Enhanced performances of nonenzymatic glucose sensors by attaching Au nanoparticles onto the surfaces of Cu2O@Cu nanocable arrays

Aiqin Zhang*a, Yakun Tiana, Meng Liua, Yuanhua Xiaoa, Dianzeng Jiab and Feng Li*abcd
aState Laboratory of Surface and Interface Science and Technology, Zhengzhou University of Light Industry, Zhengzhou, 450002, Henan, P. R. China. E-mail: zhang.aiqin@yahoo.com; lifeng696@yahoo.com; Fax: +86-371-86609676; Tel: +86-371-86609676
bInstitute of Applied Chemistry, Xinjiang University, Urumqi, 830046, Xinjiang, P. R. China. E-mail: lifeng696@yahoo.com; Fax: +86-991-8588883; Tel: +86-991-8583083
cAmerican Advanced Nanotechnology, 3519 Double Lake Dr, Missouri City, TX 77459, USA. E-mail: lifeng696@yahoo.com
dState Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, P. R. China. E-mail: lifeng696@yahoo.com

Received 18th July 2014 , Accepted 4th September 2014

First published on 8th September 2014


Nanoarchitectures (Au–Cu2O@CuNCs) have been fabricated successfully for the first time by attaching Au nanoparticles onto the surfaces of Cu2O@Cu nanocables. The hybrid material nanoelectrodes exhibit highly enhanced performances in detecting glucose nonenzymatically, compared to those made with Cu2O@Cu nanocable arrays or Au nanoparticles.


Electrochemical glucose sensors based on enzymatic and nonenzymatic detections have attracted significant attention in recent decades.1 Due to the performance of enzymatic glucose sensors being dictated by the immobilization of glucose oxidase onto the surfaces of substrates such as glass carbon and noble metals,2,3 and the fabricated electrodes being sensitive to the environment, the current research in this area has focused mainly on nonenzymatic systems. Nanosized materials, such as noble metals,4,5 metal oxides,6,7 and complex nanoarchitectures constructed with diverse materials,4,8–10 have been investigated extensively to improve the performances of glucose sensors in recent years. The majority of the devices, however, have also been made by immobilizing active materials onto the surfaces of commercially available glassy carbon electrodes11 or noble metal substrates.12 The performances of glucose sensors could therefore be affected by their stability limitations as well as the immobilization of glucose oxidase onto the surfaces of the substrates.

Recently, extensive efforts have been made to fabricate glucose sensors with porous materials and 1D nanostructures for further improving their performances.13 Metal nanowire/nanotube arrays, which have high surface area and are thus promising candidates for designing novel electrodes, for instance, have been synthesized with porous templates.14,15 Noble metal nanowire arrays including Au, Pd and Pt–Pb nanowire arrays have been applied to make glucose sensors.16–19 While glucose sensors can be made with copper and copper oxide nanomaterials for lowering their cost,6,20–23 to the best of our knowledge, there is no reports concerned with glucose sensors made with Cu2O@Cu nanocable arrays and Au nanoparticles so far.

Based on a self-assembly approach, the researches in our groups have concentrated on constructing functional nanoarchitectures with nanosized building blocks for fabricating chemical sensors and supercapacitors with highly improved performances.24–29 Herein, we report novel nonenzymatic glucose sensors made with nanoarchitectures consisting of Cu2O@Cu nanocable arrays and Au nanoparticles. Compared to the glucose sensors made with Cu2O@Cu nanocable arrays (Cu2O@CuNCs) and Au nanoparticles (AuNP@GC), respectively, Au–Cu2O@CuNCs nanoarchitectures constructed with Au nanoparticles and Cu2O@Cu nanocable arrays exhibit highly enhanced performances in sensing glucose nonenzymatically. The synergetic effect between the nanosized building blocks could lead to the exceptional performances of the devices.

