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, λ_l, 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 d_p = 20-80 nm and s = 1-50 nm is simulated. Such agglomerates shift λ_l 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (d_BET = 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.

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.

Palladium embedded in SnO2 enhances the sensitivity of flame-made chemoresistive gas sensors

Palladium is used commonly to enhance the performance of chemoresistive metal-oxide gas sensors. Typically, this enhancement is attributed to the presence of Pd clusters on the surface of their metal-oxide support (i.e. SnO₂). Possible Pd incorporation or embedding into the support rarely has been considered. Here, SnO₂ particles (15 - 21 nm in diameter measured by N₂ adsorption) with different Pd contents (0 - 3 mol%) were prepared by flame spray pyrolysis (FSP). Leaching these particles with HNO₃ and characterization by inductively coupled plasma - optical emission spectrometry (ICP-OES) indicated that only 36 - 60% of Pd have been removed (e.g., from the SnO₂ surface). The rest was embedded within the SnO₂ particles. Annealing prior to leaching decreased by ~30% that Pd surface content. Most interestingly, such SnO₂ particles (with only embedded Pd) show higher sensor response to acetone, ethanol and CO at 350 °C compared to SnO₂ particles containing both surface and embedded Pd (i.e. before leaching). As a result, such sensors can detect acetone with high (> 25) signal-to-noise ratio at levels down to 5 ppb at 50% relative humidity. Graphical abstractFlame-made SnO₂ nanoparticles with embedded and surface Pd (triangles) exhibit lower sensor response to acetone, ethanol and CO than SnO₂ from which the surface Pd had been removed by leaching (circles).

Highly selective detection of methanol over ethanol by a handheld gas sensor

Methanol poisoning causes blindness, organ failure or even death when recognized too late. Currently, there is no methanol detector for quick diagnosis by breath analysis or for screening of laced beverages. Typically, chemical sensors cannot distinguish methanol from the much higher ethanol background. Here, we present an inexpensive and handheld sensor for highly selective methanol detection. It consists of a separation column (Tenax) separating methanol from interferants like ethanol, acetone or hydrogen, as in gas chromatography, and a chemoresistive gas sensor (Pd-doped SnO2 nanoparticles) to quantify the methanol concentration. This way, methanol is measured within 2 min from 1 to 1000 ppm without interference of much higher ethanol levels (up to 62,000 ppm). As a proof-of-concept, we reliably measure methanol concentrations in spiked breath samples and liquor. This could enable the realization of highly selective sensors in emerging applications such as breath analysis or air quality monitoring.

Silica-Coated TiN Particles for Killing Cancer Cells

Photothermal therapy (PTT) using plasmonic nanoparticles for cancer treatment is on the verge of clinical application. Titanium nitride (TiN) nanoparticles offer a promising alternative to commonly used gold-based systems at a fraction of the costs. Little is known, however, about the relationship between TiN particle characteristics and their optical properties in colloidal systems. Here, TiN nanoparticles with closely controlled characteristics are prepared by nitridation of TiO₂, and their use as PTT agents is explored. Emphasis is placed on the particle surface and core oxygen content, which dominate the TiN optical properties. Colloidal suspensions were studied under UV-vis and near-infrared (NIR) laser irradiation and correlated to particle characteristics. High nitridation temperatures and long residence times lead to increased NIR light absorption. Too high nitridation temperatures, however, lead to particle aggregation that deteriorated their optical properties. This was overcome with SiO₂ coating of TiO₂ nanoparticles prior to nitridation: the resulting SiO₂-coated TiN particles exhibited increased plasmonic performance compared to bare TiN, which is attributed to reduced plasmonic coupling effects. The optimized SiO₂-coated TiN had a photothermal efficiency of 58.5% and mass extinction coefficient of 31.6 L g^-1 cm^-1, outperforming commercial gold nanoshells that are used in clinical trials. The potential of SiO₂-coated TiN for photothermal therapy was demonstrated by controllably killing HeLa cells in vitro.

