Turn on fluorescence strip based sensor for recognition of Sr2+ and CN? via lowerrim substituted calix[4]arene and its computational investigation

Article abstract of DOI:10.1016/j.saa.2020.118456

Fluorescence sensor L designed around a calix[4]arene scaffold, bearing two fluorogenic aminoquinoline moities, has been synthesized. It is found to be selective and sensitive towards Sr²⁺ and CN^? over a wide range of cations and anions in a spectrofluorometric study in acetonitrile. The ion-binding property of L was monitored by fluorescence spectroscopy, UV–vis spectroscopy, ESI-MS, ¹H NMR, FT-IR investigation and PXRD study. The host L shows a minimum detection limit which is 1.36 nM for Sr²⁺ and 1.23 nM for CN^? having concentration range 5–120 nM and 5–115 nM respectively. The calculated binding constants for L:Sr²⁺ and L: CN^? are respectively 8.859 × 10⁸ M^?1 and 8.574 × 10⁸ M^?1. Our host L has been utilised in formation of a user-friendly, affordable, and disposable paper-based analytical device (PAD) for rapid chemical screening of Sr²⁺ and CN^? ion via single strip. Moreover, the optimization of probe L has also been done by the MOPAC-2016 software package using NM7 popular method resulting ?21.71 kcals/mol heat of formation and also determined the HOMO-LUMO energy band gap for L, L:Sr²⁺ and L: CN^?. Further, molecular docking score has been calculated using HEX software. The applicability of our probe in real samples containing Sr²⁺ and CN^? has also been checked by emission study with 94–99% recovery.

Full text of DOI:10.1016/j.saa.2020.118456

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 238 (2020) 118456
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
2
+
Turn on fluorescence strip based sensor for recognition of Sr and
−
CN via lowerrim substituted calix[4]arene and its
computational investigation
a,
a
b
c
Pinkesh G. Sutariya ⁎, Heni Soni , Sahaj A. Gandhi , Alok Pandya
a
Department of Chemistry, Bhavan's Shree I.L.Pandya, Arts-Science and Smt. J.M.Shah Commerce College, Sardar Patel University, V. V. Nagar 388120, Gujarat, India
Department of Physics, Bhavan's Shree I.L.Pandya, Arts-Science and Smt. J.M.Shah Commerce College, Sardar Patel University, V. V. Nagar 388120, Gujarat, India
Department of Physical Sciences, Institute of Advanced Research, Gandhinagar 382426, Gujarat, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Fluorescence sensor L designed around a calix[4]arene scaffold, bearing two fluorogenic aminoquinoline moities,
Received 15 March 2020
Received in revised form 4 May 2020
Accepted 6 May 2020
has been synthesized. It is found to be selective and sensitive towards Sr2+ and CN− over a wide range of cations
and anions in a spectrofluorometric study in acetonitrile. The ion-binding property of L was monitored by fluores-
1
cence spectroscopy, UV–vis spectroscopy, ESI-MS, H NMR, FT-IR investigation and PXRD study. The host L shows a
Available online 08 May 2020
minimum detection limit which is 1.36 nM for Sr2+ and 1.23 nM for CN having concentration range 5–120 nM and
−
5
–115 nM respectively. The calculated binding constants for L:Sr and L: CN− are respectively 8.859 × 10 M
2
+
8
−1
Keywords:
Calix[4]arene
Paper-based sensing device
Computational study
8
−1
and 8.574 × 10 M . Our host L has been utilised in formation of a user-friendly, affordable, and disposable
paper-based analytical device (PAD) for rapid chemical screening of Sr and CN− ion via single strip. Moreover,
2+
the optimization of probe L has also been done by the MOPAC-2016 software package using NM7 popular method
2
+
resulting −21.71 kcals/mol heat of formation and also determined the HOMO-LUMO energy band gap for L, L:Sr
−
and L: CN . Further, molecular docking score has been calculated using HEX software. The applicability of our probe
in real samples containing Sr2+ and CN− has also been checked by emission study with 94–99% recovery.
