A highly selective regenerable optical sensor for detection of mercury(ii) ion in water using organic-inorganic hybrid nanomaterials containing pyrene

Article abstract of DOI:10.1039/c0nj00039f

A novel fluorescent chemosensing hybrid material (Py-SBA-15) for detecting Hg²⁺ ions in water was prepared through the functionalization of mesoporous silica (SBA-15) with a modified fluorescent chromophore, 1-(4′-hydroxyphenyl)-4-pyrenyl-2,3-diaza-1,3-butadiene (Py-OH). Py-OH covalently grafted to the coupling agent 3-(triethoxysilyl)propylisocyanate (TESPIC) was used as the precursor for the preparation of mesoporous hybrid material. The XRD data and TEM images of Py-SBA-15 confirm that the hexagonal ordered mesoporous structure is preserved after functionalization. Py-SBA-15 demonstrates a high selectivity for Hg²⁺ ions in the presence of other metal ions, which results from a dramatic fluorescence enhancement by the addition of Hg²⁺ ions upon the suspensions of Py-SBA-15. A good linearity (R² = 0.9989) between the fluorescence intensity of Py-SBA-15 and the concentration of Hg²⁺ ions is constructed, and a satisfactory detection limit of 1.7 × 10^-7 g mL^-1 is obtained. More importantly, Py-SBA-15 shows good regenerative ability due to the greatly minimized leaching effect of the sensing molecules by covalent grafting strategy. These results presented in this paper indicate that this hybrid material could be a promising fluorescence chemosensor for detecting Hg²⁺ ions.

Full text of DOI:10.1039/c0nj00039f

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PAPER
www.rsc.org/njc | New Journal of Chemistry
A highly selective regenerable optical sensor for detection of mercury(II)
ion in water using organic–inorganic hybrid nanomaterials containing
pyrenew
Yinghui Wang,ab Bin Li,*a Liming Zhang,ab Lina Liu,ab Qinghui Zuoab and
Peng Liab
Received (in Montpellier, France) 19th January 2010, Accepted 6th April 2010
DOI: 10.1039/c0nj00039f
A novel fluorescent chemosensing hybrid material (Py-SBA-15) for detecting Hg2+ ions in water
was prepared through the functionalization of mesoporous silica (SBA-15) with a modified
fluorescent chromophore, 1-(40-hydroxyphenyl)-4-pyrenyl-2,3-diaza-1,3-butadiene (Py-OH).
Py-OH covalently grafted to the coupling agent 3-(triethoxysilyl)propylisocyanate (TESPIC) was
used as the precursor for the preparation of mesoporous hybrid material. The XRD data and
TEM images of Py-SBA-15 confirm that the hexagonal ordered mesoporous structure is preserved
after functionalization. Py-SBA-15 demonstrates a high selectivity for Hg2+ ions in the presence
of other metal ions, which results from a dramatic fluorescence enhancement by the addition of
Hg2+ ions upon the suspensions of Py-SBA-15. A good linearity (R2 = 0.9989) between the
fluorescence intensity of Py-SBA-15 and the concentration of Hg2+ ions is constructed, and a
satisfactory detection limit of 1.7 ꢀ 10ꢁ7 g mLꢁ1 is obtained. More importantly, Py-SBA-15
shows good regenerative ability due to the greatly minimized leaching effect of the sensing
molecules by covalent grafting strategy. These results presented in this paper indicate that this
hybrid material could be a promising fluorescence chemosensor for detecting Hg2+ ions.
experimental methods, many efforts have been devoted to
Introduction
pursue an innovative and convenient sensor material for
detection of Hg2+ ions which can offer high sensitivity and
The development of optical sensors that can selectively
recognize and signal the presence of specific guests has been
selectivity. Among all these sensor techniques, optical sensors
are particularly well suited to detecting Hg2+ ions in environ-
extensively pushed owing to their potential for easy detection
and quantification of pollutant species in many fields of
mental analysis as they do not consume analytes, no reference
application, such as waste management, environmental chemistry,
is required, the signals are insensitive to sample flow rate or
clinical toxicology, and bioremediation of radionuclides.1
stirring speed and they allow on site, real-time qualitative or
Among these, the sensitive detection of heavy-metal ions, such
semiquantitative detection without the use of any expensive or
complicated instrumentation.3
as mercury ions, is of particular importance as they are
common environmental pollutants and highly toxic (damaging
In optical sensors, suitable fluorescence indicators that are
the central nervous system and creating neuropsychiatric
sensitive to analyte concentrations and exhibit changes in
disorders in human). As a consequence, the level of any
fluorescence intensity have been used as molecular recognition
materials.4 Pyrene derivatives are widely used as fluorescence
mercury compounds in the environment is the object of official
norms in the ppb range. To date, monitoring heavy-metals
indicators in optical sensors because they have many advantages,
ions in waste water is mainly based on atomic absorption/
such as a large Stokes shift, high quantum yield, excellent
emission spectroscopy and inductively coupled plasma-mass
photostability, and are relatively nontoxic. Kumar and
spectroscopy (ICP-MS), but the wide utilization of these
Samuelson et al. chose pyrene derivatives as fluorescence
methods is largely limited due to the expensive instruments
and complicated analysis program.2 In view of these sophisticated
indicators and used electrospinning technology to fabricate
optical sensors.3,5 Chang et al. also reported a pyrene derivative
as a fluorescence indicator which was connected to the skeleton
of crown ether to detect Hg2+ ions.6 However, these materials
a Key Laboratory of Excited State Physics, Changchun Institute of
Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,
can not be reused.
