Metal nitrides (MN) possess exceptional catalytic, electronic, and physical properties making them widely used in (opto‐)electronics and as hard coatings. When used as films in surface‐active applications, however, their performance remains limited by poor mass transfer and reduced accessibility of reactive sites. This is associated to compact film architecture yielded by conventional deposition techniques (e.g., sputtering). Herein, a template‐free method for the design of highly porous (≥84%) MN films with high compositional versatility, as shown for Cu3N, W2N, MoNx, and TiN, is demonstrated. These are obtained by conversion of fractal‐like metal oxide (MOx) agglomerated films through dry nitridation. In case of Cu3N, monocrystalline oxide nanoparticles are converted to polycrystalline nitrides, as traced by X‐ray diffraction and electron microscopy. Such films feature consistently lower resistances than their MOx counterparts, as well as high reactivity and mass transfer. This is exploited exemplarily for molecular sensing of NO2 at only 75 °C temperature, leading to up to a fivefold higher response with faster response time over more compact spin‐coated films. As a result, this approach overcomes critical mass transfer performance limitations of MN films that are also relevant for other applications like electrocatalysis and energy storage.

Wearable technologies that analyse non-conventional biological matrices, such as interstitial fluid, sweat, tears or breath, have the potential to provide longitudinal biomarker data with minimal invasiveness. These data could provide insights into physiological and behavioural patterns, in particular outside medical care facilities. Despite the success of continuous glucose monitoring, the adoption of wearable sensors for managing endocrine and metabolic diseases remains limited. This Perspective highlights five key challenges and proposes solutions. First, understanding the physiology of longitudinal biomarker profiles is crucial for uncovering rhythmic patterns and physiological interrelations in the prediction of health trajectories. Second, technical barriers currently hinder the continuous monitoring of most clinically relevant biomarkers. Third, machine learning models often struggle with the complexity of dense biomarker datasets, which increases the risk of spurious correlations. Fourth, the diagnostic value of wearable sensor data requires validation through clinical studies, and predicting treatment outcomes necessitates diverse and large patient cohorts over extended observation periods in real-world settings. Finally, most wearable devices function as isolated solutions. Thus, they lack interoperability and integration into clinical pathways, and often fail to incorporate context and user input. Addressing these challenges will be key for advancing the role of wearable sensors in endocrine and metabolic care in future health-care settings. Wearable molecular sensors hold promise for improving patient care; however, challenges in translating the technology to real-world settings remain. This Perspective outlines some of these challenges and discusses potential solutions, particularly in the context of clinical endocrinology.

The demand for highly functional chemical gas sensors has surged due to the increasing awareness of human health to monitor metabolic disorders or noncommunicable diseases, safety measures against harmful greenhouse and/or explosive gases, and determination of food freshness. Over the years of dedicated research, several types of chemiresistive gas sensors have been realized with appreciable sensitivities toward various gases. However, critical issues such as poor selectivity and sluggish response/recovery speeds continue to impede their widespread commercialization. Specifically, the mechanisms behind the selective response of some chemiresistive materials toward specific gas analytes remain unclear. In this review, we discuss state-of-the-art strategies employed to attain gas-selective chemiresistive materials, with particular emphasis on materials design, surface modification or functionalization with catalysts, defect engineering, material structure control, and integration with physical/chemical gas filtration media. The nature of material surface-gas interactions and the supporting mechanisms are elucidated, opening opportunities for optimizing the materials design, fine-tuning the gas sensing performance, and guiding the selection of the most appropriate materials for the accurate detection of specific gases. This review concludes with recommendations for future research directions and potential opportunities for further selectivity improvements.

Nitrogen dioxide (NO2) is a hazardous air pollutant with a lowered annual mean exposure limit from 40 to 10 µg/m3 (~5 parts-per-billion by volume, ppb) by the World Health Organization in 2021. This motivates the exploration of low-power and cost-efficient sensors that can detect such low concentrations of NO2, exhibit high selectivity against interfering analytes and resilience to humidity fluctuations. Here, a selective, stable and humidity-robust sensor for NO2 sensing at room temperature is presented. Flame-aerosol deposition followed by dry sulfidation results in highly porous (98%) and nanostructured WS2 films. These films exhibit a fivefold increase in response and over an order-of-magnitude reduction in response time, compared to conventional spin-coated films. Remarkable sensing performance down to 1 ppb of NO2 (with a signal-to-noise ratio of 12.9) is achieved with high selectivity (>164) towards environmental interferents including NH3, NO, acetone, H2S, benzene, CO, ethanol, methanol, N2O and toluene. We also reveal high robustness (response change ± 18%) against varying relative humidity (0 – 90%) and response stability over more than 6 months (± 10%). This sensor outperforms previously reported NO2 sensors operating at room temperature, making it well-suited for integration into devices for environmental monitoring or wearables for personal exposure assessment.

