Dual-mode, Color-tunable, Lanthanide-Doped Core-Shell

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Applications of Polymer, Composite, and Coating Materials

Dual-mode, Color-tunable, Lanthanide-Doped Core-Shell Nanoarchitectures for Anti-counterfeiting Inks and Latent Fingerprint Recognition Jun Xu, Beibei Zhang, Lei Jia, Yanping Fan, Rujie Chen, Tinghui Zhu, and Baozhong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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ACS Applied Materials & Interfaces

Dual-mode, Color-tunable, Lanthanide-Doped Core-Shell Nanoarchitectures for Anti-counterfeiting Inks and Latent Fingerprint Recognition Jun Xu,† Beibei Zhang,† Lei Jia,*,† Yanping Fan,*,† Rujie Chen,† Tinghui Zhu†, BaoZhong Liu

†

College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo,

Henan, 454000, China

KEYWORDS: dual-mode luminescence, layered lanthanide hydroxide, multi-color fluorescence, anti-counterfeiting, latent fingerprint

ABSTRACT: With the rapid development of information in modern society, the research and development of advanced anti-counterfeiting technology is becoming more and more important to protect the security and comprehensiveness of information. Therefore, fluorescent ink as an anti-counterfeiting technology and fingerprint recognition technology as a "human information identification card" has attracted the attention of 1

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many research groups. Herein, dual-mode (upconversion (UC) and down-conversion (DC))

lanthanide-doped

luminescent

nanoarchitectures

were

developed

using

Y2O3:Er3+,Yb3+ nanoparticles as a core, and layered lanthanide hydroxides (LLHs) nanomaterials as a shell. Under the irradiation of 980 nm near infrared light, the nanoarchitectures emitted a bright up-converted red light emission. While under the irradiation of 254 nm UV light, the nanoarchitectures can directly emit multi-color luminescence (from green to yellow-green, yellow, orange and red) by changing the suitable ratios of Tb3+/Eu3+ ions. The information can only be extracted when the irradiation of two kinds of excitation light sources existed at the same time, which can improve the difficulty of illegal imitation and enhance the level of anti-counterfeiting. Furthermore, these luminescent nanoarchitectures were investigated for visual latent fingerprints recognition on various substrates with high definition, high sensitivity and high anti-interference. These results indicated that the nanoarchitectures reported in this study may have great application prospects in information security and identity recognition. 1. INTRODUCTION

Counterfeiting is an increasingly serious and long-standing global problem, which is also more common in our daily life.1 Documents and valuable items, such as banknotes, diplomas and certificates, are widely and illegally copied in every country and pose a serious security threat to institutions, including individuals, companies and communities, 2


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etc.2,3 Advances in modern science and technology have contributed to counterfeiting and fraud. Therefore, anti-counterfeiting has been paid more and more attention by all countries in the world, and governments are also trying to develop advanced anti-counterfeiting technology which can protect important documents replicated.4,5 The application of fluorescence technology to high-tech anti-counterfeiting is one of the most effective technical methods. With the deepening of research, more and more optical materials, such as lanthanide-doped nanocomposites (NCs), quantum dots and metal-organic skeleton (MOF), have been used in the anti-counterfeiting marking of products.6-10 However, it is still a difficult point to produce large amounts of MOFs by chemical synthesis for anti-counterfeiting, and semiconductor quantum dots have a certain degree of toxicity. Among the above many kinds of fluorescent materials, the fluorescent materials containing lanthanide elements have the advantages of low toxicity, strong fluorescence intensity, long and stable fluorescence lifetime, and have been favored by many scientific researchers. In particular, lanthanide ions not only show the traditional Stökes type (that is, down-conversion) emission, but also have efficient antiStökes type (that is, upconversion) emission.11,12 The principle of upconversion (UC) emission is that the materials absorb low-energy photons and emit thigh-energy photons, while the principle of down-conversion (DC) emission is just the opposite of UC. The UC lanthanide ion pairs are always Yb3+/Er3+ and Yb3+/Tm3+ ions, which usually display red, green or blue colors under the irradiation of 980 nm.13 Recently, Yingliang Liu and Jianle Zhuang successfully fabricated the 3



dual-mode luminescent NaYF4:Yb,Tm@SiO2/carbon dot nanocomposites, which displayed good properties.14 The well-known DC materials, such as Tb3+ (green) and Eu3+ (red) ions doped materials can emit different colors according to the type of doping under the excitation of ultraviolet light.15 Dual-mode luminescence can integrate different fluorescent colors of UC and DC into the same luminous platform, which can improve the technical barrier of illegal counterfeiting and enhance the scientific and technological content of anti-counterfeiting technology. In the previous dual-mode anti-counterfeiting studies, a variety of rare earth elements with UC and DC properties are often uniformly mixed into the same material. Meanwhile, lanthanide cross relaxation often occurs, which leads to the decrease of luminescence intensity.16,17 An improved method is to introduce UC and DC lanthanide elements into the inner layer or outer layer of the core-shell nanostructure, respectively. Recently, Jianxiong Xu group reported lanthanide-doped NaYF4@NaGdF4 core–shell NPs with dual-mode luminescent properties, which can also be used for anti-counterfeiting printing.18 In view of the abovementioned results, a dual-mode luminescent core-shell nanoparticle can be designed and fabricated for the application in the field of anti-counterfeiting. Simultaneously, "fingerprint" is called "human identification card", which has inherent specificity. Fingerprints will also appear at some crime scenes, and will also be used as evidence to prove or establish a crime.19 At the same time, because of its specificity, 4


