An antipyrine based fluorescent probe for distinct detection of Al3+ and Zn2+ and its AIEE behaviour

Article abstract of DOI:10.1039/c9pp00472f

A simple antipyrine based fluorescent probe, 4-[(2-hydroxy-3-methoxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (OVAP), has been successfully synthesized using a one-step condensation method. It exhibits dual sensing properties to

Full text of DOI:10.1039/c9pp00472f

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2020, DOI: 10.1039/C9PP00472F.
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Number 12
December 2017
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DOI: 10.1039/C9PP00472F
An Antipyrine based Fluorescent Probe for distinct detection of
Al3+ and Zn2+ and Its AIEE Behaviour
Samir Maity, Milan Shyamal, Rakesh Maity, Naren Mudi, Paresh Hazra, Prabhat Kr. Giri,
Shashanka Shekhar Samanta, Santanu Pyne and Ajay Misra*
Department of Chemistry, Vidyasagar University,Midnapore 721 102, W.B, India.
Corresponding Author:
E-mail: ajay@mail.vidyasagar.ac.in; Tel.: +91 8967986988; Fax: +91 3222 275329
Abstract
A simple antipyrine based fluorescent probe,4-[(2-Hydroxy-3-methoxy-benzylidene)-
amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one(OVAP)has been successfully
synthesized using one step condensation method. It exhibits dual sensing property toward
Al3+ and Zn2+in presence of other relevant metal ions and also displays novel aggregation
induced emission enhancement (AIEE) characteristics in its aggregated/solid state.
Aggregated OVAP microstructures with interesting morphologies have been synthesized
using SDS as morphology directing agent. Morphologies of the particles are characterized
using optical microscopy. Photophysical properties of as synthesized OVAP hydrosol are
studied using UV–Vis absorption, steady state and time resolved fluorescence spectroscopy.
The ‘turn on’ luminescent property of OVAP is used for the selective detection of trace
amounts of Al3+ and Zn2+and a significant turn on fluorescence enhancement over ∼100-fold
is triggered via chelation-enhanced fluorescence (CHEF) through complex formation. The
1:1 stoichiometry of the each sensor metal ion complex is observed from Job’s plot based on
UV−Vis absorption titration. The LODs for Al3+and Zn2+are found to be 1.05 nM and 2.35
nM, respectively. Notably, the sensor, OVAP is further demonstrated by molecular INHIBIT
logic gate.
Keywords: OVAP; AIEE; Morphology; DFT; Dual Sensor; Molecular logic gate
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Introduction
DOI: 10.1039/C9PP00472F
Organic luminescent materials that show fluorescence in solid state have been paying
attention owing to its potential applicationsinfluorescencesensors1, semiconductorlasers,2
optoelectronicdevices3 etc .Luminescent organic aggregates such as amorphous solids,
nanocrystals or microcrystals are found to be much more photostable than isolated
molecules.4On the other hand these aggregates evoke significant interest for their potential
applications in sensors, flat panel display and illumination, etc.5-8due to their unique
photophysical properties and low cost structural design. Most organic luminescent materials
exhibits strong emission in their dilute solution, but their emissions disappeared hastily on
increasing concentrations in solution or aggregations in solid state because of strong
intermolecular π–π stacking interactions and other non-radiative decay pathways. This
phenomenon is popularly known as Aggregation Caused Quenching (ACQ) effect.9Some
anti-ACQ materials were reported by Tang10and Park11.Fluorophores showing aggregation
induced emission enhancement(AIEE) characteristics have successfully been utilized in
organic light emitting diodes(OLEDs),12 electroluminescent materials,13 photoemitters,14
mechanochromism,15 sensors16-20 etc.
In connection with the sensing of biologically and environmentally relevant ions,
fluorescence based tools are considered to have enhanced the capabilities for sensing of
specific target analytes.21,22A fluorescent probe produces a detectable fluorescent signal upon
interaction with a chemical species23 and a number of mechanisms like photoinduced
electron/energy transfer (PET),24chelation-enhanced fluorescence (CHEF),25fluorescence
resonance energy transfer (FRET),26metal–ligand charge transfer (MLCT),27excited-state
intra/intermolecular proton transfer (ESIPT),28excimer/exciplex formation ,29intramolecular
charge transfer (ICT),30and C=N isomerisation31are being used to explain the chemo sensing
processes. Compare to other transition metal ions, selective chemo sensors for Al3+are limited
due to its poor coordination power and lack of spectroscopic characteristics.32-36 It has been
observed that as a hard acid Al3+ prefers hard base containing N and O donor
atoms.37Consequently, it is highly demanding to develop simply synthesizable fluorescent
chemo sensors for selective and rapid recognition of Al3+.On the other hand, there has been
increasing interest to develop highly selective and sensitive chemo sensor for Zn2+among the
researchers. Fluorogenic chemo sensor is found to be primary choice for Zn2+ detection due
to its closed shell configuration (3d104s0)which does not produce spectroscopic or magnetic
signals.38,39Due to the importance of Al3+ and Zn2+, many fluorescent chemo sensors for
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separate detection of Al3+ ions and Zn2+ ions have been reported. Yet, most of them contain
DOI: 10.1039/C9PP00472F
potential toxic substance in their ingredients, which will pollute the environment and harm
human health. Additionally, many chemo sensors need complicated syntheses involving
harsh reaction conditions and expensive chemicals. More significantly, very few fluorescent
chemosensory can selectively detect both Al3+ and Zn2+ simultaneously.40-46So, it is desirable
to develop fluorescent chemo sensors with high sensitivity and selectivity for multi-metal
ions.
