Eu3+ based mesoporous hybrid material with tunable multicolor emission modulated by fluoride ion: Application for selective sensing toward fluoride ion

Article abstract of DOI:10.1039/c7tc01240c

A novel Eu-containing SBA-15 mesoporous hybrid material with multicolor emission modulated by fluoride ion was synthesized. First, the organic ligand 1-(2-naphthoyl)-3,3,3-trifluoroacetonate (NTA) was modified by the silane coupling agent 3-(triethoxysily

Full text of DOI:10.1039/c7tc01240c

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Eu3+ based mesoporous hybrid material with tunable multicolor
emission modulated by fluoride ion: application for selective
sensing toward fluoride ion
Received 00th January 20xx,
Accepted 00th January 20xx
Yajuan Li*, Xudong Yu, Tao Yu
DOI: 10.1039/x0xx00000x
Abstract: A novel Eu-containing SBA-15 mesoporous hybrid material with multicolor emission modulated by fluoride ion
was synthesized. First, the organic ligand 1-(2-naphthoyl)-3,3,3-trifluoroacetonate (NTA) was modified by the silane
coupling agent 3-(triethoxysilyl)propylisocyanate to form an alkene precursor (NTA-Si), which was covalently grafted onto
a mesoporous SBA-15 backbone through co-hydrolysis and co-condensation reactions; as a result, mesoporous hybrid
NTA-S15 was obtained. 2-(2-Carboxylphenyl)-imidazo[4,5-f]- 1,10-phenanthroline (L) was used as the second ligand, whose
two pyridine N atoms coordinate with europium ions and sensitize the luminescence of Eu3+, and the NH group of
imidazole ring and -COOH group formed hydrogen bonds with fluoride ions. Finally, after the coordination reaction
between Eu3+ and organic ligands (NTA-S15 and L), the mesoporous hybrid material Eu(NTA-S15)3L was obtained. Fourier
transform infrared, small-angle X-ray diffraction, N2 adsorption measurements, transmission electron microscopy, and
scanning electron microscopy were performed, and photoluminescence spectra and thermogravimetric plots were
characterized. Results show that Eu(NTA-S15)3L displays high surface area, uniform mesostructure, good thermal stability,
and excellent photophysical properties. Furthermore, Eu(NTA-S15)3L exhibits a highly sensitive sensing function toward
fluoride ion. More importantly, tunable multicolor emissions, which cover nearly the entire visible region, can be readily
achieved by varying fluoride ion concentrations under single-wavelength excitation.
www.rsc.org/
challenge.
Introduction
In addition, researches on efficient luminescent chemosensors that
Since the last century, nanosized particles emitting multiwavelength
emission color are of high importance due to their potential
applications in lightꢀemitting arrays, biological labeling, lightꢀ
harvesting systems, white emission materials, and memory devices.1ꢀ
6 Therefore, it is significant to design and synthesis of materials with
tunable multicolors. Lanthanideꢀdoped light emitting materials
increasingly arouse interest because they possess high stability,
sharp emission lines, and long lifetime of emission.6ꢀ11 In the past
years, the reported lanthanide ionꢀbased materials with tunable color
emission mainly included hybrid nanoparticles, polymer films,
organogels, and MOFs, in which, a twoꢀcomponent doped method
that involves Eu and Tb was usually utilized due to the efficient
energy transfer (ET) between Tb3+ and Eu3+.12ꢀ16 Huang et al.
recently achieved a breakthrough in this field.17 They demonstrated
that the emission color of lanthanideꢀdoped nanocrystals could be
facilely tuned by cation exchange with energy migration. However,
few studies have reported on singleꢀcomponent lanthanideꢀ
containing hybrid material with multicolor emission, and obtaining
such tunable color emission by using only a kind of anion remains a
can sense anions selectively and visually is an important subject both
in supramolecular chemistry and material chemistry due to their
roles in chemical, biological, environmental, medical, and chemical
processes.18ꢀ22 Especially fluoride selective chemosensors have
attracted a growing attention in the past decades, due to their close
relationship with the people’s health and the environmental
protection. 23ꢀ26 Great efforts have been devoted to organic receptors
and related materials, such as MOFs, silica particles for anion
recognition with hydrogen bonding interaction, which are often
accompanied by distinct changes in fluorescent property.27ꢀ32 Anion
sensing based on distinct changes in emission color is especially
attractive in visual sensing. Moreover, a recent study has indicated
that tuning ET from the host to Ln3+ is the key step in producing
materials with tunable emission color. Inspired by these concepts,
through the usage of imidazole as fluoride receptor and two pyridine
rings as coordinative units, a novel red emissive Eu3+ꢀcontaining
mesoporous hybrid material was designed and synthesized in this
study through solꢀgel method. This material could selectively sense
fluoride ion accompanying by gradual changes in emission color
from red to pink, purple, blue, green, and then to yellowish green,
allowing visual and quantitative fluoride sensing just by basing on
emission color rather than on intensity changes. Moreover, it means
that multiple colors can be systematically tuned just by varying the
fluoride ion concentrations in the THF solution of the mesoporous
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hybrid material Eu(NTAꢀS15)3L. To the best of knowledge, this is dried for 12 h at 65 °C. The copolymer surfactant P123 was removed
the first example that multiple color emission could be tuned just by through Soxhlet extraction using ethanol under reflux for 2 days to
anions concentration in the solution of mesoporous hybrid materials. yield NTAꢀS15. The sample was dried overnight at 65 °C under
vacuum.
