Sensitive and selective detection of cesium via fluorescence quenching

Article abstract of DOI:10.1039/c3dt52215f

Herein we report a robust and easy method for detecting cesium metal ion (Cs⁺) in partially aqueous solutions using the fluorescence quenching of 2,4-bis[4-(N,N-dihydroxyethylamino)phenyl]squaraine. This squaraine dye was found to be both highly sensitive (low limits of detection) and selective (limited response to other metals) for cesium ion detection. The detection is likely based on the metal complexing to the dihydroxyethanolamine moieties, which disrupts the donor-acceptor-donor architecture and leads to efficient quenching.

Full text of DOI:10.1039/c3dt52215f

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Sensitive and selective detection of cesium via
fluorescence quenching†
Cite this: Dalton Trans., 2013, 42, 16276
Received 13th August 2013,
Accepted 2nd October 2013
Bhasker Radaram, Teresa Mako and Mindy Levine*
DOI: 10.1039/c3dt52215f
www.rsc.org/dalton
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Herein we report a robust and easy method for detecting cesium sensitivity to the surrounding environment. Squaraines have
+
metal ion (Cs ) in partially aqueous solutions using the been used to detect metal ions, including mercury, silver, and
fluorescence quenching of 2,4-bis[4-(N,N-dihydroxyethylamino)- lead, as well as other transition metals, alkali metals, and
1
3
phenyl]squaraine. This squaraine dye was found to be both lanthanide metals.
highly sensitive (low limits of detection) and selective (limited
response to other metals) for cesium ion detection. The detection
is likely based on the metal complexing to the dihydroxyethanol-
amine moieties, which disrupts the donor–acceptor–donor architec-
ture and leads to efficient quenching.
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Cesium, which is found in industrial, medical and nuclear
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wastes, can cause a number of negative health effects, includ-
Compound 1 was synthesized following literature-reported
procedures. The fluorescence response of the squaraine to
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ing cardiovascular disease and gastrointestinal distress.
Current methods for detecting cesium in complex environ- various metal ions was tested by mixing a squaraine solution
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ments include atomic absorption spectroscopy (AAS), induc- in DMSO with aqueous solutions of metal ions, and compar-
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tively coupled plasma mass spectroscopy (ICP-MS), and solid ing the fluorescence spectra of the resulting solutions with the
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state sensors. While these methods are highly sensitive and squaraine fluorescence spectra in the absence of any metal ion
selective for cesium detection, they are often expensive and (but in the same DMSO–water ratio). The squaraine solution
require sample destruction.
was made fresh daily due to partial degradation under the
Fluorescence-based methods have rarely been used for experimental conditions.
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cesium detection, even though such methods have the poten-
The addition of 1.0 mM of cesium carbonate led to a sig-
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tial to be both cheaper and non-destructive, and have been nificant quenching of the squaraine fluorescence to 7.6% of
used successfully for the many analytes. In one example of its initial value, as well as a complete disappearance of the
fluorescence-based cesium detection, researchers synthesized characteristic squaraine absorption band (Fig. 1). Even as little
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0
a
substituted calixarene that bound cesium with high as 0.010 mM of cesium carbonate caused the squaraine fluo-
affinities, resulting in a significant fluorescence enhancement rescence to decrease to 92% of its initial value (red line,
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and a 0.3 µM detection limit.
Fig. 1). Plotting this fluorescence quenching as a function of
Reported herein is a highly sensitive and selective method
for cesium detection that relies on the fluorescence quenching
of a near-infrared emitting squaraine fluorophore (compound
1
). Squaraine fluorophores are used extensively in detection
schemes due to their narrow absorption and emission bands
in the near-infrared spectral region, as well as their marked
Department of Chemistry, 51 Lower College Road, Kingston, RI 02811, USA.
E-mail: mlevine@chm.uri.edu; Fax: +1 401-874-5072; Tel: +1 401-874-4243
†
Electronic supplementary information (ESI) available: Synthesis of compound
; detailed procedures for fluorescence quenching experiments and limit of
1
detection experiments, screen of all non-interacting metal ions. See DOI:
0.1039/c3dt52215f
Fig. 1 The absorbance and fluorescence spectra of compound 1 with increas-
ing amounts of cesium carbonate (0.10 mM compound 1; 650 nm excitation).
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6276 | Dalton Trans., 2013, 42, 16276–16278
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Fig. 2 Illustration of the relationship between cesium concentration and fluor-
Fig. 4 The effect of metal ion addition on the fluorescence emission spectrum
of compound 1 (1.0 mM metal ion; 0.10 mM compound 1).
escence quenching.
Fig. 3 Visual detection of cesium by color changes of compound 1.
cesium concentration yielded a plot that rapidly approached
saturation at high cesium concentrations (Fig. 2).
This complete cesium binding was easily detected by visual
Fig. 5 Partial quenching of compound 1’s emission in the presence of palla-
