Ultrathin film hydrogels with controlled swelling and viscoelastic properties deposited by nanosecond pulsed plasma induced-polymerization

Development of ultrathin film (utf) hydrogels for cutting-edge biomedical applications (i.e. artificial skins) is receiving increasing attention. Nonetheless, achieving accurate control on the structure and thickness of utf-hydrogels becomes extremely complex when assessed through conventional techniques. In this work, an atmospheric-pressure plasma-assisted deposition technique is reported, showing great thickness accuracy and versatility, to design utf-hydrogels with customized properties. For the first time, specific and independent control on the generation and nature of cross-links by only changing the plasma exposure frequency (fPE) during the synthesis process are reported. Thus, utf-hydrogels are successfully prepared with tuned swelling ratios and viscoelastic properties (ranging from 150 to 20 kPa). Moreover, a thickness accuracy of 9 nm is reported, permitting the accurate synthesis of utf-hydrogels below 150 nm. Exhaustive structural and topographical analyses allow elucidating the effects of the fPE on the cross-link generation mechanism, discarding any undesired effect on the thickness accuracy. To support the structural results obtained, quartz-crystal microbalance with dissipation (QCM-D) coupled with spectroscopic ellipsometry are put in the spotlight as an efficient and viable alternative for the characterization of the resulting properties of ultrathin film soft materials, including the presence of a hydrated layer at the interface.This work was supported by the Luxembourg National Research Fund (FNR) (C19/MS/13641732). J.S. was funded by the Margarita Salas Fellowship (Ministerio de Universidades de España and the European Union, NextGenerationEU).Peer ReviewedPostprint (published version

Electrically polarized hydroxyapatite: influence of the polarization process on the microstructure and properties

Semipermanently polarized hydroxyapatite, named SP/HAp(w), is obtained by applying a constant dc electric field of 1–10 kV/cm at 300–850 °C to the samples previously sintered in water vapor, while permanently polarized hydroxyapatite, PP/HAp(a), is produced by applying a dc electric field of 3 kV/cm at 1000 °C to the samples sintered in air. SP/HAp(w) has been used for biomedical applications, while PP/HAp(a) has been proved to be a valuable catalyst for N2 and CO2 fixation. In this work, structural differences between SP/HAp(w) and PP/HAp(a) have been ascertained using Raman microscopy, wide-angle X-ray diffraction, scanning electronic microscopy, high-resolution transmission electron microscopy, and grazing incidence X-ray diffraction. Results prove the existence of crystal distortion in the form of amorphous calcium phosphate and ß-tricalcium phosphate (ß-TCP) phases close to the surface because of the atmosphere used in the sintering process. The existence of an amorphous layer in the surface and the phase transition through ß-TCP of SP/HAp(w) are the structural factors responsible for the differences with respect to PP/HAp(a). Moreover, a superstructure has been identified in PP/HAp(a) samples, which could be another structural factor associated with enhanced conductivity, permanent polarization, and catalytic activity of this materialPeer ReviewedPostprint (author's final draft

Polarized hydroxyapatite: wew insights and future perspectives through systematic electrical characterization at the interface

Design of hydroxyapatite (HAp) with customized electrical properties is of special interest for developing technological and biomedical applications with new improved functionalities. Polarized HAp, which is obtained by applying an external electric voltage at high temperature, has been successfully shown to be an alternative to doped-HAp that is limited by the biocompatibility of the dopants used. However, many aspects about such new material remain scarcely studied, as for example the relationship between the polarization conditions and the resultant electrical enhancement, hinder a solid progress in its application. In this work, polarized HAp has been extensively characterized using electrochemical impedance spectroscopy by means of proposing a unified electrical equivalent circuit model with physical sense. This allows to explain the properties of such material by separating the bulk and the interface contributions. Moreover, the limits of the polarization mechanism have been explored, enabling a precise control on the electrical resistivity of polarized HAp above or below the intrinsic resistivity of nonpolarized HAp. Overall, necessary insights on the polarization treatment have been reported, opening an appealing avenue for generating new biomedical and technological applications based on dopant-free polarized HAp.Peer ReviewedPostprint (published version

Permanently polarized hydroxyapatite for selective electrothermal catalytic conversion of carbon dioxide into ethanol

