High Sensitively Plasmonic Fiber Optical Biosensors Based On Aptamer Conformational Amplification

Different from fiber Bragg grating,the tilted fiber Bragg grating?TFBG?enable the coupling of light to modes guided by the cladding instead of the core,resulting in a fine comb of narrowband resonances,which provides a high quality factor(Q-factor up to 10^-4).The biosensor utilizes a TFBG to excite cladding modes for surrounding refractive index measurement with no structural modifications.What's more,as the core mode calibration inherent in the TFBG,the sensor works with minimal cross-sensitivities.It is studied that the surface plasmon resonance?SPR?can be effectively excited by modifying the TFBG surface with a suitable metal film.SPR is capable of enhancing the surface localized energy over the fiber?up to 70%?.Thus,the TFBG-SPR devices are strongly sensitive to their surrounding refractive index changes.Taking advantage of the excellent electro-optical properties and absorption of biomolecule by?-?stacking,graphene can be used as functional platform for high-performance electrochemical sensors.In this paper,the monolayer graphene is transferred to a gold-coated TFBG surface by the pulling method.After that,Raman spectrum was used as a characteristic method to verify the surface morphology of graphene.At the same time,spectral data showed that the monolayer graphene do not destroy the spectral characteristics of TFBG-SPR.By integration of graphene platform and TFBG-SPR,an aptamer conformational amplification based dopamine biosensor was proposed.It is able to measure precise dopamine concentration at trace level.Dopamine is one of the crucial neurotransmitters in the human central nervous system.However,dopamine is such a small molecule that the refractive index change induced by its concentration variations is a challenge for conventional optical fiber-based biosensors to detect.Here,we functionalized a monolayer graphene over the gold film of the TFBG,which offers a stable adsorption of single-stranded DNAs aptamers by?-?stacking.When single-stranded DNAs aptamers attract and trap dopamine molecules from solution,they change the conformation?the single-stranded DNA aptamer conformation switches from a random flexible structure into a rigid stem-loop structure?,therefore amplify the surface refractive index changes at the fiber surface.It is demonstrated that the mechanism of aptamer conformational amplification enable a highly sensitive detection of dopamine.The sensor provides an extremely low limit of detection of1.6×10^-13M and a linear response to dopamine concentration from the limit of detection up to 10^-8 M.