Selective and sensitive turn-on fluorescent sensing of arsenite based on cysteine fused tetraphenylethene with AIE characteristics in aqueous media

Article abstract of DOI:10.1039/c3cc42238k

Herein, a cysteine-functionalized derivative of TPE is presented as the first fluorescent organic dye for selective, label-free, sub-ppb level detection of As³⁺ in aqueous media by taking the advantage of the aggregation induced emission feature. Besides, the first discrimination between the most toxic As³⁺ and less toxic As⁵⁺ was obtained. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc42238k

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Selective and sensitive turn-on fluorescent sensing of
arsenite based on cysteine fused tetraphenylethene
with AIE characteristics in aqueous media†
Cite this: Chem. Commun., 2013,
49, 5325
Received 27th March 2013,
Accepted 15th April 2013
Melek Baglan and Serdar Atılgan*
DOI: 10.1039/c3cc42238k
www.rsc.org/chemcomm
Herein, a cysteine-functionalized derivative of TPE is presented as highly selective and sensitive methods for detection and quan-
the first fluorescent organic dye for selective, label-free, sub-ppb titation of As is vital. Current analysis methods either rely
level detection of As3+ in aqueous media by taking the advantage heavily on chromatographic techniques which require a high
of the aggregation induced emission feature. Besides, the first cost, complicated and expensive instruments and dangerous
discrimination between the most toxic As3+ and less toxic As5+ analytical procedures.8 In addition, they are time consuming
was obtained.
and require a series of enrichment steps before analysis. Mean-
while, other methods including chemical and biological sensors
Arsenic (As) contamination of ground water poses serious based on nanomaterials were also reported for the analysis of
global human health problems.1 It induces the breaking of As with a sub-ppb detection limit recently.9 However, these
DNA strands, producing skin lesions, an increase in cellular methods have a lack of selectivity over other metals. Unlike
levels of nitric oxide and superoxide, and affects protein these, the method based on nanomaterials fabricated by Ray in
phosphorylation by binding to vicinal thiols.2 The inorganic 2009 is, so far, the only reported strategy that meets all needs of
salts of arsenic, arsenite (As3+) and arsenate (As5+), exist as the As detection, to the best of our knowledge.10a This elegant
major environmentally accessible species.3 Considering the approach demonstrates that binding of As+3 or As+5 to Cys/Glu
human health risks, As3+ exhibits the highest toxicity, while or dithiothreitol (DTT) conjugated gold nanoparticles leads to
As5+ is one tenth as toxic as arsenite.4 The upper limit recom- colorimetric change based on the aggregation of gold nano-
mended by the World Health Organization (WHO) for this particles with an excellent detection limit in aqueous media
protoplastic poison is 10 ppb in water.5 Even though both without any tagging. Likewise, L-cysteinyl-L-cysteine capped
inorganic As3+ and As5+ have different mechanisms of action gold-clusters have been introduced for application as a sensor
in the body and toxicity, the high affinity of them to the for As3+ 10b
Compared with the previously discussed gold nano-
.
sulfhydryl groups of biologically active biomolecules like particles, the latter exhibits a fluorescence emission enhance-
cysteine (Cys) and glutathione (Glu) is considered to be the ment upon As3+ addition in water. However, despite the
only effect of the toxicity.6 These effects have been investigated promising approach for many analytes in terms of sensitivity,
in great detail by many research groups and proposed selectivity, low cost and real time monitoring, the absence of a
complexes of proteins with As3+ and As5+ were correlated by turn-on fluorescent chemosensor for As3+ in aqueous media is a
many spectroscopic methods such as NMR and mass big challenge. Herein, we present the first turn-on fluorescent
spectroscopy.6a,7
sensor based on Cys modified tetraphenylethene (TPE) with
These studies suggest that As3+ coordinates with three aggregation induced emission feature (AIE) for the selective
cysteine thiolates of proteins, thus it inhibits many enzymes detection of As3+ with an excellent detection limit and selectivity
activites.7 On the other hand, As5+ oxidizes the Cys moiety over other metals in aqueous media without any tagging. More-
of proteins to form a dimer containing a disulfide bridge over, discrimination has been achieved among As3+ and As5+ for
between the two proteins.7a Owing to its threat to human life, the first time by means of their different mechanism of actions
even at the lower maximum contaminant level of As in water, towards sulfhydryl group of Cys.
