A novel ratiometric probe based on rhodamine B and coumarin for selective recognition of Fe(III) in aqueous solution

Article abstract of DOI:10.1016/j.dyepig.2013.06.024

We have developed a new ratiometric fluorescence probe based on rhodamine B and coumarin to monitor the Fe³⁺ with high sensitivity and selectivity. Upon addition of Fe³⁺ to aqueous solution of the probe, two fluorescence peaks at 580 nm and 460 nm were observed, which belong to rhodamine B and coumarin, respectively. This is a novelty design of ratiometric probe of Fe³⁺, due to CHEF process generated along with the PET process suppressed simultaneously. The fluorescence intensity at 580 nm was significantly increased about 120-fold with 5 equiv. of Fe³⁺ added in aqueous solution.

Full text of DOI:10.1016/j.dyepig.2013.06.024

Dyes and Pigments 99 (2013) 661e665
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A novel ratiometric probe based on rhodamine B and coumarin for
selective recognition of Fe(Ⅲ) in aqueous solution
Fei Ge, Hui Ye, He Zhang, Bao-Xiang Zhao*
Institute of Organic Chemistry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 April 2013
Received in revised form
We have developed a new ratiometric fluorescence probe based on rhodamine B and coumarin to
monitor the Fe with high sensitivity and selectivity. Upon addition of Fe to aqueous solution of the
3þ
3þ
probe, two fluorescence peaks at 580 nm and 460 nm were observed, which belong to rhodamine B and
8
June 2013
3þ
coumarin, respectively. This is a novelty design of ratiometric probe of Fe , due to CHEF process
Accepted 17 June 2013
Available online 26 June 2013
generated along with the PET process suppressed simultaneously. The fluorescence intensity at 580 nm
was significantly increased about 120-fold with 5 equiv. of Fe3 added in aqueous solution.
þ
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Rhodamine B
Coumarin
Ratiometric probe
CHEF
PET
3þ
Fe
1
. Introduction
Ratiometric probes can be designed to function following two
mechanisms: intramolecular charge transfer (ICT) [8e11] and
fluorescence resonance energy transfer (FRET) [12e16]. Relatively
broad fluorescence spectra are often observed for ICT fluorophores;
in a significant number of cases the broad fluorescence spectra
before and after binding target ions have a high degree of overlap
(or in an extreme case, a broad spectrum with high intensity
completely covers one with lower intensity, which makes it difficult
to accurately determine the ratio of the two fluorescence peaks.
However, probes based on FRET can avoid these disadvantages in a
certain extent [7,17]. Although some fluorescent probes for
Fe3 plays a major role in many biochemical processes at the
þ
3þ
cellular level. High levels of Fe within the body have been asso-
ciated with increasing incidence of certain cancers and dysfunction
of certain organs, such as the heart, pancreas, and liver [1,2].
Accordingly, the development of methods, enabling professionals
to spatially and temporally track intracellular Fe , is challenging
but essential to address these issues and has become the subject of
current chemical research.
3
þ
Optical cellular imaging with fluorescent probes might be the
3
þ
3þ
best choice for visualizing the intracellular Fe ion by virtue of
its high sensitivity, high-speed spatial analysis, and less cell-
damaging. However, most reported examples of fluorescent
detecting Fe have been reported [18e21], up to now, only a few
3
þ
ratiometric probe for Fe was reported [19,22] based on cyclo-
dextrin supramolecular complex and quinoline, respectively.
Moreover, the synthesis process of the probe based on cyclodextrin
superamolecular is complex, and the fluorescence intensity of
quinoline acted as donor is week.
sensing of Fe3 ions in living cells were functioned through the
enhancement of fluorescence signals [3e5]. As the change in
fluorescence intensity is the only detection signal, factors such as
instrumental efficiency, environmental conditions, and the probe
concentration can interfere with the signal output [6,7]. Ratio-
metric probes can eliminate most or all interferences by built-in
correction of two emission bands and seem to be more favorable
for imaging intracellular metal ions in comparison with fluores-
cence enhancement probes.
