Elsevier

Electrochimica Acta

Volume 260, 10 January 2018, Pages 526-535
Electrochimica Acta

Facile synthesis of Pd−Cu@Cu2O/N-RGO hybrid and its application for electrochemical detection of tryptophan

https://doi.org/10.1016/j.electacta.2017.12.125Get rights and content

Highlights

  • The Pd−Cu@Cu2O/N-RGO hybrid was synthesized via a facile wet chemical route.

  • An enhanced electrocatalytic property of the hybrid has been demonstrated.

  • The constructed sensor displays super performances for the tryptophan detection.

  • Advantages include excellent practicability with remarkable reliability.

Abstract

In this work, the N doping together with Pd−Cu@Cu2O hybridization for graphene oxide was achieved by a combined process of hydrothermal treatment and chemical reduction, based on which a novel Pd−Cu@Cu2O cubes decorated N-doped reduced graphene oxide (Pd−Cu@Cu2O/N-RGO) hybrid was obtained. The synthesized Pd−Cu@Cu2O/N-RGO was detailedly characterized by various technologies. The results show that the low Pd loading Cu@Cu2O cubes with the sizes of 300–500 nm are well dispersed on N-RGO sheets, thereby avoiding the serious aggregation and maintaining a large electroactive surface area of the attained hybrid. Benefiting from the synergistic effect of the properties of Pd−Cu@Cu2O particles and N-RGO sheets, the Pd−Cu@Cu2O/N-RGO modified electrode exhibits remarkable electrocatalytic performance on the oxidation of tryptophan with the enhanced oxidation response and the lowered oxidation overpotential. Under the optimal conditions for the electrochemical detection of tryptophan, the constructed sensor displays a wide linear range (0.01–40.0 μM) and a low detection limit (1.9 nM), outperforming most of the reported hybrid-based sensors. The proposed sensor also features good selectivity, stability and reproducibility, which has been successfully applied for the detection of tryptophan in the urine and milk samples with satisfactory recoveries. All these results suggest that the Pd−Cu@Cu2O/N-RGO hybrid could be a promising and convenient material for the fabrication of high-performance electrochemical sensors.

Introduction

Tryptophan is one of the eight essential amino acids for the human, which plays a vital role in human growth and metabolism. Metabolic disorder of tryptophan may induce hallucinations, delusions and Alzheimer's disease, and especially the strongly oxidized product of tryptophan can cause some cancers. It has been reported that early diagnosis of gastric cancer can be achieved by the tryptophan assay in gastric juice [1]. Therefore, establishing a simple and convenient method for the tryptophan detection is critically important. To date, various methods have been developed for the tryptophan determination including chromatography [2], chemiluminescence [3], colorimetric method [4] and electrochemical methods [5,6]. Since tryptophan can show desirable electrochemical response, electrochemical techniques with great sensitivity, high accuracy, fast response and simple operation have been given much more attention and expectation among these methods. Yet, it is hard to directly assay tryptophan at bare electrodes due to the high overpotential originating from the sluggishness of the electrode itself. In order to achieve an efficient electrochemical detection, the development of novel materials for the electrode surface modification is always essential and remains a great challenge.

Graphene, a single layer of carbon atoms tightly packed into a two-dimensional honeycomb crystal nanostructure, has been widely explored for the fabrication of electrode sensing materials thanks to its unique properties, such as large specific surface area, high chemical stability, excellent adsorption performance, remarkable carrier mobility and low electronic noise at room temperature [7,8]. However, the electric conductivity of graphene cannot be completely controlled because it has no bandgap. Many researches have demonstrated that the electrochemical properties of graphene can be substantially improved by regulating its existential state and tailoring its electronic structure via chemical doping [[9], [10], [11], [12]]. Especially, N doping in graphene is deemed as an efficacious way to promote the electric conductivity by lowering the semiconducting gap, improve the electron-donor performance, increase the binding active sites and enhance the biocompatibility of graphene. In addition, it can also introduce defects in the doping sites of graphene, which facilitate the analyte diffusion. These appealing merits have driven the exploitation of N-doped graphene as a novel electrode material in the field of electrochemical sensors. Unfortunately, like graphene, N-doped graphene is hydrophobic and leads to irreversibly agglomerate which frustrates its further application. To overcome its poor processability, the hybridization is always a viable strategy to boost the dispersibility and effectively alleviate the agglomeration of the resulting graphene-based hybrids.

