3.1 Characterization of materials
The SEM images of C and C/PWA samples are exhibited in Fig. 1. It can be seen from Fig. 1a that C presents the microstructure of nanoparticle clusters. Figure 1b shows the microstructure of C/PWA at the same magnification. It can be found that the C nanoparticles modified by PWA are relatively enlarged. The full binding of PWA to C nanoparticles can exert the advantage of synergistic effects. The specific surface area of the modified C material is increased, which helps to improve the conductivity of the C nanoparticles.
Figure 2 shows the XRD results of C nanoparticles and C/PWA samples. Figure 2 shows the XRD pattern of C with a cluster-like granular microstructure. The XRD pattern of C is revealed to be consistent with the one of typical C. The XRD pattern of C/PWA nanoparticles shown in Fig. 2 shows several distinct diffraction peaks characteristic of PWA (No. JCPDS 50–0655) which indicates that PWA is present in the modifier layer.
The IR spectra of C, PWA, and C/PWA are displayed in Fig. 3. Tensile vibration maxima in the C spectra can be found at 1560 cm− 1 (C = C) and 3500 cm− 1 (O-H). Peak vibrations in the PWA spectrum can be seen at 1088 cm− 1 (P-Oa), 900 cm− 1 (W-Ob-W), and 810 cm− 1 (W-Oc-W) ( While Ob and Oc are the oxygens of the W-Ob-W bridge, which connects two distinct yet identical W3O13 groups, respectively, Oa stands for the center tetrahedral PO4 oxygen). These characteristic peaks can also be observed in C/PWA which indicates the presence of PW12O403− anions in the composite structure and confirms the successful doping of PWA.
3.2 Electrochemical profile of dopamine at modified electrodes
The responses of the same concentration of DA on three different modified electrodes, GCE, C/GCE, and C/PWA/GCE, are shown in Fig. 4. The voltammogram measured by cyclic voltammetry(CV) shows the redox situation of DA at the modified electrode. As can be seen from the figure, after the modification of GCE, it can be found that the redox current response situation is significantly increased, and GCE has enhanced its conductivity by the modification of activated carbon, while after the modification of GCE by C/PWA, the current response situation is greatly increased, indicating that the synergistic effect of C and PWA makes the electron transfer efficiency increase. Figure 5 shows the differential pulse voltammetry (DPV) of DA on GCE, C/GCE, and C/PWA/GCE. In the case of bare GCE, there was almost no current response, and a significant current response appeared through the modification of GCE by activated carbon. Last but not least, C/PWA causes a noticeable rise in peak current, showing that the electrocatalytic function of PWA makes GCE treated with C and PWA more likely to experience oxidation.
3.3 Analysis of surface area and impedance
The electrochemical behavior of the electrodes in 5.0 mM K3[Fe (CN)6]/K4[Fe (CN)6] and 0.1 M KCl solutions was investigated using the electrochemical impedance spectroscopy (EIS) technique. The Nyquist curve obtained is shown in Fig. 6. Among the three different electrodes, the charge transfer resistance of the GCE modified with C/PWA was lower than that of the C/GCE, and bare GCE electrode which were C/PWA/GCE (81.4 Ω), C/GCE (179.2 Ω), and bare GCE (371.5 Ω). This result is probably due to the increased effective surface area of the electrode and the special electrocatalytic properties of PWA, which increase the electron transfer rate[35]. Based on the aforementioned findings, it can be concluded that C/PWA/GCE exhibits more effective interfacial electron transport than bare GCE and C/GCE. The modification of the GCE electrode can be confirmed by comparing the electron transfer effect of the modified electrode with that of the bare electrode. Therefore, by obtaining cyclic voltammograms at different scan rates in K3Fe (CN)6, the Randles-Sevcik equations[9, 36] for the effective surface areas of GCE, C/GCE and C/PWA/GCE were calculated as follows:
I P = (2.69 × 105 ) n3/2 A D1/2 ν1/2 C
Ip is the peak anode current (A), n is the number of transferred electrons (n = 2), A is the effective surface area of the electrode in cm2, D is the diffusion coefficient of K3Fe (CN)6 (5×10− 6 cm2 s− 1), ν is the scan rate (V/s), and C is the concentration of K3Fe (CN)6 (mol/cm3). The slope of a plot of Ip versus ν1/2 is then used to calculate A. The calculated effective surface area values for bare GCE, C/GCE and C/PWA/GCE were 0.0554 cm2, 0.0638 cm2 and 0.1939 cm2, respectively. It is evident that C/PWA/GCE has a much higher effective surface than bare GCE by a factor of 3.5, demonstrating the superiority of its electrochemical activity.
