Fabrication of Eu(III) complex doped nanofibrous membranes and their oxygen-sensing properties

Article abstract of DOI:10.1016/j.saa.2010.08.025

In this paper, we report the synthesis, characterization, and photophysical properties of Eu(TTA)₃ECIP, where TTA = 2-thenoyltrifluoroacetonate, and ECIP = 1-ethyl-2-(N-ethyl-carbazole-yl-4-)imidazo[4,5-f]1,10- phenanthroline. Its elementary application for oxygen-sensing application is also investigated by doping it into a polymer matrix of polystyrene (PS). Experimental data suggest that the 2.5 wt% doped Eu(TTA)₃ECIP/PS nanofibrous membrane exhibits a high sensitivity of 3.4 towards oxygen with a good linear relationship of R² = 0.9962. In addition, the 2.5 wt% doped Eu(TTA)₃ECIP/PS nanofibrous membrane owns a quick response of 8 s towards oxygen, along with its excellent atmosphere insensitivity and photobleaching resistance. All these results suggest that both Eu(TTA) ₃ECIP and Eu(TTA)₃ECIP/PS system are promising candidates for oxygen-sensing optical sensors.

Full text of DOI:10.1016/j.saa.2010.08.025

Spectrochimica Acta Part A 77 (2010) 885–889
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Fabrication of Eu(III) complex doped nanofibrous membranes and their
oxygen-sensing properties
Lin Songzhua,b, Dong Xiangting , Wang Jinxian , Liu Guixia , Yu Wenshen , Jia Ruokun
a,∗
a
a
a
b
a
School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, Jilin, PR China
College of Chemical Engineering, Northeast Dianli University, Jilin 132012, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 May 2010
Received in revised form 11 July 2010
Accepted 4 August 2010
In this paper, we report the synthesis, characterization, and photophysical properties of Eu(TTA) ECIP,
where TTA = 2-thenoyltrifluoroacetonate, and ECIP = 1-ethyl-2-(N-ethyl-carbazole-yl-4-)imidazo[4,5-
3
f]1,10-phenanthroline. Its elementary application for oxygen-sensing application is also investigated
by doping it into a polymer matrix of polystyrene (PS). Experimental data suggest that the 2.5 wt% doped
Eu(TTA)3ECIP/PS nanofibrous membrane exhibits a high sensitivity of 3.4 towards oxygen with a good
Keywords:
2
linear relationship of R = 0.9962. In addition, the 2.5 wt% doped Eu(TTA)3ECIP/PS nanofibrous membrane
Optical materials
Composite materials
Europium compound
Chemical synthesis
owns a quick response of 8 s towards oxygen, along with its excellent atmosphere insensitivity and pho-
tobleaching resistance. All these results suggest that both Eu(TTA)3ECIP and Eu(TTA)3ECIP/PS system are
promising candidates for oxygen-sensing optical sensors.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
cal applications, it is necessary to embed sensors into a solid matrix
acting as a supporting medium, allowing oxygen transportation
from surroundings. The commonly used matrixes are polymers,
silica-based materials, and Langmuir–Blodgett films. Indeed, the
support may have quite stringent criteria for suitable performances
[7]. For example, a high diffusion coefficient is necessary for a
rapid response, while, a highly locally effective quenching around
the sensor molecule is necessary for good sensitivity, and long
distance on-line monitoring necessitates a high degree of photosta-
bility. Thus, the exploration for sensors with intense luminescence,
excellent antijam ability, high photostability, and good compat-
ibility with supporting matrix is still a challenge for analytical
chemistry.
The determination of molecular oxygen concentration in both
gas and liquid phases is always an important analytical problem
in many fields, such as chemical and food industries, medicine,
analytical chemistry, and environmental monitoring [1,2]. Tradi-
tionally, oxygen concentration measurements are based on the
Clark electrode and the Winkler titration approach which suffer
from drawbacks such as oxygen consumption during sensing pro-
cess, long response times and the tendency of electrodes to be
poisoned by sample constituents [3,4]. In addition, both techniques
necessitate sophisticated instrumentations and require compli-
cated pretreatment procedures, making themselves unsuitable for
on-line and/or in-field monitoring. Thus, the exploration for new
oxygen-sensing systems has attracted considerable interests in the
past decades.
