Triphenyl-imidazole based reversible coloro/fluorimetric sensing and electrochemical removal of Cu2+ ions using capacitive deionization and molecular logic gates

Article abstract of DOI:10.1016/j.saa.2020.119018

A simple hydroxyl-substituted triphenyl-imidazole based receptor (HTPI) which selectively detects Cu²⁺ ion by colorimetric and fluorimetric methods was developed. HTPI detects the Cu²⁺ ions with the absorption enhancement and fluores

Full text of DOI:10.1016/j.saa.2020.119018

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Triphenyl-imidazole Based Reversible Coloro/fluorimetric
Sensing and Electrochemical removal of Cu2+ ions using
capacitive deionization and molecular logic gates
G. Prabakaran, K. Velmurugan, R. Vickram, C. Immanuel David,
A. Thamilselvan, J. Prabhu, R. Nandhakumar
PII:
S1386-1425(20)30997-5
https://doi.org/10.1016/j.saa.2020.119018
SAA 119018
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 30 May 2020
Revised date:
11 September 2020
6 September 2020
Accepted
date:
2
Please cite this article as: G. Prabakaran, K. Velmurugan, R. Vickram, et al., Triphenyl-
imidazole Based Reversible Coloro/fluorimetric Sensing and Electrochemical removal
of Cu2+ ions using capacitive deionization and molecular logic gates, Spectrochimica
Acta Part A: Molecular and Biomolecular Spectroscopy (2020), https://doi.org/10.1016/
j.saa.2020.119018
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Triphenyl-imidazole Based Reversible Coloro/fluorimetric Sensing and
2
+
Electrochemical removal of Cu ions using capacitive deionization and
molecular logic gates
a
a
a
a
b
G. Prabakaran , K. Velmurugan , R. Vickram , C. Immanuel David , A. Thamilselvan and
a
a,
J. Prabhu , R. Nandhakumar *
a
Department of Applied Chemistry, Karunya Institute of Technology and Sciences (Deemed-to-
be University), Karunya Nagar, Coimbatore - 641 114, India.
*
E-mail: nandhakumar@karunya.edu
b
Electro Organic-Division, Central Electrochemical Research Institute (CSIR-CECRI),
Karaikudi, 630 003, India
Abstract
A simple hydroxyl-substituted triphenyl-imidazole based receptor (HTPI) which
2
+
selectively detects Cu ion by colorimetric and fluorimetric methods was developed. HTPI
2
+
detects the Cu ions with the absorption enhancement and fluorescence quenching by the
possible ligand to metal charge transfer (LMCT) and the chelation-enhanced quenching (CHEQ)
2
+
approaches, respectively. HTPI showed high selectivity and sensitivity for Cu ions detection
2
+
over other interfering and competing metal ions. Interestingly, HTPI detects Cu ion (LOD) at
-9
-9
nanomolar concentrations (19 x 10 M (UV-vis) & 27 x 10 M (fluorescence), respectively),
2
+
which is lower than the permissible level of Cu ion reported by World Health Organization
(WHO). Furthermore, HTPI was applied to the molecular logic gate function by using chemical
2
+
inputs, and Cu ion was potentially removed (95 %) via Capacitive Deionization technique.
Keywords: Fluorescence; LMCT; PET, Imidazole; Copper; Electrochemical; Logic gate
1
. Introduction

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Over the years a wide range of colorimetric and fluorimetric sensors have been developed for
the specific detection of environmentally and biologically important metal ions/anions [1-9].
Among them, copper is one of the third most abundant metal ion playing an important role in our
daily life and thus used for various applications, such as printing, electrical manufacturing wires,
medical equipment’s etc. [10-14] As a result, copper can easily enter into drinking water and
cause adverse effects to humans, animals and plants [15-16]. The over-accumulation of copper in
our body leads to neurodegenerative diseases, including Alzheimer’s and Wilson’s disease, and
its deficiency causes anemia [17-21]. The American Medical Association suggests that the
presence of copper level in a normal human body should be 1.2–1.3 mg/day and our growth
development is in the order of 0.1 mg/L. In addition, WHO also recommended that the allowed
2
+
level of Cu ions in drinking water be within 15–25 μM. Therefore, the development of
2
+
coloro/fluorescent probes for the detection of Cu in environmental/biological samples is getting
more attention and highly demanding to date.
2
+
At present, there are many techniques introduced for the sensing of Cu , including
Voltammetry, atomic absorption spectroscopy (AAS), inductively coupled plasma mass
spectroscopy (ICP-MS), and colorimetric/fluorescent probe method (CFPM) [22-27]. In
2
+
comparison, CFPM is one of the most frequently used technique to the sensing of Cu , due to its
dual mode of detection, low cost, rapid response, accuracy, and high sensitivity [28]. So far,
2
+
most of the chemosensors have been developed for the sensing of Cu via either absorption or
emission spectroscopy [29-31]. Recently, there are only a few literature reports for the detection
2
+
of Cu ions by both colorimetric and fluorimetric analysis, and also, it's highly challenging.
Therefore, it is pivotal to develop a dual-channel coloro/fluorimetric probe for the detection of
2
+
Cu ion in an aqueous solution.

