# Design and Synthesis of a Fluorescent Probe Based on Copper Complex for Selective Detection of Hydrogen Sulfide

Authors: Guanglan Mao; Chenxi Liu; Nan Yang; Linlin Yang; Guangjie He
Journal: Journal of Sensors
 (2021)

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## Abstract

A novel fluorescence probe NA-LCX was rationally designed and synthesized for the sequential recognition of Cu2+ and H2S by the combination of hydroxyl-naphthalene and diformylphenol groups. The response properties of NA-LCX for Cu2+ ions and H2S with “on-off-on” manner were investigated by fluorescence emission spectra. A highly selective and sensitive response of complex NA-LCX-Cu2+ for H2S over other competing amino acids was observed with a limit of detection at 2.79 μM. The stoichiometry of NA-LCX toward Cu2+ ions was determined to be 1 : 1 by the UV-Vis absorption spectrum, and the coordination configuration was calculated by density functional theory (DFT) calculations. Moreover, probe NA-LCX was applied successfully for the recognition of Cu2+ ions and H2S in living cells.


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## Body

## 1. Introduction

Hydrogen sulfide (H2S), the simplest biomercapto compound, is not only a rotten egg smelling gas pollutant but also the third gasotransmitter and cellular signaling molecule after CO and NO [1, 2]. The endogenous H2S could regulate vascular smooth muscle tension and cardiac contractile function, anti-inflammatory and antioxidative stress, neurotransmitter transmission, and insulin signaling inhibition, which plays an important role in the physiological and pathological processes of the cardiovascular, nervous, immune, and digestive systems [3–7]. The concentrations of H2S in the normal metabolism often maintain dynamic equilibrium, while abnormal changes of the H2S level could induce serious health problems, such as heart diseases [8, 9], chronic obstructive pulmonary diseases [10, 11], cirrhosis [12, 13], and Alzheimer [14, 15]. Hence, it is crucial to exploit a highly sensitive and selective method for the detection of hydrogen sulfide in living systems.Many conventional methods for H2S detection have been developed, including colorimetric method [16, 17], electrochemical analysis [18, 19], liquid chromatography mass spectrometry [20, 21], and fluorescence analysis [22–24]. Among them, fluorescence analysis is more desirable due to its simple operation, high sensitivity, wide dynamic range, high fluorescence quantum yield, good biocompatibility, noninvasiveness, and ability of in situ real-time detection in living systems [25]. In recent years, many fluorescent probes for H2S detection have been reported on account of different types of strategies such as reduction reactions [26, 27], nucleophilic addition reactions [28, 29], dinitrophenyl ether/sulfonyl ester cleavage [30, 31], and metal sulfide precipitation reaction [32–40]. However, there are some limitations to those reaction methods as well as the products obtained via those reactions. For example, those reactions are insensitive, complex, and time-consuming; moreover, fluorescent probes prepared via those reactions are sometimes not biocompatible and sometimes unstable in the presence of biological thiols (glutathione, cysteine, etc.) [31]. The strategy by using a metal displacement approach is in high demand for its fast response and high sensitivity and selectivity. Sulfide is known to react with copper ion to form very stable CuS with a very low solubility product constant Ksp=6.3×10−36 (for cyanide, Ksp=3.2×10−20). Thus, the utilization of the higher affinity of Cu2+ towards sulfide for designing a specific Cu2+ sensor to sequentially identify H2S has received considerable attention because it can effectively eliminate the interference of other analytes in the system.Naphthalene derivatives with an electron donor-π-acceptor (D-π-A) structure have been widely used due to good optical properties, such as high fluorescence quantum yield, good biocompatibility, and light stability. Herein, we synthesized a new fluorescent probe NA-LCX based on hydroxyl-naphthalene and diformylphenol which have excellent coordination ability to metal ions. The probe showed an obvious “on-off” fluorescence quenching response toward Cu2+, and the NA-LCX-Cu2+ complex showed an “off-on” fluorescence enhancement response toward H2S in a DMSO/HEPES (3 : 2 v/v, pH=7.4). The photophysical capabilities of probe NA-LCX for Cu2+ and NA-LCX-Cu2+ for H2S were studied in details from fluorescence spectroscopy, absorption spectroscopy, and fluorescence images in vivo.


