Immobilization of stable catechol form on the SPCE surface to enhance hydrophilicity, reusability, and application for acetaminophen analysis

Abstract

This study presents the development and application of a modified screen-printed carbon electrode (SPCE) incorporating carboxylated multi-walled carbon nanotubes (MWCNT-COOH) and catechol (CA) for the electrochemical analysis of acetaminophen (ACAP). The use of CA, in combination with ultrasonic treatment, significantly improved the dispersion of MWCNT-COOH in aqueous media and promoted the formation of a stable, hydrophilic surface layer on the electrode. This modification not only enhanced the sensitivity of the sensor but also introduced a key advantage: the ability to reuse the SPCE multiple times without surface fouling or performance degradation. Differential pulse voltammetry (DPV) was employed to evaluate the electrochemical behavior of ACAP on the modified electrode, demonstrating a wide linear response range from 0.05 µM to 250 µM, with a detection limit of 15 nM and a quantitation limit of 58 nM. The selectivity of the sensor was confirmed in the presence of common tablet excipients, showing minimal interference. The method was successfully applied to the determination of ACAP content in commercial Tylenol tablets, with results (495.8 ± 1.6 mg) closely matching the declared dosage. These findings establish the modified SPCE/MWCNT-COOH/CA-U platform as a highly promising sensing tool, characterized by its simple preparation procedure, high sensitivity and selectivity, excellent stability, cost-effectiveness, and reusability, offering substantial benefits for routine pharmaceutical analysis.

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Introduction

Catechol (CA) plays a crucial role in biological systems and has been widely applied in the synthesis of food, pharmaceutical, and agrochemical products, serving also as a key industrial precursor due to its beneficial chemical properties.^1,2 CA and its derivatives exhibit strong antioxidant, chelating, and radical-scavenging capabilities, making them valuable in various scientific applications. Their ability to strongly interact with both organic and inorganic substrates has established CA as a common anchoring agent for surface modifications.^3–5

The oxidation mechanisms of CA were first explored in 1974 using dioxovanadium(V), revealing the kinetics of complex formation in acidic aqueous environments.⁶ Additionally, CA can crosslink with protein networks, enhancing mechanical properties and mimicking natural protein structures. Over the years, CA derivatives have found utility in adhesives, surface functionalization across multiple scales, biomedicine, and energy storage. As a diphenol compound, CA undergoes oxidation to semiquinone and quinone, detectable through electrochemical methods. It can also be electropolymerized into polycatechol films for electrode surface modification.⁷ The oxidized form, ortho-quinone, acts as an electrocatalyst—for instance, in the NADH oxidation using CA-encapsulated multi-walled carbon nanotube (MWCNT) composites as shown by Swetha et al.⁸

In parallel, nanotechnology has emerged as a transformative force across disciplines. At the nanoscale, materials exhibit superior surface area, reactivity, and tunable properties that drive innovations in catalysis, drug delivery, and sustainable manufacturing solutions.⁹ Carbon nanotubes (CNTs), in particular, are notable for their electrical conductivity, large surface area, and high stability. They are used in sensor development and electrode enhancement, although dispersion in organic solvents introduces toxicity concerns. Water-based dispersions present an eco-friendly alternative.¹⁰ Among CNTs, MWCNTs offer higher mechanical and chemical stability than single-walled forms. With unique sp²-bonded carbon atoms and high microporosity, carbon nanostructures like MWCNTs and graphene enhance electron transfer and are highly biocompatible, making them ideal for electrochemical sensors.^11–13 Several MWCNT-based sensors have been reported for acetaminophen (ACAP) detection.^14–17 Carboxylated MWCNTs (MWCNT-COOH) further improve hydrophilicity due to –COOH groups on the outer wall, while preserving inner wall conductivity.^18,19

CA's redox-mediating abilities also make it ideal for electrochemical sensors. Kumar et al. immobilized CA onto activated carbon electrodes via covalent bonding, enhancing redox performance.²⁰ However, this method required complex surface modifications, including oxidation and cyclic voltammetry conditioning, verified through X-ray photoelectron spectroscopy and Fourier transform infrared attenuated total reflection analyses. Although effective, the multi-step process limited its practicality. Our study presents a streamlined approach for covalent CA immobilization onto MWCNT-COOH, leveraging ultrasonic assistance to reduce complexity without compromising performance. Unlike traditional treatments of glassy carbon or screen-printed electrodes (SPCE), our method eliminates the need for elaborate activation or prolonged conditioning, while maintaining robust bonding and electrochemical sensitivity. By utilizing the intrinsic surface area and conductivity of MWCNTs, our simplified method allows for direct, stable CA attachment. This enhances sensor sensitivity, hydrophilicity, and reusability—important features for electrochemical applications.

