Biomedical subjects

Rajiv Bharadwaj

Publications and source records attributed to Rajiv Bharadwaj.

3 recordsLinked to original sources

PubMed2006

On-chip millionfold sample stacking using transient isotachophoresis.

We present a simple and robust isotachophoresis (ITP) method that can be integrated with microchip-based capillary electrophoresis (CE) devices to achieve millionfold sample stacking. We performed an experimental parametric study to show the effects of initial sample ion concentration, leading ion concentration, and trailing ion concentration on ITP stacking. We also discuss the usefulness and limitations of a simple one-dimensional nondispersive model and a scaling analysis for dispersion rate. We found that a single-column ITP configuration together with electroosmotic flow suppression and high leading ion concentration provide high-performance ITP and can be integrated readily with CE separation. We demonstrated detection of trace of 100 fM Alexa Fluor 488 (signal-to-noise ratio of 11) with a concentration increase of a factor of 2 x 10(6). Application of our ITP/CE protocol to the stacking and separation of negatively charged fluorescent tracers (Alexa Fluor 488 and bodipy) resulted in a concentration increase of 6.4 x 10(4) and a signal increase of 4.5 x 10(5). The ITP/CE protocol can be performed with a standard microchannel cross design or simple flow control. The method can be implemented with available off-the-shelf chip systems using off-the-shelf voltage control systems and buffer chemistries.

Journal Article↗

PubMed2003

Thousandfold signal increase using field-amplified sample stacking for on-chip electrophoresis.

Field-amplified sample stacking (FASS) leverages conductivity gradients between a volume of injected sample and the background buffer to increase sample concentration. A major challenge in applying FASS to on-chip assays is the initial setup of high-conductivity gradient boundaries in the region of the injected sample volume. We have designed, fabricated, and characterized a novel FASS-capillary electrophoresis (CE) chip design that uses a photoinitiated porous polymer structure to facilitate sample injection and flow control for high-gradient FASS. This polymer structure provides a region of high flow resistance that allows the electromigration of sample ions. We have demonstrated an electropherogram signal increase by a factor of 1100 in electrophoretic separations of fluorescein and Bodipy with, respectively, 2 microM and 1 microM initial concentrations.

Electrophoresis, Capillary↗

PubMed2002

Design and optimization of on-chip capillary electrophoresis.

We present a systematic, experimentally validated method of designing electrokinetic injections for on-chip capillary electrophoresis applications. This method can be used to predict point-wise and charge-coupled device (CCD)-imaged electropherograms using estimates of species mobilities, diffusivities and initial sample plug parameters. A simple Taylor dispersion model is used to characterize electrophoretic separations in terms of resolution and signal-to-noise ratio (SNR). Detection convolutions using Gaussian and Boxcar detector response functions are used to relate optimal conditions for resolution and signal as a function of relevant system parameters including electroosmotic mobility, sample injection length, detector length scale, and the length-to-detector. Analytical solutions show a tradeoff between signal-to-noise ratio and resolution with respect to dimensionless injection width and length to the detector. In contrast, there is no tradeoff with respect to the Peclet number as increases in Peclet number favor both SNR and separation solution (R). We validate our model with quantitative epifluorescence visualizations of electrophoretic separation experiments in a simple cross channel microchip. For the pure advection regime of dispersion, we use numerical simulations of the transient convective diffusion processes associated with electrokinetics together with an optimization algorithm to design a voltage control scheme which produces an injection plug that has minimal advective dispersion. We also validate this optimal injection scheme using fluorescence visualizations. These validations show that optimized voltage scheme produces injections with a standard deviation less than one-fifth of the width of the microchannel.

Diagnostic Imaging↗