Luminescent Benzothiazole-Based Fluorophore of Anisidine Scaffoldings: a “Turn-On” Fluorescent Probe for Al3+ and Hg2+ Ions

Article abstract of DOI:10.1007/s10895-017-2148-5

A new anisidine possessing benzothiaozle-based chemosensor (1) has been designed and synthesized. The chemosensor 1 was designed to provide hard base environment for ratiometric detection of comparatively less studied Al³⁺ ions. In CH₃

Full text of DOI:10.1007/s10895-017-2148-5

J Fluoresc
DOI 10.1007/s10895-017-2148-5
SHORT COMMUNICATION
Luminescent Benzothiazole-Based Fluorophore of Anisidine
Scaffoldings: a “Turn-On” Fluorescent Probe for Al3+ and Hg2+
Ions
Gargi Dhaka1 · Navneet Kaur1 · Jasvinder Singh1
Received: 8 June 2017 / Accepted: 18 July 2017
© Springer Science+Business Media, LLC 2017
Abstract A new anisidine possessing benzothiaozle-based
chemosensor (1) has been designed and synthesized. The
chemosensor 1 was designed to provide hard base environ-
ment for ratiometric detection of comparatively less studied
Al3+ ions. In CH3CN, the fluorescence spectra of chem-
osensor 1 red shifted from 368 to 430 nm with addition of
Al3+ and Hg2+ ions; while Cu2+ ions caused quenching of
emission intensity of 1. These differential changes observed
with Al3+ and Cu2+ ions addition enabled chemosensor 1 to
construct “NOR” and “TRANSFER” logic gates.
human health [5]. Similarly, the development of fluorescent
probes for heavy transition metal ions such as Hg2+ and Cu2+
is also of particular interest, due to their biological and envi-
ronmental consequences like prenatal brain damage, seri-
ous cognitive disorders and Minamata diseases [6–9]. For
these reasons, substantial efforts have been made thus far to
establish fluorescent chemosensors of different architectures
for the selective detection of Al3+, Hg2+ and Cu2+ ions [10,
11]. However, compared with other metal ions, the detec-
tion of Al3+ has always been challenging because of its poor
coordination ability, strong hydration ability and the lack of
spectroscopic characteristics [12]. Hitherto, a small number
of fluorescent chemosensors for Al3+ had been developed
[13–16]. Nitrogen and oxygen-rich coordination surround-
ings can offer a hard base environment for the hard acid Al3+
[17]. Therefore, the design and synthesis of Al3+ selective
fluorescent probes have fetched considerable attention [18,
19]. In relation to other methods [20, 21], the fluorescence
method can provide a simple and cost-effective detection
with high sensitivity, short response time, good selectivity
and real-time monitoring [22, 23]. Moreover, Benzothiazoles
are renowned to be good ligands for metal ions such as Hg2+
and Cu2+ due to the presence of N and S heteroatoms [24].
Also, the development of single molecular sensors capable
of detecting multiple metal analytes with different spectral
responses is still a great challenge [25]. So, chemosensor 1
has been designed keeping in mind the hard base environ-
ment provided by methoxy of anisidine ring and nitrogen
of benzothiazole ring which will also serve the purpose for
multi analyte detection.
Keywords Benzothiazole · Fluorescent chemosensor ·
Ratiometric · Metal ion probe · Molecular logic gates
Introduction
Trivalent metal ions have vital biological properties and are
in a straight way involved in the cell function where there is
a serious control of M3+ levels [1]. For example, Al3+ have
injurious effects on human health, as an excessive amount of
Al3+ in the brain is believed to cause neurodementia, includ-
ing neurological disorders such as Alzheimer’s disease, Par-
kinson’s disease and dialysis encephalopathy [2–4]. Thus,
the monitoring of Al3+ is imperative to control the concen-
tration levels in the biosphere and diminish direct effects on
Electronic supplementary material The online version of this
article (doi:10.1007/s10895-017-2148-5) contains supplementary
material, which is available to authorized users.
