Madhu Prakash Chatrathi

Towards disposable lab‐on‐a‐chip: Poly(methylmethacrylate) microchip electrophoresis device with electrochemical detection

A fully disposable microanalytical device based on combination of poly(methylmethacrylate) (PMMA) capillary electrophoresis microchips and thick‐film electrochemical detector strips is described. Variables influencing the separation efficiency and amperometric response, including separation voltage or detection potential are assessed and optimized. The versatility, simplicity and low‐cost advantages of the new design are coupled to an attractive analytical performance, with good precision (relative standard deviation RSD = 1.68% for n = 10). Applicability for assays of mixtures of hydrazine, phenolic compounds, and catecholamines is demonstrated. Such coupling of low‐cost PMMA‐based microchips with thick‐film electrochemical detectors holds great promise for mass production of single‐use micrototal analytical systems.

Carbon-nanotube/copper composite electrodes for capillary electrophoresis microchip detection of carbohydrates

The preparation of carbon nanotube (CNT)/copper composite electrodes, based on co-mixing CNT and Cu powders within mineral oil, is described. The new composite electrode is used for improved amperometric detection of carbohydrates following their capillary electrophoresis (CE) microchip separations. The CNT/Cu composite electrode detector displays enhanced sensitivity compared to detectors based on copper or CNT alone. The marked catalytic action of the CNT/Cu composite material permits effective low potential (+0.5 V vs. Ag/AgCl) amperometric detection, and is coupled to the renewability, bulk modification and versatility advantages of composite electrodes. The CNT/Cu composite surface also leads to a greater resistance to surface fouling compared to that observed at the copper electrode. Factors affecting the electrocatalytic activity and the CE microchip detection are examined and optimized. The CNT/Cu composite electrode is also shown to be useful for the detection of amino acids as indicated from preliminary results. While the present work has focused on the enhanced CE microchip detection of carbohydrates and amino acids, the CNT/metal-composite electrode route should benefit the detection of other important groups of analytes.

Capillary Electrophoresis Chips with Thick-Film Amperometric Detectors: Separation and Detection of Hydrazine Compounds

A miniaturized analytical system for separating and detecting toxic hydrazine compounds, based on the coupling of micromachined capillary electrophoresis (CE) chips with removable thick-film amperometric detectors is described. The study also represents the first example of using a chemically modified electrode for microchip CE applications. The microsystem with the electrocatalytic palladium-deposited detector offers a rapid (2 min) low-potential (+0.5 V vs. Ag/AgCl) simultaneous detection of hydrazine, methylhydrazine, dimethylhydrazine, and phenylhydrazine. Such compounds could be detected down to the 1.5×10–6 M level, with linearity over the 2×10–5 M–2×10–4 M range examined. While the concept of miniaturized separation/detection systems is presented within the framework of hydrazine compounds, such devices should be attractive for field monitoring of other classes of toxic contaminants. The design of the microsystem permits rapid replacement of the detector strip and a convenient surface modification (in a separate, optimal cell). The development of fast-responding miniaturized systems, with negligible waste production, holds particular promise for meeting the requirements of field ‘Green Analytical Chemistry’.

Thick-Film Electrochemical Detectors for Poly(dimethylsiloxane)-based Microchip Capillary Electrophoresis

A new poly(dimethylsiloxane) (PDMS)-based microchip capillary electrophoresis (CE) device, with a thick-film electrochemical detector, is described. The end-column design relies on screen-printing the amperometric carbon working electrode on the base plate of a PDMS microchip (opposite to the exit of the microchannel). Since the channel depth and electrode height are quite similar, this is a flow-onto/flow-by hybrid arrangement. The influence of relevant experimental variables, such as the separation and detection potentials, is reported along with the attractive analytical performance. Flat baselines and extremely low noise levels are observed even at high separation fields (approaching 700 V/cm), reflecting the effective electrical isolation of the detector. The resulting detection limits (150 nM for epinephrine and 280 nM for catechol) compare favorably with those obtained by other PDMS-based electrochemical detectors. Such coupling of low-cost and versatile PDMS chips and thick-film electrochemical detectors holds great promise for high-volume production of disposable microfluidic analytical devices.