3888 Eastern South-Lake Road, Changchun 130033, P. R. China.
E-mail: lib020@ciomp.ac.cn; Fax: +86-431-86176935
In order to develop a regenerable optical sensor, immobilizing
the fluorescence indicators onto a porous material is a feasible
way. Previous investigations on such optical sensors have
focused on incorporating the fluorescence indicators onto
sol–gel glasses because they are easily prepared and are capable
of retaining indicators inside their gel network. However, the
inhomogeneous and slow diffusion of analytes inside the matrix
is a limiting factor owing to the poorly ordered microporosity
b Graduate School of the Chinese Academy of Sciences,
Chinese Academy of Sciences, Beijing, 100039, P. R. China
w Electronic supplementary information (ESI) available: The solid-
state 29Si MAS NMR spectrum of Py-Si (Fig. S1), normalized
fluorescence response I/I0 of Py-OH (1 ꢀ 10ꢁ5 M) in acetonitrile–water
mixture (7 : 3 v/v) in the presence of various metal ions (1 ꢀ 10ꢁ3 M).
lex = 350 nm and lem = 454 nm (Fig. S2). See DOI: 10.1039/
c0nj00039f
ꢂc
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of such materials.7 After the discovery of mesoporous silicas
prepared by the cooperative organization of surfactant and
inorganic species in 1992,8 the synthesis, characterization, and
applications of the supramolecular-templated mesostructured
materials have been of widespread interest in materials science.
Recently, optical sensors using mesoporous materials as
matrix have attracted significant attention due to their high
surface area, uniform porosity, and low absorption and
emission in the visible spectrum.9 Mesoporous silica SBA-15
have so far the largest pore-size mesochannel with thick walls,
adjustable pore size from 3 to 30 nm, uniform hexagonal
channel, and high hydrothermal and thermal stability. Moreover,
the straight channel of SBA-15 is beneficial for facilitating the
entrance and diffusion of target metal ions. Its ordered structure
can be maintained during successive water dipping and drying
sensing cycles, which results from the thicker SiO2 wall
that aids in the maintenance of structural integrity. Besides,
mesoporous silica SBA-15 matrix is optically transparent in
the visible region and favorably biocompatible.10 These
advantages mentioned above make SBA-15 a promising
optical sensor matrix. However, only limited examples of such
optical sensors used mesoporous silica SBA-15 as a solid
support to detect Hg2+ ions have been reported.11
spectrophotometer (Model Bruker Vertex 70 FT-IR) with
a resolution of ꢃ4 cmꢁ1 using the KBr pellet technique.
Small-angle X-ray diffraction (SAXRD) patterns were performed
with a Rigaku-Dmax 2500 diffractometer using Cu-Ka1
(l = 0.154 05 nm) radiation at a 0.021 (2y) scanning step.
Solid-state 29Si MAS NMR spectra were recorded at 79.46 MHz,
using a Bruker Avance 400 spectrometer. Field-emission
scanning electron microscopy (FE-SEM) images were measured
on a Hitachi S-4800 microscope. The transmission electron
microscopy (TEM) images were obtained with a JEM-2010
transmission election microscope made by Japanese JEOL
Company. Nitrogen (N2) adsorption–desorption isotherms
were measured by using a Nova l000 analyzer with nitrogen.
The samples were outgassed for 6 h at 120 1C before the
measurements. Surface areas were calculated by the Brunauer-
Emmett-Teller (BET) method, and pore sizes were calculated
by the Barrett-Joyner-Halenda (BJH) method. Thermo-
gravimetric analysis (TGA) was performed on 2 mg of samples
using a Perkin-Elmer thermal analyzer. The samples were
heated from 40 to 600 1C at a heating rate of 10.0 1C minꢁ1
.
A 10 mL minꢁ1 flow of dry nitrogen was used to purge the
sample at all times. All spectrophotometic spectra of the
hybrid material were performed with a suspension of sample
dispersed in deionized water and then brought into a quartz
cell for measurement. All the photoluminescence (PL)
spectra were measured with a Hitachi F-4500 fluorescence
spectrophotometer.
Here, we intend to report the synthesis and characterization
of a low cost, solid state sensor for the detecting Hg2+ ions
based on the functionalization of mesoporous silica SBA-15
with 1-(40-hydroxyphenyl)-4-pyrenyl-2,3-diaza-1,3-butadiene
(Py-OH) as fluorescence indicator, denoted as Py-SBA-15,
which was obtained by covalently grafting Py-OH onto the
backbones of the mesoporous silica of SBA-15. The detection
of Hg2+ ions is based on the fluorescence intensity changes of
the hybrid sensing material Py-SBA-15 with the lowest detection
limit of 1.7 ꢀ 10ꢁ7 g mLꢁ1. Moreover, Py-SBA-15 demon-
strates a high selectivity for Hg2+ ions in presence of other
Synthesis of the SBA-15 mesoporous silica
The mesoporous silica SBA-15 was synthesized according to
the procedure reported by Zhao et al.12 with some minor
modifications, using P123 as a template in acidic conditions. In
a typical synthesis, P123 (4.0 g) was dissolved in deionized
water (30 g) and 2 M HCl solution (120 g) at 35 1C. To this
homogeneous solution, TEOS (8.5 g) was added under
vigorous stirring with the following molar composition:
1.0 TEOS:0.0172 P123:6 HCl:208.33 H2O. The mixture was
stirred at 40 1C for 24 h and transferred into a Teflon bottle
sealed in an autoclave, which was then heated at 100 1C for
2 days. The resultant solid was filtered off and then extensively
washed with deionized water and dried at 60 1C in vacuo for
24 h. The templating agent P123 was removed by calcination
under air at 500 1C for 6 h.
metal ions such as Ag+, Na+, K+, Ca2+, Mg2+, Ba2+, Zn2+
,
Cd2+, Cu2+ and Pb2+ ions, and possesses good regenerative
ability.
