Seoung-Jai Bai

In vivo O2 measurement inside single photosynthetic cells.

The oxygen evolution of single cells was investigated using a nano-probe with an ultra-micro electrode (UME) in a submicron sized system in combination with a micro-fluidic system. A single cell was immobilized in the micro-fluidic system and a nano-probe was inserted into the cytosolic space of the single cell. Then, the UME was used for an in vivo amperometric experiment at a fixed potential and electrochemical impedance spectroscopy to detect oxygen evolution of the single cell under various light intensities.

Conductive microtip electrode array with variable aspect ratio using combination process of reactive ion etching

A micromachining process for a conductive microtip electrode array has been developed using a combination process of reactive ion etching. A localized conical ultra-micro electrode (UME) with submicron width is realized on the tip end for electrochemical measurements. A height of several tens of microns of the silicon microtip is achieved, and the area beyond the UME is covered with dielectric material for electrical insulation. A high-aspect-ratio silicon microtip is fabricated using anisotropic and isotropic etching with varying the gap and diameter of the etch masks. Cyclic voltammetry is conducted to characterize the electrochemical properties of the UMEs. The results indicate that the electrical connection on the UME works successfully, and the measured peak currents were found to be in good agreement with the values estimated from analysis based on conical UME theory.

Microfluidic Electrochemical Impedance Spectroscopy of Carbon Composite Nanofluids

Understanding the internal structure of composite nanofluids is critical for controlling their properties and engineering advanced composite nanofluid systems for various applications. This goal can be made possible by precise analysis with the help of a systematic robust platform. Here, we demonstrate a microfluidic device that can control the orientation of carbon nanomaterials in a suspension by applying external fields and subsequently examine the electrochemical properties of the fluids at microscale. Composite nanofluids were prepared using carbon nanomaterials, and their rheological, thermal, electrical, and morphological characteristics were examined. The analysis revealed that microfluidic electrochemical impedance spectroscopy (EIS) in the device offered more reliable in-depth information regarding the change in the microstructure of carbon composite nanofluids than typical bulk measurements. Equivalent circuit modelling was performed based on the EIS results. Furthermore, the hydrodynamics and electrostatics of the microfluidic platform were numerically investigated. We anticipate that this microfluidic approach can serve as a new strategy for designing and analyzing composite nanofluids more efficiently.

Fabrication and measurements of high aspect ratio conductive microtip array with localized ultra-micro electrode at the tip end

This paper presents a fabrication and electrochemical characterization of an array of high aspect ratio conductive microtips with localized ultra-micro electrodes (UME) at the tip ends. The proposed microtip was designed to be inserted into the cytosolic space of a cell for the intracellular applications such as metabolic activity sensors and photosynthetic fuel cells. In order to be applied to these applications, the microtip structure with the localized UME was fabricated to achieve the high aspect ratio, small apex radius and the height of tens of micron scale. To fabricate the localized UME, the conductive layer on the microtip surface was revealed only at the tip end while the rest of the conductive layer was insulated electrically.

Fabrication of microtip electrode array integrated with self-aligned reference electrode

This paper presents a novel microtip-based electrode array with the reference electrode around tips. A microelectromechanical systems process for a microtip electrode array has been developed using DRIE and RIE combination process. The conductive microtip with high aspect ratio, small apex radius and the height of tens of micron scale is insulated except the tip ends. The tips are closely surrounded by the self-aligned reference electrode. The whole process was completed with a single photography mask. The fabricated microtip-based electrode array was successfully verified with the cyclic voltammetry measurements and characterized with the measurement of photosynthetic reaction of chlorella cell using microtip electrode array.

Implementation of stackable photosynthetic microbial fuel cell structure using stainless steel mesh membrane electrode assembly

This paper presents a fabrication and measurement of stackable photosynthetic microbial fuel cell (MFC) with integrating two cathode chambers in unit cell that increases surface power density compared to conventional photosynthetic MFC structures. Stackable MFC is realized by newly proposed membrane electrode assembly (MEA) structure which includes proton exchange membrane (PEM) and stainless steel mesh anode. A performance of proposed MEA and MFC is characterized by electrochemical impedance spectroscopy (EIS) and current-power curve measurements.

Nanofabrication of electrochemical planar probes for single cell analysis

Needle shaped probes with a dual electrode system in submicron size have been developed for electrochemical analyses of living single cells. The probe system is designed for local probing of the cytosolic cell environment and cell organelles using amperometric, potentiometric and impedance spectroscopic methods. Silicon nitride cantilevers with an electrode metal layer system are fabricated on four-inch wafers using conventional micro fabrication techniques. The probe needle structures with a tip in sub micron scale are patterned using Focus Ion Beam (FIB) technology. A focused ion beam is utilized to write the probe needle shape into the pre-shaped cantilever and, for a dual electrode system, the probe is divided into two parts to create two separate electrodes. Subsequently, the needle structures are released from the supporting bulk silicon during a wet etching step, and a silicon nitride layer is deposited to isolate and embed the electrode metal layer. Finally, FIB milling is used for a precise exposure of the buried metal layer by cutting the top of the tip. Electrochemical characterization of nano-probes showed full functionality of Ag/AgCl as well as of platinum transducer systems. The sharpness of the probe tip with a radius of smaller than 50nm and the mechanical robustness of the needle structure allow for a reliable penetration of cell membranes. Initial measurements of cell membrane potentials and cell membrane impedances of rat fibroblast cells using Ag/AgCl transducer probes demonstrate the analytical capability of these probes in biological environments.