Hybrid Materials: A Metareview

The field of hybrid materials has grown so wildly in the last 30 years that writing a comprehensive review has turned into an impossible mission. Yet, the need for a general view of the field remains, and it would be certainly useful to draw a scientific and technological map connecting the dots of the very different subfields of hybrid materials, a map which could relate the essential common characteristics of these fascinating materials while providing an overview of the very different combinations, synthetic approaches, and final applications formulated in this field, which has become a whole world. That is why we decided to write this metareview, that is, a review of reviews that could provide an eagle's eye view of a complex and varied landscape of materials which nevertheless share a common driving force: the power of hybridization.

The making of natural iron sulfide nanoparticles in a hot vent snail

Biomineralization in animals exclusively features oxygen-based minerals with a single exception of the scaly-foot gastropod Chrysomallon squamiferum, the only metazoan with an iron sulfide skeleton. This unique snail inhabits deep-sea hot vents and possesses scales infused with iron sulfide nanoparticles, including pyrite, giving it a characteristic metallic black sheen. Since the scaly-foot is capable of making iron sulfide nanoparticles in its natural habitat at a relatively low temperature (∼15 °C) and in a chemically dynamic vent environment, elucidating its biomineralization pathways is expected to have significant industrial applications for the production of metal chalcogenide nanoparticles. Nevertheless, this biomineralization has remained a mystery for decades since the snail's discovery, except that it requires the environment to be rich in iron, with a white population lacking in iron sulfide known from a naturally iron-poor locality. Here, we reveal a biologically controlled mineralization mechanism employed by the scaly-foot snail to achieve this nanoparticle biomineralization, through δ³⁴ S measurements and detailed electron-microscopic investigations of both natural scales and scales from the white population artificially incubated in an iron-rich environment. We show that the scaly-foot snail mediates biomineralization in its scales by supplying sulfur through channel-like columns in which reaction with iron ions diffusing inward from the surrounding vent fluid mineralizes iron sulfides.

Morphology-dependent pH-responsive release of hydrophilic payloads using biodegradable nanocarriers

The development of functional nanocarriers with stimuli-responsive properties has advanced tremendously to serve biomedical applications such as drug delivery and regenerative medicine. However, the development of biodegradable nanocarriers that can be loaded with hydrophilic compounds and ensure its controlled release in response to changes in the surrounding environment still remains very challenging. Herein, we achieved such demands via the preparation of aqueous core nanocapsules using a base-catalyzed interfacial reaction employing a diisocyanate monomer and functional monomers/polymers containing thiol and hydroxyl functionalities at the droplet interface. pH-responsive poly(thiourethane–urethane) nanocarriers with ester linkages were synthesized by incorporating polycaprolactone diol, which is susceptible to hydrolytic degradation via ester linkages, as a functional monomer in the reaction formulation. We could demonstrate that by systematically varying the number of biodegradable segments, the morphology of the nanocarriers can be tuned without imparting the efficient encapsulation of hydrophilic payload (>85% encapsulation efficiency) and its transfer from organic to aqueous phase. The developed nanocarriers allow for a fast release of hydrophilic payload that depends on pH, the number of biodegradable segments and nanocarrier morphology. Succinctly put, this study provides important information to develop pH-responsive nanocarriers with tunable morphology, using interfacial reactions in the inverse miniemulsion process, by controlling the number of degradable segments to adjust the release profile depending on the type of application envisaged.

The morphology and release properties of aqueous core nanocapsules for the pH-responsive release of hydrophilic payload was investigated by systematically varying the number of biodegradable segments.

Interfacial thiol–isocyanate reactions for functional nanocarriers: a facile route towards tunable morphologies and hydrophilic payload encapsulation

Design of nanocarriers with tunable morphology using the in situ thiol–isocyanate interfacial reactions.

Simple, common but functional: biocompatible and luminescent rare-earth doped magnesium and calcium hydroxides from miniemulsion

Highly luminescent Ca(OH)₂:Ln and Mg(OH)₂:Ln (Ln = Eu^III, Sm^III, Tb^III, Mg(Ca)/Ln = 20 : 1) nanostructures were obtained in inverse miniemulsion by exploiting the droplets to spatially confine the hydroxide precipitation in basic environment.

