Decrease in Luminescence Lifetime Indicating Nonradiative Energy Transfer from Upconverting Phosphors to Fluorescent Acceptors in Aqueous Suspensions

Utilization of energy transfer from anti-Stokes photoluminescent upconverting phosphors (UCPs) to acceptor fluorophores has been successfully described in various homogeneous bioanalytical assays with subnanomolar detection limits. However, only limited evidence has been shown concerning the nature of the energy transfer mechanism involved, especially in aqueous environment. Energy may be transferred in a photon-mediated way (e.g., photon reabsorption) or through strongly distance-dependent dipole-dipole interactions between resonant energy levels (resonance energy transfer). To study the mechanism, we determined the luminescence lifetimes of the UCP donor in aqueous suspension by using the frequency-domain measurement and examined the lifetime changes in systems involving acceptor molecules in close proximity or within undefined distance. A decreased lifetime component in the presence of close-proximity acceptors was confirmed (64-72% reduction), and strong support for the contribution of the nonradiative resonance energy transfer in the process was obtained, although photon reabsorption is always present to some extent.