An AIPE-active heteroleptic Ir(III) complex for latent fingermarks detection

Accepted Manuscript Title: An AIPE-active heteroleptic Ir(III) complex for latent fingermarks detection Authors: Rui Liu, Zhongming Song, Yuhao Li, Yang Li, Wanwan Yao, Haoling Sun, Hongjun Zhu PII: DOI: Reference:

S0925-4005(17)32455-3 https://doi.org/10.1016/j.snb.2017.12.122 SNB 23810

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

14-8-2017 19-12-2017 19-12-2017

Please cite this article as: Rui Liu, Zhongming Song, Yuhao Li, Yang Li, Wanwan Yao, Haoling Sun, Hongjun Zhu, An AIPE-active heteroleptic Ir(III) complex for latent fingermarks detection, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2017.12.122 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

An AIPE-active heteroleptic Ir(III) complex for latent fingermarks detection Rui Liua,, Zhongming Songa, Yuhao Lib, Yang Lia, Wanwan Yaoa, Haoling Suna,

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Department of Applied Chemistry, College of Chemistry and Molecular

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Engineering, Nanjing Tech University, Nanjing 211816, P. R. China.

College of Science, University of Shanghai for Science and Technology, Shanghai

200093, P. R. China. 

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Corresponding authors. E-mail addresses: [email protected] (R. Liu), [email protected] (H. Zhu)

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Graphical Abstract

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Hongjun Zhua,*

Highlights 

A heteroleptic Ir(III) complex was first used for latent fingermarks detection.

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This complex exhibits aggregation-induced phosphorescent emission features.

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This complex can define latent fingermarks up to the third levels within 5 min.

Abstract

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The existence of a sensitive, accurate and convenient method to visualize latent

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fingermarks (LFs) is of great importance for police identification. In this work, a

novel heteroleptic cationic Ir(III) complex (DX-5), which exhibits dual emission (λmax = 362 nm, Ф = 0.16 and λmax = 494 nm, Ф = 0.59) and aggregation-induced

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phosphorescent emission (AIPE) characteristics, was designed and synthesized. This

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complex was used to enhance the visualization of LFs on different surfaces of porous

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and non-porous substrates. The results illustrate that complex DX-5 can define the details of LFs up to the third level within 5 min, proving to be a candidate material for

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criminal investigations applications.

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Keywords: Latent fingermarks; AIPE; Phosphorescence; Ir(III) complex; Third

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level; visualization.

1. Introduction Human latent fingermarks (LFs) are particularly important in police investigations because they allow unique identification [1]. The existence of a sensitive, accurate and safe method to visualize LFs is of great importance [2],

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however, traditional techniques for LFs detection, such as the powder dusting,

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chemical fumes and multi-metal deposition methods, are often associated with problems that reduce their reliability. When using amino acid reagents

(ninhydrin, 1,8-diazafluoren-9-one), for example, the identification and

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visualization of LFs are often influenced by background colors, dissolution of

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the printing ink, and longtime detection [3-6]. Recently, organic dyes featuring

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aggregation-induced emission (AIE) have been investigated for visualizing LFs [7-8]. Obvious advantages of AIE dyes are their low environmental impact,

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high sensitivity and absence of fuming treatments [9]. Furthermore, previous

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works revealed that the interaction between dyes and fatty residues is essential for LFs detection [7] and lipophicity is beneficial to the targeting and

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aggregation of dye molecules to the fatty residues from the LFs. However, investigations on the lipophicity and structure of the dyes proposed for LFs

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visualization are quite limited. The development of new AIE dyes with improved lipophicity and luminescence properties is still very important for LFs imaging. Su et al. first discovered the recognition of LFs by AIE-active tetraphenylethene (TPE), which causes the appearance of green light-emitting

images of fingermarks on various surfaces when exposed to UV light [7]. Our group also reported an AIE dye (4-dimethylamino-2’-hydroxychalcone) for LFs detection that showed near infrared light-emitting images, but with a relatively dark and uneven color distribution [10]. To date, most of the AIE-active dyes

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for LFP detection are based on pure organic compounds [11]. No organometallic complex with aggregation-induced phosphorescent emission

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(AIPE) properties has been designed and applied to LFs detection. In addition, efforts are being made to obtain dyes with efficient emission at ca. 530 nm,

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which are desirable because the wavelength corresponds to the color green, to

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which the human eye is sensitive [12]. Thus, the development of a green light-

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emitting AIPE-active molecule with high quantum yields and multi-stimuli-

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responsive features for LFs recognition remains a great challenge. In contrast,

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AIPE-active Ir(III) complexes with superior properties are easy to obtain, making them promising candidates as LFs detection materials [13].

