Facile Fabrication of Metal Oxide Based Catalytic Electrodes by AC Plasma Deposition and Electrochemical Detection of Hydrogen Peroxide

: In this study, the fabrication of a metal oxide nanoparticles (NPs) dispersed catalytic electrode is described based on a new alternating current (AC) plasma deposition approach. The fabrication involves the treatment of AC plasma on a precursor solution comprised of metal salts such as CuCl 2 , FeCl 2 , and ZnCl 2 , and a monomer (acrylic acid) in the presence / absence of a cross-linker. Furthermore, the utility of such developed electrodes has been demonstrated for the electrochemical determination of hydrogen peroxide (H 2 O 2 ). The electrode materials obtained through plasma treatment was characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscope (SEM), contact angle measurements, energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry. Among the metal oxide modiﬁed electrodes prepared by the AC plasma deposition method, the copper oxide (CuO) NPs catalytic electrode exhibited signiﬁcant oxidation and reduction peaks for H 2 O 2 in phosphate-bu ﬀ ered saline solution. The catalytic electrode with CuO NPs exhibited a combination of good H 2 O 2 sensing characteristics such as good sensitivity (63.52 mA M − 1 cm − 2 ), good selectivity, low detection limits (0.6 µ M), fast sensing response (5 s), a wide linear range (0.5–8.5 mM), and good stability over 120 cycles. Based on our results, it is well demonstrated that plasma deposition could be e ﬀ ectively utilized for the fabrication of the catalytic electrode for detection of H 2 O 2 concentrations. Further, the strategy of using AC plasma for fabrication of metal oxide-based modiﬁed electrodes could also be extended for the fabrication of other kinds of nanomaterials-based sensors.


Introduction
Hydrogen peroxide (H 2 O 2 ) is a green oxidant that is widely used in various fields [1,2]. In addition, H 2 O 2 can be used as an oxidizing agent for fuel cells, and in various commercial products, such as cosmetics and medicine [3]. Recently, H 2 O 2 has attracted research attention as an important by-product of enzyme reactions in the field of biotechnology [4]. Among the various H 2 O 2 detection techniques that have been developed, which include spectrometry, fluorescence, and chromatography, the electrochemical methods offer special combinational advantages like low detection limit, high plasmas have been used to grow polymer films on different substrates [28]. Tungsten-doped TiOx nanoparticles (NPs) were obtained through the modification of commercially available TiO 2 NPs using plasma processing [29]. Tungsten trioxide NPs were synthesized by microwave plasma treatment [30]. Polyacrylic acid (PAA) has been widely used in a variety of applications including sensors due to its super hydrophilic properties, biocompatibility and biodegradability [31]. O'Hare et al. developed organic coatings with a deposition rate of 40 nm min −1 by deposition of PAA from an acrylic acid (AA) monomer through the post-discharge of a radio frequency helium plasma [32]. In the same way, Nisol et al. used a dielectric barrier discharge process to prepare plasma-polymerized PAA films by nebulizing the precursor in a helium plasma [33]. The helium and organic precursor flow rates were found to influence the fragmentation of the AA monomer and the presence of carbonyl groups in the coatings [34]. However, to the best of our knowledge, this is the first report on the simple fabrication of an electrochemical sensor using a one-step AC plasma deposition of PAA and metal oxide over the surface of an indium tin oxide (ITO) electrode.
In this study, we fabricated the metal oxide NPs-PAA composite loaded catalytic electrode by AC plasma deposition using the precursors, AA (monomer for PAA), and metal salts (CuCl 2 , FeCl 2 , and ZnCl 2 ). After the plasma treatment, the electrode surface is expected to contain PAA through polymerization of AA and respective metal oxides via reduction of metal halides. The fabricated catalytic electrodes, comprised of metal oxide-PAA film, were used for the non-enzymatic electrochemical detection of H 2 O 2 . Initially, the as-developed catalytic materials and electrodes were characterized for relevant physico-chemical properties. The sensor studies were designed in two parts: i) an initial evaluation of all the metal oxide catalyst-modified electrodes towards assessing the electrocatalytic activity, and ii) in-depth studies on the sensor performance evaluation for the best selected electrode. Briefly, CuO NPs catalytic electrodes were evaluated for the electrochemical detection of H 2 O 2. The sensing concentration range, selectivity, and stability of the fabricated electrode CuO NPs catalytic electrodes are reported. The sensor results are explained based on a plausible mechanism.

