Au/CuO nanosheets composite for glucose sensor and CO oxidation

Abstract

Au nanoparticles/mesoporous CuO nanosheets were successfully synthesized by the assembly of negatively charged AuCl₄⁻¹ and positively charged CuO nanosheets using sodium citrate and thermal reduction, respectively. The structure and morphology were characterized in detail. Both the loading amount of Au nanoparticles and the performance were controlled by the reduction process. The resulting Au/CuO NSs exhibited good catalytic activity for CO oxidation and good non-enzymatic sensitivity for glucose.

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Introduction

Various nanostructures such as nanotubes, nanowires, nanorods and nanosheets (NSs) have been extensively investigated because of their potential applications, including chemical sensing, chemical catalysis, nanoelectronics and nanophotonics.^1–4 Nanophase metal and metal oxide catalysts, with controlled particle shape and size, exhibit high surface area and densely populated unsaturated surface coordination sites, which can lead to significantly enhanced catalytic performance compared with conventional catalysts.^5–7

Cupric oxide (CuO) is an important p-type semiconductor, which has been extensively studied owing to its different applications, including glucose sensing^8,9 and as an oxidation catalyst for CO.^10–12 Gold in bulk is considered chemically inert and has often been regarded to be poorly active as a catalyst. However, when gold is small enough, surprisingly it turns out to be active for many reactions such as CO oxidation¹³ and glucose detection.¹⁴

The impregnation method (IMP) was used to prepare Au catalysts. A metal oxide support was immersed into an aqueous solution of HAuCl₄, and then water was evaporated to disperse HAuCl₄ crystallites onto the surfaces of the support. The dried precursor was then calcined in air, at a temperature above 473 K, and reduced in a diluted H₂ stream.¹³ Haruta and co-workers have developed four techniques that could deposit Au nanoparticles (NPs) over diverse types of metal oxides, including coprecipitation,¹⁵ co-sputtering,¹⁶ deposition–precipitation,¹⁷ and gas-phase grafting.^18,19

Here, we deposited Au NPs on CuO NSs by mixing NaAuCl₄ solution and CuO NSs colloidal solution. Negatively charged [AuCl₄]⁻¹ was adsorbed onto the surfaces of positively charged CuO NSs by electrostatic attraction. The precipitate of [AuCl₄]⁻¹/CuO NSs was collected and reduced to Au NPs/CuO NSs (named as Au/CuO NSs) by sodium citrate and thermal reduction, respectively. The catalytic oxidation of CO and sensitivity for glucose by using these Au/CuO NSs composites were evaluated. The Au/CuO NSs composites exhibit better performances for both catalytic oxidation of CO and detection of glucose compared to those using CuO NSs.

Experimental

Materials

Copper nitrate (Cu(NO₃)₂·5H₂O), glucose (C₆H₁₂O₆), sodium hydroxide (NaOH), potassium ferricyanide (K₃[Fe(CN)₆]), ascorbic acid (AA), uric acid (UA), sodium citrate (C₆H₅Na₃O₇·2H₂O) were purchased from SINOPHARM. Aminoethanol (NH₂CH₂CH₂OH) (AE) were purchased from Sigma-Aldrich. Sodium chloroaurate (NaAuCl₄·4H₂O) was purchased from Aladdin. Polycarbonate (PC) membranes (Whatman) with a pore diameter of 200 nm and an effective diameter of 19 mm were used for collecting the products. Deionized water (18.2 MΩ cm) was produced by a Millipore Direct-Q System, and used in all of the experiments. The chemicals were used as received without further purification.

