Chemical Visualization of Sweat Pores in Fingerprints Using GO

Jul 12, 2017 - Department of Forensic Science, People's Security University of China, Beijing, 100038, People's Republic of China. Anal. Chem. ... Sta...
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Chemical Visualization of Sweat Pores in Fingerprints Using GO-Enhanced TOF-SIMS Lesi Cai, Mengchan Xia, Zhaoying Wang, Ya-Bin Zhao, Zhanping Li, Sichun Zhang, and Xinrong Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01629 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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Analytical Chemistry

Chemical Visualization of Sweat Pores in Fingerprints Using GOEnhanced TOF-SIMS Lesi Cai,† Meng-Chan Xia,† Zhaoying Wang,† Ya-Bin Zhao,‡ Zhanping Li,*,† Sichun Zhang,† and Xinrong Zhang*,† †

Department of Chemistry, Beijing Key Laboratory of Microanalytical Methods and Instrumentation, Tsinghua University, Beijing, 100084, People’s Republic of China. ‡

Department of Forensic Science, People’s Security University of China, Beijing, 100038, People’s Republic of China

ABSTRACT: Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been used in imaging of small molecules (< 500 Da) in fingerprints, such as gunshot residues and illicit drugs. However, identifying and mapping relatively high mass molecules are quite difficult owing to insufficient ion yield of their molecular ions. In this report, Graphene oxide (GO)-enhanced TOF-SIMS was used to detect and image relatively high mass molecules such as poison, alkaloids (> 600 Da) and controlled drugs, antibiotics (> 700 Da) in fingerprints. Detail features of fingerprints such as the number and distribution of sweat pores in a ridge and even the delicate morphology of one pore were clearly revealed in SIMS images of relatively high mass molecules. The detail features combining with identified chemical composition were sufficient to establish a human identity and link the suspect to a crime scene. The wide detectable mass range and high spatial resolution make GO-enhanced TOF-SIMS a promising tool in accurate and fast analysis of fingerprint, especially in fragmental fingerprint analysis.

Dactyloscopy plays an important role in forensic science. Early analysis of fingerprint is based on pictorial comparison and matching, only providing physical shape information. Recently, there is an increasing focus on chemical imaging of fingerprint.1,2 Identification of endogenous compounds in fingerprint could offer the potential individual biological characteristics, while exogenous compounds could be used to link a suspect to a crime scene. Both spectroscopy3,4 and mass spectrometry (MS)5-7 were widely used in chemical imaging of fingerprints. MS technologies, such as desorption electrospray ionization (DESI)5,8, matrix-assisted laser desorption ionization (MALDI)9,10 mass spectrometry and time-of-flight secondary ion mass spectrometry (TOF-SIMS)11,12 offered chemical specific identification of a wide range of compounds in fingerprint. However, DESI and MALDI performed poorly in imaging the Level 3 features13 of fingerprints, such as sweat pores, due to insufficient spatial resolution14. Level 3 features include all dimensional attributes of the ridge such as pores, ridge path deviation, edge contour, breaks, creases, scars, and other permanent details, providing quantitative data supporting more accurate and robust fingerprint recognition, especially in fragmentary fingerprint comparison.13

TOF-SIMS possesses submicrometer spatial resolution15and thus could clearly reveal the level 3 features18,19. Using TOF-SIMS, a lot of contaminants including illicit drugs20,21, gunshot residues (GSR)19,22-23 and daily supplies12 have been detected in fingerprints. TOF-SIMS was also used in expansive research of fingerprints, such as determination of the deposition order of overlapping latent fingerprints24 and the age dating of fingerprints25. However, almost all of the detected compounds in fingerprints were at low molecular weight (700 Da), spermicides in condom lubricant, was detected in a fingerprint by MALDI to link the suspect to a sexual offender.29 However, identification and 17

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mapping relatively high mass molecules are quite difficult in a normal TOF-SIMS analysis owing to insufficient secondary ion yield of their molecular ions.30 Matrixenhanced SIMS (ME-SIMS) was thus introduced to realize the detection of relatively high mass molecules.31,32 We found that graphene oxide (GO) could significantly increase the secondary ion yields of intact molecular ions of lipids (> 700 Da) and peptides (> 1000 Da) as a novel matrix for TOF-SIMS.33 Moreover, GO forms an evenly continuous layer, which avoids the matrix crystallization and maintains the imaging spatial resolution. In this work, identification of relatively high mass contaminants and visualization of delicate level 3 features in fingerprints were realized by GO-enhanced TOF-SIMS. Taking alkaloids (> 600 Da) and antibiotics (> 700 Da) as examples, relatively high mass compounds left in fingerprints could be identified by intense molecular ion signals. Meanwhile, the number and distribution of pores in one ridge and even the size and shape of one pore could be clearly observed in TOF-SIMS images. The discriminatory images of such level 3 features combining with rich chemical information provide unique evidence for both criminal activity and human identification, especially in a fragmentary fingerprint analysis.

