We report on a facile method to synthesize carbon dots (CDs) using glycerol solvent as a single precursor via a pyrolysis process free from catalysts. This method is extremely simple and economical, and provides a feasible route for mass production of highly emissive CDs. For rationalization, a mechanism incorporating dehydration of glycerol, followed by acrylaldehyde formation is tentatively proposed for CD production. Further systematic improvement of particle homogeneity is made by harnessing the growth of CDs inside the mesoporous silica nanoparticles that act as a nano-reactor to regulate the size distribution. Simultaneously capping a polyethylene glycol (PEG)-derived reactant onto the CDs@SiO₂ enhances their luminescence, stability and bio-compatibility. The as-prepared CDs@mSiO₂–PEG nanocomposites are then loaded with the anti-cancer drug doxorubicin (DOX), so that the controlled release of DOX could be monitored by both time-dependent and spatially resolved ratiometric fluorescence intensity for CDs versus DOX in HeLa cells, successfully demonstrating that the CDs@mSiO₂–PEG nanocomposites are suitable for cell imaging and drug release.