Quick accessible dual mode turn-on red fluorescent chemosensor for Cu(ii) and its applicability in live cell imaging

Article abstract of DOI:10.1039/c3ra42109k

A novel fluorescent chemosensor 9,10-Diphenyl-7H-benzo[d,e]imidazo[2,1-a] isoquinolin-7-one (BIDQ-1), which exhibits rapid colorimetric and fluorimetric response for copper ions over other competitive metal ions under mild conditions has been synthesized

Full text of DOI:10.1039/c3ra42109k

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Quick accessible dual mode turn-on red fluorescent
chemosensor for Cu(II) and its applicability in live cell
imaging3
Cite this: DOI: 10.1039/c3ra42109k
Gandhi Sivaraman, Thangaraj Anand and Duraisamy Chellappa*
A novel fluorescent chemosensor 9,10-Diphenyl-7H-benzo[d,e]imidazo[2,1-a]isoquinolin-7-one (BIDQ-1),
which exhibits rapid colorimetric and fluorimetric response for copper ions over other competitive metal
ions under mild conditions has been synthesized easily by a simple one pot condensation method. On
addition of Cu(II), BIDQ-1 shows a distinct color change from yellow to green, thus permitting the probe to
be used for naked eye detection of Cu(II). The new fluorescent probe exhibits a turn-on fluorescence
response towards Cu2+ under physiological conditions with high sensitivity and selectivity. The turn-on
fluorescence behaviour of BIDQ-1 with Cu(II) was found to be via an intramolecular charge transfer (ICT)
mechanism, by TD-DFT calculations. The probe was found to be applicable for imaging intracellular Cu2+ in
living cells.
Received 29th April 2013,
Accepted 3rd July 2013
DOI: 10.1039/c3ra42109k
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preferred as not only do they require high cost analytical
instrumentation but they also have operational difficulties.
Introduction
Copper is one of the essential trace elements required for
many biochemical and physiological functions.1 Copper ions
contained in proteins act as cofactors for many enzymes such
as cytochrome oxidase, zinc-copper superoxide dismutase,
lysyl oxidase and catechol dioxygenase.2 It is involved in
critical biological processes in living organisms, such as
respiration, antioxidant defense and iron metabolism.3
However, free copper ions are toxic to the cells since they
catalyze the generation of highly reactive hydroxyl radicals,
which cause cellular damage.4 Copper ions are highly toxic to
organisms such as certain algae, fungi, many bacteria and
viruses. The interruption of the copper balance in the body
results in severe disorders such as Menkes syndrome, Wilson’s
disease, amyotrophic lateral sclerosis, prion diseases and
Alzheimer’s disease. Excessive intake of copper into the body
leads to tumours, new blood vessel growth, kidney and liver
damage, gastrointestinal disturbance and Non-Indian
Childhood Cirrhosis (NICC).5 In view of these effects it is
worth developing a suitable technique to monitor Cu(II) within
the body.The conventional methods for detection of copper
ions such as photometric methods, atomic absorption spectro-
scopy (AAS), inductively coupled plasma emission or mass
spectrometry (ICP-ES, ICP-MS), total reflection X-Ray fluori-
metry (TXRF) and anodic stripping voltammetry (ASV)6 provide
good detection limits. However, these methods are not
These techniques, though applicable for biological samples,
usually result in destruction of cells. As an alternative to these
methods fluorescent sensors have been demonstrated to be
powerful tools for the non-destructive imaging of intracellular
distribution in single cells.7 Due to the biological significance
of Cu(II), colorimetric and fluorimetric detection of copper
ions have been widely documented.8 Cu(II) is a notorious
fluorescence quencher due to its paramagnetic nature and de-
excitation pathways involving electron transfer/excited-state
energy transfer and as such, most of the earlier reports of Cu2+
sensors are based on quenching of fluorescence.9
Subsequently, the design of highly sensitive and selective
turn-on fluorescence probes for copper ions has been
reported.10 Near-infrared (NIR) or red detection of metal ions
is preferable due to its applicability in biological systems as
they minimize auto-fluorescence, cytotoxicity and photo
damage of living cells.11 Nevertheless only very few reports of
NIR/red fluorescent turn-on sensors for Cu2+ are available.12 In
the present work the receptor BIDQ-1, synthesized by a new
procedure,13 has been shown to be an excellent, sensitive and
selective turn-on sensor for Cu(II). This is the first report of
using an imidazoquinone derivative as a fluorescent sensor.
Results and discussion
School of chemistry, Madurai Kamaraj University, Madurai-625021.
