Photoluminescent Composites of Lanthanide-Based Nanocrystal

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Photoluminescent composites of lanthanide-based nanocrystalfunctionalized cellulose fibers for anti-counterfeiting applications Qing Wang, Guangxue Chen, Zhaohui Yu, Xinping Ouyang, Junfei Tian, and Mingguang Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02307 • Publication Date (Web): 06 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

Photoluminescent composites of lanthanide-based nanocrystal-functionalized cellulose fibers for anti-counterfeiting applications Qing Wang

a,c

, Guangxue Chen a*, Zhaohui Yu b, Xinping Ouyang c, Junfei Tian a,

Mingguang Yu d a.

State Key Laboratory of Pulp & Paper Engineering, South China University of

Technology, Guangzhou 510640, China b.

YUTO Research Institute, Shenzhen YUTO Packaging Technology Co., Ltd.,

Shenzhen 518000, China c.

School of Chemical and Energy Engineering, South China University of Technology,

Guangzhou 510640, China d.

School of Materials Science and Energy Engineering, Foshan University, Foshan

528000, China KEYWORDS. Cellulose, Rare earth ions, Anti-counterfeiting paper, Fluorescent property, Composite materials FULL MAILING ADDRESS. No. 381. Wushan Road, Tianhe District, Guangzhou City, Guangdong Province, China, 510641 ABSTRACT. Photoluminescent materials have been applied worldwide in

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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anti-counterfeiting and security field due to their unique performance of better security, easy identification and difficulty to duplication. Herein, a facile, green and low-cost strategy to fabricate biomass-based composites comprised of lanthanide rare earth ions-doped nanocrystals and cellulose fibers for anti-counterfeiting application was presented. The photoluminescent materials were prepared via in situ chemical deposition of rare earth ions onto bleached hardwood pulp cellulose fibers (bhpFibers) surface

using

polyvinylpyrrolidone

(PVP)

as

coupling

agent

forming

bhpFibers-PVP@LaF3:Eu3+ composites. The bhpFibers-PVP@LaF3:Eu3+ composites showed excellent luminescence and its fluorescence intensity can be simply controlled by varying the addition of the moral amount of lanthanum (La3+) and europium (Eu3+) ions in aqueous medium. Furthermore, the composites have been used as blocks to form photoluminescence paper via a suction filtration procedure. The as-prepared paper possessed excellent luminescence, high flexibility, well writable and printable properties. Moreover, the whole procedure was carried out in a mild environment without toxic reagents. This simple, green and low-cost technology presented has advantages of wholesale production of biomass based photoluminescent materials for anti-counterfeiting applications. INTRODUCTION

Counterfeited and forged products, especially counterfeit currency, drugs and food, can cause huge losses to economy, governments, companies, customers’ health and belongings security.

1-2

As for pharmaceuticals, as high as 10% of the global

medicines trade (even higher than 25% in developing countries) are counterfeits,


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reported by The World Health Organization (WHO).

3

Enhancing anti-counterfeit

technology and preventing counterfeit product from spreading has become a common concern of countries in the world. 4-6 Numerous anti-counterfeiting strategies have been developed to prevent and identify counterfeiting, such as traditional anti-counterfeiting approaches of watermarks, optically variable inks, temperature variable inks, colorimetric labels and advanced anti-counterfeiting approaches of laser coding, radio frequency identification (RFID) technology, 3D holographic imaging technology.7-9 However, easily replication, high cost equipment, relatively complex identification process or verification analysis largely limit anti-counterfeiting materials in actual applications. Therefore, it still a challenge to propose highly scalable, ecofriendly and easy to authenticate technologies for anti-counterfeiting.

Recently, the photoluminescence anti-counterfeiting technology based on various fluorescence materials, such as organics dyes, 10 carbon dots 16-17

11-15

and quantum dots

has been developed. Nevertheless, these technologies still have defects in

photobleaching behaviour, poor thermal and photo stability, strong luminescence quenching via matrix changed, which severely hinder their large-scale industrial application. 18 Lanthanide ions-doped nanocrystals, especially europium and terbium ions, have been used as luminescent materials for their pure luminescent colours, high fluorescence intensity, long fluorescence life. 19-23 Therefore, the photoluminescence materials based on lanthanide-based nanocrystal have great potential applications to anti-counterfeiting since its advantages in better security, easy identification and difficulty to duplication.



