Exploiting the higher alkynophilicity of Au-species: Development of a highly selective fluorescent probe for gold ions

Article abstract of DOI:10.1039/c2cc35083a

A new approach, involving the anchoring-unanchoring of a fluorophore, has been developed for the detection of Au-species. The fluorescent probe was found to be highly selective for sensing gold species in the presence of several other metal ions. A successful application to bioimaging has also been demonstrated with A549 lung cancer cells.

Full text of DOI:10.1039/c2cc35083a

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Exploiting the higher alkynophilicity of Au-species: development of a
highly selective fluorescent probe for gold ionsw
Nitin T. Patil,*a Valmik S. Shinde,a Milind S. Thakare,a P. Hemant Kumar,b
Prakriti. R. Bangal,*b Ayan Kumar Baruic and Chitta Ranjan Patra*c
Received 16th July 2012, Accepted 3rd September 2012
DOI: 10.1039/c2cc35083a
A new approach, involving the anchoring–unanchoring of a
fluorophore, has been developed for the detection of Au-species.
The fluorescent probe was found to be highly selective for sensing
gold species in the presence of several other metal ions. A
successful application to bioimaging has also been demonstrated
with A549 lung cancer cells.
approach, Fig. 1-a1).7 Jou et al., for the first time, reported a
Au3+ selective fluorescent probe based on the cyclization of
a rhodamine-based propargylamide.7a Almost at the same
time, Tae and coworkers disclosed a rhodamine-based probe
for the detection of Au3+ species based on intramolecular
hydroamination.7b Ahn and coworkers developed a rhodamine
based probe for Au1+ and Au3+ ions.7c Employing a gold-
catalyzed cascade reaction, Do et al. developed a Au3+
selective fluorescence turn-on probe based on the intra-
molecular hydroarylation of phenyl alkynoates.7d Peng and
Wang proposed a 1,8-naphthalimide-based probe for the
selective recognition of Hg2+ and Au3+ ions.7e Lin and
coworkers designed a fast responsive Au1+/Au3+ selective
fluorescent probe based on the hydrolysis of rhodamine-based
acyl-semicarbazides.7f In their subsequent paper, the authors
have shown that not only gold ions but gold NPs can also be
detected.7g Chang and coworkers reported a Au3+ selective
probe based on the desulfurization of thiocoumarin.7h Song
and coworkers disclosed a fluorescent probe for Au1+/Au3+
ions based on the intramolecular hydroamination of Bodipy-
derived aminoalkynes.7i Another approach is complexation
based wherein the nonfluorescent probe, containing a fluoro-
phore and a gold ion receptor, binds with Au ions triggering a
change in the fluorescence intensity (Fig. 1B). For instance,
Wang et al. developed a rhodamine based probe for selective
detection of Au3+ ions.8
During the last decade gold catalysis has emerged as a power-
ful tool in the area of organic synthesis.1 Based on the intrinsic
p-activation property of gold catalysts, either Au1+ or Au3+
species,2 a great number of novel transformations have
appeared. The high carbophilicity exhibited by gold salts,
unlike other metals in the periodic table, makes them special
and unique. Though gold in elemental form is inert, its salt
exhibits some biological effects. For instance, gold ions have
anti-inflammatory properties and are used as pharmaceuticals
in the treatment of tuberculosis, arthritis, and cancer.3 In
addition, it is well established that gold ions are known as
inhibitors of macrophages and polymorphonuclear leucocytes.4
However, gold species may tightly bind to biomolecules such
as enzymes and DNA, leading to toxicity to humans.5 Simi-
larly, certain gold salts such as gold chloride are known to
cause damage to the liver, kidneys, and the peripheral nervous
system.6
Based on the sharp increase in gold catalysis and the toxicity
associated with gold ions, it is essential to develop a chemo-
sensor to monitor the presence of gold species both in the
environment and under physiological conditions. In general,
the development of a chemosensor for the detection of gold is
based on two approaches. The first approach (reaction based,
Fig. 1A) involves a nonfluorescent probe consisting of a
fluorophore and an organic molecule which on reaction with
a gold species generates a new structure resulting in a change
in the fluorescence intensity (functional group manipulation
In this communication, we report a new approach involving
the anchoring–unanchoring of the fluorophore (Fig. 1-a2).
a Division of Crop Protection Chemicals, India.
E-mail: nitin@iict.res.in
b Inorganic and Physical Chemistry Division, India.
