Evaluación del potencial de sólidos monolíticos modificados con nanopartículas en técnicas de microextracción

1. Introducción o motivación de la tesis La Química Analítica es la ciencia que desarrolla, optimiza y aplica procesos de medida para obtener información sobre la composición y la naturaleza (bio)química de la materia. La etapa de tratamiento de la muestra es la que más afecta a la calidad de la información analítica, ya que se considera la principal fuente de error del proceso analítico. En general, las técnicas de tratamiento de muestra juegan un papel importante en la mejora de la sensibilidad, a través de la preconcentración de los analitos, y de la selectividad, por su aislamiento de la matriz de la muestra. La simplificación, automatización y miniaturización de esta etapa han sido tendencias clave que han marcado la evolución de esta disciplina en la última década. En este contexto, es necesario el desarrollo de nuevas fases extractantes más eficientes para que la reducción de dimensiones no afecte negativamente a las propiedades analíticas básicas [1, 2]. Los logros alcanzados en las técnicas de microextracción en fase sólida están en gran parte marcados por el empleo de nanopartículas y polímeros nanoestructurados [3-5]. Los sólidos monolíticos se han empleado con excelentes resultados como fases estacionarias en técnicas de separación debido a las excepcionales propiedades que les confiere su estructura continua, porosa y homogénea [6, 7]. Además, el proceso de fabricación es relativamente simple. En este contexto, la posibilidad de incorporar micro y nanomateriales permite combinar su elevada superficie específica con la alta porosidad y variabilidad funcional de los materiales monolíticos, favoreciendo la interacción con un determinado tipo de analitos [8].Teniendo en cuenta lo anteriormente expuesto, la Tesis Doctoral ha tenido como objetivo general la evaluación del potencial de los sólidos monolíticos modificados con nanoestructuras de carbono como material sorbente en técnicas de microextracción en fase sólida, culminando con el desarrollo de estructuras monolíticas formadas únicamente por nanopartículas. En concreto, se han llevado a cabo diferentes síntesis y estrategias para incorporar las nanopartículas en la matriz polimérica y, se han evaluado distintos formatos de microextracción para la resolución de problemas medioambientales, clínicos y/o de alimentos. Por último, se ha extendido el uso de estos materiales híbridos al ámbito de las separaciones cromatográficas. 2. Contenido de la investigación De acuerdo con el objetivo general, surgen diferentes objetivos específicos que se presentan a continuación: • Estudio bibliográfico como primera toma de contacto con el campo en el que se desarrolla la investigación. Esto se ha materializado en la publicación de una revisión crítica que constituye la Introducción de esta Tesis Doctoral. • Estudio de los procesos de inclusión de las nanopartículas en la matriz polimérica. Así, se estudiarán diferentes estrategias de introducción de las nanopartículas, tales como anclaje químico y fotoquímico a la superficie del polímero, así como atrapamiento de las nanopartículas en la estructura monolítica, que son ampliamente discutidas en el Bloque I. • Evaluación de diferentes formatos de microextracción, tal como se muestra en el Bloque II de esta Memoria. • Desarrollo de nuevas estrategias de síntesis que permitan obtener estructuras monolíticas formadas exclusivamente por nanopartículas de carbono. Esta investigación se recoge en el Bloque III. • Caracterización de los sólidos sintetizados mediante diversas técnicas instrumentales. Esto se ha llevado a cabo a lo largo de la Tesis Doctoral. • Aplicación de las unidades de extracción desarrolladas a la resolución de problemas medioambientales, clínicos y/o de alimentos, de manera transversal en el conjunto de publicaciones de esta Memoria. • Utilización de los sólidos sintetizados como fases estacionarias en separaciones cromatográficas, tal como se muestra en el Capítulo 2 de esta Memoria. 3. Conclusión El objetivo general y los objetivos específicos planteados en la presente Tesis Doctoral se han alcanzado con éxito. De este modo, se ha llevado a cabo la síntesis de nuevos materiales sorbentes que combinan polímeros monolíticos con nanopartículas de carbono, los cuales se han empleado como sorbentes en distintas técnicas de microextracción en fase sólida. Además, se ha realizado la caracterización de dichos materiales mediante técnicas tanto espectroscópicas como microscópicas. Las unidades de microextracción desarrolladas se han aplicado a la resolución de problemas en los ámbitos medioambientales, clínico y/o de alimentos

Nanostructured hybrid monolith with integrated stirring for the extraction of UV-filters from water and urine samples

This article presents a monolithic extraction unit with integrated stirring using carbon nanohorns and methacrylate-based compounds as monomers. The hybrid monolithic material was prepared by thermal polymerization at 70 ºC for 24 h and was applied for the extraction of UV-filters from waters and human urine samples. To achieve the integrated stirring unit, the monolith was grown over an ironware. Variables dealing with the polymerization mixture composition and the microextraction procedure were studied in depth. The resulting hybrid monolithic polymer was also characterized by scanning electron microscopy (SEM) and nitrogen intrusion porosimetry. The target analytes were quantified by UPLC-DAD, and the limits of detection were between 1 and 10 μg/L. The precision of the method (inter extraction units) expressed as relative standard deviation ranged from 5.4 and 7.9 %. Also, relative recoveries values of the analyte spiked to swimming pool water and urine samples varied in the interval 72-124 and 71-113 %, respectively

