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Oct 23, 2015 - ABSTRACT: Optical imaging of latent fingerprints (LFPs) has been widely ... and versatile photothermal LFP imaging method based on the high.
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Highly Efficient Photothermal Semiconductor Nanocomposites for Photothermal Imaging of Latent Fingerprints Jiabin Cui,†,§ Suying Xu,†,§ Chang Guo,† Rui Jiang,† Tony D. James,‡ and Leyu Wang*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemistry, University of Bath, Bath BA2 7AY, United Kingdom

S Supporting Information *

ABSTRACT: Optical imaging of latent fingerprints (LFPs) has been widely used in forensic science and for antiterrorist applications, but it suffers from interference from autofluorescence and the substrates background color. Cu7S4 nanoparticles (NPs), with excellent photothermal properties, were synthesized using a new strategy and then fabricated into amphiphilic nanocomposites (NCs) via polymerization of allyl mercaptan coated on Cu7S4 NPs to offer good affinities toward LFPs. Here, we develop a facile and versatile photothermal LFP imaging method based on the high photothermal conversion efficiency (52.92%, 808 nm) of Cu7S4 NCs, indicating its effectiveness for imaging LFPs left on different substrates (with various background colors), which will be extremely useful for crime scene investigations. Furthermore, by fabricating [email protected] NCs, a fluorescent-photothermal dual-mode imaging strategy was used to detect trinitrotoluene (TNT) in LFPs while still maintaining a complete photothermal image of LFP. ecently, visualization and identification of latent fingerprints (LFPs) have been extensively used in forensic science and for antiterrorist applications because fingerprints are one of the important long-term markers of human identity. Additionally, the secretion of drug metabolites or explosive residues left in the fingerprints can reveal additional vital information about the behavior of a particular person.1 To date, various methods,2−5 such as electrochemical techniques,6,7 optical coherence tomography,8 mass spectrometry (MS),9−11 and infrared and Raman spectroscopic techniques12 have been successfully applied for imaging of fingerprints and the detection of illicit drugs or explosive residues left in LFPs.13,14 The main problem faced in these techniques, however, is that they are hardly taken to scenes of crime for forensic investigations. Owing to the high sensitivity and added convenience, nanoparticle (NP)-based optical imaging methods, including luminescence assays, electric field microscopy,15 confocal microscopy, and colorimetry, have attracted considerable attention for LFP imaging.16−20 However, the generality of optical imaging methods still suffers from interference from background color and autofluorescence; consequently, the imaging of LFPs can only be carried out on substrates with certain colors. Functional materials that enable the conversion of electromagnetic irradiation to heat, so-called photothermal agents, have held great promise in diverse fields such as photothermal therapy. Other than the aforementioned applications, it is of great interest to explore their potentials in the photothermal imaging field because the photothermal imaging technique is nondestructive, which also has high sensitivity as photothermal imaging responses mainly result from local temperature


© 2015 American Chemical Society

changes, showing low background noise. Very recently, on the basis of the photothermal effects of polyaniline, we successfully developed a facile NIR photothermal imaging paper sensor for the sensitive detection of TNT left on the paper.21 In addition, polypyrrole,22−24 carbon-based nanomaterials,25−27 MoS2,28 WS2,29 noble metal-based nanostructures,30−33 and copper chalcogenide nanoparticles (NPs)34−37 have been widely investigated and used as photothermal agents.28,38−42 In light of applications in imaging, the photothermal agents shall display high stability, and the photothermal performance needs to be inert to local circumstances. Compared with other counterparts, Cu2−xS(Se) NPs are considered to be the most promising candidates as they have high and stable photothermal conversion efficiency, lowtoxicity, high earth abundance, and easy preparation.39,42,43 Therefore, it is very intriguing to explore the potentiality of Cu2−xS NPs for their use in photothermal imaging. However, to the best of our knowledge, the aforementioned photothermal agents, including Cu2−xS(Se)-based nanomaterials, have not yet been reported to be used for photothermal imaging of LFPs. To date, although many Cu2−xS nanocrystals have been developed, some demonstrate weak photothermal efficiency. Developing a synthetic strategy that could afford Cu2−xS nanocrystals with increased photothermal efficacy would significantly improve their sensitivity. Additionally, the Cu2−xS nanocrystals are normally prepared in organic phase, and the hydrophobic surface makes them unsuitable for the photoReceived: September 26, 2015 Accepted: October 23, 2015 Published: October 23, 2015 11592

