Developing a selective and sensitive Fluoride sensing scheme by fluorimetric and chemometric optimisations using highly fluorescent boron-doped Carbon dots with a 'turn off-on' mechanism.

Fluoride detection in water is a critical issue that has received extensive attention recently. Researchers have focused on developing practical and reliable methods for detecting Fluoride in water, and fluorescent carbon dots have emerged as a promising solution. These dots are easy to synthesise, highly fluorescent and stable, making them an ideal choice for this application. In this context, highly fluorescent boron-doped Carbon Dots (BCDs) were synthesised using Urea, Citric acid, and Boric acid via microwave synthesis. Characterisation of BCDSs was performed (Photoluminescence, HR-TEM, DLS, and FTIR), showing excellent optical properties (quantum yield = 55.4%), size (< 5 nm). The BCDSs solution was used as a fluoride sensor probe using the 'turn-off-on' property. Ferric (Fe) solution was used to inhibit (turn-off) the fluorescence of the BCDs by forming BCDs-Fe complex in the solution. The addition of a fluoride sample recovers the fluorescence (turn-on) by removing Fe from the complex to form [FeF]. Materials, BCDs, quencher volume and reaction time were optimised to develop a reliable fluoride sensing scheme, which included BCDs dilution, controlled turn-off by adjusting the volume of the quencher, and time dependence studies (2-15 s). A chemometric model was generated through PLS analysis to study the influence of each optimisation on the sensing performance. The result is a highly reproducible and reliable method for detecting Fluoride in water, obtaining high linearity (R = 0.98), low error (RMSE = 0.7) and high sensitivity (LOD and LOQ of 0.69 and 2.10 ppm, respectively) for a concentration range of 0-50 ppm. Real samples were also analysed to get an overview of sensing performance. Overall, fluorescent BCDs-based and chemometric-assisted sensor optimisation schemes have shown great promise for F detection in water. This breakthrough could open new pathways for optical-based sensor optimisations for other hazardous ions as well, which in turn have far-reaching implications for community's worldwide, helping to ensure safe and healthy drinking water for everyone.