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CuO nanorod array supported on carbon cloth for highly sensitive non-enzymatic glucose biosensing
Sensors & Actuators: B. Chemical 298 (2019) 126860
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Hierarchical Co3O4/CuO nanorod array supported on carbon cloth for highly sensitive non-enzymatic glucose biosensing
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Siyi Chenga,b,c, Steven DelaCruza,b, Chen Chenc, Zirong Tangc, Tielin Shic, Carlo Carraroa,b, ⁎ Roya Maboudiana,b, a
Berkeley Sensor & Actuator Center, University of California, Berkeley, CA 94720, United States Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720, United States c State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China b
ARTICLE INFO
ABSTRACT
Keywords: Hierarchical Co3O4@CuO Core-shell structure Carbon cloth Non-enzymatic biosensor Glucose sensor
A hierarchical core-shell Co3O4/CuO nanorod array (NRA) anchored on flexible carbon cloth (CC) has been fabricated through a stepwise process consisting of magnetron sputtering of Cu, its anodic oxidation, and chemical bath deposition of Co3O4. The structure, composition and morphology of the synthesized Co3O4/CuO NRA on CC were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The glucose sensing performance of the Co3O4/CuO NRA on CC electrode was investigated by cyclic voltammetry and chronoamperometry. The sensor exhibited a high sensitivity of 5405 μA mM−1 cm−2 with a fast response time (t90 = 1.9 s) and a low detection limit of 0.38 μM. The electrode also showed outstanding selectivity toward various interferences. These results indicate that the Co3O4/CuO nanocomposites may be promising electrode materials for electrochemical biosensing.
1. Introduction Rapid and accurate glucose detection can provide critical information in a variety of contexts, including diabetes monitoring, fermentation, and environmental protection [1–3]. To date, tremendous efforts have been made to develop glucose sensors with high sensitivity, good selectivity, fast response time, good reliability, and low cost [4,5]. Many methods have been developed for glucose detection, including ones based on electrochemical, fluorescence, surface plasmon resonance, and optical methods [3,6–8]. Among these, electrochemical methods have attracted significant attention due to such advantages as high sensitivity and selectivity, simple operation, and cost effectiveness [9,10]. Among different types of electrochemical glucose sensors, enzymatic glucose sensors, which catalyze glucose using glucose oxidase, show high sensitivity and selectivity [11]. However, low chemical stability, complicated immobilization processes and high cost limit the practical application of enzyme-based glucose sensors [12,13]. Hence, it is highly desirable to develop non-enzymatic sensors for the direct detection of glucose. Owing to great progress in nanotechnology, various nanostructures based on metals and metal oxides have been applied for the construction of non-enzymatic glucose sensors with high performance ⁎
[10,14,15]. Over the past decade, several noble metals, such as gold [16], platinum [17], silver [18] and their alloys [19], have been widely studied for glucose detection due to their high catalytic properties. However, their high cost, relatively low selectivity, and proneness to poisoning hinder their practical applications [20]. In contrast, transition metal oxides, such as CuO, NiO, Co3O4 and MnO2 have drawn increasing attention due to their low cost, enhanced selectivity, and high stability [21–24]. Among these nanomaterials, CuO possesses a high isoelectric point and excellent electrochemical performance. Hence, various CuO-based nanostructures, including nanowires [25], nanosheets [20], nanoflowers [26] and nanorods [27], have been fabricated for glucose sensing applications. However, the poor intrinsic electrical conductivity of CuO limits charge carrier transport, resulting in low sensitivity. Therefore, constructing an electrode from multiple nanomaterials and leveraging synergistic effects between its components is believed to be a promising way to improve the conductivity, electro-active surface area, and contact area between the sensing material and the substrate, leading to improved ion/electron transport inside the electrodes and better electrochemical performance. Recently, various CuO-based composite nanomaterials have been reported for glucose detection [20,28–30]. However, most of these studies have focused on the sensing performance of nanomaterials on glassy carbon
Corresponding author at: Berkeley Sensor & Actuator Center, University of California, Berkeley, CA 94720, United States. E-mail address: [email protected] (R. Maboudian).
https://doi.org/10.1016/j.snb.2019.126860 Received 15 March 2019; Received in revised form 21 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic illustration of the fabrication process for the Co3O4/CuO NRA on carbon cloth.
Fig. 1. (a) X-ray diffraction and (b) Raman spectra of the CuO NRA (black) and Co3O4/CuO NRA (red) on carbon cloth. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
electrodes, which may limit their practical applications. It was recently demonstrated that CuO-based nanostructures can be directly grown on conductive substrates without any binder [31]. This approach can reduce the resistance between the sensing materials and the substrate. Therefore, fabricating composite nanomaterials directly on conductive substrates may open a new avenue toward achieving enhanced glucose sensing performance. Herein, we report on the synthesis of a Co3O4/CuO hierarchical core-shell nanorod array (NRA) directly on flexible and conductive carbon cloth via a stepwise approach. The integration of Co3O4 nanoflakes on CuO nanorods can increase the electro-active surface area and improve the contact area between the sensing materials and the substrate, which lead to more active sites for redox reactions and faster reaction kinetics. We demonstrate that the composite nanostructures on carbon cloth exhibit high electrochemical performance with high sensitivity, fast response time, low detection limit, and good selectivity, thus proving promising for non-enzymatic glucose sensors.
2. Experimental 2.1. Fabrication of hierarchical Co3O4/CuO NRA/CC All chemicals were analytical grade and were used without further purification. Carbon cloth, purchased from Cetech Co. Ltd., was cleaned in acetone, ethanol and deionized (DI) water for 10 min each. After drying in air, carbon cloth was placed into a magnetron sputtering system. A Cu layer was sputtered on both sides of carbon cloth by using a magnetron sputtering system (TPR-450) at a power of 100 W for 30 min. Subsequently, the as-obtained sample was cut into 10 mm × 10 mm pieces and anodized in a 3 M NaOH solution at a current density of 10 mA cm−2 to form a Cu(OH)2 NRA on carbon cloth. Then, 0.075 M cobalt sulfate hydrate and 1.134 M urea were dissolved in 20 ml of DI water in a 25 mL vial and stirred to form a clear pink solution. The as-prepared Cu(OH)2 NRA/CC sample was immersed in the solution and held for 1 h at 85 °C in an oil bath. The sample was then taken out of the solution, washed with DI water, and annealed at 350 °C in air for 2 h to prepare hierarchical Co3O4/CuO NRA/CC. For comparison, the CuO NRA/CC sample was prepared by directly 2
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Fig. 2. (a) Survey scan, and (b) Co 2p, (c) Cu 2p and (d) O 1s regions of the XPS spectrum obtained on Co3O4/CuO NRA/CC.
annealing Cu(OH)2 NRA/CC in air at 350 °C.
