A novel fluorogenic hybrid material for selective sensing of thiophenols

Article abstract of DOI:10.1039/c1jm11768h

A fluorogenic organic-inorganic hybrid sensory material (denoted as material III) was successfully prepared by immobilizing disulfide-based receptors within the channels of mesoporous silica MCM-41. The material showed high sensitivity and excellent selectivity towards thiophenols due to appropriate combination of the stronger nucleophilicity of thiophenols and the secondary interactions (hydrophobicity and π-π stacking interactions) provided by the inorganic support. Fluorescence measurements revealed that the emission intensity of material III increased significantly upon addition of thiophenols, while it was not observed for aliphatic thiols and other nucleophiles. The detection limit of this material to p-toluenethiol was estimated to be 7.4 × 10^-6 M, and a good linear relationship between the fluorescence intensity and p-toluenethiol concentration (in the range of 0-110 μM) was achieved. The pronounced emission enhancement could be attributed to the following conversion process: a coumarin derivative is released from material III by cleaving the disulfide bonds in the presence of thiophenols, and then oxidized spontaneously to a fluorescent dye (coumarin 6) under ambient conditions. This novel sensing method via disulfide-based groups as the active site combining with the synergistic effect provided by the inorganic support is expected to open a new avenue for thiophenols sensing.

Full text of DOI:10.1039/c1jm11768h

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PAPER
A novel fluorogenic hybrid material for selective sensing of thiophenols†
a
a
a
a
b
Wenwen Zhao, Weimin Liu, Jiechao Ge, Jiasheng Wu, Wenjun Zhang, Xiangmin Menga
*
a
*
and Pengfei Wang
Received 22nd April 2011, Accepted 17th June 2011
DOI: 10.1039/c1jm11768h
A fluorogenic organic-inorganic hybrid sensory material (denoted as material III) was successfully
prepared by immobilizing disulfide-based receptors within the channels of mesoporous silica MCM-41.
The material showed high sensitivity and excellent selectivity towards thiophenols due to appropriate
combination of the stronger nucleophilicity of thiophenols and the secondary interactions
(hydrophobicity and p–p stacking interactions) provided by the inorganic support. Fluorescence
measurements revealed that the emission intensity of material III increased significantly upon addition
of thiophenols, while it was not observed for aliphatic thiols and other nucleophiles. The detection limit
of this material to p-toluenethiol was estimated to be 7.4 ꢀ 10ꢁ6 M, and a good linear relationship
between the fluorescence intensity and p-toluenethiol concentration (in the range of 0–110 mM) was
achieved. The pronounced emission enhancement could be attributed to the following conversion
process: a coumarin derivative is released from material III by cleaving the disulfide bonds in the
presence of thiophenols, and then oxidized spontaneously to a fluorescent dye (coumarin 6) under
ambient conditions. This novel sensing method via disulfide-based groups as the active site combining
with the synergistic effect provided by the inorganic support is expected to open a new avenue for
thiophenols sensing.
to their large surface area, high thermal stability and specific
Introduction
structure of channels. Thus far, there have been many reports
on cation sensing via grafting fluorescent molecules on the
surface or into the nanochannels of mesoporous silica.13–17
However, only limited examples of such heterogeneous
sensors for detecting anions or neutral species have been
shown.18–20
Development of sensing techniques for chemical species by
employing sophisticated molecular recognition systems has
been one of the most active areas in supramolecular chem-
istry. Among them, chromogenic and fluorogenic sensors for
detecting various cations, anions, and neutral species have
attracted great attention due to their high sensitivity, selec-
tivity, as well as their on-line and real-time analysis capabil-
ities.1–5 In recent years, organic-inorganic hybrids prepared
generally by immobilizing organic receptors on inorganic
supports of nanometre scales have been a class of extremely
promising materials for molecular recognition.6–12 Structurally
well-defined mesoporous silica materials such as MCM-41 and
SBA-15 are among the most charming solid supports owing
Recently, by utilizing the predefined chemical binding center
together with the selected properties of the mesoporous silica, an
enhanced selectivity to detect guest species with similar molec-
ular structures has been reported.21 This approach is especially
