Switchable up and down-conversion luminescent properties of Nd(III)-nanopaper for visible and near-infrared anti-counterfeiting

Article abstract of DOI:10.1016/j.carbpol.2020.117134

A series of lanthanide-based nanopaper (Nd-nanopaper) was synthesized via a neodymium organic framework (Nd-MOFs)-grafted TEMPO-oxidized cellulose nanofibrils (tCNF) using a solvothermal reaction. Not using the traditional down-conversion visible emissions of anti-counterfeiting techniques, this Nd-nanopaper achieved down-conversion near-infrared (NIR) and up-conversion visible emissions. The down-conversion luminescent property of these Nd-nanopapers exhibited characteristic NIR luminescence (λ_Em = 1080 nm) of Nd³⁺ ions with 311 nm excitation, undergoing an “antenna” effect. In contrast, the up-conversion visible light emission (λ_Em =450 nm) of Nd-nanopaper was detected under 580 nm excitation. The mechanism of up-conversion fluorescence was ascribed to excited-state absorption and energy transfer up-conversion. Interestingly, Nd-nanopaper induced both up and down-conversions for visible and NIR emissions that were completely devoid of the interference from fluorescent brighteners and background fluorescence. These switchable up and down-conversion fluorescent Nd-nanopapers with visible and NIR dual emissions or dual channels could be applied in high level anti-counterfeiting applications.

Full text of DOI:10.1016/j.carbpol.2020.117134

Carbohydrate Polymers 252 (2021) 117134
Contents lists available at ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Switchable up and down-conversion luminescent properties of Nd
(III)-nanopaper for visible and near-infrared anti-counterfeiting
,
,
,
Zhao Zhanga b,*, Ningning Maa, Xiena Kanga, Xinping Lia *, Shuangquan Yaoc **,
Wenjia Hand, Hui Change
a College of Bioresources Chemical and Materials Engineering, National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi
University of Science and Technology, Xi’an, 710021, Shaanxi, PR China
b State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, PR China
c Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, College of Light Industry and Food Engineering, Guangxi University, Nanning, 530004,
PR China
d State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan, 250353, PR China
e College of Mechanical and Electrical Engineering, Shaanxi University of Science and Technology, Xi’an, 710021, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of lanthanide-based nanopaper (Nd-nanopaper) was synthesized via a neodymium organic framework
(Nd-MOFs)-grafted TEMPO-oxidized cellulose nanofibrils (tCNF) using a solvothermal reaction. Not using the
traditional down-conversion visible emissions of anti-counterfeiting techniques, this Nd-nanopaper achieved
down-conversion near-infrared (NIR) and up-conversion visible emissions. The down-conversion luminescent
property of these Nd-nanopapers exhibited characteristic NIR luminescence (λEm = 1080 nm) of Nd3+ ions with
311 nm excitation, undergoing an “antenna” effect. In contrast, the up-conversion visible light emission (λEm
=450 nm) of Nd-nanopaper was detected under 580 nm excitation. The mechanism of up-conversion fluores-
cence was ascribed to excited-state absorption and energy transfer up-conversion. Interestingly, Nd-nanopaper
induced both up and down-conversions for visible and NIR emissions that were completely devoid of the
interference from fluorescent brighteners and background fluorescence. These switchable up and down-
conversion fluorescent Nd-nanopapers with visible and NIR dual emissions or dual channels could be applied
in high level anti-counterfeiting applications.
Lanthanide-base cellulose nanopaper
Down-conversion NIR luminescence
Up-conversion visible emission
Fluorescent brighteners
High level anti-counterfeiting
1. Introduction
et al., 2018; Li et al., 2017; Wang et al., 2020; Zhang, Liu, Chang, Li, &
Zhang, 2019; Zhang, Chen, Sun, Zhang, & Zhao, 2019). Among these
With the continuous advancement of society, counterfeit and shoddy
products are mixed into daily life (Andres, Hersch, Moser, & Chauvin,
2014). Counterfeits of paper products, such as banknotes, passports,
patents, and documents from all over the world, not only harm the in-
terests of individuals and businesses but also endanger national security
(Kaczmarek et al., 2017; Resch-Genger, 2020). Governments and busi-
nesses around the world have spent much resources, time, and energy to
protect the results and products of their intellectual property owners. At
present, researchers have developed many paper-based anti-counter-
feiting technologies, such as watermarks and thermal, holographic, and
fluorescent anti-counterfeiting technology (Arppe & Sørensen, 2017; Jin
anti-counterfeiting technologies, fluorescent materials have been widely
studied because of their advantages of easy identification and perma-
nent reversibility (Zhang, Liu et al., 2019, 2019b).
Fluorescent materials based on organic dyes (Leng, Jakubek,
Mazloumi, Leung, & Johnston, 2017), carbon dots (Jing et al., 2015;
Jiang et al., 2016), and quantum dots (Zhu et al., 2012) have been
widely reported and used extensively to produce hybrid cellulose paper
or nanopaper for visible (Vis) emission anti-counterfeiting applications.
Vis anti-counterfeiting takes advantages of naked-eye recognition, with
a
low security level that is easily imitated. In recent years,
lanthanide-based materials have attracted attention due to the excellent
* Corresponding authors at: College of Bioresources Chemical and Materials Engineering, National Demonstration Center for Experimental Light Chemistry En-
gineering Education, Shaanxi University of Science and Technology, Xi’an, 710021, Shaanxi, PR China.