Synthesis of Au/CuO NSs composites

Typically, a certain volume (500 ml) of 2 mM Cu(NO₃)₂ aqueous solution was mixed with an equal volume of 1.6 mM AE aqueous solution in a glass beaker with stirring for 1 minute at a constant temperature of 25 °C controlled by a calorstat oven. After 12 hours, CuO NSs colloid solution was obtained.²⁰ The [AuCl₄]⁻¹/CuO NSs precursor was prepared by adding 10 ml 10 mM NaAuCl₄ (pH = 7.15) to the above prepared CuO NSs colloidal solution. The suspension was stirred for 1 h and then aged for 12 h. To prepare Au/CuO NSs composite, sodium citrate reduction and thermal reduction processes were used to convert the adsorbed [AuCl₄]⁻¹ ions on CuO NSs to Au NPs on CuO NSs, respectively. In the sodium citrate reduction process, 2.5 ml, 5 ml and 10 ml of 0.1 M sodium citrate solution were individually used to reduce [AuCl₄]⁻¹/CuO NSs to metallic Au/CuO NSs composite. After stirring for about 1 hour and aging for 12 hours, the products were collected and washed by water and dried at 40 °C. Individual preparations using 2.5 ml, 5 ml and 10 ml sodium citrate were named as Au/CuO-1 NSs, Au/CuO-2 NSs and Au/CuO-3 NSs, respectively. In the thermal reduction method, the [AuCl₄]⁻¹/CuO NSs composites were thermally reduced at 200 °C for 2 and 4 hours in nitrogen, which were referred to as Au/CuO-4 NSs and Au/CuO-5 NSs, respectively. The schematic of the synthetic process is illustrated in Scheme 1.

Scheme 1 Schematic illustration of the preparation process of the Au/CuO NSs composite.

Electrochemical sensor for glucose

The electrode was prepared as follows: a glassy carbon (GC) electrode (2 mm) was carefully polished by using α-Al₂O₃ nanoparticles (20 nm) in ethanol followed by ultrasonic cleaning with distilled water and ethanol. CuO NSs or Au/CuO NSs-1 to 5 (5 mg) were dispersed in 1 ml ethanol by ultrasonication. After placing five drops (20 μl per drop) of the prepared active materials onto the surface of the GC electrode and drying in air, 20 μl of chitosan solution (0.5 wt%) was dropped onto it to encapsulate the active materials. After drying, the modified electrodes were named as CuO NSs/GC, Au/CuO-1 NSs/GC, Au/CuO-2 NSs/GC, Au/CuO-3 NSs/GC, Au/CuO-4 NSs/GC and Au/CuO-5 NSs/GC electrodes, respectively. Electrochemical measurements were performed on a model WPG100e electrochemical analyzer (WonAtech Co., Ltd Korea) using the modified GC electrode as the working electrode; moreover, cyclic voltammogram (CV) and amperometric response (AR) curves were measured in a 0.1 M NaOH solution. Note that the AR was operated at 0.6 V. Glucose was added after several seconds when the current reached a baseline in the absence of glucose. Electrochemical impedance spectroscopy (EIS) measurements were performed in a 0.1 M NaOH solution at 0.4 V by applying an AC voltage with 5 mV amplitude in a frequency range from 1 Hz to 100 kHz under an open circuit potential (Nyquist plots) condition and plotted in the form of complex plane diagrams.

Characterization

Phases of the as-prepared samples were characterized by X-ray diffraction (XRD) at room temperature using an X'Pert PRO (PANalytical, Netherlands) instrument with Cu Kα radiation. The morphologies and structures were characterized by using scanning electron microscopy (SEM) (Hitachi S-4800) and transmission electron microscopy (TEM, Philips CM200) with an accelerating voltage of 160 kV. Moreover, elemental analyses were carried out by an Oxford X-ray energy dispersive analysis spectroscopy (EDS) detector (Hitachi S-3700).

