Rapid, selective, and sensitive fluorometric detection of cyanide anions in aqueous media by cyanine dyes with indolium-coumarin linkages

Article abstract of DOI:10.1039/c4cc05412a

Cyanine dyes with indolium-coumarin linkages exhibit selective fluorescence enhancement for cyanide anions (CN^-) via the nucleophilic interaction of CN^- with their indolium carbon atoms. This facilitates rapid (detection time: 1 min) and sensitive (detection limit: 0.4 μM) sensing of CN^- in aqueous media.

Full text of DOI:10.1039/c4cc05412a

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Rapid, selective, and sensitive fluorometric
detection of cyanide anions in aqueous media by
cyanine dyes with indolium–coumarin linkages†
Cite this: Chem. Commun., 2014,
0, 11583
5
Received 14th July 2014,
Accepted 5th August 2014
Yasuhiro Shiraishi,* Masaya Nakamura, Kohei Yamamoto and Takayuki Hirai
DOI: 10.1039/c4cc05412a
www.rsc.org/chemcomm
Cyanine dyes with indolium–coumarin linkages exhibit selective
À
fluorescence enhancement for cyanide anions (CN ) via the nucleo-
À
philic interaction of CN with their indolium carbon atoms. This
facilitates rapid (detection time: 1 min) and sensitive (detection limit:
À
0
.4 lM) sensing of CN in aqueous media.
À
1
Cyanide anions (CN ) are extremely toxic to living organisms.
They strongly bind a heme unit at the active site of cytochrome c
2
and paralyze cellular respiration. The maximum permissive level
of cyanides in drinking water is therefore set at 1.9 mM by the
3
World Health Organization (WHO). Widespread use of cyanides in
4
5
industry for synthesis and metallurgy, however, inevitably causes
À
Scheme 1 Fluorometric detection of CN by a spiropyran–coumarin
linkage (1) under photoirradiation.
À
their accidental release into the environment. Quantification of CN
in biological and environmental samples is therefore necessary.
Several analytical methods such as voltammetry, potentiometry,
À
6
and chromatography have been employed for CN detection. They
Earlier, we reported that spiropyran-based dyes facilitate
À
À
can detect very low levels of CN (o0.1 mM), but need either tedious selective and sensitive fluorometric CN detection under UV
sample pretreatment or expensive instrumentation.
Design of fluorescent receptors for CN has attracted much when dissolved in aqueous media, exists in a nonfluorescent
attention because they facilitate quantitative CN detection by spirocyclic (SP) form. UV irradiation of the SP form promotes
1
1
irradiation (Scheme 1). A spiropyran–coumarin linkage (1),
À
À
simple analysis on a common fluorescence spectrophotometer. isomerization to the nonfluorescent merocyanine (MC) form.
À
À
A variety of fluorescent CN receptors have been proposed so Nucleophilic interaction of CN with the electrophilic indolium
7
À
far; however, many of them display poor selectivity or a high carbon of the MC form produces 1–CN species. This promotes
detection limit (42 mM). To the best of our knowledge, there localization of p-electrons on the coumarin moiety and creates
are only a few reports of CN receptors that selectively detect strong fluorescence at 453 nm. The fluorometric response of 1
very low amounts of CN at reliable levels [signal-to-background occurs selectively against CN , and the receptor allows accurate
S/B) ratio Z3]. These receptors, however, require relatively determination of very low levels of CN (0.5 mM at S/B 4 3).
long time (410 min) for CN detection. The design of receptors This method, however, requires preliminary UV irradiation of the
that facilitate rapid, selective, and sensitive detection of CN is solution for ca. 30 min to produce the MC form of 1. In addition,
8
À
9
À
À
1
0
À
(
À
À
À
therefore necessary.
the nucleophilic interaction of CN with 1 takes a long time
(
ca. 30 min). For practical application, the receptor must be
À
improved to react rapidly with CN without UV irradiation.
