A series of 5 randomly designed in-house cyclometalated Ir(III) complexes were examined for their application in G-pentaplex probes and the "proof-of-principle" concept in G-pentaplex-based Cs+ ions detection. The G-pentaplex-forming sequence (DNA1, 5'-T(iG)4T-3', where iG=isoguanine) is present in single strand DNA form ab initio, however, the addition of Cs+ ions lead to formation of the intermolecular G-pentaplex structure which is identified by the novel Ir(III) complex 1 afterward and produce an enhanced luminescence signal for Cs+ ions monitoring. To the best of our knowledge, this is the first G-pentaplex probe and also the first G-pentaplex-based label-free detection platform for Cs+ ions reported in the literature. The monitoring of spiked Cs+ ions in natural water samples demonstrates the potential application and technical sound of this "proof-of-principle" concept sensing platform. © 2015 Elsevier B.V.