Screen-Printed Carbon Electrode Modified with ZrO₂/Ag/GO for Simultaneous Detection of Catechol and Hydroquinone

Abstract

This study presents a straightforward process for producing a hybrid ternary composite of silver nanoparticles (Ag NPs), small graphene oxide (s-GO), and zirconia (ZrO₂) and its use as an electrode material for electrochemical sensing. The physico-chemical properties of the ternary composite were analyzed by means of field emission scanning electron microscopy (FE-SEM), ultraviolet-visible (UV-vis) and FTIR spectroscopy, X-ray Photoelectron Spectrometry (XPS) and contact angle (CA) measurements. The synthesized hybrid nanomaterial was employed as an electrode modifier in the fabrication of a modified screen-printed carbon electrode (SPCE) and used for the simultaneous electrochemical sensing of key environmental pollutants such as hydroquinone (HQ) and catechol (CAT). The developed sensor exhibited linearity in the range of 0–100 µM for both HQ and CAT, with sensitivity values of 2640 µA·mM⁻¹·cm⁻² for HQ and 5120 µA·mM⁻¹·cm⁻² for CAT. The limits of detection (LOD) were 1.5 µM for HQ and 0.72 µM for CAT, respectively. The synergistic enhancement of electron transfer kinetics, the increased electroactive surface area, the strong anti-interference capability, and excellent reproducibility and stability establish these modified electrodes as promising candidates for environmental monitoring and real sample analysis.

1. Introduction

Monitoring potentially harmful pollutants in air, soil, and water is crucial to safeguard public health and the environment against the spread of industrial and natural inorganic and organic contaminants [1,2,3,4]. Although numerous nanoparticle-based electrochemical sensors for the detection of environmental pollutants have been developed, their applicability to real samples remains limited. In particular, the realization of portable, low-cost and highly selective sensing platforms for environmental monitoring is still an open challenge. In this context, continuous efforts to optimize their performance are made [5].

Catechol (CAT) and hydroquinone (HQ) are raw chemicals used in rubber antioxidants, photographic developers, spices, insecticides, pharmaceuticals, dyes, and other high-toxicity applications [6,7]. Furthermore, CAT participates in various physiological processes, but they are difficult to degrade and can cause significant harm to both the human body and the environment. Moreover, as isomers, CAT and HQ interfere with one another. As a result, it is critical to develop a quick and straightforward approach to determining them all simultaneously. Currently, phenolic pollutants are determined using a variety of approaches, including spectrophotometry [8], chromatography [9], synchronous fluorescence spectroscopy [10], capillary electrophoresis [11], and flow injection analysis [12]. On the other hand, CAT and HQ are easily oxidized using electrochemical techniques, which makes their electrochemical determination straightforward [13]. However, the simultaneous determination of CAT and HQ is very difficult since their voltammetric peaks—corresponding to the oxidation/reduction of the two phenolic isomers—overlap significantly in many circumstances [14]. Furthermore, the competitive adsorption of phenolic isomers on the electrode surface results in a nonlinear connection between voltammetric response and concentration in the mixture. As a result, determining CAT and HQ requires selecting a suitably modified electrode [15]. In this way, the sensors detect and convert ambient analytes and chemical or physical processes into quantifiable signals [16]. Materials science breakthroughs have developed advanced sensors with increased sensitivity, specificity, and miniaturization [17,18].

Here, GO, ZrO₂, and Ag nanomaterials are used thanks to their interesting properties, both individually and in combination. Graphene oxide provides an extensive surface area and superior electrical conductivity, facilitating electron transport [19]. Zirconia is recognized for its environmental compatibility and chemical stability, which are highly resistant to chemical reactions, making it an ideal material for various industrial and environmental applications [20]. Silver nanoparticles have strong electrical conductivity and catalytic activity, which enhances sensor performance [21]. Recently, composites based on graphene and zirconia have received substantial interest as an excellent platforms for detecting biochemical species [22], while Radhakrishnan et al. studied the fabrication, characterization, and analytical performance of HQ sensors based on the PANi–Fe₂O₃–rGO composite modified glassy carbon (GC) electrode. The fabricated sensor showed high sensitivity, stability, and satisfactory reproducibility [23]. Vilian et al. studied the novel Pt/ZrO₂-RGO/GCE electrode, which showed enhanced electrochemical performance for detecting HQ and CC compared to other individually modified electrodes. The electrode was also successfully applied for the simultaneous detection of HQ and CAT in real wastewater samples, with satisfactory recoveries [18]. Graphene/metal/metal oxide binary systems (such as Ag/GO or ZrO₂/GO) have been described in literature for phenolic sensing applications [24]. However, in many cases, the improvement achieved by binary composites remains limited due to incomplete synergistic effects, poor stability, or insufficient selectivity in complex matrices.

This study examines the potential ternary combination of GO, ZrO₂, and Ag for detecting CAT and HQ in laboratory and real conditions, compared to the quantitative performance of their single/binary counterparts. We are demonstrating that the complete ZrO₂/Ag/GO ternary configuration is crucial for achieving significant improvement in electrochemical response, with respect the single or binary systems, where neither provides a substantial electrochemical improvement over the bare electrode. This highlights that sensing performance results from a synergistic interaction between Ag NPs, ZrO₂ active sites, and the conductive graphene oxide network. Furthermore, unlike previous published work based on ZrO₂/Ag/GO on conventional glassy carbon electrodes [18], our ternary composite material is integrated onto a disposable screen-printed carbon electrode (SPCE), enabling the realization of a scalable and cost-effective sensing platform.