The Cu2O@Cu nanocable arrays were produced directly in one-step by electrochemical deposition in the pores of anodic alumina membrane (AAO). FESEM image as shown in Fig. 1a reveals that the array is composed of 1D smooth nanowires of ca. 10 micron in length and 300 nm in diameter. Fig. 1b shows the XRD profile of the materials as-prepared. The main reflection peaks at 2θ = 43.3°, 50.3°and 74.2°can be indexed as [111], [200] and [220] planes of copper with cubic symmetry (JCPDS, no. 04-0836), respectively. Simultaneously, four weak reflection peaks, which can be assigned to Cu2O (JCPDS, no. 05-0667), can be observed in the profile. The microstructure of the as-prepared 1D nanostructures was further characterized with TEM and HRTEM (Fig. 1c). It can be observed clearly that the surface of the nanowire is covered with a thin layer to form cable-like structure. The lattice fringes with d-spacing of about 0.23 nm on the surface of the nanocable correspond to the [111] plane of cubic Cu2O. The results further verify that a high crystalline and thin layer of Cu2O was produced on the surfaces of Cu nanowires to form nanocables – Cu2O@CuNCs. The formation of Cu2O could be attributed to the experimental conditions, such as the pH value of the electrolyte and the applied current density.30,31


image file: c4ra07270g-f1.tif
Fig. 1 FESEM image of a Cu2O@Cu nanocable array (a) its XRD profiles (b); HRTEM image of Cu2O@Cu nanocables before (c) and after (d) modified with Au nanoparticles. The inset in (c) shows the nanocables at low magnification, the inset in (d) highlights the HRTEM image of one Au nanoparticle.

Au nanoparticles (Fig. S1) of ca. 3 nm were also synthesized with a method developed in our group32 and deposited onto the surfaces of glass carbon electrode (AuNP@GC) for detecting glucose. The surfaces of Cu2O@CuNCs were also modified with the as-prepared Au nanoparticles by dropping the colloid solution onto the surfaces of Cu2O@CuNCs to construct nanoarchitectures – Au–Cu2O@CuNCs for further tailoring the glucose sensing functionalities of the materials, based on a self-assembly technique developed in our groups.24–26 Compared to the smooth surface of Cu2O@CuNCs, Au–Cu2O@CuNCs consist of straight nanowires and circular dots (Fig. 1d). The inset in Fig. 1d shows HRTEM image of an Au nanoparticle. The d-spacing of 0.236 nm corresponding to (111) plane of Au can further verify that the Au nanoparticles have been attached onto the surfaces of Cu2O@CuNCs. Au nanoparticles have been uniformly attached onto the surfaces of nanocables successfully.

The glucose sensing performances (Fig. 2a) of AuNP@GC, Cu2O@CuNCs and Au–Cu2O@CuNCs were carefully investigated with cyclic voltammograms (CV) first. While all of the three electrodes work well to detect glucose nonenzymatically, the Au–Cu2O@CuNCs nanoelectrode shows much higher sensitivity, compared to the two others. The inset in Fig. 2a shows the CV of Au–Cu2O@CuNCs with and without glucose. The peak currents of Au–Cu2O@CuNCs electrode increase dramatically after adding glucose of 5 mM, which clearly indicates that glucose can be detected with the electrode. In comparison, the CVs of AuNP@GC and Cu2O@CuNCs electrodes with and without 5 mM glucose were also performed (Fig. S5). The value changes of peak currents were much lower than that of Au–Cu2O@CuNCs electrode, after adding 5 mM glucose into the test system. The operating potentials of the three electrodes (Fig. 2b) for detecting glucose were further optimized by the current responses to glucose as a function of potential applied in a range from 0.1 to 0.8 V (vs. SCE). The current responses of Au–Cu2O@CuNCs increase in accompanying with elevating potential initially. After reaching its maximum at +0.4 V, however, the current decreases slightly, which indicates the optimized working potential of +0.4 V of the electrode. The Au–Cu2O@CuNCs shows much higher current responses to glucose, compared to AuNP@GC and Cu2O@CuNCs.


