Lowering the limit of detection of ion-selective membranes backside contacted with a film of poly(3-octylthiophene)

Abstract

Nanometer-sized membranes (thickness of ca 200 nm) backside contacted with a film of poly(3-octylthiophene) (POT) are here interrogated by an electrochemical protocol based on the accumulation and stripping of the target ion aiming at lowering its limit of detection (i.e., in the sub-micromolar range). Thus, using a membrane based on silver ionophore IV (Sigma-Aldrich), which is one of the ionophores regularly used in ion-selective membranes presenting a large binding constant (log βAg–ionophore≈12), it is possible to detect 5 nM concentration of silver with the established methodology. Importantly, this is a 1000-fold lower concentration of silver compared with the case in which the same membrane is subjected to traditional cyclic voltammetry. Essentially, the control of the oxidation state of the POT film by applying a constant potential during a certain period of time (i.e., Eapp=0 V for 720 s) in the presence of silver ions in the sample solution (from 5 to 100 nM) allows for an enrichment of the selective membrane in silver ions. As a result, a subsequent anodic linear sweep potential generates a voltammetric peak for silver transfer across the membrane that comprises a well-defined wave for such very low concentrations of silver in high sodium ion concentration background solution (10mM NaNO3). Detection of nanomolar levels of silver in different types of natural and environmental waters is herein demonstrated and the results are validated using inductively coupled plasma mass spectrometry.