Nanoparticles for Biomedicine: Coagulation During Synthesis and Applications

Nanoparticle-based systems offer fascinating possibilities for biomedicine, but their translation into clinics is slow. Missing sterile, reproducible, and scalable methods for their synthesis along with challenges in characterization and poor colloidal stability of nanoparticles in body fluids are key obstacles. Flame aerosol technology gives proven access to scalable synthesis of nanoparticles with diverse compositions and architectures. Although highly promising in terms of product reproducibility and sterility, this technology is frequently overlooked, as its products are of fractal-like aggregated and/or agglomerated morphology. However, coagulation is a widely occurring phenomenon in all kinds of particle-based systems. In particular, protein-rich body fluids encountered in biomedical settings often lead to destabilization of colloidal nanoparticle suspensions in vivo. We aim to provide insights into how particle–particle interactions can be measured and controlled. Moreover, we show how particle coupling effects driven by coagulation may even be beneficial for certain sensing, therapeutic, and bioimaging applications.

Nd3+-Doped BiVO4 luminescent nanothermometers of high sensitivity

Neodymium-doped BiVO₄ nanoparticles are explored for luminescent nanothermometry in the first and second biological windows. The nanothermometer sensitivity can be increased by an order of magnitude through careful selection of excitation wavelength and emission peaks, leading to sub-degree resolution and penetration depth up to 6 mm in biological tissues.

Breath Sensors for Health Monitoring

Breath sensors can revolutionize medical diagnostics by on-demand detection and monitoring of health parameters in a noninvasive and personalized fashion. Despite extensive research for more than two decades, however, only a few breath sensors have been translated into clinical practice. Actually, most never even left the scientific laboratories. Here, we describe key challenges that currently impede realization of breath sensors and highlight strategies to overcome them. Specifically, we start with breath marker selection (with emphasis on metabolic and inflammatory markers) and breath sampling. Next, the sensitivity, stability, and selectivity requirements for breath sensors are described. Concepts are elaborated to systematically address these requirements by material design (focusing on chemoresistive metal oxides), orthogonal arrays, and filters. Finally, aspects of portable device integration, user communication, and clinical applicability are discussed.

Engineering the Bioactivity of Flame-Made Ceria and Ceria/Bioglass Hybrid Nanoparticles

Despite its use as a highly efficient and reusable catalyst in research and industrial settings, cerium oxide nanoparticles or nanoceria have yet to gain a foothold in the biomedical field. A variety of beneficial effects of nanoceria have been demonstrated, including its use as an inorganic nanoenzyme to mimic antioxidant enzymes, to protect mammalian cells, and to suppress microbial growth. While these properties are of high interest for wound-management applications, the literature offers contradicting reports on toxicity and enzymatic activity of nanoceria. These discrepancies can be attributed to differences between synthesis methods and insufficient physicochemical characterization, leading to incomparable studies. The activity of nanoceria is mostly governed by its Ce³⁺/Ce⁴⁺ ratio which needs to be controlled to compare different nanoceria systems. In this work, we demonstrate that liquid-feed flame spray pyrolysis offers excellent control over the oxidation state in a one-step synthesis of nanoceria. This control allows a comprehensive comparison of different types of ceria nanoparticles. We connect physicochemical characteristics to biomedically relevant properties such as superoxide dismutase and catalase mimicry, human monocyte and macrophage protection, and antimicrobial activity. Furthermore, we demonstrate how the synthesis method also allows tailoring the properties of ceria/bioglass hybrid nanoparticles, thus creating nanoparticles with manifold biomedical prospects.

