Green Synthesis Of Multi-functional Carbon-based Nanomaterials And Their Applications In Biomedical Fields

In the 21st century,non-communicable chronic diseases have become the main threat to the healthy life of human beings.The key to the treatment of non-communicable chronic diseases is the early accurate diagnosis and effective drug treatment.Nanomaterials,as contrast agents and drug carriers,have the advantages of accurate diagnosis and high efficiency of drug transport,which have become one of the hotspots in the diagnosis and treatment of non-communicable chronic diseases.Since the discovery of fullerenes in the 1980s,carbon nanomaterials have attracted great attention because of their unique optical,electronic,chemical,thermal and mechanical properties.Spherically shaped carbon nanomaterials have attracted much attention from researchers in the biomedical fields because of their advantages of small size,large specific surface area,unique optical properties and good biocompatibility.At present,the research of spherical carbon nanomaterials is mainly concentrated on the carbon quantum dots and carbon nanospheres.The optimal particle size of endocytosis is<100 nm,therefore,the porous carbon nanospheres below 100 nm can be the ideal drug carriers to carry drugs to specific disease sites of the body;In addition,the unique fluorescence characteristics of carbon quantum dots make them ideal fluorescent contrast agents and can realize accurate diagnosis in the organism.However,the present methods for preparing carbon nanospheres and carbon quantum dots still have some shortcomings,such as the used precursors are expensive,the synthesis conditions such as temperature are harsh,the preparation procedures are complex,the obtained products are difficult to purification and may have some residual chemical precursors and so on.So it is urgent to explore new raw materials and new methods for synthesizing these two kinds of carbon nanomaterials.Therefore,the aim of this study is to investigate simple,effective and green methods for synthesizing carbon nanospheres,carbon quantum dots and the hybrid quantum dots based on carbon quantum dots and to further apply the synthesized nanomaterials to biomedical applications,such as drug loading,cell imaging,and multimodal bioimaging.Based on these reasons,the main contents of this paper are as follows:?1?Green synthesis of carbon nanospheres and their pH responsive delivery of doxorubicin to cancer cellsPorous carbon nanospheres with an average particle size of about 50 nm were synthesized by using bacterial cellulose nanofibers as raw materials via hydrothermal synthesis and acidification treatment methods.After that,the drug loading and release capacity of carbon nanospheres were evaluated.On this basis,the cytotoxicity of carbon nanoparticles was tested.The distribution of drug-loaded carbon nanospheres in tumor cells was analyzed.Besides,the drug release mechanism of carbon nanospheres in tumor cells was also explored.The results show that the carbon nanospheres synthesized from bacterial cellulose nanofibers have the advantages of homogeneous size,good dispersibility of solution?containing a lot of hydrophilic functional groups?and large specific surface area(208.17 m² g^-1),and are amorphous carbon composed of many small polycyclic aromatic carbon sheets.The as-prepared carbon nanospheres are of some graphitic characteristics and can combine with the antitumor drug doxorubicin by supramolecular?-?stacking and hydrophobic interactions.They show a high drug loading capacity?the drug encapsulation efficiency is 93.4+1.6%and the drug loading efficiency is 52.3+2.1%?,besides,the encapsulation of doxorubicin in carbon nanospheres does not change the chemical structure of doxorubicin,and the doxorubicin can be released with its original structure unchanged.The carbon nanospheres have greater release in acidic and weakly acidic media,but less in neutral and alkaline media.For example,in pH 5.5 medium,the cumulative release efficiency after 48 h can reach 61.56+2.6%,while in the medium of pH 7.4,the cumulative release efficiency after 48 h is only about 18.96+2.3%.The synthesized carbon nanospheres exhibit negligible cytotoxicity within a certain concentration range,besides,they can also enter tumor cells through endocytosis and achieve a sustained and effective release of doxorubicin under the weak acidic conditions of tumor cells.The low toxicity and low cost of carbon nanoparticles enable them to become an advanced nano biological carrier for the transportation of antitumor drugs under physiological conditions.?2?Okra-derived nitrogen-doped carbon quantum dots and their bioimaging applicationCarbon quantum dots were synthesized by a simple,green and efficient hydrothermal method with natural okra juice as the precursor.By controlling the temperature of reaction,the carbon quantum dots with an average particle size of about4.2 nm were prepared.Then,a series of physicochemical properties of carbon quantum dots were characterized.The cytotoxicity of carbon quantum dots for HeLa cells?human cervical carcinoma cells?was evaluated.The fluorescence imaging of carbon quantum dots in HeLa cells and living zebrafish were also conducted.The results show that the nitrogen-doped carbon quantum dots prepared from okra juice have the advantages of excellent dispersibility,tunable fluorescence?excitation wavelength-and pH-dependent fluorescence?,good fluorescence stability,high quantum yield?about18.9%?and negligible cytotoxicity.More importantly,the synthesized carbon quantum dots can be used as effective fluorescence probes for fluorescence imaging of tumor cells and living zebrafish,indicating the potential of their application in biomedical diagnosis.?3?Preparation of C-Fe₃O₄ QDs and their application in in vivo multimodal imagingMultifunctional nitrogen-doped carbon-iron oxide hybrid quantum dots?C-Fe3O4QDs?were prepared via a series of heat treatments by using?-polyglutamic acid as the precursor and stabilizer.By controlling the parameters of heat treatment process,C-Fe3O4 QDs with good water dispersibility,wavelength-tunable fluorescence property?the emission peak shows nearly no shift when the excitation wavelength is less than410 nm,while the emission peak has an apparent red-shift from 471 nm to 544 nm when excitation wavelength changed from 420 nm to 510 nm?,high fluorescence quantum yield?about 21.6%?,good fluorescence stability,strong superparamagnetic property,high transverse relaxivity value(r₂=154.10 mM^-1 s^-1)and good biocompatibility?after incubation with various concentrations of C-Fe3O4 QDs,all the survival rates of tumor cells are above 85%and the hemolysis rates of red blood cells are lower than 0.5%?can be prepared.The as-prepared C-Fe3O4 QDs show good fluorescence and magnetic resonance imaging contrast performance in vitro,and it is interesting to find that the prepared C-Fe3O4 QDs have a certain X-ray attenuation effect for computed tomography imaging which may be due to the nitrogen doping and the synergistic effect among them.Therefore,in vivo bioimaging of C-Fe3O4 QDs in tumor-bearing nude mice were conducted,which further proved that the synthesized C-Fe3O4QDs can be efficiently used in fluorescence/magnetic resonance/computed tomography triple-modal tumor imaging.Hence,the synthesized C-Fe3O4 QDs can serve as a versatile nanoprobe for tumorous diagnostics.