A Highly K + ‐Selective Two‐Photon Fluorescent Probe

A highly K + ‐selective two‐photon fluorescent probe for the in vitro monitoring of physiological K + levels in the range of 1–100 m M is reported. The two‐photon excited fluorescence (TPEF) probe shows a fluorescence enhancement (FE) by a factor of about three in the presence of 160 m M  K + , independently of one‐photon (OP, 430 nm) or two‐photon (TP, 860 nm) excitation and comparable K + ‐induced FEs in the presence of competitive Na + ions. The estimated dissociation constant ( K d ) values in Na + ‐free solutions ( K d OP =(28±5) m M and K d TP =(36±6) m M ) and in combined K + /Na + solutions ( K d OP =(38±8) m M and K d TP =(46±25) m M ) reflecting the high K + /Na + selectivity of the fluorescent probe. The TP absorption cross‐section ( σ 2PA ) of the TPEF probe+160 m M  K + is 26 GM at 860 nm. Therefore, the TPEF probe is a suitable tool for the in vitro determination of K + .