The invention provides
ion-selective sensors capable of selectively measuring ions, e.g., Na+, K+, Cl−, etc., in the
cytosol of a single
living cell. The sensor comprises one or more
quantum dots or a fluorescent dye, a pH-sensitive dye, and optionally an
ion-selective component such as an
ionophore. These elements may, for example, be disposed in a
polymer matrix. The
polymer matrix comprises an internalizing
moiety which enables the sensor to localize within the
cytosol of a
cell. The internalizing
moiety comprises a
small molecule or
peptide such as an amine, antepennepedia, mastoparan, or
melittin that react under acidic conditions to release a sensor from the confines of a
endosome. Once in the
cytosol the sensors may detect ionic analytes by selective
ion extraction by the
polymer, thereby inducing a pH change within the sensor which in turn changes the
absorbance of the pH-sensitive dye. The change of
absorbance may in turn attenuate the intensity of detectable emissions, e.g.,
fluorescence, from the
quantum dot or dye by directly absorbing its
fluorescence emission.