Label-free physical and electrochemical imaging of latent fingerprints by water and SECM

Non-destructive and simultaneous development and enhancement of latent fingerprints on stainless steel based on the electrochromic effect of electrodeposited manganese oxides

Latent fingerprints, as one of the most frequently encountered traces in crime scene investigation and also one of the largest sources of forensic evidence, can play a critical role in determining the identity of a person who may be involved in a crime. Due to the invisible characteristic of latent fingerprints, exploring efficient techniques to visualize them (especially the ones resided on metallic surfaces) while retain the biological and chemical information (e.g., touch DNA) has become a multidisciplinary research focus. Herein we reported a new and highly sensitive electrochemical interfacial strategy of simultaneously developing and enhancing latent fingerprints on stainless steel based on synchronous electrodeposition and electrochromism of manganese oxides in a neutral aqueous electrolyte. By utilizing a specially designed device for electrochemical testing and image capture, a series of electrochemical measurements, physical characterization and image analysis have been applied to evaluate the feasibility, development accuracy and enhancement efficacy of the proposed electrochemical system. The qualitative and quantitative analysis on the in situ and ex situ fingerprint images indicates that the three levels of fingerprint features can be precisely developed and effectively enhanced. Forensic DNA typing has also been performed to reveal actual impact of the proposed electrochemical system on subsequent analysis of touch DNA in fingerprint residues. The ratio of detected loci after electrochemical treatment reaches up to 98.5 %, showing non-destructive nature of this fingerprint development and enhancement technique.

Particle Size-Tunable Polydopamine Nanoparticles for Optical and Electrochemical Imaging of Latent Fingerprints on Various Surfaces

Powder dusting method is the most widely used approach due to its low cost, simplicity, minimal instrument dependence, and extensive applicability for developing latent fingerprints (LFPs). Herein, a novel optical and electrochemical dual-mode method for high-resolution LFP enhancement has been explored based on size-tunable polydopamine (PDA) nanoparticles (NPs) and scanning electrochemical microscopy (SECM). Dark PDAs rich in functional groups and negative charges can combine with the residues of LFPs on various surfaces with high sensitivity and selectivity to realize high-resolution visual fingerprint physical patterns on various porous and nonporous substrates with light color. However, optical visualization is not feasible for LFPs on dark or multicolored surfaces. Fortunately, based on the differences in electrochemical reactivity between ridges and furrows caused by the conductivity and reducibility of PDA powders, SECM can serve as a powerful supplement to optical methods to effectively overcome background color interference and distinctly display fingerprint patterns. Intriguingly, it is noteworthy that the binding amount and particle size of PDA powder significantly affected the optical and electrochemical visualization of LFPs: more powder binding amounts provided darker ridges in optical, and more surface reaction sites (larger powder binding mass at the same particle size or smaller particle size at the same mass) provided higher currents of ridges in electrochemical imaging. It demonstrates that the PDA powder as a dual-mode developer for LFPs offers a promising method for individual identification in forensics.

Wet nitrocellulose membrane for the level 3 feature visualization of various latent fingerprints and gender determination

A facile and high-resolution enhancement of latent fingerprints (LFPs) has been developed by using a wet nitrocellulose (NC) membrane as a matrix under natural light. A clear fingerprint pattern was presented on the membrane after a fingertip touch owing to the difference in light transmittance between the ridge residues and the wet-NC-membrane background. Compared with conventional methods, this protocol can provide a higher resolution fingerprint image to extract level 3 details accurately. It is also compatible with commonly used fingerprint visualization techniques (magnetic ferric oxide powder and AgNO₃. The modified membrane could be more general to realize the high-resolution visualization of LFP transferred from various substrates, even independent of light projection. Due to the excellent feasibility and reproducibility of level 3 details extracted by the wet NC membrane, the frequency distribution of the distance between adjacent sweat pores (FDDasp) could be used to effectively distinguish the fragmentary fingerprints. Finally, the level 3 features of LFPs from females and males were conveniently extracted by the wet-NC-membrane method for gender identification. The statistical results indicated that females had a higher average sweat pore density (115/9 mm²) than males (84/9 mm²). Taken together, this approach provided a high-resolution, reproducible, and accurate imaging of LFPs, which shows great promise for forensic information analysis.

