Masato Futagawa

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.

A Miniature Integrated Multimodal Sensor for Measuring pH, EC and Temperature for Precision Agriculture

Making several simultaneous measurements with different kinds of sensors at the same location in a solution is difficult because of crosstalk between the sensors. In addition, because the conditions at different locations in plant beds differ, in situ measurements in agriculture need to be done in small localized areas. We have fabricated a multimodal sensor on a small Si chip in which a pH sensor was integrated with electrical conductivity (EC) and temperature sensors. An ISFET with a Si3N4 membrane was used for the pH sensor. For the EC sensor, the electrical conductivity between platinum electrodes was measured, and the temperature sensor was a p-n junction diode. These are some of the most important measurements required for controlling the conditions in plant beds. The multimodal sensor can be inserted into a plant bed for in situ monitoring. To confirm the absence of crosstalk between the sensors, we made simultaneous measurements of pH, EC, and temperature of a pH buffer solution in a plant bed. When the solution was diluted with hot or cold water, the real time measurements showed changes to the EC and temperature, but no change in pH. We also demonstrated that our sensor was capable of simultaneous in situ measurements in rock wool without being affected by crosstalk.

Fabrication of a 128

The charge transfer technique, which is capable of operating in charge accumulation-mode, is suitable for use in pH-imaging to detect small changes in potential. An advanced charge-transfer-type hydrogen ion image sensor consisting of 128 × 128 pixels with a 23-μm pixel pitch is fabricated. A new scanning system and high-performance drive buffer circuits are adopted to achieve high frame rates. For miniaturization of the sensor pixels, we developed an advanced new fabrication process. The pH sensitivity is 32.8 mV/pH when using standard pH solutions. Videos of the movement of hydrogen ions are clearly obtained with the 128 × 128 pixels display, and photo images are taken simultaneously with the videos of the movement of hydrogen ions. A frame rate of 58 frames per second is realized with this image sensor.

\,\times\,

The monolithic integration of a silicon-based plasmonic detector with metal- oxide-semiconductor field-effect transistors (MOSFETs) was demonstrated. The plasmonic detector consisted of a gold film with a nanoslit grating on a silicon substrate and was operated at a free-space wavelength of 1550 nm. The structure of the nanoslit grating was optimized by using the finite-difference time-domain method. The output current from the plasmonic detector was amplified by ~14 000 times using the monolithically integrated MOSFETs. In addition, dynamic operation of the integrated circuit was demonstrated by modulation of the intensity of a beam that was incident to the plasmonic detector.

128 Pixels Charge Transfer Type Hydrogen Ion Image Sensor

A new electrical conductivity (EC) sensor with Pt electrodes on a Si substrate was proposed for use in agricultural applications. The complementary metal oxide semiconductor (CMOS) logic technology-compatible sensor was successfully fabricated with a small chip (area, 25 mm2). This sensor was used for an alternating voltage, owing to the polarization of the Pt electrode and its best operation at 10 kHz frequency. Measurements were conducted on water with electrical conductivities ranging from 10-2 to 101 S/m, which shows sufficient linearity for use as a sensor for agriculture applications. It was also possible to be miniaturized. In an agricultural application, measurements were carried out with the sensor inserted in the rock wools, which was confirmed by the linearity of the values obtained using the sensor. In addition, successful real-time measurements using the sensor were conducted by inserting the sensor into the compost in the vicinity of the roots of a tomato seedling. This sensor provides a promising solution for an EC sensor for broad fields of agricultural applications.

Monolithic Integration of Surface Plasmon Detector and Metal–Oxide–Semiconductor Field-Effect Transistors

A multimodal sensor composed of an electrical conductivity (EC) sensor and a temperature sensor on the same Si chip was successfully fabricated using compatible complementary metal oxide semiconductor (CMOS) technology. The EC sensor measured the electrical conductivity between Pt electrodes, and the temperature sensor measured the forward voltage of a p–n junction diode. We confirmed the output signal in a wide dynamic range of ECs. For real-time measurement in the rumen of cows, a wireless system was fabricated. The system included a sensor, a battery, an antenna, an analog signal-processing circuit board, an AC-to-DC converter board, and a receiver–transmitter board. We adjusted the wireless system design with focus on the measurement range to adapt the system to the rumen. Real-time and simultaneous measurements of EC and temperature in the rumen using the multimodal sensor unit were performed. The wireless system successfully received signals continuously, and enabled the measurement of the rumen conditions in real time.

Miniaturization of Electrical Conductivity Sensors for a Multimodal Smart Microchip

Charge-transfer-type pH sensors can be used to improve pH sensitivity with enhanced signal-to-noise ratio by applying the charge-accumulation technique. Theoretically, the pH sensitivity improves directly with the accumulated count. However, in a conventional sensor structure, a quasi-signal resulting from low charge transfer efficiency limits the accumulated count. In this paper, an effective solution to the problem of the quasi-signal and the novel sensor structure are investigated.

A Real-Time Monitoring System Using a Multimodal Sensor with an Electrical Conductivity Sensor and a Temperature Sensor for Cow Health Control

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.

High-Sensitivity Charge-Transfer-Type pH Sensor With Quasi-Signal Removal Structure

A multi-modal sensor, with which was possible to simultaneously measure pH, electrical conductivity (EC) and temperature, was fabricated. The pH sensor, EC sensor, and temperature sensor were integrated using compatible complementary metal-oxide semiconductor (CMOS) logic technology on a small Si chip (area, 25 mm2). Simultaneous measurements of pH, EC and temperature were demonstrated, and the sensor was used successfully to measure three different kinds of signal from the sensors with a pH 4.01 buffering solution. In another measurement, the sensor was possible to be inserted in soil, and used to successfully measure EC in real time for 6 days.

Integrated Square Wave Voltammetry Redox Sensor System for Electrochemical Analysis

We have developed a CMOS-MEMS-based label-free protein sensor, which utilizes nonlinear optical transmittance change by the Fabry-Perot interference to enhance the sensitivity of surface-stress. Theoretical minimum detectable surface stress of the proposed sensor is predicted -1 μN/m which is two orders of magnitude above the peizoresistive type. A read-out tiny photocurrent from the multidimensional arrayed MEMS sensor is signal-processed by integrated source follower circuit, selector, and decoder. The integrated MEMS sensor array can be used for screening analysis for any cancers.