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fingerprint will also be used as an unlock tool or permission access tool, such as the current more popular fingerprint unlock or fingerprint identification. But in most cases, the fingerprints we find are latent, and the basic principle of fingerprint detection is to reproduce the fingerprint clearly by technical means while maintaining the integrity of the fingerprint pattern.20 In the study of visual detection of fingerprint clarity, methods such as iodophor method, ninhydrin colorimetric method, silver nitrate titration and fluorescence method have been proposed.21 Among them, the photoluminescence method has the unique advantages of high sensitivity, simple operation and suitable for on-site detection, so it has a broad application prospect in fingerprint detection.22

Scheme 1. Schematic diagram of the construction of dual-mode luminescent Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=0, 0.1, 0.2, 0.5, 1) nanoarchitectures.

5



Herein, dual-mode luminescent core-shell nanoarchitectures were designed and synthesized by a facile and green route (scheme 1) in this research. Lanthanide oxides (Y2O3), which were often considered to be excellent matrix materials for high efficiency UC luminescence due to their high chemical stability and low photon energy, were used as the inner core.23 The Y2O3:Er3+,Yb3+ core prepared in this work was monodispersed and showed strong red upconversion emission under 980 nm laser irradiation, which was ascribed to 4F9/2→4I15/2 transition of Er3+ ions. Doping Tb3+ and Eu3+ ions into the appropriate matrix materials can be used as DC luminescent shells. In recent years, layered lanthanide hydroxides (LLHs) with the general formula Ln2(OH)5X·nH2O (Ln = lanthanides and X = interlayer organic or inorganic anions), have emerged as a novel and promising matrix materials for DC luminescence, owing to their low toxicity, high stability and multi-color fluorescence tunability. As anionic clays,24,25 LLHs have a unique layered structure, similar to layered double hydroxides (LDHs),26-28 which consist of double lanthanide cations in the host layers and the exchangeable interlayer anions layers to neutralize electric charge.29-33 Therefore, a core-shell nano-architecture was developed via in situ growth of the positively charged LLHs, Ln2(OH)5(NO3)·nH2O (Ln=Gd3+, Tb3+ and Eu3+ ions) on the surface of the negatively charged surface silica modified Y2O3:Er3+,Yb3+ (Y2O3:Er3+,Yb3+@SiO2) nanomaterials. However, the parity transition and spin-forbidden transition of lanthanides resulting in the low 4f-4f excitation efficiency, considerably limit real application of LLHs in optical purposes. In addition, the layer structure of LLHs contained a large number of water molecules and hydroxyl 6


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groups, which can provide a non-radiative attenuation pathway and inhibit the emission of lanthanide activators to a great extent.34,35 Therefore, anionic organic sensitizer pyromellitic acid (PMA) was inserted into in the interlayer through ion exchange with interlayer anions to improve the luminescent intensity of LLHs. The shell nanomaterials can directly emit different luminescent color under 254 nm lamp (from green to yellow-green, yellow, orange and red) by adjusting the molar ratio of Tb3+/Eu3+ions. This dual-mode luminescent NCs effectively realized the anti-counterfeiting protection of valuables by manufacturing high-end luminous safety ink. In addition, the reported luminescent NCs here had the characteristics of high contrast, good selectivity, small background interference, which showed high advantages in visual latent fingerprints recognition. The nanoarchitectures we designed may also provide a new method to design multifunctional nanomaterial for anti-counterfeiting applications and fingerprints recognition. 2. EXPERIMENTAL DETAILS

2.1 Materials and reagents Gadolinium

nitrates

(Gd(NO3)3·6H2O,

99.95%)

(REO),

europium

nitrates

(Eu(NO3)3·6H2O, 99.99%) (REO)and terbium nitrates (Tb(NO3)3·6H2O, 99.99%) (REO) were purchased from Shanghai Energy Chemical. Pyromellitic acid (PMA, ≥98%) were purchased from Adamas Reagent Co., Ltd. Yttrium(III) nitrate hexahydrate (Y(NO3)3·6H2O, 99.99%) (REO), ytterbium(III) nitrate pentahydrate (Yb(NO3)3·5H2O,

7



99.99%) (REO) and erbium(III) nitrate pentahydrate (Er(NO3)3·6H2O, 99.99%) (REO) were also from Adamas Reagent Co., Ltd. In addtion, there were ammonia solution (25-28%), silicic acid tetraethyl ester (TEOS, >28.0%), urea (99%, Adamas Reagent) and ethanol (≥99.7%). 2.2 Measurements Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy dispersive spectrometry (EDS) and EDS mapping were obtained by using a Tecnai-G2-F30 microscope at an acceleration voltage of 300 kV. The zeta potential measurement of each samples were analyzed by Malvern Zetasizer 2000 analyzer. The surface compositions of the samples were investigated by Thermo Scientific ESCA Lab 250Xi X-ray photoelectron spectroscopy (XPS). CHN elemental analyses were performed on an Elementar Vario EL analyzer. Powder X-ray diffraction patterns (PXRD) were recorded with Rigaku-Dmax 2400 diffractometer using Cu Kα radiation over the 2θ range of 5-80°. Fourier transform infrared (FTIR) spectra of the materials were measured on a Bruker V70 FTIR spectrometer with in the 4000-400 cm-1 wavenumber range utilizing the standard KBr disk technique. Upconversion luminescence spectra were measured by using an Edinburgh FS5 with a high-power 980 nm laser diode. Down-conversion luminescence spectra in water solution were carried out on a Varian CARY Eclipse fluorescence spectrometer at the 280 nm excitation wavelength. Lifetime decay measurements were carried out with Edinburgh FS5 equipped with a cooled R928P photomultiplier tube. The quantum yields of the samples 8


