Carbazole based hemicyanine dye for both "naked eye" and 'NIR' fluorescence detection of CN- in aqueous solution: From molecules to low cost devices (TLC plate sticks)

Article abstract of DOI:10.1039/c3dt51262b

A hybrid carbazole hemicyanine dye (receptor CHD) was developed as a new visible and near infrared chemodosimeter type sensor with high ratiometric selectivity towards cyanide in the presence of other anions in aqueous solution. The chemosensor also showed excellent performance when used in the "dip stick" method, i.e. in solid phase (TLC plates). The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt51262b

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Carbazole based hemicyanine dye for both “naked eye”
−
and ‘NIR’ fluorescence detection of CN in aqueous
Cite this: Dalton Trans., 2013, 42, 10682
solution: from molecules to low cost devices
Received 14th May 2013,
Accepted 10th June 2013
(
TLC plate sticks)†
DOI: 10.1039/c3dt51262b
www.rsc.org/dalton
Shyamaprosad Goswami,* Sima Paul and Abhishek Manna
A hybrid carbazole hemicyanine dye (receptor CHD) was develo- interference from background absorption, fluorescence and
ped as a new visible and near infrared chemodosimeter type light scattering. Furthermore, they can penetrate deeply into
sensor with high ratiometric selectivity towards cyanide in the tissues with less damage. The virtues of this particular spectral
9
,10
presence of other anions in aqueous solution. The chemosensor region make the NIR probe more suitable.
Heptamethine
1
1
also showed excellent performance when used in the “dip stick” cyanine dyes are some of the most important NIR dyes and
method, i.e. in solid phase (TLC plates).
have been widely used in various fields and have been
employed as fluorescent labels in fluorescence imaging
studies of biological mechanisms.
Anion recognition is an area of growing interest in supramole-
cular chemistry due to its important role in a wide range of
environmental, clinical, chemical, and biological appli-
A colorimetric and ratiometric fluorescent chemosensor is
12
of particular interest due to its simplicity. In particular, ratio-
1
cations. Among the various anions, cyanide is one of the most
metric sensing provides a way of avoiding any misinter-
pretation of analyte-induced fluorescence quenching or
enhancement due to photobleaching, sensor concentration,
concerning anions because of its extreme toxicity to
2
mammals. It inhibits cellular respiration in mammals by
interacting strongly with a heme unit in the active site of
13
and medium effects. A ratiometric method measuring the
3
cytochrome a3. It could be absorbed through the lungs, gastro-
ratio of fluorescence intensities at two wavelengths provides an
alternative approach. However, up to now, only a limited
number of ratiometric fluorescence probes for cyanide have
intestinal track and skin, leading to vomiting, convulsion, loss
of consciousness, and eventual death. However, the use of
4
cyanide salts has remained widespread, particularly in electro-
plating, metallurgy, synthetic fibers, resins, herbicide and
14
been reported in the literature. In general, ratiometric probes
can be designed to function by following two mechanisms:
intramolecular charge transfer (ICT) and fluorescence reson-
ance energy transfer (FRET). Although a good ratiometric
response could frequently be achieved in some FRET-based
probes, the long synthetic pathways as well as the requirement
of a strong spectral overlap between the emission of the donor
and the absorption of the acceptor are necessary. On the other
hand, ICT-based ratiometric probes are structurally simple and
have advantages such as being easy to make and exhibiting a
large emission shift. Considering all the above facts for
cyanide recognition, we present here a novel NIR ratiometric
fluorescent sensor containing carbazole based hemicyanine
5
gold-extraction processes. According to the World Health
Organization (WHO), only water with cyanide concentration
6
lower than 1.9 μM is fit to drink. Thus, there is a need for an
efficient sensing system that is cheap, simple, highly sensitive
to cyanide in order to monitor cyanide from contaminant
sources. Various kinds of colorimetric or fluorometric CN
selective receptors have been proposed in organic media and
−
7
the receptors that act in aqueous media have also been pro-
posed; however, many of these show insufficient selectivity
−
8
−
towards CN . The design of CN receptors with high selecti-
vity in aqueous media is therefore currently the focus of atten-
tion. Compared to UV and visible light, a chemosensor with
a near-infrared (NIR) region at around 650–900 nm has
many obvious advantages, which can avoid or reduce the
dye (CHD) that exhibits a selective, sensitive and “naked eye”
response to CN− in aqueous solution. Due to the strong
nucleophilic nature of cyanide, we have developed a reaction
based receptor for cyanide ions to avoid the complication
induced hydrogen bonding. This receptor also has a lower
Bengal Engineering and Science University, Shibpur, Howrah 711103, India.
