Study on fabrication nano platinum modified glassy carbon electrode for application to analyze lead, cadmium in the water environment

The results of the study showed that: In the same experiment,

the relative standard deviation of RSD = 1.45% for Pb and RSD =

1.59% for Cd we concluded that this measurement was good for both

metals. At the same time, we also studied the reproducibility of the

method with the relative standard deviation (RSD) of Cd of 4.36%

and Pb of 4.65%. Compared with the maximum relative standard

deviation allowed within the laboratory by the Horwitz function

(RSDHorwitz = 32% with a concentration of 10 ppb), the RSD of the

Cd, Pb analysis method is smaller than ½ RSDHorwitz should be

internally an acceptable laboratory, ie a method with good

repeatability

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water samples using differential pulse anodic stripping voltammetry at platinum nanoflowers modified glassy electrode, Viet Nam Journal of Chemistry, 2019, 57(3), 379-383. Doi: 10.1002/vjch.201960028. 9. Lê Trường Giang, Nguyễn Thị Liễu, Đánh giá khả năng hoạt động điện hóa của các dạng tồn tại Cd, Pb trong các môi trường điện li sử dụng mô hình cânbằng hóa học, Tạp chí Hóa học, 2019, 57(6E1,2), 103-107. 10. Nguyen Thi Lieu, Pham Quoc Trung, Le Tran Thu Trang, Le Truong Giang, Simultaneous effect of pH, deposition time, deposition potential, and step potential on the stripping peak current of copper on platinum nanoflowers modified glassy carbon electrode (PtNFs/GCE) using response surface methodology, Viet Nam Journal of Chemistry, 2020, 58(3), 302-308. DOI: 10.1002/vjch.201900088. 1 INTRODUCTION 1. The urgency of the thesis Currently, in our country due to the development of industry, agriculture, daily life, transportation, etc., the environment has been seriously polluted, especially heavy metal pollution in the water environment. Heavy metals entering the human body outside the permissible level will cause serious health effects. When exposed to long-term intoxication, the metal accumulates in the body, combined with the cells, can cause cancer. Specifically, with some metals, lead is toxic to the central nervous system, peripheral nervous system, affects the enzyme system with hydrogen-containing activity groups, disturbs the hematopoietic part (bone marrow). Depending on the level of Pb poisoning, people can have abdominal pain, joint pain, kidney inflammation, high blood pressure, stroke, severe poisoning can cause death. Cadimi interferes with the activity of some enzymes, causes hypertension, lung cancer, perforation of the nasal septum, disturbs kidney function, destroys bone marrow and affects endocrine, blood, and heart. Therefore, to check and assess the level of lead, cadmium pollution of water sources, the analytical methods need to have high sensitivity and accuracy, capable of analyzing at trace content. Some methods of analyzing lead and cadmium include atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICP-MS) molecular absorption spectroscopy (UV - Vis) methods. The above methods have high sensitivity and low detection limits. However, they have disadvantages such as large equipment investment, complex operating techniques; long analysis time, only perform laboratory analysis, which requires the equipment operator to be highly qualified. Meanwhile, the electrochemical analysis method, especially the stripping voltammetry of interest is a method with many advantages such as high sensitivity, short analysis time, simple operating 2 equipment, compact, capable of analyzing samples directly in the field. For the purpose of increasing the sensitivity, selectivity, lowering the detection limit ..., the electrodes used in the stripping voltammetry method tend to be denatured by nanomaterials. From the above reasons, the thesis topic “Study on fabrication nano Platinum modified glassy carbon electrode for application to analyze lead, cadmium in the water environment” was studied 2. Research objectives of the thesis - The thesis was conducted to research and develop a new type of electrode: glass charcoal electrodes modified by flower platinum nano (PtNFs/GC) and to evaluate the properties of fabricated electrodes: electrode structure, morphology, physical and chemical properties. - Studying and orienting applicability to detecting, quantifying separately and simultaneously cadmium, lead in aqueous environment by means of stripping voltammetry method with the desire that the nanostructures of the electrodes help increase the sensitivity of the analytical method. 