FRET based selective and ratiometric 'naked-eye' detection of CN - in aqueous solution on fluorescein-Zn-naphthalene ensemble platform

Article abstract of DOI:10.1016/j.tetlet.2014.05.018

We have developed a FRET-based ratiometric fluorescent probe for the detection of CN^- using a fluorescein-Zn-naphthalene ensemble (NFH·Zn²⁺). The sensing mechanism was ascribed by displacement approach. The chemosensor exhibits high selectivity and sensibility for CN ^-. The speculation was supported by fluorescence emission spectra, UV-vis spectrum, ¹H NMR titration experiments, and mass spectra. The interconversion of probe NFH and NFH·Zn²⁺ via the complexation/decomplexation by the modulation of Zn²⁺/CN^- mimics INHIBIT gate. In addition, it also shows an excellent performance in 'dip stick' method.

Full text of DOI:10.1016/j.tetlet.2014.05.018

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
FRET based selective and ratiometric ‘naked-eye’ detection of CNÀ in
aqueous solution on fluorescein–Zn–naphthalene ensemble platform
⇑
Shyamaprosad Goswami , Sima Paul, Abhishek Manna
Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur (Formerly Bengal Engineering and Science University, Shibpur), Howrah 711103,
West Bengal, India
a r t i c l e i n f o
a b s t r a c t
We have developed a FRET-based ratiometric fluorescent probe for the detection of CNÀ using a fluores-
cein–Zn–naphthalene ensemble (NFHÁZn2+). The sensing mechanism was ascribed by displacement
approach. The chemosensor exhibits high selectivity and sensibility for CNÀ. The speculation was sup-
ported by fluorescence emission spectra, UV–vis spectrum, 1H NMR titration experiments, and mass spec-
tra. The interconversion of probe NFH and NFHÁZn2+ via the complexation/decomplexation by the
modulation of Zn2+/CNÀ mimics INHIBIT gate. In addition, it also shows an excellent performance in
‘dip stick’ method.
Article history:
Received 4 March 2014
Revised 5 May 2014
Accepted 7 May 2014
Available online xxxx
Keywords:
FRET based sensor
Zn2+ and CNÀ detection
Ensemble
Ó 2014 Elsevier Ltd. All rights reserved.
Fluorogenic detection
Naked-eye detection
The recognition of anions represent an interesting and emerg-
ing field of research due to the important roles of anions in indus-
trial and biological processes.1 Among different anions of interest,
the detection of cyanide is of great importance due to its high tox-
icity2 in physiological systems. It can affect many functions in the
human body, including the vascular, visual, central nervous, car-
diac, endocrine, and metabolic systems.3 Cyanide containing salts
are widespread chemicals found in surface water originating not
only from industrial waste but also from biological sources.4 Seri-
ous problems5 due to accidental release of cyanide into the envi-
ronment are often caused by various industrial processes such as
gold mining, electroplating etc. Therefore, various methods for
the detection of cyanide have been developed. However, all these
methods have limitations that they require time consuming proce-
dure and more sophisticated instrumentation with high detection
limits.6 Therefore, the development of optical chemosensors for
cyanide has attracted considerable attention in recent decades
because of their simplicity, low cost, and rapid measurement.7
For the optical detection of cyanide ions generally three
approaches like hydrogen-bonding interaction,8 nucleophilic addi-
tion reaction,9 and displacement approach10 have been followed.
The sensing phenomena through hydrogen-bonding interaction
mostly suffer by the interference of FÀ and AcOÀ ions. The
nucleophilic addition reactions mainly happen in organic to
semi-aqueous media. In displacement approach, almost all reac-
tions belong to copper–cyanide affinity, that is, at first, fluores-
cence of the receptor is quenched by copper with the help of its
paramagnetic character and then on addition of cyanide the
quenched fluorescence is recovered. Thus the displacement
approach is limited to a fluorescence ‘off–on’ type sensor.