E-mail: spgoswamical@yahoo.com; Fax: +91-3326682916
15
detection limit (ESI†).
The target compound was synthesized through the series of
reactions detailed in Scheme 1. The starting compound (A)
†
Electronic supplementary information (ESI) available: Details of synthetic
procedure with characterisation and spectral data. See DOI: 10.1039/c3dt51262b
1
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wavelengths, i.e. a decrease in absorbance at λmax = 493 nm
and an increase in absorbance at λmax = 285 nm, along with a
clear isosbestic point at 323 nm, indicative of anion binding.
Essentially, these changes in the absorption spectrum stopped
and the ratio of the absorbance intensities at 285 and 493 nm
−
(
A
285/A493) became constant when the amount of CN added
reached 2 equiv. It is noteworthy that the difference between
the two absorption wavelengths is very large (absorption shift
ΔA = 208 nm), which not only contributes to accurate measure-
ments of the wavelengths of two absorption peaks but also
results in a huge ratiometric value. In fact in the presence of 2
equiv. of cyanide, a 71-fold enhancement in the ratiometric
value of A285/A493 (0.30 to 21.32) is achieved with respect to the
cyanide free solution (Fig. 1).
Scheme 1 Synthetic route to CHD. Reagents and conditions: (a) octylbromide,
An apparent fluorescence enhancement was also observed
upon the addition of 2 equiv. of cyanide (Fig. 2). The cyanide
sensing fluorescence of CHD was monitored by titration in a
K
2
CO
3 3 3
, KI, CH CN, heated at 80 °C, 4 h; (b) POCl , DMF, 100 °C, 20 h; (c) ethyl
iodide, toluene, reflux, 8 h; (d) ethanol, piperidine (cat), reflux, 12 h.
mixed solution of 1 : 1 (v/v) CH
2
3
3
CN–H
50 nm. CHD showed two characteristic fluorescence bands at
82 and 600 nm in the emission spectrum, respectively. The
2
O upon excitation at
(octyl derivative of carbazole) was prepared by protecting the
NH group of the carbazole by treating it with octylbromide.
The precursor di-aldehyde of carbazole (B) was prepared from
the corresponding N-octyl-carbazole maintaining the Vilsme-
ier–Haack conditions (see the Experimental section). When
emission band at 600 nm is attributed to the ICT emission
band, while the emission band at 382 nm can be ascribed to
the carbazole moiety. With the addition of cyanide ions, the
fluorescence intensity of the solution at 600 nm gradually
decreased, but at 382 nm it gradually increased, and most
interestingly, a new peak was generated at 732 nm with a clear
isoemission point at 715 nm (Fig. S2†).
1
,3,3-trimethyl-2-methyleneindoline was treated with EtI in
acetonitrile as a solvent, the corresponding salt (C) was
1
6
obtained. Receptor CHD was obtained by Knoevenagel con-
1
7
densation. The structure of the receptor was confirmed using
1
13
The large hypsochromic shift of the ICT band from 493 to
H NMR, C NMR and ESI MS spectra (ESI†).
2
85 nm suggests that the π-conjugation and the ICT progress
The changes of the UV-vis spectra for the receptor CHD in
−
of CHD were both inhibited by the nucleophilic addition of
the absence and presence of CN in a CH
3
CN–water solution
cyanide to CHD. The absorption stabilized after the amount of
added CN reached 2 equiv. and a significant color change from
(
1 : 1, v/v) are shown in Fig. 1. The receptor CHD shows one
−
main absorption band at 493 nm, which is attributed to intra-
molecular charge transfer (ICT) transition in CHD. The various
anions tested are AcO , Cl , Br , I and F as their tetra butyl
salts; SH , H PO , PO , S , N , P O
2 7
sodium salts; and CN and ADP as their potassium salts in
red to colorless could be observed easily. In the time dependent
absorption spectra, we see that the reaction is complete within
−
−
−
−
−
0 s with a rate constant of 3.68 × 10−
s , which strongly
2
−1
−
2−
3− 2−
3−
4−
−
9
and SCN as their
2
4
4
−
supports the high reactivity of the probe (Fig. 3 and S3†).
The carbazole moiety was itself fluorescent and when it
was coupled with the ethyl salt of 1,3,3-trimethyl-2-methylene-
indoline, i.e. in CHD, fluorescence was greatly reduced due
to extensive π-conjugation and the ICT mechanism. The
−
solution. In UV-vis absorption spectrometry, only CN is found
to perturb the electronic behavior of CHD to a significant extent.
