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Dual-Mode Luminescent Nanopaper Based on Ultrathin g-C3N4 Nanosheets Grafted with Rare-Earth Up-Conversion Nanoparticles Yafei Zhao, Ruoyan Wei, Xin Feng, Lining Sun, Panpan Liu, Yongxiang Su, and Liyi Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06254 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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ACS Applied Materials & Interfaces

Dual−Mode Luminescent Nanopaper Based on Ultrathin g−C3N4 Nanosheets Grafted with Rare−Earth Up−Conversion Nanoparticles

Yafei Zhao,† Ruoyan Wei,† Xin Feng,*,† Lining Sun,† Panpan Liu,‡ Yongxiang Su,‡ Liyi Shi†

†

Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, P. R.

China

‡

Department of Chemistry, College of Science, Shanghai University, Shanghai 200444, P. R.

China

KEYWORDS: Dual−mode luminescent nanopaper; Ultrathin g−C3N4 nanosheets; Rare−earth up−conversion nanoparticles; Chitosan; Mechanical properties

ABSTRACT: Ultrathin graphite−like carbon nitride (g−C3N4) nanosheets have attracted considerable attention due to the enhanced intrinsic photoabsorption and photoresponse with respect to bulk g−C3N4. For the first time, a dual−mode of down− and up−conversion luminescent g−C3N4 nanopaper with high optical transparency and mechanical robustness was successfully fabricated through a simple thermal evaporation process using chitosan as a green crosslinking agent. The dual−mode of down− and up−conversion fluorescence emission originated from the amino terminated ultrathin g−C3N4 nanosheets functionalized with carboxylic acid modified 1

ACS Paragon Plus Environment


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multi−coloured rare−earth up−conversion nanoparticles (cit−UCNPs) via the EDC/NHS coupling chemistry. The homogenously distributed cit−UCNPs@g−C3N4 nanoconjugates with excellent hydrophilicity displayed good film−forming ability and structural integrity, thus the photoluminescence property of each ingredient were substantially maintained. Results indicated that the free−standing chitosan crosslinked cit−UCNPs@g−C3N4 luminescent nanopaper possessed of high transmittance, excellent mechanical properties and remarkable dual−mode emission. The smart design of high performance luminescent nanopaper based on ultrathin g−C3N4 nanosheets grafted with multi−coloured UCNPs offers a potential strategy to immobilize other multi−functional luminescent materials for easily recognized and hardly replicated anti−counterfeiting fields.

INTRODUCTION Since the first successful exfoliation of monolayer graphene, two−dimensional (2D) nanomaterials, such as g−C3N4,1 h−BN,2 MoS23 and SnS24 have gained renewed and accelerating interest owing to their promising practical applications in antimicrobials,5−6 transparent conducting electrodes,7 hybrid materials,8 DNA sequencings,9 solar cells,10 catalysts,11-14 and so on. Ultrathin g−C3N4 nanosheets are among the most important 2D graphene analogs because of the superior photoluminescence and enhanced photoresponse compared to their bulk counterparts. Zhang et al.1 prepared ultrathin g−C3N4 nanosheets by a “green” liquid exfoliation route from bulk g−C3N4 for the first time. The water-soluble g−C3N4 nanosheets with excellent biocompatibility and nontoxicity were utilized as a promising candidate for bioimaging and biolabelling. Significantly, the highly hydrophilic g−C3N4 nanosheets exhibited an exceptional 2


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film−forming ability.15-16 The g−C3N4 films were easily fabricated by simply drying g−C3N4 nanosheets aqueous suspensions on certain hydrophilic substrates and displayed splendid fluorescent and electrochemiluminescent activities,16 suggesting their promising applications in biosensors, flexible display and anti−counterfeiting. Most recently, anti−counterfeiting technologies

