Optical mercury sensing using a benzothiazolium hemicyanine dye

Article abstract of DOI:10.1021/ol0615580

The selectivity and sensitivity of a benzothiazolium hemicyanine dye toward mercury(II) in aqueous solutions are described. Mercury Ions coordinate to the dye forming a 1:1 complex. This interaction induces a color change in the dye at micromolar concentrations of mercury. Furthermore, the color change and quenching of the dye emission are selective for mercury when compared with other ions such as lead(II), cadmium(II), zinc(II), or iron(II).

Full text of DOI:10.1021/ol0615580

ORGANIC
LETTERS
2
006
Vol. 8, No. 17
857-3860
Optical Mercury Sensing Using a
Benzothiazolium Hemicyanine Dye
3
Sergio Tatay, Pablo Gavi n˜ a,*,† Eugenio Coronado, and Emilio Palomares*
†
†
,†,‡
Instituto de Ciencia Molecular, UniVersidad de Valencia (ICMol-UV),
Edificio de Institutos, Pol ´ı gono la Coma s/n. 46980 Paterna (Valencia), Spain
epalomares@iciq.es
Received June 25, 2006
ABSTRACT
The selectivity and sensitivity of a benzothiazolium hemicyanine dye toward mercury(II) in aqueous solutions are described. Mercury ions
coordinate to the dye forming a 1:1 complex. This interaction induces a color change in the dye at micromolar concentrations of mercury.
Furthermore, the color change and quenching of the dye emission are selective for mercury when compared with other ions such as lead(II),
cadmium(II), zinc(II), or iron(II).
Selective optical sensing is attracting strong interest due to
the use of “low-tech” spectroscopic instrumentation to detect
relevant chemical species in biological and environmental
processes.1 In particular, one aspect of environmental
monitoring is the detection of low levels of heavy metals in
aqueous solutions. Of these, mercury is of particular concern
worldwide because of its high toxicity. Mercurial species
may be released into the environment through coal-fired
power plants and into water streams from chemical, gold
mining, and chlor-alkali industrial plants.
attractive approach because of the sensitivity and mercury-
selective properties of such complexes. However, the use
3
of such complexes might be seen as a demanding step toward
the development of cheap sensing technology. For this
reason, we have decided to move forward and synthesize an
organic molecular probe (OMP) as mercury selective and
colorimetrically responsive as ruthenium dyes but less
expensive from a synthetic point of view. Furthermore, we
have taken into account the possibility of using the OMP
not only for colorimetric but also for fluorometric mercury
sensing, increasing the sensitivity by using fluorescence
emission spectroscopic methods.
,2
Recently, several colorimetric probes, so-called “naked eye
chemosensors”, have been developed for the detection of
2
4
small quantities of mercury. Several groups have shown that
On the basis of previous literature reports, we decided to
molecular sensing based on ruthenium complexes is an
synthesize a molecular probe consisting of a charge-transfer
complex whose electronic properties (UV-visible and
†
Instituto de Ciencia Molecular, Universidad de Valencia (ICMol-UV).
Present address: Institute of Chemical Research of Catalonia (ICIQ),
‡
Avda. Pa ¨ı sos Catalans, 16, 43007 Tarragona, Spain.
(3) (a) Palomares, E.; Vilar, R.; Durrant, J. R. Chem. Commun. 2004,
362-363. (b) Coronado, E.; Galan-Mascar o´ s, J. R.; Marti-Gastaldo, C.;
Palomares, E.; Durrant, J. R.; Vilar, R.; Gratzel, M.; Nazeruddin, Md. K.
J. Am. Chem. Soc. 2005, 127, 12351-12356. (c) Nazeruddin, Md. K.; Di
Censo, D.; Humphry-Baker, R.; Gratzel, M. AdV. Func. Mater. 2006, 16,
189-194.
(4) (a) Fedorova, O. A.; Fedorov, Y. V.; Vedernikov, A. I.; Gromov, S.
P.; Yescheulova, O. V.; Alfimov, M. V.; Woerner, M.; Bossmann, S.; Braun,
A.; Saltiel, J. J. Phys. Chem. A 2002, 106, 6213-6222. (b) Chen, Y.-S.;
Li, C.; Zeng, Z.-H.; Wang, W.-B.; Wang, X.-S.; Zhang, B.-W. J. Mater.
Chem. 2005, 15, 1654-1661. (c) Basheer, M. C.; Alex, S.; Thomas, G. K.;
Suresh, H. Ch.; Das, S. Tetrahedron 2006, 62, 605-610.
(
1) (a) Br u¨ mer, O.; La Clair, J. J.; Janda, K. D. Org. Lett. 1999, 1, 415-
4
18. (b) Br u¨ mer, O.; La Clair, J. J.; Janda, K. D. Bioorg. Med. Chem. 2001,
9
, 1067-1071.
(
2) (a) Sancen o´ n, F.; Mart ´ı nez-M a´ n˜ ez, R.; Soto, J. Chem. Commun. 2001,
2
1
262-2263. (b) Yang, Y.-K.; Yook, K.-J.; Tae, J. J. Am. Chem. Soc. 2005,
27 16760-16761. (c) Nolan, E. M.; Racine, M. E.; Lippard, S. J. Inorg.
Chem. 2006, 45, 2742-2749. (d) Zheng, H.; Qian, Z.-H.; Xu, L.; Yuan,
F.-F.; Lan, L.-D.; Xu, J.-G. Org. Lett. 2006, 8, 859-861. (e) Caballero,
A.; Mart ´ı nez, R.; Lloveras, V.; Ratera, I.; Vidal-Gancedo, J.; Wurst, K.;
T a´ rraga, A.; Molina, P.; Veciana, J. J. Am. Chem. Soc. 2005, 127, 15666-
1
5667.
1
0.1021/ol0615580 CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/20/2006

