A BODIPY-based reactive probe for the detection of Au(iii) species and its application to cell imaging

Article abstract of DOI:10.1039/c4cc01958j

A BODIPY-based fluorescent probe bearing a pyridyl hydrazone motif responds selectively to Au(iii) ions through an irreversible CN bond hydrolysis reaction. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01958j

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A BODIPY-based reactive probe for the detection
of Au(III) species and its application to cell
imaging†
Cite this: Chem. Commun., 2014,
50, 5884
Received 16th March 2014,
Accepted 7th April 2014
Muhammed U¨çuncu and Mustafa Emrullahoglu*
˘
¨
¨
DOI: 10.1039/c4cc01958j
www.rsc.org/chemcomm
A BODIPY-based fluorescent probe bearing a pyridyl hydrazone
motif responds selectively to Au(III) ions through an irreversible
Q
C
N bond hydrolysis reaction.
Gold species, besides their ability to catalyse chemical trans-
formations in organic synthesis,1 have significant impacts on
human health.2,3 Gold-based drugs have long been used in the
treatment of rheumatoid arthritis and other autoimmune diseases.2
At the same time, when accumulated in the biological system at
certain concentrations, gold species have the potential to disturb a
series of cellular processes by irreversible interaction with
biomolecules.3 In recent years, fluorescence-based sensing
has become an indispensable tool for sensitive and accurate
detection of trace levels of metal species in the solution.4 In
addition, with the aid of fluorescence microscopy it has become
possible to track metal species in living cells, which is of crucial
importance for elucidating their roles in the biological system.
Recently, several gold ion selective molecular sensors based
on various fluorophore units including rhodamine,5 BODIPY,6
fluorescein,7 and naphthalimide8 dyes have been reported in
the literature.9 Given the challenge in design and synthesis,
molecular sensors developed for gold species are reasonably
few at the moment. The great majority of gold ion-specific
molecular sensors reported to date share a common feature:
the ability of gold ions to activate unsaturated bonds. In other
words those sensors take advantage of the alkynophilic behaviour
of gold species. The most prominent drawback of this type of
sensing is the cross affinity with other alkynophilic metal species
such as Pd2+, Ni2+, Ag+, Cu2+ and Hg2+. To avoid this handicap,
new sensing strategies utilizing alternative recognition motifs are
urgently sought.
Scheme 1 Synthesis of the probe.
sensor that allows Au(III) ions to be detected on the basis of an
irreversible CQN bond hydrolysis-sensing mechanism. PyR-BOD
was synthesized by the synthetic route outlined in Scheme 1, and
the structure of the sensor was confirmed by 1H-NMR, 13C-NMR and
mass spectrometry. In the first step, BOD-Al was synthesized via
Vilsmeier Haack Formylation of the non-modified BODIPY.10 In the
second step, BOD-Al was treated with 2-hydrazinopyridine in the
presence of catalytic amount of glacial acetic acid to afford the title
compound, PyR-BOD, in a good yield (68%, overall).11
The structure of PyR-BOD, as displayed in Scheme 2, constitutes
a BODIPY unit as the fluorophore and a pyridine hydrazone unit as
the metal receptor site. Analyte specific fluorescent probes based on
hydrazone derivatives of BODIPY dyes have been presented in the
literature.12 In this investigation, given its high affinity to gold
species, a pyridine motif was integrated to the BODIPY fluorophore
in order to serve as the gold ion binding receptor. The BODIPY core
was chosen as the signal reporter because of its exceptional
photophysical properties such as high photo stability, high fluores-
cence quantum yield, robustness towards light and chemicals, and
long emission/absorption wavelength.13
Herein, we present the design, synthesis, spectral properties, and
cell imaging studies of PyR-BOD, a novel turn-on type fluorescent
˙
Department of Chemistry, Faculty of Science, Izmir Institute of Technology,
˙
Urla 35430, Izmir, Turkey. E-mail: mustafaemrullahoglu@iyte.edu.tr
† Electronic supplementary information (ESI) available: Absorbance and fluores-
Scheme 2 Structure of the probe and the control probe.
cence data and all experimental procedures. See DOI: 10.1039/c4cc01958j
5884 | Chem. Commun., 2014, 50, 5884--5886
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Importantly, PyR-BOD, was designed in such a way that the 537 nm belonging to the BODIPY core. In response to the
BODIPY fluorophore is non-emissive before the addition of any addition of Au(III), this band disappeared concomitantly with
metal species. As displayed in Scheme 2, the CQN functional the appearance of a new absorption band at 494 nm indicating
group incorporated into the fluorophore diminishes the fluorescence a possible structural change of the BODIPY core (Fig. 1b). The
emission because of a non-radiative deactivation process of the blue shift of the absorption wavelength is reflected in a change
excited state via isomerization of the CQN group. We envisioned in the colour of the solution from red to light orange. All
that either blocking the isomerization of the CQN bond by metal spectral changes were easily detectable by the naked eye under
binding or directly hydrolysing it with a metal species would allow visible light and UV lamp irradiation.
for the recovery of the BODIPY emission.
