Au Hierarchically Nanostructured Electrode

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A Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Non-enzymatic Glucose Sensing Yingying Su, Binbin Luo, and Jin Z. Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03396 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016

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Analytical Chemistry

A Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Non-enzymatic Glucose Sensing Yingying Su†, ‡,* , Binbin Luo‡ and Jin Zhong Zhang‡,*

†

‡

Analytical & Testing Center, Sichuan University, Chengdu 610064, China Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA

95064, USA

1

ACS Paragon Plus Environment


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ABSTRACT By electrodeposition and galvanic replacement reaction, we developed a facile, time-saving, cost-effective, and environmentally friendly, two-step synthesis route to obtain a controllable cobalt oxide/Au hierarchically nanostructured electrode for glucose sensing. The nanomaterials were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), while the sensing performance was investigated by cyclic voltammetry (CV) and amperometric response. The results revealed that this novel electrode exhibited excellent electrocatalytic performance toward glucose oxidation, with a wide double-linear range from 0.2 µM to 20 mM and a low detection limit of 0.1 µM based on a signal to noise (S/N) ratio of 3, which was mainly attributed to the ability of loading small amount of Au with good electron conductivity on the surface of cobalt oxide nanosheets with large active surface area and synergistic electrocatalytic activity of Au and cobalt oxide towards glucose electrooxidation. This facile, sensitive and selective glucose sensor is also proven to be suitable for the detection of glucose in human serum.

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INTRODUCTION Diabetes mellitus is a worldwide public health problem. The diagnosis and management of diabetes mellitus requires testing blood glucose levels one to several times in a day. At present, conventional glucose sensors on the market are mainly based on glucose oxidase (GOD) assisted electrochemical oxidation. Although the GOD-based sensors have the advantages of high sensitivity and selectivity, they still suffer from some drawbacks such as high cost of enzymes, need of a complex and tedious enzyme immobilization process, and instability due to the inherent fragility of enzymes. As a result, non-enzymatic glucose sensors based on direct electrocatalytic oxidation are expected to replace GOD based sensor by using versatile materials with ultrahigh electrocatalytic activity. To date, various noble metals (Au, Pd), their metal alloys (Pt-Au), transition metals (Ni, Cu) and their oxide (NiO, CuO, Co3O4) have been extensively used in non-enzymatic glucose sensing.1-11 Nevertheless, noble metals and their metal alloys are still not the suitable candidates for the mass production of sensors due to their high prices in terms of cost-effectiveness. Despite being cheaper, two major weaknesses of transition metal and their oxide enormously impede them from wide use. One is that they can form close-packed structures after they are modified on an electrode surface, which will reduce their specific surface area and weaken their electrochemical performance. The other is that the electronic conductivity of metal oxides is poor. To alleviate the first problem, different supporting materials (e.g. 3



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carbon nanotubes, graphene or porous carbon) have been developed to load the metal or metal oxide nanostructure to attain the larger exposed surface area and the faster electron transfer.12-15 For the second problem, except for loading metal oxide on porous or large surface supporting materials, to design composite materials by combining highly electrocatalytic materials with a conductive substance have been proved to be a neat solution.16-18 Among the transition metal oxides, cobalt oxide (Co3O4) with low cost, environmentally friendly nature, intriguing electronic, optical, electrochemical, and electrocatalytic properties have demonstrated great potential in the applications of supercapacitors, catalysts, electrochemical sensors, and Li-ion rechargeable batteries. 8,15,19

However, it had not been explored for non-enzymatic electrochemical glucose

sensing until the first study by Ding et al. in 2010.5 In their work, Co3O4 nanofibers (NFs) were fabricated by a two-step synthesis route (electrospinning and subsequent calcination), then the Co3O4 nanofibers suspension was casted onto the surface of glassy carbon electrode (GCE) and then entrapped with Nafion. The Co3O4 NFs-based glucose sensor showed a fast response within 7 s but a linear range is only up to 2.04 mM, which is lower than that in normal human blood glucose level. Afterwards, Co3O4 with different shapes (nanofiber,5 3D multilayer porous network structure,10 microspheres

with

free-standing

nanofibers8,20)

or

different

Co3O4-based

nanocomposites (graphene/Co3O4, Co3O4/PbO2 core-shell nanorod, Co3O4-ordered mesoporous carbon are being developed for non-enzymatic glucose sensing.12,13,20 In these non-enzymatic glucose sensor studies, there are two strategies to prepare 4


