Flower-like Nickel Oxide/Carbon Nanotube Nanocomposite for Sensitive Electrochemical Detection of Alfuzosin
Article information
Abstract
We propose a novel method for detecting alfuzosin hydrochloride (AFZ) using a 3D flower-like nickel oxide/carbon nanotube (NiO/CNT) nanocomposite synthesized through a hydrothermal process. The nanocomposite was extensively characterized using advanced techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). This NiO/CNT nanocomposite was subsequently employed to modify a glassy carbon electrode (GCE), enabling the development of a sensitive electrochemical sensor for AFZ detection. The fabricated sensor demonstrated a broad linear detection range of 0.5–200 μM and an impressively low detection limit (LOD) of 0.0696 μM using square wave voltammetry (SWV). Additionally, the sensor exhibited excellent selectivity, reproducibility, repeatability, and stability. Its practical utility was validated by successfully detecting AFZ in human serum samples and pharmaceutical tablets, achieving high recovery rates. These results highlight the potential of the NiO/CNT nanocomposite-based electrochemical sensor as a highly sensitive and reliable tool for AFZ detection in clinical and pharmaceutical applications.
INTRODUCTION
Benign prostatic hyperplasia (BPH), a common non-cancerous enlargement of the prostate gland, affects a significant proportion of older men [1,2]. This condition can impair kidney function by obstructing urine flow and increasing the risk of bladder dysfunction and urinary tract infections. Alfuzosin hydrochloride (AFZ) is a widely prescribed medication for managing BPH. Among the various treatment options available, alpha-adrenergic blockers remain the first-line therapy for BPH patients [3,4]. AFZ acts as a potent antagonist of second-generation alpha1-adrenoceptors (α1-AR), relaxing the smooth muscles of the bladder neck and prostate by binding to α1 adrenergic receptors. This mechanism enhances urine flow and alleviates the symptoms of prostatic hyperplasia [5,6]. Although AFZ is generally well accepted, it can have unfavorable side effects such as headache, dizziness, and hypotension. In addition, there is a small but serious risk of developing orthostatic hypotension, priapism, and hypersensitivity reactions. Therefore, precise monitoring of AFZ levels in patients is essential to ensure effective treatment and minimize risks [7].
Traditionally, AFZ quantification has relied on established techniques such as chromatography [8], spectrophotometry [9], spectrofluorometry [10], and capillary electrophoresis [11]. While effective, these methods are often complex, time-consuming, and expensive. To address these limitations, the field is witnessing a surge in the development of novel approaches that leverage extremely sensitive and electrochemical sensing platforms. These innovative methods aim to provide more efficient, cost-effective, and user-friendly solutions for AFZ quantification. Electrochemical sensors are a viable alternative for detecting AFZ owing to their ease of use, rapidity, low cost, and high sensitivity [12–15]. These sensors work by measuring electrical signals generated when AFZ interacts with the electrode surface [16–18]. The effectiveness of these sensors depends heavily on the materials used in the electrodes, particularly their conductivity, surface area, catalytic activity, and durability [19–21]. Therefore, the key to producing high-performing electrochemical sensors for AFZ detection is the development of novel electrode materials with enhanced characteristics.