Catalytic CO oxidation

The catalytic CO oxidation was carried out in a tubular quartz reactor with a diameter of 8 mm under atmospheric pressure. For comparison, the catalytic oxidation of CO over CuO NSs and Au/CuO NSs prepared by sodium citrate and thermal reduction were examined. A typical process is as follows: 35 mg of the prepared samples was loaded with a feed gas flow rate of 50 ml min⁻¹. The feed gas consists of 1.01 (vol%) CO and 7.10 (vol%) O₂ balanced with Ar. The reaction temperature was monitored by a thermocouple placed in the middle of the catalyst bed. The amounts of CO and CO₂ in the outlet streams were measured by an on-line gas chromatograph (GC 9790, Zhejiang Fuli Analysis Equipment Company).

Results and discussion

Morphology and structures

After adsorption of the [AuCl₄]⁻¹ ions on CuO NSs in the solution, the precipitated composites were converted to Au/CuO NSs composites by sodium citrate or thermal reduction as described in the Experimental section. The phases of the Au/CuO composites were characterized by XRD and the patterns are shown in Fig. 1a–g. The XRD patterns in Fig. 1g match very well with standard CuO (JCPDS standard card no. 45-0937). Moreover, Fig. 1b–f indicate that the composites are composed of Au and CuO.

Fig. 1 XRD patterns of (a) Au (PDF no. 04-0784), (b) Au/CuO-1 NSs, (c) Au/CuO-2 NSs, (d) Au/CuO-3 NSs, (e) Au/CuO-4 NSs, (f) Au/CuO-5 NSs and (g) CuO NSs, respectively.

The absence of the CuO diffraction peak in Fig. 1d could be due to the strong peaks of Au, which overlap with the peaks of CuO. All of these results demonstrate that the adsorbed [AuCl₄]⁻¹ ions were successfully converted to Au nanocrystals by sodium citrate reduction or thermal reduction; moreover, these results are further confirmed by the following SEM and TEM results. Fig. 2f shows elliptical CuO NSs with sizes ranging from 500 to 1000 nm with a clean surface. In contrast, the surface of the [AuCl₄]⁻¹/CuO NSs composite is blurry (inset in Fig. 2a). When these [AuCl₄]⁻¹/CuO NSs composites were reduced by sodium citrate, Au NPs were formed as shown in Fig. 2a–e. The bright particles are Au NPs. Fig. 2a shows the samples prepared with 2.5 ml of 10 mM sodium citrate, and a small amount of Au NPs are formed with partially blurry networks as can be seen in the [AuCl₄]⁻¹/CuO NSs precursors. When increasing the amount of sodium citrate, more Au NPS are formed by using 5 ml of 10 mM sodium citrate (Fig. 2b) and 10 ml of 10 mM sodium citrate (Fig. 2c). In addition, the Au NPs generated by a small amount of sodium citrate are isolated, but Au NPs networks are formed with higher amounts (Fig. 2c). Unlike the reduction using sodium citrate, Fig. 2d and e demonstrate that Au NPs are relatively separated on the surface of CuO NSs by the thermal reduction process. Both the XRD and SEM images indicate that the phase and morphology of CuO NSs are reserved after the reduction processes, which is further confirmed by the corresponding TEM images shown in Fig. 3.

Fig. 2 SEM images of (a) Au/CuO-1 NSs, (b) Au/CuO-2 NSs, (c) Au/CuO-3 NSs, (d) Au/CuO-4 NSs, (e) Au/CuO-5 NSs, and (f) CuO NSs respectively. The inset of (a) is the SEM image of CuO NSs absorbed with [AuCl₄]⁻¹.

Fig. 3 TEM images of (a) Au/CuO-1 NSs, (b) Au/CuO-2 NSs, (c) Au/CuO-3 NSs, (d)–(e) Au/CuO-4 NSs and (f) Au/CuO-5 NSs, respectively. The inset of (a) is a TEM image of CuO NSs. The inset of (d) is the corresponding SAED recorded from (e).

