由图5（a）可发现以超纯水为支持电解质时，所得修饰电极的循环伏安图响应

From Fig. 5(a), it can be found that when ultrapure water is used as the supporting electrolyte, the response current of the resulting modified electrode is 0 mA. This is because the ultrapure water (pH 6.88) removes the water during the treatment process. Almost all conductive media have deionized water, so there is no conductive effect, and electron transfer cannot be performed, resulting in a response current of 0 mA for the cyclic voltammogram. It was also found that the cyclic voltammogram of the modified electrode supported by 0.5 mol•L-1 Na2SO4 (pH 7.02) solution and 50 mmol•L-1PBS solution (pH 6.8, 7.2 respectively) does not have the characteristic redox peak of polyoxometalates, which may be The reason is that polyoxometalates are easily decomposed in a neutral solution, resulting in polyoxometalates ion concentration insufficient to respond to a sufficiently strong response current, so that there is no characteristic redox peak of polyoxometalates. While in 0.5mol•L-1 H2SO4 solution (pH 0.46), 0.1 mol•L-1 H2SO4 solution (pH 1.13), 0.5 mol•L-1 H2SO4 and 0.5 mol•L-1Na2SO4 mixed solution (pH is 0.73), the mixed electrode of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1Na2SO4 (pH 1.54) supported the cyclic voltammogram of the modified electrode (figure 5(b)), the characteristic redox of polyoxometalates appeared peak. And from Fig. 5(b), it can be seen that the modified electrode has the strongest response current under the support of 0.5 mol•L-1 H2SO4 solution (pH 0.46). However, considering that the target detection substance of the reaction system is tyrosinase, and the conditions required for tyrosinase to maintain activity are relatively mild, and cannot be in a strong acid and alkali environment, so 0.1 mol•L-1 H2SO4 with a pH of 1.54 is selected The mixed solution with 0.5 mol•L-1Na2SO4 is used as a reference for the next experiment, and continue to study the modification of the electrode when using 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 mixed solution with different pH as the supporting electrolyte. Chemical behavior (Figure 6a). The pH of the mixed solution of 0.1 molL-1 H2SO4 and 0.5 molL-1 Na2SO4 in Figure 6a is 1.54, 2.62, 3.86, 5.29, 6. 51. The volume ratio of the corresponding 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution in the mixed solution are respectively 1:1 (5:5), 4:6, 3:7, 2:8, 1:9. It can be seen from Figure 6(a) that as the pH of the 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution changes from 1.54 to 6.51, that is, the acidity continues to weaken, it is found that the modified electrode is in the process of cyclic voltammetry scanning The response current in the middle decreases continuously, and the redox peak potential shifts negatively, indicating that protons participate in the electrochemical reaction process. Comparing the cyclic voltammograms of the modified electrode obtained when the mixed solutions of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 at pH 1.54 and 6.51 were used as the supporting electrolyte (Figure 6(b)), we can see that the modification The electrode in two different pH supporting electrolyte solutions, although the corresponding redox peak potential and response current have changed, but still have the characteristic redox peak of polyoxometalates. And when the pH of the mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 changes from 5.29 to 6.51, that is, 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution in the mixed solution The volume ratio in the solution changes from 2:8 to 1:9, and the cyclic voltammograms under the corresponding conditions basically overlap, but when the ratio of 0.1 mol•L-1 H2SO4 solution to 0.5 mol•L-1 Na2SO4 solution is 0 : At 10 o'clock, that is, when 0.5 mol•L-1 Na2SO4 (pH 7.02) solution is used as the supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments. The volume ratio of 5 mol•L-1 Na2SO4 solution in the mixed solution is 1:1 (5:5), 4:6, 3:7, 2:8, and 1:9, respectively. It can be seen from Figure 6(a) that as the pH of the 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution changes from 1.54 to 6.51, that is, the acidity continues to weaken, it is found that the modified electrode is in the process of cyclic voltammetry scanning The response current in the middle decreases continuously, and the redox peak potential shifts negatively, indicating that protons participate in the electrochemical reaction process. Comparing the cyclic voltammograms of the modified electrode obtained when the mixed solutions of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 at pH 1.54 and 6.51 were used as the supporting electrolyte (Figure 6(b)), we can see that the modification The electrode in two different pH supporting electrolyte solutions, although the corresponding redox peak potential and response current have changed, but still have the characteristic redox peak of polyoxometalates. And when the pH of the mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 changes from 5.29 to 6.51, that is, 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution in the mixed solution The volume ratio in the solution changes from 2:8 to 1:9, and the cyclic voltammograms under the corresponding conditions basically overlap, but when the ratio of 0.1 mol•L-1 H2SO4 solution to 0.5 mol•L-1 Na2SO4 solution is 0 : At 10 o'clock, that is, when 0.5 mol•L-1 Na2SO4 (pH 7.02) solution is used as the supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments. The volume ratio of 5 mol•L-1 Na2SO4 solution in the mixed solution is 1:1 (5:5), 4:6, 3:7, 2:8, and 1:9, respectively. It can be seen from Figure 6(a) that as the pH of the 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution changes from 1.54 to 6.51, that is, the acidity continues to weaken, it is found that the modified electrode is in the process of cyclic voltammetry scanning The response current in the middle decreases continuously, and the redox peak potential shifts negatively, indicating that protons participate in the electrochemical reaction process. Comparing the cyclic voltammograms of the