Determination of membrane capacitance and cytoplasm conductivity by simultaneous electrorotation.

Membrane capacitances and cytoplasm conductivities of hematopoietic cells were investigated by simultaneous electrorotation (ROT) systems of multiple cells. Simultaneous ROT was achieved by the rotation of electric fields in grid arrays formed with three-dimensional interdigitated array (3D-IDA) electrodes that can be easily fabricated using two substrates with IDA electrodes. When AC signals were applied to four microband electrodes with a 90° phase difference to each electrode, cells dispersed randomly in the 3D-IDA device started to rotate and moved to the center of each grid. Multiple cells were simultaneously rotated at the center of grids without friction from contact with other cells and substrates. The averages and variance of ROT rates of cells at each frequency can be measured during a single operation of the device within 5 min, resulting in the acquisition of ROT spectra. Membrane capacitances and cytoplasm conductivities of hematopoietic cells (K562 cells, Jurkat cells, and THP-1 cells) were determined by fitting ROT spectra obtained experimentally to the curves calculated theoretically. The values determined by using the simultaneous ROT systems well coincided with the values reported previously. The membrane capacitances and cytoplasm conductivities of WEHI-231 cells were firstly determined to be 8.89 ± 0.25 mF m-2 and 0.28 ± 0.03 S m-1, respectively. Furthermore, the difference of the ROT rates based on the difference of the electric properties of cells was applied to discriminate the types of cells. The acquisition of rotation rates of multiple cells within a single operation makes the statistical analysis extremely profitable for determining the electrical properties of cells.

Quantitative and Single-step Enzyme Immunosensing Based on an Electrochemical Detection Coupled with Lateral-flow System.

A single-step electrochemical immunochromatography has been developed: the device was based on two pieces of nitrocellulose membrane, a sample pad with anti-mouse IgG antibody labeled with glucose oxidase (GOx-labeled antibody), a conjugate pad with glucose, and a Pt working electrode. Either antibody or antigen was immobilized on the membrane. The addition of a solution containing mouse IgG, a model target, allows for the dissolution of GOx-labeled antibody in the sample pad to form an immunocomplex. The produced immunocomplex was automatically separated by capturing to the antibody immobilized on the membrane with the sandwich structure or by passing through the membrane modified with an antigen for the competitive reaction. The separated GOx label arrived at the conjugate pad with glucose to undergo the enzyme reaction. Hydrogen peroxide generated by this reaction was detected at the Pt electrode prepared on the second nitrocellulose membrane downstream from the conjugate pad. The results demonstrated that the designed immunochromatography can be applied to quantitative detection with a single-step procedure, because both the GOx-labeled antibody for revealing the immunoreactions and the substrate for the enzyme reaction were prepared in the device. Moreover, the initial concentration of the GOx-labeled antibody permitted control of the detectable concentration for mouse IgG.

Improvement of Electrochemical Response of Cocaine Sensors Based on DNA Aptamer by Heat Treatment.

We report on a biosensor for cocaine based on the conformation change of DNA aptamer by capturing the cocaine molecules. The oxidation current of ferrocene conjugated on the terminal end of aptamer immobilized on an Au electrode increased with increasing cocaine concentration. The sensor response has been improved by a simple heat treatment after immobilization, since the aggregates of DNA aptamer generated during the immobilization step could be dissociated and rearranged on the electrode.

Development of in situ electrochemical scanning electron microscopy with ionic liquids as electrolytes.

We report that ionic liquids (ILs) can be observed by electron microscopy without any charging of the liquid. Based on this, we present an in situ electrochemical scanning electron microscopy (in situ ECSEM) system. The key technology that enables in situ ECSEM is that charges can be removed from an IL by grounding it with a Pt wire, even if the IL is in an insulating glass cell. As a first demonstration, we describe the redox reaction of a polypyrrole (PPy) film accompanied by changes in its thickness when it is polarized by the film-deposited Pt electrode in the IL. Furthermore, energy-dispersive X-ray fluorescence (EDX) analysis can be employed for the electrode polarized in the IL. The component analysis by EDX of PPy in an IL containing K+ as a marker, reveals doping of electrolyte cations into the PPy film upon the latter's reduction and dedoping of cations from the film upon oxidation.