Cost-effective CuO nanotube electrodes for energy storage and non-enzymatic glucose detection

Abstract

A facile strategy is developed for the in situ synthesis of low-cost, freestanding, binder-free CuO nanotube electrodes on a conducting Cu foil, totally eliminating non-active materials and extra processing steps. The synergy arising from the ameliorating structure, such as high porosity, large surface area and the ability for fast electron transport, make CuO nanotube electrodes ideal multi-functional electrochemical devices with excellent pseudocapacitive performance and a remarkable sensitivity to glucose for use as non-enzymatic biosensors (NGBs). The electrodes deliver remarkable specific capacitances of 442 and 358 F g⁻¹ at current densities of 1 and 20 A g⁻¹, respectively. The capacitance loss after 5000 cycles is only 4.6% at 1 A g⁻¹, reflecting the excellent cyclic stability of the supercapacitor. The biosensor made from CuO nanotubes presents an extremely rapid and accurate response to glucose in blood in a wide, linear range of 100 μM to 3 mM, with a sensitivity of 2231 μA mM⁻¹ cm⁻². These interesting discoveries may open up the potential for the further development of new, multi-functional electrodes possessing both excellent energy storage and biosensory capabilities.

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Introduction

Efficient energy storage and its economic use are central to the development of modern electronic products, transportation systems and medical devices.¹ Supercapacitors (SCs) can offer higher power densities with longer cyclic lifespans than rechargeable batteries, and can store higher energy densities than conventional capacitors,^2–6 and thus are considered as one of the most promising next-generation energy storage devices. A major bottleneck that hinders practical applications of existing SCs to date, however, is the lack of low-cost, high performance electrode materials. Porous nanostructures deposited directly on conductive metal substrates have been developed as electrodes with large active surface areas, the capability to buffer the volume change during the charge/discharge process, and involving a short pathway for electron/ion transport.^7–14 Among the various porous materials, mesoporous CuO has attracted considerable interest for SC applications, due to its low cost, excellent pseudocapacitive behaviour, environmental benignity and practical availability.^15–17

On the other hand, biosensors are important for clinical diagnostics, biological and chemical analyses and environmental monitoring.^18–20 Diabetes is a well-known global health care problem that seriously affects the daily life of hundreds of millions of people.²¹ The dynamic monitoring of the concentration of glucose in the blood is an essential component of modern diabetes management, including clinical detection and therapy.²² In the past, the vast majority of glucose monitoring was carried out using enzymatic sensors, because of their high selectivity, excellent sensitivity and low detection limit. However, enzymatic sensors suffer from instability, as well as complexity and poor reproducibility.²³ To address the above issues, significant research has been directed towards developing non-enzymatic sensors based on noble metals, such as Pt and Au, alloys and metal oxides.^24–27 Compared with noble metals and alloys, CuO-based electrodes exhibit a higher electrocatalytic oxidation ability for glucose. In particular, nanostructured CuO possesses a large specific surface area, excellent electrochemical activity, and enables high electron transfer reactions at a low overpotential,^28,29 making it a promising candidate for non-enzymatic sensor applications. For instance, CuO nanowires have demonstrated a high stability and good selectivity in glucose determination.³⁰ CuO/graphene nanocomposites have also shown good electrocatalytic activity for the oxidation of glucose in an alkaline medium at room temperature.³¹

As an important transition metal oxide, CuO has been synthesized using various methods, such as the hydrothermal method, electrodeposition, chemical bath deposition and electrostatic spinning. In order to prepare a supercapacitor or biosensor electrode using CuO powders as the active material, the CuO powder is generally mixed with binders and conducting additives to first obtain a paste. However, the electrodes obtained often suffer drawbacks of low active material utilization, due to the presence of inaccessible regions to the electrolyte. It has been demonstrated that electrodes of one-dimension arrays on a conductive substrate usually hold a larger electrochemical active surface area and have a higher utilization efficiency of the active materials than conventional powder electrodes. For example, Yang et al. reported a solution-immersion strategy for growing Cu(OH)₂/CuO nanotube arrays, which showed good non-enzymatic electrocatalytic responses to glucose in alkaline media.³²

