A simple and efficient dual optical signaling chemodosimeter for toxic Hg(II)

Article abstract of DOI:10.3762/bjoc.8.155

10-Methylthioacridone, designated as acrithion, has been employed as an easily accessible chemodosimeter for the optical targeting of toxic Hg ²⁺ in buffered aqueous DMSO. The Hg²⁺-mediated desulfurization of the probe is translated into selective dual signaling of Hg²⁺ by means of color bleaching and fluorescence amplification while several other metal ions, including potentially competing Ag+, Cu²⁺ and Pb²⁺, afford no significant interferences even in excess concentrations.

Full text of DOI:10.3762/bjoc.8.155

A simple and efficient dual optical signaling
chemodosimeter for toxic Hg(II)
Sabir H. Mashraqui*, Sapna A. Tripathi, Sushil S. Ghorpade
and Smita R. Britto
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1352–1357.
Department of Chemistry, University of Mumbai, Vidyanagari,
Mumbai-400098, India
doi:10.3762/bjoc.8.155
Received: 07 May 2012
Accepted: 20 July 2012
Published: 22 August 2012
Email:
Sabir H. Mashraqui* - sh_mashraqui@yahoo.com
*
Corresponding author
Associate Editor: J. A. Murphy
Keywords:
© 2012 Mashraqui et al; licensee Beilstein-Institut.
License and terms: see end of document.
chemidosimeter; chemosensor; fluorescence; Hg(II) selective;
UV–visible; visual detection
Abstract
1
0-Methylthioacridone, designated as acrithion, has been employed as an easily accessible chemodosimeter for the optical targeting
of toxic Hg2+ in buffered aqueous DMSO. The Hg2+-mediated desulfurization of the probe is translated into selective dual
signaling of Hg2+ by means of color bleaching and fluorescence amplification while several other metal ions, including potentially
competing Ag+, Cu2+ and Pb2+, afford no significant interferences even in excess concentrations.
Introduction
Interest continues unabatedly to design optical probes for the potentially toxic metal ions such as Cu2+, Hg2+, Pb2+ etc.
selective detection of metal ions of importance across disci- However, owing to the reversible coordination with the probes,
plines of biology, medicine and environment [1-5]. Though, these paramagnetic metal ions often result in less desirable fluo-
mercury and alkylmercury are highly poisonous, mercury ions rescence quenching effects [12-16]. Lately, the chemodosi-
are also a subject of worldwide concern because of their acute meter-based sensing has gained increasing popularity on
immune-, geno- and neurotoxic effects on human, livestock and account of the irreversible chemical intervention of analytes
marine mammals [6-11]. The poisonous effects of mercury ions with the probe molecules. Since the metal ions are disengaged
stem from their irreversible binding with the sulfhydryl- from the chemically modified dosimeters, the process results in
containing groups of proteins and enzymes, resulting in clinical the more reliable fluorescence ratiometric or switch-on
problems such as prenatal brain damage, muscle coordination responses even for the paramagnetic metal ions [17-19]. The
difficulties, lung, kidney and vision disorders.
first luminescence chemodosimeter for Hg2+ was developed by
Czarnik et al. and was based on the Hg2+-mediated desulfuriza-
To date, many chemosensors featuring assortments of ligands tion of anthracene-thioamide chromophore [20,21]. Following
and signalling units have been reported for the detection of this pioneering report, several other groups have utilized the
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exceptionally strong thiophilicity of Hg2+ to access a range of (Supporting Information File 1). The optical sensitivities of
luminescent Hg2+ chemodosimeters [22-38]. Despite impres- acrithion 2 towards various biologically and environmentally
sive advances [39,40], many known Hg2+ chemosensors suffer important metal ions as their perchlorates were evaluated on the
from multistep syntheses and delayed responses or cross affini- basis of UV–vis as well as fluorescence spectral responses.
ties, especially from Ag+, Pb2+ and Cu2+ [41-51]. Conse- Acrithion 2 (2.83 × 10−5 M) displayed absorption bands at 493,
quently, designing easily accessible Hg2+-selective chemosen- 462 and 293 nm in DMSO–H2O (70:30 v/v) at pH 7.4 (10 mM
sors with dual colorimetric and fluorescence switch-on capabili- HEPES buffer). As shown in Figure 1, the spectrophotometric
ties is deemed of interest in supramolecular research.