Ultimately, we demonstrate the sensor’s effectiveness and possible uses in environmental protection and healthcare monitoring. The analyzed nanocomposite is designed to provide outstanding performance when detecting both CAT and HQ simultaneously.

2. Results

2.1. Optical, Morphological, and Microstructural Characterization

A detailed investigation of the optical, morphological, and microstructural characteristics of the ternary sample was first carried out. Figure 1 reports the optical extinction spectrum of the ternary sample suspension in water. Optical characteristics were compared with those of the pure and binary suspensions. The optical absorption of Ag NPs shows a main band centered at about 406 nm, attributed to the surface plasmon resonance (SPR) absorption of Ag NPs, and a band at about 473 nm assigned to the acetylacetonate ligand [25]. The spectrum of the 0.1 mg/mL GO suspension is characterized by two absorption bands centered at about 228 nm and 295 nm ascribed to the carbon aromatic structure and the epoxide/peroxide subunits, respectively [26]. Finally, the spectrum of ZrO₂ suspension shows an absorption edge located at about 207 nm and ascribed to the valence band to conduction band transition [27,28]. These features were also observed in the ternary system: an intense absorption feature in the 200–400 nm range due to the GO contribution overlapping to the ZrO₂ edge located at about 207 nm as well as the Ag SPR band.

Figure 1.

UV-vis absorbance spectra of the ternary and binary systems (a) compared to those of the pure materials (b). Binary and pure systems spectra have been already reported in a previous paper [22].

Zeta potential is a critical parameter reflecting the surface charge of nanoparticles and their stability in suspension. It plays a significant role in the electrochemical sensing capabilities of materials, particularly in the detection of organic compounds such as catechol and hydroquinone. For our samples, the zeta potentials are: −45.5 mV (Ag NPs), −46.6 mV (ZrO₂), −58.2 mV (GO), and −47.7 mV (ternary hybrid composite). All the suspensions are stable, and the zeta potential value of the ternary sample indicates that the ZrO₂ material can effectively stabilize the suspensions through electro-steric mechanisms among GO/Ag nanoparticle suspensions. These results are interesting since stable dispersions ensure that the active sensing components remain well-dispersed on the electrode surface, enhancing the effective surface area available for electrochemical reactions. Since zeta potential influences the electron transfer kinetics during redox reactions, we expected that a favorable zeta potential can facilitate faster electron transfer rates, leading to higher peak currents in cyclic voltammetry (CV) measurements. This could justify the enhancement of the overall sensitivity of the sensor by allowing for quicker and more efficient detection of catechol and hydroquinone.

Figure 2a shows the FTIR spectrum of the freeze-dried ternary sample. The freeze-drying process was carried out to avoid the contribution of free –OH stretching coming from liquid water. The main features are ascribed to the O–H stretching vibration mode (3200 cm⁻¹), carbonyl (C=O) stretching (1720 cm⁻¹), C–O–C and C–OH stretching vibration modes (1613 and 1365 cm⁻¹) and the C–O stretching vibration modes (1045 cm⁻¹), confirming the presence of epoxide groups in GO layers. Conversely, the characteristic features of the optical longitudinal symmetric O–Zr–O stretching modes (expected at near 670 and 740 cm⁻¹) are not clearly visible. This can be explained by the strong infrared absorption of GO (due to functional groups such as hydroxyl and carboxyl) which may mask the weaker signals of ZrO₂ [29,30,31].

Figure 2.

FTIR (a), Raman (b), SEM (c), and EDX (d) data of the ZrO₂/Ag/GO sample. Numbers 1 and 2 represent the point in which the EDX spectra have been recorded.

Raman spectrum shown in Figure 2b is dominated by GO features, namely the typical D and G bands of the carbon-based materials, in the 1100–1650 cm⁻¹ range, as well as the typical contribution of ZrO₂ at lower wavenumber (400–700 cm⁻¹), whose intensity is enhanced thanks to SERS effects induced by Ag NPs [32]. This enhanced Raman signal amplification due to plasmonic coupling between Ag NPs, GO, and ZrO₂ species may also contribute to ensure stability. Features observed in the 800–1100 cm⁻¹ range could be associated to C–C, C–O–Ag vibrations as well as zircon (ZrSiO₄). The presence of Si–O stretching vibrations is explained since the colloids were sprayed onto a silicon substrate to carry out Raman and FTIR measurements. The C–O–Ag bond contribution, expected at around 1620 cm⁻¹ and due to the vibrations involving C–O bonds and their interaction with Ag nanoparticles into the GO matrix, is only weakly visible.

A detailed imaging of the morphology and structure of the hybrid composite was given by SEM-EDX (Figure 2c,d) analysis. Ag NPs are dispersed on GO sheets, which act as stabilizing supports due to their large surface area and oxygen-containing functional groups [33]. EDX analysis confirms the elemental composition of the hybrid composite. Peaks corresponding to silver (Ag), carbon (C), oxygen (O), and zirconium (Zr) were detected, indicating successful integration of Ag NPs, GO, and ZrO₂.