Orthogonal gas sensor arrays by chemoresistive material design

Gas sensor arrays often lack discrimination power to different analytes and robustness to interferants, limiting their success outside of research laboratories. This is primarily due to the widely sensitive (thus weakly-selective) nature of the constituent sensors. Here, the effect of orthogonality on array accuracy and precision by selective sensor design is investigated. Therefore, arrays of (2-5) selective and non-selective sensors are formed by systematically altering array size and composition. Their performance is evaluated with 60 random combinations of ammonia, acetone and ethanol at ppb to low ppm concentrations. Best analyte predictions with high coefficients of determination (R²) of 0.96 for ammonia, 0.99 for acetone and 0.88 for ethanol are obtained with an array featuring high degree of orthogonality. This is achieved by using distinctly selective sensors (Si:MoO₃ for ammonia and Si:WO₃ for acetone together with Si:SnO₂) that improve discrimination power and stability of the regression coefficients. On the other hand, arrays with collinear sensors (Pd:SnO₂, Pt:SnO₂ and Si:SnO₂) hardly improve gas predictions having R² of 0.01, 0.86 and 0.28 for ammonia, acetone and ethanol, respectively. Sometimes they even exhibited lower coefficient of determination than single sensors as a Si:MoO₃ sensor alone predicts ammonia better with a R² of 0.68. Graphical abstract Conventional arrays (red) with weakly-selective sensors span a significantly smaller volume in the analyte space than arrays containing distinctly-selective sensors (orthogonal array, green). Orthogonal arrays feature better accuracy and precision than conventional arrays in mixtures of ammonia, acetone and ethanol.

Guiding Ketogenic Diet with Breath Acetone Sensors

Ketogenic diet (KD; high fat, low carb) is a standard treatment for obesity, neurological diseases (e.g., refractory epilepsy) and a promising method for athletes to improve their endurance performance. Therein, the level of ketosis must be regulated tightly to ensure an effective therapy. Here, we introduce a compact and inexpensive breath sensor to monitor ketosis online and non-invasively. The sensor consists of Si-doped WO₃ nanoparticles that detect breath acetone selectively with non-linear response characteristics in the relevant range of 1 to 66 ppm, as identified by mass spectrometry. When tested on eleven subjects (five women and six men) undergoing a 36-h KD based on the Johns Hopkins protocol, this sensor clearly recognizes the onset and progression of ketosis. This is in good agreement to capillary blood β-hydroxybutyrate (BOHB) measurements. Despite similar dieting conditions, strong inter-subject differences in ketosis dynamics were observed and correctly identified by the sensor. These even included breath acetone patterns that could be linked to low tolerance to that diet. As a result, this portable breath sensor represents an easily applicable and reliable technology to monitor KD, possibly during medical treatment of epilepsy and weight loss.

Coercivity Determines Magnetic Particle Heating

Diseased cell treatment by heating with magnetic nanoparticles is hindered by their required high concentrations. A clear relationship between heating efficiency and magnetic properties of nanoparticles has not been attained experimentally yet due to limited availability of magnetic nanoparticles with varying size and composition. Here, versatile flame aerosol technology is used for the synthesis of 21 types of ferro-/ferrimagnetic nanocrystals with varying composition, size, and morphology for hyperthermia and thermoablation therapy. Heating efficiency, magnetic hysteresis, and first-order reversal curves of these materials are compared. The maximum heating performance occurs near the transition from superparamagnetic to single domain state, regardless of particle composition. Most importantly, the ratio between saturation magnetization and coercivity can be linked to the heating properties of magnetic nanoparticles. Magnetic interaction is controlled by changes in the architecture of the nanoparticles and closely analyzed by first-order reversal curves. Silica-coated nonstoichiometric Gd-Zn ferrite exhibits the most promising therapeutic capability at relatively low particle concentrations, as shown in vitro with cancerous prostate cells.

Impact of Humidity on Silica Nanoparticle Agglomerate Morphology and Size Distribution

