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Dual Colorimetric and Fluorescent Imaging of Latent Fingerprints on Both Porous and Nonporous Surfaces with Near-Infrared Fluorescent Semiconducting Polymer Dots You-Hong Chen, Shih-Yu Kuo, Wei-Kai Tsai, Chi-Shiang Ke, Chia-Hsien Liao, Chuan-Pin Chen, Yeng-Tseng Wang, Hsiu-Wei Chen, and Yang-Hsiang Chan Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03178 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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Analytical Chemistry

Dual Colorimetric and Fluorescent Imaging of Latent Fingerprints on Both Porous and Nonporous Surfaces with Near-Infrared Fluorescent Semiconducting Polymer Dots You-Hong Chen, Shih-Yu Kuo, Wei-Kai Tsai, Chi-Shiang Ke, Chia-Hsien Liao, Chuan-Pin Chen, Yeng-Tseng Wang,‡ Hsiu-Wei Chen,† and Yang-Hsiang Chan*,† ‡

Department of Biochemistry, College of Medicine, Kaohsiung Medical University, 100 Tzyou 1st Road, Kaohsiung Taiwan 807 † Department of Chemistry, National Sun Yat-sen University, 70 Lien Hai Road, Kaohsiung, Taiwan 80424 ABSTRACT: Semiconducting polymer dots (Pdots) have recently been proven as a novel type of ultrabright fluorescent probes that can be extensively used in analytical detection. Here we developed a dual visual sensor based on Pdots for fingerprint imaging. We first designed and synthesized two types of near-infrared fluorescent polymers and then embedded ninhydrin into the Pdot matrix. The resulting Pdot assays showed the colorimetric and fluorescent dual-readout abilities to detect latent fingerprints on both porous and nonporous surfaces. The developed fingerprints clearly revealed first-, second-, and third-level details with high contrast, high selectivity, and low background interference. We also grafted the chemical groups on the nanoparticle surface to investigate the mechanisms involved in the fingerprint development processes. We further utilized this assay in note paper and checks for latent fingerprint imaging. We believe that this dual-readout method based on Pdots will create a new avenue for research in fingerprint detection and anti-counterfeiting technology.

INTRODUCTION

adoption. As a result, it is highly desirable to design a new and universal probe with high sensitivity and low background interference for fingerprint detection on all surfaces.

In forensic science, the exchange principle proposed by Edmond Locard states that every contact leaves a trace. When a human figure contacts onto the surface of an object, a complex mixture (e.g., sebum, lipids, sweat, and contaminants)1 from the human skin was transferred to the surface, thus forming a fingerprint. Due to the uniqueness and permanence of the ridge skin patterns, fingerprints have been widely used as powerful and effective evidence for individual identification since the late 19th century. However, the most commonly found fingerprints are invisible to the naked eye and therefore additional efforts are required to develop latent fingerprints for easy visualization. To date, there have been various physical and chemical methods created for the development of latent fingerprints. These techniques include powder dusting,2 ninhydrin spraying,3 iodine/cyanoacrylate fuming,4-6 and (fluorescent) dye staining.7 Although these traditional methods are effective under ordinary circumstances, there are still numerous challenges to visualize latent fingerprints with high contrast and low background interference that can reach the requirement of modern forensic sciences. For example, powder (magnetic, metallic, fluorescent, or organic pigment powder) dusting is the most commonly used method at crime scenes but could easily damage the fingerprint details while brushing. The ninhydrin reagent can specifically react with amino acids to product a purple-blue substance but cannot be well-distinguished on a colored background. More importantly, the aforementioned techniques can be employed exclusively on either porous (e.g., ninhydrin reagents) or non-porous (e.g., cyanoacrylate fuming) surfaces, which greatly limits their widespread