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
limited due to their expensive instrumentation, not easy to operate
them on-site and lack of portability. Simultaneously, one tactic which
draws the attention of researchers is fluorometry technique. The interest
has been directed towards the making of simple π-electron-rich mole-
cule as a fluorescence sensors. In fluorescence-based sensing method,
fluorophore is tethered with an ionophore to build fluoroionophore in
order to detect molecular guest ions. The ionophore acts as a binding
site for the analyte whereas the fluorophore serves as a signalling unit
which provides optical response after detection of a recognition event.
It has emerged as one of the most robust method due to its high effi-
ciency, simplicity and ultra-sensitivity to construct an effective and eco-
nomic chemosensor.
Calixarenes [7–8], produced by the condensation of p-substituted
phenols with aldehydes, a part of macrocyclic family, paves new
avenues in diverse areas. The popularity of calixarenes, especially
calix[4]arene, arises owing to their amphiphilic nature, existence of
platform for appropriate substitution at lower and upper-rim, pres-
ence of arene core and flexible arms to provide suitable and selective
coordination environment. To broaden the area of sensing molecular
ions, the architecture of functionalized calix[4]arene [9–11] and
fluorometry technique are used. Different motifs are appended at
the upper rim as well as lower rim of calix[4]arene which act as
tweezers and chelate the guest ions. Due to this, the efficiency of
Detection of cations, anions and neutral molecules is currently stint
of major consideration in the modern research work due to their
prevalent applications in chemical, biological and environmental
2
+
assays. Though Sr is broadly used in optical industry, painting in-
dustry and military industry, it is considered as a major component
of ‘dirty bomb’. Besides, the Department of Energy entails a field
instrument that can contrivance the real-time characterization and
observation of radioactive Sr-90 inside high level waste tanks [1–2]. De-
−
spite of acute toxicity, CN is used in many industries such as synthetic
fibres and resins, herbicides, gold extraction, etc. and also can be found
in many fruits and plants. Consequently, any fortuitous release of the
extremely toxic cyanide certainly leads to water and environmental
corruptions [3–6]. The homeland security and antiterrorist activities
with the help of Sr and harmfulness of CN have elicited substantial
attentiveness in developing a convenient and easy-to-use molecular
sensor.
2
+
−
There is a wide range of instrumental techniques for the trace iden-
tification of molecular ions but the real-time use of these techniques is
⁎
Corresponding author.
E-mail address: pinkeshsutariya@gmail.com (P.G. Sutariya).
https://doi.org/10.1016/j.saa.2020.118456
386-1425/© 2020 Elsevier B.V. All rights reserved.
1

2
P.G. Sutariya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 238 (2020) 118456
chemosensor is increased which can enable researchers to detect a wide
range of chemicals with high sensitivity and fast response time using
various mechanisms [12–19].
hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) (6:4, v/v;
pH = 7.2)].
As the unique topology of calix[4]arenes offer a wide range of
scaffolds for making of platforms for sensors, a myriad study in mo-
lecular recognition have been achieved and numerous endeavours
have been devoted by our group [20–23]. In an ensuing study, by
introducing a new fluorescence probe (L) (Scheme 1), we have
tried to exploit a new approach to prepare a selective fluorescence
probe for Sr and CN . We have also completed a very challenging
task that is on-spot monitoring of recognized ions in real samples as
well as computational monitoring. A single step “dip-and-read” based
method is also developed by using the fluorescent indicator and quali-
tative strategy.
2.2. Spectroscopic extents
Absorption and emission spectra of the novel synthesized probe L
were recorded by making solution as above mentioned in Section 2.1.1.
The compound shows the absorption peak in the region between 210
and 500 nm, wherein the peak at 284 nm indicates π- π* transition
of 4-aminoquinoline system. The ligand exhibits a strong emission
peak at 575 nm with excitation at the absorption maxima of the 4-
aminoquinoline moiety that is at 284 nm.
