Photo- and Al3+- responsive fluorescent polymeric nanoparticles for multilevel information encryption and anti-counterfeiting

Stimuli-responsive fluorescent polymeric nanoparticles have gained widespread interest in information encryption and anti-counterfeiting because of their features including easy preparation, high contrast, and good photostability. However, most of reported single-responsive nanoparticles exhibited the low security of information encryption and anti-counterfeiting, and limited their application. Therefore, we reported photo- and Al³⁺-responsive fluorescent polymeric nanoparticles (PAFPNs) through a simple one-pot miniemulsion polymerization. Based on fluorescence resonance energy transfer (FRET) mechanism, Al³⁺-responsive fluorescence groups ((E)-N′-(2-hydroxybenzylidene)-4-vinylbenzohydrazide (HBVBH)) and spiropyran-linked methacrylate (SPMA) were used to construct FRET systems. Initially, the nanoparticles showed no fluorescence. Upon the addition of Al³⁺ (100 μM), the PAFPNs revealed a strong blue fluorescence, due to the HBVBH can chelate with Al³⁺ to produce a blue fluorescence via a metal chelation-enhanced fluorescence mechanism (CHEF). Subsequently, UV can induce a photoisomerization process of SPMA from a non-fluorescent spiropyran (SP) state to a merocyanine (MC) form with red fluorescence, enabling a transition in fluorescence from blue to red. Finally, the PAFPNs can return to blue fluorescence via the irradiation of visible light. PAFPNs not only exhibited the three fluorescent states of no-blue-red, but also revealed high fluorescence contrast and outstanding photoreversibility. These results implied that PAFPNs have a great potential in developing multilevel information encryption, anti-counterfeiting, and multicolor security inks.