Dispersed gold nanoparticles on NiO coated with polypyrrole for non-enzymic amperometric sensing of glucose

Abstract

A highly sensitive and selective non-enzymatic glucose sensor based on loading Au on NiO nanoparticles which were encapsulated by polypyrrole (named as NiO@PPy/Au) modified glassy carbon electrode (GCE) was constructed. Morphology images of the synthesized samples were characterized by transmission electron microscopy and the nanoparticles were characterized by Fourier transform infrared spectroscopy, UV-vis spectrometry and X-ray diffraction. Electrochemical behaviors of glucose at the NiO@PPy/Au/GCE were investigated by cyclic voltammetry and amperometry, and the modified electrode showed excellent electrochemical activity toward the oxidation of glucose. Under the optimum conditions, the calibration curve for glucose determination was linear in the range of 0.5 μM to 1.7 mM and a fast response time (<4 s), high sensitivity of 802.9 μA mM⁻¹ cm⁻² and a low detection limit (S/N = 3) of 0.15 μM were obtained. In addition, the proposed sensor was successfully applied to analyze the glucose level in human blood serum samples.

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1. Introduction

With the rapid development of modern society, glucose detection is important for the diagnosis and treatment of diabetes, as well as for various applications in the food industry.^1–3 Although third-generation enzymatic glucose biosensors with high sensitivity and selectivity have been developed, limitations such as the lack of stability, the variety of requirements for the enzyme immobilization process, low temperature storage and operation, inadequate control of pH, and high toxicity in the environment have hindered their development and motivated further exploration of non-enzymatic glucose sensors.^4–6 These non-enzymatic glucose electrodes offer high selectivity, long term stability, resistance to thermal implications with low cost and simple fabrication methods.^7–10

Among them, metal or metal oxide nanoparticles of electrochemical catalysts, such as Pd, Pt, Au, Ni, NiO, and CuO, have been studied as non-enzymatic glucose sensing materials.¹¹ However, metal oxide nanoparticles still suffer from low electrical conductivity and poor stability. To improve the electrical conductivity and mechanical properties, hybridizing the metal oxide with highly conductive material, such as graphene,¹² carbon nanotubes^13,14 or conductive polymers^15,16 is an effective method. Particularly, conductive polymer composites with metal oxide can effectively improve their electrochemical performance. Therefore, conducting polymer films have been widely studied for applications in chemical sensors and biosensors.^17–20 Polypyrrole (PPy) is one of the most extensively used in conducting polymers for construction of bioanalytical sensors and supporting matrix in electrochemical because of its good physical and electrical properties biocompatibility.^21–25 Simultaneously, PPy can support in getting a good dispersion of metal nanoparticles (NPs) for the intrinsic existence of functional groups.^21,26 Core–shell structured materials are promising for biological applications and have attracted considerable researchers' attention to this class of materials due to their unique properties such as high dispersibility, better thermal and chemical stability and less cytotoxicity.^27–31

In the present work, we used polypyrrole (PPy) to coat NiO nanoparticles and then decorated the NiO@PPy with Au NPs to construct a well conducting and highly catalytic sensing interface for non-enzymic amperometric detection of glucose. Firstly, we have used polypyrrole as shell wrap out of the nickel oxide (NiO) magnetic nano-composite, which has also provided a strict barrier between nanoparticles and reduced the magnetic-coupling effect between nanoparticles.³² Secondly, the gold nanoparticles were dispersed on the surface of polypyrrole film uniformly, which could combine its remarkable electrocatalytic performance and the prominent conductivity to enhance the electrocatalysis of NiO nanoparticles. As far as we know, this is the first time NiO@PPy/Au nano-composite was applied in glucose sensors. The electrocatalysis of NiO@PPy/Au/GCE toward the oxidation of glucose was evaluated by cyclic voltammetry (CV) and amperometry, and the proposed modified electrode exhibited excellent catalytic activity and highly sensitive for glucose determination.

