Carbon Quantum Dots as Fluorescence Turn-Off-On Probe for Detecting Fe³⁺ and Ascorbic Acid

This work describes a "turn-off-on" fluorescence probe based on carbon quantum dots for sensing Fe³⁺ and ascorbic acid. The carbon quantum dots are prepared by hydrothermal method using a biocarbon source of black sesame. When excited at 355 nm, the carbon quantum dots show a strong bright blue emission peak centered at 438 nm. Obviously, the decrease of the fluorescence intensity of carbon quantum dots can be seen upon addition of Fe³⁺. Interestingly, the fluorescence quenching can be regained after the addition of ascorbic acid. The mechanism is that the added Fe³⁺ was destroyed by reductive ascorbic acid because of the redox reaction between ascorbic acid and Fe³⁺, making the fluorescence of the system recovered. The obtained curves are linear for Fe³⁺ and ascorbic acid over the range 50-1500 μM and 32.2-987.6 μM, respectively. The detection limits for Fe³⁺ and ascorbic acid are 2.78 μM and 0.0344 μM, respectively. Thus the carbon quantum dots can be used as a dual-function fluorescent sensor to achieve sensitive detection of Fe³⁺ and ascorbic acid.