An organoselenium-based highly sensitive and selective fluorescent "turn-On" probe for the Hg2+ ion

Article abstract of DOI:10.1021/ic2023902

An organoselenium-based NSe₃ type of tripodal system 2 as a Hg²⁺-selective fluorescence "turn-on" probe is described. The "turn-on" fluorescence behavior of this selenotripod 2 is significant because it depends on Hg-Se bond formation and acts as a reporting unit for this system. The system exhibits immediate response (15 s) with a subnanomolar detection limit (0.1 nM) for the Hg²⁺ ion. It efficiently detects both aqueous and nonaqueous Hg²⁺ at 2 nM concentration.

Full text of DOI:10.1021/ic2023902

Communication
pubs.acs.org/IC
An Organoselenium-Based Highly Sensitive and Selective
Fluorescent “Turn-On” Probe for the Hg2+ Ion
Abhishek Kumar and Jai Deo Singh*
Department of Chemistry, Indian Institute of Technology Delhi (IITD), Hauz Khas, New Delhi 110016, India
S
* Supporting Information
Scheme 1. Systems Used in This Study
ABSTRACT: An organoselenium-based NSe3 type of
tripodal system 2 as a Hg2+-selective fluorescence “turn-
on” probe is described. The “turn-on” fluorescence
behavior of this selenotripod 2 is significant because it
depends on Hg−Se bond formation and acts as a reporting
unit for this system. The system exhibits immediate
response (15 s) with a subnanomolar detection limit (0.1
the presence of several other chalcophilic metal ions. The
tripod architecture6,9 and flexibility of the donor arms of
selenotripod 2 are highly favorable in achieving a monomeric
mercury species in a predictable geometry. This study is further
distinct from the fluorescence-based sensors because selenotri-
pod 2 itself exhibits fluorescence and a “turn-on” behavior on
Hg−Se bond formation.10−12 This allows us to measure the
Hg2+ ion concentration at nanomolar concentration using
fluorescence spectroscopy.
nM) for the Hg2+ ion. It efficiently detects both aqueous
and nonaqueous Hg2+ at 2 nM concentration.
n a continuation to our studies on organoselenium
Icompounds, we also directed our attention toward their
Hg2+ complexes, revealing a Hg−Se bond under certain
considerations.1 In a conventional paradigm on Hg2+ toxicity,
mercury is antagonist for selenium. It should be noted that the
selenoenzymes are required to prevent and reverse oxidative
damage throughout human body cells and the mitigating role of
selenium in mercury detoxification to a certain concentration is
one of the well-accepted examples of biological antagonism.2,3
The protective effect of selenium against mercury in human
beings is believed to lead to the loss of selenoenzyme activities
and their synthesis as a consequence of Hg2+-based seizure of
selenium. This may be further rationalized because of the fact
that binding affinities of Hg2+ for selenium are up to a million
times higher4 than those of its second-best binding partner, i.e.,
sulfur.5 Because of the selenophilic characteristics of Hg2+, one
may anticipate preferred selectivity and effective binding with
selenium.1,6 Despite the fundamental and biological significance
of Hg−Se bond formation, the potential selenium compounds
that can be used as an effective means of recognizing the Hg2+
ion or counteracting its toxicity have not been systematically
studied. Indeed, the chemistry of Hg−Se-bonded species is
limited because of either their insoluble nature or polymeric
characteristics.7 Therefore, identification of suitable organo-
selenium species and their specific interaction with Hg2+ is
highly desirable. Moreover, remediation from the adverse
effects related to Hg2+ species continues to remain a serious
challenge.8 Bearing all of these issues in mind, an interesting
prospect that deserves deeper investigation is the sensitivity,
selectivity, and entrapment behavior of organoselenium
compounds toward the detection and detoxification of Hg2+
ions. The success of such an approach mainly depends on the
identification of suitable organoselenium ligands and is yet to
be realized. We herein report our preliminary investigations
from an identified selenium-based tripod system 2 (Scheme 1)
as highly selective and sensitive for binding Hg2+ ions even in
The Hg2+-sensing studies were carried out by using a
Hg(NO3)2 salt at neutral pH under unbuffered conditions at 25
°C (298 K). The addition of an aqueous solution of Hg(NO3)2
to a solution of selenotripod 2 led to observable changes in the
UV−visible spectra (Figure 1). The peak at λ = 266 nm in 2
Figure 1. Absorption spectra of 2 (100 μM) in 80:20 (v/v) CH3CN−
H2O and changes observed upon the addition of a Hg(NO3)2 salt
solution (0−100 μM) in H2O. Inset: Job’s plot showing minima at
0.50, indicating 1:1 binding between 2 and Hg2+.
