Fluorescence Development of Latent Fingerprint with Conjugated

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Fluorescence Development of Latent Fingerprint with Conjugated Polymer Nanoparticles in Aqueous Colloidal Solution Hong Chen, Rong-liang Ma, Yun Chen, and Li-Juan Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15951 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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ACS Applied Materials & Interfaces

Fluorescence Development of Latent Fingerprint with Conjugated Polymer Nanoparticles in Aqueous Colloidal Solution Hong Chena, 1, Rong-liang Mab, 1*, Yun Chena and Li-Juan Fana* a

State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials,

Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, P. R. China b

Institute of Forensic Science, Ministry of Public Security, Beijing 100038, P. R. China

1

These authors contributed equally.

ABSTRACT: Poly(p-phenylene vinylene) (PPV) nanoparticles in aqueous colloidal solution have been prepared via a modified Wessling method, with the addition of surfactant. The fluorescent colloidal solution was used as the developing solution to develop the fingerprints on different substrates. The developing process was accomplished simply by immersing the substrates into developing solution and then taking out, followed by rinsing with deionized water. The initial study about the fingerprints on the adhesive tapes showed that the developing solution is very effective in fluorescence developing on both fresh and aged visible fingerprints; and such effect was negligibly affected by treating the fingerprints with water or other organic solvents,

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whether before developing or after. Further study on latent fingerprints (LFPs) demonstrated that PPV nanoparticles in colloidal solution have high sensitivity in developing fingerprints to give very clearly fluorescent patterns. At least 6 months of storage of the colloidal solution did not reduce the developing effect; and each developing solution (3.6 mg/mL, 5.0 mL) can be used to develop at least 30 fingerprints without sacrificing the legibility of the pattern. The preliminary mechanism investigation suggested that selectivity achieved toward the ridge of the fingerprint is very likely due to the affinity between PPV molecules and oily secretions of the fingerprints. Digital magnification of the developed fingerprints provided more details about the fingerprint.

KEYWORDS: poly(p-phenylene

vinylene) nanoparticles,

aqueous

colloidal

solution,

fluorescence development, latent fingerprint, adhesive tapes

1. INTRODUCTION

Fingerprints have been widely used as direct biometric information for personal identity storage, recognition or confirmation from ancient time, since the patterns of the fingerprint ridge are unique for everyone and immutable throughout the whole life.1,

2

Nowadays, storing and

identifying fingerprints are still regarded as a powerful tool in forensic investigation, production of identity document (ID), access control for some special regions/buildings and border entry at the immigration. Especially in the forensic fields, obtaining the fingerprint information will greatly facilitate cracking a criminal case. However, it is not easy to obtain fingerprints with enough details at crime scenes, thus forensic investigators have to find the possible latent fingerprints (LFPs) and visualizing them for further analysis.3, 4 The fingerprints components are complicated,5 vary from person to person and from time to


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time; the substrates also vary with the occasion. Therefore, different developing reagents, such as aluminum powder, superglue, ninhydrin and AgNO3 in different forms, have been employed for fingerprint development by choosing appropriate methods, like powder-brushing, fuming, immersing and spraying.3, 4, 6-11 Generally, employing fluorescent reagents to develop LFP has superiority over nonfluorescent reagents in the case of colored substrates; in addition, the sensitivity of fluorescence detection usually is high even the concentration of the analytes is very low. There are some commercially available fluorescent reagents, such as Rhodamine 6G12 and 1,8-diazafluorenone (DFO);13 however, complex developing process usually is involved when using them. Moreover, some of these commercial fluorescent compounds are expensive or/and environmentally unfriendly. Great efforts have been made by scientists in seeking new fluorescent reagents or methods for developing LFP, more effectively and efficiently.14-21 Quantum dots (QDs)18, 19 and upconversion nanoparticles (UCNPs)20, 21 have received intensive attention. However, lack of selectivity toward fingerprint components makes the additional pre-modification of the surface of QDs and UCNPs necessary, which increases the cost and reduces the developing efficiency. In addition, the toxicity of QDs, the scarcity of the non-renewable sources for UCNPs always pose problems for practical applications. Recently, carbon dots (CDs)22-24 emerge as a promising material with low toxicity and large resource, but the selectivity between the fingerprint and the substrates is still a big problem. Therefore, seeking alternative cheap and environmentally friendly fluorescent reagents for fingerprint development is still of great significance. Conjugated polymers as fluorescent materials have many advantages over other small molecular fluorophores, such as facile synthesis with relatively low cost, low toxicity, strong emission and high resistance to photobleaching as well as good mechanical stability and good



