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A time-resolved fluoro-immunoassay (TR-FIA) format is presented based on resonance energy transfer from visible emitting lanthanide complexes of europium and terbium, as energy donors, to semiconductor CdSe/ZnS core/shell nanocrystals (quantum dots, QD), as energy acceptors. The spatial proximity of the donor-acceptor pairs is obtained through the biological recognition process of biotin, coated at the surface of the dots (Biot-QD), and streptavidin labeled with the lanthanide markers (Ln-strep). The energy transfer phenomenon is evident from simultaneous lanthanide emission quenching and QD emission sensitization with a 1000-fold increase of the QD luminescence decay time reaching the hundred mus regime. Delayed emission detection allows for quantification of the recognition process and demonstrated a nearly quantitative association of the biotins to streptavidin with sensitivity limits reaching 1.2 pM of QD. Spectral characterization permits calculation of the energy transfer parameters. Extremely large Förster radii (R(0)) values were obtained for Tb (104 A) and Eu (96 A) as a result of the relevant spectral overlap of donor emission and acceptor absorption. Special attention was paid to interactions with the varying constituents of the buffer for sensitivity and transfer efficiency optimization. The energy transfer phenomenon was also monitored by time-resolved luminescence microscopy experiments. At elevated concentration (10(-)(5) M), Tb-strep precipitated in the form of pellets with long-lived green luminescence, whereas addition of Biot-QD led to red emitting pellets, with long excited-state decay times. The Ln-QD donor-acceptor hybrids appear as highly sensitive analytical tools both for TR-FIA and time-resolved luminescence microscopy experiments.

A synthetic protocol is summarized that provides access to various multi-site ligands (bipyridines, terpyridines...) bridged by alkyne spacer comprising one to four ethynyl subunits. These ditopic ligands have been used to prepare luminescent transition-metal complexes in which long-range electronic coupling between remote units has been observed. Intramolecular triplet energy transfer, photon migration and light-induced electron transfer occur by way of extremely rapid superexchange involving energetic states localized on the bridge. Very long triplet-energy excited-state have been measured in the terpyridine complexes resulting from a selective decoupling of the MLCT triplet state and higher-energy metal-centered (MC) and ligand-centered excited states (LC). The level of electronic communication along the molecular wire can be modulated by insertion of "Pt(PnBu3)2" insulating groups into the bridge. These metallo-insulating groups provide further means to manipulate the stereochemistry around the centra...