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Furthermore, fluorine substituents can be selected and introduced to tune the

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lipophicity and emission of the Ir(III) complex [14-15]. In this work, a novel heteroleptic cationic Ir(III) complex (DX-5) was

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designed and synthesized, following the procedure illustrated in Scheme 1. This complex displays green AIPE behavior and could define the details of LFs on different material surfaces within 5 min.

2. Experimental section 2.1. Materials All reagents and solvents employed were commercially available. All

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reactions were performed under a controlled nitrogen atmosphere. Toluene, MeCN and tetrahydrofuran (THF), used as solvents for chemical synthesis,

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were freshly distilled according to standard procedures. Ultra-pure water was used in the experiments. Silica column chromatography was carried out on

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silica gel (200˗300 mesh).

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2.2. Instruments

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Hydrogen and carbon nuclear magnetic resonance ( 1H NMR and 13C NMR)

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spectra of all compounds in CDCl3 or DMSO-d6 solvent, were measured on a

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Bruker Avance III HD 400 MHz and analyzed with the Bruker NMR software package-TopSpin, using tetramethylsilane (TMS) as the internal standard. High

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resolution mass spectra (HRMS) were obtained on a voyager matrix assisted

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laser desorption/ionization-time of light mass spectrometer system. Elemental analyses (C, H and N) were performed on a Vario El III elemental analyzer and

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the results were within 0.5% of the calculated value. Absorption spectra were recorded on a Shimadzu MultiSpec-1501 spectrophotometer, photoluminescence spectra and quantum yields were obtained on an Edinburgh LFS-920 fluorescence spectrometer with a slit width of 1 nm for both excitation and emission. The solid emission spectra were obtained by placing the sample

powder on a grooved quartz glass slide. Dynamic light scattering (DLS) size was performed on a Zetasizer Nano ZS90. The spectra were processed using the OriginLab OriginPro 9.0 software package. The emission lifetimes were collected using an Edinburgh ps-lifetime instrument with 355 ps laser as

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excitation source. Transmission electron microscopy (TEM) data was obtained on a FEI Tecnal G2 S-Twin. Melting points (m.p.) were taken on an X-4

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were dried in vacuum at 40 oC for 12 h before testing.

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microscope electrothermal apparatus (Taike China). All solid power samples

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2.3. Preparation of compounds

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The compounds 2-(2,3,4,5-tetrafluorophenyl)pyridine (tfppy), 2-(1-(4-

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bromobutyl)-4,5-dimethyl-1H-imidazol-2-yl) pyridine (L1), the ancillary ligands (L2) and the corresponding chloride-bridged dimers were obtained

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according to previously reported synthetic routes [20-21].

2.4. Synthesis of the compound (L1)

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Pydmi (1.00 g, 5.78 mmol) and NaH (0.46 g, 19.27 mmol) were added into

a round bottom flask at 0 oC, then the anhydrous THF (20 mL) solvent was added under N2 gas. The solution was stirred at 0 oC for 30 min, then THF solution (10 mL) with 1,4-dibromobutane (4.94 g, 23.11 mmol) was added. The reaction mixture was stirred at 25 oC for 12 h. After the solvent was filtered,

and then removed under reduced pressure, the residue was recrystallized with CH2Cl2/hexane (3:1, v/v). Yellow solid was obtained, yield 82 %, m.p. = 80.285.9 oC. 1H NMR (400 MHz, CDCl3, ppm) δ 8.54-8.52 (m, 1H, Ar), 8.14-8.11 (m, 1H, Ar), 8.72-7.68 (m, 1H, Ar), 7.70-7.66 (m, 1H, Ar), 7.17-7.14 (m, 1H,

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Ar), 4.54-4.51 (m, 2H, CH2), 3.42-3.39 (m, 2H, CH2), 2.21 (d, J = 5.4 Hz, 6H, 3CH3), 1.91-1.88 (m, 4H, 2CH2). 13C NMR (400 MHz, CDCl3) δ 150.16,

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148.09, 141.95, 136.38, 133.13, 125.66, 122.36, 121.94, 44.17, 33.02, 29.60,

29.01, 12.43, 8.86. Anal. calcd for C14H18N3Br: C 54.56, H 5.89, N 13.63, Br

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25.92; found: C 54.63, H 5.94, N 13.68, Br 25.96 %. HRMS (ESI) (M+H) +:

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m/z calcd 308.07569; found: 308.07593.

