Highly selective and sensitive phosphane sulfide derivative for the detection of Hg2+ in an organoaqueous medium

Article abstract of DOI:10.1021/ol070118l

(Chemical Equation Presented) A new fluorescent molecular sensor for Hg²⁺ based on the phosphane sulfide derivative exhibits a very low detection limit in an aqueous medium (3.8 nM) with a very high selectivity over other interfering cations. T

Full text of DOI:10.1021/ol070118l

ORGANIC
LETTERS
2007
Vol. 9, No. 6
1133-1136
Highly Selective and Sensitive
Phosphane Sulfide Derivative for the
+
Detection of Hg2 in an Organoaqueous
Medium
Minh-Huong Ha-Thi,† Mael Penhoat,‡ Ve´ronique Michelet,*,‡ and Isabelle Leray*,†
1
De´partement de Chimie, Laboratoire de Photophysique et Photochimie
Supramole´culaires et Macromole´culaires, CNRS UMR 8531, ENS-Cachan,
61 AVenue du Pre´sident Wilson, F-94235 Cachan Cedex, France, and Laboratoire de
Synthe`se Organique, CNRS UMR7573, E.N.S.C.P., 11 rue P. et M. Curie,
F-75231 Paris Cedex 05, France
icmleray@ppsm.ens-cachan.fr; Veronique-michelet@enscp.fr
Received January 16, 2007
ABSTRACT
+
A new fluorescent molecular sensor for Hg2 based on the phosphane sulfide derivative exhibits a very low detection limit in an aqueous
medium (3.8 nM) with a very high selectivity over other interfering cations. The reversibility of the complexation process was also examined
and was found to be successful.
Hg2+ is considered a highly toxic heavy metal ion causing
environmental and health problems.1 A wide variety of
symptoms are observed upon exposure including digestive,
kidney, and especially neurological diseases.1 The level of
this ion was therefore the object of strict regulation and
should not exceed 1 µg L-1.2 Although sophisticated analyti-
cal techniques (atomic absorption, atomic emission, and
inductively coupled plasma spectroscopy) are currently used
in the environment, there is a strong demand to develop
inexpensive and real-time monitoring methods for the
detection of Hg2+ and other heavy metals. In this context,
the methodology based on fluorescent molecular sensors has
attracted considerable interest because of their intrinsic
sensitivity and selectivity.3 Considerable efforts have been
devoted to the design of fluorescent molecular sensors for
mercury;4 however, only a few of them are very competitive5
in terms of sensitivity, selectivity, and measurements in
† Laboratoire PPSM, ENS-Cachan.
‡ Laboratoire de Synthe`se Se´lective Organique et Produits Naturels,
E.N.S.C.P.
(1) (a) Hutchinson, T. C.; Meema, K. M. In Lead, Mecury, Cadnium
and Arsenic in the EnVironment; John Wiley: New York, 1987. (b) Scoullos,
G. H.; Vonkeman, M. J.; Thorton, L.; Makuch, Z. In Mercury, Cadmium,
Lead: Handbook for Sustainable HeaVy Metals Policy and Regulation
(EnVironment & Policy, V. 31); Kluwer Academic: Norwell, MA, 2001.
(c) Coester, C. J. Anal. Chem. 2005, 77, 3737-3754. (d) Richardson, S.
D.; Temes, T. A. Anal. Chem. 2005, 77, 3807-3838.
(3) (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley,
A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
97, 1515-1566. (b) Valeur, B.; Leray, I. Coord. Chem. ReV. 2000, 205,
3-40.
(2) Guidelines for drinking-water quality, 3rd ed.; World Health Orga-
nization: Geneva, 2004; p 188.
10.1021/ol070118l CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/15/2007