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processability. Our group has successfully prepared a series of fluorescent materials based on conjugated polymers for different applications.25-30 Here we propose a strategy for developing fingerprint based on conjugated polymer nanoparticles (Figure 1). The fluorescent poly(p-phenylene vinylene) (PPV) nanoparticles form stable colloidal solution after the thermal elimination of the polymer precursor (pre-PPV) in aqueous environment with the aid of surfactant. The diluted colloidal solution can be used for fingerprint development by immersing the fingerprint sample into the solution for a while and then taking out, then rinsing the sample with deionized water to remove excess PPV nanoparticles. There are already some reports using colloidal solution of QDs or solution of dyes for fluorescence development via immersion-rinse procedure,31-33 but PPV nanoparticles in aqueous solution are advantageous over them with respect to the low toxicity and/or lost cost. Very recently, there appear several reports using conjugated polymers (or dots) as fingerprint developing reagents with excellent performance.17, 34-36

Our strategy still stands out in several aspects. First, the preparation of colloidal solution of

PPV nanoparticles should be of lower cost, since no expensive materials (such as catalyst, reactant and additive) or complicated procedures were employed. Second, our developing strategy is simpler and more cost-effective, since only immersion-rinse process was needed and the developing colloidal solution can be reused, while other reported systems used spraying34, 35 or brushing17 methods and the reagents are not reusable; sometimes additional post-treatment36 or fingerprint transfer process14 were needed to realize the visualization of the LFP.


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Figure 1. The schematic diagram for preparing PPV nanoparticles and the chemical structures employed in this study (a), and the procedures for fingerprint development with PPV nanoparticles in aqueous colloidal solution (b). In this study, PPV nanoparticles in the colloidal solution were prepared and characterized. The substrates for depositing fingerprint started with different types of adhesive tapes, since it is common to have LFPs on adhesive tapes in everyday life and at crime scenes, while developing LFPs on adhesive side of tape is very challenging.31, 32, 37, 38 The fingerprint development also extended to other substrates, such as cover glass and aluminum foil. In addition, the shelf life and service life for the PPV colloidal solution as developing agent were evaluated; and the developing mechanism was preliminarily investigated. At last, the digital magnification of fluorescence-developed fingerprints provided more details about some regional patterns. 2. EXPERIMENTAL SECTION 2.1 Materials All solvents and reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. They were of analytical grade and used as received unless otherwise noted. The carbon ink was of “Hero” brand. Methanol, acetone, and triethylamine were dried using 300-mesh molecular sieve. The PPV precursor (pre-PPV) was synthesized and then dialyzed according to our previous report.26 The pre-PPV aqueous solution after dialysis was stored in refrigerator for future use.



