Tesis Doctorals - Facultat - Física

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Influence of the Quasi-Biennial Oscillation on the tropical troposphere at interannual timescales
(Universitat de Barcelona, 2026-01-09) Rodrigo Sánchez, Mario; García-Serrano, Javier, 1980-; Bladé, Ileana; Universitat de Barcelona. Facultat de Física
[eng] The Quasi-Biennial Oscillation (QBO), an atmospheric phenomenon characterized by alternating westerly and easterly winds that descend through the equatorial stratosphere, is the dominant mode of tropical stratospheric variability. Several studies have explored its connection with the extratropical stratosphere and the troposphere, including its connection to El Niño-Southern Oscillation (ENSO), the main driver of interannual variability in the tropical troposphere. While it is accepted that ENSO exerts an upward impact on the QBO, manifested through changes in the QBO amplitude, period and downward propagation rate, the potential downward influence of the QBO on the tropical troposphere, and on ENSO in particular, has received less attention. Clarifying this downward influence is a difficult scientific problem because the QBO signal in the troposphere is modest and often masked by the much stronger variability associated with ENSO and other variability modes. Moreover, isolating the QBO influence requires long records and/or specifically designed model experiments to separate its effects from those of concurrent variability. This thesis examines the downward impact of the QBO on tropical convection and large-scale circulation on interannual timescales, using reanalyses and a hierarchy of climate model experiments, principally the European Consortium Earth system model (EC-EARTH). As a preliminary step, the performance of EC-EARTH in simulating the stratospheric circulation and the main features of the QBO was assessed, to provide the necessary validation for the subsequent analyses. Overall, the QBO in EC-EARTH is realistic, although its amplitude is underestimated in the lower stratosphere. Reanalysis data and coupled simulations with EC-EARTH were analyzed to examine the QBO-induced changes in the tropical circulation under climatological conditions and during El Niño events. Results show that the QBO affects upper-tropospheric divergence over the Maritime Continent, resulting in reduced outflow during the westerly phase as compared to the easterly phase. Furthermore, during El Niño events, which are characterized by a weakened zonal overturning circulation, i.e. the Walker cell, the QBO signal extends downward into the troposphere and influences El Niño evolution. The westerly QBO phase acts to further suppress summer tropical convection over the Maritime Continent and the western Pacific, thereby accentuating the weakening of the Walker circulation during El Niño. These results highlight the importance of considering the QBO for improving El Niño prediction and projection, especially for extreme events known as super El Niños. While results from that first study reveal a QBO influence, they also show that disentangling the QBO teleconnections in the tropical troposphere from the dominant influence of ENSO is challenging. To better isolate the QBO signal, an atmosphere-only experiment with climatological boundary conditions was then analyzed and treated as an ENSO-neutral state. This part of the thesis focused on identifying QBO-induced changes in temperature and zonal wind in the upper troposphere-lower stratosphere (UTLS) and their subsequent impact on static stability, wind shear and relative vorticity. The results confirm that the QBO effectively modifies vertical velocity and precipitation over the Maritime Continent region, but further show that the QBO affects both the Walker circulation and, more notably, the Hadley circulation. These impacts are highly seasonally-dependent, being strongest in summer. The vertical structure of the QBO signal in the UTLS is found to be zonally asymmetric, with anomalies descending into the upper troposphere only over the Indo-Pacific region. The timing of the QBO influence on tropical convection and precipitation is primarily due to the QBO-induced changes in static stability, which descend into the UTLS earlier than those in wind shear and vorticity. To assess how ENSO modulates the QBO teleconnection to the troposphere, two additional atmosphere-only experiments with perpetual El Niño and La Niña conditions were analysed, complementing the ENSO-neutral experiment. Results confirm that the QBO affects summer tropical convection even under a strong oceanic forcing such as ENSO, but its influence depends on the position of convection. During La Niña, when convection remains close to its climatological position but intensifies, the QBO signal closely resembles that in ENSO-neutral conditions, with the westerly QBO phase reducing summer convection north of the equator over the Maritime Continent. Instead, during El Niño, when convection shifts equatorward over the Indian-Pacific region, the QBO-related reduction in convection likewise shifts equatorward. ENSO also affects the QBO period and downward propagation rate, with a longer period and slower descent occurring under La Niña conditions, and a shorter period and faster descent under El Niño conditions. The latter causes the QBO phase to transition more quickly during El Niño, leading to a reversal of its impact on tropical convection from early to late summer: in early summer, anomalous descent dominates over the Maritime Continent, whereas anomalous ascent prevails in late summer over the western tropical Pacific. The results based on EC-EARTH thus suggest that ENSO modulates the QBO teleconnection in the tropical troposphere through changes in the position of convection and the QBO period/downward-phase propagation. In the last part of the thesis, the robustness of these results was evaluated using the multi-model ensemble from the QBO initiative (QBOi) of Atmospheric Processes And their Role in Climate (APARC). Most QBOi models reproduce similar patterns in the three experiments—ENSO-neutral, El Niño, and La Niña—though the lower-tropospheric signal is generally underestimated. The QBO impact on the tropical circulation is spatially robust across models, but its timing varies, ranging from May to November.
