Polymeric Composite including Magnetite Nanoparticles for Hydrogen Peroxide Detection

: The combination of a biopolymer and a conductive polymer can produce new materials with improved physico-chemical and morphological properties that enhance their use as sensors. Magnetite nanoparticles (MN) can be further introduced to these new matrices to improve the analytical performance. This study aimed to evaluate the electrocatalytic response of nanocomposites formed by the introduction of MN to polypyrrole (PPy) doped in the presence of cashew gum polysaccharide (CGP) and in the presence of carboxymethylated cashew gum polysaccharide (CCGP). Characterization of the nanocomposites was carried out via transmission electron microscopy (TEM) and infrared spectroscopy (FTIR) and showed that the absorption band of the blend was shifted to a higher frequency in the nanocomposites, indicating the intermolecular interaction between the blend and nanoparticles. The electrocatalytic performance of the nanocomposites was evaluated by applying a constant potential of − 0.7 V with successive additions of H 2 O 2 (1 mmol L − 1 ) in 10 mmol L − 1 phosphate buffer under agitation at pH 7.5. The nanocomposite formed by the introduction of MN to polypyrrole doped with cashew gum polysaccharide (PPy(cgp)–MN) displayed excellent electrocatalytic surface properties, with high H 2 O 2 speciﬁcity, a linear response (R2 = 0.99), high sensitivity (0.28 µ mol L − 1 ), and a low H 2 O 2 detection limit (0.072 mmol L − 1 ).


Introduction
Hydrogen peroxide (H 2 O 2 ) is widely used as an additive in products and processes in the food, personal care, pharmaceutical, and agricultural industries [1]. In the food industry, for example, H 2 O 2 is used for the preservation of raw milk due to its antiseptic property [2][3][4][5]. However, excessive use of this chemical additive is discouraged because it can cause gastrointestinal problems [6], oxidation of vitamins (such as ascorbic and folic acid) [7], and toxic responses at high concentrations in cells [8]. There is growing interest in the development of effective techniques for the rapid determination of trace H 2 O 2 in milk [1,3,4,6,9,10]. In particular, it is important to develop new technologies involving alternative methods of rapid detection in a variety of matrices to produce methods with good sensitivity, a low detection limit, easy reproducibility, and a low cost.
In particular, nonenzymatic sensors based on modified electrodes with nanodimensioned conductive polymers, such as nanostructured polypyrrole (PPy), are considered potential candidates in the development of chemical and electrochemical sensors for the detection of hydrogen peroxide due to their good electrocatalytic response to the detection of H 2 O 2 and several advantages attributed to them, such as simplicity and diversity of preparation methods, good electrical conductivity, mechanical resistance, selectivity, and chemical stability [24][25][26][27][28].
Desirable electronic and mechanical characteristics in PPy films can be enhanced by the addition of doping agents by increasing the efficiency of charge transfer by overloading the π bonding orbital along the polymeric chain [29][30][31][32]. Cashew gum polysaccharide (CGP) is a heteropolysaccharide that can be used as a polyelectrolyte in aqueous media, with a potential application as a doping agent for the development of polymeric films [32]. Cashew gum polysaccharide (CGP) is an exudate of Annarcardium occidentale, a natural heteropolysaccharide that meets the requirements of biocompatibility and biodegradability for biomedical/analytical applications [33][34][35]. As CGP is dissolved in aqueous solution, it becomes a polyelectrolyte, which can be functionalized with appropriate molecules from biological or chemical sources [32,36].
The incorporation of active redox nanoparticles in PPy films, such as magnetic iron nanoparticles (MN), positively affects the analytical performance when they are used as electrochemical signal transducers [29,37]. The presence of MN increases the stability of the film since MN generates a protective effect on the film, causing the preservation of the polaron state [38,39]. Thus, biopolymers with nanoparticles can form nanocomposites that show enhanced electrochemical performance and serve as viable options for the development of novel sensors.
In view of the above considerations, we have developed a novel technique for the electrochemical monitoring of H 2 O 2 using a glassy carbon electrode modified by the deposition of nanocomposites formed by magnetite nanoparticles incorporated into combinations of (PPy) and CGP (PPy(cgp)) or carboxymethylated CGP (PPy(ccgp)). Through appropriate characterization techniques (TEM and FTIR), the formation of nanocomposites and the molecular interactions between materials were investigated. Chronoamperometric readings conducted in buffered medium revealed that the PPy(cgp)-MN nanocomposite exhibited the best electrocatalytic properties for the detection and quantification of H 2 O 2 . The novelty of this sensor is that it is able to detect hydrogen peroxide in milk, a complex food matrix because of the presence of lipids, carbohydrates, complex proteins, antibiotics, etc. The use of hydrogen peroxide is common to mask contamination with batteries, which is why it is so important to detect this analyte because, in high concentrations (300-800 ppm), it can produce the destruction of lactoperoxidase, an enzyme that prevents the development of bacteria.

