Lanthanide-based hydrogels with adjustable luminescent properties synthesized by thiol-Michael addition

Article abstract of DOI:10.1016/j.dyepig.2019.108091

Luminescent hydrogels have been widely used in many fields because of their excellent optical properties. Although there are many mature preparation methodologies, the development of new luminescent hydrogels with adjustable fluorescence properties and good mechanical properties is still an urgent need to explore. In this paper, we describe a simple and effective way to synthesize highly luminescent PVA/AM-Ln(DPA)₃ double network hydrogels by thiol-Michael addition reaction and in situ polymerization. We firstly synthesized a bifunctional chelating ligand named as 4-(mercaptomethyl)pyridine-2,6-dicarboxylic acid, which can react with electron-deficient carbon-carbon double bonds and effectively sensitize the luminescence of Tb³⁺ and Eu³⁺ ions. And then, the luminescent lanthanide complexes were grafted onto the methacrylated polyvinyl alcohol via thiol-Michael addition reaction. Finally, the modified polymers were polymerized with acrylamide monomer to form the luminescent double network hydrogels. The obtained hydrogels possess bright luminescence, long luminescence lifetimes and good mechanical properties. Moreover, the emission colors of the hydrogels are adjustable by varying the molar ratio of Eu³⁺and Tb³⁺.

Full text of DOI:10.1016/j.dyepig.2019.108091

Journal Pre-proof
Lanthanide-based hydrogels with adjustable luminescent properties synthesized by
thiol-Michael addition
Qing-Feng Li, Shaowen Chu, Erqing Li, Meng Li, Jin-Tao Wang, Zhenling Wang
PII:
S0143-7208(19)32439-8
DOI:
https://doi.org/10.1016/j.dyepig.2019.108091
DYPI 108091
Reference:
To appear in: Dyes and Pigments
Received Date: 17 October 2019
Revised Date: 28 November 2019
Accepted Date: 29 November 2019
Please cite this article as: Li Q-F, Chu S, Li E, Li M, Wang J-T, Wang Z, Lanthanide-based hydrogels
with adjustable luminescent properties synthesized by thiol-Michael addition, Dyes and Pigments (2019),
doi: https://doi.org/10.1016/j.dyepig.2019.108091.
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Graphical Abstract
Double-network lanthanide-based hydrogels with adjustable luminescent properties
synthesized by thiol-Michael addition

Lanthanide-based hydrogels with adjustable luminescent properties synthesized
by thiol-Michael addition
Qing-Feng Lia, Shaowen Chua, Erqing Lib, Meng Lia, Jin-Tao Wanga* and Zhenling Wangc*
aHenan Key Laboratory of Rare Earth Functional Materials, International Joint Research Laboratory for
Biomedical Nanomaterials of Henan, Zhoukou Normal University, Zhoukou, 466001, Henan, China.
bCollege of Chemistry and Molecular Engineering, International Phosphorus Laboratory, International
Joint Research Laboratory for Functional Organophosphorus Materials of Henan Province, Zhengzhou
University, Zhengzhou 450001, P. R. China.
cCenter of Analysis and Testing, College of Material and Chemical Engineering, Henan University of
Engineering, Xinzheng 451191, P. R. China.
E-mail: wangjintao2565@126.com (Dr J. T. Wang); zlwang2007@hotmail.com (Prof Z. L. Wang),
+086-3948178518
Abstract
Luminescent hydrogels have been widely used in many fields because of their excellent optical
properties. Although there are many mature preparation methodologies, the development of new
luminescent hydrogels with adjustable fluorescence properties and good mechanical properties is
still an urgent need to explore. In this paper, we describe a simple and effective way to synthesize
highly luminescent PVA/AM-Ln(DPA)3 double network hydrogels by thiol-Michael addition
reaction and in situ polymerization. We firstly synthesized a bifunctional chelating ligand named
as 4-(mercaptomethyl)pyridine-2,6-dicarboxylic acid, which can react with electron-deficient
carbon-carbon double bonds and effectively sensitize the luminescence of Tb3+ and Eu3+ ions. And
then, the luminescent lanthanide complexes were grafted onto the methacrylated polyvinyl alcohol

