Hierarchy of Hybrid Materials-The Place of Inorganics- in-Organics in it, Their Composition and Applications

Abstract

Hybrid materials, or hybrids incorporating both organic and inorganic constituents, are emerging as a very potent and promising class of materials due to the diverse, but complementary nature of the properties inherent of these different classes of materials. The complementarity leads to a perfect synergy of properties of desired material and eventually an end-product. The diversity of resultant properties and materials used in the construction of hybrids, leads to a very broad range of application areas generated by engaging very different research communities. We provide here a general classification of hybrid materials, wherein organics-in-inorganics (inorganic materials modified by organic moieties) are distinguished from inorganics-in-organics (organic materials or matrices modified by inorganic constituents). In the former area, the surface functionalization of colloids is distinguished as a stand-alone sub-area. The latter area-functionalization of organic materials by inorganic additives-is the focus of the current review. Inorganic constituents, often in the form of small particles or structures, are made of minerals, clays, semiconductors, metals, carbons, and ceramics. They are shown to be incorporated into organic matrices, which can be distinguished as two classes: chemical and biological. Chemical organic matrices include coatings, vehicles and capsules assembled into: hydrogels, layer-by-layer assembly, polymer brushes, block co-polymers and other assemblies. Biological organic matrices encompass bio-molecules (lipids, polysaccharides, proteins and enzymes, and nucleic acids) as well as higher level organisms: cells, bacteria, and microorganisms. In addition to providing details of the above classification and analysis of the composition of hybrids, we also highlight some antagonistic yin-&-yang properties of organic and inorganic materials, review applications and provide an outlook to emerging trends.

Keywords: cells; hybrid; hydrogels; inorganic; lipids; nanoparticles; organic; polymers.

Graphical Abstract

Hybrid Inorganics-in-Organics Materials.

Figure 1

General classification of hybrid materials incorporating both organic and inorganic components. Functionalization of inorganic materials (the base material or matrix) by organic molecules, referred to as organics-in-organics, is shown on the left-hand side (shown in gray-dashed lines out outline the overall hierarchy of hybrids, but without being the focus of this research). Incorporation of inorganic constituents or components into organic materials (matrices) is referred to as inorganics-in-organics and is shown on the right-hand side (shown in solid dark lines, being the focus of this overview). The composition of inorganics-in-organics is outlined in a separate panel (right-hand side, in the middle). The bottom row depicts schematics of actual materials for each corresponding category of hybrids.

Figure 2

Classification of selected major classes of inorganic (left) and organic (right) components of hybrid materials as depicted by electron microscopy images. The inorganic constituents: minerals (SEM image of the calcium carbonate particles reproduced from Parakhonskiy et al., with permission Wiley-VCH), clays (TEM image of halloysites, reproduced from Fix et al., with permission Wiley-VCH), metals (TEM image of metal nanoparticles, reproduced from Simakin et al., with permission the ACS), semiconductors (TEM image of CdSe based nanocrystals, reproduced from Franzl et al., 2007), carbons (SEM image of carbon nanotubes, reproduced from Niazov-Elkan et al., with permission Wiley-VCH), ceramics (SEM images of TiO₂, which is used in ceramics and reproduced from Weir et al., with permission of the ACS). The organic matrices are represented by the following chemical: polymers (SEM image of the polycaprolactone scaffold reproduced from Savelyeva et al., with permission Wiley-Blackwell), hydrogels (an optical photograph of the Image of an DNA hydrogel removed from atubeonapipette tip reproduced from Xu et al., with permission Wiley-VCH), LbL (SEM image of a polyelectrolyte capsule reproduced from Bedard et al., with permission the Royal Society of Chemistry), brushes (AFM image of the brush polymer film, reproduced from Lemieux et al., with permission of the ACS), block copolymers [TEM image micelles formed by amphiphilic diblock co-polymer poly(ethylene glycol)-block-polystyrene-PS310 reproduced from (Geng et al., 2016) with permission of Wiley-VCH]; and biological: lipids (TEM image of liposomes, reproduced from Ruozi et al., with permission of Dove Medical Press), proteins (TEM image of the BSA, reproduced from Longchamp et al., with permission of the Natl. Acad. Sci.), carbohydrates (TEM image of pectin, reproduced from Hernandez-Cerdan et al., with permission of the ACS), nucleic acids (TEM image of DNA brick Cuboid structure assembly, reproduced from Wei et al., with permission from Wiley-VCH) materials.

Figure 3

Various modifications of organic matrices by inorganic components classified according to their applications. The left-hand schematics shows a more general range of applications of hybrid materials, in which inorganic constituents are added to organic matrices, including: biomineralization, biomimetics, retartation of flames, antibacterial properties and catalysis, fuel and solar cells, packaging and applications in dentistry, sensors and membranes, release from drug delivery vehicles, cells or delivery into cells, enhancement of mechanical properties, electrical and thermal conductivity. The right-hand images illustrate selected objects assembled by incorporating inorganic constituents in organic materials for: enhancement of mechanical properties (Optical image of the cell adhesion behavior and the film surface morphology for different AuNP surface coverage, reproduced from Schmidt et al., with permission of the ACS), sensoric functions (SEM image of BSE on hydroxyapatite with silver nanoparticles as SERS platform, reproduced from Parakhonskiy et al., with permission of Elsevier Science BV), electroconductivity (SEM images of the surface of CNT/PS nanocomposites, reproduced from Grossiord et al., with permission of Wiley-VCH), remote release by an external action of a laser (TEM images of the shell of the polyelectrolyte capsule with Ag-nanoparticles, reproduced from Skirtach et al., with permission of the ACS); biomimetics (SEM image of the Polycaprolactone scaffolds mineralized with vaterite, reproduced from Savelyeva et al., with permission of Wiley-Blackwell), catalysis (TEM images of poly(N-vinylcaprolactam-co- acetoacetoxyethyl methacrylate-co-acrylic acid) P(VCL-AAEM-AAc) microgels reproduced from (Agrawal et al., 2013) with permission of the Royal Society of Chemistry), flame retardation (SEM images of polyurethane foam, with 3-bilayer halloysite nanotubes coatings, reproduced from Smith et al., with permission from the Wiley VCH); packaging (SEM images of zein-Kaolin nanocomposites containing 2.5% Kaolin, reproduced from Arora and Padua, with permission from Wiley-Blackwell); solar cells (cross-sectional SEM image of a complete perovskite device, reproduced from Jeon et al., with permission of Nature Publish. Group).

Figure 4

Mechanical properties (Youngs' modulus) of various constituents of organic-inorganic hybrid materials in relation to those of cells, tissue, and organs. Data are based on Kuznetsova et al. (2007) and Moeendarbary and Harris (2014).

Figure 5

Antagonist (yin and yang), but complementary, properties of most common inorganic and organic compounds motivating their incorporation into hybrid materials.