Pursuit to more sensitive reagent is the eternal theme in fingerprit detection techniques. Recently, there are two types of reagents or techniques emerging in this direction: luminescent reagents and nanoparticles. Quite often, these two are integrated. Menzel et al. focused their research on the use of photoluminescent semiconductor nanocrystals (also referred to as nanocrystallites, quantum dots, nanoparticles, nanoclusters or nanocomposites), made from compounds such as ZnS, CdS, CdSe, CdTe, InP, and InAs, which yield intense luminescence with a lifetime in the desired range^{[3, 4]}. Moreover, the absorption and emission can be tailored by adjusting the nanocrystal size. Basically, CdS nanoparticles were used as the luminescence resources after Cyanoacrylate Fuming (CAF). Cadmium nitrite and sodium sulfate were added in the dendrimer solution to form the CdS/dendrimer nanocomposites. After that, exhibits with fingerprits already fumed by cyanoacrylate (CA) were dipped into a CdS/dendrimer nanocomposites solution for a number of hours (often overnight) to endure the possible reaction between the amino functionality of the dendrimer and the carboxylic acid in the fingerprint residue. An interesting observation was that this method was only effective on CA ester-fumed fingerprits, but ineffective when applied to unfumed fingerprits. This could be explained by the use of ethanol in the solution causing the fingerprint residues to be washed away. Moreover, to improve the binding of the fingerprit to the dendrimer, Bouldin et al. used diimide to pretreat the fingerprit to convert the carboxylic acid moieties of the fingerprint residue to esters that then reacted with the dendrimer amino groups to form amide linkages^[5]. The effect of temperature on the reaction of CdS/dendrimer nanocomposites with fingerprint residues was also considered and some positive results were acquired. In summary, the CdS/dendrimer nanocomposites improved the binding of nanoparticles with fingerprint residues and seemed an interesting direction in the powdering techniques, but the complexity of the operation, for example, long development time, also limited its further application. In addition, Jin et al. used CdS/PAMAM nanocomposites to develop sebaceous fingerprits on tinfoil and observed similar results^[6].

Fingerprit detection on difficult surfaces always attracts the attention of forensic scientists, for example, fingerprit detection on polymer banknotes, adhesive tapes, highly luminescent and patterned background.Jones et al. used cyanoacrylate fuming/rohdamine (6g) staining, combined with vacuum metal deposition (VMD) techniques, to detect fingerprits on Australian polymer banknotes^{[7, 8]}(Fig. 1). Sufficient fingerprits with clear ridges were detected using this method, especially for fresh fingerprit. Australian polymer banknote is a notoriously difficult surface for fingerprit detection. The coating of these banknotes is a modified polyurethane lacquer over patterned offset and intaglio printing and each of the denominations exhibited broadband luminescence.

Ma et al. applied a carbon-based suspension into the detection of fingerprits on the sticky side of adhesive tapes. Some organic solvents such as methyl ethyl ketone were also used to unravel the tangled tapes^{[9, 10]}. A carbonic ink was also used at the detection of fingerprits on the tapes since carbonic ink is a carbon-based colloid solution. Clear fingerprits with enough ridges were developed by this method in several real cases. Still, Ma et al used upconverters to detect fingerprits on difficult surfaces with background luminescence and pattern interference^{[11, 12, 13]}. Upconverter is a special material whose emission light has a shorter wavelength or higher energy than the excitation light. The result shows that upconverter successfully developed clear fingerprits on surfaces that conventional techniques hardly work, such as Australian Polymer banknotes (Fig.2).

Fig.1

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	Fig.1 Developed latent fingerprit on PVDF using the aptamer-based reagent. Overall fingerprit pattern (left) and a magnified section (right) show completely clear “ valleys” of the fingerprit, highlighting the lack of background interaction[7]

Fig.2

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	Fig.2 Fresh fingerprits (< 5 h old) on an Australian five-dollar polymer banknote, developed with NaYF4:Er, Yb, illuminated at 980nm laser light and imaged by a Rofin Poliview fitted with an IR blocking filter and exposed for approximately 15s[12]

TR and PR technology is effective for fingerprit detection that conventional luminescence techniques cannot resolve. Usually, TR/PR technology involves the use of complicated instruments such as pulsed lasers and imaging facilities. The time-resolved (TR) technique has been proposed to detect the luminescence of fingerprits for some time^{[14, 15]}. However, it is not a new technique but one developed by Murdock and Menzel in the 1990s^[16]. TR spectroscopy is a method which utilizes the difference in luminescence lifetime between a substrate and a sample. Luminescence lifetime is the average decay time of the luminescence emitted by a molecule after excitation with a short laser (or other light resource) pulse. It has been extensively applied in biology, but not widely used in fingerprit detection. So far, the TR technique has been successfully applied to fingerprit development with milli-, micro- and even nano-second resolution (Fig. 3). Generally, the TR technique needs complex and expensive devices such as laser, CCD camera, image intensifier, programmable timing generator, and the others related. This greatly limits the application of the TR technique, but it does have a significant advantage. It can address the issue caused by background luminescence interference.