## 2. Experimental Section

### 2.1. Reagents and Materials

2,6-Diformyl-4-methylphenol was purchased from Shanghai TCI Chemical Industry Development Co. Ltd. 3-Hydroxy-2-naphthoyl hydrazide was purchased from Sinopharm Chemical Reagent Co. Ltd. All the other chemicals and reagents were commercially available and were analytical grade. All solvents were purified by standard procedures. Aqueous solutions (2.0×10−2M) of perchlorates of various metal ions (Al3+, K+, Na+, Mg2+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Ag+, Pb2+, and Cu2+) and various amino acids (Asn, Glu, Cys, Phe, Pro, Gln, Arg, Trp, Asp, Tyr, Ile, Thr, His, Gly, Met, Leu, Ala, Val, Ser, Lys, Cys, GSH, Hcy, and NAC) were prepared before use.


### 2.2. Apparatus and Instruments

The following are the apparatus and instruments used in the study: UV-Vis spectrophotometer (UV-2600, Shimadzu Corporation), fluorescence spectrophotometer (FS5, Edinburgh, UK), nuclear magnetic resonance spectrometer (NMR) (Ascend™ 400, Bruker Co., USA), precision pH meter (PHS-3E, Zhengzhou Tailai Instruments Co., Ltd.), and inverted fluorescence microscope (Leica DMI8, Leica Microsystems, Germany).


### 2.3. Synthesis of the Probe NA-LCX

3-Hydroxy-2-naphthohydrazide (0.40 g, 2.0 mmol) and 2,6-diformyl-4-methylphenol (0.164 g, 1.0 mmol) were dissolved in 30 mL of ethanol, respectively. Then, the solution was mixed dropwise and refluxed for 6 hours. The obtained mixture was filtered, washed, and vacuum dried to afford ligand NA-LCX (Scheme1). 1H NMR (400 MHz, d6-DMSO), δ 8.76 (s, 2H), 8.48 (s, 2H), 7.94 (d,J=8.2Hz, 2H), 7.78 (d,J=8.2Hz, 2H), 7.61 (s, 2H), 7.53 (t,J=7.6Hz, 2H), 7.37 (dd,J=14.7, 7.1 Hz, 4H), 2.36 (s, 3H). 13C NMR (101 MHz, d6-DMSO), δ 164.23, 155.37, 154.54, 147.20, 136.36, 130.95, 130.72, 129.15, 128.76, 127.24, 126.35, 124.31, 120.70, 120.33, and 111.02, 20.41. MS: calculated [(M+Na)+] 555.1644; found 555.1638.Scheme 1
Synthesis route of ligand NA-LCX.


### 2.4. Synthesis of Complex NA-LCX-Cu2+

In a 50 mL round bottom flask, ligand NA-LCX (0.053 g, 0.1 mmol) and Cu(ClO4)2·6H2O (0.037 g, 0.1 mmol) were mixed in 15 mL methanol. After being stirred for 30 minutes, the obtained precipitate was filtered, washed, and dried to afford the NA-LCX-Cu2+ complex.


### 2.5. General Method for Cell Imaging

Human liver cancer HepG-2 cells were cultured in a 12-well plate, and when the cell saturation exceeded 80%, ligand NA-LCX and probe NA-LCX-Cu2+ solution were added. The mixture was then incubated for 3 hours in a CO2 incubator and washed three times with precooled PBS, followed by the addition of 1 mL PBS. The resulting mixture was observed under a Leica DMI8 inverted fluorescence microscope.


## 2.1. Reagents and Materials

2,6-Diformyl-4-methylphenol was purchased from Shanghai TCI Chemical Industry Development Co. Ltd. 3-Hydroxy-2-naphthoyl hydrazide was purchased from Sinopharm Chemical Reagent Co. Ltd. All the other chemicals and reagents were commercially available and were analytical grade. All solvents were purified by standard procedures. Aqueous solutions (2.0×10−2M) of perchlorates of various metal ions (Al3+, K+, Na+, Mg2+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Ag+, Pb2+, and Cu2+) and various amino acids (Asn, Glu, Cys, Phe, Pro, Gln, Arg, Trp, Asp, Tyr, Ile, Thr, His, Gly, Met, Leu, Ala, Val, Ser, Lys, Cys, GSH, Hcy, and NAC) were prepared before use.


