A luminescent lanthanide complex-based anion sensor with electron-donating methoxy groups for monitoring multiple anions in environmental and biological processes

Article abstract of DOI:10.1016/j.saa.2012.05.064

We designed a ternary europium (III) tris(2-thenoyltrifluoroacetonate) with 2-(3,4,5-trimethoxyphenyl)imidazo[4,5-f]-1,10-phenanthroline (1) ligands for the luminescent detection of various anions, such as fluoride, acetate and dihydrogen phosphate. Characterization of the sensor's photophysical properties and via NMR showed that the sensor exhibits striking emission changes to fluoride (purple), acetate (green) and dihydrogen phosphate (blue) anions, respectively. Its fluorescence lifetime was determined to be 1.10 ms for europium ions and the complex showed an overall quantum yield of 10% in DMSO. Furthermore, transparent hybrid thick films composed of the europium complex and poly-methyl methacrylate matrix were successfully prepared via copolymerization. The resulting film overall displayed intense red emissions associated with europium ions. Fluorescence microscopic evaluation showed a homogenous distribution of aggregates with average diameters of 30-50 μm throughout the film. The accordingly produced film could give rise to a luminescence change to purple in response to fluoride anions.

Full text of DOI:10.1016/j.saa.2012.05.064

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 387–394
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A luminescent lanthanide complex-based anion sensor with electron-donating
methoxy groups for monitoring multiple anions in environmental
and biological processes
Yuhui Zheng a, Chaoliang Tan a, Gregor P.C. Drummen b, Qianming Wang a,
⇑
a Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China
b Bionanoscience and Bio-Imaging Program, Cellular Stress and Ageing Program, Bio & Nano-Solutions, D-40472 Düsseldorf, Germany
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
A ternary europium (III) tris(2-thenoyltrifluoroacetonate) with 2-(3,4,5-trimethoxy phenyl)imidazo[4,5-
f]-1,10-phenanthroline (1) was prepared and it can detect various anions such as Fꢀ, AcOꢀ and H2PO4ꢀ.
Spectroscopic studies of UV–vis, Fluorescence and NMR present that the sensor exhibits striking emission
"
"
"
Ternary complex was strong red
emissive.
The sensor exhibits striking emission
changes to anions.
Luminescent could display
luminescence change for fluoride
anion.
changes to fluoride (purple), acetate anions (green) and H2POꢀ (blue), respectively. More importantly,
4
transparent hybrid thick films (poly-methyl methacrylate) show intense red emissions and give rise to
luminescence change in fluoride anion containing DMSO solution.
a r t i c l e i n f o
a b s t r a c t
Article history:
We designed
a ternary europium (III) tris(2-thenoyltrifluoroacetonate) with 2-(3,4,5-trimethoxy-
Received 5 September 2011
Received in revised form 24 May 2012
Accepted 25 May 2012
phenyl)imidazo[4,5-f]-1,10-phenanthroline (1) ligands for the luminescent detection of various anions,
such as fluoride, acetate and dihydrogen phosphate. Characterization of the sensor’s photophysical prop-
erties and via NMR showed that the sensor exhibits striking emission changes to fluoride (purple), acetate
(green) and dihydrogen phosphate (blue) anions, respectively. Its fluorescence lifetime was determined
to be 1.10 ms for europium ions and the complex showed an overall quantum yield of 10% in DMSO.
Furthermore, transparent hybrid thick films composed of the europium complex and poly-methyl
methacrylate matrix were successfully prepared via copolymerization. The resulting film overall dis-
played intense red emissions associated with europium ions. Fluorescence microscopic evaluation
Available online 2 June 2012
Keywords:
Lanthanide
Europium
Luminescence
⇑
Corresponding author. Tel.: +86 20 39310258.
E-mail address: qmwang@scnu.edu.cn (Q. Wang).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.05.064

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Y. Zheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 387–394
Anion
Sensor
Polymer films
Polymethyl methacrylate
Fluoride
showed a homogenous distribution of aggregates with average diameters of 30–50 lm throughout the
film. The accordingly produced film could give rise to a luminescence change to purple in response to
fluoride anions.
