(Invited) Analytical Models for Field Effect Control of Electrokinetic Transport Phenomena in Nanofluidics
Authors: Ma Yu , Qian Shizhi , Yeh Li-Hsien 
Published: February 2020

Naturally nanofluidics has applications demanding the samples to be handled in exceedingly small quantities due to the small size of the fluidic channels in nanofluidic devices, such as characterization of single biomolecules. Fluids confined in channels of nanometer characteristic dimensions exhibit physical behaviors not observed in large conduits. Charge properties of the nanochannel wall in contact with an aqueous solution play essential roles in the involved electrokinetic transport phenomena occurring in nanofluidic devices. In addition to tuning the charge properties of the nanofluidic channel wall by adjusting the solution properties such as pH and background salt concentration, field effect transistor (FET) with a gate electrode embedded beneath the nanochannel wall has been demonstrated to rapidly tune the surface charge condition and accordingly the electrokinetic transport phenomena in nanofluidic devices.

    First of all, a mathematical model for the charge properties of the nanofluidic channel and the electroosmotic flow (EOF) in a nanoslit gated by a FET is developed. In contrast to the previous studies, surface chemistry is considered. The obtained results demonstrated that the surface charge property as well as the direction and magnitude of the EOF can be actively tuned by the FET. The performance of FET control is more sensitive when the pH and/or the bulk electrolyte concentration are relatively low.

    Since the nanofluidics - based biosensing is based on discriminating the ionic current or conductance signal, active control of the surface charge property and accordingly the ionic current/conductance in nanofluidics is investigated. An analytical model for the surface charge property and the ionic conductance in a FET - gated silica nanochannel is developed considering practical effects of surface chemistry reactions, multiple ionic species, the Stern layer, and the EOF. The results show that the performance of the FET control on ionic conductance is more significant when the background salt concentration and pH are low.

    Experimental studies demonstrated that the streaming current in the nanochannel provides a simple and effective scenario for converting hydrodynamics to electrical power. The streaming current in a pH-regulated nanochannel gated by FET investigated. Analytical models for the streaming current/conductance with and without considering the electroviscous effects have been derived. The models take into account multiple ionic species, surface chemistry reactions, and the Stern layer effect. Results show the performance of the field effect modulation of the streaming conductance is significant for lower solution pH and salt concentration.

    At last, we extended the previous studies by considering the overlapping of the EDLs inside the nanochannel. The model takes into account the surface chemistry, Stern layer, multiple ionic species and the EDL overlapping effect. The model is validated by the existing experimental data of the ionic conductance in the silica nanochannel with significant EDL overlapping effect. Results from the model with and without considering EDL overlapping are compared.