Brigida Silvestri

Antimicrobial activity of eumelanin-based hybrids: The role of TiO2 in modulating the structure and biological performance

Eco-friendly hybrid Eumelanin-TiO2 nanostructures, recently obtained through in situ methodology based on hydrothermal route, have shown a striking antimicrobial activity, after exposure to oxidative environment, even under visible light induction condition. Nevertheless, the role of each component in defining the efficacy of these biological properties is far from being clearly defined. Furthermore, the effect of oxidative step on hybrids structure has not yet addressed. This study aims at elucidating the role of the ratio between eumelanin precursor, 5,6-dihydroxyindole-2-carboxylic acid (DHICA), and TiO2, for its polymerization in defining morphology and structural organization of TiO2-melanin nanostructures. Furthermore, tests on a Gram-negative Escherichia coli DH5α strain under UV irradiation and even visible light allowed to assess the contribution of each component, as well as of the TiO2-DHICA charge transfer complex to overall biological performance. Finally, results of biocide characterization were combined with spectroscopic evidences to prove that oxidative treatment induces a marked structural modification in melanin thus enhancing overall antimicrobial efficacy.

Titania as a driving agent for DHICA polymerization: a novel strategy for the design of bioinspired antimicrobial nanomaterials

Organic materials are widely employed to tune surface chemistry and/or as structuring agents of inorganic materials. Here, we propose a novel synthesis approach whereby TiO2 not only promotes 5,6-dihydroxyindole-2-carboxylic acid (DHICA) polymerization but also acts as a templating agent for the formation itself. Hybrid TiO2-DHICA melanin nanostructures have been produced, showing biocide activity even under visible light conditions (activation). Hybrid nanostructures have been analyzed and characterized by multiple techniques, proving that both organic and inorganic phases strongly affect each other during in situ formation, as far as it concerns both morphology and microstructure, conferring unique biocide properties to the resulting nanomaterials. This strategy ensures much more far-reaching implementation in the synthesis of hybrid nanosystems, opening new perspectives in the design of multifunctional materials.

Silica Treatments: A Fire Retardant Strategy for Hemp Fabric/Epoxy Composites

In this paper, for the first time, inexpensive waterglass solutions are exploited as a new, simple and ecofriendly chemical approach for promoting the formation of a silica-based coating on hemp fabrics, able to act as a thermal shield and to protect the latter from heat sources. Fourier Transform Infrared (FTIR) and solid-state Nuclear Magnetic Resonance (NMR) analysis confirm the formation of –C–O–Si– covalent bonds between the coating and the cellulosic substrate. The proposed waterglass treatment, which is resistant to washing, seems to be very effective for improving the fire behavior of hemp fabric/epoxy composites, also in combination with ammonium polyphosphate. In particular, the exploitation of hemp surface treatment and Ammonium Polyphosphate (APP) addition to epoxy favors a remarkable decrease of the Heat Release Rate (HRR), Total Heat Release (THR), Total Smoke Release (TSR) and Specific Extinction Area (SEA) (respectively by 83%, 35%, 45% and 44%) as compared to untreated hemp/epoxy composites, favoring the formation of a very stable char, as also assessed by Thermogravimetric Analysis (TGA). Because of the low interfacial adhesion between the fabrics and the epoxy matrix, the obtained composites show low strength and stiffness; however, the energy absorbed by the material is higher when using treated hemp. The presence of APP in the epoxy matrix does not affect the mechanical behavior of the composites.

