A light activated CMP conjugated 8-aminoquinoline turn-on fluorescent optode for selective determination of Th4+ in an aqueous environment

Article abstract of DOI:10.1039/c9dt01830a

A new dibutyl(2-oxo-2-(quinolin-8-ylamino)ethyl)phosphinate (L) was designed, synthesised and developed as a light activated optode for Th⁴⁺ determination. The sensing performance of L was studied in solution and in the polymeric membrane phase using absorbance and emission techniques. The neutral ion carrier L exhibits weak fluorescence at 400 nm in CH₃CN : H₂O (9 : 1, v/v). Upon complexation of L with Th⁴⁺, the emission intensity at 486 nm was increased 100-fold due to chelation induced enhanced fluorescence. The L-Th⁴⁺ binding interaction was studied using both Hill's and Job's plots. They indicate the formation of complex species in a 2 : 1 ratio with an estimated binding constant of 2.66 × 10⁴ M^-2. Complex formation between L and Th⁴⁺ was studied by NMR (¹H, ³¹P), FTIR and LC-MS analyses. In order to synthesize a probe for the sensing of Th⁴⁺ in 100% aqueous medium, the optode was prepared by immobilizing L as a neutral ion carrier in a PVC support using dioctyl phthalate (DOP) as a membrane solvent. The best performance of the optode was observed with a membrane composition of PVC : L : NaTPB : DOP in proportions of 35 : 5 : 3 : 57 (%, w/w) in the pH range of 4.0-8.0. The optode can detect Th⁴⁺ concentrations down to 1.1 nM with a fast response time of 15 s and the optical response remains unaltered even after 3 months; the optode sensor can be regenerated using Na₂EDTA solution. Finally, the optode was used for the quantification of Th⁴⁺ in various water samples, monazite sand, and gas mantle samples.

Full text of DOI:10.1039/c9dt01830a

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PleasDeadltoonnoTtraandsjaucsttiomnsargins
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DOI: 10.1039/C9DT01830A
ARTICLE
Light Activated CMP Conjugated 8-Aminoquinoline Turn-On
Fluorescent Optode for Selective Determination of Th4+ in
Aqueous Environment
Received 00th January 20xx,
Accepted 00th January 20xx
Selva Kumar Ra and S.K. Ashok Kumara*
DOI: 10.1039/x0xx00000x
A new dibutyl(2-oxo-2-(quinolin-8-ylamino)ethyl)phosphinate (L) was designed, synthesised and developed as a light active
optode for Th4+ determination. The sensing performance of L was studied in solution as well as polymeric membrane phase
using absorbance and emission techniques. The neutral ion carrier L exhibits weak fluorescence at 400 nm in CH3CN:H2O
(9:1, v/v). Upon complexation of L with Th4+, emission intensity at 486 nm was increased to 100-fold due to chelation induced
enhanced fluorescence. The L-Th4+ binding interaction was studied by both Hill’s and Job’s plots. It indicates, the formation
of complex species in 2:1 ratio with an estimated binding constant of 2.66 x104 M-2. The complex formation between L and
Th4+ was studied by NMR (1H, 31P), FTIR and LC-MS analysis. In order to make a probe for the sensing of Th4+ in 100% aqueous
medium, the optode was prepared by immobilizing L as a neutral ion carrier in PVC support using dioctyl phthalate (DOP) as
membrane solvent. The best performance of the optode was observed with a membrane composition of PVC:L:NaTPB:DOP
in proportions of 35:5:3:57 (%, w/w) in the pH range 4.0-8.0. The optode can detect Th4+ concentration down to 1.1 nM with
a fast response time of 15s and optical response remains unaltered even after 3 months; the optode sensing can be
regenerated using Na2EDTA solution. Finally, the optode was used for quantification of Th4+ in various water samples,
monazite sand, and gas mantle samples.