In the past few years, there have been ongoing efforts to develop a rapid, sensitive, and economical technique for the detection of latent fingerprints.8-14 Among these detection techniques, nanoparticle-based materials have attracted enormous interest due to their large surface area, enhanced stability, and high brightness. For example, quantum dots exhibit excellent fluorescence intensity and large Stokes shift, which is beneficial for the enhancement of the fingerprint contrast under ultraviolet-visible irradiation.15,16 However, the inherent toxicity of quantum dots would hinder their practical use.17-20 Metal colloids stabilized by specific surfactants could also be used to develop fingerprints but they would suffer from serious background interference on the dark surface. Recently, fluorescent upconversion nanoparticles have been successfully utilized for the visualization of fingerprints.21 These upconversion nanoparticles can be excited by nearinfrared (NIR) light to realize ultralow background interference from the substrates, although their relatively low quantum yields and small absorption cross section may lead to low fluorescence brightness.22 In recent years, semiconducting polymer dots (Pdots) have emerged as a new type of fluorescent probes due to their unique photophysical properties, including exceptional fluorescence brightness, good photostability, fast radiative rate, and minimal cytotoxicity.23-34 Besides, the facile surface functionalization of Pdots have endowed them with burgeoning applications both in vitro and in vivo.35-45 More recently, Chiu’s and our group have developed various sensing

1 ACS Paragon Plus Environment


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platforms by taking advantage of the amplified energy transfer from the excited Pdot matrix to the sensing probes.46-53 Here we report a novel strategy by use of near-infrared (NIR) fluorescent Pdots to clearly visualize latent fingerprints on different surfaces, including plastic bags, aluminum foils, acrylic sheets, colored paper, and glass. We first synthesized NIR-emitting polymers and then encapsulated ninhydrin into the Pdot matrix. After spraying the Pdot solution onto the surfaces, the latent fingerprints revealed a blue-purple color and could be directly observed by naked eyes under ambient light, and at the same time could be fluorescently imaged under the excitation of UV/blue light. Unlike conventional fingerprint visualization agents in which only the absorption change or the fluorescence emergence of the latent fingerprints could be detected, this dual-readout Pdot-based assay owns several remarkable advantages. For example, the colorimetric and fluorescent dual-readout strategy can provide complementary signals simultaneously to minimize the background interference from fluorescent substrates and/or colored surfaces. Additionally, this Pdot-based assay could be applied on all of the commonly used substrates, including porous and non-porous surfaces. More importantly, the fluorescence of these Pdots on the background emits in the NIR region which is invisible to the naked eye but shows a blue shift in the fingerprint areas, displaying a strong red emission. The NIR properties together with the ultrahigh brightness of the Pdots could greatly enhance the fingerprint contrast. We also investigated how the surface functionalization of the Pdots influenced the fingerprint development. We further employed this assay in note paper and checks to demonstrate its practical use.

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oligomers, and inorganic salts. The crude product was dissolved in CH2Cl2 and then extracted with brine for 3 times. The organic extract was separated, dried over MgSO4, and the solvent was removed under reduced pressure. The crude polymers were re-precipitated in CH2Cl2/methanol and washed with acetone. Finally, the product was collected by filtration to afford 80-120 mg of PF-BT-TBT/PF-BT-TQ polymer. Preparation of Ninhydrin-Doped PF-BT-TQ Pdots. First, 300 µL of PF-BT-TQ (1 mg/mL in THF), 60 µL of PS-PEGCOOH or PS-SH or polyoxyethylene nonylphenylether (CO520) (1 mg/mL in THF), and 6 mg of ninhydrin were mixed together in 2 mL of THF. The mixture was sonicated for 15 s and then injected into 3 mL of H2O under violent sonication. After that, THF was removed by purging with dry N2 on a 65 °C hot plate for 30 min. The resulting Pdot solution was centrifuged at 1600 rpm for 2 min to remove large aggregations formed during Pdot preparation. The supernatant was then used for the development of latent fingerprints. Characterization of Ninhydrin-Doped PF-BT-TQ Pdots. The average particle size was determined by dynamic light scattering and transmission electron microscopy (TEM). TEM images of the synthesized Pdots were acquired using a JEOL 2100 transmission electron microscope at an acceleration voltage of 200 kV. For TEM, a drop of Pdot aqueous solution was placed onto a carbon-coated grid and allowed to evaporate at room temperature. The UV-visible absorption spectra of Pdots were measured on a Dynamica Halo DB20S spectrophotometer. The fluorescence spectra were collected using a Hitachi F-7000 ﬂuorometer (Hitachi, Tokyo, Japan) under 450 nm excitation. The spectral data of Pdots on the silica surface were acquired using a combined Raman/fluorescence microscope (WITec Alpha300 R, Germany) with an Ar ion laser at 488 nm as the excitation source with a typical integration time of 36 ms/pixel. The particle diameter distributions and zeta potentials of the Pdots were determined by dynamic light scattering using a N5 submicrometer particle size analyzer (Beckman Coulter Inc., USA). For the atomic force microscopy (AFM) experiment, one drop of the Pdot suspension was placed on a freshly (3aminopropyl)triethoxysilane-coated mica substrate. After evaporation of the water solution, the mica substrate was washed with copious deionized water and then dried under a stream of nitrogen. The surface topography of the nanoparticles was imaged on a multimode AFM with the NanoScope IIIa controller (Veeco). Si3N4 cantilevers (spring constant: 4 N/m, Nanoworld) with a resonance frequency of 50 kHz were used for image collection. Images were acquired at a resolution of 256 x 256 lines at a scan rate of 2 Hz. AFM data were processed using the NanoScope analyzing software with first order flattening.