fluorescent emission spectra) can be strongly affected when
mercury is present. Moreover, the molecular probe should
contain mercury-coordinating elements to ensure high se-
lectivity in the presence of other heavy metals. Hemicyanine
dyes (Figure 1) provide such features. These molecules
The results in Figure 2 are in good agreement with the
Job’s plot analysis (Figure 3), where the binding stoichiom-
Figure 1. Molecular structure of OMP.
combine high molar extinction coefficients at the maximum
4
-1
-1
absorption wavelength (in most cases ꢀ ∼ 10 M cm in
the visible region) with excellent emission quantum yields
and good solubility in aqueous solutions.
OMP is a hemicyanine dye composed of an electron-donor
aniline moiety and an electron acceptor that in this case is a
benzothiazolium species where the sulfur atom provides the
mercury-coordinating element. This compound was easily
synthesized in two steps from 2-methylbenzothiazole, bro-
Figure 3. Job’s plot of the complexation between OMP and Hg2+.
Total concentration of OMP + Hg2 was kept constant at 10 mM
in ethanol/HEPES buffer, pH 7.0, v/v 1:10.
+
moacetic acid, and 4-diethylamino-2-hydroxybenzaldehyde
as a dark-green solid, in 54% overall yield.4b
etry for the mercury/OMP complex was found to be 1:1 with
7
-1
a calculated association constant, K
ethanol/HEPES buffer solution (1:10, v/v) at pH 7 (see
a
, of 1.0 × 10 M in
Figure 2 shows the changes on the UV-visible spectra
of OMP upon addition of mercury chloride at pH 7. As can
be observed, the addition of mercury induces a clear
isosbestic point corresponding to the presence of only two
species at a constant total concentration of OMP.
Supporting Information), showing the high affinity of OMP
2
+
for Hg .
In fact, the presence of mercury, at neutral pH, can be
detected without the use of any instrumentation as illustrated
in Figure 4. The selectivity toward mercury has been
Figure 4. Digital picture of OMP in ethanol/HEPES buffer (pH
2
+
2+
7
) solution (v/v 1:10). From left to right: no metal, Hg , Cd ,
2+
2+
2+
2+
+
2+
2+
Pb , Zn , Fe , Cu , Na , Ni , Mg , equivalent amounts of
all the cations and Hg . The cation total concentration was 0.1
mM.
2+
demonstrated even in the presence of similar amounts of
other metal cations catalogued by the Environmental Protec-
tion Agency (EPA) as toxic substances. Moreover, the
presence of common anions such as phosphate, chloride,
iodide, and fluoride also did not change the electronic
properties of OMP.
Figure 2. Changes in the UV-vis spectra of OMP (10 µM) in
aqueous solution (ethanol/HEPES buffer, pH 7.0, v/v 1:10) upon
addition of increasing amounts of HgCl
total volume was 3 mL.
2
in water (1 mM). The
Furthermore, to show the high mercury selectivity at low
metal concentrations and to determine the sensitivity of OMP
3858
Org. Lett., Vol. 8, No. 17, 2006

