β-Cyclodextrin included coumarin derivatives as selective fluorescent sensors for Cu2+ ions in HeLa cells

Article abstract of DOI:10.1039/c6ra01522k

A highly sensitive and selective fluorescent sensor for Cu²⁺ ions in water medium is reported using 4-((benzo[d]thiazol-2-ylthio)methyl)-5,7-dihydroxy-2H-chromen-2-one (1) included β-cyclodextrin, as a probe. The fluorogenic supramolecule has displayed good selectivity and affinity towards Cu²⁺ ions over other cations after examining in biological systems with intracellular Cu²⁺ ions, especially in cultured HeLa cells using fluorescence microscopic imaging. The lowest detection limit of Cu²⁺ ions observed using this probe is as low as 2.52 × 10^-10 M. The observed on-off fluorescence with the periodic addition of Cu²⁺ ion is explained via an Intramolecular Charge Transfer mechanism (ICT) and the inclusion of 1 in β-cyclodextrin is characterised by ¹H-NMR molecular modeling studies. The results show that the present β-CD:1 system, studied in HeLa cells, can be potentially used in monitoring the biological functions of Cu²⁺ ions.

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β-Cyclodextrin Included Coumarin Derivatives as Selective
Fluorescent Sensors for Cu2+ Ions in HeLa Cells
Raihana Imran Khana and Kasi Pitchumani*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Abstract
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A highly sensitive and selective fluorescent sensor for Cu2+ ion in water medium is reported using 4-((benzo[d]thiazol-2-
ylthio)methyl)-5,7-dihydroxy-2H-chromen-2-one (1) included β-cyclodextrin, as probe. The fluorogenic supramolecule has
displayed good selectivity and affinity towards Cu2+ ions over other cations after examining in biological systems having
intracellular Cu2+ ions, especially in cultured HeLa cells using fluorescence microscopic imaging. The lowest detection limit
of Cu2+ ions observed using this probe is as low as 2.52×10-10 M. The observed on-off fluorescence with the periodic
addition of Cu2+ ion is explained via an Intramolecular Charge Transfer mechanism (ICT) and the inclusion of 1 in β -
1
cyclodextrin is characterised by H-NMR molecular modeling studies. The results show that the present β-CD:1 system,
studied in HeLa cells, can be potentially used in monitoring the biological functions of Cu2+ ions.
and efflux of this ion.22-24 Much of the total cellular copper is
associated with high affinity binding proteins, and what is
considered labile copper is effectively buffered by a plethora
Introduction
Development of highly selective and efficient signaling systems
to detect biologically relevant analytes has attracted significant
attention in recent years.1-3 Highly selective chemosensors for
the recognition of metal ions such as Cu2+, Cd2+, Fe3+, Hg2+, Pb2+
and Zn2+ has always been a very important goal in view of their
environment relevance.4 Determination of copper ion is of
tremendous interest owing to the significant role of copper in
biological, environmental, and chemical fields.5 Copper is a
trace metal nutrient essential for most forms of life and is the
third most abundant transition metal in humans. Copper
serves as a structural and catalytic cofactor for many proteins
and enzymes including important metabolic factors such as
cytochrome c oxidase and copper−zinc superoxide dismutase.6-
of cellular ligands that minimize free copper ions.25-26 Live-cell
fluorescence microscopy using copper selective sensors
provides a valuable method to better understand the complex
handling of copper in cells.27-29 However, there are added
challenges posed by targeting copper ions over Zn2+ due to the
need for selectivity between different oxidation states, the
fluorescence quenching activity of Cu2+, and the fact that
sensors must have high enough affinities to compete for
copper within its biological window. As a result, copper
sensors are limited known for biologically accessible copper.30-
37
Coumarin and its derivatives are biological important and are
useful intermediates in synthesis of many heterocyclic
compounds. Coumarin derivatives have attracted attention as
some of the most popular fluorophores amenable to novel
sensor design. Recently, coumarin derivatives are also widely
utilized as excellent chromogenic and fluorogenic dyes.38 In
addition, the thiazole ring is biologically significant and
prevalent as the chemically active centre of coenzyme A, as in
10
The redox activity of copper is critical for several key
physiological processes. However, unregulated levels of
copper can induce oxidative stress and toxicity in cells.
