Results and discussion

Electrochemical behaviour of dopamine at the Co/Ni-MOF/SPCE

One of the primary experimental factors influencing the voltammetric response of DA at Co/Ni-MOF/SPCE is the pH of the supporting electrolyte. Therefore, to enhance the strength of redox peak currents, an optimal pH is required. Therefore, using the differential pulse voltammetry (DPV) technique, the voltammetric responses of DA in 0.1 mol L-1 PBS were examined over the pH range from 2.0 to 9.0. The results show that the current signal of DA at Co/Ni-MOF/SPCE increases with increasing pH from 2.0 to 7.0, then decreases as the pH rises further. As a result, pH 7.0 was determined to be the ideal pH for further research.

Cyclic voltammetry (CV) was used to study the electrochemical behaviour of DA on both the bare SPCE and a Co/Ni-MOF/SPCE in a 0.1 mol L-1, pH 7.0 PBS solution at a scan rate of 50 mV s-1 (Figure 2). The bare SPCE displays reversible behaviour for DA, as evidenced by small redox current peaks at _E_pa (anodic peak potential) = 210 mV and _E_pc (cathodic peak potential) = 75 mV. The Co/Ni-MOF/SPCE shows two distinct redox waves for DA, with _E_pa at 180 mV and _E_pc at 102 mV, as shown in Figure 2. The over-potential of DA decreases to less than that of the bare SPCE through an additional potential shift of 30 mV.

Figure 2.: CV curves of 200.0 μmol L-1 DA in 0.1 mol L-1 PBS (pH 7.0) by bare SPCE and Co/Ni-MOF/SPCE at scan rates of 50 mV s-1

Also, the redox peak currents at Co/Ni-MOF/SPCE are higher than those at the bare SPCE. These results indicate that the modification of the SPCEs with Co/Ni-MOF greatly enhanced the electrochemical behaviour of the electrode for the analysis of DA.

Influence of scan rate

The study used CV to analyse how different scan rates (v) affected the oxidative and reductive peak currents of DA (100.0 μmol L-1) at Co/Ni-MOF/SPCE. The anodic and cathodic peak signals (_I_pa and _I_pc), which were directly proportional to the scan rate over the range of 10 to 350 mV s-1, increased with the scan rate, as shown in Figure 3. The cathodic peak potentials shift in a negative direction, whereas the anodic peak potentials progressively move in a positive direction. The linear regression equation for the peak current and the square root of the scan rate (_v_1/2) is as follows: _I_pa = 1.2215 v 1/2- 1.0459 (_R_2 = 0.9993) and I_pc = 1.2887_v 1/2 + 1.3098 (_R_2 = 0.999). The results indicated that the electrochemical redox reaction of DA at the Co/Ni-MOF/SPCE was diffusion-controlled.

Figure 3.: CV curves of 100.0 μmol L-1 DA in 0.1 mol L-1 PBS (pH 7.0) by Co/Ni-MOF/SPCE at various scan rates from 10 to 350 mV s-1. Insets: (A) linear relationship between Ipa and v-1/2; (B) linear relationship between Ipc and v-1/2

Chronoamperometric studies

The working electrode potential was adjusted to 0.4 V for the different concentrations of DA (0.1 to 1.5 mmol L-1) in 0.1 mol L-1 PBS (pH 7.0) to perform chronoamperometric measurements of dopamine at Co/Ni-MOF/SPCE (Figure 4).

Figure 4.: Chronoamperograms obtained at Co/Ni-MOF/SPCE in 0.1 mol L-1 PBS (pH 7.0) for different concentrations of DA from 0.1 to 1.5 mmol L-1. Insets: (A) plots of I vs. t−1/2 obtained from chronoamperograms; (B) plot of the slope of the straight lines against DA concentration

The Cottrell equation (_I = nFAC_b_D1/2π-1/2_t-1/2) describes the current measured for the electrochemical reaction at the mass transport-limited condition for an electroactive material (DA) with a diffusion coefficient of D. _I_pa vs. t−1/2 experimental plots were used, and Figure 4A shows the best fits for various DA doses. Next, the slopes of the generated straight lines were plotted against DA concentration (Figure 4B). The mean value of the D was determined to be 5.95×10-6 cm2 s-1 based on the Cottrell equation and its associated slope.