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were determined by an absolute method using a barium sulfate coated integrating sphere on Hamamatsu Instrument Quantaurus-QY C11347. Three parallel measurements were carried out for each sample, so that the presented value corresponds to the arithmetic mean value. 2.3 Preparation of Y2O3:Er3+,Yb3+ nanospheres The synthesis of Y2O3:Er3+,Yb3+ nanospheres were reported in previous literature.36 In a typical synthetic work, 1.057 g of Y(NO3)3·6H2O, 0.067 g of Yb(NO3)3·5H2O and 0.042 g of Er(NO3)3·6H2O were accurately weighed and dissolved in 50 mL distilled water, respectively. Then the above solutions were mixed to form a transparent solution. Under vigorous stirring, 12 g of urea (first ultrasonically dispersed in 50 mL of distilled water) was added into the mixed solution. The resulting mixture was heated at 90℃for 5 hours, the precipitates were collected by centrifugation and washed with distilled water for three times. The obtained Y2O3:Er3+,Yb3+ precursor were fired at 1000℃ for 2 hours to forming the Y2O3:Er3+, Yb3+ nanospheres. 2.4 Preparation of Y2O3:Er3+,Yb3+@SiO2 nanoparticles A layer of silica was uniformly deposited on the surface of the Y2O3:Er3+, Yb3+ nanospheres using the literature method.36 100 mg of Y2O3:Er3+,Yb3+ powders were ultrasonically dispersed in 80 mL of the prepared solution containing ethanol and deionized water (v/v = 3:1 ). Then, 1.2 mL of aqueous ammonia was added into the above solution. After sonicating for 20 minutes, 0.323 mL of TEOS solution were slowly added

9



and the mixed solution was stirred for 17 h at 60 ˚C. The products were centrifuged and washed three times with a mixed solution of deionized water and ethanol solution. 2.5 Preparation of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH (x=0, 0.1, 0.2, 0.5, 1) The Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH (x=0, 0.1, 0.2, 0.5, 1) were synthesized via facile hydrothermal treatment. Typically, 100 mg of Y2O3:Er3+,Yb3+@SiO2 nanoparticles were ultrasonically dispersed in 10 mL distilled water, Gd(NO3)3·6H2O (0.50 mmol), the mixture of Eu(NO3)·6H2O and Tb(NO3)3·6H2O (0.50 mmol, the molar ratio of Eu(III)/Tb(III) = 0: 1, 0.1: 0.9, 0.2: 0.8, 0.5: 0.5, 1: 1, respectively) and KNO3 (4 mmol) were respectively dissolved in 5 mL distilled water, and then mixing with the Y2O3:Er3+,Yb3+@SiO2 aqueous solution. After stirring for 20 min, KOH (0.5 M) solution was added dropwise to the mixture for adjusting pH to 6.94. The obtained mixed solution was heated at 150 ˚C for 48 h in a Teflon-autoclave, and the final products were collected by centrifugation and washed with ethanol for three times. 2.6 Preparation of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=0, 0.1, 0.2, 0.5, 1) nanocomposites Firstly, 50 mg Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH (x=0, 0.1, 0.2, 0.5, 1) dissolved into 10 mL deionized water forming a milky white suspension, which was treated with ultrasonication for half an hour. Secondly, the aqueous solution of NaOH (1 mmol) was added into PMA (0.25 mmol) for deprotonation (pH= 7.35). The above two solution were subsequently mixed and treated hydrothermally at 180 ˚C for 5 h. After by cooling, the

10


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obtained precipitates were collected, washed with ethanol for three times to remove any impurity and then dried in vacuum. 2.7 Preparation of the security inks The hydrophilic security inks were fabricated by the following process: the optimal viscosity solution was firstly prepared with ethanol : water (v/v=10:32), which was added into 18 mL glycerol, and then stirred evenly. Subsequently, sodium dodecyl sulfate and the multicolor anti-counterfeiting material were dispersed in 5 mL of the above solution under ultrasonic for 20 min. To print a pattern on black paper, the stamp of Henan Polytechnic University was touched with multicolor inks and then pressed on black paper. 2.8 Latent fingerprints visualization All fresh latent fingerprints were taken from the same volunteer by rubbing on the forehead, and then lightly press on the different substrates, including glass petri dish, plastic petri dish, plastic sealed bag, aluminum alloy, ceramic tile, mouse and so on. In order to create latent fingerprints, the nano-powder was firstly shaken onto the surface of the whole fingerprint, and then gently swept along the fingerprint lines by using a soft feather brush until the fingerprint images were developed. The final fingerprint images were recorded under day-light or 254 nm UV irradiation. 3. RESULTS AND DISCUSSION

