Francisco del Monte

Freeze-Drying of Aqueous Solutions of Deep Eutectic Solvents: A Suitable Approach to Deep Eutectic Suspensions of Self-Assembled Structures

This work describes how the preparation of deep eutectic solvents (DES) in its pure state can be accomplished through a simple approach based on the freeze-drying of aqueous solutions of the individual counterparts of DES. DES in its pure state obtained via freeze-drying are studied by (1)H NMR, which reveals the formation of halide ion-hydrogen-bond-donor supramolecular complexes (characteristic of DES), and by cryo-etch-SEM, which provides insight about the capability of aqueous solutions of DES to be segregated in DES and ice upon freezing. The paper also explores the suitability of the freeze-drying approach to incorporate organic self-assemblies (in particular, liposomes of ca. 200 nm) in DES with full preservation of their self-assembled structure. This is not a trivial issue given that amphiphilic molecules tend to be readily dissolved (hence, disassembled) in DES. The strategy proposed in this work is based on the freeze-drying of aqueous solutions containing the individual counterparts of DES and the preformed liposomes (also known as large unilamellar vesicles or LUV). The simplicity of the method should also make it suitable for the incorporation of different self-assembled structures (such other types of vesicles and micelles) in DES in its pure state.

Deep eutectic solvents as both precursors and structure directing agents in the synthesis of nitrogen doped hierarchical carbons highly suitable for CO2 capture

Deep eutectic solvents (DESs) have been used in the synthesis of nitrogen-doped carbons exhibiting a hierarchical porous structure. The CO2 sorption capacity of these solid sorbents was extraordinary because of their relatively high nitrogen content and their bimodal porous structure where micropores provide high surface areas (ca. 700 m2 g−1) and macropores provide accessibility to such a surface. DESs were composed of resorcinol, 3-hydroxypyridine and choline chloride in 2 : 2 : 1 and 1 : 1 : 1 molar ratios. Polycondensation of resorcinol and 3-hydroxypyridine (with formaldehyde) promoted DES segregation in a spinodal-like decomposition process by the formation of a polymer rich phase and a depleted polymer phase. Thus, DESs played a multiple role in the synthetic process; the liquid medium that ensured reagents homogenization, the structure-directing agent that is responsible for the achievement of the hierarchical structure, and the source of carbon and nitrogen of the solid sorbent obtained after carbonization. Interestingly, the homogeneous incorporation of nitrogen at the solution stage of the synthetic process (rather than by post-treatment of the preformed carbon) allowed the achievement of significant nitrogen contents even in carbons obtained at relatively high temperatures (e.g. 8–12 at% for 600 °C and ca. 5 at% for 800 °C). It is worth noting that, despite thermal treatments at high temperatures tend to decrease the nitrogen content, the high surface area of the solid sorbents obtained at 800 °C contributed to a significant enhancement of CO2 capture while providing superior selectivity, recyclability and stability.

A photorefractive organically modified silica glass with high optical gain

Photorefractive materials exhibit a spatial modulation of the refractive index due to redistribution of photogenerated charges in an optically nonlinear medium. As such, they have the ability to manipulate light and are potentially important for optical applications including image processing, optical storage, programmable optical interconnects and simulation of neural networks. Photorefractive materials are generally crystals, polymers and glasses with electro-optic or birefringent properties and non-centrosymmetric structure. Here we report the photorefractive effect in both non-centrosymmetric and centrosymmetric azo-dye-doped silica glasses, in which refractive index gratings that are spatially phase-shifted with respect to the incident light intensity pattern are observed. The effect results from a non-local response of the material to optical illumination, and enables the transfer of energy between two interfering light beams (asymmetric two-beam coupling). Although the writing time for the present grating is relatively slow, we have achieved a two-beam coupling optical gain of 188 cm -1 in the centrosymmetric glasses, and a gain of 444 cm -1 in the non-centrosymmetric structures. The latter are fabricated using a corona discharge process to induce a permanent arrangement of azo-dye chromophores.

Block-Copolymer assisted synthesis of hierarchical carbon monoliths suitable as supercapacitor electrodes

Three dimensional (3D) hierarchical porous (micro-, meso- and macro-porous) carbon monoliths (HCMs) have recently been proposed as promising supercapacitor electrodes. In this work, we have further explored the use of block-copolymers as templates for the preparation of HCMs via condensation of resorcinol and formaldehyde (RF) and subsequent carbonization. The resulting HCMs exhibited a textured morphology consisting of a bicontinuous macroporous carbon network built of interconnected microporous carbon colloids, as demonstrated by nitrogen adsorption/desorption isotherms, mercury porosimetry and electron microscopy, in both scanning and transmission mode. Such a texture favored the performance of HCMs as supercapacitor electrodes, reaching remarkable values of capacitance of up to 198 F g−1 (normalized by mass) and 34.5 μF cm−2 (normalized by BET surface area). The first electrolyte infiltration into the micropore (prior capacitance measurements) was demonstrated to play a crucial role in the achievement of large capacitance values.

