Process intensification for gram-scale synthesis of N-doped carbon quantum dots immersing a microplasma jet in a gas-liquid reactor

N-doped carbon quantum dots are synthesised by immersing a microplasma jet into a gas–liquid reactor of the size of a microwell, containing an aqueous solution of folic acid (Vitamin B9). The distance of the tip of the microplasma jet to the water surface is changed in three steps, named distant, contact, and deflection modes. As a further variation, the liquid volume is either stirred or unstirred and may contain glass beads or metal flakes. In this way, the mass transfer, hydrodynamics, and the electrical field are influenced and create the specific gas–liquid interface, possibly including plasma-catalytic effects. A thermofluidic analysis confirms a uniform temperature profile and a positive temperature effect on the mass transfer. In this way, the research achieves process intensification, bringing the synthesis towards 1 g per day and maximising the intended performance, the photoluminescence intensity. Recycling further increases the mass yield via centrifugation. An analysis by optical emission spectroscopy reveals the formation of the plasma species from which a reaction mechanism is proposed