Takafumi Seto

This study delved into the complex interplay between the nanostructural characteristics of nanoparticles and their efficacy in surface-enhanced Raman scattering (SERS) for sensitive detection of trace chemical substances. Silver nanoparticles were prepared for the SERS substrate by combining laser ablation, postannealing processes (up to 500 °C), and electrostatic mobility classification, allowing high-purity silver nanoparticles with controlling their sizes (40-100 nm) and aggregate structures. These nanoparticles were then inertially deposited on the substrates to create SERS-active surfaces, employing Rhodamine B as a probe to assess the impact of particle size, shape, and deposition density on SERS effectiveness. Our findings revealed that spherical nanoparticles, especially those approximately 50 nm in diameter, controlled to a spherical structure through gas-phase annealing at 500 °C and subsequent classification, yielded the most significant SERS enhancement. This optimal can be explained by the particle size response of the surface plasmon resonance, where the enhancement of the Raman signal for particles up to 50 nm (1/10 of the laser wavelength used in this study, 532 nm) arises from a balance between the enhancement of dipole moment and the number of "hot spot" regions (respectively proportional to the cube and inverse square of the diameters in theory, leading to a linear relationship between signal intensity and particle diameter); meanwhile, in larger size region than 50 nm, the Raman signal was decreased owing to the attribution of the phase difference between the electric field and dipole moment. Furthermore, we found that a deposition density of 2 μg resulted in nearly a single layer of particles, which is crucial for maximizing SERS hotspots and, consequently, the enhancement effect.

Fine atmospheric particles, such as PM_2.5, are strongly related to the onset and exacerbation of inflammatory responses leading to the development of respiratory and cardiovascular diseases. PM_2.5 is a complex mixture of tiny particles with different properties (i.e., size, morphology, and chemical components). Moreover, the mechanism by which PM_2.5 induces inflammatory responses has not been fully elucidated. Therefore, it is necessary to determine the composition of PM_2.5 to identify the main factors causing PM_2.5-associated inflammation and diseases. In the present study, we investigated PM_2.5 from two sites (Fukue, a remote monitoring site, and Kawasaki, an urban monitoring site) with greatly different environments and PM_2.5 compositions. The results of ICP-MS and EDX-SEM indicated that PM_2.5 from Kawasaki contained more metals and significantly induced the expression of the pro-inflammatory cytokine gene IL-8 compared to the PM_2.5 from Fukue. We also verified the increased secretion of IL-8 protein from exposure to PM_2.5 from Kawasaki. We further investigated their effects on inflammatory response and cytotoxicity using metal nanoparticles (Cu, Zn, and Ni) and ions and found that the Cu nanoparticles caused a dose-dependent increase in IL-8 expression together with significant cell death. We also found that Cu nanoparticles enhanced the secretion of IL-8 protein. These results suggest that Cu in PM_2.5 is involved in lung inflammation.

We report a simple article filter-based cross flow filtration method for air purification. We have examined a monolith filter array with a cross flow ratio of 1.0 and parallel and staggered filter arrays with a cross flow ratio of about 0.7 in terms of pressure drop and collection efficiency. The monolith filter array shows a monotonic increase in pressure drop from approximately 0.9 to 2.1 kPa. The parallel and staggered filter arrays reduce the increase in pressure drop, which can be maintained as low as 0.8 kPa over the entire range of dust loadings examined. The monolith filter array shows the highest collection efficiency, which is more than 95% over the entire range of dust loadings examined. The collection efficiency of the parallel and staggered filter arrays is initially as high as 70% and decreases to 30% when the dust loading increases to 40 g m⁻² and 100 g m⁻², respectively. In particular, collected particles are more uniformly distributed on the staggered filter array than on the parallel filter array, resulting in the initial collection efficiency of 70% being sustained for a longer time. This uniform distribution in the staggered filter array may prevent a rapid increase in local pressure drop. The present study provides insight into the development of filtration systems using simple article filters without increasing the pressure drop and decreasing the collection efficiency. Graphical Abstract

We report a spray-drying method to fabricate silver nanoparticle (AgNP) aggregates for application in surface-enhanced Raman spectroscopy (SERS). A custom-built system was used to fabricate AgNP aggregates of four sizes, 48, 86, 151, and 218 nm, from drying droplets containing AgNPs atomized from an AgNP suspension. Sample solutions of Rhodamine B (RhB) at 10^–6, 10^–8, and 10^–10 M concentrations were dropped onto the AgNP aggregates as probe molecules to examine the enhancement of the Raman signals of the RhB. The ordering of the analytical enhancement factors (AEFs) by aggregate size at a 10^–6 M RhB was 86 nm > 218 nm > 151 nm > 48 nm. When RhB concentrations are below 10^–8 M, the 86 and 151 nm AgNP aggregates show clear RhB peaks. The AEFs of the 86 nm AgNP aggregates were the highest in all four aggregates and higher than those of the 218-nm aggregates, although the 218-nm aggregates had more hot spots where Raman enhancement occurred. This finding was attributable to the deformation and damping of the electron cloud in the highly aggregated AgNPs, reducing the sensitivity for Raman enhancement. When RhB was premixed with the AgNP suspension prior to atomization, the AEFs at 10^–8 M RhB rose ~ 100-fold compared to those in the earlier experiments (the post-dropping route). This significant enhancement was probably caused by the increased opportunity for the trapping of the probe molecules in the hot spots.