Results
Keeping the objective that plasma treatment can be used to modify the electrode surface, we demonstrated the feasibility of fabricating catalytic electrodes from the precursors containing a polymerizable monomer and the metal salt, through in-situ simultaneous generation of polymer and metal oxide. Typically, AA was chosen as the precursor for polymer and metal halides (CuCl 2 , FeCl 2 , and ZnCl 2 ) for metal oxide formation. The deposition of carboxylic acid-rich polymer (PAA in this case) coating causes the electrode surface to become hydrophilic, which facilitates the aqueous electrochemical reactions. Metal oxide electrocatalysts [35] have been successfully utilized for a variety of electrochemical reactions; particularly, metal oxide nanostructured modified electrodes have been demonstrated to possess excellent reduction/oxidation capabilities, and these characteristics were used for the detection of H 2 O 2 [36]. The plasma-based formation of metal oxide is known [37]. In this work, we have demonstrated the utilization of plasma treatment for the fabrication of catalytic electrodes, comprised of copper or iron or zinc oxide as the catalytic material. After characterization and further evaluation of electrocatalytic efficiency of the modified electrodes, the best modified electrode was further used for the non-enzymatic electrochemical detection of H 2 O 2 .  ITO glass, the CAs of the fabricated catalytic electrode were significantly lower. Wettability of any solid surface, electrode surface in this case, is significantly influenced by the surface characteristics such as chemistry, charge, hydrophilicity, topography, and roughness. Among the different factors, surface energy and surface roughness are expected to play dominant roles. There are two modifying components in the surface of the modified electrode, namely metal oxide and PAA. We hypothesize that the variations in the surface heterogeneity or roughness could influence the CA by changing at the point along the three-phase (solid-liquid-air) contact line. It is expected in the first principle that the hydrophilic groups (carboxylic groups (-COOH)) from PAA and the relatively hydrophillic metal oxide in different proportions in these modified electrode surfaces affect the CA on the film. A simple clue is evident from the CA measurement that the modified electrode surface is relatively hydrophilic compared to the bare electrode surface (ITO).

Characterization of the Catalytic Electrode Prepared by AC Plasma Deposition
Catalysts 2019, 9, x FOR PEER REVIEW 4 of 17 1). As can be seen, the original ITO glass is relatively hydrophobic with a high CA. Compared to original ITO glass, the CAs of the fabricated catalytic electrode were significantly lower. Wettability of any solid surface, electrode surface in this case, is significantly influenced by the surface characteristics such as chemistry, charge, hydrophilicity, topography, and roughness. Among the different factors, surface energy and surface roughness are expected to play dominant roles. There are two modifying components in the surface of the modified electrode, namely metal oxide and PAA. We hypothesize that the variations in the surface heterogeneity or roughness could influence the CA by changing at the point along the three-phase (solid-liquid-air) contact line. It is expected in the first principle that the hydrophilic groups (carboxylic groups (-COOH)) from PAA and the relatively hydrophillic metal oxide in different proportions in these modified electrode surfaces affect the CA on the film. A simple clue is evident from the CA measurement that the modified electrode surface is relatively hydrophilic compared to the bare electrode surface (ITO). On comparing the energy-dispersive X-ray spectroscopy (EDS) data of ITO and ITO/PAA, ITO/metal oxide dispersed polymer electrode surfaces, one can notice that ITO/metal oxide dispersed polymer electrode surfaces contained the additional presence of respective metal (copper, zinc and iron) signals representing the metal oxides along with carbon and oxygen arising from the polymer structure (Supplementary materials, Figure S1).  On comparing the energy-dispersive X-ray spectroscopy (EDS) data of ITO and ITO/PAA, ITO/metal oxide dispersed polymer electrode surfaces, one can notice that ITO/metal oxide dispersed polymer electrode surfaces contained the additional presence of respective metal (copper, zinc and iron) signals representing the metal oxides along with carbon and oxygen arising from the polymer structure (Supplementary materials, Figure S1).