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2.5. Synthesis of the ancillary ligand (L2). A round bottom flask was charged L1 (0.40 g, 1.30 mmol), n-But-Cz

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(0.44g, 1.56 mmol), Tetrabutylammonium bromide (TBAB, 0.04 g, 0.13

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mmol), KOH (0.37 g, 6.5 mmol) in 20 mL mixture solution (vtoluene: vwater = 3:1). The reaction mixture was refluxed under N2 for 12 h. After the solution was cooled down to room temperature, suspension was filtrated, and washed to

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remove salts. The purified product was given by silica gel column chromatography using CH2Cl2/hexane (2:1, v/v). White solid was obtained, yield 76%, m.p. = 164.8-167.6 oC. 1H NMR (400 MHz, CDCl3, ppm) δ 8.398.38 (m, 1H, Ar), 8.12-8.09 (m, 3H, Ar), 7.70-7.66 (m, 1H, Ar), 7.50-7.47 (m,

2H, Ar), 7.24 (d, J = 3.4 Hz, 2H, Ar), 7.13-7.10 (m, 1H, Ar), 4.46 (t, J = 7.6 Hz, 2H, CH2), 4.25 (t, J = 6.8 Hz, 2H, CH2), 2.21 (s, 3H, CH3), 2.08 (s, 3H, CH3), 1.94-1.87 (m, 2H, CH2), 1.84-1.77 (m, 2H, CH2), 1.46 (s, 18H, 6CH3). 13

C NMR (400 MHz, CDCl3) δ 150.99, 148.17, 142.51, 141.62, 138.89, 136.41,

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133.81, 125.76, 123.35, 122.79, 122.49, 121.85, 116.36, 108.00, 44.94, 42.62, 34.72, 32.15, 28.40, 26.14. Anal. calcd for C34H42N4: C 80.59, H 8.35, N

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11.06; found: C 80.62, H 8.41, N 11.15 %. HRMS (ESI) (M+H)+: m/z calcd 507.34822; found: 507.34830.

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2.6. Synthesis of DX-5

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A mixture of ligand L2 (0.30 g, 0.60 mmol) and dichloro-bridged diiridium

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complex [Ir(tfppy)2Cl]2 (0.38 g, 0.28 mmol) in 2-ethoxethanol (20 mL) under nitrogen with stirring was heated at 120 oC for 12 h. After the solution was

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cooled down to room temperature, KPF6 was added. The mixture was stirred

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for 1h. Then, suspension was filtrated, and washed to remove salts. The purified product was given by silica gel column chromatography using CH 2Cl2/hexane

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(1:1, v/v). Green solid was obtained, yield 80%. 1H NMR (400 MHz, DMSOd6, ppm) δ 8.27-8.22 (m, 3H, Ar), 8.18 (d, J = 1.4 Hz, 2H, Ar), 8.09-8.01 (m,

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2H, Ar), 7.95 (t, J = 2.0 Hz, 2H, Ar), 7.77 (d, J = 5.6 Hz, 1H, Ar), 7.53 (d, J = 5.2 Hz, 1H, Ar), 7.48-7.37 (m, 5H, Ar), 7.21-7.14 (m, 2H, Ar), 4.61-4.44 (m, 2H, CH2), 4.36-4.23 (m, 2H, CH2), 2.13 (s, 3H, CH3), 1.84-1.76 (m, 4H, 2CH2), 1.46 (s, 3H, CH3), 1.40 (s, 18H, 6CH3). 13C NMR (400 MHz, CDCl3) δ 164.27, 163.75, 150.45, 150.39, 149.18, 147.06, 144.38, 141.84, 140.95, 138.89,

138.83, 138.70, 135.35, 131.52, 126.10, 124.30, 123.95, 123.70, 123.59, 123.49, 123.42, 123.21, 122.68, 116.20, 108.31, 45.92, 42.08, 34.67, 32.07, 26.69, 25.79, 11.69, 9.18. Anal. calcd for IrC 56H50N6F14P: C 51.70, H 3.85, N 6.46; found: C 51.73, H 3.87, N 6.41 %. HRMS (ESI-TOF) (M-PF6⁻ ): m/z

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calcd 1151.3598; found: 1151.3637.

