Luminescent behavior of pyrene-allied calix[4]arene for the highly pH-selective recognition and determination of Zn2+, Hg2+ and I-: Via the CHEF-PET mechanism: Computational experiment and paper-based device

Article abstract of DOI:10.1039/c9nj01388a

In this article, for the first time, we have reported a novel CHEF-PET fluorescence sensor L based on calix[4]arene containing four pyrene groups as binding sites, which is highly selective and sensitive towards Zn²⁺, Hg²⁺ and I^-. This fluorescence probe was synthesized and characterized using the emission study, UV-vis titration, ¹H NMR spectroscopy and ESI-MS investigation. The linear concentration range at pH 7 of L for Zn²⁺, Hg²⁺ and I^- is 0-135 nM, 0-140 nM and 0-120 nM, respectively, with the detection limit of 6.43 nM for Zn²⁺, 2.94 nM for Hg²⁺ and 20.93 nM for I^-. The binding ability was determined through Benesi-Hildebrand equation, which was found to be 7.535 × 10⁸ M^-1 for Zn²⁺, 9.001 × 10⁸ M^-1 for Hg²⁺ and 8.139 × 10⁸ M^-1 for I^-. Further, we reported an easy-to-use, low-cost and disposable paper-based sensing device for the rapid chemical screening of Zn²⁺, Hg²⁺ and I^-. The device comprises luminescent sensing probes embedded into a cellulose matrix, where the resonance energy transfer phenomenon seems to be the sensing mechanism. It opens up new opportunities for simple and fast screening in remote settings, where sophisticated instrumentation is not always available. The MOPAC-2016 software package was used to optimize the L using the well-established PM7 method and calculate the HOMO-LUMO energy band gap for structure L and L with Zn²⁺, Hg²⁺ and I^- ion-based structures. The molecular docking study was carried out using HEX software.

Full text of DOI:10.1039/c9nj01388a

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Luminescent behavior of pyrene-allied calix[4]arene for highly pH
selective recognition and determination of Zn2+,Hg2+ and I- via
CHEF-PET mechanism: Computational experiment and paper
based device
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Pinkesh G. Sutariyaa*, Heni Sonia, Sahaj A Gandhib, Alok Pandyac
www.rsc.org/
In this article, we have reported for the first time a novel CHEF-PET fluorescence sensor L, based on calix[4]arene
containing four pyrene groups as binding sites which is highly selective and sensitive towards Zn2+, Hg2+ and I-. This
fluorescence probe has been synthesized, characterized by emission study, UV-vis titration, 1H NMR spectroscopy and ESI-
MS investigation. The linear concentration range at pH 7 of L for Zn2+, Hg2+ and I- is 0-135 nM, 0-140 nM and 0-120 nM
respectively with the detection limit of 6.43 nM for Zn2+, 2.94 nM for Hg2+ and 20.93 nM for I-. The binding ability was
determined through benesi-hildebrand equation which was found to be 7.535 × 108 M-1 for Zn2+, 9.001 × 108 M-1 for Hg2+
and 8.139 × 108 M-1 for I-. Further, we report an easy-to-use, low cost and disposable paper-based sensing device for rapid
chemical screening of Zn2+, Hg2+ and I-. The device comprises luminescent sensing probes embedded into a cellulose matrix
where the resonance energy transfer phenomenon seems to be the sensing mechanism. It opens up new opportunities for
simple and fast screening in remote settings where sophisticated instrumentation is not always available. The MOPAC-
2016 software package has been used to optimize the L using PM7 well established method and calculates the HOMO-
LUMO energy band gap for structure L and L with Zn2+, Hg2+ and I- ion based structures. The molecular docking study has
been
carried
out
using
HEX
software.
direct visual observation. To sense an ion, a chemosensor
should be more selective, rapid and cost-effective.
1. Introduction
Fluorometry technique fulfills all these criteria and
a
At present, the scientific community is taking much interest in
a large and mature family of supramolecular chemistry having
cyclodextrins, cucurbiturils, cryptands, crownethers and
calixarenes as family members. Among them, calixarenes1, a
methylene bridged cyclic oligomer of phenol, has drawn a
great attention due to its flexible arrangement. There are
variety of calixarene derivatives but calix[4]arene has been
found to be outstanding platform as the host molecule
because it can provide the space adequate in the realm of
host-guest chemistry and can be excellent and versatile
ionophore. Calix[4]arene can be modified by functionalizing at
upper and lower rim of it which turns it into an effective host
compound for a wide variety of guest molecules such as
cations, anions and neutral molecules with well-defined
chemosensor based on fluorescence acts as an attractive host
molecule. The calix[4]arene based fluorescent chemosensor
possesses mainly two parts i.e. an ionophore which is
responsible for guest ion binding and
a fluorophore
responsible for signal transduction. The mechanism of
fluorescence involved in calix[4]arene system is mainly
photoinduced electron transfer (PET)6-8
,
fluorescence
resonance energy transfer (FRET)9-11, photoinduced charge
transfer (PCT)12-13, metal-ligand charge transfer (MLCT)14,
proton transfer15, formation of an excimer16 and chelation-
enhanced fluorescence (CHEF)17 for the molecular recognition.
