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The ability to detect and size individual nanoparticles with high resolution is crucial to understanding the behaviour of single particles and effectively using their strong size-dependent properties to develop innovative products. We report real-time, in situ detection and sizing of single nanoparticles, down to 30 nm in radius, using mode splitting in a monolithic ultrahigh-quality-factor ( Q ) whispering-gallery-mode microresonator. Particle binding splits a whispering-gallery mode into two spectrally shifted resonance modes, forming a self-referenced detection scheme. This technique provides superior noise suppression and enables the extraction of accurate particle size information with a single-shot measurement in a microscale device. Our method requires neither labelling of the particles nor a priori information on their presence in the medium, providing an effective platform to study nanoparticles at single-particle resolution. Utilizing a self-referenced detection scheme based on the mode-splitting in an ultrahigh- Q microresonator, scientists realize the real-time in situ detection and sizing of single nanoparticles with radii as small as 30 nm. Labelling of the particles and a priori information on the presence of nanoparticles in the medium are not required, thus providing an effective platform for studying nanoparticles at the single-particle resolution level.

This study aimed to explain the discrepancy reported in previous studies between the observed and the calculated filtration efficiencies of a composite formed of multiple electret medium layers. After measuring the composite’s filtration efficiency using particles divided by size and electrical charge status, viz., those possessing no charge, a single charge, and a stationary charge according to the Boltzmann distribution, we traced the discrepancy to the latter attribute. Hence, to accurately predict the filtration efficiency of multi-layered electret media, we must account for the test particles’ electrical charge distribution.

The COVID-19 pandemic has underscored the importance of wearing effective facepiece filtering respirators (FFRs) to reduce infection and disease transmission. One of the reasons causing the widespread prevalence was found to be the failure of N95-Equivalent FFRs (N95-EFs), i.e., efficiency <<95 %, during the pandemic. To investigate the reasons causing the ineffectiveness of commercial N95-EFs, this study measured the efficiency of several dozens of commercially available N95-EFs following standard testing protocols. The specifications of the N95-EF including fiber diameter, solidity and surface potential of the main layer media of N95-EFs were also determined. We provide a simple method for manufacturers to quickly screen the efficiencies of their N95-EF products before distributing them to the market. We found that the failures of N95-EF are majorly attributed to overprediction of the efficiency due to i) missing neutralization of challenging particles, ii) too small or oversize of challenging particles, and iii) particle detectors with large sizing limits (>500 nm). Based on the pressure drop, respirator area, and surface potential of the N95-EFs, an empirical equation is developed to fast screen and help design effective N95-EFs.

There is a severe shortage of face masks and N95 respirators due to the current COVID-19 pandemic, particularly in countries that were not well prepared in advance. In order to help ease the supply demands of these resources, a strategy of using multiple layers of basic filtration media to construct a composite filter that can match the particle collection efficiency offered by a N95 filtering facepiece respirator (FFR) is proposed. In this study, the filtration performances of four face masks and one N95 respirator using the same test protocol (as a reference) were first compared. Composite filter samples composed of multiple layers of basic face mask and MERV13 furnace media were then constructed and the filter performance of the composite filters was investigated. As expected, the minimum particle collection efficiency of the N95 respirator media sample was higher than 95% and the efficiency of the samples from the four tested face masks varied from 71.8% to 83.6%. The Figure of Merit (FOM) values of the face mask samples were generally half that of the N95 media sample. It was found that a N95-comparable collection efficiency can be achieved by combining two/three layers of face mask media but at the expense of a higher media pressure drop. Additionally, the composite filter samples made up of three/five layers of MERV13 furnace media could approach the FOM offered by the N95 media without the increased pressure drop. It was also found that the measured collection efficiency of multiple-layered filter media was not equal to the calculated in the test particle size range. Further studies are required to identify the reason(s).

The hygroscopic growth of atmospheric aerosols plays an important role in regional radiation, cloud formation, and hence climate. Aerosol hygroscopic growth is often characterized by hygroscopic tandem differential mobility analyzers (HTDMAs), and Xie et al. (2020) recently demonstrated that hygroscopic growth measurements of a single particle are possible using a surface plasmon resonance microscope-azimuthal rotation illumination (SPRM-ARI). The hygroscopic properties of ambient aerosols are not uniform and often exhibit large relative humidity (RH) and size variabilities due to different chemical compositions and mixing states. To better understand the contribution of different aerosol components and establish a link between the apparent hygroscopic properties of single particles and bulk aerosols, we conduct combined hygroscopic growth measurements using a SPRM-ARI and an HTDMA as a case study to prove the concept (experimental information: 100–200 nm, during noontime on 28 September 2021 and 22 March 2022 in Hefei, China). According to the distinct hygroscopic growth behavior from single-particle probing using a SPRM-ARI, the individual particles can be classified into three categories defined as non-hygroscopic (NH), less hygroscopic (LH), and more hygroscopic (MH). The mean growth factor (GF) of the three categories can be utilized to reproduce the GF distribution obtained from the HTDMA measurement. The chemical compositions of individual particles from the three categories are identified to be organic carbon (OC), soot (mainly elemental carbon), fly ash, and secondary aerosols (mainly OC and sulfate), using scanning electron microscopy (SEM) with an energy-dispersive spectrometer (EDS). The coupled SPRM–HTDMA measurement suggests a size-dependent variation of aerosol chemical components, i.e., an increase of OC fraction with increasing particle sizes, which agrees reasonably well with the chemical compositions from collected aerosol samples. This likely links the hygroscopic properties of individual particles to their bulk hygroscopic growth and chemical composition.