Synthesis of Euꢀcontaining mesoporous hybrid material
(Eu(NTAꢀS15)3L). Typically, NTAꢀS15 was introduced into a
Experimental
Materials
certain amount of ethanol solution of Eu(NO3)3; subsequently,
ethanol solution of the organic ligand L was added (the molar ratio
of Eu(NO3)3:NTSꢀS15:L= 1:3:1). The mixture was refluxed at 70 °C
for 12 h followed by filtration and extensive washing with EtOH.
Finally, the Euꢀcontaining mesoporous hybrid material was obtained
and denoted as Eu(NTAꢀS15)3L. The content of Eu3+ in the
mesoporous hybrid were determined using coordination titration
method and the content of N element was determined with elemental
analysis, respectively. For Eu(NTAꢀS15)3L, Eu is 2.46% and N is
2.20%. Scheme 1 shows the detailed synthesis process and the
predicted structure of Eu(NTAꢀS15)3L.
Europium nitrate (Eu(NO3)3·6H2O) was prepared by dissolving
Eu2O3 (99.99%, J&K Scientific Ltd.) in concentrated nitric acid
(HNO3) followed by evaporation and vacuum drying. Triblock
copolymer poly(ethylene glyco)ꢀblockꢀpoly(propylene glycol)ꢀ
blockꢀpolyꢀ(ethylene glycol) (Pluronic P123, EO20PO70EO20), 3ꢀ(3ꢀ
(triethoxysilyl)propyl isocyanate (TESPIC, 96%), 1ꢀ(2ꢀnaphthoyl)ꢀ
3,3,3ꢀtrifluoroacetonate (NTA, 98%), tetraethoxysilane (TEOS,
99%), and sodium hydride (60%, dispersed in Paraffin oil) were
obtained from J&K Scientific Ltd. CO32− was obtained fromNa2CO3,
and other anions (F‾, Cl‾, Br‾, I‾, NO3‾, and SO4‾) as
tetrabutylammonium salts were purchased from Aladdin. All other
reagents were purchased from Sinopharm Chemical Reagent
Corporation (China) and used without further purification.
O
O
C
O
C
Si(OC H )
2 5 3
C
TESPIC
CH
CF
3
2
C
THF, NaH, Reflux
Si(OC H )
2 5 3
F C
C
O
3
Synthesis Procedures
NTA
NTA-Si
Synthesis of the ligand 2ꢀ(2ꢀcarboxylphenyl)ꢀimidazo[4,5ꢀf]ꢀ1,10ꢀ
phenanthroline (L). The synthesis of organic ligand L was similar
to that used in reported literature.33,34 Typically, 1,10ꢀ
phenanthrolineꢀ5,6ꢀdione (210 mg, 1 mmol) was first mixed with
ammonium acetate (2.1 g, 27 mmol) and dissolved in acetic acid (11
mL). 2ꢀCarboxybenzaldehyde (1 mmol, 150 mg) in acetic acid (4
mL) was added dropwise into the above mixture and then stirred.
The solution was heated to 95 °C in a covered flask for
approximately 5 h and quenched by water (100 mL). An aqueous
ammonium solution (30%, 12 mL) was added to neutralize the
solution to pH =7. The precipitate was collected and washed
thoroughly with deionized water. The crude product was dried
overnight at 65 °C under a vacuum environment. Further purification
was performed through flash column chromatography with alumina
(aluminum oxide 90, deactivated neutral, 70‒230 mesh) with CHCl3
/MeOH at 20/1 ratio as elution solvents. The second band was
collected to yield a title compound (169.4 mg, yield 49.6%) L as
yellow solid. 1H NMR (DMSOꢀd6): 7.85 (s, 2H), 8.13‒8.14 (d, 1H, J
= 2.0 Hz), 8.15‒8.16 (d, 1H, J = 2 Hz), 8.60‒8.64 (m, 2H), 8.94‒
8.96 (d, 2H, J = 6 Hz), 9.05 (d, J = 7 Hz, 1H), 9.06 (d, J = 7 Hz, 1H),
and 9.41 (s, 1H), 12.76 (s, 1H). Anal. Calcd for C20H12N4O2: C,
70.58; N, 16.46; H, 3.55; Found: C, 70.45; N, 16.41; H, 3.58.
HO
O
O
O
O
O
O
O
Si
Si
C
TEOS
P123, H2O
C
F C
C
O
3
O
C
HO
:
N
H
NTA-S15
EtOH
.
6H O
3NTA-S15 + Eu(NO )
+ L
3
3
2
Reflux
HO
COOH
O
O
O
O
O
O
Si
Si
N
N
O
O
C
C
N
Eu
N
H
C
CF
3
HO
3
Eu(NTA-S15) L
3
Scheme 1 The synthetic process and predicted structure of Eu(NTAꢀ
S15)3L.
