Energy transfer between conjugated-oligoelectrolyte-substituted POSS and gold nanocluster for multicolor intracellular detection of mercury ion

Although metal nanoclusters (NCs) with sizes close to the Fermi wavelength of electrons are well-known for their molecule-like luminescent behaviors, their energy-transfer properties remain unrevealed. Herein, fluorescence resonance energy transfer (FRET) between blue-fluorescent conjugated-oligomer-substituted polyhedral oligomeric silsesquioxane (POSSFF) and red-fluorescent gold NCs (R-AuNCs) is investigated and subsequently utilized for mercury-ion sensing both in solution and in cell. By virtue of their opposite charges and good spectral overlap, efficient FRET from POSSFF to R-AuNC occurs upon electrostatic complex formation, leading to dual-emissive pink fluorescence upon donor excitation. The pink fluorescence specifically turns blue in the presence of mercury ions rather than other metal ions because of the strong metallophilic Hg ²⁺/Au⁺ interaction that quenches the red fluorescence from R-AuNCs. This consequently allows for visual detection and precise quantification of mercury ions with a limit of detection of ∼0.1 nM in aqueous solution. Moreover, the whole-cell permeability of the complexes and the preserved ion-selective FRET in cells make these complexes effective for multicolor intracellular sensing of mercury ion. This study thus not only develops a promising mercury-ion nanoprobe for diagnostic and clinical applications but also provides fundamental guidelines for the design of metal-NC-based intracellular nanosensors.