formation mechanism of the Co3O4 shell can be described by the “selfassembly” and “oriented attachment” processes [33]. In our case, the Cu(OH)2 nanorod array acts as the backbone which guides the Co-based precursor deposition. XRD and Raman measurements were carried out to investigate the structure and composition of the as-synthesized materials. Fig. 1a shows representative XRD spectra obtained on CuO NRA and Co3O4/ CuO NRA on carbon cloths. The broad diffraction peaks at around 25.6° demonstrate the graphitized carbon phase originating from the carbon cloth. The peaks located at 43.2° can be ascribed to copper (JCPDS 040836), revealing that the as-deposited copper layer was not fully transformed into CuO during the anodic oxidation and annealing processes. The formation of CuO is evidenced by the peaks at 32.4, 35.5, 38.7, 48.9, 53.5, 58.2, 61.4, 66.3 and 67.9°, which correspond to the (110), (002), (111), (-202), (020), (202), (-113), (022) and (113) planes of the monoclinic CuO structure (JCPDS No. 89-5899), respectively [34]. The chemical bath deposition and calcination processes lead to the formation of the Co3O4/CuO NRA on carbon cloth. The peaks centered at 18.9, 31.1, 36.6 and 65.2° correspond to the (111), (220), (311) and (440) planes of the Co3O4 spinel phase (JCPDS No. 42‐1467), respectively [35]. The XRD result of the sample after 1 h immersion in 0.075 M cobalt sulfate hydrate and 1.134 M urea solution at 85 °C is shown in Figure S1. The peak located at 22.1° can be attributed to Co (OH)2 (JCPDS 04-0836), while the peaks located at 32.7, 38.8, and 45.7° can be ascribed to CoCO3 (JCPDS 11-692). The remaining peaks can be attributed to those of carbon cloth (25°), copper (43°), and Cu (OH)2. Therefore, the Co precursors appear to be in the form of Co(OH)2 and CoCO3, and this result corresponds to previous reported precursors for Co3O4 [36]. Raman spectroscopy was carried out to examine the difference between the CuO and the Co3O4/CuO core-shell nanostructures. Fig. 1b displays the Raman spectra of the CuO NRA and Co3O4/CuO NRA on carbon cloth. Two strong bands at 1325 and 1603 cm−1 can be indexed to the characteristic peaks of the D-band
2.2. Materials characterization The structures of the as-synthesized samples were characterized by X-ray diffraction (XRD, Bruker AXS D8 Discover with GADDS) using a Co Kα radiation source and by transmission electron microscopy (TEM) using a FEI Tecnai G2 S-TWIN. The chemical compositions of the samples were analysed by X-ray photoelectron spectroscopy (XPS, Omicron Dar400 system with an achromatic Al Kα X-ray source) and Raman spectroscopy (Horiba LabRAM confocal Raman spectrometer with an excitation laser of 632.8 nm). The morphologies and microstructures of the samples were probed by field-emission scanning electronic microscopy (FE-SEM Quanta 3D FEG). 2.3. Electrochemical measurements All electrochemical experiments were performed at room temperature with a CHI 600D electrochemical workstation. The electrochemical measurements were performed in 0.1 M NaOH using a conventional three-electrode configuration: carbon cloth samples as the working electrode, Ag/AgCl as the reference electrode and Pt wire as the counter electrode. For amperometric tests, the solution was stirred to facilitate mixing. 3. Results and discussion Scheme 1 illustrates the fabrication process of hierarchical Co3O4/ CuO core-shell structures on carbon cloth. A highly ordered Cu(OH)2 NRA was first synthesized on flexible carbon cloth via magnetron sputtering of copper and subsequent anodic oxidation [32]. Then chemical bath deposition and a follow-up calcination process were used to form a hierarchical Co3O4/CuO core-shell nanorod array. The 3
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Fig. 3. (a, b) SEM images of CuO NRA on carbon cloth with different magnifications. (c, d) SEM images of Co3O4/CuO NRA on carbon cloth with different magnifications. (e, f) TEM images of Co3O4/CuO NRA.
(indicative of the presence of defects in the hexagonal graphitic layers) and G-band (first-order scattering of the E2 g mode observed for sp2 carbon domains) of carbon cloth [37,38]. The Raman spectrum of CuO shows three peaks at 291, 347 and 623 cm−1, corresponding to the A g, B g(1) and B g(2) modes of CuO backbone, respectively [39]. As shown in the spectrum of the Co3O4/CuO NRA on carbon cloth, four peaks detected at around 190, 480, 520 and 680 cm−1 correspond to Eg, F2 g1, F2 g2 and A1g modes of Co3O4 [40]. After the growth of Co3O4, the relative peak intensity of CuO is reduced, suggesting that the CuO core is covered by the Co3O4 shell. The near-surface elemental composition and oxidation state of the Co3O4/CuO NRA/CC as well as the presence of absorbed species were analyzed by XPS. The survey spectrum (Fig. 2a) shows a series of characteristic peaks associated with copper, cobalt, oxygen and carbon elements. It is worth mentioning that the C 1s peak is weak, which may be due to the uniform growth of sensing material on the surface of the carbon substrate. In the high-resolution Co 2p XPS spectrum (Fig. 2b), the two characteristic peaks centered at 780.3 and 795.4 eV correspond
to Co 2p3/2 and Co 2p1/2, respectively, with a spin-orbit separation of around 15 eV [41,42]. The Gaussian fitted peaks positioned at the binding energies around 780.5 and 795.5 eV can be ascribed to Co3+, whereas the peaks at 781.6 and 797.4 eV confirm the presence of Co2+ [43,44]. In addition, the two shakeup satellite peaks located around 788.2 and 803.9 eV indicate that the cobalt has a spinel structure [45]. Fig. 2c displays the core level Cu 2p spectrum of the composite material. Two peaks located at the binding energies of 934.3 and 954.6 eV can be assigned to Cu 2p3/2 and Cu 2p1/2, while the shakeup satellite peak at 942.6 eV can be indexed to Cu2+ [46]. Fig. 2d shows the highresolution spectrum of the O 1s region, demonstrating the co-existence of three oxygen species marked as O1, O2 and O3. The O1 component located at a binding energy of 529.8 eV is consistent with the typical metal-oxygen bond, and the O2 component at a binding energy of 531.4 eV can be ascribed to the large density of defects with low oxygen coordination in the composite material [6]. The O3 component at a binding energy of 532.4 eV corresponds to multiplicity of physically absorbed and chemisorbed species onto and within the surface layer 4
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area than that of the CuO NRA/CC and Co3O4/CuO NRA/CC electrodes, and exhibits no redox peak after the addition of glucose (see enlarged CVs in Figure S3), implying that the contribution of the carbon cloth toward the detection of glucose can be neglected. The bare CC electrode exhibits a large oxidative tail corresponding to the onset of water breakdown observed for potentials above 0.65 V [48]. This behavior can also be observed in the CV curves of the CuO NRA/CC and Co3O4/ CuO NRA/CC electrodes. The CV result on the CuO NRA/CC shows a broad reduction peak at ˜0.42 V in the blank alkaline solution. This peak may be ascribed to the CuOOH/CuO transition [32]. However, the corresponding oxidation peak related to CuO/CuOOH is not clearly observed, which may be due to overshadowing by the oxidation peak of water-splitting [48,49]. After the addition of 1 mM glucose, an oxidation peak appears at ˜0.3 V, corresponding to the irreversible glucose oxidation reactions [32]. In comparison, on the Co3O4/CuO NRA/CC electrode, two pairs of redox peaks, one at 0.58 and 0.38 V, and one at 0.25 and 0.12 V, are observed prior to the addition of glucose, which can be ascribed to the Co3O4/CoOOH and CoOOH/CoO2 redox couples, respectively [50]. After the addition of 1 mM glucose, the oxidative peak at 0.58 V shift to 0.5 V and its magnitude increases, while the current at 0.25 V remains nearly constant. The oxidative peak at 0.5 V can be attributed to the transitions of CuO/CuOOH and CoOOH/CoO2, and the oxidative peak at 0.25 V can be assigned to the Co3O4/CoOOH transition [23]. This phenomenon indicates that the CoOOH/CoO2 couple on Co3O4 is the dominant redox reaction for the glucose catalysis process [50]. In the cathodic scan, the broad peak at 0.38 V may be attributed to the combination of CoO2/CoOOH and CuOOH/CuO transitions, while the broad peak at 0.12 V could be attributed to the Co3O4/CoOOH transition [31,51]. We hypothesize that the mechanism of the glucose oxidation in Co3O4/CuO NRA/CC electrode is similar to that of individual CuO and Co3O4 electrodes and can be explained as follows [51,52]:
Fig. 4. Cyclic voltammetry curves obtained on CC, CuO NRA/CC and Co3O4/ CuO NRA/CC in the absence and presence of 1 mM glucose in 0.1 M NaOH solution at a scan rate of 50 mV s−1.