Microstructure Analysis
X-ray photoelectron spectroscopy (XPS) survey spectra of the various fabricated electrodes by AC plasma deposition are presented in Supplementary Materials, Figure S2 and Figure S3. The survey spectra of all modified electrodes show the peak of the C1s, at 291.2 eV that originates from the skeletal structure of PAA. Likewise, survey spectra of all modified electrodes show the O1s, peak around 531.8 eV that corresponds to the presence of PAA or co-existence of PAA and metal oxides. In Figures S2 and S3, two peaks could be noticed at 933.4 eV and 953.9 eV for Cu-based PAA-modified electrodes which belong to the binding energies of Cu 2p3/2 and Cu 2p1/2. The Cu(0), Cu(I) and Cu(II) states could be distinguished from the binding energy (BE) values [38]. Specifically, the BE values of Cu(0), Cu(I) and Cu(II) states could be 932.63 eV, 932.43 eV and 933.51 eV. The presence of a higher BE peak (933.4 eV) for Cu 2p3/2 suggests the existence of Cu(II) states. Now, there could be two possibilities. The first one is that the Cu precursor, namely cupric chloride, can be present along with PAA. Alternatively, the Cu species may be transformed into copper oxide during plasma treatment. We noticed an additional satellite peak around 945.2 eV suggesting the formation of cupric oxide and

Microstructure Analysis
X-ray photoelectron spectroscopy (XPS) survey spectra of the various fabricated electrodes by AC plasma deposition are presented in Supplementary Materials, Figure S2 and Figure S3. The survey spectra of all modified electrodes show the peak of the C1s, at 291.2 eV that originates from the skeletal structure of PAA. Likewise, survey spectra of all modified electrodes show the O1s, peak around 531.8 eV that corresponds to the presence of PAA or co-existence of PAA and metal oxides. In Figures S2 and S3, two peaks could be noticed at 933.4 eV and 953.9 eV for Cu-based PAA-modified electrodes which belong to the binding energies of Cu 2p3/2 and Cu 2p1/2. The Cu(0), Cu(I) and Cu(II) states could be distinguished from the binding energy (BE) values [38]. Specifically, the BE values of Cu(0), Cu(I) and Cu(II) states could be 932.63 eV, 932.43 eV and 933.51 eV. The presence of a higher BE peak (933.4 eV) for Cu 2p3/2 suggests the existence of Cu(II) states. Now, there could be two possibilities. The first one is that the Cu precursor, namely cupric chloride, can be present along with PAA. Alternatively, the Cu species may be transformed into copper oxide during plasma treatment. We noticed an additional satellite peak around 945.2 eV suggesting the formation of cupric oxide and The survey spectra of all modified electrodes show the peak of the C1s, at 291.2 eV that originates from the skeletal structure of PAA. Likewise, survey spectra of all modified electrodes show the O1s, peak around 531.8 eV that corresponds to the presence of PAA or co-existence of PAA and metal oxides. In Figures S2 and S3, two peaks could be noticed at 933.4 eV and 953.9 eV for Cu-based PAA-modified electrodes which belong to the binding energies of Cu 2p 3/2 and Cu 2p 1/2 . The Cu(0), Cu(I) and Cu(II) states could be distinguished from the binding energy (BE) values [38]. Specifically, the BE values of Cu(0), Cu(I) and Cu(II) states could be 932.63 eV, 932.43 eV and 933.51 eV. The presence of a higher BE peak (933.4 eV) for Cu 2p 3/2 suggests the existence of Cu(II) states. Now, there could be two possibilities. The first one is that the Cu precursor, namely cupric chloride, can be present along with PAA. Alternatively, the Cu species may be transformed into copper oxide during plasma treatment. We noticed an additional satellite peak around 945.2 eV suggesting the formation of cupric oxide and the occurrence of a partially-filled d9 shell configuration in the ground state for CuO [39]. Similarly, the presence of Fe 2 O 3 and ZnO was evident from the BE peaks, 710.9 eV and 1022.1 eV, respectively.

