WEI-LUN HSU
PhD
Nanofluidics and BioMEMS
The significant advances in semiconductor manufacturing techniques and increasing demands for more efficient Micro-Total-Analysis-Systems (also referred as Lab-on-a-Chip devices or Micro-Electro-Mechanical-Systems) during the last two decades have stimulated interest of micro/nanofluidics in silica channels. These miniaturised devices have been broadly applied to biomedical science and nanotechnology due to their high efficiency and low cost characteristics over traditional instruments. Applications include bio-sensors and bio-collectors of DNA and RNA, simultaneous nanofluidic focusing and separation of proteins, micro/nanofluidic battery, nanofluidic diode and bipolar transistor, micro/nano electrokinetic pumps, and electronics cooling heat exchangers. The understanding of electrokinetics at the silica/electrolyte interfaces is needed to analyse fluid and ion behaviour in these devices and hence to optimise operating conditions. Hence, computational simulations are employed to achieve the target.
Electrokinetics
When exposed to an electrolyte solution,biomolecules are electric charged due to surface protonation/deprotonation functional groups. The behaviour of these molecules are subject to the surrounding conditions, such as electric potential, ion concentraions, temperature and pH etc. This allows us to control the molecules by changing the solution conditions.
In this work, electrokinetic behaviour of a biomolecule in an electrolyte solution in the presence of a salt concentration gradient in a nanopore is investigated using numerical simulations. The non-uniform electric double layer thickness on the molecule due to the salt gradient induces a polarisation electric field which drives the molecule towards the high or low concentration side depending on the solution conditions and pore size. The effect of electric double layer, pore radius and ion diffusivities on the diffusiophoretic behaviour of the molecules is examined.
Silica/Electrolyte Interface
The manipulation of biological macromolecules in a new breed of nanofluidic devices for applications requires a quantitative understanding of interfacial electrokinetic phenomena within silica nanochannels. It was found that several issues regarding the conventional electric double layer model at the silica/water interface exist, as evident by a lack of consistency within the literature for properties such as the (i) equilibrium constant of surface silanol groups, (ii) Stern layer capacitance, (iii) zeta potential measured by various electrokinetic methods, and (iv) surface conductivity.
In this research, a silica and water interface model is constructed based on the viscoelectric effect - that is, the increase of the local viscosity due to the polarisation of polar solvents. The model is validated by comparing theoretical results with those from previous experiments, conducted using four fundamental electrokinetic phenomena: electrophoresis, electroosmosis, streaming current and streaming potential.
Lab-on-a-Chip
During the last decade, electrokinetic phenomena originating from the electric double layer has been widely applied to biosensor technology for biomolecule manipulation and analysis. More effective but cheaper analyte focusing and separation methods are relevant to the next generation of Point-of-Care diagnostics. Using this as motivation, two protein trapping and separation methods, Concentration Gradient Focusing and Isoelectric Focusing, in rectangular and tapered nanochannels are examined using numerical simulations. The theoretical predictions are in quantitative agreement with previous experimental results validating the model.
It is shown that concentration and pH distributions in the nanochannels are largely affected by electroosmotic flow and electromigration behavior of ions. In addition, non-uniform electric double layer thickness and silica surface charge along the channel surface due to the concentration and pH gradients induce a non-uniform electroosmotic flow in the nanochannel which significantly affects the protein trapping and separation behaviour.