Herein, we develop a facile, low-cost method for the synthesis of porous CuO nanotubes through the in situ oxidation of commercial copper foils, followed by low-temperature annealing. The whole process can be scaled up for mass production. The freestanding CuO nanotube electrodes are multi-functional: they can serve as high-performance SCs, as well as high sensitivity non-enzymatic glucose biosensors (NGBs). The electrodes developed in this study have several unique material and structural features that other types of electrodes lack: (I) Cu foils offer many advantages, due to their natural abundance, excellent electrical conductivity, low cost and flexibility. (II) CuO nanotubes grown directly on the surface of current collector can be directly used as freestanding electrodes, while totally eliminating binders and conducting additives, as well as the extra steps to assemble them. The fully integrated CuO nanotube/Cu foil structure provides an extremely low contact resistance, with fast electron/ion transport and high rate capability. (III) The intrinsically large surface area of the nanotube forest offers numerous active sites and permits full utilization of the electrode materials, with a resulting high specific capacitance and excellent sensitivity to glucose detection. (IV) There are plenty of void spaces and pores between the neighbouring nanotubes, allowing full penetration of the electrolyte or glucose solution.

Experimental

Growth of CuO nanotubes on Cu foil

All the reagents were of analytical grade and used without further purification. In a typical process, 10 mm × 10 mm square Cu foils of 0.5 mm thickness and 99.9% purity (Alfa Aesar) were washed in a 3 M HCl aqueous solution for 15 min to remove the surface impurities and oxide layers. Immediately after rinsing several times with ethanol and distilled water, the Cu foils were immersed in an aqueous solution containing 2.5 M NaOH and 0.125 M (NH₄)₂S₂O₈ at room temperature for the growth of Cu(OH)₂ nanotubes. The gold colour of the Cu foil gradually changed to faint blue, while the initially colourless solution also became blue. After 20 min of treatment, the Cu foils with a layer of Cu(OH)₂ nanotubes were taken out of the solution, rinsed with deionised water and ethanol, and dried in air. To obtain CuO nanotubes, the Cu(OH)₂ nanotubes on a Cu foil were annealed at 150 °C for 1 h to complete dehydration, and then the temperature was elevated to 200 °C, and kept at this temperature for another 2 h to promote crystallization in a nitrogen atmosphere. Finally, the CuO nanotube/Cu foil composites were naturally cooled to ambient temperature.

Characterization

Thermogravimetric and differential thermal analysis (TG-DTA, SDT 2960) was conducted at a heating rate of 10 °C min⁻¹ in a nitrogen atmosphere. The phase structure of the materials were characterized by X-ray diffraction (XRD) on a D8 Focus (Bruker, Germany) automated X-ray diffractometer system with Cu-Kα radiation (λ = 1.5418 Å). Raman scattering measurements were carried out on a laser Raman spectrometer (Renishaw, UK) at room temperature. The morphologies and microstructures were characterized by field emission scanning electron microscopy (FSEM, JEOL S-4800) and transmission electron microscopy (TEM, JEOL JEM-2010). Specific surface areas were measured on a Quadrasorb SI-MP Surface Area and Porosity Analyzer (American, Quantachrome) at 77 K, using the Brunauer–Emmett–Teller (BET) method with nitrogen adsorption. The pore size distributions were obtained by means of the Barrett–Joyner–Halenda (BJH) equation using the adsorption isotherm branch. The static contact angles (CAs) were measured according to the sessile-drop method, using a contact angle analyzer (Kino SL200B, American) with water at ambient temperature. The size of the water droplet was ∼5 μL. The average CA value was obtained by measuring several different positions for the same sample.

