Study On The Construction Of DNA Functionalized Quantum Dots-Based Photoelectrochemical Biosensors And Their Novel Analytical Methods

Quantum dots(QDs),as one kind of functional nanocrystals,have been widely used for bioanalysis in DNA sensing,multiplexed immunoassay,cell imaging and in vivo observation owing to their advantages in optical and electronic aspects.To date,photoelectrochemistry(PEC)as a widely utilized analytical technology has associated with QDs as photoelectrochemical active materials for the development of novel analytical methodologies.In particularly,by integrating the peculiar properties of QDs,advantages of photoelectrochemical detection technique,and multifunctionality and programmability of DNA,QDs-based PEC sensing methods have become one of the research hotspots in life analytical community.This thesis focuses on the construction of photoelectrochemical sensors and their novel analytical methods on the basis of DNA functionalized QDs and their nanocomposites.The main works are described as follows:1."Signal-on" photoelectrochemical sensing strategy based on target-dependent aptamer conformational conversion for selective detection of lead(?)ionA "signal-on" photoelectrochemical sensing strategy for selective determination of Pb2+ is designed on the basis of the combination of Pb2+-induced conformational conversion,the amplified effect of reduced graphene oxide(RGO)and resonance energy transfer between CdS quantum dots(QDs)and gold nanoparticles(AuNPs).The RGO/CdS/aptamer platform is constructed via a stepwise modification method,and characterized by electrochemical impedance spectroscopy.In the absence of AuNP-labeled DNA,as a signal quenching element,can be introduced by hybridization with aptamer on the surface of sensing platform,which quenches the photocurrent of QDs via an energy transfer process.Upon addition of Pb2+,the aptamer is induced into a G-quadruplex structure,which can greatly hinder the hybridization between aptamer and AuNP-labeled DNA due to the competitive occupation of binding sites and steric effect,leading to the recovery of photocurrent.Under optimized conditions,this "signal-on" photoelectrochemical biosensor shows a linear relationship between photocurrent variation and the logarithm of Pb2+concentration in the range of 0.1-50 nM with a detection limit of 0.05 nM.Meanwhile,it also exhibits good selectivity for Pb2+ over other interfering ions,and is successfully applied to the detection of Pb2+ in environmental water samples.2.In situ generation of electron acceptor for photoelectrochemical biosensing via hemin-mediated catalytic reactionA novel photoelectrochemical sensing strategy is designed for DNA detection on the basis of in situ generation of electron acceptor via the catalytic reaction of hemin toward H2O2.The photoelectrochemical platform was established by sequential assembly of near-infrared CdTe quantum dots,capture DNA and hemin-labeled DNA probe to form triple-helix molecular beacon(THMB)structure on indium tin oxide electrode.According to highly catalytic capacity of hemin towards H2O2,a photoelectrochemical mechanism was then proposed,in which the electron acceptor of O2 was in situ generated on the electrode surface,leading to the enhancement of photocurrent response.The utilization of CdTe QDs can extend the absorption edge to near-infrared band,resulting in the increase of light-to-electricity efficiency.After introducing target DNA,the THMB structure is disassembled and releases hemin,and thus quenches the photocurrent.Under optimized conditions,this biosensor shows high sensitivity with the linear range from 1 to 1000 pM and detection limit of 0.8 pM.Moreover,it exhibits good performance of excellent selectivity and high stability.3.Catalytic hairpin assembly-programmed porphyrin-DNA complex as photoelectrochemical initiator for DNA biosensingA catalytic hairpin assembly(CHA)-programmed porphyrin-DNA complex is designed to trigger the chemiluminescence as photoelectrochemical initiator for DNA sensing.Firstly,the programmed double strand DNA(dsDNA)was formed using two hairpin DNAs as assembly components via target-assisted CHA reaction,and then immobilized on a capture DNA/CdS quantum dots modified electrode.The porphyrin(FeTMPyP)was conveniently assembled on dsDNA scaffold via the groove interaction.The FeTMPyP@dsDNA complex possessed high catalytic activity towards luminol oxidation to generate the desirable chemiluminescence with high stability under various temperature and alkaline condition.By integrating signal amplification capacity of CHA and in situ FeTMPyP-mediated chemiluminescence as excitation light,an amplified photoelectrochemical sensing strategy is proposed for DNA detection.Under optimized conditions,the biosensor shows a wide linear range from 5 to 10000 fM with a detection limit of 2.2 fM.Moreover,the developed photoelectrochemical device exhibits excellent selectivity,high stability and acceptable fabrication reproducibility.The CHA-programmed porphyrin-DNA strategy not only extends the applications of photoelectrochemistry,but also presents a novel methodology in bioanalysis.4.CdS/MoS2 heterojunction-based photoelectrochemical DNA biosensor via enhanced chemiluminescence excitationThis work developed a CdS/MoS2 heterojunction-based photoelectrochemical biosensor for sensitive detection of DNA under the enhanced chemiluminescence excitation of luminol catalyzed by hemin-DNA complex.The CdS/MoS2 photocathode was prepared by the stepwise assembly of MoS2 and CdS quantum dots(QDs)on indium tin oxide(ITO),and achieved about 280%increasing of photocurrent compared to pure CdS QDs electrode due to the formation of heterostructure.High photoconversion efficiency in the photoelectrochemical system was identified to be the rapid spatial charge separation of electron-hole pairs by the extension of electron transport time and electron lifetime.In the presence of target DNA,the catalytic hairpin assembly was triggered,and simultaneously the dual hemin-labeled DNA probe was introduced to capture DNA/CdS/MoS2 modified ITO electrode.Thus the chemiluminescence emission of luminol was enhanced via hemin-induced mimetic catalysis,leading to the physical light-free photoelectrochemical strategy.Under optimized conditions,the resulting photoelectrode was proportional to the logarithm of target DNA concentration in the range from 1 fM to 100 pM with a detection limit of 0.39 fM.Moreover,the cascade amplification biosensor demonstrated high selectivity,desirable stability and good reproducibility,showing great prospect in molecular diagnosis and bioanalysis.