Deregulation of copper homeostasis is associated with the
following neurodegenerative disorders; Alzheimer’s disease,11
amyotrophic lateral sclerosis,12 Menke’s disease,13-15
Parkinson’s disease,16-20 and Wilson’s disease.21 Cells must
maintain optimal concentrations and speciation of copper by
tightly regulating the uptake, distribution, storage, mobility,
luciferin of fireflies, etc., It also serves as
a fungicide
(thiabendazole), NSAID drug (meloxicam), etc.,. For the better
design of a chemosensor with biological importance, it is
relevant to consider the biocompatibility of the probe. Thus
coumarin and thiazoline units, which are widely occurring in
nature, will be free from cytotoxicity. This biocompatibility
prompted us to choose the probe 1 as a rational design for the
present study.
a.School of Chemistry, Madurai Kamaraj University, Madurai 625 021.
b*Center for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj
University, Madurai, India.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Cyclodextrins are oligosaccharides composed of six, seven or
Journal Name
eight glucose units, are toroidal in shape with a hydrophobic General procedure for syntheses of probes 1-3.
inner cavity and a hydrophilic exterior. These characteristics
In the experimental section, three sets of coumarin
facilitate them to bind selectively various organic and
derivatives were prepared which differ in the position of OH
biological guest molecules into their hydrophobic cavities to
groups. The intermediate products formed were termed
form stable host-guest inclusion complexes. The selective
as (A), (B) and (C) and the final products as (1), (2) and (3). The
association of target molecules to the hydrophobic cavity of
molecular structures for the intermediates and the final
cyclodextrin has been used to develop unique benign sensors
products are given in scheme 1. The synthetic procedure for
as cyclodextrins are environmentally friendly, water soluble
each of these three derivatives is given below:
and can improve the solubility and stability of functional
materials.39-43 Cyclodextrins (CDs) have also been extensively
Step I Preparation of the intermediates A, B and C
Benzene-1,3,5-triol was used as the precursor for the
used in molecular recognition of neutral molecules, and are
preparation of (1). 0.5 g of Benzene-1,3,5-triol (3 mmol) was
able to discriminate between guests of different shapes and
added gradually to ethyl-4-chloroacetoacetate (1 mL, 4 mmol)
dimensions. In recent times, cyclodextrins are also useful in
in H2SO4: water (3:1) mixture. This mixture was then stirred at
drug delivery and the ability of cyclodextrins to include
20 0C for 45 minutes. The reaction mixture was then poured
aromatic fluorophores, induces an increase in the fluorescence
intensity,44 and these aspects have been exploited in several
into ice cooled water immediately after the completion of the
analytical applications.45 Ueno and co-workers have shown
reaction. The white precipitate obtained was filtered using
gravimetric filtration and then dried. The white color solid
that this property could be utilized for achieving shape
selective optical chemosensors based on cyclodextrins46 which
corresponds to 4-(Chloromethyl)-5,7-dihydroxy-2H-chromen-
are also modified with several other fluorescent groups.47-49
2-one (A) was collected with an yield of 90%. The same
Among these, coumarin has been widely used,50 because of its
chemicals and procedure was used to synthesize (B) (85%)
spectroscopic and fluorescent properties.51 On account of all
and (C) (90%) except the starting material, where for (B) and
(C), the precursors used are benzene-1,4-diol and benzene-
these excellent optical and biological properties, cyclodextrin
1,3-diol respectively.
was adapted as the template for our probe synthesis.
Step II
Our ongoing efforts to realize suitable supramolecular sensing
probes52-58 with better optical and biological properties, Syntheses of Probe 1, 2 and 3
prompted us to develop an efficient and highly selective
sensing system for Cu2+using β-cyclodextrin as
4-((Benzo[d]thiazol-2-ylthio)methyl)-7-hydroxy-2H-chromen-2-
a
one (0.5g, 3mmol), was added gradually to benzothiazolidine-
2-thiol (3mmol) in DMF the presence of Et3N (5mmol). The
reaction mixture was stirred for 8h at room temperature. The
progress of the reaction was followed by thin layer
chromatography. After completion of the reaction, the DMF
was removed using rotovapor, and extracted twice with 5 mL
of dichloromethane. The organic layer was washed twice with
5 mL of distilled water, and then dried over sodium sulfate.
