Search for: All records

Fluorescent biosensors are a valuable means to report the spatiotemporal dynamics of protein activities in live cells and animals. However, biosensors affect the activities they are reporting. This can be ameliorated by increasing sensitivity, to use lower biosensor concentrations, or by choosing designs that minimize undesirable interactions. For biosensors in which fluorescent components interact to produce Forster Resonance Energy Transfer (FRET), perturbation is often due to interaction of biosensor components with nonfluorescent, endogenous proteins, rather than productive interactions that lead to FRET. Here we engineer the interface between biosensor components using charge swap and ‘knob into hole’ mutations to reduce all but desired interactions. Novel biosensors for Rac1 and Cdc42 showed reduced interactions with endogenous GTPases and effectors, normal activation by guanine nucleotide exchange factors (GEFs), and correctly reproduced previous reports of GTPase activation dynamics. Assaying concentration-dependent effects on cell motility showed substantially reduced perturbation of normal cell behavior. Computational models indicated that minimal perturbation could be achieved over a broader range of concentrations using the new ‘orthogonal’ biosensors. 

Ultrafast transient absorption microscopy of CZTSSe kesterites. 

Live Jurkat cells were trapped by dielectrophoresis on a coplanar waveguide and the resulted changes in its reflection and transmission coefficients were measured from 900 Hz to 40 GHz. The measurement confirms that the decrease of nucleus size in a cell increases its impacts on both the reflection and transmission coefficients. Being fast, compact and label free, broadband electrical sensing may be used to detect other changes of the nucleus morphology and DNA content, which could be useful for cancer diagnosis. 

Single-connection in situ calibration using biocompatible solutions is demonstrated in single-cell sensing from 0.5 to 9 GHz. The sensing is based on quickly trapping and releasing a live cell by dielectrophoresis on a coplanar transmission line with a little protrusion in one of its ground electrodes. The same transmission line is used as the calibration standard when covered by various solutions of known permittivities. The results show that the calibration technique may be precise enough to differentiate cells of different nucleus sizes, despite the measured difference being less than 0.01 dB in the deembedded scattering parameters. With better accuracy and throughput, the calibration technique may allow broadband electrical sensing of live cells in a high-throughput cytometer. 

When a colloidal suspension is dried, capillary pressure may overwhelm repulsive electrostatic forces, assembling aggregates that are out of thermal equilibrium. This poorly understood process confers cohesive strength to many geological and industrial materials. Here we observe evaporation-driven aggregation of natural and synthesized particulates, probe their stability under rewetting, and measure bonding strength using an atomic force microscope. Cohesion arises at a common length scale (∼5 μm), where interparticle attractive forces exceed particle weight. In polydisperse mixtures, smaller particles condense within shrinking capillary bridges to build stabilizing “solid bridges” among larger grains. This dynamic repeats across scales, forming remarkably strong, hierarchical clusters, whose cohesion derives from grain size rather than mineralogy. These results may help toward understanding the strength and erodibility of natural soils, and other polydisperse particulates that experience transient hydrodynamic forces.