Microscale electrokinetic sample stacking.

摘要

One key challenge, yet to be addressed by miniaturized bioanalytical devices, is the detection of analytes with nanomolar or lower initial concentrations in volumes of one microliter or less. This dissertation focuses on implementation and optimization of robust electrokinetic sample preconcentration methods to improve detection sensitivity of microscale electrophoresis systems.; We present a theoretical and experimental study of a preconcentration technique called field amplified sample stacking (FASS). FASS process is modelled as electromigration, diffusion, and advection of two background electrolyte ions and multiple sample species across a known initial concentration gradient. Regular perturbation methods and a generalized Taylor dispersion analysis are used to derive area-averaged species conservation and electric field equations. The model predictions are validated using on-chip FASS experiments. An acidified poly(ethylene oxide) (PEO) coating is used to minimize dispersion due to electroosmotic flow (EOF), and thereby evaluate the low (but finite) dispersion regime of most interest. CCD-based, quantitative, epi-fluorescence imaging is used to quantify unsteady concentration fields and validate the model. This experimentally validated model is useful in developing optimal designs of sample stacking assay devices.; In FASS, under certain conditions (e.g., sample prepared in DI water), sample ion concentration can be on the order of BGE. In these cases, sample ions can strongly affect conductivity gradients. A three-ion electromigration model is presented to investigate such cases. The model predicts two distinct regimes of concentration enhancement. The first regime is characterized by a rarefaction wave for the sample ion distribution with a final concentration enhancement which is greater than the background-to-sample solution conductivity ratio, gamma. In the second regime, the sample ion concentration wave steepens toward an ion concentration shock wave, and maximum concentration enhancement is less than gamma.; We also describe the implementation of on-chip isotachophoresis (ITP): a sample preconcentration technique based on the differences in mobility of buffer ions and sample ions. We have developed a robust and repeatable flow control method to achieve greater than 20,000-fold increase in sample concentration using this technique. Such high-performance stacking methods could lead to the development of cheap and portable electrochemical detection-based integrated microscale electrophoresis devices.