Considering the above fact, we successfully synthesized simple antipyrine based
Schiff base 4-[(2-Hydroxy-3-methoxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-
dihydro-pyrazol-3-one (OVAP) with potential hard N, O donor atoms through a simple one-
step synthetic process, where free rotating groups antipyrine and hydroxybenzene moieties
are attached to the azo group. Schiff base was found to be poorly fluorescent in good solvent,
in part due to isomerization through rotation around the C=N double bond in the excited
state.47But our synthesized OVAP as a fluorescent “turn on” chemo sensor can recognize
selectively both Al3+ and Zn2+ ion via CHEF process. On the other hand, Aggregated OVAP
exhibits AIEE effect and strong yellow fluorescence is observed under ultraviolet irradiation.
Microstructures of OVA with various morphologies have been synthesized using SDS as
morphology directing agent.
Experimental section
Materials
4-aminoantipyrine was purchased from Merck India Ltd. Ortho-Vanillin was
purchased from Sigma-Aldrich Chemical Corp. All the solvents used were of analytical
grade. Sodium dodecyl sulphate (SDS) was purchased from Merck India Ltd. and was
recrystallized from 1:1 ethanol water mixture. Purity of these chemicals was checked
spectrophotometrically. Salts(Nitrate and chloride) of all cations (Aldrich and Merck) were of
high purity and used without further purification. Triply distilled deionized water was used
throughout the experiments.
Synthesis of OVAP and OVAP Microparticles
Ortho-Vanillin (1 equiv) was added to 4-aminoantipyrine(1 equiv) in 50 mL methanol
with constant stirring for 5h. The reaction was monitored by thin-layer chromatography.
After completion, pale yellow color solid product was filtered, washed thoroughly with
methanol, recrystallized and then dried in a desiccators. Yield: 82%. Anal. Calcd for
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C19H19N3O3: C, 67.64%; H, 5.68%; N, 12.46%; Found: C, 67.28%; H, 5.29%; DNO,I: 1102.1.02393/%C9P.P00472F
FT-IR (KBr), ν, cm−1: 3431 (O − H), 1665 (C = 0), 1598 (C = N), 1459, 1411 (C = C), 1296,
1
1249 (C−O), 1136 (O − H). H NMR (400 MHz, CDCl3) δ (ppm): 13.92 (H, s (OH)), 9.84
(H, s (−CH=N)), 7.53−6.83 (8H, ArH), 4.00 (OCH3, s), 3.259 (s, 3H), 2.441 (s, 3H); 13C
NMR (400 MHz, CDCl3, δ (ppm)) δ: 10.13, 35.59, 56.07,113.60, 116.01, 118.47, 120.16,
123.61, 124.70, 127.35, 129.31, 134.33, 148.10, 149.91, 150.44, 160.24, 160.48 ppm. The 1H
NMR and 13C NMR peaks of OVAP are similar to the earlier reported data.48,49
The microstructures of OVAP were prepared by re-precipitation method and SDS was
used as morphology directing agent. In a typical preparation, small volume of 1mM OVAP in
EtOH was injected into 5 mL of continuously stirred water and aqueous SDS separately at
room temperature (25°C). Clearness of the solution was gradually disappeared and a milky
yellowish color appeared after gradual increase in concentration of OVAP. The solution was
allowed to stand for overnight. Concentration of both OVAP and SDS were varied to
synthesize various microstructures of OVAP. During the synthesis of sample-a, b, c, d, e, f, &
g concentration of SDS (3.96 mM) was kept constant and the concentration of OVAP
(sample-a: 19.6µM, sample-b: 38.4µM, sample-c: 56.6µM, sample-d: 74µM, sample-e:
90.9µM,sample-f: 122.8 µM, sample-g: 166.6µM, ) was varied to study the effect of
concentration of OVAP on aggregated microstructures. Similar method was used for the
preparation of OVAP micro structures in the absence of SDS and the samples are named as
sample-a1 to g1 respectively.
Scheme 1: Synthesis of 4-[(2-Hydroxy-3-methoxy-benzylidene)-amino]-1,5-dimethyl-2-
phenyl-1,2-dihydro-pyrazol-3-one(OVAP)
Characterization
UV-Vis spectroscopic measurements were carried out in a Shimadzu UV-1800
spectrophotometer with
a
1cm quartz cuvette. Hitachi F-7000 Fluorescence
1
Spectrophotometer was used to record the fluorescence spectra. H and 13C NMR spectra
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were recorded on a Bruker ASCEND spectrometer operating at 400MHz in CDCl3 and
DOI: 10.1039/C9PP00472F
deuterated dimethyl sulfoxide (DMSO-d6). The Fourier transform infrared (FT-IR) spectra
were obtained in the range of 4000−400 cm−1 using KBr pellets in a PerkinElmer Spectrum-
Two FTIR spectrometer. Fluorescence lifetimes were obtained by the method of Time
Correlated Single-Photon Counting (TCSPC) using Horiba, Delta Flex Modular Fluorescence
Lifetime System with diode laser excitation source (372 nm) and the detection wavelength
were 525 nm for OVAP hydrosol, 520 nm for OVAP with Al3+ and 535 nm for OVAP with
Zn2+. Lamp profiles were measured with a band-pass of 3 nm using Ludox as scatterer. The
decay parameters were analysed using non-linear iterative fitting procedure based on the
Marquardt algorithm.50 The quality of fit was assessed over the entire decay, with a plot of
weighted residuals and other statistical parameters e.g. the reduced χ2 ratio.51The signal was
collected at magic angle (54.7°) using a PPD-850, Picosecond Photon Detection Module.
