Cationic silver hydride clusters (Ag_nH⁺) can be formed by a number of physical and chemical processes, holding great promise for a range of applications including photonics, catalysis, sensing, and biomedicine, among others. Here, we present a comprehensive theoretical investigation of Ag_nH⁺ clusters (n = 1–7) using highly accurate coupled-cluster (CC) theory. Multiple low-lying isomers are identified using CC theory with single and double excitations (CCSD), whereas their relative stabilities are determined with the more accurate CCSD(T) method. The CCSD(T) results predict a pronounced odd–even alternation in relative stabilities, with Ag₂H⁺ being the most stable species, which is consistent with experimental mass spectrometry measurements. Ab initio molecular dynamics simulations show that all low-energy isomers remain structurally rigid at room temperature, whereas bonding analyses (frontier molecular orbitals, natural bond orbital, molecular electrostatic potential, quantum theory of atoms in molecules, non-covalent interaction) indicate strong ionic Ag–H interactions, weak non-covalent Ag–Ag interactions, and significant donor–acceptor stabilization in larger clusters. Electrical mobilities of these clusters, computed by the trajectory method, were labelled on experimental spectra, in order to contribute towards their interpretation. Overall, our results resolve inconsistencies from prior theoretical predictions, provide a rigorous description of cationic silver hydride clusters, and are used to improve the interpretation of earlier observations.

Methane is considered one of the cleanest energy sources as it produces fewer pollutants upon burning compared to other fossil fuels. Its accidental release during extraction, transportation, and use, however, poses significant environmental and safety risks, warranting advanced sensing technologies to monitor its concentration in ambient air. Here, we prepare nanoparticle-based materials for sensing methane at concentrations that are highly relevant in the atmospheric environment. The nanoparticle (NP) building blocks of the sensing materials are produced by spark-ablating and simultaneously quenching Sn electrodes with a N₂ flow at atmospheric pressure. The resulting Sn NPs are subsequently collected and oxidized to SnO₂ by thermal annealing in ambient air before doctor blading them onto substrates with interdigitated electrodes. The synthesized materials were characterized by X-ray diffraction and photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, as well as atomic force, transmission, and scanning electron microscopy. The results show that our sensing materials can quantify methane concentrations down to 0.2 ppm, having a signal-to-noise ratio of 58 and a theoretical limit of detection of ca. 7 ppb. What is more, they maintain excellent robustness across a relative humidity range of 20−80% and exhibit a high cycling stability and repeatability; features that render them superior compared to other metal oxide semiconducting materials reported in the literature so far. Based on our measurements, we also offer new insight into how the NP synthesis process can affect sensor sensitivity, demonstrating a correlation between spark-ablation energy and NP size, which in turn determines the crystal size, the specific surface area, as well as the fraction of adsorbed oxygen on the surface of the sensing material, and consequently its interaction with the target gas. Combined with the simplicity of their preparation, these sensing materials hold great potential for a wide range of environmental and industrial applications.

Metal nitride and metal oxide nanoparticles (NPs) provide key material components for a number of applications due to their unique properties. Here we demonstrate that spark ablation of metallic electrodes, quenched with a pure N₂ flow at atmospheric pressure, can be used as a reactive generator to synthesize metal nitride, metal oxide or pure metallic NPs depending on the material. The composition of the synthesized NPs was determined through their crystal structure using X-ray diffraction and transmission electron microscopy (TEM). Our results show that the composition of the resulting NPs strongly depends on the electrode material: Ti and Al form mixtures of metal nitride and oxide NPs, whereas Mg and Pd produce respectively only oxide and pure metallic NPs. Repeated XRD measurements of the samples after exposing them to ambient air over periods of several months showed that the stability of TiN was higher compared to that of the AIN NPs, with the first being converted to TiN_yO_x and the latter to γ-Al₂O₃ after 9 months.

This article describes a single-step method for synthesizing nanostructured materials using evaporation-condensation synthesis and inertial impaction of aerosol nanoparticles. The as-deposited films exhibit anisotropic vertical and horizontal sintering of their palladium nanoparticle building blocks, yielding vertical structures. The electrical conductivity of the films is stable and highly sensitive to the presence of hydrogen in the overlaying gas, at concentrations that range from a few hundreds parts per million to a few percent.

