SBFI is a sodium-sensitive molecule used to estimate Na⁺ gradients in isolated mitochondria, to measure intracellular NA⁺ levels, to measure Na⁺ efflux in cells, and, in combination with other fluorescent indicators used to correlate changes in intracellular Na⁺ with Ca²⁺ and Mg²⁺ concentrations, intracellular pH, and membrane potential. Although the selectivity of SBFI for Na⁺ is less than that of calcium indicators such as fura-2, it is sufficient for the detection of physiological concentrations of Na⁺ in the presence of other monovalent cations. The spectral response of SBFI 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): SBFI (∼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 dissociation constant (K_d) of SBFI for Na⁺ is 3.8 mM in the absence of K⁺, and 11.3 mM in solutions with a combined Na⁺ and K⁺ concentration of 135 mM (which approximates physiological ionic strength). SBFI is ∼18-fold more selective for Na⁺ than for K⁺.

SBFI is available as both cell-impermeant acid salt (S1262) and as cell-permeant acetoxymethyl (AM) esters (S1263, S1264). 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.