DOI: 10.1021/acs.analchem.5b03652 Anal. Chem. 2015, 87, 11592−11598


Analytical Chemistry

(S2CNBut2)2] was synthesized according to our previously reported method with some modifications.40,43 Briefly, 0.1 mmol of Cu(NO3)2·3H2O was dissolved in 1.0 mL of ethanol and then mixed with 43.1 mg of HS2CNBut2 by stirring and sonication for 15 min. The ethanol was evaporated, and then the product was transferred into oleylamine (1.0 mL). Thereafter, the mixture solution consisting of OAm (4.0 mL), ODE (6.0 mL), and DDT (0.08 mL) was slowly heated to 205 °C under a gentle nitrogen flow with magnetic stirring for 15 min. Then, the as-prepared Cu(S2CNBut2)2 precursor (1.0 mL) was injected into the above solution. The resulting black solution was further treated at 190 °C for 15 min before being naturally cooled to room temperature. Finally, the Cu7S4 semiconductor nanocrystals were obtained and dispersed in 2.0 mL of chloroform after centrifugation and purification. Synthesis of Hydrophobic ZnS:Mn2+ Red QDs.45 Briefly, 0.6 g of NaOH was dissolved in 5.0 mL of deionized (DI) water, and then 8.0 mL of ethanol and 10.0 mL of oleic acid were added under vigorous stirring. Thereafter, the aqueous solution of MnCl2 (0.1 mL, 1 mol/L) and ZnCl2 (1.9 mL, 1 mol/L), and 2.0 mL of Na2S solution (1.0 M), were added in sequence and stirred for another 5 min. Finally, the white colloidal solution was transferred to a 50 mL Teflon-lined autoclave and heated at 160 °C for 8 h. The autoclave was then allowed to cool to room temperature, and the luminescent quantum dots were collected via centrifugation and then washed with cyclohexane. All of the collected nanoparticles were dispersed into 2.0 mL of chloroform and kept refrigerated prior to use. Fabrication of Amphiphilic Nanocomposites. In a typical case, 9.6 mg of the as-prepared Cu7S4 nanoparticles and 4.5 mg of allyl mercaptan (AM) were mixed in 0.7 mL of chloroform and vortexed for 30 min. Then, 70 μL of AIBN (0.5 mg/mL) as photoinitiator was added. The mixed colloids were injected into an aqueous solution (10.0 mL) containing SDS (18.0 mg) under ultrasonication for 3 min in an ice bath. Afterwards, the afforded emulsions were optically excited at 365 nm for 10 min and then stirred overnight at room temperature. The amphiphilic nanocomposites were collected via centrifugation and washed with water. This purification process was repeated at least three times. The products were finally dispersed into 300 μL of ultrapure water for later use. It followed the same procedures for preparing other amphiphilic nanocomposites, only varying the amount of reagents being used. Latent Fingerprint Collection. Volunteers were asked to rub their fingers on their foreheads and then impress their fingers on chosen substrate surfaces that were precleaned with water and dried in air. The collected samples were aged for 48 h before being subjected to the incubation procedure. Fingerprints doped with TNT powder were collected as follows: different amounts of TNT powder were applied to the fingertips of volunteers and spread on half of the fingertips. Then, the volunteers were asked to impress their fingers on the given substrate surfaces. The dosage of TNT residues left on the fingerprints was further quantified via liquid chromatography. Latent Fingerprint Detection. The procedure for LFP incubation is same for each of the nanocomposites. Here, the incubation with Cu7S4 nanocomposite solution is taken as the example. To keep the Cu7S4 nanocomposite solution covering the surfaces containing the fingerprints, a hydrophobic pen was used to draw along the boundary of fingerprints beforehand.

