Nanoplasmonic Imaging of Latent Fingerprints with Explosive RDX

Article pubs.acs.org/ac

Nanoplasmonic Imaging of Latent Fingerprints with Explosive RDX Residues Tianhuan Peng,† Weiwei Qin,† Kun Wang,† Jiye Shi,†,‡ Chunhai Fan,† and Di Li*,† †

Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, CAS Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China ‡ Kellogg College, Oxford University, Oxford OX2 6PN, U.K.

Downloaded by UNIV OF PENNSYLVANIA on August 26, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.analchem.5b02248

S Supporting Information *

ABSTRACT: Explosive detection is a critical element in preventing terrorist attacks, especially in crowded and influential areas. It is probably more important to establish the connection of explosive loading with a carrier’s personal identity. In the present work, we introduce fingerprinting as physical personal identification and develop a nondestructive nanoplasmonic method for the imaging of latent fingerprints. We further integrate the nanoplasmonic response of catalytic growth of Au NPs with NADH-mediated reduction of 1,3,5-trinitro-1,3,5-triazinane (RDX) for the quantitative analysis of RDX explosive residues in latent fingerprints. This generic nanoplasmonic strategy is expected to be used in forensic investigation to distinguish terrorists that carry explosives.

Homeland Security (DHS) has required collecting 10 fingerprints of all non-US citizens when they apply for visas or arrive at U.S. ports of entry.11 Therefore, the capability to provide quantitative explosive information together with fingerprint images could greatly improve the possibility to distinguish terrorists. When a finger touches a surface, eccrine sweat, together with other oily substances picked up by the finger, forms an impression of the finger’s ridge pattern. Such an impression is known as a latent fingerprint (LFP).12,13 To date, there has been an intense interest to develop enhanced LFP imaging methods with various spectroscopic and microscopic techniques,10 for example, mass spectrometry,14 fluorescence microscopy,15,16 vibrational spectroscopy (infrared17 and Raman18), and electro-chemiluminescence.19 Moreover, LFPs also carry residues of various chemicals and their metabolites present in the human body, as well as exogenously doped species. Hence, LFPs carry more biological and forensic information about a person. For this purpose, many chemical imaging techniques have been designed for visualizing LFPs and detecting chemicals at the same time.20−24 For example, Maynard et al. developed a fluorescent method for visualizing LFPs with dye-modified lysozyme aptamer.25 Yuan and coworkers reported a photoluminescence technique for the imaging of LFPs and detection of lysozyme under near-infrared light excitation using Upconversion nanoparticles.26 Su et al. reported an immunological multimetal deposition (iMMD)

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he capability to detect traces of explosives is crucial for preventing terrorist attacks; however, it is still a challenge due to their extremely low vapor pressures.1 Although many techniques, such as methods based on mass spectroscopy,2 vibrational spectroscopy (Raman3 and terahertz4), and electrochemical5−7 and fluorescence sensors,8,9 have shown their capabilities for the detection of explosives, they are not amenable to miniaturization in microsystems and lack the ability to perform automated analysis, which strongly handicaps their utility for field detection. Another issue with, probably, higher importance in explosive detection is to establish the correlation between explosive information and a carrier’s personal identity. For example, ion mobility spectrometry (IMS) devices are extensively adopted worldwide for explosives detection in airports. Security staffs use a special paper disk to wipe down an item, such as a suitcase, to capture trace explosive particles, and then they stick a special adhesive tag with the sample I.D. on the suitcase to build the connection between the sample and passenger identity. This method is effective, but it has a weakness. Terrorists could easily escape detection by simply changing the paper tag with peers. The bottleneck is lack of a connection between explosives and the carrier’s identity. In the present work, we introduced fingerprints as physical personal identification and developed a nanoplasmonic method for the detection of 1,3,5-trinitro-1,3,5-triazinane (RDX) explosive residues in fingerprints. The uniqueness and invariableness of an individual’s fingerprint has long been recognized as an important physical personal identification, and they are, hence, widely used in individual credentials and forensic investigations.10 For example, the U.S. Department of © XXXX American Chemical Society

Received: June 15, 2015 Accepted: August 20, 2015

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DOI: 10.1021/acs.analchem.5b02248 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry strategy for the rapid visualization of sweat fingerprints and detection of two secreted polypeptides.27 We have developed a nanoplasmonic method for the visualizing of LFPs and detection of contact cocaine residues using aptamer-modified Au nanoparticles (NPs) as contrast and recognition reagents.28 Herein, we expanded the idea of nanoplasmonic imaging and demonstrated its ability for the detection of RDX explosive residues in LFPs.

