The illicit drug is a growing societal problem with the increasing of its supply and consumption on a global scale. In China, there were 74 670 drug cases relating to the criminals sentenced in 2011, accounting for 7.15% of all effective judgments ^[1]. The crackdown on drug-relevant crimes is always emphasized worldwide. Detection of one drug's residues and metabolites from fingerprint is evidential for combatting against the involved crimes.

David A. Russell's group has been working on the detection of drugs through which to simultaneously visualize the involved fingerprint and unveil different drugs and/or their metabolites by combining magnetic powders with different antibodies into powder-antibody conjugates ^[2]. As seen from Fig.1, both the fingerprint image and drug user's information have been successfully attained at the same time. Different drugs can be detected with different bound antibodies against, e.g., ∆ 9-tetrahydrocannabinol (THC), methadone and its metabolite, 2-ethlidene-1, 5-dimethyl-3, 3-diphenylpyrrolidine (EDDP), and benzoylecgonine (cocaine metabolite). Furthermore, a method for simultaneously detecting two drug metabolites from the same fingerprint was developed, as shown in Fig.2^[3]. When magnetic particles functionalized with antimorphine and antibenzoylecgonine antibodies were applied to fingerprint, different-color changes were respectively observed at different parts of fingerprint, corresponding to the specific drug metabolites. Besides, the group also used coenzyme immunoassay (cEIA) to detect cocaine on banknotes and latent fingerprints ^[4], thereby successfully quantifying the cocaine extracted from these substrates through optimization of their methods.

Fig.1

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	Fig.1 Detection effect of fingerprints containing THC (a), methadone and EDDP (b) and benzoylecgonine (c)[2]

Fig.2

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	Fig.2 The schematic for detection principle[3]

Frederick Rowell's group used carbon black-doped phenyltriethoxysilane powder to develop fingerprints containing drugs or their metabolites ^[5]. They utilized surface-assisted laser desorption-time-of-flight mass spectrometry (SALDI-TOF-MS) to characterize the residual substances from fingerprint that was lifted with tape beforehand. They tested codeine, diacylmorphine, cocaine, methadone and its metabolite EDDP with their so-established method, demonstrating that a specific powder is eligible as a developing reagent to detect drug residues in the remnant fingerprint survived of being lifted by tape. The Frederick Rowell group also proposed a new method ^[6] for detecting drug residues in cyanoacrylate-fumed latent fingerprints by mass spectrometry: treatment with acetone vapor plus tape extraction for mass spectrometry. Matrix-assisted laser desorption-time-of-flight mass spectrometry (MALDI-TOF-MS) and SALDI-TOF-MS were also applied to detect five drugs (cocaine, methadone, caffeine, aspirin, paracetamol) in the latent fingerprint on the slide fumed by the cyanoacrylate (CA), and the compared detection results proved that the SALDI-TOF-MS spectrum was higher of peak intensity.

Tobacco is the most common drug-containing material and consumed by millions of people around the world. Therefore, environmental tobacco smoking (ETS) is ubiquitous in many human environments and considered of serious hazard to human health. The presence of nicotine in smokers' fingerprints is of great importance for assistance to crime scene investigation.

The David A. Russell group used magnetic particles functionalized with anti-cotinine antibodies to detect nicotine, a metabolite of tobacco, in fingerprints on glass slide and porcelain substrates. The method is similar to drug detection in fingerprint ^{[7, 8]}. The Matthew Benton group, and the Frederick Rowell group alike^[9], detected nicotine and cotinine in latent fingerprints using MALDI-TOF-MS and SALDI-TOF-MS plus the submicron-sized hydrophobic silica doped with carbon black powder. This hydrophobic silica particle acted not only as a developing agent but also as a laser desorption ionization enhancer. Thus, the characteristic ion fragment peaks of nicotine and cotinine were obtained from the high energy-exciting collision-induced dissociation (CID) process of SALDI-TOF-MS, proving the correctness of this method. The Meikun Fan group evaluated the surface-enhanced Raman scattering (SERS) performance of the as-prepared SERS Q-tip and demonstrated it can be applied for the direct swab detection of 2, 4-dinitrotoluene (DNT). After 27 sequential touching onto the glass with contaminated finger, the characteristic bands of 2, 4-DNT are still clearly visible ^[10].