E-mail: prakriti@iict.res.in
c Centre for Chemical Biology, CSIR-Indian Institute of Chemical
Technology, Hyderabad, 500 007, India. E-mail: crpatra@iict.res.in
w Electronic supplementary information (ESI) available: All experi-
mental procedures, analytical data, and copies of 1H and 13C NMR
spectra of all newly synthesized products. See DOI: 10.1039/c2cc35083a
Fig. 1 Various approaches for the detection of gold ions.
Chem. Commun., 2012, 48, 11229–11231 11229
c
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Fig. 2 Approach involving the anchoring–unanchoring of the
fluorophore.
It can be seen from Fig. 1-a2, that the fluorescence of
fluorophore is turned off by anchoring with an organic sub-
strate. Once the gold ions have been sensed, the probe liberates
the highly fluorescent fluorophore with formation of the
organic product.
As a part of our ongoing interest in gold catalysis,9 we
envisioned that an organic molecule of type X would be
activated by gold ions which, in turn, would trigger a cascade
as depicted in Fig. 2 (cf. Y).10 The reaction, thus envisaged,
would liberate the highly fluorescent fluorophore11 and there-
fore the designed molecules of type X would serve as a probe
for sensing gold ions.
Fig. 3 (A) Time dependent absorption change of 5a (10 mM) upon
addition of 10 equiv. of Au1+ in CH3CN : PBS buffer (1 : 1, 0.1 M,
pH = 7.4). (B) Absorption kinetics change upon the addition of 10 equiv.
of Au1+ to 5a (10 mM) in CH3CN : PBS buffer (1 : 1, 0.1 M, pH = 7.4).
Fig. 3 shows the time dependent absorption change of 5a
(10 mM) upon addition of 10 equiv. of Au1+ ion. A new band
appeared in the visible region (lmax = 452 nm) immediately
after addition of the Au1+ ion and a large enhancement in
absorption is observed with time. The increase in absorbance
(at lmax = 452 nm) is rapid and it saturates after 40 min,
indicating the reaction is very fast. Similar spectral behaviour
has been observed to take place between 5a with the Au3+ ion
with much less enhancement in absorption (at lmax = 452 nm)
(ESIw).
The solution of probe 5a in CH3CN/PBS buffer (1 : 1,
pH = 7.4) is colorless and exhibits negligible fluorescence.
However, the addition of Au1+ ions to probe 5a triggers a
large enhancement in fluorescence (lmax = 515 nm). The time
dependent fluorescence response of probe 5a (10 mM) with
Au1+ (100 mM) showed a rapid increase in intensity up to
30 min and then saturates (Fig. 4). Similar behaviour has been
found with Au3+, but the rate at which the intensity increases
is less compared with Au1+ (ESIw).
Scheme 1 outlines the synthesis of probes 3a–b and 5a–b.
The probes 3a–b were obtained by the reaction of methoxy-
fluorescein (2) with alkynoic acids 1a and 1b, respectively. The
synthesis of probes 5a–b was achieved in two steps from
fluorescein (2). The esterification of 2-iodobenzoic acid with
2 was achieved by a conventional procedure to obtain halo-
ester 4 in 68% yield. The halo-ester 4, thus obtained, reacted
with phenyl acetylene and 1-octyne under Pd–Cu catalysis to
give the desired probes 5a and 5b in 78 and 82% yields,
respectively. As expected, all the probes 3a–b and 5a–b show
negligible fluorescence and are colorless in H2O–CH3CN-
buffer (pH = 7.4). The probes 5a–b (30 mM) on treatment
with 100 mM solution of HAuCl4 exerted strong fluorescence
at 515 nm. In addition, the solution changes from colorless to
a yellow color. Since the probes 3a–b did not give promising
results (ESIw), we considered 5a and 5b; out of which the
former was chosen for further studies because of its superiority
(ESIw).
Next, we have examined the selectivity of probe 5a towards
various other metal species. The study reveals that probe 5a
shows a special selectivity towards Au1+ and Au3+ metal ions
only. The fluorescence response of other metals such as Ag1+
Ba2+, Bi3+, Fe3+, Hg2+, Ir3+, K1+, La3+, Mn2+, Ni2+
,
,
Probe 5a in CH3CN/PBS buffer (1 : 1, pH = 7.4) has an
absorption maxima centered at 313 nm with no absorption in
the visible region (ESIw). However, the addition of Au1+ or
Au3+ ions to probe 5a induces a colorimetric change from
colorless to yellow which is visible to the naked eye (ESIw).