TEM were prepared by casting one drop of Au@Cu NPs solution onto a standard carbon-coated (200−300 Å) Formvar film on a copper grid.

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RESULTS AND DISCUSSION Inspired by NADH-reduction of RDX in degrading RDX pollutants30,31 and NADH-dependent catalytic growth of Au NPs,32−34 we are motivated to integrate the nanoplasmonic response of catalytic growth of Au NPs with NADH-reduction of RDX. The detailed principle is outlined in Figure 1. NADH


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EXPERIMENTAL SECTION Materials. Au NPs with average diameter of 50 nm were synthesized according to a seed-mediated growth method.29 HAuCl4·3H2O; NADH; picric acid; TNT; DNT; and nitrobenzene were purchased from Sigma-Aldrich. RDX was purchased from Fluka. Collecting of sebaceous LFPs. Sebaceous LFPs were collected on microscope glass slides. These slides were purified by treatment with a piranha solution (3:1 mixture of H2SO4 and H2O2) for 30 min (Caution: piranha solution should be handled with extreme care) and then rinsed thoroughly with water. Then, the cleaned glasses were blocked with casein. Prior to collect of LFPs, volunteers washed their hands carefully with soap and then dried with N2. The sebaceous LFPs were collected by gently rubbing their fingertips over their forehead and nose region and then pressing on the casein-blocked glass slides with a minimal pressure. Collecting of RDX-loaded sebaceous LFPs. After carefully washing hands, a 50 μL solution of RDX with different concentrations was drop-cast on the volunteer’s fingertips and then dried in air. The RDX-loaded sebaceous LFPs were collected with the above-mentioned method, and the RDX residues in the sebaceous LFPs were quantified. RDX was dissolved with DMSO to get a stock solution of 1 mM and then diluted with water to the desired concentration. After collecting the sebaceous LFPs loaded with RDX, 200 μL of Au-NPs was drop-caste on the glass slides and allowed to incubate for 30 min at room temperature. The Au NPscontaining glass slides were slightly washed with deionized water, dried with N2, and then treated with plasma for 10 s. These slides were incubated with NADH (0.5 mM) for 30 min and then with CuSO4 (2 mM) for another 2 h. DFM imaging and scattering spectroscopy measurements. The dark-field measurements were carried out on an inverted microscope (Olympus IX71, Japan) equipped with a dark-field condenser (0.8 < NA < 0.95) and a 4× and 60× objective lens (NA = 0.8). The sample slides were immobilized on a platform, and a 100 W halogen lamp provided a white light source to excite the Au NPs to generate plasmon resonance scattering light. The scattered light was collected with a truecolor digital camera (Olympus DP70, Japan) to generate the dark-field color images, and it was also split with a monochromator (Acton SP2300i, PI, USA) which was equipped with a grating (grating density: 300 lines/mm; blazed wavelength: 500 nm) and recorded by a spectrograph CCD (CASCADE 512B, Roper Scientific, PI, USA) to obtain the scattering spectra. The scattering spectra were integrated as 10 s. The spectra of an individual nanoparticle were corrected by subtracting the background spectra taken from the adjacent regions without the Au NPs and dividing with the calibrated response curve of the entire optical system. TEM measurement. The morphology changes of Au NPs upon coating with Cu shell were observed through TEM (FEI Tecnai, G2 F20 Super-Twin) operating at 200 kV. Samples for

Figure 1. Principle of the nanoplasmonic detection of RDX explosive.

is an efficient reducing agent for both RDX degrading and Cu2+ in the catalytic growth of Au NPs by Cu2+ deposition on Au NPs seeds. Hence, in the absence of RDX residues, NADHmediated the reduction of Cu2+ and deposition of Cu0 on Au NPs seeds (50 nm), leading to the coating of Au NPs with a shell of Cu, which results in a red shift in the localized surface plasmon resonance (LSPR) scattering spectrum, while, in the presence of RDX, NADH competes between Cu2+ and RDX. As a result, the catalytic growth of Au NPs is inhibited, leading to a weakened nanoplasmonic response. We first demonstrated the possibility of Au NPs (50 nm) as a contrast agent for the imaging of sebaceous LFPs under darkfield microscopy. Figure 2A gives a representative dark-field color image of a volunteer’s LFP with a 4× objective lens. The level 1 detail of fingerprints (ridge flow and ridge pattern) was well-resolved. Level 2 (enclosure, termination, crossover, and island), and the level 3 (pores with a diameter of 50 μm) details of the LFP were also clearly visible (Figure 2B), indicating strong and specific interactions between Au NPs and sebaceous excretions.35 A control experiment indicates that LFPs are barely visible in the absence of AuNPs (Figure S1), suggesting the effectiveness of AuNPs as contrast agents. In a highmagnification image obtained with a 60× objective lens, many green-color dots were visible (Figure 2C). The maximum wavelength (λmax) of the scattered light of these green-color dots was around 550 nm (Figure 2D), which is the characteristic LSPR of Au NPs of 50 nm.28 The λmax of 120 green color dots in Figure 2C was calculated by extracting RGB (three primary colors: red, green, and blue) chrominance information from the dark-field image and then converting wavelength with a Matlab program based on an RGB-ToWavelength (RTW) process.36 The peak distribution of λmax of B


