A simple benzimidazole styryl-based colorimetric chemosensor for dual sensing application

Article abstract of DOI:10.1016/j.saa.2019.02.009

A new colorimetric styryl-benzimidazole based receptor to recognize more than one analyte trans-2-[4′?(dimethylamino)styryl]benzimidazole) (L1) has been synthesized and fully characterized by ¹³C and ¹H NMR, elemental analysis, UV–vi

Full text of DOI:10.1016/j.saa.2019.02.009

Accepted Manuscript
A simple benzimidazole styryl-based colorimetric chemosensor
for dual sensing application
Daulat Phapale, Archana Kushwaha, Dipanwita Das
PII:
DOI:
Reference:
S1386-1425(19)30118-0
https://doi.org/10.1016/j.saa.2019.02.009
SAA 16785
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Received date: 26 October 2018
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25 January 2019
4 February 2019
Please cite this article as: D. Phapale, A. Kushwaha and D. Das, A simple benzimidazole
styryl-based colorimetric chemosensor for dual sensing application, Spectrochimica Acta
Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/
j.saa.2019.02.009
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ACCEPTED MANUSCRIPT
A simple benzimidazole styryl-based colorimetric chemosensor
for dual sensing application
Daulat Phapale, Archana Kushwaha and Dipanwita Das*
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019
(India)
Fax: (91)022-3361-1020
E-mail: dr.das@ictmumbai.edu.in
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ABSTRACT
A new colorimetric styryl-benzimidazole based receptor to recognize more than one analyte
trans-2-[4/-(dimethylamino)styryl]benzimidazole) (L1) has been synthesized and fully
13
1
characterized by C and H NMR, elemental analysis, UV-vis spectroscopy, and HRMS.
Investigation of sensing ability of receptor L1 was carried out in presence of multiple anions
−
−
−
−
−
(Br−, CN−, Cl−, ClO4 , F−, HSO4 , PF6 , NO3 , S2−, OH−, AcO− and H2PO4 ) and cations
(Cu2+, Cr3+, Al3+, Mg2+, Cd2+, Ni2+, Fe3+, K+, Fe2+, Mn2+, Ag+, Hg2+, Ca2+, Co2+, Pb2+, Na+,
and Zn2+) by using UV-vis spectroscopy. Receptor L1 showed colorimetric response towards
−
−
1
only for HSO4 ion. Receptor L1-HSO4 interaction confirmed with the help of H NMR
titration. Among various cations, L1 selectively sense the Cu2+ and Al3+ with the drastic
colour change from yellow to green and dark yellow respectively. The stoichiometric binding
−
ratio of L1 with HSO4 , Cu2+, and Al3+ found to be 1:1 by jobs method and HRMS data
proved the complex formation between L1 and Cu2+/Al3+ with very low detection limit. In
addition to explore practical applicability of L1, paper strips have been made and used to
−
detect HSO4 and Cu2+ ions, respectively, up to 10 ppm level.
Keywords:
Chemosensor; Benzimidazole; Colorimetric; Dual Sensing
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1. Introduction
The escalation of new multi-channel chemosensor selectively detect a metal cation and
anion which are biologically and physiologically important, therefore, this area of research
attract the attention of researchers due to their significant applications in biological, industrial
and environmental processes [1-8]. A single chemosensor detecting the multiple analytes
colorimetrically with the different photo-physiochemical changes is still a challenge for the
scientific community [9-11]. For the detection of analyte particularly cation or anion, the
sensor should have high selectivity and sensitivity towards a specific ion, which shows naked
eye detection, low cost and operational simplicity of the instrument and environmentally
friendly properties [12-16]. Anions have always attracted a huge interest due to their
significant role in industries such as cyanide which has been used for electroplating and
metallurgy [17-20], fluoride has been used in mouthwash [21,22], chloride has been used as a
disinfectant [23,24], phosphates, sulfate, and nitrates are used as fertilizers [25,26]. Among
−
various anions detection of HSO4 ion is an emerging area due to its potential application in
−
2−
industries, fertilizers, preservative, cosmetic and disinfectant [25-28]. HSO4 and SO4 are
also found in human plasma and utilized for the growth of the organism [29,30]. On the other
−
2−
hand, HSO4 at a higher pH dissociate into H+ and sulfate ion (SO4 ) of which sulfate is
toxic to human health causing skin and eye irritation, sometimes respiratory paralysis
−
2−
−
[11,13,31]. According to Hofmeister, bias anions such as HSO4 , SO4 , and H2PO4 having
tetrahedral geometry is difficult for detection due to their high hydration energy around
−
−1080 KJmol−1 for HSO4 [29,32-34]. Thus, its detection in a trace amount of the analyte is
−
the main concern for environment and health. The unregulated amount of HSO4 is sensitive
for health as it causes cathartic effects which include gastrointestinal infection, dehydration,
−
and skin irritation [11,29,31,35,36]. HSO4 based sensing system generally involves two
types of interaction i.e. hydrolysis and hydrogen bonding interaction [27,37-40]. A large
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variety of optical chemosensors have been reported for detection of CN− and F− due to their
small size and high electronegativity [41-44]. But, for such type of interaction, only a few
reports are available such as indole [45-47], quinoline [45], coumarins [31], Schiff-base [33],
stirylindolium [46], imidazoles [29], and rhodamine-based chemosensors [48].
Among cations, transition metals such as Cu2+, Al3+, Zn2+, and Fe3+ play a crucial role in
the biological system [1,3,4,49,50]. Copper is the third most abundant transition metal ion in
the human body and used for several physiological and metabolic activities such as for
growth and development, the formation of RBC’s, repairing injured tissues and fight against
infections [49-53] and involving many electron transfer reactions [52,54]. Copper plays a
very important role as a catalytic cofactor in many metalloenzymes along with nuclease,
tyrosinase, cytochrome c oxidase, and superoxide dismutase [49,50]. Irregular consumption
of copper causes drowsiness and high blood pressure problems [53,55-56]. According to the
United States Environmental Protection Agency (USEPA), the permissible limit of copper is
