Highly Sensitive Ratiometric Fluorescent Paper Sensors for the Detection of Fluoride Ions

Abstract

Two sensitive and ratiometric fluorescent probes (probe I and probe II) were developed for the detection of fluoride ions. Probe I can detect fluoride ions quantitatively within a range of 0-6 μM and a detection limit of 73 nM, while probe II has a range of 0-40 μM and a detection limit of 138 nM. The test strips from probe I are quickly able to recognize F^- (5 min) inside of the F^- safety level in drinking water (1.0 mg/L, ∼5 μM) under 254 nm ultraviolet light, and the test strips from probe II quickly recognize F^- (12 min) in dangerously high F^- levels in water (4.0 mg/L, ∼21 μM) under 254 nm ultraviolet light. This combination of fluorescent paper sensors from probe I and probe II can be used as a simple and convenient tool to determine whether water is safe to drink or dangerous.

Scheme 1. Synthesis of Probe I and Probe II

Figure 1

(a) Fluorescent spectra of probe I (10 μM) in different buffer solutions (pH, 3–12) with DMSO (v/v = 3:1) at 25 °C; (b) fluorescent spectra of probe I (10 μM) added to F^– (300 μM) in different buffer solutions (pH, 3–12) with DMSO (v/v = 3:1) at 25 °C; (c) fluorescent spectra of probe II (10 μM) in different buffer solutions (pH, 3–12) with DMSO (v/v = 3:1) at 25 °C; (d) fluorescent spectra of probe II (10 μM) added to F^– (300 μM) in different buffer solutions (pH, 3–12) with DMSO (v/v = 3:1) at 25 °C; (e) fluorescent spectra of probe I (10 μM) added to F^– (300 μM) in different solvents (CH₃CN, DMSO, and C₂H₅OH) with H₂O (v/v = 3:1); (f) fluorescent spectra of probe II (10 μM) added to F^– (300 μM) in different solvents (CH₃CN, DMSO, and C₂H₅OH) with H₂O (v/v = 3:1).

Figure 2

(a) Fluorescence spectra of probe I (10 μM) with F^– (0, 2, 4, 6, 8, 10, 20, 40, and 60 μM); (b) photograph of probe I solutions (10 μM) subjected to F^– (0, 2, 4, 6, 8, and 10 μM) under ambient light and 365 nm UV light; (c) fluorescence spectra of probe II (10 μM) with F^– (0, 5, 10, 15, 20, 25, 30, 35, 50, 60, 70, 80, 90, and 100 μM); (d) photograph of probe II solutions (10 μM) subjected to F^– (0, 10, 15, 20, 25, and 30 μM) under ambient light and 365 nm UV light; (e) time-dependent fluorescence spectra of probe I (10 μM) in the presence of F^– (3 μM) in DMSO with H₂O (v/v, 3:1) at 25 °C. Tests were performed in triplicate; (f) time-dependent fluorescence spectra of probe II (10 μM) in the presence of F^– (20 μM) in DMSO with H₂O (v/v, 3:1) at 25 °C. Tests were performed in triplicate.

Figure 3

(a) Fluorescence spectra of probe I (10 μM) with F^– (0, 1, 2, 3, 4, 5, and 6 μM); (b) plot of fluorescence intensity of probe I differences with F^– ranging from 0 to 6 μM; (c) fluorescence spectra of probe II (10 μM) with F^– (0, 5, 10, 15, 20, 25, 30, and 35 μM); (d) plot of fluorescence intensity of probe II differences with F^– ranging from 0 to 35 μM.

Figure 4

(a) Fluorescence intensity change of probe I (10 μM) upon addition of various species (10 μM for each. 1, blank; 2, Cu²⁺; 3, Ca²⁺; 4, K⁺; 5, Na⁺; 6, Mg²⁺; 7, HS^–; 8, HSO₃^–; 9, SO₃^2–; 10, SO₄^2–; 11, Fe²⁺; 12, NH₄⁺; 13, Cl^–; 14, Br^–; 15, I^–; 16, H₂O₂; 17, Cys; 18, GSH; 19, Hcy; 20, Fe³⁺; 21, Glu. 4 μM for F^–). Tests were performed in triplicate; (b) fluorescence intensity change of probe II (10 μM) upon addition of various species (10 μM for each. 1, blank; 2, Cu²⁺; 3, Ca²⁺; 4, K⁺; 5, Na⁺; 6, Mg²⁺; 7, HS^–; 8, HSO₃^–; 9, SO₃^2–; 10, SO₄^2–; 11, Fe²⁺; 12, NH₄⁺; 13, Cl^–; 14, Br^–; 15, I^–; 16, H₂O₂; 17, Cys; 18, GSH; 19, Hcy; 20, Fe³⁺; 21, Glu. 20 μM for F^–). Tests were performed in triplicate.

Figure 5

¹H NMR titration spectra of probe I-F^–.

Figure 6

HPLC spectra of probe II-F^–.

Scheme 2. Mechanism for Reaction of Probe I and Probe II with F^–

Figure 7

(a) Photograph of the test strips PI-A subjected to F^– (0, 1, 2, 3, 4, 5, and 6 μM) under ambient light and 254 nm UV light; (b) photograph of the test strips PI-B subjected to F^– (0, 1, 2, 3, 4, 5, and 6 μM) under ambient light and 254 nm UV light; (c) photograph of the test strips PII subjected to F^– (0, 3, 6, 13, 21, and 27 μM) under ambient light and 254 nm UV light; (d) photograph of the test strips PII subjected to F^– (0, 3, 6, 13, 21, and 27 μM) under ambient light and 365 nm UV light.