High resolution transmission electron microscopy: A key tool to understand drug release from mesoporous matrices

doi:10.1016/j.micromeso.2016.01.019

Microporous and Mesoporous Materials

Volume 225, 1 May 2016, Pages 399-410

https://doi.org/10.1016/j.micromeso.2016.01.019 Get rights and content

Highlights

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Treating SBA15_DPT with 12M HCl produces –PO₃H₂ groups but damages the mesostructure.
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HCl-SBA15_DPT presents uncontrolled drug burst release compared with SBA15_DPT.
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XRD and N₂ adsorption are not enough to detect mesostructural damages in HCl-SBA15_DPT.
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HRTEM is essential to understand drug release from mesoporous materials.

Abstract

This work demonstrates that high resolution transmission electron microscopy (HRTEM) is an essential tool to understand drug delivery performance of mesoporous silica materials, mainly those submitted to functionalization processes involving harsh conditions that may affect the mesostructure. Herein an SBA-15-type mesoporous material bearing triple bond Si(CH₂)₂P(O)(OCH₂CH₃)₂ groups was synthesized following the co-condensation route. Then, the resulting material was treated with 37 wt% HCl to convert ethylphosphonate groups to ethylphosphonic acid groups. The proper dealkylation of ethoxy groups following acid treatment was confirmed by FTIR and CP-MAS ¹H → ¹³C solid state NMR, which indicated the presence of triple bond Si(CH₂)₂P(O)(OH)₂ functionalities in the treated sample. Characterization of mesoporous materials by XRD diffraction and N₂ adsorption points to well-ordered SBA-15 structures in both untreated and acid-treated samples. Nonetheless, a deep study by HRTEM reveals that the acid-treatment provokes noticeable loss of mesostructural order, only remaining small crystalline domains. This structural damage does not influence cargo loading but it severely affects the release of molecules confined into the mesopores, as concluded from in vitro delivery tests using cephalexin as model drug. Thus, whereas untreated sample showed a sustained diffusion-controlled drug release during more than 2 weeks, 100% of the loaded drug was released only after 10 h from treated sample. This abrupt burst effect cannot be explained on the basis of the existing matrix–drug interactions, whose nature and extension is quite similar under the release conditions for both samples. Thus, it can be only understood on the basis of the mesostructural damage revealed by HRTEM studies.

Graphical abstract

Introduction

Since silica-based ordered mesoporous materials entered the drug delivery landscape back in 2001 [1], they have received growing attention by the scientific community [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Mesoporous silicas are attractive drug carriers due to their outstanding properties, including: i) low toxicity and biocompatibility; ii) an ordered pore arrangement, with narrow pore size distributions that allow controlling drug loading and release kinetics; iii) high surface area that provides high potential for drug adsorption; iv) great pore volume to house high amount of therapeutic molecules; v) a silanol-containing surface, which can be functionalized to achieve higher control over drug loading and release.

Understanding drug delivery profiles from mesoporous matrices is essential to exploit their potential as controlled release systems. Drug release process obeys four sequential steps [15]: penetration of the release medium into the pore network, which is governed by osmotic pressure derived from concentration gradients; drug dissolution in the release medium; drug diffusion through the porous cavities due to concentration gradients; and drug diffusion and convection within the delivery medium. The structural and textural properties of mesoporous materials together with the chemical nature of their surface are the driving factors that govern the release of molecules and determine drug delivery profiles [2], [5]. Thus, pore diameter is a limiting factor for the diffusion of the molecules to the delivery medium, thus regulating the release rate [1], [16]. Pore connectivity and structure also influence drug release kinetics [17], [18], [20]. For instance 3D-bicontinuous cubic mesostructures allow easy fluid accessibility and fast molecular transport than 2D-hexagonal array of pores [17]. However, organic modification or functionalization of mesoporous matrices is cornerstone in the performance of these materials as drug delivery systems [2], [5], [19], [20]. Commonly, functionalization strategies of mesoporous silicas rely on the covalently grafting of organic silanes ((RO)₃SiR′) [21]. This process permits the modification of the silica surface by grafting organic groups selective to the chemical nature of the drug to be hosted. In most cases, organic modification allows increasing host–guest interaction between the mesoporous matrix and the drug molecule. Matrix–drug interactions are usually established through electrostatic or Coulombic interaction, hydrogen bonding, and apolar interaction [5], [22], [23], offering many possibilities to control drug adsorption and release. Some functionalization processes require post-synthesis treatments under rather harsh conditions, highlighting those involving strong acids, which could affect the mesostructural order [24]. For instance, the treatment of SBA-15 functionalized with –CN groups followed by treatment with 48 wt.% H₂SO₄ to oxidize cyanide groups to carboxylic acid groups (–COOH) [25], [26]; or the treatment of SBA-15 functionalized with –P(O)(OCH₂CH₃)₂ groups followed by the treatment with 37 wt.% HCl to dealkylate phosphonate groups to phosphonic acid groups (–P(O)(OH)₂) [27]. Whatever the functionalization method used to organically modify the surface of mesoporous matrices, but highlighting those involving aggressive conditions, it is indispensable to deeply study the properties of the resulting materials by using suitable characterization techniques. Actually, any alteration in the mesostructural order could strongly influence drug release process leading to unforeseen delivery profiles, albeit, to the best of our knowledge, this has not been reported yet.