Ó 2012 Elsevier B.V. All rights reserved.
Acetate
Dihydrogen phosphate
Introduction
methoxy-phenyl)imidazo[4,5-f]-1,10-phenanthroline (1) for
a
europium-based Eu-(TTA)3-1 anion sensor. This novel complex
(Fig. 1A) could be applied as an effective receptor and sensor for
the luminescent detection of acetate (AcOꢀ), fluoride (Fꢀ) and
Since different anions such as fluoride, hydrogen sulfate and
acetate are critical components in environment, biological and
metabolic processes. The specific structural and reactive receptors
composed of organic fluorophores were designed and prepared in
monitoring particular anions [1–8].
The trivalent lanthanide ions are well-known for their lumines-
cent properties in the visible (Eu, Tb) and near-infrared region (Yb,
Pr, Nd) of the electromagnetic spectrum and long emission lifetime
ranging from microseconds (e.g. Yb, Nd) to milliseconds (e.g. Eu
and Tb). Thus autofluorescence from the much shorter lived bio-
molecules (sub-microseconds) can easily be filtered via time-gated
techniques and the penetration depth for imaging purposes can be
maximized using NIR emitting probes. Unfortunately, trivalent
lanthanide ions inherently suffer from weak light absorption
capabilities because their f–f transitions are Laporte-forbidden
dihydrogen phosphate (H2POꢀ) anions. Since hybrid materials of
4
polymers and lanthanide complexes display beneficial properties,
such as mechanical flexibility and durability, thermal stability,
and easy processability, such configurations have attracted much
attention, particularly for applications in electronics and optics
[17,18]. Because simple embedding of lanthanide ions in the poly-
mer matrix often leads to inhomogeneous distribution of the probe
and associated luminescence artifacts [9,19,20], such as local
quenching, we opted to covalently couple the Eu-(TTA)3-1 anion
sensor monomers via co-polymerization to the polymer host
[9,17,21]. We thus managed to introduce the europium-based
anion sensor into a polymeric host, poly-methyl methacrylate
resulting in molar absorption coefficients
(e) smaller than
10 L molꢀ1 cmꢀ1 for most of the absorption transitions [9]. This is
extremely low compared to organic chromophores, which are gen-
erally at least 5000 time larger [10]. Thus an obvious solution to
this problem is to introduce an organic chromophore that acts as
an ‘‘antenna’’ and has the capacity to absorb more light because
of its more intense absorption bands to subsequently transfer the
excitation energy to the lanthanide ion by intramolecular energy
transfer (indirect excitation) [9,11].
Not surprisingly, lanthanide-organic chromophore complexes
have caught the attention of many researchers in various disci-
plines over the past few decades, not in the least because of their
extraordinary properties. Their sharp and intense emission bands
and versatile color changes are sensitive to external interaction
with guest molecules such as anions. Lanthanide complexes are
markedly oxophilic and positively charged, thereby providing
strong electrostatic interactions with anions [12]. Furthermore,
the aforementioned long emission lifetimes, obviation of practical
problems associated with autofluorescence and Rayleigh scattering
in biological samples make them desirable probes in biological and
biomedical research.
A number of groups have previously reported various lantha-
nide-based luminescent sensors with various sensitivities and
specificities. Parker et al. [11], Faulkner et al. [13], and Gunnlaugs-
son et al. [14] developed multidentate cyclen-based lanthanide
complexes for the selective detection of various anions, including
bicarbonate. Tsukube’s group reported lanthanide complexes of
tris(2-pyridylmethyl)amines [15] and a series of N3, O-mixed
donor chiral tripods that formed stabile lanthanide complexes
[16] as anion-responsive luminescent compounds for anions such
as chloride and nitrate. Ziessel and co-workers [12] prepared com-
plexes of europium and terbium with bis-[(60-carboxy-2,20-bipyri-
dine-6-yl)]phenylphosphine oxide donor ligands for the detection
of ADP3ꢀ, ATP4ꢀ, and phosphate anions. However, because there
are few sensors available that detect multiple anions and, to the
best of our knowledge, no sensor that is capable of detecting dihy-
drogen phosphate anions, we aimed to synthesize a europium-
based sensor that would accomplish this.
