A fluorescence turn-on detection of cyanide in aqueous solution based on the aggregation-induced emission

Article abstract of DOI:10.1021/ol900376r

The nucleophilic addition of cyanide to the trifluoroacetylamino group in 2 yields an amphiphilic species which would induce the aggregation of silole 1, and as a result, the fluorescence of the ensemble increases largely. Accordingly, a fluorescence turn

Full text of DOI:10.1021/ol900376r

ORGANIC
LETTERS
2009
Vol. 11, No. 9
1943-1946
A Fluorescence Turn-on Detection of
Cyanide in Aqueous Solution Based on
the Aggregation-Induced Emission
Lihua Peng, Ming Wang, Guanxin Zhang, Deqing Zhang,* and Daoben Zhu
Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
dqzhang@iccas.ac.cn
Received February 22, 2009
ABSTRACT
The nucleophilic addition of cyanide to the trifluoroacetylamino group in 2 yields an amphiphilic species which would induce the aggregation
of silole 1, and as a result, the fluorescence of the ensemble increases largely. Accordingly, a fluorescence turn-on detection of cyanide in
aqueous solution can be established with the ensemble of silole 1 and compound 2. Also, this ensemble displays good selectivity toward
cyanide over other common inorganic anions.
Cyanide binds to the ferric form of cytochrome-c and inhibits
the mitochondrial electron-transport chain.1 For this reason,
cyanide is one of the most toxic anions and harmful to human
health and the environment. A large proportion of fatalities
among fire victims is due to cyanide poisoning, as blood
cyanide concentrations reach a level of 23-26 µM.2 Fur-
thermore, cyanide concentrations in drinking water have been
required to be lower than 1.9 µM.3 However, the use of
cyanide salts remains widespread, particularly in gold mining,
electroplating, and metallurgy.4 Also, cyanide salts have been
widely applied in the production of organic chemicals and
polymers including nitriles, nylon, and acrylic plastics.5
Despite the increasing level of monitoring and strict control,
accidental releases of cyanide into the environment do occur.
Therefore, it is highly desirable to develop sensitive and
selective chemical sensors for cyanide. By making use of
the coordination ability and nucleophilic reactivity of cyanide,
a number of chemical sensors for cyanide were developed.6
For instance, Sessler et al. have recently described a selective
cyanide indicator based on the benzyl cyanide reaction.7
However, drawbacks exist for these cyanide sensors. These
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Raymo, F. M. J. Org. Chem. 2006, 71, 744–753. (f) Todd, W.; Gabba,
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F. Cyanide in Biology; Academic Press: London, 1981.
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10.1021/ol900376r CCC: $40.75
Published on Web 04/03/2009
 2009 American Chemical Society

