Fabrication of triazine based colorimetric and electrochemical sensor for the quantification of Co2+ ion

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Fabrication of Triazine Based Colorimetric and Electrochemical Sensor for the Quantification of Co2+ Ion J.Jone Celestina , P. Tharmaraj , A. Jeevika , C.D. Sheela PII: DOI: Reference:

S0026-265X(19)32980-7 https://doi.org/10.1016/j.microc.2020.104692 MICROC 104692

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Microchemical Journal

Received date: Revised date: Accepted date:

21 October 2019 8 January 2020 2 February 2020

Please cite this article as: J.Jone Celestina , P. Tharmaraj , A. Jeevika , C.D. Sheela , Fabrication of Triazine Based Colorimetric and Electrochemical Sensor for the Quantification of Co2+ Ion, Microchemical Journal (2020), doi: https://doi.org/10.1016/j.microc.2020.104692

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Highlights    

Design and synthesis of triazine based chromophore for selective sensing of Co2+ was investigated by both colorimetrical and electrochemical methods. Triazine moiety shows the limit of detection of 0.03 µM (electrochemical) and 0.05 µM (colorimetrical) towards Co2+ ion. Intra molecular charge transfer mechanism was proposed for BNHNTA- Co2+ complex with the help of NMR, HR-LCMS and job’s plot. Real sample analysis was done in different water samples and shows an excellent recovery of Co2+.

Fabrication of Triazine Based Colorimetric and Electrochemical Sensor for the Quantification of Co2+ Ion J. Jone Celestinaa, P. Tharmaraja*, A. Jeevikaa and C. D. Sheelab a*

PG and Research Department of Chemistry, Thiagarajar College, Madurai 625009, Tamilnadu, India

b

PG and Research Department of Chemistry, The American College, Madurai 625002, Tamilnadu, India

*Corresponding author: Mobile. No: +91- 9442555836 E-mail address: [email protected]

Abstract Triazine based Schiff base was synthesized by sonochemical method and it was explored for the robust selective and sensitive determination of Co2+ by both colorimetry and electrochemical sensing. Selectivity studies were carried out with different metal ions and found that the proposed chemosensor has significant selectivity for the detection of Co2+ with the colour change from yellow to blue owing to the formation of complex with Co 2+.The colorimetric sensitivity studies of BNHNTA shows maximum detection value of 0.05 µM towards the quantification of Co2+. In addition the amperometric studies exhibit selective electrochemical sensing of Co2+ by BNHNTA and the detection limit was calculated to be 0.03 µM. The HOMO-LUMO diagram clearly shows mechanism of charge transfer from ligand to metal (LMCT) between the triazine and Co2+ ion. In addition to that, Density functional theoretical (DFT) study provides the structural insights about cobalt (II) complex, which confirms the square pyramidal geometry of the formed complex. Theoretical studies confirm the formation of 1:1 receptor-metal ion complex in detecting Co2+ ion. High resolution mass spectrometry (HRMS) measurements confirm the above complex structure. Job’s plot measurements also justify the same. Keywords: Cobalt, Triazine, Schiff base, Colorimetric, Electrochemical, DFT.

1.

Introduction Nitrogen heterocycles play an essential role in pharmaceutical chemistry due to their

