INTRODUCTION:
One of the foremost issues of environmental drinking
water is metallic contamination due to the continuous discharge of a mixture of
heavy metals that are generated from mining, metal processing, agricultural
drugs, chemical wastewater, domestic sewage, and so on1-2. Heavy
metals such as Lead (Pb), Mercury (Hg), Copper (Cu), Cadmium (Cd), Chromium
(Cr) and Manganese (Mn) are exceedingly toxic even at very low quantity in water
and food. The consumption of these heavy metals by living organism causes a
serious threat, notably causes vomiting, increased blood pressure, hemolytic
anemia, Meknes and Wilson diseases, Alzheimer's and prion disease neuron
degenerative and neurotoxicity in human beings3-8. As a result
numerous detection methods have been studied to develop sensors to detect metal
ions in different sources to monitor environmental pollution 9-14.
All
the above techniques have limitations in terms of complicated operating
systems, the want of trained operators, interferences due to other ions,
time-consuming and cost-ineffective 15-19. On the other hand,
optical detection methods are noteworthy due to their low cost, high
selectivity, simplicity, ease of operation, direct visual perception, quick
response, on-site detection and high sensitivity 20-25.
Copper
is an essential micronutrient for most living organisms and the third among the
most abundant transition metals26. Excess amount of copper in
drinking water can cause vomiting, nausea, gastric complaints, diarrhoea and
headaches27. As with most metals, excess Cd2+is also
toxic. Cadmium and Copper are one of the determined contaminants, in the environmental
water and food, imparting serious adverse effect on human health and other
living beings. The presence of water-soluble Cd2+ and Cu2+
in drinking water is still the most common. Hence, it is imperative to monitor
selectively Cd2+ and Cu2+ levels with high sensitivity in
aqueous system.
Recently,
the expansion of research to fabricate sensitive fluorescent probes for
the qualitative and quantitative identification of Cd2+has been
reported28-29. Colorimetric probe-based detection techniques have also
been studied to determine Cd2+ in water based on photochemical
oxidation reaction of probe accelerated by the contaminant Cd2+ and
Cu2+ 30-32. The optical active sensing molecules/polymers exhibit
fluorescence behaviour or changes in the colour (λmax) upon interaction
with the heavy metal ions. Recently researchers studied the detection of Cu(II)
ion using fluorescence chemosensors which involves the mechanism of molecular
self absorption as understood from observed Stokes shift 33-35. A
fluorescent molecule namely bis(1,2,3-triazole) amino receptor (BODIPY) was
studied as probe which rapidly binds to Cu2+ and Hg2+
ions in CH3CN/H2O (5:1 v/v) . The sensing studies
revealed that this BODIPY probe is significantly valuable for the distinctive
sensing of Hg2+ and Cu2+ ions in living cells
[36]. A fluorescent sensor based on gold nanoclusters (AuNCs) and carbon dots
(CDs) loaded metal-organic frameworks (ZIF-8) to form CDs/AuNCs@ZIF-8
nanocomposites was developed by Tan et al. The MOF based system was
found to effective ratiometric fluorescence nano probe for selective detection
of Cu2+ in the concentration range of 10−3-103
μM, with a lowest sensing limit of 0.3324 Nm37. Bekhradnia et
al developed a new coumarin-based fluorescent organic molecule as probe for the
detection of copper (II). The coumarin probe was able to detect Cu2+
in the concentration range of 0-16 μM and the lowest limit of detection
was reported to be 62 nM 38.
Arylidene
malonitriles are promising optical sensing molecules and are used to study the
sensing of hydrazine,39-40 cyanide41-42, Silver ion 43
and Pb2+ 44. In the present work, the detection of
Manganese (II) and Copper (II) was studied using fluorescent molecules namely
arylidenemalononitrile, the results are compared in detail using the data
resulted from photoluminescence (PL) spectroscopy and UV-Vis spectral analysis.
EXPERIMENTAL
SECTION:
Materials:
Benzaldehyde,
Cinnamaldehyde, Thiophenealdehyde, Pyridinealdehyde, Malanonitrile, were
purchased from Avra chemicals. The UV-Vis absorption behaviour of probes with varying
concentrations of Cd and Cu was studied by using Shimadzu UV-vis 1800
spectrophotometer. The fluorescence emission intensity was monitored during the
gradual addition of metal ion solution into the probe solution. Perkin Elmer
LS45 fluorescence spectrometer was employed to study the photoluminescence behavior
of probes with metal ions; the spectra were recorded between 360nm to 900nm
wavelength by exciting the probes at a wavelength of 350nm.