11



3.1

Morphology

and

structure

properties

Page 12 of 42

of

Y2O3:Er3+,Yb3+@SiO2@

in

well-defined

LGdEuxTb1-xH-PMA core-shell nanocomposites To

realize

efficient

dual-mode

luminescence

core-shell

nanoarchitectures, Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=0, 0.1, 0.2, 0.5, 1) NCs were successfully synthesized via layer-by-layer self-assembly strategy. TEM and EDX spectroscopy were adopted to characterize the as-synthesized NCs in order to investigate the microstructure, composition and shape evolution of this core-shell NCs. It was observed in Figure 1a that Y2O3:Er3+,Yb3+ nanoparticles were monodisperse spherical structure with smooth surface, and the diameter was around 120 nm. The lattice fringes shown in the HRTEM image (Figure 1a, inset) indicated the formation of polycrystalline Y2O3 with good crystallinity. The observed lattice fringes of 0.30 nm, 0.27 nm and 0.43 nm from the HRTEM image can be assigned to the d-spacing of (222), (400) and (211), respectively.36 After being coated by a thin silica layer (Figure 1b), the morphological features of the nanospheres became rough as compared with pristine Y2O3:Er3+,Yb3+ nanoparticles and the average size of nanospheres was found to be around 130 nm. Due to Y2O3:Er3+,Yb3+@SiO2 nanospheres with the negatively charged surface, the positively charged LLHs precursor can be deposited on their surface via electrostatic interactions, resulting in the formation of core-shell structure. As shown in Figure 1c, the LGdEu0.5Tb0.5H-PMA shell layer fully covered the surface of the core layer. Moreover, the energy dispersive X-ray spectroscopy spectra (EDS, Figure 1d) revealed the presence of Y(III), Yb(III), Er(III), Si(IV), Gd(III), Tb(III) and Eu(III) in the 12


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synthesized

core-shell

NCs,

further

Y2O3:Er3+,Yb3+@[email protected]

confirming

core-shell

that

nanoarchitecture

the was

successfully obtained.

Figure 1. TEM images of (a) Y2O3:Er3+,Yb3+, (b) Y2O3:Er3+,Yb3+@SiO2, (c) Y2O3:Er3+,Yb3+@[email protected]

and

(d)


NCs.

Inset:

EDS HRTEM

spectra images

of of

Y2O3:Er3+,Yb3+ and Y2O3:Er3+,Yb3+@SiO2 shown in part a and b. Furthermore,

the

EDS

mappings

of

elemental

distributions

of

Y2O3:Er3+,Yb3+@[email protected] were shown in Figure 2. Figure 2b-d revealed that Y(III), Yb(III) and Er(III) were uniformly distributed in the core, and Figure

13



2e displayed elemental map of Si along the shape of the core, revealing that the core was homogeneously coated by the SiO2 encapsulation layer. Figure 2f-h described that Gd(III), Tb(III) and Eu(III) were uniformly distributed in the shell, which was consistent with the designed elemental distribution, and further validated the synthesis of core-shell structure.

Figure 2. (a) STEM image and the corresponding STEM elemental mapping for (b) Y(III), (c) Er(III), (d) Yb(III), (e) Si(IV), (f) Gd(III), (g) Eu(III) and (h) Tb(III) of Y2O3:Er3+,Yb3+@[email protected] NCs. The zeta potential measurement of each samples were analyzed by Malvern Zetasizer 2000 analyzer. The Y2O3:Er3+,Yb3+ nanoparticles had a positive zeta potential of +31.2 mV, which was benefical to the coating of the negatively charged silica shell using a modified Stöber method. After modifying with silica shell, the zeta potential decreased to -8.6 mV, which facilitated the deposition of positively charged LGdEu0.5Tb0.5H precursor on their surface through electrostatic interactions. The zeta potential value for the obtained Y2O3:Er3+,Yb3+@[email protected] was +3.4 mV. These zeta 14


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potential measurements strongly implied a successful surface functionalization of both silica shell and LGdEu0.5Tb0.5H-PMA to the Y2O3:Er3+,Yb3+ nanoparticles. XPS characterization was further employed to investigate the element composition of Y2O3:Er3+,Yb3+ and Y2O3:Er3+,Yb3+@[email protected] (Figure 3a). It can be observed that the peaks at 157.0, 168.5.0 and 183.9 eV for Y2O3:Er3+,Yb3+ were assigned to Y-3d, Er-4d and Yb-4d, respectively. Compared with pure Y2O3:Er3+,Yb3+, the intensity of the Y-3d, Er-4d and Yb-4d peaks of Y2O3:Er3+,Yb3+@SiO2@LGdEuTbH-PMA decreased, while the Si-2P, Gd-3d, Eu-3d and Tb-3d peaks appeared, indicating the formation of silicon and LGdEuTbH shells on the surface of the Y2O3:Er3+,Yb3+ core. The insertion of PMA in the Y2O3:Er3+,Yb3+@SiO2@LGdEuTbH can be demonstrated by XPS N1s spectra (Figure S1). The N1s spectra of Y2O3:Er3+,Yb3+@SiO2@LGdEuTbH located at 406.7 eV was characteristic of NO3- with an oxidation state of +5.37 However, it disappeared after exchanging with PMA anions, indicating that the nitrate anions had been replaced by PMA anions. This was consistent with the results from elemental analysis. The chemical composition of the corresponding intercalated materials was also determined by the content of C, H and N (Table S1). It was found that the content of N element was greatly reduced and the content of C element was correspondingly increased after exchanging with PMA anions, indicating that the PMA anions replaced the nitrate anions in the interlayer.

15



Figure

3.

