Covalent Organic Polymers for Rapid Fluorescence Imaging of Latent

5 hours ago - Here, we synthesized a series of covalent organic polymers (COPs) with colorful fluorescence (from blue to green, pale yellow, bright ye...
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Covalent Organic Polymers for Rapid Fluorescence Imaging of Latent Fingerprints Meng Wang, Lin Guo, and Dapeng Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05213 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Covalent Organic Polymers for Rapid Fluorescence Imaging of Latent Fingerprints Meng Wang #, Lin Guo # and Dapeng Cao * State Key Lab of Organic-Inorganic Composites, and Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China #

Equally contributed to this work, *Corresponding Author. Email: [email protected]

ABSTRACT: Rapid, simple and highly sensitive identification of latent fingerprints (LFPs) is an important issue related to national security and recognition of potential crimes. Here, we synthesize a series of covalent organic polymers (COPs) with colorful fluorescence (from blue to green, pale yellow, bright yellow and red), and further investigate its performance for fluorescence imaging of LFPs. Results indicate that the COP materials can be used as fluorescence probes to rapidly visualize the precision substructure of LFPs within 5 s by simply spraying method, and tunable fluorescent color makes the COP probes have a high contrast and low interference for fluorescence imaging of LFPs on different substrates (including glass slides, paper, aluminum foil, plastic, ironware) in different backgrounds. We also further reveal the mechanism of COP probes for fluorescence imaging of LFPs. Importantly, the COP probes show high stability and could successfully achieve the fluorescence imaging for LFPs after aged for 45 days or washed by water. In short, this is the first report on the porous polymers for fluorescence imaging of LFPs, and expected that it can be also applied to the fluorescence imaging of other fields. KEYWORDS: Porous organic polymers; Fluorescence imaging; Latent fingerprints; Background interference; Visualization 1

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 INTRODUCTION Latent fingerprints (LFPs) are the most commonly encountered forms of fingerprints, which also brings an identification problem due to its invisibility by the naked eyes. Rapid and highly sensitive visualization of LFPs of a person is an important issue, because it is closely related to immigration inspection, forensic investigation and identification of potential crime, and even involves in national security. 1-4 Currently, it is still a big challenge for rapid and high-resolution imaging of the LFPs on different substrates in different backgrounds even the LFPs washed by water. Therefore, developing a high efficient and low background interference fluorescent probe to achieve the LFP imaging on universal substrates is highly desirable and significantly important. The basic principle for LFP imaging is to produce a strong optical contrast between the fingerprint ridge and the fingerprint deposited surfaces.5 At present, a lot of methods have been developed for the LFP imaging, such as powder dusting,6 iodine fuming,7 electrochemistry,8 vacuum metal deposition,9 mass spectrometry,10 fluorescence spectroscopy,11-12 etc. Among these methods, the powder dusting method is considered as the simplest and most commonly used one for LFP imaging at crime scenes due to its high efficiency and simplicity.13 However, it also suffers some disadvantages like low contrast, low selectivity and strong background interference. Meanwhile, it can be learned from previous report that fluorescence spectroscopy method has higher sensitivity and selectivity than other LFP identification methods, which was commonly achieved through selectively identifying some components in the fingerprint secretions.5, 14-16 Until now, a series of fluorescent-based materials have been reported for fluorescence imaging of LFPs, 2

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such as up-conversion nanoparticles,5 metal-organic frameworks,17-18 quantum dots,19 gold nanoparticles,20 carbon dots21 and semiconductor polymer dots.22 Although many researches have been performed for the LFP imaging, some problems still exist. For example, Roux et al. pointed out that background development interference is still one of the major issues of LFP visualization imaging.

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Moreover, some materials previously

reported for LFP visualization, cannot be imaged clearly due to their weak fluorescence intensity.21 In addition, many methods and materials reported in the literature require multiple combinations and modifications, which may also requires post-processing, such as high temperature heating or surfactants or solution incubating.24-25 Even in some cases, the heating and solution incubation may destroy the surface and structure of the LFPs, which limits the practical application of these methods and materials. In general, the degree of fingerprint identification can be divided into three levels. In the first-level, LFPs can be imaged but no specific parts can be used as an evidence to identify individuals. In the second-level, LFPs can be imaged more clearly than the first level, and the image may include whorl, bifurcation, termination, crossover etc., but some details are still ambiguous, like sweat pores and edge contours. In the third-level, LFPs can be imaged and all details can be identified, including sweat pores and edge contours, which is always used to the forensic identification.