Learning From Nature: Bioinspired Strategies Towards Antimicrobial Nanostructured Systems

Microbial contamination still remains a major issue of the modern era, due to the widespread of drug-resistant pathogens. This has prompted researchers to come up with novel antimicrobial systems that could overcome antibiotic-resistance. In this context, nature can provide inestimable source of inspiration to design high-performance multifunctional materials with potent activity against drug-resistant pathogens. Actually, integrating the bio-inspired-approach with nanotechnology can provide cutting-edge solutions for drug-resistant infections. In this context, this review will examine recent advances in the development of bio-inspired antimicrobial nanostructures. Advantages of bioinspired approach to nanomaterials over conventional routes have been highlighted. Generally, bionspired synthesis can be carried out either by mimicking the functions of natural materials/ structures or by mimicking the biological processes employed to produce substances or materials. The review provides an overview of both strategies as applied to the synthesis of inorganic, organic as well as hybrid nanostructures. Antimicrobial efficacy and biological properties of these systems have been highlighted. Antimicrobial and antibiofouling nanostructured surfaces are also discussed.

Probing the Eumelanin–Silica Interface in Chemically Engineered Bulk Hybrid Nanoparticles for Targeted Subcellular Antioxidant Protection

We disclose herein the first example of stable monodispersed hybrid nanoparticles (termed MelaSil-NPs) made up of eumelanin biopolymer intimately integrated into a silica nanoscaffold matrix and endowed with high antioxidant and cytoprotective effects associated with a specific subcellular localization. MelaSil-NPs have been fabricated by an optimized sol-gel methodology involving ammonia-induced oxidative polymerization of a covalent conjugate of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) with 3-aminopropyltriethoxysilanes (APTS). They displayed a round-shaped (ca. 50-80 nm) morphology, exhibited the typical electron paramagnetic resonance signal of eumelanin biopolymers, and proved effective in promoting decomposition of hydrogen peroxide under physiologically relevant conditions. When administered to human ovarian cancer cells (A2780) or cervical cancer cells (HeLa), MelaSil-NPs were rapidly internalized and colocalized with lysosomes and exerted efficient protecting effects against hydrogen peroxide-induced oxidative stress and cytotoxicity.

A New Sol‐Gel Ru‐Nb‐Si Mixed‐Oxides Bifunctional Catalyst for the Hydrogenation of Levulinic Acid in Aqueous Phase

A mixed‐oxide nanomaterial with composition (RuO2)0.038(Nb2O5)0.024(SiO2)0.938 was prepared by a one‐pot sol–gel route. The synthesis was entirely performed at room temperature, by using easy‐to‐handle precursors and avoiding the employment of any toxic and/or polluting reactant. One of the samples was synthesised in the presence of a non‐ionic surfactant acting as both pore directing agent and metal complexing agent, obtaining a high‐specific‐surface‐area material characterized by a very good dispersion of the metallic species. In both cases, nanomaterials characterized by a complete reducibility of Ru species were obtained. The reduced nanomaterials acted as effective bifunctional catalysts in the hydrogenation of levulinic acid (LA) to γ‐valerolactone, performed under mild conditions in the aqueous phase. The catalyst prepared without surfactant showed good stability and no activity loss during all the performed cycles. By contrast, some deactivation phenomena took place with the surfactant‐templated catalyst. These phenomena were ascribed to both the partial surface oxidation and the surface adsorption of LA molecules giving deactivation during the second run.

Agglomeration-free silica NPs in dry storage for PBT nanocomposite

Dispersion of nanoparticles actually plays a key role in preparing high-performance nanocomposites. Within sol–gel procedures, the Stöber method is widely used to produce monodisperse systems of silica particles with controlled size and morphology. However, if stored as dried, the Stöber silica nanoparticles form stable agglomerates that no longer resuspend. Herein, we propose a novel straightforward methodology that overcomes the irreversible aggregation of particles, ultimately leading to a very good dispersion of the filler within the polymeric matrix without any coupling agent, even long time after their preparation. This synthesis approach has been exploited to produce PBT/SiO2 nanocomposites, as a model system. The produced nanocomposites have been analyzed and characterized by multiple techniques proving a fine dispersion of the filler within the matrix, as well as a significant increase in both thermal and dynamic mechanical properties. The proposed strategy ensures high compatibility with current industrial compounding facilities and far-reaching implementation in the preparation of polymer nanocomposites.Graphical Abstract

The fate of silica based Stöber particles soaked into growth media (RPMI and M254): A DLS and ζ-potential study.