## 2.2. Apparatus and Instruments

The following are the apparatus and instruments used in the study: UV-Vis spectrophotometer (UV-2600, Shimadzu Corporation), fluorescence spectrophotometer (FS5, Edinburgh, UK), nuclear magnetic resonance spectrometer (NMR) (Ascend™ 400, Bruker Co., USA), precision pH meter (PHS-3E, Zhengzhou Tailai Instruments Co., Ltd.), and inverted fluorescence microscope (Leica DMI8, Leica Microsystems, Germany).


## 2.3. Synthesis of the Probe NA-LCX

3-Hydroxy-2-naphthohydrazide (0.40 g, 2.0 mmol) and 2,6-diformyl-4-methylphenol (0.164 g, 1.0 mmol) were dissolved in 30 mL of ethanol, respectively. Then, the solution was mixed dropwise and refluxed for 6 hours. The obtained mixture was filtered, washed, and vacuum dried to afford ligand NA-LCX (Scheme1). 1H NMR (400 MHz, d6-DMSO), δ 8.76 (s, 2H), 8.48 (s, 2H), 7.94 (d,J=8.2Hz, 2H), 7.78 (d,J=8.2Hz, 2H), 7.61 (s, 2H), 7.53 (t,J=7.6Hz, 2H), 7.37 (dd,J=14.7, 7.1 Hz, 4H), 2.36 (s, 3H). 13C NMR (101 MHz, d6-DMSO), δ 164.23, 155.37, 154.54, 147.20, 136.36, 130.95, 130.72, 129.15, 128.76, 127.24, 126.35, 124.31, 120.70, 120.33, and 111.02, 20.41. MS: calculated [(M+Na)+] 555.1644; found 555.1638.Scheme 1
Synthesis route of ligand NA-LCX.


## 2.4. Synthesis of Complex NA-LCX-Cu2+

In a 50 mL round bottom flask, ligand NA-LCX (0.053 g, 0.1 mmol) and Cu(ClO4)2·6H2O (0.037 g, 0.1 mmol) were mixed in 15 mL methanol. After being stirred for 30 minutes, the obtained precipitate was filtered, washed, and dried to afford the NA-LCX-Cu2+ complex.


## 2.5. General Method for Cell Imaging

Human liver cancer HepG-2 cells were cultured in a 12-well plate, and when the cell saturation exceeded 80%, ligand NA-LCX and probe NA-LCX-Cu2+ solution were added. The mixture was then incubated for 3 hours in a CO2 incubator and washed three times with precooled PBS, followed by the addition of 1 mL PBS. The resulting mixture was observed under a Leica DMI8 inverted fluorescence microscope.


## 3. Results and Discussion

### 3.1. UV-Vis Spectrum Recognition of Probe NA-LCX toward Cu2+

UV-Vis absorption spectra of ligand NA-LCX (20μM) in the presence of Cu2+ ions in DMSO : HEPES (3 : 2, v/v, pH=7.4) at various concentrations were performed in Figure 1. Upon the addition of Cu2+ ions, the UV-Vis absorption intensities of ligand NA-LCX at 312 nm and 366 nm decreased gradually, while the intensities at 346 nm and 438 nm increased with a change in color from yellowish to orange. A plateau was reached for the 438 nm wavelength upon the addition of about 1.5 equivalents of Cu2+ ions. To further determine the molar ratio of probe NA-LCX to Cu2+ ions, Job’s plot fitting was performed and the molar ratio of ligand NA-LCX to Cu2+ was determined as 1 : 1 based on the continuous changes in absorbance at 438 nm (Figure S4).Figure 1
UV-Vis spectrum of ligand NA-LCX (20μM) with the addition of Cu2+ ions in DMSO : HEPES (3 : 2, v/v). Inset figure: the absorption intensity of ligand NA-LCX at 438 nm with the addition of Cu2+ ions.


