Introduction

Due to manganese’s special electronic structure (3d54s2)1, which consists of five unpaired electrons, has several unique properties, including multiple valence states, Forming species with a high oxidation potential, and rich redox chemistry compared to other d-block elements2. Manganese with the ability to form multiple bonds with oxygen through spontaneous oxidation has created a diverse family of manganese oxides3. Their diverse redox properties, crystalline structures, morphologies, and surface nanoarchitecture have made them an interesting class of transition metal oxides. Various synthetic strategies can be employed to prepare manganese oxides with specified stoichiometry, crystal structure, morphology, size, and surface properties4,5,6,7. The electrochemical performance of manganese oxides depends heavily on their structural characteristics, crystal modification, and defect chemistry8,9. Manganese oxides with similar structural features exhibit a variety of different electrochemical properties depending on a wide variety of morphology, porosity, crystal structures, crystal defects, and textures1,2,10. These structural parameters are crucial in determining and optimizing their electrocatalytic properties for the performance of manganese oxides as sensor materials. Wide efforts have been devoted to tailoring the synthesis conditions to prepare manganese oxides with desirable morphologies to improve the electrochemical performance properties11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30. Among all preparation techniques, electrochemical deposition is a simple, low-cost method of depositing thin films of manganese oxide on conductive substrates31. Despite their low cost, abundance, and environmental friendliness, manganese oxides are used less as a based material in the preparation of sensors for electrochemical measurements because of their insufficient conductive properties32,33,34, still, limitations can be overcome by choosing a suitable strategy for electrodeposition that can reduce the charge transfer resistance.

The use of manganese oxides in the preparation of electrochemical sensors has been widely reported35,36,37,38,39,40,41. Paracetamol (PAR), 4-aminophenol (4-APh), and 4-nitrophenol (4-NP) are among the analytes determined by electrochemical sensors modified with manganese oxide42,43,44,45.

PAR is one of the most popular nonprescription drugs available today. Among its uses are colds, coughs, fevers, mild to moderate pain, headaches, joint pain, toothaches, etc42. During the production or storage of paracetamol, it is probable that hydrolytic degradation will take place. 4-APh is one of the main byproducts and is hard to remove. 4-APh can cause some diseases, including eye irritation, liver damage, skin problems, and kidney failure46. The inset of Fig. 1 shows the mechanism of the electrochemical oxidation of paracetamol PAR (a) and 4-APh (b)47.

Fig. 1
figure 1

Preparation procedures of MnO2(nh) and MnO2 electrodes, inset: Mechanism of the electrochemical oxidation of paracetamol PAR (a) and 4-APh (b).

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4-NP is a nitroaromatic compound utilized in the production of pesticides, fungicides, plasticizers, dyes, and pharmaceuticals (being a step in the creation of paracetamol, where 4-NP is converted to 4-APh and then acetylated with acetic anhydride). 4-NP is classified as a priority pollutant by the Environmental Protection Agency (EPA) in the U.S. because of its harmful effects (carcinogenic, teratogenic, and mutagenic properties) and its long-lasting presence. Negative impacts of 4-NP on humans range from mild symptoms like headaches and fever to more severe ones such as difficulties in breathing and even mortality when exposed at high levels48,49,50.

The European Pharmacopoeia (Ph. Eur.) defines the specification limit for 4-APh as 50 ppm. Drug products mostly have higher specification limits, such as 1000 ppm 4-APh/PAR51. It is therefore crucial to develop analytical methods that are efficient, simple, low-cost, biocompatible, and available for the simultaneous detection of PAR, 4-APh, and 4-NP. Due to their electroactive nature, much effort has been focused on creating modified electrodes to determine three compounds52,53. In this work, Manganese oxides have become the subject of our research interest to increase their electrical conductivity. This focus is due to its affordability, wide availability, environmentally friendly properties, and electrocatalytic properties that can aid in the decomposition of electroactive compounds. In this work, we presented a synthesis process that can increase the conductivity of manganese oxides and use it to determine analytes. We applied the cyclic voltammetry method for the electrodeposition of MnO2 on Fluorine-doped Tin Oxide (FTO) and illustrated that heat treatment during the deposition process significantly improves the conductivity of all three manganese oxides. In such a way, under certain conditions, Rct of MnO2/FTO decreased by 53.0% (see Sect. 3.3). Next,

we compared the performance of MnO2/FTO in the measurement of 4-NP and the simultaneous measurement of PAR and 4-APh with a differential voltammetry technique. The electrosensing performance of the developed electrode is ultimately promoted by the increment electrical conductivity of manganese oxides.