E-mail: dcmku123@gmail.com
One pot condensation of acenaphthoquinone, benzil with
ammonium acetate in ethanol in the presence of acetic acid
Electronic supplementary information (ESI) available: Spectral datas and
3
computational information. See DOI: 10.1039/c3ra42109k
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Scheme 1 Synthesis of BIDQ-1.
under refluxing conditions furnished the product in an
excellent yield (ESI ) (Scheme 1).
3
Fig. 2 Fluorescence spectra of BIDQ-1 in the presence of Fe3+, Ag+, Ca2+, Cd2+
,
Co2+, Cu2+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+and Zn2+ (10 mM) in aqueous
PBS buffer (10 mm PBS pH 7.54). Excitation was performed at 440 nm.
Excitation/Emission Slit width: 5/5.
The structure of BIDQ-1 was confirmed by spectro-
analytical techniques. The response action of BIDQ-1 towards
different metal ions was investigated by UV-vis and fluores-
cence spectrophotometric techniques. The UV-visible absorp-
tion spectrum of BIDQ-1 in CH3CN:H2O (1 : 1, v/v; PBS buffer,
pH = 7.54) shows bands at 423, 347, 333, 290 nm. In order to
check the selective sensing ability of BIDQ-1 metal ions such
as Na+, K+, Mg2+, Ca2+,Ba2+, Zn2+, Cd2+, Hg2+, Pb2+, Ni2+, Co2+,
Fe3+, Mn2+, and Ag+ were added to BIDQ-1 and found no
observable changes in its uv-vis spectrum. On addition of Cu2+
ions the band at 423 nm is suppressed along with the
appearance of a new broad absorption band at 397 nm with
the isobestic point at 429 nm. (Fig. 1).
The appearance of the isobestic point centred at 429 nm
was reliable with equilibrium between BIDQ-1 and copper
complexes in solution. UV-vis studies clearly show that the
probe BIDQ-1 shows a selective and high binding affinity for
Cu2+ ions even in the presence of other metal ions. The
fluorescence spectra of the probe BIDQ-1 (1 mM) in the
presence of tested metal ions other than Cu(II) ions exhibit a
delicate emission at 588 nm, in CH3CN:H2O (1 : 1, v/v; PBS
buffer, pH = 7.54) medium when excited at 440 nm. (Fig. 2).
The weak fluorescence band of BIDQ-1 is probably due to
the ICT-type excited state. The gradual incremental addition of
Cu2+ ions led to a gradual enhancement of the fluorescence
intensity at 610 nm with a slight red shift whereas bare BIDQ-1
shows an emission band at 588 nm (Fig. 3). This intense
enhancement of the fluorescence intensity of BIDQ-1 induced
by Cu2+ ions is attributed to the improved ICT effect.
The binding constant14 of BIDQ-1 with Cu2+ ions was
estimated to be 5.5 6 102 mol L21 from the fluorescence
titration and the limit of detection (LoD)15 was found to be 2.1
6 1029 mol L21. The other metal ions such as Na+, K+, Mg2+,
Ca2+,Ba2+, Zn2+, Cd2+, Hg2+, Pb2+, Ni2+, Co2+, Fe3+, Mn2+, and
Ag+ were added to BIDQ-1 but insignificant changes were
observed in the fluorescence spectra. To elucidate the binding
stoichiometry between BIDQ-1 and Cu2+, a continuous varia-
Fig. 1 UV-vis absorption spectra of BIDQ-1 (10 mM) in the presence of Cu2+, Ag+,
Ba2+, Ca2+, Cd2+, Co2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+and Zn2+ (10
mM) in aqueous PBS buffer (10 mM PBS pH 7.54).
Fig. 3 Fluorescence emission spectrum of BIDQ-1 (1 6 1025 mol L21) upon
addition of Cu2+ (0–1 equivalent) in aqueous PBS (10 mM PBS, pH = 7.54)
Excitation was performed at 440 nm. Excitation/Emission Slit width: 5/5.
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tion method (Job’s) was employed using the fluorescence
emission data. The maxima was observed when the mole
fraction of Cu(II) reached 0.5 exposed 1 : 1 binding. (ESI ) This
3
stoichiometry was further supported by the nonlinear fit of the
fluorimetric titration and ESI mass spectral analysis. The peak
3
at m/z = 436.29 clearly indicates the formation of 1 : 1 BIDQ-1
2 Cu2+ complex (ESI ). In order to achieve excellent recogni-
3
tion of the analyte over potential competing species, selectivity
of the probe is a key factor. The turn-on fluorescence
recognizing efficiency of BIDQ-1 for Cu(II) is not affected in
the presence of common cations, such as Na+, K+, Mg2+, Ca2+,
Ba2+, Zn2+, Cd2+, Hg2+, Pb2+, Ni2+, Co2+, Fe3+, Mn2+and Ag+. The
effect of pH on the probe BIDQ-1 was tested and the probe was
found to be unaffected by pH. The fluorescence enhancement
of BIDQ-1 in the presence of Cu(II) was studied at different
Fig. 5 Frontier molecular orbital diagram of BIDQ-1 and BIDQ-1 + Cu2+
.
time intervals and quick response times were revealed (ESI ).
