A highly sensitive and selective chemosensor for Pb2+ based on quinoline-coumarin

Abstract

In this study, a highly sensitive and selective fluorescent chemosensor, ethyl(E)-2-((2-((2-(7-(diethylamino)-2-oxo-2H-chromene-3-carbonyl)hydrazono)methyl)quinolin-8-yl)oxy)acetate (1), was synthesized and characterized by ¹H NMR, ¹³C NMR and ESI-MS. Sensor 1 showed an "on-off" fluorescence response to Pb²⁺ with a 1 : 1 binding stoichiometry in CH₃CN/HEPES buffer medium (9 : 1 v/v). The detection limit of sensor 1 to Pb²⁺ was determined to be 0.5 μM, and the stable pH range for Pb²⁺ detection was from 4 to 8.

Fig. 1. Variation of fluorescence intensity of sensor 1 (10 μM) in CH₃CN/HEPES buffer medium (9 : 1 v/v, pH = 7.4) with and without Pb²⁺ ions (10 equiv.) with pH at room temperature.

Fig. 2. (a) UV-vis spectra of 1 (10 μM) with the addition of different metal ions (10 equiv.) in CH₃CN/HEPES buffer medium (9 : 1 v/v, pH = 7.4). Inset: the color changed of probe 1 (10 μM) in the absence and presence of Pb²⁺ (20 equiv.).

Fig. 3. UV-vis spectra of sensor 1 (10 μM) with the addition of various concentrations [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15 and 20 equiv.] of Pb²⁺ in CH₃CN/HEPES buffer medium (9 : 1 v/v, pH = 7.4).

Fig. 4. (a) Fluorescence spectra of sensor 1 (10 μM) with the addition of different metal ions (10 equiv.) in CH₃CN/HEPES buffer medium (9 : 1 v/v, pH = 7.4) with an excitation at 440 nm. Inset: the fluorescence changed of probe 1 (10 μM) in the absence and presence of Pb²⁺ (20 equiv.) under UV light at 365 nm.

Fig. 5. Fluorescent intensity of sensor 1 (10 μM) with selected cations (10 equiv.) in the absence (red bars) or presence (black bars) of Pb²⁺ (10 equiv.).

Fig. 6. Fluorescence spectra of sensor 1 (1 μM) with the addition of various concentrations [0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10 equiv.] of Pb²⁺ in CH₃CN/HEPES buffer medium (9 : 1 v/v, pH = 7.4) with an excitation at 440 nm. Inset: the Job plots with fluorescence titrations showed an intersection point at about 0.5 mol fractions, indicating that sensor 1 formed a 1 : 1 complex with Pb²⁺.

Scheme 1. Synthesis of sensor 1.

Fig. 7. The Benesi–Hildebrand plot of sensor 1 with Pb²⁺. Linear equation: Y = 5.09 × 10⁻⁹ × X + 1.04 × 10⁻⁵, R² = 0.9973, K_a = 2043 M⁻¹.

Fig. 8. The plot of the intensity at 485 nm for a mixture of sensor 1 and Pb²⁺ in CH₃CN/HEPES (9 : 1, v/v) buffer (pH = 7.4) in the range 0.1–1 equiv. (λ_ex = 440 nm). The calculated detection limit of 1 is 5 × 10⁻⁷ M.

Scheme 2. The possible mechanism of sensor 1 with Pb²⁺.