attractive due to the employment of molecular-solid synergistic
effects that are difficult to achieve by only molecular-based
systems or in nanoscopic solids. In the reported sensory systems,
mesoporous supports could provide some additional interac-
tions, such as size-sieving effect, hydrophobic/hydrophilic
property, p–p interaction, or their combined effect.18,22–24 As
a prominent enhancement of selectivity based on the synergistic
effect, Lin et al. designed and synthesized the OPTA-derived
mesoporous silica materials, and demonstrated that the substrate
accessibility could be modulated simultaneously by creating
a different pore micro-environment. As a result, its selectivity to
the similar bio-molecules such as dopamine and glucosamine was
aKey Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing, 100190, P. R. China. E-mail: wangpf@
mail.ipc.ac.cn; Tel: +010-82543435
bCenter of Super-Diamond and Advanced Films (COSDAF) and
Department of Physics and Materials Sciences, City University of Hong
Kong, Hong Kong SAR, P. R. China. E-mail: apwjzh@cityu.edu.hk
† Electronic supplementary information (ESI) available: Preparation of
material I; Synthesis and characterization of compounds 2 and 3;
Analysis of the TGA of materials; Calculation of the detection limit
and measurements of the response time for material III towards
p-toluenethiol; ESI-MS and 1H NMR spectrum analysis to prove the
formation of coumarin 6. See DOI: 10.1039/c1jm11768h
enhanced.22 Martınez-Manez et al. also prepared several sensory
materials for the selective detection of long-chain carboxylates,
specific amines,24,25 and ATP (adenoisine 50-triphosphate)18,26
among their analogues by coupling the selected organic
ꢀ
ꢀ~
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molecules with the additional interactions provided by meso-
porous solids.
Experimental
Reagents and materials
Herein, we report a new organic–inorganic hybrid sensory
system (material III as shown in Scheme 1a) which enables
a highly selective detection of thiophenols over aliphatic thiols
and other nucleophiles. Thiophenols have been widely used as
a chemical intermediate for pesticides, pharmaceuticals, and
amber dyes.27–30 Long-term exposure to thiophenols can cause
severe damage to the central nervous and other nervous systems
including shortness of breath, muscular weakness, paralysis of
the hind limbs, coma, and even death.31–33 The median lethal
dose (LC50) of thiophenols ranges from 0.01 to 0.4 mM for
fish,34 and thiophenol has been added to the priority lists of
pollutants by the United States Environmental Protection
Agency (USEPA) (EPA waste code: P014).35,36 Although
a number of fluorescent probes for sensing thiols have been
reported,37–44 they exhibit poor selectivity towards thiophenols
and aliphatic thiols. Thus far, there are only few examples of
fluorescent probes showing high selectivity toward thio-
phenols,45–47 and they are all built with relatively simple
molecular systems. Therefore, it is significant to develop new
sensory systems to discriminate thiophenols over aliphatic thiols
in environmental analysis. In our current approach, the disul-
fide-based group is selected as the receptor which can be selec-
tively cleaved by thiol-containing compounds.39,48–50 We expect
that anchoring of this unique receptor on the pore surface of
mesoporous silica MCM-41 could provide some additional
interactions in the solid surface, including hydrophobic and p–
p stacking interactions, which could further enhance its sensi-
tivity and selectivity to thiophenols. The experimental results
demonstrate that addition of thiophenols results in a significant
emission enhancement of material III, while there is no gain for
aliphatic thiols and other nucleophiles under the same condi-
tions. To illustrate clearly the necessity of the solid supporting
matrix, model compound 4 (Scheme 1b) was also synthesized
for a comparable illustration of its fluorescent response towards
p-toluenethiol.
2,20-Dipyridyl disulfide, 2-aminothiophenol, tetraethoxysilane
(TEOS), cetyltrimethylammonium bromide (CTAB), 3-mercap-
topropyltrimethoxysilane (MPTES), and all analytical chem-
icals, including cysteine, gluotathione, glycine, phenol, aniline,
n-propylmercapatan, p-nitrothiophenol, p-chlorobenzenethiol,
p-methoxythiophenol and p-toluenethiol, were purchased from
Alfa Aesar. Other reagents were purchased from Beijing Chem-
ical Regent Co. All reagents and chemicals were AR grade and
used directly without further purifications unless otherwise
noted. Ethanol used for measurement is of chromatographic
grade, and water was purified by Millipore filtration system. The
metal cations for interference determination were all of
perchlorate salts.