Following the pioneering reports of Tang,11a the AIE effect,
has gained a good reputation with many successful examples.
Department of Chemistry, Suleyman Demirel University, Isparta, Turkey.
E-mail: serdaratilgan@sdu.edu.tr; Fax: +902462371106; Tel: +902462114168
Considering its unusual photophysical property, tetraphenyl-
† Electronic supplementary information (ESI) available: Experimental proce-
ethene (TPE), an important class of AIE active compound, is an
attractive starting point in such constructions because of its
unique properties in that it is easily prepared with inexpensive
dures, characterization data including 1H-NMR, 13C-NMR and MALDI-TOF of
Cys-TPE, complex and reaction intermediates, spectrophotometric studies. See
DOI: 10.1039/c3cc42238k
c
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Fig. 1 Schematic view of the fluorescence activation of As(TPECys)3.
Scheme 1 Synthesis routes to TPE-Cys, 7.
reagents and easily structurally modifiable with various func-
tional groups through simple reactions.11 Thus, by simple
incorporation of chelating groups to the TPE core, many
judiciously designed chemosensors were presented for both
anions and cations, so far, thanks to its AIE feature.11c–f The
interaction of analytes with the TPE functionalized chemo-
sensor yields a fluorescence enhancement depending on the
Fig. 2 (a) Fluorescence titration of 7 (10 mM) with As3+ in 1% THF-distilled water
at 298. The excitation wavelength was 330 nm and the emission intensity was
measured at 470 nm. (b) Digital photographs of the THF-water (1 : 99) solution of
7 (a) in the absence of and (b) presence of 5 ppb As3+ and (c) 100 ppb As3+ after
three hours under UV light (365 nm) illumination.
transition from a molecularly dissolved fluorogen to aggregated units in the complex, we investigated the recognition of As3+
microcrystals either by self-organization between analyte and with 7 by fluorescence techniques. The fluorescence response
dye11e or polarity changes on the dye.11f Our approach involves of 7 in the presence of As3+ was measured at room temperature
self-organization of As species with free thiols on a Cys-modified in distilled water. As shown in Fig. 2a, the aqueous solution
TPE fluorophore. The synthetic strategy is straightforward and of 7 was almost non-fluorescent based on hydrophilic Cys
effective as seen in Scheme 1. The functionalization of the TPE substituents of 7 which would promote water solubility.
core with Cys was achieved according to the work by Katritzky Upon the addition of As3+, an increase in the fluorescence
et al.12 To yield TPE-Cys 7, the acylation of acid on 5 with Cys intensity at 470 nm was observed. Thus, the emission spectrum
through the thiol group was successfully achieved by the is indicating the validity of the aforementioned model. Remark-
treatment of Cys with 5 under basic pH media in MeCN–H2O ably, one of the major consequences of fluorescence studies
at room temperature.
is the excellent detection limit, that is 0.5 ppb, which is
Investigations clearly show that the free SH group of Cys is much lower than the limit given in the WHO5 guidelines.
the binding site of As3+ through an As–S bond. Analysis of the However, an estimate of the maximum response is provided
complex structure illustrates that As3+ is bound to three Cys, with 100 ppb As3+ for 7; above this level, fluorescence quench-
resulting in the formation of an As(Cys)3 complex in a trigonal ing was recorded based on the increase in the ratio of the
pyramidal geometry.7a,13b Encouraged by these findings, we hydrophobic part in the aggregate which lead to the compound
envisioned that As+3 binding to 7 would be a facile event precipitating from solution. Considering the lower and upper
that could result in the formation of symmetric As(CysTPE)3. detection limits of 7 to As3+ and the WHO guidelines, we
Considering the structure of the complex, in aqueous media thought that this appears to be not a discouraging event for
the hydrophobic surfaces of TPE in complex would promote our work.
