Hybrid organic–inorganic silica based particles for latent fingermarks development: A review

Synthetic Metals 222 (2016) 124–131

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Hybrid organic–inorganic silica based particles for latent fingermarks development: A review Adam Lesniewski Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 November 2015 Received in revised form 29 January 2016 Accepted 23 March 2016 Available online 31 March 2016

Recent progress in nanotechnology has led to the emergence of many new methods of latent fingermarks development. However, there is still need for a material that is easy to detect and has a high affinity to the latent fingermarks ridges. One way to combine these two features are hybrid organic–inorganic silica based composite materials. The chemical nature of silica lets for its simple modification with various organic moieties by organosilanes chemistry. Moreover, nanoparticles, quantum dots, molecules etc. can be easily embedded in the silicate matrix. In this work, the use of silica based materials for fingermarks development have been summarized. Three main groups of the materials have been isolated: SiO2 based composites, porous phosphate heterostructures and desorption/ionization facilitating agents for surfaceassisted laser desorption ionization time-of-flight mass spectrometry. ã 2016 Elsevier B.V. All rights reserved.

Keywords: Latent fingermarks Development Silica Hybrid SALDI Particles

1. Introduction Fingermarks are one of the most important group of physical evidence used for an individual identification in forensic science. Their usage for person identification is based on the fact that there was no individual ever found who has ridge patterns on the hands and feet identical with any other person [1]. This unique property of human ridge patterns makes fingermarks useful tool to link an exhibit with a certain person. The Locard’s Principle claims that it’s impossible to act with the intensity related with a criminal action without leaving multiple traces [2]. The crime scene is usually full of traces. The biggest challenge is to find them and to understand their meaning. There are several types of fingermarks. Only few of them can be seen with a naked eye. The vast majority, are so called latent fingermarks that need to be developed before they can be seen, recorded and used in an investigation process. Up to now, there have been various methods proposed for latent fingermarks development. The most frequently used ones are: powder dusting, small particle reagent (SPR), cyanoacrylate and iodine fuming, methods based on ninhydrine and its analogs, physical developer (PD), vacuum metal deposition (VMD), multi metal deposition (MMD), single metal deposition (SMD) etc. [1]. The multiplicity of the methods is caused by the diversity of the latent fingermarks that can be found on a crime scene. The fingermarks can be deposited on porous or

E-mail address: [email protected] (A. Lesniewski). http://dx.doi.org/10.1016/j.synthmet.2016.03.032 0379-6779/ã 2016 Elsevier B.V. All rights reserved.

non-porous surface but also on adhesives, metals, fabric, human skin and many more. They can also be blood-based. Fingermarks can be also divided by the residue composition. Most of fingermarks are composed of eccrine and sebaceous fractions. Eccrine fraction is mostly composed with inorganic compounds, amino acids, proteins, lipids and miscellaneous compounds such as lactate, urea, creatine, creatinine, glucose and many others. The sebaceous part are fatty acids, phospholipids, wax esters, sterols, squalene and other organic compounds [1]. One should bear in mind that the composition of human sweat and sebum is an individual feature. Moreover, it changes with an individual’s age, health condition, diet and so on. The fingermark already deposited on the surface also changes with time. The volatile components of the mark evaporates. The composition changes with light exposure, temperature and humidity [3]. In the case of porous surfaces, fingermark components diffuse with time into the material. Various components move with different velocity, what causes the changes in fingermark surface composition [4]. To manage the above mentioned diversity many different methods have been proposed to develop various types of latent fingermarks over the years. The classic, latent fingermarks development methods, have been already reviewed in several papers [5] and textbooks [1,6]. New, non-conventional methods for latent fingermarks development are still being introduced [7]. Among the others, various electrochemical methods have been proposed recently. There are several papers on latent fingermarks development by electrodeposition of conducting polymers such as polyaniline [8], poly(3,4-