Instruments and spectroscopic measurements
1H NMR and 13C NMR spectra were obtained on a Bruker
Advance 400 NMR spectrometer using tetramethylsilane (TMS)
as an internal reference. Electron impact (EI) mass spectra were
recorded on a Waters GCT Premier mass spectrometer and
electrospray ionization (ESI) mass spectra on a Shimadzu LC-
MS2010 instrument. FT-IR spectra (4000–400 cmꢁ1) were
collected on a Varian Excalibur 3100 FTIR spectrometer.
Thermogravimetric analysis of the samples was carried out on
a TA TGA 2050 with a nitrogen flow of 100 mL minꢁ1 at a rate of
10 ꢂC minꢁ1 from room temperature to 900 ꢂC. Elemental
analysis (C, H, N) was performed on an Elementar Vario EL III
analyzer. Small-angel XRD measurements were performed on
a X’Pert PRO X-ray diffractometer using Cu-Ka radiation (l ¼
ꢁ
1.5418 A) at 40 kV and 10 mA. TEM spectra were recorded on
a JEOL JEM-2100 at an acceleration voltage of 150 kV. The
nitrogen adsorption-desorption isotherms were measured at 77
K on a QuadraSorb SI automated surface area and pore size
analyzer. And the samples were degased under vacuum at 423 K
Scheme 1 Structures of a) material III and b) model compound 4, and c) the proposed sensing mechanism for material III towards thiophenols.
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for 4 h before measurements. Surface areas were calculated based
on the Brunauer-Emmett-Teller method, and the pore size
distribution were calculated using the Barrett–Joyner–Halenda
(BJH) model on the desorption branch. The pore volumes were
taken at the P/P0 z 0.99. UV-visible spectra and fluorescence
spectra were obtained with Hitachi U-3010 and F-4500 spec-
trophotometers, respectively. All spectrophotometric spectra of
hybrid sensory material III, including fluorescence titration and
the selectivity, were performed with a suspension of sample
dispersed in a citric acid (0.1 M)-phosphate (0.2 M) buffer (pH 3,
ethanol/water ¼ 1/4, v/v), and all data were acquired at 30 min
after the addition of target analytes. (Fig. S1†)
colorless solution turned to yellow immediately after the addition
of 2-aminothiophenol due to the formation of pyridine-2-thione.
The mixture was stirred at room temperature for 3 h. After
removal of the solvent by rotary evaporation, the residue was
dropped into 50 mL of petroleum ether under vigorous ultra-
sonic vibration, and then was filtered. The filtrate was evaporated
to produce the crude product, which was purified by column
chromatography on silica gel (Eluent: dichloromethane/ethyl
acetate ¼ 100/2, v/v) to give 1 (1.1 g, 47%) as a yellowish liquid.
1H NMR (CDCl3, 400 MHz, d): 8.51 (d, 1H, pyridyl-H), 7.49–
7.58 (m, 3H, pyridyl-H), 7.08–7.16 (m, 2H, Ar-H), 6.61–6.70 (m,
2H, Ar-H), 4.92 (br, 2H, NH2). 13C NMR (CDCl3, 100 MHz, d):
159.0, 149.6, 149.2, 136.8, 136.2, 131.6, 121.1, 118.2, 118.0, 115.7.
MS (ESI, m/z): [M + H]+, calcd for C11H10N2S2, 234.0, found,
235.0.
Synthesis of material I
Material I was prepared according to reported procedures.51 The
details for the synthetic process are described in the ESI†.
Synthesis of 2 and 3
The processes for synthesis of organosilane 2 and compound 3
were similar to the process for compound 1 described above. The
details for the synthetic process and characterization are
described in the ESI†.
Synthesis of the amine-functionalized material II
Material II was synthesized by co-condensation under basic
conditions using CTAB as surfactant templates, TEOS and
organosilane 2 as co-condensed precursors. The molar ratio of
reagents was CTAB : NaOH : TEOS : organosilane 2 : H2O ¼
1.0 : 4.9 : 10.1 : 0.97 : 9356.8. As a typical procedure, CTAB
(0.1 g) was dissolved in deionized water (48 g), and then NaOH
(0.7 mL, 2 M) was added with vigorous stirring at room
Synthesis of (E)-7-(diethylamino)-3-((2-(propyldisulfanyl)
phenylimino)methyl)-2H-chromen-2-one (4)
Compound 4 was synthesized from a condensation reaction of
compound 3 and 3-formyl-coumarin. A mixture of 3-formyl-
coumarin (98 mg, 0.4 mmol) and 3 (0.12 g, 0.6 mmol) in ethanol
(10 mL) with glacial acetic acid (1 mL) was refluxed for 8 h, and
then cooled to room temperature. The dark red precipitates were
collected, washed with cold ethanol, and purified by recrystalli-
zation from ethanol to yield 68 mg (40%) of 4 as a wine red solid.