** Corresponding author.
E-mail addresses: zhangzhaoqg@sust.edu.cn (Z. Zhang), lixp@sust.edu.cn (X. Li), yaoshuangquan@gxu.edu.cn (S. Yao).
https://doi.org/10.1016/j.carbpol.2020.117134
Received 23 July 2020; Received in revised form 6 September 2020; Accepted 18 September 2020
Available online 29 September 2020
0144-8617/© 2020 Elsevier Ltd. All rights reserved.

Z. Zhang et al.
Carbohydrate Polymers 252 (2021) 117134
luminescence of lanthanide (Ln3+) ions, large Stokes shifts, line-shaped
spectra and microsecond lifetime (Wang, Batentschuk, Osvet, Pinna, &
Brabec, 2011). In particular, Nd3+, Yb3+, and Er3+ ions extend emissions
to the near-infrared (NIR) regions or emit Vis light through
up-conversion, which greatly improves anti-counterfeiting characteris-
tics. Zhao et al. have obtained nanopapers with up-conversion lumi-
nescence characteristics for anti-counterfeiting using cellulose
nanofibrils (CNFs) doped with NaYF4:Yb, Er nanoparticles (Zhao et al.,
2014). You et al. have reported lanthanide up-conversion fluorescence
based on 3D quick response (QR) codes fabricated by inkjet printing
technology for drug anti-counterfeiting (You et al., 2016). Da Luz et al.
have demonstrated photoluminescent lanthanide-organic frameworks
(Ln-MOFs) printed onto flexible substrates (paper and plastic foils) with
a conventional inkjet printer (Da Luz et al., 2015). To date, there appear
to have been few reports on the development of lanthanide-based ma-
terials with dual-emission mixed (dual-channel) fluorescence for
anti-counterfeiting. On the other hand, these lanthanide-based fluores-
cent materials doped into or coated/printed onto nanopaper, give rise to
poor stability and easy response losses. The fluorescent material modi-
fication of nanocellulose via coordination, covalent, or ionic bonds is
helpful for improving material stability and, in comparison, coordina-
tion bond-modified nanocellulose is simple and the driving force of the
reactions comes from coordination abilities between Ln3+ ions and co-
ordination sites (Miao et al., 2015). It is very interesting to use
Ln-MOF-coordinate bonding TEMPO-oxidized CNF (tCNF) for preparing
Vis and NIR dual-emission mixed dual-channel anti-counterfeiting ma-
terials (Gao, Han, & Wang, 2018; Liu, Ai, & Lu, 2011).
2. Experimental section
2.1. Materials
The needle bleached kraft pulp (NBKP) is purchased from Tianjin
Woodlf Biotechnology Co., Ltd. The main ingredients of NBKP are cel-
lulose (65.5 %) and hemicelluloses (31.5 %), which also contain a small
amount of lignin (1.8 %). N, N-dimethylformamide was purchased from
Sinopharm Co., Ltd. Sodium bromide (NaBr, AR), sodium hypochlorite
(NaClO, AR), absolute ethanol (CH3COOH, AR), glacial acetic acid
(CH3COOH, AR), acetone (CH3COCH3, AR), sulfuric acid (H2SO4, 98 %)
and potassium hydroxide (KOH, AR) was purchased from Kermel
Chemical Reagents Co., Ltd (Tianjin, China). Reagents grade,
NdCl3•6H2O and 1,3,5-Benzenetricarboxylic acid (H3BTC) were pur-
chased from Aladdin Reagents Co., Ltd (Shanghai, China) and 2,2,6,6-
tetramethyl-piperidine-1-oxyl (TEMPO, 98 %) bought from Sigma-
Aldrich Inc (St. Louis, MO, USA). All chemicals were used without any
further purification. All coordination polymers were formed in a Teflon-
lined stainless-steel vessel under autogenous pressure and afterward
gradually cooled to room temperature.
2.2. Synthesis of Nd-MOFs
A mixture of NdCl3•6H2O (0.006 mmol), H3BTC (0.02 mmol), N,N-
dimethylformamide (10 mL), and deionized water (10 mL) were well
mixed in a sealed vial. Then, the vial was heated at 115 ◦C for 8 h. The
final purple product [Nd(BTC)]•(H2O)] (Nd-MOFs) were separated by
filtration, washed with ethanol, and then dried under vacuum at 80 ◦C
for 12 h.
In this study, Nd-MOFs with up and down-conversion luminescent
characteristics were first prepared and then grafted to tCNF using a
solvothermal reaction. After coordinate bond coupling of tCNF carboxyl
group and lanthanide ions, a heterogeneous network structure was
formed, thereby preparing a series of fluorescent nanopapers via
Buchner funnel filtering. This novel up-conversion Vis and down-
conversion NIR emission nanopaper successfully avoided the emission
range of fluorescent brighteners, and these dual channels could be
switched via excitation control. In addition, fluorescent brighteners in
Nd-nanopaper@FB achieved down-conversion Vis emission, that offer-
ing a dual emission system with Nd-nanopaper’s down-conversion NIR
emission. This dual emission and dual-channel anti-counterfeiting sys-
tem within nanopaper could further improve the security of anti-
counterfeiting (Fig. 1).