To overcome this limitation our choice is a ratiometric sensor.
Ratiometric sensor is exceptionally useful compared to a conven-
tional ‘off–on’ sensor, as the former allows the measurement of
emission intensities at two different wavelengths, and thereby,
the correction for environmental effects.11 This can be achieved
by the use of FRET based receptor moiety. A FRET based process
is independent of the concentration of a single fluorescent dye
and one can quantify the analyte concentration by using the ratio
of intensities of the well resolved fluorescence peaks with reason-
able intensities at two different wavelengths for analyte-free and
analyte-bound probe.12 Normally, dyes based on FRET processes
are usually linked by a non-conjugated spacer, and the energy
transfer occurs through space. In FRET-based energy cassettes large
pseudo-Stokes shifts have occurred and overlap between acceptor
and donor moiety is a necessary condition. To design small-mole-
cule FRET ratiometric fluorescent probe for ratiometric detection of
anions, it is required to formulate a FRET platform, which consists
of an energy donor and an energy acceptor. In general, the emis-
sion spectrum of the donor should have reasonable overlap with
the absorption spectrum of the acceptor.13 In most of the cases
FRET process is turned on due to binding but here we report the
reverse process, that is, FRET process is turned off (Scheme 1).
⇑
Corresponding author. Tel.: +91 3326684561–3; fax: +91 3326682916.
E-mail address: spgoswamical@yahoo.com (S. Goswami).
http://dx.doi.org/10.1016/j.tetlet.2014.05.018
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Goswami, S.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.018

2
S. Goswami et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Toward this end, herein we wish to disclose the first FRET based
ratiometric fluorescent probe (fluoroscein–Zn–naphthalene
ensemble, or Zn complex of naphthalene fluorescein hybrid, i.e.,
NFH) following displacement approach for the detection of CNÀ
in aqueous media.
The synthesis of chemosensor (NFH) is outlined in Scheme 2. 2-
Hydroxy-1-naphthaldehyde, fluorescein hydrazine and chemosen-
sor (NFH) are synthesized according to reported procedure.14 The
structure of the receptor is confirmed by 1H NMR and HRMS
spectra.
In NFH chemosensor naphthalene and fluorescein chromoph-
ores act as an energy donor and an acceptor, respectively. First,
the emission of the naphthalene chromophore and the absorption
of the ring-opened fluorescein dye were measured (Fig. 1). From
this figure, it is clear that there is an overlap between these two
spectra, indicating that the FRET from the naphthalene chromo-
phore to the fluorescein moiety took place.
Figure 1. The overlap (shown with green shade) between the emission and
absorption spectra of the donor (naphthalene) and acceptor (fluorescein) moieties.
Figure 2a shows the absorption spectra of NFH as a function of
Zn2+ concentration in aqueous solution at room temperature. Addi-
tion of Zn2+ ion the absorption band at 370 nm is decreased rapidly
and the absorption intensity at 436 nm is increased with an isos-
bestic point at 397 nm, which corresponds to the absorption of
fluorescein. The absorption stabilized after the amount of added
Zn2+ reached 1 equiv and a significant color change from colorless
to yellow could be observed easily by the naked eye (Fig. 2a, inset).
Job’s plot shows a 1:1 stoichiometry between NFH and Zn2+
(Fig. S1).
emission at 524 nm at the expense of the emission at 431 nm,
exhibited a clear green fluorescence (Fig. 2b). Almost complete
enhancement happened when 1 equiv of Zn2+ was added to the
solution of NFH (Supporting information).
The sensor NFH–Zn2+ for cyanide detection was prepared in situ
by addition of 1 equiv of ZnCl2 to CH3OH/H2O (3:7, v/v) solution of
NFH. Now, the UV–vis and fluorescence studies were carried out at
pH 7.1. The yellow color of the NFHÁZn2+ disappeared upon addi-
tion of an increasing concentration of CNÀ (Fig. 3, inset). The absor-
bance at 436 nm decreased and it increased at 370 nm with an
The probe NFH, when excited at 371 nm, exhibits a weak emis-
sion at 415 and 524 nm. In the absence of Zn2+, the excited energy
of the naphthalene donor could not be transferred to the fluores-
cein acceptor, as the fluorescein acceptor is in the closed form.