The effect of humidity on flame-made metal oxide agglomerate morphology and size distribution is investigated, for the first time to our knowledge, and compared to that on soot, which has been widely studied. Understanding the impact of humidity on such characteristics is essential for storage, handling, processing, and eventual performance of nanomaterials. More specifically, broadly used agglomerates of flame-made silica nanoparticles are humidified at various saturation ratios, S = 0.2-1.5, and dried before characterization with a differential mobility analyzer (DMA), an aerosol particle mass (APM) analyzer, and transmission electron microscopy. At high humidity, the constituent single and/or aggregated (chemically bonded) primary particles (PPs) rearrange to balance the capillary forces induced by condensation-evaporation of liquid bridges between PPs. Larger agglomerates restructure more than smaller ones, narrowing their mobility size distribution. After humidification at S = 1.5 and drying, agglomerates collapse into compact structures that follow a fractal scaling law with mass-mobility exponent D_fm = 3.02 ± 0.11 and prefactor k_m = 0.27 ± 0.07. This critical S = 1.5 for silica agglomerates is larger than the 1.26 obtained for soot because of the hydrophilic surface of silica that delays water evaporation. The relative effective density, ρ_eff/ρ, of collapsed agglomerates becomes invariant of mobility diameter, d_m, similar to that of fluidized and spray-dried granules. The average silica ρ_eff/ρ = 0.28 ± 0.02 is smaller than the 0.36 ± 0.04 measured for the humidified-dried soot because of the larger size of silica aggregates, d_m/ d_p, and number of constituent primary particles, n_p, of diameter d_p. This is verified by tandem-DMA (TDMA) measurements, yielding maximum d_m = 3 d_p or 5 d_p and n_p = 13 or 36 for the soot or silica aggregates studied here, in good agreement with those reported from microscopy and high-pressure agglomerate dispersion. A scaling law relating the initial d_m,o to d_m, D_fm, and k_m after condensation-drying is developed. The mass-mobility relationship of collapsed silica and soot agglomerates obtained by combining this law with fast TDMA measurements is in excellent agreement with that measured by the direct, but tedious, DMA-APM analysis.

Reactive polycyclic aromatic hydrocarbon dimerization drives soot nucleation

Nucleation is an important, yet poorly understood step in soot formation. Here, the importance of reactive PAH dimerization in reducing soot nucleation reversibility is investigated by simulating soot formation in a so-called "nucleation" flame (P. Desgroux et al., Combust. Flame, 2017, 184, 153-166). There, inception of soot particles is prolonged at minimal subsequent growth. With only reversible PAH dimerization, the simulated soot concentration is negligible. Accounting however for PAH chemical bond formation after physical dimerization, stabilizes dimers by covalent bonds and increases the soot concentration by four orders of magnitude, in good agreement with Laser Induced Incandescence measurements. In particular, dimers of benzene with benzene, phenylacetylene, naphthalene, toluene, acenaphthylene and cyclopentapyrene make significant contributions to the total soot concentration. The abundance of dimers with small PAHs highlights the dominant role of PAH concentration over their size and dispersion forces on dimer formation. Higher collision factors are used for irreversible dimerization models using larger PAHs because of their lower concentrations and not their larger dispersion forces leading to reduced reversibility and more stable dimers. The qualitative trend of main peaks agrees well with stochastic simulations and aerosol mass spectra measured in the above "nucleation" as well as premixed flames highlighting the abundance of PAHs with five-membered rings and substituted aliphatic chains in incipient soot. The predicted number of trimers is very low, i.e. less than 3% of the total soot nuclei formed, indicating that covalently bonded PAH dimers can be the main contributors to soot nucleation.

Sniffing Entrapped Humans with Sensor Arrays

Earthquakes are lethal natural disasters frequently burying people alive under collapsed buildings. Tracking entrapped humans from their unique volatile chemical signature with hand-held devices would accelerate urban search and rescue (USaR) efforts. Here, a pilot study is presented with compact and orthogonal sensor arrays to detect the breath- and skin-emitted metabolic tracers acetone, ammonia, isoprene, CO2, and relative humidity (RH), all together serving as sign of life. It consists of three nanostructured metal-oxide sensors (Si-doped WO3, Si-doped MoO3, and Ti-doped ZnO), each specifically tailored at the nanoscale for highly sensitive and selective tracer detection along with commercial CO2 and humidity sensors. When tested on humans enclosed in plethysmography chambers to simulate entrapment, this sensor array rapidly detected sub-ppm acetone, ammonia, and isoprene concentrations with high accuracies (19, 21, and 3 ppb, respectively) and precision, unprecedented by portable sensors but required for USaR. These results were in good agreement (Pearson's correlation coefficients ≥0.9) with benchtop selective reagent ionization time-of-flight mass spectrometry (SRI-TOF-MS). As a result, an inexpensive sensor array is presented that can be integrated readily into hand-held or even drone-carried detectors for first responders to rapidly screen affected terrain.

