An oxidation–reduction fluorescent switch system is prepared based on low-cost and easily prepared poly-(2,6-diaminopyridine) (PdAP) nanoparticles (NPs) and bovine hemoglobin (Hb). As a fluorescent sensing platform, the PdAP NPs can enhance the PL intensity of Hb markedly. The system is triggered by the redox process of PdAP NPs catalyzed by Hb. This switch system is highly sensitive and selective with high signal-to-noise ratio and can be used as a fluorescent probe for vitamin C and H2O2 (ppb level). All geometries and energies were investigated by generalized gradient approximation of density functional theory.