thermal imaging of LFP. Thus, it is still a challenge to obtain Cu2−xS nanocrystals40,44 with good and stable photothermal effects as well as proper surface properties for photothermal imaging of LFPs. Here, we demonstrate a new and facile strategy for the synthesis of Cu7S4 nanocomposites (NCs) with high and stable photothermal efficacy as well as appropriate amphiphilic surfaces functionalized with amphiphilic polythioether via photoinduced thiol−ene click chemistry. On the basis of these amphiphilic Cu7S4 NCs, we developed a general and portable strategy capable of photothermal imaging of LFPs from different people and on different substrates with various background colors. Moreover, by using the fluorescentphotothermal dual modal imaging strategy based on [email protected] NCs, trinitrotoluene (TNT) residues could be detected via fluorescence quenching while the photothermal images of LFPs remain unaffected, allowing for simultaneous TNT detection and complete LFP imaging. For the first time, the photothermal imaging technique was used for LFP imaging, and the near-infared light responsive photothermal agents allow for low background noise from local circumstances, resulting in a sensitive and facile method for LFP imaging.

EXPERIMENTAL SECTION Reagents and Chemicals. Oleic acid (OA), oleylamine (OAm), and 1-octadecene (ODE) were purchased from Alfa Aesar. Sodium dodecylsulfonate (SDS), ZnCl2, MnCl2, NaOH, Na2S·9H2O, cyclohexane, chloroform, acetonitrile, methanol, thioacetamide, dodecanethiol (DDT), Cu(NO3)2·3H2O, and ethanol were supplied by Beijing Chemical Factory. 2,2′Azobis(isobutyronitrile) (AIBN) was obtained from Tianjin Chemical Factory (China). N′N′-Dibutyldithiocarbamic acid (HS2CNBut2) was supplied by Pacific Ocean United (Beijing) Petro-Chemical Company, Ltd. [email protected] quantum dots (QDs) were kindly supplied by Wuhan Jiayuan Quantum Dots Company, LTD (China). All the reagents were of analytical grade and used as received without further purification. Ultrapure Milli-Q water (Millipore) was used throughout the experiments. An Agilent 1200 liquid chromatographer (LC) was applied for the quantitation of TNT residues left on a fingerprint. Characterization. The size and morphology of the nanomaterials were characterized by the H-800 transmission electron microscope (TEM). FTIR spectra were performed on a Nexus 670 Fourier-transform infrared spectrophotometer (Nicolet, USA). X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-7000 X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å). A commercial ZF-1 UV lamp with excitation light at 254 and 365 nm was used in optical imaging of fingerprints. A digital single-lens reflex camera (Nikon, D3200, Japan) equipped with a macrolens (Nikon, AF-S VR Micro-Nikkor 105 mm f/2.8G IF-ED, Japan) was used for recording photographs. CEL-HX (100 mW) photoreaction system from Beijing Taught Jinyuan Technology Company, LTD was used to carry out the photoinduced in situ polymerization. The photothermal imaging and photothermal effect tests were carried out via a Flir A615 camera (Sweden). Absorption spectra were recorded using a UV-3600 UV−vis− NIR spectrophotometer (Shimadzu) equipped with a plotter unit. Synthesis of Hydrophobic Cu7S4 Semiconductor Nanocrystals. Before the preparation of the hydrophobic Cu7S4 semiconductor nanocrystals, the single precursor [Cu11593

DOI: 10.1021/acs.analchem.5b03652 Anal. Chem. 2015, 87, 11592−11598


Analytical Chemistry Then, 200 μL of the Cu7S4 nanocomposite colloidal solution (1.0 mg/mL) was added to the given area where LFPs were deposited. The LFPs were incubated with the solution for 3 min at room temperature in a wet chamber. Thereafter, the excess solution was carefully removed with a micropipette. Finally, the pretreated LFPs were applied for imaging to give the visible ridge patterns of the fingerprints.