Highly Selective and Rapid Breath Isoprene Sensing Enabled by Activated Alumina Filter

Isoprene is a versatile breath marker for noninvasive monitoring of high blood cholesterol levels as well as for influenza, end-stage renal disease, muscle activity, lung cancer, and liver disease with advanced fibrosis. Its selective detection in complex human breath by portable devices (e.g., metal-oxide gas sensors), however, is still challenging. Here, we present a new filter concept based on activated alumina powder enabling fast and highly selective detection of isoprene at the ppb level and high humidity. The filter contains high surface area adsorbents that retain hydrophilic compounds (e.g., ketones, alcohols, ammonia) representing major interferants in breath while hydrophobic isoprene is not affected. As a proof-of-concept, filters of commercial activated alumina powder are combined with highly sensitive but rather nonspecific, nanostructured Pt-doped SnO₂ sensors. This results in fast (10 s) measurement of isoprene down to 5 ppb at 90% relative humidity with outstanding selectivity (>100) to breath-relevant acetone, ammonia, ethanol, and methanol, superior to state-of-the-art isoprene sensors. Most importantly, when exposed continuously to simulated breath mixtures (four analytes) for 8 days, this filter-sensor system showed stable performance. It can be incorporated readily into a portable breath isoprene analyzer promising for simple-in-use monitoring of blood cholesterol or other patho/physiological conditions.

Mobility and settling rate of agglomerates of polydisperse nanoparticles

Agglomerate settling impacts nanotoxicology and nanomedicine as well as the stability of engineered nanofluids. Here, the mobility of nanostructured fractal-like SiO₂ agglomerates in water is investigated and their settling rate in infinitely dilute suspensions is calculated by a Brownian dynamics algorithm tracking the agglomerate translational and rotational motion. The corresponding friction matrices are obtained using the HYDRO++ algorithm [J. G. de la Torre, G. del Rio Echenique, and A. Ortega, J. Phys. Chem. B 111, 955 (2007)] from the Kirkwood-Riseman theory accounting for hydrodynamic interactions of primary particles (PPs) through the Rotne-Prager-Yamakawa tensor, properly modified for polydisperse PPs. Agglomerates are generated by an event-driven method and have constant mass fractal dimension but varying PP size distribution, mass, and relative shape anisotropy. The calculated diffusion coefficient from HYDRO++ is used to obtain the agglomerate mobility diameter d_m and is compared with that from scaling laws for fractal-like agglomerates. The ratio d_m/d_g of the mobility diameter to the gyration diameter of the agglomerate decreases with increasing relative shape anisotropy. For constant d_m and mean d_p, the agglomerate settling rate, u_s, increases with increasing PP geometric standard deviation σ_p,g (polydispersity). A linear relationship between u_s and agglomerate mass to d_m ratio, m/d_m, is revealed and attributed to the fast Brownian rotation of such small and light nanoparticle agglomerates. An analytical expression for the u_s of agglomerates consisting of polydisperse PPs is then derived, u_s=1-ρ_fρ_pg3πμmd_m (ρ_f is the density of the fluid, ρ_p is the density of PPs, μ is the viscosity of the fluid, and g is the acceleration of gravity), valid for agglomerates for which the characteristic rotational time is considerably shorter than their settling time. Our calculations demonstrate that the commonly made assumption of monodisperse PPs underestimates u_s by a fraction depending on σ_p,g and agglomerate mass mobility exponent. Simulations are in excellent agreement with deposition rate measurements of fumed SiO₂ agglomerates in water.

Facile meltPEGylation of flame-made luminescent Tb3+-doped yttrium oxide particles: hemocompatibility, cellular uptake and comparison to silica

MeltPEGylation constitutes an elegant one-pot route for the efficient PEGylation of metal oxide nanoparticles with improved hemo- and cytocompatibility.