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ethylenedioxythiophene) (PEDOT) [9], polypyrrole [10] and poly (pyrrole-co-EDOT) copolymer [11]. Other materials such as gold or silver nanoparticles [12], prussian blue [13], graphene [14] or copper [15] have been also electrodeposited in order to develop latent fingermarks. An interesting group of methods are those utilizing electrochemiluminescence (ECL) [16–18]. ECL have been successfully used to detect explosive residues [19]. Recently, a highly sensitive method for latent fingermark development have been designed by combining ECL with immunodetection [20]. Scanning electrochemical microscopy (SECM) have been used to visualize latent fingermarks pre-modified with metallic deposits [21,22]. Fingermarks developed with vacuum deposited Al-doped ZnO thin film has been also visualized with SECM [23]. Finally it was demonstrated that SECM is a suitable tool to develop latent fingermarks on conductive surfaces without previous modification [24]. An alternative approach is designing of a new, effective developing agents. In the late 80s a new group of such agents based on nanomaterials starts to grow. Nanoparticles based methods have been already summarized in several review papers [25–27]. Briefly, the first group of methods—multi metal deposition (MMD,) based on gold nanoparticles [28] was introduced in 1989 [29] and later modified [30,31]. The original method was further modified resulting in single metal deposition (SMD) [32] and one-step single metal nanoparticles deposition (SND) where glucose stabilized gold nanoparticles have been used [33]. Modification of gold nanoparticle surfaces with a ZnO layer enabled observation of fingermark luminescence [34]. Gold nanoparticles can be selectively bound to the fingermark [35] or to the porous surface [36,37]. The latter approach helps to overcome problems with variable fingermarks composition. The affinity of gold nanoparticles to the latent fingermark surface can be increased by their modification with antibodies [38]. Besides of gold, also nanostructures from different materials have been used. Standard physical developer (PD) can be used as an example of silver nanoparticles utilization for latent fingermark development [39]. CdS quantum dots modified with dendrimeric polymers [40] and CdSe/ZnS modified with octadecylamine [35] have been described. Chitosan stabilized CdS have been successfully used for latent fingermarks development on aluminum foil [41]. CdSe

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nanoparticles stabilized with mercaptoacetic acid have been used for latent fingermarks development on the sticky side of adhesive tape [42] while CdTe have been used for bloody fingermarks development on non-porous surfaces [43]. The diversity of fingermarks’ types is a real challenge for the forensic science. Despite nanotechnology, which brought many new tools to forensic laboratories, there is still need for the new materials for latent fingermarks development. When searching for a new latent fingermark developer one should take into account two main features of the material. First of all, the material should be easy to detect. It may be colorful, fluorescent, phosphorescent etc. Secondly, it should have a diverse affinity to the fingermark ridges and to the material on which the fingermark have been deposited. It is a difficult task to find a single material that combines the two above mentioned features. The solution to this issue are hybrid, organic–inorganic composites. A suitable base for this type of materials are silica particles. The silicate matrix can be modified with, an easy-to-detect elements such as quantum dots, organic dyes etc. while their surface properties can be easily tuned with organosilanes chemistry. Hybrid silica-based particles of various sizes, structure and functional groups have been used in numerous applications [44], but only recently they have been tested as a novel latent fingermark developing agents. The aim of this review is to summarize the above mentioned efforts. 2. SiO2 based composites The first two papers on silica based composites for latent fingermarks development have appeared in 2008 [45,46]. Liu et al. [45] have entrapped Eu3+ sensitizer complex in sol–gel processed SiO2 matrix. Authors have tested 1,10-phenantroline (OP) and thenoyltrifluoroacetone (TTFA) as sensitizers. The Eu3+/sensitizer silica xerogels have been obtained by reacting sol–gel precursors such as tetraethoxysilane (TEOS), tetramethoxysilane (TMOS) and tetrakis (2-hydroxyethyl) orthosilicate (TKIS) in the presence of complex followed by drying procedure. The synthesis was found out to be time consuming. Xerogel fabrication process lasts for 28 days. In order to facilitate fingermarks dusting, resulting xerogels were grounded with mortar into a fine powder and mixed with magnetic powder. The best results have been obtained