m.p: 128–130 ꢂC. 1H NMR (DMSO-d6, 400 MHz, d): 8.56 (s, 1H,
coumarin-H), 8.50 (s, 1H, CH]N), 7.76 (dd, 1H, J ¼ 7.5 Hz, J ¼
1.0 Hz, benzo-H), 7.73 (d, 1H, J ¼ 8.9 Hz, coumarin-H), 7.28 (m,
2H, benzo-H), 7.16 (dd, 1H, J ¼ 7.4 Hz, J ¼ 1.0 Hz, benzo-H),
6.82 (dd, 1H, J ¼ 2.2 Hz, J ¼ 8.9 Hz, coumarin-H), 6.62 (d, 1H,
J ¼ 2.1 Hz, coumarin-H), 3.50 (q, 4H, J ¼ 7.0 Hz, 2NCH2), 2.73
(t, 2H, J ¼ 7.2 Hz, SCH2), 1.63(m, 2H, SCH2CH2), 1.16 (t, 6H,
J ¼ 7.0 Hz, 2NCH2CH3), 0.93 (t, 3H, J ¼ 7.2 Hz, SC2H4CH3),
13C NMR (CDCl3, 100 MHz, d): 162.4, 157.9, 154.4, 152.2, 148.8,
142.0, 133.1, 131.3, 126.8, 126.8, 125.4, 117.8, 115.2, 109.8, 109.1,
97.3, 45.2, 40.8, 22.6, 13.3, 12.6. MS (HR-EI, m/z): [M]+, calcd
for C23H26N2O2S2, 427.1436, found, 427.1440.
ꢂ
temperature. After the solution was heated at 80 C for 30 min,
a mixture of TEOS (0.6 g) and organosilane 2 (0.1 g) was poured
into the solution while stirring vigorously. A yellowish precipi-
tate was observed after 2 min and the mixture was continuously
stirred for 2 h. After cooling to room temperature, the white solid
products were isolated by filtration, washed thoroughly with
water, and dried under vacuum for 4 h at 40 ꢂC. The surfactant
was removed by acid/solvent extraction in a methanol (15 mL)
containing aqueous HCl (0.15 mL, 37.5%) solution. The mixture
was stirred at room temperature for 24 h, washed with methanol
several times after being filtered, and then dried in a vacuum.
Elemental analysis, found: N, 1.545%, C, 19.35%, H 2.910%.
Synthesis of the fluorophore-grafted material III
Material III was prepared by anchoring the fluorophore to
material II. 3-Formyl-coumarin (24.5 mg, 0.1 mmol) and mate-
rial II (0.1 g) were dissolved in ethanol (10 mL). The color of the
solution turned to bright red from light yellow immediately. The
mixture was stirred for 8 h at room temperature. Then the
resulting red solid was collected by centrifugation and washed
with ethanol for about ten times to remove unreacted 3-formyl-
coumarin. Elemental analysis, found: N, 1.301%, C, 15.24%, H,
2.685%.
Results and discussion
Synthesis and characterization of materials I–III
The synthetic pathway is shown in Scheme 2. To explore the
characters of the organic-inorganic hybrid sensory material III,
we prepared a typical MCM-41-type material I as a contrast
according to the literature, as shown in Scheme 2a.51 Amine-
functionalized material II was synthesized from the co-conden-
sation of tetraethoxysilane (TEOS) and organosilane 2. The co-
condensation method allows the organic functional groups to be
anchored homogeneously into the mesopore channels,52,53 which
is beneficial to the creation of a unique pore micro-environment.