EXPERIMENTAL SECTION Chemicals. All reagents were purchased from SigmaAldrich or Alfa Asear and used as received unless indicated elsewhere. Polystyrene graft ethylene oxide functionalized with carboxylic end group (PS-PEG-COOH, Mn = 6500 Da of PS moiety; 4600 Da of PEG-COOH; polydispersity, 1.3) and thiol-terminated polystyrene (PS-SH, Mn = 11000; polydispersity, 1.25) were purchased from Polymer Source, Inc. (Dorval PQ, Canada) and used as received. High purity water (18.2 MΩ•cm) was used throughout the experiment. Synthesis of PF-BT-DBT and PF-BT-TQ polymers. PFBT-DBT and PF-BT-TQ polymers were synthesized according to our reported literature by using Suzuki coupling reaction.37 Briefly, monomer fluorene (0.5 mmol), monomer 4,7Dibromo-2,1,3-benzothiadiazole (0.45 mmol), and monomer 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(3(hexyloxy)phenyl)quinoxaline or 4,7-Bis(2-bromo-5-thienyl)2,1,3-benzothiadiazole (0.05 mmol) were dissolved in 10 mL, and then 4-6 mg (0.02 mmol) of tetra-n-butylammonium bromide (Bu4NBr) and 3 mL of Na2CO3 (2 M) was added. The mixture solution was purged with nitrogen for 1 h. After that, the mixture solution was degassed and refilled with N2 (repeated 4 times) before and after the addition of Pd(PPh3)4 (7 mg, 0.006 mmol). The reactants were stirred at 100 °C for 48 h and then 50 mg of phenylboronic acid dissolved in 1 mL of THF was added. After 2 h, 0.5 mL of bromobenzene was added and further stirred for 3 h. The mixture was poured into 120 mL of methanol. The precipitate was filtered, washed with methanol and acetone to remove monomers, small

Fingerprints Development. Nonporous substrates including glass, aluminum foil, plastic bag, and acrylic sheet were selected for detecting latent fingerprints. Porous surfaces used here were printing paper and colored paper. Volunteers were asked to wash hands thoroughly with the soap and then blown dry. Fingertips were gently rubbed across the forehead and nose and then pressed onto different surfaces. For nonporous substrates, 0.2-0.5 mL of Pdot solution was added dropwise onto the substrates and then waited till the water was dry at room temperature. For porous substrates, the Pdot


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solution was added into a spray bottle together with 5-20% (v/v) of acetone to accelerate water evaporation, and then sprayed onto the substrates. The resulting samples were placed inside an oven at 60 °C for 10 min. The developed fingerprints were photographed by use of a Nikon D5500 digital camera equipped with AF-S DX NIKKOR 18-55mm f/3.5-5.6G VR lens and an orange filter under blue light (450 nm LED flashlight) illumination. The orange filter is used to filter out the blue excitation light.

