Short communicationPlasticizers free polyvinyl chloride membrane for metal ions sequestering
Graphical abstract
Introduction
Rapid and accurate detection of solute species, such as metal ions, is of major importance in several fields, from food analysis to pharmaceutics production. The removal or detection of contaminating metals can be obtained using different tools [1], [2], [3]. Nevertheless, ion selective membranes (ISMs) still remain the most used tool to detect the presence of metal ions in liquid phases. The earliest ISMs were developed in 1960s and worked basing on the principle of ion exchange thanks to the presence of polymeric systems enriched with phosphate derivatives or rare-earth doped single crystals. Actually, ISMs can be distinguished in two sub-groups, membranes working on ion coordination through electrostatic interactions or on size exclusion effect [4], [5]. Despite this difference, all ISMs are composed of a supporting material, commonly high molecular weight polyvinyl chloride (PVC), a plasticizer, an ion excluder and the ionophore. PVC is largely used because of its good performances in a wide range of temperatures (from −40 °C to 125 °C) and its stability under physical (UV light) and chemical agent attacks (i.e. chemicals, oil and gasoline). Moreover, it shows a high compatibility with biological fluids [6]. Contrarily, several plasticizers have been tested to improve the response of the membranes. Gupta et al. used dibutyl butyl phosphonate (DBBP), as plasticizer in a PVC matrix, to realize a membrane suitable for AsO2− estimation [7]. Tris (2-ethylhexyl) phosphate (TEHP) was also used in membranes for detecting uranium [8]. Bala et al. tested several plasticizers, namely bis(2-ethylhexyl) sebacate (DOS), o-nitrophenyl octyl ether (o-NPOE), 2-fluorophenyl 2-nitrophenyl ether (2-FPNPE) and chloroparaffin (CP) for the preparation of sensors for fluoride determination [9]. The plasticizer is the most critical component of ISMs since by it depends the ionophore adsorption in the membrane and hence, sensor performance [4]. Indeed, the plasticizer provides elasticity to PVC decreasing its glass temperature (Tg) and so guaranteeing a proper ion and ionophore mobility in the membrane. Some plasticizers can also act as a solvent for the ionophore in the membrane. Several attempts were used to avoid the use of plasticizer. Mahanty et al. used a multiple diglycolamide (DGA) as ionophore that permitted them to eliminate plasticizer since part of the ionophore acts as plasticizer [10].
The objective of this work is the realization of an ion-capturing membrane, suitable for analysis of aqueous solutions, avoiding the plasticizer thanks to microcavities that allow the ionophore to easily penetrate inside the material. This leads to a better sensitivity thanks to the increased active surface. Protophorphyrin IX was used as ionophore. Protoporphyrins are a class of macrocyclic organic compounds that are often applied to ion-selective membranes thanks to their selectivity. Moreover, metalloporphyrins show intensive absorption bands in the visible region making metal detection easily executed [11].
Section snippets
Materials
All reagents and solvents were purchased from Fluka Sigma Aldrich (Switzerland). Polyvinyl chloride (high molecular weight PVC) and polyvinyl alcohol (PVA: 125 kDa; hydrolysis degree: 98%) were used without further treatment.
Membranes preparation
Two different procedures were set-up to obtain membranes.
Procedure 1: PVC (10 g) was dissolved in the minimum amount of Tetrahydrofuran (THF) and PVA crystals or NaHCO3 crystals were added in 1% or 10% concentration (PVC-c-PVA1 and PVC-c-PVA10; PVC-s-NaHCO31 and PVC-s-NaHCO3
Membranes preparation
Two procedures were developed to prepare porous PVC membranes. In procedure 1, PVC was completely dissolved in THF, a solvent that allows the best solubility conditions for PVC. Porogenic agents (PVA or NaHCO3), in the forms of crystals, were then added and the enriched solution casted·THF facilitates PVC casting process, thanks to its low boiling point (i.e. 66 °C), without dissolving crystals as also verified by Yang et al. [20].
Procedure 2 is aimed to obtain PVC-PVA blend. DMA-LiCl system
Conclusions
Porous PVC-based membranes enriched with Protoporphyrin IX were prepared as single-use system to capture Zinc ions. Porosity was realized by mixing PVC with PVA as a blend and removing the water soluble component by washing. The procedure permitted to obtain a uniformly distributed porosity (≈2 µm). FTIR analysis demonstrated both the presence and the removal of the porogenic elements. The presence of ionophore was also verified by FTIR. DSC analysis showed that the effect of porogenic agents
Data availability
Upon request to corresponding author
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.
Acknowledgement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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