Highly selective and sensitive simultaneous nanomolar detection of Cs(i) and Al(iii) ions using tripodal organic nanoparticles in aqueous media: The effect of the urea backbone on chemosensing

Article abstract of DOI:10.1039/d0ra03171b

Chemosensing plays a very important role in the detection of essential/pollutant ions in aqueous media. In this manuscript, two tripodal ligands, i.e., 1-(2-hydroxybenzyl)-3-(4-nitrophenyl)-1-phenylurea (ligand 1) and 1-(2-hydroxybenzyl)-3-(4-nitrophenyl)

Full text of DOI:10.1039/d0ra03171b

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Highly selective and sensitive simultaneous
nanomolar detection of Cs(I) and Al(III) ions using
tripodal organic nanoparticles in aqueous media:
the effect of the urea backbone on chemosensing†
Cite this: RSC Adv., 2020, 10, 22691
c
d
Jayanti Mishra,ab Manpreet Kaur,c Navneet Kaur
and Ashok K. Ganguli
*
*
Chemosensing plays a very important role in the detection of essential/pollutant ions in aqueous media. In
this manuscript, two tripodal ligands, i.e., 1-(2-hydroxybenzyl)-3-(4-nitrophenyl)-1-phenylurea (ligand 1)
and 1-(2-hydroxybenzyl)-3-(4-nitrophenyl)-1-phenylthiourea (ligand 2) have been synthesised, which
differ in the linker molecule, i.e., urea and thiourea in ligand 1 and ligand 2, respectively. The ligands
were characterised by NMR, IR and mass spectroscopic techniques. Ligands 1 and 2 (2 mM) were further
employed for the generation of their organic nanoparticles (ONPs) (0.01 mM) of size 20–25 nm and 30–
35 nm, respectively, by the reprecipitation method. The chemosensing properties of 1-ONP and 2-ONP
solutions were investigated. 1-ONP showed simultaneous recognition behaviour towards Cs(I) and Al(III)
with the limits of detection of ꢀ220 and ꢀ377 nM, respectively, in an aqueous medium, while 2-ONP did
not show any recognition behaviour towards any ion.
Received 8th April 2020
Accepted 19th May 2020
DOI: 10.1039/d0ra03171b
rsc.li/rsc-advances
Cd, Hg, and Pb can displace the required metals in the enzymes,
resulting in their denaturing.4 This manuscript discusses the
Introduction
synthesis of two new tripodal organic ligands (1 and 2), the
development of their nanoparticles (1-ONP and 2-ONP, respec-
tively) by an economical and simple reprecipitation method,
and the application of 1-ONP in the highly selective and sensi-
tive simultaneous chemosensing of Cs(I) and Al(III) by different
mechanisms in aqueous media up to the nanomolar level.
A sensor responds to any physical, chemical or binary input
parameter (like temperature, light, heat, motion, moisture, and
pressure) and converts it into an electrical or electronic output
readable signal.5–10 There are various types of sensors such as
chemical, biological, potentiometric, electrochemical, chemo-
mechanical, colourimetric, voltammetric, piezoelectric, optical
(i.e., uorescence), electromechanical, and thermal sensors.11–15
A good sensor should be cost-effective, selective, sensitive, and
specic, have no interference from other ions and should be
stable over a wide pH range with an accurate and precise
detection of the analyte.16–19 The interaction between the host
and the guest should be effective enough for successful detec-
tion with a low detection limit and response time.16–19 Fluores-
cence spectroscopy is a very sensitive optical transduction
technique. The efficiency of a chemosensor depends upon rapid
sensitisation, intensity decay lifetime, chemical and photo-
stability, fast target delivery and suitable solubility.20,21 A typical
uorescent chemosensor bears a recognition site linked to
a uorophore that behaves as the signal source, which trans-
forms the recognition behaviour of the sensor into a uores-
cence signal.22
The qualitative and quantitative detection of metal ions, irre-
spective of whether they have a positive or negative impact, is
very crucial for life on the Earth. If the presence of essential
metal ions is below the required limit, the deciency will cause
malnutrition. On the other hand, if such metal ions are more
than required, the excess will cause abnormalities as well. For
example, copper is very important for the lives of plants and
animals because it is a part of several vital enzymes and proteins
like cytochrome oxidase, ceruloplasmin, monoamine oxidase,
and tyrosinase.1 On the other hand, a genetic disorder, Wilson's
disease, causes the excess accumulation of copper in the body,
further causing hepatic and neurological decits like dystonia
and Parkinsonism.2 The excess of one metal ion induces the
irregularity and absorption of other metal ions in plants and
animals.3 Similarly, if harmful or heavy metal ions exceed the
limit, it would create danger for terrestrial and aquatic life by
posing a threat to the functioning of enzymes. Heavy metals like
aCentre for Nanoscience and Nanotechnology (UIEAST), Panjab University,
Chandigarh, 160014, India. E-mail: jayanti.mishra2001@gmail.com
bDepartment of Chemistry, East Point College of Engineering and Technology, Virgo
Nagar Post, Avalahalli, Bengaluru, 560049, Karnataka, India
cDepartment of Chemistry, Panjab University, Chandigarh, 160014, India. E-mail:
navneetkaur@pu.ac.in
dDepartment of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi,
110016, India. E-mail: ashok@chemistry.iitd.ac.in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra03171b
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The excess and deciency of metal ions are both hazardous
for the environment. Na/K-dependent ATPase is activated by
Cs(I) in the absence of K(I).23 Cs(I) can also be a substitute for K(I)
in muscles and erythrocytes.24 Cs(I) easily binds to the anionic
intracellular components of erythrocytes, resulting in
a decrease in the ability to release oxygen in tissues.25
Aluminium inhibits glutamate dehydrogenase and interferes
with the Kreb's cycle. Aluminium excess can also interfere with
the glycolysis pathway, bioenergetic pathways in mitochondria
and haematopoiesis. It can trigger Alzheimer's disease by
affecting the protein folding.26 Thus, recognition of metal ions
is very necessary for checking and controlling their presence
and amount for the maintenance of balance.27
Many macrocyclic compounds, like naphthalene,28,29 calix-
arenes bearing dansyl groups,30 squaraine,31 dioxycoumarin,32
anthracene33 etc. have been reported for cesium ion detection.