Since the 1990s, terrorist attacks have had a severe trend of spreading rapidly around the world. Explosion is a common means for terrorist attack. Terrorists carrying explosives are usually disguised, yet the explosive traces left on their hands are not easy to clean. Therefore, police or security agencies can rely on a simple and fast latent fingerprint (LFP) development method for simultaneously identifying individuals and screening the explosive carriers.

The Hong-Yuan Chen ^[11] group developed a nanocomposite for simultaneous visualization of fingerprints and detection of TNT in fingerprints. The red Cu-doped ZnCdS (Cu-ZnCdS) quantum dots were embedded in the silica nanoparticles, and the green ZnCdS quantum dots were anchored on the surface of the silica nanoparticles, and the polyallylamine (PAA) was used to be further functionalized. The fluorescence of green quantum dots can be quenched by TNT, while the quantum dots that emit red light are inert. Thus, the nano-mixture exhibited a traffic light type fluorescent color change (green-yellow-red) to TNT concentration in the range of 40-400 mM. This method is expected to be used for security screening in public places such as airports and railway stations. Instrumental analysis methods have also been attempted to detect explosives in fingerprint. The Wayne Rabalais ^[12] group used attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to unveil three explosive particles (trinitrotoluene (TNT), trinitrobenzene (TNB) and ammonium nitrate (AN)) in latent fingerprints deposited on stainless steel. ATR-FTIR spectroscopy can provide morphology and composition of particles, and compare the acquired spectra of individual finger residues to distinguish fingerprint residues from those of the explosive. Moreover, the fingerprints analyzed by ATR-FTIR are non-destructive without any disturbance from chemical reagents or powders. The explosive residues can be identified through the infrared spectrum library searching, highlighting a major advantage compared with gas chromatography-mass spectrometry, high performance liquid chromatography and capillary electrophoresis. Hence, the method can be used for further analysis and identification.

Sexual assault is an especially problematic crime to prosecute. On one hand, sexual assault victims are instinctive to try to expel the sense of violation by washing themselves and their clothes; on the other hand, an increasing use of condoms by sexual offenders has been observed, resulting in difficulty to collect the biological evidence from the offender and/or victim. Yet, in such cases, condom lubricants, if detected at the crime scenes, are helpful for the suspect to be accused.

Simona Francese ^[13] group demonstrated a new matrix-assisted laser desorption/ionization mass spectrometry imaging method (MALDI-MSI) that can simultaneously develop fingerprint ridge patterns and detect condom lubricants on fingerprints to identify sexually assault suspects, therewith successfully having tested two brands of condoms. The fingerprints aged for 1 month were also detected, and the fingerprint patterns were legible and can be further enhanced by image software. The results demonstrate the possibility of linking suspects (through fingerprint ridge pattern recognition) to crime (detection of condom lubricants) in one analysis via MALDI-MSI, enabling forensic scientists to provide evidence of stronger support in sexual assault cases. The Simona Francese ^[14] group also combined MALDI-MSI, MS, Raman microscopy and ATR-FTIR spectroscopy, which are complement to each other and provide additional information to retrieve “ condom brand spectral fingerprints” for facilitating condom brand recognition and detecting a range of condom brands/types for adding intelligence to the cases under investigation.

The sex of the donor is forensically important information, and possibly capable of being used as a first step to reveal the identity of the donor. At present, the identification of gender by fingerprint detection is mainly divided into two major categories: statistical methods by determining the density of fingerprints to distinguish gender; biochemical choices by detecting amino acid content.

Dr. Sudesh Gungadin ^[15] selected fingerprints from 500 donors (250 males and 250 females) in the 18-60 age scope. After extraction of the fingerprints from all ten fingers, the ridges were counted at the upper portion of the boundary of each fingerprint and their average was calculated. The results showed that fingerprints of ≤ 13 ridges/25 mm² were more likely to be from males, and fingerprints of ≥ 14 ridges/25 mm² were more likely to be from females. Table 1 shows the probability of densities and likelihood ratios that were derived from the observed ridge count. The study established a relationship between gender and fingerprint ridge density, successfully supporting the assumption that females' ridge density is often larger than males'.

