Recognition of Latent Fingerprints and Ink-Free Printing Derived from

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Functional Nanostructured Materials (including low-D carbon)

Recognition of Latent Fingerprints and Ink-Free Printing Derived from Interfacial Segregation of Carbon Dots Cai-Feng Wang, Rui Cheng, Wen-Qing Ji, Kangzhe Ma, Luting Ling, and Su Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13545 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Materials & Interfaces

Recognition of Latent Fingerprints and Ink-Free Printing Derived from Interfacial Segregation of Carbon Dots Cai-Feng Wang, Rui Cheng, Wen-Qing Ji, Kangzhe Ma, Luting Ling, and Su Chen* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, and Jiangsu Key Laboratory of Fine Chemicals and Functional Polymer Materials, Nanjing Tech University, Nanjing 210009, China KEYWORDS: carbon dots, fluorescence, fingerprint recognition, ink-free printing, interfacial segregation ABSTRACT: Carbon dots (CDs) have attracted increasing interest in recent years owing to their desirable properties. Despite the availability of diverse elaborate CDs, the function and application of CDs are far to be fully exploited. Here, biomass-derived carbon dots dispersed in a polymer matrix are found to behave as ink-free patterned substrates, which are demonstrated to be useful for nondestructive collection and recognition of latent fingerprints (LFPs), as well as printing. The coating of CDs/polyvinyl alcohol (PVA) solution on a LFP, yields a flexible transparent film; a stable fluorescent fingerprint with clear ridge details enabling personal identification is formed on this film. Encouragingly, this method can be applied to nondestructively lift and recognize long-timely exposed LFPs from various surfaces. The

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mechanism for LFP collection and visualization is proposed, which should be ascribed to the interfacial segregation of CDs in polymer matrix during the film forming process. This mechanism is further validated by and utilized for the applications of CDs/polymer composites in relief printing, intaglio printing and micro-trace transferring. INTRODUCTION Carbon dots (CDs, also known as C-dots), as a kind of carbon-based nanomaterials, show favorable attributes including unique tunable photoluminescence (PL), low toxicity, high watersolubility, good biocompatibility, and low-cost preparation.1-4 Such nanomaterials have received increasing attention since their first discovery in 2004,5 and stand out as a potential candidate for bioimaging,6-9 optoelectronic devices,10-15 sensing,16-19 and catalysis.20-23 Particularly, significant progress has been made in fabricating CDs, to discover various precursors for CDs (e.g. graphite,24 citric acid or its salt,8, leaves16,

31),

13, 17, 25-27

phenylenediamines,28-29 eggs,30 glass and plant

develop diverse synthetic routes (e.g. electrochemical,24,

32

microwave,33 and

hydrothermal synthesis,25, 28-29 plasma treatments,30 and pyrolysis method10, 26), and even realize the scaling-up production of CDs.34-36 Great advances in the availability of elaborate CDs evoke the development of CDs for commercial use, however, which has yet to be achieved. The huge impact achievements on CDs have not been revealed owing to the unexplored wide practical use of CDs in some important industrial aspects. Therefore, it is worth discovering an available route, allowing CDs to show extensive commercial applications. In view of this point, we demonstrate herein a further benefit of CDs unexploited before, that is, CDs/polymer composites can act as ink-free patterned substrate films useful for picking-up and recognition of latent fingerprints (LFPs), and also for printing industry. Fingerprint recognition has been widely applied in many domains such as personal identification, access


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control and forensic investigation.37 The pattern formed by the ridges and furrows of a human finger is unique to each individual; however, its contact impression, namely latent fingerprint (LFP), is invisible to naked eyes. To visualize LFPs, various elegant physical, chemical, spectroscopic, and combined techniques have been developed.37-40 Among those reported, several efforts suggest that CDs could be low-toxicity materials promising for fingerprinting ink and fingerprint recovery.41-47 In this respect, however, issues of easily destructive fingerprint patterns, and significant diminishing of fluorescence for CD-based fingerprint dusting are ongoing problems. The exploration of facile approaches for LFP recognition continues to be an important scientific endeavor. For the application of CD composites as ink-free patterned substrates in printing industry, it still hasn’t been investigated, even though some fluorescent patterns have been realized with use of CDs as ink via handwriting, inkjet printing, or silk-screen printing by our groups30-31 and other researchers.9, 17, 41, 48 Here we develop biomass-derived CDs dispersed in a polymer matrix as ink-free patterned substrates useful for LFP recognition and printing (Figure 1a). The CDs are generated from an abundant nontoxic intestine material as new affordable green carbon source in a one-step heating route. The synthesis process is simple, low-cost, and environmentally friendly. The as-prepared CDs, without need of surface treatment, show good amphiphilicity with excellent solubility in various aqueous and organic solvents, meaningful for further applications. Such robust fluorescent nanomaterials can be well dispersed in polyvinyl alcohol (PVA) to form transparent composite films as ink-free substrates for patterns. These composite films can be directly applied to nondestructively collect LFPs on various surfaces, and then visualize them under UV light (Figure 1b). The fluorescent fingerprints developed on the composite films are stable and clear,