The inset in Fig. 3a shows the uniform and elliptical CuO NSs with clean edges and surface. After reduction, due to the heavy metal Au, the dark particles are Au NPs in the TEM images. Fig. 3a–c indicate that the size of Au NPs reduced by sodium citrate is about 20 nm. Fig. 3e and f indicate that the size of the Au nanoparticles formed by thermal reduction had a broad distribution in the range from 10 to 35 nm. The inset in Fig. 3d is the corresponding selected area electron diffraction (SAED) pattern recorded from Fig. 3e, which demonstrates the coexistence of Au and CuO of the composite. The corresponding high resolution TEM image (Fig. 3d) shows the lattice structures of CuO (002) planes and Au (111) planes. Note that all of the above results confirms that the Au NPs are actually formed on the surface of CuO NSs.

The loading amount of Au was characterized by EDS. Table 1 shows the corresponding weight and atomic percentages of Au, as well as the Cu and O elements recorded from the Au/CuO NSs composites. The EDS results indicate that more Au was loaded on CuO NSs when sodium citrate was used as the reduction agent. This may be attributed to more Au atoms generated from the [AuCl₄]⁻¹ ions in the solution that nucleated on the Au NPs/CuO NSs in the solution during the reduction process using sodium citrate. However, in the case of thermal reduction, since the precursors are the same with the same loading amount of [AuCl₄]⁻¹ ions, the loading amounts Au NPs in the thermally reduced Au/CuO NSs-4 and 5 are very close. Only some aggregation was found in the longer thermally reduced sample of Au/Cu NSs-5 (Fig. 3e and f).

Table 1 The weight and the atomic percentage of Au, Cu and O elements in Au/CuO-1, Au/CuO-2 NSs, Au/CuO-3 NSs, Au/CuO-4 NSs and Au/CuO-5 NSs

Sample	Element	Weight (%)	Atom (%)
Au/CuO-1 NSs	O	13.25	48.26
	Cu	41.24	39.53
	Au	45.51	12.20
Au/CuO-2 NSs	O	10.27	44.03
	Cu	33.81	36.50
	Au	55.92	19.47
Au/CuO-3 NSs	O	9.10	52.26
	Cu	5.48	7.91
	Au	85.42	39.83
Au/CuO-4 NSs	O	19.71	57.94
	Cu	34.66	8.28
	Au	43.17	19.69
Au/CuO-5 NSs	O	21.89	58.24
	Cu	54.85	36.74
	Au	23.26	5.03

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Nonenzymatic sensor for glucose

CuO and Au nanostructures have been reported for glucose sensors.^8,9,14 Herein, we also examined the prepared CuO and Au/CuO NSs for nonenzymatic glucose detection. The CV curves of the above prepared samples were acquired in 50 ml NaOH (0.1 M) solution in the presence of different concentrations of glucose as shown in Fig. 4. There is no peak that can be observed at the bare GC electrode. Nevertheless, a gradual increase in current can be observed at around 0.5 V (vs. Ag/AgCl) on the CuO NSs and Au/CuO NSs electrodes when glucose was added (Fig. 4b–f), which indicates that both CuO NSs and Au/CuO NSs have good electrocatalytic ability towards glucose oxidation.

Fig. 4 CVs of (a) bare GC electrode, (b) CuO NSs/GC electrode, (c) Au/CuO-2 NSs/GC electrode, (d) Au/CuO-3 NSs/GC electrode, (e) Au/CuO-4 NSs/GC electrode and (f) Au/CuO-5 NSs/GC electrode ran at 50 mV s⁻¹ in 0.1 M NaOH (50 ml) with different concentrations of glucose, respectively.

