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Development and empirical optimization of an electrochemical analysis cell for the visualization of latent fingerprints and their chemical adhesives

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Fingerprint analysis played a major role in the investigation of criminal offences for the past 100 years and is often the sole means of criminal identification [YA04]. Electrochemical analysis can yield important additional evidence like fingerprint age, biological age and gender of its creator as well as chemical adhesives [GRW12]. Additional gained characteristics through electrochemical analysis can supplement latent or incomplete fingerprints. In previous work a ruthenium-complex based solution was used as illuminant. Since luminol is readily available and is used in many forensic applications, the presented paper will focus on luminol as an alternative chemical for the ECL-aided visualization of fingerprints. Experiments were conducted by creating an electrochemical reaction inside a purpose build analysis cell. Eccrine, sebaceous glandlike and vaseline contaminated fingerprints were created on a stainless-steel plate placed inside the cell and investigated while applying direct current. Aim of this research was to investigate which kind of fingerprints can be visualized and which quality of the resulting images can be reached using luminol as illuminant. The used laboratory power supply created a strong light reaction at the start of each experiment revealing potential for further enhancement of the image quality. Eccrine dactyloscopic evidence showed no visible results. For sebaceous glandlike fingerprints age was discovered to significantly influence image quality.
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A. Br¨
omme, C. Busch, A. Dantcheva, K. Raja, C. Rathgeb and A. Uhl (Eds.): BIOSIG 2020,
Lecture Notes in Informatics (LNI), Gesellschaft f¨
ur Informatik, Bonn 2020 1
Development and empirical optimization of an
electrochemical analysis cell for the visualization of latent
fingerprints and their chemical adhesives
Tommy Bergmann1,Sebastian Gottschall2,Enrico Fuchs2
,Oliver Berlipp1, Dirk Labudde1,3
Abstract:
Fingerprint analysis played a major role in the investigation of criminal offences for the past 100
years and is often the sole means of criminal identification [YA04]. Electrochemical analysis can
yield important additional evidence like fingerprint age, biological age and gender of its creator
as well as chemical adhesives [GRW12]. Additional gained characteristics through electrochemical
analysis can supplement latent or incomplete fingerprints. In previous work a ruthenium-complex
based solution was used as illuminant. Since luminol is readily available and is used in many forensic
applications, the presented paper will focus on luminol as an alternative chemical for the ECL-aided
visualization of fingerprints. Experiments were conducted by creating an electrochemical reaction
inside a purpose build analysis cell. Eccrine, sebaceous glandlike and vaseline contaminated finger-
prints were created on a stainless-steel plate placed inside the cell and investigated while applying
direct current. Aim of this research was to investigate which kind of fingerprints can be visualized
and which quality of the resulting images can be reached using luminol as illuminant. The used
laboratory power supply created a strong light reaction at the start of each experiment revealing
potential for further enhancement of the image quality. Eccrine dactyloscopic evidence showed no
visible results. For sebaceous glandlike fingerprints age was discovered to significantly influence
image quality.
Keywords: latent fingerprints (LFP), electrochemoluminescence (ECL), luminol, chemical adhe-
sives (substances), gender determination, age determination, information of fingerprints, forensic
science.
1 Introduction
1.1 Background
The use of forensic dactyloscopy for suspect indentification is as old as criminalistic itself.
References to this can be seen in ”System und Praxis der Daktyloskopie” from Heindl.