(a)

XPS

survey

of

Y2O3:Er3+,Yb3+

Page 16 of 42

(red)

and

Y2O3:Er3+,Yb3+@[email protected] (blue) samples; (b) PXRD patterns of Y2O3:Er3+,Yb3+ (A), Y2O3:Er3+,Yb3+@SiO2 (B), Y2O3:Er3+,Yb3+@[email protected] (C) and Y2O3:Er3+,Yb3+@[email protected] NCs (D). The successful assembly of the Y2O3:Er3+,Yb3+@[email protected] nanoparticles can also be confirmed by the FT-IR spectra. Figure S2 showed the FTIR spectra of a series of materials, such as Y2O3:Er3+,Yb3+, Y2O3:Er3+,Yb3+@SiO2, Y2O3:Er3+,Yb3+@[email protected]

and

Y2O3:Er3+,Yb3+@

[email protected]. Compared with Y2O3:Er3+,Yb3+, Y2O3:Er3+,Yb3+@SiO2 had obvious Si-O-Si characteristic peaks at 1078 cm-1 and 801 cm-1, and obvious Si-OH characteristic peaks at 954 cm-1, indicating that Y2O3:Er3+, Yb3+ core surface was successfully covered with a layer of SiO2 shell.38-41 The intense band at 1384 cm-1 was corresponded to the ν3 mode of the nitrate species appeared in the FTIR spectra of Y2O3:Er3+,Yb3+@[email protected], while it was correspondingly weakened after exchanging with PMA anions.29,42 Moreover, the symmetric stretching vibration of the

16


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–COO- group appeared at 1520 cm-1, which illustrated that PMA molecules successfully displaced the nitrate ions and intercalated into the LRH gallery. The successful insertion of PMA in the Y2O3:Er3+,Yb3+@SiO2@LGdEuTbH can also be verified by the PXRD pattern. The PXRD patterns of Y2O3:Er3+,Yb3+, Y2O3:Er3+,Yb3+@SiO2, Y2O3:Er3+,Yb3+@[email protected], and Y2O3:Er3+,Yb3+@ [email protected] samples were displayed in Figure 3b. It can be seen that all samples had the main characteristic peaks at the planes (211), (222), (400), (440) and (622), which can be attributed to the standard diffraction peaks of cubic phased Y2O3 (JCPDS #01-0831)36,43,44. Compared with the Y2O3:Er3+,Yb3+@SiO2, a strong (002) reflection

at

9.7°

appeared

in

the

PXRD

pattern

of

Y2O3:Er3+,Yb3+@[email protected], indicating the formation of LGdEuTbH layered phase on the surface of Y2O3:Er3+,[email protected] The exchange reaction between NO3− and PMA anions in the interlayer space of LGdEuTbH led to the formation of LGdEuTbH-PMA.

The

(002)

reflection

of

Y2O3:Er3+,Yb3+@[email protected] shifted toward lower reflection angle, which was due to the increase of the interlayer distance. This expansion of basal spacing from ∼9.12 to ∼13.33 Å indicated that the PMA were inserted into the LGdEuTbH gallery. 3.2 The upconversion photoluminescence and down-conversion properties of the Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=0, 0.1, 0.2, 0.5, 1) nanocomposites

17



Figure

4.

(a)

Upconversion

luminescence

Page 18 of 42

emission

spectra

of

Y2O3:Er3+,Yb3+@[email protected] NCs with excitation at 980 nm; (b) energy transfer

mechanism

diagram

of

upconversion

emission

for

Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=0, 0.1, 0.2, 0.5, 1). In order to explore the upconversion luminescence properties, the upconversion luminescence

emission

spectra

of


core-shell NCs were studied under the excitation at 980 nm continuous wave laser diode (Figure 4a). The emission spectra consisted of two primary emission bands: one with strong red emission band at 654-683 nm and another with weak green emission band at 521-564 nm, which were respectively ascribed to 4F9/2→4I15/2 and 2H11/2/4S3/2→4I15/2 transition of Er3+.36 The typical energy transfer mechanism of the upconversion emission in Er3+/Yb3+ co-doping core was shown in Figure 4b. First, the Yb3+ electrons located in 2

F7/2 level (ground state) were excited by the absorption of 980 nm light photon energy to

reach the 2F5/2 level (excited state), and then the energy was absorbed by the electrons of Er3+ ions, resulting in energy transitions toward the 4I11/2 level (excited state). Although Er3+ can be also directly excited under 980 nm laser irradiation, Yb3+ can absorb photons 18


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energy more easily. In addition, Er3+ electrons can receive one more photon from Yb3+ energy and be promoted from the 4I11/2 level to the 4F7/2 level by transferring energy. Finally, the electrons decayed to 4F9/2 and 2H11/2/4S3/2 level from the 4F7/2 level by photons nonradiative relaxation, which produced red emission band and green emission band, respectively.46 Due to the green emission band was much weaker and thus red upconversion luminescence emission can be easily seen with the naked eye (inset in Figure 4a). Furthermore, the absolute UC quantum yield was measured to be 2.3% by using

an

integrating

sphere.