To our delight, no obvious spectral changes could be
The fluorescence-sensing behaviour of PyR-BOD towards detected for the competing alkynophilic metal species such as
addition of AuCl3 was comprehensively surveyed upon excitation Ag+, Ni2+, Pd2+, Hg2+, Cu2+, and for the other metal species such
at 460 nm. As shown in Fig. 1a, metal-free PyR-BOD was non- as Mg2+, Mn2+, Pb2+, Zn2+, Cd2+, Fe3+, K+, Li+, Ba2+, Ca2+, Bi3+,
emissive due to a non-radiative deactivation process. As envisioned, In3+, Ce3+, Nd3+, Sm3+. Especially notable was the observed
the addition of AuCl3 (20 mM, 2 equiv.) to the probe solution (0.1 M selectivity for Au(III) over Au(I), which can be hardly seen for
phosphate buffer pH 7.0/EtOH, v/v, 1 : 1) resulted in a significant other gold ion responsive molecular sensors. Interference of
increase of the emission band centred at 512 nm. The increase in other metal ions to the selectivity of PyR-BOD was also
the emission intensity was proportional to the amount of Au(III) explored. The spectral response induced by Au(III) ions was
with a good linear correlation and the minimum amount of Au(III) not affected in the presence of any other metal species.11 These
that can be detected under these conditions was evaluated to be results indicated that PyR-BOD could properly detect Au(III) ions
44 nm based on S/N = 3.11 The emission intensity reached its in the mixtures of other related species.
maximum when 20 equiv. of Au(III) was added, with an enhance-
To check the reversibility of the sensing process, the probe
ment factor over 300 fold. Notably, the spectroscopic response was solution PyR-BOD/Au3+ (1 : 1) was treated with a cyanide ion
fast and saturation of the emission intensity was observed within source. The addition of Bu4NCN did not alter the emission
minutes (Fig. S8, ESI†).
The UV-Vis spectra of PyR-BOD displayed a similar sensing irreversible in nature and based on a chemical reaction.
behaviour towards the addition of AuCl3. The UV spectrum of The outcome of the reaction-based sensing process could be
pattern of PyR-BOD/Au3+ indicating the sensing process to be
metal-free PyR-BOD displayed one single absorption band at easily monitored using Thin Layer Chromotography (TLC).11
The appearance of a green fluorescent compound on the TLC
plate was a strong evidence for the formation of a new BODIPY
derivate (Fig. S10, ESI†). By the aid of NMR spectroscopy, the
structure of this new compound was easily confirmed to be the
hydrolysis product (BOD-Al) of PyR-BOD.
The sensing mechanism, as illustrated in Scheme 3, is
proposed to proceed via coordination of Au(III) to both pyridyl and
imine nitrogen atoms forming an intermediate (PyR-BOD–Au3+),
which is rapidly hydrolysed by the attack of water. Optimization
experiments revealed that the content of water in the sensing media
has a dramatic role in the sensing efficiency.11 While there were no
spectral changes in pure organic solvent (i.e., ethanol), increasing
the content of water in the sensing media (H2O/EtOH, v/v, 1 : 1)
dramatically enhanced the response of the probe towards Au(III),
which established the obvious need of water for the hydrolysis
step. However, it is worth noting that, at higher percentages of
water (e.g. H2O/EtOH, v/v, 9 : 1), Au(III) loses its activity by
forming aggregates in the solution.
Fig. 1 (a) Fluorescence and (b) absorbance spectra of PyR-BOD (10 mM) in the
presence of increasing amount of Au3+ (0–200 mM) and 0.1 M phosphate
Scheme 3 Gold mediated hydrolysis of PyR-BOD.
buffer/EtOH (pH 7.0, v/v, 1 : 1). Inset: calibration curve. (lex: 460 nm at 25 1C).
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and adherent morphology. This preliminary cell imaging study
suggested that PyR-BOD is cell-membrane permeable and can
be efficiently used for in vitro imaging of Au(III) in living cells.
In summary, we have developed a fluorescent molecular
sensor that shows a turn-on type emission enhancement
towards Au(III) ions with high sensitivity and selectivity over other
metal ions. This novel probe operates through an irreversible
CQN hydrolysis reaction and thus can be classified as a chemo-
dosimeter for Au(III) ions.
˙
We thank Izmir Institute of Technology (IZTECH) and
TUBTAK (113Z601) for financial support.
Fig. 2 Fluorescence images of Human Lung Adenocarcinoma cells
(A549). (a) Fluorescence image of A549 cells treated with only PyR-BOD
(5 mM); (b) fluorescence image of cells treated with PyR-BOD (5 mM) and
Au3+ (10 mM) (lex = 460 nm); (c) fluorescence image of cells treated with
DAPI (control); (d) merged images of frames (b) and (c).
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carefully. PyR-BOD displayed exceptional stability over a wide
pH range, while remaining non-emissive over this range. Even in
strong acidic and basic environments there were no indications
for any decomposition of the probe structure. In the same way,
the response of PyR-BOD towards the addition of Au(III) ions was
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addition of Au(III) (10 mM) and incubation for another 20 min.
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(1.0 mM)) for another 10 min. With the aid of fluorescence
microscopy, the turn-on response towards Au(III) was clearly
monitored in the cells (Fig. 2). The fluorescence images of cells
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5886 | Chem. Commun., 2014, 50, 5884--5886
This journal is ©The Royal Society of Chemistry 2014