High Surface Area Mesoporous Silica Nanoparticles with Tunable Size in the Sub-Micrometer Regime: Insights on the Size and Porosity Control Mechanisms.

Federica Rizzi,Marino Lavorgna,Elisabetta Fanizza,Roberto Comparelli,Rachele Castaldo,Tiziana Latronico,Marinella Striccoli,Gennaro Gentile,Maria Lucia Curri,Angela Agostiano,Pierluigi Lasala,Nicoletta Depalo

doi:10.3390/molecules26144247

Abstract

Mesoporous silica nanostructures (MSNs) attract high interest due to their unique and tunable physical chemical features, including high specific surface area and large pore volume, that hold a great potential in a variety of fields, i.e., adsorption, catalysis, and biomedicine. An essential feature for biomedical application of MSNs is limiting MSN size in the sub-micrometer regime to control uptake and cell viability. However, careful size tuning in such a regime remains still challenging. We aim to tackling this issue by developing two synthetic procedures for MSN size modulation, performed in homogenous aqueous/ethanol solution or two-phase aqueous/ethyl acetate system. Both approaches make use of tetraethyl orthosilicate as precursor, in the presence of cetyltrimethylammonium bromide, as structure-directing agent, and NaOH, as base-catalyst. NaOH catalyzed syntheses usually require high temperature (>80 °C) and large reaction medium volume to trigger MSN formation and limit aggregation. Here, a successful modulation of MSNs size from 40 up to 150 nm is demonstrated to be achieved by purposely balancing synthesis conditions, being able, in addition, to keep reaction temperature not higher than 50 °C (30 °C and 50 °C, respectively) and reaction mixture volume low. Through a comprehensive and in-depth systematic morphological and structural investigation, the mechanism and kinetics that sustain the control of MSNs size in such low dimensional regime are defined, highlighting that modulation of size and pores of the structures are mainly mediated by base concentration, reaction time and temperature and ageing, for the homogenous phase approach, and by temperature for the two-phase synthesis. Finally, an in vitro study is performed on bEnd.3 cells to investigate on the cytotoxicity of the MNSs.

Highlights

Silica-based mesoporous nanoparticles (MSNs) have attracted increasing attention as nanocontainers [1,2,3] thanks to their high specific surface area, easy and versatile surface chemistry modification [4], narrow pore size distribution, tunable characteristics of pore network, with pore size ranging from 2 to 50 nm, and excellent biocompatibility with 4.0/).minimal non-specific or adverse effects [5,6]
According to the commonly reported procedures (see reactions (1) and (2) below), the basic pH provided by NaOH promotes the hydrolysis of tetraethyl orthosilicate (TEOS), bursting the nucleation of the Mesoporous silica nanostructures (MSNs) and supporting the further condensation of
All the reactions have been advantageously carried out at temperature not higher than 50 ◦ C and using small reaction medium volumes. Such a simple reaction scheme, inspired by the Stöber approach, have been demonstrated to offer the potential of size tuning the MSNs in the sub-micrometer range by purposely balancing the reaction mixture composition and synthesis conditions

Summary

Introduction

Minimal non-specific or adverse effects [5,6] These features make these structures potential candidates for delivery of active payloads useful in several fields of application [7]. Most of the synthetic approaches used to prepare MSNs derive from the pioneering work of Stöber [13], based on a sol-gel process of an alkoxysilane in water-alcohol solution, at ambient conditions in the presence of ammonia, as base catalyst, and modified via addition of a pore-structuring agents. Most common methodologies [14] are based on quaternary alkylammonium surfactants (i.e., cetyltrimethylammonium bromide, CTAB) as SDA, and are performed under strongly basic conditions, in the presence of tetraethyl orthosilicate (TEOS) as silica source, resulting in porous structures with pores size of nearly 3 nm. Spherical worm-like MSNs with mesopores or hierarchical porous structures, including hollow [16,17,18], yolk-shell, stellate nanoparticles (NPs) [19]

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