A highly selective colorimetric and fluorescent chemosensor for Cr2+in aqueous solutions

Article abstract of DOI:10.1016/j.tetlet.2016.12.038

A new rhodamine-based chemosensor was synthetized through a modified copper-catalyzed [3+2]-cycloaddition of an azidocoumarin with an alkynyl-rhodamine. Its sensing properties toward various metal cations in aqueous solutions were investigated by colorimetric changes, UV–vis and fluorescence spectroscopies. The sensor exhibited a high selectivity for Cr²⁺over Cr³⁺and other divalent cations such as Cu²⁺, Mg²⁺, Zn²⁺, Ca²⁺, Cd²⁺, Co²⁺, Hg²⁺and Ni²⁺. The linear range of detection by fluorescence spectroscopy is 0.07–3.5?mM, with a detection limit of ca. 64?μM. The binding mode of Cr²⁺with the sensor was rationalized through experimental evidences.

Full text of DOI:10.1016/j.tetlet.2016.12.038

Accepted Manuscript
A highly selective colorimetric and fluorescent chemosensor for Cr2+ in aqueous
solution
Jordi Rull-Barrull, Martin d'Halluin, Erwan Le Grognec, François-Xavier Felpin
PII:
S0040-4039(16)31673-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.12.038
TETL 48450
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
19 November 2016
9 December 2016
14 December 2016
Please cite this article as: Rull-Barrull, J., d'Halluin, M., Le Grognec, E., Felpin, F-X., A highly selective colorimetric
and fluorescent chemosensor for Cr2+ in aqueous solution, Tetrahedron Letters (2016), doi: http://dx.doi.org/
10.1016/j.tetlet.2016.12.038
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Jordi Rull-Barrull, Martin d’Halluin, Erwan Le Grognec, François-Xavier Felpin

1
Tetrahedron Letters
A highly selective colorimetric and fluorescent chemosensor for Cr2+ in aqueous
solution
Jordi Rull-Barrulla,†, Martin d’Halluina,†, Erwan Le Grogneca, François-Xavier Felpina,b,*
a Université de Nantes, UFR Sciences et Techniques, UMR CNRS 6230, CEISAM, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France. Fax: (+33)
251 125 40; E-mail: fx.felpin@univ-nantes.fr
b Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A new rhodamine-based chemosensor was synthetized through a modified copper-catalyzed
[3+2]-cycloaddition of an azidocoumarin with an alkynyl-rhodamine. Its sensing properties
toward various metal cations in aqueous solutions were investigated by colorimetric changes,
UV-vis and fluorescence spectroscopies. The sensor exhibited a high selectivity for Cr2+ over
Cr3+ and other divalent cations such as Cu2+, Mg2+, Zn2+, Ca2+, Cd2+, Co2+, Hg2+ and Ni2+. The
linear range of detection by fluorescence spectroscopy is 0.07-3.5 mM, with a detection limit of
ca. 64 µM. The binding mode of Cr2+ with the sensor was rationalized through experimental
evidences.
Available online
Keywords:
Chemosensor
Chromium
Fluorescent
Colorimetric
Click reaction
2009 Elsevier Ltd. All rights reserved.
Chromium is an intriguing heavy metal displaying a
contrasted biological profile according to its oxidation
degree. While Cr(VI) is an extremely toxic, mutagenic and
carcinogenic metal contaminant, trivalent chromium Cr(III)
is an essential nutrient for mammals, acting as a co-factor of
insulin and playing a vital role in the metabolism of
carbohydrates, lipid, proteins and nucleic acid.1 However, at
high concentration chromium is one of the heavy metals of
concern for public health since upon repeated exposure
through water supply and seafood, humans are highly
susceptible to be affected by severe disorders.1 Therefore,
detecting natural or anthropogenic chromium contamination
of the aquatic environment is an important requirement to
prevent excessive chromium concentration in the food
supply chain. In occupational settings, the detection of
heavy metals, including chromium, is accurately addressed
by inductively coupled plasma mass spectrometry (ICP-MS)
or inductively coupled plasma atomic emission spectroscopy
(ICP-AES). While these analytical tools provide very low
limit of detection, in the ppb level, they are not suitable for
on-site sensing and require both tedious sample preparation
and highly skilled operators. Chemosensing allowing
colorimetric and fluorimetric detection of heavy metals is an
alternative technology exhibiting many benefits such as
operational simplicity, high sensitivity, real-time detection
and direct visual perception for on-site sensing.2-6 While
many efforts have been devoted to the development of
selective chemosensors for both Cr(III)7-20 and Cr(VI)
cations,21-26 the detection of Cr2+ species has been
overlooked. Since both Cr2+and Cr3+ can be readily oxidized
to highly toxic hexavalent chromium, the development of a
selective probe for Cr2+ is a highly appealing challenge for
complementing the detection of all oxidation degrees of
chromium species.
In continuation of our recent efforts to design new
chemosensors for various pollutants,27-28 herein we disclose
the first highly selective rhodamine-based chemosensor for
colorimetric and fluorimetric detection of Cr2+ in aqueous
conditions.
The new chemosensor 3 was prepared through a copper-
catalyzed [3+2]-cycloaddition of the azidocoumarin 1 with the
alkynyl-rhodamine 2 using a recently developed procedure
(Scheme 1).29 Azidocoumarin 1 was prepared in two steps by
etherification of commercially available umbelliferone with 1,3-
dibromopropane, followed by a bromine-azide exchange. The
alkynyl-rhodamine partner 2 was synthetized by treatment of
rhodamine B with ethanolamine followed by alkylation of the
newly introduced alcohol with propargyl bromide. The click
ligation of azide 1 with alkyne 2 was carried out using cellulose
paper grafted with thioglycolic acid (Cell-SH) as biomimetic
reducing agent.29 This heterogeneous sodium ascorbate surrogate
allows not only reducing Cu(II) to catalytically active Cu(I)
nanoparticles, but also sequestering copper species to give metal-
free products after
a single filtration. This approach is
particularly relevant for applications requiring the use of metal-
free compound, which is obviously the case for the preparation of
metal chemosensor. We anticipated that in the presence of a
suitable metal ion, both the rhodamine and coumarin moieties
should be involved in the coordination, resulting in different UV-
vis absorption phenomena (vide infra).

