Dual humidity-pH responsive cellulose-based phosphorescent paper for multimodal anti-counterfeiting

Herein, a facile strategy for fabricating cellulose-based paper with dual humidity-pH responsive room-temperature phosphorescence (RTP) through molecular excited-state modulation was reported. The phosphorescent system was constructed by immobilizing ethylenediamine-mediated condensation product of 4-bromo-1,8-naphthalic anhydride (forming 1,8-naphthalimide derivatives) onto cellulose matrix via hydrogen-bond rigidification. Post-papermaking and drying processes, the developed material exhibits remarkable excitation-dependent dual emission: orange-red phosphorescence ( τ  = 6.3 ms) under ambient conditions and blue fluorescence ( τ  = 3.3 ns) under saturated humidity. Notably, the humidity-responsive luminescence switching demonstrates excellent reversibility after drying cycles. Furthermore, the incorporated naphthalimide moiety preserves its inherent pH sensitivity, enabling simultaneous pH detection through emission wavelength shifts. Systematic characterization reveals that the humidity response originates from water-molecule regulated intersystem crossing efficiency, while the pH response stems from protonation-induced electronic structure modifications. This work establishes a novel paradigm for designing multi-stimuli responsive smart materials through rational manipulation of molecular excited states, demonstrating potential applications in paper-based anti-counterfeiting technologies.