We report on the design and application of fluorescent nanoprobes based on the covalent linking of L-glutathione-capped CdSe@ZnS quantum dots (QDs) to newly synthesized unsymmetrically substituted nickel mercaptosuccinic acid triazatetra-benzcorrole (3) and phthalocyanine (4) complexes. Fluorescence quenching of the QDs occurred on conjugation to complexes 3 or 4. The nanoprobes were selectively screened in the presence of different cations and Hg(2+) showed excellent affinity in "turning ON" the fluorescence of the nanoprobes. Experimental results showed that the sensitivity of QDs-4 towards Hg(2+) was much higher than that of QDs-3 nanoprobe. The mechanism of reaction has been elucidated based on the ability of Hg(2+) to coordinate with the sulphur atom of the Ni complex ring and apparently "turn ON" the fluorescence of the linked QDs.