Rapid Imaging of Latent Fingerprints Using Biocompatible Fluorescent

Article pubs.acs.org/Langmuir

Rapid Imaging of Latent Fingerprints Using Biocompatible Fluorescent Silica Nanoparticles Young-Jae Kim,† Hak-Sung Jung,*,‡ Joohyun Lim,§ Seung-Jin Ryu,∥ and Jin-Kyu Lee*,§ †

Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 08826, South Korea ‡ Laboratory of Molecular Biophysics, National Heart, Lung, and Blood Institute, National Institutes of Health, 50 South Drive, Building 50, Room 3517, Bethesda, Maryland 20892, United States § Department of Chemistry, Seoul National University, Seoul 08826, South Korea ∥ Forensic Chemistry Laboratory, National Forensic Service, Seoul 08036, South Korea S Supporting Information *

ABSTRACT: Fluorescent silica nanoparticles (FSNPs) are synthesized through the Stöber method by incorporating silane-modified organic dye molecules. The modified fluorescent organic dye molecule is able to be prepared by allylation and hydrosilylation reactions. The optical properties of as-prepared FSNPs are shown the similar optical properties of PR254A (allylated Pigment Red 254) and have outstanding photostability. The polyvinylpyrrolidone (PVP) is introduced onto the surface of FSNP to enhance the binding affinity of PVP-coated FSNP for latent fingerprints (LFPs) detection. The simple preparation and easy control of surface properties of FSNPs show potential as a fluorescent labeling material for enhanced latent fingerprint detection on hydrophilic and hydrophobic substrates in forensic science for individual identification.

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photochemical stability, and high fluorescence intensity. Although the use of fluorescent nanomaterials, including quantum dots, carbon dots, and upconversion nanoparticles, for improving the detection limit of LFPs has been attempted, there are still concerns regarding their low detection efficiency, complicated process involved, photobleaching, and toxicity.16−21 In order to overcome the aforementioned limitations, hybrid organic/inorganic nanoparticles containing organic dyes and amorphous silica have been proposed.22−24 As a matrix material, silica provides physicochemical stability, protecting the encapsulated dye from external perturbations. Moreover, silica is biocompatible and has an easily functionalized surface in itself.24−26 In particular, interactions between silica nanoparticles and LFPs can be enhanced by surface modification of the former, leading to improved detection. In this regard, the use of silica nanomaterials with organic ligands for LFP detection has been demonstrated.27 However, there is still an information deficiency on the correlation between surface modification of silica nanoparticles and the efficiency of LFP development. Therefore, surface modification with suitable

INTRODUCTION Fingerprints have long been utilized as powerful physical evidence, providing additional donor information such as gender, presence of human metabolites, and evidence of contact with explosives or substances of abuse.1−4 In most cases, however, latent fingerprints (LFPs) are not easily detected because of their poor optical contrast when viewed with the naked eye. Therefore, physical or chemical treatment of LFPs is needed to enable their detection. In the early days, methods of LFP detection included treatment with ninhydrin solution and iodine/benzoflavone spray.5,6 Thereafter, various methods, such as powder dusting, metal deposition, cyanoacrylate/iodine fuming, and fluorescence staining, have been developed for detecting and visualizing LFPs.7−12 Among these methods, powder dusting has been widely used as an extremely simple and effective method for LFP detection on diverse surfaces,13,14 employing luminescent, metallic, and magnetic materials. Even though this method is effective for the development of LFPs under some prevalent conditions, it is still subject to several limitations such as difficulty of application on some surfaces, low contrast, low selectivity, high background interference, and toxicity.7,15,16 Therefore, the aforementioned problems of the powder-dusting method have to be addressed. With this in mind, fluorescent nanomaterials are attractive alternatives owing to their unique physical and chemical properties such as ease of preparation, small particle size, good © XXXX American Chemical Society

Received: May 24, 2016 Revised: July 19, 2016

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Scheme 1. Schematic Illustrations of (a) Synthesis of PR254@SiO2, (b) Surface Modification of PR254@SiO2 with PVP, and (c) Latent Fingerprints Development Using PR254@SiO2@PVPs

Figure 1. TEM images and size distributions of (a, c) PR254@SiO2 and (b, d) PR254@SiO2@PVPs.