inherent potential against the harmful microorganisms. Especially 1, 2, 4- triazines and their metal complexes plays a vital role, as they exhibit an extensive assortment of pharmaceutical activities like anti-inflammatory, anti-viral, anti-bacterial, anti-cancer and anti-HIV activities owing to the presence of donor atoms of N [1-3]. They also act as a major pesticides, herbicides and agents for de-toxcification. Additionally, organic s- triazine compounds exhibit non-linear optical activity (NLO) due to its excellent accepting property of the chromophores. Owing to all the above, S-Triazine was studied for selective sensing using various metal salts [4]. Besides the above properties of triazine systems, the complexation of the triazine Schiff bases with d- block metals via intramolecular charge transfer (ICT) [5] or ligand to metal charge transfer (LMCT) transitions, the azomethine bond (-C=N-) of Schiff bases binds itself with various metals thereby they can be employed in the development of chemosensors with naked eye detection of various analytes [6-8]. This leads to the coordinative conjugated π-electron system which can act as both an acceptor and a donor through a strong host–guest interaction. Due to the above charge transfer nature it paves a way for the selective and sensitive determination of transition metal ions in the surroundings of the environment [9-11]. According to Environmental Protection Agency (EPA) United States, the permissible limit of cobalt in air is 0.0004 µg/m3 and drinking water is 2 µg/L [12]. Due to high exposure of Co2+, various disabilities arise such as asthma, gastrointestinal disturbance, liver damage, kidney damage and cardiac enlargement [13]. Hence, quantification of Co2+ in biological and environmental samples using simple techniques is highly needed. Many analytical techniques including atomic emission spectrometry, atomic absorption, fluorescence spectroscopy, Naked-eye detection and electrochemical methods have been practised for the determination

of Co2+ [14]. Among them, colorimetry and electrochemical methods are gaining more interest due to its easy fabrication, more accuracy, in-field detection and low-cost [15-20]. Therefore, developing a simple and novel method for the detection of Co2+ ion with high selectivity and sensitivity are desired. In this paper, a novel triazine based chemosensor 4-2-(4-nitrobenzylidene) hydrazinyl)-6-(2-(4-nitrobenzylidene) hydrazinyl)-N-(4-nitrophenyl)-1, 3, 5-triazin-2-amine (BNHNTA) was developed for the selective quantification of Co2+ ion. BNHNTA was prepared by sonochemical method which acts as receptor for detection of Co2+ ion. It exhibits high selective and sensitive activity towards the detection of Co2+ ion via colour change from yellow to blue. BNHNTA has a detection limit of 0.05 and 0.03 µM by using colorimetry and electrochemical methods, respectively. The pair of oxidation– reduction peaks equivalent to the oxidation–reduction couples Co (III)/Co (II) suggests a quasi-reversible electrode process taking place in bonding of BNHNTA with Co2+. In addition, the geometrical parameters and structural insights of the synthesized chemosensor are analysed, which supports the experimental data obtained in this work. 2. Experimental 2.1. Materials Cyanuric chloride, p-nitroaniline, hydrazine hydrate and 4-nitrobenzaldehyde were procured from Merck chemicals. The metal salts and ethylenediamine tetra acetic acid (EDTA) were procured from Merck chemicals. Tetra butyl ammonium perchlorate (TBAP) was purchased from Molychem, Distilled methanol was used as solvent throughout the experiments.

2.2. Synthesis of Schiff base (BNHNTA) A mixture of cyanuric chloride (1 mmol) and P-nitroaniline (0.5 mmol) dissolved in acetone (15 mL) and stirred for 3 hrs. The yellow precipitate obtained was dried and recrystallized. To the above, hydrazine hydrate (3 mL) was added and refluxed for 1 hr. The resulting yellow precipitate was cooled and dried. This hydrazine hydrate derivative (1 mmol) was added with P-nitrobenzaldehyde (2 mmol) and sonicated for 30 min at 60 °C. The yellow precipitate obtained was filtered and characterized. The schematic representation of BNHNTA synthesis was depicted in scheme 1. Yield of the formed Schiff base was calculated to be 81 %. Scheme 1. 2.3. Characterization UV- Visible spectrum was recorded using Jasco (V-530) spectrophotometer in the range of 200 - 800 nm. FT-IR spectrum was analysed using KBr pellet from the range of 4000 to 400 cm-1. 1H-NMR was recorded using DMSO d6 as solvent in 400 MHz Bruker nuclear magnetic resonance (NMR) instrument. DFT studies were performed in Gaussian 08 at B3LYP level with double basis set LANL2DZ for cobalt and 6-311+G (d, p) basis set for other atoms present in the ligand. The liquid chromatography mass spectrometry (LCMS) and high resolution mass spectrometry (HR-LCMS) were recorded in Bruker mass spectrometer using acetonitirile as the solvent. 2.4. Colorimetric studies of Co2+ Methanolic solution of ligand (BNHNTA) (1 µM) and all the metal salts (0.1 µM) were prepared. The BNHNTA (1 µM) was taken in various vials and variety of metal ion was added, shaked well and kept for observations for 3 min. The colour change was noticed. Then

