Tunable white-light emission by supramolecular self-sorting in highly swollen hydrogels

Article abstract of DOI:10.1039/c7cc08822a

Fluorescence-tunable hydrogels especially emitting white-light were achieved by swelling hydrogels in solutions containing two kinds of dyes. The fluorescence of the dyes was enhanced by the orthogonal supramolecular complexation with different binding si

Full text of DOI:10.1039/c7cc08822a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Tunable White-Light Emission by Supramolecular Self-sorting in
Highly Swollen Hydrogel
Received 00th January 20xx,
Accepted 00th January 20xx
a
a, b
a, b
a, b,
Qian Zhao , Yong Chen , Sheng-Hua Li and Yu Liu
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Fluorescence-tunable hydrogels especially emitting white-light with different fluorophores in an orthogonal way for emitting
were achieved by swelling hydrogels in the solutions containing different colors. Herein we reported a hydrogel-I containing two
two kinds of dyes. The fluorescence of dyes was enhanced by the binding sites, i.e. the adamantyl (Ad) and the sulfonatocalix[4]arene
orthogonal supramolecular complexation with different binding (SC[4]A) moieties. The hydrogel-I exhibited excellent swelling
sites in the hydrogels.
property and orthogonal supramolecular recognition of β-
cyclodextrin-modified tetraphenylethene (TPECD) and 4-[4-
Photo-luminescent gels with multicolor emissions, especially white
light, have attracted more attention due to their potential
(
dimethylamino)styryl]-1-methylpyridinium iodide (DASPI). As a
result, the regulation of the fluorescence emission was conveniently
achieved, which was changed from blue to yellow and especially
including white light.
1
applications in chemosensors, light-emitting materials, etc . Most of
those gels are supramolecular gels, in which the small fluorescent
molecules and polymers chelate as 3D networks through
noncovalent interactions such as π-π interactions, hydrogen
Hydrogel-I was produced by one-pot UV-initiated free-radical
copolymerization of acrylamide (AAm), 1-adamantyl acrylamide
2
1
0
11
bondings, and metal coordinations (Fig 1a Strategy I). But once gels
(
AAmAd) , 4-(allyloxy)sulfonatocalix[4]arene (SC[4]AA) and N,N'-
were formed in these“in situ” ways, it is not easy to regulate the
emission colors because the fluorophores have already been
chelated in the gels. Considering networks of gels can swell and
3
absorb some of substances dissolved in the solvent , making the
hydrogels swell with two or more fluorophores might be a
promising way for developing multifluorescent materials (Fig 1a
4
Strategy II) . This pre-formed method is more convenient to
manipulate the emission colors by swelling different fluorophores in
different ratios whenever necessary.
Macrocycle-based supramolecular hydrogels, have been widely
applied in stimuli-responsive and self-healing materials owing to
distinct host-guest interactions between macrocyclic host and guest
3
c, 5
moieties . Different host-guest pairs usually show high binding
affinity and selectivity for molecular recognitions which do not
6
interact with each other. Those “self-sorting” behaviors could make
7
the supramolecular systems self-assemble in “orthogonal” way at
8
molecular levels, and even on macroscopic-scale . Our group has
developed several self-sorting systems which showed potential
applications in molecular machines, supramolecular polymers and
9
biomedicines . But it remains challenging to make a hydrogel swell
a
College of Chemistry, State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, P. R. China.
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Nankai University, Tianjin 300071, P. R. China
E-mail: yuliu@nankai.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Fig. 1 a) Different strategies for fabricating multicolor fluorescent hydrogels (Strategy I:
Sol-gel transformations, chelating fluorophores to gels; strategy II: swelling with
fluorophores, self-sorting of fluorophores in gel phase). b) Synthesis of hydrogel-I by
copolymerization.
b
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a
Table 1 Weight of substrates for I ~ VI
shrinking state (diameter: 1.2 cm, weight: 0.25 g). That means, the
hydrogel gelator ratio was eased from 74 % to 1.60 % when
hydrogel-I fully swollen in the water. The scanning electron
microscopy (SEM) images showed the morphology of fully swollen
hydrogel-I as a porous structure (Fig S7), while the shrunk state as a
dense structure (Fig S8). Compared to the fully swollen hydrogel-II
b
AAm AAmAd SC4AA bisAAm Initiator
Hydrogel-I
Hydrogel-II
150
150
20
20
0
10
0
5
5
5
0
0
0
5
5
5
5
5
5
Hydrogel-III 150
10
10
10
0
(
diameter: 2.6 cm, weight: 3.3 g), the better swelling properties of
Polymer-IV
Polymer-V
Polymer-VI
150
150
150
20
0
hydrogel-I and hydrogel-III (diameter: 3.6 cm, weight: 8.3 g) were
undoubted attributed to the presence of SC4A. As shown in Fig S9,
accompanied by the increased amounts of SC4AA (0, 5, 10, 15
mg/mL) in the polymers, the weight of fully swollen hydrogel was
even up to 24.2 g with the water content as 99.2%.
