An all-solid-state pH sensor based on hydrogen storage alloy

Abstract

A hydrogen storage alloy-based all-solid-state pH sensor is disclosed, comprising a counter electrode, a working electrode, a reference electrode, and a flexible substrate. The counter electrode is a platinum electrode, the working electrode is a hydrogen storage alloy electrode, the reference electrode is a silver / silver chloride electrode, and the flexible substrate is PET. Using the hydrogen-storage titanium-iron alloy electrode as the working electrode and an Ag|AgCl electrode as the reference electrode, the potential of the test solution is measured, and the pH value of the test solution is obtained through a standard curve. This invention explored the optimal ratio of titanium and iron elements through a series of experiments, enabling the sensor to simultaneously achieve both accuracy and stability. While ensuring good hydrogen ion response, optimal stability is obtained by adjusting the titanium-iron element ratio, making it suitable for long-term real-time online in vivo pH detection.

Description

Technical Field

[0001] This invention relates to a pH sensor, and more specifically to an all-solid-state pH sensor based on a hydrogen storage alloy. Background Technology

[0002] Driven by the era of "big health," the concept of "prevention is better than cure" has taken root. Compared to the later stages of disease treatment, disease prevention is, in a sense, more important. The pH value of the human body's internal environment can be monitored in real-time through bodily fluids such as saliva, blood, urine, sweat, tissue fluid, and tears, enabling early prediction and diagnosis of various important diseases, truly achieving "prevention before the disease occurs." Flexible wearable sensors, with their advantages of easy miniaturization, low power consumption, and real-time monitoring, meet these requirements and have become a hot area in the era of "big health." Classic modern commercial pH meters integrate a glass electrode for sensing and a reference electrode (RE) for setting a standard potential into a single device, creating a complete redox electrode to obtain a stable solution potential difference. However, the ion-selective membrane and conductive substrate of the pH meter are connected by an internal filling fluid, making the electrode's stability susceptible to factors such as evaporation and contamination of the internal filling fluid. Furthermore, the electrode cannot be miniaturized, requiring regular calibration to ensure stable operation. Furthermore, the manufacturing process of such electrodes is cumbersome, and testing requires a large sample volume, making them unsuitable for testing minute samples and significantly limiting their application in certain scenarios. In particular, being made of glass, their lack of flexibility prevents them from being used in in-situ wearable systems, limiting their application to non-in-situ measurements via in vitro collection. To achieve in-situ measurement, flexible wearable pH sensors have become an important development direction for medical monitoring.

[0003] Existing reports indicate that existing sensors suffer from drawbacks such as complex fabrication, poor stability, and difficulty in miniaturization, failing to meet the practical needs of real-time in vivo monitoring. According to current reports, titanium-based hydrogen storage alloys are abundant, low-cost, and relatively simple in design, but they are difficult to activate, require high temperatures and pressures for hydrogen absorption and desorption, and are susceptible to poisoning by gaseous impurities such as CO and O2, exhibit significant hysteresis, short cycle life, and performance degradation after repeated hydrogen absorption. Furthermore, the influence and mechanism of the hydrogen storage capacity and hydride conformation type of titanium-based hydrogen storage alloys on pH response characteristics are not yet fully understood. This severely limits the optimized design and fabrication process of pH sensors, hindering the precise design and practical application of wearable flexible pH sensors. Therefore, there is an urgent need to develop and design a novel wearable oral pH sensor with superior performance and easy miniaturization. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of traditional pH sensors, such as poor stability and poor biocompatibility, by applying electrochemical hydrogen storage technology to titanium materials with good biocompatibility, and to provide a pH sensor with superior sensing performance that can be used for in vivo, wearable, and online real-time pH monitoring.

[0005] Existing glass pH meters and all-solid-state pH sensors face problems such as signal drift due to environmental interference during long-term measurements, directly affecting sensor stability. This is especially true for flexible wearable pH sensors used for oral microenvironment monitoring, which require considerable time for real-time dynamic detection. This poor stability is a significant technological bottleneck, severely limiting the practical application and commercialization of wearable pH sensors. Hydrogen storage alloys generally consist of a hydrogen-absorbing element (A) and an element with low or no hydrogen absorption (B). A controls the hydrogen storage capacity and is a key element in the alloy; B controls the reversibility of hydrogen absorption and desorption, adjusting the heat of generation and decomposition pressure. This invention uses a method of dynamically controlling element B to reduce the reversibility of the hydrogen storage alloy electrode, thereby improving stability and solving the bottleneck problem.

