Luminescent terbium(iii) complex-based titania sensing material for fluoride and its photocatalytic properties

Article abstract of DOI:10.1039/c2pp05380b

A new terbium 2-isopropylimidazole-4,5-dicarboxylic acid complex was prepared and incorporated into titanium dioxide matrix by mild sol-gel method. Then we fabricated a terbium luminescent hybrid material, which displayed striking green emission even in pure water. It was interesting to find that this target material exhibited highly selective and fast (1 s) quenching effect to F^- compared with CH₃COO^-, Cl^-, Br^-, I^-. We recognized that the hydrogen bonding interactions between fluoride and ligand resulted in the recognition process. More significantly, this hybrid titania material prepared under low temperature (80°C) could be used in photodegradation of methyl orange in aqueous environment.

Full text of DOI:10.1039/c2pp05380b

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PAPER
Luminescent terbium(III) complex-based titania sensing material for fluoride
and its photocatalytic properties†
Zhan Zhou, Qianming Wang,* Shumin Huo and Yaqing Yang
Received 15th November 2011, Accepted 20th January 2012
DOI: 10.1039/c2pp05380b
A new terbium 2-isopropylimidazole-4,5-dicarboxylic acid complex was prepared and incorporated into
titanium dioxide matrix by mild sol–gel method. Then we fabricated a terbium luminescent hybrid
material, which displayed striking green emission even in pure water. It was interesting to find that this
target material exhibited highly selective and fast (1 s) quenching effect to F− compared with CH3COO−,
Cl−, Br−, I−. We recognized that the hydrogen bonding interactions between fluoride and ligand resulted
in the recognition process. More significantly, this hybrid titania material prepared under low temperature
(80 °C) could be used in photodegradation of methyl orange in aqueous environment.
investigated by photo-degradation methyl orange under the
irradiation of mercury lamp for the first time.
Introduction
Chemical sensing using fluorescence to signal analytes has
received much attention in the fields of industrial, physiological
Experimental
and environmental engineering due to the necessities for rapid
and low-cost testing method.1–5 Lanthanide luminescence-based
sensors have been developed based on their unique photophysi-
cal properties such as long lifetimes, sharp emission peaks and
large Stokes shifts.6,7 Parker et al.,8,9 Tsukube et al.,10–12 Gunn-
laugsson et al.13–15 and Wang and Tamiaki16 utilized rare earth–
acridone complexes, β-diketonate complexes, supramolecular
lanthanides or imidazo[4,5-f]-1,10-phenanthroline complex to
detect anions respectively. Unfortunately, most lanthanide com-
plexes were restricted by instability in water, because they are
very easy to be quenched by high frequency hydroxyl groups.
Therefore, we have recently developed several novel silica
hybrid materials, which can selectively recognize anions in water
via fluorescence.17,18 However, few publications have been
devoted to the design of smart responsive luminescent titania
materials.19–22 It is attractive to investigate the photophysical
properties of TiO2 composites and the activities of their photo-
catalytic applications.23–25
In this article, tetra-n-butyl titanate (TNBT) was used as the
host, the terbium 2-isopropylimidazole-4,5-dicarboxylic acid
complex was introduced and a novel terbium titania xerogel with
strong green luminescence was assembled as solid sensing
material in water (Fig. 1). More importantly, we studied the rec-
ognition abilities of this material by adding various anions, such
as F−, Cl−, Br−, I− and CH3COO−. Additionally, the photocata-
lytic properties of this novel terbium xerogel were also
All the starting materials were obtained from commercial suppli-
ers and used as received. 1H-NMR spectra were recorded at
293 K using Varian 400 (400 MHz) with TMS as an internal
standard. Fluorescence spectra were measured using a Hitachi-
2500 spectrophotometer with a 150 W xenon lamp as light
source. The scan speed was fixed at 300 nm min−1. Both exci-
tation and emission slit widths were set as 5.0 nm. Visible
absorption spectra and photocatalytic degradation were obtained
with an Agilent 8453 spectrophotometer and BL-GHX-V photo-
chemical reaction instrument, respectively. The fluorescence
images were taken using a Nikon Eclipse TS100 inverted fluor-
escence microscope system (Japan), equipped with a 50 W
mercury lamp source. LC-MS was measured by Thermo Finni-
gan LCQ Deca XP Max equipment. Thermogravimetric analysis
(TGA) was carried out on a STA409PC system under air at a rate
of 10 °C min−1. IR spectra was measured by Fourier transform
infrared. SEM was measured using a Tescan 5136MM scanning
electron microscope. Tetra-n-butyl titanate (TNBT) was provided
by Aladdin Company.
The synthesis of 2-isopropyl-benzoimidazole, the detailed
process was similar to reported in ref. 26. A mixture of o-pheny-
lendiamine (2.16 g, 0.02 mol) and isobutyric acid (1.76 g,
0.02 mol) was refluxed for 1 hour in hydrochloric acid (4 M,
35 ml). The pale green solution was neutralized by ammonia.