[47]. These results clearly show that the surface of the as-synthesized Co3O4/CuO NRA contains Co2+, Co3+ and Cu2+ ions, indicating the composition consists of CuO and Co3O4. Carbon cloth, which consists of interconnected carbon fibers with an average diameter of ˜10 μm (Figure S2a), was used as the conductive substrate. As shown in Figure S2b, the bare carbon cloth possesses high flexibility under bending, making it a good substrate for flexible sensing applications. The morphologies of the CuO NRA and core-shell Co3O4/ CuO NRA on carbon cloth were examined by SEM. Fig. 3a shows that the CuO nanorods were grown uniformly and vertically on carbon fiber. The magnified SEM image (Fig. 3b) indicates that the as-synthesized CuO nanorods possess lengths of a few micrometers and an average diameter of ˜100 nm. Fig. 3c-d presents the SEM images of the coreshell Co3O4/CuO NRA under different magnifications. It can be seen that the deposited Co3O4 layer consists of nanoflakes which are interconnected to each other and form a highly ordered 3D nanostructure. The detailed structures of the Co3O4/CuO NRA were recorded by TEM. Fig. 3e shows the TEM image of the Co3O4/CuO NRA. It can be seen that the CuO nanorod is compactly wrapped by interconnected Co3O4 nanoflakes. Fig. 3f displays the TEM image of Co3O4 at high resolution, revealing lattice fringes with the interplanar gap of 0.24 nm, corresponding well to the (311) plane of Co3O4. Electrochemical tests for different working electrodes were performed in a three-electrode cell system. Fig. 4 displays the cyclic voltammograms (CVs) of different electrodes in 0.1 M NaOH solution in the absence and presence of 0.5 mM glucose under a scan rate of 50 mV s−1. The bare carbon cloth electrode shows a much smaller CV curve
Co3 O4 + OH + H2 O
CoOOH+ OH
CoO2 + H2 O+ e
2CoO2 + glucose CuO+ OH
3 CoOOH+ e
2 CoOOH+ gluconolactone
CuOOH+ e
2 CuOOH+ glucose
2 CuO+ gluconolactone+ H2 O
(1) (2) (3) (4) (5)
In addition, the oxidation and reduction peaks of the Co3O4/CuO NRA/CC electrode show higher intensity than those of the CuO NRA/ CC electrode, demonstrating that the composite electrode has better electrochemical behavior toward the electro-oxidation of glucose. The improved electrochemical performance can be ascribed to the large amount of Co3O4 nanoflakes firmly and uniformly fixed to the surface of the CuO NRA electrode, providing greater electroactive surface area for the detection of glucose.
Fig. 5. (a) CV responses of Co3O4/CuO NRA/CC electrode in 0.1 M NaOH with different scan rates. (b) Corresponding calibration curve of current vs. scan rate. 5
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Fig. 6. (a) Amperometric responses of the CuO NRA/CC (red) and Co3O4/CuO NRA/CC (black) electrodes under the successive addition of 50 μM glucose at the applied potential of 0.55 V. (b) Corresponding calibration curves for glucose detection. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (a) Amperometric response of the Co3O4/CuO NRA/CC electrode upon successive additions of glucose with select concentrations marked. Inset: the enlarged view in low concentrations. (b) Calibration curve for glucose of the Co3O4/CuO NRA/CC electrode.
much higher than that of CuO NRA/CC electrode (0.0035 m F/cm2), indicating the higher electrochemical surface area of the Co3O4/CuO NRA/CC electrode (Figure S4c). To better understand the electrochemical behavior of the Co3O4/ CuO NRA/CC electrode, CV tests were also conducted at different scan rates. Fig. 5a displays the CVs of the Co3O4/CuO NRA/CC electrode in the presence of 1 mM glucose with scan rates (υ) of 5, 10, 20, 30, 40, 50 mV s−1. It can be seen that the anodic (Jpa) and the cathodic (Jpc) peak current densities increase with increasing scan rate. The redox peak current shows good linearity with the scan rate, and the regression equations for the anodic and cathodic peak current can be expressed as Jpa(mA cm-2) = 1.16 + 0.53υ (mV s−1) (R2 = 0.999) and Jpc(mA cm-2) = -0.11 - 0.27υ (mV s−1) (R2 = 0.998), respectively. This linearity indicates that the electro-oxidation of glucose on the Co3O4/CuO NRA/ CC electrode is a diffusion-controlled process [56,57]. Fig. 6a shows the amperometric response curves of the CuO NRA/ CC and Co3O4/CuO NRA/CC electrodes after successive injection of glucose into 0.1 M NaOH under continuous stirring at an applied potential of 0.55 V. After glucose addition, the Co3O4/CuO NRA/CC electrode exhibits a larger response current compared to the CuO NRA/ CC electrode. The slope of the calibration curve (Fig. 6b) also confirms the higher sensitivity of the Co3O4/CuO NRA/CC electrode. The amperometric results are consistent with the CV results. The improved response current achieved with the Co3O4/CuO NRA/CC could be ascribed to the larger electro-active surface area, which provides more active sites for the oxidation of glucose. Thus, Co3O4/CuO core-shell nanostructures possess better electrochemical performance than the bare CuO nanorods, making them promising materials for the detection of glucose. Fig. 7a shows a typical amperometric response of the Co3O4/CuO NRA/CC electrode to the successive addition of glucose into 0.1 M NaOH with an applied potential of 0.55 V. After the injection of glucose,
Fig. 8. Amperometric responses of the Co3O4/CuO NRA/CC electrode to the successive addition of glucose and interference species.
The electrochemical surface area (ECSA) of the Co3O4/CuO NRA/CC and CuO NRA/CC electrodes were estimated through electrical doublelayer capacitance (EDLC) tests in 0.1 M NaOH solution as the doublelayer capacitance (Cdl) is proportional to the ECSA [53]. The CV plots were obtained within ± 0.05 V of open circuit potential, where the current response should only be due to the charging of the double layer [54]. The CV curves of Co3O4/CuO NRA/CC and CuO NRA/CC electrodes at various scan rates (in 2–10 mV/s range) are shown in Figures S4a and b, respectively. At each scan rate, the Co3O4/CuO NRA/CC electrode exhibits much higher anodic (ja) and cathodic (jc) current densities than the CuO NRA/CC electrode. As shown in the reported works, the linear slope in such plots is equal to 2Cdl [55]. The calculated capacitance of the Co3O4/CuO NRA/CC electrode (0.03 m F/cm2) is 6
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the current increases, reaching 90% of its steady-state value (t90) in 1.9 s (Figure S5), demonstrating the rapid response of the Co3O4/CuO core-shell structure. The corresponding calibration curve of the Co3O4/ CuO NRA/CC electrode is displayed in Fig. 7b. The peak current increases linearly with increasing glucose concentration within the range of 1–500 μM (R2 = 0.997), and the electrode demonstrates a high sensitivity of 5405 μA mM−1 cm−2 and a low detection limit of 0.38 μM (S/N = 3). When the glucose concentration is above 500 μM, the slope of the fitting curve is slightly reduced (Figure S6), which may be ascribed to a condition where the glucose diffusion rate exceeds the consumption rate at high glucose concentration [58]. The Co3O4/CuO NRA/CC electrode exhibits excellent glucose sensing performance, with the highest sensitivity, lower detection limit and response time, and a comparable linear range, when compared to the performance of recently reported CuO and Co3O4 based glucose sensors (Table S1). The superior performance can be attributed to the following aspects: (1) The Co3O4 shell can act as a stable matrix to encapsulate and connect the core CuO nanorods, thereby generating more channels for electron transport between the CuO nanorods and the substrate; (2) The uniform hierarchical Co3O4/CuO core-shell nanostructures can offer electro-active sites for glucose oxidation; (3) The core-shell structures are supported on highly conductive carbon cloth without binder, which can further improve the electron transportation efficiency. Selectivity is one of the most important aspects for judging the performance of a glucose sensor. The selectivity of the Co3O4/CuO NRA/CC electrode was investigated by adding sucrose, fructose, uric acid (UA), ascorbic acid (AA), and dopamine (DA) into the 0.1 M NaOH solution at a potential of 0.55 V. Additionally, the influence of chloride ions was studied because they can poison non-enzymatic glucose sensors and lower the activity significantly [20]. The ratio of glucose and interferences in the human blood is more than 30:1 at a physiological level [59]. In order to evaluate the selectivity of the glucose sensor, a 10:1 concentration ratio of glucose to interferences was used to simulate the physiological level. Herein, the interference test was conducted by first adding 1 mM glucose and then adding 100 μM of the various interfering substances sequentially. As shown in Fig. 8, with the addition of glucose, the current density increased significantly, whereas negligible current responses can be observed following the injection of the interferences. Therefore, it can be concluded that the Co3O4/CuO NRA/CC electrode possesses excellent selectivity as a non-enzymatic glucose sensor. The stability of the Co3O4/CuO NRA/CC electrode was evaluated by measuring the amperometric current response of Co3O4/ CuO NRA/CC electrode with the addition of 0.1 mM glucose during a period of 20 days at a time interval of 2 days, as shown in Figure S7. After 20 days storage in air, the Co3O4/CuO NRA/CC electrode maintained approximately 97.6% of its original current response, implying a good stability of the Co3O4/CuO core-shell nanostructure. The Co3O4/ CuO NRA /CC electrode shows better stability than some recently reported CuO based glucose sensors [60–62].
