Lanthanide complexes as anion-responsive luminescent probes
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Eu(III) 4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane ( S-THP ) was studied as an optical sensor for biologically relevant anions. Binding of anions was studied by using europium (III) excitation, ratiometric analysis of emission peaks, and time resolved luminescence spectroscopy. The europium (III) excitation and emission spectroscopy give spectra that are unique to the different anions bound to the Eu(III) center, while time resolved luminescence data gives insight as to the type of binding mode. The time resolved luminescence suggests the formation of both outer and inner sphere complexes upon the addition of phosphate or carbonate. The steady state excitation and emission spectra also support the proposed formation of an inner and outer sphere complex upon the binding of phosphate and carboxylate containing anions. Ratiometric analysis of emission peaks was used to determine anion concentration independent of probe concentration. Binding constants determined by plotting a ratio of the intensity of the peaks within the ΔJ=2 and ΔJ=1 manifold, 622/593 nm, showed weaker interactions towards the monoanionic anions acetate and lactate, followed by the dianionic anions hydrogenphosphate and methylphosphate, and had the strongest interaction with the trianionic anion, citrate. The positive charge of Eu(S-THP) +3 correlates with larger affinity to polyanionic ligands. CdSe quantum dot sensitization a macrocyclic complex with pendent amide groups and a thiol containing linker, Ln +3 , was studied. Tb(III) emission was detected upon excitation of the CdSe quantum dots and a time delay of 100 μs, indicating the transfer of energy from the CdSe to the Tb(III).