Introduction

For many purposes, including the detection of environmental contaminants, medicinal medicines, biomolecules, and metal ions, electrochemical sensing has emerged as a crucial resource (Laurila et al. 2017; Ramnani et al. 2016; Hefnawy et al. 2021a). Paracetamol, also known as N-acetyl-P-aminophenol (PS), is an analgesic and antipyretic that is widely used to treat a variety of painful conditions around the globe (Teker and Aslanoglu 2020; Avinash et al. 2019; Chetankumar et al. 2021). Many approaches were applied for detecting paracetamol in biological systems, such as liquid chromatography-mass spectrometry (LC–MS), chemiluminescence, high-performance liquid chromatography (HPLC), spectrofluorimetry, and electrochemical techniques.

Because of its superior speed, low cost, high sensitivity, and great selectivity in comparison with other detection methods like spectroscopy, electrochemical detection has found widespread application in the determination of pharmaceuticals, neurotransmitters, and metal ions (Wring and Hart 1992; Zen et al. 2003).

However, despite its widespread use in the electrochemical detection of drugs and neurotransmitters, metal oxide is categorized as a semiconductor or insulator due to its huge bandgap. Metal oxide is routinely functionalized with carbonaceous chemicals to increase its electrical conductivity (Hefnawy et al. 2023a, b, c; Eliwa et al. 2023a, b; Bashal et al. 2023; Gamal et al. 2024). A number of metal oxide composites, such as MoS2/TiO2 (Kumar et al. 2019a), Pt/CeO2/Cu2O (Rajamani and Peter 2018), Fe2O3 (Vinay and Nayaka 2019), Bi2O3 (Zidan et al. 2011), and MnO2 (Xu et al. 2019), have been described as excellent electrocatalysts for acetaminophen detection.

Zinc oxide (ZnO) is an n-type semiconducting material that has been extensively utilized as an electrocatalyst in applications such as fuel cells (Shah et al. 2019), water splitting (Rahimi and Moshfegh 2021), UV light emitters (Tsai et al. 2021), solar cells (Chen et al. 2021; Degefa et al. 2021; Ge et al. 2021), and electrochemical sensors (Kang et al. 2021; Zhang et al. 2021; Jaballah et al. 2021; Al-Kadhi et al. 2023a). Thus, the ZnO nanoparticles were widely mentioned as electrocatalyst for efficient acetaminophen detection like CeO2–ZnO–chitosan (Almandil et al. 2019), GC/ZnO NPs (Hanabaratti et al. 2020), ZnO-MoO3 (Liu et al. 2021), carbon dots-ZnO nanoflowers (Hatamluyi et al. 2020).

Zinc oxide nanocubes were extensively used in electrochemical detection of species owing to high surface area and excellent electrical properties. Chitosan, a natural polymer, is frequently used as the matrix for metal oxide in medical and electrochemical sensors due to its magnetic properties, such as its film-forming ability, remarkable biocompatibility, nontoxicity, and high mechanical strength (Hefnawy et al. 2022a; Medany and Hefnawy 2023; Alamro et al. 2023). Several chitosan composites were reported as efficient electrocatalysts for electrochemical detection of acetaminophen, such as Ti/Chitosan@Au (Sadeghi and Shabani-Nooshabadi 2021), Co@Chitosan-CNT (Akhter et al. 2018), chitosan/TiO2 (Jazini et al. 2020), and Au/RGO/chitosan (Rahman et al. 2023). The presence of metal oxides in the chitosan matrix led to enhancement of the activity toward electrochemical detection. The enhancement of the activity of the electrode is explained by high ability of the chitosan to adsorb the paracetamol.

Density functional theory (DFT) has become an essential theoretical tool to support the researcher's work and estimate work efficiency (Wang et al. 2016; Hefnawy et al. 2021, 2023d). Consequently, the DFT calculation is employed for predicting the interaction between the metal oxide and polymer for using polymer composite in different applications (Hefnawy et al. 2021a; Daoulas et al. 2005; Hanifehpour et al. 2021; Gwon et al. 2016). Several theories were employed to explain the adhesion phenomena, such as mechanical interlocking, diffusion, electronic, and adsorption (McBain and Hopkins 2002; Voyutskii 1971; Derjaguin and Smilga 1967; Semoto et al. 2011).

