only a few hundred microliters of gas samples are required for analysis, resulting in short sample transient times facilitating a real-time monitoring of gaseous species in virtually any concentration range. In this review, we give an overview of recent advancements and achievements since their introduction eight years ago, focusing on the development of iHWG-based mid-infrared sensor technologies. Highlighted applications ranging from clinical diagnostics to environmental and industrial monitoring scenarios will be contrasted by future trends, challenges, and opportunities for the development of next-generation portable optical gas-sensing platforms that take advantage of a modular and tailorable device design.

Recent technological and scientific developments concerning the use of infrared spectroscopy for point-of-care applications

In this contribution we review selected point-of-care applications of infrared spectroscopy and the technological innovations they are based on. After a short introduction summarizing the general idea behind point-of-care applications we introduce the reader to important infrared spectroscopy sensing principles on a very basic level. We discuss the role of optical components like quantum cascade lasers, supercontinuum sources, waveguides and how they are potentially going to revolutionize point-of-care applications. First, we focus on the technological solutions of some principal problems like increasing the pathlength in a transmission cell to enhance the sensitivity for solutes in aqueous solutions and discuss indirect methods which circumvent the problem of low transmittance. In the second part we show how the technological progress of the last decades enabled scientific progress leading to selected concrete and outstanding point-of-care solutions and applications based on infrared spectroscopy. These include the detection and quantification of malaria parasitemia, early recognition of Alzheimer's disease long before the onset of clinical symptoms and a non-invasive method for testing the blood glucose content. The selected examples demonstrate and showcase that infrared spectroscopy is on the way to become an indispensable technique for point-of-care applications.

Metal oxide-based gas sensors for detection of exhaled breath markers

Exhaled breath test is a typical disease monitoring method for replacing blood and urine samples that may create discomfort for patients. To monitor exhaled breath markers, gas biomedical sensors have undergone rapid progress for non‐invasive and point‐of‐care diagnostic devices. Among gas sensors, metal oxide‐based biomedical gas sensors have received remarkable attention owing to their unique properties, such as high sensitivity, simple fabrication, miniaturization, portability and real‐time monitoring. Herein, we reviewed the recent advances in chemoresistive metal oxide‐based gas sensors with ZnO, SnO₂ and In₂O₃ as sensing materials for monitoring a range of exhaled breath markers (i.e., NO, H₂, H₂S, acetone, isoprene and formaldehyde). We focused on the strategies that improve the sensitivity and selectivity of metal oxide‐based gas sensors. The challenges to fabricate a functional gas sensor with high sensing performance along with suggestions are outlined.

An eNose-based method performing drift correction for online VOC detection under dry and humid conditions

Our recently demonstrated innovative concept of electronic nose (eNose) based on a combination of gas sensors is further tested and benchmarked in the present study. The system is a test bed for gas sensors of different principal technologies distributed within three compartments, which share a compact, very low volume, temperature-controlled sensing chamber. Here, the eNose-based analyser contains three sensing arrays of commercially available semiconducting metal oxide (MOX) gas sensors: one compartment contains 8 analog MOX sensors, while the other two compartments comprise 10 digital MOX sensors. The presented instrument is explicitly tested for the discrimination between mid-range (3-18 ppm) concentrations of different volatile organic compounds (VOCs) including acetaldehyde, acetone, ethanol, ethyl acetate, isoprene and n-pentane under dry and humid conditions, which are all considered relevant gases in future breath diagnostic applications. Since the experiments were performed in periods of time separated by around 20 days, they are affected by drift. For this reason, we explore the opportunity of drift mitigation using methods based on component removal computed by linear discriminant analysis, partial least squares discriminant analysis and direct orthogonalization, which lend themselves to future in-field applications of the developed device and sensing methodology.