A New, Extremely Sensitive, Turn-Off Optical Sensor Utilizing Schiff Base for Fast Detection of Cu(II)

Abstract

Throughout this research, a unique optical sensor for detecting one of the most dangerous heavy metal ions, Cu(II), was designed and developed. The (4-mercaptophenyl) iminomethylphenyl naphthalenyl carbamate (MNC) sensor probe was effectively prepared. The Schiff base of the sensor shows a "turn-off" state with excellent sensitivity to Cu(II) ions. This innovative fluorescent chemosensor possesses distinctive optical features with a substantial Stocks shift (about 114 nm). In addition, MNC has remarkable selectivity for Cu(II) relative to other cations. Density functional theory (DFT) and the time-dependent DFT (TDDFT) theoretical calculations were performed to examine Cu(II) chelation structures and associated electronic properties in solution, and the results indicate that the luminescence quenching in this complex is due to ICT. Chelation-quenched fluorescence is responsible for the internal charge transfer (ICT)-based selectivity of the MNC sensing molecule for Cu(II) ions. In a 1:9 (v/v) DMSO-HEPES buffer (20 mM, pH = 7.4) solution, Fluorescence and UV-Vis absorption of the MNC probe and Cu(II) ions were investigated. By utilizing a solution containing several metal ions, the interference of other metal ions was studied. This MNC molecule has outstanding selectivity and sensitivity, as well as a low LOD (1.45 nM). Consequently, these distinctive properties enable it to find the copper metal ions across an actual narrow dynamic range (0-1.2 M Cu(II)). The reversibility of the sensor was obtained by employing an EDTA as a powerful chelating agent.

Keywords: chemical analysis; copper; fluorescence; optical properties; sensor; synthesis.

Scheme 1

Synthesis of the Schiff Base carbamate.

Figure 1

MNC (a) UV-Vis; (b) emission and excitation spectra.

Figure 2

(a) The UV-Vis spectra of MNC throughout the gradual adding Cu(II); (b) the relation of the band ratios at 273 and 378 nm (A₂₇₃/A₃₇₈) against [Cu(II)].

Figure 3

(a) Luminescence of MNC 10 μM in a 1:9 (v/v) DMSO-HEPES buffer (20 mM, pH = 7.4) during gradual adding Cu(II) of 0–1.5 μM range, λ_exc 312 nm; (b) calibration curve of MNC intensity versus [Cu(II)].

Figure 4

The influence of the pH on the fluorescence intensities of MNC ligand under λ_exc 312 nm.

Figure 5

Stern Volmer plot for Cu(II)-MNC.

Figure 6

(a) Maximum luminescence intensities of the MNC sensing molecule (I) in the presence of various metal ions (blue bar) and (II) in the presence of the same metal ions and Cu(II) ions (red bar). The ratio of (MNC: Cu(II): M(II) has a 1:1:1 ratio (λ_ex = 312 nm); (b) Job’s plot of MNC with Cu(II) ions.

Figure 7

Reversibility of MNC-Cu(II) and EDTA mutually, λ_ex = 312 nm and λ_em = 426 nm, respectively.

Figure 8

Stern-Volmer plot for calculating K_b of MNC-Cu(II).

Figure 9

Benesi-Hildebrand plots for K_b of MNC- Cu(II).

Figure 10

Model structures for (a) MNC and (b) MNC-Cu(II).

Figure 11

Frontier orbitals and their energies for L and Cu-L.