A novel nonenzymatic amperometric hydrogen peroxide sensor based on CuO@Cu2O nanowires embedded into poly(vinyl alcohol)

Author’s Accepted Manuscript A novel nonenzymatic amperometric Hydrogen peroxide sensor based on CuO@Cu2O nanowires embedded into poly(vinyl alcohol) Chirizzi Daniela, Guascito Maria Rachele, Filippo Emanuela, Malitesta Cosimino, Tepore Antonio www.elsevier.com/locate/talanta

PII: DOI: Reference:

S0039-9140(15)30330-1 http://dx.doi.org/10.1016/j.talanta.2015.09.038 TAL15971

To appear in: Talanta Received date: 22 June 2015 Revised date: 10 September 2015 Accepted date: 12 September 2015 Cite this article as: Chirizzi Daniela, Guascito Maria Rachele, Filippo Emanuela, Malitesta Cosimino and Tepore Antonio, A novel nonenzymatic amperometric Hydrogen peroxide sensor based on CuO@Cu2O nanowires embedded into poly(vinyl alcohol), Talanta, http://dx.doi.org/10.1016/j.talanta.2015.09.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A novel nonenzymatic amperometric hydrogen peroxide sensor based on CuO@Cu2O nanowires embedded into poly(vinyl alcohol) Chirizzi Daniela,a Guascito Maria Rachele,b* Filippo Emanuela,a Malitesta Cosimino,b Tepore Antonioa a

Department of Cultural Heritage, University of Salento, Via Birago, 73100 Lecce, Italy.

b

Department of Di.S.Te.B.A., University of Salento, Via Monteroni, 73100 Lecce, Italy.

* [email protected]. Tel: +39 0832297075 Fax: +39 0832297100

ABSTRACT A new, very simple, rapid and inexpensive nonenzymatic amperometric sensor for hydrogen peroxide (H2O2) detection is proposed. It is based on the immobilization of cupric/cuprous oxide core shell nanowires (CuO@Cu2O-NWs) in a poly(vinyl alcohol) (PVA) matrix directly drop casted on a glassy carbon electrode surface to make a CuO@Cu2O core shell like NWs PVA embedded (CuO@Cu2O-NWs/PVA) sensor. CuO nanowires with mean diameters of 120–170 nm and length in the range 2–5 μm were grown by a simple catalyst-free thermal oxidation process based on resistive heating of pure copper wires at ambient conditions. The oxidation process of the copper wire surface led to the formation of a three layered structure: a thick Cu2O bottom layer, a CuO thin intermediate layer and CuO nanowires. CuO nanowires were carefully scratched from Cu2O layer with a sharp knife, dispersed into ethanol and sonicated. Then, the NWs were embedded in PVA matrix. The morphological and spectroscopic characterization of synthesised CuO-NWs and CuO@Cu2O-NWs/PVA were performed by transmission electron microscopy (TEM), selected area diffraction pattern (SAD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analysis. Moreover a complete electrochemical characterization of these new CuO@Cu2O-NWs/PVA modified glassy carbon electrodes was performed by Cyclic Voltammetry 1

(CV) and Cronoamperometry (CA) in phosphate buffer (pH=7; I=0.2) to investigate the sensing properties of this material against H2O2. The electrochemical performances of proposed sensors as high sensitivity, fast response, reproducibility and selectivity make them suitable for the quantitative determination of hydrogen peroxide substrate in batch analysis.

Keywords: Electrochemical nonenzymatic sensor, nanowires, hydrogen peroxide, TEM, XPS.

Dedication: "Dedicated to Professor Pier Giorgio Zambonin (Bari, Italy) on the occasion of his 80th birthday".