2. Experimental

2.1. Chemicals and reagents

NiCl₂·6H₂O was purchased from Tianjin BASF Chemical Co. Ltd; ethylene glycol, urea, SDS (sodium dodecyl sulfate), pyrrole, trisodium citrate (99%) and chloroauric acid (98%) were supplied by Sinopharm Chemical Reagent Co., Ltd; Chitosan (>90% deacetylation), APS (ammonium persulfate), p-TSA (p-toluenesulfonic acid) were got from Shanghai Yuanju Biotechnology Co., Ltd and was served as binding agent; Glucose, uric acid (UA), fructose, L-cysteine (CySH) and all other chemicals were of analytical grade and were used without further purification; sodium hydroxide (NaOH, 0.1 M) was used as the supporting electrolyte. Deionized water was used in all experiments.

2.2. Apparatus

All of the electrochemical experiments were performed with a CHI660D electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd, China) using a three-electrode system, where a standard saturated calomel electrode (SCE) served as the reference electrode, a platinum wire electrode as the auxiliary electrode, and the modified electrode GCE (3 mm in diameter) as the working electrode. Fourier transform infrared spectroscopy (FTIR) was conducted on a NEXUS670. UV-vis adsorption spectra were investigated by Cary 50 scan UV-vis spectrophotometer (Varian, Australia). Powder X-ray diffraction (XRD) patterns of the product were collected on a Rigaku XRD-6000 by use of Cu-Kα radiation at 40 kV, 30 mA. Transmission electron micrographs (TEM) were collected on an E.M. 912 Ω energy filtering TEM (120 kV).

2.3. Preparation of the nanohybrid films coated electrode

2.3.1. Synthesis of NiO nanoparticles. The NiO precursor were prepared according to the reported method.³³ Briefly, NiCl₂·6H₂O (1.0 g), sodium hydroxide (8.0 g) and SDS (sodium dodecyl sulfate) (1.0 g) were dissolved in 10 mL deionized water which contained 1 mL ethylene glycol. The obtained homogeneous solution was further transferred into an autoclave reactor, which was heated in an oven at 100 °C for 4 h, then cooled naturally to the room temperature. The product was washed with deionized water and ethanol for several times, respectively, then dried at 60 °C in an oven overnight. Finally, the product was calcined at 350 °C for 2 h to get the final product.

2.3.2. Synthesis of NiO@PPy nanocomposite. The as-prepared NiO nanoparticles were encapsulated by polypyrrole (PPy) via an in situ polymerization in water and alcohol mixture at 0–5 °C.³⁴ 30 mL of anhydrous alcohol p-TSA (0.08 M) was mixed with 0.1 mL pyrrole monomer and stirred for 10 min to form a uniform mixture A. This step is allowed the pyrrole monomer to diffusion in the surface of the NiO, forming a uniform distribution of the pyrrole monomer. At the same time, Solution B (volume of solution B: 20 mL) is 0.026 M of APS aqueous solution. Subsequently, solution B was added dropwise into the solution A (0.5 mL min⁻¹). The mixture was shielded from light for 24 h before rinsing with ethanol and deionized water successively to remove residues. Then the final sediment was gathered by centrifugation, washed with distilled water to neutral, subsequently dried in an oven at 65 °C for 10 hours.

2.3.3. Synthesis of NiO@PPy/Au nanocomposite. The Au NPs were synthesized by the citrate reduction of HAuCl₄^.35,36 Briefly, HAuCl₄·H₂O solution was added to ultrapure water and heated to a boil while stirring; then trisodium citrate solution was added quickly, which resulted in a change in solution color from pale yellow to deep red. After the color change, the solution was boiled for an additional 15 min and allowed to cool to room temperature. Thereafter, a certain amount of NiO@PPy nanocomposites (1.0 mg) were dispersed into the gold nanoparticle suspensions (1.0 mL) evenly. The desired product NiO@PPy/Au nanocomposite were generated by self-assembly.

2.3.4. Electrode modification. Prior to the modification, the GCE was polished with 1.0 and 0.3 μm alumina slurry to obtain mirror like surface, respectively, and rinsed with doubly distilled water, followed by sonication in ethanol solution and doubly distilled water successively. Then, the GCE was allowed to dry in a stream of nitrogen. Last, the nanohybrid (5 μL) were delivered to a treated GCE surface and dried with air. The target working electrode was obtained based on these works above. For comparison, NiO and NiO@PPy nanocomposites (1.0 mg) were respectively dispersed into chiston (1.0 mL) evenly. NiO/GCE and NiO@PPy/GCE were made in the same way and dried at room temperature. These modified electrodes were stored at 4 °C for further use.

