A phenylene-diimine-capped conjugate of lower rim 1,3-calix[4]arene (L) was synthesized, characterized, and shown to selectively bind to Mg2+ using its capped arms. This results in a selective recognition of Mg 2+ through eliciting fluorescence enhancement of ∼70 fold with a detection limit of 40 ± 5 ppb. However, in the presence of blood serum, the lowest detection limit is 209 ± 10 ppb (0.2 μM). The binding of Mg2+ to L is authenticated by absorption and 1H NMR data. The Jobs plot derived on the basis of the absorption data showed 1:1 stoichiometry between the receptor and Mg2+. The 1:1 species was further confirmed through ESI MS, that is, being supported by the isotope peak pattern authenticating the presence of Mg2+ in the complex. The L binds Mg2+ octahedrally using the tetradentate L2- and two additional acetic acid moieties by bringing conformational changes as studied on the basis of MM computations. The conformational changes that occur in the capped arms upon Mg2+ binding were supported experimentally by NOESY. AFM and SEM studies showed that spherical particles of L are modified into flower and chain type aggregates upon complexation with Mg2+, confirming the supramolecular behavior of the species formed. © 2014 American Chemical Society.