Adsorption energy calculations are the most widely accepted general theory of adhesion, whereas adhesive and adherend can adhere via forces acting between atoms in the interface region when intermolecular contact is attained (Semoto et al. 2011; Wake 1982; Al-Kadhi et al. 2023b; Hefnawy et al. 2022b).

In numerous applications, the metallic interface with polymers is regarded as a fundamental strategy (Mahani 2020). Various computational methods, including Monte Carlo (MC) (Daoulas et al. 2005), molecular dynamics (MD) (Rissanou et al. 2015), and dissipative particle dynamics (DPD) (Semoto et al. 2011), were used to estimate the interaction between polymer and metal surfaces (Gooneie et al. 2016).

Herein, ZnO and ZnO@Chitosan matrices for the electrochemical detection of acetaminophen were prepared. Comparative research was conducted on ZnO and ZnO@Chitosan. Utilizing DFT calculations, the adsorption of acetaminophen on the chitosan surface was determined. Alternately, the interaction energy and stability of ZnO on the chitosan surface were investigated at various ZnO faces. The detection of acetaminophen was conducted in various pH conditions. At various acetaminophen concentrations, cyclic voltammetry was used to examine the calibration curve. Electrochemical impedance spectroscopy (EIS) was utilized to compare the charge transfer resistance of ZnO and ZnO@Chitosan for the detection of acetaminophen.

Experimental

Synthesis of ZnO nanocubes

First, zinc oxide was synthesized from two distinct solutions: solution (A) (3.73 mmol of zinc acetate dihydrate dissolved in 40 ml of ethanol) and solution (B) (7.22 mmol of NaOH dissolved in 320 L of bi-distilled water and then in 25 mL of ethanol). Under 2.25 h of vigorous agitation at 55 °C, solution (B) was added drop by drop to solution (A). The As-synthesized ZnO nanoparticles were collected by centrifugation after 24 h and then rinsed with pure ethanol. Two hours were spent redispersing ZnO NPs in ethanol or drying them at 60 °C.

Preparation of ZnO@Chitosan composite

Briefly, one gram of ZnO material was dissolved in one hundred milliliters of acetic acid at a concentration of one percent. After adding 1.0 g of chitosan to the solution, the mixture was sonicated for 20 min. The pH of the solution was altered drop by drop with 1.0 M NaOH. The product was then filtered and rinsed multiple times with distilled water before being dried in an oven at 60 °C for 5 h.

Computational calculations

Using density functional theory, the geometry optimization for chitosan, acetaminophen and chitosan-acetaminophen was estimated (DFT). To investigate the equilibrium geometry of chitosan and chitosan-acetaminophen, calculations were performed using the Gaussian 09 program (Frisch 2009) at the B3LYP/6-31G level of theory.

To calculate the interaction energy between ZnO and chitosan polymer, a DFT study was conducted. The research was conducted using Forcite modules (Lippa et al. 2005). The simulation procedure utilized the force field of COMPASS (condensed phase optimized molecular potentials for atomistic simulation studies). Temperature is equilibrated in all simulations using the Andersen algorithm (Andersen 1980). This is the first ab-initio force field method to be validated by condensed-phase characteristics (Sun 1998). The ZnO crystal lattice consisted of three layers, forming a 3 \(\times\) 3 supercells monolith. An oligomer chain of chitosan composed of eight monomer units of N-acetyl glucosamine was utilized to study chitosan. The adhesion interaction energy (Eint) was calculated using the following equation:

$$E_{\text{int}} = E_{{\text{total}}} -\left( {E_{{\text{polymer}}} + E_{{\text{surface}}} } \right)$$
(1)

Eint is the energy of interaction between the polymer and metal oxide (kcal/mol). Etotal is total energy of the polymer and zinc oxide layers (kcal/mol); Epolymer is energy of the polymer layer (kcal/mol). Esurface is the energy of the metal oxide layer without polymer (kcal/mol).