Materials and methods

Reagents and instrumentation

All agents were purchased from Sigma-Aldrich with analytical grade and used as received without further purification. All the solutions were prepared with redistilled water. 0.1 M Phosphate buffers with different pHs were prepared by mixing appropriate solutions of 0.1 M KH2PO4, and K2HPO4.

Powder XRD patterns were achieved on an X-ray diffraction Spectrometer (Philips X’Pert Pro., Almelo, Netherlands), using a Cu-Kα line as a radiation source. SEM images were recorded by a scanning electron microscope (Mira 3 Tescan, Czech Republic). The energy dispersive spectroscopy (EDX) model Mira 3 Tescan, Czech Republic was used for elemental analysis of the electrode surface. A calibrated pH-meter model 827 pH lab–Metrohm (Herisau, Switzerland) was used to measure the pH of solutions.

FTO pretreatment and MnO2/FTO electrode preparation

FTO (< 7 Ω/sq) glass substrates were purchased from Latech Co. (Singapore). Before use, FTO glass substrates were rinsed with ethanol and redistilled water successively. Cyclic voltammetry (CV) was used to electrodeposition of MnO2 on FTO substrate in an aqueous solution of 0.1 M containing Na2SO4 and MnSO4, ranging from 0.4 V to 1.3 V versus Ag/AgCl (3 M KCl), at a scanning rate of 10 mVs− 1using the following two procedures:

  1. (1)

    40 cycles of discontinuous voltammogram were applied in such a way that after every 10 cycles, the electrode was heated for 1 h at 200 °C. In total, it was heated 4 times at 200 °C and each time for 1 h. We call the electrode obtained in this procedure MnO2/FTO.

  2. (2)

    40 cycles of voltammogram, without any heat treatment, were continuously applied on the FTO substrate. We call the electrode obtained in this procedure MnO2(nh)/FTO.

After deposition of the MnO2 and MnO2 (nh) on the FTO substrate, electrodes were thoroughly rinsed with redistilled water and dried in ambient conditions (see all these procedures in Fig. 1).

Electrochemical measurements

Electrochemical investigations were conducted on an Autolabs potentiostat/galvanostat (MetrohmAutolab, Netherlands). A typical three-electrode setup was used to perform differential pulse voltammograms and cyclic voltammograms. The reference electrodes in this study were Ag/AgCl (3.0 M KCl). MnO2/FTO or MnO2 (nh)/FTO were applied as the working electrodes, and a platinum bar was used as an auxiliary electrode. The simultaneous determination of 4-APh and PAR was achieved using differential pulse voltammetry in the potential window from − 0.1 to 0.6 V. Determination of 4-NP was obtained using differential pulse voltammetry in the potential window from − 0.8 to -0.4 V.

The pH of the buffer was optimized as 6.0. At least three replicates were run for each electrochemical experiment and all error bars in the Figs. represent one standard deviation.

Results and discussion

Electrodeposition of MnO2 and MnO2(nh) on FTO

Figure S1 shows the cyclic voltammograms of the electrodeposition of MnO2 (a) and MnO2(nh) (b) on the FTO substrate. As can be seen, heat treatment does not change the CV profile. The clear rise in peak currents for reduction and oxidation during the potential scan cycle suggests that the FTO glass substrate’s surface changes as a result of the repeated potential sweep, leading to the formation of new substances.