ACAP or paracetamol is a widely used analgesic and antipyretic, effective for pain relief and fever control in various medical contexts and approved by the WHO. While generally safe, overdose leads to toxic metabolite accumulation and potential liver damage.^21,22 Additionally, ACAP contributes to growing pharmaceutical pollution.^23,24 Detection methods include spectrophotometry,^25–27 chemiluminescence,^28,29 titrimetry,³⁰ capillary electrophoresis,^31,32 thin-layer chromatogramphy,^33,34 HPLC,^35–37 colorimetry,^38,39 and electrochemical techniques.^40–42 Electrochemical detection is particularly appealing for its simplicity and rapid response, especially for electroactive compounds like ACAP, which undergoes irreversible oxidation via a two-electron, two-proton transfer on glassy carbon electrodes.⁴³

This study introduces a practical, low-cost method for preparing modified electrodes using MWCNT-COOH and CA. The entire preparation relies on ultrasonic assistance, creating a surface-modified electrode nanocomposite mixture with advantages such as chemical stability and good dispersion in water—an environmentally friendly solvent. The resulting electrodes exhibit excellent analytical performance for ACAP, with high accuracy, reproducibility, and reuse potential, making this system a strong candidate for pharmaceutical sensor development.

Materials and methods

Reagent

All chemicals were of the highest analytical grade available as shown in Table S1. MWCNT-COOH was purchased from Dropsens and all other chemicals obtained from Acros (USA) were used without any further purification. All solutions were prepared using deionized water from a Milli-Q ultrapure water system with a resistivity of 18 MΩ cm. The phosphate buffer solutions (0.1 M) were prepared by dissolving sodium phosphate in distilled water, and concentrated HCl and KOH solutions were used to adjust the solution acidity.

Instruments

Voltammetric experiments were conducted using a CHI electrochemical workstation (Model CHI-621C). A screen-printed carbon electrode SPCE (0.2 cm²) from Zensor R&D (Taichung, Taiwan) was rinsed with deionized water before use. A platinum wire served as the counter electrode, and a homemade Ag|AgCl|KCl (sat.) electrode acted as the reference. All potentials are reported relative to this reference. Experiments were conducted at room temperature (25 ± 2 °C). pH measurements were made using a Thermo Scientific Orion pH meter (Model 420) using a Mettler Toledo pH electrode (Model Inlab 439/120). Atomic force microscopy (AFM) images were obtained with a CSPM 5500 atomic force microscope (Benyuan Co. Ltd, Beijing, China) in tapping mode. Water contact angles were measured using a KRÜSS EasyDrop instrument. Fourier transform infrared attenuated total reflection (FTIR/ATR) data for the electrodes were collected using a Nicolet IS20, Thermo. Scanning electron microscopy images were taken on a HITACHI FE-SEM S-480. Elma Elmasonic S 180 H was used for preparing surface-modified electrode mixtures.

Preparation of the MWCNT-COOH/CA-U mixture

A dispersion containing 1.0 mg of MWCNT-COOH and 5.0 mg of CA was prepared in 1.0 mL of distilled water. The dispersion was first purged with nitrogen gas for 5 minutes to remove any dissolved oxygen, followed by sonication for 30 minutes to ensure thorough mixing and proper dispersion of the components.

Modified electrode preparation

The SPCE was electrochemically cleaned by performing potential cycling between −0.4 V and +1.0 V in 0.1 M phosphate buffer solution (PBS, pH 7.0). Following cleaning, 5 µL of the MWCNT-COOH/CA-U mixture was applied to the electrode surface and allowed to dry under an infrared lamp for 10 minutes. The modified electrode was then subjected to repeated potential cycling between −0.2 V and +1.0 V in PBS (pH 7.0) until a stable electrochemical background was obtained (continuous 50 cycles) (Fig. S1). The modified electrode is referred to as SPCE/MWCNT-COOH/CA-U to emphasize the incorporation of ultrasound in the electrode fabrication process. The electrode fabrication process is delineated in Scheme 1.

Scheme 1 Schematic illustration of the electrode modification process.

Results and discussion

Distributed state stability

The –COOH functional group in MWCNT-COOH forms hydrogen bonds with water, preventing MWCNT-COOH from agglomerating. As a result, it maintained a more stable dispersed state in water compared to that of MWCNTs.¹⁰ The presence of additional –OH groups on the surface of MWCNT enhances its dispersibility.¹⁸ The agglomeration could be observed by monitoring changes in a mixture over time or under the influence of ultrasound. After 7 days, MWCNT-COOH tended to agglomerate at the bottom of the container (Fig. S2A), while MWCNT-COOH/CA showed no signs of agglomeration (Fig. S2B). Notably, when both mixtures were centrifuged at 5000 rpm, only MWCNT-COOH agglomerated. Conversely, when CA was immobilized on the surface of MWCNT-COOH, it introduced –OH groups on the surface, resulting in a stable dispersed state even at a high centrifugal speed of 13 000 rpm (Fig. S2C). The reason for this phenomenon is the smaller size of –OH groups compared to –COOH facilitating hydrogen bonding.⁴⁴ The process of adding –OH groups to the surface of MWCNT-COOH using CA was straightforward. However, it's important to note that CA is susceptible to degradation in ambient air.³

The stability of both SPCE/MWCNT-COOH/CA-E and SPCE/MWCNT-COOH/CA-U electrodes was evaluated by immersing them in pH 7 PBS buffer solution and stirring at 1000 rpm for 15 minutes. The electrochemical signals recorded before and after stirring-both in PBS and in 1 mM ACAP solution-were nearly identical, confirming the formation of a polymer layer on the electrode surface that is both electrochemically stable and mechanically adherent.