Surface Composition and Crystallinity of Coalescing Silver-Gold Nanoparticles

Bimetallic nanoparticles exhibit catalytic, optical, electronic, and magnetic synergy between their constituent metals. Typically, that synergy is traced to the domain structure and surface characteristics of such particles. Here these characteristics of coalescing Ag-Au nanoparticles of various initial sizes and morphologies (segregated or alloys) are investigated by atomistic molecular dynamics (MD) at different temperatures. Silver atoms exhibit increased mobility over Au and occupy gradually the surface of the coalesced (or sintered) bimetallic particle, consistent with scanning electron microscopy and selective O₂ chemisorption experiments for heterogeneous catalysis of ethylene oxidation. The characteristic sintering time of equally sized Ag-Au nanoparticles is similar to that of pure Au but shorter than that of Ag nanoparticles. When the latter coalesce with substantially bigger Au ones, a patchy Ag layer is formed at the Au particle surface. However, when Ag nanoparticles are bigger, then Au is rather embedded into Ag, consistent with microscopy data. Most notably, X-ray diffraction (XRD) patterns of Ag-Au nanoparticles are obtained by MD, distinguishing segregated from alloyed ones. The latter exhibit a weaker XRD reflection of the (200) crystalline plane and, most distinctly, form smaller crystal size (highly polycrystalline) than coalescing pure and segregated Ag and Au nanoparticles, quantitatively explaining the structure of flame-made Ag-Au nanoparticles for biomaterial applications.

Noninvasive Body Fat Burn Monitoring from Exhaled Acetone with Si-doped WO3-sensing Nanoparticles

Obesity is a global health threat on the rise, and its prevalence continues to grow. Yet suitable biomedical sensors to monitor body fat burn rates in situ, to guide physical activity or dietary interventions toward efficient weight loss, are missing. Here, we introduce a compact and inexpensive breath acetone sensor based on Si-doped WO₃ nanoparticles that can accurately follow body fat burn rates in real time. We tested this sensor on 20 volunteers during exercise and rest and measured their individual breath acetone concentrations in good agreement with benchtop proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS). During exercise, this sensor reveals clearly the onset and progression of increasing breath acetone levels that indicate intensified body fat metabolism, as validated by parallel venous blood β-hydroxybutyrate (BOHB) measurements. Most importantly, we found that the body fat metabolism was especially pronounced for most volunteers during fasting for 3 h after exercise, with strong variation between subjects, and this was displayed correctly by the sensor in real-time. As a result, this simple breath acetone sensor enables easily applicable and hand-held body fat burn monitoring for personalized and immediate feedback on workout effectiveness that can guide dieting as well.

The silanol content and in vitro cytolytic activity of flame-made silica

HYPOTHESIS

The surface chemistry of synthetic amorphous silicas is essential for their applicational performance and for understanding their interactions with biological matter. Synthesis of silica by flame spray pyrolysis (FSP) allows to control the content and type of hydroxyl groups which also affects the cytolytic activity.

EXPERIMENTS

By controlling the FSP process variables, silica nanoparticles with the same specific surface area but different surface chemistry and content of internal silanols are prepared by combustion of hexamethyldisiloxane sprays, as characterized by Raman and infrared spectroscopy, thermogravimetric analysis, and titration with lithium alanate. Cytolytic activity is assessed in terms of membrane damage in human blood monocytes in vitro.

FINDINGS

Unlike commercial fumed silica, FSP-made silicas contain a significant amount of internal silanol groups and a high surface hydroxyl density, up to ∼8OH/nm², similar to silicas made by wet-chemistry. Increasing the residence time of particles at high temperature during their synthesis reduces the internal and surface hydroxyl content and increases the relative amount of isolated silanols. This suggests incomplete oxidation of the silica matrix especially in short and "cold" flames and indicates that the silica particle formation pathway involves Si(OH)₄. The surface chemistry differences translate into lower cytolytic activity for "cold-" than "hot-flame" silicas.

