Microplasma nanoengineering of emission-tuneable colloidal nitrogen-doped graphene quantum dots as smart environmental-responsive nanosensors and nanothermometers

Nitrogen-doped graphene quantum dots (NGQDs) with controlled emission properties are useful materials for fundamental research and applications. However, the scalable and sustainable production of NGQDs with defined nanostructures from biomass precursors remains elusive. Here a bottom-up microplasma synthesis of emission-controlled colloidal NGQDs used as the multifunctional nanosensors for temperature and selective metal ion sensing is demonstrated. The emission-tuneable NGQDs are synthesized by engineering the N-doping configurations and surface functional groups on the NGQDs surfaces by carefully controlling the plasma conditions and electrolyte chemistry. In situ optical emission and UV–Vis absorbance spectroscopy measurement reveals that the plasma-induced hydroxyl radicals and solvated electrons define the N-dopant configuration and surface functionalization of NGQDs, leading to emission-tuneable NGQDs. The emission-tuneable NGQDs are applied as multifunctional nanosensors for selective Fe3+, Cu2+, and Hg2+ detection revealing broad linear detection range (0.5–300 μM) and low limit of detection (LOD) of 47.9 nM for Hg2+ and sensitive temperature detection from 10 to 80 °C. The developed PL sensing method shows high throughput of 5000 detections per hour. Our work demonstrates the possibility of the plasma-enabled nanoengineering of NGQDs with controlled optical properties and a step towards sustainable and scalable synthesis of graphene nanomaterials from renewable bioresources.