2.3. Preparation of tCNF
According to the literature, this method mainly induces the oxidation
of C6-aldehyde groups of CNFs to C6-carboxyl groups via the TEMPO/
NaBr/NaClO oxidation system (Chen, Geng, Jing, Tong, & Chen, 2017).
First, a quantitative amount of needle bleached kraft pulp (NBKP) were
dispersed in aqueous solution and subjected to ultrasonic treatment. In
addition, a mixed aqueous NBKP dispersion system was obtained by
adding a certain ratio of aqueous solutions of TEMPO and NaBr aqueous
solution. The pH was adjusted to 10.5 by adding KOH solution to the
above solution, with NaClO solution gradually added as an oxidizing
agent, to carry out the oxidation reaction, and stirred for 3 h. Finally, the
pH was adjusted to 7 using hydrochloric acid to prepare oxidized tCNF
Fig. 1. Schematic diagram for the preparation of Nd-nanopaper and Nd-nanopaper@FB.
2

Z. Zhang et al.
Carbohydrate Polymers 252 (2021) 117134
and the obtained suspension filtered and washed with pure water and
subjected to high pressure homogenization 20 times to obtain a uniform
tCNF suspension. The main ingredients of tCNF were cellulose (81.81 %)
and hemicelluloses (14.94 %), with a small amount of lignin (0.78%).
Further, the surface charge (ꢀ 1.59 × 10ꢀ 3 eq/g) of tCNF was deter-
mined using a colloidal charge titrator.
(Fig. S2). For Nd-nanopaper, the absorption peak at 1620 cmꢀ 1 was due
–
to stretching vibrations of tCNF carboxyl (C O) groups. The peak at
–
1620 cmꢀ 1 was red-shifted by 20 cmꢀ 1 compared to pure tCNF (nano-
paper), which was attributed to coordination of carboxyl groups and
lanthanide ions and suggested coupling reactions between tCNF and Nd-
MOFs. Therefore, FT-IR results demonstrated that Nd-MOFs had been
coordinate-bonded with tCNF, resulting in the formation of Nd-
nanopaper. By analyzing FT-IR spectra of Nd-nanopaper@FB, the
2.4. Preparation of Nd-MOFs-grafted-nanopaper
broad band at 3412 cmꢀ 1 was assigned to O H stretching vibrations
–
and the band at 1616 cmꢀ 1 to COOꢀ stretching (Fig. S3). Furthermore,
A mixture of NdCl3⋅6H2O (0.06 mmol), 1,3,5-benzenetricarboxylic
acid (H3BTC, 0.02 mmol), tCNF (2.5 mL), and deionized water (10
mL) were evenly added to a sealed vial. The reaction vessel was heated
the peak located at 1350 cmꢀ 1 was assigned to stretching vibrations of
ꢀ 1
–
–
C
H and the band at 1028 cm to O H in-plane bending vibrations.
◦
These results were basically consistent with the position of absorption
peaks of Nd-nanopaper IR spectra. Therefore, the addition of fluorescent
brightener was just impregnation into the nanopaper without chemical
reaction and new bond formation. It also did not affect peak positions in
the present Nd-nanopaper IR spectrum.
at 105 C for 3 h and the resulting suspension filtered to prepare Nd-
MOFs-grafted-nanopaper (Nd-nanopaper) using Buchner funnel
filtering. Then, the Nd-nanopaper was peeled off the filter, pressed be-
tween smooth glass plates, and allowed to dry naturally at ambient at-
mosphere for 24 h.
The dispersion uniformity of fluorescent brighteners in nanopaper is
particularly important, because it determines the luminescence unifor-
mity. Laser scanning confocal microscopy (LSCM) images showed that
fluorescent dots distribution was relative uniform (Fig. S4), indicating
that fluorescent brighteners were evenly distributed in the nanopaper.
As the fluorescent brighteners were just associated in the Nd-nanopaper
without any chemical reaction, therefore, its concentration in the
nanopaper was determined via an external standard method. The
absorbance curves at 5 different concentrations for each fluorescent
brightener and standard curves of 7 kinds of fluorescent brighteners
were obtained (Fig. S5). A 2.1 mg mass of Nd-nanopaper@FB was
dispersed into 9 mL of solvent (Nd-nanopaper@FB1-6 DMF and Nd-
nanopaper@FB7 H2O) and their maximum absorbance measured. The
fluorescent brightener content in the Nd-nanopaper@FB was calculated
using the equation y = ax + b, where y is the absorbance and x the mass
concentration. The coordinates of mass concentration and absorbance of
the fluorescent brighteners were also noted and the mass concentration
and absorbance of the seven Nd-nanopaper@FB nanopapers showed
corresponding coordinates of Nd-nanopaper@FB1-(0.076, 0.89), Nd-
nanopaper@FB2-(0.091, 0.72), Nd-nanopaper@FB3-(0.073, 0.85), Nd-
nanopaper@FB4-(0.098, 0.95), Nd-nanopaper@FB5-(0.088, 0.76), Nd-
nanopaper@FB6-(0.070, 0.87), and Nd-nanopaper@FB7-(0.118, 0.89).