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electrodes. One is casting the as-prepared Co3O4 onto the surface of GCE, and then entrapping them with Nafion or dropping Co3O4 Nafion/water suspension on to GCE, and then drying in air to obtain modified electrodes.21 The preparation process of these electrodes is non-trivial and the active materials could fall off from the electrodes easily. The other strategy is growing Co3O4 to the substrates (graphene-coated micropipette, carbon cloth, solid/nanoporous gold) directly. Although it provides an accessible means without the pretreatment and modification of the electrode, the synthesis strategies of Co3O4 generally require high reaction temperature, expensive equipment, complex steps, and surfactants or harmful organic reagents, which might further hinder their application. Therefore, it is highly desirable to explore facile synthesis strategies which are easy to operate, time-saving, cost-effective, and environmentally friendly (without any organic reagent, surfactant and Nafion) to grow Co3O4 on the conductive substrate directly. Galvanic replacement is a simple, low-cost, well-established and non-hazardous chemical way to fabricate metal (especially noble metal) nanostructures. Through a galvanic replacement reaction between a cobalt sacrificial template and gold ions, hollow gold nanostructures (HGNs) were first synthesized by Liang et al.22 Subsequently, the synthesis and application of HGNs was continually studied by our group.23-25 Herein, by combining electrodeposition with the above galvanic replacement reaction, we have developed a facile, time-saving, cost-effective, and environmentally friendly, two-steps synthesis rout to directly obtain a controllable cobalt oxide/Au hierarchically nanostructured electrode for glucose sensing. Firstly, 5



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flower-like cobalt particles formed directly on FTO by cyclic voltammetry. Then, cobalt particles deposited on FTO were put into chlorauric acid solution to form cobalt oxide/Au composite by galvanic replacement reaction. The advantage of this method is the ability of loading low amount of Au with good electron conductivity on the surface of cobalt oxide nanosheet with high active surface area, which not only cuts the cost in the case of expensive and rare Au but also improves the transportation of electrons between glucose and cobalt oxide to greatly enhance the sensitivity of the sensor. Additionally, owing to the synergistic electrocatalytic activity of Au and cobalt oxide towards glucose electrooxidation, more negative onset potential effectively avoids the interferences of endogenous species (such as uric acid (UA), dopamine (DA) and ascorbic acid (AA)) which are usually eliminated by adding Nafion. This facile, sensitive and selective glucose sensor is also proven to be suitable for the detection of glucose in human serum.

EXPERIMENTAL SECTION Chemicals and materials. Cobalt sulfate heptahydrate (>99%), L-ascorbic acid (AA), dopamine (DA) and uric acid (UA) were purchased from Sigma Aldrich, boric acid (99.5%), chloroauric acid trihydrate (ACS reagent grade), sodium hydroxide and D-glucose (C6H12O6) were obtained from Fisher Scientific. All water used in the experiments was 18 MΩ Milli-Q filtered. Fluorine doped tin oxide coated glass slide (FTO, L×W×D 100 mm×100 mm×2.3 mm, surface resistivity ～ 7 Ω/sq) was purchased from Sigma Aldrich and cut small parts (25 mm×6.25 mm×2.3 mm) evenly. 6


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A human serum sample was purchased form Sigma Aldrich and a whole blood sample was provided by School Infirmary of Sichuan University. The human whole blood was pretreated by centrifugation to get serum. In the centrifugation process, the plasma was separated from 2.0 mL of the whole blood by centrifugation at 3000 rpm for 10 min at 4˚C. The supernatant (serum) was collected for further experiments, and the pellet was discarded.

Apparatus

and

electrochemical

measurements.

Low

resolution

electron

microscopy was performed on an FEI Quanta 3-D dual bean microscope with accelerating voltages of 5.00 and 30.00 kV at the Keck Center of the Nanoscale Optofluidics of the University of California Santa Cruz. The elemental composition was analyzed with EDS (JSM-5900LV). Raman spectrum was collected using a Raman spectrometer (Renishaw) with a laser of 633 nm wavelength. The XPS was acquired on a XSAM 800 Electron Spectrometer (Kratos) using monochromatic Al Ka radiation. All electrochemical experiments were conducted on an electrochemical measurement unit (Solarton S1 1280B). A three-electrode system consisted of FTO modified with cobalt oxide/Au composite as the working electrode, an Ag/AgCl (with saturated KCl) electrode as the reference and a platinum wire as the auxiliary electrode. In this work, all potentials were referred to the Ag/AgCl elecrode. Amperometric measurements were carried out at an appropriate potential on the working electrode by successive injection of glucose into 0.1 M NaOH under stirring.