Kaur et al. have prepared a chemosensor for Cs(I) applicable in
aqueous medium bearing mixed imine and amide linkage.34
Arvand et al. prepared zeolite-modied sol–gel electrodes for
the potentiometric determination of Cs(I) ions.35
Jiang et al. synthesised a uorescent sensor based on Schiff
base directed 8-hydroxyquinoline-5-carbaldehyde for Al(III)
detection with a detection limit of 10ꢁ7 M in weakly acidic
aqueous medium.36 Bera et al. reported a neutral imidazol
carrier i.e. 2-(4,5-dihydro-1,3-imidazol-2-yl)phenol-based liquid
membrane electrode in a poly(vinyl chloride) (PVC) matrix for
the potentiometric sensing of Al(III) having a detection limit of 7
ꢂ 10ꢁ7 M.37 Maity and Govindaraju reported a conformationally
constrained (coumarin–pyrrolidinyl–triazolyl–bipyridyl) uo-
roionophore conjugate using click chemistry, applicable as an
Al(III) chemosensor for up to submicromolar detection (1.0 ꢂ
10ꢁ7 M) by internal charge transfer in CH3CN.38 Mashhadizadeh
and Talemi synthesised a potentiometric sensor for Al(III) in the
presence of gold nano-particles using a carbon paste electrode
modied with silica sol–gel and mercaptosuccinic acid (MSA),
having limit of detection of 1.6 ꢂ 10ꢁ7 M. Without gold nano-
particles, it showed sensing for Cu(II) with a limit of detection of
4.0 ꢂ 10ꢁ7 M.39 Gholivand et al. prepared an electrochemical
sensor for Al(III) using PVC membrane and a Schiff base, i.e.
N,N0-bis(salicylidene)-1,2-phenylenediamine (salophen), as the
membrane carrier having a limit of detection of 6.0 ꢂ 10ꢁ7 M.40
Many recognition studies have been reported but they are
Experimental
Materials and instruments
Analytical grade chemicals were used. Salicylaldehyde was
purchased from Loba Chemie, aniline and sodium borohydride
were purchased from SDFC and used without further purica-
tion. Glass slides coated with silica gel (Kieselgel 60 PF254,
Merck) were used for TLC studies. Chloroform was puried by
simple distillation over K2CO3. Metal nitrates (Sigma-Aldrich)
and tetrabutylammonium salts of anions were used as sources
of cations and anions, respectively.
Characterisation of ligands was done by NMR, mass and IR
spectroscopic methods. NMR studies were performed by
BRUKER SPECTROSPIN at 300 MHz. Mass studies were per-
formed on a BRUKER MICROTOF-QII instrument as well as
WATERS Q-TOF MICROMASS instrument. FT-IR studies were
performed using a Thermo Fisher Scientic NICOLET iS50 FT-
IR. Elemental analysis was performed on a FLASH 2000
Organic Elemental Analyzer from Thermo Fisher Scientic. The
optical properties of compounds were studied by UV-Vis spec-
troscopy, i.e. Shimadzu 2600 UV spectrophotometer for
absorption spectra, and uorescence spectroscopy, i.e. SHI-
MADZU RF-5301PC spectrouorophotometer for emission
spectra. Fluorescence spectroscopic studies were conducted on
a
SHIMADZU RF-5301PC spectrouorophotometer having
a xenon lamp as the excitation source using quartz cells (1 cm
path length).
The size, shape and distribution of ONPs were investigated
using a scanning electron microscope (SEM) (JSM-IT300 JEOL)
operated at 20 kV voltage, with silicon wafers as substrates to
coat the samples followed by gold coating (to make the samples
conducting for analysis) and a transmission electron micro-
scope (TEM) (JEM-2100 JEOL) operated at an accelerating
voltage of 200 kV using a 300-mesh copper grid mounted with
2–3 drops of a distilled water dispersion of ONPs. Dynamic light
scattering spectroscopy (DLS) (Malvern ZETASIZER NANO ZSP)
was used for the characterisation of organic nanoparticles
(ONPs).
Synthetic procedures
Synthesis of ligand 1 and ligand 2. Salicylaldehyde (5 mM,
largely applicable in organic medium. Successful chemosensing 531 ml) was mixed with aniline (5 mM, 456 ml) solution dropwise
in environmental and biological media needs sensing in in methanol. The mixture immediately turned pale and was
aqueous media. Taking inspiration from the reports discussed reuxed at 60–80 ꢃC for 5–6 hours for (–C]N) Schiff base
above combined with the applicability in aqueous media, we formation. Excess NaBH4 (20 mM, 758 mg) was added slowly for
developed tripodal compounds and generated their organic the reduction of the Schiff base, which resulted in the disap-
nanoparticles by the reprecipitation method, for the efficient pearance of colour and led to effervescence. Aer sometime,
and simultaneous recognition of multiple metal ions. The ONPs when the effervescence stopped, the mixture was covered and
having 0.01 mM concentration of ligands are sufficient for the stirred for 3–4 hours at room temperature. Methanol was
selective and sensitive binding of metal ions in an aqueous evaporated from the mixture and distilled water was added
medium. 1-ONP shows a strong and selective simultaneously followed by of chloroform then shaken vigorously, which
response for Cs(I) and Al(III) in comparison with 2-ONP. The 1- resulted in the formation of two layers. The aqueous layer
ONP can effectively determine Cs(I) and Al(III) to limits of contained excess NaBH4 while the chloroform layer contained
detection of 220 and 377 nM, respectively, in an aqueous the reduced Schiff base. The chloroform layer was extracted and
medium.
evaporated to obtain the saturated or reduced Schiff base
product. The synthesis of ligand 1 was carried out by adding 4-
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nitrophenyl isocyanate (5 mM, 820.6 mg) to dry chloroform and obtain a uniform ONPs solution (0.01 mM). The size of organic
reuxing at 75–85 ꢃC for 5–6 hours. A yellow precipitate nanoparticles can be maintained by controlling the rate of
appeared which was ltered and dried to obtain ligand 1. It was injection of the ligand solution, speed of stirring and
characterised by ESI-MS (m/z value) ¼ 364.1305 [M ꢁ H]ꢁ and temperature.
