Electrochemical And Electrochemiluminescent Biosensing Based On Functional Nanomaterials

Graphene,a monolayer of carbon atoms closely packed into a two-dimensional honeycomb lattice,has attracted tremendous attentions because of its extraordinary properties,such as large specific surface area,strong mechanical strength,high electronic conductivity,and excellent chemical stability.It holds great promise for potential applications in a wide variety of technological fields,for instance,nanoelectronics,sensors,supercapacitors,composite materials,and energy storage.Quantum dots have become an area of growing interest because of their unique properties,including size-tunable emission profiles,high photoluminescence quantum yields,excellent photochemical stability,and resistance to photobleaching.Biosensor is an analytical device which combines bioactive substances(e.g.enzymes,antibodys,cells,tissues,nucleic acids,etc.)with physicochemical detectors.Taking advantages of high sensitivity,good selectivity,easy operation,low cost and real-time monitoring,biosensors are widely used in clinical diagnosis,food safety,environmental monitoring,and biological medicine.The applications of functional nanomaterials not only favor the design of various biosensing platforms,but also exhibit many excellent properties.This thesis focuses on the studies of the preparation,functionalization,and application of graphene composites,graphitic-phase carbon nitride composites,and quantum dots in the field of biosensors(including electrochemical and electrochemiluminescent biosensors).The main contents were summarized as follows:(1)Biofunctionalized reduced graphene oxide-gold nanoparticle(AuNP)hybrids were prepared using a facile approach of in situ growth,with homogeneous distribution of AuNPs on the reduced graphene oxide nanosheets.Hemoglobin(Hb)was immobilized on the reduced graphene oxide-AuNP composites to fabricate biosensors for determination of nitrite(NO2-).The electrochemical oxidation of nitrite was preferred with the final product of nitrate,since main limitations in reduction of nitrite such as the interferences from several products depending on electrode condition and nature of catalysts employed could be avoided.The novel biosensors exhibited many advantages,such as wide linear response range(from 0.05 to 1000 mM,R2=0.997),low detection limit(0.01 mM,a signal to noise ratio of 3),high sensitivity(0.15 ?A ?M-1 cm-2),and excellent selectivity.These constructed biosensors were further used for determination of nitrite in pickled radish.The results obtained were in good agreement with those using spectrophotometry based on the National Food Safety Standard(GB 5009.33-2010),which indicates that these novel and sensitive biosensors have promising application for determination of nitrite in food.(2)Sensitive electrochemical sensors were fabricated with reduced graphene oxide-supported Au@Pd(Au@Pd-RGO)nanocomposites by one-step synthesis for individual and simultaneous determination of ascorbic acid(AA),dopamine(DA),and uric acid(UA)with low detection limits and wide concentration ranges.From the Au@Pd-RGO-modified electrodes,well-separated oxidation peaks and enhanced peak currents of AA,DA,and UA were observed owing to the superior conductivity of RGO and the excellent catalytic activity of Au@Pd nanoparticles.For individual detection,the linear responses of AA,DA,and UA were in the concentration ranges of 0.1-1000,0.01-100,and 0.02-500 ?M with detection limits of 0.02,0.002,and 0.005 ?M(S/N=3),respectively.For simultaneous detection by synchronous change of the concentrations of AA,DA,and UA,the linear response ranges were 1-800,0.1-100,and 0.1-350 ?M with detection limits of 0.28,0.024,and 0.02 ?M(S/N=3),respectively.The fabricated sensors were further applied to the detection of AA,DA,and UA in urine samples.The Au@Pd-RGO nanocomposites have promising applications in highly sensitive and selective electrochemical sensing.(3)A novel ratiometric electrochemiluminescence(ECL)biosensor was constructed for sensitive detection of glucose in combination of glucose oxidase(GOx)-immobilized graphitic-phase carbon nitride-supported Au nanocomposites(GOx/Au-g-C3N4)and luminol without additional coreactant.In the absence of glucose,the cathodic ECL emission of the g-C3N4 nanosheets was only acquired with dissolved oxygen in neutral aqueous solution as a coreactant.In the presence of glucose,the in situ generation of H2O2 and the consumption of dissolved oxygen simultaneously occurred,and H2O2 was further catalyzed by the Au nanoparticles to generate reactive oxygen species,which resulted in the increase of the anodic ECL signal of luminol concomitant with the decrease of the cathodic ECL signal.The in situ generation and conversion of coreactants were related to the excellent catalytic activities of GOx and Au nanoparticles.The constructed ratiometric ECL biosensor showed high sensitivity and selectivity for the detection of glucose with a low detection limit of 0.05 ?M(S/N=3)and a wide concentration range from 0.1 to 8000?M.Furthermore,this constructed ratiometric ECL sensing platform was successfully applied to detect glucose in real samples devoid of interference of other biomolecules.This facile,reliable,and sensitive ratiometric ECL biosensor shows a promising application in clinical diagnostics.(4)A novel electrochemiluminescence(ECL)ratiometric sensing strategy was designed for sensitive detection of target DNA(T-DNA)on the basis of target-mediated G-quadruplex/hemin(G4-hemin)followed by catalytic oxidation of hydroquinone(HQ)and luminol using H2O2 as an oxidant.First,the ECL biosensor was constructed via a stepwise assembly of CdS quantum dots(QD)and hairpin DNA(H-DNA)probe on glassy carbon electrode.Upon addition of T-DNA,a stable G4-hemin DNAzyme was generated by the specific recognition of T-DNA and H-DNA in the presence of hemin,which could not only stimulate biocatalytic precipitation onto the electrode surface for reducing cathodic ECL signal of CdS QD,but also trigger luminol-based chemiluminescence for increasing anodic ECL signal,respectively.Under optimized conditions,the biosensor showed a wide linear range from 1 to 10000 fM with a detection limit of 0.2 fM(S/N=3).Moreover,the developed ratiometric ECL device exhibited excellent selectivity.