Time resolution of the experimental setup was ~80 ps. Scanning electron microscopic study
of gold coated dried OVAP hydrosols were done using Carl Zeiss EVO 18 scanning electron
microscope. Optical microscopic study were carried out using an NIKON ECLIPSE
LV100POL upright microscope with CCD camera (model no. Nikon DS-Fi l), polarizer-
analyser assembly and 100W mercury lamp as excitation source for emission study.
Particlesize distribution was measured by dynamic light scattering (DLS) experiments on a
Malvern Zetasizer NanoZS90 instrument.
Limit of Detection (LOD)
The LOD was calculated on the basis of the fluorescence titration.Standard deviation
of the blank measurement was obtained from the plot of fluorescence intensity of OVAP as a
function of its increasing concentrations. This procedure was repeated five times to get its
average. The slopes (k) of 3σ method for sensing Al3+ and Zn2+ by OVAP was obtained from
the plot of OVAP PL emission intensity as a function of increasing Al3+& Zn2+
concentrations at 520 nm and 535 nm respectively. The detection limit was calculated using
the following equation.52
LOD = 3σ/k
Where, σ is the standard deviation of blank measurement, and k is the slope of the calibration
curve obtained from linear dynamic plot of fluorescence intensity versus increasing
concentration of metal ions.
Fluorescence Quantum Yield
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We also measured the fluorescence quantum yield of OVAP, OVAP-Al3+ and OVAP-
DOI: 10.1039/C9PP00472F
Zn2+ in ethanol and OVAP aggregates in water using the following equation, where quinine
sulphate was used as standard with λex=350 nm.
Φs = Φr (ArFs/AsFr) (ηs2 /ηr2)
As and Ar are the absorbance of the sample and reference solutions, respectively, at the same
excitation wavelength, Fs and Fr are the corresponding relative integrated fluorescence
intensities, ηs and ηr are the refractive index of the sample and reference solutions
respectively.
Results and Discussion
Synthesis of OVAP is depicted in Scheme-1 and synthesis details are described in
Experimental Section. It was completely characterized by physicochemical and spectroscopic
1
analysis. Structure and purity of the probe were confirmed using H NMR (Fig.S1) and13C
NMR (Fig.S2) spectra. A vibrational band at 1598 cm−1 was observed in the IR spectra
(Fig.S3) of OVAP and it is due to the stretching vibrational mode of imine(−CH=N−) group
presentin OVAP.
SEM &Optical Microscopic Study
Scanning electron microscopic images of gold coated dried OVAP hydrosol (sample-
e, f & g) are shown in Fig. 1.Fig. 1 shows rod shaped particles are formed and the length (1-
D growth) of the particles increases with increasing concentration of the particles in the
samples.
Fig. 1: SEM images of OVAP microstructures prepared in presence of 4 mM SDS (i) sample-
e (90.0µM OVAP),(ii) sample-f (122.8 µM OVAP) , (iii) sample-g (166.6µM OVAP).
Morphologies of the particles as observed by fluorescence microscopy are shown in
Fig.2. At lower concentration of OVAP bar shaped morphologies are clearly observed for the
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sample-e, f, g. It shows rod-shaped microstructures with greenish yellow emission upon UV
DOI: 10.1039/C9PP00472F
excitation. There is a clear indication that the dimensions of particles increases along its
length with increasing concentration of OVAP.
Fig.2: Optical fluorescence microscopy images (under UV excitation) of OVAP
microstructure prepared in presence of 4 mM SDS (a) sample-e (90.0µM OVAP), (b) sample-
f (122.8 µM OVAP) (c) sample-g (166.6µM OVAP).
UV-Vis Study
UV–Vis absorption spectra of OVAP in EtOH and the as prepared hydrosol of OVAP
(sample a, b,c,d,e,f,g) are shown in Fig.3.Absorption spectrum of OVAP in EtOH has two
distinct regions at 210-280 nm and 300-400 nm respectively. Comparison of UV-Vis spectra
of OVAP with that of vanillin and antipyrine moieties in ethanol(Fig.S4) suggests that the
structured absorption bands in the region 210-280 nm are due to aromatic π- π* absorption
band of O-vanillin and antipyrine group present in OVAP. In order to understand the nature
of absorption band at 300-400 nm, we did the UV-Vis absorption study of OVAP and O-
vanillin in presence of base (Fig. S5). Gradual increase of base concentration (pH= 8 to 11), a
new red shifted broad absorption band (λmax = 390 nm) for OVAP appear with an isosbestic
point at 372 nm (Fig.S5a). Similarly, absorption study of O-vanillin (Fig. S5b), and
comparison with OVAP revels that the broad absorption band of OVAP at 300-400 nm is due
to intramolecular proton transfer absorption band between phenolic-OH and imine N atom in
OVAP.