Among various transition metals, Zn2+ is the second most
abundant transition metal ion in human body after iron. It is
very essential element in many biological processes such as
protein synthesis, neural transduction, gene transcription,
immune functions, brain function and pathology and
mammalian reproduction. Failure to maintain zinc homeostasis
causes alzheimer’s disease, epilepsy, low blood sugar, ischemic
stroke and infantile diarrhea18-20. The detection of heavy
metals is receiving ever-increasing attention by researchers
nowadays. Mercury is considered as one of the most notorious
toxic metals to environment which causes severe neurotoxic,
genotoxic and immunotoxin effects and thus poses severe risk
binding core2-5
.
Considerable efforts have been done on exploring efficient and
economic artificial molecular receptor for targeted cations,
anions and neutral molecules recognition and sensing through
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for human beings and other organisms. In the human body, a (Scheme 1) which shows turn on/off/on CHEF-PET phenomena
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DOI: 10.1039/C9NJ01388A
-
small amount of mercury may cause a variety of diseases like for the selective recognition of Zn2+, Hg2+ and I .
prenatal brain damage, serious cognitive and motion disorders 2. Experimental
and minamata disease. Because of its wide distribution in air, 2.1 Synthesis of compound (A-C) is described in ESI.
water and soil, it
2.1.2 Synthesis of compound D
A solution of C (0.5 g), DCC (0.25 g), DMAP (0.15 g) and 1-
aminopyrene (0.27 g) in DCM solvent was stirred for 48 h. The
actual reaction time was considered by taking thin layer
chromatography (tlc) at regular interval of time by using
mixture of chloroform: methanol (3.5:1.5). The solvent was
then evaporated under vacuum. The synthesis route has been
shown in Scheme 1. Yield: 0.672 g, (90 %). Anal calcd for
C116H100N4O8: calcd C, 83.03; H, 6.01; O, 7.63; N, 3.33. Found:
C, 83.01; H, 5.94; N, 3.30. FT-IR *Fig. S1 (ESI†)+: 1678 cm-1 (-
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1
C=O), 3442 cm-1 (-NH); H NMR (600 MHz, DMSO-d6) [Fig. S2
(ESI†)+: 8.26 (s, 4H, –4CONH), 8.11 (d, 4H, J = 9.4 Hz, pyrene-
H), 8.07 (d, 4H, J = 6.1 Hz, pyrene- H), 7.98 (d, 4H, J = 9.0 Hz,
pyrene-H), 7.80-7.90 (m, 4H, pyrene-H), 6.55(d, 4H, J = 8.3 Hz,
pyrene-H), 7.70-7.80 (m, 16H, pyrene-H), 6.34-6.42 (m, 8H, Ar-
H), 2.98 (s, 8H, –4OCH2CO), 2.65 (s, 8H, 4ArCH2Ar), 1.23 (s,
36H, -4C(CH3)3). ESI MS (m/z): found 1664.7 (m+1) [Fig. S3
(ESI†)+.
2.2 Photo physical evaluation
The photo physical properties of the newly synthesized L was
evaluated by UV-visible experiment and fluorescence
spectroscopy in all the solvents in which it is soluble, however
acetonitrile was selected for further studies since acetonitrile
is miscible with water. The UV-visible profile exhibits
absorption peak in the region between 250-380 nm. The peak
at 276 nm indicates π- π* transition of 1-aminopyrene system
and the emission profile shows a strong luminescence peak at
548 nm in acetonitrile with excitation at the absorption
maxima (λmax) of the 1-aminopyrene moiety, which is at 200-
450 nm.
Scheme 1: Synthetic route for 5,11,17,23-tetra-tert-butyl-25,26,
27,28
tetramethoxy–bis[(1-pyrenylaminocarbonyl)methoxy]
calix[4]arene (L)
is necessary to develop a sensor for it21-22. As anions are
ubiquitous and very important in chemical, biological and
environmental processes, its recognition is now growing field
of research23
. Among various anions, iodide is one of the
essential anion due to its vital role in biological processes in
human body such as neurological process and thyroid gland
function. Furthermore, it is also used in making of organic
chemicals, dyes and medicines. The importance of these ions
has enlightened us to design and develop a fluorescence
sensor which can easily and rapidly sense these ions.
