Microfluidic paper-based analytical devices (µPADs) are a promising technology to enable accurate and quantitative in situ assays. Paper’s inherent hydrophilicity drives the fluids without the need for external pressure sources. However, controlling the flow in the porous medium has remained a challenge. This study addresses this problem from the nature of the paper substrate and its design. A computational fluid dynamic model has been developed, which couples the characteristics of the porous media (fiber length, fiber diameter and porosity) to the fluidic performance of the diffusion-based µPAD sensor. The numerical results showed that for a given porous membrane, the diffusion, and therefore the sensor performance is affected not only by the substrate nature but also by the inlets’ orientation. Given a porous substrate, the optimum performance is achieved by the lowest inlets’ angle. A diffusion-based self-referencing colorimetric sensor was built and validated according to the design. The device is able to quantify the hydronium concentration in wines by comparison to 0.1–1.0 M tartaric acid solutions with a 41.3 mM limit of detection. This research showed that by proper adjustments even the simplest µPADs can be used in quantitative assays for agri-food applications.

Recycling and revalorization of waste are currently essential for sustainable growth. Mill scale, a waste product from steel production industries, which contains high levels of iron and minimal impurities, is proposed in this study as the source to synthesize magnetite nanoparticles (Fe₃O₄) for an enhancement of a lactate biosensor range. The synthesized Fe₃O₄ nanoparticles were coated with polydopamine (PDA) to prevent aggregation and degradation, creating a stable platform for immobilizing lactate oxidase enzyme (LOx) on their surfaces. The characterization of the Fe₃O₄@PDA material was carried out using transmission electron microscopy (TEM), dynamic light scattering (DLS), and measurement of the polydispersity index (PdI). The Fe₃O₄@PDA-LOx material was then deposited on a screen-printed carbon electrode modified with Prussian blue (SPCE-PB) for lactate detection. The biosensor exhibited a broad, dual linear concentration-response range, one from 0.1 to 4.62 mM with a limit of detection of 0.32 mM and sensitivity of 1.54 $\mathsf{\mu }\mathrm{A}{\mathrm{m}\mathrm{M}}^{-1}{\mathrm{c}\mathrm{m}}^{-2}$, and another one from 4.62 to 149.21 mM with a limit of detection of 6.31 mM and sensitivity of 0.08 $\mathsf{\mu }\mathrm{A}{\mathrm{m}\mathrm{M}}^{-1}{\mathrm{c}\mathrm{m}}^{-2}$. The dual-range concentration response of the biosensor makes it an ideal tool for lactate determination in various applications, including sports medicine, clinical diagnosis, and industrial bioprocessing.

Microfluidic paper-based analytical devices (µPADs) are leading the field of low-cost, quantitative in-situ assays. However, understanding the flow behavior in cellulose-based membranes to achieve an accurate and rapid response has remained a challenge. Previous studies focused on commercial filter papers, and one of their problems was the time required to perform the test. This work studies the effect of different cellulose substrates on diffusion-based sensor performance. A diffusion-based sensor was laser cut on different cellulose fibers (Whatman and lab-made Sisal papers) with different structure characteristics, such as basis weight, density, pore size, fiber diameter, and length. Better sensitivity and faster response are found in papers with bigger pore sizes and lower basis weights. The designed sensor has been successfully used to quantify the ionic concentration of commercial wines with a 13.6 mM limit of detection in 30 s. The developed µPAD can be used in quantitative assays for agri-food applications without the need for any external equipment or trained personnel.

Plastic pollution, particularly from microplastics, poses serious risks to ecosystems and human health. Polyethylene terephthalate (PET), a widely used polymer, commonly leaches into food and water sources, necessitating effective monitoring. This study presents a simple electrochemical sensor for trace detection of PET microplastics in water. The sensor was developed by modifying a screen-printed gold electrode (SPAuE) with magnetite (Fe₃O₄) nanoparticles valorized from mill scale. Electrochemical analysis revealed a fully irreversible, diffusion-controlled oxidation response. Under optimized conditions, the SPAuE/Fe₃O₄ sensor successfully detected PET in the concentration range of 6.25 to 500 mg/L, with a detection limit of 3.6 mg/L. The improved performance is attributed to the high conductivity and surface area of Fe₃O₄ nanoparticles. The sensor exhibited excellent sensitivity, selectivity, reproducibility, stability, and practical applicability in both synthetic and real water samples, supporting its potential as a simple, portable, and cost-effective tool for monitoring PET pollution in aquatic environments.