Acknowledgements The authors thank Jinhu Fan for help with the magnetron sputtering. The support of the industrial members of the Berkeley Sensor & Actuator is gratefully acknowledged. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. S.C. acknowledge additional support through the China Scholarship Council. C.C., Z.T. and T.S. acknowledge the National Basic Research Program of China with Project No. 2015CB057205, the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT13017), and the National Science Foundation of China (No. 51775218). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126860. References [1] E. Vargas, M. Ruiz, S. Campuzano, A. Reviejo, J. Pingarrón, Non-invasive determination of glucose directly in raw fruits using a continuous flow system based on microdialysis sampling and amperometric detection at an integrated enzymatic biosensor, Anal. Chim. Acta 914 (2016) 53–61. [2] A.J. Bandodkar, J. Wang, Non-invasive wearable electrochemical sensors: a review, Trends Biotechnol. 32 (2014) 363–371. [3] W. Wu, N. Mitra, E.C. Yan, S. Zhou, Multifunctional hybrid nanogel for integration of optical glucose sensing and self-regulated insulin release at physiological pH, ACS Nano 4 (2010) 4831–4839. [4] D. Bruen, C. Delaney, L. Florea, D. Diamond, Glucose sensing for diabetes monitoring: recent developments, Sensors 17 (2017) 1866. [5] D. Rodbard, Continuous glucose monitoring: a review of successes, challenges, and opportunities, Diabetes Technol. Ther. 18 (2016) S2-3-S2-13. [6] S. Liu, K. Hui, K. Hui, Flower-like copper cobaltite nanosheets on graphite paper as high-performance supercapacitor electrodes and enzymeless glucose sensors, ACS Appl. Mater. Interfaces 8 (2016) 3258–3267. [7] H. Yuan, W. Ji, S. Chu, S. Qian, F. Wang, J.-F. Masson, et al., Fiber-optic surface plasmon resonance glucose sensor enhanced with phenylboronic acid modified Au nanoparticles, Biosens. Bioelectron. 117 (2018) 637–643. [8] J. Chang, H. Li, T. Hou, W. Duan, F. Li, Based fluorescent sensor via aggregation induced emission fluorogen for facile and sensitive visual detection of hydrogen peroxide and glucose, Biosens. Bioelectron. 104 (2018) 152–157. [9] J. Kim, A.S. Campbell, J. Wang, Wearable non-invasive epidermal glucose sensors: a review, Talanta 177 (2018) 163–170. [10] D.-W. Hwang, S. Lee, M. Seo, T.D. Chung, Recent advances in electrochemical nonenzymatic glucose sensors–a review, Anal. Chim. Acta (2018). [11] J. Wang, Electrochemical glucose biosensors, Chem. Rev. 108 (2008) 814–825. [12] M. Liu, R. Liu, W. Chen, Graphene wrapped Cu2O nanocubes: non-enzymatic electrochemical sensors for the detection of glucose and hydrogen peroxide with enhanced stability, Biosens. Bioelectron. 45 (2013) 206–212. [13] M. Chowdhury, F. Cummings, M. Kebede, V. Fester, Binderless solution processed Zn doped Co3O4 film on FTO for rapid and selective non‐enzymatic glucose detection, Electroanalysis 29 (2017) 578–586. [14] M. Rahman, A. Ahammad, J.-H. Jin, S.J. Ahn, J.-J. Lee, A comprehensive review of glucose biosensors based on nanostructured metal-oxides, Sensors 10 (2010) 4855–4886. [15] X. Niu, X. Li, J. Pan, Y. He, F. Qiu, Y. Yan, Recent advances in non-enzymatic electrochemical glucose sensors based on non-precious transition metal materials: opportunities and challenges, RSC Adv. 6 (2016) 84893–84905. [16] S.-L. Zhong, J. Zhuang, D.-P. Yang, D. Tang, Eggshell membrane-templated synthesis of 3D hierarchical porous Au networks for electrochemical nonenzymatic glucose sensor, Biosens. Bioelectron. 96 (2017) 26–32. [17] C. Chen, R. Ran, Z. Yang, R. Lv, W. Shen, F. Kang, et al., An efficient flexible electrochemical glucose sensor based on carbon nanotubes/carbonized silk fabrics decorated with Pt microspheres, Sens. Actuators B Chem. 256 (2018) 63–70. [18] Y. Liu, S. Liu, R. Chen, W. Zhan, H. Ni, F. Liang, An antibacterial nonenzymatic glucose sensor composed of carbon nanotubes decorated with silver nanoparticles, Electroanalysis 27 (2015) 1138–1143. [19] S. Nantaphol, T. Watanabe, N. Nomura, W. Siangproh, O. Chailapakul, Y. Einaga, Bimetallic Pt–Au nanocatalysts electrochemically deposited on boron-doped diamond electrodes for nonenzymatic glucose detection, Biosens. Bioelectron. 98 (2017) 76–82. [20] J. Lv, C. Kong, Y. Xu, Z. Yang, X. Zhang, S. Yang, et al., Facile synthesis of novel CuO/Cu2O nanosheets on copper foil for high sensitive nonenzymatic glucose biosensor, Sens. Actuators B Chem. 248 (2017) 630–638. [21] J. Huang, Y. Zhu, X. Yang, W. Chen, Y. Zhou, C. Li, Flexible 3D porous CuO nanowire arrays for enzymeless glucose sensing: in situ engineered versus ex situ piled, Nanoscale 7 (2015) 559–569.
4. Conclusions In summary, the hierarchical core-shell Co3O4/CuO NRA were synthesized on carbon cloth via a stepwise fabrication process including magnetron sputtering, anodic oxidation and chemical bath deposition. This core-shell nanostructure favors mass ion/electron transport and reversible redox reactions. The as-obtained sensor showed excellent electrochemical performance with a fast response time of 1.9 s, a low detection limit of 0.38 μM and a high sensitivity of 5405 μA mM−1 cm−2. These sensing properties are comparable to or better than many recently reported non-enzymatic glucose sensors. The results demonstrate that the Co3O4/CuO NRA/CC is promising for biosensing applications.
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Siyi Cheng received the B.S. degree from Beijing University of Chemical Technology in 2010, and the M.S. degree from Beijing University of Chemical Technology in 2014, he is working toward the Ph.D. degree in Mechanical Engineering in Huazhong University of Science and Technology. Currently he is working as a visiting graduate student in Department of Chemical Engineering at the University of California Berkeley. His research focuses on synthesis of semiconductor nanostructures for gas sensing and electrochemical applications. Steven DelaCruz received his B.S. in Chemical Engineering from University of NevadaReno in 2015. He is currently a graduate student researcher in Chemical Engineering at the University of California Berkeley. His research concerns the development and characterization of electrode materials for thermionic energy converters. Chen Chen received the B.Sc. degree in Engineering Machinery from Chang’an University in 2016. Currently She is working towards her Ph.D degree in Mechatronics Engineering at Huazhong University of Science and Technology. Her current research activities are the fabrication of micro/nano-materials. Zirong Tang is a professor in the State Key Laboratory for Digital Manufacturing Equipment and Technology at Huazhong University of Science and Technology. He obtained his Ph.D. in Material Science and Engineering in 2001 from the University of California, Irvine, USA. His current interests are in MEMS, electrochemical sensors, biosensors and micro/nanofabrication technologies. Tielin Shi is the director of the State Key Laboratory for Digital Manufacturing Equipment and Technology at Huazhong University of Science and Technology. He obtained his Ph.D in Mechanical Engineering in 1999 from Huazhong University of Science and Technology. His interests are in micro/nanofabrication technologies and developing advanced fabrication systems for microelectronic industry. Carlo Carraro is a researcher and a lecturer in the Department of Chemical and Biomolecular Engineering at the University of California, Berkeley. He received his bachelor's degree from the University of Padua, Padua, Italy, and his Ph.D. degree from California Institute of Technology in Pasadena, California. His research interests are in the physics and chemistry of surfaces and synthesis of novel thin-film materials and processes. Roya Maboudian is a Professor of chemical engineering at the University of California, Berkeley. She received her Ph.D. in applied physics from California Institute of Technology. Her current research interests are in the areas of surface/interfacial science and engineering of micro-/nanosystems, and thin-film science and technology.
