Embedding Pd into SnO2 drastically enhances gas sensing.

Combustion aerosol processes can uniquely embed noble metals into semiconducting particles. Here, monocrystalline SnO2 particles embedded with Pd and/or PdOx were made by flame spray pyrolysis (FSP) of appropriate precursors through microexplosions by droplet-to-particle conversion as the crystal size was proportional to the cube root of precursor solution concentration, C. These particles were air-annealed and leached with nitric acid for removal of metallic Pd from their surface. The SnO2 crystal size varied from 11 to 24 nm and was in close agreement with the primary particle size determined by nitrogen adsorption. The embedded fraction of Pd ranged from about 30 to 80% of the nominal Pd-content. This was achieved by judiciously varying the C, Pd content and the ratio of precursor solution to dispersion oxygen flowrates during FSP. The response of sensors made by doctor blading films of such particles to 1 ppm of acetone and CO was evaluated at 350 °C and 50% relative humidity. Embedding Pd/PdOx into SnO2 significantly increased the sensor response: 2-6 times over that of pure or conventionally-made Pd-containing SnO2 sensors at low nominal Pd-contents (0.2 mol%). For higher ones (i.e. 1 mol% Pd), the sensor response was enhanced by up to two orders of magnitude. This is attributed to Pd atoms in the SnO2 lattice near the particle surface and/or Pd/PdOx clusters acting as nanoelectrodes into SnO2 films and altering their transducing properties as shown by high resolution electron microscopy, XPS and baseline resistance measurements of pure and Pd-embedded SnO2 sensing films.

Toward Elimination of Soot Emissions from Jet Fuel Combustion.

Soot from jet fuel combustion in aircraft engines contributes to global warming through the formation of contrail cirrus clouds that make up to 56% of the total radiative forcing from aviation. Here, the elimination of such emissions is explored through N2 injection (containing 0-25 vol % O2) at the exhaust of enclosed spray combustion of jet fuel that nicely emulates aircraft soot emissions. It is shown that injecting N2 containing 5 vol % of O2 enhances the formation of polyaromatic hydrocarbons (PAHs) that adsorb on the surface of soot. This increases soot number density and volume fraction by 25 and 80%, respectively. However, further increasing the O2 concentration to 20 or 25 vol % enhances oxidation and nearly eliminates soot emissions from jet fuel spray combustion, reducing the soot number density and volume fraction by 87.3 or 95.4 and 98.3 or 99.6%, respectively. So, a judicious injection of air just after the aircraft engine exhaust can drastically reduce soot emissions and halve the radiative forcing due to aviation, as shown by soot mobility, X-ray diffraction, Raman spectroscopy, nitrogen adsorption, microscopy, and thermogravimetric analysis (for the organic to total carbon ratio) measurements.

Dynamic Breath Limonene Sensing at High Selectivity.

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.

Enhanced Light Absorption and Radiative Forcing by Black Carbon Agglomerates.

The climate models of the Intergovernmental Panel on Climate Change list black carbon (BC) as an important contributor to global warming based on its radiative forcing (RF) impact. Examining closely these models, it becomes apparent that they might underpredict significantly the direct RF for BC, largely due to their assumed spherical BC morphology. Specifically, the light absorption and direct RF of BC agglomerates are enhanced by light scattering between their constituent primary particles as determined by the Rayleigh-Debye-Gans theory interfaced with discrete dipole approximation and recent relations for the refractive index and lensing effect. The light absorption of BC is enhanced by about 20% by the multiple light scattering between BC primary particles regardless of the compactness of their agglomerates. The resulting light absorption agrees very well with the observed absorption aerosol optical depth of BC. ECHAM-HAM simulations accounting for the realistic BC morphology and its coatings reveal high direct RF = 3-5 W/m2 in East, South Asia, sub-Sahara, western Africa, and the Arabian peninsula. These results are in agreement with satellite and AERONET observations of RF and indicate a regional climate warming contribution by 0.75-1.25 °C, solely due to BC emissions.

Light Extinction by Agglomerates of Gold Nanoparticles: A Plasmon Ruler for Sub-10 nm Interparticle Distances.

