Rational Design of a Polymer-Based Ratiometric K+ Indicator for High-Throughput Monitoring Intracellular K+ Fluctuations.

Highly selective fluorescent K+ sensors are of great importance for monitoring K+ fluctuations in various biological processes. In particular, highly efficient ratiometric K+ sensors that can emit in dual wavelengths and facilitate the quantitative determination of K+ are highly anticipated. Herein, we present the first polymer-based ratiometric fluorescent K+ indicator (PK1) for quantitatively detecting K+ in aqueous solutions and high-throughput monitoring K+ fluctuations in living cells. PK1 was synthesized by conjugating a small molecular K+ probe and a red emission reference dye to a hydrophilic polymer skeleton. The newly synthesized PK1 can form highly stable nanoparticles in aqueous solutions and work in 100% water without the aid of any organic solvents or surfactants. PK1 is sensitive to K+ with a fluorescence enhancement of sevenfold after interactions with K+ at 1000 mM and inert to other metal ions, physiological pH, or dye concentration vibrations. More importantly, the fluorescence intensity ratio at 572 and 638 nm is linearly correlated with log [K+] in the range of 2-500 mM (R2 = 0.998), which will facilitate the quantitative detection of K+. Practical application of PK1 in detecting different K+-rich samples demonstrates its great potential in quantitative detection of K+. PK1 can be quickly internalized by live cells and shows no obvious cytotoxicity. We also demonstrate that PK1 could be used for monitoring K+ fluctuations under different stimulations by using a confocal microscope and especially a microplate reader, which is high throughput and time saving. The rational design of PK1 will broaden the design concept of ratiometric fluorescent K+ sensors and facilitate the quantitative detection of K+.