Figure 3 reports some STEM images of the hybrid nanocomposite. The spherical Ag nanoparticles, appearing brighter due to their higher atomic number compared to zirconia ones, are distributed on the surface of GO. Although the morphological features of ZrO₂ are not well defined, zirconia is integrated onto GO sheets as indicated by EDX data, suggesting intimate interfacial contact between the components.

Figure 3.

STEM images of the ZrO₂/Ag/GO sample recorded in different points with different scales and magnification: 2 μm and 4.81 KX (a), 1 μm and 11.53 KX (b), 400 nm and 21.36 KX (c), and 400 nm and 22.42 KX (d).

The interaction between carbon (C), silver (Ag), and zirconium (Zr) involves complex chemical and physical processes that could favor the formation of carbides. Therefore, in order to further investigate the composition and the sample surface chemical bonding configurations, we carried out XPS measurements. Figure 4 shows high resolution XPS profiles. The binding energies of Zr 3d around 182.5 eV suggest that zirconium is mainly present in an oxidized state. The Ag 3d binding energy, centered approximately at 368.1 eV, indicates substantial oxidation of silver. In addition, the slightly pronounced shoulder at lower binding energy (about 366–367 eV) could be attributed to the formation of Ag–C bonds [34]. The C 1s deconvoluted profile is typical of carbon found in graphene oxide (GO) structures, where multiple peaks can occur due to the presence of various functional groups on the GO surface. Finally, the O 1s spectrum exhibits a band centered at around 530.5 eV, which can be attributed to Zr–O bonds in zirconia composites, along with contributions from hydroxyl and carbonyl groups associated with GO. On the overall, according to literature data, we found that Ag NPs interact strongly with the epoxy functional groups and neighboring sp² carbon atoms on GO rather than hydroxyl groups. Epoxy groups on GO are known to be highly reactive. They can interact strongly with metal nanoparticles like silver (Ag NPs) due to their ability to form covalent bonds or coordinate with metal ions [35,36].

Figure 4.

XPS analysis on the ZrO₂/Ag/GO sample. Survey spectrum (a), HR Zr 3d (b) HR Ag 3d (c), HR O 1s (d), deconvoluted HR C 1s (e) and corresponding percentage of the different functional groups (f).

The morphological and microstructural analysis of electrode materials constitutes a pivotal phase in the development of electrochemical sensing devices, given that these attributes significantly impact the device’s overall performance. Hence, Scanning Electron Microscopy (SEM) images were acquired on the SPCE-based sensor. SEM images in Figure 5 display a heterogeneous morphology characterized by surface irregularities and a widespread distribution of nanostructures. These features create promising active sites for facilitating electrochemical reactions. Complementary energy-dispersive X-ray (EDX) analysis confirms the presence of carbon, oxygen, zirconium, and silver species on the electrode surface, although their distribution is non-uniform.

Figure 5.

SEM images of the ZrO₂/Ag/GO sample deposited on the SPCE electrode acquired in different points with different scales and magnification: 10 μm and 396 X (a), 1 μm and 5.52 KX (b), 1 μm and 7.20 KX (c), and 200 nm and 21.23 KX (d). The table in (d) reports the elemental percentages obtained by EDX analyses on the two points (1 and 2) highlighted in (c).

The contact angle (CA) is a measure of the wettability of a surface by a liquid, with angles below 90° indicating good wettability and those above 90° indicating poor wettability. In our case, the contact angle would likely be influenced by the hydrophilic nature of GO and the properties of the ZrO₂ and Ag nanoparticles, affecting the surface energy and thus the contact angle. Figure 6 shows the water droplets formed on the surface of the ternary system deposited on the SPCE (Figure 6d) compared to that of the bare SPCE (Figure 6a) and to those of GO/ZrO₂ and Ag NPs/GO systems (panels b and c, respectively). The Figure 6e reports the measured contact angles (CAs) values for all the studied systems deposited on the SPCE starting just from the bare electrode. Data show that the bare SPCE is hydrophobic, and that water adhesion is enhanced with all the as-deposited samples. In particular, the GO/ZrO₂ sample shows the highest degree of wettability (CA of about 74°) while the ternary system exhibits an intermediate CA value of 84°. The presence of oxygenated groups on the sample surface favors the formation of hydrogen bonds with water molecules thereby enhancing the wettability degree.

Figure 6.

Contact angle values for the bare SPCE (a), two binary systems deposited on the SPCE, ZrO₂/GO, and Ag/GO ((b) and (c), respectively), the ternary system (d) and those measured for all intermediate systems (e). Solid lines represent the fitted ellipses, while the dashed lines represent the tangent evaluated in the circled points.

2.2. Electrochemical Detection of Environmental Analytes

Preliminary experiments to evaluate the ternary hybrid’s applicability have been carried out for the monitoring of environmental organic pollutants such as hydroquinone and catechol [37]. Figure 7 illustrates the current responses of different electrode configurations, including the bare SPCE, individual components (GO, ZrO₂, and AgNPs), binary combinations (ZrO₂/GO, ZrO₂/Ag, Ag/GO), and the ternary hybrid (ZrO₂/Ag/GO) toward the electrochemical detection of hydroquinone (HQ), and catechol (CAT), at a concentration of 100 µM. The ternary hybrid electrode exhibited the highest current responses for both analytes. Notably, on all single and binary composites, no improvement was observed concerning bare SPCE, suggesting that a synergistic effect between Ag NPs, ZrO₂, and GO is necessary for boosting the sensitivity towards these analytes.

