1.D.70 The Metallic (Au/Ag/Pt/graphene) Nanopore (MetNP) Family
The introduction of metallic elements into microfluidic devices that support electrokinetic transport creates several fundamental issues relative to the high conductivity of the metal, which can act as a current shunt, causing profound effects on a transport process. Piruska et al. 2010 examined the use of Au-coated nanocapillary array membranes (Au NCAMs) as electrically addressable fluid control elements in multi-layer microfluidic architectures. Three alternative methods for fluid injection across Au NCAMs were presented: electrokinetic injection across NCAMs with Au coated on one side (asymmetric NCAM), electrokinetic injection across NCAMs with an embedded Au layer (symmetric NCAM), and field-free electroosmotic flow (EOF) pumping across either type of Au NCAM. Injection efficiency across asymmetric NCAMs depended on the orientation of the asymmetric membrane relative to the driving potential. Efficient injections were enabled when the Au coating was on the receiving side of the membrane. These results for asymmetric membranes agreed qualitatively with two-dimensional numerical simulations of injections across a single slit pore, which suggested that the direction-selective transport behavior was related to electrophoretic transport of the anionic fluorescein probe. EOF pumping could be an alternative to electrokinetic injections in some applications, but this approach is only useful for relatively large pore sizes (>400 nm) (Piruska et al. 2010).
As noted above, nanopore sensors embedded within thin dielectric membranes are single molecule sensitive and can detect large ranges of analytes, from DNA and proteins, to small molecules and particles. Cecchini et al. 2013 utilized a metallic Au solid-state membrane to translocate and rapidly detect single Au nanoparticles functionalized with 589 dye molecules using surface-enhanced resonance Raman spectroscopy (SERRS). They showed that, due to the plasmonic coupling between the Au metallic nanopore surface and the nanopore, signal intensities were enhanced when probing analyte molecules bound to the nanopore surface. This nanopore sensing scheme benefits from the ability of SERRS to provide vibrational information on the analyte, improving on current nanopore-based electrical and optical detection techniques. Cecchini et al. 2013 showed that the full vibrational spectrum of the analyte can be detected with ultrahigh spectral sensitivity and a rapid temporal resolution of 880 μs. Graphene nanopores can be utilized for protein sequencing (Wilson et al. 2016).
Two functionalized graphene nanopore models (i.e., co_5 and coo_5) inspired by the characteristic structural features of Mg2+ channels exhibit higher preferences for Mg2+ than Li+, with selectivity ratios higher for coo_5 than for co_5 at all studied transmembrane voltages (Zhu et al. 2017). An evaluation of the effect of coordination on the ionic hydration microstructures for both nanopores showed that the positioning of the modified groups could better fit a hydrated Mg2+ than a hydrated Li+, as if Mg2+ was not dehydrated according to hydrogen bond analysis of the ionic hydration shells. This condition led to a lower resistance for Mg2+ than for Li+ when traveling through the nanopores. Moreover, a distinct increase in hydrogen bonds occurred with coo_5 compared with co_5 for hydrated Li+, which made it more difficult for Li+ to pass through coo_5. These findings provide design principles for developing artificial Mg2+ channels, which have potential applications as Mg2+ sensors (Zhu et al. 2017).