### 3.2. Fluorescence Spectroscopy Recognition of Probe NA-LCX toward Cu2+

The sensitivity of probe NA-LCX for Cu2+ was investigated by fluorescence titration. Probe NA-LCX showed a strong fluorescence emission peak at 575 nm upon excitation at 312 nm. As shown in Figure 2, the fluorescence intensity of the ligand NA-LCX gradually decreased upon the addition of Cu2+ ions and became constant until about 2 equiv. of Cu2+ ions were added. The quenching rate was extremely high, indicating that probe NA-LCX was highly sensitive to Cu2+, which could be due to the photoinduced electron transfer of Cu2+ ions and/or the d-d electron paramagnetic quenching effect [41–43]. Moreover, the fluorescence emission intensity showed a good linear relationship (R2=0.99) with the concentration of Cu2+ ions in the range of 1~20 μM. The quenching constant value of probe NA-LCX with Cu2+ ions was determined from the titration plots. The corrected Stern-Volmer fitting indicated the value of 2.6×104mol−1·L (Figure S5). The fluorescence response of probe NA-LCX to other metal ions in DMSO : HEPES (3 : 2, v/v) was shown in Figure S6. It could be found that many other metal ions, such as Co2+, Fe3+. It could be found that many other metal ions, such as Co2+, Fe3+, Fe2+, Ni2+, Zn2+, Cd2+, and Mn2+, also exhibited a similar fluorescence quenching response.Figure 2
Fluorescence spectra of Cu2+ ions with probe NA-LCX (20 μM) in DMSO : HEPES (3 : 2, v/v) at an excitation of 312 nm. Inset figure: F/F0 for probe NA-LCX at 575 nm upon the addition of different equivalents of Cu2+ ions.


### 3.3. Fluorescence Spectra Response of Complex NA-LCX-Cu2+ toward H2S

The complex formed by probe NA-LCX and Cu2+ was used as a new sensor NA-LCX-Cu2+ for sequential recognition of H2S. Upon the addition of Na2S, as shown in Figure 3, the fluorescence intensity was gradually increased and remained unchanged up to 10 equiv. The probe NA-LC-Cu2+ released Cu2+ ions due to the strong reaction between sulfide and copper ions, which restored the original fluorescence of the probe. Furthermore, the detection limit was 2.79 μM according to the formula LOD=3σ/S (Figure S7). The responses of ligand NA-LCX to other metal ions such as Co2+, Cd2+, Zn2+, Ni2+, Fe2+, Mn2+, and Fe3+ and the subsequent addition of 10 equiv. of Na2S are shown in Figure S8. It could be found that the complex NA-LCX-Cu2+ possess the highest fluorescence intensity ratio of recovery/quenching, and hence, the probe NA-LCX-Cu2+ was chosen to pursue H2S detection.Figure 3
Fluorescence spectra change of probe NA-LCX-Cu2+ (20 μM) with the titration of Na2S in DMSO : HEPES (3 : 2, v/v) solution at an excitation of 312 nm. Inset figure: F/F0 intensity for NA-LCX-Cu2+ at 575 nm vs. different equivalents of Na2S.To further explore whether probe NA-LCX-Cu2+ could be used as a highly selective H2S sensor, the fluorescence response of probe NA-LCX-Cu2+ (20 μM) to different amino acids was tested. As described and shown in Figure 4, only the addition of H2S instantly caused an obvious fluorescence enhancement. The fluorescence intensity of probe NA-LCX-Cu2+ remains unchanged in the presence of 10 equiv. of different mercapto-amino acids such as glutathione, cysteine, N-acetyl-L-cysteine, homocysteine, and non-mercapto-amino acids. And other reactive sulfur species (S2O32-, SO42-, and HSO3-) also did not cause obvious fluorescence changes. The competitive experiments further showed significant fluorescent enhancement without being interfered by other amino acids and reactive sulfur species, which further indicated the good selectivity of the probe NA-LCX-Cu2+ for H2S detection.Figure 4
Blue: fluorescence response of probe NA-LCX-Cu2+ (20 μM) toward 10 equiv. of various amino acids (NA-LCX+Cu2+, Na2S, NAC, GSH, Hcy, Cys, Asn, Glu, Phe, Pro, Gln, Arg, Trp, Asp, Tyr, Ile, Thr, His, Gly, Met, Leu, Ala, Val, Ser, and Lys) and other reactive sulfur species (Na2S2O3, Na2SO4, and NaHSO3). Orange: fluorescence response of probe NA-LCX-Cu2+ after addition of 10 equiv. of Na2S in each of the above solutions.