Deep Eutectic Solvents in Polymerizations: A Greener Alternative to Conventional Syntheses

The use of deep eutectic solvents (DESs) that act as all-in-one solvent-template-reactant systems offers an interesting green alternative to conventional syntheses in materials science. This Review aims to provide a comprehensive overview to emphasize the similarities and discrepancies between DES-assisted and conventional syntheses and rationalize certain green features that are common for the three DES-assisted syntheses described herein: one case of radical polymerization and two cases of polycondensations. For instance, DESs contain the precursor itself and some additional components that either provide certain functionality (e.g., drug delivery and controlled release, or electrical conductivity) to the resulting materials or direct their formation with a particular structure (e.g., hierarchical-type). Moreover, DESs provide a reaction medium, so polymerizations are ultimately carried out in a solventless fashion. This means that DES-assisted syntheses match green chemistry principles 2 and 5 because of the economy of reagents and solvents, whereas the functionality incorporated by the second component allows the need for any post-synthesis derivatization to be minimized or even fully avoided (principle 8). DESs also provide new precursors that favor more efficient polymerization (principle 6) by decreasing the energy input required for reaction progress. Finally, the use of mild reaction conditions in combination with the compositional versatility of DESs, which allows low-toxic components to be selected, is also of interest from the viewpoint of green chemistry because it opens up the way to design biocompatible and/or eco-friendly synthetic methods (principle 3).

Three-dimensional microchanelled electrodes in flow-through configuration for bioanode formation and current generation

Three-dimensional microchannelled nanocomposite electrodes fabricated by ice-segregation induced self-assembly of chitosan-dispersed multiwall carbon nanotubes are shown to provide a scaffold for growth of electroactive bacteria for use as acetate-oxidizing bioanodes in bioelectrochemical systems. The hierarchical structure provides a conductive surface area available for G. sulfurreducens colonization, with a flow through configuration along the electrode providing a substrate for bacterial colonization and bio-electrochemical processes. This configuration, whilst resulting in sub-monolayer biofilm coverage over the three-dimensional surface, is capable of providing acetate oxidation current densities of up to 24.5 A m−2, equating to a volumetric current density of 19 kA m−3, in the flow-through configuration. Such bioanodes, when operated in non-optimized flow-through microbial fuel cell configuration, provide a maximum power density of 2.87 W m−2, which is equivalent to 2.0 kW m−3 volumetric power density.

Biocompatible MWCNT scaffolds for immobilization and proliferation of E. coli

Ultralightweight (specific gravity 8.0 × 10–2) and highly conductive (1.4 S cm–1) MWCNT scaffolds exhibited remarkable biocompatibility for E. coli which allows for bacteria immobilization and proliferation within its microchanneled structure. The above-mentioned features make these scaffolds potentially useful as electrodes in microbial fuel cells (MFCs).

PPO15-PEO22-PPO15block copolymer assisted synthesis of monolithic macro- and microporous carbon aerogels exhibiting high conductivity and remarkable capacitance

Ultralightweight (specific gravity 5 × 10−2) and highly conductive (2.5 S/cm) monolithic carbon aerogels exhibiting a three-dimensionally continuous micro- and macroporous structure have been prepared through a PPO-PEO-PPO block copolymer assisted route. The resulting carbon aerogels were highly suitable as electrodes of electric double layer capacitors, with remarkable values of capacitance of up to 225 F/g (normalized by mass) and 31 µF/cm2 (normalized by BET surface area).

Phosphate-Functionalized Carbon Monoliths from Deep Eutectic Solvents and their Use as Monolithic Electrodes in Supercapacitors

Porous carbon materials have received considerable attention in the last years because of their outstanding performance as substrates in a number of separation and catalytic processes as well as electrodes in fuel cells, supercapacitors, and batteries. 2] In sustainable terms, these latter electrochemical processes are especially attractive for the future of our society because of their capability to either provide an alternative to combustion of fossil fuels for energy production (e.g. , fuel cells) or to store energy (e.g. , supercapacitors and batteries). Particularly interesting are those nanostructured porous carbon materials or those carbon materials showing three-dimensional (3D) hierarchical porous textures (containing pores at different scales, from micropores to mesopores, up to macropores) that combine high specific surface areas with proper channels, allowing good diffusion of any substance (e.g. , analytes, adsorbates, electrolytes, etc.) to the entire surface of the material. A number of synthetic routes using different carbonaceous precursors in the presence of either hard or soft templates has been explored to date with the aim to control the textural properties of the resulting carbon materials. Among them, the use of ionic liquids (ILs) and deep-eutectic solvents (DESs) is especially interesting in sustainable terms due to their recent application to a number of green processes, both synthetic and catalytic. Sustainability is emphasized with those ILs and DESs capable to play multiple roles (e.g. , from reaction medium to carbonaceous precursors up to structure-directing agents) in the synthetic process and hence, promoting a significant economy of reagents. Thus, ILs and DESs have proved efficient in tailoring not only the textural properties, but also the composition of the resulting carbon materials. For instance, nitrogen-functionalized carbon materials have exhibited a remarkable capacity for CO2 adsorption [9] and they hold much potential as electrodes for supercapacitors. With regard to this application, the use of phosphate-functionalized carbon materials open interesting perspectives because of the widening of the operational voltage window with aqueous electrolytes and the subsequent increase of the energy density that can be attained. However, neither ILnor DES-assisted synthesis have yet been described for the preparation of phosphate-functionalized carbon monoliths (PfCMs). Herein, we describe the preparation of PfCMs by polycondensation of formaldehyde with a resorcinol-based DES—composed of resorcinol, choline chloride, and glycerol—and using phosphoric acid as catalyst. The ternary DES was prepared in a closed container by the complexation of resorcinol, choline chloride, and glycerol in a molar ratio 1:1:1 at 90 8C (see the Experimental Section). The inset in Figure 1 a shows that the resulting DES is a viscous liquid (the static viscosity is 314 cP at 22 8C, as determined by