UV-induced functionalization of poly(divinylbenzene) nanoparticlesvia efficient [2 + 2]-photocycloadditions

A facile postmodification strategy for the surface functionalization of nanoparticles is presented based on [2 + 2] photoconjugation with particles made from miniemulsion polymerization of divinylbenzene.

Development of star polymers as unimolecular containers for nanomaterials

Star polymers containing one central core surrounded by multiple radiating arms represent an intriguing type of globular platform to be used as unimolecular containers and reactors. The core domain can encapsulate guest "cargos", whereas protective shell and chain ends can be functionalized with reactive groups and ligands. This Feature Article highlights the recent development on using core-shell structured amphiphilic star polymers as unimolecular containers for applications in drug delivery, catalysis, and template of hybrid nanomaterials. As compared with dendrimers, star polymers enjoy advantages of facile synthesis, flexible compositions, and tunable sizes, which allow them being able to carry more and multiple "cargos" within one molecule.

Toward designer magnetite/polystyrene colloidal composite microspheres with controllable nanostructures and desirable surface functionalities

An effective method was developed for synthesizing magnetite/polymer colloidal composite microspheres with controllable variations in size and shape of the nanostructures and desirable interfacial chemical functionalities, using surfactant-free seeded emulsion polymerization with magnetite (Fe(3)O(4)) colloidal nanocrystal clusters (CNCs) as the seed, styrene (St) as the monomer, and potassium persulfate (KPS) as the initiator. The sub-micrometer-sized citrate-acid-stabilized Fe(3)O(4) CNCs were first obtained via ethylene glycol (EG)-mediated solvothermal synthesis, followed by 3-(trimethoxysilyl)propyl methacrylate (MPS) modification to immobilize the active vinyl groups onto the surfaces, and then the hydrophobic St monomers were polymerized at the interfaces to form the polymer shells by seeded emulsion radical polymerization. The morphology of the composite microspheres could be controlled from raspberry- and flower-like shapes, to eccentric structures by simply adjusting the feeding weight ratio of the seed to the monomer (Fe(3)O(4)/St) and varying the amount of cross-linker divinyl benzene (DVB). The morphological transition was rationalized by considering the viscosity of monomer-swollen polymer matrix and interfacial tension between the seeds and polymer matrix. Functional groups, such as carboxyl, hydroxyl, and epoxy, can be facilely introduced onto the composite microspheres through copolymerization of St with other functional monomers. The resultant microspheres displayed a high saturation magnetization (46 emu/g), well-defined core-shell nanostructures, and surface chemical functionalities, as well as a sustained colloidal stability, promising for further biomedical applications.

Gold nanoparticles generated by thermolysis of "all-in-one" gold(I) carboxylate complexes

Consecutive synthesis methodologies for the preparation of the gold(I) carboxylates [(Ph(3)P)AuO(2)CCH(2)(OCH(2)CH(2))(n)OCH(3)] (n = 0-6) (6a-g) are reported, whereby selective mono-alkylation of diols HO(CH(2)CH(2)O)(n)H (n = 0-6), Williamson ether synthesis and metal carboxylate (Ag, Au) formation are the key steps. Single crystal X-ray diffraction studies of 6a (n = 0) and 6b (n = 1) were carried out showing that the P-Au-O unit is essentially linear. These compounds were applied in the formation of gold nanoparticles (NP) by a thermally induced decomposition process and hence the addition of any further stabilizing and reducing reagents, respectively, is not required. The ethylene glycol functionalities, providing multiple donating capabilities, are able to stabilise the encapsulated gold colloids. The dependency of concentration, generation time and ethylene glycol chain lengths on the NP size and size distribution is discussed. Characterisation of the gold colloids was performed by TEM, UV/Vis spectroscopy and electron diffraction studies revealing that Au NP are formed with a size of 3.3 (±0.6) to 6.5 (±0.9) nm in p-xylene with a sharp size distribution. Additionally, a decomposition mechanism determined by TG-MS coupling experiments of the gold(i) precursors is reported showing that 1(st) decarboxylation occurs followed by the cleavage of the Au-PPh(3) bond and finally release of ethylene glycol fragments to give Au-NP and the appropriate organics.