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.