Table 1

Table 1

Table 1 Probability of densities and likelihood ratios derived from the observed ridge count[15] Ridge count Probability of density Likelihood ratio Favored odds Male (C) Female (C) LR (C/C1) LR (C1/C) Males Females 11 0.06 0.001 600.0 0.002 0.99 0.01 12 0.3 0.004 75.0 0.01 0.98 0.02 13 0.43 0.08 5.4 0.19 0.84 0.16 14 0.18 0.36 0.5 2.0 0.33 0.66 15 0.02 0.42 0.05 21.0 0.05 1.00 16 0.001 0.13 0.01 130.0 0.00 1.00

Table 1 Probability of densities and likelihood ratios derived from the observed ridge count[15]

The Jan Halá mek ^[16] group proposed a bio-affinity-based sensor-system dual-enzyme cascade test that uses the concentration of amino acids present in the fingerprint to distinguish male and female fingerprint samples. The initial experiment of the sample concluded that there is a 99% probability of determining the correct gender of the fingerprint provider through analyzing fingerprints by statistical methods. Since the reaction of ninhydrin with the amino acids in the fingerprint produces the Ruhemann violet with blue-violet color, Jan Halá mek ^[17] has proposed an improved method for the traditional ninhydrin method, quantitatively analyzing amino acids by chromatography. After the amino acid extracted from the fingerprint on polyethylene film was reacted with ninhydrin, the signal corresponding to the Ruhemann purple concentration in the fingerprint sample was determined by spectrophotometry. The results showed that the absorbance of the female sample was higher than that of the male sample, having proved to be useful for gender identification of the fingerprints found from crime scenes. However, the method of extracting water-soluble amino acids in the fingerprint will cause a certain degree of damage to the fingerprint itself.

As seen from the above discussion, scientists and criminal investigators have paid great attention to detect components in fingerprints that can depict criminal suspects. In China, the researches on detecting fingerprint residues also have been focused ^{[18, 19, 20]}. However, the current methods are still not perfect, for example, the problem of long detection period for the biochemical methods and possible damage to the fingerprint caused by some detection handlings. The instrumental analysis is neither convenient nor feasible for the “ on-the-spot” work by local forensic laboratories due to the large size and high cost of these methods. Therefore, it is necessary to develop a more facile and cost-effective detection method for detecting both the diverse substances and special ones in fingerprints quickly, efficiently, and conveniently without damaging the fingerprint.

Regarding with using biochemical methods to detect substances in fingerprints that can depict suspect, the main problem to be overcome is not to damage the fingerprint. Although damage is permitted in some cases, it is necessary to retain the physical evidence as completely as possible for future verification. In addition, what needs to be paid attention to is that the efficiency of detection should be improved. Shortening the detecting duration can save time for the police and greatly improve the efficiency of solving the case. When it comes to biochemical reagents, one problem that cannot be ignored is the toxicity of the reagents. For the future research, the toxicity of the reagents should be minimized to ensure the health of the testers. Moreover, biochemical reagents are generally expensive, hence developing cost-effective detection reagents is expected. For instrumental analysis, future study should make the test results more visualized and easy to analyze, and replace the complicated and expensive instruments with common simple alternatives. Either way, developing a method that can simultaneously detect multiple substances is an attractive tendency.

Fingerprint-harbored endogenous substances are affected of their composition and content by many factors, e.g., gender, age, ethnicity, drug use, health status, mental state, metabolic status and diet, thus potential to provide a lot of important information about the suspect. Nevertheless, through suspects' habit of taking drugs and/or smoking, can the drugs or nicotine residues in the fingerprints not certainly attest that the suspects use drugs and/or smoke because they may come from contact with the outside, yet their metabolites can prove the use of related substances, for example, such metabolites of THC, EDDP, benzoylecgonine and cotinine are respective from marijuana, methadone, cocaine and nicotine. In addition, the metabolites of some drugs can be tested to determine whether the suspect has a related disease, such as the detection of metabolites of antihypertensive drugs and/or hypoglycemic drugs implying the suspect is of hypertension and/or diabetes. From the analysis of fingerprint components to identify the gender and the age of suspects is a topic of concern. At present, the identification of gender mainly focuses on the detection of amino acid content, along with fatty acids, sterols, steroids and waxes. Male have higher levels of fatty acids than female, who otherwise has higher levels of sterols and steroids. Wax fats also have gender-related different concentrations. Even, fingerprint composition changes significantly between neonate, adolescence and the old age with which to vary of volatile components, sebum excretion rates and organic ingredients. The eating habits of the suspects can also be clued by the analysis of fingerprint components. For instance, the fingerprints of vegan contain higher levels of alanine, soy and serine. The detection of the endogenous substances mentioned above can be expected to depict the relating suspect's characteristics.