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On the continuation of our previous work [32], we realized that hydroxyl-substituted probe is
2
+
necessary for the selective sensing of Cu ion. Accordingly, we have designed and synthesized
the triphenyl imidazole derivatives HTPI and TPI by the treatment of benzil with
salicylaldehyde and benzaldehyde, respectively. Probe HTPI comprising of benzil act as a
fluorophore, and imidazole and phenolic –OH act as ionophores, thus separated by C-C single
bond spacer. This probe was utilized for the sensing of metal ions via dual modes, i.e., UV-vis
and fluorimetric, respectively, and compared with TPI (absence of -OH). Furthermore, HTPI
2
+
was employed as a complexing agent for effective removal of Cu ion via capacitive
deionization method, and chemical inputs based logic gates were also demonstrated.
2
2
. EXPERIMENTAL METHODS
.1 Synthesis of HTPI
Salicylaldehyde (0.50 g, 4.09 mmol), benzil (2.14 g, 10.22 mmol), and ammonium acetate
(2.52 g, 32.72 mmol) were dissolved in 30 ml of acetic acid in 100 ml round bottom flask. The
reaction mixture was refluxed with vigorous stirring for 18 hours and cooled to room
temperature. The reaction mixture was poured into ice-cold water and the white precipitate
formed was filtered, dried, and purified by column chromatography using petroleum ether –
-1
1
ethyl acetate as eluents. Yield: 78 %. FT-IR ((KBr), (cm )): 1593, 3051 and 3197. H NMR
(
200 MHz, CDCl , ppm): 7.55-7.58 (m, 4H), 7.25-7.48 (m, 8H), 7.0-7.10 (d, 1H), 6.91-6.94 (d,
3
1
3
1
1
H). C NMR (125 MHz, CDCl , ppm):157.5, 145.7, 130.5, 128.2, 128.1, 123, 127.3, 118.9,
3
17.8, 112.4. Elemental analysis: C H N O; calcd. (%); C, 80.75; H, 5.16; N, 8.97 and found
2
1
16
2
+
+
+
(
%); C, 80.70; H, 5.10; N, 8.90. LC-MS calcd.; C H N O: [M ] 311.36, found; [M +H]
21 16 2
3
12.40.
2
.2. Synthesis of TPI

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1
Similar procedure was followed for the synthesis of TPI. Yield: 78 %, H NMR (200 MHz,
CDCl , ppm): 7.9-7.94 (m, 2H), 7.26-7.51 (m, 14H). Elemental analysis: C H N ; calcd (%); C,
3
21 16
2
8
5.11; H, 5.44; N, 9.45, found (%); C, 85.00; H, 5.35; N, 9.35, LC-MS calcd.; for C H N :
21 16 2
+
+
+
[
M ] 296.37, found; [M -H] 295.49.
3
3
. Results and discussion
.1. Synthetic design of HTPI & TPI
Triphenyl-imidazole based probes HTPI and TPI were synthesized by a one-step simple
condensation reaction between respective aldehydes (salicylaldehyde and benzaldehyde) and
benzil in the presence of ammonium acetate, respectively as shown in Scheme 1. These probes
were well-characterized by the spectroscopic and analytical techniques (Fig. S1-S5). For an
efficient chemosensor, signalling and ion-binding units are highly indispensable. Accordingly,
we have introduced the imidazole unit as an ionophore, where the presence of a donor nitrogen
atom within the ring makes good interactions with metal ions [33-35]. Also, introducing the
hydroxyl group on the phenyl ring covalently linked with conjugated imidazole-benzil scaffold
2
+
provides a suitable coordination environment for the detection of Cu ion by various
photophysical pathways.
Scheme 1
3
3
.2 Colorimetric Studies
2
+
.2.1 Selectivity of HTPI towards Cu
Initially, probe HTPI was treated with different solvents, such as THF, DMSO, and
DMF, and studied their polarity by using UV-vis and fluorescence techniques (Fig. S15). From
these results, HTPI showed the higher absorbance and emission intensities in THF than other

