Thermo-temporal physisorption in metal-organic frameworks probed by cyclic thermo-ellipsometry.

Temperature-induced sorption in porous materials is a well-known process. What is more challenging is to determine how the rate at which temperature is varied affects these processes. To address this question, we introduce a methodology called "cyclic thermo-ellipsometry" to explore the thermo-kinetics of vapor physisorption in metal-organic framework films.

Advanced Characterization Methodology to Unravel the Biodegradability of Metal-Organic Framework Nanoparticles in Extremely Diluted Conditions.

Porous iron(III) carboxylate metal-organic frameworks (MIL-100; MIL stands for Material of Institute Lavoisier) of submicronic size (nanoMOFs) have attracted a growing interest in the field of drug delivery due to their high drug payloads, excellent entrapment efficiencies, biodegradable character, and poor toxicity. However, only a few studies have dealt with the nanoMOF degradation mechanism, which is key to their biological applications. Complementary methods have been used here to investigate the degradation mechanism of Fe-based nanoMOFs under neutral or acidic conditions and in the presence of albumin. High-resolution STEM-HAADF coupled with energy-dispersive X-ray spectroscopy enabled the monitoring of the crystalline organization and elemental distribution during degradation. NanoMOFs were also deposited onto silicon substrates by dip-coating, forming stable thin films of high optical quality. The mean film thickness and structural changes were further monitored by IR ellipsometry, approaching the "sink conditions" occurring in vivo. This approach is essential for the successful design of biocompatible nano-vectors under extreme diluted conditions. It was revealed that while the presence of a protein coating layer did not impede the degradation process, the pH of the medium in contact with the nanoMOFs played a major role. The degradation of nanoMOFs occurred to a larger extent under neutral conditions, rapidly and homogeneously within the crystalline matrices, and was associated with the departure of their constitutive organic ligand. Remarkably, the nanoMOFs' particles maintained their global morphology during degradation.

Revealing the Dependency of Dye Adsorption and Photocatalytic Activity of ZnO Nanoparticles on Their Morphology and Defect States.

ZnO is an effective photocatalyst applied to the degradation of organic dyes in aqueous media. In this study, the UV-light and sunlight-driven photocatalytic activities of ZnO nanoparticles are evaluated. A handheld Lovibond photometer was purposefully calibrated in order to monitor the dye removal in outdoor conditions. The effect of ZnO defect states, i.e., the presence of zinc and oxygen defects on the photocatalytic activity was probed for two types of dyes: fuchsin and methylene blue. Three morphologies of ZnO nanoparticles were deliberately selected, i.e., spherical, facetted and a mix of spherical and facetted, ascertained via transmission electron microscopy. Aqueous and non-aqueous sol-gel routes were applied to their synthesis in order to tailor their size, morphology and defect states. Raman spectroscopy demonstrated that the spherical nanoparticles contained a high amount of oxygen vacancies and zinc interstitials. Photoluminescence spectroscopy revealed that the facetted nanoparticles harbored zinc vacancies in addition to oxygen vacancies. A mechanism for dye degradation based on the possible surface defects in facetted nanoparticles is proposed in this work. The reusability of these nanoparticles for five cycles of dye degradation was also analyzed. More specifically, facetted ZnO nanoparticles tend to exhibit higher efficiencies and reusability than spherical nanoparticles.

In Operando Spectroscopic Ellipsometry Investigation of MOF Thin Films for the Selective Capture of Acetic Acid.