Characterizations
Fourier Transform Infrared (FTIR) spectra were measured using a
Nicolet 6700 spectrophotometer with the KBr pellet technique from
4000ꢀ400 cmꢀ1 wavenumber range. 1HNMR spectra were recorded in
Synthesis of NTAꢀfunctionalized SBAꢀ15 mesoporous hybrid
material (NTAꢀS15). The modified precursor NTAꢀSi was
synthesized via the reaction between NTA and TESPIC in THF as
previously described.35 NTAꢀS15 was synthesized according to our
reported method.36 Typically, P123 (2.0 g) was first dissolved in
deionized water (15 g) and 2 M HCl solution (60 g) at 35 °C. A
TEOS and NTAꢀSi mixture was added into the above solution,
resulting in a molar ratio of 0.0172 P123 : 0.96 TEOS : 0.04 NTAꢀSi
6 HCl : 208.33 H2O. The mixture was stirred at 35 °C for 24 h,
transferred into a Teflon bottle, and placed in an autoclave, which
was heated to 100 °C for 48 h. The solid product was recovered
through filtration, washed thoroughly with deionized water, and airꢀ
DMSOꢀd6 on
a
Bruker AVANCEꢀ500 spectrometer with
tetramethylsilane (TMS) as an internal reference. Elemental analysis
(CHN) was performed on a Vario EL elemental analyzer. Small
angle Xꢀray powder diffraction patterns (XRD) were recorded on a
Rigaku D / max2500VB+ / PC diffractometer equipped with a Cu
anode in a 2θ range from 0.6 ˚ to 6 ˚. Nitrogen adsorptionꢀdesorption
isotherms were carried out by a Nova 2000 analyzer at the liquid
nitrogen temperature. The surface areas were calculated by
BrunauerꢀEmmettꢀTeller (BET) method, and the Barrett–Joyner–
Halenda (BJH) model was used to evaluate the pore size
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distributions from the desorption branches of the nitrogen isotherms. Characterization of Eu(NTA-S15)3L
Transmission electron microscope (TEM) experiments were
Spectroscopic evidence for the formation of the precursor and
mesoporous hybrid is also seen in the FTIR spectra. Fig. S1†
compares the FTIR spectra data for NTA(A), precursor NTAꢀSi (B)
and NTAꢀfunctionalized SBAꢀ15 mesoporous hybrid material NTAꢀ
S15 (C), whose main absorptions and assignments are shown in
Table S1†. All of these are coincident with the results reported in
previous inference.36 Fig. 1 presents the FTIR spectra of pure SBAꢀ
15 (a), and the Euꢀcontaining mesoporous hybird material Eu(NTAꢀ
S15)3L (b). In the SBAꢀ15 host material (a), the evident bands
appearing at 1074 cmꢀ1 is attributed to asymmetric Si–O stretching
vibration modes (νas, SiꢀO), and the band at 801 cmꢀ1 is due to the
symmetric Si–O stretching vibration (νs, SiꢀO). The Si–O–Si
bending vibration (δ, SiꢀOꢀSi) can also be observed at 459 cmꢀ1, and
the band located at 967 cmꢀ1 is associated with silanol (Si–OH)
stretching vibrations of surface groups. Moreover, the band at 3440
cmꢀ1 confirms the presence of hydroxyl group. Compared with SBAꢀ
15, the Euꢀcontaining mesoporous hybrid material Eu(NTAꢀS15)3L
shows the similar infrared absorption bands as the silica framework.
Because of the strong signal background arising from the SBAꢀ15
host matrix, bands originating from the organic ligands are relatively
weak. Nevertheless, Fig.1 (b) shows a prominent band at 1383 cm−1
derived from the imidazole rings of organic ligang L.34 Furthermore,
the weak absorption bands located about 2979 and 2932 cmꢀ1 which
are attributed to methylene groups of 3ꢀ(3ꢀ(triethoxysilyl)propyl
isocyanate (TESPIC) are observed. They all suggest that the NTAꢀSi
has been grafted on to the wall SBAꢀ15 and the mesoporous hybrid
material Eu(NTAꢀS15)3L has formed.
performed using a JEOL2011 microscope operated at 200 kV.
Scanning electronic microscope (SEM) images were obtained by a
TS 5136 MM instrument. Luminescence (excitation and emission)
spectra were carried out on an Hitachi Fꢀ4600 phosphorimeter using
a xenon lamp as the excitation source. And the luminescent decay
properties were studied on an Edinburgh FLS920 phosphorimeter.
Thermoꢀgravimetric analysis (TGA) was carried out on a Perkinꢀ
Elmer Pyris Diamond thermal analyzer at a heating rate of 10
°C/min under a nitrogen atmosphere.
