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

摘要

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 highland 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.