it was transferred to the cuvette and the optical property of BNHNTA was studied at room temperature using UV-Vis absorption spectroscopy. 2.5. Electrochemical studies of Co2+ The electroanalytical studies of BNHNTA were carried out using CHI- Instruments electrochemical workstation (Model-660E, USA) with two compartment three electrodes system. Glassy carbon electrode (GCE) used as working electrode, Platinum wire was used as counter electrode and Ag/AgCl/saturated KCl were used as reference electrode for all the electrochemical experiments. Tetra butyl ammonium Perchlorate (TBAP) (0.1 M) solution was used as supporting electrolyte for all the electrochemical experiments. For this electrochemical study, BNHNTA was deposited over the surface of carbon electrode and desiccated at room temperature. It acts as working electrode for the quantification of Co2+. 2.6. DFT study DFT calculations were done using Gaussian 08 package. The geometric parameters of the ligand and the cobalt(II) complex were fully optimized without symmetrical constraints using Becke’s three-parameter hybrid functional with Lee-Yang-Parr correlation functional (B3LYP) and 6-311+G(d,p) was used for the ligand atoms and LanL2DZ for cobalt atom. 2.7. Real sample analysis Real samples were collected from our College campus and from Vaigai River and removed of impurities. After filtration through Whattman No.1 filter paper, the centrifuged water samples were spiked into the BNHNTA+ Co2+. After carrying out the experiments the recovery percentage was calculated.

3. Results and Discussion 3.1 Characterization of BNHNTA The ligand BNHNTA was characterized by various spectral methods. The infrared spectrum Fig. S1 (a) exhibit imine peak at 1601 cm-1. The broad band observed at 3133 cm-1 is characteristic of ν (-NH) stretching vibration and the sharp intense band observed at 1466 cm-1 is attributed to ν (C=N) stretching vibration of triazine ring [21]. The peaks appeared at 1399 cm-1 and 1342 cm-1 are assigned as the stretching vibration of ν (N-O) [22,23]. 1

H-NMR spectrum for BNHNTA as in Fig.S1 (b), exhibit characteristic imine peak at

8.2 (S, 2H), all the aromatic protons are appeared as a multiplet from 7.86-7.87 (m, 4H), from 8.0-8.13 (m, 4 H) and 8.23-8.25 (d, J=4.8 Hz, 4H) a doublet corresponds to the presence of NH present in the synthesized compound [24]. The HR-MS mass spectrum of BNHNTA (positive ion mode) (Fig.S1 (c)) shows the single mass peak at m/z 544.48 (M + 1H) which clearly indicate the neat formation of ligand [25]. 3.2 Colorimetric detection of Co2+ 3.2.1 Effect of pH The pH effect on the detection of Co2+ using BNHNTA was determined by recording the absorbance spectra by varying the pH from 2-14. It was observed that the absorbance intensity of BNHNTA was very low at pH=8 (Fig.S2) compared to its cobalt complex. It was noticed that the increase of pH lead to enhancement of absorption intensity and become constant between the pH 6-8 and a gradual decrease was observed between pH 10-12 leading to the formation of cobalt hydroxide [26]. It clearly confirms the complex formation and the complex remains stable from pH 6 – 8, and pH 8 is selected for further sensor studies.

