Fluorimetric analysis was performed in quartz cuvettes using a using a spectrofluorimeter (LS55 Luminescence, PerkinElmer, Massachusetts). First, the nanosensor was diluted to 100 nM in an extracellular-like solution containing (mM): NaCl 160, -gluconate 0-50, 1.3, 0.81, 0.78, HEPES 20 adjusted to pH 7.4 with NaOH. For calibration curves, fluorescence intensities of the emission maxima were plotted against known concentrations. For this, K-gluconate (1M) was titrated in the cuvette over the range of 0 to 50 mM, and the results were corrected for dilution. For experiments, extracellular-like solution, as described above, was used but with 5 mM KCl and 135 mM NaCl. NaCl (2M) was then titrated to gather data in the range of 135 to 165 mM and were also corrected for dilution. For , and experiments, , , and (adjusted to pH 7.4) solutions were titrated in extracellular-like solution (5 mM ), as described above. All spectral characterizations are an average of three separate scans.