formation of p–p aggregation which would result in turn-on
On the other hand, the photographs given in Fig. 2b clearly
fluorescence based on the nature of the AIE effect. Moreover, exhibit the non-emissive 7 and emissive aggregated species of
intramolecular hydrogen bonding between the amine and the As(CysTPE)3 complex, respectively, when illuminated
carboxylic acid may have a positive influence on the formation with UV light. To investigate the selectivity of 7 towards As3+
,
of a symmetric As(CysTPE)3 complex, thus the aggregation a comparative study of other cations including 10 ppb and
tendency of TPEs increased, and this is associated with the 100 ppb of Cu2+, Fe2+, Hg2+, K+, Ag+, Na+, Zn2+, Cd2+, and Pb2+
strongest fluorescence. The schematic model depicted in Fig. 1 was performed. As shown in Fig. 3, only As3+ induced a high
was developed, in which p–p stacking of TPEs leads to turn-on emission intensity ratio under the given conditions, revealing
fluorescence emission after interaction of As3+with 7.
that 7 is highly selective for As+3 (Fig. S2a, ESI†). Similar results
7 shows an absorption maximum at 325 nm both in THF were obtained for the solution containing 100 ppb of each the
and aqueous media. (Fig. S1, ESI†) To verify the aforementioned metals (Fig. S2b, ESI†). It is also important to note that Hg(II)
turn-on fluorescence tendency of the aggregate structure of TPE has almost no effect on fluorescence intensity. Thus, 7 is
c
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chemical environment and physical properties, such as polarity
with its monomer 7. Thus, we concluded that the formation of a
dimer does not appear to significantly affect the photophysical
properties as compared with 7 in terms of solution emission
spectra.
In conclusion, we have developed the first label-free visual
off/on organic fluorescent dye for the most toxic form of
arsenic, As3+, in aqueous solution by taking the advantage of
the AIE feature of TPE. The sensor operates simply, detects at
the ppt level without any tagging, and requires no sample
preparation. The experimental studies clearly demonstrate the
selectivity of 7 for As3+ detection when compared with other
metals, particularly Hg(II). In the latter case, besides its fasci-
nating ability to detect As3+, the first example of discrimination
of the most toxic As3+ from less toxic As5+ is presented with 7.
This work was financially supported by scientific and
Technical Council of Turkey (TUBITAK-111T089). M. B. thanks
TUBITAK for a scholarship.
Fig. 3 Fluorescent responses of 7 (10 mM) in 1%THF/distilled water upon the
addition of various metal ions (10 ppb).
markedly superior than reported literature examples of As3+
selective molecules regarding the selectivity. On the other
hand, cross-contamination tests have failed due to precipitate
formation.
The absorption spectrum of the As(CysTPE)3 complex shows
the characteristic peak of TPE at 325 nm, as expected. (Fig. S1,
ESI†) The proof of concept and desired complex was further
achieved by 1H-NMR and mass spectrometry. When comparing
the NMR spectra of 7 and the As(CysTPE)3 complex, (Fig. S12,
ESI†) it is clearly seen that, the protons attached to methylene
group (b-CH2) bonded to the sulfhydryl group of Cys moiety of 7
which resonate at 2.9 ppm shift downfield upon formation of
As(CysTPE)3 complex. This result clearly exhibits that 7 is
attached to As3+ through the thiol group. In addition, minor
changes in a-CH protons and aromatic phenyl protons were
observed, i.e., the protons appeared as a broadening multiplet
signal at 4.5 ppm in 7 splitting to broadening doublet in the
As(CysTPE)3 1H-NMR spectrum and some signal broadening
together with a low-field shift was recorded for Ar–H based on
aggregation and electrostatic interaction between the carboxylic
acid and amine moieties of 7. These findings are in close
agreement with the presented literature examples7 of As3+ and
Cys interactions. The data collected from mass analysis also
confirmed the formation of the expected complex. The mass
spectrum of As(CysTPE)3 manifested a peak at m/z 1532.359
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