Results and discussion

Surface, structural and spectral characterization

The FTIR spectra of ZnO and chitosan revealed various absorption bands for identifying the distinct functional groups detected in the mid-infrared (4000–400 cm−1). Figure 1 depicts the infrared spectra of the two compounds. At 3445.44 cm−1, the stretching vibrations of the O–H bond in the prepared chitosan were measured. The C–H transition was detected at 2937.41 cm−1. The absorption peaks at 1635.84, 1571.05, 1434.48 and 1370.84 cm−1 are the C=O stretching of the amide I band while bending the N–H, C–H and O–H, respectively (Wang et al. 2016b; Esquivel et al. 2015; Tyliszczak et al. 2016; Lichawska et al. 2019). The peak at 1159.45 cm−1 was attributed to anti-symmetric stretching of the (C–O–C) bridge; 1085.43 and 1022.32 cm−1 were predicted for skeletal vibrations involving C–O stretching (Yasmeen et al. 2016). For the ZnO spectrum of the synthesized ZnO nanoparticles, the fundamental vibration mode at 3424.3 cm−1 is referred to as O–H stretching and deformation, respectively, and is attributed to the metal surface's water adsorption. Zn's tetrahedral coordination is responsible for its 875 cm−1 absorption. Observed peaks in the frequency range of 731.9–608.6 cm−1 indicate the ZnO particle's vibrations of stretching (Mahalakshmi 2020; Silva-Neto et al. 2019; Kołodziejczak-Radzimska et al. 2012).

Fig. 1
figure 1

IR spectra of chitosan and ZnO

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X-ray diffraction was used to determine the structure of modified ZnO@Chitosan materials, as depicted in Fig. 2. Accordingly, seven maxima for ZnO were observed at 2θ = 31.78, 34.41, 36.26, 47.54, 56.62, 62.86 and 69.10 for miller indices (100), (002), (101), (102), (110), (103) and (112) (Khorsand Zak et al. 2011). In addition, the peak at 2θ ~ 20 corresponds to the miller indices (110) for chitosan (Esquivel et al. 2015; Tyliszczak et al. 2016).

Fig. 2
figure 2

XRD of ZnO@Chitosan

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As depicted in Fig. 3a, b, the surface morphology of ZnO@Chitosan was characterized using a scanning electron microscope (SEM). In the SEM images of ZnO nanoparticles embedded in chitosan sheets, hexagonal nanocubes were observed. In addition, the defined and uniform distribution of ZnO on the chitosan surface facilitates acetaminophen adsorption on the electrode surface. Typically, transmission electron microscopy (TEM) was used to measure the size of ZnO nanoparticles. The estimated average particle size of zinc oxide was approximately 80 nm (see Fig. 3c).

Fig. 3
figure 3

a, b Different magnifications of SEM of ZnO@Chitosan, c TEM of ZnO@Chitosan, d EDX

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Energy-dispersive X-ray spectroscopy (EDX), a form of elemental analysis, was used to detect the presence of elements such as phosphorus (Zn, C, O and N). The ratios of Zn and O atoms indicate the presence of ZnO and the absence of contamination from other elements in the samples (see Fig. 3d).

Electrochemical detection of acetaminophen

Cyclic voltammetry was used to assess the electrochemical activity of ZnO and ZnO@Chitosan toward acetaminophen detection. The electrochemical analysis was carried out at a scan rate of 50 mV s−1 (vs. Ag/AgCl) in a solution containing 50 µM of acetaminophen and 0.1 M PBS at a pH of 7.4. ZnO and a composite made of ZnO and chitosan were compared. Acetaminophen's noticeable oxidation peak was seen at potentials between + 510 and 470 mV (vs. Ag/AgCl), as shown in Fig. 4. The irreversible behavior for electrooxidation of acetaminophen on ZnO electrode can be noticed by decrease in 170 mV.

