A label-free detection of K+ was developed using G-quadruplex DNA (c-Myc) modulated fluorescence enhancement of tetrakis-(diisopropylguanidino) zinc phthalocyanine (Zn-DIGP). Upon the addition of increasing concentrations of potassium, a detection limit of 0.8 μM for K+ was easily achieved. Comparative titrations using sodium, lithium, ammonium, transition metal, or alkali earth salts revealed that the fluorescence enhancement was highly specific for potassium ions. This system has, for the first time, provided a means for detecting 40 μM of K+ even in the presence of a 3500-fold excess of Na+ ions.