Results and discussion

Characterization of NiO nanostructures

The XRD pattern of NiO nanostructures is exhibited in Figure 1. The observed peaks in the XRD pattern of NiO nanostructures located at 2_θ_ = 37.1, 43.3, 62.6, 75.5 and 79.3° can be indexed as (111), (200), (220), (311) and (222) to a cubic structure of NiO (JCPDs No. 01-075-0197).

Figure 1.: XRD pattern of NiO nanostructures

Electrochemical behaviour of epinephrine at the NiO/screen-printed carbon electrode

The impact of pH variation within the buffer solution on the peak current response during EP detection was examined. This study employed a NiO/SPCE over a pH range of 2.0 to 9.0, using DPV for the measurements. Based on the findings, the anodic peak current (_I_pa) increased with elevated pH, reaching its maximum at pH 7.0, beyond which it declined with further increases in alkalinity. Accordingly, all subsequent experimental procedures were conducted with EP solutions formulated in 0.1 mol L-1 PBS at pH 7.0.

An unmodified SPCE (a) and NiO/SPCE (b) were utilized for the electrochemical evaluation of EP. CV measurements were conducted with a sweep rate of 50 mV s-1 in 0.1 mol L-1 PBS at pH7.0. The Cu concentration of 100.0 μmol L−1 EP in 0.1 mol L−1 PBS at pH 7.0 is presented in Figure 2, which compares the responses of the unmodified electrode and NiO/SPCE, and shows the CV data for this comparison along with some results from a modified electrode control experiment. A noticeable oxidation peak at 450 mV was found for the unmodified SPCE electrode with EP concentration of 100.0 μmol L−1 In sharp contrast, the NiO/SPCE exhibited only a well-defined oxidation peak at 370 mV. The respective _I_pa measured were 5.7 μA and 12.0 μA for unmodified SPCE and NiO/SPCE, respectively. This notable increase in _I_pa at the NiO/SPCE is probably attributable to the larger electroactive surface area afforded by the presence of nanostructured NiO.

Figure 2.: CV curves of 100.0 μmol L-1 EP in 0.1 mol L-1 PBS (pH 7.0) by bare SPCE and NiO/SPCE at scan rates of 50 mV s-1

Influence of scan rate

To examine the effect of scan rate, cyclic voltammograms of the NiO/SPCE were recorded at scan rates (v) ranging from 10 to 450 mV s-1 (Figure 3).

Figure 3.: CV curves of 100.0 μmol L-1 EP in 0.1 mol L-1 PBS (pH 7.0) by NiO/SPCE at diverse scan rates from 10 mV s-1 to 400 mV s-1.; inset: the linear relationship between Ipa and the v-1/2

A positive shift in the peak potential was observed with increasing scan rate. The anodic peak current (_I_pa) linearly increased with an increase in the square root of scan rate (v-1/2) and followed the following calibration equation: _I_pa = 1.2713_v_1/2 + 3.0465 with _R_2 = 0.9998 (Inset Figure 3). In conclusion, the linear relationship between _I_pa and v-1/2 indicates that the electrochemical reaction is governed by a diffusion-controlled electron-transfer process.

Chronoamperometric studies

The chronoamperometric analysis was performed by holding the potential of the NiO/SPCE sensor at 420 mV for a solution containing 0.1-1.0 mmol L-1 EP in 0.1 mol L-1 PBS at pH 7.0, as illustrated in Figure 4. The current responses (_I_pa) for the diffusion-controlled electrochemical oxidation of the electroactive species (EP) were analysed using Cottrell's equation.

Figure 4.: Chronoamperograms received at NiO/SPCE in PBS (pH 7.0; 0.1 mol L-1) for diverse concentrations of EP from 0.1 to 1.0 mmol L-1; insets: A) plots of Ipa against t−1/2; B) graph of the linear plot slopes against EP concentration

As shown in this Equation, the parameter D / cm2 s-1 represents the diffusion coefficient of the target analyte, while _C_b / mol mL-1 denotes its bulk concentration. By plotting _I_pa against the inverse square root of time (t⁻1/2), a linear relationship was derived from the chronoamperometric data for various concentrations of EP, as shown in Figure 4A. Subsequently, the slopes of these linear plots were graphed as a function of EP concentration, as presented in Figure 4B. Consequently, the diffusion coefficient for EP was calculated to be 4.0×10-5 cm2 s-1.

