Fabrication of Sb3+ sensor based on 1,1′-(-(naphthalene-2,3-diylbis(azanylylidene))bis(methanylylidene))bis(naphthalen-2-ol)/nafion/glassy carbon electrode assembly by electrochemical approach

A new Schiff base named 1,1′-(-(naphthalene-2,3-diylbis(azanylylidene))bis (methanylylidene))bis(naphthalen-2-ol) (NDNA) derived from 2,3-naphthalenediamine and 2-hydroxy-1-naphthaldehyde was synthesized by condensation reaction and then characterized by spectroscopic techniques for structure elucidation. In addition to spectroscopic techniques, the molecular structure of NDNA was clearly confirmed by single-crystal X-ray diffraction study. A thin film of NDNA was fabricated onto glassy carbon electrode (GCE) using 5.0% ethanolic nafion solution as a conducting binder in order to develop the cationic electrochemical sensor (NDNA/nafion/GCE) for the sensing of heavy-metal cations in aqueous systems by electrochemical technique. This newly designed sensor exhibited higher sensitivity and selectivity towards antimony (Sb3+) in the presence of other interfering heavy metal cations, as well as long-term stability. Fascinating analytical parameters such as limit of detection (LOD = 0.075 nM, SNR of 3), limit of quantification (LOQ = 0.25 nM) and sensitivity (12.658 × 10−4 μA μM−1 cm−2) were calculated from the calibration curve plot, which shows a linear dynamic range (LDR) of Sb3+ ion concentration from 0.1–10.0 mM. This work presents a new approach towards the development of sensitive, efficient as well as selective toxic cationic electrochemical sensors in the environmental and healthcare fields. Hence, this newly designed NDNA/nafion/GCE presents cost-effective and efficient outcomes and can be used as a practical substitute for the efficient detection and removal of Sb3+ ions from water samples.


Introduction
Heavy-metal ions badly affect the aquatic environment, consequently damaging human health. The pollution by heavy metals occurs via many routes, such as industrial effluents, reneries, waste-treatment plants, groundwater and rainwater. 1 Therefore, studies are growing toward making suitable devices for sensing and detecting such traces in the environment. [2][3][4] In this interest, the Schiff base, which is the condensation product of aldehyde (or ketone) and amine-containing compounds, 5,6 seems to be a viable ligand for sensing metal ions. 4,7,8 Schiff bases have shown several biological activities such as antibacterial, antifungal, antimalarial, anti-inammatory, and antipyretic. 9 These properties are mainly due to the imine group present in Schiff bases, which could also be altered by varying the substituents in the molecules. [10][11][12] Furthermore, several studies have explored the properties of Schiff bases as catalysts, [13][14][15][16] uorescent materials, [17][18][19] electroluminescent materials, [20][21][22] etc. Since Schiff bases are known by their strong coordinative ability as a family of ligands, with the formation in most cases of 1 : 1 transition metal complexes, 23 they have been used to develop various chemosensors. [24][25][26] Chemosensors can be classied into three categories according to the nature of the signal emitted by the signaling subunit: (i) colorimetric sensors related to change in electronic properties in the form of intra/intermolecular charge transfer (ICT), [27][28][29] (ii) uorogenic sensors related to photoinduced electron transfer (PET), 27,30 excited-state intramolecular proton transfer (ESIPT), 31 uorescence resonance energy transfer (FRET), 32,33 bond energy transfer (TBET), 34 excimer-exciplex formation, 35 etc., and (iii) electrochemical sensors related to measurement of changes in redox potential or electrical responses. Because of the great worth and commercial applications of the Schiff bases, the intention of this study is to design an ion-selective cationic electrochemical sensor based on a newly synthesized, non-reported Schiff base as tetradentate ligand in order to probe heavy metal ions in an aqueous system by electrochemical approach. For this reason, 2,3-naphthalenediamine and 2-hydroxy-1-naphthaldehyde were selected as precursors to synthesize the novel Schiff base, named as NDNA. It was observed that the newly designed cationic electrochemical sensor based on the above Schiff base is selective for Sb 3+ in the presence of other interfering heavy metal ions in aqueous system.
The toxicity of antimony on health includes keratitis, dermatitis, gastritis, and conjunctivitis, and long-term exposure can cause the development of cardiac problems and lung cancer. 36,37 Aquatic environments are polluted by Sb through rock weathering, soil runoff and anthropologic activities. 38,39 The toxicity of antimony strongly depends on its oxidation state, as (Sb 3+ ) is about ten times more toxic than (Sb 4+ ). 40,41 The maximum permissible concentrations in drinking water as recommended by the U.S. Environmental Protection Agency (EPA) and World Health Organization (WHO) are 6 and