Fig. 4
figure 4

Cvs of GC/ZnO and GC/ZnO@Chitosan modified electrodes to determine 50 μM acetaminophen at 0.1 M PBS (pH = 7.4) at scan rate 50 mV s−1

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However, adding ZnO to chitosan caused the oxidation peak of acetaminophen to shift to a higher negative potential, indicating that the reaction should be more thermodynamically advantageous. Otherwise, the oxidation of acetaminophen on modified ZnO@Chitosan recommended to be reversible due the peak at a potential of − 50 mV. Scheme 1 estimates two electro-redox reactions as the acetaminophen electrooxidation process (Niedziałkowski et al. 2019; Karikalan et al. 2016; Tavakkoli et al. 2018; Fan et al. 2011; Nematollahi et al. 2009).

Scheme 1
scheme 1

The electrochemical oxidation mechanism of acetaminophen

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In a solution of 50 µM of acetaminophen and 0.1 M PBS (pH = 7.4), the diffusion coefficient of the drug was assessed using cyclic voltammetry for various modified electrodes, including GC/ZnO and GC/ZnO@Chitosan, at various scan rate ranges (5–200 mV s−1). Using the Randles–Sevcik equation, the diffusion coefficient was determined as follows (Hefnawy et al. 2022c):

$$i_{\rm p} = 2.69 \times 10^5 n^{3/2} AD^{1/2} C\nu^{1/2}$$
(2)

where A is the electrode surface area, D is the diffusion coefficient, C is the bulk concentration, and v is the scan rate.

The Cvs of the modified electrodes GC/ZnO and GC/ZnO@Chitosan are shown in Fig. 5a, c. Figure 5b, d shows the linear relationship between the square root of scan rate and the anodic oxidation peak current. For ZnO and ZnO@Chitosan, the calculated diffusion coefficients were 3.61 × 10–5 and 6.83 × 10–5 cm2 s−1, respectively.

Fig. 5
figure 5

a Cvs of different modified GC/ZnO at different scan rate range (5–200 mV s−1), b linear relation between ν1/2 and ip for GC/ZnO. c Cvs of different modified electrode GC/ZnO@Chitosan at different scan rate range (5–200 mV s−1), d linear relation between ν1/2 and ip for GC/ZnO@Chitosan. A solution of 50 μM of acetaminophen and 0.1 M PBS at different scan rate ranges (5–200 mV s−1)

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The electrochemical impedance (EIS) was used to evaluate the impedance parameters such as charge transfer resistance, diffusional component and capacitive nature of the modified electrodes. Figure 6 shows the Nyquist plot of modified GC/ZnO and GC/ZnO@Chitosan in 50 μM of acetaminophen and 0.1 M PBS (pH 7.4) at constant AC voltage + 0.5 V. The result of Nyquist plots was fitted as circuit inset Fig. 6 in the fitting circuit, solution resistance (Rs) is connected to outer later capacitor (C1) and charge transfer resistance (Rct), while the C1 is connected to the cell of diffusion element (W) and inner capacitance element (C2). The value of charge transfer resistance (Rs) reflects the improvement of chitosan addition to ZnO. Table 1 shows the decrement in resistance was observed for GC/ZnO@Chitosan electrode. On the other hand, the higher capacitance of GC/ZnO@Chitosan electrode corresponds to the higher adsorption rate of acetaminophen and efficient redox process. The semi-circuit behavior of the electrode is regarding to the charge transfer process of drugs sensing. Thus, the lower diameter of semi-circuit is related to higher activity. Consequently, the lower resistance for ZnO@chitosan is corresponding to the higher activity of modified chitosan electrode toward electrochemical detection compared to the pristine ZnO modified electrode.

Fig. 6
figure 6

The EIS of modified electrode GC/ZnO and GC/ZnO@Chitosan for electrochemical detection of acetaminophen

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Table 1 EIS fitting parameters for acetaminophen detection
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Effect of concentration

Figure 7a, c shows the changing in the acetaminophen concentration's using cyclic voltammetry (CV) for different modified surfaces ZnO and Zn@Chitosan. The concentration range (10 × 10–6 to 50 × 10–6 M) is at a pH 7.4 PBS solution at a scan rate of 50 mV s−1. The sensor was discovered to have a linear response as the following equations, as illustrated in Fig. 7b, d:

$$I_{\text{p}} \;\left( {{{\upmu }}A} \right) = 1.4C_{({\text{ZnO}})} \left( {{{\upmu }}{\text{M}}} \right) + 4.9\quad \quad R^2 = 0.989$$
(3)
$$I_{\text{p}} \;({{\upmu }}{\text{A}}) = 0.56C_{({\text{ZnO}}@{\text{Chitosan}})} \left( {{{\upmu }}{\text{M}}} \right) + 1.51 \quad \quad R^2 = 0.991$$
(4)
Fig. 7
figure 7

