(120d) Study of Fet Flow Control and Electrostatic Response of Charged Molecules in Nanofluidic Channels

Using scanning laser confocal fluorescence microscopy (SL-CFM) and multiple internal reflection Fourier transform infrared spectroscopy (MIR-FTIRS), we have studied the field-effect-transistor (FET) flow control of charged dye molecules (Alexa 488 and Rhodamine B) in a parallel array (≈10⁵) of nanochannels during electroosmosis. For fluidic FET, a DC potential is applied to the gate surrounding an isolated mid-section of the channels. The gate potential controls the surface charge on SiO₂ channel walls and therefore the ζ-potential. Depending the polarity and magnitude, the gate potential can accelerate, decelerate, or reverse the flow. We observe that the isolated gate, which is heavily doped with B (≈10¹⁹ cm^-3), shows more pronounced control than applying the bias to the entire substrate. We also demonstrate a pH shift in nanochannels, when bulk electrolyte solutions enter the channels. This outcome illustrates that the solution pH can be further controlled by the gate bias. To improve the controllability of flow and to introduce a pH gradient along the channels for isoelectric focusing, our latest nanochannel device contains multiple gates. A different potential is applied to each gate to differentially control the surface charge on the SiO₂ channel walls and to create a pH gradient along the channels. Since the nanochannels are integrated into a MIR infrared waveguide, we can also probe the molecular orientation, segregation, and reaction of charged molecules in nanochannels in response to the gate bias. For instance, the xenthene-skeletal C-C vibrational modes at 1617 and 1552 cm^-1 of Rhodamine B shift upon gate biasing, indicating a conformational change. The control of pH gradient as a function of gate bias as well as molecular orientation will be further discussed in this presentation.