Sensor technology is much simpler in instrumental implemen-
tation and sample preparation. Owing to the advantages of simple,
rapid, and non-destructive characteristics, many oxygen-sensing
systems have been reported. Among them, the development of
optical oxygen sensors has attracted considerable attentions in
recent years, as such sensors can offer advantages in terms of size,
electrical safety, costs, not requiring a reference element, and the
fact that analytical signals is free of influence from electromagnetic
filed as well as easy to transmit over a long distance [5,6]. For practi-
It seems that rare-earth (RE)-based emitters which are usually
excellent emitters can well satisfy the above requirements. Due to
the unique excitation mechanism of antenna effect and f–f radia-
tive transitions, RE3+-based emitters can generate characteristically
sharp and narrow emissions without being affected by surround-
ing environment or reagents, offering an excellent antijam ability.
In addition, RE3+-based emitters’ good solubility in common sol-
vents allow themselves to be easily embedded into supporting
matrixes. All these excellent characters make RE3 -based emit-
ters promising candidates for oxygen-sensing materials. Guided by
above results, in this paper, we report the synthesis, characteriza-
+
tion, and photophysical properties of Eu(TTA) ECIP. Its elementary
3
application for oxygen-sensing application is also investigated by
doping it into a polymer matrix of PS. Experimental data suggest
that both Eu(TTA) ECIP and Eu(TTA) ECIP/PS system are promising
∗ Corresponding author. Tel.: +86 431 85583010.
E-mail address: xiangtingd@163.com (D. Xiangting).
3
3
candidates for oxygen-sensing optical sensors.
1
386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.08.025

8
86
L. Songzhu et al. / Spectrochimica Acta Part A 77 (2010) 885–889
Scheme 1. A synthetic procedure for ECIP ligand and its corresponding Eu complex of Eu(TTA)3ECIP.
of CH Cl2 and hexane to get yellow crystals. 1H (300 MHz, CDCl ),
2 3
2
. Experimental
ı: 9.06 (2H); 8.94 (1H); 8.92 (1H); 8.63 (1H); 8.31 (1H); 7.88 (3H);
7.84 (1H); 7.82 (1H); 7.55 (1H); 7.28 (1H); 4.70 (2H); 4.55 (2H); 1.48
(3H); 1.41 (3H).
A synthetic procedure for the ligand of 1-ethyl-2-(N-ethyl-
carbazole-yl-4-)imidazo[4,5-f]1,10-phenanthroline
(referred
as ECIP) and its corresponding Eu(III) complex with 2-
thenoyltrifluoroacetonate (referred as TTA) as the auxiliary
ligand is shown in Scheme 1.
2
.2. Synthesis of Eu(TTA) ECIP
3
1
,10-Phenanthroline, TTA, 9H-carbazole, bromoethane, and
Eu(TTA) ECIP was synthesized according the literature proce-
3
polystyrene (referred as PS) were purchased from Aldrich Chem-
ical Co. and used without further purifications. Organic solvents,
including N,N -dimethylformamide (DMF) and CH Cl , were puri-
dure except that the auxiliary ligand was replaced by TTA [9]. Anal.
Calcd. for C53H35F O S N5Eu: C, 50.64, H, 2.81, N, 5.57. Found: C,
9
6 3
ꢀ
2
2
50.48, H, 2.89, N, 5.67.
fied through standard procedures. 1,10-phenanthroline-5,6-dione
referred as Phen-O) and 9-ethyl-9H-carbazole-2-carbaldehyde
were synthesized according to literature procedures [7,8].
(
2
.3. Fabrication of Eu(TTA) ECIP/PS nanofibrous membranes
3
A
typical procedure for the electrospinning composite
2
.1. Synthesis of ECIP
nanofibers is described as follows. PS with a number-average
molecular mass of 100,000 was dissolved in DMF to form a 22 wt%
solution. Then Eu(TTA)3ECIP was added into the solution under
stirring to form Eu(TTA)3ECIP/PS homogeneous solutions. The
final solutions were then electrospun to be composite nanofibrous
membranes of Eu(TTA)3ECIP/PS.
