Bisindole anchored mesoporous silica nanoparticles for cyanide sensing in aqueous media

Article abstract of DOI:10.1039/c1cc13481g

For CN^- recognition, a series of bisindolyl compounds 1-3 were prepared, and their chromodosimetric color changes toward anions were investigated. Nucleophilic addition of the cyanide ion to the meso position of the bisindolyl group gave rise to breaking of the double bond conjugation, thereby inducing spectroscopic changes in the compound. Mesoporous silica nanoparticles 3 also gave color changes from deep orange to yellow in response to the cyanide ion.

Full text of DOI:10.1039/c1cc13481g

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COMMUNICATION
Bisindole anchored mesoporous silica nanoparticles for cyanide sensing in
aqueous mediaw
a
b
Hyun Jung Kim,z Hyejin Lee,z Ji Ha Lee,b Dong Hoon Choi,ac Jong Hwa Jung*b and
Jong Seung Kim*a
Received 13th June 2011, Accepted 31st August 2011
DOI: 10.1039/c1cc13481g
For CNꢀ recognition, a series of bisindolyl compounds 1–3 were
prepared, and their chromodosimetric color changes toward anions
were investigated. Nucleophilic addition of the cyanide ion to the
meso position of the bisindolyl group gave rise to breaking of the
double bond conjugation, thereby inducing spectroscopic changes in
the compound. Mesoporous silica nanoparticles 3 also gave color
changes from deep orange to yellow in response to the cyanide ion.
Silica-based nanoparticles are of great interest for biomedical
and environmental research applications such as bio-separation,
drug targeting, cell isolation, enzyme immobilization, and
stability against degradation.13 It is clear that the receptor-
immobilized nanoparticles have some important advantages as
solid chemosensors. First, immobilized receptors on inorganic
supports can liberate the organic guest molecules (metal ions or
anions) from the pollutant solution. Second, the functionalized
nanomaterials, combined with a chemodosimeter such as a
chromophore, display extremely high selectivity and sensitivity
to absorption changes compared to spherical structures because
of their larger surface areas and well-defined pores. The homo-
geneous porosity and large surface area of mesoporous silica
make it a promising inorganic support.14
Anion and cation sensing have received considerable attention
because of their important roles in biological, industrial, and
environmental processes.1 Cyanide, known especially for being
a detrimental anion, can cause biological and environmental
poisoning.2 The sources of cyanides in water are discharged
from metal mining processes, organic chemical industries, iron
and steel plants or manufacturers, and wastewater treatment
facilities.3 Other cyanide sources include vehicle exhaust,
release from certain chemical industries, burning of municipal
waste, and use of cyanide-containing pesticides. When humans
are exposed to cyanide, the cyanide anion rapidly binds to
enzymes and to other proteins that contain ferric iron, resulting
in inactivation and loss of proper function. In particular,
cyanide has been also used as a chemical warfare agent, and
even as a terror material.4–6
In this regard, sensing the cyanide anion through coordination7
or covalent bonding8 has received particular interest by chemists.
However, most of cyanide anion receptors reported to date
have mostly relied on hydrogen-bonding motifs and, as a
consequence, have generally displayed moderate selectivity
over other anions.9 To overcome this limitation, reaction based
receptors (chemodosimeters) have been developed recently;
these include oxazines,10 cationic borane derivatives,11 and
acridinium salts.12
Motivated by this approach for a cyanide sensor, we herein
report a new Michael type reaction based chromogenic bisindole
derivative 1 and its mesoporous silica-immobilized 3 for the
purpose of cyanide detection. Numerous bisindolyl derivates
have been isolated from various terrestrial and marine natural
sources exhibiting important biological activities, which lead
chemists to synthesize the bisindole derivatives.15,16 H atom in
the meso position of the bisindolyl group was known to be
unstable and easily oxidized to give a corresponding conjugated
product. We therefore hypothesized that the meso position of
the bisindolyl group would immediately react with CNꢀ, which
may cause distinct color changes. The dosimeter 1 and its
mesoporous silica-immobilized 3 selectively displays drastic
changes in UV-vis absorption wavelength for CNꢀ in H2O/
CH3CN (7 : 1, v/v).