3.2.2 Sensitivity and Interference studies In order to determine the absorbance capacity of BNHNTA, UV-Visible absorption spectral studies were performed. The binding efficiency of BNHNTA was determined by using different metal ions (transition and alkali earth metal cations and anions) and its corresponding absorption spectra are shown in Fig.2 (a, b) respectively. The free BNHNTA has an absorbance at 297 nm and 301 nm due to π–π* transition and an absorption band appeared at 346 nm corresponding to n-π* transition. The results obtained were in good accordance with the previously reported works [27, 28]. BNHNTA (1 µM) and respective chloride salts (0.1 µM) in methanol were taken and absorbance was recorded by mixing the metal salts with BNHNTA. No colour change was observed with metal ions such as Zn2+, Cd2+, Cu2+, Cr3+, Pb2+, Mg2+, Fe2+, Fe3+, Al3+, Hg2+, Na+, K+, As2+, Ni2+, Ca2+, Bi2+, So42-, Cr2O72-, Co32- and No3- ion. At the same time, a distinct colour change from yellow to blue was observed for Co2+ on interaction with BNHNTA as shown in Fig.1 which signposts that BNHNTA could acts as a robust and potent ‘naked - eye’ chemosensor. Two absorbance peaks, observed at 516 and 654 nm in Fig.2 (b) which fascinates the spectral change due to dd transition and of the complexation of Co2+ with BNHNTA [29]. The possible mechanism of Co2+ detection was given in scheme 2. The interference in sensing ability of BNHNTA from other employed metal ions towards Co2+ was studied. Absorbance spectrum was recorded for BNHNTA (1µM) and Co2+ (0.1µM) along with 1.5 µM of other metal ions. Fig.1 Fig.2 The Bar diagram showing the peaks due to interaction of BNHNTA with Co2+ (red bars) and other metal ions as given in Fig.3 (c). It is noteworthy that in sensing of Co2+ by BNHNTA has no interference of co-existing metal ions. The competitive heights of the red and black bars indicate that BNHNTA can selectively sense Co2+even in presence of 1.5 µM

of other metal ions [30]. The binding nature of Co2+ with BNHNTA was further studied by analysing the absorbance at different concentration. The absorbance at 516 nm gradually increases while the peak at 654 nm decreases as the concentration of Co2+ increases from 0.1 – 1.2 µM. An unblemished isobestic point was detected at 560 nm, indicating that only one product was generated from BNHNTA upon binding with Co2+ and confirms the formation of complex between BNHNTA and Co2+ [31, 32]. Since, the absorbance range was in the visible region and shows a bathochromic shift confirming both Intramolecular Charge Transfers (ICT) and ligand to Metal Charge Transfer (LMCT) taking place [33]. The detection limit for the sensor system was calculated by the formula following LOD = 3.3S/b

[34]

Where S stands for standard deviation corresponding to lowest concentration of [BNHNTA + Co2+] and b stands for the slope of the calibration curve obtained from absorbance measurements. The possible lowest detection was around 0.03 µM which is comparatively lower than reported works. The table 1 indicate the comparative LOD of various sensing ligands studied. The binding constants of BNHNTA with Co2+ were predicted as 2.08 x 105 µM (R2 =0.983) as in Fig. 3(a). On the basis of Benesi–Hildebrand equation using the formula [1/A-A°] with the excitation wavelength (λ max) was calculated [35]. The formation of cobalt complex was further evidenced by various characterization studies are displayed in Fig.S3 (a) & (b). IR spectrum confirms the formation of cobalt complex as evidenced by the shifting of (N=CH) from 1601 cm-1 -1595 cm-1 , –NH from 3133 cm-1 to 3138 cm-1 and  (C=N) from 1466 to cm-1 to 1460 cm-1 [37,38].  (Co-N) vibration was observed at 581 cm-1 in the complex spectra confirming the binding of metal with ligand. Fig.S3 (b) depicts the mass spectrum of BNHNTA along with cobalt which was observed at m/z 673.25 corresponding to the formation of metal complex with pentacoordination structure [34].

