A novel fluorescent probe for Au(iii)/Au(i) ions based on an intramolecular hydroamination of a Bodipy derivative and its application to bioimaging

Article abstract of DOI:10.1039/c1cc16128h

A novel fluorescent probe for gold ions (Au³⁺/Au⁺) is reported through blocking photoinduced electron transfer, in which a boron dipyrromethene (Bodipy) derivative reveals high selectivity and sensitivity in a gold-catalyzed intramol

Full text of DOI:10.1039/c1cc16128h

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COMMUNICATION
A novel fluorescent probe for Au(III)/Au(I) ions based on an
intramolecular hydroamination of a Bodipy derivative and its
application to bioimagingw
Jian-Bo Wang,a Qing-Qing Wu,a Yuan-Zeng Min,b Yang-Zhong Liub and Qin-Hua Song*a
Received 1st October 2011, Accepted 16th November 2011
DOI: 10.1039/c1cc16128h
A novel fluorescent probe for gold ions (Au3+/Au+) is reported
through blocking photoinduced electron transfer, in which a boron
dipyrromethene (Bodipy) derivative reveals high selectivity and
sensitivity in a gold-catalyzed intramolecular hydroamination,
and is successfully applied to fluorescence imaging of Au3+ in
living cells.
reported a ratiometric fluorescent sensor for dual-analytes
(Au3+ and Hg2+) through pH tuning in different aqueous
solutions, e.g. MeOH–H2O (95: 5 v/v pH 9.0) for Au(III) ions.8
However, it is still a challenge to extend a new fluorescent
probe with high sensitivity and selectivity for gold ions.
In this communication, we report a Bodipy-based fluorescent
probe for highly selective and sensitive detection of gold ions,
and its successful application to cell imaging of Au3+
.
In the past, gold-catalyzed chemistry has received extensive
attention, and gold ions have been known to activate carbon–
carbon multiple bonds, especially alkynes, toward nucleophilic
addition. Due to the characteristic alkynophilicity of gold ions,
many organic transformations have been investigated by
employing gold ions.1 Gold ions also have anti-inflammatory
properties and are used as drugs for dealing with diseases such
as arthritis, tuberculosis and cancer.2 However, their salts such
as gold chloride are known to cause damage to the liver,
kidneys, and the peripheral nervous system.3 Some gold ions
such as Au(III) are so tightly bound to a certain enzyme that
they are reported to cause cell toxicity in living organisms.4
For this reason, it is of urgent necessity to develop fluorescent
probes to assess the quantity of environmental gold ions by
luminescence methods.
Our strategy is based on the blocking of photoinduced
electron transfer (PET) process. The synthesized Bodipy
derivative 1 is non-fluorescent resulting from a photoinduced
intramolecular electron transfer from the aniline unit to the Bodipy
moiety. However, in the presence of Au3+/Au+ compound 1
can convert to 2, in which the 20-ethynyl biphenylamine unit is
transformed to the phenanthridine moiety by gold-ion-promoted
intramolecular hydroamination.9 Because of the loss of the
electron-donating amino group, the PET process cannot
occur, and thus strong green fluorescence of the Bodipy is
released. This process is shown in Scheme 1.
The synthetic route to probe 1 is depicted in Scheme 2. A very
important building block, diphenylaldehyde derivative 5, was
synthesized by using 2-bromo-1-iodo-4-methylbenzene as starting
material undergoing reaction of the Sonogashira coupling,
hydroxy methylation, boronization and the Suzuki coupling.
The Bodipy derivative 6 was obtained through the TFA catalyzed
condensation reaction of intermediate 5 with 3,5-dimethyl-4-
(ethoxycarbonyl)pyrrole according to a routine Bodipy formation
procedure. At last, the target probe 1 was afforded by reduction
reaction of 6 with SnCl2. The detailed experimental procedure
and characterization data are provided as the ESI.w
Recently, a few fluorescence probes for gold ions have been
reported based on gold-ion-promoted cyclizations5 from the
non-fluorescent spirocyclic rhodamine to the strongly fluorescent
rhodamine6 or from the non-fluorescent apocoumarin to the
strongly fluorescent coumarin.7 Among the former, Lin and
co-workers have ingeniously designed a new FRET-based,
ratiometric probe through linking a Bodipy unit to the non-
fluorescent rhodamine, that is, before the Au3+-mediated
cyclization, the Bodipy unit gives a short-wave emission, and
after, the excited Bodipy unit transfers energy to the fluorescent
rhodamine to emit a long-wave emission.6e In addition, based
on Au3+-promoted oxidizing reaction, Wang and Peng et al.
The treatment of probe 1 with a catalytic amount of AuCl3
in ethanol solution results in the appearance of a strong green-
fluorescent solution. After the reaction was completed, the
fluorescent compound 2 was isolated by column chromatography,
a Department of Chemistry, Joint Laboratory of Green Synthetic
Chemistry, University of Science and Technology of China, Hefei,
230026, P. R. China. E-mail: qhsong@ustc.edu.cn;
Fax: +86 551 3601592; Tel: +86 551 3607524
b CAS Key Laboratory of Soft Matter Chemistry, University of
Science and Technology of China, Hefei, 230026, P.R. China
