Synthesis of novel benzothiazole based fluorescent and redox-active organic nanoparticles for their application as selective and sensitive recognition of Fe3+ ions

Article abstract of DOI:10.1016/j.inoche.2021.108648

A novel benzothiazole based compound (1) has been synthesized and further employed for the fabrication of fluorescent as well as redox active organic nanoparticles (1-ONPs) by adopting a nano-re-precipitation method. The formation of 1-ONPs was confirmed by Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM) techniques, where the size was found to be ~42.84 nm in the optimized conditions. The resulting 1-ONPs acted as selective and sensitive sensor for recognition of Fe³⁺ ions in dual transduction mode. The nanoparticles underwent quenching of fluorescence intensity as well as an amplified stripping current response in the presence of Fe³⁺. The high surface charge density worked as a tool for enhancing the current response of nanoparticles over bulk solution mode and hence is extended to in-field analysis.

Full text of DOI:10.1016/j.inoche.2021.108648

Inorganic Chemistry Communications 129 (2021) 108648
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Synthesis of novel benzothiazole based fluorescent and redox-active
organic nanoparticles for their application as selective and sensitive
recognition of Fe3 ions
+
*
*
Navneet Kaur , Randeep Kaur, Ranjeet Kaur, Shweta Rana
Panjab University, Chandigarh, Punjab, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel benzothiazole based compound (1) has been synthesized and further employed for the fabrication of
fluorescent as well as redox active organic nanoparticles (1-ONPs) by adopting a nano-re-precipitation method.
The formation of 1-ONPs was confirmed by Dynamic Light Scattering (DLS) and Transmission Electron Micro-
scopy (TEM) techniques, where the size was found to be ~42.84 nm in the optimized conditions. The resulting 1-
Benzothiazole
Organic nanoparticles (ONPs)
Fluorescence sensing
Electrochemical sensing
ONPs acted as selective and sensitive sensor for recognition of Fe3 ions in dual transduction mode. The
+
nanoparticles underwent quenching of fluorescence intensity as well as an amplified stripping current response
in the presence of Fe3 . The high surface charge density worked as a tool for enhancing the current response of
+
nanoparticles over bulk solution mode and hence is extended to in-field analysis.
1
. Introduction
et al. [11] have also designed fluorescent organic nanoparticles based
chemosensors using rhodamine derivative. The chemosensor experi-
enced fluorescent enhancement on chelation with Hg2 ions stated as
+
Introduction of nano materials to the scientific word has unveiled
large number of possibilities for engineering different types of materials
of great interest [1]. The exceptional combination of small size and
prominent properties of nano-materials is continuously carving their
path towards their involvement in every type of research area such as
medicine [2], analytical [3], electronics [4], chemosensors [5] etc.
Along with the outstanding advantages of nano-materials, there are
some biological issues related to their biodegradability. Inorganic nano-
materials possess very high stability due to which they are very difficult
to decompose. Along with this, they are highly penetrating in living
bodies [6]. On the other hand, the stability of organic nano particles
chelation enhanced fluorescence (CHEF).
In sensing, electroanalytical methods pose advantages of ease of
operation, portability, simple instrumentation and sample preparation.
However, very limited research work has been reported by authors
utilizing the electrochemical properties of ONPs for metal ion sensing.
3
4
ꢀ
Billing et al. [12] have developed electrochemical sensor for PO
ions
utilizing ONPs derived from Schiff’s base receptor. The method offered
good detection limit ranged in nM and was exploited to determine the
amount of phosphate in biologically important samples. Recently,
Kumar et al. [13] utilized ONPs of Schiff base for voltammetric detection
Cu(II) ions. However, to the best of our knowledge till date, stripping
voltammetric approach [14,15] has not been utilized for the metal ions
sensing via ONPs, which is assumed to be better in terms of sensitivity
and selectivity for the detection of metal ions [16] in comparison to
other voltammetric techniques.
