Hg2+ Sensor Development Based on (E)- N'-Nitrobenzylidene-Benzenesulfonohydrazide (NBBSH) Derivatives Fabricated on a Glassy Carbon Electrode with a Nafion Matrix

Abstract

Three novel derivatives of (E)-N'-nitrobenzylidene-benzenesulfonohydrazide (NBBSH) were synthesized by a condensation method from nitrobenzaldehyde and benzenesulfonylhydrazine reactants in low to moderate yields, which crystallized in methanol, acetone, ethyl acetate, and ethanol. NBBSH derivatives were totally characterized using various spectroscopic techniques, such as Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, proton nuclear magnetic resonance spectroscopy (¹H NMR), and carbon-13 nuclear magnetic resonance (¹³C NMR) spectroscopy. The molecular structure of the NBBSH derivates was confirmed by the single crystal X-ray diffraction method and used for potential detection of a selective heavy metal ion, mercury (Hg²⁺), by a reliable I-V method. A thin coating of NBBSH derivatives was deposited on a glassy carbon electrode (surface area = 0.0316 cm²) with a binder (nafion) coating to modify a sensitive and selective Hg²⁺ sensor with a short response time in phosphate buffer. The modified cationic sensor exhibited enhanced chemical performances, such as higher sensitivity, linear dynamic range, limit of detection (LOD), reproducibility, and long-term stability toward Hg²⁺. The calibration curve was found to be linear over a wide range of Hg²⁺ concentrations (100.0 pM-100.0 mM). The sensitivity and LOD were considered to be ∼949.0 pA μM^-1cm^-2 and 10.0 ± 1.0 pM (S/N = 3), respectively. The sensor was applied to the selective measurement of Hg²⁺ in spiked water samples to give acceptable and satisfactory results.

Scheme 1. Synthesis of NBBSH Molecules by a Condensation Method

Figure 1

Crystal structures of NBBSH molecules.

Figure 2

Unit cell diagram of NBBSH derivatives (focused on inter- and intramolecular hydrogen bonding interactions).

Scheme 2. Schematic Representation of Sensor Fabrication and Detection Mechanism of Hg²⁺ Ion with a 3-NBBSH/GCE Sensor

Figure 3

(a) I–V responses of various compounds (2-NBBSH, 3-NBBSH, and 4-NBBSH) coated on a GCE (at pH = 7.0); (b) selectivity study at 100.0 nM in the presence of various cations including mercury; (c) control experiment with various synthesized compounds under identical conditions, [analyte concentration = 100.0 nM, pH = 7.0, amount: 25.0 μL, surface area of GCE = 0.0316 cm², method: I–V, delay time = 1.0 s]; and (d) bar diagram presentation of selectivity optimization at +1.2 V.

Figure 4

I–V responses. (a) Bare GCE, nafion-coated GCE, and 3-NBBSH-coated nafion/GCE, (b) absence and presence of Hg²⁺ with different electrode modifications of synthesized compounds, (c) concentration variation, and (d) calibration curve at +0.7 V (range of target Hg²⁺ concentration = 100.0 pM ∼ 100.0 mM; pH = 7.0; potential range = 0.0 ∼ 1.5 V; technique: I–V).

Figure 5

(a) Plot of LDR, (b) plot of r.t. of mercury cations at 100.0 nM (inset: expansion of 0–20 s), (c) repeatability study at 100.0 nM of Hg²⁺ [RA calculated at the calibration potential, +0.7 V], and (d) real-sample analysis (industrial effluent, red sea water, and tap water).

Figure 6

Interference effect of other metal ions toward the proposed Hg²⁺ ionic sensor. (a) Comparative study in the presence of interfering cationic metal ions, (b) Bar-diagram presentation at +1.2 V with error limit [PB, pH = 7.0; amount: 25.0 μL; method: I–V; delay time = 1.0 s, and potential range: 0.0 ∼ +1.5 V].