In recent times, the advances in chemosensing field have
attracted huge attention in the construction of photonic
devices [26] that function as molecular level logic gates [27,
28] based on the optical sensing of particular analytes. The
customized chemosensors can be sighted as computational
* Navneet Kaur
neet_chem@yahoo.co.in; neet_chem@pu.ac.in
1
Department of Chemistry, Panjab University,
Chandigarh 160014, India
Vol.:(0123456789)
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devices that use physical or chemical inputs to generate out-
puts based on a set of logical operators. In the molecular
recognition field, the objective is often selectivity, that is,
a specific analyte persuade specific change in color or fluo-
rescence. In contrast, molecular logic gates looks for more
complex systems, which usually enclose multiple binding
sites and show certain exclusive response patterns, demon-
strating the desired logic operation in the presence of either
or both inputs [29]. The chemically encoded information was
transformed into the optical changes as the output, which
outcomes in development of molecular logic gates such as
OR, NOR, AND, INHIBIT, YES, NOT, XOR, and XNOR
logic gates. The integration of these molecular logic gates
at molecular stage is allied to the integration and process-
ing of binary data of conventional microprocessor systems
[30]. The rapidity of the conventional computers can be
improved by miniaturizing electronic components to a very
tiny micron-size scale so that those electrons have to travel
only very short distances within a very short time [31].
(ppm): 162.7, 157.2, 152.2, 136.2, 131.6, 129.7, 125.8,
124.5, 122.9, 122.5, 121.2, 121.1, 111.5, 55.5; ESI-MS:
m/z (relative abundance (%), assignment) = 242.1 [100,
(M+1)+]. (Found C, 69.59; H, 4.62; N, 5.76%. C14H11NOS
requires C, 69.68; H. 4.59, N, 5.80%).
General Procedure for Spectroscopic Analysis
Preparation of Stock Solutions
Stock solutions of metal ions (10− 1 M) were prepared in
DMSO using their perchlorate salts. Stock solutions of the
receptor 1 (10− 2 M) was prepared in DMSO, which were
further diluted with CH3CN (99:1, CH3CN:DMSO, v/v) and
used further for different spectroscopic experiments.
Fluorescence and UV–vis Absorption Selectivity
Experiments
For the different absorption and fluorescence experiments,
all the metal ions (Na+, K+, Mg2+, Al3+, Mn2+, Fe2+, Co2+,
Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+) were added as their per-
chlorate salts. Aliquots of ions under investigation were then
injected into the sample solution through a rubber septum
in the cap. The solutions were allowed to get stabilized after
each addition, and were then scanned. Excitation wavelength
was fixed at 300 nm in case of 1.
Experimental Section
Reagents and Apparatus
All reagents and chemicals were purchased from Aldrich and
used without further purification. Solvents used for spectro-
scopic studies were purified by standard procedures before
use. 1H NMR spectra were recorded on a BrukerAvance II
400 at 400 MHz spectrometer with TMS (trimethylsilane) as
internal standard, and the chemical shifts (δ) were expressed
in ppm and coupling constants (J) in Hertz. Absorption spec-
tra were recorded on Shimadzu UV-240 spectrophotometer.
Fluorescence spectra of solutions were recorded on Hitachi
F–7000 equipped with 220–240 V Xe lamp.
Results and Discussion
Scheme 1 outlines the synthesis of the chemosensor 1 and
the desired product was explicitly characterized by NMR
spectra (1H NMR, 13C NMR), FT-IR and Mass spectrum
(Fig. S1–S2).
The photophysical properties of 1 (10 µM) was investi-
gated in CH3CN solution by recording both UV–vis. and
fluorescence spectra. The UV–vis spectra of 1 showed an
absorption maximum at 321 nm, which remained unper-
turbed upon addition of different cations such as Na+, K+,
Mg2+, Al3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and
Hg2+ (Fig. S3).
