As a unique information feature that can serve as personal “ID cards” and “information banks”, fingerprints are valuable evidences in criminal cases. Latent fingerprint (LFP) is the most common type of fingerprint on crime scenes and hardly invisible by the naked eye. Hence, the development of LFP is crucial for the detection of criminal cases. However, the existing developers suffer from some shortcomings such as inferior security, inconvenience, poor universality and low resolution, which result in being harmful to the detector's health and showing inferior detection efficiency in practical application.
Figure 1. (A) Schematic of the synthesis of TPA-1OH and the AIE property. (B) Fluorescence photographs (under 405 nm irradiation) of characters “LFPs” written with different substances common in LFPs and sprayed with a TPA-1OH aqueous solution.
Professor Zhu's group designed and synthesized an amphiphilic AIE molecule TPA-1OH (Figure 1A). Based on the hydrophilic-hydrophobic characteristics of TPA-1OH, the safe, convenient and efficient fluorescence visualization of LFP was implemented by simply soaking method or spraying method with TPA-1OH aqueous solution (30 μM, non-toxic) (Figure 1A). The main merits of this developer are: 1) high practicability due to simple-to-prepare, easy-to-operate; 2) easy-to-store of the dye aqueous solution; 3) splendid safety due to visible light excitation and non-toxic; 4) high-contrast and high-resolution development; 5) excellent universality due to suitable for various substrates, even substrate with rough surface. It was found that TPA-1OH only interacted with lipid secretions by staining different components in LFP with TPA-1OH aqueous solution(Figure 1B). The in situ real-time fluorescence development mechanism of AIE is as follows: The pyridine cation moiety, the hydrophilic end in the TPA-1OH structure, improves the hydrophilicity, making the molecules soluble in water in a certain range of concentrations. In aqueous solution, TPA-1OH shows no fluorescence due to non-radiative transition. The triphenylamine moiety is the hydrophilic end of TPA-1OH, which could rapid combine with lipid secretions in LFP due to the hydrophobic-hydrophobic interaction. Therefore, TPA-1OH molecules are able to emit strong fluorescence due to restriction of intramolecular motion (RIM) induced by the viscosity effect of lipid secretions. Hence, LFP can be quickly and efficiently developed without background interference by this "off-on" fluorescence probe.
Figure 2. (A) Real-time fluorescence in situ development of LFPs on tinfoil in a TPA-1OH aqueous solution under 405 nm irradiation. (B) Time-resolved changes of fluorescence intensity in the white circle regions. (C) Variations of fluorescence intensity contrast between the fingerprint ridges and furrows across the green line at different times.
Based on this "off-on" fluorescence probe, the in situ real-time fluorescence visualization of LFP was implemented. It is shown that clear, complete and high-contrast fingerprint pattern can be obtained within 30 s by analyzing time-resolved changes of fluorescence intensity and the signal changes between the fingerprint ridges and furrows at different time (Figure 2). In addition, the LFPs on different substrates, even substrates with rough surface (paper, brick, wall, etc.), were developed with high quality, indicating the universality of TPA-1OH probe. Legible, complete, high-contrast fingerprint patterns were captured by a SLR camera with a cut-off filter JB510 under 405 nm illumination. Level 1 detail (whorl), level 2 detail (lake, short ridge, bifurcation and ridge termination) and level 3 detail (pore) of the fingerprint were liable to be fetched in the captured patterns (Figure 3).
Figure 3. Photographs (under 405 nm irradiation) of LFP on different substrates developed by TPA-1OH aqueous solution. Level 1 (whorl), Level 2 (lake, short ridge, bifurcation and ridge termination) and Level 3 (pore) details of local LFP on tinfoil.
Furthermore, the level 3 microscopic characteristics of fingerprint, such as the width of ridge, the morphology and distribution of pores, the distance between pores, were successfully obtained base on fluorescence microscope (Figure 4). These level 1-3 details effectively provide individual identification and identity information, and powerful evidence for the investigation of criminal cases. To obtain more detailed information, LFP were further studied by optical imaging, SEM imaging and super-resolution imaging. It is distinctly shown that the LFP ridge is composed of many lipid secretions with different sizes. Fluorescence images and super-resolution images prove that the AIE molecule TPA-1OH indeed combine with lipid secretions.
Figure 4. (A) Fluorescence microscopic images for partial region of the latent fingerprints and the analysis of Level 3 microscopic details (the width of ridge, the morphology and distribution of pores, the distance between pores). (B) Number and location distribution of sweat pores on the bifurcation of the real fingerprint (top) on the finger and its developed fingerprint (bottom) on the substrate.
The highlight of this paper is that the design concept of the safe, convenient and efficient fluorescence visualization of LFP based on the hydrophilic-hydrophobic feature of amphiphilic AIE molecule, which is of guiding significance for the design and synthesis of a new amphiphilic/ water-soluble AIE probes and their application.
On March 30th，Journal of the American Chemical Society published the latest research results of Prof. Zhu Mingqiang's team at Wuhan National Laboratory for Optoelectronics, “Real-Time Fluorescence In Situ Visualization of Latent Fingerprints Exceeding Level 3 Details Based on Aggregation-Induced Emission”. This work was completed by Ya-Long Wang, Chong Li (co-first author and co-corresponding author) Hong-Qing Qu, Chen Fan, Peng-Ju Zhao, Rui Tian and Ming-Qiang Zhu (co-corresponding author) from the Energy Photonics Functional Laboratory of Wuhan National Center for Optoelectronics.
This work was supported by the National Basic Research Program (973) of China, the National Science Foundation of China, the Fundamental Research Funds for Central Universities, and the Natural Science Foundation of Hubei Province.
Full text link: https://pubs.acs.org/doi/10.1021/jacs.0c00124。
