Fig. 2.

Spectral properties and sensitivity of potassium nanosensors. (a) Single-photon excitation and emission spectra of the APG4 PAMAM-PEG nanosensor measured in a spectofluorimeter. (b) In vitro emission spectra (${\lambda }_{\mathrm{ex}}$ 515 nm) of APG4 PAMAM-PEG with a ${\mathrm{K}}^{+}$ titration in the range 0 to 50 mM, corrected for dilution. (c) Comparison of sensitivity of three different ${\mathrm{K}}^{+}$-sensitive fluorescent indicators encapsulated in PAMAM dendrimers. The respective responsivity of APG4, APG2, and PBFI is shown for the same ${\mathrm{K}}^{+}$ concentration range. (d) Differential sensitivities of the ${\mathrm{K}}^{+}$-indicator PBFI to ${\mathrm{K}}^{+}$ in solutions mimicking the extracellular or the intracellular milieu (${\lambda }_{\mathrm{ex}}$ 330 nm) in response to a ${\mathrm{K}}^{+}$ titration ranging from 0 to 150 mM. Intracellular-like solutions contained 15 mM ${\mathrm{Na}}^{+}$, whereas extracellular-like solution contained 150 mM ${\mathrm{Na}}^{+}$. (e) APG4 PAMAM-PEG fluorescence in response to an increasing ${\mathrm{Na}}^{+}$ concentration spanning the extracellular physiological range, and (f) to the divalent cations ${\mathrm{Ca}}^{2+}$, ${\mathrm{Mg}}^{2+}$, and ${\mathrm{Ba}}^{2+}$ measured at a fixed 5 mM ${\mathrm{K}}^{+}$ concentration emission (${\lambda }_{\mathrm{ex}}$ 515 nm, ${\lambda }_{\mathrm{em}}$ 537 nm).