A multicolor-tunable fluorescent hydrogel for information encryption

Although fluorescent hydrogels have been gaining significant attention for their potential application in optical encryption owing to their smart responsiveness, their complex preparation processes have hindered their practical applications. This paper reports the fabrication of a multicolor-tunable fluorescent hydrogel, annotated as GA/ASA/Eu(PDA)3, from gelatin (GA) as the gel matrix in addition to 5-aminosalicylic acid (ASA) and a lanthanide complex (Eu(PDA)3, pyridine-2,6-dicarboxylic acid (PDA) was chosen as a ligand for sensitizing the luminescence of europium ions (Eu3+)) as the fluorescent centers. Upon UV light excitation, ASA emitted blue-green fluorescence, while Eu(PDA)3 showed red fluorescence. The difference between the maximum excitation wavelengths of the fluorescent groups was approximately 100 nm. Therefore, the fluorescence color of the GA/ASA/Eu(PDA)3 hydrogel could be changed to red, pink, white, light blue, or blue by simply altering either the concentrations of the fluorescent groups or the excitation light wavelengths. Furthermore, the prepared hydrogel was responsive to metal ions, and the energy transfer between PDA and Eu3+ ions could be interrupted by Cu2+ or Mn2+ ions, thus resulting in quenching of the red fluorescence. Finally, the designed hydrogel successfully enabled storage encryption. The results of this study can help in the design and manufacturing of novel smart optical materials for advanced information encryption applications.