Numerous sensors were reported in the past few decades to
detect various analytes, but most of them suffer to work in pure
aqueous media which restrict their real applications.29–32 Such a
sensors can be applied in a fully aqueous medium with an
additional advantage of reusability by developing a polymeric
optode. In this system, immobilizing the sensing material
(chromoionophores or fluoroionophores) with or without an
extractant in the polymeric backbone support is a highly
diversified method.33–37
Recently, some chromogenic and fluorogenic optodes for
thorium have been reported using arsenazo III,38 thorin,39
xylenol orange,40 triazene-1,3-di(2-methoxyphenyl),41 4-(p-
nitrophenyl azo)–pyrocatechol,42 and 2-(acetyloxy)-N-(5-nitro-
2-thiazolyl)-benzamide43 as sensing materials but some
limitations such as poor sensitivity, limited working pH range,
serious interference (Al3+, UO22+, Ti4+, Zr4+, and lanthanides) due
to similar chemical properties need to be improved. Hence, the
fabrication of highly selective optode for the quantification of
Th4+ with good sensitivity to use under wide pH range and
negligible interference from serious interfering ions are the
main requirement for thorium sensor.33,44,45
Introduction
The progress of artificial optical receptors designed for the
selective binding to actinides is an active area of research.1–11
Particularly, thorium is an important metal for diverse
applications such as the manufacture of ceramics, carbon arc
lamps, and catalyst in the chemical industry,12 its alloys are used
in jet engine spare parts,13 and in mantles for lanterns. It is also
used as nuclear reactor fuels.14 Due to the extensive use of
Th15,16 and its toxic properties, contamination of air, water, and
soil components are likely. Hence, the development of simple
and efficient techniques for the determination of thorium is
most needed.16 Many techniques have been used for
determination of thorium such as ICP-MS,17 ion
chromatography,
voltammetry,20
fluorescence, 23 laser fluorescence,24 alpha spectrometry,25 and
gamma spectrometry.26 Yet, most of these techniques are
expensive, skilled supervision and laborious procedure.27
Optical chemosensors with target-selective ligands and
luminescent signaling systems for direct measurement of
change in emission intensities arising upon analyte recognition
have gained prime importance in current analytical research.28
18
neutron
activation
radiometry,22
analysis,19
X-ray
potentiometry,21
Herein, ligand
L was prepared by reacting 2-chloroquinolinyl
acetamide with tributyl phosphate. As a part of our ongoing
research on the development of new chemosensors for actinide
ions,35,46–48 the neutral carrier
L was used for the fabrication of
a.Department of Chemistry, School of Advanced Sciences, Vellore Institute of
Technology, Vellore-632014, Tamil Nadu, India.
† E-mail: ashok312002@gmail.com.
Th4+ selective polymeric optode and applied for the estimation
of Th4+ in various samples.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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149.57, 138.44, 137.44, 134.04, 128.36, 127.48, 123.28, 122.81,
117.43, 44.13 (Fig. 3S). DEPT 135 NMR (100 MHz, DMSO-d6) δ
positive 149.58, 137.44, 127.49, 123.28, 122.81, 117.44 and
negative 44.13 (Fig. 4S). HR-MS (M+) calculated: 220.0403 and
obtained: 220.0400 (Fig. 5S).
Experimental Section
Material and instruments
View Article Online
DOI: 10.1039/C9DT01830A
Dioctyl phthalate (DOP), polyvinyl chloride (PVC), sodium
tetraphenylborate (NaTPB), 8-amino quinolone, tributyl
phosphate, lanthanides as chloride salts, other metal ions as
nitrate salts and required were purchased from Sigma Aldrich,
SD fine and Merck chemicals.
L
: (C19H27N2O4P): FTIR (ATR, cm-1) 3336 (-NH), 2958 (-CH, Ar),
2873 (-CH, Ali), 1685 (C=O), 1527 (P=O), 1246 (N-C), 1018 (P-O)
Fig. 6S). 1H NMR (400 MHz, CDCl3) δ 10.40 (s, 1H), 8.84 (d, J =
(
Absorbance and emission spectral responses were recorded on
a JASCO V-730 UV-Vis spectrophotometer and JASCO FP-8200
spectrofluorometer. Fluorescence lifetime measurement was
carried using a HORIBA Jobin-Yvon instrument. FTIR spectra
were recorded on a Shimadzu Affinity FTIR spectrometer in the
range of 4000-400 cm-1. NMR spectra were recorded using a 400
MHz Advance Bruker using TMS as an internal standard. Mass
analysis was analysed using Agilent Ultivo LC/TQ. Spin coating
carried out using Spin NXG-M1 manual spin coating unit.