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Until now, most of the LFP

identification are in the first and the second levels, which cannot be used as the evidence for identify individuals. Therefore, it needs to develop a new fluorescent material to identify LFPs in the third-level. Recently, as a new kind of polymer materials, covalent organic polymers (COPs) 3

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have been developed and widely used in gas separation and storage,28 photocatalysis,29 energy storage,30 electrochemistry catalysts,31-34 and fluorescent sensors35 due to its versatile structures, robust porosity, large specific surface area, good hydrothermal stability and abundant π conjugated system. Previous studies indicate that the functional group modification and tailoring of fluorescent intensity and color can manipulate the photoluminescence (PL) properties of the COP materials efficiently.36-38 For example, functional group modification can improve the selectivity of fluorescence probe for the analytes,36, 39-40 while high fluorescent intensity can enhance the sensitivity of COP probes. Although there are many studies on COP fluorescent probes for detections of metal ions, explosives, small organic molecules, the investigation on the COP probe for the fluorescence imaging of LFPs and cell imaging is not reported yet. Herein, we synthesize a series of COP materials (COP-101 ~ COP-105) with high thermal stability, high fluorescence intensity and tunable fluorescent color. These color-tunable COP fluorescent probes exhibit excellent performance for rapid fluorescence imaging of LFPs without any thermal/chemical treatments. We also further explore the applicability of these COP probes for fluorescence imaging of LFPs on different substrates (including glass slides, paper, aluminum foil, plastic, ironware) in different backgrounds, and the LFP imaging mechanism. Finally, some conclusions are drawn and some discussion is addressed.  EXPERIMENTAL SECTION The synthetic route of the COPs (COP-101 ~ COP-105) was shown in Scheme 1, in which the COPs were synthesized by using 1,4-Dibromonaphthalene (DN) and 4

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1,3,6,8-tetrabromopyrene (TBP) as double monomers with different ratios (1:0, 16:1, 8:1, 2:1, 0:1) by Ni-catalyzed Yamamoto-type coupling reaction, and the detailed experimental process and specific monomer usage are presented in the Supporting Information and Table S1. All of the experiments were carried out at room temperature, and the experimental conditions remain the same.

Scheme 1 Synthetic route of COP-101 ~ COP-105 by using 1,4-Dibromonaphthalene (DN) and 1,3,6,8-tetrabromopyrene (TBP) as double monomers with different ratios (1:0, 16:1, 8:1, 2:1, 0:1) by Ni-catalyzed Yamamoto-type Ullmann cross-coupling reaction.

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 RESULTS AND DISCUSSION The successful synthesis of these COPs can be confirmed by FTIR spectra (see Figure S1), because the absence of C-Br stretching peak around 500 cm-1 in the FTIR spectra marked by yellow rectangular box means that the Br functional groups in the reactants have been consumed completely by phenyl-phenyl coupling. The SEM images of these materials were shown in Figure 1, where all the five COPs are spherical particles. TGA curves of the COPs in a nitrogen atmosphere suggested that all the five COPs have no obvious weight loss before 380 °C (Figure 2a), which indicates that these COPs possess high thermal stability. The porous properties of these COPs have been characterized by N2 adsorption at T=77 K (Figure 2b ~ 2f). The BET specific surface areas of COP-101 ~ COP-105 are 55.46, 70.40, 210.87, 474.67, 1231.88 m2·g-1, respectively. Figure S2 shows the pore size distribution of the COPs, and detailed pore volume and pore size of the materials are listed in Table S2. We can see that the pore size of COPs is mainly distributed in 0 - 25 nm, indicating that these COPs are micro- and meso-porous materials.

Figure 1 SEM images of the five COP materials. (a) COP-101; (b) COP-102; (c) COP-103; (d) COP-104; (e) COP-105.