For Nd-nanopapers, the homogeneous distribution of Nd-MOFs on
the tCNF matrix played an important role in material photophysical
properties. Images showed that Nd-MOF possessed tiny purple crystal
particles with beautiful crystal clear luster (Fig. S6a). Further, the
morphologies of Nd-MOFs was captured by scanning electron micro-
scopic (SEM) images (Figs. S6b-S6c). Nd-MOFs were clearly seen to be
2.5. Preparation of Nd-nanopaper encapsulated fluorescent brighteners
Based on the Nd-nanopapers described above, a series of Nd-
nanopaper encapsulated fluorescent brighteners (Nd-nanopaper@FB)
were prepared using a similar method. The fluorescent brighteners were
1,1’-biphenyl-4,4’-bis[2-(methoxyphenyl)ethenyl] (FB-1), 4.4-bis(5-
methyl-2-benzoxoazol)-ethylene (FB-2), 1,4-bis(benziazolyl-2-yl-)naph-
thalene (FB-3), 4,4’-(2-sulfostyryl)biphenyl disodium (FB-4), 4,4’-bis(2-
benzoxazolyl)-stilbene (FB-5), 4,4’-(1,4-phenylenebis(ethene-2,1-dlyl))
(FB-6), and 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (FB-7). A
mixture of NdCl3⋅6H2O (0.06 mmol), H3BTC (0.1 mmol), tCNF (2.5 mL),
and deionized water (10 mL) was well mixed in a sealed vial. Then, 4.26
mg of fluorescent brighteners were dissolved in DMF (1 mL) and added
to the above solution. Finally, the reaction vessel was heated at 105 ◦C
for 3 h, the aqueous suspension filtered to prepare Nd-nanopaper using
Buchner funnel filtering, and the product washed three times with
alternating water and ethanol. Then, the Nd-nanopaper@FB was peeled
off, pressed between smooth glass plates, and allowed to dry naturally at
ambient atmosphere for 24 h.
3. Results and discussion
3.1. Preparation and characterization of Nd-nanopaper and Nd-
nanopaper@FB
The tCNF was prepared by TEMPO-mediated oxidization of NBKP.
Subsequently, Nd-MOFs were assembled on tCNF surfaces via coordi-
nate bonding and then Nd-nanopaper produced by filtering with a
Buchner funnel. Wide-angle powder X-ray diffractometric analysis
(PXRD) was performed on Nd-MOFs, pristine nanopaper, and Nd-
nanopaper. The resulting XRD patterns showed Nd-MOFs with sharp
peaks present at 10.12, 16.8, 17.3, 29.2, and 33.8◦, which indicated a
crystalline material (Fig. 1S). In their PXRD patterns, pristine nano-
papers showed peaks at 15.6 (11‾0) and 22.8◦ (002), which were
attributed to the crystalline structure of tCNF (Lin & Dufresne, 2014).
Nd-nanopapers showed the prominent peaks at 15.6, 22.8, 29.2, and
33.8◦, illustrating that Nd-nanopapers were composed of a combination
of Nd-MOFs and pristine nanopaper. The crystallinity index of pristine
(67%) and Nd-nanopapers (51 %) were calculated using the equation Ic
= 1- I1/I2, where I1 is the amorphous region diffraction intensity 2θ =
18◦ and I2 the intensity at 2θ = 22.8◦, related to the ordered (crystalline)
area of tCNF (Abraham et al., 2016). These results indicated that the
obtained Nd-MOFs were successfully combined with tCNF (Missoum,
Bras, & Belgacem, 2012) and, by virtue of Nd-MOFs binding occurring
solely at tCNF surfaces, tCNF crystalline structure was preserved.
FT-IR spectra of Nd-nanopaper, Nd-MOFs, and nanopaper demon-
strated characteristic signal peaks at 3341 and 2901 cmꢀ 1, which were
attributed to stretching vibrations of -OH groups and C-H, respectively
rod-shaped, with 1–8 μm diameter and 15–90 μm length. Due to the
influence of nano-size effects of tCNFs, there were no large-sized Nd-
MOFs particles in Nd-nanopaper. As expected, Nd-nanopaper appeared
dense and its surface smooth, which was similar to nanopaper (Fig. 2a
and b). To our best knowledge, cellulose density was packed to form a
layer-like structure and then layers connected by hydrogen bonds within
the nanopaper. Through cross-sectional analysis, the layers between
nanopaper and Nd-nanopaper sections were found to be very compact,
with Nd-nanopaper sections appearing a little rougher than nanopaper,
which might be attributed to Nd-MOF grafted tCNFs destroying
hydrogen bonds and affecting its layered structure (Fig. 2c and d).
The elemental composition of Nd-MOFs and Nd-nanopaper was
determined by X-ray photoelectron spectroscopy (XPS) analysis. The
XPS spectra of Nd-MOFs and Nd-nanopaper illustrated that Nd-MOFs
and Nd-nanopaper had similar compositions, primarily of C, O, and
Nd (Table 1, Fig. 3a). The high-resolution XPS spectrum of C-1s
exhibited five peaks at about 282.5, 282.8, 283.1, 284.2, and 286.6 eV,
–
–
–
–
– – – –
C O, and O C O,
which were attributed to C C, C C, C O, O
–
respectively (Fig. 3b, Wang et al., 2014; Zor, Alpaydin, Arici, Saglam, &
Bingol, 2017). The analysis of O-1s illustrated that oxygen was divided
–
–
C
into two peaks at 527.95 eV for O Nd bonds and 529.25 eV for O
3

Z. Zhang et al.
Carbohydrate Polymers 252 (2021) 117134
Fig. 2. SEM micrographs of nanopaper (a) and Nd-nanopaper (b) in the dispersion state (scale bar = 1
μ
m); the fracture surface of nanopaper (c) and Nd-nanopaper
(d) (scale bar =5 μm).
