Visualization of Sweat Fingerprints on Various Surfaces Using a

Research Article www.acsami.org

Visualization of Sweat Fingerprints on Various Surfaces Using a Conjugated Polyelectrolyte Joon-Hyun Yoon,† Young-Jae Jin,† Toshikazu Sakaguchi,‡ and Giseop Kwak*,† †

School of Applied Chemical Engineering, Major in Polymer Science and Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu 702-701, Korea ‡ Department of Materials Science and Engineering, University of Fukui, Bunkyo, Fukui 910-8507, Japan S Supporting Information *

ABSTRACT: A conformation-variable conjugated polyelectrolyte responding to oppositely charged biomolecules was examined as an imaging agent for the detection of latent fingerprints (LFPs). Sulfonated poly(diphenylacetylene) (SPDPA) produces high-resolution fluorescence (FL) LFP images by simple wetting of the target objects with the polymer solution without any additional treatment. SPDPA readily interacts with LFP sweat components (especially amino acids) via electrostatic interactions, leading to significantly enhanced FL images in a “turn-on” mode. The FL emission enhancement was examined in a model reaction between SPDPA and an amino acid standard. Visualization with SPDPA is effective on various surfaces, including both rough (paper) and smooth (glass and plastic) ones. Moreover, SPDPA readily interacts with extremely thin sweat LFPs, especially on smooth glass surfaces. KEYWORDS: conjugated polyelectrolyte, latent fingerprint, sweat, imaging probe, visualization

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INTRODUCTION Latent fingerprints (LFPs) are crucial clues in criminal investigations.1−3 In forensic science, considerable efforts have been devoted to exploring high-resolution visualization methods for the detection of LFPs, such as chemical staining, powder dusting, and spectroscopic analysis.4−7 Recently, many state-of-the-art reagents and methods have been developed, such as nanoparticle reagents,8,9 nucleic acid recognition reagents, 10−12 electrochemical/electrochemiluminescence methods,13,14 and aggregation-induced fluorescence (FL).15,16 Among them, the reactive detection method using chemical reagents has been most widely used.17−22 Some chemical reagents such as ninhydrin (NH) and 1,8-diazafluoren-9-one (DFO) have been commercialized and are widely available in spray cans and ready-to-use bottles.23 Cyanoacrylate (CA) is often fumed onto LFPs to produce highly resolved images. Despite their huge contribution in criminal investigations, these probe materials still have several drawbacks owing to their intrinsic physicochemical properties. First, their uses are limited by the properties and morphologies of the surfaces on which LFPs are present. For example, NH and DFO are effective exclusively for the detection of LFPs on the rough paper surfaces, while CA works only for LFPs on smooth surfaces such as glass, plastic, and metal. Second, the development of LFP images often takes several minutes or hours, since both NH and DFO need to be sufficiently heated to undergo a reaction with LFPs, while CA inevitably requires fuming for its deposition on the latter. Finally, fluorescent probes exhibit © XXXX American Chemical Society

superior detectability compared to colorimetric ones, but NH and CA are not fluorophores in themselves. Thus, they often require additional spraying of a luminescent stain for better LFP visualization. Electrostatic interactions can be exploited for attaching fluorescent probes to LFPs.24,25 Conjugated polyelectrolytes (CPEs), consisting of hydrophobic π-conjugated backbones and hydrophilic ionic side groups, are substitutes that might overcome the problems mentioned above. CPEs are watersoluble and interact with oppositely charged species such as surfactants, proteins, and metal ions via strong electrostatic interactions in water. These interactions induce considerable variation of the chain conformation and packing structure of CPEs.26−29 Complexes of CPEs and charged species often exhibit FL emission completely different from that of pristine CPEs.30−34 Owing to this unique optical characteristic in aqueous environments, CPEs have recently been used as key materials in the fields of bioimaging probes and biological sensors.35−39 From this viewpoint, CPEs may also be used in the detection of LFPs containing various charged biomolecules. In our previous study, sulfonated poly(diphenylacetylene) (SPDPA, Figure 1) readily interacted with oppositely charged proteins and surfactants via electrostatic interactions, leading to very unusual FL “turn-on” signals.40,41 This unusual FL Received: May 10, 2016 Accepted: August 25, 2016