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Anti-counterfeiting materials containing trivalent lanthanide rare earth ions has been extensively applied in anti-counterfeiting for its excellent luminescence intensity and free from harmful radiation. 24-26 The functional matrix was made of the rare-earth luminescent materials and film-forming polymers, combined with functional additives by a melt spinning process.

27-29

The film-forming polymers used such as nylon,

polypropylene, polyethylene, polyvinyl chloride, polyester had ostensibly led to higher resource exhausting and environmental pollution caused by petroleum waste. 30 Moreover, the poor thermal stability of rare earth-doped nanocrystals largely restricted its application when melt spinning with polymers at high temperatures. 31-32

Cellulose fibers, as the most abundant natural polymer, possessed plenty of advantages, including high mechanical strength, large surface area, renewability, recyclability and biodegradability.

33-35

Benefiting from these unsurpassed

quintessential physical and chemical properties, cellulose fibers can be used as an ideal natural polymer matrix to manufacture photoluminescence anti-counterfeiting materials for security applications. 36-38 Lately, a variety of lanthanide ions-doped nanocrystals incorporated with cellulose have been studied.

39-42

Fluorescent

functionalization of cellulose has been fabricated by incorporation of lanthanide complex39,

42

and rare-earth doped nanoparticles

40, 41

as fluorescent dopants.

Nevertheless, multistep pre-treated methods are necessary to modify cellulose fibers or fluorophores which lead to complicated processes and complex attachments.


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Herein, a novel and green technique fabricating lanthanide rare earth ions-doped nanocrystals organic-inorganic composites based on bhpFibers for anti-counterfeiting applications by the aid of polyvinylpyrrolidone (PVP) was reported. The effects of rare earth amount on structure, morphology, and luminescence intensity of the as-prepared composites were studied in detail. The photoluminescence paper prepared using bhpFibers-PVP@LaF3:Eu3+ composites as blocks via a suction filtration procedure also exhibited excellent luminescence, good writable, printable and flexibility properties, which would be a promising candidate for large-scale in anti-counterfeiting applications.

EXPERIMENTAL SECTION

Materials. Hardwood bleached pulp was provided by Guangzhou Chenhui Paper Co., Ltd. Polyvinylpyrrolidone (130 kg/mol, PVP-130), Europium chloride hexahydrate (99.99%), Lanthanum chloride hexahydrate (99.99%), sodium fluoride (NaF, 98.0%) were obtained from Shanghai Aladdin Co., Ltd. All chemicals were used as supplied.

Pretreated of Bleached hardwood pulp. The bleached hardwood pulp was first defibering (100rpm/min) with fluffer so that the interwoven fibers can be separated in water without changing the original structure of the fibers. Then bhpfibers with beating degree of 75 °SR could obtained after thrice loop mechanically beating with a1000 revolutions PFI-mill.

Synthesis

of

bhpFibers-PVP@LaF3:Eu3+


composites

were

composites.

synthesized


through

The direct


co-precipitation of rare earth ions (La3+, Eu3+) and fluoride ion (F-) in an aqueous suspension containing PVP modified pulp fibers at room temperature. In a typical procedure, 0.15 g PVP was first added into 60 mL deionized water under vigorous stirring (300rpm) for 15 min. Then, 1 g of dried bhpFibers was transferred into the above solution. After stirring mechanically for 30 min, bhpFibers-PVP were obtained. Then, 10 mL of rare earth aqueous solution (70% mol of La3+ and 30% mol of Eu3+ for LaF3:Eu3+ nanocrystal) with certain amounts (0.2 mmol, 0.4 mmol and 0.8 mmol) was introduced and stirred at room temperature for 1h to allow lactam group in PVP to adsorb RE3+ ions to form the bhpFibers-PVP@RE3+ complexes. Subsequently, NaF aqueous solution (3.2 mmol of NaF dissolved in 30 g of water) was dropwise added in 30 min and then conducted for 3 hours at room temperature, directly causing bhpFibers-PVP@LaF3:Eu3+ composites. The reaction solution was centrifuged (4000rpm, 5min) and rinsed with deionized water for 3 times to purify products.

