Carbon-Based Solid-Contact Calcium Ion-Selective Electrodes

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membranes Article Carbon-Based Transducers for Solid-Contact Calcium Ion-Selective Electrodes: Mesopore and Nitrogen-Doping Effects Yirong Zhang, Yitian Tang, Rongfeng Liang, Lijie Zhong *, Jiexian Xu, Huici Lu, Xiaofeng Xu, Tingting Han, Yu Bao, Yingming Ma, Shiyu Gan and Li Niu * Citation: Zhang, Y.; Tang, Y.; Liang, R.; Zhong, L.; Xu, J.; Lu, H.; Xu, X.; Han, T.; Bao, Y.; Ma, Y.; et al. Carbon-Based Transducers for Solid-Contact Calcium Ion-Selective Electrodes: Mesopore and Nitrogen-Doping Effects. Membranes 2022,12,903. https://doi.org/ 10.3390/membranes12090903 Academic Editor: Bernhard Schuster Received: 13 August 2022 Accepted: 13 September 2022 Published: 19 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Guangzhou Key Laboratory of Sensing Materials & Devices, Center for Advanced Analytical Science, School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China * Correspondence: Correspondence: ccljzhong@gzhu.edu.cn (L.Z.); lniu@gzhu.edu.cn (L.N.) Abstract: Solid-contact ion-selective electrodes (SC-ISEs) exhibit great potential in the detection of routine and portable ions which rely on solid-contact (SC) materials for the transduction of ions to electron signals. Carbon-based materials are state-of-the-art SC transducers due to their high electrical double-layer (EDL) capacitance and hydrophobicity. However, researchers have long searched for ways to enhance the interfacial capacitance in order to improve the potential stability. Herein, three representative carbon-based SC materials including nitrogen-doped mesoporous carbon (NMC), reduced graphene oxide (RGO), and carbon nanotubes (CNT) were compared. The results disclose that the NMC has the highest EDL capacitance owing to its mesopore structure and N-doping while maintaining high hydrophobicity so that no obvious water-layer effect was observed. The Ca2+-SC- ISEs based on the SC of NMC exhibited high potential stability compared with RGO and CNT. This work offers a guideline for the development of carbon-material-based SC-ISEs through mesoporous and N-doping engineering to improve the interfacial capacitance. The developed NMC-based solid- contact Ca2+-SC-ISE exhibited a Nernstian slope of 26.3 ± 3.1 mV dec−1 ranging from 10 μM to 0.1 M with a detection limit of 3.2 μM. Finally, a practical application using NMC-based SC-ISEs was demonstrated through Ca2+ ion analysis in mineral water and soil leaching solutions. Keywords: ion-selective electrode; potentiometric sensors; mesopore structure; N-doping; water layer 1. Introduction Ion-selective electrodes (ISEs) act as a type of classic electrochemical sensor that enable the detection and analyses of over 60 ions [1–4]. However, traditional liquid- contact ISEs have encountered difficulties in miniaturization and instrumentation [5,6]. To overcome this challenge and satisfy practical requirements, solid-contact ion-selective electrodes (SC-ISEs) have been proposed, which extend the application to ion detection in complex environments, for example, marine salts [7–10], human biofluids [11–13], and even biomolecules [14–16]. Advances in SC-ISEs stem from the development of state-of-the-art SC materials [17], which involves the two core issues of interfacial capacitance and hydrophobicity [18]. The need for reasonable consideration of capacitance and hydrophobicity was expounded upon in our previous report [19]. In principle, a high interfacial capacitance reduces the effect of ion flux and thus achieves the purpose of stabilizing the potential. Currently, a variety of SC materials have been developed, including conductive polymers (CPs) [20–24], carbon- based materials [17,18], transition metal oxides [25] or sulfides [26], redox couples [27–31] and noble-metal nanoparticles [32–35]. Among these, CPs bear a large redox capacitance but face interference from water layer and gas effects [36]. Attempts at overcoming this issue have included adjusting the thickness of the CPs [37] and designing hydrophobic CPs [38,39]. Noble metals have a relatively high application value. Carbon-based materials Membranes 2022, 12, 903. https://doi.org/10.3390/membranes12090903 https://www.mdpi.com/journal/membranes

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