1. INTRODUCTION Electrodes modified with nanomaterials are of great interest to develop nonenzymatic electrochemical sensors for important applications in chemical/biochemical field with a high selectivity, reproducibility, stability and fast response time. The new nanomaterials are promising tools because they could facilitate electron-transfer reactions and this could be coupled with easy miniaturization of sensing devices to nanoscale dimensions. In particular, electrochemically active nanomaterials such as metals [1], carbon nanotubes [2] and metal oxide [3] have been extensively studied because they have high surface-to-volume ratio and highly effective catalytic properties [47]. Of these, Cu oxide NWs, like the other nanoparticles, not only could increase the electrochemical activity but also are relatively inexpensive and have a good biocompatibility. Moreover they are chemically more stable respect to metallic copper and so they are suitable for various application including electrochemical hydrogen peroxide sensors [8, 9]. The detection of H2O2 is of great importance in food and environmental monitoring, diagnosis, and biological studies [10]. Conventional techniques for hydrogen peroxide determination such as fluorimetry [11], chemiluminescence [12], fluorescence [13] and spectrophotometry [14] are

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complex, costly and time consuming. In comparison, electrochemistry can offer simple, rapid, sensitive, and cost effective means since H2O2 is an electroactive analyte [15-20]. In this work, for the first time, a new amperometric sensor based on GC modified electrode by direct drop casting of the mixture containing CuO@Cu2O-NWs embedded into poly(vinyl alcohol) (PVA) was developed. The dispersion of CuO@Cu2O-NWs into a polymeric support prevents the aggregation of nanoparticles. PVA material is a low cost, inert and water soluble polymer depending on its degree of hydrolysis, molecular weight, and tendency to hydrogen bond [21]. It shows, also a inherent good biocompatibility and desirable physical properties, such as elastic nature and good film forming properties [22]. Moreover, different method of synthesis have been reported to make also stable PVA and PVA-composite materials suitable to be used in sensing applications in aqueous solutions [23]. To the best of our knowledge, there is no report on the hydrogen peroxide electrochemical sensing using GC conventional electrode material modified and/or based on CuO@Cu2O-NWs dispersed in PVA. The morphology, the chemical composition and the structure of the copper nanowires surface as synthesized and after PVA immobilization were performed by transmission electron microscopy (TEM), selected area electron diffraction (SAD), and XPS analysis. Electrochemical characterization of CuO@Cu2O/PVA/GC electrodes was performed by CV and CA. The collected data evidenced that the presence of nanowires was responsible for an increment of reduction currents in presence of H2O2, and promote a remarkable, stable and reproducible response at very low reduction potentials for H2O2 of -0.200 V vs. SCE in batch analysis.

2. EXPERIMENTAL 2.1 Chemicals and Apparatus Hydrogen peroxide 30%, Na2HPO4, NaH2PO4, PVA film (product number Z300381), alumina powder were analytical grade reagents furnished from Sigma. As substrate, copper wires

3

(99.999%, Aldrich Chemical, ϕ = 0.25 mm) were used. Ultrapure water (Millipore Milli-Q, 18.2 MΩcm-1) was used. Diluted H2O2 standard solution were freshly prepared directly before use. All electrochemical experiments were carried out by using a µStat400 DropSens electrochemical workstation controlled by computer. In batch experiments conventional three-electrode system with a GC disk (0.07065 cm2) as working electrode, a Pt wire as counter electrode and a saturated calomel electrode (SCE) as reference were used. All electrochemical experiments were carried out in phosphate buffer pH=7.0 and I=0.2 bubbled with N2 for fifteen minutes to avoid oxygen interference and a nitrogen atmosphere was maintained in the cell throughout the measurements. The surface of GC electrode was initially polished with alumina powder (0.05µm) on a soft polishing cloth (CH Instruments). After the electrode was sonicated in water for 10 min. Then it was washed with deionised water and dried with N2 flow. The morphological characterization of the samples was performed by scanning electron microscope (SEM, Tescan) operated at 25kV equipped with an advanced Quantax EDX microanalysis system (Bruker). Transmission electron microscopy observations and electron diffraction investigations were carried out using a Hitachi H-600 microscope operating at 100kV. XPS spectra were recorded using a Leybold LHS10 upgraded by a PHOIBOS 100 Analyzer/Detector system (SPECS, Berlin, Germany) equipped with a twin anode (Mg Kα/Al Kα) non-monochromatized source (operating at 200 W). Survey spectra (FRR, B=30) and detailed spectra (FAT, E0=50 eV) for C1s, Cu2p, O1s and valence band (VB) regions were collected at 0.1eV step intervals. High-resolution spectra were referenced to the C1s main component binding energy of the adventitious hydrocarbon at 285.0eV. All XPS spectra were recorded at an electron take-off-angle (TOA) of 90◦ relative to the sample surface, giving the maximal sampling depth 3λ of about 4nm is1.25 nm) [24, 25]. XPS spectra were collected for CuO-NWs as synthesised and as CuO@Cu2O-NWs/PVA for comparison. No X-ray induced decomposition of samples was observed during XPS data

