Development of a molecular K+ probe for colorimetric/fluorescent/photoacoustic detection of K.

The potassium ion (K+) plays significant roles in many biological processes. To date, great efforts have been devoted to the development of K+ sensors for colorimetric, fluorescent, and photoacoustic detection of K+ separately. However, the development of molecular K+ probes for colorimetric detection of urinary K+, monitoring K+ fluxes in living cells by fluorescence imaging, and photoacoustic imaging of K+ dynamics in deep tissues still remains an open challenge. Herein, we report the first molecular K+ probe (NK2) for colorimetric, fluorescent, and photoacoustic detection of K+. NK2 is composed of 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) as the chromophore and phenylazacrown-6-lariat ether (ACLE) as the K+ recognition unit. Predominate features of NK2 include a short synthetic procedure, high K+ selectivity, large detection range (5-200 mM), and triple-channel detection manner. NK2 shows good response to K+ with obvious color changes, fluorescence enhancements (about threefold), and photoacoustic intensity changes. The existence of other metal ions (including Na+, Mg2+, Ca2+, Fe2+) and pH changes (6.5-9.0) have no obvious influence on K+ sensing of NK2. Portable test strips stained by NK2 can be used to qualitatively detect urinary K+ by color changes for self-diagnosis of diseases induced by high levels of K+. NK2 can be utilized to monitor K+ fluxes in living cells by fluorescent imaging. We also find its excellent performance in photoacoustic imaging of different K+ concentrations in the mouse ear. NK2 is the first molecular K+ probe for colorimetric, fluorescent, and photoacoustic detection of K+ in urine, in living cells, and in the mouse ear. The development of NK2 will broaden K+ probes' design and extend their applications to different fields. Graphical abstract.