Reagents
All chemicals used in this study were of analytical grade and used without further purification. Deionized water was processed in Millipore's Milli-Q system (

Isolation and Purification of Polysaccharides
Raw cashew gum was collected from the tree trunk of Anacardium occidentale L. cultivated in the experimental field of Embrapa Tropical Agroindustry (coordinates: 4 • 11 26.62 S and 38 • 29 50.78 W). Isolation and subsequent purification were performed according to the procedure reported by Torquato et al. [40], with adaptations. The gum was milled, dissolved in distilled water at a concentration of 30% (g L −1 ), and kept at room temperature (25 • C) for 24 h. Afterward, it was centrifuged for 10 min. at 10,000 rpm at 25 • C (to remove tree bark fragments) and precipitated with commercial ethanol (96%) at a ratio of 1:3 (v/v). The precipitated cashew gum polysaccharide (CGP) was filtered, washed with ethanol, and dried in an oven at 60 • C for 24 h. Finally, the extracted CGP was milled to a fine powder, vacuum packed, and kept at room temperature.

Carboxymethylation of Cashew Gum Polysaccharide
The isolated and purified CGP was subjected to the carboxymethylation reaction according to the methodology reported by Melo et al. and Silva et al. [36,41]. A mixture containing 5 g of CGP was mixed with 5 mL of deionized water until completely homogenized. We then added 10 mol L −1 NaOH (6.2 mL) to the homogeneous solution and stirred for 10 min. The reaction was started after the careful addition of monochloroacetic acid (5.25 g). The mixture was heated at 55 • C for 3 h. The mixture containing carboxymethylated cashew gum polysaccharide (CCGP) was neutralized with 1 mol L −1 HCl and purified by dialysis against deionized water until the elimination of starting reagents or added salt (monitored for water conductivity). The CCGP was subjected to the lyophilization process and stored in vacuum-sealed packages.

Electrochemical Synthesis of Magnetite Nanoparticles (MN)
Magnetite nanoparticles Fe 3 O 4 were synthesized using a flow cell with parallel electrodes according to the procedure developed by Lozano et al. [42]. Eight low-carbon steel sheets (100 mm × 45 mm) of 1 mm thickness and 99.8% pure iron were used as electrodes. The polished electrodes were immersed in 0.04 mol L −1 NaCl. A peristaltic pump operating at 30 rpm with a flow rate of 50 mL min −1 was used, and the total cell volume was 460 mL. A direct current of 700 mA was supplied to the parallel monopolar electrodes. The pH value was measured in the outflow cell. The nanoparticles were collected by the magnetic precipitation method using a neodymium magnet placed at the cell bottom. The supernatant was discarded. The nanoparticles were washed several times with distilled water, collected with a magnet, and then centrifuged at 9000 rpm for 15 min. at room temperature. Drying was carried out in a vacuum system for a period of 24 h.