via thiol-Michael addition reaction. Finally, the modified polymers were polymerized with
acrylamide monomer to form the luminescent double network hydrogels. The obtained hydrogels
possess bright luminescence, long luminescence lifetimes and good mechanical properties.
Moreover, the emission colors of the hydrogels are adjustable by varying the molar ratio of
Eu3+and Tb3+.
Keywords: luminescence; lanthanide complexes; Michael addition reaction; hydrogels
1. Introduction
Luminescent hydrogels have gained considerable interest due to their excellent optical properties
and inventive applications in controlled drug release, sensing, 3D printing and tissue engineering
[1-4]. Various luminophores, including organic dyes, luminescent quantum dots, metal complexes
are frequently used as emitting sources to fabricate luminescent hydrogels [5-9]. Recently, much
effort has been directed towards hybridizating hydrogels with lanthanide complexes through
non-covalent interaction or covalent connection. This is related to some feature of lanthanide
complexes [10,11]: (1) Strong designability. Lanthanide complexes with different functional
groups can be prepared by organic synthesis and coordination assemble technology, which is
beneficial to realize the hybridization of lanthanide complexes with hydrogels; (2) Outstanding
emission properties. including wide range of emission from ultraviolet to near-infrared regions,
high color purity, long luminescence lifetime; (3) Small size. Thus, hybridization of lanthanide
complexes with hydrogels at the molecular level is easy to achieve, which is helpful to obtain a
luminescent hydrogel with uniform structure and stable performance; (4) Unique paramagnetic
properties, which make them potentially useful in magnetic resonance imaging and paramagnetic
labeling of proteins.

Over the years, several different approaches, including click chemistry, photo-initiated/free radical
polymerization and Michael-like addition reaction, have been explored for preparation of chemical
crosslinking hydrogels [12-15]. Michael-like addition reaction, by contrast, are especially
appealing due to its high atom economy (no by-products), environmental friendliness, and mild
reaction conditions (no need for heating or ultraviolet light). For instance, Lensen and coworkers
developed a PEG-based hydrogel with tunable stiffness and post gelation chemical functionality
via amine-Michael addition reaction [16]. Misra et al reported a pH and reduction
dual-stimuli-responsive hydrogel synthesized by thiol-Michael addition reaction for anticancer
drug delivery [17]. Recently, we have synthesized a bifunctional chelating ligand named as
4-(mercaptomethyl)pyridine-2,6-dicarboxylic acid (L) which contains: a free thiol group that can
react with electron-deficient carbon-carbon double bonds via a Michael-like addition reaction [18,
19] and a 2,6-pyridinedicarboxylic acid moiety that can effectively sensitize the luminescence of
Tb3+ and Eu3+ ions [20]. Moreover, three such ligands can bind to one lanthanide ion so as to form
a trithiol complex (Ln·L3). This work has aroused our interest to explore its potential applications
in the synthesis of novel luminescent hydrogels.
Herein, we developed a novel method for synthesis of luminescent lanthanide double network
hydrogels via thiol-Michael addition reaction and in situ polymerization (Scheme 1). Firstly,
methacrylated polyvinyl alcohol (PAV-GMA) was crosslinked by the trithiol complexes (Ln·L3)
via thiol-Michael addition reaction to obtain a network polymer which was used to polymerize
with acrylamide monomer to form the luminescent double network hydrogels. It should be noted
that Ln·L3 has a good water solubility which enables its homogeneous distribution in the
hydrogels. In addition, we used weak base (catalyst) instead of photoinitiator to accelerate the

