Cross-Linked Curcumin/Kaolin Composite: Comprehensive Forensic Application and a Mathematical Model-Validated Method for Quantifying Fingerprint Fluorescence Contrast and Tunable Image Enhancement

Abstract

Fluorescent materials have unlocked promising avenues for developing latent fingerprints (LFPs), owing to their distinctive photoluminescent properties. However, existing fluorescent dyes face challenges in complex synthesis, potential toxicity, and fluorescence quenching. Meanwhile, as photoluminescent materials are increasingly used in LFP visualization, a significant gap remains in scientific methodologies to quantify fingerprint fluorescence contrast. In this study, green-emissive micron-sized curcumin/kaolin composites were synthesized via a facile one-step physical cross-linking method. Based on physical characterization, the formation of hydrogen bond network between curcumin and kaolin was verified, and the varied photoluminescent properties of the composites were clarified from the perspective of energy band. Furthermore, the curcumin/kaolin powders exhibit exceptional performance in imprint development, including high selectivity, sensitivity, versatility, stability, and contrast, realizing the development of not only LFPs on various substrates, but also knuckle prints, palm prints, and footprints, with clear three-level details. More notably, the powders successfully developed LFPs aged 30 days and even up to 100 days. Afterward, a novel method designated as fingerprint fluorescence intensity ratio (FFIR) was developed to quantify the contrast between fingerprint signal and background noise and compare the effects of different developing agents, which even achieved tunable multi-color image enhancement. The Shapiro-Wilk Normality Test-Based Paired Sample Wilcoxon Signed-Rank Test model was conducted to validate the feasibility of FFIR and its accuracy in tracking image capture parameters. The newly synthesized composites for imprint development and the mathematical model-validated method for quantifying the developed contrast and image enhancement have profound practical significance in comprehensive forensic application.

Keywords: Composite material, Photoluminescence, imprint development, contrast, tunable image enhancement, mathematical model