Europium(III) β-diketone complex as portable luminescent chemosensor for naked eye Cu2+ detection and recyclable on-off-on vapor response

Article abstract of DOI:10.1039/c5ra19710d

A novel Eu³⁺ complex coordinated by polymers with β-diketone pendant groups (Eu³⁺-PDKMA) has been synthesized for Cu²⁺ cation and acid-base vapor detections. Its sensitive and selective response to Cu²⁺ is confi

Full text of DOI:10.1039/c5ra19710d

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Europium (III) β-Diketone Complex as Portable Luminescent
Chemosensor for Naked Eye Cu2+ Detection and Recyclable On–
Off–On Vapor Response
Received 00th January 20xx,
Accepted 00th January 20xx
Fangyi Cao a, Zheng Yuan a, Junhua Liu a, Jun Ling*a
DOI: 10.1039/x0xx00000x
A novel Eu3+ complex coordinated by polymers with β-diketone pendant groups (Eu3+-PDKMA) has been synthesized for
Cu2+ cation and acid-base vapor detections. Its sensitive and selective response of Cu2+ is confirmed by fluorescence
quenching of characteristic emission of Eu3+ at 612 nm wavelength. The fluorescence intensity of Eu3+-PDKMA obeys linear
relationship at the presence of Cu2+ with the concentration in the order of 10-7 mol/L. The detection limit of Cu2+ is as low
as 2.0 × 10−8 mol/L at natural pH, which makes Eu3+-PDKMA an excellent luminescent sensor. Portable strips containing
Eu3+-PDKMA can be easily prepared to detect Cu2+ in aqueous solution. Moreover, the same strips can also be used to
recognize and sense acidic and basic environments by naked eye. The red emission can be switched “off” and “on” when
the strips contact acid and base vapors, respectively. The produced strips exhibit relatively fast response time less than 10
s, and good reusability over 10 times. The Eu3+-PDKMA materials are promising in biological and environmental
applications.
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terms of narrow and highly structured emission bands with
large Stokes shift.19-22 Moreover, Eu3+ complexes are valuable
alternative sensor for conventional dyes because Eu3+
Introduction
Detection of metal ions has been attracting increasing
complexes cannot be quenched by O2, which makes the
detection possible in non-degassed solution.17 These
advantages make Eu3+ complexes attractive sensor for the
fluorescent detection of Cu2+ cation.
attention due to the concern of human health, environmental
2
issue and industrial process.1, Among these metal ions,
copper is the third most abundant essential trace element for
growth in human body.3 However, Cu2+ cation has toxic and
persistent characteristics.4-6 The sensitive sensing and accurate
monitoring of Cu2+ in environment is very important.
On–off–on acid-base dependent fluorescence switching of
smart luminescent materials has received unprecedented
attentions.23-26 Actually, acid–base gas-responsive fluorescent
materials were designed in an effective strategy of tuning
intermolecular reactions, for instance, reversible hydrogen
bonding.27, 28 They have applications in sensors and biological
assay.24-26 Some materials have described a rapidly on–off
luminescent switch in acidic and basic vapors.24, 25, 29 From the
viewpoint of application in acid-base vapors, polymeric
materials have attractive advantages including the ease of
fabrication, high mechanical strength, and low cost compared
with small molecular complexes. For instance, Higuchi et al.
Although present detecting methods based on atomic
absorption emission spectroscopy, such as inductively coupled
plasma optical emission spectrometry (ICP-OES) and
inductively coupled plasma-mass spectrometry (ICP-MS), can
directly determine trace Cu2+ and reach sub-ng/mL limit,7-10 the
expensive equipment and time-consuming pretreatment hold
back their wide and routine applications. Direct fluorescent
detection of Cu2+ using a chromophore sensor becomes a
practical and economical method because most of the
methods are highly selective, sensitive, fast responsive, simple,
and capable for on-line and on-the-spot monitoring.11-13
Recently, many sensors with strong fluorescence have been
designed to recognize Cu2+ by means of fluorescent change.13-
reported
a
europium-based
metallo-supramolecular
luminescence sensor for switchable acid-base vapors.26 They
introduced Eu3+ complexes into polymer system in order to
overcome the poor photo-stability of small molecular Eu3+
complexes in water or at high temperature.