4.2, Hz, 1H), 8.76 – 8.72 (m, 1H), 8.16 (dd, J = 8.3, 1.5 Hz, 1H),
7.53 (d, J = 4.2 Hz, 1H), 7.46 (dd, J = 8.3, 4.2 Hz, 1H), 7.44 (d, J =
4 Hz, 1H ) 4.20 – 4.13 (m, 4H), 3.23 (d, J = 21.2 Hz, 2H), 1.73 –
1.64 (m, 4H), 1.38 (m, 4H), 0.87 (t, J = 7.4 Hz, 6H) (Fig. 7S). 13C
NMR (100 MHz, CDCl3) δ 162.71, 148.37, 138.51, 136.25,
134.44, 127.93, 127.25, 121.96, 121.65, 116.79, 66.68, 66.62,
38.11, 36.81, 32.46, 32.39, 18.66, 13.56 (Fig. 8S). 31P NMR (162
MHz, CDCl3) δ 21.47 (Fig. 9S). DEPT 135 NMR (100 MHz, CDCl3)
δ Positive 148.37, 136.27, 127.25, 121.98, 121.65, 116.81, 13.56
and negative 66.70, 66.63, 38.10, 36.80, 32.45, 32.39, 18.66
Synthesis and characterization of L
(
Fig. 10S). HR-MS (M+): calculated: 378.1708 and obtained:
The synthesis of
L follows two steps reactions; in the first,
378.1705 (Fig. 11S).
preparation of intermediate product (2-chloro-N-(quinolin-8-
yl)acetamide); in the second step, the reaction of intermediate
product with tributyl phosphate, leading to the formation of
Quantum yield
Fluorescence quantum yield of L
and L-Th4+ was carried out as
per the Nature method reported by Resch-Genger.50 The
standard compound quinine sulfate was used and it's quantum
yield is 0.546 in 0.5 M H2SO4.51 In order to calculate quantum
yield, the UV-Vis absorbance and emission spectra of five
final compound L (Scheme 1).
An intermediate product was synthesized by adding
chloroacetyl chloride (0.8 mL, 1.2 eq.) dissolved in dry
dichloromethane (DCM) which is slowly added to a stirred
solution of quinoline-8-amine (1.16 g, 1 eq.) and triethylamine
different concentrations of quinine sulfate,
taken in acetonitrile. Further, the quantum yield was calculated
using the equation (i).
L
and L-Th4+ were
o
(0.3 mL) in DCM at 0 C under nitrogen atmosphere. After the
completion (as monitor by TLC), the reaction was quenched
with saturated NaHCO3 solution. The mixture was then
extracted with 20 mL of DCM for about three times and the
collected organic layer was washed with dil. HCl, dried with
Na2SO4. The solvent was removed under reduced pressure, and
the obtained residue was further purified by silica gel column
chromatography (60-120 mesh silica gel, EA/Hex = 3:7). The
obtained yield was 1.63g of 92%.
Фf,s = Фf,r (Fs/Fr) × (ƒr/ƒs) (Ƞs )(λem)/(Ƞr2)(λem)----(i)
2
Where s and r, denoted as sample (L,
L-Th4+) and reference
quinine sulfate respectively. Fs and Fr is the integral photo flux
and ƒs and ƒr are the absorption factors of sample and reference
and ղis the refractive index of the solvent.
In the second step, the synthesized 2-chloro-N-(quinolin-8-yl)
acetamide (1.10 g, 5mM) and tributyl phosphate (2 mL) were
Fabrication of optode
o
refluxed at 120 C for overnight under nitrogen atmosphere.49
The PVC-based optode was prepared by thorough mixing of
L
The reaction progress was monitor by TLC. The obtained
product was further purified by silica gel column
chromatography (60-120 mesh silica gel, EA/Hex = 5:5) and the
obtained yield was 1.61g of 85% yellow oily-liquid.
(0-6%, w/w), NaTPB (0-6%, w/w), DOP (65-53%, w/w) and PVC
(35%, w/w) for 200 mg batch added slowly in 2 mL of
tetrahydrofuran (THF) in a 50 mL beaker to get complete
homogenous mixture. Then, the mixture was allowed to
evaporate at room temperature to remove the excess of THF
solvent to get a consistent viscous solution. To improve the
adhesion of the membrane on the quartz plate was activated by
washing with concentrated nitric acid for 12 h followed by
hydrofluoric acid (3%) and hydrogen peroxide (10%) each for 30
min, then washed with distilled water and ethanol. The mixture
(100μL) was placed using micropipette on a quartz plate of
dimension: 16 mm (l) x 12 mm (w) x 3 mm (t), the quartz plate
was placed on a spin-on device and rotated at 2000 rpm for
about 2 min to achieve uniform distribution of the membrane
matrix on the quartz plate. The quartz plate coated with the
polymeric membrane was taken out from the spin-on device
and allowed to stand at ambient condition for 1 h before use.
