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It is important to understand the nanomaterials intracellular trafficking and distribution and investigate their targeting into the nuclear area in the living cells. In our previous study, we firstly observed penetration of nonmodified positively charged carbon dots decorated with quaternary ammonium groups (QCDs) into the nucleus of mouse NIH/3T3 fibroblasts. Thus, in this work, we focused on deeper study of QCDs distribution inside two healthy mouse NIH/3T3 and L929 cell lines by fluorescence microspectroscopy and performed a comprehensive cytotoxic and DNA damage measurements. Real-time penetration of QCDs across the plasma cell membrane was recorded, concentration dependent uptake was determined and endocytic pathways were characterized. We found out that the QCDs concentration of 200 µg/mL is close to saturation and subsequently, NIH/3T3 had a different cell cycle profile, however, no significant changes in viability (not even in the case with QCDs in the nuclei) and DNA damage. In the case of L929, the presence of QCDs in the nucleus evoked a cellular death. Intranuclear environment of NIH/3T3 cells affected fluorescent properties of QCDs and evoked fluorescence blue shifts. Studying the intracellular interactions with CDs is essential for development of future applications such as DNA sensing, because CDs as DNA probes have not yet been developed. [ABSTRACT FROM AUTHOR]

Tissue regeneration based on stem cell therapy is one of the most rapidly developing fields of modern medicine. Several properties of human mesenchymal stromal cells (MSCs), such as tropism toward a tumor or injury site, make them promising candidates for regenerative medicine, targeted therapy, or treating injured tissues. However, to fully understand the role of stem cells in therapeutic function, their visualization in vivo is essential. Here, we describe, for the first time, the use of biocompatible quaternized carbon dots (QCDs) as a novel stem-cell tracking probe for in vivo fluorescence imaging of transplanted human MSCs. By studying the in vitro cytotoxicity, intracellular distribution, and precise uptake mechanism, we showed that QCDs had a high biocompatibility and excellent fluorescence properties after 24 h incubation with MSCs. Further to demonstrate the in vivo feasibility of the system, QCD-labeled MSCs (100 μg/mL of QCDs, 24 h incubation time) were transplanted subcutaneously into an immunodeficient mouse and visualized by optical in vivo imaging. The labeled cells were strongly fluorescent, allowing their semi-quantitative detection. Moreover, the homing of intravenously transplanted QCD-labeled MSCs into the solid tumor was clearly shown. The results demonstrated that QCD-labeling of human MSCs is a highly promising approach for in vivo tracking during stem cell therapy. Image 1 [ABSTRACT FROM AUTHOR]