A sensitive pyrimethanil sensor based on porous NiCo₂S₄/graphitized carbon nanofiber film

Highlights

• NiCo₂S₄ nanoneedles growed on graphitic nanofiber film (NiCo₂S₄/GCNF) was prepared.

• NiCo₂S₄ arrays provide abundant active sites exposed to the electrolyte.

• NiCo₂S₄/GCNF sensor showed good sensing properties toward pyrimethanil detection.

Abstract

With the extensive use of pesticides, the problem of pesticide residues has become people's concern. In this work, NiCo₂S₄ nanoneedle arrays grown on an electrospun graphitized carbon nanofiber film (NiCo₂S₄/GCNF) is successfully prepared by a simple two-step hydrothermal method, and further applied to detection of fungicide pyrimethanil (PMT). NiCo₂S₄ arrays exhibit a unique core-shell structure with rough surface, providing abundant electrochemically active sites exposed to the electrolyte. The NiCo₂S₄/GCNF modified electrode displays excellent electrocatalytic activity, and the electrode surface is controlled both by diffusion and adsorption processes. When applied to PMT determination, NiCo₂S₄/GCNF sensor displays wide linear range from 0.06 to 800 μM with low detection limit (20 nM). Furthermore, the as-proposed sensor also displays other outstanding advantages, including simple preparation, low cost, perfect reproducibility and good application in practical samples. Such attracting analytical properties could be attributed to high electrocatalytic activity of NiCo₂S₄ and superior electrical conductivity of GCNF frameworks. In addition, the detailed oxidation mechanism of PMT at NiCo₂S₄/GCNF electrode was also studied. The results indicate that NiCo₂S₄/GCNF is a promising platform for PMT sensors.

Introduction

Pyrimethanil (PMT) is a common aniline pyrimidine fungicide. PMT could prevent the infestation of bacteria and kill bacteria by inhibiting the secretion of bacterium [1]. Commonly, PMT was widely used to control gray mold and blight of crops, such as cucumber, tomato, strawberry and pea [2,3]. Although PMT is less toxic, it can remain in agricultural products or drinking water, and posing a hazard to human health if it is used improperly or not collected at safe intervals. Therefore, developing a sensitive PMT detection method is very urgent. The current methods including gas chromatography-mass spectrometry (GC-MS) [4], high performance liquid chromatography (HPLC) [5,6], liquid chromatography-electrospray tandem mass spectrometry (LC-ET/MS) [7]，ultra performance liquid chromatography-triple quadrupole mass spectrometry (UPLC-MS) [8], and gas chromatography (GC) [9]. These methods have the advantages of high precision and palmary reproducibility. However, the inevitable disadvantages of expensive apparatus, complicated operation and time-consuming limit their wide applications.

Electrochemical sensors were widely used to quantify pesticide residues, which was due to its high sensitivity, fast response as well as low cost. Generally, the properties of electrode materials play a significant role in the electrochemical sensors. Therefore, seeking for novel electrode materials with good characteristic is great important. Over the past years, modified electrodes with noble metal materials (e.g. Pd [10], Au [11]) and alloy metals (Pt-Pd [12]) have been studied. However, their widespread application was limited by the high cost of electrode materials. Recently, the cost-effective transition metal oxides, such as MnO₂ [13], TiO₂ [14]，and binary transition metal oxides (NiCo₂O₄ [15,16] and MnCo₂O₄ [17]) were widely explored as novel electrode materials. Owing to the electronegativity of sulfur is smaller than that of oxygen, the interlayer elongation prevents the structure from disintegrating and facilitates electron transport [18], which promotes the wide application of binary metal sulfides in various fields. For instance, Yang et al. reported an anion exchange method to synthesized a ball-in-ball NiCo₂S₄ hollow structure for the construction of glucose sensor [19]. Zhu et al. obtained mesoporous NiCo₂S₄ nanoparticles by solvothermal route as supercapacitor electrode material with ultra-high capacitance [20]. Wu et al. synthesized NiCo₂S₄ nanotube arrays for lithium battery anode material [21]. Hence, NiCo₂S₄ nanostructures became the focus of research as a promising binary metal sulfide.

Herein, a needle-like NiCo₂S₄ arrays grown on an electrospun graphitized carbon nanofiber film (NiCo₂S₄/GCNF) was synthesized by a two-step hydrothermal method. The morphology and electrochemical behavior were characterized in details. The three-dimensional (3D) self-supporting structure of NiCo₂S₄/GCNF prevented the agglomeration of NiCo₂S₄ and increased the specific surface area. Additionally, its excellent electrocatalytic performance was promoted by the synergistic effect of NiCo₂S₄ and GCNF. Furthermore, it was successfully applied to the construction of PMT electrochemical sensor with good sensor performances.