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Figure 2. (a) TGA result of SiO2, PR254@SiO2, and PR254@SiO2@PVPs and (b) FT-IR comparison of PVP, PR254@SiO2, and PR254@SiO2@ PVPs.

because of the presence of a highly polar amide group in the pyrrolidone ring and nonpolar methylene and methine groups in the ring and along its backbone. Therefore, PVP-coated PR254@SiO2 (PR254@SiO2 @PVPs) is expected to enhance adhesion to diverse surfaces. The size of PR254@SiO2 and PVP-modified PR254@SiO2 was precisely characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements (Figure 1). TEM images of PR254@SiO2 and PR254@ SiO2@PVP did not show any significant size and shape alterations after surface modification. Moreover, the TEM images clearly showed that both PR254@SiO2 and PR254@ SiO2@PVPs were well dispersed. Size measurements by DLS show an increase of the PR254@SiO2@PVPs hydrodynamic radius in ethanol from 94.6 to 95.2 nm; the size obtained by DLS was usually larger than that obtained by TEM because of surrounding solvent molecules and the swelling of surface molecules.33 Thermogravimetric analysis (TGA) results show that the additional 2% weight loss detected for PR254@SiO2 could be attributed to the thermal degradation of organic fluorescent dye in silica matrices, and the extra 5.6% weight loss of PR254@SiO2@PVPs could be ascribed to the thermal degradation of PVP on the surface of PR254@SiO2 (Figure 2a). In the formed PR254@SiO 2 , the relative amount of incorporated fluorescent organic dye in relation to TEOS was calculated to be 0.29 mol % based on TGA results, which coincided with the corresponding mixing proportions (Materials and Methods). Figure 2b shows FT-IR spectra of PVP, PR254@SiO2, and PR254@SiO2@PVPs. In the case of PVP, the IR spectrum clearly shows characteristic absorption bands for the CO groups of pure PVP at 1670 cm−1. Meanwhile, the IR spectrum of PR254@SiO2 shows a band at 1631 cm−1, assigned to the scissor bending vibration of molecular water.34 The PVP-coated PR254@SiO2 show an adsorption band at 1654 cm−1 due to an overlap between the scissor bending vibration of molecular water and the absorption bands of PVP CO groups (shifted owing to interaction with the OH groups on the surface of silica nanoparticles). In addition, the surface charge of PVP-coated PR254@SiO2 decreased to −29.5 mV (compared to −40.5 mV for PR254@SiO2), attributed to the presence of PVP molecules on the surface of PR254@SiO2 (Table 1).

organic molecules is needed to improve the nanoparticle−LFP interaction. In this study, we report polyvinylpyrrolidone (PVP)-coated fluorescent silica nanoparticles (FSNPs) as a powder-dusting material for effective LFP detection, obtained by incorporating a silane-modified organic dye into the silica shell using the Stöber method. The chemical binding of the organic dye to the dense silica matrix resulted in stable optical properties, with no photodegradation observed under irradiation by UV light. In order to enhance the LFP detection limit, FSNPs were coated with PVP to improve their binding affinity to LFPs. It is wellknown that PVP has been used as adhesive layer or binder in pharmaceutical industry because of its binding affinity to polar and nonpolar molecules contributed to intrinsic amphiphilic property of PVP.28−30 Thus, compared to bare FSNPs, their PVP-modified counterparts showed much higher contrast LFP images on glass and polystyrene (representing hydrophilic and hydrophobic substrates) because of their high binding affinity. LFP detection using PVP-coated FSNPs as fluorescent labeling markers can be well-defined in terms of finger ridge details without background staining, resulting in a good definition of enhanced detection.

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RESULTS AND DISCUSSION Fluorescent silica nanoparticles were synthesized using the Stöber method.31 In order to incorporate the organic dye into the silica matrix, silane groups were grafted onto Pigment Red 254 (PR254) by allylation and subsequent hydrosilylation in the presence of a Pt catalyst (platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane) (Figure 1S). The silane-modified fluorescent organic dye was successfully encapsulated into silica nanoparticles by co-condensation with tetraethyl orthosilicate (TEOS) in ethanolic solution containing NH4OH to yield PR254@SiO2 (Scheme 1a). Specific synthetic procedures for the fluorescent organic dye and PR254@SiO2 are described in the Materials and Methods section. In order to enhance the adhesion of PR254@SiO2 to latent fingerprints, the surface of the as-prepared PR254@SiO2 was modified with PVP, as illustrated in Scheme 1b. Adsorption of PVP was achieved by hydrogen bonding between the carbonyl groups of the pyrrolidone ring and the hydroxyl groups on the surface of [email protected] Moreover, PVP plays a crucial role in adhesive interactions with the fingerprint residue, being amphiphilic C