The mixture of 10 mmol of 1,10-phenanthroline-5,6-dione,
0 mmol of 9-ethyl-9H-carbazole-2-carbaldehyde, 15.4 g of NH Ac,
1
4
◦
and 30 mL of HAc was stirred at 90 C for 4 h. Then the mix-
ture was poured into water and extracted with CH Cl . The
2
2
crude product was used directly for next step without further
purification. The mixture of 10 mmol of 1-H-2-(N-ethyl-carbazole-
yl-4-)imidazo[4,5-f]1,10-phenanthroline, 10 mL of EtBr, and 0.3 g
2.4. Measurements
◦
of NaH in 15 mL of DMF was stirred at 120 C for 25 h. Then the
Luminescence lifetimes were obtained with a 355 nm light gen-
erated from the Third-Harmonic-Generator pumped, using pulsed
Nd:YAG laser as the excitation source. The Nd:YAG laser possesses
mixture was poured into cold water and extracted with CH Cl ,
2
2
the crude product was purified by recrystallization from a mixture

L. Songzhu et al. / Spectrochimica Acta Part A 77 (2010) 885–889
887
Fig. 1. A SEM image of the 2.5 wt% doped Eu(TTA)3ECIP/PS nanofibrous membrane.
a line width of 1.0 cm−1, pulse duration of 10 ns and repetition fre-
Fig. 2. Absorption spectra of pure PS, pure Eu(TTA)3ECIP, and Eu(TTA)3ECIP/PS
nanofibrous membranes. Inset: a clear view for pure Eu(TTA)3ECIP.
quency of 10 Hz. A Rhodamine 6G dye pumped by the same Nd:YAG
laser was used as the frequency-selective excitation source. All pho-
toluminescence (PL) spectra were measured with a Hitachi F-4500
fluorescence spectrophotometer. UV–vis absorption spectra were
recorded using a Shimadzu UV-3101PC spectrophotometer. Scan-
ning electron microscopy (SEM) images were obtained on a Hitachi
indicating that the increasing dopant concentrations and conse-
quently the increasing intermolecular actions exert no obvious
effect on molecular absorption. Correspondingly, the typical exci-
tation spectrum of Eu(TTA)3ECIP/PS nanofibrous membranes peaks
at 348 nm, attributing to TTA ␲ → ␲* transitions. In addition, it is
observed that there is a shoulder peak of 260 nm corresponding to
PS ␲ → ␲* transitions, which means that there may be intermolec-
ular energy transfer from PS matrix to Eu(TTA)3ECIP molecules.
Thus, we come to a conclusion that Eu(TTA)3ECIP molecules are
successfully embedded into PS matrix.
1
S-4800 microscope. H NMR spectra were obtained with a Varian
INOVA 300 spectrometer. Elemental analysis was performed on a
Carlo Erba 1106 elemental analyzer. Oxygen-sensing performances
were measured on the basis of steady emission intensity quenching.
All measurements were carried out in the air at room temperature
without being specified.
3
. Results and discussion
3.3. Photophysical properties of Eu(TTA)3ECIP/PS nanofibrous
membranes
3
.1. Morphology of Eu(TTA) ECIP/PS nanofibrous membranes
3
Fig. 3 shows the PL spectra of the four samples upon excita-
tion wavelength of 350 nm. Not surprisingly, character lines of Eu ,
3+
For practical applications in optical oxygen-sensing devices, it is
5
7
5
7
necessary to embed sensors into a solid matrix acting as a support-
ing medium, allowing oxygen transportation from surroundings,
and the supporting medium may also have quite stringent crite-
ria for suitable performances. Here, we select PS which has been
proved to be an excellent host material for electrospinning as the
supporting matrix for our primitive research [10]. To begin with, we
try four dopant concentrations of 1.5, 2, 2.5, and 3 wt%, respectively.
Fig. 1 shows a typical SEM image of the 2.5 wt% doped sample. The
composite fibers with an average diameter of ∼1 ␮m are randomly
distributed on the substrate, showing a smooth and uniform mor-
phology. No branch structure is observed for the composite fibers.
Those composite fibers own a large surface-area-to-volume ratio
which is two orders of magnitude larger than that of continuous
thin films, providing an excellent matrix with high diffusion coef-
ficient for a rapid response, which will be demonstrated later [10].
peaking at 577 nm for D0 → F0, 590 nm for D0 → F1, and 610 nm
5 7
for D0 → F2, respectively, are obtained. No emission the PS matrix
is detected within the region from 400 to 750 nm, indicating that
there is an efficiency energy transfer from PS to Eu(TTA)3ECIP.
