Sou Takahashi

Directly amplified redox sensor for on-chip chemical analysis

In recent years, many groups have studied redox sensors for chemical analysis. A redox sensor has certain powerful advantages, such as its ability to detect multiple ions inside the sensing area, and its ability to measure concentrations of materials by using voltage and current signals. However, the output current signal of a redox sensor decreases when either concentration or sensing area decreases. Therefore, we propose the use of an amplified redox sensor (ARS) for measuring small current signals. The proposed sensor consists of a working electrode combined with a bipolar transistor. In this study, we fabricated an ARS sensor and performed low-concentration measurements using current signal amplification with an integrated bipolar transistor. The sensor chip successfully detected a potassium ferricyanide (K3[Fe(CN)6]) concentration of as low as 10 µM using cyclic voltammetry.

Flexible Neural Electrode Arrays With Switch-Matrix Based on a Planar Silicon Process

Herein, we fabricate a flexible microelectronic system using a conventional silicon (Si) integrated circuit process. The fabricated device is a -thick film flexible 7 × 8 (56 ch) switch-matrix microelectrode array, which can be used to record the electrical activity from numerous three-dimensional biological tissues. The embedded Si-nMOSFETs/(111) in a polyimide flexible film exhibit a controlled threshed voltage with a leakage current of 10-11 A and a subthreshold swing of 123 mV/decade at a 50-mV drain voltage. The electrical characteristics between the flat and bent (with a 3-mm curvature radius) devices do not significantly change in a saline environment. These results indicate that the proposed method, which does not utilize conventional transfer printing technology, may be used to fabricate high-performance flexible electronics via a high-resolution lithography process. Such flexible electrode arrays may be applicable to high spatial-resolution recordings of neuronal signals from three-dimensional tissues, such as the brain surface, retina, and peripheral nerves.

Integrated Square Wave Voltammetry Redox Sensor System for Electrochemical Analysis

An integrated square wave voltammetry (SWV) redox sensor has been developed on the basis of a standard complementary metal–oxide–semiconductor (CMOS) process technology. The sensor consists of a square wave (SW) pulse generator, a voltage controller, and two electrodes for electrochemical analysis. Our proposed sensor is the first integrated sensor system of a SW pulse generator. Potassium ferricyanide solution was measured to obtain the characteristics of the proposed sensor. We confirmed that the dynamic ranges of potassium ferricyanide concentration and SW frequency were obtained from 0.6 to 6 mM and from 20 to 500 Hz, respectively. To verify the accuracy of the proposed sensor system, we performed a comparison between the fabricated sensor and an electrochemical analyzer.

Development of amperometric ion sensor array for multi-ion detection

We have successfully integrated an amperometric ion sensor array on an Si chip which simultaneously detects potassium ion (K+) and sodium ion (Na+). The sensor array has two kinds of selective electrodes which exhibit high sensitivity for K+ over Na+ (K+-selective electrode) and for Na+ over K+ (Na+-selective electrode), respectively. Ion transfer voltammetry method was adopted for the detection of these ions utilizing ionophore-doped PVC (polyvinyl chloride) membrane because it is difficult to detect these ions using redox reaction. K+- and Na+-selective electrodes have sensitivity of -5.1 and -4.0 nA/μM, respectively, while the differences in the sensitivity were 2.83 and 1.54 times in the K+- and Na+-selective electrodes, respectively. Consequently, selectivity on detection of multiple ions was demonstrated by an integrated sensor array on a Si chip, leading to miniaturized amperometric ion sensor array systems.

On-chip square wave voltammetric pulse generator for redox measurement employing array structure.

A square wave voltammetric pulse generator circuit has been developed with CMOS technology and it was integrated with a redox array sensor. The pulse generator circuit consists of a presetter, two counters, a selector, a D flip-flop, an R-2R ladder DAC, and a voltage shifter. We can control the frequency, the square wave amplitude, and the step increment of square wave voltammetry. We confirmed that the square wave pulse and the 64 peak currents are obtained using the pulse generator circuit and 8 × 8 array working electrodes with potassium ferricyanide solution. Each read-out signal of array electrodes is read in sequence using the shift resistor.

Integrated 8 × 8 array redox sensor system employing on-chip square wave voltammetric circuit for multi point and high-speed detection

A square wave voltammetric measurement system with an 8 × 8 array of sensing pixels is integrated within a single chip for multi point and high-speed electrochemical analysis. The sensor was designed to operate at 400 ms/frame under the condition of 20 mV and 500 Hz of step increment and square wave frequency, respectively. Peak currents from the 8 × 8 pixel array of potassium ferricyanide were obtained at the potential range from −0.5 to 0.5 V. We also obtained a small variation of the measured concentration result of all 64 pixels under the concentration range from 0.6 to 6 mM of potassium ferricyanide standard solution.

Fabrication of an Integrated Square Wave Voltammetry (SWV)-Redox Sensor

An integrated square wave voltammetry (SWV) redox sensor has been developed on a 5 x 5 mm 2 Si chip. The sensor consists of a SWV pulse generator, a voltage controller, and two electrodes for electrochemical analysis. Our proposed sensor is the first integrated sensor system of SWV pulse generator. Potassium ferricyanide solution was measured to obtain characteristics of proposed sensor. We confirmed that the dynamic range of concentration and frequency was obtained from 0.6 to 6 mM and from 20 to 500 Hz of potassium ferricyanide.