Detection of Extreme Acidity Based on Carbon Dots: One‐Step Gram‐Scaled Syntheses, Ratiometric Emission Photoluminescent Probe, and Hand‐Held Visual Detection

Abstract It is a significant challenge to develop acidity probes suitable for use in extremely acidic conditions, especially to achieve real‐time on‐site detection. In this work, carbon dots (CDs) are synthesized on a gram‐scale under mild conditions using 3‐phenylpropionaldehyde as precursors. The CDs have excellent robustness, showing distinguished anti‐photobleaching and ionic stability. Dramatically, a visual transition of the fluorescent color of CDs from cyan to orange is observed when the concentration of H⁺ ([H⁺]) increased under high acidity conditions, owing to the generation of another emission peak in the red‐light region. Taking advantage of the optical properties, CDs are designed as a ratiometric sensing platform for the detection of extreme acidity. Furthermore, the platform for [H⁺] possesses high selectivity and excellent anti‐interference capability. In extremely acidic wastewater, I_{595 nm}/I_{480 nm} and [H⁺] maintain a very good linear relationship (R² = 0.996). Moreover, a smartphone is employed to obtain the red‐green‐blue (RGB) values of the samples to realize hand‐held detection. By measuring the R/G, the platform exhibits ultrasensitive recognition of [H⁺] in the range of 1–9 mol L⁻¹ with R² of 0.992. This work paves an avenue for the design of rapid and on‐site CDs based sensors for critical environmental applications.