Gold nanoparticles decorated reduced graphene oxide for detecting the presence and cellular release of nitric oxide
Introduction
Nitric oxide (NO) was first identified as an endothelium-derived relaxing factor (EDRF) in 1970s [1]. Since then, the research on the regulatory roles of NO in biological systems has been fervent. The studies have revealed that NO regulates many physiological functions including blood vessel dilation [2], anti-coagulation [3], neurotransmission [4], and anti-inflammation [5]. The excess or deficiency of NO can result in various pathological conditions such as tumor angiogenesis [6], atherosclerosis[7], Parkinson's disease [8] and diabetes [9], [10]. Therefore, it is of great importance to accurately quantify nitric oxide level for study of cell functions and diagnosis. However, this is challenged by the low physiological concentration of NO and its short life time (∼5 s) due to its rapid conversion to NOx- by oxygen and superoxides present in biofluids.
Compared to the detection methods based on fluorescence measurement [11], [12] and liquid chromatography [13], electrochemical detection allows simple, fast, real-time and quantitative detection with high sensitivity. Recently, efforts have been made to increase the sensitivity for electrochemical detection of NO by engineering the electrode with functional nanomaterials [14]. Among them, graphene (a monolayer of carbon atoms two-dimensionally arranged in a honeycomb structure) is of particular interest, due to its high conductivity, large surface area, wide electrochemical detection window, and chemical inertness [15].
Gold nanoparticles (AuNPs), which are biocompatible, have been used in conjunction with graphene for sensor applications [16], [17]. AuNPs also exhibit excellent catalytic property toward NO oxidation. Therefore, AuNPs have been composited with various nanomaterials for the development of NO sensors [18], [19], [20], [21], [22], [23]. However, they have not been employed for physiological measurements and there is still room to simplify the fabrication process and improve the performance of NO sensors. And in the current developments, hazardous chemical reagents (e.g. hydrazine) are used to reduce graphene oxide (GO) in graphene preparation. In comparison to such chemical reduction, electrochemical reduction of GO is faster and more environmental-friendly [24].
In this work, we report a hybrid film of electrochemically reduced graphene oxide (ERGO) and gold nanoparticles (AuNPs) simply made by electrophoretic deposition of GO sheets followed by in situ electrochemical reduction and subsequent in situ electrochemical growth of AuNPs. We have also demonstrate the use of such hybrid film modified electrode for sensitive detection of NO and its dynamic release from live cells.
Section snippets
Materials
Sulfuric acid (H2SO4), nitric acid (HNO3), sodium nitrite (NaNO2), potassium hydroxide (KOH), potassium chloride (KCl) and acetylcholine were purchased from Sigma–Aldrich.). Phosphate buffer solution (PBS) (pH 7.4) was used as the electrolyte solution for all experiments. All solutions were prepared with deionized water purified from Millipore Milli-Q system.
Equipment
Electrochemical characterizations and measurements were conducted with a CHI-660D electrochemical workstation (CH Instruments, China)
The AuNP-ERGO hybrid electrode
Usually, graphene modified electrodes are made by drop-casting GO dispersion onto the supporting electrode followed by chemical reduction of GO [29], [30]. Here, we deposit GO sheets onto a positively-biased glassy carbon electrode via electrophoresis because GO nanosheets bear abundant negatively charged hydroxyl, ketone carbonyls, epoxide, and carboxyl groups [31]. As compared to drop-casting and other coating methods, electrophoretic deposition can easily and reproducibly make uniform and
Conclusion
In summary, we present a method to electrochemically prepare a hybrid thin-film electrode made of electrochemically reduced graphene oxide (ERGO) and gold nanoparticles (AuNPs). Comparing to the commonly used methods which usually involve functionalization of graphene oxides (GOs), drop-casting GOs onto the supporting electrode, and reduction of GOs using harsh chemicals [29], [30], [49], the herein reported method is simple, fast, environmental friendly, and able to reproducibly fabricate
Acknowledgments
This study is supported by AcRF Tier 2 grants from Ministry of Education of Singapore (MOE2011-T2-2-010, MOE2012-T2-2-049), and a grant from NNSF of China (61328401). We also thank GlobalFoundries (Singapore) for the scholarship provided to S.L. Ting.
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