Figure 7.

Comparison of current responses for bare, individual, binary, and ternary hybrid-modified SPCEs toward the detection of HQ and CAT at a 100 µM concentration.

To elucidate the improved electrochemical performances of the ZrO₂/Ag/GO-modified SPCE, electrochemical characterization was performed by using [Fe(CN)₆]³⁻/⁴⁻ as a redox probe through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The well-defined redox peaks of the bare SPCE and ZrO₂/Ag/GO-modified electrodes treated with 10 mM [Fe(CN)₆]³⁻/⁴⁻ at a scan rate of 50 mV s⁻¹ were compared (Figure 8), revealing that the bare SPCE showed lower anodic and cathodic peak currents and higher peak-to-peak separation (ΔE_p) than the modified electrode. The ΔE_p for the bare SPCE was 324 mV, whereas for the ZrO₂/Ag/GO/SPCE, it was 309 mV. Contextually, the anodic (I_pa) and cathodic (I_pc) peaks intensity, for the bare SPCE, were 98.1 μA and −133.2 μA, respectively, while, for the ZrO₂/Ag/GO/SPCE, they assume the value of 127.2 μA and −158.8 μA, respectively. The Randles–Sevcik formula was employed to find out the active surface area of the SPCE and modified SPCE by using [Fe(CN)₆]³⁻/⁴⁻ as a redox probe [38]:

$$I_p=2.69\times {10}^5{n}^{3/2}A{D}^{1/2}C{v}^{1/2}$$

where I_p is the current maximum in amps, n is the number of electrons transferred in the redox event, A corresponds to electrode area in cm², D is the diffusion coefficient in cm²/s, C is the concentration in mol/cm³, and ν is the scan rate in V/s.

Figure 8.

Cyclic voltammograms of bare SPCE and modified SPCE with 10 µL of the ZrO₂/Ag/GO ternary composites. Scan rate 50.0 mV/s in 0.1 M PBS solution containing 10.0 mM K₃[Fe(CN)₆].

The electrochemically active surface area (ECSA) increased by 29.6% for the ZrO₂/Ag/GO/SPCE compared to the unmodified SPCE (Figure 8 and Table 1). The bare SPCE displayed typical redox peaks for the Fe(CN)₆³⁻/⁴⁻ pair, while the ZrO₂/Ag/GO-modified SPCE exhibited reduced ΔE_p and higher peak current, indicating enhanced electron transfer [39]. This enhancement was achieved through the synergistic combination of Ag nanoparticles, ZrO₂, and GO, which collectively improved the SPCE’s conductivity, increased its electroactive surface area, and facilitated faster electron transport [22]. Consequently, the screen-printed carbon electrode modified with ZrO₂/Ag/GO demonstrates enhanced catalytic performance compared to unmodified SPCE (Figure 8).

Table 1.

Experimental results for the simultaneous determination of HQ and CAT in a real sample.

Sensor	Analyte	Expected (µM)	Found (µM)	Recovery (%)
ZrO2/Ag/GO/SPCE	HQ	50	47.36	94.72
		50	49.42	98.84
		50	50.17	100.34
ZrO2/Ag/GO/SPCE	CAT	50	47.22	94.44
		50	48.39	96.78
		50	49.14	98.28

Cyclic voltammetry in 10 mM [Fe(CN)₆]³⁻/⁴⁻ was also carried out at scan rates ranging from 0.025 to 0.6 V/s. The linear increase in both anodic and cathodic peak currents with scan rate (Figure 9) indicates a redox process involving surface-confined species. The symmetric peak shapes and consistent peak separation (ΔE_p) further support this mechanism. However, the observed shifts in peak potentials toward more positive (anodic) and negative (cathodic) values at higher scan rates indicate emerging kinetic limitations and increased irreversibility, likely due to charge-transfer resistance or ohmic losses [40]. The electrochemical data show a linear relationship between peak current (I_p) and the square root of the scan rate (ν¹/²), which confirms that the reaction is primarily diffusion-controlled; the current depends on how quickly molecules move through the solution to the electrode [41]. However, at higher scan rates, the peaks broaden, suggesting minor influences from surface reactions (e.g., slow electron transfer) or mass transport limitations [42].

Figure 9.

(a) Effect of scan rate from 0.025 to 0.6 Vs⁻¹ on cyclic voltammograms of the ZrO₂/Ag/GO ternary composites in 0.1 M PBS solution containing 10.0 mM K₃[Fe(CN)₆]; (b) Current intensity of the oxidation peaks (black square) and reduction peaks (red dots) as a function of the square root of scan rate.