Fig. 1. Ternary europium complex-based luminescent anion sensor. (A) Molecular
structure of Eu(TTA)3-1, consisting of the chromophoric antenna 2-(3,4,5-trimeth-
oxy-phenyl)imidazo[4,5-f]-1,10-phenanthroline and the ternary europium com-
plex; R: tris(2-thenoyltrifluoroacetonate). (B) Photo visualizing the color change in
response to the addition of 5 eq. of different anions (tetrabutyl ammonium salts).
Excitation was performed with a portable UV-lamp (254 and 365 nm).
In this paper, we describe the two step synthesis, initial charac-
terization, and application of a novel antenna ligand, 2-(3,4,5-tri-

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389
(PMMA), to create transparent polymeric films that were sensitive
to fluoride anions and showed homogenous emission signals.
The preparation of EuTTA3ꢁ2H2O was performed according to
the method described by Lenaerts et al. [17]. Synthesis of Eu-
TTA3-1: Compound 1 (16.6 mg, 0.05 mmol) was dissolved in
10 ml ethanol, to which EuTTA3ꢁ2H2O (40.8 mg, 0.05 mmol) was
added. The whole mixture was refluxed for 3 h and allowed to cool
to room temperature. The resulting precipitate was collected and
washed twice with distilled water to give the titled complex
(50 mg, 83%) as a yellow powder. Infrared spectroscopic analysis
Experimental
Materials and techniques
showed: IR (KBr)
t = 3696, 1603, 1535, 1306, 1246, 1180, 1105,
Tetrabutyl ammonium fluoride trihydrate (97%), tetrabutyl
ammonium chloride (97%), tetrabutyl ammonium bromide (98%),
tetrabutyl ammonium iodide (98%), tetrabutyl ammonium hydrog-
ensulfate (97%), tetrabutyl ammonium dihydrogen phosphate
(98%), tetrabutyl ammonium acetate (97%) were purchased from
Sigma–Aldrich company. Other chemicals and materials were ac-
quired from local commercial suppliers, used as received and were
of the highest purity available. 1H-NMR spectra were recorded at
293 K on a Varian MR 400 MHz NMR spectrometer (Agilent Tech-
nologies Ltd., Beijing, China) using tetramethylsilane (TMS) as an
internal standard. Visible and luminescence spectra, emission life-
time, and absolute quantum yields were measured on an Agilent
8453 UV–visible spectrophotometer (Agilent Technologies Ltd.,
Beijing, China) and Edinburgh FLS920 spectro-fluorimeter (Edin-
burgh Instruments Ltd., Livingston, UK), respectively. The errors
for the lifetime measurements were within 10%. Absolute quantum
yields (/) were determined at ambient temperature according to
Wrighton et al. [22] and further described by Carlos et al. [23] were
calculated as / = A/(Rs ꢀ RH), where A is the area under the complex
emission spectrum, and RS and RH are the diffuse reflectance (fixed
wavelengths) of the complex and of the reflecting standard. The
deviations in the quantum yields were estimated to be 25% [22].
The luminescence of PMMA films was measured by using a
front-surface accessory to clamp the samples. LC-MS was per-
formed on an Agilent 1100 HPLC-MS with an electrospray ioniza-
tion source. Thermogravimetric analysis (TGA) was carried out on
a STA 409 PC system (Netzsch-Gerätebau GmbH, Selb, Germany)
under air at a rate of 10 °C/min. Dynamic light scattering was
measured with a BI-200SM Laser Light Scattering Goniometer
(Brookhaven Instruments Ltd., Redditch, UK). The fluorescence
images were taken using a Nikon Eclipse TS100 inverted fluores-
cence microscope system (Japan), equipped with a 50 W mercury
lamp source.
897, 723 cmꢀ1. EA found: C, 45.88%; H, 2.31%; N, 4.81%, Anal. Calcd.
for C46H30EuF9N4O9S3 = EuTTA3-1: C, 45.97%; H, 2.52%; N, 4.66%.