ishes, and when a basic pH is used, the UV-visible spectra
has a bathochromic shift of 25 nm. We expected this pH
effect because of the nature of the charge-transfer band of
the hemicyanine molecule, which depends on its protonation
state.
As can be seen in Figure 7, the presence of mercury
chloride induces changes on the absorbance of OMP.
2+
However, as expected, the response toward Hg is different
depending on the pH of the aqueous solution, obtaining the
highest sensitivity at neutral pH.
Figure 5. UV-visible titration of OMP in an aqueous solution
ethanol/HEPES buffer, pH 7 solution, v/v 1:10) in the presence
of selected cations. The absorption wavelength was fixed at λ )
38 nm.
(
5
toward mercury, we used UV-visible spectroscopy. Figure
shows that in the presence of other metals the selectivity
5
and sensitivity of our molecular probe OMP is barely
affected.
Taking into account that most of the common metal cations
could precipitate in basic pH conditions, we decided to
perform some additional experiments using acidic and neutral
pH controlled solutions. To evaluate the photophysical
properties of the dye in acidic or basic media, we measured
the UV-visible absorbance as a function of pH. Our
experiments showed that the maximum of the UV-vis
spectra was hardly affected in acidic and neutral conditions
Figure 7. UV-vis response of OMP toward Hg2 at different
+
pH values registered at λ ) 535 nm. The OMP concentration was
0.1 mM.
(
Figure 6); however, in highly acidic media (pH < 3.5), the
Turning to the fluorescent emission properties of OMP,
molar extinction coefficient for the organic molecule dimin-
it is well-known that fluorescent emission spectroscopy is
more sensitive toward small changes that affect the electronic
properties of molecular receptors. Several authors have based
5
their chemosensors on this approach. To check this method,
we measured the fluorescent emission properties of OMP
in the presence of mercury.
As illustrated in Figure 8, mercury has a strong quenching
effect, where almost 85% of luminescence is quenched at
room temperature. Moreover, the emission yield is barely
affected by other toxic cations commonly used for mercury-
interference experiments. Thus, OMP not only shows
excellent colorimetric properties for mercury sensing but also
can be used as a “turn-off” fluorometric molecular probe
with high selectivity toward mercury and a detection limit
6
of 100 ppb.
(
5) (a) Nolan, E. M.; Lippard, S. J. J. Am. Chem. Soc. 2003, 125, 14270-
4271. (b) Chen, Y.; Zeng, D. X. Chem. Phys. Chem. 2004, 5, 564-566.
c) Wang, J.; Qian, X. Chem. Commun. 2006, 109-111. (d) Matsushita,
M.; Meijer, M. M.; Wirsching, P.; Lerner, R. A.; Janda, K. D. Org. Lett.
1
(
2
005, 7, 4943-4946. (e) Zhang, H.; Han, L.-F.; Zacharlasse, K. A.; Jiang,
Figure 6. Absorbance of OMP at different pH values. The ligand
total concentration was 0.1 mM.
Y.-B. Org. Lett. 2005, 7, 4217-4220. (f) Ono, A.; Togashi, H. Angew.
Chem., Int. Ed. 2004, 43, 4300-4302. (g) Rurack, K.; Kollmannsberger,
M.; Resch-Genger, U.; Daub, J. J. Am. Chem. Soc. 2000, 122, 968-969.
Org. Lett., Vol. 8, No. 17, 2006
3859

