Resonance energy transfer from β-cyclodextrin-capped ZnO:MgO nanocrystals to included Nile Red guest molecules in aqueous media

Abstract

Core-shell ZnO:MgO nanocrystals have been synthesized by a sequential preparative procedure and capped with carboxymethyl β-cyclodextrin (CMCD) cavities, thereby rendering the surface of the nanocrystals hydrophilic and the particles water-soluble. The water-soluble CMCD-capped ZnO:MgO nanocrystals emit strongly in the visible region (450-680 nm) on excitation by UV radiation and are stable over extended periods and over a range of pH values. The integrity of the cyclodextrin cavities is preserved on capping and retains their capability for complexation of hydrophobic species in aqueous solutions. Here we report the use of the water-soluble cyclodextrin-capped ZnO:MgO nanocrystals as energy donors for fluorescence resonance energy transfer studies. The organic dye Nile Red has been included within the anchored cyclodextrin cavities to form a noncovalent CMCD ZnO:MgO-Nile Red assembly in aqueous solution. Significant Nile Red fluorescence at 640 nm is observed on band gap excitation of the ZnO:MgO in the UV, indicating efficient resonance energy transfer (RET) from the nanocrystals to the included dye. The number of acceptor molecules interacting with a single donor in the CMCD ZnO:MgO-Nile Red assembly may be altered by controlling the filling up of the anchored cavities by Nile Red, leading to a variation in the efficiency of resonance energy transfer. The donor-acceptor distance was estimated from the efficiency measurements. The Nile Red emission following RET shows a pronounced thermochromic shift, suggesting the possible use of the CMCD ZnO:MgO-Nile Red assembly as thermometers in aqueous solutions.