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We present the design, fabrication, and characterization of submicronic grating couplers integrated on Si₃N₄ rib waveguide Mach-Zehnder interferometers (MZIs) for biosensing applications working in the visible spectral range for both TE and TM polarizations. Depending on the waveguide structure, a maximum of 11.5% of coupling efficiency has been experimentally obtained at 658 nm, while a limit of detection of 1.6 × 10^-7 in refractive index unit is achieved for the biosensor. These results represent an important milestone toward the achievement of a truly portable and multiplexed point-of-care platform using the integrated MZI sensor.