Joseph Wang

This review addresses recent advances in carbon‐nanotubes (CNT) based electrochemical biosensors. The unique chemical and physical properties of CNT have paved the way to new and improved sensing devices, in general, and electrochemical biosensors, in particular. CNT‐based electrochemical transducers offer substantial improvements in the performance of amperometric enzyme electrodes, immunosensors and nucleic‐acid sensing devices. The greatly enhanced electrochemical reactivity of hydrogen peroxide and NADH at CNT‐modified electrodes makes these nanomaterials extremely attractive for numerous oxidase‐ and dehydrogenase‐based amperometric biosensors. Aligned CNT “forests” can act as molecular wires to allow efficient electron transfer between the underlying electrode and the redox centers of enzymes. Bioaffinity devices utilizing enzyme tags can greatly benefit from the enhanced response of the biocatalytic‐reaction product at the CNT transducer and from CNT amplification platforms carrying multiple tags. Common designs of CNT‐based biosensors are discussed, along with practical examples of such devices. The successful realization of CNT‐based biosensors requires proper control of their chemical and physical properties, as well as their functionalization and surface immobilization.

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 × 10⁻⁴ cm² 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 × 10⁻⁴ cm² 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.

The attractive features of a boron‐doped diamond (BDD) thin‐film detector for microchip capillary electrophoretic (CE) separations of dye‐related amino‐substituted aromatic compounds are described. The diamond electrode was employed in the end‐column amperometric detection of 4‐aminophenol (4‐AP), 1,2‐phenylenediamine (1,2‐PDA), 2‐aminonaphthalene (2‐AN), 2‐chloroaniline (2‐CA), and o‐aminobenzoic acid (o‐ABA), and its attractive behavior was compared to commonly used screen‐printed carbon and glassy‐carbon electrodes. These conventional electrode materials exhibit a significant degree of passivation and low sensitivity to the above‐mentioned environmental pollutants. The diamond‐based electrochemical detection system displayed a favorable analytical performance, including lower noise levels, higher peak resolution with enhanced sensitivity, and improved resistance against electrode passivation. Factors influencing the on‐chip analysis were assessed and optimized. The diamond detector displayed detection limits of 2.0 and 1.3 μM for 4‐AP and 2‐AN, respectively, and a wide linear response for these compounds over the 2–50 μM range. The enhanced stability was demonstrated by relative standard deviation (RSD) values of 1.4% and 4.7% for 100 μM 1,2‐PDA and 200 μM 2‐CA, respectively, for repetitive detections (n = 7). Besides, the simultaneously observed current decrease was 2.4 and 9.1% for 1,2‐PDA and 2‐CA, respectively (compared to 21.8 and 41.0% at the screen‐printed carbon electrode and 28.3 and 34.1% at the glassy carbon electrode, respectively). The favorable properties of the diamond electrode indicate great promise for environmental applications in CE and other microchip devices.

A microchip protocol for the capillary electrophoresis separation and electrochemical detection of thiol-containing degradation products of V-type nerve agents is described. The microchip assay relies on the derivatization reaction of 2-(dimethylamino)ethanethiol (DMAET), 2-(diethylamino)ethanethiol (DEAET), and 2-mercaptoethanol (ME) with o-phthaldialdehyde in the presence of the amino acid valine along with amperometric monitoring of the isoindole derivatives. Both off-chip and on-chip derivatization reactions have led to highly sensitive and rapid detection of the thiol degradation products. Various parameters influencing the derivatization, separation, and detection processes were examined and optimized. These include the amino acid co-reagent, reagent-mixing ratio, reaction time, injection time, separation voltage, and detection potential. The chip microsystem offers a rapid (<4 min) simultaneous detection of micromolar concentrations of DMAET, DEAET, and ME. Linear calibration plots were observed for the V-type nerve agent thiol degradation products, along with good stability and reproducibility (RSD < 8.0%). Detection limits of 5 and 8 microM were obtained for the off-chip reaction of DMAET and DEAET, respectively, following a 2-s injection. The suitability for assays of environmental matrixes was demonstrated for the determination of DMAET and DEAET in untreated tap and river water samples. The favorable analytical performance makes the new microfluidic device attractive for addressing the needs of various security scenarios.