Manganese dioxide based electrochemical sensor for the detection of nitro-group containing organophosphates in vegetables and drinking water samples

Highlights

• A non-enzymatic strategy for 4-NPP detection using α-MnO2 electrocatalysis.

• Synthesis of α-MnO₂ nanorods by potentiodynamic electrodeposition onto Platinum electrode.

• Adsorption controlled electrochemical reduction of 4-NPP on MnO₂/Pt electrode.

• Good linear response in the concentration range 100 nM to 900 nM and 1 μM to 25 μM and a limit of detection of 9.22 nM.

• Appreciable selectivity against metal cations and other organophosphates pesticides such as Quinalphos and Dimethoate.

Abstract

The widespread use of organophosphates in agricultural farms for pest control has raised serious concerns over the quality of food and water available to the common public. As an effort to fabricate a sensitive, selective, cost-effective and non-toxic sensor to detect nitro-group containing organophosphates in and vegetable washings, a simple manganese dioxide based sensor was developed. α-Manganese dioxide nano-rods were electrodeposited on platinum disk electrode (MnO₂/Pt) and is employed to detect 4-nitrophenyl phosphate (4-NPP). 4-NPP is a model compound that well represents widely used nitro-group containing organophosphates such methyl parathion, parathion, fenitrothion, methyl paraoxon and paraoxon in aqueous medium. Determination of 4-NPP at nanomolar levels was achieved using the fabricated sensor using cyclic voltammetry. The developed sensor was found to show a linear response in the concentration range 100 nM to 900 nM with a Limit of Detection (LOD) of 10 nM and a high sensitivity of 11.68 μA μM⁻¹. The sensor showed good selectivity against many of common inorganic ions and two of the major organophosphates: Quinalphos and Dimethoate but the selectivity is poor among other nitro-group containing aromatics.

Introduction

Acute and chronic toxicity associated with organophosphates which are widely used in vegetable and fruit farms all over the world for pest control has serious impacts on human health [1]. Irreversible phosphorylation of the carboxylic ester hydrolase, acetylcholinesterase by organophosphates at its serine residue accounts for their severe toxicity posed to the humans. The acetylcholinesterase inhibition disrupts acetylcholine metabolism resulting in the accumulation of acetylcholine in synapses and neuromuscular junctions causing the acetylcholine receptors to get over-stimulated and eventually get damaged [2]. The stability of the phosphorylated acetylcholinesterase against hydrolysis depends upon the alkoxy groups attached to the phosphorus [3]. Therefore, the level of toxicity varies from one organophosphate to another. With the exception of fenitrothion (moderately toxic), all nitro-group containing organophosphates such as methyl parathion, parathion, EPN (O-Ethyl O-(4-nitrophenyl) phenylphosphonothioate) are classified under “Class Ia (extremely hazardous)” category by the ‘WHO Recommended Classification of Pesticides by Hazard’ [4]. Apart from their extreme inherent toxicity, wide spread usage of them, especially methyl parathion and parathion at considerable levels by the farm owners and availability of multiple routes of entry for these into the human system (dermal contact and inhalation and ingestion of organophosphate contaminated food and water) [3] further aggravates the problem of pesticide poisoning.

Number of electrochemical [[6], [7], [8], [9]] and optical sensing [[10], [11], [12], [13]] strategies that employs biological recognition elements for organophosphate has been reported till now. Many have developed electrochemical organophosphate sensors based on electrocatalysis that does not make use of a biological recognition element such as an enzyme. The reported electrocatalyst materials include silicon carbide nanoparticles [14], zirconia nanoparticles [15,16] gadolinium-prussian blue–graphene composite [17], polyacrylamide-MWCNTs composite [18], nano-platinum intercalated Ni/Al layered double hydroxides [19], nanosilica-graphene composite [20], SWCNTs with ionic liquid composite [21], ionic silsesquioxane film immobilized on silica [22], polyaniline nanofibers-SWCNTs composite [23], gold nanoparticles-MWCNTs composite [24,25], gold nanoparticles-graphene nanocomposite [26], gold nanoparticles-nafion film composite [27], palladium-MWCNTs composite [28] and cerium oxide-reduced graphene oxide composite [29]. Yet the number of electrocatalyst materials explored remains relatively lower than the non-electrocatalysis based works such as enzymatic sensors and molecularly imprinted polymer based sensors [30].

Metal oxides are well known for their affinity towards phosphate groups. This affinity is seen to be utilized in “Metal Oxide Affinity Chromatography (MOAC)” as many metal oxides such as titanium dioxide, zirconium dioxide aluminium oxide, gallium oxide, iron oxide, niobium oxide, tin oxide, hafnium oxide and several others have been used as stationary states in liquid chromatography [31]. Good absorption rates of organophosphates ensure efficient electrocatalysis by reducing the adsorption component of the activation energy [30]. Some of the aforementioned metal oxides such as ZrO₂ [15,33], CeO₂ [29,34,35] and TiO₂ [36,37] have already been used to develop electrochemical organophosphate sensors. To the best of our knowledge, the ability of manganese dioxide in behaving as an electrocatalyst for organophosphate detection has not been studied yet. This is probably because unlike many other metal oxides such as clay minerals, hydrous iron(III) oxides and hydrous aluminium oxides, that develop a positive surface charge at near-neutral pH, manganese dioxide develops a negative surface charge [[38], [39], [40]]. Thus manganese dioxide has low adsorption rates at neutral pH. Interestingly, the manganese oxide attains a positive surface charge in the presence of alkaline earth metal cations (Ba²⁺, Sr²⁺ Ca²⁺, Mg²⁺) and transition metal cations (Mn²⁺, Co²⁺ and Ni²⁺) and therefore exhibits strong phosphate adsorption rates in the pH range 6–9 [41]. Synthesis of manganese dioxide having a high population of Mn²⁺ ions is therefore an appropriate strategy to allow high rates of organophosphate adsorption onto the oxide surface at neutral pH.

In this work, cyclic voltammetry is employed to deposit α-manganese dioxide nanorods on a platinum disk electrode in a three-step electrodeposition process. Potentiodynamic nature of the electrodeposition process and the stabilization steps involved are assumed to introduce a number of Mn²⁺ ions in the deposited oxide rendering its surface positively charged at neutral pH, thereby aiding in providing high adsorption rates. Cyclic voltammetry is exclusively employed in this work for the determination of 4-NPP to ensure that Mn²⁺ ions on the oxide surface are preserved.