Experimental
Materials
Analytical grade solvents and compounds were used for
preparation. Tetraethoxysilane (TEOS, Tianjin Chemicals Co.),
3-(triethoxysilyl)-propyl isocyanate (TESPIC, Aldrich), triblock
copolymer poly(ethylene glyco)-block-poly(propylene glycol)-
block-poly(ethylene glycol) (PluronicP123; EO20PO70EO20;
Mw = 5800, Aldrich), 4-hydroxy-benzaldehyde (Aldrich) and
1-pyrenecarboxaldehyde (Aldrich) were used as received.
Mercury nitrate and the other inorganic metal salts were
purchased from Shanghai Chemical Company (Shanghai,
China). The solvent toluene was used after desiccation with
anhydrous sodium sulfate (Beijing, China). Concentrated HCl
was obtained from Shanghai Chemical Company (Shanghai,
China). The water used in our work was deionized.
Synthesis of 1-(40-hydroxyphenyl)-4-pyrenyl-2,3-diaza-1,3-
butadiene (Py-OH)
To a solution of 4-hydroxy-benzaldehyde (0.230 g, 1.00 mmol)
and 1-pyrenecarboxaldehyde (0.122 g, 1.00 mmol) in absolute
ethanol (25 ml), hydrazine hydrate was added dropwise and
then refluxed for 10 h. The solvent was evaporated under
reduced pressure yielding 1-(40-hydroxyphenyl)-4-pyrenyl-2,3-
1
diaza-1,3-butadiene as a yellow solid, in 58% yield. H NMR
(DMSO, 400 MHz), d (ppm): 7.16 (2H, d), 8.14 (2H, d),
8.20 (1H, d), 8.22–8.26 (4H, m), 8.29 (1H, t), 8.37 (1H, d),
8.41(1H, d), 8.69 (1H, s), 8.72 (1H, d), 8.76 (1H, s). Anal.
Calcd for C24H16N2O3: C, 82.74; H, 4.64; N, 8.04%. Found:
C, 82.72; H, 4.68; N, 8.03%.
Characterization
The Fourier transform infrared (FT-IR) spectra were
recorded within a 400–4000 cmꢁ1 region using an FT-IR
ꢂc
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Synthesis of pyrene-functionalized SBA-15 mesoporous hybrid
material (Py-SBA-15)
mesoporous material Py-SBA-15 (D) are shown in Fig. 1.
The absorption band at 3362 cmꢁ1 in Fig. 1A corresponds to
the strong vibration of hydroxyl. The emergence of a series of
bands at 2975, 2929, 2885 cmꢁ1 due to the vibrations of
methylene –(–CH2)3– and the disappearance of the stretch
vibration of the absorption peaks at 2250–2275 cmꢁ1 for
NQCQO of TESPIC in Fig. 1B indicate that Py-OH has
been successfully grafted on to TESPIC.15 Furthermore, in
Fig. 1B the spectrum of Py-Si is dominated by n (C–Si,
1208 cmꢁ1) and n (Si–O, 1080 cmꢁ1) absorption bands,
characteristic of trialkoxylsilyl functions, proving that the
–Si(OEt)3 group is preserved after reaction. The band centered
at 3383 cmꢁ1 which corresponds to the stretching vibration of
grafted –NH– groups and the bending vibration d (NH,
1563 cmꢁ1) indicates the formation of amide groups. New
peaks at 1724 and 1622 cmꢁ1 were due to the CQO absorptions
of TESPIC, further proving that Py-OH has been successfully
grafted on to TESPIC. In addition, the formation of the
Si–O–Si framework is evidenced by the bands of absorption
of siloxane bonds located at 1080 cmꢁ1 (nas, Si–O),
803 cmꢁ1(ns, Si–O), and 468 cmꢁ1 (d, Si–O–Si) (n represents
stretching, d in plane bending, s symmetric, and as asymmetric
vibrations) in Fig. 1D comparing the spectrum of SBA-15 host
material (Fig. 1C). The n (Si–C) vibration located at 1208 cmꢁ1 is
still observed in the IR spectrum of Py-SBA-15 (Fig. 1D), which
is consistent with the fact that Si–C bond is reserved during
reactions. 29Si MAS NMR spectrum of Py-SBA-15 in the solid
state is displayed in Fig. S1 of the ESIw. The peaks corresponding
to the various siloxane Qm [Qm = Si(OSi)m(OH)4ꢁm, m = 2–4]
and organosiloxane Tn [Tn = RSi(OSi)nOH3ꢁn, n = 1–3] species
can be identified clearly. The relative integrated intensities of the
organosiloxane T1, T2, and T3 NMR signals can be employed to
estimate the degree of hydrolysis-condensation of organic
functional groups. The predominance of T3 [the ratio
T3 : (T3 + T2 + T1) = 0.55], compared with T1 and T2, suggests
that the hydrolysis and condensation of Py-Si is nearly complete
in the order structure. These results indicate the successful
immobilization of Py-Si within the channels of SBA-15.