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering,
Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531,
Japan. E-mail: shiraish@cheng.es.osaka-u.ac.jp; Fax: +81 6 6850 6273;
Tel: +81 6 6850 6271
À
Here we report rapid CN detection in 1 min without UV
irradiation, accomplished by simple modification of the
spiropyran–coumarin linkage. As shown in Scheme 2, we synthe-
sized cyanine dyes with indolium–coumarin linkages (2 and 3).
These dyes possess electrophilic indolium carbon due to the
intramolecular charge transfer (ICT) from the coumarin to the
†
Electronic supplementary information (ESI) available: Experimental methods,
calculation details, supplementary data (Tables S1 and S2, Fig. S1–S14) and
Cartesian coordinates for the respective compounds. See DOI: 10.1039/
c4cc05412a
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À
Scheme 2 Nucleophilic reaction between CN and cyanine dyes with
indolium–coumarin linkages.
indolium moiety, as does the MC form of 1 (Scheme 1), thus
À
facilitating nucleophilic reaction with CN under the dark
Fig. 2 Absorption spectra of 3 measured with 20 equiv. of each respec-
tive anion in a buffered water/MeCN mixture (7/3 v/v; pH 9.0) at 25 1C. The
analytical conditions are identical to those in Fig. 1.
conditions. These dyes exhibit selective emission enhancement
against CN and allow sensitive CN sensing (detection limit:
.4 mM). In addition, substitution of an electron-withdrawing
À
À
0
–
Cl group (3) increases the electrophilicity of the indolium
À
À
Addition of CN leads to a decrease in this band, along with an
carbon and accelerates the reaction with CN , allowing rapid
CN detection within 1 min.
Iodide salts of cyanine dyes (2 and 3) were prepared by
condensation of 1,2,3,3-tetramethyl-3H-indolium iodides and
7
respectively. They were confirmed by H, C NMR and FAB-MS
analysis (Fig. S1–S6, ESI†).
Fig. 1 shows the fluorescence spectra of 3 (5 mM) measured in a
buffered water/MeCN mixture (7/3 v/v, pH 9.0) at 25 1C, when
photoexcited at 415 nm. 3 itself shows almost no emission
À
appearance of a new blue-shifted band at 423 nm. Addition of
other anions scarcely changes the spectrum, again suggesting
that 3 selectively reacts with CN . A similar spectral change is
À
observed for compound 2 (Fig. S8, ESI†). As shown in Fig. S10
(ESI†), time-dependent changes in the absorption spectra of 2
-diethylaminocoumarin-3-aldehyde with 47% and 39% yields,
1
13
À
and 3 after CN addition show isosbestic points at 463 and
4
67 nm, respectively, suggesting that reaction of both 2 and 3
À
with CN produces a single component. The respective products
were concentrated by evaporation, washed by diethyl ether, and
subjected to FAB-MS analysis. As shown in Fig. S11 and S12
(ESI†), the products of 2 and 3 show peaks at m/z 427.4 and
61.3, assigned to [2 + CN –I ] and [3 + CN –I ] species,
respectively. This indicates that both 2 and 3 react with CN
in a 1 : 1 stoichiometry. These data suggest that, as is the case for
compound 1 (Scheme 1), nucleophilic reaction of CN with the
indolium carbon of 2 and 3 produces 2–CN and 3–CN species,
respectively, as shown in Scheme 2; this creates strong aqua
blue fluorescence (Fig. 1).