Scheme 1
aqueous media, and there still is a need for alternative
available 4-bromotrimethylsilylphenyl acetylene led to the
diphenylphosphanoethane (dppe) derivative in 55% yield.
Treatment with elemental sulfur (1:2 mole ratio) in toluene
gave the thiophosphano product in 88% yield, which was
then engaged in the desilylation step using K2CO3 in a
mixture of CH2Cl2/MeOH. The corresponding phosphane
sulfide 2 was obtained in 100% yield. The Sonogashira
coupling between the thiophosphano product 2 and the iodide
39 was then performed and afforded the corresponding DPPS
4 in 50% yield. This methodology is particularly interesting,
as it would allow the synthesis of analogues.
systems. In the course of our recent ongoing program directed
toward the production of novel fluorophores6 and on the basis
of recent work from Lobana et al.,7 we envisaged that
phosphane sulfide derivatives might be excellent candidates
for complexation and detection of Hg2+. Because there is
no precedent on such a fluorescent family, we therefore wish
to describe our preliminary results on the synthesis and
applications of a novel sensor.
An issue for the success was the design and the easy
preparation of the fluorescent molecule. Anticipating that 1,2-
bis(diphenyl thiophosphinophenyl)ethane could form a strong
complex with mercury, we decided to prepare diphosphane-
bearing polyphenylethynyl fluorophores substituted by an
ether group and to study their complexing and photophysical
properties.8 Scheme 1 outlines the synthesis of the DPPS
ligand 4. The reaction between the 1,2-bis(dichlorophospha-
no)ethane and the Grignard derivative of the commercially
The DPPS derivative 4 shows an intense absorption band
in the UV region (ꢀ ) 1.5 × 105 L mol-1 cm-1) and a
fluorescence quantum yield (ΦF) of 0.1 in acetonitrile. The
emission maxima are indeed strongly dependent on the
(4) (a) Kim, I. B.; Ergodan, B.; Wilson, J. N.; Bunz, U. H. F. Chem.-
Eur. J. 2004, 10, 6247-6254. (b) Caballero, A.; Martinez, R.; Lloveras,
V.; Ratera, I.; Vidal-Gancedo, J.; Wurst, K.; Tarraga, A.; Molina, P.; Veiana,
J. J. Am. Chem. Soc. 2005, 127, 15666-15667. (c) Wang, J; Qian, X. Chem.
Commun. 2006, 109-111. (d) Winkler, J. D.; Bowen, C. M.; Michelet, V.
J. Am. Chem. Soc. 1998, 120, 3237-3242. (e) Metivier, R.; Leray, I.; Valeur,
B. Chem.-Eur. J. 2004, 10, 4480-4490. (f) Metivier, R.; Leray, I.; Lebeau,
B.; Valeur, B. J. Mater. Chem. 2005, 15, 2965-2973. (g) Zheng, H.; Qian,
Z.-H.; Xu, L.; Yuan, F.-F.; Lan, L.-D.; Xu, J.-G. Org. Lett. 2006, 8, 859-
861. (h) Zhao, Y.; Zhong, Z. J. Am. Chem. Soc. 2006, 128, 9988-9989.
(5) (a) Ono, A.; Togashi, H. Angew. Chem., Int. Ed. 2004, 43, 4300-
4302. (b) Yoon, S.; Albers, A. E.; Wong, A. P.; Chang, C. J. Am. Chem.
Soc. 2005, 127, 16030-16031. (c) Yang, Y. K.; Yook, K. J.; Tae, J. J. Am.
Chem. Soc. 2005, 127, 16760-16761. (d) Nolan, E. M.; Racine, M. E.;
Lippard, S. J. Inorg. Chem. 2006, 45, 2742-2749. (e) Nolan, E. M.; Lippard,
S. J. J. Am. Chem. Soc. 2003, 125, 14270-14271.
(6) (a) Metivier, R.; Amengual, R.; Leray, I.; Michelet, V.; Genet, J. P.
Org. Lett. 2004, 6, 739-742. (b) Ha-Thi, M. H.; Souchon, V.; Hamdi, A.;
Metivier, R.; Alain, V.; Nakatani, K.; Lacroix, P. G.; Genet, J. P.; Michelet,
V.; Leray, I. Chem.-Eur. J. 2006, 12, 9056-9065.
(7) (a) Lobana, T. S.; Verma, R.; Singh, A.; Shikha, M.; Castineiras, A.
Polyhedron 2002, 21, 205-209. (b) Lobana, T. S.; Sandhu, M. K.; Tiekink,
E. R. J. Chem. Soc., Dalton Trans. 1988, 1401-1403.
(8) For a recent review on organophosphorus π-conjugated materials,
see: Baumgartner, T.; Re´au, R. Chem. ReV. 2006, 106, 4681-4727 and
references cited therein.
Figure 1. Absorption of DPPS 4 (3.3 µM) in the presence of an
increasing concentration of Hg2+ in CH3CN-H2O (80:20 v/v) at
pH ) 4. Inset: titration curves at 305 and 345 nm.
1134
Org. Lett., Vol. 9, No. 6, 2007