þ
Thus, we designed a coumarin-rhodamine system L as a ratio-
3
þ
.
metric probe for Fe
Two fluorescence peaks which belong to coumarin and
3
þ
rhodamine exist simultaneously. Upon addition of Fe , a new
fluorescence emission peak appeared at 580 nm. This wavelength
3
þ
change allows the ratiometric detection of Fe ions in ethanol/
water solution. To the best of our knowledge, this is a novelty
3
þ
based on the conjugated
design of ratiometric probe of Fe
link of the rhodamine and coumarin. Herein, we develop a new
*
Corresponding author. Tel.: þ86 531 88366425; fax: þ86 531 88564464.
E-mail addresses: bxzhao@sdu.edu.cn, sduzhao@hotmail.com (B.-X. Zhao).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.06.024

662
F. Ge et al. / Dyes and Pigments 99 (2013) 661e665
Scheme 1. Synthesis of probe L.
ratiometric probe L for Fe3þ, which differs from reported ratio-
metric probe.
2.4. Synthesis of N-(3 ,6 -bis(diethylamino)-3-oxo-4a’,9a’-
0
0
0
dihydrospiro [isoindoline-1,9 -xanthen]-2-yl)-7-(diethylamino)-2-
oxo-2H-chromene-3-carboxamide (L)
2
. Experimental sections
Compound 3 (0.279 g, 1.0 mmol) and 4 (0.547 g, 1.2 mmol)
was dissolved in 20 mL of acetonitrile. The mixture was stirred
for 8 h under nitrogen at room temperature. Concentration of
the mixture under reduced pressure gave a yellow solid. The
residue was purified by silica gel column using petroleum ether:
ethyl acetate ¼ 1:1 as eluent to obtain an orange solid L (0.51 g)
2
.1. Materials and methods
Deionized water was used throughout the experiment. All the
reagents were purchased from commercial suppliers and used
without further purification. All samples were prepared at room
temperature, shaken for 10 s and waited for 18 h before UVevis and
fluorescence determination. The solutions of metal ions were pre-
pared from NaNO
Cr(NO $9H O, Fe(NO
Cu(NO O, Zn(NO
HgCl $H O, Pb(NO , respectively, and were dissolved in distilled
ꢀ
ꢁ1
in 73% yield; m.p. 305 C. IR (KBr, cm ): 3461.7, 2971.4, 2928.4,
1725.4, 1692.3, 1615.9, 1582.3, 1512.7, 1353.4, 1215.8, 1117.1,
1
3
,
Mg(NO
$9H O, Co(NO
$6H O, AgNO , Cd(NO
3 2
) $6H
2
O, KNO
3
,
Ca(NO
O, Ni(NO
$4H
3
)
2
$4H
2
O,
O,
819.0, 787.6; H NMR (300 MHz, DMSO-d
6
)
d
(ppm) 1.05e1.15
)
3 3
2
3
)
3
2
3 2
) $6H
2
3
)
2
$6H
2
(18H, m, CH
3
), 3.30e3.38 (8H, m CH
2
), 3.44e3.51 (4H, m, CH ),
2
3 2
)
$3H
2
3
)
2
2
3
3
)
2
2
O, Ba(NO
3
) ,
2
6.33 (2H, d, J ¼ 2.4 Hz, ArH), 6.37e6.40 (2H, m, ArH), 6.57 (1H, d,
J ¼ 1.8 Hz, ArH), 6.62 (1H, s, ArH), 6.65 (2H, s, ArH), 6.78e6.82
(1H, m, ArH), 7.02e7.05 (1H, m, ArH),7.51e7.56 (2H, m, ArH),
2
2
3 2
)
water. Thin-layer chromatography (TLC) was conducted on silica gel
0F254 plates (Merck KGaA). HEPES buffer solutions (pH 7.2) were
prepared using 20 mM HEPES, and proper amount of aqueous sodium
13
6
7.57-7.65 (1H, m, ArH), 7.84e7.87 (1H, m, NH),
(75 MHz, DMSO-d (ppm) 12.4, 43.6, 65.2, 97.2, 103.8, 107.0,
118.0, 128.5, 132.0, 145.0, 148.4, 149.0, 152.8, 157.5, 161.1, 162.2,
C NMR
6
) d
1
13
hydroxide under adjustment by a pH meter. H NMR and C NMR
spectra were recorded on a Bruker Avance 300 spectrometer, using
DMSO as solvent and tetramethylsilane (TMS) as internal standard.