Conversion of CO2 into valuable chemicals is not only a very challenging topic but also a socially demanding issue. In this work, permanently polarized hydroxyapatite obtained using a thermal stimulated polarization process is proposed as a highly selective catalyst for green production of ethanol starting from CO2 and CH4.Peer ReviewedPostprint (author's final draft

Regulating the superficial vacancies and OH- orientations on polarized hydroxyapatite electrocatalysts

Smart designs of hydroxyapatite (HAp) materials with customized electrical properties are drawing increasing attention for their wide range of potential applications. Such enhanced electrical properties directly arise from the number and orientation of OH- groups in the HAp lattice. Although different polarization treatments have been proposed to enhance the final conductivity by generating vacancies at high temperatures and imposing specific OH- orientations through electric voltages, no direct measurement showing the evolution that OH- groups undergo has been described yet. In this article, the first direct empirical observation that allows the characterization of both the generation of vacancies and the polarization of OH- groups is reported. The mechanisms behind the electrical enhancement are elucidated allowing to distinguish between charge accumulation at the crystal grains, which is due to the formed vacancies, and charge accumulation in the boundaries of particles. In addition, a linear dependence between the number of vacancies and the superficial charge is observed. Therefore, it is demonstrated that the charge accumulation at the micrometric grain boundaries has a great impact on the catalytic properties of the thermally stimulated polarized HAp. These results will be used for further optimization of the catalyst properties.Peer ReviewedPostprint (author's final draft

Analysis of nitrogen fixation by a catalyst capable of transforming N2, CO2 and CH4 into amino acids under mild reactions conditions

The processes related to the fixation of nitrogen ina catalyst able to produce glycine and alanine from a N2, CO2and CH4gas mixture at mild reaction conditions have been studied by combining experimental and theoretical investigations.Results have allowed to understand the role of different elements of the catalyst, which is constituted by permanently polarized hydroxyapatite (p-HAp), zirconia, and aminotris(methylenephosphonic acid)(ATMP). ATMP attractsN2moleculestowards the surface,maintainingthem close to the zirconiaand p-HAp componentsthatare the most active from a catalytic point of view. On the other hand, the associative mechanismisthermodynamicallyfavouredunder mild reaction conditionswith respect to the dissociative one,whichis limited by the barrier associated to the N–Nbondcleavage. Because this reaction mechanism is similar to that employed in the nitrogen fixation bynitrogenase enzymes, thesefindingsprovide an opportunity to designnew bioinspired catalystsPostprint (author's final draft

Optimization of permanently polarized hydroxyapatite catalyst. Implications for the electrophotosynthesis of amino acids by nitrogen and carbon fixation

The enhanced catalytic activity of permanently polarized hydroxyapatite, which is achieved using a thermally stimulated polarization process, largely depends on both the experimental conditions used to prepare crystalline hydroxyapatite from its calcium and phosphate precursors and the polarization process parameters. A mineral similar to brushite, which is an apatitic phase that can evolve to hydroxyapatite, is found at the surface of highly crystalline hydroxyapatite. It appears after chemical precipitation and hydrothermal treatment performed at 150 °C for 24 h followed by a sinterization at 1000 °C and a polarization treatment by applying a voltage of 500 V at high temperature. Both the high crystallinity and the presence of brushite-like phase on the electrophotocatalyst affect the nitrogen and carbon fixation under mild reaction conditions (95 °C and 6 bar) and the synthesis of glycine and alanine from a simple gas mixture containing N2, CO2, CH4 and H2O. Thus, the Gly/Ala ratio can be customized by controlling the presence of brushite on the surface of the catalyst, enabling to develop new strategies to regulate the production of amino acids by nitrogen and carbon fixation.Peer ReviewedPostprint (author's final draft

