POLYMERS IN CRIMINALISTICS: LATENT FINGERPRINT DETECTION AND ENHANCEMENT – FROM IDEA TO PRACTICAL APPLICATION

Summary : The identification of latent fingerprints is crucial for the security agencies in order to identify persons suspected of having committed criminal acts. Latent prints often remain as random impressions onto the surfaces of various objects. Huge progress has been made in the identification processes using rolled and plain impressions fingerprints, while detection and comparison of latent prints still remains a challenging task. In addition to the existing standardized methods, in this paper the possible utilization of natural and synthetic polymers and their conjugates with the same goal will be reviewed, by means of latent fingerprints detection and development


Introduction
Identification of latent fingerprints is crucial for law enforcement agencies in identifying suspected persons. 2 Nowadays, many different methods for revealing and developing the latent fingerprints have been used.The utilization

Polymer-based Conjugates as Systems for Latent Fingerprints Development and Detection
The materials that go through changes of color in response to the external physical (heat, light, pressure, electricity) and the chemical/biochemical (solvent, pH, ligand-receptor interaction) stimuli may be used as key components sensors, switches or memory devices. 8,9,10 Aong colorimetric substances sensitive to external stimuli tested so far, conjugated polymers have attracted certain attention due to their unique optical properties associated with the presence of delocalized π-electrons. 11When localized stimulation of conjugated polymers results in a change in the absorption and emission characteristics, these materials could have been effectively used as sensor matrices. 12, 13 As written in the article published in Nature Communications 14 a polymer that exhibits instantaneous fluorescence and color change in response to a small amount of water can be useful material for individual sweat pores detection.It has been confirmed that this method is sensitive enough to detect individual sweat pores of a human hand that leads to detailed color prints.The idea of using sweat pores in order to detect latent fingerprints was proposed earlier, but cheap, fast and reliable methods of pores mapping are still difficult to achieve.Jong-Man Kim et al. 15 described a simple polymer-based material that gives an immediate, sensitive enough response to distinguish functional pores of those that are inactive (Figure 1) when finger was presented.As known, the ridges are generally used for inducing fingerprints, but a reliable analysis sometimes requires large print portions.Sweat pores allow identification of a pattern and with very small part of the latent trace, suggesting that this method can be used as a reliable method for detection of latent fingerprints.Mapping of active sweat pores was performed by using a fluorescent microscope excitation at 510-550 nm.Unlike conventional methods, which largely depend on the friction of papillae ridges over the surface, and thus require large areas of latent prints for reliable analysis, the proposed method can be used to identify even when there is a relatively small portion of latent fingerprint. 16n order to achieve stable and improved performance of the synthesized system (conjugate), and thus to obtain the efficient transfer of electrons, good color retention, good photosensitivity and high photostability, the researchers more often apply the immobilization of an active ingredient (usually different colors) conjugates into a various materials (carriers), such are hydrophobic bentonite particles17 or hydrogels. 18For instance, some fluorescence positively charged dyes, Rhodamine 6G (R6G) or Methylene Blue (MB), showing different fluorescence lifetime 19 may be suitable as an agent for developing the latent fingerprints in forensics. 20he polymer surfaces play an important role in many technological processes, such as catalysis, corrosion or adhesion. 21These processes depend on the chemical composition of the surface but also on the analytical techniques (spectroscopy or microscopy), with an aim of the characterization of the surface chemical composition. 