Ion-selective optode nanospheres as heterogeneous indicator reagents in complexometric titrations.

Traditionally, optical titrations of inorganic ions are based on a rapid and visible color change at the end point with water-soluble organic dyes as indicators. Adequate selectivity is required for both the indicator and the complexing agent, which is often limited. We present here alternative, heterogeneous ionophore-based ion-selective nanospheres as indicators and chelators for optical titrations. The indicating nanospheres rely on a weaker extraction of the analyte of interest by ion-exchange, owing to the additional incorporation of a lipophilic pH indicator in the nanosphere core. Ca(2+) titration was demonstrated as a proof-of-concept. Both the chelating and the indicating nanospheres showed good selectivity and a wide working pH range.