Research on Improvement of Detection of Pb2+ in Environment Waters by Graphene/Nafion Modified Glassy Carbon Electrode

An electrochemical determination of Pb2+ by cyclic voltammetry, differential pulse voltammetry and differential pulse anodic stripping voltammetry in various waters using a glassy carbon electrode modified with graphene/Nafion mixture has been developed. A series of optimum conditions such as graphene concentration, modifying amount, Nafion concentration, scanning speed, pH, accumulation potential and time were optimized. Under optimal experiments, some evaluation indexes were analyzed. Graphene/Nafion/GCE was successfully used in the detection and quantification of Pb2+ in various real water samples using three methods above. Differential pulse anodic stripping voltammetry method that is adopted to study the linear range and the detection limit for Pb2+ proved to be more effective than the other two. The wider range is 0.02∼24.15 μM, the lower detection limit is 5.1 mM and the recovery is 96.1%∼109.0%. To sum up, it will provide a theoretical basis and the practical method for the analysis of heavy metal polltants in different kinds of water.

Because of its wide sources and hardly degradable characters, lead will gradually accumulate in the living organisms.It constitutes a major threat to humans or even affects the normal functioning, including the abnormity of peripheral nerves, human body metabolism and so on. 1,2Therefore, the pollution caused by the toxic lead has attracted extensive attention. 3,4As is known to all, the quantitative determination method of lead mainly relies on general spectrometric approaches including atomic absorption spectrometry, 5 atomic fluorescence spectrometry, 6 mass spectrometry, 7 etc.However, these methods would be difficult to realize on-site rapid analysis as a result of the weakness of high cost, large volume and professional operators of the instrument.Compared with common spectrometric techniques, the electrochemical methods using various electrodes were developed for determination of toxic lead metal. 8,9The main advantages consist in low cost, simplicity, fast analysis speed and high sensitivity. 10,11anomaterials have shown great application prospects in the field of sensors because of their unique properties such as large specific surface area and good electrical conductivity.At present, nano-sensors with better design performance are used in environmental, biological and other fields.For example, Hassan et al. 12 constructed ZIF-8/Co/rGO/C 3 N 4 hybrid nanocomposites biosensor through layer-by-layer fabrication pathway.The proposed biosensor successfully quantified pendimethalin in a wide concentration range of 0.01-35 μM, with a limit of detection (LOD) value of 8.0 nM.The pendimethalin monitoring capability of the fabricated biosensor in real samples including rice, wheat, tap, and river water samples was verified with a recovery range of 98.2∼105.6%.Muniandy et al. 13 developed a reduced graphene oxide-titanium dioxide (rGO-TiO 2 ) nanocomposite based aptasensor to detect Salmonella enterica serovar Typhimurium.The optimized aptasensor exhibited high sensitivity with a wide detection range (108∼ 101 cfu mL −1 ), low LOD (101 cfu mL −1 ) and good selectivity for Salmonella bacteria.Moreover, the electrochemical sensor based on Cu-MOF with excellent sensitivity in this study has been successfully applied to the simultaneous determination of Tl (I) and Hg (II) ions in drinking water and swimming pool water without any interference. 14All research to date shows that there is a need for advanced materials engineering and design electrochemical sensors with higher sensitivity to meet the detection needs in different fields.There is still a need for studies on the creation of hybrids of carbon-based materials such as graphene, graphene oxide and carbon nanotubes, which are frequently used at present, with different advanced materials. 15As cheap and easily available material, graphene will be used as modified electrode material in this experiment.7][18] However, there are few reports on the detection of heavy metal lead.Furthermore, appropriate Nafion amount represents good affinity and immobilization towards nanomaterials on glassy carbon electrode (GCE), 19 which enhance the sensitivity towards heavy metal, various novel voltametric sensors were fabricated based on nanomaterials combined with Nafion modified on GCE. 20n this paper, cyclic voltammetry (CV), differential pulse voltammetry (DPV) and differential pulse anodic stripping voltammetry (DPASV) were employed to discuss and analyze the characteristic of the fabricated graphene/Nafion/GCE, respectively.The experimental results demonstrated that the graphene/Nafion/GCE can improve the redox process of lead ions, respectively.The sensitive and accurate voltammetry method for lead determination established in this study was successfully used in detecting lead content in lake water, waste water, Yellow River, pond water and tap water.In a word, it has important practical significance for evaluating water quality and protecting human health.
All electrochemical experiments were performed with CHI620E potentiostat (Shanghai, China).A series of electrochemical method z E-mail: mengpeijun79@163.com;luoluo_80@yeah.netECS Advances, 2023 2 030505 including cyclic voltammetry, differential pulse voltammetry (DPV) and differential pulse anodic stripping voltammetry (DPASV) were employed to analyze the peak current response of lead ions using a three-electrode system.A bare or modified GCE (3 mm diameter), Ag/AgCl and platinum wire were served as working electrode, reference electrode and auxiliary electrode, respectively.Moreover, the bare electrode was polished with Al 2 O 3 (0.05 μm) slurry before each experiment and cleaned ultrasonically with the solvent (1:1 nitric acid, alcohol and distilled water in turn).
Preparation of graphene suspension and modified electrode.-First of all, graphene dispersion was prepared by mixing graphene and 0.5% Nafion with ethanol.Then a homogeneous turbid liquid was obtained after being sonicated for 30 min.Graphene/Nafion modified electrode was performed using dip-coating technique.15 μL graphene turbid liquid was dropped on the bare glassy carbon electrode surface and dried under infrared light.Correspondently, the Nafion modified glassy carbon electrode was allowed to prepare by the same methods as above.Normally, a new fabricated electrode was activated in the 0.2 mol•L −1 buffer solution (pH4.6).It was stored in this solution at 4 °C before use.All measurements were performed at uniform room temperature.
Sample treatment.-Inour experiment, samples including waste water, tap water, Yellow River and pond water were collected in the surrounding factory in Baotou, laboratory of school, the section of Yellow River in Baotou and near fishponds, respectively.Before to use, all water samples using a series of plastic bottle were placed for a few days; then, they were handled using centrifugal rotation.Supernatant fluid separating from the mixture after microfiltration membrane were performed for next experiment.

