Highly stabile ZnGa₂O₄:Eu nanocrystals as a fluorescence probe for bio-imaging

Acetylacetonate (acac) passivated and europium incorporated ZnGa₂O₄ nanocrystals (ZGO:Eu NCs) were successfully prepared via a facile one-step non-injection process in oleylamine at a relatively low reaction temperature of 300 °C under N₂ ambient. The resultant ZGO:Eu NCs exhibited a well-dispersed morphology with an average particle size of 4 nm. The enhanced and broad absorption band from 250 to 450 nm is attributed to the synergistic effect between the acac as an antenna and the coexistence of Eu³⁺ and Eu²⁺. Under 405 nm excitation, a full spectrum emission covering from 400 to 720 nm was observed. The sharp red emission coming from Eu(III) ⁵D₀ → ⁷F_J (J = 1–4) transitions can be enhanced by adjusting Eu concentrations and reaches the optimum at 20 at% Eu. Subsequently, the as-prepared NCs were transferred into an aqueous media via a ligand exchange by polyethyleneimine (PEI) to obtain the hydrophilic ZGO:Eu@PEI NCs. The initial fluorescent properties are preserved well. Finally, the strong red fluorescence emission, suitable excitation window, high photo-stability, excellent water solubility, and prominent bio-compatibility make ZGO:Eu@PEI NCs as high quality bio-imaging probes successfully identify living KB cells. The method mentioned in this paper overcomes the bottle-neck of the inorganic bulk phosphors as probes in bio-imaging, and as a result, expands their applications in optical detection and medical diagnosis fields.