To study the electrochemical activity of GO/AuNPs/MCPE compared to the unmodified electrode, square wave voltammetric (SWV) responses in the presence of uric acid and tyrosine solution with concentrations of 19.05 µmol L-1 and 23.2 µmol L-1 in 0.2 mol L-1 phosphate buffer solution with pH 2.0 and prepotential of 0.42 mv was investigated (Fig. 1).
The voltammetric response of the unmodified electrode is shown in Figure (1-a), which shows how the electrode reacts to the solution containing analytes. It is noted that the electrode's response to analytes is relatively low, and a peak of low current is observed, which indicates that it is not efficient. The voltammogram in Figure (1-b) illustrates the response of the modified electrode, indicating that the peak current of the modified electrode is increased compared to the voltammogram in Figure (1-a). An investigation of the effects of different factors on the simultaneous determination of tyrosine and uric acid was conducted in order to optimize and obtain the best possible response of the electrode.
The effect of pH
The pH of the environment is one of the most important factors in electrochemical investigations and can have a significant impact on electrochemically determining tyrosine and uric acid oxidation currents. In this analysis, the effect of pH on peak potential is analyzed to determine the protons versus electrons ratio. Therefore, the effect of pH on uric acid and tyrosine oxidative behavior was evaluated separately by linear sweep voltammetry (LSV) technique in 0.1 mol L-1 phosphate solution for the modified electrode at concentrations of 14.15 µmol L-1 and 48.70 µmol L-1 in the pH range of 1.0–7.0. Figure 2 and Fig. 4S illustrate the results.
The pH affects the current and peak potential of uric acid, as shown in Figure (2-b, c). Eq. (1) indicates a negative shift in anodic peak potential with an increase in pH, which indicates deprotonation during oxidation is facilitated at higher pH, as indicated by the slope of -0.053 V. According to the slope value, the same number of protons and electrons participate in the tyrosine oxidation reaction, which is consistent with previous studies [33, 34].
Epa(V) = -0.053 pH + 0.7007 (R2 = 0.9933) Eq. (1)
As can be seen from Figure (4S-b, c) that tyrosine's response current and peak potentials are both affected by pH. As pH rises in the buffer solution, the anodic peak potential values decrease, indicating the participation of protons in the tyrosine It oxidation process. Eq. (2) shows a negative shift in the anodic peak potential with a slope of -0.0638 V. The slope obtained from Eq. (2) and its comparison with the theoretical value of -0.06 (m/n), where “m” is the number of protons and “n” is the number of electrons participating in the reaction, shows that the number of protons participating in the tyrosine oxidation reaction is equal to the number of electrons, in agreement with previous reports [35, 36].
Epa(V) = -0.0638 pH + 1.0407 (R2 = 0.9816) Eq. (2)
The oxidation peak current for tyrosine and uric acid is maximum at pH 2.0 and pH 3.0, respectively, and decreases as the pH of the current environment increases. These curves also show two peaks for tyrosine and uric acid, indicating two pka. For simultaneous determination of tyrosine and uric acid on the surface of the modified electrode, all voltammetric measurements were conducted in phosphate buffer solution with pH 2.0 as carrier electrolyte. On the surface of the modified electrode, the proposed mechanism for oxidizing tyrosine and uric acid can be seen in Figure (5S-A, B).
The effect of scan rate
As part of the investigation of the effect of potential sweep speed on uric acid and tyrosine electrochemical behavior on the surface of the modified electrode, the LSV was recorded for each of the solutions of uric acid and tyrosine in 0.1 mole L-1 phosphate buffer solution with pH 2.0, containing concentrations of 3.46 and 29.7 µmole L-1, respectively, with scanning rates ranging from 50 to 130 milliseconds per second and 10 to 100 milliseconds per second, respectively. The voltammograms (8-a) and (9-a) show that the anodic current increases with an increase in scanning rate, as can be seen in the voltammograms (Fig. 6S-a) and (Fig. 7S-a). Based on Figures (6S-b) and (7S-b) as well as the results obtained from the analysis of the graph of changes in anodic peak current versus potential sweep speed, and the graph of logarithmic anodic peak current versus potential sweep rate in Figures (6S-c) and (7S-c), the mechanism of oxidation of uric acid and tyrosine is influenced by absorption, according to the results of the analysis.