Characterization of MnO2 nanostructured film

Figure 2 shows the XRD patterns of the FTO substrate, MnO2(nh)/FTO, MnO2/FTO, and MnO2/FTO after being used 12 times. It appears from Fig. 2b,c that XRD patterns of MnO2/FTO and MnO2(nh)/FTO are quite similar that confirms heat treatment (200 °C, 1 h) during the synthesis process did not affect the crystal structure of MnO2. Furthermore, after 12 times analyte measurements by MnO2/FTO, XRD was performed again and showed that the structure had not changed and the electrode had good stability (Fig. 2d). As-deposited MnO2 films without crystalline impurities can be exhibited as -MnO2 (JCPDS Card, No 44–0142)54,55.

Fig. 2
figure 2

X-ray diffraction patterns of FTO (a) MnO2(nh)/FTO (b) and MnO2/FTO (c) films and MnO2/FTO (after 12 times measurements) (d).

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SEM images and corresponding EDX in Fig. 3 and Fig. S2 provide additional insights into the structure of the as-prepared samples. Figure 3a shows a low-magnification SEM image of MnO2(nh). As can be seen, MnO2(nh) film has a sheet-like morphology and is uniformly distributed. This film formed a network of interconnected nanosheets that create porous microstructures.

Fig. 3
figure 3

SEM images of MnO2(nh) (a,b) and MnO2 (c,d) films deposited on FTO substrate.

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Figure 3b shows a high-magnification SEM image of MnO2 nanosheets with a sheet thickness of about 12 nm. Spaces between nanosheets could facilitate electrolyte access to the well-distributed nanosheets. Figure 3 (c, d) shows low and high-magnification SEM images of MnO2 which are electrodeposited with heat treatment. As can be seen, there are no structural differences between the two unheated and heated films. Fig. S2 displays the EDX spectrum of MnO2/FTO and MnO2(nh)/FTO. The EDX spectrum confirms the presence of Mn, O, and Sn in the samples.

Electrochemical Impedance spectroscopy

Electrochemical Impedance spectroscopy (EIS) is a proven technique for the characterization of electrochemical systems. Data from the EIS can be related to several important physical properties, including charge transfer resistance and structural features56,57. In this study, we used the EIS to investigate the effect of heat treatment during the electrodeposition process in improving the charge transfer resistance and consequently in improving the sensing performance of manganese oxide films.

EIS was run on MnO2/FTO and MnO2(nh)/FTO electrodes in 0.1 M KCl containing 5.0 mM Fe (CN)63−/4− with a frequency range of 100 kHz to 0.01 kHz.

The Nyquist plots for the MnO2/FTO and MnO2(nh)/FTO are given in Fig. 4a. With Z-view software, the diameter of high-frequency semi-circles in the EIS Nyquist diagram was fitted to estimate charge transfer resistance (Rct). Just as we can observe in Fig. 4a, Rct was obtained 245 and 115 Ω for MnO2(nh)/FTO and MnO2/FTO, respectively, which shows about 53.0% reduction in the resistance of the MnO2/FTO compared to MnO2(nh)/FTO.

Fig. 4
figure 4

EIS curves of FTO, MnO2(nh)/FTO, and MnO2/FTO in 5.0 mmol/L [Fe(CN)6]3-/4- containing 0.1 mol/L KCl (a), Comparative DPVs of 4-AP and PAR in PBS solution (pH 6.0) containing 50 µM 4-AP and 50 M PAR at MnO2(nh)/FTO and MnO2/FTO (b), The effect of pH on the peak current of 12 µM 4-APh and 40 µΜ PAR in PBS at MnO2/FTO and its corresponding potential-pH plot (c,d).

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The significant decrease in all three comparisons is probably due to the elimination of layer-by-layer hydration sites during the electrodeposition process, which results in a more uniform electron transport network.

