Synthesis and characterization of magneto-fluorescent nanoparticles with excited-state intramolecular proton transfer

E.M.N. Oliveira, F.L. Coelho, L.F. Campo, M.L. Zanini, R.M. Papaléo
Pontifical Catholic University of Rio Grande do Sul,
Brazil

Keywords: magnetic nanoparticles, fluorescent nanoparticles

Summary:

Superparamagnetic iron oxide nanoparticles (SPION) coated with different shells are hybrid materials that can be used in biomedical imaging, due its excellent biocompatibility and biodistribution in tissue. Different dyes have been attached to the nanoparticles surface to act as optical markers, especially those that emit in the near infrared [1]. However, the use of a fluorescent compound that fluoresces by excited-state intramolecular proton transfer (ESIPT) has been little explored. In this work, we report the synthesis and characterization of a fluorescent silica-coated SPION (SPION@SiO2-HBT). The synthesis encompassed 3 stages. Firstly, the SPION core was synthesized by coprecipitation in a basic medium. Subsequently, a silica coating was added following the Stöber method. At the end, the fluorescent compound 2-[5′-N-(3-triethoxysilyl)propylurea-2′-hydroxyphenyl] benzothiazole (HBT) was linked to the SiO2 shell of the nanoparticles. The mean diameter of the bare SPION was 7.9 nm as indicated by TEM. The SPION@SiO2-HBT system has a total diameter of ~225 nm. The hydrodynamic diameters in aqueous solutions were 49.3 nm for the SPION and 248.7 nm for the SPION@SiO2-HBT system. The composite nanoparticles presented good stability in aqueous solutions showing no significant changes with pH (in the range of 2 to 14). Despite of the relatively large silica shell the nanoparticles showed superparamagnetic behavior. Pure HBT, in apolar solvents, presents very fast ESIPT reaction to produce a keto tautomer, with an emission band located at 538 nm (Stokes shift of ~185 nm) and a weak emission of enol tautomer at 436 nm (Stoke shift ~83 nm) [2]. On the other hand, in polar protic solvents HBT exhibits only a main emission band located at 440 nm related to the enol tautomer. The fluorescence emission of HBT in SPION@SiO2-HBT showed only an emission band located at 550 nm assigned to the same keto tautomer emission observed for pure solid state HBT or HBT in apolar solvents. On the other hand, when successive aliquots of bare SPION were added in the HBT solution, quenching of fluorescence occurs until complete elimination at high iron oxide concentrations. Thus, the silica shell provide protection against the annihilation of HBT emission in SPION@SiO2-HBT, resulting in a magneto-fluorescent compound with great potential for use as a multimodal imaging nanoprobe due to its large Stokes shift and high optical stability. References [1] X. Wu, M. Wu, J. X. Zhao. Nanomedicine: nanotechnology, Biology, and Medicine, 2014, 10, 297-312. [2] F. L. Coelho, F. S. Rodembusch, L. F. Campo. Dyes Pigm. 2014, 110, 134