Modified electrode characterization

The electrode modification process is detailed in Scheme 1. Material mixtures with the assistance of ultrasonic treatment, prepared according to the methodology described in the section “Preparation of the MWCNT-COOH/CA mixture”, were applied to the surfaces of the SPCE electrodes. Subsequently, the modified electrodes were dried under infrared light to ensure readiness for the ensuing experimental procedures.

The active o-diphenol group of CA is capable of being oxidized by atmospheric oxygen and participates in electrochemical polymerization in various electrolytes and on different electrode surfaces. At the SPCE/MWCNT-COOH electrode, typical background current responses were observed (Fig. 1, dashed line), with two distinct redox peaks appearing during cyclic voltammetry in a mixture of MWCNT-COOH and CA in phosphate buffer solution (PBS, pH 7) (Fig. 1, solid line). The electrode modified with the MWCNT-COOH/CA mixture exhibited two distinct pairs of redox peaks, labeled A₁/C₁ (E_pc = −0.01 V; E_pa = +0.06 V) and A₂/C₂ (E_pc = +0.16 V; E_pa = +0.24 V), when immersed in phosphate-buffered solution (PBS, pH 7.0). This electrochemical profile aligns with previous studies in which electrodes were modified by cyclic voltammetric scanning in CA solutions, confirming the successful incorporation and redox activity of CA on the electrode surface. The modified electrode is henceforth referred to as SPCE/MWCNT-COOH/CA-U. These findings are consistent with previous studies,^20,45,46 where CA-encapsulated MWCNT-modified glassy carbon electrodes or CA immobilized on activated carbon electrodes demonstrated stable and well-defined surface-confined redox behaviors. Our results underscore the pivotal role of the ultrasonic process in electrode modification. Notably, the incorporation of an ultrasonic step yields performance comparable to traditional electrochemical methods, yet with greater procedural simplicity. In conclusion, ultrasonication enhances the dispersion of MWCNT-COOH and promotes more uniform mixing with CA. This process likely facilitates partial polymerization of CA prior to electrode deposition, as evidenced by the immediate appearance of the A₁/C₁ peak in the first electrochemical cycle. In contrast, the non-sonicated mixture requires multiple cycles to initiate polymer formation. Thus, ultrasonication plays a critical role in accelerating the formation of the stable polymer layer of electrode modification.

Fig. 1 Cyclic voltammogram of SPCE/MWCNT-COOH-U (dashed line) and SPCE/MWCNT-COOH/CA-U (solid line) in pH 7 PBS buffer solution.

AFM and water contact angle measurements were utilized to investigate the surface morphology and wettability of the modified electrodes. AFM imaging (Fig. 2A) demonstrated that the unmodified SPCE exhibited a higher average surface roughness (R_a = 39.1 nm) compared to the SPCEs modified with MWCNT-COOH; R_a = 11.2 nm) and with both MWCNT-COOH and CA (R_a = 13.4 nm), indicating that surface smoothness increased following modification. Complementary surface area analysis further supported the successful modification of the electrode surfaces. Additionally, water contact angle measurements (Fig. 2B) revealed a significant shift in surface wettability: the initially hydrophobic nature of the bare SPCE (θ = 144.9°) transitioned to a more hydrophilic character upon modification, as evidenced by reduced contact angles of 98.0° and 50.7° for SPCE/MWCNT-COOH and SPCE/MWCNT-COOH/CA-U, respectively. These results validate the existence of polar substituents, whose bond characteristics contribute to the increased hydrophilicity of the material.

Fig. 2 (A) AFM images and (B) water contact angle of bare SPCE, SPCE/MWCNT-COOH and SPCE/MWCNT-COOH/CA-U electrodes.

These findings from AFM imaging and water contact angle measurements are further corroborated by the morphological analysis provided by SEM, as shown in Fig. 3. The SEM image of the unmodified SPCE (Fig. 3A) reveals a rough, porous surface characteristic of SPCE. Upon modification with MWCNT-COOH (Fig. 3B), the surface morphology changes markedly, with the porous network increasingly filled by the carbon nanotube matrix. This transformation is even more pronounced in the SPCE/MWCNT-COOH/CA-U electrode (Fig. 3C), where the surface appears smoother and more uniform, indicative of the successful integration of CA into the MWCNT framework. The observed reduction in porosity and enhancement in surface smoothness support the conclusions drawn from AFM and water contact angle measurements, affirming that CA is effectively immobilized within the MWCNT-COOH network, contributing to both surface modification and improved hydrophilicity. Further morphological insights are provided by the SEM image shown in Fig. 3D, which depicts an electrode modified via an electrochemical approach. In this method, the MWCNT-COOH-containing electrode was immersed in a CA solution and subjected to cyclic voltammetry (SPCE/MWCNT-COOH/CA-E), enabling surface modification through electrochemical polymerization. The resulting SEM image reveals a noticeable reduction in surface porosity compared to the unmodified electrode, likely due to the formation of poly(catechol). This polymer layer is known to rapidly foul electrode surfaces, which may explain the observed decrease in porosity.²⁰ These results highlight the differences in surface characteristics arising from ultrasound versus electrochemical modification techniques and further underscore the importance of the chosen modification strategy in optimizing electrode performance.