conversion. A slight decrease in the photothermal efficiency was observed for amphiphillic Cu7S4 NCs, and the temperature reached 50 °C under identical conditions; however, it is still efficient enough for photothermal imaging of LFPs. The slight decrease of photothermal efficiency can be attributed to the weak heat conduction of the polymer. Inspired by the high photothermal efficacy, for the first time, we explored the potentials of Cu7S4 NCs for photothermal imaging of LFPs (Figure 1e). Prior to photothermal imaging application of these nanocomposites, the photothermal conversion efficiency was first investigated. As shown in Figure S2a, it was found that the photothermal effect was dependent on the power density being used. When the irradiation power density was 2.0 W/cm2, the final temperature of the Cu7S4 NC powder increased to 72 °C from room temperature (20 °C) within 1.5 min. For further study of the photothermal performance of the Cu7S4 NCs, the photothermal transduction efficiencies of the NCs were then measured by a modified method similar to that reported previously.46 To be specific, an 808 nm laser (1.0 W) was used to continuously illuminate the Cu7S4 NC dispersions until the temperature reaches plateau, and then the irradiation light was switched off. The real-time temperature was recorded during this process and lasts until the temperature returns to a steady state (Figure S2b).The photothermal conversion efficiency (ηT) was calculated to be 52.9% using eq 1.

RESULTS AND DISCUSSION From the transmission electron microscope (TEM) image (Figure 1a) of the newly synthesized Cu7S4 nanoparticles

ηT =

τS =

hA(Tmax − Tamb) − Q 0 I(1 − 10−Aλ) mDC D hA



where h is the heat transfer coefficient, A is the surface area of the container, and the value of hA can be obtained from eq 2. Tmax and Tamb refer to the maximum system temperature and ambient surrounding temperature, separately, where in this case, (Tmax − Tamb) is 25.3 °C according to Figure S2b. I is the laser power (in units of mW; here, I = 1000 mW), and Aλ means the absorbance of Cu7S4 NC dispersions (Aλ = 0.357) at an excitation wavelength of 808 nm. Q0 (in units of mW) is defined as the rate of heat input due to light absorption by the solvent. By means of measuring a quartz cuvette cell containing pure water, Q0 was determined to be 351.2 mW. In eq 2, τs is the sample system time constant, which could be calculated, as shown in Figure S2c; mD and CD are the mass (1.09 g) and heat capacity of deionized water, respectively. Aside from the high photothermal efficiency, the photothermal stability of these NCs is another key parameter for photothermal therapy and photothermal imaging; therefore, we also carried out photothermal stability tests. These are unlike the gold nanostructures, whose photothermal efficiency would greatly decrease under long-term irradiation because their shape and size changes with long-term irradiation.47,48 After continuous irradiation for 1 h, the shape, size, and photothermal efficiency of our Cu7S4 nanocomposites have not changed (Figure S3), suggesting very good photothermal stability, which is more suitable for long-term imaging than the organic dyes and quantum dots whose fluorescence often suffers from photobleaching. As mentioned above, the surface properties of NPs are crucial to achieve LFPs imaging. Thus, the polymer coating on the NCs was characterized using Fourier transform infrared (FTIR) spectroscopy (Figure 2a) and thermal gravimetric

Figure 1. (a) TEM images of hydrophobic Cu7S4 NPs and (b) amphiphilic Cu7S4 NCs, (c) fabrication strategy for Cu7S4 NCs, (d) photothermal effects of Cu7S4 NCs under a given power density versus the irradiation time, and (e) scheme for photothermal imaging of fingerprints. The photothermal tests were conducted on the thin film of Cu7S4 NC powder with excitation at 808 nm.

(Cu7S4 NPs), it can be seen that Cu7S4 forms monodisperse nanoplates with an average size of 9 ± 1 nm. The crystal structure of these nanoplates was further characterized using high resolution TEM and X-ray diffraction techniques (Supporting Information Figure S1). Considering that LFPs contain water, inorganic salts, and oily substances, the surfaces of the LFPs display amphiphillic properties. Proper surface modification of these inorganic NPs is required to offer appropriate affinities toward the LFPs, making them attach well to the fingerprints for LFP imaging. By using a facile photoinduced in situ polymerization strategy, hydrophobic Cu7S4 NPs functionalized with allyl mercaptan were prepared and successfully incorporated into the amphiphilic particle− polymer NCs (Figure 1c) with an average size of 61 ± 2 nm (Figure 1b). Our novel fabrication strategy allows for the preparation of amphiphilic NCs without complicated synthetic procedures and more importantly, for the most part, the photothermal effects are maintained. As shown in Figure 1d, under irradiation at 808 nm with a power density of 1.0 W/cm2, the temperature of the hydrophobic Cu7S4 NP powder reaches 57 °C within 1.5 min, demonstrating excellent photothermal effects, which could be ascribed to efficient light-to-heat 11594