Electrochemical analyses

CuO nanotubes on a Cu foil were directly used as electrodes without adding any conductive agent or binder. The average mass loading of CuO on a Cu foil was ∼2.5 mg cm⁻². The pseudocapacitive performances of the CuO nanotube/Cu foil electrodes were investigated on an electrochemical workstation, CHI660E (Chenhua, P. R. China), using a three-electrode system. The composite electrode was used as the working electrode, while Pt foil and Hg/HgO served as the counter electrode and the reference electrode, respectively, with a solution of 1 M KOH as the electrolyte at room temperature. Cyclic voltammetry (CV) analyses were performed between −0.5 and 0.1 V vs. Hg/HgO at scan rates from 5 to 100 mV s⁻¹. Galvanostatic charge/discharge tests were conducted in a stable potential window at different current densities of 1–20 A g⁻¹. Electrochemical impedance spectroscopy (EIS) was performed at an AC voltage of 5 mV in the frequency range from 0.01 Hz to 100 kHz.

The electrocatalytic activities of the CuO nanotube/Cu foil electrodes for the oxidation of glucose were investigated in 0.1 M KOH electrolyte solution on a CHI660E electrochemical workstation under ambient conditions. A standard three-electrode system was employed, and consisted of a CuO nanotube working electrode, a Pt foil counter electrode and a Ag/AgCl reference electrode. Linear-sweep voltammetry measurements were carried out in the potential range of 0–0.8 V vs. Ag/AgCl. A sufficient amount of glucose was successively added into the 0.1 M KOH solution while continuously stirring to perform the amperometric study.

Results and discussion

Structures and morphologies

Fig. 1a schematically illustrates the synthesis process of CuO nanotubes. The growth of Cu(OH)₂ and CuO nanotubes can be visually tracked by the color changes of the Cu foil. The fresh Cu foil exhibited a gold color with a glossy surface (Fig. 1b). During the treatment in the alkaline solution, the surface color gradually changed to light blue and lost its luster (Fig. 1c), consistent with the in situ growth of Cu(OH)₂ nanotubes directly on the Cu foil substrate. The Cu(OH)₂ nanotubes were finally transformed to black CuO nanotubes after annealing (Fig. 1d). The corresponding SEM images reveal that CuO consisted of a myriad of nanotubes 150–350 nm in diameter, over 2 μm in length and tens of nanometers in wall thickness. They were derived directly from the unique structural feature of Cu(OH)₂ in the form of a nanotube forest. Fig. S1† clearly indicates that the surface morphology gradually evolved with the reaction time. The cleaned Cu foil exhibited a rough surface with some directionality (Fig. S1a and b†). After 5 min of treatment, solid nanorods started appearing sparsely on the surface (Fig. S1c and d†). The nanorods became thicker and longer, covering the surface in a directional manner with less blank surface after another 5 min of reaction (Fig. S1e†). More interestingly, the solid rod tips started opening up at this stage (Fig. S1f†). Finally, these nanorods were transformed entirely into nanotubes with irregularly-shaped ends after 20 min of reaction, which densely covered the Cu foil surface (Fig. S1g and h†). These numerous nanotubes were grown in the form of a forest with ample space in between, such that the electrolyte could have full access to and direct contact with, the nanotubes for electrochemical reactions.

Fig. 1 (a) Schematic of the synthesis process of CuO nanotubes; (b–d) optical and SEM images of pristine copper foil, Cu(OH)₂ nanotubes on Cu foil, and CuO nanotubes on Cu foil.

The TEM image of a single CuO nanotube shows a porous structure with a uniform wall thickness (Fig. S2a†). The lattice fringe spacings of 0.233 and 0.26 nm marked in Fig. S2b† correspond to the (111) and (002) planes of monoclinic CuO, respectively. However, the lattice fringes were not continuous, which is consistent with the porous nature of the nanotube. The selected area electron diffraction (SAED) pattern (inset of Fig. S2b†) indicates that the porous nanotubes had a relatively ordered internal structure with the same orientation.^33,34