### 3.4. DFT Calculation

To gain further insight into the nature of coordination configuration and optical response of sensor NA-LCX toward Cu2+, the different coordination structures of NA-LCX-Cu2+ were examined by density functional theory calculation. All calculations were performed by Gaussian 09 program. The geometries were optimized at the B3LYP/6-31G(d)/SDD level, and the interaction energies were calculated based on the single point energies obtained at the B3LYP/6-31+G(d)/SDD level. As shown in Figure S9, it was obvious that the interaction energy of structure C was higher than structures A and B, which verified the experimental results and presumed the complexation mode of probe NA-LCX with Cu2+.


### 3.5. Effect of pH on the Performance of Probe NA-LCX and Complex NA-LCX-Cu2+

To investigate the effect of pH value, fluorescence intensity of probe NA-LCX, complex NA-LCX-Cu2+, and complex NA-LCX-Cu2+ in the presence of S2- was investigated in a wide range of pH values. No significant changes in fluorescence intensity were found at lower pH (pH≤6) (Figure S10). However, It could be observed that significant fluorescence changes in ligand NA-LCX and NA-LCX-Cu2+-Na2S at pH>6, indicating potential of probe NA-LCX-Cu2+ to detect H2S in physiological environments.


### 3.6. Cell Imaging Experiments

Inspired by the excellent selectivity at physiological pH levels, the cell imaging application of sensor NA-LCX for detection of Cu2+ and H2S was further investigated. Prior to the cell imaging experiment, the MTT cell toxicity assay for probe NA-LCX-Cu2+ was performed in human liver cancer cells (HepG-2) shown in Figure S11, and no significant cytotoxicity was found in the range of 0~10 μM, even after incubating for 24 h. As shown in Figure 5, significant intracellular green fluorescence in the HepG-2 cells was observed in the presence of probe NA-LCX when excited with blue light (Figure 5(a)), indicating that the sensor NA-LCX was well permeable. However, the complex NA-LCX-Cu2+ was added to the wells, and the green fluorescence in HepG-2 cells was quenched to a large degree, as expected (Figure 5(b)). Upon subsequent addition of 2 and 5 equiv. of Na2S solution, obvious fluorescence recovery was observed (Figures 5(c) and 5(d)). The fluorescence imaging results suggested the potential of probe NA-LCX-Cu2+ for in vivo detection of H2S.Figure 5
The fluorescence images of HepG-2 cells with the incubation of (a) ligand NA-LCX (3μM), (b) complex NA-LCX-Cu2+ (3 μM), (c) complex NA-LCX-Cu2+ with the addition of 2 equiv. Na2S, and (d) complex NA-LCX-Cu2+ with the addition of 5 equiv. Na2S and their corresponding bright field images.
(a)(b)(c)(d)


## 3.1. UV-Vis Spectrum Recognition of Probe NA-LCX toward Cu2+

UV-Vis absorption spectra of ligand NA-LCX (20μM) in the presence of Cu2+ ions in DMSO : HEPES (3 : 2, v/v, pH=7.4) at various concentrations were performed in Figure 1. Upon the addition of Cu2+ ions, the UV-Vis absorption intensities of ligand NA-LCX at 312 nm and 366 nm decreased gradually, while the intensities at 346 nm and 438 nm increased with a change in color from yellowish to orange. A plateau was reached for the 438 nm wavelength upon the addition of about 1.5 equivalents of Cu2+ ions. To further determine the molar ratio of probe NA-LCX to Cu2+ ions, Job’s plot fitting was performed and the molar ratio of ligand NA-LCX to Cu2+ was determined as 1 : 1 based on the continuous changes in absorbance at 438 nm (Figure S4).Figure 1
UV-Vis spectrum of ligand NA-LCX (20μM) with the addition of Cu2+ ions in DMSO : HEPES (3 : 2, v/v). Inset figure: the absorption intensity of ligand NA-LCX at 438 nm with the addition of Cu2+ ions.