Fig. 1. Results obtained after application of SiO2–COOH NPs at pH 6. For the right half, EDC/NHS is used to mediate the reaction. On the right is a schematic illustration of the bond formation between carboxyl groups present on SiO2 NPs surface and amine groups found in the fingermark secretion. The amide linkage is mediated by the use of EDC/ NHS [48]. (reprinted with permission)

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Fig. 2. Schematic illustration of the preparation of the nanohybrid of green/red QDs for simultaneous latent fingermark development and TNT detection [57]. (reprinted with permission)

for Eu3+/OP modified powder. Eccrine fingermarks have been developed on metal foil, glass, plastic, colored paper, and a green leaf. In the same year, Theaker et al. [46] have developed silica nanoparticles’ synthesis from tetraethoxysilane (TEOS), phenyltriethoxysilane (PTEOS) and aminopropyltriethoxysilane (APTES). Two parallel approaches have been introduced. Particles have been fabricated in a top-down approach—by crushing the bulk material in a mortar or in a bottom-up sol–gel synthesis. During the synthesis process particles have been doped with organic dyes (namely: ethidium bromide, crystal violet, bromothymol blue, fluorescein, thiazole orange, oxazine perchlorate, methylene blue, basic yellow 40, basic red 28, rhodamine B and rhodamine 6G),

carbon black, titanium dioxide or magnetite. Two alternative techniques of particles application have been proposed—powder dusting and suspension application. Both of them lead to the latent fingermarks development on non-porous surfaces such as glass, metal and black laboratory bench top. Fresh and 40 days old marks have been developed. Another approach is modification of latent fingermarks with columnar thin films (CTF) of CaF2 and SiO2 by physical vapour deposition technique. The resulting deposits have been further modified by spin-coating of 1,2-indanedione–alanine solution or rhodamine 6 G solution. The method was found suitable for developing latent fingermarks on non-porous surfaces [47].

Fig. 3. PPH-SH modification with quantum dots—scheme [62]. (reprinted with permission)

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Fig. 4. (a) Functional chemical groups on the surface of the different silica particles: particle (A) formed from mixture of PTEOS and TEOS will contain a mixture of phenyl and (B) groups (silicate, hydroxyl and ethyl groups); particle (B) formed from tetraethoxysilane (TEOS) starting product will contain groups from (B) (silicate, hydroxyl and ethyl groups); particle (C) formed from 3-(triethoxysilyl)propylsuccinic anhydride (TPSA) will contain succinic acid groups and those from (B); particle (D) formed from N-(trimethoxysilylpropyl)ethylene-diamine triacetic acid (EDTA) will contain EDTA groups and those from (B); particle (E) formed from 3aminopropyltriethoxysilane (APTES) will contain amine groups and those from (B); particle (F) formed from p-aminophenyltrimethoxysilane (APhTES) will contain paminophenyl groups and those from (B). (b) Chemical groups covalently attached to carbon black networks; (i) underivatised carbon black without any surface functional groups; (ii) phenyl functional groups on carbon black network; (iii) 2carboxyphenyl functional groups on carbon black network; (iv) 3-carboxyphenyl functional groups on carbon black network; (v) 4 carboxyphenyl functional group on carbon black network [69]. (reprinted with permission)

In 2011, a separate section of the review on the use of stains to detect fingermarks have been devoted to silica nanoparticles [25]. Authors have proposed modification of stain-doped silica nanoparticles surface with organosilanes bearing amine, carboxyl or thiol functionalities in order to increase their affinity to latent fingermarks ridges. This concept has been further developed in order to understand the general mechanisms responsible for fingermark ridges— nanoparticles interactions [48]. Silica nanoparticles modified with rhodamine 6G have been synthesized by reverse microemulsion method. Carboxylic, succinic anhydride, methylphosphonate and

