Real-time monitoring of ozone in air using substrate-integrated hollow waveguide mid-infrared sensors

Parametric investigation and RSM optimization of DBD plasma methods (direct & indirect) for H2S conversion in the air

Hydrogen sulfide (H2S) is known as a harmful pollutant for the environment and human health, and its emission control is a high priority. Non-thermal plasma is an effective technology in this field. In this study, for the first time, the performance of direct and indirect H2S plasma conversion methods was compared, optimized, and modeled with the CCD method. H2S was diluted in zero air, and the study investigated the effect of discharge power, relative humidity, total flow rate, initial H2S concentration, and their interactions. ANOVA results showed that the models for H2S conversion efficiency and energy yield were significant and efficient. The direct method achieved a maximum conversion efficiency of 56 % and energy yield of 3.43 g/kWh, while the indirect method produced 68 % conversion efficiency and 1.59 g/kWh energy yield. According to the process optimization results, the direct conversion method is more optimal than the indirect conversion method due to the presence of active species and high-energy electrons in the plasma treatment, and it is a better choice if there are suitable working conditions.

Mid-infrared pulsed Er:ZBLAN fiber laser producing mode-switchable cylindrical vector beams

We demonstrate the generation of both continuous-wave (CW) and Q-switched cylindrical vector beams (CVBs) from a mid-infrared Er3+-doped ZBLAN (Er:ZBLAN) fiber laser at ∼ 2.8 µm. A customized S-waveplate is incorporated as the intracavity mode converter to achieve the mid-infrared CVBs. Switchable modes of CVBs between the radially and azimuthally polarized beam can be realized easily by manipulating the cavity conditions. A maximum output power of ∼250 mW is achieved for the CW CVBs. In the short-pulsed CVBs operation regime, both the active and passive Q-switching modes are realized with a pulse duration of hundreds of nanoseconds. The proposed mid-infrared cylindrical vector lasers can have significant potential for applications in biomedicine, optical trapping, material processing and optical communication.

Structural stability and optical properties of 1D photonic crystals based on porous anodic alumina after annealing at different temperatures

Porous anodic alumina (PAA) photonic crystals with a photonic stop-band (PSB) placed in the mid-infrared (MIR) spectral region represent a promising approach for increasing of gas sensors sensitivity. An onion-like layered distribution of anionic impurities is a hallmark of PAA, and its presence is generally considered to demarcate the boundary between transparent and opaque ranges in the infrared spectral region. Here, we study the effect of annealing in the temperature range of 450 °C-1 100 °C on the structural stability and optical properties in photonic crystals based on PAA fabricated by pulse anodization in oxalic acid. Pulse sequences were selected in a way to obtain photonic crystals of different periodic structures with a PSB located in visible and MIR spectral regions. The first photonic crystal was composed of layers with gradually changing porosity, whereas the second photonic crystal consisted of a sequentially repeated double-layer unit with an abrupt change in porosity. We investigated the response of alumina with rationally designed porosities and different arrangements of porous layers for high-temperature treatment. The microstructure (scanning electron microscopy), phase composition (x-ray diffraction), and optical properties (optical spectroscopy) were analysed to track possible changes after annealing. Both photonic crystals demonstrated an excellent structural stability after 24 h annealing up to 950 °C. At the same time, the evaporation of the anionic impurities from PAA walls caused a shift of the PSB towards the shorter wavelengths. Furthermore, the annealing at 1 100 °C induced a high transparency (up to 90%) of alumina in MIR spectral region. It was shown thus that properly selected electrochemical and annealing conditions enable the fabrication of porous photonic crystals with the high transparency spanning the spectral range up to around 10μm.

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.

Innovative Substrate-Integrated Hollow Waveguide Coupled Attenuated Total Reflection Sensors for Quantum Cascade Laser Based Infrared Spectroscopy in Harsh Environments

An innovative mid-infrared spectroscopic sensor system based on quantum cascade lasers has been developed. The system combines the versatility of substrate-integrated hollow waveguides (IHWGs) with the robustness of attenuated total reflection (ATR) crystals employed as internal reflection waveguides for evanescent field sensing. IHWGs are highly reflective metal structures that propagate infrared (IR) radiation and were used as light pipes for coupling radiation into the ATR waveguide. The combined IHWG-ATR device has been designed such that the utmost stability and robustness of the optical alignment were ensured. This novel assembly enables evanescent field absorption measurements at yet unprecedently harsh conditions, that is, high pressure and temperature. Combining these advantages, this innovative sensor assembly is perfectly suited for taking ATR spectroscopy into the field where the robustness of the assembly and optical alignment is essential.

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.