386.1122 [M ꢁ Na]+; 1H NMR (300 MHz, CDCl3 : DMSO-d6
Chemosensing studies of 1, 2-ONPs. Chemosensing studies
(4 : 1), 25 ꢃC) peaks of 1: d ¼ 8.81 (s, 1H, –OH), 7.54 (s, 1H, –NH), were done by adding various cations (5 mM, 50 ml) i.e. Li(I),
7.21 (d, 2H, ArH), 6.78 (d, 2H, ArH), 6.53 (m, 2H, ArH), 6.46 (m, Na(I), K(I), Cs(I), Mg(II), Ca(II), Ag(I), Zn(II), Cr(III), Hg(II), Al(III),
1H, ArH), 6.36 (d, 2H, ArH), 6.22 (m, 1H, ArH), 6.0 (m, 1H, ArH), Pb(II), Ni(II), Sr(II), Co(II), Ba(II), Cu(II) and Mn(II) into 5 ml ONP
5.81 (m, 2H, ArH) and 4.02 (s, 2H, –CH2). 13C NMR (300 MHz, solution. The solutions were shaken and le for a few minutes
CDCl3 : DMSO-d6 (4 : 1), 25 ꢃC) peaks of 1: d ¼ 160.07 (1C, ArC– to allow for binding. Their emission spectra were recorded by
OH), 150.77 (1C, CO), 146.86 (1C, ArC–NH), 145.81 (1C, ArC– exciting the aqueous solutions under the spectro-
NO2), 135.24 (1C, ArC–N), 134.65 (1C, ArC), 133.98 (1C, ArC), uorophotometer having the xenon lamp source at the wave-
132.88 (2C, ArC), 132.58 (1C, ArC), 129.31 (2C, ArC), 127.80 (1C, length of maximum absorption. If any kind of anomalous
ArC), 124.11 (2C, ArC), 123.82 (2C, ArC), 121.13 (1C, ArC), 117.96 behaviour was shown for any ONP towards any cation, the
(1C, ArC) ꢁa1nd 54.51 (1C, –CH2) ppm. FTIR peaks at conrmation of binding was done by performing the uoro-
3341.36 cmꢁ1 (–NH stretching), 3067.91 cmꢁ1 (–OH stretching), metric titration of that ONP as a function of the concentration
1643.85 cm (–C]O stretching), 1593.21 cmꢁ1 (–NH bending), of the cation. The maxima of the uorescence emission inten-
1533.11 cmꢁ1 (aromatic –C]C– bending), 1485.26 cmꢁ1 (Ar- sity recorded at each concentration during uorometric titra-
NO2) and 1381.27 cmꢁ1 (aliphatic –CH bending). Elemental tion were joined and a linear t was obtained to get the limit of
analysis: C ¼ 65.25%, N ¼ 10.14%, H ¼ 4.62% and O ¼ 19.99% detection using the following formula:45
for C: 66.11%; H: 4.72%; N: 11.56% and O: 17.61% [Fig. 7S–
10S†].
Limit of detection ¼ 3s/m
Similarly, for the synthesis of ligand 2, the reaction of the
where, ‘s’ is the standard deviation of the intercept, and ‘m’ is
reduced Schiff base with 4-nitrophenyl isothiocyanate (5 mM,
the slope of the curve.
900.9 mg) was carried out in dry chloroform and reuxed at 75–
For further insight, the interference studies, pH studies,
response time studies and ionic strength environment studies
were done. The stability of the ONPs was observed under
various pH values as well as under the high ionic strength of
a heavy salt, i.e. tetrabutylammonium perchlorate. The stability
of the ONP–analyte complex was investigated with respect to
time at various concentrations of the analytes. The interference
study was performed to investigate the perturbation in the
uorescence emission spectra of the host–analyte complex
caused by other metal ions if any. At rst, the complex of the
ONP–analyte (metal ions) was formed, aer which, other
cations/anions were added separately, shaken and allowed to
displace the earlier metal ion from the complex, which was
tracked by recording the uorescence emission spectra of the
solutions. If the uorescence emission spectrum deviated from
an earlier one, the added metal ion was able to displace the
earlier bound ion from the complex, thus interfering with the
complex; otherwise, the added metal ion is not able to disturb
the complex.
ꢃ
85 C for 5–6 hours. The chloroform was allowed to evaporate,
leaving behind a dark maroon liquid that was le overnight,
followed by the addition of chilled diethyl ether. This was
shaken vigorously and kept in the refrigerator. The liquid
turned into a yellow powder aer 5–6 hours and ether was
evaporated, leaving behind ligand 2. It was characterised by 1H
NMR (300 MHz, CDCl3 : DMSO-d6 (4 : 1), 25 ꢃC) peaks of 2: d ¼
8.71 (s, 1H, –OH), 8.28 (s, 1H, –NH), 7.21 (d, 2H, ArH), 6.8 (d, 2H,
ArH), 6.46 (m, 2H, ArH), 6.4 (m, 1H, ArH), 6.28 (d, 1H, ArH), 6.17
(t, 1H, ArH), 6.09 (m, 1H, ArH), 5.96 (m, 1H, ArH), 5.80 (m, 2H,
ArH) and 4.47 (s, 2H, –CH2) ppm [Fig. 11S†]. 13C NMR (300 MHz,
CDCl3 : DMSO-d6 (4 : 1), 25 ꢃC) peaks of 2: d ¼ 181.10 (1C, –CS),
154.39 (1C, ArC–OH), 146.19 (1C, ArC–NH), 143.06 (ArC–NO2),
142.24 (ArC–N, 1C), 129.92 (1C, ArC), 129.41 (2C, ArC), 128.64
(1C, ArC), 127.48 (2C, ArC), 126.25 (1C, ArC), 124.86 (2C, ArC),
123.43 (1C, ArC), 121.51 (2C, ArC), 119.02 (1C, ArC), 115.31 (1C,
ArC) and 53.51 (–CH2, 1C) ppm [Fig. 12S†]. ESI-MS (m/z value) ¼
378.2277 [M ꢁ H]ꢁ [Fig. 13S†]. FTIR peaks at 3468.91 cmꢁ1 (–NH
stretching), 3272.77 cmꢁ1 (–OH stretching), 1595.79 cmꢁ1
(aromatic –C]C– bending), 1515.42 cmꢁ1 (Ar-NO2),
1421.62 cmꢁ1 (NCN stretch), 1301.81 cmꢁ1 (Ar–NO2),
1180.63 cmꢁ1 (–C]S) and 853.73 cmꢁ1 (para-substituted
benzene) [Fig. 14S†]. Elemental analysis: C ¼ 60.34%, N ¼
Results and discussion
Synthesis and characterisation of ligand 1 and 2
10.81%, H ¼ 5.33%, S ¼ 7.89% and O ¼ 15.62% for C: 63.31%; Ligands 1 and 2 were synthesised in multiple steps [Scheme 1].
H: 4.52%; N: 11.07%; S: 8.45% and O: 12.65%.