solvent polarity. The unresolved vibronic structure and the
large Stokes shift observed in the polar solvent suggest the
formation of an intramolecular charge transition (see Sup-
porting Information).10 In view of practical applications, the
effect of cation complexation was then studied in CH3CN/
H2O (80:20 v/v) at pH 4, and the DPPS 4 exhibits still the
same fluorescence quantum yield (ΦF ) 0.1). Figure 1 and
Figure 2 display the evolution of the absorption and emission
The detection limit calculated as three times the standard
deviation of the background noise was found to be 0.75 µg
L-1 (3.8 nM) which is lower than the level defined by the
World Health Organization.2
Careful analysis of the absorption spectra and the emission
spectra upon mercury addition by means of the SPECFIT
program reveals that a 1:2 complex, a 1:1 complex, and a
2:1 complex (metal-ligand) are successively formed with
very high stability constants, which is in complete agreement
with the work from Lobana’s group.7a
The titration curves by considering the following com-
plexes are shown in the insets of Figure 1 and Figure 2,
respectively. The stability constants were found to be log
K12 ) 15.4 ( 0.5, log K11 ) 8.4 ( 0.3, and log K21 ) 13.8
( 0.4, respectively.
Figure 2. Corrected emission spectra of DPPS 4 (1.1 µM) in the
presence of an increasing concentration of Hg2+ in CH3CN-H2O
(80:20 v/v) at pH ) 4. λexc ) 350 nm. Inset: titration curve at 410
nm.
Figure 3. Binding model of DPPS 4 with Hg2+ leading to the
formation of the complex ML2, ML, and M2L.
spectra of DPPS 4 upon complexation with Hg2+, respec-
tively.
Addition of mercury induces a 15 nm bathochromic shift
of the absorption spectra, which may be rationalized by an
enhancement of the electron-withdrawing character of the
thiophosphano group due to the interaction of the sulfur atom
with the Hg2+.
A strong decrease of the fluorescence is observed upon
mercury complexation, which can be explained in terms of
electron transfer between the excited fluorophore to the
complexed mercury cation. The electrochemistry of the
excited state of DPPS (E(DPPS•+/DPPS*) ) -3 V/SCE)11
clearly shows that the excited state of DPPS is able to reduce
Hg2+ (E ) 0.68 V/SCE). A linear response of the fluores-
cence intensity as a function of the mercury complexation
was observed from 0 to 140 µg L-1.
Figure 3 shows the possible structures of the formed
complexes. The formation of these different complexes is
in good agreement with the different possible coordination
numbers in the case of the mercury complexation.12 Con-
sidering the value of the stability constants, the linear
coordination with the two sulfur atoms is clearly favored in
the case of the mercury complex.
The selectivity of DPPS 4 over other cations such as Ca2+,
Na+, K+, Mg2+, Pb2+, Cd2+, Cu2+, Ag+, and Zn2+ was
evaluated. No significant change of the fluorescence was
observed upon addition of a large excess of these interfering
cations. Except for Ag+, the stability constants of the
complexes of these ions are too low to be determined. The
stability constants obtained in the case of the Ag+ complexes
were log K11 ) 4.6 ( 0.5 and log K21 ) 8.6 ( 0.5. The
selectivity toward Hg2+, expressed as the ratio of the stability
constants, was found to be higher than 6800.
(9) Hajduk, P. J.; Sheppard, G.; Nettesheim, D. G.; Olejniczak, E. T.;
Shuker, S. B.; Meadows, R. P.; Steinman, D. H.; Carrera, G. M.; Marcotte,
P. A.; Severin, J.; Walter, K.; Smith, H.; Gubbins, E.; Simmer, R.; Holzman,
T. F.; Morgan, D. W.; Davidsen, S. K.; Summers, J. B.; Fesik, S. W. J.
Am. Chem. Soc. 1997, 119, 5818-5827.
(10) (a) Yuan, Z.; Taylor, N. J.; Marder, T. B.; Williams, I. D.; Kurtz,
S. K.; Cheng, L.-T. Chem. Commun. 1990, 1489-1490. (b) Yuan, Z.;
Taylor, N. J.; Marder, T. B.; Williams, I. D.; Cheng, L.-T. J. Organomet.
Chem. 1993, 449, 27-37.
(11) This value was estimated by considering the electrochemical
potential in the ground state, E ) 0.5 V, and the energy of the first excited
state.
Moreover, the competition-based fluorescence effect pro-
files for these cations are shown in Figure 4. No significant
change of the response of the sensor is observed upon
addition of a large excess of interfering cations at concentra-
tions compatible with that obtained in the real medium. The
(12) Colton, R.; Ebner, J.; Hoskins, B. F. Inorg. Chem. 1988, 27, 1993-
1999.
Org. Lett., Vol. 9, No. 6, 2007
1135