Thus, the FRET should be off in the free NFH. Upon addition of an
increasing amount of Zn2+
, a dramatic enhancement of the
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
700
600
500
400
300
200
100
0
280
350
420
490
400 450 500 550 600
Wavelength (nm)
Wavelength (nm)
Figure 2. (a) UV–vis absorption titration spectra and (b) Fluorescence spectra of
NFH (c = 2.0 Â 10À5 M) in presence of Zn2+ (c = 2.0 Â 10À4 M, 0–200
ll) at pH 7.1 in
Scheme 1. Approach for the detection of cyanide ions on the basis of Zn-complex.
CH3OH/H2O (3:7, v/v).
CHO
OH
H2N N
OH
NH2NH2
NaOH, CHCl3
OH
reflux, 6 hr
MeOH, reflux, 3hr
(A)
(C)
O
COOEt
O
NH2
COOH
N
H2SO4
NH2NH2
EtOH, reflux
MeOH, reflux, 12hr
HO
OH
O
HO
O
(D)
HO
O
O
(B)
HO
O
(C)
N
(A)
N
(D)
(B)
EtOH, reflux, 18hr
Ethanol, reflux, 12 hr
HO
OH
O
(NFH)
Scheme 2. Reaction scheme for preparation of NFH.
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S. Goswami et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
(a)
(b)
(a)
(b)
1.5
30
25
20
15
10
5
1.5
1.0
0.5
0.0
1.2
Y = 0.096 - 0.0406
R2 = 0.99
0.5
0.9
0.4
0.3
0.2
0.1
0.0
0.6
0.3
0.0
0
5
10 15 20 25
-
[ CN] in micromolar
Figure 5. (a) Ratiometric response of NFH–Zn2+ (2.0 Â 10À5 M) toward anions in
UV–vis titration. (b) Fluorescence response of NFH–Zn2+ (2.0 Â 10À5 M) toward
anions (1 equiv) with emission spectroscopy.
300
350
400
450
500
Wavelength (nm)
Figure 3. (a) Absorbance spectra of Zn2+ ensemble upon addition of CNÀ at pH 7.1.
The naked eye color change of NFH + Zn2+ with addition of CNÀ (inset) (b)
Ratiometric response of NFH + Zn2+ ensemble (2.0 Â 10À5 M) toward CNÀ.
600
500
400
300
200
100
0
Figure 6. Color changes on test paper (a) NFH; (b) NFH in presence of Zn2+; (c) NFH
Zn2+ in presence of CNÀ under ambient and UV light.
420
490
560
630
Wavelength (nm)
and the CNÀ concentration. Moreover, when the same equiv of
Zn2+ was added for its interaction with CNÀ, which showed fluores-
cence recover for 6 switching cycles (Fig. S4). The equilibrium com-
petition constant15 was 170 MÀ1 on the basis of the fluorescence
titration data in HEPES buffer solution at pH 7.1. The data fit to a
displacement model that provides an equilibrium competition
constant (Fig. S5). Finally, the detection limit was measured to be
Figure 4. Fluorescence emission spectra of Zn2+ ensemble upon addition of CNÀ at
pH 7.1 (kex = 371). The fluorescence color change of NFH + Zn2+ with addition of CNÀ
(inset).
isosbestic point at 397 nm upon the addition of an increased con-
centration of CNÀ (Fig. 4). The addition of 1.2 equiv of CNÀ to
NFHÁZn2+ turned the original yellow colored solution into a color-
less solution. The decrease of yellow color of the solution contain-
ing the NFHÁZn2+ complex strongly suggested that the ring opened
amide form of NFHÁZn2+ was converted to the spirolactam form of
NFH in the presence of CNÀ. The selectivity of the UV response of
NFHÁZn2+ was verified in the presence of different anions such as
BrÀ, ClÀ, IÀ, FÀ, ADP, ATP, PPi, OAcÀ, Pi, SHÀ, SCNÀ, N3À (Fig. 5a).