Design and Synthesis of Semiconducting Polymers for Development of Latent Fingerprints. In an effort to obtain bright NIR fluorescent Pdots, we first synthesized two types of semiconducting polymers with NIR emissions and high brightness. As shown in Figure 1A, two NIR-emitting polymers, PF-BT-DBT and PF-BT-TQ, were synthesized using Suzuki polymerization. Their corresponding Pdot aqueous solutions were prepared via nanoprecipitation method and their absorption and emission spectra in the Pdot form were shown in Figure 1A, exhibiting large Stokes shifts (> 200 nm) and dominant UV to visible absorption (360-500 nm). The emission of PF-BT-DBT Pdots centered at 655 nm with a fluorescence quantum yield of 0.29, while the emission of PFBT-TQ Pdots centered at 680 nm with a fluorescence quantum of 0.32. The average particle size of the PF-BT-TQ Pdots was determined by AFM to be 16 nm (Figure 1B), and their average hydrodynamic diameter was measured by DLS to be 28 nm (Figure 1C). The size discrepancy can be attributed to the collapse of the nanoparticles once deposited onto the coverslip during AFM measurements. The morphology of the Pdots was also characterized by TEM, showing a spherical morphology (inset in Figure 1C). Based on the above results together with our previous studies that these NIR-emitting Pdots are at least 3-5 times brighter than inorganic quantum dots,35,37 the ultrahigh brightness of these Pdots should offer high contrast and excellent sensitivity for fingerprint imaging.

RESULTS AND DISCUSSION Our aim was to design and synthesize NIR fluorescent Pdots with high brightness and large Stokes shifts and then use these Pdots for latent fingerprint detection. The NIR emission of the Pdots can effectively lower the background interference because NIR light (λmaxemi > 660 nm) is invisible to the naked eye. In the fingerprint regions, on the contrary, the emission of the Pdots would blueshift (λmaxemi < 660 nm) due to the interactions between the Pdots and the fingerprint residues (vide infra), making the Pdots visible to the naked eye. In this scenario, the Pdots in the fingerprints could selectively “light up” for easy visualization. Moreover, the Stokes shift of the NIR-fluorescing Pdots should also be large enough to prevent any cross-talk between the excitation light and the fluorescence signals. PF-BT-TQ

PF-BT-DBT

C8H17

C8H17

N

S

N

N

S

S 0.5

C8H17

N

C8H17

N

S

C6H13O

OC6H13

N

N

0.45

0.05

0.5

n

0.45

0.05

n

Absorbance

PF-BT-DBT PF-BT-TQ

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400

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700

Number(%)

Occurrence

15 10 5 0

0

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18 14

22

x102

PF-BT-TQ With Fingerprint W/O Fingerprint NIR windows

18 14

10

10 640

680

720

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Selection of Pdots with Optimal Fluorescence for NakedEye Fingerprint Visualization. Once we synthesized NIR fluorescent polymers, we would like to choose the optimal Pdots for fingerprint visualization. Figure 2A displays the confocal fluorescence spectra of PF-BT-DBT Pdots on silica surface in which the emission exhibits an enhancement and a blueshift in the fingerprint regions as compared to that in the regions of no fingerprints. The emission blueshift (~14 nm) accompanied by the fluorescence enhancement (~ 1.1 times) could be attributed to the hydrophobic interactions between the fatty residues of the fingerprints and the semiconducting polymers10,54, in which these phenomena are very similar to the commonly observed solvent effect.55 Due to the emission enhancement and blueshift of Pdots upon contact with the fatty components of fingerprints, the majority of the Pdot

10 25

With Fingerprint W/O Fingerprint NIR windows

Figure 2. (A) Emission spectra of PF-BT-DBT Pdots measured by a confocal fluorescence microscope on silicon surface in the regions with (solid black line) and without (dashed black line) fingerprints. The inset in the upper-left corner shows the photograph of the latent fingerprint developed with PF-BT-DBT Pdots. (B) Emission spectra of PF-BT-TQ Pdots measured by a confocal fluorescence microscope on silicon surface in the regions with (solid red line) and without (dashed red line) fingerprints. The inset in the upper-left corner shows the photograph of the latent fingerprint developed with PF-BT-TQ Pdots.

40

0 20

PF-BT-DBT

Wavelength (nm)

(C)

20

22

x102

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(B)

(B)

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(A)

N

Intensity ( a. u.)