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Figure 1. a) Schematic synthesis of CDs from small intestine of pig and preparation of CDs/PVA solutions for ink-free patterned substrate film. b) Sketch process of LFP collection and recognition by means of CDs/PVA film. c) Illustration of the applications of CDs/PVA composites in relief printing, intaglio printing and silk screen printing. enabling personal identification. Comparing to traditional methods, the outstanding feature of this new detection method is that the materials of the film are easily available and nontoxic, and have stable color-tunable properties convenient for further identification analysis, whilst possessing robust nondestructive LFP collection and recognition functions. This method can be applied to nondestructively lift and recognize LFPs from various surfaces, and to repeatedly lift a LFP that has been left on a surface for a long time. Moreover, we propose the visualization mechanism of LFPs, interfacial segregation of CDs in polymer matrix during the film forming


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process, and further apply the principle to printing industry (Figure 1c). Interestingly, we can easily transfer relief patterns or groove images from various surfaces onto the CDs/PVA composite films in an ink-free way, useful for relief printing and intaglio printing. Also, the CDs/PVA composites can be used as fluorescent ink for inkjet or silk-screen printing. Therefore, we present an available platform that enables CDs to be included in various applications such as fingerprint identification, robust fluorescent printing, anti-counterfeiting labelling, or micro-trace transferring areas. RESULTS AND DISCUSSION Syntheses and Characterizations. The fluorescent CDs were fabricated from pyrolysis of the small intestine of pig under 300 °C for 2h without any surface treatment. Figure 2a shows the transmission electron microscope (TEM) image of CDs. The as-prepared CDs are spherical and well dispersed, with a mean particle diameter of 3.36 nm (Figure S1). High-resolution TEM (HRTEM) image suggests that CDs have lattice fringes with an interplanar spacing of 0.21 nm, corresponding to the (100) diffraction facet of graphite carbon (inset in Figure 2a).9, 30 Powder Xray diffraction (XRD) pattern displays an obvious broad peak around 2 = 25o (Figure S2), indicating interlayer spacing (002) of 0.356 nm. The increase in interlayer spacing (compared to 0.335 nm for that in graphite) should be due to steric effect of functional groups of CDs.12, 49-50 The overall XRD pattern reveals partial graphitization of CDs.49-51 The Raman spectrum shows two peaks at around 1564 and 1349 cm1 (Figure S3), attributed to the G band and D band,10, 12, 30, 35

respectively, indicating the presence of both sp2- and sp3-hybridized carbon atoms in CDs.

To further analyze the chemical compositions and structures of CDs, elemental analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy were also performed. Elemental analysis reveals the composition of C (65.17%), H (5.29%), N


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(13.28%) and O (16.26%, calcd.) for CDs. The full spectrum of XPS shows three typical peaks, which are attributed to C 1s (285 eV), N 1s (400 eV), and O 1s (532 eV) signals, respectively (Figure S4a). Other two small peaks at 152 and 100 eV, should be assigned to Zn and Si elements coming from the trace elements in the biomass material. The high-resolution C 1s band can be divided into three peaks centered at 284.6, 286.0 and 288.4 eV, respectively, corresponding to CC/C=C, CN/CO and C=O, respectively (Figure S4b).16, 30-31 In the N 1s spectra, three peaks located at 398.5, 399.3, and 400.4 eV indicate the presence of pyridine-like CN, amino N, and N(C)3, respectively (Figure S4c).29, 52 Besides, the O 1s band consists of two peaks at 531.6 and 533.1 eV, assigned to C=O and CO, respectively.29, 53 The evidences for surface functional groups of CDs mentioned above were further confirmed by FT-IR spectroscopy (Figure S5). The above TEM, XRD, Raman, XPS, and FT-IR spectroscopy results demonstrate the carbonization of intestine biomass material into CDs with good surface functionalization, as schematically illustrated in Figure S6. The abundant functional groups on surfaces of CDs enable CDs to be well dispersed in water and most organic solvents, and also affect the optical properties of CDs. Optical Properties. The optical properties of CDs were investigated. By varying calcination temperature (250, 300, 350 and 400 °C, respectively), the optimal pyrolysis temperature of 300 °C for CDs to obtain highest PL intensity was confirmed (Figure S7). The as-prepared CDs show UV-vis absorption at around 325 nm, and exhibit photoluminescence (PL) emission centred at 450 nm under UV-light excitation (370 nm) (Figure 2b). Thus, the aqueous solution of CDs is colorless under daylight but displays bright blue fluorescence under irradiation with a 365 nm UV-light lamp (Inset in Figure 2b). Furthermore, the CDs behave typical excitation-wavelengthdependent PL emission. As shown in Figure 2c, as the excitation wavelength


