Suk Tai Chang

A biosensor for the detection of gas toxicity using a recombinant bioluminescent bacterium

A whole-cell biosensor was developed for the detection of gas toxicity using a recombinant bioluminescent Escherichia coli harboring a lac::luxCDABE fusion. Immobilization of the cells within LB agar has been done to maintain the activity of the microorganisms and to detect the toxicity of chemicals through the direct contact with gas. Benzene, known as a representative volatile organic compound, was chosen as a sample toxic gas to evaluate the performance of this biosensor based on the bioluminescent response. This biosensor showed a dose-dependent response, and was found to be reproducible. The immobilizing matrices of this biosensor were stored at 4 degrees C and were maintained for at least a month without any noticeable change in its activity. The optimal temperature for sensing was 37 degrees C. A small size of this sensor kit has been successfully fabricated, and found to be applicable as a disposable and portable biosensor to monitor the atmospheric environment of a workplace in which high concentrations of toxic gases could be discharged.

Soil biosensor for the detection of PAH toxicity using an immobilized recombinant bacterium and a biosurfactant

A biosensor for detecting the toxicity of polycylic aromatic hydrocarbons (PAHs) contaminated soil has been successfully constructed using an immobilized recombinant bioluminescent bacterium, GC2 (lac::luxCDABE), which constitutively produces bioluminescence. The biosurfactant, rhamnolipids, was used to extract a model PAH, phenanthrene, and was found to enhance the bioavailability of phenanthrene via an increase in its rate of mass transfer from sorbed soil to the aqueous phase. The monitoring of phenanthrene toxicity was achieved through the measurement of the decrease in bioluminescence when a sample extracted with the biosurfactant was injected into the minibioreactor. The concentrations of phenanthrene in the aqueous phase were found to correlate well with the corresponding toxicity data obtained by using this toxicity biosensor. In addition, it was also found that the addition of glass beads to the agar media enhanced the stability of the immobilized cells. This biosensor system using a biosurfactant may be applied as an in-situ biosensor to detect the toxicity of hydrophobic contaminants in soils and for performance evaluation of PAH degradation in soils.

Gel-Based Self-Propelling Particles Get Programmed To Dance

We present a class of gel-based self-propelling particles moving by the Marangoni effect in an oscillatory mode. The particles are made of an ethanol-infused polyacrylamide hydrogel contained in plastic tubing. These gel boats floating on the water surface exhibit periodic propulsion for several hours. The release of ethanol from the hydrogel takes place beneath the liquid surface. The released ethanol rises to the air-water interface by buoyancy and generates a self-sustained cycle of surface tension gradient driven motion. The disruption of the ethanol flux to the surface by the bulk flows around the moving particle results in their pulsating motion. The pulse interval and the distance propelled in a pulse by these gel floaters were measured and approximated by simple expressions based on the rate of ethanol mass-transfer through and out of the hydrogel. This allowed us to design a multitude of particles performing periodic steps in different directions or at different angles of rotation, traveling in complex preprogrammed trajectories on the surface of the liquid. Similar gel-based self-propelling floaters can find applications as mixers and cargo carriers in lab-on-a-chip devices, and in various platforms for sensing and processing at the microscale.

Novel Synthesis, Coating, and Networking of Curved Copper Nanowires for Flexible Transparent Conductive Electrodes

