A Novel Non-Enzymatic Efficient H₂O₂ Sensor Utilizing δ-FeOOH and Prussian Blue Anchoring on Carbon Felt Electrode

Abstract

In this study, an efficient H₂O₂ sensor was developed based on electrochemical Prussian blue (PB) synthesized from the acid suspension of δ-FeOOH and K₃[Fe(CN)₆] using cyclic voltammetry (CV) and anchored on carbon felt (CF), yielding an enhanced CF/PB-FeOOH electrode for sensing of H₂O₂ in pH-neutral solution. CF/PB-FeOOH electrode construction was proved by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), and electrochemical properties were verified by impedance electrochemical and CV. The synergy of δ-FeOOH and PB coupled to CF increases electrocatalytic activity toward H₂O_2, with the sensor showing a linear range of 1.2 to 300 μM and a limit of detection of 0.36 μM. Notably, the CF/PB-FeOOH electrode exhibited excellent selectivity for H₂O₂ detection in the presence of dopamine (DA), uric acid (UA), and ascorbic acid (AA). The calculated H₂O₂ recovery rates varied between 93% and 101% in fetal bovine serum diluted in PBS. This work underscores the potential of CF/PB-FeOOH electrodes in progressing electrochemical sensing technologies for various biological and environmental applications.

1. Introduction

Hydrogen peroxide is a potent oxidizing agent widely applied in the food, cosmetic, and textile industries [1]. Additionally, it is a vital molecule within the human body, implicated in immunological responses, inflammation, and cell signaling [2]. The scientific literature characterizes hydrogen peroxide as a reactive oxygen species, with a concentration exceeding 50 µM in biological fluids, signaling metabolic dysfunction to diseases like Alzheimer’s, Parkinson’s, and diabetes, among other health conditions [3,4]. In the environmental context, hydrogen peroxide concentration in aquatic ecosystems, including rivers, lakes, and oceans, indicates the health of fauna, flora, and overall water and air quality. In the food and beverages industry, hydrogen peroxide is a technological aid for diminishing the presence of bacteria [2]. Nonetheless, following processes such as pasteurization, sterilization, and packaging, the H₂O₂ content in the final product must not exceed 147 µM. Thus, it is crucial for health, safety, and the environment to ensure that the level of hydrogen peroxide remains within a safe range [2,5,6].

Considering the significance and wide-ranging applications of H₂O₂ in industrial and biological contexts, several techniques, including spectrophotometry, chemiluminescence, fluorescence, and chromatography, are employed to measure its concentration. Nonetheless, these techniques necessitate expert operators, rendering them unsuitable for point-of-care scenarios. Moreover, the extensive analysis duration and the equipment’s cumbersome nature preclude their use in remote locations [7].

Conversely, electrochemical methods present a promising alternative due to their numerous benefits, such as sensitivity, portability, rapid response time, cost-effectiveness, and an environmentally friendly analytical approach; however, they often lack specificity [8]. The electrochemical sensor is an accessible tool that non-specialist technicians can utilize for point-of-care detection and continuous monitoring. Since H₂O₂ is a reactive molecule, its detection is enhanced by its interaction with the working electrode’s surface. Modifying this surface with an electrocatalytic material can substantially increase the sensor’s specificity [9].