It is well-known that Pd, an important noble metal, possesses distinctive heterogeneous catalytic and electrocatalytic activities together with other excellent chemical and physical properties, including ruggedness, corrosion resistance, versatility and non-toxicity, so the integration of Pd nanocrystals on the graphene sheets has attracted enormous interest in the fields of catalysis, adsorption and sensing. Currently, lots of Pd-based nanomaterials have been synthesized and adopted as sensing materials in the electrochemical detections of hydrogen peroxide [13], desipramine [14], taxifolin [7], rutin [8] and isoquercitrin [8]. Nevertheless, due to the high cost of Pd, its application in numerous fields on a large industrial scale is restricted. Therefore, searching for relatively cheap low-Pd catalysts is extremely urgent for the commercialization of electrochemical sensors. A survey in related publications indicates that the main strategy for developing low-Pd catalysts is realized with the reduction of the loading of Pd by adding other cheap metals and/or transition-metal oxides [1517]. With this in mind, Cu and Cu2O are believed as good additives not only owing to their low cost and significant catalytic activities, but also attributing to their excellent chemical stability, low toxicity, abundance, and ease of preparation and functionalization. It has been reported that the excellent structural flexibility of Cu and Cu2O can also be beneficial to the promotions of their intrinsic electronic and catalytic properties [15,16,18,19]. Based on these advantages, the Cu@Cu2O combined with Pd hybrid (Pd−Cu@Cu2O) has been reported for the glucose electrocatalytic oxidation [15,20], ethanol electrocatalytic oxidation [18] and H2O2 electrocatalytic reduction [16]. These instances have also verified that in comparison with individual nanocrystals, Pd−Cu@Cu2O nanostructures exhibit much better catalytic performances via their synergy in electrochemistry. The synergy effect of Pd−Cu@Cu2O may emanate from the following aspects: (i) the electronic coupling contribution of Cu@Cu2O and Pd nanoparticles can greatly enhance the catalytic activity; (ii) upon introducing Pd, the conductivity of Pd−Cu@Cu2O composite is sharply boosted in comparison with Cu2O alone; and (iii) Cu@Cu2O can alleviate the aggregation of Pd nanoparticles, resulting in increases of chemical stability and catalyst utilization. Inspired by these pioneering works, incorporation of Pd−Cu@Cu2O particles into N-doped reduced graphene oxide (Pd−Cu@Cu2O/N-RGO) may pave a new avenue to improve the electrochemical properties of the graphene-based composites, on the basis of which it is worth anticipating to developing high-performance electrochemical sensors for the tryptophan assay. It is also worth noting that the catalytic activity and utilization efficiency of metal nanocrystals are inevitably associated with their size and morphology. Nowadays, there is a steady trend toward the exploration of metal nanocrystals with well-defined morphologies for electrocatalysis [15,16,1820]. In particular, the controlled fabrication of nanocrystals with various morphologies including nanospheres, nanorods, nanoflowers, nanowires and nanocubes is quite attractive but very challenging for researchers. It is generally accepted that the controllable synthesis primarily depends on the synthetic method and conditions. So far the approaches for the controllable syntheses of metal@Cu2O hybrids mainly comprise microwave synthesis [19], electrodeposition [18], sonoelectrochemical method [21], heating metal substrates at elevated temperature [22], galvanic replacement reaction [20] and seed-assisted growth in organic solvents [17]. Apart from these methods, the wet chemical routes can offer direct ways for the fabrication of the desired morphology and crystallinity by virtue of the variations of reaction conditions. To this end, a wet chemical method has been developed in this work for the preparation of Cu2O cubes with high dispersion on N-RGO succeeded by the introduction of Pd nanoparticles together with Cu onto the cubes by a co-reduction process to form the Pd−Cu@Cu2O/N-RGO hybrid. To our best knowledge, employing the Pd−Cu@Cu2O/N-RGO hybrid as an enhanced electrocatalyst for the rapid and effective detection of tryptophan has not yet been documented.

The aim of this work is to exploit the application of Pd−Cu@Cu2O/N-RGO hybrid in constructing high-performance tryptophan electrochemical sensors. As a precondition, the Pd−Cu@Cu2O/N-RGO hybrid was successfully prepared by a combined process of hydrothermal treatment and chemical reduction. To be specific, this protocol firstly utilized cheap and green N precursors to synthesize N-RGO sheets via a hydrothermal treatment, and then Pd−Cu@Cu2O particles were anchored onto the N-RGO sheets using glucose and ascorbic acid as the green reducing agents in aqueous solution without intensive energy input and rigorous reaction requirements. The received Pd−Cu@Cu2O/N-RGO hybrid affords desirable activity for the electrocatalytic oxidation of tryptophan with higher sensitivity due to the synergistic interactions among different components. More strikingly, the novel hybrid holds extraordinary electroanalytical performances such as superior selectivity, repeatability and stability accompanied with a low detection limit when used for tryptophan sensing.