Melting points were determined on an XD-4 digital micro melting
point apparatus. IR spectra were recorded with an IR spectropho-
tometer VERTEX 70 FT-IR (Bruker Optics). HRMS spectra were recor-
ded on a Q-TOF6510 spectrograph (Agilent). UVevis spectra were
recorded on a U-4100 (Hitachi). Fluorescent measurements were
recorded on a PerkinElmer LS-55 luminescence spectrophotometer.
þ
þ
5
164.0; HRESIMS calcd for [M þ H]
42 46 5
C H N O : 700.3499
found: 700.3422.
2.2. Synthesis of ethyl 7-(diethylamino)-2-oxo-2H-chromene-3-
carboxylic acid (2)
A mixture of 1 (1.15 g, 4 mmol) and NaOH (0.48 g 12 mmol) in
EtOH (20 mL) was refluxed for 1 h, after which an orange solid was
precipitated. When the reaction was cooled to room temperature,
the mixture was added to a breaker containing 150 mL water. The
orange solid was dissolved and adjust the pH of the solution to
3
w 4. An orange solid 7-(diethylamino) coumarin-3-carboxylic
ꢀ
acid 2 (0.845 g) was obtained in 81% yield; m.p. 90e92 C.
2.3. Synthesis of 7-(diethylamino)-2-oxo-2H-chromene-3-carbonyl
chloride (3)
The acid chloride 3 was synthesized by the reaction of the 7-
diethylamino) coumarin-3-carboxylic acid 2 with thionyl chloride
at room temperature in 80% yield.
3þ
Fig. 1. Absorption spectra of 10
mM L upon the addition of Fe (0e4 equiv.) in buffered
(
EtOH/HEPES ¼ 99:1 solution at pH 7.2. The inset shows the ratio of the absorbance
3
þ
(555 nm/425 nm) as a function of Fe concentration.

F. Ge et al. / Dyes and Pigments 99 (2013) 661e665
663
form of the rhodamine B units was confirmed by the presence of a
13
peak at w65.2 ppm in the C NMR spectra.
The UV/Vis spectrum of L solution (EtOH/HEPES 99:1, pH 7.2)
showed only the absorption profile of the coumarin at 425 nm,
which indicates that L exists as a spirocycle-closed form (Fig. S1)
[
24,25]. An obvious absorption at 560 nm induced by addition of
3
þ
Fe confirmed the ring-opened of rhodamine. We can also find
2
þ
2þ
2þ
3þ
that upon addition of Cu , Co , Ni and Fe , the solution of L
display an obvious jacinth color at the micromolar level and other
ions show little interference (Fig. S2).
To get more insight on the binding mode of L and Fe , ab-
sorption spectra titration was performed as shown in Fig.1. It can be
observed that the absorbance peaks at 425 nm and 555 nm
3þ
3
þ
increased upon gradual addition of Fe and remained constant
after 2 equiv. of Fe added. A significant color change from green to
3
þ
pink could be observed easily by naked-eyes (Fig. S2). The non-
3þ
linear fit of the data revealed that the binding of L to Fe was
most probably of 1:1 stoichiometry with an association constant
4
ꢁ1
(
Ka) of about Ka ¼ 1.172 ꢂ 10 M (Fig. S3). This binding mode was
also supported by the data of Job’s plots evaluated from the ab-
3
þ
sorption spectra of L and Fe with a total concentration of 20
Fig. S4).
Excitation at 425 nm introduced an emission at 460 nm. Upon
mM
(
3
þ
addition of Fe , a new fluorescence peak appeared at 580 nm
3
þ
attributed to rhodamine B part (Fig. 2). With the addition of Fe
the peaks at 460 nm and 580 nm increased gradually. And the ratio
of fluorescence intensity at 580 nm and 460 nm (I580nm/I460nm
peaked when 2 equiv. Fe was added. The fluorescence clearly
changed from blue to orange. Finally, the concurrence of fluores-
)
3
þ
cence of rhodamine B part (
part (
Moreover, a 155 nm of strokes shift (425 nme580 nm) of probe L
l
em ¼ 580 nm, red color) and coumarin
l
em ¼ 460 nm, blue color) resulted in an orange color.