The filtrate was concentrated under vacuum and the residue
was purified by column chromatography on silica (20% ethyl
acetate in petroleum ether). Detail NMR spectrum for Probe 1,
2 and 3 are in supporting information Fig S1 to Fig S11. A
brown colored solid were obtained with 75%, 80% and 90%
yields, Melting point 230 0C, 210 0C and 210 0C.
supramolecular host and 4-((benzo[d]thiazol-2-ylthio)methyl)-
5,7-dihydroxy-2H-chromen-2-one (1) as a fluorescent guest
molecule. We also present here the biological applications of
β-CD:1to monitor Cu2+ in cultured cells.
2. Materials and Methods
The cations Ag+, Ca2+, Fe3+, Co2+, Cu2+, Ni2+, Zn2+, Hg2+, Cd2+,
Al3+, Cr3+, In3+, and Mg2+ were obtained (either as the
corresponding chlorides and acetates) from Sigma–Aldrich and
Merck and used without further purification. Doubly distilled
water and HPLC grade solvents were used for all spectral
measurements. All other chemicals and solvents were
purchased from either Aldrich (99% purity) or Merck (GR
Grade) and used without further purification.
The fluorescent quantum yields (Φ)
Fluorescent quantum yields (Φ) of the β-CD:1(Φ1) and β-
CD:1:Cu2+ complex (Φ1:Cu2+) were compared, with the
fluorescent quantum yield of Quinine sulfate (0.1 M H2SO4)
solution with as reference compound. (ΦQs = 0.54 %),via Eq. (5)
and Eq. (6).59
UV–Vis absorption spectra were recorded using JASCO-(V-550)
double beam spectrophotometer using quartz cell with a
volume of 2 mL and 1 cm path length at room temperature.
All fluorescence measurements were recorded on
a
Fluoromax-4 Spectrofluorometer (HORIBA JOBIN YVON) with
excitation and emission slit width set at 5.0 nm band pass
Φ1 = Φ(GradQs/Grad1) (ή2 / ή2
)
Qs
………. (5)
………..(6)
1
in 1 cm
spectrometry (ESI-MS) analysis was performed in the negative
ion mode on liquid chromatography- ion trap mass
× 1 cm quartz cell. Electrospray ionization mass
Φ1:Cu2+ = (GradQs/Gradβ-CD:1:Cu2+) (ή2
2+/ ή2
)
Qs
β-CD:1:Cu
a
Where, Φ1 and Φ1:Cu2+ are the quantum yields of β-CD:1 and
β-CD:1:Cu2+ complex; ΦQs is the quantum yield of reference
dye Quinine sulfate (0.1 M H2SO4) solution (ΦQs = 0.54),
spectrometer (LCQ Fleet, Thermo Fisher Instruments Limited,
USA). Nuclear magnetic resonance (NMR) spectra were
acquired on a Bruker DRX-300 (300 MHz) instrument using
TMS as an internal standard.
2+
respectively. η1 and η1:Cu were the refractive indexes of the
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solvents of β-CD:1and β-CD:1:Cu2+ complex; ηQs is the
refractive index of the reference solvent respectively.
Cell Incubation
HeLa cells were cultured in Dulbecco’s modified Eagle’s
medium (DMEM,) supplemented with 10% fetal calf serum , 50
μg/mL penicillin/streptomycin at 37 °C in a 5/95% CO2/air
incubator. The cells were cultured on a well plate covered
cover slip. The cultured cells were treated with CD:1 (10 micro
molar in PBS buffer) for 30 minutes. After the incubation the
cells were washed with PBS buffer for 5 times to ensure the
non-availability of the CD:1 in extra cellular region. The CD:1
treated cells were again incubated with Cu2+ ions in two
different concentrations (10 micromolar and 50 micromolar)
for 10 minutes and the cells were subjected to the
Figure
1 (a) Absorption and (b) fluorescence spectra of 1(20 and 5µM
respectively) upon addition of various concentration of CuCl2
[0,5,10,15,20,25,30,35,40,45, and 50 µM respectively] in aqueous solution with
excitation at 350 nm.
Fluorescence Imaging. Fluorescence imaging experiments were The fluorescence changes of β-CD:1 were recorded in the
performed on an Olympus FV-1000 fluorescence microscopy range of 0-50 µM of [Cu2+] and it was observed that a
system.
detection limit of 0.5 µM is sufficient to sense Cu2+ ion in the
blood samples. Using the fluorescence titration data, the
binding constant of β-CD:1
with Cu2+ in aqueous solution was
found to be (1×10-6 mol L-1). The detailed absorption and
emission changes are shown in figures S12-S16 in supporting
information.