The development of novel electrochemical sensors leveraging the exceptional properties of nanomaterials has garnered significant attention from researchers. Nanoparticles, with their extraordinary physical, chemical, optical, and electrical properties, often outperform bulk materials, making them highly desirable in various applications [22]. Among these materials, transition metal oxide (TMO) nanostructures have gained prominence due to their outstanding capabilities in optical, magnetic, and electrical applications. TMOs possess unique attributes such as wide band gaps, high dielectric constants, and highly reactive electronic transitions, which make them ideal for advanced sensor technologies [23,24]. Nickel oxide (NiO), a well-known p-type TMO, is particularly attractive due to its superior catalytic properties, environmental friendliness, chemical stability, biocompatibility, and high adsorption capacity [25]. These properties contribute to it being an attractive material for several uses, such as solar energy cells [26], gas sensors [27], photo-electrocatalysis [28,29], capacitors [30], and absorbents [31]. Some research explored the electrochemical performance of NiO to be strongly affected by its structural characteristics and shape [32], which include various forms such as nanoparticles, hollow nanostructures, nano microspheres, nanoflowers, and nanowires [33]. Among these, flower-like NiO nanostructures stand out due to their expansive, three-dimensional surfaces, which provide numerous catalytically active sites, enhancing their efficiency in electrochemical applications. Furthermore, their highly porous structures and large active surface areas enhance diffusion and adsorption efficiency for analytes [34]. However, the inherent low conductivity of NiO poses a significant limitation, hindering its electron transfer rates and, consequently, its potential in electrochemical sensing applications. To address this challenge, hybridizing NiO with highly conductive materials such as carbon nanotubes (CNTs) has proven effective in enhancing electron transfer rates and electrocatalytic activity. CNTs, one of the most widely used carbon nanomaterials, are renowned for their exceptional conductivity, strong mechanical and physical properties, efficient catalytic activity, and resistance to surface fouling. These attributes make CNTs an ideal choice for highly sensitive electrochemical sensors [35,36]. Therefore, the improved electron transfer rate of the NiO/CNT composite compared to CNTs is due to the synergistic effect of NiO and CNTs. NiO provides abundant redox-active sites, while CNTs offer excellent conductivity, creating efficient electron pathways. The 3D flower-like structure of the composite increases the surface area and active sites, enhancing electron transfer kinetics. Additionally, better contact and connectivity between NiO and CNTs reduce charge transfer resistance [32,34,37–39].
In this study, we synthesized a 3D flower-like NiO/CNT nanocomposite using a hydrothermal approach. The resulting nanocomposite was used to fabricate a modified electrode (NiO/CNT/GCE) by drop-casting the NiO/CNT material onto a glassy carbon electrode (GCE). This modified electrode was employed, for the first time, for the highly sensitive detection of alfuzosin hydrochloride (AFZ) in commercial tablets and human serum samples. The NiO/CNT/GCE electrode, developed through a simple, cost-effective, and environmentally friendly method, exhibited superior performance characteristics. These included enhanced conductivity, reduced charge transfer resistance, improved stability, excellent reproducibility, and a low detection limit for AFZ electrochemical sensing. Furthermore, the incorporation of CNTs as a support material effectively addressed the inherent low conductivity of NiO while simultaneously improving AFZ adsorption capabilities, thereby enhancing the electrocatalytic properties of the electrode [40–43]. With its potential to significantly advance the management of benign prostatic hyperplasia (BPH), this work offers a robust and highly efficient platform for the quantification of AFZ. By enabling accurate and real-time monitoring of AFZ levels, this innovation contributes to improved patient safety and therapeutic outcomes.
EXPERIMENTAL
Chemicals
Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), cetyltrimethylammonium bromide (CTAB), urea (NH2CONH2), alfuzosin hydrochloride (AFZ), ascorbic acid (AA), uric acid (UA), glucose (C6H12O6), potassium hydroxide (KOH), sodium chloride (NaCl), iron (II) sulfate heptahydrate (FeSO4·7H2O), calcium sulfate (CaSO4), N,N-dimethylformamide (DMF), human serum, methanol, sodium dihydrogen phosphate (NaH2PO4), and disodium hydrogen phosphate (Na2HPO4) were obtained from Sigma-Aldrich, USA. Without any additional purification, all chemicals were used exactly as they were collected. Milli-Q water (Millipore) was used for preparing all the solutions. Na2HPO4 and NaH2PO4 were dissolved in Millipore water in order to produce 0.1 M phosphate buffer saline (PBS), which has a pH range of 5–9. The solution was then neutralized using 0.1 M HCl or NaOH to achieve the desired pH range.
Instrumentation
Several methods were employed to characterize the synthesized catalysts to comprehend their structure and characteristics. X-ray diffraction (XRD) on a (Rigaku MiniFlex600, Japan) revealed their crystal structure. X-ray photoelectron microscopy (XPS) using a (K-Alpha, USA) confirmed the chemical composition of the NiO/CNT nanocomposite by analyzing the binding energies of key elements (nickel, oxygen, carbon). Scanning electron microscopy (SEM) with a (ZEISS Sigma 500, Germany) provided initial observations of the surface morphology and microstructure. Further details were obtained using field emission SEM (FE-SEM, TESCAN MIRA LMH, Czech Republic) for higher resolution and transmission electron microscopy (TEM, JEOL JEM-1010, Japan) for the most intricate structural features. Furthermore, the materials electrochemical characteristics were assessed using a CHI 770E electrochemical workstation (from USA) in a typical threeelectrode setup. A platinum wire was used as the counter electrode, an Ag/AgCl as the reference electrode, and either a bare GCE or one modified with the NiO/CNT nanocomposite as the working electrode.