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keV Bi3+ LMIG. The Bi3+ current was 0.8 pA (6000. A flood gun with low energy electrons was used to compensate for charge buildup on sample surface. Micrograph of fingertip for comparison. A micrograph of intact fingertip was obtained using a Zeiss Stemi 508 stereomicroscope. Image size (scaled) was 8.65 mm × 6.49 mm. The same finger (right middle finger of donor 1) was used to further investigate chemical imaging in TOFSIMS. Ethics. Two fingerprint donors used in this study were volunteers, who gave their fingerprints on the understanding that the fingerprints would only be used for research purposes. RESULTS AND DISCUSSION

METHODS Chemicals and Materials. Antibiotics standard samples including roxithromycin, azithromycin and erythromycin were purchased from Sigma Aldrich (St. Louis, MO, USA). Commercial drugs, roxithromycin capsules were purchased from Zhuhai United Laboratories (Zhongshan) Co., Ltd. (Guangdong, China) and azithromycin capsules were purchased from Jiangsu Fubang Pharmaceutical Co., Ltd. (Nanjing, China). Hypaconitine, mesaconitine and aconitine were purchased from Shanghai shifeng biological technology Co., Ltd. (Shanghai, China). Graphene oxide was purchased from Nanjing XFNANO Materials Tech Co., Ltd. (Nanjing, China). Chloroform, methanol, dichloromethane, and acetone were at HPLC grade and the water was prepared from a Milli-Q water (Millipore, Billerica, MA, USA) purification system. Sample preparation. A Si wafer (14 mm × 10 mm) was washed by water, dichloromethane, acetone and methanol. For the preparation of GO layer on Si wafer, a Si wafer was immersed in 2 mg/mL GO solution and then dried on oven at 60 ℃ for 6 hours. The thickness of GO layer was about 2 μm, measured by a commercial step profiler (see Figure S1). In standard solution test, 10 μL solution was dropped on GO layer or Si wafer and air-dried. As for fingerprint analysis, before presenting fingerprints on Si wafer or GO layer, a fingertip was smeared by drugs solutions and airdried for a few seconds. Laid the fingertip on GO layer gently to avoid the distortion of fingerprint pattern. Mass Spectrometry. TOF-SIMS analysis was carried out on a TOF-SIMS 5 (ION-TOF GmbH, Münster, Germany), equipped with a Bi liquid metal ion gun (LMIG). TOF-SIMS spectra and images were acquired using a 30

Enhancement of Relatively High Mass Molecular Ion Signals on GO Layer. A smooth and even GO layer on Si substrate was easily obtained by immersing a cleaned Si wafer into GO solution and then drying (see Figure 1a). Using GO layer as matrix, intense molecular ion ([M + H]+) signals of alkaloids and antibiotics were observed in TOF-SIMS spectra. The [M + H]+ signals at m/z 616.3, 632.3 and 646.3 from hypaconitine, mesaconitine and aconitine achieved significant enhancement compared with only using Si wafer as substrate (see Figure 1b). The intensity of molecular ion signals increased >10fold for all alkaloids (see Table S1). As for antibiotics, similarly, the [M + H]+ signals at m/z 734.5, 749.5 and 837.5 from erythromycin, azithromycin and roxithromycin were largely increased on GO layer (see Figure 1c). The intensity of molecular ion signals increased >10-fold, 20-fold and 30-fold for erythromycin, azithromycin and roxithromycin respectively (see Table S1). Other relatively high mass molecules, such as lipids (700-900 Da) and peptides (9001100 Da) could also be detected with increased molecular ion signals on GO layer (see Figure S2), consistent with previous research33. The results showed that molecular ion signals from different kinds of relatively high mass chemicals could be significantly increased with the use of the GO layer. Although the mechanism of the enhancement effect brought by matrix31,32 is still not completely understood, a possible explanation is that using GO could facilitate the desorption/ionization process and relatively reduce fragmentation of the analyte. As a matrix, GO helps to transfer the energy of cascade collision between high-energy primary ion and matrix molecules to the analyte on surface or several top layers. The energy transferred from GO to the analyte facilitates the desorption/ionization process

