Synthesis,Characterization And Biomedical Applications Of Fluorescent Calcium Phosphate/Carbon Dot Hybrid Composites

As the main inorganic component of bone and teeth,calcium phosphate is an important biomineral.Owing to its excellent biocompatibility and biodegradability,calcium phosphate has been exploited in biomedical field such as gene delivery,tissue engineering and drug delivery,etc.In addition,fluorescent calcium phosphate enables the tracking of calcium phosphate or the visualization of a tumor in line with drug delivery,which has been prepared by incorporation of organic dyes or lanthanide ions.However,the poor photostability and optical blinking of organic dyes may limit their biomedical applications.On the other hand,lanthanide-doped calcium phosphate with high crystallinity may be too large for cellular uptake.Typically,particle with a size less than 200 nm is able to avoid the recognition by the immune system and can be easily taken up by cells.Therefore,the aim of this study is to synthesize fluorescent calcium phosphate with good photoluminescent property and a size less than 200 nm.As the new form of fluorescent nanomaterial,carbon dots have distinct advantages such as good aqueous solubility,excellent photostability,low cytotoxicity,and non-blinking.Therefore,carbon dots have attracted tremendous interest in various applications including fluorescence sensing,bio-imaging,optoelectronic devices,catalysis and nanomedicine.We speculate that carbon dots could mediate the biomimetic formation of calcium phosphate,owing to the abundance of oxygen-containing functional groups on the surface.Meanwhile,carbon dots could serve as the fluorescent label to enable the calcium phosphate to emit fluorescence under a UV excitation.1.Synthesis and characterization of carbon dotsCarbon dots with a quantum yield over 65 % were synthesized by using citric acid as carbon source and polyethylenimine as passivation agent.The high resolution transmission electron microscopy image reveals that the as-prepared carbon dots are well-separated nanodots with a diameter of 2 3 nm in the aqueous solution.Due to the presence of oxygen-containing functional groups on the surface,the carbon dots could be utilized as the template to mediate the biomimetic synthesis of calcium phosphate.2.Synthesis and characterization of calcium phosphate/ carbon dot hybrid compositesFluorescent calcium phosphate/carbon dot hybrid composites were then obtained by a biomimetic method using carbon dots and carboxymethylcellulose as the dual templates.Transmission electron microscopy images indicate that the calcium phosphate/carbon dot hybrid composites are in needle-like shape with a length of 50 130 nm and a width of 10 30 nm.The X-ray powder diffraction pattern and Fourier transform infrared spectrum indicate the crystalline phase of calcium phosphate is hydroxyapatite.As the phosphor,carbon dots enable the calcium phosphate/carbon dot hybrid composites to emit blue luminescence under UV irradiation.3.Detection of metal ions with calcium phosphate/carbon dot hybrid compositesThe effects of metal ions and common anion ions on the fluorescence of calcium phosphate/carbon dot hybrid composites were explored.The fluorescence of the calcium phosphate/carbon dot hybrid composites may be effectively quenched by Fe³⁺ and Cu²⁺ ions,while other ions have no obvious effect on the fluorescence intensity.There is a good linear correlation between the quenching efficiency of calcium phosphate/carbon dot hybrid composites and the concentration of Fe³⁺ and Cu²⁺.The applicability of calcium phosphate/carbon dot hybrid composites for the selective detection of Cu²⁺ was examined in drinking water,and the interference of Fe³⁺ could be eliminated by addition of masking reagent.4.Cell labeling of calcium phosphate/carbon dot hybrid compositesThe cytotoxicity of calcium phosphate/carbon dot hybrid composites towards He La cells was evaluated by CCK-8 assay.The cytotoxicity of calcium phosphate/carbon dot hybrid composites is negligible in the concentration range from 0.1 to 2 mg/m L.Moreover,the calcium phosphate/carbon dot hybrid composites can efficiently label He La cells,suggesting their potential application in cell imaging.5.Synthesis,characterization and cell labeling of green luminescent carbon dotsIn the case of cell labeling,the green luminescent carbon dots might be more favorable due to the visible absorption and green fluorescence.Herein,a microwave-assisted method was employed to prepare green luminescent carbon dots with a quantum yield over 25 % through carbonization of citric acid.Moreover,these green luminescent carbon dots could be serving as nanoprobes for cell imaging,benefit from the green fluorescence and low cytotoxicity.In summary,fluorescent calcium phosphate with a dimension less than 200 nm were acquired by using carbon dots and carboxymethylcellulose as the dual templates.Moreover,the potentials of the as-prepared calcium phosphate/carbon dot hybrid composites for detection of copper ions in drinking water and cell labeling were testified.To ease the damage of ultraviolet and interference of autofluorescence,carbon dots with a fascinating character of green fluorescence and visible absorption were synthesized by the microwave-assisted method.In addition,these green fluorescent carbon dots could be efficiently uptaken by cells and exhibited a low cytotoxicity,suggesting their potentials in bio-imaging applications.