Hajar Maleki

Synthesis of mechanically reinforced silica aerogels via surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization

Mechanically reinforced polymer–silica aerogels have been successfully prepared by using surface-initiated reversible addition-fragmentation chain transfer polymerization. With this approach, well-defined polystyrene (PSt) and poly(butyl acrylate) (PBA) with low polydispersities grew on the silica surface and improved the mechanical strength in relation to native aerogels. Moreover, it allowed establishing a structure–property relationship between the grafted polymer molecular weight and physical properties of the hybrid aerogels, thereby enabling the preparation of composites with tailored properties. The aerogel composites here obtained exhibited a low density of 0.13–0.17 g cm−3, high thermal insulation performance of 0.03–0.04 W m−1 K−1 and a high specific surface area of 350–780 m2 g−1, with approximately one order of magnitude improvement in the compression strength over the non-reinforced aerogels.

Synthesis and biomedical applications of aerogels: Possibilities and challenges

Aerogels are an exceptional group of nanoporous materials with outstanding physicochemical properties. Due to their unique physical, chemical, and mechanical properties, aerogels are recognized as promising candidates for diverse applications including, thermal insulation, catalysis, environmental cleaning up, chemical sensors, acoustic transducers, energy storage devices, metal casting molds and water repellant coatings. Here, we have provided a comprehensive overview on the synthesis, processing and drying methods of the mostly investigated types of aerogels used in the biological and biomedical contexts, including silica aerogels, silica-polymer composites, polymeric and biopolymer aerogels. In addition, the very recent challenges on these aerogels with regard to their applicability in biomedical field as well as for personalized medicine applications are considered and explained in detail.

High Antimicrobial Activity and Low Human Cell Cytotoxicity of Core–Shell Magnetic Nanoparticles Functionalized with an Antimicrobial Peptide

Superparamagnetic iron oxide nanoparticles (SPIONs) functionalized with antimicrobial agents are promising infection-targeted therapeutic platforms when coupled with external magnetic stimuli. These antimicrobial nanoparticles (NPs) may offer advantages in fighting intracellular pathogens as well as biomaterial-associated infections. This requires the development of NPs with high antimicrobial activity without interfering with the biology of mammalian cells. Here, we report the preparation of biocompatible antimicrobial SPION@gold core-shell NPs based on covalent immobilization of the antimicrobial peptide (AMP) cecropin melittin (CM) (the conjugate is named AMP-NP). The minimal inhibitory concentration (MIC) of the AMP-NP for Escherichia coli was 0.4 μg/mL, 10-times lower than the MIC of soluble CM. The antimicrobial activity of CM depends on the length of the spacer between the CM and the NP. AMP-NPs are taken up by endothelial (between 60 and 170 pg of NPs per cell) and macrophage (between 18 and 36 pg of NPs per cell) cells and accumulate preferentially in endolysosomes. These NPs have no significant cytotoxic and pro-inflammatory activities for concentrations up to 200 μg/mL (at least 100 times higher than the MIC of soluble CM). Our results in membrane models suggest that the selectivity of AMP-NPs for bacteria and not eukaryotic membranes is due to their membrane compositions. The AMP-NPs developed here open new opportunities for infection-site targeting.

Compressible, Thermally Insulating, and Fire Retardant Aerogels through Self-Assembling Silk Fibroin Biopolymers Inside a Silica Structure—An Approach towards 3D Printing of Aerogels

Thanks to the exceptional materials properties of silica aerogels, this fascinating highly porous material has found high-performance and real-life applications in various modern industries. However, a requirement for a broadening of these applications is based on the further improvement of the aerogel properties, especially with regard to mechanical strength and postsynthesis processability with minimum compromise to the other physical properties. Here, we report an entirely novel, simple, and aqueous-based synthesis approach to prepare mechanically robust aerogel hybrids by cogelation of silk fibroin (SF) biopolymer extracted from silkworm cocoons. The synthesis is based on sequential processes of acid catalyzed (physical) cross-linking of the SF biopolymer and simultaneous polycondensation of tetramethylorthosilicate (TMOS) in the presence of 5-(trimethoxysilyl)pentanoic acid (TMSPA) as a coupling agent and subsequent solvent exchange and supercritical drying. Extensive characterization by solid-state 1H NMR, 29Si NMR, and 2D 1H-29Si heteronuclear correlation (HETCOR) MAS NMR spectroscopy as well as various microscopic techniques (SEM, TEM) and mechanical assessment confirmed the molecular-level homogeneity of the hybrid nanostructure. The developed silica-SF aerogel hybrids contained an improved set of material properties, such as low density (ρb,average = 0.11-0.2 g cm-3), high porosity (∼90%), high specific surface area (∼400-800 m2 g-1), and excellent flexibility in compression (up to 80% of strain) with three orders of magnitude improvement in the Youngs modulus over that of pristine silica aerogels. In addition, the silica-SF hybrid aerogels are fire retardant and demonstrated excellent thermal insulation performance with thermal conductivities (λ) of 0.033-0.039 W m-1 K-1. As a further advantage, the formulated hybrid silica-SF aerogel showed an excellent printability in the wet state using a microextrusion-based 3D printing approach. The printed structures had comparable properties to their monolith counterparts, improving postsynthesis processing or shaping of the silica aerogels significantly. Finally, the hybrid silica-SF aerogels reported here represent significant progress for a mechanically customized and robust aerogel for multipurpose applications, namely, as a customized thermal insulation material or as a dual porous open-cell biomaterial used in regenerative medicine.

Towards improved adsorption of phenolic compounds by surface chemistry tailoring of silica aerogels

The high toxicity/volatility and low biodegradation of phenolic compounds are serious concerns in terms of environmental and health impact—their recommended max. value for drinking water is 0.005 mg/L. They are usually removed from effluents by adsorption, but they show a complex interaction behavior with adsorbents, because the hydroxyl group and the hydrophobic aromatic ring are very close. In this work, the versatility of Si chemistry was explored to tailor the surface chemistry of silica aerogels and improve their adsorption performance towards phenolic compounds. Methyltrimethoxysilane and tetramethylorthosilicate were combined to adjust the hydrophobicity of the obtained aerogels. In the next stage, β-cyclodextrin, with its highly hydrophobic cavity, was grafted into the gels to improve the capturing of aromatic rings. For a sustainable linkage of β-cyclodextrin to silica, the methyltrimethoxysilane/tetramethylorthosilicate precursor system was modified by adding an epoxy functionalized silane. A first screening of the adsorption performance shows a 1.5–2-fold increase of the adsorption capacity and removal efficiencies of the epoxy-cyclodextrin-modified aerogel toward phenol and p-cresol when compared to aerogel counterpart without modification. Freundlich isotherm model was the most suitable to describe the equilibrium data of aerogels with or without β-cyclodextrin, with the curves showing favorable profiles, more evident in the case of aerogels with β-cyclodextrin. Apart from the improving of the sorption capacity for phenolic compounds (achieving a maximum of 60 mg g−1 in the case of p-cresol), the utilization of the biodegradable β-cyclodextrin moiety obtained from natural and sustainable resources is a further asset of the epoxy-cyclodextrin-modified aerogel.Graphical Abstract