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Mucosal antibodies in the upper respiratory tract are the earliest and most critical responders to prevent respiratory infections, providing an indication for the rapid evaluation of immune protection. Here, we report a microfluidic particle counter that directly visualizes mucosal antibody levels in nasal mucus. The mucosal anti-SARS-CoV-2 spike receptor binding domain (RBD) antibodies in nasal secretions first react with magnetic microparticles (MMPs) and polystyrene microparticles (PMPs) that are surface-modified to form a "MMPs-anti-spike RBD IgG-PMPs" complex when RBD is present. After magnetic separation and loading into the microfluidic particle counter, the free PMPs, which are reduced with increasing anti-spike RBD IgG antibody levels, are trapped by a microfluidic particle dam and accumulate in the trapping channel. A sensitive mode [limit of detection (LOD): 14.0 ng mL-1; sample-to-answer time: 70 min] and an equipment-free rapid mode (LOD: 37.4 ng mL-1; sample-to-answer time: 20 min) were achieved. Eighty-seven nasal secretion (NS) samples from vaccinees were analyzed using our microfluidic particle counter, and the results closely resemble those of the gold-standard enzyme-linked immunosorbent assay (ELISA). The analysis shows that higher antibody levels were found in convalescent volunteers compared to noninfected volunteers. Together, we demonstrate a rapid kit that directly indicates immune status, which can guide vaccine strategy for individuals and the government.

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The intracellular sodium ion is one of the crucial elements for regulating physiological functions such as action potential and muscle contractions. However, detecting sodium ions in live cells is challenging because false signals may arise from the abundant sodium ions in the extracellular environment when introducing the detection agents. To minimize it, we report a DNAzyme-based detection of sodium ions in live cells via activation by endogenous mRNA. The substrate strand of DNAzyme first hybridizes to a blocking strand that prevents undesired cleavage of DNAzyme when delivered. Once entering cells, an endogenous mRNA biomarker binds to the toehold region of the blocking strand and displaces it, allowing the proper formation of the DNAzyme, which specifically catalyzes the cleavage of the substrate strand in the presence of intracellular Na+ and produces fluorescence signals. Using differentiating skeletal muscle cells as the model system, we demonstrated the successful delivery and phenotype-specific detection of intracellular sodium ions only in differentiated myotubes with highly-expressed myosin heavy chain mRNA. Moreover, using a drug cocktail to increase the permeability of the cell membrane, elevated levels of intracellular sodium ion was observed. This platform offers a broad and promising strategy for detecting intracellular metal ions, suggesting a great potential for understanding its role in cell/tissue physiology.

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In December 2019, coronavirus disease 2019 became a pandemic affecting more than 200 countries and territories. Millions of lives are still affected because of mandatory quarantines, which hamstring economies and induce panic. Immunology plays a major role in the modern field of medicine, especially against virulent infectious diseases. In this field, neutralizing antibodies are heavily studied because they reflect the level of infection and individuals' immune status, which are essential when considering resumption of work, flight travel, and border entry control. More importantly, it also allows evaluating the antiviral vaccine efficacy as vaccines are still known for being the ultimate intervention method to inhibit the rapid spread of virulent infectious diseases. In this Review, we first introduce the host immune response after the infection of SARS-CoV-2 and discuss the latest results using conventional immunoassays. Next, as an enabling platform for detection with sufficient sensitivity while saving analysis time and sample size, the progress of microfluidic-based immunoassays is discussed and compared based on surface modification, microfluidic kinetics, signal output, signal amplification, sample matrix, and the detection of anti-SARS-CoV-2 antibodies. Based on the overall comparison, this Review concludes by proposing the future integration of visual quantitative signals on microfluidic devices as a more suitable approach for general use and large-scale surveillance.

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Cell chirality is observed with diverse forms and coordinates various left-right (LR) asymmetry in tissue morphogenesis. To give rise to such diversity, cell chirality may be coupled with cell differentiation. Here, using micropatterned human mesenchymal stem cells (hMSCs), an early committed clockwise (CW) cell chirality that can itself upregulate the adipogenic differentiation is reported. hMSC chirality enables a positively tilted chiral orientation on micropatterned stripes. When cultured as single cells on circular micropatterns, an anticlockwise (ACW)-biased nucleus rotation and swirling pattern of actin filament are observed. Interestingly, with adipogenic induction for 3-6 days, such chirality is reversed to negative chiral orientation and CW-biased rotation, which is earlier than the maturation of other differentiation markers, and consistently expressed in terminally differentiated adipocytes. Using latrunculin A (LatA), cytochalasin D (CD), and nocodazole (Noco) that forces a CW-biased actin filament and nucleus rotation resembling the early differentiated chirality upon adipogenic induction, an upregulation of adipogenic differentiation is found. The result demonstrates that the early differentiated chirality may serve as a mechanical precursor to engage the lineage commitment, suggesting a feedback mechanism of chiral actin in regulating cell differentiation and LR morphogenesis.

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Lead contamination in drinking water is a primary concern in public health, but it is difficult to monitor by end-users. Here, we provide a rapid and power-free microfluidic particle dam which enables visual quantification of lead ions (Pb2+) by the naked eye. GR-5 DNAzyme with extended termini can connect magnetic microparticles (MMPs) and polystyrene microparticles (PMPs) by DNA hybridization, forming "MMPs-GR-5-PMPs". When Pb2+ is present, GR-5 is cleaved, resulting in an increasing number of free PMPs. To visually count the free PMPs, the solution is loaded to a capillary-driven microfluidic device that consists of a magnetic separator to remove the MMPs-GR-5-PMPs, followed by a particle dam that traps and accumulates the free PMPs into a visual bar with growing length proportional to the concentration of lead. The device achieved a limit of detection at 2.12 nM (0.44 ppb), high selectivity (>20,000-fold) against other metal ions, high tolerance to different pH and water hardness, and is compatible with tap water with a high recovery rate, enabling visual quantification and user-friendly interface for rapid screening of water safety.

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