Physical visualization and squalene-based scanning electrochemical microscopy imaging of latent fingerprints on PVDF membrane

Fingerprints have long been the gold standard for personal identification in forensic science. However, realizing the high-resolution enhancement of eccrine LFPs is difficult using the traditional methods and the label-free detection of fingerprint residue information is also challenging. Herein, we propose two enhancement strategies for LFPs on PVDF membrane (LFPs/PVDF) using blue-black ink staining and scanning electrochemical microscopy (SECM). The blue-black ink staining method was used for the first time to develop three types (sebaceous, natural and eccrine) of LFPs/PVDF based on the difference in wettability between the fingerprint residues and PVDF membrane. The enhanced fingerprints clearly displayed levels 1-3 features with high contrast and low background interference. Furthermore, we achieved chemical imaging of the LFP/PVDF samples, where their possible visualization mechanisms were ascribed to the electrochemical reactivity of squalene and difference in wettability between the LFP and PVDF membrane, which was first proposed and investigated by SECM imaging and water contact angle (WCA) measurements, respectively. Significantly, SECM imaging not only provided fingerprint patterns without any labelling but also revealed the spatial distribution information of squalene in LFPs simultaneously. In addition, it was also demonstrated that LFPs deposited on various surfaces were first successfully transferred to the PVDF membrane, and then further developed with both methods, making them general for personal identity-related applications. Taken together, the blue-black ink staining method can easily and quickly obtain level 3 features of LFPs/PVDF and the SECM approach can non-invasively image the topography and chemical information of LFPs/PVDF, and thus they can be potentially selected according to various requirements in forensic scenarios.

A sweat-responsive covalent organic framework film for material-based liveness detection and sweat pore analysis

Covalent organic frameworks have shown considerable application potential and exceptional properties in the construction of stimulus-responsive materials. Here, we designed a sweat-responsive covalent organic framework film for material-based fingerprint liveness detection. When exposed to human sweat, the COF_TPDA-TFPy film can transform from yellow to red. The COF_TPDA-TFPy film, when touched by living fingers, can produce the naked-eye-identified fingerprint pattern through the sweat-induced color change, while artificial fake fingerprints cannot. This technique, which we named material-based liveness detection, can thus intuitively discern living fingers from fake fingerprints with a 100% accuracy rate. Additionally, the distribution of sweat pores on human skin can also be collected and analyzed by shortening the contact time. By merely washing them with ethanol, all the samples can be utilized again. This work inventively accomplished material-based liveness detection and naked-eye-identified sweat pore analysis and highlighted their potential for use in clinical research and personal identification.

New Horizons for Estimating the Time Since Deposition of Fingermarks: Combining Label-Free Physical Visualization and Electrochemical Characterization

The time since deposition (TSD) of latent fingermarks (LFMs) serves as "witnesses" for crime scene reconstructions. Nevertheless, existing TSD prediction approaches focused on either physical or chemical aging parameters leading to inaccurate estimation. A novel label-free protocol has been developed, where both physical ridge patterns and lipid oxide (LipOx) degradation kinetics are realized using optical microscopy and scanning electrochemical microscopy (SECM) and combined for TSD prediction. Specifically, the surface interrogation (SI)-SECM titration was utilized to monitor the LipOx degradation in LFM arrays aligned by hole array masks, through which we derived the LipOx degradation function. After establishing the relationship between several titration parameters and titrated area by experimental and numerical simulation methods, the titrated area could be reasonably estimated and subsequently used to calculate the surface coverage of LipOx. Results demonstrated that the tip transient revealed the LipOx coverage of deposited LFMs. Notably, LipOx coverage was found to increase during the first day and then decrease over time, whose degradation rate was susceptible to light. Thus, TSD candidates of an LFM could be limited to two values through the established function. Due to the nonmonotonic trend of LipOx aging, a physical parameter "the gray value ratio (GVR) of furrows to ridges" was proposed to exclude irrelevant TSD through support vector machine (SVM) classification. Ultimately, we predicted TSDs of seven LFMs with estimation errors of 2.2-26.8%. Overall, our strategy, with the outperformed capability of gleaning physical and electrochemical information on LFMs, can provide a truly label-free way of studying LFMs and hold great promise for multidimensional fingerprint information analysis.

Flexible Disk Ultramicroelectrode for High-Resolution and Substrate-Tolerable Scanning Electrochemical Microscopy Imaging

