Electrochemiluminescence Properties And Applications Of Functionalized Nitrogen-doped Carbon Quantum Dots

Electrochemiluminescence（ECL）is a process that produces chemiluminescence by electrode excitation,and the method of quantitative analysis based on ECL signals is called ECL analysis.With the advantages of low background signal,simple instrumentation and high sensitivity,ECL analysis has become an essential analytical tool in the fields of food,environmental monitoring and bioanalysis.Carbon quantum dots（CQDs）are zero-dimensional carbon conformation quasi-spherical nanoparticles less than 10 nm in size,which have the advantages of simple synthesis,high electron mobility,and environmental friendliness.Since the first report of the ECL phenomenon of carbon quantum dots in 2009,CQDs have been widely considered as a promising ECL material.However,simple CQDs have low luminescence efficiency and lack selectivity,and often require doping or surface modification to improve their luminescence efficiency and selectivity and further expand the application range.In this thesis,we first selected suitable precursors and prepared nitrogen-doped CQDs（N-CQDs）by hydrothermal method,and then modified different molecular groups on their surfaces to obtain surface-functionalized CQDs:Dopamine-modified N-CQDs（DA-N-CQDs）and L-Cysteine modified N-CQDs（L-Cys-N-CQDs）.Based on the good ECL performance of N-CQDs and selective recognition of surface groups,enhanced and quenched ECL sensors were constructed and successfully applied to the highly sensitive and selective detection of glutathione in FBS and TC in milk,respectively.The details of the study are as follows:1.Synthesis and properties study of N-CQDsN-CQDs were prepared by hydrothermal method using citric acid（CA）and melamine as raw materials,and then their morphology was observed by transmission electron microscopy（TEM）,their surface composition was tested by X-ray photoelectron spectrometry（XPS）,and their optical properties were characterized by fluorescence spectrophotometer and UV spectrophotometer.The ECL performance was investigated and the synthesized N-CQDs were shown to emit strong and stable ECL signals in the presence of the co-reactant potassium persulfate（K₂S₂O₈）.Further studies showed that the presence of gold nanoparticles（Au NPs）quenches the ECL signal of N-CQDs,but the ECL signal is restored when GSH is added to the system,and a sensitive detection of GSH was achieved based on this feature,the linear response range was 1.0×10^-9mol·L^-1to 1.0×10^-4mol·L^-1,and the detection limit was8.0×10^-10mol·L^-1.This strategy can also be used for the detection of other-SH-containing biothiols.2.DA-N-CQDs and the selective detection to GSHIn our previous work,we achieved a sensitive detection of GSH using synthetic N-CQDs,but this detection is based on the interaction of Au NPs with-SH on GSH,and other substances that contain-SH will also respond and thus interfere.To solve this problem,we introduced dopamine（DA）molecules on the surface of N-CQDs to obtain DA-N-CQDs;in the presence of K₂S₂O₈,DA on the surface of quantum dots is oxidized to dopaquinone,and internal energy transfer between dopaquinone and N-CQDs can occur,leading to a decrease in the ECL signal of N-CQDs;after the addition of GSH,GSH undergoes Michael addition reaction with dopaquinone,while converting the quinone group to phenol,cutting off the energy transfer of N-CQDs,allowing their ECL signals to be restored.Since the reaction between GSH and dopaquinone has high specificity and other co-existing substances do not interfere,this method not only provides sensitive detection of GSH,but also improves the detection selectivity.The actual sample analysis also proved that the coexisting substances do not interfere.In our experiments,we prepared an ECL solid-state sensor by immobilizing DA-N-CQDs on a glassy carbon electrode（GCE）,and achieved a sensitive and selective detection of GSH using the above principle,with a linear response range of1×10^-11mol·L^-1to 1×10^-9mol·L^-1and a detection limit of 1.0×10^-12mol·L^-1.The sensor was used for the detection of GSH in FBS with satisfactory results,this method is simpler,more sensitive,and more selective than other GSH assays.3.ECL sensors based on L-Cys-N-CQDs for sensitive detection of TCThe ECL performance of CQDs is influenced by a number of factors,of which the surface condition is an important one;modification or passivation of the surface of CQDs can change their ECL properties.In this study,we used L-Cys as a surface modifier and introduced L-Cys on the surface of N-CQDs to obtain L-Cys-N-CQDs.TC is rich in amides and hydroxyl groups,which can bind to L-Cys on the surface of L-Cys-N-CQDs through various interactions.The surface state of L-Cys-N-CQDs is changed after binding to TC,causing changes in its ECL signal,thus achieving a sensitive detection of TC,with a linear response range of 1.0×10^-10mol·L^-1to1.0×10^-8mol·L^-1and a detection limit of 1.9×10^-11mol·L^-1.The method was applied to the determination of TC in milk with satisfactory results.