Scheme 1 indicates a synthetic route to compounds 1–3. The
bisindolyl group at the para-position of toluene (4) was introduced
by the coupling reaction of indole with p-tolualdehyde in CH3OH.
Compound 1 was then prepared by treating 4 with DDQ in
CH3CN. Chromogenic 2 was also synthesized by the pathway
shown in Scheme 1. In consideration of extending its usefulness,
3 was fabricated by a sol–gel reaction of 2 with mesoporous
silica possessing the isocyanate group (6). Immobilization of
receptor 2 was conducted under reflux conditions for 24 h in
toluene to give 3. In this process, the hydroxyl group of
2 undergoes hydrolysis and is covalently immobilized to the
isocyanate group attached onto the surface of mesoporous
silica. After cooling to room temperature, the red solid
product was filtered, washed with THF, and then dried
a Department of Chemistry, Korea University, Seoul 136-701, Korea.
E-mail: jongskim@korea.ac.kr; Fax: +82-2-3290-3121;
Tel: +82-2-3290-3143
b Department of Chemistry and Research Institute of Natural Science,
Gyeongsang National University, Jinju 660-701, Korea.
E-mail: jonghwa@gnu.ac.kr; Fax: +82-55-758-6027;
Tel: +82-55-751-6027
c Research Institute for Natural Sciences, Korea University,
Seoul 136-701, Korea. E-mail: dhchoi8803@korea.ac.kr;
Fax: +82-2-3290-3121; Tel: +82-2-3290-3540
w Electronic supplementary information (ESI) available: Experimental
procedures and additional figures. See DOI: 10.1039/c1cc13481g
z Equally contributed to this work.
c
10918 Chem. Commun., 2011, 47, 10918–10920
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CNꢀ showed a peak at m/z 361.4, assigned to the [1 + CNꢀ]
ion. The reaction mechanism is depicted in Scheme 2.
1
We then investigated H NMR and 13C NMR spectra of 1
in the presence of cyanide anions and compared them with
that of reactant 1 (Fig. S4, ESIw). When 3 equiv. of cyanide
anions were added to a solution of 1, a new marked carbon
peak of the chemodosimetric product of 1 was clearly observed
at 44.6 ppm (Fig. S4c, ESIw). We newly found that the CQC
bond conjugated to the bisindolyl system is labile to a good
nucleophile such as CNꢀ, similar to a Michael type reaction,
to form a C–C single bond. To the best of our knowledge, the
bisindolyl group we report has never been previously exploited
for this Michael reaction-based anion sensing.18,19 1H NMR,
13C NMR, and FAB-MS analysis upon addition of the
cyanide anion clearly indicates that the cyanide reacts with
the meso-position of the bisindole compound.
Scheme 1 Synthetic pathways of 1–3.
overnight. The solid product 3 was characterized by transmission
electron microscopy (TEM), FT-IR, BET isotherms, and TOF-
SIMS.
Spectral changes of the CH3CN solution of 1 (2.0 ꢁ 10ꢀ5 M)
in the presence of various anions are shown in Fig. S1 (ESIw).
In the presence of 30 equiv. of Fꢀ, CNꢀ, AcOꢀ and H2PO4
,
ꢀ
Various thiols including cystein have been added to a
solution of 1 to see any sensing event for thiols which are
known as a good nucleophile for Michael reaction. We then
noticed that addition of thiols to a solution of 1 gives rise to
similar spectroscopic changes to that of the cyanide ion, but
the reaction time is much slower than the case of CNꢀ.
