Lanthanides-based invisible multicolor luminescent hydrogels and films for anti-counterfeiting

Developing high-level luminescent anti-counterfeiting materials with simple preparation and portability is still challenging. Herein, the luminescent anti-counterfeiting hydrogels were acquired via the coordination of 1,2,4,5-benzene tetracarboxylic acid (H4BTC) to Tb(NO3)3 and/or Eu(NO3)3 in water. Multicolor luminescence from green to yellow, orange, and red can be easily obtained by controlling the molar ratio of Tb(NO3)3 to Eu(NO3)3. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the coordination between H4BTC and lanthanides. Emission spectra and optical images confirmed that luminescent hydrogels could be functionalized as luminescent inks for painting and writing with the help of a writing brush. Importantly, the information fabricated by hydrogels cannot be seen under daylight and can only be seen under UV light. Noteworthy, multicolor luminescence can be designed to encode information in multiple ways, resulting in increasing the complexity of information decryption. It can prevent the tampering and forging of information effectively. Moreover, the inks have good stability and adhesion. And the H4BTC-Ln hydrogels can be dried to powders for storage and recovered to hydrogels by adding water. Meanwhile, the multicolor luminescent films were easily prepared by dispersing the corresponding powders of H4BTC-Ln into polyacrylamide (PAAm) matrix. The acquired films have high transmittance as well as good luminescent properties and excellent mechanical properties. They were applied as luminescent anti-counterfeiting labels. This work provides a simple and convenient method for preparing high-level lanthanides-based luminescent anti-counterfeiting materials.