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Preparation of the Cobalt oxide/Au nanostructures arrays electrode. Our electrode fabrication strategy briefly illustrated in Figure 1 is to combine the electrodeposition and galvanic replacement reaction methods, through which the preparation of flower-like cobalt particles and cobalt oxide/Au hierarchical nanostructures modified on FTO (25 mm×6.25 mm×2.3 mm) are consecutively carried out. Typically, the FTO glass was washed with acetone, ethanol and twice-distilled water for 10 min in an ultrasonic cleaner, respectively. An aqueous solution prepared from 0.1M CoSO4 was used as the electrolyte and added 0.025 M H3BO3 as a buffering agent and the pH value was 5.4. Then the air of the solution was removed by high purity nitrogen gas for at least 10 min prior to electrolysis and maintained

under

nitrogen

atmosphere

during

experiments.

The

cobalt

elecrodeposition was performed by applying a cyclic voltammogram (CV) in the potential range of -0.2 to -1.2 V at 20 mV/s for 20 cycles. After rinsed with water, the strips were immersed in 1.0 mM HAuCl4 solution for 30 min at 50°C. These prepared cobalt oxide/Au hierarchical nanostructures were rinsed by water and dried in air.

RESULTS AND DISCUSSION Characterization of the films of cobalt, cobalt oxide and cobalt oxide/Au. The cobalt particles were first deposited on FTO glass surface using a cyclic voltammetric method. Cobalt oxide/Au hierarchical nanostructures were subsequently prepared 8


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using the reaction of HAuCl4 aqueous solution with the cobalt particles. Figure 1A and Figure 1B show typical SEM images of FTO surface and flower-like cobalt particles on FTO glass stripe, respectively. The cobalt particles were sparsely and randomly distributed on the surface of substrate without aggregation. The diameter of flower-like cobalt was approximate 700 nm. These results indicate that the electrochemical reduction is regarded as a facile synthesis method to obtain large sized particles immobilized on solid substrate without any organic stabilizers and linkers. The SEM image of cobalt oxide/Au was shown in Figure 1C. The vertical nanosheets with irregular nanoparticles structured material were obtained.

The TEM

image also indicates that the Co3O4/Au hierarchically nanostructured material is composed of mainly nanoparticles with relatively uniform distribution and a small amount of nanosheets (Figure 2A). The high-resolution TEM was employed to further characterize the nanoparticles. As can be seen from Figure 2C, which was obtained from an area of Figure 2B, the nanoparticles with a lattice spacing of about 0.23 nm correspond to the (111) planes of Au while nanoparticles with a lattice spacing of 0.26 nm correspond to the interspacing of the (311) planes of Co3O4 . Based on over 20 TEM images, most Au nanoparticles are less than 5 nm in diameter, even though there are a few that are larger than 10 nm (Figure 2D). In order to further characterize the materials, Raman Spectroscopy, EDS and XPS

measurements

were

performed.

Firstly,

the

material

obtained

by

electrodeposition was measured using Raman Spectroscopy as soon as possible (Figure 3, curve a). No obvious peaks could be found, indicating that the material is 9



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likely mainly cobalt metal. However, when we repeated the same measurement after the material was exposed to air for 6 hours, two weak peaks at 483 and 683cm−1 were observed, as shown in Figure 3 (curve b) and could be attributed to cobalt oxide, indicating that a small amount of cobalt was oxidized in the air. For the material obtained by the electrodeposition and galvanic replacement reaction successively, five characteristic peaks located at 193, 475, 516, 615, and 675 cm−1 were observed, which result from the F2g, Eg, F2g, F2g , and Ag vibrational modes of the crystalline Co3O4 (Figure 3, curve c).26 EDS was employed to investigate the composition of the two materials (Figure 3). Figure 4A clearly shows peaks corresponding to Co, O and Sn, in which Co-related peaks came from Co, Sn/O-related came from the SnO2, which is main composition of FTO. The atomic percentage of Co is 88.41%, which suggests cobalt is the principal product of the material obtained by electrodeposition. Apart from Co, O and Sn, Figure 4B reveals peaks corresponding Au, and the atomic percentage of O being greatly increased (from 7.76 to 43.35) than that in Figure 4A. Thus, the results indicate that Au can be obtained by the galvanic replacement reaction and Co3O4 can be obtained by oxidization of cobalt in the water bath. XPS is an effective technique to gain information about surface element composition, metal oxidation states, and adsorbed species of a solid material. In the XPS survey spectrum (Figure 5A) of Co3O4/Au nanostructure arrays, a series of sharp features corresponding to the characteristic peaks of C 1s, O 1s, Co 3s, Co 3p, and Co 2p, Au 4f indicate the existence of carbon, oxygen, cobalt and gold elements. Among 10