3
In order to investigate their molecular structure and possible
electronic transitions in BIDQ-1 and BIDQ-1 + Cu2+, DFT
calculations were carried out using the Gaussian03 program.16
The ground state structure optimization of BIDQ-1 and BIDQ-1
+ Cu2+ were carried out using B3LYP and LANL2DZ functio-
nalised with 6-31G* basis set respectively.
The lowest energy minimized structures for BIDQ-1 and
BIDQ-1 + Cu2+ are shown in Fig. 4. The TD-DFT/B3LYP/6-31G*
calculations based on the optimized structure of BIDQ-1,
predict that electron density in HOMO is spread over the
whole p-system while that of LUMO is localised on the
acenaphthene ring The transition between HOMO to LUMO
excitations (490.29 nm) has very low oscillator strength, f =
0.00624. This suggests that the probability of HOMO to LUMO
transition is very low. Furthermore, the first excited state
optimization of BIDQ-1 was carried out using the CIS/B3LYP-6-
31G* method.
Both ground states as well as the excited state DFT
calculations predict the low probability for HOMO to LUMO
transition. This in-turn leads to the inference that fluorescence
will also unlikely be observed. The large degree of electron
transfer between the phenyl unit and acenaphthoquinone in
the first excited state was observed.(Fig. 5) The fluorescence
enhancement of BIDQ-1 by Cu2+ could be illustrated by means
of frontier orbitals obtained from the TD-DFT calculations.
The HOMO–LUMO excitation (462.25 nm, f = 0.3164) was
found to be relevant to the charge transfer, and the
contribution to the lowest energy excitation was 40%. This
excitation corresponds to charge transfer from the excited
imidazo quinoline moiety to the Cu2+ center. It is clearly
evidencing that the internal charge transfer, occurred during
the addition of Cu(II), leads to the enhancement of fluores-
cence. TD-DFT and frontier molecular orbital calculations
clearly support the observed fluorescence intensity enhance-
ment is due to the ICT mechanism. The applicability of bio
sensing probes for selective detection of guest species in live
cells is of great interest and importance for biological
application in the present scenario. Due to the quick response
and high sensitivity of BIDQ-1 towards Cu2+ ion, its wide
application in imaging Cu2+ in HeLa cells was studied. The
HeLa cell lines were incubated with the probe BIDQ-1 for 30
min at 37 uC (5.0 mM in H2O/DMSO (2 : 1, v/v) buffered with
PBS, pH = 7.54) in a Dulbecco’s Modified Eagle’s Medium for
30 min at 37 uC.
The TD-DFT results using excited state optimised geometry
of BIDQ-1 clearly shows weak HOMO to LUMO transition
(585.88 nm, f = 0.0962). This non radiative excited state
transition leads to the weak fluorescence nature of the probe.
In order to remove the extra cellular probe BIDQ-1, the
HeLa cells was again washed with a phosphate-buffered saline
buffer. The fluorescence images of HeLa cells were recorded.
BIDQ-1 shows a hardly detectable fluorescence signal in living
cells. The cells were incubated again with the Cu2+ (10.0 mM in
H2O) for 10 min to observe a bright red fluorescence in living
cells.(Fig. 6) The fluorescence imaging results suggested that
BIDQ-1 can permeate the cell membrane and can be applied
for in vitro imaging of Cu2+ in living HeLa cells.
Fig. 4 Energy optimized geometry of BIDQ-1 and BIDQ-1 + Cu2+ by the B3LYP/
6-31G and LANL2DZ methods respectively.
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Computational details
Density functional theory (DFT) calculations Were carried out
with 6-31G* basis set using Gaussian 03 program in order to
understand the fluorescence enhancement performance of
BIDQ-1 on complexation with Cu2+
. The ground state
geometries of BIDQ-1 and BIDQ-1 + Cu2+ complex were
optimized by DFT-B3LYP using 6-31G* and LANL2DZ basis
sets respectively. The excited state geometry of BIDQ-1 was
optimised using CIS-B3LYP using 6-31G* method. Using the
ground state and excited state optimized geometries absorp-
tion behaviour, emission behaviour and corresponding transi-
tions of BIDQ-1 and BIDQ-1 + Cu2+ complex obtained from TD-
DFT using above basis sets.
