Diffusivity of sub-1,000 Da molecules in 40 nm silicon-based alumina pores

Abstract

Porous anodic alumina has been of an increasing interest to applications of on-chip biosensors and bioseparations. However, the characteristics of molecular diffusion in alumina nanopores have not been fully explored. Here, we have investigated an ultra-thin freestanding alumina membrane fabricated on a silicon substrate as a new on-chip diffusion system. Sub-1,000 Da molecules such as caffeine were diffused through alumina pores with a pore diameter of 40 nm and a pore length of 1.2 μm. The diffusion dynamics was characterized by modeling the molecular transport as one-dimensional convective Fickian flow. The diffusion coefficients were calculated to be on the order of 10−8 cm2 s−1. The transport rate of coions was enhanced by increasing the ionic strength of diffusion solution. Relative to thick alumina membranes, the thin-film alumina was able to achieve a significantly higher flux rate, making it more favorable for rapid molecular transport. The characterizations we present here have contributed important experimental data to alumina nanofluidics, and are believed to be valuable for applications such as drug deliveries, molecular separations, and membrane biosensors.