Results and Discussion
Electrochemical behavior of modified surfaces by cyclic voltammetry.-Theelectrochemical behavior of Pb 2+ in 0.2 mol•L −1 HAC-NaAC buffer solution (pH4.6) were studied by using cyclic voltammetry at different electrodes (Fig. 1).When the Pb 2+ was added into HAC-NaAC solution, a weak oxidation peak (Ip = 0.8 μA) was observed at the GCE, presenting a Ep = −0.50V.When the Nafion was added into HAC-NaAC solution, the peak current increases by 6 times as much as the current obtained on the glassy carbon electrode.This voltammetric response could be largely due to the negatively charged Nafion, which absorbed positively charged lead ions to increase the signal.The result was consistent with the reported literature. 21When the glassy carbon electrode was modified with graphene/Nafion, the oxidation peak current of the lead ion increased significantly, which was as much as 23 fold that of naked GCE and 4 fold that of Nafion/GCE.It could be attributed to the presence of Nafion (negatively charged) and graphene (electrocatalytic activity), leading to the sensitivity of the experiment increased significantly.This would further show that the lead ion could be determined by constructing graphene-modified electrodes.
The cyclic voltammograms was obtained in the lead and blank buffer, as showed in Fig. 1 inset.From the inset, no peak current occurs in the blank solution, and the well peak signal was founded in the lead solution.This phenomenon indicated that the electrochemical analysis of the real sample could be conducted by the modified electrode.
Optimization of experimental conditions.-Toperform the analytical methodology, different experimental conditions (graphene or Nafion concentration, modification amount, pH, scanning speed), the different chemical and electrochemical behavior of the lead ions at the graphene/Nafion/GCE have been performed.Some defined signals and some optimal parameters from the lead ions had obtained.The first parameter to consider is graphene concentration.Various graphene dispersions, including 2 mg•ml −1 , 4 mg•ml −1 , 8 mg•ml −1 , 12 mg•ml −1 , 16 mg•ml −1 , 20 mg•ml −1 were investigated using CV, as showed in Fig. 2a.The results were compared and a defined oxidation peak at −0.50 V was observed.Fig. 2b showed an increase on the signal of lead ion as a function of the graphene concentration, reached the maximum at 12 mg•ml −1 .From the Fig. 2b, the peak current decreased with the continuous increase of graphene concentration, which was attributed to be difficult to attach thick graphene film to the electrode.On the other hand, the membrane could prevent the electron transfer from solution to electrode surface.
Another effect of Nafion concentration was studied by CV in Fig. 2c.Fig. 2d illustrated that the stripping peak signal of lead ion increased first and then decreased with increasing Nafion concentration, which was consistent with literature reports. 22herefore, 0.25% of Nafion was selected as the optimal ratio in this study.
The influence of the modification amount of graphene was also investigated in the range of 3 to 15 μL.The results were showed in Fig. 2e.When the amount increased, the stripping peak currents enhanced significantly in a certain range, owing to the electrocatalytic activity (resulting from the graphene).The increase in modification amount on the electrode decreased the diffusion rate of lead ions, and therefore, there is a decrease in terms of current.As a result, 8 μL of graphene dispersion was chosen as the optimum amount for subsequent measurements.
The influence of pH on the electrochemical response of 2.42 × 10 −5 mol•L −1 of lead ion was evaluated using graphene/Nafion/GCE between values of 3.8 and 5.4.Fig. 2f showed that the highest peak current intensity was obtained at a pH of 4.2.When considering the well defined peak shape and the stripping peak current, pH 4.2 was chosen as the detection solution in following experiments.
The scan rate was another important parameter for the determination of lead ion.As can be seen in Fig. 3a, the effect of scan rate on electrochemical response at the graphene/Nafion/GCE was studied in the range from 0.01 to 0.20 V•s −1 by cyclic voltammetry.The peak current was proportional to the square roots of the scan rate, as showed in Fig. 3b.This was indicated that the oxidation process was typical diffusion-controlled process in the selected scan rate range.When the scanning speed is slow, the peak current is weak.The peak current increases with the increase of sweep speed, yet the background current also increases.When considering the well defined peak shape and the stability of peak current, it is found that the value 0.07 V•s −1 was chosen as the optimum sweep speed in following experiments.Once the above parameters were optimized, the effect of accumulation potential and time is crucial to the dissolution peak current on the lead ions.As showed in Fig. 4a, the effect of the accumulation potential on the 2 μmol•L −1 peak current signal of Pb 2+ using GN/Nafion GCE with was studied between −1.2 V and −0.8 V.And it is observed that the intensity of peak current increases firstly but decrease subsequently, reaching a maximum at −1.0 V.At potentials lower than −1.0 V, a decrease can be seen of the peak current signal attributed to the discharge of H 2 .Moreover, while the accumulation potential is closer to the oxidation potential of lead ions, the unstable signal will interfere with the enrichment of lead ions on the electrode surface.This phenomenon is consistent with the literature reports. 