Magnetic and fluorescent core–shell nanoparticles for ratiometric pH sensing

This paper describes the preparation of nanoparticles (NPs) with a magnetic core surrounded by two successive silica shells embedding two fluorophores which allow ratiometric pH-measurments in the biologically relevant range. Iron oxide magnetic NPs (between 10-20 nm in diameter) were formed by the coprecipitation reaction of ferrous and ferric salts. Then, they were added to a water-in-oil microemulsion where the hydrophilic silica shells were obtained through hydrolysis and condensation of tetraethoxyorthosilicate (TEOS), together with the corresponding silylated dye derivatives - a sulforhodamine was embedded in the inner silica shell and used as the reference dye while a pH-sensitive fluorescein was incorporated in the outer shell as the pH indicator. The morphology of the resulting NPs was characterized by Transmission Electron Microscopy (TEM) and Dynamic Light Scattering (DLS). The results were in agreement between the two techniques, showing nano-objects of very uniform size (60-70 nm in diameter) and shape. In an attempt to confirm that the magnetic cores are inside the NPs, wide-angle X-ray diffraction (XRD) patterns were recorded. This technique showed, for uncoated as well as coated NPs, the typical peaks for the spinel structure of ferrimagnetic iron oxides at the same diffraction angle for both diffractograms. Finally, the magnetic NPs were characterized by vibrating sample magnetometry (VSM) obtaining the magnetization curves for our ferrimagnetic NPs and the typical magnetization parameters as saturation magnetization (Ms), coercivity (Hc), and relative remanence (Mr/Ms). Even decreasing after silica coating, the resulting magnetic nanosensors show a magnetization suitable for magnetic targeting, which decreases after silica coating. Changes in fluorescence intensity of the pH indicator in different pH buffered solutions are observed within few seconds indicating an easy accessibility of the embedded indicator to protons through the pores of the silica shell. The relationship between the analytical parameter, that is, the ratio of fluorescence between sensing and reference dye, and pH was adjusted to a sigmoidal fit using a Boltzmann type equation giving a pKa value of 6.8. The fluorescence intensity of the reference dye does not change significantly (~3.0%) by modifying the pH of the NPs dispersion in the pH range 5–8.