According to the Judd–Ofelt theory, the radiative transitions within
the [Xe]4f6 configuration of Eu are parity forbidden and consist
mainly of weak magnetic dipole (MD) and induced electric dipole
3+
5
7
(ED) transitions [11]. The MD transition of D0 → F1 is independent
of the influence from micro-environment around Eu3 . On the other
+
3
.2. UV–vis absorption of Eu(TTA) ECIP/PS nanofibrous
3
membranes
Fig. 2 shows the absorption spectra of the four samples with
various dopant concentrations above mentioned. Compared with
the absorption spectra of pure PS and pure Eu(TTA) ECIP shown
3
in Fig. 2, it can be observed that each absorption spectrum of
nanofibrous membranes is composed of two absorption bands:
a high-energy band centering at ∼260 nm originated from PS
␲
→ ␲* transitions mixed with Eu(TTA) ECIP absorption, and a
3
low-energy band peaking at ∼350 nm which is attributed to TTA
␲
→ ␲* transitions of Eu(TTA) ECIP complex. The absorption inten-
3
Fig. 3. PL spectra of the four samples upon excitation wavelength of 350 nm. Inset:
emission intensity decay curves of the 2.5 wt% doped sample and pure Eu(TTA)3ECIP
powder upon excitation wavelength of 350 nm.
sity at 350 nm increases with the increasing dopant concentrations.
The absorption band shape, however, shows no spectral shift,

8
88
L. Songzhu et al. / Spectrochimica Acta Part A 77 (2010) 885–889
hand, the ED transition of 5D → F is hypersensitive towards
7
0
2
3+
the micro-environment around Eu . Since no spectral splitting is
detected for both insensitive and hypersensitive transitions, it is
thus estimated that all Eu(TTA) ECIP molecules localize in a homo-
3
geneous micro-environment within PS matrix, which is critical for
a good linear response and will be further confirmed later. In addi-
tion, it is observed that the micro-environment within PS matrix
has no obvious influence on the emission peaks of Eu(TTA) ECIP
3
because the partially filled 4f orbitals are shielded from environ-
ment by the filled 5d and 5p orbitals, offering a good antijam ability
towards environment effect.
As mentioned above, high photostability is a desired charac-
ter for sensing materials. The inset of Fig. 3 demonstrates the
emission intensity variations of the 2.5 wt% doped sample and
pure Eu(TTA) ECIP powder upon excitation wavelength of 350 nm.
3
Clearly, both samples suffer from photobleaching caused by photo-
induced structural decomposition. What’s more, it is observed that
the composite sample owns a largely improved photodurability
Fig. 4. Emission spectra of the 2.5 wt% doped sample under various oxygen concen-
trations from 0% to 100% with an interval of 10%.
compared with pure Eu(TTA) ECIP powder, which can be explained
3
as follows. The PS matrix may provide a rigid environment and thus
a shield for Eu(TTA) ECIP molecules, restraining the photo-induced
3
structural decomposition. In addition, it is observed that the 2.5 wt%
doped sample shows a better photodurability upon excitation
wavelength of 260 nm compared with that under 350 nm radiation,
which can be explained as follows. The PS absorption at 260 nm
is stronger than that at 350 nm, consequently, the largely domi-
interval of 10%. It is observed that the emission intensity at 610 nm
decreases with the increasing oxygen concentrations. Here, oxy-
gen sensitivity is defines as I0/I100, where I0 is the luminescence
intensity under 100% N2 atmosphere and I100 is that under 100%
O2 atmosphere. The corresponding values for the four samples
are measured to be 2.45 for the 1.5 wt% doped sample, 2.68 for
the 2 wt% doped sample, 3.40 for the 2.5 wt% doped sample, and
2.57 for the 3 wt% doped sample. Correspondingly, the 5D0 → F2
lifetimes for the four samples are measured to be 380 ␮s for the
1.5 wt% doped sample, 420 ␮s for the 2 wt% doped sample, 440 ␮s
for the 2.5 wt% doped sample, and 420 ␮s for the 3 wt% doped
sample. The 2.5 wt% doped sample exhibits a higher sensitivity
and a longer lifetime than the others, which means that the sen-
sitivity and the 5D0 → F2 lifetime are not simply proportional
to the loading concentration. It is thus reasonable to expect that
there are at lest two opposite factors affecting sample sensitivity:
emission intensity from probe molecules and adverse interac-
tion between probe molecules (aggregation, for example). A low
loading concentration leads to a weak emission from the probe
and correspondingly the low sensitivity. On the contrary, a much
higher loading concentration may result in the intermolecular
aggregation which also decreases the sensitivity. This hypothesis
is supported by the luminescence lifetimes analyses mentioned
above. The two opposite factors may achieve a balance in the
2.5 wt% doped sample, resulting in the maximum sensitivity of
3.40.