The white precipitation that formed was filtered and recrystal-
lized from water. Yield (2.822 g, 72%).
The preparation of 2-isopropyl-imidazole-4,5-dicarboxylic
acid, the detailed process was similar to reported reference.26
A
School of Chemistry and Environment, South China Normal University,
Guangzhou 510006, P. R. China. E-mail: qmwang@scnu.edu.cn;
Fax: +86-20-39310187; Tel: +86-20-39310258
†Electronic supplementary information (ESI) available: Fluorescence
spectra of the sensing material. See DOI: 10.1039/c2pp05380b
solution of 30% hydrogen peroxide (8 ml) was added dropwise
to a solution of 2-isopropyl-benzoimidazole (1 g, 6.25 mmol) in
concentrated sulfuric acid (8 ml) at 110 °C. Hydrogen peroxide
738 | Photochem. Photobiol. Sci., 2012, 11, 738–743
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Fig. 2 1H-NMR spectra measured by titration of a DMSO-d6 solution
of pure 1 (1 mM) with 1 equiv. of [Bu4N] F.
centrifuged to remove the suspended solid. Finally, the absor-
bance of the methyl orange solution was detected by a UV-vis
spectrophotometer. And the photodegradation rate of methyl
orange was calculated by the formula as follows: ηt = (A0 − A)/
A0 × 100%.
Results and discussion
The FT-IR spectra of the hybrid material were displayed in
Fig. S1†. The broad band located at 3138 cm−1 was attributed to
the remaining –OH vibration. The absorption bands at 1626 and
1401 cm−1 may be interpreted as the asymmetric and symmetric
stretching vibrations of COO−, which are identical with the fact
of the coordination reaction with terbium ions. The band cen-
tered at 628 cm−1 can be ascribed to the titania framework.27
1H-NMR spectroscopy was used to study anion binding
affinity to ligand 1. After addition of 1 equiv. fluoride anion, the
protons of isopropyl of 1 shifted upfield due to the recognition
process (from 3.23 and 1.28 to 3.11 and 1.09 ppm, respectively)
(Fig. 2), suggesting that 1 bound to fluoride ions in terms of
hydrogen binding formation. It is proposed that the anion
addition will increase the electron density of ligand 1, generating
a stronger shielding effect and leading to the protons shifted to
the upfield. We also tried analogous NMR experiments of Cl−,
Br−, I−, and CH3COO−, but 1 did not show signal changes
(figures not given).
Fig. 1 Synthetic process of hybrid material.
(30%) was slowly added for 1 h. The reaction mixture was
heated at 110 °C after a couple of hours and then poured into ice
water (ca. 30 ml). The precipitate was filtered and washed with
water and then recrystallized from water. Yield, (580 mg, 47%).
1H-NMR (DMSO-d6) δ = 3.23 (1H, m, H1), 1.28 (6H, d, J =
6.8Hz, H2); MS (LCMS) found: m/z 197.87 [M − H]−.
The preparation of terbium 2-isopropyl-imidazole-4,5-dicar-
boxylic acid based hybrid material was similar to the reference:20
TNBT (408 mg, 1.2 mmol) was dissolved in 10 ml water at a
50 ml beaker, then we added hydrochloric acid until the solution
became transparent. After 1 hour, 2-isopropyl-imidazole-4,5-
dicarboxylic acid (47.5 mg, 0.24 mmol) and Tb(NO3)3·6H2O
(54.4 mg, 0.12 mmol) were dissolved in 10 ml DMSO. Finally,
ammonia (5 ml) was added into the above solution. The mixture
was stirred under room temperature for around 4 hours. After
that, it was dried at 80 °C for 24 hours and the resulting precipi-
tate was collected to give the titled complex (260 mg) as white
powder.
The photocatalytic degradation of methyl orange was
measured under the irradiation of mercury lamp (500 W) at room
temperature. 0.1 g of the terbium hybrid material was dispersed
in 100 ml of 10 mg L−1 methyl orange aqueous solution. Then
the mixture was put into test tube with magnetic stirring. The
samples were taken out at an interval of about 20 min and
The luminescent binary complex Tb12 presents intense green
fluorescence. The excitation spectrum of the complex was
achieved by monitoring the emission wavelength at 545 nm. In
the excitation spectra, a broad band covering from 250 to
325 nm can be observed (Fig. 3). When the excitation wave-
length was fixed at 262 nm, its emission spectra displayed
characteristic terbium luminescence. But the emission will be
rapidly terminated by the O–H oscillator in case of aqueous sol-
ution (Fig. 3). In order to strengthen the stability of the complex,
we introduced the complex into the inorganic host (TNBT) and
obtained a more stable hybrid material (Ha), which displays a
more intense green emission in water compared with Tb12
(Fig. 3). Consequently, the target material was possibly taken as
luminescent probe for monitoring anions in aqueous
environment.