exhibited an inhibition of absorbance, i.e., a hypochromic effect
upon the addition of a Hg2+ solution. An isosbestic point at λ =
281 nm was also observed along with a hyperchromic effect at λ
= 300 nm. The spectral changes were observed until attainment
of a 1:1 equivalence between 2 and Hg2+, as shown by Job’s
Received: November 4, 2011
Published: December 21, 2011
© 2011 American Chemical Society
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Communication
plot minima for [Hg2+]/([2] + [Hg2+]) at 0.50, indicating the
formation of a 1:1 species between system 2 and the Hg2+ ion
in solution.
The fluorescence spectrum of 2 in acetonitrile−water
(80:20) shows a weak emission at λ = 302 nm. However, a
4-fold enhancement in fluorescence was observed upon the
addition of 1 equiv of an aqueous Hg2+ solution (Figure 2). The
response toward Hg2+ ions. Comparatively, no significant
changes were observed upon the addition of Hg2+ into a
solution of 1 or 3 (Figures S15−17, SI).
Because the coordination chemistry of some of the
chalcophilic metal ions (Ag+, Cu2+, Pb2+, Cd2+, Zn2+, Ni2+,
and Co2+) with organoselenium donors has been studied
earlier, the fluorescence response of system 2 toward possible
interference of these metal ions along with K+ and Fe3+ ions
was also examined (Figure 3). No appreciable changes were
Figure 2. Emission spectra of 2 (1.5 μM) in 80:20 (v/v) CH3CN−
H2O and changes observed upon the addition of a Hg(NO3)2 salt
solution (0−1.5 μM) in H2O. Inset: Enhancement in the fluorescence
intensity observed at 302 nm upon the addition of a Hg2+ solution
(λexcitation = 270 nm; λemission = 285 nm).
Figure 3. Changes observed in the emission spectrum of 2 (1.5 μM)
in 80:20 (v/v) CH3CN-H2O upon the addition of various metal
nitrate salts (25 equiv) in H2O except Hg2+ (1 equiv) (λexcitation = 270
nm; λemission = 285 nm).
fluorescence enhancement (10%) of system 2 for an aqueous
solution of Hg2+ at the 2 nM level was a remarkable response
over many of the fluorescence-based probes.10,11 This further
indicates that the Hg−Se bond formation process can be
monitored by enhancement in fluorescence spectroscopy.
Moreover, a linear response to Hg2+ in the concentration
range 2−165 nM can be utilized at best for the practical
estimation of the Hg2+ ion present in solution (Figure S9, SI).
Further, in comparison to most of the other fluorescent probes
reported to date for Hg2+ detection, an immediate fluorescence
enhancement due to the Hg−Se bond was observed in the
present case upon mixing of a Hg2+ solution with selenotripod
2 (15 s). Additionally, no appreciable changes in the emission
spectra were noticed from 30 min to 24 h in all of the observed
cases, which indicates that the initially formed species in
solution is irreversible. The detection limit of this new
chemodosimeter 2 was found to be 1.0 × 10−10 M (or 0.1
nM), which is 100 times less than the EPA standards (10
nM)13 and is the lowest detection limit observed so far, lower
than any chemodosimeter reported for Hg2+-ion detection.10
The anion effect on this Hg2+ sensing was also found negligible
because fluorescence studies repeated with mercuric perchlo-
rate and chloride salts under identical conditions revealed
observations similar to those obtained with mercuric nitrate salt
(Figure S10, SI). Over the pH range tested, a stable and
sensitive fluorescence response in the pH range of 4.0−9.0 was
observed, affording the possibility for detection of the Hg2+ ion
by 2 under both acidic and basic conditions (Figures S11 and
12, SI). The Hg2+ species formed upon binding of selenotripod
2 with Hg2+ also exhibited thermal stability in the temperature
range of 25−45 °C (Figure S13, SI). Further, no change in the
fluorescence response was observed under oxidizing conditions
(Figure S14, SI). For comparison purposes, sulfur (1) and
tellurium (3) analogues of 2 have also been examined for their
observed upon the addition of even 25 equiv of these metal
ions except Ag+. A nearly 2-fold rise in fluorescence was
observed upon the addition of 1 equiv of Ag+, but no further
changes were observed even upon the addition of up to 25
equiv of Ag+. However, the addition of 1 equiv of a Hg2+
solution into this 2·Ag+-containing solution immediately
showed fluorescence enhancement as observed for 2·Hg2+.