Section snippets

Materials and apparatus

The full information about materials and apparatus can be found in Supporting Information (SI).

Synthesis of Pd−Cu@Cu2O/N-RGO hybrid

Graphene oxide (GO) was prepared from graphite powders by a modified Hummer's method [23]. Then, N-RGO was synthesized according to our previous report [24], in which a hydrothermal method was employed with GO as the starting material. Eventually, Pd−Cu@Cu2O particles were further in situ grown on the N-RGO sheets through a green, facile and two-step chemical reduction method. Specifically, the first

X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FT-IR) characterizations

The formation and crystalline structure of the PdCu@Cu2O/N-RGO hybrid can be confirmed by XRD (Fig. 1A). As shown from Fig. 1A, the diffraction peaks at 36.2°, 42.5°, 61.5° and 73.2° are attributed to the (111), (200), (220) and (311) planes of cubic Cu2O (JCPDS, No. 05–0667), respectively [16,26]. The diffraction peaks at 43.3°, 50.4° and 74.1° are assigned to the (111), (200) and (220) planes of metallic Cu (JCPDS, No. 04–0836), respectively [26]. The weak reflection peaks at 46.8°, 68.2°,

Conclusions

A tryptophan electrochemical sensor was successfully constructed employing the Pd−Cu@Cu2O/N-RGO hybrid as an advanced electrocatalyst, which was prepared through in-situ growth of Pd−Cu@Cu2O crystals on the N-RGO sheets by a simple wet chemical method. The N-RGO sheets with a large surface area that can function as an effective support matrix to disperse or stabilize Pd−Cu@Cu2O crystals, resulting in a substantial enhancement for the catalytic performance of the resulting hybrid. The

Acknowledgments

This work was financially supported by National Natural Science Foundation of China (No. 21505035, 21171174, 21472038), Provincial Natural Science Foundation of Hunan (No. 2016JJ3028, 09JJ3024), Scientific Research Projects of Education Department of Hunan Province (No. 16A029, 15A027) and Hengyang Normal University (No. 16D04), and the Aid Programs for Innovative Team and Key Discipline in Education Department of Hunan Province (No. 2014207).

References (41)

  • F.H. Cincotto et al.

    Electrochemical sensor based on reduced graphene oxide modified with palladium nanoparticles for determination of desipramine in urine samples

    Sens. Actuators B

    (2017)
  • Y. Ji et al.

    3D porous Cu@Cu2O films supported Pd nanoparticles for glucose electrocatalytic oxidation

    Electrochim. Acta

    (2017)
  • H. Rostami et al.

    An electrochemical method to prepare of Pd/Cu2O/MWCNT nanostructure as an anode electrocatalyst for alkaline direct ethanol fuel cells

    Electrochim. Acta

    (2016)
  • B. Wang et al.

    Rapid synthesis of Cu2O/CuO/rGO with enhanced sensitivity for ascorbic acid biosensing

    J. Alloy. Compd.

    (2017)
  • P. Liu et al.

    Non-enzymatic glucose biosensor based on palladium-copper oxide nanocomposites synthesized via galvanic replacement reaction

    Sens. Actuators B

    (2017)
  • H. Ashassi-Sorkhabi et al.

    Sonoelectrosynthesized polypyrrole-graphene oxide nanocomposite modified by carbon nanotube and Cu2O nanoparticles on copper electrode for electrocatalytic oxidation of methanol

    J. Taiwan Inst. Chem. E

    (2016)
  • R.C. Wang et al.

    Cu, Cu-Cu2O core-shell, and hollow Cu2O nanodendrites: structural evolution and reverse surface-enhanced Raman scattering

    Acta Mater.

    (2011)
  • J. Li et al.

    High-sensitivity paracetamol sensor based on Pd/graphene oxide nanocomposite as an enhanced electrochemical sensing platform

    Biosens. Bioelectron.

    (2014)
  • S. Mao et al.

    Sensitive electrochemical sensor of tryptophan based on Ag@C core-shell nanocomposite modified glassy carbon electrode

    Anal. Chim. Acta

    (2012)
  • J.V. Kumar et al.

    Green synthesis of a novel flower-like cerium vanadate microstructure for electrochemical detection of tryptophan in food and biological samples

    J. Colloid Interface Sci.

    (2017)
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