thiol-Michael addition reaction because the competition absorption of excited light energy
between photoinitiator and Ln·L3 has strong adverse effects on the luminescence quality of
Ln·L3.Thus, the prepared hydrogels exhibited excellent luminescent properties. We expect that
this work will provide a simple way for fabricating luminescent lanthanide hydrogels by means of
thiol-Michael addition reaction.
2. Material and methods
2.1. Preparation of 4-(mercaptomethyl)pyridine-2,6-dicarboxylic acid (L)
4-(bromomethyl)pyridine-2,6-dimethyl dicarboxylate (0.56 g, 2.0 mmol) and thiourea (0.20 g,
2.6mmol) were added to 20 mL methanol. The resulting solution was refluxed under an
atmosphere of N2 for 8 h, and then cooled down to room temperature. The solvent was removed
by vacuum distillation. The remaining residue was redispersed in acetone (20 mL), filtered and
dried under vacuum. The obtained solid together with NaOH (0.62 g) was added in 20 mL H2O.
The mixture was heated at 80 for 2 h under N2 atmosphere. The pH of the above mixture was
adjusted to 2 by H+ ion exchange resin. After filtering out the resin, the filtrate was evaporated to
dryness. Finally, 4-(mercaptomethyl)pyridine-2,6-dicarboxylic acid was obtained as a pale yellow
solid in a 63% yield.1H-NMR (300 MHz, D2O, pD>14) δ ppm: 7.73 (s, 2H), 3.53 (s, 2H).
13C-NMR (300/4 MHz, D2O, pD>14) δ ppm: 173.6, 159.6, 152.8, 124.4, 28.0 (Fig. S1). ESI-MS:
M/Z calcd for C8H7NO4S: 213.0096; [M-1]-, found: 212.0036 (Fig. S2).
2.2. Preparation of lanthanide complexes (Ln·L3)
4-(mercaptomethyl)pyridine-2,6-dicarboxylic acid (0.0478 mmol) and LnCl3·6H2O (0.0159 mmol
Ln = Tb or Eu) were firstly dissolved in distilled water. And then the pH of the mixture was
adjusted to about 7.0 (total volume: 2.0 mL). Finally, the prepared sample was stored in a

refrigerator at -20 for use later.
2.3. Synthesis of methacrylated polyvinyl alcohol (PVA-GMA)
Polyvinyl alcohol (PVA, 5.0 g) was dissolved in dimethyl sulfoxide (DMSO, 100 mL) to form a
clear solution. Glycidyl methacrylate (0.32 g) and 4-dimethylaminopyridine (DMAP) were added
into the PVA solution. The mixture was stirred at 60 for 6 h, and then diluted to 600 mL with
acetone. After centrifugation, the obtained sample was washed three times with acetone, dried
under vacuum and stored at -5 in the dark.
2.4. Synthesis of luminescent double network (DN) hydrogels
PVA-GMA (0.05 g) was dissolved in H2O (1.0 mL) to form a clear solution. Then, aqueous
ammonia (2.5 µL, 3wt%) and Eu·L3 (37.5 µL) were mixed with the above solution under stirring.
After standing under N2 atmosphere for one day, acrylamide monomer was added. The reaction
system was cooled in an ice bath and bubbled with N2 for 20 min. Finally, N,N,N',N'-tetramethyl
ethylene diamine (5.0µL) and ammonium persulfate (2.0 mg) were added to the reaction solution.
The Eu3+-containing DN hydrogel (PVA/AM-Eu(DPA)3) was formed after the mixture was kept at
room temperature for 5 min. The Tb3+-containing DN hydrogel (PVA/AM-Tb(DPA)3) was
prepared according to a similar procedure except that Eu·L3 was replaced by Tb·L3. Hydrogels
with adjustable luminescent color were prepared by varying the molar ratio of Eu·L3 and Tb·L3
during the synthesis (the total volume of Eu·L3 and Tb·L3 is 37.5 µL). PVA/AM was synthesized
through the same process without adding lanthanide complexes for comparison.
2.5. Synthesis of PVA-Eu(DPA)3 hydrogels
PVA-GMA (0.05 g) was firstly dissolved in H2O (1.0 mL) to form a clear solution. Then, aqueous
ammonia (2.5 µL, 3wt%) and Eu·L3 (37.5 µL) were added under stirring. After standing under N2

