Carbon-based Nanosensors for Salicylate Determination in Pharmaceutical Preparations

Thiourea derivative-based carbon paste electrode (TUD1-CPE) was constructed as a potentiometric sensor for the determination of salicylate anion in pharmaceutical formulations, Aspocid® and Aspirin®. The optimized CPE contained 45.5 % graphite, 0.5 % reduced graphene oxide (rGO), 46.0 % nitrophenyl octyl ether (NPOE) plasticizer, 5.0 % TUD1 ionophore, and 3.0 % tridodecylmethyl ammonium chloride as additive. The incorporation of NPOE of high dielectric constant, and rGO in electrode caused better performance of the sensor; Nernstian response of 59.0 mV decade⁻¹ in the concentration range of 10⁻¹–10⁻⁵ mole L⁻¹, a detection limit of 1×10⁻⁵ mole L⁻¹ in a very short response time of 6 seconds. The prepared sensor showed high selectivity against similar anions (i. e. ClO^-₄, benzoate, I⁻, SCN⁻). Selectivity was confirmed by calculating the formation constant (K_β) using sandwich membrane method, where K_β for TUD1-salicylate is 10^0.43. Theoretical calculations at DFT-B3LY/6-31G** level of theory were performed to find interaction mechanism, Energies of HOMO and LUMO orbitals, non-linear optical (NLO) properties (the electronic dipole moment (μ), first-order hyperpolarizability (β), the hyper-Rayleigh scattering (β_HRS) and the depolarization ratio (DR)), and other global properties; these calculations showed lower values of β and DR, higher value of β_HRS, and the shortest lengths of the four N−H bonds between TUD1 and salicylate which confirm their strong complexation and salicylate-selectivity. Also, all the studied anion-TUD1 exhibited relatively high NLO properties, and these results were considered as a preliminary study for investigating new types of NLO bearing materials. The sensors were applied successfully for the determination of salicylate anion in Aspocid® and Aspirin®.