Fabrication of fluorescent anti-counterfeiting paper using bhpFibers-PVP@ LaF3:Eu3+composites. The fluorescent anti-counterfeiting paper was fabricated with a vacuum filtration equipment. Typically, 0.05g bhpFibers-PVP@LaF3:Eu3+ composites were dispersed in 100mL water to form stable aqueous suspension under ultrasonic (300W, 50 KHz) for 5min. Then, the suspension was poured onto the filtration funnel with a Ф45 mm filter paper. After vacuum dried at 45 °C, the LaF3:Eu3+ composites modified fluorescent anti-counterfeiting paper was prepared.


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Characterization. Chemical structures of cellulose fibers before and after modification were characterized by FT-IR spectra (Spectrum 2000 PerkinElmer spectrometer, USA). Chemical compositions were performed on X-Ray photoelectron spectroscopy (XPS) (Escalab 250 spectrometer, Thermo Electron Corporation). The morphology and elemental mapping analysis (EDS) of the surfaces were recorded on a ﬁeld emission scanning electron microscope (FEI Quanta 400F) (FESEM). The photoluminescence

(PL)

spectra

were

carried

out

with

a

fluorescent

spectrophotometer (HORIBA Jobin Yvon). Crystal structures were performed on X-ray diffraction (XRD) using a Philips PW3040/00 X’Pert MPD diffractometer with Cu Kα radiation at 40 kV range between 10° and 90°. The crystallinity index (CI) of the bhpFibers before and after modification were calculated from the reflected intensity data using Segal et al. method.

43,44

Thermal gravimetric analyses (TGA)

were performed on a TA Instruments Q600 instrument heating from 40 °C to 600 °C using a heating rate of 20 °C/min in a nitrogen atmosphere. The folding endurance was tested on a PTI folding endurance tester according to ISO5626 at 20 °C.

Scheme 1. Schematic representation of the fabrication of bhpFibers-PVP@ LaF3:Eu3+



Page 8 of 30

composites (a), and the fluorescent anti-counterfeiting paper (b). RESULTS AND DISCUSSION

As is well known, PVP is a water soluble polymer obtained from free radical polymerization of N-vinylpyrrolidone. And the -C-N-C=O groups of PVP had good adsorption capability to different substrates including metal, metal oxides, polymers and cellulose. 45-46 Very recently, we reported a simple, general, and green method to microspheres and cotton fibers surface modification with SiO2 nanoparticles, which was based on the use of PVP. 47-49 In this study, we show how PVP can adsorbed onto bhpFibers and then played vital importance in the stability of rare earth ions (RE3+). As shown in Scheme 1a, the bleached hardwood pulp fibers were pretreated with PVP solutions to obtained bhpFibers-PVP. The presence of PVP chains interlaced covered on bhpFibers surface ensured strong static adsorption of RE3+ due to the highly polar -C-N-C=O groups within the pyrrolidone ring of PVP.

50

Under

continuous addition of NaF, LaF3:Eu3+ nanoparticles were in situ chemical deposition onto the surface of bhpFibers forming bhpFibers-PVP@LaF3:Eu3+ organic/inorganic hybrid materials.


