A ratiometric fluorescent probe for cyanide based on FRET

Article abstract of DOI:10.1039/c1ob05387f

On the basis of FRET from 4-(N,N-dimethylamino)benzamide to fluorescein, a new ratiometric fluorescence probe bearing a hydrazone binding unit was developed for highly selective and sensitive detection of CN^- in aqueous solution. The Royal Soci

Full text of DOI:10.1039/c1ob05387f

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A ratiometric fluorescent probe for cyanide based on FRET†
Xin Lv, Jing Liu, Yunlong Liu, Yun Zhao, Maliang Chen, Pi Wang and Wei Guo*
Received 12th March 2011, Accepted 5th May 2011
DOI: 10.1039/c1ob05387f
On the basis of FRET from 4-(N,N-dimethylamino)benzamide to fluorescein, a new ratiometric
fluorescence probe bearing a hydrazone binding unit was developed for highly selective and sensitive
detection of CN- in aqueous solution.
as instrumental efficiency, environmental conditions, and the
Introduction
probe concentration can interfere with the signal output. The
ratiometric method, measuring the ratio of fluorescence intensities
at two wavelengths, provides an alternative approach that can
overcome the drawbacks of intensity-based measurements by
built-in correction of two emission bands and seems to be more
favorable for sensing target ions in comparison with fluorescence
intensity-based probes.
In general, ratiometric probes can be designed to function
following two mechanisms: intramolecular charge transfer (ICT)
and fluorescence resonance energy transfer (FRET). ICT probes
have been frequently reported and some work well. However,
these spectra-shift type probes, in many cases, show relatively
broad emission spectra with a high degree of overlap before and
after binding target ions, which makes it difficult to accurately
determine the ratio of the two fluorescence peaks. Theoretically,
the problem can be avoided by using a FRET-based sensor for
which the single excitation wavelength of a donor results in
emission of the acceptor at a longer wavelength. However, up to
now, although some ratiometric fluorescence probes for CN- have
been reported in the literature,11 to our knowledge, ratiometric
fluorescence probes for CN- based on FRET are considerably
limited.11e
Anion recognition is an area of growing interest in supramolec-
ular chemistry due to its important role in a wide range of
environmental, clinical, chemical, and biological applications.1
Among the various anions, cyanide is one of the most important
anions due to it being widely used in synthetic fibers, resins,
herbicides, and the gold-extraction process.2 Unfortunately, the
cyanide anion is extremely detrimental and can be absorbed
through the lungs, gastrointestinal track and skin, leading
to vomiting, convulsion, loss of consciousness, and eventual
death.3 Thus, there is a need for an efficient sensing system for
cyanide, to monitor the cyanide concentration from contaminant
sources.
In the past few years, a variety of colorimetric and fluorescent
probes for cyanide have been reported. One of the common
approaches is utilizing cyanide complexes or addition with Zn2+–
porphyrin,4a Ru2+–pyridine,4b boronic acid derivatives4c or CdSe
quantum dots.4d Other strategies have also been used, such as
hydrogen bonding interactions,5 copper–cyanide affinity6 and
single-electron transfer reaction.7 Recently, nucleophilic addition
of cyanide has also been adopted for sensing cyanide. This type
of recognition mode takes advantage of a particular feature
of the cyanide ion: its nucleophilic character, and enables the
recognition system with some characteristic features such as
CN--specific response and little competition from the aqueous
media. Based on this idea, nucleophilic addition of cyanide to
oxazine, pyrylium, squarane, trifluoroacetophenone, acyltrazene,
acridinium, salicylaldehyde and carboxamide has been reported,8
in which the interference by other anions, such F- and AcO-, can
be efficiently minimized.