2
+
−
2.3. Preparation of real sample assay
2
. Experimental
The tap water was taken directly for the analysis without doing puri-
fication. The lake water collected from the local lake (Gomati Lake, Dakor)
was first filtered through nylon microporous filter to remove unnecessary
suspension. The industrial waste water samples (100 ml) were collected
from industrialized sewage (vatava). The water sample was exposed to
extraction procedure. Our compound was soluble in chloroform as well
as in acetonitrile but as acetonitrile is miscible with water, chloroform
was preferred to prepare the solution of the compound. Then in a separat-
ing funnel, we procured 60 ml of L (10 nM) solution and 40 ml of water
2
.1. Synthesis of compound (A-E) and its characterization [Scheme 1,
Figs. S1–S2 are described in ESI†
2
.1.1. General procedure for spectroscopic study
As acetonitrile is miscible with water, we have prepared stock solu-
+
+
+
tions of analytes such as perchlorate salts of metal ions M (Li , Ag ,
Na , K ), M
Ni , Mn , Pb
Cr ), M
+
+
2+
(Zn , Ca , Cd , Fe , Ba , Co , Hg2+, Cu
2+
2+
2+
2+
2+
2+
2+
,
2
+
2+
2+
2+
3+
(Fe , La , As , Nd , Ce3+,
3+
3+
3+
3+
2+
and Sr ), M
sample and extracted Sr by shaking for half an hour. Then we separated
organic layer and dehydrated organic phase with anhydrous Na SO . The
2 4
3
+
4+
4+
(Zr ) and tetrabutyl ammonium (TBA) salts of anions
−
−
−
−
−
−
−
−
2−
−
(
F , Cl , Br , I , NO
3
, HSO
4
, CN , H
2
PO
4
, S
and CH
3
COO ) in
extraction was repetitive done by three to four times. Then different con-
centrations of cyanide (10 nM, 20 nM, 30 nM, 40 nM, 50 nM and 100 nM)
were spiked to the solution. In a similar manner, industrial waste water
−
6
acetonitrile with the concentration of 1 × 10 M. Simultaneously, the
stock solution of ligand L was also made in acetonitrile with the concen-
tration of 1 × 10 M. Then the stock solutions were diluted to desired
concentration when needed. The spectroscopic properties of L were
examined in mixed aqueous organic medium [acetonitrile/4-(2-
−8
2+
samples for Sr were taken for the analysis and above-mentioned proce-
dure was done. Then the fluorescence intensity for each sample was
recorded for quantification of developed method.
Scheme 1. Schematic representation showing synthesis pathway of 5, 11, 17, 23-tetra t-butyl- 26, 28 dimethoxy 25, 27 amidoquinoline calix[4]arene (L).

P.G. Sutariya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 238 (2020) 118456
3
2
.4. Construction of paper-based device
fluorescence spectral investigation was performed in acetonitrile pro-
duced by addition of several perchlorate salts of metal ions and
tetrabutyl ammonium salts of anions mentioned in Fig. 1. It shows
that out of a large number of metal ions and anions, L performs with
The outline and photograph of a single paper-based analytical device
2
+
−
(
PAD) for Sr and CN detection are shown in [Fig. S3 (ESI†)]. It works
high selectivity to Sr2 and CN by enhancing fluorescence intensity
+
−
in the principle of capillary flow mechanism in paper. We have per-
formed simple fabrication technique to create the specific patterned
layouts of single or multi well patterns onto the nitrocellulose mem-
brane (NCM). The fabrication is performed mainly three components
i.e. backing card, NCM, punched NCM. Initially, the desired control
compared to the original spectra of L. We have kept equal concentration
−8
(1 × 10 M) for L, various aforesaid perchlorate salts and tetrabutyl
ammonium salts as mentioned above in acetonitrile. The overall emis-
sion changes were dramatic: 1.76 fold for Sr2 and 1.89 fold CN− with
L [Fig. 1 A and B]. Two aminoquinoline groups are linked to calix[4]
arene in such a way that carbonyl groups of the fluorophore interacts
with Sr2 and –NH linkage participates in the complexation process
+
(
C) hole, test hole (T1 and T2) was created in NCM using punch tool.
