Perturbation of Microfluidic Transport Following Electrokinetic Injection through a Nanocapillary Array Membrane: Injection and Biphasic Recovery

Ionic transport in nanopores is dependent on the nature of the electrical communication between the pores and the surrounding environment. A particularly useful fluidic device structure uses nanopores in nanocapillary array membranes (NCAMs) as electrically switchable valves between vertically separated microfluidic channels. In the off-state, the gate isolates the fluidic environments in the microchannels, but when the appropriate forward-bias voltage is applied, it selectively allows ions and analytes to move between the microchannels. However, the populations of species in the microfluidic channels are perturbed from their steady-state values due to ion accumulation and depletion effects. Experiments conducted here characterize the electrical conduction along the length of a microfluidic channel, and laser-induced fluorescence probes the formation of a high- and low-concentration regions of fluorescent dye before and after application of forward- and reverse-bias voltage pulses in both small ( a = 10 nm) and large ( a = 100 nm) pore NCAMs. In all cases, switching from injection (transport across the NCAM) to microfluidic flow (transport only in the microfluidic channel) results in a multiphasic current recovery profile, signifying the presence of ion accumulation and depletion regions at the microfluidic−nanofluidic boundary, that is, in the region adjacent to the NCAM. The behavior is consistent with a model in which a volume of altered ion concentration is created at the microfluidic−nanofluidic boundary upon injection. Switching back to microfluidic flow causes this altered conductivity region to be swept from the microfluidic channel, re-establishing the steady state conduction properties.