Also, Heindl clearly describes the historical development of dactyloscopy, which has al-
ready undergone several innovations in the course of its development [He22]. Today, fin-
gerprints are even considered more valuable evidence than deoxyribonucleic acid (DNA)
1University of Applied Sciences, Computer-and Biosciences, Technikumpl. 17, 09648 Mittweida, Deutschland,
Email: {firstname.name}@hs-mittweida.de
2Fraunhofer-Institut f¨
ur Verfahrenstechnik und Verpackung IVV, Institutsteil Verarbeitungstechnik, Heidel-
berger Straße 20, 01189 Dresden, Deutschland, Email: {firstname.name}@ivv-dd.fraunhofer.de
3Fraunhofer-Institut f¨
ur Sichere Informationstechnology SIT, Rheinstrasse 75, 64295 Darmstadt, Deutschland
2 T. Bergmann, S. Gottschall , E. Fuchs, O. Berlipp, D. Labudde
[HS07]. A new approach for the visualization of latent fingerprints is the use of elec-
trochemical luminescence (ECL) reactions. In ECL reactions the luminescence is gener-
ated electrochemically by applying an electrical potential e.g. to a luminol, ruthenium, or
rubrene solution. The resulting intermediates are subjected to an immense exergonic reac-
tion in order to reach an energetically higher state. In the further course of the process, the
relaxation leads to a transition to the energetically lower state, whereby the energy differ-
ence can be observed in the form of light. ECL reactions are already proven in analytical
applications because they are highly sensitive and can be used selectively by applying
a potential [GA13, FBK09, Va16, PS74]. For example, Beresford et al. describe visual-
ization by spatially selective deposition of an electrochromic polymer (polyaniline). The
electrochemical process is inhibited by the fingerprint and a negative image is created. The
advantages of their method is an increase in contrast by varying the applied electric po-
tential. Also, the electrochromic coating results in a longevity of the evidence [BH10]. In
the work of Jasuja et al. an aqueous electrolyte solution was used, which made it possible
to visualize latent fingerprints on deformed surfaces (aluminum foil) [Ja15]. Additional
examples are provided in the review‘s of Su et al. [Su16] and Yamashita et al. [YF11].
The work of Xu et al. shows that the combination of electrochemistry and forensic dacty-
loscopy has a considerable advantage. For example, explosive residues can be detected
[Ad11, LZJ06]. Due to the difference in brightness on the electrode caused by this reac-
tion and the fingerprint residue lying thereon and blocking the electron exchange a contrast
is generated which results in high-resolution images of the fingerprint.
Fig. 1: [a] Experimental setup with recommended chemicals published by Xu et al.; Reproduced by
our research group [b] high-contrast and high-resolution images of fingerprints based on the method
described by Xu et al.
Furthermore, electrochemical scanning methods are used to detect spatial differences in
the electrochemical reactivity and surface exit work of fingerprints. It is of special interest
to reach a resolution which makes pores visible as demonstrated by Xu et al. [Xu12]. We
could reproduce this high quality images using their recommended chemicals (tris(2,2’-
bipyridyl) ruthenium(II) ([Ru(bpy)3]2+) and tri-n-propylamine (TPrA) as illustrated in
Fig. 1. All minutiae (islands, inclusions, branches, bridges, etc.), even sweat pores within
the papillary ridges, are clearly visible. However the high price and poor availability of the
used ruthenium-solution leads to the demand for alternatives. The use of luminol to make
blood evidence visible with the help of a catalyst has been a common practice since 1937
[Sp37]. It is also used in immunoassays as a part of an antibody reaction [Ji13]. Due to
Visualization of latent fingerprints through ECL reaction 3
its wide usage in forensics using luminol for ECL-reactions is a cost efficient and obvious
approach. Therefore the presented research will focus on the development of luminol as
an alternative chemical for the ECL-aided visualization of fingerprints.
2 Materials and Methods
Development of an electrochemical analysis cell An analysis cell with a two electrode
system was constructed out of available stainless steel components, a petri dish and com-
pleted by a purpose built part 3 D printed from polyethylene terephthalate (PETG). A plate
electrically connected through a screw forming the base electrode was placed at the bot-
tom of the petri dish. The insulation distance of 1 mm between both electrodes was realized
with the 3 D printed part enclosing the second electrode as well as restricting the cells ac-
tive area to a circular diameter of 28 mm while reducing the necessary liquid volume to
2.4 ml. Fig. 2 illustrates the construction of the cell.