Fig. 3

3.3 Electrochemical studies Detection of Co2+ using fluorimetry is comparably difficult due to the paramagnetic nature of Co2+ which leads the fluorescence quenching [40]. So far, only a few reports were available for the detection of Co2+ based on fluorimetry. Compared to fluorimetry, electrochemical sensors have attracted more due to its accuracy, cost effectiveness which offers both qualitative and quantitative information. The cyclic voltammetry behaviour of BNHNTA on interaction with Co2+ was studied under room temperature using 0.1 M TBAP as supporting electrolyte [41]. Cyclic voltammogram (CV) of BNHNTA were performed under the electro active range from -0.2 V to 1.0 V using methanol as the solvent at the scan rate of 50 mVs−1 [42]. Fig.4 (a) shows two well-defined oxidation peaks at 0.45 V, 0.72 V and reduction peak at -0.08 V on binding of Co2+ with ligand. The oxidation peak at 0.72 V shifting from the original position strongly confirms the formation of complex between BNHNTA and Co2+ [43]. The observance of the Co2+ complex was well established by a reversible redox peak, assigning to the consecutive conversion of Co(II)/Co(III) peak at 0.72V and Co(III)/Co(II) reduction peak at -0.08V [44]. The pair of redox peaks of Co (III)/Co (II) suggests that, quantification of Co2+ using BNHNTA was a quasi-reversible electrode process [45]. 3.3.1 Effect of scan rate The electrochemical behaviour on different scan rate of BNHNTA with Co2+ was examined in glassy carbon electrode. It shows the CV response of BNHNTA in (0.1 M) TBAP with 0.1 µM of Co2+ at different scan rates from 50- 350 mVs−1 as in Fig.4 (b). The redox peak current of [BNHNTA + Co2+] complex increased with increasing in the scan rate

from 50 to 350 mVs−1 [46]. Additionally, an increment of scan rate leads to shifting of peak potential towards the positive and negative direction. On the other hand, the redox peak potential currents of the [BNHNTA + Co2+] complex shows the linear increment to the square root of scan rate in the range from 50 to 350 mVs−1. As evidenced from the correlation coefficient measurements R2=0.99 in Fig. 4 (c) also supports the strong bonding of ligand with cobalt [47,48]. As a result of the above the sensitivity in selective sensing of Co2+ by BNHNTA and complex formation has been confirmed [49]. Fig. 4 3.3.2 Amperometric studies The specificity of BNHNTA towards the Co2+ was appraised by means of amperometric technique as depicted in Fig. 5 (a). It is clearly showing the amperometric i−t curve obtained for selectivity of Co2+ with applied reduction potential of -0.08 V at 0.1 M TBAP (pH 8.0). On, successive addition of 0.1 µM of Co2+ at 50 s time intervals the current potential was decreased gradually due to the interaction of Co2+ with BNHNTA which leads to the complex formation. The linearity in relationship between the response current with respect to concentration of [BNHNTA + Co2+] has an effective correlation coefficient of 0.9937 as portrayed in Fig. 5 (b). Fig. 5 3.4 DFT studies Theoretical Density Functional Theory computations were investigated for [Co (BNHNTA) Cl2] complex in order to acquire the mechanism of Cobalt binding to BNHNTA [50]. It explains the one electron excitation from ground state to the first excited state through (HOMO) to (LUMO) [51]. The difference between the (ELUMO–EHOMO) is accounted on excitation energy. The optimized structure, HOMO and LUMO plots of [Co (BNHNTA)Cl2] complex drawn by DFT–B3LYP method are given in Fig. 6 (a). The HOMO levels are