Plasmon rulers relate the shift of resonance wavelength, λl, of gold agglomerates to the average distance, s, between their constituent nanoparticles. These rulers are essential for monitoring the dynamics of biomolecules (e.g., proteins and DNA) by determining their small (<10 nm) coating thickness. However, existing rulers for dimers and chains estimate coating thicknesses smaller than 10 nm with rather large errors (more than 200%). Here, the light extinction of dimers, 7- and 15-mers of gold nanoparticles with diameter dp = 20-80 nm and s = 1-50 nm is simulated. Such agglomerates shift λl up to 680 nm due to plasmonic coupling, in excellent agreement with experimental data by microscopy, dynamic light scattering, analytical centrifugation, and UV-visible spectroscopy. Subsequently, a new plasmon ruler is derived for gold nanoagglomerates that enables the accurate determination of sub-10 nm coating thicknesses, in excellent agreement also with tedious microscopy measurements.

Handheld Device for Selective Benzene Sensing over Toluene and Xylene.

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.

Highly selective gas sensing enabled by filters.

Portable and inexpensive gas sensors are essential for the next generation of non-invasive medical diagnostics, smart air quality monitoring & control, human search & rescue and food quality assessment to name a few of their immediate applications. Therein, analyte selectivity in complex gas mixtures like breath or indoor air remains the major challenge. Filters are an effective and versatile, though often unrecognized, route to overcome selectivity issues by exploiting additional properties of target analytes (e.g., molecular size and surface affinity) besides reactivity with the sensing material. This review provides a tutorial for the material engineering of sorption, size-selective and catalytic filters. Of specific interest are high surface area sorbents (e.g., activated carbon, silica gels and porous polymers) with tunable properties, microporous materials (e.g., zeolites and metal-organic frameworks) and heterogeneous catalysts, respectively. Emphasis is placed on material design for targeted gas separation, portable device integration and performance. Finally, research frontiers and opportunities for low-cost gas sensing systems in emerging applications are highlighted.

Acetone Sensing and Catalytic Conversion by Pd-Loaded SnO2.

Noble metal additives are widely used to improve the performance of metal oxide gas sensors, most prominently with palladium on tin oxide. Here, we photodeposit different quantities of Pd (0-3 mol%) onto nanostructured SnO2 and determine their effect on sensing acetone, a critical tracer of lipolysis by breath analysis. We focus on understanding the effect of operating temperature on acetone sensing performance (sensitivity and response/recovery times) and its relationship to catalytic oxidation of acetone through a packed bed of such Pd-loaded SnO2. The addition of Pd can either boost or deteriorate the sensing performance, depending on its loading and operating temperature. The sensor performance is optimal at Pd loadings of less than 0.2 mol% and operating temperatures of 200-262.5 °C, where acetone conversion is around 50%.

Bi2O3 boosts brightness, biocompatibility and stability of Mn-doped Ba3(VO4)2 as NIR-II contrast agent.

Deep-tissue fluorescence imaging remains a major challenge as there is limited availability of bright biocompatible materials with high photo- and chemical stability. Contrast agents with emission wavelengths above 1000 nm are most favorable for deep tissue imaging, offering deeper penetration and less scattering than those operating at shorter wavelengths. Organic fluorophores suffer from low stability while inorganic nanomaterials (e.g. quantum dots) are based typically on heavy metals raising toxicity concerns. Here, we report scalable flame aerosol synthesis of water-dispersible Ba3(VO4)2 nanoparticles doped with Mn5+ which exhibit a narrow emission band at 1180 nm upon near-infrared excitation. Their co-synthesis with Bi2O3 results in even higher absorption and ten-fold increased emission intensity. The addition of Bi2O3 also improved both chemical stability and cytocompatibility by an order of magnitude enabling imaging deep within tissue. Taken together, these bright particles offer excellent photo-, chemical and colloidal stability in various media with cytocompatibility to HeLa cells superior to existing commercial contrast agents.

The Electrophilicity of Surface Carbon Species in the Redox Reactions of CuO-CeO2 Catalysts.