Metastable nanostructures are kinetically trapped in local energy minima featuring intriguing surface and material properties. To unleash their potential, there is a need for non-equilibrium processes capable of stabilizing a large range of crystal phases outside thermodynamic equilibrium conditions by closely and flexibly controlling atomic reactant composition, spatial temperature distribution and residence time. Here, we demonstrate the capture of metastable pseudo-binary metal oxides at room temperature with scalable combustion-aerosol processes. By a combination of X-ray diffraction, electron microscopy and online flame characterization, we investigate the occurrence of metastable CoCu$_2$O$_3$ with controlled crystal size (4-16 nm) over thermodynamically stable CuO and Co$_3$O$_4$. Immediate practical impact is demonstrated by exceptional sensing and catalytic performance for air pollutant detection (e.g., 15 parts-per-billion benzene). This approach can be extended to various binary, ternary and high entropy oxides with even more components to access novel materials also promising for actuators, energy storage or solar cells.

Dermal interstitial fluid (ISF) is a rich source of biomarkers (e.g., glucose) that can be used for continuous health monitoring with wearable sensors. Hollow microneedle devices are a promising solution to extract ISF on demand by penetrating the skin with minimal pain. However, they rely on inserting bio-incompatible materials (e.g., silicon) into individuals, limiting the application time. Here, the direct 3D printing of polymer hollow microneedles on silicon-based microfluidic devices and the successful in-vivo extraction of ISF are demonstrated. Our additive manufacturing approach enables the versatile combination of materials and rapid prototyping of microneedle geometry. After improving the design through finite element modeling, a hollow microneedle geometry was printed by two-photon polymerization and experimentally characterized with mechanical and fluidic tests. Microneedles were fabricated with high accuracy (i.e., 997 +/- 2 um) and reliably interfaced with the microfluidic chip (i.e., centerline alignment within 5 The needles demonstrated sufficient mechanical strength (i.e., 411 +/- 3 mN per needle) to endure at least 10 consecutive insertions into simulated skin. Biocompatibility and ISF extraction were demonstrated in an in-vivo 72-hour test, showing the safety and reliability of our approach. Such a platform is promising for minimally invasive, continuous monitoring of biomarkers in ISF, aiding in medical diagnoses and personalized health treatments.

Polymeric bags are a widely applied, simple, and cost-effective method for the storage and offline analysis of gaseous samples. Various materials have been used as sampling bags, all known to contain impurities and differing in their cost, durability, and storage capabilities. Herein, we present a comparative study of several well-known bag materials, Tedlar (PVF), Kynar (PVDF), Teflon (PTFE), and Nalophan (PET), as well as a new material, ethylene vinyl copolymer (EVOH), commonly used for storing food. We investigated the influences of storage conditions, humidity, bag cleaning, and light exposure on volatile organic compound concentration (acetone, acetic acid, isoprene, benzene, limonene, among others) in samples of exhaled human breath stored in bags for up to 48 h. Specifically, we show high losses of short-chain fatty acids (SCFAs) in bags of all materials (for most SCFAs, less than 50% after 8 h of storage). We found that samples in Tedlar, Nalophan, and EVOH bags undergo changes in composition when exposed to UV radiation over a period of 48 h. We report high initial impurity levels in all the bags and their doubling after a period of 48 h. We compare secondary electrospray ionization and proton transfer reaction mass spectrometry in the context of offline analysis after storage in sampling bags. We provide an analytical perspective on the temporal evolution of bag contents by presenting the intensity changes of all significantm/zfeatures. We also present a simple, automated, and cost-effective offline sample introduction system, which enables controlled delivery of collected gaseous samples from polymeric bags into the mass spectrometer. Overall, our findings suggest that sampling bags exhibit high levels of impurities, are sensitive to several environmental factors (e.g. light exposure), and provide low recoveries for some classes of compounds, e.g. SCFAs.

