Simple, rapid and green one-step strategy to synthesis of graphene/carbon nanotubes/chitosan hybrid as solid-phase extraction for square-wave voltammetric detection of methyl parathion

Highlights

• One-step electrodeposition strategy to construct GR-based hybrid.

• One-step, green, simple and rapid.

• Highly efficient to solid-phase extraction of organophosphate.

• Low detection limit; enzymeless sensor.

• High reproducibility, long-time stability and good anti-interference ability.

Abstract

Simple, rapid, green and one-step electrodeposition strategy was first proposed to synthesis of graphene/carbon nanotubes/chitosan (GR/CNTs/CS) hybrid. The one-step electrodeposition approach for the construction of GR-based hybrid is green environmentally, which would not involve the chemical reduction of graphene oxide (GO) and therefore result in no further contamination. The whole procedure is simple and needs only several minutes. Combining the advantages of GR (large surface area, high conductivity and good adsorption ability), CNTs (high surface area, high enrichment capability and good adsorption ability) and CS (good adsorption and excellent film-forming ability), the obtained GR/CNTs/CS composite could be highly efficient to capture organophosphate pesticides (OPs) and used as solid phase extraction (SPE). The GR/CNTs/CS sensor is used for enzymeless detection of OPs, using methyl parathion (MP) as a model analyte. Significant redox response of MP on GR/CNTs/CS sensor is proved. The linear range is wide from 2.0 ng mL⁻¹ to 500 ng mL⁻¹, with a detection limit of 0.5 ng mL⁻¹. Detection limit of the proposed sensor is much lower than those enzyme-based sensors and many other enzymeless sensors. Moreover, the proposed sensor exhibits high reproducibility, long-time storage stability and satisfactory anti-interference ability. This work provides a green and one-step route for the preparation of GR-based hybrid, and also offers a new promising protocol for OPs analysis

Introduction

Graphene (GR), a perfect two-dimensional carbon material, has attracted much attention since the first report in 2004 [1], [2]. GR possesses many novel properties, such as exceptional thermal and mechanical properties, high surface areas (calculated value, 2630 m²/g), strong adsorptive ability and excellent electrical conductivity. To combine well unique properties of individual nanostructures, recently GR-based hybrids, as a multifunctional assembly, have been highly concerned in sensing applications [3], [4], [5], [6]. Various GR-based hybrids, including GR combined with metal nanoparticles (NPs), metallic oxide NPs, or chitosan, etc., have been fabricated as enhanced electrochemical sensing platforms [5], [6], [7], [8], [9]. It has been demonstrated that GR-based hybrids as sensing platforms display extraordinary activity. For sensing applications, how to construct a high-performance GR-based sensing platform in a simple, rapid, green and controllable way has become a key issue.

So far, GR-based hybrids are usually obtained from chemical reduction of graphene oxide (GO) sheets and drop-casted onto electrodes directly [10]. Such a preparation methodology has intrinsic limitations such as lack control of the film thickness, not so reproducible, and moreover, toxic chemicals are involved [11]. These severely restrict the further applications of GR. Recently, electrochemical reduction of GO to GR has drawn much attention due to its rapid, green and film thickness controllable nature. However, the electrochemical synthesis of GR was carried out via two steps, namely, GO being first assembled on the electrodes by solution deposition methods, and then being subjected to electrochemical reduction [6], [12]. More recently, it is further reported that GR can be prepared directly from GO dispersions by one-step electrodeposition under controlled potential [11]. Obviously, this one-step electrodeposition approach has several clear advantages: simpler and faster than the typically two steps electrodeposition approach; no toxic solvents are used and therefore will not result in contamination of the product; the high negative potential used can overcome the energy barriers for the complete reduction; and the final solid film of GR can be further used in sensor [6], [12]. In particular, the one-step electrodeposition provides an effective strategy for nanofabrication by deliberately introducing the functional building block into the assembly.

Recently, Wu et al. have reported that GR could be used as sorbent for direct electrochemical stripping analysis of nitroaromatic organophosphorus compounds (OPs), the large delocalized π electron system of GR could form strong π–π stacking interaction with the aromatic ring in the methyl parathion (MP), facilitating the strong bounding of MP on its surface [13]. Moreover conveniently, carbon nanotubes (CNTs), another perfect carbon material and similarly novel properties to GR, have attracted tremendous attention [14]. Their porosity and heterogeneity essentially allow them to interact with some organic compounds, (polynuclear) aromatic compounds, in particular, through π–π electronic and hydrophobic interactions [15], [16]. Previous studies on the adsorption of some hazardous organic molecules have shown that nitroaromatic OPs could strongly bind to the CNTs surface, due to the strong affinity of CNTs for phosphoric group [15], [16], [17], [18]. Du et al. further demonstrated the feasibility for electrochemical stripping analysis of OPs using CNTs as the host of OPs [17]. However, GR and CNTs, both of which can interact with OPs through π–π electronic stacking and hydrophobic interactions, are used separately until now. No reports concern the combination of GR and CNTs together for OPs sorption. Inspired by these, we first propose a simple, rapid and green one-step electrodeposition strategy to construct CNTs-decorated GR hybrid coating on a glassy carbon electrode, without involving the chemical reduction of GO and therefore result in no further contamination. Moreover, in this paper, we have found that CNTs can be skeleton to prop open GR film and greatly increase the surface-to-volume ratio of GR film, thus GR/CNTs/CS composite can be highly efficient to capture more OPs and used as SPE.

Chitosan (CS), obtained by deacetylation of nature chitin, is a linear hydrophilic polysaccharide. It is an attractive biocompatible, biodegradable, and nontoxic natural biopolymer that exhibits excellent film-forming ability. CS has been widely used to disperse nanomaterials and can provide a good biocompatible microenvironment to construct sensors.

Therefore, the resulting GR/CNTs/CS hybrid, combining the advantages of GR (large surface area, high conductivity and good adsorption for OPs), CNTs (high surface area, high enrichment capability and good adsorption for OPs) and CS (good adsorption and excellent film-forming ability), is believed to be highly efficient to capture OPs and used as SPE. Recently, a combination of solid phase extraction (SPE) with square-wave voltammetric analysis resulted in a low-cost, fast, sensitive, selective and stable electrochemical method for determination of OPs [13], [17], [18], [19], [20], [21]. In SPE procedure, the choice of appropriate sorbent is a critical factor to obtain full recovery and high-enrichment factor [22], [23], [24]. Therefore, although it has not been reported yet, it is reasonable to expect that the resulting GR/CNTs/CS hybrid could dramatically facilitate the enrichment and adsorption ability of OPs, effectively accelerate the electron transfer and realize their rapid, stable and sensitive stripping voltammetric detection.

To the best of our knowledge, this is the first report on one-step electrodeposition fabrication of GR/CNTs/CS hybrid. The obtained hybrid is very stable, shows strong adsorption ability of OPs, and can be further used as SPE for enzymeless detection of OPs, using MP as a model analyte. The detection limit of the proposed sensor was lower than many previously reported enzyme-based electrodes. Moreover, the detection limit and linear response range of the proposed sensor were also better than many other enzymeless electrochemical sensors. In addition, the simple, rapid and green one-step electrodeposition strategy provided here could be also used to construct more GR-based hybrids. And the GR-based hybrids might be a new series of highly efficient SPE, which offering new opportunity for simple, rapid, sensitive and enzymeless analysis of OPs.