[0006] An all-solid-state pH sensor based on a hydrogen storage alloy includes: a counter electrode, a working electrode, a reference electrode, and a flexible substrate. The counter electrode is a platinum electrode, the working electrode is a hydrogen storage alloy electrode, the reference electrode is a silver / silver chloride electrode, and the flexible substrate is PET.

[0007] The hydrogen storage alloy electrode is prepared by the following method:

[0008] Step 1: Basic Electrode Preparation

[0009] The basic working electrode is a copper electrode, the reference electrode is a silver / silver chloride electrode, and the counter electrode is a platinum electrode.

[0010] Step 2: Electroplating hydrogen storage alloy

[0011] (1) Plating solution formulation: main salt: FeSO4 0.1-0.2mol / L, Ti(SO4)2 0.4-0.5mol / L; buffer: NaH2PO4 0.1mol / L, complexing agent: sodium citrate 0.05-0.1mol / L, plating solution pH: 4.6±0.2; add the corresponding weight of the compound directly to the water, and after dissolution, it becomes a plating solution containing the corresponding concentration of main salt, buffer and complexing agent;

[0012] (2) Electroplating method: A three-electrode system was used in an H-type electrolytic cell, and cathodic polarization and cyclic voltammetry curves were tested. The basic working electrode was a copper electrode, and the reference electrode was an Ag|AgCl electrode. The plating solution was heated by a heating rod and controlled within a specified temperature range (56±2℃). The thoroughly cleaned electrodes were placed close to the tank wall, and alloy electroplating was performed in the three-electrode system of the electrochemical workstation using the chronopotential method. During electroplating, the current was kept constant, and the pH value was kept stable between 4.6±0.2. The concentration difference was eliminated by stirring. Plating was carried out for 10-20 minutes, and then the electrodes were removed, cleaned, dried, and stored.

[0013] Step 3: Electrochemical hydrogen storage treatment

[0014] The prepared titanium-iron alloy electrode was placed in 0.1-0.2 mol / L HCl and subjected to electrochemical hydrogen storage treatment using a constant potential electrolysis method on an electrochemical workstation. The hydrogen storage potential was -1.7V, the hydrogen storage time was 1-2h, and the hydrogen storage was terminated when the electrolysis charge reached 3C. The electrode was then cleaned, dried, and stored for later use.

[0015] pH measurement

[0016] Using a hydrogen-storage titanium-iron alloy electrode as the working electrode and an Ag|AgCl electrode as the reference electrode, the potential of the test solution was measured, and the pH value of the test solution was obtained through a standard curve. The response principle is shown in the following equation.

[0017]

[0018] In the formula, φ e Indicates electrode potential, Φe θ P represents the standard electrode potential, with units of V; θ The pressure is standard atmosphere; P(H2) is the partial pressure of hydrogen, in Pa; T represents temperature, in K; R represents the gas constant, 8.314 J / (mol·K); F represents the Faraday constant, 96485 C·mol. -1 ;a H+ This indicates the concentration of hydrogen ions in the solution, expressed in mol / L.

[0019] The beneficial effects of this invention are:

[0020] This invention overcomes the shortcomings of traditional pH sensors, such as poor stability and poor biocompatibility. Titanium is a metallic material with excellent biocompatibility and has been successfully and widely used in many fields, such as titanium alloy porcelain crowns and titanium alloy artificial bones. Moreover, titanium is a good hydrogen storage material, which provides a theoretical possibility for applying titanium alloy hydrogen ion sensors in vivo. The hydrogen storage alloy is composed of two elements, A and B. The main function of element A (titanium in this invention) is hydrogen storage, and the main function of element B (iron in this invention) is to regulate the reversibility of the hydrogen storage material, i.e., the stability of the sensor. This invention explored the optimal ratio of elements A and B through a series of experiments, so that the sensor can simultaneously take into account the two important parameters of accuracy and stability. While ensuring good hydrogen ion response capability, the optimal stability is obtained by adjusting the ratio of elements A and B, making it suitable for long-term real-time online in vivo pH detection. Attached Figure Description

[0021] Figure 1 pH response curve of the all-solid-state pH sensor based on hydrogen storage alloy of the present invention;

[0022] Figure 2 Stability was continuously tested for 24 hours under different pH conditions;

[0023] Figure 3 A schematic diagram of the pH sensor of the present invention; in the figure: 1-counter electrode, 2-working electrode, 3-reference electrode, 4-flexible substrate. Detailed Implementation

[0024] Example

[0025] A hydrogen storage alloy-based all-solid-state pH sensor, such as Figure 3 As shown, it includes: a counter electrode, a working electrode, a reference electrode, and a flexible substrate. The counter electrode is a platinum electrode, the working electrode is a hydrogen storage alloy electrode, the reference electrode is a silver / silver chloride electrode, and the flexible substrate is PET.

