Fluorescent Cdots(N)-Silica composites: Direct synthesis and application as electrochemical sensor of fenitrothion pesticide

https://doi.org/10.1016/j.mseb.2021.115084Get rights and content

Highlights

  • A new nanocomposite of silica and Cdots-N was obtained by sol–gel process Electroanalytical.

  • Determination of fenitrothion with good sensitivity.

  • A very low limit of detection value (0.0011 μM) have been obtained for Fenitrothion.

  • Determination of Fenitrothion was successfully tested in real samples.

Abstract

In this work, we reported the synthesis of fluorescent and electrochemically active Cdots(N)-Silica composites by very simple route based on classical sol–gel techniques. The microporous composites exhibited luminescence associated with Cdots emitting centers, and were characterized by Ultraviolet–visible, photoluminescence, Fourier Transform InfraRed, Raman, X-Ray Diffraction, Scanning Electron Microscopy, Adsorption/desorption N2, Atomic force microscopy and electrochemical techniques. They were applied as electrochemical sensor, by the glassy carbon surface modification with Cdots(N)-Silica films, showing a consistent electrocatalytic performance for the analysis of fenitrothion pesticide, as determined by differential pulse voltammetry. The GCE/Cdots(N)-Silica composite sensors exhibited a detection limit of 1.05 × 10-9 mol L-1 associated with a high selectivity, and have been successfully applied for the determination of the fenitrothion pesticide in orange juice samples.

Introduction

Carbon quantum dots, more commonly known as Cdots, have aroused the interest of researchers in several areas. As a newest member of the carbon nanomaterials family Cdots has 0D structure with typical sizes smaller than 10 nm, encompassing amorphous (C sp3) or crystalline (C sp2), depending on the synthesis route employed. Its relevant properties are biocompatibility, solubility in water and photoluminescence. Different functional groups can be associated in its structure according to the synthetic route used, with hydroxyls and carboxylate groups being the most common. In addition, there is also the possibility of incorporating different elements in the carbon core, such as nitrogen and oxygen [1] directly linked to the carbon atoms. Such possibility can be achieved by the use of compounds that contain nitrogen atoms added to the reaction medium, or through the use of carbon sources that already have different elements present in the structure. The presence of different elements in the carbon core, can have strong influence on the emission properties, shifting the wavelengths to different regions of the visible spectrum when excited with ultraviolet sources. Such fluorescent phenomenon is directly related to the quantum confinement effect (electron-hole pair), and is sensitive to the functional groups and different elements present in the carbon core. Cdots are also being used as reducing agents for different metals, most commonly gold and silver [2], [3] and also to reduce graphene oxide [4] due to their ability to donate electrons, under the influence of the functional groups [5], [6]. In addition to being used as reducing agents, Cdots obtained by hydrothermal means have recently been used in association with a sol gel process, in the presence of a basic catalyst [7]. However, the direct generation of Cdots(N)-Silica nanocomposites, using only Cdots as catalyst, has not yet been reported. Therefore, in this work we describe the direct synthesis of the Cdots(N)-Silica microporous composite, and its use in the construction of new electrochemical sensors, here applied for the determination of the fenitrothion pesticide. Silica-based materials are being used in the development of electrochemical sensors with excellent performance for application in different areas, due to their chemical inertness, high porosity, thermal stability and biocompatible properties [8], [9], [10], [11]. For the production of silica gel coatings the most common method is the Stober process [12].

The indiscriminate use of pesticides with the objective of increase of production can promote serious problems in the health of humans. The fenitrothion pesticide (o,o-dimethyl o-(3-methyl-4-nitrophenyl) phosphorothioate, scheme 1) that belong, class of organophosphates, present action directly on the parasites' central nervous system and it is also considered very toxic to living beings by acting on the brain neurotransmitter levels, thyroid hormone levels, and impairing gonadal and sensory deprivation [13]. Fenitrothion belongs the ‘Red List’ of pesticides according to the UK Environmental Agency, can easily contaminate soils, rivers and springs and can cause serious damage to the environment and living beings and, toxic action of fenitrothion is due to the formation of linkage of proteins and metabolites of species with the P = O functional groups of pesticide resulting in the formation of the adducts [14], [15]. Several methods are employed to the determination of fenitrothion as spectroscopic [16] Surface plasmon resonance [17] chromatograph [18] and electrochemical methods [19]. It is important to say that the Cdots(N)-Silica composite obtained has presented different properties in the comparison with the Silica and Cdots individually, basically due to the synergic effect between the main characteristics that they show, for example, size nano, quantum confinement and functional groups of Cdots(N), and porosity and chemical stability of silica. In this regard, this work shows a new and simple route of synthesis to obtain Cdots(N)-Silica, a new composite material, that has been used for the development of an electrochemical sensor with good electrocatalytic activities, aiming to improve the control and detection of fenitrothion in real samples.

Section snippets

Reagents

The reagents used were of high purity: fenitrothion (Aldrich), glyphosate (Aldrich), oleymine (Aldrich), TEOS (Aldrich), NaOH (Aldrich), sodium acetate (Synth), KCl (Synth), potassium ferrocyanide and ferricyanide (Synth), hydroquinone (Aldrich), sodium carbamate (Aldrich)

Synthesis of Cdots-N

The electrochemical synthesis of carbon quantum dots, containing N heteroatom in its structure, Cdots(N), was carried out by adapting a previously reported procedure [20]: In a 200 ml Becker containing 150 ml of oleylamine,

UV–vis spectra

Cdots(N) and Cdots(N)-Silica are colorless and white materials, respectively, and their UV–vis absorption spectra are shown in Fig. 1. The Cdots(N) particles exhibited a main band located at 204 nm and a shoulder at 288 nm. The 204 nm intense band can be associated with π − π * transitions due to the presence of the C = Csp2 bonds. The shoulder at 288 nm is consistent with a n → p * transition of C = O functional groups on the surface [23], [24]. The presence of C = N bonds were expected to

Electrochemical measurements

The electrochemical behavior of the Cdots(N) and Cdots(N)-Silica materials has been investigated as glassy carbon modified electrodes (GCE) in the presence of the fenitrothion analyte, and the typical response can be seen in Fig. 10. The electrochemical measurements were performed in the cathodic direction, because of the expected reduction reaction of the NO2 group yielding hydroxylamine, NH(OH).

The GCE electrodes cleaned and modified with Cdots(N) and Cdots(N)-Silica exhibited

Conclusion

The combination between silica and Cdots(N) that resulted in the Cdots(N)-Silica composite formation has been proved, mainly by FTIR and Raman spectroscopy. The GCE modified with Cdots(N)-Silica composite has presented an excellent electrocatalytic property for the determination of fenitrothion in real samples. Such good performance was compatible with the special distribution of the Cdots(N) nanoparticles in the silica matrix, allowing effective interaction with the GCE surface and the

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.

Acknowledgements

RC acknowledges mackpesquisa and T.C.C acknowledges Mackpesquisa for Research grant.

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