Fabrication and characterization of nanofluidic channels for ion and mass transport: graphene and piezoelectric-based materials

Recent developments in nanotechnology including fabrication, measurement and characterization have enabled the discovery of interesting and novel phenomena at the nanoscale, in particular transport through fabricated nanochannels. There are many advantages of scaling down the laboratory below the sub-millimeter range, such as faster reaction rates due to the larger surface-to-volume ratio, smaller sample volume, high throughput, potentially low cost by utilizing mass production, and capability for compact and portable analysis systems. As the channel dimension goes down to the nanometer scale, unique properties emerge, for example, overlapped electric double layers, reduced electroosmotic flow, increased viscosity, ion current rectification, ion enrichment and depletion. These novel transport properties of solvents and solutes/ions through nanochannels emerge from the combination of strong confinement and interactions with the channel inner surfaces on the nanoscale. As a prerequisite to the development of nanofluidic devices and discovering novel transport phenomena, various nanofabrication techniques have been investigated by many researchers for decades. However, the available technologies used to fabricate nanochannels are often expensive, time consuming, or simply not accurate enough. This creates a need for controlled yet simple, cost-effective, rapid, and scalable fabrication technologies. In this dissertation, we report three novel phenomena discovered by fabrication and/or characterization of nanofluidics with graphene and piezoelectric-based materials. In the first approach, a dry and rapid (∼10 s) method was used to deagglomerate bulk, unbound multi-walled carbon nanotube (CNT) bundles into individual CNTs aligned along a common axis using surface acoustic waves (SAW) and bulk shear along a piezoelectric substrate. The underlying mechanism was carefully investigated. The process first forms 1 μm CNT bundles from extremely large (∼10 Mm/s2 ) mechanical accelerations due to the SAW; these bundles are consequently susceptible to SAW-induced evanescent, quasistatic electric fields that couple into the bundles and form a mat of long (1−10 μm) individual CNTs on the substrate surface. These CNTs were then aligned along the direction of shear provided by sliding a glass cover slip 10 mm across the CNT mat. This alignment is notably independent of the SAW propagation direction. Further, the intrinsic structure of the nanotubes was unaffected as verified using Raman spectroscopy. Uniquely simple, the approach avoids the many shortcomings of other CNT deagglomeration techniques−particularly surface modification and suspension in solution−to rapidly separate and align large numbers of CNTs, thereby overcoming a key limitation in their use for a diverse range of applications. In the second approach, we discovered a quite anomalous rectification behaviour in nanochannels that make up the interstitial layers between the sheets that comprise a thin graphene oxide film. Structurally symmetric, two-dimensional multilayered graphene oxide films were shown for the first time to exhibit peculiar ion current rectification and nonlinear current-voltage characteristics below a critical electrolyte concentration. We attribute the unexpected rectification behavior to the fore-aft asymmetry (i.e. asymmetry across the length of film) that arises in the diffusion boundary layer on both sides of the millimeter long film upon reversal between the high resistance positive bias state and the low resistance negative bias state, the asymmetry being primarily a consequence of the trapping and release of counterions within the film, compounded by the nonuniform electric field that occurs in the tortuous nanochannels within the film. In addition, the influence of applied bias voltage and the electrolyte concentration and pH on the ion transport and ectification factor were studied. Furthermore, we showed how the nonlinear current-voltage characteristics and, in particular, ion current rectification in these platforms can be enhanced about 2-3 times through simple ways through which the asymmetry in the system can be additionally increased. Three sources of asymmetries were introduced to the system, including geometrical asymmetry, solution pH asymmetry, and solution ionic concentration asymmetry. Last but not least, we demonstrated, for the first time, the possibility for the generation of osmotic power through salinity gradients using these planar graphene-based devices. These first graphene-based nanofluidic rectifiers, which are easily synthesized, therefore offer a flexible, robust, low cost, and facile large-scale alternative to conventional nanochannels that require elaborate and sophisticated nanofabrication. In the third approach, for the first time, study of transport phenomena in a nanogap confining the fluid on a vibrating solid surface, which has an extremely high surface acceleration, became possible by integrating surface acoustic waves generated on a piezoelectric substrate (Lithium Niobate, LN) and planar nanofluidic channels milled in the same piezoelectric material. A facile deep reactive ion etching technique was used to form various high aspect ratio (width to depth), planar nanochannels in LN with depths down to the nanometer scale. A LN-LN bonding method was developed to seal the nanochannel on the vibrating surface. Finally, the sealing of the final device was tested by pumping water and fluorescent dye through the nanochannel. Fluid pumping and formation of chaotic flow in this acoustic nanofluidic system are reported. In addition, using transparent piezoelectric materials enables both electrical and optical characterization of fluids within the nanochannels. This acoustic nanofluidic system can be used for rapid mixing, detection and fast mass or ions transport at the nanoscale overcoming the major drawback of most micro- nanofluidic systems: the slow and highly laminar flow present inside the nanochannels.