In Situ Monitoring of the Deposition of Flame-Made Chemoresistive Gas-Sensing Films

Flame-deposited semiconducting nanomaterials on microelectronic circuitry exhibit exceptional performance as chemoresistive gas sensors. Current manufacturing technology, however, does not monitor in situ the formation of such nanostructured films, even though this can facilitate the controlled and economic synthesis of these sensors. Here, the resistance of such growing films is measured in situ during fabrication to monitor the creation of a semiconducting nanoparticle network for gas sensors. Upon formation of that network, the film resistance drops drastically to an asymptotic value that depends largely on the film structure or morphology rather than on its thickness and size of nanoparticle building blocks. Precursor solutions of various concentrations enable the flame deposition of Sb-doped SnO₂ sensing films of different morphologies, each of which exhibit a characteristic in situ resistance pattern. Low precursor concentrations (1 mM) lead to thin (ca. 0.16 μm) films with slender columnar structures of increasing diameter (up to 25 nm) after prolonged deposition (up to 6 min) and show an oscillating in situ resistance during their fabrication. On the other extreme, high precursor concentrations (100 mM) lead to thick (up to 80 μm) dendritic and porous films consisting of nanoparticles with relatively small primary particle diameter (around 7 nm) that remain invariant of deposition duration, which is in agreement with the stable in situ resistance. Such dendritic films exhibit a sensor recovery time that is an order of magnitude longer than that of those made at lower concentrations. The above understanding enables the rapid and economic flame synthesis of thin gas sensors consisting of minimal semiconducting nanomaterial mass possessing a tuned baseline resistance and exhibiting excellent response to ethanol vapor.

Nanogenerator power output: influence of particle size and crystallinity of BaTiO3

Lead-free piezoelectric nanogenerators made with BaTiO₃ offer an attractive energy harvesting solution towards portable, battery-free medical devices such as self-powered pacemakers. Here, we assembled nanogenerators made of thin, flexible poly(vinylidene fluoride-co-hexafluoropropylene) films containing either polycrystalline BaTiO₃ nanoparticles of various sizes or commercial monocrystalline particles of 64 or 278 nm in average diameter. The nanoparticles were prepared by hydrogen-driven flame aerosol technology and had an average diameter of 24-50 nm with an average crystal size of about 10 nm. The rapid cooling during nanoparticle formation facilitated the synthesis of polycrystalline, multi-domain, piezoelectrically active tetragonal BaTiO₃ with a high c/a lattice ratio. Using these particles, 2 μm thin polymer nanocomposites were formed, assembled into nanogenerators that exhibited a 1.4 V time-averaged output, almost twice that of the best commercial BaTiO₃ particles. That output was maintained stable for over 45 000 cycles with each cycle corresponding to a heartbeat of 60 bpm. The exceptional piezoelectric performance of these nanogenerators is traced to their constituent polycrystalline nanoparticles, having high degree of domain orientation upon poling and exhibiting the flexoelectric effect, polarization induced by a strain gradient.

Developing a tissue glue by engineering the adhesive and hemostatic properties of metal oxide nanoparticles

Despite decades of research, wound complications remain a major cause of postoperative mortality, especially in the face of multiple comorbidities. Addressing the issue of anastomotic leakages and impaired wound healing from a new angle is of great interest with the prospect of having direct impact on patient outcome. Recently, aqueous suspensions of silica and iron oxide nanoparticles have been employed to connect biological tissue by serving as an adhesive layer eventually leading to macroscopic gluing of tissue. In this work, we explore the prospects of this effect by introducing bioactive tissue adhesives composed of nanoparticles produced via scalable and sterile flame spray pyrolysis. We investigate six different metal oxides on cytocompatibility, hemostatic activity and adhesive properties in a small intestine lap joint model. While bioglass nanoparticles show exceptionally strong procoagulant and adhesive properties, the cell membrane integrity is impaired at high particle concentrations. Interestingly, when bioglass is combined with ceria, a material that has well-documented cytoprotective effects, the resulting hybrid particles exhibit the same beneficiary effects as bioglass while featuring superior cytocompatibility. Taken together, we demonstrate highly modular synthesis of nanoparticles expressing adhesive properties in conjunction with tailored bioactivity. Such bioactive nanoparticles as adhesion nuclei in wound healing have a wide range of potential applications in surgical wound care and regenerative medicine.