Electrolyte-controllable synthesis of Cu_xO with novel morphology and their application in glucose sensors

Abstract

The Cu_xO micro/nanomaterials with various novel morphologies have been synthesized on the surface of copper foil by a simple electrochemical method. The morphology of the Cu_xO crystal could be controlled by adjusting the electrolyte solution. The structure and morphology of as-prepared Cu_xO micro/nanomaterials were determined by scanning electron microscopy, elemental analysis, X-ray photoelectron spectroscopy and X-ray powder diffraction. The formation mechanism of these Cu_xO micro/nanomaterials was discussed. The Cu_xO micro/nanomaterial electrodes showed good electrocatalytic activity toward glucose oxidation and the performance of nonenzymatic glucose sensors based on these Cu_xO micro/nanomaterials was investigated in detail. As a typical representative, the nonenzymatic glucose sensor based on the Cu_xO_{NaClO4+CoCl2+SDS} micro/nanomaterials showed a wide linear range of 0.025–9.05 mM and a low detection limit of 14.3 ± 0.6 μM. The green synthetic method of simply adjusting the electrolyte is not only suitable for synthesis of Cu_xO with various morphologies, but also provides an easy way to prepare other metal oxide with different morphologies for constructing nonenzymatic glucose sensors.

---

1. Introduction

Diabetes is a worldwide chronic disease and affects more than 220 million people.¹ Thus, quantitative determination of glucose concentration both in blood and in other sources such as foods and pharmaceuticals is very important in biological and clinical analysis. Sensors based on nanomaterials are likely to become clinical and laboratory diagnostic tools because they are significantly smaller, easier-to-use, and less expensive than spectrometry or spectroscopy.² In the past, the enzymatic glucose biosensors based on glucose oxidase (GODx) have attracted most attention due to their good sensitivity and selectivity.³ However, an enzyme-based sensor is expensive and lacks long-term stability because the activity of GODx is vulnerable to temperature, pH, humidity and chemical poisoning. What's more, enzyme-based sensor involves complicated, multi-step immobilization procedures.⁴ On the contrary, the nonenzymatic glucose sensors can overcome these problems and accordingly become one of the most appealing approaches for the determination of glucose. To date, various metal or metal oxide nanomaterials, including CuO,⁵ Pt,⁶ Au,⁶ etc., are widely used in nonenzymatic glucose sensor. Among these materials, Cu or Cu_xO micro/nanomaterials, which are currently drawing much attention because of their intriguing catalytic activity, low-cost and capacity of resisting interference.^7–10 For example, Jiang et al. previously reported CuO nanoparticles (NPs) onto the vertical multi-walled carbon nanotubes (MWCNTs) was used to detect glucose, showing a low detection limit, high sensitivity, good reproducibility, good selectivity and wide linear range.¹¹ CuO/CuO_x modified nonenzymatic electrochemical sensor for the detection of glucose in alkaline medium displayed high sensitivity, low detection limit, and excellent linearity.⁴

Generally, the catalytic activity of Cu_xO micro/nanomaterials is mainly dependent on its morphology.¹² Thus, it is very important to control the morphology of Cu_xO micro/nanomaterials for enhancing their performance. In the past few years, many research groups have synthesized various Cu_xO with various morphologies. For example, Sui et al. used glucose to reduce the mixed liquor of citric acid copper and polyvinylpyrrolidone (PVP) and obtained polyhedral Cu₂O NPs, then the Cu₂O polyhedron were transformed into Cu₂O nanoframe by oxidation corrosion at room temperature.¹³ Other Cu_xO micro/nanomaterials with novel structure including nanorods,¹⁴ gear-like CuO nanostructure,¹⁵ dandelion,¹⁶ microspheres,⁷ etc. have also been synthesized. Accordingly, many methods have been developed to synthesize Cu_xO micro/nanomaterials with various morphologies. For example, D. Barreca et al. synthesized a series of Cu_xO nanosystems by chemical vapor deposition (CVD) approach, which were successfully used in lithium batteries, photocatalytic H₂ production and so on.^17–21 Some other methods, such as electrospinning,²² hydrothermal,²³ chemical reduction,²⁴ were also developed to synthesize Cu_xO micro/nanomaterials. However, all of these methods often took a long time and required very strict reaction condition. Another important issue in electrochemical nonenzymatic glucose sensors is tailoring the morphology of the Cu_xO micro/nanomaterials as well as immobilizing the resulted materials on the electrode surface. The conventional electrode preparing method always involved the synthesis of active material and complex immobilization steps, in which process the active material might be lost or aggregated on the electrodes surface, which reduced the electrocatalytic activity of the Cu_xO micro/nanomaterials.