Materials and methods
All solvents and reagents were purchased from Sigma-Aldrich Company and used as received. 1 H and 13 C NMR spectra were recorded in DMSO-d6 solutions on a Bruker Avance 850 MHz spectrometer. Infrared spectra were performed on a Perki-nElmer spectrum 100 FTIR spectrometer. FT-IR spectra were recorded as neat on a Thermo Scientic NICOLET iS50 FT-IR spectrometer (Thermo Scientic, Madison, WI, USA). Melting points were determined in open capillary tubes in a Stuart Scientic melting point apparatus (SMP3) and are uncorrected.

X-ray crystallography of compound I (NDNA)
To observe the geometry of molecules and their interactions in the unit cell, we have crystalized the compound I (NDNA). Goodlooking crystals were taken to the microscope for the nal selection of sample for mounting on the diffractometer. The selected crystal was xed over the tip of a thin glass ber adsorbed in wax on a copper rod with a magnetic base. This holder was mounted on an Agilent SuperNova (dual source) Agilent Technologies Diffractometer, equipped with graphitemonochromatic Cu/Mo Ka radiation source for data collection. The data collection was accomplished using the CrysA-lisPro soware 63 at 296 K under Cu Ka radiation. The structure solution was performed and rened by full-matrix least-squares methods on F 2 using SHELXL-97, 64 in-built with WinGX. 65 All non-hydrogen atoms were rened anisotropically by full-matrix least-squares methods. 64 Figures were drawn using PLATON 66 and ORTEP-3. 67 All the aromatic hydrogen atoms were positioned geometrically and treated as riding atoms with C-H ¼ 0.93Å and U iso (H) ¼ 1.2 U eq (C) carbon atoms. The N-H ¼ 0.99(6)Å and O-H ¼ 0.93(6)Å; the hydrogen atoms were located with the difference Fourier map and rened with U iso (H) ¼ 1.2 U eq (N) and U iso (H) ¼ 1.5 U eq (O), respectively. The CIF of the NDNA has been submitted to the Cambridge Crystallographic Data Centre (CCDC).

Fabrication of electrochemical sensor
A very easy and effortless method was applied to fabricate the GCE with newly synthesized non-reported Schiff base as chelating agent. A very small amount (approximately 1.5 mg) of NDNA was mixed with 0.1 mL of ethanol in order to make the slurry, which was then applied onto the at surface of the GCE, with one drop of 5% ethanolic naon as an adhesive conducting binder, Scheme 1. Herein, naon was used as an adhesive conducting polymer in order to adhere NDNA to the at surface of GCE and allow the conduction of electrons between NDNA and GCE against the potential (V) applied in the system during the detection of toxic cations. Aer coating, it was dried at room temperature for 45 minutes to obtain the completely dried, evenly coated and stable NDNA/naon/GCE as an efficient and selective cationic electrochemical sensor for Sb 3+ . A Pt wire was also used as counter electrode in addition to the newly designed NDNA/naon/GCE as working electrode to measure the I-V response. In this way, a lab-made electrochemical cell was formed which accommodates the two electrodes (working and counter electrodes) to dip into phosphate buffer solution (PBS) of pH ¼ 7.0, in a 15 mL beaker. The volume of PBS was kept constant at 10 mL in this lab-made electrochemical cell throughout the study. The PBS was prepared by mixing 39 mL of 0.2 M Na 2 HPO 4 and 61 mL of 0.2 M NaH 2 PO 4 in a 200 mL measuring cylinder, diluting to the mark by deionized (DI) water. The stock solution of Sb 3+ was used to make the different concentrations of Sb 3+ (full concentration range: 0.1 nM to 0.1 M) in DI water for use as our target analyte. Linear dynamic range (LDR), regression coefficient r 2 , sensitivity, limit of detection (LOD) at S/3N, and limit of quantication (LOQ) for Sb 3+ was calculated from the slope of the calibration curve (from current versus concentration plot). Keithley electrometer was used as a constant voltage source for I-V measurement in a simple two-electrode system.