Understanding the mechanisms involved in the interaction of biological systems with inorganic materials is of great importance and interest in both fundamental and applied disciplines in several different fields such as astrobiology, ecology, biology, biotechnology, engineering, and medicine. In this context, this paper investigates the interaction of biomacromolecules with submicrometric silica gel particles (NP) obtained through the Stöber method. Surprisingly, particles size reduction is observed after their dispersion into two different reconstituted growth media (RPMI and M254). This effect was related to the nature of the Stöber particles and the mechanism of their formation. The experimental results can be explained arguing that a biomacromolecule corona rapidly forms on NP incubated in both RPMI and M254 growth media. The results suggest that hydrolytic attack at incompletely condensed internal surface valence sites as well as interactions between NPs surface and the components of the growth media reverse the aggregation process, giving smaller disaggregated particles surrounded by a biomacromolecule corona. Moreover it was assessed that, at longer incubation time, the particles slightly grow probably due to interlocking of biomacromolecules in the corona. Furthermore, experimental results confirm that formation of this corona is a competitive and dynamic process. The present paper shows that the described effects (as size changes) are strongly dependent on the nature of the growth medium.

Bioinspired hybrid eumelanin–TiO2 antimicrobial nanostructures: the key role of organo–inorganic frameworks in tuning eumelanin's biocide action mechanism through membrane interaction

Intrinsic biocide efficacy of eumelanins can be markedly enhanced through a templated formation in the presence of a TiO2-sol, leading to hybrid TiO2–melanin nanostructures. However, mechanisms and processes behind biocide activity still remain poorly understood. This paper discloses the fundamental mechanism of action of these systems providing mechanistic information on their peculiar interaction with Escherichia coli strains. To this purpose biocide characterization is combined with Electron Paramagnetic Resonance (EPR) spectroscopy to investigate radical species produced by the hybrids as well as their interactions with Gram(−) external bacterial membranes. Experimental results indicate that TiO2 mediated eumelanin polymerization leads to a peculiar mechanism of action of hybrid nanostructures, whose strong interactions with bacterial membranes enhance the action of reactive oxygen species (ROS) produced by eumelanin degradation itself, also concurring with the final biocide action. These findings provide strategic information for the development of eumelanin-based systems with enhanced activity against drug-resistant strains.

From Nanocomposite to Blend Hybrid pHEMA/SiO2 Hydrogels: The Effect of Chemical Coupling on Thermal Stability, Swelling and Bioactivity

Hybrids of poly(2-hydroxyethylmethacrylate) (pHEMA), a polymer widely employed for biomedical applications, and silica gel, exhibiting a well-known bioactivity, were produced by in- situ sol-gel synthesis using two different procedures. Only Tetraethylortosilicate (TEOS) was used as inorganic precursor in the former, whereas both TEOS and methacrylate monomers bearing an alkoxysilyl unit, prepared by Michael addition of 2-hydroxyethylmethacrylate (HEMA) to 3-Aminopropyltriethoxysilane (APTS), were employed in the latter. A final concentration of 30% w/w of silica gel to the mass of polymer was obtained in both procedures. The samples prepared through the former route will be referred as T03, those obtained the latter route will be named as AT03. AT03 hybrids are optically transparent, whereas T03 samples show phase separation between the polymer and the silica gel. Synthesis procedure strongly affects hybrids structure: the silica phase mean diameter changes from 500 nm (T03) to about 50 nm when the coupling hybrid monomer is employed (AT03). Both hybrids keep swelling properties and show improved thermal stability than the as-prepared polymer, indeed AT03 exhibits the highest decomposition temperature and T03 show the highest swelling ratio. Moreover, silica gel has hydroxyapatite formed on both hybrids surface, when soaked in SBF.