Fig.3

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	Fig.3 The intensified image (left) and the fluorescence lifetime one (right) of the blitz-green-treated fingerprit on postcard substrate[15]

A more complicated technique named the phase-resolved (PR) technique was also applied with the similar theoretical principle^{[17, 18]}. The PR technique is based on the phase shift that occurs in luminescence emissions. In this method, the sample (fingerprint) is excited with an intensity-modulated laser. The luminescence emissions from the sample and the background will be phase-shifted with respect to the excitation. Then the heterodyne technique is applied to acquire the frequencies of different emissions from the fingerprints and the background. The obtained heterodyne signal will be further processed to resolve the phase information by electronically mixing it with a square wave pulse. Finally, the fingerprint is “ separated” from the background. The PR technique can image the luminescence emission even when the fingerprit’ s luminescence lifetime is shorter than that of the background, which is not at all possible with the TR technique. Moreover, the PR technique offers better contrast for fingerprit detection than the TR technique^[17]. However, the PR technique also faces many of the same disadvantages as the TR technique, being limited its further application.

Chemical (or hyperspectral) imaging is the most powerful weapon that fingerprint experts ever acquired. Chemical imaging include infrared (IR), UV and visible, Raman, and X-ray fluorescence imaging. When these images are obtained by the chemical imaging facilities, the component of the fingerprint residues can be acquired as well^[19]. In theory, the chemical imaging can even solve the most challenging problems in fingerprit detection.

Infrared chemical imaging was firstly applied for fingerprit detection on a variety of surfaces by Tahtouhand coworkers^{[20, 21, 22]}. Usually, the fingerprits detected by this method should be treated before the application of chemical imaging. Several types of cyanoacrylate esters with strong absorption peaks in the infrared region were synthesized and applied for the fuming of fingerprits on various surfaces such as Australian polymer banknotes. The results showed that very clear fingerprits even with tertiary features were developed by this method on Australian banknote which was impossible using other techniques (Fig. 4). After this work, advances have been achieved by chemical imaging in many areas in forensic science, together with the application in fingerprit detection, for example, trace evidence and document examination.

Fig.4

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	Fig.4 Ethyl-cyanoacrylate-fumed mark on $5 note: (a) White light photograph (b) Infrared spectrum of fingerprit-ridge showing peak at 1760 cm-1 used to generate image (c) Monochrome representation of infrared chemical image (d) Fig. 4c with contrast and brightness adjustment [20]

It is a challenging issue on how to develop fingerprits on the exhibits that are polluted by bio-, chem- and/or nuclear hazardous materials. This issue was raised after the ‘ Anthrax attack’ that happened in the US immediately after the ‘ 911 attack’ .

Hoile et al. conducted research using a number of porous and nonporous items contaminated with viable anthrax spores and marked with latent fingerprits. The test samples were then subjected to a standard formulation of formaldehyde gas for decontamination. After decontamination, latent fingerprits were recovered using a range of methods. It was found that the formaldehyde gas fumigation was effective at destroying viable spores, but also contributed to the degradation of amino acids, leading to loss of ridge details. Finally, a new protocol for formaldehyde gas decontamination was developed, allowing for the destruction of viable spores and the successful recovery of latent marks, all within a rapid response time of less than one hour^[23](Fig. 5).

The use of antibody (immune technology) and aptamer for fingerprit detection is an important approach that could lead to the breakthrough to current detecting methods. Fingerprit detection using antibodies is not a brand new technology as Ishayam and coworkers had tried some antibodies to develop fingerprits on the adhesive tapes as early as in 1977. From then on, many scientists applied this method on fingerprit detection and some gave quite positive results. Spindler and coworkers successfully developed fingerprits on a series of surfaces by the antibodies targeting the amino acid in fingerprint residues^[24]. The primary antibody reacted with the amino acid by the help of nanogold to dramatically increase the reaction sites, and the secondary antibody was then connected with the primary antibody. Two types of luminescent tags attached to the secondary antibody were used for imaging, but the Fluorescent Red 610 was proved slightly better. This work is important because it proved that the immune method could be a universal method for fingerprit detection since amino acid is an ordinary constituent in human sweat.

