PBFI is a potassium-sensitive molecule used measure intracellular K⁺ fluxes in animal cells and in plant cells and vacuoles. Although the selectivity of PBFI for K⁺ is less than that of calcium indicators such as fura-2, it is sufficient for the detection of physiological concentrations of K⁺ in the presence of other monovalent cations. The spectral response of PBFI upon ion binding permit excitation ratio measurements, and this indicator can be used with the same optical filters and equipment used for fura-2.

Learn more about ion indicators including calcium, potassium, pH, and membrane potential indicators ›

Fluorescent Ion Indicators Specifications:
• Label (Ex/Em): PBFI (∼340,380/500 nm)
• Lyophilized product may be dissolved in distilled water or aqueous buffer for use
• Product is typically loaded into cells by diffusion from a patch pipette for correlated fluorescence imaging and electrophysiological recording

Selectivity Considerations and Cell Loading Strategies
The K_d of PBFI for K⁺ is strongly dependent on whether Na⁺ is present, with a value of 5.1 mM in the absence of Na⁺ and 44 mM in solutions with a combined Na⁺ and K⁺ concentration of 135 mM (which approximates physiological ionic strength). In buffers in which the Na⁺ is replaced by tetramethylammonium chloride, the K_d of PBFI for K⁺ is 11 mM; choline chloride and N-methylglucamine are two other possible replacements for Na⁺ in the medium. Although PBFI is only 1.5-fold more selective for K⁺ than for Na⁺, this selectivity is often sufficient because intracellular K⁺ concentrations are normally about 10 times higher than Na⁺ concentrations.

PBFI is available as both cell-impermeant acid salt (P1265MP) and as cell-permeant acetoxymethyl (AM) esters (P1267MP). The anionic acid forms can be loaded into cells using our Influx™ pinocytic cell-loading reagent (I14402, Chelators, Calibration Buffers, Ionophores and Cell-Loading Reagents—Section 19.8), or by microinjection, patch-pipette infusion or electroporation. For AM ester loading (Loading and Calibration of Intracellular Ion Indicators—Note 19.1), addition of the Pluronic™ F-127 (P3000MP, P6866, P6867) or PowerLoad™ (P10020) dispersing agents as well as relatively long incubation times—up to four hours—are typically necessary.

Find Fluorescent Indicators for Na⁺ and K⁺
We offer a number of fluorescent indicators for measuring Na⁺ and K⁺. Review Fluorescent Na+ and K+ Indicators—Section 21.1 in the Molecular Probes™ Handbook for more information on these products.

For Research Use. Not for human or animal therapeutic or diagnostic use.