| Electrochemical sensors has advantages of simplicity,low cost,fast response and high stability,which have been widely used in analytical chemistry.They can not only be used to detect heavy metals,but also can be used for the detection of small molecules,proteins,drugs and so on.Furthermore,due to the introduction of aptamers,the selectivity of the electrochemical sensors has been greatly improved.Recently,in order to pursue higher sensitivity,various nanomaterials and enzymes have been used in the electrochemical sensors.The unique electrochemical properties of the nanomaterials and the catalysis ability of enzymes may improve the analytical performances of the sensors by amplifying response signals.Based on different signal response strategies,several novel electrochemical sensors have been developed in this thesis.Also,by using cyclodextrin,ordered mesoporous carbon and exonuclease,the performance of the sensors has been improved due to the amplification of the signals.In addition,a new triple signaling amplification strategy has been developed.The main points of this thesis are summarized as follows:(1)An electrochemical sensor for zinc ion analysis was developed based on nitrogen doped porous carbon nanopolyhedrons.The sensor can greatly improve the efficiency of preconcentration of zinc ions and reduce the detection limit.By using square-wave anodic stripping voltammetry as the analytical method,a calibration curve was established between the oxidation peak current of zinc and its concentration,exhibiting a wide linear range(10-600 nM,R2= 0.9964),low detection limit(2.6 nM)and acceptable recovery(92-100%).The electrode also exhibited satisfactory specificity and reproducibility.(2)A novel ratiometric electrochemical aptasensor for analysis of thrombin has been developed based on aptamer-target-aptamer sandwich structure.In the absence of thrombin,the methylene blue(MB)tagged thrombin aptamer(MB-TBA)was introduced on the electrode through the hybridization with the immobilized DNA sequences.After the addition of thrombin,the specific thrombin-aptamer interaction drove the conformational change of MB-TBA(from the unfolded conformation to the G-quadruplex),resulting in the MB tag far from the electrode surface.The second thrombin aptamer with ferrocene(Fc)tagged on both chain ends(Fc-TBA-Fc)interacted with thrombin to form another G-quadruplex conformation.Such sandwich structure and the G-quadruplex conformation acted cooperatively to allow the two Fc tags to be close to the electrode surface.The logarithmic value of IFc/IMB was linear with the logarithm of thrombin concentration.The developed aptasensor shows good response toward the target with a wide linear range from 1 nM to 600 nM and a low detection limit of 170 pM.(3)A novel competitive host-guest strategy regulated by protein biogate was developed for analysis of prion protein.The methylene blue(MB)-tagged prion aptamer(MB-Apt)was introduced to the multiwalled carbon nanotubes-?-cyclodextrins(MWCNTs-?-CD)composites-modified glassy carbon(GC)electrode through the host-guest interaction between ?-CD and MB.In the absence of prion,MB-Apt could be displaced by ferrocenecarboxylic acid(FCA)due to its stronger binding affinity to ?-CD,resulting in a large oxidation peak of FCA.However,in the presence of prion,the specific prion-aptamer interaction drove the formation of protein biogate to seal the cavity of ?-CD,which hindered the guest displacement of MB by FCA and resulted in the oxidation peak current of MB increased and that of FCA decreased.The developed aptasensor showed good response towards the target(prion protein)with a low detection limit of 160 fM.By changing the specific aptamers,this strategy could be easily extended to detect other proteins,showing promising potential for extensive applications in bioanalysis.(4)Based on ordered mesoporous carbon(OMC)probe,exonuclease III(Exo III)and competitive cyclodextrin—guest interaction,a novel label-free and cascaded dual—signaling amplified electrochemical strategy was developed for analysis of cellular prion protein(PrPc).OMC probe was fabricated by sealing the electroactive ferrocenecarboxylic acid(Fc)into its inner pores and then using single-stranded DNA(DNA3)as the gatekeeper.In the presence of PrPc,DNA1 could bind the target protein and free DNA2.More importantly,DNA2 could hybridize with DNA3 to form a rigid duplex DNA and thus triggered the exonuclease ?(Exo ?)cleavage process to realize the DNA2 recycling,accompanied by opening more biogates and releasing more Fc.The released Fc could be further used as a competitive guest of?-cyclodextrin(?-CD)to displace the Rhodamine B(RhB)on the electrode.The detection limit was 7.6 fM(3?).This strategy has a great potential for bioanalysis,disease diagnostics,and clinical biomedicine applications.(5)A novel electrochemical ratiometric aptasensor has been developed based on a new dual-ratiometric readouts strategy for detection of bisphenol A(BPA).The thiolated ferrocene-labeled aptamer probe(Fc-P)as an internal control was designed to hybridize with the methylene blue-modified DNA probe(MB-P)on gold electrode to form rigid duplex DNA.In the presence of BPA,the interaction between BPA and the aptamer led to the dissociation of the duplex DNA structure and thereby the release of the MB-P from the sensing interface.As a result,a sharp decrease of the oxidation peak current of MB and an obvious increase of the oxidation peak current of BPA were obtained,while the Faradaic current from the Fc tag was almost unchanged since its distance to the electrode surface remained unchanged.The detection limit is 0.42 pM.Furthermore,the developed method showed good reliability and reproducibility,and high sensitivity.By changing the specific aptamers,this new strategy could be easily extended to detect other targets with redox activity,showing promising potential for extensive applications.(6)Based on a triple-signaling strategy,a novel electrochemical aptasensor has been developed for detection of bisphenol A(BPA).The thiolated ferrocene(Fc)-modified BPA-binding aptamer probe(Fc-P)was immobilized on the gold electrode and then hybridized with the methylene blue(MB)-modified complementary DNA probe(MB-P)to form a rigid double-stranded DNA(ds-DNA).The specific interaction between BPA and Fc-P led to the release of MB-P from the sensing interface and the conformational change of Fc-P.As a result,the oxidation peak currents of Fc and BPA increased with the increase of the concentration of target(BPA)according to the "signal-on" mode while that of MB decreased with the increase of the BPA concentration according to the“signal-off" mode.By superimposing the triple signal changes,BPA was detected sensitively with a linear range from 1 pM to 100 pM.The detection limit is 0.19 pM.The aptasensor also exhibited satisfactory specificity,selectivity,reproducibility and stability.By changing the specific aptamers,this strategy could be easily extended to detect other redox targets,showing promising applications in environmental analysis,food safety monitoring,and bioanalysis. |
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