Synthesis of Nanocomposites
The nanocomposites were synthesized based on the study developed by Ayad et al. [43] with some adaptations. The chemical polymerization of pyrrole (Py) occurred with either CGP or CCGP in aqueous reaction media and iron oxide nanoparticles (MN) as the oxidizing agent. For each synthesis, solutions of the polysaccharide (1 w/v) were prepared by dissolving 0.5 g in 50 mL of deionized H 2 O. In all cases, 0.25 mL of predistilled pyrrole was added, and the resulting solutions were kept for 45 min. under magnetic stirring. Then, 1.41 g of MN was added to the aqueous solutions of each polysaccharide in an ice bath to obtain the respective products PPy (cgp) -MN and PPy (ccgp) -MN. The mixtures were stirred overnight, and NaOH solution (50 mL, 0.1 mol L −1 ) was slowly added to the mixtures. The polymeric nanocomposite (PN) from each synthesis was washed five times with deionized water in an ultrasonic bath for 3 min. and recovered by centrifugation at 9000 rpm/15 min. The last wash was performed with methanol and collected by centrifugation at 9000 rpm/15 min. The PN was finally vacuum dried for 40 h.

Instrumentation and Characterization
The morphologies of the nanocomposites were studied by transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). For TEM, a single drop (10 µL) of the alcoholic solution containing the PN was placed on a copper grid coated with a carbon film. Photomicrographs were obtained using a JEM 1010 microscope (JEOL, Tokyo, Japan) operating at an acceleration voltage of 100 kV. FTIR was employed to confirm the incorporation of MN in the different matrices using an IFS66v instrument (Bruker, Karlsruhe, Germany).

Electrochemical Measurements
Measurements involving voltammetric and chronoamperometric techniques were recorded on an Autolab PGSTAT302N potentiostat/galvanostat (Metrohm AG, Herisau, Switzerland) controlled by Nova 11.1 software. A glass cell of 10 mL and a conventional system with three electrodes were adopted, using a vitreous carbon electrode (VC) (Φ = 2.7 mm) as the substrate for the deposition of the different nanocomposites as working electrodes (WE), an Ag/AgCl electrode (3.0 mol L −1 KCl) as the reference electrode, and a platinum helical electrode as the auxiliary electrode.

Preparation and Modification of the Electrocatalytic Surface
According to the methodology developed by Jaime et al. [44] and with modifications, the different nanocomposites that were synthesized were added directly to the vitreous carbon surface using chemical adsorption and carbon paste preparation. The electrode was gently polished in 0.3 and 0.05 µm alumina powder for 10 min. and carefully washed ultrasonically in absolute ethanol and water for 10 min. each. The electrode was then rinsed with deionized water and dried. The electrode was submitted to cyclic scans of anodic and cathodic potentials in 0.5 mol L −1 H 2 SO 4 in a potential range of −0.8 to 2 V at 100 mV s −1 . The electrode surface and the assembly steps of the sensors were evaluated by voltammetric measurements at 5.0 mmol L −1 K 3 [Fe(CN) 6 ] and 0.10 mol L −1 KCl in a potential range of −0.7 to 0.75 V. To prepare the carbon paste, we used 1 mL of isopropyl alcohol (4:1), 1 mg of vulcan carbon, and 1 mg of the PN synthesized by the method described above, solubilized for 10 min. in an ultrasonic bath. Subsequently, 8 µL of NAFION ® was added to the mixture and sonicated for an additional 30 min. The working electrode was prepared with 4 µL of the homogenized suspension on the polished and activated VC surface and then dried overnight.

Analytical Response
A phosphate buffer solution (PBS) (0.10 mol L −1 , pH 7.5) was used as a support electrolyte to evaluate the performance of different electrocatalytic surfaces. Sensor performance was tested by applying a constant potential of −0.7 V with successive additions of 1 mmol L −1 H 2 O 2 every 60 s in PBS. The procedure was performed in quintuplicate under continuous stirring.
The sensor performance in samples containing hydrogen peroxide was evaluated for the following parameters: specificity/selectivity, linearity, sensitivity, accuracy, precision (repeatability and reproducibility), and detection limit (LD). For this evaluation, the following equations were used: LD = 3SD/slope, linearity = 10SD/slope, and sensitivity = slope/S, where SD (n = 3) and S are the electrode surfaces.