23Based on this background, −1.0 V is chosen as the optimal accumulation potential.On the other hand, Figure 4b shows that the accumulation time between 30 s and 280 s has an influence on the dissolution peak current of the lead ions.It is observed that there is an increase in the peak current between 30 s and 240 s.Subsequently, a plateau appears, perhaps induced by ECS Advances, 2023 2 030505 reaching of an adsorption equilibrium at the electrode-solution interface.Therefore, the intensity of peak current would not increase significantly.Above all, a time of 240 s is chosen to develop the analytical methodology.
Determination of Pb2+.-The determination of Pb 2+ was performed at the graphene/Nafion/GCE by pH 4.2, v = 0.05 V•s −1 .Three electrochemical methods (CV, DPV and DPASV) were employed to examine the Pb 2+ in the experiment.As shown in Fig. 5, voltampere response and linearity curves of different concentrations of Pb 2+ on graphene/Nafion/GCE by using various electrochemical methods were obtained, respectively.It was found that there was line relation between the peak current of Pb 2+ and concentration(c).And with increasing the Pb 2+ concentration, the peak current obtained increased by the above three methods.For CV method, as showed in Figure 5a, the oxidation peak current of Pb 2+ was proportional to the  ).The ratio of 3 times the standard deviation of the intercept on the slope of the calibration curve was used as the limit of detection (LOD), So the LOD value obtained from three methods were calculated as 3.22 μM, 0.0478 μM and 0.0051 μM, respectively.Therefore, compared with CV and DPV method, DPASV method has higher sensitivity and lower detection limit.On the other hand, the results obtained by using DPV and DPASV methods show that the detection level of the lead ion is less than the allowed level established by Sanitary Standard for Drinking Water (GB 5749-2006), which the limit value of lead in tap water was 0.048 μM.The obtained results were comparable to the values reported in the literature, as showed in Table I.It could be seen that the experimental result was indicative of a great advantages for a wider linear range and a lower LOD in the Reproducibility, stability and interference experiments.-Underoptimized conditions, the reproducibility was estimated in the presence of 0.483 μM and 9.66 μM Pb 2+ in HAc-NAc buffer solution using the same modified electrode made five consecutive measurements, respectively.The results obtained were shown in Table II.The relative standard deviation (RSD) in the above two lead ion solutions were 2.0% and 2.1% respectively.The low RSD value indicated that the modified electrode had excellent reproducibility.When the modified electrode was stored in a refrigerator at   4 °C for 1d, 3d, 5d, 7d and 10d, respectively, the oxidation peak current for 2.41 μM Pb 2+ had a relatively small decrease, as shown in Fig. 6.After 7d, the peak current change was observed with the signal change was less than 10% for Pb 2+ .The experiments results showed that the modified electrode had good stability for the determination of Pb 2+ .In addition, the interference was evaluated in the analysis of some common inorganic ions containing a known concentration of 0.966 μM for Pb 2+ .From Figs. 7a-7c, it was found that only a small change of the oxidation peak current with a deviation of less than 10% was observed after adding 10, 20 and 50-fold Cd 2+ , K + , Zn 2+ , Fe 3+ , Mg 2+ , Ca 2+ interference.However, as showed in Fig. 7d, when 30-fold Cu 2+ was added, the peak current dipped to 12%.If 40 or 50-fold Cu 2+ was introduced into the buffer solution, the peak current decreased obviously to 17.5% or 21.8%, separately.Furthermore, when adding 100 times Cu 2+ , the decrease was more obvious, reaching 41.7%.The results indicated that 30-fold Cu 2+ has had certain influence on experimental determination and only the high concentration of Cu 2+ can cause greater interference.In a word, it was considered that the modified electrode had good antiinterference ability for the determination of Pb 2+ .Sample determination.-Inorder to evaluate the feasibility of the method established by using graphene/Nafion/GCE modified electrode in the analysis of Pb 2+ in real samples.The standard addition method was employed, taking into account possible matrix effects the presence of other components in the real samples.Recovery experiment was carried out on tap water.As shown in Table III, the average recovery for Pb 2+ was 96.1%∼100.5%, and the relative standard deviation was less than 10%, indicating that the reliability and applicability of the modified electrode used in this research.