Design of catalysts based on hydroxyapatite for carbon and nitrogen fixation

(English) Permanently polarized hydroxyapatite (p-HAp) has been recently proposed as a novel green catalyst. p-HAp is obtained by applying an electrical and thermal stimulation polarization process (TSP), which consists on a DC electric voltage of 500 Vat 1000 °C to previously sintered hydroxyapatite. The objective of this PhD Thesis has been to settle down a solid understanding of p-HAp through the investigation of its structure and physico-chemical features, that results in enhanced electrical and catalytical properties that have allowed to develop novel and outstanding applications in the field of green catalysis. We have focused the research in decarbonization and nitrogen conversion processes that result in high added value molecules. The TSP not only increases the crystallinity, reducing the defects at the crystal lattice, but also creates charges that accumulate at the crystalline boundaries and at the surface of microscopic grains, boosting the electrical conductivity. Hence, the utilization of p-HAp in the catalytic fixation of carbon and nitrogen from CO2, CH4 and N2 gases, has been explored reporting the successful formation of different products of chemical and industrial interest (e.g. amino acids, ethanol and ammonia) as a function of the reaction conditions (i.e. feeding gases and presence/absence of UV illumination). In addition, the catalyst plasticity has been explored to enhance the selectivity towards the desired reaction products. In conclusion, we reveal that p-HAp exhibits important advantages with respect to other consolidated catalysts, drastically increasing the final energetic net balance of the reactions, a fact that opens new avenues in the green chemistry catalysis field.(Català) La hidroxiapatita polaritzada permanentment (p-HAp) s'ha proposat recentment com a nou catalitzador verd. La p-HAp s'obté a partir d'un procés de polarització d'estimulació elèctrica i tèrmica (TSP), que consisteix en aplicar una tensió elèctrica constant (DC) de 500 V i 1000 °C a la hidroxiapatita prèviament sinteritzada. L'objectiu d'aquesta tesi doctoral ha estat establir una sòlida comprensió de la p-HAp mitjançant la investigació de la seva estructura i característiques fisicoquímiques, que es tradueixen en unes propietats elèctriques i catalítiques millorades, que a la vegada han permès desenvolupar noves i destacades aplicacions en el camp de la catàlisi verda. Hem centrat la recerca en processos de descarbonització i conversió de nitrogen que donen lloc a molècules d'alt valor afegit. El TSP no només augmenta la cristal·linitat, reduint els defectes a la xarxa cristallina, sinó que també crea càrregues que s'acumulen als límits cristal·lins i a la superfície dels grans microscòpies, augmentant la conductivitat elèctrica. Així doncs. s'ha explorat la utilització de la p-HAp en la fixació catalítica de carboni i nitrogen a partir dels gasos CO2, CH4 i N2, reportant la formació exitosa de diferents productes d’interès químic i industrial (per exemple, aminoàcids, etanol i amoníac) en funció de les condicions de reacció (es a dir. gasos d’alimentació i presència/absència d’il·luminació UV). A més, s'ha explorat la plasticitat del catalitzador per millorar la selectivitat cap als productes de reacció desitjats. En conclusió, postulem que la p-HAp presenta avantatges importants respecte a altres catalitzadors consolidats, augmentant dràsticament el balanç net energètic final de les reaccions, fet que obre noves vies en el camp de la catàlisi de la química verda.DOCTORAT EN POLÍMERS I BIOPOLÍMERS (Pla 2012