4

acquisition, as manifested by changing peak areas or shifting of positions. Data analysis of highresolution spectra was performed using the fitting program GOOGLY [26].

2.2 Synthesis of nanowires and Preparation of the CuO@Cu2O-NWs/PVA/GC The synthesis of CuO nanowires was carried out by direct resistive heating of copper wire in ambient conditions applying a potential difference 2.8V to 20 cm long Cu wires for 40 minutes. Such a method doesn’t require any high-priced and complicated equipment and it requires a growth time of a few seconds and energy values which is thousand times less than the energy consumed during conventional synthesis methods as reported in literature [27]. Ambient air conditions corresponded to regular laboratory and atmospheric pressure conditions with a temperature from 21 to 24 ◦C and a relative humidity from 30 to 45%. In our previous work [24], we observed that the oxidation process of the copper wire surface led to the formation of a three layered structure: a thick Cu2O bottom layer, a CuO thin intermediate layer and CuO nanowires. CuO nanowires were carefully scratched from CuO layer with a sharp knife, dispersed into ethanol in an ultrasonic bath for about 10 minutes. In the second step the filtered nanowires (1 mg) were dissolved in 1mL aqueous solution of 10% PVA in an ultrasonic bath for 5 min. Aqueous solution of 10% PVA was prepared by dissolving PVA in hot deionized water (100 C°), kept at room temperature for 6 h and then stored at -18 °C for 48 h, according to the procedure already developed in our laboratory [28, 29]. It was observed that it was necessary to keep the PVA solution in such conditions to obtain a stable PVA matrix to embed copper oxide NWs. This behavior was in agreement with literature data reporting that the subjection of aqueous PVA solutions to freezing–thawing treatment led to reinforced gels owing to the densification of the macromolecular structure [30]. Modified electrodes were made by drop casting of 5 µL of CuO@Cu2O-NWs/PVA mixture directly on GC electrode surface to make a modified electrode with a very reproducible and not contemned surface. Then, the suspension was let to evaporate at room temperature. After 5

CuO@Cu2O-NWs/PVA/GC modified electrodes were cycled in PB between -0.400V and +0.400 V at 0.020 Vs-1 until a steady-state current was obtained to make a stable electrochemical response. Modified electrodes were washed after preparation and tested as amperometric sensor to detect hydrogen peroxide both in CV and crono-amperometric experiments in a fresh prepared PB solution.

3. RESULTS AND DISCUSSION 3.1 Morphologic and Spectroscopic Characterization A typical TEM image of a single CuO-NW as synthesized is shown in Figure 1A. It can be seen that nanowires are straight with smooth surfaces and uniform diameters in average of 150 nm. Figure 1B shows the corresponding selected area electron diffraction (SAD) pattern that would be indexed as monoclinic CuO structure with the lattice constants, a=0.469 nm, b=0.343 nm, c=0.513 nm and β=99.55°. SAD pattern evidenced that CuO NWs are single crystals with preferential growth along (111) direction. SEM observations clearly showed that as synthesized NWs were preferentially oriented perpendicularly to the substrate surface layer and they had typical lengths between 2-5 µm and diameter in the range 120-170 nm (Fig. 1C). SEM images (Fig. 1D) shows nanowires carefully encapsulated into PVA matrix evidencing that some longer NWs extended above the polymer matrix by few hundred nanometers. Such a polymer formed a protective and mechanically tough sheath that enabled simple and macroscopic manipulation to be carried out in a straightforward fashion without expensive nanomanipulation techniques, circumventing a major challenge to the large scale integration.