can eliminate the interference of most or all ambiguities by self-
3
þ
calibration of two emission bands. The solution of L-Fe
sesses a quantum yield of 0.33 in EtOH/H O (1:1, v/v, pH 7.2) and
0.38 in ethanol, based on optically matching solution of rhodamine
B standard (
F ¼ 0.69 in ethanol) at an excitation wavelength of
25 nm and emission at 580 nm. The detection limit, based on the
pos-
2
m
M L upon the addition of 0e4 equiv Fe3 in
þ
Fig. 2. a) The fluorescence spectra of 10
buffered EtOH/HEPES ¼ 99:1 solution at pH 7.2. The inset shows the fluorescence
F
3þ
photo of L and L-Fe . b) The ratio of fluorescence intensity (580 nm/460 nm) of L as a
4
3
þ
function of Fe concentration.
ꢁ7 ꢁ1
M , which can be
definition by IUPAC (CDL ¼ 3Sb/m), is 5.24 ꢂ 10
obtained from the liner of response range covering a concentration
3
þ
ꢁ6
ꢁ6
ꢁ1
M (Fig. S5). The
3
. Result and discussion
range of Fe from 4.0 ꢂ 10 to 10.0 ꢂ 10
fluorescence intensity enhanced approximately 120 fold at 580 nm.
According to the theory of FRET, coumarin excimer emission
should decrease when such energy transfer takes place. However,
unexpected phenomenon was observed (Fig. 2). To explain the
phenomenon, a mechanism is proposed (Scheme 2). For coumarin
part, the N atom with an unshared electron pair quenches both the
coumarin monomer and excimer emissions strongly in the absence
Probe L was synthesized by the reaction of 7-(diethylamino)-2-
oxo-2H-chromene-3-carbonyl chloride 3 and rhodamine B hydra-
zine 4 in ethanol at room temperature for 12 h in 73% yield (Scheme
1
). Compound 3 was easily obtained by the reaction of 7-(dieth-
ylamino)-2-oxo-2H-chromene-3-carboxylic acid 2 and sulfurous
dichloride at room temperature for 4 h. Compound 4 was synthe-
sized according to our previous report [23]. The structure of L was
3
þ
of Fe due to a photoinduced electron/energy transfer (PET) pro-
cess [26,27]. However, comparing with literature, the fluorescence
1
13
determined by IR, H NMR, C NMR and HRMS. The spirolactam
Scheme 2. Proposed complex mechanism of L-Fe3
þ
.

664
F. Ge et al. / Dyes and Pigments 99 (2013) 661e665
was not quenched completely, so the PET process was not obvious.
When Fe3 is added to the solution of L, the N atom participates in
þ
3
þ
the coordination process of probe L and Fe . So PET process is
suppressed and the fluorescence increased. For rhodamine part,
fluorescence also increased, due to the opening of spirolactam and
chelation-enhanced fluorescence (CHEF). Thus, for the fluorescence
3
þ
phenomenon of the L-Fe
complex, CHEF may play a more
important role than FRET [28]. Enhanced fluorescence of both
3
þ
the coumarin monomer and rhodamine B upon addition of Fe is
observed. So the fluorescence of rhodamine (w580 nm) and
coumarin (w460 nm) enhanced simultaneously.
3þ
Probe L shows an excellent selectivity toward Fe . The fluores-
cence spectra (
ex ¼ 425 nm) of L with respective metal cations are
shown in Fig. 3a. Without cation, L revealed no fluorescence at 500e
50 nm. In fact, L also did not give any observable response for many
l
6
2
þ
þ
þ
2þ
2þ
2þ
2þ
3þ
2þ
2þ
metal ions such as Mg , Na , Mn , Ni , Pb , Zn , Ba , Ca ,
Cd , Co , Cu , or K . However, addition of Fe
2þ
2þ
2þ
created a
remarkable fluorescence enhancement at 580 nm and a little
enhancement at 460 nm. A ratio of fluorescence intensity at 580 nm
3þ
Fig. 4. The effect of pH (4.0e10.0) on the fluorescence intensity ratio (580 nm/460 nm)
of 10 mM probe L with 5.0 equiv. Fe in buffered EtOH/HEPES solution.
vs. 460 nmwas shown in Fig. 3b. Fe can be detected clearly by probe
L. Moreover, an ability to resist interference is an important
index for a cationic probe. The enhancement of the fluorescence in-
3þ
eyes. All of these results indicated the high selectivity of L for Fe3
þ
3þ
tensity resulting from the addition of Fe was little influenced
2
þ
2þ
over other competing cations in EtOH/H O (99:1, v/v, pH 7.2) solution.