A great challenge of designing such artificial chemosensor lead
us to develop various calix[4]arene armed fluorescence sensor
for recognition and determination of Zn2+, Cu2+, Cd2+, Sr2+, Fe3+,
F-, phosphate ions, L-Tryptophan and L-Histidine24-30. Previous
research by our group and others have inspired us to
contribute our efforts into the direction of developing paper
based analytical device and some theoretical computational
work. To expand the area of its application and validate our
method, we have also done on-spot monitoring of recognized
ions in real samples. The need of multi-ion selective
calix[4]arene based fluorescence sensor lead us to be
committed for making a convenient, fast responsive, highly
selective and sensitive fluorescence sensor; so here we are
proposing calix[4]arene linked fluorescence probe 5,11,17,23-
2.3 General procedure for the spectroscopic study of L
The calix[4]arene based fluorescence sensor L was investigated
for its interaction with numerous perchlorate salts of metal
ions (Zn2+, Cd2+, Fe2+, Fe3+, La3+, As3+, Nd3+, Zr4+, Ca2+, Ce3+, Li+,
Ag+, Ba2+,Co2+, Hg2+, Na+, K+, Cu2+, Ni2+, Mn2+, Cr3+, Pb2+ and
Sr2+) and tetrabutylammonium (TBA) salts of anions (F-, Cl-, Br-,
-
-
I-,S2-,CN-,NO2 ,HSO4- CH3COO- and H2PO4 ) through fluorescence
spectroscopy, UV-vis spectroscopy, 1H NMR and ESI-MS
investigation. The stock solutions of the compound L (1 x 10-8
M) and that of perchlorate salts (1 x 10-6M) of aforesaid metal
ions and TBA salts of anions (1 x 10-6 M) were prepared in
freshly purified acetonitrile as acetonitrile is miscible with
water. To transfer 2 mL stock solution of L and 2 mL stock
solution of cation-anion salts in a 5 mL volumetric flask, micro
syringe was utilized. The spectra of the cation-anion added
solutions were compared with that of the original solution.
Further, the solution of L was titrated with Zn2+, Hg2+ and I- by
addition of increasing equivalent of respected ions. Then the
spectroscopic properties of L were examined in mixed aqueous
organic
medium
[acetonitrile/4-(2-hydroxyethyl)-1-
tetra-tert-butyl-25,26,
27,28
tetramethoxy–bis[(1-
piperazineethanesulfonic acid (HEPES) (6:4, v/v; pH = 7.2)].
2.4 Fabrication of paper based analytical device
pyrenylaminocarbonyl)methoxy] calix[4]arene (L
) receptor
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We have performed a wax printing technique to create the due to the inhibition of PET mechanism which can be
View Article Online
specific patterned layouts of single or multi well patterns onto maximized in a system that has a fairly rDeOduI:c1i0n.1g03a9m/Cid9NeJg0r1o38u8pA.
the dried cellulose paper (CP) forming hydrophobic barriers The fluorescence spectra of the compound was recorded in
made of wax. Initially, the desired patterns (single or multi well acetonitrile in presence of 100-fold excess of various cations
design) were created in microsoft office. Furthermore, the CP and anions which were prepared in acetonitrile. We evaluated
was then attached uniformly on the wax paper and kept for the interaction of ligand with Zn2+, Hg2+ and I- in the presence
melting for 120 seconds at 1000C. The wax will be printed onto of other cations and anions. To explore sensing ability of our
the CP. After that, it was detached from the paper and used fluoroionophore, we added 1× 10-6 M solutions of Zn2+, Hg2+
for detection of Hg2+, Zn2+, and I- ion as test hole as shown in and I- into 1× 10-8 M solution to the fluoroionophore (L). From
[Fig.S4 (ESI†)+. The detection was typically performed as the fluorescence spectra, we observed that the intensity of L
following: 10 μL of the L sample was added into CP and kept it was enhanced because of CHEF effect between L and Zn2+. This
for 15 min for drying. And then, 10 μL of the solution of Hg2+, kind of interaction was also occurred in the case of ligand L
Zn2+, and I- ion at different concentration was added. After the and I- which leads to enhancement of ligand intensity whereas
solution totally soaked in 5 min, the test results could be read Hg2+ quenched the fluorescence intensity of L via PET.
by naked eye under UV light (365 nm) illumination, and the
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corresponding fluorescent images were captured directly by a 3.2 Selectivity of Zn2+, Hg2+ and I- with L based CHEF-PET
samsung digital camera.