The method used for the synthesis of alkoxysilane modified
pyrene (Py-Si) was according to the procedure described in the
literature.13 Py-OH (0.348 g, 1.00 mmol) was dissolved in
20 mL chloroform and excess TESPIC (1.7 mL, 6.37 mmol)
was added while the solution was heated at 60 1C in a covered
flask for 48 h. Cold hexane was added to precipitate a yellow
solid from the mixture. The final filtered-off precipitate was
washed with several portions of cold hexane and dried
in vacuo. Py-Si: IR –CONH– 1563, 1622 cmꢁ1, –(–CH2)3–
2929 cmꢁ1, Si–O 1080 cmꢁ1. Then the hybrid materials
(Py-SBA-15) were prepared via a reported pseudo-one-pot
synthesis route depicted in Scheme 1.14 Py-Si (5.00 g) was
reacted with activated SBA-15 (5.00 g, 5 h at 160 1C under high
vacuum) in dry refluxing toluene (50 mL) with mechanical
stirring for 48 h under a nitrogen atmosphere. The resulting
modified mesoporous silica (Py-SBA-15) was filtered off and
washed with toluene (2 ꢀ 30 mL), ethanol (2 ꢀ 30 mL) and
diethyl ether (2 ꢀ 30 mL). Finally, the product was heated for
4 h at 110 1C under vacuum.
Results and discussion
To synthesize Py-SBA-15, SBA-15 was first synthesized as a
starting material following the literature procedure.12 Py-Si
was then easily synthesized through the hydrogen-transfer
nucleophilic addition reaction between the hydroxyl group
of Py-OH and the isocyanate group of TESPIC, and the
as-obtained molecular precursor Py-Si was used as a bridge
molecule both response to Hg2+ ions and form an inorganic
Si–O network with SBA-15 (Scheme 1). The successfully
covalent-grafting of the Py-OH to SBA-15 can be supported
by the FT-IR spectra. The IR spectra for Py-OH (A), Py-Si
(B), SBA-15 (C) and Py-OH functionalized hybrid
The quantity of Py-OH molecules attached to the mesoporous
silica surface is estimated from the result of the TGA measurement
which is presented in Fig. 2. Three weight losses are observed
by TGA analysis in the temperature ranges 40–100 1C,
Fig. 1 IR spectra for Py-OH (A), Py-Si (B), SBA-15 (C) and
Scheme 1 Synthesis procedure of Py-SBA-15.
Py-OH-functionalized hybrid mesoporous material Py-SBA-15 (D).
ꢂc
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Fig. 3 Small angle powder X-ray diffraction patterns of (a) SBA-15,
Fig. 2 Thermogravimetric analysis data of Py-SBA-15.
(b) Py-SBA-15.
160–330 1C and 350–600 1C, respectively. The first weight loss
exhibits a weight loss of ca. 3.2% associated with an endothermic
peak with a maximum around 80 1C is owing to the removal of
physisorbed water molecules. From 160 to 330 1C, there was a
large mass loss of ca. 10.2% (DTGmax = 300 1C), which is
mainly attributed to the decomposition of the organic moiety.
The weight loss occurring between 350 and 600 1C is attributed
to the thermal degradation of the organosilicate frameworks,
involving Si–C, C–C, and C–N bond cleavage.16 From this
second weight loss, the amount of Py-OH immobilizing onto
the SBA-15 is calculated to be about 10%, and the corres-
type IV isotherms with H1-type hysteresis loops at high
relative pressure according to the IUPAC classification,19
which is characteristic of mesoporous materials with highly
uniform size distributions. The sharp inflection between the
relative pressure P/Po 0.5 and 0.8 observed in the two
isotherms, representing a capillary condensation of nitrogen
within the uniform mesopore structure. The structure data of
the pure SBA-15 and Py-SBA-15 (BET surface area, total pore
volume, and pore size, etc.) are summarized in Table 1. It is
known from Table 1 that the Brunauer-Emmett-Teller surface
area (SBET) of Py-SBA-15 decreases from 750 m2 gꢁ1 (SBA-15)
to 545 m2 gꢁ1. The pore volume and pore diameter are smaller
than that of the pure SBA-15, and the channel wall of Py-SBA-15
becomes thicker. These results mean that the Py-SBA-15 has a
smaller specific area and a slightly smaller pore size and pore
volume in comparison with the pure SBA-15, which might be
due to the presence of fluorescence indicator Py-OH on the
pore surface. Anyway, it is reasonable to believe that the
introduction of the fluorescence indicator Py-OH on the inner
surface of SBA-15 strongly affects the adsorption of nitrogen
and changes the physicochemical parameters of matrix
material.
ponding molar amount is about 0.2 mmol gꢁ1
.