Figure 1 here

XPS spectra were acquired for studying the surface properties of both CuO-NWs and CuO@Cu2O-NWs/PVA materials. The high resolution region of Cu2p was employed to investigate 6

the Cu surface oxidation. Figure 2 shows the main and the satellite peaks of Cu2p3/2 and Cu2p1/2 of CuO-NWs as synthesized (Fig. 2A) and after encapsulated in PVA (Fig. 2B). For both samples, typically, the two components and the shake-up (SH) peaks, in which each Cu2p3/2 and Cu2p1/2 signal have been deconvoluted, are indicated. In detail, the broad Cu2p peaks components have been fitted with two peak pairs which are marked as A and A' and B and B' peaks, respectively. Also the shake-up peak pair is present: SHA and SHA'. For Cu2p3/2, the component at higher BE (peak A) corresponds to CuO species, as clearly confirmed by the peaks values at 936.6±0.1eV and at 934.6±0.1eV for CuO-NWs and CuO@Cu2O-NWs-PVA respectively, according to binding energy values already reported for nano-sized CuO particles [31]. Consistently, the shape of the photoelectronic peaks shows characteristic shake-up (SH) features at 944.0±0.1 eV and 942.6±0.1 eV in both samples, that cannot be attributed to Cu2O and/or Cu(0) [31]. The lower BE component (peak B) of the Cu2p3/2 at 933.6±0.1 eV and 932.2±0.1 eV were attributed to Cu2O species. For a better attribution of these peak components to discriminate between Cu(I) and Cu(0) eventually present species, the values of α' [32] parameter has been calculated. The α' values obtained were 1848.6eV for CuO-NWs and 1847.9 eV for CuO-NWs/PVA, allowing to exclude the formation of Cu(0) species for both samples, for which a value of α' parameter of 1850.0±0.1 eV is attended [33]. A typical red shift in binding energy, was observed from CuO-NW to CuO-NWs/PVA, confirming an interaction of CuO-NWs with PVA matrix. As far as it is well know that XPS chemical shift and α' parameter changes are dependent of the local electronic structure on the atomic environment [34]. Moreover, the typical CuO/Cu2O ratio obtained from the Cu2p3/2 fitting area of about 0.76 measured for CuO-NWs was further reduced up to 0.38 for CuO-NWs capped in PVA, suggesting that the CuO and Cu2O species are not uniformly distributed along the surface depth probed by the XPS photoelectrons. In fact, if the composition of the surface layer of the NWs was homogenous along the whole thickness analyzed in the absence of PVA, the CuO/Cu2O ratio decrement could not be observed when capped in PVA film, as the photoelectronic peaks of both copper species are characterized by almost the same kinetic energy, and then they will pass through the same PVA 7

film thickness with equal efficiency. Consistently the observed CuO/Cu2O ratio decrement is related to a different surface composition of the NWs enriched in Cu2O with respect CuO core. According the NWs CuO core was or capped in a Cu2O external top layer (Cu2O shell) or otherwise diluted along the analysed depth from the inner to the top of the NWs surfaces. The Cu2O coverage (θA) has been approximately estimated in monolayer (ML) from the area under the Cu2p peaks (IA) attributed to Cu2O (adsorbate) relative to that of Cu2p peaks (IB) attributed to CuO (substrate), as a function of emersion potential according to equation (1): [24]

(

)(

(

(

)