Herein, we present
a 4-(N,N-dimethylamino)benzamide–
fluorescein FRET “off-on” system 1 as a ratiometric fluorescence
probe for CN-. The probe was designed to involve three parts: a 4-
(N,N-dimethylamino)benzamide energy donor, a salicylaldehyde
hydrazone binding unit,12 and a fluorescein spirolactone unit
(potential fluorescein monoanion energy receptor). It is expected
that nucleophilic attack by cyanide toward the activated hydrazone
functionality, followed by fast proton transfer of the acidic phenol
proton to the developing nitrogen anion of this compound, will
trigger ring-opening inthe fluoresceinspirolactone unit to generate
a long-wavelength fluorescein monoanion fluorophore13 that can
act as the energy acceptor (Fig. 1). Although the efficiency of
FRET is affected by the distance between the donor and the
acceptor and the relative orientation of transition dipoles of both
the donor and acceptor, it is mainly determined by the extent of
the spectral overlap between the donor emission and the acceptor
absorption.14 Thus, 4-(N,N-dimethylamino)benzamide was cho-
sen as the energy donor in our work, because its fluorescence
Fluorescence spectroscopy has become a powerful tool for
sensing and imaging trace amounts of samples because of its
simplicity and sensitivity.9 However, most of the reported examples
of fluorescent sensing of CN- function by fluorescence quenching
or fluorescence enhancement.4b,4d,6a,8e,8h,8j–8l,10 As the change in
fluorescence intensity is the only detection signal, factors such
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan
030006, China. E-mail: guow@sxu.edu.cn; Tel: +86 351 7011600
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1ob05387f
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Scheme 1 Synthesis of compound 1.
Fig. 1 A plausible mechanism of response of 1 to CN-.
temperature, and could complete within 5 s (Fig. S1†), suggesting
the rapid detection ability of 1 for CN-.
spectrum matches well with the absorption spectrum of fluorescein
monoanion, fulfilling a favorable condition for FRET (Fig. 2). In
the absence of CN-, the fluorescein moiety adopts a closed, non-
fluorescent spirolactone form. As a result, FRET is suppressed,
and only the blue emission of the donor is observed upon excitation
of the 4-(N,N-dimethylamino)benzamide chromophore. Binding
to CN- induces opening of the fluorescent fluorescein monoanion
moiety, corresponding to intense absorption in the 4-(N,N-
dimethylamino)benzamide emission region. Spectral overlap is en-
hanced, and excitation of the 4-(N,N-dimethylamino)benzamide
chromophore results in strong emission of fluorescein monoanion
owing to FRET.
Changes in the UV-vis spectra for 1 in the absence and presence
of CN- in DMF–water solution (9 : 1, v/v) are shown in Fig. 3.
The UV-vis spectrum of 1 shows mainly the absorption profile of
the donor (4-(N,N-dimethylamino)benzamide group), which has
a maximum at 350 nm. In addition, a weak absorption band at
515 nm is also observed, which resulted from the trace ring-opened
form of the fluorescein unit in 1. This result suggests that 1 exists
mainly in the fluorescein spirocyclic form. The characteristic peak
of the spirocycle carbon (C-9) shift of 1 in the alkyl region (near 81
ppm) in the 13C NMR also supports this consideration (Fig. S10†).
When CN- was introduced into the sensing system, the band at
350 nm was attenuated and the band at 515 nm sharply increased
with the increase in the CN- concentration, indicating that the
addition of CN- can promote the formation of the ring-opened
form of 1 from the spirocyclic form. The absorption stabilized after
the amount of added CN- reached 20 equiv. and a significant color
change from colorless to brown could be easily observed by eye.
The absorption titration profile at 515 nm versus concentration
of CN- is shown in Fig. 3 inset. The association constant for
CN- was estimated to be 4.78 ¥ 104 M-1 (R2 = 0.9974) on the
basis of nonlinear fitting of the titration curve, assuming 1 : 1
stoichiometry.16 Job plot analysis of the UV-vis titrations revealed
a maximum at about 0.5 mole fraction (Fig. S2†), in accord with
the proposed 1 : 1 binding stoichiometry.
Fig.
2
Spectral overlap of 4-(N,N-dimethylamino)benzoylhydrazine
emission (blue) with fluorescein absorption (red).15
Results and discussion
Probe 1 is synthesized via a Reimer–Tiemann formylation re-
action of fluorescein followed by condensation with 4-(N,N-
dimethylamino)benzoylhydrazine in refluxing ethanol (Scheme 1).
