An outcladding sensitized label-free DNA biosensor is developed based on tilted fiber Bragg gratings. The biosensor, functionalized with peptide nucleic acid (PNA) probes, is based on a double tilted fiber Bragg grating that forms a modified Fabry-Perot core-cladding closed-loop cavity. Interference is set up between an injected guided mode and the reflected core mode and cladding modes created by the light scattered by the tilted gratings, leading to the generation of interference fringes within both these spectral notches. When the DNA binds to the PNA probes attached onto the fiber cladding, a refractive index change occurs at the cladding-PNA interface and the fringe visibility changes accordingly. Real-time spectral measurement results are reported, showing that a 10-nM DNA solution induces a 10% modulation of the corresponding fringes visibility. Cycling tests are performed for measuring and checking the repeatability of the sensor.