A polyanionic dendritic fluorophore for selective detection of Hg 2+ in triton X-100 aqueous media

Article abstract of DOI:10.1021/ol900929g

A serles of water-soluble fluorescent dendritic compounds composed of phenylene-ethynylene repeating units and anionic carboxylate or catlonlc ammonium peripheral groups were synthesized. The first generation fluorescent dendrlmer containing nine phenylene-ethynylene units and six carboxylate peripheral groups exhibited a highly selective fluorescence quenching by Hg ²⁺ Ions. The Stern-Volmer constant (K_8v) was 33,700 M ^-1 In aqueous media In the presence of Triton X-100 surfactant.

Full text of DOI:10.1021/ol900929g

ORGANIC
LETTERS
2009
Vol. 11, No. 13
2768-2771
A Polyanionic Dendritic Fluorophore for
Selective Detection of Hg2+ in Triton
X-100 Aqueous Media
Nakorn Niamnont,† Warathip Siripornnoppakhun,† Paitoon Rashatasakhon,† and
Mongkol Sukwattanasinitt*,†,‡
Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science, and
Center for Petroleum, Petrochemicals and AdVanced Materials, Chulalongkorn
UniVersity, Bangkok 10330, Thailand
smongkol@chula.ac.th
Received April 29, 2009
ABSTRACT
A series of water-soluble fluorescent dendritic compounds composed of phenylene-ethynylene repeating units and anionic carboxylate or
cationic ammonium peripheral groups were synthesized. The first generation fluorescent dendrimer containing nine phenylene-ethynylene
units and six carboxylate peripheral groups exhibited a highly selective fluorescence quenching by Hg2+ ions. The Stern-Volmer constant
(Ksv) was 33,700 M-1 in aqueous media in the presence of Triton X-100 surfactant.
Mercury is one of the most toxic environmental pollutants
generated from industrial sources. The most abundant ionic
form of this element is the mercuric ion (Hg2+), which can
be accumulated in the organs of human or animal bodies
through the food chain.1 Inorganic mercury has been reported
to produce harmful effects at a concentration of 5 ppb.2
Development of highly sensitive and selective Hg2+ sensors
that can provide direct determinations of the amount of Hg2+
in aqueous media is therefore of great interest.3 Chemosen-
sors for Hg2+ have been progressively improved using
redox,4 chromogenic,5 or fluorogenic6 changes as the means
of detection.
Fluorescence-based methodologies have attracted much
interest due to their intrinsic sensitivity and selectivity.7
Considerable efforts have been devoted to the design of
fluorescent compounds to be used as sensors for mercury.
However, their poor solubility in water and low fluorescence
quantum yield due to aggregation have limited the satisfac-
tory application of these compounds in aqueous media.8
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5042. (b) Caballero, A.; Espinosa, A.; Ta´rraga, A; Molina, P. J. Org. Chem.
2008, 73, 5489–5497.
† Department of Chemistry, Faculty of Science.
‡ Center for Petroleum, Petrochemicals and Advanced Materials.
(1) (a) Stern, A. H.; Hudson, R. J. M.; Shade, C. W.; Ekino, S.;
Ninomiya, T.; Susa, M.; Harris, H. H.; Pickering, I. J.; George, G. N. Science
2004, 303, 763–766. (b) Nendza, M.; Herbst, T.; Kussatz, C.; Gies, A.
Chemosphere 1997, 35, 1875–1885.
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Zhao, Y.; Zhong, Z. Q. Org. Lett. 2006, 8, 4715–4717. (c) Zhao, Y.; Zhong,
Z. J. Am. Chem. Soc. 2006, 128, 9988–9989. (d) Huang, C.-C.; Chang,
H.-T. Anal. Chem. 2006, 78, 8332–8338. (e) Huang, C.-C.; Yang, Z.; Lee,
K.-H.; Chang, H.-T. Angew. Chem., Int. Ed. 2007, 46, 6824–6828. (f) Nolan,
E. M.; Lippard, S. J. Chem. ReV. 2008, 108, 3443–3480. (g) Liu, C. W.;
Huang, C. C; Chang, H. T. Langmuir 2008, 24, 8346–8350.
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10.1021/ol900929g CCC: $40.75
Published on Web 06/04/2009
 2009 American Chemical Society