Firstly, we synthesised the Schiff base from the reaction of sal-
1
Preparation of organic nanoparticles (1-ONPs and 2-ONPs) icylaldehyde with aniline and this was conrmed by H NMR,
of ligand 1, 2. The reprecipitation method35 was used to produce which showed the characteristic chemical shi at 8.651(s) ppm
organic nanoparticles (ONP). Here, 4 mg of ligand 1 and ligand of the Schiff base proton of the (–HC]N–) group [Fig. 1S†],
2 were separately dissolved in an organic solvent, i.e. N,N- while the chemical shi (d) at 76.33 ppm in the 13C NMR
dimethylformamide (DMF) and 1 ml of this solution (2 mM) was spectrum represents the carbon atom of the (–C]N–) group
slowly injected into 200 ml of double-distilled water while [Fig. 2S†]. The Schiff base formation was also conrmed by
sonicating. Sonication was carried out for 5–10 minutes more to mass spectroscopy [Fig. 3S†]. Aer that, excess NaBH4 was
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ligand 1 shows the (m/z) value at 364.1305 and 386.1122 corre-
sponding to [M ꢁ H]ꢁ and [M ꢁ Na]+ peaks, respectively
[Fig. 9S†]. In the FT-IR spectrum of ligand 1, the major peaks at
3341.36 cmꢁ1, 3067.91 cmꢁ1, 1643.85 cmꢁ1 and 1485.26 cmꢁ1
marked the presence of the stretching vibrations of the amine,
hydroxyl, carbonyl and nitro groups, respectively [Fig. 10S†].
Further, the purity of ligand 1 was conrmed by the elemental
analysis which showed the presence of carbon, hydrogen,
nitrogen and oxygen in 65.25%, 4.62%, 10.14% and 19.99%,
respectively; this matches closely with the theoretical values of
C: 66.11%; H: 4.72%; N: 11.56% and O: 17.61%. Similarly,
ligand 2 was fully characterised by standard spectroscopic
1
techniques. In the H NMR spectrum of ligand 2, a singlet at
4.47 ppm denotes two aliphatic protons. The chemical shis (d)
at 6.46, 6.4, 6.09, 5.96 and 5.80 ppm show multiplets repre-
senting the presence of 7 aromatic protons. The doublets at (d)
7.21 and 6.8 ppm marked the presence of 4 protons from the
substituted p-nitrophenyl ring (2 close to –NO2 group and 2
close to –NH group), while 6.28 ppm showed the presence of 1
proton of the substituted phenol ring [Fig. 11S†]. The 13C NMR
spectrum showed a chemical shi (d) at 53.51 ppm, which
depicts the presence of 1 aliphatic carbon, while the chemical
shis (d) at 181.10 and 154.39 ppm denote the (–C]S) group
and the aromatic carbon atom attached to the (–OH) group,
respectively [Fig. 12S†]. The m/z value at 378.2277 denotes the
[M ꢁ H]ꢁ peak, which conrmed the synthesis of ligand 2
[Fig. 13S†]. The major peaks at 1515.42 cmꢁ1 and 1301.81 cmꢁ1
in the FT-IR spectrum of ligand 2 conrmed the presence of the
aromatic (–NO2) group, while 3468.91 cmꢁ1, 3272.77 cmꢁ1 and
1180.63 cmꢁ1 indicated the presence of the (–C]S), (–OH) and
(–NH) groups, respectively [Fig. 14S†]. The elemental analysis
conrmed the presence of carbon, hydrogen, nitrogen, sulphur
and oxygen atoms as 60.34%, 5.33%, 10.81%, 7.89% and
15.62%, respectively which is close to the theoretical outcome,
i.e. C: 63.31%; H: 4.52%; N: 11.07%; S: 8.45% and O: 12.65%.
Scheme 1 Synthesis of ligand 1, i.e., 1-(2-hydroxybenzyl)-3-(4-nitro-
phenyl)-1-phenylurea and ligand 2, i.e., 1-(2-hydroxybenzyl)-3-(4-
nitrophenyl)-1-phenylthiourea.
added for the reduction of the Schiff base and the reduced
Schiff base was further characterised by 1H NMR, 13C NMR and
mass spectroscopy [Fig. 4S–6S†]. The chemical shis (d) in the
1H NMR at 1.341(t), 1.672(s), 4.028(bs), 4.483(d), 6.932(m),
7.301(m), 8.5(bs) ppm represent the hydrogen atoms of the
reduced product [Fig. 4S†], while in the 13C spectrum, chemical
shis (d) at 48.75, 147.23 and 156.8 ppm represent the carbon
atom belonging to the (–CH2), (–NH) and (–OH) groups,
respectively. In the mass spectrum, (m/z) peaks at 198.0915 and
200.1072 conrmed the Schiff base and the reduced product,
respectively [Fig. 3S and 6S†]. The error percentage is ꢁ0.2 ppm
as calculated from HRMS for both the Schiff base and the
reduced product. This reduced product was further reacted with
4-nitrophenyl isocyanate or 4-nitrophenyl isothiocyanate to get
ligands 1 [0.900 g, yield 55.95%] and 2 [1.549 g, yield 81.74%],
respectively [Scheme 1].
Development and characterisation of 1, 2-ONPs
The reprecipitation method41 was used to produce organic
nanoparticles (ONP). Organic ligands were dissolved in an
organic solvent, i.e. N,N-dimethylformamide (DMF), followed by
the injection of a small amount of ligand solution in DMF into
double distilled water (with sonication), which was further
sonicated for 5–10 minutes to get a uniform ONP solution. The
formation of ONPs was conrmed by evaluating the average size
of particles by dynamic light scattering (DLS) and surface
morphology by using transmission electron microscopy (TEM)
and scanning electron microscopy (SEM). The optical properties
of ONPs were also studied with different spectrophotometric
methods. 1-ONP was characterised by both SEM-EDX and TEM-
EDX while 2-ONP was characterised by SEM-EDX only.
1-ONP showed a blue shi in its absorbance as compared to
ligand 1, and it had much higher uorescence emission
intensity than ligand 1 [Fig. 1(A) and (B)]. 1-ONP and ligand 1
absorb at 335 nm and 346 nm, respectively. 1-ONP emits at
362 nm, while ligand 1 emits at 352 nm when excited at 260 nm.
1-ONP has spherical morphology as shown by SEM and TEM
Ligands 1 and 2 were fully characterised by standard spec-
troscopic techniques, for example NMR, mass and FTIR spec-
1
troscopy. The H NMR spectrum of ligand 1 shows a singlet at
4.02 ppm, which indicates the presence of 2 aliphatic protons.
The chemical shi (d) values at 6.53, 6.46, 6.22, 6.0 and 5.8 ppm
denote 7 aromatic protons. The two aromatic protons of the
substituted benzene ring are denoted by a doublet at 6.36 ppm,
while four aromatic protons of the substituted p-nitrophenyl
ring are shown by doublets at 6.78 and 7.21 ppm [Fig. 7S†]. The
13C NMR spectrum of ligand 1 shows the chemical shi at
54.51 ppm, which indicates the presence of one aliphatic
carbon. The aromatic carbons attached to the –NH, –NO2 and
–N– groups are given by 146.86, 145.81 and 135.24 ppm,
respectively. The chemical shis at 160.07 and 150.77 ppm
indicated the presence of carbon atoms attached to the –OH
and –C]O group, respectively [Fig. 8S†]. The mass spectrum of
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Fig. 3 Comparison of ligand 2 and 2-ONP by (A) UV-Vis and (B)
fluorescence emission spectroscopy.