2
by subsequent addition of other metal ions except Cu and Ni
3þ
A pH titration showed that probe L had stable fluorescence
property over a wide pH span of 4e10 (Fig. 4), which suggests that
probe L is suitable for application under physiological conditions.
The curves (Fig. S7) of time response showed that the probe can
(
Fig. S6). However, the solution of L-Fe turned a deeper red upon
addition of Cu and Ni , which can be distinguished well by naked
2þ
2þ
3
þ
complex with 5 equiv. Fe completely in 7 min, after which the
fluorescence intensity peaks.
4. Conclusion
In summary, we developed a novel ratiometric fluorescence
probe based on rhodamine and coumarin. The probe displayed a
3
þ
high sensitivity and selectivity toward Fe even coexistent with
other metal ions. The probe differs from reported ratiometric probe
in fluorescence mechanism.
Acknowledgment
This study was supported by 973 Program (2010CB933504) and
National Natural Science Foundation of China (20972088 and
90813022).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.06.024.
References
[
[
[
[
1] D’Autreáux B, Tucker NP, Dixon R, Spiro S. A non-haem iron centre in the
transcription factor NorR senses nitric oxide. Nature 2005;437:769e72.
2 2
2] Lee JW, Helmann JD. The PerR transcription factor senses H O by metal
catalysed histidine oxidation. Nature 2006;440:363e7.
3] Xiang Y, Tong A. A new rhodamine-based chemosensor exhibiting selective
III
Fe -amplified fluorescence. Org Lett 2006;8:1549e52.
4] Weerasinghe AJ, Schmiesing C, Varaganti S, Ramakrishna G, Sinn E. Single-
and multiphoton turn-on fluorescent Fe3 sensors based on bis(rhodamine).
þ
J Phys Chem B 2010;114:9413e9.
[
5] She M, Yan Z, Yin B, Zhang J, Gu J, Yin W, et al. A novel rhodamine-based
fluorescent and colorimetric “off-on” chemosensor and investigation of the
recognizing behavior towards Fe3 . Dye Pigme 2012;92:1337e43.
þ
[
6] Zhang X, Xiao Y, Qian X. A ratiometric fluorescent probe based on FRET for
Fig. 3. a) The fluorescence spectra of 10
m
M L in buffered EtOH/HEPES ¼ 99:1 solution
2þ
imaging Hg ions in living cells. Angew Chem Int Ed 2008;47:8025e9.
2
þ
2þ
2þ
2þ
þ
2þ
þ
þ
3þ
at pH 7.2 with 5 equiv. metal ions: Ba , Ca , Cd , Co , K , Mg , Na , Ag , Fe ,
[7] Han ZX, Zhang XB, Li Z, Gong YJ, Wu XY, Jin Z. Efficient fluorescence resonance
2þ
2þ
2þ
2þ
2þ
Pb , Hg , Ni , Cu ions and blank. b) The ratio of fluorescence intensity (580 nm/
60 nm) of L and L with metal ions.
energy transfer-based ratiometric fluorescent cellular imaging probe for Zn
4
using a rhodamine spirolactam as a trigger. Anal Chem 2010;82:3108e13.

F. Ge et al. / Dyes and Pigments 99 (2013) 661e665
665
[
8] Xu Z, Qian X, Cui J. Colorimetric and ratiometric fluorescent chemosensor with
a large red-shift in emission: Cu(II)-only sensing by deprotonation of sec-
ondary amines as receptor conjugated to naphthalimide fluorophore. Org Lett
[18] Mao J, He Q, Liu W. A rhodamine-based fluorescence probe for iron(III) ion
determination in aqueous solution. Talanta 2010;80:2093e8.
[19] Wang S, Meng X, Zhu M. A naked-eye rhodamine-based fluorescent probe for
Fe(III) and its application in living cells. Tetrahedron Lett 2011;52:2840e3.