A

DOI: 10.1021/acsami.6b05573 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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be 1.3 by chemical titration. NH, DFO, and CA were purchased from Sigma-Aldrich and were used without further purification. All solvents were purchased from Sigma-Aldrich or Tokyo Chemical Industry and were used without further purification. Glass slides, paper (cellulose filter paper), paper cups, and plastic (PET films) were purchased from Marienfeld, Advantec, Goodcup, and Fancylobby, respectively. Hand cream was purchased from Unilever and was used for the preparation of cream-applied latent fingerprints. Preparation of LFPs and Probe Solutions. (1) Sweat-based LFPs: One 26-year-old male participated in this study as a donor. The hands of the donor were washed with soap and successively rinsed with water, propanol, and ethanol. After drying in air, the donor wore double latex gloves to induce perspiration under a hot-air blower (∼60 °C) and then deposited his fingerprints on the substrates. (2) Sebumbased LFPs: The hands of the donor were washed with soap and successively rinsed with water, propanol, and ethanol and finally dried in air. Subsequently, the donor deposited his fingerprints on the substrates after rubbing his thumbs against an oily part of his face or neck. (3) Probe solutions: SPDPA was dissolved in propanol to a concentration of 0.50 wt %. NH was dissolved in acetone to a concentration of 0.79 wt %, while DFO was first dissolved in an acetone/acetic acid mixture and then diluted in acetone to a concentration of 0.95 wt %. LFP Detection Procedure. Approximately 0.3 mL of the probe solution was dropped on a Petri dish. The surface bearing LFPs was brought in contact with the solution for a few seconds and slowly lifted up. Paper substrates required attention, as these objects should not get wet more than necessary. In the case of glass or plastic, the residual solution on the surfaces gathered at edges and was then removed by wiping. About 0.1 g of CA was dropped onto a hot plate coated with aluminum foil. The substrate bearing LFPs was fixed on the top of a glass beaker and placed over the CA. Subsequently, the hot plate was heated to 60 °C and left for 15 min. Model Reaction. (1) Aqueous SPDPA solution: SPDPA was dissolved in water to a concentration of 1.0 wt %. (2) Aqueous amino acid standard (AAS) solution: The AAS solution was prepared as described previously.46−48 Glycine (3.71 mg), threonine (1.91 mg), alanine (2.60 mg), histidine (2.01 mg), serine (7.91 mg), and glutamic acid (2.13 mg) were dissolved in 100 mL of water to a concentration of 0.02 wt %. (3) Aqueous metal cation standard (MCS) solution: The MCS solution was prepared as described previously.46−48 NaCl (0.30 g) and KCl (0.04 g) were dissolved in 100 mL of water to a concentration of 0.14 wt %. (4) Preparation of films for FL emission comparison: The SPDPA film (∼5 μm thick) was prepared by casting 1.0 wt % aqueous SPDPA onto a glass slide. The SPDPA−AAS complex film (∼5 μm thick) was prepared by mixing 1.0 wt % aqueous SPDPA and 0.02 wt % aqueous AAS in a volume ratio of 90.9:9.1 and subsequently casting the mixture on a glass slide. The SPDPA−MCS complex film (∼5 μm thick) was similarly prepared by mixing 1.0 wt % aqueous SPDPA and 0.14 wt % aqueous MCS in a volume ratio of 90.9:9.1 and casting the mixture on a glass slide. Measurements. LFP images were recorded with a digital camera (SONY Alpha A6000) under room light (usual daylight fluorescent lamp) or UV light (hand-held UV lamp, excitation wavelength of >365 nm). FL emission spectra were recorded using a JASCO FP-6500 spectrofluorometer. Fluorescence CCD images were recorded on a Nikon Eclipse E600 fluorescence microscope equipped with a Nikon DS-Fi1 digital camera.

Figure 1. Chemical structure of SPDPA.

response was explained by the charged species acting as internal plasticizers in the complex materials, loosening the intramolecular phenyl−phenyl stack structure of SPDPA42−44 and consequently restricting vibrational relaxation,45 which leads to efficient radiative emission decay of the electronic transition (Figure 2). Based on this observation, SPDPA was expected to act as an imaging probe for the detection of LFPs.

Figure 2. Schematic diagram of the interaction between SPDPA and charged species.

Biogenic substances secreted by the human skin, such as sweat and sebum, are necessarily present in LFPs as a mixture, despite differences among individuals. Sweat mainly comprises water-soluble charged biomolecules such as proteins, amino acids, and metal ions,46−48 while sebum is an oily mixture of triglycerides, wax esters, squalene, cholesterol, and cholesterol esters.49,50 Although SPDPA is expected to preferentially interact with sweat components owing to their charge, SPDPA-assisted visualization of LFPs was initially undertaken for sebum components in a previous study.51 SPDPA nanoparticles in water were transferred to the oily components of LFPs with an aid of a surfactant, which enabled material transfer between the aqueous and oil phases, producing highly resolved FL images. However, the application of this method was limited to smooth glass surfaces and also required attention to avoid situations wherein LFPs were removed from the substrates due to the usual detergent effect of surfactants. If sweat, and not sebum, components are targeted in the visualization of LFPs, the strong electrostatic interactions of the former with SPDPA may be more useful for fingerprint detection. To pursue this idea, we examined the FL response behavior of SPDPA to sweat LFPs in comparison with NH, DFO, and CA. Herein, we describe the visualization of LFPs on various surfaces based on microscopic observations and spectroscopic analyses.

