Fluorescent-based solid sensor for HSO4- in water

Article abstract of DOI:10.1111/j.1751-1097.2010.00795.x

In this report, we have shown that the encapsulation of the terbium 2-methylimidazole-4,5-dicarboxylic acid complex into inorganic host tetraethoxysilance is considered to be an efficient way for the design of anion sensors. Strong green emission still can be observed when it disperses in pure water. It was found that the luminescence of hybrid material was selectively turned off rapidly (1 s) by hydrogen sulfate compared with the addition of different anions such as F^-, Cl^-, Br^- and I^-. Thin film was successfully prepared and also could be a promising tool for recognizing HSO₄^-.

Full text of DOI:10.1111/j.1751-1097.2010.00795.x

Photochemistry and Photobiology, 2010, 86: 1191–1196
)
Fluorescent-based Solid Sensor for HSO4 in Water
Chaoliang Tan, Qianming Wang* and Lijun Ma
School of Chemistry and Environment, South China Normal University, Guangzhou, China
Received 4 June 2010, accepted 2 August 2010, DOI: 10.1111/j.1751-1097.2010.00795.x
investigated by addition of various anions, such as HSO4), F),
ABSTRACT
Cl), Br) and I) to water suspension of the assayed bulk material.
Highly selective quenching of the hybrid material by hydrogen
sulfate anion was observed. According to the spectroscopic
analyses, we regard that the quenching process was because of
the hydrogen-bonding interaction between the hydrogen sulfate
anion and the ligand of the hybrid material. Moreover, thin films
were prepared by the same material, which also give rise to
luminescence quenching in the presence of hydrogen sulfate
anion and could be used as a convenient tool for anion sensing in
real-life applications.
In this report, we have shown that the encapsulation of the
terbium 2-methylimidazole-4,5-dicarboxylic acid complex into
inorganic host tetraethoxysilance is considered to be an efficient
way for the design of anion sensors. Strong green emission still
can be observed when it disperses in pure water. It was found that
the luminescence of hybrid material was selectively turned off
rapidly (1 s) by hydrogen sulfate compared with the addition of
different anions such as F), Cl), Br) and I). Thin film was
successfully prepared and also could be a promising tool for
)
recognizing HSO4
.
MATERIALS AND METHODS
INTRODUCTION
All the starting materials were obtained from commercial suppliers and
used as received. 1H-NMR and 13C-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 spectropho-
tometer with a 150 W xenon lamp as light source. All the scan speed
was fixed at 300 nm min)1. Both excitation and emission slit widths
were 2.5 nm. Luminescence lifetime measurements were carried out
using an Edinburgh FLS920 spectrometer with a pulse width of 3 ls.
LC-MS was measured by Thermo Finnigan LCQ Deca XP Max
equipment. Thermogravimetric analysis was carried out using a
STA409PC system under air at a rate of 10ꢀC min)1, IR spectra was
measured by Fourier transform infrared, and nitrogen adsorp-
tion ⁄ desorption isotherms were measured at the liquid nitrogen
temperature, using an ASAP2020 analyzer. Surface areas were
calculated by the Brunauer–Emmett–Teller (BET) method and pore
size distributions were evaluated from the desorption branches of the
nitrogen isotherms using the Barrett–Joyner–Halenda (BJH) model.
In the field of chemistry, clinical biology, environmental
sciences and biomolecules, ion sensors for the detection of
biological process or hazardous chemicals have been exten-
sively studied (1–4). Among many other features, fluorescence
was chosen as the most powerful approach to record the
chemical recognition event. Therefore, lanthanide complexes
have attracted considerable attention as promising sensory
materials because of their narrow emission bands and long-
luminescence lifetimes, which allow time-resolved measure-
ments to rule out the influence of autofluorescence noise (5–7).
Parker and Gunnlaugsson developed cyclen-based lantha-
nide complexes for the detection of a group of anions (8–11).
Tsukube (12–14) presented that lanthanide tris(b-diketonates)
and tripod-lanthanide complexes are available as anion-
responsive luminescent compounds. Despite the development
of the above research works, there are very limited examples of
lanthanide luminescence changes in terms of hydrogen-bonding
formation with specific anions (15,16). Furthermore, the
prepared lanthanide complexes have relatively lower
thermo ⁄ photostabilities and are not suitable for the repeated
analyses in solutions (17).