[0026] The hydrogen storage alloy electrode is prepared by the following method:

[0027] Step 1: Basic Electrode Preparation

[0028] The sampling was conducted on a screen-printed electrode based on PET flexible material. The printed electrode included a working electrode, a reference electrode, and a counter electrode. The overall dimensions of the printed electrode were 12×34mm (width×length) and the thickness was 0.3mm. The basic working electrode was a copper electrode with a diameter of 4mm, the reference electrode was a silver / silver chloride electrode, and the counter electrode was a platinum electrode. The electrodes were ultrasonically cleaned and then put into use.

[0029] Step 2: Electroplating hydrogen storage alloy

[0030] Plating bath formulation: Main salts: FeSO4 0.1 mol / L, Ti(SO4)2 0.4 mol / L; Buffer: NaH2PO4 0.1 mol / L; pH of plating bath: 4.6 ± 0.2; Complexing agent: sodium citrate 0.05 mol / L. The copper electrode was subjected to titanium-iron alloy electroplating in a three-electrode system using an electrochemical workstation. The plating was performed for 10 minutes using a chronopotential bonding method, after which the electrode was removed, cleaned, dried, and stored.

[0031] Step 3: Hydrogen Storage Treatment

[0032] The prepared titanium-iron alloy electrode was placed in 0.1 mol / L HCl and subjected to electrochemical hydrogen storage treatment using a constant potential electrolysis method on an electrochemical workstation. The hydrogen storage potential was -1.7 V, the hydrogen storage time was 1-2 h, and the hydrogen storage was terminated when the electrolysis charge reached 3 C. The electrode was then cleaned, dried, and stored for later use.

[0033] pH measurement

[0034] After modification, titanium and iron hydrides have formed on the electrode surface, giving it a characteristic responsiveness to hydrogen ions in the solution. The natural potential between the working electrode (hydrogen storage titanium-iron alloy electrode) and the reference electrode (silver / silver chloride reference electrode) is measured in a two-electrode system. Substituting this natural potential into the Nernst equation allows for the calculation of the solution pH. The specific testing procedure is as follows:

[0035] A modified titanium-iron alloy electrode was used as the working electrode, and an Ag|AgCl electrode was used as the reference electrode to form a two-electrode system. The pH sensor's response slope, response time, detection range, detection limit, selectivity, stability, and repeatability were comprehensively evaluated. The evaluation methods are as follows:

[0036] ①Selectivity: When an electrode responds to a primary ion, it may be interfered with by other ions, including ions with the same and opposite charges. This invention proposes to use a mixed solution method for testing and to evaluate selectivity using a selectivity coefficient (1).

[0037]

[0038] In the formula, E B E represents the hydrogen ion electrode potential. B ' represents the hydrogen ion conditional electrode potential, R represents the gas constant, T represents the temperature, n represents the number of electrons transferred, F represents the Faraday constant, and a B Indicates the activity of interfering ions, a A K represents the activity of the target ion being detected. pot B,A This represents the selectivity coefficient.

[0039] ② Response Slope: The electrode has a wide measurement range, typically several orders of magnitude. Based on the formula for membrane potential, plotting the potential against the logarithm of ion activity yields a response line. The slope of this line is the response slope, indicating the degree to which the actual relationship curve between the sensor's output and input deviates from the fitted straight line. It is defined as the maximum deviation Δmax between the actual characteristic curve and the fitted straight line over the full range, and the full-scale output value Y. FS The ratio. Linearity, also known as nonlinearity error, is denoted by γ.

[0040]

[0041] In the formula, Δmax is the maximum deviation value, Y FS This represents the full-scale potential.

[0042] ③ Response time: The response time of an electrode can be expressed in different ways. Generally, the immersion method is used for measurement. The response time refers to the time from the moment the electrode comes into contact with the solution until it reaches a stable potential value (±1mV). It is generally evaluated by the time required for the potential change to reach 90% (t90) or the time required for the potential change to reach 95% (t95).

[0043] ④ Sensitivity: Theoretical calculations show that for monovalent ions, a measurement error of 1 mV will result in a relative concentration error of ±4%. As the ion valence increases, the error also increases exponentially. Sensitivity is an important indicator of the static characteristics of a sensor. It is defined as the ratio of the increment of the output quantity Δy to the corresponding increment of the input quantity Δx that causes that increment. Sensitivity is represented by S (3), i.e.

