A sensitive and selective ratiometric near IR fluorescent probe for zinc ions based on the distyryl-bodipy fluorophore

Article abstract of DOI:10.1021/ol801554t

(Chemical Equation Presented) A novel distyryl-substituted boradiazaindacene (bodipy) dye with an emission peak moving hypsochromically from 730 to 680 nm on Zn(II) ion binding seems to be promising as one of the very few water-soluble fluorescent chemosensors emitting in the near IR region.

Full text of DOI:10.1021/ol801554t

ORGANIC
LETTERS
2008
Vol. 10, No. 18
4065-4067
A Sensitive and Selective Ratiometric
Near IR Fluorescent Probe for Zinc Ions
Based on the Distyryl-Bodipy
Fluorophore
Serdar Atilgan,†,‡ Tugba Ozdemir,† and Engin U. Akkaya*,§
Department of Chemistry, Middle East Technical UniVersity, Ankara, Turkey TR-06531,
Department of Chemistry, Suleyman Demirel UniVersity, Isparta, Turkey TR-32260,
and Department of Chemistry and UNAM-Institute of Materials Science and
Nanotechnology, Bilkent UniVersity, Ankara, Turkey, TR-06800
eua@fen.bilkent.edu.tr
Received July 9, 2008
ABSTRACT
A novel distyryl-substituted boradiazaindacene (bodipy) dye with an emission peak moving hypsochromically from 730 to 680 nm on Zn(II) ion
binding seems to be promising as one of the very few water-soluble fluorescent chemosensors emitting in the near IR region.
The design of fluorescent chemosensors for ions implicated
in important biological processes, especially linked to human
disease states, is an active research area. Zinc(II) ions are
involved in the biology of Alzheimer’s disease1 and diabe-
tes,2 among others.3 It is now well established that large
amounts of “free” zinc are sequestered in the secretory
granules of different types of mammalian cells, and its release
is under the precise control of secretagogues.4 For fluorescent
chemosensors aimed at cellular imaging or real-time visu-
alization of ion concentrations, additional constraints would
apply. Long wavelength excitability and emission are very
important for reduced scattering and securing low back-
(2) Taylor, C. G. Biometals 2005, 18, 305–312.
(3) (a) Frassinetti, S.; Bronzetti, G.; Caltavuturo, L.; Cini, M.; Croce,
C. D. J. EnViron. Pathol. Toxicol. Oncol. 2006, 25, 597610. (b) Lim, C. L.;
Freake, H. D.; Bru¨ckner, C. Chem.-Eur. J. 2005, 11, 38–39.
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Miyake, N.; Hiai, H.; Chiba, T. Biochim. Biophys. Acta 2001, 1540, 43–
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† Middle East Technical University.
‡ Suleyman Demirel University.
§ Bilkent University.
(1) Noy, D.; Solomonov, I.; Sinkevich, O.; Arad, T.; Kjaer, K.; Sagi, I.
J. Am. Chem. Soc. 2008, 130, 1376–1383.
10.1021/ol801554t CCC: $40.75
Published on Web 08/15/2008
2008 American Chemical Society

ground emissions. In addition, signal transduction in the
wavelength ratiometric mode is highly advantageous.
Scheme 1
.
Synthesis of the Near IR Emitting, Water-Soluble
DS-Bodipy-Based Chemosensor 4
In recent literature, a large number of fluorescent probes
for zinc have been proposed based on different fluorophores
such as quinoline,5 boradiazaindacene (bodipy),6 dansyl,7
fluorescein,8 anthracene,9 acridine,10 benzofuran,11 naph-
thalimide,12 cyanine,13 and squaraines.14
Unfortunately, for most of them, water solubility is limited,
thus various solvent mixtures are used in characterization.
This fact, together with the requirement for shorter wave-
length excitation, hinders biological applications.
As part of our ongoing interest in rational design of
ratiometric fluorescent chemosensors, emitting especially in the
red end of the visible spectrum, we are particularly intrigued
by the distyryl-bodipy15 dyes. These novel derivatives of well-
known bodipy dyes are emerging as promising red emitting
dyes with interesting applications as sensitizers for photody-
namic therapy15a and ion sensing.15b The distyryl-bodipy
dyes can be synthesized with water solubilizing groups,
which increases the likelihood of biological applications.
When the styryl groups carry dialkylamino groups, the ICT
process can be modulated by cation binding which provides
ample opportunities of cation sensing.16
With these considerations, we set out to synthesize
compound 4 (Scheme 1). The synthesis requires stepwise
Knoevenagel condensations with two different aldehydes to
yield the distyrl-bodipy. The TEG (triethyleneglycol) groups
are placed on a gallic acid derived, 3,4,5-trihydroxyphenyl
unit. The metal chelator is a well-known Zn(II) specific
ligand, dipicolylamine (DPA). The target compound 4 was
obtained in satisfactory yield and characterized by H and
13C NMR and HRMS. Six TEG groups are clearly enough
to make this molecule compatible with water, and the spectral
data to be discussed from this point on are obtained in 5:95
ethanol-aqueous buffer solutions.
The absorption spectrum (Supporting Information) displays
a broad peak centered at 680 nm, and the molar extinction
coefficient is 72 000 M-1cm-1 at this wavelength. The
emission spectrum obtained in aqueous buffer solution (5%
ethanol was added as a cosolvent) shows a peak centered at
726 nm (Figure 1). Gradual addition of Zn(II) ions to this
1
(5) (a) Fahrni, C. J.; O’Halloran, T. V. J. Am. Chem. Soc. 1999, 121,
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Talanta 2001, 51, 693–699.
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Figure 1. Change in the emission spectrum of the chemosensor 4
(13) Kiyose, K.; Kojima, H.; Urano, Y.; Nagano, T. J. Am. Chem. Soc.
2006, 128, 6548–6549.
(2.5 µM) in response to increasing concentrations of Zn(II) in an
ethanol-aqueous buffer mixture (5% in ethanol, HEPES 0.1 M,
pH ) 7.2). Zinc concentrations were varied in the following order:
0, 0.25, 0.5, 1, 2, 5, 10, 20, 50, 100 µM. Excitation was at 630 nm,
with slit widths of 5 nm.
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3724. (b) Carol, P.; Sreejith, S.; Ajayaghosh, A. Chem. Asian J. 2007, 2,
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8488. (b) Atilgan, S.; Ekmekci, Z.; Dogan, A. L.; Guc, D.; Akkaya, E. U.
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14475. (c) Peng, X.; Du, J.; Fan, J.; Wang, J.; Wu, Y.; Zhao, J.; Sun, S.;
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solution results in a blue shift to 625 nm with a concomitant
increase in emission intensity. This clearly shows that the
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Org. Lett., Vol. 10, No. 18, 2008