Methanol is a toxic alcohol contained in alcoholic beverages as a natural byproduct of fermentation or added intentionally to counterfeits to increase profit. To ensure consumer safety, many countries and the EU have established strict legislation limits for methanol content. Methanol concentration is mostly detected by laboratory instrumentation since mobile devices for routine on-site testing of beverages in distilleries, at border stations or even at home are not available. Here, we validated a handheld methanol detector for beverage analysis in an ISO 5725 interlaboratory trial: A total of 119 measurements were performed by 17 independent participants (distilleries, universities, authorities, and competence centers) from six countries on samples with relevant methanol (0.1, 1.5 vol%). The detector was based on a microporous separation filter and a nanostructured gas sensor allowing on-site measurement of methanol down to 0.01 vol% (in the liquid) within only 2 min by laymen. The detector showed excellent repeatability (<5.4%), reproducibility (<9.5%) and small bias (<0.012 vol%). Additional measurements on various methanol-spiked alcoholic beverages (whisky, rum, gin, vodka, tequila, port, sherry, liqueur) indicated that the detector is not interfered by environmental temperature and spirit composition, featuring excellent linearity (Pearson’s R²>0.99) down to methanol concentrations of 0.01 vol%. This device has been recently commercialized (Alivion Spark M-20) with comparable accuracy to the gold-standard gas chromatography and can be readily applied for final product inspection, intake control of raw materials or to identify toxic counterfeit products.

Humanity endeavors to resume crewed missions to the Moon and prepares for the exploration of Mars. These missions will require sustained human presence in space for longer periods than ever before. Space exposes astronauts to demanding conditions, including microgravity, radiation, rapid light-dark cycles, and hazardous chemicals. Gas sensors will be pivotal in preserving astronaut health by providing critical health data (e.g., through breath analysis) and space-resolved environmental information. Here, we explore the recent progress of gas sensors to meet the key needs of space exploration. First, the fundamental sensing principles of electrochemical, chemoresistive, mass-sensitive, and optical sensors are briefly introduced. Then, we connect space-related health challenges with suitable breath markers and sensor solutions, encompassing areas like gut microbiome, muscle activity, cardiovascular health, hepatic and renal function, and circadian rhythm. Finally, environmental exposure guidelines and suitable sensor innovations for distributed air quality monitoring in space vehicles and habitats are presented.

Metastable nanostructures are kinetically trapped in local energy minima featuring intriguing surface and material properties. To unleash their potential, there is a need for non-equilibrium processes capable of stabilizing a large range of crystal phases outside thermodynamic equilibrium conditions by closely and flexibly controlling atomic reactant composition, spatial temperature distribution and residence time. Here, the capture of metastable pseudo-binary metal oxides at room temperature is demonstrated with scalable combustion-aerosol processes. By a combination of X-ray diffraction, electron microscopy and on-line flame characterization, the occurrence of metastable CoCu2O3 is investigated with controlled crystal size (4-16 nm) over thermodynamically stable CuO and Co3O4. Immediate practical impact is demonstrated by exceptional sensing and stable catalytic performance for air pollutant detection (e.g., 15 parts-per-billion benzene) shown for, at least, 21 days. This approach can be extended to various binary, ternary and high entropy oxides with even more components. Also, secondary phases can be loaded on such metastable nanocrystals to access novel materials promising for actuators, energy storage or solar cells.

Liver diseases (e.g., cirrhosis, cancer) cause more than two million deaths per year worldwide. This is partly attributed to late diagnosis and insufficient screening techniques. A promising biomarker for noninvasive and inexpensive liver disease screening is breath limonene that can indicate a deficiency of the cytochrome P450 liver enzymes. Here, we introduce a compact and low-cost detector for dynamic and selective breath limonene sensing. It comprises a chemoresistive sensor based on Si/WO3 nanoparticles pre-screened by a packed bed Tenax separation column at room temperature. We demonstrate selective limonene detection down to 20 parts per billion over up to three orders of magnitude higher concentrated acetone, ethanol, hydrogen, methanol, and 2-propanol in gas mixtures, as well as robustness to 10-90% relative humidity. Most importantly, this detector recognizes the individual breath limonene dynamics of four healthy volunteers following the ingestion (swallowing or chewing) of a limonene capsule. Limonene release and subsequent metabolization are monitored from breath measurements in real time and in excellent agreement (R2 = 0.98) with high-resolution proton transfer reaction mass spectrometry. This study demonstrates the potential of the detector as a simple-to-use and noninvasive device for the routine monitoring of limonene levels in exhaled breath to facilitate early diagnosis of liver dysfunction.