In this work, Cu_xO micro/nanomaterials directly grew on the surface of copper foil were synthesized by a simple electrochemical method with several minutes. Copper sheet here not only was served as precursor for synthesis of Cu_xO micro/nanomaterials, but also was used as transducer for collecting electron. Unlike conventional electrode preparing method, the route we introduced here accomplished the preparation and immobilization in one step. The morphology of the Cu_xO micro/nanomaterials could be controlled by simply adjusting the electrolyte solution. The resulted Cu_xO micro/nanoelectrodes showed good electro-catalytic activity toward glucose oxidation as compared with other Cu_xO materials and the performance of nonenzymatic glucose sensors based on these Cu_xO micro/nanomaterials were investigated in detail. The green synthetic method by adjusting the electrolyte to prepare nanomaterials with different morphology was not only suitable for the synthesis of Cu_xO with various morphologies, but also provided a facile way for the synthesis of other metal oxide with different morphology.

2. Experimental

2.1 Chemicals and reagents

The D(+)-glucose was purchased from Aladdin. Sodium dodecyl sulfate (SDS) was obtained from Sigma-Aldrich (Milwaukee Wisconsin). High purity copper (>99.95%) was purchased from Sangon Biotech Company (Shanghai, China). Other reagents were purchased from Beijing Chemical Reagent Factory (Beijing, China). All reagents were of analytical grade and used without further purification. All solutions were prepared with ultrapure water, purified by a Millipore-Q system (18.2 MΩ cm⁻¹).

2.2 Synthesis of Cu_xO on Cu foils by electrochemical method

The synthesis of Cu_xO onto Cu foils was described as follows. First, Cu foils (0.45 cm × 0.2 cm) were tailored and washed in dilute sulfuric acid solution (H₂SO₄ : H₂O, 1 : 5, v/v), ultrapure water and absolute ethanol by ultrasonicating for 5 min, 10 min and 5 min respectively to remove the surface impurities and the oxide layers. Second, two pieces of as-processed Cu foils were connected to the positive and the negative of the DC power, respectively. Then the two Cu foils were inserted in a corresponding electrolyte solution (purged with high purity nitrogen for 20 min beforehand and a nitrogen atmosphere was kept over the solution during experiments) to constitute a loop. Finally, a voltage (5 V) was applied to the two Cu foils to generate Cu_xO micro/nanomaterials with various morphologies on the Cu foils. Each kind of Cu_xO micro/nanomaterials derived from certain electrolyte solution was abbreviated as Cu_xO_electrolyte.

2.3 Apparatus

The scanning electron microscopy (SEM) analysis was taken using a VEGA3 TESCAN SEM at an accelerating voltage of 20 kV equipped with a Phoenix energy dispersive X-ray analyzer. The chemical compositions of the synthesized Cu_xO micro/nano materials on Cu foils were determined by energy dispersive X-ray spectroscopy (EDXS) using a HITACHI S-3400N attached to the SEM. X-ray powder diffraction (XRD) data were collected on a D/Max 2500 V/PC X-ray powder diffractometer using Cu Kα radiation (λ = 0.154056 nm, 40 kV, 200 mA). The preparation of electrodes was carried out with a DELIXI WYJ-30V5A DC stabilized power supply. X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALAB-MKII spectrometer (VGCo., United Kingdom) with Al Kα X-ray radiation as the X-ray source for excitation.

All electrochemical measurements were performed on a CHI 660C electrochemical workstation (Shanghai, China) at ambient temperature. A conventional three-electrode system was employed including a bare or Cu_xO micro/nanomaterials modified Cu foils electrode as the working electrode, a platinum wire as the auxiliary electrode and a SCE electrode (saturated KCl) as the reference electrode. The cyclic voltammetric experiments were performed in a quiescent solution. The amperometric experiments were carried out under a continuous stirring. 0.1 M NaOH as the supporting electrolyte solution was purged with high purity nitrogen for 15 min prior to each measurement then a nitrogen atmosphere was kept over the solution during measurements.