The experimental details and characterization data for 1 are given
in the Experimental section and ESI.† CN- detection using probe
1 was found to be sensitive to water content. Increasing the water
content resulted in a decrease in the fluorescence intensity of the
1–CN- complex. In view of the solubility and sensitivity of 1,
a DMF–water (9 : 1, v/v) solution was used in the subsequent
investigation. The time course studies of the absorption and
fluorescence responses in DMF–water (9 : 1, v/v) solution revealed
that the binding process of CN- to 1 was very fast at room
Fig. 3 Change in absorption spectra of 1 (20 mM) measured in DMF–
water (9 : 1, v/v) upon addition of CN-. Inset: absorption titration profile
(at 515 nm) versus concentration of CN- for 1.
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Fig. 4 shows the fluorescence spectra of 1 exposed to DMF–
water solution (9 : 1, v/v) containing different concentrations of
CN-. The spectrum of 1 without CN- exhibits a blue emission at
450 nm of donor (4-(N,N-dimethylamino)benzamide group) and
a weak yellow emission at 530 nm of energy acceptor. Similar to
the case of absorption spectrum of 1, the latter can be ascribed to
the trace ring-opened form of fluorescein unit in 1. Upon addition
of CN-, the donor emission at 450 nm decreased, and the acceptor
emission band gradually increased in intensity with a substantial
red shift from 530 nm to 550 nm, indicating that the configuration
transformation of the fluorescein moiety from the spirocyclic form
to a ring-opened form. These changes in the fluorescence spectrum
stopped and the ratio of the emission intensities at 550 nm and
450 nm (I550/I450) became constant when the amount of CN- added
reached 26 equiv., and the emission intensity ratio, I550/I450, varied
from 0.60 to 16.6. The color of the fluorescence changed from
blue to yellow. It was clear that the FRET process was switched
on by CN- as excitation of the 4-(N,N-dimethylamino)benzamide
group at 350 nm resulted in the emission of fluorescein with a
maximum of 550 nm. In addition, the excitation spectra obtained
by collecting emission data of 1–CN- adduct (1 + 20 equiv. CN-)
at 550 nm shows the donor and the acceptor bands (Fig. S3†),
respectively, indicating that both the transitions participate in the
emission process and an efficient energy transfer can occur from
the 4-(N,N-dimethylamino)benzamide donor to the fluorescein
monoanion receptor. Of note, the large wavelength shift (200 nm)
between the donor excitation and the acceptor emission can
eliminate any influence of excitation backscattering effects on
the fluorescence assay. When examining the emission changes at
550 nm upon gradual addition of CN-, a good linear working
range from 0–90 mM was observed (Fig. S4†). The detection limit
was measured to be 4.4 ¥ 10-7 M at S/N = 3. According to the
World Health Organization (WHO), cyanide concentrations lower
than 1.9 mM are acceptable in drinking water.17 This means that
our proposed fluorescent method based on probe molecule 1 is
sensitive enough to monitor cyanide concentrations in drinking
water.
The spectral response of 1 in the absence and presence of CN- at
different pH values was also evaluated (Fig. 5). At pH values lower
than 3, no obvious characteristic absorption of the fluorescein
chromophore could be observed regardless of the presence and
absence of CN-, indicating that no reaction occurs between 1 and
CN-. This is possibly because the protonation of CN- decreases
the actual concentration of CN- in the sample solution. Between
pH 4 and 8, free 1 showed a weak absorption band at 515 nm
due to the trace ring-opened form of the fluorescein unit in 1.
However, upon addition of CN-, 1 responded stably to CN-
without any interference by protons. At pH > 8, the spectral
response also occurred upon addition of CN-, but the absorbance
of free 1 increased with increasing pH. This could be ascribed to
the deprotonation of the phenol OH group in 1 in basic conditions,
resulting in the ring-opening of the spirolactone moiety of 1. The
results indicate that 1 can successfully react with CN- and allows
CN- detection at a range of pH values (pH 4–8).
Fig. 5 Changes in absorption (515 nm) of 1 (20 mM) in DMF–water (9 : 1,
v/v) measured with and without CN- (20 equiv.) as a function of pH.