(
ONPs) is dependent on various factors due to which their working
environment is very limited. Due to this nature, ONPs are unlikely to
persist in solution with unoptimized physical parameters such as tem-
perature, pH and solvent system that tend to agglomerate in unfriendly
conditions [7]. Therefore, ONPs do not pose major threat to the living
bodies unlike inorganic nanoparticles. Evidences of ONPs employed for
sensing purpose can be found in literature [8,9]. Kaur et al. [10]
Among the various metal ions present in the biological systems, Fe3
+
ion dictates its importance by catalysing various biological reactions
that are fundamental to life of a living specie. One of the examples is
haemoglobin, the organometallic protein of high importance, requires
iron for performing its function of oxygen transport to all body parts.
explored rhodamine based ONPs for sensing of Al3 and adenosine
+
monophosphate. The sensor showed fluorescence enhancement in the
presence of Al3 ion through spirolactam ring opening process. Mahajan
+
*
Corresponding authors.
E-mail addresses: neet_chem@pu.ac.in (N. Kaur), shweta0402@pu.ac.in (S. Rana).
https://doi.org/10.1016/j.inoche.2021.108648
Received 14 March 2021; Received in revised form 22 April 2021; Accepted 29 April 2021
Available online 4 May 2021
1
387-7003/© 2021 Elsevier B.V. All rights reserved.

N. Kaur et al.
Inorganic Chemistry Communications 129 (2021) 108648
The low iron content in the body leads to various problems like
including anaemia and decrease in cognitive abilities. Whereas the
excessive presence of Fe in the body can give birth to health issues like
hereditary hemochromatosis. Iron is taken by our body through various
types of iron rich foods for which the ultimate source of iron is the
environment [17]. Therefore, to keep a check on the presence of iron in
the water samples around us, sensing of iron is necessary. Dong et.al.
had synthesized benzimidazole functionalized Zr-UiO-66 nanocrystals
compound 1 had poor solubility in water, so their organic nanoparticles
2
have been fabricated in DMSO:H O (1:99, v/v) solvent system. The re-
precipitation method, being advantageous in terms of being extremely
efficient, less time consuming and energy efficient, has been used for the
fabrication of organic nanoparticles (ONPs). The rapid mixing of hy-
drophobic organic probe, 1, and large volume of water changed the
microenvironment of 1 and consequent precipitation of 1 led to for-
mation of stable dispersion of nanoparticles in aqueous medium [21,22].
Due to the unavoidable effects of various physical parameters such as
nature of organic solvent, concentration of organic compound, tem-
perature and sonication time on formation and stability of ONPs, opti-
mization of these parameters was conducted. Amongst various tested
for sensing of Fe3 ion that work efficiently in aqueous system [18]. For
+
3
+
fast and highly sensitive detection of Fe ion, Xu et al. had synthesized
a two-dimensional metal organic framework nanosheets. The high sur-
face area of nano-porous sheets was responsible for high sensitivity of
the sensor [19]. 5 Keeping this in mind, here organic nanoparticles
based sensor has been designed using a dual mode response mechanism
3 3
organic solvents (CH CN, CHCl , DMF, DMSO and THF), DMSO was
found to be the most suitable for formations of 1-ONPs with smallest
nanoparticle size of 82.09 nm and least polydispersity index value of
0.180, as measured using DLS (Table 1).
of optical and redox behaviour for the very first time to sense Fe3 . The
tailoring methodology offered here facilitates the formation of ONPs due
to increase in the possibility of ᴨ-ᴨ interactions between the molecules in
solution phase [20].
+
Concentration of organic probe also plays significant role in the
synthesis of organic nanoparticles, so, different concentrations of
organic compound 1 (0.25 mM, 0.5 mM, 1 mM and 2 mM) have been
used for fabricating 1-ONPs in DMSO as the optimized organic solvent.