Fig. 3 SEM images of (A) SPCE, (B) SPCE/MWCNT-COOH, (C) SPCE/MWCNT-COOH/CA-U and (D) SPCE/MWCNT-COOH/CA-E electrodes.

The results obtained from FTIR spectroscopy further support the modifications observed in the electrode materials. As presented in Fig. 4, the FTIR spectra reveal characteristic differences between the unmodified and modified electrodes, highlighting the chemical changes associated with the surface functionalization process. The presence of CA increases the signal strength of the –OH group in the region of 3000–3600 cm⁻¹,²⁰ increasing the hydrophilicity of the SPCE/MWCNT-COOH/CA-E (Fig. 4B) and SPCE/MWCNT-COOH/CA-U surface (Fig. 4C). The disappearance of the signal of the C=O group at 1731 cm⁻¹ on the surface of the bare electrode (Fig. 4A), accompanied by the appearance of peaks at 1656 cm⁻¹ and 1747 cm⁻¹ indicates the C=C of sp²-hybridized carbon and the C=O of carboxyl groups in MWCNT-COOH^44,47 (Fig. 4B–D), when the electrode surfaces were coated with MWCNT-COOH or MWCNT-COOH/CA, proved that the process of coating the material on the electrode surface was successful. The slight changes in the intensity and position of the signals corresponding to the C=C and C=O groups of SPCE/MWCNT-COOH/CA-E, SPCE/MWCNT-COOH/CA-U, and SPCE/MWCNT-COOH (Fig. 4B–D) also indicate structural changes. The interaction of polar functional groups on the surface of bare electrodes, MWCNT-COOH, and CA created a stable adhesion on the electrode surface, even for those adsorbed on the outer surface that were not diffused into the solution even under vigorous stirring.⁴⁶ However, this layer underwent rapid transformation under the influence of atmospheric oxygen or during CV scanning, whereas the CA form corresponding to the A₁/C₁ signal remained stable. For the MWCNT-COOH/CA-U electrode, along with the appearance of the A₁/C₁ signal, the peak at 1747 cm⁻¹ was almost absent (Fig. 4C). Therefore, the –COOH group could play an important role in forming a stable CA layer on the electrode surface; this result is consistent with previous research.²⁰

Fig. 4 FTIR/ATR spectra of (A) bare SPCE, (B) SPCE/MWCNT-COOH/CA-E and (C) SPCE/MWCNT-COOH/CA-U, and (D) SPCE/MWCNT-COOH electrodes.

Experimental observations suggest that the interaction between CA and MWCNT-COOH involves more than simple physical adsorption. Specifically, the appearance of a stable A₁/C₁ redox peak pair during repeated electrochemical cycling, along with the retention of the electrochemical signal after vigorous stirring in pH 7 PBS buffer, indicates the formation of a robust and adherent layer on the electrode surface. This behavior is consistent with covalent or chemisorptive bonding, likely facilitated by oxidative polymerization of CA. Supporting this, FTIR spectral analysis reveals the emergence of characteristic vibrational bands corresponding to C=O, C=C, and O–H groups following the modification process, suggesting the formation of a stable polymeric layer covalently anchored to MWCNT-COOH.

Electrochemical behavior of modified electrodes

The electrochemical activity of the modified electrode surfaces was evaluated using K₃Fe(CN)₆ and Ru(NH₃)₆Cl₃ as redox probes. CV in K₃Fe(CN)₆ solution (Fig. 5A) showed quasi-reversible redox peaks with a high peak-to-peak potential separation (ΔE_p) of 206 mV for bare SPCE, indicating poor electron-transfer kinetics. In contrast, SPCE/MWCNT-COOH and SPCE/MWCNT-COOH/CA-U exhibited significantly improved kinetics, with ΔE_p values of 92 mV and 91 mV, respectively. The current intensity also increased significantly. This enhancement in electrochemical activity confirms effective surface modification. The Randles–Sevcik equation was employed using CV data obtained in 1.0 mM [Fe(CN)₆]^3−/4− with 0.1 M KCl as a supporting electrolyte at a scan rate of 100 mV s⁻¹ (Fig. 5A). The anodic peak currents were used to calculate ECSA for each electrode.^48,49 The calculated surface areas for bare SPCE, SPCE/MWCNT-COOH, and SPCE/MWCNT-COOH/CA were 0.0348, 0.1411, and 0.1387 cm², respectively. These results confirm that both the MWCNT-COOH and CA modifications substantially increased the active surface area compared to the unmodified SPCE (by approximately fourfold). The enhancement in ECSA contributes directly to the improved electrochemical performance of the modified electrodes in ACAP detection.⁵⁰ Electrochemical impedance spectroscopy (EIS) further characterized electron-transfer properties. The impedance spectra for all electrodes were analyzed and fitted using the Randles equivalent circuit, added and shown in Fig. 5B (inset). This model includes the solution resistance (R_s), charge transfer resistance (R_ct), and double-layer capacitance (dl).⁵¹

Fig. 5 (A) Cyclic voltammograms (CV) and (B) Nyquist plot of (a) bare SPCE, (b) SPCE/MWCNT-COOH and (c) SPCE/MWCNT-COOH/CA-U in 0.1 M KCl electrolyte solution containing 1.0 mM Fe(CN)₆³⁻. Inset: Equivalent circuit model (Randles circuit) used to fit the Nyquist plots, including solution resistance (R_s), charge transfer resistance (R_ct), and double-layer capacitance (C_dl).