Synthesis
The idea behind the present work is to have a bifunctional fused aromatic Schiff base that would be suitable for making a stable lm on an electrochemical device. For this purpose, NDNA Schiff base was easily obtained (Scheme 2) in excellent yield by the condensation reaction of 2-hydroxynaphthaldehyde and 2,3naphthalenediamine. ATR-FTIR, 1 H NMR, 13 C and single-crystal X-ray spectroscopy measurements were made to conrm the chemical structure of NDNA. The FTIR spectra show several characteristic peaks, for the azomethine group (CH]N) at 1618 and aromatic rings (C]C) at 1607, 1593, 1568, and 1542 cm À1 . Additionally, C-H aromatic stretching vibration is clearly observed at 3054 cm À1 . Other bands appear at 1490, 1350, and 1307 cm À1 , attributed to C-H bending vibration, C-N stretching vibration and C-O stretching vibration, respectively. An orthodi-substituted aromatic band appears at 739 cm À1 . The phenolic O-H stretching vibration appears weak, with a broad peak centered at 3675 cm À1 . This behavior might be due to intramolecular hydrogen bonding with the azomethine nitrogen atom. The proton NMR data show the imine and OH protons as singlet, far downeld at 9.83 and 15.08 ppm, respectively. This downeld effect is attributed to the formation of intramolecular hydrogen bonds, which results in further deshielding of the OH group protons. Also, the creation of a sixmembered ring as a result of intramolecular hydrogen bonds (see Scheme 3) may further impact the OH proton by the magnetic anisotropic effect. Previous studies indicated that intramolecular hydrogen bonds have a very signicant effect on phenolic protons, with chemical shis in the range of 4.5 to 19 ppm. 22 Good correlation between doublet and triplet protons with their corresponding coupling constants are clearly indicative of the neighboring aromatic protons as presented in the experimental section. Additionally, 13 C NMR recorded all carbons present in the NDNA, with a special attention to the characteristic peak of aromatic azomethine carbon at 169.36 ppm.

Single-crystal X-ray description of synthesized compound
The CCDC number (1812136) obtained for the NDNA is mentioned in Table 1. There are three naphthalene rings (C1-C10), (C12-C21) and (C23-C32) in the molecule (Fig. 1). The  Fig. 1. 68 The ring motif generated through the atoms (O1/C13/C12/C11/N1/H1n) have the r.m.s. deviation of 0.0301Å, and its dihedral angle with its fused naphthalene ring is 3.298 (9) . The second ring motif generated through the atoms (N2/C22/C23/C24/O2/H1o) has almost the same r.m.s. deviation, i.e. 0.0325Å, and is twisted at a dihedral angle of 7.727 (1) with respect to the naphthalene ring (C23-C32). The intermolecular hydrogen bonding interactions observed in the crystal structure of compound I are weak, but they connect the molecules to generate the intricate network. Atom C(6) in the molecule occupied at (x, y, z) takes part in hydrogen bonding and acts as donor atom via H(6) to atom O(1) at (1 À X, 1 À Y, 1/2 + Z). This interaction connects the molecules along the c-axis to form innite long chains ( Fig. 2 and S1, † Table 2). On the other hand, the atom C(22) at (x, y, z) acts as donor via H(6) to atom O(1) at (X, 1 + Y, Z) and connects the molecules along the b-axis ( Fig. 2 and S1, † Table 2). Both interactions produce the network along the bc plane.