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Langmuir Table 1. Hydrodynamic Size and Zeta-Potentials of PR254@ SiO2 and PR254@SiO2@PVPs PR254@SiO2 PR254@SiO2@PVPs a

sizea (nm)

potential (mV)

94.6 95.2

−40.5 ± 1.05 −29.5 ± 1.07

Size was determined by DLS.

Figures 3a,b show the optical properties of fluorescent organic dye molecules, PR254@SiO2, and PR254@SiO2@ PVPs. The primary absorption bands of PR254A, PR254@ SiO2, and PR254@SiO2@PVPs are at 469 nm. The photoluminescence maxima of PR254@SiO2 and PR254@SiO2@ PVPs were slightly red-shifted from 532 nm (for free fluorescent organic dye) to 537 nm in ethanol solution, when excited at 450 nm. The change in emission spectral properties, such as red-shifting of the emission band and bandwidth change, was due to different environments provided by the solvent and silica, including hydrogen bonding and polarity.35,36 PR254@SiO2 was exposed to UV light under a tungsten halogen lamp to verify their photostability (Figure 4). The photoluminescence of PR254A in EtOH dramatically decreased after 30 h of UV light exposure, being almost completely quenched. This means that the organic fluorescent dye was discolored due to photodegradation. However, in the case of PR254@SiO2, the photoluminescence intensity almost retained its initial level and slightly increased. The encapsulated fluorescent dye cannot be easily removed by external perturbations, such as extraction with a large amount of solvent or applying sonication, as it is covalently bound to the silica matrix. In addition, because of the minimized movement of dye molecules incorporated in the silica matrix, they are not easily photobleached, as reported in the literature.24,37 The slight increase in photoluminescence may be result from the condensation of the remaining silanol groups in the silica matrix during the photostability test.23 More rigid structure could make dye molecule more fixed in the silica matrix, which reduce a free rotation and the nonradiative decay of excited electron.38−40 This optical stability can be beneficial for latent fingerprint detection using PR254@SiO2. Based on their high brightness, PR254@SiO2 and PR254@ SiO2@PVPs were evaluated in LFP detection on glass and polystyrene surfaces, chosen to represent hydrophilic and hydrophobic substrates, respectively. Sebaceous fingerprints from a 30-year-old male were deposited on each substrate. After

Figure 4. Photostability of PR254A and PR254@SiO2 as a function of light exposure time in EtOH.

applying PR254@SiO2 and PR254@SiO2@PVPs on the substrates, the excess of each powder was removed by air blower for 90 s. After that, the specimen was excited with a 365 nm light source, and images were recorded with a camera. Figure 5 shows fluorescence images of LFPs developed by PR254@SiO2 and PR254@SiO2@PVPs. PR254@SiO2@PVPs show much better and efficient imaging quality for both hydrophilic and hydrophobic substrates, while PR254@SiO2 display poor resolution because of nonspecific bonding or meager adhesion. This difference demonstrates the important role of PVP on the surface of PR254@SiO2 for well-defined LFP detection, providing clear evidence that the interactions of PVP-coated PR254@SiO2 with the fingerprint residues are determined by their surface properties. Generally, the main component of the fingerprint is water when fingerprint is deposited on a surface. After the water evaporate from the deposit, the fingerprint residue has hundreds of compounds such as salt, amino acids, proteins, nonpolar lipids, and diverse metabolite.41 Meanwhile, the PVP is known as having a high binding affinity for water and several molecules because the highly polar amide groups in PVP forms hydrogen bonds.29 In addition, PVP has ability to bind with protein by hydrophobic interaction contributed to nonpolar methylene and methine group in the backbone.30 Therefore, amphiphilic property of PVP leads increase in the adhesive interaction between

Figure 3. (a) UV−vis absorption and (b) photoluminescence spectra of PR254A, PR254@SiO2, and PR254@SiO2@PVPs in EtOH. D

