Synthesis, surface modification, and photophysical studies of Ln₂O₂S:Ln'³⁺ (Ln=Gd, Tb, Eu; Ln'=Tb and/ or Eu) nanoparticles for luminescence bioimaging

It has been demonstrated that by using doping and co-doping strategies the lifetimes as well as luminescence intensities of the doping species in the lanthanide oxysulfide nanoparticles (NPs) Ln₂O₂S:Ln'³⁺ (Ln=Gd, Tb, Eu; Ln'=Tb, Eu, 1% and 5%) could be tuned. The lanthanide oxysulfide NPs, i.e., Gd₂O₂S, Tb₂O₂S and Gd₂O₂S:Tb³⁺(5%) were synthesized as matrices and doped with Eu³⁺(1% or 5%), by the thermal decomposition method and characterized by TEM, XRD, photoluminescence, cellular and in vivo animal imaging studies for potential luminescence bioimaging applications. Of these materials, Gd₂O₂S:Eu³⁺(5%) NPs possess the highest photo-stability and strongest luminescent intensity at 625 nm with a lifetime 853 μs in hexane. Surface modification of the Gd₂O₂S:Eu³⁺(5%) NPs with mPEG-APTES to increase its aqueous solubility resulted in almost complete luminescence quenching. Calcination of the host Gd₂O₂S:Eu³⁺(5%) NPs at 400 °C to increase their crystallinity and maintained some of their luminescence properties in aqueous solution. Further surface modification of the Gd₂O₂S:Eu³⁺(5%)-APTES NPs with mPEG and Alexa Fluor 660 allowed their effective cellular and in vivo animal luminescence imaging with low bio-toxicity. These novel imaging materials with tunable lifetimes would be potentially useful for luminescence bioimaging applications, particularly in the time-resolved, up-conversion, and/or multiplex modes.