Fig. 2a shows the powder XRD patterns of Cu(OH)₂ and CuO nanotubes in the wide angle region. Both materials showed two strong peaks at 43.3° and 50.4°, arising from the Cu foil (JCPDS file no. 65-9026). All the other diffraction peaks for the Cu(OH)₂ nanotubes are indexed to orthorhombic Cu(OH)₂ (JCPDS file no. 35-0505). After heat treatment, the peaks located at 32.5°, 35.5°, 38.7°, 38.9°, and 48.7° are assigned to the (110), (002), (111), (200), and (202) planes of the CuO phase (JCPDS file no. 45-0937), respectively. There were no other sharp peaks due to impurities, confirming that high purity CuO nanotubes were obtained in this study using the mild in situ reaction strategy. The high purity nanotubes were further evidenced by the Raman result, as seen in Fig. 2b. The broad peak with a relatively high intensity at 295 cm⁻¹ is assigned to the A_g band, while the two peaks at 342.8 cm⁻¹ and 628.5 cm⁻¹ are assigned to 2B_g. The significant intensities of these peaks indicate a single phase and high crystallinity of CuO nanotubes, which is in good agreement with the previous reports.^35,36

Fig. 2 (a) XRD patterns of as-prepared Cu(OH)₂ and CuO nanotubes directly grown on Cu foil; and (b) Raman spectrum of CuO nanotubes.

The results from the thermogravimetric and differential thermal analyses (TG-DTA) of Cu(OH)₂ nanotubes are shown in Fig. S3.† The gradual weight loss of 1.4% between 26 °C and 150 °C resulted from the removal of physically adsorbed and chemically bound water. The rapid weight loss of 12.4% between 150 °C and 180 °C is ascribed to the decomposition of Cu(OH)₂ to become CuO nanotubes. The DTA curve showed a strong heat absorption peak at ∼170 °C, consistent with the decomposition of Cu(OH)₂ nanotubes.³⁷ A further increase in temperature beyond 190 °C did not present any weight loss, indicating complete decomposition below 190 °C, which is considered an appropriate calcination temperature.

N₂ adsorption/desorption isotherm curves were obtained to investigate the porous characteristics and textural properties. According to the IUPAC classification of hysteresis loops,³⁸ Fig. 3 plots type IV isotherms with type H3 hysteresis loops. They do not exhibit any limited adsorption at relative pressures from 0 to 1, proving the presence of a typical hierarchical porosity.^39,40 An increase in slope at 0.6, especially for CuO nanotubes, corresponds to capillary condensation, which is typical of mesoporous materials, while the further increase in adsorbed volume at higher relative pressures indicates inter-particle porosity. The BET surface area was much larger for CuO nanotubes (109.02 m² g⁻¹) than for Cu(OH)₂ nanotubes (41.84 m² g⁻¹). The average pore diameters of the two materials were positioned in the mesopore region: the maxima were centered between 3.76 nm and 6.54 nm for Cu(OH)₂ nanotubes, and between 4.15 nm and 10.34 nm for CuO nanotubes. Such well-developed pore structures are advantageous for energy storage and electrocatalysis applications, since large pore channels permit rapid electrolyte transport, while small pores offer more active sites for redox reactions.

Fig. 3 Nitrogen adsorption/desorption isotherms of (a) Cu(OH)₂ nanotubes and (b) CuO nanotubes: the insets show the corresponding BJH pore size distributions.

The wettability test result shown in Fig. S4a† presents a very low CA of 18.57° on the surface of Cu(OH)₂. The CA with the CuO surface remains very low, at 27.01° (Fig. S4b†). The low CAs are attributed to the hydrophilic nature of both Cu(OH)₂ and CuO, along with the mesopores covering their surfaces,⁴¹ which confirms the excellent wettability of the nanotubes by an aqueous electrolyte when they are employed as the electrode.⁴²

Excellent energy storage performance

The above ameliorating surface properties of the porous CuO nanotubes, including the extremely large BET surface area, pore volume and excellent wettability, offer significant synergy towards the pseudocapacitive performance of the electrode, as shown in Fig. 4. The CV curves of the three different electrode materials obtained at a scan rate of 10 mV s⁻¹ (Fig. 4a) clearly indicate that the capacitive current of the neat Cu foil was much lower than that of the Cu(OH)₂ and CuO nanotube electrodes, which confirms the negligible capacitance contribution of the Cu foil to the composite electrodes. The CuO nanotube electrode exhibited more distinct redox peaks and a much larger enclosed area of the CV curve than those of the Cu(OH)₂ electrode. These findings imply that the former electrode would have a higher capacitance than the latter, as the specific capacitance is proportional to the area of the CV curve.⁴³ The CV profiles given in Fig. 4b and c reveal that the capacitance characteristics of both the nanotube electrodes are obviously different from that of the ideal rectangular shape for electric double-layer capacitance, suggesting that their capacitances originate primarily from the Faradic redox reactions. The redox reaction involved in the transition between Cu⁺ and Cu²⁺ species associated with OH⁻ ions is:⁴⁴

