Preparation Of Several Carbon-based Fluorescent Nanomaterials And Their Applications In Biochemical Analysis

Fluorescent materials have broad application prospects in biochemical analysis and biomimetic fields due to their unique optical properties.Compared with traditional fluorescent materials（such as semiconductor quantum dots and fluorescent dyes）,carbon-based fluorescent nanomaterials have the advantages of cheap and environmentally friendly which make them become the best choice for biochemical analysis and bioimaging applications.The graphene oxide quantum dots（GO-dots）,carbon dots（C-dots） and graphitic carbon nitride（g-C₃N₄） in carbon-based fluorescent nanomaterials have been widely used by the researchers because of their good water solubility,high chemical stability,stable fluorescence performance and good biocompatibility.At present,there are still some shortcomings of the synthesis of these three materials,such as the complex operation,strict condition and time-consuming.In addition,many excellent properties of them have not been fully utilized in their application,the related sensors also have some shortcomings.Therefore,it is of great significance to develope new fabricated methods and expand the application of these materials.In this paper,we have reviewed the preparation and application of GO-dots,C-dots and g-C₃N₄,and developed a series of green and efficient electrochemical and hydrothermal methods for synthesis of GO-dots,C-dots and g-C₃N₄.Furthermore,based on the different properties of the three materials,we builded several high-performance chemical/biosensors.The main research are as follows:（1） Establishing a green and simple method to synthesis of GO-dots.By using BC as the precursor and H₂O₂ as green oxidant,GO-dots was successfully synthesized through a one-pot hydrothermal oxidation method.We discussed the synthetic mechanism of GO-dots and found that oxidative active substances such as hydroxyl radicals and oxygen radicals play an important role in the preparation of GO-dots.The as-synthesized GO-dots have excellent fluorescence stability,low toxicity and excellent biocompatibility and can be applied in vitro bioimaging directly.The proposed preparation method not only avoids the use of strong concentrated acid and introduction of metal impurities contamination but also does not need any other post-processing steps.It is the fastest and the simplest method at present.（2） Founding new properties of C-dots.We successfully synthesized hydroxyl-rich C-dots by one-pot electrochemical carbonization of ethylene glycol.The as-synthesized C-dots have excellent reducibility,which can reduce the metal salts into metal nanoparticles.It is a simple and fast method without the use of any additional reductant,stabilizing agent and external energy.We discussed the reduction mechanism of C-dots and found the reducibility of C-dots is universal that can be used to synthesize gold nanoparticles（AuNPs）,silver nanoparticles（AgNPs） and gold/silver nanocomposites（Au@AgNPs）.We expanded the application fields of C-dots.In addition,the AuNPs exhibit excellent peroxidase-like activity,which can be used for colorimetric detection of glucose.（3） Expanding the new application fields of C-dots.Based on the ability of C-dots reducing Au³⁺ to form AuNPs,and the high affinity and etching effect between thiocyanate（SCN^-） and AuNPs,we found that SCN^- can inhibit the growth of the AuNPs.Therefore,an optical sensor is developed for the detection of SCN^-based on measuring the plasmon resonance absorption peak change of the AuNPs.This sensor exhibits excellent sensitivity（the limit of detection is 0.16 μM） and selectivity toward SCN^-.Meanwhile,this method can detect SCN^- in raw milk with satisfactory results.We provide a new approach for the sensing application of C-dots.（4） Establishing electrochemical method for the synthesizing ultrathin g-C₃N₄.We used melamine as the precursor,two platinum sheets as the positive and the negative electrode to synthesize ultrathin g-C₃N₄ with a thickness of about 2 nm.The structure of ultrathin g-C₃N₄ was characterized by a series of characterization methods,and the possible mechanism was discussed.Additionally,the ultrathin g-C₃N₄ exhibits excellent peroxidase-like activity that can catalyze H₂O₂ to oxidize 3,3’-5,5’-tetramethylbenzidine（TMB） for colorimetric detection of H₂O₂ and uric acid.The method has the advantages of operating at room temperature,eliminating the needs of dangerous chemical reagents and short preparation time,and provides a new method for the preparation of ultrathin g-C₃N₄.（5） Establishing a simple electrochemical “tailoring” process to synthesize the fluorescence graphitic carbon nitride nanodots（g-C₃N₄-dots）.The g-C₃N₄-dots with the average diameters of 5-8 nm was obtained by electrolysis for 3 hours with the bulk g-C₃N₄ as the precursor,the alkaline substance was the electrolyte,two platinum electrodes acted as the positive electrode and the negative electrode.The method is simple and avoids the use of complex instruments and toxic chemical reagents.Compared with the bulk g-C₃N₄,the preparation of g-C₃N₄-dots not only has a stronger fluorescence and better water solubility,but also can reduce Ag⁺ to AgNPs at 60 ℃.Based on the specific effect of biothiols on Ag⁺,we developed a sensor to detect biothiols by monitoring the change of absorbance of AgNPs.The proposed method exhibits excellent sensitivity and selectivity to biothiols and has been successfully applied for the detection of biothiols in human serum with satisfactory results.We expanded the new application field of g-C₃N₄.