probe
3. Results and Discussion
To check the importance of calix[4]arene cavity role for
recognition of molecular ions we have synthesised model
3.1 Fluorescence spectra of L towards Zn2+, Hg2+ and I-
To examine the spectroscopic properties of ligand L, we have compound without calix[4]arene moiety. We observed that
used [acetonitrile/4-(2-hydroxyethyl)-1- there was no significant change in emission in absence of
piperazineethanesulfonic acid (HEPES) (6:4, v/v; pH = 7.2)]. calix[4]arene molecule [Fig. S5 (ESI†)]. Furthermore we have
Pyrene and its derivatives have been comprehensively carried out ion-binding study by using various perchlorate salts
examined for the recognition of molecular ions 31. In the of metal ions (Zn2+, Cd2+, Fe2+, Fe3+, La3+, As3+, Nd3+, Zr4+, Ca2+,
present study, we are reporting the first example of a Ce3+, Li+, Ag+, Ba2+, Co2+, Hg2+, Na+, K+, Cu2+, Ni2+, Mn2+, Cr3+,
calixarene-based fluoroionophore which is armed by four Pb2+ and Sr2+) and tetrabutyl ammonium salts of anions (F-, Cl-,
-
-
-
pyrene groups as binding sites. We have noted an on/off/on Br-, I-, S2-, CN-, NO2 , HSO4 , CH3COO- and H2PO4 ) with ligand L.
fluorescent effect by using our new ligand
L
having In this experiment, we have kept equal concentration (1 × 10-8
amidopyrene as four armed binding sites. In the absence of M) for L, various aforesaid perchlorate salts and tetrabutyl
Zn2+ and I-, the fluorescence is partially quenched because of ammonium salts as mentioned above in acetonitrile. In the
photoinduced electron transfer (PET) effect. In this system, the fluorescence spectra, only Zn2+, Hg2+ and I- display noticeable
amide linkage of L ring not only participates in the selective changes when it is compared to the spectra of original ligand L
binding but also serves as a probe for the chelation-enhanced while other cations and anions were seemed inert to the
fluorescence (CHEF) effect. Upon complexation, the nitrogen supramolecular interactions of ligand L. The ligand L displayed
lone pairs will not participate in PET, causing CHEF effect. ON-OFF-ON behaviour as Zn2+, Hg2+ and I- induced obvious
However, studies indicate that this CHEF effect was observed fluorescence changes by increasing, decreasing and again
A
B
Figure 1 (A-B): (A) Selectivity plot of L (1×10-8M) with various cations from top to bottom (La3+, Mn2+, Cd2+, Fe2+, Fe3+, Pr3+, Zr4+, Ca2+,
Ce3+, Li+, Ag+, Ba2+,Co2+, Na+, K+, Ni2+, Pb2+, Sr2+ and Cu2+) (B) Selectivity plot of L (1×10-8M) with various anions from top to bottom F-,
-
-
Cl-,Br-, NO2 , S2- HSO4 , H2PO4- and CN-) (1×10-8M) in acetonitrile; λex = 480 nm, λem = 540 nm, pH = 7.2.
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increasing intensity of ligand L. The initial fluorescence calibration curve [Fig. S7-S9(ESI†)] shows good linearity with
View Article Online
DOI: 10.1039/C9NJ01388A
2+
2+
intensity of L is 110000; on addition of Zn2+, Hg2+ and I- correlation coefficient of 0.9907 for Zn , 0.9901 for Hg and
fluorescence intensity is changed. Now, it appears 400000 for 0.9906 for I-. We have also calculated limit of detection (LOD)
Zn2+, 10400 for Hg2+ and 172000 for I- (Fig. 1 A and B). We as per 3σ IUPAC criteria for synthesized probe from this
believed that we were simply observing fluorescence emission study. The LOD was found to be 6.43 nM for Zn2+,
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Figure 2 (A-C): Absorption spectral changes of L (1×10-6M) ligand in the presence of Zn2+, Hg2+ and I-
quenching via a PET process. CHEF on Zn2+ addition thus 2.94 nM for Hg2+ and 20.93 nM for I-.
results when ion (Zn2+) complexation of the amide lone pairs of 3.4 Binding constant of L based CHEF-PET probe
L decreases amide oxidizability such that the amide cannot Binding constants for fluoroionophore were determined by
reduce the amidopyrene excited state. It is because of the using emission titration data following the previously reported
conjugation of electron donating (-NH) and electron articles33-35. The plots of Log [(F0 -F)/ (F-Fœ)] vs. Log [M] for
withdrawing group (-C=O) in the same molecule which selected ions with L. The binding constants were calculated by
undergoes intermolecular charge transfer from donor to fluorescence titration data. Here, we have presented
acceptor. The consequent change of dipole moment leads to representative spectra showing the changes observed in
stoke shift, which is influenced by the microenvironment of emission intensities upon the addition of increasing
the fluorophore. This can be a further evidence for the concentration of ions. According to this procedure, the
“Molecular Taekowndo” procedure32 through the fluorescence fluorescence intensity (F) scales with the metal ion
change. Therefore it is predicted that Hg2+, Zn2+ and I- closely concentration ([M]) through (F0-F)/(F-Fœ) = ([M]/Kdiss)n. The
interact with L and cause obvious change in fluorescence binding constant (Ks) is obtained by plotting Log[(F0-F)/(F-Fœ)]
intensities with respect to other large number of cations and vs Log[M], where F0 and Fœ are the relative fluorescence
anions. Hence, for quantitative analysis we have selected these intensities without addition of guest metal ions and with
three ions (Zn2+, Hg2+ and I-) for further examination.