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Figure 2. a) TEM image of CDs. Inset: HRTEM image of a single dot. b) UV-Vis absorption and PL emission spectra of aqueous solution of CDs. Inset: photographs of aqueous solution of CDs taken under daylight (left) and under irradiation with a 365 nm UV light (right). c) Normalized PL emission spectra for aqueous solution of CDs obtained at different excitation wavelengths starting from 340 nm to 480 nm in 20 nm increments. d) Time-resolved fluorescence decay curve of CDs. The data were fitted by biexponential decay function. increases from 340 to 480 nm, the PL emission peak red-shifts from 425 to 540 nm. No obvious variation in PL intensity was observed for aqueous solution of CDs under UV irradiation (1 W, 365 nm) for 30 h, suggesting high photostability of CDs (Figure S8). Besides, time-resolved fluorescence decay curve indicates CDs have lifetime of  = 3.96 ns (Figure 2d). We also investigated the PL intensity of CDs dispersed in different solvents, including toluene, ethanol,


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N, N-dimethylformamide (DMF), and water (Figure S9). The PL shows enhancement in the intensity without peak shift for CDs in the solvent with higher polarity, and the CDs have highest PL intensity in water. We believe the abundant functional groups on CD surfaces maintain good solubility for CDs in various solvents; the CDs show better solubility in polar solvents and hence give stronger PL emission. Collection and Recognition of Latent Fingerprints. With merit of good solubility and fluorescence, CDs were well dispersed in polymer matrix to provide CDs/polymer composites useful for recognition of latent fingerprints (LFPs). The schematic procedure for LFP collection and visualization is presented in Figure 1b, and Figure S10 shows the real operation procedure for the nondestructive collection of a LFP on a glass slide by CDs/PVA composites. We dropped a droplet of CDs/PVA aqueous solution onto a LFP-deposited glass slide. A free-standing flexible transparent film formed after solvent evaporation, which was then easily peeled from the glass slide, impressed with distinct ridge and furrow details of the LFP (Figure 3a). In this process, thanks to the water insolubility of lipid residues in fingerprints, the LFP pattern deposited on substrate wouldn’t be damaged upon the addition of CDs/PVA aqueous solution, so that fingerprint could be preserved intact and picked up nondestructively by the CDs/PVA film. Subsequently, such film was put under a UV light, and a well-developed fingerprint with clear ridge structure could be observed (Figure 3b). This fingerprint collected on the CDs/PVA film was scrutinized by scanning electron microscopy (SEM) to provide its micromorphology, from which we can see that high-resolved ridge and furrow details, including ridge ending and bifurcation, are well preserved on the film (Figure 3c), which are meaningful for personal identification. PL measurement suggests that the PL intensity of the ridge area is twice higher than that of the furrow area (Figure 3d). The difference in PL intensity between the ridge and


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furrow areas results in the fluorescent fingerprint pattern (Figure 3b), enabling identification of the individual. It is worth noting that the polymer matrix used here is not limited to PVA; other hydrophilic polymers with good film-forming performance, which can form free-standing transparent films, are also applicable, for instance, waterborne polyurethane (Figure S11). Recently, it has been reported that CDs prepared from such as p-phenylenediamine, citric acid or citrate, could be used as fluorescent dusting or solution for LFP enhancement.41-47 Particularly, Xiong’s