Incremental addition of CN clearly showed the CN−
induced changes in the absorption spectrum of CHD at two
−
−5
Fig. 1 UV-vis absorption titration spectra of CHD (c = 2.0 × 10 M) in the pres-
−
−4
Fig. 2 Titration spectra of CHD (c = 2.0 × 10−5 M) in the presence of 2 equiv. of
ence of 2 equiv. of CN (c = 2.0 × 10 M) at pH 7.1 in CH
3 2
CN–H O (1 : 1, v/v)
CN (c = 2.0 × 10−4 M) at pH 7.1 in CH
−
−
(
left) and absorbance ratio changes (A285/A493) of CHD upon gradual addition
3
2
CN–H O (1 : 1, v/v) and plot of [CN ] vs.
−
of CN (2 equiv.) (right).
Fl. Intensity at two different wavelengths.
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Fig. 3 Time-dependent absorption intensity of CHD at 285 nm (in red) and
4
93 nm (in black) in the presence of CN− (2 equiv.) (left) and the time vs. absor-
bance at a fixed wavelength (285 nm) plot using the first order rate equation
right).
Fig. 5 Fluorescence responses of CHD (c = 2.0 × 10− M) to CN− (2 equiv.) con-
5
(
taining 10 equiv. of various anions. λex = 250 nm (right).
addition of cyanide to the CHD results in the formation of new
strongly fluorescent chromophores due to the breaking of the
1
8
conjugation and the ICT (Fig. S4†). Additionally, the absor-
bance intensity ratio at 285 and 493 nm and the emission
intensity ratio at 732 and 600 nm showed drastic changes from
0
.17 to 22.17 and from 0.12 to 11.43 in the presence of 2 equiv.
of cyanide respectively (Fig. 4). The detection limit of CHD
toward CN− was obtained as 0.54 μM (ESI†), which is
−
sufficiently low for the detection of the CN found in many
chemical systems. Thus, CHD is a promising fluorescence
ratiometric sensor for the detection of low levels of cyanide
ions in samples and the selectivity of the fluorescence test was
very high because the intensity enhancement was not observed
in a solution containing all other anions before the addition of
cyanide (Fig. 5).
Fig. 6 Photograph of CHD towards various concentrations of CN− (×10−5 mol)
of (A) 0, (B) 2, (C) 20, (D) 100, and (E) 200 in solution and in TLC plates.
Motivated by the favourable features of this system in solu-
tion, we prepared test strips by immersing TLC plates coated
with CHD which were immersed in a CH CN–H O (1 : 1, v/v)
3 2
solution of KCN in different concentrations and then dried in
air to determine the suitability of a “dip stick” method for the
−
detection of CN . We found that an instant bleaching of the
red color was observed with increasing concentration of
−
cyanide at concentrations as low as 20 μM of CN (Fig. 6).
Development of such dipsticks is useful as instant qualitative
Fig. 7 H-NMR spectra of (a) CHD and (b) CHD + CN− in CDCl
1
.
3
information is obtained without resorting to instrumental
analysis.
1
Furthermore, we investigated the H-NMR spectra of CHD
in the presence of CN− and compared them with that of the
sensor itself (Fig. 7). The protons of the benzene ring in hemi-
cyanine dye in CHD at around δ 7.99 (Ha), 7.51 (Hb) and 7.42 (Hc)
Fig. 4 Ratiometric response of CHD (2.0 × 10−5 M) towards anions (2 equiv.)
with naked eye color changes and changes in emission spectroscopy (F732/F600).
1
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ppm dramatically shifted upfield toward δ 6.88 (Ha), 6.52 (Hb)
Warfare, ed. R. F. Sidell, E. T. Takafuji and D. R. Franz,
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ization, Geneva, 1996.
−
and 6.51 (Hc) ppm respectively upon CN addition, indicating
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13
2
3
nine sp C) is shifted to 70.35 (becomes sp C), confirming the
formation of the cyano adduct. In high resolution ESI TOF MS
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CN adduct, which confirmed the formation of a binuclear
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1
there is a new peak at 2250 cm , concluding the presence of
cyanide in the CHD–CN adduct. All of these results are consist-
ent with our proposed mechanism (ESI†).
In conclusion, we have successfully devised a novel NIR
probe (CHD) towards cyanide in aqueous acetonitrile solution.
CHD exhibits a unique colorimetric and fluorescence enhance-
ment only with cyanide ions even in the presence of excess
amounts of other anions, demonstrating its excellent selecti-
vity compared to other anions. The detection limit of CHD was
estimated to be 0.54 μM. The sensitivity is lower than the
maximum permissive level in drinking water according to the
World Health Organization (WHO). The significant changes in
color can be observed with the naked eye.
The authors thank the DST and CSIR (Govt. of India) for
financial support. S.P. and A.M. acknowledge the UGC and
CSIR respectively for providing fellowships.
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