A microfluidic device includes a first substrate including at least one microfluidic channel and a plurality of microwells, as well as a cooperating second substrate defining multiple split-walled cell trap structures that are registered with and disposed within the plurality of microwells. A method for performing an assay includes flowing cells and a first aqueous medium into a plurality of microwells of a microfluidic device, wherein each microwell includes a cell trap structure configured to trap at least one cell. The method further comprises flowing a nonpolar fluid with low permeability for analytes of interest through a microfluidic channel to flush a portion of the first aqueous medium from the microfluidic channel while retaining another portion of the first aqueous medium and at least one cell within each microwell. Surface tension at a non-polar/polar medium interface prevents molecule exchange between interior and exterior portions of microwells.