Electronic metal-support interactions (EMSI) describe the electron flow between metal sites and a metal oxide support. It is generally used to follow the mechanism of redox reactions. In this study of CuO-CeO2 redox, an additional flow of electrons from metallic Cu to surface carbon species is observed via a combination of operando X-ray absorption spectroscopy, synchrotron X-ray powder diffraction, near ambient pressure near edge X-ray absorption fine structure spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy. An electronic metal-support-carbon interaction (EMSCI) is proposed to explain the reaction pathway of CO oxidation. The EMSCI provides a complete picture of the mass and electron flow, which will help predict and improve the catalytic performance in the selective activation of CO2 , carbonate, or carbonyl species in C1 chemistry.

Detecting methanol in hand sanitizers.

The coronavirus disease 2019 (COVID-19) pandemic has increased dramatically the demand for hand sanitizers. A major concern is methanol adulteration that caused more than 700 fatalities in Iran and U.S.A. (since February 2020). In response, the U.S. Food and Drug Administration has restricted the methanol content in sanitizers to 0.063 vol% and blacklisted 212 products (as of November 20, 2020). Here, we present a low-cost, handheld, and smartphone-assisted device that detects methanol selectively in sanitizers between 0.01 and 100 vol% within two minutes. It features a nanoporous polymer column that separates methanol selectively from confounders by adsorption. A chemoresistive gas sensor detects the methanol. When tested on commercial sanitizers (total 76 samples), methanol was quantified in excellent (R2 = 0.99) agreement to "gold standard" gas chromatography. Importantly, methanol quantification was hardly interfered by sanitizer composition and viscosity. This device meets an urgent need for on-site methanol screening by authorities, health professionals, and even laymen.

Screening Methanol Poisoning with a Portable Breath Detector.

Methanol poisoning outbreaks after consumption of adulterated alcohol frequently overwhelm health care facilities in developing countries. Here, we present how a recently developed low-cost and handheld breath detector can serve as a noninvasive and rapid diagnostic tool for methanol poisoning. The detector combines a separation column and a micromachined chemoresistive gas sensor fully integrated into a device that communicates wirelessly with a smartphone. The performance of the detector is validated with methanol-spiked breath of 20 volunteers (105 breath samples) after consumption of alcoholic beverages. Breath methanol concentrations were quantified accurately within 2 min in the full breath-relevant range (10-1000 ppm) in excellent agreement (R2 = 0.966) with benchtop mass spectrometry. Bland-Altman analysis revealed sufficient limits of agreement (95% confidence intervals), promising to indicate reliably the clinical need for antidote and hemodialysis treatment. This simple-in-use detector features high diagnostic capability for accurate measurement of methanol in spiked breath, promising for rapid screening of methanol poisoning and assessment of severity. It can be applied readily by first responders to distinguish methanol from ethanol poisoning and monitor in real time the subsequent hospital treatment.

Superior Acetone Selectivity in Gas Mixtures by Catalyst-Filtered Chemoresistive Sensors.

Acetone is a toxic air pollutant and a key breath marker for non-invasively monitoring fat metabolism. Its routine detection in realistic gas mixtures (i.e., human breath and indoor air), however, is challenging, as low-cost acetone sensors suffer from insufficient selectivity. Here, a compact detector for acetone sensing is introduced, having unprecedented selectivity (>250) over the most challenging interferants (e.g., alcohols, aldehydes, aromatics, isoprene, ammonia, H2, and CO). That way, acetone is quantified with fast response (<1 min) down to, at least, 50 parts per billion (ppb) in gas mixtures with such interferants having up to two orders of magnitude higher concentration than acetone at realistic relative humidities (RH = 30-90%). The detector consists of a catalytic packed bed (30 mg) of flame-made Al2O3 nanoparticles (120 m2 g-1) decorated with Pt nanoclusters (average size 9 nm) and a highly sensitive chemo-resistive sensor made by flame aerosol deposition and in situ annealing of nanostructured Si-doped ε-WO3 (Si/WO3). Most importantly, the catalytic packed bed converts interferants continuously enabling highly selective acetone sensing even in the exhaled breath of a volunteer. The detector exhibits stable performance over, at least, 145 days at 90% RH, as validated by mass spectrometry.

Selective formaldehyde detection at ppb in indoor air with a portable sensor.