3. The main research contents of the thesis - Study on fabrication nano Platinum modified glassy carbon electrode by chronoamperometry - Evaluate surface morphology of nano Platinum electrodes made by SEM, AFM - Evaluate physical and chemical properties of electrodes by methods: XRD, EDX, CV. - Develop the process of analyzing Cd and Pb separately and simultaneously on PtNFs /GC electrodes made by the ASV method. - Application of ASV method using PtNFs /GC electrodes to analyze Cd and Pb in some practical samples 4. The structure of the thesis 3 The thesis consists of 134 papes with 75 figures, 41 tables. The thesis includes the following sections: Introduction (3 pages); Chapter 1: Overview (26 papes); Chapter 2: Research methods and experiment (11 papes); Chapter 3: Results and discussion (75 papes); Conclusions (1 papes); Novel scientific contributions of the thesis; List of publication; References and appendices. CHAPTER 1: OVERVIEW Chapter 1 includes a general introduction of heavy metals and their harms, methods of determining trace amounts of Cd, Pb, the basis of the stripping voltammetry method, introducing some electrodes working in the method. The chapter also focuses on the presentation of Pt nanofabrication methods and the applications of Pt nanoparticles in special fields of analysis; a general overview of the research situation at home and abroad related to the topic. CHAPTER 2: RESEARCH METHOD AND EXPERIMENT 2.1. Chemistry CH3COONa, CH3COOH, H2PtCl6.6 H2O, H2SO4, K3[Fe(CN)6], NaOH, K4[Fe(CN)6], HNO3, K2HPO4, KH2PO4, H3PO4, H3BO3, KCl, HCl, CTAB, TritonX-100. Working solutions of metals: Pb 2+ , Cd 2+ , Zn 2+ , Cu 2+ , Fe 3+ ... prepared from standard solution for AAS with a concentration of 1000 ppm. 2.2. Fabrication electrodes Glassy carbon (GC) was chosen as the ground electrode for platinum nano. First, the GC electrode (d = 3.0 mm) is treated by polishing the surface with ultrafine sandpaper, rinsing with distilled water in an ultrasonic bath. The electrodes are then electrically cleaned by applying a voltage of E = -1.0 V (with Ag / AgCl) in a 0.5 M H2SO4 acid solution for 200 s. Platinum nanoparticles were precipitated on the GC surface from the H2PtCl6 solution with a concentration of 1mM in the phase of 0.1 M H2SO4 by chronoamperometry method, using 3 electrodes 4 RE, CE, WE: GC, the fixed voltage is applied to the system for a certain period. Figure 2.2. Simulation of electrode fabrication process PtNFs/GC - Deposition potential (EPt) investigated: -0.5 V; -0.3 V; -0.2 V; 0.0 V; 0.2 V with tPt = 150 s with stirring of solution. - Deposition time (tPt) investigated 50 s; 100; 150 s; 200 s; 300 s with EPt = -0.2 V with stirring of solution. - The stirring and not stirring the solution: EPt = -0.2 V; tPt = 150 s 2.3. Characterizations Characterizations techniques: XRD, EDX, SEM, AFM 2.4. Application of Cd and Pb analysis on PtNFs/GC electrodes - The enrichment process by electrodeposition then dissolving sweep is carried out right in the solution containing Cd, Pb (different pulse anodic stripping voltammetry method - DPASV) - Optimize the analysis conditions including electrolytic solution, pH, preconcentration potential, accumulation time, pulse amplitude, step potential, electrode cleaning mode, interfering substances. 