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Analytical Chemistry

Figure 2. (A) Dark-field image of representative sebaceous LFPs obtained with a 4× objective lens. (B) Dark-field images of LFPs showing level 2 details including enclosure 1, termination 2, crossover 3, and island 4, and level 3 details (pores in 1, 2, 3, and 4). (C) Magnified dark-field image of representative LFPs obtained with a 60× objective lens showing the presence of Au NPs. (D) LSPR scattering spectra of three randomly selected green-color dots in Figure 2C.

120 dots was shown in Figure S2. 90% of the calculated λmax was in the range of 550 ± 5 nm, suggesting the Au NPs were monodispersed. Next, we used a dark-field microscope (DFM) combined with plasmonic resonance Rayleigh scattering (PRRS) spectroscopy28 to quantitate RDX residues in LFPs. Previous work suggested that NADH acts as a sufficient reducing agent for RDX in enzymatic remediation and detoxification of groundwater containing nitroaromatic pollutants. The redox potential of NADH/NAD+ is −0.32 V (vs NHE);37 hence, NADH does not reduce RDX with a redox potential of −0.55 V (vs NHE) in homogeneous solution.38,39 Enzymes, such as nitroreductases, however, are needed as catalysts.40 Interestingly, Willner et al. found that Zn2+ might activate the nitro functionalities of RDX toward hydride transfer and demonstrated the possibility of Zn2+ as catalysts in an enzyme-free reduction of RDX.41 We indeed found that the reduction of RDX proceeded effectively in the presence of Zn2+ ions (Figure S3). In addition, NADH also acts as a reducing agent for the catalytic deposition of Cu0 on Au NPs seeds in a concentration-dependent manner (Figure S4).34 Therefore, NADH acts as reducing agent for the reduction of both Cu2+ and RDX; this motivated us to develop a competitive assay for the detection of RDX residues in LFPs. Briefly, RDX solutions (with a net mass of RDX residues ranging from 0 g up to 5.5 μg) were drop-cast on a volunteer’s finger. After drying in air, the LFPs of this finger were collected and then incubated with Au NPs. The RDX and Au NPs carrying LFPs were incubated with NADH and ZnSO4 for 30 min for the degrading of RDX and then for another 2 h with CuSO4 for the catalytic growth. Figure 3A depicts the magnified dark-field images of LFPs containing different loadings of RDX residue obtained with a 60× objective lens. Figure 3B shows the end-point PRRS spectra of representative Au NPs in Figure 3A. In the absence of RDX (Figure 3A (a)), we observed many red dots with λmax at 585 nm. The 35 nm red shift of the plasmon band was attributed to the synthetic effect of enlargement of Au NPs and the change of the dielectric environment by the Cu shell.34 We thus defined this 35 nm of red shift as Δλmax. With the increase

Figure 3. (A) Magnified dark-field images of LFPs containing different loadings of RDX. (a) 0 g, (b) 1.1 μg, (c) 2.2 μg, (d) 3.3 μg, (e) 4.4 μg, (f) 5.5 μg. (B) The end-point PRRS spectra of representative Au NPs in Figure 3A. (C) The relationship between RDX loadings and plasmon band shift (Δλ). (D) The relationship between RDX loadings and ((Δλmax − Δλ)/Δλmax). In Figure 3C and D, error bars represent standard deviations for measurements taken from at least 20 individual dots in three parallel experiments. (E) TEM image of a single Au@Cu core−shell nanoparticle. In all experiments, the concentration of NADH is 0.5 mM.