the level defined by the World Health Organization. This
fluorescent molecular sensor displays reversible response
upon mercury complexation and exhibits a very high
selectivity for the mercury ion over other interfering cations.
In view of practical application, this new fluorescent mo-
lecular sensor can be incorporated into a microfluidic flow-
injection analysis device with fluorometric detection. A
device that works in the case of potassium detection has been
already developed in our group,13 and work is in progress
to apply it in the case of the DPPS 4 to detect Hg2+ directly
in water.
Acknowledgment. This work was supported by the
French Research Ministry program “ACI Jeune Chercheur
2004-2006”. We are grateful to F. Miomandre (UMR 8531
ENS-Cachan) for electrochemical potential measurements
and to B. Valeur (UMR 8531 ENS-Cachan) for fruitful
discussions. M.-H. Ha-Thi and M. Penhoat are, respectively,
grateful to the Ministe`re de l’Education et de la Recherche
and C.N.R.S. for grants (2004-2007 and 2005-2006).
Figure 4. Response IF/I0 of DPPS 4 (0.88 µM) in CH3CN-H2O
(80:20 v/v) at pH ) 4 in the presence of Hg2+ (1.4 µM) and
interfering ions K+, Na+, Mg2+, Zn2+, Cu2+, and Cd2+ at 1 mM,
Pb2+ at 0.1 mM, and Ag+ at 1.5 µM (λexc) 324 nm, λem ) 408
nm).
reversibility of the complexation process was also examined.
After addition of 1 equiv of Hg2+, the fluorescence was
quenched immediately, and a switch on of the fluorescence
was observed after addition of 50 equiv of a heavy metal
ion chelator (2,3-dimercapto-1-propanol) (see Supporting
Information).
In conclusion, we have described the synthesis and the
photophysical and complexing properties of a new fluores-
cent molecular sensor DPPS for Hg2+ in an aqueous medium.
The obtained detection limit (3.8 nM) is compatible with
Supporting Information Available: Experimental pro-
cedures for the synthesis, full analyses of the synthesized
products, and the photophysical measurements are described.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL070118L
(13) Destandau, E.; Lefevre, J.-P.; Chouai Fakhr Eddine, A.; Desportes,
S.; Jullien, M. C.; Hierle, R.; Leray, I.; Valeur, B.; Delaire, J. A. Anal.
Biochem. 2007, accepted.
1136
Org. Lett., Vol. 9, No. 6, 2007