Water Diffusion Modulates the CEST Effect on Tb(III)-Mesoporous Silica Probes

Fabio Carniato,Lorenzo Tei,Giuseppe Ferrauto,Mónica Muñoz-Úbeda

doi:10.3390/magnetochemistry6030038

Abstract

The anchoring of lanthanide(III) chelates on the surface of mesoporous silica nanoparticles (MSNs) allowed their investigation as magnetic resonance imaging (MRI) and chemical exchange saturation transfer (CEST) contrast agents. Since their efficiency is strongly related to the interaction occurring between Ln-chelates and “bulk” water, an estimation of the water diffusion inside MSNs channels is very relevant. Herein, a method based on the exploitation of the CEST properties of TbDO3A-MSNs was applied to evaluate the effect of water diffusion inside MSN channels. Two MSNs, namely MCM-41 and SBA-15, with different pores size distributions were functionalized with TbDO3A-like chelates and polyethylene glycol (PEG) molecules and characterized by HR-TEM microscopy, IR spectroscopy, N2 physisorption, and thermogravimetric analysis (TGA). The different distribution of Tb-complexes in the two systems, mainly on the external surface in case of MCM-41 or inside the internal pores for SBA-15, resulted in variable CEST efficiency. Since water molecules diffuse slowly inside silica channels, the CEST effect of the LnDO3A-SBA-15 system was found to be one order of magnitude lower than in the case of TbDO3A-MCM-41. The latter system reaches an excellent sensitivity of ca. 55 ± 5 μM, which is useful for future theranostic or imaging applications.

Highlights

The distinctive properties of mesoporous silica nanoparticles (MSNs) in terms of their tunable mesoporosity, high thermal stability, chemical versatility, and good biocompatibility, have come to receive increasing attention by the scientific community [1,2]
MCM-41 and SBA-15 MSNs with particle sizes in the 100–200 nm and 500–600 nm range characterized by an order array of hexagonal channels (qualitatively evaluated by transmission electron microscopy (TEM) images at low (Supplementary Figure S1) and high magnifications (Figure 1) were functionalized with amino groups, following a post-synthesis approach optimized in our lab and reported in the literature [30,31]
The IR spectra of MCM-41 and SBA-15, collected at room temperature under vacuum, showed a typical band at 3745 cm−1 due to the stretching of isolated silanol groups (Si-OH) and a wide absorption centered at 3500 cm−1 attributed to the Si-OH groups involved in hydrogen bonds [37]

Summary

Introduction

The distinctive properties of mesoporous silica nanoparticles (MSNs) in terms of their tunable mesoporosity, high thermal stability, chemical versatility, and good biocompatibility, have come to receive increasing attention by the scientific community [1,2]. A number of molecules have been investigated as possible CEST agents Both diamagnetic (e.g., polysaccarides, peptides, etc.) [12,15,16] and paramagnetic (Ln-based PARACEST complexes) [12,13,17,18,19] have been described as efficient for in vitro and in vivo contrast generation and as responsive MRI probes [20,21,22,23,24]. The peculiarity of such agents is that they generate a frequency-encoded contrast, the contrast arises only if the proper radiofrequency pulse is applied

Methods

Results

Discussion

Conclusion