MA Rongliang

(Institute of Forensic Science, Ministry of Public Security, Beijing 100038, China)

Fingerprint Techniques: the Current and Trend

MA Rongliang

(Institute of Forensic Science, Ministry of Public Security, Beijing 100038, China)

This article tries to summarize the recent advances of fingerprint technology and demonstrates ten possible developing directions in the future: 1. more sensitive reagents; 2. fingerprit detection on surfaces difficult to handle; 3. timeresolve (TR) and phase-resolve (PR) technology; 4. chemical imaging technology; 5. fingerprit detection on the exhibits polluted by bio-, chem- and/or nuclear-hazardousness materials; 6. immune and aptamer technology; 7. forensic intelligence from fingerprit detection; 8. the use of 3rd level characteristics in fingerprint identification; 9. age estimation for fingerprits; 10. more powerful Automatic Fingerprint Identification System (AFIS).

fingerprint; fingerprit detection; time-resolve (TR) and phase-resolve (PR) technology; bio-, chem-, nuclearhazardousness materials; forensic intelligence; 3rd level characteristics; age estimation; Automatic Fingerprint Identification System (AFIS)

Fingerprint techniques are in mainstream of forensic science. For over a hundred years, fingerprint evidence has been applied in criminal investigation and judicial process[1]. Nowadays, with the help and rapid development of Automated Fingerprint Identification System (AFIS), fingerprint techniques have been widely used as among the most important tools in crime investigation. Generally, fingerprint techniques are divided into three categories: detection, identification and AFIS techniques[2]. Fingerprint detection develops latent (or invisible) fingerprits into visible ones by the physical, chemical and biological methods; fingerprint identification compares the fingerprints collected from crime scenes with those of the suspects to decide if they are identical. AFIS is the application of information technology into fingerprint identification, more virtual of the information technology rather than fingerprint technology.

The advances of fingerprint techniques are slow, compared to other more ‘modern' fields in forensic science, for instance, DNA and drug detection techniques. However, since around 2000, there is great development in fingerprint techniques, especially in fingerprit detection techniques. Based on these observations, we can draw the conclusion that fingerprint techniques will take progress in the following areas.

1 More Sensitive Detection Reagent

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].

2 Fingerprit Detection on Difficult Surfaces

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 coat-ing of these banknotes is a modifi ed 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 fi ngerprits 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 fi ngerprits on diffi cult surfaces with background luminescence and pattern interference[11-13]. 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 fi ngerprits on surfaces that conventional techniques hardly work, such as Australian Polymer banknotes (Fig.2).

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

3 Time-resolve (TR) and Phase-resolve (PR) Technology

TR and PR technology is effective for fi ngerprit 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 fi ngerprit 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 intensifi er, programmable timing generator, and the others related. This greatly limits the application of the TR technique, but it does have a signifi cant advantage. It can address the issue caused by background luminescence interference.

Fig.3 The intensifi ed image (left) and the fl uorescence lifetime one (right) of the blitz-green-treated fi ngerprit 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 (fi ngerprint) 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 fi ngerprints 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 fi ngerprint 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 fi ngerprit 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.

4 Chemical Imaging Technology

Chemical (or hyperspectral) imaging is the most powerful weapon that fi ngerprint 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 fi ngerprint residues can be acquired as well[19]. In theory, the chemical imaging can even solve the most challenging problems in fi ngerprit detection.

Infrared chemical imaging was firstly applied for fingerprit detection on a variety of surfaces by Tahtouhand coworkers[20-22]. Usually, the fi ngerprits 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 fi ngerprits on various surfaces such as Australian polymer banknotes. The results showed that very clear fi ngerprits 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 fi ngerprit detection, for example, trace evidence and document examination.

Fig.4 Ethyl-cyanoacrylate-fumed mark on $5 note: (a) White light photograph (b) Infrared spectrum of fi ngerprit-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]

5 Fingerprit Detection on the Exhibits Polluted by Bio-, Chem- and/or Nuclear Hazardous Materials

It is a challenging issue on how to develop fi ngerprits 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 fi ngerprits. The test samples were then subjected to a standard formulation of formaldehyde gas for decontamination. After decontamination, latent fi ngerprits 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).

6 Immune and Aptamer Technology

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 fi ngerprit detection and some gave quite positive results. Spindler and coworkers successfully developed fi ngerprits on a series of surfaces by the antibodies targeting the amino acid in fi ngerprint 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 fi ngerprit 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 fi ngerprits aginst lysozyme in human sweat that the 5'-end of the aptamer was connected with CAL-Flour orange 560 fl uorescent 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 fi ngerprit detection methods in the future.

7 Fingerprint Intelligence

Extracting information of certain suspect from fi ngerprint is an emerging attempt as one important part of forensic intelligence. From fi ngerprint 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 fi ngerprits for smokers[26-27]. The anti-cotinine antibodies were connected to the gold nanoparticles with other conjugates to react with the fi ngerprit materials[27]. Then the anti-cotinine antibody complex reacted with the secondary antibody tagging fl uorescence. The fi ngerprits of smokers show red or green luminescence with different luminescent markers, but the fi ngerprit of nonsmoker 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 i maging, 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 fi ngerprits. 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 fi ngerprint information is fundamental in police data, especially useful for the detection of serious and/or volume crimes.