Fig. 1 Comparison of ligand 1 and 1-ONP by (A) UV-Vis and (B)
fluorescence emission spectroscopy.
presence of C, N and O in 1-ONP [Fig. 15S(A) and (B)†]. The
hydrodynamic diameter and zeta potential of 1-ONPs were ob-
tained as 572.3 nm and ꢁ24.2 mV, respectively [Fig. 15S(B) and
(C)†]. The higher value was obtained in the DLS measurement
as compared to the TEM measurement because of the inclusion
of the solvent sphere in the DLS measurement, which was
avoided in TEM images.
images [Fig. 2(A) and (B)]. The TEM image of 1-ONPs revealed
the diameter of the spheres as approximately 20–25 nm
[Fig. 2(B)]. The SEM-EDX and TEM-EDX spectra showed the
Ligand 2 absorbs at 266 nm, 357 nm and 452 nm, i.e. at 3
wavelengths, while 2-ONP absorbs over 320–327 nm with a wide
absorption band [Fig. 3(A)]. Ligand 2 emits at 339 nm, while 2-
ONP emits at 350 nm when excited at 270 nm. The 2-ONP
solution shows a redshi in its uorescence emission spectrum
as compared to ligand 2 [Fig. 3(B)]. The spherical morphology of
2-ONP can be clearly seen from its SEM and TEM images
[Fig. 4(A) and (B)].
2-ONPs are spherical structures with a diameter of approxi-
mately 30–35 nm, as obtained from the TEM image [Fig. 4(B)].
The EDX spectrum conrmed the presence of C, N, O and S
atoms in 2-ONP [Fig. 16S(A)†]. The amounts of the ligands (4
mg) used for the preparation of ONPs were very small. In
addition, theoretically, the sulphur atom was present in only
8.45% by molecular weight as per the molecular formula in
ligand 2. As such, the signals corresponding to sulphur atoms
are very weak as compared to other atoms in 2-ONP in its SEM-
EDX spectrum.
The DLS study showed the hydrodynamic diameter and zeta
potential of 2-ONP as 593.6 nm and 0.944 mV, respectively
[Fig. 16S(B) and (C)†]. The zeta potential is the potential
difference between the dispersion medium and the stationary
layer of uid attached to the dispersed particles. The higher the
value (irrespective of being positive or negative), the higher will
Fig. 2 (A) SEM and (B) TEM images of 1-ONP.
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Fig. 4 (A) SEM and (B) TEM images of 2-ONP.
Fig. 5 Cation recognition chemosensing study of (A) 1-ONP and (B) 2-
ONP.
be the stability of the colloidal dispersion. If the value of the
zeta potential of any colloidal dispersion is from 0 to ꢄ5 mV, it
means that the dispersion has rapid coagulation, while if the
zeta potential is near ꢄ30 mV, it is moderately stable. The zeta
potential of 2-ONP is 0.944 mV while that of 1-ONP is ꢁ24.2 mV,
which means that 1-ONP is more stable than 2-ONP.
The optical properties are very much dependent on the size
of the particles. As we move from the bulk material to the
nanomaterial, the energy band gap increases. As the energy
band gap increases, the wavelength decreases, which results in
the blue shi of the wavelength in the absorbance spectra of the
ONPs. Similarly, an increase in the energy band gap in the
nanoparticles resulted in greater absorption energy while going
from the ground state to the excited state, and hence the greater
uorescence emission intensity on coming back from the
excited state to the ground state.
The ONPs showed the blue shi as compared to the bulk
ligands in their respective absorption spectra. Also, the emis-
sion intensities of ONPs were 600–700 times greater than that of
the respective ligands. This means that ONPs are uorescent
even in very low concentrations as compared to that of their
respective ligands. The absorption and emission spectra
support the formation of ONPs from their respective ligands.
investigate the chemosensing behaviour of 1-ONP. Except for
Al(III), the other metal ion solutions, i.e., Li(I), Na(I), K(I), Cs(I),
Mg(II), Ca(II), Sr(II), Ba(II), Cu(II), Co(II), Ag(I), Cd(II), Ni(II), Pb(II),
Zn(II), Cr(III), Hg(II) and Mn(II) with 1-ONP emit around 410 ꢄ
5 nm with variation in uorescence emission intensity in
between 205 to 300 a.u. only. 1-ONP showed distinct chemo-
sensing behaviour towards Cs(I) and Al(III) ions, while 2-ONP did
not show any recognition behaviour towards any metal ion
[Fig. 5(A) and (B)]. The 1-ONP–Cs(I) complex showed much
higher intensity emission as compared to other 1-ONP–metal
complexes. The 1-ONP–Al(III) complex showed a blue shi and
emitted around 377 nm with lower uorescence emission
intensity. The 1-ONP–Cs(I) complex emitted with higher inten-
sity at ꢀ411 nm as compared to other cationic solutions with 1-
ONP and follows the photoinduced electron transfer (PET) off
phenomenon. Earlier, before binding of Cs(I) with 1-ONP, the
receptor of the ONP was free and the HOMO of the free receptor
was above the HOMO of the uorophore of ONP, which resulted
in electron transfer from the receptor to the uorophore in the
presence of a light source (the Xe lamp of the spectro-
uorophotometer). This was evidenced by the uorescence
emission intensity of 262.117 a.u. at ꢀ411 nm.
Aer binding, the 1-ONP–Cs(I) complex was stabilised and its
energy decreased, which resulted in the lowering of the HOMO
of the Cs(I) bound receptor and thus electrons of the receptor of
1-ONP were no longer available for its uorophore, which
increased the uorescence emission intensity of the 1-ONP–
Chemosensing studies
Chemosensing studies of 1-ONPs. Aer the separate addi-
tion of various cations to 1-ONP solutions, the solutions were
excited at 295 nm and the emission proles were observed to
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Cs(I) complex. The binding behaviour of 1-ONP towards Cs(I)
The limit of detection calculated from this plot was
was conrmed by uorometric titration of the 1-ONP–Cs(I) ꢀ220 nM. The stability of the 1-ONP–Cs(I) complex was
complex with increasing concentration of Cs(I) ions [Fig. 6(A)]. observed by investigating the uorescence emission spectrum
The binding of Cs(I) and 1-ONP followed a linear pattern of the 1-ONP–Cs(I) complex at various concentrations of Cs(I),
(adjusted R2 value ¼ 0.9968) at 411 nm [Fig. 6(B)].
i.e., 10, 30, 50 and 100 mM. At each Cs(I) ion concentration, the
uorescence emission spectrum of the 1-ONP–Cs(I) complex
was recorded for 5 measurements. The plot between the relative
uorescence intensity and the concentration of Cs(I) in the 1-
ONP–Cs complex showed its stability during the measurements.