The fluorescence enhancement of 2 and 3 upon reaction
with CN is promoted by localization of p-electrons on the
coumarin moiety. This is confirmed by ab initio calculation based
on the time-dependent density functional theory (TD-DFT) per-
formed within the Gaussian 03 program. As shown in Table S1
(fluorescence quantum yield: F = 0.02). Addition of 20 equiv. of
F
À
CN to this solution, however, creates a strong aqua blue fluores-
cence at 484 nm (F = 0.32). In contrast, addition of other anions
shows almost no emission enhancement. In addition, as shown in
Fig. S7 (ESI†), the CN -induced emission enhancement is scarcely
affected by other anions, suggesting that 3 selectively detects
CN by fluorescence analysis as does compound 1. As shown
in Fig. S8 (ESI†), dye 2 (F = 0.03) also shows selective emission
enhancement by CN (F = 0.26). These findings suggest that both
2
À
À
À
À
4
F
À
À
1
1
À
À
À
À
11
F
À
F
À
and 3 dyes selectively detect CN without UV irradiation.
Fig. 2 shows the absorption spectra of 3 measured with each
À
respective anion. 3 itself shows an absorption band at 581 nm.
(
ESI†), singlet electronic transition of 3 mainly consists of
HOMO - LUMO (S - S ) transition. Its energy (2.50 eV,
95 nm) is similar to the observed lmax of the absorption
0
1
4
spectrum of 3 (581 nm, Fig. 2). As shown in Fig. 3 (left), the
p-electrons of both HOMO and LUMO of 3 are delocalized over
the molecule. This indicates that the 581 nm absorption (Fig. 2)
12
is assigned to the ICT band from the coumarin to the indolium
11
moiety, as is the case for the MC form of 1. As shown in Table S1
À
(
ESI†), the electronic transition of 3–CN species is mainly
0 1
contributed by HOMO - LUMO (S - S ) transition. Its energy
is 3.46 eV (358 nm) and is close to lmax of the absorption
À
spectrum of 3–CN species (423 nm, Fig. 2). As shown in
Fig. 1 Fluorescence spectra (lex = 415 nm) of 3, measured with 20 equiv.
Fig. 3 (right), p-electrons of the HOMO and the LUMO for the
+
of each respective anion (as n-Bu
4
N
salts) in
a
buffered water/
À
3
–CN species are localized on the coumarin moiety, while the
MeCN mixture (7/3 v/v; CHES 100 mM, pH 9.0) at 25 1C. All of the
measurements were carried out after stirring the solution with respective
anions for 5 min. The spectra measured in solutions with water content are
shown in Fig. S9 (ESI†).
electrons on 3 are delocalized over the molecule (left). This
À
indicates that nucleophilic interaction of CN with the indolium
carbon of 3 leads to localization of p-electrons. This suppresses
1
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Fig. 5 Time-dependent change in fluorescence spectra of 3 (5 mM), measured
À
with 50 equiv. of CN in a buffered water/MeCN mixture (7/3 v/v; CHES
100 mM, pH 9.0) at 25 1C. (inset) Change in normalized fluorescence intensity
of (white circle) 3 at lem = 484 nm, (triangle) 2 at lem = 484 nm, and (black circle)
1
at lem = 453 nm. The data for 1 were obtained after UV (334 nm) irradiation for
À
À
Fig. 3 Energy diagrams for main orbitals of (left) 3 and (right) 3–CN
90 min followed by addition of CN with continued irradiation.
species, calculated at the DFT level (B3LYP/6-31G*). Gray, white, blue, red,
and yellow-green atoms indicate C, H, N, O, and Cl atoms. Green and deep
red parts on the molecular orbitals refer to the different phases of
molecular wave functions (isovalue: 0.02 a.u.).
À
9
À
fluorescent CN receptors reported so far. In the case of CN
11
detection by compound 1, preliminary UV irradiation for ca.
0 min is necessary for isomerization to the MC form (Scheme 1).
3
The black circle keys in the inset of Fig. 5 show the time-
dependent change in the emission intensity of 1, measured after
À
UV irradiation for 90 min followed by the addition of CN . The
intensity change terminates in ca. 30 min. This indicates that 3
À
indeed facilitates very rapid CN sensing. In addition, as shown
À
by the white triangle keys, compound 2 requires 2 min for CN
detection, which is quite longer than that required by 3 (1 min).