Scheme. 1. Reagents and conditions to synthesize
L
2-chloro-N-(quinolin-8-yl) acetamide (C11H9ClN2O): FTIR (ATR,
cm-1) 3269 (-NH), 2968 (-CH, Ar), 2899 (-CH, Ali), 1678 (C=O),
1
1240 (C-N) (Fig. 1S). H NMR (400 MHz, DMSO-d6) δ 10.91 (s,
1H), 8.87 (d J = 4.2, 1H), 8.75 (d J = 7.5 Hz, 1H), 8.19 (dd, J = 8.3,
1.3 Hz, 1H), 7.75 (d, J = 7.7 Hz, 2H), 7.49 (dd, J = 8.3, 4.2 Hz, 1H),
4.31 (s, 2H) (Fig. 2S). 13C NMR (100 MHz, DMSO-d6) δ 165.50,
2 | J. Name., 2012, 00, 1-3
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The fluorescence measurement was carried out by keeping the colourless to pale yellow was noticed with Al3+ and UO2 . But,
2+
View Article Online
DOI: 10.1039/C9DT01830A
quartz plate in a quartz cuvette cell containing 2 mL of pH 6.5 under UV light, the selective cyan colour fluorescence was
Tris-HCl buffer solution.52 To this solution, Th4+ was added and noticed with Th4+ and slight yellow-orange fluorescence was
corresponding emission intensity was recorded at 486 nm by noticed with Ti4+ and Zr4+. While other tested metal ions, it did
exciting at 315 nm.
not give such visible changes under similar conditions.
The UV-Vis spectral studies of (50 µM) shows two absorption
maximum at 240 nm and 315 nm with a molar absorptivity of
L
Fluorescence titration
The fluorescence titration was carried out to check the 2.0x104 and 4.0x103 L.M-1.cm-1, respectively (Fig. 2a). Both
sensitivity of the optode-10 sensor by keeping the optode-10 absorbance peaks are due to π-π* and n-π* transitions on the
diagonally inside quartz cuvette containing 2 mL of pH 6.5 tris- quinoline ring system. When
L
interacts with Th4+, both peaks
HCl buffer solution. To this solution an incremental amount of are experiencing a blue shift to 233 nm and 310 nm with a molar
Th4+ (1 mM) was added using micropipette. After each addition absorptivity of 1.65x104 and 3.94 x103 L.M-1.cm-1 respectively
of Th4+, emission spectra was recorded upon excitation at 315
nm
(
Fig. 12S). In order to check the sensitivity of the response, UV-
Vis titration of
L
with Th4+ was carried out. The absorbance peak
at 240 nm starts to undergo blue shift with slight hypochromic
shift and band at 315 nm undergo slight hyperchromic shift with
Response time determination
The optode response time was carried out by using the optode- a clear isosbestic point at 238 nm, 310 nm, and 345 nm, which
10 placing diagonally inside cuvette cell containing in 2 mL clearly indicated the new species formation during the
buffer solution. To this solution added 20 μM of Th4+ solution interaction (Fig. 2b, 13S). Due to the low molar absorption
and the fluorescence intensity at 486 nm was recorded at every coefficient of L-Th4+ complex, the quantification of Th4+ using
2 seconds time interval upon an excitation at 315 nm.
L
at low concentration becomes uncertain. However, L-Th4+
Estimation Th4+ in water samples
Initially, the 1 mM Th4+ stock solution was prepared by using
double distilled water, tap water and well water collected from
VIT University campus. The optode-10 sensor was immersed in
2 mL of buffer solution, to this solution added known amount
of Th4+ solution. The emission intensity at 486 nm was recorded
upon the excitation at 315 nm for all spiked samples. The spiked
concentration of Th4+ was calculated using the calibration curve
obtained using standard solution of Th4+ using same optode
sensor.
