A highly specific single-atom Fe-NC nanozyme-based colorimetric probe for ultrasensitive visual detection of fluoride

• Silica-encapsulated SA Fe-NC was prepared for sensitive F - colorimetric detection. • The activity of nanozyme was released through the specific etching of SiO2 by F - . • A portable paper sensor with smartphone was designed for visual analysis of F - . • An economical method was offered for detecting F - in complex real samples. As the smallest and most electronegative anion, fluoride ion (F − ) requires precise concentration monitoring due to its significant implications for environmental analysis. Thus, it is necessary to develop a suitable method for the sensitive detection of F − in complex matrices. In this work, the single-atom (SA) Fe-NC nanozyme with high oxidase-like activity were prepared and encapsulated within a SiO 2 shell layer to form the nanozyme@SiO 2 core-shell structure. The active sites of SA Fe-NC@SiO 2 nanozyme were exposed through specific etching of the unsintered SiO 2 shell in the presence of fluoride ions, thereby gradually releasing the enzymatic activity to catalyze the oxidation of chromogenic substrate TMB into oxTMB, yielding a visual color change from colorless to deep blue. In the most favorable conditions, the oxTMB absorbance exhibited a good linear correlation with the logarithm F − concentration (10–1,000 nM), enabling sensitive quantitative detection of F − with a detection limit as low as 4.5 nM. Notably, this proposed SA Fe-NC@SiO 2 nanozyme colorimetric probe displayed remarkable specificity towards F − over several potentially interfering ions in environmental water, allowing it applicable for complex real samples such as pond water and toothpaste. A portable paper sensor incorporating SA Fe-NC@SiO 2 was subsequently developed, which facilitated an intuitive on-site detection of F − in complex matrices by integrating with smartphone-based analysis.