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them, carbon likely came from some contamination in the sample handling process. In the high resolution Co 2p XPS spectrum (Figure 5B), two characteristic peaks centered at 780.6 and 796.1eV correspond to the Co 2p 3/2 and Co 2p 1/2 spin-orbit peaks of the Co3O4 phase, respectively.27 The high resolution O 1s spectrum (Figure 5C) shows that the medium binding energy (531±0.1eV) is associated with O2- ions in oxygen-deficient regions within the matrix of Co3O4.19 As shown in Figure 5D, The Au 4f XPS spectra showed two sets of Au 4f 7/2 and Au 4f 5/2 signals. The ones at BE = 83.5 and 87.2 eV could be ascribed to the surface metallic gold (Au 0).28 Electrochemical characterization of the different electrodes. The electrochemical behaviors of the different electrodes were characterized by CV. CVs of bare FTO, the prepared CoOX/FTO and Au/ Co3O4/FTO electrodes were first investigated in neutral solution (PBS buffer, pH 7.4), no obvious peaks can be obtained, consistent with a previous report.5 When studied in alkaline solution (0.1 M NaOH), two pairs of well-defined redox peaks were observed at CoOx/FTO and Au/ Co3O4/FTO electrodes, as shown in Figure 6 (curve b and c), while no obvious response was found at the bared FTO electrode (Figure 6, curve a), indicating the necessity of OH- in the electrochemical redox reaction of Co3O4. Meanwhile, the peaks obtained with the Au/Co3O4/FTO electrode are much higher than those of the CoOx/FTO electrode, which can be attributed to enhanced conductivity by Au. Similar to the previous report, the two pairs of redox peaks (OI/RI and OII/RII) are presumably result of the reversible transformation between Co3O4 and CoOOH (OI/RI) and further conversion between CoOOH and CoO2(OII/RII), respectively.29 11



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In addition, we studied the effect of scan rates on CV responses (Figure 7). Both redox peak currents increased with the increase of scan rate. And the redox peak currents increased linearly with the scan rate within the range from 10 to 200 mV/s, indicating a surface-controlled electrochemical process.12 Electrooxidation behavior of glucose at the cobalt oxide and cobalt oxide/Au electrodes. To further study the electrocatalytic activity of CoOx/FTO and Au/Co3O4/FTO electrodes for glucose oxidation, CVs for different concentrations of glucose were acquired. The significant oxidation of glucose starts at ca. 0.40V with the CoOx/FTO electrode for glucose oxidation, an obvious increase of ipa at peak OII was observed in 0.1 M NaOH solution (Figure 8A, curve b) upon the addition of 0.01 mM glucose ( Figure 8A, curve c). Further, the ipa values increased with the increase of glucose concentration from 0.01 mM to 0.1 mM (Figure 8A, curves c-f). While for the Au/Co3O4/FTO electrode, the significant oxidation of glucose starts at ca. 0.25V, which is more negative, an obvious increase of ipa at peak OI and OII was observed in 0.1 M NaOH solution (Figure 8B, curve b) upon the addition of 0.01 mM glucose (Figure 8B, curve c). Further, the ipa values increased with the increase of glucose in the concentration range of 0.01 mM to 0.1 mM (Figure 8B, curves c-f). However, there was no obvious current change for the bare FTO in 1.0 mM glucose (Figure 8A and B, curve a). These results indicate the excellent electrocatalytic activity of the Co3O4 for glucose oxidation. It is widely accepted that glucose can be oxidized to gluconolactone through a two-electron electrochemical reaction.30 The mechanism of electrochemical oxidation of glucose catalyzed by Co3O4 has been proposed to be that 12


