Stimuli-Responsive Phenanthrene–Naphthol Fluorescent Marker for Security and Forensic Authentication

Abstract

Smart functional materials with tunable optical features are highly attractive for anticounterfeiting and forensic applications. In this study, a phenanthrene–naphthol hybrid (PNH) was successfully synthesized and structurally characterized. PNH exhibited good solid-state fluorescence, photostability, intramolecular charge transfer characteristics, solvent-responsive luminescence, and aggregation-induced emission, along with reversible acido- and mechanofluorochromism. Quantitative pressure-dependent studies revealed a progressive and reproducible fluorescence response of PNH to mechanical stimuli. An eco-friendly water-based security ink with PNH as functional pigment was formulated, and the screen-printed patterns exhibited excellent UV stability and abrasion resistance. The prints enabled multimodal authentication through UV-induced fluorescence, reversible mechanical and acid/base vapor switching, and solvent-induced rear-side fluorescence. Notably, the chromic responses retained >90% of their initial fluorescence after 50 switching cycles, demonstrating excellent reversibility and operational durability. Further, PNH enabled high-contrast visualization of latent fingerprints on nonporous, semiporous, complex, and same-background substrates, facilitating the identification of level 1–3 ridge characteristics. Grayscale intensity profiling and three-dimensional surface mapping further confirmed enhanced ridge–furrow contrast and distinct ridge–valley morphology. Molecular dynamics simulations and spectroscopic analysis revealed favorable supramolecular hydrogen bonding and other noncovalent interactions between PNH and representative fingerprint residues, supporting its effective adhesion to latent-print ridges. Its forensic utility was further demonstrated for aged prints and samples exposed to elevated temperature and high relative humidity, where characteristic ridge and groove features remained discernible despite reduced fluorescence intensity. In addition, PNH facilitated clear visualization of groove patterns and minutiae in latent lip prints, indicating significant promise for cheiloscopic analysis. Comparative assessment with recently reported multifunctional fluorescent materials highlights the unique integration of AIE, reversible mechano- and acidofluorochromism, photostability, security-printing capability, and latent finger- and lip-print visualization in a single PNH fluorophore. Overall, PNH represents a versatile platform for next-generation security printing, anticounterfeiting, and forensic latent-print visualization.