Bacterial chemotaxis-enabled autonomous sorting of nanoparticles of comparable sizes

High throughput sorting of micro/nanoparticles of similar sizes is of significant interest in many biological and chemical applications. In this work, we report a simple and cost-effective sorting technique for separation of similarly-sized particles of dissimilar surface properties within a diffusion-based microfluidic platform using chemotaxis in Escherichia coli bacteria. Differences in surface chemistry of two groups of similarly-sized nanoparticles in a mixture were exploited to selectively assemble one particle group onto motile E. coli, through either specific or non-specific adhesion, and separate them from the remaining particle group via chemotaxis of the attached bacteria. To enable optimal operation of the sorting platform, the chemotaxis behavior of E. coli bacteria in response to casamino acids, the chemoeffector of choice was first characterized. The chemical concentration gradient range within which the bacteria exhibit a positive chemotactic response was found to be within 0.25 × 10−7–1.0 × 10−3 g ml−1 mm−1. We demonstrate that at the optimum concentration gradient of 5.0 × 10−4 g ml−1 mm−1, a sorting efficiency of up to 81% at a throughput of 2.4 × 105 particles per min can be achieved. Sensitivity of the sorting efficiency to the adhesion mechanism and particle size in the range of 320–1040 nm was investigated.