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Analytical Chemistry

suggest that the appropriate surface modification of hydrophobic Cu7S4 NPs is essential to ensure successful photothermal imaging of LFPs. Additionally, the capability of this method for imaging different types of LFPs, including eccrine and sebaceous sweat fingerprints, were investigated. It was found that the as-prepared amphiphilic Cu7S4 nanocomposites displayed better affinity toward sebaceous fingerprints, as shown in Figure S9. Again, it indicated the crucial importance of the surface properties. It is also known that the LFPs would degrade along with time, and it is always challenging to visualize old LFPs. Here, we attempted to apply the proposed strategy on LFPs with differently aged times, which suggested that this method is suitable for LFPs aged for approximately 48 h (Figure S10). Once placed under 808 nm excitation, the ridge patterns of LFPs treated with Cu7S4 NCs were immediately visualized, which reflects the local temperature changes induced mainly by the photothermal effect of Cu7S4. As shown in Figure 3, clear LFP images were acquired independent of the background

Figure 2. (a) FTIR and (b) TGA characterization of Cu7S4 NCs; photothermal images of LFPs using (c) Cu7S4 NCs, (d) hydrophobic Cu7S4 NPs, and (e) hydrophilic Cu7S4 NPs as photothermal agents. All of the images were conducted under the same irradiation conditions.

analysis (TGA) (Figure 2b). Comparing the FTIR spectra of hydrophobic Cu7S4 NPs and amphiphilic Cu7S4 NCs, the main differences are located at 3200, 2915, 2846, 1620, 1460, 1380, and 1020 cm−1. The absorptions at 3200 and 1620 cm−1 result from the vibration of υ(C−H) and υ(CC), respectively, which can be attributed the unpolymerized ally mercaptan attached to the Cu7S4 NPs. The 2915, 2846, and 1460 cm−1 absorptions can be assigned to the vibration of (−CH2), and 1020 cm−1 is attributed to υ(C−S). All the results suggest the existence of abundant [-S-(CH2)3-] units in the polymer on NCs produced by thiol−ene click chemistry (Figure 1c).49,50 This polythioether shell renders the NCs amphiphilic and provides excellent affinity for LFPs.51 According to the TGA analysis, a weight loss of ∼20 wt % was observed and attributed to the polymer coating in the NCs. The polymer shell was clearly observed in the TEM image of Cu7S4 NCs when more ally mercaptan was used (Figure S4), indicating that photoinitiated polymerization is feasible. As a control, the Cu7S4 nanospheres were fabricated in the absence of allyl mercaptan by the same photoinduced polymerization strategy. However, these nanospheres flocculated and decomposed during the purification process (Figure S5), indicating that allyl mercaptan is essential for preparing amphiphilic polymer coatings. Using the same method, ZnS:Mn2+, [email protected], and [email protected] NCs were also successfully fabricated. All the hydrophobic NPs and amphiphillic NCs were characterized using TEM (Figure S6). The fluorescence of NPs was well retained within the NCs (Figure S7). Using the amphiphillic Cu7S4 NCs, a photothermal image of LFPs was readily obtained (Figure 2c). As a control, hydrophobic Cu7S4 NPs dispersed in chloroform were also used for the photothermal imaging of LFPs. However, because organic solvents such as chloroform used for dissolving the hydrophobic Cu7S4 NPs could also dissolve the oily substance presented in LFPs, the LFPs were destroyed, and the photothermal imaging was not possible (Figure 2d). Furthermore, using hydrophilic nanoparticles functionalized with polysuccinimide52 (Figure S8) resulted in a weak and blurry photothermal image, which may be attributed to a low affinity for LFPs (Figure 2e). All of these results