of RDX loadings, a gradual red-to-green color change was observed (Figure 3A (b−f)), indicating the competitive reaction of NADH with RDX and Cu2+. The plasmon band shift induced by RDX loadings was defined as Δλ. The relationship between RDX loadings and degree of red shift of the plasmon band Δλ and ((Δλmax − Δλ)/Δλmax) is shown in Figure 3C and D, respectively. In Figure 3C and D, error bars represent standard deviations for measurements taken from at least 20 individual dots in three parallel experiments to eliminate the influence caused by the uniformity of different AuNPs. The limit of detection (LOD) of RDX loadings in LFPs was calculated as 1 μg (>3σ). The Au@Cu core−shell nanostructure was further confirmed by TEM (Figure 3E), and a shell structure with a different contrast around Au NPs was clearly observed. The selectivity of the nanoplasmonic assay was examined by monitoring its response over other nitroaromatic explosives, including 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), nitrobenzene, picric acid, and mixtures of TNT and RDX. Magnified dark-field images of LFPs containing different types of explosives are shown in Figure 4A. Green dots were observed only in RDX loaded LFPs (Figure 4Aa), suggesting good selectivity toward RDX (Figure 4B). The redox potential C


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Analytical Chemistry

assay to provide quantitative information on RDX explosive residues in LFPs by exploring the competitive reduction of Cu2+ and RDX by NADH. In addition to the optical advantages of nanoplasmonic imaging of LFPs,28 the nanoplasmonic detection of RDX residues in LFPs possesses several superiorities. First, the proposed method used an objective lens of different magnifications (4× and 60×) to obtain LFPs images and RDX loadings, respectively. Hence, it is nondestructive, which is ideal when dealing with forensic evidence. Second, the proposed method uses a halogen lamp as light source to excite Au NPs in the detection of RDX, thereby avoiding the possible laser-induced ignition of explosives in laser-involved assays, such as fluorescence, Raman, and mass spectroscopy-based methods. Third, the nanoplasmonic LFPs imaging method generally suffers from high background noise. Perspiration and natural secretion residues in LFPs also scatter light, resulting in more red dots as high background noise; thereby, a clean fingerprint is usually required.28 However, the present RDX assay is based on a competitive reaction; hence, it is, in principle, a signal-off model, or in another word, increased loadings of RDX lead to more green dots. We count green dots as the readout instead of red dots; as a result the false-negative rate being dramatically depressed. Therefore, we expect that our nanoplasmonic strategy might be used in forensic investigations to distinguish terrorists that carry explosives.

Figure 4. (A) Magnified dark-field images of LFPs containing different types of nitroaromatic explosives. (a) RDX (11.11 μg), (b) TNT (22.7 μg), (c) RDX:TNT = 4:1 (8.88 μg RDX and 2.27 μg TNT), (d) RDX:TNT = 1:1 (5.55 μg RDX and 5.68 μg TNT), (e) RDX:TNT = 1:4 (2.22 μg RDX and 9.08 μg TNT), (f) DNT (18.2 μg), (g) nitrobenzene (12.3 μg), (h) picric acid (11.15 μg). (B) Selectivity of the nanoplasmonic assay toward RDX over other nitroaromatic explosives. Error bar represent standard deviations for measurements taken from at least four independent experiments.

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* Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02248. Experimental details, Figures S1−S4. (PDF)

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of these nitroaromatic explosives is listed in Table 1. Clearly, all these explosives could not reduce NADH, especally RDX, with

RDX TNT DNT nitrobenzene picric acid

Redox potential (E° vs NHE) −0.55 −0.31 −0.41 −0.49 −0.39

V V V V V

AUTHOR INFORMATION

Corresponding Author

Table 1. Redox Potential of Several Nitroaromatic Explosives Compound

ASSOCIATED CONTENT

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*E-mail: [email protected]. Author Contributions ref

T. Peng and W. Qin contributed equally to this work

38 39 39 39 38

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 program, 2012CB82580, 2012CB932603, and 2013CB933802), the National Natural Science Foundation of China (21222508, 61378062, 21390414, and 91313302), the Shanghai Municipal Commission for Science and Technology (13QH1402300), and the Youth Innovation Promotion Association CAS.

the most negative potential. To resolve this enigma, we realized that numerous biomimetic studies reported that the reduction of different substrates (i.e., ketones or NO2 groups) by NADH model systems required the coaddition of metal ions such as Zn2+.42−44 It was demonstrated that these metal ions act as Lewis acids that bind the reducible functionality and activate it toward hydride transfer from the NADH-model compound. In addition, many RDX sensors based on electron donor and acceptor require the presence of Zn2+.31,45−47 Therefore, we speculate that Zn2+ could activate the reducing of RDX, which might facilitate the formation of a donor−acceptor complex between RDX and NADH that cooperatively assists the activation of the NO2 groups by the Zn2+ Lewis acid−base complex RDX.

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CONCLUSIONS In summary, we have developed a method for imaging sebaceous LFPs. Meanwhile, we designed a nanoplasmonic D


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Nanoplasmonic Imaging of Latent Fingerprints with Explosive RDX

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