Eco-Friendly Carbon Paste Sensor Modified with Multi-Walled Carbon Nanotubes, Cesium Chloride, and Sodium Dodecyl Sulfate for Ultra-Sensitive Voltammetric Detection of Levocabastine in Artificial Tears and Nasal Spray: A Promising Approach to Ocular Pharmacokinetic Studies

This work reports a novel, ultra-sensitive electroanalytical sensor based on a carbon paste electrode modified with multi-walled carbon nanotubes, cesium chloride, and sodium dodecyl sulfate (CPE/MWCNT-CsCl/SDS) for the determination of the antihistamine levocabastine (LC). The structural, morphological, and surface properties of the nanocomposite and modified electrode were thoroughly characterized. Cyclic voltammetry (CV) and square wave voltammetry (SWV) showed that combining MWCNTs and CsCl markedly enhanced the peak current by providing a porous, high-surface-area network, while the addition of SDS enabled remarkable preconcentration via electrostatic interactions and conferred antifouling properties. The oxidation of LC was an irreversible, predominantly diffusion-controlled process with a one-electron, one-proton rate-determining step. Under optimized conditions, the sensor exhibited a wide linear dynamic range (4.0 nM–22 µM in buffer) with a limit of detection of 0.9 nM in artificial tear fluid, and was successfully applied to a commercial nasal spray, with recoveries of 99.15–100.80% (RSD ≤ 1.12%). As a result, we were able to detect LC either alone or in the presence of azelastine HCl (AZE) or emedastine difumarate (EMD) using square wave voltammetry. The AGREE metric gave a score of 0.79, indicating the greenness of the overall analytical procedure. The sensor is suitable for routine quality control and ocular pharmacokinetic studies.