Electrochemical impedance spectroscopy (EIS) was employed to assess the interfacial charge-transfer characteristics of the modified electrode (Figure 10). The Nyquist plot of ZrO₂/Ag/GO/SPCE exhibits a reduced semicircle diameter compared to bare SPCE, accompanied by a lower charge-transfer resistance (R_CT) of 65.1 Ω, confirming significantly enhanced interfacial conductivity [39]. This improvement arises from the synergistic effects of the ternary nanocomposite: (i) graphene oxide (GO) provides an extended conjugated structure for efficient electron transport [22], (ii) Ag nanoparticles enhance catalytic activity and conductivity [39], and (iii) ZrO₂ contributes stability and additional active sites [43]. The composite’s hierarchical ZrO₂/Ag/GO nanostructure amplifies performance by increasing the electroactive surface area, which improves mass transport and enhances analyte accessibility to active sites. This architecture also facilitates faster interfacial electron exchange, consistent with the observed reduced charge-transfer resistance (Rct) and strengthens the overall electrocatalytic response during HQ and CAT oxidation. Consequently, the combined benefits—lower Rct, enhanced electrocatalysis, and optimized surface morphology—highlight the ZrO₂/Ag/GO/SPCE’s superior capability for simultaneous detection of hydroquinone and catechol [22]. This improvement is driven by clear synergistic effects among the three components: GO provides a conductive, high-area scaffold that promotes charge percolation and prevents particle aggregation; Ag nanoparticles act as efficient electron-mediator/catalytic centers that accelerate oxidation kinetics; and ZrO₂ supplies chemically active adsorption sites (surface hydroxyl/defect sites) that favor analyte preconcentration and stabilize the response. In addition, Ag–GO coupling reduces the electron-transfer barrier, while ZrO₂–Ag heterointerfaces combine adsorption and catalytic functions, increasing current response and selectivity under mixed-analyte conditions. Overall, this behavior corroborates established structure–property relationships in metal oxide/graphene hybrid materials reported in the literature [22].

Figure 10.

EIS of bare SPCE and modified SPCE with 10 µL of the ZrO₂/Ag/GO ternary composites.

Overall, our data suggest that the improved electron transfer kinetics and increased surface area of the ternary hybrid are the key parameters contributing to the enhanced electrochemical response.

2.3. Electroanalytical Determination of HQ and CAT

Based on the obtained results, the developed sensor was employed for the electroanalytical determination of HQ and CAT using differential pulse voltammetry (DPV) in 0.01 M phosphate buffer solution (PBS, pH 7.4) at a scan rate of 50 mV/s, with HQ and CAT concentrations ranging from 0 to 100 µM. DPV responses to HQ recorded on the bare screen-printed carbon electrode (SPCE) and the ZrO₂/Ag/GO/SPCE-modified electrode are shown in Figure 11a,b. In the presence of HQ, both electrodes exhibited a well-defined anodic peak at −0.1 V, corresponding to the oxidation of HQ. However, the modified ZrO₂/Ag/GO/SPCE demonstrated significantly enhanced electrocatalytic activity toward HQ oxidation compared to the bare SPCE, indicating improved electron transfer kinetics and higher sensitivity. The calibration curve for the ZrO₂/Ag/GO/SPCE revealed a linear increase in peak current intensity upon increasing HQ concentration in the range of 0 to 100 μM (Figure 11c).

Figure 11.

DPV responses for the electroanalytical determination of Hydroquinone (HQ) for both the bare screen-printed carbon electrode (SPCE) (a) and the ZrO2/Ag/GO/SPCE-modified electrode (b), and corresponding calibration curves (c).

Notably, the modified sensor demonstrated nearly a two-fold increase in sensitivity compared to the bare SPCE (Figure 11c). The sensitivity, extracted from the slope of the calibration plot, was determined to be 5776 μA·mM⁻¹·cm⁻², with a determination coefficient (R²) of 0.998. The limit of detection (LOD) was calculated as 3.11 μM, based on a signal-to-noise ratio (S/N) of 3.

Additionally, the sensor exhibited superior performance also for CAT detection (Figure 12a,b). The DPV curves recorded with the modified sensor were well-defined even at low CAT concentrations, showing an increase in anodic peak current compared to the bare SPCE.

Figure 12.

DPV responses for the electroanalytical determination of catechol (CAT) for both the bare screen-printed carbon electrode (SPCE) (a) and the ZrO₂/Ag/GO/SPCE-modified electrode (b) and corresponding calibration curves (c).

The linear relationship (R² = 0.994) between peak current variation and CAT concentration enabled the calculation of the sensitivity and LOD of the ZrO₂/Ag/GO/SPCE sensor, which were determined to be 6688 µA·mM⁻¹·cm⁻² and 1.85 µM (S/N = 3), demonstrating the sensor’s high analytical capability.

Kinetically, the oxidation reaction of hydroquinone and catechol detected electrochemically by the modified electrode primarily depends on the adsorption of hydroquinone and catechol on the electrocatalyst surface and the transfer of electrons from the electrocatalyst to the analytes. Based on the previously reported mechanism, the electrocatalytic oxidation of hydroquinone and catechol in the presence of the ZrO₂/Ag/GO-modified SPCE can be described as follows:

HQ → benzoquinone + 2e⁻ + 2H⁺

Catechol → o-quinone + 2e⁻ + 2H⁺

In this process, hydroquinone and catechol adsorb onto the electrode surface, where they lose two electrons and two protons, forming benzoquinone and o-quinone, respectively. The enhanced conductivity and electrocatalytic activity of the modified electrode facilitate efficient electron transfer, improving the sensitivity of the oxidation reaction.