Cu₂O + 2OH⁻ ↔ 2CuO + H₂O + 2e⁻ (1)

Fig. 4 (a) CV curves of three electrode materials measured at 10 mV s⁻¹; CV curves of (b) Cu(OH)₂ and (c) CuO nanotube electrodes at different scan rates; (d) plots of specific capacitance of Cu(OH)₂ and CuO nanotube electrodes vs. scan rate; (e) galvanostatic charge/discharge curves at 1 A g⁻¹; galvanostatic charge/discharge curves of (f) Cu(OH)₂ and (g) CuO nanotube electrodes at different current densities; (h) plots of specific capacitance of Cu(OH)₂ and CuO nanotube electrodes vs. current density; and (i) cyclic performance of electrodes at 1 A g⁻¹.

With increasing the scan rate, the current density increased, while the shape of the CV curves remained largely unchanged, except for the small shift of the peak position, presenting prominent electrochemical reversibility and apparent high-rate performance. The specific capacitances (C_s) were calculated from the CV curves according to the equation:⁴⁵

where I (A) is the response current, ν (V s⁻¹) is the potential scan rate, ΔV (V) is the potential window, and m (g) is the mass of the active electrode material. It is noted that the specific capacitance of the CuO nanotube electrode gradually decreased from 564 to 419 F g⁻¹ when the scan rate was changed from 5 to 100 mV s⁻¹ (Fig. 4d). The Cu(OH)₂ nanotube electrode presented an essentially similar trend with capacitances lower by 80–90 F g⁻¹ than the corresponding values of the former electrode. The reduction in capacitance at high charge/discharge rates means that not all active species of the electrode were involved in the redox process.

The galvanostatic charge/discharge curves obtained at a current density of 1 A g⁻¹ (Fig. 4e) gave discharge times 5.04, 226.2 and 265.2 s for the Cu foil, Cu (OH)₂ and CuO nanotube electrodes, respectively. This finding further demonstrates that Cu foil had a negligible capacitance, while the CuO electrode exhibited better electrochemical performance than the Cu(OH)₂ electrode. Both the curves of the Cu(OH)₂ and CuO electrodes were not ideal straight lines, suggesting the dominance of the pseudocapacitive behaviour. In addition, the sudden potential drop in the initial discharge period was caused by the internal resistance of the electrodes.⁴⁶ Fig. 4f and g show the galvanostatic charge/discharge curves of the Cu(OH)₂ and CuO electrodes at different current densities, respectively. The specific capacitances (C_s) were calculated at different applied current densities using the following equation:³

C_s = IΔt/ΔVm (3)

where I, Δt, ΔV and m refer to the constant discharge current, the total discharge time, the potential window and the mass of the electroactive materials, respectively. When the current density was changed from 1 A g⁻¹ to 20 A g⁻¹, the specific capacitance of the CuO nanotube electrode gradually decreased from 442 F g⁻¹ to 358 F g⁻¹ and likewise from 377 F g⁻¹ to 221 F g⁻¹ for the Cu(OH)₂ electrode, with remarkable capacity retention ratios of ∼81% and ∼59%, respectively (Fig. 4h). The higher the current density used, the greater the difference in capacitance between the two electrodes. This result further confirms that the porous CuO electrode with a larger surface area was more competent to boost electron transport when charged/discharged at faster rates. The common capacitance decrease resulting from the increase in current density is most likely caused by the increase in potential drop due to the electrode resistance and the relatively lower utilization of the active material at high current densities.⁴⁷