EuTTA3-1 doped poly-methyl methacrylate (PMMA) thick films
were prepared as follows: compound EuTTA3-1 was dissolved in
MMA monomer (concentration from 0.1 mg to 0.9 mg/5 ml
MMA). Azobisisobutyronitrile (AIBN) was used as the radical poly-
merization initiator and added to the above mentioned solution to
a final concentration of 0.1%. The mixture was heated in an oil-bath
at 75 °C for 10 min under stirring. After the monomers became vis-
cous, the pre-polymers were quickly cast onto a glass substrate.
Further polymerization was conducted in an oven at 70 °C over-
night until the transparent films peeled off. The luminescence
sensing capacity of the final product was evaluated with solutions
of different anion types (10ꢀ4 M) in dimethylsulfoxide (DMSO).
Standard procedures were used to calculate means and stan-
dard deviations. Image processing was performed with ImageJ
ver. 1.44p (NIH, USA).
Results and discussion
We aimed to synthesize a lanthanide complex-based lumines-
cent sensor for the detection of various anions relevant to biologi-
cal and environmental processes. The newly synthesized
lanthanide complex with electron-donating methoxy groups,
Eu-(TTA)3-1, is structurally depicted in Fig. 1A. Eu-(TTA)3-1 was
characterized by NMR and MS and its properties to detect various
anions evaluated. The sensor’s photophysical properties were
determined via UV–vis and fluorescence spectroscopy in DMSO
as a solvent. Fig. 1B shows that Eu-(TTA)3-1 could be excited by
both the 254 and 365 nm excitation wavelengths of a portable ul-
tra-violet light, which resulted in an intense red emission. It fur-
ther visualizes that responses to various anions and concomitant
color changes can easily be detected, even by the human eye.
The absolute quantum yield and emission lifetime were deter-
mined to be 10% and 1.10 ms for europium ions (Ex = 356 nm) in
pure DMSO, respectively.
Synthesis procedure
Synthesis of 2-(3,4,5-trimethoxy phenyl)imidazo[4,5-f]-1,10-
phenanthroline (ligand (L)) was essentially performed as described
in more detail previously [24]. Briefly, 1,10-Phenanthroline-5,6-
dione (50 mg, 0.24 mmol) was mixed with ammonium acetate
(0.5 g, 6.5 mmol) and subsequently dissolved in 3 ml glacial acetic
acid. Under stirring, 3,4,5-trimethoxybenzaldehyde (47 mg,
0.24 mmol) in acetic acid (1.5 ml) was added to the previously
prepared mixture. The solution was heated to 90 °C for 2 h and
quenched with 50 ml distilled water. An aqueous ammonium solu-
tion (30%, 3 ml) was added to neutralize the solution to pH 7. The
precipitate was collected and washed with distilled water. The
crude product was dried in vacuo overnight. Further purification
steps were performed by flash column chromatography with alu-
mina (aluminum oxide 90 deactivated neutral) and CHCl2/MeOH
(10/1) as elution solvents. The second band was collected as the
titled compound (70 mg, 75%) and the product presented itself as
a yellow solid. Subsequent NMR and MS analysis showed: 1H-
NMR (DMSO-d6) d = 9.05 (2H, d, J = 4.0 Hz, Ha), 8.94 (2H, d,
J = 8.0 Hz, Hc), 7.86 (2H, m, J = 8.0 Hz, Hb), 7.62 (2H, s, Hd), 4.00
(6H, s), 3.76 (3H, s); MS analysis (LCMS) showed that the major
product was: m/z 387.5 (MH+).
The luminescent ternary complex Eu-(TTA)3-1 was dissolved in
DMSO, to a final concentration of 1 ꢂ 10ꢀ5 M to give a colorless
solution with two distinguished UV bands at 283 and 346 nm
and a single broad and a series of sharp emission bands (Fig. 2).
The peak at 346 nm may be attributed to both the thenoyltrifluoro-
acetonate ligands [25] and the phenanthroline ring of ligand 1.