(A)

Emission

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40

Figure 1. Characterization of two types of NIR-fluorescing Pdots. (A) UV-visible spectra of PF-BT-DBT (dashed black line) and PF-BT-TQ Pdots (dashed red line) in water; emission spectra of PF-BT-DBT (solid black line) and PF-BT-TQ Pdots (solid red line) in water when excited at 450 nm. (B) Particle height histogram of PF-BT-DBT Pdots as measured by AFM. (C) Hydrodynamic diameters of PF-BT-DBT Pdots as determined by DLS. The inset in the upper-right corner shows the corresponding TEM image. Scale bar represents 100 nm.



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Scheme 1. Schematic Showing the Detection of Latent Fingerprints with Dual-Readout Assay Based on Pdotsa OC6H13

C6H13O C8H17

C8H17

N

S

N

N

N

Pdot

S 0.5

+

S 0.05

0.45

Co-precipitation

n

PF-BT-TQ

O OH OH O

Ninhydrin

m

+

PS-PEG-COOH O O

OH

n O O

360-500 nm L

Ambient Ligh

y Pdots

aFirst,

semiconducting polymer PF-BT-TQ, carboxyl-functionalized polystyrene PS-PEG-COOH, and ninhydrin dyes were mixed well in THF and then coprecipitated in water under vigorous sonication to form ninhydrin-embedded PF-BT-TQ Pdots. The Pdot solution was then sprayed onto latent fingerprint regions to image fingerprints with dual (colorimetric and fluorometric) readouts.

emission became visible to naked eyes. As a result, the latent fingerprints emerged with red fluorescence under UV-visible light (360-500 nm) irradiation while the background (i.e., the regions without fingerprints) appeared in NIR emission and remained invisible to naked eyes as shown in the inset of Figure 2A. This strategy offers a high contrast and low background interference for latent fingerprint development. Similar phenomena but a much larger blueshift of ca. 50 nm as well as an improved fluorescence enhancement by a factor of 1.8 could be observed for PF-BT-TQ Pdots (Figure 2B). This could be ascribed to the increased hydrophobic properties of the PF-BT-TQ polymers provided by the extended aromatic rings and long-chain alkyl groups (Figure 1A). Accordingly, the contrast of the developed fingerprints by using PF-BT-TQ Pdots is much higher than that by using PF-BT-DBT Pdots as shown in the insets of Figure 2. Based on these results, we thus chose PF-BT-TQ Pdots as our optimal fingerprint developing agent for the following studies. Preparation of NIR Fluorescent Pdots with Dual Readouts for Imaging of Latent Fingerprints. Once we have selected the PF-BT-TQ polymer, we intended to prepare a dual-readout (colorimetric and fluorometric) assay based on this polymer for fingerprint detection. Scheme 1 illustrates our strategy for the preparation of Pdot-based dual-readout assay by embedding ninhydrin into the matrix of Pdots via the nanoprecipitation method. One of the three types of polystyrene polymers: Carboxyl-functionalized polystyrene (PS-PEG-COOH, thiol-terminal polystyrene (PS-SH), and polyoxyethylene nonylphenylether (CO-520) was selectively blended with the PF-BT-TQ polymers to tailor the functional

groups of the resulting Pdots. PS-PEG-COOH was taken as an example in Scheme 1. The hydrodynamic diameters of the ninhydrin-embedded Pdots functionalized with carboxyl, thiol,

(A)

(B)

(D)

(E)

(C)

(F)

(G)

Figure 3. Images of latent fingerprints developed with Pdots on (A) white paper, (B) blue color paper, (C) red color paper, (D) green color paper, and (E) purple color paper. The left panels of each image represent the photographs of latent fingerprints under daylight, while the right panels represent that under 450-nm light irradiation with an orange filter. The magnified images in the squares of (C) and (E) are shown in (F) and (G), respectively.


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and hydroxyl groups were measured to be 26 nm, 44 nm, and 69 nm, respectively. The effect of the functional groups on the performance of the fingerprint detection was studied in the next sections. Here we added the Pdot solution into a spray bottle to uniformly spray Pdots onto the substrates with latent fingerprints. We expected that ninhydrin could specifically react with the amino acids excreted by eccrine glands to form a non-fluorescent purple product (i.e., Ruhemann’s pruple) with an absorption maximum at ~560 nm.1 The nonfluorescent property of ninhydrin as well as the mismatch between its absorption profile and the emission spectrum of PF-BT-TQ could minimize its optical interference with the fluorescence from PF-BT-TQ, thereby ensuring the dualreadout capability of this assay.