The long-term cyclic stability of the electrodes was measured at a current density of 1 A g⁻¹ (Fig. 4i). Interestingly, the specific capacitance of both electrodes increased in the first several hundred cycles, which can be attributed to the gradual activation of the electrodes before reaching the full activation. The capacitance retention of the Cu(OH)₂ and CuO electrodes after 5000 cycles was 95.4% and 91.9% of the maximum value, respectively. The pseudocapacitive behaviour is associated primarily with the redox reactions of the cations in electrode materials.⁴⁸ Thus, the charge storage mechanisms of both electrodes in SCs should be similar.

Judging from the fact that both electrodes have the same Cu²⁺ and similar nanotube structures, they should show a similar electrochemical performance. Because the Cu(OH)₂ electrode possessed a relatively smaller surface area and lower porosity than the CuO counterpart, the kinetics of the ions should be slower, with an inevitably lower capacitance. The difference in electrochemical performance of these two electrodes can also be understood from the Nyquist plot (Fig. 5a). The two impedance spectra were similar, being composed of one semicircle component each at high frequencies and a linear component each at low frequencies, further demonstrating the long-term electrochemical stability of these electrode materials.⁴⁹ The internal resistance (R_s) is the sum of the ionic resistance of electrolyte, the intrinsic resistance of active materials and the contact resistance at the active material/current collector interface,⁵⁰ and can be obtained from the intercept of the plots on the real axis. The semicircle of the Nyquist plot corresponds to the faradaic reactions, and its diameter represents the interfacial charge transfer resistance (R_ct). The inset of Fig. 5a gives the equivalent circuit used to fit the EIS curves to measure R_s and R_ct, where Z_w and CPE are the Warburg impendence and the constant phase element, respectively.⁵¹ The results are shown in Table S1,† confirming the much lower R_s and R_ct values for the CuO electrode than for the Cu(OH)₂ electrode. Furthermore, the CuO electrode presented a higher slope and a shorter line in the low frequency region, which suggests faster OH⁻ diffusion rates and a smaller variation of diffusion paths. All these observations are directly related to the better overall electrochemical properties of the CuO electrode over the Cu(OH)₂ counterpart, as discussed above with reference to Fig. 4.

Fig. 5 (a) Electrochemical impedance spectra (EIS) of electrodes at the open circuit potential; and (b) schematic of the charge storage advantages of porous CuO nanotube electrodes.

Summarizing, the mechanism of charge transfer in the CuO nanotube electrode is schematically shown in Fig. 5b. The CuO nanotubes offer reliable one-dimensional pathways for charge carrier transport. The open-end nanotube structure, combined with a large surface area and ample empty space in-between, allows the relief of strains induced during the charge/discharge process and provides extremely large reaction sites between the active materials and the electrolyte.

To further demonstrate the long-term stability of the CuO nanotube electrode, the changes in morphology and impedance after 5000 cycles at 1.0 A g⁻¹ were evaluated, and the results are shown in Fig. S5 and S6.† It can be clearly seen that the nanotube forest was largely intact and was attached firmly onto the Cu foil current collector after the repeated phase conversion reactions (Fig. S5†). There was virtually no change in the detailed nanotube structure. When the Nyquist plots obtained after 1 cycle and 5000 cycles are compared (Fig. S6†), the R_s of the CuO electrode show only a negligible increase from 1.71 Ω to 1.83 Ω, manifesting the excellent conductivity of the electrolyte and very low internal resistance of the composite electrode (Table S2†). R_ct, however, increased slightly more after 5000 cycles, probably as a result of the gradual formation of solid electrolyte interface films. Nevertheless, the CuO nanotube electrode retained remarkable cyclic stability, as seen from Fig. 4i.

