Woo-Jin Chang

Nickel nanoparticle–chitosan-reduced graphene oxide-modified screen-printed electrodes for enzyme-free glucose sensing in portable microfluidic devices

A facile one-step strategy is reported to synthesize nanocomposites of chitosan-reduced graphene oxide-nickel nanoparticles (CS-RGO-NiNPs) onto a screen-printed electrode (SPE). The synthesis is initiated by electrostatic and hydrophobic interactions and formation of self-assembled nanocomposite precursors of negatively charged graphene oxide (GO) and positively charged CS and nickel cations (Ni(2+)). The intrinsic mechanism of co-depositions from the nanocomposite precursor solution under cathodic potentials is based on simultaneous depositions of CS at high localized pH and in situ reduced hydrophobic RGO from GO as well as cathodically reduced metal precursors into nanoparticles. There is no need for any pre- or post-reduction of GO due to the in situ electrochemical reduction and the removal of oxygenated functionalities, which lead to an increase in hydrophobicity of RGO and successive deposition on the electrode surface. The as-prepared CS-RGO-NiNPs-modified SPE sensor exhibited outstanding performance for enzymeless glucose (Glc) sensing in alkaline media with high sensitivity (318.4µAmM(-1)cm(-2)), wide linear range (up to 9mM), low detection limit (4.1µM), acceptable selectivity against common interferents in physiological fluids, and excellent stability. A microfluidic device was fabricated incorporating the SPE sensor for real-time Glc detection in human urine samples; the results obtained were comparable to those obtained using a high-performance liquid chromatography (HPLC) coupled with an electrochemical detector. The excellent sensing performance, operational characteristics, ease of fabrication, and low cost bode well for this electrochemical microfluidic device to be developed as a point-of-care healthcare monitoring unit.

Influence of ionic liquids on the growth ofEscherichia coli

Ionic liquids are compounds that composed only of ions and are liquid at room temperature. Thus, it is normally named room temperature ionic liquid (RTIL). In this study, the application of RTILs to the extractive fermentation of biomaterials was investigated as a substitute of organic solvents. The relative toxicity of the RTILs on the growth ofE. coli was tested. The inhibition of cell growth in the presence of various ionic liquids was measured using solid and liquid culture, and EC50 of each RTILs was calculated. The number of viable and total cells was measured by the number of colonies and optical density, respectively. Effective concentrations of toxicity (EC50) in these tested systems were similar with conventional solvents, such as acetone, acetonitrile, and ethanol. The viability ofE. coli was affected by the polarity and ionic properties of ionic liquids. The resistance of the microorganisms against ionic liquids was different with the cations and anions composing ionic liquids. No general influence of the anionic compound of the ionic liquids was found on toxicity comparing with distinctive influence of cationic moiety.

Poly(dimethylsiloxane) (PDMS) and Silicon Hybrid Biochip for Bacterial Culture

In this study, a novel PDMS/silicon hybrid microfluidic biochip was fabricated and tested for the long-term batch culture of bacterial cells. The PDMS (poly(dimethylsiloxane)) cover with 3-dimensional micro-channels for flow was fabricated using Teflon tubing and hole-punch techniques, without photolithographic methods. The PDMS/silicon hybrid biochip was prepared by bonding of PDMS cover and a silicon chip that had electrodes and micro-fluidic channels defined. The absorption of liquid into PDMS cover was characterized and conditions to prevent drying of nutrient media within the micro-chamber were shown. The absorption of liquid from micro-chambers into the PDMS cover was reduced up to 2.5 times by changing the mixing ratio of PDMS and curing agent from 10 : 1 to 2.5 : 1. In addition, pre-saturation of the PDMS cover with media prior to the incubation resulted in the preservation of liquid in the micro-chambers for up to 22 hours. Optimization of the mixing ratio and pre-saturation of the PDMS cover reduced the drying time 10 times when compared to the unsaturated PDMS cover composed of 10 : 1 ratio of PDMS and curing agent. Listeria innocua and a strain of Escherichia coli, expressing green fluorescent protein (GFP), were successfully cultured in batch mode within the PDMS/silicon hybrid biochip.

On-chip magnetic separation and encapsulation of cells in droplets

Single cell study is gaining importance because of the cell-to-cell variation that exists within cell population, even after significant initial sorting. Analysis of such variation at the gene expression level could impact single cell functional genomics, cancer, stem-cell research, and drug screening. The on-chip monitoring of individual cells in an isolated environment would prevent cross-contamination, provide high recovery yield, and enable study of biological traits at a single cell level. These advantages of on-chip biological experiments is a significant improvement for a myriad of cell analyses methods, compared to conventional methods, which require bulk samples and provide only averaged information on cell structure and function. We report on a device that integrates a mobile magnetic trap array with microfluidic technology to provide the possibility of separation of immunomagnetically labeled cells and their encapsulation with reagents into picoliter droplets for single cell analysis. The simultaneous reagent delivery and compartmentalization of the cells immediately following sorting are all performed seamlessly within the same chip. These steps offer unique advantages such as the ability to capture cell traits as originated from its native environment, reduced chance of contamination, minimal use of the reagents, and tunable encapsulation characteristics independent of the input flow. Preliminary assay on cell viability demonstrates the potential for the device to be integrated with other up- or downstream on-chip modules to become a powerful single-cell analysis tool.