3. Results and discussion

3.1 SEM characterization of Cu_xO micro/nanomaterials synthesized in different electrolyte

Cu_xO micro/nanomaterials with various morphologies were obtained in different electrolyte by a simple electrolysis. The typical morphology of the products synthesized in different electrolyte solution was characterized using SEM and the results were shown in Fig. 1. Fig. 1A showed the SEM image of Cu_xO_NaClO4 micro/nanomaterials prepared by electrolyzing Cu foils in 0.05 M NaClO₄ for 10 min, and these Cu_xO micro/nanocrystals were irregular star shapes. It was very interesting that some small octahedral crystals preferentially grew on the angle of some big star crystal (Inset in Fig. 1A). These star crystals crossed each other together and uniformly distributed on the surface of copper foils to form a three-dimensional structure. When the Cu foils was electrolyzed in 0.01 M CoCl₂ solution for 5 min, the product was composed of large Cu_xO nanosheet which self-assembled into planar triangle and the large Cu_xO nanosheet further vertically grew on the edge of the planar triangle to set up triangular fence structure (Fig. 1B). The enlarged complete triangular fence (inset in Fig. 1B) indicated that the unit of the triangular fence is Cu_xO nanosheet, and its undersurface was composed of many Cu_xO nanosheets assembling into a triangular structure. Some Cu_xO nanosheets were distributed surrounding the plane triangle and arranged into a nearly equilateral triangular fence orderly. Fig. 1C showed the Cu_xO micro/nanomaterials prepared by electrolyzing Cu foils in 0.05 M NaClO₄ + 0.01 M CoCl₂ solution for 10 min. The products were irregular tetrahedron with large porosity in the middle. There were a number of Cu_xO NPs distributed uniformly on the irregular tetrahedron to form Cu_xO micro/nanomaterials. After the Cu foil was electrolyzed in 0.05 M NaClO₄ + 0.01 M CoCl₂ + 0.025 M SDS solution for 10 min, the products were Cu_xO cube with size of 100–800 nm (Fig. 1D). The growth process of the Cu_xO micro/nanomaterials with reaction time was investigated by SEM in detail (Fig. S1–4, ESI†).

Fig. 1 SEM images of Cu_xO crystals synthesized by electrochemistry method in different electrolyte (A) 0.05 M NaClO₄; (B) 0.01 M CoCl₂; (C) 0.05 M NaClO₄ + 0.01 M CoCl₂; (D) 0.05 M NaClO₄ + 0.025 M SDS + 0.01 M CoCl₂. (The reaction time was (A) 10 min, (B) 5 min, (C) 10 min and (D) 10 min, respectively).

The crystal structure and the phase purity of the as-prepared Cu_xO micro/nanomaterials were further characterized by XRD and the results were shown in Fig. 2A. A series of characteristic diffraction peaks at 43.32°, 50.49° and 74.16° were indexed as the diffractions of the (111), (200) and (220) crystalline planes of Cu substrate (JCPDS, no. 65-9026), respectively. For Cu_xO_NaClO4, the characteristic diffraction peak at 36.42° was indexed as the diffractions of the (111) of cuprite Cu₂O (JCPDS, no. 05-0667) and no other impurities could be detected in the XRD pattern of the materials. When the electrolyte was CoCl₂, XRD spectra indicated that CuO and Cu₂O micro/nanomaterials were obtained. Two characteristic diffraction peaks at 28.48° and 36.29° were indexed to the diffractions of (110) and (111) of cuprites Cu₂O (JCPDS, no. 05-0667). The characteristic diffraction peak at 65.81° was indexed to the diffractions of the (220) of the black copper CuO (JCPDS, no. 48-1548). For Cu_xO_NaClO4+CoCl2, two characteristic diffraction peaks at 28.57° and 36.40° were indexed to the diffractions of (110) and (111) of cuprites Cu₂O (JCPDS, no. 05-0667). Another two characteristic diffraction peaks at 47.53° and 65.81° were indexed to the diffractions of (202̄) and (021) of the black copper CuO (JCPDS, no. 48-1548). When the electrolyte was NaClO₄ + CoCl₂ + SDS, there were two characteristic diffraction peaks at 28.47° and 36.26° which were indexed to the diffractions of (110) and (111) of cuprites Cu₂O (JCPDS, no. 05-0667). One characteristic diffraction peak at 65.90° was indexed as the diffractions of (022) of the black copper CuO (JCPDS, no. 48-1548). The result indicated that when the electrolyte was NaClO₄ + CoCl₂ + SDS, CuO and Cu₂O micro/nanomaterials were obtained. We have calculated the NPs size on the Cu_xO micro/nanomaterials according to the Scherrer's formula²⁵ and the values of Cu_xO_NaClO4, Cu_xO_CoCl2, Cu_xO_NaClO4+CoCl2, and Cu_xO_{NaClO4+CoCl2+SDS} were 40 nm, 56 nm, 46 nm and 65 nm, respectively. The NPs size on the Cu_xO micro/nanomaterials was impacted by the ion concentration. Generally, the particle size increased with the increase of ion concentration. However, the NPs size on Cu_xO_NaClO4 and Cu_xO _NaClO4+CoCl2 were smaller than that of Cu_xO_CoCl2. It might be ascribed to the fast nucleation which gave a large number of nuclei, and shortened the crystal growth stage, leading to small-sized particles. As shown in Fig. S1–S4,† the density of Cu_xO_NaClO4 and Cu_xO _NaClO4+CoCl2 crystal was higher than Cu_xO_CoCl2 nanostructures.