SAXRD patterns and nitrogen adsorption–desorption
isotherms are popular and efficient methods to characterize
highly ordered mesoporous material with hexagonal symmetry
of the space group P6mm. Fig. 3 displays the X-ray diffraction
patterns of a pure SBA-15(a), Py-SBA-15(b) within the 2y
range of 0.5–51. Both of the SBA-15 type samples show at least
three well-resolved broad Bragg reflections that can be indexed
as d100, d110 and d200, which are the characteristics of a
well-ordered hexagonal mesostructure. For the pure SBA-15,
the intense (100) peak reflects a d spacing of 10.89 nm,
corresponding to a large unit-cell parameter (a0 = 12.57 nm).
After the introduction of functionalized organic indicator, the
(100) peak appears at slightly larger 2y value with d100 = 10.63 nm
and a0 = 12.27 nm. The close d100 spacing values of the two
samples indicate that their ordered hexagonal framework has
been preserved after the introduction of Py-OH. It is worth
noting that the intensity of characteristic diffraction peaks of
SBA-15 in the pyrene functionalized mesoporous materials
decreases as compared with the pure SBA-15, that is interpreted
as larger contrast in density between the silica walls and the
open pores than that between the silica walls and the organic
functional groups, which provides evidence that grafting
occurs inside the mesopore channels.17 The presence of
fluorescence indicator on the mesoporous framework of
SBA-15 results in the decrease of crystallinity but not the
collapse in the pore structure of mesoporous materials.18
The pore structure of SBA-15 and Py-SBA-15 mesoporous
materials was further characterized by using the nitrogen
sorption experiment. The nitrogen adsorption–desorption
isotherms of a pure SBA-15 mesoporous silicon (a) and
Py-SBA-15 (b) are presented in Fig. 4. Both of them display
The powder morphology of Py-SBA-15 mesoporous material
is shown in Fig. 5. The SEM images shown in Fig. 5a reveal
that a hexagonal cylinder shape is the dominant morphology.
Fig. 4 Nitrogen adsorption–desorption isotherms of (a) SBA-15,
(b) Py-SBA-15 surface area, V is the total pore volume, DBJH is
the average pore diameter, and hw is the wall thickness calculated by
a0 ꢁ DBJH
.
ꢂc
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Table 1 Structural data of SBA-15 and Py-SBA-15a
Sample
d100/nm
a0/nm
SBET/m2 gꢁ1
V/cm3 gꢁ1
DBJH/nm
hw/nm
SBA-15
Py-SBA-15
10.89
10.63
12.57
12.27
750
545
1.03
0.86
6.31
5.5
6.26
6.77
pffiffiffi
a
d100 is the d(100) spacing, a0 is the cell parameter ða0 ¼ 2d100
=
3Þ, SBET is the BET surface area, V is the total pore volume, DBJH is the average
pore diameter, and hw is the wall thickness calculated by a0 ꢁ DBJH
.
The detailed morphology can be seen under higher magnification
in Fig. 5b. The hexagonal cylinder-particles have a diameter of
about 500–700 nm and length up to 1–2 mm, which is
typical morphology for SBA-15 mesoporous materials.20
Complementary to the SEM images, the TEM images of
Py-SBA-15 demonstrate highly ordered hexagonal arrays
(Fig. 5c) of mesopores with one-dimensional channels
(Fig. 5d) throughout the sample. It is further approved that
the hexagonal structure of SBA-15 is preserved after
functionalization.
obvious and steady fluorescence increase between pH 3.0 and
8.0, indicating that acidity does not affect the determination of
Hg2+ with Py-SBA-15. Thus, Py-SBA-15 can detect Hg2+ in a
large pH range from 3.0 to 8.0. This result suggests that no
buffer solutions are required for the detection of Hg2+, and
this is convenient for practical application.
In order to test the recognition ability of Py-SBA-15 for
Hg2+ ions, fluorescence spectra were recorded following
excitation at 350 nm at room temperature upon the gradual
addition of small amounts of Hg2+ ions into the aqueous
solution of the suspended Py-SBA-15 (20 mg Lꢁ1). Fig. 7
illustrated the detailed fluorescence response of the chemosensor
Py-SBA-15 to the Hg2+ concentration. In absence of Hg2+
ions, the fluorescence emission intensity of Py-SBA-15 is very
weak. Upon the addition of increasing concentrations of Hg2+
ions, the suspension of Py-SBA-15 remarkably shows an
enhancing fluorescence emission centered at 454 nm. A
complexation sensor mechanism proposed in Scheme 2 is
responsible to the fluorescence remarkable enhancement.
Because the pyrene molecular is electron-deficient, the lone
pair electrons of the nitrogen atoms transfer to it, resulting the
quenching of the pyrene emission in the free state.21 With
gradual addition of small amounts of Hg2+, it could be
expected that, upon complexation with Hg2+ ion, the lone
pairs no longer participate in the quenching process, causing
the recovery of the fluorescence.
Fluorophores are usually disturbed by the proton in the
detection of metal ions, so the effect of pH on the luminescence
is extremely expected. In order to explore the utility of
Py-SBA-15 as an ion selective fluorescence sensor for Hg2+
,
the proper pH condition of this new hybrid material was also
evaluated. A solution of the high concentration of Hg2+ might
cause precipitation of HgO in the alkaline condition, so these
experiments were carried out at a pH range from 3.0 to 8.0.