These results are suggestive that Ag+ bonded to 2 in solution
can be exchanged by the Hg2+ ion. This was further confirmed
separately when an isolated Ag+ species with 2 in a 1:1 ratio
dissolved in CH3CN and treated with 1 equiv of Hg2+ led to
isolation of 2·Hg2+ species. The recovery of 99.9% Ag+ ion was
also confirmed by a solvent-extraction process. The other metal
ions were also tested for their likely interference in Hg2+-ion
detection by 2. In all of the cases, fluorescence enhancement of
selenotripod 2 upon the addition of 1 equiv of the Hg2+ ion
into a solution containing the other tested metal ions (25
equiv) was identical with that observed for the Hg2+ ion alone
(Figure S18, SI).
The 77Se NMR spectra of a 1:1 species of selenotripod 2 and
Hg2+ species revealed a peak at δ 206.7 ppm (Figure S19, SI).
This is a significant upfield shift in comparison to that of
selenotripod 2 (δ 279.1 ppm)14 and is in contrast to the usual
downfield shift observed upon metal−ligand coordination. This
may be attributed to a significant charge transfer between the
mercury and selenium centers upon binding. This charge
transfer between the mercury and selenium centers makes 2 a
unique system to be directly monitored by fluorescence
spectroscopy. No additional peaks were observed in the 77Se
NMR spectrum even at higher scans (10 K). The dissociation
of the complex with time (7 days) as a consequence of the
solvent-exchange effect was found negligible. It can be
suggested that the existing species in solution is coordinatively
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Communication
intact. It is interesting to note here that, because of very high
solvation of the mercury ion, their direct recognition in both
aqueous and nonaqueous solutions is quite difficult. For
example, in a solution of HgCl2 (at 25 °C), an equilibrium
ASSOCIATED CONTENT
* Supporting Information
Synthetic procedure and characterization data for 1−3, scanned
copies of spectral data of 2 and 3, various emission spectra and
other information mentioned in this Communication, and a
CIF file of 2·Hg(NO3)2. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
2−n
between 12 mercury species was observed, namely, HgCln
with n = 0−4, HgOHCl, Hg(OH)n2−n with n = 1−3, HgOH3+,
Hg3(OH)33+, and HgO(s).15 Therefore, the solution obtained
from the fluorescence titration experiment was further analyzed
by mass spectroscopy to observe whether any mercury species
formed in an aqueous Hg2+ solution had been left out of the
estimation by selenotripod 2. It exhibited a molecular-ion peak
at m/z 933.7816 ([2 + Hg(NO3)2 + K]+). However, no peak
corresponding to any solvated mercury ion was observed in the
mass spectrum. Moreover, the fluorescence experiment carried
out with an acetonitrile solution of Hg(NO3)2 also provided a
fluorescence enhancement response identical with that
obtained with an aqueous Hg(NO3)2 solution (Figure S20,
SI). In fact, the mass experiment repeated after 7 days also
returned identical results. The structural determination of a
crystal obtained from the solution upon slow evaporation was
also attempted. The X-ray structure obtained from this crystal is
shown in Figure 4. The three selenium-donor arms of 2
AUTHOR INFORMATION
Corresponding Author
*E-mail: jaideo@chemistry.iitd.ernet.in.
■
ACKNOWLEDGMENTS
A.K. thanks CSIR, New Delhi, India, for a research fellowship.
■
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