20
a
Preparation of hydrogels and polymers with various substrates (mg) in 1 ml DMSO as
b
list in the Table 1. Hydroxycyclohexylphenylketone was used as initiator.
Rheology experiments showed that the fully swollen hydro-gel-I
has certain mechanical strength. The dynamic strain sweep curves
showed that G' was always larger than G" with the strain from 0.1%
to 100% (Fig S10), indicating that the hydrogel-I was stable and
remained undamaged. The further increasing strain made G'
decrease and G'' increase dramatically and came across at the strain
methylenebisacrylamide (bisAAm) (mass ratio at 30:1:4:2) with
hydroxycyclohexylphenylketone as photoinitiator (Fig 1b). After
polymerization, the homogeneous solution yielded gel, which was
purified by washing with DMSO and water several times for
removing unreacted monomers. Finally, hydrogel-I was easily
obtained by solvent replacement in excess amount of water. As
shown in Tables 1 & S1, other referential hydrogels and polymers
1
2
of 900%, indicating the damage of the hydrogel-I. The dynamic
frequency sweep curves showed that G' kept larger than G" and did
not change obviously with the fixed 1% strain (Fig S11), indicating
the good stability of hydrogel towards the frequency oscillation.
To investigate the self-sorting assembly behaviors of hydrogels
with two binding sites (Fig 3), corresponding dyes TPECD and DASPI
1
were prepared in the same way. As shown in the H NMR spectrum
of polymer-IV (Fig S5), the upfield shifted proton signals of phenyl
rings and the disappearance of allyl proton signals suggested the
copolymerization of SC4AA with acrylamide species.
13, 14
were synthesized by reported methods
and their structures
All of resulting hydrogels were transparent and colorless, with a
water content ranging from 26% to 98%. Photographs of completely
shrunken hydrogel-I, fully swollen hydrogel-I, -II and -III were
presented in Fig S6. The weight of the fully swollen hydrogel-I was
were verified by NMR and HRMS (Fig S12-15). As a class of dyes
with special fluorescent properties "aggregation-induced emission
(
AIE)", TPE derivatives gave stronger fluorescence emission when
1
5
the "restriction of intramolecular rotation (RIR)" happened. DASPI
11.5 g with a diameter of 4.2 cm, which was 46 folds higher than its
Fig 2. The schematic illustration of the orthogonal supramolecular recognition in the luminescent hydrogel during swelling process.
2
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is a typical twisted intramolecular charge transfer (TICT) molecule,
Vis absorption band of DASPI shows a good overlap with the
which can shine brightly when falling down in a confined region. The fluorescence band of TPECD, so a fluorescence resonance energy
fluorescence properties of TPECD and DASPI based on molecular transfer (FRET) process could be involved in this hydrogel phase.
recognitions with polymers were first investigated in aqueous Energy-transfer efficiency (ΦET) was calculated as 70.3%. To further
solution. Upon the addition of an excess amount of polymer-VI confirm the self-sorting processes in hydrogel phase, similar swollen
(involving Ad), the fluorescence of TPECD enhanced 5 times (Fig S18, experiments were carried out for hydrogel-II and -III. When
S19), while the addition of polymer-V (without Ad) did not affect the hydrogel-II (without Ad) was swelled in TPECD solution, the hydrogel
intensity (Fig S16). So the fluorescence enhancement was attributed did not give blue emission owing to no RIR effect on TPECD (Fig
to the complexation of Ad with CD, which further restricted the S23b). Analogously, when hydrogel-III (without SC4A) was swelled in
intramolecular rotation of TPE moiety for reducing the non-radiative DASPI solution, the yellow fluorescent hydrogel could not be
1
3
relaxation. As shown in Fig S17, the fluorescence intensity of obtained either because DASPI was not confined (Fig S23d).
DASPI greatly enhanced (34 times) in the presence of excess The trajectory of color changes based on the fluorescence
polymer-V (involving SC4A) due to the host-guest complexation spectra of hydrogel-I swollen with different ratios of TPECD and
16b
between DASPI and SC4A,
and was less changed (4 times) with DASPI solution was marked out on a CIE program (Fig 3f). When the
polymer-VI (without SC4A), which was attributed to the TPECD : DASPI ratio is 8:1, the hydrogel gave dual emission colors
hydrophobic microenvironment aroused by penetrating into under 365 nm excitation which could merge white light (Fig 3d). The
1
7
polymers micelles (Fig S20, S21). Those results indicated that the same phenomena were also observed in the laser scanning confocal
enhanced fluorescence of TPECD and DASPI could little interfered microscopy (LSCM) images. The lyophilized hydrogel-I swollen with
each other in this systems.