atmosphere for one day, the reaction system was cooled in an ice bath and bubbled with N2 for 20
min. Finally, N,N,N',N'-tetramethyl ethylene diamine (5.0µL) and ammonium persulfate (2.0 mg)
were added to the above solution. The hydrogel (PVA-Eu(DPA)3) was formed after the mixture
was kept at room temperature for 10 min.
2.6. Swelling measurements
The swelling properties of the as-prepared hydrogels were determined by gravimetric analysis.
The dried hydrogels were weighed accurately and immersed in an excess amount of deionized
water at 37 . At specific intervals, the swollen hydrogels were removed from water and weighted
after carefully wiping off excess surface water. The swelling ratio (SR) of the as-fabricated was
calculated by using the following equation (1):
SR = (Ws - Wd)/Wd × 100%
(1)
where Ws and Wd are the weight of the swollen and dried hydrogels, respectively.
2.7. Characterization
1H-NMR and 13C-NMR spectra were recorded on a Bruker AV-300 or AV-400 spectrometer. Mass
spectrum was acquired on an Agilent 6520 Q-TOF mass spectrometer. The photoluminescence and
lifetime measurements were measured on an Edinburgh instruments FLS920 spectrofluorometer.
Quantum efficiency was measured by using an integrating sphere whose inner face was coated
with Benflect. FT-IR spectrum was carried out using a Thermo Nicolet 5700 spectrophotometer.
The compression test of the prepared hydrogels (cylinder-shape, diameter × height = 16.0 mm × 5
mm) were performed at a constant velocity of 1.0 mm/min on a Shimadzu universal testing
machine (AGS-X500N) with a 500 N load cell. Rheological tests were carried out by an Anton
Paar MCR302 rheometer. Scanning electron microscopy (SEM) measurements were performed on

a FEI Quanta200 electron microscope.
3. Results and discussion
Scheme 1 Proposed structures and synthetic route of the lanthanide double network hydrogels.
The yellow curve represents PVA and the blue one represents polyacryamide.
PVA-GMA was synthesized from polyvinyl alcohol and glycidyl acrylate and characterized by
1H-NMR. As shown in Fig. 1, the signals at δ = 5.98 and 5.62 ppm were assigned to the olefinic
protons on C9. The signal at δ = 1.86 ppm corresponds to the methyl protons on C8 (Fig. 1a) [21].
These results confirm that methacryloyl group has been successfully grafted onto the polyvinyl
alcohol by using glycidyl acrylate. Moreover, the degree of substitution of -OH groups by
1
methacryloyl groups was determined from H-NMR. Before that, the alcoholysis degree of

polyvinyl alcohol was firstly measured from the integral area ratio of the protons on C11 (δ =
4.68ppm) and C13 (δ = 3.81 ppm) (Fig. 1b). And an alcoholysis degree of 84% was obtained
1
according to the H-NMR analysis. The presence of ester groups in PVA was also confirmed by
FT-IR spectrum. The absorption peak at 1714 cm-1 (Fig. S3) can be ascribed to C=O stretching
vibration of aliphatic eater [22]. And then the degree of substitution was calculated from the
relative integral area of olefinic protons on C9 (δ = 5.98 ppm) and the methyl protons on C3 (δ =
1.86 ppm). By calculation, the degree of substitution was determined to be 1 %.
Fig.11H-NMR spectra of PVA-GMA (a) and PVA (b) in DMSO-d6.
4-(mercaptomethyl)pyridine-2,6-dicarboxylic
acid
was
prepared
using
4-bromomethylpyridine-2,6-dimethyl dicarboxylate and thiourea as the raw materials. The
coordination stoichiometry between 4-(mercaptomethyl)pyridine-2,6-dicarboxylic acid and
lanthanide ions was determined by fluorescence titration [23]. As shown in Fig. 2, upon addition
of Eu3+ to ligand solution, the characteristic emission bands of Eu3+with sharp peaks at 589, 594,
615, 649 and 694 nm appeared and increased gradually until a Eu3+ to ligand molar ration of 1:3
was reached. However, further increasing of Eu3+ concentration leaded to a decrease in
fluorescence intensity. The inflection point occurred at a molar ration of 0.33, corresponding to a

