| DNA electrochemical biosensor is noted widely in recent years for rapid development. It has been used in many fields such as disease diagnosis, pharmaceutical analysis and environmental monitoring due to the high sensitivity, better specificity, rapid response, low cost, easy handling and miniaturization, etc. Four New-typed DNA electrochemical biosensors were prepared, and the detection of Avian Influenza Virus genotype and chemical and biological small molecules were studied in this paper. The dissertation is divided into four main parts:(1) An electrochemical method for the detection of DNA hybridization using a novel electroactive, cationic, water-soluble branched polyethyleneimine(BPEI)-cobalt(III)- phenanthroline(phen) polymeric indicator and single-stranded neutral PNA probe on the Au electrode was investigated. The indicator possesses both some free amine groups and cationic cobalt complex moieties in the polymer chain. It does not bind to neutral PNA capture probes alone but interacts strongly with the negatively charged backbone of the complementary oligonucleotide bound to the PNA probes through electrostatic interactions, and the coordination spherical moieties of indicator which will embed double-helix structure of PNA–DNA, allowing transduction of hybridization into a clear electrical signal in DPV. The discrimination against non-complementary DNA, three-base and one-base mismatch DNA was sharp, and the signal of indicator for target DNA was a linear relationship in the concentration range 5.0×10-9 2.5×10-7 M (R = 0.9940) with a detection limit of 5.6×10-10 M.(2) A novel electrochemical method of detecting DNA hybridization is presented based on the change in flexibility between the single and double stranded DNA. A recognition surface based on GNPs is firstly modified via mixing self-assembled monolayer of thiolated probe DNA and 1,6-hexanedithiol. The hybridization and electrochemical detection are performed on the surface of probe-modified GNPs and electrode, respectively. Here in our method the Rct signal is enhanced by blocking the surface of electrode with DNA covered GNPs. The GNPs will be able to adsorb on the gold electrode when covered with flexible ssDNA. On the contrary, it will be repelled from the electrode, when covered with stiff dsDNA. Therefore, different Rct signals are observed before and after hybridization. The hybridization events are monitored by EIS measurement based on the Rct signals without any external labels. This method provides an alternative route for expanding the range of detection methods available for DNA hybridization.(3) A sensitive electrochemical method for the detection of avian influenza virus H5N1 gene sequence using DNA aptamer immobilized hybrid nanomaterials modified electrode was developed. The hybridization events were monitored by DPV measurement. In order to enhance the selectivity and sensitivity, the modified electrode was assembled by MWNT, PPNWs and GNPs. It could provide a porous structure with large effective surface area, highly electrocatalytic activities and electronic conductivity. Therefore, it could largely increase the immobilization amount of DNA aptamer, maintain the accessibility of detection target and lead to an increase of the detection signal. The signal was approximately linear to log(concentration) of complementary DNA over the range of 5.0×10-12 to 1.0×10-9 M (R = 0.9863) with a detection limit of 4.3×10-13 M. These studies showed that the new hybrid nanomaterials (MWNT/PPNWs/GNPs) and DNA aptamer could be used to fabricate an electrochemical biosensor for gene sequence detection. Moreover, this design strategy was expected for further extensive applications in other biosensors.(4) A novel bio-composite film which contains MWNT-Chit/PAMAM nanocomposite along with the incorporation of DNA modified Au electrode as a biosensor for determination of DA and UA under coexistence of AA was fabricated by layer-by-layer modification. EIS and CV were used to characterize the electrochemical properties of the modified electrodes. The biosensor was applied to detect DA and UA in the presence of AA. It not only exhibited strong catalytic activity toward the oxidation of DA and UA but also separated the originally overlapped signals of UA, DA and AA oxidation at the bare electrode into three well-defined peaks. The peak separation between AA and DA, AA and UA was 179 mV and 288 mV, respectively. In the presence of 1.0 mM AA, two linear relationships were obtained for DA over the concentration range from 0.2 to 10μM (R = 0.9984) and 10 to 100μM (R = 0.9957) with a detection limit of 0.03μM (s / n = 3). The anodic peak current of UA was also a linear relationship in the concentration range 0.5 100μM (R = 0.9971) with a detection limit of 0.07μΜ. Moreover, the modified electrode surface has very good reproducibility and stability.
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