Furthermore, backing card is being used to stick plain NCM on it.
Then, on NCM stacked backing card, punched NCM were uniformly
stacked to perform to capillary flow of sample solution. This fabricated
+
−
with CN . Consequently, this binding results in enhancement of fluo-
PAD is used for detection of Sr2 and CN ion as test strip as shown in
+
−
rescence intensity which is introduced by Chelation Enhanced Fluores-
cence (CHEF) mechanism. The reason behind CHEF process is that the
binding of Sr2 to ligand L reduces the electron-donating ability of N
and O to aromatic rings, causing a remarkable increment in the emission
intensity. Moreover, our probe has coordination site having N, O and –
[
Fig. S3 (ESI†)]. The detection was typically performed as following:
+
for single ion detection: 5 μL of the L sample was immobilized into
NCM and kept it for 15 min for drying. And then, dip in 5 ml of solution
of Sr2 and CN ion at different concentration was added. After the so-
lution totally soaked in 5 min, the test results could be read by naked eye
under UV light (365 nm) illumination, and the corresponding fluores-
cent images were captured directly by a VIVO digital camera. Similar
method is performed for the detection of dual ion in single test strip.
+
−
2+
CONH linkage as a binding site which can detect hard acid Sr easily.
−
Additionally, CN interacts with –CONH linkage via hydrogen bonding
which leads to the enhancement of fluorescence intensity. Therefore,
it is predicted that Sr2 and CN closely interact with L and cause obvi-
ous change in fluorescence intensities with respect to other large num-
ber of cations and anions. Hence, for quantitative analysis we have
+
−
3
. Results and discussion
selected these two ions (Sr2 and CN ) for further examination.
+
−
3
.1. Fluorescence interaction studies of L
3.2. Sensing properties of probe L
In this feature work, we have employed aminoquinoline [24] motif
as a fluorophore with calix[4]arene for the detection of molecular ions.
To fully harness the binding abilities of fluorescent probe L, the
In order to acquire swift and sensitive analysis of our fluoroionophore,
we added 1× 10 M solutions of Sr and CN into 1× 10 M solution
−6
2+
−
−8
Fig. 1. (A-D). (A-B) Fluorescence spectral changes of L (1 × 10 M) caused by binding with different cations (from top to bottom, Zn , Fe , Fe , Mn2+, La3+, Nd3+, As3+, Cd2+, Pr3+
−
8
2+
2+
3+
,
Zr , Ag , Ba , Co , Ca , Ce , Li , Hg , Na , K , Ni , Pb2+, and Cu ) and anions (from top to bottom, F−, Cl , Br , I , HSO
spectral traces obtained during the titration of L (1 × 10 M) with increasing concentration 5–120 nM and 5–115 nM of Sr and CN respectively at pH 7.
4+
+
2+
2+
2+
3+
+
2+
+
+
2+
2+
−
−
−
−
, H
2
PO
−
, NO
−
, S2− and CH
−
4
4
3
3
COO ). (C-D) Fluorescence
−8
2+
−

4
P.G. Sutariya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 238 (2020) 118456
to the fluoroionophore (L). Fluorescence titration has explored by gradu-
respectively. A and Astd are the areas under relative absorbance of the
Sr and CN ions complexes with L and standard L at the excitation
ally varying the concentration of Sr2 (5–120 nM) and CN (5–115 nM)
and keeping the concentration of L constant because sensitivity is crucial
part for any sensor [Fig. 1 (C-D)]. The calibration curve shows good line-
+
−
2+
−
wavelength, respectively. η and ηstd are the refractive indices of solvent
2+
−
(acetonitrile) used for the Sr , CN and standard (L), respectively. The
arity with correlation coefficient of 0.9922 for Sr2 and 0.9904 for CN
+
−
Sr , CN and the standard (L) were excited at the same wavelength.
