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A modified glassy carbon electrode is used to examine the mass transport characteristics of the neurotransmitter dopamine (DA) in sodium montmorillonite films. The apparent diffusion coefficient (Dapp) of DA in the film is measured by chronocoulometry and cyclic voltammetry to be 2.3×10–9and 5.1×10–9 cm2/s. According to the Nicholson theory, the standard rate constant (ks) of heterogeneous electron transfer reaction for DA is estimated to be 2.7×10–6 cm/s by cyclic voltammetry. Montmorillonite has been modified on carbon fiber electrodes, and voltammetric response of some neurotransmitters and their metabolites has been examined. The variation of DA concentration in a rat brain with decreasing blood supply has been detected when the modified microelectrode is used in in vivo experiments.

A bilayer coated-wire electrode incorporating xanthine oxidase is reported to estimate fish freshness. The modified electrode is constructed with two membranes, a silk fibroin membrane with immobilized enzyme and a cellulose acetate membrane to eliminate the interference from high molecular weight compounds such as proteins. The biosensor is based on detecting hydrogen peroxide released from the enzymatic reaction. It shows high sensitivity and long-time stability for fish samples. The minimum concentration of hypoxanthine that could be detected was 1×10 −7 mol l −1 . Due to the protection of the cellulose acetate membrane, the electrode was stable for six weeks or 400 assays of fish tissue samples. The results were in good agreement with those of traditional assays for determination of the freshness of some kinds of river fishes.

Cell-based microfluidic devices have attracted interest for a wide range of applications. While optical cell counting and flow cytometry-type devices have been reported extensively, sensitive and efficient non-optical methods to detect and quantify cells attached over large surface areas within microdevices are generally lacking. We describe an electrical method for counting cells based on the measurement of changes in conductivity of the surrounding medium due to ions released from surface-immobilized cells within a microfluidic channel. Immobilized cells are lysed using a low conductivity, hypotonic media and the resulting change in impedance is measured using surface patterned electrodes to detect and quantify the number of cells. We found that the bulk solution conductance increases linearly with the number of isolated cells contributing to solution ion concentration. The method of cell lysate impedance spectroscopy is sensitive enough to detect 20 cells mu L-1, and offers a simple and efficient method for detecting and enumerating cells within microfluidic devices for many applications including measurement of CD4 cell counts in HIV patients in resource-limited settings. To our knowledge, this is the most sensitive approach using non-optical setups to enumerate immobilized cells. The microfluidic device, capable of isolating specific cell types from a complex bio-fluidic and quantifying cell number, can serve as a single use cartridge for a hand-held instrument to provide simple, fast and affordable cell counting in point-of-care settings.

In this paper, we present a new impedance-based method to detect viable spores by electrically detecting their germination in real time within microfluidic biochips. We used Bacillus anthracis Sterne spores as the model organism. During germination, the spores release polar and ionic chemicals, such as dipicolinic acid (DPA), calcium ions, phosphate ions, and amino acids, which correspondingly increase the electrical conductivity of the medium in which the spores are suspended. We first present macro-scale measurements demonstrating that the germination of spores can be electrically detected at a concentration of 10(9) spores ml(-1) in sample volumes of 5 ml, by monitoring changes in the solution conductivity. Germination was induced by introducing an optimized germinant solution consisting of 10 mM L-alanine and 2 mM inosine. We then translated these results to a micro-fluidic biochip, which was a three-layer device: one layer of polydimethylsiloxane (PDMS) with valves, a second layer of PDMS with micro-fluidic channels and chambers, and the third layer with metal electrodes deposited on a pyrex substrate. Dielectrophoresis (DEP) was used to trap and concentrate the spores at the electrodes with greater than 90% efficiency, at a solution flow rate of 0.2 ml min(-1) with concentration factors between 107-109 spores ml(-1), from sample volumes of 1-5 mu l. The spores were captured by DEP in deionized water within 1 min (total volume used ranged from 0.02 ml to 0.2 ml), and then germinant solution was introduced to the flow stream. The detection sensitivity was demonstrated to be as low as about a hundred spores in 0.1 nl, which is equivalent to a macroscale detection limit of approximately 10(9) spores ml(-1). We believe that this is the first demonstration of this application in microfluidic and BioMEMS devices.

In this study, we demonstrated a micro-fluidic system with multiple functions, including concentration of bacteria using dielectrophoresis (DEP) and selective capture using antibody recognition, resulting in a high capture efficiency of bacterial cells. The device consisted of an array of oxide covered interdigitated electrodes on a flat silicon substrate and a y16 mm high and y260 mm wide micro-channel within a PDMS cover. For selective capture of Listeria monocytogenes from the samples, the channel surface was functionalized with a biotinylated BSA– streptavidin–biotinylated monoclonal antibody sandwich structure. Positive DEP (at 20 Vpp and 1 MHz) was used to concentrate bacterial cells from the fluid flow. DEP could collect y90% of the cells in a continuous flow at a flow rate of 0.2 ml min21 into the micro-channel with concentration factors between 102–103, in sample volumes of 5–20 ml. A high flow rate of 0.6 ml min21 reduced the DEP capture efficiency to y65%. Positive DEP attracts cells to the edges of the electrodes where the field gradient is the highest. Cells concentrated by DEP were captured by the antibodies immobilized on the channel surface with efficiencies of 18 to 27% with bacterial cell numbers ranging from 101 to 103 cells. It was found that DEP operation in our experiments did not cause any irreversible damage to bacterial cells in terms of cell viability. In addition, increased antigen expression (antigens to C11E9 monoclonal antibody) on cell membranes was observed following the exposure to DEP.

In this study, growth of Listeria monocytogenes in a low conductivity growth medium (LCGM) was simulaneously monitored by conductivity and pH measurements. Detection times obtained from the conductivity and pH growth curves were inversely related to the initial concentration of L. monocytogenes in the medium. Linear responses were found by plotting detection times obtained from both conductivity and pH growth curves as a function of initial cell concentration in the range of 10^2 to 10^7 cfu/mL. The detection time was approximately 12 and 2 h for 10^2 and 10^7 cfu/mL of viable L. monocytogenes, respectively, using the conductivity growth curves, whereas it was approximately 1 h less using the pH growth curves. This dual detection system was used for valuating the growth of acid-, temperature-, and salt-treated L. monocytogenes in the medium. Acid stress at pH 2 and 3 for 3 h caused approximately 12 and 4 h delay in the detection time on pH growth curves, while stress at pH 5 for 3 h did not cause a significant delay in detection time. Delay in detection times was also observed for L. monocytogenes cells exposed to 45 degrees C for more than 1 h (2 and 6 h). Exposure to 10% NaCI for 3 h did not cause visible delay in the detection time. These observations on detection times for stressed L. monocytogenes had a consistent trend with the cell number decrease determined by surface plating method.