Silicate can accommodate a series of photophysically active
centers and we focused on the construction of lanthanide-based
organic–inorganic hybrid material (18–25). Inspired by the
combined merits of both systems, in this work, we introduced a
terbium 2-methylimidazole-4,5-dicarboxylic acid complex into
an inorganic matrix and developed a novel hybrid material into a
solid-sensing material (Fig. 1). As expected, the hybrid material
still gives rise to strong green emission in pure water solution.
More importantly, the sensing abilities of this material were
X-ray powder diffraction was measured using a Y-2000 X-ray
diffractometer of Dandong Aolong Company, China.
For synthesis of 2-methylbenzimidazole, the detailed process was
similar to that reported previously (26). A mixture of o-phenylenedi-
amine (3.24 g, 0.03 mol) and acetic acid (1.80 g, 0.03 mol) was refluxed
for 1 h in HCl (4 M, 35 mL). The green solution was neutralized by
ammonia. The white precipitate formed was filtered and recrystallized
from water (yield: 3.53 g, 70%).
For preparation of 2-methylimidazole-4,5-dicarboxylic acid (L), the
detailed process was similar to that reported previously (26). A
solution of chromium trioxide (1.5 g, 0.015 mol) in water (5 mL) was
added dropwise to a solution of 2-methylbenzimidazole (2.64 g,
0.02 mol) in glacial acetic acid (15 mL) at 90ꢀC. The reaction mixture
was heated at 100ꢀC for a further 5 min and then poured into water
(ca 200 mL). The precipitate formed was discarded. The filtrate was
extracted using chloroform, and the combined extracts were dried
(Na2SO4) and evaporated. Crystallization of the residue from benzene
gives 2-methylimidazole-4,5-dicarboxylic acid [yield: 1.848 g, 70%;
1H-NMR (DMSO-d6) d = 2.87 (3H, s, Ha); 13C-NMR (DMSO-d6)
d = 159.7 (Ca), d = 146.2 (Cc), d = 128.2 (Cb), d = 11.61 (Cd); and
MS found: m ⁄ z 171.7].
The preparation of terbium 2-methylimidazole-4,5-dicarboxylic
acid complex was performed as follows (24): 2-Methylimidazole-4,5-
dicarboxylic acid (33.8 mg, 0.2 mmol) was dissolved in 8 mL of
*Corresponding author email: qmwang@scnu.edu.cn (Qianming Wang)
ꢁ 2010 The Authors. JournalCompilation. The AmericanSocietyofPhotobiology 0031-8655/10
1191

1192 Chaoliang et al.
O
H
N
NH2
NH2
OH
N
CrO3
Si
Si
Si
Si
Si
Si
Si
Si
O
O
O
O
O
O
O
O
H
N
O
O
3+
TEOS
Cd
Ha
HO
HO
Ca
Tb
O
O
O
HN
Cc
O
N
Tb(NO3)3.6H2O
N
Cb
O
N
H
N
O
O
ligand 1
Si
Si
Si
Si
Si
Si
Si
Si
O
O
O
O
O
O
O
Figure 1. Synthetic process of the hybrid material.
ethanol in a 50 mL round bottom flask, and Tb(NO3)3Æ6H2O (45.3 mg,
0.1 mmol) dissolved in 2 mL ethanol was added to the flask followed
by the addition of ammonia (1 mL). The mixture was stirred at 90ꢀC
for 1 h. The crude product was washed using ethanol and water, then
dried in vacuo overnight and the resulting precipitate was collected to
give the complex (TbL2Æ2H2O (60 mg) as white powder. Elemental
analysis found: C, 27.18; H, 2.34; N, 10.59%; Anal. Calcd for
C12H12N4O10Tb: C, 27.13; H, 2.28; N, 10.55%.
a
3450
1917
b
2553
The preparation of terbium 2-methylimidazole-4,5-dicarboxylic
acid containing hybrid material was as follows (24): 2-methylimidaz-
ole-4,5-dicarboxylic acid (67.6 mg,0.4 mmol) was dissolved in 5 mL
DMSO in a 50 mL beaker, then tetraethoxysilane (TEOS; 416 mg,
2 mmol) and Tb(NO3)3Æ6H2O (90.6 mg, 0.2 mmol) were dissolved in
10 mL water and ammonia (5 mL) was added into the above solution.