The absorption at 283 nm is mainly caused by the aromatic
moieties or the phenanthroline heterocyclic ring (Fig. 3). Upon
titration with 3 equivalents fluoride anions (tetrabutylammonium
salt), the two peaks remained relatively stable. However, the aro-
matic ring of ligand 1 was obviously affected after an eight-fold
fluoride addition, as deduced from changes in the spectrum at
283 nm, since this band gradually decreased and concomitantly a
new peak at around 294 nm emerged. This indicated that the sen-
sor complex formed hydrogen bonds with fluoride anions at rela-
tively high concentrations. Similar effects were observed during
titration of 3–10 equivalents of tetrabutylammonium acetate
([Bu4N]+AcOꢀ) to the sensor complex, indicating interaction with
acetate anions and a recognition effect for these (Fig. S1). Further
addition, however, caused little change in the absorption curves.

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1.2
Absorption
Emission
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength / nm
Fig. 2. Absorption and emission spectra of 1 ꢂ 10ꢀ5 M Eu(TTA)3-1 in DMSO. The emission spectrum was recorded by exciting the chromophore at 346 nm.
0.6
0.5
3 F
0.4
0 F
8 F
10 F
0.3
0.2
0.1
0.0
300
350
400
Wavelength / nm
450
500
Fig. 3. UV–vis spectra of 10ꢀ5 M Eu(TTA)3-1 in DMSO in the presence of 0–10 equivalents of fluoride anions.
Analꢀogously, we also measurꢀed the binding behavior and affinity of
HSO4 , Clꢀ, Brꢀ, Iꢀ and H2PO4 anions for Eu-(TTA)3-1. On all occa-
sions the ultra-violet absorption lines remained almost constant
at high concentrations, showing that there is insufficient binding
to cause changes in the absorption spectrum (Fig. S2).
strengthened the electron-rich p-cloud of aromatic ring, directly
affect the electron distribution of the adjacent imidazole moiety.
As a result, the hydrogen atom of imidazole is much less electron
positive and it becomes harder to accept the attack of a lone elec-
tron pair from fluoride ꢀor other anions. NMR analysis of ligand 1 in
combination with HSO4 , Brꢀ, Iꢀ and Clꢀ, but 1 did not give rise to
signal changes in each proton (data not shown).
As shown in Figs. 2 and 5, the Eu(III) ternary complex in DMSO
solution mainly displays characteristic metal-centered emission.
When the emission wavelength was fixed at 615 nm, the excitation
spectrum shows a broad band from 250 to 400 nm with two peaks
at 275 and 353 nm, which were virtually consistent with their
absorption lines (Fig. S4). The shape of the emission spectrum
can be interpreted as follows: the excited 5D0?7FJ transitions exhi-
bit five main components for J = 0–4, respectively and no splitting
of 7FJ level could be observed. After addition of 3 eq. of fluoride an-
ions, the red emission from europium ions decreased and the blue
emission of the ligand changed slightly. As a consequence, we ob-
served a combination of red and blue luminescence resulting in the
1H-NMR spectroscopy was used to study the anion binding
affinity to ligand 1 (Fig. 4). The addition of 5 eq. fluoride anions
caused the aromatic protons Hd of 1 nearest to the imidazole ring
(Fig. 1A) to shift to low field (from 7.62 to 7.71 ppm) as a result of
the recognition process (Fig. 4). Furthermore, the protons of the
phenanthroline ring Ha and Hc (Fig. 1A) have enlarged the dis-
tances between each other, suggesting that 1 binds fluoride ions
via hydrogen interactions. Interestingly, ligand 1 responded not
only to fluoride anions, but acetate and H2POꢀ anions could also
4
take part in hydrogen bond formation (Fig. S3).
Previously, we found that an imidazole-ring covalently bound
to hydroxyl substituted benzene could recognize fluoride anions
stoichiometrically (1:1) [24]. In the current Eu(TTA)3-1 complex,
the three electron donating methoxy groups (Fig. 1A), which

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391
Fig. 4. 1H-NMR spectra measured by titration of a DMSO-d6 solution of pure 1 (1 mM) with 0–5 eq. of [Bu4N]F.