The

photoluminescence

decay

time

of

Y2O3:Er3+,Yb3+@[email protected] NCs were measured at room temperature and the obtained 4F9/2 lifetime value was 667 μs (Figure S3), which was compatible with the reported values.47 To fabricate the dual-mode luminescent NCs, we designed and modified color-tunable layered down-conversion luminescent lanthanide hydroxides on the surface of Y2O3:Er3+,Yb3+@SiO2. In order to demonstrate that inserted pyromellitic acid (PMA) can improve the luminescent intensity of layered down-conversion luminescent lanthanide hydroxides

(LLHs),

Y2O3:Er3+,Yb3+@SiO2@LGdTbH

and

Y2O3:Er3+,Yb3+@SiO2@LGdEuH examples were used to determine the fluorescence spectra before and after inserting PMA (Figure S4 and Figure S5). Due to the parity transition and spin-forbidden transition of lanthanides, no efficient energy can transfer to lanthanide ions and the lanthanides were excited directly via the intra-4f6 level, which made the emission intensity of LLHs very weak.48 After inserted by PMA anions, PMA 19



can efficiently transfer energy to the lanthanide ions. The characteristic emission intensity of Eu3+ ions and Tb3+ ions significantly enhanced. According to the ratio of Tb3+/Eu3+ ions, Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=0, 0.1, 0.2, 0.5, 1) NCs exhibited a variety of luminescent colors from green to red under 280 nm excitation (Figure 5a). The characteristic emission peaks at 593, 617, 651, and 699 nm can be assigned to the 5

D0→7FJ (J = 0, 1, 2, 3, 4) transition of Eu3+ ions. Similarly, the sharp emission peaks

observed at 489, 545, 586, and 621 nm can be attributed to 5D4→7FJ (J= 6, 5, 4, 3, 2) transition of Tb3+ ions.49-51 For the samples containing both Tb3+ and Eu3+ ions, the emission spectrum contains the characteristic emission peaks of the two ions. According to the relative concentrations of Tb3+ and Eu3+ ions, the emission spectra of the samples were basically similar, but the relative emission intensity was slightly different. Concentration dependence of the absolute emission intensity of the Tb3+ ions, Eu3+ions, the Tb3+/Eu3+ ratio and DC quantum yields (Φ) were listed in Table S2 and Figure S6. It can be found that the emission intensity of Tb3+ ions were reduced obviously, even when the content of Eu3+ ions was small. Accordingly, the red emission of Eu3+ ions was becoming predominant. The quantum yield of NCs decreased with the decrease of Tb3+/Eu3+ ratio, which can be attributed to both the reduced Tb3+ emission and the Tb3+-sensitized Eu3+ emission.32

20


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Figure 5. (a) Down-conversion luminescence emission spectra (the upper right corner was the digital luminescent photographs of samples with different Tb3+/Eu3+ ratios under 254 UV lamp), (b) CIE chromaticity diagram with (x, y) emission color coordinates, and (c) energy transfer mechanism diagram of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=0, 0.1, 0.2, 0.5, 1) NCs with UV excitation at 280 nm. (d) Photoluminescence decay curves of 5D0 of Eu3+ in samples Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x= 0.2, 0.5, 1; ex= 280 nm, em= 617 nm).

In addition, the digital luminescent photographs of samples with different Tb3+/Eu3+ ratios (the upper right corner of Figure 5a) for these NCs were also displayed under UV

21



irradiation (254 nm). As illustrated in Table S2 and Figure 5b, the 1931 Commission Internationale de l'éclairage (CIE) chromaticity coordinates of Y2O3:Er3+,Yb3+@SiO2@ PMA-LGdEuxTb1-xH (x=0, 0.1, 0.2, 0.5, 1) NCs under excitation at 280 nm were calculated and marked in the CIE chromaticity diagram. When the concentration of Eu3+ ions increased, the luminescent color shifted to the red region, indicating that the fluorescence colors of the NCs can be adjusted by changing the ratio of Tb3+/Eu3+ ions. Importantly, the photoluminescence of Y2O3:Er3+,Yb3+@SiO2@ -LGdEuH-PMA NCs were weak in the absence of Tb3+. It was generally believed that the triplet energy level of PMA was 26310 cm-1,42,52 higher than the 5D0 level of Eu3+ (17300 cm-1, resonance energy level ) and the 5D4 level of Tb3+ (20500 cm-1). The empirical rules supposed that energy transfer was favorable when energy gap of Eu3+ or Tb3+ (T1→5DJ) was 2500-4500 cm-1. Therefore, the distance between the triplet energy level of PMA and the resonance energy level of Eu3+ ions was too high to transfer energy to europium ion sufficiently. Without terbium ion, the strong emission of Eu3+ can be observed, which was mainly caused by the energy transfer from Tb3+ to Eu3+ and the mechanism was shown in Figure 5c. During this energy transfer process, electrons in the organic sensitizers (PMA) can capture photon energy under 280 nm excitation, and transfer from the ground state (S0) to the first excited state (S1) and further to the lowest excitation triplet state (T1).42 Subsequently, the electrons can directly transfer to the lowest excited state (5D4) of Tb3+, while the lowest excited state (5D0) electrons of Eu3+ may be derived from T1 of the ligand and 5D4 of Tb3+. Finally, the excited electrons returned to the ground state of RE3+ 22


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ions, which resulted in red emission (5D0→7FJ transition at 616 nm) and green emission (5D4→7FJ) transition at 545 nm), respectively. It can be further confirmed by the following fluorescence lifetime experiments. The decay curves of 5D0 emission of Eu3+ ions were monitored at 612 nm under the excitation at 280 nm. All these curves can be fitted by bi-exponential equation:

I  I 0  A1 exp(t / 1 )  A2 exp(t /  2 )