modified electrode obtained when the mixed solutions of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 at pH 1.54 and 6.51 were used as the supporting electrolyte (Figure 6(b)), we can see that the modification The electrode in two different pH supporting electrolyte solutions, although the corresponding redox peak potential and response current have changed, but still have the characteristic redox peak of polyoxometalates. And when the pH of the mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 changes from 5.29 to 6.51, that is, 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution in the mixed solution The volume ratio in the solution changes from 2:8 to 1:9, and the cyclic voltammograms under the corresponding conditions basically overlap, but when the ratio of 0.1 mol•L-1 H2SO4 solution to 0.5 mol•L-1 Na2SO4 solution is 0 : At 10 o'clock, that is, when 0.5 mol•L-1 Na2SO4 (pH 7.02) solution is used as the supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments. 51, that is, the acidity continues to weaken, and it is found that the modified electrode responds to a continuous weakening of the current during the cyclic voltammetry scan, and the redox peak potential shifts negatively, indicating that the electrochemical reaction process involves protons. Comparing the cyclic voltammograms of the modified electrode obtained when the mixed solutions of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 at pH 1.54 and 6.51 were used as the supporting electrolyte (Figure 6(b)), we can see that the modification The electrode in two different pH supporting electrolyte solutions, although the corresponding redox peak potential and response current have changed, but still have the characteristic redox peak of polyoxometalates. And when the pH of the mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 changes from 5.29 to 6.51, that is, 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution in the mixed solution The volume ratio in the solution changes from 2:8 to 1:9, and the cyclic voltammograms under the corresponding conditions basically overlap, but when the ratio of 0.1 mol•L-1 H2SO4 solution to 0.5 mol•L-1 Na2SO4 solution is 0 : At 10 o'clock, that is, when 0.5 mol•L-1 Na2SO4 (pH 7.02) solution is used as the supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments. 51, that is, the acidity continues to weaken, and it is found that the modified electrode responds to a continuous weakening of the current during the cyclic voltammetry scan, and the redox peak potential shifts negatively, indicating that the electrochemical reaction process involves protons. Comparing the cyclic voltammograms of the modified electrode obtained when the mixed solutions of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 at pH 1.54 and 6.51 were used as the supporting electrolyte (Figure 6(b)), we can see that the modification The electrode in two different pH supporting electrolyte solutions, although the corresponding redox peak potential and response current have changed, but still have the characteristic redox peak of polyoxometalates. And when the pH of the mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 changes from 5.29 to 6.51, that is, 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution in the mixed solution The volume ratio in the solution changes from 2:8 to 1:9, and the cyclic voltammograms under the corresponding conditions basically overlap, but when the ratio of 0.1 mol•L-1 H2SO4 solution to 0.5 mol•L-1 Na2SO4 solution is 0 : At 10 o'clock, that is, when 0.5 mol•L-1 Na2SO4 (pH 7.02) solution is used as the supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments. The cyclic voltammogram of the modified electrode obtained when the 5 mol•L-1 Na2SO4 mixed solution was used as the supporting electrolyte (Figure 6(b)) shows that the modified electrode is in two different pH supporting electrolyte solutions, although the corresponding redox peaks The potential and its response current have changed, but they still have the characteristic redox peaks of polyoxometalates. And when the pH of the mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 changes from 5.29 to 6.51, that is, 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution in the mixed solution The volume ratio in the solution changes from 2:8 to 1:9, and the cyclic voltammograms under the corresponding conditions basically overlap, but when the ratio of 0.1 mol•L-1 H2SO4 solution to 0.5 mol•L-1 Na2SO4 solution is 0 : At 10 o'clock, that is, when 0.5 mol•L-1 Na2SO4 (pH 7.02) solution is used as the supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments. The cyclic voltammogram of the modified electrode obtained when the 5 mol•L-1 Na2SO4 mixed solution was used as the supporting electrolyte (Figure 6(b)) shows that the modified electrode is in two different pH supporting electrolyte solutions, although the corresponding redox peaks The potential and its response current have changed, but they still have the characteristic redox peaks of polyoxometalates. And when the pH of the mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 changes from 5.29 to 6.51, that is, 0.1 mol•L-1 H2SO4 solution and 0.5 mol•L-1 Na2SO4 solution in the mixed solution The volume ratio in the solution changes from 2:8 to 1:9, and the cyclic voltammograms under the corresponding conditions basically overlap, but when the ratio of 0.1 mol•L-1 H2SO4 solution to 0.5 mol•L-1 Na2SO4 solution is 0 : At 10 o'clock, that is, when 0.5 mol•L-1 Na2SO4 (pH 7.02) solution is used as the supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments. 02) When the solution is a supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments. 02) When the solution is a supporting electrolyte, its cyclic voltammogram does not have the characteristic redox peak of polyoxometalates. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decompose and the redox peak decreases. Therefore, in order to ensure the detection sensitivity, a mixed solution of 0.1 mol•L-1 H2SO4 and 0.5 mol•L-1 Na2SO4 with a pH of 6.51 was selected as the supporting electrolyte for subsequent experiments.