2.5. Experimental modeling studies the simultaneous effect of pH, tdep, Edep and Ustep on the stripping peak current (Ip) of Cd, Pb 2.6. Evaluate the reliability of the method and process empirical data 2.7. How to prepare the actual sample for the analysis process of Cd, Pb 5 CHAPTER 3: RESULTS AND DISCUSSION 3.1. Fabrication electrodes 3.1.1. Survey manufacturing conditions Figure 3.1. The cyclic voltammograms of a glassy carbon electrode in 0.1 M H2SO4 solution and in 0.1 M H2SO4 solution containing 10.0 mM H2PtCl6 solution (a); a glassy carbon electrode and Pt/GCE in 0.1 M H2SO4 solution (b), scanning rate of 0.1 V/s Figure 3.1a is the CV curve of the GC electrode in a 10 mM H2PtCl6 solution + 0.1 M H2SO4 with three potential regions: the hydrogen region (from -0.2 to + 0.15 V) characterized by the presence of adsorbed hydrogen on the electrode surface that shows peaks corresponding to the adsorption/desorption of hydrogen with different energies, a broad oxidation peak for the Pt – oxide formation (commences at ca. 0.8 V and extends up to 1.2 V), and a single reduction peak at 0.5 V corresponding to the reduction of Pt(IV) to Pt(0) on the electrode surface. Our results were in agreement with other observational studies. From the results in Figure 3.1a shows that to create platinum particles on the GC by the static potential method, the applied voltage has a negative value more than 0.3 V. The effect of deposition potential (EPt) and time (tPt) to composition, properties of Pt layer will be shown in the following studies. 6 From figure 3.1b shows that when the granular layer of platinum is precipitated onto the GC to form a Pt/GC electrode, the obtained circular potential sweep has a characteristic line shape of the platinum material with the reduction puck at about 0.5 V. (compared to Ag/AgCl), this is entirely consistent with previous similar claims. This result proves that platinum has been put above GC. 3.1.2. Formation of PtNFs/GCE 3.1.2.1. Images of electrode surfaces and SEM images of Pt / GC; GC Images of the electrode surface before (GC) and after making platinum nanolayer (Pt/GC) are shown in Figure 3.2. Figure 3.2. GC electrode before (a) and after platinum nanoparticle creation (b) Figure 3.3. SEM image of GC electrode before (a) and after platinum nanoparticle creation (b) 7 According to the SEM images, the GCE surface has a smooth and homogeneous morphology. The GCE surface is densely coated with Pt nanoflowers. After electrodeposition, a large number of Pt nanoflowers are produced on the GCE surface, the surface of PtNFs/GCE was rougher with irregular cubic nanoflowers shape whose size varies in the range (50 – 400 nm). 3.1.2.2. X-ray diffraction pattern of Pt/GC Figure 3.4. The X-ray diffraction pattern of platinum nanoparticles showing the face-centered cubic (fcc) crystal structure The X-ray diffraction pattern of platinum nanoparticles on GC is shown in Figure 3.4. There were three well-defined characteristic diffraction peaks at 39.9º, 46.2º, and 67.5º respectively, indexed to reflections from (111), (200), and (220) planes of face-centered cubic (fcc) crystal structure of metallic platinum This result exhibits that Pt exists on the GCE surface. 3.1.2.3. EDX spectra for Pt/GCE EDX spectra (Figure 3.5) shows that the composition of the Pt/ GC electrode surface beside the elements C, Pt does not appear any 8 other element. The results showed that the presence of two elements C, Pt with a mass ratio of 88.58% and 11.42% respectively (Pt/GC electrode made at a voltage of 0.2 V). This result is consistent with the X-ray diffraction diagram and is the evidence for the presence of platinum particles on the GC ground electrode. Figure 3.5. EDX spectra for Pt/GC prepared at the deposition potential of 0.2 V 3.2. Characteristics of electrode properties 3.2.1. Effect of deposition potential (EPt) 3.2.1.1. Effect of EPt on electrode surface morphology According to the SEM images, Pt was formed separately in nanoparticles shape at the deposition potential of 0.2 V and 0.0 V platinum particles begin to be formed and distributed on the electrode surface. When the voltage is -0.2 V, the GC surface is covered with a platinum layer with a flower-shaped structure without agglomeration, retaining the status of individual particles with size of 50 - 400 nm. It 9 is the structure of these flowers that leads to the increased electrochemical active area of the electrode. At the deposition potential of -0.3 V, however, gathering together, and gradually filling the