The synthesis and application of (E)-N′-(benzo[d]dioxol-5-ylmethylene)-4-methyl-benzenesulfonohydrazide for the detection of carcinogenic lead

In this study, noble ligands of (E)-N′-(benzo[d]dioxol-5-ylmethylene)-4-methyl-benzenesulfonohydrazide (BDMMBSH) were prepared via a simple condensation method using benzo-[d][1,3]-dioxole carbaldehyde, benzenesulfonylhydrazine (BSH), and 4-methyl-benzenesulphonylhydrazine (4-MBSH) in good yield, which were crystallized in acetone, EtOAc, and EtOH. The BDMMBSH derivatives were characterized using different spectroscopic techniques, such as 1H-NMR, 13C-NMR, FTIR, and UV-Vis spectroscopy, and their crystal structures were analyzed using the single crystal X-ray diffraction method (SCXRDM). Subsequently, the BDMMBSH compounds were used for the significant detection of the carcinogenic heavy metal ion, lead (Pb2+), via a reliable electrochemical approach. A sensitive and selective Pb2+ sensor was developed via the deposition of a thin layer of BDMMBSH on a GCE with the conducting polymer matrix Nafion (NF). The sensitivity, LOQ, and LOD of the proposed sensor towards Pb2+ were calculated from the calibration curves to be 2220.0 pA μM−1 cm−2, 320.0 mM, and 96.0 pM, respectively. The validation of the BDMMBSH/GCE/NF sensor probe was performed via the selective determination of Pb2+ in spiked natural samples with a satisfactory and rational outcome.


Introduction
Currently, toxic metal ions are a signicant source of pollution in nature, which originate from impure water, foodstuffs, and soil. 1 Among them, the presence of lead ions (Pb 2+ ) is a key concern to human health owing to their extremely poisonous characteristics at low concentrations. Exposure to Pb 2+ in humans (children) may cause different adverse effects and toxicity, for example brain issues, behavioral problems, cardiovascular problems, learning disability, muscle paralysis, memory failures, irritability, reproductive issues, and hearing disorders. 2,3 Pb 2+ can accumulate in the kidneys and bones, and can lead to damage of the CNS and renal activities. Traditional methodical procedures such as electrochemical techniques, GFAAS, ICP-MS, ICP-AES, ASV, and X-ray uorescence spectrometry (XRFS) have been reported in previous studies for the detection of Pb 2+ . However, these methods require costly operation, complicated instrumentation, skilled manpower, and complicated sample preparation. 4,5 Thus, it is necessary to design a new approach with good simplicity, sensitivity, and selectivity for the detection of Pb 2+ . The electrochemical approach (sensor) is an investigative tool that consists of nding elements such as biological, chemical, and metal ions and detector particles (i.e. transducers). A transducer can shi the response of the desired elements into a detectable signal that can be made from different materials (electrodes, nanoparticles, small organic molecules, and thin lms). Electrochemical sensors represent a capable advancement to complement the previously published procedures owing to their easy instrumentation, low cost, high selectivity and sensitivity, and signicant properties for the successful detection of heavy metal ions. [6][7][8][9][10][11][12][13][14] In this research, a simple, sensitive, and selective Pb 2+ sensor was developed based on (E)-N 0 -(benzo[d]dioxol-5-ylmethylene)-4-methyl-benzenesulfonohydrazide derivatives modied on a GCE with 5% conducting Naon polymer matrix via an electrochemical approach under ambient conditions.

Materials and methods
Chemicals including benzo-[d] [1,3]-dioxole carbaldehyde, BSH, 4-MBSH, AgNO 3 , AuCl 3 , CaCl 2 , FeCl 3 , MgCl 2 , Pb(NO 3 ) 2 , ZnSO 4 , SnCl 2 , EtOH, NaH 2 PO 4 , Na 2 HPO 4 , and NF (5.0 weight% in water and lower aliphatic alcohol containing 45.0% water) were purchased from Sigma Aldrich Company, KSA and used as received. A mother solution of toxic metal ions (100.0 mM and 10.0 mL) was made from the respective purchased chemical with distilled water, DSTW (10.0 mL). 1 H-NMR and 13 C-NMR spectra were recorded at 300 K on an ASCEND NMR machine (400 and 850 MHz, respectively) and chemical shis reported in ppm with a common solvent signal as a reference. FTIR and UV-Vis experiments were conducted using a Thermo Scientic NICOLET iS50 and Evolution 300 UV-Vis spectrophotometer. Electrochemical (I-V) examination was performed to detect Pb 2+ at a selective point with a BDMMBSH/GCE/NF sensor using a Keithley electrometer.