Figure 1. Structures of dendritic molecules 1, 2, and 3.
Several methods have been used to prevent aggregation and
increase quantum efficiencies, for example, the use of
polyelectrolytes or surfactants.
of dendritic molecules. We thus decided to synthesize
dendritic compounds 1-3 (Figure 1) and study their fluo-
rescent sensing applications.
Conjugated polyelectrolytes have been applied to the
detection of metals due to their efficient static quenching
properties with quenchers.9 In some recent studies, surfac-
tants were also used to enhance the fluorescence quantum
yields of hydrophobic conjugated polymers containing hy-
drophilic side chains.10 The dominant hydrophobic interac-
tions between surfactants and fluorophores can significantly
reduce the aggregation. An application of surfactants for
amplification of the quenching effect between the fluorescent
polymers and quenchers has also been reported.11
The fluorescent linear polymers poly(phenylene-ethy-
nylenes) substituted by carboxylate groups have been
reported to exhibit a quenching effect by Hg2+ in aqueous
mediaintheabsenceandpresenceofavidin,withStern-Volmer
constant (Ksv) values of 104 and 105 M-1, respectively.12
Despite offering high sensivity, the unpredictable secondary
and tertiary structures in solution, due to a wide number of
repeating fluorophores and random molecular conformations
in linear conjugated polymers, can lead to inconsistent
quenching effects and inexplicable behaviors.
Diphenylacetylene is selected as the repeating fluorescent
unit for its known high fluoresence quantum yield and
structural rigidity. To make the dendritic compounds water-
soluble, carboxyl or quaternary ammonium groups have been
installed as the peripheral groups. The compounds were
synthesized by a convergent approach as outlined in Scheme
1. The reactive core, 4,4′,4′′-triiodotriphenylamine 4, was
prepared from the iodination of triphenylamine using ben-
zyltrimethylammonium iododichloride (BnMe3-ICl2).13 The
peripheral building blocks, methyl 4-ethynyl benzoate 5 and
N,N-dimethyl-4-ethynylaniline 6, were obtained through the
Sonogashira coupling14 of trimethylsilylacetylene with the
corresponding aryl iodide and a subsequent base-catalyzed
desilylation. With the required building blocks in hands, we
proceeded with the Sonogashira coupling between 4 and 5
followed by the hydrolysis of triester 7 to afford ionizable
fluorophore 1. Similarly, the reaction between 4 and 6 gave
rise to the triamine 8, which was treated with an excess of
MeI to provide the polycationic fluorescent compound 2. In
order to obtain the first generation fluorescent dendrimer 3,
we carried out a reaction of the core 4 with 1 molar equiv
of trimethylsilylacetylene to obtain the branch building blocks
9. The Sonogashira coupling of 9 with 5 followed by
desilylation gave the dendron 10, which was coupled with 4
to afford hexaester 11. The hydrolysis of 11 eventually
afforded first generation fluorescent dendrimer 3 in moderate
yield.15
In comparison with linear polymers, the numbers of
fluorophore units in dendrimers can be controlled by a
stepwise synthesis. This should reflect in the more predictable
fluorescence property and other structure-related behaviors
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100, 2537. Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. ReV 2007,
107, 1339–1386.
The effects of surfactants on the photophysical properties
of 1-3 were studied. Without the surfactant, compounds 1-3
displayed absorption peaks around 370-375 nm. In the
presence of the surfactant Triton X-100, these bands were
slightly red-shifted to 379-383 nm. The spectral shifts
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P.; Yue, H; Wu, M.; Liu, M.; Treece, J.; Waldeck, D. H.; Xue, C.; Liu, H.
J. Phys. Chem. B 2007, 111, 8589–8596. (e) Pu, K.-Y.; Pan, S. Y.-H.; Liu,
B. J. Phys. Chem. B 2008, 112, 9295–9300.
(13) Kajigaeshi, S.; Kakinami, T.; Moriwaki, M.; Fujisaki, S.; Maeno,
K.; Okamoto, T. Synthesis 1988, 545–546.
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3878.
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1996, 118, 9635–9644. (b) Pesak, D.; Moore, J. S. Macromolecules 1997,
30, 6467–6482. (c) Nantalaksakul, A.; Dasari, R. R.; Ahn, T.-S.; Al-Kaysi,
R.; Bardeen, C. J.; Thayumanavan, S. Org. Lett. 2006, 8, 2981–2984. (d)
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11, 1257–1260.
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Macromolecules 2005, 38, 4560–4562. (b) Kim, I.-B.; Bunz, U. H. F. J. Am.
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U. H. F. Macromolecules 2007, 40, 814–817.
Org. Lett., Vol. 11, No. 13, 2009
2769