2.4. Simultaneous Electroanalytical Determination of HQ and CAT

In some real conditions, the simultaneous electrochemical detection of HQ and CAT is essential. This property characterizes our sensor: DPV curve (Figure 13a,b) displays two distinct anodic peaks, corresponding to the oxidation of HQ and CAT, at −0.08 V and 0.05 V, respectively, thus confirming the sensor’s ability to detect both analytes with high sensitivity and selectivity. The linear ranges (LR), sensitivities (S), and LODs of the ZrO₂/Ag/GO/SPCE sensor are (i) LR_HQ = 0–100 μM/LR_CAT = 0–100 μM, (ii) S_HQ = 2640 μA·mM⁻¹·cm⁻²/S_CAT = 5120 μA·mM⁻¹·cm⁻², and (iii) LOD_HQ = 1.5 μM/LOD_CAT = 0.72 μM. Comparing the calibration curves for the HQ and CAT detection alone or simultaneously, the simultaneous detection of these two analytes appears to be the most suitable, providing higher sensitivity and a lower limit of detection.

Figure 13.

DPV responses for the simultaneous detection of HQ and CAT for both the bare screen-printed carbon electrode (SPCE) (a) and the ZrO₂/Ag/GO/SPCE-modified electrode (b).

Selectivity is another critical parameter for the practical application of electrochemical sensors, especially in complex biological and environmental samples. The selectivity of the ZrO₂/Ag/GO-modified SPCE towards the simultaneous detection of HQ and CAT and in the presence of other potential interfering analytes, including 200 µM of ascorbic acid (AA), folic acid (FA), and resorcinol (RES), was evaluated using DPV (Figure 14). Since these species are commonly found alongside HQ and CAT in biological fluids, food products, and pharmaceutical formulations, their coexistence can complicate accurate detection [44,45]. As shown in Figure 14, the anodic peaks corresponding to HQ and CAT remained well-defined and almost unchanged even in the presence of these interfering compounds. This suggests that the modified electrode exhibits a strong anti-interference capability, allowing for the reliable and simultaneous detection of HQ and CAT in complex samples.

Figure 14.

Differential pulse voltammetry (DPV) responses of ZrO₂/Ag/GO-modified SPCE in 0.01 mM PBS and in the presence of 100 µM HQ, 100 µM CAT, 200 µM AA, 200 µM RES, and 200 µM FA.

Finally, the stability, repeatability, and reproducibility of the ZrO₂/Ag/GO-modified SPCE were each evaluated in triplicate in the presence of 100 µM CAT and 100 µM HQ (Figure 15). A single electrode was used for both the stability and repeatability tests, whereas three independently prepared electrodes were used for the reproducibility assessment. Stability testing over five days showed minimal deviation in the current response, with RSD values of 1.37% for CAT and 3.10% for HQ, confirming the electrode’s ability to maintain consistent performance over time. The electrode was stored in a controlled environment when not in use, ensuring minimal degradation. Repeatability measurements demonstrated low RSD values of 0.55% for CAT and 1.34% for HQ, indicating high measurement precision under identical conditions. To further confirm the reliability of the sensor, additional tests were conducted by fabricating and testing three modified electrodes at different time intervals, but under identical conditions for CAT and HQ detection. All electrodes exhibited highly comparable oxidation current responses, with RSD values of 0.12% for CAT and 1.27% for HQ, confirming the reproducibility of the fabrication process and the sensor’s robustness. The consistently low RSD values across all parameters demonstrate that the ternary composite modification significantly enhances electrode stability, repeatability, and reproducibility. These findings establish the proposed sensor as a robust candidate for electrochemical sensing applications, ensuring reliable and stable performance over extended periods.

Figure 15.

Stability (a), repeatability (b), and reproducibility (c) of the ZrO₂/Ag/GO-modified SPCE for the simultaneous detection of hydroquinone (HQ) and catechol (CAT).

2.5. Real Sample Analysis

Detecting harmful substances, including phenolic compounds, in wastewater is essential for environmental and public health. Monitoring is crucial due to the toxicity, persistence, and bioaccumulative nature of these pollutants. Therefore, enhancing trustworthy sensing technology has emerged as a primary challenge in water quality management [46]. Using wastewater from the Office National de l’Assainissement (ONAS) treatment station in Sfax, Tunisia, the synthesized ZrO₂/Ag/GO/SPCE electrochemical sensor was evaluated for its accuracy, reliability, and usefulness in the simultaneous detection of HQ and CAT. Wastewater is a complex mixture of various organic and inorganic compounds, heavy metals, and other pollutants; therefore, it is considered a good medium for assessing the sensor’s selectivity and sensitivity under environmental conditions [46]. To further investigate the effect of potential interfering agents in the real wastewater matrix, the standard addition method was used by spiking real samples with 50 µM of HQ and CAT. The recoveries of the spiked wastewater samples were 94.72–100.34% for hydroquinone and 94.44–98.28% for catechol (Table 1), indicating that the sensor is capable of accurately quantifying the substances of interest in the presence of other potential interfering agents found in real wastewater samples. This also provides evidence of the strong adsorption and catalytic interaction of the analytes with the ZrO₂/Ag/GO nanocomposite, which promotes electron transfer kinetics on the electrode surface.

