= Solution
One design is an \b[<anthracene>–methylene–<aza-15-crown-5> sensor], with the <tertiary amine> of the aza-crown linked through $\mathrm{CH_2}$ to <anthracene>. The crown supplies the sodium-binding <oxygen>/nitrogen cavity, the <amine> supplies an <electron donor> and <anthracene> supplies the <fluorophore>. A sufficiently rigid sodium-sized <cryptand> could improve discrimination further.
Without a bound <cation>, <photoinduced electron transfer> from the <amine> can quench excited <anthracene>. <sodium> coordination lowers the electron-donating ability of that lone pair, suppressing this quenching route and increasing <fluorescence>. The methylene spacer allows electronic communication by <electron> transfer while limiting direct conjugation between the binding site and <fluorophore>.
A cavity appropriate to <sodium> is preferable to an 18-crown-6-like potassium-sized cavity. \b[Selectivity must be tested, not inferred perfectly from cavity size.] Potassium binding, including possible sandwich complexes of flexible crowns, can still compete. At the intended <concentrations> the design should satisfy $K_{\mathrm{Na}}[\mathrm{Na^+}]\gg K_{\mathrm K}[\mathrm{K^+}]$ and should give a <fluorescence> change on <sodium> binding. Rigidity, cavity size and suppression of competing stoichiometries are useful design controls. <fluorescence> calibration in the actual potassium-containing <solvent> establishes the usable detection range.
Back to article page