Electrochemical sensors can be constructed by immobilizing biological materials on working electrodes, such as enzymes or inorganic materials, including carbon nanomaterials, metal complexes, metal oxide hydroxides, etc. [10,11,12]. These materials act as catalysts for H₂O₂ reactions on surface electrodes. While enzymatic sensors exhibit selectivity and sensitivity, their catalytic activity is affected by pH, temperature, and the reaction medium, which impact sensor performance quality and limit these sensors’ shelf life [10]. In contrast, non-enzymatic sensors are more attractive because of their low cost and stability. Different groups of scientists have demonstrated various non-enzymatic sensors for H₂O₂ using organic molecules and other inorganic compounds. Ranni et al. demonstrated a sensor for H₂O₂ with gold nanoparticle-decorated copper cross-linked pectin on a glassy carbon electrode [13]. Yin et al. developed a biochar H₂O₂ sensor using crab shells as the carbon precursor through pyrolysis [14]. Wang et al. constructed a photoelectrochemical H₂O₂ sensor based on pillar [5] arene-functionalized Au nanoparticles and MWNTs hybrid BiOBr heterojunction [15]. An electrochemical sensor modified with cerium oxide nanocubes and carbon black as a highly active non-enzymatic H₂O₂ catalyst was proposed by Shen et al. [16]. Researchers have demonstrated that semiconductor δ-FeOOH, a material with good catalytic performance and practical application in photocatalysis [17,18], produces non-enzymatic sensors with excellent electrocatalytic activity and electronic conduction properties. For instance, Afonso and colleagues constructed an all-plastic disposable carbon electrochemical cell modified with δ-FeOOH and silver nanoparticles, which exhibited an electrocatalytic response for H₂O₂ with a limit of detection of 71 µM [19]. Further research demonstrated an improvement in the same system with the introduction of carbon black, achieving a limit of detection of 22 µM [20].

Another important inorganic compound used to modify electrodes for electrocatalytic purposes is Prussian blue (PB). PB is known as an artificial enzyme capable of catalyzing H₂O₂ reactions. Extensive research involving PB-based composite materials for electrochemical sensing of H₂O₂ has focused on synthesizing and depositing PB nanoparticles on various conductive surfaces to enhance their electrocatalytic and conductivity properties [21,22,23,24]. However, enhancing individual properties does not guarantee the material’s overall performance, necessitating better structural integration of the components.

Moreover, PB, as an electrocatalyst for H₂O₂ reaction, faces several limitations. The penetration of H₂O₂ and counter ions into PB structure can lead to volume changes and mechanical stress, resulting in reduced stability [25]. Side reactions caused by lower electrical conductivity and decomposition of PB structure in a medium with pH-neutral or basic are effects that reduce the stability of PB [26,27,28]. Therefore, it is essential to develop methods that improve the intimate contact between components of the sensing surface and enhance the physical and electrochemical stability.

Scholtz and colleagues demonstrated that Prussian blue (PB) strongly interacts with the goethite surface (α-FeOOH) due to the prevalence of positive charge groups in a medium with a pH below 9 [29]. This interaction facilitates PB binding to the surface’s negative charges and hexacyanoferrate anions, which provides remarkable stability for Prussian blue. Significantly, this property can be extended to other iron oxide hydroxides, making them effective traps for Prussian blue and its analogs. By integrating sensitive components for H₂O₂ detection with suitable conductive substrates, the resulting sensor’s sensitivity, selectivity, and practicality can be significantly enhanced.

Flexible electrodes, particularly carbon felt, stand out due to their adaptability to substrate shapes, robust mechanical properties, and superior electrical conductivity [30,31]. These characteristics are instrumental in elevating the efficacy of electrochemical sensors. Carbon felt is especially noteworthy among flexible electrodes for its extensive electrochemical surface area, chemical stability, ease of regeneration, swift charge and ion transport, and affordability [32]. While various electrochemical sensors employing carbon felt have been utilized to detect H₂O₂ and other analytes using diverse nanomaterials [23,33,34,35,36], the development of a non-enzymatic sensor that synergizes the properties of PB and δ-FeOOH remains an area not yet studied. δ-FeOOH is a relatively new material in sensor development, and its unique properties and potential applications are still being explored.

In this study, we have developed a novel non-enzymatic and accurate sensor for the electrochemical sensing of H₂O₂. PB was electrochemically synthesized and deposited on CF from a δ-FeOOH acid suspension, which also was adsorbed on CF, resulting in the formation of the CF/PB-FeOOH electrode. The synergistic effect of PB and δ-FeOOH enhances the electrode’s performance, as demonstrated by its excellent electrochemical reduction in H₂O₂, rapid response, high sensitivity, low limit of detection, and strong applicability in biological samples.