3. Discussion

The ZrO₂/Ag/GO composite significantly enhances the sensitivity of electrochemical sensors for detecting catechol and hydroquinone through several key factors associated to the increased surface area and enhanced conductivity. The combination of silver nanoparticles (Ag), graphene oxide (GO), and zirconium dioxide (ZrO₂) results in a composite with a high specific surface area. This increased surface area allows for more active sites for the adsorption of catechol and hydroquinone, facilitating more efficient electron transfer during the electrochemical reactions [47]. When silver nanoparticles are incorporated into the GO matrix, they improve the overall conductivity of the composite, since the conductive pathways created by Ag nanoparticles enable faster electron transfer, leading to improved current responses during voltammetric measurements [48]. However, mainly the interaction between Ag, GO, and ZrO₂ creates a synergistic effect that optimizes the electrochemical properties of the sensor, allowing to detect low concentrations of the target analytes. For instance, GO provides a platform for Ag nanoparticles to be dispersed uniformly, and it serves as an excellent conductive support, facilitating electron transfer during redox reactions of catechol and hydroquinone. Instead, ZrO₂ contributes to stabilizing and enhancing electrocatalytic activity, even in real/complex matrices, mainly helping to mitigate interference from other substances, thus improving the specificity of the sensor [48,49].

We outline that the morphology of ZrO₂/Ag/GO composites plays a crucial role in enhancing the electrochemical sensing capabilities for catechol and hydroquinone. ZrO₂ nanostructures promote a high surface area that is beneficial for electrochemical reactions; as reported in literature [24], composites that do not incorporate ZrO₂ might experience more interference from other compounds in complex samples, thereby reducing their sensitivity. On the other hand, Ag nanoparticles can create a rough surface on the electrode, which increases the active sites available for electrochemical reactions, improving charge transfer rates [50]. Specifically, the presence of Ag in the composite not only improves conductivity but also catalyzes the oxidation reactions of catechol and hydroquinone. This leads to higher sensitivity in detection, allowing for lower detection limits.

To provide a complete overview, the performances (linear range, sensitivity, and limit of detection) of the proposed ZrO₂/Ag/GO/SPCE-modified electrode-based sensor have been compared to most of the recently reported HQ and CAT electrochemical sensors based on Ag, ZrO₂, and GO (Table 2).

Table 2.

Comparison of the electroanalytical performances of the developed ZrO₂/Ag/GO/SPCE sensor with previously reported HQ and CAT sensors.

Sensors	Analytes	Range (µM)	LOD (µM)	Ref.
AuNPs/Fe3O4-GO/GCE	HQ	3–137	1.1	[51]
	CAT	2–145	0.8	[51]
Et-CG	HQ	0.2–360	1.0	[52]
	CAT	0.3–360	1.1	[52]
RGO-MWNTs	HQ	8–391	2.6	[53]
	CAT	5.5–540	1.8	[53]
PE-CNT/GCE	HQ	0–300	1.5	[54]
	CAT	0–300	0.7	[54]
MWCNT@rGONR	HQ	15–921	3.89	[55]
	CAT	15–1101	1.73	[55]
ZrO2/Ag/GO/SPCE	HQ	0–100	1.5	This work
	CAT	0–100	0.72	This work

The sensitivity of our ZrO₂/Ag/GO composite in electrochemical sensing applications can differ significantly from other nanocomposites, such as those based solely on carbon materials or metal oxides without silver, since they may not achieve the same level of conductivity, potentially leading to lower sensitivity in detecting analytes like catechol and hydroquinone

4. Materials and Methods

4.1. Synthesis of Electrode Materials

Silver acetylacetonate, purchased from Sigma-Aldrich (St. Louis, MO, USA), was used for synthesizing Ag NPs. These latter were prepared according to a simple and low-cost synthesis, relying only on Ag(acac) (acac = 2,4-pentanedionate) as both the reagent and precursor for metallic silver [22,56,57]. A 0.2 mM solution of Ag(acac) in ultrapure water was prepared at room temperature (RT) and magnetically stirred for at least 48 h, as reported in Figure 1a. The amount of Ag NPs (about 10.3 µM) was estimated by atomic absorption measurements.

Graphene oxide (<35 mesh) was purchased from Layer One, and ZrO₂ was purchased from Acros Organics (98.5%). Starting from these materials, two different water suspensions based on ZrO₂ (0.1 mg/mL) and GO (0.1 mg/mL) were prepared at RT. Following previous results, the ZrO₂ and GO suspensions were sonicated at 40 kHz for 10 and 60 min, respectively, to ensure uniform dispersion of the materials minimizing aggregation in aqueous medium [22]. Figure 16b,c illustrates the ultrasound’s effect on the commercial ZrO₂ and GO sheets. After sonication, a size reduction of ZrO₂ particles and GO sheets—from large “l” to small “s” GO—promotes a better dispersion in water [22,57].

Figure 16.

Schematic representation of Ag NPs (a), ZrO₂ (b), and s-GO (c) pure precursor suspensions; (d) the ZrO₂/Ag/GO ternary composites are prepared by mixing equal volumes of the precursor materials.

Finally, a suspension containing all three materials was prepared by combining equal volume ratios of ZrO₂, Ag NPs, and s-GO suspensions, resulting in a ZrO₂/Ag/GO 1:1:1 v/v mixture (Figure 16d).

4.2. Characterization Techniques

The UV-vis optical absorption response of the samples was investigated in the colloidal phase immediately after the preparation processes, with a Perkin-Elmer Lambda 750 UV-vis spectrometer (Shelton, CT, USA) operating in the 200–900 nm range. High-resolution Scanning Electron Microscopy operating in transmission mode (HR-STEM) was performed by using a Zeiss (Oberkochen, Germany) field emission scanning electron microscope (Gemini II model) with an accelerating voltage of 30 kV. A few drops of each suspension were put on 400 mesh, holey, carbon-coated copper grids. In addition, SEM analyses were performed with the same scanning electron microscope, operating at 30 kV and at a working distance of 4 mm.