However, with gradual increase of water fraction, aggregation starts and the absorption
maximum at 332 nm diminishes (Fig.3). On the other hand, a new shoulder band at ̴264 nm is
appeared and the absorbance of the band increases with increasing volume percentage of
water. In addition, the overall shift of the spectral baseline of samples a-f relative to the
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monomer(Fig.S6) is attributed to the scattering of light by the larger aggregated structurVeieswiAnrticle Online
DOI: 10.1039/C9PP00472F
solution.
Fig. 3: UV-Vis absorption spectra of OVAP (47 µM) in (i) 0% water (OVAP in EtOH), (ii)
20% water, (iii) 40% water, (iii) 60% water, (iv) 80% water, (v) 90% water and (vi) 99%
water.
Steady State Emission Study
PL spectra of OVAP in EtOH and its aggregated hydrosol (in 4mM SDS)with
different amount of OVAP is shown in Fig.4a. It is observed that the dilute solution (6 μM)
of OVAP in EtOH exhibits very weak broad emission band with peak maximum at 550nm
upon photo excitation at 360 nm. But the aggregated hydrosol of OVAP in SDS (4 mM)
medium exhibits nearly 25 nm blue shifted intense structured emission with a maximum at
525 nm. It is also observed that the fluorescence intensity of OVAP hydrosol in the region
475-650 nm increases gradually with the increasing concentration of OVAP. This is due to
aggregation-induced emission enhancement (AIEE)effect. Energy optimized structure of
OVAP (Fig.S7) suggests that the molecule OVAP as a whole is floppy in nature. Now upon
aggregation in its aggregated hydrosol, the floppiness of the molecule decreases, resulting the
restriction of intramolecular rotation (RIR) to be operative to show AIEE emission. Also a
similar observation in PL study using different concentration of OVAP without SDS is shown
in Fig. 4b.It shows that the PL intensity of the aggregated hydrosol of OVAP without SDS is
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lower compared to the hydrosol in SDS medium but there is a clear signature of AIEE
DOI: 10.1039/C9PP00472F
compare to OVAP in EtOH.
Fig.4c shows the fluorescence emission spectra of OVAP at different volume
fractions of water for a fixed concentration of OVAP(47 μM). It is observed that PL intensity
increases gradually up to 90% water fraction, indicating the fabulous AIEE properties of
OVAP. Buta sharp change of PL spectra with increased intensity are observed as the volume
percentage of water changes from 90% to 99%. Fluorescence emission study at different
interval of time of OVAP hydrosol shows that it takes nearly 20 minutes to attain stable
emission intensity. This is due to the time required to attain thermodynamically stable
aggregates of OVAP in 99% volume percentage of water. PL spectrum of the aggregated
hydrosol at 99% v/v is blue shifted and it is due to comparatively less polar environment
faced by OVAP in its aggregated state. A plot of PL emission intensity as a function of
volume fraction of water is show in Fig. 4d and it shows a sharp rise in PL intensity at > 90%
volume fraction of water. Time evolution of emission intensity of OVAP hydrosol in 99%
volume fraction of water shows that it takes nearly twenty minutes to attain the maximum
stable intensity of hydrosol (Fig. S8). On the other hand, we also measured the solid state
emission of OVAP which is similar to that of the aggregated hydrosol of OVAP (Fig.S9).
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Fig.4: (a) Fluorescence emission spectra of (i) OVAP in EtOH, and OVAP hydrosol prepared
DOI: 10.1039/C9PP00472F
in the presence of 4 mM SDS (ii) sample-a, (iii) sample-b, (iv) sample-c, (v) sample-d, (vi)
sample-e, (vii) sample-f, (viii) sample-g. (b) The Fluorescence emission spectra of (i) OVAP
in EtOH and OVAP hydrosol prepared in the absence of SDS (ii) sample-a1, (iii) sample-b1,
(iv) sample-c1, (v) sample-d1, (vi) sample-e1, (vii) sample-f1, (viii) sample-g1. All emission
spectra were taken with 360 nm excitation. (c) Emission spectra of freshly prepared OVAP
(62 μM) in 0% - 99% water. λex: 360 nm. (d) Plot of relative variation of PL intensity Vs
water volume percentage in the EtOH/water mixture. Insets: Fluorescence images of OVAP
(0% and 99% H2O) under the illumination of 366 nm light.
Time-Resolved Photoluminescence (TRPL)Measurements
In order to understand the Photophysical behaviour of OVAP and its hydrosol, time
resolved photo luminescence (TRPL) were carried out as shown in Fig.5. Measurement of
fluorescence lifetime is more robust compare to fluorescence intensity, because it depends
neither on the excitation photon density nor on the concentration of the fluorophore. Fig.5
shows the fluorescence decay of OVAP in pure EtOH and EtOH-H2O (99%) mixture.