Chronoamperometry was utilized to ascertain the detection of glucose. Note that 50 ml of 0.1 M NaOH solution was used as the supporting solution with successive injection of 0.5 mM glucose. The potential was set at 0.6 V (vs. Ag/AgCl), and the results are shown in Fig. 5. As is expected from the CV curves (Fig. 4), the GC electrode exhibited a low response to glucose, while the CuO NSs and Au/CuO NSs showed a highly enhanced response to glucose. After plotting the response vs. the glucose concentration (Fig. 5), it is seen that CuO NSs/GC, Au/CuO-1 NSs/GC, Au/CuO-2 NSs/GC, Au/CuO-3 NSs/GC, Au/CuO-4 NSs/GC and Au/CuO-5 NSs/GC electrodes provided steady sensitivities of 319.47, 138.22, 628.34, 16.56, 320.53 and 312.10 μA mM⁻¹ cm⁻² with a detection limit of 8.23, 75.7, 7.33, 133.5, 7.70 and 7.40 μM, respectively. For the thermal reduction prepared samples, both the sensitivities and detection limits of Au/CuO NSs-4 and Au/CuO-5 NSs/GC are very similar. However, among the samples prepared by sodium citrate reduction, the Au/CuO-2 NSs/GC electrode showed the best performance with a sensitivity of 628.34 μA mM⁻¹ cm⁻² and a detection limit of 7.33 μM, which is significantly higher than that of the CuO NSs/GC electrode. However, the sensitivity of Au/CuO-1 NSs/GC electrode and Au/CuO-3 NSs/GC electrodes were much lower than that of the CuO NSs/GC electrode. This is because the presence of sodium citrate was not enough to convert the adsorbed [AuCl₄]⁻¹ to Au for Au/CuO-1 NSs, which may have blocked the active surface of CuO NSs and resulted in a worse performance. As for the Au/CuO-3 NSs/GC, the surfaces of CuO NSs are covered by Au NPs networks as shown in Fig. 2c. Note that excess of gold may not be good for the oxidation of glucose. These indicate Au/CuO-2 with optimal amount of Au NPs demonstrates the best performance among the prepared samples. Table 2 shows that the best performance of our samples is comparable to that of other glucose sensors, which have previously reported.^9,22–24

Fig. 5 (a) Amperometric response to successive additions of 0.5 mM glucose at a potential of 0.6 V (vs. Ag/AgCl) and (b) the corresponding calibration sensitivity plot of the bare GC electrode, pure CuO NSs/GC electrode, and Au/CuO NSs/GC electrodes. The amperometric response to successive additions of 3 mM glucose, 0.1 mM AA and UA to 0.1 M NaOH at potential of 0.6 V (vs. Ag/AgCl) of (c) the CuO NSs/GC electrode and (d) Au/CuO-2 NSs/GC electrode.

Table 2 The performances of different nanomaterials-based glucose sensors

Electrodes	Detection limit (μM)	Weight (mg)	Sensitivity (μA mM^−1 cm^−2)	Potential (V)	Ref.
Nanoporous gold film	8.7	—	66	+0.2	24
Nafion/CuO-NFs film/GC	0.8	—	431.3	+0.4	22
Nafion/CuO nanospheres/GC	1	0.25	404.53	+0.6	23
Nafion/CuO nanorods/G	4	1	371.43	+0.6	9
Nafion/CuO flowers/G	4	1	709.52	+0.6	9
Chitosan/Au/CuO-2 NSs/GC	7.4	0.1	628.34	+0.6	This work

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For monitoring the concentration of glucose in human blood, the possible cross-sensitivity from interfering biomolecules should be considered. For example, uric acid (UA) and ascorbic acid (AA) normally coexist with glucose in human blood. Taking account of the concentration of glucose in the human blood, which is about 30 times that of AA or UA,^21,22 the amperometric responses toward the addition of 3 mM glucose, 0.1 mM AA and UA were examined in 50 ml NaOH (0.1 M) solution at a potential of 0.6 V (vs. Ag/AgCl). As shown in Fig. 5c and d, the CuO NSs and Au/CuO NSs modified electrodes can effectively inhibit the oxidation of AA and UA. In comparison with the response to 3 mM glucose, the interferences from 0.1 mM AA and 0.1 mM UA are less than 5%. This indicates that the specific selectivity of CuO NSs and Au/CuO NSs towards glucose is quite good.

