Subnanomolar Detection of Ions Using Thin Voltammetric Membranes with Reduced Exchange Capacity

Abstract

Herein, we report on a new strategy to improve the limit of detection of ionophore-based thin membranes interrogated under accumulation/stripping electrochemical protocol. Accordingly, we demonstrate subnanomolar detection of silver ion (Ag+) in water samples by re-formulating the membrane content with a reduced amount of the cation exchanger sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na+TFPB–), i.e. 10 mmol kg–1 compared to 40 mmol kg–1 commonly used in previous thin cation-selective membranes. Thoughtfully, by decreasing the amount of NaTFPB in the membrane phase, a diminution of its total ion-exchange capacity is to be seen. Essentially, a lower exchange capacity causes that the saturation of the membrane occurs at a lower concentration of Ag+, allowing us to reach a lower limit of detection. This effect is indeed promoted by achieving the total replacement of the Na+ present in the membrane by Ag+ entering from the solution (even at the subnanomolar level) at shorter accumulation times in the readout protocol. For the silver-selective electrode, we found a linear range of response with the peak current from 0.05–10 nM Ag+ concentration. The developed silver-selective electrode is successfully applied to the determination of Ag+ at the (sub)nanomolar level in different water samples (i.e., tap water, seawater and freshwater samples), with the results validated using inductively coupled plasma mass spectrometry as well as recovery studies. In addition, the electrode is suitable for dynamic studies involving the interaction of Ag+ with compounds forming natural organic matter in aquatic resources such as humic acid.