Effects of heterogeneous electron-transfer rate on the resolution of electrophoretic separations based on microfluidics with end-column electrochemical detection

We demonstrate here that the electrode kinetics of an electrochemical detector contributes greatly to the resolution of the analyte bands in microchip electrophoresis systems with amperometric detection. The separation performance in terms of resolution and theoretical plate number can be improved and tailored by selecting or modifying the working electrode and/or by controlling the detection potential. Such improvements in the separation performance reflect the influence of the heterogeneous electron‐transfer rate of electroactive analytes upon the post‐channel band broadening, as illustrated for catechol and hydrazine compounds. The electrode kinetics thus has a profound effect not only on the sensitivity of electrochemical detectors but on the separation efficiency and the overall performance of microchip electrochemistry systems.

Micromachined Separation Chips with Post‐Column Enzymatic Reactions of “Class” Enzymes And End‐Column Electrochemical Detection: Assays of Amino Acids

Microchip devices integrating eletrophoretic separations, post-column enzymatic derivatization reactions, and amperometric detection, have been developed. The performance of the new integrated microfabricated chip system is demonstrated for on-chip assay of amino acids based on their electrophoretic separation, post-column reaction with amino-acid oxidase and amperometric detection of the hydrogen peroxide product. Factors influencing the response are examined and optimized, and the analytical performance is characterized. The concept can be extended to different target analytes based on the post-column reactions of other “class” enzymes.

Bulk modification of polymeric microfluidic devices

The surface properties of microfluidic devices play an important role in their flow behavior. We report here on an effective control of the surface chemistry and performance of polymeric microchips through a bulk modification route during the fabrication process. The new protocol is based on modification of the bulk microchip material by tailored copolymerization of monomers during atmospheric-pressure molding. A judicious addition of a modifier to the primary monomer solution thus imparts attractive properties to the plastic microchip substrate, including significant enhancement and/or modulation of the EOF (with flow velocities comparable to those of glass), a strong pH sensitivity and high stability. Carboxy, sulfo, and amino moieties have thus been introduced (through the incorporation of methylacrylic acid, 2-sulfoethyl-methacrylate and 2-aminoethyl-methacrylate monomers, respectively). A strong increase in the electroosmotic pumping compared to the native poly(methylmethacrylate)(PMMA) microchip (ca. electroosmotic mobility increases from 2.12 to 4.30 x 10(-4) cm(2) V(-1) s(-1)) is observed using a 6% methylacrylate (MAA) modified PMMA microchip. A 3% aminoethyl modified PMMA microchip exhibits a reversal of the electroosmotic mobility (for example, -5.6 x 10(-4) cm(2) V(-1) s(-1) at pH 3.0). The effects of the modifier loading and the pH on the EOF have been investigated for the MAA-modified PMMA chips. The bulk-modified devices exhibit reproducible and stable EOF behavior. The one step fabrication/modification protocol should further facilitate the widespread production of high-performance plastic microchip devices.

Microchip-Based Electrochemical Enzyme Immunoassays

In this chapter a microchip-based electrochemical enzyme immunoassay is developed and its performance is demonstrated for the determination of monoclonal mouse IgG as a model analyte. Such a direct homogeneous immunoassay requires the integration of electrokinetic mixing of alkaline phosphatase (ALP)-labeled anti-mouse IgG antibody (Ab-E) with the mouse IgG antigen (Ag) analyte in a precolumn reaction chamber, injection of immunochemical products into the separation channel, followed by rapid electrophoretic separation of enzyme-labeled free antibody and enzyme-labeled antibody-antigen complex. The separation is followed by a postcolumn reaction of enzyme tracer with p-aminophenyl phosphate (p-APP) substrate (S) and downstream amperometric detection of p-aminophenol (p-AP) product. Factors influencing the reaction, injection, separation, and detection processes are optimized. We have characterized the microchip-based immunoassay protocol. The resulting attractive analytical performance, along with distinct miniaturization and portability advantages of the electrochemical microsystem, offer considerable promise for designing self-contained and disposable chips for decentralized clinical diagnostics.