Motion of Enzyme-Powered Nanomotors in Complex Media
(Universitat de Barcelona, 2025-12-19) Ruiz González, Noelia; Sánchez Ordóñez, Samuel; Universitat de Barcelona. Facultat de Física
[eng] In recent years, the field of NMs has experienced rapid growth in biomedical applications, largely due to their ability to navigate complex physiological environments. However, unlocking their full clinical potential requires a deeper understanding of NMs modulate with and interact with biological interfaces. This doctoral thesis aims to address this need by focusing on the design, synthesis, and evaluation of enzyme-powered NMs capable of propelling in viscous biological environments. To this end, this PhD thesis investigates a variety of NPs platforms, ranging from inorganic and polymer-based nanocarriers, functionalized with enzymes to produce enzyme-powered NMs. These NMs are evaluated across clinically relevant scenarios, including navigation in biological barriers, tissue regeneration or drug delivery. The first part of the thesis focuses on the development of inorganic-based NMs, such as those based on MSNPs, demonstrating a dual-enzyme NMs strategy to enhance diffusion through highly viscous SF. For that, two distinct NM actuating in “troops” were developed, capable of reducing SF viscosity and self-propel more effectively. The synergistic interaction between these NM systems significantly improved the transport of macromolecules across SF-mimicking environments, offering a potential strategy for intra-articular drug delivery in joint diseases. Building on these findings, the second part of this thesis continues within the same biomedical application framework but introduces key advancements in NM design. In this part, a new generation of nanogel-based NMs (NGs-NMs) composed of synthetic polymers are introduced to substitute the inorganic core of NMs. These NGs are soft and flexible, and enable motion in viscoelastic fluids while preserving the bulk rheological properties of SF. Their structural flexibility and tunable responsiveness to environmental changes (e.g., temperature, pH, or redox conditions) are achieved through controlled polymer crosslinking, enabling dynamic modulation of size and density allowing for improved interaction with ECMs. After surface functionalization with urease, NGs-NMs achieved efficient propulsion in viscous media at low urea concentrations. Importantly, they exhibited rapid cellular uptake and were further evaluated as active transport carriers for growth factors, as IGF-1, preserving the bioactivity of the conjugated IGF-1 and demonstrated pro-regenerative effects in chondrogenic cell model. The third part of this thesis further explores the application of NG-NMs for antimicrobial therapy, extending their use to other biomedical applications that require navigation across mucosal barriers for effective drug delivery. In this case, hyaluronic acid (HA), a naturally occurring mucoadhesive biopolymer, was employed as the core material to construct enzyme-powered nanomotors (HA-NMs) functionalized with urease. These HA-NMs were designed to actively cross mucin environments and deliver therapeutic agents, such as antibiotics. Their performance was assessed in mucosal models, highlighting their potential to overcome barriers associated, for instance, with antimicrobial resistance and enable targeted treatment at mucosal interfaces. Their performance was evaluated in vitro using mucosal models, including transwell assays, which confirmed their ability to penetrate mucin barriers and significantly reduced bacterial proliferation. When loaded with antibiotics, HA-NMs exhibited enhanced antibacterial activity against Escherichia coli compared to both free antibiotic and reference MSNPs-NMs from the first part of the thesis. The results presented in this thesis highlight the transformative potential of enzyme-powered NMs as highly versatile platforms for targeted therapeutic delivery. Their ability to actively navigate complex and viscous biological environments and enhance therapeutic efficacy represents an important breakthrough in the field of nanomedicine. These findings open the door for a new generation of self-propelled nanotherapeutics, bringing us closer to their clinical translation and opening new avenues for precision treatment in challenging pathological contexts.