The evolution of the fluorescence emission of Py-OH (10 mM),
Py-SBA-15 only (20 mg Lꢁ1) and Py-SBA-15 with the addition
of Hg2+ (10 mM) in aqueous solution as a function of pH is
shown in Fig. 6. When pH was below 5, the fluorescence
intensity of Py-OH increased, which can be explained that the
protonation of nitrogen atoms inhibited the electron transfer
from nitrogen atoms to pyrene molecular to quenching of
the pyrene emission. However, when Py-OH was grafted into
the SBA-15, the fluorescent intensity varied less than 5% in the
pH range from 3.0 to 8.0, so the effect of pH on the
luminescence of the Py-SBA-15 can be negligible. This may
be due to the protective effect on pH variation of silica
network structure. In the presence of Hg2+, there was an
To further prove the complexation sensor mechanism of
Py-SBA-15 to Hg2+ ions, the complexing and photophysical
properties of Py-OH were researched. Fig.
8 showed
the complexation by Hg2+ induced a strong fluorescence
enhancement of Py-OH in an acetonitrile–water mixture
(7 : 3 v/v). Job’s plot analysis was conducted to determine
the binding stoichiometry of the Py-OH–Hg2+ complex, by
maintaining the total Py-OH and Hg2+ ions constant (50 mM)
Fig. 5 Scanning electron micrographs of Py-SBA-15 and TEM
images (c) in the direction of the pore axis and (d) in the direction
perpendicular to the pore axis.
Fig. 6 pH dependent fluorescence response of Py-OH (K), Py-SBA-15
only (’, 20 mg Lꢁ1) and Py-SBA-15 (m) with the addition of Hg2+
(10 mM) in aqueous solution. lex = 350 nm and lem = 454 nm.
ꢂc
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Scheme 2 Sensor mechanism of Py-SBA-15 to Hg2+ ions.
Fig. 8 Emission spectra of Py-OH in acetonitrile–water mixture
(7 : 3 v/v) upon additions of Hg2+. lex = 350 nm.
Fig. 7 Emission spectra of Py-SBA-15 (20 mg Lꢁ1) in the presence of
increasing concentration of Hg2+ in water. lex = 350 nm.
and the mole fraction of Py-OH changing from 0 to 1.22 The
fluorescence emission was measured and plot of fluorescence
intensity versus the molar fraction of Py-OH was shown in
Fig. 9. It is showed that the fluorescence intensity goes through
a maximum at a molar fraction of about 0.5 from Job’s plot of
Py-OH, indicating that a 1 : 1 stoichiometry is most possible
for the binding mode of Hg2+ and Py-OH.
The plot of fluorescence intensity of Py-SBA-15 against the
concentration of added Hg2+ is depicted in Fig. 10. A good
linearity between I/I0 and concentration of Hg2+ in the range
of 10ꢁ5 to 10ꢁ7 mol Lꢁ1 is obtained with a linearly dependent
coefficient R2 of 0.9989. The detection limit of this hybrid
chemosensor for monitoring Hg2+ ion is calculated to be
about 1.7 ꢀ 10ꢁ7 g mLꢁ1 using 20 mg Lꢁ1 Py-SBA-15 in the
Fig. 9 Job’s plot for determining the stoichiometry of Py-OH and
Hg2+ ions. (The total concentration of Py-OH and Hg2+ ions was
50 mM) lex = 350 nm and lem = 454 nm.
water. On the basis of 1 : 1 stoichiometry and fluorescence
Hg2+
concentration of 1 ꢀ 10ꢁ3 M, to evaluate the selectivity of the
chemosensor Py-SBA-15 towards Hg2+ ions. It was found
from Fig. 11 that these metal ions result in the slight change of
the fluorescence intensity at 454 nm of Py-SBA-15. However, a
dramatic increase in fluorescence is detected upon the addition
of Hg2+ ions of 1 ꢀ 10ꢁ5 M, much difference from the other
metal ions. These results indicate that the hybrid sensor
titration data (Fig. 10), the association constant (Ka
)
of Py-SBA-15 with Hg2+ ions in water was found to be
5.4 ꢀ 105 Mꢁ1 22
.
In order to examine the selectivity of Py-SBA-15, the other
biologically and environmentally important cation species,
such as Ag+, Na+, K+, Ca2+, Mg2+, Ba2+, Zn2+, Cd2+
,
Cu2+ and Pb2+ ions, were used as the competitive ions, at the
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Fig. 12 Time-dependent fluorescence response of Py-SBA-15
(20 mg Lꢁ1) upon addition of 50 ml of Hg2+ (1.0 mM), followed by
the addition of 100 mM of EDTA (sodium salt). lem = 454 nm.
Fig. 10 The plot of normalized fluorescence response I/I0 of
Py-SBA-15 (20 mg Lꢁ1) upon additions of Hg2+ from 0.5 to 20 mM
in water. lex = 350 nm and lem = 454 nm.
with increasing concentrations of Hg2+ ions, which contributed to
the complexation of Py-SBA-15 with a Hg2+ ion, resulted
in the lone pair electrons of the nitrogen no longer participate
in the quenching process, causing the recovery of the fluorescence.
material Py-SBA-15 can recognize Hg2+ ions from the metal
ions (even those that exist in high concentration) mentioned
above, which is similar to that of Py-OH (see ESI, Fig. S2w).
This result enables this solid fluorescence chemosensor a wide
potential application in environment protection.
A
good linearity between the fluorescence intensity of
Py-SBA-15 and the concentration of Hg2+ ions is constructed.
