Thermodynamic Studies of Ester-Based Gemini Surfactant and Nitrogen-Doped Graphene Quantum Dot Coacervation and Its Application in Quercetin Detection

Coacervation is spontaneous phase separation into colloid-rich and dilute phases. Here, we report self-assembled luminescent coacervate droplets formed by mixing nitrogen-doped graphene quantum dots (NGQDs) with an ester-based cationic gemini surfactant in water. Confocal laser scanning microscopy (CLSM) confirmed the formation of intrinsically fluorescent spherical droplets. Double-logarithmic plot analysis and Van’t Hoff thermodynamic studies revealed that the spontaneous formation of stable luminescent coacervate droplets is primarily driven by hydrogen bonding and van der Waals interactions. Fluorescence, UV–vis spectroscopy, lifetime measurements, and temperature-dependent Stern–Volmer studies showed that quercetin-induced fluorescence quenching of the coacervate droplets proceeds through a synergistic combination of static quenching, PET-mediated dynamic quenching, and the inner filter effect (IFE), providing a simple and selective fluorometric sensing approach. The sensor exhibited a linear response in the range of 5–50 μM, with detection limits of 5.7 nM in deionized (DI) water and 7.1 nM in tap water. The fluorescent probe effectively measured quercetin in biological samples (human serum, plasma, whole blood) and real samples (Java plum, tomato) with recovery rates ranging from 94.9% to 108.3%. This turn-off droplet-based platform provides a simple, rapid, and effective method for quercetin detection, demonstrating the potential of hybrid nanomaterials in advanced sensing applications.