Manipulation and sensing of ions and molecules in nanofluidic devices

This thesis focuses on exploration of phenomena that arise in nanofluidic channels with the aim of developing fluidic devices with new functionalities that are not accessible to microfluidics. The size of nanofluidic channels (1-100 nm) is comparable to the Debye length that characterizes the range of electrostatic interactions in aqueous solutions. As a result of electrostatic interactions, surface charge can govern ionic concentrations in nanofluidic channels. The effect of surface charge was characterized using fluorescence and ionic conductance measurements and compared with theoretical predictions. Two regimes of ionic conductance were observed---surface charge-governed regime at low ionic concentrations and geometry-governed regime at high ionic concentrations.; Based on these electrostatic interactions, a nanofluidic transistor consisting of a metal gate electrode patterned on a nanofluidic channel was developed for field-effect flow control. Using fluorescence and ionic conductance measurements, it was shown that the gate voltage in a nanofluidic transistor controls the concentration of ions and biomolecules in the channel and hence controls their transport. Flow control of the protein avidin was demonstrated in a simple nanofluidic circuit consisting of a reservoir connected by two nanofluidic transistors with chemically modified surfaces to decrease nonspecific interactions. This device also demonstrated the capability for integration using the current fabrication scheme.; The effects of surface reactions and modifications on ionic conductance were studied in order to develop a sensor based on surface charge-governed and geometry-governed ionic conductance regimes in nanofluidic channels. While reactions involving small molecules could be detected by their effect on surface charge, binding of the protein streptavidin changed surface charge and also blocked part of the channel. These observations reflect interplay between the competing effects of streptavidin charge and size on the ionic conductance of nanofluidic channels.; Patterning of molecules or surface properties inside nanofluidic channels is important for flow control and integration of different functions in a single device. A new method of diffusion-limited patterning (DLP) that exploits the high surface area-to-volume ratio in nanofluidic channels was developed. This technique is self-aligning and is simple to implement for patterning multiple species. DLP was demonstrated by patterning alternating bands of fluorescently labeled and unlabeled streptavidin in biotin-functionalized nanofluidic channels. A theoretical analysis was developed to describe pattern formation and resolution.; A nanofluidic diode was fabricated using DLP to illustrate flow control with surface charge patterning. This device consisted of half the channel functionalized with the protein avidin, the other half being neutral biotin. Current rectification in the nanofluidic diode was characterized and compared with a theoretical model.; Nanofluidic devices described here are amenable to integration with other nanofluidic and microfluidic components. Integration of the devices developed here into micro/nanofluidic systems may greatly expand the capabilities of current microfluidic and nanofluidic technologies for applications in biochemical processing and analysis.