White-Emitting Protein Nanoparticles for Cell-Entry and pH Sensing

Facile synthesis of white-emitting, protein-based, metal-free, stable, nontoxic, and pH sensitive, advanced functional nanoparticles (GlowDots), as alternatives to quantum dots, is reported here. Controlled cross-linking of bovine serum albumin resulted in facile formation of spherical nanoparticles of 35 nm in diameter with a sharp size distribution (+/- 10 nm), which were then conjugated with specific dyes to produce white-emitting particles with tunable excitation wavelengths. Chemical novelty is that the particle size, size distribution, stability, surface chemistry, and emission properties are under full chemical control where the size and absorption/emission properties are independently tuned. Up to 100 dye molecules were attached to each particle, on an average, and hence, particles acquired strong absorption cross-sections as well as high brightness. White fluorescence of GlowDots is strongly sensitive to pH over a range of pH 2-11, and pH-induced emission changes are fully reversible. The particles readily entered HeLa cells and emission color depended on particle location in the live cells, which is most likely due to the local environment surrounding the particles. These are the very first reports of white-emitting advanced functional nanoparticles that are biodegradable, sensitive to pH, and amenable for live cell imaging to probe the subcellular compartments.