Efficient facemask decontamination via forced ozone convection

The COVID-19 crisis has taken a significant toll on human life and the global economy since its start in early 2020. Healthcare professionals have been particularly vulnerable because of the unprecedented shortage of Facepiece Respirators (FPRs), which act as fundamental tools to protect the medical staff treating the coronavirus patients. In addition, many FPRs are designed to be disposable single-use devices, creating an issue related to the generation of large quantities of non-biodegradable waste. In this contribution, we describe a plasma-based decontamination technique designed to circumvent the shortages of FPRs and alleviate the environmental problems posed by waste generation. The system utilizes a Dielectric Barrier Discharge (DBD) to generate ozone and feed it through the fibers of the FPRs. The flow-through configuration is different than canonical ozone-based sterilization methods, in which the equipment is placed in a sealed ozone-containing enclosure without any flow through the mask polymer fibers. We demonstrate the rapid decontamination of surgical masks using Escherichia coli (E. coli) and Vesicular Stomatitis Virus (VSV) as model pathogens, with the flow-through configuration providing a drastic reduction in sterilization time compared to the canonical approach. We also demonstrate that there is no deterioration in mask structure or filtration efficiency resulting from sterilization. Finally, we show that this decontamination approach can be implemented using readily available tools, such as a plastic box, a glass tube, few 3D printed components, and the high-voltage power supply from a plasma globe toy. The prototype assembled for this study is portable and affordable, with effectiveness comparable to that of larger and more expensive equipment.

Sec-Eliminating the SARS-CoV-2 by AlGaN Based High Power Deep Ultraviolet Light Source

The world-wide spreading of coronavirus disease (COVID-19) has greatly shaken human society, thus effective and fast-speed methods of non-daily-life-disturbance sterilization have become extremely significant. In this work, by fully benefitting from high-quality AlN template (with threading dislocation density as low as ≈6×10⁸ cm^-2) as well as outstanding deep ultraviolet (UVC-less than 280 nm) light-emitting diodes (LEDs) structure design and epitaxy optimization, high power UVC LEDs and ultra-high-power sterilization irradiation source are achieved. Moreover, for the first time, a result in which a fast and complete elimination of SARS-CoV-2 (the virus causes COVID-19) within only 1 s is achieved by the nearly whole industry-chain-covered product. These results advance the promising potential in UVC-LED disinfection particularly in the shadow of COVID-19.

Gas Detection Using Portable Deep-UV Absorption Spectrophotometry: A Review

Several gas molecules of environmental and domestic significance exhibit a strong deep-UV absorption. Therefore, a sensitive and a selective gas detector based on this unique molecular property (i.e., absorption at a specific wavelength) can be developed using deep-UV absorption spectrophotometry. UV absorption spectrometry provides a highly sensitive, reliable, self-referenced, and selective approach for gas sensing. This review article addresses the recent progress in the application of deep-UV absorption for gas sensing owing to its inherent features and tremendous potentials. Applications, advancements, and challenges related to UV emission sources, gas cells, and UV photodetectors are assessed and compared. We present the relevant theoretical aspects and challenges associated with the development of portable sensitive spectrophotometer. Finally, the applications of UV absorption spectrometry for ozone, NO₂, SO₂, and aromatic organic compounds during the last decades are discussed and compared. A portable UV absorption spectrophotometer can be developed by using LEDs, hollow core waveguides (HCW), and UV photodetectors (i.e., photodiodes). LED provides a portable UV emission source with low power input, low-intensity drifts, low cost, and ease of alignment. It is a quasi-chromatic UV source and covers the absorption band of molecules without optical filters for absorbance measurement of a target analyte. HCWs can be applied as a miniature gas cell for guiding UV radiation for measurement of low gas concentrations. Photodiodes, on the other hand, offer a portable UV photodetector with excellent spectral selectivity with visible rejection, minimal dark current, linearity, and resistance against UV-aging.

Carbon Nanofiber-Based Functional Nanomaterials for Sensor Applications

Carbon nanofibers (CNFs) exhibit great potentials in the fields of materials science, biomedicine, tissue engineering, catalysis, energy, environmental science, and analytical science due to their unique physical and chemical properties. Usually, CNFs with flat, mesoporous, and porous surfaces can be synthesized by chemical vapor deposition and electrospinning techniques with subsequent chemical treatment. Meanwhile, the surfaces of CNFs are easy to modify with various materials to extend the applications of CNF-based hybrid nanomaterials in multiple fields. In this review, we focus on the design, synthesis, and sensor applications of CNF-based functional nanomaterials. The fabrication strategies of CNF-based functional nanomaterials by adding metallic nanoparticles (NPs), metal oxide NPs, alloy, silica, polymers, and others into CNFs are introduced and discussed. In addition, the sensor applications of CNF-based nanomaterials for detecting gas, strain, pressure, small molecule, and biomacromolecules are demonstrated in detail. This work will be beneficial for the readers to understand the strategies for fabricating various CNF-based nanomaterials, and explore new applications in energy, catalysis, and environmental science.