are

increasingly

important

in

secrecy

fields.17

Among

the

different

anti−counterfeiting technologies, fluorescent anti−counterfeiting technology has attracted extensive attention in every realm due to its potential applications in covert or invisible tagging. Up to now, invisible anti−counterfeiting films have been produced by utilizing certain transparent substrates to greatly increase the difficulty of its duplications by counterfeiters. Zhao et al.18 developed transparent luminescent cellulose−based nanopaper functionalized with rare−earth up−conversion luminescent nanoparticles (UCNPs) as a prospective candidate for multimodal anti−counterfeiting. Miao et al.19 synthesized multi−luminescent anti-counterfeiting nanopaper printed Quick response (QR) codes by conjugating three kinds of lanthanide complexes [Eu(dbm)3(H2O)2, Sm(dbm)3(H2O)2, Tb(tfacac)3(H2O)2]. Campos-Cuerva et al.20 applied spherical gold, silver and magnetite nanoparticles as multiple anti−counterfeiting tags with optical and magnetic signals on different paper supports. Although the cellulose−based paper is particularly suitable for printing with fluorescent materials, some disadvantages such as the extremely slow dewatering and poor mechanical robustness under high moisture conditions due to the inherent hydrophilic nature of cellulose limit its further practical application. The exfoliated g−C3N4 nanosheets based thin films are emerging alternative substrates with tailored mechanical and optical properties for anti−counterfeiting. Chitosan (CS) is a linear polysaccharide with high concentrations of primary amino groups along the chains,21 which can be 3



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efficiently employed as a significant reinforcing material for the formation of high strength thin films according to covalent bonding. Recently, artificial nacre was synthesized based on graphene oxide crosslinked with CS through synergistic interactions of hydrogen and covalent bonding.22 Results indicated that the tensile strength and toughness was 4 and 10 times higher than that of natural nacre, respectively. Analogous to graphene, MoS2 nanosheets were firstly surface modified with thioglycolic acid to provide them with carboxyl groups and then incorporated with CS. The resulting MoS2/CS composite films displayed dramatic enhancement in mechanical properties.23 Thus it is expected that the mechanical strength of layered structure g−C3N4 nanocomposites can also be improved by covalently crosslinking g−C3N4 nanosheets with CS.24 Herein, a novel dual−mode of down− and up−conversion photoluminescent g−C3N4 nanopaper with excellent optical transparency and mechanical properties was firstly reported. In our proof−of−concept prototype, citric acid modified multi−coloured up−conversion nanoparticles (cit−UCNPs) were grafted onto the surface of blue light down−conversion g−C3N4 nanosheets via a covalent crosslinking. The dual−mode luminescence was witnessed in cit−UCNPs@g−C3N4 nanoconjugates under different excitations. Most importantly, the mechanical properties of luminescent nanopaper with high optical transparency and dual−mode photoluminescence were strengthened by hydrogen bonding and electrostatic attraction with CS. This strategy may provide a new bi−functional anti−counterfeiting effect in improving the practical applications such as paper documents, certificates, packaging, artworks and banknotes.

4


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EXPERIMENTAL SECTION Chemicals and Materials. All the chemicals were used as received without further purification. N-hydroxysuccinimide

(NHS,

97%),

1-(3-dimethylamino-propyl)-3-ethylcarbodiimide

hydrochloride (EDC, 98%), melamine (99%) and citric acid (99.5%) were provided from Sigma Aldrich. Oleic acid (OA, 90%) and 1-octadecene (ODE, 90%) were obtained from Alfa Aesar. MnCl2, YCl3·6H2O (99.99%), YbCl3·6H2O (99.99%), ErCl3·6H2O (99.99%), NaOH (96%), NH4F (98%), Chitosan (CS, medium molecular weight, over 95% deacetylated), and diethylene glycol (98%, DEG) were purchased from Aladdin Company. Deionized water was used throughout. Preparation of Ultrathin g−C3N4 Nanosheets. Ultrathin g−C3N4 nanosheets were achieved by liquid exfoliating of bulk g−C3N4 in deionized water. Firstly, the bulk g−C3N4 was synthesized by direct pyrolysis of melamine according to a previously reported method.1 In detail, melamine (20 g) was placed in an alumina crucible with a cover and then heated at 650 °C for 4 h with a heating rate of 5 °C min-1, and the yellow g−C3N4 powders were obtained. Subsequently, 50 mg of bulk g−C3N4 powders were dispersed in 100 mL deionized water and ultrasonicated for about 10.5 h. The colour of the reaction solution gradually changed from an initial yellow to white in exfoliating process. The resultant suspension was centrifuged at 8000 rpm for 8 min and washed three times to remove the residual unexfoliated g−C3N4. The concentration of the nearly transparent solution was estimated to be about 0.17 mg mL-1.