3. Results and Discussion
Compounds 1-3 were synthesized in 75%, 76%, and 85% yields
respectively via a two-step procedure, (Scheme 1). The
1
structures of 1-3 were confirmed by H-NMR, 13C- NMR, and
To have a clear insight into the role of 1, which behaves as an
additional binding site for Cu2+ ion along with β-cyclodextrin,
we have synthesized β-CD:2 and β-CD:3 complexes, bearing 6-
hydroxy and 7-hydroxy units on the coumarin moiety,
respectively. The above complex was tested for fluorescence
changes upon addition of copper ion as well. As seen in Figure
2, unlike β-CD:1, neither of them showed any distinct
absorption or fluorescence changes upon addition of Cu2+ ion.
This powerfully supports that the hydroxyl group in the 5th -
position of the coumarin moiety plays an important role for
the Cu2+ complexation.
ESI-MS data.
Scheme 1 Synthetic pathways to 1-3
The absorption and fluorescence properties of compound 1, 2
and
3 were studied in water, which showed the dual
absorption band at 278 nm and 336 nm (Figure S12).
Remarkably, the fluorescence bands of 1 were located at 465
nm when excited at 346 nm (Figure S12). Compound
1undergoes inclusion complex formation with β-cyclodextrin
(β-CD:1), which can induced Intramolecular Charge Transfer
(ICT). Figure 1 shows changes in absorption and fluorescence
spectra of an aqueous solution for β-CD:1 spectra upon
addition of Cu2+. Addition of 50 equivalents of Cu2+ produced a
ratiometric response in its absorption. The absorption maxima
increased at 278 nm and decreased at 336 nm, with an
isobestic point at 295 nm. Also the absorbance at 278 nm
remains constant, upon further addition of copper ions. On the
other hand, the fluorescence maximum at 465 nm was
quenched upon addition of Cu2+ ions in contrast to other metal
cations, where no significant fluorescence changes were
observed.
Figure 2 (a) Fluorescence spectra of 1-3 (5 µM) their β-CD complexes with
addition of CuCl2 (10 equiv. respectively) in aqueous solution with excitation at
350 nm. (b) Relative response at 475 nm of 1-3 (5 µM respectively) with addition
of CuCl2 (10 equiv. respectively) in aqueous solution with excitation at 350 nm.
The binding constant of CD:1 with Cu2+ ions was estimated to
be 5.5×102 mol L-1 from the fluorescence titration and the limit
of detection (LoD) was found to be 2.52×10-10 mol L-1 (Fig S18
in Supporting Information). The other metal ions such as Ag+,
Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Hg2+, Pd2+, Cd2+,
Sn2+, Pb2+, Al3+, Cr3+, Fe3+ and In3+ were added to CD:1 but
insignificant changes were observed in the fluorescence
spectra. To elucidate the binding stoichiometry between CD:1
and Cu2+, a continuous variation method (Job’s plot) was
employed using the fluorescence emission data. The maxima
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are observed when the mole fraction of Cu2+ reached 0.5, primary side of β-CD cavity (mode A, figure 4) and coumarin
which indicates 1:1 binding. (Figs. S17-S18). The heavy metal group of 1stays outside (mode B, figure 4) has a lower
ions such as Cu2+, Cd2+, Hg2+, and Pb2+ tend to quench the complexation energy (-E = -83.4251 kcal. M-1) is preferred over
luminescence through electron- and/or energy-transfer the other mode such as coumarin group penetrates inside the
processes.10 Additional evidence was also obtained from ESI- primary side of β-CD cavity (mode B, figure 4), (-E = -72.8448
MS spectrum (Figure S19) wherein initially a peak at 395.22 kcal. M-1). The relevant details are given (figure S21) in
[M+K]+ value was observed which corresponds to
1. When 1.0 supporting information.
equiv. of β-cyclodextrin and Cu2+ ions (1×10-6 C) was added, The geometry of probe 1 was optimized using DFT-B3LYP 6-
the peak at 395.22 [M+K]+ disappeared and a new peak 31G level respectively using Gaussian 03 package. The sensor
appeared at 1592.09 corresponding to [β-CD:
adduct].