The bare SPCE exhibited the highest R_ct (5.8 kΩ), indicating sluggish electron transfer. Modification with MWCNT-COOH reduced R_ct to 3.6 kΩ, confirming improved conductivity and interfacial properties. Further addition of CA resulted in the lowest R_ct value (2.4 kΩ), suggesting enhanced charge transfer kinetics due to better surface hydrophilicity and synergistic redox activity.^52,53

For the positively charged Ru(NH₃)₆Cl₃ redox probe, SPCE/MWCNT-COOH and SPCE/MWCNT-COOH/CA-U showed enhanced current values compared to bare SPCE (Fig. S3A), likely due to stronger electrostatic attraction. After potential scanning in Ru(NH₃)₆Cl₃ solution and subsequent CV scanning in 0.1 M KCl, adsorption behavior was observed on both modified electrodes but not on the bare SPCE surface (Fig. S3B).

Electrochemical behavior of ACAP on the SPCE/MWCNT-COOH/CA-U electrode

The CV response of ACAP at various electrode configurations in a 0.1 M phosphate-buffered saline solution (pH 7.0) is depicted in Fig. 6. The CV analysis revealed that the bare SPCE exhibited a relatively low oxidation current (curve a), indicating poor electrochemical activity towards ACAP. Similarly, the SPCE modified with catechol (SPCE/CA), fabricated by drop-coating a CA solution followed by drying under an infrared (IR) lamp, also demonstrated limited electrochemical performance, as evidenced by curve b. In contrast, significant enhancement in the redox signals of ACAP was observed when employing the SPCE modified with carboxyl-functionalized multi-walled carbon nanotubes (SPCE/MWCNT-COOH), as well as the SPCE incorporating both MWCNT-COOH and CA through ultrasonic dispersion (SPCE/MWCNT-COOH/CA-U), as shown by curves c and d, respectively. This substantial improvement can be attributed to the superior electrical conductivity inherent to MWCNT-COOH and CA, which facilitates more efficient electron transfer processes. Especially, curve c (blue) represents the electrode modified solely with MWCNT-COOH, whereas curve d corresponds to the electrode modified with a mixture of CA and MWCNT-COOH. The results indicate that, although the oxidation peak current of ACAP for curve c is higher than that for curve d, the ΔE value of the redox peak pair is notably larger for curve c than for the CA-containing electrode in curve d.

Fig. 6 Cyclic voltammograms of 1.0 mM ACAP in 0.1 M PBS (pH 6) solution at different electrodes (a) bare SPCE (black solid line), (b) SPCE/CA (red dashed line), (c) SPCE/MWCNT-COOH (blue line) and (d) SPCE/MWCNT-COOH/CA-U (purple line).

Reusability

The electrochemical performance and reusability of SPCEs are critical factors in analytical applications. In this study, we investigated the role of CA in enhancing the electrochemical performance and reusability of SPCEs when co-immobilized with MWCNT-COOH. The ACAP redox signal remained stable over 10 CV scans for SPCE/MWCNT-COOH/CA-U (Fig. 7A), while it decreased to 78.5% after 5 scans for MWCNT-COOH modified electrodes (Fig. 7B). This indicates that CA, when confined with MWCNT-COOH as a homogeneous and well-bonded material, reinforces electrode stability and accelerates the electrochemical stability of ACAP. This stabilization contrasts with the deactivation observed with MWCNTs only.⁵⁴ The electrode without CA demonstrates significant signal fluctuations and instability upon reuse, with a markedly larger ΔE value for the redox peak pair. This instability is attributed to surface contamination that occurs after only a few measurements. This approach aligns with previous findings where the incorporation of carbon-based nanomaterials improved the electrochemical properties of electrodes. Moreover, the use of ultrasonic dispersion during material preparation may facilitate better interaction between CA and MWCNT-COOH, leading to a more homogeneous and stable composite. The ability to reuse SPCEs multiple times without significant loss of performance is a notable advancement, considering that these electrodes are typically designed for single-use applications. Our findings suggest that the MWCNT-COOH/CA-U composite not only enhances the electrochemical response but also extends the functional lifespan of SPCEs, offering a cost-effective and sustainable solution for repeated measurements. To assess long-term stability, the sensor was tested after 5 days of storage under ambient conditions. The results showed that the ACAP signal remained stable, with no decrease in intensity, indicating excellent reusability and performance over time. The electrode incorporating CA exhibits high reusability, excellent stability, and remarkable signal repeatability, with no observable surface contamination even after 10 consecutive measurements. This strategy presents a promising avenue for developing durable and efficient electrochemical sensors. These findings clearly demonstrate that the absence of CA in the electrode modification mixture severely limits the ability to reuse the electrode, due to rapid degradation of surface integrity.