Preparation and modication of GCE with BDMMBSH molecules
A series of phosphate buffer (PB) from slightly acidic to basic pH ¼ 5.7, 6.5, 7.0, 7.5, and 8.0 was prepared from NaH 2 PO 4 , Na 2 HPO 4, and DSTW and the amount of PB ¼ 10.0 mL was kept constant throughout the experiment. Primarily, a GCE was cleaned systematically with DSTW and acetone, and subsequently le in open air to dry (1.0 h). The dried GCE was deposited with a slurry (EtOH + BDMMBSH) and le again in open air (1.5 h) to dry. NF was added dropwise to the dried deposited electrode and it again was le in open air (2.0 h) for harmonized lm development with comprehensive drying. The custom-made GCE and Pt wire were used as the working and counter electrode, respectively, to investigate the electrochemical (I-V) responses upon TMI exposure. Organic functional compounds have attracted signicant attention due to their chemical, structural, electrochemical, and optical properties in terms of the attachment of active functional groups. Sensitivity and detection ability are directly dependent on the This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 5316-5327 | 5319 functional anchoring groups attached into the main frame carbon skeleton, which are prepared by reactant precursors under ambient conditions. BDMMBSH and BDMBSH were synthesized via a simple condensation method from benzo-[d] [1,3]-dioxole carbaldehyde, benzenesulfonylhydrazine (BSH) and 4-methyl-benzenesulphonylhydrazine (4-MBSH) in good yield, which were crystallized in acetone, EtOAc, and EtOH systematically. The optical and functional properties of BDMMBSH and BDMBSH compounds have a huge signicance as electrochemical sensor probes for selectivity to heavy metal ions and are superior to other ligands. The functional groups in BDMMBSH on its main carbon strands are responsible for enhancing the capture of heavy metal ions for the selective determination of Pb 2+ cations.

Studies of BDMMBSH derivatives
Two new title ligands (4 and 5) were prepared from benzo[d] [1,3] dioxole-5-carbaldehyde (1), BSH (2), and 4-MBSH (3) in good yield via a simple route (Scheme 1). 19 Using different spectroscopic procedures, molecules 4 and 5 were characterized, and their structures were conrmed nally via SCXRDM. The purity of the derivatives was conrmed from their spectra, which helped us to classify the existing protons in the molecules using the chemical coupling constant (J) and shi (d). The N-H proton of the marked molecules 4 and 5 exhibited a singlet at d 11.16 and 11.26, respectively. The aromatic protons in benzo[d] [1,3] dioxole and phenyl groups of the as-prepared molecules (4 and 5) exhibited different signals (d: 7.94-6.05 and 7.83-6.06) for 4 and 5, respectively. One more singlet was observed at d 2.27, corresponding to three protons, which may be due to CH 3 in the preferred molecule 5. 13 C-NMR was also performed and the carbon atoms in the aromatic region were observed (Fig. S1-S4 †). FTIR was also performed in the wavenumber range 4000-400 cm À1 to conrm the presence of the functional groups, which presented different bending and stretching peaks in the spectra. UV-Visible was conducted in DMSO in the wavelength range of 200-800 nm and l max was found to be 316.0 and 314.0 nm (Fig. S5 and S6 †), respectively.

Analysis of crystal structure
Spectroscopic analysis of the as-synthesized molecules (4 and 5) was performed via SCXRDM with the aim to determine the van der Waals interactions and geometry of the compounds in their unit cells. The structures of 4 and 5 were determined to be in a monoclinic and hexagonal crystal system with the space group P2 1 /c and P6 5 , respectively (Table 1). Both molecules belong to a homologous series since there is a difference in -CH 2between their molecular formulas. In general, the O-S-O angle in the sulfonamide family forces the other angles to adopt a tetrahedral geometry. [20][21][22][23] The <O1-S1-O2 angles around the S atoms are 119.07 (8) and 119.41 (19) for molecules 4 and 5, respectively. The dihedral angle between the aromatic rings and 1,3-benzodioxol system are 72.46 (4) and 76.26 (1) (Fig. 1 and Table S1 and S2 †). The root mean square deviation values for the tted atoms of the dioxol ring are 0.0273 and 0.0369Å for molecules 4 and 5, respectively. The two fused rings (benzodioxol) are twisted at 1.219 (7) Scheme 2 Proposed mechanism of BDMMBSH-Pb 2+ complex formation and the respective electrochemical responses.
hydrogen bonding was observed between the N-H of hydrazine and oxygen of the SO 2 moiety in molecule 5. This linkage connects the molecules along the c-axis and forms an innite chain in 5, and this arrangement produced ball-like shapes when viewed along the ab plane ( Fig. 2 and Table 2).