Sensitive biosensory capability

The electrochemical performance of the CuO nanotube electrode as a non-enzymatic glucose biosensor was also investigated. Fig. 6a shows the linear-sweep voltammogram of the electrode in 0.1 M KOH solution, measured at a scan rate of 0.05 V s⁻¹. Broad catalytic current peaks with a peak potential of about +0.5 V were observed, which might correspond to the Cu²⁺/Cu³⁺ redox couple. The amperometric response linearly increased with increasing the glucose concentration at all applied potentials. Furthermore, glucose oxidation occurred in the potential range of 0.4–0.8 V, where the oxidation wave for Cu²⁺/Cu³⁺ was demonstrated.³¹ All these observations prove that the CuO electrode was electrocatalytically active for glucose oxidation. Fig. 6b shows a typical amperometric response curve of the electrode when the glucose concentration was successively increased in a stepwise manner. The electrode exhibited a stable and immediate amperometric response to the changes in glucose concentration. As expected, the signal noise increased when increasing the glucose concentration, due to the accumulation of adsorbed intermediate species on the electrode surfaces.⁵² The corresponding calibration curve is plotted in Fig. 6c, where a linear region was found between the concentrations 100 μM and 3 mM, with the equation: I (mA) = 0.1267 + 2.231C (mM), and a correlation coefficient of 0.999. The electrode presented an excellent sensitivity of 2231 μAm M⁻¹ cm⁻² and a low detection limit of 1.07 μM (with a signal/noise ratio of 3). It should be noted that the detection limit is at least three orders of magnitude lower than the normal blood glucose level of 4.4–6.6 mM,⁵³ indicating that the CuO nanotube electrode is perfectly suited to the electrochemical detection of blood glucose. Note that the sensitivity of our present sensing system is higher than that of other CuO materials,^54–57 as shown in Table S3.† To test the stability and reproducibility of the sensor, we performed 20 successive measurements at a glucose concentration of 0.1 mM. It was found that the relative standard deviation (R. S. D.) was 2.3%, which indicated an acceptable reproducibility.

Fig. 6 (a) Linear-sweep voltammograms collected for the CuO nanotube electrode at different glucose concentrations; (b) amperometric response of the CuO nanotube electrode to the changes in glucose concentration in 0.1 M KOH at 0.50 V vs. Ag/AgCl; (c) current-glucose concentration calibration curve; and (d) anti-interference property of the CuO nanotube electrode upon the stepwise addition of 10 μM AA, UA, urea, lactose and NaCl, followed by the addition of 100 μM glucose solution.

Another important analytical factor of a glucose sensor is the ability to discriminate interference species from the real sample, such as ascorbic acid (AA), urea, lactose, uric acid (UA), and sodium chloride (NaCl), all of which usually coexist with glucose in human blood. In view of the fact that the concentration of glucose in human blood is more than 30 times the interfering species,⁵⁸ the interference experiment was carried out by the successive addition of 0.1 mM of glucose and 0.01 mM of interfering species in the 0.1 M KOH solution. As shown in Fig. 6d, a distinct glucose response was observed, while the responses to the interfering species were negligible, compared to glucose addition. This observation confirms the satisfactory anti-interference ability of the CuO nanotube-based glucose sensor.

Conclusions

We present a facile strategy for the in situ synthesis of low-cost, free-standing, binder-free CuO nanotube electrodes on a highly conductive Cu foil. The electrode material offered many advantages, including highly porous structure, large surface area, abundant empty space, short and fast electron transport paths and the total elimination of both inactive materials and extra processing steps. These advantages make them ideal electrodes for SCs and NGBs. The electrode exhibited excellent electrochemical properties, including a remarkable specific capacitance of 382 F g⁻¹ at a current density of 10 A g⁻¹ and a response to glucose at a linear range of 100 μM to 3 mM, with an outstanding sensitivity of 2231 μA mM⁻¹ cm⁻². The above findings may open up new avenues for the practical application of CuO nanotube electrodes in the next-generation SCs and NGBs.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (nos U1304108, U1204501 and 21373107), the Innovative Research Team (in Science and Technology) in University of Henan Province (no. 13IRTSTHN018).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: TG-DTA curves, hydrophilicity test, FESEM images of the growth process of Cu(OH)₂ nanotubes, FESEM images and electrochemical impedance spectra of CuO nanotube electrodes. See DOI: 10.1039/c4ra08230c

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