Synthesis of BIDQ-1
Fig. 6 (a) Fluorescence microscopic image of HeLa cells treated with BIDQ-1 (5.0
mM) for 20 min at 37 uC; (b) Bright field image of BIDQ-1 treated HeLa cells. (c)
Fluorescence microscopic image of BIDQ-1 (5.0 mM) treated HeLa cells then Cu2+
(d) bright field image of HeLa cells treated with BIDQ-1 (5.0 mM) and Cu2+ (10.0
mM) lex = 440 nm lem = 550–650 nm.
1 mmol of acenaphthoquinone, 1 mmol of benzil and 4 mmol
of ammonium acetate was refluxed in ethanol in an oil bath for
15 min in the presence of catalytic amount of acetic acid.
The reaction mixture color was suddenly changed to red. The
reaction mixture was allowed to reach room temperature. The
precipitated red colour solid was filtered. Further purification of
the crude product was carried out by column chromatography
using Petether-EtOAc (9 : 1) as eluent yields 9,10-Diphenyl-
Conclusions
In conclusion, an imidazoquinoline BIDQ-1 has been synthe-
sized very easily by a simple one pot condensation method and
was well characterized. BIDQ-1 acts as a highly selective
fluorescent and colorimetric sensor for Cu2+ in an aqueous
acetonitrile medium. It is observed that BIDQ-1 displays
‘‘Turn-on’’ fluorescence and color change towards Cu2+, which
may be due to the ICT mechanism. The DFT, TD-DFT
calculations on ground and excited state structures of BIDQ-
1, BIDQ-1 + Cu(II) by 6-31G and LANL2DZ methods obviously
indicate that the fluorescence enhancement can be attributed
to the ICT process. The probe BIDQ-1 has great potential
applications in live cell imaging of Cu2+ due to its excellent
sensitivity and good selectivity.
7H-benzo[d,e]imidazo[2,1-a]isoquinolin-7-one (BIDQ-1) as
a
1
reddish yellow solid. H-NMR (300 MHz,CDCl3): ppm d 8.777–
8.750 (dd, 1H, J = 1 Hz), 8.600–8.572(dd, 1H, J = 1 Hz), 8.241–
8.213 (d, 1H, J = 8.5 Hz), 8.052–8.025 (d, 1H, J = 8.5 Hz),
7.79–7.69(m, 3H), 7.624–7.597 (dd, 3H, J = 9.6 Hz), 7.532–7.497
(m, 4H),7.304–7.211(m, 2H). 13C-NMR (75 MHz, CDCl3): ppm d
160.3, 141.1, 135.3, 135.1, 133.1, 132.0, 131.9, 131.2, 130.8,
130.3, 128.5, 128.2, 128.194, 127.7, 127.5, 127.3, 126.6, 125.8,
125.6, 123.2, 120.925. MS (ESI): 373.1707(M + H+) calculated:
372.0412.
Cell culture and fluorescence imaging
HeLa cells were grown in modified Eagle’s medium supple-
mented with 10% FBS (fetal bovine serum) at 37 uC. The cells
were washed with PBS buffer. The HeLa cells were then
incubated with BIDQ-1 (5.0 mM in H2O/DMSO (2 : 1, v/v)
buffered with PBS, pH = 7.54) in the culture medium for 30
min at 37 uC. After washing with PBS three times to remove
any excess of the probe BIDQ-1 in the cells, the cells were
further incubated with CuCl2 (10.0 mM in H2O) for 10 min at 37
uC and imaged with a Zeiss LSM 510 META confocal
fluorescence microscope.
Materials and methods
Acenaphthoquinone obtained from Aldrich were used as such.
Benzil, ammonium acetate and metal chloride salts were
procured from Merck. Absorption spectra were recorded using
a JASCO V-650 spectrophotometer while fluorescence analyses
were done using F-4500 Hitachi fluorescence spectrofluori-
meter. The excitation and emission slit width were kept
constant as 5 nm. NMR spectra were recorded on a Bruker
(Avance) 300 MHz NMR instrument. Electrospray ionisation
mass spectral (ESI-MS) analysis was performed in the positive
ion mode on a liquid chromatography-ion trap mass spectro-
meter (LCQ Fleet, Thermo Fisher Instruments Limited, US).
Microanalysis (C, H, and N) was performed using a Perkin-
Elmer 4100 elemental analyser. Fluorescence microscopic
images were taken using Zeiss LSM 510 META confocal
fluorescence microscope.
Acknowledgements
GS thanks UGC senior research fellowship. GS, TA, DC
acknowledge DST-IRHPA, FIST, and PURSE for instrumental
facilities and funding.
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