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Figure 6. Fluorescence images of latent fingerprints developed using PR254@SiO2@PVPs on polystyrene. Fluorescence images of latent fingerprints show specific details including short ridge 1, enclosure 2, bifurcation 3, and termination 4.

istics of the patterns, including short ridge, enclosure, bifurcation, and termination. As fingerprint minutiae are generally stable and robust to impression conditions, these detailed features identified in the fingerprint form the basis of personal identification. Furthermore, it has been reported that the fluorescent dye incorporated silica nanoparticle exhibited nontoxicity and biocompatibility.22,43,44 Meanwhile, PVP has been also used in wide range of application such as pharmaceutical products, cosmetics, foods, and adhesive due to its excellent chemical stability, low toxicity, and biocompatibility.45 In the case of PVP-coated SiO2, they exhibited nontoxic effect even at high concentration.46 For this reason, PVP-coated PR254@SiO2 is expected to be biocompatible and nontoxic. Therefore, we suggest that the PR254@SiO2@PVPs has great potential as regards use as fluorescent labeling maker for fingerprint detection, considering safety of the examiner.

Figure 5. Fluorescence images of latent fingerprints developed using (a) PR254@SiO2 on glass, (b) PR254@SiO2@PVPs on glass, (c) PR254@SiO2 on polystyrene, and (d) PR254@SiO2@PVPs on polystyrene.

PR254@SiO2@PVPs and the composition of latent fingerprint residue compared that of PR254@SiO2. The improvement of the interaction can provide high sensitivity and reliability in latent fingerprint imaging. Fingerprint features can usually be grouped into three levels.42 Level 1 features are described by fingerprint ridge flow and general morphological information such as ridge orientation field, ridge pattern types, and singular points. Traditionally, level 1 features are useful for fingerprint type classification and indexing, but their characteristics are not adequate for accurate comparative analysis. Level 2 features provide pattern matching followed by the detection of individual fingerprint ridges and fingerprint ridge events such as minutiae. These features are not only discriminative but also stable and robust to fingerprint imprint conditions. Level 3 details are defined as all attributes of a ridge, including shape, width, pores, curvature, and other permanent minutiae such as dots. Figure 6 shows a well-defined ridge flow, ridge orientation field, and ridge pattern types (level 1) visualized on a polystyrene substrate by PR254@SiO2@PVPs. Moreover, several magnified images of this fingerprint are shown on the right side of Figure 6, clearly demonstrating level 2 character-

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CONCLUSION The silane-modified fluorescent organic dye, obtained by consecutive allylation and hydrosilylation reactions, was successfully incorporated into silica nanoparticles using the Stöber method. As the derivatized fluorescent dye was covalently bound to silica nanoparticles, they showed superior photostability. The surface of the obtained PR254@SiO2 could be easily modified with PVP (using hydrogen bonding between the carbonyl groups of pyrrolidone ring and hydroxyl groups on the surface of PR254@SiO2) to improve LFP affinity. The high brightness, large Stokes shift, and excellent surface properties of PR254@SiO2@PVPs resulted in greater sensitivity, selectivity, and contrast of LFP detection compared to PR254@SiO2. The E

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samples were photographed under a UV (∼365 nm) lamp illumination.

PVP-coated PR254@SiO2 are thus promising candidates for potential applications in forensic sciences owing to their outstanding performance based on biocompatibility, optical properties, and ease of surface treatment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01977. Reaction scheme for the silane-modified organic dye; 1H NMR spectra of PR254A and PR254H; 13C NMR spectra of PR254A (PDF)