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Co3O4 was oxidized to CoOOH, and then CoOOH was further oxidized to CoO2 which oxidized glucose to generate gluconolactone and CoOOH (Eq. (1)-(3)).10 Thus, the ipa increases with the addition of glucose. Co3 O4 + OH − + H 2 O ↔ 3CoOOH + e −

(1)

CoOOH + OH − ↔ CoO2 + H 2 O + e −

(2)

2CoO2 + C 6 H 12 O6 ↔ 2CoOOH + C 6 H 10 O6

(3)

Amperometric detection of glucose. To obtain optimal amperometric response to glucose, the effect of different applied potentials on the response current of Au/Co3O4/FTO was investigated. The amperometric response of the electrode to injection of 0.1 mM glucose into the continuously stirred 0.1M NaOH at different applied potentials increased slightly from 0.30 V to 0.60 V. High potential can activate interfering substances and generate many intermediates that can interact with the electrode and result in interference for the determination of glucose. As a result, the optimal potential of 0.30 V was chosen as the working potential in the following experiments. While for CoOX/FTO, the oxidation current of glucose increased sharply at 0.60 V. Therefore, 0.60 V was chosen as the working potential for this electrode. The relationship between the amperometric currents of glucose and the thickness of the film of Au/Co3O4 was also investigated. The thickness of the film was controlled by applying a cyclic voltammogram (CV) in the potential range of -0.2 to -1.2 V at 20 mV/s for different cycles from 5 cycles to 50 cycles. The amperometric currents increased with increasing cycles. However, the film peels easily when the 13



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cycles are over 20. Therefore, 20 cycles were selected as the optimal electro-deposition parameter. The thickness of the film is 221.7±13.2 nm (n=10) which can be obtained by the SEM image of the side of the electrode. Amperometric measurements were carried out at 0.60V on CoOX/FTO and 0.30 V on Au/Co3O4/FTO by successive injection of glucose (Figure 9A, curve a and b) into 0.1 M NaOH under stirring, respectively. There were two linear ranges for the two kinds of electrodes, respectively. For CoOX/FTO, one is from 2 µM to 0.2 mM (R2=0.9931), the other is from 1 mM to 20 mM (R2=0.9967) (Figure 9B, C). The detection limit was estimated to be 1 µM (S/N=3).The limit of detection (LOD) was calculated as LOD=3s/m (s is standard deviations which was estimated from a small segment of the amperometric response curve (Fig. 9A), obtained before the first addition of glucose, and m is the slope of the calibration curve at low glucose concentrations. For Au/Co3O4/FTO, one is from 0.2 µM to 0.2 mM (R2=0.9999), and the other is from 0.5 mM to 20 mM (R2=0.9946) (Figure 9B, C). The detection limit was estimated to be 0.1 µM (S/N=3), which is lower than 1 µM for CoOX/FTO. Additionally, the amperometric responses of cobalt oxide and cobalt oxide/Au for the oxidation of glucose were investigated (Table 1). The performance of various cobalt oxide or Au nonenzymatic glucose sensors reported was also shown in Table 1. Compared to the other reported glucose sensors based cobalt oxide and gold, the sensitivities of the two kinds of glucose sensors in this present work are excellent. The comparison between cobalt oxide and cobalt oxide/Au shows that cobalt oxide/Au exhibited a better catalytic activity and sensitivity for the oxidation of glucose. The 14


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good performance likely results from excellent conductivity of Au and synergistic electrocatalytic activity of Au and cobalt oxide. The vertical array structure of cobalt oxide/Au avoided formation of close-packed structure on the surface and increased the effective loading of the cobalt oxide catalyst to some extent. As is well known, diabetes mellitus is diagnosed by a blood glucose concentration, which is higher than the normal range of 4.4～6.6 mM. The sensor developed by us can be used to evaluate the level of blood glucose without dilution. Moreover, the Au with more negative oxidation potential for glucose oxidation can avoid much interference in real samples. Additionally, in this work, the preparation procedure of the sensors is simple, fast (within 1 hour), and does not involve any complicated processes or use of any organic solutions. These features are important for practical glucose detection applications.