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Analytical Chemistry

Figure 1. a) Preparation of a GO layer on Si substrate. Intact molecular ion signals in positive ion mode of b) alkaloids, left to right: hypaconitine at m/z 616.3, mesaconitine at m/z 632.3 and aconitine at m/z 646.3 and c) antibiotics, left to right: erythromycin at m/z 734.5, azithromycin at m/z 749.5 and roxithromycin at m/z 837.5 on GO layer (up line) or Si substrate (down line).

of relatively high mass molecules but not induces additional fragmentation. In addition, the heavily oxygenated chemical structure of GO may change the way of electron transfer between the matrix and analyte molecules, which helps to increase the ionization yield of analyte molecules. In this way, secondary ion yields of intact molecular ions are significantly increased especially for relatively high mass molecules. However, there need more experimental data to prove the mechanism discussed above. Chemical Imaging of Aconitine and Antibiotics in Fingerprints. GO-enhanced TOF-SIMS was then applied to detect relatively high mass contaminants in fingerprints. In Figure 2a, b and c, imaging molecular ion signals of three antibiotics, erythromycin, azithromycin and roxithromycin presented delicate patterns when a fingerprint was left on a GO layer. In Figure 2d, imaging intense molecular ion signals of aconitine also revealed the clear fingerprint pattern on GO layer. Such images offered chemical composition information and pattern information simultaneously for fingerprint analysis. However, molecular ion signals of such relatively high mass molecules were much lower when a fingerprint was directly left on a Si substrate, which were insufficient to obtain a clear fingerprint image. This result showed the importance of GO layer as a matrix to increase the secondary ion yields of the molecular ions from relatively high mass molecules in fingerprints. Identification of the chemical composition of contaminants in fingerprints were performed by contaminating a finger with commercial drug capsule solution and leaving fingerprints on GO layer. When a

Figure 2. Spectra of molecular ion signals in positive ion mode and their 2D SIMS images of fingerprints after contact with a) erythromycin at m/z 734.5, b) azithromycin at m/z 749.5, c) roxithromycin at m/z 837.5 and d) aconitine at m/z 646.3 left on a GO layer or Si substrate. Scan area: 2.0 × 2.0 mm2.

finger was contaminated by azithromycin capsule solution, only one intense peak at m/z 749.5 was observed in spectrum which was identified as the molecular ion peak of azithromycin. The fingerprint images could be obtained only at m/z 749.5 channel. When contaminated by a mixture of two drug capsules, there were two intense peaks at m/z 749.5 and m/z 837.5 in the spectrum, which were identified as the molecular ion peak of azithromycin and roxithromycin respectively. The fingerprint images could be obtained at both m/z 749.5 and 837.5 channels (Figure 3). These results showed the superior capability of GO-enhanced TOF-SIMS to identify the chemical composition of contaminants in fingerprint, especially for relatively high mass molecules according to their molecular ion signals. In this way, multi-components in fingerprint could be detected at the same time (an example of three compositions detection in one fingerprint was showed in Figure S4). On GO layer, overlapping fingerprints containing different drug molecules could also be distinguished by imaging the molecular ions of drugs (see figure S5). Moreover, relatively high mass molecules in fingerprints from an assumed crime scene could be detected by TOF-SIMS, via a simple transfer from other substrates such as glass and wood to a GO layer (see Figure S6). Therefore, GO-enhanced TOF-SIMS holds potentials in such applications as identification of chemical composition in fingerprints.

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Figure 3. SIMS spectra and images of molecular ion signals of azithromycin and roxithromycin in fingerprints. The fingertip was contaminated by a) a solution of commercial azithromycin capsule or b) a mixture solution of azithromycin and roxithromycin capsules. Scan area: 5.0 × 5.0 mm2.