Fig. 2 (A) XRD patterns and (B) EDXS analysis of as-synthesized Cu_xO micro/nano materials: Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode (curve a), Cu_xO/Cu_NaClO4+CoCl2 electrode (b), Cu_xO/Cu_CoCl2 electrode (curve c), and Cu_xO/Cu_NaClO4 electrodes (curve d).

EDXS was employed to evaluate the composition of the as-prepared Cu_xO micro/nanomaterials (Fig. 2B). It revealed the peaks corresponding to Cu, C, O Cl and Au element, in which Cu, O-related peak came from CuO and Cu₂O, the presented Cl-related peaks came from the electrolyte due to the strong adsorption ability of chlorine,²⁶ the C-related peaks came from the conductive tapes and additional Au-related peaks came from the Au spraying. The result also suggested that no other impurities were formed in the process.

The XPS spectrum of Cu 2p3 in Cu_xO micro/nanomaterials could be fitted into three kinds of Cu species including Cu₂O (932.4 eV and 932.9 eV), CuO (933.9 eV) and Cu(OH)₂ (934.6 eV). Agreeing with the result of XRD, the XPS spectrum of Cu_xO/Cu_NaClO4 in Fig. 3A didn't find the characteristic peak of CuO. Both the peaks of CuO and Cu₂O were observed in the spectra of Cu_xO/Cu_CoCl2 (Fig. 3B), Cu_xO/Cu_NaClO4+CoCl2 (Fig. 3C) and Cu_xO/Cu_{NaClO4+CoCl2+SDS} (Fig. 3D). The Cu_xO nanostructure prepared in CoCl₂ solution had highest ratio of CuO to Cu₂O, suggesting that the Co²⁺ was favorable to form CuO. When Cu foils was electrolyzed in CoCl₂ solution, Co(OH)₂ flocks appeared around the cathode. Since some OH⁻ was combined with Co²⁺ to Co(OH)₂ and accordingly resulted in the decrease of OH⁻ concentration, some Cu⁺ could not transform into CuOH. As a result, some Cu⁺ was further oxidized into Cu²⁺ and transformed into CuO finally. It explained why only Cu₂O was formed in NaClO₄ electrolyte, while Cu₂O and CuO mixtures were formed in other electrolytes. Furthermore, the characteristic peak of Cu(OH)₂ which was never observed in XRD pattern appeared in all the XPS spectra.

Fig. 3 XPS spectra of (A) Cu_xO/Cu_NaClO4 nanostructure, (B) Cu_xO/Cu_CoCl2 nanostructure, (C) Cu_xO/Cu_NaClO4+CoCl2 nanostructure and (D) Cu_xO/Cu_{NaClO4+CoCl2+SDS} nanostructure.

