Development of an Inner Filter Effects-Based Upconversion Nanoparticles–Curcumin Nanosystem for the Sensitive Sensing of Fluoride Ion

This paper describes a novel ratiometric fluorescence-based sensor for the detection of fluoride ion. Yb³⁺, Er³⁺, and Tm³⁺ codoped NaYF₄ upconversion nanoparticles (UCNPs), which can emit fluorescence at 546, 657, 758, and 812 nm under the 980 nm single wavelength excitation, were synthesized, amino-modified and applied as the fluorescent signal indicator. The natural chemical curcumin served as specific recognition element and mixed with UCNPs to make a nanosystem. In this nanosystem, the absorption peak of curcumin shows a bathochromic shift when F^- was added, causing an upconversion fluorescence quenching at 546 and 657 nm through inner filter effects (IFE), whereas the upconversion emission at 758 and 812 nm remained unchanged. Thus, the fluorescence ratio I₅₄₆/I₇₅₈ was inversely proportional to F^- concentration. Meanwhile, the large absorption bathochromic shift also lead to a color change, based on the colorimetric analysis of F^- by the naked eye. Under the optimized conditions, the developed UCNPs-curcumin mixed system achieved the colorimetric and ratiometric fluorescence sensing toward F^- in the linear range of 25-200 μM and 5-200 μM, with the detection limits as low as 25 μM (ca. 0.48 ppm) and 5 μM (ca. 0.10 ppm), respectively. The developed nanosystem also has high selectivity and antijamming ability. Furthermore, this method showed promising practical applications in spiked real samples (ex., tap water and milk) with recoveries of 79.58% to 134.02% and RSD values in the range of 0.94% to 22.11%, which confirmed its great potential in harmful substance detection.