I ꢁ Io
Relative fluorescence intensity ¼
Io
where I ¼ the emission intensity of the host ONP–analyte
complex solution at a specic time interval, and Io ¼ the
emission intensity of the host ONP solution.
It was observed that the 1-ONP–Cs(I) complex was stable at
all concentrations for at least 5 measurements [Fig. 6(C)]. The
interference of other cations with the 1-ONP–Cs(I) complex was
studied by adding Cs(I) to the 1-ONP (5 ml) solution, followed by
the addition of solutions of other cations. It was observed that
only Li(I) and Al(III) interfered to some extent with Cs(I) at
411 nm [Fig. 6(D)]. 1-ONP also showed chemosensing behaviour
towards Al(III) via the intramolecular charge transfer (ICT)
mechanism [Fig. 5(A)].
As discussed above, when other metal ions were added to the
1-ONP solution, the uorescence emission was observed at
around 410 nm, except for Al(III) which had its uorescence
emission at around 377 nm, i.e. a blue shi of 36 nm with
respect to 1-ONP. Al(III) was bound to the receptor (donor) and
charge transfer occurred from the receptor of the host to its
acceptor on the uorophore end. Here, the receptor acts as
a donor, which binds to the Al(III). Aer the binding of Al(III)
with 1-ONP, the LUMO shied higher, which resulted in the
increase in the band gap and hence a blue shi occurred. To
conrm the binding behaviour of 1-ONP with Al(III), uoro-
metric titration as a function of Al(III) concentration was per-
formed [Fig. 7(A)]. The binding pattern of the 1-ONP–Al(III)
complex was found to be moderately linear (adjusted R2 value ¼
0.9587) at 377 nm [Fig. 7(B)]. From this plot, the limit of
detection was calculated as ꢀ377 nM. Fluorescence titration
revealed that the blue shi in the emission was due to intra-
molecular charge transfer (ICT) along with in increase and
decrease in the uorescence emission intensity at around 384–
372 nm and 377–398 nm, respectively, because of which perfect
linearity was not observed at all wavelengths in binding.
The emission intensity of the old peaks gradually decreased
while new peaks were increasing, which collectively resulted in
the ratiometric sensing of Al(III) by 1-ONP. For the response time
study, the emission behaviour of the 1-ONP–Al(III) complex was
observed as a function of various concentrations of Al(III), i.e.,
10, 20, 30 and 50 mM. At each concentration, the 1-ONP–Al(III)
complex was stable for at least 5 measurements under the
radiation source (Xe lamp) [Fig. 7(C)].
Fig. 6 (A) Fluorometric titration of 1-ONP with increasing concen-
tration of Cs(I). (B) The linear fit of the fluorometric titration maxima of
1-ONP with increasing concentration of Cs(I). (C) Response time study
of the 1-ONP–Cs(I) complex at various concentrations of Cs(I). (D)
Interference study of other cations with the 1-ONP–Cs(I) complex.
For the interference study, Al(III) solution was added to the 1-
ONP (5 ml) solution, followed by the addition of other metal
nitrate solutions. It was observed that only Cs(I) and Li(I) ions
were interfering with Al(III) to some extent at 377 nm [Fig. 7(D)].
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acidic and basic medium, respectively, so it is applicable in the
pH range of 5.7 to 11 [Fig. 17S(A)†]. The effect of the heavy ionic
strength on 1-ONP was also checked by the gradual addition of
various concentrations of a heavy salt, i.e. tetrabutylammonium
perchlorate followed by the observation of its uorescence
emission spectra [Fig. 17S(B)†]. It was seen that the uorescence
spectrum did not change in any signicant manner until the
concentration of the heavy salt in 1-ONP was 600 mM; i.e.,
uorescence emission intensity difference was seen at around
50 a.u. only.
Chemosensing study of 2-ONP. The chemosensing study of
2-ONP was done by adding various cations, i.e. Li(I), Na(I), K(I),
Cs(I), Mg(II), Ba(II), Cu(II), Co(II), Ca(II), Sr(II), Ag(I), Pb(II), Cd(II),
Ni(II), Zn(II), Cr(III), Hg(II), Mn(II) and Al(III), and recording their
uorescence emission spectra. Solutions of various cations were
separately added to 2-ONP solutions (5 ml), shaken well and le
for a few minutes to allow binding between 2-ONP and the
cations. The solutions were excited at 270 nm and their uo-
rescence emission spectra were recorded. 2-ONP did not show
any recognition behavior towards any cation [Fig. 5(B)]. The pH
effect on 2-ONP was studied by creating acidic and basic envi-
ronments by the gradual addition of HCl solution and NaOH
solution, respectively into 2-ONP solutions. The uorescence
emission spectrum of 2-ONP solutions at various pH values
revealed that it is applicable in the pH range of 3.07 to 9.95. In
the acidic medium, a dip of ꢀ43 a.u. was rst seen when the pH
of the host solution (pH 7.3) was adjusted to pH 5.6, and then it
was stable until pH 3.07. In the basic medium, it was stable till
pH 9.95 [Fig. 18S(A)†]. The effect of strong ionic strength on 2-
ONP was also investigated by observing the uorescence spectra
of the 2-ONP–ClO4(I) complex with increasing concentration of
ClO4(I). At rst, when 10 ml of ClO4(I) solution (5 mM) was added
to the 2-ONP solution, the uorescence emission prole of the
2-ONP–ClO4(I) mixture was slightly changed by an increase of
ꢀ88 a.u. in its intensity, aer which no change was observed till
1000 ml were added. Hence, initially, 2-ONP was slightly affected
by heavy salt but aer that, no effect was observed [Fig. 18S(B)†].
Theoretical study
The density functional theory (DFT) study42–44 showed that the
HOMO and LUMO of molecule 1 were previously located on its
95th and 96th orbitals with energy at ꢁ5.827 eV and ꢁ4.806 eV,
respectively. Aer binding with Cs(I), it got shied to the 122nd
and 123rd orbitals with energies of ꢁ5.686 eV and ꢁ3.926 eV,
respectively, upon binding with Cs(I) [Fig. 8(A)–(D)]. DFT study
suggested that three arms of molecule 1 occupied three
different planes in the space to avoid steric repulsion. The
HOMO electron density of molecule 1 was majorly located on
the phenol group, which shied to the (–NO2) group of the p-
nitrophenyl moiety aer complexation with Cs(I), while the
LUMO electron density of molecule 1 was majorly located on the
p-nitrophenyl moiety, which was not altered aer complexation.