Preparation of macroscopic carbon nanohorn-based monoliths in polypropylene tips by medium internal phase emulsion for the determination of parabens in urine samples

A porous monolithic solid based on single-walled carbon nanohorns dahlia-like structure, produced from a medium internal phase emulsion (MIPE), was prepared in a polypropylene tip using UV energy. Thus, single-walled carbon nanohorns (SWNHs) were added to the organic phase where they polymerized in the presence of a radical initiator. A cross-linker (ethylene dimethacrylate, EDMA) was also used in order to obtain a more robust structure. On the other hand, aqueous phase was the responsible for generating the pores in the final solid being inside the droplets generated by the surfactant (Pluronic L121) used to stabilize the polymerization emulsion. Variables related to the formation of the monolithic phase including the stability and composition of emulsion mixture, size of pores, solvent flow resistance and robustness, were studied in detail. In addition, the potential of the SWNH-monolith as extractant phase was evaluated using parabens as target analytes. The LODs ranged from 1 to 7 μg·L-1, while the linear range was extended up to 5000 μg·L-1. The reproducibility of the extraction procedure with different batches of emulsions was acceptable with RSD values <16% and one prepared SWNH-tip can be used for more than 100 times without apparent extraction losses. The microextraction unit yielded an enrichment factor of 20 for all analytes (extraction efficiency of 100%), with recovery values between 80-116 % in human urine samples

Advanced polymeric solids containing nano- and micro-particles prepared via emulsion-based polymerization approaches. A review

This review critically summarizes the latest contributions in the preparation of advanced nano/microparticle-contained polymers from emulsions. The nano- or micro-particles can be dispersed in the phase where the polymerization reaction takes place and, consequently, once the solid is formed, the particles are embedded in the final polymeric structure. This results in the formation of hybrid materials, which combine the unique properties of nano/micro-particles with the inherent properties of the polymers (which depend mainly on the selected monomers). In addition to this, some nano- and micro-particles can be used as solid stabilizers in emulsions. This kind of emulsions, called ‘Pickering emulsions’, presents some additional advantages over those prepared with conventional surfactants (e.g., non-ionic polymers) such as higher emulsion stability. In this way, the nano/micro-particles fulfill a double mission. On the one hand, they are responsible for forming and stabilizing the emulsion. On the other, they are part of the final solid, obtaining polymeric materials with new functionalities. In this context, this review aims to describe the most innovative strategies for the incorporation of nano- and micro-particles in polymers through the direct addition of them to the emulsion in which the polymerization is carried out. Also, the effect of the addition of these nano/micro-particles in the emulsions (e.g., size of droplets, type of emulsion and stability), the type of solids obtained (e.g., monolithic polymers or individual particles), morphology (e.g., open- or closed-cell polymers) and functionality of the final solid will be also commented on

Improved microextraction of selected triazines using polymer monoliths modified with carboxylated multi-walled carbon nanotubes

This article reports on the enhancement of the capacity of an acrylate-based monolithic solid sorbent by anchoring carboxylated multi-walled carbon nanotubes (c-MWCNTs) in its pores and on its surface. Monolithic poly(butyl acrylate-co-ethyleneglycol dimethacrylate) [poly(BA-co-EGDMA)] was synthetized inside a fused silica capillary via free-radical polymerization, and an ethanolic dispersion of c-MWCNTs was passed through the capillary. The resulting poly(BA-co-EGDMA-c-MWCNTs) monolith was characterized by scanning electron microscopy to confirm the presence of the c-MWCNTs. The effect of using three different kinds of carbon nanoparticles and the microextraction step were studied using triazine herbicides as model compounds. The use of c-MWCNTs resulted in best performance in terms of extraction enhancement (compared to carboxylated single-walled carbon nanotubes and oxidized single-walled carbon nanohorns). The use of these carbon nanoparticles improved the extraction of triazines in any case when compared to using a bare poly(BA-co-EGDMA) monolith. The triazines were then quantified by gas chromatography with mass spectrometric detection. Detection limits ranged from 0.03 to 0.1 µg·L-1 (except for simazine; 0.6 µg·L-1), and the precision (relative standard deviation) varied between 3.0 and 11.4%. The reproducibility between units is <14.3% (expressed as relative standard deviation) which demonstrates the robustness of the method. The method was applied to analyze an unknown sample of orange juice and gave a value of 0.18 µg·L-1 for prometryn. Finally, the analysis of spiked samples of water and orange juices yielded recoveries ranging from 81 to 113% and 75 to 125%, respectively

Preparation and evaluation of micro and meso porous silica monoliths with embedded carbon nanoparticles for the extraction of non-polar compounds from waters

