Preparation of Monolithic Silica Nanocomposite Plates Embedding Fluorescent Graphene Quantum Dots via an Aqueous Sol–Gel Process

Phloroglucinol (PG)-derived graphene quantum dots (GQDs) exhibit narrow-band emission in solution; however, they suffer from severe aggregation-induced photoluminescence (PL) quenching in the solid state. Here, we report the preparation of monolithic silica nanocomposite plates incorporating GQDs via a tetramethylammonium silicate (TMAS)-based sol–gel route. Crude GQDs (C-GQDs) were synthesized through a glycothermal reaction and purified by dialysis. C-GQDs were further purified by silica-gel column chromatography to yield P-GQDs. Both C-GQDs and P-GQDs showed stable dispersion under strongly alkaline conditions owing to electrostatic repulsion between deprotonated surface hydroxy groups at the edges of GQDs. The TMAS sol–gel method enabled homogeneous GQD dispersion within silica, producing transparent plate-like monoliths that emitted blue PL at ∼470 nm. PL quantum yields of 20.4% and 23.7% were obtained at the optimal loading amounts of C-GQDs and P-GQDs, respectively. Purification enhanced spectral clarity and PL intensity, although partial PL quenching occurred during gelation due to the significant pH decrease induced by ester hydrolysis. This study demonstrates the first example of PG-derived GQDs incorporated into monolithic silica plates, providing insight into dispersion behavior, matrix-nanoparticle interactions, and remaining factors limiting PL efficiency in solid-state carbon-based nanophosphors.