The zeta potential values of the colloids were determined by a Horiba SZ-100 Nanoparticle Analyzer (Lyon, France) based on the laser Doppler method, which measures electrophoretic mobility under an applied electric field.

The spectroscopic characterizations were carried out using the FTIR Perkin Elmer spectrometer (Spectrum 100 model) (Shelton, CT, USA). FTIR spectra were acquired by placing the sample directly on the ATR crystal, performing four scans with an acquisition time of 60 s to achieve optimal signal quality.

Raman spectra were acquired by using the 532 nm laser excitation line, for an integration time of 60 s and an accumulation time of 3. Moreover, a 50× objective was used to collect the backscattered signal sent to a charge-coupled device (CCD) detector. The Raman setup is the XploRA HORIBA spectrometer (Lyon, France).

The compact and fully integrated surface analysis K-Alpha X-ray Photoelectron Spectrometer (XPS) was adopted to estimate the sample surface composition and chemical bonding configurations with high performance and productivity. Specifically, the system is equipped with the monochromatic Al-Kα source (1486.6 eV), operating in a constant analyzer energy (CAE) mode with a pass energy of 20 eV for high-resolution spectra and a spot size of 400 µm.

Static contact angle (CA) measurements were performed using a specialized setup that included a high-intensity LED lamp (6500 K, 580 lumens), a USB IDS uEye camera (Obersulm, Germany, model UI-146xLE-C featuring a 1/2” CMOS sensor), and a convex lens with a focal length of 16 mm and a maximum relative aperture of 1:1.4. A 2 µL droplet of water was dispensed onto the ZrO₂/Ag/GO-coated SPCE surface using a micrometer-controlled micro-syringe.

4.3. Sensing Test

Electrochemical tests were performed using a screen-printed carbon electrode (SPCE) commercial platform (DropSens C110, Metrohm, Varese, Italy). This electrode consists of a ceramic substrate with a 4 mm working electrode, a silver reference electrode, and a carbon counter electrode, and was used as received without any additional processing before modification.

To fabricate the modified sensors, each material (1 mg/mL in distilled water) was ultrasonically dispersed, and 10 µL of the resulting suspension was drop-cast onto the carbon working electrode. The electrodes were then air-dried at room temperature. Electrochemical performance of the ZrO₂/Ag/GO ternary composite modified SPCE for the oxidation/reduction of CAT and HQ was investigated. Our sensor is designed for multiple uses; after each measurement, the electrode is gently rinsed with water and then electrochemically cleaned using the CV technique in PBS. Following these cleaning steps, the sensor consistently produced data comparable to those obtained during its first use, confirming its stability and repeatability. Figure 17 shows a schematic pathway of the modified electrochemical sensor manufacturing.

Figure 17.

Schematization of the fabrication of ZrO₂/Ag/GO ternary composites-modified SPCE. The electrochemical reactions that can occur in the presence of catechol and hydroquinone are highlighted.

Electrochemical measurements were carried out in 0.01 M phosphate-buffered saline (PBS, pH 7.4), at a scan rate of 50 mV s⁻¹ in the potential range of −0.4 to 1 V. Cyclic voltammetry (CV) and Electrical Impedance Spectroscopy (EIS) were performed by using a galvanostatic potentiostat from Metrohm Autolab. Differential Pulse Voltammetry (DPV) tests were performed by a dropSens µStat 400 Potentiostat equipped with DropView 8400 software (version 3.6 20B0514) for data acquisition. The wastewater sample was collected from the Office National de l’Assainissement (ONAS) treatment station in Sfax, Tunisia.

5. Conclusions

A hybrid ternary composite of Ag nanoparticles (NPs), s-GO, and ZrO₂ and its use as an electrode material for electrochemical sensing has been proposed. The developed sensor not only provides quick and reliable detection of catechol and hydroquinone but also highlights the potential of the as-synthesized nanocomposite material in enhancing sensor performance. The development of ZrO₂/Ag/GO composite modified screen-printed carbon electrodes represents a significant step forward in electrochemical sensing technology. Future research will focus on optimizing composite formulation to further improve efficiency and performance.

Abbreviations

The following abbreviations are used in this manuscript:

SPCE	Screen-printed carbon electrode
GO	Graphene oxide
CAT	Catechol
HQ	Hydroquinone

Author Contributions

Conceptualization, E.F. and G.N.; software, C.C.; formal analysis, S.C. (Sabrine Chelly) and M.C.; investigation, S.C. (Sabrine Chelly), M.C., E.F., C.C., G.M.A. and D.M.; resources, E.F., S.C. (Sabrina Conoci), G.N. and D.M.; data curation, S.C. (Sabrine Chelly), M.C., E.F., C.C. and G.M.A.; writing—review and editing, S.C. (Sabrine Chelly), M.C., S.B.H.F., E.F., C.C., S.C. (Sabrina Conoci), G.N. and D.M.; supervision, E.F., C.C., S.C. (Sabrina Conoci), G.N. and D.M.; funding acquisition, G.N. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding Statement

This work has been partially funded by European Union (NextGeneration EU), through the MUR-PNRR project SAMOTHRACE (grant number ECS00000022).