Graphene quantum dots (GQDs) are zero-dimensional materials that exhibit characteristics of both graphene and quantum dots. Herein, we report a rapid, relatively green, one-pot synthesis of size-tunable GQDs from graphene oxide (GO) by a sonochemical method with intermittent microwave heating, keeping the reaction temperature constant at 90 °C. The GQDs were synthesized by oxidative cutting of GO using KMnO₄ as an oxidizing agent within a short span of time (30 min) in an acid-free condition. The synthesized GQDs were of high quality and exhibited good quantum yield (23.8%), high product yield (>75%), and lower cytotoxicity (tested up to 1000 μg/mL). Furthermore, the as-synthesized GQDs were demonstrated as excellent fluorescent probes for bioimaging and label-free sensing of Fe(III) ions, with a detection limit as low as 10 × 10^–6 M.