3.2 Electrocatalytic oxidation of glucose on the Cu_xO/Cu electrode

The electrocatalytic activity of the typical Cu_xO micro/nanomaterials towards the oxidation of glucose was firstly investigated by CVs as shown in Fig. 4. The CVs of Cu_xO/Cu_NaClO4 obtained by electrolyzing Cu foils in 0.05 M NaClO₄ for 1 min was recorded in Fig. 4A. An obvious catalytic oxidation current (about 800 μA) was observed after adding 4.0 mM glucose. The catalytic oxidation current decreased with electrolytic time prolonging because small NPs gradually grew up and aggregated (Fig. S1† and inset in Fig. 4). Thus, Cu foils electrolyzed in 0.05 M NaClO₄ for 1 min was assumed as the optimum Cu_NaClO4 electrode. Analogously, it took 3 min, 30 s and 30 s to construct the optimum Cu_xO/Cu_CoCl2, Cu_xO/Cu_NaClO4+CoCl2 and Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode, respectively. The different electrolysis time to obtain the optimum electrodes might result from the different concentration of used electrolyte solution. Since the concentration of Co²⁺ in the CoCl₂ electrolyte decreased with the Co(OH)₂ depositing, the conductivity of solution decreased which resulted in more electrolysis time to construct Cu_xO/Cu micro/nanomaterials. Obviously, the coverage of Cu_xO crystal on Cu_xO/Cu_NaClO4 electrode was higher than that on Cu_xO/Cu_CoCl2 electrode as shown in Fig. S1 and S2† at the same electrolysis time. The electrolytes to construct Cu_xO/Cu_NaClO4+CoCl2 and Cu_xO/Cu_{NaClO4+CoCl2+SDS} contained higher concentration of ions, and accordingly it took much less time to fabricate the optimum electrodes. The optimum electrodes obtained a maximum catalytic oxidation current of 1600 μA (Fig. 4B), 1650 μA (Fig. 4C) and 1700 μA (Fig. 4D), respectively. The different maximum catalytic oxidation current might be ascribed to the different morphologies of Cu_xO/Cu nanomaterials which resulted in different catalytic activity. The smaller particle size and better dispersion might increase the electroactive surface area and electron transfer rate, which mainly do help to decrease the applied potential and enhance catalytic activity^27–30 Meanwhile, larger electroactive surface area provided more electroactive sites and larger surface area for glucose molecules to adsorb and react.³¹ Furthermore, the difference of oxidation peak potential has been detected, which might be ascribed to a kinetic effect by an increase in the electroactive surface area and the electron transfer rate from the glucose to the Cu_xO/Cu electrodes. All in all, as compared with other method such as electrospinning,²² hydrothermal³² chemical deposited method³³ and CVD,^17–21 the novel method is superior in energy saving, speed and controllability.

Fig. 4 CVs of optimized electrodes in the absence (a, black curve) and presence (b, red curve) of 4.0 mM glucose in 0.10 M NaOH: (A) Cu_xO/Cu_NaClO4 electrodes, (B) Cu_xO/Cu_CoCl2 electrode, (C) Cu_xO/Cu_NaClO4+CoCl2 electrode and (D) Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode. Scan rate: 100 mV s⁻¹. Inset: effect of electrolytic time towards catalytic current.

Amperometric measurements of the optimum Cu_xO/Cu micro/nanomaterial electrode were carried out at their own peak potential by successive injection of glucose into a stirring 0.1 M NaOH (Fig. 5). It was noticeable that as the glucose concentration increased, the noise also increased, which might be caused by more and more intermediate species adsorbing onto the Cu_xO/Cu electrode. As shown in Fig. 5, the optimum Cu_xO_{NaClO4+CoCl2+SDS} exhibited the widest linear range. The oxidation current was proportional to glucose concentration in the range of 0.025–9.05 mM (r = 0.9987) with a slope of 452.4 ± 5.6 μA mM⁻¹. The detection limit was estimated to be 14.3 ± 0.6 μM based on the criterion of a signal-to-noise ratio of 3. The electrocatalytic performances of other optimum Cu_xO micro/nanomaterials electrode toward the oxidation of glucose were also investigated and the results were summarized in Table S1.†

Fig. 5 (A) Typical amperometric response of the optimized Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrodes to successive injection of glucose into the stirred NaOH (pH = 13). (B) The calibration curve of Cu_xO/Cu_{NaClO4+CoCl2+SDS}. (C) Typical amperometric response of the optimized Cu_xO/Cu_NaClO4+CoCl2 electrode (blue curve), Cu_xO/Cu_CoCl2 electrode (green curve) and Cu_xO/Cu_NaClO4 electrode (red curve). (D) The calibration curve of Cu_xO/Cu_NaClO4+CoCl2 electrode (blue curve), Cu_xO/Cu_CoCl2 electrode (green curve) and Cu_xO/Cu_NaClO4 electrode (red curve).