3.3 Sensitivity of L based CHEF-PET probe
maximum concentration of metal ions (when no further
change in emission intensity takes place), respectively. The
Fluorescence titration has been investigated by gradually value of Log [M] at Log[(F0-F)/(F-Fœ)]= 0 gives the value of
varying the concentration of Zn2+ (0-135 nM), Hg2+ (0-140 nM) log(Kdiss), the reciprocal of which is the binding constant (Ks).
and I- (0-120 nM) and keeping the concentration of L constant The observed binding constant from fluorescence spectra for
because sensitivity is crucial part for any sensor [Fig. S6(A- Zn2+, Hg2+ and I- are respectively 7.535 × 108 M-1, 9.001 × 108
C)ESI†]
.
For the mechanism of emission quenching, stern– M-1 and 8.139 × 108 M-1 [Fig. S10-S12 (ESI†)]
.
3.5 Quantum yield of L with Zn2+, Hg2+ and I-
volmer plots are worthwhile and that is why they are used to
analyze the nature of the quenching process in the chelating
complex L-Hg2+. As shown in equation (1), the determination
of dynamic or static quenching process can be examined by
plotting relative emission intensities (I0/I) against quencher
The quantum yield36 was calculated by following equation (2)
(2)
concentration [Q] in which the slope of the plotted line yields where F and Fstd are the areas under the fluorescence emission
curves of the Zn2+, Hg2+ and I- ions complexes with L and only
Ksv (the static quenching constant).
standard L respectively. A and Astd are the areas under relative
absorbance of the Zn2+, Hg2+ and I- ions complexes with L and
standard L at the excitation wavelength, respectively. η and
ηstd are the refractive indices of solvent (acetonitrile) used for
I0/I = 1 + Ksv[Q]
(1)
In our case, typical linear plots for L-Hg2+ suggest a pure
fluorescence static quenching because of a non-fluorescence
ground state complex between ligand L and Hg2+. The
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the L:Zn2+, L:Hg2+, L:I- and standard (L), respectively. The L:Zn2+,
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L + Zn2+
L
L
L + Hg2+
Figure 3: ESI mass spectrum showing the isotopic peak pattern of molecular ion peak for 1:1 complex formed between L + Zn2+ and L +
Hg2+
L:Hg2+, L:I- and the standard (L) were excited at the same
3.6 UV-Visible experiment
wavelength. The quantum yield of fluoroionophore L was
obtained using emission spectra of standard fluorophore
(pyrene). The reported quantum yield of pyrene37 was
approximately 0.32 and that of fluoroionophore with L:Zn2+,
L:Hg2+, L:I- ion complex were found to be 1.42, 0.04 and 0.62
respectively. Thus, when Zn2+ and I- were bound to L, the
quantum yield increases whereas for Hg2+; quantum yield
decreases. Hence L provides a sensitive means to recognize
Zn2+, Hg2+ and I- by direct fluorescence measurement.
In supporting evidence to fluorescence spectroscopy, we
further conducted UV-vis experiment of ligand L with Zn2+,
Hg2+ and I-. The original absorbance band of ligand L was at
276 nm. When we added 5 nM of Zn2+, Hg2+ and I- solutions, it
shifted effectively and intensity variation rises at 261 nm, 262
nm and 263 nm respectively which indicates hypsochromic
effect or blue shift as displayed in Fig.2 (A, B and C). The
interaction of anion with amide linkage of L by forming
hydrogen bonding and cation binding with carbonyl group of
amide linkage are the causes of blue shift. To know the
detection limit of L towards Zn2+, Hg2+ and I-, we have also
performed UV-vis titration by varying concentration of
selective ions (0 nM- 100 nM) ([Fig. S13(A-C)(ESI†)]. From this
investigation, we can assure that when it is matter of
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sensitivity and low detection limit, fluorescence technique is reason of the recognition process of cation andVaienwioArnticlienOntlhinee
DOI: 10.1039/C9NJ01388A
more reliable than absorption study.
same molecule.
