A iridium(III) complex-based ‘turn-on’ fluorescent probe with two recognition site for rapid detection of thiophenol and its application in water samples and human serum
Graphical abstract
Introduction
Thiophenol and its derivatives, as common intermediate in organic synthesis, are widely used in the preparation of pesticides, pharmaceuticals, and polymers [[1], [2], [3]]. Nevertheless, according to the United States Environmental Protection Agency (USEPA waste code: P014) [4], thiophenol has been defined as a major pollutant due to its high toxicity. For instance, the median lethal concentration of thiophenol in fish range from 0.01 to 0.4 mM [5], and the range of median lethal dose in mouse is as low as 2.15–46.2 mg kg−1 [6]. Moreover, thiophenol in water and soil also can be probably absorbed by human being through various biological and physicochemical pathways, further lead to a series of health problems including the central nervous system injury, muscular weakness, headache, nausea, vomiting, and even death [7,8]. Therefore, it is very necessary to develop an efficient and sensitive method to detect thiophenol in organisms or environment.
In the past few decades, a variety of detection methods have been reported, such as optical analysis, high performance liquid chromatography, gas chromatography, nonlinear spectroscopy and nanophase material sensor [[9], [10], [11], [12], [13]]. Fluorescence sensors, based on changes of fluorescent signal before and after detection, have become the most popular detection approach due to its simple operation, high sensitivity, cheap cost, low detection limit [14]. At present, although many fluorescent probes for thiophenol have been reported, most of them are based on organic small molecule [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]], and possess some shortcomings like singlet emission and emitting via a short-lived fluorescence modality. In contrast, probes based on transition-metal complexes in the second- and third-row of the d-block typically display triplet emission and exhibit long-lived phosphorescence, as well as high luminescence quantum efficiency and large Stokes shift [32].
Zhang and co-workers designed and synthesized a series of Ru(II) complex-based phosphorescent probes to detect thiophenols for the first time [33]. These probes showed large Stokes shift and long luminescence lifetime, due to the triplet metal-to-ligand charge-transfer transitions (3MLCTs) excited state. But long response time (30 min) was their obvious disadvantage. Transition metal complex-based probes for thiophenol have frequently emerged since then [[34], [35], [36], [37], [38], [39]].
We noted that ruthenium complexes usually only possess 3MLCT excited state, but iridium complexes exhibited multiple excited states, such as 3MLCT, 3LLCT, and 3IL excited states, which may lead to their excellent photophysical and photochemical properties with long lifetimes. So the octahedral structure of iridium(Ⅲ) complexes had attracted more interest as fluorescence probe [[40], [41], [42], [43]]. Thus we designed and synthesized a novel iridium(Ⅲ) complex-based ‘turn-on’ fluorescent probe Ir-DNBS with two 2,4-dinitrobenzenesulfonate(DNBS) moieties as recognization sites to detect thiophenol and its derivatives. Herein, two electron withdrawing group DNBS in the two C^N ancillary ligands (phenylpyridine) could completely quench the fluorescence of iridium(Ⅲ) complex-based probe Ir-DNBS via internal charge transfer (ICT) mechanism [44,45]. Upon addition of thiophenol to Ir-DNBS in PBS/DCM/DMSO solution (pH = 7.4), the two recognition units (DNBS) could be cut off by nucleophilic elimination reaction (SNAr) to form strong fluorescent complex Ir-1 with obvious emission band centered at 620 nm. The results demonstrated that probe Ir-DNBS could rapidly (response time: 9 min), sensitively (LOD: 62.5 nM) and selectively detect thiophenol without interference of aliphatic thiols. Additionally, it also could quantitatively recognize thiophenol in actual water samples and efficiently detect thiophenol in human serum, implying that probe Ir-DNBS could be potentially applied in environment and clinic fields.
Section snippets
Chemicals and apparatus
Unless otherwise specified, all chemicals used were purchased from commercial suppliers without purification. The human serum was provided by health volunteers. The solvents were dried in standard methods before use. All reactions were carried out under the dry argon atmosphere protection using the Schleck operation. The solution of Ir-DNBS (1 mM) was prepared in chromatographic grade CH2Cl2. Standard solution of various analytic samples, such as C6H5SH, p-Cl-C6H5SH, p-CH3-C6H5SH, p-CH3O–C6H5
Preparation and characterization
The synthetic route of probe Ir-DNBS was displayed in Scheme 1. The N^N ancillary ligand 4-(4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenyl)morpholine was obtained by simple Debus-Radziszewski imidazole synthesis using 1,10-phenanthroline-5,6-dione to react with 4-morpholinobenzaldehyde in the presence of ammonium acetate [[56], [57], [58]]. Chloro-bridged dimer Ir(Ⅲ) complex 1 was synthesized through reaction of 2-(4-methoxyphenyl)pyridine with anhydrous IrCl3 without further purification.
Conclusion
A novel ‘turn-on’ fluorescent probe Ir-DNBS based on iridium (Ⅲ) complex with two recognition site (DNBS) for detection of thiophenol had been designed, synthesized and characterized. The two fluorescence quencher 2,4-dinitrobenzenesulfonate (DNBS) in probe Ir-DNBS could completely quench the fluorescence. After upon addition with thiophenol, the two DNBS groups were both removed to form fluorescent complex Ir-1 and significant fluorescence enhancement (about 35-fold) was observed. The probe
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful for the financial support from the National Natural Science Foundation of China (21701065, 41867053), the Young Talents Project of Jiangxi Science and Technology Normal University (2019QNBJRC005).
References (68)
- et al.
J. Chromatogr. B Biomed. Sci. Appl.
(1994) - et al.
Toxicol. Lett.
(1984) - et al.
J. Chromatogr. A
(2002) - et al.
Dyes Pigments
(2015) - et al.
Dyes Pigments
(2016) - et al.
Dyes Pigments
(2019) - et al.
Dyes Pigments
(2020) - et al.
J. Org. Chem.
(2020) - et al.
Organometallics
(2015) - et al.
Dyes Pigments
(2020)