The Randles-Sevcik equation can be used to calculate the effective surface area based on electrochemical measurements58:

ip= 2.69 × 105n3/2AD1/2C1/2.

where ip, n, and A, stand for anode peak current (A), the number of electrons transferred in the redox event (n = 1), and effective surface area (cm2), respectively. D is the diffusion coefficient of Fe(CN)63−/4− known as 6.5 × 10− 6 cm2/s, C is concentration of Fe(CN)63−/4− (0.5mmol/cm3in 0.1 M KCl) and ν is scan rate in V/s. The effective surface area of MnO2(nh)/FTO, and MnO2/FTO was found to be 0.071 cm2 and 0.079 cm2, respectively.

The electro-sensing performance of developed electrodes was investigated by DPV technique in 0.1 M phosphate buffer containing 50 µM PAR and 20 µM 4-APh to evaluate the electrocatalytic behavior of these electrodes. Figure 4b illustrates that there is a difference between the oxidation peaks of 4-APh and PAR in all three electrodes, the oxidation peaks of 4-APh are at 0.09, 0.085, and 0.11 V for MnO2/FTO, Mn2O3/FTO, and Mn3O4/FTO, respectively. The oxidation peaks of PAR are at 0.35, 0.36, and 0.31 V for Mn2O, Mn2O3, and Mn3O4, respectively. A continuous increase in oxidation peak currents was observed in electrodes that were subjected to heat during the electrodeposition process. Specifically, a 1.9-fold increase in MnO2/FTO current responses over MnO2(nh)/FTO was seen in both analytes. It is noticeable that the reduction in Rct due to heating during synthesis results in more intense peaks for 4-APh and PAR currents.

Influence of pH

The influence of pH value on the determination of 4-NP and simultaneous determination of 4-APh and PAR were evaluated in a pH range of 2.0–8.0 using MnO2/FTO. Figure 4c, Fig. S3, and Fig S4 show that as pH increased, analytes exhibited an increased peak current and then decreased, reaching the maximum response at pH 6.0.

From Fig. 4d the corresponding linear regression equations were y = -0.0178x + 0.4451 (R2 = 0.9945) and y = 0.0288 x + 0.266 9 (R2 = 0.9818). pH can greatly affect the catalytic activity of MnO2/FTO due to the involvement of protons in the electrode reaction. 4-APh and PAR are weak acids, therefore, they can ionize protons. The balance shifts to the right when the base is added. Too much hydroxide ion present can impede a reaction from reaching equilibrium. Therefore, a pH of 6.0 was chosen as the optimal value for achieving maximum sensitivity and specificity.

Simultaneous determination of 4-APh and PAR

The 4-APh and PAR concentrations were determined using MnO2/FTO under optimal experimental conditions. Individual determination entails a change in one substance’s concentration while another substance’s concentration remains constant. Figure 5a shows no considerable change in the signal current of 4-APh with a concentration of 10 µM in the presence of PAR from 2.0 to 55.0, and the peak current of PAR is linearly correlated to its concentration. According to Fig. 5b the increase of I(4−APh) is correspond to the linear equation of I(PAR) = 0.8322 C + 1.0367 and R2 = 0.9967.

Fig. 5
figure 5

DPVs of different concentrations of PAR (2.0–55.0µM) and 10 µM PAR in 0.1 M PBS, pH 6.0, and its corresponding calibrating plot (a,b).DPVs of different concentrations of 4-AP (0.8–22.0 µM) and 30 µM PAR in 0.1 M PBS, pH 6.0, and its corresponding calibrating plot (c,d).

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4-APh also exhibits a rise in peak current with an increasing concentration of 4-APh from 0.8 to 22.0 µM, and the presence of 30 µM PAR does not interfere with it (Fig. 5c). According to Fig. 5d increase of I(4−APh) fits the linear equation of I(4−APh) = 2.8451 C- 1.9047 and R2 = 0.9971. The simultaneous determination of 4-APh and PAR in 0.1 M PB at pH 6.0 was carried out with the MnO2/FTO electrode using the DPV technique. As can be seen in Fig. 6a Peak currents of both compounds increased linearly without interfering when their concentrations increased simultaneously. The corresponding linear regression equations are I(4−APh) = 2.8049 C − 2.5518 (R2 = 0.9983) and I(PAR) = 0.8865 C -0.0301 and R2 = 0.9945 (Fig. 6b). At S/N = 3, detection limits of 0.19 and 0.60 were acquired for 4-APh and PAR, respectively. Table 1 Presents a comparison of some developed sensors for the determination of 4-APh and PAR. Detection limits for determining 4-APh and PAR are relatively lower and have a wider linear range than in previous experiments using electrochemical methods.