Dioxin and Related Compound Detection: Perspectives for Optical Monitoring

Dioxins and related compounds are environmental xenobiotics that are dangerous to human life, due to the accumulation and persistence in the environment and in the food chain. Cancer, reproductive and developmental issues, and damage to the immune system and endocrine system are only a few examples of the impact of such substances in everyday life. For these reasons, it is fundamental to detect and monitor these molecules in biological samples. The consolidated technique for analytical evaluation is gas chromatography combined with high-resolution mass spectrometry. Nowadays, the development of mid-infrared optical components like broadband laser sources, optical frequency combs, high performance Fourier-transform infrared spectroscopy, and plasmonic sensors open the way to new techniques for detection and real time monitoring of these organic pollutants in gaseous or liquid phase, with sufficient sensitivity and selectivity, and in short time periods. In this review, we report the latest techniques for the detection of dioxins, furans and related compounds based on optical and spectroscopic methods, looking at future perspectives.

A Hyphenated Preconcentrator-Infrared-Hollow-Waveguide Sensor System for N2O Sensing

Following the Kyoto protocol, all signatory countries must provide an annual inventory of greenhouse-gas emission including N₂O. This fact associated with the wide variety of sources for N₂O emissions requires appropriate sensor technologies facilitating in-situ monitoring, compact dimensions, ease of operation, and sufficient sensitivity for addressing such emission scenarios. In this contribution, we therefore describe an innovative portable mid-infrared chemical sensor system for quantifying gaseous N₂O via coupling a substrate-integrated hollow waveguide (iHWG) simultaneously serving as highly miniaturized mid-infrared photon conduit and gas cell to a custom-made preconcentrator. N₂O was collected onto a solid sorbent material packed into the preconcentrator unit, and then released via thermal desorption into the iHWG-MIR sensor utilizing a compact Fourier transform infrared (FTIR) spectrometer for molecularly selective spectroscopic detection with a limit of detection (LOD) at 5 ppbv. Highlighting the device flexibility in terms of sampling time, flow-rate, and iHWG design facilitates tailoring the developed preconcentrator-iHWG device towards a wide variety of application scenarios ranging from soil and aquatic emission monitoring and drone- or unmanned aerial vehicle (UAV)-mounted monitoring systems to clinical/medical analysis scenarios.

polyHWG: 3D Printed Substrate-Integrated Hollow Waveguides for Mid-Infrared Gas Sensing

Gas analysis via mid-infrared (MIR) spectroscopic techniques has gained significance due to its inherent molecular selectivity and sensitivity probing pronounced vibrational, rotational, and roto-vibrational modes. In addition, MIR gas sensors are suitable for real-time monitoring in a wide variety of sensing scenarios. Our research team has recently introduced so-called substrate-integrated hollow waveguides (iHWGs) fabricated by precision milling, which have been demonstrated to be useful in online process monitoring, environmental sensing, and exhaled breath analysis especially if low sample volumes (i.e., few hundreds of microliters) are probed with rapid signal transients. A logical next step is to establish ultralightweight, potentially disposable, and low-cost substrate-integrated hollow waveguides, which may be readily customized and tailored to specific applications using 3D printing techniques. 3D printing provides access to an unprecedented variety of thermoplastic materials including biocompatible polylactides, readily etchable styrene copolymers, and magnetic or conductive materials. Thus, the properties of the waveguide may be adapted to suit its designated application, e.g., drone-mounted ultralightweight waveguides for environmental monitoring or biocompatible disposable sensor interfaces in medical/clinical applications.

iHEART: a miniaturized near-infrared in-line gas sensor using heart-shaped substrate-integrated hollow waveguides

A novel heart-shaped substrate-integrated hollow waveguide (hiHWG) was integrated with a near-infrared micro-spectrometer (μNIR) for sensing natural gases, resulting in an ultra-compact near-infrared gas sensing system - iHEART. The iHEART system was evaluated using two different μNIR spectrometers, and the performance was compared with a laboratory NIR spectrometer for gas analysis based on an acousto-optic tunable filter (AOTF). The spectral data were pre-processed using the 1(st) derivative Savitzky-Golay algorithm, and then used for establishing multivariate regression models based on partial least squares (PLS). The root mean square errors of prediction (RMSEPs) obtained for major components of natural gas with both iHEART systems were similar to those associated with the AOTF spectrophotometer combined with a conventional long-path measurement cell. It was demonstrated that the iHEART system has significant potential for the development of compact in-line gas sensing systems, thus facilitating monitoring of (petro)chemically relevant processes and products. However, the flexibility and modularity of the system also allows tailoring iHEART to a wide range of other relevant analytical measurement scenarios requiring short response times and minute gas sample volumes.

