Synthesis of stable carboxy-terminated NaYF4:Yb3+, Er3+@SiO2 nanoparticles with ultrathin shell for biolabeling applications

Here, a two-step method has been developed for synthesizing carboxy-terminated NaYF4:Yb3+, Er3+@SiO2 core@shell nanoparticles (UCNP@SiO2) with ultrathin shell (1.5 nm). First, the NaYF4:Yb3+, Er3+ upconverting nanoparticles (UCNPs) were prepared using solvothermal technology; then, silica shells (SiO2) were deposited on the nanocrystals to form core-shell structures by the hydrolysis of tetraethylorthosilicate (TEOS). The ultrathin SiO2 shell was obtained by increasing surfactant amount and decreasing TEOS amount in the reaction mixture. Carboxyethylsilanetriol (CTES) was used to generate the carboxy group on the particle surface. The carboxy-terminated UCNP@SiO2 are ideally suited for biolabeling and bioimaging applications because the as-prepared nanoparticles have extreme colloidal and optical stabilities, and the carboxy groups on the particle surface easily react with amino residues of biomolecules. As an example, we reported on the interactions of Ricinus Communis Agglutinin (RCA 120) conjugated UCNP@SiO2 with HeLa cells. The excellent performance of the RCA 120 conjugated UCNP@SiO2 in cellular fluorescence imaging was demonstrated.