Electrochemical sensing of hydrogen peroxide on a carbon paste electrode modified by a silver complex based on the 1,3-bis(1H-benzimidazole-2-yl)propane ligand

2.1 Materials and instruments

All chemicals and solvents were reagent grade and were used without further purification. C, H, and N contents were determined using a Carlo Erba 1106 elemental analyzer. IR spectra were recorded from 4000 to 400 cm⁻¹ with a Nicolet FT-VERTEX 70 spectrophotometer using KBr pellets. Electronic spectra were recorded on a Lab-Tech UV Bluestar instrument. ¹H NMR spectra were obtained with a Mercury plus 400 MHz NMR spectrometer with TMS as internal standard. All electrochemical experiments were performed on a LK2005A electrochemical workstation using a conventional three-electrode system.

2.2 Synthesis of BBP

The BBP ligand was synthesized according to the literature method [30], [31], [32], [33]. Yield: 1.08 g, 78%. M.p. 270 °C. – IR (KBr pellet, cm⁻¹): ν = 3431(w), 1439(s), 1268(m), 747(s). – UV/Vis (DMF): λ = 277, 283 nm – ¹H NMR (400 MHz, DMSO-d ₆): δ = 7.12 (m, 4H, H-ph), 7.48 (q, 4H, H-ph), 2.93 (d, 4H, CH₂), 2.31 (q, 2H, CH₂). – Elemental analysis for C₁₇H₁₆N₄ (276.14): calculated (%) C 73.89, H 5.84, N 20.27; found C 73.80, H 5.83, N 20.26.

2.3 Synthesis of [Ag₂(BBP)₂](pic)₂·CH₃OH

A solution of BBP (0.0138 g, 0.05 mmol) in CH₃OH (3 mL) was carefully layered onto a solution of silver(I) picrate (0.0168 g, 0.05 mmol) in CH₂Cl₂-CH₃OH (6 mL, v/v = 1 : 1) in a glass tube. The solution was left to stand for two weeks at room temperature in the dark, and block colorless crystals were obtained. Yield: 49.5 mg, 79%. – IR (KBr pellet, cm⁻¹): ν = 3436(w), 1444(s), 1358(s), 1163(w), 788(s), 738(m), 702(m). – UV/Vis (DMF): λ = 275, 281, 380 nm – Elemental analysis for C₄₇H₄₀Ag₂N₁₄O₁₅ (1254.09): calculated (%) C 44.92, H 3.21, N 15.60; found C 45.01, H 3.23, N 15.58.

2.4 X-ray structure determination

A suitable single crystal of the Ag(I) complex was mounted on a glass fiber, and the intensity data was collected on a Bruker APEX II area detector with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) at T = 153 K. Data reduction and cell refinement were performed using the Smart and Saint programs. The absorption corrections were carried out by an empirical method [34], [35], [36], [37]. The crystal structure of the Ag(I) complex has been solved by Direct Methods and refined by full-matrix least-squares against F ² of the data using the Olex 2 program [38, 39]. All hydrogen atoms were located geometrically and were subsequently refined in a riding-model approximation with C–H distances from 0.97 to 0.99 Å. The crystal data and experimental parameters relevant to the structure determination are listed in Table 1. Selected bond lengths and angles are presented in Table 2.

CCDC 2088230 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

2.5 Preparation of the CPE modified by the Ag(I) complex

Ag-CPE was prepared by manually grinding 10 mg of the Ag(I) complex and 50 mg of carbon powder with a mortar and pestle to make them uniformly mixed. Afterwards, 20 μL of liquid paraffin was added dropwise and stirred sufficiently to form a carbon paste. The resulting paste was filled into the inner cavity of the tetrafluoroethylene electrode (3 mm diameter). A copper rod of appropriate size was inserted into one end of the tetrafluoroethylene electrode tube to make it conductive, and the carbon paste surface was polished with butter paper. For the preparation of an unmodified electrode (CPE) the amount of Ag(I) complex was replaced with the same amount of graphite.

2.6 Electrochemical responses toward H₂O₂

Electrochemical measurement was carried out using a conventional three-electrode system, with Ag-CPE as the working electrode, Ag/AgCl as the reference electrode, and a platinum plate as the auxiliary electrode. The supporting electrolyte was 0.2 m PBS (pH = 6). The voltage was selected for electrochemical detection of H₂O₂, and different concentrations of H₂O₂ are added to the stirred 0.2 m PBS (pH = 6). The corresponding current response signal was recorded by chronoamperometry.

3.1 General characterization and crystal structure

The Ag(I) complex was obtained by the interface reaction between the flexible ligand BBP and silver picrate. The results of elemental analysis of the Ag(I) complex agree with the theoretical composition. The compound is soluble in polar aprotic solvents such as DMF and DMSO, partly soluble in ethanol, methanol and ethyl acetate, but insoluble in Et₂O and petroleum ether. The IR and UV spectra show that the characteristic absorption peaks are significantly shifted compared with that of the BBP ligand, which confirms that the nitrogen atom from BBP ligand is coordinated to the central metal silver atom [40], [41], [42], [43], [44], [45], [46], [47], [48].

The X-ray diffraction studies have shown that the Ag(I) complex crystallizes in the triclinic space group P 1 ‾ ; with Z = 2. As is shown in Figure 1a, the crystal structure features a binuclear cluster. The asymmetric unit of the crystal is containing a Ag₂(bbp)₂ dimer, two picrate anions and a methanol molecule. The Ag₂(bbp)₂ dimer is formed by two BBP ligands bridging two Ag(I) centers and a Ag–Ag interaction with an Ag–Ag distance of 3.0875 Å (Figure 1b) [49]. Each Ag(I) atom is two-coordinated by two N atoms from two BBP ligands. However, the bond angles at the two non-equivalent Ag(I) atoms are different, where the angle of N(1)–Ag(1)–N(2) is 170.90° (Figure 1c), the angle of N(3)–Ag(2)–N(4) is 175.90° (Figure 1d). The picrate anions do not participate in the coordination and only act as counter anions for charge equilibrium.

