The water-soluble fluorescent organic nanoparticles (PDAPLP NPs) was synthesized by the oxidation of dopamine in the presence of vitamin B₆ cofactor pyridoxal 5′-phosphate (PLP). The fluorescent intensity of PDAPLP NPs was quenched in the presence of MnO₂ nanospheres due to the energy transfer process occurred from the PDAPLP NPs to MnO₂ nanospheres. The fluorescence was restored upon addition of GSH to the PDAPLP_MnO₂ nanocomposite due to the redox transformation of Mn(IV) to Mn(II). The fluorescence turn-on response allowed to detect GSH down to 0.31 µM. The developed nanoprobing approach was fast (response time < 2 min), simple and highly selective. The practical utility of the PDAPLP_MnO₂ nanoprobe was validated by quantifying GSH in real saliva samples. In addition, the PDAPLP NPs was applied for the fluorescence sensing of pH. The cyan-blue fluorescent PDAPLP NPs between pH 2.0 to 7.0 turned to green-fluorescent above pH 7.0 in the basic pH.