pH Measurement at Elevated Temperature with Vessel Gate and Oxygen-Terminated Diamond Solution Gate Field Effect Transistors

At a Glance

Metadata	Details
Publication Date	2022-02-25
Journal	Sensors
Authors	Shuto Kawaguchi, Reona Nomoto, Hirotaka Sato, Teruaki Takarada, Yu Hao Chang
Institutions	Waseda University
Citations	7
Analysis	Full AI Review Included

Executive Summary

The research details the development and performance of Oxygen-Terminated Boron-Doped Diamond Solution Gate Field Effect Transistors (C-O BDD SGFETs) for high-temperature pH sensing, specifically targeting applications in the food industry.

High-Temperature Operation: The C-O BDD SGFETs demonstrated stable electrical operation in harsh environments up to 95 °C, significantly exceeding the limits of commercial glass electrodes (which fail near 100 °C).
Boron Activation Effect: At high temperatures (80 °C), the BDD SGFET channel became pH-insensitive (sensitivity dropped to 4.27 mV/pH) due to the thermal activation of boron acceptors, increasing the hole concentration.
Novel System Configuration: A highly pH-sensitive system was realized by combining the pH-insensitive C-O BDD SGFET with a pH-sensitive Stainless Steel Vessel Gate (SUS304).
High Sensitivity Achieved: The combined system achieved a high pH sensitivity of -54.6 mV/pH at 80 °C.
Nernst Efficiency: This sensitivity corresponds to 77.9% of the theoretical Nernst response (70.1 mV/pH at 80 °C), confirming the viability of the robust, all-solid-state system for high-temperature use.
Mechanism of Sensitivity: The vessel gate configuration utilizes the opposite sensitivity direction of the vessel surface relative to the FET channel, allowing the pH-sensitive vessel gate to dominate the overall system response.

Technical Specifications

Parameter	Value	Unit	Context
Maximum FET Operating Temperature	95	°C	Demonstrated stable I_DS-V_DS characteristics.
Maximum pH Sensing Temperature	80	°C	Highest temperature tested for sensitivity measurement.
pH Sensitivity (RT, Ag/AgCl)	69.6 / 16.8	mV/pH	Acidic region / Alkaline region (SGFET alone).
pH Sensitivity (80 °C, Ag/AgCl)	4.27	mV/pH	SGFET alone, demonstrating pH insensitivity at high T.
pH Sensitivity (80 °C, Vessel Gate System)	-54.6	mV/pH	Combined pH-insensitive SGFET and pH-sensitive vessel gate.
Nernst Response (80 °C)	70.1	mV/pH	Theoretical maximum response at 80 °C.
System Efficiency (80 °C)	77.9	%	Achieved sensitivity relative to Nernst response.
Transconductance Increase (95 °C)	3	Times higher	Compared to room temperature (due to boron activation).
Activation Energy of Boron (E_i)	0.15	eV	Calculated between RT and 95 °C.
Substrate Material	Polycrystalline Diamond	N/A	Substrate for BDD SGFET fabrication.
Channel Dimensions (L x W)	200 µm x 5 mm	N/A	Length (L) and Width (W) of the FET channel.
Electrode Thickness (Au)	100	nm	Source and Drain electrode thickness.

Key Methodologies

The C-O BDD SGFETs were fabricated on polycrystalline diamond substrates and integrated into a novel sensing system:

Boron-Doped Layer Deposition: A boron-doped layer was deposited onto the polycrystalline diamond substrate using Microwave Plasma Chemical Vapor Deposition (CVD).
Surface Cleaning and Hydrogen Termination: Substrates were cleaned in a mixture of nitric acid (HNO₃) and sulfuric acid (H₂SO₄) at 200 °C, followed by full hydrogen termination (C-H) using microwave-enhanced plasma CVD.
Electrode Metallization: Gold source and drain electrodes (100 nm thick) were deposited via electron-beam evaporation, defining a channel region of 200 µm (L) by 5 mm (W).
Passivation: Conductive epoxy resin was used to connect wires, and insulating epoxy resin covered all conductive parts except the channel.
Oxygen Termination: The final channel surface was terminated with oxygen (C-O) using a plasma reactor in an oxygen atmosphere.
Gate Electrode Implementation: Two gate types were tested: (1) a standard Ag/AgCl glass electrode and (2) a Stainless Steel Vessel Gate (SUS304), which served as both the container and the gate electrode.
pH Measurement: Carmody buffer solutions (pH 2 to 12) were used. Temperature was controlled using a digital hot plate (HP-2SA).
Sensitivity Calculation: pH sensitivity was derived by calculating the shift in gate voltage (V_GS) required to maintain a constant drain current (I_DS) across different pH values.

Commercial Applications

The robust, high-temperature capability of the diamond SGFET system, particularly when combined with the corrosion-resistant stainless steel vessel gate, makes it highly suitable for industrial environments:

Food and Beverage Processing: Essential for monitoring pH during high-temperature sterilization, pasteurization, and cooking processes (up to 95 °C), where SUS304 is standard and glass electrodes fail.
Industrial Chemical Monitoring: Use in harsh, corrosive, or high-temperature chemical reactors and manufacturing lines where traditional sensors degrade rapidly.
Bioprocess Engineering: Applications requiring stable pH control in bioreactors or fermentation tanks operating at elevated temperatures.
Wastewater Treatment: Monitoring aggressive effluent streams where both high temperature and chemical resistance are required.
All-Solid-State Sensing Systems: Provides a robust, miniaturizable alternative to bulky, fragile glass electrodes, enabling integration into complex flow systems and IoT sensor networks.

View Original Abstract

Diamond has many appealing properties, including biocompatibility, ease of surface modification, and chemical-physical stability. In this study, the temperature dependence of the pH-sensitivity of a oxygen-terminated boron-doped diamond solution gate FET (C-O BDD SGFET) is reported. The C-O BDD SGFET operated in an electrolyte solution at 95 °C. At 80 °C, the pH sensitivity of C-O BDD SGFET dropped to 4.27 mV/pH. As a result, we succeeded in developing a highly sensitive pH sensing system at −54.6 mV/pH at 80 °C by combining it with a highly pH sensitive stainless-steel vessel.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s22051807

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