Finally, by the reaction of the amine groups of material II to the
Synthesis of 2-(pyridin-2-yldisulfanyl)benzenamine (1)
Compound 1 was synthesized from a thiol-disulfide exchange
reaction. In a typical preparation process, 2,20-dipyridyl disulfide
(2.2 g, 10 mmol) was dissolved in dry CH2Cl2 (30 mL) and
a solution of 2-aminothiophenol (1.5 mL, 12 mmol) in dry
CH2Cl2 (15 mL) was injected dropwise at room temperature
within about 40 min under nitrogen atmosphere. The initially
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stretching and bending vibrations, suggesting the successful
functionalization of material II with compound 2 by the co-
condensation method. For material III, in addition to C–H
vibrations bands at 2800–3000 and 1300–1500 cmꢁ1, the C]N
and C]O stretching vibrations at 1607 and 1708 cmꢁ1, respec-
tively, and the peaks at 1574 and 1532 cmꢁ1 originating probably
from the C]C stretching of the phenyl ring, are also observed.
The above results provide a clear evidence that 3-formyl-
coumarin is indeed attached to the pore surface of material II.
The content of incorporated coumarin-residues was determined
by thermogravimetric analysis (TGA) and elemental analysis
(EA). According to the EA results, material III was estimated to
contain approximately 3.91 wt.% of coumarin-residues, which
agreed well with the content from thermogravimetric analysis
(Fig. S2†). As a result, 100 ppm material III in aqueous solution
corresponds to about 1.8 ꢀ 10ꢁ5 M of receptor.
Fig. 2 presents small-angle X-ray diffraction (SAXRD)
patterns of materials I–III. Three well-resolved diffraction peaks
of 100, 110, 200 and a weak peak of 210 were observed in
material I, suggesting a 2-dimensional (2D) hexagonal symmetry
(p6mm) and a long-range ordering of the mesoporous structure.59
For the functionalized material II, the similar diffraction peaks
at 100, 110 and 200 remain observable, indicating the same 2D
hexagonal mesoporous structure of material II synthesized by
the co-condensation method. Compared with material II, the
diffraction peak 100 of material III keeps its original shape,
indicating that the 2D-hexagonal ordering has been retained
after grafting with the fluorophore. However, obvious decreases
in intensity of all the peaks were observed in material III. Such
a decrease might be ascribed to the lowering of local mesoporous
ordering, such as the variations in the wall thickness, or it might
be due to the reduction of scattering contrast between the
channel walls and pore-filling material.52 Thus, it can be under-
stood as an indirect evidence for the successful coupling of flu-
orophores to the aniline groups inside the channels.
Scheme 2 Synthetic pathways for materials I, II, III, and model
compound 4. (i) 2-aminothiophenol, CH2Cl2, rt, 4 h; (ii) 3-mercapto-
propyltriethoxysilane, CH2Cl2, rt, 4 h; (iii) TEOS, H2O/NaOH (2 M, aq),
80 ꢂC, 2 h; (iv) HCl/MeOH, rt, 24 h; (v) 3-formyl-coumarin, EtOH/
HOAc, 60 ꢂC, 8 h; (vi) CH3(CH2)2SH, CH2Cl2, rt, 4 h; (vii) 3-formyl-
coumarin, EtOH, rt, 8 h.
aldehyde groups of 3-formyl-coumarin, as illustrated in Scheme
2b, material III was obtained as a bright red solid. Moreover,
model compound 4 (Scheme 1b) was designed and synthesized
via a two-step thiol-disulfide exchange reaction (Scheme 2c).54,55
Compound 2 was synthesized following the similar procedure of
that of compound 4 and the fluorophore 3-formyl-coumarin was
prepared according to the literature.56 Further details for
synthesizing materials I–III and the compounds 1–4 are
described in the experimental section or ESI†.
Fourier transform infrared (FTIR) spectra of materials I–III
are shown in Fig. 1. A peak at 1632 cmꢁ1 is observed in all the
spectra, which is ascribed to the –OH bending vibration of
adsorbed H2O.57 The typical Si–O–Si band in materials I–III
appears as three peaks: one broad and strong peak centered at
1084 cmꢁ1, and two narrow and relatively weak peaks near 801
and 468 cmꢁ1, which are associated with the condensed silica
network.58 Compared with the FTIR spectrum of material I,
material II exhibits several new peaks at about 2929, 2856, 1484,
1447, and 1411 cmꢁ1, which are assignable to aliphatic C–H
Fig. 3a displays the nitrogen adsorption-desorption isotherms
for samples II and III. The isotherm of material II shows two
sharp capillary condensation steps (at intermediate and high
P/P0 values) related to a bimodal pore system. The first step
originates from the condensation of the nitrogen into the meso-
pores and the second one from the condensation inside the large
Fig. 1 FTIR spectra of materials I, II, and III.
