Fortunately, the multifunctional gel has been successfully prepared by compositing the PVA with MOF and DBCZ. We have screened various concentrations of MOF and DBCZ, 10 mg MOF and 2 mg DBCZ per 1 g PVA was found to be appropriate to ensure strong fluorescence intensity as well as obvious visually blue fluorescence under neutral conditions and red fluorescence under alkaline conditions. The composite film is constructed using the drop film method, and the required composite gel film can be obtained by evenly coating and drying. The presence of DBCZ and porphyrins can be observed through UV-Vis absorption spectroscopy (Figure S4). It is worth noting that PVA is specially treated. Commercially available PVA was highly susceptible to water erosion without exception although we have attempted many times, which undoubtedly hinders the construction of composite materials. Fortunately, choosing the method of glutaraldehyde crosslinking to improve the water erosion resistance of PVA could solve this problem. At the same time, considering the crosslinking efficiency and the influence of PVA embrittlement resulting from the concentration is too high, the concentration of glutaraldehyde in the crosslinking solution is 1.25%. It is confirmed that the film was completely immersed in the cross-linking solution for 5 min, there was no obvious water erosion on the gel film. There is no obvious change of surface morphology before and after treatment through microscope photos (Figure S5). In order to further characterize the success of crosslinking, the Fourier transform infrared spectroscopy was carried out before and after crosslinking. The C-O-C stretching vibration at 1142 cm-1 is clearly visible, demonstrating the formation of acetals (Figure S6). Based on above, stimulus responsive films with good water resistance and specific response ability have been successfully synthesized.
Fluorescence behavior of Y-TCPP/DBCZ PVA gel film at different pH values
The luminous properties of composite material are the key to the design of multifunctional film. The emission of Y-TCPP/DBCZ PVA gel film with pH changes was recorded under 365 nm excitation. The fluorescence of DBCZ remains basically unchanged at pH greater than 6 (Figure S7), while the fluorescence intensity of MOF increases with the increase of OH- concentration as previously reported (Figure S8). The mechanism of fluorescence enhancement of MOF is attributed to the photo induced electron transfer (PET)(Figure 2a). It is worth noting that the MOF emission is obviously change until the pH up to 13 (Figure S9). The fluorescence photos are corresponding well to the results of spectroscopy. The blue fluorescence was observed at pH=6-12, and tune to the red when pH become 13, which may be due to that the weak fluorescence intensity of MOF at pH of 6-12 and the blue fluorescence emission of DBCZ is dominant. When pH=13, prohibited PET results in the red fluorescence emission of the film (Figure 2b) thus red is more visible to the naked eye. All above experimental results indicate that there is a mutation in the fluorescence color of the composite film from pH=12 to pH=13.