[0044]

[0045] In the formula, Δy is the potential difference and Δx is the hydrogen ion concentration difference.

[0046] ⑤ Repeatability: Repeatability refers to the degree of inconsistency in the characteristic curves obtained when the input quantity of a sensor changes continuously multiple times in the same direction across its full range. Repeatability error is a type of random error, commonly calculated using the standard deviation σ, or it can be calculated using the maximum repeatability difference ΔRmax in both forward and reverse strokes.

[0047]

[0048] Y FS ΔRmax represents the full-scale potential, and ΔRmax represents the maximum repeatability difference during forward and reverse strokes.

[0049] or

[0050]

[0051] In the formula, Y FSΔRmax represents the full-scale potential, ΔRmax represents the maximum repeatability difference in forward and reverse strokes, and σ is the standard deviation.

[0052] ⑥ Electrical signal drift: Sensor drift refers to the change in sensor output over time when the input remains constant. There are two main causes of drift: one is the sensor's own structural parameters; the other is the surrounding environment (such as temperature and humidity). The most common type of drift is temperature drift, which is the change in output caused by changes in ambient temperature. Temperature drift mainly manifests as zero-point drift and temperature sensitivity drift. Temperature drift is usually expressed as the ratio (ξ) of the change in output value when the sensor's operating environment temperature deviates from the standard ambient temperature (generally 20℃) to the change in temperature.

[0053]

[0054] In the formula, ξ represents the ratio of the change in output value to the change in temperature; y t This represents the potential value at the operating ambient temperature; y 20 This represents the potential value at ambient temperature (20℃), where Δt represents the temperature difference between the working ambient temperature and the standard ambient temperature (20℃). For example... Figure 1-2 As shown, the slope of the response of this invention to hydrogen ions in aqueous solution is -56mV dec. -1 The response time is 5s, and the stability during continuous 24-hour testing is high, with potential fluctuations within ±3mV. It was compared with a traditional glass pH meter in actual sample testing, and the results are shown in Table 1.

[0055] Table 1 Comparison of the pH sensor of this invention with that of a traditional glass pH meter

[0056]

[0057] The results show that the pH sensor prepared by the method of the present invention has highly consistent test results with those of the glass pH meter, and the sensor prepared by the method of the present invention can be used in actual detection work.

Claims

1. A hydrogen storage alloy-based all-solid-state pH sensor comprising: The counter electrode, the working electrode, the reference electrode and the flexible substrate are characterized in that the counter electrode is a platinum electrode, the working electrode is a hydrogen storage alloy electrode, the reference electrode is a silver / silver chloride electrode, and the flexible substrate is PET; the hydrogen storage alloy electrode is prepared by the following method: Step 1: Preparation of base electrode The base working electrode is a copper electrode, the reference electrode is a silver / silver chloride electrode, and the counter electrode is a platinum electrode; Step 2: Electroplating hydrogen storage alloy (1) Plating solution formula: main salt: FeSO4 0.1-0.2 mol / L, Ti(SO4)2 0.4-0.5 mol / L; buffer: NaH2PO40.1 mol / L, complexing agent: sodium citrate 0.05-0.1 mol / L; (2) Electroplating method: a three-electrode system is used in an H-type electrolytic cell, the plating solution is heated by a heating rod and controlled within a temperature range of 56±2℃; the thoroughly washed electrode is placed close to the tank wall, and the alloy is treated by chronopotentiometry in a three-electrode system of an electrochemical workstation; during electroplating, the current is kept constant, the pH value is stabilized at 4.6±0.2, and the concentration difference is eliminated by stirring; Step 3: Electrochemical hydrogen storage treatment The titanium-iron alloy electrode obtained after electroplating in step 2 is placed in 0.1-0.2 mol / L HCl, and the alloy electrode is treated by constant potential electrolysis using an electrochemical workstation.

2. A hydrogen storage alloy-based all-solid-state pH sensor according to claim 1, wherein In (2) of step 2, the electrode is removed after plating for 10-20 min, and then washed, dried and stored.

3. A hydrogen storage alloy-based all-solid-state pH sensor according to claim 1, wherein In step 3, the hydrogen storage potential is -1.7 V, the hydrogen storage time is 1-2 h, the hydrogen storage is terminated when the electrolytic capacity reaches 3 C, and then the electrode is washed, dried and stored.

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