Luminescent lanthanide coordination polymers for the detection of nitro explosives and efficient identification of latent fingerprints

Abstract

The uniquely rich photophysical properties of luminescent lanthanide probes and their complementary favorable ionic interactions with hard nitro groups in nitroaromatic compounds (NACs) make them highly desirable optical probes for the detection of dangerous nitroaromatic explosives and the detection and visualization of latent fingerprints (LFPs). We present two new coordination polymers (CPs) utilizing a bottom-up approach: [Eu₂(ctpy)₆·3H₂O]_n (Eu-CP) and [Tb₂(ctpy)₆·3H₂O]_n (Tb-CP) (Hctpy = 4′-carboxy-4,2′:6′,4′-terpyridine). The molecular structures of both CPs were systematically analyzed using a range of solid- and solution-based material characterization techniques. The Eu-CP and Tb-CP displayed bright red and green emissions, respectively, attributed to the ⁵D_J → ⁷F_J f–f transitions from Eu(III) and Tb(III) upon photo-excitation with UV light via ctpy as the antenna. The Eu-CP probe can be used to detect nitro explosives in the solution and vapor phases by employing turn-off luminescence in the presence of 2,4,6-trinitrotoluene (TNT). Through several spectroscopic studies and observations, a plausible mechanism of interaction was established, wherein the TNT molecules quench the potential PeT-mediated energy transfer (EnT) pathway from ctpy to Eu(III). The results demonstrate that the Eu-CP exhibits high selectivity and sensitivity, with an LOD of 18 ppm. Eu-CP-impregnated solid-state films can be washed to remove TNT and reused for up to 8 times, demonstrating the practical applicability of the recyclable solid-state film-based sensors. Additionally, the intense red emissions allowed the use of the Eu-CP as phosphor powder for the sensitive detection, development, and effective visualization of latent fingerprints (LFPs) demonstrating its practical applicability in forensic science. The Eu-CP phosphor powder offers permanent and direct colorful visualization of LFPs with minimal background interference and allows improved detection and capture of fingerprints on various surfaces compared to the existing cumbersome approaches. Notably, the Eu-CP exhibits exceptional sensitivity and can detect aged fingerprints with up to level-3 high-resolution traits. The Eu-CP phosphor powder was tested using dusting for the development of aged and charged fingerprints on various surfaces for practical use in forensic science. The Eu-CP could be suitable for developing reliable fingerprint databases, quickly comparing individuals with hallmark differentiation and deriving deep insights utilizing big data analytics-based artificial intelligence (AI) for bio-authentication and forensic applications.