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Figure 2 (a) TGA trace of COPs; (b) ~ (f) N2 adsorption isotherms at T =77K. Black and red line represent adsorption and desorption isotherms of the COPs.

Figure 3 (a) ~ (e) are the excitation (black line) and emission (red line) spectra of COP-101, COP-102, COP-103, COP-104 and COP-105, respectively. (a') ~ (e') are the fluorescent photographs of solid powder of COP-101, COP-102, COP-103, COP-104 and COP-105 under an UV lamp, λex=365 nm. (a'') ~ (e'') are fluorescent images of LFPs after treated with COP-101, COP-102, COP-103, COP-104 and COP-105 under an UV lamp (λex=365 nm), respectively.

The solid-state photoluminescence (PL) spectra of the five COP materials and two monomers at room temperature were shown in Figure 3 a ~ e and Figure S3, respectively, and the detailed excitation and PL emission data were listed in Table S1. Compared to the 7

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monomers DN and TBP, the fluorescent intensities of all COP materials increased at different degrees. In addition, the positions of the maximum PL emission peaks of these five COP materials also exhibit apparent red-shift with different levels compared to TBP, which can also be highlighted by the normalized PL emission spectra as shown in Figure S4. These phenomenon can be attributed to the increase of the π-π conjugate systems produced by the phenyl-phenyl coupling reaction, and the stable structure of the COP materials could reduce the energy consumption during vibrational relaxation and internal conversion, which is the same as the AIE effect.41-42 And with the increase of the amount of TBP, the π-π conjugate degree of the COP materials increases, which makes that the maximum PL emission peaks of the COPs show greater red-shifts. Moreover, all the five COP materials exhibit strong solid state fluorescence with different emitting colors under an UV lamp (see Figure 3 a' ~ e'), ranging from blue (COP-101), to green (COP-102), pale yellow (COP-103), bright yellow (COP-104) and red (COP-105), which is consisted with the PL emission spectra. It is worth mentioning that both the fluorescent color and intensities of the as-synthesized COPs can be manipulated by adjusting the ratio of the two monomers. This excellent color-tunable fluorescent properties of the COP materials are a good priority for imaging of LFPs in different environments, because they can provide the high contrast and low background interference by manipulating the fluorescence color. Figure S5 shows that the PL intensity of COPs keeps no change after 40 days, indicating that the COPs possess extremely high photostability and are an excellent candidate for long-term LFPs detection. Thus, the practicability of the COP materials for the LFP imaging was investigated systematically. 8

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First, the performance of the COP materials for fluorescent imaging of LFPs was explored and evaluated by visualizing LFPs collected on the glass slide. A smooth glass slide was chosen as a representative surface for LFP imaging because the glass substances are very common encountered in daily life and criminal suspects can always be identified by fluorescence imaging of LFPs on glass substances in forensic investigations.43 As shown in Figure 3a'' ~ e'', the images of LFPs on the glass slide can be real-time visualized under an UV lamp, where λex = 365 nm, and the image ridges of LFPs exhibit high contrast between the fluorescent ridges and non-fluorescent furrows without any interference. It should be mentioned that the whole imaging process of LFPs is very fast and could be finished in 5 s. In previous study, a series of complicated post-processing operations were needed to visualize LFPs, such as heating with hot air, incubating in material solutions for a period of time, drawing fingerprint contours with a hydrophobic pen, etc,44-46 which would greatly reduce the efficiency of LFP recognition. Compared to previous methods, it is simpler and time-saving to spray COP powder to visualize LFP images because no post-processing operations are required. Of course, the entire imaging process is also more efficient and convenient, because it efficiently combines the advantages of the fluorescence imaging and powder dusting method. To highlight the efficiency of COP probes and make a comparison, we first use the two monomers to carry out the LFP imaging experiment, as shown in Figure S6. Obviously, the intrinsic properties and weak fluorescence intensity of monomers cannot make the LFP ridge pattern displayed clearly, indicating that the two monomers cannot achieve the visualization of LFPs. As expected, the COP fluorescence probes can real-time 9

