Determination of 1-Naphthol Concentration on Electrode Modified with Electrochemically Polymerized β-Cyclodextrin Film

A glassy carbon electrode (GCE) modified with poly β-cyclodextrin (β-CD) film (β-CD/GCE) was prepared. The electrochemical properties of the β-CD/GCE were explored by cyclic voltammetry (CV) and AC impedance measurement, and the obtained results indicate significant improvements in impedance and electrocatalysis behavior. Further experimental results show that in HAc-NH4Ac solution (pH 5.55), the electrocatalysis of 1-naphthol is very clear on the polymerized β-CD/GCE. The peak current increased linearly with the concentration of 1-naphthol in the range of 2.05 × 10−3–1.07 × 10−6 mol·L−1, and the detection limit was 3.06 × 10−7 mol·L−1 at an SNR of 3:1. The modified electrode was applied, with satisfactory results, to the determination of 1-naphthol concentration in tap water, and its recovery rate was between 97.3 and 103%.


Introduction
1-Naphthol, which is used as the raw material of the insecticide Sevin, is now in great demand. It is a large-scale industrial chemical that is widely used in the manufacture of dyes and insecticides, and is also a secondary product of chemical, paint, paper, and pesticide industries. In addition, it is a kind of phenolic compound with high toxicity and thus has a profound effect on the environment and health of people. (1)(2)(3) Currently, the common methods of determining the concentration of 1-naphthol include high-performance liquid chromatography (HPLC), (4)(5)(6) the use of quartz crystal microbalance (QCM), (7) immune cytochemistry, (8) fluorescence methods, (9)(10)(11)(12)(13)(14) and synchronous-derivative phosphorimetric determination. (15) These methods have their own characteristics: some of them require expensive instruments and complicated sample processing, and they are not convenient to use in real-time field testing. Therefore, a rapid and accurate determination of 1-naphthol concentration is of practical significance. At present, the field environment real-time detection, mobile laboratory, portable detection instruments, and so on have a good prospect of development. The electrochemical method with simple and rapid features is gradually widely used. Therefore, in this study, we aim to establish a rapid and simple electrochemical method for the determination of 1-naphthol. β-cyclodextrin (β-CD) is a functional supramolecule with a hydrophobic interior and a hydrophilic exterior, and may selectively recognize some organic or inorganic small molecules through nonbonding intermolecular forces. (16) 1-Naphthol molecules entering into the cavity of β-CD tend to accumulate on one side, which enhances the nonbonding forces between 1-naphthol and β-CD, resulting in an enhancement of the electrochemical response of 1-naphthol on a glassy carbon electrode modified with poly β-CD film (β-CD/GCE). In addition, β-CD, which has a special cylindrical structure with external hydrophilic and internal hydrophobic chambers, can be combined with 1-naphthol to form the host-and guest-containing compounds. Therefore, it can be applied to the detection of 1-naphthol. In a preliminary work, we carried out extensive research on polymer electrodes, such as an electrode modified with a Prussian blue analog doped with copper (II) ions (17,18) and a poly(methyl red)-modified electrode. (19) We have also conducted some research on β-CD-film-modified electrodes by electrochemical polymerization. (20) Using the results of previous work, we developed a new test system using the β-CD/GCE prepared by electrochemical polymerization, which has made the rapid and accurate determination of the concentration of 1-naphthol a reality. Moreover, linear calibration ranges are wider and the detection limit is higher than literature values. (21,22)

Preparation of β-CD/GCE electrode
The GCE (Φ = 4 mm) was polished with emery paper, then washed ultrasonically for 3 min each in 1:1 NaOH solution, 1:1 HNO 3 solution, pure alcohol, and distilled water, successively. The cleaned GCE was put in a 0.5 mol·L −1 solution of sulfuric acid, then cyclic voltammetry (CV) experiments were carried out in a potential range from −1.0 to 1.0 V with a scan rate of 0.10 V/s until the peak current became stable, thus attaining an activation electrode. Next, the activation electrode was put in a saturated solution of β-CD containing 0.1 mol·L −1 potassium chloride and 0.01 mol·L −1 sodium hydroxide for cyclic voltammetric experiments in a potential range from −1.0 to 1.0 V with a scan rate of 0.05 V/s until the peak current became stable. The working electrode was taken out, rinsed, and then dried in air. A baby-blue film was observed on the surface of the working electrode, indicating the successful preparation of a steady β-CD/GCE.

Analytical procedure
A conventional three-electrode system, including a β-CD/GCE or glassy carbon working electrode, a platinum-wire counter electrode, and a reference SCE, was employed using the CHI660C electrochemical workstation. Impedance from 0.1 MHz down to 1 Hz was measured. The amplitude of the sinusoidal voltage was chosen to be 5 mV. Cyclic voltammograms were recorded from −0.8 to 0.9 V under various conditions, for example, scan rate, electrolyte, and 1-naphthol concentration. All the experiments were carried out at room temperature, and nitrogen was bubbled into the sample to remove the oxygen.