The emission of polar volatile organic compounds (VOCs) is a major worldwide concern of air quality and equally impacts the preservation of cultural heritage (CH). The challenge is to design highly efficient adsorbents able to selectively capture traces of VOCs such as acetic acid (AA) in the presence of relative humidity (RH) normally found at storage in museums (40-80%). Although the selective capture of VOCs over water is still challenging, metal-organic frameworks (MOFs) possess highly tunable features (Lewis, Bronsted, or redox metal sites, functional groups, hydrophobicity, etc.) suitable to selectively capture a large variety of VOCs. In this context, we have explored the adsorption efficiency of a series of MOFs thin films (ZIF-8(Zn), MIL-101(Cr), and UiO-66(Zr)-2CF3) for the selective capture of AA based on a UV/vis and FT-IR spectroscopic ellipsometry in operando study (2-6% of relative pressure of AA under 40% of RH), namely conditions close to the realistic environmental storage conditions of cultural artifacts. For that purpose, optical quality thin films of MOFs were prepared by dip-coating, and their AA adsorption capacity and selectivity were evaluated under humid conditions by measuring the variation of the refractive index as a function of the vapor pressures while the chemical nature of the coadsorbed analytes (water and AA) was identified by FT-IR ellipsometry. While thin films of ZIF-8(Zn) strongly degraded upon exposure to AA/water vapors, films of MIL-101(Cr) and UiO-66(Zr)-2CF3 present a high chemical stability under those conditions. It was shown that MIL-101(Cr) presents a high AA adsorption capacity due to its high pore volume but exhibits a poor AA adsorption selectivity under humid conditions. In contrast, UiO-66(Zr)-2CF3 was shown to overpass MIL-101(Cr) in terms of AA/H2O adsorption selectivity and AA adsorption/desorption cycling stability because of its high hydrophobic character, suitable pore size for adequate confinement, and specific interactions.

Optical-Quality Thin Films with Tunable Thickness from Stable Colloidal Suspensions of Lanthanide Oxysulfide Nanoplates.

In modern laser technologies, there is a need for coatings that would be compatible with flexible substrates while retaining the advantages of inorganic compounds in terms of robustness. As a first step in this direction, we developed here thin films of lanthanide oxysulfide, of optical quality, prepared by low-temperature dip coating. As a model compound in the family of oxysulfides, (Gd,Ce)2O2S anisotropic nanoplates were used. The films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and in situ UV and IR spectroscopic ellipsometry, showing that the band gap of the materials was preserved through the deposition process. The thickness of the films was tuned in a broad range, from a few nanometers to 150 nm, using different concentrations of the colloidal suspensions as well as single-layer and multilayer deposition. Lastly, thermal treatment of the thin films was optimized to remove the stabilizing organic ligands of the nanoparticles while preserving their integrity, as confirmed by SEM and XRD.

High-Entropy-Alloy Nanocrystal Based Macro- and Mesoporous Materials.

High-entropy-alloy (HEA) nanoparticles are attractive for several applications in catalysis and energy. Great efforts are currently devoted to establish composition-property relationships to improve catalytic activity or selectivity. Equally importantly, developing practical fabrication methods for shaping HEA-based materials into complex architectures is a key requirement for their utilization in catalysis. However, shaping nano-HEAs into hierarchical structures avoiding demixing or collapse remains a great challenge. Herein, we overcome this issue by introducing a simple soft-chemistry route to fabricate ordered macro- and mesoporous materials based on HEA nanoparticles, with high surface area, thermal stability, and catalytic activity toward CO oxidation. The process is based on spray-drying from an aqueous solution containing five different noble metal precursors and polymer latex beads. Upon annealing, the polymer plays a double role: templating and reducing agent enabling formation of HEA nanoparticle-based porous networks at only 350 °C. The formation mechanism and the stability of the macro- and mesoporous materials were investigated by a set of in situ characterization techniques; notably, in situ transmission electron microscopy unveiled that the porous structure is stable up to 800 °C. Importantly, this process is green, scalable, and versatile and could be potentially extended to other classes of HEA materials.

MIL-100(Fe) Sub-Micrometric Capsules as a Dual Drug Delivery System.

Nanoparticles of metal-organic frameworks (MOF NPs) are crystalline hybrid micro- or mesoporous nanomaterials that show great promise in biomedicine due to their significant drug loading ability and controlled release. Herein, we develop porous capsules from aggregate of nanoparticles of the iron carboxylate MIL-100(Fe) through a low-temperature spray-drying route. This enables the concomitant one-pot encapsulation of high loading of an antitumor drug, methotrexate, within the pores of the MOF NPs, and the collagenase enzyme (COL), inside the inter-particular mesoporous cavities, upon the formation of the capsule, enhancing tumor treatment. This association provides better control of the release of the active moieties, MTX and collagenase, in simulated body fluid conditions in comparison with the bare MOF NPs. In addition, the loaded MIL-100 capsules present, against the A-375 cancer cell line, selective toxicity nine times higher than for the normal HaCaT cells, suggesting that MTX@COL@MIL-100 capsules may have potential application in the selective treatment of cancer cells. We highlight that an appropriate level of collagenase activity remained after encapsulation using the spray dryer equipment. Therefore, this work describes a novel application of MOF-based capsules as a dual drug delivery system for cancer treatment.