Another important method is the use of aptamer targeting lysosome in human sweat. Aptamers are, for example, the short single-stranded oligonucleotides of either ssDNA or RNA exhibiting specific and complex stable three-dimensional structures that allow aptamers to bind to numerous targets. Wood et al used DNA aptamers to detect fingerprits aginst lysozyme in human sweat that the 5’ -end of the aptamer was connected with CAL-Flour orange 560 fluorescent tag for imaging^{[7, 25]}. Some fingerprits on PVDF were successfully detected, but on other substrate, the effect was not promising. However, it proves that aptamer has great selectivity and could be revised and used together with other fingerprit detection methods in the future.

Extracting information of certain suspect from fingerprint is an emerging attempt as one important part of forensic intelligence. From fingerprint residues, the detection of cotinine, a metabolite of nicotine in the cigarette, is a particularly interesting topic for forensic scientists. Russell and his coworkers used anti-cotinine antibodies to detect fingerprits for smokers^{[26, 27]}. The anti-cotinine antibodies were connected to the gold nanoparticles with other conjugates to react with the fingerprit materials^[27]. Then the anti-cotinine antibody complex reacted with the secondary antibody tagging fluorescence. The fingerprits of smokers show red or green luminescence with different luminescent markers, but the fingerprit of non-smoker is not luminescent. The importance of the result is the recoginition of whether the offender is a smoker or not. There are some reports that explosives or drugs can be detected by chemical imaging including FTIR imaging, especially by near-infrared imaging^[20]. Some researchers also use UV and visible light imaging to discern exogenous materials, mainly drugs and explosives, when detecting fingerprits. But the identifying capability of UV and visible light imaging is much less than that of near-infrared imaging. Whereas, the FTIR imaging facilities are far more expensive. Moreover, the AFIS information can also been used for intelligence aims. The fingerprint information is fundamental in police data, especially useful for the detection of serious and/or volume crimes.

The use of 3rd level characteristics in fingerprint identification remains a topic of interest. In fingerprint identification process, the number of the 2nd level characteristics is usually taken as the key criterion. Nevertheless, there is no solid standard on the numbers of 2nd level characteristics used into identification in countries like China, US and Australia, though general 7 or 8 of 2nd level characteristics are the conventional standard, albeit not declared by law. It is quite common that a fingerprint collected from crime scene shows only 5 or 6 stable characteristics under which many fingerprint experts resort to the help of 3rd level characteristics. The 3rd level characteristics mainly include sweat pores and the micro-shape of ridges. Some research groups have studied the 3rd characteristics and their preliminary results showed that the 3rd characteristics have the practical value in fingerprint identification. There were some reports about the use of 3rd characteristics in real cases, e.g., the ‘ cannabis leaf case’ happened in Sydney, Australia. In this case, the fingerprit expert only found seven 2nd level characteristics on the stamp of an envelope containing some cannabis inside. At that time, the High Court in New South Wales ruled that a person cannot be identified by fingerprint with less than 10 of 2nd characteristics. However, there were many sweat pores in the fingerprit developed on the stamp. In the court, the expert of Australian Federal Police showed 20 sweat pores plus seven 2nd level characteristics which both matched the suspect’ s fingerprint. Finally, the jury accepted the identification using this 3rd level features, and also this led the NSW high court changed the regulation that fingerprint cannot be identified with less than ten 2nd level features(Fig.6).

Fig.5

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	Fig.5 Development of print using physical developer after decontamination treatment with formaldehyde (standard concentration) [23]

Fig.6

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	Fig.6 Upconverter-developed fingerprint showing third level properties (magnification times: 50x)[11]