2. Materials and Methods

2.1. Reagents and Apparatus

The experiment used only chemicals of analytical grade. Ammonium iron (II) sulfate hexahydrate (NH₄)₂Fe(SO₄)₂·6H₂O, uric acid, dopamine hydrochloride, ascorbic acid, and fetal bovine serum (FBS) were purchased from Sigma-Aldrich (St. Louis, MO, USA); potassium hexacyanoferrate (III) (K₃[Fe(CN)₆]), potassium hexacyanoferrate (II) trihydrate (K₄[Fe(CN)₆]), and iron chloride (III) (FeCl₃) were purchased from A.C.S. scientific (Sumaré, SP, Brazil); potassium phosphate monobasic (KH₂PO₄) and potassium phosphate dibasic (K₂HPO₄) were obtained from Dinâmica Química (Indaiatuba, SP, Brazil); 37% v/v hydrogen chloride (HCl) was supplied by Labsynth (Diadema, SP, Brazil); sodium hydroxide (NaOH) was purchased from Anidrol (Diadema, SP, Brazil); 30% v/v hydrogen peroxide (H₂O₂) was purchased from Alphatec (Santo André, SP, Brazil); and potassium chloride was obtained from C.R.Q. (Diadema, SP, Brazil). Ultrapure water from a Millipore Direct Q^® system (Billerica, MA, USA) was used in this research.

The electrochemical experiments were conducted on a potentiostat/galvanostat Autolab PGSTAT128N ( Utrecht, The Netherlands) connected to a computer running NOVA 2.0 software. The 50 mL one-compartment electrochemical cell was equipped with a working electrode made of carbon felt (CF) (Fuel Cell Store, TX, USA) cut into small ribbons with 2.5 cm² areas, a platinum counter electrode, and Ag|AgCl immersed in 3 M KCl reference electrode. The morphology of the PB nanoparticles was evaluated by scanning electron microscopy using a Philips XL-30 FEG microscope equipped with an energy-dispersive X-ray spectrometer (EDS, Bruker, Singapore). The structural analysis of PB-FeOOH film was carried out by XRD measurements using an X-ray diffractometer with Rigaku Miniflex 600 diffractometer, which emitted Cu Kα radiation (λ = 1.54056 Å) at a scanning speed of 2° min⁻¹ at 30 kV.

2.2. Synthesis of δ-FeOOH

The synthesis of δ-FeOOH followed the procedure reported by Pereira et al. [37]. δ-FeOOH was prepared by adding 200 mL of 2 M NaOH to 200 mL of 0.71 mM (NH₄)₂Fe(SO₄)₂·6H₂O solution with stirring. After forming the green precipitate, 5 mL of H₂O₂ was added and magnetically stirred for 30 min. The reddish-brown solid formed indicated the attainment of the δ-FeOOH phase. The as-made material was washed with deionized water, centrifuged at 3600 rpm for 5 min, and dried under vacuum at room temperature.

2.3. Carbon Felt Electrode Preparation

We assessed three different approaches to preparing the CF surface for PB synthesis: (i) the CF was exposed to an air atmosphere in the oven, maintained at 100 °C for 2 h; (ii) the CF was immersed in 1 M H₂SO₄ solution for 2 h, followed by a thorough rinse with distilled water and subsequent drying in the oven at 100 °C for 2 h; (iii) we cleansed the CF with distilled water and left to dry at room temperature.

2.4. Synthesis of PB-FeOOH Film

PB-FeOOH film was formed by electrochemically synthesizing PB nanoparticles on CF using cyclic voltammetry at a scan rate of 50 mV·s⁻¹ and a voltage range of –0.2 V to 0.7 V vs. Ag|AgCl. The synthesis was performed by placing CF in a precursor solution containing 5 mM of K₃[Fe(CN)₆], 22 mg of δ-FeOOH, and 0.1 M KCl as a supporting electrolyte, forming the PB-FeOOH film on CF. The quantity of δ-FeOOH in precursor solution was varied in 11, 22, and 44 mg of δ-FeOOH. The conditions for forming the PB-FeOOH film, such as the pH values of the precursor solution and the number of cyclic voltammetric scans, were also assessed. The effect of the pH value of the precursor solution from 2.0 to 6.0 was studied by adjusting the solution with 0.1 M HCl. Finally, CV scan numbers were evaluated using 15, 30, 50, 100, and 150 scans. For comparison, PB nanoparticles using a CF surface previously optimized were synthesized by applying 100 cyclic voltammetric in 2 mM FeCl₃, 2 mM K₃[Fe(CN)₆], 0.1 M KCl, and 0.1 M HCl solution based on the modified method published by De Mattos [38].

