Plasticizers free polyvinyl chloride membrane for metal ions sequestering

Highlights

• A porous polyvinyl-chloride membrane able of sequestering metal ions was realized.

• Microporosity (≈2 µm) was obtained adding Poly vinyl alcohol (PVA) as blend.

• Protophorphyrin IX was used as ionophore.

• The presence of PVA and ionophore does not decrease the thermal field of application.

• The effective complexion with metal ions was verified by UV.

Abstract

The purpose of this work is to create a porous polyvinyl-chloride (PVC) membrane able of sequestering metal ions in solution avoiding the use of plasticizers that can affect ionophores distribution and performance. In the sensor here developed, the distribution of ionophore (i.e. protoporphyrin IX) is no longer due to plasticizers but to microcavities that allow it to easily penetrate inside the material. Microporosity was obtained adding water soluble components (Poly vinyl alcohol or NaHCO₃) as blends or crystals to PVC. Only blends show a uniformly distributed porosity (≈2 µm), that allows a rapid exchange thanks to the increased active surface. Membranes were characterized by Infrared Spectroscopy (IR), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM). The effective complexion with metal ions was verified by IR and UV Spectroscopy.

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].