1
+ Cu2+ ion + K+ β-CD:1 effective binding sides to form a 1:1 complex with Cu2+
and this supports the experimental finding obtained from Job’s
Complexation of β-CD:
studies (Figure S20). In that aromatic protons in
are compared, where in β-CD: a continuous downfield shift the
1
was also confirmed by NMR titration plot and ESI-MS analysis of the complex. DFT-calculations60,
1
and β-CD:
1
show that the HOMO of β-CD:
(Fig. 4b) whereas LUMO is with the coumarin part of the
(Fig. 4b) indicating that the intramolecular charge transfer
(ICT) takes place from thiazolidine unit to the coumarin unit.
1 rests with the thiazoline unit of
1
1
from 6.526 to 8.323 ppm occurs when 1 undergoes inclusion
1
into β-cyclodextrin cavity.
As selectivity is an important parameter for evaluating the Thus when Cu2+binds with sulphur atom in 2-position of
performance of any fluorescence sensing system, the thiazolidine ring, thiazolidine nitrogen, the 5-hydroxyl group of
selectivity studies were also extended to various other metal coumarin moiety and
ions, such as Ag+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, cyclodextrin, the ICT is inhibited resulting in remarkable
Hg2+, Pd2+, Cd2+, Sn2+, Pb2+, Al3+, Cr3+, Fe3+ and In3+. From the fluorescence quenching. In the titration of β-CD: with Cu2+,
a primary hydroxyl group of β–
1
results, shown in Figure 3(a), it can be seen that Cu2+ ions the quenching process is consistent with the coordination
induced a fluorescence quenching, whereas other metal ions sulphur atom in 2-position of thiazolidine ring, thiazolidine
led to a very slight or no fluorescence change. This result nitrogen, the 5-hydroxyl group of coumarin moiety and a
indicates clearly that the present β-CD:1 based sensor has primary hydroxyl group of β–cyclodextrin. DFT calculations on
good selectivity towards Cu2+ ion over other competitive
1
and its HOMO, LUMO form indicated that electron transfer
cations.
takes place from thiazolidine group to coumarin group by
The effect of other coexisting cations on copper ion sensing Intramolecular Charge Transfer (ICT) process.
was also determined. The fluorescence responses of the
sensing system toward Cu2+ ions in the presence of alkali,
alkaline earth and other transition metal ions are shown in
Figure 3(b). The presence of the selected metal ions does not
interference with Cu2+ binding to the probe, indicating that
these co-existing ion have negligible interference effects on
Cu2+ sensing by the β-CD:1 system.
Figure 3 a) Bar chart illustrating fluorescence response of CD:1 and one equiv. of
various metal ions in aqueous solution (λex = 350 nm, λem = 475 nm, slit: 5 nm/5 nm). b)
Figure 4 (a) CVFF optimized inclusion complex of β-CD:1. In mode A. Inclusion of
Bar chart illustrating fluorescence response in the selectivity of CD:1 (6.0 × 10-6 mol L-1
)
thiazolidine part in β-cyclodextrin. In mode B: inclusion of coumarin part in β-
cyclodextrin. (b) HOMO and LUMO of 4-((benzo[d]thiazol-2-ylthio)methyl)-5,7-
dihydroxy-2H-chromen-2-one (1) calculated with DFT/TD-DFT at B3LYP/6-31G (d)
level using Gaussian 03.
for Cu2+ ion in the presence of other metal ions (6.0 × 10-6 mol L-1). The blue bars
represent the fluorescence intensity of CD:1 in the presence of one equivalent of the
other metal ions. The brown bars represent the change in fluorescence intensity that
occurs upon subsequent addition of one equivalent of Cu2+ to the solution containing
CD:1 and the other metal ions in aqueous solution (λex = 350 nm, λem = 475 nm, slit: 5
nm/5 nm).
The proposed sensing of β-CD:1 in present system based on
ICT mechanism. Before the addition of β-CD, compound 1
shows the emission at 465 nm. After addition of β-CD, the
emission intensity has increased and also a red shift has
occurred (from 465 nm to 475 nm), owing to the interaction
between the probe 1 with β-CD. The (ICT) from the electron
The proposed structure for β-CD:1 also finds strong support
from molecular modeling studies. In the proposed structure of
β-CD:1, the mode of binding of 1 to β -CD plays a crucial role.
Inclusion in which the thiazolidine group penetrates inside the
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rich portion (thiazolidine group) to the electron withdrawing complex (
portion coumarin aided by the presence of β-CD takes place using the fluorescent quantum yield of quinine sulfate as
Φ
1:Cu2+) were compared, which were calculated
and the whole process is shown in scheme 2.
reference, in 0.1 M H2SO4 solution (ΦQs = 0.54). It was found
that the Φ1 had decreased from 0.52 to 0.06 (Φ1:Cu2+) upon
addition of equimolar Cu2+ ion. These observations clearly
support the suppression of ICT process upon binding of Cu2+
ion to β-CD:1.
