Fluorescent boron and phosphorus Co-doped graphene quantum dots for bovine serum albumin sensing in cow's milk with smartphone-enabled real-time readout

-doped graphene quantum dots (BP-GQDs) for the quantitative detection of BSA directly in milk samples. The BP-GQDs were synthesized through a simple hydrothermal method, producing uniformly distributed nanodots with strong blue fluorescence emission. Their structural, morphological, and optical properties were systematically examined using UV-vis spectroscopy, high-resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), steady-state fluorescence spectroscopy, and time-correlated single-photon counting (TCSPC). Co-doping with boron and phosphorus led to a noticeable enhancement in fluorescence efficiency and quantum yield while maintaining good biocompatibility. The sensing performance was governed by a photoinduced electron transfer (PET) - based fluorescence quenching mechanism arising from the interaction between BP-GQDs and BSA, resulting in high selectivity and excellent sensitivity, with a detection limit of 7.861 nM. Also, integration of this sensor with a smartphone-assisted platform enabled rapid, on-site quantification of BSA in milk, demonstrating its potential for field-level dairy quality monitoring and improved protein analysis compared with conventional colorimetric and immunological methods.