8 The Application of 3rd Level Characteristics in Fingerprint Identifi cation

The use of 3rd level characteristics in fi ngerprint identifi cation remains a topic of interest. In fi ngerprint identifi cation 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 fi ngerprint collected from crime scene shows only 5 or 6 stable characteristics under which many fi ngerprint experts resort to the help of 3rd level characteristics. The 3rd level characteristics mainly include sweat pores and the microshape of ridges. Some research groups have studied the 3rd characteristics and their preliminary results showed that the 3rd characteristics have the practical value in fi ngerprint identifi cation. 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 identifi ed by fi ngerprint with less than 10 of 2nd characteristics. However, there were many sweat pores in the fi ngerprit 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 fi ngerprint. 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 identifi ed with less than ten 2nd level features(Fig.6).

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

Fig.6 Upconverter-developed fi ngerprint showing third level properties (magnifi cation times: 50x)[11]

9 The Age Estimation of Fingerprints

The age estimati on of fi ngerprints 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 fi ngerprint found at a crime scene. However, no method currently exists that can reliably predict the age of a latent fi ngerprint. In the past, only some explicit rules were argued to evaluate the approximate age of fi ngerprint residues. For example, when fi ngerprits left over some time are diffi cult to develop with powdering method, these fi ngerprits are meant to be possibly aged. Furthermore, the image of a fi ngerprit will become dim or less visible if it is not fresh. However, the age dating of a latent fi ngerprit is affected by such many factors that it is almost impossible to work out the age of the fi ngerprit. Recently, Muramoto and Sisco seemed to give a real scientifi c methodto solve this problem[28]. In their manuscript, time-offlight secondary ion imaging mass spectrometry (TOFSIMS) 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.

10 The Application of More Powerful Automatic Fingerprint Identification System (AFIS)

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 crossprovincial 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 crossprovince 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.

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指紋顯現(xiàn)技術的現(xiàn)狀與發(fā)展趨勢

馬榮梁

（公安部物證鑒定中心，北京 100038）

本文旨在總結指紋技術的新發(fā)展，并從以下10個方面分析指紋技術未來發(fā)展的可能方向。1. 更靈敏的顯現(xiàn)試劑。以熒光試劑和納米粒子為代表的兩種技術最為重要和突出。納米粒子較常規(guī)粉末吸附性好，而熒光試劑具有高靈敏度且能克服背景干擾的優(yōu)點，二者結合表現(xiàn)更加顯著，有關研究表明其前景廣闊。2. 疑難客體上的指紋顯現(xiàn)技術。指皮膚、膠帶粘面、塑料紙幣及背景發(fā)熒光的材質等表面上的指紋顯現(xiàn)。3. 時間分辨和相分辨技術。它們都涉及到一系列復雜儀器的使用，另外與傳統(tǒng)熒光使用不同，時間分辨技術是根據(jù)指紋試劑及背景的熒光壽命或者相位的不同，通過復雜儀器分辨出該微小差異并加以放大，從而將指紋顯現(xiàn)出來。此方法能顯現(xiàn)傳統(tǒng)熒光法處理不了的指紋。4. 光譜成像技術。包括紅外、紫外及可見光、拉曼成像等。光譜成像技術特別是紅外光譜成像技術在顯現(xiàn)指紋的同時，能夠分辨出指紋物質的成分，比如手上粘附的一些外源性物質像毒品、爆炸殘留物等。因而，光譜成像技術可能是迄今為止最為有效的能解決一些疑難指紋顯現(xiàn)的技術，但光譜成像一般需要大型昂貴的儀器設備。5. 生化核危害性物質污染的檢材上的手印顯現(xiàn)。甲醛處理生化類污染的指紋檢材有報道，但核污染材料本文未涉及。6. 免疫學和適配體技術。使用抗原抗體的免疫學反應來顯現(xiàn)指紋也是人們探索的重要方向之一。免疫學和適配體技術都具有高靈敏度和選擇性強的優(yōu)點，但反應條件較為苛刻。7. 指紋來源的情報信息獲取。從指紋中探測毒品、爆炸殘留物等信息屬于指紋信息學的范疇。此外，指紋自動識別系統(tǒng)的指紋信息也是偵查破案所需的重要情報信息。8. 指紋鑒定及三級特征的應用。現(xiàn)有指紋鑒定是以二級特征數(shù)量為標準的，但在實際案件中，常有二級特征不足的情形，汗孔及指紋紋線微小形狀等三級特征可作為重要輔助特征而幫助鑒定。9. 指紋遺留時間的判斷。該技術和方法具有重大意義，但也一直存有難點。有報道通過測定指紋遺留物質中棕櫚酸的擴散速度，初步確定出其與指紋遺留時間的相關性。但影響判斷指紋遺留時間的因素太多，建立通用的指紋遺留時間判斷模型仍需艱苦的工作。10. 計算機指紋自動識別技術（AFIS）。AFIS在中國發(fā)展很不均衡。國家層面沒有統(tǒng)一的AFIS，而是由各省分別建立，這導致了指紋工作總體效率較低。為此，公安部建立了指紋協(xié)查平臺，出臺了系統(tǒng)認證等辦法，部分解決了既有難題。指紋大庫建設現(xiàn)正在準備和論證中。

指紋；指紋顯現(xiàn)；時間分辨與相分辨；生化核危害性物質；物證情報；第3級特征；指紋遺留時間的估測；計算機指紋自動識別系統(tǒng)

DF794.1

A

1008-3650(2016)04-0302-07

2016-01-27

格式：Ma Ronglaing.Fingerprint Techniques: the Current and Trend[J].刑事技術，2016,41(4):302-308.

DF794.1 Document Code: A Article ID: 1008-3650(2016)04-0302-07

10.16467/j.1008-3650.2016.04.012

Author: MA Rongliang（1980—）, male, PhD, Associate Professor, a researcher of fingerprint techniques. Email: marl2013@163.com