This suggests that substitution of the –Cl group onto the indolium
À
moiety accelerates the reaction with CN .
À
The rapid CN sensing by 3 is due to the strong nucleophilic
À
14
interaction of CN with 3. This is confirmed by fluorescence
Fig. 4 Change in the ratio of fluorescence intensity (lex = 415 nm; lem
=
titration analysis. Fig. 6 shows the result of fluorescence titration of
À
4
84 nm) of 3 (5 mM) as a function of CN concentration.
À
3
with CN . As shown by the white circle keys in the inset,
nonlinear fitting of the titration data based on 1 : 1 reaction of 3
À
5
À1
ICT from the coumarin to the indolium moiety and allows with CN determines the association constant to be 4.5 Â 10 M
.
À
fluorescence emission from the photoexcited coumarin moiety. In contrast, the association constants for 1 and 2 with CN are
As shown in Table S2 (ESI†), TD-DFT calculations of 2 and
À
À
2
–CN species show the results similar to those of 3 and 3–CN
species. This supports the emission mechanism upon inter-
À 13
action with CN .
À
The cyanine dyes, 2 and 3, facilitate sensitive CN detection.
Fig. 4 shows the change in the ratio of fluorescence intensity
À
(
FI/FI ) of 3 with CN concentration. A linear relationship is
0
À
2
À
observed in the range of 0.3–5 mM CN (R = 0.999). The CN
concentration at the S/B ratio (= FI/FI ) = 3 is determined to be
.4 mM. The detection limit, 0.4 mM, is similar to that obtained
by compounds 1 (0.5 mM) and 2 (0.4 mM, Fig. S13, ESI†),
suggesting that both 2 and 3 successfully detect very low levels
0
0
1
1
À
of CN in aqueous media.
À
Another striking aspect is that fluorometric CN detection Fig. 6 Results of fluorescence titration (lex = 415 nm) of 3 (5 mM) with
À
CN in a water/MeCN mixture (7/3 v/v; pH 9.0) at 25 1C. (inset) Change in
by 3 can be carried out within only 1 min. Fig. 5 shows the time-
normalized fluorescence intensity of (white circle) 3 at lem = 484 nm,
dependent change in fluorescence spectra of 3 obtained after
addition of CN . As plotted by the white circle keys in the inset,
the change in the emission intensity of 3 almost terminates
(
triangle) 2 at lem = 484 nm, and (black circle) 1 at lem = 453 nm, where the
À
lines are the nonlinear fitting curves based on 1 : 1 association. The data for
À
1
were obtained after UV (334 nm) irradiation for 90 min followed by CN
within 1 min, which is the shortest assay time among the addition with continued irradiation.
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À
may contribute to the design of more efficient CN receptors
and open a new strategy for the creation of chemosensors
for anions.
This work was supported by the Grant-in Aid for Scientific
Research (No. 26289296) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan (MEXT).
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À
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À
À
CN within only 1 min.
4
In conclusion, we found that cyanine dyes with indolium–
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À
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selective, and sensitive CN detection. These dyes when dissolved
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strong fluorescence via a nucleophilic interaction of CN with
their indolium carbons. Substitution of the electron-withdrawing
Cl group increases the electrophilicity of the indolium carbon
and promotes strong interaction with CN . This thus facilitates
rapid (detection time: 1 min) and sensitive (detection limit:
.4 mM) sensing of CN in aqueous media. The basic concept
presented here based on cyanine dyes with an indolium moiety
5
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À
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1
3 Fig. S14 (ESI†) shows the fluorescence spectra of 3 measured by
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fluorescence at 635 nm assigned to the emission from the excited
–
À
À
ICT state. Addition of CN quenches this emission, whereas addition
of other anions does not. This suggests that nucleophilic addition of
À
CN to the indolium carbon indeed suppresses ICT.
À
0
1
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1
1586 | Chem. Commun., 2014, 50, 11583--11586
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