The quantum yield of fluoroionophore L was obtained using emission
spectra of standard fluorophore (4-aminoquinoline). The reported
quantum yield of 4-aminoquinoline [26] was approximately 0.23 and
2+
−
[Fig. S4 (A-B) (ESI†)]. The limit of detection (LOD) and the limit of
quantification (LOQ) were calculated by the minimum level at which
2
+
−
the solution of Sr and CN can be readily quantified with accuracy.
LOD and LOQ were calculated according to the 3.3 σ/s and 10 σ/s criteria
respectively where “σ” is the standard deviation of intercept of regression
equation and “s” is the slope of the corresponding calibration curve. The
that of L with Sr2 and CN ion complex were found to be 0.41 and
+
−
2+
−
0.45 respectively. Thus, when Sr and CN were attached to L, the
quantum yield increases. Hence L provides a sensitive means to recog-
LOD was found to be 1.36 nM for Sr2 and 1.23 nM for CN . The limit
+
−
nize Sr and CN by direct fluorescence measurement.
2+
−
of quantification (LOQ) has also been calculated for L and was found to
2+
−
be 4.52 nM for Sr and 3.74 nM for CN .
3.5. UV–visible spectral observations of L
3
.3. Determination of association constants
To examine the chromogenic behaviour and selectivity of L, UV–
visible titration experiment was carried out for Sr2 and CN . The orig-
+
−
Association constants for fluoroionophore were calculated by using
fluorescence titration data following the previously reported articles
25]. Here, we have displayed representative spectra showing the
changes observed in emission intensities upon the addition of increas-
ing concentration of ions. According to this procedure, the fluorescence
intensity (F) scales with the metal ion/anion concentration ([M])
inal absorbance band of ligand L was at 284 nm. It is noteworthy that
when we added 5 nM of Sr2 and CN solutions, it shifted affectedly
and a new band rises at 315 nm and 293 nm respectively which indicates
bathochromic effect or blue shift as displayed in [Fig. 3(A-B)]. The inter-
+
−
[
−
action of CN with amide linkage of L by forming hydrogen bonding and
2+
Sr binding with ketonic group via electrostatic interaction of scaffolds
2
+
through (F
obtained by plotting Log[(F
0
\\F)/(F-Fœ) = ([M]/Kdiss
)
n
. The binding constant (Ks) is
)] vs Log[M], where F and F
are the causes of blue shift. To know the detection limit of L towards Sr
−
0
\\F)/(F-F
œ
0
œ
and CN , we have also performed UV–vis titration by varying concentra-
tion of selective ions (0 nM- 100 nM) [Fig. 3(C-D)]. From this exploration,
if we compare both techniques- fluorescence and UV–vis experiment,
we can state that fluorescence technique is more unswerving than ab-
sorption study in terms of sensitivity and low detection limit.
are the relative fluorescence intensities without addition of guest
metal ions and with maximum concentration of metal ions (when no
further change in emission intensity takes place), respectively. The
value of Log [M] at Log[(F
0 œ
\\F)/(F-F )] = 0 gives the value of log
(
K
diss), the reciprocal of which is the binding constant (Ks). The ob-
2+
−
served binding constant from fluorescence spectra for Sr and CN
3.6. Determination of complex stoichiometry
are respectively 8.859 × 10 M and 8.574 × 10 M−1[Fig. 2(A-B)].
8
−1
8
To determine the stoichiometric ratio of L with Sr2+ and CN , we
have implemented different studies like ESI-MS, job's plot method, FT-
IR and powder XRD investigation. The formation of 1:1 complex was
confirmed by ESI-MS, where the spectrum shows molecular ion peak
m/z at 1047.8 whereas in the presence of Sr2 show peak at 1136.4
[Fig. S5 (ESI†)]. Further in this direction, the stoichiometry of the com-
plex formed (1: 1) has also been derived based on the Job's plot by
performing UV–visible experiment [Figs. S6-S7 (ESI†)]. After the confir-
−
.4. Quantum yield calculation of L with Sr and CN−
The quantum yield was calculated by following equation
ðF ꢀ Astd ꢀ ηÞ
2+
3
+
ϕ ¼ ϕstd
ðFstd ꢀ A ꢀ ηstdÞ
where F and Fstd are the areas under the fluorescence emission curves
mation of 1:1 complex formation ratio, we have also recorded FT-IR
2
+
−
2+
of the Sr
and CN ions complexes with L and only standard L
spectra of pure ligand L, L+ Sr
by potassium bromide (KBr) disk
Fig. 2. (A-B). Association constant diagram for (A) Sr2+ and (B) CN− with L ligand from fluorescence titration experiment.