Analysis of fluorescence decay curve shows that the fluorescence lifetime value increases
from 96 ps for OVAP in EtOH to 1.72 ns for OVAP aggregates in 99% volume fraction of
water. The measured emission quantum yield of OVAP in EtOH and aggregated hydrosol of
OVAP are ΦOVAP(EtOH)=0.009 and ΦOVAP (agg)= 0.172 respectively. On the basis of
radiative rate constant (Kr) and emission quantum yield, we have calculated the non- radiative
rate constant (Knr) and the values are listed in Table-1. There is almost 21times decreased of
Knr value is observed upon aggregation of OVAP. Thus the increased PL lifetime of OVAP
aggregates is the results of decreases of non-radiative process due to restrictions of
intramolecular rotational(RIR) motion upon aggregation.
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DOI: 10.1039/C9PP00472F
Fig.5:Time-resolved fluorescence spectra of OVAP in ethanol (0% water; red), OVAP
hydrosol (99% water; blue) and Lamp (black). ex= 372 nm and em= 550 nm (OVAP in
ethanol), em= 525 nm (OVAP hydrosol).
DLS Measurement
DLS measurements were also performed in support of average particle size distribution of
OVAP aggregates in the presence of 99% water volume fraction in H2O/EtOH mixture (Fig.
S10). DLS studies revealed that the diameter of the aggregated particles lies between 200 nm
to 400 nm and the average calculated diameter of OVAP aggregates is found to be ~310nm
and this is in the range of previously reported many other AIEE systems [21,22].
Analytical Performance: Sensing Knack toward Al3+& Zn2+ Ions
UV-Vis Study of OVAP in the Presence of Metal Ions
Interaction of OVAP with various metal ions (Na+, K+, Ag+, Mg2+, Ca2+, Co2+, Ni2+,
Cu2+, Cd2+, Zn2+, Pb2+, Hg2+, Cr3+, Fe3+, Al3+ and Mn2+) was performed in EtOH medium.
The absorption spectrum of chemo sensor OVAP exhibits two weak absorption bands at 229
nm and 272 nm, a broad band at 334 nm originating due to intramolecular proton transfer
absorption of OVAP (Fig.6). The position of absorption band remained unchanged with most
of the metal ions (1 equivalents of each)except Al3+, Co2+, Cu2+, Ni2+, Cd2+, Mn2+and
Zn2+where a broad absorption peak appear at 400-470 nm with a concomitant decreases of
334 nm band. Titration experiments with gradual addition of Al3+& Zn+2ions resulted in
diminishing of the peaks at 229, 272 and 334 nm along with the appearance of a new peak at
406 nm for Al3+ and 425 nm for Zn2+(Fig.6), and these new peaks are attributed to the strong
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ligand-to-metal charge-transfer (LMCT) absorption band. The presence of isosbesDtiOcI: p10o.1i0n3t9s/Ca9PtP00472F
380 and 250 nm established the transformation of a free receptor to its aluminium complex as
shown in Fig.6a. Titration experiments with Zn2+also yielded almost similar results as in the
case of Al3+(Fig.6b). Interestingly, a colour change from colorless to light yellow and deep
yellow was observed with Al3+ and Zn2+, respectively (inset Fig.6a and 6b). Job’s plot
obtained from the titration experiments yielded 1 : 1 stoichiometry of the complex between
OVAP and each of metal ions Al+3& Zn+2(Fig. S11a & b) respectively.
Fig. 6:Changes in absorption spectra of OVAP (47µM) with (a)increasing concentration (4.9
µM to 47 µM) of Al3+, (b)increasing concentration (4.9 µM to 47 µM) of Zn2+.Inset: colour
change upon the addition of Al3+and Zn2+ to OVAP in ethanol.
Fluorescence Study of OVAP in the Presence of Metal Ions
PL spectra of OVAP in presence of various metal ion in solution are shown in
Fig.7.OVAP displays a weak emission peak at 550 nm on excitation at 360 nm. In the
presence of various metal ions (Na+, K+, Ag+, Mg2+, Ca2+, Co2+, Ni2+, Cu2+, Cd2+, Zn2+, Pb2+,
Hg2+, Cr3+, Fe3+, Al3+, Mn2+ in ethanolic solution), a remarkable enhancement of the emission
spectra of OVAP was observed only with Al3+ and Zn2+peaking at two distinct wavelengths,
520 nm and 535 nm, respectively ( Fig.7a & 7b). Green fluorescence was observed in the
case of Al3+ whereas yellowish green fluorescence was observed in presence of Zn2+ (inset
Fig.7a & 7b). These results also validated the selectivity of OVAP towards Al3+ and Zn2+.
Fluorescence titration experiments were also carried out with gradual addition of Al3+ and
Zn2+ to the solution of OVAP. In both the cases, a gradual increase in emission intensity of
OVAP in presence of metal ions (Al3+& Zn2+) was observed (Fig.7). Relative fluorescence
colour changes of OVAP in presence of Al3+ and Zn2+ are shown in the inset of Fig.7a & 7b
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respectively. Fluorescence titration using the equimolar mixture of Al3+ and Zn2+ (Fig. VSie1w2Ar)ticle Online
DOI: 10.1039/C9PP00472F
were also carried out. Two distinct emission maxima ( 520nm & 535 nm) separated by only
15 nm are observed and it further supports that OVAP can be an efficient fluorescence probe
for simultaneous sensing of Al3+ and Zn2+ in a mixture.