In this work, a whole manufacturing process of the curved copper nanowires (CCNs) based flexible transparent conductive electrode (FTCE) is reported with all solution processes, including synthesis, coating, and networking. The CCNs with high purity and good quality are designed and synthesized by a binary polyol coreduction method. In this reaction, volume ratio and reaction time are the significant factors for the successful synthesis. These nanowires have an average 50 nm in width and 25-40 μm range in length with curved structure and high softness. Furthermore, a meniscus-dragging deposition (MDD) method is used to uniformly coat the well-dispersed CCNs on the glass or polyethylene terephthalate substrate with a simple process. The optoelectrical property of the CCNs thin films is precisely controlled by applying the MDD method. The FTCE is fabricated by networking of CCNs using solvent-dipped annealing method with vacuum-free, transfer-free, and low-temperature conditions. To remove the natural oxide layer, the CCNs thin films are reduced by glycerol or NaBH4 solution at low temperature. As a highly robust FTCE, the CCNs thin film exhibits excellent optoelectrical performance (T = 86.62%, R(s) = 99.14 Ω ◻(-1)), flexibility, and durability (R/R(0) < 1.05 at 2000 bending, 5 mm of bending radius).

Remotely powered distributed microfluidic pumps and mixers based on miniature diodes

We demonstrate new principles of microfluidic pumping and mixing by electronic components integrated into a microfluidic chip. The miniature diodes embedded into the microchannel walls rectify the voltage induced between their electrodes from an external alternating electric field. The resulting electroosmotic flows, developed in the vicinity of the diode surfaces, were utilized for pumping or mixing of the fluid in the microfluidic channel. The flow velocity of liquid pumped by the diodes facing in the same direction linearly increased with the magnitude of the applied voltage and the pumping direction could be controlled by the pH of the solutions. The transverse flow driven by the localized electroosmotic flux between diodes oriented oppositely on the microchannel was used in microfluidic mixers. The experimental results were interpreted by numerical simulations of the electrohydrodynamic flows. The techniques may be used in novel actively controlled microfluidic-electronic chips.

Aqueous soft matter based photovoltaic devices

We present a new type of photovoltaic systems based on aqueous soft gel materials. Two photosensitive ions, DAS− and [Ru(bpy)3]2+, were used as photoactive molecules embedded in aqueous gel. The hydrogel photovoltaic devices (HGPVs) showed performance comparable with or higher than those of other biomimetic or ionic photovoltaic systems reported recently. We suggest a provisional mechanism, which is based on a synergetic effect of the two dye molecules in photocurrent generation. We found an efficient replacement of the expensive Pt counter-electrode with copper coated with carbon materials, such as carbon nanotubes, carbon black or graphite. These Cu electrodes coated with carbon layers could drastically reduce the cost of such hydrogel devices without efficiency loss. Thus, a new class of low cost and flexible photovoltaic cells made of biocompatible matrix was demonstrated. Biologically derived photoactive molecules, such as Chlorophyll and Photosystem II, were successfully operated in aqueous gel media of such HGPVs.

Highly Stretchable and Transparent Microfluidic Strain Sensors for Monitoring Human Body Motions.

We report a new class of simple microfluidic strain sensors with high stretchability, transparency, sensitivity, and long-term stability with no considerable hysteresis and a fast response to various deformations by combining the merits of microfluidic techniques and ionic liquids. The high optical transparency of the strain sensors was achieved by introducing refractive-index matched ionic liquids into microfluidic networks or channels embedded in an elastomeric matrix. The microfluidic strain sensors offer the outstanding sensor performance under a variety of deformations induced by stretching, bending, pressing, and twisting of the microfluidic strain sensors. The principle of our microfluidic strain sensor is explained by a theoretical model based on the elastic channel deformation. In order to demonstrate its capability of practical usage, the simple-structured microfluidic strain sensors were performed onto a finger, wrist, and arm. The highly stretchable and transparent microfluidic strain sensors were successfully applied as potential platforms for distinctively monitoring a wide range of human body motions in real time. Our novel microfluidic strain sensors show great promise for making future stretchable electronic devices.