Fig. 5. Emission spectra of 10ꢀ6 M Eu(TTA)3-1 in DMSO, excited at 356 nm in the presence of 0–10 eq. of [Bu4N]+Fꢀ.
observed purple color shown in Fig. 1B. With increasing addition of
fluoride anions, gradual changes in the emission spectrum became
apparent culminating in the appearance of a new band centered at
500 nm. Further addition of the same anion beyond 10 eq. did not
change the luminescence spectrum significantly.
the emission of the europium ions (Fig. 7). Concomitantly, the
broad band attributed to the phenanthroline ligand dramatically
increased in the blue emission region upon addition of H2POꢀ an-
4
ions. Hence, we could visually observe strong blue luminescence
when exciting the solution with UV light (Fig. 1B).
Similarly, the binding capability of acetate anions was evalu-
ated and upon addition of 3 eq. of acetate anions, the red emission
from europium ions almost disappeared and only a broad band
from 400 to 550 nm was observed. Interestingly, the blue emission
of the ligand (around 450 nm) decreased and a new band around
500 nm increased stepwise (5–10 eq. addition), which again dem-
onstrates that acetate anions were involved in hydrogen bonding
(Fig. 6). Accordingly, it is possible to observe green luminescence
visually.
Liu et al. [26] recently reported a calix[4]arene-Ru(II) polypyri-
dine sensor with imidazo[4,5-f]-1,10-phenanthroline groups,
which displayed bright green emission upon binding of fluoride
anions. They argue that deprotonation of the NH group of the imid-
azole ring causes charge redistributions within the complex and
that fluoride anions bind via hydrogen bonds. In our series of
experiments, we have observed similar effects and consequently
agree with their description of a possible mechanism for the
changes in luminescence upon fluoride binding. As for dihydrogen
phosphate anion binding, we suggest that the observed lumines-
cence effects are caused by displacement of the phenanthroline
Additionally, we investigated the effect of titration of Eu(TTA)3-
1 with various concentrations of H2POꢀ on the emission spectrum.
4
In stark contrast to the aforementioned anions, the addition of
dihydrogen phosphate to the solution instantaneously quenched
ligand by H2POꢀ and as such displays phenanthroline’s original
4
blue emission.

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Y. Zheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 387–394
Fig. 6. Emission spectra of 10ꢀ6 M Eu(TTA)3-1 in DMSO, excited at 356 nm in the presence of 0–10 eq. of [Bu4N]+AcOꢀ.
Fig. 7. Emission spectra of 10ꢀ6 M Eu(TTA)3-1 in DMSO, excited at 356 nm in the presence of 0–10 eq. of [Bu4N]+ H2POꢀ4
.
Analogous experiments were carried out with Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ
flexible and robust, and could easily be bent under application of
external forces whilst retaining its structural integrity.
and the emission was monitored as presented in Fig. S5. The previ-
ous UV–vis absorption and 1H-NMR measurements indicated that
no hydrogen bonding interactions occurred with these anions,
whilst the fluorometry measurements showed that the variation
in the europium emission was relatively low in the presence of
an excess amount of these anions, suggesting that some quenching
of the signal at 615 nm occurred, which was most pronounced for
iodide and hydrogen sulfate anions (Iꢀ > HSO4ꢀ > Brꢀ ꢃ Clꢀ).
To explore the potential of the complexes for electronic, pho-
tonic and advanced (bio) imaging applications and to enhance
processability and mechanical stability, it is useful to incorporate
lanthanide-complex sensors into an inert matrix, such as synthetic
polymers. We opted to use poly-methyl methacrylate since this is
presently the cheapest and most commonly used support for the
development of chemical sensors. Furthermore, PMMA nanoparti-
cles can be synthesized via emulsion polymerization for the deliv-
ery of payloads to cells and tissues [27] and, in combination with a
luminescent complex, for imaging and sensing purposes. There-
fore, we incorporated the europium complex into PMMA via co-
polymerization from which thick films could be made by casting
on a glass support. The resulting composites were all transparent,
We determined that 0.9 mg Eu(TTA)3-1/5 ml methyl methacry-
late (MMA) doping concentration resulted in optimal emission
intensities. Furthermore, both the overall quantum yield (18%)
and lifetime of europium (III) ions (1.30 ms) increased in this poly-
meric matrix compared with the pure complex (Ex = 343 nm). The
emission spectra showed significant suppression of the blue emis-
sion of the organic ligands, indicating a more efficient energy
transfer to the europium complex (Fig. 8). We also studied the
initiator concentration dependence of the emission spectra by
Eu-complex doping films (Fig. S6). It was observed that the amount
of initiator would not induce dramatic luminescence changes
when the concentration was less than 0.3%. If it was increased to
0.5%, the emission peak of Eu-complex reduced significantly. It
shows that numerous resultant shorter chains (high initiator con-
centration) may restrict the complexation between Eu(III) ions
and the coordination ligands. To test if the films were sensitive
to anions and as such detect these, we immersed the films into var-
ious DMSO solutions containing a variety of anions (10ꢀ4 M) and
determined their luminescence or loss thereof. The red emitting
film lost its luminescence only in response to fluoride anions over

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393
Fig. 8. Emission spectra of Eu(TTA)3-1 (0.1, 0.5 and 0.9 mg) entrapped in PMMA films (kex = 343 nm).
