Education: A modular approach to microfluidics in the teaching laboratory.

This article seeks to educate the reader about the role played by the microfluidics teaching lab in the education of science, technology, engineering and mathematics for students of all ages. The discussion is intended to serve as a general guide to educators about the lab philosophy, goals, lab experiments and required equipment and reagents necessary for a successful microfluidics teaching laboratory. We hope that this article will stimulate other groups and companies to describe what they are doing to encourage education in this sector. At LabSmith we have developed a modular approach for teaching and demonstrating microfluidics that allows the end user to tailor the laboratory to course goals without an impact on the package of experimental equipment required and available to them. Thus, it is possible to educate students either in the art of microfluidics or use microfluidics to educate students about fundamental physical, chemical, or biological principles. The laboratory experiments discussed here are for students with educational experience at high school, undergraduate, graduate, and post-graduate levels.

Determination of multiresidue pesticides in green tea by using a modified QuEChERS extraction and ion-trap gas chromatography/mass spectrometry.

The pesticide residues in exported and imported tea products must not exceed the maximum residue limits (MRLs) regulated by the import countries. Tea is a complex matrix that obfuscates the determination of pesticide residues. Many available methods for multiresidue pesticide analysis of tea are time-consuming and require many cleanup steps. The objective of this study was to develop a simple multiresidue method by using a modified quick, easy, cheap, effective, rugged, and safe (QuEChERS) extraction and ion-trap GC/MS/MS, which can identify, confirm, and quantify pesticides in complex matrixes. A tea product was homogenized with water, and the pesticides were extracted with acetonitrile containing 1% acetic acid. The extract was subjected to centrifugation, initial cleanup with dispersive SPE (dSPE), solvent exchange, and final cleanup with dSPE. Diethyl-d10-parathion and triphenyl phosphate were used as the internal standard and surrogate, respectively. The final extract was injected into an ITQ 700 gas chromatograph/mass spectrometer. Quantitation of individual pesticides was based on matrix-matched calibration curves with a correlation coefficient of > 0.9930 for the 22 pesticides selected for the study. The recoveries of the 22 pesticides ranged from 78 to 115%, except those for diazinon (130%) and malathion (122%), with an average RSD of 8.7%. The LOD values of all of the pesticides, except for terbufos, were below the MRLs set by the European Union and Japan.

Emerging pesticide residue issues and analytical approaches.

The 46th Annual Florida Pesticide Residue Workshop of 2009 (FPRW 2009) held in St. Pete Beach, FL, is the latest in an annual tradition drawing scientists from U.S. federal and state government laboratories, industry, and other laboratories worldwide. In 2009, selected FPRW presenters were invited to contribute to this special issue of the Journal of Agricultural and Food Chemistry with a section devoted to emerging pesticide residue issues and analytical approaches. What follows is the written record of what should become a scientific conversation launched at FPRW 2009. There are two distinct approaches to organic residue analysis: instrumental methods and assays. In much of the world, scientists primarily rely on laboratories equipped with instrumentation for analysis, usually gas chromatography and liquid chromatography with some type of selective detector. In the discussion of instrumental approaches, the focus is on chromatography with mass spectrometry as a detection method. Approaches such as biomonitoring and assays fall outside the traditional instrumental method approach to residue analysis. Assays that do not require laboratory equipment are of greater interest for screening and are well-suited to field use. Regardless of the analytical method, the success of multiresidue analysis relies on the appropriate choice of sample preparation and cleanup methodologies. Many new sample preparation and cleanup approaches used for pesticide and other small molecule contaminant residue analyses in a variety of complex sample matrices are discussed in this special issue. The goal of these approaches is to reduce overall analysis time and solvent consumption without compromising the analytical results.

Performance impact of dynamic surface coatings on polymeric insulator-based dielectrophoretic particle separators.

Efficient and robust particle separation and enrichment techniques are critical for a diverse range of lab-on-a-chip analytical devices including pathogen detection, sample preparation, high-throughput particle sorting, and biomedical diagnostics. Previously, using insulator-based dielectrophoresis (iDEP) in microfluidic glass devices, we demonstrated simultaneous particle separation and concentration of various biological organisms, polymer microbeads, and viruses. As an alternative to glass, we evaluate the performance of similar iDEP structures produced in polymer-based microfluidic devices. There are numerous processing and operational advantages that motivate our transition to polymers such as the availability of numerous innate chemical compositions for tailoring performance, mechanical robustness, economy of scale, and ease of thermoforming and mass manufacturing. The polymer chips we have evaluated are fabricated through an injection molding process of the commercially available cyclic olefin copolymer Zeonor 1060R. This publication is the first to demonstrate insulator-based dielectrophoretic biological particle differentiation in a polymeric device injection molded from a silicon master. The results demonstrate that the polymer devices achieve the same performance metrics as glass devices. We also demonstrate an effective means of enhancing performance of these microsystems in terms of system power demand through the use of a dynamic surface coating. We demonstrate that the commercially available nonionic block copolymer surfactant, Pluronic F127, has a strong interaction with the cyclic olefin copolymer at very low concentrations, positively impacting performance by decreasing the electric field necessary to achieve particle trapping by an order of magnitude. The presence of this dynamic surface coating, therefore, lowers the power required to operate such devices and minimizes Joule heating. The results of this study demonstrate that iDEP polymeric microfluidic devices with surfactant coatings provide an affordable engineering strategy for selective particle enrichment and sorting.