1:3 complexation stoichiometry between Eu3+ and L. Previous studies have shown that 1:3, 1:2 or
1:1 Eu3+/DPA coordination complexes can be formed by addition of Eu3+to DPA solution with
different molar ratio. As for 1:2 or 1:1 Eu3+/DPA coordination complexes, its luminescence would
be quenched to a certain level by energy transfer from excited state of Eu3+ to the higher O-H
vibration overtones of coordination H2O. Therefore, an inflection point was observed on the
fluorescence induction curve.
Fig. 2 Fluorescence spectra of L (0.02 mM, 3mL) upon the addition of different amounts of Eu3+
(0.3 mM). Inset: the fluorescence intensity changes at 615 nm vs Eu3+/L molar ratio. (λex=280 nm).
As shown in Fig. 3, simple mixing of PVA-GMA and Eu·L3 under base condition only gave a
clear solution (Fig. 3a) which could be further polymerized to form a hydrogel with ammonium
persulfate as the initiator (Fig. 3b). Whereas in situ polymerization of acrylamide in the presence
of PVA-GMA or Eu·L3/PVA-GMA led to the formation of transparent double network hydrogels,
which could easily be molded into free-standing object with a certain shape (Fig. 3c to e). The
Eu3+-containing hydrogel exhibited the characteristic red emission under a 254 nm UV lamp (Fig.
3f). During this process, the thiol-Michael addition reaction between PVA-GMA and Eu·L3 can be
monitored by Ellman's reagent (DTNB, 5,5'-dithiobis-(2-nitrobenzoic acid)) [24]. The reaction of
free thiol groups with DTNB produces a yellow 5-mercapto-2-nitrobenzoic acid anion with strong

absorption at 412 nm (Fig. S4). Therefore, a naked color change from colorless to bright yellow
was observed upon the addition of DTNB to the solution of Eu·L3. However, when DTNB were
added into the mixture of PVA-GMA and Eu·L3, no obvious color changes were observed,
indicating that the thiol groups were consumed by the Michael addition reaction. Meanwhile, the
above inference was also confirmed by UV-vis absorption spectra. As shown in Fig. S5, an
obvious absorption peak at 412 nm belonging to 5-mercapto-2-nitrobenzoic acid was observed
(Fig. S5a), showing that Eu·L3was reacted with DTNB by thiol-disulfide exchange reaction.
However, similar absorption peak did not appear in Fig. S5b, suggesting the absence of free thiol
groups in the reaction solution of PVA-GMA and Eu·L3. In fact, according to the alcoholysis
degree and degree of substitution of PVA, the minimum molar ratio of Michael receptor
(electron-deficient double bonds) and donor (thiol groups) in this thiol-Michael addition reaction
was found to be 10:1, which ensured that thiol groups were completely consumed in this reaction.
Fig. 3g showed the swelling behaviors of PVA-Eu(DPA)3, PVA/AM and PVA/AM-Eu(DPA)3.
Obviously, PVA/AM and PVA/AM-Eu(DPA)3 exhibited a smaller swelling ratio than
PVA-Eu(DPA)3, indicating that PVA/AM and PVA/AM-Eu(DPA)3 have a more compact network
structure [25]. In addition, the mechanical properties of PVA-Eu(DPA)3, PVA/AM and
PVA/AM-Eu(DPA)3 were studied by compression tests (Fig. 3h). The compressive strength at
strain of 80% was 0.489 MPa for PVA/AM and 0.464 MPa for PVA/AM-Eu(DPA)3, respectively.
However, PVA-Eu(DPA)3 was apt to fracture under the relatively low compress load (Strain: 68%,
Stress: 0.08 MPa). Compared to PVA-Eu(DPA)3, the prepared double network hydrogels
(PVA/AM and PVA/AM-Eu(DPA)3) showed improved mechanical performance.