For the optimum Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode, we further investigated its catalytic kinetics, anti-interference ability, stability, etc. The catalytic rate constant (K_cat) on the as-prepared optimum Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode was measured with double steps chronoamperograms by setting the working electrode potentials to proper values.³⁴ Fig. 6A showed double steps chronoampergrams for Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode in the absence and presence (curve a: 0 mM, curve b: 3 mM, curve c: 5 mM, curve d: 7 mM) of glucose. The applied potential steps were set to 0.35 V and 0.10 V, respectively. Plot of net current with respect to the minus square roots of time was shown by Fig. 6B, presenting a linear dependency. It demonstrated that the electrocatalytic oxidation of glucose was a diffusion-controlled process. The diffusion coefficient (D) and the catalytic rate constant (K_cat) of glucose were estimated according to Cottrell equation.³⁴ The mean value of D and K_cat for glucose was calculated to be 4.85 × 10⁻³ cm² s⁻¹ and 2.89 × 10⁶ cm³ mol⁻¹ s⁻¹, respectively.

Fig. 6 (A) Double steps chronoamperograms of the optimized CuxO/Cu_{NaClO4+CoCl2+SDS} electrode in 0.1 M NaOH with different concentrations of glucose: (a) 0, (b) 3, (c) 5 and (d) 7 mM. Potential steps were 0.35 V and 0.1 V, respectively. (B) Dependency of transient current on t^−1/2. (C) Dependency of I_cat/I_d on t^1/2.

A comparison study was shown in Table S1.† Up to now, many sensors have been developed for the detection of glucose based on Cu or Cu_xO nanomaterials, and all of them have some advantages and limitations. Taking CuO nanocubes–graphene/GCE³⁵ as an example, the detection limit was lower (0.70 μM) than that of the as-prepared Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode (14.3 ± 0.6 μM), but the linear range was not very wide, and the applied potential was higher (0.55 V (vs. SCE)) than that of the as-prepared Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode (0.35 V (vs. SCE)). For Cu_xO/PPy/Au electrode,³⁶ the linear range was not very wide (up to 4.5), and the applied potential was higher (0.60 V (vs. SCE)). By comparing, it could be clearly seen that the Cu_xO/Cu_{NaClO4+CoCl2+SDS} offered a reasonable linear range and detection limit, higher sensitivity and the lowest applied potential among these sensors. The high sensitivity might be ascribed to good conductivity and large surface area of Cu_xO/Cu_{NaClO4+CoCl2+SDS} nanomaterials. As shown in Fig. S1–S4,† the Cu_xO crystal on the optimized Cu_xO/Cu_{NaClO4+CoCl2+SDS} electrode showed the best dispersity among four optimized materials and the reduced size (about 200 nm) increased the specific surface area. Interestingly, although the size of Cu_xO crystals on Cu_xO/Cu_NaClO4 electrode was the smallest, the Cu_xO/Cu_NaClO4 electrode had the highest applied potential and lowest sensitivity. The result could be due to the composition of Cu_xO. As mentioned above, different from other three Cu_xO/Cu electrodes which consisted of CuO and Cu₂O, the Cu_xO/Cu_NaClO4 electrode consisted of pure Cu₂O. The mechanism and the optimized ratio of Cu₂O to CuO need to explore in the future work.

An important parameter for a sensor was its ability to discriminate between the interfering species and the target analyte. Some interference studies in our work were shown in Fig. 7. Inorganic chemicals such as SO₄²⁻, NO₂⁻ and Cl⁻ in 10-fold concentration did not show obvious interference to glucose detection, and SO₃²⁻ in 2-fold concentration showed a weak interference for the glucose detection. Amperometric response of organic chemicals showed that the addition of 0.1-fold uric acid (UA) hardly provided notable interference for glucose sensing, and 0.1-fold fructose, D-galactose, mannose and ascobic acid (AA) only marked a poor increase of currents (<10%). These results implied the good selectivity of the resulted sensor. The applied potential for electrocatalytic oxidation of glucose played a key role in improving the selectivity of sensor because the interference could be weaken at more negative potential. Another important factor for selectivity is the sensitivity to the packing of atoms on the surface or the exposed facets of a nanocrystal. Of course, the facets exposed on a nanocrystal have a strong correlation with the shape. Many other examples clearly illustrate the importance of shape control to the efficient utilization of nanocrystals. In this work, the differences in shapes and intensity of the peaks of XRD patterns indicated the distinctions of facets exposed on four Cu_xO nanocrystals, which was inferred to effect on the selectivity of Cu_xO/Cu electrodes.

Fig. 7 (A) The interference of some organic and inorganic substances on glucose detection. Applied potential: 0.35 V. (B) The current ratio of corresponding organic and inorganic substance.

