Reversible solid optical sensor based on acyclic-type receptor immobilized SBA-15 for the highly selective detection and separation of Hg(II) ion in aqueous media

Article abstract of DOI:10.1039/b806019c

An acyclic receptor immobilized mesoporous silica (ARMS) was prepared by sol-gel reaction and its optical sensing ability was studied upon the addition of heavy metal ions in aqueous solution. The Royal Society of Chemistry.

Full text of DOI:10.1039/b806019c

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Reversible solid optical sensor based on acyclic-type receptor immobilized
SBA-15 for the highly selective detection and separation of Hg(II) ion in
aqueous mediaw
Eunjeong Kim, Hee Eun Kim, Soo Jin Lee, Shim Sung Lee, Moo Lyong Seo and
Jong Hwa Jung*
Received (in Cambridge, UK) 9th April 2008, Accepted 16th May 2008
First published as an Advance Article on the web 2nd July 2008
DOI: 10.1039/b806019c
An acyclic receptor immobilized mesoporous silica (ARMS) was
prepared by sol–gel reaction and its optical sensing ability was
studied upon the addition of heavy metal ions in aqueous
solution.
to the acid type receptor attached onto mesoporous silica after
hydrolysis). We herein report the fabrication and coloration of
acyclic receptor 1 immobilized mesoporous silica (ARMS),
which selectively changes color upon the addition of Hg2+
.
The synthesis of the target compound 1 is difficult due to the
polymerization of the triethoxyl group during hydrolysis,
which removes the ethyl groups of 5 by pathway A in Scheme
1. Thus, we tried to obtain 1 by hydrolysis of 5 in acidic
conditions after immobilization of receptor 5 onto the surface
of mesoporous silica by pathway B. To prepare a selective
chromogenic receptor for Hg2+, we began with 2, then formed
o-anisidine (3) by alkylation using two equivalents of ethyl
bromoacetate in the presence of potassium dihydrogen phos-
phate and potassium iodide in CH3CN. The azobenzene ester
4 was made by the reaction of the diazonium salt of o-
aminobenzoic acid with 3. Subsequently, treatment with 4
and aminopropyltriethoxysilane as precursor for the sol–gel
reaction afforded compound 5 as a yellow powder. Then,
immobilization of the chromogenic receptor 5 to mesoporous
silica was conducted under reflux for 24 h in toluene. In this
process, the triethoxylsilyl group of 5 attached onto mesopor-
ous silica undergoes hydrolysis and attaches covalently to the
Chemical sensors are molecular receptors that transform their
chemical information into analytically useful signals upon binding
to specific guests. These sensors are attracting attention owing to
their potential for easy detection and quantification of pollutant
species in fields such as waste management, environmental chem-
istry, clinical toxicology and bioremediation of radionuclides.1–9
Among these, the selective detection of metal ions such as
mercury and lead is critical for environmental monitoring, as
these are highly toxic and common environmental pollutants.
Very recently, organic–inorganic hybrid materials have been
investigated in the search for new methodologies for ion recog-
nition and sensing. As solid chemosensors, receptors immobi-
lized on inorganic nanomaterials such as SiO2, Al2O3 and TiO2
have important advantages when used in the heterogeneous
solid–liquid phase.10–17 First, immobilized receptors on an
inorganic support can remove the guest molecules (toxic metal
ions or anions) from the pollutant solution. Second, these oxides
can be fabricated as functionalized porous-type nanomaterials.
When combined with chemophores, this allows for extremely
highly selective, sensitive absorption or fluorescence changes
compared with macro-sized spherical structures because of the
larger surface areas and well-defined pores of nanomaterials.
In particular, the homogeneous porosity and large surface
area of mesoporous silica (SBA-15 and MCM-41) make it a
promising inorganic support. However, only limited examples of
such heterogeneous sensors have been reported,10–17 despite the
fact that they take advantage of the independent solubility
properties of the receptor in water and organic solvents. Based
on this idea, we synthesized an azobenzene-coupled acyclic
receptor 5 (Scheme 1) and immobilized 5 onto the surface of
mesoporous silica as a new approach to the development of
nanomaterial chemosensors and adsorbent. We then trans-
formed the ester of receptor 5 into the acid of the receptor 1
attached onto mesoporous silica by hydrolysis (receptor 1 refers
Scheme
1 Synthetic method. Reagents and conditions: (i)
Department of Chemistry and Research Institute of Natural Science,
Gyeongsang National University, Jinju, 660-701, (Korea). E-mail:
jonghwa@gnu.ac.kr
w Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b806019c
BrCH2CO2Et, 60 1C; (ii) diazonium salt, DMF, 0 1C; (iii) 3-amino-
propyltriethoxysilane, DCC, DMAP, EA, RT; (iv) mesoporous silica,
reflux; (v) Na2CO3, NaNO3, 90 1C; (vi) H2O; DMF = N,N0-dimethyl-
formamide, DCC = 1,3-dicyclohexyl carbodiimide, DMAP = N,N-
dimethyl-4-pyridinamine, EA = ethyl acetate.
