Profiling pH gradients across nanocapillary array membranes connecting microfluidic channels

Nanocapillary array membranes (NCAMs), comprised of thin (d∼ 5-10 μm) nuclear track-etched polycarbonate sheets containing ∼10⁸ cm² nearly parallel nanometer-diameter capillaries, may act to gate fluid transport between microfluidic channels to effect, for example, sample collection. There is interest in H⁺-transport across these NCAMs because there is significant practical interest in being able to process analyte-containing samples under different pH conditions in adjacent layers of an integrated microfluidic circuit and because protons, with their inherently high mobility, present a challenge in separating microfluidic environments with different properties. To evaluate the capability of NCAMs to support pH gradients, the proton transport properties of NCAMs were studied using laser scanning confocal fluorescence microscopy (LSCFM). Spatiotemporal maps of [H⁺] in microfluidic channels adjacent to the NCAMs yield information regarding diffusive and electrokinetic transport of protons. The NCAMs studied here are characterized by a positive zeta potential, ζ gt; 0, so at small nanocapillary diameters, the overlap of electrical double layers associated with opposite walls of the nanocapillary establish an energy barrier for either diffusion or electrokinetic transport of cations through the nanometer-diameter capillaries due to the positive charge on the nanocapillary surface. Proton transfer through an NCAM into microchannels is reduced for pore diameters, d ≤ 50 nm and ionic strengths I ≤ 50 mM, while for large pore diameters or solution ionic strengths, the incomplete overlap of electric double layer allows more facile ionic transfer across the membranes. These results establish the operating conditions for the development of multilevel integrated nanofluidic/microfluidic architectures which can support multidimensional chemical analysis of mass-limited samples requiring sequential operations to be implemented at different pH values.