Milk Analysis
Whole and skimmed ultrahigh-temperature (UHT) milk samples were purchased in local stores. Both kinds of milk were subjected to amperometric readings without previous dilution, and only the whole milk sample was previously centrifuged to remove fat (5000 rpm at 25 • C for 20 min.) before each electrochemical reading. Amperometric curves and subsequent performance parameters were obtained after applying a constant potential of −0.7 V with successive additions of 1 mmol L −1 H 2 O 2 every 60 s under agitation conditions.

Characterization of Nanocomposites
Magnetic polymer nanocomposites containing Fe 3 O 4 were produced after pyrrole polymerization in the presence of CGP isolated and modified as doping agents. The VC surfaces modified by nanocomposites were subjected to electrochemical readings for the determination of hydrogen peroxide in buffered media and in milk samples.
The structural elucidation confirmed by FTIR spectroscopy reveals the effectiveness of the reaction mixture to form the nanocomposite. The different nanocomposites of PPy (cgp ) and PPy (ccgp) with iron oxide nanoparticles incorporated in the matrix in aqueous medium have similar bands, e.g., at 3700-3000 cm −1 (typical for O-H stretch), 2924 and 2860 cm −1 (corresponding to the symmetrical and asymmetric stretch vibration of CH of carbon sp 3 ), and 1640 cm −1 (OH vibration for the water molecules bound to the polysaccharide) [45,46] ( Figure 1). The absorption peaks present at 1420 cm −1 and 1380 cm −1 are related to the symmetrical angular deformation of CH 2 and CH 3 in polysaccharides [47,48]. The peaks present in the characteristic absorption band between 1200 and 900 cm −1 , more specifically at 1160, 1070, and 1020 cm −1 , in the different samples can be attributed to carbohydrate and different vibrations of COC and cyclic aliphatic secondary alcohol (C-OH) of glycosidic bonds [47,49]. The spectral difference between the two pure polysaccharides, as reported in the literature, and their respective synthesized nanocomposites is observed by the presence of the adsorption band at 579 cm −1 derived from the MN incorporated into the conducting polymer matrices, corresponding to the band typical of elongation vibration from the Fe-O bond [47,50]. It is noticeable that the typical peak for Fe-O in Fe 3 O 4 in the nanocomposites is displaced to higher wavelengths when compared to the pure oxide spectrum. This strongly implies effective mixing between the blend segment and the metal oxide particles, which leads to effective intermolecular interactions between the metal oxide particles and the organic groups of polysaccharides, such as hydroxyl or amine groups. A possible molecular structure of the nanocomposite is presented in the inset of Figure 2 based on studies of other polysaccharides [33,43,50] and suggests the intermolecular interactions that occur between the metal oxide particles and the organic groups of the CGP, PPy, and MN. This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, and the experimental conclusions that can be drawn.
TEM images revealed the efficiency of incorporating MN in the different conductive polymeric matrices and reinforced the strong intermolecular interaction between functional groups and MN. The incorporation of MN (30 ± 8.8 nm) in the matrices of PPy (cgp) and PPy (ccgp) can be seen in Figure 2b,c, respectively. It is clearly observed that a polymeric shell is formed around the MN with the formation of nanocomposites and showed an average thickness of 6.9 ± 1.78 nm and 5.4 ± 2.1 nm for matrices of PPy (cgp) and PPy (ccgp) (average obtained by the shell thickness in different areas of the picture), and this same phenomenon does not occur with the MN obtained electrochemically (Figure 2a). This phenomenon proves a stable structural system because the blend segment remains unbroken after successive washes performed in an ultrasonic bath and further reinforces the strong effective molecular interaction between the doped polymer, the polysaccharides, and the inorganic nanoparticles. Both nanocomposites showed a morphology with dispersed spherical particles of sizes 38.05 ± 11 nm and 33.3 ± 5.9 nm for nanocomposites formed by PPy (cgp) -MN and PPy (ccgp) -MN in aqueous medium, respectively. TEM images revealed the efficiency of incorporating MN in the different conductive polymeric matrices and reinforced the strong intermolecular interaction between functional groups and MN. The incorporation of MN (30 ± 8.8 nm) in the matrices of PPy(cgp) and PPy(ccgp) can be seen in Figure 2b,c, respectively. It is clearly observed that a polymeric shell is formed around the MN with the formation of nanocomposites and showed an average thickness of 6.9 ± 1.78 nm and 5.4 ± 2.1 nm for matrices of PPy(cgp) and PPy(ccgp) (average obtained by the shell thickness in different areas of the picture), and this same phenomenon does not occur with the MN obtained electrochemically (Figure 2a). This phenomenon proves a stable structural system because the blend segment remains unbroken after successive washes performed in an ultrasonic bath and further reinforces the strong effective molecular interaction between the doped polymer, the polysaccharides, and the inorganic nanoparticles. Both nanocomposites showed a morphology with dispersed spherical particles of sizes 38.05 ± 11 nm and 33.3 ± 5.9 nm for nanocomposites formed by PPy(cgp)-MN and PPy(ccgp)-MN in aqueous medium, respectively.   TEM images revealed the efficiency of incorporating MN in the different conductive polymeric matrices and reinforced the strong intermolecular interaction between functional groups and MN. The incorporation of MN (30 ± 8.8 nm) in the matrices of PPy(cgp) and PPy(ccgp) can be seen in Figure 2b,c, respectively. It is clearly observed that a polymeric shell is formed around the MN with the formation of nanocomposites and showed an average thickness of 6.9 ± 1.78 nm and 5.4 ± 2.1 nm for matrices of PPy(cgp) and PPy(ccgp) (average obtained by the shell thickness in different areas of the picture), and this same phenomenon does not occur with the MN obtained electrochemically (Figure 2a). This phenomenon proves a stable structural system because the blend segment remains unbroken after successive washes performed in an ultrasonic bath and further reinforces the strong effective molecular interaction between the doped polymer, the polysaccharides, and the inorganic nanoparticles. Both nanocomposites showed a morphology with dispersed spherical particles of sizes 38.05 ± 11 nm and 33.3 ± 5.9 nm for nanocomposites formed by PPy(cgp)-MN and PPy(ccgp)-MN in aqueous medium, respectively.