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Different substrates were employed for depositing fingerprint, such as adhesive tapes, cover glass and aluminum foil. The adhesive tapes include three types, the special transparent tape for fingerprint extraction (T-tape), the yellow tape for packaging (Y-tape) and the black electrical tape (B-tape). The former two types of tapes in this study were kindly provided by Institute of Forensic Science, Ministry of Public Security of the People’s Republic of China. The black electrical tape was obtained from Minnesota Mining and Manufacturing Company (3M). The cover glass and aluminum foil were obtained from Suzhou Tessler Tech. Co., Ltd. 2.2 Preparation of PPV Nanoparticles The aqueous solution of pre-PPV (5.0 mL), deionized water (15.0 mL) and sodium dodecyl sulfonate (SDS, 0.2000 g, 7.40×10-4 mol) were successively placed into a 50.0 mL two-necked round-bottomed flask, which was deoxygenated three times by vacuum-argon cycling in advance. The mixture was stirred at 50 °C until transparent solution was obtained. After adding triethylamine (TEA, 0.2 mL, 1.40×10-3 mol) into the flask, the mixture was heated to 80 °C for 3 hours with continuously stirring. Finally, the resulting PPV colloidal solution (18.0 mL) as the stock solution was transferred into a glass bottle for future use. The concentration of PPV nanoparticles in the aqueous colloidal solution (stock solution) was found to be about 18.0 mg/mL (see Supporting Information for the detailed calculation). The colloidal solution for fingerprint developing (developing solution, 3.6 mg/mL) was prepared by taking out 1.0 mL of stock solution into a suitable flask, which was diluted by adding 4.0 mL of deionized water. 2.3 Fingerprint Collecting and Development Process Regular fingerprint samples for developing were prepared with the same procedures. Before the fingerprints were deposited on the substrates (the adhesive side of the tapes, the aluminum foil, or the cover glass), the donor should wash her/his hands in water and then rub the fingers from


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oily parts of the body like retroauricular region. The collected fingerprints on the substrates were stored in desk drawer under ambient condition for further use. In some cases, special treatments may be applied on the fingerprint samples, which will be specified in detail when necessary. The fingerprints on different substrates were developed with the same procedures. First, the substrate with fingerprint was immersed directly into the developing solution for 5-10 min; then the substrate was taken out and quickly rinsed by deionized water; afterwards, it was dried at room temperature or with a dryer depending on the weather. Then the fingerprint patterns was recorded by camera under natural light or ultraviolet light (365 nm). 2.4 Instruments and Methods All the characterizations about PPV nanoparticles were carried out using the developing solution. The size distribution of the nanoparticles was characterized by dynamic light scattering (DLS) on a Zetasizer Nano ZS granulometer of Malvern Instruments Ltd. The Zeta potential of nanoparticles was also measured on the Zetasizer Nano ZS granulometer. Transmission electron microscope (TEM) images were obtained on a Tecnai G20 microscope (Tecnai G20, FEI company, US); and the sample for TEM was prepared by dropping 8 µL of developing solution onto cupper grid and dried in drying chamber under 26 °C. Fluorescence and adsorption spectra were obtained on a Shimadzu RF-5301 PC spectrofluorophotometer and a Shimadzu UV-1800 spectrophotometer, respectively. The digital photos were taken by Nikon D5100 camera. FTIR spectra were obtained with a Vertex 70 (Bruker) spectrophotometer by attenuated total reflection (ATR). 3. RESULTS AND DISCUSSION 3.1 Preparation and Characterizations of PPV Nanoparticles The modified Wessling method to synthesize PPV was carried out referring to our previous



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studies about preparing PPV microspheres or nanofibers.25-30 Before the thermal elimination to convert pre-PPV into PPV with triethylamine as the catalyst, surfactant SDS was also added in this study to prevent the resulting hydrophobic PPV aggregates from precipitating out of the aqueous environment. The size of PPV aggregates is affected by many factors, such as the type and concentration of the surfactant, the heating temperature and the stirring rate. The parameters described in the experimental section were adopted after optimization. The possible mechanism for the formation of nanoparticles may be as following description. SDS molecules formed micelles in the aqueous environment, since the concentration of SDS was much larger than the critical micelle concentration (more detailed discussion see Supporting Information). During the thermal elimination process, the hydrophilic pre-PPV chains gradually transformed into the hydrophobic PPV chains with the elimination of the positive side groups; once reaching certain extent of elimination, the polymer chains entered into the micelle; finally, the PPV nanoparticles formed after the full elimination, suspending in the water and surrounded by SDS molecules. The detailed investigation about mechanism for the formation of nanoparticles has not been carried out since it is not of our prime interest once we obtained the desirable nanoparticles. The as-prepared aqueous colloidal solution (stock solution) before using was stored in a sealed glass bottle to prevent pollution or water evaporation. The bottle was kept in dark to avoid light induced instability. The colloidal solution for fingerprint developing (developing solution) was diluted (as noted in the experimental section) from the stock solution to avoid polluting the substrates with high concentration of PPV nanoparticles. The characterizations about PPV nanoparticles were performed on the developing solution, as shown in Figure 2. In macroscopic view, the developing solution displayed greenish yellow color under natural light, and emitted strong green fluorescence under UV-light (excited at 365 nm); and clear Tyndall phenomenon