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Hierarchical Co3O4/CuO nanorod array supported on carbon cloth for highly sensitive non-enzymatic glucose biosensing
T
Siyi Chenga,b,c, Steven DelaCruza,b, Chen Chenc, Zirong Tangc, Tielin Shic, Carlo Carraroa,b, ⁎ Roya Maboudiana,b, a
Berkeley Sensor & Actuator Center, University of California, Berkeley, CA 94720, United States Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720, United States c State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China b
ARTICLE INFO
ABSTRACT
Keywords: Hierarchical Co3O4@CuO Core-shell structure Carbon cloth Non-enzymatic biosensor Glucose sensor
A hierarchical core-shell Co3O4/CuO nanorod array (NRA) anchored on flexible carbon cloth (CC) has been fabricated through a stepwise process consisting of magnetron sputtering of Cu, its anodic oxidation, and chemical bath deposition of Co3O4. The structure, composition and morphology of the synthesized Co3O4/CuO NRA on CC were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The glucose sensing performance of the Co3O4/CuO NRA on CC electrode was investigated by cyclic voltammetry and chronoamperometry. The sensor exhibited a high sensitivity of 5405 μA mM−1 cm−2 with a fast response time (t90 = 1.9 s) and a low detection limit of 0.38 μM. The electrode also showed outstanding selectivity toward various interferences. These results indicate that the Co3O4/CuO nanocomposites may be promising electrode materials for electrochemical biosensing.
1. Introduction Rapid and accurate glucose detection can provide critical information in a variety of contexts, including diabetes monitoring, fermentation, and environmental protection [1–3]. To date, tremendous efforts have been made to develop glucose sensors with high sensitivity, good selectivity, fast response time, good reliability, and low cost [4,5]. Many methods have been developed for glucose detection, including ones based on electrochemical, fluorescence, surface plasmon resonance, and optical methods [3,6–8]. Among these, electrochemical methods have attracted significant attention due to such advantages as high sensitivity and selectivity, simple operation, and cost effectiveness [9,10]. Among different types of electrochemical glucose sensors, enzymatic glucose sensors, which catalyze glucose using glucose oxidase, show high sensitivity and selectivity [11]. However, low chemical stability, complicated immobilization processes and high cost limit the practical application of enzyme-based glucose sensors [12,13]. Hence, it is highly desirable to develop non-enzymatic sensors for the direct detection of glucose. Owing to great progress in nanotechnology, various nanostructures based on metals and metal oxides have been applied for the construction of non-enzymatic glucose sensors with high performance ⁎
[10,14,15]. Over the past decade, several noble metals, such as gold [16], platinum [17], silver [18] and their alloys [19], have been widely studied for glucose detection due to their high catalytic properties. However, their high cost, relatively low selectivity, and proneness to poisoning hinder their practical applications [20]. In contrast, transition metal oxides, such as CuO, NiO, Co3O4 and MnO2 have drawn increasing attention due to their low cost, enhanced selectivity, and high stability [21–24]. Among these nanomaterials, CuO possesses a high isoelectric point and excellent electrochemical performance. Hence, various CuO-based nanostructures, including nanowires [25], nanosheets [20], nanoflowers [26] and nanorods [27], have been fabricated for glucose sensing applications. However, the poor intrinsic electrical conductivity of CuO limits charge carrier transport, resulting in low sensitivity. Therefore, constructing an electrode from multiple nanomaterials and leveraging synergistic effects between its components is believed to be a promising way to improve the conductivity, electro-active surface area, and contact area between the sensing material and the substrate, leading to improved ion/electron transport inside the electrodes and better electrochemical performance. Recently, various CuO-based composite nanomaterials have been reported for glucose detection [20,28–30]. However, most of these studies have focused on the sensing performance of nanomaterials on glassy carbon
Corresponding author at: Berkeley Sensor & Actuator Center, University of California, Berkeley, CA 94720, United States. E-mail address: [email protected] (R. Maboudian).
https://doi.org/10.1016/j.snb.2019.126860 Received 15 March 2019; Received in revised form 21 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic illustration of the fabrication process for the Co3O4/CuO NRA on carbon cloth.
Fig. 1. (a) X-ray diffraction and (b) Raman spectra of the CuO NRA (black) and Co3O4/CuO NRA (red) on carbon cloth. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
electrodes, which may limit their practical applications. It was recently demonstrated that CuO-based nanostructures can be directly grown on conductive substrates without any binder [31]. This approach can reduce the resistance between the sensing materials and the substrate. Therefore, fabricating composite nanomaterials directly on conductive substrates may open a new avenue toward achieving enhanced glucose sensing performance. Herein, we report on the synthesis of a Co3O4/CuO hierarchical core-shell nanorod array (NRA) directly on flexible and conductive carbon cloth via a stepwise approach. The integration of Co3O4 nanoflakes on CuO nanorods can increase the electro-active surface area and improve the contact area between the sensing materials and the substrate, which lead to more active sites for redox reactions and faster reaction kinetics. We demonstrate that the composite nanostructures on carbon cloth exhibit high electrochemical performance with high sensitivity, fast response time, low detection limit, and good selectivity, thus proving promising for non-enzymatic glucose sensors.
2. Experimental 2.1. Fabrication of hierarchical Co3O4/CuO NRA/CC All chemicals were analytical grade and were used without further purification. Carbon cloth, purchased from Cetech Co. Ltd., was cleaned in acetone, ethanol and deionized (DI) water for 10 min each. After drying in air, carbon cloth was placed into a magnetron sputtering system. A Cu layer was sputtered on both sides of carbon cloth by using a magnetron sputtering system (TPR-450) at a power of 100 W for 30 min. Subsequently, the as-obtained sample was cut into 10 mm × 10 mm pieces and anodized in a 3 M NaOH solution at a current density of 10 mA cm−2 to form a Cu(OH)2 NRA on carbon cloth. Then, 0.075 M cobalt sulfate hydrate and 1.134 M urea were dissolved in 20 ml of DI water in a 25 mL vial and stirred to form a clear pink solution. The as-prepared Cu(OH)2 NRA/CC sample was immersed in the solution and held for 1 h at 85 °C in an oil bath. The sample was then taken out of the solution, washed with DI water, and annealed at 350 °C in air for 2 h to prepare hierarchical Co3O4/CuO NRA/CC. For comparison, the CuO NRA/CC sample was prepared by directly 2
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Fig. 2. (a) Survey scan, and (b) Co 2p, (c) Cu 2p and (d) O 1s regions of the XPS spectrum obtained on Co3O4/CuO NRA/CC.
annealing Cu(OH)2 NRA/CC in air at 350 °C.