Optical Particle Sizers (OPSs) are widely used for measuring size distributions of particles larger than ca. 0.2 μm. To do so, they use mirrors or lenses to gather light scattered by particles passing through a focused beam, directing it to a photo-detector that produces electric pulses from the scattering events. Considering their ability to provide near real-time measurements with minimal attendance and maintenance, and to expand the networks of Particulate Matter (PM) monitoring, several manufacturers have developed low-cost and compact OPS systems. Despite that low-cost OPSs are already available in the market and employed for monitoring PM concentrations, their reported values typically deviate from those of high-end instruments, warranting further efforts to improve their performance. In this work, we designed and built a custom-made yet inexpensive OPS optical system, and studied its performance using a combination of computational and experimental methods at different flow conditions. Our results demonstrate the importance of the flow field within the OPS optical system, and how this can affect its counting and sizing ability. The overall performance of our OPS optical system is very similar to that of high-end instruments, exhibiting a counting efficiency of 50% for particles having a diameter of 320 nm, and a sizing resolution of below 15% for 500-nm particles, complying with the ISO 21501-1 and 21501-4 standards.

Aircraft emissions of (ultra)fine particles during landing and take-off operations pose increasing human health hazards for airport employees and near-airport communities. Measurements of in-operation aircraft are therefore crucial for characterizing real-world aircraft emissions, and their variability. In this work, we develop an approach that enables the gathering of large quantities of data on real-world aircraft-specific emissions. We use three types of portable PM sensors located ca. 200 m downwind of an operational runway at Amsterdam Airport Schiphol, over different seasons, to characterize the plumes from ca. 500 specific operations covering most aircraft types of the global flying fleet. High concentration peaks (in the order of 10⁶ particles/cm³) of sub-25 nm particles are observed in the near field. While departure plumes exhibit higher particle number concentrations than arrival plumes, the values do not necessarily scale with aircraft size or engine thrust rating. We find large variability among aircraft types and engine models, highlighting the importance of incorporating real-world observations when assessing the impacts of aviation on the atmospheric composition and human health.

The stability of nanoparticle (NP) production by atmospheric-pressure spark ablation was studied and found to depend on the composition of the electrodes and the carrier gas (here N₂ or Ar). For materials that do not react with N₂, such as Pd and Ni, NP production was rather stable regardless of the carrier gas employed. In contrast, for materials that can easily produce nitride species (e.g., Al and Mg), both the concentration and size of the resulting NPs exhibited noticeable fluctuations, when ablating them in N₂, which are more pronounced when the electrical energy input to the system is low. The variation in concentration and particle size is attributed to the formation of a metal-nitride region on the face of the electrodes where the sparks hit, as a result of its reaction with the carrier gas, altering the electrical and thermal conductivity, and consequently the ablatability of the electrode at that region. This explanation was corroborated by offline analysis of the face surface of the electrodes, showing two chemically distinct regions: one with high content of N and one without. In addition, the concentration of the Al and Mg NPs produced in N₂ decreased gradually over time until it reached a plateau after several hours. When using Ar, the fluctuation and decreasing trend in NP production, and consequently the formation of nitride compounds on the face surface of the electrodes, were negligible, providing an effective solution for stable ablation of materials that can easily react with N₂.

Positive and negative ions produced by radioactive sources and corona discharges in gases find a number of applications, including charging aerosol particles prior to their measurement by electrical and/or electrical mobility techniques. The degree to which these ions can charge aerosol particles depends on their mobility and mass; properties that are strongly affected by the composition of the carrier gas and the impurities that it contains. We show that when the purity of the carrier gas is increased, the mobility of both positive and negative ions increases by more than 50%, whereas the respective masses reduce by more than 50%. In most cases, the dominant positive species is N ₄ ⁺, whereas NO ₂ ^- and NO ₃ ^- prevail for the negative polarity. Differences in ion mobility and mass resulting from the two ionization methods (i.e., radioactive source and corona discharges) remain limited. When volatile methyl siloxanes (VMS) are introduced deliberately to the gas, the mobility of the cations decreases by 39% and their mass increases by 385%, while the dominant mobility and mass peaks of the negative ions remains almost unaffected. Interestingly, introduction of VMS also leads to consistent and reproducible positive ion properties across all variations of the experiments, which can be especially relevant for charging aerosol particles in a reproducible manner. Taken together, the new measurements we report in this paper corroborate prior knowledge that the composition and purity of the carrier gas strongly influence the properties of positive and negative ions generated in aerosol neutralizers, and provide new evidence regarding their evolution in the presence of impurities.