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can be observed with a laser pointer. Both TEM and DLS suggested that the size of these particles is in the range of tens of nanometers, for average. The photophysical study showed that the solution have absorption between 300-480 nm, with a peak around 330 nm and a number of shoulders; and emission among 470-600 nm with a peak around 500 nm and a shoulder around 530 nm. The multiple absorption peaks may due to the existence of nanoparticles in various sizes, which resulted in different chromophores with various bandgaps originated from different conformations and aggregations of PPV conjugated backbones; and possible multiple vibrational structures for each chromophore. The emission spectrum displayed a typical profile for PPV, very likely due to the absorbed energy from all the chromophore migrated to the part with largest conjugated system with two vibrational structures. The absorption and emission wavelengths are consistent with the direct observation of the PPV colloidal solution.

Figure 2. (a) Digital photos of the developing solution under natural light with a laser pointer light going through (left) and UV light of 365 nm (right); (b) the TEM image of PPV nanoparticles; (c) the DLS distribution of PPV nanoparticles; (d) the normalized spectra of adsorption and emission (excited at 365 nm) of the developing solution. 3.2 Fluorescence Development of Fingerprints on the Transparent Tape The composition of the fingerprint is complicated. Generally, fingers are always full of lipid



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because of sebaceous excretion and continuously touching oily parts of body like face or forehead inadvertently. Many methods for fingerprint development were based on the introduction of the affinity between the oily components from the fingerprint and the hydrophobic compounds in the developing reagent. The pre-treatment of the donors’ fingers, as described in the experimental section, further ensured the fingerprints in this study were lipid fingerprints. As the first try, the adhesive side of transparent fingerprint tape (T-tape), a kind of special tape to lift fingerprint in crime scene, was used for depositing fingerprints.

Figure 3. The digital photos of fingerprints on the adhesive side of T-tape: undeveloped fingerprints stored for 0 day (top), 26 days (middle) and 50 days (bottom) under natural light (left); the corresponding developed fingerprints under natural light (middle) and UV light (right).


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Figure 3 showed undeveloped and developed images for fresh (stored for 0 day) or aged (stored under ambient environment for 26 days or 50 days) fingerprints on the T-tape. The pattern for undeveloped fresh fingerprint (stored for 0 day) was already clear under natural light (the photos of undeveloped fingerprints under UV light were shown in Figure S1a), very likely due to the transparency of T-tape and its high extracting power for fingerprint components. The developed fingerprint has similar legibility under natural light as the undeveloped one, but the fluorescent photo under UV light apparently displayed enhanced legibility due to the greater contrast between the fluorescent ridge and non-fluorescent furrow. For the aged fingerprints, both stored for 26 days and 50 days, the advantage of fluorescence development is more obvious since the resolution/legibility of developed fluorescent image of the fingerprint pattern is much higher than the undeveloped ones. For the fingerprints stored for 26 days, it displayed dimmer fluorescence under UV light, very likely due to the loss of some of the components on the ridge with the time went on. However, the fluorescent image displayed even higher resolution, which probably due to the reduced adsorption of PPV nanoparticles and thus weakened the halo effect of the fluorescence. When the fingerprints were stored for even longer time, such as 50 days, the fingerprint pattern under natural light was much less clear, for both undeveloped and developed, due to the further loss of the fingerprint components and contamination by dust during storage; however, the developed fingerprint displayed very clear pattern under UV light. Therefore, PPV nanoparticles in colloidal solution not only can be used to develop fresh fingerprints but also aged ones. For comparison, we also developed fingerprints by carbon ink and Crystal Violet (sometimes called Gentian Violet）solution, two commonly used reagents for developing the latent fingerprint on sticky side of adhesive tape employing “immersion-rinse process”.37,