As can be seen in Fig. 3, the terbium xerogel was dispersed in
pure water at a concentration as low as 0.1 mg ml−1. It also
exhibited the characteristic metal-centered emission and could be
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Fig. 3 Excitation (λem = 545 nm) spectra of Tb(1)2; emission (λex = 262 nm) and spectra of Tb(1)2 and Tb(1)2, Ha in water.
Fig. 4 Emission spectra of hybrid material (0.1 mg ml−1 in aqueous) excited at 262 nm upon addition of 10−5–10−4 M of [Bu4N]F. Right cuvette:
terbium hybrid material (0.1 mg) was dispersed in 1 ml H2O. Left cuvette: addition of [Bu4N]F 1 × 10−4
.
very susceptible to anion interactions. When emission wave-
length was fixed at 545 nm, the excitation spectrum showed a
broad band from 240 to 285 nm with maximum at 268 nm
(Fig. S2†). The emission spectra can be explained as follows:
the excited 5D4 → 7FJ transitions included four main components
for J = 6, 5, 4 and 3 respectively (excited at 262 nm). The lumi-
nescence of Ha decreased gradually and almost quenched at last
upon addition of fluoride anion from 10−5 to 10−4 mol L−1. Con-
sequently, we could observe the sharp and dramatic change by
the naked eye under the excitation at 254 nm ultraviolet light
(Fig. 4). Parallel experiments were applied by Cl−, Br−, I−,
CH3COO−, 10−3 mol L−1 titration of corresponding [Bu4N]+
salts were recorded by fluorescence spectroscopy (Fig. S3†). We
could hardly observe changes (less than 10%) in the emission
spectra. Accordingly, the emission investigations indicated that
the target hybrid material can be applied as a chemical sensor
more appropriate for fluoride anions. According to the report of
ref. 12, we considered that fluoride anion could give rise to lumi-
nescence quenching through hydrogen bonding interactions
(10−4 M).
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Fig. 5 Thermogravimetric analysis traces of terbium complex and hybrid material.
Fig. 6 Scanning electron microscopy of hybrid material.
The thermal properties of both the hybrid and the pure Tb12
complex have been recorded depending on TGA (Fig. 5). We
could clearly observe that terbium complex-containing hybrid
material has a much higher thermal stability than the pure
complex in the range of 0–700 °C. Below 200 °C, the weight
loss can be attributed to the removal of water (including coordi-
nation water) and solvents. In the range of the thermal decompo-
sition temperature (300–600 °C), the weight loss of the terbium
complex was much faster than that of the hybrid material. This
showed that when the complex was introduced into the TNBT,
the inorganic host matrix will protect the organic ligand and its
decomposition temperature is enhanced. Therefore, we recog-
nized that the introduction of the inorganic host matrix could
increase the thermal stability and extend the application area of
the terbium complex.
67.42 m2 g−1 and 5.24 nm (pore diameter), respectively. These
particular small pores might be applied for the carrier of special
substances.
The fluorescence microscopy experiment was performed in
order to probe the existence of emission species (Fig. S5†). The
green luminescence image clearly confirmed the localization of
terbium complex in TNBT matrices. Meanwhile, the terbium
complex was dispersed in hybrid materials and assembled into
large aggregates. Moreover, the morphology of the hybrid
material was explored by the scanning electron micrographs
(Fig. 6). This figure demonstrated that the homogeneous
materials and the micro-meter size particles were obtained. We
could see the dimensions are around 5 μm.
In addition, in order to investigate the potential applicability
of the novel terbium hybrid material in photocatalysis, we
studied its behavior for decomposition of methyl orange (Fig. 7).
From the UV-vis spectra, we can obviously see that the absor-
bance of methyl orange decreased with the increasing irradiation
Classical N2 adsorption–desorption isotherms of the hybrid
material are given in Fig. S4†. The specific area and the pore
size calculated by the BET formula and BJH model were
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Fig. 7 UV-vis spectra of the hybrid dispersed in 100 ml of 10 mg L−1 methyl orange aqueous solution in the range of 0–180 min. Inset: the photo-
catalytic performance of the hybrid based on TiO2 irradiated by a 500 W mercury lamp for degrading methyl orange solution.
time. It is shown that the terbium hybrid material can remove
46% methyl orange in relative longer period (around 3 hours).
Generally speaking, TiO2 for photocatalytic purposes was pre-
pared through heat treatment (for example 600 °C) before utiliz-
ation. In contrast, the terbium titania hybrid material in this
report was fabricated under low temperature (sol–gel process
without calcination treatment). Although the decomposition rate
and time were not very satisfactory, it opens a new way to under-
stand the photocatalytic properties of luminescent titania sol–gel
materials.
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