Herein we demonstrate that high resolution transmission electron microscopy (HRTEM) is an essential tool to understand drug release kinetics from mesoporous matrices. For this purpose, SBA-15 incorporating ethylphosphonate functions (–(CH₂)₂P(O)(OCH₂CH₃)₂) was synthesized by co-condensation route and then was treated with concentrated HCl under reflux to achieve ethylphosphonic acid moieties (–(CH₂)₂P(O)(OH)₂). Although X-ray diffraction (XRD) and N₂ adsorption studies point to well-ordered 2D-hexagonal structures in both samples, in vitro drug delivery profiles are dramatically different, despite of exhibiting similar host–guest interactions under the release conditions. This behavior can be only explained if a deep characterization of samples by HRTEM is performed. Such study reveals that there is a noticeable loss of mesostructure in acid-treated sample, which would account for the rapid and uncontrolled drug release kinetics.

Section snippets

Synthesis of materials

SBA-15 type mesoporous silica material containing nominal 15 mol% (based on silicon) phosphonic acid diethyl ester groups (SBA15_DPT) was synthesized by the co-condensation route using diethylphosphatoethyltriethoxysilane (DPT, 92%, ABCR) (Scheme 1). Briefly, 8.0 g of Pluronic^® P123 block copolymer (PEO₂₀PPO₇₀PEO₂₀ kindly provided by BASF Co.) was added to a mixture of 276 mL of H₂O and 20.6 mL of concentrated HCl (37 wt.%, Aldrich) [28]. The solution was moderately stirred at 35 °C until total

Materials characterization

The chemical nature of samples was investigated by using FTIR, CHNS elemental chemical analysis, XRF and solid-state NMR spectroscopy. Fig. 1A displays FTIR spectra of samples before (SBA15_DPT) and after (HCl-SBA15_DPT) being submitted to the HCl treatment. All spectra show a broad band at around 3400 cm⁻¹ corresponding to the overlapping of the O–H stretching bands of hydrogen-bonded water molecules (H–O–H) and SiO–H stretching of surface silanols hydrogen bonded to molecular water (SiO–H⋯OH₂).

Conclusions

Deep characterization of mesoporous materials by HRTEM is indispensable to understand their performance as drug delivery devices. This tool becomes of foremost relevance to study functionalized mesoporous samples that have to be submitted to harsh post-synthesis treatments to attain certain surface organic moieties. This fact has been proved by synthesizing an SBA-15-type mesoporous material functionalized with ethylphosphonate groups and subsequently treating with concentrated HCl to afford

Acknowledgments

This study was supported by research grants from Ministerio de Economía y Competitividad (MINECO), Spain, through the projects MAT2012-35556 and CSO2010-11384-E (Agening Network of Excellence). M. Martínez-Carmona also thanks Moncloa Campus of International Excellence (UCM-UPM) for a PICATA predoctoral fellowship. We also thank X-ray Diffraction and Elemental Chemical Analysis C.A.I., UCM.

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High resolution transmission electron microscopy: A key tool to understand drug release from mesoporous matrices

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of materials

Materials characterization

Conclusions

Acknowledgments

Microporous Mesoporous Mater

Biomaterials

Microporous Mesoporous Mater

Eur. J. Pharm. Sci.

Microporous Mesoporous Mater.

J. Control. Release

Microporous Mesoporous Mater.

J. Control. Release

Microporous Mesoporous Mater

Microporous Mesoporous Mater

Chem. Mater

Angew. Chem. Int. Ed.

Chem. Soc. Rev.

Small

Adv. Mater.

Chem. Soc. Rev.

Chem. Mater.

Chem. Res. Toxicol.

Biomedical Applications of Mesoporous Ceramics: Drug Delivery, Smart Materials and Bone Tissue Engineering

Nat. Mater

Expert Opin. Drug Deliv.

J. Mater. Chem. B

Chem. Soc. Rev.