Selective Packaging of Ferricyanide within Thermoresponsive Microgels

Abstract

This study effectively demonstrates that thermoresponsive, cationic poly(N-isopropylacrylamide-co-methacrylamidopropyltrimethylammonium chloride) P(NIPAM-co-MAPTAC) microgels act as selective, closable carriers for trivalent hexacyanoferrate(III) (ferricyanide). At the same time, the microgel disregards even higher charged hexacyanoferrate(II) (ferrocyanide). This is seen by investigating the electrochemistry of hexacyanoferrates in the presence of porous microgel particles with help of cyclic voltammetry (CV), hydrodynamic voltammetry (rotating disk electrode, RDE), and electrochemical impedance spectroscopy (EIS). For analysis, temperature-corrected parameters for each technique are introduced. Assuming incorporation/complexation between hexacyanoferrates and microgels, different limiting scenarios for the electron pathway are proposed by discussing different life times of the hexacyanoferrates within the microgel: fast exchange (scenario 1: full electrochemical addressability of all counterions), permanent entrapment (scenario 2: still full addressability of all counterions by injection of electrons into the microgels), and full entrapment (scenario 3: only remaining free counterions are addressable). Also, negligible interaction between hexacyanoferrates and microgels can be postulated, as found experimentally for ferrocyanide [Fe(CN)6]4–. In contrast for ferricyanide [Fe(CN)6]3–, temperature even allows a switching between a dominant scenario 1 (fast exchange) in the cold and the scenario 3 (full entrapment) in the heat. In more detail, the attraction between ferricyanide and microgel is enhanced at elevated temperatures due to the collapse and increasing charge density induced by the thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) component, which in turn acts more as an insulator in the heat. Hence, only the free hexacyanoferrates are electrochemically accessible in the heat. In addition, EIS and CV indicate only a minor contribution of permanent entrapment (scenario 2) during charge transport.