Subsequently, the selectivity of 1 for CN- was evaluated (Fig. 6).
Changes in the fluorescence spectra of 1 caused by CN- and
miscellaneous anions generally used in the literatures, including
-
-
-
F-, Cl-, Br-, I-, AcO-, N3 , SCN-, ClO4 , NO3 , SO42-, and
H2PO4- as their sodium salts, are recorded in Fig. 6a. Most of the
competitive anions did not lead to significant fluorescence changes,
whereas F-, AcO- and H2PO4 had a slight interference due to
-
the deprotonation reaction of 1 by these weakly basic anions to
partly trigger the ring-opening of the fluorescein unit. For anions,
2-
-
such as Cl-, Br-, SO4 and NO3 , which can be present in higher
concentration in practice, no significant interference was observed
even if their concentration reached 10 mM (Fig. S5†). For more
basic anions, such as PO43- andCO32-, the interference is significant
due to the deprotonation reaction (Fig. S6†). In addition, the
competition experiments were also measured by addition of 25
equiv. of CN- to the aqueous solutions of 1 in the presence of
3-
2-
25 equiv. of miscellaneous anions except for PO4 and CO3
Fig. 4 Fluorescence titration spectra of 1 (5 mM) in DMF-water (9 : 1)
upon gradual addition of CN-. Inset: Fluorescence intensity ratio changes
(I550/I450) of 1 upon gradual addition of CN-. lex = 350 nm. Slits:
5 nm/5 nm.
(Fig. 6b). The competitive anions had no obvious interference with
the detection of CN-, indicating the system was hardly affected by
these coexistent anions.
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Fig. 7 1H NMR spectral change of 1 in DMSO-d6 upon addition of
cyanide anions.
547.0 (calcd = 547.1) corresponding to [1 + CN-]- was observed
(Fig. S8†). All of these results are consistent with our proposed
mechanism.
Conclusions
In summary, we have developed a FRET-based ratiometric probe
1 that can selectively and sensitively detect CN- in aqueous
solution. It exhibits a clear CN--induced change in the intensity
ratio of the two well-separated emission bands of 4-(N,N-
dimethylamino)benzamide group and fluorescein. The significant
changes in the color and fluorescence can be observed by the naked
eye.
Experimental section
Fig. 6 (a) Fluorescence responses of 1 (5 mM) upon addition of 25 equiv.
of various other anions in DMF–water (9 : 1) solution. (b) Fluorescence
responses of 1 (5 mM) to CN- (25 equiv.) containing 25 equiv. of various
anions. lex = 350 nm. Slit: 5 nm/5 nm.
Materials and general methods
All reagents and solvents were purchased from commercial
sources and were of the highest grade. Solvents were dried
according to standard procedures. All reactions were magnetically
stirred and monitored by thin-layer chromatography (TLC). Flash
chromatography (FC) was performed using silica gel 60 (200–
300 mesh). Absorption spectra were taken on an Agilent 8453
spectrophotometer. Fluorescence spectra were taken on Varian
With regards to the mechanism, the nucleophilic attack of CN-
to hydrazone functionality in 1 followed by the ring-opening of the
spirocyclic moiety of fluorescein (Fig. 1) is likely to be responsible
for the spectral change. The result obtained from a Job plot
supports the formation of a 1 : 1 1–CN- adduct (Fig. S2†). In
addition, a clear isosbestic point at 368 nm and a well-defined
isoemissive point at 506 nm in the absorption and fluorescence
spectra, respectively, upon addition of CN- to the solution of
1 indicates that the binding of 1 with CN- produces a single
Cary Eclipse fluorescence spectrometer. The H NMR and 13C
1
NMR spectra were recorded at 300 and 75 MHz, respectively. The
following abbreviations were used to explain the multiplicities: s =
singlet; d = doublet; t = triplet; q = quartet; m = multiplet; br =
broad. High resolution mass spectra were obtained on a Varian
QFT-ESI mass spectrometer.