gaps among the flowers onto the GC surface. It is observed in Fig.3.6e that there are some defects at which no Pt occupied resulting from the attachment of a large H2 bubble at those sites. At the deposition potential of -0.5 V, the Pt flower pieces developed slapping whereby they aggregated into a film, they had not been single, free flower-shaped structures at nanoscale any longer. Also, the stronger hydrogen bubble release at that potential should significantly prevent the formation of Pt on the electrode surface, thus, it can be seen many sites of glassy carbon surface (black spots) that have no Pt occupying at (Fig.3.6f). 10 Figure 3.6. SEM images of GCE (a); PtNFs/GCE deposited at potential of 0.2 V (b), 0.0 V (c), -0.2 V (d), -0.3 V (e), -0.5 V (f) 3.2.1.2. Effect of EPt on electrode surface composition Table 3.1. The composition of Pt /GC electrode surface at different deposition potential (EPt) E Pt (V) -0.5 -0.3 -0.2 0.0 0.2 Pt weight ratio (%) 6.30 22.54 19.26 15.28 11.42 C weight ratio (%) 93.70 77.46 80.74 84.72 88.58 Tổng 100.00 According to the results of EDX spectra, there are no other elements on the surface of Pt /GC electrode surface besides C, Pt elements. Based on Table 3.1, the percentage of Pt mass increases when EPt is from 0.2 V to -0.3 V and then decreases at a voltage of -0.5 V. 3.2.1.3. The effect of EPt on electrode surface structure 11 Figure 3.8. AFM images show the characteristic surface morphology form GCE (a); PtNFs/GCE deposited at the potential of -0.2 V (b); 0.2 V(c) According to the AFM results, the GC electrode has an average roughness value (RMS) of 672.82 nm, the Pt/GC electrode surface of -0.2 V has a high RMS value of 777.09 nm. 0.2 V more than the Pt/GC electrode surface (730.53 nm). Figure 3.9. Relationship between RMS roughness and Ahd of the electrode 12 And based on the graph (Figure 3.9) shows that the greater the roughness of the surface, the larger the electrochemical active area (Ahd) of the electrode. This means that the Pt /GC electrode made at -0.2 V will give a better electrochemical operating area than the Pt/GC electrode made at 0.2 V and GC. 3.2.1.4. Diffuse and reversible properties of electrochemical reactions on Pt / GC electrodes The electrochemical active area (Ahd) of Pt/GC electrodes fabricated at different EPt is calculated based on the Randles-Sevcik equation. The current value is measured on the Von-Ampe line (the peak is used to calculate the reduction of Fe 3+ → Fe2+, the value of the current is Ipc). In this study, the electrolyte solution used is K3[Fe (CN)6] 5 mM phase in 0.2 M phosphate buffer, pH = 7 with parameters used for calculation according to Randle-Sevcik equation is : C = 5.10 -6 (mol /cm 3 ); D = 5.69.10 -6 (cm 2 /s); v = 0.10 (V /s); n = 1. The result is shown in Figure 3.11. Figure 3.11. Cyclic voltammograms obtained at a bare GCE and PtNFs/GCE prepared in different electrodeposition potentials (b): 0.2 V, 0 V, -0.2 V in 0.2 mol·L -1 phosphate buffer solution (PBS), pH 7.0 containing 5 mM K3Fe(CN)6 at a scan rate of 0.1 V·s -1 13 The electrochemical activity area of Pt/GC electrodes gradually increases in the decreasing direction of the precipitated potential (EPt) from 0.2 V to -0.2 V and then decreases to the voltage of -0.5 V. This can be explained by the fact that when the negative voltage is more than -0.2 V, the rapid precipitation phenomenon can cause Pt crystallization on the surface to increase, the particle size also increases accordingly leading to surface area reduced. 3.2.1.5. The stripping peak current intensity of Pb on Pt/GC electrodes made at EPt varies The results show that the stripping peak current (Ip) of Pb on Pt/GC electrode fabricated at -0.2 V is the largest, the reason may be due to this condition the electrode is flower-shaped, so the electrochemical activity area of the electrode is the largest, so -0.2 V is chosen as the optimal EPt for modified GC by platinum nanoparticles and applied to the analysis of heavy metal ions in water. 