5



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Preparation of Green Emission OA−UCNPs and cit−UCNPs. The monodispersed oleic acid−capping UCNPs (OA−UCNPs, NaYF4:Yb(20%),Er(2%)) were prepared via a solvothermal method according to our previous method.25 In brief, 2 mmol of LnCl3 (Ln: 78 mol%Y + 20 mol%Yb + 2 mol%Er) in deionized water were added into 100 mL flask, and then the mixture was heated to 110 °C to remove water to get white solids. Then 12 mL OA and 30 mL ODE were added to the solution in sequence and heated to 150 °C to form a homogeneous transparent solution. After adding 20 mL methanol containing NaOH (5 mmol, 0.2 g) and NH4F (8 mmol, 0.3 g), the solution was stirred at 110 °C to evaporate methanol and water. Finally, the mixture was maintained at 300 °C for 1 h under argon atmosphere and then cooled to room temperature. The as−obtained products were washed with cyclohexane and acetone for several times, and subsequently dispersed in a small amount of cyclohexane. The detailed experimental procedures for the synthesis of UCNPs with blue, yellow and red CUL emission are available in Supporting Information. The monodispersed hydrophobic OA−UCNPs were treated with citric acid to introduce functional carboxyl groups and render them hydrophilicity according to the ligand exchange method reported previously.26-29 Citrate acid (1.05 g) was added into the solution of DEG (30.0 mL), and then heated to 110 °C for 30 min under argon atmosphere. When the solution was cooled down to 60 °C, 25 mg OA−UCNPs dispersed in chloroform and toluene (v/v = 3:2) solution (10 mL) was injected into the above mixed solution. Then, the mixture was heated to 130 °C for 1 h to evaporate chloroform and toluene. Finally, the system was further heated to 175 °C for 2 h until the solution became yellow under argon atmosphere. The resulting solution was cooled down to room temperature and collected by centrifugation (15000 rpm, 15 min). The precipitates were 6


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washed three times with ethanol and deionized water (v/v = 1:1) solution (30 mL), and the final citric acid capped UCNPs (cit−UCNPs) were dispersed in 5 mL demonized water. Preparation of cit−UCNPs@g−C3N4 Nanoconjugates. In the present work, standard procedures were employed with minor modifications to crosslink cit−UCNPs and amino terminated g−C3N4 nanosheets. In brief, the as−synthesized cit−UCNPs (10 mL, 5 mg mL−1) were dissolved in 5 mL containing EDC (40 mg) and NHS (30 mg) aqueous solution for 2.5 h in an ice−bath to activate the surface carboxylic acid group. Afterwards, the as−exfoliated g−C3N4 (200 mL, 0.17 mg mL-1) was added to the above solution under strong stirring for 12 h. The excess of cit−UCNPs were removed by centrifugation in low rotate speeds (2000 rpm). After repeated washing with deionized water, the cit−UCNPs@g−C3N4 nanoconjugates were further dispersed in deionized water with a concentration of 3 mg mL-1. Fabrication of Dual−mode Luminescent cit−UCNPs@g−C3N4/CS Nanopaper. The CS was dispersed in 2% acetic acid solution with a concentration of 10 mg mL-1 and stirred for 24 h to achieve a homogeneous solution.22 Then 2 mL cit−UCNPs@g−C3N4 (3 mg mL-1) was dissolved in 3 mL CS solution (10 mg mL-1) under constantly stirring for 30 min. Subsequently, the cit−UCNPs@g−C3N4/CS suspension was degassed and thrown onto a flat dish at 45 °C for 5 h to form a transparent nanopaper. The nanopaper (cit−UCNPs@g−C3N4/CS=20 wt%) was peeled off from the disk and hereinafter referred to as F−20. The different contents of cit−UCNPs@g−C3N4 (0 wt%, 20 wt%, 40 wt%, 60 wt%, 90 wt%) were corresponding to F−0, F−20, F−40, F−60, F−90, respectively. All the different loading nanopaper with ca. 15 µm in thickness and ca. 45 mm in diameter were ultimately obtained. Characterization. The particle sizes and morphologies were characterized using a JEOL 7



