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

Highlights

• A “turn on” probe based on Ir(III) complex with two recognition units (DNBS) for detection of thiphenol was synthesized.

• It could be used to detect thiophenol in real water samples and human serum.

• It could apply in wide pH range (7–11) of environments and physiological conditions.

Abstract

A new iridium(Ⅲ) complex-based near-infrared ‘turn-on’ fluorescent probe Ir-DNBS was synthesized for fast (9 min), highly selective and sensitive (LOD: 62.5 nM) thiophenol detection. Photophysical and spectral characterization (NMR, HRMS) results demonstrated that the recognization mechanism of probe for thiophenol was based on thiolate-mediated nucleophilic elimination reaction (S_NAr). Upon addition with thiophenol to PBS/DCM/DMSO solution (pH = 7.4) of probe Ir-DNBS, obvious orange red fluorescence with maximum emission wavelength at 620 nm was observed with a large Stokes shift (200 nm) and long lifetime (135 ns). The practical utility of probe Ir-DNBS was also investigated by thiophenol detection in real water samples and human serum.

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 (S_NAr) 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.