Among various sensing systems for cyanide, ensemble based
demetallization of copper ion by cyanide anion has been
utilized, because the cyanide reacts with copper ions to produce
stable [Cu(CN)x]nꢀ species.20 To address any possibility of the
Cu2+ interference toward CNꢀ detection in our chemodosimetric
system, we added Cu2+ to a solution of 1–CNꢀ, and tested
their absorption and emission spectral changes. The resulting
blue-shifted absorption bands of 1 occurred by addition of
CNꢀ and showed no absorption changes upon addition of
Cu2+ ions, demonstrating that unlike other chemosensors, 1
undergoes an irreversible chemical reaction with CNꢀ
(Fig. 2A).
the absorption band at 422 nm diminished, while a new
red-shifted absorption band appeared at 518 nm with concomitant
color change from yellow to red. This effect is probably due to the
H-bonding between the tested anion and the hydrogen atom of the
bisindole. We found that the selectivity of 1 towards specific
anions varies with the solvent system. Upon addition of a
considerable amount of water, e.g. H2O/CH3CN (7 : 1, v/v),
the absorption band of 1 at 492 nm decreased, but a new band
at 450 nm concomitantly appeared upon the addition of CNꢀ
as seen in Fig. 1A.ꢀOther anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ,
H2PO4ꢀ, and HSO4 as their tetrabutylammonium salts) did
not cause significant absorption changes of 1 under the
identical condition (Fig. 1B). Selectivity changes of 1 towards
the CNꢀ reaction can be explained by the hydration energy of
the corresponding anions in an aqueous environment. In
H2O/CH3CN (7 : 1, v/v) solution, a very high solvation energy
of Fꢀ (DHhyd = ꢀ505 kJ molꢀ1), Clꢀ (DHhyd = ꢀ363 kJ molꢀ1),
Brꢀ (DHhyd = ꢀ336 kJ molꢀ1), Iꢀ (DHhyd = ꢀ295 kJ molꢀ1),
The TEM image of 3 revealed the mesopores with a narrow
size distribution (ca. 4–5 nm) (Fig. 2B). For further proof of
the new bond formation, we acquired FT-IR and TOF-SIMS
spectra of 3. From the FT-IR spectrum of 3, in addition to the
bands attributed to the mesoporous silica nanoparticles
themselves, new strong bands at 3420, 2900, 2670, 2100,
1700, 1651, 1510, 1480, 1010, 998, 751 and 502 cmꢀ1 appeared,
which originate from receptor 2, in accordance with 2 residing
on the mesoporous silica nanoparticles (Fig. S6, ESIw). The
TOF-SIMS spectrum of the fragment obtained from 3
displayed the characteristic fragment of the bisindole moiety
(m/z = 337), also giving us a firm evidence that the bisindole
moiety was anchored onto the surface of the mesoporous silica
nanoparticles (Fig. S7, ESIw).
ꢀ
HSO4 (DHhyd = ꢀ1103 kJ molꢀ1), CH3COOꢀ (DHhyd
=
ꢀ375 kJ molꢀ1), and H2PO4ꢀ (DHhyd = ꢀ260 kJ molꢀ1) could
account for the very weak/negligible reaction to receptor 1.17
On the other hand, the much lower hydration energy of CNꢀ
(DHhyd = ꢀ67 kJ molꢀ1) primarily contributed to the product
formation (1–CN) with high selectivity.
Fig. S2 (ESIw) shows the absorption spectral changes of 1 on
the gradual addition of CNꢀ. The solution displayed a distinct
color change from deep orange to yellow simultaneously. The
isosbestic points at 414 and 457 nm suggest that the reaction of
1 with CNꢀ produces a single component. The 1 : 1 reaction is
confirmed by FAB-MS (Fig. S3, ESIw); a solution of 1 with
Fig. 1 (A) UV/vis spectra of 1 (20.0 mM) upon addition of various
anions (30 equiv.) in H2O/CH3CN (7 : 1, v/v). (B) Absorption changes
(A0 ꢀ A) of 1 upon addition of 30 equiv. of various anions. A0:
absorbance of free 1. A: absorbance of 1–anion complexes.
Scheme 2 Reaction mechanism of CNꢀ for 1 and 3.