The stability, repeatability and reproducibility of the optimized Cu_xO/Cu_{NaClO4+CoCl2+SDS} sensor were also investigated in this work. After the sensor was stored at room temperature for 30 days, the current response to 4.0 mM glucose decreased 2.68% of the original current. The repeatability of successive amperometric measurements for five different 0.1 mM glucose carried out with the same biosensor was checked. A relative standard deviation value (RSD) of 3.14% was calculated for the steady current. The reproducibility of the response to 0.1 mM glucose obtained with five different biosensors was also evaluated with a RSD of 3.1%.

The determination of glucose in human serum samples was also performed on the optimized Cu_xO_{NaClO4+CoCl2+SDS} micro/nanomaterials electrode. In brief, the blood samples obtained from the hospitalized patients were first diluted with 100 mL of the PBS buffer (pH 7.4), and then the Cu_xO micro/nanomaterials electrode was used to monitor the glucose content. The concentration of glucose in human blood serum sample was calibrated by the standard colorimetric enzymatic procedure as a reference for checking the biosensor accuracy. The results obtained from the glucose sensor agreed well with those obtained by the standard colorimetric enzymatic method. The RSD listed in Table S2† indicated most of the results were accurate and credible. Thus, it could be concluded that the developed sensors could performed very well in the detection of glucose in serum samples. The outstanding performance of the sensor based on Cu_xO_{NaClO4+CoCl2+SDS} micro/nanomaterials electrode in the determination of glucose in human serum samples than other has developed sensor might be ascribed to their good properties. As mentioned above, GODx based biosensors always showed higher cost, worse reproducibility and poorer long-term stability than the Cu_xO/Cu based nonenzymatic glucose sensors. The nonenzymatic glucose sensors based on carbon-based materials and precious metals often involved in high cost, low sensitivity, poor selectivity, and the intermediates are easily adsorbed onto the electrode surface resulting in catalyst poisoning as compared with the sensor in our work. The carbon/metal oxide composite sensor involved in multi-steps and challenge of immobilization.

4. Conclusions

The Cu_xO micro/nanomaterials have been synthesized on the surface of Cu foils by electrochemical method. The morphology of the resulted crystal was controllable by adjusting electrolyte solution. A variety of different structure of Cu_xO micro/nanomaterials were synthesized rapidly based on this method. This method by simply adjusting the electrolyte to prepare completely different morphology materials was not only suitable for other Cu_xO morphology synthesis, but also provided a facile way for other metal oxide with variable morphology. This work also studied the growth process of various materials with different morphology, and the cubic crystal Cu_xO was further used for nonenzymatic glucose sensor. The results showed that the as-prepared sensor had wide linear range, high sensitivity and good stability, which made such Cu_xO modified electrode to be a promising candidate for novel nonenzymatic sensor.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (21065005, 21165010 and 21101146), Young Scientist Foundation of Jiangxi Province (20112BCB23006 and 20122BCB23011) and the State Key Laboratory of Electroanalytical Chemistry (SKLEAC201310), Foundation of Jiangxi Educational Committee (GJJ13243 and GJJ13244), the Open Project Program of Key Laboratory of Functional Small organic molecule, Ministry of Education, Jiangxi Normal University (KLFS-KF-201214 and KLFS-KF-201218).