group

sprayed

the

hydrochloric

acid

aqueous

solution

of

p-

phenylenediamine/phosphorus acid-derived CDs to LFPs on solid substrates, to obtain clear fluorescent fingerprints stable for 1 day.45 However, despite great efforts, issues remain in this area, such as fluorescence quenching of CDs and easily destructive fingerprint patterns. And the nondestructive collection of LFPs has not been demonstrated previously by CDs materials. In this case, biomass-derived CDs dispersed in PVA are employed to lift LFPs on various surfaces, through that LFPs are easily developed onto a portable flexible transparent film to show clear fluorescent ridge details, which is meaningful for identifying LFPs having strong background interference issues and for further analysis. Moreover, the fluorescent fingerprints developed on the composite films are very stable that they could survive upon exposure in atmosphere for 2.5 years (Figure S12). The universal application of CDs/polymer composites for practical detection of LFPs was assessed. One of the main challenges in recovering fingerprints lies in identifying LFPs on diverse surfaces. Using the procedure (Figure S10) described above, we tested the applicability of CDs/PVA composites for identification of LFPs on different surfaces. As shown in Figure 3e3h, all fingerprints on CDs/PVA films, transferred from polypropylene film, aluminum sheet, stainless steel, and carnelian, are well resolved and clearly visualized under UV light. We noted


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Figure 3. a) Photograph of a CDs/PVA film picking up a LFP on a glass slide. b) Photograph of the collected fingerprint on the CDs/PVA film taken under irradiation with a 365 nm UV light. c) SEM image of a random zone of the fingerprint shown in b). d) PL emission spectra of furrow zone (black, weak) and ridge zone (red, strong) of the fingerprint under excitation at 400 nm. Fluorescence images of fingerprints collected from different surfaces of e) polypropylene surface, f) aluminum sheet, g) stain steel, and h) carnelian, and collected from a glass slide where a LFP exposed prior in air atmosphere for i) 30 days, and j) 60 days. Fluorescence microscopy images of fingerprints developed on PVA/CDs film from glass slide under different excitation wavelengths: k) 360 nm, l) 430 nm, and m) 530 nm.


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that this drop-casting technique is not suitable to detect LFPs on surface with high waterabsorbing property such as paper. Another challenge in this regime is to recognize LFPs that have been left on a surface for a long time, since the volatile components in LFPs would be gone after long-term exposure, making many chemical detection methods invalid. In this case, we employed CDs/PVA composites to collect a LFP that was deposited on a glass slide and prior to expose in air for 30 days (Figure 3i), and repeated the collection procedure on the same LFP after 60 days (Figure 3j). Clear and fadeless fingerprints were obtained on both CDs/PVA films. The results suggest that this method could be applied to detect LFPs left over from the past, and the success in repeated collection of fingerprint reveal the collecting operation wouldn’t damage the original LFPs. Furthermore, we found the fingerprint developed by the CDs/PVA film could not be expunged by external force. In addition, thanks to the excitation-dependent PL characteristic of CDs, the developed fingerprints show color-tunability. As shown in Figure 3k-3j, when irradiated by lights of 360, 430 and 530 nm, respectively, the fingerprint shows blue, green, and red colors, respectively. Therefore, the fingerprint recovery strategy developed here has advantages that LFPs on various surfaces, even left over from the past, could be easily and repeatedly collected, and then identified as stable multicolor fluorescent fingerprints. Mechanism Study and Ink-Free Printing. Significantly, the mechanism for the visualization of LFPs was revealed and then applied to printing field. The success in repeated collection of clear fingerprint structure on one long-timely exposed LFP (Figure 3i-3j), implies that, the enhanced visualization of LFPs on CDs/PVA films might be not attributed to chemical reaction between the LFPs and CDs/PVA composites, but caused by the spatial distribution of CDs in the polymer film formed during the LFP collecting process. To testify this proposal, we built a magnified model analogous to the structure of a fingerprint on a substrate, as shown in Figure 4a


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and 4b. We made a relief template, where the relief parts are corresponding to the ridges of fingerprint. Figure 4c shows the relief printing process for Chinese words of “printing” in a clerical script style by using CDs/PVA composites as the ink-free patterned substrate, which is similar to the procedure for LFP recognition (Figure 1b). An aqueous solution of CDs/PVA composites was coated onto the relief template, followed by the evaporation of solvents, to yield a flexible transparent film which was then easily peeled from the template; fluorescent words were successfully displayed on the film, where brighter fluorescence was observed in the edge areas of the words. During the evaporation process, coffee-ring effect,45, 54-55 caused by outward capillary flow, induces the migration of the CDs to the edges of the relief parts/fingerprint ridges (Figure 4d). Also, nanoparticles tend to migrate to the film-substrate interface owing to an entropic “depletion attraction” in the nanoparticle/polymer composite system.56-59 Therefore, as the CDs/PVA film forms over the fingerprint or relief template, the segregation of CDs to the interfaces between PVA film and ridges/relief parts occurs. Meanwhile, the molecular interactions such as hydrogen bonding between PVA and surface functional groups of CDs circumvent CD aggregation and fluorescence quenching. Hence, relative CD enrichment and stronger PL intensity could be observed for the interfacial area in the film (Figure 3d, 4e), enabling visualization of LFPs (Figure 3b) and relief patterns (Figure 4c, right). Based on this visualization mechanism, we further realized relief printing at large scale, for instance, to develop a well-known Chinese poem on the CDs/PVA film, “prelude to water melody”, wrote by a poet Dongpo Su in the Song dynasty in China (1076 A.D.) (Figure 4f). Similarly, relief patterns could also be transferred and visualized through this relief printing technique.