MATERIALS AND METHODS

Pigment Red 254 and TEOS (tetraethyl orthosilicate) were purchased from TCI. Ethanol (EtOH) was purchased from J.T. Baker. Allyl bromide, potassium carbonate (K2CO3), chloroform, polyvinylpyrrolidone (PVP), trimethoxysilane (HSi(OMe)3), a solution of the platinum(0)−1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex (Pt(dvs)), and N,N-dimethylformamide (DMF) were purchased from Aldrich. CDCl3 was purchased from Cambridge Isotope Laboratories for use as an NMR solvent. Dichloromethane (CH2Cl2), anhydrous magnesium sulfate (MgSO4), methanol, and ammonium hydroxide solution (NH4OH) were purchased from Samchun Chemical Co. All organic solvents were used without any further purification. Synthesis of Allylated Pigment Red 254 (PR254A). PR254A was prepared using a modification of the previously reported method.47 Pigment red 254 (86 mg, 0.24 mmol) and K2CO3 (350 mg, 2.5 mmol) were dissolved in DMF (8 mL). After heating to 130 °C under a N2 atmosphere, a solution of allyl bromide (0.23 mL, 2.8 mmol) in DMF (2 mL) was dropwise added, followed by stirring for 5 h at 130 °C. After cooling to room temperature, the mixture was extracted with CH2Cl2 and washed twice with brine and water. The organic layer was dried over MgSO4, filtered, concentrated using a rotary evaporator, and dried in vacuo. Purification by column chromatography (SiO2, eluent: CH2Cl2) yielded the product as an orange powder (58 mg, 56%). Synthesis of Hydrosilylated Pigment Red 254A (PR254H). PR254A (35 mg, 0.08 mmol) was added to anhydrous CHCl3 (5 mL), followed by the addition of HSi(OMe)3 (0.04 mL, 0.32 mmol) and a catalytic amount of Pt(dvs). The mixture was stirred at 60 °C for 12 h. After cooling to room temperature, the mixture was filtered through a Celite pad. The solvent and excess HSi(OMe)3 were removed in vacuo. PR254H was obtained as orange sticky oil (46 mg, 85%) and was immediately used for the next process after confirmation of the disappearance of allyl groups by NMR (Figure S2). Synthesis of Fluorescent Silica Nanoparticles (PR254@SiO2). The trimethoxysilane-modified fluorescent organic dye (6 mg, 0.17 mM) was dissolved in EtOH (45 mL), followed by the sequential addition of TEOS (1.08 mL, 0.097 M), water (1.6 mL), and ammonium hydroxide (1.6 mL, 0.23 M). The mixture was stirred at 400 rpm for 12 h at room temperature. The resulting dispersion was centrifuged for 20 min at 20 000 rpm, and the precipitate was redispersed in EtOH. This procedure was repeated more than three times. The dispersion was further centrifuged at 3800 rpm for 10 min, and the supernatant was collected and used for the next process. The PR254@SiO2 powder, meanwhile, was obtained by removing residual EtOH from the obtained pellet. Surface Modification of Fluorescent Silica Nanoparticles with PVP (PR254@SiO2@PVPs). 29K PVP (50 mg) was dissolved in a solution of PR254@SiO2 (50 mg of PR254@SiO2 in 5 mL of EtOH), and the mixture was stirred for 12 h at room temperature and then centrifuged for 20 min at 20 000 rpm. The precipitate was redispersed in EtOH, and the dispersion was centrifuged again. The precipitate was dried in vacuo to obtain the PR254@SiO2@PVPs powder. Development of Latent Fingerprints. Glass and polystyrene were selected as representative hydrophilic and hydrophobic substrates, respectively, for detection of LFPs. All fingerprints were collected from the same 30-year-old donor. To develop the LFPs, PR254@SiO2 and PR254@SiO2@PVPs powders were carefully introduced onto the substrates. The excess of each powder was removed by air blower for 90 s. The mild air flow could primarily remove nonspecific interaction or weak interaction between latent fingerprint and each powder. Therefore, this process should facilitate more selective and sensitive visualization of latent fingerprint. The final

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-K.L.). *E-mail: [email protected] (H.-S.J.). Author Contributions

Y.-J.K. and H.-S.J. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the availability of facilities of the Materials Chemistry Laboratory at Seoul National University. This study was supported by a grant of the Korea Health Technology R&D project, Ministry of Health & Welfare, Republic of Korea (HI15C0979) allocated to H.-S. Jung.

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ABBREVIATIONS FSNPs, fluorescent silica nanoparticles; FSNPs@PVP, PVPcoated fluorescent silica nanoparticles; PVP, polyvinylpyrrolidone; LFPs, latent fingerprints; TEOS, tetraethyl orthosilicate; PR254A, allylated Pigment Red 254; PR254H, hydrosilylated Pigment Red 254A.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b01977 Langmuir XXXX, XXX, XXX−XXX