Fig. 3 The digital photos of the mixture of PVA and Eu·L3 (a), PVA-Eu(DPA)3 (b), PVA/AM (c),
PVA/AM-Eu(DPA)3 under sunlight (d and e) or a 254 nm UV lamp (f). The swelling kinetics (g)
and compressive properties (h) of the prepared hydrogels.
To gain insight into the dynamic mechanical behavior of the as-prepared hydrogels, we performed
some rheological tests on two representative samples at different frequencies and strains. As seen
from Fig. 4a and 4b, the storage modulus (G') value was larger than the loss modulus (G'') value
over the whole investigated frequency range, indicating that the as-prepared double network
hydrogels has strong gel-like behavior [23]. Moreover, the value of G''/G'(tanδ) was not
frequency-independent, showing that both of the hydrogel systems were kept stable over a broad
frequency range. The corresponding strain sweep of the Eu3+-containing hydrogel was also carried
out (Fig. 4c). Interestingly, the storage modulus remained steady when the strain increased from 1%
to 20%, indicating a satisfactory damage resistance capacity of the Eu3+-containing hydrogel.
However, the loss modulus exceeded the storage modulus when the strain increased to more than
200%, implying that gel-sol transition behavior appeared. Therefore, the strain was fixed at γ=1%
in the rheology measurements. Furthermore, the continuous step tests at γ=100% and at γ=0.1%

with ω=10 rad/s were used to evaluate the recovery property of the prepared hydrogel (Fig. 4d). It
is clearly seen that the Eu3+-containing hydrogel possessed similar storage modulus and loss
modulus when exposed to higher strain (100%), resulting in the formation of a metastable fluid
state. However, a sudden increase in storage modulus and decrease in loss modulus were observed
when the strain decreased to 0.1%, indicating that the hydrogel was reformed. The storage
modulus and loss modulus immediately returned to their initial values when the strain was
increased from 0.1% to 100%. This rheology behavior confirmed excellent self-recovery property
of the Eu3+-containing hydrogel [26].
Fig. 4 Rheological properties. Frequency (ω) sweep tests at ω=0.05-100 rad/s and strain (γ)=1% of
PVA/AM (a), PVA/AM-Eu(DPA)3 (b). Strain sweep tests of PVA/AM-Eu(DPA)3 at ω=10 rad/s,
γ=0.1-1000% (c). Continuous step strain tests of PVA/AM-Eu(DPA)3 at γ=100% (black) and 0.1 %
(red) with ω=10 rad/s.
The surface morphology of the prepared hydrogels was investigated by SEM. It should be noted
that the samples were first frozen in liquid nitrogen, and then dehydrated by freeze drying process
before investigation. Fig. 5 showed that both of the hydrogels have a rough surface morphology
with a number of irregular pores. However, the pore diameter of the double network hydrogel was

increased slightly with the incorporation of lanthanide complexes. And many smaller pores with
diameter below 1µm were observed in PVA/AM, which indicated that PVA/AM-Eu(DPA)3 has a
lower crosslinking density than PVA/AM [27-29]. This result was also consistent with the analysis
from the swelling and compression measurements.
Fig. 5 SEM images of PVA/AM-Eu(DPA)3 (a, b) and PVA/AM (c, d) with low and high
magnifications.
Benefitting from the high color purity and high luminescence efficiency of Ln·L3, the
subsequently obtained lanthanide-containing hydrogels exhibited excellent optical properties
under UV lamp illumination. Fig. S6 showed that Tb3+and Eu3+-containing hydrogels displayed a
broad excitation band ranging from 240 to 310 nm with a maximum at 280 nm [30-32], which
indicated that these two kinds of hydrogels can be excited by the UV light of the same wavelength.
The emission spectrum of Tb3+-containing hydrogel exhibited four characteristic emission lines at
490, 542, 581 and 620 nm, which can be assigned to the 5D4→7FJ (J=6, 5, 4, 3) transitions of Tb3+,
in which the emission line corresponding to 5D4→7F2 is the strongest and responsible for the green
emission (Fig. 6a) [33-37]. The emission spectrum of Eu3+-containing hydrogel showed four sharp