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sulfonate groups have been introduced on the particles surface. Fingermarks containing both sebaceous and eccrine fraction deposited on aluminum foil have been developed by immersion into the hybrid particles suspension. In the paper the commonly accepted hypothesis that various nanoparticles interact with fingermark residue by electrostatic forces have been questioned. The alternative mechanism, chemical reaction between carboxylic groups of the nanoparticles with amine groups of the fingermark ridges, have been proposed. (Fig. 1) Moreover the paper have shown the usefulness of the SiO2 nanoparticles’ surface for chemical modifications that can enhance the interactions between the particle and the ridges. Mercaptorpropyltrimethoxysilane (MPS) [49] and APTES [50] have been used to modify Fe3O4 nanoparticles in order to facilitate their further decoration with Ag nanoparticles. Subsequently the organosilane modified Fe3O4 nanoparticles have been modified with Ag nanoparticles and finally with a continuous Ag shell. Resulting Fe3O4@Ag nanoeggs have been found suitable for development of latent fingermarks on porous and non-porous surfaces such as ceramic, paper, plastic and glass. The Fe3O4@Ag nanoeggs have been casted on an exhibit surface in the form of suspension and as a dry powder. Both methods have shown promising results. Unfortunately the details of the development and fingermarks deposition procedures have not been provided. Highly luminescent nanophosphors, based on lanthanide doped, yttrium zirconate nanoparticles embedded in silicate matrix have been synthesized by the sol–gel process [51]. Eu3+, Tb3+, Sm3+, Dy3+ or Pr3+ have been used as lanthanide dopants. They have been mixed with zirconium oxychloride, TMOS and other additives and whole mixture was left to form a gel. The resulting material have been calcinated in 1100  C. The highest pure color emission and photoluminescence lifetime have been observed for europium and terbium doped materials. The resulting Ln3+:Y2Zr2O7/SiO2 composite nanoparticles have been used as a dusting powder for latent fingermarks development. Fingermarks deposited on non-porous surfaces such as glass, aluminum foil, plastic bag and CD have been successfully developed. The research have been continued and Eu3+:Y2Ti2O7/SiO2 nanoparticles have been synthesized in a similar way [52]. The new dusting powder have been used to develop latent fingermarks on non-porous surfaces, namely glass, plastic, CD and aluminum foil. The promising results have been also obtained for fingermarks deposited on porous surfaces. The method was demonstrated to be suitable for development of 7 days old marks on non-porous surfaces. In another experiment [Eu(Phen)2]3+ complex have been incorporated into synthetic hectorite and natural montmorillonite clay materials. Resulting nanohybrids have been utilized to develop latent fingermarks on glass [53]. Some attempts to combine quantum dots (QD) with a silicate matrix have been made. Core–shell-structured CdTe@SiO2 have been synthesized by hydrolizing of TEOS in the presence of NH2NH2 and NH3. Resulting powder have been applied to develop latent fingermarks on non-porous and porous surfaces [54]. 50 nm SiO2 nanobeads have been decorated with 3–10 nm QD (CdTe, PbTe or PbS) covered with 1–2 nm SiO2 in order to obtain SiO2– QDs@SiO2 composite nanostructures [55]. The surface of resulting nanostructures was further functionalized with carboxylic groups by organosilane chemistry in order to improve the material affinity to the latent fingermark ridges. SiO2–QDs@SiO2–COOH water solution have been used to develop a fingermark deposited on a glass slide. An interesting composite material have been obtained by embedding CdTe quantum dots in silicate matrix. Resulting beads have been subsequently decorated with Ag nanoparticles. CdTe@SiO2/Ag nanocomposite have been used for latent fingermarks visualization on various non-porous surfaces. Material’s antimicrobial properties can be utilized for a long-term

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Table 1 Latent fingermarks development methods based on hybrid organic–inorganic silica based particles. Developing agent

Application technique

Development time

Visualization technique

Suitable for porous surfaces?

