Ratiometric Fluorescent Sensor for Al³⁺ Based on the Inner Filter and Static Quenching Effects of Carbon Dots Obtained from Neem Leaves

The level of Al³⁺ is an indicator for many diseases of human, so the selective and sensitive detection of Al³⁺ is critically important for human health. In this study, a simple and sensitive ratiometric fluorescent sensor for the detection of Al³⁺ in the aqueous solution was developed based on the inner filter effect (IFE) and static quenching effect (SQE) of the carbon dots (CDs) and fluorescein. The preparation of CDs was facile and green by a one-step hydrothermal method, using the fresh Melia azedarach L. leaves as the raw material. The fluorescence of CDs at 430 nm was quenched owing to the combination of IFE and SQE while the fluorescence of fluorescein at 515 nm was enhanced upon adding fluorescein. Meanwhile, fluorescein interacted with Al³⁺ resulting in the quenching of the fluorescence of fluorescein at 515 nm and the recovering of the fluorescence of CDs at 430 nm with the addition of Al³⁺ into the system of CDs-fluorescein. The linear range of the ratiometric fluorescent sensor (F₅₁₅/F₄₃₀) for Al³⁺ detection was 0–500 μM and the detection limit was 0.216 μM. Importantly, it had been successfully applied in the detection of Al³⁺ in the drinking and environmental water.