Fig. 2. (a) UV-Vis spectral response of L
(50 µM) and L-Th4+(2eq.) in
CH3CN:H2O (9:1, v/v) (b) UV-Vis spectral titration of
L (50μM) with
Th4+ (0-2 eq.) in CH3CN:H2O (9:1, v/v).
exhibits emission property shown Fig.1b
In order to study fluorescence properties of
emission spectra were recorded for in the presence and
absence of different metal ions in CH3CN:H2O (9:1, v/v) media
by exciting at 315 nm. Accordingly, alone shows a weak
emission band at 400 nm. However, shows a strong emission
band at 486 nm upon interaction with Th4+ while Ti4+, Zr4+
exhibits a weak emission intensity to an extent of 10-fold
increase while for other metal ions, emission intensity increased
to a factor of 3-4 fold (Fig. 3a). Besides, a large Stokes shift (171
nm) (Fig. 3b) and intensity increased by 100-fold with Th4+ (15au
.
Results and discussion
In order to establish selective polymeric optode for Th4+,
systematically characterized in solution phase using absorption
and emission techniques.
L
and its complex,
L
L is
L
L
UV-Visible and fluorescence spectral response of L
The metal binding ability of
L (50 µM), the colorimetric changes
of were performed with twenty three different metal ions
L
such as Na+, Ca2+, Mg2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+,
Cd2+, Hg2+, Ag+, Al3+, In3+, Bi3+, Pb2+, La3+, Ce3+, Lu3+, Gd3+, Sm3+,
Zr4+, Ti4+, UO22+ and Th4+ (10 eq.) in CH3CN:H2O (9:1, v/v) shown
in Fig. 1a. Under visible light, a very weak colour changes of
to 1500 au). On excitation of
L and its metal complexes at 240
nm,
L
did not show any emission band while complexes of Ti4+,
Fig. 1. Colorimetric performance of
ions (10 eq.) under (a) visible light (b) UV light.
L (50 µM) with various metal
Fig. 3. Fluorescence response of
L (50 µM) with (a) various metal
ions (10 eq) upon the excitation at 315 nm (b) Absorbance and
emission spectral response of L-Th4+ in CH3CN:H2O (9:1, v/v).
This journal is © The Royal Society of Chemistry 20xx
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Zr4+, and Th4+ showed weak emission intensity at 486 nm. The quantum yield of
Journal Name
L
and L-Th4+ was calculated by standard
View Article Online
DOI: 10.1039/C9DT01830A
Hence, all sensing ability of
studied by exciting at 315 nm (Fig. 14S).
The non-fluorogenic nature of is due to the photoinduced time-resolved fluorescence decay measurements were
electron transfer (PET) occurred from a phosphoryl oxygen to performed to calculate lifetime (1.27 ns and 3.47 ns), radiative
the quinoline ring system (Scheme 2) 53,54 In the case of L-Th4+ decay process (Kr= 8.11 x107 s-1) and non-radiative (Knr= 70.6
complex, the significant fluorescence enhancement of
is due x107 s-1) for and decay process (Kr= 7.89 x107 s-1) and non-
to the participation of carbonyl oxygen, phosphoryl oxygen, and radiative (Knr= 20.9 x107 s-1) for L-Th4+ respectively (Fig. 5a
5b).
L with selected metal ions were procedure and it was found to be 0.103 and 0.274, respectively.
In order to establish fluorescence properties of
L ,
and L-Th4+
L
.
L
L
,
quinoline nitrogen in coordinate bond formation with Th4+ Results show that L-Th4+complex exhibits an increase in Kr and
which triggered inhibits the PET process.55
decrease in Knr recommends strong fluorescence emission
associated with L-Th4+ complex.
Scheme 2 The possible mechanism of fluorescence in L
and L-Th4+
Fig. 5. Fluorescence lifetime decay profile of (a) L (50 μM, λex= 315
nm and λem = 400 nm) and (b) L-Th4+ complex (50 μM, λex = 315 nm
and λem = 486 nm) in CH3CN.
To elucidate the sensitivity of
was carried out by sequential addition of Th4+ to a fixed quantity
of and measured emission intensity at 486 nm (Fig. 4a, 15S).
