Fluorescent Boron and Phosphorus Co-Doped Graphene Quantum Dots for Bovine Serum Albumin Sensing in Cow's Milk with Smartphone-Enabled Real-Time Readout

Abstract

Accurate and rapid quantification of milk proteins is essential for dairy quality control and nutritional assessment. However, many conventional analytical techniques remain limited by high operational cost, labor-intensive sample pretreatment, and inadequate sensitivity in complex milk matrices. Among milk proteins, bovine serum albumin (BSA) is a valuable quality indicator because its concentration can reflect the physiological condition of dairy animals and may also indicate adulteration in milk products. Here, we report a sensitive fluorescence nanosensor based on boron and phosphorus codoped graphene quantum dots (BP-GQDs) for quantitative BSA detection directly in milk. BP-GQDs were synthesized via a simple hydrothermal route, yielding uniformly distributed nanodots with strong blue emission. Their structural, morphological, and optical properties were systematically characterized by UV-Vis spectroscopy, high-resolution transmission electron microscopy, powder X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, steady-state fluorescence spectroscopy, and time-correlated singlephoton counting. Boron and phosphorus co-doping enhanced fluorescence efficiency and quantum yield while maintaining good biocompatibility. The sensing response was attributed to a Forster resonance energy transfer-based quenching process arising from BP-GQD-BSA interactions, providing high selectivity and excellent sensitivity with a limit of detection of 7.861 nM. In addition, integration with a smartphone-assisted platform enabled rapid, on-site quantification of BSA in milk, demonstrating the potential of this approach for field-level dairy quality monitoring and improved protein analysis compared with conventional colorimetric and immunological methods.