The mixture was stirred under room temperature for approximately
5 h. Then it was dried at 80ꢀC for 24 h and the resulting precipitate
was collected to give the titled complex (300 mg) as white powder.
For preparation of the terbium complex containing TEOS thin
films, the detailed process was similar to that reported previously (22).
[TbL2] (5 mg) was dissolved in 2 mL of ethanol. This solution was
mixed with TEOS (1 mL) and water (1 mL). Two drops of ammonia
were then added to the above solution. The mixed solution was stirred
at room temperature for 48 h. The glass substrates were cleaned first
by rinsing them in distilled water containing a surfactant. As a next
step, they were cleaned ultrasonically for 30 min in distilled water
containing surfactant, then ultrasonically for 10 min in acetone and
finally they were boiled for 10 min in 2-propanol. Finally, the sol-gel
solutions were cast on the prepared glass substrates, dried in open air
for a week and the thin films were formed.
3535
c
1575
1384
1083
1581
1384
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Figure 2. The IR spectra of the ligand (a), terbium complex (b) and
hybrid material (c).
through oxygen atoms of carboxyl groups. After incorporation
into the silica network, it can be observed that the formation
of the silica framework is given by the intense band at
1083 cm)1 (Fig. 2c) (21).
RESULTS AND DISCUSSION
1H-NMR spectroscopy was used to study the binding
affinity of anions to ligand 1. Upon addition of 1 equivalent of
hydrogen sulfate ion, the Ha protons shifted to downfield
(from 2.87 to 3.18 ppm) (Fig. 3), suggesting the binding of
ligand 1 to hydrogen sulfate ions in terms of hydrogen
interactions to the imidazole-ring NH. Similarly, we also
found the shifting of proton to downfield with the addition of
1 equivalent of fluoride ion (Fig. S1; see Supporting Informa-
tion). We tried analogous NMR experiments of Cl), Br) and
I), but ligand 1 did not give rise to signal changes in each
proton (data not shown).
The IR spectra for ligand 1 (a), TbL2 (b) and the hybrid
material containing TbL2 (c) are shown in Fig. 2. The broad
bands at 3535 and 3450 cm)1 were assigned to be –OH
stretching vibrations. Intramolecular hydrogen-bonding for-
mation and molecular packing were detected by the broad
band located between 2400 and 2800 cm)1. The intense band
at 1917 cm)1 is attributed to C=O vibration in COOH groups
(19,20). In Fig. 2b, the coordination interactions between the
carboxylate and the terbium ion are supported by the bands
located at 1575 and 1384 cm)1 that is ascribed to the
asymmetric (tas) and symmetric (ts) stretching vibrations of
carboxylate. In the meantime, the COOH peak at 1917 cm)1
vanished completely. The difference between tas and ts is
191 cm)1, showing that the ligand was coordinated with Tb3+
There are considerable research reports concerning aro-
matic carboxylic acids and terbium ions because the former
have suitable triplet state energy levels for resonant emissive
energy of the latter (24,25,27). However, we recently found

Photochemistry and Photobiology, 2010, 86 1193
10000
8000
6000
4000
2000
0
*
5D4
7F5
Hybrid material
Terbium complex
5D4
7F6
Figure 3. 1H-NMR spectra measured by titration of a DMSO-d6
solution of pure 1 (1 mM) with 2 equivalents of [Bu4N] HSO4.
Terbium complex in water
5D4 7F4
10000
8000
6000
4000
2000
0
5D4
7F3
450
500
550
600
650
Wavelength (nm)
Figure 5. Emission spectra of TbL2, TbL2 in water and the hybrid
material powder excitation at 282 nm (w: false peak).
800
5D4
7F5
pure
700
600
500
400
300
200
100
0
10-5 mol/L
5D4
7F6
5x10-5
7x10-5
250
300
350
400
Wavelength (nm)
Figure 4. Excitation spectra of the hybrid material powder emission at
545 nm.