inspection of the squaraine solution (Fig. 3). The addition of dium(II) chloride.
.0 mM of cesium carbonate rapidly turned the light blue
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squaraine solution colorless, whereas other common metal
ions displayed no measurable color change.
fluorescence quenching, albeit significantly less than the
CO (at 1.0 mM metal ion: 7.6%,
ing the limit of detection. Although this limit is typically 28% and 29% of initial fluorescence was observed for Cs CO ,
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The sensitivity of this method was determined by calculat- quenching observed for Cs
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3
2
defined as three times the standard deviation of the back-
ground noise (or ten times the standard deviation for quantifi-
K
2
CO
3 2 3
and Na CO , respectively).
The squaraine fluorescence was also partially quenched in
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cation limits), in this case that definition led to a value that the presence of palladium chloride, with the addition of
was effectively zero. Using 30 times the standard deviation of 1.0 mM of palladium chloride leading to a 22% decrease in
the background noise for these calculations led to a limit of the squaraine emission (Fig. 5). In this case, the addition of
detection of 0.096 µM, which provides an upper boundary for any amount of palladium led to approximately the same
the detection limit. This detection method is more sensitive degree of fluorescence quenching (17% quenched at [PdCl
than previously reported fluorescence-based methods for 0.010 mM vs. 22% quenched at [PdCl ] = 1.0 mM), indicating a
purely aqueous solutions, as well as for cesium ion detection non-specific fluorescence quenching mechanism.
2
] =
2
1
1
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in mixed solvent systems.
The mechanism of cesium-induced fluorescence quenching
The selectivity of this detection method was determined by is currently under investigation, but the following conclusions
screening a wide variety of other metals, including transition can already be drawn: (a) the lack of any bathochromic or hyp-
metals, alkali metals, and alkali earth metals in a variety of sochromic shift in the squaraine fluorescence spectra indicates
oxidation states. Most of these ions led to no significant that metal-induced aggregation is unlikely to be a significant
changes in the squaraine’s fluorescence spectra (Fig. 4).
contributing factor; (b) the relatively linear relationship
Preliminary experiments indicate that both the carbonate between cesium concentration and fluorescence quenching
anion and cesium cation are necessary for the highly efficient (for low cesium concentrations) indicates that a well-defined
fluorescence quenching, as no fluorescence quenching was ion–squaraine interaction is occurring; and (c) Job plot experi-
observed for the following species: Cs SO , CsNO and CsI. ments do not yield a clear stoichiometry between the metal
2
4
3
2 3 2 3
Both K CO and Na CO
induced some degree of squaraine ion and the squaraine fluorophore.
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The observed fluorescence quenching is in line with a lit-
erature report of a crown-ether containing squaraine whose
fluorescence was quenched in the presence of alkali and alkali
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earth metals. In that report, the authors concluded that the
metal ions caused fluorescence quenching through binding in
the crown ether moieties, which disrupted the donor–acceptor–
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donor nature of the squaraine chromophore.
Similarly,
in this case we expect that the cesium cation binds strongly to
diethanolamine, attenuating its strongly donating character
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and leading to highly efficient fluorescence quenching.
9
(a) C. Li and G. Shi, ACS Appl. Mater. Interfaces, 2013, 5,
503; (b) S. W. Thomas, G. D. Joly and T. M. Swager, Chem.
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4
Conclusions
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0 (a) I. Grabchev, D. Staneva and R. Betcheva, Curr. Med.
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U. Simonsen, Curr. Anal. Chem., 2012, 8, 485.
In conclusion, reported herein is a sensitive and selective
method for detecting cesium via fluorescence quenching of a
squaraine fluorophore. This detection method has a number
of advantages compared to previously-reported systems,
including (a) low limits of detection; (b) marked insensitivity
to a variety of other common metals; (c) ease of operation; and
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1 V. Souchon, I. Leray and B. Valeur, Chem. Commun., 2006,
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(d) monitoring of a fluorescent signal in the near-infrared
spectral region, which has limited interference from other ana-
lytes and will enable detection in complex media. The mechan-
ism of cesium-induced fluorescence quenching and the
applicability of this quenching in multiple environments are
currently under investigation, and results will be reported in
due course.
264.
1
3 (a) M. C. Basheer, S. Alex, K. George Thomas, C. H. Suresh
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Acknowledgements
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This research was partially supported by an undergraduate
research grant from the University of Rhode Island Foundation
to Ms Teresa Mako, and by a research grant from the Gulf of
Mexico Research Initiative.
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6278 | Dalton Trans., 2013, 42, 16276–16278
This journal is © The Royal Society of Chemistry 2013