13564 | J. Mater. Chem., 2011, 21, 13561–13568
Fig. 2 SAXRD patterns of materials I, II, and III.
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Fig. 4 TEM images of a) material II, and b) material III. Insets are the
corresponding images obtained along the rod axis directions.
Fig. 3 a) N2 adsorption (solid)-desorption (hollow) isotherms, and b)
pore size distributions of materials II and III.
SAXRD result. After grafting of fluorophores (material III), the
image of lattice fringes and pore-ordering remain observable
(Fig. 4b), which confirms that hexagonal ordering has been
preserved after the introduction of fluorophores.
interparticle pores.23,59 Consequently, pore size distribution
(PSD) of this sample reveals bimodal pore diameters of ca. 2.21
and 32.22 nm, respectively (Fig. 3b). Comparing material III
with material II, the isotherm and PSD pattern are almost
retained, suggesting that the structural ordering of the channels
was unchanged after grafting. But the decrease in the peak
intensity and corresponding pore diameter (1.93 and 31.65 nm
for material III) clearly suggests that the chromophores are
grafted inside the mesopores, which agrees well with the obser-
vation in SAXRD. The corresponding structural parameters are
summarized in Table S1†. Comparing material III with II, the
Brunauer–Emmett–Teller (BET) surface area decreases from
621.0 to 270.3 m2 gꢁ1 and pore volume shrinks from 0.92 to
0.65 cm3 gꢁ1. The remarkable decreases in surface area and pore
volume also support the conclusion that the fluorophores are
indeed attached to mesopore surfaces and the void space is
occupied.
Fluorescence response of material III towards thiophenols
Fluorescence titration experiments were carried out to illustrate
the emission response of material III to the various concentra-
tions of p-toluenethiol, as shown in Fig. 5a. The emission peak
intensity at ca. 500 nm increases monotonically with a ‘‘red shift’’
with the increase of p-toluenethiol concentration. Fig. 5b shows
the dependence of fluorescence intensity at 512 nm on the
p-toluenethiol concentration. A linear relationship (R ¼ 0.994)
for the concentrations ranging from 0 mM to 110 mM was
observed (inset in Fig. 5b), implying that material III is very
promising in the quantitative analysis of thiophenols. Further-
more, the detection limit of material III for sensing p-toluenethiol
was determined to be as low as 7.4 ꢀ 10ꢁ6 M (Fig. S3†), which
can meet the practical requirement for detecting thiophenols on
the submillimolar level in environmental analyses.34
The microstructures of materials II and III were studied by
transmission electron microscopy (TEM), as shown in Fig. 4. It is
interesting to note that material II is in the morphology of heli-
calmesostructured rods. The image obtained with the incident
beam perpendicular to the pore axis illustrates the appearance of
lattice fringes, indicating the presence of chiral channels within
the helical rods.60,61 Observation along the pore axis direction
reveals well-aligned mesopores in a highly-ordered 2D-hexag-
onal symmetry (inset in Fig. 4a), which is well consistent with the
The proposed sensing mechanism of material III to p-tolue-
nethiol is depicted in Scheme 1c. Upon the addition of thio-
phenols, the disulfide bond is facilely cleaved by thiophenols and
a coumarin-based compound (2H-coumarin 6) is released to the
solution, which is spontaneously oxidized to a laser dye
(coumarin 6) under ambient conditions. To confirm the forma-
tion of coumarin 6, electrospray ionization (ESI) mass
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Fig. 6 Fluorescence emission intensity (lem ¼ 512 nm) of material III
(100 ppm) in the absence and presence of 170 mM p-toluenethiol in
ethanol/water (1 : 4, v/v) under different pH conditions. The excitation
wavelength was at 450 nm. The pH values were maintained by using the
following solutions: KCl (0.2 M)-HCl (0.2 M) buffer (pH 1.5), citric acid
(0.1M)-phosphate (0.2 M) buffer (pH 2.4–6), HEPES (0.01 M) buffer (pH
7 and 8), Tris-HCl (0.01 M) buffer (pH 9).