Figure 1:

(a) The crystal structure of [Ag₂(BBP)₂](pic)₂·CH₃OH with ellipsoids shown at the 30% probability level (hydrogen atoms and methanol are omitted for clarity). (b) The simplified frame-shaped structure and the Ag1–Ag2 distance. Coordination model of the (c) Ag1 and the (d) Ag2 atoms.

3.2 Cyclic voltammetry studies

Cyclic voltammetry (CV) responses of different electrodes have been investigated in 0.2 m PBS (pH = 6) at a scan rate of 0.025→0.5 Vs⁻¹. As shown in Figure 2a, Ag-CPE has an anode peak at 0.39 V and a cathode peak at −0.05 V, which can be attributed to the redox couple of Ag⁺/Ag²⁺. Obviously, the two well-defined redox peak currents increase proportionally with the gradual increase in scan rate. The linear relationship between the peak current and the square root of the scan rate indicates that the electrochemical reaction on the Ag-CPE is mainly a process controlled by diffusion (Figure 2b) [50].

Figure 2:

(a) Cyclic voltammograms of an Ag-CPE in 0.2 m PBS (pH = 6) at different scan rates. (b) Peak current versus square root of scan rate (0.025→0.5 Vs⁻¹).

3.3 Electrochemical detection of H₂O₂

Optimization of the experimental parameters. Since the current response of the modified electrode is closely related to the potential, the induced voltage for H₂O₂ was screened by chronoamperometry. In the voltage range of −0.1 to −0.5 V, the influence of the applied potential on the ampere response of the Ag-CPE when 1 mm H₂O₂ was continuously added to 0.2 m PBS (pH = 6) was studied (Figure 3). With the increase of the applied potential, the current response continues to improve. When the potential reaches −0.2 V, the electrochemical signal shows negligible noise and is accompanied by an obvious signal generation. Therefore, it is necessary to select −0.2 V as the applied potential for detecting H₂O₂.

Figure 3:

The current-time response of the Ag-CPE with continuously adding 1 mm H₂O₂ into 0.2 m PBS (pH = 6) at −0.1 ∼ −0.5 V.

3.3.1 Chronoamperometric titration

For the continuous addition of different concentrations of H₂O₂ to a stirred 0.2 m PBS solution (pH = 6) under an applied potential of −0.2 V, the electrochemical response of the Ag-CPE to the measured amount of H₂O₂ is depicted in Figure 4a [51], [52], [53]. When adding 5 μm H₂O₂, the curve shows a significant decline, which indicates the current response at low H₂O₂ concentration. The current response achieves more than 98% of the steady-state current within 4 s, indicating a speedy amperometric response to H₂O₂. Simultaneously, the response current gradually increases with the increase of the H₂O₂ concentration. According to the current value analysis, the linear relationship between H₂O₂ concentration and response current can be fitted as shown in Figure 4b [54], [55], [56], [57]. The sensor displays a linear response to H₂O₂ in the concentration range between 0.5 μm and 4.0 mm with a correlation coefficient of 0.999. The detection limit is estimated as 0.39 μm and a higher sensitivity of 6.77 μA mm ⁻¹ is found (S/N = 3). The concentration and response current show a good linear relationship, which indicates that the Ag(I) complex synthesized using BBP as a ligand can be used as a modifier for the construction of H₂O₂ electrochemical sensors [58]. The Michaelis-Menten constant (K _M) is calculated to be 0.289 mm (Figure 5) from the slope and the intercept of the plot of the reciprocals of the steady-state current versus H₂O₂ concentration based on the Lineweaver-Burk equation [59]. It is well known that the smaller K _M indicates higher catalytic activity of the electrode, and the low K _M value in this work indicates that the Ag-CPE exhibits a high catalytic affinity [60].

Figure 4:

(a) Current response of the Ag-CPE for the addition of H₂O₂ (concentration range from 0.5 to 4 mm) into 0.2 m PBS (pH = 6) at −0.2 V. (b) Linear fitting curve obtained from 0.5 to 4 mm.

Figure 5:

Lineweaver-Burk plot of current⁻¹ versus C H 2 O 2 − 1 based on the data from Figure 4(b).

3.3.1.1 Anti-interference and stability

A chronoamperometry curve was recorded in a 0.2 m PBS (pH = 6) containing 1.0 mm H₂O₂ (Figure 6a). When 1.0 mm H₂O₂ was added dropwise again after 900 s, the corresponding response current was 98.2% of the initial response current, proving that the Ag-CPE has excellent stability. In order to test the anti-interference performance of the Ag-CPE, dopamine, uric acid, glucose, ascorbic acid, d-fructose and adenine were sequentially introduced as interference substances. After adding 1.0 mm H₂O₂, the current has a significant and clear response. When the interfering substance is added 1 min later (Figure 6b), the current has no obvious response. The anti-interference experiment certificated that the Ag-CPE still has good stability in the presence of interference.

Figure 6:

For a stirred 0.2 m PBS solution (pH = 6) containing 1 mm H₂O₂ at −0.2 V, is shown (a) the stability test of the Ag-CPE electrode, and (b) the current response of the Ag-CPE when 1 mm interfering substance was continuously introduced.