Synthesis of flower-like NiO/CNT nanocomposite
Prior to the synthesis of NiO/CNT nanocomposite, the multi-walled carbon nanotubes (MWCNTs) were subjected to an acid treatment. In a typical procedure, 2 g of MWCNTs were well-dispersed under stirring in a mixture of concentrated H2SO4 and HNO3 (3:1 v/v ratio) for 4 hours at 25–30°C [44]. The resulting mixture was thoroughly washed with deionized (DI) water and ethanol until a neutral pH of 7 was achieved. Finally, the pretreated MWCNTs were dried in an oven at 70°C overnight.
3D flower-like NiO/CNT nanocomposite were prepared by a facile hydrothermal method. In a typical synthesis, Ni(NO3)2·6H2O (0.5089 g) and CTAB (0.319 g) were dissolved under magnetic stirring for 20 minutes in a 50 mL mixed solvent of DI water and ethylene glycol (1:1 v/v). Subsequently, 20 mg of the pretreated MWCNTs were added to the above solution. 0.4 g of urea was added after 30 minutes of stirring. After that, the solution mixture was transferred in a 100 mL autoclave lined with Teflon coating and hydrothermally treated for 12 hours at 180°C. The autoclave was then allowed to cool down to room temperature naturally. The product was then collected by centrifugation and carefully cleaned using three cycles of DI water and ethanol. Eventually, the precipitates were vacuum dried for 15 hours at 70°C and then annealed for 4 hours at 450°C in muffle furnace. On the other hand, the same process was used to synthesize NiO nanoflowers, but CNTs were not included. A graphical representation of the synthesis of the 3D flower-like NiO/CNT nanocomposite is shown in Scheme 1.
Graphical illustration of the synthesis of the 3D flower-shaped NiO/CNT nanocomposite.
Preparation of the modified electrode
In order to attain a clean GCE surface prior to construction, the GCE was polished in stages using alumina slurries with progressively lower particle sizes (0.3, 0.1, and 0.05 µm). After that, the polished GCE was cleaned by repeatedly sonicating it with ethanol and deionized water. A 1 mL solution of deionized water was mixed with 1 mg of the produced NiO/CNT nanocomposite to create a stable suspension for electrode modification. Afterwards, vortex mixing and ultrasonication were used to homogenize the suspension. Following that, 6 µL of the NiO/CNT nanocomposite suspension was dropped over the cleaned GCE surface, and it was dried under an IR lamp. The resulting fabricated electrode was named as NiO/CNT/GCE, and it was used for the electrochemical detection of AFZ in this research.
RESULTS AND DISCUSSION
Physical characterization of nanocomposite
The crystal structure and phase composition of the synthesized materials were analyzed using XRD. As shown in Fig. 1A, the XRD pattern of the as-prepared NiO nanoflowers corresponds to the standard reference spectrum (JCPDS No. 25-0581). The diffraction peaks associated with the (111), (200), and (220) planes exhibited strong intensities, while the (311) and (222) planes showed weaker intensities, confirming the cubic crystal structure and high crystallinity of the NiO nanoflowers [45]. The XRD patterns of the synthesized NiO/CNT composites retained the characteristic peaks of NiO while also exhibiting the distinct peak at 25.76°, attributed to the (002) plane of graphitic CNTs (Fig. 1A). The variations in peak intensity between NiO and NiO/CNT confirmed the successful incorporation of CNTs into the composite. Additionally, the absence of impurity peaks in the patterns indicated the high phase purity of both NiO and NiO/CNT composites [46]. XPS analysis (Fig. 1B–E) revealed the elemental composition and chemical state of the NiO/CNT nanocomposite. The survey spectrum (Fig. 1B) showed that the three most noticeable elements in the composite are nickel (Ni 2p), oxygen (O 1s), and carbon (C 1s). High-resolution XPS spectra for Ni 2p (Fig. 1C) displayed two main peaks at 854.8 eV (Ni 2p3/2) and 871.77 eV (Ni 2p1/2), along with prominent satellite peaks at 860.19 eV and 878.05 eV, which arise from shake-up processes characteristic of Ni2+ ions in NiO. These shake-up peaks confirm the presence of the Ni2+ oxidation state, validating the successful formation of NiO in the composite [46]. The presence of these satellite peaks is significant for the NiO/CNT composite's electrochemical properties. The Ni2+ ions provide abundant redox-active sites, facilitating efficient electron transfer reactions. This directly contributes to the improved catalytic activity of the composite for alfuzosin hydrochloride detection. Furthermore, these peaks reflect the proper chemical integration of NiO with CNTs, ensuring the structural quality of the nanocomposite. The deconvoluted O 1s spectrum (Fig. 1D) identified three distinct oxygen species: Oiv and Oi corresponding to metal-oxygen bonds at 527.93 eV and 529.28 eV, respectively; low oxygen coordination defects are responsible for Oii at 531.09 eV, while physically and chemically adsorbed water molecules are indicated by Oiii at 532.16 eV [47–50]. Furthermore, C 1s spectrum (Fig. 1E) revealed five peaks at 282.87 eV (Ni–C), 284.91 eV (C–C/C=C), 286.72 eV (C–O), 288.21 eV (C=O), and 289.09 eV (O–C=O), signifying the various carbon bonding environments within the nanocomposite [51–54].