less than 1.3 ppm [57-59]. An excess amount of copper causes hemolyticanemia,
gastrointestinal infections, hemopoiesis, and liver damage [60-62].
The third most abundant element found in earth crust is aluminum and broadly used in
cooking appliances, production of light alloys, water purification, food processing,
pharmaceutical industries, and construction areas. WHO (World Health Organization)
recommends the daily intake limit of aluminum for the human body is approximately 3-10
mg per day and weekly intake is around 7 mg/kg of body weight [63-65]. However, the
determination of aluminum has always been complicated because Al3+ ion displays a weak
coordination, high hydration capability, and scarcity towards spectroscopic characteristics
[66]. The excess amount of either Al3+ or Cu2+ causes damage to the human central nervous
system and results in severe neurodegenerative disorders such as Alzheimer’s, Parkinson,
Menkes, and Wilson diseases [67-70]. The aluminum production factory workers may cause
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serious lung problems due to inhalation of aluminum dust. Now a days, aluminum waste
concentrations have been increased in lakes and river, which acidifies water. Hence,
determination of such ions in trace amount is remarkably significant. Therefore, a rational
design of chemosensor which can act as a dual sensor for these ions is a great demand due to
environment and health concern. But till date, only a few chemosensors have been reported
−
which can depict the multifunctional sensing with HSO4 .
Herein, we report a simple, one-step synthesis and characterization of trans-2-[4/-
(dimethylamino)styryl]benzimidazole) (L1) receptor, which shows selective colorimetric
−
chemosensor for Cu2+, Al3+ cations and HSO4 anion in presence of other inspected ions.
1
Based on the H NMR study, electronic and HRMS spectral data, it was revealed that Cu2+,
−
Al3+, and HSO4 effectively interacted with receptor L1.
2. Experimental
2.1. Material and Instrumentation
For synthesis, all the chemicals and solvents were purchased from a commercially
available supplier and used without further purification. For absorption studies of receptor L1
UV-vis spectroscopic grade methanol was used. Britton-Robinson buffer (BR) solution was
used for the pH study, adjusting with a diluted aqueous solution of HCl/NaOH. Electronic
spectra were recorded by using Shimadzu UV-1800 spectrophotometer at room temperature
with quartz cuvette of 1cm pathlength and 2 cm3 solutions throughout the study. 1H NMR and
13C spectra were recorded on a Mercury Plus 300 and 101 MHz NMR spectrometer using
CD3OD as a solvent respectively. HRMS (High-resolution mass spectra) data were obtained
from 1290 Infinity UHPLC system from Agilent Technologies. Theabsorption study of L1
monitored by addition of different analytes such as Cu2+, Cr3+, Al3+, Mg2+, Cd2+, Ni2+, Fe3+,
K+, Fe2+, Mn2+, Ag+, Hg2+, Ca2+, Co2+, Pb2+, Na+, and Zn2+ with their nitrate salts and Br−,
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−
−
−
−
−
CN−, Cl−, ClO4 , F−, HSO4 , PF6 , NO3 , S2−, OH−, AcO− and H2PO4 with their TBA
(Tetrabutylammonium) salts of 10 mM stock solution in methanol.
2.2. Synthesis of receptor trans-2-[4’-(dimethylamino)styryl]benzimidazole) (L1)
The receptor L1 was synthesized according to the previously reported literature [71] with
some modification. 4-(dimethylamino)cinnamaldehyde (1.0 mmol) and o-phenylenediamine
(1.05 mmol) were mixed in 10 ml distilled water, stirred for 10 min then boric acid (1.5
mmol) was dissolved separately in water and added slowly to the reaction mixture. The
reaction was stirred for overnight at room temperature, completion of the reaction was
monitored by TLC (Ethylacetate: Pet Ether 2:8 v/v). Excess water was added to the reaction
mixture and then the crude product was separated and collected by vacuum filtration, which
was further purified by silica column chromatography (1% Chloroform : Methanol). Yield:
69 %, Anal. calcd. for C17H17N3: C, 77.54; H, 6.51; N, 15.96, Found: C, 77.59; H, 6.65; N,
1
16.01, UV–vis (MeOH) λ/nm (ε mol-1cm-1) 269 (26950), 394 (10950), H NMR (300 MHz,
CD3OD) δ7.52 – 7.38 (m, 5H), 7.19 (m, 3H), 6.83 (d, J = 16.2 Hz, 1H), 6.70 (d, J = 8.2 Hz,
13
2H), 2.94 (s, 6H). C NMR (101 MHz, CD3OD) δ152.05, 151.42, 137.82, 136.86, 128.24,
123.42, 122.40, 113.67, 111.86, 109.61, 38.94. HRMS (ESI–MS) m/z: [M + H] + calcd. For
C17H17N3: 264.1422, found: 264.1484.
2.3. UV –Vis Measurement
.
Receptor L1(13.1571 mg, 0.05 mmol), Cu(NO3)2 6H2O( 12.08 mg, 0.05 mmol),
.
Al(NO3)3 9H2O (18.7565 mg, 0.05 mmol), HSO4 − (16.9765 mg, 0.05 mmol) and remaining
all cations/anions 5 mL (10 mM) stock solution were prepared in methanol and used for
further studies. For the systematic titration, the final concentrations 20 µM for receptor L1,
−
10 mM and 1 mM for Cu2+/ HSO4 and Al3+ were prepared in methanol and used for
absorption study. For the stoichiometric determination of receptor L1: guest ion (guest ion:
−
Cu2+, Al3+, and HSO4 ), Job’s plots were carried out. A sequential solution of guest ion 0.0,
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1.2, 2.4, 3.6, 4.8, 6.0, 7.2, 8.4, 9.6, 10.8, and 12.0 μL was added to the solution of L1 12.0,
10.8, 9.6, 8.4, 7.2, 6.0, 4.8, 3.6, 2.4, 1.2 and 0.0 μL of 10 mM stock solution respectively, the
final volume was kept constant 3 mL by adding 2.998 mL of methanol to each vial and then
absorption spectra were taken after few minutes at room temperature. A graph is plotted
between corrected absorbance vs. mole fraction of ions. Binding constant was further
calculated using Benesi-Hildebrand (B-H) equation (1) from the systematic UV-vis titration
−
of receptor L1 with Cu2+, Al3+ and HSO4 at 420 nm, 430 nm, and 527 nm absorbance
respectively.
1
1
1
=
+
(1)
푛
(퐴 −퐴 )
퐾(퐴
−퐴 )[G]
(퐴
−퐴 )
max 0
표푏푠
0
max
0
Where, A0 is the absorbance of native receptor L1, Aobs is the absorbance obtained in presence
of ion added, Amax is maximum absorbance obtained with the excess amount of ions, [G]n is
the concentration of ions (guest) added, n is the stoichiometric quotient and K is the binding
constant which is determined from slope of straight line plot of (Amax ̶ A0)/(Aobs ̶ A0) vs
−
1/[G]n. To calculate the limit of detection of receptor L1 with HSO4 , Al3+ and Cu2+, the