Micro-Cantilever (MCL) is a thin film structure that is applied for aerosol particle mass sensing. Several modifications to the rectangular MCL (length-to-width ratio, slots at the anchor, serrations at its side edges) are made to deduce the role and influence of the shape of rectangular MCL-based aerosol mass sensors and reduce gas damping. A finite element fluid-structure interaction model was used to investigate the performance of MCL. It is found that (I) the mass sensitivity and quality factor decline with the increasing of length-to-width ratio which alters the resonant frequency of the MCL. The optimum conditions, including the length-to-width ratio (σlw = 5) and resonant frequency (f0 = 540.7 kHz) of the MCL, are obtained with the constant surface area (S = 45,000 μm2) in the frequency domain ranging from 0 to 600 kHz. (II) The slots can enhance the read-out signal and bring a small Q factor drop. (III) The edge serrations on MCL significantly reduce the gas damping. The results provide a reference for the design of aerosol mass sensor, which makes it possible to develop aerosol mass sensor with high frequency, sensitivity, and quality.

Opposing-pulse-jet technology has been proposed as a solution for regenerating filtration media by minimizing the incomplete cleaning of pleated filter cartridges. In this study, we investigated the effects of a pleated filter cartridge’s overall length and outer diameter ( OD ) on the performance of opposing-pulse-jet cleaning via numerical modeling. For each pleated filter cartridge, the delay time, Δ t (defined as the delay in launching the secondary nozzle), was varied to analyze the intensity and the uniformity of the static pressure distribution in the core of the cartridge. It was found that two opposing jet flows collided much more intensely in the core when the length of the overall cartridge, L , was short, and the OD was small. For a given L , the pressure pulse’s performance was maximized by varying Δ t . The pulse intensity and uniformity in the filter core can be represented as a bimodal (double-peaked) function of Δ t . In general, the pulse intensity was greater when Δ t > 0 s than when Δ t < 0 s (when the secondary jet was launched before the primary jet).

This is the first comprehensive study to evaluate the cytotoxicity, biochemical mechanisms of toxicity, and oxidative DNA damage caused by exposing human bronchoalveolar carcinoma-derived cells (A549) to 70 and 420 nm ZnO particles. Particles of either size significantly reduced cell viability in a dose- and time-dependent manner within a rather narrow dosage range. Particle mass-based dosimetry and particle-specific surface area-based dosimetry yielded two distinct patterns of cytotoxicity in both 70 and 420 nm ZnO particles. Elevated levels of reactive oxygen species (ROS) resulted in intracellular oxidative stress, lipid peroxidation, cell membrane leakage, and oxidative DNA damage. The protective effect of N -acetylcysteine on ZnO-induced cytotoxicity further implicated oxidative stress in the cytotoxicity. Free Zn 2+ and metal impurities were not major contributors of ROS induction as indicated by limited free Zn 2+ cytotoxicity, extent of Zn 2+ dissociation in the cell culture medium, and inductively-coupled plasma-mass spectrometry metal analysis. We conclude that (1) exposure to both sizes of ZnO particles leads to dose- and time-dependent cytotoxicity reflected in oxidative stress, lipid peroxidation, cell membrane damage, and oxidative DNA damage, (2) ZnO particles exhibit a much steeper dose–response pattern unseen in other metal oxides, and (3) neither free Zn 2+ nor metal impurity in the ZnO particle samples is the cause of cytotoxicity.

An aerosol-assisted process for continuously producing sodium tungsten bronze particles in one step was proposed. The effects of the precursor solution, solvent, heating temperature, and moisture concentration on the quality (i.e., crystallinity) of the products were systematically investigated. The mechanisms for the particle formation and the chemical reactions involved in this process were also studied. By varying the atomic ratio of sodium to tungsten in the precursor solution, sodium tungsten bronze particles in the cubic (Na 0.70 WO 3 ) and tetragonal (Na 0.57 WO 3 ) phases were monophasically produced. Particles in both phases showed near infrared (NIR) shielding properties. However, cubic-phase particles (Na 0.70 WO 3 ) possessed higher visibility in the visible light range due to their lower impurity content and higher crystallinity.

A cost-effective, miniature electrical ultrafine particle sizer (mini-eUPS) has been developed for future UFP (ultrafine particle) monitoring networks in cities with high traffic density and in communities close to freeways, airports or stationary combustion sources. The mini-eUPS mainly consists of a mini-plate unipolar particle charger, a mini-plate differential mobility classifier and a mini-particle Faraday cage with a sensitive electrometer. A custom-made circuit board was designed for operating the mini-eUPS. In addition to the primary function of measuring the UFP size distribution, the mini-eUPS records the temperature, relative humidity, pressure, altitude, timing and location for each measured data set. The operational status of the mini-eUPS is also registered for data quality assurance. Due to the built-in features of self-diagnosis, malfunction reporting and wireless networking, this particle sizer is suitable for UFP monitoring network. The prototype, which possesses overall dimensions of 6.5′′ (L) × 5.0′′ (W) × 4′′ (H) and a total weight of ~1.0 kg, measures particles ranging in size from 5 to 200 nm. To evaluate the performance of the mini-eUPS, size distribution measurements of lab-generated particles in both unimodal and bimodal distributions and of UFPs emitted from a low-cost 3D printer and diesel engine were compared with those taken by a Scanning Mobility Particle Sizer (SMPS; TSI Inc.). A reasonably good agreement between the mini-eUPS and SMPS data was found.