Less is not Always More: Creating Super Bright and Robust Polymer-Based Fluorescent Nanomaterials

Polymer-based fluorescent nanoparticles (NPs) are the brightest tool in developing the fields of bioimaging, diagnostics, and sensing. By translating existing fluorophores to a nanoparticle platform, photostability and the fluorescent signal can be drastically increased, while enabling the use of lipophilic dyes in aqueous media. This expands the fluorescent probe toolbox and dramatically increases signal fidelity. Here, we use perylene diimide (PDI) loaded polystyrene nanoparticles as a platform to investigate the effects of surfactant capping and dye loading, and we demonstrate nanoparticle stability, pH invariability, and increased emission intensity. The target is PDI loaded NPs that provide properties identical to a perfectly water soluble PDI dye, that is, a component with a nanosecond lifetime and bright emission in the orange. This was achieved by increasing the PDI loading of the NPs beyond where quenching was expected, which resulted in a new class of nanoparticles that are 100 times brighter than PDI and stable for months. These findings indicate that the usual design paradigm: "Less is more" for fluorescent dye loading is not always the optimal solution to achieve the maximal signal.