L
towards Th4+, spectral titration
L
Performance of optodes
The titration data reveals that linear correlation between the
concentrations of Th4+ and a measured intensity in the range of
Any sensor works completely in the water phase and reusability
always good for the sensor. These features encourage us to
develop and characterize the optode sensor for Th4+
determination. The fluorescent optode used in this study is
1-25 µM of Th4+. The binding stoichiometric ratio of with Th4+
L
was determined by using Job’s plot 56 by maintaining the sum of
the concentrations of Th4+ and
L constant and varying the molar
ratio of Th4+ from 0.1 to 0.9. As seen in Fig. 4b, it shows that
based on the immobilization of
NaTPB as a lipophilic anionic additive. Since
L
in a plasticized PVC containing
is neutral ion
when a molar fraction of Th4+ reached 0.3, it exhibits maximum
L
intensity which reflects a 2:1 stoichiometry between L
and Th4+.
carrier and co-agent (NaTPB) lipophilic anions provide ion-
exchange properties to optode membrane.32,33 The possible
response mechanism of the present optical sensor can be
supported by the following ion-exchange mechanism shown in
equ-ii.59,60
Further to support Job’s method, the binding stoichiometry was
calculated using Hill’s plot57,58 shown in Fig. 4c. The slope
calculated to be 2.085 indicating the involvement of 2L binding
with Th4+. The binding constant was calculated from the
fluorescence titration data using a non-linear curve fitting of 2:1
binding model on BindFit v0.5 program. The estimated
-
Th4+(aq) +L(org)+ NaTPB(org)
⇌
L-Th(org)+TPB(org)+ Na+(aq)+ NO3 (aq)---(ii)
association constant was found to be 2.66 x104 M-2
(Fig. 4d).
For the preparation of an ion-selective optical sensor, the ligand
will be incorporated into a hydrophobic membrane such as a
plasticized PVC membrane. Then, the sensor is used in contact
with a solution containing a primary ion (Th4+). This ligand
can
L
L
extract Th4+ from aqueous solution into the organic membrane
phase and form a complex which exhibits fluorescence emission
at 486 nm. As can be seen from Fig. 6a, the fluorescence
intensity of the optode membrane increases with increasing
concentration of Th4+, which constitutes the basis for the
determination of Th4+. Various membranes were prepared by
dissolving various membrane ingredients (in % weight) such as
L
(0-6), DOP (55-65), NaTPB (2-6) and PVC (35) in 2 mL of THF.
The total membrane composition was fixed to 200 mg. The ratio
of and different membrane ingredients were optimized to give
a better response in terms of working concentration range, and
response time. From Table 1, it is clear that as the amount of
L
Fig. 4. (a) Fluorogenic titration of L
(50 µM) Th4+(0-30 µM) Binding
stoichiometric study using (b) Job’s plot (c) Hill’s plot (d) BindFit
plot in CH3CN:H2O (9:1, v/v) (λex = 315 nm and λem = 486 nm).
L
4 | J. Name., 2012, 00, 1-3
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Table 1. Optimization of optode membrane ingredients
PVC
(%, w/w)
35
DOP
(%, w/w)
65
L
NaTPB
(%, w/w)
Working Range
(M)
Response
Time (s)
-
S.No.
(%, w/w)
1
2
-
-
-
No response
35
64
1
2
3
4
5
6
5
5
5
5
5
1 x 10-5 to 5 x 10-3
8 x 10-6 to 10 x 10-5
1 x 10-6 to 2.5 x 10-4
5 x 10-7 to 1 x 10-4
5 x 10-8 to 2.5 x 10-5
1 x 10-7 to 2.5 x 10-5
8 x 10-7 to 1 x 10-5
5 x 10-8 to 2.5 x 10-5
2 x 10-9 to 5 x 10-4
1 x 10-8 to 2.5x 10-5
5.5 x 10-9 to 1.5 x 10-5
60
3
35
63
-
22
4
35
62
-
35
5
35
61
-
25
6
35
60
-
30
7
35
59
-
25
8
35
59
1
2
3
4
5
35
9
35
58
20
10
11
12
35
57
15
35
56
20
35
55
25
increases from 0 to 6 %, optical response of optode also
increases and attained highest sensitivity at 5% of . Further, the
effect of lipophilic salt on carried out from 0 to 6%, results The response of pH on fluorescence intensity of optode-10
Effect of pH
L
L
show that at 3%, the optode gave the best response in terms of studied in the presence and absence of Th4+ (20 µM). The
detection limit (1.1 nM) and response time (15s) (Fig. 16S).33 required pH 1.0- 12.0 adjusted using 0.1N HCl/NaOH solution.