Periodic Nanoporous Inorganic Patterns Directly Made by Self-Ordering of Cracks.

Solution-processed inorganic nanoporous films are key components for the vast spectrum of applications ranging from dew harvesting to solar cells. Shaping them into complex architectures required for advanced functionality often needs time-consuming or expensive fabrication. In this work, crack formation is harnessed to pattern porous inorganic films in a single step and without using lithography. Aqueous inks, containing inorganic precursors and polymeric latexes enable evaporation-induced, defect-free periodic arrays of cracks with tunable dimensions over several centimeters. The ink formulation strategy is generalized to more than ten inorganic materials including simple and binary porous oxide and metallic films covering a whole spectrum of properties including insulating, photocatalytic, electrocatalytic, conductive, or electrochromic materials. Notably, this approach enables 3D self-assembly of cracks by stacking several layers of different compositions, yielding periodic assemblies of polygonal shapes and Janus-type patterns. The crack patterned periodic arrays of nanoporous TiO2 diffract light, and are used as temperature-responsive diffraction grating sensors. More broadly, this method represents a unique example of a self-assembly process leading to long-range order (over several centimeters) in a robust and controlled way.

Block-Copolymers Enable Direct Reduction and Structuration of Noble Metal-Based Films.

Noble metal nanostructured films are of great interest for various applications including electronics, photonics, catalysis, and photocatalysis. Yet, structuring and patterning noble metals, especially those of the platinum group, is challenging by conventional nanofabrication. Herein, an approach based on solution processing to obtain metal-based films (rhodium, ruthenium (Ru) or iridium in the presence of residual organic species) with nanostructuration at the 20 nm-scale is introduced. Compared to existing approaches, the dual functionality of block-copolymers acting both as structuring and as reducing agent under inert atmosphere is exploited. A set of in situ techniques has allowed for the capturing of the carbothermal reduction mechanism occurring at the hybrid organic/inorganic interface. Differently from previous literature, a two-step reduction mechanism is unveiled with the formation of a carbonyl intermediate. From a technological point of view, the materials can be solution-processed on a large scale by dip-coating as polymers and simultaneously structured and reduced into metals without requiring expensive equipment or treatments in reducing atmosphere. Importantly, the metal-based films can be patterned directly by block-copolymer lithography or by soft-nanoimprint lithography on various substrates. As proof-of-concept of application, the authors demonstrate that nanostructured Ru films can be used as efficient catalysts for H2 generation into microfluidic reactors.

Metal-Organic Framework Photonic Balls: Single Object Analysis for Local Thermal Probing.

Due to their high porosity and chemical versatility, metal-organic frameworks (MOFs) exhibit physical properties appealing for photonic-based applications. While several MOF photonic structures have been reported, examples of applications thereof are mainly limited to chemical sensing. Herein, the range of application of photonic MOFs is extended to local thermal and photothermal sensing by integrating them into a new architecture: MOF photonic balls. Micrometric-sized photonic balls are made of monodispersed MOFs colloids that are self-assembled via spray-drying, a low-cost, green, and high-throughput method. The versatility of the process allows tuning the morphology and the composition of photonic balls made of several MOFs and composites with tailored optical properties. X-ray nanotomography and environmental hyperspectral microscopy enable analysis of single objects and their evolution in controlled atmosphere and temperature. Notably, in presence of vapors, the MOF photonic balls act as local, label-free temperature probes. Importantly, compared to other thermal probes, the temperature detection range of these materials can be adjusted "on-demand." As proof of concept, the photonic balls are used to determine local temperature profiles around a concentrated laser beam. More broadly, this work is expected to stimulate new research on the physical properties of photonic MOFs providing new possibilities for device fabrication.

The origin of the high electrochemical activity of pseudo-amorphous iridium oxides.

