Described herein are optical agents, including compositions, preparations and formulations, for monitoring the pH of a fluid. Optical agents described herein include photonic nanostructures and nanoassemblies including supramolecular structures, such as shell cross-linked micelles, that incorporate at least one linking group comprising one or more photoactive moieties that provide functionality as exogenous agents for a range of pH monitoring applications. Optical agents described herein comprise supramolecular structures having linking groups imparting useful optical and structural functionality. In an embodiment, for example, the presence of linking groups function to covalently cross link polymer components to provide a cross-linked shell stabilized supramolecular structure, and also impart useful optical functionality, for example by functioning as a fluorophore.