The age estimation of fingerprints is becoming possible with the development of modern analysis techniques. Age estimation of fingerprints could have a significant impact in forensic science, as it has the potential to facilitate the judicial process by assessing the relevance of a fingerprint found at a crime scene. However, no method currently exists that can reliably predict the age of a latent fingerprint. In the past, only some explicit rules were argued to evaluate the approximate age of fingerprint residues. For example, when fingerprits left over some time are difficult to develop with powdering method, these fingerprits are meant to be possibly aged. Furthermore, the image of a fingerprit will become dim or less visible if it is not fresh. However, the age dating of a latent fingerprit is affected by such many factors that it is almost impossible to work out the age of the fingerprit. Recently, Muramoto and Sisco seemed to give a real scientific method to solve this problem^[28]. In their manuscript, time-of-flight secondary ion imaging mass spectrometry (TOF-SIMS) was used to measure the diffusivity of saturated fatty acid molecules from a fingerprint on a silicon wafer. A mathematic model was established to evaluate the time when a fingerprit was left. The results showed that on a clean silicon substrate, the age of a fingerprint (t ≤ 96 h) could reliably be obtained through the extent of diffusion of palmitic acid. However, it is only a very preliminary work to decide the age of a latent fingerprit, and it is still a long way to go in the future.

The application of more powerful Automatic Fingerprint Identification System (AFIS) will profoundly improve the efficiency of crime investigation and stimulate the research of fingerprit detection techniques. Currently, there is no national AFIS or central fingerprint database in China. Oppositely, each province has its own AFIS that is from several companies, leading to the inefficiency compared to a national database, such as the Integrated Automated Fingerprint Identification System (IAFIS) run by the FBI of US. Automated Fingerprint Identification System (AFIS) is one of the fundamental projects built by China Police, and also one of the most important scientific approaches for criminal investigation. AFIS is now applied universally in policing in China. Presently, there are about one hundred and ten million suspects’ fingerprints in the database under China Police, together with about two million fingerprints from the crime scenes. In 2011-2012, over two hundred thousand criminal cases were solved through AFIS at provincial level in China. In addition, more than twenty thousand criminal cases have been uncovered by cross-provincial assistance of AFIS (or national level) in China.

For historical and other reasons, AFIS in China was built on provincial level and there is no National Automated Fingerprint Identification System (NAFIS). Now there are several types of AFIS from different manufacturers in the total 31 provinces of Chinese mainland. The AFIS operators can make fingerprint searches freely inside each province, but the fingerprint search across different provinces is not available yet.

In order to solve the cross-provincial fingerprint searches, the China’ s Ministry of Public Security (MPS) established an Assistant Fingerprint Search Platform (AFSP) in 2010. In the AFSP system, the Provincial Police forwards the request of fingerprint search to the AFSP where the search task is then distributed to all of the other provinces. The responsible policeperson in other provinces will make the search and the results will be returned to the AFSP in different intervals according to the seriousness of the crimes. At the top, the AFSP officiers will make statistics on all the results of cross-provincial matches and maintain the normal operation of this system.

However, the barriers among the different manufactures still significantly affect the in-depth application of AFIS. To solve the problem, the MPS orders the accreditation of the provincial AFIS so as to standardize the basic characteristics of fingerprints, strengthen the cooperation in criminal investigation using AFIS from different manufacturers, eliminate the barriers of different software and hardware, and finally improve the information communication among different AFIS. The MPS promulgated a series of standards that all of the provincial AFIS have to comply with, for example, all of the AFIS must have the input ports permitted of the accredited livescan system to transfer fingerprint data, connected to the Central Management System and operated by the eligible people in other provinces. Only when the above requirements were met could the AFIS pass the accreditation organized by MPS. The AFIS that does not pass the accreditation will be banned to sell, install and use within the China Police.

The accreditation has two stages: the first one is the software accreditation and the 2nd the on-site test. In the first stage, the software is required to test all of the demanded features in laboratory and this task was already completed in 2014. Most of the AFIS vendors have passed the accreditation. In the 2nd stage, all of the AFIS should be tested with the effectiveness of fingerprint check after installed and updated in local Police. This 2nd stage work had been done in 2015.

The AFSP and the accreditation for AFIS can partially solve the difficulty of fingerprint search of cross-province and inter-manufacturer, but there still remains the strong need to establish a NAFIS in China for crime investigation. MPS eventually decided to found the NAFIS about which the investigation has been started. MPS plans that there will be more than one hundred million people’ s fingerprints and more than ten million fingerprits to be collected from crime scenes in NAFIS’ database. The daily search will surpass twelve thousand times and the equipment for live scan will be over fifty thousand sets. The feasibility of this plan is still under investigation and the invaluable advice is welcomed from the experts all over the world.

In summary, fingerprint techniques will be very likely to boom again with the development of fundamental sciences, such as biochemistry and analytical chemistry, in the near future.