Fluorene-vanillin hybrid: A multifunctional fluorophore for latent fingerprint detection and sustainable offset security ink development

Abstract

Fluorophores play a pivotal role in forensic science and anticounterfeit applications due to their unique and distinct emission features, which enable visualization and authentication. In the pursuit for advanced fluorescent pigments, ortho vanillin-fluorene Schiff base (OAF) with bright orange fluorescence was synthesized using a cost-effective, simple, and one-step method. This fluorophore was highly effective in developing latent fingerprints (LFPs), enabling visualization of level 1–3 features on nonporous glass and semi-porous leather under 365 nm UV light. The interaction between glycine in the fingerprint residue on glass substrate and OAF was analyzed through computational studies and further validated using ATR-FTIR analysis. The FESEM images of the latent prints revealed detailed surface morphology and particle distribution on the ridges of the fingermarks. Further, fluorescent images of LFPs were successfully developed on sticky paint, moist glass, multicolored compact disc, and uneven leaf surface, demonstrating the material's versatility on challenging substrates. Moreover, the powder dusting of composite powder of OAF and JUP-AS120, a commercially available pigment enabled enhanced LFP visualization on glass under both 365 and 980 nm light sources for better contrast and minimal background fluorescence, while minimising degradation of fingerprint residues over time. In addition, an OAF spray was formulated using nitrocellulose resin, ethyl acetate and carbitol for broader forensic application, which facilitated LFP visualization down to fine minutiae details on porous, semi-porous and non-porous substrates. Furthermore, to address the growing rates of counterfeited products/banknotes/security documents, an ecofriendly composite security ink was formulated using OAF and JUP-AS120 to obtain offset prints with excellent resistance to photobleaching and scuffing on paper substrate. The ink film exhibited multilevel authentication features: orange fluorescence under a 365 nm light source, and green and red emissions without and with a 610 nm band pass filter, respectively under 980 nm laser source, enhancing security and making forgery more challenging. Additionally, biocompatible OAF can be incorporated in ink pads to offer a dual layer of validation for fingerprints on security documents: optical authentication by leveraging the ink's fluorescence properties and biometric recognition through fingerprint patterns. © 2025 The Author(s)