emission lines at 593, 615, 650 and 694 nm, assigned to 5D0→7FJ (J=1, 2, 3, 4) transitions of Eu3+.
5
Among these emissions, the emission line corresponding to D0→7F2 is prominent, resulting in a
red luminescence (Fig. 6a) [38-41]. The addition of DPA ligands to Tb3+ solution brings about
noticeable alterations in spectral shape. And the appearance of splitting peaks at 490, 542 and 582
nm indicates the formation of 1:3 Tb3+/Ligand coordination complexes [42]. Moreover, the
emission colors of the hydrogels can be tuned by altering the molar ratio of the two types of
lanthanide luminophores in hydrogels. The intensity ratio of red to green emission (I615nm/I542nm
)
decreased with the increasing Tb3+ concentrations. Thus, a color change from red to green though
yellow was observed (Fig. 6b and Fig. S7). To further investigate the optical performance of these
luminescent hydrogels, the corresponding decay curves were measured. It is clear that the decay
curves were well fitted into a single-exponential function described as I= I0exp(-t/τ) (Fig. S8),
implying that the coordination environment of Tb3+ or Eu3+ in hydrogel is uniform [43]. According
to the fitting results of Fig. S8, the lifetime values of Tb3+ in PVA/AM-Tb(DPA)3 and Eu3+ in
PVA/AM-Eu(DPA)3 were determined to be 1.41 ms and 1.46 ms, respectively. The quantum yield
of PVA/AM-Tb(DPA)3 and PVA/AM-Eu(DPA)3 are 3.02% and 7.85%, respectively. Furthermore,
the excitation/emission wavelength (Fig. S9), lifetimes (Fig. S10) and quantum efficiency of
Ln·L3 also measured and listed in Table S1. It can be clearly seen that the polymer matrixes used
in PVA/AM-Ln(DPA)3 have a little effect on the luminescent properties of the lanthanide
complexes. Moreover, in order to determine the stability of the hydrogel, a simple experiment was
carried out. The prepared PVA/AM-Eu(DPA)3 was first immersed into deionized water (10 mL)
for two days, and then removed from the deionized water. However, when the soaking solution
was excited by a UV lamp (280 nm), no obviously emission peak was found, showing that the

luminophore did not escape from the hydrogel. (Fig. S9). These result also shows that it is feasible
to synthesize lanthanide complexes-based hydrogels using our strategy.
Fig. 6 Emission spectra of the lanthanide-containing double network hydrogels (a), the
corresponding CIE 1931 chromaticity diagram, (Eu3+/Tb3+ = 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7,
2:8, 1:9, 0:10) (b).
4. Conclusions
In summary, we developed a novel and simple strategy to prepare luminescent hydrogels by a
combination of thiol-Michael addition reaction and free radical polymerization. Luminophore was
successfully fixed into the hydrogel by employing a bifunctional organic ligand. Lanthanide

complexes and double network structures were used to improve the luminescence properties and
mechanical strength of hydrogels, respectively. The obtained PVA/AM-Tb(DPA)3 and
PVA/AM-Eu(DPA)3 double network hydrogels exhibit the characteristic emission of Tb3+ (green)
and Eu3+ (red), long luminescence lifetimes and high quantum efficiency. Moreover, emission
color-tunable luminescent hydrogels were obtained by adjusting the molar ratio of Eu3+ and Tb3+.
Acknowledgment
This work was supported by the National Natural Science Foundation of China [grant numbers
21805319, 51572303, 21401218] and Key Research projects of Henan Education institutions [grant
number 19A150050].
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Research Highlights
ò
ò
A trithiol luminescent lanthanide complex was developed.
The trithiol luminescent lanthanide complex was grafted onto the methacrylated polyvinyl
alcohol via Michael-like addition reaction.
ò
The obtained hydrogels displayed multi-color luminescence and good mechanical properties..

Qing-Feng Li: Conceptualization, Investigation, Writing-Original Draft. Shaowen Chu:
Investigation. Erqing Li: Formal analysis. Meng Li: Resources. Jin-Tao Wang: Writing-Review
and Editing. Zhenling Wang: Supervision.

Declaration of interests
☑The authors declare that they have no known competing financial interestsor personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: