Separation and concentration effect of f-MWCNTs on electrocatalytic responses of ascorbic acid, dopamine and uric acid at f-MWCNTs incorporated with poly (neutral red) composite films

Abstract

A novel conductive composite film containing functionalized multi-walled carbon nanotubes (f-MWCNTs) with poly (neutral red) (PNR) was synthesized on glassy carbon electrodes (GC) by potentiostatic method. The composite film exhibited promising electrocatalytic oxidation of mixture of biochemical compounds such as ascorbic acid (AA), dopamine (DA) and uric acid (UA) in pH 4.0 aqueous solutions. It was also produced on gold electrodes by using electrochemical quartz crystal microbalance technique, which revealed that the functional properties of composite film were enhanced because of the presence of both f-MWCNTs and PNR. The surface morphology of the polymer and composite film deposited on transparent semiconductor tin oxide electrodes were studied using scanning electron microscopy and atomic force microscopy. These two techniques showed that the PNR was fibrous and incorporated on f-MWCNTs. The electrocatalytic responses of neurotransmitters at composite films were measured using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). These experiments revealed that the difference in f-MWCNTs loading present in the composite film affected the electrocatalysis in such a way, that higher the loading showed an enhanced electrocatalytic activity. From further electrocatalysis studies, well separated voltammetric peaks were obtained at the composite film modified GC for AA, DA and UA with the peak separation of 0.17 V between AA–DA and 0.15 V between DA–UA. The sensitivity of the composite film towards AA, DA and UA in DPV technique was found to be 0.028, 0.146 and 0.084 μA μM⁻¹, respectively.

Introduction

Electrochemically functional polymer films and carbon nanotubes (CNTs) films have been of great interest for the past decade in the electrocatalysis reactions because of their unique chemical and electrochemical properties. Previous studies demonstrated conjugated polymers showing interesting electrocatalytic activities such as, reduction of acetylene in poly-3-methylthiophene, reduction of dioxygen at 4,4′,4″,4‴-tetraaminophthalocyanine macrocycles, oxidation of glucose by polypyrrole and oxidation of NADH, reduction of IO₃⁻, BrO₃⁻ at poly (neutral red) (PNR) films [1], [2], [3], [4]. On the other hand, CNTs have shown interesting electrocatalytic activities for both bioorganic and inorganic compounds such as glutathione, homocysteine, carbohydrates, NADH, nitric acid, hydrogen peroxide, etc. [5], [6], [7], [8], [9], [10], [11], [12]. In addition CNTs were also used for methanol oxidation in the fuel cells, detection of insulin and oxidation of acetic acid. Their wide varieties of applications have developed a great interest in the electrocatalysis field [13], [14], [15]. Even though the electrocatalytic activity of the CNTs and conjugated polymer films were individually good, some properties like mechanical stability, sensitivity for different techniques and electrocatalysis for multiple compounds were not efficient. So, new studies have been developed in the past decade for the preparation of the composite films, which are composed of both CNTs and conducting polymers with enhanced electrocatalytic activity [16], [17], [18].

The rolled-up graphene sheets of carbon, i.e., CNTs exhibit a π-conjugative structure with a highly hydrophobic surface. This unique property of the CNTs allows them to interact with some organic aromatic compounds through π–π electronic and hydrophobic interactions to form new structures [19], [20], [21]. These interactions were used for preparing composite sandwiched films for electrocatalytic studies such as selective detection of dopamine in the presence of ascorbic acid [22] and in the designing of nano devices with the help of non-covalent adsorption of enzyme and proteins on the side walls of CNTs. This resulted in a novel CNTs based nanostructures, which contain biochemical units in them [23]. The electrodeposition of CNTs with polypyrrole composite films too was studied in detail [24]. Electrodes modified with composite films were widely used in capacitors, battery and material science, photo electrochemistry, fuel cells, chemical sensors and biosensors [25], [26].

Ascorbic acid (AA) being a constituent of the cell has momentous biological functions and is an analyte of great importance. It is found in fruits, vegetables and in beverages especially those derived from fruit juices [28], which mainly act as a preventer of respiratory viral infections [29]. Dopamine (DA) is one of the most important neurotransmitters in the central nervous system of the mammals, which exist in the tissues and body fluids in the form of cations for controlling the nervous system [30]. Uric acid (UA) and other oxypurines are the principal final products of purine metabolism in the human body [31]. Abnormal levels of UA cause symptoms of several diseases, including gout, hyperuricemia and Lesch-Nyan disease [32]. However AA, DA and UA coexist in biological fluids such as blood and urine. Due to their crucial role in neurochemistry and industrial applications, several traditional methods have been used for their determination. Among those, electrochemical methods have more advantage over the other because; in these the electrodes sense the neurotransmitters, which are present with in the living organisms [27]. Electrochemical analysis on the unmodified electrodes, for example glassy carbon has limitations because of the overlapping of oxidation potentials of AA, DA and UA and hence often suffers from a pronounced fouling effect that results in rather a poor selectivity and reproducibility [33], [34]. For electrochemical analysis, multilayer films of shortened negatively-charged multi-walled carbon nanotubes (MWCNTs) were homogeneously and stably assembled on glassy carbon (GC) electrodes by layer-by-layer (LBL) method using electrostatic interaction of positively-charged poly(diallyldimethylammonium chloride) (PDDA) [22]. In the past, several modified electrodes were used for the simultaneous determination of AA and DA [35], [36], specifically PNR modified electrodes too were used [37]. Further, the simultaneous determination of AA and UA were also reported [38].

The electropolymerization of neutral red (NR) from aqueous solution and its catalytic activity towards biomolecules and different inorganic compounds were already reported. The structure of the neutral red monomer (C₁₅H₁₇N₄⁺) and the possible composition of the PNR film showing the tetramer were in the Scheme 1(a) and (b), respectively [39]. Further the literature survey reveals that previously there were no attempts made for the synthesis of composite film using CNTs with PNR. In this article, we report a novel composite film (f-MWCNTs-PNR) made of functionalized mutiwall carbon nanotubes (f-MWCNTs) incorporated with PNR, along with its characterization. Different concentration effects of f-MWCNTs in the composite film, the composite film's enhancement in functional properties, film stability, peak current and electrocatalytic activity are also reported. Not only limited to these, we also report its application on effective separation of biochemical compounds such as AA, DA and UA, even at very wide concentrations. The film formation process involved the modification of glassy carbon electrode (GC) with uniform well dispersed f-MWCNTs aqueous solution and then electropolymerization of the neutral red from neutral aqueous solution on the f-MWCNTs modified GC.