Combining high activity and stability, iridium oxide remains the gold standard material for the oxygen evolution reaction in acidic medium for green hydrogen production. The reasons for the higher electroactivity of amorphous iridium oxides compared to their crystalline counterpart is still the matter of an intense debate in the literature and, a comprehensive understanding is needed to optimize its use and allow for the development of water electrolysis. By producing iridium-based mixed oxides using aerosol, we are able to decouple the electronic processes from the structural transformation, i.e. Ir oxidation from IrO2 crystallization, occurring upon calcination. Full characterization using in situ and ex situ X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy allows to unambiguously attribute their high electrochemical activity to structural features and rules out the iridium oxidation state as a critical parameter. This study indicates that short-range ordering, corresponding to sub-2nm crystal size for our samples, drives the activity independently of the initial oxidation state and composition of the calcined iridium oxides.

Biomimetic Tough Gels with Weak Bonds Unravel the Role of Collagen from Fibril to Suprafibrillar Self-Assembly.

Biological tissues rich in type I collagen exhibit specific hierarchical fibrillar structures together with remarkable mechanical toughness. However, the role of collagen alone in their mechanical response at different structural levels is not fully understood. Here, it is proposed to rationalize such challenging interplay from a materials science perspective through the subtle control of this protein self-assembly in vitro. It is relied on a spray-processing approach to readily use the collagen phase diagram and set a palette of biomimetic self-assembled collagen gels in terms of suprafibrillar organization. Their mechanical responses unveil the involvement of mechanisms occurring either at fibrillar or suprafibrillar scales. Noticeably, both modulus at early stage of deformations and tensile toughness probe the suprafibrillar organization, while durability under cyclic loading and stress relaxation reflect mechanisms at the fibril level. By changing the physicochemical environment, the interfibrillar interactions are modified toward more biomimetic mechanical responses. The possibility of making tissue-like materials with versatile compositions and toughness opens perspectives in tissue engineering.

Replacing Metals with Oxides in Metal-Assisted Chemical Etching Enables Direct Fabrication of Silicon Nanowires by Solution Processing.

Metal-assisted chemical etching (MACE) has emerged as an effective method to fabricate high aspect ratio nanostructures. This method requires a catalytic mask that is generally composed of a metal. Here, we challenge the general view that the catalyst needs to be a metal by introducing oxide-assisted chemical etching (OACE). We perform etching with metal oxides such as RuO2 and IrO2 by transposing materials used in electrocatalysis to nanofabrication. These oxides can be solution-processed as polymers exhibiting similar capabilities of metals for MACE. Nanopatterned oxides can be obtained by direct nanoimprint lithography or block-copolymer lithography from chemical solution on a large scale. High aspect ratio silicon nanostructures were obtained at the sub-20 nm scale exclusively by cost-effective solution processing by halving the number of fabrication steps compared to MACE. In general, OACE is expected to stimulate new fundamental research on chemical etching assisted by other materials, providing new possibilities for device fabrication.

Nanoconfined water vapour as a probe to evaluate plasmonic heating.

Engineering photothermal effects in plasmonic materials is of paramount importance for many applications, such as cancer therapy, chemical synthesis, cold catalysis and, more recently, metasurfaces. The evaluation of plasmonic heating at the nanoscale is challenging and generally requires sophisticated equipments and/or temperature-sensitive probes such as fluorescent molecules or materials. Here, we propose to use water vapor as a probe to evaluate the local heating around plasmonic nanoparticles. We demonstrate the concept for the case of a plasmonic colloidal film characterized by bi-modal nanoporosity. In particular, we exploit the thermal and light water liquid-vapor phase transitions taking place in the nanoporous medium that can be triggered by external stimuli, such as heating or irradiation, to obtain structural and optical variations in the film. The local temperature is then estimated using spectroscopic ellipsometry data acquired by a multimodal chamber. More generally, this method offers a simple and general approach to determine the local temperature that only requires a nanoporous material and water vapor, such as environmental humidity. In addition, this approach can be further generalized to other materials, vapor molecules or optical techniques.

Following in Situ the Degradation of Mesoporous Silica in Biorelevant Conditions: At Last, a Good Comprehension of the Structure Influence.

