Photosensitizer-Conjugated Ultrasmall Carbon Nanodots as Multifunctional Fluorescent Probes for Bioimaging

Abstract

Highly luminescent ultrasmall carbon nanodots (CDs) have been prepared by one-step microwave-assisted pyrolysis and functionalized with fluorescein photosensitizer by a diazo-bond. The absorption edge of such prepared fluorescein–NN–CDs was red-shifted in comparison with the bare one. Nevertheless, the emission signal induced by the nanoparticle quantum-sized graphite structure was quenched due to photoisomerization of the diazo group at the photoexcited state. In order to restrict the photoisomerization, i.e., rotation around the nitrogen–nitrogen bond, the diazo group was fixed by a metal cation to form a complex compound or chelate. The obtained metal complex of fluorescein–NN–CDs shows an absorbance maximum the same as bare CDs but a recovered emission signal from the nanoparticle moiety, which was bathochromically shifted. They exhibit lower quantum yield in comparison with the bare CDs but better photostability toward emission quenching in nutrition cell culture. The formed photosensitizer-conjugated nanoprobes were proposed as multifunctional fluorophores for intracellular <i>in vivo</i> imaging due to their attractive photophysical attributes and tunable and excitation-dependent emission. The bioapplication of photosensitizer-conjugated CDs was demonstrated as fluorescent tracers for endocytosis pathways in cultured Tobacco cells. Their successful staining and lower toxicity to the plant cells were compared with conventional quantum dots (CdSe/ZnS core–shell type, which caused an acute toxicological <i>in vivo</i> effect)