Magnetic Eu3+/sensitizer- doped SiO2-based xerogel powder [45] Dye-doped hydrophobic particles [46]

Magnetic brushing

Immediately

Photoluminescence

Yes

Immersion in the particles suspension/ powder dusting Physical vapour deposition spincoating Immersion in the particles suspension

2–10 min/ immediately 20 min

White light photography, photoluminescence Photoluminescence

No

30–60 min

Photoluminescence

No

Rinsing with particles suspension/ magnetic brushing Rinsing with particles suspension/ magnetic brushing Powder dusting Powder dusting Powder dusting Powder dusting Suspension deposition and drying Powder dusting

Immediately/ immediately Immediately/ immediately Immediately Immediately Immediately Immediately 120 min Immediately

Optical

Yes

Optical

Yes No Yes No Yes No No

Immersion in the particles suspension Pressing the mark against developing agent layer Powder dusting/rinsing with particles suspension Powder dusting Powder dusting Powder dusting Powder dusting Powder dusting Powder dusting

0.5 min Immediately

Photoluminescence Photoluminescence Photoluminescence Photoluminescence Photoluminescence Photoluminescence/white light photography Photoluminescence Optical

Immediately/ immediately Immediately Immediately Immediately Immediately Immediately Immediately

Optical

No

Optical Photoluminescence Photoluminescence Photoluminescence White light photography Mass spectrometry

No Yes Yes No No No

Magnetic brushing

Immediately

Mass spectrometry

No

Magnetic brushing

Immediately

Mass spectrometry

No

Dye-doped columnar thin film [47] Functionalized SiO2 nanoparticles ( OPO2CH3, OH, COOH) [48] Fe3O4-core@Ag-shell nanoeggs [49] Fe3O4@Ag Nanoeggs [50] Ln3+:Y2Zr2O7/SiO2 nano-phosphore [51] Eu3+:Y2Ti2O7/SiO2 nano-powder [52] [Eu(Phen)2]3+-clay mineral hybrids [53] Core–shell-structured CdTe@SiO2 QD [54] SiO2–QDs@SiO2 nanostructures [55] CdTe@SiO2/Ag [56] Green/red QD nanohybrid [57] Silica gel G [58] ZnO–SiO2 nanopowder [59] Amphiphilic silica nanoparticles [60] CdS QD decoreted PPH [63] CdSe QD decoreted PPH [62] CdSe QD decoreted PPH [64] Carbon black doped silica [65] Carbon black doped hydrophobic silica powder [66] Carbon black doped magnetisable hydrophobic powder [67] Carbon black doped silica nanoparticles [71]

preservation of fingermarks [56]. Recently, the system for simultaneous fingermark development and TNT detection have been described [57]. Red emitting Cu doped Zinc’s QD have been embedded in silicate matrix. Next, the surface of red QDs@silica nanocomposite surface have been decorated with green emitting Zinc’s QDs by EDC/NHS coupling. Resulting red QDs@silica@green QD nanocomposite surface has been functionalized with polyallylamine (PAA)—a TNT receptor. The resulting dual emission nanocomposite suspension was applied to develop sebaceous latent fingermarks on non-porous surface—aluminum foil. In the case when the TNT was present in the fingermark the luminescence color of the nanocomposite changes from green to red due to green emission quenching by the Meisenheimer complex that forms between TNT and PAA (Fig. 2). Contrary to the described above complicated, multi-component systems, also much simpler silica based materials have been tested as potential latent fingermark developing agents. Silica gel G have been demonstrated to be comparably effective in developing of latent fingermarks on various, porous and non-porous, surfaces to the conventional dusting powders [58]. The ZnO–SiO2 nanopowder have been synthesized by precursors heating method and grinding the resulting material in a mortar [59]. Zn–SiO2 can be used as a dusting agent as well as in the water suspension form. This material can be used to develop fingermarks on various nonporous and semi porous surfaces. It was demonstrated that the method is capable to develop more details of the fingermark than the conventional dusting powders. However, the comparison experiment have been conducted on two separate fingermarks, but not on two parts of the same fingermark. Recently, monodisperse silica nanoparticles of various sizes have been synthesized by ammonium catalyzed sol–gel method. The surface of the resulting particles have been grafted with 4-(chloromethyl)