coordination of Zn(II) ions effectively blocks excited-state
charge transfer from the dipicolylphenyl group. Hill plot
analysis of the data obtained in the titration of 4 with Zn(II)
yielded a 1:1 stoichiometry (slope ) 0.97) with a dissociation
constant (Kd) of 2.0 × 10-5 M. The fluorescence emission
of the Zn(II) complex is bright red, whereas weaker intensity
emission of the free chemosensor is hardly visible since it
is at the red end of the visible spectrum, extending to near
IR. Not surprisingly, the dipicolylamine ligand imparts
remarkable metal ion selectivity onto this chemosensor
(Figure 2). The chemosensor 4 generates the largest signal
Figure 3. Results of the competition experiments between Zn(II)
and selected metal ions. The free dye 4 (chemosensor) concentration
was set at 5.0 µM. Excitation was at 630 nm; emission intensity
values at 680 nm were collected; and all metal ions were added at
200 µM concentration. Solvent: ethanol-aqueous buffer mixture
(5% in ethanol, HEPES 0.1 M, pH ) 7.2).
rational design for novel ratiometric chemosensors. The
chemosensor discussed in this communication is a prototype
Figure 2. Normalized emission intensities at 680 nm (the emission
intensity of the free dye ) 1) of the chemosensor (1.2 µM) in the
presence of selected metal ions (1.0 mM). Excitation was done at
630 nm with 5 nm slit widths. Solvent: ethanol-aqueous buffer
mixture (5% in ethanol, HEPES 0.1 M, pH ) 7.2).
Figure 4. Digital photographs of the chemosensor solutions (5.0
µM) in the presence of different metal ions at 100 µM concentration.
The upper plate is taken under ambient light, and the bottom one
under UV illumination at 360 nm. Solvent: ethanol-aqueous buffer
mixture (5% in ethanol, HEPES 0.1 M, pH ) 7.2).
in the presence of Zn(II) ions, but Hg(II) and Cd(II) also
show some response. Fortunately, in biological media (unlike
environmental samples), these three ions rarely compete, and
our data show that most likely biological interferants such
as Ca(II), K(I), Na(I), and Mg(II) have no effect on the
emission spectrum. The results of a competition experiment
between Zn(II) and selected metal ions is shown in Figure
3. Digital photographs (Figure 4) of the aqueous solutions
of the chemosensor with added Zn(II) clearly show very large
enhancement of the red emission under UV illumination at
360 nm, with no perceivable changes in the presence of other
biologically relavant ions.
for such molecular ion sensors to be developed via this
approach.
Acknowledgment. This work was supported by the
Turkish Scientific and Technical Research Council (TUBI-
TAK) and the Turkish Academy of Sciences (TUBA). The
authors gratefully acknowledge Mr. Ilker Kutuk (METU) for
his assistance with the synthesis work.
In conclusion, we have demonstrated that versatile bodipy
chemistry allows the synthesis of long wavelength emitting,
water-soluble dyes through stepwise reaction of the methyl
groups at the 3 and 5 positions of the bodipy core. Thus,
multiple and distinct functional groups can easily be placed
on the chromophore, offering many opportunities for the
Supporting Information Available: Syntheses, experi-
mental details, and additional spectroscopic data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL801554T
Org. Lett., Vol. 10, No. 18, 2008
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