Results and discussion

Electrochemical behaviour of doxorubicin at the Zn-Ni MOF NSs@GO/SPCE

The oxidation of DOX in PBS solution depends on pH. CV measurements were conducted to investigate how different pH levels in 0.1 M PBS, ranging from 2.0 to 9.0, affected DOX oxidation at the Zn-Ni MOF NSs@GO/SPCE. The CV data show that DOX exhibits its highest anodic peak current at pH 7.0, which decreases as pH increases. Therefore, PBS buffer at pH 7.0 was identified as the optimal pH for DOX detection using the Zn-Ni MOF NSs@GO/SPCE system.

The Zn-Ni MOF NSs@GO/SPCE showed better sensing performance for DOX 100.0 μM when tested against the bare SPCE electrode (Figure 1). The Zn-Ni MOF NSs@GO/SPCE exhibits separate oxidation and reduction peaks at 400 and 240 mV, while the SPCE bare displays a broad oxidation peak that peaks at 470 mV with a reduction peak at 200 mV. The Zn-Ni MOF NSs@GO nanocomposite shows catalytic activity, evidenced by increased DOX oxidation peak current and a shift of the oxidation peak potential towards more negative values.

Figure 1.: CV curves of 100.0 μM DOX in 0.1 M PBS (pH 7.0) by bare SPCE (red curve) and Zn-Ni MOF NSs@GO /SPCE (black curve) at scan rates of 50 mV s-1

Influence of scan rate

In CV, the relationship between peak current and scan rate (v) typically provides useful information about the electrochemical process. Consequently, CV was used to examine how scan rates affected the DOX peak currents at the Zn-Ni MOF NSs@GO/SPCE in 0.1 M PBS (pH 7.0) (Figure 2).

Figure 2.: Zn-Ni MOF NSs@GO/SPCE CV curves of 30.0 μM DOX in 0.1 M PBS (pH 7.0) at different scan rates from 10 to 400 mV s-1. Inset: The linear relationship between redox currents (Ipa and Ipc) and the v1/2

Consequently, Figure 2 also displays the electrochemical behaviour of 30.0 μM DOX at various scan rates ranging from 10 to 400 mV s-1. The scan rates and the peak current heights (_I_pa and _I_pc) have a strong linear relationship. Additionally, there is a minor variation in the peak potentials (both _E_pa and _E_pc) with potential scan rates. Figure 2 (Inset) shows a linear relationship between the square root of the scan rate (_v_1/2) and peak current heights (_I_pa and _I_pc). This suggests that a diffusion mechanism governs DOX redox process at the Zn-Ni MOF NSs@GO/SPCE.

Chronoamperometric studies

By setting the working electrode potential at 440 mV for the different concentrations of DOX (0.1 to 1.0 mM) in 0.1 M PBS (pH 7.0), chronoamperometric analyses of DOX at Zn-Ni MOF NSs@GO/SPCE were carried out (Figure 3). The Cottrell equation (I = nFAD_1/2_C_b_π-1/2_t_-1/2) describes the current measured for the electrochemical reaction under the mass transport constrained condition for an electroactive substance (in this example, DOX) with a diffusion coefficient of D. The best fits for various DOX doses were found using experimental plots of _I_p vs. t-1/2 (Figure 3A). Plotting the slopes of the obtained straight lines against DOX concentration was the next step (Figure 3B). From the resulting slope and Cottrell equation, the mean value of the D was found to be 6.24×10-5 cm2 s-1.

Figure 3.: Chronoamperograms for various DOX concentrations ranging from 0.1 to 1.0 mM acquired at Zn-Ni MOF NSs@GO/SPCE in 0.1 M PBS (pH 7.0). (A) Plots of I versus t-1/2 is shown as an inset. (B) A plot of the straight-line slopes against the concentration of DOX

Quantitative measurements of doxorubicin at Zn-Ni MOF NSs@GO /SPCE sensor using differential pulse voltammetry

The DPV response of the Zn-Ni MOF NSs@GO/SPCE sensor to DOX was examined by varying the DOX concentration from 0.004 to 190.0 μM. The calibration plot shows a linear segment from 0.004 to 190.0 μM, and Figure 4 shows that the peak current increases with DOX concentration. The related linear regression equation is _I_pa = 0.1705 _C_DOX + 0.7944 (_R_2 = 0.9998), with a sensitivity of 0.1705 μA μM-1. The limit of detection (LOD) was determined to be 0.001 μM using the formula LOD = 3_S_b/m, where _S_b is the blank standard deviation and m is the calibration curve's slope.

Figure 4.: Zn-Ni MOF NSs@GO/SPCE in PBS (0.1 M; pH 7.0) with varying DOX concentrations (0.004 to 190.0 μM by DPVs. Plot of the Ipa vs. DOX concentrations is shown in the inset

Application of the Zn-Ni MOF NSs@GO /SPCE platform for doxorubicin analysis in real sample

To verify the use of Zn-Ni MOF NSs@GO/SPCE, the suggested sensor's applicability and reliability in real samples were examined. The DPV technique was used to determine DOX in DOX injection samples after sample preparation and appropriate dilution. Table 1 provides a summary of the analysis's findings. The test recoveries fell between 97.1 and 102.0 % categories. Zn-Ni MOF NSs@GO/SPCE may be effectively employed to determine DOX in doxorubicin injection samples, according to the recovery findings shown in Table 1.

DOX concentration, μM | Recovery, % | RSD, %
Added | Found
0 | 2.9 | - | 3.3
2.0 | 5.0 | 102.0 | 1.9
4.0 | 6.7 | 97.1 | 2.2
6.0 | 9.0 | 101.1 | 2.7
8.0 | 10.8 | 99.1 | 2.3