3.4 Conditions optimization
3.4.1 pH effect
In phosphate buffer, the effects of pH on the oxidation peak current values of DA (100 M) were investigated. The results are presented in Fig. 7. Figures 7a and 7c show that the peak current increases starting at pH 6 and climbs until it reaches a maximum at pH 7.5. The next experiment used phosphate buffer solution to bring the solution's pH to 7.5. Furthermore, when the pH of the buffer solution was varied from 6 to 8, With rising pH, the maximal oxidation potential of DA moved toward being lower, indicating the involvement of protons in these oxidation processes. The linear relationship between Ep and pH is shown in Fig. 7b, and the linear equation is Ep = -0.0648pH + 0.5832 (R2 = 0.998), with the absolute value of the slope approximately equal to the theoretical value of 0.059 VpH− 1, demonstrating that at the C/PWA/GCE modified electrode, the amount of electrons and protons transported during the redox process of DA is equivalent, which is consistent with the literature[16]. Figure 8 depicts the potential electrochemical oxidation of DA at the C/PWA/GCE modified electrode.
3.4.2 Effect of C/PWA content
By altering the C/PWA content, the impact of the modifier content on the DA oxidation peak current was examined. Different masses of C/PWA (1, 2, 3, 4 and 5 mg) were dispersed in a specific volume of Nafion solution to test the impact of mass. Figure 9 illustrates the peak currents attained by the modified electrode as a result of the dispersed modifier's drop coating approach of modifying the bare GCE. The data showed that as the dispersion mass grew from 1 to 5 mg, the oxidation peak current of DA initially increased and subsequently declined. The peak current is increased because of the increase in C/PWA content and the gradual increase in the number of electrons transferred. The later peak current decreases because of the increase in C/PWA content and the thicker coating due to the drop coating method of modifying the electrode, which has a hindering effect on electron transfer, and therefore the peak current response is reduced. Therefore, 3 mg was chosen as the optimum dispersion mass in C/PWA.
3.4.3 Effect of the modification time
The C/PWA enrichment time was investigated under the conditions of optimal C/PWA dispersion mass, and the results of the investigation are shown in Fig. 10. From the analysis of the graph, it is known that when the enrichment time increases from 3 min to 7 min, the peak current reaches a higher level at 4 min, so 4 min is chosen as the best enrichment time.
3.5 Electrochemical analysis of the modified electrodes
3.5.1 Analysis of scan rate
The effect of the scan rate on the peak current and peak potential of C/PWA/GCE in the presence of DA concentration of 100 µM was tested using cyclic voltammetry under the above optimal test conditions. The CV for different scan rates (20-400mVs− 1) were tested as shown in Fig. 11a. As the scan rate increases from 20mVs− 1 to 400mVs− 1, the peak anode and cathode currents of the DA also increase. It was shown that the peak current increases linearly with the square root of the sweep speed (ʋ1/2) (Fig. 11b). The linear regression equations are Ιpa (µA) = 17.88ʋ1/2-89.05 and Ιpc (µA) = -16.54ʋ1/2+94.36 with a 0.996 correlation coefficient, which indicates that the electron transfer process is diffusion controlled[13].