Fig. 7 Cyclic voltammogram of 1.0 mM ACAP in 0.1 M PBS (pH 7) solution at (A) SPCE/MWCNT-COOH/CA-U after 10 times reuse and (B) at SPCE/MWCNT-COOH after 5 times reuse.

Optimization experiments were systematically conducted to identify the most favorable conditions for the electrochemical detection of ACAP using modified electrodes. In particular, the ratio of MWCNT-COOH and CA incorporated in the electrode matrix was varied to determine its effect on the current response. The results demonstrated that a mass ratio of 1 : 10 for MWCNT-COOH:CA produced the highest and most reproducible current signals, as illustrated in Fig. S4A. This ratio appears to provide an optimal balance between sufficient surface area and effective electron transfer pathways, thereby enhancing the electrochemical response of ACAP.

Additionally, the influence of immersion time on ACAP adsorption at the surface of the modified electrode was investigated to maximize analyte accumulation and signal intensity. It was found that an immersion period of 5 minutes yielded the greatest adsorption efficiency and consequently the strongest current response (Fig. S4B). Shorter immersion times resulted in incomplete surface coverage, while longer durations did not produce further improvements, indicating that equilibrium adsorption was reached within this time frame. These optimization parameters, the mass ratio of 1 : 10 for MWCNT-COOH : CA and the 5-minute adsorption time, are critical for achieving sensitive and reliable electrochemical detection of ACAP under the experimental conditions employed.

The influence of pH solution on the voltammetric behavior of ACAP was systematically investigated using the SPCE/MWCNT-COOH/CA-U modified electrode. Experiments were performed in a 1.0 mM ACAP solution across a broad pH range, spanning from highly acidic (pH 2.0) to strongly alkaline (pH 12.0) conditions, to comprehensively evaluate the electrochemical response under varying proton concentrations (Fig. 8). Two distinct trends were observed: a nonlinear dependence of the oxidation peak current (I_pa) and a linear shift in the oxidation peak potential (E_pa) with increasing pH. The data revealed a pronounced dependence of the ACAP oxidation peak current on the pH value of the supporting electrolyte, with the highest peak current observed at neutral pH 7.0 (Fig. 8A). This optimal pH was consequently selected for all subsequent electrochemical determinations, as it provided the most sensitive and reliable analytical signal.

Fig. 8 (A) The pH effect on the voltammetric responses of ACAP at SPCE/MWCNT-COOH/CA-U. (B) Plot of anodic peak potential and current vs. pH.

Furthermore, as shown in Fig. 8B, the oxidation current increased progressively from pH 2.0 to a maximum at pH 7.0, and then declined in more alkaline media. This trend response reflects the pH-dependent ionization state of ACAP. At low pH, excessive protonation suppresses ACAP oxidation, while at high pH (above 7), deprotonation of the phenolic group or increased OH⁻ competition may inhibit adsorption or alter reaction kinetics. Therefore, neutral pH provides the most favorable balance between analyte solubility, electrochemical activity, and surface interaction, making it the optimal condition analysis for analytical measurements. In contrast, the anodic peak potential (E_pa) shifted linearly with pH, with a slope of −55.5 mV pH⁻¹ unit (R² = 0.9982), in good agreement with the Nernstian slope of 59 mV pH⁻¹ expected for a two-proton, two-electron redox process. This confirms that the ACAP oxidation involves equal numbers of electrons and protons, supporting a 2e⁻/2H⁺ mechanism at the modified electrode.^55,56 Together, these trends provide a consistent understanding of ACAP behavior under varying proton concentrations and validate the choice of pH 7.0 for optimized detection. Such mechanistic understanding not only validates the electrochemical behavior of ACAP on the novel electrode but also guides the optimization of experimental parameters for enhanced sensor performance.

The proposed mechanism for the electrocatalytic oxidation of ACAP at the MWCNT-COOH/CA modified electrode surface follows a two-electron, two-proton transfer pathway. Initially, ACAP undergoes a one-electron oxidation to form a radical cation localized on the amide nitrogen (–NH–). This intermediate then undergoes deprotonation, yielding a resonance-stabilized radical species. A second electron transfer and proton loss subsequently lead to the formation of N-acetyl-p-benzoquinone imine, the final oxidized product (Scheme S1).

The effect of scan rate on the electrochemical response of ACAP at the SPCE/MWCNT-COOH/CA-U modified electrode was thoroughly investigated to elucidate the underlying kinetics and control mechanisms governing the redox process. CV experiments were conducted over a broad range of scan rates, from 5 mV s⁻¹ to 300 mV s⁻¹, to monitor the corresponding changes in the peak current. The double logarithmic plots of the anodic and cathodic peak currents (log i_pvs. log scan rate) exhibited slopes of 0.9415 and 0.9926, respectively (Fig. S5B). These values, being close to unity, indicate that the electrochemical oxidation of ACAP on the SPCE/MWCNT-COOH/CA-U electrode is predominantly adsorption-controlled.⁵⁷ This behavior suggests strong interaction between ACAP molecules and the modified electrode surface, facilitated by the π–π stacking and functional groups of MWCNT–COOH and CA, which promote surface accumulation and efficient charge transfer.