This novel mesoporous material Py-SBA-15 demonstrates the
high selectivity for Hg2+ ions in presence of other metal ions
(even those that exist in high concentration) with the lowest
detection limit of 1.7 ꢀ 10ꢁ7 g mLꢁ1. Furthermore, the sensing
detection of Hg2+ ions is very fast and reversible. The
combined physicochemical properties and sensing per
formance of Py-SBA-15, including regenerative ability, response
time, selectivity, stability, as well as the low detection limits,
indicating that this hybrid martial could be a promising
fluorescence chemosensor for detecting Hg2+ ions.
The regenerative ability of the hybrid optical sensors is of
particular interest in developing recyclable chemosensing
material, highlighting their important characteristic features
and desirability for industrial applications.23 In this work, the
regenerative ability is clearly demonstrated in Fig. 12. After
adding specific concentration of EDTA, the fluorescence
intensity decreases instantly because EDTA removes the
Hg2+ ions, which further affirming the complexation sensor
mechanism of Py-SBA-15 to Hg2+ ions. The fluorescence is
quenching by the lone pair electrons of the nitrogen atoms
under this condition. Furthermore, the response time of only a
few seconds is observed upon addition Hg2+ ions.
Acknowledgements
Conclusions
The authors gratefully thank the financial supports of the
National Natural Science Foundations of China (Grant No.
50872130).
In summary, a novel hybrid mesoporous sensing material
Py-SBA-15 for detecting Hg2+ ions in water is designed and
synthesized by covalent grafting organic fluorescent molecule
Py-OH within the channel of SBA-15. As fluorescence
enhancement-based optical sensor, Py-SBA-15 was efficiently
recognized by Hg2+ ions and fluorescence intensities increased
References
1 G. K. Walkup and B. Imperali, J. Am. Chem. Soc., 1996, 118,
3053–3054; A. Miyawaki, J. Llopis, R. Helm, J. M. McCaffery,
J. A. Adams, M. Ikura and R. Y. Tsien, Nature, 1997, 388,
882–887; M. M. Henary and C. J. Fahrni, J. Phys. Chem. A,
2002, 106, 5210–5220; J. Homola, S. S. Yee and G. Gauglitz, Sens.
Actuators, B, 1999, 54, 3–15; F. Turner, Science, 2000, 290,
1315–1317; P. Chen and C. He, J. Am. Chem. Soc., 2004, 126,
728–729; P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001,
40, 486–516; M. Schmittel and H. W. Lin, Angew. Chem., Int. Ed.,
2007, 46, 893–896; H. Komatsu, T. Miki, D. Citterio, T. Kubota,
Y. Shindo, Y. Kitamura, K. Oka and K. Suzuki, J. Am. Chem.
Soc., 2005, 127, 10798–10799; D. C. Magri, G. J. Brown,
G. D. McClean and A. P. de Silva, J. Am. Chem. Soc., 2006,
128, 4950–4951; T. Gunnlaugsson, J. P. Leonard and
N. S. Murray, Org. Lett., 2004, 6, 1557–1560; J. H. Popline,
R. P. Swatloski, J. D. Holbrey, S. K. Spear, A. Metlen,
M. Gratzel, M. K. Nazeeruddin and R. D. Rogers, Chem.
¨
Commun., 2007, 2025–2027; J. Wang and X. Qian, Chem.
Commun., 2006, 109–111.
2 E. Curdova, L. Vavruskova, M. Suchanek, P. Baldrian and
J. Gabriel, Talanta, 2004, 62, 483–487; C. L. He, F. L. Ren,
X. B. Zhang and Z. X. Han, Talanta, 2006, 70, 364–369.
Fig. 11 Normalized fluorescence response I/I0 of Py-SBA-15 (20 mg
ꢁ1) in the aqueous solution in the presence of various metal ions
(1 ꢀ 10ꢁ3 M). lex = 350 nm and lem = 454 nm.
L
ꢂc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010
1952 | New J. Chem., 2010, 34, 1946–1953

View Online
3 X. Wang, C. Drew, S. H. Lee, K. J. Senecal, J. Kumar and
L. A. Samuelson, Nano Lett., 2002, 2, 1273–1275; R. Metivier,
I. Leray and B. Valeur, Chem.–Eur. J., 2004, 10, 4480–4490;
E. Palomares, R. Vilar and J. R. Durrant, Chem. Commun.,
2004, 362–363; J. Liu and Y. Lu, J. Am. Chem. Soc., 2004, 126,
12298–12305; J. Liu and Y. Lu, Anal. Chem., 2004, 76, 1627–1632;
J. M. Nam, C. S. Thaxton and C. A. Mirkin, Science, 2003, 301,
1884–1886.
