An Adaptive Anti-Brownian Electrokinetic Trap with Real-Time Information on Single-Molecule Diffusivity and Mobility

We present the design and implementation of an adaptive Anti-Brownian ELectrokinetic (ABEL) trap capable of extracting estimates of the diffusion coefficient and mobility of single trapped fluorescent nanoscale objects such as biomolecules in solution. The system features rapid acousto-optic scanning of a confocal excitation spot on a 2D square lattice to encode position information on the arrival time of each detected photon, and Kalman filter-based signal processing unit for refined position estimation. We demonstrate stable trapping of multisubunit proteins (D ≈ 22 μm(2)/s) with a count rate of 6 kHz for as long as 15 s and small single-stranded DNA molecules (D ≈ 118 μm(2)/s) at a 15 kHz count rate for seconds. Moreover, we demonstrate real-time measurement of diffusion coefficient and electrokinetic mobility of trapped objects, using adaptive tuning of the Kalman filter parameters.