A new palladium complex as a dual fluorometric and colorimetric probe for rapid determination of sulfide anion

Highlights

• A BODIPY based Pd (II) complex, Pd-BDPA was synthesized as dual fluorometric and colorimetric probe for sulfide determination

• The interaction affinity of Pd (II) with sulfide is the basis for sulfide determination

• Highly sensitive, immediate response with excellent selectivity in the presence of wide range of other anions

• Pd-BDPA was applied successfully for the detection of sulfide anion by using paper-based test strips

• The method was successfully utilized to determine the sulfide in the real water samples

Abstract

A new BODIPY based Pd (II) complex, Pd-BDPA has been developed as a dual fluorometric and colorimetric probe for the detection of sulfide anion. The sensing ability is based on the interaction affinity of the Pd (II) towards the sulfide anion which is highly selective and immediate with a detection limit of 240 nM. The interaction between Pd (II) and sulfide is not only highly selective over the other common anions but also not interfered by their coexisting. This selective interaction between them leads to the regeneration of free ligand resulting in the dramatic changes in both fluorescence and absorption spectra. The fluorescent changes lead to quenching the emission spectra which is linear with the concentration of sulfide in the range of 1-10 μM. In addition to fluorescence quenching, the interaction also resulted in the significant color switching from purple to blue which was easily distinguished by the naked eye. Another important advantage is that the Pd-BDPA coated paper-based strips show the sulfide selectivity and sensitivity by the color changes as is in the solution form, revealing its practical utility more conveniently. In addition, the Pd-BDPA probe can also successfully be utilized for the determination of sulfide in real water samples with satisfactory recovery results indicating its feasibility for environmental samples.

Introduction

During the last few years, the development of selective and efficient fluorescent sensors for anion recognition has gained too much attention due to their important role in biological, industrial, and environmental processes [1,2]. Since sulfide is one of the important anions from the biological and environmental point of view, so it is frequently used in various industrial processes like conversion into sulfur, preparation of sulfuric acid and dyes, cosmetics manufacturing, wood pulp production etc. Along with these, some other industries such as paper, petrochemical and leather are also releasing the sulfide in routine as a toxic environmental pollutant. [3,4]. In addition to the industrial processes, another origin of the generation of sulfide anion is microbial reduction of sulfate by anaerobic bacteria or from the sulfur-containing amino acids in meat proteins [4]. More toxic and caustic HS⁻ or H₂S are formed if the sulfide anion is protonated [5]. As a result, the high-level exposure of sulfide is not only related to different types of risks such as unconsciousness, irritation of mucous membranes and respiratory problems but also closely associated with various diseases including Alzheimer's disease, Down's syndrome and cirrhosis of the liver [[6], [7], [8], [9]]. Therefore, there is a need to develop a selective and sensitive method for the immediate detection of sulfide anion in the aqueous media.

Presently, a large number of detection approaches have been established for sulfide anion. These methods include titration [10,11], fluorimetry [[12], [13], [14], [15]], chemiluminescence [16,17], spectrophotometry [18,19], inductively coupled plasma atomic emission spectroscopy [20], hydride generation atomic fluorescence spectrometry [21], electrochemistry [22] and chromatography [23]. Among these, fluorimetry has received a reasonable attentiveness owing to its enhanced sensitivity and ease of detection [24,25]. Compared with a large numbers of cation chemosensors, to design a sensitive and selective chemosensor for a specific anion is still a difficult task. This is due to little knowledge about the interactions between probes and anions compared with the interaction of cations and ligands as their coordination chemistry is well known. However, generally the anions are detected through hydrogen-bonding interactions [[26], [27], [28], [29]], electrostatic interactions [30,31], coordination through metal ions [[32], [33], [34]] etc. The determination of sulfide by the fluorescent sensing in aqueous media is still a difficult job due to its strong hydration nature as an anion, which results in the weakening of its interaction with probes [35]. This problem can be sidestepped by the metal displacement method, which is based on competitive binding of an indicator and the analyte to a receptor [2,[36], [37], [38]]. Up to now, a large number of sulfide detection probes have been developed by utilizing the Cu²⁺ ion displacement strategy in organic/water mix-solvent [35,[39], [40], [41], [42], [43], [44], [45], [46], [47], [48]]. In addition to Cu²⁺, palladium is also one of the important transition metals which has the ability to coordinate with nitrogen and oxygen like electron donor atoms. The Pd (II) has the highest formation constant with heavy halide and sulfur ligands [49] and hence has the ability to form stable complexes with a variety of sulfur compounds [50]. By utilizing the similar type of interaction affinity of sulfide with Pd (II) as is with Cu²⁺in displacement approach, a fluorescent probe for the sulfide determination was reported on the basis of fluorescent changes near 400 nm [51]. Inspiring from the natural affinity of Pd (II) towards sulfide, in the present work, an attempt was made to develop a compound which can explore their interaction not only in the form of fluorescent changes but also in the significant variation of the color. By keeping in mind this highly selective and immediate interaction between Pd (II) and sulfide, a Pd (II) based fluorophore has been developed as dual functional probe. This fluorescent and colorimetric probe having extreme sensitivity and selectivity would certainly be useful not only for sulfide determination but also for the improvement of some features of the already existing sulfide detection probes. A BODIPY core was selected for this purpose due its strong absorption and high emission quantum yield in the visible region [52]. A ligand bis(pyridine-2-ylmethyl) amine (DPA) was introduced to BODIPY core through α-styryl group which not only shifts the spectra towards NIR due to ICT [53], but also acts as chelator to coordinate with Pd(II). The synthesized ligand (BDPA) when coordinated with Pd (II), a new Pd-BDPA complex was synthesized with high emission intensity and different color compared with the free ligand. The interaction of sulfide with Pd-BDPA, extracts Pd (II) from complex similar to the displacement approach, leading to regeneration of the free BDPA ligand with decreasing of its emission intensity and changing of its color. The resulting decrease in fluorescence intensity and the color change is proportional to the sulfide concentration and provides the basis for its determination. In addition, similar to Pd-BDPA, a Pt (II) complex, Pt-BDPA was also synthesized in order to compare the effect of metal ion in the complex.