Among the thermally reduced samples, the electrochemical sensor performance for glucose of Au/CuO NSs-5 was similar to that of Au/CuO NSs-4. Among the sodium citrate reduced samples, Au/CuO-2 shows the best performance. To study the interface properties of the CuO NSs/GC electrode, Au/CuO-2 NSs/GC electrode, Au/CuO-5 NSs/GC electrode, and CuO NSs electrode were investigated using EIS with frequencies ranging from 1 to 100 kHz at 0.4 V (vs. Ag/AgCl) in 0.1 M NaOH solution. The resulting Nyquist complex plane plots of these three electrodes are shown in Fig. 6. After immobilizing the CuO NSs composites on the GC electrode, the semicircle diameter of EIS increased, indicating the firm contact between CuO NSs composite and the GC electrode.^25–27 It also shows that the resistance of the charge transfer (Ret) of the Au/CuO-5 NSs/GC electrode is smaller than that of the other samples, indicating that the electron transfer speed of the Au–CuO-5 NSs/GC electrode is the fastest and thus it gives the best electrochemical performance. The order of the electron transfer ability with the three samples is Au/CuO-5 NSs/GC electrode > Au/CuO-2 NSs/GC electrode > CuO NSs/GC electrode.

Fig. 6 EIS of bare GC electrode, CuO NSs/GC electrode, Au/CuO-2 NSs/GC electrode, Au/CuO-5 NSs/GC electrode in 0.1 M NaOH at of 0.1 V (vs. Ag/AgCl), and inset shows a magnified plot of high frequency zone.

Catalytic oxidation of CO by CuO NSs and Au/CuO NSs

Both CuO nanostructures and Au nanoparticles have demonstrated good catalytic oxidation of CO.^12,28,29 Fig. 7 shows that catalytic CO oxidation over CuO NSs and Au/CuO NSs conducted at 70 °C and 120 °C, respectively. At 70 °C, the CO conversion efficiency of Au/CuO NSs was higher than that of the CuO NSs. This should be due to the loading of Au NPs on CuO NSs, which promotes the catalytic activity, since it has been reported that Au NPs could initiate the catalytic oxidation of CO at relative low temperature.²⁹ However, when the temperature was increased up to 120 °C, the promotion by Au NPs seems inconspicuous. This is because when the temperature increases, the catalytic activity of CuO NSs is remarkably increased and results in high conversion efficiency.²⁰ Among the different samples that were prepared, the Au/CuO NSs prepared by thermal reduction demonstrated the best catalytic oxidation efficiency for CO at 70 °C. This might be due to the uniform distribution of Au NPs on CuO nanosheets as well as the strong adhesion between Au NPs and CuO NSs since they were formed at 200 °C.

Fig. 7 CO conversion efficiency of CuO NSs, Au/CuO-2 NSs, Au/CuO-3 NSs, Au/CuO-4 NSs and Au/CuO-5 NSs at 70 °C and 120 °C.

Conclusion

In summary, Au/CuO NSs composite have been successfully synthesized by the electrostatic assembly of negatively charged [AuCl₄]⁻¹ and positively charged CuO NSs, which are subsequently reduced by sodium citrate and thermal reduction, respectively. Au/CuO NSs synthesized by these two methods exhibited better catalytic activity in the CO oxidation reaction at 70 °C than pure CuO NSs. When the loading amount of Au on CuNSs was optimal, the Au/CuO NSs presented high electrochemical non-enzymatic sensitivity for glucose. The process reported here may be extended to prepare other metal/metal oxide composites for advanced catalysis as well as for nonenzymatic glucose sensors.

Acknowledgements

This work was supported by the National Natural Science Foundations of China (NSFC 21271154), the National Basic Research Program of China (2015CB655302), Natural Science Foundation for Outstanding Young Scientist of Zhejiang Province, China (LR14E020001), Doctoral Fund of Ministry of Education of China (20110101110028), and the project-sponsored by SRF for ROCS, SEM.

Notes and references

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