10-4 mol/L
5D4 7F4
that imidazole derivative as a new type of chelating ligands
could firmly bind to terbium ions and efficient intramolecular
energy transfer occurs. The luminescent binary complex TbL2
gave very strong green color emission, but when it was
immersed into the water solution, the light was completely
quenched by the O–H oscillator of the water. Consequently, to
improve the stability of the complex, we incorporated the
complex into the inorganic host (TEOS). A more stable hybrid
material, which also has a strong green color emission, was
fabricated. When emission wavelength was fixed at 545 nm,
the excitation spectrum shows a broad band covering from 250
to 310 nm with two peaks at 275 and 290 nm (Fig. 4). Hence,
the hybrid is a broad wavelength excitation material. Its
emission spectra are shown in Fig. 5. The emission can be
5D4
7F3
*
400
450
500
550
600
650
Wavelength (nm)
Figure 6. Emission spectra of the hybrid material (10 lg mL)1 in
water) excited at 281 nm upon addition of 0–10 · 10)5 M of [Bu4N]
HSO4 (w: false peak).
ions decreased a little. When 5 · 10)5 M hydrogen sulfate
anions were added, the characteristic emission of the terbium
complex was quenched by 50%. Further quenching was seen in
the fluorescence emission spectra upon addition of 7 · 10)5
M
interpreted as follows: the excited 5D4
fi
7FJ transitions
of hydrogen sulfate anions. When the concentration of
hydrogen sulfate anions increased to 10)4 M, the luminescence
of the hybrid material was completely shut down. As a
consequence, we could observe the sharp and distinguished
changes by the naked eye under the excitation of the ultraviolet
light (Fig. 7). Continuous addition of the same anion resulted
in no change in the luminescence spectra. The above data
firmly suggested that this terbium-containing material showed
affinity to hydrogen sulfate even in competitive media such as
water. Analogous experiments were performed with F), Cl),
exhibit four main components for J = 6, 5, 4 and 3,
respectively. Under the current situation, no splitting of 7FJ
level could be observed.
As shown in Fig. 6, the hybrid material was dispersed in
water with the concentration as low as 10 lg mL)1, which was
very efficient in heterogeneous solid–liquid phase. It mainly
displayed the characteristic metal-centered emission and could
be sensitive to anion interactions. When emission wavelength
was fixed at 545 nm, the excitation spectrum shows a sharp
peak at 281 nm, which was not very similar to the solid state
(Fig. S2; see Supporting Information). After addition of
10)5 M hydrogen sulfate anions, green emission from terbium
Br), I) and H2PO4 (10)3 M of corresponding [Bu4N] salts)
or within HBF4 acidic condition (10)3 M) (Fig. S3; see
Supporting Information). Approximately 10% decrease could
)
)

1194 Chaoliang et al.
800
600
400
200
0
ON
Figure 7. Photograph of hybrid material solution (10 lg mL)1 in
water) and addition of 10)4 M [Bu4N] HSO4 (excited by ultraviolet
light lamp at 254 nm).
OFF
be detected in the emission spectra for halides and HBF4,
indicating that the recognition process has less relationship to
1
2
3
4
Cycles
)
acidity in aqueous solution. H2PO4 has led to the largest
Figure 8. Plot of the fluorescence of the hybrid material with alter-
nated dipping in 10)4 M aqueous solution of hydrogen sulfate ions
(OFF) and distilled water (ON). The cyclic index is the number of
alternating dipping–rinsing cycles, with the vertical axis showing the
position of the emission intensity of the hybrid material.
change (about 20% reduction in intensity), showing that this
specific tetrahedral anions might also interact with the
receptor. However, we still observed weak green luminescence
upon addition of 10)3 M anions. Therefore, the emission
investigation confirmed that the present hybrid material could
be applied as a chemical sensor more suitable for hydrogen
sulfate anion.
100
80
60
40
20
0
It is worth mentioning that the fluorescence quenching
observed following dipping of powder material into hydrogen
sulfate anion solution was found to be almost fully reversible
when it was rinsed thoroughly in distilled water. Reusability
was evaluated by repeated dipping–rinsing cycles, with the
hybrid material fluorescence spectrum being recorded after
each step. Typical data are shown in Fig. 8. However, the
powder is not very easy to recover for recycling purposes.
Subsequently, we coated the hybrid material onto the glass
substrate and prepared the thin film for use as a convenient
tool as anion sensor. As shown in Fig. S4 (see Supporting
Information), the left one is the freshly prepared film. Strong
green luminescence can be seen when excited at 254 nm by
ultraviolet lamp (the middle one). We can find that it lost the
luminescence when it was placed in 10)4 M hydrogen sulfate
anions solution (the right one). Similar to the powder, the film
has no responses in the presence of other anions at the same
concentration. Therefore, we can use this film as a tool in real-
life applications, such as environmental analysis. The as-
prepared films could also be recycled for at least three times.