Synthesis
of arylidenemalononitriles:
1mmol
of arylaldehyde and 1mmol of Malanonitrile was poured carefully into the test
tube with constant shaking for one hour at room temperature. Then the contents
were transferred into a 50 mL of D.I water and kept at stagnation for one day
to obtain a solid product. Then solid mass was washed a number of times with
water to eliminate the unreacted materials, and the product was air dried at
room temperature and purified over column chromatography.
RESULTS
AND DISCUSSIONS:
The
target probe molecules are synthesized according to a reported procedure45-46
which follows Knoevenagel condensation reaction between malononitrile and
aldehydes by collision that can be done via vigorous shaking at 28°C in the
absence of catalyst and solvents. The characterization data is presented in
Table 1.
Probe-1:
2-benzylidenemalanonitrile
The
emission spectrum of the 2-benzylidene malanonitrile with various metal ions
(0.1M) was recorded and a schematic bar diagram depicting the response of the
probe has been presented in Figure 1. It is observed from the
photoluminescence studies that the probe exhibited enhanced emission intensity
for Cu when compared to other metal ions. This indicates that the probe is
suitable to be used as optical sensor probe for the sensing of Copper (II) ions
in aqueous samples and the photoluminescence response of the probe was studied
by varying the concentration of Cu2+.
Figure
1: Bar diagram of the photoluminescence intensity of 2-benzylidenemalanonitrile
with various metal ions.
The
Figure 2. presents the PL spectrum of 2-benzylidenemalanonitrile which shows a
strong fluoroscence maximum at 362nm. The addition of Copper solution into the
2-benzylidenemalanonitrile causes an enhancement in the fluorescence intensity
at λmax=362 nm which confirms the availability of Cu2+
in the aqueous solution. Further, the raise in the fluorescence intensity was
found to be proportional to the quantity of added Cu2+ ions, thus
forms the basis for quantitative estimation of Cu2+. In the present
study the lowest limit of detection for Cu2+ ions is 1.0
10-12M.
Figure
2: PL spectra of 2-benzylidenemalanonitrile with various concentration of Cu2+solution.
In
addition to the photoluminescence studies we have also carried out the
detection studies using UV-Vis spectrometry. The Figure 3 UV spectrum of
2-benzylidenemalanonitrile showed anstrong absorption maximum at λmax=692nm.
The adding up of copper solution into the solution of probe-1 found to increase
the intensity of the peak λmax =692 nm. Thus, conforms the
presence of copper in the test solution. Further, the increase in absorption
was determined to be comparative to the quantity of Cu2+ ions added
to the solution, thus forms the basis for quantitative estimation of Cu2+.
In the present study the probe-1 exhibited a lowest detection limit of 1.910-9M for Cu2+ in water under
UV-Vis absorption measurements.
Figure
3: UV Visible analysis of 2-benzylidenemalanonitrile with various concentration
of Cu
Probe
2: 2-((E)-3-phenylallylidene) malanonitrile
It
is observed from the photoluminescence studies of probe-2 revealed that the
probe-2 have better response for Cu when compared to other metal ions and hence
we have studied the photoluminescence response of the probe by varying the concentration
of Cu2+.The PL spectrum of 2-((E)-3-phenylallylidene) malanonitrile
shown in Figure 4. showed a strong fluorescence maximum at λmax=426nm
and medium peak at 490nm. The addition of Copper solution into the probe-2
molecule imparted an increase in the fluorescence intensity, particularly at
λmax value at 426 and 490nm. Further, the fluorescence
intensity was increased with respect to the content of added Cu2+ ions,
thus forms the basis for quantitative estimation of Cu. The probe 2 provided the
lowest detection limit of Cu2+ is 2.410-9M as observed from probe-2 in
photoluminescence studies.
Figure
4: PL spectral analysis of 2-((E)-3-phenylallylidene) malanonitrile with
various concentration of Copper solution.
The
UV spectrum of 2-((E)-3-phenylallylidene) malanonitrile showed a absorption
band, presented in Figure 5. with a strong absorption maximum at λmax=354nm.
The addition of Copper solution into the2-((E)-3-phenylallylidene) malanonitrile
resulted in increase in absorption maximum, which is relative to the
concentration of added Cu2+ ions. The results indicated that the
probe-2 detects the Cu2+ions in water as low as 2.4910-9M concentration.