As can be seen from Fig. S1a, the results supported 1 mM concentration
of 1 as the optimum concentration for nanoparticles fabrication. As the
nucleation process involved in nanoparticles formation is dependent on
temperature, therefore, synthesis of 1-ONPs was carried out at different
temperatures. The studies revealed that the nanoaggregates developed
were of appropriate size range at the temperature equal to or less than
2
. Experimental
2
.1. Materials and instrumentation
The detailed information about materials used for synthesis of
compound 1, its nanoparticles 1-ONPs, evaluation of its optical studies
along with quantum yield determination as well as redox studies and
instruments used for its structural elucidation has been given in SI.
◦
10 C (Fig. S1a). The size of ONPs started increasing with increase in the
temperature of the solution. Size of the resultant ONPs was also
dependent on the ratio of DMSO to water, where it was observed that
increasing amount of DMSO led to the formation of large size particles
due to aggregation (Fig. S1b). The effect of pH on the organic nano-
particle formation was recorded and as it is clear from Table S1, the best
pH value for the synthesis of organic nanoparticles of required size range
is 6.86, which is the pH of pure deionized water (D.I.) taken during the
experiment.
2
.2. Synthesis of benzene-1,3-bis(benzothiazole) (1) and its organic
nanoparticles 1-ONP)
Compound 1 has been synthesized by reaction of 2-aminothiophenol
and benzene-1,3-dicarbaldehyde using microwave conditions (Scheme
1
). The simple and cost-effective re-precipitation method was used to
prepare an aqueous suspension of ONPs of compound 1 (1-ONPs). The
detailed synthesis has been given in SI.
3
.2. DLS and TEM analysis of fabricated 1-ONPs
2
.3. General procedure of fluorescence and electrochemical
measurements for metal ion recognition using 1-ONPs
The formation and stability of 1-ONPs was monitored by DLS and
The details for the general procedure of fluorescence and electro-
chemical measurements for metal ion recognition of 1-ONPs has been
given in SI.
Table 1
Particle size and polydispersity index value of fabricated 1-ONPs in various
solvents.
Solvent
CH CN
Particle size (in nm)
Polydispersity Index (PDI)
3
. Results and discussion
3
229.3
356.2
307.6
82.09
147.7
0.481
0.708
0.721
0.180
0.874
CHCl
DMF
3
3
.1. Fabrication of organic nanoparticles (1-ONPs) of compound 1
DMSO
THF
For real time applications of sensors in environmental and biological
domains, aqueous medium plays an important role. Since organic
Scheme 1. Synthesis of compound 1 using solvent free approach.
2

N. Kaur et al.
Inorganic Chemistry Communications 129 (2021) 108648
TEM techniques. Under optimized conditions, the DLS experiment
showed that the particles have size of 42.84 nm. The DLS study provides
hydrodynamic diameter of the particles due to the presence of solvent
shell around the nanoparticles and therefore lacks the actual size
determination. Therefore, to find the nearest obtainable size of nano-
particles, TEM analysis was performed. The particles size was approxi-
mately ~16 nm, obtained from the size intensity curve provided in the
inset in Fig. 1b. This difference in the sizes from both the studies could
be attributed to the difference in defining principle of both the tech-
niques. To ensure the reproducibility of the 1-ONPs, the procedure was
repeated four times and the obtained results approved the synthetic
procedure.
of 1-ONPs. The designed nano-chemosensor displayed linear responses
in two different ranges of Fe3 ion concentration. The low concentration
+
plot ranges from 0 to 33.3 equiv and the high concentration plot ranges
from 33.3 to 88.8 equiv of Fe3 ions (Fig. S3a). The limit of detection
+
+
(LOD) value of 16.99
equation LOD = 3 /s, where,
the calibration curve between fluorescence intensity and concentration
μ
M for Fe3 detection was calculated using
σ
σ
is standard deviation and s is the slope of
of Fe3 ions. The binding ability of 1-ONPs with the Fe was deter-
mined with the help Benesi Hildebrand plot (Fig. S3b) and the associ-
ation constant value which is equal to intercept/slope was obtained as
+
3+
3
ꢀ 1
3.3 × 10 M . The equation employed is as follows [25]
1
I ꢀ I
1
1
0
= I
max ꢀ I
+
(Imax ꢀ I
0
0
a
)K [Q]
3
.3. Fluorescence recognition studies with organic nanoparticles (1-
where I is the observed fluorescence intensity, I
0
is the fluorescence
ONPs)
3
+
intensity of 1-ONPs in the absence of Fe ions and Imax is the intensity
3
+
of 1-ONPs in the presence of excess Fe ions. [Q] represents the con-
The emission spectra of organic probe 1 and 1-ONPs were analysed
centration of quencher, here it is Fe3 ions and K
+
represents association
a
in order to see the spectral difference between organic compound 1 and
its organic nanoparticles. The fluorescence emission peak underwent
significant changes with reduced intensity, red shift and broadening of
the emission peaks (Fig. S2). These changes are in agreement with the
constant.
