Fine Sieving of Atmospheric Particles in a Collected Air Sample Using Oil Electrophoresis

To solve the challenge of extracting nanoto micrometer-sized atmospheric particles from a mixed sample, we developed an electrostatic sieve system, the Fine Sieving of Collected Atmospheric Particles using Oil Electrophoresis (iSCAPE), based on the application of an electrostatic field to a non-conductive mineral oil. Using atmospheric samples, which were collected from different cities, in addition to soil and road dust samples, we tested this system under different conditions and found that the “iSCAPE’d” particles moved rapidly at varying velocities and in two opposite directions. The diverse origins of the sample—ambient air, soil, or road dust—exhibited specific charged properties, and clearly affected the electrical mobility, as demonstrated by the graphs, of the particles following the “iSCAPEing,” which lasted from seconds to minutes. We also observed an increased abundance of particles in specific mobility ranges. Furthermore, according to our adenosine triphosphate (ATP) monitoring results, the iSCAPE is capable of separating bacterial particles by size and electrical mobility. The experimental data suggests that the iSCAPE relies heavily on the electrostatic field strength, mineral oil viscosity, and run time. In theory, this method can extract any targets from a complex sample, thus creating many research opportunities in environmental, biomedical, and life sciences.


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microbiology, the method of gel electrophoresis has been extensively used in  of different sizes out from collected atmospheric particle mixture based on their

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5 electrical mobility differences.

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7

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In this work, for different samples, we took samples from various points away 136 from the sample feed points, e.g., 0.5, 1, 1.5, 2 and 3 cm. The samples were further 137 subjected to microscopic analysis using a microscope (BX 63, Olympus Co., Tokyo, 138 Japan). In addition, using a slightly modified iSCAPE system, the particle 139 electrophoresis was also directly conducted on a microscopic slide (S2112, as an example analysis we have calculated their electrical mobility, performed 146 microscopic imaging, and bacterial ATP measurements. The particle electrical 147 mobility was calculated using the following equation (Hinds, 1999): where μd is the particle electrical mobility (m 2 /(V*s), Vd (m/s) is the particle velocity, 150 E (V/m) is the uniform electrostatic field, K is a constant, Q is the particle charge, d is   sizes (13.9 μm/s). In addition to these sampling points, we have provided particle 227 separation information (iSCAPEd for 3 min) along the particle moving line in videos

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12 (File S1 and S2, Supporting Information) from which imaged particles can be seen at  To further validate the iSCAPE system, we have also performed the same tests

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13 with Beijing soil and road dust samples (Fig. S2, S3, and Fig. 5) at 3.17 kV/ cm for 2 246 min. As observed in Fig. S2, and S3 along with videos (File S3 S4, S5, and S6), the to air, these data showed that the iSCAPE system can be also applied to many other 260 samples, and the particle sieving can be fine controlled by adjusting the electrostatic 261 field strength, particle charge and the run time. The particle electrical mobility per 262 equation (1) is a function of particle charge, electrophoresis medium viscosity and 263 particle diameter (Hinds, 1999). The bacterial particle charge can be attributed to two The efficiency of the separation by the iSCAPE technology depends on the 271 particle size distribution and particle charges. Here, we described the particle . Also, they reported that the particle morphology had an 288 influence on the bipolar charge distribution, but much weaker than the mobility

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15 equivalent size. In this work, the particle mixture to be analyzed is complex, 290 consisting varying sizes of particles of different origins and materials. Of course the 291 particle charge distribution is also unknown. Here, we intended to demonstrate an 292 approach to separate particles with different electrical mobility from an already 293 collected particle mixture. This technology opens up opportunities for many air 294 pollution and its health effect experts to study collected ambient particles by size or 295 their mobility attributes, thus adding additional capability to current available 296 methods. In future efforts, similar to the literature, particle charging for the mixture 297 needs to be externally modified in order to convert the particle mobility into particle 298 size distribution or directly separate the particles from the mixture by size. An 299 advantage from this effort is that once the particles can be separated solely by size, 300 they can be used for many size-resolved post analysis including their toxicity for an 301 already collected ambient particle sample. This will significantly reduce the reliance 302 on size-resolved ambient particle sampling, and make the size-resolved ambient 303 particle analysis widely possible to academic communities and government agencies.

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Nonetheless, the iSCAPE system could be negatively impacted by the moistures in 305 the sample and possible ions in the mineral oil. Additionally, for those non-charged 306 particles they could remain at the sample feeding location without moving, and such a 307 fraction needs to be determined for samples collected under different atmospheric 308 conditions which play important roles in particle charging.

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In this work, we report an invention (the iSCAPE) that can be used to fine sieve, 313 enrich and extract desired particles including bacteria, fungi, pollen and viruses, out 314 of a particle mixture based on their electrical mobility. For particle health or haze 315 formation mechanism study, the iSCAPE can be used to extract selected particles 316 from an air sample using pre-determined operating parameters. The iSCAPE system 317 also holds a potential in separation and purification of protein and chemical molecules 318 from a biological sample. In principle, the iSCAPE system can be used to extract any 319 desired targets from a sample of environmental or medical origin, e.g., for an 320 improved PCR detection, by modifying electrical field, mineral oil viscosity, run time 321 and particle charge. Different from the measurements directly from the air, particle