## 3.2. Fluorescence Spectroscopy Recognition of Probe NA-LCX toward Cu2+

The sensitivity of probe NA-LCX for Cu2+ was investigated by fluorescence titration. Probe NA-LCX showed a strong fluorescence emission peak at 575 nm upon excitation at 312 nm. As shown in Figure 2, the fluorescence intensity of the ligand NA-LCX gradually decreased upon the addition of Cu2+ ions and became constant until about 2 equiv. of Cu2+ ions were added. The quenching rate was extremely high, indicating that probe NA-LCX was highly sensitive to Cu2+, which could be due to the photoinduced electron transfer of Cu2+ ions and/or the d-d electron paramagnetic quenching effect [41–43]. Moreover, the fluorescence emission intensity showed a good linear relationship (R2=0.99) with the concentration of Cu2+ ions in the range of 1~20 μM. The quenching constant value of probe NA-LCX with Cu2+ ions was determined from the titration plots. The corrected Stern-Volmer fitting indicated the value of 2.6×104mol−1·L (Figure S5). The fluorescence response of probe NA-LCX to other metal ions in DMSO : HEPES (3 : 2, v/v) was shown in Figure S6. It could be found that many other metal ions, such as Co2+, Fe3+. It could be found that many other metal ions, such as Co2+, Fe3+, Fe2+, Ni2+, Zn2+, Cd2+, and Mn2+, also exhibited a similar fluorescence quenching response.Figure 2
Fluorescence spectra of Cu2+ ions with probe NA-LCX (20 μM) in DMSO : HEPES (3 : 2, v/v) at an excitation of 312 nm. Inset figure: F/F0 for probe NA-LCX at 575 nm upon the addition of different equivalents of Cu2+ ions.


## 3.3. Fluorescence Spectra Response of Complex NA-LCX-Cu2+ toward H2S

The complex formed by probe NA-LCX and Cu2+ was used as a new sensor NA-LCX-Cu2+ for sequential recognition of H2S. Upon the addition of Na2S, as shown in Figure 3, the fluorescence intensity was gradually increased and remained unchanged up to 10 equiv. The probe NA-LC-Cu2+ released Cu2+ ions due to the strong reaction between sulfide and copper ions, which restored the original fluorescence of the probe. Furthermore, the detection limit was 2.79 μM according to the formula LOD=3σ/S (Figure S7). The responses of ligand NA-LCX to other metal ions such as Co2+, Cd2+, Zn2+, Ni2+, Fe2+, Mn2+, and Fe3+ and the subsequent addition of 10 equiv. of Na2S are shown in Figure S8. It could be found that the complex NA-LCX-Cu2+ possess the highest fluorescence intensity ratio of recovery/quenching, and hence, the probe NA-LCX-Cu2+ was chosen to pursue H2S detection.Figure 3
Fluorescence spectra change of probe NA-LCX-Cu2+ (20 μM) with the titration of Na2S in DMSO : HEPES (3 : 2, v/v) solution at an excitation of 312 nm. Inset figure: F/F0 intensity for NA-LCX-Cu2+ at 575 nm vs. different equivalents of Na2S.To further explore whether probe NA-LCX-Cu2+ could be used as a highly selective H2S sensor, the fluorescence response of probe NA-LCX-Cu2+ (20 μM) to different amino acids was tested. As described and shown in Figure 4, only the addition of H2S instantly caused an obvious fluorescence enhancement. The fluorescence intensity of probe NA-LCX-Cu2+ remains unchanged in the presence of 10 equiv. of different mercapto-amino acids such as glutathione, cysteine, N-acetyl-L-cysteine, homocysteine, and non-mercapto-amino acids. And other reactive sulfur species (S2O32-, SO42-, and HSO3-) also did not cause obvious fluorescence changes. The competitive experiments further showed significant fluorescent enhancement without being interfered by other amino acids and reactive sulfur species, which further indicated the good selectivity of the probe NA-LCX-Cu2+ for H2S detection.Figure 4
Blue: fluorescence response of probe NA-LCX-Cu2+ (20 μM) toward 10 equiv. of various amino acids (NA-LCX+Cu2+, Na2S, NAC, GSH, Hcy, Cys, Asn, Glu, Phe, Pro, Gln, Arg, Trp, Asp, Tyr, Ile, Thr, His, Gly, Met, Leu, Ala, Val, Ser, and Lys) and other reactive sulfur species (Na2S2O3, Na2SO4, and NaHSO3). Orange: fluorescence response of probe NA-LCX-Cu2+ after addition of 10 equiv. of Na2S in each of the above solutions.


