We present a compact portable biosensor to measure arsenic As(III) concentrations in water using Escherichia coli bioreporter cells. Escherichia coli expresses green fluorescent protein in a linearly dependent manner as a function of the arsenic concentration (between 0 and 100μg/L). The device accommodates a small polydimethylsiloxane microfluidic chip that holds the agarose-encapsulated...