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2.7. Theoretical calculation

The singlet ground state (S0) were investigated using density functional theory (DFT) calculation with B3LYP level at the basis sets level of 6-31G*

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and the effective core potential (ECP) LANL2DZ. The lowest-lying triplet state

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(T1) was optimized at spin-unrestricted B3LYP level. Additionally, the

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excitation energies were obtained using time dependent-density functional

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theory (TD-DFT). All results were performed with Gaussian 09 package.

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3. Results and discussion

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3.1. Photophysical properties In the range of concentrations in CH2Cl2 solution studied (1×10−6 – 1×10−4

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mol/L), the UV-Vis absorption of DX-5 obeys Lambert-Beer’s law, suggesting that no dimerization or oligomerization occurs under these conditions. The UV-Vis absorption and emission spectra of DX-5 are illustrated in Fig. 1 and the absorption band maxima and molar extinction coefficients are listed in Table 1. As shown in Fig.1a, the major absorption bands of DX-5 in CH2Cl2 appearing for

wavelengths lower than 300 nm are assigned to spin-allowed π-π* transitions from the ligands, the weaker bands, observed for wavelengths starting from 350 nm and extending to the visible region, are ascribed to metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) transitions

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[13,16]. Meanwhile, the solution shows dual emissions, at λmax=362 nm and λmax=494 nm, which originate from the ligand-centered (LC) 1π-π* and

MLCT/3LLCT emitting state, respectively [16-20]. The energy transfer process

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is evidenced by the strong overlaps between the absorption and emission

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spectra. The emission quantum yields (Фsolution) of the complex in CH2Cl2 are

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0.16 and 0.59 (Table 1), respectively. The absolute quantum yield (Фsolid) in the

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solid-state is 0.28. As shown in Fig. 1b, the solid-state exhibits strong green

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emission at λmax=511 nm, which is close to the value of 530 nm to which the

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human eye is sensitive [12]. The HOMO of DX-5 is localized on cyclometalated ligands and iridium atom. The LUMO is located on the

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cyclometalated ligands, iridium atom and 2-(4,5-dimethyl-imidazol-2-

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yl)pyridine of the ancillary ligand (Fig. S11). The energy levels of HOMO and LUMO are -5.71 eV and -2.45 eV, respectively, resulting in gap of 3.26 eV. The results indicate that attaching fluorine to the cyclometalated ligands

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significantly influences the energy level, high quantum yield and bright-green emission.

3.2. AIPE properties

The emission of DX-5 in mixtures characterized by diff erent MeCN:water ratios was investigated. As shown in Fig. 2a, the emission intensity of DX-5 increases continuously with the amount of water in the solution, consistently with typical AIPE properties. The addition of water also causes a red-shift of 3-

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4 nm, due to the intermolecular interactions, and the formation of an amorphous state [22]. The AIPE properties of DX-5 are attributed to the characteristics of

ILCT and the RIR mechanism, which are consistent with the characteristics of

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AIPE complexes reported in literature [16-20]. In addition, because of the

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higher amount of -F substituents present and of intermolecular interactions (C–

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H∙∙∙π, C–F∙∙∙π, C–H∙∙∙F, C–H∙∙∙N, etc.) in the aggregated state, this complex

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shows much stronger emission than that of the similar Ir(III) complex reported

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previously [20]. TEM and DLS, which reveal the nanostructure in the

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aggregated state, were used to characterize the samples (complex containing 90% water and 10% MeCN, c = 1.0 ×10-5 mol/L). As shown in Fig. 2b, the

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addition of water causes the formation of spherical aggregates and an average

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particle diameter of about 50 nm. 3.3. Lipophicity study

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To demonstrate the lipophicity between the fatty residues of fingermarks

and DX-5, emission spectra are recorded before and after covering the diff erent substrate surfaces with DX-5 (Fig. 3). The red-shift (~15 nm) in finger-marked regions compared to those not finger-marked indicate the presence of interactions between the complex and fatty residues of the fingermarks [23].