[20] Dalapati S, Jana S, Alam MA, Guchhait N. Multifunctional fluorescent probe
selective for Cu(II) and Fe(III) with dual-mode of binding approach. Sensor
Actuat B 2011;160:1106e11.
[21] Liu SR, Wu SP. New water-soluble highly selective fluorescent chemosensor
for Fe (III) ions and its application to living cell imaging. Sensor Actuat B
2012;171e172:1110e6.
2005;7:3029e32.
[
9] Yang H, Liu Z, Zhou Z, Shi E, Li F. Highly selective ratiometric fluorescent
sensor for Cu(II) with two urea groups. Tetrahedron Lett 2006;47:2911e4.
[
[
10] Liu Z, Zhang C, He W, Yang Z, Gao X, Guo Z. A highly sensitive ratiometric
2
þ
fluorescent probe for Cd
detection in aqueous solution and living cells.
Chem Commun 2010;46:6138e40.
11] Goswami S, Sen D, Das NK. A new highly selective, ratiometric and colori-
2
þ
metric fluorescence sensor for Cu with a remarkable red shift in absorption
[22] Xu M, Wu S, Zeng F, Yu C. Cyclodextrin supramolecular complex as a water-
soluble ratiometric sensor for ferric ion sensing. Langmuir 2010;26:4529e34.
[23] Liu WY, Li HY, Zhao BX, Miao JY. Synthesis, crystal structure and living cell
and emission spectra based on internal charge transfer. Org Lett 2010;12:
856e9.
imaging of a Cu2 -specific molecular probe. Org Biomol Chem 2011;9:4802e5.
[24] Xiang Y, Tong A, Jin P, Ju Y. New fluorescent rhodamine hydrazone chemo-
sensor for Cu(II) with high selectivity and sensitivity. Org Lett 2006;8:2863e6.
[25] Li M, Lu HY, Liu RL, Chen JD, Chen CF. Turn-on fluorescent sensor for selective
þ
[
[
[
[
[
12] Kim HJ, Park SY, Yoon S, Kim JS. FRET-derived ratiometric fluorescence sensor
for Cu2 . Tetrahedron 2008;64:1294e300.
þ
13] Lee SY, Kim HJ, Wu JS, No K, Kim JS. Metal ion-induced FRET modulation in a
bifluorophore system. Tetrahedron Lett 2008;49:6141e4.
detection of Zn2 , Cd , and Hg in water. J Org Chem 2012;77:3670e3.
þ
2þ
2þ
14] Zhou Z, Yu M, Yang H, Huang K, Li F, Yi T. FRET-based sensor for imaging
chromium(III) in living cells. Chem Commun 2008;29:3387e9.
15] Long L, Lin W, Chen B, Gao W, Yuan L. Construction of a FRET-based ratio-
metric fluorescent thiol probe. Chem Commun 2011;47:893e5.
16] Sun Y, Zhong C, Gong R, Mu H, Fu E. A ratiometric fluorescent chemo-
dosimeter with selective recognition for sulfite in aqueous solution. J Org
Chem 2009;74:7943e6.
[26] Li Z, Zhang L, Li X, Guo Y, Ni Z, Chen J, et al. A fluorescent color/intensity
changed chemosensor for Fe3 by photo-induced electron transfer (PET) in-
þ
hibition of fluoranthene derivative. Dye Pigme 2012;94:60e5.
[27] Sung K, Fu HK, Hong SH. A Fe3 /Hg -selective anthracene-based fluorescent
þ
2þ
PET sensor with tridentate ionophore of amide/b-amino alcohol. J Fluoresc
2007;17:387e9.
[
17] Liu Y, Lv X, Zhao Y, Chen M, Liu J, Guo W. A naphthalimide-rhodamine
[28] Othman AB, Lee JW, Wu JS, Kim J, Abidi S, Thuéry RP, et al. Calix[4]arene-bsed,
2þ
2þ
ratiometric fluorescent probe for Hg based on fluorescence resonance en-
ergy transfer. Dye Pigme 2012;92:909e15.
Hg -iduced intramolecular fluorescence resonance energy transfer chemo-
sensor. J Org Chem 2007;72:7634e40.