Enhanced fluorescence sensing of hydroxylated organotins by a boronic acid-linked Schiff base

Article abstract of DOI:10.1039/b906467b

A simple Schiff base, 2-(2′,4′-dihydroxybenzylidene) aminobenzeneboronic acid, was found to show a fluorescence enhancement in the presence of hydroxylated organotins in aqueous solution.

Full text of DOI:10.1039/b906467b

Chemical Communications
www.rsc.org/chemcomm
Number 28 | 28 July 2009 | Pages 4133–4304
ISSN 1359-7345
COMMUNICATION
FEATURE ARTICLE
Shun-Hua Li, Jin-Gou Xu et al.
Enhanced fluorescence sensing of
hydroxylated organotins by a boronic
acid-linked Schiff base
Gwilherm Evano, MathieuToumi
and Alexis Coste
Copper-catalyzed cyclization reactions
for the synthesis of alkaloids

COMMUNICATION
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Enhanced fluorescence sensing of hydroxylated organotins by a boronic
acid-linked Schiff basew
Shun-Hua Li,* Fei-Ran Chen, Yue-Feng Zhou, Jia-Ni Wang, Hong Zhang and Jin-Gou Xu*
Received (in Cambridge, UK) 1st April 2009, Accepted 28th April 2009
First published as an Advance Article on the web 19th May 2009
DOI: 10.1039/b906467b
A simple Schiff base, 2-(20,40-dihydroxybenzylidene)amino-
benzeneboronic acid, was found to show a fluorescence enhancement
in the presence of hydroxylated organotins in aqueous solution.
aniline. A variety of metal species were tested in aqueous
solutions of 2 at physiological pH. Fig. 1 shows a distinct
fluorescence response of 2 toward BuSn(OH)2Cl, a model
complex for HOTs in relation to the widely-concerned toxicity
of tributyltins (TBTs).4b Emission of 2 (lem = 474 nm) is
dramatically enhanced upon addition of BuSn(OH)2Cl,
whereas no obvious change is observed upon addition of other
Fluorescent chemosensors offer an essential strategy for real-time
and real-space monitoring or imaging. Many fluorescent
chemosensors have been reported for transition metal species
of biological or environmental interest.1 While organotin
compounds (OTCs) are known for their high toxicity yet wide
use,2 selective and sensitive fluorescent chemosensors for
OTCs remain largely unexplored. Hydroxylated organotins
(HOTs) of high biological activities3 are important OTC
metabolites, which result mainly from the dealkylation and
hydrolysis processes in OTC biodegradation.4 Sensitive optical
sensors for HOTs should therefore be of great use for studying
the absorption, accumulation, degradation and delayed
toxicity of the related OTCs in aquatic organisms. Herein,
we report a simple fluorescent chemosensor for HOTs, operative
in aqueous solutions of physiological pH.
metal species, including K+, Na+, Mg2+, Ca2+, Ba2+, Al3+
Cd2+, Ag+, Mn2+, Ni2+, Zn2+, Pb2+, Co2+, Fe3+, Cr3+
,
,
Cu2+, Bu3SnCl, Bu2SnCl2, BuSnCl3, SnCl4, SnCl2, Me2SnCl2
and Ph2SnCl2, despite the Hg2+-induced little quenching.
Fluorescence quenching of 2 induced by Hg2+ binding might
result from the heavy-atom effect. Under the test conditions,
an approximately 13-fold increase in the relative fluorescence
quantum yield of 2 (F0 = 0.017) was induced by addition of
excess BuSn(OH)2Cl, using quinine sulfate as a reference
standard for the determination. Obviously noted in Fig. 1,
highly selective and sensitive fluorescence enhancement was
obtained in HOT sensing.
The HOT binding mechanism underlying the observed
binding selectivity and fluorescence enhancement was studied.
The sensory molecule, 2, was designed to combine an
o-hydroxyl Schiff base moiety with a boronic acid group as a
cooperative HOT binding site (Scheme 1). Many Schiff bases,
especially those containing an o-hydroxyl group,5 have proven
to be efficient ligands for the tin centre in OTCs. On the other
hand, boronic acid is known to have a high affinity for the
vicinal diol group.6 It has recently been reported that boronic
acid was also able to bind to transition metal ions.7 This
enlightened us to examine boronic acid as a binding group
for hydroxylated transition metals. Rational conjugation of
boronic acid group with o-hydroxyl Schiff bases was assumed
to achieve highly selective HOT receptors. Because transition
metal cations are usually known to be efficient fluorescence
quenchers, an operating mode with fluorescence enhancement
is preferable in sensing transition metal species, due to selectivity
reasons. In this design, the binding of a HOT substance would
lead to conformational restriction of the receptor and that
might result in fluorescence enhancement due to the strong
increase of molecular rigidity.
The characteristic bands of O–H bond stretching (3373 cmꢀ1
)
and Sn–Cl bond stretching (682, 612 and 511 cmꢀ1) in the IR
spectrum of BuSn(OH)2Cl disappeared upon reaction with 2
(Fig. S3 in ESIw). This indicates that both the coordination
substitution of Sn–Cl by the Schiff base and the boronic
acid–diol interaction occur in the binding reaction. The
combined binding mode described in Scheme 1 was also
supported by characterising the 2–BuSn(OH)2Cl adduct using
mass spectrometry and NMR studies (Fig. S4 in ESIw). The
cooperation of the o-hydroxyl Schiff base and boronic acid
group of 2 was assumed to contribute to its higher binding
constant for HOTs than for other OTCs, which explains the
high selectivity in its sensing response. Cooperative participation
of the o-hydroxyl Schiff base and boronic acid group in HOT
binding induces a conformational restriction in 3, leading to a
2 was synthesized by direct treatment of 2,4-dihydroxybenz-
aldehyde with 2-aminobenzeneboronic acid, while Schiff base
analogue 1 as a control complex was similarly derived from
Department of Chemistry, College of Chemistry and Chemical
Engineering, and Key Laboratory of Analytical Science, Xiamen
University, Xiamen 361005, China. E-mail: lishua@xmu.edu.cn,
jgxu@xmu.edu.cn; Fax: (+86) 592-2186401;
Tel: (+86) 592-2180307
w Electronic supplementary information (ESI) available: Details of
syntheses, characterization and sensing performance of the chemosensor.
See DOI: 10.1039/b906467b
Scheme
1 Synthesis and sensing reactions of the investigated
o-hydroxyl Schiff base.
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4179–4181 | 4179