w Electronic supplementary information (ESI) available: Experimental
details, characterization data and NMR spectra of new compounds.
See DOI: 10.1039/c1cc16128h
Scheme 1 Hydroamination of 1 in the presence of Au3+/Au+ in EtOH.
c
744 Chem. Commun., 2012, 48, 744–746
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Fig. 2 Fluorescence changes of probe 1 (5 mM) with various amounts
Scheme 2 Synthetic route to probe 1. Reagents and conditions:
(a) Pd(PPh3)2Cl2, CuI, Et3N, propargyl alcohol, 40 1C; (b) NaH,
MeI, THF; (c) (i) n-BuLi, THF, ꢁ78 1C; (ii) B(OCH3)3, ꢁ78 1C;
(d) Pd(PPh3)4, K2CO3, 4-bromo-3-nitrobenzaldehyde, toluene, 80 1C;
(e) (i) cat. TFA, DDQ, CH2Cl2; (ii) Et3N, BF3ꢂEt2O, toluene, r.t.;
(f) SnCl2, conc. HCl (aq), THF/EtOH = 1 : 1.
of Au3+ (0, 0.2, 0.4, 0.6, 0.8, 1, 1.4, 1.8, 2.6, 3.4, 4, 5, 6, 7 equiv.) in
EtOH/0.01 M PBS buffer (1 : 1, v/v, pH 7.4), recorded after 30 min,
excitation at 480 nm. Inset: fluorescence intensity changes (at 511 nm)
of probe 1 with various amounts of Au3+
.
The fluorescence response of probe 1 in the aqueous
solution toward various amounts of Au3+ was examined.
The fluorescence intensity was measured after 30 min for each
addition of Au3+. As shown in Fig. 2, the fluorescence
intensity at 511 nm increases rapidly with the concentration
of Au3+, and reaches the saturation of intensity at about
4 equiv. Au3+. It is a linear relationship between the fluorescent
intensity and the concentration of Au3+ in the range from
0.1 mM to 0.6 mM (Fig. S3, ESIw). Based on those data, the
detection limit of probe 1 was estimated to be 320 nM (63 ppb)
on the basis of the signal-to-noise ratio of 3. This shows that
probe 1 has a high sensitivity for detecting gold ions.
which was confirmed as an intramolecular hydroamination
1
product by H NMR and mass spectral analyses.
The probe 1 in EtOH/0.01 M PBS buffer (1 : 1, v/v, pH 7.4)
(5 mM) exhibits almost no fluorescence due to the effective
photoinduced electron transfer (PET) quenching process.
Upon addition of 5.0 equiv. of Au3+ ions to the solution, the
fluorescence intensity increased rapidly and an approximately
116-fold fluorescence enhancement after 60 min was observed
(Fig. 1). This shows that the gold-ion-mediated intramolecular
hydroamination occurs. Similar fluorescence enhancements at
511 nm were observed in the presence of Au+ ions under the
same conditions (Fig. S1, ESIw), however, a longer time was
required to reach fluorescence intensity maximum relative to
Au3+. A pseudo-first-order rate constant of probe 1 (5 mM)
conversion to 2 in the aqueous solution was measured in the
presence of Au3+ (5 equiv.), and the observed rate constant
(kobs) is obtained to be 9.2 ꢀ 10ꢁ4 sꢁ1. The kobs value of Au+ is
Next, Ag+, Hg2+, Zn2+, Ni2+, Cd2+, Cr3+, Fe3+, Cu2+
,
Co2+, Ca2+, Mg2+ and Pd2+ ions were used to measure the
selectivity of probe 1 toward gold ions in EtOH/0.01 M PBS
buffer (1 : 1, v/v, pH 7.4) and fluorescence spectra were
recorded after 60 min upon the addition of 5 equiv. of those
metal ions. As shown in Fig. 3a, those ions display almost no
fluorescence except Pd2+. Hence, probe 1 displays excellent
selectivity toward gold ions.
6.0 ꢀ 10ꢁ4
s
ꢁ1, which is lower than that of Au3+ (Fig. S2,
ESIw), and this may be because of a stronger interaction
between highly charged Au3+ and the alkyne than that of
Au+, in a protic solvent.6c These rate constants are faster than
The interference experiments were also carried out for the
selectivity of probe 1 toward Au3+ ions in the presence of those
metal ions. The fluorescence was turned on again when the
Au3+ ions were added to the solutions of probe 1 and those
metal ions, which indicated that those ions have little effect on
the fluorescent probe 1 except Hg2+ and Pd2+ (Fig. 3b). Both
Hg2+ and Pd2+ can activate alkynes especially terminal
alkynes,1d,e,10 so we had hoped to avoid their interference by
designing the probe molecule 1 as a non-terminal alkyne.
However, it is surprising that probe 1 is still interfered by
the two ions. The detail mechanism remains to be investigated.
Fluorescence microscopic imaging for the Au3+ mediated
probe 1 in HeLa cells was performed. HeLa cells were seeded
in a 24-well size plastic-bottom dish at a density of 5 ꢀ 104 cells
per well in a culture medium overnight. The cells were treated
with 10 mM of probe 1 for 1 h and washed 3 times with PBS
buffer solution. Then cells were incubated with 20 mM AuCl3
for 1 h and the cell cultures were washed with PBS to remove
the residue Au3+ ions. The images of the cells were obtained
by a confocal laser scanning microscope, shown in Fig. 4.
The cells exposed to both 1 and Au3+ display a green
emission, while those treated only with 1 are dark.
those for propargylamine-derived rhodamine (4.5 ꢀ 10ꢁ4 sꢁ1 6a
)
and apocoumarin (3.31 ꢀ 10ꢁ5 sꢁ1),7 and show that probe 1 can
respond rapidly to gold ions.
Fig. 1 Time-dependent fluorescence spectral changes of probe 1
(5 mM) with addition of 5.0 equiv. of Au3+ in EtOH/0.01 M PBS
buffer (1 : 1, v/v, pH 7.4), excitation at 480 nm. Inset: time dependent
fluorescence intensity changes (at 511 nm) of probe 1 with Au3+
.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 744–746 745