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JEM−2010F low−to−high resolution transmission electron microscope (TEM) operated at 120 kV. The microstructures and surface morphologies were observed using scanning electron microscopy (SEM, Hitachi S−4800, Hitachi, Japan). Powder X−ray diffraction (XRD) measurements were performed on a 3 KW D/MAX2200 V PC diffractometer using Cu kα radiation (60 KV, 80 mA) at a step width of 8° min−1. Fourier transform infrared spectroscopies (FT−IR) were performed in the spectral range from 4000 to 400 cm−1 with a Thermo Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA) by using the pressed KBr pellet technique. Up−conversion luminescence spectra were recorded on an Edinburgh LFS−920 fluorescence spectrometer with the excitation of an external 0−2 W adjustable continuous wave laser (980 nm, Connect Fiber Optics, China). The Light transmittance was measured on a UV−vis spectrophotometer (UV−2501PC, Shimadzu, Japan).

Photoluminescence

(PL)

spectra

were

measured

on

a

Shimadzu

RF−5301

spectrophotometer. Mechanical properties were conducted via a Shimadzu AGS−X Tester (XLM(EC), Labthink Instruments Co. Ltd, China). All data were expressed as the mean result ± standard deviation (SD), and all figures were acquired from three independent experiments with consistent results.

RESULTS AND DISSCUSSION Characterization of Ultrathin g−C3N4 Nanosheets. Ultrathin g−C3N4 nanosheets were explored as transparent substitutes to house a range of multi−coloured UCNPs and meanwhile as the down−conversion luminescent materials for preparing dual−mode anti−counterfeit nanopaper. The g−C3N4 nanosheets with high PL intensity (quantum yield=18.9%) were successfully prepared via a liquid exfoliation process.1 The as−exfoliated g−C3N4 consist mainly of transparent 8


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graphene−like ultrathin 2D nanosheets (Figure S1A, Supporting Information). Due to the limitation of the top−down stripping method, the lateral dimension of resulting g−C3N4 nanosheets is not uniform, ranging from approx. 100 nm to 1.5 µm. In addition, the obvious Tyndall effect of the g−C3N4 nanosheets in water indicates that the colloidal suspensions are very stable (inset of Figure S1B, Supporting Information). Furthermore, FT−IR spectrum displays characteristic peaks of the as−exfoliated ultrathin g−C3N4 nanosheets (Figure S2, Supporting Information). The transmission band at 813 cm−1 represents the out−of−plane bending vibration characteristic of heptazine rings, which is the characteristic mode of the triazine units of g−C3N4. The two absorption peaks at 3265 and 3170 cm-1 are characteristics of N−H stretching vibrations. Besides, the peaks between 1000 and 1800 cm-1 are ascribed to stretching vibration of connected units of C–N(–C)–C (full condensation) or C–NH–C (partial condensation).30 As shown in Figure 1, the luminescence properties of the ultrathin g−C3N4 nanosheets were investigated by PL spectroscopy.31 The PL spectrum of the ultrathin g−C3N4 nanosheets shows a blue shift of ~20 nm compared with bulk g−C3N4, which may be attributed to the quantum confinement effect with the conduction and valence band shifting in opposite directions.30 Interestingly, the fluorescence emission wavelength of the ultrathin g−C3N4 nanosheets is around 449 nm, showing no significant shift upon different excitation wavelengths (Figure S3, Supporting Information). This PL property is quite different from other down−conversion fluorescent material (e.g. carbon dots,32-34 graphene quantum dots35). This excitation−independent PL behavior may depend on the uniform surface states/electronic structure, which leads to inter−system crossing and adjacent vibrational relaxation of excited electrons, therefore triggering fluorescence emission with corresponding energy.36 Obviously, the excitation−independent fluorescent g−C3N4 nanosheets conjugated with 9



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multi−coloured UCNPs can further enhance anti−counterfeiting effect in the reality application.

