K+ is the most abundant metal ion in cells, and changes of [K+] around cell membranes play important roles in physiological events. However, there is no practical method to selectively visualize [K+] at the surface of cells. To address this issue, we have developed a protein- coupled fluorescent probe for K+, TLSHalo. TLSHalo is responsive to [K+] in the physiological range, with good selectivity over Na+ and retains its K+-sensing properties ...