Judging from the current development status of glucose sensors at home and abroad, there are still many difficulties for glucose sensors to enter practical clinical applications. In addition to low sensitivity, poor stability, and short life, low linear range is also a difficult problem to solve. problem. Generally, the linear range of glucose sensors is only a few mM, while the blood glucose concentration of diabetic patients can reach 33.3 mM. At present, the main way to solve this problem is to cover the surface of the enzyme membrane with a selectively permeable polymer film as a diffusion-limiting membrane. The purpose is to expand the response linear range of the glucose sensor and reduce the interference of telephone substances. Why can the introduction of a diffusion-limiting membrane expand the response linear range of a glucose sensor? According to the response kinetics of glucose oxidase to glucose, the response of glucose oxidase to glucose concentration is a linear response only in the lower glucose concentration range. One function of the diffusion limiting membrane is to allow glucose molecules in the blood to diffuse through this selectively permeable polymer film to the surface of the glucose oxidase membrane, thereby reducing the diffusion speed of glucose molecules and making the diffusion process of glucose molecules as a whole During the control process of the reaction, the glucose concentration in the enzyme membrane is always maintained within a lower concentration range to achieve the purpose of expanding the linear range. Another function of the diffusion limiting membrane is to prevent or reduce electroactive substances such as ascorbic acid in the blood from reaching the surface of the glucose oxidase membrane and interfering with the measurement of the glucose sensor. Although glucose oxidase is specific for glucose, this specificity is limited by the electrode. Reduced by poor selection, at a voltage of 650mV, not only hydrogen peroxide has a response current, but also ascorbic acid in the blood has a response current [1]. These electroactive substances interfere with the function of the sensor through oxidation of the electrode or reaction with hydrogen peroxide [ 2]. Diffusion-limiting membranes can reduce the dependence of the electrode on enzyme kinetics and can also reduce the generation of hydrogen peroxide [3]. Diffusion-limiting membranes can reduce the dependence of glucose sensors on oxygen pressure [4]. The diffusion-limiting membrane material should be chosen to limit glucose diffusion. In addition, the effect of the diffusion-limiting membrane material on glucose oxidase and the erosion of the electrode under physiological conditions is required to be as small as possible. The selection of diffusion-limiting membrane materials must also meet conditions such as nontoxicity, good biocompatibility, high tolerance, and high chemical stability (under physiological conditions). The diffusion limiting membrane materials currently used at home and abroad include cellulose acetate (CA) [5], polyurethane (PU) [6], polyvinyl chloride (PVC) [7], polycarbonate (PC) [8], Nafion [9] and polytetrafluoroethylene (PTFE) [10] etc. Because PU has good biocompatibility and good film-forming properties, it is suitable as a diffusion-limiting film. To this end, research on PU diffusion limiting films was conducted. 1． Experimental part 1.1. Experimental instruments and drugs Glucose oxidase (GOD, EC 1.1.3.4) are produced by Sigma Chemical Company, glutaraldehyde is imported and packaged by Chengdu Meile Medical and Health Products Co., Ltd., polyurethane is synthesized by the Polymer Materials Department of our school, and the microcurrent meter is Designed by the Institute of Photochemistry, Chinese Academy of Sciences. All other drugs are of analytical grade. 1.2. Preparation of glucose sensor Chitosan was used as the immobilization matrix for immobilization of glucose oxidase, and glucose oxidase was immobilized by cross-linking method. Prepare chitosan (3,, W/V, deacetylation degree 83, dissolved in glacial acetic acid) solution. Pipette 25 μl GOD solution (5 mg/ml) into a 5 ml beaker, stir in one direction with a platinum wire, slowly add 2 ml of chitosan solution and continue stirring with a platinum wire, slowly add 100 μl glutaraldehyde (1, V/ V), start timing when glutaraldehyde is added, soak the platinum wire for 8-10 minutes, take it out and dry it and then immerse it in the polyurethane solution (polyurethane is dissolved in a solvent of tetrahydrofuran: N, N-dimethylformamide = 9:1) Take it out quickly around 2a and let it stand in the air for 5 minutes, then put it into phosphate buffer solution (pH=6.8) and store it in a refrigerator at 4°C for later use. The thickness of the outer diffusion-limiting membrane was controlled by different dipping times and different polyurethane solution concentrations. 1.3. Glucose sensor response test The test conditions are pH=6.8, temperature t=37°C, and working voltage=0.7V.
The reference electrode is an Ag/AgCl electrode, which is stabilized in the glucose sensor phosphate buffer for 2 h before testing to obtain a stable background current. The glucose solution is prepared and left overnight to develop optical activity. 2． Results and Discussion The response curves of the glucose sensor to glucose under different polyurethane concentrations and different immersion times are shown in the figure. Figure 1 shows the response curve of a glucose sensor immersed for different times with a polyurethane concentration of 5 (W/V). It can be seen from the figure that as the number of immersions increases, the linear range of the response gradually increases, and the linear range of the response reaches 16.7mM after 4 times of immersion, while the response current decreases as the number of immersions increases. Figure 2 shows the response curve of a glucose sensor immersed for different times with a polyurethane concentration of 6.5 (W/V). It can be seen from the figure that as the number of immersions increases, the response linear range gradually increases, and the response linear range reaches 22.2mM after 4 times of immersion, while the response current decreases as the number of immersions increases. Figure 3 shows the response curve of a glucose sensor immersed for different times with a polyurethane concentration of 8 (W/V). It can be seen from the figure that as the number of immersions increases, the linear range of the response gradually increases. The linear range of the response reaches 27.8mM after immersing 4 times. The response current decreases as the number of immersions increases. Figure 4 is the response curve of a glucose sensor immersed for different times with a polyurethane concentration of 10 (W/V). It can be seen from the figure that as the number of immersions increases, the linear range of the response gradually increases. The linear range of the response after immersing 4 times reaches 33.3mM. The response current decreases as the number of immersions increases. For the glucose sensor with the same number of immersions, as the polyurethane concentration increases, the response linear range gradually increases.