Abstract
Potassium ions (K+) are one of the most abundant ions in intracellular fluid, affecting a wide variety of cellular processes in living organisms. In humans, irregularities in extracellular K+ levels contribute to pathologies including cardiovascular disease, immunological diseases, and some cancers. Despite the role of K+ ions, the detection, quantification, and monitoring of K+ remains difficult. While fluorescent indicators exist that can provide a fast, easy readout for K+ concentration, they are often nonspecific, particularly to ions with similar charge states. To address this issue, we developed a vesicle-based sensor that harnesses membrane channels to gate access of K+ ions to an encapsulated fluorescent indicator. We assembled phospholipid vesicles that incorporated valinomycin, a K+ specific membrane transporter, and that encapsulated benzofuran isophthalate (PBFI), a K+ sensitive dye that nonspecifically fluoresces in the presence of sodium and calcium ions. The specificity, kinetics, and reversibility of PBFI fluorescence was monitored and the nanosensors were added to E. coli bacterial culture supernatant to evaluate K+ levels in media as a function of cell density. We demonstrate that this nanosensor can selectively detect K+ in the presence of other important biological cations, and can detect changes in extracellular K+ concentration in bacterial cultures. The approach presented here could be extendable to a range of ions, which can be customized by altering the ion transporter and ion indicator. We expect our methods will enable a new generation of ion sensors that will reveal new information about extracellular ion variations during normal and pathological functions.