(A)

Glass

Aluminum Foil

Acrylic Sheet

Plastic Bag

(B)

Dual Colorimetric and Fluorescent Detection of Latent Fingerprints with High Resolution. The experimental procedures of latent fingerprint development using ninhydrinencapsulated PF-BT-TQ Pdots were recorded as a video and shown in Supporting Information. Figure 3 displays colorimetric and fluorescent images of the fingerprints developed by ninhydrin-embedded PF-BT-TQ Pdots on different colored paper (porous substrate). The developed fingerprints exhibited a purple color which could be clearly observed by naked eyes on white, red, and green paper, but could be barely distinguished on blue and purple paper due to the strong background interference. On the other hand, the fluorescence of the developed fingerprints could be apparently visualized on all of the colored paper under 450-nm light illumination with negligible background interference. The high contrast of the fluorescent signals of Pdots was attributed to the low fluorescent background from the substrates. It is worth mentioning that because it is rare to find an object in everyday use that exhibits red fluorescence, PF-BT-TQ Pdots are especially suitable for fingerprint development without background interference. The enlarged images of fingerprints on red and purple paper were shown in Figure 3F and 3G, respectively. We found that the resolution of the latent fingerprints detected by the fluorescent strategy is higher than that by the colorimetric strategy even though the fluorescent method requires an excitation source (e.g., LED UV/blue flashlight). Owing to the strong absorption of PF-BT-TQ Pdots in the UV-visible regions, the fluorescence of the fingerprints could also be imaged by UV light.

(C) Ridge Ending

Core

Island

Bifurcation

Scar

Pore

Figure 4. (A) Images of latent fingerprints developed with Pdots on glass, aluminum foil, acrylic sheet, and plastic bag. (B) Comparison of the fingerprints labeled by a red inkpad (left) and Pdots (right). (C) High-resolution fluorescence images of latent fingerprints showing level 1-3 details including core, ridge ending, bifurcation, island, scar, and pore. All fluorescence images were excited by a 450-nm LED flashlight with an orange filter.

for recognition. Level 2 details refer to minutiae points such as ridge endings and bifurcations, which are the most distinctive features. Level 3 features are defined as the dimensional attributes of ridges including sweat pores, ridge path deviations, and edge contours, which are able to provide quantitative data for accurate fingerprint recognition. Figure 4C shows three levels of fingerprint features including core (level 1), ridge ending (level 2), bifurcation (level 2), island/dot (level 2), scar (level 3), and pore (level 3). The high resolution of level 1-3 details in the fingerprint developed by the Pdot-based agent demonstrated its feasibility for fingerprint recognition.

To further assess the performance of this Pdot-based assay on non-porous surfaces, we developed latent fingerprints on four commonly used substrates including metallic and polymeric substrates. Figure 4A displays the fluorescence images of developed latent fingerprints on four non-porous substrates with high contrast and quality. The results indicate that this dual-readout fingerprint agent can be essentially applied on both porous and non-porous surfaces although ninhydrin is unsuitable for non-porous surfaces. Figure 4B shows the comparison of fingerprints produced by using a red inkpad (left) and by Pdots (right). The identical fingerprint patterns in these two images confirm the high accuracy, high sensitivity, and high reliability of this Pdot-based assay. Fingerprint recognition highly relies on the match of minutiae points and pores. Therefore, a high-resolution fingerprint with detailed characteristics is required for identification. Fingerprint features are usually classified at three levels of details.56 Level 1 details are macro features of fingerprints including cores and deltas, which are not distinctive enough

Effect of Functional Groups of Pdots on the Performance of Fingerprint Development. The interactions between the Pdots and the chemical components in the fingerprints can be attributed to both hydrophobic and electrostatic interactions.57 The hydrophobic interactions originated from the interactions between the fatty compounds (e.g., fatty acids, wax esters, squalene, and cholesterol) and the semiconducting polymers, while the electrostatic interactions stemmed from the sweat residues (e.g., inorganic salts, water, amino acids, proteins, glucose, and urea)



-SH

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-COOH

-OH

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Pdot-SH

50 30

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70

-30

-OH

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-50

clearly seen for Pdot-OH. The results indicate that the Pdots with a compact particle size and a negative zeta potential are beneficial for fingerprint development.