Ús de discos compactes com a nanoreactors per a aplicacions fotocatalítiques

This project presents a novel work developing cheap nanoreactors with Au/TiO2 photocatalyst for H2 production; using the morphology of compact discs (CD) as the fundamental matrices to set up nanochannels for the reactors. Nanochannels of 200 nm depth have been coated with the photocatalyst using electrophoretic deposition and then covered and sealed with plasma enhanced polydimethylsiloxane (PDMS). The nanoreactor has been tested at room temperature and atmospheric pressure under different conditions of flow and irradiance. Activity of the catalyst for hydrogen production from gaseous water - ethanol mixture has been calculated. Results show a strong improvement on the activity of the photocatalyst, with respect to latest pulications. This improvement is attributed to a higher surface/volume ratio of the catalyst because of the nanometric sizes of the channels.Este proyecto presenta un trabajo innovador en el desarrollo de nanorectores baratos utilizando un fotocatalizador de Au / TiO2 para la producción de H2; a partir la morfología de los discos compactos (CD) como matrices fundamentales para los nanocanales de los reactores. Se han recubierto los nanocanales de 200 nm de profundidad con el fotocatalizador mediante deposición electroforética y después se han cubierto y sellado con polidimetilsiloxano (PDMS) mejorado por plasma. El nanorector se ha probado a temperatura ambiente y presión atmosférica bajo diferentes condiciones de flujo e irradiancia. Se ha calculado la actividad del catalizador para la producción de hidrógeno a partir de la mezcla en gas de agua-etanol. Los resultados muestran una importante mejora en la actividad del fotocatalizador, respecto a las últimas publicaciones. Esta mejora se atribuye a una mayor relación de superficie / volumen del catalizador debido a los tamaños nanométricos de los canales.Aquest projecte presenta un treball innovador en el desenvolupament de nanorectors barats utilitzant un fotocatalitzador de Au / TiO2 per a la producció d'H2; a partir la morfologia dels discs compactes (CD) com a matrius fonamentals per als nanocanals dels reactors. S'han recobert els nanocanals de 200 nm de profunditat amb el fotocatalizador mitjançant deposició electroforètica i després s?han cobert i sellat amb polidimetilsiloxà (PDMS) millorat per plasma. El nanorector s'ha provat a temperatura ambient i pressió atmosfèrica sota diferents condicions de flux i irradiància. S'ha calculat l'activitat del catalitzador per a la producció d'hidrogen a partir de la barreja en gas de aigua-etanol. Els resultats mostren una important millora en l'activitat del fotocatalitzador, respecte a les últimes publicacions. Aquesta millora s'atribueix a una major relació de superfície / volum del catalitzador a causa de les mides nanomètriques dels canals

Ús de discos compactes com a nanoreactors per a aplicacions fotocatalítiques

This project presents a novel work developing cheap nanoreactors with Au/TiO2 photocatalyst for H2 production; using the morphology of compact discs (CD) as the fundamental matrices to set up nanochannels for the reactors. Nanochannels of 200 nm depth have been coated with the photocatalyst using electrophoretic deposition and then covered and sealed with plasma enhanced polydimethylsiloxane (PDMS). The nanoreactor has been tested at room temperature and atmospheric pressure under different conditions of flow and irradiance. Activity of the catalyst for hydrogen production from gaseous water - ethanol mixture has been calculated. Results show a strong improvement on the activity of the photocatalyst, with respect to latest pulications. This improvement is attributed to a higher surface/volume ratio of the catalyst because of the nanometric sizes of the channels.Este proyecto presenta un trabajo innovador en el desarrollo de nanorectores baratos utilizando un fotocatalizador de Au / TiO2 para la producción de H2; a partir la morfología de los discos compactos (CD) como matrices fundamentales para los nanocanales de los reactores. Se han recubierto los nanocanales de 200 nm de profundidad con el fotocatalizador mediante deposición electroforética y después se han cubierto y sellado con polidimetilsiloxano (PDMS) mejorado por plasma. El nanorector se ha probado a temperatura ambiente y presión atmosférica bajo diferentes condiciones de flujo e irradiancia. Se ha calculado la actividad del catalizador para la producción de hidrógeno a partir de la mezcla en gas de agua-etanol. Los resultados muestran una importante mejora en la actividad del fotocatalizador, respecto a las últimas publicaciones. Esta mejora se atribuye a una mayor relación de superficie / volumen del catalizador debido a los tamaños nanométricos de los canales.Aquest projecte presenta un treball innovador en el desenvolupament de nanorectors barats utilitzant un fotocatalitzador de Au / TiO2 per a la producció d'H2; a partir la morfologia dels discs compactes (CD) com a matrius fonamentals per als nanocanals dels reactors. S'han recobert els nanocanals de 200 nm de profunditat amb el fotocatalizador mitjançant deposició electroforètica i després s?han cobert i sellat amb polidimetilsiloxà (PDMS) millorat per plasma. El nanorector s'ha provat a temperatura ambient i pressió atmosfèrica sota diferents condicions de flux i irradiància. S'ha calculat l'activitat del catalitzador per a la producció d'hidrogen a partir de la barreja en gas de aigua-etanol. Els resultats mostren una important millora en l'activitat del fotocatalitzador, respecte a les últimes publicacions. Aquesta millora s'atribueix a una major relació de superfície / volum del catalitzador a causa de les mides nanomètriques dels canals