include poor selectivity,8 particularly in the presence of
fluoride or acetate, and the fact that the detection of cyanide
cannot be performed in aqueous solution in some cases.
Fluorescent probes for cyanide were reported,9 but fluores-
cence turn-on detection still remains rare. Thus, there remains
a need for improved sensing ensembles for cyanide. In this
paper, we will report a fluorescence turn-on sensing ensemble
for cyanide in aqueous solution by making use of the
aggregation-induced-emission (AIE) feature of silole (sila-
cyclopentadiene) compounds.
Scheme 1. (A) Chemical Structure of Silole 1 with an
Ammonium Group; (B) Chemical Structure of Compound 2 and
the Reaction with Cyanide; (C) Illustration of the Design
Rationale for the Fluorescence Turn-on Detection of Cyanide by
Making Use of the AIE Feature of Silole Compounds
It is known that silole derivatives are weakly fluorescent
in solution but become highly fluorescent after aggregation.
This intriguing phenomenon was referred to as aggregation-
induced enhanced emission (AIE) first reported by Tang,
Zhu, and their co-workers in 2001.10 Chemo-/biosensors have
been reported on the basis of the AIE features of silole and
relevant compounds.11 We have recently established a
fluorescence turn-on detection of heparin in serum,12 DNA
and label-free fluorescence nuclease assay,13 and continuous
on-site label-free ATP fluorometric assay14 by taking ad-
vantage of the AIE feature of silole compounds. The present
sensing ensemble toward cyanide contains silole 1 and
compound 2 with a trifluoroacetylamino group. The design
rationale is illustrated in Scheme 1 and is explained as
follows: (1) silole 1 with a positive ammonium group shows
weak fluorescence in aqueous solution but in the presence
of an amphiphilic compound with a negatively charged group
aggregation would occur due to the intermolecular electro-
static and hydrophobic interactions schematically shown in
Scheme 1;15 (2) cyanide can easily react with the trifluoro-
acetylamino group16 in compound 2, leading to formation
of an amphiphilic compound with a negative headgroup. As
a result, it is anticipated that coaggregation of silole 1 and
compound 2 would occur in aqueous solution after addition
of cyanide, and accordingly, the fluorescence of silole 1
would increase. The results show that a fluorescence turn-
on detection of cyanide in aqueous solution can be estab-
lished with the ensemble of silole 1 and compound 2.
Silole 1 was prepared according to a previous report.13,17
Compound 2 was obtained simply by the reaction of
4-hexylanine with trifluoroacetic anhydride (see the Sup-
porting Information). Silole 1 can be dissolved in pure water,
and the solution exhibits rather weak fluorescence as
expected. Compound 2 is not soluble in water, but it can be
dissolved in DMSO. A clear solution of 1 (7.5 × 10-5 M)
and 2 (4.0 × 10-4 M) in a mixture of DMSO and water
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ca. 1.0 equiv of CN- was estimated to be 0.35 mM. For the fluorescence
measurements, the concentration of 2 was 0.4 mM. Therefore, it is probable
that the reaction adduct of 2 with CN- forms micelles in aqueous solution
and the hydrophobic part of 1 may enter into the hydrophobic core of the
micelles. Moreover, such micelles may be further interconnected due to
the electrostatic interactions between the ammonium group in 1 and anionic
part of the reaction adduct.
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Figure 1. Fluorescence spectra (λex ) 370 nm) of the ensemble of
silole 1 (7.5 × 10-5 M) and compound 2 (4.0 × 10-4 M) in DMSO/
H2O (1/75, v/v) in the presence of different concentrations of sodium
cyanide (from 0 to 1.4 × 10-4 M) at room temperature. The solution
was detected immediately when sodium cyanide was added at room
temperature; the pH value was changed from 6.7 to 8.7 after
addition of cyanide (1.2 × 10-4 M). Inset: photos of the solutions
of silole 1 (A), mixture of silole 1 (7.5 × 10-5 M) and compound
2 (4.0 × 10-4 M) (B), and the mixed solution of silole 1 and
compound 2 in the presence of 2.0 equiv of NaCN (C) under UV
light (365 nm) illumination.
Figure 3
.
Partial 1H NMR spectra of compound 2 (4.0 × 10-3 M)
in DMSO-d6/D2O (5/1, v/v) in the presence of different amounts
of NaCN.
is a nearly linear relation between [I476 - I0]/I0 and the
concentration of cyanide (y ) 0.66283x - 0.28899, r )
0.99822, n ) 12). The detection limit of cyanide with the
ensemble of silole 1 and compound 2 under these conditions
was estimated to be 7.74 µM, which is lower than concentra-
tions of cyanide in the blood of fire victims.
(1/75, v/v) was prepared for the following studies. The mixed
solution of silole 1 and compound 2 shows negligible
emission (see Figure 1). However, the fluorescence intensity
of the solution increased gradually after addition of different
amounts of NaCN as shown in Figure 1. For instance, the
fluorescence intensity of the solution at 476 nm was enhanced
8 times after the concentration of cyanide in the solution
reached 1.2 × 10-4 M. Indeed, such fluorescence enhance-
ment after introducing cyanide to the solution can be
distinguished by the naked eye as shown in the inset of
Figure 1, where the photos of the aqueous solutions with
and without addition of cyanide under UV light (365 nm)
illumination are shown.
The fluorescence enhancement after addition of cyanide