由图5（a）可发现以超纯水为支持电解质时，所得修饰电极的循环伏安图响应电流为0 mA，这是由于超纯水（pH为6.88）在处理的过程中除去了水中几乎所有的导电介质，对水进行了去离子化，所以没有导电效果，不能进行电子的转移，导致循环伏安图的响应电流为0 mA。并且发现在0.5 mol•L-1 Na2SO4（pH为7.02）溶液和50mmol•L-1PBS溶液（pH分别为6.8、7.2）支持下的修饰电极的循环伏安图没有polyoxometalates的特征氧化还原峰，可能原因是polyoxometalates在中性溶液中易分解，导致polyoxometalates离子浓度不足以响应足够强的响应电流，从而没有polyoxometalates的特征氧化还原峰。而在0.5mol•L-1 H2SO4溶液（pH为0.46）、0.1 mol•L-1 H2SO4溶液（pH为1.13）、0.5 mol•L-1 H2SO4和0.5 mol•L-1Na2SO4的混合溶液（pH为0.73）、0.1 mol•L-1 H2SO4和0.5 mol•L-1Na2SO4的混合溶液（pH为1.54）支持下的修饰电极的循环伏安图（图5（b））均出现了polyoxometalates的特征氧化还原峰。并且从图5（b）中可以看出，修饰电极在0.5 mol•L-1 H2SO4溶液（pH为0.46）支持下的响应电流最强。但考虑到该反应体系的目标检测物质是酪氨酸酶，而酪氨酸酶保持活性要求的条件比较温和，不能处于强酸强碱的环境，所以选择pH为1.54的0.1 mol•L-1 H2SO4和0.5 mol•L-1Na2SO4的混合溶液作为下一步实验的参考，并继续研究以不同pH的0.1 mol•L-1 H2SO4和 0.5 mol•L-1 Na2SO4混合溶液为支持电解质时，修饰电极的电化学行为（图6a）。图6a中0.1 mol•L-1 H2SO4和 0.5 mol•L-1 Na2SO4混合溶液的pH分别为1.54、2.62、3.86、5.29、6.51。对应的0.1 mol•L-1 H2SO4溶液和 0.5 mol•L-1 Na2SO4溶液在混合溶液中的体积配比分别为1：1（5：5）、4：6、3：7、2：8、1：9。从图6（a）中可以看出随着0.1 mol•L-1 H2SO4溶液与0.5 mol•L-1 Na2SO4溶液的pH由1.54至6.51，即酸性不断减弱，发现修饰电极在循环伏安扫描过程中响应电流不断减弱，氧化还原峰电位发生负移，表明该电化学反应过程有质子的参与。对比分别以pH为1.54和6.51的0.1 mol•L-1 H2SO4和0.5 mol•L-1 Na2SO4混合溶液做为支持电解质时所得的修饰电极的循环伏安图（图6（b））可知，修饰电极在两种不同pH的支持电解质溶液中，虽然相应氧化还原峰电位及其响应电流发生了变化，但仍都具有polyoxometalates的特征氧化还原峰。而且当0.1 mol•L-1 H2SO4溶与0.5 mol•L-1 Na2SO4混合溶液的pH由5.29变化为6.51时，即0.1 mol•L-1 H2SO4溶液和 0.5 mol•L-1 Na2SO4溶液在混合溶液中的体积配比由2：8变化为1：9，其对应条件下的循环伏安图基本重合，但是当0.1 mol•L-1 H2SO4溶液与0.5 mol•L-1 Na2SO4溶液的比例为0：10时，即以0.5 mol•L-1 Na2SO4（pH为7.02）溶液为支持电解质时，其循环伏安图并没有polyoxometalates的特征氧化还原峰。这是由于polyoxometalates对酸度敏感，当酸性减弱，pH增大到一定值时，polyoxometalates分解，氧化还原峰减少。因此为了保证保证检测的灵敏度，选择pH为6.51的0.1 mol•L-1 H2SO4与0.5 mol•L-1 Na2SO4混合溶液为支持电解质进行后续的实验。

It can be seen from Fig. 5 (a) that when ultrapure water is used as the supporting electrolyte, the response current of the cyclic voltammetry of the modified electrode is 0 ma. This is because ultrapure water (pH 6.88) removes almost all the conductive media in the water and deionizes the water in the process of treatment, so there is no conductive effect and electronic transfer, resulting in the response current of the cyclic voltammetry is 0 mA。 And it was found that at 0.5 mol · L-1 The cyclic voltammograms of modified electrodes supported by Na2SO4 (pH 7.02) and 50mmol · l-1pbs (pH 6.8 and 7.2, respectively) do not have the characteristic redox peak of polyometalates. The possible reason is that polyometalates are easy to decompose in neutral solution, so the ionon concentration of polyometalates is not enough to respond to strong response current, so there is no characteristic oxidation of polyometalates Reduction peak. But in the mixed solution of 0.5mol · L-1 H2SO4 (pH 0.46), 0.1mol · L-1 H2SO4 (pH 1.13), 0.5mol · L-1 H2SO4 and 0.5mol · l-1na2so4 (pH 0.73), 0.1mol · L-1 H2SO4 and 0.5 The cyclic voltammograms (Fig. 5 (b)) of the modified electrode supported by the mixed solution of mol · l-1na2so4 (pH 1.54) all show the characteristic redox peaks of polyoxometalates. It can be seen from