Here, we report for the first time on photoswitchable nanospheres containing spiropyran (Sp) for reversible release and uptake of metal ions. K+ is used as a model ion to demonstrate the chemical principle of this approach. Valinomycin is incorporated in the nanospheres to stabilize K+. Upon UV illumination, Sp transforms to the more basic ring-opened merocyanine form, which takes up H+ from the surrounding aqueous solution and expels K+ from the nanospheres. The process can be reversed by irradiation with visible light to reduce the surrounding K+ concentration.