Results and discussion
In this paper, firstly NTA was selected as organic ligand; on one
hand, its diketone group modifies and adheres onto mesoporous
matrix; on the other hand, modified NTA chelates with rare earth
ions and sensitizes well their luminescence. Then an organic ligand
containing both imidazole group and pyridine N atoms was designed
and synthesized as the second ligand, where pyridine N atoms
chelate with rare earth ions and sensitize their luminescence, and NH
of imidazole group bond with fluorine ions, resulting in detection of
fluorine ions. Finally a material displaying orderly mesoporous
structures, favorable luminescence property, and selective
identification of fluorine ions was obtained through coordination
between rare earth Eu ions and the two ligands. It’s surprising that
the color of the solution changes because of the difference of
fluorine ion concentration in Eu solution. Hence, multicolor
solutions of different wavelengths were obtained by regulating
fluorine ion contents of Eu solution. Multicolor nearly covered the
entire visible light region. This work is the first to synthesize rare
earth mesoporous hybrid materials with multicolor emission through
regulation of anion contents; thus, this paper introduces a new
approach to regulate multicolor system.
a
801
967
459
Scheme 1 presents the synthesis process and the predicted
composition of the mesoporous hybrid Eu(NTAꢀS15)3L. Describing
the exact structure of this type of nonꢀcrystalline hybrid material is
generally difficult. However, the Eu3+ and N content are unlike the C
or H content, which cannot change with the differences in the sol–
gel process. Thus, the molar ratio of Eu3+ and N can be used to
predict the main composition of the mesoporous hybrids and
especially the coordination environment surrounding Eu3+.
According to the Eu3+ and N content, the molar ratio of Eu3+ and N
=1: 10 can be predicted. Of the ten N atoms, six are from three
NTAꢀS15 units, and the other four is from one L unit, so the ratio
can demonstrate the composition of the mesoporous hybrid system.
According to rare earth chemistry principles and the properties of the
functional group in the hybrid system, the coordination behavior
should also be predicted. The organic ligand L provides two
chelating nitrogen atoms, and the three NTAꢀS15 units provide six
3440
1074
b
1383
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber / cm-1
Fig. 1 FTIR spectra of pure SBAꢀ15 mesoporous silica (a), and the
Euꢀcontaining mesoporous hybrid Eu(NTAꢀS15)3L (b).
The smallꢀangle Xꢀray diffraction (SAXRD) pattern is a popular
and efficient method for highly ordered mesoporous material with
hexagonal symmetry in the space group P6mm. Fig. 2 shows the
chelating carbonyl groups; these eight groups occupy the eight SAXRD patterns of the pure mesoporous silica SBAꢀ15 (a), NTAꢀ
coordination positions around 3+. This coordination behavior is not
S15 (b), and Eu(NTAꢀS15)3L (c). The pure mesoporous silica SBAꢀ
15 material shows three wellꢀresolved diffraction peaks within the
0.6° < 2θ < 2° range, and these peaks are indexed as (100), (110),
difficult to understand, because it is most stable for the lanthanide
complex to exist with the coordination number of 8 or 9, and the
coordination behavior of NTAꢀS15 and phen derivative is also well and (200) planes that are associated with twoꢀdimensional hexagonal
symmetry. It can be seen that the characteristic reflections were still
known.
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observed at about the same positions in NTAꢀS15 and Eu(NTAꢀ
N2 sorption experiments were conducted to investigate the pore
S15)3L, demonstrating that the longꢀrange hexagonal symmetry was structures of these mesoporous materials. Fig. 3 shows the N2
well preserved. However, it is worth noting that a clear peak shift in adsorptionꢀdesorption isotherms of the pure mesoporous silica SBAꢀ
the first reflection peak towards to a higher angle is clearly observed 15 (a), NTAꢀS15 (b), and Eu(NTAꢀS15)3L (c). All of these materials
in the sample Eu(NTAꢀS15)3L compared with the sample NTAꢀS15. show Type IV isotherms with H1ꢀtype hysteresis loops under high
This suggests the dꢀspacing value decreased，and there is a small relative pressure according to the IUPAC classification,
shrinkage of the mesoporous silicate network as consequence of characteristic of ordered mesoporous materials with highly uniform
introducing Eu3+. The detailed d100 spacing values of SBAꢀ15, NTAꢀ size distributions. By BrunauerꢀEmmettꢀTeller (BET) and Barrettꢀ
S15, and Eu(NTAꢀS15)3L were listed in Table 1. In addition, the JoynerꢀHalenda (BJH) methods, the surface area and the pore sizes
diffraction intensity of Eu(NTAꢀS15)3L slightly decreased compared have been calculated. The structure data (surface area, total pore
with that of NTAꢀS15, and this phenomenon is common to materials volume, and pore sizes, etc.) of the mesoporous materials are
containing guest species. This finding is probably due to the summarized and listed in Table 1. It can be clearly seen that the pure
reduction of Xꢀray scattering contrast between the silica walls and SBAꢀ15 showed a high specific surface area (752 m2/g), a large pore
the covalently bonded europium complexes in the mesoporous volume (0.95 cm3/g), and a BJH pore diameter of 5.68 nm. After
materials but not due to the collapse in pore structure of mesoporous functionalization with NTA, the SBET, V, and DBJH values in
materials.37
mesoporous material NTAꢀS15 decreased, consistent with the result
obtained in the presence of organic ligands in pore channels of SBAꢀ
15. Furthermore, Eu(NTAꢀS15)3L exhibits a smaller surface area and
a slightly smaller pore size and pore volume than NTAꢀS15, which is
consistent with the presence of anchored Eu(NTA)3L moieties in the
SBAꢀ15 mesoporous channels. The phenomenon originates from the
dispersion of the europium complexe on the surface of the NTAꢀS15
material, since the covalently complex that dispersed inside the
channel will always enhance the roughness of the pore surfaces and
then make the surface area, pore volume, and pore size of the
material much smaller than those of the NTAꢀS15 material. 38ꢀ40
100
110
200
a
b
c
Table 1 Structural parameters of SBAꢀ15, NTAꢀS15, Eu(NTAꢀS15)3phenꢀ
COOHa
sample
d100
(nm)
a0
(nm)
SBET
V
D
(nm)
t
(m2/g) (cm3/g)
(nm)
1.0
1.5
2.0
θ / degree
2.5
3.0
SBAꢀ15
10.14
9.92
9.81
11.71
11.45
11.33
752
565
453
0.95
0.77
0.61
5.68
5.39
5.21
6.03
6.06
6.12
2
NTAꢀS15
Fig. 2 SAXRD patterns of the pure mesoporous silica SBAꢀ15 (a),
NTAꢀfunctionalized SBAꢀ15 mesoporous hybrid material NTAꢀS15
(b), and the Euꢀcontaining mesoporous hybrid Eu(NTAꢀS15)3L (c).
