Fundamental Studies of Nanofluidics: Nanopores, Nanochannels, and Nanopipets

Ion, particle, and fluid transport in nanofluidic devices has received considerable attention over the past two decades due to unique transport properties exhibited at the nanoscale.1,2 Phenomena such as double layer overlap, high surface-to-volume ratios, surface charge, ion-current rectification, and entropic barriers can influence transport in and around nanofluidic structures because the length scales of these forces and the critical dimensions of the device are similar. Advances in micro- and nanofabrication techniques provide the ability to design a variety of well-defined nanofluidic geometries to study these phenomena and their effects on ion and fluid transport. Integration of micro- and nanofluidic structures into lab-on-a-chip devices permits increased functionality that is useful for a range of analytical applications.3,4 This Review focuses on recent advances in nanofabrication techniques as well as studies of fundamental transport in nanofluidic devices. Nanopores, nanochannels, and nanopipets are three common nanofluidic structures that have been influential in studying nanofluidic transport. Because of space limitations, we have limited the scope of this Review to studies with these three structures, and we focus our attention primarily on work published between January 2011 and August 2014. We do not discuss work with carbon nanotubes,5 nanomeshes,6 or nanowires.7 Figure ​Figure11 shows examples of the three nanofluidic geometries discussed here. Nanopores are typically formed perpendicular to the plane of a substrate and are characterized by a critical limiting dimension, which is measured by scanning electron microscopy (SEM), transmission electron microscopy (TEM), or conductance measurements. Pores are fabricated in a variety of materials, e.g., poly(carbonate), poly(ethylene terephthalate), or silicon nitride, and can have an asymmetric (Figure ​(Figure1a)1a) or symmetric (Figure ​(Figure1b)1b) shape, depending on the fabrication technique. Symmetric pores are either cylindrically shaped with a constant critical dimension determined by electron microscopy or hourglass-shaped with a critical dimension at the center of the pore. Although electron microscopy is capable of measuring exterior pore dimensions, the exact inner geometry is often unknown and may contain an asymmetry between two symmetric features, e.g., cigar-shaped pores. Asymmetric nanopores typically have a narrow tip and a wide base with a funnel-shaped geometry along the pore axis. Tip and base dimensions are measured by SEM, but the exact pore geometry is often unknown. Nanochannels often refer to in-plane structures with either symmetric (Figure ​(Figure1c)1c) or asymmetric (Figure ​(Figure1d)1d) geometries. Channels may be confined to the nanoscale in depth, width, or both, depending on the fabrication method. Nanochannels are commonly fabricated in glass and polymer substrates and characterized by SEM and atomic force microscopy (AFM). The in-plane nature of these channels allows the integration of well-defined features into more complex geometries, and any two-dimensional (2D) channel architecture can be designed. Nanopipets are specialized nanopores fabricated from pulled glass or fused-silica capillaries (Figure ​(Figure1e,f). The1e,f). The geometry of a nanopipet is conically shaped with a critical tip diameter of tens to hundreds of nanometers, which can be measured by electron microscopy. Unlike nanopores and nanochannels, nanopipets can be easily coupled with position control, which allows the tip of the nanopipets to be positioned in specific locations or used in scanned probe microscopies. Figure 1 Nanopores, nanochannels, and nanopipets are three common nanofluidic platforms. Nanopores are typically out-of-plane structures and have either an asymmetric or symmetric geometry. Conical nanopores have a wide base as shown in panel a that tapers to ...