oxygen atom of the phenolic group, forcing the hemicyanine
to adopt a cis configuration. In the cis-OMP isomer, both
chromophores are nonplanar, inducing a hypsochromic shift
on the UV-visible spectrum. Other groups have reported a
similar stabilization of a related cis-merocyanine isomer via
4
a,7
intramolecular chelation to transition metals.
This hy-
pothesis is supported by the fact that in a highly acidic media
the sensitivity of OMP toward mercury decreases. Most
probably, the nitrogen atom of the donor moiety will be
protonated, inhibiting the thermal trans/cis isomerization of
the central double bond.
In conclusion we have demonstrated selective colorimetric
and fluorometric mercury sensing using a benzothiazolium
hemicyanine dye in aqueous solution with a response limit
in the case of fluorescence spectroscopy of 100 ppb. The
high selective nature of the molecular probe toward mercury
is probably due to the presence of the sulfur atom and to the
fact that mercury ions induce a geometric change from the
trans isomer to the cis form, as can be deduced from the
results of simple spectroscopic measurements. Further efforts
toward understanding and controlling the mercury uptake by
light switching between both dye stereoisomers are currently
in progress. We see these initial results as the first step toward
light-induced molecular sensors in the presence of mercury
ions.
Figure 8. Fluorescent emission spectra under normal conditions
of OMP in an aqueous solution (ethanol/HEPES buffer, pH 7
solution, v/v 1:10) (excitation wavelength λ ) 530 nm). As can be
seen, the emission is only strongly quenched by Hg2 (∼15 µM).
+
Finally, we would like to consider the chemical nature of
1
the high selectivity of OMP toward mercury. H NMR
3
experiments of OMP in CD OD at different temperatures
Acknowledgment. E.P. and P.G. wish to thank the
Spanish Ministerio de Educaci o´ n y Ciencia (MEC) for the
Ramon y Cajal Fellowships and the Ph. D. grant for S.T.
E.P. also thanks the EU for the project Heteromolmat and
Bioreply S.L. for financial support. The authors thanks
Amparo Forneli for excellent technical assistance and the
referees for the useful comments.
showed that the ligand is present as a mixture of trans- and
cis-cyanines (trans-OMP and cis-OMP) in dynamic equi-
librium, with trans-OMP being the most stable isomer due
to a minimum of steric repulsions. The planarity of the trans-
OMP isomer optimizes the conjugation of the donor and
acceptor moieties resulting in a strong coloration. In the
2
+
presence of Hg , OMP forms a relatively stable metal
complex with a 1:1 stoichiometry, involving the sulfur atom
of the benzothiazolium moiety and most probably also the
Supporting Information Available: Experimental details
and characterization data for OMP and mercury binding
constant determination. This material is available free of
charge via the Internet at http://pubs.acs.org.
(
6) The response limit was calculated as the minimum concentration
that can be determined with 99% of confidence that the true concentra-
tion is greater than zero. See: Guidelines establishing test procedures
for the analysis of pollutants. Appendix B: Definition and Procedure
of the Determination of the Method Detection Limit (MDL). http://
ecfr.gpoaccess.gov.
OL0615580
(7) Wojtyk, J. T. C.; Kazmaier, P. M.; Buncel, E. Chem. Mater. 2001,
13, 2547-2551.
3860
Org. Lett., Vol. 8, No. 17, 2006