Conclusions
In this study, the determination of Pb 2+ at the graphene/Nafion/ GCE was studied not only various electrochemical methods but also comprehensive optimization of experimental conditions.The experimental results demonstrated that graphene combined with Nafion could synergistically increase the oxidation peak current of Pb 2+ .The fabricated electrode presented excellent sensitivity, high selectivity and nice stability.It was essential that wide linear range of 0.02∼24.15μM and low detection limit were obtained by using DPASV method.Under optimal conditions, the obtained LOD values (0.0051 μM for Pb) satisfy the threshold limit (0.048 μM for Pb) established by the WHO for normal content of lead in drinking water.Furthermore, the applicability of the graphene/ Nafion/SCE for measuring of Pb 2+ ions in different real water samples was successfully explored.All these findings prefigure great prospect graphene-based sensor in efficient and reliable quantification of lead ions.Moreover, hybrids of graphene-based nanomaterials for the fabrication of electrochemical sensors with different advanced material will pave the way for further studies in environmental application.

Figure 2 .
Figure 2. The influence of graphene concentrations, Nafion concentrations, modification amount and buffer pH on peak current.

Figure 3 .
Figure 3.The CV diagrams of different scanning speeds (a); the linear diagrams of the oxidation peak current and the reduction peak current with the square root of sweep speed (b).

Figure 7 .
Figure 7. (a) 10 times the concentration of interference; (b) 20 times the concentration of interference.(c)50 times the concentration of interference; (d) Different multiples of Cu 2+ .

Table II .
Reproducibility test with many modified electrodes (n = 5).

Table III .
Simultaneous determination results of Pb 2+ in real water sample (n = 3).