The emission features are consistent with those reported for MeCN-H2O mixtures in Fig. 2a. These results demonstrate that fluorine substituents can profoundly change the lipophicity of DX-5, indicating this dye is a potential

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3.4. Detection of LFs on non-porous materials surfaces

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candidate for LFs detection [14].

Based on the AIPE and lipophicity properties of DX-5, a new technology to

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develop fingermarks images was designed. The schematic diagram of the

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process is shown in Fig. 4a. First, the sebaceous fingermarks (sebum-rich) were

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stamped on the materials’ surface. Then, a MeCN-water solution (1:9, v/v, c =

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1.0 × 10-4 mol/L) of DX-5 was used to cover the fingermarks using a pipette.

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After about 5 min, the fingermarks were carefully washed with an ethanolwater solution (1:8, v/v) and then dried with flowing air at 25 oC. Finally, the

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LFs images were captured using a digital camera under 365 nm UV light.

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Diff erent substrate surfaces, such as stainless steel and glass and plastics, often encountered in criminal investigations, were used. It is obvious that the LFs details are not clear and distinguished with difficultly by the naked eye before

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being covered with the DX-5 solution (Fig. 4b, 4c and S12), however, clear fingermarks images can be easily distinguished 5 min after covering. In the field of forensic chemistry and crime detection, fingermarks details are divided in three levels [23]. At the first level, only the macroscopic features

of fingermarks, including cores and deltas, are distinguishable and the information is not sufficient for recognition. The second level details are the minutiae of points such as ridge endings and bifurcations, which are the most distinctive features. The third level details are defined as the dimensional

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attributes of ridges, including sweat pores, ridge path deviations and edge contours, which provide quantitative data for accurate fingermarks recognition.

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By depositing a fresh latent fingermark on different substrate surfaces, the images obtained exhibit all three levels of fingermarks details. The high

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recognition of fingermarks details demonstrates its usefulness for recognition.

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Furthermore, to investigate the versatility of the complex, LFs from several

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volunteers were gathered. Detection of the LFs based on the sample is still the

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most widespread method of identification (Fig. S13). It is worth noting that

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aged fingermarks are commonly encountered in practical applications. In this study, three aged fingermarks could also be clearly detected (aged 10, 20, and

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40 days, Fig. S14).

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3.5. Detection of LFs on porous paper money Considering the large diffusion of paper money in most countries, a study

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was also carried out to investigate the possible use of the proposed complex in the detection of LFs on banknote. As shown in Fig. 5a, LFs cannot be observed on the (Renminbi) RMB cash surface. However, after stamping the sample and illuminating it with 365 nm UV light, clear fingermarks images with bright green-emissions appeared. The three levels of features (core, bifurcation, ridge

ending, and enclosure) were all clearly visible. The comparison of fingermarks produced by the sample (left) and by a red inkpad (right), shows that the minutiae, consisting of 14 dots, are completely matched (Fig. 5b and 5c). The excellent performance in different situations indicates that DX-5 can be

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successfully applied in LFs detection.

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4. Conclusions

In this work we demonstrated the APIE activity of the DX-5 dye and its application for LFs detection. This Ir(III) complex exhibits dual emissions, high

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quantum yields and significant green AIPE features. Having confirmed its

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favorable properties, the complex was used in LFs detection. Fingermarks

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details up to the third level can be clearly observed on different material surfaces. Notably, some aspects still need to be improved. The higher cost of

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transition metal Ir and synthetic routes would restrict the large-scale production

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of DX-5. More simpler procedures of LFs detection should be explored. In any case, all the results indicate that this complex is a potential candidate for

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applications in criminal investigations. Acknowledgements

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The authors greatly acknowledge the financial support in part by National Natural Science Foundation of China (21602106), Natural Science Foundation of Jiangsu Province-Outstanding Youth Foundation (BK20170104), “Six Talent Peaks Project” of Jiangsu Province (XCL-037), Industry--cademy-Research Prospective Joint Project of Jiangsu Province (BY2016005-06) and National Key Research and Development Program of China (2017YFB0307202).