Figure 3. (a1−a5) Photothermal images of LFPs treated with Cu7S4 NCs under irradiation at 808 nm; (b1−b5) optical images of LFPs under daylight treated with Cu7S4 NCs; (c1−c5) red fluorescence images of LFPs treated with ZnS:Mn2+ NCs with excitation at 365 nm; (d1−d5) green fluorescence images of LFPs treated with [email protected] NCs with excitation at 365 nm. The background colors under daylight (top) and 365 nm light (bottom) (from left to right) are white, orange, red, green, and black. FLI: fluorescence imaging. 11595

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Analytical Chemistry color (Figure 3a1−a5). However, optical images of LFPs are hardly visible under daylight conditions because the background colors interfere with the successful imaging of LFPs (Figure 3b1−b5). It is noteworthy that a contrast-enhanced latent-fingerprint image was obtained with a black background because the Cu7S4 NCs display an off-white appearance (Figure 3b5). As comparisons, we also used ZnS:Mn2+ red QDs (Figure 3c1−c5) and [email protected] green QDs (Figure 3d1−d5) for LFP imaging on glass slides with different background colors. As mentioned above, the main interference faced by optical imaging of LFPs is the background color, including autofluorescence and reflection. The QD-based LFP imaging system was dramatically impaired by interference from the background color; only the substrate with black color displayed clear LFPs images (Figure 3c 5 and 3d 5). To further demonstrate the generality of this method, we performed the photothermal imaging on backgrounds with a series of different colors (Figure S11). All the results suggest that photothermal imaging is a facile and general strategy for the rapid and sensitive imaging of LFPs without interference from background colors or autofluorescence. Meanwhile, by employing the proposed photothermal imaging strategy, not only were well-resolved ridge patterns with good separation between furrows and ridges of the LFP images observed, but detailed information on a fingerprint, including bifurcation, whorl, termination, and crossover, could also be recognized in high-magnification images (Figure 4). It is

performance of optical imaging techniques. Therefore, developing and imaging LFPs that have been left on sticky tape is extremely useful. Figure 5 reveals that clear patterns of

Figure 5. (a−d) Photothermal images of LFPs on iphone, USB disk, marble, and clear sticky tape (the adhesive side was used to peel off the LFPs left on substrates that are not suitable for imaging and further used for imaging).

LFPs with enough details for personal identification have been achieved, demonstrating the versatility of the photothermal imaging protocol. Subsequently, LFPs from different volunteers were also successfully imaged (Figure S13), which further indicates the wide applicability of this method. As discussed above, the detection of trace explosives,45,53−57 such as trinitrotoluene (TNT), in LFPs is important for forensic analysis and homeland security. To date, the detection of drugs or explosives in LFPs has been achieved using luminescence; however, this results in incomplete imaging of fingerprints. Therefore, it is important to develop a novel strategy for the detection of TNT residues in LFPs while maintaining the completeness of the fingerprint image. To fulfill this goal, we fabricated bifunctional nanocomposites ([email protected]) enriched with amine groups for simultaneously imaging LFPs using the photothermal effects of Cu7S4 NCs and recognizing TNT residues in LFPs using the fluorescence quenching of [email protected] QDs. Fingers with TNT residues were impressed onto sticky tape and then incubated with the [email protected] NCs colloidal solution followed by rinsing with water. As a control, a fingerprint without TNT was visible by green fluorescence (Figure 6) upon irradiation at 365 nm, and the complete photothermal image of the LFPs was obtained (Figure 6a1). As for the fingerprint with TNT residues, the area that contained TNT (right half of the fingerprint) displays obvious fluorescence quenching compared to that without TNT residues (Figure 6b). It reasons that the acid−base pair (Meisenheimer complex) between electron-rich amine ligands on the NCs and the electron-deficient aromatic rings of TNT accounts for the fluorescence quenching (Figure S14).58 Such sensing protocols have been extensively employed in designing nitroaromatic sensing platforms. When more TNT was used, the fluorescence images of the LFPs with TNT contamination were almost completely destroyed (Figure 6c). However, the photothermal images were well-maintained independent of the

