We report a simple new approach for green preparation of gallic acid supported reduced graphene oxide encapsulated gold nanoparticles (GA-RGO/AuNPs) via one-pot hydrothermal method. The as-prepared composites were successfully characterized by using Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray powder diffraction techniques (XRD), scanning electron microscope (SEM), high resolution transmission electron microscopy (HRTEM) and elemental analysis. The GA-RGO/AuNPs modified electrode behaves as a hybrid electrode material for sensitive and selective detection of dopamine (DA) in presence of ascorbic acid (AA) and uric acid (UA). The GA-RGO/AuNPs modified electrode displays an excellent electrocatalytic activity towards the oxidation of DA and exhibits a wide linear response range over the DA concentrations from 0.01–100.3 μM with a detection limit (LOD) of 2.6 nM based on S/N = 3. In addition, the proposed sensor could be applied for the determination of DA in human serum and urine samples for practical analysis.
Dopamine (DA, 3,4-dihydroxy phenylalanine) is an essential biological small molecule, which plays an important biological roles in human metabolism as well as central nervous system of mammalian1,2,3. The higher concentration of DA is a cardiotoxic including death of heart muscles, high blood pressure and high heart rate. Besides, the higher concentration of DA will cause a serious of neurological disorders such as Parkinson’s disease, hypertension, and schizophrenia4,5,6. Therefore, the low level detection of DA in biological samples has been much important in analytical and biomedical applications. To date, the electrochemical methods have been widely considered for the determination of DA owing to their remarkable properties such as simplicity, low cost, eco-friendly and higher sensitivity2. Over the past decays, various modified electrodes have been utilized for the determination of DA in the literature. However, these modified electrodes have not suitable for the determination of DA due to poor selectivity, sensing in higher oxidation potential and fouling to the signal response7,8. Hence, there is a demand to develop the modified electrodes with those requirements that can be employed by exploiting the hybrid nanomaterials.
So far, numerous hybrid nanomaterials have been constructed for the electrochemical sensor and biosensor applications. Specifically, reduced graphene oxide (RGO) is a well known sp2 hybridized 2D planar carbon nanosheets, have been received much attention for assembling different nanomaterials due to its structural, electrical and conducting properties9,10,11. However, the chemical modifications and/or functionalizations have been used frequently to improve their structural, physiochemical and conducting properties by introducing the new functional groups12,13,14. Meanwhile, the noble metal nanoparticles such as Pt, Au, and Ag have been attained much attention in catalysis owing to their unique physical and chemical properties15. In particular, gold nanoparticles (AuNPs) have been widely adapted in various potential applications such as electrocatalysis and biosensor due to their excellent conductivity, specific large surface area and good biocompatibility16,17,18. Gallic acid (GA, 3,4,5-trihydroxybenzoic acid) is a polyphenolic compound consists of three adjacent hydroxyl groups at benzene ring which facilitates considerable ability to reduce the GO19,20. Besides, the GA has been covalently attached with the edge plane of GR sheets through H-bonding between the OH groups of GA21,22. Here, GA acts as a functionalization and reducing as well as stabilizing agent for the preparation of GA-RGO/AuNPs. In previous reports, Stathi et al. reported the immobilization of gallic acid on graphene oxide for XPS and EPR Study with long term stability in both solid form and aqueous solution21. Li et al. studied the dispersion ability of reduced graphene oxide at room temperature using GA as a reducing and stabilizing agent with excellent dispersion ability both in water and organic solvents20. Xu et al. was prepared reduced graphene oxide by GO was deoxygenated with GA for Li-storage applications with satisfactory storage performance23. To the best of our knowledge, there are no reports available for the determination of DA using GA supported RGO/AuNPs.
Conversely, the analytical performance of the proposed sensor such as linear response range and detection limit (LOD) for DA has been compared with previously reported DA sensor using AuNPs and RGO nanohybrids. For example, Liu et al. used Layer-by-layer assembled multilayer films of reduced graphene oxide/gold nanoparticles for the electrochemical detection of dopamine. The sensor presented the response range is 1–60 μM with a detection limit of 20 nM based on S/N = 324. Rao et al. utilized the glassy carbon electrode modified with MnO2, graphene oxide, carbon nanotubes and gold nanoparticles for DA sensor. The sensor showed the response range for DA is 0.5 μM to 2.5 mM with a detection limit of 0.17 μM25. Thanh et al. prepared the seed assisted synthesis of gold nanoparticle/nitrogen-doped graphene nanohybrids for electrochemical detection of glucose and DA. The sensor displayed the response range for DA is 38 nM to 48 μM with a detection limit of 10 nM18. Khudaish et al. fabricated the glassy carbon electrode modified with poly(2,4,6-triaminopyrmidine) decorated AuNPs for DA sensor. The sensor exhibited the response range for DA is 0.15–1.5 μM with a detection limit of 17 nM26. Baig et al. prepared the direct electrochemical reduced graphene oxide and AuNPs for simultaneous detection of DA and uric acid which shows the response range for DA is 0.1–25 μM with a detection limit of 24 nM27. Kwak et al. used thermally reduced AuNPs/graphene nanocomposite for electrochemical detection of DA. The sensor showed the response range for DA is 0.1–100 μM with a detection limit of 95 nM28. Hou et al. used the hierarchical nanoporous AuAg alloy for the sensitive detection of DA and uric acid. The sensor displayed the response range for DA is 5–335 μM with a detection limit of 0.2 μM29. Vinoth et al. prepared AuNPs/RGO nanocomposite using a facile “single-step one-pot approach” for electrochemical detection of DA and uric acid. The sensor exhibited the response range for DA is 0.05–11 μM with a detection limit of 20 nM30. Among them, the advantage of our proposed sensor showed a much lower detection limit (LOD) and good linear response range towards the detection of DA.
In this study, we report a one-pot facile synthesis of GA-RGO/AuNPs composite for sensitive and selective electrochemical detection of DA for the first time. The functionalization of GA-RGO was achieved by the bond formation between the carboxylic group of graphite oxide and the phenolic group of GA. The as-prepared GA-RGO/AuNPs composite was highly stable on the electrode surface and possessed excellent electrocatalytic activity and good sensing performance towards the detection of DA. The GA-RGO/AuNPs modified electrode was successfully applied for the determination of DA via differential pulse voltammetry (DPV), with excellent analytical performance such as good linear response, lower detection limit (LOD) and better sensitivity. The sensor was achieved a good recovery for the determination of DA in human serum and urine samples. The overall procedure of the preparation of GA-RGO/AuNPs composite was shown in Fig. 1.
Results and Discussion
The structural morphology of the as-prepared hybrid materials were characterized by SEM and TEM analysis. Figure 2 shows the SEM and TEM images of (A) GO, (B) RGO and (C) GA-RGO/AuNPs composites. As can be seen that the GO showed a crumpled and folded sheet like structures (Fig. 2A). After the hydrothermal reaction, these folded sheets of GO were well reduced and exfoliated sheet like morphology has been observed which leads to the better surface area (Fig. 2B). The similar kind of morphology has been observed in the literatures, suggesting the successful formation of RGO31,32.
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