Systematic approach in Mg²⁺ ions analysis with a combination of tailored fluorophore design

A systematic study of a series of diaza-18-crown-6 8-hydroxyquinoline (DCHQ) chemosensors, devoted to Mg(II) ion detection, was performed. Functionalization of DCHQ by peripheral substituents allowed the development of novel all-solid-state optodes via inclusion inside PVC-based polymeric films. The influence on the DCHQ-based optode response of the lipophilic sites functionalization and of the nature of the plasticizer, was investigated. Fluorimetric studies on optodes sensitivity towards a number of different metal cations (Ca²⁺, Na⁺, K⁺, Li⁺, Co²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Hg²⁺, Zn²⁺) and NH₄⁺ were carried out. The results demonstrated the suitability of the DCHQ optodes to perform fast monitoring (<10s) of magnesium (II) ions. Emission light signal was sufficiently brilliant to be captured by a low-cost computer webcam. The phenyl-substituted DCHQ-Ph derivative showed the best performance with a wide range for Mg(II) ion determination between 2.7 × 10^-7 and 2.2 × 10^-2 mol/L. It was possible, therefore, to determine the concentrations of Mg(II) in commercial fertilizer samples by DCHQ-Ph-based optodes with acceptable results: recoveries of 96.2-104.9% and relative standard deviation (RSD, n = 6) less than 5%. Moreover, in comparison to single sensors, the use of an array composed of five optodes (the ones showing the best performances in the preliminary tests) has allowed to reduce the RSD of magnesium determination in real samples (down to 3.7% with respect to 5.5% for single optodes) and to achieve a detection limit (estimated by s/n = 3 method) as low as 4.6 × 10^-7 mol/L.