formation mechanism of the Co3O4 shell can be described by the “selfassembly” and “oriented attachment” processes [33]. In our case, the Cu(OH)2 nanorod array acts as the backbone which guides the Co-based precursor deposition. XRD and Raman measurements were carried out to investigate the structure and composition of the as-synthesized materials. Fig. 1a shows representative XRD spectra obtained on CuO NRA and Co3O4/ CuO NRA on carbon cloths. The broad diffraction peaks at around 25.6° demonstrate the graphitized carbon phase originating from the carbon cloth. The peaks located at 43.2° can be ascribed to copper (JCPDS 040836), revealing that the as-deposited copper layer was not fully transformed into CuO during the anodic oxidation and annealing processes. The formation of CuO is evidenced by the peaks at 32.4, 35.5, 38.7, 48.9, 53.5, 58.2, 61.4, 66.3 and 67.9°, which correspond to the (110), (002), (111), (-202), (020), (202), (-113), (022) and (113) planes of the monoclinic CuO structure (JCPDS No. 89-5899), respectively [34]. The chemical bath deposition and calcination processes lead to the formation of the Co3O4/CuO NRA on carbon cloth. The peaks centered at 18.9, 31.1, 36.6 and 65.2° correspond to the (111), (220), (311) and (440) planes of the Co3O4 spinel phase (JCPDS No. 42‐1467), respectively [35]. The XRD result of the sample after 1 h immersion in 0.075 M cobalt sulfate hydrate and 1.134 M urea solution at 85 °C is shown in Figure S1. The peak located at 22.1° can be attributed to Co (OH)2 (JCPDS 04-0836), while the peaks located at 32.7, 38.8, and 45.7° can be ascribed to CoCO3 (JCPDS 11-692). The remaining peaks can be attributed to those of carbon cloth (25°), copper (43°), and Cu (OH)2. Therefore, the Co precursors appear to be in the form of Co(OH)2 and CoCO3, and this result corresponds to previous reported precursors for Co3O4 [36]. Raman spectroscopy was carried out to examine the difference between the CuO and the Co3O4/CuO core-shell nanostructures. Fig. 1b displays the Raman spectra of the CuO NRA and Co3O4/CuO NRA on carbon cloth. Two strong bands at 1325 and 1603 cm−1 can be indexed to the characteristic peaks of the D-band
2.2. Materials characterization The structures of the as-synthesized samples were characterized by X-ray diffraction (XRD, Bruker AXS D8 Discover with GADDS) using a Co Kα radiation source and by transmission electron microscopy (TEM) using a FEI Tecnai G2 S-TWIN. The chemical compositions of the samples were analysed by X-ray photoelectron spectroscopy (XPS, Omicron Dar400 system with an achromatic Al Kα X-ray source) and Raman spectroscopy (Horiba LabRAM confocal Raman spectrometer with an excitation laser of 632.8 nm). The morphologies and microstructures of the samples were probed by field-emission scanning electronic microscopy (FE-SEM Quanta 3D FEG). 2.3. Electrochemical measurements All electrochemical experiments were performed at room temperature with a CHI 600D electrochemical workstation. The electrochemical measurements were performed in 0.1 M NaOH using a conventional three-electrode configuration: carbon cloth samples as the working electrode, Ag/AgCl as the reference electrode and Pt wire as the counter electrode. For amperometric tests, the solution was stirred to facilitate mixing. 3. Results and discussion Scheme 1 illustrates the fabrication process of hierarchical Co3O4/ CuO core-shell structures on carbon cloth. A highly ordered Cu(OH)2 NRA was first synthesized on flexible carbon cloth via magnetron sputtering of copper and subsequent anodic oxidation [32]. Then chemical bath deposition and a follow-up calcination process were used to form a hierarchical Co3O4/CuO core-shell nanorod array. The 3
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Fig. 3. (a, b) SEM images of CuO NRA on carbon cloth with different magnifications. (c, d) SEM images of Co3O4/CuO NRA on carbon cloth with different magnifications. (e, f) TEM images of Co3O4/CuO NRA.
(indicative of the presence of defects in the hexagonal graphitic layers) and G-band (first-order scattering of the E2 g mode observed for sp2 carbon domains) of carbon cloth [37,38]. The Raman spectrum of CuO shows three peaks at 291, 347 and 623 cm−1, corresponding to the A g, B g(1) and B g(2) modes of CuO backbone, respectively [39]. As shown in the spectrum of the Co3O4/CuO NRA on carbon cloth, four peaks detected at around 190, 480, 520 and 680 cm−1 correspond to Eg, F2 g1, F2 g2 and A1g modes of Co3O4 [40]. After the growth of Co3O4, the relative peak intensity of CuO is reduced, suggesting that the CuO core is covered by the Co3O4 shell. The near-surface elemental composition and oxidation state of the Co3O4/CuO NRA/CC as well as the presence of absorbed species were analyzed by XPS. The survey spectrum (Fig. 2a) shows a series of characteristic peaks associated with copper, cobalt, oxygen and carbon elements. It is worth mentioning that the C 1s peak is weak, which may be due to the uniform growth of sensing material on the surface of the carbon substrate. In the high-resolution Co 2p XPS spectrum (Fig. 2b), the two characteristic peaks centered at 780.3 and 795.4 eV correspond
to Co 2p3/2 and Co 2p1/2, respectively, with a spin-orbit separation of around 15 eV [41,42]. The Gaussian fitted peaks positioned at the binding energies around 780.5 and 795.5 eV can be ascribed to Co3+, whereas the peaks at 781.6 and 797.4 eV confirm the presence of Co2+ [43,44]. In addition, the two shakeup satellite peaks located around 788.2 and 803.9 eV indicate that the cobalt has a spinel structure [45]. Fig. 2c displays the core level Cu 2p spectrum of the composite material. Two peaks located at the binding energies of 934.3 and 954.6 eV can be assigned to Cu 2p3/2 and Cu 2p1/2, while the shakeup satellite peak at 942.6 eV can be indexed to Cu2+ [46]. Fig. 2d shows the highresolution spectrum of the O 1s region, demonstrating the co-existence of three oxygen species marked as O1, O2 and O3. The O1 component located at a binding energy of 529.8 eV is consistent with the typical metal-oxygen bond, and the O2 component at a binding energy of 531.4 eV can be ascribed to the large density of defects with low oxygen coordination in the composite material [6]. The O3 component at a binding energy of 532.4 eV corresponds to multiplicity of physically absorbed and chemisorbed species onto and within the surface layer 4
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area than that of the CuO NRA/CC and Co3O4/CuO NRA/CC electrodes, and exhibits no redox peak after the addition of glucose (see enlarged CVs in Figure S3), implying that the contribution of the carbon cloth toward the detection of glucose can be neglected. The bare CC electrode exhibits a large oxidative tail corresponding to the onset of water breakdown observed for potentials above 0.65 V [48]. This behavior can also be observed in the CV curves of the CuO NRA/CC and Co3O4/ CuO NRA/CC electrodes. The CV result on the CuO NRA/CC shows a broad reduction peak at ˜0.42 V in the blank alkaline solution. This peak may be ascribed to the CuOOH/CuO transition [32]. However, the corresponding oxidation peak related to CuO/CuOOH is not clearly observed, which may be due to overshadowing by the oxidation peak of water-splitting [48,49]. After the addition of 1 mM glucose, an oxidation peak appears at ˜0.3 V, corresponding to the irreversible glucose oxidation reactions [32]. In comparison, on the Co3O4/CuO NRA/CC electrode, two pairs of redox peaks, one at 0.58 and 0.38 V, and one at 0.25 and 0.12 V, are observed prior to the addition of glucose, which can be ascribed to the Co3O4/CoOOH and CoOOH/CoO2 redox couples, respectively [50]. After the addition of 1 mM glucose, the oxidative peak at 0.58 V shift to 0.5 V and its magnitude increases, while the current at 0.25 V remains nearly constant. The oxidative peak at 0.5 V can be attributed to the transitions of CuO/CuOOH and CoOOH/CoO2, and the oxidative peak at 0.25 V can be assigned to the Co3O4/CoOOH transition [23]. This phenomenon indicates that the CoOOH/CoO2 couple on Co3O4 is the dominant redox reaction for the glucose catalysis process [50]. In the cathodic scan, the broad peak at 0.38 V may be attributed to the combination of CoO2/CoOOH and CuOOH/CuO transitions, while the broad peak at 0.12 V could be attributed to the Co3O4/CoOOH transition [31,51]. We hypothesize that the mechanism of the glucose oxidation in Co3O4/CuO NRA/CC electrode is similar to that of individual CuO and Co3O4 electrodes and can be explained as follows [51,52]:
Fig. 4. Cyclic voltammetry curves obtained on CC, CuO NRA/CC and Co3O4/ CuO NRA/CC in the absence and presence of 1 mM glucose in 0.1 M NaOH solution at a scan rate of 50 mV s−1.