3.2.2. Effect of deposition time (tPt) 3.2.2.1. Effect of EPt on electrode surface morphology When the deposition time (tPt) in 50 seconds, platinum particles begin to form and are scattered on the electrode base. As the deposition time increased to 100 s, many new crystal sprouts were formed on the surface of the electrode, so the density of platinum nanoparticles was denser, but it could not cover the surface of glass charcoal. As the deposition time increases to 150 s, this time the nano platinum takes the shape of flowers, the alternating density of the flowers is thicker and evenly distributed throughout the surface. When the time increased to 200 s, 300 s, the size of platinum flowers increased and uneven, at this time the phenomenon of platinum aggregation, clustering of platinum particles formed many layers of particles, overlapping one another. 14 Figure 3.13. SEM images of PtNFs/GCE deposited at different deposition time: (a) 0 s; (b) 50 s; (c) 100 s; (d) 150 s; (e) 200 s; (f) 300 s 3.2.2.2. Effect of tPt on electrode surface composition According to the EDX spectra (EDX) of the Pt/GC electrode surface according to platinum generation time (tPt), it is shown that when platinum generation time (tPt) increases, the percentage of Pt mass on the electrode surface increases. 15 Figure 3.13. EDX spectra for Pt/GC prepared at different deposition time (tPt) 3.2.2.3. The electrochemical active area of Pt/GC electrodes fabricated at different tPt The electrochemical active area (Ahd) of Pt/GC electrodes increases with the increasing trend of tPt from 50 s to 150 s, after which the value decreases when tPt is 200 s, 300 s. Figure 3.15. Cyclic voltammograms obtained at Pt/GC prepared in different deposition time in 0.2 mol·L -1 phosphate buffer solution (PBS), pH 7.0 containing 5 mM K3Fe(CN)6 at a scan rate of 0.1 V·s -1 16 3.2.2.4. The stripping peak current intensity of Pb on Pt/GC electrodes made at tPt varies The result is that the stripping peak current (Ip) of Pb on Pt/GC electrode fabricated at 150 s is the largest 3.2.3. The effect of stirring solution on the electrode surface structure When there is a stirring solution, Pt particles form a flower- shaped structure arranged evenly on the surface of the GC electrode. Thus, the requirement for electrodes denaturation by platinum nanoparticles is to have a stirring solution to obtain the desired surface and well cover the substrate. 3.3.2. Optimization of factors affecting the signal of Cd, Pb by the univariate method 3.3.2.1. Effect of the electrolytic solution The effect of the various electrolytic solution including acetate buffer solution, Britton – Robinson buffer solution, phosphate buffer solution, KCl/HCl on the stripping peak currents of Pb, Cd are studied. Figure 3.24. DPASV curve of PtNFs /GC electrode in different electrolytic solution 17 The results showed that the peak current (Ip) of lead and cadmium in acetate buffer solution was the highest, followed by Britton-Robinson buffer, KCl/HCl and phosphate buffer. To explain this problem, we rely on the existing forms of lead, cadmium in the electrolytic solution and the ability to participate in the electrolysis of each form in each electrolyte. 3.3.2.2. Effect of solution Figure 3.26. DPASV curve of Cd, Pb at different pH Figure 3.27. Effect of pH on Ip of Cd, Pb Experimental results obtained Ip of lead, cadmium highest at pH = 4.5 and stable in the pH range = 4.5 ÷ 4.75. Therefore, to facilitate the analysis, we choose the most suitable buffer pH value for recording DPASV of Cd, Pb is 4.5. 3.3.2.3. Effect of electrochemical technologies The obtained results show that the DP-SV technique has a higher sensitivity of Cd and Pb than SW-SV (higher Ip). Therefore, we choose the DP-SV recording technique for further studies. 3.3.2.4. Effect of preconcentration time (tdep) 18 Figure 3.29. DPASV curve of Cd, Pb at different tdep Figure 3.30. Effect of tdep on Ip of Cd, Pb Therefore, to expand the linear range, give good measurement repetition and minimize the time for the analysis process as well as avoid the possible accumulation of foreign ions such as In, Zn, Cu ..., obstructing the accumulation capacity of Pb, Cd on the surface of the working electrode, so we chose 120 s as the electrolytic time (tdep) for future experiments. 