18
Among them, probes with europium (III) cation (Eu3+)
complexes possess well-defined photophysical properties in
Among the reported Eu3+ complexes, β-diketone ligands are
well-known as a sensitizer of Eu3+ cation. Furthermore, β-
diketone-coordinated Eu3+ complexes can easily dissociate by
Cu2+ and rapidly respond with acid-base gas. By this approach,
β-diketone-coordinated Eu3+ complexes are able to act as
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chemosensor for both Cu2+ detection and acid-base vapors
response. In this contribution, we polymerize vinyl monomer
test paper strips. The elemental analyses were performed on a
Vario MICRO cube (Elementar).
containing pendant
β-diketone functional group via RAFT
Methods
method, and fabricate dual-responsible chemosensor after
coordinating Eu3+. The polymeric chemosensor is exclusively
sensitive to Cu2+ and acid-base vapors in both solution and
solid state. Application of portable test paper is also
demonstrated. To the best of our knowledge, this is the first
example of both cations and acid-base vapor sensors based on
Eu3+-polymer complex.
4-(3-Oxo-3-phenylpropanoyl)phenyl methacrylate (DKMA).
DIK (5.0 g, 20.4 mmol) was dissolved in 50 mL dry THF followed
by adding TEA (3.4 mL, 25.1 mmol). A 10 mL THF solution of
methacryloylchloride (2.37 mL, 24.5 mmol) was added
dropwise at 0 °C, then the reaction was carried out for 30 min
followed by another 8 h at room temperature. A dilute
ammonium chloride aqueous solution was added. After stirred
for 15 min, the mixture was concentrated and the THF was
evaporated. After that, aqueous phase was extracted 3 times
with ethyl acetate, and washed by 1 mol/L HCl, saturated
sodium bicarbonate and brines successively. The organic layer
was dried by anhydrous sodium sulfate and the solution was
evaporated. The residue was purified by column
chromatography (1/10 EtOAc/Hexanes), recrystallized and
dried in vacuum at 30 °C. DKMA was obtained as white crystal
(5.35 g, yield = 88%).
Experimental
Materials
1-(4-Hydroxyphenyl)-3-phenylpropane-1,3-dione (DIK) and 4-
(3-oxo-3-phenylpropanoyl)phenyl methacrylate (DKMA) were
prepared according to our previous report.30 1-(4-
hydroxyphenyl)ethanone (Energy Chemical), methyl benzoate
(Sinopharm Chemical Reagent), 2,2′-Azoisobutyronitrile (AIBN,
98%, Aldrich) was recrystallized twice from ethanol. Cumyl
dithiobenzoate (CDB) was synthesized as reported.31
Methacryloylchloride (Mingxing Chemical Co.) was distilled
under reduced pressure before use. Eu2O3 (99.9%, Beijing
Founde Star Science and Technology Co. Ltd.) were
transformed to Eu3+ through reaction with diluted nitric acid at
elevated temperature. Triethylamine (TEA) was distilled over
CaH2 and stored in argon atmosphere at room temperature.
Poly[4-(3-oxo-3-phenylpropanoyl)phenyl
(PDKMA).
methacrylate]
PDKMA was synthesized via RAFT polymerization using CDB as
chain transfer agent (CTA) and AIBN as initiator. In a typical
homo-polymerization, DKMA (1.70 g, 5.52 mmol), CDB (17.99
mg, 0.066 mmol), and AIBN (2.71 mg, 0.017 mmol) were
dissolved in 5.5 mL anisole to form a crimson transparent
solution. The monomer concentration was 1.0 mol/L. After
deoxygenated by bubbling Ar for 30 min, the flask was sealed
and immersed into a preheated oil bath at 70 °C to start the
polymerization. After a predetermined time, the reaction
mixture was cooled to room temperature and the product was
precipitated from cold diethyl ether. PDKMA was obtained by
centrifugation and dried in vacuum at 35 °C for 24 h (1.34 g,
yield = 78.2%).
Tetrahydrofuran
(THF)
was
refluxed
over
potassium/benzophenone ketyl prior to use. All other solvents
and chemicals were of analytic grade and used as received.
