Aqueous biphasic system in differential extraction of arseno and phospho molybdenum blue: Consequent sensing of glutathione in acid-free medium

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Abstract

Polyoxometalates (POMs) are the oxyanion clusters of early transition metals (mostly molybdenum (VI), tungsten (VI) and vanadium (V)) and they show interesting properties particularly in the field of catalysis and sensing chemistry. In this work molybdenum blue (MB), phosphomolybdenum blue (PMB) and arsenomolybdenum blue (AsMB) are prepared using glutathione (GSH) as reducing agent in acid–free condition. The MB species are further characterised by UV–vis spectroscopy, Raman spectroscopy, XPS, powder XRD and FTIR spectroscopy. The prepared MB solutions showed an exciting behaviour in Aqueous Biphasic Systems (ABS) using PEG#4000 and Na2SO4 as phase forming components. MB and PMB partition to the micellar medium of PEG upto 44 % and 66 % respectively but AsMB is not at all partitioned. Therefore the method is useful for differentiating PMB and AsMB. PEG has been recovered using ultra-filtration technique after the ABS. The experiment also reveals that GSH, a biomolecule with high physiological impact, can be detected at trace concentrations by PMB formation method both in water and blood serum media.

Introduction

Polyoxometalates (POMs) are oxygen anion clusters, mainly composed of early transition metals such as V(V), Nb(V), Ta(V), Mo(VI), and W(VI). These metals are involved in self-assembly processes by bridging the oxide anions [[1], [2], [3], [4]]. POMs exist in two types, isopolyoxo metalates, and hetropolyoxo metalates. Isopoly anions are solely made of d-block elements and oxide anions, whereas heteropoly anions contain hetero atoms such as P, As, Ge and Si along with the d-block element and oxide linkages [[1], [2], [3], [4], [5]]. The general formula of heteropolyoxo metalates are [XM12O40]n− and [X2M18O62]n which can have Keggin type or Dawson type structures respectively, where ‘M’ is an addendum atom like Mo or W while ‘X’ is a heteroatom such as P, As, Si and Ge [6]. These metals in POMs are involved in self aggregations using the oxide ions, molybdenum, in particular shows dramatic and interesting self aggregation phenomena in POM chemistry. The Mo-centres are self assembled to form mainly three different shapes such as wheel (154 and 176 Mo centres), Keplerate (102-Mo centres) or lemon (368-Mo centres) [[7], [8], [9]].

Molybdenum based POMs are widely used both as catalysts and sensors. MB nano rings effectively catalyse the partial oxidation of cyclohexane to form cyclohexanol [10]. Phospho molybdic acid is also reported to catalyze Friedel-Crafts type dehydrative coupling reaction [11] and act as an oxidation catalyst [12]. Not only in the field of catalysis, molybdenum blue is also popular in analytical chemistry to detect phosphorus and arsenic by formation of phosphomolybdenum blue (PMB) and arsenomolybdenum blue (AsMB) respectively [[13], [14], [15]].

Arsenate and arsenite are the most commonly occurring arsenic species in natural waters [16,17]. Various conventional analytical techniques are applied for both detection and quantification of arsenic species in environmental samples viz., inductively coupled plasma mass spectrometry (ICP-MS), atomic fluorescence spectrometry (AFS), high performance liquid chromatography (HPLC), cathodic stripping voltammetry (CSV) and atomic absorption spectrometry (AAS) [18]. Some colorimetric methods, such as arseno molybdenum blue formation technique [16,17], Fe3O4 nanoparticles [18], europium-coordinated gold nanoparticles [19], S-layer functionalized gold nanoparticles [20] etc. are also used to detect arsenate.

Adequate phosphorus balance is vital for maintaining the basic cellular functions, ranging from energy metabolism to signalling in human cells [21]. Phosphate can be detected using gold nanoparticles [22], carbon black nanoparticles [23], etc. Spectrophotometric detection of phosphate is also done by Amplitude-Modulated Flow Analysis Coupled with Malachite Green Method [24]. For colorimetric detection of phosphate, molybdenum blue formation is another established method [2].