Characterization of β-CD/GCE electrode
In 1.01 × 10 −3 mol·L −1 potassium ferricyanide solution containing 0.1 mol·L −1 potassium chloride, the β-CD/GCE and activation electrode were used as the working electrodes and the cyclic voltammograms were recorded from −0.8 to 0.9 V with a scan rate of 0.10 V/s. The obtained results are presented in Fig. 1. Compared with the CV response on a bare GCE, a large well-defined peak appears on the β-CD/GCE. The peak potentials are 0.015 and 0.268 V. These results indicate that the β-CD/GCE shows good electrocatalytic action for potassium ferricyanide, but a bare GCE does not. The polymerized β-CD/GCE was successfully prepared by electrochemical polymerization, and the electrocatalytic property of this modified GCE showed a significant change. Typical complex impedance plots for the β-CD/GCE and GCE in the 1.01 × 10 −3 mol·L −1 potassium ferricyanide solution containing 0.1 mol·L −1 potassium chloride are shown in Fig. 2. There is a high-frequency semicircle on the GCE and a quarter-circle on the β-CD/GCE. These results show that poly-(β-CD) was successfully modified on the GCE by electrochemical polymerization, resulting in a clear change in impedance. By combining Figs. 1 and 2, we can see that the peak current of potassium ferricyanide is low for a bare GCE, and the corresponding impedance is larger. However, for the β-CD/GCE, the peak current of potassium ferricyanide markedly increases and its impedance decreases. These results show that the poly-(β-CD) participates in the electron transfer process on the electrode surface and enhances the electron conductivity. In the 1.03 × 10 −3 mol·L −1 1-naphthol solution, the CV response of 1-naphthol on the β-CD/GCE exhibits a large well-defined oxidation peak whose peak current is −1.871 × 10 −5 A. Compared with the 1-naphthol peak current of −4.779 × 10 −6 A for the bare GCE, the peak current has almost quadrupled. This indicates that the electrochemical polymerization of β-CD on the GCE greatly improved the electrochemical reactivity of 1-naphthol and markedly enhanced the electrochemical response of 1-naphthol on the β-CD/GCE.

Effect of solution pH
With a scan rate of 0.12 V/s, we carried out CV analysis on the β-CD/GCE in the HAc-NH 4 Ac buffer solution with 1.03 × 10 −3 mol·L −1 1-naphthol at different pH values of 4.00, 5.55, 7.50, 8.52, and 10.03. The cyclic voltammograms are presented in Fig. 4. The plot of Ep vs pH is shown in the inset of Fig. 4. A marked electrocatalytic action of 1-naphthol on the β-CD/GCE is observed in the electrolyte at any pH. Ep and Ip are affected by the pH of the electrolyte. When the pH is 4.00, the peak shape is imperfect and the peak current is low, preventing the quantitative analysis of 1-naphthol. When the pH is above 8.52, 1-naphthol decomposes, resulting in a large measurement error. Ip is maximal and the electrocatalytic action is the most marked when the HAc-NH 4 Ac buffer solution of pH 5.55 is used in the CV analysis. The pH dependence of Ep obeys the equation Ep = −0.06128 pH + 0.86273 (R = 0.9985). The slope of 61.28 mV/pH shows that equal numbers of protons and electrons were involved in the oxidation of 1-naphthol. (23)    This result shows that the electrocatalytic process was an irreversible process controlled by adsorption. When the scan rate reached 0.30 V/s, the peak shape changed and the background current became higher, which was not suitable for the determination of peak current. Therefore, the scan rate of 0.25 V/s was used in the CV analysis. Under optimized test conditions, the cyclic voltammograms of 1.03 × 10 −3 mol·L −1 1-naphthol on the β-CD/GCE were recorded every day for a week. After testing, the β-CD/GCE was rinsed and preserved in HAc-NH 4 Ac buffer solution of pH 5.55. Three weeks later, the peak current remained roughly stable, indicating that the β-CD/GCE has good stability. The results are shown in Table 1. Under the same experimental conditions, a relative standard deviation of 1.35% was obtained for ten repetitions of measuring 1.03 × 10 −3 mol·L −1 1-naphthol on the β-CD/GCE. Therefore, the β-CD/GCE also has good reproducibility.

Interference study
Under the same experimental conditions as above and within the error allowed, the factors affecting the determination of 1.03 × 10 −3 mol·L −1 1-naphthol were studied. The results indicate that the 500-fold higher concentrations of Al 3+ , Zn 2+ , Ca 2+ , Mg 2+ , SO and Na + , the 100-fold higher concentrations of p-nitrophenol and p-aminophenol, and the 10-fold higher concentrations of hydroquinone, pyrocatechol, phenol, and bisphenol A did not affect the determination of 1-naphthol. However, a 50-fold higher concentration of 2-naphthol interfered with the determination. The results are shown in Table 2.

Standard Addition Analysis of Samples
A standard solution of 1.52 × 10 −3 mol·L −1 1-naphthol was prepared in pH 5.55 HAc-NH 4 Ac buffer solution. Then, three samples were obtained by transferring 2.5, 5.0, and 10.0 mL of 1.52 × 10 −3 mol·L −1 1-naphthol standard solution to a 50 mL volumetric flask, diluting with pH 5.55 HAc-NH 4 Ac buffer solution to make 50 mL volume, and mixing. The samples were labeled from 1 to 3, respectively. The standard addition analysis of the samples was carried out by CV with scanning from −0.8 to 0.9 V at a scan rate of 0.12 V s −1 . The recovery rate was from 97.3 to 103%. The results are shown in Table 3.

Conclusions
A GCE modified with a poly β-CD film was prepared by electrochemical polymerization. The sensor was developed for use in a new test system for the measurement of the 1-naphthol concentration. The peak current of CV increased linearly with the 1-naphthol concentration in the range of 2.05 × 10 −3 -1.07 × 10 −6 mol·L −1 , and its detection limit was 3.06 × 10 −7 mol·L -1 (3SNR). This method may also hold promise for potential applications in environmental analysis and measurement.