Figure 1. Normalized photoluminescence spectra of bulk g−C3N4 and the as−exfoliated ultrathin g−C3N4 nanosheets excitated with 365 nm; the insets show photographs of the colour change of ultrathin g−C3N4 nanosheets solution before and under UV light illumination. Characterization of OA−UCNPs and cit−UCNPs. Hydrophobic OA−UCNPs was converted to hydrophilic cit−UCNPs for efficiently conjugating with the hydrophilic –NH2/−HN terminated ultrathin g−C3N4

nanosheet. The successful hydrophobic/hydrophilic conversion from

OA−UCNPs to cit−UCNPs is illustrated by the FT−IR spectra (Figure S4C, Supporting Information). Both OA−UCNPs and cit−UCNPs exhibit a broad band at around 3437 cm-1, corresponding to O−H stretching vibration. From the FT−IR spectrum of the OA−UCNPs, the absorption bands located at 2921 cm-1 and 2851 cm-1 are attributed to the asymmetric and symmetric stretching vibrations of methylene (CH2) in long alkyl chain of OA, respectively, and the bands at approximate 1557 cm-1 and 1452cm-1 are assigned to the stretching vibration of ester groups (–COO−) in OA.37 By contrast to OA−UCNPs and citric acid, the peaks (2921cm-1, 2851cm-1) in the curve of cit−UCNPs apparently decreased after ligand exchange. In addition, the emergence of a new peak at 1633 cm−1 corresponding to the group −C=O originated from the citric acid, suggesting the successful replacement of oleic acid by citrate ligand on the surface of 10


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UCNPs. Both OA−UCNP and cit−UCNPs have an average diameter of approximately 30 nm (Figure S4A−B, Supporting Information), indicating that the ligand−exchange process did not change its initial morphology. The HRTEM image of uniform and well−dispersed OA−UCNP (inset of Figure S4A, Supporting Information) clearly shows that the distance between lattice fringes was 0.51 nm, which is ascribed to the (100) plane of the hexagonal phase. The result was further verified by the XRD patterns as depicted in Figure 2. The pattern of cit−UCNPs demonstrates that all the diffraction peaks are well indexed to the hexagonal phase of β−NaYF4 crystals (JCPDS: 16−0334). The OA−capping UCNPs is easily dispersed in cyclohexane attributed to the long alkyl chains of oleic acid coordinated with UCNPs (inset of Figure S4D, Supporting Information).

cit-UCNPs g-C3N4@cit-UCNPs g-C3N4

Intensity (a.u.)

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♦

10

20

30

40

50

60

70

80

90

2θ (degree) Figure 2. XRD patterns of cit−UCNPs, g−C3N4 nanosheets, cit−UCNPs@g−C3N4 nanoconjugates and the standard card of β−NaYF4 (JCPDS: 16−0334). The striking up−conversion luminescent (UCL) properties of OA−UCNPs and cit−UCNPs dispersed in cyclohexane and deionized water, respectively, upon excitation with near−infrared (NIR) light were investigated (Figure S4D, Supporting Information). It can be obviously observed that the emission spectra of OA−UCNPs and cit−UCNPs all exhibit three clear up−conversion 11



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emission peaks at 521, 540, and 654 nm, attributed to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4

I15/2 transitions of Er3+ ions, respectively.38 On the other hand, the significant decrease of the UCL

intensity of cit−UCNPs compared to OA−UCNPs is ascribed to the change of solvent and ligand.39

Scheme 1. Schematic illustration of the synthetic route of cit−UCNPs@g−C3N4 nanoconjugates.

Characterization of cit−UCNPs@g−C3N4 Nanoconjugates. The nanoconjugation of the cit−UCNPs with the ultrathin g−C3N4 nanosheets is schematically illustrated in Scheme 1. The strategy for the preparation of cit−UCNPs@g−C3N4 nanoconjugates was firstly constructed by the covalent reaction between −NH2 (or −NH) of ultrathin g−C3N4 nanosheet and –COOH of UCNPs via EDC/NHS coupling chemistry. XRD patterns of the cit−UCNPs, as−exfoliated ultrathin g−C3N4 nanosheets and cit−UCNPs@g−C3N4 nanoconjugates are displayed in Figure 2. It is obvious that the marked peak at 2θ=27.6° (d002=0.32 nm) in cit−UCNPS@g−C3N4 XRD pattern is indexed to the characteristic peak of (002) of g−C3N4 crystal, indicating that the cit−UCNPs was successfully assembled on the as−exfoliated ultrathin g−C3N4 nanosheets. Furthermore, the successful attachment of cit−UCNPs to ultrathin g−C3N4 nanosheets was also further illustrated by FT−IR spectra. As shown in Figure 3, two obvious peaks at 2922 and 2852 cm−1 appeared in the spectrum of the cit−UCNPs@g−C3N4 ascribed to the asymmetric and symmetric stretching vibration of methylene (CH2), which possibly originated from the outer layer hydrophilic citric acid ligand in UCNPs. Moreover, both the disappearance of the infrared absorption peaks at 3170 and 3265 cm-1 corresponding to dangling N−H group of g−C3N4, and the appearance of the broad 12


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peak located at 3455 cm-1 after the conjugation of cit−UCNPs on ultrathin g−C3N4 nanosheet are assigned to the strong stretching vibration of O−H (possibly from citric acid ligand) and N−H, further implying the successful conjugation of cit−UCNPs to ultrathin g−C3N4 nanosheets.

cit-UCNPS g-C3N4@cit-UCNPs

Transmittance (a.u.)