## 3.4. DFT Calculation

To gain further insight into the nature of coordination configuration and optical response of sensor NA-LCX toward Cu2+, the different coordination structures of NA-LCX-Cu2+ were examined by density functional theory calculation. All calculations were performed by Gaussian 09 program. The geometries were optimized at the B3LYP/6-31G(d)/SDD level, and the interaction energies were calculated based on the single point energies obtained at the B3LYP/6-31+G(d)/SDD level. As shown in Figure S9, it was obvious that the interaction energy of structure C was higher than structures A and B, which verified the experimental results and presumed the complexation mode of probe NA-LCX with Cu2+.


## 3.5. Effect of pH on the Performance of Probe NA-LCX and Complex NA-LCX-Cu2+

To investigate the effect of pH value, fluorescence intensity of probe NA-LCX, complex NA-LCX-Cu2+, and complex NA-LCX-Cu2+ in the presence of S2- was investigated in a wide range of pH values. No significant changes in fluorescence intensity were found at lower pH (pH≤6) (Figure S10). However, It could be observed that significant fluorescence changes in ligand NA-LCX and NA-LCX-Cu2+-Na2S at pH>6, indicating potential of probe NA-LCX-Cu2+ to detect H2S in physiological environments.


## 3.6. Cell Imaging Experiments

Inspired by the excellent selectivity at physiological pH levels, the cell imaging application of sensor NA-LCX for detection of Cu2+ and H2S was further investigated. Prior to the cell imaging experiment, the MTT cell toxicity assay for probe NA-LCX-Cu2+ was performed in human liver cancer cells (HepG-2) shown in Figure S11, and no significant cytotoxicity was found in the range of 0~10 μM, even after incubating for 24 h. As shown in Figure 5, significant intracellular green fluorescence in the HepG-2 cells was observed in the presence of probe NA-LCX when excited with blue light (Figure 5(a)), indicating that the sensor NA-LCX was well permeable. However, the complex NA-LCX-Cu2+ was added to the wells, and the green fluorescence in HepG-2 cells was quenched to a large degree, as expected (Figure 5(b)). Upon subsequent addition of 2 and 5 equiv. of Na2S solution, obvious fluorescence recovery was observed (Figures 5(c) and 5(d)). The fluorescence imaging results suggested the potential of probe NA-LCX-Cu2+ for in vivo detection of H2S.Figure 5
The fluorescence images of HepG-2 cells with the incubation of (a) ligand NA-LCX (3μM), (b) complex NA-LCX-Cu2+ (3 μM), (c) complex NA-LCX-Cu2+ with the addition of 2 equiv. Na2S, and (d) complex NA-LCX-Cu2+ with the addition of 5 equiv. Na2S and their corresponding bright field images.
(a)(b)(c)(d)


## 4. Conclusion

In this study, a novel two-armed naphthalene derivative probe NA-LCX was synthesized and its spectral performance for sequential recognition of Cu2+ and H2S was studied. The probe NA-LCX showed an obvious “on-off-on” fluorescence response toward Cu2+ and H2S. The probe NA-LCX showed a 1 : 1 binding stoichiometry to Cu2+ with a complexation constant of 2.6×104M−1. Fluorescence study indicated sensitivity and selectivity of the probe NA-LCX-Cu2+ for H2S detection without interference from other amino acids. The detection limit for H2S was calculated to be 2.79 μM. The cell imaging results further showed the potential of the probe for Cu2+ and H2S detection in living cells.



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*Source: 8822558-2021-02-16.xml*