The XRD analysis was performed using conducting fluorine-doped tin oxide (FTO)-coated glass where PB–FeOOH was deposited. Before deposition, the substrate was washed with acetone and deionized water in an ultrasonic bath for 15 min.

2.5. Electrochemical Measurement

CF/PB–FeOOH electrodes were electrochemically characterized in a 0.1 M KCl solution at pH 2.0 and a 1 mM equimolar mixture of K₃[Fe(CN)₆]/K₄[Fe(CN)₆ containing 0.1 M KCl as a supporting electrolyte by cyclic voltammetry. The solution used for amperometric measurement was 0.1 M PBS at pH 7.0, under stirring, with or without adding H₂O₂. All solutions were N₂-saturated for 15 min. The electrochemical impedance spectroscopy was conducted in the hexacyanoferrate solution, applying an open-circuit potential with a 5 mV amplitude and a frequency range from 100 kHz to 0.01 Hz.

3. Results and Discussion

According to the previous literature, CF surface treatments using acid solutions or heating in an air atmosphere have been shown to improve the electrochemical activity of the CF surface [34,35]. Therefore, we evaluated three approaches to prepare the CF surface by studying the magnitude of redox peaks of PB nanoparticles on CF. Figure 1A shows typical CV curves of PB nanoparticle synthesis, which occurs during the potential scanning in 5 mM of K₃[Fe(CN)₆], 22 mg of δ-FeOOH, and 0.1 M KCl at pH 2.0, applying 30 cycles. We observed increasing redox peaks at 0.39 V and 0.12 V, along with a slight potential shift with the number of scans, suggesting the growth of stable PB nanoparticles [39] on the CF electrode. Furthermore, δ-FeOOH was also deposited on CF due to electrostatic interaction [29]. Therefore, the electrode was designated CF/PB-FeOOH. CV tests were conducted in a 0.1 M KCl solution pH 2.0 to evaluate the electrochemical properties of CF/PB-FeOOH electrodes (Figure 1B). The characterization was maintained in a solution with pH 2 to ensure the structure and properties of the PB. The CF/PB-FeOOH electrodes exhibit two pairs of sharp redox peaks related to Prussian white (PW)/PB near 0.3/0.2 V with a potential separation of 100 mV; this was observed for three different CF surfaces prepared, indicating a fast charge transfer in CF (Reaction (R1)). Among the approaches used to prepare the CF surface, a better signal was achieved by washing CF with distilled water and drying it at room temperature (curve c). Contrary to the literature, which claims that the electrochemical activity of CF electrodes improves with chemical and thermal treatments—treatments that increase the functional groups as hydroxyl and carbonyl—we did not observe this in our findings. The interaction of FeOOH with CF was more effective; therefore, we conducted the following experiments with CF prepared by method (iii).

Fe₄^III [Fe^II(CN)₆]₃ + 4K⁺ + 4e⁻ ⇌ K₄Fe₄^II[Fe^II(CN)₆]₃
PB PW

(R1)

The electrochemical synthesis of PB on electrodes involves several controllable factors to achieve PB deposited with good electrocatalytic activity, such as deposition time, concentration of reagents, and effect of solution pH [40]. Figure 2A shows the CV of CF/PB-FeOOH electrodes in 0.1 M KCl solution pH 2.0 constructed in precursor solution with pH ranging from 2 to 6. The CF/PB-FeOOH electrode constructed under pH 2.0 exhibited notably more significant two pairs of redox peaks than those constructed under other pH conditions, indicating the successful electrochemical formation of PB from δ-FeOOH in an acid solution. Subsequently, we assessed the effect of δ-FeOOH mass in the precursor solution. Figure 2B shows the electrochemical performance of the CF/PB-FeOOH electrode in 0.1 M KCl prepared with 11, 22, and 44 mg of FeOOH. The optimal amount of FeOOH was 22 mg, which was used in the subsequent experiments. To analyze the electrochemical behavior of the CF/PB-FeOOH electrode obtained with different deposition cycles, we obtained the CV curves of the CF/PB-FeOOH electrodes in 0.1 M KCl generated with 15, 30, 50, 100, and 150 cycles. The peak current enhanced as the number of scans increased until 100 cycles but decreased with 150 cycles, indicating that the electrochemical properties of PB deteriorated with a high number of scans (Figure 2C). Based on the best electrochemical performance of the optimized CF/PB-FeOOH electrode, we proceed with the upcoming experiments under these conditions.

