A novel nanocomposite electrochemical sensor based on green synthesis of reduced graphene oxide/gold nanoparticles modified screen printed electrode for determination of tryptophan using response surface methodology approach

https://doi.org/10.1016/j.microc.2020.104634Get rights and content

Highlights

  • Reduced graphene oxide was synthesized for the first time using E. tereticornis leave extract during a mild reaction condition.

  • Gold nanoparticles could be synthesized uniformly using tereticornis leave as an environmentally friendly reducing reagent.

  • SPE/rGO/AuNPs has a selective and sensitive signal for oxidation of tryptophan.

  • A simple, affordable and fast method was developed for pretreatment of saliva sample.

  • Tryptophan was determined successfully in spiked saliva, serum and plasma samples by using the fabricated electrochemical biosensor.

Abstract

In this study, a new method is presented for the direct determination of L-tryptophan (Try), using E. tereticornis leave as an environmentally friendly reducing agent for synthesis of reduced graphene oxide/gold nanoparticles (rGO/AuNPs). Scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy analyses showed a uniform distribution of gold nanoparticles on thin layers of reduced graphene oxide. The synthesized nanocomposite was used as a sensitive and electroactive substrate on the surface of screen-printed electrode for electrochemical oxidation of Try in biological samples. Differential pulse voltammetry (DPV) and cyclic voltammetry (CV) showed a reliable analytical signal for the oxidation of Tryptophan at 0.65 V. Response surface methodology (RSM) model was utilized for improving the sensitivity of detection based on central composite design. Under the optimum conditions (Britton Robinson buffer, pH: 6.0; AuNPs/rGO ratio (W/W): 4 and drop casted nanocomposite volume: 5 µl), the calibration curve of Try is linear between 0.5–500 µmol/L (R = 0.9976) with detection limit (LOD) and limit of quantification (LOQ) of 0.39 and 1.32 µmol/L, respectively. In addition, the SPE/rGO/AuNPs electrochemical sensor showed good reproducibility, high sensitivity and good selectivity towards the determination of Try, which makes it suitable for the analysis of Try in human plasma, serum and saliva.

Introduction

Lung cancer is one of the most common cancers in the world and is the leading cause of cancer death in humans. Tryptophan is a specific and important indicator in the diagnosis of lung cancer with a clear detection rate of 92%. The results show that the concentration of tryptophan in blood lung cancer patients is significantly lower than healthy individuals. L-tryptophan (Try) is one of the important standard amino acids that is vital for human life by controlling the nitrogen balance in body. In addition, it is responsible for producing Niacin for synthesis of neurotransmitter serotonin. Therefore, Tryptophan detection in biological fluids has significance in clinical studies [1]. There are many methods for determination of Try in various biological fluids, including fluorescence spectroscopy, capillary electrophoresis (CE), high-performance liquid chromatography (HPLC) and chemiluminescence (CL) approaches [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. These techniques have some drawbacks in terms of sample size, analysis time and expert operator [15]. Therefore, the need for a reliable, fast and sensitive method for determination of tryptophan in biological fluids is in high demand.

Electrochemical techniques are simple, portable, fast, cost-effective and sensitive for trace analysis of biomolecules [16]. Because of excellent electron transfer capability and large surface area of reduced graphene oxide (rGO), it is a suitable choice for using in electrochemical sensors. Generally, gold nanoparticles (AuNPs) is used to enhance the sensitivity of electrochemical sensors as it offers great catalytic and conductivity properties [17]. rGO/AuNPs nanocomposite can be easily synthesized and has various potential applications [17], [18], [19], [20]. Generally, rGO is decorated with AuNPs by an indirect or a direct deposition process. In the process of direct deposition, rGO surface is directly coated with AuNPs by the reduction of chloroauric acid, while during the process of indirect deposition, a covalent bond is created between AuNPs and GO during the process of nanocomposite production [21]. Jin et al. reported that a rGO/AuNPs nanocomposite can be used to facilitate electron transfer with free diffusion of redox species [22]. Nonetheless, the production of the mentioned nanocomposite is limited in certain cases because of the tendency of AuNPs which can block the active surface area of GO.