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moiety, to phenanthridine, resulting in a loss of the electron-
donating ability, thus blocking the intramolecular PET
process to emit a strong green fluorescence. This is the first
time a fluorescent probe for gold ions has been constructed in
this way. It provides a new idea for designing fluorescent
probes for gold ions. The fluorescent probe with a Bodipy
chromophore possesses a short response time and a low
detection limit.
This work was supported by the National Natural Science
Foundation of China (Grant No. 30870581, 20972149).
Notes and references
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Fig. 3 (a) Fluorescence spectra of probe 1 (5 mM) with Au3+/Au+
ions and other metal ions (5 equiv.) including Ag+, Hg2+, Zn2+
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Ni2+, Cd2+, Cr3+, Fe3+, Cu2+, Co2+, Ca2+, Mg2+ and Pd2+ in
EtOH/0.01 M PBS buffer (1 : 1, v/v, pH 7.4). (b) Relative fluorescence
responses at 511 nm of 1 in the presence of Au3+ (5 equiv.) and various
metal species (5 equiv.), where F0 is the intensity in the absence of Au3+
.
In summary, we have designed a novel fluorescent probe for
gold ions based on the gold-ion-mediated intramolecular
hydroamination, which converts the electron donor, the aniline
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746 Chem. Commun., 2012, 48, 744–746
This journal is The Royal Society of Chemistry 2012