receptor (Fig. S5 in ESIw). This confirms that the high
selectivity of 2 in HOT recognition lies heavily on the covalent
interaction of boronic acid with tin-centered diol. The
HOT-induced fluorescence enhancement was suppressed to a
high degree when 2 was pretreated with 1 equiv. of catechol
(Fig. 2). In this test, the catechol protected boronic acid
from reacting with the tin-centered diol. Consequently, the
binding of tin can only lead to a restriction of the CQN
conformation,10 whereas it cannot conjugate the Schiff
base moiety with the benzeneboronic acid group to form a
highly rigid fluorophore. It is therefore concluded that the
binding-induced increase in molecular rigidity, but not the
suppression of CQN isomerization, is mainly responsible for
the HOT-induced fluorescence enhancement of 2.
The fluorescent receptor, 2, was successfully applied to
the determination of HOT in aqueous media. The solution
fluorescence of 2 increases rapidly and substantially after
addition of HOTs. Importantly, a stable fluorescence response
can be achieved in 1 min at physiological pH (Fig. S6 in ESIw).
Fig. 3 shows the spectral evolution of 2 upon titration with
BuSn(OH)2Cl in H2O–EtOH (90 : 10, v/v) at pH 7.4. The
binding constant of 2 for BuSn(OH)2Cl was calculated to
be 8.6 ꢂ 103 Mꢀ1. A good linearity is observed between the
relative fluorescence intensity and HOT concentration in the
Fig. 1 The fluorescence response of 2 to a variety of metal species
in H2O–EtOH (90 : 10, v/v). 2: 1.0
10ꢀ6 M; metal species:
5.0 ꢂ 10ꢀ5 M; pH: 7.4, buffered by 0.01 M Tris-HCl; excitation
wavelength: 403 nm. Inset: photographs showing fluorescence of the
sensory solution of 2 before and after addition of BuSn(OH)2Cl.
ꢂ
Fig. 2 The emission spectra of 2 in H2O–EtOH (90 : 10, v/v).
2: 1.0 ꢂ 10ꢀ6 M; pH: 7.4, buffered by 0.01 M Tris-HCl; excitation
wavelength: 403 nm. A:
2 only; B: 2 + catechol (1 equiv.);
C:
2 + catechol (1 equiv.) + BuSn(OH)2Cl (50 equiv.);
D: 2 + BuSn(OH)2Cl (50 equiv.).
dramatic enhancement in the fluorescence. This is reflected by
the observed vibrational structure in the fluorescence spectrum
of 3 (Fig. 1). Fluorescence enhancement sensing based on the
binding-induced rigidity increase of boronic acid-linked
flexible fluorophores has been previously reported.8 The
increased rigidity of sensing product might be anticipated
to significantly reduce non-radiative decay.9 Wang et al.10
recently reported a new fluorescence signaling mechanism
based on CQN isomerization. It was similarly assumed that
coordination of the unbridged CQN structure with transition
metals would suppress CQN isomerization in the excited
states and result in fluorescence enhancement. This is likely
to be involved in the signaling mechanism of the binding-
induced fluorescence enhancement in this design.
Control experiments were carried out to identify the
important role of the boronic acid group in the binding and
signaling mechanisms. No optical response selectivity has been
observed for BuSn(OH)2Cl against other investigated OTCs
when 1, bearing no boronic acid group, was employed as the
Fig. 3 The fluorescence evolution of 2 in H2O–EtOH (90 : 10, v/v)
upon titration with BuSn(OH)2Cl at increasing concentration.
2: 1.0 ꢂ 10ꢀ6 M; pH: 7.4, buffered by 0.01 M Tris-HCl; excitation
wavelength: 403 nm. The calibration curve was obtained by measuring
the solution fluorescence at 474 nm.
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This journal is The Royal Society of Chemistry 2009
4180 | Chem. Commun., 2009, 4179–4181