But the responsive time (3–5 h according to the thickness) was
longer than the powder material. We thought that the reason
might be the as-derived film is too thick. When it was
immersed into the aqueous solution, the luminescent complex
on the surface was quenched immediately. However, the
internal structure was still giving rise to green emission as it
needs more time for the anion penetration effect. Thus, we will
try to use the spin-coating technique to immobilize the
fluorescent probes onto the substrate in the future.
10
20
30
40
50
60
70
2θ / degree
Figure 9. X-ray diffraction patterns of the hybrid material.
hybrid (29,30). In this figure, we cannot observe existence of
any types of crystalline patterns such as lanthanide salts,
complexes or 2-methylimidazole-4,5-dicarboxylic acid ligand.
Adsorption–desorption isotherms of N2 were recorded
to assess the textural properties of the hybrid material (Fig. S5;
see Supporting Information). The specific area and the pore size
are calculated by the BET method and BJH model, respectively.
It displays a decent surface area (SBET, 100.66 m2 g)1; pore
volume, 0.356 cm3 g)1; pore diameter, 14.17 nm) without the
construction of amphiphilic surfactant templates such as hex-
adecyl trimethylammonium bromide. The above small pores
might be useful as a carrier of particular substances.
The room-temperature X-ray diffraction pattern from 10ꢀ
to 70ꢀ of the hybrid material is shown in Fig. 9. Broad signals
are observed and it could be caused by the existence of a short-
range order in the material (28). The broad peak centered on
25ꢀ in the XRD pattern of the material was ascribed to be the
diffraction signal of the amorphous silica moieties of the
Thermal gravimetric analysis provides a reliable way for
measuring the thermal stability of the hybrid material. At less
than 270ꢀC, which is below the commonly used temperature,
terbium complex containing hybrid materials gave a relatively

Photochemistry and Photobiology, 2010, 86 1195
100
80
60
40
20
TEOS. It is a breakthrough for lanthanide-based chemical
sensor and is possible for practical use. More importantly,
upon addition of HSO4), the unique green emission peak of
the present terbium complex was significantly quenched
through the hydrogen-bonding interaction between ligand 1
and HSO4). However, the addition of excess amounts of F),
Cl), Br) and I) showed minor changes of emission bands of
terbium ions. This kind of hybrid material can be used several
times. In addition, sol-gel film was successfully prepared,
which could be directly applied in aqueous solution as a tool
for hydrogen sulfate ion recognition. Efforts to apply these
results to real-time monitoring and in situ detection are
currently underway in our laboratory.
Solid: hybrid material
Dot: terbium complex
Acknowledgements—This work was supported by the National
Natural Science Foundation of China (21002035) and Start funding
of South China Normal University G21117.
100
200
300
400
500
600
700
oC
Temperature (
)
Figure 10. Thermogravimetric analysis traces of terbium complex and
hybrid material.
SUPPORTING INFORMATION
Additional Supporting Information may be found in the online
version of this article:
Figure S1. 1H-NMR spectra measured by titration of a
DMSO-d6 solution of pure 1 (1mM) with 1 equiv. of [Bu4N] F.
Figure S2. Excitation spectra of the hybrid material
(10 lg ⁄ ml in water) emission at 545 nm.
Figure S3. Emission spectra of the hybrid material
(10 lg ⁄ ml in water) excited at 281 nm upon addition of
10)3 mol ⁄ L of [Bu4N] F, Cl, Br, I, H2PO4 and HBF4 acid
(*: false peak).
Figure S4. Photograph of thin films (left: room light;
middle: UV 254 nm light; right: dipping in 10)4 M HSO4)).
Excited by UV light lamp at 254 nm.
Figure S5. The nitrogen adsorption–desorption isotherms of
the hybrid material.
Please note: Wiley-Blackwell are not responsible for the
content or functionality of any supporting information sup-
plied by the authors. Any queries (other than missing material)
should be directed to the corresponding author for the article.
Figure 11. Fluorescence microscope of the hybrid material.
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complex was homogeneously dispersed in hybrid materials and
assembled into large aggregates with a diameter of approxi-
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