Calixarene-doped PVC polymeric films as size-selective optical sensors: Monitoring of salicylate in real samples

Preparation of novel salicylate-selective optical sensors (bulk optodes) was performed and applied successfully for salicylate determination in pharmaceutical formulations, Aspirin® and Aspocid®. t-butyl calix[4]arene ionophore was incorporated in a plasticized poly (vinyl-chloride) membrane containing the chromoionophore ETH5294 (O₁) or ETH7075 (O₅). The optical response to salicylate was due to size-selective extraction of salicylate from the aqueous solution to the optode bulk through formation of hydrogen bond accompanied by chromoionophore protonation, that resulted in the optical response at 680 or 540 nm for O₁ or O₅, respectively. Reliable size-selectivity was measured for salicylate over other anions; The calculated selectivity coefficients of O₅ optode were found to be: -4.4, -2.0 and - 3.7 for iodide, benzoate and perchlorate, respectively. The hydrogen bonding mechanism and selectivity pattern were ensured and explained by IR and ¹H NMR spectroscopy. For the same purpose, a molecular recognition constant of β_sal=10^0.043 was calculated using sandwich membrane method, and its small value ensured that hydrogen bonding interaction is responsible for the optode response. The detection limits of O₁ and O₅ in salicylate buffered solutions were 9.0 × 10^-5 and 8.9 × 10^-5 M with response times of 5 and 3 min, respectively, and with very good reversibility. The practical utility of the developed sensors was ensured by salicylate determination in Aspirin® and Aspocid®. Beyond the observed analytical performance, the present work aims not only to effectively apply Calixarene without derivatization, but also to estimate the strength of the size-dependent hydrogen bonding and comprehensively study the interaction mechanism.