(A) XRD spectrum of CNT, NiO and NiO/CNT nanocomposite, (B–E) XPS spectrum analysis of NiO/CNT, (B) full survey and high resolutions of (C) Ni 2p, (D) O 1s, and (E) C 1s, respectively.
Surface morphological investigations
FESEM images revealed the formation of flower-shaped NiO structures at both low and high magnifications (Fig. 2A, B). These NiO flowers exhibit a hierarchical assembly with a high surface area to volume ratio. Each flower is constructed from nanosheets of 2D NiO, resembling flower-like structures. The nanosheets are curved and well-organized, forming circular assemblies with bulbous exteriors. These assemblies come together to create the overall flower-like structure. Fig. 2C shows the morphology of MWCNTs, while Fig. 2D illustrates the NiO/CNT nanoflower composite. The incorporation of CNTs into the NiO matrix significantly altered the morphology of the composite. The resulting NiO/CNT composite exhibited sharper, petal-like nanosheets compared to the original NiO nanoflakes (Fig. 2A, B). EDS analysis (Fig. 2I and S1) verified the elemental composition and distribution within the composite. The presence of nickel (Ni), oxygen (O), and carbon (C) was confirmed, with atomic percentages of 29.74% (C), 15.54% (O), and 54.72% (Ni) as indicated in the spectrum (Fig. 2I). EDS mapping (Fig. S1) further revealed the uniform distribution of these elements throughout the nanocomposite, validating the successful integration of NiO and CNTs. These findings demonstrate that the synthesis process effectively combines the components with consistent dispersion. Further morphological insights were obtained using TEM analysis (Fig. 2E). TEM images revealed that the NiO/CNT nanocomposite consists of nanoflakes that assemble into a three-dimensional, flower-like structure. HRTEM (Fig. 2F) further demonstrated lattice fringes with spacings of 0.24 nm and 0.34 nm, corresponding to the (111) crystal plane of NiO and the (002) plane of CNT, respectively [46]. This confirms the presence of crystalline NiO and CNT in the composite. To evaluate the crystallinity of the NiO/CNT nanoflowers, additional analyses were performed using fast Fourier transform (FFT) (Fig. 2G) and selected area electron diffraction (SAED) patterns (Fig. 2H). The well-defined diffraction spots in both patterns confirmed the crystalline structure of the NiO/CNT composite. These combined analyses validated the successful synthesis, integration, and crystallinity of the NiO/CNT nanoflower composite.
(A, B) FESEM images of NiO nanoflower, (C) CNT, (D) NiO/CNT nanoflower composite, (E) TEM image, (F) HRTEM of NiO/CNT nanoflower composite. (G) FFT pattern, (H) SAED pattern and (I) EDS analysis of the NiO/CNT nanoflower composite.
