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. First, the length scale of electrostatic interactions in aqueous solutions becomes comparable to nanochannel size. 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 compared to rates of diffusive transport. These phenomena enable us to control transport of ions and molecules in unique ways that are not possible in larger channels. Surface charge can govern ionic transport in nanochannels, enabling surface reactions and biological binding events to be sensed via their effect on nanochannel conductance. The effect of biomolecular charge is dominant at low ionic concentrations, whereas the effect of biomolecular size is dominant at higher ionic concentrations. It is also possible to locally control ionic concentrations and transport inside nanochannels through field effect in a nanofluidic transistor, which is analogous to the metal-oxide-semiconductor field effect transistor. Rapid surface reactions due to the large surface area-to-volume ratios enable a new technique of diffusion-limited patterning. This technique is useful for patterning of biomolecules and surface charge in nanochannels and can be used to fabricate a nanofluidic diode with a surface charge discontinuity for rectification of ionic transport. These experiments suggest that there is potential for controlling flows inside nanochannels for operations such as analyte focusing, pH and ionic concentration control, and biosensing.