Measurements
Nuclear magnetic resonance spectroscopy (NMR) were
recorded on a Bruker Avance III 400 spectrometer (1H: 400
MHz and 13C: 100 MHz) at room temperature using CDCl3 as
solvent and tetramethylsilane (TMS) as internal reference. Gas
chromatography-time of flight mass spectrometry (GC-TOFMS)
spectrum was measured by GCT Premier, WATERS. Molecular
weights (MWs) and polydispersity index (Đ) were determined
by size exclusion chromatography (SEC) on a Waters-150C
apparatus equipped with Waters Styragel HR3 and HR4
columns and a Waters 2414 refractive index detector. THF was
used as the eluent with a flow rate of 1.0 mL/min at 40 °C.
Commercial polystyrene samples were used as the MW
calibration standards. FT-IR spectra were recorded on Bruker
VECTOR 22. The measurements were performed on tablets
containing KBr and sample. The absorbance spectra of the
polymers and Eu3+ hybrid materials were recorded in the range
between 3500 and 400 cm-1. UV-Vis spectra were conducted
on a UV-2450 UV-Vis spectrophotometer (SHIMADZU). The
absorbance and transmittance spectra of the Eu3+-coordinated
polymer were recorded in the range from 380 to 680 nm.
Photoluminescence (PL) emission spectra of the samples were
Solution of Eu3+-coordinated PDKMA complex.
Eu3+-chelated polymer complexes solution was prepared by
mixing the Eu(NO3)3∙6H2O with PDKMA in alkaline THF solution
while the molar ratio of [Eu]:[PDKMA]:[C2H5ONa] was 1:3:3. In
a
typical chelation process, Eu3+ hybrid complex was
manufactured in 4 mL THF at initial concentrations of 6.25
mg/mL PDKMA in a spawn bottle. After 1 mL of Eu(NO3)3 in
THF containing 12.06 mg Eu(NO3)3∙6H2O was added with
continuous stirring, 5.2 mg C2H5ONa was added at room
temperature. The mixture was allowed for 2 h vigorous
stirring. Homogeneous Eu3+ hybrid complex solution was
obtained by passing a filter with 0.45µm pores.
Test paper of Eu3+ hybrid PDKMA.
Paper-based strips containing Eu3+-coordinated polymer
complex (PEP-strips) were prepared by simply immersion of
filter paper in Eu3+-PDKMA solution and then dried. In brief, as-
prepared homogeneous Eu3+-PDKMA solution was put into a
round glass tray with a plastic cover, and 30 pieces of
Whatman filter paper sheets (50 × 10 mm) were immersed
into the solution for 10 min and air-dried for approximately
recorded by using
a
FluoroMax-4 spectrofluorometer
(HORIBA). A UV lamp operating at 365 nm was applied to
record the luminescence images of solution samples, films and
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half an hour. They were dried in vacuum at 30 °C for 2 h. The spectrum confirms the composition of DKMA (calculated:
above was repeated two more times. Finally, the test paper 308.1049 g/mol, found: 308.1048 g/mol).
strips were stored in air-tight bags.
Copper (II) cation detection in solution.
To evaluate the selectivity of Eu3+-PDKMA complex on Cu2+
detection, the quenching ability of other metal ions on the
fluorescence of Eu3+-PDKMA complex were also examined. A
20 µL THF solution of Eu3+-PDKMA solution (5 mg/mL) was
added into 2 mL aqueous solutions of Mg2+, Ca2+, Fe3+, Fe2+,
Ni2+, Cu2+, Zn2+, Cd2+, Co2+, Al3+, Cr3+ and Pb2+, respectively. The
concentrations of the metal ions were set as 2.0 × 10-5 mol/L.
The muddy mixture was ultrasonic oscillating for 1 min at
room temperature, and then fluorescence intensity were
measured by spectrometers.
A mixture solvent of THF and deionized water with the
volume ratio of 9:1 was used in the measurement of Cu2+
sensing ability of Eu3+-PDKMA complex. A 20 µL THF solution of
Eu3+-PDKMA (5 mg/mL) was added to 2 mL THF/H2O solvent in
quartz cuvette. 2 µL Cu2+ aqueous solutions with different
concentrations were added, ultrasonic oscillated for 1 min, and
examined by fluorescence photometer directly.
Copper (II) cation detection by PEP-strips.
Paper strips containing Eu3+-PDKMA (PEP-strips) were simply
immersed into aqueous solutions of different metal ions for
approximately 45s under ambient condition. The ions included
Mg2+, Ca2+, Fe3+, Fe2+, Pb 2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+, and
Cr3+. The PEP-strips were dried under a warm air stream and
1
Figure 1. (I) Synthesis of PDKMA. (II) H NMR spectra of DKMA
then examined by naked eye under a standard UV lamp with
(A) and PDKMA67 (B) with assignments. *: residue CHCl3 in
365 nm excitation light.
solvent CDCl3.
