A Metal Nano-cavity Based Fiber Optic Surface Plasmon Resonance Biosensor For The Rapid Detection Of Plant Hormone Abscisic Acid

As an important research direction of photonics,optical surface plasmon resonance(SPR)has attracted a great deal of attention in the application of biosensing.The SPR biosensor based on optical fiber is sensitive to the biological medium attached to the fiber surface.The free electrons on the metal surface is oscillated regularly by the light excitation of the fiber core,which forms surface plasmon wave(SPW).The change of the surrounding environment of the optical fiber will cause the change of the transmission information of the optical field.By changing the fiber type or the surface plasmon material,the sensitivity of fiber SPR sensor will be improved highly.The nano-structure can make the surface plasmon polariton(SPP)localized and the localized surface plasmon resonance(LSPR)is formed around the structure.The application of sub-wavelength nanoparticles based on LSPR for assign of single molecular has been reported.In order to further improve the performance of optical fiber SPR sensor,SPP propagation characteristics of metal-medium interface in Dtype fiber SPR sensor are studied in this paper.In combination with the characteristics of metal nanoparticles in local surface plasma resonance,the sensing performance of optical fiber surface plasmon resonance is enhanced by using metal nano-cavities.The main studies are listed as follows:1.The sensitivity enhancement of D-shaped fiber SPR sensor with symmetric metal cavity has been proposed.The symmetric metal cavity platform is developed with first excitation metal-coated D-shaped fiber as the sensing device,while thin coupling metal layer is situated under the inner wall of the substrate as a second metal layer to construct the symmetric metal cavity.With the high reflection characteristic of metal in visible-near infrared wavelength,the excitation of evanescent field on the fiber surface makes the free electron oscillations in the metal layer,then SPW is inhibited from spreading in the medium by coupling metal layer,and limited within the cavity sandwich.Due to the electromagnetic coupling conditions of cavity structure in the micro-nano size metal waveguide,the excitation metal layer and the coupling metal finish the second wave vector matching in cavity,and the high-order SPP mode makes the electric field inside the cavity enhanced greatly.The symmetric metal cavity can significantly improve D-shaped fiber SPR sensor sensitivity,and the sensitivity of the refractive index(RI)from 1.33 to 1.36 is ~ 4000 nm/RIU.Compared with the ordinary D-shaped fiber SPR sensor,the sensitivity is improved by more than 1.5 times.The optimized sensitivity can be further enhanced to 25000 nm/RIU.This new concept of enhancement of sensitivity with limited measurand volume will open a new designing methodology for optical fiber biosensors.2.We proposed the D-shaped fiber surface plasmon resonance model based on locally asymmetric metal cavity.The nano-cavity gap is formed by placing metal particle above the D-shaped fiber coated metal film.The surface plasmon resonance from excited metal layer is enhancing in the cavity and the mode field and the model electric filed is shift laterally to outside along the nanostructure boundary.The polarization cleavage of plasmon model appears under this locally asymmetric structure,then surface plasmon resonance coupling is formed in the cavity and the localized surface plasmon field is formed at the edge of the nanoparticle.The strength of these modes is influenced by the interface shape of the metal cavity,there is strong resonance in the local cavity with round interface under the influence of polarization direction of the round structure,while the plasmon power is converged easily at the tip in the square and triangle interface.By expanding the single local metal cavity to more than one,the mode loss of D-shaped fiber is obvious at the resonance wavelength.The sensing sensitivity of proposed sensor has been improved about one time,and it can be applied to the detection of smaller molecules using the biological properties of nanoparticles.3.A nano-coated optical fiber biosensor based on surface plasmon resonance is proposed and proved clearly for plant hormones abscisic acid functionalized detection.The surface plasmon resonance sensor is designed to conjugate nanoparticles onto gold thin layer coated on the surface of high plastic cladding fiber core.After coupling gold layer with nanoparticles,a nanoscale local structure will be formed between the two.The electric field intensity in the polarization direction of the particles is significantly enhanced,and the sensor under the change of the external environment refractive index is sensitive.The free gold nanoparticles are easily self-assembled on the surface of the sulfhydryl fiber to form the nano-coating of the surface plasmon resonance sensor.The particles with 10 nm size move by 10 nm at the resonance wavelength of 0~5 mmol?L-1 recognition,and the average sensing sensitivity of the overall sensor to the refractive index change is 2000 nm?RIU-1.After using nucleic acid aptamer-functionalized gold nanoparticles,low concentration of plant hormone abscisic acid molecules can specifically bind to the aptamer.The formant position shifts to 3 nm in the concentration change of 0.1~1.6 ?mol?L-1,the average detection sensitivity is 1.87?106 nm?M-1 and the resolution of detection is 0.535?10-6 M?nm-1.This nano-coated fiber surface plasmon resonance sensing method has simple preparation steps and high sensitivity detection response,which is expected to play a role in the detection of other small biological molecules.