One-step Preparation Of Proton-functionalized G-C₃N₄ And Its Application In Biosensing

Graphitic carbon nitride(g-C3N4)represents an important class of conjugated polymer semiconductors with the fascinating electronic band structure and excellent absorption capacity of visible light.Moreover,it also has high chemical stability,outstanding photocatalytic performance and good fluorescence properties.Although it has been widely utilized in the field of photoelectronics and catalysis,especially as the metal-free photocatalyst for water splitting,g-C3N4 has barely been used in direct biosensing applications.In general,the bulk g-C3N4 materials can be easily prepared by the thermal treatment of organic precursors,for example,cyanamide,dicyandiamide and melamine.However,the above process always needs a high reaction temperature,a long reaction time or a tedious purification step.Therefore,there're some problems need to be overcomed in the synthesis of g-C3N4.Herein,we report a one-step appraoch,which uses thiourea and nitric acid as source materials,to prepare proton-functionalized g-C3N4.By 4 h sonication-exfoliation,g-C3N4 nanosheets are produced thereafter.The optical properties are thoroughly investigated and a hemin sensing approach is thus developed.The specific contents are as follows:In chapter 1,brief reviews are given to the properties,preparation approachs and the modifications of g-C3N4,and their applications in the fields of catalysis and sensing as well.In chapter 2,the proton-functionalized bulk g-C3N4 is prepared and characterized.A one-step approach for the preparation of proton-functionalized bulk g-C3N4 has been proposed by 3 min microwave heating of the mixture of thiourea and nitric acid.g-C3N4 nanosheets are further obtained by sonication-exfoliation for 4 h with a yield of 16.8%.The reaction product is characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and fourier transform infrared spectra(FT-IR),respectively,confirming the resulting product is indeed g-C3N4,with typical tri-s-triazine structure and small amount of S;Meanwhile,the XRD results also reveals that the layered structure reduces after the sonication-exfoliation process,indicating the bulk g-C3N4 is exfoliated into nanosheet structure.The transmission electron microscopy(TEM)results indicate that the obatined g-C3N4 nanosheets are ultrathin and show an irregular lamellar structure,with a size of ca.60-90 nm.The atomic force microscopy(AFM)measurement confirms the thickness of about 3.3 nm,consistent with the TEM observation.The Zeta potential proves the prepared g-C3N4 shows a positive charge,i.e.,it is proton-functionalized.In chapter 3,the photoluminescence(PL)properties of the as-prepared g-C3N4 nanosheets are investigated.The obtained g-C3N4 nanosheets exhibit excitation-independent PL behavior in a certain range,with a decrease in the fluorescence intensity as the excitation wavelength increases.The excitation/emission maximum is 270 nm/380 nm,respectively.The PL intensity of g-C3N4 nanosheets remain very stable within the pH range of 1-12,and an ionic strength up to 2.0 mol L-1 KCl and irritating time of 2 h hardly pose any effect on the PL of g-C3N4.Based on the quenching effect of hemin on the fluorescence of the g-C3N4 nanosheets,a sensitive and fast sensing approach for the measurement of hemin is developed,with a LOD of 0.17 ?g mL-1 and a linear range of 0.5-6 ?g mL-1.The approach is successfully applied in the analysis of hemin in drug and human serum samples,demonstrating the viability of this approach.