[47]. These results clearly show that the surface of the as-synthesized Co3O4/CuO NRA contains Co2+, Co3+ and Cu2+ ions, indicating the composition consists of CuO and Co3O4. Carbon cloth, which consists of interconnected carbon fibers with an average diameter of ˜10 μm (Figure S2a), was used as the conductive substrate. As shown in Figure S2b, the bare carbon cloth possesses high flexibility under bending, making it a good substrate for flexible sensing applications. The morphologies of the CuO NRA and core-shell Co3O4/ CuO NRA on carbon cloth were examined by SEM. Fig. 3a shows that the CuO nanorods were grown uniformly and vertically on carbon fiber. The magnified SEM image (Fig. 3b) indicates that the as-synthesized CuO nanorods possess lengths of a few micrometers and an average diameter of ˜100 nm. Fig. 3c-d presents the SEM images of the coreshell Co3O4/CuO NRA under different magnifications. It can be seen that the deposited Co3O4 layer consists of nanoflakes which are interconnected to each other and form a highly ordered 3D nanostructure. The detailed structures of the Co3O4/CuO NRA were recorded by TEM. Fig. 3e shows the TEM image of the Co3O4/CuO NRA. It can be seen that the CuO nanorod is compactly wrapped by interconnected Co3O4 nanoflakes. Fig. 3f displays the TEM image of Co3O4 at high resolution, revealing lattice fringes with the interplanar gap of 0.24 nm, corresponding well to the (311) plane of Co3O4. Electrochemical tests for different working electrodes were performed in a three-electrode cell system. Fig. 4 displays the cyclic voltammograms (CVs) of different electrodes in 0.1 M NaOH solution in the absence and presence of 0.5 mM glucose under a scan rate of 50 mV s−1. The bare carbon cloth electrode shows a much smaller CV curve
Co3 O4 + OH + H2 O
CoOOH+ OH
CoO2 + H2 O+ e
2CoO2 + glucose CuO+ OH
3 CoOOH+ e
2 CoOOH+ gluconolactone
CuOOH+ e
2 CuOOH+ glucose
2 CuO+ gluconolactone+ H2 O
(1) (2) (3) (4) (5)
In addition, the oxidation and reduction peaks of the Co3O4/CuO NRA/CC electrode show higher intensity than those of the CuO NRA/ CC electrode, demonstrating that the composite electrode has better electrochemical behavior toward the electro-oxidation of glucose. The improved electrochemical performance can be ascribed to the large amount of Co3O4 nanoflakes firmly and uniformly fixed to the surface of the CuO NRA electrode, providing greater electroactive surface area for the detection of glucose.
Fig. 5. (a) CV responses of Co3O4/CuO NRA/CC electrode in 0.1 M NaOH with different scan rates. (b) Corresponding calibration curve of current vs. scan rate. 5
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Fig. 6. (a) Amperometric responses of the CuO NRA/CC (red) and Co3O4/CuO NRA/CC (black) electrodes under the successive addition of 50 μM glucose at the applied potential of 0.55 V. (b) Corresponding calibration curves for glucose detection. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (a) Amperometric response of the Co3O4/CuO NRA/CC electrode upon successive additions of glucose with select concentrations marked. Inset: the enlarged view in low concentrations. (b) Calibration curve for glucose of the Co3O4/CuO NRA/CC electrode.
much higher than that of CuO NRA/CC electrode (0.0035 m F/cm2), indicating the higher electrochemical surface area of the Co3O4/CuO NRA/CC electrode (Figure S4c). To better understand the electrochemical behavior of the Co3O4/ CuO NRA/CC electrode, CV tests were also conducted at different scan rates. Fig. 5a displays the CVs of the Co3O4/CuO NRA/CC electrode in the presence of 1 mM glucose with scan rates (υ) of 5, 10, 20, 30, 40, 50 mV s−1. It can be seen that the anodic (Jpa) and the cathodic (Jpc) peak current densities increase with increasing scan rate. The redox peak current shows good linearity with the scan rate, and the regression equations for the anodic and cathodic peak current can be expressed as Jpa(mA cm-2) = 1.16 + 0.53υ (mV s−1) (R2 = 0.999) and Jpc(mA cm-2) = -0.11 - 0.27υ (mV s−1) (R2 = 0.998), respectively. This linearity indicates that the electro-oxidation of glucose on the Co3O4/CuO NRA/ CC electrode is a diffusion-controlled process [56,57]. Fig. 6a shows the amperometric response curves of the CuO NRA/ CC and Co3O4/CuO NRA/CC electrodes after successive injection of glucose into 0.1 M NaOH under continuous stirring at an applied potential of 0.55 V. After glucose addition, the Co3O4/CuO NRA/CC electrode exhibits a larger response current compared to the CuO NRA/ CC electrode. The slope of the calibration curve (Fig. 6b) also confirms the higher sensitivity of the Co3O4/CuO NRA/CC electrode. The amperometric results are consistent with the CV results. The improved response current achieved with the Co3O4/CuO NRA/CC could be ascribed to the larger electro-active surface area, which provides more active sites for the oxidation of glucose. Thus, Co3O4/CuO core-shell nanostructures possess better electrochemical performance than the bare CuO nanorods, making them promising materials for the detection of glucose. Fig. 7a shows a typical amperometric response of the Co3O4/CuO NRA/CC electrode to the successive addition of glucose into 0.1 M NaOH with an applied potential of 0.55 V. After the injection of glucose,
Fig. 8. Amperometric responses of the Co3O4/CuO NRA/CC electrode to the successive addition of glucose and interference species.
The electrochemical surface area (ECSA) of the Co3O4/CuO NRA/CC and CuO NRA/CC electrodes were estimated through electrical doublelayer capacitance (EDLC) tests in 0.1 M NaOH solution as the doublelayer capacitance (Cdl) is proportional to the ECSA [53]. The CV plots were obtained within ± 0.05 V of open circuit potential, where the current response should only be due to the charging of the double layer [54]. The CV curves of Co3O4/CuO NRA/CC and CuO NRA/CC electrodes at various scan rates (in 2–10 mV/s range) are shown in Figures S4a and b, respectively. At each scan rate, the Co3O4/CuO NRA/CC electrode exhibits much higher anodic (ja) and cathodic (jc) current densities than the CuO NRA/CC electrode. As shown in the reported works, the linear slope in such plots is equal to 2Cdl [55]. The calculated capacitance of the Co3O4/CuO NRA/CC electrode (0.03 m F/cm2) is 6
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the current increases, reaching 90% of its steady-state value (t90) in 1.9 s (Figure S5), demonstrating the rapid response of the Co3O4/CuO core-shell structure. The corresponding calibration curve of the Co3O4/ CuO NRA/CC electrode is displayed in Fig. 7b. The peak current increases linearly with increasing glucose concentration within the range of 1–500 μM (R2 = 0.997), and the electrode demonstrates a high sensitivity of 5405 μA mM−1 cm−2 and a low detection limit of 0.38 μM (S/N = 3). When the glucose concentration is above 500 μM, the slope of the fitting curve is slightly reduced (Figure S6), which may be ascribed to a condition where the glucose diffusion rate exceeds the consumption rate at high glucose concentration [58]. The Co3O4/CuO NRA/CC electrode exhibits excellent glucose sensing performance, with the highest sensitivity, lower detection limit and response time, and a comparable linear range, when compared to the performance of recently reported CuO and Co3O4 based glucose sensors (Table S1). The superior performance can be attributed to the following aspects: (1) The Co3O4 shell can act as a stable matrix to encapsulate and connect the core CuO nanorods, thereby generating more channels for electron transport between the CuO nanorods and the substrate; (2) The uniform hierarchical Co3O4/CuO core-shell nanostructures can offer electro-active sites for glucose oxidation; (3) The core-shell structures are supported on highly conductive carbon cloth without binder, which can further improve the electron transportation efficiency. Selectivity is one of the most important aspects for judging the performance of a glucose sensor. The selectivity of the Co3O4/CuO NRA/CC electrode was investigated by adding sucrose, fructose, uric acid (UA), ascorbic acid (AA), and dopamine (DA) into the 0.1 M NaOH solution at a potential of 0.55 V. Additionally, the influence of chloride ions was studied because they can poison non-enzymatic glucose sensors and lower the activity significantly [20]. The ratio of glucose and interferences in the human blood is more than 30:1 at a physiological level [59]. In order to evaluate the selectivity of the glucose sensor, a 10:1 concentration ratio of glucose to interferences was used to simulate the physiological level. Herein, the interference test was conducted by first adding 1 mM glucose and then adding 100 μM of the various interfering substances sequentially. As shown in Fig. 8, with the addition of glucose, the current density increased significantly, whereas negligible current responses can be observed following the injection of the interferences. Therefore, it can be concluded that the Co3O4/CuO NRA/CC electrode possesses excellent selectivity as a non-enzymatic glucose sensor. The stability of the Co3O4/CuO NRA/CC electrode was evaluated by measuring the amperometric current response of Co3O4/ CuO NRA/CC electrode with the addition of 0.1 mM glucose during a period of 20 days at a time interval of 2 days, as shown in Figure S7. After 20 days storage in air, the Co3O4/CuO NRA/CC electrode maintained approximately 97.6% of its original current response, implying a good stability of the Co3O4/CuO core-shell nanostructure. The Co3O4/ CuO NRA /CC electrode shows better stability than some recently reported CuO based glucose sensors [60–62].