Fig. 7: Fluorescence emission spectra of OVAP (47μM) (a) with the increasing concentration
(4.9 µM to 47 µM) of Al3+. [Inset: colour change upon the addition of Al3+ to OVAP under
UV light irradiation,  = 366 nm]. (b) with increasing concentration (4.9 µM to 47 µM) of
Zn2+[Inset: colour change upon the addition of Zn2+ to OVAP under UV light irradiation,  =
366 nm]. (c) Plot of emission intensity at 520 nm vs. [Al3+]. (d) Plot of emission intensity at
535 nm vs. [Zn2+]. Excitation wavelength for all the emission spectra is 360 nm.
Time-Resolved Photoluminescence(TRPL)Study
The TRPL measurements were also carried out to better understand the mechanism of
the ‘turn-on’ emission responses of OVAP toward Al3+ and Zn2+ ion as shown in Fig.8. The
fluorescence lifetime (τ) of OVAP in EtOH is 96 ps, whereas longer average fluorescence
lifetimes of 2.23 ns and 2.32 ns were detected in the presence of 1:1 equiv of Al3+&
Zn2+respectively. Our measured fluorescence quantum yield for OVAP, OVAP-Al3+ and
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OVAP-Zn2+ in EtOH are 0.009, 0.68 and 0.47 respectively (Table-1). We furthDeOrI:c10a.l1c03u9l/aCt9ePP00472F
the non-radiative rate constant of OVAP and its complex with Al3+& Zn2+ (Table-1). On the
basis of the measured fluorescence quantum yield and fluorescence lifetime, the non-
radiation rate constant of OVAP in EtOH, OVAP hydrosol, OVAP-Al3+ and OVAP-Zn2+
were obtained as 10.32, 0.48, 0.14 and 0.22 S-1 respectively. The observed non-radiative rate
constant of OVAP is nearly 20,50, and 100 times higher compare to aggregated hydrosol,
OVAP-Zn2+ and OVAP-Al3+ complexes respectively. This higher Knr vale of OVAP is
responsible for its much lower steady state fluorescence intensity. On the other hand a
comparison of Knr value of OVAP-Zn2+ (0.22 S-1) and OVAP-Al3+ (0.14 S-1) shows that Knr
of OVAP-Al3+ is about 1.6 times higher than OVAP-Zn2+ complex. This lower Knr value of
OVAP-Al3+ complex is responsible for higher steady sate fluorescence intensity of OVAP-
Al3+ complex than OVAP-Zn2+ complex for the similar concentration of OVAP and metal
complexes. The Knr value upon complexation decreases nearly 73 time in OVAP-Al3+ and 47
times in OVAP-Zn2+ complex. Thus the enhance fluorescence lifetime is due to possible
decreases of Knr upon chelation between metal ions and OVAP.
Fig. 8:Time resolved fluorescence spectra of OVAP (47 μM) in EtOH and OVAP with Al3+&
Zn2+ respectively. Lamp (black), OVAP in EtOH (red), 1:1 equiv OVAP &Al3+(blue); 1:1
equiv OVAP & Zn2+ (pink); (λex = 372 nm, λem = 520 nm for Al3+& OVAP and 535 nm for
Zn2+& OVAP).
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DOI: 10.1039/C9PP00472F
Table-1:
Sample
τf
Φf
Kr (S-1)
Knr (S-1)
χ2
(avg. ns)
0.096
(Φf/ τf) [(1/τf)- Kr]
OVAP in EtOH
0.009
0.172
0.68
0.093
0.1
10.32
0.48
0.14
0.22
1.27
1.12
1.13
1.11
Aggregated OVAP (99% H2O)
OVAP+Al3+ (1:1) in EtOH
OVAP+Zn2+ (1:1) in EtOH
1.72
2.23
2.32
0.304
0.202
0.47
Metal Ions Competition Study
The selectivity of OVAP towards Al3+ and Zn2+ was also established through
experiments in the presence of competing metal ions (Fig.9)through UV-Vis and fluorescence
emission study. Absorption spectrum of chemo sensor OVAP shows two weak absorption
bands at 272 and 334 nm (Fig. 6a).The position of absorption band remained unchanged with
most of the metal ions except Al3+, Co2+, Cu2+, Zn2+, Ni2+, Cd2+, Mn2+ where a new broad
band 400-470 nm is appeared. It is interesting to note that among all the tested metal ions
only Al3+ and Zn2+ rendered significant fluorescence ‘turn-on’ responses, whereas other
relevant competing metal ions has no noteworthy effect in the emission spectra(Fig.9b).In
order to establish the specific selectivity of OVAP toward Al3+ and Zn2+, we also carried out
the single and dual metal competitive analysis (Fig.S13& S14). Bar diagram plot for the
measurement of selectivity of OVAP for Al3+, Zn2+ ions in the presence of other competitive
metal ions are shown in Fig.10. The photographs of OVAP with different metal ions (under
normal light and UV light irradiations) well-verified its sensitivity by strong blue
fluorescence as depicted in Fig.11.
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DOI: 10.1039/C9PP00472F
Fig. 9: a)UV–Vis absorption spectra of OVAP (47µM) in theabsences and presence of
different metal ions (45µM) at 25◦C. b) Fluorescence emission spectra of OVAP (47µM) in
Ethanol in the presenceof different metal ions (45µM) such as Na+, K+, Ag+, Mg2+, Ca2+,
Co2+, Ni2+, Cu2+, Cd2+, Zn2+, Pb2+, Hg2+, Cr3+, Fe3+, Al3+ andMn2+. All the emission spectra
were taken with excitation at 360 nm.