The effect of accumulation time
In each measurement, the modified electrode was immersed in a solution containing tyrosine and uric acid in order to study the effect of accumulation time on the anodic peak current. According to the results of this experiment, changing the accumulation time did not have any effect.
The effect of pre-potential time
It also investigated whether the time at which the prepotential is applied affects the electrochemical response. An electrode with GO/AuNPs/MCPE modified was exposed to 0.42 mv of prepotential at different times in 0.1 mol L-1 phosphate buffer solution at pH 2.0 in the presence of 79.42 µmol L-1 uric acid. The square wave voltammetry technique showed an increase in the current of the electrode between 5 and 10 s, but no significant change was found beyond 10 s. It was therefore determined that 10 s would be the ideal time to apply the pre-potential (Fig. 8S).
Electrochemical impedance spectroscopy (EIS) measurement
In order to investigate the surface resistance of modified electrode (GO/AuNPs/MCPE), the electrochemical impedance spectroscopy response in the presence of [Fe(CN)6]-4/-3 couple as a redox probe was evaluated using the unmodified electrode and modified electrode (Fig. 9S).
Measurement of carbon paste electrode surface
We measured electrode areas by LSV using [Fe(CN)6]-4/-3 as the probe in 0.2 mol L-1 phosphate buffer solution with pH 7.0 and different scanning rates. Randles-Sevcik equation is used for this reaction at T = 298 K.
𝐼𝑝=2.69×105 n3/2 𝐴 D01/2 C0* 𝑣1/2 Eq. (3)
Eq (3) deals with the "anodic peak current," the electron transfer (n = 1), the electrode surface, the diffusion coefficient, the scanning velocity, and the concentration of Fe(CN)6-4/-3.
Using 0.07 µmol L-1 solution of [Fe(CN)6]-4/-3, the surface of the electrode in 0.2 mol L-1 phosphate buffer solution, T = 298 K, C0*=7×10− 4 M, D = 5.0810-6 cm2mol-1, n = 1, F = 96485 Cmol-1, R = 8.314 jk-1 mol-1 was obtained. The surface area of the unmodified electrode and the modified electrode was 0.129 cm2 and 0.190 cm2, respectively, using Eq. (3).
Repeatability
The square wave voltammograms of the modified electrode was performed using 4 consecutive tests using a concentration of 44.9 µmol L-1 uric acid and 45.3 µmol L-1 tyrosine solution in 0.2 mol L-1 phosphate buffer solution at pH 2.0 at a scanning rate of 100 mv/s for evaluating its repeatability behavior. Tyrosine had a relative standard deviation of 3.51% and uric acid had a relative standard deviation of 5.77%.
Reproducibility
The same synthesis procedure was used to reproduce the oxidation current of three different modified electrodes (To prepare the electrode, gold nanoparticles-modified graphene oxide (8% w/w), graphite powder, and ecosan as an adhesive were mixed for 20 minutes at 40°C to obtain a homogenous mixture). Then, it was transferred to a Teflon tube with an inner diameter of 5 mm and an outer diameter of 2 mm, and its connection was established via a copper wire located in the middle of the Teflon tube) was calculated in uric acid and tyrosine solution with a concentration of 44.9 µmol L-1 and 45.3 µmol L-1 in 0.2 mol L-1 phosphate buffer solution, pH 2.0, and 0.1 mol L-1 potassium chloride at a scanning rate of 100 mv/s. Calculating the standard deviation, we found that tyrosine had a relative standard deviation of 5.13% and uric acid had a relative standard deviation of 8.42%, respectively.