Cyclic Voltammetry
Cyclic voltammetry was used to evaluate the electrochemical performance of the fabricated catalytic electrodes. Cyclic voltammograms (CVs) of these electrodes were recorded in 0.1M phosphate buffer solution (PBS) (pH = 7.4) for the bare ITO, PAA/ITO electrodes and similarly fabricated metal oxide/PAA modified electrodes. The bare ITO and PAA/ITO electrodes ( Figure 4) did not show any electrochemical redox peaks inferring weak electroactivity for the electrodes. However, metal oxide-PAA/ITO electrodes. Figure 4 also displayed redox peaks that correspond to the respective metal oxides. The redox peaks of Fe 2 O 3 -PAA/ITO and ZnO-PAA/ITO electrodes had very low peak current values. On the other hand, CuO-PAA/ITO electrode exhibited a remarkable redox peak current as compared to Fe 2 O 3 -PAA/ITO and ZnO-PAA/ITO electrodes ( Figure 4). From these results, we considered the CuO-PAA/ITO modified electrode as the best one amongst the metal oxide-PAA/ITO electrodes prepared in this study. Therefore, we used the CuO-PAA/ITO modified electrode for further electrochemical studies towards the electrochemical detection of H 2 O 2 . However, we could observe that the electrochemical stability of the CuO-PAA/ITO modified electrode was not good enough to proceed for long-time electrochemical measurements because the redox peak current rapidly decayed with the number of potential cycles. We attributed the deterioration of electroactivity of the CuO-PAA/ITO electrode to the leaching of the CuO from the electrode surface. Hence, we wanted to prevent leaching of CuO during electrochemical measurements and to have stable anchoring of CuO onto the surface of the electrode. In the precess of improving the stability of the working electrodes, we tested the electrochemical stability of the CuO-PAA(MAA)/ITO electrode over the long extended potential cycle conditions. The redox currents corresponding to the CuO did not show significant variation over the long time of cyclic voltammetric measurements. Hence, we selected the CuO-PAA(MAA)/ITO electrode for studying the electrochemical detection of H 2 O 2 .
Catalysts 2019, 9, x FOR PEER REVIEW 6 of 17 the occurrence of a partially-filled d9 shell configuration in the ground state for CuO [39]. Similarly, the presence of Fe2O3 and ZnO was evident from the BE peaks, 710.9 eV and 1022.1 eV, respectively.

Cyclic Voltammetry
Cyclic voltammetry was used to evaluate the electrochemical performance of the fabricated catalytic electrodes. Cyclic voltammograms (CVs) of these electrodes were recorded in 0.1M phosphate buffer solution (PBS) (pH = 7.4) for the bare ITO, PAA/ITO electrodes and similarly fabricated metal oxide/PAA modified electrodes. The bare ITO and PAA/ITO electrodes (Figure 4) did not show any electrochemical redox peaks inferring weak electroactivity for the electrodes. However, metal oxide-PAA/ITO electrodes. Figure 4 also displayed redox peaks that correspond to the respective metal oxides. The redox peaks of Fe2O3-PAA/ITO and ZnO-PAA/ITO electrodes had very low peak current values. On the other hand, CuO-PAA/ITO electrode exhibited a remarkable redox peak current as compared to Fe2O3-PAA/ITO and ZnO-PAA/ITO electrodes ( Figure 4). From these results, we considered the CuO-PAA/ITO modified electrode as the best one amongst the metal oxide-PAA/ITO electrodes prepared in this study. Therefore, we used the CuO-PAA/ITO modified electrode for further electrochemical studies towards the electrochemical detection of H2O2. However, we could observe that the electrochemical stability of the CuO-PAA/ITO modified electrode was not good enough to proceed for long-time electrochemical measurements because the redox peak current rapidly decayed with the number of potential cycles. We attributed the deterioration of electroactivity of the CuO-PAA/ITO electrode to the leaching of the CuO from the electrode surface. Hence, we wanted to prevent leaching of CuO during electrochemical measurements and to have stable anchoring of CuO onto the surface of the electrode. In the precess of improving the stability of the working electrodes, we tested the electrochemical stability of the CuO-PAA(MAA)/ITO electrode over the long extended potential cycle conditions. The redox currents corresponding to the CuO did not show significant variation over the long time of cyclic voltammetric measurements. Hence, we selected the CuO-PAA(MAA)/ITO electrode for studying the electrochemical detection of H2O2.    with an anodic to cathodic peak separation of 200 mV and an anodic to cathodic peak current ratio of~0.60. The deviation from reversibility in terms of peak current ratio is attributed to the possible presence of agglomerated CuO NPs [40]. The wider anodic to cathodic separation revealed that Cu-based electron transfer processes occur at an insulating polymer matrix, possibly diminishing the rate of the electron transfer process. window as compared to CuO-MBA-PAA/ITO electrode. Particularly, the CuO-(MBA)PAA/ITO electrode showed well-defined oxidation-reduction peaks that corresponded to the oxidation and reduction of Cu(I)/Cu(II) couple. The electrochemical response of the CuO-(MBA)PAA/ITO electrode was quasi-reversible with an anodic to cathodic peak separation of 200 mV and an anodic to cathodic peak current ratio of ~0.60. The deviation from reversibility in terms of peak current ratio is attributed to the possible presence of agglomerated CuO NPs [40]. The wider anodic to cathodic separation revealed that Cu-based electron transfer processes occur at an insulating polymer matrix, possibly diminishing the rate of the electron transfer process.  window as compared to CuO-MBA-PAA/ITO electrode. Particularly, the CuO-(MBA)PAA/ITO electrode showed well-defined oxidation-reduction peaks that corresponded to the oxidation and reduction of Cu(I)/Cu(II) couple. The electrochemical response of the CuO-(MBA)PAA/ITO electrode was quasi-reversible with an anodic to cathodic peak separation of 200 mV and an anodic to cathodic peak current ratio of ~0.60. The deviation from reversibility in terms of peak current ratio is attributed to the possible presence of agglomerated CuO NPs [40]. The wider anodic to cathodic separation revealed that Cu-based electron transfer processes occur at an insulating polymer matrix, possibly diminishing the rate of the electron transfer process.