Sub-10 nm metal-based nanoparticles have garnered immense interest due to their unique properties and versatile applications. In this study, we created sub-10 nm Ag-based particles with a spark discharge generator and explored the parameters impacting their size distribution and charging properties, including carrier gas flow rates, spark discharge voltage, electrode gap distances, and capacitance. Our findings illuminate that there is a comparable influence of different factors on both self-charged and neutral particles. Among the different factors, carrier gas flow rates emerging as a paramount determinant in particle size. While increasing spark discharge voltage and capacitance within the spark circuit increases particle concentrations, the associated changes in particle size prove to be less straightforward. Significant differences between the concentration of positive and negative self-charged particles manifest when the carrier gas flow rate surpasses 5.0 L min⁻¹, with positive particles ranging from 0.8 to 1.2 nm and negative particles spanning 0.8 to 3.0 nm. Self-charged particles close to 1 nm tend to exhibit positive charges, whereas those larger than 2 nm tend to acquire negative charges, which suggests the growth of negative particles is faster than positive ones in the spark chamber. Nevertheless, these disparities between bipolar particles diminish with the increase of residence time, leading to the observation of similar particle size distributions. Positive particles consistently bear a single charge, while some negative particles exceeding 3 nm exhibit multiple charges, primarily under carrier gas flow rates exceeding 7.5 L min⁻¹. This study provides insights into the control of properties of nano-sized metal particles, which are crucial for their practical utilization.

Inhalation of nanosized metal oxides may occur at the workplace. Thus, information on potential hazardous effects is needed for risk assessment. We report an investigation of the genotoxic potential of different metal oxide nanomaterials. Acellular and intracellular reactive oxygen species (ROS) production were determined for all the studied nanomaterials. Moreover, mice were exposed by intratracheal instillation to copper oxide (CuO) at 2, 6, and 12 μg/mouse, tin oxide (SnO₂) at 54 and 162 μg/mouse, aluminum oxide (Al₂O₃) at 18 and 54 μg/mouse, zinc oxide (ZnO) at 0.7 and 2 μg/mouse, titanium dioxide (TiO₂) and the benchmark carbon black at 162 μg/mouse. The doses were selected based on pilot studies. Post-exposure time points were 1 or 28 days. Genotoxicity, assessed as DNA strand breaks by the comet assay, was measured in lung and liver tissue. The acellular and intracellular ROS measurements were fairly consistent. The CuO and the carbon black bench mark particle were potent ROS generators in both assays, followed by TiO₂. Al₂O₃, ZnO, and SnO₂ generated low levels of ROS. We detected no increased genotoxicity in this study using occupationally relevant dose levels of metal oxide nanomaterials after pulmonary exposure in mice, except for a slight increase in DNA damage in liver tissue at the highest dose of CuO. The present data add to the body of evidence for risk assessment of these metal oxides.

Spark ablation is a highly effective and versatile method for producing nanoparticles from bulk conductive electrode materials. For a number of applications, however, the production throughput of the process needs to be increased with respect to the current state of the art. Here we show that this can be achieved by decreasing the diameter of the employed bulk-material electrodes from ca. 12 to 0.15 mm, corroborating previous observations, and demonstrate that the throughput is associated with the ablation efficiency (i.e., the energy spent to produce nanoparticles per total input energy) that respectively increases by a factor of 10. It is also shown that the commonly used theory for predicting the mass of nanoparticles produced by spark ablation cannot capture this effect, and thus we extend it to account for heat losses that affect the process when electrode diameter reduces below ca. 2 mm. Through this exercise we also show that reduced heat losses associated with thinner electrodes provide an effective recipe to increase the ablation efficiency, also referred as the nanoparticle production yield. The new extended theory for estimating spark ablation nanoparticle mass production throughput is also accompanied by an empirical equation predicting its dependence on electrode diameter.

Portable instruments that can measure the number concentration and size of airborne nanoparticles are very useful for assessing their impacts on human health and climate, mainly because they can enable personal monitoring when carried by individuals, and/or 2- or 3-dimensional mappings when employed onboard mobile platforms. Partector 2 (P2), which is a lightweight and portable instrument manufactured by Naneos Particle Solutions GmbH (Windisch, Switzerland), can determine the concentration (up to 10⁶ #/cm³) and average diameter of aerosol particles having sizes from 10 to 300 nm, making it an excellent candidate for such measurements. Although its performance has been investigated at standard conditions (i.e., ground level pressure and room temperatures), it has not been assessed under reduced pressure and temperature conditions that are typically encountered at higher altitudes; e.g., when employed outdoors in mountainous environments and/or onboard Unmanned Aerial Systems; UASs. Here we assess the counting and sizing capabilities of P2 at temperatures from ca. 22 down to 4 °C, and pressures from 1013 down to 710 hPa that correspond to altitudes from sea level to ca. 3 km. Our results show that the performance of the instrument is not substantially affected when operated at these conditions, remaining within the accuracy thresholds of ±30% reported by the manufacturer. P2, therefore, qualifies for outdoor use at higher altitudes, and can be employed in such environments to determine the number concentration and mean size of sub-300 nm aerosol particles, complementing existing portable optical particle counters that are already employed onboard aerial systems.