1
component. Furthermore, we investigated the H NMR spectra
of 1 in the presence of CN- and compared it with that of the
sensor itself (Fig. 7). The imine proton (Ha) at around d 9.22 ppm
was dramatically shifted upfield toward d 5.60 ppm upon CN-
addition, indicating that the CN- functions as a nucleophile. Also,
upon addition of CN- to the DMSO-d6 solution of 1, the signal
of C-9 (spirocyclic carbon atom, 81 ppm) disappeared (Fig. S7†),
which supports the ring-opening of the spirocyclic structure of
1 due to the nucleophilic attack of the CN- anion.18 The solid
evidence of the 1–CN- interaction mode comes from the MS
spectra of the adduct of 1 with CN-, in which the peaks at m/z
Synthesis of compound 2
Fluorescein-monoaldehyde 2 was synthesized according to the
previous report.19 Fluorescein (4 g, 12 mmol), 10 mL CHCl3,
6 mL MeOH and 0.06 g 15-crown-5 were placed in a 100 mL
flask. Then 20 g 50% NaOH solution was carefully added while the
reaction temperature was maintained at 55 C. The mixture was
stirred at this temperature for 5 h. After cooling, the mixture was
acidified with 10 M H2SO4. The precipitates were collected and
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dried in vacuo. Chromatography on silica gel (EtOAc : CH2Cl2,
5 (a) P. Anzenbacher, Jr., D. S. Tyson, K. Jurs´ıkova´ and F. N. Castellano,
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15 : 85) and recrystallization in CH3OH afforded 1.5 g product as
1
pale yellow solid. Yield: 34%. H NMR (CDCl3) d (ppm): 12.10
(s, 1H), 10.59 (s, 1H), 8.00 (d, J = 7.5, 1H), 7.63 (m, 2H), 7.12 (d,
J = 7.2, 1H), 6.85 (d, J = 9.0, 1H), 6.76 (s, 1H), 6.58 (m, 3H).
7 M. R. Ajayakumar and P. Mukhopadhyay, Org. Lett., 2010, 12, 2646.
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Synthesis of compound 1
A solution of 4-(N,N-dimethylamino)benzoylhydrazine (54 mg,
0.3 mmol) and fluorescein-monoaldehyde 2 (108 mg, 0.3 mmol)
in ethanol (50 mL) was heated under reflux for 3 h. After cooling,
the precipitated crystals were filtered and washed with ethanol to
give pure 1 as yellow solid (130 mg, 83%). Mp: > 250 C; H
NMR (300 MHz, DMSO-d6) d 9.22 (s, 1H), 8.05 (d, J = 7.2, 1H),
7.73–7.98 (m, 4H), 7.37 (d, J = 7.5, 1H), 6.64–6.85 (m, 8H), 3.06 (s,
6H); 13C NMR (75 MHz, DMSO-d6) d 167.6, 161.4, 158.5, 151.8,
151.2, 150.2, 148.3, 141.8, 134.6, 129.2, 128.3, 125.0, 123.8, 123.1,
117.1, 112.2, 110.0, 108.4, 105.0, 101.0, 81.5; MALDI-TOF MS:
calcd. for (M + H)+ 522.166, found 522.109 (M + H)+; Anal. calcd
for C30H23N3O6: C, 69.09; H, 4.45; N, 8.06. Found: C, 69.11; H,
4.48; N, 8.17%.
◦
1
Procedure for ion sensing
Deionized water was used throughout all the experiments. All
anions were prepared from their sodium salts. A stock solution of
1 (5 mM) was prepared in DMF. The stock solution of 1 was then
diluted to the corresponding concentration (20 mM, 5 mM) with
the solution of DMF–water (9 : 1, v/v). The sodium cyanide stock
solution of 1.0 ¥ 10-1 M was diluted to 1.0 ¥ 10-2 M and 1.0 ¥
10-3 M with deionized water for spectra titration studies. Spectra
data were recorded in an indicated time after the addition.
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Acknowledgements
We acknowledge the Natural Science Foundation of China (NSFC
No. 20772073 and 21072121) and the Natural Science Foundation
of Shanxi Province (2008011015) for support of this work.
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4958 | Org. Biomol. Chem., 2011, 9, 4954–4958
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