Toward Organic Mimics of Metallic Nanoparticles: Tuning and Enhancing Fluorescence in Binary Dye-Based Nanoparticles.

A key characteristic of binary metallic nanoparticles is the extreme tuneability of their optical properties through small changes in their composition and structure. Organic mimics of such binary metallic nanoparticles with similar properties would be of major interest. Here, we present Binary Fluorescent Organic Nanoparticles (hereafter termed вFONs) made from two structurally similar but optically complementary polar and polarizable dyes (PPDs) (i.e., energy donor D and energy acceptor A) as such potential mimics. We designed dye-based core-shell and alloy вFONs with varying composition (i.e., D versus A content) and investigated their fluorescence properties. In both core-shell and alloy вFONs, the donor dyes efficiently harvest energy-through Förster Resonance Energy Transfer (FRET)-resulting in substantial brightness values (up to 1.3 10⁸ M^-1cm^-1 per nanoparticle). However, вFONs' luminescence properties were found to strongly depend on their nanostructuration. Core-shell вFONs exhibit a Nano Interfacial Emission Enhancement (NIEE) effect, consisting in a blue-shifted fluorescence and enhanced FRET-mediated fluorescence quantum yield, compared to alloy вFONs of identical composition-but devoid of a core-shell topology. Moreover, we demonstrate for the first time that the extent of this NIEE effect directly depends on вFONs composition, and increases with the D/A ratio. Furthermore, bioimaging in live cells reveals that вFONs retain their binary nature and unique fluorescence properties in cellular environments.