(1)

where I is the luminescence intensity; A1 and A2 are constants; t is the time, and τ1 and τ2 are rapid and slow lifetimes for exponential components, respectively. Since the multiexponential decay curves which are usually observed in the solid composites, the average lifetime τav was calculated as:

 av  ( A112  A2 22 ) / ( A11  A2 2 )

(2)

According to the above formula (2), the average decay times of Eu3+ ions were listed in Figure 5d. The average decay time of Eu3+ ion increased with the increase of Tb (III) concentration, indicating that there was obvious energy interaction in the NCs.30 3.3. The Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA NCs as Anti- Counterfeiting Fluorescent Ink

The rapid development of modern science and technology and the increasingly rampant activities of counterfeiting and forgery have promoted the development of various anti-counterfeiting technologies. Among the many anti-counterfeiting technologies, fluorescent encryption anti-counterfeiting technology has become a widely used

23



anti-counterfeiting

technology because

of

its

good

Page 24 of 42

concealment

and

strong

anti-counterfeiting strength. In order to encrypt the anti-counterfeiting information, we dispersed the synthesized dual-mode multicolor luminescent Y2O3:Er3+,Yb3+@ SiO2@PMA-LGdTbH NCs in the glycerol and ethanol-water solution to prepare anti-counterfeiting ink, and then draw anti-counterfeiting numbers (Figure 6 and Figure S7). As seen in Figure 6, the number “1” had been handwritten on the normal pattern by using the synthesized fluorescence security inks and the markings were almost invisible under normal light. When “1” was irradiated with 254 nm light, it can emit green light, and the fluorescence of “1” was red under the irradiation by a 980 nm laser. The anti-counterfeiting information can be extracted under the irradiation of two kinds of excitation light sources (254 nm and 980 nm). Its characteristic was that only the two luminescent colors were completely correct to confirm the authenticity of anti-counterfeiting printing, making it difficult to forge. Patterning played an essential role in anti-counterfeiting process, so the multicolor luminescence patterns (20mm, 40mm in size) consisting of Chinese characters (Henan Polytechnic University) and the abbreviation “HPU” were demonstrated in Figure 7 at high resolution. More interestingly, the NCs we synthesized as fluorescent ink can also be used in the field of multicolor fluorescence anti-counterfeiting. By adjusting the proportion of Eu3+ and Tb3+ ions, a variety of fluorescence colors can be obtained for different anti-counterfeiting requirements.

24


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Figure 6. (a) The normal pattern with hand-written Arabic numeral of “1” in the red region (Inset: the photographs of the as-prepared ink under 254 nm (left) and 980 nm (right) excitation). (b) Photograph of the purple square region under 254 nm UV lamp illumination. (c) Photograph of the red circle region under 980 nm laser irradiation.

Figure 7. Multi-color luminescent patterns by using Y2O3:Er3+,Yb3+@SiO2@ LGdEuxTb1-xH-PMA (x=1 (a), 0.5 (b), 0.2 (c) and 0 (d)) NCs under 254 nm UV irradiation. All the figures have the same size and the scale bar was 0.5 cm. 25



In

order

to

illustrate

the

stability

of

Page 26 of 42

ink,

we

selected

Y2O3:Er3+,Yb3+@SiO2@LGdTbH-PMA NCs doped ink as an example and tested the fluorescence stability. Upon exposure of the ink to ambient surroundings for 10 days, the fluorescence of the ink had almost no difference both under 980 nm and 254 nm UV light excitation (Figure S8), demonstrating that these dual-mode luminescent NCs can serve as an excellent stable source for enhancing anti-counterfeiting performance. 3.4 Detection of Latent Fingerprint The Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA NCs were selected to develop latent fingerprints because of their relatively small nanometer size and high fluorescence brightness. The preparation method of the fluorescent anti-counterfeiting nanomaterial we designed was relatively simple, more rapid and effective. We only need to gently apply the NCs to the surface where there were suspicious fingerprints, and then gently blew away the excess materials. In order to avoid the damage of the NCs to the potential fingerprint as much as possible, we used a very soft brush throughout the experiment. To explore the detection of potential fingerprints, the same volunteer provided a number of fingerprints deposited on different substrates, including hydrophilic and hydrophobic substrates. Moreover, the developing methods for latent fingerprints, including contrast, selectivity, sensitivity and applicability, were investigated in detail. In general, fingerprint features can be divided into three levels details.53 Level 1 described in detail the overall direction of the fingerprint ridge, which was not sufficient to be identified, but can be used to exclude. Level 2 indicated in detail the specific ridge paths, including termination 26


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and bifurcation, which were the most discriminative features. Level 3 provided quantitative data for accurate fingerprint recognition, including ridge path deviations, pores, and edge shape.

Figure 8. The images of latent fingerprints on glass petri dish (a-e) by using Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=1, 0.5, 0.2, 0) NCs under bright light (a), and with UV irradiation (b-e). (f) The image of a fingermark labeled by Y2O3:Er3+,Yb3+@[email protected] NCs with UV irradiation and (g) the corresponding magnified images.

