Overview The range of electrostatic interactions in ionic solutions is characterized by the Debye length, which is typically 1-100 nm in aqueous solutions. When a fluidic channel is scaled down to dimensions comparable to the Debye length, electrostatic interactions govern the ionic environment in such nanofluidic channels. Based on this unique property, we have developed a nanofluidic transistor for electrokinetic flow control of ions and molecules. Analogous to a field-effect transistor, the gate voltage in a nanofluidic transistor controls the concentration of ions and biomolecules in the nanofluidic channel, and hence controls their transport. While the nanofluidic transistor can dynamically control flow, we have also developed a new technique to pattern surface charge for spatially modulating electrokinetic flow. In addition, we have demonstrated that biological binding reactions inside nanofluidic channels block the channel and change its charge environment, which can be sensed by simply measuring the channel conductance. With the capabilities of dynamic flow control and sensing combined with ease of fabrication, our system is a promising tool for processing and analysis of complex solutions.  Control of Protein Transport Proteins in a solution typically interact with a solid surface through various interactions such as van der Waals, hydrophobic interactions, electrostatic interactions, etc. Only when the nanochannel surface is passivated such that electrostatic interactions are dominant can biomolecule transport be controlled in a nanofluidic transistor.  Characterization of Surface Charge Effects In addition to a gate voltage, modification of surface charge also changes the ionic environment and controls transport through nanochannels. This effect also enables sensing of surface reactions in nanochannels. We have examined the effects of surface charge using ionic conductance. At low bulk ionic concentrations counter-ions accumulate in the nanofluidic channel to neutralize surface and conductance is governed by surface charge. Figure 1 below (left) shows the measured ionic conductance of the nanofluidic channels along with theoretical predictions, which confirms surface charge-governed transport in our devices. At 1 M KCl, the ionic conductance depends only on bulk properties and channel geometry. As predicted theoretically, ionic conductance saturates at low bulk concentrations, and varies linearly with concentration at high concentrations.   Affinity-Based Protein Capture in Nanochannels In the regime of low electrolyte concentration, where surface charge governs the ionic concentration inside the channel, any functionalization of nanofluidic channel surfaces with different surface groups can be expected to change surface charge and hence the nanofluidic channel conductance. In the high concentration regime, where conductance depends on channel height, biomolecules with sizes comparable to the channel size will result in a change in channel geometry and hence in a change in nanofluidic channel conductance. In both regimes, measurement of electrical conductance of nanofluidic channels offers means of probing biological reactions and modifications on surfaces, as illustrated above in Fig. 2 (right). Our group has used biotin-streptavidin system to conceptually demonstrate the existence of those two regimes. Diffused Limited Patterning Diffusion-limited patterning (DLP) is a new technique that enables patterning multiple labile molecular species inside channels in solution phase and without the need for re-alignment. We demonstrated DLP by patterning alternating bands of fluorescently labeled and unlabeled streptavidin in biotin-functionalized nanofluidic channels with spatial resolution better than 1 m.. DLP could be used as a cost-effective method for biomolecular patterning and flow control in micro/nanofluidics. It may also emerge as a new analytical tool in biochemistry since the generated pattern reflects the analyte concentration, reaction kinetics and interfacial transport.  Figure 4. Diffusion-limited patterning (DLP). (a) DLP requires a channel that is accessible to the bulk solution only from its entrance. The surface is functionalized, so that reactants can bind to the channel surface. (b) Once a reactant (red) is introduced into the bulk solution, it diffuses into the channel, forming a sharp reaction front under certain conditions. (c) When a second reactant (blue) is introduced, it reacts with the region of the channel beyond the first reactant. (d) Repeating this process with different reactants results in patterning of the reactants inside the channel.  Figure 5. DLP with unlabeled and fluorescently labeled streptavidin diffusing for equal durations of time. Channels used, concentrations of streptavidin and time durations for each step are (a) Type I, 1 mg/mL, 5 min (b) Type II, 1 mg/mL, 2 min (c) Type I, 100 μg/mL, 10 min and (d) Type II, 100 μg/mL, 10 min. Scale bar 10 μm. (e) Progress of reaction front with time, shown for the above patterns. The position of the reaction front measured from the channel end is denoted by x, and x2 is plotted on the Y-axis for comparison with Equation (3). Dashed lines passing through the origin are fit to data (symbols).
Nanofluidics
Members: Chuanhua Duan and Yusra Satoglu
FIG 1.  Nanofluidic device: Thirty nanochannels, 120 m long, run left to right and are connected by two microchannels. Three gate electrodes run vertically across the nanochannels. Nanochannels are made by etching a 35 nm thick and patterned poly-silicon layer (inset). The microchannels allow for easy access to the nanochannels.
FIG 2 (top).  A Metal-Oxide-Solution nanofluidic transistor can control ionic concentration by the application of a gate voltage.
 
FIG 3 (bottom).  The concentration of a negatively charged dye was controlled and monitored fluorescently.