Advances in Mid-Infrared Spectroscopy for Chemical Analysis

Infrared spectroscopy in the 3-20 μm spectral window has evolved from a routine laboratory technique into a state-of-the-art spectroscopy and sensing tool by benefitting from recent progress in increasingly sophisticated spectra acquisition techniques and advanced materials for generating, guiding, and detecting mid-infrared (MIR) radiation. Today, MIR spectroscopy provides molecular information with trace to ultratrace sensitivity, fast data acquisition rates, and high spectral resolution catering to demanding applications in bioanalytics, for example, and to improved routine analysis. In addition to advances in miniaturized device technology without sacrificing analytical performance, selected innovative applications for MIR spectroscopy ranging from process analysis to biotechnology and medical diagnostics are highlighted in this review.

Sensing hydrocarbons with interband cascade lasers and substrate-integrated hollow waveguides

Tunable diode laser absorption spectroscopy (TDLAS) is an excellent analytical technique for gas sensing applications.

Infrared spectroscopy via substrate-integrated hollow waveguides: a powerful tool in catalysis research

Monitoring catalyst performance via substrate-integrated hollow waveguide (iHWGs) assisted infrared spectroscopy.

Towards the determination of isoprene in human breath using substrate-integrated hollow waveguide mid-infrared sensors

Selected volatile organic compounds (VOCs) in breath may be considered biomarkers if they are indicative of distinct diseases or disease states. Given the inherent molecular selectivity of vibrational spectroscopy, infrared sensing technologies appear ideally suitable for the determination of endogenous VOCs in breath. The aim of this study was to determine that mid-infrared (MIR; 3-20 µm) gas phase sensing is capable of determining isoprene in exhaled breath as an exemplary medically relevant VOC by hyphenating novel substrate-integrated hollow waveguides (iHWG) with a likewise miniaturized preconcentration system. A compact preconcentrator column for sampling isoprene from exhaled breath was coupled to an iHWG serving simultaneously as highly miniaturized gas cell and light conduit in combination with a compact Fourier transform infrared spectrometer. A gas mixing system enabled extensive system calibration using isoprene standards. After system optimization, a calibration function obtaining a limit of quantification of 106 ppb was achieved. According to the literature, the obtained sensitivity is sufficient for quantifying middle to high isoprene concentrations occurring in exhaled breath. Finally, a volunteer breath sample was analysed proving comparable values of isoprene in a real-world scenario. Despite its fundamental utility, the proposed methodology contains some limitations in terms of sensitivity and temporal resolution in comparison with the readily available measurement techniques that should be addressed during future optimization of the system. Nonetheless, this study presents the first determination of endogenous VOCs in breath via advanced hollow waveguide MIR sensor technology, clearly demonstrating its potential for the analysis of volatile biomarkers in exhaled breath.

Monitoring of hydrogen sulfide via substrate-integrated hollow waveguide mid-infrared sensors in real-time

Hydrogen sulfide is a highly corrosive, harmful, and toxic gas produced under anaerobic conditions within industrial processes or in natural environments, and plays an important role in the sulfur cycle. According to the U.S. Occupational Safety and Health Administration (OSHA), the permissible exposure limit (during 8 hours) is 10 ppm. Concentrations of 20 ppm are the threshold for critical health issues. In workplace environments with human subjects frequently exposed to H2S, e.g., during petroleum extraction and refining, real-time monitoring of exposure levels is mandatory. Sensors based on electrochemical measurement principles, semiconducting metal-oxides, taking advantage of their optical properties, have been described for H2S monitoring. However, extended response times, limited selectivity, and bulkiness of the instrumentation are common disadvantages of the sensing techniques reported to date. Here, we describe for the first time usage of a new generation of compact gas cells, i.e., so-called substrate-integrated hollow waveguides (iHWGs), combined with a compact Fourier transform infrared (FTIR) spectrometer for advanced gas sensing of H2S. The principle of detection is based on the immediate UV-assisted conversion of the rather weak IR-absorber H2S into much more pronounced and distinctively responding SO2. A calibration was established in the range of 10-100 ppm with a limit of detection (LOD) at 3 ppm, which is suitable for occupational health monitoring purposes. The developed sensing scheme provides an analytical response time of less than 60 seconds. Considering the substantial potential for miniaturization using e.g., a dedicated quantum cascade laser (QCL) in lieu of the FTIR spectrometer, the developed sensing approach may be evolved into a hand-held instrument, which may be tailored to a variety of applications ranging from environmental monitoring to workplace safety surveillance, process analysis and clinical diagnostics, e.g., breath analysis.