The amount of PB deposited on the CF/PB-FeOOH electrode following 100 electrodeposition cycles was determined by CV collected in 0.1 M KCl using Equation (1).

$${\Gamma }_{\Gamma }={\displaystyle \frac{\mathrm{Q}}{\mathrm{n}\mathrm{F}\mathrm{A}}}$$

(1)

Q is the charge equivalent to the reduction peak at 0.16 V, n represents the number of electrons in the redox process (n = 1), F stands for the Faraday constant, A is the electroactive area (3.0 cm²), and ${\Gamma }_{\Gamma }$ is the amount of PB on the active surface. The quantity of PB deposited was calculated to be 4.4 × 10⁻⁹ mol/cm².

The structural property of PB-FeOOH nanoparticles was evaluated by X-ray diffraction using FTO as substrate. Figure 3 shows the XRD pattern with only a characteristic strong peak (JCPDS#1-239) at 17.65° corresponding PB phase (100). This result suggests that the PB-FeOOH film has a predominantly amorphous structure [41], indicating no crystalline peaks other than the characteristic peak of PB.

Figure 4 shows the SEM images of the surface morphology of CF and CF/PB-FeOOH electrodes, respectively. The bare CF electrode (Figure 4A,B) has a fibrous and smooth surface, while the CF/PB-FeOOH electrode (Figure 4C,D) has a rough surface with nanometric cubic structures [40]. In addition, some irregular microstructures can be seen on the surface. The EDS analysis in Figure 4E,F shows C, O, Fe, N, K, and Cl elements, confirming the successful electrosynthesis of the PB on CF.

We use CV and EIS to assess the electrocatalytic activity of bare CF and CF/PB-FeOOH in a ferri-ferro cyanide solution. In Figure 5A, CV was carried out by cycling the potential between −0.2 and 0.7 V at a scan rate of 100 mV·s⁻¹. The peak current in the redox probe at CF/PB-FeOOH (curve b) was higher than that at CF (curve a) due to the good electrochemical properties of the modified electrode. For the CF/PB-FeOOH electrode, the I_a/I_C has an average of 0.86, and ΔE_p is +0.21 V, showing a quasi-reversible electrochemical response. However, for the bare CF electrode, ΔEp is 0.12 V. This better value is probably due to the different thermodynamic conditions of the surface.

Furthermore, the effect of scan rate was studied. Figure S1 shows CVs obtained with different scan rates from 10 to 300 mV·s⁻¹ for CF and CF/PB-FeOOH electrodes and the magnitudes of CV peak current plotted against the square root v^1/2. The results exhibited a linear relationship for both anodic and cathodic current enhancement along with an increase in scan rate for the CF and CF/PB-FeOOH, indicating a diffusion-controlled process at the surface electrode. From these data, we experimentally determined the electroactive area for CF and CF/PB-FeOOH electrodes using the Randles–Ševčík equation. The electroactive areas were 3.0 ± 0.1 cm² and 8.2 ± 0.4 cm² for CF and CF/PB-FeOOH electrodes, respectively. CF modified with PB exhibited a large surface area and improved CF’s electroactive properties. These findings were aligned with EIS experiments. Figure 5B shows the Nyquist plot for CF (curve a) and CF/PB-FeOOH (curve b). The circuits used to obtain the EIS data were R_s(R_ct[C_dl]) for CF and R_s(CPE[R_ctW]) for CF/PB-FeOOH, where R_s is solution resistance, R_ct is electron transfer kinetics as charge transfer resistance, C_dl is double layer capacitance, CPE is constant phase element, and W is mass transfer element. The R_ct for CF and CF/PB-FeOOH were 324.2 Ω and 1.28 Ω, respectively. From the EIS data, the value of the apparent heterogeneous electron transfer constant (k_app) was calculated using Equation (2).