The HOMO and LUMO of molecule 2 were previously located
on its 99th and 100th orbitals with energies of ꢁ5.739 eV and
ꢁ4.854 eV, respectively, as shown by their density functional
theory (DFT) calculations37–39 [Fig. 9(A) and (B)]. It was suggested
Fig. 7 (A) Fluorometric titration of 1-ONP with increasing concen-
tration of Al(III). (B) The linear fit of the fluorometric titration maxima of
1-ONP with increasing concentration of Al(III). (C) Response time study
of the 1-ONP–Al(III) complex at various concentrations of Al(III). (D)
Interference of other cations with the 1-ONP–Al(III) complex.
Thus, 1-ONP has shown signicant recognition behaviour
towards Cs(I) and Al(III) by the PET-off phenomenon and ICT
phenomenon, respectively.
The applicability of 1-ONP under various pH conditions was
checked in acidic and basic media (HCl and NaOH solutions,
respectively). It was found that the uorescence emission
spectrum of 1-ONP was not disturbed till pH 5.7 and pH 11 in
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uorometric titration of Cs(I) and Al(III), showed linear behav-
iour with the adjacent R2 value of >95%. The limits of detection
for Cs(I), and Al(III) were calculated as ꢀ220 and ꢀ377 nM,
respectively, in the chemosensing study of 1-ONP. The response
time study proved the stability of the ONPs against the irradi-
ation source and time, while the competitive binding studies
proved the stability of the analyte–host complex in the presence
of other cations/anions. 1-ONP was stable in the pH range of 5.7
to 11, while 2-ONP was stable in the pH range of 3.07 to 9.95.
Theoretical studies proved the change in the electron density
and orbitals aer the interaction of analytes with host mole-
cules. 1-ONP was proved to be highly efficient in very small
amounts for the simultaneous detection of 2 cations, i.e., Cs(I)
and Al(III) by different mechanisms of detection, while 2-ONP
did not recognise any ion in aqueous medium. Cs(I) is highly
electropositive and its d-electrons are unavailable for bonding.
Fig. 8 The DFT-optimised pictorial representation of (A) the HOMO of
1; (B) the LUMO of 1; (C) the HOMO of the 1–Cs(I) complex; (D) the It has a larger size but less polarizing power, so it behaves as
LUMO of the 1–Cs(I) complex (red and green colour for the HOMO,
blue and yellow colour for the LUMO).
a hard acid. Due to the high positive charge and small size, Al(III)
also behaves like a hard acid. Oxygen behaves as a hard base, so
both Cs(I) and Al(III) prefer to bind with the oxygen-bearing
complex, i.e. 1-ONP, as per the HSAB concept. The orientation
of the atoms in 1-ONP along with the cavity size also supported
its complex formation with Cs(I) and Al(III). The proper cavity
size and orientation of linker molecules in 1-ONP favoured this
highly selective, sensitive and simultaneous nanomolar che-
mosensing behaviour in 1-ONP. This work demonstrates the
effect of urea and thiourea linker molecules on the chemo-
sensing properties of the ONPs.
This manuscript describes the detection of multiple cations
at the same time in the aqueous medium with detection limits
Fig. 9 The DFT-optimised pictorial representation of (A) the HOMO of
2; (B) LUMO of 2 (red and green colour for the HOMO, blue and yellow
colour for the LUMO).
up to the nano level, as well as the effect of the architecture of
the linker molecule on the type of detection.
Conflicts of interest
that three arms of molecule 2 were oriented in three different
planes in the space. The HOMO electron density of molecule 2
was majorly located on the p-nitrophenyl moiety and the thio-
urea group along with a small distribution on the phenol group,
while the LUMO electron density was majorly located on the p-
nitrophenyl and thiourea group.
There are no conicts to declare.
Acknowledgements
Authors thank Department of Science and Technology (DST),
Government of India for nancial support. JM thanks Sophis-
ticated Analytical Instrumentation Facility (SAIF), Panjab
University, Chandigarh, Indian Institute of Technology (IIT),
Delhi and Institute of Nano Science and Technology (INST),
Mohali for instrumentation facilities.
Conclusions
Two tripodal ligands 1 and 2 were synthesised and charac-
terised by NMR, mass and IR spectroscopy. Ligand 1 bears the
urea group, while ligand 2 bears the thiourea group. Their
organic nanoparticles (ONP) were prepared by the reprecipita-
tion method under ultrasonic waves. 1-ONP and 2-ONP have
spherical morphologies with diameters of approximately 20–
25 nm and 30–35 nm, respectively. The chemosensing proper-
ties of 1-ONP and 2-ONP were investigated, which revealed that
1-ONP simultaneously and clearly recognised Cs(I) and Al(III)
ions, while 2-ONP did not recognise any cation in the aqueous
medium. In the chemosensing study of 1-ONPs, the recognition
of Cs(I) followed the PET-off mechanism path and Al(III) ion
recognition followed the ICT mechanism path. The
References
1 P. A. Walravens, Clin. Chem., 1980, 26(2), 185–189.
2 O. Bandmann, K. H. Weiss and S. G. Kaler, Lancet Neurol.,
2015, 14(1), 103–113.
3 P.
Karimi,
R.
A.
Khavari-Nejad,
V.
Niknam,
F. Ghahremaninejad and F. Naja, Sci. World J., 2012,
615670, DOI: 10.1100/2012/615670, 6 pages.
4 R. Singh, N. Gautam, A. Mishra and R. Gupta, Indian J.
Pharmacol., 2011, 43(3), 246–253.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 22691–22700 | 22699

View Article Online
RSC Advances
Paper
5 B. Adhikari and S. Majumdar, Prog. Polym. Sci., 2004, 29, 23 (a) J. C. Skou, Biochim. Biophys. Acta, 1960, 42, 6–23; (b)
699–766. R. Whittam and M. E. Ager, Biochem. J., 1964, 93(2), 337–348.
6 A. N. Shipway, E. Katz and I. Willner, ChemPhysChem, 2000, 24 J. Bramham and F. G. Riddell, J. Inorg. Biochem., 1994, 53,
1, 18–52. 169–176.
7 W. Gopel, T. A. Jones, M. Kleitz, J. Lundstorm, T. Seiyama, 25 P. Melnikov and L. Z. Zanoni, Biol. Trace Elem. Res., 2010,
J. Hesse and J. N. Zemel, Sensors, A Comprehensive survey, 135, 1–9.
Wiley-VCH, Weinheim, 2008, vol. 2, Part I, Chemical and 26 (a) D. Krewski, R. A. Yokel, E. Nieboer, D. Borchelt, J. Cohen,
biochemical sensors, print ISBN: 9783527267682, online
ISBN: 9783527620135, DOI: 10.1002/9783527620135.