Fig. 10: Selectivity of OVAP for Al3+, Zn2+ions in the presence of other competitive metal
ions, excitation wavelength 360 nm and emission maxima are 520 nmand 535 nm for Al3+
and Zn2+respectively.
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DOI: 10.1039/C9PP00472F
Fig. 11: Visual color change observed with the addition of different metal ions to OVAP as
seen under daylight (upper) and UV light (lower) (λ = 366 nm).
Dissociation Constant
The dissociation constant (Kd) of the corresponding OVAP compounds with Al3+/
Zn2+ have been determined by fitting the absorption data as a function of metal ion
concentration using the following equation,53
I = I0 + (Ilim - I0)/2C0 [C0 + CM + Kd – [(C0 + CM + Kd)2 – 4C0CM]1/2]
where I0 and I are the absorbance of OVAP at 406 nm in the absence and presence of Al3+and
at 425 nm in the absence and presence of Zn2+ .respectively; C0is the total concentration of
OVAP, CM is the concentration of metal ion i.e Al3+ or Zn2+; Ilim is the limiting value of the
absorption intensity in the presence of excess metal ion. The evaluated Kd values using
absorption spectral studies for Al3+ and Zn2+ are (1.07 ± 0.06) × 10-5 M and (6.16 ± 0.47) ×
10-6 M respectively (Fig.S15). Kd values of the corresponding divalent ions with OVAP have
also been obtained by the non-linear fitting of corresponding fluorescence intensity at 520 nm
for Al3+ and 535 nm for Zn2+using similar type of equation where fluorescence intensity is
used instead of absorption value in the above equation and the values are (1.06 ± 0.14) × 10-5
M for Al3+ and (2.53 ± 1.71) × 10-6 M for Zn2+ respectively(Fig.S16). Our observed Kd values
obtained from both absorption and emission data are in excellent agreement and it proves the
self consistency of our spectral data.
Detection Limit
Sensitivity of the probe are examined by measuring limit of detection (LOD) for each
cation. LOD has been determined by following the 3σ method. The LOD values are 1.05 nM
for Al3+, 2.35 nM for Zn2+(Fig.S17). LOD values are exceptionally low compared to the
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others.54-60A comparative study of OVAP and other reported dual Al3+& Zn2+ sensors are
DOI: 10.1039/C9PP00472F
shown in Table-S1. Study shows that the LOD of OVAP for Al3+& Zn2+is comparable to that
reported by Alam et. al.60 These data indicate that OVAP is highly sensitive towards Al3+ and
Zn2+ ions.
We also demonstrate the visual detection of Al3+ and Zn2+ by sharp fluorescence
changing response on TLC plate upon UV irradiation. We have prepared fluorescent test
strips by dip-coating solution of OVAP in methanol on TLC plate simply followed by drying
the test strips under vacuum condition. OVAP absorbed on the test strips, show no colour
upon UV illumination at 365 nm. But as we irradiated 365 nm light after adding few drops
(30 µM) of Al3+ and Zn2+ each, strong yellow luminescence appears from the strip containing
OVAP+Al3+ and green luminescence appears from OVAP+Zn2+ containing strip (Fig. S18).
This clearly signifies the solid state detection of Al3+ and Zn2+ at a very low concentration.
Binding Sites and Reversibility
Stoichiometry and binding sites of OVAP with Al3+& Zn2+ were determined by1H
NMR titration experiment (Fig.S19& Fig.S20). Upon addition of 0.5 equiv of Al3+, the signal
of the −OH proton decreases in intensity and finally disappeared at 1.0 equiv. of Al3+. A
1
slight down field shift of H NMR signal of imine proton (HC=N) from 9.68 ppm to 9.69
ppm was observed, which strongly suggests the involvement of heteroatoms (O and N) and
their chelation to form the Al3+-OVAP complex. On the other hand, addition of 0.5 and 1.0
equiv of Zn2+, the proton of the hydroxyl group at 13.039 ppm decreases in intensity and the
imine proton (9.68 ppm) was shifted slightly up-field at 9.67 ppm . These results indicated
that the nitrogen atom in the imine group and the oxygen atom in the hydroxyl group might
coordinate to Zn2+ion.No considerable changes in the position of proton signals on further
addition ( >1.0 equivalent) of Al3+and Zn2+was observed which supports the 1:1 metal to
ligand ratio of the Al3+-OVAP & Zn2+-OVAP complex and it was further supported by the
Job’s plot. Again chelation site of metal ions with OVAP was supported by DFT based
computational study. It has been shown that the possible location of metal ions at the imine
chelation site has the lowest optimized energy structure for both Al3+-OVAP & Zn2+-OVAP
complex (Fig.S21). Bond distances between metal ions (Al3+ and Zn2+) and the binding sites
of OVAP are shown in table-S2. The observed bond distance between the metal ion and
atoms in the chelating sites of OVAP as observed from DFT study show that the binding
with Al3+ is more rigid than Zn2+ and it is reflected in the lower knr values in Al3+-OVAP
complex than Zn2+–OVAP complex.