Applying Laviron's equation for irreversible systems, the apparent electron transfer coefficient (α) was estimated to be approximately 0.152, assuming a two-electron process. This relatively low α value is consistent with the irreversible nature of ACAP oxidation on the modified SPCE and further supports our earlier mechanistic interpretation involving a 2e⁻/2H⁺ transfer process.

Interference

The potential interference of commonly encountered excipients on the electrochemical analysis of ACAP was systematically investigated to ensure the reliability and specificity of the proposed sensing method. These interfering agents were selected based on their frequent inclusion in pharmaceutical tablet formulations. The study focused on evaluating the electrochemical behavior of ACAP in the presence of these substances using the SPCE/MWCNT-COOH/CA-U modified electrode, with both ACAP and the interfering agents maintained at an identical concentration of 50 µM. The resulting data are summarized in Table 1.

Table 1 Interference effects on the determination of 5 × 10⁻⁶ M ASP in pH 7.0 PBS

No.	Interference	Concentration (M)	Signal change (%)	RSD (%)
1	Croscarmellose sodium	5 × 10^−5	−4.6	1.8
2	Ascorbic acid	5 × 10^−5	−4.8	2.4
3	Magnesium stearate	5 × 10^−5	+0.4	1.2
4	Lactose	5 × 10^−5	−5.1	3.2
5	Dopamine	5 × 10^−5	+1.7	2.6
6	Uric acid	5 × 10^−6	−2.9	2.3
7	Aspirin	5 × 10^−6	+1.3	1.7
8	Caffeine	5 × 10^−7	+2.5	2.8
9	2-Aminophenol	5 × 10^−6	−5.3	3.6
10	Phenol	5 × 10^−6	−1.4	1.2

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As illustrated by the results, most of the tested excipients—including L-ascorbic acid, dopamine, uric acid, magnesium stearate, croscarmellose sodium (CMS), 2-aminophenol, phenol and lactose—exhibited no significant impact on the voltammetric response of ACAP. Their presence did not alter the peak current or shift the potential of the ACAP signal, thereby affirming the selectivity of the sensor in complex sample matrices. These findings indicate that the developed sensor system demonstrates high selectivity and robustness in the presence of most typical excipients. The results confirm the practical applicability of the SPCE/MWCNT-COOH/CA-U sensor in real-world pharmaceutical analysis. The minimal interference from most excipients supports its use in routine ACAP determination, while the identified sensitivity to corn starch highlights the importance of method optimization for specific sample matrices.

Electrochemical determination of ACAP using the SPCE/MWCNT-COOH/CA-U electrode

The electrochemical behavior of ACAP at varying concentrations was systematically investigated using differential pulse voltammetry (DPV) at a screen-printed carbon electrode (SPCE) modified with MWCNT-COOH and CA. The resulting DPV responses are illustrated in Fig. 9A. A clear correlation between peak current and ACAP concentration was observed, indicating the suitability of this sensor configuration for quantitative analysis.

Fig. 9 (A) DPV responses for various concentration of ACAP at SPCE/MWCNT-COOH/CA-U electrodes in pH 7.0 PBS. (B) Plot of I_pavs. ACAP concentrations.

As shown in Fig. 9B, the calibration plot revealed two well-defined linear regions. The first linear range extended from 0.05 to 50 µM, with an excellent correlation coefficient (R² = 0.9995), while the second range spanned from 50 to 250 µM, also showing a strong linearity (R² = 0.9962). These dual linear ranges demonstrate the sensor's capacity to maintain sensitivity across a broad concentration spectrum. The sensitivity of the modified SPCE electrode was estimated to be 9.4602 µA µM⁻¹; this high sensitivity value confirms the sensor's strong analytical response and suitability for ACAP trace-level quantification.

Furthermore, the analytical performance of the method was evaluated by determining the limit of detection (LOD) and limit of quantitation (LOQ), which were calculated to be 15 nM and 58 nM, respectively. These values underscore the high sensitivity of the sensor, making it capable of detecting trace levels of ACAP. The use of MWCNT-COOH in conjunction with CA and ultrasonication technique provides an enhanced electrochemical interface, likely due to increased surface area, improved electron transfer kinetics, and favorable interactions with ACAP molecules.

The proposed sensor performance is compared with recent ACAP sensors reported in the literature, as summarized in Table 2. While our detection limit of 15 nM is not the lowest among existing methods, it is still within a competitive range for trace detection. Notably, our sensor offers a broader linear range (0.05–250 µM) compared to most previously reported sensors, which often exhibit narrower ranges. This combination of low detection limit and wide linearity highlights the potential of our modified electrode as a versatile and sensitive platform for ACAP analysis across a wide concentration spectrum.