Eu(NTAꢀS15)3L
a
d100 is the d(100) spacing, a0 the cell parameter (a0=2 d100/√3), SBET the BET
surface area, V the total pore volume, DBJH the average pore diameter, and t
the wall thickness, calculated by a0ꢀD.
Fig. 4 shows the highꢀresolution TEM images of the pure
mesoporous silica SBAꢀ15, NTAꢀS15, and the Euꢀcontaining
mesoporous hybrid material Eu(NTAꢀS15)3L. From fig. 4, we can
observe that all the materials shows the regular hexagonal array of
uniform channels characteristic of mesoporous SBAꢀ15 material,
demonstrating that Eu(NTAꢀS15)3L maintains its mesoporous
structure well after organic modification and complexation. It is
worth noting that the Euꢀcontaining mesoporous hybrid material
Eu(NTAꢀS15)3L exhibits smaller dꢀspacing compared with pure
mesoporous silica SBAꢀ15, which also further confirmed the
presence of anchored Eu(NTA)3L moieties in the SBAꢀ15
mesoporous channels. In addition, the dꢀspacing values measured
from highꢀresolution TEM images of three materials are substantial
agreement with the values determined from the corresponding XRD
analysis (Table 1).
a
600
b
400
c
200
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure
(
P/P )
0
Fig. 3 N2 adsorptionꢀdesorption isotherms of the pure mesoporous
silica SBAꢀ15 (a), NTAꢀfunctionalized SBAꢀ15 mesoporous hybrid
material NTAꢀS15 (b), and the Euꢀcontaining mesoporous hybrid
Eu(NTAꢀS15)3L (c).
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whereas the 5D0→7F1 transition corresponds to a parityꢀallowed
magnetic dipole transition, which is practically independent of the
host material. Therefore, the intensity ratio (R
) of 5D0→7F2 to
5D0→7F1 is sensitive to the symmetry around Eu3+ and provides
valuable information on the chemical microenvironment change of
anions coordinating the Eu3+ ion.41ꢀ43 The calculated
R value for
Eu(NTAꢀS15)3L is 7.8. Such a high ratio is only possible when Eu3+
ions do not occupy a site with inversion symmetry. The strong red
luminescence under UVꢀlight irradiation (inset in Fig. 5) indicates
the occurrence of antenna effect, that is, energy migration takes
place upon ligand absorption followed by intersystem crossing
S1→T1 and antenna T1→f transfer, generating the fꢀf emissions of
Eu3+.
Fig.4 HighꢀresolutionTEM images of SBAꢀ15 (a, b), NTAꢀS15 (c, d)
and Eu(NTAꢀS15)3L(e, f). (a, c, e) imaged along the [110] zone axe
and (b, d, f) imaged along the [100] zone axe, respectively.
Moreover, the surface morphology of Eu(NTAꢀS15)3L was also
examined by scanning electron microscope (Fig. S2†). It can be
observed that the resulting material consists of relatively uniform
columnar wormꢀlike particles, which are approximately 10 ꢁm in
length and with an average width of 3 ꢁm. This finding demonstrates
that a homogeneous, molecularꢀbased material (Eu(NTAꢀS15)3L)
was obtained, wherein no phase separation was observed because
strong covalent bonds exist between the organic ligand L or
mesoporous hybrid material NTAꢀS15 and europium ions.
Compared with the hybrid materials with doped lanthanide
complexes, which generally display phase separation phenomena,
the inorganic and organic phases in this work display their distinct
properties in the synthesized mesoprous hybrid materials containing
covalent bonds.
Fig. 5 Emission spectra of the Euꢀcontaining mesoporous hybrid
Eu(NTAꢀS15)3L. The inset shows the corresponding luminescence
image under UVꢀlight irradiation of 365 nm.
The luminescence lifetime decay curves and luminescent quantum
efficiency of Eu(NTAꢀS15)3L were measured to further investigate
the luminescence properties. The mesoporous hybrid Eu(NTAꢀ
S15)3L presents reasonably long lifetimes (0.31 ms) and high
quantum yields (14%), further demonstrating the effective ET from
the ligands to Eu3+. The luminescence time decay curves for the
sample Eu(NTAꢀS15)3L is given in Fig. S4†.
