A kind of label-free refractive index biosensor based on long-period gratings (LPGs) inscribed in microstructured optical fibers (MOFs) is theoretically studied. The finite element method is used to visualize the mode field distribution along the grapefruit MOF. The transmission spectrum of LPG in the air or liquid is simulated by the coupled-mode theory. An immobilized biorecognition molecules layer on the inner walls of the holes in the MOF provides the ability to capture analyte molecules selectively. Numerical results show that an ultrahigh refractive index sensitivity of approximately 397.3 nm/RIU over the refractive index range of 1.4-1.5 is achieved. Moreover, the sensitivity of the grapefruit MOF can be improved by reducing the periods of gratings. In addition, the shift of the resonant wavelength is linear to the analyte thickness with good sensitivity about 0.6 nmm. It is expected to play a great role in theoretical guidance for further development of biosensors.
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