Pd2+, Pt2+, Pt4+, Ru3+, Sc3+, Yb3+, Zn2+, Cd2+ and Cr3+
is negligible. Further the selectivity of probe 5a towards Au1+
in the presence of other competing metal species has been
tested (ESIw). It is evident that the interference of other metal
species is minimal, so this probe can be used as a Au1+ sensor
in the presence of other metal species. The fluorescence
Fig. 4 Time dependent fluorescence change obtained for a mixture of
5a (10 mM) and Au1+ (100 mM) in CH3CN : PBS buffer (1 : 1, 0.1 M,
pH = 7.4). lexcit = 452 nm. Inset: plot of fluorescence intensity at
515 nm against time.
Scheme 1 Synthesis of fluorescein-based probes.
11230 Chem. Commun., 2012, 48, 11229–11231
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Fig. 5 Fluorescence images of A549 cells: (a–b) bright field images,
(a1–b1) fluorescence images and (a2–b2) DAPI stained images of
A549 cells. The figures a, a1 and a2 indicate the bright field, fluorescent
and DAPI stained images of control untreated A549 cells. Similarly, b,
b1 and b2 indicate the bright field, fluorescent and DAPI stained images
of A549 cells treated with 20 mM of Au1+ and 50 mM of probe 5a.
response of the probe shows an excellent linear relationship
towards Au1+ in the range of 5–80 mM, indicating that the
probe can be used for the quantitative determination of Au1+
.
4 C. J. Fleming, E. L. Salisbury, P. Kirwan, D. M. Painter and
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The favourable features of 5a such as a fast response, high
selectivity and fluorescence under physiological pH encour-
aged us to further examine the potential of the sensor for
imaging Au1+ in living cells. The lung cancer cell A549 and
Au1+ were chosen for studies. The bright field images of
control untreated A549 cells are depicted in Fig. 5a. The
corresponding fluorescence image and the blue color DAPI
stained image of untreated control A549 cells are shown in
Fig. 5a1 and a2, respectively. As can be seen, the green
fluorescence of untreated A549 cells was not observed in the
absence of 5a and Au1+ (Fig. 5a1). However, the presence of
green fluorescence was noticed when the 5a was treated with
Au1+ (Fig. 5b1) even after extensive washing with DPBS.
These observations clearly indicate that the probe 5a can sense
Au1+ ion in living cells.
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In summary, we have developed a new approach involving
the anchoring–unanchoring of a fluorophore, for the detection
of gold ions.12 We believe that this new strategy will attract the
attention of the scientific community and therefore many
probes based on the present approach will appear in the
future. Bioimaging studies have also been successfully demon-
strated with A549 lung cancer cells. We anticipate that this
study could provide the basis for the future development of a
biodegradable fluorescent probe for cancer diagnostics.
Generous financial support by the Council of Scientific and
Industrial Research (CSIR, MLP0010C), New Delhi, India
and the Department of Science and Technology (DST,
GAP-0303) is gratefully acknowledged. NTP is grateful
to Dr V. J. Rao, Head, CPC division, CSIR-IICT for his
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support and encouragement. VSS thanks UGC for
a
senior research fellowship. C. R. P. is grateful to the DST,
New Delhi for financial support and the award of Ramanujan
fellowship (SR/S2/RJN-04/2010, GAP 0305\DST\CP). This
paper is dedicated to Dr J. S. Yadav on the occasion of his
62nd birthday.
11 Fluorescein was chosen as a fluorophore because of its bright
fluorescence combined with its non-toxicity, see: (a) P. D.
McQueen, S. Sagoo, H. Yao and R. A. Jockusch, Angew. Chem.,
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1703–1704.
12 One of the referee’s suggested that the Au(I) ion may undergo
disproportionation to Au(0) and Au(III) in the presence of
CH3CN–H2O, see: R. Kissner, G. Welti and G. Geier, J. Chem.
Soc., Dalton Trans., 1997, 1773–1777. While such a kind of
disproportion is possible in our case, the time dependant study
in MeOH–H2O also reveals similar kinetics (Fig. S11 and S12,
ESIw).
Notes and References
1 Selected recent reviews: (a) A. S. K. Hashmi and M. Rudolph,
Chem. Soc. Rev., 2008, 37, 1766–1775; (b) E. Jimenez-Nu´ nez
´
and A. M. Echavarren, Chem. Rev., 2008, 108, 3326–3350;
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11229–11231 11231