$$k_{app}={\displaystyle \frac{RT}{n^2{F}^{2}A{R}_{Ct}C}}$$

(2)

where R stands for the ideal gas constant (8.414 Joule/(mol·K)), T is the temperature absolute (298 K), n is the number of electrons transferred during reaction redox (n = 1), F is the constant of Faraday (96,485 C/mol), A stand for the active area of the electrode (cm²); R_ct is electron transfer kinetics as charge transfer resistance (Ω); C is the concentration of the probe redox (mol/cm³). In the ferri-ferro solution, the K_app calculated were 0.25 × 10⁻⁴ and 1 × 10⁻⁷ cm·s⁻¹ for CF/PB-FeOOH and CF, respectively. The K_app value obtained for CF/PB-FeOOH is 254 folds higher than that for CF; this improvement agrees with other K_app values for modified electrodes reported in the literature [20,42].

Cyclic voltammograms were recorded for the CF/PB-FeOOH-modified electrode in a 0.1 M PBS solution to simulate the pH of the biology environment, with and without 1.0 mM of H₂O_2, at a scan rate of 50 mV/s. For comparison, we also evaluated the performance of PB deposited on CF using iron (III) chloride and ferricyanide as precursor solution (CF/PB). Figure 6A,B display CF/PB-FeOOH and FC/PB CVs, revealing redox peaks at 0.28 and 0.21 V and 0.46 and 0.43 V in PBS, respectively. The capacitive current of CF/PB was higher than that of CF/PB-FeOOH in PBS. This behavior is likely due to the larger CF/PB film thickness than CF/PB-FeOOH. PB thick films provide easier ion exchange between PB nanoparticles and the solution interface, improving the electroactivity of the electrode [43,44]. However, this property did not enhance PB’s electrochemical catalytic activity toward H₂O₂. After adding H₂O₂, CF/PB and CF/PB-FeOOH voltammetric profiles changed, especially for the CF/PB-FeOOH electrode. Evident alterations in current values were observed at 0.0 V, −0.1 V, and −0.2 V, amounting to 35%, 28%, and 29% for CF/PB, and 31%, 66%, and 85% for CF/PB-FeOOH, respectively. The difference between the current response in PBS and PBS with H₂O₂ was more pronounced for CF/PB-FeOOH, demonstrating greater sensitivity to H₂O₂. The H₂O₂ reduction by CF/PB-FeOOH can be explained by considering the synergistic effect of the Fenton-like reaction at the FeOOH (Reaction (2a–c)) surface and the PB electrocatalytic process according to Reaction (2d) [19,45]. Based on this comparison, we demonstrate the positive influence of FeOOH in the electrochemical properties of PB deposited on CF to the reduction in H₂O₂ and its potential application in electroanalytical. Therefore, we evaluated the analytical performance of the CF/PB-FeOOH electrode by amperometry at 0.0 V, −0.1 V, and −0.2 V, which were assessed for various H₂O₂ concentrations. The current response increased with H₂O₂ concentration. Figure S2 shows the calibration curve constructed taken from the three potentials. The linear regression equations were as follows: Y (μA) = 0.122 × C (μM) + 0.800 (R² = 0.977); Y (μA) = 0.250 × C (μM) + 2.33 (R² = 0.940); and Y (μA) = 0.164 × C (μM) + 0.150 (R² = 0.998), and the limits of detection (LOD) calculated were 0.91 μM, 0.55 μM, and 0.36 μM (S/N = 3) for 0.0 V, −0.1 V, and −0.2 V, respectively. We observed that the current value at −0.2 V exhibited outstanding linearly and showed low LOD. The current CF/PB-FeOOH electrode values for H₂O₂ detection showed a linear range of 1.2 to 300 μM, and the limit of quantification (LOQ) was 1.19 μM (Figure 7A,B). Our results show that the CF/PB-FeOOH electrode exhibits a low limit of detection comparable to those previously reported. Additionally, it is constructed with inexpensive materials (see Table S1 [19,20,38,46,47,48,49,50]).