Designing highly reactive surface clusters at the nanoscale on metal-oxide supports enables selective molecular interactions in low-temperature catalysis and chemical sensing. Yet, finding effective material combinations and identifying the reactive site remains challenging and a key obstacle for rational catalyst/sensor design. Here, we demonstrate the low-temperature oxidation of formaldehyde with CuOx clusters on Co3O4 nanoparticles yielding an excellent sensor for this critical air pollutant. When fabricated by flame-aerosol technology, such CuOx clusters are finely dispersed onto the surface, while some Cu ions are incorporated into the Co3O4 lattice enhancing thermal stability. Most importantly, infrared spectroscopy of adsorbed CO and temperature-programmed reduction in H2 identified Cuδ+ species in these clusters as active sites. In fact, its surface concentration correlated with the apparent activation energy of formaldehyde oxidation (Spearman’s coefficient ρ = 0.89) and sensor response (0.96). At optimal composition, such sensors detected even the lowest formaldehyde levels of 3 parts-per-billion at 75 °C, superior to the state-of-the-art sensors. Also, selectivity to other aldehydes, ketones, alcohols, and inorganic compounds, robustness to relevant humidity levels and stable performance over 4 weeks were achieved, rendering such sensors promising as low-power gas detectors in air and food quality control as well as in health monitoring.

Designing reactive surface clusters at the nanoscale on metal-oxide supports enables selective molecular interactions in low-temperature catalysis and chemical sensing. Yet, finding effective material combinations and identifying the reactive site remains challenging and an obstacle for rational catalyst/sensor design. Here, the low-temperature oxidation of formaldehyde with CuOx clusters on Co3 O4 nanoparticles is demonstrated yielding an excellent sensor for this critical air pollutant. When fabricated by flame-aerosol technology, such CuOx clusters are finely dispersed, while some Cu ions are incorporated into the Co3 O4 lattice enhancing thermal stability. Importantly, infrared spectroscopy of adsorbed CO, near edge X-ray absorption fine structure spectroscopy and temperature-programmed reduction in H2 identified Cu+ and Cu2+ species in these clusters as active sites. Remarkably, the Cu+ surface concentration correlated with the apparent activation energy of formaldehyde oxidation (Spearman's coefficient ρ = 0.89) and sensor response (0.96), rendering it a performance descriptor. At optimal composition, such sensors detected even the lowest formaldehyde levels of 3 parts-per-billion (ppb) at 75°C, superior to state-of-the-art sensors. Also, selectivity to other aldehydes, ketones, alcohols, and inorganic compounds, robustness to humidity and stable performance over 4 weeks are achieved, rendering such sensors promising as gas detectors in health monitoring, air and food quality control.

Formaldehyde is a toxic and carcinogenic indoor air pollutant. Promising for its routine detection are gas sensors based on localized surface plasmon resonance (LSPR). Such sensors trace analytes by converting tiny changes in the local dielectric environment into easily readable, optical signals. Yet, this mechanism is inherently non-selective to volatile organic compounds (like formaldehyde) and yields rarely detection limits below parts-per-million concentrations. Here, we reveal that chemical reaction-mediated LSPR with nanohybrids of Ag/AgOx core-shell clusters on TiO2 enables highly selective formaldehyde sensing down to 5 parts-per-billion (ppb). Therein, AgOx is reduced by the formaldehyde to metallic Ag resulting in strong plasmonic signal changes, as measured by UV/Vis spectroscopy and confirmed by X-ray diffraction. This interaction is highly selective to formaldehyde over other aldehydes, alcohols, ketones, aromatic compounds (as confirmed by high-resolution mass spectrometry), inorganics, and quite robust to relative humidity changes. Since this sensor works at room temperature, such LSPR nanohybrids are directly deposited onto flexible wristbands to quantify formaldehyde between 40-500 ppb at 50% RH, even with a widely available smartphone camera (Pearson correlation coefficient r = 0.998). Such chemoresponsive coatings open new avenues for wearable devices in environmental, food, health and occupational safety applications, as demonstrated by an early field test in the pathology of a local hospital.

The Spark M-20 is a nanotechnology-based, handheld device that detects toxic methanol in beverages and sanitizers, which may soon also be applied for intoxication screening in human breath. Here, we share our pathway and experiences during the translation of this university-originated innovation into a commercial product that today is serving customers in 23 countries on 6 continents.

Isoprene has received widespread attention in breath research because of its potential to serve as a sensitive and non-invasive biomarker for the detection and monitoring of several metabolic effects. To date, research activity on breath isoprene focused on mass spectrometry-based measurement techniques, which are not portable and require skilled operators. Here, we show, for the first time to our knowledge, selective isoprene monitoring in exhaled breath (148 breath samples, 60–1250 parts-per-billion, ppb) with an inexpensive, user-friendly and compact filter–sensor device. This detector is based on a previously developed concept comprising a sorption filter of activated alumina that removes hydrophilic volatiles ahead of a micro gas sensor consisting of chemoresistive Si-doped WO3 nanoparticles to quantify the isoprene down to few ppb concentrations. When tested on humans during exercise and at rest, the detector accurately followed breath isoprene dynamics in linear (Pearson’s coefficient 0.89) correlation to proton transfer reaction mass spectrometry measurements. Most importantly, the output from the device is not interfered by high and variable concentrations of other breath volatile compounds, specifically acetone, ethanol and methanol. This isoprene detector can be readily applied for online monitoring of physical activity.