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Furthermore, complexation behaviour was well-supported by the reversibiDliOtyI: 1o0.f10A39l/C3+9-PP00472F
OVAP and Zn2+-OVAP. Reversibility is one of the most important features to convince the
demand of a novel chemo sensor mainly for improving practical real time applications of a
probe. To examine the reversible binding of OVAP with Al3+ and Zn2+, we used EDTA as a
strong chelating ligand at identical reaction conditions(Fig.12). The fluorescence intensity of
OVAP in the presence of Al3+(at 520 nm)and Zn2+(at 535 nm)decreased immediately when
excess EDTA was added due to dechelation of Al3+and Zn2+from the corresponding complex
i.e. the sensor complexes, (Al3+-OVAP) and (Zn2+-OVAP) was found to be reversible with
free metal ions & OVAP. We have checked the reversibility study up to 10th cycle for each
metal ions (fig.12) and after the end of 10th cycle we obtained almost same fluorescence
intensity as in case of 1st cycle. Thus the probe OVAP has excellent stability for reuse.
Fig. 12: Reversibility cycles of OVAP metal complexes with EDTA.
Molecular Logic Operation
An INHIBIT logic gate is made of particular combination of the logic operation of
NOT and AND functions, i.e. out-put signal is inhibited by only one type of input. To
demonstrate INHIBIT logical function, one can choose two inputs Al3+/ Zn2+ (input-1) and
EDTA (input-2) and the emission intensity at 520nm/ 535nm as the out-put. The high value
of emission intensity (>31at 520nm (Fig.13a)) & (>17at 535nm(Fig.13b)) has been assigned
as 1(ON) and the low emission intensity (<17at both 520nm & 535nm) has been assigned as
0 (OFF). We further defined the absence of Al3+/ Zn2+ and EDTA as ‘0’ and their presence as
‘1’. The out-put signal corresponding to four different inputs i.e. (0,0), (1,0), (0,1) & (1,1) are
shown in the truth table (Fig.13c ). It is obvious from the truth table, when only input-1 is
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present a significant enhancement of emission intensity at 520nm/535nm takes place
DOI: 10.1039/C9PP00472F
indicating the 1 (ON) state. On the other hand, when only input-2 is present the output
emission intensity becomes very weak indicating OFF state. Therefore it is necessary to apply
NOT gate with input-2. In presence of both inputs the emission intensity again low indicating
OFF state as listed in the truth table (Fig.13c). Thus INHIBIT function logic gate(Fig. 13d)
can be constructed by the sequential application of these two input and it greatly expands the
application of chemo-sensor, OVAP.
Fig. 13: (a)Fluorescence spectra of OVAP (47 μM) in the presence of Al3+ before and
after the treatment with excess EDTA (λex = 360 nm), (b) Fluorescence spectra of OVAP (47
μM) in the presence of Zn2+ before and after the treatment with excess EDTA (λex = 360 nm),
(c) Truth table of logic gate, (d) General representation of an INHIBIT logic gate circuit.
20

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Conclusions
DOI: 10.1039/C9PP00472F
In summary, herein we have reported the synthesis of a simple and low-cost
fluorescent probe, 4-[(2-Hydroxy-3-methoxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-
1,2-dihydro-pyrazol-3-one (OVAP) which is weakly emissive in solution state, but exhibits
interesting aggregation-induced emission enhancement (AIEE) properties in aggregated/ solid
state. Microstructures of OVAP with various morphologies are synthesized using
reprecipitation method. Again, the ‘turn on’ fluorescence of OVAP in presence of Al3+
&
Zn2+ has been utilized for sensing single as well as simultaneous presence of Al3+ and Zn2+ in
aqueous solution with LODs 1.05 nM and 2.35 nM respectively. Due to strong overlap of
emission band, it is suggested that the band edge emission intensity can be used to detect Al3+
and Zn2+ simultaneously present in water sample. Further development of dual metal sensing
probe showing negligible overlap of emission upon complexation with metal ions are in
progress in our lab. It is further demonstrated that the fluorescence ‘turn on’ emission of
OVAP in presence of Al3+/ Zn2+ and ‘turn off’ in presence of EDTA can be utilized to
fabricate molecular INHIBIT logic gate.
Supplementary Information (SI) Available
1H NMR Study, 13C NMR Study, FT-IR Study, UV-Vis study, solidstateemission
study, Job’s plot, Competitive analysis study, Dissociation Constant study, Fluorescence Life
1
Time Table, LOD study, H NMR titration, Comparative table with the previously reported
dual sensor.
Acknowledgement
S.M (Award No.: 09/599(0065)/2015-EMR-I), N.M (Award No.: 09/599(0075)/2018-
EMR-I), S.S.S (Award No.: 09/599(0084)/2019-EMR-I) thanks CSIR, New Delhi, Indiaand
P.H (Ref. No.: 204/(CSIR-UGC NET JUNE 2017), P.K.G (Ref. No.: 228/(CSIR-UGC NET
DEC 2017) thank UGC, New Delhi, India for their individual fellowship. Departmental
instrumental facilities from DST FIST and UGC SAP programs are gratefully acknowledged.
We also acknowledge the help render by USIC,Vidyasagar University for doingoptical
microscopic study and both steady state and time resolved fluorescence study.
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DOI: 10.1039/C9PP00472F
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Graphical Abstract