The absence of lipophilic salt affects the working concentration The emission intensity was recorded at 486 nm upon excitation
range and delay the fluorescence response. Furthermore, all at 315 nm. As seen from Fig. 7, fluorescence intensity remains
other studies were carried out using optode-10. As shown in Fig. same in the pH range 4-8 and above pH 8 and below pH 4
6a, the free optode shows no fluorescence emission upon the decreased due to formation polymeric thorium hydroxide
complexation with Th4+ it shows remarkable fluorescence formation Thx(OH)y.62 and protonation of heteroatoms.
change. The lowest detection limit (LOD) and lowest detecting However, in the case of free optode, it didn’t give any change in
quantity (LOQ) was calculated according to IUPAC definition fluorescence intensity by varying the pH from 1.0 to 12.0 due to
using the linear equation obtained from calibration plot (Fig. the non-fluorogenic nature of
6b).61 The LOD and LOQ were found to be 1.1 nM and 3.69 nM
respectively. The membrane optode was regenerated by
treating with a strong chelating agent such as Na2EDTA. The
Th4+-EDTA complex stability constant was comparatively high,
so it is expected to easily decomplex the L-Th4+ complex and
L.
leaves the free neutral carrier
L in the membrane phase. For
subsequent Th4+ measurements, the optode should be kept in a
pH 6.5 buffer solution for about 10 min (Fig. 17S). To calculate
the lifetime of the optode-10, every ten days of time interval,
fluorescence response was recorded and plotted calibration
graph to find the detection limit. The result reveals that the
detection limit of Th4+ does not change till 90th day use. After
90th day, the response of the optode start to decrease (Fig. 18S).
This shows that optode can used for about 3 months.
Fig. 7. Effect of pH on the performance of optode-10 and optode
with 20 μM of Th4+ in various buffer solutions (λex = 315 nm and λem
= 486 nm).
Interference study
The specificity of optode-10 as a probe for the quantification of
Th4+ (0.5 eq) in the presence of various interfering metal ions (5
eq.) was explored in a fully aqueous medium. The emission
intensity was recorded at 486 nm upon excitation at 315 nm. As
seen from Fig. 8, the intensity of optode in the presence of Th4+
is unperturbed in the presence of 10 eq. of interfering ions; this
Indicates an excellent selectivity of Th4+ over competing ions.
The response of optode-10 for Th4+ detection in the presence of
UO22+, Ce3+, Al3+, and group IV-B elements such as Ti4+ and Zr4+
Fig. 6. (a) Fluorescence response of optode-10 with and without
Th4+ (25μM) in pH 6.5 buffer (b) Calibration curve (λex = 315 nm and
λem = 486 nm).
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Table 2. Fluorogenic optode performance towards Th4+ recognition and its comparison with previously reported work
View Article Online
DOI: 10.1039/C9DT01830A
Name of chromogenic/fluorogenic
ionophore
Response
Time (s)
λmax
(nm)
Matrix
Mode
LOD
pH
Interference
4-(p-nitrophenyl azo)–
pyrocatechol37
2+
PVC
Abs
Abs
Abs
600
60
500
370
540
6 µM
1.15 μM
2.6 µM
3.5
3.0
3.0
Ga3+, UO2
2+
Triazene-1,3-di(2-methoxyphenyl)41
PVC
PEI-
PCMs
PEI-
PCMs
PEI-
PCMs
UO2
Arsenazo III38
30
NR
NR
NR
Thorin38
Abs
Abs
Abs
50
60
483
606
554
2.7 µM
0.97 µM
1.85 µM
3.0
6.0
3.0
Thionin38
La3+, As3+,
Sb3+
Thorin–methyltrioctylammonium39
CTA
420
2-(acetyloxy)-N-(5-
nitro-thiazolyl)-benzamide43
Xylenol orange40
L
420/48
2
576
315/48
6
Sol-gel
PAN
Em
Abs
Em
120
600
15
78 nM
0.2 µM
1.1 nM
4.0
4.0
6.5
NR
NR
No
PVC
(Present work)
Interference
NR- not reported, PEI -polyethylenimine, PCMs -pluronic polycarbamates, CTA-cellulose triacetate, PAN-polyacrylonitrile, Abs-absorbance
and Em -emission.
is relatively low but clearly, quantification can be made (Fig. 8a). are in the L-Th4+ complex are an unsymmetrical environment of
Further, the effect of selected anions response was investigated distracted octahedron geometry.