Application: detection of Sb 3+ ions with NDNA/naon/ GCE probe
The impressive application of NDNA is the electrochemical sensing of heavy metal cations in aqueous solution by using I-V approach. The fabrication of GCE with NDNA as chelating agent has been discussed in the experimental section. The creation of the NDNA/naon/GCE as an efficient and selective cationic electrochemical sensor in a two-electrode system (working and  counter electrodes) is a new report as per our knowledge, and no other reports are found in the literature. Moreover, the sensor was found to be very selective and sensitive for Sb 3+ in the presence of other interfering heavy metal cations. The notable change in current response of the newly designed NDNA/naon/ GCE against applied potential was observed in the presence of our target analyte in aqueous solution upon being adsorbed on the surface of the working electrode. Aer the fruitful result of a change in the current response of the modied GCE, analytical parameters such as LDR, LOD, LOQ, etc. were also studied in order to optimize the newly designed NDNA/naon/GCE as an efficient, sensitive and as well as selective cationic electrochemical sensor for Sb 3+ . Initially, the change in current response with this newly designed NDNA/naon/GCE against the applied potential was observed and compared with the bare GCE in the absence of target analyte. It was noticed that the fabricated GCE has a high current response as compared to the bare GCE, which reects the high electron communication feature between the active site of NDNA and GCE (Fig. 3a). Subsequently, a selectivity study was carried out in the presence of various heavy metal cations such as Sb 3+ , As 3+ , Ce 3+ , Cr 3+ , Cu 2+ , Hg 2+ , Ni 2+ , Sn 2+ and Y 3+ , and it was found to be very selective only for Sb 3+ (Fig. 3b). For selectivity study, 25 mL of 0.1 mM concentration of toxic cations was used. Moreover, the current response in the presence and absence of Sb 3+ was also observed in order to conrm the fabricated GCE's affinity with Sb 3+ , and it was found that it shows very good response in the presence of Sb 3+ (Fig. 3c). It also reects the very good adsorption and absorption capability of the Sb 3+ analytes onto the coated surface of the GCE. Similarly, the I-V response of naon/GCE in the presence and absence of Sb 3+ analytes was also observed in order to conrm that naon, as conducting binder, also has no signicant effect without NDNA for the sensing of Sb 3+ in the system. It was noticed that the response of naon/GCE without NDNA was almost same and has no signicant effect in the presence and absence of Sb 3+ (Fig. 3d). PBS solutions of different pH values were also investigated in order to optimize the operational condition of the newly designed GCE (Fig. S1 presented in ESI †). It was found that this newly designed GCE gives good response at pH 7.0 and 1.0 second as delay time in the presence of Sb 3+ .
Similarly, statistical approach was also applied to study the interference effect of other heavy metal cations (As 3+ , Ce 3+ , Cr 3+ , Cu 2+ , Hg 2+ , Ni 2+ , Sn 2+ and Y 3+ ) on NDNA/naon/GCE in the presence of Sb 3+ analytes, in the form of I-V response at +1.0 V at normal condition, and results are presented in Fig. 4 and Table 3. The concentration and volume of all cations were kept constant, and the amounts were taken as 25 mL of 0.1 mM cation in PBS (7.0 pH). From the interference study, it was also concluded that NDNA/naon/GCE is very selective toward Sb 3+ detection. It does not exhibit any major change in current response towards the other interfering heavy metal cations compared to Sb 3+ . For the Sb 3+ , it has given a 3-fold current response compared to Ni 2+ , which gave the second highest current response.
The proposed mechanism of Sb 3+ detection by using NDNA/ naon/GCE is shown in Scheme 4, with I-V graphical responses. Table 4 Comparison of proposed electrochemical method with different previously reported analytical methods for the detection of Sb 3+a  I-V response of the newly designed NDNA/naon/GCE in the presence of Sb 3+ ions is functional in PBS at normal condition, and good response is observed. The current response of NDNA/ naon/GCE in the absence of Sb 3+ is also presented in Scheme 4a in order to compare the current response in the presence of Sb 3+ (Scheme 4b). When we add a small amount of Sb 3+ into the system, there is a small increase in the current response of NDNA/naon/GCE because a smaller surface of coated GCE is covered by the Sb 3+ analytes (p-p* interaction). In this way, the surface reaction proceeds continuously but slowly and surely. Similarly, current response increases progressively with gradual increase of the concentration of Sb 3+ analytes in the system (pp* interaction). Subsequently, we can get a better idea about the increases in current response from the concentration variation plot (Fig. 5a). The surface reaction does not stop at this stage but increases progressively as a function of the concentration of our target analytes. Thus, we observe a greater current response of the NDNA/naon/GCE-lm as more surface is covered by our target analytes, along with increasing p-p* interaction of the functional groups of NDNA and Sb 3+ (Scheme 4b). The p-p* interaction could be approached as inter-molecular and intramolecular interactions of the functional compound. Similarly, the same phenomena have also been reported for the detection of toxic chemicals in the literature. 2,69 Finally, Sb 3+ analytes on the NDNA/naon/GCE surface reach the ideal saturated stage, which results in a regular increment of current response (Scheme 3c). In order to optimize the NDNA/naon/GCE as an efficient cationic electrochemical sensor for Sb 3+ , current responses at different concentrations of Sb 3+ analytes in aqueous system against the applied potential were also observed. It was noticed that current increases as a function of Sb 3+ concentration from lower to higher values (0.1 nM to 0.1 M) at normal condition (Fig. 5a). Analytical parameters such as linear dynamic range (LDR), regression coefficient (r 2 ), limit of detection (LOD) and limit of quantication (LOQ) as well as sensitivity were found by plotting the calibration curve at the potential of +1.0 V from various concentrations of Sb 3+ analytes (Fig. 5b). From the plot, the linear dynamic range (LDR), regression coefficient (r 2 ), sensitivity, and limits of detection (LOD) and quantication (LOQ) were calculated as 0.1 nM to 10.0 mM, (r 2 ) ¼ 0.9710, 12.658 Â 10 À4 mA mM À1 cm À2 , 0.075 nM and 0.25 nM, respectively.

