In this study, we develop a room-temperature, single-step and chemically clean oxido-reductive top - down method to prepare highly pure graphene quantum dots (GQDs) of size 3.3 ± 0.7 nm by noncontact gamma radiolysis of aqueous dispersion of graphene oxide (GNO). The strong oxidising hydroxyl radicals and reducing nascent hydrogen are in situ generated by gamma radiolysis of water, and these radicals simultaneously react with GNO to produce green photoluminescent GQDs possessing some interesting photophysical and electrochemical properties. Both the steady-state and excited-state photophysics of GQDs suggest the involvement of two prominent energy states, which are deeply rooted with its molecular functionalities. By virtue of this novel synthesis approach, the GQDs possess several core-confined p-units together with edge-functional groups and thus show high intrinsic outer-sphere electrode kinetics to reversible electrochemical benchmark redox mediator (viz. K₃[Fe(CN)₆]/K₄[Fe(CN)₆]) of sensors through electrochemical impedance spectroscopy and cyclic voltammetry. The GQDs synthesised through gamma radiolysis show three-order faster intrinsic heterogeneous electron transfer kinetics than hydrothermally synthesised GQDs for that redox couple. This study is expected to open up a new direction for the chemically clean and sustainable synthesis of pure GQDs as impending electrocatalyst for the development of efficient electrochemical sensors.