Highly Sensitive Detection and Removal of Lead Ions in Water Using Cysteine-Functionalized Graphene Oxide/Polypyrrole Nanocomposite Film Electrode

We synthesized cysteine-functionalized graphene oxide (sGO) using carbonyldiimidazole as a cross-linker via amide and carbamate linkages. The sGO/polypyrrole (PPy) nanocomposite film was grown on the working electrode surface of a screen-printed electrode (SPE) via controlled one-step electrochemical deposition. The sGO/PPy-SPE was used to detect lead ions (Pb(2+)) in water by first depositing Pb(2+) on the working electrode surface for 10 min at -1.2 V, and then anodic stripping by differential pulse voltammetry (DPV). The DPV signals were linear in the ranges of 1.4-28 ppb (R(2) = 0.994), 28-280 ppb (R(2) = 0.997), and 280-14 000 ppb (R(2) = 0.990) Pb(2+). The measurable detection limit of the sensor is 0.07 ppb (S/N = 3), which is more than 2 orders of magnitude below the 10 ppb threshold for drinking water set by the World Health Organization. The average removal efficiency of Pb(2+) deposited on the electrode was 99.2% (S/N = 3), with relative standard deviation (RSD) of 3.8%. Our results indicate good affinity of sGO/PPy nanocomposite to Pb(2+), which can be used to effectively adsorb and remove Pb(2+) in water samples. Therefore, sGO/PPy nanocomposite we synthesized is useful for highly sensitive on-site and real-time monitoring of heavy metal ions and water treatment.

“Bottom-up” approach for implementing nano/microstructure using biological and chemical interactions

The “Bottom-up” approach for implementing nano/microstructure using biological self-assembled systems has been investigated with tremendous interest by many researchers in the field of medical diagnostics, material synthesis, and nano/microelectronics. As a result, the techniques for achieving these systems have been extensively explored in recent years. The developed or developing techniques are based on many interdisciplinary areas such as biology, chemistry, physics, electrical engineering, mechanical engineering, and so on. In this paper, we review the fundamentals behind the self-assembly concepts and describe the state of art in the biological and chemical self-assembled systems for the implementation of nano/microstructures. These structures described in the paper can be applied to the implementation of hybrid biosensors, biochip, novel bio-mimetic materials, and nano/microelectronic devices.

Dielectrophoretic technique for measurement of chemical and biological interactions.

We present a novel dielectrophoretic technique that can be used to characterize molecular interactions inside a microfluidic device. Our approach allows functionalized beads which are initially at rest on a functionalized surface to be pulled away from the surface by the dielectrophoretic force acting on the beads. As a result, the interaction between the molecules on the surface and the beads can be quantitatively examined. We report detailed experimental results and validate the results with a model to show that the technique can be used to measure forces of interaction between molecules under various experimental conditions.

Microfluidic Multifunctional Probe Array Dielectrophoretic Force Spectroscopy with Wide Loading Rates

The simultaneous investigation of a large number of events with different types of intermolecular interactions, from nonequilibrium high-force pulling assays to quasi-equilibrium unbinding events in the same environment, can be very important for fully understanding intermolecular bond-rupture mechanisms. Here, we describe a novel dielectrophoretic force spectroscopy technique that utilizes microsized beads as multifunctional probes for parallel measurement of intermolecular forces with an extremely wide range of force rate (10(-4) to 10(4) pN/s) inside a microfluidic device. In our experiments, various forces, which broadly form the basis of all molecular interactions, were measured across a range of force loading rates by multifunctional probes of various diameters with a throughput of over 600 events per mm(2), simultaneously and in the same environment. Furthermore, the individual bond-rupture forces, the parameters for the characterization of entire energy landscapes, and the effective stiffness of the force spectroscopy were determined on the basis of the measured results. This method of determining intermolecular forces could be very useful for the precise and simultaneous examination of various molecular interactions, as it can be easily and cost-effectively implemented within a microfluidic device for a range of applications including immunoassays, molecular mechanics, chemical and biological screening, and mechanobiology.

Highly Selective Mercury Detection at Partially Oxidized Graphene/Poly(3,4-Ethylenedioxythiophene):Poly(Styrenesulfonate) Nanocomposite Film-Modified Electrode

Partially oxidized graphene flakes (po-Gr) were obtained from graphite electrode by an electrochemical exfoliation method. As-produced po-Gr flakes were dispersed in water with the assistance of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). The po-Gr flakes and the po-Gr/PEDOT:PSS nanocomposite (po-Gr/PEDOT:PSS) were characterized by Raman spectroscopy, Fourier transform-infrared spectroscopy (FT-IR), UV-Vis spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, we demonstrated the potential use of po-Gr/PEDOT:PSS electrode in electrochemical detection of mercury ions (Hg2+) in water samples. The presence of po-Gr sheets in PEDOT:PSS film greatly enhanced the electrochemical response for Hg2+. Cyclic voltammetry measurements showed a well-defined Hg2+ redox peaks with a cathodic peak at 0.23 V, and an anodic peak at 0.42 V. Using differential pulse stripping voltammetry, detection of Hg2+ was achieved in the range of 0.2 to 14 µM (R2 = 0.991), with a limit of detection (LOD) of 0.19 µM for Hg2+. The electrode performed satisfactorily for sensitive and selective detection of Hg2+ in real samples, and the po-Gr/PEDOT:PSS film remains stable on the electrode surface for repeated use. Therefore, our method is potentially suitable for routine Hg2+ sensing in environmental water samples.

A nanoscale Ti/GaAs metal-semiconductor hybrid sensor for room temperature light detection

We report an individually addressable Ti∕GaAs metal-semiconductor hybrid optical nanosensor with positive photoresistance and a sensitivity that increases as the device dimensions shrink. The underlying physics relates to the crossover from ballistic to diffusive transport of the photoinduced carriers and the geometric enhancement of the effect associated with a Schottky-barrier-coupled parallel metal shunt layer. For a 250 nm device under 633 nm illumination we observe a specific detectivity of D(*)=5.06×10(11) cm √Hz∕W with a dynamic response of 40 dB.