(a) FeOOH + H₂O₂ → (FeOOH)H₂O₂
(b) Fe(III)_(surf)(H₂O₂) + e⁻ → Fe(II)_(surf) + HOO• + H⁺
(c) Fe(II)_(surf) + HOO• + H⁺ → Fe(III)_(surf) + H₂O + ½ O₂
(d) H₂O₂ + 2e⁻ → 2OH⁻

(R2)

The reproducibility of the three independent CF/PB-FeOOH electrodes was tested by measuring the reduction current of 80 μM H₂O₂ under the same experimental conditions. Figure S3 shows no change in reduction current, and the relative standard deviation (RSD) is calculated to be 3.3%. This value typically represents the acceptable reproducibility of the CF/PB-FeOOH electrode. The selectivity of the CF/PB-FeOOH electrode was evaluated in PBS in the presence of electroactive species usually found in biological samples, such as dopamine (DA), uric acid (UA), and ascorbic acid (AA). According to the literature, (DA) is found in blood plasma in a concentration range of 0.24 to 23 µM [51], (UA) from 200 to 390 µM [52], and (AA) at 30 µM [53]. Therefore, we used 100 µM of ascorbic acid, 400 µM of uric acid, 40 µM of dopamine, and 80 µM of H₂O₂ in the selectivity assay. Figure 8 displays the amperometric response in PBS to adding H₂O₂ and electroactive interferents. A significant reduction current was observed upon introducing H₂O₂ at the initial and final stages of the experiment. In contrast, a weak current was detected upon adding AA despite the concentration being three times higher than usually found in blood plasma. Moreover, no discernible reduction current is detected for DA and UA. These results demonstrate that the CF/PB-FeOOH electrode possesses high selectivity.

The long-term stability of the CF/PB-FeOOH electrode was estimated by storing it at 25 °C in a room. After being stored and tested eighteen days later, the sensor’s current response maintained 93% of its initial response. This characteristic, combined with the steady electrochemical response of the CF/PB-FeOOH electrode, makes it useful in sensing H₂O₂ levels in biological samples.

Living cells can be induced to release H₂O₂ into a culture medium enriched with FBS, which supplies a range of nutrients essential for cell growth [54]. Different concentrations of H₂O₂ standard solution were spiked into PBS with 10% FBS to assess the practical application potential of the CF/PB-FeOOH electrode in biological samples. The calculated H₂O₂ recovery rates varied between 93% and 101%, as shown in

Table 1. Spinking and recovery of H₂O₂ in PBS with 10% of FBS.

Sample	Added (μM)	Found (μM)	Recovery (%)
1	5	4.66	93
2	15	14.30	95
3	30	30.28	101

. These results underscore the suitability of the CF/PB-FeOOH electrode for detecting H₂O₂ in biological samples.

4. Conclusions

We have developed a novel non-enzymatic electrochemical sensor utilizing PB and δ-FeOOH to determine H₂O₂. PB was successfully synthesized electrochemically from δ-FeOOH acid suspension and deposited on CF, yielding a CF/PB-FeOOH electrode. XRD analysis suggests that PB-FeOOH film has a predominantly amorphous structure, and SEM images revealed nanocubes deposited on CF. The synergic effect of PB and δ-FeOOH enhanced the electroactivity of the CF/PB-FeOOH electrode, showing sensitivity, selectivity, and reproducibility to determine H₂O₂. CF/PB-FeOOH achieved greater sensitivity to H₂O₂ than the CF/PB electrode, which was used for comparison. Moreover, the CF/PB-FeOOH electrode detected H₂O₂ in 10% fetal bovine serum. This work underscores the potential of CF/PB-FeOOH electrodes in progressing electrochemical sensing technologies for various biological and environmental applications.