Routine detection of health parameters is desirable to recognize the early onset of metabolic diseases (e.g., diabetes mellitus) and to personalize their treatment. Promising are non-invasive, affordable and portable technologies, such as breath sensors. Yet, the selective monitoring of breath markers (e.g., acetone for lipolysis) with sensors to track metabolic changes that can reveal disease-related abnormalities remains challenging. Here, subtle breath acetone changes during fasting, exercise and glucose ingestion are tracked in two model situations: Patients suffering from type-1 diabetes mellitus (T1DM) and healthy subjects (total: 19 volunteers) were monitored using chemoresistive sensors based on Si/WO3 nanoparticles. Specifically, each subject cycled after overnight fasting to stimulate fatty acid oxidation followed by an oral glucose tolerance test (OGTT), as moni-tored by capillary blood glucose and beta-hydroxybutyrate (BOHB) concentrations. The sensor recognized accu-rately the individual breath acetone patterns before and after OGTT (both R2 = 0.9) at negligible interference, for instance, from glucose ingestion-associated volatiles (e.g., ethanol) or isoprene, as confirmed by high-resolution mass spectrometry. Furthermore, distinct differences in the breath acetone patterns of T1DM over healthy subjects were revealed including higher (t-test, p = 0.006) breath acetone ratio 2 h after starting the OGTT. Worth noting is that after glucose intake, breath isoprene steadily increased for T1DM subjects while it remained rather constant for healthy ones, an intriguing observation that requires more research to clarify its biochemical origin and medical relevance.

Combustion aerosol technology has distinct advantages for the assembly of chemoresistive gas sensors compared to their traditional wet chemistry synthesis. These advantages are traced to combustion's steep temperature gradients and high particle concentrations during sensing particle formation and film deposition. This gives direct access to a plethora of material compositions (e.g. metastable phases, solid solutions, mixed oxides) and fractal-like porous but rigid structures that can lead to unique sensor selectivity, sensitivity and stability along with short response and recovery times. Here, flame-made gas sensors are reviewed tutorially and compared quantitatively. First, their basics are introduced focusing on the relationship between gas sensing and particle morphology (e.g. agglomerated vs. aggregated) and heterogeneity (e.g. noble metal surface clusters) including the embedding of noble metals into the chemoresistive metal oxides, a unique feature of flame-made particles. Then, sensors are distinguished between those made by conventional wet-deposition of flame-made sensing particles and those made by direct flame deposition onto sensing substrates. The fundamentals of combustion synthesis of sensing particles are traced to those of ceramic particles with emphasis on direct flame deposition of sensing films as their assembly can be monitored in situ, another unique feature of combustion processes. This is followed by a presentation of the evolution of flame-made gas sensor compositions (e.g. based on SnO2, WO3, ZnO, TiO2 and other materials) with respect to selective sensing of key analytes (ethanol, NO2, CO, acetone, isoprene, H2 etc.). Finally, sensor systems (arrays and catalytic or gas chromatographic filters) and their integration into devices with validation under realistic conditions are presented. Examples like carcinogenic formaldehyde monitoring in indoor air, fat metabolism monitoring in human breath or the distinction of toxic methanol from ethanol in alcoholic beverages and hand sanitizers are elaborated to demonstrate the immediate practical impact of flame-made gas sensors.

More than 1 million workers are exposed routinely to carcinogenic benzene, contained in various consumer products (e.g., gasoline, rubbers, and dyes) and released from combustion of organics (e.g., tobacco). Despite strict limits (e.g., 50 parts per billion (ppb) in the European Union), routine monitoring of benzene is rarely done since low-cost sensors lack accuracy. This work presents a compact, battery-driven device that detects benzene in gas mixtures with unprecedented selectivity (>200) over inorganics, ketones, aldehydes, alcohols, and even challenging toluene and xylene. This can be attributed to strong Lewis acid sites on a packed bed of catalytic WO3 nanoparticles that prescreen a chemoresistive Pd/SnO2 sensor. That way, benzene is detected down to 13 ppb with superior robustness to relative humidity (RH, 10-80%), fulfilling the strictest legal limits. As proof of concept, benzene is quantified in indoor air in good agreement (R2 ≥ 0.94) with mass spectrometry. This device is readily applicable for personal exposure assessment and can assist the implementation of low-emission zones for sustainable environments.