(1)

))

where λA (EA) is the mean free path of the inelastically scattered photo electrons with kinetic energy EA of the adsorbate Cu2O (λCu2p=1.86 nm (8 ML) at EA= 552 eV) [35]; β is the escape angle of the photo electrons (β=0 in our case); αA is the atom size for Cu ~0.192 nm [36], AB (0.019 nm2) is the averaged surface unit cell area of CuO substrate and IA∞ and IB∞ are the atomic sensitivity factors of overlayer and substrate copper species (IA∞ = IB∞). The averaged XPS-derived coverage of Cu2O was almost 3-4 ML with thickness between 0.75-1.0 nm [35]. A similar behaviour, according to structural and bulk chemical analysis characterizations by SEM and TEM, has confirmed the fabrication of nanowires with a CuO core and a thin Cu2O outer layer, segregated in the top of layer of nanowires, mimicking a core-shell like structure CuO@Cu2O-NWs/PVA [37]. Cu2O phase formation on the top layer of NWs could be attributed to the more symmetric cubic Cu2O phase more stable than the monoclinic CuO phase at nanosize [38].However, as already stated in literature, many surface properties of copper oxides are not well understood particularly at nano scale [31].

Figure 2 here

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3.2 Electrochemical Characterization Cyclic voltammetry was used to study the main electrochemical properties of the CuO@Cu2ONWs/PVA/GC electrodes in PB. GC bare electrodes are electrochemically inactive in the potential range between -0.400 V and +0.400 V so they can be used as conductive substrates (Fig. 3A dot curves). Moreover, a substantial difference was evident between the electrochemical activity recorded on CuO@Cu2O-NWs/PVA/GC electrodes at 0.020 Vs-1 scan rate in a potential range between -0.400 V and +0.400 V, when used as prepared (Fig. 3A, solid curve first cycles) and after reaching the steady-state condition (Fig. 3A, solid curve 15th cycle). Infact, in the first CV cycles, two not well resolved peaks were observed in reduction, respectively at -0.210 and -0.265 V, moreover in oxidation only a peak at -0.020 V was observed, as expected for a totally irreversible surface controlled electrochemical process [39]. According to copper Pourbaix diagram at pH 7 [40], initially these red/ox processes, probably involve CuO, Cu2O (see also XPS analysis) and Cu(0) species in line with the following simplified scheme. In reduction: 2CuO + 2H+ + 2e- = Cu2O + H2O (EpI = -0.210 V)

(2)

Cu2O + 2H+ + 2e- = 2Cu + H2O (EpII = -0.265 V)

(3)

In oxidation: Cu + H2O = CuO + 2H+ + 2e- (EpIII = -20mV)

(4)

However, in steady-state conditions, reached after almost fifteen cycles, in reduction it is now possible to observe only a peak at -190 mV attributed to the electrochemical reduction of CuO to Cu2O. The Cu2O re-oxidation, was always observed at peak potential of 0.00 V. It is evident that in the actual experimental conditions the most stable electro active red-ox pair was the CuO/Cu2O for CuO@Cu2O-NWs PVA embedded. The modified electrodes, obtained in stady-state conditions, were washed and transferred in a fresh prepared PB. Figure 3B shows for comparison typical GC and CuO@Cu2O-NWs PVA base lines obtained in the same experimental conditions used for H2O2 detection. The intersystem electrochemical reproducibility of the sensor obtained in steady-state 9

conditions was calculated by valuing the variation of chatodic peak currents (Ip) and related peak potentials (Ep) measured on five independently prepared sensors cycled in PB in the same potential range used for H2O2 sensing measurements. The coefficients of variation was 10% for the Ip and 3% for Ep.