3.5.2 Chronoamperometric studies
The diffusion coefficient (D) of DA in the electrochemical process was determined using chronoamperometric measurements. In a phosphate buffer solution containing a specific concentration of DA, a constant voltage of 1 V was supplied to the modified working electrode, and its current was measured as a function of time. Since the mass transfer process in the solution is completely controlled by diffusion, the generated I-t curve displays changes in the gradient of concentration close to the electrode surface. This situation entails the diffusion layer gradually expanding as the electroactive species are gradually removed. Consequently, the Cottrel equation can be used to express the current's strength as follows:
\(\)\(I=\frac{nFAC{D}^{1/2}}{{\pi }^{1/2}{t}^{1/2}}=K{t}^{-1/2}\)
I is the current (A), C is the volume concentration (mol cm− 3), A is the electrode surface area (cm2), t is the time (s), and D is the diffusion coefficient (cm2 s− 1). I equals the current (A), C equals the volume concentration (mol cm− 3), A equals the electrode surface area (cm2), t equals the time (s), and D equals the diffusion coefficient (cm2 s− 1). Figure 12 shows the timing current plots of C/PWA/GCE for three different concentrations of DA (50, 100 and 150 µM) at potential steps of 1 V. The related I vs. t− 1/2 plots are displayed in inset a. The slope of these plots was used to extract the diffusion coefficients. For this purpose, the slope of the resulting line is depicted as a function of DA concentration (inset b). According to the resulted slope, the D of DA is calculated to be equal to 7.57 × 10− 7 cm2 s− 1.
3.6 Analytical performances
By measuring DA under ideal experimental conditions using C/PWA/GCE in phosphate buffer solution and the DPV method, the calibration curve for the modified sensor was created. Figure 13 shows the voltammograms obtained for the different concentrations. According to the calibration curve (inset Fig. 13), the current is linear with concentration from 10 × 10− 6 to 600 × 10− 6 M (R2 = 0.9988) and the linear equation is Ip (µA) = 0.42059C (µM) + 196.315. The detection limit (S/N = 3) for 5 replicate measurements of the blank solution is 0.1 × 10− 6M.
3.7 Reproducibility and repeatability
The C/PWA/GCE sensor's repeatability measurements for five consecutive measurements are shown in Fig. 14a. The associated peak current measurements display an RSD value of 0.56%, which indicates that the sensor has good repeatability. The reproducibility measurements of the C/PWA/GCE sensor are given in Fig. 14b. Five different C/PWA/GCE sensors were manufactured by the same fabrication method and the same test method was used to obtain the peak currents under the same conditions. These several DA sensors' peak current responses exhibit good agreement and minimal fluctuation. These peak current measurements had an RSD value of 1.1%, demonstrating the C/PWA/GCE sensors' strong repeatability.
3.8 Interference effects
The effect of ions that may have the ability to interfere during DA measurements, which are usually components of pharmaceutical or biological media, was examined. Interference effects were examined by measuring 100 µM DA solutions containing various concentrations of foreign substances under optimal conditions. The data obtained showed no significant effect of Na+, Ca2+, Mg2+, K+, Cl¯, NO3−, PO43−, CO32− ions (500-fold concentration) as well as uric acid, ascorbic acid and glucose. The results showed a good selectivity of C/PWA/GCE.
3.9 Analytical applications
To evaluate the practicality of the proposed sensor, the concentration of DA in dopamine hydrochloride injection was determined by this sensor. The purchased dopamine hydrochloride injection (10 mg/ml) from Shanghai He Feng Pharmaceutical Co., Ltd. was diluted to a certain concentration and assessed under the optimal experimental conditions with the addition of 50 µM and the detection amount of 51.2 ± 1.8 µM with the recoveries of 98.8%-105% and the relative standard deviations less than 5%. This indicates that C/PWA/GCE has good prospects for use in the detection of DA.
3.10 Comparison of the sensors in this study with those in the literature
The linear range, detection limit, and sensitivity of the developed electrochemical sensor (as three important analytical factors) and previously reported linear range, detection limit, and sensitivity for measuring DA are shown in Table 1. The given data demonstrate that the constructed sensor is superior to other sensors in determining the detection range of DA. The better properties of this sensor can be attributed to the combination of the excellent electrocatalytic properties of polyoxometalates and the electrochemical modification properties of activated carbon, which have higher sensitivity than other modifiers in the modified electrodes.
Table 1
Comparison of different modified sensors for DA detection.
Electrode/modifier | Linear range (µM) | Detection limit (µM) | Ref. |
GR/GCE | 4.0-100.0 | 2.64 | [9] |
1PdNPs/CNF/CPE | 0.5–160 | 0.2 | [37] |
gold nanosheets/GCE | 2-298 | 0.28 | [16] |
Pd@Fe3O4/GCE | 0.96–107 | 0.41 | [11] |
MWCNTs/GCE | 3-200 | 0.8 | [8] |
C/PWA/GCE | 10–600 | 0.1 | This work |