Evaluation of the Analytical Response
Chronoamperometry was used as a technique for electrochemical measurement of the H 2 O 2 decomposition process. The choice of the best electrocatalytic surface for this process was made through the modification of the vitreous carbon substrate by chemical adsorption using different nanocomposites formed by PPy (cgp) and PPy (ccgp) with nanoparticles in aqueous media.
The amperometric responses showed good performance as H 2 O 2 sensors, according to Figure 3. When 1 mmol L −1 H 2 O 2 was added to pH 7.5 phosphate buffer under conditions controlled by diffusion, an initial increase in peak current was noted, demonstrating effective electrocatalytic reduction of H 2 O 2 . Subsequent additions of H 2 O 2 at regular intervals resulted in a drastic change in electrical current because the increase in the magnitude of the reduction peak was proportional to the H 2 O 2 concentration and steadystate formation.
It was also observed in the chronoamperogram that throughout the reading time and successive additions of H 2 O 2 , a slight decline in the reduction current amplitude in the final min. was noted for the two nanocomposites formed by PPy (cgp) -MN and PPy (ccgp) -MN. This can be associated with the relationship between the increased H 2 O 2 concentration and the possible electrocatalytic surface poisoning by the analyte. At low levels of H 2 O 2 , the concentration next to the electrode surface is rapidly depleted as the substrate is converted to the product by the catalytic action of the nanoparticle, resulting in a high sensitivity of the electrode response [51]. Successive additions of the analyte solution to the same electrochemical system cause an increase in the concentration of H 2 O 2 and, consequently, a saturation of the system by the excessive reaction product, thereby bringing about surface poisoning because there will be less deposition of the substrate to act on the catalytic activity of H 2 O 2, and as a result, a lower slope is found [3].
Chronoamperometry was used as a technique for electrochemical measurement of the H2O2 decomposition process. The choice of the best electrocatalytic surface for this process was made through the modification of the vitreous carbon substrate by chemical adsorption using different nanocomposites formed by PPy(cgp) and PPy(ccgp) with nanoparticles in aqueous media.
The amperometric responses showed good performance as H2O2 sensors, according to Figure 3. When 1 mmol L −1 H2O2 was added to pH 7.5 phosphate buffer under conditions controlled by diffusion, an initial increase in peak current was noted, demonstrating effective electrocatalytic reduction of H2O2. Subsequent additions of H2O2 at regular intervals resulted in a drastic change in electrical current because the increase in the magnitude of the reduction peak was proportional to the H2O2 concentration and steady-state formation. It was also observed in the chronoamperogram that throughout the reading time and successive additions of H2O2, a slight decline in the reduction current amplitude in the final min. was noted for the two nanocomposites formed by PPy(cgp)-MN and PPy(ccgp)-MN. This can be associated with the relationship between the increased H2O2 concentration and the possible electrocatalytic surface poisoning by the analyte. At low levels of H2O2, the concentration next to the electrode surface is rapidly depleted as the substrate is converted to the product by the catalytic action of the nanoparticle, resulting in a high sensitivity of the electrode response [51]. Successive additions of the analyte solution to the same electrochemical system cause an increase in the concentration of H2O2 and, consequently, a saturation of the system by the excessive reaction product, thereby bringing about surface poisoning because there will be less deposition of the substrate to act on the catalytic activity of H2O2, and as a result, a lower slope is found [3].
The significant improvement in the electrocatalytic performance of the sensors is observed in the nanocomposites that have the greatest reduction current for H2O2, and this improvement can be attributed to the protective effect that the oxide nanoparticles have The significant improvement in the electrocatalytic performance of the sensors is observed in the nanocomposites that have the greatest reduction current for H 2 O 2, and this improvement can be attributed to the protective effect that the oxide nanoparticles have on the sensor platform with polypyrrole. The H 2 O 2 electrons, when penetrating the polymer, reduce the volume fraction of the conductive particles and, therefore, reduce the number of conducting pathways for charge carriers, causing an increase in the electrical resistance of the polymer and, consequently, a decrease in the current crossing the polymer [4,37,52]. MN contributes to the preservation of the state of the polaron in the polypyrrole, and consequently, it will have greater stability and conductivity, positively affecting the sensitivity and detection limit of the H 2 O 2 sensor platform [29,38].