To affirm the contrast and selectivity of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA (x=1, 0.5, 0.2, 0) fluorescent NCs in latent fingerprint development, transparent glass was chosen as the representative smooth substrate because glass was very common in people's 27



daily life. In forensic investigations, the suspect can often be identified by fluorescence imaging of potential fingerprints on the glasses.54 The fingerprints labeled with fluorescent NCs showed medium contrast when the background color was dark (Figure 8a), and the contrast was enhanced markedly because of the strong fluorescence intensity with different fluorescence colors (Figure 8b-e), which were consistent with the luminescence emission spectrum (Figure 5a). In addition, these fluorescent fingerprint images clearly exhibited the flow of the colored ridges and the colorless furrows (Level 1) due to their small size and suitable affinity. Only papillary ridges was stained by the fluorescent NCs, and no background staining were observed, making the ridge details of the developed fingerprints easy to be identified by the naked eye and thus indicating a high selectivity. It was interesting that the fluorescence color and intensity of fluorescent NCs can be adjusted by changing the proportion of rare earth ions (Eu3+/Tb3+). The rich fluorescence color tunability of rare earth fluorescent nanomaterials as mentioned above had great potential application value for the ultra-sensitive recognition of suspicious fingerprints, because these fluorescent composites can reduce the background interference and improve the accuracy of fingerprint identification by changing the intensity and color of the fluorescence signal.

In previous reports, conventional

fluorescent powders55,56 showed lower contrast in complicated background environments. Therefore, Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA fluorescent NCs reported here can indeed effectively provide high contrast and selectivity for developing latent fingerprints. 28


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To affirm the sensitivity of fluorescent NCs in latent fingerprint development, we also enlarged the representative image of Y2O3:Er3+,Yb3+@[email protected] NCs in Figure 8f-g to investigate the details of latent fingerprints. The left image in Figure 8f displayed a well-resolved ridge flow and pattern structure (level 1). The four expanded images of the fingerprint were shown in the right side of Figure 8g, which can clearly visualize level 2 details, including termination (1), bifurcation (2), core (3), island (4). In addition, the level 3 details called the sweat pores (1), were also visible because of their small size and optimal affinity, which was rare to see such a fine fingerprint analysis in other research work.57,58 Overall, these details can apparently lead to individualization and can thus demonstrated its feasibility for fingerprint recognition. It was worth mentioning that all the fingerprint details in this work were obtained without the help of external equipment or any chemical method, which would greatly improve the speed of potential fingerprint identification. To affirm the applicability of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1-xH-PMA fluorescent NCs in latent fingerprint development, various substrates commonly handled in daily life, including drink glass, black mouse, white ceramic tile, knife, wrench and wood (Figure 9) were used as the substrates for verification. Furthermore, we also demonstrated latent fingerprints comparison images using the fluorescent NCs on plastic Petri dish, black mouse, plastic sealing bag, aluminum alloy and black ceramic tile. As shown in Figure S9, the images with magnification on substrates displayed bright fluorescent fingerprints patterns under 254 nm UV lamp, and the details of fingerprint ridges were visual by 29



naked eyes with high contrast, well selectivity and low background interference, which provided the direct evidence for the practicability of our synthesized fluorescent NCs for TaTablepersonal identification in forensics.

Figure 9. Latent fingerprints on drinking glass. (a, a’), mouse (b, b’), white ceramic tile (c, c’), knife (d, d’), (e, e’) wrench and wood (f, f’) labeled by multicolor Y2O3:Er3+,Yb3+@SiO2@ LGdEuXTb1-XH-PMA (x=1, 0.5, 0) NCs under bright light (a-f), and under 254 nm UV irradiation (a’-f’).

4. CONCLUSIONS

30


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In summary, new dual-mode luminescent nanoarchitectures had been well designed and applied in the information security and information recognition. The nanoarchitectures, which were the composites of upconversion fluorescent materials Y2O3:Er3+,Yb3+ and down-conversion fluorescent materials LGdEuTbH-PMA, were capable of producing multi-color fluorescence emission under the excitations of 254 nm and 980 nm wavelengths, respectively. This kind of safe ink formed by the intelligent integration of nanoarchitectures with dual-mode luminescent functions can provide a strong anti-counterfeiting effect for files or banknotes that need to be protected. Meanwhile, the nanoarchitectures were successfully used as effective fluorescence markers to establish highly sensitive potential fingerprints on a variety of substrates with great contrast, high selectivity, and low background interference. In a word, the nanoarchitectures with dual-mode fluorescence properties reported here may have potential value in security applications.

Corresponding Author *E-Mail: [email protected] (L. J.). *E-Mail: [email protected] (Y. F.).

Author Contributions

31



The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This work was supported in part by the National Natural Science Foundation of China (No. 51773052, 51871090, U1804135 and 51671080), the Program for Science & Technology Innovation Talents in Universities of Henan Province (19HASTIT040), the Plan for Scientific Innovation Talent of Henan Province (194200510019) and the Program for Innovative Research Team (in Science and Technology) in the University of Henan Province (16IRTSTHN005).

Supporting Information XPS N1s spectra of Y2O3:Er3+,Yb3+@SiO2@LGdEuTbH before and after exchanging with PMA anions, infrared spectra of the product of each step, photoluminescence decay curves, DC luminescence emission spectra, Concentration dependence of the intensity ratio of Tb3+ (545 nm) to Eu3+ (616 nm), anti-counterfeiting application on different printed matter, photographs of Y2O3:Er3+,Yb3+@SiO2@LGdTbH-PMA doped inks at indicated time periods under 980 nm and 254 nm excitation, fingerprint identification on different matrices, elemental analysis of the nanocomposites, the color coordinates of CIE chromaticity diagram, luminescence intensity of various transition, the intensity ratio (ITb/IEu) and DC quantum yield of the nanocomposites.

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Dual-mode, Color-tunable, Lanthanide-Doped Core-Shell

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