Figure 3 here

3.3 Electrochemical response to H2O2 on CuO@Cu2O-NWs/PVA/GC electrode Cyclic voltammetry was used to study the main electrochemical properties of the CuO@Cu2ONWs/PVA/GC electrode respect to H2O2. All experiments were carried out in PBS N2 saturated, scan rate 0.020 Vs-1. In figure 4 voltammograms recorded on bare GC (Fig. 4A) and on CuO@Cu2O-NWs/PVA/GC (Fig. 4B) electrodes are reported respectively in the absence (dot curves) and in presence of H2O2 2 mM (solid curves). Substantial differences were evident between the electrochemical activity recorded on bare GC in the presence and in the absence of CuO@Cu2O-NWs/PVA. As expected, relevant processes related to H2O2 reduction were present only on CuO@Cu2O-NWs/PVA/GC electrodes (Fig. 4B). It is known that the oxidation of H2O2 does not show a clearly defined wave at a bare GC electrode [41] and our results supported this. The increment of currents, observed only in reduction on the CuO@Cu2O-NWs/PVA/GC, is easily attributable to an irreversible electrocatalytic process of hydrogen peroxide reduction. Consistently, the electro reduction of hydrogen peroxide took place during both direct and reverse scans, as expected for a catalytic process [42]. In figure 5A CV recorded on CuO@Cu2O-NWs/PVA/GC electrodes are reported respectively in the absence (black curve) and in presence of H2O2 at different concentrations (coloured curves), ranging between 0.01 and 5.00 mM. The catalytic currents Ipc in reduction, obtained after subtracting the relative baselines recorded in PBS, were found to increase linearly (Fig. 5B) with H2O2 concentration up to 2 mM, with a 10

determination coefficient of 0.997 deduced by the linear regression analysis carried out with the following equation: Ipc (µA) = −0.729 (µA) − 0.535 (µA mM−1) C (mM)

(5)

where C is H2O2 bulk concentration. Based on these results probably in a simplified scheme the electro-catalytic reaction involved firstly the electrochemical reduction of CuO to form Cu2O (6) which reacted chemically with H2O2 to re-generate CuO starting electro-active material (7). 2CuO + 2e- + 2H+= Cu2O + H2O

(6)

Cu2O + H2O2 = 2CuO + H2O

(7)

Figure 5 here

The analytical behavior of CuO@Cu2O-NWs/PVA/GC electrodes to consecutive additions of H2O2 was investigated also by amperometric experiments at a static potential in stirred PBS. The effect of the applied potential was studied on the modified electrodes by cronoamperometric measurements, in potentiodynamic conditions, between 0.00 V and - 0.400 V vs SCE, at H2O2 100 M (Fig. S1). The optimal value (-0.200 V) was selected as a compromise between the highest current output and the stability of the signal. In figure 6A the typical amperometric measurement at a static potential of -0.200 V is reported regarding CuO@Cu2O-NWs/PVA/GC sensor response to successive H2O2 injections. The inset shows how it is sensitive even at very low concentrations (1-50 µM). Moreover, sensor response time is very fast and it reaches a stable value approximately in 5s after successive H2O2 injections. For a comparison, also amperometric responses of GC bare electrode are investigated. Calibration curves for the CuO@Cu2O-NWs/PVA/GC sensor were obtained by plotting the average value of catalytic current (I), measured at each successive H2O2 injection (step) performed in triplicate on three different electrodes, against its concentration (results are presented in figure 6B). The role played by CuO@Cu2O-NWs as promoters of H2O2 reduction is showed by the 11