2
Tetrahedron
Figure 1. (a) Color changes of sensor 3 in CH3CN/H2O (2/5) at 24
µM upon addition of various metal cations at 0.7 mM. (b) UV-vis
absorption spectra of sensor 3 in CH3CN/H2O (2/5) at 24 µM upon
addition of various metal chloride salts at 0.7 mM.
The UV-vis spectra of the samples containing various
concentration of Cr2+ cations, from 70 µM to 3.5 mM are
reported in Figure 2a.
Scheme 1. Preparation of chemosensor 3.
We investigated the colorimetric detection of various metal
cations including Li+, Na+, Ag+, Cu+, Cu2+, Mg2+, Zn2+, Ca2+,
Cd2+, Co2+, Cr2+, Cr3+, Hg2+ and Ni2+. Colorimetric sensing is the
simplest technology for on-site naked-eye detection even for
unskilled people since it allows qualitative and quantitative
detection by visual comparison with a color chart. A photograph
of the colorimetric response of 3 (24 µM) in CH3CN/H2O (2/5,
v/v) to metal cations (0.7 mM) is depicted in Figure 1. An
obvious rosy color appeared in the vial containing Cr2+cations
while no other metals induced a color change, attesting for the
high metal and oxidation degree selectivity even in the presence
of Cr3+ cations or other divalent metal cations such as Cu2+, Mg2+,
Zn2+, Ca2+, Cd2+, Co2+, Hg2+ and Ni2+. This high selectivity for
Cr2+over Cr3+is also noteworthy given that many rhodamine-
based chemosensors have been described for the selective
detection of Cr3+ cations.30-43 In a second time, the photochemical
properties of 3 were also assessed through its UV-vis absorption
spectra upon addition of 0.7 mM of various metals (Figure 1b).
While the spectrum of 3 displays a single band centered at 320
nm attributed to the characteristic absorbance of the double bond
of the benzopyrone ring in the absence of metal, a strong band
with a maximum at 562 nm, attributed to the open form of
rhodamine as a monomer, was observed in the presence of Cr(II).
The shoulder at ca. 531 nm is characteristic of the presence of
aggregated forms of rhodamine.44 Hg2+, Cu2+ and Cr3+were
marginally detected but their contributions are negligible.
Interestingly, while the maximum absorption for Cr2+, Hg2+ and
Cu2+ was recorded at ca. 562 nm, a 10 nm blue shift of the
absorption band was observed for Cr3+ at ca. 552 nm, suggesting
that a different complex was formed in a marginal amount in the
presence of this trivalent cation.
Figure 2. (a) UV-vis absorption spectra of sensor 3 in CH3CN/H2O
(2/5) at 24 µM upon addition of Cr2+ cations at various
concentrations. (b) Calibration curve for the determination of the
detection limit.
Upon decreasing the concentration of Cr2+, the band
intensity at λ = 562 nm decreases. The isobestic point
observed at 335 nm indicates that only one complex was
formed for chemosensor
3
upon binding to Cr2+ cations. The
limit of detection by UV spectroscopy was determined on
the absorption band centered at 562 nm with the set of data
collected in Figure 2a. The value of the absorbance at 563
nm of the samples representing different concentrations of
Cr2+ was normalized between the blank and the solution
containing 3.5 mM Cr2+. As depicted in Figure 2b, a linear
regression curve was fitted to the concentration values 0.07–
3.5 mM and the equation was found to be y = 0.5296x +
0.388 (R2 = 0.96). The detection limit of 0.18 mM, was
determined by extrapolation of the straight line on the
abscissa (blue circle).
Sensor giving turn-on fluorescence signals are usually
preferred over analytical devices promoting fluorescence
quenching (turn-off) due to increased sensitivity. Indeed,
fluorescence quenching generates a weaker signal output
upon complexation with an analyte and restrains the
temporal separation of structurally related complexes with
time-resolved fluorometry.45 When a solution of Cr2+ cations
at 0.7 mM in CH3CN/H2O (2/5) in the presence of probe
24 µM was excited at 525 nm, a strong fluorescence band
was observed at 587 nm, indicating that is an excellent
3 at
3