An insulator-based (electrodeless) dielectrophoretic concentrator for microbes in water.

Dielectrophoresis (DEP), the motion of a particle caused by an applied electric field gradient, can concentrate microorganisms non-destructively. In insulator-based dielectrophoresis (iDEP) insulating microstructures produce non-uniform electric fields to drive DEP in microsystems. This article describes the performance of an iDEP device in removing and concentrating bacterial cells, spores and viruses while operated with a DC applied electric field and pressure gradient. Such a device can selectively trap particles when dielectrophoresis overcomes electrokinesis or advection. The dielectrophoretic trapping behavior of labeled microorganisms in a glass-etched iDEP device was observed over a wide range of DC applied electric fields. When fields higher than a particle-specific threshold are applied, particles are reversibly trapped in the device. Experiments with Bacillus subtilis spores and the Tobacco Mosaic Virus (TMV) exhibited higher trapping thresholds than those of bacterial cells. The iDEP device was characterized in terms of concentration factor and removal efficiency. Under the experimental conditions used in this study with an initial dilution of 1 x 105 cells/ml, concentration factors of the order of 3000x and removal efficiencies approaching 100% were observed with Escherichia coli cells. These results are the first characterization of an iDEP device for the concentration and removal of microbes in water.

The zeta potential of cyclo-olefin polymer microchannels and its effects on insulative (electrodeless) dielectrophoresis particle trapping devices.

While cyclo-olefin polymer microchannels have the potential to improve both the optical detection sensitivity and the chemical resistance of polymer microanalytical systems, their surface properties are to date not thoroughly characterized. These surface properties dictate, among other things, electrokinetic effects when electric fields are present. Here, we report the measurement of the zeta potential of cyclo-olefin polymers (injection-molded and hot-embossed Zeonor 1060R and 1020R) microchannels as a function of pH, counter-ion concentration, storage conditions, and chemical treatment in aqueous solutions both with and without EOF-suppressing additives. In contrast with previous reports, significant surface charge is measured, consistent with titration of charged sites with pK(a) = 4.8. Storage in air, acetonitrile, or aqueous solutions has relatively minor effects. While the source of the surface charge is unclear, chemical functionalization has shown that carboxylic acid groups are not present at the surface, consistent with the chemical structure of Zeonor. EOF-suppressing additives (hydroxypropylmethylcellulose) and conditioning in perchloric acid allow the surface charge to be suppressed. We demonstrate dielectrophoretic particle trapping devices in Zeonor 1060R substrates that show reduced trapping voltage thresholds as compared to previous implementations in glass.

Insulator-based dielectrophoresis for the selective concentration and separation of live bacteria in water.

Insulator-based dielectrophoresis (iDEP) was utilized to separate and concentrate selectively mixtures of two species of live bacteria simultaneously. Four species of bacteria were studied: the Gram-negative Escherichia coli and the Gram-positive Bacillus subtilis, B. cereus, and B. megaterium. Under an applied direct current (DC) electric field all the bacterial species exhibited negative dielectrophoretic behavior. The dielectrophoretic separations were carried out in a glass microchannel containing an array of insulating posts. The insulating posts in the microchannel produced nonuniformities in the electric field applied along the channel. Mixtures of two species of bacteria were introduced into the microchannel and the electric field was applied. The bacterial species exhibited different dielectrophoretic mobilities under the influence of the nonuniform field. From these experiments a trapping order was established with E. coli trapping at the weakest applied electric field, while the Bacillus species were trapped at different characteristic threshold fields. At stronger applied electric fields, the two different species of bacteria in the microchannel were dielectrophoretically trapped into two spatially distinct bands. The results showed that iDEP has the potential to selectively concentrate and separate different species of bacteria.

Dielectrophoretic concentration and separation of live and dead bacteria in an array of insulators.

Insulator-based (electrodeless) dielectrophoresis (iDEP) is an innovative approach in which the nonuniform electric field needed to drive DEP is produced by insulators, avoiding problems associated with the use of electrodes. Live and dead Escherichia coli were concentrated and selectively released by applying stepped DC voltages across a microchannel containing an array of insulating posts etched in glass. The only electrodes present were two platinum wires placed in the inlet and outlet reservoirs, producing mean electric fields of up to 200 V/mm across the insulators. The cells were labeled with Syto 9 and propidium iodide and imaged through a fluorescent microscope. Cell trapping and release were controlled by modifying the relative responses of electrokinesis and DEP by adjusting the magnitude of the applied voltage. Dead cells were observed to have significantly lower dielectrophoretic mobility than live cells, whereas the electrokinetic mobilities of live and dead cells were indistinguishable. The locations of the bands of differentially trapped cells were consistent with predictions. In addition, cells were selectively trapped and concentrated against backgrounds of 1- and 0.2-microm carboxylate-modified polystyrene particles. This first application of iDEP for simultaneous live/dead bacteria separation and concentration illustrates its potential as a front-end method for bacterial analysis.