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solvents and thus used as a solvent for all the photophysical experiments. The UV-vis spectrum
of HTPI (THF-H O, 1:1 v/v, solution HEPES = buffer, pH=7.4) displayed two absorption bands
2
at 236 nm and 321 nm, which were attributed to π-π* and n- π* transitions of the phenyl and
2
+
imidazole rings. Upon the addition of Cu to HTPI, a 6.2-fold enhancement of the absorption
band intensity was observed, with a notable large blue shift (321 nm to 293 nm) in the absorption
spectrum. Besides, the naked-eye observation showed that the colorless solution of HTPI
instantly changed into sky blue color, which unquestionably proved that HTPI coordinates with
2
+
Cu ion in THF-H O solution. It is noteworthy to mention that the significant shift in the
2
spectral wavelength assisted with the respective color changes are mainly due to the ligand to
2+
metal charge transfer (LMCT) process, thus resulting in a reduction of Cu ions by HTPI.
2
+
Furthermore, during the HTPI+Cu complexation, the C-C single bond free rotation between
the phenyl and imidazole scaffolds was arrested which eventually pushes the sensor into a
different conformation making the molecule more rigid and causing a drastic absorption
enhancement. Unfortunately, however, this complex did not exhibit a d-d band in the visible
region and may be due to the high solvating nature. Under identical conditions, the addition of
various cations to HTPI did not produce any significant spectral changes in the absorption
2
+
2+
spectrum except Hg (Fig. 1a). When HTPI interacts with Hg , the band intensity was slightly
enhanced but there is no substantial color change in this solution. On the other hand, probe TPI
did not exhibit any notable changes in the absorption bands with any of the metal ions (Fig. 1b).
Therefore, these results advocated that the additional hydroxyl group substituted HTPI played a
2
+
crucial role in the selective sensing of Cu ion in THF-H O solution.
2
Fig. 1.

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3
.2.2 Competitive complexation experiment
To study the possible interaction of HTPI+Cu complex with other competing metal ions
2
+
+
3+
2+
2+
2+
2+
3+
3+
2+
3+
2+
+
3+
2+
2+
+
2+
(
Ag , Al , Ba , Ca , Cd , Co , Cr , Ce , Fe , Fe , Hg , K , La , Mg , Mn , Na , Ni ,
2
+
2+
2+
Pb and Zn ) interference experiments were performed. HTPI mixed with Cu ion (100
equiv.) in the presence of same equivalents of other metal ions did not show any significant
changes in the absorption band. This proved that there is no influence of other metal
2
+
2+
ions on the detection of Cu ions. Hence, the probe HTPI selectively detects Cu ion in
the presence of other metal ions and can be potentially utilized for on-site monitoring works.
Fig. 2.
3
.2.3 Binding Stoichiometry
To get further insights into the binding stoichiometry between HTPI (host) and Cu (guest),
UV−vis titrations were performed in THF-H O ((1:1 (v/v), HEPES 50 mM, pH = 7.4) solution.
2
+
2
2
+
The absorbance of HTPI was gradually increasing by increasing the concentration of Cu (0-
1
00 equiv.) and associated with a large blue shift, got saturated during the addition of 100 equiv.
2
+
2+
of Cu . This result indicated that HTPI has strong interactions with Cu ions. Furthermore, to
2
+
confirm the stoichiometric ratio of HTPI-Cu (H-G) complex, Job’s plot method was
implemented and the maximum mole fraction was observed at 0.49 (Fig. 4), revealed a 1:1
2
+
binding stoichiometry between HTPI and Cu . In addition, mass spectrum showed a prominent
2
+
-
peak at 455.36 (m/z) for [HTPI + Cu + NO + H O] complex directly evidenced that 1:1
3
2
2
+
stoichiometric ratio between the HTPI and Cu (Fig. S6).
Fig. 3(a) & (b) & Fig. 4.