A novel hybrid micro and meso porous silica monolith with embedded carbon nanoparticles (Si-CNPs monolith) was prepared inside a fused silica capillary (3 cm in length) and used as a sorbent for solid-phase microextraction. The hybrid monolithic capillary was synthetized by hydrolysis and polycondensation of a mixture of tetraethoxysilane (TEOS), ethanol, and three different carbon nanoparticles such as carboxylated single-walled carbon nanotubes (c-SWCNTs), carboxylated multi-walled carbon nanotubes (c-MWCNTs), and oxidized single-walled carbon nanohorns (o-SWNHs) via a two-step catalytic sol-gel process. Compared with silica monolith without carbon nanoparticles, the developed monolithic capillary column exhibited a higher extraction efficiency towards the analytes which can be ascribed to the presence of the carbon nanoparticles. In this regard, the best performance was achieved for silica monolith with embedded c-MWCNTs. The resulted monolithic capillaries were also characterized by scanning electron microscopy (SEM), elemental analysis and nitrogen intrusion porosimetry. Variables affecting to the preparation of the sorbent phase including three different carbon nanoparticles and extraction parameters were studied in depth using polycyclic aromatic hydrocarbons (PAHs) as target analytes. Gas chromatography-mass spectrometry was selected as instrumental technique. Detection limits range from 0.1 to 0.3 µg·L-1, and the inter-extraction units precision (expressed as relative standard deviation) is between 5.9 and 14.4 %

Synthesis, characterization, and application of chemically interconnected carbon nanotube monolithic sorbents by photopolymerization in polypropylene caps

A facile and convenient approach for the preparation of interconnected multiwalled carbon nanotubes (MWCNTs) monolithic sorbents in recycled plastic caps has been developed. The method, which was based on the photopolymerization of the individual MWCNTs via the formation of a W/O medium internal phase emulsion (40/60 w/w%), provides control over size of pores, rigidity and the mechanical stability of the final solid. Pluronic L121 was used as a surfactant containing the water phase inside it and consequently, the organic and non-polar phase, in which the MWCNTs and the cross-linker were trapped, remained on the outside of the droplets. Optical microscopy and scanning electron microscopy (SEM) were employed to characterize the morphology of both the emulsions and the final solids, respectively. In addition, nitrogen intrusion porosimetry was performed in order to study how the specific surface area of the final monolithic solid changes (from 19.6 to 372.2 m2 g-1) with the variables involved in the polymerization step. To exemplify the great sorbent potential of the synthesized material, a colorimetric assay based on the retention of methylene blue within the interconnected MWCNTs monolithic structure was carried out. Finally, following the positive results the carbon nanotube-monolithic stirred caps were applied for the determination of chlorophenols in a biological matrix such as a human urine, obtaining excellent recovery values (91-98 %) and good precision (5.4-9.1 %) under optimized extraction conditions

Molecularly Imprinted Polymer Micro- and Nano-Particles. A review

In recent years, molecularly imprinted polymers (MIPs) have become an excellent solution to the selective and sensitive determination of target molecules in complex matrices where other similar and relative structural compounds could coexist. Although MIPs show the inherent properties of the polymers, including stability, robustness, and easy/cheap synthesis, some of their characteristics can be enhanced, or new functionalities can be obtained when nanoparticles are incorporated in their polymeric structure. The great variety of nanoparticles available significantly increase the possibility of finding the adequate design of nanostructured MIP for each analytical problem. Moreover, different structures (i.e., monolithic solids or MIPs micro/nanoparticles) can be produced depending on the used synthesis approach. This review aims to summarize and describe the most recent and innovative strategies since 2015, based on the combination of MIPs with nanoparticles. The role of the nanoparticles in the polymerization, as well as in the imprinting and adsorption efficiency, is also discussed through the review

Monolithic Solid Based on Single-Walled Carbon Nanohorns: Preparation, Characterization, and Practical Evaluation as a Sorbent

A monolithic solid based solely on single walled carbon nanohorns (SWNHs) was prepared without the need of radical initiators or gelators. The procedure involves the preparation of a wet jelly-like system of pristine SWNHs followed by slow drying (48 h) at 25 C. As a result, a robust and stable porous network was formed due to the interaction between SWNHs not only via - and van der Waals interactions, but also via the formation of carbon bonds similar to those observed within dahlia aggregates. Pristine SWNHs and the SWNH monolith were characterized by several techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), confocal laser scanning microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and nitrogen intrusion porosimetry. Taking into account the efficiency of carbon nanoparticles in sorption processes, the potential applicability of the SWNH-monolith in this research field was explored using toluene; m-, p-, and o-xylene; ethylbenzene; and styrene, as target analytes. Detection limits were 0.01 g L�����1 in all cases and the inter-day precision was in the interval 7.4–15.7%. The sorbent performance of the nanostructured monolithic solid was evaluated by extracting the selected compounds from different water samples with recovery values between 81.5% and 116.4%