Aqueous Synthesis of Multi-Carbon Dot Cross-Linked Polyethyleneimine Particles with Enhanced Photoluminescent Properties

Heavy-metal-free fluorescent nanoparticles with high photostability and low toxicity are highly desirable as imaging probes for biological applications. Here, a novel synthetic strategy to prepare ultrabright multi-carbon dot cross-linked PEI particles, namely CDs@PEI, through self-assembly of hydrophobically modified PEI and in situ generations of carbon dots within residual monomer-swollen micelles is reported. The resulting particles consist of numerous carbon dots, which are individually and homogeneously embedded within the PEI network, and have an average hydrodynamic diameter of approximately 100 nm with zeta-potential above +35 mV. The CDs@PEI particles possess the synergistic effect of photoluminescent carbon dot and crosslink-enhanced emission of PEI, giving the particles superior optical properties such as high fluorescence quantum yield (up to 66%) in the aqueous system, excitation-dependent emission phenomenon, stable fluorescence in a wide pH range, and resistance to photobleaching.