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To verify the binding stoichiometry and affinity, Benesi-Hildebrand method was performed for
2
+
HTPI-Cu complex by using the following equation [36].
ꢄ
ꢆ
ꢇꢈ[ꢀ ꢁꢀ ][ꢉꢊꢋꢌ]
ꢂ
(1)
ꢀꢅꢁꢀꢂ
ꢅ
ꢀ
ꢁꢀꢂꢃ
2
+
Where, A , A, and Aꢍwere represented as the absence, presence and saturated absorbance of Cu
0
2+
2+
2+
with HTPI, and [Cu ] is the concentration of Cu ion. Plotting of ꢎ /ꢃꢏꢐꢑ versus 1/[Cu ]
0
showed a linear relationship, which also strongly supported the 1:1 complex stoichiometry of
2
+
4
-1
HTPI+Cu (Fig. 3b), and the association constant (Ka) was calculated to be 9.84 × 10 M . The
-9
LOD was found to be 19×10 M by using the equation 3δ/S, where δ denotes the standard
deviation of the free probe, and S denotes the slope of the linear regression plot obtained from
the titration spectrum [37].
3
.2.4. Effect of pH, time and reversibility of HTPI
For practical utility, the effect of pH, time, and reversibility of the sensor plays a vibrant role
2
+
in the selectivity of metal ions. Hence, the sensing ability of HTPI with Cu ions was evaluated
by different pH in THF-H O (1:1 v/v) solution. Though, at higher acidic and basic pH ranges,
2
hydroxyl substituted HTPI could have been involved in the protonation and deprotonation
processes, respectively, it did not much affect the absorbance intensity, and showed a weak
2
+
absorption intensity in all the pH range. At the same time, surprisingly, in the case of HTPI-Cu
complex, absorption enhancement was observed in the wide range of pH (Fig. S7) and there
were no notable changes in the intensity at the higher acidic and basic conditions. Therefore,
2
+
HTPI detects Cu ion in diverse pH ranges, and for our photophysical experiments, the

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physiological pH was selected as a working condition. The effect of time response for the
2+
complexation of HTPI with Cu ion was monitored by UV-vis analysis (Fig. S8). It specified
2
+
that the recognition of Cu ions by HTPI accomplished within two minutes and persisted
steadily for a further 1 h and no changes in their intensity was noted even after a period of 24 h.
2
+
Therefore, HTPI could be suitably utilized to detect Cu ions in a short span of time. Besides,
2
+
the reversibility of HTPI was investigated in the presence of Cu and EDTA as shown in Fig. 5.
2
+
As already discussed that HTPI showed the maximum absorbance in the presence of Cu ion
2
+
(
100 equiv.), while introducing EDTA (100 equiv.) to HTPI + Cu (100 equiv.) solution, the
2
+
absorbance gets quenched and reached to the original HTPI signal, which is due to EDTA-Cu
complexation eventually displacing HTPI from coordination. Moreover, free HTPI again
2
+
2+
participates in the Cu recognition process by the addition of Cu ion in cyclic manners. The
intensity almost remained the same upto fourteen cycles.
Fig. 5.
3
3
.3 Fluorimetric Studies
.3.1. Metal ion selectivity
Based on the absorption spectrum, the excitation wavelengths of HTPI and TPI were fixed at
30 nm and 315 nm, respectively, and the emission studies were carried out. Free HTPI showed
3
2
+
strong fluorescence intensity (436 nm) caused by its ionized form. Upon the addition of Cu
ions to HTPI, the fluorescence intensity got considerably quenched (3.4-fold) with a slight blue
shift from 436 nm to 429 nm. This quenching process may possibly be due to the paramagnetic
2
+
nature of Cu ion and Chelation Enhanced Quenching (CHEQ) and are produced from charge
2
+
transfer from HTPI to Cu (LMCT) via reverse PET process. However, HTPI treated with
+
3+
2+
2+
2+
2+
3+
3+
2+
3+
2+
+
3+
other metal ions, such as Ag , Al , Ba , Ca , Cd , Co , Cr , Ce , Fe , Fe , Hg , K , La ,