Interference study. As is well known, chloride can poison electrodes in many cases. So the anti-poison study should be done in order to develop a glucose sensor. Various amounts of chloride ions in the range of 25-500 mM were added to the standard solutions of glucose for at least 5 minutes to evaluate the interference. The result indicated that the chloride ion does not significantly interfere with the measurement even when its concentration is as high as 500 mM. Thus, the method is suitable for human blood serum samples containing chloride ions up to 500 mM. Additionally, some easily oxidative species, such as AA, UA and DA, co-exist with glucose in human blood serum and their concentrations are generally less than 0.1 mM. Thus, the interference effect of AA, UA and DA on the determination of glucose was examined. 15



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The equilibration time is also more than 5 minutes. A foreign species was considered not to interfere with the determination if it caused a relative error less than 10% during the determination of 1mM glucose. The maximal tolerable concentration of the co-existing at 1 mM glucose was 0.2 mM for AA, UA and DA.

Reproducibility and stability. The reproducibility of developed sensor based on controllable cobalt oxide/Au hierarchically nanostructured array electrode was examined by measuring the current response of glucose oxidation at seven equally fabricated electrodes under the same conditions. The relative standard deviation (RSD) of the current response was only 3.7%, demonstrating a good reproducibility. Furthermore, five successive measurements on each sensor gave a RSD of about 1.7-2.4%. The long-term stability of glucose sensor is a critical factor in practical detection application. The stability of the electrode was investigated by amperometric response containing 1.0 mM glucose solution. After 60 days storage at room-temperature, the current response to 1 mM glucose only decreased 6.7 % of its initial response current. The excellent long-term storage stability could be attributed to the chemical stability of cobalt oxide/Au on the substrate in basic solution.

Validation of analytical principle. In an attempt to evaluate the feasibility of the Au/Co3O4/FTO electrode for routine analysis, the sensor was applied to a glucose assay with standard human blood serum sample and human whole blood samples. Quantitative determination of glucose was analyzed by the standard addition method. The standard human blood serum sample was studied in detail. The concentration of 16


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glucose in the human blood serum sample measured by our electrochemical method agrees well with the certificated value and the recoveries for sample determination were in the range of 95.5%-102.8%. For the human whole blood sample, the concentration of glucose measured by our electrochemical method agrees well with that obtained in the hospital. The results were listed in Table 2, indicating that the sensor could be used practically for routine analysis of glucose in real biological samples.

CONCLUSIONS In summary, a novel electrochemical sensor for glucose, based on controllable cobalt oxide/Au hierarchical nanostructures arrays electrode, has been successfully demonstrated for the detection of glucose in human blood samples. This sensor has the following advantages: (1) it can be used to determine the blood glucose level directly; (2) it avoids much interference in the real samples with more negative oxidation potential; (3) it has good reproducibility and stability due to the catalyst grown on the conductive substrate directly. More importantly, compared with the other electrochemical senors reported, the preparation of the senor is very simple, cost-effective (without any complicated process, such as Hummers' method, calcinations, hydrothermal process and chemical vapor deposition), environmentally friendly (without any organic solutions, such as Nafion, chitosan, PVP and CTAB), and fast (within only about 1 hour).

It is therefore promising for practical glucose

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detection applications.

The principles demonstrated could be applicable to sensing

of other molecules based on nanostructured electrodes.

AUTHOR INFORMATION Corresponding Authors * YYS. E-mail: [email protected] * JZZ. E-mail: [email protected]

ACKNOWLEDGMENTS YYS acknowledges the financial support from Sichuan University Scholarship Council. JZZ is grateful to the US NSF and Delta Dental Associates for financial support.

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Table 1. Comparison of the performance of various cobalt oxide or Au nonemzymatic glucose sensors. Modified electrode

Detection

potential

DL (µM)

Linear range (mM)

Sensitivity (µAcm-2mM-1)

Ref.