titration of acrithion 2 (2.83 × 10−5 M) with increasing Hg2+
induced a new blue-shifted maximum at 404 nm, while the
Acridone, a photostable fluorophore [52], has been designed to probe’s maxima at 493 and 462 nm gradually diminished and
function as metal ion and anion sensor [53-57]. In contrast to disappeared at the limiting 2.83 × 10−4 M of Hg2+. At the
moderately fluorescent acridone, its thione analog is poorly limiting concentration of Hg2+, the final UV–vis spectrum was
emissive presumably on account of the intersystem crossing essentially similar to that of 1 [60], an observation that supports
process of the C=S bonding [58]. Keeping this in mind, we have the Hg2+-induced desulfurization of the probe [60]. An inde-
presently conceived the design of simple and easily accessible pendent confirmation of 1 was also obtained from the 13C NMR
1
0-methylthioacridone – designated as acrithion 2 – as a new spectrum of the product isolated from the reaction of acrithion 2
chemodosimeter for the highly selective colorimetric and fluo- with Hg(ClO4)2 in aqueous DMSO (Supporting Information
rescence signaling of Hg2+.
File 1).
The chemodosimeter mechanism, illustrated in Scheme 1, is
proposed to proceed via chelation of Hg2+ with the thione group
of acrithion 2, forming an intermediate thioacridinium–Hg+
complex. This phenomena would polarize the C=S bond and
therefore facilitate nucleophilic attack by water at the electro-
philic C9 position. Subsequently, the hydrolytic desulfurization
is expected to form compound 1 – which is more fluorescent –
at the expense of poorly emitting acrithion 2. In addition to the
fluorescence amplification, the difference in the absorbance
wavelengths between acrithion 2 and the desulfurization prod-
uct 1 would also afford colorimetric naked-eye signaling of
Hg2+.
Results and Discussion
Acrithion 2 is synthesized in good yield by treatment of readily
Figure 1: Spectrophotometric titration of acrithion 2 (2.83 × 10−5 M)
with Hg2+ (0–2.83 × 10−4 M) in DMSO–H2O (70:30 v/v) at pH 7.4.
available 10-methyl acridone 1 with P2S5 in pyridine [59]
As depicted in Figure 2, no detectable changes in the
absorbance profile of acrithion 2 could be seen up on adding
2
.83 × 10−3 M of Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+,
Cu2+, Zn2+, Cd2+, and Pb2+ even after 20 min, implying the
absence of substantive ground state interaction. On the other
hand, the exposure of the probe to Ag+ at 2.83 × 10−3 M led to
partial reductions of the probe’s maxima at 493 and 462 nm,
accompanied by the appearance of a new, weakly absorbing
band at 347 nm. However, unlike Hg2+, the desulfurization
product 1 could not be detected with Ag+. Though it has not
been investigated in detail, the findings mentioned above imply
that the interaction of Ag+ is not only relatively weaker, but also
of a different nature than observed with Hg2+. The addition of
Hg2+ turned the deep yellow solution of acrithion 2 into a color-
Scheme 1: Synthesis of acrithion 2 and its chemodosimeter mecha-
nism for Hg2+ recognition.
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Beilstein J. Org. Chem. 2012, 8, 1352–1357.
Figure 2: Spectrophotometric investigation of acrithion 2
(
2.83 × 10−5 M) with metal ions (2.83 × 10−3 M) in DMSO–H2O
Figure 3: Fluorimetric titration of acrithion 2 (2.5 × 10−6 M) with Hg2+
(0–5.1 × 10−5 M) in DMSO–H2O (70:30 v/v) at pH 7.4.
(70:30 v/v) at pH 7.4.
less one, permitting ready naked-eye detection of this ion. probe is unsuitable to measure 1 μg L−1 of Hg2+ – the lowest
While weakly interacting Ag+ induced only a slight dilution of permission quantity in drinking water – however, there might be
the yellow color, no perceptible color modulations were notice- potential for measuring micromolar concentrations of Hg2+ in
able with the several background metal ions which were exam- environmental samples with the probe.
ined (Supporting Information File 1).