References

1. X. Wang, C. Hu, H. Liu, G. Du, X. He and Y. Xi, Sens. Actuators, B, 2010, 144, 220–225.
2. G. Konvalina and H. Haick, Acc. Chem. Res., 2014, 47, 66–76.
3. O. Yehezkeli, R. Tel-Vered, S. Raichlin and I. Willner, ACS Nano, 2011, 5, 2385–2391.
4. J. Tian, Q. Liu, C. Ge, Z. Xing, A. M. Asiri, A. O. Al-Youbi and X. Sun, Nanoscale, 2013, 5, 8921–8924.
5. T. G. Satheesh Babu and T. Ramachandran, Electrochim. Acta, 2010, 55, 1612–1618.
6. K. E. Toghill and R. G. Compton, Int. J. Electrochem. Sci., 2010, 5, 1246–1301.
7. Z. Zhang, H. Chen, X. She, J. Sun, J. Teo and F. Su, J. Power Sources, 2012, 217, 336–344.
8. K. P. S. Prasad, Microporous Mesoporous Mater., 2013, 172, 77–86.
9. K. P. Musselman, A. Marin, L. Schmidt-Mende and J. L. MacManus-Driscoll, Solar Cells, Adv. Funct. Mater., 2012, 22, 2202–2208.
10. M. Vaseem, A. R. Hong, R.-T. Kim and Y.-B. Hahn, J. Mater. Chem. C, 2013, 1, 2112–2120.
11. L. C. Jiang and W. D. Zhang, Biosens. Bioelectron., 2010, 25, 1402–1407.
12. F. Bayansal, H. Çetinkara, S. Kahraman, H. Çakmak and H. Güder, Ceram. Int., 2012, 38, 1859–1866.
13. Y. Sui, Angew. Chem., Int. Ed., 2010, 49, 4282–4285.
14. K. Rajeshwar, N. R. Tacconi, G. Ghadimkhani, W. Chanmanee and C. Janáky, ChemPhysChem, 2013, 14, 2251–2259.
15. L. Yu, Y. Jin, L. Li, J. Ma, G. Wang, B. Geng and X. Zhang, CrystEngComm, 2013, 15, 7657.
16. B. Liu and H. C. Zeng, J. Am. Chem. Soc., 2004, 126, 8124–8125.
17. D. Barreca, A. Gasparotto, C. Maccato, E. Tondello, O. Lebedev and G. V. Tendeloo, Cryst. Growth Des., 2009, 9, 2471–2480.
18. D. Barreca, G. Carraro, A. Gasparotto, C. Maccato, O. Lebedev, A. Parfenova, S. Turner, E. Tondello and G. V. Tendeloo, Langmuir, 2011, 27, 6409–6417.
19. D. Barreca, P. Fornasiero, A. Gasparotto, V. Gombac, C. Maccato, T. Montini and E. Tondello, ChemSusChem, 2009, 2, 230–233.
20. D. Barreca, G. Carraro, A. Gasparotto, C. Maccato, M. Cruz-Yusta, J. L. Gomez-Camer, J. Morales, C. Sada and L. Sanchez, ACS Appl. Mater. Interfaces, 2012, 4, 3610–3619.
21. D. Barreca, G. Carraro, E. Comini, A. Gasparotto, C. Maccato, C. Sada, G. Sberveglieri and E. Tondello, J. Phys. Chem. C, 2011, 115, 10510–10517.
22. W. Wang, Z. Li, W. Zheng, J. Yang, H. Zhang and C. Wang, Electrochem. Commun., 2009, 11, 1811–1814.
23. C. Li, Y. Su, S. Zhang, X. Lv, H. Xia and Y. Wang, Biosens. Bioelectron., 2010, 26, 903–907.
24. K. M. E. Khatib and R. M. A. Hameed, Biosens. Bioelectron., 2011, 26, 3542–3548.
25. C.-W. Kung, C.-Y. Lin, Y.-H. Lai, R. Vittal and K.-C. Ho, Biosens. Bioelectron., 2011, 27, 125–131.
26. J. L. Stickney, C. B. Ehlers and B. W. Gregory, Langmuir, 1988, 4, 1368–1373.
27. H. Wei and E. Wang, Chem. Soc. Rev., 2013, 42, 6060.
28. M. Segev-Bar, G. Shuster and H. Haick, J. Phys. Chem. C, 2012, 116, 15361–15368.
29. N. Gozlan, U. Tisch and H. Haick, J. Phys. Chem. C, 2008, 112, 12988–12992.
30. Y. Zilberman, U. Tisch, G. Shuster, W. Pisula, X. Feng, K. Müllen and H. Haick, Adv. Mater., 2010, 22, 4317–4320.
31. X. Lu, Y. Ye, Y. Xie, Y. Song, S. Chen, P. Li, L. Chen and L. Wang, Anal. Methods, 2014, 6, 4643–4651.
32. R. Khan, M. Vaseem, L. Jang, J. Yun, Y. Hahn and I. Lee, J. Alloys Compd., 2014, 609, 211–214.
33. M. Sahooli, S. Sabbaghi and R. Saboori, Mater. Lett., 2012, 81, 169–172.
34. Y. Song, Z. He, H. Zhu, H. Hou and L. Wang, Electrochim. Acta, 2011, 58, 757–763.
35. L. Luo, L. Zhu and Z. Wang, Bioelectrochemistry, 2012, 88, 156–163.
36. F. Meng, W. Shi, Y. Sun, X. Zhu, G. Wu, C. Ruan, X. Liu and D. Ge, Biosens. Bioelectron., 2013, 42, 141–147.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06682k

---