Fig. 5 (b) that the response current of the modified electrode is the strongest under the support of 0.5 mol · L-1 H2SO4 solution (pH 0.46). However, considering that the target substance of the reaction system is tyrosinase, and the conditions for keeping the activity of tyrosinase are relatively mild, and it can not be in the environment of strong acid and strong alkali, the mixed solution of 0.1 mol · L-1 H2SO4 and 0.5 mol · L-1 Na2SO4 with pH of 1.54 is selected as the reference for the next experiment, and the further research is to take 0.1 mol · L-1 H2SO4 and 0.5 mol · L-1 with different pH as the reference When Na2SO4 mixed solution is a supporting electrolyte, the electrochemical behavior of the modified electrode (Fig. 6a). In Fig. 6a, the pH of 0.1 mol · L-1 H2SO4 and 0.5 mol · L-1 Na2SO4 mixed solution is 1.54, 2.62, 3.86, 5.29 and 6.51, respectively. The volume ratios of the corresponding 0.1 mol · L-1 H2SO4 solution and 0.5 mol · L-1 Na2SO4 solution in the mixed solution are 1:1 (5:5), 4:6, 3:7, 2:8 And 1:9, respectively. From Fig. 6 (a), it can be seen that with the pH of 0.1 mol · L-1 H2SO4 solution and 0.5 mol · L-1 Na2SO4 solution from 1.54 to 6.51, namely, the acidity is constantly weakened. It is found that the response current of the modified electrode is continuously weakened during the cyclic voltammetry scanning process, and the redox peak potential is negatively shifted, which indicates that the electrochemical reaction process involves the participation of protons. Compare 0.1 mol · L-1 H2SO4 and 0.5 mol · L-1 with pH of 1.54 and 6.51, respectively The cyclic voltammogram of the modified electrode obtained when Na2SO4 mixed solution is used as the supporting electrolyte (Fig. 6 (b)) shows that the modified electrode in two different pH supporting electrolyte solutions, although the corresponding redox peak potential and its response current have changed, still has the characteristic redox peak of polyoxyometalates. Moreover, when the pH of 0.1 mol · L-1 H2SO4 solution and 0.5 mol · L-1 Na2SO4 solution changes from 5.29 to 6.51, that is, the volume ratio of 0.1 mol · L-1 H2SO4 solution and 0.5 mol · L-1 Na2SO4 solution in the mixed solution changes from 2:8 to 1:9, the cyclic voltammograms under the corresponding conditions basically recombine, but when 0.1 mol · L-1 H2SO4 solution and 0.5 mol · L-1 When the ratio of Na2SO4 solution is 0:10, i.e. 0.5 mol · L-1 Na2SO4 (pH 7.02) solution is used as supporting electrolyte, there is no characteristic redox peak of polyoxyometalates in the cyclic voltammogram. This is because polyoxometalates are sensitive to acidity. When the acidity weakens and the pH increases to a certain value, polyoxometalates decomposes and the redox peak decreases. Therefore, in order to ensure the sensitivity of detection, the mixed solution of 0.1 mol · L-1 H2SO4 and 0.5 mol · L-1 Na2SO4 with pH of 6.51 was selected as the supporting electrolyte for subsequent experiments.<br>