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2
+
2+
+
2+
2+
2+
Mg , Mn , Na , Ni , Pb and Zn did not produce the substantial changes in the emission
spectra (Fig. 6a). Similarly, TPI was treated with all the above metal ions, and did not display
considerable selectivity with any of the metal ions (Fig. 6b). These results indicated that the
2+
supplementary hydroxyl group substituted HTPI has contributed an essential part in Cu
binding process.
Fig. 6 (a) & (b)
3
.3.2. Competitive complexation experiment
Similar to colorimetric studies, competition experiments of HTPI were also carried out to
2
+
investigate whether the other metal ions influence the sensing of Cu ions via fluorimetric
2
+
analysis. A fixed concentration of Cu (100 equiv.) with HTPI was treated with an identical
concentration of competing metal ions and their emission spectrum was monitored (Fig. 7).
There were no significant changes in the emission spectra in the experiment carried out.
Therefore, these results specified that the other potentially competing metals ions do not affect
2
+
the sensing property of probe HTPI while detecting Cu ion and hence the sensor could be
utilized for practical purposes.
Fig. 7.
3
.3.3. Binding Stoichiometry
From the titration analysis, it was observed that the HTPI emission intensity gradually
2+
2+
decreases by increasing the concentrations of Cu and gets saturated at 100 equiv. of Cu ions
2
+
added, which prove that both HTPI and Cu ion is involved in the chelation process. With the
support of the Benesi-Hildebrand equation and fluorescence titrations, the association constant

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4
-1
2+
was found to be Ka = 1.98×10 M . In addition, Job’s plot of HTPI-Cu showed a maximum
2
+
mole fraction at 0.5, revealed a 1:1 (HTPI-Cu ) binding stoichiometry (Fig. 8b & 9). The limit
-9
of detection (LOD) was found to be 27×10 M [38], which is lower than the recommended level
2
+
2+
of Cu ion in drinking water set by WHO. To evaluate the nature of quenching in HTPI by Cu
4
-1
ion, the Stern-Volmer quenching constant (Ksv) was calculated to be 4.2467× 10 M (Fig. S9).
2
+
It indicates that HTPI has a high binding affinity with Cu ion leading to the static quenching of
fluorescence. For more confirmation, HTPI and TPI quantum yield (φ) were calculated to be
2+
0
.14 and 0.09, and HTPI with Cu was decreased to 0.03.
Fig. 8 (a) & (b)
Fig. 9
3
.3.4. Effect of pH, time and Reversibility
Fluorescence intensities of HTPI were quite stable in pH 6 to 8 and slightly changed in higher
acidic and basic pH ranges, possibly due to protonation and deprotonation of hydroxyl
2
+
substituted HTPI, respectively. In the case of HTPI + Cu complex emission intensities were
almost quenched and there are no observable changes in pH 5 to 9 (Fig. S10). Therefore, we
have selected the physiological pH for all fluorimetric experiments. As a part of the experiments,
2
+
the effect of time response of HTPI with Cu ion was performed. The emission intensity of the
probe HTPI was completely quenched within 7 min (Fig. S11) which indirectly proves that
2
+
HTPI complex with Cu in a short time. To know the reversibility of HTPI, an alternate
2+
2+
addition of Cu and EDTA were added into the solutions. Briefly, the addition of Cu (100
equiv.) to HTPI, fluorescence intensity was decreased and the addition of EDTA (100 equiv.) to
2
+
HTPI+Cu , the intensity gets enhanced and reached to the original state of HTPI via EDTA-

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2
+
Cu complexation effectively displacing the probe (Fig. 10). These results stated that HTPI
2
+
assisted Cu recognition process is reversible and could be used for many number of cycles.
Fig. 10.
3
.3.5. FT-IR and microscopic studies
FT-IR spectra of HTPI and HTPI+Cu complex were recorded to get information about the
2
+
2
+
mode of binding between the HTPI and Cu ions. HTPI showed the characteristic absorption
-1
bands at 1593, 3051 and 3197 cm corresponds to the imidazole C=N, secondary N-H, and
2
+
phenyl -OH, respectively [39-41]. After the coordination of HTPI with Cu , the absorption
-1
band was upward shifted to 1606 cm and the other bands were almost disappeared. These
changes specified that the HTPI containing C=N and -OH groups involved in the binding with
2
+
Cu ion (Fig. S14). To gain a better understanding of the surface topography changes, scanning
2
+
electron microscopy (SEM) images of free HTPI and HTPI-Cu were observed (Fig. 11).
Eventually, free HTPI exhibited the rod-like structures stacked in order, which describes that the
2
+
molecules are uniformly arranged. In the case of HTPI-Cu complex displayed broken rod-like
structures with irregular cubic particles have increased surface roughness. Therefore, based on
the above studies, including colorimetric, fluorimetric studies, Job’s plot, Benesi−Hildebrand
non-linear curve fitting, mass spectra, and IR analysis, the proposed binding mechanism was
2
+
illustrated in the Scheme 2. When the chemosensor HTPI interacts with Cu , there is an
enhancement in the absorbance intensity assisted with a large blue shift. This may possibly be
2
+
due to the LMCT process, thus resulting in a reduction of Cu ion by HTPI. Also, carbon-
carbon single bond free rotation between the phenyl and imidazole scaffold was blocked after the
coordination, which eventually leads to a change in conformation of the complex and make the
molecule more rigid which may be another reason for the absorption enhancement. On the other

