Carbon Quantum Dots-based Nanomaterials For The Application Of Biosensor And Electrocatalysis

Carbon quantum dots(CQDs),as a new member of the carbon-based nanomaterials,have drawn increasing attention in recent years owing to their fascinating optical properties(namely,size-and wavelength-dependent luminescence emission,resistance to photobleaching,ease of bioconjugation),excellent chemical inertness,favorable biocompatibility,and more importantly,their unique electrochemiluminescence(ECL)characters.Unfortunately,the preparations of CQDs usually involve expensive precursors,intricate processes or low quantum yields(QY),while the exact mechanism of the photoluminescence is still not clear.Furthermore,carbon dots as a fluorescent probe were applied to amino acid,protein or active oxygen still show negative results.The CQDs in ECL are still at an early stage;the detailed reasons and rules for ECL generation have not yet been explored thoroughly and systematically compared to QDs,for example.Consequently,much work is still needed to exploit these nanomaterials in biosensing and bioimaging.Very recently,nitrogen-doped carbon dots(N-Cdots)or graphene quantum dots(N-GQDs)have also been proposed as potential metal-free catalysts for ORR.Nevertheless,due to the low electrical conductivity of the Cdots or GQDs with high percolation threshold values,the enhanced electrocatalytic activity is limited.Therefore,it is urgent to develop a green,highly efficient,and environmentally benign route to synthesize N-doped carbon-based materials with high ORR catalytic activity.In conclusion,the dissertation is mainly focused on following four parts:1.Highly photoluminescent nitrogen-doped carbon quantum dots was synthesized for biosensing and bioimagingNitrogen-doped carbon quantum dots(N-CQDs)with high fluorescence quantum yield(QY)(46.2%)were successfully synthesized through one-pot hydrothermal carbonization approach assisted by ethylenediamine as surface passivation agent,wherein alanine acted as a single carbon source.To gain more insight into the origin of the PL behavior for explaining the high QY of N-CQDs,more accurately,the reaction conditions such as the concentration of reactants,temperature and time were found to be very important.The obtained biocompatible N-CQDs could effectively label both the MCF-7 cell membranes and the nucleus,suggesting the potential application in biolabeling and bioimaging.Furthermore,the addition of dihydronicotinamide adenine dinucleotide(NADH)could dramatically quench the fluorescence of N-CQDs.Therefore,a novel protocol for NADH determination was established and its mechanism was discussed.This proposed fluorescence sensor displayed an enhanced performance for NADH determination with the linear range up to 80?M and the detection limit down to 25.1nM.2.The synthesis of carbon quantum dots/AuNCs nanohybrid for ratiometric sensing cadmium(?)ions and 1-ascorbic acidFor the first time,a novel carbon quantum dots/gold nanoclusters(CQDs/AuNCs)nanohybrid based ratiometric fluorescent probe for sensitive and selective sensing of cadmium(?)ions and 1-ascorbic acid(AA)has been established.The blue emissive CQDs,prepared by a one-step hydrothermal treatment with alanine and histidine,play the role of reference fluorophore.The red emissive AuNCs,capped by 11-mercaptoundecanoic acid(MUA),serve as a specific recognition element.Different from MUA-capped AuNCs,the proposed CQDs/AuNCs nanohybrid exhibits high selectivity and sensitivity to cadmium(?)ions with a detection limit of 32.5nM,which is attributed to the static quenching and the inner filter effect(IFE)of Cd2+ to CQDs/AuNCs nanohybrid.Unbelievable,in the presence of AA,the above-quenched sensor is recovered gradually.The proposed fluorescence sensor displays an enhanced performance for AA determination in the range from 0.15 to 15 ?M(R=0.995)with a detection limit down to 0.105 ?M.More importantly,this fluorescent "on-off-on" method also offers the excellent performance to Cd2+ and AA detection in lake water and in human serum,respectively.3.Ferrocyanide-ferricyanide redox couple induced electrochemiluminescence amplification of carbon dots for ultrasensitive sensing of glutathioneA novel solid-state ECL sensor for ultrasensitive sensing of glutathione(GSH)based on ferrocyanide-ferricyanide redox couple(Fe(CN)63-/4-)induced electrochemiluminescence(ECL)amplification of carbon dots(C-dots).The electropolymerization of C-dots and(11-Pyrrolyl-1-yl-undecyl)triethylammonium tetrafluoroborate(A2)enabled immobilization of the hydrophilic C-dots on the surface of glassy carbon electrode(GCE)perfectly,while the excellent conductivity of polypyrrole was exploited to accelerate electron transfer between them.The Fe(CN)63-/4-can expeditiously convert the C-dots and S2O82-to C-dot�-and SO4�-,5respectively.High yields of the excited state C-dots(C-dots*)were obtained and a?10-fold ECL amplification was realized.The C-dots*obtained through the recombination of electron-injected and hole-injected processes may be impeded due to the interference of GSH to K2S2O8.Therefore,the constructed sensor for GSH showed a detection limit down to 54.3 nM and a wide linear range from 0.1?1.0?M with a correlation coefficient of 0.997.4.One-pot synthesis of nitrogen-rich carbon dots decorated graphene oxide as metal-free electrocatalyst for oxygen reduction reactionNitrogen-rich carbon dots decorated graphene oxide(N-Cdots/GO)hybrid,as a metal-free electrocatalyst for oxygen reduction reaction(ORR),is synthesized by a facile and low-cost hydrothermal treatment for the first time.Specifically,as a nitrogen-rich bridge-molecule,urea can trap free citric acid on planar planes,edges,and wrinkle sections of GO through amidation reaction in the initial stage.Then,polymer-like intermediates are in situ formed and carbonized to N-Cdots on GO.Owing to the simultaneous N-doping into both Cdots and GO characterized by XPS,as well as the formation of defective graphitic structures analyzed by Raman spectra,the as-prepared hybrid displays a remarkable electrocatalytic performance with more positive onset potential(0.13 V vs.Ag/AgCl)and high kinetic current density(up to 18.4 mA cm-2 at-0.70V)comparable to commercial Pt/C catalyst.Furthermore,it demonstrates significantly higher catalytic efficiency(nearly 100%of four-electron ORR pathway)than those of N-Cdots or N-Cdots/GO mixture with an outstanding cyclic stability and superior methanol tolerance capability in alkaline solution.