nant matrix may transfer energy from PS matrix to Eu(TTA) ECIP
3
molecules upon excitation wavelength of 260 nm, which is also
7
helpful for photodurability improvement of Eu(TTA) ECIP.
3
5
7
The D → F emissions of the four samples follow an exhibit
0
2
single-exponential decay pattern with lifetimes of 380 ␮s for
the 1.5 wt% doped sample, 420 ␮s for the 2 wt% doped sample,
4
40 ␮s for the 2.5 wt% doped sample, and 420 ␮s for the 3 wt%
5
7
doped sample, respectively. The D → F luminescence lifetime
0
2
7
shows an increase tendency with increasing concentrations from
.5 to 2.5 wt%, which means that the intermolecular interaction
1
between Eu(TTA) ECIP molecules is so weak and the increasing
3
concentrations of Eu(TTA) ECIP molecules are positive to depress
3
nonradiative decay within the matrix. Upon an even higher dopant
concentration of 3 wt%, the 5D → F luminescence lifetime tends
7
0
2
to decrease due to the adverse intermolecular aggregation. Those
long luminescence lifetimes make the samples vulnerable and sen-
sitive towards energy-acceptors such as molecular oxygen. What
is more, those long luminescence lifetimes make the elimination
of fluorescence originated from surrounding impurity such as pro-
tein possible and practicable, offering an antijam ability towards
fluorescence pollution.
5
7
3+
Despite of the long D → F lifetime of Eu center (440 ␮s) as
0
2
3
.4. Oxygen-sensing properties of Eu(TTA) ECIP/PS nanofibrous
mentioned above, the samples’ sensitivity values are much lower
than those of sensing systems based on Ru2+ and Cu+ complexes
whose excited lifetimes are greatly shorter (<100 ␮s) [7,10]. It is
expected that the luminescence mechanism difference between
3
membranes
Considering the long luminescence lifetimes mentioned above,
we intend to investigate their PL responses towards molecular oxy-
3
+
+
2+
Eu -based and Cu /Ru -based emitters should be responsible
+
2+
gen and thus explorer the possibility of using Eu(TTA) ECIP/PS
for this phenomenon. As for typical Cu /Ru -based emitters, the
highest occupied molecular orbital has a predominant metal d
character, while the lowest unoccupied molecular orbital is essen-
tially ␲* orbital of the ligands. Their emission corresponds to the
lowest triplet T1, and the excited state electron localizes on the
3
nanofibrous membranes as oxygen-sensing materials. According
to the literature reports, molecular oxygen is an efficient killer for
excited state emitters: energy transfer between molecular oxygen
and excited state emitters happens, and the quenching mechanism
is a dynamic one which can be presented as follows [7,12]:
+
2+
ligands, which makes Cu /Ru -based emitters open for molec-
ular oxygen attack, leading to a high sensitivity. While, as for a
Eu(TTA) ECIP ∗ + O → Eu(TTA) ECIP + O ∗
(1)
3
2
3
2
3+
Eu -based emitter, the emission originates from metal-centered
where “*” indicates an excited state.
.4.1. Sensitivity of Eu(TTA) ECIP/PS nanofibrous membranes
Fig. 4 shows the emission spectra of the 2.5 wt% doped sam-
ple under various oxygen concentrations from 0% to 100% with an
f–f transitions. The emissive center is covered by surrounding lig-
ands and outer filled orbitals of Eu3+, preventing molecular oxygen
from closing in, leading to the low sensitivity, even though the
excited state lifetime is much longer than those of Cu /Ru -based
emitters.
