Off-on BODIPY-based chemosensors for selective detection of Al3+ and Cr3+versus Fe3+ in aqueous media

Article abstract of DOI:10.1039/c3ra46845c

Two new off-on BODIPY-based chemosensors that are highly sensitive for trivalent cations in aqueous solutions are described. Compound 2 exhibits selective sensing of Al³⁺ and Cr³⁺versus Fe³⁺ through two different channels

Full text of DOI:10.1039/c3ra46845c

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Off–on BODIPY-based chemosensors for selective
detection of Al3+ and Cr3+ versus Fe3+ in aqueous
media†
Cite this: RSC Adv., 2014, 4, 8962
b
ab
Andrea Barba-Bon,ac Laura Calabuig, Ana M. Costero,
Salvador Gil,ab
Received 19th November 2013
Accepted 16th January 2014
*
ac
*
Ramon Martınez-Manez
´˜
and Felix Sancenonac
´ ´
´
´
DOI: 10.1039/c3ra46845c
www.rsc.org/advances
Two new off–on BODIPY-based chemosensors that are highly sensi-
Based in these concepts, and bearing in mind our interest in
tive for trivalent cations in aqueous solutions are described. the design of chemosensors, we report herein two new probes (1
Compound 2 exhibits selective sensing of Al3+ and Cr3+ versus Fe3+ and 2, see Scheme 1) based in dipyrromethene boron diuoride
through two different channels (UV-vis and fluorescence).
(BODIPY) scaffold, for a simple optical detection of trivalent
cations. BODIPY dyes are a class of well-known uorophores
with widespread applications as uorescent probes due to their
valuable characteristics, such as high molar absorption coeffi-
cients and high quantum yields leading to intense absorption
and uorescence bands.7 In this context, although a large
number of BODIPY dyes have been designed and prepared for
detecting metal cations,8 very few examples display sensing
features in aqueous solutions.9
Probe 1 was synthesized through a bicondensation of
N-methyl-N-(2-hydroxyethyl)-4-aminobenzaldehyde and 2,4-
dimethylpyrrole in the presence of triuoroacetic acid (TFA) as
catalyst,10 followed by oxidation with p-chloranil. The boron
diuoride bridge was introduced by treatment with boron
triuoride diethyl etherate (BF3ꢀEt2O) in the presence of triethy-
lamine (TEA). For the synthesis of probe 2, the BODIPY deriv-
The design of new probes for transition and p-block metal
cations is an important subject within the eld of supramo-
lecular chemistry because of their impact on the environment
and human health. In this area, although a number of che-
mosensors for divalent transition metal cations have been
described, there are a relatively small number of probes able to
selectively respond to triple-charged metal cations1 and even
less in aqueous environments.2 However, trivalent cations have
important properties and play signicant roles in different
elds. For instance chromium is an essential element in
human nutrition and has a huge impact on the metabolism of
carbohydrates, fats, proteins and nucleic acids and has been
reported to disturb glucose levels and lipid metabolism.3 On
the other hand Fe3+ also plays a key role in many biochemical
processes at the cellular level and it is indispensable for most
organisms, and both its deciency and overload can induce
various disorders.4,5 Finally, it is well-known that Al3+ plays
important roles in cells and environmental food chains and for
instance was found to kill shes in acidied water and cause
damages to the central nerve system of human beings.6
Therefore, the design of new probes for the simple and easy
detection of these metal cations in a number of different
situation is of much interest.
ative
I
was prepared from 2,4-dimethylpyrrole and
benzaldehyde (see ESI†) following the same procedure as
above.10 Condensation of I and N-methyl-N-(2-hydroxyethyl)-4-
aminobenzaldehyde in benzene in presence of acetic acid and
piperidine7b,11 yielded 2.
Probe 1 shows in water : CH3CN (80 : 20 v/v) an intense
absorption band at 490 nm (3 ¼ 77600 cmꢀ1 Mꢀ1) yet it is
scarcely uorescent (F ¼ 0.002 using aqueous uorescein as
a
´
Centro de Reconocimiento Molecular y Desarrollo Tecnologico (IDM), Unidad Mixta
´
Universidad Politecnica de Valencia-Universidad de Valencia, Spain
b
´
´
`
Departamento de Quımica Organica, Universitat de Valencia, Dr Moliner 50, 46100,
Burjassot, Valencia, Spain. E-mail: ana.costero@uv.es
c
´
´
Departamento de Quımica, Universidad Politecnica de Valencia, Camino de Vera s/n,
46022, Valencia, Spain. E-mail: rmaez@qim.upv.es
† Electronic supplementary information (ESI) available: Experimental details and
spectroscopic data; NMR, uorescence and UV titrations of 1 and 2 with Al3+, Cr3+
and Fe3+. See DOI: 10.1039/c3ra46845c
Scheme 1 Chemical structure of the BODIPY derivatives 1 and 2.