To investigate further the practical applicability of 1-ONPs as Fe3
+
selective fluorescent sensor, competitive experiments were performed in
the presence of various metal ions. To carry out this experiment, 100
equiv. of all interfering metal ions were introduced into the mixture
supra-molecular self-organization and intermolecular
typically observed in organic nanoparticles [23].
π
- interactions
π
3
+
containing 10 M of 1-ONPs solution and 100 equivalents of Fe ions.
μ
The metal ion binding ability of synthesized 1-ONPs was determined
As is clear from fig S4, the quenching of 1-ONPs is almost identical to
by fluorescence emission spectroscopy in the absence and presence of
that observed in the presence of Fe3 ions alone, pointing to the fact that
+
+
+
2+
3+
2+
2+
3+
2+
various metal ions such as Na , K , Mg , Al , Mn , Fe , Fe , Co ,
3
+
2
+
2+
2+
2+
2+
2+
fabricated 1-ONPs could serve as potential candidate as selective Fe
Ni , Cu , Zn , Cd , Hg and Pb (added as their perchlorate
salts). When excited at 280 nm wavelength, the organic nanoparticles
displayed emission maxima at 406 nm with an observable stoke’s shift of
ion sensor in practical environmental applications.
3
.4. Electrochemical analysis
The electrochemical response has been explored by recording cyclic
1
26 nm (Φ = 0.14). Upon addition of 100 equivalents of each of the
S
above-mentioned ions, the fluorescence spectra of 1-ONPs underwent
perturbations with addition of only Fe3+ ions (Fig. 2a). This feature
revealed that 1-ONPs can serve as selective fluorescent nano-
voltammograms (CV) of chemosensor 1 and 1-ONPs within wide po-
tential window of ꢀ 2.0 V to +2.0 V at a scan rate of 50 mV/s. CV of
3
+
chemosensor for Fe ions in an aqueous system.
ꢀ
5
chemosensor 1 (Fig. S5) with concentration of 10 M is displaying
dismayed oxidation-reduction peaks while it is clearly observable that
conversion of chemosensor 1 (bulk supramolecule) to ONPs has resulted
in the emergence of new redox peaks along with amplified current
response. In 1-ONPs (Fig. S5.), the new anodic peaks are seen at +0.62 V
and +1.13 V along with a weak anodic hump at +0.13 V. In addition to
this shift in cathodic peaks was also observed. The amplified current
response is accredited to the increased charge transfer rate and addi-
tional active sites generated due to enhanced surface to volume ratio
after conversion of chemosensor 1 to ONPs [26].
Fluorescence titration analysis was performed in order to study the
binding ability of 1-ONPs with Fe3 ions. The fluorescence emission
intensity was measured for each subsequent addition of Fe3+ ions to 1-
+
ONPs and it was observed that peak intensity decreased with increasing
concentration of Fe3 ions (0–88.8 equiv) in 1-ONPs (Fig. 2b). Fe is a
+
3+
well known paramagnetic ion with an empty d shell and has the ability
to quench the fluorescence of a fluorophore via a photoinduced metal-
to-fluorophore electron or energy transfer mechanism [24]. Moreover,
3
+
Fe has high thermodynamic affinity for ligands with “N” or “O” atoms
leading to formation of stable complex. So, large fluorescence quenching
was observed at 406 nm upon addition of Fe3 ions to aqueous solution
+
Fig. 1. (a) DLS graph of fabricated 1-ONPs and (b) TEM images of fabricated 1-ONPs with intensity curve showing average size of 16 nm.