Scheme 1
.
Synthesis of 1, 2, and 3
Table 1. Photophysical Properties of 1-3 in 50 mM Phosphate
Buffer (pH 8.0) without and with Triton X-100
absorption
λmax (nm)
ε (M-1 cm-1
fluorescence
a
compd
)
λmax (nm)
ΦF
Without Triton X-100
1
2
3
374
370
375
5900
26823
63500
454
485
489
0.097
0.14
0.037
With Triton X-100
9592
1
2
3
383
382
379
434
438
421
0.47
0.46
0.65
21310
49820
a Quinine sulfate in 0.1 M H2SO4 (ΦF ) 0.45) was the reference.
by Hg2+ ions (Figure 2a), whereas there are no significant
effects on the signals of 1 and 2. With Triton X-100, the
fluorescence signal of 1 was quenched by all metal ions listed
above, but the signal of 2 was not affected. On the other
hand, the fluorophore 3 still exhibited a selective quenching
by Hg2+ ions.
A Stern-Volmer plot was made in order to access a
quantitative measurement of fluorescence quenching. Ksv for
the system with Triton X-100 was 33,700 M-1, whereas it
was only 5,800 M-1 for the system without Triton X-100
(Figure 3). It is clear that Triton X-100 could amplify this
selective quenching effect.
The nonselective quenching of 1 suggested that positively
charged metal ions reduce the electrostatic repulsion between
observed in the emission spectra were in the opposite
direction and of more significance. The emission peaks of
1, 2, and 3 were at 454, 485, and 489 nm in the absence of
Triton X-100. The surfactant caused the emission bands to
blue-shift by 20, 47, and 68 nm, respectively (Table 1).
The blue shift and increase in fluorescence quantum yields
(ΦF) suggested that the deaggregation of 1-3 was caused
by the addition of the surfactants. The enhancement of ΦF
of 3 was greater than that of 1 implied that compound 1, a
smaller molecule, was only partially deaggregated.
We also investigated the fluorogenic behaviors of 1-3 in
the presence of metal ions in the +2 oxidation state, such as
Cr2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Hg2+, and Pb2+. Without
the surfactant, the fluorescence signal of 3 can be quenched
Figure 2. Emission spectra of the solutions of 1-3 (10 µM) upon
the addition of metal ions (40 µM): without Triton X-100 for (a)
1, (b) 2, and (c) 3; with 0.1 mM Triton X-100 for (d) 1 (1 µM), (e)
2 (0.1 µM), and (f) 3 (0.1 µM).
(15) The spectroscopic data (1H and 13C NMR, MALDI-TOF MS, and
ESI-MS spectra) of every compound are available in Supporting Informa-
tion.
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Org. Lett., Vol. 11, No. 13, 2009

Figure 4. Emission spectra of 3 (0.1 µM) upon the addition of
Figure 3. Stern-Volmer plots for fluorescence quenching of 3 (0.1
µM) with and without of Triton X-100 (0.1 mM).
Hg2+ (40 µM) and EDTA (100 µM).
the partially deaggregated negatively charged fluorophores
1, resulting in the enhancement of self-quenching. Con-
versely, the charge repulsion among positively charged
fluorophore 2, having a comparable molecular size to 1,
cannot be reduced by the metal ions and therefore exhibits
no enhancement of the self-quenching effect.
convergent approach. We have demonstrated that these
diphenylacetylene based dendritic molecules with a charge
decorated periphery constitute a new intriguing class of
fluorophores useful for sensing applications in aqueous
media. The quantum efficiencies of the fluorophores in water
were considerably enhanced by the nonionic sufactant Triton
X-100. In the presence of Triton X-100, the fluorescent signal
of the first generation dendrimer containing a carboxylate
periphery could be selectively quenched by Hg2+. A wide
linear fluorescence quenching response to Hg2+ concentration
was observed in the range of 2-80 µM (0.4-16 ppm). Other
applications along this line are currently under investigation,
and the results will be disclosed in due course.
The selective fluorescence quenching of 3 by Hg2+ is more
complicated and difficult to rationalize. With the high
selectivity for Hg2+, the common metal ion enhanced self-
quenching mechanism is unlikely to play the key role. The
quenching effect may involve selective formation of 3·Hg2+
complex and efficient energy transfer between the fluorescent
units in 3 to this complex at the periphery.16 To test if the
proposed complex could be reversed, a strong chelator,
EDTA, was added. The addition of 2.5 molar equiv of EDTA
could restore the fluorescent signal of 3 to its original level
(Figure 4). The result supports the above hypothesis to some
extent.
Acknowledgment. This work is supported by the Thailand
Research Fund (TRF) and the 90th Anniversary of Chula-
longkorn University Fund (Ratchadaphiseksomphot Endow-
ment Fund). The measurement of fluorescent spectra was
performed in the laboratory of Prof. Thawatchai Tuntulani.
In summary, a series of diphenylacetylene dendritic
compounds containing negatively and positively charged
peripheral groups were successfully synthesized through a
Supporting Information Available: Spectroscopic data
and detailed experimental procedures. This information is
available free of charge via the Internet at http://pubs.acs.org.
(16) (a) Sandanaraj, B. S.; Demont, R.; Aathimanikandan, S. V.;
Savariar, E. N.; Thayumanavan, S. J. Am. Chem. Soc. 2006, 128, 10686–
10687. (b) Jiwpanich, S.; Sandanaraj, B. S.; Thayumanavan, S. Chem.
Commun. 2009, 806–808.
OL900929G
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