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a) bhpFibers

b) bhpFibers-PVP

1667 c) bhpFibers-PVP@LaF3:Eu3+

418 1636 4000

3500

3000

2500

2000

1500

Wavenumber (cm-1)

1000

500

Figure 1. FTIR spectrum of pristine bhpFibers (a), bhpFibers-PVP (b), and bhpFibers-PVP@LaF3:Eu3+ composites (c) Structures of bhpFibers, bhpFibers-PVP and bhpFibers-PVP@LaF3:Eu3+ composites were characterized by FTIR spectra. Comparing with the spectra of pristine bhpFibers in Figure 1a, appeared the stretching vibration absorption peak of –C=O in amide carbonyl at 1657 cm-1, and the stretching vibration absorption peak of –C-N- in 1290 cm-1. These infrared spectra analysis proved that PVP was successfully covered onto the surface of bhpFibers. After RE3+ modificaiton, the –C=O peak in 1667 cm-1 shift to lower wave number of 1636 cm-1,

51

the displacement in 1495 cm-1 of the

absorption peaks of –C=O in amide carbonyl is not obvious (Figure 1c). Besides, in 418 cm-1 appeared the absorption peak of RE-O group. These changes of peaks indicated that the C=O group in PVP take part in coordination to the metal ions, which confirmed successful synthesis of bhpFibers-PVP@LaF3:Eu3+ composites.

The

surface

composition

of

pristine

bhpFibers,

bhpFibers-PVP

and

bhpFibers-PVP@LaF3:Eu3+ composites were also confirmed with XPS spectra (Figure 2, Table S1). As depicted in Figure 2a, the binding energy spectra at about



Page 10 of 30

284-288 eV and 533.2 eV in the XPS spectra of pristine bhpFibers were the signals of elements of carbon and oxygen, respectively. 1s)

appeared

after

surface

40

New binding energy of 399.8 eV (N

modification

bhpFibers

with

PVP.

For

the

bhpFibers-PVP@LaF3:Eu3+ composites, new peaks at 142.5 eV (Eu 4d) and 837.6 eV (La 3d) were appeared (Figure 2b, 2c), which demonstrated that the LaF3:Eu3+ nanoparticles were chemically presented within the near-surface region of bhpFibers.

High resolution spectra of C 1s of pristine bhpFibers, bhpFibers-PVP and bhpFibers-PVP@LaF3:Eu3+ composites were performed in Figure 2d-2f. In Figure 2d, the observed peaks at 288.1, 286.8 and 284.8 eV were attributed to typical signals of carbon atoms of -C=O, C-O/C-N and C-C/C=C, respectively.

40, 52

After

modification with PVP, the relative intensity of the peak for C-C/C=C and C=O increase due to the C-C structure in PVP chains. For bhpFibers-PVP@LaF3:Eu3+ composites, the relative intensity of the peak for C=O increase, while the relative of the peak for C-O decrease. The C 1s XPS spectra indicate that the oxygen-containing functional groups (-C-N-C=O) of PVP can provide numerous surface reactive sites and greatly enhances the interaction between Re3+ and bhpFibers. These results indicated

that

the

biomass-based

rare

earth

fluorescent

materials

bhpFibers-PVP@LaF3:Eu3+ were successfully synthesized. (a)

(b)

O1s


Eu4d (c)

bhp-Fibers bhp-Fibers@PVP@LaF3:Eu3+

bhp-Fibers bhp-Fibers@PVP@LaF3:Eu3+

La3d

F1s La3d C1s bhpFibers-PVP

N1s

Eu4d La4d

bhpFibers

1200

1000

800

600

400

Binding Energy (eV)

200

0 120

125

130

135

140

145

830

Binding Energy (eV)


835

840

Binding Energy (eV)

845

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(d)

292

bph-Fibers C1s

Raw Total Background C-C/C=C C-O/C-N C=O

290

288

286

284

282

(e)

292

Binding Energy (eV)

Figure

2.

XPS


290

bph-Fibers@PVP

(f)

C1s

288

286

284

282

292

spectra

of

290

288

bhp-Fibers@PVP@LaF3:Eu3+ C1s

286

284

282

280

Binding Energy (eV)