Synthesis of 2-(2-methoxy-phenyl)benzothiazole (1)
2-Aminothiophenol (250 mg, 3.5 mmol) and 2-methoxy-
aldehyde (275 mg, 3.5 mmol) were mixed thoroughly and
taken in a round bottom flask (RBF, 50 mL) placed in an
alumina bath inside a microwave oven and irradiated under
640 W for 10 min with an interval of 2 min between two
halves of the duration [32]. After completion of reaction
(TLC monitoring), the crude product was directly recrystal-
lized from ethyl acetate to yield light brown colored product
1. Yield (%) 82; m.p. (°C) 106–110, FT-IR (cm−1) 2923 m
(ν Ar-H str.), 2029 m (νAr−C=C str.νAr−H str.), 1620s (νC=N),
1494 m (νC−N); 1H NMR (CDCl3, 300 MHz) δ (ppm): 3.95
(s, 3H, - OCH3), 6.93 (d, 1H, J=6.0 Hz, Ar-H), 7.04 (t, 1H,
J1 =J2 =6.0 Hz, Ar-H), 7.24– 7.41 (m, 3H, Ar-H),7.82 (d,
1H, J=6.0 Hz, Ar-H), 7.99 (d, 1H, J=6.0 Hz, Ar-H),8.47
(d, 1H, J = 6.0 Hz, Ar-H); 13C NMR (CDCl3, 75 MHz) δ
On the other hand, the fluorescence spectra of 1 (10 µM;
λex 300 nm) displayed emission maximum at 368 nm.
S
SH
H3CO
CHO
MW
N
H3CO
1
NH2
Scheme 1 Synthetic pathway of chemosensor 1
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Addition of Hg2+ and Al3+ ions to solution of 1 resulted
in new red shifted emission band at 430 nm; whereas Cu2+
addition resulted in quenching of fluorescence intensity of
1 (Fig. 1a). Moreover, presence of other metal ions did not
perturb the fluorescence emission spectra of 1. The fluores-
cence changes observed with addition of Al3+ / Hg2+ / Cu2+
ions were also associated with fluorescent color changes
under illumination with UV lamp at 365 nm (Fig. 1b).
For quantitative analysis of interaction of 1 with different
metal ions, fluorescence titrations of 1 with Al3+/ Hg2+/ Cu2+
ions were carried out. Upon gradual addition of Al3+ metal
ion (0–50 equiv.) to the solution of 1 (10 µM; λex300 nm),
the emission spectra showed ratiometric behavior in which
the band centered at 368 nm decreased, while the intensity
of a new band at 430 nm enhanced concomitantly (Fig. 2a).
The appearance of isoemissive point at 400 nm clearly sup-
ported the existence of more than one species in the medium.
Similar type of ratiometric change has been observed
upon addition of Hg2+ ions to solution of 1 (Fig. 2b). The
observed significant red shift in the photophysical behavior
of 1 upon interaction with Al3+/Hg2+ ions has attributed to
the transformation from the flexible structure of 1 to a rigid
one upon metal complexation, which was most likely caused
by increased intramolecular charge transfer (ICT) transition
within the whole molecule arising due to Al3+ / Hg2+ ion
binding as well as the inhibition of photoinduced electron
transfer (PET) from nitrogen donors [33]. The Job’s plot [34]
suggested a 1:1 stoichiometry for a 1.Mn+ (Mn+ = Al3+ or
Hg2+) interaction (Fig. S4).The binding constants have been
estimated through the Benesi–Hildebrand (B–H) method
[35] and were found to be Ka =2.9×106 and 8.8×105 M−1
,
respectively. The limit of detection (LOD) of 1 for Al3+and
Hg2+ ions was then evaluated based on the titration profiles
using the equation LOD=3σ/slope and was estimated to be
31 and 40 µM, respectively [36].
Figure 3a shows the emission titration spectra of 1
with incremental addition (0–60 equiv.) of Cu2+ ions. The
decrease in fluorescence (~ 70%) intensity was observed
at 368 nm because Cu2+ ions typically have a pronounced
quenching effect on fluorophores by mechanisms inherent
Fig. 1 a Fluorescence spectra and b Fluorescence color changes of 1 (10 µM; λex300 nm) upon addition of 100 equiv. of various metal ions as
their perchlorate salts in CH3CN and under illumination with UV lamp at 365 nm
Fig. 2 Fluorescence spectra of 1 (10 µM; λex300 nm) upon addition of (a) Al3+ and (b) Hg2+ ions; Inset: Binding isotherms at 368 and 430 nm
in CH3CN
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Fig. 3 a Fluorescence spectra of 1 (10 µM; λex 300 nm) upon addition of Cu2+ions; Inset: Binding isotherms at 368 nm and b Job’s plot of 1 for
Cu2+ ions in CH3CN
to the paramagnetic species [37]. The stoichiometry of the
fluorescence quenching interaction between receptor 1 and
Cu2+ was further evaluated by Job’s plot method and found
to be in the ratio of 1:1 (Fig. 3b). The binding constant
and LOD were also determined to be 2.4 × 105 M− 1 and
28 µM, respectively.