localized mainly on the site of metal while the LUMO levels on the π-system of BNHNTA. The orbital densities are significant for Co2+ and coordinated Cl-atoms in case of [Co (BNHNTA) Cl2]. Hence, it is clear that electronic transition may be due to mixed π-π* and d-d transitions [52]. The calculated energy gap value ∆E=1.9576 eV (EHOMO - ELUMO) determines the ease of electronic transition (ICT) from HOMO to LUMO as depicted in Fig.6 (b). According to the experimental results, the complex of [Co(BNHNTA)Cl2] displayed bathochromic shift in the absorption which clearly shows the Ligand to Metal Charge Transfer (LMCT) taking place and confirms the square pyramidal structure of the proposed metal complex. Thus, the theoretical calculation results suits up well with that of the experimental results obtained for the above mentioned complex. The calculated geometrical parameters are in agreement with the previously reported literature data [53, 54]. HOMO-LUMO diagram in the Cobalt (II) complex formation with BNHNTA clearly explains the electronic properties, bond energies and ligand to metal charge transfer properties (LMCT). Fig.6 3.5 Reversibility with EDTA The absorption intensity of BNHNTA cobalt complex was found to be enhanced by the addition of Co2+. The influence of EDTA on cobalt complex of BNHNTA was studied by the addition of 10 mm of EDTA solution [55]. Upon addition of EDTA solution, the peak at 548 nm was found to disappear due to the liberation of Co2+ and the appeared peak is only due to BNHNTA as shown in Fig.S4 [56]. The peak was found to be appeared by the addition of small amount of Co2+ solution. This result helps us to assume that the binding of Co2+ with BNHNTA was chemically reversible. This behaviour is useful to fix the concentration of Co2+ solution in the study of real samples [57, 58].

3.6 Real sample analysis The synthesized BNHNTA chemosensor was evaluated for its practical application towards the determination of Co2+. BNHNTA was tested with different water samples like (tap water and river water). The samples were gathered from various sources in Madurai city namely, vaigai River and our campus drinking water, ground water and RO water. In order to remove the impurities from the collected samples, centrifugation was done for about 15 min and it was filtered using whattman No.1 filter paper. Then 2 µM of Co2+ was spiked in different water samples and recovery values were calculated which are displayed in Table. 2. From the obtained results, it was confirmed that the synthesized BNHNTA chemosensor shows as excellent quantification of Co2+ in practical applications. 4. Conclusion The sensing behaviour of the synthesized chemosensor was investigated by means of visual and electrochemical methods. It exhibits high selectivity towards the detection of Co2+ in methanol solution among various metals employed. The visible colour change from yellow to blue and a sharp reduction peak in cyclic voltammogram and the amperometric study acts as a splendid proof for the enhanced sensing of Co2+ by the synthesized probe. Furthermore, the mechanism of sensing in detection of Co2+ was explained by theoretical calculations via job’s plot and benesihilderm plot which gives the stoichiometry of the metal complex as 1:1 and the binding constant value is 2.38 x105 provides the strong binding nature of the BNHNTA with Co2+. DFT studies of the formed metal complex shows the LMCT taking place in the complex. The detectable limit was intended and found to be of 0.03 µM (Electrochemical) and 0.05 µM (Colorimetry). From these results, it confirmed that synthesized chemosensor selectively sensed the Co2+ with high reproducibility, repeatability and long-time stability which could be used both in natural and industrial samples.

Author Statement

P. Tharmaraj: Conceptualization, Methodology, Writing- Original draft preparation, Reviewing and Editing the manuscript J. Jone Celestina: Methodology, Experimental, data collection, data analysis, draft preparation. A. Jeevika: Formal analysis, Methodology, Visualization, Investigation. C. D. Sheela : Resources, electrochemical instrumentation, data analysis, Data Curation Declaration of interests

x

The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.