Electrochemical characterization of the modified electrode
The electrochemical performance of various modified electrodes was evaluated using cyclic voltammetry (CV). As shown in Fig. S2A, the NiO/CNT/GCE electrode exhibited the highest peak current response (Ipa = 130.7 µA) and the lowest peak-to-peak separation potential (ΔEp = 0.092 V) among the tested electrodes, which included NiO/GCE (Ipa = 87.1 µA, ΔEp = 0.098 V) and CNT/GCE (Ipa = 107.6 µA, ΔEp = 0.095 V). These measurements were performed in a solution containing 0.1 M KCl with 5 mM [Fe(CN)6]3−. The superior performance of NiO/CNT/GCE suggests that the synergistic interaction between CNTs and NiO nanoparticles significantly enhances electron transfer kinetics at the electrode interface, enabling more efficient redox reactions. Electrochemical impedance spectroscopy (EIS) further corroborated the exceptional conductivity and electron mobility within the NiO/CNT composite. The EIS spectra (Fig. S2B) revealed that NiO/CNT/GCE exhibited a significantly lower transfer resistance (Rct = 32 Ω) compared to bare GCE (216 Ω), NiO/GCE (180 Ω), and CNT/GCE (72 Ω). The smaller semicircle in the Nyquist plot for NiO/CNT/GCE indicates simplified electron transmission at the electrode/electrolyte interface. This improved charge transfer can be attributed to the synergistic effect of NiO and CNTs: NiO nanoparticles provide abundant redox-active sites through Ni2+/Ni3+ transitions, while CNTs create a highly conductive network that facilitates efficient electron transport. The redox reaction of [Fe(CN)6]3− at the NiO/CNT/GCE was determined to be diffusion-controlled, as demonstrated by the CV results obtained at different scan rates (20–200 mV/s) ((Fig. S2C). An increase in the scan rate (ʋ1/2) resulted in a proportional rise in the redox peak currents (Ipa/Ipc), confirming the diffusion-controlled nature of the reaction. This was further validated by the linear relationship observed between the peak currents and the square root of ʋ1/2 ((Fig. S2D). Additionally, NiO/CNT/GCE showed significantly higher peak currents compared to other modified electrodes ((Fig. S3), highlighting its superior electrocatalytic activity. The electrochemical active surface area (EASA) of the electrodes was calculated using the Randles–Sevcik equation (Ip = (2.69 × 105) n3/2 D1/2 A ʋ1/2 C) [55]. Here, ‘D’ stands for the diffusion coefficient (cm2/s), ‘A’ for the active surface area of the electrode (cm3), ‘Ip’ for the peak current (Ipa/Ipc, µA), ‘n’ for the electrons involved (n=1), and ‘ʋ’, ‘C’ for the scan rate (mV/s) and concentration (mol/cm3) of the redox probe [Fe(CN)6]3−, respectively. The EASA of NiO/CNT/GCE (0.1486 cm2) exceeded those of CNT/GCE (0.0842 cm2), NiO/GCE (0.0739 cm2), and bare GCE (0.0676 cm2). This larger EASA, combined with lower resistance and smaller peak separation, underscores the exceptional electrochemical performance of NiO/CNT/GCE.
In summary, the strategic integration of NiO and CNT nanoparticles onto the GCE surface resulted in an electrode material with enhanced electron transfer kinetics, higher current response, and a significantly expanded active surface area. These characteristics establish NiO/CNT/GCE as a promising candidate for the fabrication of advanced electrochemical sensors.