Cvs curves of the modified electrode; a GC/ZnO, b GC/ZnO@Chitosan in the solution of 0.1 M PBS at wide concentration ranges (10–50 μM) of acetaminophen at scan rate 50 mV s−1. c, d Calibration curve for each compound

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The ZnO@Chitosan has a smaller zero concentration current (non-Faradic current) than the ZnO composite because of the previous equation. The following relation was used to estimate the limit of detection for each composite:

$${\text{LOD}} = 3 \, D/S$$
(5)

where D is the standard deviation, and S is the slope of the calibration curves.

The detection limits were estimated as 0.94 and 0.71 μmol L−1 for ZnO and ZnO@Chitosan. The efficiency of the ZnO and ZnO@Chitosan sensors was compared with other reported electrocatalysts for acetaminophen detection, as explained in Table 2.

Table 2 Comparison between different electrodes for the detection of acetaminophen
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Effect of different pH values

The pH of the solution is important for drug detection. The detection of acetaminophen was investigated at several pH ranges between 5 and 9. Chitosan's solubility in an extremely acidic media is its principal disadvantage when used in electrochemical detection. As a result of the unstable surface, pH 5 had the greatest impact on pH.

The linear sweep voltammetry for modified glassy carbon electrodes with ZnO and ZnO@Chitosan is shown in Fig. 8a, b at a scan rate of 50 mV s−1 and in a solution of 50 µM of acetaminophen in 0.1 M PBS at varied pH values (5 up to 9). The oxidation peaks demonstrate the clear correlation between the potential for oxidation and the pH of the solution, with the peak beginning to move toward a more negative value as the pH rises. According to the following equations, it was also discovered that the anodic Ep was shifted more negatively with increasing pH (Fig. 8c, d):

$$E = 0.937 - 0.069{\text{ pH}}_{{\text{ZnO}}} \quad \quad R^2 = \, 0.996$$
(6)
$$E = 0.923 - 0.063 \, pH_{{\text{ZnO}}@{\text{Chitosan}}} ,\quad \quad R^2 = \, 0.993$$
(7)
Fig. 8
figure 8

a, b LSV curves of modified electrode GC/ZnO, GC/ZnO@Chitosan in solution 0.1 M PBS and 50 μM of acetaminophen at different pH values range (5–9) at scan rate 50 mV s−1. c, d Relation between pH vs. anodic peak potential (Ep) for GC/ZnO and GC/ZnO@Chitosan, respectively. e, f Relation between pH vs. anodic peak current (Ip) for GC/ZnO and GC/ZnO@Chitosan, respectively

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The detection of acetaminophen at GC/ZnO and GC/ZnO@Chitosan demonstrated Nernstian behavior, with the slope of the pH vs. Ep relation equaling 0.069 and 0.063 for GC/ZnO and GC/ZnO@Chitosan, respectively. This is due to Eqs. 6 and 7. The graph in Fig. 8e, f shows the relationship between pH and anodic peak current. For GC/ZnO electrode, the decrease of activity of ZnO is at highly basic medium (pH = 9) due to instability of ZnO (i.e., ZnO + OH− + H2O \(\to\) Zn \({({\text{OH}})}_{3}^{-}\)−3) (Liu et al. 2018). The ZnO@Chitosan surface, on the other hand, displayed limited efficiency in the acidic medium, despite the fact that chitosan is easily soluble in the acidic medium.