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reference).12 This low emission is tentatively attributed to a
photo-electron transfer (PET) from the lone pair of the amino
group to the photo-excited BODIPY group.13 Fig. 1 shows the
uorescence spectrum (lexc ¼ 480 nm) of 1 alone and in the
presence of 1 equiv. of different metal cations. Addition of
Fe2+, Cu2+, Zn2+, Cd2+, Co2+, Ni2+, Li+, Hg2+ and Ru3+ did not
modify the emission of 1, whereas trivalent cations Al3+, Fe3+
and Cr3+ led to a very remarkable enhancement of the uo-
rescence emission (FAl ¼ 0.29; FFe ¼ 0.17; FCr ¼ 0.24) at
515 nm.
Moreover, no colour modulations in the presence of metal
cations were found for 1. This was an expected result bearing in
mind the presence of methyl groups in the pyrrole units that
most likely impose a twist position of the phenyl ring that
interrupts the conjugation between the N-methyl-N-(2-hydroxy-
ethyl) coordination site and the signaling unit.14
In contrast, the signaling unit and binding site in probe 2 are
electronically connected and therefore changes both in colour
and emission were found (vide infra). In water : CH3CN (40 : 60
v/v), 2 exhibited a strong absorbance with a maximum at 603 nm
(3 ¼ 38400 M-1 cmꢀ1). This band is bathochromically shied by
ca. 100 nm when compared with the parent BODIPY uo-
rophore due to the styryl extension at the a-position. Moreover
probe 2 was poorly uorescent (lexc ¼ 530 nm, F ¼ 0.007) most
likely due to an efficient ICT quenching of the excited state of
the BODIPY-chromophore from the electron-donating amino
moiety.
Fig. 2 UV-Vis spectra (a) and fluorescence emission spectra (lex
¼
530 nm) (b) of 2 (10ꢀ5 M) upon addition of 1 eq. of Fe3+, Fe2+, Cu2+
,
Zn2+, Cd2+, Co2+, Ni2+, Li+, Hg2+,Cr3+ and Al3+ in water : CH3CN
(40 : 60 v/v).
Addition of Fe2+, Zn2+, Cd2+, Co2+, Ni2+, Li+, Cu2+, Hg2+, Ru3+,
or Fe3+ to solutions of 2 in water : CH3CN (40 : 60 v/v) did not
induce any change neither in the UV-Vis nor in the uorescence
spectra. By contrast, in the presence of the trivalent cations Cr3+
and Al3+ the colour of the solutions changed dramatically from
Furthermore in competitive experiments it was found that
blue to pink due to the appearance of a new band at 560 nm (see probes 1 and 2 respond to the presence of Al3 +, Cr3+ and Fe3+
Fig. 2). Probe 2 also shows some colour change in the presence (for 1) and to Al3+and Cr3+ (for 2) in the presence of Hg2+, Li+,
of Fe3+ but only when CH3CN alone or mixtures with a Na+, K+, Ag+, Ca2+, Mg2+, Ni2+, Zn2+, Cd2+, Fe2+, Co2+ and Cu2+
maximum of 8% water were used. Interestingly probe 2 also cations. In addition, limits of detection (LOD) were determined
displays a remarkable strong uorescence emission at 563 nm from the equation LOD ¼ K ꢁ Sb1/S, where K ¼ 3, Sb1 is the
in water : CH3CN (40 : 60 v/v) upon addition of the metal standard deviation of the blank solution and S is the slope of
cations Cr3+ and Al3+ (FAl ¼ 0.33; FCr ¼ 0.30).
the calibration curve.15 The obtained results were 0.14, 019 and
0.10 mM for Al3+, Cr3+ and Fe3+ respectively with ligand 1 and
0.08 and 0.18 mM for Al3+ and Cr3+ with ligand 2 using in both
cases emission measurements. Moreover LOD of 0.18 and 0.52
mM were calculated for Al3+ and Cr3+ using 2 from UV-vis
titrations.
Titration experiments of 1 and Fe3+, Cr3+ and Al3+ in
water : CH3CN (80 : 20 v/v) (by uorescence spectroscopy) and
of 2 with Cr3+ and Al3+ in water : CH3CN (40 : 60 v/v) (by either
UV-Vis or uorescence) were carried out in order to determine
the complexation constants by using the soware Spect
program.16 As an example Fig. 3 shows the uorescence titration
3+
of 1 with Al
.
A stepwise addition of Al3+ led to an enhancement of the
band at 515 nm, which is saturated upon the addition of 1
equiv. of Al3+, strongly, suggesting the formation of 1 : 1 ligand-
to-metal complexes. This was also demonstrated via the corre-
sponding Job's plot and MS. The same stoichiometry was
observed for Cr3+ and Fe3+ with 1.