The potential of biosensing molecules to selectively examine a
guest species in living cells is of great significant for biological
applications.12 The cytotoxicity of CD:1 to HeLa cells was
evaluated by standard MTT (3-(4,5- dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide) assay. The results clearly
indicate that the CD:1 was non-toxic to the HeLa cells under
the experimental conditions (fig S24). The HeLa cell culture is
utilized to find out the Cu2+ sensing by fluorescence
microscope imaging technique (Figure 5 a-d). The CD:1 treated
cells showed enhanced fluorescence in the intracellular region,
which implicates the fluorescence behavior of the CD:1. After
treatment using 10 micromolar solution of Cu2+, the
fluorescence in the cellular region decreased, which shows
that after forming complex of Cu2+, the probe becomes non-
fluorescent complex. Similarly, addition of 50 micromolar
solution of Cu2+, also leads to the quenching of fluorescence.
Scheme 2 Proposed mechanism of Cu2+ sensing by β-CD probe 1 system
The red shift maybe interpreted as an indication of the
existence of charge transfer in the excited state involving the
promotion of the lone pair of electrons of thiazolidine group
into a π-π* anti bonding orbital of the coumarin ring with help
of β-CD. When Cu2+ is added, it forms a complex involving
sulphur atom in 2-position of thiazolidine ring, thiazolidine
nitrogen, the 5-hydroxyl group of coumarin moiety and a
primary hydroxyl group of β–cyclodextrin. Due to this
complexation, the intramolecular charge transfer (ICT) is
suppressed. Control experiments show that when hydroxyl
groups are present in the 6th and 7th position of coumarin
moiety as in probe (2) and (3), respectively, no change in
fluorescence intensity is observed when Cu2+ ion is added. This
indicates that only with β-CD:1, strong binding of Cu2+ ion has
occurred.
Upon addition of β-cyclodextrin to 1, the absorption band at
278 nm has slightly increased, and this indicates formation of
inclusion complex β-CD:1. Similarly the intensity of the band at
336 nm has decreased which undergoes a significant increase,
compared to other metal ions indicating a more efficient
coordination of Cu2+ ion with sulphur atom in 2-position of
thiazolidine ring, thiazolidine nitrogen, the 5-hydroxyl group of
coumarin moiety and
a primary hydroxyl group of β–
Figure 5 Bright field images and fluorescence images of HeLa cells. (a) Bright-field
image of HeLa cells (b) Fluorescence image of HeLa cells were treated with CD:1
(10 micromol in PBS buffer) for 30 minutes (c) The CD:1 treated cells were again
incubated with Cu2+ ions in 10 micromol (d) 50 micromol concentrations for 10
minutes.
cyclodextrin. Control experiments in the absence of β-
cyclodextrin there is with 1 alone indicate that, there is no
negligible change in fluorescence intensity, when Cu2+ is
added.
When Cu2+ ion is added the charge transfer was arrested due
to their strong complexation of Cu2+ with β-CD:1. In the case of
other metal ions, the absence of significant binding with the
sulphur atom is the main reason for the absence of any
significant emission response. In addition, it is well known that
Cu2+ ion (a borderline acid), preferentially interacts with the
nitrogen, sulphur and hydroxyl atom according to Pearson’s
HSAB theory. Thus the changes of fluorescence spectrum,
which is induced through the interaction of Cu2+ ion with the
lone-pair of electrons on the nitrogen, sulphur and –OH atom
are clearly justified.
The above results clearly represent that β-CD:
increasing the cell viability, which was decreased by the
addition of Cu2+. Thus, the present β-CD:
system can be
1 is helpful for
1
employed as a suitable fluorescence chemosensing probe for
Cu2+ ions in biological systems.
4. Conclusions
A highly sensitive and selective fluorescent sensing of Cu2+ ion
is reported using the inclusion complex of 4-((benzo[d]thiazol-
In order to find out ICT suppression process, fluorescent
2-ylthio)methyl)-5,7-hydroxy-2H-chromen-2-one
1 with β -
quantum yields (
Φ) of the β-CD:1 (Φ1) and β-CD:1
:Cu2+
cyclodextrin (β-CD:
1
) in aqueous medium. To the best of our
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