Luminescence properties of Eu(NTA-S15)3L
The solidꢀstate luminescent properties of Eu(NTAꢀS15)3L were
studied at room temperature. The fluorescent excitation and emission
spectra of Eu(NTAꢀS15)3L are shown in Figs. S3† and 5,
respectively. The excitation spectrum was obtained by monitoring
the strongest emission of the Eu3+ at 615 nm, which is dominated by
a broad band centered at 379 nm and a narrow peak centered at 395
nm; the former band is ascribed to the π→π* electron transition of
the organic ligand, whereas the latter peak is attributed to the
absorption transition (7F0→5L6) of the fꢀf transition of Eu3+.
Absorption of organic ligand is considerably stronger than f–f
transition, suggesting that luminescence sensitization via excitation
of ligands is preferred over direct excitation of Eu3+. Under
excitation at 379 nm, the emission spectra of Eu(NTAꢀS15)3L
revealed the characteristic emission peaks of Eu3+. The emission
spectra show all the transition of Eu3+ from the emitting 5D0 level to
the groundꢀstate manifold (579 nm, 5D0→7F0; 593 nm, 5D0→7F1; 615
nm, 5D0→7F2; 650 nm, 5D0→7F3 and 695 nm, 5D0→7F4. It is
obviously observed that a very intense 5D0→7F2 transition at 615 nm
is the dominant peak in the emission spectra, indicating that the
chemical environment around Eu3+ ion is without an inversion center.
Sensing of F‾ ̄ anions
The anionꢀsensing properties of the mesoporous hybrid Eu(NTAꢀ
S15)3L were investigated through fluorescence emission
spectroscopy in THF at a concentration of as low as 1 mg/mL, and
the results are shown in Fig. 6. The anions examined include F‾, Cl‾,
2
3
Br‾, I‾, NO3‾, SO4
̄ , CO3 ‾, and PO4 ‾. The results show that most of
2
3
these anions, namely, Cl‾, Br‾, I‾, NO3‾, SO4
̄
, CO3 ‾, and PO4 ‾,
exhibit slight influence on the emission intensities of the hybrid
Eu(NTAꢀS15)3L, and changes in fluorescent color were not observed;
It is presumed the main reason for the slight emission intensity
changes is that there formed very weak hydrogen bonding
interactions between ligand L with these anions, which slightly
affected the efficiency of energy transfer efficiency from L to Eu3+.
however, the fluorescent spectrum of the hybrid Eu(NTAꢀS15)3L
significantly changes upon addition of F‾, and an obvious fluorescent
5
As is well known, the D0→7F2 transition is a typical electric dipole
transition and is very sensitive to local symmetry of europium ions,
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DOI: 10.1039/C7TC01240C
ARTICLE
Journal Name
color change (from red to green) was easily observed with the naked
eye under UVꢀlight irradiation (inset in Fig. 6).
O 10-3
(a)
3
N 9
M 8
L 7
K 6
J 5
×
×
×
×
×
×
×
×
×
×
10-4
10-4
10-4
10-4
10-4
10-4
pure
-
F
-
Cl
I
4
-
Br
-
H 3
G 2
F 1
10-4
10-4
10-4
I
-
NO3
10-5
CO32
-
E 7
SO42
D 5×
10-5
10-5
10-5
-
PO43
-
C 3
×
B 2
×
A 10-5
350 400 450 500 550 600 650 700 750
Wavelength / nm
360
420
480
540
600
660
720
4
(b)
Wavelength / nm
Y=-0.0056+4.658X
Fig. 6 Emission spectra of Eu(NTAꢀS15)3L in THF solutions (1 mg/
mL) upon addition of different anions (10‾3 mol/L). The inset shows
the corresponding luminescence under UVꢀlight irradiation at 365
nm.
2
3
2
1
R =0.9985
Furthermore, the interference of other anions (Cl‾, Br‾, I‾, NO3‾,
2
3
NO3‾, SO4̄, CO3 ‾ and PO4 ‾) on the detection of F‾ was also
determined. As shown in Fig. 6, the emission intensity of the Eu
mesoporous hybrid only exhibits slight changes in the presence of
other anions. Upon introduction of F‾ into the mixture of Eu
mesoporous hybrid and other anions, an obvious fluorescent spectra
change was observed (Fig. S5†). This result shows that the
interference caused by other anions can be neglected, further
demonstrating that the Eu mesoporous hybrid sensor can efficiently
recognize and sense F‾; this property is very important for practical
applications in biological and environmental fields.
0
0.00
0.15
0.30
0.45
0.60
ꢀ
0.75
0.90
Fig. 7 (a) Emission spectra of Eu(NTAꢀS15)3L in THF solutions
upon addition of 1.0 × 10−5 to 10−3 mol/L F‾ under excitation at 340
nm; (b) The ratio of intensity of L / Eu (IL / IEu) for Eu(NTAꢀS15)3L
in THF solutions with the F‾ concentration range from 1.0×10ꢀ5 to
8.0×10ꢀ4 mol/L.