P.G. Sutariya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 238 (2020) 118456
5
Fig. 3. (A-B) UV–visible absorption spectral changes of L (1 × 10 M) caused by binding with Sr2+ and CN−, (C-D) The UV–visible spectra of L (1 × 10−6 M) after adding different
−6
concentration of Sr and CN− (0, 10 nM, .0.100 nM) respectively.
2+
method and scanned at the wave number region 4000–600 cm−1.
19.05° (21−2) and 38.63° (102) respectively as depicted in Fig. 4. From
Moreover, FT-IR comparative experiment was also executed in which
the overly of PXRD patterns of metal complex, new peaks and peak shift
2
+
2+
the binding of Sr with ligand L was evaluated. The broadening of –
CH aromatic stretching peak, change in –C=O (ketone) peak in pres-
were observed which give reassurance of L: Sr binding. Table S1 re-
veals that ligand L and Sr2 has monoclinic crystal system with space
group P2/m. Analytical indexing of the powder patterns for ligand L
and L with Sr2 are summarized in Table S2 and Table S3 respectively.
+
2
+
2+
ence of Sr will give assurance of effective binding of Sr with L via
+
electrostatic interaction. [Fig. S8 (ESI†)]. Besides, for more confirmation
2+
of binding L: Sr , the PXRD study has also been conducted. In the Fig. 4,
) and L: Sr2
Sr ) are presented. The experimental 2θ range is 5.00 to
0.00° with a step size of 0.01° and a counting time of 60s per step.
+
3.7. Recognition of CN− with L via hydrogen binding
78 6 4
the powder diffraction patterns of the ligand L (C68H O N
(C H O N
68 78 6 4
2
+
The complexion mechanism between L and CN− was investigated by
H NMR titration study in DMSO‑d . Different amounts of CN was added
6
8
2+
1
−
The PXRD patterns of L and L: Sr exhibited intense peaks at 2θ values
Fig. 4. The plot shows results of PXRD investigation of the ligand L and L with Sr2+
.

6
P.G. Sutariya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 238 (2020) 118456
to a solution of L in DMSO‑d
addition of 10-fold excess CN into L ligand, the –NH peak at δ =
.71 ppm disappears because of the hydrogen bonding with –NH group
6
and then the spectrum was recorded. Upon
addition method was applied to evaluate the validity of the proposed
−
sensor. Table S4 and Table S5 demonstrate analytical results of them
+
−
8
with excellent recovery of spiked Sr2 and CN ranged from 94 to
99%, proving the validity of the developed technique. We have also car-
ried out selectivity factor calculation in presence of different ions with
of amide linkage of ligand L [Fig. S9 (ESI†)]. The fading of –NH peak will
−
give assurance of binding of CN ion with amide linkage of L.
L + Sr2 (10 nM) and L + CN (10 nM) at pH 7 (Table S6 and S7,
+
−
3
.8. pH evaluation of L
ESI). The proposed technique is compared with reported methods for
determination of Sr2 and CN (Table S8). This comparison undoubt-
+
−
It is indispensable to check at which pH our fluorescence probe works
edly endorses that proposed fluorometric method is superior to others
2+
effectively. The effect of pH on fluorescence spectrum L in the presence
in terms of sensitivity, selectivity and binding ability to recognize Sr
2+
−
−
and absence of Sr and CN was studied by measuring fluorescence in-
and CN [27–31].
tensity of L at different pH values. It was observed that the interaction of L
with Sr2 and CN was the most effective at pH 7 as shown in [Fig. S10
+
−
3.11. Outcomes of paper based analytical device (PAD)
(A-B) (ESI†)] by exhibiting maximum enhancement which indicates that
our chemosensor will act efficiently in neutral condition.
