Electrochemical investigation of Fe²⁺: trioctylphosphine oxide coordination reaction at a micro liquid|liquid interface

Abstract

Dissolved Fe is an important micronutrient for marine biota in the worlds' oceans that typically exists at sub-nanomolar concentrations. Many microorganisms such as phytoplankton have evolved specialized ligands for Fe uptake. Thus, spatiotemporal oceanic mapping of dissolved Fe and understanding Fe-ligand binding are critical areas of research. Herein, we have employed the micro (25 μm in diameter) interface between two electrolyte solutions (ITIES) as a platform to investigate Fe²⁺ coordination with trioctylphosphine oxide (TOPO) via an electrochemically controlled facilitated ion transfer reaction. Based on established thermodynamics and using differential pulse voltammetry, the metal ion:ligand stoichiometry was determined to be ~15, and overall binding coefficient (β) of Fe²⁺-TOPO was calculated 4.4×10⁴⁵, respectively. It is speculated that the high stoichiometry may be owing to the generation of pseudo-micelle since n greatly exceeds the typical Fe-ligand stoichiometry. Na⁺ and K⁺ binding towards TOPO were also examined since they are major electrolyte components in seawater. However, they demonstrated high binding affinities through much lower Na+: TOPO or K⁺: TOPO stoichiometries of roughly 1.18 and 2.52 as well as β9s of 3.3×10¹⁴ and 2.4×10¹⁵, respectively.