remarkable increments registered during experiments involving CuO@Cu2O-NWs/PVA/GC sensor in contrast to bare GC. Two main regimes of linear variation of I towards concentration may be identified for CuO@Cu2O-NWs/PVA/GC sensor. The first range at low H2O2 levels (1.0 µM-3.0 mM) was relevant to a linear variation of I towards concentration with a good value for the slope (2.793 μAmM-1), the second linear range was determined at higher H2O2 concentrations (3.0 - 10.0 mM) with a slight decrease in slope (-1.218 μAmM-1). The proposed sensor showed then a sensitivity of 39.5 μAcm-2mM−1 between 1 µM- 3 mM and 17.3 μAcm-2mM−1 between 3.0-10.0mM and a estimated detection limit of 0.35 μM (S/N=3). The existence of two linear range on nonenzymatic metallic devices was already observed for H2O2 detection [43] and also reported for copper nanoparticles [44]. According to Ghateri et al., 2014 [43], this behaviour could indicate different mechanism at these ranges: at low concentrations of hydrogen peroxide, the electrocatalytic mechanism is dominant, but at higher concentrations, the direct reduction of hydrogen peroxide on the surface can play an important role in the analytical signal. Analytical performances of CuO@Cu2O-NWs/PVA/GC were compared to other reported nonenzymatic sensors for H2O2 reduction in neutral buffer based on GC copper nanomaterial modified electrodes, at potential range between -0.300 V and -0.110 V (Table 1). The reported LOD's values for these sensors, typically may vary over three orders of magnitude (i.e. from 0.16 to 20 M of H2O2). In comparison, the proposed sensor with a LOD of 0.35 M showed to be close to the minimum threshold. Moreover, linear ranges shown from the previous reported sensors extend between a minimum interval of several M (i.e. 3.9 M to 22 M) to a maximum of 12000 M (i.e. 50 M to 12000 M). The sensor proposed in this work has a first linear range that extends for about 3000 M (i.e. 1 M to 3000 M) and a second linear range between 3000 M and 10000 M, making it suitable to be used in a dynamic range that covers almost four orders of magnitude, in line with most of those already reported and in some cases more performing. Similarly, the sensitivity of the sensor is comparable to or greater. Moreover, a good reproducibility (RSD 4.17%) was obtained for the response to H2O2 1mM recorded on three biosensors independently prepared. 12

Figure 6 here

Table 1 here

Sensor selectivity was evaluated by monitoring amperometric response after consecutive injection of 100µM H2O2, 100µM of each interfering species tested (glucose, fructose, dopamine and ascorbic acid) and again 100µM H2O2. These results were showed in figure S2. The species under test do not influence sensor performance exhibiting a negligible response that indicate a high selectivity of the proposed sensor.

4. Conclusions For the first time, we have immobilized a new core shell CuO@Cu2O nanowires into PVA inert matrix which has a inherent good biocompatibility and desirable physical properties, such as elastic nature and good film forming properties. PVA formed a protective layer mechanically resistant which facilitates the handling of the nanowires. The as prepared CuO@Cu2O-NWs/PVA hybrid material has been used to develop an amperometric nonenzymatic H2O2 sensor. The electrochemical results showed that the CuO@Cu2O-NWs/PVA/GC modified electrodes have a larger current to the reduction of H2O2 than bare GC one, with wide responding range, high sensitivity, good reproducibility, low detection limit and a suitable applied redox potential (useful to avoid the influence of many interfering species) at neutral pH, which might be attributed to the electrocatalytic properties of the copper reaction center. Therefore the morphological and electrochemical characterization showed that CuO@Cu2O core shell nanowires embedded into poly(vinyl) alcohol matrix provide a promising start platform for the development and characterization of innovative amperometric nonenzymatic H2O2 detector with easy miniaturization as request to realize sensing devices at nanoscale dimensions. 13

Acknowledges The authors are grateful to A.R. De Bartolomeo for her technical assistance. Authors wish to thank Prof. A. Salvi (University of Basilicata) and Prof. J.E. Castle (University of Surrey) for the permission to use the fitting program Googly.

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15

FIGURE AND CAPTIONS

Figure 1. Typical TEM images of an isolated as-synthesized CuO nanowire (A) with its corresponding SAD pattern (B). Typical SEM images of Cu nanowires before (C) and after (D) encapsulation into PVA matrix.

Figure 2. Cu2p high resolution XPS spectra acquired for the CuO-NWs (A) and CuO-NWs (B) embedded in PVA.

