Bioimaging, pH sensing, and fluorescence lifetime imaging microscopy using polyethyleneimine coated carbon dots and gold nanoparticles

The unique fluorescent nanomaterials known as carbon dots (CDs) are highly resistant to photobleaching, have low toxicity, and are well soluble in water. Polyethyleneimine (PEI) coated CDs are a novel fluorophore with good biocompatibility and pH sensing ability. Here, p-phenylenediamine (p-PD) is used as a carbon source and hyperbranched PEI is used as a surface passivation agent in a simple, one-step hydrothermal synthesis process. The CDs optical characteristics are pH-responsive due to the presence of different amine groups on PEI, which is functional polycationic polymer. The limits of techniques based on fluorescence intensity can be overcome by fluorescent lifetime imaging microscopy (FLIM), a very sensitive method for detecting a microenvironment. In this study, FLIM was used to measure pH with pH-sensitive CDs. These molecules are nontoxic to the cells, and the positively charged CDs have the potential for nuclear targeting, allowing for electrostatic contact with DNA in the nucleus. Higher wavelengths have a larger penetration depth of electromagnetic radiation and low tissue autofluorescence, hence CDs emitting at these wavelengths are used for biolabeling applications. However, the quantum yield of these synthesized red-emissive CDs is lower. In order to enhance it, they are conjugated with gold nanoparticles(AuNPs) for metal enhanced fluorescence (MEF). Through a potent covalent bond between them, the AuNPs are linked to CDs surfaces. These gold-CDs nanoconjugate can be used in the future for targeted imaging applications.