No abstract

Visualization of latent blood fingerprints (LBFPs) at violent crime scenes is very important for identifying criminals, while current reagents/methods for developing LBFPs still have more or less limitations. Here, we advance a new strategy for LBFP development based on the dye-soaked cotton pads as "ensemble" materials. The cotton pad was first soaked in the dye solution and then placed on the substrate with deposited blood fingerprint for development. After peeling off the pad, uniform and legible fingerprint patterns were obtained in all cases where different dyes and substrates were employed. The dye-soaked pads displayed superior developing effect compared with those using conventional methods (immersing, smearing and spraying). The pad-based method exhibited many other merits, such as little damage to the fingerprint patterns and less dye stains left on fingerprint and substrate, as well as the reusability. This pad-based method was also applicable for developing sebaceous latent fingerprints by replacing the small molecular dyes with fluorescent conjugated polymer nanoparticles. In all, the dye-soaked cotton pad developing protocol is feasible, cost-effective and universally applicable for developing different types of latent fingerprints on various substrates.

Both latent sebaceous and blood fingerprints may provide valuable information for forensic investigation. To detect both types of fingerprints with no need to predistinguish them, a new adaptive developing strategy was proposed. A cationic conjugated polymer with poly­[p-(phenylene ethylene)-alt-(thienylene ethynylene)] backbone (PPETE-NMe3 +) was synthesized, which was dissolved in N,N-dimethylformamide (DMF) to form the developing solution. Fingerprints were developed by a simple dropping and incubating process without any pre-/post-treatments. Fluorescent photographs of the developed fingerprints on various substrates demonstrated that this developing strategy was effective for both types of fingerprints on nonporous substrates. Gray value analysis further confirmed the enhancement of the legibility of the fingerprint images. The preliminary mechanism exploration suggested that certain weak interactions, such as hydrophobic interaction and electrostatic interaction, may synergistically contribute to the interaction between the polymer and fingerprint components. The molecular design of the polymer combined with an appropriate solvent endowed the developing system the adaptiveness toward different types of fingerprints. This adaptive developing strategy made the fingerprint-developing process more efficient and may be further extended to more practical application scenes.

To detect neomycin, one of the common antibiotics in the environment and in food and which may pose harm to human health, a fluorescence sensing system based on an amphiphilic conjugated polyelectrolyte (CPE) was constructed. The CPE, PFPE-COONa, with poly[fluorenyl-alt-p-phenyleneethynylene] (PFPE) as the backbone and carrying four pendant hydrophilic carboxylate groups and two long hydrophobic alkoxyl chains in each repeat unit, was successfully synthesized. The photophysical study found that PFPE-COONa emission was solvent dependent, displaying a 58 nm red shift in wavelength and a nearly 150-fold enhancement in intensity for the emission maximum, by varying the ratios of the mixed solvent. Based on an understanding of the photophysics, the sensing condition was optimization to dissolve PFPE-COONa in the THF/H 2 O mixed solvent with a ratio of 1:1. The sensing system displayed excellent sensing performance toward neomycin at a neutral pH value. The blue-to-green fluorescence color change of the polymer solution, which can be clearly observed by the naked eye, upon around 0.7 μM of neomycin and the limit of detection (LOD) was calculated to be 0.78 nM. In addition, other possible interferents had no significant influence on the detection of neomycin. The mechanism study confirmed that the polymer aggregation, which induced strong electrostatic static interaction between the carboxylate groups of high density in the polymer and the dense positively charged primary amino groups in neomycin, was the main reason for such a visually fluorescent color change. This study provides a general strategy for the future design of sensing systems for organic pollutants.

Developing a new anti-counterfeiting strategy is of great significance to combating the global counterfeiting problem. Here we report the construction of a dynamic fluorescence response system for anti-counterfeiting by combining the photochromism induced by the ring-opening of spiropyran (SP) to merocyanine (MC) with the fluorescence resonance energy transfer (FRET) between the conjugated polymer and MC. After elucidating the design principle, a new conjugated polymer, PPETE-SP, consisting of a poly­[p-(phenylene ethynylene)-alt-(thienylene-ethynylene)] (PPETE) backbone with pendant SP, was synthesized and characterized. With poly­(methyl methacrylate) (PMMA) as the matrix, the PPETE-SP/PMMA fibrous membrane was prepared via electrospinning. Under the irradiation of UV light, the fluorescent color of the membrane dynamically changed from green to light green, then light pink, and finally pink, and this process was reversible under visible light. The fluorescence emission switch was examined for 10 cycles and proved to have good repeatability, indicating that the membrane can be directly used as an anti-counterfeiting label for multiple verifications. The FRET efficiency was found to be about 61% based on the FRET study with confocal laser scanning microscopy. The covalent bonding between PPETE backbone and SP, the confinement of PPETE-SP chains in the fibrous membrane, as well as employing PMMA as the matrix were demonstrated to be crucial in realizing the photochromism and the FRET. Different anti-counterfeiting modes were proposed, providing rich selections for operation of verification. Such facile-to-operate and hard-to-imitate dynamic fluorescent responsive materials are very promising for use in practical anti-counterfeiting applications.