Acid–base gas switching test by PEP-strips and Eu3+-PDKMA
film.
Well-defined PDKMAs with different chain lengths are
A PEP-strip was exposed to acidic vapor of HCl gas and basic
synthesized by RAFT polymerization. MWs measured by SEC, 1H
ammonia vapor consecutively in a hood. Each time the strip
NMR and 13C NMR spectroscopy (Table 1 and Figures 1, S3 and S4 in
was kept for about 10 s. The PEP-strip was examined by naked
ESI†) agree well with the corresponding theoretical values
eye under a standard UV lamp (365 nm). We also made a Eu3+
indicating well-controlled RAFT polymerizations. As shown in
hybrid PDKMA film on a quartz plate by spin coating. The
Figures 1B and S3, the ratio of enol/ketone in PDKMA67 is calculated
polymer film also underwent the acid-base vapors switching
as 94.6:5.4 by the integrals of proton signals at 16.87 and 4.50 ppm,
from HCl and ammonia vapors. The corresponding
which indicates that the equilibrium is not changed after
fluorescence spectra were recorded for each cycle.
polymerization. The protons of aromatic rings (Hf’, g’, h’, i’, j’ at 7.86-
a’
7.18 ppm) and backbone (Hb’, between 2.44 and 1.43 ppm) of
PDKMA67 are observed and carefully assigned. The C, H and N
Results and Discussion
percentages of obtained homo-polymers are determined by
Polymerizations and characterizations of PDKMA
elemental analysis consistent with the theoretical values. As a
The β-diketone-containing monomer, i.e. DKMA, is prepared typical result of PDKMA67, C, H and N percentages are 73.09%, 5.45%
from aldol reaction and followed by esterification with and 0%, respectively, and the calculated values for PDKMA67 are
methacrylic chloride. The overall yield can reach 74%. DKMA 73.96%, 5.25% and 0%.
has been fully characterized by H NMR and 13C NMR (Figures
1
PDKMAs with MWs ranging from 12.8 to 16.3 kDa and moderate
1A, S1 and S2 in ESI†). The characteristic signals of
methacrylate protons (Hb and Ha) are observed at 6.39, 5.80
ratio of [DKMA]/[CDB] (Table 1 and Figure S4). The symmetrical and
polydispersity indices (Đ ≤ 1.30) are obtained by varying the feed
and 2.08 ppm. The singlet signals of enol protons (Hd and He at
mono-modal peaks indicate that polymerization showed
16.87 and 6.84 ppm) and methyne protons (Hc at 4.62 ppm)
satisfactory results in both reactivity and dispersity. We keep
present equilibrium between enol and ketone forms with the
monomer conversions applicable (45-85%) for better
molar ratio of 96.0:4.0. Meanwhile, high resolution mass
polymerization control.
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Table 1. RAFT homo-polymerization of DKMA in anisole at 70 °C.
c)
d)
Time
(h)
Conv.
(%)
Mn, NMR.
(kDa)
21.5
Mn,SEC
Sample
[DKMA]/[CDB]/[AIBN]a)
DPb)
Đd)
(kDa)
1
2
3
300/3/1
333/4/1
300/3/1
10.0
6.5e)
7.5
69
69
67
50
16.3
15.3
12.8
1.30
1.25
1.30
81
20.9
49.5
15.7
a) Molar ratio in feed. The concentration of DKMA is 0.5 mol/L. b) Degree of polymerization (DP) was calculated by ([DKMA]/[CDB]) ×
Conversion. c) Determined by 1H NMR according to trioxymethylene as internal reference, Mn,NMR = MWmonomer × ([DKMA]/[CDB]) ×
Conversion + MWCDB. d) Determined from SEC in THF. e) The concentration of DKMA is 1.0 mol/L.
Eu3+ coordination with PDKMA
The coordination of Eu3+ with PDKMA is supported by FT-IR
DKMA is performed by means of density functional theory
(DFT). The calculaꢀon details are included in ESI†. Figure 2
illustrates the optimized geometry at B3LYP/6-31G(d,p). Three
DKMA repeating units evenly distribute in space around Eu3+
and the dihedral angle of two carbonyl (C=O) groups on each
DKMA is about 6° in proximity to a plane. All the bond lengths
between Eu3+ and oxygen atom of C=O group in each DKMA
are nearly 2.30 Å indicating a strong coordinating interaction.
