RESUMEN
Low-frequency AC electrophoresis lies in a regime between DC microchannel electrophoresis and dielectrophoresis, which typically utilizes frequencies above 1000 Hz. Although few electrophoretic methods have been reported in this ≤100 Hz range, traveling wave electrophoresis (TWE) and transverse AC electrophoresis (TrACE) operate in this frequency range, and use low voltages to avoid bubble formation from water electrolysis. TWE provides molecular separations with enhanced control and TrACE provides multiplexed, multiparameter particle characterization. However, two related fundamental questions remain about the mechanisms of electrophoretic migration in these systems. First, particle electrophoresis in TrACE is largely captured by a simple model that combines the alternating electric field with DC electrokinetics, but a deviation from the model is observed for applied square electric field waves that increases with decreasing frequency. Second, although electrode charging is believed to drive ion migration in TWE, the estimated electrode charging time is about 2-3 orders of magnitude faster than the wave period. In this study, a 1D finite numerical model that excludes Faradaic reactions simulates ion and particle migration across the microchannel width in TrACE. The 1D model results show good agreement with both particle and ion migration in TrACE systems. Furthermore, although ion migration between the pair of electrodes slows during each excursion of a 1 Hz square wave, there is substantial ion migration throughout the 0.5 s half-period. This modeling result agrees with experimental observations in TWE. Therefore, the clarification of the mechanisms of ion migration in these low-frequency and low-voltage AC electrophoresis is expected to expand their applications.
RESUMEN
The ability to measure the charge and size of single particles is essential to understanding particle adhesion and interaction with their environment. Characterizing the physical properties of biological particles, like cells, can be a powerful tool in studying the association between the changes in physical properties and disease development. Currently, measuring charge via the electrophoretic mobility (µep) of individual particles remains challenging, and there is only one prior report of simultaneously measuring µep and size. We introduce microfluidic transverse AC electrophoresis (TrACE), a novel technique that combines particle tracking velocimetry (PTV) and AC electrophoresis. In TrACE, electric waves with 0.75 to 1.5 V amplitude are applied transversely to the bulk flow and cause the particles to oscillate. PTV records the particles' oscillating trajectories as pressure drives bulk flow through the microchannel. A simple quasi-equilibrium model agrees well with experimental measurements of frequency, amplitude, and phase, indicating that particle motion is largely described by DC electrophoresis. The measured µep of polystyrene particles (0.53, 0.84, 1, and 2 µm diameter) are consistent with ELS measurements, and precision is enhanced by averaging â¼100 measurements per particle. Particle size is simultaneously measured from Brownian motion quantified from the trajectory for particles <2 µm or image analysis for particles ≥2 µm. Lastly, the ability to analyze intact mammalian cells is demonstrated with B cells. TrACE systems are expected to be highly suitable as fieldable tools to measure the µep and size of a broad range of individual particles.
RESUMEN
Correction for 'Measuring the electrophoretic mobility and size of single particles using microfluidic transverse AC electrophoresis (TrACE)' by M. Hannah Choi et al., Lab Chip, 2023, https://doi.org/10.1039/D3LC00413A.
RESUMEN
The SARS-CoV-2 omicron variant presented significant challenges to the global effort to counter the pandemic. SARS-CoV-2 is predicted to remain prevalent for the foreseeable future, making the ability to identify SARS-CoV-2 variants imperative in understanding and controlling the pandemic. The predominant variant discovery method, genome sequencing, is time-consuming, insensitive, and expensive. Ultraperformance liquid chromatography-mass spectrometry (UPLC-MS) offers an exciting alternative detection modality provided that variant-containing peptide markers are sufficiently detectable from their tandem mass spectra (MS/MS). We have synthesized model tryptic peptides of SARS-CoV-2 variants alpha, beta, gamma, delta, and omicron and evaluated their signal intensity, HCD spectra, and reverse phase retention time. Detection limits of 781, 781, 65, and 65 amol are obtained for the molecular ions of the proteotypic peptides, beta (QIAPGQTGNIADYNYK), gamma (TQLPSAYTNSFTR), delta (VGGNYNYR), and omicron (TLVKQLSSK), from neat solutions. These detection limits are on par with the detection limits of a previously reported proteotypic peptide from the SARS-CoV-2 spike protein, HTPINLVR. This study demonstrates the potential to differentiate SARS-CoV-2 variants through their proteotypic peptides with an approach that is broadly applicable across a wide range of pathogens.