For better understanding the response of fluorescence of the Eu
hybrid to F ̄, luminescence titration upon addition of F‾ into Eu
hybrid was further conducted at 298 K in THF (europium material
concentration is 1 mg/mL).
Fig.7b gives the typical variations in emission intensity ratio of L /
Eu (IL / IEu) as a function of the concentration of F‾. Where the IL is
Fig 7a presents the emission spectra of Eu(NTAꢀS15)3L in THF
solutions upon addition of 1.0 × 10−5 to 10−3 mol/L F‾ under
excitation at 340 nm. The emission spectra for organic ligand L
(c=10ꢀ3 mol/L) and ligand NTA (c=10ꢀ3 mol/L) in THF solution
obtained under the same measure conditions are shown in Fig. S6†.
As shown in Fig. 7a, when THF solution of Eu(NTAꢀS15)3L was
the intensity value of the emission peak from organic ligand L; and
the IEu is the emission intensity value of the 5D0→7F2 transition. It is
interesting to find that the correlation of the emission intensity ratio
of L / Eu (IL
8.0 × 10−4 mol/
−0.0056 + 4.658X as shown by the leastꢀsquares fitting method, and
the correlation coefficient (
2) is >0.99. The limit of detection
/
IEu) versus F‾ concentration in the range of 1.0 × 10−5‒
fit well with the simple linear equation Y =
L
irradiated at 340 nm, we can find five characteristic
R
5
(LOD) for F‾ was 1.8 × 10−4 M as determined by the equation LOD
= 3SD/slope. SD refers to the standard deviation of the blank sample
(obtained by 10 consecutive scans of the above blank sample) and
slope is the calibration curve slope.
intraconfiguration f
states to the different
-
f transitions of europium ions from D0 excited
J
levels of the ground term 7FJ (
J = 0 to 4) in
the emission spectra. With further addition of fluoride ions, the
characteristic fluorescence emission intensity of europium ion
gradually declines, and a new band centered at 416 nm derived from
organic ligand L appeared, and this emission band increased and
Interestingly, the photoluminescence colors of the mesoporous
hybrid Eu(NTAꢀS15)3L in THF solution can be finely tuned from red
to pink, purple, blue, green, and yellowish green by varying the
gradually red shifted. L
Finally, after addition of 8.0 × 10−4 mol/
fluoride ions, the red emission from europium ions was nearly
completely quenched, going with the emission peak from organic
ligand L red shifted to 550 nm, and the fluorescent color of the
solution turned from red to green.
concentration of F‾ as illustrated in CIE (Commission International
de L'Eclairage) chromaticity diagram (Fig. 8). The corresponding
CIE color coordinates (x, y) are listed in Table 2. The THF solution
of mesoporous hybrid material Eu(NTAꢀS15)3L realize tunable
6 | J. Name., 2012, 00, 1-3
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ARTICLE
multicolor emissions only by changing fluoride ion concentrations, gradually showed a downfield shift from 9.19 ppm to 9.47 ppm,
which has practical applications in color displays, fluorescent lamps, suggesting that strong hydrogen bonding interactions exist between
and LEDs.
the ꢀNH and ꢀCOOH groups and F−. Upon addition of 30 equiv. of
F−, ꢀNH disappeared, and other H atoms of aromatic ring showed an
upfield shift, indicating the occurrence of deprotonation process. The
above results indicated that the ligand L reacted with fluoride ions
via two processes: 1) hydrogen bonding formation and 2)
deprotonation.
-
30 eq.F
-
3 eq.F
-
2 eq.F
-
1 eq.F
-
0.5 eq.F
pure
14
13
12
11
δ
10
/ ppm
9
8
7
Fig. 8 CIE chromaticity diagram showing the location of multicolor
Fig. 9 1 H NMR spectra of ligand L (bottom) in the presence of
luminescence for AꢀO.
various equivalents of F− in DMSOꢀd6
Thermal stability of Eu((NTA-S15)3L
100
.
Table 2 CIE coordinates and emission colors of AꢀO
Color
F‾ concentration
(mol/L)
1×10ꢀ3
2×10ꢀ5
3×10ꢀ5
5×10ꢀ5
7×10ꢀ5
1×10ꢀ4
2×10ꢀ4
3×10ꢀ4
4×10ꢀ4
5×10ꢀ4
6×10ꢀ4
7×10ꢀ4
8×10ꢀ4
9×10ꢀ4
1×10ꢀ3
Number
CIE (x,y)
0.00
A
B
C
D
E
F
G
H
I
(0.54, 0.30)
(0.48, 0.29)
(0.46, 0.28)
(0.42, 0.26)
(0.38, 0.23)
(0.33, 0.20)
(0.26, 0.17)
(0.23, 0.17)
(0.21, 0.22)
(0.22, 0.29)
(0.24, 0.37)
(0.26, 0.37)
(0.28, 0.48)
(0.29, 0.51)
(0.31, 0.54)
Red
Pink
Pink
Purplishꢀpink
Purplishꢀpink
Purple
Purple
Violet
95
90
85
80
75
70
-0.02
-0.04
-0.06
-0.08
514oC
Blue
Bluishꢀgreen
Green
J
K
L
M
N
O
Green
200
400
Temperature / oC
600
800
Yellowishꢀgreen
Yellowishꢀgreen
Yellowishꢀgreen
Fig. 10 Thermogravimetry trace (—) and differential thermoꢀ
gravimetry trace (ꢀꢀꢀ) curves (DTG) of the europium mesoporous
hybrid material Eu(NTAꢀS15)3L.