3. Results and discussion

3.1. Characterization of the NiO@PPy/Au

The TEM images of NiO nanoparticles, NiO@PPy nanocomposites and NiO@PPy/Au nanocomposites are presented in Fig. 1 respectively. Fig. 1A and B shows that the size of the synthesized NiO nanoparticles was about 80–100 nm in diameter. From the TEM image of NiO/PPy nanocomposites (Fig. 1C and D) it was observed that the dark NiO nanoparticles were coated by some light shaded shell structure. This indicated that NiO nanoparticles were covered with PPy which confirmed the core shell morphology of NiO@PPy nanocomposites. The average thickness of conducting PPy layer over NiO core was about 10 nm. Spherical structure in the surface of NiO@PPy/Au nanocomposites (Fig. 2E and F) indicated the presence of Au nanoparticles and also confirmed the successful adsorption of Au nanoparticles on the surface of NiO@PPy nanocomposites.

Fig. 1 TEM images of nanocomposites: (A, B) NiO, (C, D) NiO@PPy and (E, F) NiO@PPy/Au.

Fig. 2 (A) FTIR, (B) UV-vis and (C) XRD spectra of (a) PPy, (b) NiO, (c) NiO@PPy, and (d) NiO@PPy/Au nanocomposites.

The molecular structure of the obtained NiO, PPy, NiO@PPy and NiO@PPy/Au nanocomposites were characterized by FTIR spectroscopy. As shown in Fig. 2A, the outstanding sharp peak at 472 cm⁻¹ is due to M–O stretching vibration mode. Characteristic absorption peaks of PPy could also been observed, which are located at 1590 and 1460 cm⁻¹ corresponding to the stretching mode of the pyrrole ring. The peak at 3351 cm⁻¹ is attributed to N–H and C–H stretching vibrations. The peak at 1046 and 786 is related to the in-plane vibrations of C–H and C–H wagging vibration respectively. UV-vis spectrometry is an effective method to monitor the evolution of metal species in the synthesis of metal nanoparticles. Fig. 2B shows UV-vis spectrum of NiO, PPy, NiO@PPy and NiO@PPy/Au nanocomposites. An adsorption peak at about 520 nm is observed, similar to that of bare Au nanoparticles, indicating the existence of Au. No adsorption band attributed to NiO is observed and also most literatures did not report characteristic adsorption band for NiO. To confirm the composition of the nanoparticles, the XRD pattern of NiO, PPy, NiO@PPy and NiO@PPy/Au nanocomposite are shown in Fig. 2C. The data showed diffraction peaks at 2θ = 37.3°, 43.3°, 63.0°, 75.6°, and 79.6°, which can be indexed to (111), (200), (220), (311), and (222) crystal planes of NiO with face-centered cubic phase respectively. These peaks are in agreement with NiO nanoparticles. Another five diffraction peaks (at 2θ = 38.3°, 44.4°, 64.8°, 77.9°, and 82.0°), revealed indices corresponding to (111), (200), (220), (311), (222), planes of gold in a cubic phase respectively. On the other hand, the weak reflection centered at a 2θ value of 24° was characteristic of the doped PPy.³⁷ XRD measurement confirmed the presence of Au and NiO in the NiO@PPy/Au nanocomposites.

3.2. Electrochemical properties of NiO@PPy/Au

Electrochemical impedance spectroscopy (EIS) was a powerful tool for studying the interfacial properties of surface-modified electrodes. As can be seen from Fig. 3, the bare GCE had a smaller semicircle diameter (curve a), implying low Ret to the redox probe dissolved in electrolyte solution. After NiO was modified onto the electrode, the resistance increased compared with bare GCE (curve b). The reasons were that chitosan which was used as fixative to immobilize nanomaterials would hinder the diffusion of ferricyanide toward the electrode surface. After PPy was wrapped on NiO, a remarkable decrease in the semicircle diameter was observed (curve c), which was ascribed to the presence of conducting polymer films PPy. However, when Au NPs was immobilized onto NiO@PPy, the Ret value was reduced owing to the excellent conductivity of Au NPs that decreased the impedance of the electrode. The results were indicating that the NiO@PPy/Au could efficiently enhance the electron transfer efficiency.

