To explore the ability of repeated discoloration for DQBD, we first investigated the fluorescence characteristic of DQBD solution (Group 1,
Fig. 4a) with addition of difference concentrations of 10
-7 mol/L (1), 10
-4 mol/L (2), 10
-3 mol/L (3) and 10
-2 mol/L (4) of HCl. As anticipated from the above results, gradual fluorescence colors were observed for these solutions due to the different pH values. In this case, when we added sodium hydroxide into these solutions to adjust its neutral property, similar fluorescence color was observed under the neutral conditions (Group 2,
Fig. 4b). Further adding HCl into the Group 2 solutions, reversible florescence colors were observed again that was similar to that of Group 1, suggesting the its good reversibility for pH sensor applications. In addition, by absorbing DQBD onto thin-layer silica gel plates and then dropping HCl with different concentrations, similar florescence color changes with that in solution were also observed (
Fig. 4a, below), suggesting its potential use for pH paper. It should be stated that it is difficult for DQBD films to achieve quick response for the detection of HCl gas because of the inadequate protonation effect within a relatively short time.
Fig. 4d shows the reversable changes for fluorescence intensity under different pH values which demonstrates good stable and reversibility.
Fig. 4e shows the corresponding FL spectra of DQBD solutions in accordance with that shown in
Fig. 4a. Although different acids were substituted, the spectral morphology was similar to that of the buffer solution. As shown in Fig. S7 (Supporting information), FL spectra of four samples of
Fig. 4d (Cycle 1, Cycle 2, Cycle 7, Cycle 8) almost coincide at the same acid – base properties. The results show that the emission wavelength of the reused solution is basically the same under the same concentration of HCl. In summary, after eight times of acid - base changing, DQBD can still be used with high accuracy, which promises a potential application of DQBD on reusable and accurate pH sensors. In addition, the pH testing ability of DQBD for other acids such as acetic acid, trifluoroacetic, nitric acid and sulfuric acid has also been investigated in our experiment. As shown in Fig. S6 (Supporting information), obvious emission changes could be easily observed under a certain concentration, suggesting its wide applications in various fields.