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Figure captions

Scheme 1. Synthesis of BNHNTA Fig. 1 Photographic image of BNHNTA in the presence of different metal ions with the concentration of 1.0 μM . Fig. 2(a) Absorbance responses of BNHNTA (1.0 μM) upon addition 0.1μM of metal cations and anions in methanol. Insert image: Absorbance of BNHNTA+ Co2+. (b) UV–vis spectral changes of BNHNTA (1.0 μM) upon addition of different concentrations of Co2+ (0.1-1.2 μM) in methanol solution. Insert image: Absorbance of BNHNTA+ Co2+ (0.1 μM) Scheme 2. Plausible sensing mechanism for BNHNTA in sensing Co2+ Fig. 3 (a) Benesi–Hildebrand plot of BNHNTA with Co2+ with absorbance changes. (b) Job’s plot of BNHNTA with Co2+ showing 1:1 stoichiometric ratio and (c) Interferences study by various metal ions on Co2+ Fig. 4 (a) Cyclic voltammogram of BNHNTA in methanol and [Co(BNHNTA)Cl2] with 0.1 M TBAB as supporting electrolyte. (b) Different scan rate of Co2+ (0.2-0.1.4 μM) in 0.1 M TBAB (pH 8.0) at scan rate 50 mVs−1 and (c) The calibration plot for the linear dependence of Co2+ vs Epc and Epa Fig. 5 a) Amperometric response curve for sensing of Co2+ by BNHNTA in 0.1 M TBAB at pH 8 and (b) linear curve for the determination of Co2+ Fig.6 DFT computed (a) optimized structure of [Co (BNHNTA)Cl2] complex (b) HOMO and LUMO diagrams

Figures

Scheme 1. Synthesis of BNHNTA

Fig. 1. Photographic image of BNHNTA in the presence of different metal ions with the

concentration of 1.0 μM .

Fig.2 (a) Absorbance responses of BNHNTA (1.0 μM) upon addition 0.1μM of metal cations

and anions in methanol. Insert image: Absorbance of BNHNTA+ Co2+. (b) UV–vis spectral changes of BNHNTA (1.0 μM) upon addition of different concentrations of Co2+ (0.1-1.2 μM) in methanol solution. Insert image: Absorbance of BNHNTA+ Co2+ (0.1 μM)

Scheme 2. Plausible sensing mechanism for BNHNTA in sensing Co2+

Benesi–Hildebrand plot of BNHNTA with Co2+ with absorbance changes. (b) Job’s plot of BNHNTA with Co2+ showing 1:1 stoichiometric ratio and (c) Interferences study by various metal ions on Co2+ Fig. 3 (a)

Fig. 4 (a) Cyclic voltammogram of BNHNTA in methanol and [Co(BNHNTA)Cl 2] with 0.1

MTBAB as supporting electrolyte. (b) Different scan rate of Co2+ (from 0.2-0.1.4 μM) in 0.1M TBAB (pH 8.0) at scan rate 50 mVs−1 and (c) The calibration plot for the linear dependence of Co2+ vs Epc and Epa

Fig. 5 a) Amperometric response curve for sensing of Co2+ by BNHNTA in 0.1 M TBAB at

pH 8 and (b) linear curve for the determination of Co2+

Fig.6 DFT computed (a) optimized structure of [Co (BNHNTA)Cl2] complex (b) HOMO and LUMO diagrams

Table 1. Comparative studies of various sensors for the detection of Co2+ ion

Probe

Method

Solvent

Colorimetric

Aqueous medium

Colorimetric

DMSO

Colorimetric

Linear Range 0.1-1.0 µM

LOD

Ref

1.0 µM

[1]

0-18 µM

1.5 µM

[9]

Aqueous medium

0-1 µM

1.28 µM

[20]

Colorimetric

CH3CN/Water

0-10 µM

0.31 µM

[22]

Colorimetric

AcetonitrileHEPES buffer

0-1.3 µM

0.85 µM

[4]

Colorimetric

Ethanol: water (1:1)

0-10 µM

0.45 µM

[24]

Colorimetric

Ethanol/water (1:9)

10-300 µM

7.09 µM

[29]

Colorimetric

Water

0-1 µM

1.8 µM

[30]

0-1.2 µM

0.05 µM 0.03 µM

Present work

Colorimetric Ethanol Electrochemic al sensor

Table 2. Real sample analysis of BNHNTA+Co2+ with different water samples

Samples Vaigai river water

Added (µM) 2.0

Found (µM) 2.035 ± 0.001

Recovery (%) 101.75± 0.05

Ground Water

2.0

2.015 ± 0.002

100.75 ± 0.1

Drinking tap water in our Campus RO Water

2.0

1.991± 0.001

99.55 ± 0.05

2.0

1.980 ± 0.001

99 ± 0.05