Fig. 6
figure 6

DPVs of different concentrations of 4-AP (0.8–22.0 µM) and PAR (2.0–55.0 µM) in 0.1 M PBS, pH 6.0, and its corresponding calibrating plot (a,b), DPVs of different concentrations of 4-NP (0.1–250.0 µM) in 0.1 M PBS, pH 6.0, and its corresponding calibrating plot (c,d).

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Table 1 Comparison of different electrodes for determining AC and 4-AP [* differential pulse voltammetry, ** square wave voltammetry, *** amperometry].
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Determination of 4-NP

4-NP concentrations were determined using MnO2/FTO under optimal experimental conditions (Fig. 6c). The calibration plots were obtained with the DPV technique in PBS (pH 6.0). Figure 6d shows that the increase of I (4−NP) corresponds to the linear equation of I (4−NP) = 0.2076 C -2.1047 and R2 = 0.9937. At S/N = 3, The linear range of 0.1–250 µM and detection limit of 0.01 was acquired.

Selectivity, repeatability, and reproducibility of the MnO2/FTO

The selectivity of MnO2/FTO was investigated by comparing DPV signals before and after adding different organic compounds and ions to 15.0 µM 4-APh and 45.0 µM PAR. Figure 7 shows that response signals, with a significant level of 0.05, are unaffected with 3mM (equivalent to 200-fold of 4-APh) of dopamine, uric acid, ascorbic acid, L-dopa, L-cysteine, and glucose.

Fig. 7
figure 7

Current responses towards 12 µM 4-APh and 45 µM PAR in the presence and absence of 0.5 mM interferents in 0.1 M PBS (pH 6.0) at MnO2/FTO.

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The repeatability of MnO2/FTO was studied with the same electrode to detect 15.0 µM 4-APh and 45.0 µM PAR 12 times. 4-APh and PAR had respective relative standard deviations (RSD) of 1.79% and 2.22%.

To evaluate reproducibility, five independent MnO2/FTO electrodes were fabricated, yielding RSDs of 3.95% for PAR and 4.46% for 4-APh. During a sustained two-week period, the same electrode was used to detect PAR and 4-APh every day, while the electrode was stored at ambient conditions. After 15 days, MnO2/FTO retained 98.2% of its original current. The results verified good selectivity, repeatability, and reproducibility, for MnO2/FTO.

Real sample determination

Paracetamol 40 mg/ml oral solution was chosen as a real matrix. In 0.1 M PBS (pH 6.0), 10, 20, 30, and 50 µM of PAR were prepared. The MnO2/FTO sensor was used to analyze matrixes with the DPV technique. The achieved recovery values are shown in Table 2.

Table 2 Real sample electrochemical determination of acetaminophen at MnO2/FTO (n = 5) [*ND  not detected].
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The recovery values of 4-APh and PAR were 95.0-102.5%. The result shows that the method is highly accurate and The MnO2/FTO sensor is excellent for the simultaneous determination of 4APh and PAR in drugs.

Conclusions

We proved that with a simple step-by-step heating technique during the electrochemical synthesis of MnO2 on an FTO substrate, its electron transfer resistance can be reduced and its electrochemical performance can be improved. This is due to the elimination of hydration sites in the film layers. Then, heated MnO2/FTO was used to simultaneously determine 4-APh and PAR. After that, analytes PN, PA, and UA were also investigated separately. The results proved that the prepared electrode has an excellent electrocatalytic activity for the simultaneous determination of 4-APh and PAR as well as 4-NP. In conclusion, the prepared electrode provides an efficient, economical, eco-friendly, and available sensing platform for the determination of 4-NP and simultaneous determination of 4-APh and PAR.