Amperometric H2O2 Detection and Sensor Performances
The amperometric analysis was used to evaluate the electrocatalytic activity of the CuO-PAA(MBA)/ITO electrode for different concentrations of H2O2. Figure 7

Selectivity Measurement of the Fabricated Catalytic Electrode
Selectivity is one of the most crucial factors that needs to be considered while fabricating an electrochemical sensor for a selected analyte. In this study, the selectivity of the catalytic electrode with CuO NPs was evaluated in the presence of three electrochemical interfering substances for H 2 O 2 , namely sodium chloride, ascorbic acid, and sodium nitrite, at the 0.1 mM level, respectively ( Figure 8). As can be seen in Figure 8, the current value of CuO-PAA-MBA/ITO electrode is not significantly influenced in the presence of any of the three selected interfering substances. These results indicate that the CuO-PAA(MBA)/ITO electrode, has a good selectivity towards the detected H 2 O 2 .
Selectivity is one of the most crucial factors that needs to be considered while fabricating an electrochemical sensor for a selected analyte. In this study, the selectivity of the catalytic electrode with CuO NPs was evaluated in the presence of three electrochemical interfering substances for H2O2, namely sodium chloride, ascorbic acid, and sodium nitrite, at the 0.1 mM level, respectively ( Figure  8). As can be seen in Figure 8, the current value of CuO-PAA-MBA/ITO electrode is not significantly influenced in the presence of any of the three selected interfering substances. These results indicate that the CuO-PAA(MBA)/ITO electrode, has a good selectivity towards the detected H2O2.
It is known that oxygen is an another important interferent during the reduction process of H2O2. Hence, the supporting electrolyte needs to be de-oxygenated by bubbling nitrogen for sufficient length of time (~20 minutes) to avoid the most common possible interferents, namely dissolved oxygen in the electrolyte system [41]. Importantly, in the present work, the reduction potential of H2O2 on CuO-PAA-MBA/ITO electrode was around -0.4 V, which is far beneath the thermodynamic electrode potential for oxygen reduction at standard conditions. Hence, the interference from oxygen is negligible in this work. All the above results have indicated that the proposed H2O2 sensor could be used in vivo measurements.

