Continuous hydrothermal flow synthesis of blue-luminescent, excitation-independent nitrogen-doped carbon quantum dots as nanosensors

Blue-luminescent N-doped carbon quantum dots (NCQDs) exhibiting rarely observed excitation independent optical properties are synthesised from citric acid in the presence of ammonia via a Continuous Hydrothermal Flow Synthesis (CHFS) approach. CHFS is an eco-friendly, rapid synthetic approach (within fractions of a second) facilitating ease of scale-up industrialization as well as offering materials with superior properties. The synthesised NCQDs readily disperse in aqueous solution, have an average particle size of 3.3 0.7 nm, with highest emission intensity at 441 nm (and a narrow full width at half maximum, FWHM 78 nm) under a 360 nm excitation wavelength. N-doped carbon quantum dots, without any further modification, exhibited a high selectivity and sensitivity as a nano-sensor for the highly toxic and carcinogenic chromium( vi ) ions. The nano-chemo-sensor delivers significant advantages including simplicity of manufacturing via a continuous, cleaner technology (using targeted biomass precursor), high selectivity, sensitivity and fast response leading to potential applications in environmental industry as well photovoltaics, bio-tagging, bio-sensing and beyond. We present the first report of continuous hydrothermal flow synthesis of N-doped carbon quantum dots, with excitation-independent optical properties. They display high selectivity and sensitivity for Cr( vi ), and are thus suitable for environmental applications and beyond.