Fig. 2: [a] Overview of the components, [b] construction sketch cross-section and [c] structured anal-
ysis cell. The construction as seen in the picture consists of a petri-Dish, they contain two electrodes
(stainless steel -plate, -nut), separated by a plastic insulator.
Both electrodes were connected using crocodile clip equipped wires. While the wire to
the stainless steel nut was constantly connected to a laboratory power supply the second
electrode was switched by plugging and unplugging it. The influence of ambient light
fluctuation was eliminated by conducting all experiments in a darkroom and using a cam-
era for observation. Additionally, two ultra violet (UV) lamps [see Tab.1] illuminating the
fingerprints at 45 degree angle were positioned left and right of the camera.
Materials Manufacturer
Laboratory power supply unit: ”PPS-11360” VOLTCRAFT
Fine balance: ”New Classic MF” METTLER TOLEDO
Camera: ”VCXU-51C” BAUMER
Lens: TV ZOOM Lens S6x11 11.5-69mm SPACECOM
Drying cabinet ”VC 0020” V¨
otsch Industrietechnik
UV lamp: Synergy 21 LED Prometheus UV V2 ALLNET GmbH
Tab. 1: laboratory equipment
4 T. Bergmann, S. Gottschall , E. Fuchs, O. Berlipp, D. Labudde
Luminol Solution In the experiments undiluted (0.025 mol/ l) and diluted luminol solu-
tion was tested. For the preparation of the solution, 0.44 g luminol was dissolved in 3 ml
hydrogen peroxide (NaOH) with a purity of 50 % [Ea11]. Subsequently, 97 ml deionized
water was added. For the further experiments, this luminol solution was used as the basis
for the undiluted version or, with the addition of 100ml deionized water, for the diluted
version.
Preparation of the fingerprints For the following experiments the fingerprints (thumb
and index finger) of a single person were used. Before the application of the fingerprints,
the person was instructed to wash their hands with soap and then rinse them with lukewarm
water. Drying was done by air. For the transfer of eccrine fingerprints, powder-free rubber
gloves were worn for 10 minutes to stimulate sweat production. To get sebum with finger-
prints, the person touched the forehead, the lateral nostrils, and the areas behind the ears
with their fingers. Finally, to produce vaseline-containing fingerprints, contact was made
with commercially available vaseline, which was wiped off on external surfaces before the
fingerprint was transferred. All images of fingerprints listed in chapter 3 were transferred
to a stainless steel plate, which then was included in the electrochemical analysis cell. For
the transfer, the contact time was about one minute.
Fingerprint visualization The analysis cell was aligned so that the fingerprint was in
the centre of the stainless steel nut and a reference picture was taken under UV exposure.
Then 5 ml of the luminol solution was added to the fingerprint. Furthermore 0.25 ml of
the hydrogen peroxide solution was added and a current of 2 amperes (A) was applied
and a picture was taken. The voltage was between 8V and 10 V in all experiments. This
resulted in emission of light that occured everywhere in the solution, except at the adhesion
(fingerprint) itself.
3 Results
In the experiments light emission could be observed everywhere in the solution except for
the adhesion or the sebaceous fingerprint. The intensity of the emitted light shortly peaked
at the start of each experiment when the power supply was connected. The ECL reaction
resulted in a useful contrast only when the fingerprint was placed onto the anode.
Fig. 3 shows a comparison of the visibility of the characteristics of differently aged se-
baceous fingerprints. In Fig. 3 [a] a partial impression of the fresh fingerprint with char-
acteristic values and optical anomaly (scar) is shown. In Fig 3 [b] a 16 h aged fingerprint
is visible, including more detailed anatomic features. The Comparison of the visibility of
the anatomic features of the fresh and the aged sebaceous fingerprint revealed that the
aged fingerprint created better optical results. The picture in Fig. 3 [a] was taken several
seconds delayed to the application of the electrical current resulting in a vivid reaction
growing form the outside inwards.