Acknowledgements The authors thank Jinhu Fan for help with the magnetron sputtering. The support of the industrial members of the Berkeley Sensor & Actuator is gratefully acknowledged. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. S.C. acknowledge additional support through the China Scholarship Council. C.C., Z.T. and T.S. acknowledge the National Basic Research Program of China with Project No. 2015CB057205, the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT13017), and the National Science Foundation of China (No. 51775218). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126860. References [1] E. Vargas, M. Ruiz, S. Campuzano, A. Reviejo, J. Pingarrón, Non-invasive determination of glucose directly in raw fruits using a continuous flow system based on microdialysis sampling and amperometric detection at an integrated enzymatic biosensor, Anal. Chim. Acta 914 (2016) 53–61. [2] A.J. Bandodkar, J. Wang, Non-invasive wearable electrochemical sensors: a review, Trends Biotechnol. 32 (2014) 363–371. [3] W. Wu, N. Mitra, E.C. Yan, S. Zhou, Multifunctional hybrid nanogel for integration of optical glucose sensing and self-regulated insulin release at physiological pH, ACS Nano 4 (2010) 4831–4839. [4] D. Bruen, C. Delaney, L. Florea, D. Diamond, Glucose sensing for diabetes monitoring: recent developments, Sensors 17 (2017) 1866. [5] D. Rodbard, Continuous glucose monitoring: a review of successes, challenges, and opportunities, Diabetes Technol. Ther. 18 (2016) S2-3-S2-13. [6] S. Liu, K. Hui, K. Hui, Flower-like copper cobaltite nanosheets on graphite paper as high-performance supercapacitor electrodes and enzymeless glucose sensors, ACS Appl. Mater. Interfaces 8 (2016) 3258–3267. [7] H. Yuan, W. Ji, S. Chu, S. Qian, F. Wang, J.-F. Masson, et al., Fiber-optic surface plasmon resonance glucose sensor enhanced with phenylboronic acid modified Au nanoparticles, Biosens. Bioelectron. 117 (2018) 637–643. [8] J. Chang, H. Li, T. Hou, W. Duan, F. Li, Based fluorescent sensor via aggregation induced emission fluorogen for facile and sensitive visual detection of hydrogen peroxide and glucose, Biosens. Bioelectron. 104 (2018) 152–157. [9] J. Kim, A.S. Campbell, J. Wang, Wearable non-invasive epidermal glucose sensors: a review, Talanta 177 (2018) 163–170. [10] D.-W. Hwang, S. Lee, M. Seo, T.D. Chung, Recent advances in electrochemical nonenzymatic glucose sensors–a review, Anal. Chim. Acta (2018). [11] J. Wang, Electrochemical glucose biosensors, Chem. Rev. 108 (2008) 814–825. [12] M. Liu, R. Liu, W. Chen, Graphene wrapped Cu2O nanocubes: non-enzymatic electrochemical sensors for the detection of glucose and hydrogen peroxide with enhanced stability, Biosens. Bioelectron. 45 (2013) 206–212. [13] M. Chowdhury, F. Cummings, M. Kebede, V. Fester, Binderless solution processed Zn doped Co3O4 film on FTO for rapid and selective non‐enzymatic glucose detection, Electroanalysis 29 (2017) 578–586. [14] M. Rahman, A. Ahammad, J.-H. Jin, S.J. Ahn, J.-J. Lee, A comprehensive review of glucose biosensors based on nanostructured metal-oxides, Sensors 10 (2010) 4855–4886. [15] X. Niu, X. Li, J. Pan, Y. He, F. Qiu, Y. Yan, Recent advances in non-enzymatic electrochemical glucose sensors based on non-precious transition metal materials: opportunities and challenges, RSC Adv. 6 (2016) 84893–84905. [16] S.-L. Zhong, J. Zhuang, D.-P. Yang, D. Tang, Eggshell membrane-templated synthesis of 3D hierarchical porous Au networks for electrochemical nonenzymatic glucose sensor, Biosens. Bioelectron. 96 (2017) 26–32. [17] C. Chen, R. Ran, Z. Yang, R. Lv, W. Shen, F. Kang, et al., An efficient flexible electrochemical glucose sensor based on carbon nanotubes/carbonized silk fabrics decorated with Pt microspheres, Sens. Actuators B Chem. 256 (2018) 63–70. [18] Y. Liu, S. Liu, R. Chen, W. Zhan, H. Ni, F. Liang, An antibacterial nonenzymatic glucose sensor composed of carbon nanotubes decorated with silver nanoparticles, Electroanalysis 27 (2015) 1138–1143. [19] S. Nantaphol, T. Watanabe, N. Nomura, W. Siangproh, O. Chailapakul, Y. Einaga, Bimetallic Pt–Au nanocatalysts electrochemically deposited on boron-doped diamond electrodes for nonenzymatic glucose detection, Biosens. Bioelectron. 98 (2017) 76–82. [20] J. Lv, C. Kong, Y. Xu, Z. Yang, X. Zhang, S. Yang, et al., Facile synthesis of novel CuO/Cu2O nanosheets on copper foil for high sensitive nonenzymatic glucose biosensor, Sens. Actuators B Chem. 248 (2017) 630–638. [21] J. Huang, Y. Zhu, X. Yang, W. Chen, Y. Zhou, C. Li, Flexible 3D porous CuO nanowire arrays for enzymeless glucose sensing: in situ engineered versus ex situ piled, Nanoscale 7 (2015) 559–569.
4. Conclusions In summary, the hierarchical core-shell Co3O4/CuO NRA were synthesized on carbon cloth via a stepwise fabrication process including magnetron sputtering, anodic oxidation and chemical bath deposition. This core-shell nanostructure favors mass ion/electron transport and reversible redox reactions. The as-obtained sensor showed excellent electrochemical performance with a fast response time of 1.9 s, a low detection limit of 0.38 μM and a high sensitivity of 5405 μA mM−1 cm−2. These sensing properties are comparable to or better than many recently reported non-enzymatic glucose sensors. The results demonstrate that the Co3O4/CuO NRA/CC is promising for biosensing applications.
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Siyi Cheng received the B.S. degree from Beijing University of Chemical Technology in 2010, and the M.S. degree from Beijing University of Chemical Technology in 2014, he is working toward the Ph.D. degree in Mechanical Engineering in Huazhong University of Science and Technology. Currently he is working as a visiting graduate student in Department of Chemical Engineering at the University of California Berkeley. His research focuses on synthesis of semiconductor nanostructures for gas sensing and electrochemical applications. Steven DelaCruz received his B.S. in Chemical Engineering from University of NevadaReno in 2015. He is currently a graduate student researcher in Chemical Engineering at the University of California Berkeley. His research concerns the development and characterization of electrode materials for thermionic energy converters. Chen Chen received the B.Sc. degree in Engineering Machinery from Chang’an University in 2016. Currently She is working towards her Ph.D degree in Mechatronics Engineering at Huazhong University of Science and Technology. Her current research activities are the fabrication of micro/nano-materials. Zirong Tang is a professor in the State Key Laboratory for Digital Manufacturing Equipment and Technology at Huazhong University of Science and Technology. He obtained his Ph.D. in Material Science and Engineering in 2001 from the University of California, Irvine, USA. His current interests are in MEMS, electrochemical sensors, biosensors and micro/nanofabrication technologies. Tielin Shi is the director of the State Key Laboratory for Digital Manufacturing Equipment and Technology at Huazhong University of Science and Technology. He obtained his Ph.D in Mechanical Engineering in 1999 from Huazhong University of Science and Technology. His interests are in micro/nanofabrication technologies and developing advanced fabrication systems for microelectronic industry. Carlo Carraro is a researcher and a lecturer in the Department of Chemical and Biomolecular Engineering at the University of California, Berkeley. He received his bachelor's degree from the University of Padua, Padua, Italy, and his Ph.D. degree from California Institute of Technology in Pasadena, California. His research interests are in the physics and chemistry of surfaces and synthesis of novel thin-film materials and processes. Roya Maboudian is a Professor of chemical engineering at the University of California, Berkeley. She received her Ph.D. in applied physics from California Institute of Technology. Her current research interests are in the areas of surface/interfacial science and engineering of micro-/nanosystems, and thin-film science and technology.
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