Density Functional Theory (DFT) calculations for the adsorptive voltammetric determination of Meloxicam using a paste electrode made of Functionalized Carbon nanotubes

Meloxicam (MLC) is a nonsteroidal anti-inflammatory drug (NSAID) frequently used for the treatment of osteoarthritis and arthritis. In the present work, a simple and sensitive differential pulse voltammetric (DPV) method has been investigated for the determination of MLC drug using functionalized carbon nanotubes paste electrode (f-CNTs-PE). The acid treated CNTs were investigated by TEM, FT-IR and XPS techniques, which showed carboxylated functional groups in their structure. An adsorptive stripping voltammetric (AdSV) technique was employed to determine MLC in a 0.25 M Na₂HPO₄ solution at pH 9 using accumulation time (t_a) of 5 min at open circuit potential. The functionalized electrode exhibited good eletrocatalytic activity for the oxidation of MLC at an oxidation potential of +0.45V, owing to the strong interaction ability of the surface functional groups. Under optimized conditions, the proposed electrode demonstrated high sensitivity toward MLC, with good linear performance (R² = 0.9944) between peak current and MLC concentration in the range of 0.1–50 μM and a limit of detection (LOD) of 5 × 10⁻⁷ M. The interaction ability of the f-CNTs for MLC was also confirmed through density functional theory (DFT) calculations at the ωB97XD/6-31G (d,p) level using Gaussian 09 package. The high values of interaction energies (E_int) and the low HOMO-LUMO energy gap (E_H-L) of MLC@CNTs-COOH + HN₂ complex confirmed the sensitivity of CNTs towards MLC. Furthermore, the natural bond orbital (NBO) analysis and the charge transferred from MLC to CNTs were consistent with the voltammetric oxidation of MLC at the CNTs surface. Finally, the voltammetric method showed good selectivity for MLC in the presence of common interfering agents. The low cost and easily renewable CNTs-PE was successfully applied for the determine MLC in pharmaceutical tablets and in a human urine sample with better recoveries.