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RESULTS AND DISCUSSION LFPs were prepared separately from sweat and sebum components, as described in the Experimental Section. First, sweat-based LFPs on rough paper surfaces were visualized. SPDPA produced apparent FL images under UV light by simple short wetting of the target LFPs with the polymer solution and subsequent drying in air (Figure 3). Conversely, the two conventional chemical probes, NH and DFO, needed an additional heat treatment for better visualization, producing

EXPERIMENTAL SECTION

Materials. SPDPA was synthesized by polymerization and sulfonation of the obtained polymer according to a previously reported procedure.52 The degree of sulfonation was determined to B

DOI: 10.1021/acsami.6b05573 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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Notably, SPDPA also worked well on smooth glass and plastic (PET) surfaces to produce apparent fingerprint images (Figure 5a), whereas NH and DFO worked well on paper or

Figure 3. Sweat LFPs on paper stained using SPDPA (0.50 wt % in propanol), NH (0.79 wt % in acetone), and DFO (0.95 wt % in acetone).

fingerprint images under room light (for NH and DFO) or UV light (only for DFO) (Figure 3). In the cases of NH and DFO, visualization was attributed to the chemical reactions of these species with the amino acids and/or proteins of the LFPs to produce colored and/or fluorescent species.2 In the case of SPDPA, on the other hand, the appearance of FL images was probably due to the electrostatic interaction of anionic SPDPA with positively charged amino acids and/or proteins to enhance FL emission, as is schematically shown in Figure 2.40,41 The model reaction of SPDPA with the AAS supports this inference. When an appropriate amount of amino acids was added to aqueous SPDPA, the SPDPA−AAS complex film cast from the solution showed a significantly enhanced FL emission relative to that of the pristine SPDPA film (Figure 4). Unlike the case

Figure 5. Features of sweat LFPs on various surfaces (paper, paper cups, glass, and plastic) stained using (a) SPDPA (0.50 wt % in propanol), (b) NH (0.79 wt % in acetone), and (c) DFO (0.95 wt % in acetone) solutions and fumed with (d) CA.

paper cups (Figure 5b,c) but did not produce any images on smooth surfaces (Figure S3). The successful visualization on smooth surfaces using SPDPA could be explained by the hypothesis that complexes resulting from electrostatic interaction with sweat no longer dissolved in propanol that was used as solvent; hence, the intact complexes remained attached to the surfaces. The resolution of fingerprint images was as high as that obtained by CA fuming (Figure 5d). The failed visualization on smooth surfaces using NH and DFO is probably due to the lack of surface humidity, unlike the case of paper. Moreover, even if colored species were produced by the reaction between NH or DFO with the amino acids, they would dissolve in the organic solvent (acetone) and be removed from the surface during the wetting process. Not surprisingly, the SPDPA−AAS film prepared in the model reaction hardly dissolved in propanol. Such significant changes in solubility, accompanied by the FL response due to electrostatic interactions, make SPDPA more reliable and universal for the detection of sweat LFPs. The FL response of SPDPA to sweat LFPs was examined in more detail by varying the amounts of the sweat component in comparison with NH, DFO, and CA (Figure 6). FL images of LFPs on paper were clearly visible to the naked eye up to the third deposition, similarly to the case of DFO. Moreover, the LFPs on glass surfaces show highly resolved FL images up to the fifth deposition, as in the case of CA fuming. These results indicate that SPDPA readily interacts with extremely thin sweat LFPs, especially on smooth surfaces.

Figure 4. Model reactions: features and FL emission spectra of films cast from 1.0 wt % aqueous SPDPA, 1.0 wt % aqueous SPDPA containing an equivalent AAS, and 1.0 wt % aqueous SPDPA containing an equivalent MCS.

of AAS, SPDPA did not show any FL enhancement after its reaction with MCS, indicating FL inactivity in response to metal cations (Figure 4). Next, SPDPA was also tested for the detection of sebum LFPs in comparison with NH and DFO. However, in this case, all probes were almost FL-inactive (Figure S1), since sebum LFPs seldom contain amino acids. Therefore, SPDPA did not respond to LFPs when the hands were completely dried or covered with cream (Figure S2). Hence, the FL response of SPDPA is highly specific to amino acids, is easier to handle, and is more convenient for highresolution visualization of sweat LFPs rather than sebum ones.

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CONCLUSIONS SPDPA readily interacted with sweat components, especially charged biomolecules such as amino acids, in LFPs on rough paper surfaces, resulting in highly resolved FL images. The high-resolution visualization of LFPs was successfully achieved C

DOI: 10.1021/acsami.6b05573 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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by simple short wetting of the target objects with SPDPA solution. Unlike conventional chemical reagents, SPDPA did not require any additional treatment for high-resolution visualization. SPDPA also worked on the smooth surfaces of glass and plastic very efficiently. The basic concept for the present detection method of sweat LFPs using SPDPA is its specific FL response to charged biomolecules due to electrostatic interactions between the polymer and sweat components. If the molecular design of the LFP imaging probe is further improved based on this concept, other CPEs or charged fluorophores may also be used for the high-resolution visualization of LFPs.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05573. Characteristics of LFPs from sebum-covered hands, completely dried hands, and cream-covered hands on paper, stained using SPDPA, NH, and DFO; features of LFPs on glass and plastic, stained using NH and DFO (PDF)

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Figure 6. Features of a consecutive set of five LFPs on paper and glass stained using SPDPA (0.50 wt % in propanol), NH (0.79 wt % in acetone), and DFO (0.95 wt % in acetone) and fumed with CA.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grants funded by the Korea government (MEST) (2014R1A2A1A11052446). D

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