Fluorometric probes for dual ion detection are commonly used in
cuvette-based tests employing conventional spectrometers. This ap-
proach is inconvenient and cannot be used for on-site detection. To over-
come this problem, we have developed a paper-based analytical device
(PAD) which is based on fluorescent calix[4]arene (L) immobilized on
PAD as sensing probes contained within NCM membrane as illustrated
in Fig. 7. NCM was selected as substrate due to its superior ability to
wick aqueous or organic solutions by capillary action, among other rec-
ognized qualities that make pumps or high-power source requirements
not necessary. The sensing mechanism observed here is based on chela-
tion enhanced fluorescence effect where the fluorescence enhancement
of probe L occurs simultaneously due to its distinctive interaction of de-
signed calix[4]arene for cations and anions. Fig. 7 elucidates the fluores-
cent calix[4]arene (L) based probe which is immobilized on PAD and can
produce blue fluorescent colour under UV light. Thus, to aid the determi-
nation of fluorescent changes with portable, handheld and test strip
reader, we have tried to prepare a simple dip test strip-based assay
3
.9. Interference study
As in real samples, various other metal ions and anions are also pres-
2+
−
ent along with the Sr and CN and can affect the sensing process, it is
necessary to investigate functional properties and selective nature of L to-
2+
−
wards Sr and CN in the presence of co-existing cations and anions.
This interference study was carried out using 10 equivalents of other cat-
2+
ions with Sr separately. In a same way, 10 equivalents of different an-
−
ions were taken with CN to check whether it affects the fluorescence
intensity or not. The results of this experiment lead us to note that no
other competitive ions displayed any significant interference. The fluores-
2+
−
cence intensity of L: Sr and L: CN did not change drastically upon ad-
dition of aforementioned cations and anions as depicted in [Fig. 5(A-B)].
Plausible binding mechanism through hydrogen bonding and electro
static interaction with L ligand by Sr2 and CN is shown in Fig. 6.
+
−
which can detect Sr2 and CN ions in neat and low molar concentration
aqueous solution. A drop of L solution was placed at delineated area test
hole (T1 and T2) along with control, dried at 37 °C, and a fluorescent
change was measured. As shown in [Fig. 11(A) (ESI†)] after the addition
+
−
3
.10. Application of the method to real sample assay
After studying the sensing properties of our fluorescence probe L
of different concentration of Sr2 ranging from 10 to 10 M, excited-
+
−6
−3
with various molecular ions, we have also investigated its properties
in real sample assays such as different industrial waste water samples
state photons from the calix[4]arene are transferred to the analyte
2
+
−
2+
towards Sr and CN by using fluorescence titration. The standard
displaying a proficient enhancement in presence of Sr . Likewise,
Fig. 5. (A-B). (A) The histogram shows relative changes in fluorescence intensity of L (1 × 10 M) with Sr in presence of other cations (a = Ligand + Sr2+, b = a + Zn2+, c = a + La3+
−
8
2+
,
d = a + Mn , e = a + Cd , f = a + Ba , g = a + Co , h = a + Ni , i = a + Zr , j = a + Ca2+, k = a + Ce , l = a + Li+, m = a + Ag+, n = a + Fe2+,o = a + Fe3+a + Co2+,
2
+
2+
2+
2+
2+
4+
3+
p = a + Hg , q = a + Na , r = a + K+,s = a + Nd3+,t = a + Pb , u = a + Pr , v = a + Cu and w = a + As ), (B) The histogram shows relative changes in fluorescence intensity
2
+
+
2+
3+
2+
3+
of L (1 × 10− M) with CN in presence of other cations (a = Ligand + CN , b = a + F−, c = a + Cl−, d = a + Br−, e = a + I−, f = a + HSO
8
−
−
−
, g = a + H
PO
−
, h = a + NO
−
, i = a + S
2−
,
4
2
4
3
−
j = a + CH
3
COO ).

