Transport of Ions and Molecules in Nanofluidic Devices

Interesting transport phenomena arise when fluids are confined to nanoscale dimensions in the range of 1–100 nm. We examine three distinct effects that influence ionic and molecular transport as the size of fluidic channels is decreased to the nanoscale. First, the length scale of electrostatic interactions in aqueous solutions becomes comparable to nanochannel size and the number of surface charges becomes comparable to the number of ions in the channel. Second, the size of the channel becomes comparable to the size of biomolecules such as proteins and DNA. Third, large surface area-to-volume ratios result in rapid rates of surface reactions and can dramatically affect transport of molecules through the channel. These phenomena enable us to control transport of ions and molecules in unique ways that are not possible in larger channels. Electrostatic interactions enable local control of ionic concentrations and transport inside nanochannels through field effect in a nanofluidic transistor, which is analogous to the metal-oxide-semiconductor field effect transistor. Furthermore, by controlling surface charge in nanochannels, it is possible to create a nanofluidic diode that rectifies ionic transport through the channel. Biological binding events result in partial blockage of the channel, and can thus be sensed by a decrease in nanochannel conductance. At low ionic concentrations, the effect of biomolecular charge is dominant and it can lead to an increase in conductance. Surface reactions can also be used to control transport of molecules though the channel due to the large surface area-to-volume ratios. Rapid surface reactions enable a new technique of diffusion-limited patterning (DLP), which is useful for patterning of biomolecules and surface charge in nanochannels. These examples illustrate how electrostatic interactions, biomolecular size, and surface reactions can be used for controlling ionic and molecular transport through nanochannels. These phenomena may be useful for operations such as analyte focusing, pH and ionic concentration control, and biosensing in micro- and nanofluidic devices.