Detection of Pb 2+ using BDMMBSH by electrochemical method
The development of a custom-made electrode with small organic molecules is the initial application as a TMI sensor. Thus, the BDMMBSH-modied GCE was examined in PB for the sensitive detection of the TMI, Pb 2+ . The as-synthesized two ligands worked as conjugate molecules to detect Pb 2+ , where their nitrogen and oxygen atoms donate electrons to Pb 2+ because they possess lone pairs of electrons (Scheme 2). Here, two molecules were synthesized to observe the electrochemical responses toward heavy metal ions. Based on the electrochemical method, the current signals (CS) of the BDMMBSH/ GCE/NF sensor extensively changed throughout the adsorption of Pb 2+ . Accordingly, the proposed mechanism for the electrochemical detection of Pb 2+ using the I-V performance based on the BDMMBSH-Pb 2+ complex arrangement is presented in Scheme 2.
Initially, the effect of the phosphate buffer pH (10.0 mL) was examined with respect to BDMBSH to determine the most appropriate conditions to detect the toxic metal ion, and pH ¼ 5.7 showed the highest response towards the tailored electrode compared with other PB pH (Fig. 3a). Accordingly, BDMBSH and BDMMBSH were optimized in PB (pH ¼ 5.7 and 10.0 mL), and BDMMBSH showed higher responses (Fig. 3b), where the bar diagram of the molecule optimization at +1.2 V with error bars of 10.0% is presented in Fig. 3c. The I-V responses for the uncoated GCE, GCE with Naon, and GCE with Naon coated with BDMMBSH working electrodes are presented in Fig. 3d, where differences in the current signal among the tailored electrodes can be observed, and the signal was the highest with the BDMMBSH/GCE/NF sensor compared with the bare GCE and GCE with Naon.
Toxic metal ions (25.0 mL and 1.0 mM) including Ag + , Au 3+ , Ca 2+ , Fe 3+ , Mg 2+ , Pb 2+ , Sn 2+ , and Zn 2+ were examined in phosphate buffer (10.0 mL, pH ¼ 5.7, and 100.0 mM) using the unique active surface area containing different custom-made electrodes in order to determine the maximum current signal towards the BDMMBSH/GCE/NF sensor, and accordingly it was distinctly found that the sensor was more selective towards Pb 2+ compared to other toxic metal ions (Fig. 4a). A control experiment was performed at 1.0 mM (25.0 mL) in phosphate buffer (10.0 mL, pH ¼ 5.7, and 100.0 mM) using different deposited electrodes to exhibit that the BDMMBSH derivatives and BDMMBSH showed excellent responses towards Pb 2+ (Fig. 4b). Fig. 4c shows the bar diagram for the control experiment at +1.2 V with error bars of 10.0%. The current signals without Pb 2+ (black) and with Pb 2+ (grey and pink) were also examined (Fig. 4d). An increase in the current signal was observed for the BDMMBSH/GCE/NF sensor in the presence of Pb 2+ , which possesses a massive external area, resulting in enhanced exposure for the potential incorporation and adsorption of the favored toxic metal ions in the holey BDMMBSH molecule surfaces.
The current signals of the TMI, Pb 2+ with different concentrations (100.0 pM-100.0 mM) towards the BDMMBSH/GCE/NF sensor were examined to determine the variation in the current signals of the modied electrode for the function of Pb 2+ detection under the standard conditions. It was observed that the current signals increased at a regular basis from a lower to higher conc. of Pb 2+ [SD ¼ 0.42, RSD ¼ 26.75% at +0.4 V, n ¼ 10, and EB ¼ 10.0%] (Fig. 5a). A good range of Pb 2+ conc. was evaluated from a lower to higher potential (0.0-+1.5 V) serially to determine the probable analytical upper limits. The calibration curve was plotted at +0.7 V as a function of Pb 2+ conc. (100.0 pM-100.0 mM), which was found to be linear [R 2 ¼ 0.9965 and SD ¼ 2.24 at n ¼ 10] (Fig. 5b). The sensitivity, LOQ, and LOD were calculated from the calibration curvature and found to be 2220.0 pA mM À1 cm À2 , 320.0 mM, and 96.0 pM, respectively. 24,25 The LDR (100.0 pM-10.0 mM) was calculated from the calibration curve and was also found to be linear, R 2 ¼ 0.8133 (Fig. 5c). The response time of Pb 2+ towards the BDMMBSH/ GCE/NF sensor was measured at (1.0 mM and 25.0 mL) in buffer (10.0 mL, pH ¼ 7.5, and 100.0 mM), which was determined to be 8.0 s (Fig. 5d).