Fig. 3 EIS of (a) bare GCE, (b) NiO/GCE, (c) NiO@PPy/GCE and (d) NiO@PPy/Au/GCE in 5.0 mM [Fe(CN)₆]^4−/3− containing 0.1 M KCl from 10⁵ to 10⁻² Hz at amplitude of 5 mV.

The electrocatalytic activity of NiO@PPy/Au/GCE towards glucose oxidation was investigated by cyclic voltammograms (CVs). Fig. 4 shows CVs of bare GCE, NiO modified GCE, NiO@PPy/GCE and NiO@PPy/Au/GCE without and with 3.0 mM glucose in 0.1 M NaOH, respectively. We can see from the Fig. 4, Compared with NiO, NiO@PPy, NiO@PPy/Au showed higher current response and discernible current peak, meaning integration of unique properties of NiO, PPy and Au NPs was superior to that of NiO. NiO@PPy/Au showed a remarkable catalytic current peak about 0.23 mA in intensity at +0.55 V, indicating that the Au NPs supported on the NiO@PPy exhibited excellent catalytic performance toward the glucose oxidation.

Fig. 4 CVs of (a) bare GCE, (b) NiO/GCE, (c) NiO@PPy/GCE and (d) NiO@PPy/Au/GCE in 0.1 M NaOH in the absence and presence of 0.3 mM glucose at a scan rate of 0.05 V s⁻¹.

The catalytic activity of NiO@PPy/Au nanocomposites by changing the concentration of glucose as shown in Fig. 5, it can be seen that apparent enhancements in the oxidation peak currents while decreases in the cathodic peak currents depended on increased glucose concentrations. These results indicated that glucose had been oxidized by the active NiO@PPy/Au nanohybrid films via a cyclic mediation redox process. The redox transition can be described as follows:³⁸

NiO + OH⁻ → Ni(OH)₂

Ni(OH)₂ + OH⁻ → NiOOH + H₂O + e⁻

NiOOH + glucose → Ni(OH)₂ + glucolactone

Fig. 5 CVs of the NiO@PPy/Au/GCE modified electrode with different glucose concentration in 0.1 M NaOH solution (from (a) to (g): 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 mM glucose).

Then, the electrochemical responses of the NiO@PPy/Au/GCE were further investigated with the scan rates (v) ranging from 10 mV s⁻¹ to 70 mV s⁻¹ and the results are shown in Fig. 6. We can see that the peak current was proportional to the scan rate from Fig. 6, inset (correlation coefficients of 0.9990 and 0.9982 for anodic and cathodic peaks, respectively). The results indicate that the electrochemical kinetics is a surface-controlled process rather than a diffusion-controlled process at these scan rates.

Fig. 6 CVs of the NiO@PPy/Au modified electrode with different scan rate, from (a) to (h): 10, 20, 30, 40, 50, 60, 70, 80 mV s⁻¹ (inset: plots of peak current vs. scan rate).

3.3. Amperometric detection of glucose

To improve the performance of the sensor, the effect of the applied potential on the response current of the sensor is illustrated in Fig. 7. When the applied potential was increased from 0.35 to 0.6 V, very little current was observed at an applied potential 0.4 V; however, a large increase was observed from 0.5 to 0.6 V. Therefore, 0.55 V was selected as the optimum applied potential for the amperometric detection of glucose in subsequent studies. It is how we choose the working potential in conducting the experiment.

Fig. 7 Steady-state current–time responses of the NiO@PPy/Au films to 0.1 mM glucose in 0.1 M NaOH at various potentials.

Fig. 8A displays the amperometric current–time curve of the NiO@PPy/Au/GCE under the optimized experimental conditions with successive additions of glucose. It took less than 4 s to achieve the steady-state current upon addition of glucose to the stirring support electrolyte, indicating the fast amperometric response of the modified electrode. The calibration curve for the glucose sensor is shown as Fig. 8B. The modified electrode gave a linear dependence in the glucose concentration range from 0.5 μM to 1.7 mM with a correlation coefficient of 0.9986, a sensitivity of 809.3 μA mM⁻¹ cm⁻² and a detection limit of 0.15 μM. Comparison of some efficient non-enzymatic glucose sensors are presented in Table 1. It can be seen that the proposed glucose sensor shows a good superiority in terms of sensitivity, linear range and detection limit. This is attributed to the synergetic effects by combining NiO with AuNPs, which greatly increase the electrical conductivity and catalytic performance of nanohybrid and further promote the sensitivity to oxidation of glucose.

