Nuevas estrategias electrocinéticas y nuevas aportaciones a la detección electroquímica en microchips de electroforesis capilar basadas en el empleo de nanotubos de carbono y otras nanoestructuras

Resumo

The miniaturization of laboratories (lab-on-a-chip) is, without any doubt, one of the most promising, dynamic and multidisciplinary fields of research in the present scientific scene being the Analytical Chemistry pioneer in this revolution with the introduction of the micro total analysis system (µ-TAS) concept in the early 1990s. Indeed, the decrease of the scale could potentially provide some unique benefits for the analysis such as low analysis time, high throughput, low sample and reactive consumption, low waste generation and portability (possibility to perform the analysis in situ). Nowadays, after two decades, some of these potential benefits have been managed and they form part of the scientific actuality. Nevertheless, there are still some aspects that have to be solved despite the large volume of research. On the other hand, the capillary electrophoresis, in connection with labon-a-chip world, has been the main protagonist (CE- microchips) due to its easiness of miniaturization without losing analytical performance (analysis time can be reduced to seconds and high separation efficiencies can be achieved using extremely low volume samples). Likewise, the electrokinetic flow in multiple channels on a CE-microchip can be obtained and controlled easily using a few electrodes, avoiding the microfabrication and integration of complex components (micropumps, microvalves, etc.) necessary for hydrodynamic flow. In addition, the discovery of novel materials, process and phenomena at the nanoscale and the development of new experimental and theoretical techniques for research provide fresh opportunities for the development of innovative nanosystems and nanostructured materials. The application of the nanomaterials and, in general, of the nanotechnology in the field of microfluidics is also revolutionizing the analytical microsystems. Taking into account that one of most important problem of microfludic systems, in general, and CE- microchip, in particular, is the low sensitivity due to the extremely small volume of sample introduced (nL-pL), the main objective of this Thesis Doctoral has been to evaluate the possibilities offered by nanomaterials, specially, carbon nanotubes to increase the sensibility in μCE-ED looking for synergies between both lab-on-a-chip technology and analytical nanotechnology. The results showed in this Thesis cover not only the developing of new methods for the determination of the couple arbutin-hydroquinone, vitamins (pyridoxine, ascorbic and folic acids) and polyphenols (arbutin, phloridzin, catechin, rutin and isoflavones) in pharmaceutical and food samples but, also, the improvement of the analytical performance of µCE-ED by introducing of new methodologies or strategies on-chip. Firstly, the technical possibility of working indiscriminately and/or sequentially with both simple cross-injectors in glass CE microchip was demonstrated. In this way, strategies were designed to carry out a methodological innovation integrating both calibration and analysis on the chip in only one run. The strategy consisted in sequentially using both reservoirs (named calibration and analysis reservoirs) as well as a calibration factor (defined as signal/concentration of standard). The analytical route required 350s in the overall protocol (employing 130 s in calibration plus 130 s in analysis) for water-soluble vitamin determination (B and C vitamin groups) and an improvement over the times used in both conventional and microchip protocols was obtained. On the other hand, the synergy of CE microchips with carbon nanotube detectors in food analytical science has been explored. Two different carbon electrodes (glassy carbon electrode (GCE) and screen printed electrode (SPE)) were modified with multiwalled carbon nanotubes (MWCNTs) and their electrochemical behaviour was evaluated as detectors in CE microchip using water-soluble vitamins (pyridoxine, ascorbic acid and folic acid) in different pharmaceutical formulations. SPE modified with CNTs have become the best approach because of their improvement of sensitivity and lowered LODs with the advantages of easy surface modification, inherent miniaturization and disposability. Moreover, it was demonstrated that carbon nanotubes (CNTs) possess preferential “electrocatalytic” properties over the oxidation of enediol groups, establishing a relationship between electrocatalysis and chemical structure, being MWCNT the only material that perfectly joined both a decrease in overpotentials and an enhanced analytical signal selectively over enediol groups. This fact could open up new perspectives in application domains since their inherent behaviour permits direct electrochemical sensing, simplifying the overall detection processes. Finally, in this research work, fabrication strategies have also been developed for metallic nanowires and their preliminary characterization and potential applications for barcoding and electroanalysis have also been explored. Taking into account the whole of results obtained in this research work we can conclude (i) carbon nanotubes coupled to the advantages of microchips capillary electrophoresis mentioned above are opening new frontiers in the field of food and pharmaceutical analysis developing new applications due to the synergic effect of analytical miniaturization and nanotechnology and (ii) the fabrication and design of multichannel microchip combined with new methodologies as showed in this work will permit the creation of high throughput miniaturized analytical systems able to carry out simultaneously a great number of analysis for food and pharmaceutical control.