appeared more complicated, appearing at ~400 and 520 ◦C, which
corresponded to the loss of organic compound groups and the formation
of metal oxides. TG curves for Nd-nanopapers also showed a three-stage
thermal decomposition process, with the first stage from 50 to 200 ◦C
with a small weight loss caused by water evaporation. In the second
stage, the temperature range from 200 to 391 ◦C, and it was decomposed
rapidly at 345 ◦C with a mass loss rate of ~5.56 %/min. A third stage
exothermic enthalpy change at 391 ◦C was observed in Nd-MOFs
(Fig. S8). After assembly of Nd-MOFs on tCNF surfaces, a similar
exothermic enthalpy change occurred for the material at 372 ◦C, which
was a 19 ◦C decrease compared to Nd-MOFs alone. The third stage, from
Table 1
Specific elemental data of Nd-MOFs and Nd-nanopaper.
Classification
Nd-MOFs
Name
Binding Energy/
eV
FWHM/
eV
Area
At%
C 1 s
O 1 s
Nd 3d
C 1 s
283.00
530.00
1003.00
283.00
2.872
3.215
5.871
3.567
334566.22
258453.48
30253.25
73.85
19.47
6.68
Nd-
349267.06
69.77
nanopaper
O 1 s
530.00
3.184
2.867
398007.11
15462.57
27.14
3.09
Nd 3d
1003.00
◦
391 to 600 C, resulted in the formation of residues due to the degra-
dation of the nanocellulose backbone. Notably, the degradation tem-
perature was very close to that of pristine nanopaper, which was
distinctly shown from the DTG curves (Fig. S9). Compared with pristine
nanopapers, Nd-nanopapers had higher residue amounts, which was
likely to be related to the introduction of Nd-MOF centers. These
TG-DTG-DSC results further indicated that Nd-nanopaper had excellent
thermal stability from 0 to 200 ℃ and could be used in security
anti-counterfeiting paper.
bonds (Fig. 3c). These results confirmed the presence of
–
oxygen-containing groups in the prepared Nd-MOFs, including COOH
–
and OH, which was in good agreement with FT-IR spectra. The C-1s
peaks were deconvoluted into five parts at about 285.9–1, 286–.41,
–
287.81, 288.61 and 290.01 eV and assigned to C C, C C, C O,
–
–
– –
–
O
C
O, and O
C
O respectively (Fig. 3d). The analysis of O-1s
–
showed that oxygen was divided into three peaks at 532.86 eV for
–
–
O
Nd bonds, 531.26 eV for O-H bonds, and 532.46 eV for O C bonds
(Fig. 3e). In addition, high-resolution XPS spectra for Nd of Nd-MOFs
and Nd-nanopaper were both concentrated at 1003.00 eV (Fig. 3f),
which further confirmed the success of using trivalent state Nd3+ and
indicated a higher tendency to form Nd-O bonds in Nd-MOFs and
Nd-nanopaper.
3.2. Photophysical properties of Nd-MOFs and Nd-nanopaper
UV-Vis absorption spectra of samples showed that Nd-MOFs
exhibited a narrow-band absorption peak corresponding to the 4f elec-
tron transition of Nd3+ ions in the range of 350 to 1000 nm (Fig. S10).
There was one strong absorption peak in Nd-MOFs and Nd-nanopapers
Thermogravimetric, differential thermogravimetric, and differential
scanning calorimetric (TG, DTG, and DSC, respectively) curves of sam-
ples are shown in the Supporting Information. For security anti-
counterfeiting paper, good thermal stability is essential (Zhang, Chang
et al., 2018, Zhang, Zhang et al., 2018). The thermal stabilities of
Nd-MOFs, pristine nanopaper, and Nd-nanopaper were analyzed by TG
curves (Fig. S7). Nd-MOFs exhibited a three-step thermal decomposition
process, with the first stage, at ~120 ◦C, resulting in a weight loss of 24.6
%. The polycrystalline structures lost DMF and H2O when heated to 120
◦C and then remained stable, as a second stage, until the temperature
reached 400 ◦C. Additional thermal decomposition, the third stage,
at 580 nm, which corresponded to the absorption transition of 4I9/2
→
4G5/2. The strong absorption at wavelengths <350 nm were attributed to
the absorption of the H3BTC ligand. Notably, Nd-MOFs exhibited a
strong ligand π-π* absorption band at 200–350 nm, which showed a
significant red shift compared with the free ligand π-π* absorption band,
indicating that Nd-MOFs were formed. The conjugation and planarity of
the post-substitution system appeared to be enhanced. At the same time,
the main characteristic of Nd3+ was the characteristic absorption band
from 350 to 1000 nm. In addition to the characteristic absorption of Nd-
MOFs between 400 and 750 nm, Nd-nanopaper exhibited almost no
4

Z. Zhang et al.
Carbohydrate Polymers 252 (2021) 117134
Fig. 3. XPS pattern of Nd-MOFs and Nd-nanopaper (a). The high-resolution XPS spectra of Nd-MOFs and Nd-nanopaper of C 1s (b) and (d), O 1s (c) and (e), Nd 3d (f),
respectively.
other absorption peaks (Fig. S11). Compared with nanopaper, the light
transmittance of Nd-nanopaper decreased slightly at 400 nm, which was
attributed to the likely light absorption of Nd-MOFs and light scattering
of the relatively rough nanopaper (Foster et al., 2018).
with their theoretical values (1081 and 1379 nm), which was the result
of the adjustment of the system’s internal energy levels after formation
of Nd-MOFs emitting centers (Lupei & Lupei, 2000).