Both electrodes modified by nanocomposites PPy (cgp) -MN and PPy (ccgp) -MN showed good electrochemical responses to H 2 O 2 with linearities of 0.996 and 0.994, respectively, and sensitivities of 2.8 × 10 −4 and 3.8 × 10 −4 µAmM −1 . The chromoamperometric curves of the two nanocomposites showed no significant difference in electrocatalytic activity in relation to hydrogen peroxide at concentrations of 0.1 at 0.9 mmol L −1 . Therefore, it was decided to continue only with the PPy (cgp) -MN nanocomposite to evaluate the electrocatalytic potential of the sensor platform, as the addition of a carbonyl group in the polysaccharide structure, acting as a doping agent, did not generate a significant improvement in the catalytic activity against H 2 O 2 for this application. For these reasons, the performance evaluation study was conducted only on the nanocomposite based on CGP.
The PPy (cgp) -MN platform presented a linear increase in current and reached the maximum steady-state current in 2 to 4 s. The sensor curve (Figure 4a) presented a detection limit of 71.6 µM, estimated by the equation from the standard deviation of the blank and the angular coefficient (slope) of the analytical curve [43]. The linear behavior follows the equation y = 3.8 × 10 −4 x + 6.34 × 10 −5 . The detection limit shown by the PPy (cgp) -MN nanocomposite outstrips previous reports on a magnetite sensor by Cao et al.
with a detection limit of 1 mmol L −1 [19] and a microfluidic paper-based analytical device by Lima et al. with a detection limit of 3.54 × 10 −4 mol L −1 [10]. It is worth noting that the detection limit expressed by the sensor platform developed herein is low enough to satisfy the requirement for the determination of H 2 O 2 residues in food [52] the catalytic activity against H2O2 for this application. For these reasons, the performance evaluation study was conducted only on the nanocomposite based on CGP.
The PPy(cgp)-MN platform presented a linear increase in current and reached the maximum steady-state current in 2 to 4 s. The sensor curve (Figure 4a) presented a detection limit of 71.6 µM, estimated by the equation from the standard deviation of the blank and the angular coefficient (slope) of the analytical curve [43]. The linear behavior follows the equation y = 3.8 × 10 −4 x + 6.34 × 10 −5 . The detection limit shown by the PPy(cgp)-MN nanocomposite outstrips previous reports on a magnetite sensor by Cao et al. with a detection limit of 1 mmol L −1 [19] and a microfluidic paper-based analytical device by Lima et al. with a detection limit of 3.54 × 10 −4 mol L −1 [10]. It is worth noting that the detection limit expressed by the sensor platform developed herein is low enough to satisfy the requirement for the determination of H2O2 residues in food [52] Figure 4b, reveals that the presence of MN increases the electron transfer, the ratio between peak currents with only nanoparticles and with the composite increases by a value greater than 2.5 (I p (VCPPy (cgp) -MN/I p ) (VC-MN)), and consequently improves the electrocatalytic properties of the matrix based on cashew gum and polypyrrole (PPy (cgp) ). The presence of MN generates an increase in the electrode surface area, which is confirmed by the difference in the increase in electron transfer among the different phases of the electrocatalytic platform development process. For this reason, the amperometric response in relation to H 2 O 2 is visibly improved because MN favors a better electrocatalytic reduction of H 2 O 2 [1]. With these characteristics, the PPy (cgp) -MN nanomaterial is promising for applications in electrochemical devices, such as sensors and capacitors.