3.10 Interference study
3.7 ESI-MS investigation
The chemosensor should not be considered as a perfect sensor
6
For further binding confirmation of Zn2+,Hg2+ and I- with ligand until it is practically used for selective ions in the presence of
L, we have implemented ESI-MS technique. The complex was other cations and anions. We can easily define the selective
dissolved in CH3CN (3 mL) and 10 equivalent amount of the nature of ligand L towards Zn2+, Hg2+ and I- ions from above
desired cations as perchlorate salt and iodide as TBA salts were described techniques but to identify the real potential ability
added into the solutions. The reaction mixtures were stirred at of L, we have pursued an interference study. [Fig. S23-S24
room temperature for 2 h, the solutions were then filtered off (ESI†)+ depicts the fluorescence response of the chelated
and the filtrates were analysed by ESI-MS instrument. The complex (L-Zn2+, L-Hg2+ and L-I-) with other competing ions.
formation of 1:1 complex was confirmed by ESI-MS, where the The results showed an obvious fluorescence enhancement and
spectrum shows molecular ion peak m/z at 1664 whereas in quenching as aforementioned technique even in the presence
the presence of Zn2+ and Hg2+ show peak at 1729.7, 1864.02 of other cations and anions. From this experiment, we can
respectively (Fig. 3); for I- it appears at 1791.3 (Fig. S14, ESI†). conclude that except Zn2+, Hg2+ and I- ions other cations and
These results will give assurance of binding of Zn2+ and Hg2+ anions are inert toward ligand L. Proposed binding mechanism
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with L ligand via electrostatic interaction.
through hydrogen bonding and electro static interaction with L
3.8 1H NMR experiment for I- with L
ligand by Zn2+, Hg2+ and I- is shown in [Fig. S25 (ESI†)].
Our whole proposed mechanism is based on the strong 3.11 Real sample analysis
interaction of iodide ion with –NH subunits. In supporting
This CHEF-PET probe is applied for its important applicability to
1
evidence to this, we have carried out H NMR investigation of sense Hg2+ in industrial waste water sample by using
chelating complex L-I-. In this experiment, the solutions fluorescence titration and also confirmed the stability of
containing 2 mg of complex, dissolved in 0.5 mL of DMSO-d6, metal-complex. The standard addition method was applied to
1
were used for H NMR spectra. The solutions of the tetrabutyl evaluate the validity of the proposed sensor. The result gained
ammonium salts of I- was then we added 5 molar equivalent of with excellent recovery of spiked Hg2+ ranged from 93 to 99 %,
TBAI solution to ligand. The interaction of the anion with the proving the validity of the developed technique. Furthermore,
ionophore was observed from spectral changing in the this fluoroionophore has been applied for salt samples from
comparison of the spectra of the anion added solutions to that different brand have been used for the detection of iodide ion
of the original compounds. Upon addition of 5 fold excess I- by this probe and Table 2 depicts the results of them. The
into L ligand, the –NH peak at δ =8.26 ppm disappears because concentration of each sample was measured by the CHEF-
of the hydrogen bonding with –NH group of amide linkage of PET sensor, using the calibration method. The proposed
ligand L [Fig. S15 (ESI†)+. The similar type of experiment has technique is compared with reported methods for
been carried out for Zn2+ and Hg2+ in presence of L but we determination of Zn2+, Hg2+ and I- (Table S1 (ESI†)). This
could not find any major spectral changes. It is just because of comparison undoubtedly endorses that proposed fluorometric
electrostatic interaction of Zn2+ and Hg2+ with carbonyl group method is superior to others in terms of sensitivity, selectivity
of amide linkage *Fig. S16 (ESI†)+.
and binding ability to recognize Zn2+, Hg2+ and I-39-43
.
3.9 pH study and job’s plot
Sample
Spiked Found by
Found by Recovery
The supramolecular interaction of metal ions and anions with L
is often sensitive while pH is varying. Therefore, it is needful to
achieve an optimum pH at which our fluorescence probe can
work effectively. By doing screening process of L at various pH,
we found maximum enhancement for Zn2+ and I- and
maximum quenching for Hg2+ at 7 pH as shown in [Fig. S17-S19
(ESI†)+. The reason behind this phenomenon is amide linkage
of L which makes it neutral in nature. The pKa value of L has
been calculated by following emission data38 and it was found
7.2±0.2. By performing UV-visible experiment, the
stoichiometry of the complex formed (1:1) has been also
derived based on the Job’s plot [Fig. S20-S22 (ESI†)+. The
reason behind enhancement and quenching in fluorescence
intensity of ligand L is that Zn2+ and Hg2+ interact with ketonic
group of L while I- makes strong hydrogen bonding with –NH
group of L ligand. Keto-enol nature of L ligand is the main
Hg2+
AAS (nM) proposed
(%)
(nM)
sensor
(nM)
Industrial
Water Sample 1
Industrial
10
20
9.78
9.34
93.4
92.8
97.8
96.4
99.34
18.89
29.67
38.89
99.78
18.56
29.34
38.56
99.34
Water Sample 2
Industrial
Water Sample 3
Industrial
Water Sample 4
Industrial
Water Sample 5
30
40
100
Table 1: Determination of Hg2+ in industrial water samples.
