A facile, economic and green one-step microwave synthesis route towards photoluminescent carbon dots is proposed. The preparation requires a carbohydrate (glycerol, glycol, glucose, sucrose, etc.) and a tiny amount of an inorganic ion, and can finish in just a few minutes, no surface passivation reagent is needed. The carbon dots are biologically compatible and show favorable optical properties and have potential applications in biolabeling and bioimaging.