No

No Yes

phenyltrichlorosilane in order to facilitate particles adhesion to the ridges of sebaceous fingermarks [60]. The new amphiphilic hybrid nanoparticles have been successfully used for development of sebaceous fingermarks on glass surface by dusting process. 3. Porous phosphate heterostructures An interesting group of composite materials that were recently applied for fingermark development are porous phosphate heterostructures (PPH). They are materials composed of expanded zirconium phosphate with silica galleries [61]. Their mesoporous structure combined with the reactivity of the silica surface makes them promising candidates for further modifications (Fig. 3). Mercaptopropyl (PPH-SH) and propionitrile (PPH-CN) groups bearing PPH have been synthesized. The resulting materials have been decorated with CdS quantum dots. PPH–S–CdS nanocomposite has been used for latent fingermark development on plastic, glass, steel, ceramic and wood. Problems with background fluorescence in the case of paper surface have been reported by authors [63]. The system was further investigated and cadmium selenide sulphide quantum dots have been incorporated into the material, what results in PPH–S–CdSe composite formation [62]. The new material have been used to develop latent fingermarks on various surfaces such as a credit card magnetic band, plastic, methacrylate, LCD screen, aluminum, steel, silica gel and wood. In the subsequent paper amino groups have been introduced onto the PPH silica galleries surface [64]. The PPH–NH2 material have been further modified with CdSe quantum dots. Resulting PPH– NH2@CdSe material have been demonstrated to be useful for latent fingermarks development especially on non-porous surfaces. All composite materials described above are attractive latent fingermark developing agents due to their strong

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photoluminescence, high photo-stability and a good adherence to the fingermarks’ ridges. 4. Surface-assisted laser desorption ionization time-of-flight mass spectrometry (SALDI-TOF-MS) The characteristic ridge pattern are the most common feature of fingermark to be used in forensic science. However, fingermarks can bear much more information, than that—chemical composition. Based on the fingermark composition one can tell a lot about the donor. Unfortunately, the extraction of the chemical information usually means, that the fingermark have to be dissolved. A surface-assisted laser desorption ionization time-of-flight mass spectrometry (SALDI-TOF-MS) a variant of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOF-MS) have been proposed as a non-destructive method to analyze the chemical composition of the latent fingermarks [65]. In the paper, the hydrophobic silica particles described earlier by Theaker et al. [46] have been used as an agent facilitating desorption and ionization of the compounds of interest from the fingermarks lifted with a standard lifting tape [66]. The detection of codeine, diacetylmorphine, cocaine, methadone, morphine, noscapine and papaverine has been demonstrated. Moreover, it was possible to create the maps showing the distribution of the compound of interest over the fingermark. The hydrophobic silica hybrid particles has been also used as an enhancing agent in SALDI detection of nicotine and cotinine [67]. The transfer of the compounds through the hands shaking, touching the same surface by smoker and no-smoker and through the smoke have been investigated [68]. The work on the development of the hybrid silica based SALDI enhancing aged have been continued [69]. The collection of silica particles with a surfaces modified with various chemical groups have been synthesized. The versatility of silica chemistry allows for phenyl, silicate, hydroxyl, ethyl, succinyl, ethylenediaminetetraacetic acid (EDTA), amine and p-aminophenyl groups introduction. The hybrid particles have been doped with phenylated carbon black derivatives, polyaniline or graphite (Fig. 4). Obtained hybrid particles have been used to detect a set of amino acids and others polar and non-polar endogenous metabolites. An interesting insight into the role of chemical groups in desorption/ionization mechanism have been presented. A polar and non-polar components of the fingermark residues have been separated with magnetic, carbon black doped, hybrid silica particles bearing amino – hydrophilic and phenyl – hydrophobic groups on their surface. The separated compounds have been further analyzed with SALDI-TOF-MS [70]. Magnetized carbon black doped silica particles have been also used for development of latent fingermarks. It was possible to detect terbinafine in the fingermarks donated by the person who was medically advised to use this drug [71]. Recently magnetic carbon black doped silica particles have been used to detect 6monoacetylmorphine (6-MAM), diacetylmorphine (heroin), methadone, nicotine and noscapine in the fingermarks lifted from the surface with a conventional lifting tape. No drugs were detected, while regular dusting powders were used for development. Moreover, it was demonstrated that fingermarks already developed with traditional dusting powder can be treated with carbon black doped silica particles in order to enhance the SALDI effect [72].