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Fig. 1 (A) TEM image of ARMS. (B) The nitrogen adsorption–
desorption isotherms of (a) mesoporous silica and (b) ARMS.
surface of the mesoporous silica.17 After cooling to room
temperature, the yellow solid product was filtered off, washed
with THF and dried. Finally, a chemosensing material was
obtained as a light yellow solid by hydrolysis of 5 attached
onto mesoporous silica under acidic conditions. The acyclic
receptor 1 immobilized mesoporous silica (ARMS) was char-
acterized by transmission electron microscopy (TEM), UV-Vis
spectroscopy, thermogravimetric analysis (TGA) and FT-IR.
In Fig. 1 and S1 (ESIw), TEM images of ARMS and
mesoporous silica clearly show the formation of a well-ordered
hexagonal arrangement of mesoporous channels before and
after attaching the receptor 1. To investigate the porosity
changes of the mesoporous silica induced by introduction of
the azo-coupled derivative 1, we measured the surface area, pore
volumes and pore diameters of both mesoporous silica and
ARMS using nitrogen adsorption–desorption isotherms (Fig.
1(B)). The mesoporous silica has a BET (Brunauer–Em-
mett–Teller) surface area of 1119.72 m2 gꢁ1 and a pore volume
of 0.49 cm3 gꢁ1. On the other hand, we observed that the ARMS
has a BET surface area of 673.21 m2 gꢁ1 and a pore volume of
0.26 cm3 gꢁ1. The mesoporous silica and the ARMS have BJH
(Barrett–Joyner–Halenda) pore diameters of 2.24 and 2.15 nm,
respectively (Fig. S2, ESIw). The decreased surface area and pore
diameter in ARMS are attributable to the attachment of the
acyclic azo-coupled derivative to the mesoporous silica. Further-
more, from the results of the TGA measurement (Fig. S3, ESIw),
we determined that the ARMS consists of only 18.0 wt% of 1.
For further structural proof of the ARMS, we carried out IR
spectroscopy of both mesoporous silica and ARMS. For the
Fig. 2 The colorimetric response of (A) dried and (B) H2O suspen-
sion samples of ARMS (5.0 mg) in the (a) absence and the presence of
(b) HgCl2, (c) CoCl2, (d) CdCl2, (e) PbCl2, (f) ZnCl2, (g) FeCl3 and (h)
CuCl2.
complex with strong coordination bonding between the
bridgehead nitrogen atom of aniline group of
1 and
Hg2+ 10,18 Except for Hg2+, no significant color changes were
.
observed in the experiments using other metal ions (Fig.
2(A)(c–i) and Fig. S5, ESIw)). These findings confirm that
ARMS can be useful as a colorimetric sensing material for
selective detection of Hg2+ in the presence of other metal ions.
This is a rare example of chromogenic sensing for a specific
metal ion by functional inorganic nanomaterials.
To develop the ARMS as a general mercury cation sensor
which is independent of the anion present, we investigatꢁed the
anion effect by addition of other anions such as NO3 and
ꢁ
ClO4 (Fig. S6, ESIw). As observed for HgCl2 solution, the
color of the ARMS also changed from yellow to red. This
finding indicates that the ARMS can be employed for the
detection of Hg2+ independently of the other anion(s) present.
The Hg2+-loaded ARMS was isolated to confirm the bind-
ing efficiency of Hg2+ by ARMS. Then, the solid UV-Vis
spectrum for the Hg2+-loaded ARMS was compared with that
for ARMS alone (Fig. 3A). The Hg2+-loaded spectrum of
ARMS exhibits an absorption maximum at 495 nm, whereas
the absorption peak appears at 310 nm for the Hg2+-free
ARMS, indicating that the Hg2+ is efficiently bound to 1
attached in the ARMS by covalent bonds.10
mesoporous silica, IR peaks appear at 3450, 1658 and 1084 cmꢁ1
.
For ARMS (Fig. S4, ESIw), peaks appear at 3382, 2976, 2933,
2884, 1626, 1615, 1570, 1471, 1446, 1428 and 1382 cmꢁ1. These
new peaks originate from the acyclic azo-coupled receptor 1,
providing solid evidence that 1 is indeed attached to the surface of
the mesoporous silica.
In addition, we confirmed the reversibility of the color
change of ARMS by removing the Hg2+ ion bound to ARMS
by treatment with EDTA. As expected, the red color of ARMS
in the presence of Hg2+ ion was changed into light yellow
upon the EDTA treatment (Fig. 3(B)). Once again, the color
change was fully reversible by the addition of EDTA. Because
Hg2+ ion bound to ARMS is dissociated by EDTA, the
ARMS can be repeatedly used by renewing with EDTA
(Fig. 3(C)).