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visualize the LFP images. Figure 4 shows a representative image of LFPs by spraying COP-104 powder onto the glass slide with a LFP. As shown in Figure 4b, the ridges and contours of the LFPs were displayed clearly and can be identified easily by the naked eye, while the LFPs cannot be visualized without spraying COP-104 powder (see Figure 4a). To determine the sensitivity of COP probes for LFP imaging, we also amplify the image in Figure 4b to explore the details of the LFPs in three levels. As shown in Figure 4c, the second-level details at different regions of the LFPs can be visualized clearly at a higher magnification including whorl (1), bifurcation (2), termination (3) and crossover (4). Moreover, the third-level details, namely the sweat pores, were also observed clearly along the ridges in Figure 4c, which was fairly difficult to visualize these details by many other reported methods.26-27 It is worth mentioning that all the LFP details were obtained without adding any external conditions (thermal or chemical post-treatment). Overall, the successful identification of LFPs clearly demonstrates the practicability of spraying COP materials for rapid LFP imaging, and the details make the LFPs unique and can provide valuable information for personal identification.

Figure 4 (a) and (b) are fluorescent images of the LFPs before and after spraying COP-104 under an UV lamp, where excited at 365 nm; (c) The fingerprint details of the LFPs at higher 10

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magnification: whorl (1), bifurcation (2), termination (3), and crossover (4).

To study the mechanism of fluorescence imaging of LFPs by spraying COP powder onto the substrates with a LFP, we further performed comparative experiments, fluorescence microscope imaging, SEM imaging, and selective experiment and so on. As shown in Figure S7 a ~ c, the fluorescent pictures are the LFPs with secretion, the LFPs with secretion after spraying the COP probe, and the LFPs without secretion under an UV lamp, respectively. Clearly, only the LFPs with secretion after spraying the COP can be visualized, suggesting that there exist some strong interactions between the COP probe and the secretions of fingers in the LFPs. Furthermore, the SEM images and highresolution fluorescence imaging of LFPs (see Figure 5), can confirm that the COP probes have a strong interaction with the secretions from the ridges in the LFPs rather than the furrows in the LFPs.

Figure 5 (a) SEM images of deposited LFPs after spraying the COP-104; (b) SEM images of ridges at higher magnification; (c) SEM images of furrows at higher magnification; (d) Fluorescence image of the deposited LFPs after spraying the COP-104.

As shown in Figure 5a ~ 5c, it can be seen clearly that there is material adhesion at the ridge of the fingerprint while hardly no material exists in the furrows of the fingerprint. 11

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And this also can be further evidenced by LFP image under fluorescence microscope, as shown in Figure 5d, where we can see clearly that the COP probe is attached to the fingerprint ridge shown bright green, while the furrows show a black color due to the absence of COP probe, indicating that the COP probe does have a strong interaction with the fingerprint secretion. To find out which components of secretions in fingerprints have strong interaction with the COP probe, we considered several most common components in secretions, including water, lysozyme, squalene, oleic acid, palmitate, glucose. Protein gelatin, bovine serum albumin, glycerides and wax esters were also used as component selection. Small amount of each standard substance (1 mg) in the form of aqueous solutions were placed onto the glass slides by glass capillary sample tube and left to dry naturally. Then the samples were subjected to the same visualization processes of the LFPs. As shown in Figure 6, letter “A” is clearly visible only on the squalene-containing glass slide while others cannot, suggesting that squalene in secretions is the key substance for fluorescence imaging of LFPs. We also prepared the solution containing squalene (1 µg) to examine whether it can interact with the COP probe for fluorescence imaging. As shown in Figure S8, the COP probe can still achieve the fluorescence imaging under an UV lamp even at an extremely lower concentration of squalene, which may be attributed to the porosity-induced strong affinity of COP probe towards squalene. These observations clearly illustrate that the COP probe has a strong interaction with squalene that can be used as excellent candidate for fluorescence imaging of LFPs.

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Figure 6 The fluorescent images under an UV lamp (where λex=365 nm) after depositing different substances on the glass slides. (a) ~ (j) are water, lysozyme, glycerides, oleic acid, palmitate, glucose, protein gelatin and bovine serum albumin, wax esters, squalene, respectively.

