Addition of Cu 2+ into a solution of 4 leads to a quench of fluorescence at 434 nm due to the formation of 6. The fluorescence was recovered by the addition of polyP8 into the solution of 6, leading to the release of 4. Five alternating additions of Cu 2+ and polyP8 showed interconversion of 4 to 6 and back, which indicates that 4 can be used as a reversible fluorescent 'turn-off-turnon' probe for Cu 2+ and polyP8 (Fig. ). The concentration of Cu 2+ and polyP8 had to be adjusted in each cycle to obtain reversibility (see details in SI section 4).