Mesoporous silica nanoparticles (MSNs) have seen a fast development as drug delivery carriers thanks to their tunable porosity and high loading capacity. The employ of MSNs in biomedical applications requires a good understanding of their degradation behavior both to control drug release and to assess possible toxicity issues on human health. In this work, we study mesoporous silica degradation in biologically relevant conditions through in situ ellipsometry on model mesoporous nanoparticle or continuous thin films, in buffer solution and in media containing proteins. In order to shed light on the structure/dissolution relationship, we performed dissolution experiments far from soluble silicate species saturation. Via a complete decorrelation of dissolution and diffusion contributions, we proved unambiguously that surface area of silica vectors is the main parameter influencing dissolution kinetics, while thermal treatment and open mesoporous network architecture have a minor impact. As a logical consequence of our dissolution model, we proved that the dissolution lag-time can be promoted by selective blocking of the mesopores that limits the access to the mesoporous internal surface. This study was broadened by studying the impact of the organosilanes in the silica structure, of the presence of residual structuring agents, and of the chemical composition of the dissolution medium. The presence of albumin at blood concentration was found affecting drastically the dissolution kinetics of the mesoporous structure, acting as a diffusion barrier. Globally, we could identify the main factors affecting mesoporous silica materials degradation and proved that we can tune their structure and composition for adjusting dissolution kinetics in order to achieve efficient drug delivery.

Self-Assembled Collagen Microparticles by Aerosol as a Versatile Platform for Injectable Anisotropic Materials.

Extracellular matrices (ECM) rich in type I collagen exhibit characteristic anisotropic ultrastructures. Nevertheless, working in vitro with this biomacromolecule remains challenging. When processed, denaturation of the collagen molecule is easily induced in vitro avoiding proper fibril self-assembly and further hierarchical order. Here, an innovative approach enables the production of highly concentrated injectable collagen microparticles, based on collagen molecules self-assembly, thanks to the use of spray-drying process. The versatility of the process is shown by performing encapsulation of secretion products of gingival mesenchymal stem cells (gMSCs), which are chosen as a bioactive therapeutic product for their potential efficiency in stimulating the regeneration of a damaged ECM. The injection of collagen microparticles in a cell culture medium results in a locally organized fibrillar matrix. The efficiency of this approach for making easily handleable collagen microparticles for encapsulation and injection opens perspectives in active tissue regeneration and 3D bioprinted scaffolds.

Interdiffusive Surfactant Procedure for the Preparation of Nanoarchitectured Porous Films: Application to the Growth of Titania Thin Films on Silicon Substrates.

Herein is reported the preparation of nanostructured mesoporous supported films, in this case, titanium dioxide nanoparticles on silicon wafer, according to a new approach taking place in two consecutive deposition steps: (i) coating of a homogeneous and continuous layer of a surfactant on the selected support and (ii) building up of a second layer of the fresh metal-oxide gel precursor, followed by thermal treatment to generate porosity. This approach represents an alternative way to soft-template procedures, as for instance, the largely applied evaporation-induced self-assembly (EISA) method, which typically consists of a single-step deposition of the mixture of gel precursor and surfactant used as a soft template to create porosity. The main advantage of the procedure reported here compared to the EISA method is the possibility of reaching tunable textural characteristics along the growing film (pore size, shape, and distribution of pores) by using gels with nanoparticles preformed at different stages via a simple regulation of the residence time of the precursors deposited on the support containing the surfactant.

Hollow zeolite microspheres as a nest for enzymes: a new route to hybrid heterogeneous catalysts.

In the field of heterogeneous catalysis, the successful integration of enzymes and inorganic catalysts could pave the way to multifunctional materials which are able to perform advanced cascade reactions. However, such combination is not straightforward, for example in the case of zeolite catalysts for which enzyme immobilization is restricted to the external surface. Herein, this challenge is overcome by developing a new kind of hybrid catalyst based on hollow zeolite microspheres obtained by the aerosol-assisted assembly of zeolite nanocrystals. The latter spheres possess open entry-ways for enzymes, which are then loaded and cross-linked to form cross-linked enzyme aggregates (CLEAs), securing their entrapment. This controlled design allows the combination of all the decisive features of the zeolite with a high enzyme loading. A chemo-enzymatic reaction is demonstrated, where the structured zeolite material is used both as a nest for the enzyme and as an efficient inorganic catalyst. Glucose oxidase (GOx) ensures the in situ production of H2O2 subsequently utilized by the TS-1 zeolite to catalyze the epoxidation of allylic alcohol toward glycidol. The strategy can also be used to entrap other enzymes or combination of enzymes, as demonstrated here with combi-CLEAs of horseradish peroxidase (HRP) and glucose oxidase. We anticipate that this strategy will open up new perspectives, leveraging on the spray-drying (aerosol) technique to shape microparticles from various nano-building blocks and on the entrapment of biological macromolecules to obtain new multifunctional hybrid microstructures.