Evaluation of sensor efficiency
The sensing efficiency of the BDMMBSH/GCE/NF sensor was examined up to a few days to estimate its reproducibility (RP). Accordingly, a series of six consecutive cycles for the detection of Pb 2+ at (1.0 mM and $25.0 mL) using different sensors in the same environment in buffer (10.0 mL, pH ¼ 7.5, and 100.0 mM) was performed, and the BDMMBSH/GCE/NF sensor yielded good responses (RP ¼ 67.0%, SD ¼ 0.44 at +0.7 V, and n ¼ 6) (Fig. 6a and Table S3 †). The current signal of the BDMMBSH/ GCE/NF sensor was measured with respect to storage time to examine its extended storage ability. Accordingly, the storage ability of the BDMMBSH/GCE/NF sensor was examined (1.0 mM and 25.0 mL) in buffer (10.0 mL, pH ¼ 7.5, and 100.0 mM) under the standard conditions using the same sensor, and the repeatability (RA) at the calibrated potential (+0.7 V) was found to be 98.0% towards Pb 2+ , [SD ¼ 0.02, RSD ¼ 1.58%, and n ¼ 6] (Fig. 6b and Table S3 †). The sensitivity remained almost similar as the initial responses up to a few days, and aer that the responses of the BDMMBSH/GCE/NF sensor declined gradually. This distinctly demonstrated that the proposed sensor can be used without any primary changes in its sensitivity for up to few days. A comparison of the Pb 2+ detection using different organic compound-based sensors [26][27][28][29][30][31][32][33] is discussed and presented in Table 3. According to the comparative study of previous sensors    with various organic materials, the highest sensitivity value was found with our fabricated sensor probe (BDMMBSH/GCE/NF) for the detection of lead ions. Various inorganic materials were also used for the selective detection of lead ions via optical and electrochemical methods. [34][35][36][37][38][39][40][41][42]

Assessment of interference effect
Investigation of the interference effects is one of the extensive practices in analytical science to determine the effect of different interfering metal ions having a similar cationic nature. Cd 2+ , Ce 2+ , Co 2+ , Cu 2+ , Ni 2+ , Sn 2+ , and Zn 2+ are generally used as interfering metal ions in the detection of Pb 2+ electrochemically. The current signals of the BDMMBSH/GCE/NF sensor upon the addition of Pb 2+ (1.0 mM and 25.0 mL) and interfering metal ions such as Co 2+ , Cu 2+ , Ni 2+ , Sn 2+ , and Zn 2+ (10.0 mM and 25.0 mL) in buffer (10.0 mL, pH ¼ 7.5, and 100.0 mM) were examined using the same sensor. The interference effect of the interfering metal ions towards Pb 2+ was calculated at the calibrated potential (+0.7 V), where the interfering metal ion of Pb 2+ was considered to be 100.0% (Fig. 7a and b and Table 4). It was evident that BDMMBSH/GCE/NF sensor did not demonstrate any notable responses towards the IMI. Thus, the proposed sensor (BDMMBSH/GCE/NF) is appropriate for the detection of Pb 2+ with good sensitivity.

Analysis of real environmental samples
A group of natural samples such as coal water, industrial effluent, Red Sea water, rice water, tap water, and well water was utilized as real samples to estimate the I-V performance of the BDMMBSH/GCE/NF sensor. A standard addition procedure was used to determine the concentration of Pb 2+ . A xed quantity ($25.0 mL) of each natural sample was examined in phosphate buffer (10.0 mL, pH ¼ 7.5, and 100.0 mM) using the modied BDMMBSH/GCE/NF sensor. [43][44][45][46][47][48][49] The concentrations were calculated at the calibrated potential (+0.7 V) for the detection of Pb 2+ in coal water, industrial effluent, Red Sea water, rice water, tap water, and well water, which absolutely demonstrated that the I-V method is proper, reliable, and appropriate for the analysis of real samples. Fig. 8 shows the bar diagram presentation of the natural sample analysis at +0.7 V with error bars of 10.0%. The results show that the conc. of Pb 2+ in the well water was the greatest among the natural samples (Table 5).

Conclusion
Herein, two new BDMMBSH compounds were synthesized, characterized, and used to selectively detect heavy metal ions via an electrochemical method. Good sensor performances for the selective Pb 2+ sensor were demonstrated in terms of sensitivity, LOQ, LOD, LDR, response time, RP, and RA. This new device can be a suitable analytical tool to design sensitive and selective TMI detection platforms with BDMMBSH-embedded GCE using conducting polymer matrix. Finally, the innovative sensor probe was introduced from this new approach for the detection of toxic TMIs in the environmental and healthcare elds on a broad scale.

Conflicts of interest
The authors declared that there is no conict of interest to be reported in this research work.