Photoluminescence (PL) spectra were utilized to investigate the
luminescent properties of Nd-MOFs and Nd-nanopapers. The up and
down-conversion PL spectra of Nd-nanopapers and Nd-MOFs are dis-
played in Figs. 4a and S12, respectively. The PL spectra of Nd-MOFs and
Nd-nanopaper excited at 580 nm showed up-conversion fluorescence
peaks both at 450 nm with a 5 nm half-band widths. At the same time,
when 311 nm excited Nd-MOFs and Nd-nanopapers, light emission
appeared in the NIR region. The corresponding down-conversion fluo-
rescence peaks appeared at 903, 1060, and 1334 nm, and their distinct
3.3. Up- and down-conversion mechanisms of Nd-MOFs and Nd-
nanopaper
The up and down-conversion PL mechanisms of Nd-MOFs and Nd-
nanopaper were further studied by examining the energy transfer pro-
cess of Nd-MOFs and Nd-nanopaper (Fig. 4b). In the final analysis, as the
luminescence of Nd-nanopaper was Nd-MOF luminescence, only Nd-
MOFs were discussed here. The electronic transition mechanisms of
Nd-MOFs at different excitation wavelengths have been summarized,
including 311, 580, and 808 nm for green, red, and yellow lines,
respectively (Tai, Li, & Pan, 2018). The up-conversion luminescence
mechanism has been studied in lanthanide ion systems, corresponding to
the red line arrow. The present up-conversion luminescence mechanism
was thus attributed to two explanations for excited state absorption
4
4
emission bands deduced to represent 4F3/2 → I9/2, 4F3/2 → I11/2, and
4F3/2 → 4I13/2 transitions of Nd3+, respectively. Clearly, the appearance
of the Nd3+ f-f emission band in Nd-MOF centers was due to more
effective ligand sensitization to Nd3+. In addition, the two emission
bands of the Nd3+ at 1060 and 1334 nm were blue-shifted compared
Fig. 4. Up-conversion and down-conversion fluorescence spectrum of Nd-nanopaper excited by 580 nm laser line and 311 nm laser line (a); energy level diagram for
the fluorescent emission of the Nd-MOFs (b).
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Carbohydrate Polymers 252 (2021) 117134
(ESA) and energy transfer up-conversion (ETU) (Chuang & Verdun,
1996; Courrol et al., 2004). The former was a single Nd3+ ion, which
continuously absorbed two 580 nm photons and reached the 4D5/2 en-
maximum sensitization range of organic ligands (ΔE = 2100–3200
cmꢀ 1) proposed by Sato et al. (Sato & Wada, 1970), it was here known
that the excited state energy level of the ligand must have been higher
than the co-transfer of energy and occurred only when the excited state
energy level was within a certain range. From the absorption and fluo-
rescence emission spectra of Nd-MOFs, the ligand was concluded to have
been effectively sensitized Nd3+, which is the "antenna effect". This
indicated that the triplet (T1) energy level of the H3BTC ligand of the
transition to Nd3+ might have been effective. At this time, the ligand
electrons returned from the lowest vibration energy level of the first
excited singlet state S1 to the optical radiation generated by the vibra-
tion energy levels of the ground state S0, which was fluorescence. When
4
ergy level through the excited state energy of G5/2. The higher the
excited state energy level was, the more transitions back to the ground
state that radiated photons. The latter involved two Nd3+ ions absorbing
two 580 nm photons to reach the excited state of 4G5/2, with one Nd3+
ion transferring energy to another Nd3+ ion and returning to ground
state. Another Nd3+ ion received energy and jumped to a higher excited
state, inducing up-conversion fluorescence (λ
=450 nm), corre-
Em
sponding to the Nd3+ ion 2D5/2 → 4I11/2 transition.
According to the solid sensitization theory of Dexter (1953) and the
Fig. 5. Emission spectra (a) and CIE chromaticity graphs (b) of Nd-nanopaper@FB under 311 nm excitation; up-conversion fluorescence spectra (c) and CIE
chromaticity graphs (d) of Nd-nanopaper@FB excited by 580 nm laser line; the photos of seven Nd-nanopaper@FB under natural light and UV-irradiation under 365
nm excitation (e).
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Carbohydrate Polymers 252 (2021) 117134
the stimulated electron dropped to the lowest vibration energy level of
S1, it did not emit fluorescence, but instead reached the T1 vibration
energy level through the intersystem transition (ISC) and relaxed to the
lowest vibration energy level of T1 after vibration relaxation, from the
lowest vibration energy of T1. The optical radiation emitted by each
vibration level back to the ground state was phosphorescence. Finally,
the ligand transitioned the excited state energy to the 2P1/2 excited state
of Nd3+. Then, for each energy level transition of Nd3+, the
down-conversion fluorescent emission energy level diagram of Nd-MOFs
corresponded to the green line. The electronic energy level transition
that converted the fluorescence (λEx =311 nm) to NIR were 903 nm for
fluorescent peaks of Nd-nanopaper@FB from 450 to 550 nm (Fig. S14).