DFT studies and compatibility between ZnO and Chitosan

The interaction between the polymer and the metal oxide surfaces is one of the crucial key aspects. In order to evaluate the adherence of chitosan on the ZnO(100), ZnO(101) and ZnO(002) surfaces, XRD used various zinc oxide facets. Setting up two layers of metal oxides and chitosan with various spacing between them allowed for the calculation to be done (10–30Å). Setting up two layers of metal oxides and chitosan with various spacing between them allowed for the calculation to be done. Using the Forcite model, the energy was computed for ZnO, polymer and polymer-ZnO layers. As a result, interaction energies are valued and presented in Table 3. The ability to adsorb metal oxide on the surface of the polymer is shown by the interaction energy's negative value. It was discovered that Zn(100) had a greater interaction energy value than other crystal slabs. As represented in Fig. 9a–c, two layers of different ZnO facets are at layer spacing 30Å.

Table 3 Comparison between interaction energies for different layer spaces and different facets
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Fig. 9
figure 9

DFT study of the interaction between ZnO and chitosan polymer at different metal oxide facets a ZnO100, b ZnO101, c ZnO002. d The optimized geometry of chitosan and chitosan-acetaminophen

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Acetaminophen is better able to bind to the electrode surface thanks to the ZnO@Chitosan composite. The DFT approach was used to examine how chitosan and acetaminophen interacted. Using the B3LYP/6-31G level of theory, the structures of chitosan, acetaminophen and chitosan-acetaminophen were optimized. The following equation was then used to determine the interaction between chitosan and acetaminophen as a function of adsorption energy:

$$\Delta E_{{\text{ad}}} = \Sigma E_{{\text{Chitosan}} - {\text{acetaminophen}}} - \Sigma E_{{\text{Chitosan}}} + E_{{\text{acetaminophen}}}$$
(8)

Figure 9D, The adsorption energy of the acetaminophen at the chitosan surface was equaled − 0.76 eV, where the negative value of the energy reflects that the adsorption process is energetically favored.

Effect of interference

Investigations of the electrode's acetaminophen selectivity were made in comparison with other species that can interfere in biological fluids, such as ascorbic acid, dopamine, KCl, citric acid, glucose and glutamine. After adding the interfering species at high concentrations (10 times that of acetaminophen), linear sweep voltammetry was carried out both with and without the interfering species. The normalized current of ZnO and ZnO@chitosan composite in the presence of various interfering species is shown in Fig. 10a, b. The minimal current changes were between 96 and 98%. As a result, in the presence of various interfering species in biological fluids, the ZnO and ZnO@Chitosan nanocomposites demonstrated high selectivity for acetaminophen.

Fig. 10
figure 10

The study of the interference for 50 μM acetaminophen in the presence of (10-folds) of interfering species at a GC/ZnO, b GC/ZnO@Chitosan

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Real sample determination

By analyzing the spiked acetaminophen samples, it was possible to examine the recovery of acetaminophen detection at various electrode surfaces. Initially, medicines containing paracetamol (such as the 500 mg Panadol tablet with acetaminophen) were bought on the open market and used to make stock solutions. In 0.1 M PBS with a pH of 7.4, various acetaminophen concentrations were produced. By using linear sweep voltammetry, the anodic current of each sample was studied. The predicted recovery values for the various electrode surfaces (GC/ZnO and GC/ZnO@Chitosan) are shown in Table 4. The results of the calibration curve were used to compare the linear sweep voltammetry measurements of the anodic peak current.

Table 4 Real sample electrochemical detection of acetaminophen at the surface of ZnO and ZnO@Chitosan
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Conclusion

At a modified glassy carbon electrode, the acetaminophen detection was investigated using ZnO and ZnO@Chitosan composites. It was discovered that adding ZnO to chitosan improved the detection of acetaminophen in a synergistic manner. Better criteria, such as a lower detection limit and a higher diffusion coefficient, were demonstrated by ZnO@Chitosan (LOD changed from 0.94 to 0.71). High recovery properties for the ZnO@chitosan electrode were discovered. In the presence of the various interfering species, the composite of ZnO@Chitosan was also found to have a good anti-interference capacity. The availability of acetaminophen to be adsorbed on chitosan has been demonstrated by the negative value of the interaction between acetaminophen and chitosan. The ZnO and chitosan layer's interaction and computability were demonstrated by the DFT experiments, supporting the electrochemical findings.