Synthesis and functionalization of monodisperse carbon nano-onions (CNOs) for glucose sensing applications. Display omitted •A simple, scalable and inexpensive metal catalyzed synthesis of CNOs is reported.•The CNOs based enzymatic glucose sensor shows a sensitivity value of 26.5 µA cm−2 mM−1.•A non-enzymatic sensor is designed by decorating 2.5 nm Pt NPs on the surface of CNOs.•The non-enzymatic sensor has good sensitivity, a wide linear range and low detection limit. A high sensitive glucose sensing characteristic has been realized in carbon nano-onions (CNOs). The CNOs of mean size 30 nm were synthesized by an energy-efficient, simple and inexpensive combustion technique. These as-synthesized CNOs could be employed as an electrochemical sensor by covalently immobilizing the glucose oxidase enzyme on them via carbodiimide chemistry. The sensitivity achieved by such a sensor is 26.5 µA mM−1 cm−2 with a linear response in the range of 1–10 mM glucose. Further to improve the catalytic activity of the CNOs and also to make them enzyme free, platinum nanoparticles of average size 2.5 nm are decorated on CNOs. This sensor fabricated using Pt-decorated CNOs (Pt@CNOs) nanostructure has shown an enhanced sensitivity of 21.6 µA mM−1 cm−2 with an extended linear response in the range of 2–28 mM glucose. Through these attempts we demonstrate CNOs as a versatile biosensing platform.