The molecular switching behavior of chemosensor 1 with
two chemical inputs (20 equiv. of Al3+ and Cu2+) can be
demonstrated with the help of binary logic. The input can
take values of 0 and 1 corresponding to the absence or the
presence of input. The output, the fluorescence intensity at
a given wavelength, can be of values 0 and 1 depending on
the fluorescence intensity value below or above a certain
threshold.
The differential behavior of chemosensor 1 towards
Al3+ and Cu2+ ions enabled chemosensor 1 to elaborate
molecular logic gates i.e. NOR and TRANSFER at two
different wavelengths. Now a days, unconventional com-
puting based on chemical reactions has stimulated interest
in the development of molecular logic gates and devices
at the molecular level. Molecular logic gates convert input
stimulations into output signals with intrinsic protocols. It
follows that the principles of binary logic can be applied
to the signal transduction operated by molecular switches.
For the analysis of their logic behaviors, first of all, thresh-
old values are assigned and then logic convention is used
for their inputs and outputs signals [29].
There are two input signals, viz. In1 (Al3+) and In2
(Cu2+), whereas the output signals are, Out1 (Fl368 nm) and
out2 (Fl430 nm). Figure 4a shows different fluorescence spec-
tra observed with addition of two inputs. Monitoring the
absorption changes at 368 and 430 nm i.e. the addition of
Al3+, Cu2+ and an equimolar mixture of Al3+ and Cu2+, lead
to “NOR” and “TRANSFER” logic gates, respectively as
represented in truth table (Fig. 4b).
“NOR” gate is a combination of OR gate followed by an
inverter. The output of NOR gate is high when both inputs
are low [38]. The fluorescence intensity at 368 nm was
decreased with addition of either Al3+ / Cu2+/ both of the
a
b
In1
In2
Out1
Out2
(Al3+) (Cu2+) (Fl368
)
(Fl430)
0
1
0
1
0
0
1
1
1
0
0
0
0
1
0
1
Al3+
Cu2+
Fl380
TRANSFER
NOR
Fig. 4 a Molecular scale implementation of different logic gates; (i).blank; (ii).1+Al3+; (iii).1+Cu2+; (iv).1+Al3++Cu2+ and b Truth table for
“NOR” and “TRANSFER” gates
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ions i.e. the output was read as “0” in the presence of Al3+
/ Cu2+ ions (Fig. 4).
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Moreover, by changing the observation wavelength from
368 to 430 nm, the spectral changes led to “TRANSFER”
logic gate [39]. The output of TRANSFER gate is high
when either one of the inputs or both of the inputs are high
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Conclusion
In conclusion, we have designed and synthesized simple tai-
lor made benzothiazole chemosensor possessing anisidine
ring (1) for providing hard base environment for ratiometric
detection of Al3+ ions. Apparently, 1 could be used as single
molecular multianalyte fluorescent probe for the determina-
tion of Al3+ and Hg2+ ions via fluorescent enhancement;
while Cu2+ ions via fluorescence quenching in a wide con-
centration range along with different color changes. These
differential absorption changes were responsible for con-
struction of “NOR” and “TRANSFER” logic gates.
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Acknowledgements The authors are greatly thankful to SAIF, Pan-
jab University Chandigarh for recording the NMR and Mass spectra
and are grateful to DST (grant no. SR/FT/CS-36/2011), UGC (grant
no. AB2/12/3115) and DST PURSE-II (Grant no. 48/RPC) for the
fellowship.
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