Microfluidic device for analyzing preferential chemotaxis and chemoreceptor sensitivity of bacterial cells toward carbon sources

We present a novel microfluidic device that enables high sensitive analyses of the chemotactic response of motile bacterial cells (Escherichia coli) that swim toward a preferred nutrient by sorting and concentrating them. The device consists of the Y-shaped microchannel that has been widely used in chemotaxis studies to attract cells toward a high concentration and a concentrator array integrated with arrowhead-shaped ratchet structures beside the main microchannel to trap and accumulate them. Since the number of accumulated cells in the concentrator array continuously increases with time, the device makes it possible to increase the sensitivity of detecting chemotactic responses of the cells about 10 times greater than Y-shaped channel devices in 60 min. In addition, the device can characterize the relative chemotactic sensitivity of chemoreceptors to chemoeffectors by comparing the number of cells in the concentrator array at different distances from the channel junction. Since the device allows the analysis of both the chemotactic responses and the sensitivity of chemoreceptors with high resolution, we believe that not only can the device be broadly used for various microbial chemotaxis assays but it also can further the advancement of microbiology and even synthetic biology.close9

Two-Dimensional Dirac Fermions Protected by Space-Time Inversion Symmetry in Black Phosphorus

We report the realization of novel symmetry-protected Dirac fermions in a surface-doped two-dimensional (2D) semiconductor, black phosphorus. The widely tunable band gap of black phosphorus by the surface Stark effect is employed to achieve a surprisingly large band inversion up to ~0.6 eV. High-resolution angle-resolved photoemission spectra directly reveal the pair creation of Dirac points and their moving along the axis of the glide-mirror symmetry. Unlike graphene, the Dirac point of black phosphorus is stable, as protected by spacetime inversion symmetry, even in the presence of spin-orbit coupling. Our results establish black phosphorus in the inverted regime as a simple model system of 2D symmetry-protected (topological) Dirac semimetals, offering an unprecedented opportunity for the discovery of 2D Weyl semimetals