In contrast to the sizeable fluorescence amplification observed
Excitation of acrithion 2 at 404 nm in DMSO–H2O (70:30 v/v) with Hg2+, the weakly interacting Ag+ induced only less
at pH 7.4 gave rise to a weak emission band at 424 nm with a marked emission enhancement of only 3-fold at a concentration
quantum yield (ФF) of 0.021, calculated with respect to which was nearly 10 times the limiting concentration of Hg2+
anthracene (ФF = 0.27) [61]. Fluorimetric titration (Figure 3) (Supporting Information File 1). Consistent with their lack of
revealed linear enhancements in the emission intensity with a ground state interaction, metal ions viz. Li+, Na+, K+, Mg2+,
significant 17-fold enhancement observed at a limiting Ca2+, Ba2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ up to
5
.1 × 10−5 M of Hg2+ [60]. Based on the linear response 100 equiv did not reveal any significant fluorescence modula-
observed in emission intensity with respect to increasing Hg2+ tions (Supporting Information File 1).
concentration, we calculated the limit of detection (LOD) of
Hg2+ to be 1.06 × 10−8 mol L−1 or 2.12 μg L−1 of Hg2+ As shown in Figure 4, the addition of Hg2+ to a solution of
(
Supporting Information File 1). Though, this LOD level of the acrithion 2 generated a brilliant blue fluorescence whereas other
Figure 4: Changes in fluorescence of acrithion 2 upon addition of metal ions in DMSO–H2O (70:30 v/v) at pH 7.4.
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Beilstein J. Org. Chem. 2012, 8, 1352–1357.
metal ions showed no detectable fluorescence responses. To
verify the high selectivity of Hg2+ over other metal ions, we
first measured the fluorescence response of the probe
(
2.5 × 10−6 M) in the presence of a matrix consisting of
00 equiv of each Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+,
Cu2+, Zn2+, Cd2+, Ag+ and Pb2+. The matrix gave rise to about
-fold fluorescence enhancement compared to that of the free
1
3
probe. The addition of Hg2+ (5.1 × 10−5 M) to the matrix solu-
tion described above caused ca. 15-fold fluorescence enhance-
ment, a value comparable to that induced by Hg2+ alone at the
same concentration. This experiment (Figure 5) clearly demon-
strates the superior chemodosimeter action of hazardous
mercury ions in samples containing several background metal
ions.
Figure 6: Fluorimetric response of acrithion 2 (2.5 × 10−6 M) in the
presence of background metal ions (2.5 × 10−4 M each) in red and
after addition of Hg2+ (5.1 × 10−5 M) in blue.
reaching an optimum value within ca. 30 min and then
remaining steady beyond this point. The response time of
acrithion 2 is comparable to many known Hg2+ chemodosime-
ters, however, this delayed time for Hg2+ signaling may limit its
practical applications.
Figure 5: Emission spectra of acrithion 2 (2.5 ×10−6 M), acrithion 2 +
matrix consisting of 2.5 × 10−4 M each of Li+, Na+, K+, Mg2+, Ca2+,
Ba2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Ag+ and Pb2+ and acrithion 2 +
matrix + Hg+2 (5.1 × 10−5 M).
In addition to the matrix experiment described above, we
studied the effects of individual interferents on the fluorescence
response of mercury ions. Therefore, the fluorescence spectra of
the probe in the presence of individual metal ions (2.5 × 10−4 M
each) without and with added merucury ions at 5.1 × 10−5 M
were recorded. As shown in Figure 6, the background metal
ions exhibited either none or insignificant perturbations on the
Figure 7: Time course plot of the change in emission intensity of
acrithion 2.
selective fluorescence response observed upon adding mercury Conclusion
ions. In conclusion, 10-methylthioacridone constitutes one of the
simplest and most easily accessible chemodosimeters affording
Next, to evaluate the response time, we plotted a time-depen- dual colorimetric and fluorescence switch-on responses for the
dent evolution of fluorescence with respect to the limiting selective and sensitive targeting of toxic Hg2+. Other merits of
concentration of Hg2+ at 20 °C. As shown in Figure 7, a the chemodosimeter include visible-light excitation and absence
substantial gain in the emission intensity (ca. 75 % of the total of significant interferences, especially from the competing Ag+,
enhancement) is recorded under 20 min of contact time. There- Cu2+ and Pb2+, the features which may prove conducive to
after, the fluorescence enhancement occurs relatively slowly, Hg2+-sensing applications in certain circumstances.
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