Quantitative measurements of epinephrine at NiO/screen-printed carbon electrode sensor using differential pulse voltammetry

A calibration curve for EP in PBS (pH 7.0; 0.1 mol L-1) was constructed using DPV measurements with the NiO/SPCE under optimized conditions. Figure 5 displays representative DPVs recorded at various EP concentrations. The relationship between _I_pa and EP concentration demonstrated that the _I_pa scaled linearly with increasing EP concentration. The DPV results also indicated a linear response for EP over the range of 0.01 to 400.0 μmol L-1, as represented by the calibration equation _I_pa = 0.1167_C_EP + 0.5278. The LOD was determined to be 0.005 μmol L-1.

Figure 5.: DPV responses obtained at the NiO/SPCE in PBS (pH 7.0; 0.1 mol L-1) containing various concentrations of EP (0.01 to 400.0 μmol L-1); inset: plot of the Ipa vs. EP concentrations

The simultaneous determination of epinephrine and acetaminophen at NiO/screen-printed carbon electrode sensor

Simultaneous detection of EP and AC was performed using DPV in PBS (pH 7.0, 0.1 mol L-1) with the NiO/SPCE sensor. As illustrated in Figure 6, two clearly separated and well-defined oxidation peaks corresponding to EP and AC were observed. The peak currents for both analytes increased linearly with concentration, indicating the absence of significant interferences between them. As shown in Figure 6, two distinct peaks were observed at about 360 mV and 545 mV in the DPV signals for EP and AC, respectively. A peak separation of 185 mV between EP and AC enables their simultaneous detection via DPV. The sensitivity of the NiO/SPCE sensor for the oxidation of EP in a mixture containing both compounds was determined to be 0.1169 μA L μmol-1 (Inset A). This value is very close to the sensitivity measured for EP alone (0.1167 μA L μmol-1), indicating minimal interference from AC. This result confirms the feasibility of simultaneously quantifying EP and AC using the NiO/SPCE sensor.

Figure 6.: DPVs accepted NiO/SPCE in PBS (pH 7.0; 0.1 mol L-1) containing diverse concentrations of EP (from 0.5 μmol L-1 to 400.0 μmol L-1) and AC (from 0.8 μmol L-1 to 500.0 μmol L-1). Insets: A) plot of the Ipa versus. EP concentrations. B) Plot of the Ipa versus. AC concentrations

Application of the NiO/screen-printed carbon electrode platform for epinephrine and acetaminophen analysis in real water sample

The practical utility of the NiO/SPCE sensor for detecting trace amounts of EP and AC in real samples, specifically, urine and pharmaceutical products, was confirmed through recovery studies. As presented in Table 1, excellent recovery values, ranging from 96.7 to 103.4 %, were achieved for both urine samples and pharmaceutical formulations. These analytical recovery results demonstrate that the NiO/SPCE sensor is suitable for accurately measuring EP and AC concentrations in urine samples and pharmaceutical products.

Sample | Added concentration, μmol L-1 | Found concentration, μmol L-1 | Recovery, % | RSD, %
Human urine | EP | AC | EP | AC | EP | AC | EP | AC
0 | 0 | - | - | - | - | - | -
5.0 | 5.5 | 4.9 | 5.6 | 98.0 | 101.8 | 1.9 | 3.3
7.0 | 7.5 | 7.1 | 7.3 | 101.4 | 97.3 | 3.1 | 2.1
9.0 | 9.5 | 8.8 | 9.8 | 97.8 | 103.2 | 2.6 | 2.9
11.0 | 11.5 | 11.3 | 11.4 | 102.7 | 99.1 | 2.2 | 3.2
Epinephrine Injection | 0 | 0 | 1.9 | - | - | - | 3.0 | -
3.0 | 4.0 | 4.8 | 4.1 | 98.0 | 102.5 | 2.7 | 3.3
4.0 | 6.0 | 6.1 | 5.8 | 103.4 | 96.7 | 2.2 | 2.6
5.0 | 8.0 | 7.0 | 7.8 | 101.4 | 97.5 | 3.5 | 2.1
6.0 | 10.0 | 7.8 | 10.1 | 98.7 | 101.0 | 1.8 | 2.4