Electrochemical behavior of AFZ on NiO/CNT/GCE
The electroanalytical performance of NiO/CNT/GCE was evaluated and compared to other electrodes (bare GCE, NiO/GCE, CNT/GCE) for the detection of AFZ (0.5 mM) using CV in 0.1 M PBS (pH 7) at a scan rate of 100 mV/s. As shown in Fig. 3A, NiO/CNT/GCE exhibited the highest anodic peak current (Ipa = 142.6 µA) and a lower anodic peak potential (Epa = 0.849 V) compared to the other electrodes. This superior performance can be attributed to the synergistic interaction between NiO and CNT, which enhances the catalytic activity, conductivity, and electron transfer kinetics of the composite. The electrochemical behavior of NiO/CNT/GCE toward AFZ oxidation was further analyzed by investigating the effect of solution pH. As shown in Fig. 3B, the peak current increased as the pH of the electrolyte rose from 5 to 7 and then decreased at pH values above 7. The highest oxidation peak current was observed at pH 7, which was selected as the optimal condition for subsequent experiments (Fig. 3C). Additionally, the peak potential showed a slight negative shift with increasing pH, likely due to the protonation dynamics of AFZ molecules. A linear relationship between the oxidation peak potential and pH was observed (Fig. 3D), described by the equation Epa = −0.0506pH + 1.202 (R2 = 0.997), with a slope close to the theoretical Nernstian value (–0.059 V/pH) [56]. This result indicates that the electro-oxidation of AFZ at NiO/CNT/GCE involves an equal number of electrons and protons. The effect of scan rate on AFZ detection was also studied at NiO/CNT/GCE using CV (Fig. 3E). As the scan rate increased (20–200 mV/s), the oxidation peak current rose proportionally, indicating a diffusion-controlled process for AFZ oxidation. This was further validated by a linear relationship between the peak current (Ipa) and the square root of the scan rate (ʋ1/2) (Fig. 3F), described by the equation Ip (μA) = 13.078 ((mV/s)1/2) – 24.91 (R2 = 0.998). The remarkable electrochemical performance of NiO/CNT/GCE was further supported by its higher peak current and lower peak potential compared to the other electrodes. These characteristics highlight the superior electrocatalytic activity of NiO/CNT/GCE, driven by the synergistic effects of NiO’s catalytic properties and CNT’s high conductivity and large surface area. The composite offers enhanced sensitivity, efficient electron transfer, and improved electrochemical behavior, making it an ideal platform for AFZ detection.
(A) CV curves of bare GCE, NiO/GCE, CNT/GCE and NiO/CNT/GCE in 500 μM AFZ containing 0.1 M PBS (pH 7.0). (B) CV curves of NiO/CNT/GCE in 500 μM AFZ at various pH ranges from 5 to 9. (C, D) Plots of I pa and E pa versus pH for NiO/CNT/GCE, respectively. (E) CV curves of NiO/CNT/GCE for various scan rates ranging from 20 to 200 mV/s in the presence of 500 μM AFZ containing 0.1 M PBS (pH 7.0) at. (F) Plot showing the relationship between peak current (Ipa) and the square root of the scan rate (ʋ1/2).
Analytical performance of NiO/CNT/GCE toward AFZ detection
The analytical performance of the fabricated electrode for AFZ determination under optimal conditions was examined using square wave voltammetry (SWV), a technique recognized for its simplicity, rapid response, wide linear range, high sensitivity, and low detection limits [57]. Fig. 4A shows the SWV responses of the NiO/CNT/GCE electrode to various concentrations of AFZ in 0.1 M PBS (pH 7). Well-defined anodic peaks at a potential of 0.84 V, corresponding to AFZ oxidation, were observed, demonstrating the suitability of the NiO/CNT/GCE electrode for AFZ sensing. As shown in Fig. 4A, the anodic peak current (Ipa) increased proportionally with AFZ concentration in the range of 0.5 to 200 µM. The calibration plot (Fig. 4B) revealed three distinct linear regions, reflecting the varying response rates at different concentration ranges. At lower AFZ concentrations, the response was faster due to the rapid transport of molecules to the electrode surface, while higher concentrations showed slower responses caused by diffusion limitations. The linear equations obtained were: Ipa (μA) = 0.474 C [μM] + 0.092 (R2 = 0.984) for 0.5–20 µM and Ipa (μA) = 0.254 C [μM] + 4.574 (R2 = 0.999) for 20–60 µM, and Ipa (μA) = 0.0761 C [μM] + 15.791 (R2 = 0.977) for 60–200 µM. The limit of detection (LOD) was calculated to be 0.0696 µM using the formula (LOD = 3 S/m), where ‘S’ is the standard deviation of the lowest concentration of AFZ in the calibration curve and ‘m’ is the slope of the calibration curve [58]. A comparison of the NiO/CNT/GCE sensor with previously reported AFZ sensors is presented in Table 1, highlighting its superior performance. The selectivity of the NiO/CNT/GCE sensor for AFZ detection was evaluated in the presence of potential interferents, including common biological metabolites (such as glucose, ascorbic acid, and uric acid) and inorganic substances (such as K+, Na+, Cl–, NO3–, SO42–, Co3+, Ca2+, and Fe2+). These interferents were added in 20-fold excess to a solution containing 500 µM AFZ in 0.1 M PBS (pH 7). The results showed only minor shifts in the sensor’s peak potential, indicating that the detection of AFZ was not significantly affected by the interferents. A bar graph (Fig. 4C) displaying the oxidation peak current responses to the interferents showed a relative error of less than 10%, confirming the high selectivity of the NiO/CNT/GCE sensor for AFZ monitoring.