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g-C3N4

C-H

O-H

triazine units

N-H 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Figure 3. FT−IR spectra of the as−prepared cit−UCNPs, g−C3N4 nanosheets and cit−UCNPs@g−C3N4 nanoconjugates. The morphology and structure of individual cit−UCNPs@g−C3N4 nanoconjugates were further examined by using HRTEM and elemental mappings (Figure 4). From HRTEM image, it can be clearly seen that cit−UCNPs are uniformly distributed on the surface of ultrathin g−C3N4 nanosheets without obvious aggregation, suggesting that the cit−UCNPs were well−bonded with g−C3N4 via EDC/NHS coupling chemistry. EDS mapping characterization further verifies the formation of cit−UCNPs@g−C3N4 nanoconjugates, as elemental signals from both the cit−UCNPs (O, F, Na, Er, Y, Yb) and the ultrathin g−C3N4 (C, N) nanosheets can be clearly identified. Noteworthy, the oxygen element was attributed to citrate acid ligands (−COOH), which further indicated that the citric acid was successfully coordinated with UCNPs.

13



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Figure 4. HRTEM image of cit−UCNPs@g−C3N4 and the corresponding EDS elemental mapping images (C, N, O, F, Na, Er, Y, Yb).

The

predominant

down−

and

up−conversion

luminescence

properties

of

the

cit−UCNPs@g−C3N4 nanoconjugates in aqueous solution were investigated by fluorescence spectroscopy, as shown in Figure 5. It is obvious that the cit−UCNPs@g−C3N4 nanoconjugates have both down− and up−conversion emission spectra under different excitation light source, indicating the luminescence properties of cit−UCNPs and ultrathin g−C3N4 nanosheets were well maintained in the nanoconjugates (blue for g−C3N4 and green for cit−UCNPs). Remarkably, the outstanding bi−functional properties of cit−UCNPs@g−C3N4 nanoconjugates can be suitable for anti−counterfeiting tags or labels.

14


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Fluorescence Intensity (a.u.)

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Excitation by 330 nm Excitation by 980 nm

400

500

600

700

800

Wavelength (nm) Figure 5 Down− and up−conversion luminescence spectra of cit−UCNPs@g−C3N4 nanoconjugates (5 mg mL−1), excited with 330 and 980 nm, respectively; The insets: digital photographs of cit−UCNPs@g−C3N4 nanoconjugates in aqueous solution under excitation of 365 nm (left), visible (middle) and 980 nm (right) light. Characterization of Dual−mode Luminescent cit−UCNPs@g−C3N4/CS Nanopaper. CS is known as an ideal and green crosslinking agent to construct composite films with enhanced mechanical

strength.

Thus,

the

free−standing

CS

crosslinked

cit−UCNPs@g−C3N4

(cit−UCNPs@g−C3N4/CS) nanopaper was fabricated through a simple thermal evaporation method. The surface and cross section SEM images of cit−UCNPs@g−C3N4/CS nanopaper with different cit−UCNPs@g−C3N4 loadings (F−0, F−40, F−90) were presented in Figure 6. It is clearly observed that the network structure of nanopaper is tightly packed after solvent evaporation. The surface of F−0 is quite smooth, but the flatness decreases with the addition of cit−UCNPs@g−C3N4. The island−shaped materials are cit−UCNPs@g−C3N4 which well dispersed and entangled with each other in the CS matrix. From the cross−sectional SEM image of F−40, micro−topographies reserve uniform configuration with the loading of 40 wt% (Figure 6E), implying a homogeneous distribution and good adhesion between cit−UCNPs@g−C3N4 and CS. However, with the loading increasing to 90 wt%, excessive multilayer stacks were observed in 15