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Figure 4. a) Schematic illustration of CDs/PVA composite film-forming on a) a fingerprint on a substrate, and b) a relief template. c) Relief printing process for Chinese words of “printing” in a clerical script style by using CDs/PVA composites as the ink-free patterned substrate. d) Scheme of CD flows within evaporating CDs/PVA aqueous solution covering the relief structure. e) PL emission spectra of light area (black, stronger) and dim area (red, weaker) of Chinese word pattern developed on the CDs/PVA composite film. f) Fluorescence image of “Prelude to water melody” Chinese poem developed on the CDs/PVA film by relief printing.


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To further validate the LFP visualization mechanism, and also thoroughly exploit its application in printing field, the interfacial segregation of CDs in polymer films was also investigated in intaglio printing which may also suit for line painting. Figure 5a shows the intaglio printing process for a picture of “Chinese pandas” with use of CDs/PVA composites as the ink-free patterned substrate. A xylograph with such specific pattern was manufactured beforehand as template, where the dark areas are the groove parts and the light areas are the smooth parts (Figure 5a, left). Then a CDs/PVA solution was coated onto the intaglio template, followed by the solvent evaporation, to form a flexible transparent film. Such film could be easily peeled from the xylograph (Figure 5a, middle), to display a well-developed profile of the panda pattern under irradiation with a UV light (Figure 5a, right). We noted that stronger PL emission in the interface between the groove parts and the smooth parts is responsible for the formation of fluorescent profile on the film (Figure 5a, right; Figure 5b), allowing the success in intaglio printing. This feature confirms the interfacial segregation of CDs in PVA film during the intaglio printing process. Furthermore, by intaglio printing, fluorescent letters “NJT” were developed on a CDs/PVA composite film (Figure 5c). Micro-IR spectroscopy was used to analyze the distribution of functional groups in the designate micro-scale region of letter “T” on the film, including –OH groups (3401cm-1), –COO groups (1643 cm-1 and 1397 cm-1, respectively) and C–O groups (1246 cm-1), through color variation that colors ranging from blue to red represent increasing intensity. We found the relatively stronger intensity of these functional groups (and hence the enriched distribution of CDs and brighter fluorescence) in the selected letter region on the film (and therefore in the micro-scratch area during film forming). Besides intaglio printing, this phenomenon also implies the promising application of CDs in micro-trace transferring area. In addition, the CDs/PVA composite solution could be used as


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fluorescent ink for inkjet printing (Figure S13) and silk-screen printing (Figure S14), to obtain fluorescent patterns useful for optoelectronic devices, anti-counterfeiting, etc.

Figure 5. a) Intaglio printing operation process for a pattern of “Chinese pandas” by using CDs/PVA composites as the ink-free patterned substrate. b) PL emission spectra of bright blue area (black line, stronger) and dark blue area (red line, weaker) in the fluorescent pattern shown in a). c) Fluorescence image of letters “NJT” on CDs/PVA film (top, left), and optical image (bottom, left) and IR images (right) of the designate region of letter “T”. CONCLUSIONS This work demonstrates a new green route to realizing fingerprint recognition and printing. Carbon dots prepared from an abundant nontoxic biomass intestine material were incorporated into polyvinyl alcohol (PVA), to form flexible and transparent films as ink-free substrates for collection and recognition of latent fingerprints, relief printing, intaglio printing and micro-trace transferring. Encouragingly, this route for fingerprint recovery has the unique advances that the materials involved are easily available and nontoxic, and it can be applied to nondestructively lift