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to design a material which selectively binds to the fingermark ridges but not to the surface. This is especially an issue concerning porous surfaces. Besides selective adherence, an ideal fingermark developer have to be easy to detect. It can be colorful, luminescent etc. One of the ways to combine both features in one material is use of hybrid organic–inorganic silica based composite materials. The chemical properties of silica, make it natural candidate for designing hybrid materials. Despite the fact, that this type of materials are well known and broadly applied in live sciences connected applications, their potential in the latent fingermarks development have been noticed only recently. Several papers describing development of latent fingermarks using various silica based composite materials, have been published in the last few years. The hybrid organic–inorganic structures used for the latent fingermark development can be organized into three main groups—SiO2 based composites, porous phosphate heterostructures and SALDI-TOF-MS desorption/ionization facilitating agents. The basic experimental facts on the latent fingermarks development methods based on hybrid organic–inorganic silica based particles have been summarized in Table 1. Most of the new developers have been applied on the latent fingermarks by simple powder dusting or magnetic brushing. The main advantage of this simple methods is a fact that one can get developed fingermark almost immediately. This method of application is based on the differences in adsorption of the developing agents on the fingermark ridges and on the substrate. This may be a problem especially in the case of porous substrates. Majority of the developers adhere to the porous substrates comparably well as to the fingermark ridges what limits their use to non-porous surfaces only. However, there are few examples where powder dusting have been successfully applied to the porous surfaces. The development is usually of a poorer quality than for non-porous surfaces, nevertheless, it was demonstrated to be possible. In my opinion, it is worthy to investigate those systems deeper in order to understand the selective adhesion mechanism, and utilize it in new developing agents design. There are only few examples, of application of the developing agent onto the fingermark in the form of suspension. This approach is more complex and time consuming than simple brushing but it has one great advantage. It allows to utilize the chemical interactions between developing agent and latent fingermark ridges. Experimental efforts should be put in modifying the hybrid structure surface in a way that it will react selectively with a fingermark but not with the substrate or the other way around. As it was demonstrated, SiO2 is a perfect candidate for chemical modification. One could consider using aptamers, antibodies, peptides or other selective individuals. Resulting materials could be possibly applied to develop latent fingermarks on porous substrates, but it will need to be applied in suspension form. Most of the described developing agents have been visualized using their optical properties such as light absorption or more frequently photoluminescence. This approach may be problematic in the case of surfaces with a strong background luminescence. It would be worthy to try to overcome this problem possibly by using upconverting materials or non-light based visualization techniques such as e.g. SECM. One can suppose, that the pioneering papers collected in this review will become foundations for the new research on latent fingermarks development, as well as their composition analysis.

5. Conclusions Acknowledgment The chase for an ideal latent fingermarks developing agent still continuous. The progress in nanotechnology have led to the introduction of several new latent fingermarks developing methods based on nanoparticles. However, it is still a challenge

This work was financed by the National Science Center, Poland under the project 2015/17/D/ST5/01320.

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