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2
+
hand, during the HTPI interaction with Cu , the fluorescence intensity was quenched and also
2
+
associated by a slight shift in the emission profile, due to the paramagnetic nature of Cu ions
2
+
and CHEQ, produced due to the charge transfer from HTPI to Cu (LMCT) via reverse PET
processes [42-46]. As a result, HTPI displays turn “off-on” (colorimetric) and “on-off”
2
+
(
fluorimetric) mechanisms proposed based on the sensing of Cu ions.
Fig. 11. & Scheme 2.
4
4
. Application studies
.1. Electrochemical removal of Cu ions via Capacitive Deionization
2
+
2
+
Probe HTPI was potentially utilized to remove Cu ions from the aqueous solution via
Capacitive deionization (CDI) technique [47]. In order to prove the removal efficiency, we have
collected two different water samples, one from Karunya Nagar groundwater sample (Sample
A), and the other one river water sample (Sample-B) from the Siruvani River, Coimbatore. These
samples were spiked with Cu(NO ) solution concentrations of 4 and 8 ppm, respectively, and
3
2
-5
monitored their removal efficiency in the presence and absence of HTPI (2×10 M). The CDI
containing the carbon aerogel electrodes exhibited the higher electrosorption capacitance even
without the addition of HTPI (Fig. 12). In the absence of HTPI, both water samples showed
2
+
moderate removal efficiency (62.5 to 70 %) over Cu ions, due to CDI containing carbon
aerogel electrodes have higher electrosorption capacitance. In the presence of HTPI added water
2
+
samples increased the rejection rate of Cu ion to 95% (Table 1). These results suggested that
2
+
HTPI could be played a vital role to remove Cu from the contaminated river water and
groundwater with an aid of CDI technique
Table 1. & Fig. 12.

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4
.2. Molecular Logic Gate Function:
Based on the photophysical responses of HTPI towards Cu and EDTA, we have constructed
2
+
2
+
the IMPLICATION logic gate by both colorimetric and fluorimetric experiments. Here, Cu and
EDTA were used as two inputs, and the presence and absence of inputs are represented as “0”
2
+
and “1”, respectively. After the inclusion of Cu and EDTA to HTPI, the absorbance and
fluorescence intensities were used as outputs at 321 nm and 430 nm. For outputs, weak and
strong fluorescence intensities were taken as “0” and “1”, respectively. When inputs were (0, 0)
and (0, 1), respectively the fluorescence intensity was extremely higher and the output was “1”.
2
+
Similarly, with the inclusion of Cu (1, 0), fluorescence intensity was strongly quenched due to
its binding with HTPI and output was “0”. These results concluded that there is no signal
2
+
activated in output when Cu ion only present in this system. On the other hand, colorimetric
2
+
responses were used as two inputs, such as Cu and EDTA represents the INHIBIT logic
.
function [48-50]. The absence and presence of inputs are represented as “0” and “1” For outputs,
strong and weak absorptions were represented as “1” and “0”, respectively. With inputs were (0,
0
), (0, 1) and (1, 1), respectively, there were no changes in the absorption and output was “0”.
2+
With the addition of Cu (1, 0), the absorption intensity was high due to its binding with HTPI,
and output was “1” (Table 2 and Fig. 13).
Table 2 & Fig. 13
2
+
Comparison of HTPI with recently reported chemosensors for Cu
Table 3
5
. Conclusion
In summary, we have designed and synthesized a hydroxyl-substituted triphenyl-imidazole
2+
based probe HTPI, which detects Cu ions at nanomolar concentrations by dual-channel