Quantitative measurements of dopamine at Co/Ni-MOF/SPCE sensor using DPV method

DA was oxidized at a Co/Ni-MOF/SPCE electrode in 0.1 mol L-1 PBS (pH 7.0) by adjusting its concentration between 0.01 and 660.0 μmol L-1. The plot of _I_pa vs. DA concentration is shown in Figure 5 and indicates a linear relationship. It also indicates that peak current increases as DA concentration rises. _I_pa = 0.065_C_DA + 1.018 (R = 0.9999) is the linear fitted equation, and the linearity is found in the range of 0.01 to 660.0 μmol L-1. The LOD was computed using the formula: LOD = 3_S_b m-1, where m is the calibration curve's slope and _S_b is the blank's standard deviation. The LOD for the Co/Ni-MOF/SPCE sensor was 0.007 μmol L-1 based on the standard deviation of ten repeat readings (n = 10).

Figure 5.: DPVs accepted Co/Ni-MOF/SPCE in PBS (0.1 mol L-1; pH 7.0) containing diverse concentrations of DA (from 0.01 to 660.0 μmol L-1). Inset: plot of the Ipa vs. DA concentrations

The simultaneous determination of dopamine and tyrosine at Co/Ni-MOF/SPCE sensor

The simultaneous determination of DA and Tyr was the primary focus of this investigation. The DPVs for the DA and Tyr combination on Co/Ni-MOF/SPCE in 0.1 mol L-1 PBS (pH 7.0) are shown in Figure 6 after the concentrations of DA and Tyr were changed synchronously. With a peak difference of 150 mV, the current responses to the oxidation of Tyr (at 330 mV) and DA (at 180 mV) were shown to grow linearly, with _R_2 = 0.9982 and 0.9999, respectively. The Co/Ni-MOF/SPCE's sensitivity to the oxidation of DA in the presence of Tyr was found to be approximately 0.0648 μA L μmol-1, which was very close to the obtained value (0.065 μA L μmol-1) in the absence of Tyr. This suggests that the oxidation of these compounds on the Co/Ni-MOF/SPCE is independent, enabling simultaneous determination of their mixtures without significant interference.

Figure 6.: DPVs accepted Co/Ni-MOF/SPCE in PBS (0.1 mol L-1; pH=7.0) containing diverse concentrations of DA (from 1.0 μmol L-1 to 600.0 μmol L-1) and Tyr (from 1.0 μmol L-1 to 600.0 μmol L-1). Insets: A) plot of the Ipa versus. DA concentrations. B) Plot of the Ipa versus. Tyr concentrations

Application of the Co/Ni-MOF/SPCE platform for dopamine and tyrosine analysis in real sample

To assess the use of the Co/Ni-MOF/SPCE sensor for determining DA and Tyr in real samples (DA injection and urine), analytical experiments were conducted. Table 1 displayed the findings. Relative standard deviations (RSDs) ranged from 1.9 to 3.5 %, while recoveries ranged from 97.1 to 102.5 %. These findings show that the sensor developed in this work can detect DA and Tyr in real samples with high sensitivity and selectivity.

Sample | Added concentration, μmol L-1 | Found concentration, μmol L-1 | Recovery, % | RSD, %
DA injection | DA | Tyr | DA | Tyr | DA | Tyr | DA | Tyr
0 | 0 | 2.9 | - | - | - | 3.3 | -
2.0 | 5.0 | 4.8 | 5.1 | 98.0 | 102.0 | 2.7 | 2.6
4.0 | 7.0 | 7.0 | 6.8 | 101.4 | 97.1 | 1.9 | 3.5
Urine | 0 | 0 | - | - | - | - | - | -
5.5 | 6.0 | 5.6 | 5.9 | 101.8 | 98.3 | 3.4 | 2.1
7.5 | 8.0 | 7.3 | 8.2 | 97.3 | 102.5 | 2.2 | 3.0