3
turn-on fluorescent sensor due to the presence of the
rhodamine moiety in the ring-opened form. By contrast, the
probe showed no fluorescent response to Li+, Na+, Ag+, Cu+,
Cu2+, Mg2+, Zn2+, Ca2+, Cd2+, Co2+, Cr2+, Cr3+, Hg2+ and Ni2+
and the corresponding spectra were not discernable from the
blank. Only two exceptions contravened to this rule since
Hg2+ and Cu2+ cations induced a slight fluorescence
response, albeit with a maximum intensity more than 10-
fold weaker than the Cr2+-induced fluorescence.
form evidenced by a strong absorption band at 563 nm,
while the coumarin absorbance is quenched through the
coordination, allowing a dual recognition.
Further insights on the binding mode of Cr2+ cations with
sensor 3, were obtained through the Job’s method of
continuous variations as depicted in Figure 5. The
absorbance value attained a maximum when the molar
fraction reached 0.5, suggesting the formation of a 1 : 1
complex between the chemosensor
3
and Cr2+.
Figure 5. Job plot of sensor 3 and Cr2+ in CH3CN/H2O (2/5, v/v).
The total concentration of 3 and Cr2+ was 100 µM and the monitored
wavelength was 562 nm.
Figure 3. Emission spectra of sensor 3 in CH3CN/H2O (2/5, v/v) at
24 µM upon addition of various metal cations at 0.7 mM, λex = 525
nm.
Based on these informative experimental evidences and
considering that Cr2+ ions usually adopt a tetrahedral or
square planar arrangement, we propose the following
binding mode depicted in Scheme 2 where both the
rhodamine and coumarin units participate to the
coordination sphere of Cr2+. It is premature at this stage to
The fluorescence titration of Cr2+ was conducted in a
solution of sensor
3 in CH3CN/H2O (2/5, v/v) at 24 µM.
Addition of Cr2+ cations from 0 to 3.5 mM resulted in an
obvious fluorescence increase with a maximum emission
intensity at 587 nm.
explain the high selectivity of the chemosensor
3
for Cr2+
with respect to other divalent metal cations. The selectivity
may result of the conformational arrangement of the
chemosensor
3
in the presence of Cr2+ which could perfectly
fit with the size of the cation. Modelling studies are
currently conducted in our laboratory on a series of related
sensors and should help us in the understanding of the
bonding mode.
Figure 4. (a) Emission spectra of sensor 3 in CH3CN/H2O (2/5, v/v)
at 24 µM upon addition of various Cr2+ cations at various
concentrations. (b) Calibration curve for the determination of the
detection limit.
As depicted in Figure 4b, the linear regression curve
fitted to the concentration values 0.14–3.5 mM was found to
be y = 0.5989x + 0.7143 (R2 = 0.97) and allowed the
determination of the detection limit with a value computed
at 64 µM, by extrapolation of the straight line on the
abscissa (blue circle). This result indicates that the
fluorescence spectroscopy has a much increased sensibility
for detecting Cr2+ cations, giving an improved limit of
detection compared to absorption spectroscopy.
The absorbance behavior of sensor
3 in the presence of
various concentration of Cr2+ cations is particularly
informative on the sensing mechanism (Figure 2a). As
expected, both the rhodamine and coumarin moieties are
involved in the coordination of Cr2+. In metal free-solutions
Scheme 2. Proposed binding mode.
the absorption spectrum of
3 displays a single band centered
In summary, we unveiled the first chemosensor for the
colorimetric, spectrophotometric and fluorimetric detection
of Cr2+ cation. This technology, consisting in a rhodamine-
based chemosensor, complements the detection of Cr(III)
and Cr(VI) species. The chemosensor was prepared through
at ca. 320 nm attributed to the characteristic absorbance of
the double bond of the benzopyrone ring the coumarin unit
while the rhodamine moiety do not absorb in the range of
300-750 nm. By contrast, in the presence of Cr2+ cations the
rhodamine unit opens into a strongly colored and fluorescent

4
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5
Highlights
 A highly selective chemosensor for Cr2+ in
aqueous solution has been developed
 The chemosensor allows colorimetric,
spectrophotometric and fluorimetric
detection
 The chemosensor was designed with
rhodamine and coumarin moities
 The chemosensor was prepared through a
copper catalyzed click cycloaddition