22Yu-Juan et al. 23 synthesized uniformed and well dispersed photoluminescent semiconductor cadmium sulfide (CdS) quantum dots in situ within 4.0 Generation-NH 2 polyamidoamine (PAMAM) dendrimers in methanol and a mixture of methanol and water in a volume ratio of 1:9 respectively, used to detect cyanoacrylate ester fumed/deposited fingerprints on tinfoil.When applying the ultraviolet excitation of 365 nm a light yellow-green and orange luminescence appears (Figure 2) respectively, and fumed fingerprints were successfully detected with completely described reaction mechanism. 24hese systems have recently attracted huge attention as they possess unique optical properties due to the effect of quantum restrictions because of its structure.CdS quantum dots are one of the most investigated semiconductors as they possess unique chemical, physical and optoelectronic properties which make them potentially useful in microelectronics, catalysis and bioanalytical applications.) with the assistant of a yellow light filters. 23S/dendrimer nanocomposites can be useful for the production of new optical sensors, microelectronic devices and luminescent biological probes, 26 and their application is justified in terms of selective latent prints labeling, with both physical and chemical mechanisms, with adjustable excitation and color luminescence by synthesis of nanocrystals of desired size having strong luminescence.The fumed fingerprints can be detected more efficiently than when applying traditional CdS/2-mercaptoethanol nanocomposites.
[138] NBP • Žurnal za kriminalistiku i pravo Dilag et al. examined the possibility of applying high photoluminescent CdS quantum dots encapsulated in a cheap chitosan polymer matrice for detection of latent fingerprints. 27Quantum dots represent semiconductor nanocrystals that have optical and electrical characteristics that depend on particle size due to the effect of quantum limitations. 28Cadmium ions are chelated within a chitosan matrice by rapid addition of sodium sulfide to form particles of CdS/ chitosan nanocomposite. 29Fresh deposited non-fumed cyanoacrylate latent fingerprint on the aluminum foil was clearly observed under Rofin 30 light at 450 nm when spattered with prepared suspension together with surfactant (Tergitol 31 ), allowing formation of powder surfactant/quantum dot (Figure 3).Quantum dots powder has been successfully used to develop latent fingerprints on aluminum foil, with distinctive characteristics of the ridges.
Furthermore, chitosan is very much used in the preparation of microparticles for sustained release of the active substances.However, its utilization in forensic applications has not been studied enough so far.Issa et al. 32 examined the possibility of application of chitosan microparticles crosslinked with sodium tripolyphosphate (TPP) in the presence of red color, to ensure better visualization, deposited on the latent fingerprints enabling their identification, by binding of synthesized microparticles to fatty acids residues of latent fingerprints (Figure 4).It points to the fact that a large number of factors influence the effectiveness of the application of the proposed system, such as the degree of deacetylation of chitosan, chitosan/TPP ratio, storage temperature, molar mass, charge, etc. 34 Lipophilic, polycationic polymer, chitosan, can be used as substrate for latent fingerprints detection.Chitosan is the second most abundant natural polymer, and according to the literature data, it is mainly used as a carrier for various active components. 35The scientific community is aware of the fact that chitosan can also be used for the deposition of gold nanoparticles, and Islam et al. 36 examined the possibility of application of the proposed system for detection of latent fingerprints (Figure 5).In this way cheap but effective improvement of the optical contrast was achieved, which greatly facilitated the identification process itself.[140] NBP • Žurnal za kriminalistiku i pravo About 99% of the fingerprints represents water, which evaporates very quickly leaving behind the remains of the order of micrograms.About half of these residues are complex mixtures of organic substances such as fats, amino acids, vitamins, etc. 37 Schnetz and Margot 38 were the first to report the possibility of using gold nanoparticles that depended on a large number of parameters, both on the porous and non-porous surfaces, but the reaction mechanism of gold nanoparticles with the sweat residues was not entirely determined.This technique has been significantly improved by Islam et al., and the synthesized system was applied in order to bind chitosan to fats which remain in a latent (finger)print.