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Biographies

Vitae

Rui Liu received his Ph.D. in applied chemistry from Nanjing University of

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Technology, China in 2010. He is currently working as an associate professor at Nanjing Tech University. His current research interests focus on organic synthesis, optical materials and chemical sensors. Zhongming Song received his science bachelor in applied chemistry in 2014 from Nanjing Tech University, China. He is currently pursuing his PhD degree in Nanjing

Tech University, China. His research interests are optoelectronic applications and the recognition of the latent fingerprints. Yuhao Li received his Ph.D. in applied chemistry from Nanjing University of Technology, China in 2011. He is currently working as an associate professor at University of Shanghai for Science and Technology. His research interests are

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nonlinear optical materials, nanomaterials-based sensors, and biomaterials. Yang Li received her science bachelor in applied chemistry in 2016 from Nanjing

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Tech University, China. He is working for her master’s degree in Nanjing Tech University, China. His research interests are electrochemical biosensor.

Wanwan Yao received her science bachelor in applied chemistry in 2017 from

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Nanjing Tech University, China. She is working for her master’s degree in Nanjing

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Tech University, China. Her research interests are biosensor.

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Haoling Sun received her science bachelor in applied chemistry in 2017 from

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Nanjing Tech University, China. He is working for her master’s degree in Nanjing Tech University, China. His research interests are optoelectronic applications.

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Hongjun Zhu is currently a professor at Nanjing Tech University of China. He has a broad research interest, such as the research and development of OLEDs materials,

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the fluorescent molecular probes, new pesticide, water treatment agent and the

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organic synthesis methodology.

Table 1. Photophysical data for DX-5 in solution and solid-state. Absorption

Emission

Complex

λmaxa /nm

DX-5

265 (8.38), 298 (7.38), 350 (3.11), 406 (0.13)

4

-1

λmaxa /nm

-1

(ε/10 M cm )

(τ/μs, Фsolution ) b

Measured in CH2Cl2 (1.0 × 10-5 mol/L) solution at 298K under air. b Determined by using tryptophan (ФPL = 0.14 in water, pH=7.2, 25 oC) and quinine sulfate (ФPL = 0.54 in 0.1 mol/L H2SO4) as a standard. c Absolute phosphorescence quantum yield in solid state determined by calibrated integrating sphere system.

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a

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0.28

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362(-, 0.16), 494 (1.61, 58.68%; 9.40, 41.31%, 0.59)

Фsolidc

Scheme 1 The synthesis of complex DX-5.

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 200

0.0

300 400 500 600 Wavelength (nm)

4000

(b)

3000 2000 1000 0

450 500 550 600 650 700 Wavelength (nm)

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Absorption 1.0 Emission

(a)

PL Intensity (a.u.)

1.0

1.2

Normalized PL Intensity

Normalized Absorption

1.2

Fig. 1. (a) UV-vis absorption and emission spectra (λexc = 305 nm) of DX-5 in

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6000

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(a)

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4500

500

550

600

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0 to 90%

1500 0 450

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3000

650

Wavelength (nm)

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PL Intensity (a.u.)

CH2Cl2. (b) Emission spectra of DX-5 in solid-state.

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Fig. 2. (a) Emission spectra of DX-5 (c = 1.0 × 10-5 mol/L) in MeCN-water with different water fraction (0-90%), the inset and the photograph show the complex in different water fraction mixtures under 365 nm UV lamp. (b) Particle size distributions of DX-5 in MeCN-water mixture (1:9, v/v).

nm

3000

Without LFs With LFs on steel With LFs on paper With LFs on glass With LFs on plastic

2500 nm

2000

500

510 520 530 Wavelength (nm)

540

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1500 490

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Emission Intensity (a.u.)

3500

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Fig. 3. Emission spectra of the solid-state DX-5 without fingermarks (solid black line) and the fingermarks with DX-5 on different material's surface (steel: solid red line, paper: solid blue line, glass: solid yellow line, plastic: solid green line).

Fig. 4. (a) Procedures and photographs of LFs detection. LFs images on different substrate surfaces with sample under 365 nm UV lamp. (b) Stainless steel and (c) glass.

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Fig. 5. (a) LFs images with sample on paper money under 365 nm UV lamp. The fingermarks labeled by DX-5 (b) and a red inkpad (c).