The mechanism for the emission changes of the target mesoporous
hybrid material Eu(NTAꢀS15)3L could be ascribed to the formation
of hydrogenꢀbonding interaction between ꢀNH and ꢀCOOH groups
of organic ligand L and F−. In detail, the strong fluoride binding
changed the coordination conformation and affected the efficiency of
energy transfer form ligand L to central Eu ions. To further confirm
the proposed fluoride ion sensing mechanism, 1H NMR titration
To investigate the thermal stability of the europium mesoporous
hybrid Eu(NTAꢀS15)3L, thermogravimetric analyses (TGA) was
performed at a heating rate of 10 °C/min−1 under a nitrogen
atmosphere. Fig. 10 shows the TG weight loss curve and differential
weight loss (DTG) curve of Eu(NTAꢀS15)3L. The TG curve shows
that decomposition of the hybrid Eu(NTAꢀS15)3L proceeds in three
stages. The first stage, which occurred between 27 °C and 145 °C
with a mass loss of 5.5%, is ascribed to the physically absorbed
water and residual solvent without any decomposition of the
experiments of the ligand L with F− were carried out in DMSOꢀ
d6 at
room temperature. Fig. 9 shows the spectral changes in L in the
presence of various equivalents of different F− ions in DMSOꢀd6. It
was observed that the proton signal of the ꢀCOOH group entirely
disappeared upon addition of 1 equiv. of F−. With the increased
concentration of F− from 0 equiv. to 3 equiv., ꢀNH of imidazole
chemical bonds. This stage is followed by
(approximately 5.7%) that occurred between 145 °C and 355 °C, and
a weight loss
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ARTICLE
Journal Name
this stage is attributed to the decomposition of the incompletely 9. G. F. DeSá, O. L. Malta, C. D. Donegá, A. M. Simas, R. L.
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,
removed surfactant. Finally, weight loss (approximately 12.9%)
peaked at 514 °C is due to the decomposition of organic ingredients.
Moreover, the weight loss peak of pure complex Eu(NTA)3L
occurred at approximately 343 °C (Fig. S7†). This findings indicates
that the thermal stability of the europium complex is enhanced when
the complex is covalently bonded to SBAꢀ15 mesoporous matrix.
,
567ꢀ576.
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L. Zhang, Langmuir, 2011, 27, 5616ꢀ5623.
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In conclusion, a novel covalently bonded mesoporous hybrid
material Eu(NTAꢀS15)3L with tunable multicolor emission was
designed and synthesized. This material possesses the combined
merits of europium complex and mesoporous silica, which does not
only preserve mesoscopically ordered structure and high thermal
stability but also displays the characteristic emission of Eu3+. In
addition, this material is a highly selective and sensitive fluorescence
sensor used to detect F ̄. Most importantly, the tunable multicolor
17. S. Y. Han, X. Qin, Z. F. An, Y. H. Zhu, L. L. Liang, Y. Han, W.
emissions, including red, pink, purplishꢀpink, purple, violet, blue,
bluishꢀgreen, green, and yellowish green, can be obtained by
precisely controlling the fluoride ion concentrations. Therefore,
Huang and X. G. Liu, Nat. Comm., 2016, 7, 13059.
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quantifying F ̄ via visual inspection is feasible. This is the first
example of a material based on a lanthanide functionalized
mesoporous hybrid material to display tunable multicolor emission
under singleꢀwavelengthen excitation. The versatile luminescence,
good thermal stability, and high selective for F ̄, together with
tunable multicolor, demonstrate that this material present potential
applications in fluorescent sensor field and luminescent devices for
display or lighting.
GonzalezꢀNoya and M. R. Bermejo, Angew. Chem., Int. Ed.
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,
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (21401040 and 21301047), the Natural Science
Foundation of Hebei Province (B2014208160 and B2014208091),
the Key Foundation of Hebei Province Department of Education
Fund (ZD2016059), and the Young Talent Plan of Hebei province.
,
,
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3
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Journal of Materials Chemistry C
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DOI: 10.1039/C7TC01240C
Graphical Abstract
Eu3+ based mesoporous hybrid material with tunable multicolor
emission modulated by fluoride ion: application for selective sensing
toward fluoride ion
Yajuan Li*, Xudong Yu, Tao Yu
We first synthesize a lanthanide functionalized mesoporous hybrid material with tunable multicolor
emission and F- sensing property.
O 10-3
N 9
×
10-4
10-4
10-4
M 8
×
L 7
×
K 6
J 5
×
×
×
×
×
×
×
×
×
×
10-4
10-4
10-4
-
F
I
4
H 3
G 2
F 1
E 7
D 5
10-4
10-4
10-4
10-5
10-5
10-5
10-5
C 3
B 2
A 10-5
350 400 450 500 550 600 650 700 750
Wavelength / nm