Figure 7 The fluorescent images of LFPs on paper, aluminum foil, plastic and ironware before and after spraying COP materials. 13

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To extend the practicability of the COP probe for fluorescence imaging of LFPs, we also explored the performance of COP probe for imaging of LFPs deposited on a series of different substrates, including paper, aluminum foil, plastic, ironware. Figure 7 shows the fluorescence images of LFPs on these substrates under an UV lamp by spraying the COP probe onto the substrates. All these cases shows bright fluorescent patterns and the details of fingerprint ridges could be recognized clearly with a high contrast and almost no background interference. We also demonstrated the practicability of the COP probe for fluorescence imaging of LFPs from different persons. As shown in Figure S9, the LFPs from six volunteers were displayed clearly under an UV lamp. Furthermore, we also studied the LFP imaging of COP probes in different color backgrounds (see Figure S10). The LFPs deposited on different color backgrounds can still achieve a clear fluorescence imaging owing to the adjustable color and high fluorescence intensity of COP probes, which overcomes the serious background interference issues as mentioned in previous literature and provides a great advantage for application of COP probes in the practical environments. In fact, the LFPs encountered in real situations are often aged, which had left for a long time instead of the fresh deposited. A major challenge for imaging of aged LFPs is the loss of water, chloride ions and other substances in fingerprints, which often makes imaging technologies ineffective. Thus, we also examine the performance of the COP probe for fluorescence imaging of aged LFPs which had been placed for 45 days, and Figure S11 shows the images of the aged LFPs on the glass slide. All of the aged LFPs can 14

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be displayed, and the ridges and second-level details of the fingerprint could be identified clearly without noticeable loss of resolution, indicating that not only fresh LFPs but also aged LFPs can be imaged clearly by the COP probes. In addition, LFPs in the actual scene may be also affected by natural weather. For example, LFPs after washed by rain may be incomplete, which is relatively difficult to LFP recognition. To this end, we also simulated the rainy environment by soaking the fingerprints deposited on the glass slide in a water environment and shaking for a few minutes, and then performed the fluorescence imaging of the LFPs by spraying the COP probe. Interestingly, the LFPs washed by rain can be still visualized clearly, as shown in Figure S12. This observation indicates that the COP probes possess strong anti-interference ability in the fluorescence imaging of the LFPs, which also exhibits the extensive applications of COP probes for the fluorescence imaging in the actual conditions.  CONCLUSIONS In summary, we have synthesized a series of COP materials (COP-101 ~ COP-105) with considerable stability and tunable color (ranging from blue to green, pale yellow, bright yellow and red) as effective probes for fluorescent imaging of LFPs. To our knowledge, it is the first report that successfully combines the advantages of fluorescence recognition and the powder dusting method in LFPs imaging based on COP materials. Compared with the previous reports, our method is single-step without any thermal/chemical treatment during visualizing process. Excellent LFP images were obtained by spraying COP probe within 5 s and the second/third-level substructures of the LFPs can be visualized clearly. Further studies revealed the mechanism of fluorescence 15

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imaging of LFPs, which is attributed to the strong interaction of the COP probes with squalene in the fingerprint secretions. This method developed in this work exhibited a universal applicability for visualization of LFPs on various substrates (including glass slides, paper, aluminum foil, plastic, and ironware) as well as different persons and backgrounds. Moreover, the COP probes possessed strong anti-interference ability and can clearly identify LFPs even after aged for 45 days or washed by water. This work makes a good beginning about the porous polymer probes for fluorescence imaging of LFPs, and expected that it can be also applied to the fluorescence imaging of other fields.  ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS website http://pubs.acs.org The experimental details on synthesis and characterization of COP materials, including FTIR spectra, SEM, TGA, N2 adsorption isotherms at 77 K, PL emission spectra, fluorescent image of the LFPs in different conditions.

 AUTHOR INFORMATION Corresponding Author *Email: [email protected] Meng Wang and Lin Guo equally contributed to this work. Notes The authors declare no competing financial interest. 16

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 ACKNOWLEDGEMENTS This work is supported by National Science Fund for Distinguished Young Scholars (No. 21625601) and Outstanding Talent Fund from BUCT.

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