Digital, ultrasensitive, end-point protein measurements with large dynamic range via Brownian trapping with drift.

This communication shows that the concept of Brownian trapping with drift can be applied to improve quantitative molecular measurements. It has the potential to combine the robustness of end-point spatially resolved readouts, the ultrasensitivity of digital single-molecule measurements, and the large dynamic range of qPCR; furthermore, at low concentrations of analytes, it can provide a direct comparison of the signals arising from the analyte and from the background. It relies on the finding that molecules simultaneously diffusing, drifting (via slow flow), and binding to an array of nonsaturable surface traps have an exponentially decreasing probability of escaping the traps over time and therefore give rise to an exponentially decaying distribution of trapped molecules in space. This concept was tested with enzyme and protein measurements in a microfluidic device.