obtained absorption band at particular wavelength was plotted against the concentration of
analyte added which getting linear calibration curve and calculated by using equation (2).
퐿푂퐷 = 3 × 푆퐷/퐵
(2)
Where SD is the Standard deviation for blank receptor L1 solution and B is the slope of the
straight line.
3. Results and Discussion
3.1. Synthesis of receptor L1
The preparation of L1 receptor was carried out by condensation of 4-
(dimethylamino)cinnamaldehyde and o-phenylenediamine in water and boric acid was used
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1
13
as a catalyst (Scheme 1) and the confirmation of structure was done by H and C NMR,
UV-vis, and HRMS spectroscopic techniques (Figures S1-S4).
Scheme 1. Synthetic representation of receptor L1.
3.2. Anion sensing response of L1
The receptor L1 shows two absorption maxima at 269 and 394 nm corresponding to the
π ̶ π* transition. To appraise the sensing ability of receptor L1 towards different anions such
−
−
−
−
−
as Br−, CN−, Cl−, ClO4 , F−, HSO4 , PF6 , NO3 , S2−, OH−, AcO− and H2PO4 , the UV-vis
absorption study was carried out in methanol as shown in Fig. 1(a). After addition of different
−
anions, only HSO4 ion shows appreciable changes in the absorption spectrum (Fig. 1(a)).
−
The absorption titration of receptor L1 with stepwise addition of HSO4 was extensively
examined. As shown in Fig. 1(b) intensity of 394 nm absorption band decreases along with
−
527 nm absorption band gradually increasing. After addition of 40 equivalents of HSO4 , 394
nm band completely diminished, and absorption spectra get saturated with huge 133 nm
bathochromic shift with a clear isobestic point at 455 nm representing the chemical
−
interaction of receptor L1 with HSO4 ion. The large bathochromic shift arises because there
may be intramolecular charge transfer induced intermolecular hydrogen bonding between
−
receptor L1 and HSO4 ion [72]. In addition to that, with increasing polarity of solvent
(methanol or acetonitrile) the protonation occurs at imidazole nitrogen of receptor L1 and
hydrogen bonding takes place with the solvent molecules which also favored a red shift in
−
absorption spectra [73]. The binding constant was found to be 4.17 × 103 M−1 with HSO4 ion
−
shown in the Fig. S5. The stoichiometry between receptor L1 and HSO4 ion is calculated by
using jobs plot method. The plot between corrected absorbance against the mole fraction of
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−
HSO4 ion has maxima at 0.5, which indicates that 1:1 stoichiometry between receptor
−
−
L1:HSO4 ion (Fig. S6). The limit of detection (LOD), lower detectable amount of HSO4
was found to be 7.96 × 10−6 M (Fig. S7), which is much lower concentration than the
−
maximum contaminant level of HSO4 ion (250 mg/L) according to U. S. Environmental
Protection Agency (USEPA, 1994a). The receptor L1 is very sensitive to pH; at lower pH (6
̶
3) red shift was observed and the new peak appearing at 434 nm (Fig. S8).
−
For the confirmation of hydrogen bonding interaction between receptor L1 and HSO4
ion in solution, 1H-NMR titration has been carried out in a CD3OD solvent. As shown in Fig.
1
2, the H-NMR spectra of receptor L1 shows four signals (labeled as a, b, c and d) but after
−
−
the addition of HSO4 ion, the six 1H-NMR signals appeared. As shown in Scheme 2, HSO4
not only interact with imidazole proton but also showed hydrogen bonding with one of the
styryl proton. The 5-H multiplets signal (b, 7.52 ̶ 7.58 ppm) get separated with addition of
−
HSO4 ion, the downfield signal at 7.66 ppm (b*, J = 15.6 Hz) arises corresponds to a styryl
−
proton which interacted with HSO4 ion and other styryl proton (c at 6.83 ppm, J = 16.7 Hz)
−
which remains unchanged (does not interact with HSO4 ion). Both the styryl proton showed
J value in between 15 ̶ 17 Hz suggesting the trans coupling occurs between them. The
imidazole N
̶
H proton signal merged with the 3-H signal (d, 2H) at 7.19 ppm in L1 spectra
−
which get deprotonated in presence of HSO4 ion. The shifting of downfield (bʺ, 2H at 7.58
ppm) and upfield (bʹ, 2H at 7.3 ppm) because of imidazole N ̶ H deprotonation and charge
delocalization in the imidazole attached benzene ring. The significant upfield shifting was
observed from 6.7 ppm (a, 2H) to 6.33 ppm (aʹ, 2H) due to dimethyl aniline electron pushing
−
effect which clearly supports that the receptor L1 shows strong interaction with HSO4 ion
(Scheme 2).
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−
Scheme 2. The Proposed binding mechanism of receptor L1 with HSO4 .
3.3. Cation sensing response of L1
The investigation of cation sensing behavior of receptor L1 towards the 10 equivalents of
various metal ions (Cu2+, Cr3+, Al3+, Mg2+, Cd2+, Ni2+, Fe3+, K+, Fe2+, Mn2+, Ag+, Hg2+, Ca2+,
Co2+, Pb2+, Na+, and Zn2+) has been carried out by using UV-vis absorption spectroscopic
study in methanol. As can be seen in Fig. 3, only Cu2+ and Al3+ show remarkable changes in
the absorption spectra. Upon systematic addition of Cu2+ and Al3+ in receptor L1 solution
separately, the 394 nm absorption band decreasing with concomitant appearance of 420 and
430 nm absorption bands respectively (Fig. 4(a) and 4(b)). For Cu2+ and Al3+, 26 and 36 nm
red shift was observed in absorption spectra of receptor L1. Meanwhile, an isobestic point at
404 nm for Cu2+ and 400 nm for Al3+ were observed, which indicates the clean formation of
receptor L1-Cu2+ and L1-Al3+ complexes.
To explore the detail chemistry regarding L1-Cu2+ complex, the higher concentration
absorption study was performed. As shown in Fig. 4(c), in presence of Cu2+ the low energy
band in the visible region at 765 nm increases simultaneously as the 623 nm absorption band
decreased. This low energy 765 nm absorption band is not observable at low concentration
due to very low absorption coefficient value (La Porte partially allowed d-d transition, ε =
280) comparatively UV transitions [60]. The 1:1 stoichiometry between receptor
L1:Cu2+/Al3+ has been confirmed by Job’s plot (Fig. S9). To investigate the binding affinity
between L1 and Cu2+/Al3+, Benesi-Hildebrand equation was used and binding constant
determined to be 3.3056 × 103 M−1 for Cu2+ and 5.8139 × 103 M−1 for Al3+ by using UV-vis
10

