Label Free Fiber Optic Biosensor Based On Cladding Mode Ultra Narrow Resonance

The diagnosis and control of diseases,environmental monitoring,drug development and food safety issues have become a worldwide concern.The number of new coronavirus infected reached 140 million and the death rate reached 3 million.The Japanese government announced that it would start to discharge Fukushima nuclear waste water into the sea and other incidents,which posed a serious threat and impact on people's life and health,production and life.The current urgent needs promote the continuous development of biosensors,but also put forward higher requirements for biosensors.A high reflectivity FBG biosensor based on ultra narrow band cladding mode resonance is proposed.Due to the coupling of millimeter wave forward propagating core mode and fiber Bragg grating backward propagating cladding mode,the transmission spectrum of the sensing structure presents a dense comb resonance.The FWHM of narrow-band resonance is as narrow as 40 pm and the Q factor is as high as 38500.Based on the coupled mode theory,the coupling among the core mode,cladding mode and radiation mode is described and numerically analyzed.The simulated spectrum of FBG sensor shows multiple transmission resonance depressions due to the coupling between the bounded modes,which is in good agreement with the experimental spectrum.The resonance wavelength of cladding mode is sensitive to Sri and shows high stability in blank buffer samples.Subsequently,the FBG sensor was functionalized with anti IgG to detect different concentrations of IgG.According to the function relationship between wavelength response and IgG concentration,the LOD value was evaluated as 360 pm.The specificity of FBG sensor was verified by detecting serum IgG.Then graphene oxide(go)was selected as the sensitizing material of FBG sensor to further improve the sensitivity.A uniform HRI layer is formed on the surface of FBG by dip coating.As a kind of high RI membrane,go membrane has evanescent wave enhancement effect,and its functional groups promote the biosensor's biological function through covalent bonding.Finally,the FBG sensor with go coating achieves high sensitivity and ultra-low LOD(32 ? m)It is one order of magnitude lower than the uncoated FBG sensor.The high reflectivity FBG biosensor proposed in this paper only involves the standard FBG manufacturing process and fiber fusion technology,thus ensuring cost-effectiveness and mass production.Compared with the optical fiber biosensor reported currently,it has the advantages of simplicity,low cost and high performance,and is expected to be popularized and applied in the field of biosensor.