detection (LOD) was measured by LOD = 3SD (standard devia-
tion)/slope of the linear function (LOD here is 1.45 ꢂ 10ꢀ5 M).
TGA provides a reliable way to measure the thermal stability of
europium containing polymers. Polymer films are subject to tem-
peratures less than 180 °C in daily practical use. The europium
complex doped PMMA films showed a significantly higher thermal
stability compared with the pure complex (Fig. S8). According to
Wang et al. [21], the coordination number of europium ions was
unsaturated and further complexation between oxygen atoms of
the methacrylate moiety and europium ions will occur. This
cross-linking effect reduces the flexibility of the polymer chains
and concomitantly the decomposition temperature will increase.
Fluorescence microscopic evaluation of the film revealed an
overall homogenous bright red luminescent signal (Fig. 9A), which
clearly confirms the presence of the europium complex in the
PMMA matrix. The lanthanide complex was homogeneously dis-
persed within the polymer matrix (Fig. 9A and B) and assembled
into large aggregates with average diameters from 30–50
lm and
seldom large than 100 m, which nonetheless led to a homoge-
l
nous signal (Fig. 9B). Overall, approximately 70% of surface area,
as calculated from Fig. 9B, showed emission intensities above 0.4
normalized emission, of which 28% exhibited the maximum
intensity emission.
Conclusions
With the intention of exploiting the unique properties of lan-
thanide luminescence and based thereon to synthesize a sensor
sensitive to different types of relevant anions, we prepared a red
luminescent ternary europium-chromophore-sensing ligand com-
plex (ternary europium (III) tris(2-thenoyltrifluoroacetonate) with
2-(3,4,5-trimethoxyphenyl)imidazo[4,5-f]-1,10-phenanthroline
ligands). The sensor itself exhibited a bright red luminescence with
an absolute quantum yield of 10% and a lifetime of 1.10 ms. In
response to fluoride anions this color changed to an intense purple
color and upon acetate titration gave rise to light green emission.
Additionally, for the first time, we developed a sensor of this type
Fig. 9. Fluorescence microscopic evaluation of the luminescence of Eu(TTA)3-1
(0.9 mg) doped PMMA film. (A) Fluorescence micrograph of the doped film
((kex = 343 nm) show a homogenous distribution of the complex with some larger
clusters. (B) A surface plot of the fluorescence intensities in (A) reveal that in
addition to the areas of maximal intensity, significant emission between these exist.
capable of recognizing H2POꢀ via a sharp change to blue lumines-
4
cence. The addition of excess amounts of Brꢀ, Iꢀ and Clꢀ showed no
shift in the emission maxima with only minor quenching of the
major emission bands. Europium complex doped polymeric PMMA
films not only guaranteed mechanical robustness, but showed a
strong red luminescent signal that responded to fluoride anions
and as such could be used as fluoride sensor. Further validation
all the others (including acetate and H2POꢀ anion) and turned to an
intense purple after a relatively long period of time (10 h). The
quenching constant was calculated from the Stern–Volmer equa-
tion [28] (Y = 10351X + 1.0281, R2 = 0.9996, Fig. S7). The limits of
4

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