P.G. Sutariya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 238 (2020) 118456
7
Fig. 6. Plausible binding model of the sensor L, L: Sr2+ and L: CN−
.
after the addition of different concentration of CN ranging from 10−7 to
−
most stable molecular geometry. The heat formation energy was gener-
ated −21.71 kcals/mol using NM7 method. MOPAC calculations pro-
duced visual models of the most stable molecular geometry in ball and
stick configuration are displayed in [Fig. S12 (ESI†)].
−4
1
0
M L exhibits a remarkable enhancement depicted in [Fig. S11
B) (ESI†)]. It can be envisioned by naked eye. These results also indicate
(
−6
that naked eye, this sensor is visibly able to detect ion up to the 10
M
for Sr2 ion and 10 M for CN ion with better sensitivity. It is also sig-
+
−7
−
2+
−
nificantly less than the recommended Sr and CN levels mentioned by
3.12.1. HOMO and LUMO analysis
2+
−
EPA guidelines. Design for onsite monitoring of Sr and CN levels
using low cost test papers has been shown in Fig. 7.
The most important orbitals in a molecule are the frontier molecular
orbitals, called highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO). These orbitals determine the
way the molecule interacts with other species. The Eigen values of
HOMOs (π-donor) and LUMOs (π-acceptor) and their energy gap reflect
the chemical activity and the energy gap between them which has
been used to prove the bioactivity from keto-enol charge transfer of L.
Homo and Lumo orbitals of the L and L with Sr2 and CN− complexes
are plotted in Fig. 8. The change in HOMO-LUMO energy band gap
suggested that the charge transfer of molecules and its explanation of
3
.12. Binding nature of L by computational studies
Molecular orbital package (MOPAC) software has used to optimize
geometry of ligand L as optimization of the structure is an imperative vi-
sion of computational studies [32]. This semi-empirical approach has
been executed by applying a Z-matrix of the molecule which was then
transformed into its MOPAC input data. Optimization of these data has
delivered the specific bond distance, bond angle, dihedral angle of the
+
2+
fluorescence enhancement during the recognition event of Sr and
Fig. 7. (A-C). Fluorescent L immobilized dual plasmonic sensor, (A) L containing test strip under fluorescent light (B) Dip in Sr and CN− containing sample (C) Read under fluorescent
2+
light (365 nm) indicating the presence of Sr2+ (T1) and CN−(T2).

8
P.G. Sutariya et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 238 (2020) 118456
Fig. 8. Results of Homo - Lumo orbital analysis of free ligand L, L + Sr 2+ and L + CN− complexes.
−
CN . The band gap (Fig. 8) reveals that the energy band gap in free li-
Declaration of competing interest
There are no conflicts to declare.
Acknowledgements
2+
gand L is −0.11363 eV whereas for L-Sr has shown −0.01193 eV
band gap. The band gap of complex L: CN is −0.11308 eV, which
supports the enhancement effect in fluorescence intensities of Sr
and CN with L.
−
2+
−
P. G. Sutariya would like to thank the Department of Science and
Technology – Science and Engineering Research Board (DST-SERB) for
providing financial assistance under the Young Scientist Scheme (YSS/
3
.12.2. Molecular docking investigation
To explore new possibilities of L, we have also carried out molecular
docking investigation of our synthesized probe L. For the molecular
docking study, we have collected protein structure receptors (Homo
sapiens) 5ADG, 5VUV, 5VUW, 5UVX, 5VVZ and 5VV1 from protein data
bank and calculated energy score of molecular docking by Hex 8.0 soft-
ware [33]. It was evaluated using molecular dynamics, by their binding
affinities, free energy simulations and relative stabilities. The energy
values obtained by docking study tabulated in Table S9 and the molec-
ular docking pose of different protein receptors with L is presented in
2015/00128) (New Delhi).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.saa.2020.118456.
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