Figure 5. The developed latent fingerprints with varying amounts
of residues collected from different individuals by applying the proposed (optimized) method. 37is method represents a non-destructive method, while at the same time the removal of any of the characteristics of physical evidence was avoided.In comparison with more developed techniques for fingerprints detection, such as gas chromatography, mass spectrometry, X-ray fluorescence, elemental analysis, the degree of identification does not depend on the material being used.An excellent contrast is obtained when applying 1% chitosan solution after 30 minutes, and its further development in the agglomerated colloidal solution of gold, for the period of 6-7 hours.The resulting contrast clearly differentiated the ridges, and hence this method has the potential to be applied for forensic identification. 39lmong et al. 40 examined the potential application of genipin, the hydrolytic product of geniposide extracted from gardenia fruit, as a reagent for detection of latent fingerprints found on the brown wrapping paper.One of the great advantages of genipin is the fact that it is completely harmless and environmentally friendly reagent 41 and developed fingerprints appear as blue traces (impressions) that fluoresce when exposed to the UV light of wavelength 590 nm. 42he same research team examined the optimal conditions for genipin utilization taking into account the presence of a different solvent, concentration, temperature, humidity of the surroundings and the heating mode.The possibility of sequential developing of traces was also examined by using 1,8-diazafluoren-9-one (DFO) and ninhydrin, 43 and the obtained results were compared in terms of performance, and the spectrofluorometric data of reaction of genipin with several amino acids were generated.The circumstances under which genipin takes precedence over DFO and ninhydrin were specified, like on the brown wrapping paper or on documents written using fluorescent dye (Figure 6).It is well documented fact that DFO is more sensitive to amino acids than ninhydrin, and that its use is recommended in series with ninhydrin, but before ninhydrin application. 44Unlike ninhydrin, the reaction mechanism of DFO and its analogs (Figure 7) have not gained much attention since its introduction in forensic application. 45a) (b) (c) 1,2-Indandione first came into use in 1997, as a reagent for developing fingerprints on porous surfaces giving the final pink colored product (reminiscent of ninhydrin), while at the same time being fluorescent (reminiscent of DFO).46

Some New Systems Used for Latent Fingerprints Detection
In addition to the foregoing materials, different suspensions based systems also containing cadmium are in use nowadays.Wang et al. 47 in 2009 addressed the possibility of applying the suspension of photostable cadmium selenide (CdSe) nanoparticles for developing of latent fingerprints on the sticky side of the adhesive, synthesized in an aqueous solution in the presence of mercaptoacetic acid as a stabilizer, 48 and the results showed that the fluorescence intensity was significantly increased. 49 2014, the same laboratory examined the possibility of employing suspensions of CdSe for detection of latent fingerprints on non-porous and semi-porous surfaces, 51 also described by Bécue and Cantú. 52

Conclusion
Contemporary detection methods include the use of modern (nano)materials, nanocrystals or nanobelts for developing of latent fingerprints and with high sensitivity, excellent contrast, high selectivity and efficiency, low background noise, and on the various surfaces. 53Synthesis and application of these materials requires further improvement of the existing performances, but also provides the possibility for synthesis of entirely new systems for detection of latent fingerprints.

Figure 1 .
Figure 1.Mapping of sweat pores.(A) Photograph of a fingerprint on poly(10,12-pentacosadiynoic acid-cesium (PCDA-Cs)) film (thickness about 3.8 mm); (B) optical and (c) fluorescence micrograph of magnified area in the image (a) obtained with excitation at 510-550 nm; (D) images with increased contrast -dots inside the blue circles are pores that do not secrete sweat. 1

Figure 2 .
Figure 2. Fingerprint left on the tinfoil, fumed with cyanoacrylate ester fumes a) CdS/G4.0PAMAM nanocomposite; b) CdS/G4.0PAMAM nanocomposite in a methanol solution; c) CdS/G4.0PAMAM nanocomposite in a methanol : water (1 : 9) solution; and d) CdS/ mercaptoacetic nanocomposite in water solution (without additional yellow filter), taken with a digital camera (Nikon D80) with the assistant of a yellow light filters.23

Figure 3 .
Figure 3. Photos of latent fingerprints on the aluminum foil surface spattered using (a) powdered quantum dots, (b) white powder, and (c) aluminum powder 9

Figure 6 .
Figure 6.Half palmprints developed on notebook paper by using (a) genipin and (b) ninhydrin, recorded under white light.