to the mixture of silole 1 and compound 2 can be understood
as follows: (1) addition of cyanide to the trifluoroacetylamino
group in compound 2 leads to the formation of an am-
phiphilic compound with a negative headgroup (see Scheme
1); (2) coaggregation of silole 1 and the new amphiphilic
compound generated from compound 2 would occur in
aqueous solution18 as schematically shown in Scheme 1, and
as a result the fluorescence of silole 1 would be enhanced.
1
Both H NMR and dynamic light scattering (DLS) experi-
ments support these assumptions. Figure 3 shows the partial
1H NMR spectra of compound 2 in the presence of different
amounts of cyanide. Clearly, the signals due to the aromatic
protons were upfield shifted. The signals at 7.55 and 7.22
ppm were shifted to 7.34 and 6.98 ppm, respectively, when
1.0 equiv of cyanide was added. This is likely due to the
transformation of the trifluoroacetylamino group in com-
pound 2 into an anionic group in the presence of cyanide.
1
But, the H NMR signals kept almost unchanged if more
than 1.0 equiv of cyanide was introduced to the solution.
This implies the 1:1 reaction stoichiometry between com-
pound 2 and cyanide, which is in agreement with the previous
reports.19 13C NMR, 19F NMR, and mass spectroscopic data
Figure 2. Plot of ([I476 - I0]/I0), where I476 and I0 refer to the
fluorescence intensity of the ensemble of silole 1 and compound 2
at 476 nm in the presence and absence of cyanide, respectively, vs
the concentration of cyanide.
(18) (a) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B
1998, 102, 1528–1538. (b) El-Hachemi, Z.; Mancini, G.; Ribo, J. M.;
Sorrenti, A. J. Am. Chem. Soc. 2008, 130, 15176–15184.
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He, J. Q.; Cheng, J. P. Org. Biomol. Chem. 2008, 6, 3038–3040. (b) Niu,
H. T.; Jiang, X. L.; He, J. Q.; Cheng, J. P. Tetrahedron Lett. 2008, 48,
6521–6524.
Figure 2 shows the plot of [I476 - I0]/I0, where I0 refers to
the fluorescence intensity at 476 nm before addition of
cyanide vs the concentration of cyanide. Of interest, there
Org. Lett., Vol. 11, No. 9, 2009
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also supported the reaction of 2 with CN- (see the Supporting
Information).
A DLS signal corresponding to particles with sizes around
10 nm was detected for the solution of silole 1 and compound
2 in DMSO/H2O (1/75, v/v). This is likely due to the self-
aggregation of compound 2 because of its hydrophobic
feature. However, the DLS measurement (see Figure S2,
Supporting Information) indicated that aggregates with sizes
around 100 nm were formed for the solution of silole 1 and
compound 2 after addition of cyanide. As discussed above,
the intermolecular hydrophobic and electrostatic interaction
between silole 1 and the amphiphilic compound generated
from compound 2 may induce the formation of such
molecular aggregates.
In order to demonstrate the selectivity of the ensemble of
silole 1 and compound 2 toward cyanide, fluorescence spectra
of the ensemble in the presence of excess amounts of NaOAc,
NaBr, NaCl, NaF, NaH2PO4, NaHSO4, NaN3, and NaNO3
were recorded respectively (see Figure S1, Supporting
Information). As shown in Figure 4, where the respective
fluorescence intensities at 476 nm of the ensemble in the
presence of 10.0 equiv NaCN and 1. equiv other salts are
shown, significant fluorescence enhancement was detected
only in the presence of cyanide. Therefore, it can be
concluded that the ensemble of silole 1 and compound 2
can function as a selective chemosensor for cyanide. This
may be understandable by considering the fact that cyanide
is more nucleophilic reactive than other anions such as
fluoride and acetate in aqueous solution.
In summary, we have successfully established a selective
and relatively sensitive chemosensor for cyanide by making
use of the AIE feature of silole compounds and high
nucleophilic reactivity of cyanide in aqueous solution. It
should be noted that the present sensing ensemble for cyanide
has the following advantages: (1) it is a fluorescence turn-
on sensor; (2) the detection can be carried out in aqueous
solution; (3) silole 1 and compound 2 are easily accessible.
The present investigation provides a good avenue for sensing
Figure 4. Variation of the fluorescence intensity at 476 nm for the
ensemble of silole 1 (7.5 × 10-5 M) and compound 2 (4.0 × 10-4
M) in DMSO/H2O (1/75, v/v) after addition of 1.0 equiv of cyanide
and 10 equiv of CN-/AcO-/Br-/Cl-/F-/ H2PO4-/HSO4-/N3-/NO3
.
-
cyanide, and further optimization of the ensemble, in
particular the structure of compound 2, is in progress with
the aim of improving the sensitivity toward cyanide.
Acknowledgment. The present research was financially
supported by the NSFC, Chinese Academy of Sciences, and
State Key Basic Research Program. We appreciate the kind
help of Dr. Junfeng Xiang and Prof. Yilin Wang. We also
thank the anonymous reviewers for their suggestions.
Supporting Information Available: Synthesis and char-
acterization data of compound 2; DLS data for the ensemble
of 1 and 2 in the absence and presence of cyanide; isothermal
titration microcalorimetry (ITC), ESI-mass, and NMR spec-
tral studies of 2 after reaction with CN-. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL900376R
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Org. Lett., Vol. 11, No. 9, 2009