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2+
colorimetric and fluorimetric methods. The HTPI interactions with Cu displayed an enhanced
absorbance (6.2-fold) followed by a significant blue shift caused by LMCT, thus resulting in a
2
+
reduction of Cu ion by HTPI along with the arresting of the C-C single bond rotation between
2
+
2+
imidazole and phenyl rings during the HTPI-Cu complex. On the other hand, HTPI-Cu
complex drastically quenches the fluorescence (3.4-fold) because of the paramagnetic nature of
2
+
Cu ion and CHEQ via reverse PET process. Furthermore, the static nature of quenching
mechanism was proved by Stern-Volmer plot, which displayed the strong interaction between
2
+
2+
HTPI and Cu ion. The binding stoichiometry was found to be 1:1 between the HTPI and Cu
4
-1
4
-1
ion, and the association constants were calculated as Ka = 9.84× 10 M and Ka =1.98×10 M ,
by colorimetric and fluorimetric analysis, respectively. HTPI was successfully used to remove
2
+
Cu ions by Capacitive Deionization technique, and chemical inputs based molecular logic gates
were illustrated. Therefore, HTPI based colorimetric/fluorimetric sensor can be used for the
2
+
naked-eye detection of Cu ions in environmental monitoring applications.
Associated content
Author Information
G.P, K.V. & R.N. – Design, synthesis, and characterization of the sensor along with writing,
discussion, and revising the manuscript. R.V, C.I, & J.P- are taking part in the photometric
studies, discussion, and revising manuscript.
A.T – performed the capacitive deionization studies and related discussions. All authors were
involved in the writing, revising, and the completion of the manuscript.
Corresponding Authors
*
E-mail: nandhakumar@karunya.edu
Acknowledgement

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This work is supported by the SERB-EMR grant by the DST (Sanction No. SERB-
EMR/2016/005692).
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Scheme. 1. Synthesis of receptors TPI and HTPI
(
a) Benzaldehyde, CH COONH , CH COOH, reflux, 18 h.
3 4 3
(
b) Salicylaldehyde, CH COONH , CH COOH, reflux, 18 h
3
4
3
−
5
Fig. 1. Absorption changes of the probes (a) HTPI and (b) TPI [(4 × 10 M) in THF-H O (1:1
2
v/v, 50 mM (HEPES), pH = 7.4) in the presence of various metal ions (100 equiv.).
2
+
Fig. 2. Absorption changes of the HTPI with other metal ions (Cu and other stated metal ions)
in THF-H O (1:1 v/v, 50 mM (HEPES), pH = 7.4). The error bar suggests that the standard
2
deviation of three measurements.
−
5
Fig. 3 (a) UV−vis spectrum of HTPI (4 × 10 M) in THF-H O (1:1 v/v, 50 mM (HEPES), pH =
7
2
2
+
.4) with different concentrations (0−100 equiv.) of Cu . (b) Benesi−Hildebrand non-linear
curve fitting plot (absorbance at 236 nm) of HTPI assuming 1:1 binding stoichiometry with
2
+
Cu . The error bar suggests that the standard deviation of three measurements.
2
+
Fig. 4. Job’s plot for the complexation of the [HTPI + Cu ] system in THF-H O (1:1 v/v, 50
2
mM (HEPES), pH = 7.4). The error bar suggests that the standard deviation of three
measurements.
−
5
Fig. 5. UV−vis spectrum of HTPI (4 × 10 M) in THF-H O (1:1 v/v, 50 mM (HEPES), pH =
7
2
2+
.4) upon addition of Cu (100 equiv.) and EDTA (100 equiv.).
−
6
Fig. 6. Fluorescence spectra of (a) HTPI and (b) TPI [(4 × 10 M) in the presence of various
cations (100 equiv. λ = 330 nm & λ = 315 nm, respectively) in THF-H O (1:1 v/v, 50 mM
ex
ex
2
(HEPES), pH = 7.4).
2
+
Fig. 7. Fluorescence response of the HTPI + Cu complex on addition of other metal ions
+
3+
2+
2+
2+
2+
3+
3+
2+
3+
2+
+
3+
2+
2+
+
2+
(
Ag , Al , Ba , Ca , Cd , Co , Cr , Ce , Fe , Fe , Hg , K , La , Mg , Mn , Na , Ni ,
2
+
2+
Pb , and Zn ;100 equiv. λ = 330 nm) in THF-H O (1:1 v/v, 50 mM (HEPES), pH = 7.4). The
ex
2
error bar suggests that the standard deviation of three measurement.
−
6
Fig. 8. (a) Fluorescence titration spectra of HTPI (4 × 10 M) upon the incremental addition of
2
+
Cu ions (0−100 equiv., λ = 330 nm) in THF-H O (1:1 v/v, 50 mM (HEPES), pH = 7.4). (b)
ex
2
2
+
Benesi−Hildebrand plot of the 1:1 complex of HTPI and Cu ions. The error bar suggests that
the standard deviation of three measurements.
2
+
Fig. 9. Job’s plot of HTPI with Cu ions in THF-H O (1:1 v/v, 50 mM (HEPES), pH = 7.4).
2
The error bar suggests that the standard deviation of three measurements.
-
6
Fig. 10. Fluorescence spectral changes of HTPI (4× 10 M) in THF-H O (1:1 v/v, 50 mM
2
2
+
(
HEPES), pH = 7.4) on the addition of Cu ions and EDTA (100 equiv. of each, λ = 330 nm).
ex.
2
+
Fig. 11. SEM images of a) HTPI and b) HTPI + Cu (scale bar = 5 µm)
2+
Scheme. 2. Plausible binding mechanism of HTPI with Cu