TD-DFT calculation at B3LYP/6-311+(d,p) level provides the
frontier molecular orbitals of excited states. It suggests a 3.44
eV energy gap between HOMO and LUMO. Its corresponding
excitation wavelength is 360.8 nm quite close to the
experimental value of 350 nm. Thus the chelation between the
three DKMA repeating units and Eu3+ is proved.
analysis. Figure S5 in ESI† shows an example of PDKMA67 and
the corresponding Eu3+ complex. Strong absorbent bands at
1600 and 1495 cm-1 in PDKMA67 attribute to the stretching
vibrations of C=O and C=C (enol isomer) in β-diketone side
groups, respectively.32 Their red shifts to 1595 and 1475 cm-1 in
Eu3+-PDKMA67 complex evidence the coordination of carbonyl
group to Eu3+.33
UV and fluorescence of Eu3+ Chelated PDKMA Complex.
The UV-Vis absorption spectra of PDKAM with and without
Eu3+ in THF are shown in Figure S6 ESI†. In a typical UV-Vis
profile of PDKMA67, a broad absorption band is observed
between 300 and 420 nm with maximum of 340 nm. With the
increasing concentration of Eu3+, the π-π* transition bands
centered on the
ca. 10-15 nm compared with that of free
chromophore, indicating an electronic interaction between the
light-absorbing aryl
-diketone and the light-emitting Eu3+
β
-diketone moiety experience a red shift of
β
-diketone
β
center.34 Photoluminescent spectra of Eu3+-polymer in
aqueous solution consist of two groups of emission peaks after
being excited at 350 nm.30, 35-38 The first group from 400 to 550
nm contains the emission bands originated from π-π*
transitions of ligand in PDKMA. The second one is composed of
typical narrow-band Eu3+ luminescence in the region of 570-
680 nm at 579 (J = 0), 590 (J = 1), 612 (J = 2) and 650 (J = 3) nm
(antenna effect), which are originated from Eu3+ ions
corresponding to 5D0→7FJ f-f transiꢀons (Figure S7 in ESI†).39
When the concentration of PDKMA67 reaches 0.05 mg/mL, the
luminescence of Eu3+ is the largest. After that, the intensity of
Eu3+ photoluminescence decreases quickly due to the
precipitation of hydrophobic PDKMA67. Therefore, we choose
0.05 mg/mL as the standard concentration of PDKMA67 for
subsequent analysis.
Figure 2. Optimized geometries of chelation of Eu3+ with three
DKMA repeating units and the frontier molecular orbitals.
Quantum chemical calculation of coordination of Eu3+ with
Fluorescence of Eu3+-PDKMA in various pH conditions
To explore the stability of Eu3+-PDKMA in aqueous solution, we
investigated the luminescent features of Eu3+-PDKMA at
PDKMA
In order to further understand the details of chelation
between DKMA and Eu3+, a theoretical calculation of Eu3+-
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drastically. To further quantify the turn-off efficiencies, the
fluorescence intensities of Eu3+-PDKMA at the presence of Cu2+
various pH values. The pH values of the solutions are tuned by
adding either HNO3 or NaOH solutions. The plot of
photoluminescence intensity vs. pH over a range between 2.0 with various concentrations shows a linear range from 0 to
10-6 mol/L (In set of Figure 4), where I0 and I are the
and 10.0 shows a non-linear trend with a peak near pH = 7.2-
1×
8.2 (Figure S8). The dissociation between Eu3+ and diketone in
acidic medium quenches chromophore fluorescence. On the
contrary, high basic solution also destroys the coordination of
Eu3+ and diketone to form Eu(OH)3 precipitation and decreases
the fluorescent emission.40 At neutral pH from 6 to 9, Eu3+-
PDKMA complexes exhibit good stability and strong light
emission. The pH values in all the following metal detection
are fixed under neutral condition.
luminescent intensities before and after metal ion
incorporation, respectively. The detection limit of Cu2+ by Eu3+-
PDKMA is 2
to 5.
