Study Of The Fluorescent Aptasensors

DNA aptamers are synthetic single-strand nuceleic acids that have high affinity and specificity to the targets ranging from small molecules to large proteins and even cells. Some new methods based on the special recogination of aptamers have been developed for effieient and fast detection in biochemical and biomedical fields. Recently, aptasensors have gained their popularity in clinical diagnosis, environmental montiring, and pharmaceutical study.Optical aptasensors can change biological interation between aptamer and the target into various measurable optical signal. According to the different optical principles, optical aptasensor can be classified as florescent aptasensor, spectroscopic aptasensor, electrochemical aptasensor and surface plasmon resonance aptasensor. Among these aptasensors, florescent aptasensor shows many advantades due to the sensitive and miltipe fluorescent changes(fluorescence intensity, wavelength shift, and fluorescent polarization) upon the aptamer-target combination. In this dissertation, several aptasensors with high sensitivitye and good selectivity are designed-using conjugated polyelectrolytes, graphene oxide and thioflavin T for the florescent detection of lead(II) ion, thrombin and hemin.Firstly, a novel lead(II) ion sensing platform is designed based on polycationic polymer(poly{[9,9-bis(6′-(N,N,N-diethylmethylammonium)-hexyl)-2,7-fluorenylene ethynylene]-alt-co-[2,5-bis(3′-(N,N,Ndiethylmethylammonium)-1′-oxapropyl)-1,4-phenylene]tetrai odide}(PFEP)/aptamer system. A fluorescein-labeled oligonucleotide with specific sequence, TBAA(5′-GGAAGGTGTGGAAGG-3′) was selected as a recognition probe because it can eliminate the interference of Hg2+, was selected as a recognition probe. In the presence of lead(II) ion, TBAA can form the G-quadruplex DNA with higher charge density. Then the electrostatic attractions between CCP and DNA brings the fluorescein closer to the CCP than the ss-DNA, thus, a significantly increased fluorescence resonance energy transfer(FRET) signal can be observed. The difference in the FRET efficiency can be used for simple, selective and sensitive detection of lead(II) ion in the presence of Hg2+ with an improved sensitivity of 0.06 nM.. Secondly, graphene oxide is introduced to the florescent detection of tumor markers. A “turn-on” thrombin aptasensor is developed based on the remarkable quenching of GO to the fluorescence of dye via efficient electronic/energy transfer(ET). In the absence of target protein, the fluorescence of Apt15-A/Apt15-B was quenched by GO through the π–π stacking effect between GO and single-stranded DNA. However, when thrombin was introduced into the system, a target induced G-quadruplex was formed with the two split aptamer fragments and thrombin. Fluorescence of G-quadruplex recovered due to weak interaction with the GO surface and releases from the surface of GO. Compared with the strategy using intact aptamer, probe concentration was lowered, and an improved sensitivity of 1 nM was obtained.Finally, a label-free hemin florescent aptasensor is developeded based on the aptamer of hemin and and the property that ThT can induce and combine with G-quadruplex to further capture hemin. With the formation of more stable hemin–PS2.M/G-quadruplex complex, a noticeable light-up fluorescence in ThT on binding to theG-quadruplex grants the sensor excellent sensitivity, which allows for the selective determination of hemin in the range of 0.1 nM~1000 nM. The label-free hemin aptasensor is simple, time-saving, and costeffective.