Formaldehyde is a carcinogenic indoor air pollutant emitted from wood-based furniture, building materials, paints and textiles. Yet, no low-cost sensor exists for on-site monitoring to fulfill stringent current and upcoming (e.g., 8 parts-per-billion by volume, ppb, in France by 2023) exposure guidelines. Here, we present an inexpensive and handheld formaldehyde detector with proven performance in real indoor air. Selectivity is achieved by a compact packed bed column of nanoporous polymer sorbent that separates formaldehyde from interferants present in ambient air. Downstream, a highly sensitive nanoparticle-based chemoresistive Pd-doped SnO2 sensor detects formaldehyde in the relevant concentration range down to 5 ppb within 2 min. As a proof-of-concept, we measured formaldehyde in indoor air and from different wood product emissions, in excellent agreement (R2 > 0.98) with high-resolution proton-transfer-reaction time-of-flight mass spectrometry. This detector is simple-in-use and readily applicable for on-site formaldehyde exposure monitoring at home or work. It is promising for internet-of-things (IOT) sensing networks or even wearables for personal exposure assessment.

Adsorption and activation of molecular oxygen over atomic copper(I/II) site on ceria.

Supported atomic metal sites have discrete molecular orbitals. Precise control over the energies of these sites is key to achieving novel reaction pathways with superior selectivity. Here, we achieve selective oxygen (O2) activation by utilising a framework of cerium (Ce) cations to reduce the energy of 3d orbitals of isolated copper (Cu) sites. Operando X-ray absorption spectroscopy, electron paramagnetic resonance and density-functional theory simulations are used to demonstrate that a [Cu(I)O2]3- site selectively adsorbs molecular O2, forming a rarely reported electrophilic η2-O2 species at 298 K. Assisted by neighbouring Ce(III) cations, η2-O2 is finally reduced to two O2-, that create two Cu-O-Ce oxo-bridges at 453 K. The isolated Cu(I)/(II) sites are ten times more active in CO oxidation than CuO clusters, showing a turnover frequency of 0.028 ± 0.003 s-1 at 373 K and 0.01 bar PCO. The unique electronic structure of [Cu(I)O2]3- site suggests its potential in selective oxidation.

Simultaneous Nanothermometry and Deep-Tissue Imaging.

Bright, stable, and biocompatible fluorescent contrast agents operating in the second biological window (1000-1350 nm) are attractive for imaging of deep-lying structures (e.g., tumors) within tissues. Ideally, these contrast agents also provide functional insights, such as information on local temperature. Here, water-dispersible barium phosphate nanoparticles doped with Mn5+ are made by scalable, continuous, and sterile flame aerosol technology and explored as fluorescent contrast agents with temperature-sensitive peak emission in the NIR-II (1190 nm). Detailed assessment of their stability, toxicity with three representative cell lines (HeLa, THP-1, NHDF), and deep-tissue imaging down to about 3 cm are presented. In addition, their high quantum yield (up to 34%) combined with excellent temperature sensitivity paves the way for concurrent deep-tissue imaging and nanothermometry, with biologically well-tolerated nanoparticles.

Thickness Optimization of Highly Porous Flame-Aerosol Deposited WO3 Films for NO2 Sensing at ppb.

Nitrogen dioxide (NO2) is a major air pollutant resulting in respiratory problems, from wheezing, coughing, to even asthma. Low-cost sensors based on WO3 nanoparticles are promising due to their distinct selectivity to detect NO2 at the ppb level. Here, we revealed that controlling the thickness of highly porous (97%) WO3 films between 0.5 and 12.3 µm altered the NO2 sensitivity by more than an order of magnitude. Therefore, films of WO3 nanoparticles (20 nm in diameter by N2 adsorption) with mixed γ- and ε-phase were deposited by single-step flame spray pyrolysis without affecting crystal size, phase composition, and film porosity. That way, sensitivity and selectivity effects were associated unambiguously to thickness, which was not possible yet with other sensor fabrication methods. At the optimum thickness (3.1 µm) and 125 °C, NO2 concentrations were detected down to 3 ppb at 50% relative humidity (RH), and outstanding NO2 selectivity to CO, methanol, ethanol, NH3 (all > 105), H2, CH4, acetone (all > 104), formaldehyde (>103), and H2S (835) was achieved. Such thickness-optimized and porous WO3 films have strong potential for integration into low-power devices for distributed NO2 air quality monitoring.