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titration (Fig. S10). Interestingly, the LOD of Cu2+ and Al3+ were found to be 4.09 × 10−7 M
and 15.7 × 10−9 M respectively which is much lower than the permitted amount of USEPA
and WHO guidelines (Fig. S11).
The study of competing metal ions on the sensing ability of receptor L1 with Cu2+ and
Al3+ were done by using UV-vis spectroscopic techniques. This study was executed by
mixing receptor L1 with interfering metal ions such as Cr3+, Mg2+, Cd2+, Ni2+, Fe3+, K+, Fe2+,
Mn2+, Ag+, Hg2+, Ca2+, Co2+, Pb2+, Na+, and Zn2+ then into that Cu2+ and Al3+ solutions were
added separately and the solution mixed uniformly then absorption spectra were taken at
room temperature. The absorption spectrum of interfering cations, similar change observed to
that of absorption spectra obtained after addition of Cu2+ and Al3+ to the receptor L1 (Fig. 5).
The achieved outcome about competitive selectivity experiment of receptor L1 with Cu2+ and
Al3+ suggest that the receptor L1 is selective in sensing Cu2+ and Al3+ ions with competing
metal ions. Furthermore, High-resolution mass spectroscopic study was employed to inspect
the binding mode of receptor L1 towards Cu2+ and Al3+. The positive-ion mass spectrum of
receptor L1 with Cu2+ indicated that the peak at m/z: 461.26 assignable to
− +
[L1+Cu2++NO3 ] +4H2O calcd.m/z: 461.13 (z = 1) (Fig. 6) and Al3+, peak at m/z: 344.20
− 2+
designated to [L1+Al3++NO3 ] +4H2O+3CH3OH calcd. m/z: 344.24 (z = 2) (Fig. 7)
(Scheme 3).
Scheme 3. The Proposed complex formation mechanism of receptor L1 with Cu2+ and Al3+.
−
For naked eye detection, only HSO4 (yellow to dark red), Cu2+ (yellow to green) and
Al3+ (yellow to dark yellow) ion showed colour change, however with other ions, no
significant colour change was observed (Fig. 8). The drastic colour change of receptor L1 in
11