range of 0–40 mM. It is therefore made clear that HOTs can be
conveniently detected at low concentration levels, with a
detection limit down to 4.9 ꢂ 10ꢀ7 M. It must be noted that
pH greatly affects the HOT-sensing response of 2 (Fig. S7 in
ESIw). Protonation of the o-hydroxyl Schiff base at low pH
leads to sharp decreases of its fluorescence quantum yield and
metal-binding affinity, while binding of hydroxide ions on the
boronic acid group occurs at basic pH. In accordance with
the binding and signaling mechanisms described above, the
fluorescence response of 2 is dramatically weakened in acidic
or basic solution. Therefore, high sensitivity in fluorometric
determination of HOTs is conditioned at near neutral pH.
The performance of HOT sensing was further evaluated in
the coexistence of other metal species (Fig. 4). The presence of
alkali or alkali-earth metal ions at high concentration
(50 equiv. tin) caused no obvious interference. Absorption
spectral studies indicated the binding affinity of the chromogenic
o-hydroxyl Schiff base moiety towards transition metals such
as Cr3+, Fe3+, Cu2+ and other organotins (Fig. S8 in ESIw),
although this was not reported by fluorescence spectral
changes in 2. This explains the puny drifts of the HOT-sensing
signal of 2 in the presence of these interfering species. It
was observed that fluorescence responses resulting from the
addition of these interfering species followed by the addition
of BuSn(OH)2Cl to 2 were similar to those obtained in a
reverse way. These results indicate that the binding affinity of
HOT is higher than that of the investigated interfering species,
ensuring the high selectivity of the designed HOT receptor.
Standard addition methods confirmed that HOTs in synthetic
samples could be determined with satisfactory recoveries
(96–105%) at mM concentration level.
receptor nicely combines the binding character of an o-hydroxyl
Schiff base for the transition metal and boronic acid for the
vicinal diol. To the best of our knowledge, this is the first
example of a fluorescent receptor for hydroxylated metal
species. We also demonstrated that boronic acid acted as an
efficient receptor group for metal-centered diols. This may
lead to important applications of boronic acid in molecular
recognition in further studies.
This work was supported by the National Natural Science
Foundation of China (No. 20705029 and 20835005),
the Natural Science Foundation of Fujian Province
(No. A0610028) and the Science & Technology Project of
Fujian Province of China (No. 2005J001).
Notes and references
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Fig. 4 The influence of coexistence of other metal species on the
fluorescence response of 2 to 20 equiv. of BuSn(OH)2Cl in H2O–EtOH
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l
ex/lem: 403/474 nm. K+, Na+, Mg2+, Ca2+, Ba2+, Al3+: 1000
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100 equiv.; Hg2+, Cu2+, Bu3SnCl, Bu2SnCl2 BuSnCl3, SnCl4, SnCl2,
Me2SnCl2, Ph2SnCl2: 20 equiv.
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Chem. Commun., 2009, 4179–4181 | 4181