Conductance Interplay in Ion Concentration Polarization across 1D Nanochannels: Microchannel Surface Shunt and Nanochannel Conductance.

Abstract

Ion concentration polarization has received much interest in the past decade for lab-on-a-chip applications, primarily preconcentration of biomolecules and water desalination. Studying the basic phenomenon in microfluidics has also generated new knowledge, which could be pivotal in the design of novel devices. Many studies, however, have focused on designs featuring nanoslits/slots or surface-patterned ion-selective membranes whereas the characteristics of 1D nanochannels are still lacking. Here, we report on ion concentration polarization across highly ordered 1D nanochannels in isolation as well as in array formation. Intriguingly, the nanochannels in isolation exhibit a linear current-voltage characteristic at low salt concentrations despite the confirmed presence of ion-depletion zone, which is associated with the diffusion-limited transport and the consequent nonlinearity in the classical sense. The characteristic in array formation breaks away from the linearity with a peculiar dip in current for a critical salt concentration in the dilute limit. We describe these findings based on the interplay between the nanochannel conductance and the conductance of the neighboring microchannel walls (the so-called surface shunt). Also, the nanochannel transport is identified with the mobility of protons more closely than that of salt cations. We attribute fast transport to phosphorus-doped silicate glass, the nanochannel material known to have very fine pores likely to be populated with protons as a result of moisture and carbon dioxide adsorption from the air. The nanochannels possess a tubular profile 70 nm in nominal diameter and fabricated through thermal reflow of doped glass on silicon without using advanced lithography.