Aerosol processing: a wind of innovation in the field of advanced heterogeneous catalysts.

Aerosol processing is long known and implemented industrially to obtain various types of divided materials and nanomaterials. The atomisation of a liquid solution or suspension produces a mist of aerosol droplets which can then be transformed via a diversity of processes including spray-drying, spray pyrolysis, flame spray pyrolysis, thermal decomposition, micronisation, gas atomisation, etc. The attractive technical features of these aerosol processes make them highly interesting for the continuous, large scale, and tailored production of heterogeneous catalysts. Indeed, during aerosol processing, each liquid droplet undergoes well-controlled physical and chemical transformations, allowing for example to dry and aggregate pre-existing solid particles or to synthesise new micro- or nanoparticles from mixtures of molecular or colloidal precursors. In the last two decades, more advanced reactive aerosol processes have emerged as innovative means to synthesise tailored-made nanomaterials with tunable surface properties, textures, compositions, etc. In particular, the "aerosol-assisted sol-gel" process (AASG) has demonstrated tremendous potential for the preparation of high-performance heterogeneous catalysts. The method is mainly based on the low-cost, scalable, and environmentally benign sol-gel chemistry process, often coupled with the evaporation-induced self-assembly (EISA) concept. It allows producing micronic or submicronic, inorganic or hybrid organic-inorganic particles bearing tuneable and calibrated porous structures at different scales. In addition, pre-formed nanoparticles can be easily incorporated or formed in a "one-pot" bottom-up approach within the porous inorganic or hybrid spheres produced by such spray drying method. Thus, multifunctional catalysts with tailored catalytic activities can be prepared in a relatively simple way. This account is an overview of aerosol processed heterogeneous catalysts which demonstrated interesting performance in various relevant chemical reactions like isomerisation, hydrogenation, olefin metathesis, pollutant total oxidation, selective oxidation, CO2 methanation, etc. A short survey of patents and industrial applications is also presented. Our objective is to demonstrate the tremendous possibilities offered by the coupling between bottom up synthesis routes and these aerosol processing technologies which will most probably represent a major route of innovation in the mushrooming field of catalyst preparation research.

Dynamic Shaping of Femtoliter Dew Droplets.

Herein, we show that wetting properties such as giant wetting anisotropy and dynamic shaping can be observed when femtoliter (submicron scale) dew droplets are condensed on nanopatterned mildly hydrophilic surfaces. Large-scale, optically transparent, nanopatterned TiO2 surfaces were fabricated by direct nanoimprinting lithography of sol-gel-derived films. Square, infinitely elongated, or circular droplets were obtained with square, line, or concentric patterns, respectively, and were visualized in situ during formation and recession using optical microscopy and environmental scanning electronic microscopy. We first describe how extremely elongated droplets could form on mildly hydrophilic surfaces, naturally contaminated in real environmental conditions. In this configuration, the dew droplet shape can be dynamically and reversibly varied by controlling the out-of-equilibrium conditions associated with condensation/evaporation kinetics. As an example of the application, we propose a "morphological" sensor that exploits the shape of the dew droplets as a transduction mode for detecting organic vapors in the outer atmosphere. Importantly, this study is underlining that environmentally stable, purely hydrophilic surfaces can be smartly engineered to induce wetting phenomena at very small scale never observed so far for hydrophobic or heterogeneous surfaces. Our versatile approach based on nanoimprinted, transparent sol-gel films could open great perspectives for the implementation of environmentally stable, mildly hydrophilic materials for "dew engineering" applications such as open microfluidics, fuming for fingerprints, vapor sensing, or water harvesting on glass windows, for instance.