DNA Strand Displacement Signal Amplification Strategies For High-performance Photoelectrochemical Biosensors

MicroRNAs（miRNAs）are noncoding RNA that typically have a length of 19-23nucleotides,which play a key role in the regulation of gene transcription as well as gene expression.Therefore,miRNAs have been regarded as an ideal biomarker for cancer diagnosis,prognosis,and therapy.As a new analytical method,photoelectrochemical sensing exhibits the significant advantages as follows:simplicity,rapidity,high sensitivity and good biocompatibility.Nanomaterials such as superparamagnetic Fe₃O₄@SiO₂,Fe₃O₄@SiO₂@CdS with core-shell structures were prepared with Fe₃O₄ as cores.Besides,CdSequantum dots（CdSeQDs）,Au nanorods（Au NRs）and nanoparticles（Au NPs）were also prepared and used to construct high-performance photoelectrochemical biosensors for miRNA-196a,miRNA-1246and 17β-estradiol.The high sensitivity benefits from entropy-driven chain replacement and other biological signal amplification techniques.The main work in the thesis are as follows:（1）Superparamagnetic Fe₃O₄@SiO₂@CdS core-shell structures were prepared and used to construct high-performance photoelectrochemical biosensors for the detection of miRNA-196a.First,amino group was introduced on the surface of Fe₃O₄@SiO₂@CdS nanomaterials through chitosan,and then the Fe₃O₄@SiO₂@CdS-NH₂ was linked to P6 chains with carboxyl（COOH）by amide reaction and resulted in Fe₃O₄@SiO₂@CdS-P6 nanocomposites.At the same time,amino group was introduced on the surface of Cu S through chitosan,then it was connected with amino group modified chain P5 by glutaraldehyde.Then,the above nanocomposites were assembled on the ET DNA tetrahedra to form a ETC composite in Tris-HCl buffer.At this point,the photocurrent of the complex composite is almost zero in the absence of the target.However,in the presence of targets,which triggered the operation of entropy-driven amplification process,and leaded to the proportional Fe₃O₄@SiO₂@CdS-P6 composites released,and then the improved photocurrent.The sensitivity of as-designed biosensor benefits from the entropy-driven amplification technique.And the selectivity results from the application of superparamagnetic Fe₃O₄@SiO₂@CdS structures.（2）A highly sensitive photoelectrochemical biosensor for miRNA-1246 was constructed based on ZnO nanorods arrays as a good photoelectric beacon.First,a thin layer of ZnO nanoarray was formed on ITO conductive glass by electrodeposition.Second,the amino group were introduced on the Fe₃O₄@SiO₂ surface and then connected to the DNA in a solution to capture Hg²⁺using the T-Hg²⁺-T structure,and then the magnetic spheres were removed from the solution.Following,the target and HP DNA were added into the reaction system.When the Hg²⁺ions released from the DNA chain,the ZnO nanoarray was placed in a solution containing Hg²⁺with UV irradiation.The produced Hg elementary substance prevents the transfer of photogenerated electrons from ZnO nanorods,resulting in the photoelectric signal decrease.The as-designed photoelectrochemical biosensor has good performance in simple fabrication,high sensitivity,good reproducibility,and rapid detection.（3）The functionalized nanomaterials such as CdSeQDs,Au NPs,and Au NRs were synthesized and used to construct a high-performance photoelectrochemical and photothermal dual-signal mode biosensor for 17β-estradiol.First,output DNAs were released from entropy-driven amplification system.Meanwhile,amino was introduced on the surface of the Fe₃O₄@SiO₂microsphere by APTES,and then Fe₃O₄@SiO₂-NH₂ was connected with the Au-DNA-CdSeQDs-COOH by amino reaction.And then it was used to capture Ag⁺ions in the solution using the C-Ag⁺-C structure.Meanwhile,the distance between the CdSeQDs and the Au NPs is shortened,resulting in the exciton-plasmon interactions（EPI）occuring and then the weak photoelectric signal.Following,the Ag⁺ions captured by hairpin DNAs were released when the output DNAs were introduced into the system,and then the distance between the CdSeQDs and the Au NPs was increased.This makes the photoelectric signal recover proportionally with the increase concentration of targets.Meanwhile,the released Ag⁺ions were reduced to Ag particles on the surface of Au nanorods with the reduction of ascorbic acid.Then,the photothermal detection was carried out.The results indicated that the temperature decreased linearly with increased concentration of targets.The two-signal mode biosensor has advantages of high sensitivity,good selectivity,satisfied reproducibility.Especially,the reliability of results can be self-confirmed by the two-signal expressions.