Some minor deviations occurred because fluorescent brightener addi-
tion to Nd-nanopaper inevitably caused some diffuse reflections and
weak interaction. The up-converted Vis light fluorescence spectrum
(λmax =450 nm) of Nd-nanopaper@FB at an excitation wavelength of
580 nm and the fluorescence intensity and peak position were similar
(Fig. 5c). The up-conversion fluorescence of Nd-nanopaper@FB
showed CIE coordinates that were all localized at (0.1542, 0.02;
Fig. 5d). When irradiated separately under natural light, these
Nd-nanopaper@FB samples did not fluoresce. However, under UV light
at 365 nm, the seven fluorescent brighteners were excited, emitting
blue fluorescence (Fig. 5e). Clearly, Nd-nanopaper and nanopaper@FB
were colorless and without light emission under natural light. How-
ever, under the excitation of 365 nm ultraviolet light, the
Nd-nanopaper emitted NIR luminescence (invisible to the naked eye),
while nanopaper@FB emitted blue (Vis) fluorescence (Fig. S15).
As is known, fluorescent materials or FB are prone to photo-
bleaching, when exposed to long-term ultraviolet radiation, and their
molecular structure irreversibly damaged. NIR luminescence of Nd3+
ions originates from ligand sensitization, called the “antenna effect”.
Therefore, the longevity of Nd-MOFs and FB emitters under UV excita-
tion depends on H3BTC and FB’s photostability, respectively. The UV
absorption curves of a solution of the H3BTC and representative FB3
were measured every 30 min 10 times and it was observed that neither
H3BTC nor FB3 absorbance decreased, which demonstrated the good
photostability of H3BTC and FB (Figs. S16–S17).
4F3/2 → 4I9/2, 1060 nm for 4F3/2 → 4I11/2, and 1334 nm for 4F3/2 → 4I13/2
.
3.4. NIR-visible dual emission and dual channel anti-counterfeiting of Nd-
nanopapers under fluorescent brighteners environment
Fluorescent brighteners (Santos, Batlle, Salafranca, & Nerín, 2005;
Shu & Ding, 2009) appear very frequently in our daily lives, being
widely used in a variety of applications in the textile, paper, detergent,
and plastic industries. In the paper industry, fluorescent brighteners
are one of the most important additives. Consequently,
anti-counterfeiting nanopapers must avoid the effects of fluorescent
brighteners. From the Vis light fluorescent spectrum of
Nd-nanopaper@FB excited at a wavelength of 311 nm, it was inferred
that there must be a down-conversion in the NIR fluorescent spectrum
consistent with Nd-nanopaper nanoparticles (λmax = 1080 nm,
Fig. S13). Furthermore, Vis spectra were interpreted with the aid of
Commission Internationale deL’Eclairage (CIE) chromaticity
coordinates in a chromaticity diagram. The fluorescence emission
ranges of the seven Nd-nanopaper@FB were between 425 and 550 nm
(Fig. 5a). For the fluorescence of these seven Nd-nanopaper@FB under
311 nm excitation, the corresponding CIE values were
The security anti-counterfeiting model and its rules were studied
based on switchable up and down-conversion luminescent properties of
lanthanide-based cellulose nanopapers (Fig. 6) (Zhang, Chang et al.,
2018, Zhang, Zhang et al., 2018). The detailed security code information
was explained below. For the present Nd-nanopaper, it not only could
produce down-conversion NIR fluorescence under excitation at 311 nm,
but also emitted up-conversion Vis light under excitation at 580 nm.
Thus, switchable dual-channel emission was achieved (Fig. 6a). For
nanopaper@FB, it only emitted down-conversion Vis light under exci-
tation at 311 nm (Fig. 6b). Nd-nanopaper@FB integrated the fluorescent
characteristics of the above two nanopaper types (Fig. 6c). First,
Nd-nanopaper@FB was excited by 311 nm, which generated a dual
Nd-nanopaper@FB1-(0.1665,
0.1825, 0.1765),
Nd-nanopaper@FB4-(0.1854,
0.2346),
Nd-nanopaper@FB2-(
Nd-nanopaper@FB3-(0.2005,
0.2677),
0.2002),
Nd-nanopaper@FB5
-(0.1659, 0.1529), Nd-nanopaper@FB6-(0.1601, 0.1449), and
Nd-nanopaper@FB7-(0.1604, 0.0922; Fig. 5b). The fluorescent peaks
of pure fluorescence brighteners were slightly different from the
Fig. 6. The schematic illustration for anti-counterfeiting element and mechanism of Nd-nanopaper (a), nanopaper@FB (b) and Nd-nanopaper@FB (c).