Binding Energy (eV)

survey


bhpFibers,

bhpFibers-PVP

and

bhpFibers-PVP@LaF3:Eu3+ composites (a), high resolution XPS signals of Eu 4d (b), La 3d (c), C 1s of bhpFibers (d), bhpFibers-PVP (d) and bhpFibers-PVP@ LaF3:Eu3+ (f). SEM was used to illustrate the morphologies of unmodified and modified bhpFibers samples. As shown in Figure 3a, the pristine bhpFibers had a smooth and uniform surface with shallow channels, which provided larger surface area for loading more LaF3:Eu3+ nanoparticles. After surface modification with rare earth ions with certain amounts and subsequent in situ chemical deposition reaction (Figure 3b-3d), the coralloid LaF3:Eu3+ nanoparticles with size distribution of ca. 30 nm were arranged well on the bhpFibers surface, confirming high affinity between rare earth particles and PVP modified bhpFibers. Moreover, with the increase of RE3+ions, the amounts of LaF3:Eu3+ nanoparticles on the surface of bhpFibers also increased. When the addition of RE3+ ion was 0.8 mmol, the coralloid LaF3:Eu3+ nanoparticles were not only covered on the surface of bhpFibers, but also formed a large number of aggregates in the voids of the bhpFibers. Besides, EDS depicted in Figure 4 showed that each kind of elements was evenly distributed on the surface of fibers, which also proved that bhpFibers were successfully coated with rare-earth-doped coralloid LaF3:Eu3+ nanoparticles.



Figure 3. SEM images of pristine bhpFibers (a), bhpFibers-PVP@LaF3:Eu3+ composite with 0.2 mmol rare earth ions (b), bhpFibers-PVP@LaF3:Eu3+ composite with 0.4 mmol rare earth ions (c), bhpFibers-PVP@LaF3:Eu3+ composite with 0.8 mmol rare earth ions (d).

Figure 4. EDS elemental mapping images of the bhpFibers-PVP@LaF3:Eu3+ composites. The crystalline structure of pristine bhpFibers and bhpFibers-PVP@LaF3:Eu3+ composite with different molar amount (0.2 mmol, 0.4mmol and 0.8 mmol) were investigate by XRD shown (Figure 5). Figure 5a showed three typical cellulose peaks of pristine bhpFibers at 2θ values of 14.6°, 22.4° and 34.6° corresponding to the


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reflexions planes (101), (002) and (400).

53

After surface modification, the estimated

crystallinity index for bhpFibers modified with different amount of RE3+ was found decreased from 65.9% to 60.2%, 57.7% and 55.48%, respectively. The ordered structure of the crystalline region on the remaining cellulose was slightly disrupted by the rare earth doped nanoparticles incorporation. Besides, the reflection planes of (002), (110), (111), (112), (300), (113), (302), (221), (223), (304), (411) at 2θ =24.2°, 24.8°,

27.6°,

34.9°,

43.7°,

44.8°,

50.5°,

52.5°,

64.3°,

68.2°,

70.6°of

bhpFibers-PVP@LaF3:Eu3+ composite (shown in Figure 5b-5d) were recognized from the XRD pattern of LaF3 nanocrystal which had good agreement with the data of JCPDS 32-0483 of single-phase hexagonal crystals. 54 Moreover, with the addition of the rare earth doped nanoparticles, the peak strength gradually increased. These results indicated that the rare earth doped LaF3:Eu3+ nanoparticles have crystallized well and embedded in modified bhpFibers without changed, which ensured the luminescent properties of the bhpFibers-PVP@LaF3:Eu3+ composite. The size of the LaF3:Eu3+ nanoparticles obtained by the Scherrer equation was about 39-44 nm which was accordance with SEM observation. (111) (300) (113) (302) (112) (221)

(110)

(304) (411) (223)

(d) (c) (b) (a)

10

20

30

40

50

60

70

2θ (Degree)


80

90


Figure 5. XRD patterns of the as-prepared bhpFibers-PVP@LaF3:Eu3+ composites: (a) pristine bhpFibers, (b) bhpFibers-PVP@LaF3:Eu3+ composite with 0.2 mmol rare earth ions, (c) bhpFibers-PVP@LaF3:Eu3+ composite with 0.4 mmol rare earth ions, (d) bhpFibers-PVP@LaF3:Eu3+ composite with 0.8 mmol rare earth ions. TGA was used to quantitate the amount of LaF3:Eu3+ nanoparticles deposited on bhpFibers shown in Figure 6. The residue of pristine bhpFibers was 20.95 wt%, due to the inorganic ash content present (Figure 6a).