Surface Energy Engineered, High‐Resolution Micropatterning of Solution‐Processed Reduced Graphene Oxide Thin Films

Graphene, an atomically thin layer of two-dimensional carbon nanostructure, has received intense attention in recent years because of its extraordinary optoelectronic properties and potential applications in microelectronics. [ 1–4 ] While high-quality graphene has been produced by chemical vapor deposition (CVD) on metallic surfaces [ 5 , 6 ] and graphitization of a single crystal SiC, [ 7 ] reduced graphene oxide (rGO) is also considered as a promising electronic nanomaterial because of its solution processability, residual chemically active sites, and high-volume production at low cost. [ 4 , 8 , 9 ] In the form of a single-layer sheet or fi lms of a few layers, rGO has been employed in various electronic devices including chemical/biological sensors, [ 10 , 11 ] fi eldeffect transistors (FETs), [ 8 , 12 ] transparent electrodes, [ 13 , 14 ] and photovoltaics. [ 15 ] However, previous studies have largely focused on a single electronic device or sensor. To fabricate practical and reproducible rGO-based microelectronics, a scalable and effective method for high-resolution rGO micropatterns on various substrates is highly desirable. Top-down lithographic techniques have been widely used to create rGO micropatterns by selectively etching parts of rGO thin fi lms. [ 12 , 16–18 ] Although a variety of well-defi ned rGO patterns can be obtained from such lithographic methods, they are time-consuming, involve complex procedures, and give rise to undesirable contamination of the patterned surface from contact with sacrifi cial masks. Alternatively, rGO patterning has been explored with nonlithographic routes such as micromolding in capillaries [ 19 , 20 ] and solvent evaporation-driven self-assembly process. [ 21 ] These methods, however, are often limited to simple patterned structures such as stripes, because the assembly of GO fl akes occurs in a restricted geometry. In addition, although various printing techniques including inkjet printing, [ 22 , 23 ] transfer printing [ 24 , 25 ] and imprinting [ 26 ] have also been applied for rGO patterning, the production of highresolution and reproducible rGO micropatterns on a large scale still remains a challenging task.

Ion‐Current Diode with Aqueous Gel/SiO2 Nanofilm Interfaces

Non-linear ion transport through nanoscale structures can be used in new types of microcircuit, inspired by biological processes such as adenosine triphosphate synthesis and neural transmission. The non-linear current resulting from preferential direction of ion flow can be used for signal transmission as well as signal processing, for example, ionic current rectification and amplification as in electronic circuits. Electronic signal-processing systems using ionic current conduction are capable of operation in aqueous phase, which makes them suitable for biomedical and chemical applications such as molecular delivery and sensing. Recent advances in nanofabrication have enabled control of selective ionic transport by designing various nanostructures of controlled surface charge, such as aligned nanochannels, nanopippettes, and cone-shaped nanotubes. Ionic flows can be selectively gated by electrostatic field at the charged channel walls. Furthermore, asymmetric distribution of charges on two surfaces of the nanofluidic channel walls creates an ionic junction in the channels and engenders onedirectional ionic flows. On a larger scale, the so-called ‘‘electrolyte diodes’’ provide an example of device-like structures operating on ionic conductance. Aqueous electrolyte junctions between acidic (e.g., HCl) and basic (e.g., KOH) solutions exhibit rectifying properties analogous to the junction of nand p-type semiconductors. Our group reported earlier a practical ionic rectifier – hydrogel-based polyelectrolyte diode constructed by bringing into contact two gels doped with polyelectrolytes of opposite charges. The junction formed at the interface between the gel layers rectifies

Microlitre scale solution processing for controlled, rapid fabrication of chemically derived graphene thin films

We report a new class of rapid solution processes for fabricating highly uniform chemically derived graphene thin films with control over the thickness on the subnanometre scale. The film deposition directly on various substrates is driven by dragging a meniscus of microlitre graphene oxide (GO) suspension trapped between two plates. The fine tuning of the optoelectronic properties of the graphene thin films is achieved by simply varying the number of depositions, GO concentration, dragging speed, and angle between two plates. This coating technique is simple, inexpensive, and easy to scale for large-area graphene films used as transparent electrodes with a significant reduction of the material consumption.