The selectivity of the sensors in the presence of interfering species in pH 7.5 phosphate buffer was evaluated in this study ( Figure 5). The results indicate no change in the catalytic capacity for ascorbic acid, glucose, urea, ethanol, sodium chloride, and lithium perchlorate at concentrations that were ten times higher than H 2 O 2 . These results confirm that the surface modified with the PPy (cgp) -MN nanocomposite exhibited excellent electrochemical performance due to the absence of any appreciable interference signal in the electrolyte, thus demonstrating excellent selectivity for the analyte of interest. In parallel to this result, we also justify the choice of a working potential of −0.7 V because this potential does not cause the oxidation of possible interfering substances. chemical performance due to the absence of any appreciable interference signal in the electrolyte, thus demonstrating excellent selectivity for the analyte of interest. In parallel to this result, we also justify the choice of a working potential of −0.7 V because this potential does not cause the oxidation of possible interfering substances. The reproducibility of the modified surface with chemical adsorption of PPy(cgp)-MN was investigated by the amperometric responses. The electrocatalytic behavior of four successive assemblies was observed, providing a relative standard deviation (RSD) of 4.66% after nine injections of 1 mmol L −1 H2O2 in PBS buffer per assembly (Figure 6a). Therefore, the reproducibility is acceptable when compared to other studies already reported by Ping et al. [4], where the RSD was less than 4.7%. The reproducibility of the modified surface with chemical adsorption of PPy (cgp) -MN was investigated by the amperometric responses. The electrocatalytic behavior of four successive assemblies was observed, providing a relative standard deviation (RSD) of 4.66% after nine injections of 1 mmol L −1 H 2 O 2 in PBS buffer per assembly (Figure 6a). Therefore, the reproducibility is acceptable when compared to other studies already reported by Ping et al. [4], where the RSD was less than 4.7%.
The possible reuse of the PPy (cgp) -MN sensor was evaluated using the same electrode for five successive measurements (Figure 6b) with the addition of 1 mmol L −1 H 2 O 2 every 60 s. The coefficients of variance (CV) were 1.80%, 0.24%, 0.21%, 0.25%, 0.23%, 0.47%, 0.27%, 2.44%, and 0.27% for the respective concentrations of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 mmol L −1 H 2 O 2 . The low CV values indicate that the sensors maintain satisfactory electrochemical stability and that they can be reused for different measurements. The increase in current with the use of the electrode could be due to the conditioning of the electrode when it is used at different times.
The viability of the amperometric sensor proposed for the detection of H 2 O 2 in milk samples was obtained through the amperometric curve in current time and constant potential using the CV electrode modified with PPy (cgp) -MN under optimal conditions (Figure 7). The nanocomposite showed a lower baseline than the PBS readings, which can be attributed to electrode surface fouling in milk samples [4]. The sensor showed sensitivities of 5.6 × 10 −5 and 1.7 × 10 −4 µA mM −1 and detection limits of 0.53 and 0.13 mmol L −1 for skimmed and whole milk, respectively. These data represent satisfactory performance for the determination of H 2 O 2 in the two different types of milk because of the rapid response and the significant increase in the reduction current after the consecutive addition of 1 mmol L −1 H 2 O 2 . Thus, the new materials developed from the cashew gum polysaccharide behaved as excellent sensor platforms for the testing of H 2 O 2 in foods. The possible reuse of the PPy(cgp)-MN sensor was evaluated using the same electrode for five successive measurements (Figure 6b) with the addition of 1 mmol L −1 H2O2 every 60 s. The coefficients of variance (CV) were 1.80%, 0.24%, 0.21%, 0.25%, 0.23%, 0.47%, 0.27%, 2.44%, and 0.27% for the respective concentrations of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 mmol L −1 H2O2. The low CV values indicate that the sensors maintain satisfactory electrochemical stability and that they can be reused for different measurements. The increase in current with the use of the electrode could be due to the conditioning of the electrode when it is used at different times.
The viability of the amperometric sensor proposed for the detection of H2O2 in milk samples was obtained through the amperometric curve in current time and constant potential using the CV electrode modified with PPy(cgp)-MN under optimal conditions (Figure 7). The nanocomposite showed a lower baseline than the PBS readings, which can be attributed to electrode surface fouling in milk samples [4]. The sensor showed sensitivities of 5.6 × 10 −5 and 1.7 × 10 −4 µA mM −1 and detection limits of 0.53 and 0.13 mmol L −1 for skimmed and whole milk, respectively. These data represent satisfactory performance for the determination of H2O2 in the two different types of milk because of the rapid response and the significant increase in the reduction current after the consecutive addition of 1 mmol L −1 H2O2. Thus, the new materials developed from the cashew gum polysaccharide behaved as excellent sensor platforms for the testing of H2O2 in foods.