Fig. 8 (A) Typical amperometric response of the modified electrode to successive addition of glucose into the stirring 0.1 M NaOH solution (E = 0.55 V); (B) calibration curve of I–C.

Table 1 Comparison of the present sensor electrode with other nonenzymatic glucose sensors

Sensors	Sensitivity (μA mM^−1 cm^−2)	Linear range (mM)	Detection limit (μM)	Literature
a Carbon ionic liquid electrode (CILE).b Indium tin oxide(ITO).c Carbon nanotubes (MWNTs).d Carbon paste electrode.e Carbon nanofiber paste electrode (CFPE).
Ni(OH)2 modified CILE^a	202	0.05–23	6	39
NiO–SWCNTs/ITO^b	907	0.001–0.9	0.3	40
Ni–MWNTs^c	67.19	3.2–17.5	0.98	41
NiO/CPE^d	—	0.001–1	0.3	42
Ni–NiO/CFPE^e	420.4	0.002–2.5	1.0	43
Ni(OH)2/Au/GC	371.2	0.005–2.2	0.98	33
Coreshell NiO/C nanobelts	149.11	0.001–0.17	9	44
NiO@PPy/Au	802.9	0.0005–1.7	0.15	This work

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3.4. Anti-interference of the NiO@PPy/Au/GCE electrodes

It is known that some easily oxidative species, such as DA, UA, AA, and other carbohydrate compounds, usually co-exist with glucose in biological sample and consequently interfere with the detection of glucose. Therefore, anti-interference property is an important factor for the practical use of glucose sensors. The electrochemical response of the interfering species was examined at the NiO@PPy/Au film electrode. As shown in Fig. 9, the current response of the interfering species to the glucose is negligible, respectively, indicating the high selectivity of our present NiO@PPy/Au sensor for glucose detection with the normally co-existing electroactive interfering species.

Fig. 9 Amperometric response of the NiO@PPy/Au/GCE to successive addition of glucose, UA, AA, DA, Fru and AAP (0.1 mM, respectively) in 0.1 M NaOH solution at 0.55 V.

3.5. Application of the sensor in human serum samples

To evaluate its applicability in routine analysis, the glucose concentration in the real sample was detected using the standard addition method, with three times the addition of pure glucose to solutions containing serum samples (diluted 20-fold concentration). The results in Table 2 showed that the sensor produced recoveries in the range of 97.3–101.5%, indicating that the as-prepared NiO@PPy/Au nanohybrid films hold great potential in real sample analyses.

Table 2 Glucose detection in blood serum samples by the proposed sensor

Sample	Glucose added (mM)	Glucose found (mM) (n = 3)^a	Recovery (%)
a Average of three determinations (±relative standard deviation).
Plasma sample	—	0.25	—
Sample 1	1.25	1.24	99.2
Sample 2	1.50	1.46	97.3
Sample 3	2.00	2.03	101.5

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4. Conclusions

In summary, we have developed a high sensitive non-enzymatic glucose sensors based on that loaded Au NPs on the surface of polypyrrole film as shell wrap out of NiO use a quite simple and valid way to improve the electrocatalysis performance of NiO in this study successfully. The as-fabricated NiO@PPy/Au films exhibited notable catalytic performance for oxidation of glucose and the sensor constructed by NiO@PPy/Au film electrodes showed high sensitivity, wide detection range, low detection limitation to glucose as well as good selectivity, which possesses a good overall performance while compared to other non-enzymatic glucose sensors. Therefore, the proposed research strategy has not only broadened the scope for assembling metal nanoparticles over the active channels of but also has extended their applications to the electrochemical quantification of glucose.

Acknowledgements

The authors gratefully acknowledge the financial support of this project by the National Science Foundation of China (No. 21575113 and 21275116), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20126101120023), the Natural Science Foundation of Shaanxi Province in China (2013JM2006, 2013KJXX-25 and 2012JM2013), the Scientific Research Foundation of Shaanxi Provincial Key Laboratory (2010JS088, 11JS080, 12JS087, 13JS097, 13JS098) and the Fund of Shaanxi Province Educational Committee of China (12JK0576).

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