(A) SWV curves of the NiO/CNT/GCE at varying AFZ concentrations (0.5–200 µM) in 0.1 M PBS (pH 7.0). (B) Calibration plot showing the relationship between AFZ concentrations and the anodic peak current. (C) Bar plot of oxidation peak currents in the presence of various interfering species. (D) Repeatability analysis, (E) reproducibility evaluation, and (F) stability assessment of the NiO/CNT/GCE sensor for AFZ monitoring.
Comparison of the NiO/CNT/GCE sensor with previously reported methods for AFZ detection
The repeatability, reproducibility, and stability of the fabricated NiO/CNT/GCE sensor are critical factors influencing its reliability and practical applicability. As shown in Fig. 4D, the voltammetry responses from five consecutive measurements displayed minimal variation, indicating excellent repeatability. Similarly, Fig. 4E highlights the sensor’s reproducibility, with consistent voltammetry responses observed across five independently fabricated electrodes. The stability of the NiO/CNT/GCE sensor was evaluated over a 20-day period, during which the sensor was stored at 5°C in 0.1 M PBS (pH 7). Measurements were conducted every five days, and as depicted in Fig. 4F, the sensor retained 90% of its initial peak current after 20 days with no significant degradation in performance. These results demonstrate the robust and consistent performance of the NiO/CNT/GCE sensor, underscoring the reliability of this fabrication technique for practical applications.
Real sample analysis
To validate the feasibility of the proposed sensing method for detecting AFZ in real-world conditions, the performance of the NiO/CNT/GCE sensor was tested on actual samples. The sensor’s real-time applicability was assessed using AFZ-containing pharmaceutical tablets ((Fig. S4A) and AFZ-free human serum samples (Fig. S4C) via the standard addition method. For the pharmaceutical tablet analysis, 10 mg of an AFZ tablet, obtained from a local pharmacy, was ground into a fine powder using an agate mortar and dissolved in 100 mL of 0.1 M PBS (pH 7) to prepare the sample within the calibration range of standard AFZ. The solution was thoroughly shaken to ensure uniform concentration and then used for analysis. For the human serum analysis, serum samples were collected, refrigerated, and diluted 10-fold in 0.1 M PBS (pH 7) prior to testing. The recovery results for pharmaceutical tablets and human serum samples are summarized in Table 2. Additionally, Fig. S4B and S4D show the linear relationship between the peak current and the AFZ concentration in the real samples. These findings demonstrate that the NiO/CNT/GCE sensor provides accurate recovery rates, indicating its suitability for monitoring AFZ in real-world samples.
Determination of AFZ concentrations in real samples using NiO/CNT/GCE (n=3)
CONCLUSIONS
This study successfully developed a fast, simple, and efficient method for the detection of AFZ using a NiO/CNT nanocomposite as the sensing platform. The flower-shaped NiO/CNT nanocomposite was synthesized via a hydrothermal method and thoroughly characterized using various spectroscopic and microscopic techniques, including XRD, XPS, SEM, FESEM, HRTEM, and EDS analysis. The NiO/CNT/GCE sensor demonstrated excellent electroanalytical performance, enabling the first successful detection of AFZ with a low LOD of 0.0696 μM and a wide linear detection range of 0.5–200 μM, representing a significant improvement over existing methods. The sensor exhibited remarkable selectivity, resistance to interference, stability over time, repeatability, and reproducibility, attributed to the synergistic interaction between NiO and CNT. Furthermore, the NiO/CNT/GCE sensor achieved high recovery rates during real-time AFZ detection in biological serum and pharmaceutical tablet samples, underscoring its practical applicability. These results highlight the potential of this cost-effective and environmentally friendly sensor material for future applications in electrochemical sensing and therapeutic monitoring.
Notes
ACKNOWLEDGEMENTS
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (NRF-2021R1A6A1A03039503). Additional support was provided by the Korea Basic Science Institute (National Research Facilities and Equipment Center) through a grant funded by the Ministry of Education (2022R1A6C101B794) and the ‘Regional Innovation Mega Project’ program through the Korea Innovation Foundation, funded by the Ministry of Science and ICT (2023-DD-UP-0007). This work was also supported by the Soonchunhyang University Research Fund.