The methods for detecting endogenous substances can be mainly divided into three categories: biochemical methods, instrumental analysis and their combination. For the detection of drugs/nicotine or drug metabolites, biochemical methods can be used to detect metabolites while developing fingerprints. In terms of chemical materials, priority should be placed to develop fingerprint. Due to the advantages of fluorescent materials, the new fluorescent dyes such as fluorescent conjugated polymers, up-conversion nanoparticles, quantum dots, carbon particles and fluorescent probes, can be explored. In terms of fingerprint extraction, various forms of materials, e.g., nano-films, fiber membranes and gel membranes, can be researched so as to have the fingerprints simultaneously lifted and transferred. The biological component bound onto the chemical material is mainly coupled with the antibody against the endogenous substance to be detected. The on-going development revealed that the aptamer is also a good choice. Compared with the antibody-antigen mixture, aptamer is more advantageous with its flexible design, mild synthesis conditions, easy modification, good biochemical stability, high specificity and attractiveness. The instrumental analysis may select surface enhanced Raman spectroscopy (SERS), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), infrared spectroscopy (IR), and/or their eligible combination with biochemical methods. For the detection of gender, age and diet habit that need to analyze the content of the components, the main focus is on instrumental analying choices such as IR, gas chromatography/mass spectrometry (GC/MS), high performance liquid chromatography (HPLC), laser-assisted thin-layer chromatography (LA-TLC) and analytical electrospray ionization mass spectrometry (DESI-MS). Furthermore, the combination of biochemical methods and instrumental analysis should be used with lessons drawn from the medical to detect various substances.

The detection of exogenous substances will also provide information about suspects. There may be many exogenous substances in the fingerprints such as drugs, explosives, gun residues, nicotine, condom lubricants, food residues and cosmetics. There is no practical way to detect food residues and cosmetics since these substances contain fatty acids, which are able to interfere with endogenous fatty acids in fingerprints. Therefore, it will be an important advance for forensic scientists to develop the methods that can detect food residues and cosmetics. Nicotine is not the same of detection as its metabolite, cotinine, though its detection can be used to evaluate the smoking habits of fingerprint donors. Similarly, the detection of gun residues can assist to judge the fingerprint donor's gun-holding habit, which can make investigators depict criminal suspects more detailed and accurately. The detection of exogenous substances is relatively simple, compared to the detection of endogenous substances with complex biochemical methods being required. Generally, only instrumental analysis or its combination with chemical methods can work well for the detection. If fingerprint was analyzed by SERS or FTIR, a complete standard spectrum database should be established to have the fingerprint spectrum compared. Definitely, the combination of chemical method with the instrumental analysis can also play its role. Targeting the fingerprint developed by a chemical reagent, further detection can be carried out with the analysis method such as SALDI-MS, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MSI).

The detection of the aged time of fingerprint can help determine the time of the case occurrence and deduce whether the suspect is present on the time of the incident. It is a very critical area in the fingerprint component identification. The composition of fingerprint changes over time, yet currently there is no reliable and acceptable method to track it. The fingerprint components changing along with time should be studied so that the change rate of the old and new components could be explored. The specific data of each component content changing can facilitate the subsequent detection and comparison. To detect substances (e.g., amino acids and albumin) that are stable in the fingerprint, the reagents specific for those substances can be used to develop old fingerprints. Through detecting diffuse substances such as chlorides in fingerprints, their diffusion rate and diffusion shape can be applied to explore the deposited time of fingerprints. For liquid components, such as fatty acid or cholesterol, there are mainly long-chain fatty acids in fresh fingerprints and short-chain fatty acids in old fingerprints, and the content of cholesterol in old fingerprints is reduced.

There are various methods for detecting the aged time of fingerprint. Statistical method can be used to detect changes in ridge characteristics such as the ridge width, though there is a difficulty in practice since the information of fresh fingerprint such as width, sweat pore size and distribution needs to be grasped. Alternately, when developing fingerprints with chemical reagents, the identification of the fingerprint changing with time after development is feasible from the used powder and/or fluorescent reagents. Truly, instrumental analysis is also a credible choice. Firstly, the fingerprint aging curve is generated with the instrument detection (such as FTIR, SERS and MALDI-MSI), and then the old fingerprint is tested, finally, the test result is compared with the aging curve to determine the aged time of the fingerprint.