IPG-4 AM, a membrane-permeable fluorescent indicator, specifically binds to potassium ions (K+) with a dissociation constant (Kd) of 7 mM. This compound exhibits excitation and emission maxima (λ) at 525 nm and 545 nm, respectively, and displays a 100:1 selectivity for K+ over sodium ions (Na+). It is utilized for real-time detection of intracellular K+ levels, as well as monitoring extracellular K+ changes in vivo.