Exposure to polycyclic aromatic hydrocarbons (PAHs) are a cause of chronic cough occurrence in adult patients. In order to clear the relationship between transboundary transport of PAH and health effects, this study investigates the relationship between atmospheric particulate PAHs (p-PAHs), cough occurrence by epidemiological research, and meteorological conditions using a chemical transport model. Source receptor relationship (SRR) analysis revealed that a higher cough occurrence was caused by exposure to high p-PAH levels in air masses transported from central China (CCHN, 30–40° N) under westerly conditions. The p-PAHs transported from northern China (NCHN, >40° N) and the eastern part of Russia (ERUS) under north-westerly conditions also contributed to cough occurrence. The low equivalent potential temperature (ePT) and geopotential height anomaly suggested that the p-PAHs emitted near the surface were suppressed to upward transport under the colder air mass but were instead transported horizontally near the surface in the boundary layer, resulting in high p-PAH concentrations arriving in Kanazawa. Our study’s findings suggest that the air mass transport pattern associated with meteorology strongly influences the high p-PAH concentrations causing adult chronic cough occurrence.

A water-based particle size magnifier (WPSM) was designed and applied to the wet sampling of airborne nanoparticles at the air-liquid interface (ALI). A T-shaped micromixer was employed to supersaturate the water vapor by instantaneous turbulent mixing between a cold aerosol and hot water vapor-laden gas. The nucleation efficiency of the size-selected test particles of sodium chloride (10 to 40 nm) and lithium sulfate (30 to 150 nm) was evaluated by changing the operation parameters of the WPSM. It was found that over 80% of the test particles (>30 nm) successfully grew into droplets larger than 2.3 µm, which is the cutoff diameter of the impactor. The condensational growth of the water droplets was also confirmed experimentally using an optical particle sizer as well as theoretical calculations. To realize stable deposition at the ALI, a cold wall condenser was employed for draining the excess water vapor from the WPSM. As a result of inertial deposition on the water surface, airborne particles were transported from air to the liquid phase through the ALI, and aqueous ions (Li⁺) were successfully detected by ion chromatography (IC). The detection limit of the WPSM-IC system was as low as 0.33 µg m⁻³-air, which corresponded to the 10-minute sampling of 30-nm particles with a number concentration of approximately 10,000 particles mL⁻¹. Copyright © 2021 American Association for Aerosol Research

The charge reduction of multiply charged macromolecular ions via recombination with small ions in the gas phase is commonly employed to modulate the charge on macromolecules prior to mass spectrometric and mobility analyses. We employ a recently developed continuum-Molecular Dynamics (MD) calculation approach to determine the recombination rate coefficient of multiply charged (1 to 7 excess positive charged) polyethylene glycol ions (mass of 4600 Da) with smaller singly charged anions, modeled as NO₂ ⁻ ions. The continuum-MD approach accounts explicitly for the influence of the background gas on the recombination process, accounts explicitly for ion translational, vibrational, and rotational motion, and enables recombination rate coefficient calculation in nitrogen near atmospheric pressure, wherein neither low pressure nor high pressure recombination theories are strictly applicable. Continuum-MD simulations yield recombination rate coefficients near 3.9 × 10⁻¹⁴ m³ s⁻¹ for singly charged ions, increasing to 3.0 × 10⁻¹¹ m³ s⁻¹ for the +7 ion. Pre-existing collision rate coefficient expressions for rigid ions are found to be within a factor of 2–5 of calculations for all charge states, but their use requires knowledge of an appropriate collision distance, which is not well-defined for flexible polymer ions. Continuum-MD-inferred rate coefficients are incorporated into a model of charge reduction, and the charge state distribution versus anion concentration determined with it is compared to charge reduction measurements made via atmospheric pressure differential mobility analysis. Good agreement is observed between simulations and experiments; although results are extremely sensitive to the recombination rate coefficients, experimental results are bound by models utilizing rates within a factor of 2 (0.5–2.0×) of the continuum-MD rates.

A new residual-free atomizer was designed to transfer colloidal nanoparticles measuring less than 100 nm into aerosol phase. Miniaturization of droplet size distribution successfully reduced background aerosol concentration of particles sized greater than 2.5 nm to 400 particles·mL⁻¹ of gas, which corresponded to an NaCl-equivalent impurity concentration of less than 100 ppb. Direct injection of colloid suspension enabled precise control of aerosol number concentrations by colloidal concentration (10⁵–10¹¹ particles·mL⁻¹ of liquid). Correlations between the size distributions of colloid and aerosol were also investigated using aqueous suspensions of the standard nanoparticles sized 10–100 nm. It was found that the aerosol size distribution was in very good agreement (i.e., less than 1 nm accuracy) with that measured by scanning electron microscopy.

High-performance air filters composed of a hybrid structure of nanofiber/microfiber were fabricated using wet paper processing. Two types of nanofibers (NF) with average diameters of 180 and 234 nm were mixed with a suspension of microfibers (11.5 and 11.7 µm) in various mixing fractions. Then, the suspension was filtered to fabricate hybridized fiber sheets with a known nanofiber/microfiber composition. The effects of NF diameter and mixing fraction on the performance of the hybrid filters were experimentally investigated. With increasing NF fraction, both the particle collection efficiency and the pressure drop increased. The quality factor (Q_f) was used to evaluate the performance of the prepared filters. As predicted by the single fiber filtration theory, the experimentally obtained Q_f was almost independent of the mixing fraction of the NF. The collection efficiency and pressure drop of the hybrid filters could be controlled by the NF fraction at the same Q_f. Moreover, the inhomogeneity factor of fiber packing (δ) did not significantly affect Q_f over the δ range from 3 to 23 for our filters. This implies that the lower particle capturing efficiency due to heterogeneous packing could be compensated by a decrease in the pressure drop, resulting in the same Q_f value. Therefore, Q_f for particles smaller than 100 nm, which are in the diffusion-controlled regime, can be increased by reducing the NF diameter.