Fig. 1 Fluorescence emission spectra of 1 (10 ꢀ5 M) upon addition of 1
eq. of Fe3+, Fe2+, Cu2+, Zn2+, Cd2+, Co2+, Ni2+, Li+, Hg2+, Ru3+, Cr3+
and Al3+ in water : CH3CN (80 : 20 v/v) (lex ¼ 480 nm). Inset: UV-Vis
spectra of 1 (10ꢀ5 M) upon addition of 1 eq. of the different cations in
water : CH3CN (80 : 20 v/v).
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Fig. 3 Fluorescence response (lexc. ¼ 480 nm) for 1 (10ꢀ5 M) to
increasing amounts of Al3+ in water : CH3CN (80 : 20 v/v) The inset
shows the corresponding Job plot.
Fig. 4 1H-NMR spectra of 1 and 1 + increasing amounts of Al3+ in
CD3CN.
Moreover, similar uorescence titration studies with 2 also
indicated the formation of 1 : 1 ligand-to-metal complexes with
the trivalent metal cations Cr3+ and Al3+. Moreover UV-vis
spectroscopy titrations with 2 also resulted in similar results
(see Table 1).
group that in the free ligand appears at 3.44 ppm show a
downeld shi of 0.20 ppm and the signal corresponding to the
hydroxymethylene protons, at 3.58 ppm, displayed a different
behavior with an upper eld shi of 0.18 ppm. Finally, there
were no changes in the protons of the pyrrole units (at
6.15 ppm). These data strongly suggest the direct involvement
of the amino group in the Al3+ coordination.
A similar behavior was observed with ligand 2 (see ESI†)
upon addition of Al3+. Remarkable changes were observed in
the N-methyl-N-(2-hydroxyethyl)styryl group, especially in the
ortho-protons to the amino group. On the other hand, changes
in the protons of the pyrrole units and phenyl moiety in the 8
position of the BODIPY were negligible.
All these results are consistent with a sensing mechanism in
which the metal cations form complexes with 1 and 2 via
coordination with the N-methyl-N-(2-hydroxyethyl) moiety. The
interaction of the cation with the lone pair on the nitrogen atom
in 1 results in an inhibition of the PET process giving rise to the
observed enhancement of the uorescence emission. On the
other hand, the binding of the cation with the lone pair on
the nitrogen atom in 2 results in a reduction of the electron-
donating ability of the nitrogen atom of N-methyl-N-(2-hydroxy-
ethyl)-styryl group which is in conjugation to the BODIPY core,
thus suppressing the ICT process causing the blue shi of the
absorption spectrum band and an enhancement of the
uorescence.
Conclusions
In summary, we have successfully synthesized and character-
ized two BODIPY-based probes which show a “turn-on”
response with trivalent cations of interest whereas the probe
To conrm the propose sensing mechanism, 1H NMR
experiments were carried out. Thus, 1H NMR spectra of ligand 1
free and the presence of different amounts of Al3+ were recorded
in CD3CN (Fig. 4). The most important modications of the
signals of 1 aer complexation were observed in the phenyl
moiety (7.10 and 6.84 ppm) which underwent a signicant
downeld shi upon the addition of Al3+ (0.52 and 0.90 ppm
respectively). The change is especially important in the ortho-
protons to the amino group. On the other hand the methylene
remained silent in the presence of competitive cations (Hg2+
,
Li+, Na+, K+, Ag+, Ca2+, Mg2+, Ni2+, Zn2+, Cd2+, Fe2+, Co2+, Ru3+,
Fe2+, and Cu2+). This sensing behavior is highly selective and it
is observed in mixed aqueous solutions. Compound 1 can be
only used in uorescence studies whereas compound 2 gives
sensing response through two different channels, UV-vis and
uorescence. The observed colour change or enhanced uo-
rescence emission can be attributed to the binding of M3+ to the
Table 1 Complexation constants for probes 1 and 2 with trivalent 2-aminoethanol moiety which reduces the electron-donating
cations for the formation of 1 : 1 ligand-to-metal complexes
ability of the nitrogen atom conjugated to the BODIPY core.
Finally, selective sensing of Al3+ and Cr3+ versus Fe3+ was
Ligand 1a
log Kc
Ligand 2b
log Kd
observed with ligand 2.
Cation
log Kc
Al3+
Cr3+
Fe3+
5.9 ꢂ 0.2
6.5 ꢂ 0.3
4.8 ꢂ 0.2
5.9 ꢂ 0.7
5.3 ꢂ 0.6
—
5.2 ꢂ 0.3
5.6 ꢂ 0.3
—
Acknowledgements
Financial support from the Spanish Government (Project
MAT2012-38429-C04) and the Generalitat Valencia (Project
PROMETEO/2009/016) is gratefully acknowledged. A.B.B.
thanks for a pre-doctoral FPI fellowship.
a
b
Determined in water : CH3CN (80 : 20 v/v).
Determined in
c
water : CH3CN (40 : 60 v/v).
Determined by UV-vis titrations.
d
Determined by uorescence titrations.
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