Direct electrochemical determination of carbaryl using a multi-walled carbon nanotube/cobalt phthalocyanine modified electrode

Abstract

The electrochemical detection of carbaryl at low potentials, in order to avoid matrix interferences, is an important challenge. This study describes the development, electrochemical characterization and utilization of a glassy carbon (GC) electrode modified with multi-wall carbon nanotubes (MWCNT) plus cobalt phthalocyanine (CoPc) for the quantitative determination of carbaryl in natural waters. The surface morphology was examined by scanning electron microscopy, enhanced sensitivity was observed with respect to bare glassy carbon and electrocatalytic effects reduced the oxidation potential to +0.80 V vs. SCE in acetate buffer solution at pH 4.0. Electrochemical impedance spectroscopy was used to estimate the rate constant of the oxidation process and square-wave voltammetry to investigate the effect of electrolyte pH. Square-wave voltammetry in acetate buffer solution at pH 4.0, allowed the development of a method to determine carbaryl, without any previous step of extraction, clean-up, or derivatization, in the range of 0.33–6.61 μmol L⁻¹, with a detection limit of 5.46 ± 0.02 nmol L⁻¹ (1.09 ± 0.02 μg L⁻¹) in water. Natural water samples spiked with carbaryl and without any purification step were successfully analyzed by the standard addition method using the GC/MWCNT/CoPc film electrode.

Introduction

Carbamate compounds are the most widely used pesticides due to their high insecticidal activity and relatively low persistence [1]. Carbaryl (1-naphthyl methylcarbamate) was the first successful carbamate insecticide; it is broad-spectrum and used to control over 100 species of insect on crops, on lawns, ornamental plants, trees, in forestry and even on animals. It is also used as a molluscicide and acaricide [2]. Considered a short-lived pesticide, the mode of action of carbaryl in vertebrates and insects is based on inhibition of the activity of acetylcholinesterase (AChE) enzyme in the hydrolysis of the neurotransmitter acetylcholine, which is responsible for the transmission of nervous impulses [3].

The indiscriminate use of carbaryl can cause its bioaccumulation in food and water sources with subsequent bio-concentration through the food chain. In humans, acute and chronic occupational exposure has been observed to cause inhibition of cholinesterase and reduced enzyme activity in the blood, leading to neurological effects, nausea, vomiting, coma, respiratory failure and death [4]. The maximum residue levels of pesticides in food, water and vegetable samples are regulated by government agencies of most countries. Thus, sensitive and rapid determination of these compounds is important for the protection of the environment and of human health.

Most of the analytical methods employed for the determination of carbaryl have been based on chromatographic techniques using different detection systems [5], [6]. These methods do not easily allow continuous in situ analysis and often require several previous steps in sample preparation, which include an extraction and clean-up procedure in order to obtain a final extract fully compatible with chromatographic determination. These techniques usually generate waste-containing organic solvents, which makes the procedure more complicated and more expensive.

Electrochemical methods have the advantage of low cost, high sensitivity, easy operation, potentiality for miniaturization and automation, construction of simple portable devices for fast screening purposes and in-field/on-site monitoring. Several electrochemical methods to quantify carbaryl have been reported. These indirect methods are based on amperometric or voltammetric detection using the inhibition of acetylcholinesterase enzyme [7], [8]. However, the use of enzymes as modifiers of electrodes needs special care with respect to pH, temperature, applied potential, storage, and enzyme activity, which makes these systems more difficult to work with and less robust.

The direct electrochemical detection of carbaryl without derivatization was first reported by Rao et al. [9]. In 2006, Codognoto et al. [10] developed a methodology to quantify carbaryl in natural waters. In both cases, a boron doped diamond (BDD) electrode was used to oxidize carbaryl at the amide nitrogen in the carbamate molecule, that usually occurs at high potentials [11]. The carbaryl oxidation potential of 1.4 V at BDD is thus subject to the interference of many substances, especially in environmental matrices. In this sense, exploring the use of carbon nanotubes becomes an interesting alternative, as an electrocatalytic material to reduce the oxidation potential, for the direct electrochemical determination of carbaryl.

Since their discovery in 1991 [12], carbon nanotubes (CNTs) have been the target of numerous investigations, owing to their extraordinary mechanical, chemical and electronic properties [13]. The CNT mechanism of charge transport can vary from semiconducting-type to metallic-type depending on their radius and helical structure and ability to mediate electron transfer reactions with electroactive species in solution [14]. They have been studied extensively for a large variety of applications, particularly for solid-state chemical and biological sensors [15]. The development of new materials for electrochemical sensors with improved reproducibility, sensitivity and stability is one of the areas of greater and faster growth in materials science [16].

Metallophthalocyanine complexes (MPc) are well recognized for their excellent electrocatalytic activity in many reactions, which is highly dependent on the central metal atom (as well as on substituents in the case of aza-macrocyclic compounds). The change in the geometry of the complex during the redox process is one of the most important factors to be considered [17]. It is possible for phthalocyanines to be a convenient substitute for natural proteins, since they can act as the active centre of an enzyme molecule with the same efficiency and selectivity [18].

It is known that amino-substituted metallophthalocyanine complexes can be covalently linked to CNTs via amide bond formation [19] while non-substituted MPc complexes are non-covalently adsorbed onto CNTs via π–π interactions. It has been observed that phthalocyanine–CNT complexes have the excellent catalytic properties of phthalocyanines without losing any of the electronic properties of carbon nanotubes [20], [21]. In recent work, Moraes et al. development an electrochemical device using phthalocyanine-functionalized carbon nanotubes as a sensor for dopamine contaminated with a high concentration of ascorbic acid, reaching a detection limit of 2.6 × 10⁻⁷ mol L⁻¹ [22].

In the study reported here, a glassy carbon (GC) electrode surface-modified with multi-walled carbon nanotubes plus cobalt phthalocyanine (MWCNT/CoPc) was employed for the direct oxidative determination of carbaryl in pure and natural water samples using square-wave voltammetry (SWV).