3

N. Kaur et al.
Inorganic Chemistry Communications 129 (2021) 108648
3
+
Fig. 2. Fluorescence spectra of 1-ONPs (a) in the presence of 100 equivalents of different metal ions and (b) with different concentrations of Fe ions in
aqueous medium.
3
.5. Electrochemical recognition studies with organic nanoparticles (1-
utilized for further studies. At a fixed deposition potential of ꢀ 0.8 V,
deposition time was also varied from 20 s to 60 s. where a maximum in
stripping peak arises at 45 s. Thus, the optimized deposition potential of
ꢀ 0.8 V and deposition time of 45 s was approved for further studies.
(Fig. S6).
ONPs)
The electro-analytical response of 1-ONPs in presence of different
3
+
2+
2+
2+
2+
3+
2+
+
2+
2+
metal ions (Al , Cd , Co , Cu , Fe , Fe , Hg , K , Mg , Mn ,
+
2+
2+
2+
Na , Ni , Pb and Zn ) was monitored to check for the selectivity
Fig. 3). For the recognition experiments, square wave anodic stripping
voltammetry (SWASV) was carried out between the potential range of
1.5 V to +1.0 V using 1-ONPs (concentration used during experiments
3.5.2. Analytical performance of ONPs towards Fe3 metal ions:
To check the legitimacy of the proposed SWASV method for the
determination of Fe3 ions, the stripping peak current response was
+
(
+
ꢀ
ꢀ 5
3+
is 10 M) to which 100 equivalents of metal ions were added. After
adding the 100 equivalents of each metal ion to ONPs solution, the
mixture was left undisturbed to ensure complete binding. It is clearly
evident from Fig. 3a, that maximum change in amperometric current
response was observed in Fe3 ion and hence further optimization was
carried out to obtain high sensitivity for the developed method. Usually,
benzothiazole moieties offered either formation of new complex or un-
dergo protonation behavior. The anodic peaks in SWASV of Fig. 3b
showed the appearance of new peaks (at ꢀ 0.967 V and ꢀ 0.683 V)
pointing to the formation of stable complex due to high thermodynamic
affinity of Fe3 for ligands with “N” or “O” atoms.
monitored at different concentrations of Fe ions (Fig. 4). The gradual
3
+
increase in peak current was seen with increasing concentration of Fe
M to
M were observed with limit of detection of
ions (from 0.3 µM to 8 µM). Also, a set of linear ranges from 0.3
μ
0.95
μM and 1.0
μM to 8
μ
+
ꢀ 8
ꢀ 7
5.67 × 10 M and 5.17 × 10 M respectively.
The improved limit of detection of 1-ONPs using electrochemical
mode rather than fluorescence here indicated its pre-eminence for the
detection of Fe3 ions. The calculated limit of detection value is com-
+
parable with the previously reported values (Table S2).
+
3.5.3. Interference studies
3
+
To monitor the effects of interferents, equal amounts of Fe and
3
+ 2+ 2+ 2+ 2+ 3+ 2+ +
3
.5.1. Optimization of stripping analysis parameters:
To obtain maximum sensitivity of ONPs towards Fe3 , optimization
different metal ions (Al , Cd , Co , Cu , Fe , Fe , Hg , K ,
+
2+
2+
+
2+
2+
2+
Mg , Mn , Na , Ni , Pb and Zn ) were administered to 10 ml of
ꢀ
5
of deposition potential and deposition time was carried out. For this,
deposition potential was varied from ꢀ 0.9 V to ꢀ 0.1 V where maximum
anodic stripping current response was obtained at ꢀ 0.8 V and thus
ONPs (10 M) solution. Fig. S7 indicates the least variation in stripping
current response of Fe3 ions in presence of these metal ions. Further,
the analysis of interference was done using the equation:
+
ꢀ 5
Fig. 3. (a) Bar graphs depicting the change in stripping current response of 1-ONPs (10 M) after addition of 100 equivalent of different metal ions and (b) Square
ꢀ 5
3+
wave stripping response of 1-ONPs (10 M) after addition of 100 equivalent of Fe ions.