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-
the presence of excess HSO4 ion could be explained due to the NH proton dissociation
followed by the intramolecular charge transfer (ICT) through conjugation and the
deprotonated species absorb the appropriate visible region light which leading to the colour
change in the system. The metal ions (Cu2+ and Al3+) bind to the receptor L1, affects the
electronic ground and excited state of receptor L1 with respect to the corresponding
electronic state of uncomplexed receptor L1. The other possibility could be that charge
transfer from metal to ligand takes place via π conjugation due to the electron richness of
metal ion in metal complexes.
3.4. Test paper strip application
To investigate the applicability of receptor L1, test paper strip experiments were carried
out. The test paper strip was prepared by dipping paper into the receptor L1 (0.001 M) in
dichloromethane and air dried at room temperature. Then paper strips were immersed in the
−
solutions of HSO4 and Cu2+ ions with different concentrations in methanol, exhibit
significant colour change which can be visualized by naked eyes (Fig. 9)
4. Conclusion
In conclusion, among all anions and cations, the styryl-imidazolium based receptor L1
−
provided an easy, simple and effectual way for sensitive recognition of HSO4 , Cu2+, and Al3+
−
ions by using UV-vis spectroscopy. The mechanistic and binding study of L1 with HSO4 ,
Cu2+ and Al3+ ions was confirmed by 1H NMR titration and ESI-MS data. Receptor L1 found
−
to be an excellent candidate for the detection of HSO4 , Cu2+ and Al3+ ions over the other
−
competitive ions with a very low limit of detection (7.96 µM for HSO4 , 0.4 µM for Cu2+ and
15.7 nm for Al3+) and distinct colour change was visualized by naked eyes. For practical
−
applications, paper strips have been successfully made for the naked eye detection of HSO4
and Cu2+ ions.
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Acknowledgment
This work financially supported by DST-INSPIRE Faculty research grant (IFA–13
CH–94) and DST/SERB/YSS/000241. AK and DP are grateful to the Department of Science
and Technology INSPIRE, Govt. of India for project fellowship. Special thanks to the
Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology, Bombay
(IITB), for providing the ESI–MS facility and Department of Pharmaceutical Science and
1
13
Technology, ICT, Mumbai, for providing H and C NMR facilities. Institute of Chemical
Technology (ICT) is acknowledged for providing the research facilities.
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J
o
Fig. 1. Changes in UV-vis absorption spectra of receptor L1 (2 × 10−5 M) (a) with the
addition of 10 equiv. of various anions (0.01 M) as their TBA salts in CH3OH and (b)
−
systematic titration with HSO4 (0.01 M) in CH3OH. Inset: Plot of absorbance at 527 nm
−
versus equivalent of HSO4 added.
14

