Synthesis Of Fluorescent Carbon Dots And Their Applications For Bioimaging And Cancer Therapy

Carbon dots(CDs),a novel type of carbon nanomaterials with a size smaller than 10 nm,are usually named fluorescent carbon dots due to their excellent fluorescence properties.Compared with traditional fluorescent organic dyes,CDs show various merits,including cost-effective and facile preparation,wide and continuous excitation and emission spectra,excitation-dependent and multicolor fluorescence emission,and excellent photostability for long-term fluorescence tracking.Besides,due to the remarkable biocompatibility,CDs also exhibit great advantages over traditional semiconductor quantum dots commonly synthesized with toxic heavy metal elements.These fascinating properties make CDs an ideal alternative to organic fluorescent dyes and semiconductor quantum dots in numerous areas,such as catalysis,light-emitting diodes(LED),biochemical detection,bioimaging,drug delivery,cancer therapy,and antibacterial treatments.In this thesis,we have developed novel CDs for live/dead differentiation of bacterial and fungal cells,subcellular imaging,and drug delivery.The details are shown below:(1)In the first work,we synthesized a novel type of CDs using bacterial cells,including Escherichia coli(E.coli)and Staphylococcus aureus(S.aureus).The as-synthesized CDs showed a highly negatively-charged surface,which might be related to the high negative charges on the bacterial surfaces.After incubation with live microbial cells,it was found that the CDs could not enter the cells due to the electrostatic repulsion between the CDs and the live microbial cells.However,for dead microbial cells,owing to their compromised cell surfaces,the CDs could enter the cells and light them with strong fluorescence.Consequently,we realized the live/dead differentiation of bacterial and fungal cells using the bacteria-derived CDs,which possessed great advantages over the commercial dye PI,including superior biocompatibility,lower fabrication cost,and better aqueous dispersity.(2)With the carbon sources of chitosan,ethylenediamine,and mercaptosuccinic acid,we synthesized N,S-doped multicolor fluorescent carbon dots(termed CEM CDs)via a one-pot hydrothermal treatment.CEM CDs exhibited various merits,including low cytotoxicity,easy and low-cost synthesis,good photostability,and modifiable surface.After imaging of tumor cells with CEM CDs,it was found that the CDs could directly target mitochondria without extra modification of mitochondria-targeted ligands.The mechanism of the mitochondria-targeting ability of the CEM CDs was verified to be their positive surface charges and conjugated structures,and it was found that after targeting mitochondria,the CDs could in turn reduce the negative charge of the mitochondrial outer membrane.Afterward,we further achieved the mitochondria-targeted delivery of a photosensitizer,rose bengal(RB),via the conjugation between CEM CDs and RB,which finally enhanced the anticancer therapeutic efficacy of RB due to the higher susceptibility of mitochondria toward the singlet oxygen produced in the RB-based photodynamic treatment.This work suggests the feasibility of using mitochondria-targetable CDs for mitochondria-targeted drug delivery and enhanced cancer therapy.(3)Using m-phenylenediamine and cysteine as the carbon sources,we synthesized green-emissive CDs(named m PC CDs).It was confirmed that the CDs could enter the nucleus and realize high-quality nucleolar imaging at a low CD concentration of 5 μg/m L and a short staining duration of 5 min.Compared with the commercial nucleolus imaging dye SYTO RNASelect,the m PC CDs showed various advantages of lower cost,simpler staining process,higher photostability,superior imaging quality,and richer surface functional groups for drug conjugation and delivery.In addition,compared to SYTO RNASelect that could only stain the nucleoli of fixed cells,the CDs could realize the nucleolar imaging of both live and dead cells.The mechanistic investigation revealed that the selective RNA binding ability was the main factor determining the nucleolus-targeting performance of m PC CDs as numerous RNAs exist in nucleolar area.We then conjugated a common photosensitizer protoporphyrin IX(Pp IX)onto the m PC CDs and demonstrated that the CDs could successfully carry Pp IX into the nucleus to realize nucleus-located photodynamic therapy.This work highlights the possibility of using CDs for nucleus-targeted drug delivery and cancer treatment.(4)Using p-phenylenediamine as the carbon source,we systematically explored the effect of metal ion introduction on the properties of the synthesized CDs and found that the metal ions could largely affect the fluorescence quantum yield of the CDs.Specifically,Ni,Pd,and Fe ions could increase the quantum yield,while Cu,Pt,and Ag ions decreased it.More importantly,all the CDs synthesized using p-phenylenediamine and one type of the metal ions mentioned above did not contain metal elements,indicating the catalytic role of these metal ions and ensuring the excellent biocompatibility of prepared CDs.Afterward,we found that Ni-pPCDs(the CDs synthesized using Ni ions and p-phenylenediamine)possessed the highest red fluorescence quantum yield of 45.6%(in dimethyl sulfoxide),good photostability and water dispersity,polarity-responsive and excitation-independent fluorescence emission,and excellent nucleolus-targeting capability.Benefiting from their unique fluorescence properties,Ni-pPCDs could also be utilized for the stimulated emission depletion(STED)super-resolution fluorescence imaging of the nucleoli with a resolution of ～150 nm.Besides,the excellent in vivo imaging performance of Ni-pPCDs in mice and zebrafish was also confirmed,suggesting the great potential of the CDs for in vivo fluorescence tracking.