High resolution simulations of isotachophoresis and experimental studies of indirect detection and identification of analytes using fluorescent carrier ampholytes [electronic resource]
Keywords: Aeronautics and Astronautics

The physical regimes of microfluidics enable innovative methods for miniaturization, automation, parallelization, control, and analysis of chemical and biochemical processes. This dissertation focuses on both modeling and applications of an on-chip electrokinetic process called isotachophoresis (ITP). ITP uses discontinuous electrolyte properties to create sharp ion concentration gradients allowing transport and separation of ionic species in microchannels. Sample ions can be concentrated at these sharp interfaces by more than a million fold. Although the method itself dates back over 60 years, there remain significant challenges in modeling ITP, and in leveraging it for innovative applications and functionality. In the first part of the dissertation, we present the development, formulation, and performance of a new simulation tool for electrophoretic transport, with emphasis on isotachophoresis. The code solves the one-dimensional (e.g., area-averaged) transient advection-diffusion equations for multiple multi-valent weak electrolytes (including ampholytes) and includes a model for pressure driven flow and Taylor-Aris dispersion. The code uses a new approach for the discretization of the equations; consisting of a high resolution compact scheme which is combined with an adaptive grid algorithm. We show that this combination allows for accurate resolution of sharp concentration gradients at high electric fields, while at the same time significantly reducing the computational time. We demonstrate smooth, stable, and accurate solutions at high current densities with as much as a 75-fold reduction in computational time compared with equivalent uniform grid techniques. The code is available open-source for free at http://microfluidics.stanford.edu. In the second part, we present a novel method for fluorescence-based indirect detection and identification of analytes and demonstrate its use for label-free detection of chemical toxins in a hand-held device. We fluorescently label a mixture of low-concentration carrier ampholytes and introduce it into an isotachophoresis (ITP) separation. The carrier ampholytes provide a large number of fluorescent species with a wide range of closely spaced effective electrophoretic mobilities. ITP physics cause analytes to focus and displace subsets of these carrier-ampholytes. The analytes are detected indirectly and quantified by analyzing the gaps in the fluorescent ampholyte signal. The large number (order 1000) of carrier ampholytes enables detection of a wide range of analytes, requiring little a priori knowledge of their electrophoretic properties. We discuss the principles of the technique and demonstrate its use in the detection of various analytes. We then present a new signal analysis method, based on normalized integrals of the associated fluorescent signal, which enables the extraction of fully-ionized mobility and dissociation constant of analytes. Finally, we present the integration of the technique into a self-contained, hand-held device and demonstrate detection of chemical toxins, an explosive, and a herbicide in tap water and river water, with no sample preparation steps.