3
3
+
2+

L. Songzhu et al. / Spectrochimica Acta Part A 77 (2010) 885–889
889
to 100% N2 atmosphere. The corresponding values of the four sam-
ples are summarized in Table 1. The inset of Fig. 5 demonstrates a
PL intensity response of the 2.5 wt% doped sample when exposed
to periodically varied 100% N2 and 100% O2 atmospheres. Corre-
spondingly, the 2.5 wt% doped sample renders a response time of
only 8 s and a recovery time of 21 s. The quick response towards O2
and N2 suggests that the 2.5 wt% doped sample is highly sensitive
towards O due to the large surface-area-to-volume ratio of nanofi-
2
brous membranes as mentioned above. In addition, it is found that
the recovery time is obviously longer than the response time, which
can be explained by the diffusion-controlled dynamic response and
recovery behavior of a hyperbolic-type sensor reported by Mills
and Lepre [14]. The 2.5 wt% doped sample exhibits reversible time
response plots, with slightly intensity decrease which is called pho-
tobleaching. For future studies, we are hoping to further improve
the photostability and eliminate photobleaching by covalently
grafting the probe into silica matrix.
Fig. 5. Stern–Volmer plots of the four Eu(TTA)3ECIP/PS nanofibrous membranes at
various oxygen concentrations. Inset: PL intensity response of the 2.5 wt% doped
sample when exposed to periodically varied 100% N2 and 100% O2 atmospheres.
4. Conclusion
In this paper, we report the synthesis, characterization, and pho-
Table 1
tophysical properties of Eu(TTA) ECIP. Its elementary application
3
Oxygen-sensing properties of Eu(TTA)3ECIP/PS nanofibrous membranes.
for oxygen-sensing application is also investigated by doping it into
a polymer matrix of PS. Experimental data suggest that the 2.5 wt%
KSV (O2% 1
−
)
R2
Response Recovery
Sample (wt%)
I0/I100 (a.u.)
time (s)
time (s)
doped Eu(TTA) ECIP/PS nanofibrous membrane exhibits a high sen-
3
1
2
2
3
.5
.5
2.45
2.68
3.40
2.57
0.0142
0.0156
0.0228
0.0156
0.9991
0.9968
0.9962
0.9996
8
9
8
8
24
27
21
24
sitivity of 3.4 towards oxygen with a good linear relationship of
2
R = 0.9962. In addition, the 2.5 wt% doped Eu(TTA) ECIP/PS nanofi-
3
brous membrane owns a quick response of 8 s towards oxygen,
along with its excellent atmosphere insensitivity and photobleach-
ing resistance. All these results suggest that both Eu(TTA) ECIP
3
3
.4.2. Stern–Volmer plots of Eu(TTA) ECIP/PS nanofibrous
and Eu(TTA)3ECIP/PS system are promising candidates for oxygen-
sensing optical sensors.
3
membranes
It has been reported that the quenching behavior of a probe
is affected by the microstructure of matrix and the micro-
environment in which the probe is located. Generally, in a
homogeneous medium with a single-exponential decay, the inten-
sity form of Stern–Volmer equation with dynamic quenching is
described as follows [13]:
Acknowledgement
Support of this research by the National Natural Science Foun-
dation of China (Grant No. 50972020), the Research Project of Jilin
Province (Grant No. 20070402, 20060504) and the Research Project
of Changchun City (Grant No. 2007045).
I0
=
1 + KSV [O2]
(2)
I
References
where I is luminescent intensity. The subscript 0 denotes a value in
the absence of quencher, KSV is the Stern–Volmer constant, and [O2]
is O2 concentration. A plot of I /I versus [O ] should be linear with
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0
2
identical slopes of KSV. Fig. 5 shows the Stern–Volmer plots of the
four samples at various oxygen concentrations. It is observed that
all plots fit well with Eq. (1) and exhibit good linear relationships
with increasing oxygen concentrations as depicted by Table 1. It is
[
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3
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.4.3. Response/recovery time and photodurability of
5
Eu(TTA) ECIP/PS nanofibrous membranes
3
[11] B.R. Judd, Phys. Rev. 127 (1962) 750–761.
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ple to lose 95% of its initial emission intensity when changed from
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1
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