Stability Measurement of the Fabricated Catalytic Electrode
The stability of the CuO-PAA(MBA)/ITO electrode towards detection of H2O2 was investigated by CVs by monitoring the reduction current of 0.5 mM H2O2 for different numbers of cycles. As shown in Figure 9, the reduction current value of the catalytic electrode showed a marginal difference for initial cycles but decreased by ~ 23% after 120 cycles. The stable peak currents at the initial cycles is acceptable for a practical sensor. The decrease in current after a large number of continuous cycles (>100) may be due to the leaching of CuO from the electrode surface. Continuously keeping the electrode at a longer period during the experiment may be the reason for the leaching of catalytically active material from the electrode surface. Further optimization of the PAA (MBA) forming condition could possibly avoid the problem related to leaching of catalytically active particle from the electrode surface. It is known that oxygen is an another important interferent during the reduction process of H 2 O 2 . Hence, the supporting electrolyte needs to be de-oxygenated by bubbling nitrogen for sufficient length of time (~20 min) to avoid the most common possible interferents, namely dissolved oxygen in the electrolyte system [41]. Importantly, in the present work, the reduction potential of H 2 O 2 on CuO-PAA-MBA/ITO electrode was around −0.4 V, which is far beneath the thermodynamic electrode potential for oxygen reduction at standard conditions. Hence, the interference from oxygen is negligible in this work. All the above results have indicated that the proposed H 2 O 2 sensor could be used in vivo measurements.

Stability Measurement of the Fabricated Catalytic Electrode
The stability of the CuO-PAA(MBA)/ITO electrode towards detection of H 2 O 2 was investigated by CVs by monitoring the reduction current of 0.5 mM H 2 O 2 for different numbers of cycles. As shown in Figure 9, the reduction current value of the catalytic electrode showed a marginal difference for initial cycles but decreased by~23% after 120 cycles. The stable peak currents at the initial cycles is acceptable for a practical sensor. The decrease in current after a large number of continuous cycles (>100) may be due to the leaching of CuO from the electrode surface. Continuously keeping the electrode at a longer period during the experiment may be the reason for the leaching of catalytically active material from the electrode surface. Further optimization of the PAA (MBA) forming condition could possibly avoid the problem related to leaching of catalytically active particle from the electrode surface.

Electrode Fabrication by Plasma Treatment
In the case of PAA/ITO and PAA (MBA)/ITO electrode fabrication through plasma treatment, we envisage the deposition of polymer layer on ITO through the following sequence of reactions. Initially, the physical state of vinyl monomer (AA) or mixture of AA and MBA, upon treatment of plasma, changes from liquid state to vapor phase ( Figure 10). Subsequently, the formation of reactive fragments, such as radicals from the vinyl monomer(s) and conversion of them to polymer are expected to happen in the gas phase. The polymer (PAA or PAA(MBA)) is deposited on the ITO surface, thus creating a plasma-deposited polymer coating on the surface. Since the vinyl monomer is fragmented in the plasma, the created polymer layer will not necessarily have a structure and composition which is similar to polymers achieved by conventional polymerization of the same monomer. Plasma polymerization can take place with many monomers in the vapor phase, even if they do not have unsaturated bonds or cyclic structures. The use of MBA along with generates a network, PAA (MBA), which was eventually deposited on the entire surface of the ITO [28].

Electrode Fabrication by Plasma Treatment
In the case of PAA/ITO and PAA (MBA)/ITO electrode fabrication through plasma treatment, we envisage the deposition of polymer layer on ITO through the following sequence of reactions. Initially, the physical state of vinyl monomer (AA) or mixture of AA and MBA, upon treatment of plasma, changes from liquid state to vapor phase ( Figure 10). Subsequently, the formation of reactive fragments, such as radicals from the vinyl monomer(s) and conversion of them to polymer are expected to happen in the gas phase. The polymer (PAA or PAA(MBA)) is deposited on the ITO surface, thus creating a plasma-deposited polymer coating on the surface. Since the vinyl monomer is fragmented in the plasma, the created polymer layer will not necessarily have a structure and composition which is similar to polymers achieved by conventional polymerization of the same monomer. Plasma polymerization can take place with many monomers in the vapor phase, even if they do not have unsaturated bonds or cyclic structures. The use of MBA along with generates a network, PAA (MBA), which was eventually deposited on the entire surface of the ITO [28].