Addition of CNÀ to the solution of the complex of NFHÁZn2+
brought the reverse change in the fluorescence spectra (Fig. 4)
via reverse FRET phenomenon. The fluorescence of NFHÁZn2+ at
524 nm dramatically decreased along with small enhancement at
415 nm with a clear isoemission point at 452 nm upon the addition
of CNÀ. The changes of the emission intensities became constant
and enhanced fluorescence of NFHÁZn2+ was recovered by 98%
eventually when the amount of CNÀ (2.0 Â 10À4 M) added reached
1.2 equiv).
As there were more errors when the volume of the mixture was
gradually increasing, it could not achieve better fluorescence
recovery later. So, it is clear that all of the anions, except CNÀ,
are practically insensitive to the fluorescence of the NFHÁZn2+
complex and binding of CNÀ ions to Zn2+ led to regeneration of
the cyclic lactam form and confirms the reversible binding of fluo-
rescein dyad to Zn2+. One of the most important features of NFH–
Zn2+ is its high selectivity toward CNÀ over other competitive
anions. Only ADP and ATP interfere a little more in this case
(Fig. S3). The sensitivity for CNÀ was calculated on the basis of
the linear relationship between the emission intensity at 552 nm
0.509 lM. According to the World Health Organization (WHO),
CNÀ concentrations lower than 1.9
lM are acceptable in drinking
water.16 This means that this system is sensitive enough to moni-
tor cyanide concentrations in drinking water (Fig. S6).
Now, we became interested in studying the binding affinity of
NFH–Zn2+-ensemble for CNÀ using 1H NMR and 13C NMR spectros-
copy. Interestingly, the peak at 11.493 ppm (for the naphthalene
OH proton) shifted to 11.508 ppm due to binding with CNÀ. More-
over, the peak at 8.569 ppm shifted to 8.304 ppm and most of the
aromatic protons shifted to upfield due to formation of NFH. In the
case of 13C NMR, the peaks at 206.65 ppm (for spirolactum carbon
atom), 201.37 ppm (for emine carbon atom) and 171.51 ppm (9
carbon in fluorescein), respectively, shifted to 163.33 ppm,
152.24 ppm, and 79.18 ppm, respectively, when CNÀ was added
to NFHÁZn2+ ensemble. Thus, from 1H NMR and 13C NMR titration
we conclude that NFH–Zn2+ ensemble behaves as a very smart
probe for CNÀ.
To further look into the nature of the interaction between NFH–
Zn2+ and CNÀ, HRMS mass spectrometric experiments were carried
out. When 1 equiv of CNÀ was added to this complex, the peak at
m/z 562.0517 corresponding to (NFHÀH++Zn2+ +
) disappeared and
only the peak at m/z 501.1495 corresponding to [NFH + H+]+ was
observed. This suggests that the binding affinity of CNÀ toward
Zn2+ is much more than that of NFH toward Zn2+ and thus the
removal of Zn2+ from the complex by the CNÀ induced ring-closure
reaction.
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S. Goswami et al. / Tetrahedron Letters xxx (2014) xxx–xxx
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tion limit of CNÀ is quite low (0.509
lM) using this ensemble. The
color changes would be useful for visual detection which is cost-
effective and convenient without resorting to any instrument. This
system also mimics INHIBIT gate.
Acknowledgments
DST and CSIR (Govt. of India) are gratefully acknowledged by
the authors for financial support and fellowship. S.P. and A.M.
acknowledges UGC and CSIR, respectively, for their fellowships.
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Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.05.
018.
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