(V) Cobalt oxide/Au /FTO

0.30 (vs.Ag/AgCl)

0.1

0.0002-0.2; 0.5–20

6000

This work

Cobalt oxdie /FTO

0.60 (vs.Ag/AgCl)

1

0.002–0.2; 1-20

1330

This work

Co3O4 Nanofibers/nafion/GCE

0.59 (vs.Ag/AgCl)

0.97

Up to 2.04

36.25

5

CTAB–Co3O4/Nickel foam

0.55 (vs.Hg/HgO),

0.08

0.005–12

1440

8

3D graphene/Co3O4

0.58 (vs.Ag/AgCl)

0.025

Up to 0.08

3390

31

graphene/Co3O4

0.60(vs.Ag/AgCl)

〈10

50–300

Co3O4 Nanoparticle/GCE

0.59 (vs.SCE)

0.13

0.005–0.8

520.7

21

Co3O4 nanostructure /GCE

0.47(vs.Ag/AgCl)

0.8

0.5–5.0

27.33

32

Co3O4/PbO2/carbon cloth

0.55

0.31

0.005–1.2

460.3

20

Co3O4/nanoporous Au nanowire

0.26 (vs.Ag/AgCl)

0.005

Up to 70

12500

26

AuNPs/ graphene oxide nanoribbons

0.20 (vs.Ag/AgCl)

5

0.05–4.92

59.1

33

0.55 (vs.Ag/AgCl)

1

0.01–6.1

198

34

12

/Carbon sheet Au/ Ni-Al layered double hydroxide/ single-walled carbon nanotubes/ graphene nano-composite/nafion/GCE

22


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Table 2. Analytical result of glucose in human blood serum. Sample

Human blood serum Human blood

Certificate/Hospital Proposed (mM) methoda (mM) 5.44 5.37±0.13

5.08

Spiked Founda (mM) (mM) 2.00 4.00 5.00

7.35±0.16 95.5 9.39±0.20 98.8 10.58±0.18 102.8

5.21±0.18

a: Values include mean values of three determinations and the standard deviation.

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Recovery (%)


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Figure 1. Scheme for fabrication of cobalt oxide/Au hierarchically nanostructured electrode. (a) FTO (b) cobalt particles (c) and Co3O4/Au hierarchically nanostructured material.

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Figure 2 TEM images of Co3O4/Au hierarchically nanostructured material

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Figure 3. Raman spectra of Co particles (a), a in the air for 6 hours (b) and Co3O4/Au hierarchically nanostructured material (c)

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Figure 4. EDS analysis of Co particles (A) and Co3O4/Au hierarchically nanostructured material (B).

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Figure 5. XPS survey spectrum (A) and deconvoluted high resolution XPS spectra of Co 2p (B), O 1s (C), and Au 4f(D) for the Co3O4/Au sample.

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Figure 6. CVs of bared FTO electrode in 0.1 M NaOH solution (a), CoOx/FTO electrode in 0.1 M NaOH solution (b) Au/Co3O4/FTO electrode in 0.1 M NaOH solution (c). Scan rate, 20 mV/s.

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Figure 7. (A) CV curves of the CoOx/FTO electrode in 100 mM NaOH at different Scan rates ranging from 10 mVs−1 to 200 mVs−1 (from bottom to up: 10 mVs−1, 20 mVs−1, 40 mVs−1, 80 mVs−1, 100 mVs−1, 200 mVs−1); (B) CV curves of the Au/Co3O4/FTO electrode in 100 mM NaOH at different Scan rates ranging from 10 mVs−1 to 200 mVs−1 (from bottom to up: 10 mVs−1, 20 mVs−1, 40 mVs−1, 80 mVs−1, 100 mVs−1, 200 mVs−1); (C) plot of peak currents on CoOx/FTO electrode vs. scan rate; (D)plot of peak currents on Au/Co3O4/FTO electrode vs. scan rate.

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Figure 8. . (A) CVs of FTO in 0.1 M NaOH solution containing 1 mM glucose (a), CoOx /FTO in 0.1 M NaOH solutions containing 0 (b), 0.01 (c), 0.02 (d), 0.05(e) and 0.1 mM (f) glucose. Scan rate, 20mV/s; (B) CVs of FTO in 0.1 M NaOH solution containing 1 mM glucose (a), Au/Co3O4 /FTO in 0.1 M NaOH solutions containing 0 (b), 0.01 (c), 0.02 (d), 0.05(e) and 0.1 mM (f) glucose. Scan rate, 20 mV/s.

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Figure 9. (A) Amperometric response of the cobalt oxide without Au(a) and with Au(b) modified FTO electrode on successive droppings of the glucose solution of different concentrations into 0.1 M NaOH solution; (B) the calibration curves for the low concentrations of glucose; (C) the calibration curves for the high concentrations of glucose. Error bars represent one standard deviation of the means.

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For TOC only

33


Au Hierarchically Nanostructured Electrode

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