(English) The transition toward low-cost, portable, and environmentally conscious analytical technologies has intensified the pursuit of sustainable alternatives to conventional laboratory instrumentation. This thesis develops paper-based and electrochemical sensing platforms that prioritize circular-economy principles by employing renewable and waste-derived materials. Non-wood cellulose fibers were selected as substrates for microfluidic paper-based analytical devices (μPADs), while industrial mill scale was valorized to synthesize magnetite nanoparticles for electrode modification, demonstrating that sustainability and high analytical performance can be synergistic. In the first study, μPADs fabricated from alternative cellulose sources were evaluated. Their fiber morphology and porosity strongly influenced capillary flow and colorimetric responses. Compared to commercial cellulose papers, non-wood substrates enabled substantially faster wicking and significantly reduced detection time, underscoring their suitability for rapid, low-resource diagnostics. The second study focused on lactate detection using magnetite-modified electrodes. Wastederived Fe₃O₄ nanoparticles enhanced electron transfer and enzyme immobilization, enabling an exceptionally broad detection range alongside high sensitivity and a low detection limit. To the best of our knowledge, this work represents the first demonstration of a lactate biosensing platform that simultaneously achieves such a wide dynamic range while retaining high analytical sensitivity, making it suitable for applications from trace physiological monitoring to highly concentrated food and fermentation environments. Finally, a novel electrochemical strategy was developed for polyethylene terephthalate (PET) microplastic quantification in water. Leveraging the natural affinity between PET and magnetite nanoparticles, the approach transitions from the traditional use of magnetite for magnetic pre-concentration toward direct and quantitative electrochemical measurement, successfully validated in synthetic and real water matrices. Overall, this thesis demonstrates that renewable and waste-derived materials from non-wood cellulose to mill-scale-derived magnetite can serve as functional components in advanced sensing platforms, advancing sustainable analytical technologies for biomedical and environmental applications. (Català) La transició cap a tecnologies analítiques portàtils, de baix cost i ambientalment responsables ha intensificat la recerca d’alternatives sostenibles a la instrumentació de laboratori convencional. Aquesta tesi desenvolupa plataformes de detecció electroquímiques i basades en paper que prioritzen els principis de l’economia circular mitjançant l’ús de materials renovables i derivats de residus. Es van seleccionar fibres de cel·lulosa no llenyosa com a substrat per a dispositius microfluídics analítics de paper (μPADs), mentre que l’escorça de laminació industrial es va valoritzar per sintetitzar nanopartícules de magnetita per a la modificació d’elèctrodes, demostrant que la sostenibilitat i l’alt rendiment analític poden ser sinèrgics. En el primer estudi, es van avaluar μPADs fabricats a partir de fonts alternatives de cel·lulosa. La seva morfologia fibrosa i porositat van influir fortament en el flux capil·lar i en les respostes colorimètriques. En comparació amb els papers de cel·lulosa comercials, els substrats no llenyosos van permetre una absorció molt més ràpida i una reducció significativa del temps de detecció, evidenciant la seva idoneïtat per a diagnòstics ràpids en entorns amb recursos limitats. El segon estudi es va centrar en la detecció de lactat mitjançant elèctrodes modificats amb magnetita. Les nanopartícules de Fe₃O₄ derivades de residus van millorar la transferència electrònica i la immobilització enzimàtica, permetent un rang de detecció excepcionalment ampli juntament amb una alta sensibilitat i un baix límit de detecció. Fins on arriba el nostre coneixement, aquest treball representa la primera demostració d’una plataforma biosensora de lactat capaç d’assolir simultàniament un rang dinàmic tan ampli mantenint una elevada sensibilitat analítica, fent-la adequada per a aplicacions que van des del monitoratge fisiològic fins a entorns alimentaris i de fermentació d’alta concentració. Finalment, es va desenvolupar una nova estratègia electroquímica per a la quantificació de microplàstics de polietilè tereftalat (PET) en aigua. Aprofitant l’afinitat natural entre el PET i les nanopartícules de magnetita, aquest enfocament evoluciona de l’ús tradicional de la magnetita per a la preconcentració magnètica cap a una mesura electroquímica directa i quantitativa, validada amb èxit en mostres d’aigua sintètica i real.En conjunt, aquesta tesi demostra que materials renovables i derivats de residus des de la cel·lulosa no llenyosa fins a la magnetita obtinguda de l’escorça de laminació poden actuar com a components funcionals en plataformes de detecció avançades, impulsant tecnologies analítiques sostenibles per a aplicacions biomèdiques i mediambientals. (Español) La transición hacia tecnologías analíticas portátiles, de bajo coste y respetuosas con el medio ambiente ha intensificado la búsqueda de alternativas sostenibles a la instrumentación de laboratorio convencional. Esta tesis desarrolla plataformas de detección electroquímica y basada en papel que priorizan los principios de la economía circular...