Visualization of latent fingerprints through ECL reaction 5
Fig. 3: Images created with ECL of two sebaceous fingerprints [a] fresh and [b] aged
As with Xu et al., it could be experimentally confirmed in Fig. 4 [a], [b] that vaseline ad-
hesion can also block signals. Details of the anatomic features were not visible, only the
outline of the fingerprint. All areas of the fingerprint coated with vaseline showed a specific
reaction (a bubble-like pattern). Fig. 4 demonstrates the results of fingerprints covered with
vaseline. The experiment was carried out as described in chapter 2. The undiluted luminol
solution [see 2 Luminol Solution] was used for this purpose.
Fig. 4: Images created with ECL of fingerprints with vaseline adhesion.
The visualization of the eccrine fingerprints was not possible with our method. In subse-
quent processes, it is necessary to either pre-treat the eccrine impression fingerprint or to
implement a different methodology in the prototype specifically for this evidence.
4 Discussion and future work
The observed peak in intensity right at the start of each experiment was caused by the
output capacity of the used power supply creating a short burst in current. Utilizing this
effect in the millisecond range requires an optimal timing between applying the current
and image acquisition.
In summary, the visualization of sebaceous fingerprints and fingerprints with adhesions
was possible with the presented method. The vaseline adhesion complicated the visibility
6 T. Bergmann, S. Gottschall , E. Fuchs, O. Berlipp, D. Labudde
of anatomic features and also caused a bubble-like pattern which could be specific for this
kind of adhesion and should be further investigated. The solubility of eccrine fingerprints
prevented a successful visualization. It can be assumed that fresh fingerprints are better
soluble in water than old ones. The ridges of the old trace are optically smaller than by the
fresh one. As a result, it’s easier to see the anatomic features. Our presented method there-
fore works better with traces that already dried up. Furthermore, fingerprints with other
adhesions are to be investigated, preferably with criminally relevant background. Subse-
quent image processing may be one way to improve the results.
It should also be noted that there is a need to add hydrogen peroxide to the luminol solu-
tion, as it acts as a catalyst, even though it increases the formation of bubbles and should
therefore be kept to a minimum.
The current approach is limited to smooth conductive surfaces. In future work transfering
fingerprints from various surfaces to the ECL analysis cell should be tested. Future work
should focus on the visualization of fingerprint adhesions. Forensic relevant information
like gender and age can be determined from the ratio of different amino acids and fatty
acids. Those adhesions can also serve as a hint for the usage of drugs or fire accelerators
[AIA12, Du17, Gi16]. In summary, it is desirable to gain more information of a fingerprint
than the anatomic features. ECL approves to be a good approach to reach this goal. Those
information can be used to increase the succes rate of identification. The linkage between
dactyloscopy and ECL can change forensic casework in terms of duration and quality, but
needs further scientific analysis to develop its full potential.
5 Acknowledgement
This projekt was founded by the ”Bundesministerium f¨
ur Wirtschaft und Technologie
(BMWi) - F¨
ordermodul Koperationsprojekte (KF)” within the framework of the ”Zen-
trales Innovationsprogramm Mittelstand” (ZIM).
We would like to thank all participants of the project partners of IVV Fraunhofer Dresden
and the company Helling GmbH as well as the students Maria Izaber for their assistance.
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... These developed solutions culminated in the creation of an initial integrated solution comprising a modified camera and electrolytic cell, capable of implementing the desired functionalities. The high-contrast, high-resolution photos generated through this method could already be examined in the prototype using suitable image processing and evaluation programs (Figure 4) [14]. ...
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System und Praxis der Daktyloskopie und der sonstigen technischen Methoden der Kriminalpolizei
  • Robert Heindl
Heindl, Robert: System und Praxis der Daktyloskopie und der sonstigen technischen Methoden der Kriminalpolizei. De Gruyter, Berlin and Boston, 1922.