An ''off-on-off'' sensor for sequential detection of Cu2+ and hydrogen sulfide based on a naphthalimide-rhodamine B derivative and its application in dual-channel cell imaging

Abstract

A novel colorimetric and fluorometric sensor with unique dual-channel emission to sequentially detect Cu²⁺ and hydrogen sulfide (H₂S) was synthesized from naphthalimide-rhodamine B through the PET and FRET mechanism. The sensor showed a selective "off-on" fluorescence response with a 120-fold increase toward Cu²⁺, and its limits of detection were 0.26 μM and 0.17 μM for UV-vis and fluorescence measurements, respectively. In addition, 1-Cu²⁺ was an efficient "on-off" sensor to detect H₂S with detection limits of 0.40 μM (UV-vis measurement) and 0.23 μM (fluorescence measurement), respectively. Furthermore, the sensor can also be used for biological imaging of intracellular staining in living cells. Therefore, the sensor should be highly promising for the detection of low level Cu²⁺ and H₂S with great potential in many practical applications.

Scheme 1. Structure and synthesis of 1.

Fig. 1. Spectroscopic changes of 1 (20 μM) with the addition of various metal ions in CH₃CN–H₂O (9/1, v/v) solution. (A) Absorption spectral changes. (B) Fluorescence emission intensity changes. (C) The fluorescence intensity changes at 610 nm (black bars: 1 with other ions, red bars: 1 with Cu²⁺ and other metal ions). (D) The color image with the addition of various metal ions. (E) The fluorescence image with the addition of various metal ions.

Fig. 2. (A) Time-dependent fluorescence intensity changes of sensor 1 to Cu²⁺ in CH₃CN–H₂O (9/1, v/v) solution. (B) Relative fluorescence intensity at 610 nm as a function of time in CH₃CN–H₂O (9/1, v/v) solution.

Fig. 3. Absorbance spectra (A) and fluorescence spectra (B) of 1 (20 μM) in the presence of different amounts of Cu²⁺ in CH₃CN–H₂O (9/1, v/v) solution. Inset of (A): the ratio of absorbance at 564 nm and absorbance at 425 nm (red dots line) and the ratio of absorbance at 357 nm and absorbance at 425 nm (black dots line) as a function of Cu²⁺ concentration. Inset of (B): the fluorescence intensity at 528 nm and 610 nm as a function of Cu²⁺ concentration.

Fig. 4. (A) Spectroscopic changes of 1–Cu²⁺ complex (20 μM) with the addition of various anions in CH₃CN–H₂O (7/3, v/v) solution. (A) Absorption spectral changes. (B) Fluorescence emission intensity changes. (C) The fluorescence intensity changes at 610 nm (black bars: 1–Cu²⁺ complex with other anions, red bars: 1–Cu²⁺ complex with H₂S and other anions). (D) The color image with the addition of various anions. (E) The fluorescence image with the addition of various anions.

Fig. 5. (A) Time-dependent fluorescence intensity changes of sensor 1–Cu²⁺ to H₂S in CH₃CN–H₂O (7/3, v/v) solution. (B) Relative fluorescence intensity at 610 nm as a function of time in CH₃CN–H₂O (7/3, v/v) solution.

Fig. 6. (A) Absorbance and (B) fluorescence spectra of 1–Cu²⁺ complex in the presence of different amounts of H₂S in CH₃CN–H₂O (7/3, v/v) solution. Inset of (A): the ratio of absorbance at 564 nm and absorbance at 425 nm (red dots line) and the ratio of absorbance at 357 nm and absorbance at 425 nm (black dots line) as a function of Cu²⁺ concentration. Inset of (B): the fluorescence intensity at 528 nm and 610 nm as a function of Cu²⁺ concentration.

Fig. 7. (A) Job's plot for the complexation of 1 with Cu²⁺, indicating the formation of a 1 : 1 complex. The total [1] + [Cu²⁺] = 40 μM. (B) ESI-MS spectrum of 1–Cu²⁺. (C) ¹³C NMR spectra of 1 upon addition of Cu²⁺ in CD₂Cl₂.

Fig. 8. Partial FT-IR spectra of 1 (A), 1–Cu²⁺ (B) and 1–Cu²⁺ + H₂S (C).

Fig. 9. The proposed mechanism of 1 to sense Cu²⁺ and 1–Cu²⁺ to sense H₂S.

Fig. 10. Confocal fluorescence images of HeLa cells: (A) bright field image (B) dark field image of channel 1 (emission collected at 460–560 nm, pseudo-color: green); (C) dark field image of channel 2 (emission collected at 560–670 nm, pseudo-color: red); (D) merged images of A, B and C. Fluorescence emissions were collected at an excitation wavelength of 405 nm with a [40×] objective.