7

Z. Zhang et al.
Carbohydrate Polymers 252 (2021) 117134
epoxynanocrystalline cellulose (CNC) nanocomposites. ACS Applied Materials &
Interfaces, 8(41), 28086–28095.
emission. In the Vis region, fluorescent colors of fluorescent brighteners
were able to be identified directly by the naked eye. Conversely, NIR
luminescence could not be recognized with the naked eye and only
detected using some advanced analytical instruments. Moreover, here,
as both the fluorescent brightener excitation spectra (Fig. S18) and Nd3+
down-conversion NIR excitation spectrum overlapped in their absorp-
tion spectra (Fig. S19), fluorescent brightener Vis (450 nm) emissions
and Nd3+ NIR (1060 nm) dual emission were generated under 311 nm
excitation. Second, when Nd-nanopaper@FB was excited at 580 nm,
only one band of fluorescence emission is produced, which meant that
the naked eye could directly recognize the fluorescent of Nd3+ fluores-
cence in the Vis light region. The fluorescent brighteners did not fluo-
resce, which negated interference by background fluorescence from
fluorescent brighteners. Therefore, the Nd3+ emitter generated
up-converted fluorescence under 580 nm excitation and
down-converted fluorescence under 311 nm excitation (Wang, Xie,
Suehiro, Takeda, & Hirosaki, 2018). Thereby, this material could be
used in Vis and NIR dual emission and dual channel anti-counterfeiting
applications.
Andres, J., Hersch, R. D., Moser, J., & Chauvin, A. (2014). A new anti-counterfeiting
feature relying on invisible luminescent full color images printed with lanthanide-
based Inks. Advanced Functional Materials, 24(32), 5029–5036.
Arppe, R., & Sørensen, T. J. (2017). Physical unclonable functions generated through
chemical methods for anti-counterfeiting. Nature Reviews Chemistry, 1(4), 1–13.
Chen, Y., Geng, B., Jing, R., Tong, C., & Chen, J. (2017). Comparative characteristics of
TEMPO-oxidized cellulose nanofibers and resulting nanopapers from bamboo,
softwood, and hardwood pulps. Cellulose, 24(11), 1–14.
Chuang, T., & Verdun, H. R. (1996). Energy transfer up-conversion and excited state
absorption of laser radiation in Nd:YLF laser crystals. IEEE Journal of Quantum
Electronics, 32(1), 79–91.
Courrol, L. C., Lima, B. L. S. D., Kassab, L. R. P., Cacho, V. D. D., Tatumi, S. N. H.,
Gomes, L., et al. (2004). Up-conversion losses in Nd3+ doped lead fluoroborate
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These Nd-nanopapers were synthesized by a one pot solvothermal
method. Shifting away from traditional down-conversion Vis emission
anti-counterfeiting, the present Nd-nanopapers achieved down-
conversion NIR emission and up-conversion Vis emission. The down-
conversion luminescent property of Nd-nanopaper exhibited character-
istic NIR luminescence (λEm = 1080 nm) of Nd3+ ions when excited by
311 nm, which underwent an “antenna” effect by the H3BTC ligand. In
contrast, up-conversion Vis light emission (λEm =450 nm) of Nd-
nanopaper was detected under 580 nm excitation, which was ascribed
to excited-state absorption and energy transfer up-conversion. Nd-
nanopaper induced both up and down-conversion for Vis and NIR
emissions that completely avoided interference by fluorescent bright-
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conversion fluorescent Nd-nanopaper with Vis and NIR dual emission
or dual channel could be applied in high level anti-counterfeiting
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Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgments
Missoum, K., Bras, J., & Belgacem, M. N. (2012). Organization of aliphatic chains grafted
on nanofibrillated cellulose and influence on final properties. Cellulose, 19(6),
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transparent and multi-luminescent TEMPO-oxidized nanofibrillated cellulose
nanopaper functionalized with lanthanide complexes. Journal of Materials Chemistry
C, 3(11), 2511–2517.
This work was supported by National Natural Science Foundation of
China (21703131, 31370578), Doctoral Scientific Research Foundation
of Shaanxi University of Science & Technology (2016BJ-40), State Key
Laboratory of Pulp and Paper Engineering, South China University of
Technology (201821), Special Research Fund by Shaanxi Provincial
Department of Education (18JK0122), Chinese Postdoctoral Science
Foundation (2018M643707), Natural Science Basic Research Plan in
Shaanxi Province of China (2019JQ-516), Guangxi Key Laboratory of
Resch-Genger, M. S. J. K. (2020). Near-IR to Near-IR upconversion luminescence in
molecular chromium ytterbium Salts. Angewandte Chemie-International Edition.
https://doi.org/10.1002/anie.202007200
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infant clothes and paper materials by ion-pair chromatography and fluorescence
detection. Journal of the Chinese Chemical Society, 56(4), 797–803.
Tai, Y., Li, X., & Pan, B. (2018). Efficient near-infrared down conversion in Nd3+-Yb3+ co-
doped transparent nanostructured glass ceramics for photovoltaic application.
Journal of Luminescence, 195, 102–108.
Clean Pulp
&
Papermaking and Pollution Control (2019KF18,
2019KF33), State Key Laboratory of Biobased Material and Green
Papermaking (KF201918).
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doped up-conversion materials for photovoltaic applications. Advanced Materials, 23
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Appendix A. Supplementary data
Wang, J., Song, B., Tang, J., Hu, G., Wang, J., Cui, M., et al. (2020). Multi-modal anti-
counterfeiting and encryption enabled through silicon-based materials featuring pH-
responsive fluorescence and room-temperature phosphorescence. Nano Research, 13
(6), 1614–1619.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.carbpol.2020.117134.
Wang, L., Xie, R. J., Suehiro, T., Takeda, T., & Hirosaki, N. (2018). Down-conversion
nitride materials for solid state lighting: recent advances and perspectives. Chemical
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