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and recognize long-timely exposed LFPs from various surfaces. This work also indicates that the success in above-mentioned applications is benefited from the mechanism of interfacial segregation of carbon dots in polymer films. Therefore, we present a facile platform that enables carbon dots to be included in various applications such as fingerprint identification, robust inkfree printing, or micro-trace transferring areas. EXPERIMENTAL SECTION Materials. Small intestine of pig was bought from local market in Nanjing, China, and was washed with ethanol and water and then dried prior to use. Polyvinyl alcohol (PVA) was purchased from standard sources. Solvents including N, N-Dimethylformamide (DMF), ethanol and toluene were of analytical grade and purchased from standard sources. Preparation of CDs. CDs were fabricated via pyrolysis method. Small intestine was loaded into a quartz boat, then put into a tube furnace and was pyrolyzed at 250, 300, 350 and 400 oC, respectively, for 2 h at a heating rate of 5 oC min-1 under an atmosphere of purified nitrogen. After cooled to room temperature, the dark black products were milled into powders. Subsequently, 0.1 g of such powder was dispersed in water with ultrasonic for 60 min. Then, the CDs were collected by centrifuging the suspension at 10000 rpm for 10 min in order to remove black deposit. The transparent brown supernatant was filtrated with a filtration membrane (0.2 m) three times to remove large particle and other impurities. Fabrication of Aqueous Solution CDs/PVA Composites. 20 g of PVA solid and 100 g of deionized water were added into a three-necked, round-bottomed flask with a stirrer. The solution was heated to 98 oC under stirring for 2 h. 10 g of CDs aqueous solution (CDs 0.01 g/mL) was then added into the above solution. After string for 30 min, the aqueous solution CDs/PVA composites was obtained.


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CDs/PVA Composites as Ink-Free Patterned Substrates. An aqueous solution of CDs/PVA composites was drop- casted onto a surface deposited with a latent fingerprints (LFP) or pattern, followed by the evaporation of solvents, to yield a flexible transparent film. Such film was then peeled from the surface, and a stable fluorescent fingerprint or pattern was displayed on this film. Characterization. Transmission electron microscopy (TEM) images of the CDs were obtained by using a JEOL JEM-2100 electron microscope. Morphologies of the fingerprint on CDs/PVA film were measured by scanning electron microscope (SEM) with a QUANTA 200 instrument (Philips-FEI, Holland). Photoluminescence (PL) was performed with a Varian Cary Eclipse spectrophotometer. UV-vis absorption spectra were measured on a Perkin-Elmer Lambda 900 UV-vis spectrometer. Fourier-transform infrared (FT-IR) spectra were carried out by a Nicolet 6700 FT-IR spectrometer. The Raman spectrum was measured on a Horiba HR 800 Raman system equipped with a 514.5 nm laser. The X-ray diffraction (XRD) pattern was investigated by Bruker AXS D8 ADVANCE X-ray diffractometer with Cu K radiation at a scanning speed of 60 rpm over a 2 range of 10–80o. X-ray Photoelectron Spectroscopy (XPS) was recorded on ES-CAIAB250 XPS system with Al/Ka as the source, and the energy step size was set as 0.100 eV. Fluorescence decay time was measured by using a 405 nm as excitation based on the Leica SP5 FILM system. The fluorescence microscopy images were obtained via a laser scanning confocal microscope (LSCM). IR images of the crack were performed using Thermo Scientfic Nicolet iN10 infrared microscope equipped with a liquid nitrogen cooled MCT detector (Thermo Electron Corporation, USA). ASSOCIATED CONTENT Supporting Information. Additional characterization data including size distribution, XRD pattern, Raman, XPS, FT-IR and PL spectra, PL photostability, schematic formation, operation


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procedure for nondestructive LFP collection, stability of fluorescent fingerprints, schematic illustrations of inkjet printing and silk-screen printing, and more fluorescence images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (2016YFB0401700), National Natural Science Foundation of China (21706122 and 21474052), Fund of State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201716), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Qing Lan Project. REFERENCES (1) Lim, S. Y.; Shen, W.; Gao, Z. Carbon Quantum Dots and Their Applications. Chem. Soc. Rev. 2015, 44, 362-381. (2) LeCroy, G. E.; Yang, S.-T.; Yang, F.; Liu, Y.; Fernando, K. A. S.; Bunker, C. E.; Hu, Y.; Luo, P. G.; Sun, Y.-P. Functionalized Carbon Nanoparticles: Syntheses and Applications in Optical Bioimaging and Energy Conversion. Coord. Chem. Rev. 2016, 320-321, 66-81.


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SYNOPSIS TOC.

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Recognition of Latent Fingerprints and Ink-Free Printing Derived from

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