S. Dai, M. C. Burleigh, Y. Shin, C. C. Morrow, C. E. Barnes and
Z. Xue, Angew. Chem., Int. Ed., 1999, 38, 1235–1239; A. Stein,
B. J. Melde and R. C. Schroden, Adv. Mater., 2000, 12, 1403–1419.
11 E. Kim, H. E. Kim, S. J. Lee, S. S. Lee, M. L. Seo and J. H. Jung,
Chem. Commun., 2008, 3921–3923; J. Wang, L. Huang, M. Xue,
L. Liu, Y. Wang, L. Gao, J. Zhu and Z. Zou, Appl. Surf. Sci., 2008,
254, 5329–5335; R. Metivier, I. Leray, B. Lebeau and B. Valeur,
´
J. Mater. Chem., 2005, 15, 2965–2973.
4 M. M. F. Choi and D. Xiao, Anal. Chim. Acta, 1999, 387, 197–205.
5 S. H. Lee, J. Kumar and S. K. Tripathy, Langmuir, 2000, 16,
10482–10489; X. Wang, S. H. Lee, B. C. Ku, L. A. Samuelson and
J. Kumar, J. Macromol. Sci., Part A: Pure Appl. Chem., 2002, 39,
1241–1249; X. Wang, C. Drew, S. H. Lee, K. J. Senecal, J. Kumar
and L. A. Samuelson, J. Macromol. Sci., Part A: Pure Appl. Chem.,
2002, 39, 1251–1258.
12 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson,
B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548–552.
13 B. F. Lei, B. Li, H. R. Zhang, S. Z. Lu, Z. H. Zheng, W. L. Li and
Y. Wang, Adv. Funct. Mater., 2006, 16, 1883–1891.
14 T. Lancaster, S. Lee and J. Ying, Chem. Commun., 2005,
3577–3579; R. Clarke and I. Shannon, Chem. Commun., 2001,
1936–1937.
6 S. H. Kim, K. C. Song, S. Ahn, Y. S. Kang and S. K. Changa,
Tetrahedron Lett., 2006, 47, 497–500.
7 C. Rottman, M. Ottolenghi, R. Zusman, O. Lev, M. Smith,
G. Gong, M. L. Kagan and D. Avnir, Mater. Lett., 1992, 13,
293–298.
8 J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz,
C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson,
E. W. Sheppard, S. B. McCullen, J. B. Higgins and
J. L. Schlenker, J. Am. Chem. Soc., 1992, 114, 10834–10843;
C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and
J. S. Beck, Nature, 1992, 359, 710–712.
9 G. Wirnsberger, B. J. Scott and G. D. Stucky, Chem. Commun.,
2001, 119–120; N. Nivarlet, S. Martinquet and B. L. Su, Stud. Surf.
Sci. Catal., 2007, 165, 873–876; T. Balaji, M. Sasidharan and
H. Matsunaga, Analyst, 2005, 130, 1162–1167; J. V. Ros-Lis,
15 J. L. Liu and B. Yan, J. Phys. Chem. B, 2008, 112, 10898–10907.
16 T. Tien and L. K. Chau, Chem. Mater., 1999, 11, 2141–2147.
17 B. Marler, U. Oberhagemann, S. Voltmann and H. Gies,
Microporous Mesoporous Mater., 1996, 6, 375–383; A. Vinu,
V. Murugesan and M. Hartmann, J. Phys. Chem. B, 2004, 108,
7323–7330; D. P. Quintanilla, I. Hierro, M. Fajardo and I. Sierra,
J. Mater. Chem., 2006, 16, 1757–1764.
18 L. N. Sun, J. B. Yu, H. J. Zhang, Q. G. Meng, E. Ma, C. Y. Peng
and K. Y. Yang, Microporous Mesoporous Mater., 2007, 98,
156–165.
19 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou,
R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl.
Chem., 1985, 57, 603–619; M. Kruk and M. Jaroniec, Chem.
Mater., 2001, 13, 3169–3183; W. H. Zhang, X. B. Lu, J. H. Xiu,
Z. L. Hua, L. X. Zhang, M. Robertson, J. L. Shi, D. S. Yan and
J. D. Holmes, Adv. Funct. Mater., 2004, 14, 544–552.
20 Z. Y. Wang, F. X. Zhang, Y. L. Yang, B. Xue, J. Cui and
N. J. Guan, Chem. Mater., 2007, 19, 3286–3293.
R. Casasu
R. Martınez-Ma
Haskouri, N. Garro
´
s, M. Comes, C. Coll, M. Dolores Marcos,
nez, F. Sancenon, J. Soto, P. moros, J. El
and K. Rurack, Chem.–Eur. J., 2008, 14,
´
´
´
´
´
8267–8278; M. H. Lee, S. J. Lee, J. H. Jung, H. Lim and J. S. Kim,
Tetrahedron, 2007, 63, 12087–12092; W. Yantasee, G. E. Fryxell,
M. M. Conner and Y. Lin, J. Nanosci. Nanotechnol., 2005, 5,
1537–1540; E. Kim, H. J. Kim, D. R. B, S. J. Lee, E. J. Cho,
M. R. Seo, J. S. Kim and J. H. Jung, New J. Chem., 2008, 32,
1003–1007.
21 S. R. Davidson, Adv. Phys. Org. Chem., 1983, 19, 1–130.
22 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer
Academic/Plenum, New York, 1999; W. A. E. McBryde, Talanta,
1974, 21, 979–1004; W. D. Likussar and D. F. Boltz, Anal. Chem.,
1971, 43, 1265–1272.
23 K. A. Connors, Binding Constants—The Measurement of
Molecular Complex Stability, John Wiley & Sons, New York,
1987, ch. 4.
10 L. Gao, Y. Wang, J. Wang, L. Huang, L. Shi, X. Fan, Z. Zou,
T. Yu, M. Zhu and Z. Li, Inorg. Chem., 2006, 45, 6844–6850;
ꢂc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010 New J. Chem., 2010, 34, 1946–1953 | 1953