Polymer-Mediated Extraction for Microplate Reader Spectrophotometric Determination of ppb-Level Copper(II) Ion in Water

Abstract

A negatively charged metal chelate, bis(bathocuproinedisulfonic acid)-copper(I), was formed and stably solubilized in an aqueous solution of a water-soluble polymer, poly(N-isopropylacrylamide) (PNIPAAm), at room temperature. When the solution was warmed at 40°C, PNIPAAm became water-insoluble. It then formed a gum-like aggregate (polymer phase) having a very small volume. The chelate was difficult to be extracted to the polymer phase in the absence of counter ions, but was efficiently incorporated when appropriate amounts of quaternary ammonium ion are present as the counter ions. The recovery of the chelate increased with increasing the concentration or the hydrophobic properties of the counter ions, indicating the ion-pair extraction of the charged chelate into the polymer phase. In the presence of 1 mM dodecyltrimethylammonium chloride, the recovery of the chelate was quantitative. After the polymer phase was dissolved by 1-propanol in the hole of a 96-hole microplate, the absorbance at 450 nm of copper(I) chelate was measured. A slightly curved calibration was obtained in the copper(II) concentration range from 5×10^-9 M to 5×10^-7 M. The standard deviation was 6% (n=5) for 5×10^-8 M copper(II) ions.