We report the development of a new type of flexible electrochemical biosensors based on graphene paper loaded with closely-packed Au@Pt core-shell nanoparticles as a freestanding cell culture substrate for real-time monitoring cell secretion of nitric oxide. The hybrid electrode was fabricated through a modular approach in which 2D-assembly of nanoparticles formed at the oil-water interface was transferred onto graphene paper by dip-coating. We have shown that the independently optimized metal nanostructures and graphene paper were integrated into functional electrodes with high electrocatalytic activity. When used for the detection of nitric oxide, the flexible electrodes have demonstrated high sensitivity, a wide linear range, and a low detection limit, which, in combination with its biocompatibility, offer unique opportunities for the real-time monitoring of nitric oxide secretion by human endothelial vein cells grown on the electrode. These interesting findings collectively demonstrate the potential of our modular approach for designing high-performance flexible electrodes with tailored surface properties.