Monitoring nanoparticle dissolution via fluorescence-colour shift

[La(OH)]$^{2+}$[ICG]$^{−}$$_{2}$ and [La(OH)]$^{2+}$$_{2}$[PTC]$^{4−}$ inorganic–organic hybrid nanoparticles (IOH-NPs) with indocyanine green (ICG) and perylene-3,4,9,10-tetracarboxylate (PTC) as fluorescent dye anions are used for emission-based monitoring of the dissolution of nanoparticles. Whereas ICG shows a deep red emission in the solid [La(OH)]$^{2+}$[ICG]$^{−}$$_{2}$ IOH-NPs, the emission of PTC in the solid [La(OH)]$^{2+}$$_{2}$[PTC]$^{4−}$ IOH-NPs is completely quenched due to π-stacking. After nanoparticle dissolution, the emission of freely dissolved ICG is weak, whereas freely dissolved PTC shows intense green emission. We report on the synthesis of IOH-NPs and nanoparticle characterization as well as on the fluorescence properties and how to avoid undesirable energy transfer between different fluorescent dyes. The emission shift from red (intact solid nanoparticles) to green (freely dissolved dye anions), indicating nanoparticle dissolution, is shown for aqueous systems and verified $\textit{in vitro}$. Based on this first proof-of-the-concept, the IOH-NP marker system can be interesting to monitor nanoparticle dissolution in cells and tissues of small animals and to evaluate cell processes and/or drug-delivery strategies.