specific environmental monitoring of thiophenols, for instance in
highly acidic soils with pH values of 3–4.66
To evaluate the selectivity of our sensory system, the fluores-
cent response of material III to a variety of thiols including thi-
ophenols and aliphatic thiols and other common nucleophiles
were studied, as illustrated in Fig. 7a. It reveals that only the
addition of thiophenols such as p-chlorobenzenethiol,
p-methoxythiophenol, thiophenol and p-toluenethiol could result
in a prominent enhancement in the fluorescence intensity at 512
nm. In contrast, negligible variations in the emission intensity
were observed upon the addition of aliphatic thiols such as
cysteine, glutathione and n-propylmercapatan. As we know,
compared with aliphatic thiols, thiophenols have stronger
nucleophilicity and higher affinity to hydrophobic environments.
The stronger nucleophilicity could make thiophenols more
reactive in the reaction of the disulfide reduction. And the
hydrophobicity and p–p stacking interactions provided by
the mesopores maybe serve as significant forces to enhance the
accessibility of thiophenols over the aliphatic thiols. For other
common nucleophiles such as phenol and potassium iodide,
there are nearly no fluorescence responses due to the specific
reduction of disulfide by thiols. In addition, the emission
enhancement by p-nitrothiophenol is much smaller than those by
other thiophenols. In this case, the thiol nucleophilicity is
probably reduced by the electron-withdrawing nitro group, and
as a result its reactivity for the SNAr reaction is weakened. The
above observation suggests evidently that our sensory system of
material III displays a high selectivity for thiophenols.
Fig. 5 a) Fluorescence emission spectra (lex ¼ 450 nm) of material III (100
ppm) recorded with the addition of p-toluenethiol with different concen-
trations in a citric acid (0.1 M)-phosphate (0.2 M) buffer (pH ¼ 3, ethanol/
water ¼ 1/4, v/v). b) Fluorescence intensity at 512 nm against the concen-
tration of p-toluenethiol. Inset: a linear portion from 0 mM to 110 mM.
spectrometry was carried out to analyze the solution of material
III titrated with excess p-toluenethiol (Fig. S4a†). The peak at m/
z 351.2, corresponding to [coumarin 6 + H]+ (Calc. ¼ 351.1), was
clearly observed, indicating that coumarin 6 is released to the
solution from material III. The reaction product was further
isolated and identified to be coumarin 6 by 1H NMR (Fig. S4b†).
The conversion mechanism is similar to that reported in our
previous work.62
The effects of pH values on the sensing performance of our
sensing system were investigated. The fluorescence intensity at
512 nm for material III with and without p-toluenethiol as
a function of pH values was shown in Fig. 6. It is found that the
emission of material III is relatively stable for the pH values
ranging from 3 to 9. However, upon the addition of p-toluene-
thiol at a constant concentration of 170 mM, the pH value of the
solution affects the emission intensity significantly. The fluores-
cence enhancement drops a lot as the pH value varies from 3 to 9.
This phenomenon could be attributed to the catalysis effect of
H+, which accelerates the oxidization reaction of 2H-coumarin 6
to coumarin 6.62,63 On the other hand, the fluorescence
enahncement also decreases at pH < 3 (e.g., pH 1.5), this may be
because the 7-diethylamino group of coumarin derivatives is
protonized at very acidic pH value, which decreases the fluo-
rescence intensity of coumarin derivatives including coumarin
6.64,65 Therefore, to improve the signal/noise ratio of the sensing
system, a citric acid (0.1 M)-phosphate (0.2 M) buffer at pH 3
was chosen as the detection medium. The finding also implies
that material III is a particularly attractive chemosensor for some
High selectivity for the analyte in the presence of competing
species is a challenging task for practical applications. In this
work, we also conducted competition experiments to detect
p-toluenethiol that was mixed with cysteine, glutathione, n-pro-
pylmercapatan, glycine, aniline, phenol and potassium iodide, as
well as in a mixture of common metal ions such as K+, Ca2+, Na+,
Zn2+, and Cu2+, respectively. As shown in Fig. 7b, whether or not
the competing species exist, obvious emission intensity changes
are observed for material III upon the addition of p-toluenethiol,
13566 | J. Mater. Chem., 2011, 21, 13561–13568
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Fig. 8 Profiles of fluorescence intensity (lex ¼ 450 nm) variations at 512
nm of compound 4 (2 ꢀ 10ꢁ5 M) reacted with p-toluenethiol or cysteine
(Cys) with different concentrations or reaction time in a citric acid (0.1
M)-phosphate (0.2 M) buffer (pH 3, ethanol/water ¼ 1/4, v/v). (1) 170 mM
Cys after 30 min; (2) 170 mM p-toluenethiol after 30 min; (3) 170 mM Cys
after 10 h; (4) 170 mM p-toluenethiol after 10 h; (5) 1700 mM p-toluene-
thiol after 30 min; (6) 1700 mM p-toluenethiol after 10 h.
