## 3 Results and Discussion
The influence of the above-mentioned molecules on the Cu-ETTC MOF sheet was then analyzed using a more intuitive histogram and peak-axis location distribution diagram. As shown in Figure 5a, molecules 1-3 mainly contain carboxyl and hydroxyl groups, which cannot effectively awaken the fluorescence of the MOF sheet. Molecules 4-10 are various amino acids, which can restore the fluorescence of the MOF sheet, but the different groups attached have a great influence on the fluorescence intensity. Molecules 11-13 are molecules with sulfhydryl groups, which can also effectively awaken the fluorescence of the MOF sheet. The coexistence of amino (imine) and sulfhydryl groups in molecules 14-15 can arouse the fluorescence of the MOF sheet strongly. Therefore, it may be a more effective method to reduce the intermolecular LMCT process in the MOF structure by using molecules with coexisting amino and sulfhydryl groups as electron donors. These results have certain guiding significance for adjusting the conversion route of S1 excitation energy and enhancing the fluorescence intensity of MOF materials. Interestingly, the peak-axis location of the Cu-ETTC MOF sheet also conforms to certain statistical laws. As shown in Figure 5b, the peak positions of the above-mentioned molecules are mainly distributed around 470, 490, and 510 nm, with a difference of about 20 nm, and the peak distribution positions are basically independent of the peak intensity. In addition to the amino and sulfhydryl groups that can restore the fluorescence of the MOF sheet, some molecules with multiple carboxyl groups and multiple hydroxyl groups can also effectively reduce the intermolecular LMCT process in the Cu-ETTC structure and turn on the MOF sheet fluorescence switch. As shown in Figure 5c,d, citric acid and L(+)-ascorbic acid can arouse the fluorescence of the Cu-ETTC MOF sheet, although the specific electron-donating mechanism is still unclear. It is worth mentioning that the peak axes of citric acid and L(+)-ascorbic acid are located at 488 and 510 nm, respectively, which are still within the above distribution range. Figure 4: (**a**–**c**) Fluorescence intensity curves obtained by interaction of different molecules with the Cu-ETTC MOF sheet. The concentrations of the Cu-ETTC MOF sheet and other molecules were both 50 \(\upmu\)mol/L. (**d**) Serial number and corresponding molecular structure. Compared with other molecules, glutathione has a stronger fluorescence-awakening effect on the Cu-ETTC MOF sheet. Therefore, the prepared a Cu-ETTC sheet with a large specific surface area may be viable for the sensitive detection of glutathione, because such MOF sheets have more active sites for copper. Glutathione mainly uses its electron-donating ability to arouse the fluorescence of the MOF sheet, so whether the amount of glutathione is sufficient becomes a major factor affecting the fluorescence intensity of the Cu-ETTC sheet. As shown in Figure 6b, under the condition of sufficient glutathione, the fluorescence intensity of the MOF enhanced with it showed a linear relationship at the concentration of about 0\(\sim\)25 umol/L. When continuing to increase the content of the MOF sheet, the increase in fluorescence decreased, thus showing a parabolic profile (Figure 6c), which may be due to the difficulty for glutathione to provide enough electrons to reduce the LMCT process in the Cu-ETTC MOF structure. Therefore, based on this mechanism of action between Cu-ETTC and glutathione, the MOF sheet can only have a linear relationship with glutathione within a certain matching range. In addition, the instrument voltage used in the test has a considerable influence on its sensitivity. As shown in Figure 6a, the fluorescence intensity of the Cu-ETTC MOF sheet showed a huge transition with the voltage of the instrument, and even increased by about 10 times. Therefore, the use of larger voltages is an effective way to increase the detection sensitivity.