38

The developed


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fingerprints together with the one using our colloidal solution of PPV nanoparticles are shown in Figure S1b. Apparently, our developing reagent displayed much superior effect than the traditional reagents. To investigate if the external interferences will affect the developing effect or the preservation of developed fingerprints, the fingerprints were pre-treated before the development or post-treated after the development, by rinsing the sample with about 2 mL of solvent for three times; the solvents included water, ethanol, chloroform, THF, acetone and toluene. All the fingerprint photos under natural light and UV light are shown in Figure S2. For representative, the samples treated by ethanol and chloroform are selected and shown in Figure 4. Though some of fluorescent images were slightly dimmer compared with the counterparts without such treatments, all the fingerprint patterns are still clear after different treatments. Therefore, our developing solution should be still effective in developing fingerprints contaminated by water or common organic solvents; and the developed fingerprints can be preserved even being washed over organic solvents. The reason for varied degree of change in the intensity of the fluorescence will be discussed later in the section of preliminary mechanism study.

Figure 4. Representative photos under natural light (the nonfluorescent images) and UV light


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(the fluorescent images) for developed fingerprints being pre-treated before developing or post-treated after developing by different solvents.

3.3 Development of Latent Fingerprints Above fluorescence development of the fingerprints on the T-tape enhanced the legibility of the fingerprints compared with the undeveloped fingerprint collected. However, it would be of even greater significance if the latent fingerprints (LFPs) can be visualized. Therefore, further study will be carried out on two types of latent fingerprints. First, visualizing latent fingerprints on the transparent T-tape was tried. A series of fingerprints were successively deposited onto the adhesive side of T-tape with the same condition (supposed to have the same original legibility) for comparison. The fingerprint sample on the tape was adhered to the sticky side of another piece of T-tape and then peeled off; this adhesion-peeling off process was repeated for certain times. The whole series of the undeveloped and developed fingerprints are shown in Figure S3, and selected photos are shown in Figure 5. Obviously, the pattern of fingerprint became more and more indiscernible under natural light when increasing the number of times for repeating adhesion-peeling off process, since more and more fingerprint components in this process were removed, which resulted in a series of the latent fingerprints on the T-tape. After development, the indistinctness of fingerprints was similar to the undeveloped ones under natural light. However, very clear patterns were observed for all fingerprints under UV light of 365 nm, though the fluorescence seems became slightly weaker with increasing the times for repeating adhesion-peeling off process. These results indicate that, for LFPs with greatly reduced fingerprint components, excellent fluorescence developing effect can still be achieved using our developing solution.



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Figure 5. The representative photos of fingerprints obtained after the deposited fingerprints on the T-tape treated by the adhesion-peeling off process with blank T-tape for different times: the undeveloped fingerprints under natural light (left); the corresponding developed fingerprints under natural light (middle) and UV light (right).

Second, visualizing latent fingerprints on colored tapes was also tried. Another type of latent fingerprints was created by depositing fingerprint on colored adhesive tapes used in our everyday life, such as yellow packaging tape (Y-tape) and the black electrical tape (B-tape) for further study. The visual observation of undeveloped fingerprint pattern becomes impossible due to the interference from the colored substrates; after being developed by PPV nanoparticles, the fingerprint pattern was still invisible under natural light, but showed very clear pattern in fluorescent images under 365 nm UV light (Figure 6). Therefore, our developing solution was effective in developing LFP on the adhesive side of colored tapes with negligible interference on the legibility of the fluorescent pattern from the colored substrates.