over the aliphatic thiols. All these studies show that the inorganic
support indeed presents the synegistic effects and efficiently
improves the sensitivity and selectivity of material III towards
thiophenols.
Fig. 7 a) Profiles of fluorescence intensity (lex ¼ 450 nm) variations of
material III upon the addition of 170 mM of different thiols and nucle-
ophiles. b) The selectivity of material III towards thiols and other
nucleophiles. Gray bar: the profiles of fluorescence intensity changes at
512 nm in a single analyte at 170 mM with material III (100 ppm); Black
bar: the profiles of fluorescence intensity changes of a mixture of
a nucleophilic reagent at 170 mM and p-toluenethiol at 170 mM with
material III (100 ppm). (1) material III only, (2) Cys, (3) GSH, (4)
n-propylmercapatan, (5) glycine, (6) aniline, (7) phenol, (8) potassium
iodide, (9) cations, (10) p-nitrothiophenol, (11) p-chlorobenzenethiol,
(12) p-methoxythiophenol, (13) thiophenol, (14) p-toluenethiol.
Conclusions
In summary, we have developed an organic-inorganic hybrid
chemosensor material (material III) to detect thiophenols based
on covalently immobilizing a disulfide-based receptor within the
mesopores of MCM-41. Comprehensive characterization tech-
niques demonstrate that the organic fluorophore is grafted inside
the mesopores. The material shows high sensitivity and excellent
selectivity towards thiophenols over aliphatic thiols and other
nucleophiles due to the synergistic effects of the stronger nucle-
ophilicity of thiophenols and the secondary interactions
provided by the inorganic support. The detection limit for
p-toluenethiol is determined to be as low as 7.4 ꢀ 10ꢁ6 M, which
fulfills the requirement for practical applications in the envi-
ronmental and biological analysis. In the sensing mechanism it
was suggested that the disulfide bond is facilely cleaved by thi-
ophenols, and a coumarin-based compound (2H-coumarin 6) is
released and then spontaneously oxidized to a laser dye
(coumarin 6) under ambient conditions. We expect that this work
may provide a strategy to design chemical and biological sensors
enabling highly selective detection of analytes.
which demonstrates that the detection of thiophenols by material
III is not affected in the presence of common competing species.
To further illustrate the role of the solid support, controlled
experiments using a small molecule 4 as a model compound
(Scheme 1b) were performed. Fig. 8 displays the fluorescence
response of 4 towards p-toluenethiol and cysteine (Cys) with
different concentrations or reaction time. Compound 4 shows
negligible response for neither p-toluenethiol nor Cys under the
same measuring condition as that of material III (pH 3, 170 mM,
30 min), while similar emission enhancements are observed for p-
toluenethiol or Cys by prolonging the reaction time to 10 h. On
the other hand, the reaction time could be shortened efficiently
upon the addition of p-toluenethiol with higher concentration
(1700 mM). These results indicate that simple molecular 4
exhibits very poor selectivity and sensitivity towards p-toluene-
thiol. Compared with the simple organic molecular 4, material
III could provide a large surface area and hydrophobic envi-
ronment, which could lead to a strong enrichment effect for
hydrophobic thiophenols. In addition, the p–p stacking inter-
actions between the coumarin moiety in material III and thio-
phenols could enhance strongly the accessibility of thiophenols
Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (Grant No. 21073213, 20903110),
the Main Direction Program of Knowledge Innovation of
Chinese Academy of Sciences, the National Basic Research
Development Program of China (973 Program) (Grant No.
2007CB936001), and Research Grants Council of Hong Kong
SAR (project No. CityU 123607).
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 13561–13568 | 13567

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