Ion-Sensitive Stainless Steel Vessel for All-Solid- State pH Sensing System Incorporating pH-Insensitive Diamond Solution Gate Field-Effect Transistors

An all-solid-state pH sensing system utilizing a stainless steel vessel (SUS304) and a pH-insensitive diamond solution-gate field-effect transistor (SGFET) is presented here to explain the interaction between stainless steel vessel and field-effect transistor (FET) pH sensors for the first time. A pH-sensitive ion-sensitive field-effect transistor (ISFET) was first used to show the change of the sensing behavior from 47.78 to −4.73 mV/pH when using an Ag/AgCl electrode and stainless steel vessel as the gate. This intriguing sensing behavior was investigated by developing large- and small-signal equivalent circuit models in a transistor circuit for both the Ag/AgCl and the stainless steel vessel gate. The result shows that the targeted ion change $\Delta {Q}_{h}$ corresponding to the pH sensitivity has been offset, which explains the phenomenon observed when using the pH-sensitive ISFET with the stainless steel vessel. We then hypothesize that combining a pH-insensitive device with the stainless steel vessel gate should show a pH sensitivity close to the Nernst response. To validate this, a pH-insensitive diamond SGFET was then fabricated and combined with the stainless steel vessel for pH measurements. The system demonstrates a high pH sensitivity at −54.18 mV/pH across a wide range of pH solutions (pH 2–12) and remains stable in elevated temperatures when measured with a potentiostat setup at 80 °C. The results also suggest that this all-solid-state sensing system has great potential to be used in the food and beverage industry where stainless steel is widely employed due to its excellent corrosion resistance, low cost, and high sensing capabilities.


I. INTRODUCTION
S TAINLESS steel has attracted much attention in the field of biosensing and food industry due to its fast response, low cost, and high sensing capabilities [1], [2], [3], [4], [5]. Several studies have reported the use of stainless steel (SUS304 and SUS316) for sensing applications and addressed some existing issues of glass sensors [6], [7], [8]. These glass sensors have been extensively used because of their stable and high sensing capability, high reliability, and intuitive operation. Nevertheless, these sensors are fragile and large in size and require constant recalibration due to contamination accumulation and liquid junction [9]. The stainless steel surface demonstrates close to ideal Nernstian pH sensitivity due to the films of mixed metal oxides on its surface. Upon further investigation, we found intriguing results when using the stainless steel as the gate in combination with either diamond solution-gate field-effect transistors (SGFETs) with different surface functional groups or commercially available ion-sensitive field-effect transistors (ISFETs). While most reports focus on using a small piece of stainless steel for sensing or using the stainless steel as the backbone for other pH-sensitive coatings, we utilize a stainless steel vessel connected with the gate, thus the name stainless steel vessel gate, in this work. The advantages of this all-solid-state sensing system include a large surface sensing area, direct sensing on the stainless steel vessel surface, and excluding the use of any glass-related components (glass electrodes, glass beakers, and so on). This all-solid-state system also shows great potential to be used in the food and beverage industry where stainless steel vessels are widely used. In addition, the pH insensitivity of the diamond SGFET used is of great importance in this all-solid-state sensing system.
Diamond is suitable for biosensing and pH sensing applications due to its excellent properties including wide potential window, chemical inertness, and ease of surface modification. We have reported diamond SGFETs where the semiconductor surface was directly immersed in electrolyte solutions and the drain current was controlled by an electric doublelayer capacitor at the diamond surface [10]. We have also reported diamond SGFETs utilizing various functional groups, including hydrogen, oxygen, nitrogen, and fluorine to achieve excellent pH sensing capabilities or complete pH insensitivity [11], [12], [13], [14], [15]. Like most ISFETs, a silver/silver chloride (Ag/AgCl) reference electrode has been commonly used as the gate electrode for diamond SGFETs. These glass electrodes have been widely used because of their lasting stability, fast response time, and a wide range of pH sensing capabilities. However, the risk of potassium chloride (KCl) internal solution leakage causing sample contamination, the problem concerning constant calibration required for maintaining the pH sensitivity, or breakage of the glass electrode have prevented it from being used in several industries [9]. One solution to this problem is to develop an all-solid-state pH sensing system, excluding the use of Ag/AgCl glass electrodes.