Figure 3. (A) Typical voltammograms recorded on bare GC electrodes respectively in absence (dot curve) and in presence of CuO@Cu2O-NWs/PVA (solid curves) as prepared (1st cycle) until reaching the steady-state condition (15th cycle). Potential range between -0.400 V - +0.400 V. (B) Voltammograms recorded in clean PBS on bare GC bare electrodes (dot curve) and on modified electrodes, obtained in stady-state conditions, washed and transferred in a fresh prepared PB (solid curve) respectively. Potential range between -0.500 V - +0.400 V. Measurements were performed in PBS N2 satured. Scan rate: 0.020 Vs-1.

Figure 4. (A) Cyclic voltammograms recorded: (A)on bare GC electrode in absence and in presence of H2O2 2 mM, respectively; (B)on CuO@Cu2O-NWs/PVA/GC electrode in absence and in presence of H2O2 2 mM, respectively. Potential range between -0.500 V - +0.400 V. Measurements were performed in PBS N2 satured. Scan rate: 0.020 Vs-1.

Figure 5. (A) Cyclic voltammogram recorded on CuO@Cu2O-NWs/PVA/GC electrode to consecutive additions of hydrogen peroxide (0.01 – 5 mM). (B) Peak currents plot Ipc (in reduction) against hydrogen peroxide concentration up to 5 mM. Measurements were performed in PBS N2 satured. 16

Figure 6. (A) Amperometric responses of bare GC (curve a) and CuO@Cu2O-NWs/PVA/GC (curve b) after successive hydrogen peroxide additions between 1.0 µM to 3.0 mM in stirred PB N2 sutured. Applied potential −0.200 V vs SCE. Insert shows responses at H2O2 low concentration (1 µM - 50 µM). (B) Calibration plot of catalytic current I vs. hydrogen peroxide concentrations recorded on CuO@Cu2O-NWs/PVA/GC modified electrode.

Table 1. Analitycal performance of CuO@Cu2O-NWs/PVA/GCE compared with other nonenzymatic H2O2 sensors based on Cu nanimaterials on GC electrode (3 mm diameter). Sensitivity (µA/mM)

Detection Limit (µM)

Linear Range (µM)

Potential work (V) vs Ag/AgCl

pH

Ref

8.7

2.6

10-45

/

/

[45]

Cu2ONFs

3.693

0.039

3.9-22

-0.2

7.4

[46]

Cu2S/OMC

2.59

0.2

1-3030

-0.1

7.3

[47]

Octahedral Cu2O

6.13

6.4

10-4900

-0.2

7.4

[48]

Aggregate CuO

8.7

6

10-400

-0.2

7.4

[49]

/

1.6

5-180

-0.2

7

[50]

CuO-MWCNTs

1.13

0.16

0.5-82

-0.3

/

[51]

Cu/Psi-CPE

13.09

0.27

0.5-3780

-0.2

7

[52]

/

20

50-12000

-0.2

7

[53]

2.793 1.218

0.35

1-3000 3000-10000

-0.2

7

This work

Electrode

Cu2O

CuO/GCE

Cu/chitosan/CNTs CuO@Cu2O-NWs/PVA

17

Highlights:



A new, very simple, rapid and low-cost CuO@Cu2O-NWs/PVA/GC sensor core shell nanowires (CuO@Cu2O-NWs) embedded into PVA is prepared.



CuO@Cu2O-NWs/PVA is fully characterized by SEM, TEM, XPS, CV and CA techniques.



A simple and reproducible H2O2 enzyme less sensor based on CuO@Cu2O-NWs/PVA/GC is obtained.



CuO@Cu2O-NWs/PVA/GC sensor has remarkable catalytic ability toward H2O2 reduction.



Sensor has wide dynamic range, low LOD, high selectivity and good sensitivity.

18

19

Graphical Abstract

A schematic outlook of the electrocatalytic mechanism of CuO@Cu2O-NWs/PVA for the reduction of H2O2.

20