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fracture surface of F−90 (Figure 6F). This could result in the slipping of cit−UCNPs@g−C3N4 and poor interfacial adhesion between cit−UCNPs@g−C3N4 and CS, failure to transfer the load from CS matrix to cit−UCNPs@g−C3N4.40

Figure 6 Typical SEM images of the cit−UCNPs@g−C3N4/CS nanopaper with different contents of cit−UCNPs@g−C3N4: F−0 (A, D); F−40 (B, E); F−90 (C, F). The dual−mode luminescence properties of cit−UCNPs@g−C3N4/CS nanopaper were investigated by fluorescence spectroscopy. Figure 7A shows the UCL spectra (excited with 980 nm)

of

pure

CS

(F−0)

and

cit−UCNPs@g−C3N4/CS

nanopaper

with

different

cit−UCNPs@g−C3N4 loadings (F−20, F−40, F−60 and F−90). With the increasing loading contents of cit−UCNPs@g−C3N4, the relative emission intensity increases. Then, it is logical to speculate in a qualitative way that no quenching effects were detected with the increased loading of cit−UCNPs@g−C3N4 in the luminescent nanopaper, as the experimental conditions (such as excitation power and detection slits) were kept constant during the entire set of measurements . Figure 7B shows the corresponding down−conversion luminescence spectra of pure CS (F−0) and cit−UCNPs@g−C3N4/CS nanopaper (excited with 365 nm). Due to the well-defined homogeneous distribution of cit−UCNPs@g−C3N4 in F−40 nanopaper, the optimal down−conversion fluorescence intensity is given to the weight ratio of 40 wt%. On the other hand, the occurrence of 16


Page 17 of 27

fluorescence quenching over 40 wt% loading maybe ascribed to the out−of−plane accumulation or agglomeration of cit−UCNPs@g−C3N4 in the luminescent nanopaper (F−60 and F−90).

(A)

400

(B)

F-0 F-20 F-40 F-60 F-90

500

600

700

F-0 F-20

PL Intensity (a.u.)

UCL Intensity (a.u.)

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


800

F-40 F-60 F-90

400

450

500

550

600

Wavelength (nm)

Wavelength (nm)

Figure 7. Up−conversion luminescence spectra (A) and Down−conversion luminescence spectra (B) of pure CS (F−0) and cit−UCNPs@g−C3N4/CS nanopaper with different cit−UCNPs@g− C3N4 loading (F−20, F−40, F−60, F-90). The relationship between the optical transparency and

the loading amounts of

cit−UCNPs@g−C3N4 in the dual−mode luminescent nanopaper was also investigated. Figure 8A displays the digital pictures of the cit−UCNPs@g−C3N4/CS nanopaper with different cit−UCNPs@g−C3N4 loadings (F−0, F−40, F−90). Generally, light scattering effect is actually reduced as the dimensions are close to nanoscale.18,19,41 All of the cit−UCNPs@g−C3N4/CS nanopaper exhibit high optical transparency. It can be concluded that the high degree of transparency is ascribed to the high dispersity of cit−UCNPs@g−C3N4 in the CS matrix. With the increased loadings of cit−UCNPs@g−C3N4 to CS, the school emblem of Shanghai University is gradually becoming obscure. Figure 8B shows the optical transmittance plotted against the wavelength of the luminescent nanopaper with different dosages of cit−UCNPs@g−C3N4. The result further indicates that the luminescent nanopaper possess high transmittance in the visible range. A slight decrease of transmittance is attributed to the excessive accumulation of UCNPs@g−C3N4.42 Noteworthily, a regular ultraviolet shielding effect at the wavelength of 320 nm can be detected, which is probably attributed to the light absorption of cit−UCNPs@g−C3N4. 17



It’s also worth noting that F-40 nanopaper achieves an excellent transmittance of 93.8%@550 nm. The stress−strain curves of the dual−mode luminescent nanopaper with different contents of cit−UCNPs@g−C3N4 are displayed in Figure 8C. With increasing content of cit−UCNPs@g−C3N4, the tensile strength is dramatically enhanced, which is ascribed to hydrogen bonding or electrostatic interaction.22,24 It can be demonstrated that the F−40 nanopaper possess excellent tensile strength. On the other hand, the decrease of the tensile strength for over 40% loading of cit−UCNPs@g−C3N4 may be attributed to the multilayer stacks or aggregation. The results are in good agreement with the cross−section SEM analysis above mentioned.