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−
Fig. 2. 1H-NMR spectral titration of receptor L1 with HSO4 ion in CD3OD.
15

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Fig. 3. Changes in absorption spectra of receptor L1 (2 × 10−5 M) with the addition of 10
equiv. of various cations (0.01 M) as their nitrate salts in CH3OH.
16

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Fig. 4. Systematic absorption spectral titration of (a) receptor L1 (2 × 10−5 M) with Cu2+
(0.01 M, 0-40 µL) in CH3OH. Inset: Plot of absorbance at 420 nm versus equivalents of
Cu2+added (b) receptor L1 (2 × 10−5 M) with Al3+ (0.001 M, 0-80 µL) in CH3OH. Inset: Plot
of absorbance at 430 nm versus equivalents of Al3+ added(c) receptor L1 (1× 10−3 M) with
Cu2+ (0.5 M, 0-40 µL) in CH3OH.
17

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Fig. 5. Competitive selectivity absorption spectral changes of receptor L1 (1 × 10−4 M)
towards (a) Cu2+ (0.01 M) (b) Al3+ (0.01 M) (10 equiv) in presence of different cations (10
equiv).
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Fig. 6. HRMS of receptor L1 in presence of Cu2+ in CH3OH.
19

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Fig. 7. HRMS of receptor L1 in presence of Al3+ in CH3OH.
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Fig. 8. Visual colour change of receptor L1 (1 × 10−4 M) in presence of different anions and
cations (20 equiv.) in CH3OH.
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Fig. 9. Visual colour change paper strip applications of receptor L1 (0.001 M) in presence of
−
different concentrations of (a) 0, 10, 20, 50, 100 ppm HSO4 ion and (b) 0, 10, 50, 100, 200
ppm Cu2+ ion in methanol.
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