Table 2 Comparison of analytical performances for some ACAP reported methods

Sensor	Technique	Linear range (µM)	LOD (nM)	Ref.
Ag/Au-GCE^a	DPV	0.4–10.2	540	58
GCP^b@Bi2O3	DPV	0.05–12	10	59
β-Cyclodextrin/GCE	DPV	0.1–80	97	60
GCP-RGO^c	DPV	1.2–220	31	61
CPE^d/MWCNTs/ZnCrFe	DPV	0.1–368	9	62
CPE/CuO-Gr^e	DPV	0.025–5.3	8	63
SPCE/MWCNT-COOH/CA-U	DPV	0.05–250	15	This work
	CV	10–2500

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Overall, the proposed method represents a robust and efficient strategy for ACAP detection. Its wide dynamic linear range, coupled with low detection limits and strong signal stability, highlights its potential for application in pharmaceutical quality control, clinical diagnostics, and environmental monitoring. The incorporation of nanomaterials such as MWCNT-COOH further enhances the versatility and performance of the electrochemical sensing platform, supporting its broader application in analytical chemistry.

Real sample analysis

The quantification of ACAP content in commercially available tablet formulations was conducted to evaluate the applicability and accuracy of the proposed electrochemical sensing method. Tylenol tablets were initially weighed and then meticulously ground into a fine powder to ensure homogeneity. A measured quantity of this powdered sample was subsequently dissolved in double-distilled water to prepare a solution suitable for analysis. To generate sample concentrations falling within the established range of the calibration curve, appropriate dilutions were carried out. These prepared solutions were then subjected to differential pulse voltammetry (DPV) analysis using proposed modified electrodes (SPCE/MWCNT-COOH/CA-U). The voltammetric data obtained were statistically evaluated using a paired sample t-test to compare the measured acetaminophen (ACAP) concentrations with the values indicated on the pharmaceutical label. At a significance level of α = 0.05, no statistically significant difference was observed, indicating that the results derived from the proposed electrochemical method are in close agreement with the labeled ACAP contents. The analysis yielded an average ACAP content of 495.8 ± 1.6 mg per tablet. These findings underscore the analytical reliability and potential applicability of the SPCE/MWCNT-COOH/CA-U electrode system for accurate determination of acetaminophen in commercial pharmaceutical formulations.

The proposed analytical method demonstrated enhanced performance in the determination of ACAP, particularly when evaluated against previously reported techniques. Notable improvements were observed in three critical analytical parameters: a broader linear dynamic range, heightened sensitivity, and a significantly lower detection limit. A comparative summary of these performance metrics is systematically presented in Table 2.

Conclusions

In this study, SPCE was successfully modified using a composite of MWCNT-COOH and CA, forming a hybrid material aimed at enhancing the electrochemical properties of the sensor platform. The incorporation of CA into the MWCNT-COOH matrix, aided by the application of ultrasonic dispersion techniques, significantly improved the solubility and dispersion of the carbon nanotubes within the aqueous phase. This enhanced dispersion facilitated the uniform coating of the electrode surface and enabled the stabilization of the catechol functional group on the SPCE surface. The presence of catechol contributed to an increase in surface hydrophilicity, which is critical for efficient analyte interaction, and notably improved the reusability and overall stability of the modified electrode.

The electrochemical performance of the modified electrode was evaluated using the DPV technique, which is known for its high sensitivity and resolution in detecting trace levels of analytes. The SPCE/MWCNT-COOH/CA-U sensor demonstrated excellent analytical capabilities for the detection of ACAP, offering a remarkably broad linear detection range spanning from 0.05 µM to 250 µM. The method also achieved low limits of detection (LOD) and quantification (LOQ), calculated to be 15 nM and 58 nM, respectively. These figures underscore the method's potential for accurate trace-level analysis, making it highly suitable for pharmaceutical quality control and clinical diagnostics.

To further evaluate the applicability of the developed sensor in real-world scenarios, the potential interference from commonly used pharmaceutical excipients was assessed. The results confirmed minimal interference. This reinforces the robustness of the method when applied to complex sample matrices. The sensor was then utilized to determine the ACAP content in commercially available Tylenol tablets produced in the United States. The measured ACAP concentration was 495.8 ± 1.6 mg per tablet, which closely corresponds with the labeled value of 500 mg. This level of agreement validates the accuracy and practical reliability of the proposed electrochemical sensing approach.

Author contributions

T-T. H., A. V. T. L. and S-H. C. designed and directed the research. T-T. H., A. V. T. L., D. V. A. D., conceived and planned the experiments. T-T. H., A. V. T. L., D. V. A. D., K-L. K. carried out the experiment and analysed the data., S-H. C., T-T. H. and A. V. T. L. wrote the manuscript with inputs from all authors. A. V. T. L., S-H. C. are supervisors. All authors discussed the results and contributed to the final manuscript. All authors read and approved the final manuscript.

Conflicts of interest

There are no conflicts to declare

Data availability

The data supporting this article has been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03786g.

Acknowledgements

This research was supported by the Foundation for Science and Technology Development of Dalat University, Research no. 19TĐ/1420/QĐ-ĐHĐL from Dalat University, Vietnam, and grant number 106-2113-M-260-005 from the National Science and Technology Council, Taiwan.

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