For generation of molybdenum blue species, previous reports indicate essentiality of acidic medium using reducing agents (e. g. B2H6, N2H4, NaBH4, H2S, S2O32−, NH2OH, S2O42−, SO32−, SO2, MoOCl52−, SnCl2, MoCl5, etc.) [2]. In the present work, glutathione (GSH) is used as a reducing agent to synthesize molybdenum blue species. Glutathione is a tripeptide, composed of cysteine, glutamic acid and glycine, and is the principle nonprotein molecule present in all mammalian tissues [25]. Being a ubiquitous molecule, GSH is produced in all organs, especially in the liver [25]. It shows brilliant antioxidant property in cell defence system and its active group is thiol (-SH) which belongs to its cystein residue [25]. The concentration range of GSH lies in between 1–10 mM in mammalian tissues [25,26]. Besides protecting the cell membrane from oxidative stress, GSH plays an important role in the functions of central nervous system as well as is involved in protein synthesis [25].

Aqueous biphasic system (ABS) is a greener alternative of traditional liquid-liquid extraction. ABS is composed of two mutually incompatible aqueous solutions of polymer-polymer, polymer-salt, ionic liquid-salt, etc., at a certain thermodynamic condition such as pH, temperature, etc [27,28]. ABS is the greener tool to extract metals, biological substrates as well as to study the molecular interactions [29,30]. It has been reported already that ABS is used to extract molybdate species [31] and there are very few reports which narrate the extraction of molybdenum blue species using ABS [32]. In this work we are not only able to extract the MB species but also successfully distinguish the phosphomolybdenum blue (PMB) and arsenomolybdenum blue (AsMB) using ABS.

The present work deals with synthesis of MB, PMB and AsMB using GSH as a biocompatible reducing agent without the use of any strong mineral acid making the process greener. The synthesized MB species is then characterised by several analytical techniques such as UV–vis spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), powder XRD and FTIR spectroscopy. In order to distinguish the PMB and AsMB, aqueous biphasic extraction was carried out. As the ABS requires high concentration of phase forming components, we have also regenerated the polymer rich phase of ABS after extractions. It was also possible to detect glutathione in the both serum and aqueous media at physiological concentrations using the molybdenum blue technique. Overall we can say that, this research work provides three major findings, (i) synthesis of MB species using GSH and their characterisation using a range of analytical techniques, (ii) developing a method to distinguish between AsMB and PMB employing ABS and (iii) effective detection of glutathione has been carried out in water and blood serum medium by applying the PMB synthesis reaction which can enrich the analytical detection method of GSH.

Section snippets

Materials

Sodium molybdate dihydrate (Merck, ≥99.0 %), glutathione reduced (GSH) (Sisco Research Laboratories Pvt. Ltd., 99 %), sodium dihydrogen phosphate dihydrate (Merck, 98.0–100.5 %), polyethylene glycol 4000(Merck), sodium arsenate AR (S D FINE −CHEM LIMITED, 99.0–102.0 %), sodium chloride (Merck), Amberlite IR-120 (Sigma-Aldrich), Amberlite IRA-400 chloride (Sigma-Aldrich), assayed chemistry control human serum (HS) (Lypochek, Bio-Rad laboratories) were used as received, and all the chemicals were

Optimization of the time required for molybdenum blue formation

Synthesis of MB using GSH needs an optimization for the reaction conditions such as reaction time and ratio of the reagents used. Optimization of time of the reaction was performed as discussed in experimental section. Fig. 1 shows the UV–vis spectral study to optimize the time required to form molybdenum blue. It shows three characteristic peaks at 335 nm, 700 nm and 865 nm. The peak at 335 nm appears due to ligand to metal charge transfer (LMCT) of Mo-O-Mo bond [33,34]. The broad peak at 700

Conclusion

We report analytical and bio-analytical applications of MB species in this work. Phosphomolybdenum blue and arsenomolybdenum blue were also prepared and their extraction properties in aqueous biphasic system were studied. We have noticed that the percentage of extraction of phosphomolybdenum blue is 66 % whereas arsenomolybdenum blue does not get extracted in ABS. Therefore this observation generates a method to distinguish between phosphate and arsenate species which have otherwise similar

CRediT authorship contribution statement

Laboni Das: Methodology, Data curation, Writing - original draft. Santanu Ray: Conceptualization, Writing - original draft. Sreyan Raha: Visualization, Data curation. Debarati Dey: Visualization, Investigation. Kamalika Sen: Visualization, Investigation, Supervision, Writing - review & editing.

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

K. S. acknowledges UGC CAS V, India for funding. L. D. acknowledges UGC-Ref. No.: 259/ (CSIR-UGC NET JUNE 2017) for funding. Authors would also like to acknowledge the funding received under Global Challenge Research Fund (GCRF) Business Case through the University of Brighton, UK.

References (43)

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