Research Of Biosensor Based On Excessively Tilted Fiber Grating Integrated With Graphene Oxide

In recent years,biosensors based on fiber Bragg gratings have been widely studied and applied in biomedical,biochemistry and other fields.Comparing with other types of fiber gratings,excessively tilted fiber gratings?ExTFG?have higher refractive index?RI?sensitivity and lower temperature cross-sensitivity effect,so they have more advantages in the field of detection/monitoring of biological immune response,where requiring high sensitivity and stability.Nevertheless,in the actual biological reaction solution,the RI sensitivity shown by ExTFG is only¹20nm/RIU,which needs to be further improved.Due to its rich oxygen-containing functional groups and large specific surface area,graphene oxide?GO?,a new two-dimensional nanomaterial,can be combined with fiber grating sensors to improve the modification effect of biological molecules and thus improve the detection performance of the biosensor.However,there are few reports about GO integrated ExTFG biosensor.This paper aims to improve the RI sensitivity of ExTFG sensor and the detection performance of GO integrated ExTFG biosensor.The main research contents are as follows:?1?Based on fiber Bragg grating?FBG?coupled-mode theory and tilted fiber Bragg grating?TFG?coupled-mode theory,the coupled-mode theory of ExTFG was demonstrated in detail.According to the phase matching condition and mode coupling principle of ExTFG,the theoretical analysis results of spectral response characteristics showed that RI sensitivity of ExTFG could be improved by reducing the radius of ExTFG cladding.?2?Experimental study on spectral and refractive index sensing characteristics of cladding etched ExTFG.ExTFG with cladding diameters ranging from¹25?m to¹5?m was prepared by HF etching,and its spectral response and RI sensing characteristics were studied experimentally.The experimental results show that the number of cladding modes decreases,and the spacing of adjacent resonance peaks becomes larger and larger with the reduction of the cladding diameter in the observed wavelength range of 1250¹650nm.The average RI sensitivity in the index region of1.33¹.38,the one near 1.33,and the one at around 1.38 of the etched ExTFG with a diameter of 15?m is⁶.3,⁵.3,and⁶.67 fold than those of the no-etched ExTFG,respectively.Moreover,the RI sensitivity around 1.38 could reach¹600nm/RIU.?3?The experimental study of immunosensor based on GO integrated ExTFG.Initially,GO film was coated on the surface of ExTFG by hydrogen bonding.Secondly,bovine serum albumin?BSA?monoclonal antibodies?MAbs?were used as the biometric units,which was combined with GO film in the form of covalent bond.Finally,the prepared GO-ExTFG immunosensor was used for the specific detection of BSA antigen molecules.The effectiveness and uniformity of GO coating were evaluated by field emission scanning electron microscopy?SEM?and energy spectrum.The spectral modulation characteristics of GO to ExTFG were analyzed,experimental results show that GO can modulate the wavelength of TM mode more effectively than TE mode.On the contrary,the peak intensity modulation of TE mode is much more obvious than that of TM mode.By recording the spectral evolution of the surface modification process and the dynamic process of BSA antigen detection,it can be seen that the proposed immunosensor has the characteristics of large detection range and rapid response for BSA antigen.The detection range is 1.5nM⁷5nM and the response time is less than 5min.The limit of detection?LOD?is⁰.88nM when the wavelength resolution of spectrometer is 0.03nm.The immunoassay followed the Langmuir adsorption model,and the affinity coefficient K_A and dissociation coefficient K_D of the immunosensor for BSA antigen were calculated to be¹.5?10⁸M^-11 and⁶.66?10^-9M,respectively.?4?Based on the above research background,the GO integrated ExTFG immunosensor was optimized.The structural design of the sensor was improved by integrating GO into ExTFG with a cladding diameter of⁸0?m,and the RI sensing characteristics and the detection performance of biological immune response were analyzed.Experimental results show that the detection sensitivity,limit of detection?LOD?and affinity coefficient K_Aof the sensor have been significantly optimized.