Conclusions
In this work, we designed and fabricated a new nonenzymatic sensor from a PPy(cgp)-MN nanocomposite. This material provided a better electrocatalytic surface than the other surfaces tested in this study and exhibited good sensitivity and specificity for H2O2 detec-

Conclusions
In this work, we designed and fabricated a new nonenzymatic sensor from a PPy (cgp) -MN nanocomposite. This material provided a better electrocatalytic surface than the other surfaces tested in this study and exhibited good sensitivity and specificity for H 2 O 2 detection, including milk contaminated with H 2 O 2 . This sensor demonstrated good performance with a low detection limit, good reproducibility, and potential reuse for consecutive measurements.
The fast response and higher reduction current of H 2 O 2 by nanocomposite-modified electrodes stand out in relation to other electrocatalytic surfaces. This superior performance for the decomposition of the analyte of interest verified that the presence of Fe 3 O 4 nanoparticles in the composition of the electrocatalytic nanomaterial, together with the conductive properties derived from the conductive composite, were effective and contributed favorably to the reduction of H 2 O 2 . Thus, our materials are electrochemically more stable and conductive and can increase the surface area of the electrodes.
Furthermore, in this work, we used cashew gum, a renewable natural resource of high availability but relatively low value, to produce the nanocomposite sensor, together with the incorporation of magnetite nanoparticles and polypyrrole. The novel use of cashew gum for this application adds potential economic value to the cashew production chain and is beneficial to cashew gum producers and their countries.