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2
+
Fig. 12. Electrosorption performance of HTPI in the removal of Cu by CDI process
Fig. 13. Logic gate circuits of HTPI constructed by colorimetric and fluorimetric techniques

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2
+
Table 1. Electrochemical removal of Cu ion in carbon aerogel electrode with HTPI
-5
(
2 x 10 M)
2
+
Amount of Cu in permeate
(ppm)
Rejection (%)
Amount of
Cu present
in blank
2
+
Samples
Before
Before
addition of
solution
After addition
addition of
of HTPI
After addition
(ppm)
of HTPI
HTPI
HTPI
Sample-A
Sample-A
Sample-B
Sample-B
4
8
4
8
1.4
3.2
1.2
3
0.2
0.4
0.2
0.4
65
60
95
95
95
95
70
62.5
2
+
Table 2. Truth table of HTPI+Cu complex
TRUTH TABLE
INPUT-1
INPUT-2
OUTPUT
Fluorescence Intensity
at 430nm
Absorbance at 321nm
2
+
Cu
EDTA
(
INHIBIT)
(
IMPLICATION)
0
0
1
1
0
1
0
1
1
1
0
1
0
0
1
0
2
+
Table 3. Comparison of HTPI with recently reported chemosensors for Cu
Mode of
detection
Detected
ions
Detection
medium
−1
Detection Interfering
Compound
K (M )
Ref
a
limit
ions
4
-
-
5
.7×10 M
-9
1
9.52× 10
2
+
2+
colorimetric
and
fluorimetric
M
&
15.1×10
M
Cu
Fe
[42]
DMSO/H O
&
2
(
3:7)
-9
4
1
.5×10 M
1
2
+
−6
-
Cu
[51]
MeOH/H O
1.8 × 10
M
2
fluorimetric
-
(
7:3)

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−6
8
×10
M
2
+
colorimetric
and
fluorimetric
Cu
-
[52]
&
.7×10
M
Water
-
−7
5
2
+
9
-7
-
fluorimetric
fluorimetric
Cu
[53]
[54]
CH CN-
HEPES (1:9)
7.63×10
2.19×10
M
3
-
2
M
2
+
HEPES
BUFFER
SOLUTION
4
-
Cu
-
3
.09×10
M
0.29×10
-1
6
M
4
4
9
.84 x 10
M
-9
-
1
19 x10
M
2
+
colorimetric
and
fluorimetric
HTPI
Cu
(THIS
WORK)
THF-Water
2+
&
Hg
(
1:1)
&
1.98 x 10
-9
-
1
27×10 M
M

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Research Highlights



A Triphenyl-imidazole derived dual channel fluorescent chemosensor HTPI for the
2
+
detection of Cu ions.
2
+
The chemosensor shows high selectivity towards Cu in both colorimetric and
fluorimetric sensing.
A reversible sensor with a “Turn-on” process in colorimetric and “Turn-off” process in
fluorimetric.


A plausible mechanism has been demonstrated with a 1:1 binding stoichiometry
2+
Removal of Cu ions from the environmental water samples and molecular logic gate
were performed.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6