Nanocomposite of copper oxide nanoparticles and multi-walled carbon nanotubes as a solid contact of a copper-sensitive ion-selective electrode: intermediate layer or membrane component–comparative studies

Abstract

In this paper, ion-selective electrodes sensitive to copper(II) ions are presented, in which new composite, synthesized from copper(II) oxide nanoparticles (CuONPs) and multi-walled carbon nanotubes (MWCNTs), was used as a solid contact. For comparison, electrodes obtained using separate components of the nanocomposite, i.e., CuONPs and MWCNTs, as well as unmodified electrodes, were also studied. The tested nanomaterials have been applied in two ways: as an intermediate layer placed between the ion-sensitive membrane and the internal electrode, and as an additional component of the ion-selective membrane mixture. To investigate the influence of the electrode’s structure modification, the selected analytical parameters obtained by potentiometric measurements (slope, linearity range, detection limit, potential stability, and reversibility) and electrochemical impedance spectroscopy measurements (membrane resistance and charge transfer resistance as well as double layer capacitance) were determined and compared. It was found that the use of all nanomaterials improves the properties of the electrodes, with the effect being the strongest for electrodes modified with the CuO-MWCNTs nanocomposite. The nanocomposite-based electrodes, both those with an intermediate layer and those with a nanocomposite-modified membrane, showed a Nernstian slope of the characteristic, a wider working range and a lower detection limit compared to unmodified electrodes. Moreover, application of all nanomaterials, especially nanocomposite resulted in improvement of both, stability and reversibility of the sensor potential. Modification of the electrodes did not make them sensitive to changing external measurement conditions (lighting, presence of gasses, redox potential). The electrode with the best parameters (based on nanocomposite) was successfully used to determine the Cu2+ ions content in tap water and mineral water, obtaining satisfactory results.

Application of Metal Oxide Nanoparticles in the Field of Potentiometric Sensors: A Review

“…A year later, the group of Wardak presented the CuO-MWCNT material, applied as a solid-contact layer in Cu 2+ -selective electrodes [112]. These electrodes were characterized by a low detection limit (1.5 × 10 -8 mol L -1 ) and very good potential stability.…”

Application of ionic liquids in ion‐selective electrodes and reference electrodes: A review

“…[28] This modification improved the electrical performance, i. e. increasing the electrical capacitance of the double layer, as well as reducing the resistance of the membrane, which in turn resulted in enhanced ion-to-electron conductivity, consequently obtaining a stable and reversible potential. [29] If highly hydrophobic layers are used, there is a high probability that a water layer (causes potential drift) will not form between the ISM and the intermediate layer or electrode contact. [30] Before a material can serve as a solid contact layer, it must meet certain requirements and exhibit (1) sufficiently high hydrophobic properties, (2) chemical inertness, (3) chemical stability, (4) good conductivity and electrical capacitance.…”

“…[30] Before a material can serve as a solid contact layer, it must meet certain requirements and exhibit (1) sufficiently high hydrophobic properties, (2) chemical inertness, (3) chemical stability, (4) good conductivity and electrical capacitance. [31] The most commonly used components of intermediate layers include carbon materials, e. g. graphene, [32] multi-walled carbon nanotubes (MWCNTs), [33][34][35] single-walled carbon nanotubes (SWCNTs), [36][37][38] fullerenes, [39] carbon nanofibers (CNFs); [40] con- ducting polymers, e. g. poly(3,4-ethylenedioxythiophene) (PEDOT), [41] polyaniline (PANI), [26,40,42] poly(3-octylthiophene) (POT); [43,44] nanoparticles of metals and metal oxides, e. g. nanoparticles of platinum, [28] silver, [45] copper oxides; [29] ionic liquids, [46] as well as composite materials consisting of two or more of the materials listed above. [31,47,48] In order to improve the performance of ion-selective electrodes, they are not only modified by the introduction of an intermediate layer (in liquid-contact ISEs this is not done, because solid contact does not occur there), but also additional components are introduced as additives to the membrane mixtures.…”

“…Consequently, the solution to improve these parameters was sought, and for this reason, the application of an intermediate layer between the electrode substrate and the ion‐selective membrane was proposed [28] . This modification improved the electrical performance, i. e. increasing the electrical capacitance of the double layer, as well as reducing the resistance of the membrane, which in turn resulted in enhanced ion‐to‐electron conductivity, consequently obtaining a stable and reversible potential [29] . If highly hydrophobic layers are used, there is a high probability that a water layer (causes potential drift) will not form between the ISM and the intermediate layer or electrode contact [30]…”

Graphene Oxide–Poly(vinyl alcohol) Hydrogel-Coated Solid-Contact Ion-Selective Electrodes for Wearable Sweat Potassium Ion Sensing

“…Such characteristics make it popular in recently focused wearable sensors toward noninvasive sweat analysis. − The solid-contact ISEs (SC-ISEs) are composed of a substrate, SC transducer, and ion-selective membrane (ISM). The SC transducer and ISM are mostly individual layers but also could be in a single-piece structure. − The SC transducers functionalize the transformation of ion concentration to an electronic signal. The top layer of the ISM plays a role in ion recognition through crucial components of ionophores.…”