Material
In order to validate this newly designed NDNA/naon/GCE, the repeatability test was carried out at the concentration of 0.1 mM with a sequence of nine to ten successive measurements (Fig. 6a). No signicant changes were observed in current responses in addition to the absence of electrode poisoning and erosion during the detection of Sb 3+ at normal condition. I-V response remained almost the same as the initial response aer washing for each experiment. On the other hand, Fig. 6b shows the plot of response time of the newly designed NDNA/naon/ GCE recorded as current (mA) vs. time (sec) (I-T) measurement. It took 10 to 13 seconds to reach the saturated steadystate current. This plot indicates that the newly designed NDNA/naon/GCE is very efficient for the detection of Sb 3+ analytes even at very low concentration and within a short response time.
The sensitivity of the newly designed NDNA/naon/GCE is due to its outstanding absorption (assembly of NDNA-modied GCE) and adsorption (surface of NDNA/naon/GCE) properties. In addition, high catalytic decomposition characteristics and high electron communication features in the active sites of NDNA and GCE make it sensitive. The sensitivity and detection limit of this cationic sensor are in good according with the previously reported methods for the detection of Sb 3+ . The NDNA/naon/GCE system introduces a very simple and reliable method to detect toxic chemicals, and it also reveals a signicant access to a large group of chemicals for wide-ranging applications in environmental and healthcare elds. This study is the initial report for the sensitive and selective detection of Sb 3+ by I-V technique based on the NDNA/naon/GCE as compared to other previously reported methods. Table 4 shows the evaluation of the proposed I-V technique for the detection of Sb 3+ using NDNA/naon/GCE compared with other previously reported analytical techniques. It indicates that this outstanding proposed method, with our designed NDNA/ naon/GCE as a sensor for probing the Sb 3+ , is more sensitive and efficient than previously reported methods.

Real sample analysis
Real sample analysis was carried out in order to validate this proposed I-V method by using NDNA/naon/GCE through the standard addition method. 70 Industrial effluents, a baby feeding bottle, a PVC food packing bag, and a mineral water bottle were used for this purpose. A xed amount of each sample (25.0 mL) was analyzed in PBS (0.1 M, pH ¼ 7.0). The results (Table 5) obtained by this method were in good accord with the proposed I-V technique, which indicates that this technique is also reliable, satisfactory and stable for analyzing real samples with the newly designed NDNA/naon/GCE as an efficient and selective Sb 3+ cationic electrochemical sensor.

Conclusions
A new NDNA Schiff base was easily synthesized, and its molecular structure was conrmed by singly-crystal X-ray diffraction studies to have the dimensions of a ¼ 20.7011 (17) A, b ¼ 6.2155(4)Å, c ¼ 18.0683(13)Å, space group ¼ Pca2 and Z ¼ 4. The nal R 2 was 0.1369 (all data), and R 1 was 0.0543 (I > 2\s(I)). We observed inter-and intra-molecular hydrogen bonding interactions which provide stability to the crystal structure. The N-H/O and O-H/N type intramolecular interactions form the six-membered ring motifs. The NDNA was used to fabricate a new electrochemical sensor (NDNA/naon/ GCE) for the detection of Sb 3+ from simulated as well as real water samples in the presence of other interfering heavy metal cations by using the I-V method. This novel study gives a good proposal for the selective and very sensitive detection of Sb 3+ analytes by using NDNA/naon/GCE as an antimony probe with short response time and good reproducibility as well as repeatability. Hence, this technique launches a new approach that can be applied for the development of new electrochemical sensors for probing other heavy metal cations in environmental and healthcare elds.

Conflicts of interest
The authors declare no conict of interest.