A simple and universal strategy for fabricating flexible 25 μm platinum (Pt) disk ultramicroelectrodes (UMEs) was proposed, where a pulled borosilicate glass micropipette acted as a mold for shaping the flexible tip with flexible epoxy resin. The whole preparation procedure was highly efficient, enabling 10 or more probes to be manually fabricated within 10 h. Intriguingly, this technique permits an adjustable RG ratio, tip length, and stiffness, which could be tuned according to varying experimental demands. Besides, the electroactive area of the probe could be exposed and made renewable with a thin blade, allowing its reuse in multiple experiments. The flexibility characterization was then employed to optimize the resin/hardener mass ratio of epoxy resin and the tip position during HF etching in the fabrication process, suggesting that more hardener, a larger RG value, or a longer tip length obtained stronger deformation resistance. Subsequently, the as-prepared probe was examined by optical microscopy, cyclic voltammetry, and SECM approach curves. The results demonstrated the probe possessed good geometry with a small RG ratio of less than 3 and exceptional electrochemical properties, and its insulating sheath remained undeformed after blade cutting. Owing to the tip's flexibility, it could be operated in contactless mode with an extremely low working distance and even in contact mode scanning to achieve high spatial resolution and high sensitivity while guaranteeing that the tip and samples would suffer minimal damage if the tip crashed. Finally, the flexible probe was successfully employed in three scanning scenarios where tilted and 3D structured PDMS microchips, a latent fingerprint deposited on the stiff copper sheet, and soft egg white were included. In all, the flexible probe encompasses the advantages of traditional disk UMEs and circumvents their principal drawbacks of tip crash and causing sample scratches, which is thus more compatible with large specimens of 3D structured, stiff, or even soft topography.

Recent Progress in Visualization and Analysis of Fingerprint Level 3 Features

Fingerprints provide sufficient and reliable discriminative characteristics which have been considered one of the most robust evidence for individualization. The limitation of current minutiae-based fingerprint technology seems to be solved with the development of level 3 features since they can offer additional information for problematic fingerprint recognition and even donor profiling. So far, tremendous efforts have been devoted to detecting and analysing the third-level details. This review summarizes the advances in level 3 details with an emphasis on their reliability assessment, visualization methods based on physical interaction, residue-response, mass spectrometry and electrochemical techniques, as well as the potentiality for individualization, donor profiling and even other application scenarios. In the end, we also give a personal perspective on the future direction and the remaining challenges in the third-level-detail-related field. We believe that the new exciting progress is expected in the development of level 3 detail detection and analysis with continued interest and attention to this field.

Chemical-Based Surface Plasmon Resonance Imaging of Fingerprints

Fingerprints are extremely useful in personal identification; however, they are usually based on physical rather than chemical images because it remains a challenge to reveal a clear chemical fingerprint easily and sensitively. Herein, a surface plasmon resonance imaging (SPRi) method, combined with a chemically selective stepwise signal amplification (CS³A) strategy, is proposed to chemically image fingerprints with adjustable sensitivity and clarity. High-fidelity glucose-associated fingerprint images were obtained at five to seven cycles of CS³A based on the recognition reaction of concanavalin A (ConA) with dextran. The method is also extendable to image substances that possess and/or can be tagged with ConA- or dextran-recognizable groups. For demonstration, SPRi of carboxylic substances was conducted by amidating the carboxyl group with glucosamine to enable the ConA-based CS³A. Glucose- and carboxyl-based fingerprints were simultaneously and clearly imaged, allowing us to perform quantitative analysis of the representative of either glucose or amino acid (e.g., serine) or both. The curves measured from the standard spots were linear in the ranges of 1-4000 μM for glucose and 3.2-4000 μM for serine, with linear correlated coefficients of 0.9979 and 0.9962, respectively. It was then applied to the study of metabolic secretions in fingerprints during running exercise, yielding variation tendencies similar to those measured from sweat samples in the literature. As a noninvasive tool, the CS³A-coupled SPRi reveals both clear images of fingerprints and quantitative chemical information, and it is anticipated to become a competitive new method for chemically imaging fingerprints.

Fast and quantitative analysis of level 3 details for latent fingerprints

Level 3 details play essential roles in practical latent fingerprint (LFP) identification. To reliably extract reproducible and identifiable level 3 features, high-resolution images of fingerprints with adequate quality are required. Conventional methods for acquiring level 3 details often involve specific pretreatment, intricate peripheral, leading to time-consuming analysis. Herein, we simply used water to develop the sebaceous LFPs deposited on nitrocellulose (NC) membranes with only one step, and then the high-resolution (2048 pixels per inch) optical micrographs were captured to reflect the live fingertip with high fidelity. From the pictures, level 3 features, including all dimensional attributes of the ridges and pores such as number, size, location, shape, and edge contour can be extracted accurately and reproducibly. Among them, qualitative features (the structures of ridge edges) and several quantitative characteristics (the number and the relative location of sweat pores) exhibit good reproducibility. Remarkably, we proposed a new parameter termed "frequency distribution of the distance between adjacent sweat pores", short form "FDDasp", which was further proved highly identifiable in different individuals, enabling the successful distinguishment between two fragmentary fingerprints with similar level 2 structures. We believe that this methodology provides a fast and quantitative analytical paradigm for latent fingerprint identification at level 3 details.