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Iodide
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Found by
proposed
sensor (nM)
9.52
Recovery (%)
View Article Online
DOI: 10.1039/C9NJ01388A
3.12 1. Detection of Hg2+, Zn2+, and I- ions in single-strip paper
In this study, the paper-based screening device (PAD) is
proposed which is based on fluorescent calix[4]arene (L)
immobilized on PAD as sensing probes contained within a
cellulose paper. Paper was selected as substrate due to its
Salt (Brand 1)
Salt (Brand 2)
Salt (Brand 3)
95.2
94.2
97.7
18.96
29.32
Table 2: Determination of I- in different salt samples.
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where the L fluorescence quenching or enhancement occurs
simultaneously due to its distinctive interaction of designed
calixarene for cations and anions.
Fig. 4A illustrates the fluorescent L immobilized on PAD which
is able to produce fluorescent colored in blue under UV light.
Samples were placed at the delineated area where the sensing
later occurs. As shown in Fig. 4(A) after the addition of
-7
different concentration of Hg2+ ranging from 10 to 10-5 M,
excited-state photons from the calix[4]arene are transferred to
the analyte displaying an efficient quenching in presence of
Hg2+. Similarly, after the addition of different concentration of
-7
Zn2+ and I- ranging from 10 to 10-5 M displaying an efficient
enhancement which the detection of Zn2+ and I- as shown in
Fig. 4 (B-C). It can visualize by naked eye. These results also
indicate that this sensor is visibly able to detect ion up to
Figure 4(A-C): (A) L-PAD and after detection of Hg2+, at the
concentration ranging from 10 -7 to 10-5 M (B) after detection
-7
the10 M concentration which is better the existing sensor.
Moreover, these all results in a solid platform (paper) correlate
well with experiments performed in aqueous solutions using a
fluorimeter. Furthermore, we also demonstrate the ready-to-
use single-strip paper based sensor for multiplex ion detection
of Hg2+, Zn2+, and I- . Wherein, we have performed one test of
multiple ion testing. The multi well design desired patterns
were created in microsoft office word. The CP were then
attached uniformly on the wax paper and kept for melting for
120 seconds at 1000C. The wax will be printed onto the CP.
After that, it was detached from the paper and used for
detection of Hg2+, Zn2+, and I- as shown in Fig.5A. On designed
multiwell pattern, 5 µL of 10-5 M of TAAC–PAD is immobilized
on sample hole of paper and then 5 µL of 10-5 M Hg2+, Zn2+,
and I- ion is added. As a result, we find more clearly
distinguishable fluorescence changes after addition of Hg2+,
Zn2+, and I- as shown in Fig. 5B. In presence of Hg2+, ligand get
quenched and in presence of Zn2+, and I- , fluorescence of
ligand get enhanced. These results indicate that the sensor is
suitable for sensitive
-7
of Zn2+ at the concentration ranging from 10 to 10-5 M (C)
after detection of I-, at the concentration ranging from 10 -7to
Figure 5 (A-B): (A) fluorescence L immobilized multiplex
plasmonic sensor, (B) Under fluorescent light fluorescence L
immobilized multiplex plasmonic sensor after addition of 10-5
M Hg2+, Zn2+, and I-.
superior ability to wick aqueous or organic solutions by
capillary action, among other recognized qualities that make
pumps or high power source requirements not necessary. The
sensing mechanism observed here is based on CHEF-PET
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detection of these ions even under the coexistence, which can orbitals – HOMOs (π-donor) and lowest unoccupiVeiedwmArtoiclleecOunllianer
DOI: 10.1039/C9NJ01388A
not only determine all three kinds of ions successively but also orbitals – LUMOs (π-acceptor) and their energy gap reflect the
of detecting in one test. On the occasions where Hg2+ is the chemical activity and the energy gap between HOMO and
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Figure 6: Homo - Lumo orbital analysis of free ligand (L), ligand L with Zn2+, Hg2+ and I-complexes.
target ion, masking the interference of Zn2+, and I- is necessary. LUMO have been used to prove the bioactivity from
This precautions is been taken all detections methods.