and results indicate that there is no significant change in The 31P NMR spectra of
L
and L-Th4+ given in Fig. 9b, here,
alone exhibits a peak at 22.39 ppm, which experience shielding
the present optode was compared with the previously reported effect (21.60 ppm) upon complexation with Th4+. Overall, the
Th4+ selective optodes.21–26 The present optode is significantly change in NMR signal suggests that the binds with Th4+
improved in terms of response time, detection limit and through coordination of phosphoryl oxygen (P=O), carbonyl
L
intensity in the presence of other anions (Fig. 8b). In Table 2
,
L
interference.
carbon (C=O), and with quinoline nitrogen (-N=). Further, as
seen in Fig. 9d, the FTIR spectra of neat
L shows peak at 3336
cm−1 for amide νN-H, 2958 cm−1 for aromatic νC-H, 2874 cm−1 for
aliphatic νC-H, carbonyl (C=O) 1682 cm−1 for the νC=O, imine (-N=)
1527 cm-1 for νC=N, phosphine oxide (P=O) 1246 cm−1 for νP=O
,
975 cm−1 for νP-O-C stretching and 556 cm-1 for νP-O-C bending
vibrations. After interacting with Th4+, it exhibits all peaks are
experiencing shifts from the original vibrations. Accordingly
Fig. 8. Bar diagram representing performance of optode-10 with
20μM of Th4+ in the presence of various (a) metal ions (100 μM) (b)
anions (100 μM) in pH 6.5 buffer (λex = 315 nm and λem = 486 nm).
Sensing mechanism
The binding mechanism of
the 1H, 13C and 31P NMR for neat
d6. As seen in Fig. 9a, neutral carrier
H) peak at 10.36 ppm and bridging methyl (-CH2) at 3.52 ppm.
When
interacted with Th4+, amine (N-H) peak experience
L
with Th4+ was studied by recording
and L-Th4+ complex in DMSO-
alone exhibits amide (N-
L
L
L
deshielding effect (at 10.41 ppm) whereas bridging methyl (CH2)
experience shielding effect (at 3.46 ppm). The 13C NMR spectra
for L shows (Fig. 9c), the carbonyl carbon peak at 163 ppm, upon
complexation with Th4+, the carbonyl peak exhibits as two
signals with small deshielding effect (at 164 ppm). The splitting
of the carbonyl carbon signal suggests that the carbonyl groups
Fig. 9. Establishment of binding mechanism of
L
with Th4+ using (a)
L
and L-Th4+ complex in DMSO-
1H NMR (b) 31P NMR (c) 13C NMR for
d6 (d) FTIR spectra.
6 | J. Name., 2012, 00, 1-3
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shifted to 3315 cm−1 for amide νN-H, 2960 cm−1 for aromatic νC-
H, 2873 cm−1 for aliphatic νC-H, 1687 cm−1 for the νC=O, 1506 cm-1
for νC=N, 1168 cm−1 for νP=O, 1020 cm−1 for νP-O-C stretching and
557 cm-1 for νP-O-C vibrations. The FTIR study reveals the
participation of C=O, P=O and -N=C are involved in the bond
formation with Th4+.
Conflicts of interest
There are no conflicts to declare.
View Article Online
DOI: 10.1039/C9DT01830A
Acknowledgements
Authors thank Board of Research in Nuclear Sciences (BRNS),
GOI for supporting the work through the project grant
(34/14/11/2015/BRNS). Authors are also thankful to DST-VIT-
FIST for NMR and Vellore Institute of Technology, Vellore for
other research facilities
Further, LC-MS analysis of L-Th complex exhibits two major
peaks appeared at 1006.4204 (m/z) and 1011.3639 (m/z)
which corresponds to species involvement of 2
+1Th+Na with a calculated mass of 1006.4114 (m/z) and
L
+1Th+NH4+ and
2
L
1011.3695 (m/z) respectively (Fig. 19S). Finally, LC-MS analysis
also supporting the formation of 2:1 ligand to metal complex.
Notes and references
Analytical applications
The practical utility of the optode-10 was studied to estimate
the Th4+ concentration in different water samples such as
double distilled water, bore well water and well water. The
results are given in Table 3, it shows good agreement between
the spiked and recovered concentration of the thorium.
Besides, the same optode was successfully used for the direct
quantification of thorium in monazite sand and lantern gas
mantle samples and results are compared with ICP-MS analysis.
As it is clear from Table 3, there is a satisfactory agreement
between the thorium detected by optode-10 and those
determined with the ICP-MS method.
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