×
10-8 mol/L when the signal/noise ratio decreases
Cu2+ detection by Eu3+-PDKMA
Eu3+-PDKMA shows a strong red emission at 612 nm under the
excitation light of 350 nm. Cu2+, a typical transition metal
cation, can quench its luminescence to make Eu3+-PDKMA a
42
good chemosensor.41,
The fluorescence turn-off switch
probably contributes to the reason that Cu2+ can replace Eu3+
in coordinating β-ketone groups, and quench excitation energy
through heavy atom effect.43 Eu3+-PDKMA solution exhibits
very good selectivity for Cu2+. Other common cations including
Mg2+, Ca2+, Fe2+, Pb2+, Co2+, Ni2+, Zn2+, Cd2+, Al3+ and Cr3+ in 2
×
10-5 mol/L aqueous solution do not quench the luminescence
at 612 nm of Eu3+-PDKMA67 as shown in Figure 3. The highest
quenching intensity is less than 15% while Cu2+ and Fe3+ show
the effect as high as 66.8% and 48.5%, respectively.
Fortunately in practice Fe3+ can be easily reduced to Fe2+ which
is negative in the luminescence quench of Eu3+-PDKMA. And
thus the interference of Fe3+ is not a problem.
Figure 4. Luminescence spectra of Eu3+-PDKMA67 in THF/water
solution with different Cu2+ concentrations under excitation at
350 nm at 25 °C. Inset: the photoluminescence intensity ratios
at 612 nm as a function of various Cu2+ concentrations.
Since Eu3+-PDKMA probe is highly sensitive and selective in
Cu2+ recognition, it provides numerous advantages to fabricate
portable test paper or films, such as effective reagent
immobilization, simple preparation, versatility, and good
mechanical properties.44,
45
With a simple immerse-and-dry
method (see the Experimental section), we develop
fluorescence test paper (PEP-strip) for rapid detection of Cu2+
in aqueous solution. The inset of Figure 3 shows an obvious
emission color change from red to blue by naked eye when the
test paper immerses into Cu2+ solution with the concentration
of 2.0 × 10−5 mol/L. On the contrary, no color change is found
for other cations. The response of PEP-strips to Cu2+ is fast
within 45 s. All the above suggest PEP-strip an excellent
candidate for fast and selective Cu2+ detection.
5
Figure 3. Photoluminescence quenching intensity of the D0→7F2
Recyclable On–Off–On vapoluminescent sensor.
transition (612 nm) as a function of various cationic species in
aqueous solution. Insets: photographs of PEP-strips with different
cations under sunlight (top) and UV light of 365 nm (bottom), Blank:
deionized water.
Luminescent On–Off–On response is based on the
coordination of Eu3+ with diketone groups assisted by alkali.
The classical Eu3+ emission is turned on in basic condition. Eu3+
dissociates from polymer if the diketone groups are
protonated by extra acid, which turns off the red luminescent
emission of Eu3+ and shows blue emission of diketone itself.
The photoluminescent properties of Eu3+-PDKMA67 at
This color-tunable mechanism makes Eu3+-PDKMA
a
various Cu2+ concentrations in the range between 0 and 2
×
10-
mol/L in a mixture of THF and water (VTHF / Vwater = 9/1) are
5
vapoluminescent sensor. A video in ESI† illustrates the
reversible color change of PEP-strip by the two vapors which is
similar with other reported vapoluminescent materials.24-26
recorded and summarized in Figure 4. With the increase of
Cu2+ concentration, the characteristic Eu3+ emission decreases
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From the viewpoint of device applications, reversibility and
Polymer Chemistry
Financial support by National Natural Science Foundation of
China (21374093) is acknowledged.
response time are the most important factors. Although PEP-
strip works well in the color change, we use a Eu3+-PDKMA
coated quartz substrate to quantitative examine the
reversibility. This film shows bright-red emission excited under
a UV lamp at 365 nm after brief exposure with NH3 and drifted
back to the blue emission when treated with HCl gas (the inset
of Figure 5). The spectral changes at 612 nm are monitored
upon alternating exposure to NH3 and HCl gases over several
NH3–HCl cycles as shown in Figure 5. The Eu3+-PDKMA67 film
exhibits good reversibility at least 10 times suggesting that
PEP-strip shows a good recyclable Off–On–Off response. In
addition, the response time of PEP-strip is also rapid about 10
s. Such a naked eye distinguishable sensor of Eu3+-PDKMA will
be of importance in a variety of applications.
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4. D. R. Brown and H. Kozlowski, Dalton Transactions, 2004,
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