4

N. Kaur et al.
Inorganic Chemistry Communications 129 (2021) 108648
3
+
3+
Fig. 4. (a) Titrimetric analysis of ONPs in the presence of Fe from 0.3
μM to 8
μ
M and (b) Calibration plot between current response and Fe concentration in
organic nanoparticles solution.
Interference(%) = (I
0
ꢀ ꢀ I)/I
0
*100
4. Conclusions
3
+
where, I refers to the peak current for Fe ion and I refers to the peak
0
New selective and sensitive nano sensor has been developed for the
3
+
3+
current for Fe and the interferents (Other metal ions) [27]. There is no
fluorescent as well as redox sensing of Fe ions in aqueous solution
2
+
significant interference from any of these metal ions except Pb
7.90%) that hints the applicability of the proposed sensor to real time
analysis.
using benzothiazole based organic nanoparticles (1-ONPs). The nano-
re-precipitation method was used for the preparation of nanoparticles
and 1-ONPs were well characterized by DLS and TEM techniques. The
nanoparticles exhibited “on-off” type fluorescent changes with high
(
selectivity in the presence of Fe3 ions, most probably due to its para-
magnetic nature as well as its high affinity towards ligands with “N” or
“O” atoms. An enhanced current response due to high surface to volume
ratio facilitated by the increase in the number of active sites on con-
+
3
.5.4. Real time monitoring:
The designed 1-ONPs were examined for its reliability for Fe3
+
sensing in iron tablets. For this, different solutions of iron tablets were
analysed by using Fluorescence spectroscopy and SWASV. The response
obtained was noted down and compared with the calibration curves
version to ONPs has resulted in better sensitivity for Fe3 in comparison
+
3
+
obtained with different concentrations of Fe to calculate the amount
to optical mode. Besides these, the designed 1-ONPs were also examined
3
+
3+
of Fe present in the prepared samples. The expected amount of iron in
prepared samples was in close agreement with that of amount found as
shown in Tables 2 and 3. The appreciably high recovery (%) of 90 to
for their real time application for Fe sensing in iron tablets accentu-
ating on their expediency.
1
03.7% indicated the applicability of prepared ONPs for real time
CRediT authorship contribution statement
monitoring.
Navneet Kaur: Conceptualization, Resources. Randeep Kaur:
Methodology, Visualization, Investigation, Data curation, Writing -
original draft. Ranjeet Kaur: Investigation, Data curation. Shweta
Rana: Supervision, Conceptualization, Resources, Funding acquisition.
Table 2
3
+
Real sample analysis of Fe in iron tablets and their recovery (%) using Fluo-
rometric analysis.
Declaration of Competing Interest
Iron tablet
Expected (M)
Found (M)
Recovery (%)
RSD (%)
5
ꢀ 5
ꢀ 5
ꢀ 5
Sample 1
Sample 2
Sample 3
32 × 10ꢀ
30.12 × 10
43.13 × 10
58.75 × 10
94.1
90.0
91.8
1.09
2.13
3.46
ꢀ
5
5
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
48 × 10
64 × 10ꢀ
Table 3
Acknowledgements
3
+
Real time analysis of Fe in iron tablets and their recovery (%) using SWASV.
Iron tablet
Expected (
μM)
Found (
μM)
Recovery (%)
RSD (%)
Financial grant from Department of Science and Technology-Science
and Engineering Research Board (Grant No. SERB/2016/000046) is
gratefully acknowledged by the authors.
Sample 1
Sample 2
Sample 3
0.4
0.6
0.8
0.39
0.55
0.83
97.5
91.0
103.7
1.2
3.1
1.4
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.inoche.2021.108648.
5

N. Kaur et al.
Inorganic Chemistry Communications 129 (2021) 108648
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