Fluorescent ion indicators are widely used to measure ion concentrations in living cells. However, despite considerable
efforts in synthesizing new compounds, no ratiometric sodium indicator is available that can be excited at visible
wavelengths. Ratiometric indicators have an advantage in that measured fluorescence intensities can be corrected for
fluctuations of the indicator concentration and the illumination intensity, which is not possible when non-ratiometric
indicators are used. One way to circumvent this problem is to measure fluorescence lifetimes, which are independent of
these factors. Another way to overcome the disadvantages of a
non-ratiometric indicator dye is to embed it, together with
a reference dye, into nanoparticles. By relating the indicator fluorescence to the fluorescence of the reference dye,
inhomogeneities in the nanosensor concentration or the illumination intensity can be cancelled out reliably. In this study
we compare the benefits and drawbacks of these approaches.
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