3.13 Computational studies
intramolecular charge transfer (ICT). Computational analysis of
energy levels of HOMO and LUMO orbitals were calculated and
The important objective of computational studies has to the frontier orbitals (HOMO, LUMO) of compound L and
optimize geometry of title molecule L using molecular orbital compound L with Zn2+, Hg2+ and I- complexes. The HOMO,
package (MOPAC) software45. This semi-empirical approach LUMO energy and the results of the energy gap values were
was done by creating a Z-matrix of the molecule which was reported in Table S2 (ESI†). Homo and Lumo orbitals energy
then converted to its MOPAC input data. Optimization of these band gap of the title compound L and compound L with Zn2+,
data had provided the specific bond distance, bond angle, Hg2+ and I- complexes were plotted in Fig. 6. The change in
dihedral angle of the most stable molecular geometry. The HOMO-LUMO energy band gap (Fig. 6), suggested that the
heat formation energy was generated 1182.4434 kcals/mole charge transfer within the molecules and its explanation for
using PM7 method. MOPAC calculations generated visual fluorescence enhancement and quenching phenomenon of L
models of the most stable molecular geometry of L in ball and with Zn2+, Hg2+ and I-. [Fig. S27 (ESI†)+ shows that the only free
stick configuration as shown in [Fig. S26 (ESI†)+
.
ligand compound L has band gap -0.034 eV, on addition of Zn2+
cation into L, the band gap changed to -0.009 eV whereas for
3.13.1 HOMO and LUMO analysis
Computational studies of the molecules, like molecular orbital Hg2+ and I- ; band gap appears at -0.006 eV and
-0.011
and their properties are very valuable to the physicists and eV respectively. The prominent result shows that in free ligand
chemists. The frontier electron density castoff for predicting L and complexes of Zn2+, Hg2+ and I- , band gaps are almost
the most reactive position in π and σ electron system and zero and the charge accommodate on HOMO and LUMO
explained several types of reaction in conjugated system. The bearing at opposite side of amide linkage of L.
electronic absorption corresponds to the transition from the 3.11.2 Molecular docking study
ground to the first excited state is mainly described by on Molecular docking of the free ligand L with different receptors
electron excitation from the highest occupied molecular were carried out with Hex 8.0 software46. Docking was the
orbital (HOMO) to the lowest unoccupied molecular orbital process of fitting together of two molecules in 3-dimensional
(LUMO). The Eigen values of highest occupied molecular space. It explored ways in which two molecules, such as drugs
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and protein receptor fit together and docked to each other industrial waste water sample for selective deteVciteiwonArtioclfe OHnglin2e+
well. The molecule binding to a receptor, inhibit its function, and salt sample for selective detection of I with 92 to 99 %
-
DOI: 10.1039/C9NJ01388A
and thus act as drug. The collection of different protein recovery. Computational work has supported the fluorescence
receptors of human lung cancer cell (4ZSE, 5HG5, 5HG7, 5UGA enhancement and quenching. The molecular docking results
and 5UGC) from the protein data bank were docked with displayed that the good interaction between title molecule L
ligand L. It was evaluated using molecular dynamics, by their and protein receptor of human lung cancer cell 4ZSE with
binding affinities, free energy simulations and relative energy value of -646.46 Kcal/mol. This highly sensitive,
stabilities. The energy values obtained by docking study was selective, easy and cost-effective fluorometric method will
tabulated in Table S3 revealing the best ligand structure fitting provide great interest for routine analysis of Zn2+, Hg2+ and I-.
was due to 4ZSE (-646.46 Kcal/mol) compare to other
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receptors. The molecular docking pose of 4ZSE, 5HG5, 5HG7,
Conflicts of interest
5UGA and 5UGC with molecule L as shown in [Fig. S28 (ESI†)+
.
“There are no conflicts to declare”.
4. Conclusions
Acknowledgements
In conclusion, we have successfully synthesized and
characterized a lower rim functionalized calix[4]arene armed
with four amidopyrene groups which work as fluorogenic
units. The probe displays CHEF-PET phenomena when it comes
in contact with Zn2+, Hg2+ and I-. The sensing properties of our
fluorescence probe is confirmed by fluorescence titration as
well as UV-vis titration and competitive study. Proposed
fluorescence probe has lower sensing limit as well as high
selectivity towards 6.43 nM for Zn2+, 2.94 nM for Hg2+ and
20.93 nM for I-. Binding constants with strongly interacting
ions were determined by fluorescence titration via PET and
CHEF mechanism. The possible molecular structure of the Zn2+,
Hg2+ and I- complex is proposed on the basis of ESI-Mass and
1H NMR data. Furthermore, the present system has been
applied for
P. G. Sutariya would like to thank Department of Science and
Technology- Science and Engineering Research Board (DST-
SERB) for providing financial assistance under Young Scientist
Scheme (YSS/2015/001258) (New Delhi).
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