Electrochemiluminescence Properties And Applications Of Nitrogen-sulfur Double-doped Graphene Quantum Dots

In recent years,carbon-based luminescent nanomaterials have gained widespread attentions due to the their unique electrical and optical properties.As a new type of carbon nanomaterials,graphene quantum dots（GQDs）have the characteristics of good optical stability,abundant surface modification,large specific surface area,low toxicity,high biocompatibility and unique electronic transition ability.However,pristine GQDs have a major shortcoming of low quantum yield that limits their applications in microanalysis and ultramicroanalysis.In this paper,in order to improve the quantum yield of GQDs,we introduced nitrogen and sulfur into graphene quantum dots and then characterized their morphologies and optical properties.Based on their ECL properties,we constructed new ECL systems to detect Pb²⁺and epinephrine successfully.（1）The synthesis and characteristic of N,S:GQDsN,S:GQDs were prepared by a one-step hydrothermal carbonization of citric acid and thiourea.Their morphologies and optical properties were characterized by Transmission electron microscope（TEM）,X-ray photoelectron spectroscopy（XPS）,UV-vis spectrophotometer and fluorospectrometer,It is proved that N and S atoms are successfully introduced and the obtained N,S:GQDs are uniform in size,and exhibite excellent optical properties.（2）Investigation on ECL Performance of N,S:GQDsThe ECL signals of N,S:GQDs was investigated with the three-electrode system in the phosphate buffer medium and it was found N,S:GQDs could produce an obvious ECL signal at+2.5 V（vs Ag/AgCl）,more interesting is that the ECL signal could be greatly amplified 11 times by CdS QDs.The enhancement mechanism was studied by spectroscopy and electrochemical method and it was speculated that CdS QDs can reduce the electron transfer barriers of N,S:GQDs and accelerate their electron transfer velocity（3）Establishment of CdS QDs/N,S:GQDs ECL System and Its application for Pb²⁺detectionThe influence of buffer solution pH and the amount of CdS QDs on the ECL signal of N,S:GQDs was investigated and the system of CdS QDs/N,S:GQDs was investigated.The combination of Pb²⁺with S atoms on the surface of N,S:GQDs can change the surface state of quantum dots and reduce their ECL signal.Based on the quenching effect we established a method for sensitive determination of Pb²⁺.The linear response range was 1.0×10^-95.0×10^-7mol/L with a detection limit of 0.29nmol/L.Compared with other methods of Pb²⁺determination,this method is simpler,cheaper,and more convenient.（4）The construction of CdS QDs/N,S:GQDs ECL sensor and the application for Pb²⁺detectionImmoblized the luminescent material on the surface of electrode to form a sensor,which can not only save the reagents and improve the sensitivity,but also enable rapid online analysis.CdS QDs and N,S:GQDs are immobilized to the electrode surface by the coupling reaction between materials in order to improve the conductivity and sensitivity of the sensor.Compared with the liquid detection in first chapter,the sensor has a wider linear range and lower limit for detection Pb²⁺.The linear response range was 1.0×10^-91.0×10^-7mol/L with a detection limit of 0.14nM.（5）Double-potential electrochemiluminescence ratiometric strategy based on N,S:GQDs and Ru（bpy）₃²⁺and its application for epinephrine determinationA novel dual-potential ratiometric electrochemiluminescence（ECL）sensor was designed with Ru（bpy）₃²⁺as the cathodic emitter and N,S:GQDs as the anodic emitter.Based on the influence of epinephrine on the ratio of the two ECL signals,the determination of epinephrine was achieved with a linear range from 1.0×10^-9 to 5.0×10^-5mol/L and detection limit of 5.3 nM（S/N=3）.This method can effectively prevent interference from the environment and instrument compared to the single-signal ECL method,and has a wide linear range and lower detection limit.