-OH -SH

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Bifurcation

Scar

Ridge Ending

Figure 6. Images of latent fingerprints developed with Pdots on (A) note paper and (B) a personal bank check. The upper panels of each image represent the photographs of latent fingerprints under ambient light, while the middle panels represent that under 450-nm light irradiation with an orange filter. The bottom panels show the enlarged fluorescence images of latent fingerprints.

Figure 5. Effect of Pdot functional groups on the fingerprint development. (A) Zeta potentials and hydrodynamic diameters of Pdot-COOH, Pdot-SH, and Pdot-OH. (B) Gel electrophoresis of Pdot-COOH, Pdot-SH, and Pdot-OH. (C) Images of latent fingerprints developed with Pdot-COOH (left), Pdot-SH (middle), and Pdot-OH (right) on white paper. The upper panels of each image represent the photographs of latent fingerprints under ambient light, while the bottom panels represent that under 450nm light irradiation with an orange filter.

Application of the Pdot-Based Assay for Latent Fingerprint Development. We also assessed the applicability of this Pdot-based assay for detection of latent fingerprints in real samples. Figure 6A shows the latent fingerprint detection on a piece of note paper with Chinese characters printed in black ink. Both the purple color produced by ninhydrin and the fluorescence generated by Pdots in the fingerprint patterns could be clearly observed. Although the blank ink might inevitably interfere the visualization of latent fingerprints, the high resolution and high contrast of the fingerprint details could still be distinctly recognized. The same results could also be seen on a personal bank check as shown in Figure 6B. These results demonstrated that this Pdot-based assay can be applied to a variety of samples with complex backgrounds and will have a widespread use in forensic sciences.

and the functional groups of the Pdots. To better understand the mechanisms of latent fingerprint detection, we fabricated three different functional groups onto the surface of Pdots and then investigated how the functional groups affected the detection of fingerprints. We first coprecipitated polymers with different functional units (PS-PEG-COOH, PS-SH, and CO-520) during nanoprecipitation to generate Pdots with carboxyl (Pdot-COOH), thiol (Pdot-SH), and hydroxyl (Pdot-OH) groups as illustrated in Figure 5. We then measured their zeta potentials and found that Pdot-COOH possessed the most negative surface potentials at -39 mV, followed by Pdot-SH (-30 mV), and Pdot-OH (-27 mV). On the contrary, their particle sizes showed an opposite trend in which the hydrodynamic diameters of 26 nm, 44 nm, and 69 nm were obtained for Pdot-COOH, Pdot-SH, and Pdot-OH, respectively. These results suggest that the particle size of Pdots is highly dependent on their zeta potentials, which is consistent with the reported data.36 Figure 5B shows the result of gel electrophoresis in which Pdot-COOH moved faster than both Pdot-SH and Pdot-OH, further confirming the observation from zeta potential measurements. The images of latent fingerprints developed by these three Pdots on white paper are displayed in Figure 5C. We found that the latent fingerprint developed by Pdot-COOH offered the highest quality of fingerprints as compared to Pdot-SH and Pdot-OH. Small particle aggregates were observed for Pdot-SH while large particle aggregates were

CONCLUSIONS In summary, we have developed a sensitive, selective, rapid, and efficient approach for latent fingerprint imaging based on ninhydrin-embedded NIR fluorescent Pdots. By taking advantages of the dual-readout ability of this Pdot-based assay, we could detect latent fingerprints with high contrast, high resolution, and low background interference virtually on all smooth surfaces. The high fluorescence brightness, large Stokes shift, dominant UVvisible absorption, and NIR emission of Pdots contributed to the high-resolution imaging of latent fingerprints with distinct level 13 details. We have also successfully applied this assay in


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developing latent fingerprints on note paper and personal bank checks. We anticipate this Pdot-based fingerprint developing agent to find broad utility in research of latent fingerprint detection and further anti-counterfeiting applications.

ASSOCIATED CONTENT Supporting Information. High-resolution images of Figure 3-6, and video of fingerprint developing processes. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT We would like to thank the Ministry of Science (105-2113-M110-012-MY3), NSYSU-KMU Joint Research Project (105P001), and National Sun Yat-sen University.

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