An AIE-active multi-stimuli responsive fluorophore with solvatochromic, acidochromic, and ultrasensitive Cu2+-sensing capabilities: anti-counterfeiting, latent fingerprint, bioimaging and food sample analysis

Abstract

The fluorescence of AIEE materials changes characteristically in response to stimuli such as heat, solvent environment, or applied mechanical force. In this work, a new Schiff base fluorophore (HP), derived from a 9-fluorenone hydrazone, was synthesized. The molecular structure of HP was validated using FT-IR, NMR, and ESI-MS techniques. Photophysical investigations showed that HP displays typical AIEE characteristics, emitting intense yellowish-green fluorescence in 90% water fractions. Interestingly, HP exhibited multi-stimuli-responsive fluorescence behavior. A significant solvatochromic response was observed, with the emission properties changing progressively with increasing solvent polarity from hexane to DMSO, which was attributed to ICT processes. Moreover, HP showed reversible acidochromic behavior resulting from protonation of the imine moiety, which induced a red-shift in emission along with a decrease in energy band gap; this feature was further exploited in the development of a fluorescent anti-counterfeiting system. In addition, HP demonstrated excellent sensitivity toward Cu²⁺ ions, achieving a low LOD of 4.7 nM. Binding studies indicated a 2 : 1 stoichiometric interaction between HP and Cu²⁺ ions, with a binding constant of 1.03 × 10⁴ M⁻¹. Moreover, HP was successfully employed for latent fingerprint visualization. The probe also proved effective for Cu²⁺ detection in real food samples and living cells. Overall, this study presents a new strategy for the development of AIEE-active, multi-stimuli-responsive fluorescent materials with promising applications in metal ion sensing, fingerprint analysis, and bioimaging investigations.