image file: c4ra07270g-f2.tif
Fig. 2 (a) Cyclic voltammograms in PBS solution (pH = 6.94) with glucose of 5 mM. The inset shows the cyclic voltammograms of Au–Cu2O@CuNCs electrode with and without glucose of 5 mM. Scan rate: 50 mV s−1. (b) Dependences of current response on the applied potentials to glucose of 1 mM in PBS solution (pH = 6.94). (c) Amperometric responses to successive increase of glucose concentration in PBS solution (pH = 6.94) at +0.4 V (vs. SCE). The inset shows the steady-state calibration curve for the Au–Cu2O@CuNCs electrode. (d) Current–time responses of Au–Cu2O@CuNCs electrode to successive addition of 1 mM glucose, the mixture of 1 mM glucose and 0.002 mM AA, the mixture of 1 mM glucose and 0.002 mM UA, and 0.5 mM glucose, respectively.

Fig. 2c shows typical steady-state amperometric responses of AuNP@GC, Cu2O@CuNCs and Au–Cu2O@CuNCs on successively increasing glucose concentrations at an applied potential of +0.4 V (vs. SCE). Au–Cu2O@CuNCs shows linearly increased responses to the change of glucose concentrations. In contrast, the current responses of two others were much lower and only show slight increases in accompanying with adding glucose into the cell. The results are consistent with those obtained from cyclic voltammograms. The calibration curve for Au–Cu2O@CuNCs is presented in the inset of Fig. 2c. The glucose sensors show linear dependence in the glucose concentration range of 0.05 to 1.5 mM with a correlation coefficient of 0.996, a sensitivity of 0.0001 A mM−1, and a detection limitation of 5 μM at a signal-to-noise ratio of 3. The novel Au–Cu2O@CuNCs glucose sensors exhibit high sensitivity, low detection limitation and fast response time in less than 5 s.

Because easily oxidizable species such as uric acid (UA) and ascorbic acid (AA) usually co-exist with glucose in bio-samples, we further examined the electrochemical responses (Fig. 2d) of Au–Cu2O@CuNCs to them by successively adding 1 mM glucose, the mixture of 1 mM glucose and 0.002 mM AA, the mixture of 1 mM glucose and 0.002 mM UA, and 0.5 mM glucose into the test system. The responses of the interfering species are insignificant. The results indicate that Au–Cu2O@CuNCs can detect glucose selectively. The excellent sensing performances of Au–Cu2O@CuNCs could be resulted from their much higher electrocatalytic performances due to the synergetic effect between Au nanoparticles and Cu2O@Cu nanocables. On the one hand, Cu2O@Cu nanocables have big surface area, which could load more electroactive Au nanoparticles. On the other hand, the 3D nanoarchitectures, which could facilitate the electron transportation and ion diffusion, could also contribute to their exceptional performances in glucose detections.

In summary, novel Au–Cu2O@CuNCs glucose sensors with highly enhanced performances sensors have been fabricated successfully by attaching Au nanoparticles onto the surfaces of Cu2O@Cu nanocable arrays to construct 3D nanoarchitectures. The Cu2O@Cu nanocables can not only act as carriers for immobilizing Au nanoparticles onto their surfaces, but also catalyze the oxidation of glucose for releasing H2O2 together with Au nanoparticles, because of the synergetic effect between the nanosized building blocks. Compared to the glucose sensors based on noble metal nanowire arrays reported previously, the Au–Cu2O@CuNCs nanoarchitectures show apparent advantages in their excellent performances of glucose detection at much lower cost. Due to the microstructures and composition of the nanoarchitectures could be further tailored conveniently, the performances of the devices could be optimized further in the future.

Acknowledgements

We acknowledge National Natural Science Foundation of P. R. China (NSFC.21071130 and 21371157), Outstanding Scholar Program of Henan Province (114200510012) P. R. China, and Key Program of Henan Province for Science and Technology (132102210424) for the supports to this work.

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Footnote

Electronic supplementary information (ESI) available: Experimental section, characterization and supporting figures of electrochemical performance. See DOI: 10.1039/c4ra07270g

This journal is © The Royal Society of Chemistry 2014