Calibration curve and Figures of merit
Following an evaluation of the parameters affecting the determination of tyrosine and uric acid, these two analytes were analyzed simultaneously, and a calibration curve was plotted. In order to accomplish this, uric acid and tyrosine solutions were prepared with different concentrations (0.06–141.00 µmol L-1 uric acid, 0.14–340.00 µmol L-1 tyrosine) in 0.2 mol L-1 phosphate buffer solution, pH 2.0, and 0.1 mol L-1 potassium chloride at a scanning rate of 100 mv/s. A SWV technique was then used to determine the optimal conditions for using the modified carbon paste electrode. Considering the high sensitivity of this method, the SWV method was used to determine the limit of detection and linear range for the simultaneous determination of both tyrosine and uric acid. Tyrosine and uric acid concentrations are clearly related to the anodic peak current, and the peak current increases as tyrosine and uric acid concentrations increase. In Figure (3), you can see the SWV for the simultaneous evaluation of tyrosine and uric acid. The concentration range of 0.14–340.00 µmol L-1 for tyrosine (Figure (3-b)) and the concentration range of 0.06–141.00 µmol L-1 (Figure (3-c)) for uric acid were obtained. The Method detection limit (MDL) of tyrosine and uric acid was obtained 0.0060 µmol L-1 and 0.0037 µmol L-1, respectively, and the equation of line for each of them is given as Eqs. (4) and (5) below.
IµA = 0.39843 CM-1.293 R2 = 0.9958 (Eq. 4)
IµA = 1.2813 CM-3.4023 R2 = 0.9844 (Eq. 5)
As a measure of the efficiency of the electrochemical method with the proposed electrode, the limit of detection and linear range obtained by this electrode was compared with some previously published literature (Table 1). In this comparison, it was found that the sensor prepared using the proposed method was highly efficient and cost-effective.
Application of the proposed sensor in real sample
Real samples were used to evaluate the efficiency and reliability of this MCPE. As part of this process, the urine sample was prepared in 0.2 mol L-1 phosphate buffer solution and 0.1 mol L-1 potassium chloride with pH 2.0, considering the DLR in simultaneous determination. Specific amounts of tyrosine and uric acid were added to the sample, and an SWV technique was utilized to record the voltammogram (Fig. 4-a). The plot of concentration versus flow indicates that this method has a detection limit of 0.0067µ moles L-1 and 0.001 µmole L-1, a correlation coefficient of 98.35% and 97.79%, and a sensitivity of 0.3117 and 0.6589 for tyrosine and uric acid, respectively, in real urine samples (Fig. 4-b,c). Based on these results, the proposed method is applicable to the analysis of biological samples.
Investigating the possible interference effect of species
This method was tested in different matrixes in order to investigate its selectivity and efficiency. A quantitative study was conducted using solutions containing uric acid and tyrosine with concentrations of 0.014 mol L-1 and 0.07 mol L-1, respectively, as well as different concentrations of interfering species in pH 2.0 phosphate buffer solution 0.2 mol L-1 and 0.1 mol L-1 potassium chloride at Table (1S), the results confirm the feasibility of using the proposed method for both tyrosine and uric acid determinations.
Table 1. Comparison of the proposed method with other methods.
Analyte
|
Analytical
method
|
DLR
|
MLD
|
reference
|
|
DPV
|
30.0-150.0 µM
|
0.01 µM
|
[37]
|
Tyrosine
|
DPV
|
0.9-95.4 µM
|
0.19 µM
|
[38]
|
|
DPV
|
0.1-400.0 µM
|
0.046 µM
|
[39]
|
|
SPE1
|
13.70-303. 50 µM
|
3.860 µM
|
[40]
|
|
Fluorescence
|
0.07-230.0 µM
|
0.034 µM
|
[41]
|
|
Fluorescence
|
0.5-35.0 µM
|
0.370 µM
|
[42]
|
|
SWV
|
0.14-340.0 µM
|
0.006 µM
|
This work
|
|
DPV
|
0.01-100 µM
|
0.032 µM
|
[39]
|
Uric acid
|
SWV
|
0.60-100.0 µM
|
0.13 µM
|
[43]
|
|
DPV
|
1.00-680 µM
|
0.090 µM
|
[44]
|
|
UV-Visible
|
100.0-450.0 µM
|
38.40 µM
|
[45]
|
|
HILIC2
|
1.189-59.48 µM
|
0.356 µM
|
[46]
|
|
SWV
|
0.06-141.0 µM
|
0.0037 µM
|
This work
|
1Solid-phase extraction
2Hydrophilic interaction liquid chromatographic