Single-Nanoparticle Thermometry with a Nanopipette.

Thermal measurements at the nanoscale are key for designing technologies in many areas, including drug delivery systems, photothermal therapies, and nanoscale motion devices. Herein, we present a nanothermometry technique that operates in electrolyte solutions and, therefore, is applicable for many in vitro measurements, capable of measuring and mapping temperature with nanoscale spatial resolution and sensitive to detect temperature changes down to 30 mK with 43 µs temporal resolution. The methodology is based on local measurements of ionic conductivity confined at the tip of a pulled glass capillary, a nanopipettete, with opening diameters as small as 6 nm. When scanned above a specimen, the measured ion flux is converted into temperature using an extensive theoretical support given by numerical and analytical modeling. This allows quantitative thermal measurements with a variety of capillary dimensions and is applicable to a range of substrates. We demonstrate the capabilities of this nanothermometry technique by simultaneous mapping of temperature and topography on sub-micrometer-sized aggregates of thermoplasmonic nanoparticles heated by a laser and observe the formation of micro- and nanobubbles upon plasmonic heating. Furthermore, we perform quantitative thermometry on a single-nanoparticle level, demonstrating that the temperature at an individual nanoheater of 25 nm in diameter can reach an increase of about 3 K.

Rapid and Selective NH3 Sensing by Porous CuBr.

Fast and selective detection of NH3 at parts-per-billion (ppb) concentrations with inexpensive and low-power sensors represents a long-standing challenge. Here, a room temperature, solid-state sensor is presented consisting of nanostructured porous (78%) CuBr films. These are prepared by flame-aerosol deposition of CuO onto sensor substrates followed by dry reduction and bromination. Each step is monitored in situ through the film resistance affording excellent process control. Such porous CuBr films feature an order of magnitude higher NH3 sensitivity and five times faster response times than conventional denser CuBr films. That way, rapid (within 2.2 min) sensing of even the lowest (e.g., 5 ppb) NH3 concentrations at 90% relative humidity is attained with outstanding selectivity (30-260) over typical confounders including ethanol, acetone, H2, CH4, isoprene, acetic acid, formaldehyde, methanol, and CO, superior to state-of-the-art sensors. This sensor is ideal for hand-held and battery-driven devices or integration into wearable electronics as it does not require heating. From a broader perspective, the process opens exciting new avenues to also explore other bromides and classes of semiconductors (e.g., sulfides, nitrides, carbides) currently not accessible by flame-aerosol technology.

Catalytic Filter for Continuous and Selective Ethanol Removal Prior to Gas Sensing.

Ethanol is a major confounder in gas sensing because of its omnipresence in indoor air and breath from disinfectants or alcoholic beverages. In fact, most modern gas sensors (e.g., graphene, carbon nanotubes, or metal oxides) are sensitive to ethanol. This is challenging because ethanol is often present at higher concentrations than target analytes. Here, a simple and modular packed bed filter is presented that selectively and continuously removes ethanol (and other alcohols like 1-butanol, isopropanol, and methanol) over critical acetone, CH4, H2, toluene, and benzene at 30-90% relative humidity. This filter consists of catalytically active ZnO nanoparticles (dBET = 55 nm) made by flame aerosol technology and annealing. Continuous oxidation of ethanol to CO2 and H2 was observed at filter temperatures above 260 °C while below that, unwanted acetaldehyde was formed. Most remarkably, ethanol concentrations up to 185 ppm were removed from exhaled breath in preliminary tests with an alcohol intoxicated volunteer, as confirmed by mass spectrometry. At the same time, almost 4 orders of magnitude lower (e.g., 0.025 ppm) acetone concentrations were preserved. This was superior to previous catalyst filters (e.g., CuO, SnO2, and Fe2O3) with overlapping ethanol and acetone conversions and related to ZnO's surface basicity. The ZnO filter performance was stable (±2.5% conversion variability) for, at least, 21 days. Finally, when combined with a Si-doped WO3 sensor, the filter effectively mitigated ethanol interference when sensing acetone without compromising the sensor's fast response and recovery times. Such catalytic filters can be combined readily with all gas sensors.