Fluorometric detection of pH and quercetin based on nitrogen and phosphorus co-doped highly luminescent graphene-analogous flakes

Highly luminescent nitrogen and phosphorus co-doped graphene-analogous flakes (NPCGFs) were synthesized by a one-pot simple hydrothermal reaction using β-cyclodextrin (β-CD), vinylphosphoric acid (VPA), and o-phenylenediamine (oPD) as the precursors. VPA, as an important organic P-containing monomer, was selected as the phosphorus source to generate additional conjugated and effective binding sites on the surface of the NPCGFs. This synthetic strategy not only allows enhancement of structural rigidity, but also effectively eliminates surface traps of the NPCGFs, resulting in an improved fluorescence quantum yield (FL QY) of the NPCGFs. Additionally, oPD simultaneously acts as a nitrogen source and enables amino functionalisation of the NPCGF surface in the synthesis process. The NPCGFs (QY, 32.49%) are irregularly shaped with a typical diameter of approximately 54 nm and display strong fluorescence, with excitation/emission maxima of 360/445 nm. It was found that the NPCGFs can serve as a multifunctional FL probe for pH measurement and quercetin (Qc) detection. A linear relationship exists between the decrease in FL intensity and the concentration of Qc in the range from 0.35 to 30 μg mL−1 as well as the pH variation between 4.0 and 7.0. The probe was further applied to the determination of Qc in living cells.