55

The residue content slightly

increased to 35.78 wt% after rare earth doped LaF3:Eu3+ nanoparticles surface modification (Figure 6b). As shown in Figures. 6(c, d), with increasing amount of RE3+ ions incorporation from 0.4 mmol to 0.8 mmol, the final residue weight ratio of bhpFibers-PVP@LaF3:Eu3+ composites increased from 49.44 wt.% to 61.13 wt.%, demonstrating that more LaF3:Eu3+ nanoparticles were incorporated into bhpFibers, which was accordance with the results of the scanning electron microscopy in Figure 3. This can validate that the as-prepared bhpFibers-PVP@LaF3:Eu3+ composites were composed of cellulose and rare earth doped LaF3:Eu3+ nanoparticles, both of them constituted an organic-inorganic hybrid composite. 100

Cellulose 0.2 mmol 0.4 mmol 0.8 mmol

80

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

d 60

c b

40

a 20 100

200

300

400

Temperature (°C)


500

600

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Figure 6. TG curves of pristine bhpFibers (a), bhpFibers-PVP@LaF3:Eu3+ composite with 0.2 mmol rare earth ions (b), bhpFibers-PVP@LaF3:Eu3+ composite with 0.4 mmol rare earth ions (c), bhpFibers-PVP@LaF3:Eu3+ composite with 0.8 mmol rare earth ions (d).

Figure 7. Excitation spectra (left) of bhpFibers-PVP@LaF3:Eu3+ composites aqueous solution and emission spectra (right) of bhpFibers-PVP@LaF3:Eu3+ composites aqueous solution with different amounts of rare earth ions, (a) 0.2 mmol (b) 0.4 mmol and (c) 0.8 mmol. The insert image in the middle was the luminescence photograph of the bhpFibers-PVP@LaF3:Eu3+ composites aqueous solution with 0.4 mmol rare earth ions amount under sunlight and 365nm excitation. The excitation and emission spectra of the bhpFibers-PVP@LaF3:Eu3+ composites aqueous solution measured at room temperature were shown in Figure 7. Seen from the excitation spectrum in the left of Figure 7, the energy levels of Eu3+ in bhpFibers-PVP@LaF3:Eu3+ composites at 361 nm, 376 nm, 380 nm, 397nm, 414 nm and 464 nm were corresponded to 5D4, 5G6, 5G2, 5L6, 5D3 and 5D2. 25 The dominating excitation spectrum at 397 nm was assigned to the 7F0→5L6 transition of Eu3+ ions. In



the emission spectrum of bhpFibers-PVP@LaF3:Eu3+ composites under the excitation at 397 nm, the emission spectrum of Eu3+ at 595nm (5D0 level to 7F1), 615nm (5D0 level to 7F2), 650nm (5D0 level to 7F3) and 697nm (5D0 level to 7F4) were observed, which exhibited red emissions. 27 The main fluorescence peak of Eu3+ was at 595 nm, assigning to 5D0→7F1 magnetic dipole transition, which conformed that Eu3+ was doped into the inverse symmetry position of LaF3 nanocrystals, which were consistent with the results of the other literatures previously.56 Moreover, the luminescence photographs of the bhpFibers-PVP@LaF3:Eu3+ composites under UV lamp (∼365 nm) showed red color (inset in the middle of Figure 7), indicating that the as prepared bhpFibers possessed good fluorescent properties. From Figure 7, the bhpFibers-PVP@LaF3:Eu3+ composites with different amounts of RE3+ ions (0,2mmol, 0.4mmol and 0.8mmol RE3+ ions) all showed good fluorescence properties. With the dosage of rare earth ions increased from 0.2 mmol to 0.4 mmol, the fluorescent intensity of the main peak 5D0 → 7F1 transition of Eu3+ in the bhpFibers-PVP@LaF3:Eu3+ composite was significantly enhanced. However, further increase the dosage of rare earth ions to 0.8 mmol, the fluorescent intensity decreased accordingly. This mainly be caused by the concentration self-quenching mechanism. 57 The concentration increases of RE3+ ions leaded to activation of cross relaxation between particles and resonance enhancement. With rare earth ions concentration increased, fluorescence intensity increased, but after reach to a certain degree, it decreased badly due to the non-radiative transition of resonance energy transfer existing between adjacent Eu3+ ions.