Figure 4. Photothermal images of LFPs: bifurcation (1), whorl (2), termination (3), and crossover (4).

reasoned that the specific binding interactions are due to the amphiphilic nature of the polymers coating the NCs, which facilitate the deposition along the ridges through the electrostatic and hydrophobic interactions.18 Additionally, the feasibility of the Cu7S4 NC-based photothermal imaging method for fingerprints was evaluated on different substrates as well as LFPs from different volunteers. It is of great practical significance to rapidly develop LFP images on substrate surfaces that are frequently touched in daily life. Nevertheless, most techniques for LFP imaging to date have certain requirements for the substrates, for example, electrochemical methods prefer conductive substrates and photoluminescence approaches favor substrates with a black color whose autofluorescence is relatively weak. Here, the proposed method was exploited to image LFPs left on nonporous substrate surfaces including cellphones, USB disks, marble floors, and clear sticky tape. The commonly used sticky tape gives rise to autofluorescence (Figure S12), which impairs the 11596

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Analytical Chemistry



S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b03652. HRTEM image and XRD pattern of Cu7S4 nanocomposties were described and listed in Figure S1; Figure S2 presents the photothermal conversion efficiency evaluation and photothermal stabilities; Figure S3 shows the TEM images of nanocomposites before and after irradiation with 808 nm NIR light; the surface modification investigation (Figure S4 and S5), TEM images (Figure S6) and photo images (Figure S7) after phase transformation of QDs; Cu7S4 with different surface properties (Figure S8); photothermal images of different types of LFPs, LFPs with different aging times, and LFPs with different background colors (Figure S9− 11); autofluoresence of tape (Figure S12); photothermal images of LFPs from different volunteers (Figure S13); and schematic illustration for recognizing TNT (Figure S14) (PDF)

Corresponding Author

Figure 6. (a−c) Green fluorescence and (a1−c1) photothermal imaging of LFPs with different amounts of TNT residues on the sticky tape. TNT contents: (a and a1), 0 μg; (b and b1): 5.6 μg; (c and c1): 10.7 μg. The TNT powders in the LFPs were collected by dissolving in acetonitrile and then analyzed with high performance liquid chromatography (HPLC).

*E-mail: [email protected] Author Contributions §

J.C. and S.X. contribute equally to this work.


The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21475007, 21275015 and 21505003) and the Fundamental Research Funds for the Central Universities (YS1406). We also thank the support from the “Innovation and Promotion Project of Beijing University of Chemical Technology”, the “Public Hatching Platform for Recruited Talents of Beijing University of Chemical Technology” High-Level Faculty Program of Beijing University of Chemical Technology (buctrc201507).

explosive residues (Figure 6b1 and c1). These observations demonstrate the ability of the proposed dual-modal imaging method for simultaneous detection of TNT residues and imaging of LFPs.



In summary, the Cu7S4 nanocomposite-based photothermal imaging technique has been demonstrated to be a convenient, effective tool for LFP imaging, even for LFPs left on different substrates with various background colors, which is particularly important for forensic investigations at the scene of a crime. Through the construction of bifunctional nanocomposites (Cu7S4−[email protected]) and with the assistance of high photothermal efficiency of Cu7S4 NPs, not only could the characteristic details of fingerprints be provided with high resolution, but the TNT residues left on the LFPs were also detected using the fluorescence quenching of [email protected] QDs without impairing the quality and completeness of the photothermal image of the LFPs. In addition, by variation of the target responsive moieties (e.g., aptamers and antibodies) on the nanocomposites, it is possible to recognize other exogenous substances of interest, such as illicit drugs, demonstrating the massive potential of this design principle in the construction of novel multifunctional LFP imaging nanoprobes, which will find extensive applicability for forensic investigations, biomedical diagnostics, and even photothermal therapy.


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DOI: 10.1021/acs.analchem.5b03652 Anal. Chem. 2015, 87, 11592−11598


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DOI: 10.1021/acs.analchem.5b03652 Anal. Chem. 2015, 87, 11592−11598

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