High Resolution Images of Detail Features in Fingerprints. As a powerful chemical imaging tool, TOFSIMS possess submicrometer spatial resolution to reveal the detail features of fingerprints. Delicate morphology of fingerprint was well preserved on the smooth GO layer. Mapping molecular ions of drugs could show the fingerprint details in a hierarchical order. Figure 4a showed pattern features of two fingerprints, revealing two pattern types, loop (left) and double loop (right). Compared to other chemical imaging tools, TOF-SIMS performs better in imaging Level 3 features, such as sweat pores. In Figure 4b, the number and position of pores could be easily observed in a ridge and the precise distance between two pores could also be obtained in such SIMS images. Furthermore we could clearly observe the size, shape, position and other characteristics of one pore (Figure 4c-f). Figure 4c, d and e showed three closed pores. In Figure 4c and d the pores are nearly rounds with different size while in Figure 4e is nearly a triangle. Figure 4f showed an open pore, which located at the edge of a ridge. These features are unique for each fingerprint and thus provide discernible information for fingerprint recognition. Fragmentary Fingerprint Recognition by Level 3 Details and Chemical Information. No two pores are identical in fingerprints. Two donors were asked to touch different drug solutions and leave their fingerprints on GO layers. SIMS images showed that the number and distribution of pores in a ridge and the size and shape of a

Figure 4. 2D SIMS images of molecular ions of roxithromycin at m/z 837.5 in fingerprints contaminated by roxithromycin solution. a) Scan area: 5.0 × 5.0 mm2 in MacroRaster mode, 128 × 128 pixels, b) Scan area: 1.2 × 0.6 mm2 in MacroRaster mode, 128 × 128 pixels and c)-f) Scan area: 400 × 400 μm2, 256 × 256 pixels in normal 2D mode.

pore are unique and different in two fingerprints from different people (Figure 5a and b). At the same time, roxithromycin was detected in the fingerprint of donor 1, while azithromycin in the fingerprint of donor 2. Level 3 features combining with identified chemical composition offered a unique information for a fingerprint, forming accurate and robust evidence in criminal investigation. When a fingerprint is not intact, analysis of Level 3 features of fingerprints become more important. Figure 5c showed a SIMS image of a fraction of fingerprint from donor 1. By analyzing the number and distribution of a few pores and a ridge branch, this SIMS image was finely matched with a micrograph of the same part, which was from an intact fingerprint profile. As we know, using SIMS imaging to find out a specific delicate region may take too much time and effort. Although optical imaging offered only pattern information of fingerprint, it would be a fast way to pre-locate the interested area before chemical composition analysis by SIMS. GO-enhanced SIMS imaging guided by optical imaging would largely speed up the analysis process. In chemical composition analysis, intense molecular ion peak at m/z 749.5 was observed in spectrum of the SIMS image, which meant donor 1 had

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Analytical Chemistry could thus be clearly revealed in SIMS images. A few such Level 3 features combining with identified chemical composition offered a unique information, which is sufficient to establish a human identity and link the suspect to a crime scene. GO-enhanced TOF-SIMS holds potentials in such applications as more accurate and fast analysis of fingerprints, especially for a fragmental fingerprints analysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author Zhanping Li*, E-mail: [email protected] Xinrong Zhang*, E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

Figure 5. Imaging a fraction of a ridge and a pore on this fraction using molecular ions of a) roxithromycin at m/z 837.5 in fingerprint from donor 1 and b) azithromycin at m/z 749.5 in fingerprint from donor 2. The molecular ion peaks in spectra corresponding to single pore images. c) 2D SIMS image of a part of fingerprint using molecular ions of azithromycin at m/z 749.5. A camera picture of the same fingertip was obtained by a stereomicroscope. The same part was chosen for comparison with SIMS image.

touched azithromycin before. Early studies also showed a few Level 3 features were sufficient to establish a human identity.34 This protocol not only enabled the fingerprint recognition by Level 3 features but also provided the chemical information to link the suspect to a special substances, thus to a crime scene or crime activities. Especially when the fingerprint left by a suspect was fragmentary, only Level 3 features were available to establish the individuality of fingerprint, such additional chemical information provided by GO-enhanced SIMS could help to identify the suspect in a crime. CONCLUSIONS In this work, GO-enhanced TOF-SIMS was firstly used in fingerprint analysis. The detectable mass range was enlarged by using GO as a matrix to increase the molecular ion signals of relatively high mass molecules in fingerprints. In this way, more exogenous species in fingerprints, such as drugs, poisons, biological agents or other relatively high mass molecules from various crime scenes could be detected and imaged by TOF-SIMS. Meanwhile, the even and smooth GO layer ensures the well preservation of delicate morphology of fingerprint and holds the high spatial resolution of SIMS imaging. The detail features of fingerprint such as number and distribution of pores in a ridge and even the size and shape of a pore

XR Zhang and LS Cai thank to the financial support provided by the National Natural Science Foundation of China (21390410) and the 973 program (2013CB933804); SC Zhang and ZP Li thank to the National Natural Science Foundation of China (21621003).

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