In the case of ISFETs, an all-solid-state pH sensing system consists of a pH-sensitive ISFET, a pH-insensitive reference FET (REFET), and a metal used for the quasi-reference electrode (QRE) [16], [17]. While various types of REFETs have been reported, achieving enough pH insensitivity for the all-solid-state pH sensing system to perform effectively while not sacrificing any pH sensing capability has been challenging. In the case of diamond SGFETs, hydrogen termination or fluorine termination incorporated diamond SGFETs have shown pH-insensitive results and are excellent potential candidates as REFETs for all-solid-state pH sensing systems. We have reported such a pH sensing system using a fluorineterminated pH-insensitive diamond SGFET in combination with an oxygen-terminated diamond SGFET and platinum as the QRE [14]. The result shows moderate pH sensitivity and provides us insight on utilizing pH-insensitive diamond SGFETs as REFETs.
In this work, an all-solid-state pH sensing system is realized by utilizing pH-insensitive diamond SGFETs and a stainless steel vessel as the gate. We have reported a similar system utilizing pH-sensitive boron-doped diamond SGFETs with the stainless steel vessel [18]. However, much attention has been focused on the correlation between boron-doped diamond SGFETs and high temperatures. Here, large-and small-signal equivalent circuits are developed to explain the working mechanism of this all-solid-state pH sensing system. A potentiostat measurement is also conducted to observe the changes of the stainless steel vessel in elevated temperatures.

II. MATERIAL AND METHODS
Polycrystalline diamond substrates were used for fabricating the diamond SGFETs in this work, while the Si ISFETs were purchased from Winsense Company Ltd. [19], [20]. The diamond SGFET device design is similar to those reported in our previous reports and is shown in Fig. 1 [12], [15]. The polycrystalline diamond substrates were first cleaned in a mixture of nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ) with a ratio of 1:3 at 200 • C for 30 min. The substrates were then cleaned for 10 min each in the order of deionized water, ethanol, acetone, ethanol, and deionized water in an ultrasonic washer to remove any organic residues. Hydrogen termination was conducted to the substrate surface under 600 • C in a hydrogen atmosphere at 20 torr for 30 min in a chemical vapor deposition (CVD) system. Contact lithography was used to form patterns on the substrate's surface. After depositing Au with an electron beam evaporator, source and drain electrodes were formed by lifting off the deposited pattern on the channel region. The Au electrodes were then encapsulated with negative resists (SU-8) except regions where wires were later attached to and bonded using conductive paste (Chemtronics). Finally, the entire device except the channel region was encapsulated using epoxy resin (Huntsman Advanced Materials) to prevent any leakage when measured in electrolyte solutions. The finalized hydrogen-terminated diamond SGFET device has a channel width W = 5 mm, channel length L g = 30 µm, and L gd = L gs = 10 µm, where L gd or L gs refers to the distance between the end of a channel to the source or drain.
Carmody buffer solutions were prepared by mixing boric acid (BH 3 O 3 ), citric acid monohydrate (C 6 H 8 O 7 ), and trisodium phosphate dodecahydrate (Na 3 PO 4 ·H 2 O) solutions. The pH solutions from pH 2 to 12 were procured by measuring and calibrating the Carmody buffer solutions with a digital pH meter (Yokogawa Electric Corporation), which was carefully calibrated using a three-point calibration method using phthalate pH standard solutions. The pH sensitivity of the system was measured with a semiconductor device parameter analyzer (Keysight) with the LabVIEW program and was calculated based on the shift of V TH for every pH solution. Two different gates were used: Ag/AgCl electrode and commercially available stainless steel vessel cup (SUS304). When using the Ag/AgCl electrode as the gate, the device and the electrode were both immersed in Pyrex beakers containing the corresponding pH solutions. In the case of using stainless steel vessel as the gate, the device was directly immersed in the stainless steel vessel containing the corresponding pH solutions, as shown in Fig. 2. A potentiostat measurement was used to observe the behavior of the stainless steel vessel in  elevated temperatures. The Ag/AgCl glass electrode, Pt electrode, and stainless steel vessel were used as the reference electrode, counter electrode, and working electrode, respectively, while potentiostat measurements were taken from pH 2 to 12 at room temperature and 80 • C. Fig. 3 shows the pH sensitivity of the ISFET when using two different gates. The ISFET shows a high pH sensitivity of −47.78 mV/pH when combined with the Ag/AgCl gate electrode. However, the pH sensitivity decreases to −4.73 mV/pH and shows no pH sensitivity when using the stainless steel vessel as the gate. The pH sensitivity difference between the two gates is around 53 mV/pH and is close to that of the Nernst response. The underlying mechanism of this sensing behavior can be explained using large-and smallsignal equivalent circuit models in a transistor