8 G. Aragay, F. Pino and A. Merkoci, Chem. Rev., 2012, 112,
5317–5338.
9 M. Britschgi, S. von Greyerz, C. Burkhart and W. J. Pichler,
Curr. Drug Targets, 2003, 4(1), 1–11.
J. Harry, S. Kacew, J. Lindsay, A. M. Mahfouz and
V. Rondeau, J. Toxicol. Environ. Health, Part B, 2007,
10(Suppl. 1), 1–269, DOI: 10.1080/10937400701597766; (b)
P. Zatta, E. Lain and C. Cagnolini, Eur. J. Biochem., 2000,
267, 3049–3055.
27 R. R. Crichton, Biological Inorganic Chemistry, Elsevier,
Oxford, 2nd edn, 2012, ISBN 978-0-444-53782-9.
10 N. Barsan, G. Gauglitz, A. Oprea, E. Ostertag, G. Proll,
K. Rebner, K. Schierbaum, F. Schleifenbaum and 28 R. Azadbakht and J. Khanabadi, Spectrochim. Acta, Part A,
U. Weimar, Chemical and Biochemical Sensors, 1. 2015, 139, 279–285.
Fundamentals. Ullmann's Encyclopedia of Industrial 29 M. Shamsipur, S. Y. Kazemi, H. Sharghi and K. Niknam,
Chemistry, Wiley-VCH, 2016, pp. 1–81, DOI: 10.1002/
14356007.b06_121.pub2.
11 (a) E. Shoji and M. S. Freund, J. Am. Chem. Soc., 2001, 123,
Fresenius. J. Anal. Chem., 2001, 371, 1104–1108.
30 E. D. Roper, V. S. Talanov, M. G. Gorbunova, R. A. Bartsch
and G. G. Talanova, Anal. Chem., 2007, 79, 1983–1989.
3383–3384; (b) Z.-H. Sheng, X.-Q. Zheng, J.-Y. Xu, W.-J. Bao, 31 B. Radaram, T. Mako and M. Levine, Dalton Trans., 2013, 42,
F.-B. Wang and X.-H. Xia, Biosens. Bioelectron., 2012, 34,
125–131.
12 (a) H.-J. Schneider, K. Kato and R. M. Strongin, Sensors, 2007,
16276–16278.
32 V. Souchon, I. Leray and B. Valeur, Chem. Commun., 2006,
4224–4226.
7, 1578–1611; (b) H. S. Tzou and C. I. Tseng, J. Sound Vib., 33 J. S. Benco, H. A. Nienaber, K. Dennen and
1990, 138(1), 17–34.
13 (a) X. Luo, A. Morrin, A. J. Killard and M. R. Smyth,
W. G. McGimpsey, J. Photochem. Photobiol., A, 2002, 152,
33–40.
Electroanalysis, 2006, 18(4), 319–326; (b) J. Liu and Y. Lu, 34 H. Sharma, N. Kaur, A. Singh, A. Kuwar and N. Singh, J.
Angew. Chem., 2006, 118, 96–100. Mater. Chem. C, 2016, 4, 5154–5194.
14 (a) A. S. Klymchenko and A. P. Demchenko, J. Am. Chem. Soc., 35 M. Arvand, M. Moghimi and M. A. Bagherinia, Anal. Lett.,
2002, 124, 12372–12379; (b) D. W. Kimmel, G. LeBlanc, 2009, 42, 393–408.
M. E. Meschievitz and D. E. Cliffel, Anal. Chem., 2012, 84, 36 X.-H. Jiang, B.-D. Wang, Z.-Y. Yang, Y.-C. Liu, T.-R. Li and
685–707. Z.-C. Liu, Inorg. Chem. Commun., 2011, 14, 1224–1227.
15 (a) R. C. Jorgenson and S. S. Yee, Sens. Actuators, B, 1993, 12, 37 R. K. Bera, S. K. Sahoo, S. K. Mittal and A. Kumar, Int. J.
213–220; (b) H. Raisanen, US Pat. 7216417B2, 15 May 2007. Electrochem. Sci., 2010, 5, 29–38.
16 W. Gopel, J. Hesse, J. N. Zemel, T. Grandke and W. H. Ko, 38 D. Maity and T. Govindaraju, Inorg. Chem., 2010, 49(16),
Sensors, A Comprehensive Survey, Wiley-VCH Weinheim, 7229–7231.
2008, vol. 1, Fundamentals and General Aspects, print 39 M. H. Mashhadizadeh and R. P. Talemi, Anal. Chim. Acta,
ISBN: 9783527267675, online ISBN: 9783527620128, DOI:
10.1002/9783527620128.
17 S. Chopra, N. Singh, P. Thangarasu, V. K. Bhardwaj and
N. Kaur, Dyes Pigm., 2014, 106, 45–50.
18 G. Kaur, T. Raj, N. Kaur and N. Singh, New J. Chem., 2016, 40,
10536–10544.
2011, 692, 109–115.
40 M. B. Gholivand, F. Ahmadi and E. Raee, Electroanalysis,
2006, 18(16), 1620–1626.
41 H. Kasai, H. S. Nalwa, H. Oikawa, S. Okzada, H. Matsuda,
N. Minami, A. Kakuta, K. Ono, A. Mukoh and
H. Nakanishi, Jpn. J. Appl. Phys., 1992, 31, L1132.
19 S. Kaur, A. Kaur, N. Kaur and N. Singh, Org. Biomol. Chem., 42 Accelrys, Materials Studio Release Notes, Release 4.4, Accelrys
2014, 12, 8230–8238. Soware Inc., 2008.
20 A. P. Demchenko, Introduction to Fluorescence Sensing, 43 R. D. L. Johnstone, A. R. Lennie, S. F. Parker, S. Parsons,
Springer International Pulishing, Switzerland, 2nd edn,
2015, print ISBN: 978-3-319-20779-7, online ISBN: 978-3-
319-20780-3, DOI: 10.1007/978-3-319-20780-3.
E. Pidcock, P. R. Richardson, J. E. Warren and P. A. Wood,
CrystEngComm, 2010, 12, 1065–1078.
44 J. P. Perdew and Y. Wang, Phys. Rev. B: Condens. Matter
Mater. Phys., 1992, 45, 13244–13249.
45 A. Shrivastava and V. Gupta, Chron. Young Sci., 2011, 2, 21–
25.
21 H. Sharma, A. Singh, N. Kaur and N. Singh, ACS Sustainable
Chem. Eng., 2013, 1, 1600–1608.
22 J. M. Garcia, F. C. Garcia, F. Serna and J. L. de la Pena, Polym.
Rev., 2011, 51, 341–390.
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