In this research, we synthesized a nanocomposite of AuNPs and rGO using E. tereticornis solution (extracted from Eucalyptus tree) as an environmentally friendly reducing agent for simultaneous reduction of carboxyl groups of GO and HAuCl4 under mild temperature and short reaction time with uniform distribution of gold nanoparticles prior to the fabrication process. E. tereticornis solution is one of the green chemistry reducing reagents [23]. The fabricated nanocomposite by was used as a sensitive and electroactive substrate on the surface of screen printed electrodes for electrochemical oxidation of tryptophan. In this work, Response Surface Methodology (RSM) is utilized to explore the relationships between numerous explanatory variables and response variables [24]. RSM is defined as methods that combine some statistical techniques in experimental design, model development, evaluating the main effective factors and variables and predicting the responses at optimized conditions [25], [26], [27]. The conventional methods in which a parameter changes at one time while the other variables are kept constant is not able to evaluate the impact and interactions of parameters on the response [28]. RSM can find the optimum values of all the parameters affecting the response and therefore it has the capability of reducing the number of experiments [29] for optimization. In addition to these, the above method is able to recognize the main factors with the greatest influence on the response and also the overall effect of all the variables and factors on the final response. RSM methods enable us to distinguish the importance of each parameter on the desired response. To the extent of our knowledge, this is the first report on the application of SPE/rGO/AuNPs based electrochemical sensors in the determination of tryptophan in biological fluids, including saliva, plasma and serum, utilizing RSM methodology and sustainable chemistry.

Section snippets

Materials and apparatus

Graphene oxide, gold (III) chloride hydrate (99.9%) and Tryptophan (98%) were supplied by Sigma–Aldrich (TCI, China). Phosphoric acid (≥85 wt.%, Sigma–Aldrich), Acetic acid ((≥99 wt.%, Sigma–Aldrich), Boric acid (≥99.5 w.t.%, Sigma–Aldrich) and sodium hydroxide ((≥98 w.t.%, Sigma–Aldrich) were supplied and used for preparation of Britton–Robinson buffer. Serum, plasma and saliva samples were taken from healthy volunteers and stored frozen at −20°C before using.

A portable potentiostat (Drop

Morphological and elemental composition studies

The morphology of the SPE before and after modification with rGO/AuNPs was studied by FESEM to confirm the incorporation of rGO with AuNPs. Fig. 1 shows the SEM images of bare SPE (a), modified SPE with rGO (b) and modified SPE with rGO/AuNPs (c). The white network of deposits in Fig. 1(a) is carbon, as mentioned by Jian et al. [31]. The grooved surface of the sheets in Fig. 1(b) is rGO after treatment with E. tereticornis reagent as the environmentally friendly reducing agent. In Fig. 1(c),

Conclusions

A new and effective electrochemical sensor for determination of Try residue in biological samples (saliva, serum and plasma) was presented by using an environmentally friendly reducing agent (E. tereticornis). Raman spectroscopy showed that E. tereticornis is a promising green reducing agent for graphene and AuNPs synthesis. The green synthesized nanocomposite has multiple promising properties for selective analysis of tryptophan (free from the effect of interfering species) in biological

Author statement

It is with excitement that I resubmit to you a revised version of manuscript MICROC_2019_2999, “A Novel Nanocomposite Electrochemical Sensor based on Green Synthesis of Reduced Graphene Oxide/Gold Nanoparticles Modified Screen Printed Electrode for Determination of Tryptophan using Response Surface methodology Approach” for the Microchemical Journal. Thank you for giving us the opportunity to revise and resubmit this manuscript. In keeping with my last conversation with you, I am resubmitting

CRediT authorship contribution statement

Somayeh Nazarpour: Formal analysis, Writing - original draft. Reza Hajian: Methodology, Supervision, Project administration, Writing - review & editing. Mina Hosseini Sabzvari: Resources, Software.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to thank Azad university of Gachsaran and Omidiyeh sites for supporting this project.

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