Deep-Eutectic-Solvent-Assisted Synthesis of Hierarchical Carbon Electrodes Exhibiting Capacitance Retention at High Current Densities

Nature provides a wide range of entities and/or systems with different functions that may serve as a source of bioinspiration for material chemists. Actually, materials exhibiting a 3D porous texture (combining pores at different scales, from macroto mesoup to micropores) mimic the hierarchical structure of different systems found in living organisms (e.g., the blood circulation or the respiratory system in mammalians). Structural organization at different scales is ultimately responsible for the outstanding properties offered by hierarchical materials (not only as stationary phases in separation and catalytic processes, but also as electrodes in fuel cells and capacitors) because they offer not only large surface areas, but also accessibility to such a surface. A number of synthetic routes have been explored by using different carbonaceous precursors and either exo or endo templates to modulate the porous texture of the resulting carbon structures. Recent efforts have also been focused on the preparation of porous carbon composites containing graphitic carbon entities (e.g., carbon nanotubes and nanohorns, or even graphene oxide) the challenge of which is double and resides in 1) the achievement of a homogenous dispersion of these entities throughout the monolith structure and 2) the preservation of high surface areas. Baumann and co-workers have recently performed a quite extensive and stimulating work on single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) synthesized by resorcinol–formaldehyde polycondensation. Particularly interesting in terms of conductivity and surface area were those composites based on DWCNTs, although the authors expressed the convenience of substituting the surfactants used for carbon-nanotube (CNT) dispersion. Ionic liquids (ILs) and deep eutectic solvents (DESs, a new class of ILs obtained by complexion of quaternary ammonium salts with hydrogen-bond donors, such as acids, amines, and alcohols) have lately been the solvent of choice in a number of chemical processes because of special features: for example, they are nonreactive with water, nonvolatile, and biodegradable as well as excellent solvents for a wide variety of solutes, such as different substrates, enzymes, and even microorganisms of catalytic and biocatalytic interest. Of particular interest for the purpose of this work are those processes for which the capability both as solvents for CNT dispersion and structure-directing agents in the synthesis of different materials was demonstrated. Actually, ILs and DESs have been used as solvents for preparation of CNT-based carbon composites and even as carbonaceous precursors of both nontextured and textured carbons. In particular, we have recently described the preparation of DESs based on mixtures of resorcinol and choline chloride, the rupture of which (via resorcinol polycondensation and subsequent segregation of choline chloride) resulted in the formation of bimodal porous carbons. In this case, hierarchy was obtained through a synthetic mechanism that combined aspects from those original works used for synthesis of zeolites (i.e., based on DES rupture and controlled delivery of an organic template to the reaction mixture) and those used for synthesis of macroporous structures (i.e. , based on spinodal-like processes) . It is also worth noting the “green” character of the process as a result of the absence of residues and/or byproducts eventually released after the synthetic process, that is, one of the components forming the DES (e.g., resorcinol) becomes the material itself, whereas the second one (e.g., choline chloride) is fully recovered and can be reused in subsequent reactions. Based on these previous results, we considered that the use of DESs could open an interesting path for the preparation of hierarchical porous CNT composites. Herein, we describe the preparation of hierarchical porous multiwalled CNT (MWCNT) composites exhibiting high surface areas and outstanding conductivities through furfuryl alcohol (FA) condensation catalyzed by a protic DES based on complexes of para-toluene sulfonic acid [a] Dr. M. C. Guti rrez, Dr. D. Carriazo, Dr. R. Jim nez, Dr. J. M. Rojo, Dr. M. L. Ferrer, Dr. F. del Monte Instituto de Ciencia de Materiales de Madrid-ICMM Consejo Superior de Investigaciones Cient ficas-CSIC Campus de Cantoblanco, 28049-Madrid (Spain) E-mail : mcgutierrez@icmm.csic.es delmonte@icmm.csic.es [b] Dr. A. Tamayo Instituto de Ceramica y Vidrio-ICV Consejo Superior de Investigaciones Cient ficas-CSIC. Campus de Cantoblanco, 28049-Madrid (Spain) [c] Dr. F. Pic Centro Nacional de Investigaciones Metalurgicas-CENIM Consejo Superior de Investigaciones Cient ficas-CSIC Av. Gregorio del Amo s/n, 28040-Madrid (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101679.