3.3.2.5. Effect of preconcentration potential ( Edep) Figure 3.32. DPASV curve of Cd, Pb at different Edep Figure 3.33. Effect of tdep on Ip of Cd, Pb The lowest relative standard deviation was obtained at -1.1 V for both metals. Moreover, if the negative voltage is applied more easily the reaction of reducing the solution to form H2 gas will affect the electrode surface. Therefore, we choose Edep = -1.1 V for further studies. 19 3.3.2.6. Effect of pulse amplitude (ΔE) To satisfy the condition (sharp, beautiful peaks, low baselines), select ∆E = 60 mV and this value are chosen for further studies. 3.3.2.7. Effect of step potential (Ustep) With a good dissolution signal, at a scan speed of 0.267 V / s (Ustep = 8 mV) was selected for further surveys. 3.3.2.8. Influence of electrode cleaning To reduce the error of the method, it is necessary to perform cleaning of the electrode with two-step cleaning were better than those of the non-cleaning electrode and the one-step cleaning electrode: firstly, the voltage on the WE is set at -1.1 V for 30 s, to eliminate the fraction of the particles metal ions present at the electrode surface; Next, applying a potential of +0.2 V to the electrode for 30 seconds to dissolve the metal on the electrode surface into the solution. 3.3.2.9. Influence of interferences - Cd and Pb do not affect each other's signals - Zn, Fe has negligible influence on the signal of Cd, Pb - Cu has a significant effect on Cd, Pb signals. Overcome the effects of Cu by adding K4[Fe (CN) 6] at a concentration 5 times higher than Cu - Surfactants affect the signal of Cd, Pb. Eliminate the effect of surfactants by UV irradiation. - Some anions Cl - , SO4 2- , NO3 - have a negligible influence on the signal of Cd, Pb 3.3.3. Experimental modeling, simultaneous effects of pH, tdep, Edep and Ustep on the stripping peak current (Ip) of Cd, Pb Experimental modeling built the regression equation describing the simultaneous effect of all factors and determining the optimal conditions of simultaneous influence of pH, tdep, Edep and Ustep on Ip of Cd, Pb: pH = 4.72, tdep = 120 s, Edep = -1.14 V và Ustep = 7 mV 20 3.3.4. Investigate the durability of the electrode According to research results, after 30 days % Ip obtained maintained around 91%. Therefore the Pt/GC electrode has good stability and can be used to analyze heavy metal ions within 30 days. Figure 3.51. %Ip of Pb obtained every 2 days for 30 days Simultaneously after 50 measurements for solution containing Pb or containing simultaneously Pb, Cd is shown in Figure 3.52 for the repeatability of RSDPb = 2.16% and RSDCd = 2.12%, after 100 measurements for the repeatability of RSDPb = 6.14% (Pb), RSDPb = 5.93%, after 200 measurements for the repeatability of RSD = 12.02% (Cd), 12.83% (Pb) proves the electrode have good durability. Figure 3.52. DPASV curve examines the durability of the electrode after 50 measurements 21 3.3.5. Evaluate analytical methods 3.3.5.1. Repeatability The results of the study showed that: In the same experiment, the relative standard deviation of RSD = 1.45% for Pb and RSD = 1.59% for Cd we concluded that this measurement was good for both metals. At the same time, we also studied the reproducibility of the method with the relative standard deviation (RSD) of Cd of 4.36% and Pb of 4.65%. Compared with the maximum relative standard deviation allowed within the laboratory by the Horwitz function (RSDHorwitz = 32% with a concentration of 10 ppb), the RSD of the Cd, Pb analysis method is smaller than ½ RSDHorwitz should be internally an acceptable laboratory, ie a method with good repeatability. 3.3.5.2. Accuracy of the method The accuracy of the method is assessed by the recovery when analyzing the spiked sample. Analyze the spiked sample at 4 concentration levels 3 ppb, 5 ppb, 10 ppb, 20 ppb. The result is obtained in table 3.30. Table 3.30. Survey results of method accuracy Analyte Sample Available in the test sample (ppb) Added (ppb) Found (ppb) Recovery (%) Pb 1 8.59 3 11.32 91.00 2 5 13.88 105.80 3 10

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