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An ultrasensitive biosensor based on hybrid structure and composed of long-range surface plasmon polariton (LRSPP) and dielectric planar waveguide (PWG) modes is proposed. Both PWG and LRSPP modes have strong resonances to form strong coupling between the two modes, and the two modes can couple to enhance sensitivity of sensors. In the hybrid structure, PWG is composed of cytop–Si–cytop multilayers and the LRSPP configuration is composed of cytop–metal–sensing medium multilayer slabs. The highest imaging sensitivities of 2264 and 3619 RIU^−1 were realized in the proposed sensors based on Au and Al-monolayer graphene, respectively, which are nearly 1.2 and 1.9 times larger than the 1910 RIU^−1 sensitivity of the conventional LRSPR sensor (LRSPP sensor). Moreover, it is demonstrated that the PWG-coupled LRSPP biosensor is applicable to the sensing medium, with refractive …