Next, we investigated spectroscopic properties of the ARMS
towards the metal ions K+, Ca2+, Sr2+, Co2+, Cd2+, Pb2+
,
Zn2+, Fe3+, Cu2+ and Hg2+ in aqueous solution (pH = 7.4,
all as chlorides, 5.0 equiv. with respect to 1 anchored to the
mesoporous silica). Interestingly, upon the addition of Hg2+
in H2O suspension, ARMS resulted in a color change from
light yellow to red within 10 s (Fig. 2(A)(b) and (B)(b)). In this
category of chromophore, photoexcitation causes a net elec-
tronic charge transfer from the donor end (bridgehead nitro-
gen) to the acceptor end within the chromophore. Thus, the
observed color change is ascribed to the formation of a
In order to understand the coordination behavior between
receptor 1 attached to ARMS and Hg2+, we made repeated
attempts to obtain the crystal structure of complex 1 with
Hg2+, but were not successful. As an alternative, we measured
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The extraction ability of the ARMS was also estimated by
measuring the amount of Hg2+ adsorbed on the ARMS by ICP,
resulting in 90% of Hg2+ ion being extracted by ARMS. This
result suggests that the ARMS is potentially useful as a stationary
phase for separation of Hg2+ in liquid chromatography. Also, the
adsorption capacity of the ARMS was measured through solid
extraction using solutions of binary metal ions (Hg2+/Fe3+
,
Hg2+/Co2+, Hg2+/Cd2+, Hg2+/Pb2+ and Hg2+/Zn2+) result-
ing in 85.7–90.5% of Hg2+ being adsorbed by ARMS (Fig. 4). In
contrast, other metal ions such as Zn2+, Cd2+, Pb2+ and Zn2+
were extracted into the solid phase at percentages of only
2.5–5.2% in the competition system. These results suggest that
ARMS is a useful adsorbent for the selective separation of Hg2+
over a range of transition and heavy metal ions.
In conclusion, we have fabricated ARMS using a functional
azobenzene-coupled acyclic receptor. The ARMS recognized
and separated Hg2+ with a high degree of selectivity among
heavy metal ions in aqueous solution. Beyond its immediate
applications in environmental cleanup, ARMS provides a
unique opportunity to introduce molecular binding sites and
to rationally design the surface properties of inorganic nano-
materials. We believe the combination of well-defined inor-
ganic nanomaterials and functionalized organic receptors can
play a pivotal role in the development of a new generation of
hierarchical structures and functionalized composites.
This work was supported by Korea Ministry of Environ-
ments as ‘‘The Eco-technopia 21 Project’’ and ’’KOSEF(2008-
01014)’’.
Fig. 3 (A) Solid UV-Vis spectra of ARMS (5.0 mg) in the (a) absence
and (b) the presence of HgCl2 (5.0 equiv.), and (c) after addition of
EDTA (10 mmol, 2 mL). (B) Picture and (C) proposed structure of
ARMS–Hg2+ before and after treatment of EDTA.
the UV-Vis spectra of ARMS with the addition of Hg2+ to
confirm the stoichiometry between 1 attached onto ARMS
and Hg2+ ion. The spectral variation of ARMS in H2O was
observed upon the gradual addition of HgCl2. As a function of
the Hg2+ concentration, a new absorption band centered at
495 nm leading to obvious color change from light yellow to
red was observed. The red-shift from 310 to 495 nm of the
absorption of ARMS is attributed to a strong binding affinity
between the nitrogen atom of 1 attached to ARMS and Hg2+
(Fig. S7, ESIw).10 The stoichiometry for the complex of 1 with
Hg2+ was examined by a Job plot. As shown in Fig. S8,
ESI,wit was found that the 1–Hg2+ complex concentration
approaches a maximum when the molar faction of [1]/[1] +
[Hg2+] is about 0.5, indicating that Hg2+ forms a 1 : 1
complex with 1 attached to ARMS as shown in Fig. 3(C).
Fig. S9, ESI,w shows standard calibration data (Abs. vs.
[Hg2+]) for ARMS. A linear response is observed (between
1.0–10 mM) with a sensitivity of B1.0 mM. This sensitivity is
equivalent to those previously reported for spectrophotometric
sensors anchored to mesoporous aluminosilicates.19 Furthermore,
we examined the pH effect on Hg(II) uptake by measuring
absorbance of ARMS treated with Hg(II) solutions (pH
1.0–12.0). As shown in Fig. S10, ESI,wno significant absorbance
or color changes were observed between pH 2–9, suggesting the
proposed ARMS sensor can be used in this pH range.
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Fig. 4 Adsorption capacities of ARMS for (a) single and (b) binary
metal ions in H2O.
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