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The potential reach of point-of-care (POC) diagnostics into daily routines for exposure to reactive oxygen species (ROS) and Cu in aerosolized particulate matter (PM) demands that microfluidic paper-based analytical devices (μPADs) take into consideration the simple detection of these toxic PM components. Here, we propose μPADs with a dual-detection system for simultaneous ROS and Cu(II) detection. For colorimetric ROS detection, the glutathione (GSH) assay with a folding design to delay the reaction yielded complete ROS and GSH oxidation, and improved homogeneity of color development relative to using the lateral flow pattern. For electrochemical Cu(II) determination, 1,10-phenanthroline/Nafion modified graphene screen-printed electrodes showed ability to detect Cu(II) down to pg level being low enough to be applied to PM analysis. No intra- and inter-interference affecting both systems were found. The proposed μPADs obtained LODs for 1,4-naphthoquinone (1,4-NQ), used as the ROS representative, and Cu(II) of 8.3 ng and 3.6 pg, respectively and linear working ranges of 20 to 500 ng for ROS and 1 × 10 -2 to 2 × 10 2 ng for Cu(II). Recovery of the method was between 81.4 and 108.3% for ROS and 80.5-105.3% for Cu(II). Finally, the sensors were utilized for simultaneous ROS and Cu(II) determination in PM samples and the results statistically agreed with those using the conventional methods at 95% confidence., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.)

Flow field-flow fractionation (FlFFF) with inductively coupled plasma mass spectrometric (ICP-MS) detection was applied for estimating the hydrodynamic diameter of gold nanoparticles (AuNPs). Hydrodynamic diameters of AuNPs of the same core diameter but with different surface coatings were different because the coating agents and their properties were different. The challenge of this work is due to the fact that AuNPs with various types of surface coatings exhibited different interactions in the FlFFF channel, leading to different retention behaviors. Therefore, we are interested in finding suitable FlFFF conditions for estimating the hydrodynamic diameter of AuNPs with various types of electrostatic stabilizing agents [tannic acid (TA) and citrate (CT)] and steric stabilizing agents [polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and branched polyethylene imine (BPEI)]. Different types of carrier liquids (DI water, 0.02% FL-70, 0.05% SDS, and 30 mM Tris buffer) and membrane materials [regenerated cellulose (RC) and polyethersulfone (PES) membranes] were investigated. Generally, FlFFF was applied for size characterization of nanoparticles based on FlFFF theory but the interactions between AuNPs and membrane affected the retention and the experimentally obtained hydrodynamic diameters of AuNPs from the FlFFF system. With DI water as a carrier liquid with RC or PES membranes, the hydrodynamic diameters of negatively charged particles (TA-, CT-, PVP-, and PEG-stabilized AuNPs) from FlFFF corresponded well with the hydrodynamic diameters from dynamic light scattering (DLS). Interestingly, it was possible to estimate hydrodynamic diameters of AuNPs in the mixture by using FlFFF whereas it was not possible with the use of DLS within the size range studied. This work summarized the possible interactions between AuNPs with various coating agents and membrane materials in different carrier liquids to give guidelines on the suitable conditions of FlFFF for further applications on AuNP hydrodynamic diameter estimation.

Thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) was applied to investigate the time-dependent absorption peak profile of various forms of silver. The thermospray flame furnace was set up with a 10-cm-long nickel tube with six holes, each 2.0 mm in diameter, to allow the flame to enter, and this nickel tube acted as a furnace. A sample of 300 μL was introduced into this furnace by use of water as a carrier at a flow rate of 0.5 mL min -1 through the ceramic capillary (0.5-mm inner diameter and 2.0-mm outer diameter), which was inserted into the front hole of the nickel tube. The system was applied to examine atomization behaviors of silver nanoparticles (AgNPs) with particle sizes ranging from 10 to 100 nm. The atomization rate of AgNPs was faster than that of the dissolved silver ion. With increased amount of silver, the decay time observed from the time-dependent absorption peak profile was shortened in the case of dissolved silver ion, but it was increased in the case of AgNPs. With the particle size ranging from 10 to 100 nm, the detection sensitivity was indirectly proportional to the particle size, suggesting that TS-FF-AAS may offer insights into the particle size of AgNPs provided that the concentration of the silver is known. To obtain quantitative information on AgNPs, acid dissolution of the particles was performed before TS-FF-AAS analysis, and recoveries of 80-110% were obtained.

The potential of biased cyclical electrical field-flow fractionation (BCyElFFF), which applies the positive cycle voltage longer than the negative cycle voltage, for characterization of submicron particles, was investigated. Parameters affecting separation and retention such as voltage, frequency, and duty cycle were examined. The results suggest that the separation mechanism in BCyElFFF in many cases is more related to the size of particles, as is the case with normal ElFFF, in the studied conditions, than the electrophoretic mobility, which is what the theory predicts for CyElFFF. However, better resolution was obtained when separating using BCyElFFF mode than when using normal CyElFFF. BCyElFFF was able to demonstrate simultaneous baseline separations of a mixture of 0.04-, 0.1-, and 0.2-μm particles and near separation of 0.5-μm particles. This study has shown the applicability of BCyElFFF for separation and characterization of submicron particles greater than 0.1-μm in size, which had not been demonstrated previously. The separation and retention results suggest that for particles of this size, retention is based more on particle size than on electrophoretic mobility, which is contrary to existing theory for CyElFFF.