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Figure 6. The digital photos of fingerprints on the adhesive side of Y-tape (top) and B-tape (bottom): undeveloped latent fingerprints under natural light (left); the corresponding developed fingerprints under natural light (middle) and UV light (right).

3.4 The Shelf Life of the Stock Solution and the Service Life of the Developing Solution To obtain an idea about how long the PPV nanoparticles can be stored once prepared, the fingerprint developments were carried out using developing solution diluted from stock solution freshly prepared and stored for 6 months, for comparison. Negligible difference in the profile was observed in the normalized emission spectra of the developing solution diluted from fresh prepared or stored stock solution (Figure S4, left). As noted in the experimental section, each developing solution was prepared by diluting the 1.0 mL of stock solution by adding 4.0 mL of deionized water. The clear and strong fluorescent patterns of the developed fingerprints under UV light (Figure S4, right) also showed that the PPV nanoparticles in colloidal solution after 6 months’ storage were still very effective to be used in fingerprint development.



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The developing solution can be reused for fingerprint development. To explore the possible times of reuse, 30 freshly deposited fingerprints on the T-tape were developed one by one using the same developing solution. The photos under UV light (365 nm) of several samples were selected as the representatives and shown in Figure S5. All the fingerprint samples displayed very clear patterns, but the fluorescence intensity became slightly weaker (or the contrast between the fluorescence part and the background became smaller) when increasing the times of reusing the developing solution. Such phenomenon is reasonable since the concentration of the effective developing agent (PPV) decreased gradually. However, even after 30 times of reuse, the fluorescent fingerprint pattern is still very discernible. Therefore, the shelf life of the stock solution should be 6 months or even longer and the service life of the developing solution should be more than 30 times. In addition, inexpensive starting materials were used for synthesizing PPV and the relatively easy work-up was required; moreover, very small volume of stock solution was needed for preparing the developing solution. Therefore, the current strategy for fingerprint development is very cost-effective. 3.5 Preliminary Study of Development Mechanism As demonstrated above, good selectivity and high sensitivity were realized in developing visible and latent fingerprints on the adhesive side of tapes. Obviously, the PPV nanoparticles seem to have good affinity toward the ridge of fingerprint, while not toward the furrow or substrate. The high sensitivity should come from the strong fluorescence possessed by PPV nanoparticles, which emitted strong fluorescence even at very low concentration (or very small amount adsorbed) and no significant fluorescence quenching occurred upon interaction with the fingerprints. Efforts have been carried out to explore the development mechanism. At first, the influence


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from the substrates can be ruled out. Though the adhesive tapes can adsorb many substances, but apparently the nanoparticles didn’t stain the substrate and the furrow when the tapes were immersed in the developing solution. Moreover, our developing strategy also worked well for other substrates, such as cover glass and aluminum foil (Figure S6), with similar developing effect. Second, the adsorption simply due to the surface and interface effect of nanomaterials cannot be the main developing mechanism, since such adsorption usually does not have high selectivity toward different surfaces (with or without the fingerprint residue). In addition, QDs and CDs, as the typical nanomaterial, were found to lack selectivity; and sometimes the selectivity was still not very good even after surface modification.23, 32, 33 Moreover, it seems that the size has no significant influence according to the literatures where particles ranges from several nanometer to several hundreds of nanometer ,20, 33 though the influence from size of the nanoparticles on the developing effect has not been explored systematically. Third, to explore if the presence of the fingerprint pattern will affect the adsorption, a special sample with the fingerprint components but no pattern was produced by depositing the fingerprint several times onto the same area and then deliberately rubbing the fingerprint area covered with a plastic film. As shown in Figure S7, fluorescence can be observed from the whole area without pattern, which suggested that the developing process, very likely, depended more on the components than the pattern of fingerprint. Therefore, our attention moved to the interaction between the chemical components of fingerprints and nanoparticles. There are several possible strong or weak interactions, such as electrostatic interaction, chemical reactions among functional groups, hydrogen bonding, π-π interaction, and hydrophobic interaction. The IR spectra for the developed fingerprints were measured, trying to find any possible changes due to the interaction, comparing with the