TPECD gave signals at channel 435-475 nm suggesting the blue
Fortunately, multicolor hydrogel was easily obtained by swelling emission, and that with DASPI gave signals at channel 570-610 nm
fluorophores based on the self-sorting assembly of TPECD-Ad and suggesting the yellow emission (Fig S24). While the white-light
DASPI-SC4A pairs in hydrogel phase (Fig 3). Via soaking in the TPECD emission hydrogel-I showed strong signals both at 435-475 nm (Fig
or DASPI solution, the shrunken hydrogel-I was expanded and gave S25a) and 570-610 nm (Fig S25b) in the dark field, which can be
blue (454 nm) or yellow (595 nm) emissions under 365 nm assigned to the luminescence of TPECD and DASPI respectively, and
excitation (Fig 3b & 3c). Compared to the fluorescence spectrum of the merged image exhibited the strong white light (Fig S25d). In
TPECD solution with the maxima peaked at 470 nm, a 16 nm blue addition, the porous morphology was also observed in LSCM images,
shift suggested the efficient binding of TPECD-Ad rather than the which coincided well with the SEM image.
free existing (Fig S22). The yellow emission also suggested the
strong interaction of DASPI-SC4A in hydrogel phase. When the show high binding affinities with SC4A (binding constants are 9.33 х
The cationic pesticides, such as paraquat (PQ) and diquat (DQ),
4
-1
5
-1
18
hydrogel-I was swelled in aqueous solution with different ratios of 10 M and 7.95 х 10 M respectively ), which are stronger than
4
-1 16b
TPECD and DASPI, the fluorescence spectra of hydrogels exhibited that of DASPI with SC4A (binding constant is 1.3 х 10 M ). So we
two peaks around 454 and 595 nm which could be assigned to could use the fluorescent hydrogel-I to detect PQ and DQ. As shown
emissions of TPECD and DASPI respectively. The fluorescence in figure 4a, the white light was quenched to the weak blue light
emissions at 454 nm (TPECD) gradually decreased and around 595 when the thin layer of hydrogel-I was immersed with paraquat or
nm (DASPI) increased slowly (Fig 3e). As shown in Fig S26, the UV- diquat solution. But when the hydrogel-I emitting blue light was
immersed with the pesticide solution, there was no obvious change
in the fluorescence spectra (figure 4b), indicating that PQ and DQ
could not affect the fluorescence of TPECD. So the quenching of the
white light was mainly attributed to the energy transfer from TPECD
to the free DASPI as a result of the competitive binding of
SC4A DASPI with pesticides.
In summary, a copolymer hydrogel-I of AAm, AAmAd and SC4AA
-
4
-1
Fig. 3 Photos of hydrogels I swollen with (a) water, (b) TPECD (1x10 mol L ), (c) DASPI
Fig. 4 (a) Fluorescence emission spectra and photos taken at 365 nm light (insert) of the
-
5
-1
-4
-1
-5
-1
(
1.25x10 mol L ) (d) mixture of TPECD (1x10 mol L ) and DASPI (1.25x10 mol L )
thin layer of hydrogel-I emitting white light (black) and after immersing with PQ (red) or
taken under 365 nm light; (e) fluorescence emission spectra of hydrogel swollen with
TPECD and DASPI at different molar ratios (top-down are 1:0, 32: 1, 16:1, 8:1, 4:1); (f)
CIE diagram showing the trajectory of the color changes based on the fluorescence
emission spectra.
-
5
-1
DQ (blue) solution (1x10 mol L ). (b) Fluorescence emission spectra and photos taken
at 365 nm light (insert) of hydrogel-I emitting blue light (black) and after immersing
with PQ (red) or DQ (blue) solution.
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with bisAAm as cross-linker was prepared and showed good
swelling property and certain mechanical strength. Owing to
orthogonal supramolecular recognitions of TPECD-Ad and DASPI-
SC4A pairs in hydrogel-I, the multi-color fluorescence emissions
including white light were achieved by swelling hydro-gel-I in
aqueous solution of TPECD and DASPI with different ratios. This
integrative self-sorting system in hydrogels made different
fluorophores emit light efficiently due to the fluorescence
enhancement and FRET induced by host-guest complexations.
Comparing with the well-known method of pre-gelation
modification of luminescent species for gels, our post-gelation
modification method was more convenient for fabricating tunable
fluorescent hydrogels with multi-color emissions. We do believe
this post-modification method based on integrative self-sorting
concept would greatly promote the preparation of multicolor
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1 SC[4]AA was obtained via efficient substitution reaction of
sulfonatocalix[4]arene with allyl bromide (Scheme S2), and its
luminescent displayers or optical devices.
1
Financial support from the National Natural Sciences Foundation of
China (21432004 21672113, 21772099 and 91527301), and the
China Postdoctoral Science Foundation (2016M591380) are
acknowledged.
"cone" conformation was confirmed by the appearance of single
resonance peak of the methylene hydrogen signal in the 1H
NMR spectrum (Fig S2), which suggested the reservation of
hydrophobic cavity of the calix[4]arene. 13C NMR (Fig S3) and
mass spectra (Fig S4) also performed to verify the structure of
SC[4]AA.
Conflicts of interest
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2 K.-P. Wang, Y. Chen, Y. Liu. Chem. Commun., 2015, 51, 1647–
1
649.
3 Q. Zhao, Y. Chen, M. Sun, X.-J. Wu, Y. Liu. RSC Adv., 2016, 6,
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The authors have no conflicts of interest to declare for this
communication.
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