Electrode Fabrication by Plasma Treatment
In the case of PAA/ITO and PAA (MBA)/ITO electrode fabrication through plasma treatment, we envisage the deposition of polymer layer on ITO through the following sequence of reactions. Initially, the physical state of vinyl monomer (AA) or mixture of AA and MBA, upon treatment of plasma, changes from liquid state to vapor phase ( Figure 10). Subsequently, the formation of reactive fragments, such as radicals from the vinyl monomer(s) and conversion of them to polymer are expected to happen in the gas phase. The polymer (PAA or PAA(MBA)) is deposited on the ITO surface, thus creating a plasma-deposited polymer coating on the surface. Since the vinyl monomer is fragmented in the plasma, the created polymer layer will not necessarily have a structure and composition which is similar to polymers achieved by conventional polymerization of the same monomer. Plasma polymerization can take place with many monomers in the vapor phase, even if they do not have unsaturated bonds or cyclic structures. The use of MBA along with generates a network, PAA (MBA), which was eventually deposited on the entire surface of the ITO [28]. The precursor solution for the metal oxide/PAA or metal oxide/PAA(MBA) deposition contained either a mixture of acrylic acid and the respective salts, or acrylic acid, cross-linking agent (MBA), and the respective salts as in Table 1. Complexes of polymers derived from natural amino acids or carboxylic-containing polymers such as PAA with various charge density along the main chain, have been extensively studied [42]. Based on that, we envisage there could be possibility for the formation of complexes between metal salts or the polymer after the plasma treatment. Keeping the fact that the primary coordination number of the divalent metals used in this work is predominantly four, we presume that PAA-divalent metal complexes could be a square planar or tetrahedral structure as proposed earlier [42]. In the complexation proposal, the two coordination sites of the metal ion can be satisfied by the two carboxyl anions, and the residual two coordination sites can be satisfied by the oxygen of two carboxyl groups in the polymer. In our case, our goal was to fabricate a modified electrode. The plasma treatment, as we have used in this work, was favorably used to fabricate the metal oxide anchored PAA or PAA(MBA)-based electrodes. However, in the case of metal oxide/PAA/ITO electrode, the metal oxide was found to be leached out. Hence, we presume that the removal of metal ions from the complexes could be possible when the PAA structure favors planar complexes. On the other hand, the electrochemical stability of metal oxide/PAA(MBA) electrode was superior ( Figure 4). We therefore envisage that the three-dimensional polymer network generated by MBA can facilitate three-dimensional interlocked metal ions within PAA(MBA) and could be the reason for the extended stability for metal oxide/PAA(MBA)/ITO over metal oxide/PAA electrodes.  In plasma environments, the metal oxide NPs could be formed and included in PAA(MBA) film. A schematic presentation is shown in Figure 11 that presents how electrocatalytic reduction of H 2 O 2 at the CuO-PAA(MBA)/ITO electrode could be used for the fabrication of sensor electrodes. The mechanism comprises the electrochemical reduction of CuO (Cu(II) state) to Cu2O (Cu(I) state), which subsequently converts H 2 O 2 to a hydroxyl ion or water along with the regeneration of the catalyst [43]. The precursor solution for the metal oxide/PAA or metal oxide/PAA(MBA) deposition contained either a mixture of acrylic acid and the respective salts, or acrylic acid, cross-linking agent (MBA), and the respective salts as in Table 1. Complexes of polymers derived from natural amino acids or carboxylic-containing polymers such as PAA with various charge density along the main chain, have been extensively studied [42]. Based on that, we envisage there could be possibility for the formation of complexes between metal salts or the polymer after the plasma treatment. Keeping the fact that the primary coordination number of the divalent metals used in this work is predominantly four, we presume that PAA-divalent metal complexes could be a square planar or tetrahedral structure as proposed earlier [42]. In the complexation proposal, the two coordination sites of the metal ion can be satisfied by the two carboxyl anions, and the residual two coordination sites can be satisfied by the oxygen of two carboxyl groups in the polymer. In our case, our goal was to fabricate a modified electrode. The plasma treatment, as we have used in this work, was favorably used to fabricate the metal oxide anchored PAA or PAA(MBA)-based electrodes. However, in the case of metal oxide/PAA/ITO electrode, the metal oxide was found to be leached out. Hence, we presume that the removal of metal ions from the complexes could be possible when the PAA structure favors planar complexes. On the other hand, the electrochemical stability of metal oxide/PAA(MBA) electrode was superior ( Figure 4). We therefore envisage that the three-dimensional polymer network generated by MBA can facilitate three-dimensional interlocked metal ions within PAA(MBA) and could be the reason for the extended stability for metal oxide/PAA(MBA)/ITO over metal oxide/PAA electrodes.  In plasma environments, the metal oxide NPs could be formed and included in PAA(MBA) film. A schematic presentation is shown in Figure 11 that presents how electrocatalytic reduction of H2O2 at the CuO-PAA(MBA)/ITO electrode could be used for the fabrication of sensor electrodes. The mechanism comprises the electrochemical reduction of CuO (Cu(II) state) to Cu2O (Cu(I) state), which subsequently converts H2O2 to a hydroxyl ion or water along with the regeneration of the catalyst [43]. Figure 11. A plausible mechanism for the electrocatalytic reduction of H 2 O 2 at the CuO-loaded ITO electrode. Table 2 provides the summary of fabricated CuO-modified sensors and preparation conditions of CuO-loaded ITO electrodes. In brief, the CuO-loaded ITO electrode exhibits a wider linear range (0.5-8.5 mM), a lowest detection limit (0.6 µM) and high sensitivity (63.52 mA M −1 cm −2 ) for the electrochemical detection of H 2 O 2 . The fabrication of CuO-loaded electrode is simple (one-step AC plasma deposition) and easy to prepare.  6 .3H 2 O), potassium ferricyanide (K 3 Fe(CN) 6 , and hydrogen peroxide (H 2 O 2 , 30%) were purchased from Duksan Pharmaceutical Co., Ltd. (Ansan, Korea). Acrylic acid and hydrochloric acid were supplied by Alfa Aesar (Ward Hill, MA, USA). Indium tin oxide (ITO)-coated glass as working electrode (25 mm × 25 mm × 1.1 mm, 30-60 Ω/sq) was purchased from Sigma Aldrich Co. (Sigma-Aldrich, St. Louis, MO, USA). Phosphate buffer solution (PBS) was prepared by mixing of 0.1M Na 2 HPO 4 and 0.1M NaH 2 PO 4 , and then adjusted pH to the value of 7.4. Solutions for the experiments were prepared with water purification by a Milli-Q plus water purification system (Millipore Co. Ltd., Burlington, MA, USA, the final resistance of water was 18.2 MΩ cm −1 ).
Plasma devices were required to be small, simple, and portable for various applications. We chose an AC-type plasma device that is closest to these requirements, and the power can be controlled in the range of 200 to 250 W. Most of the experiments were carried out in the region of 200 W. Torches used in this experiment were conical and the diameter of the torch tip was 2.0 mm. Nitrogen as the working gas was supplied from a tank connected to the plasma generator and a flow controller inside. The plasma generator was used to keep the pressure at a constant level. The stock solution was injected to the AC plasma jet by a syringe pump in a down-stream region. ITO substrate was placed under the AC plasma jet at a distance of 1 cm and could be moved manually during the deposition process as shown in Figure 10.

Fabrication of Catalytic Electrode by AC Plasma Deposition
The precursor solutions for the experiment were prepared as follows. Firstly, a precise amount of a metal salt was dissolved in 0.5 mL purified water, then mixed with 5 mL of acrylic acid. Three metal salts, namely copper (II) chloride, ion (III) chloride, and zinc (II) chloride were used in this experiment. A blank sample which has only acrylic acid was also prepared for comparison. The suggested amounts of those precursor solution are shown in Table 1.
ITO substrate was pre-cleaned using ethanol and ultra-sonicated for 10 min before AC plasma deposition. During the deposition process, the ITO substrate was moved slowly in the x and y directions and the deposition time was five minutes. The precursor solutions were injected into the AC plasma jet by the syringe pump at a flow rate of 0.3 mL/min and the gas pressure was kept constantly at 0.018 MPa. After the deposition processes finished, the fabricated electrodes were cleaned with water and methanol to remove the unreacted precursors then dried and stored at 4 • C before application.

Conclusions
A metal oxide (CuO) dispersed catalytic electrode was successfully fabricated by a one-step AC plasma deposition method, and further utilized for the electrochemical detection of hydrogen peroxide (H 2 O 2 ). Polymer and metal oxide were simultaneously formed on the electrode surface, thus making the modified electrode fabrication simple. The stability of the polymer-metal oxide modified surface can be made electrochemically stable through a cross-linking process during plasma treatment. Our results indicated that electrochemical sensing of H 2 O 2 could be achieved through electrocatalytic reduction with a reasonable sensitivity, selectivity and stability. More importantly, the simple strategy, namely plasma treatment of relevant precursors, which we used in this work, could be conveniently extended to the fast and simple fabrication of electrochemical sensors based on other kinds of nanomaterials (example: metallic nanoparticles).