circuit [21]. The large-signal equivalent circuit is composed of a fixed term (dc) and a small-signal term (ac). Note that the ac term in this case is representative of the time-dependent part that is not necessary in the form of sinusoidal wave. Fig. 4 shows the large-and small-signal equivalent circuits of the Si ISFET or diamond SGFET system when using the Ag/AgCl electrode, where C EDL is the electric double-layer capacity, C i is the insulating film capacity in the Si ISFET or the inversion layer capacitance in the diamond SGFET, and Q h is the number of protons corresponding to the changes in pH solutions [10]. Since the ions in the pH solution are close to the surface of the insulating film, Q h is generated between C EDL and C i . Fig. 4(b) shows the large-signal equivalent circuit with a fixed voltage source V GS (dc part) and a fluctuating small signal (ac part). When transforming the large-signal equivalent circuit to a small-signal equivalent circuit, voltage sources are shorted [ Fig. 4(c)], and the two capacitors can be positioned in parallel. In the small-signal equivalent circuit, the electric charge Q h is divided between C EDL and C i in the form of parallel capacitance [ Fig. 4(d)], and only N accumulates charges in C i . The equation for N is shown as follows [22]:

III. RESULTS AND DISCUSSION
where C EDL for Si ISFETs and diamond SGFETs is 5-10 µF/cm 2 and C i in Si ISFETs and diamond SGFETs is approximately 0.1-0.5 µF/cm 2 with SiO 2 gate oxide (≈10 nm in thickness) and 5 µF/cm 2 , respectively [10]. Ci is considered to be constant since the carrier was enough at channel above V T . In typical Si ISFETs, careful derivation of threshold voltage change is required as shown in the work by Xu et al. [23] due to having a gate insulator thicker than 10 nm and an additional passivation layer on top to prevent ion intrusion. For diamond SGFETs and very thin gate oxide ISFETs, however, the charge can be considered as a 2-D sheet on the channel since sensing happens directly on the channel surface. This allows the change in gate potential caused by changes in the pH solutions to simply be proportional to the number of carriers ( N ∼( Q h )/(2)) [10]. qN (a large signal) is now the total charge of the FET channel and is composed of a first term (a fixed dc part at a certain V GS point) where C EDL and C i are series capacitances in Fig. 4(b) and a second term (a small signal) with N , and the equation can be expressed as follows [10]: When C EDL is much greater than C i , (2) can be approximated to the following: Since Q h is inversely proportional to pH, meaning that the value of Q h is larger in low pH and smaller in high pH, the threshold voltage value increases and shifts to the positive side as pH increases. Fig. 5 shows the large-and small-signal equivalent circuits of the ISFET when using the stainless steel vessel. Since sensing happens directly on the surface of the stainless steel vessel in contact with the pH solutions, an electric double layer is considered for the vessel gate. The sensing area of the stainless steel vessel is attributed as k times larger than that of ISFET, and the capacity is expressed as kC EDL . The transformed small-signal equivalent circuit can be short-circuited of dc voltage sources because both sides of the circuit are grounded, and the potentials are the same.
Since there are two signal sources, the analysis of V T shift has been carried out by two superpositioned circuits with one signal source at each other, as shown in Fig. 6. The charges can be calculated by considering the independent two circuits separately when grounding either Q h or k Q h . Fig. 6 shows the Q h grounded circuit where N is expressed as follows [21]: The total charge can then be expressed as follows: Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply.  When kC EDL is much larger than C i , (5) can be simplified and approximated to (6) as shown in the following: In this case, the threshold voltage value decreases and shifts to the negative side as pH increases. When grounding k Q h , since the sensitivity of the stainless steel vessel is not involved, the equation for this circuit is similar to (2) and (3) when using the Ag/AgCl electrode. By combining the approximated [(3) and (6)] calculated for the two grounded circuits, the Q h term corresponding to the pH sensitivity is canceled out as shown in the following equation: This explains why the combination of an ISFET and stainless steel vessel gate offsets the pH sensitivity. Therefore, we hypothesize that combining a pH-insensitive device with the stainless steel vessel gate should show a pH sensitivity close to the Nernst response. A hydrogen-terminated diamond SGFET was used as the pH-insensitive device with the stainless steel vessel gate to test this hypothesis, as shown in Fig. 7. Fig. 8 shows the pH sensing results of the hydrogenterminated diamond SGFET when using different gates.  The sensitivity was investigated based on the shift of V TH for every pH solution. The pH sensitivity can be approximated by first determining an I ds value. V gs of each pH at that I ds is then calculated and plotted against pH. The pH sensitivity can then be calculated by finding the slope of the V gs versus pH graph.
Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply. The hydrogen-terminated device demonstrates close to zero pH sensitivity (0.60 mV/pH) when using Ag/AgCl as the gate. This pH-insensitive result is expected as explained in our previous report [8]. When using the stainless steel vessel as the gate, the system shows a high pH sensitivity at −54.18 mV/pH close to the Nernst response. The result confirms our hypothesis and shows excellent pH sensing capability. Clearly, a pHinsensitive SGFET, such as the hydrogen-terminated diamond SGFET used in this work, is preferred for obtaining a high pH sensitivity result when using the stainless steel vessel gate. While a detailed selectivity study on this all-solid-sensing system has yet to be conducted, a selectivity study on heattreated stainless steel SUS304 and SUS316 as pH sensors has been reported by Nomura and Ujihira [1]. In their report, the heat treated stainless steel (SUS304) shows excellent hydrogen ion selectivity over a wide range of pH solutions (pH 1-13) coexisting with various alkali-metal ions including Li+, Na+, and K+. However, it shows an unstable pH response when immersed in 0.1-and 0.5-M NaCl solutions below pH 4. A similar selectivity study should be conducted in the future to understand the limitation of this all-solid-state sensing system with the untreated commercially available stainless steel SUS304. In addition, surface characterizations, including X-ray photoelectron spectroscopy (XPS), will help clarify whether there is any difference between the current commercially available stainless steel and others reported in the literature [1], [2]. Fig. 9 shows the potentiostat results and the pH sensitivity of the stainless steel vessel at room temperature and at 80 • C. The results suggest that the stainless steel vessel remains stable while demonstrating a high pH sensitivity at −59.4 and −63.8 mV/pH at room temperature and 80 • C, respectively. Due to partial channel oxidation of the hydrogen-terminated diamond SGFETs in elevated temperatures and limitations of the component used during our device fabrication, using the all-solid-state sensing system in higher temperatures has resulted in unstable measurements and inconsistent results. Alternative insensitive surface modifications, including fluorine termination that can withstand higher temperatures and heat-resistant wires, should be considered in future research.

IV. CONCLUSION
In summary, we have fabricated an all-solid-state pH sensing system using a hydrogen-terminated diamond SGFET and a stainless steel vessel as the gate. The results show a high pH sensitivity close to the Nernst response (−54.18 mV/pH). We have also shown pH sensing results of an ISFET connected to Ag/AgCl electrode or stainless steel vessel as the gate and explained the working mechanism of this system by using large-and small-signal equivalent circuits. The stainless steel vessel has shown excellent stability and pH sensitivity in both room temperature and 80 • C when measured in a potentiostat setup. Future research will focus on investigating the use of other pH-insensitive diamond SGFET such as the fluorine-terminated diamond SGFETs with the stainless steel vessel gate, response time and the performance in elevated temperatures, and surface characterizations of the stainless steel vessel gate sensing system. Teruaki Takarada received the B.Eng. and M.Eng. degrees in electronic and physical systems from Waseda University, Tokyo, Japan, in 2020 and 2022, respectively.
His research interests include diamonds, field-effect transistors, and seawater wireless communications.
Mr. Takarada received the Best Poster Session Award at the 33rd Diamond Symposium in 2019 and the Nano Academy Award at the Nano Tech International Nanotechnology Exhibition and Conference in 2020. Mohd Syamsul is a Researcher in the field of wide bandgap materials, specifically GaN, diamond, and carbon-related materials. He has made several contributions to the research of nanoelectronics, with a particular focus on developing diamond for biosensors and power devices. Previously, he worked as a Postdoctoral Researcher and a Doctoral Student at Waseda University, Tokyo, Japan, where he studied diamonds and their potential applications as power devices. Currently, he is a Senior  He is a Professor with the Department of Social System Science, Chiba Institute of Technology, Narashino, Japan. His laboratory currently focuses on two research areas: social implementation of advanced technologies and technology management. In the advanced technology area, he mainly focuses on diamond semiconductors, the IoT, and robotic digital twins, while in the technology management area, he mainly focuses on innovation management and research and development organizational control. Prior to joining the university, he led research and development at several companies, including Yokogawa Electric Corporation, Tokyo, where he was responsible for the research and development of factory visualization technologies, and 3M, where he was responsible for the research and development of smart factories based on robotic digital twin technology.