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undeveloped ones. However, IR spectra for undeveloped fingerprints (Figure S8) varied from time to time, very likely due to that the secretion varied from time to time and from person to person, which makes the comparison between the spectra for the developed and undeveloped ones meaningless. A closer look at the chemical structures involved in PPV nanoparticles, we can see there is PPV itself only have conjugated π structure without any other functional group, while the surfactant SDS has the alkyl chain ended with negatively charged sulfonate group. To investigate if SDS plays an important role in the developing process, such as providing electrostatic interaction or other interaction/reactions with the fingerprint components, we also prepared the nanoparticles with positive charged surfactant, cetyltrimethyl ammonium bromide (CTAB), or without any surfactant, instead of using positive charged surfactant SDS as above. The Zeta potentials of nanoparticles prepared with SDS or CTAB was measured to be -45.4 mV and 43 mV, respectively (raw data showed in Figure S9). In the case of using CTAB, colloidal solution of PPV nanoparticles can be obtained with similar stability as the PPV colloidal solution prepared with SDS; in the case of not using any surfactant, PPV nanoparticles can still be prepared but they gradually precipitated out of the aqueous environment due to the hydrophobic nature of PPV. As shown in Figure 7, similar good developing effect was also realized in the case of using CTAB; and still clear pattern with relatively weak fluorescence can be observed in the case of not using any surfactant. In the latter case without any surfactant, the weaker fluorescence should be due to that the concentration of PPV nanoparticles suspending in the water is low, even after ultrasonic treatment. Therefore, the electrostatic interaction or other interaction/reactions based on surfactants should not be the main developing mechanism in this study.


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Figure 7. The photos of developed fingerprints on the T-tape under UV light by PPV nanoparticles prepared with CTAB to replace SDS (left) and without any surfactant (right).

The focus of mechanism study was then moved to PPV itself. As seen from its structure (Figure 1) and possible components of fingerprints,5 hydrogen bonding and chemical reaction seem not to be the prime reasons. The typical characteristics for PPV molecules in this study are rigid conjugated chains with rich π electrons, hydrophobic but nonsoluble in any solvent; and such characteristics have been demonstrated in our previous studies.25, 28, 30 It was reported that there are a few fingerprint components containing aromatic ring, such as phenol, vitamins and several kinds of amino acid, but in very lower concentration. The investigation was carried out to see if the π-π interaction between fingerprint components and PPV nanoparticles play an important role for the developing. Similar developing process was applied on the tape with p-benzenediol dropped on the adhesive side, but no PPV nanoparticles was adsorbed. Therefore, π-π interaction cannot be the main reason for the selectivity. Hydrophobic interaction was employed as the developing mechanism in many literatures.35, 39,



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The fingerprints employed in this study were lipid fingerprints full of grease. The adsorption

of PPV nanoparticles without any surfactant suggested that hydrophobic interaction was possible. In addition, in the previous section using different solvents to rinse the fingerprints before developing, the fingerprints pre-treated with chloroform, THF, acetone and toluene displayed relatively weak fluorescence, and while those pre-treated with water and ethanol have less influence. Such observation can also be a support for the hydrophobic interaction between PPV nanoparticles and the oily components in the fingerprint since chloroform, THF, acetone and toluene might wash off the oily substances while water and ethanol very likely not. The good developing effect suggests that the interaction between the nanoparticles and the finger secretion is larger than that between nanoparticles and the alkyl chain end of the surfactant. Therefore, the surfactant molecules may adjust their position during developing and leave the surface of nanoparticles after developing, though at the beginning they surrounded PPV nanoparticles to make the particles stable in the colloidal solution. Moreover, the fact that toluene had similar effect as chloroform, THF and acetone on the developing effect, also suggests that the π-π interaction cannot be the dominating reason for the developing mechanism. Therefore, we propose that the selective adsorption of PPV nanoparticles onto the fingerprint ridge may be due to the hydrophobic interaction between PPV and the oily component in the finger secretion. Further study on the mechanism is still needed to give a more confirmative conclusion.