(B)

100

F-0 F-20 F-40 F-60 F-90

80

Normalized Inthesity

60 40 20 0 200

1.0 0.8 0.6

F-0 F-20 F-40 F-60 F-90

80 60 40 20

0.2 0.0

F-0 F-20 F-40 F-60 F-90

Loading (%) 400

(C)

0.4

320 nm

300

Stress (MPa)

100

Transmittance (%)

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 18 of 27

500

600

700

0

800

0

Wavelength (nm)

1

2

3

4

Strain (%)

Figure 8. Characterization of the as−prepared cit−UCNPs@g−C3N4/CS nanopaper: (A) Digital pictures; (B) Optical transmittance curves (inset, normalized light transmittance intensity in the 550 nm); (C) The typical stress−strain curves. In addition, in order to improve the complexity of practical anti−counterfeiting applications, an extended

approach

for

incorporating

multi−coloured

up−conversion

nanoparticles

or

multifunctional luminescent materials is to be available. Figure 9 shows different colours (excited with 980 nm) dual−mode luminescent nanopapers by doping different rare−earth ions to UCNPs 18


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(Figure S5 and Figure S6, Supporting Information). It can be obviously observed entire blue fluorescence and punctual point UCL fluorescence under 365 nm and 980 nm, respectively. The green, yellow and red UCL emission were attributed to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, and the blue UCL emission was arisen from the 1G4→ 3H6 transitions of Tm3+. Therefore, the novel dual−mode luminescent nanopaper possessing down− and up−conversion photoluminescence properties simultaneously can be easily applied for anti−counterfeiting tags and labeling fields.

Figure 9. Digital pictures of different colours of the cit−UCNPs@g−C3N4/CS nanopaper (F−20) in the bright fields and dark fields in the excitation of 365 nm and 980 nm, respectively.

CONCLUSIONS The novel dual−mode luminescent nanopaper (cit−UCNPs@g−C3N4/CS) for anti−counterfeiting possessed down− and up−conversion photoluminescent properties simultaneously was successfully achieved via a simple evaporation. To fabricate the proof−of−concept nanopaper, blue luminescence g−C3N4 nanosheets were functionalized with monodispersed multi−coloured UCNPs. Hydrophobic OA−UCNPs were surface modified using citric acid via ligand exchange to 19



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Page 20 of 27

introduce carboxyl groups on the surface of UCNPs, and then combined with –NH2/−HN terminated ultrathin g−C3N4 nanosheets by the EDC/NHS crosslinking reaction. The luminescent nanopaper had a dual−mode anti−counterfeit effect of both down− and up−conversion emission spectra under different excitation light sources. With the optimal loading of 40 wt%, the luminescent

nanopaper

(cit−UCNPs@g−C3N4/CS)

exhibited

high

optical

transmittance

(93.8%@550 nm) and excellent mechanical properties. More importantly, we offer a smart strategy for other multi−functional luminescent materials to conjugate with ultrathin g−C3N4 nanosheets used in anti−counterfeiting fields.

ASSOCIATED CONTENT Supporting Information Experimental details of blue, yellow, red exmission OA−UCNPs; Quantum yield measurements of ultrathin g−C3N4 nanosheets; HRTEM and SEM image of ultrathin g−C3N4 nanosheets; FT−IR spectrum of ultrathin g−C3N4; Photoluminescence spectra of bulk g−C3N4 and the as-exfoliated ultrathin g−C3N4 nanosheets; TEM image, FT−IR spectra and up−conversion luminescence spectra of OA−UCNPs and cit−UCNPs nanoparticles; TEM images of OA−UCNPs with different emission colours; Up−conversion luminescence spectra of OA−UCNPs with different emission colours.

AUTHOR INFORMATION Corresponding Authors * E−mail: [email protected] (X. Feng). 20


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Notes The authors declare no competing financial interest.

Acknowledgements This work was financially sponsored by the Natural Science Foundation of Shanghai (13ZR1415100) and National Natural Science Foundation of China (21571125). We are also grateful to Instrumental Analysis & Research Center of Shanghai University.

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Dual-Mode Luminescent Nanopaper Based on ... - ACS Publications

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