Chem. Commun., 2011, 47, 10918–10920 10919
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intensity (Fig. S9b, ESIw), signifying that the solid 3 can be
useful as a chemosensor for CNꢀ at pH 4 5.
In order to extend the above performance to a portable
chemosensor kit, a disk-type pellet has been prepared from the
solid 3 (Fig. 3B). The orange color of the disk-type pellet produced
from solid 3 was changed to yellow when dipped in CNꢀ (0.01 M)
aqueous solution. On the other hand, no significant changes in
absorption were observed in the parallel experiments in the case of
Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ and HSO4ꢀ solutions (0.01 M).
In addition, the solid absorption spectrum of the disk-type pellet
of 3 with CNꢀ was the same as that obtained from 3 dispersed in
aqueous solution. It is notable that the disk-type pellet prepared
from solid 3 is applicable as a portable chemosensor for the
detection of CNꢀ in biological and environmental fields. In
addition, from the plot of absorbance at 515 nm versus increasing
quantities of CNꢀ added to 3 suspension at pH 7, a detection limit
of ca. 3 mM was determined (Fig. S11, ESIw).
This work was supported by a grant from the CRI program
(No. 2010-0000728) (JSK) and World Class University (WCU)
Program (R32-2008-000-20003-0) (JHJ) from the National
Research Foundation of Korea. DHC also acknowledges
Priority Research Centers Program through the National
Research Foundation of Korea (NRF) funded by the Ministry
of Education, Science and Technology (NRF2011-0018396).
Fig. 2 (A) Absorption spectra of 1 (black), 1 + 3.0 equiv. of CNꢀ
(green) and 1 + 10.0 equiv. of Cu2+ + 3.0 equiv. of CNꢀ (red) in
H2O/CH3CN (7 : 1, v/v), (B) TEM image of the solid product 3.
Solid 3 shows some advantages over the parent 2. Whereas 2
is insoluble in water, solid 3 can be used in aqueous media. The
sensing ability of solid 3 was studied in water through assays
involving the addition of 30 equivalents (with respect to
receptor 2 anchored to solid 3) of the corresponding guest to
water suspensions of the assayed solid. The suspension color
of 3 was red in the absence of specific analytes. However, in the
presence of CNꢀ, the suspension of 3 showed a remarkable
color change from red to yellow (Fig. 3). In these colorimetric
changes, we noticed that the solid 3 reveals a high selectivity
for CNꢀ ions over other anions, showing a similar spectroscopic
response to that of 1 obtained in the solution system. The results
imply that the solid 3 is considerably applicable to the environ-
mental field as a new organic–inorganic hybrid sensor for the
detection of CNꢀ ions.
Notes and references
As studied in the solution system, we also observed interface
effects of the Cu2+ ion toward CNꢀ detection in the solid
chemosensor 3. We added Cu2+ to the suspension of the solid
3–CNꢀ, and observed their absorption spectral change. The
resulting blue-shifted absorption bands of the solid 3 caused
by CNꢀ showed no absorption changes upon addition of
Cu2+ ions, implicating that solid chemodosimeter 3 proceeds
in an irreversible chemical reaction upon addition of CNꢀ,
same as that observed in 1 (Fig. S8, ESIw).
For biological and environmental applications as a colori-
metric sensor, the sensing should be effective over a wide pH
range. The effect of pH on the solid 3 in the absence of CNꢀ
was examined (Fig. S9a, ESIw). At pH o 5, the absorption
intensity is largely decreased, presumably due to protonation
of the nitrogen atoms of the bisindoyl group. However,
minimal or no significant absorption changes were observed
even between pH 6–9. This result clearly demonstrates that the
solid 3 can be used in physiological environments where a pH 4 5
occurs. Moreover, upon addition of CNꢀ ions, a blue-shift
occurred over the full range of pH 6–9 with the same absorption
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Fig. 3 (A) UV/vis spectra and (B) photograph of the pellet prepared
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c
10920 Chem. Commun., 2011, 47, 10918–10920
This journal is The Royal Society of Chemistry 2011