3.6 Digital Magnifications of Fluorescence-Developed Fingerprint Recognition of personal identity can be achieved by matching the stored fingerprints in the database with the fingerprint acquired with the aid of computer. Usually the whole pattern of the fingerprint will first be compared to narrow down the searching range. Then some regional


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details, if available, will be favorable for further confirmation. Such process becomes more and more convenient with the increasingly powerful computer and information analysis techniques, and sometimes combining with the manual comparison. To take advantage of the high-resolution in fingerprint images obtained employing our developing strategy, digital magnification of the developed fingerprints was carried out. As a representative, one typical fluorescence-developed fingerprint, is shown here (Figure 8 left, the same as in Figure S2, rinsed by water before developing). The magnified regional developed fingerprints are shown in Figure 8 right. Obviously, the details of friction ridge skin features can be clearly seen after magnification. These details satisfied the needs of fingerprint identification.

Figure 8. The digital photos of fluorescence-developed fingerprint and magnified regional images with some specific details. 4. CONCLUSIONS The fluorescent PPV nanoparticles in aqueous colloidal solution have been successfully prepared via a very simple strategy. DLS and TEM results demonstrated that the average size of particles was around tens of nanometers with uniform size and roundness; and the photophysical study showed that colloidal solution has desirable absorption and emission to be used as fingerprint developing reagent. The developing process was demonstrated very fast and easy to carry out.



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Study with visible fingerprints on the adhesive side of transparent fingerprint tape showed that such PPV nanoparticles in colloidal solution are effective in fluorescence development both fresh and aged fingerprints, with little influence from components reduction and dust coverage during storage. Treating the fingerprints by different solvents before or after developing had the negligible effect on the legibility of resulted fluorescent patterns of the fingerprints. Thus certain external contaminates on undeveloped fingerprint would not affect the developing effect; and the developed images can also be well-preserved against them. Two types of latent fingerprints on the adhesive tapes, invisible due to the reduced fingerprints components or interference from the colored substrates, were successfully developed with high legibility of the pattern due to the high sensitivity and strong emission of fluorescent materials. Further study showed that the colloidal solution of PPV nanoparticles can be stored over a long period and each developing solution can be reused for many times, with similar legibility achieved in fluorescent pattern of fingerprint. Such fluorescence development was also successful for the fingerprints on other substrates such as cover glass and aluminum foil, suggesting the substrates have no influence on the selectivity during developing. From other experimental results or theoretic analysis, the size effect from the nanomaterials, electro-static interaction, chemical reaction or π-π interaction between the fingerprint secretion and the PPV nanoparticles cannot be the main reason for the developing mechanism, while the hydrophobic may account for the selectivity of developing solution toward the components in the fingerprint ridge. The digital magnifications of the developed photo provided more clear details about some regional patterns of the fingerprint, which makes the identification of the personal information much easier. In all, our strategy in fingerprint developing has been demonstrated as a facile, cost-effective and environmentally friendly method. This strategy is very promising in developing fingerprints in real forensic investigation


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or fabricating ID card with personal identity information in the further. ASSOCIATED CONTENT Supporting Information The experimental section, photos of whole series of fingerprints, fluorescence and IR spectra, Zeta potential. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Fan, L. J.); [email protected] (Ma, R. L.) Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 21374071) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES (1) Polson, C. J. Finger Prints and Finger Printing. An Historical Study. J. Crim. Law Criminol. 1950, 41, 495-517. (2) Hale, A. R. Morphogenesis of Volar Skin in the Human Fetus. Am. J. Anat. 1952, 91, 147-181. (3) Champod, C.; Lennard, C.; Margot, P.; Stoilovic, M. Fingerprints and Other Ridge Skin



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Covalent

Patterning

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Visualization

of

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Fingerprints

with

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TOC


Fluorescence Development of Latent Fingerprint with Conjugated

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