27

That is to say, with rare earth ions


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concentration increased, the fluorescence strength increased firstly and then decreased after exceeding a certain concentration because of the process of concentration quenching. The stability of the as-prepared bhpFibers-PVP@LaF3:Eu3+ composites with time was investigated. Figure S1 displayed the emission spectrum of the as-prepared composites at different time intervals. Fluorence emission spectra showed that the relative intensities of peaks changed rarely, indicating that the composites were storage stable within 15 days. Besides, the stability of the composites at different pH values (range from 2 to 12) were also studied. The corresponding fluorescence emission spectrums at different pH values were given in Figure S2. No obvious variations were observed of the as-prepared composites with different pH values, suggesting that the composites showed good acid and alkali resistance.

Figure 8. (a) Optical images of the fluorescent paper using 0.4 mmol RE3+ ions of bhpFibers@LaF3:Eu3+ composites. A circular free-standing paper with written letters



Page 18 of 30

before and after UV (365nm) lamp excitation (a), (b); (c) and (d) the as-prepared fluorescent paper showed high flexibility before and after UV (365nm) lamp excitation (c), (d). A fluorescent paper can commonly be used as an anticounterfeiting packaging, labels or display materials, which possessed great application value in life and industry. Herein, a new kind of plant cellulose based fluorescent paper for anticounterfeiting application via a simple filtration method was prepared (Scheme 1b). Figure 8 showed the optical images of the fluorescent paper. Similar to conventional paper, the as-prepared fluorescent paper presented a high-quality white colour under sunlight, while turned red under UV (365nm) lamp excitation. Moreover, the fluorescent paper showed good writable and printable ability on which English letters can be easily and clearly written . Furthermore, the paper exhibited high flexibility which can withstand 300 times repeated bending (Figure S3). Based on this mechanism, other Fibers/REDNPs hybrids-based paper can also be obtained by changing the kinds of rare earth doping ions (cerium, terbium, dysprosium).

CONCLUSIONS

In summary, a facile, green and economical method was proposed to fabricate biocompatible bhpFibers-PVP@LaF3:Eu3+ composites which could further be used for the fabrication of photoluminescent anti-counterfeiting paper. The functionalized bhpFibers-PVP@LaF3:Eu3+ composites were confirmed by FT-IR, XPS, SEM, EDS elemental

mapping,

XRD,

TGA

and

photoluminescence


spectra.

The

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bhpFibers-PVP@LaF3:Eu3+ composites showed stable luminescence within 15 days or

with

different

pH

values

(2

to

12).

Furthermore,

the

as-prepared

photoluminescence paper exhibited high flexibility, good writable and printable properties. On the basis of this approach, various Fibers/ Rare earth ions-doped nanocrystal composites along with several other RE3+ ions can be synthesized. Therefore, this facile and green synthesis approach for fabrication photoluminescent cellulose fibers hold promising applicative value in anticounterfeiting material in large-scale.

AUTHOR INFORMATION Corresponding Author *(Guangxue Chen). E-mail: [email protected]. Phone: +86-022-22236485. Fax: +86-022-22236485.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work is supported by the Science and Technology Planning Project of Guangdong Province (No.2017B090901064), the Science and Technology Project of Guangzhou City (No.2016070220045), China Postdoctoral Science Foundation (2018M633054) and the High-Level Talent Start-Up Research Project of Foshan University (gg040945).



Page 20 of 30

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For Table of Contents Use Only

Lanthanide-based nanocrystal-functionalized cellulose fibers with good stability and fluorescence intensity for photoluminescent anti-counterfeiting papers was reported.


Page 30 of 30

Photoluminescent Composites of Lanthanide-Based Nanocrystal

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