Figure
5: UV spectral analysis of 2-((E)-3-phenylallylidene) malanonitrile with
various concentration of Copper solution.
Probe
3: 2-((thiophen-2yl) methylenemalononitrile
PL
spectrum of 2-((thiophen-2yl) methylene malononitrile exhibited a fluorescence
emission maximum at λmax=316nm. It is observed from Figure 6.
that the probe-3 has better response for Cd2+ when compared to other
metal ions and hence we have studied the photoluminescence response of the
probe-3 by addition of Cd2+. The Figure 7. presents the variation of
photoluminescence response of probe-3 by the addition of Cadmium solution,
which imparted an increase in the emission intensity at λmax=316nm.
It is also observed from Figure 7 that the increase in fluorescence intensity
was found to vary with respect to the quantity of Cd2+ ions in
solution, thus forms the basis for quantitative estimation of Cd2+.
In the present study, the probe-3 showed appreciable response to the added Cd2+
ions in aqueous samples with a lowest sensing limit of is 2
10-10M.
Figure
6: PL spectra of 2-((thiophen-2yl) methylenemalononitrile with various metals
Figure
7: PLspectral analysis of 2-((thiophen-2yl)methylenemalononitrile with various
concentration of Cadmium
Probe
4: 2-((Pyridin-3yl) methylenemalononitrile
The
UV spectrum of probe-4 shows intense absorption peak with peak maximum at λmax=274nm
and λmax=303nm. It is observed from Figure 8. The addition of
Copper solution into probe-4 resulted increasing the absorption maximum,
however further increase in concentration of Cu found to lower the absorption.
The observable response was found for Cu2+ is 1.610-12M showing higher sensitivity than the
probe-2.
Figure
8: UV spectral analysis of 2-((Pyridin-3yl) methylenemalononitrile with various
concentration of Copper solution.
The
PL spectrum of probe-4 showed an intense emission at λ max=284nm.
The addition of Copper solution into the 2-((Pyridin-3yl) methylene
malononitrile resulted in increase of the fluorescence intensity λmax=284nm.
It is observed from Figure 9.
Figure
9: PL spectra of 2-((Pyridin-3yl) methylenemalononitrile with various concentration
of Copper solution.
In
addition, the increment in fluorescence intensity of probe-4 due to the added
Cu2+ ions were monitored as a function of Cu2+
concentration, thus provides quantitative estimation of Cu in aqueous solution.
The probe-4 can be used to detect Cu2+as low as 1.610-12M.
Probe
5: 2-((Pyridin-4yl) methylene
malononitrile
The
PL spectrum of probe-5 is shown in Figure 10. which showed emission maximum at
λmax=330nm. The addition of Cu2+ ion solution into
the probe-5 exhibited a decrease in emission intensity, however, an increase in
emission is observed for copper.
Figure
10: PL spectra of 2-((Pyridin-4yl) methylenemalononitrile with various metal
ions
Figure
11: PL spectra of 2-((Pyridin-4yl) methylene malononitrile with various
concentration of Copper solution.
The
PL spectrum of 2-((Pyridin-4yl) methylene malononitrile presented in Figure 11.
showed a increase of the emission intensity at 330nm was noted for the addition
of copper solution into the probe-5 solution. Further, the Figure 11, revealed
that the emission intensity of probe-5 was amplified proportional to the Cu2+
ions concentration, the probe-5 can detect Cu2+ as low as 410-12M. The observed data in respect of all
the five probes are tabulated in Table. 2.
CONCLUSION:
In
summary, sensitive optical sensing ability of few small organic molecules,
namely arylidenemalanonitriles, towards the detection of Cd2+ and Cu2+
was studied. The selectivity and sensitivity of the proposed sensor was
evaluated from the observed fact that the increase in concentration of Cd2+
and Cu2+ resulted in gradual enhancement of the emission intensity
of synthesized probes. The lowest detection limit of the probes was found to be
1.910-9M and 1.610-12M for Cd2+ and Cu2+
ions respectively, thus, the developed fluorescent sensor might find possible
utility in the detection of cadmium and copper in the environmental samples.
The developed protocol assay is convenient and cost effective.
ACKNOWLEDGEMENT:
The authors acknowledge the financial
support of SERB, Department of Science and Technology, New Delhi, India,
through Grant No. EEQ/2016/000049.
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