Hyperbolic Cartography of Complex Networks. Designing Maps for Unipartite, Bipartite, and Feature-Enriched Graphs
(2025-09-30) Jankowski, Robert; Serrano Moral, Ma. Ángeles (María Ángeles); Boguñá, Marián; Universitat de Barcelona. Facultat de Física
[eng] Cartography is the science and practice of creating and utilizing maps. Originating in ancient times, it flourished notably during the Age of Discovery and remains essential today through digital mapping applications. Historically, maps have served both to depict the world generally and to facilitate navigation and wayfinding. Nowadays, we regularly rely on tools such as Google Maps to commute within cities through transportation networks, including metro, bus, and tram systems. These physical networks are naturally embedded in Euclidean space. However, many social, biological, or technological networks lack such apparent spatial structures. In this Thesis, we develop methods to chart multidimensional hyperbolic maps for a diverse range of complex systems, encompassing unipartite, bipartite, and feature-enriched networks. These maps provide new means to interpret, analyze, and visualize these systems. Studying interactions between discrete entities has emerged as the study of complex networks in recent years. Many networks share similar topological properties, such as the small-world effect, heavy-tailed degree distributions, high clustering coefficients, and sparsity. The network geometry framework was proposed to explain these properties. In this approach, nodes are located in the hidden latent space, where each node’s coordinates reflect its similarity to other nodes and its intrinsic popularity, thereby intertwining the geometry with the network topology. Models from this family are able to explain many properties of real networks. These latent-space models accurately replicate empirical observations by interpreting connection probability as a decreasing function of hyperbolic distance. The quest to reverse-engineer these geometric models gave rise to model-based hyperbolic embeddings. By projecting complex networks into low-dimensional hyperbolic spaces, graph operations, such as community detection, node classification, and link prediction, become efficient vector computations. To date, however, these methods have been limited to one dimension, even though there is no reason for this to be the case. In this Thesis, we develop three multidimensional hyperbolic embed-dings: D-Mercator for unipartite networks, FiD-Mercator for feature-enriched networks, and B-Mercator for bipartite networks, demonstrating that these embeddings significantly enhance our understanding of network structures. With D-Mercator, we estimated the intrinsic dimensionality of real networks in terms of navigability and community structure. FiD-Mercator emphasized the importance of quantifying the correlations among node labels, graph topology, and node features for downstream tasks. Whereas, B-Mercator revealed patterns of linguistic and geographic diversity, and also improved the node classification task. Moreover, we bridged the gap between network geometry and graph machine learning by introducing HypBench, a benchmarking framework for graph neural networks. HypBench provided concrete guidance for selecting the optimal model for specific graph datasets. Together, these contributions establish a flexible, multidimensional framework for mapping and analyzing complex networks, offering novel insights and practical tools for advancing research across multiple disciplines.
Development of a fast-timing detector for Time-of-Flight Applications: enabling new opportunities in Positron Emission Tomography and Velocity Map Imaging Mass Spectrometry
(Universitat de Barcelona, 2025-11-14) Mariscal Castilla, Antonio; Guberman, Daniel; Gómez Fernández, Sergio; Universitat de Barcelona. Facultat de Física
[eng] Silicon photomultipliers (SiPMs) have driven recent advances in time-of-flight radiation detectors owing to their single-photon sensitivity, excellent time resolution, and compact form factor. To support the development of fast and scalable detection systems, new front-end electronics are required that can process the fast signals from SiPMs while maintaining low power consumption and minimal size. Application-specific integrated circuits (ASICs) represent the most promising solution, offering a high degree of customization that enables the design of power-efficient, compact, and cost-effective electronic systems. This thesis presents the design and testing of a novel SiPM-based radiation detector that employs the FastIC ASIC. FastIC is an analog front-end that extracts the arrival time and peak amplitude from eight SiPM channels, with a power consumption of approximately 12 mW per channel. This allows for the development of compact and scalable detector systems. The performance of the proposed detector is evaluated in two applications: time-of-flight positron emission tomography (TOF-PET) and time-of-flight mass spectrometry, with a particular focus on velocity map imaging mass spectrometry (VMImMS). In time-of-flight positron emission tomography (TOF-PET), the com-bination of SiPMs and fast scintillators has enabled the achievement of coincidence time resolutions (CTR) below 200 ps FWHM in commercial scanners and below 100 ps FWHM in laboratory settings. However, sub-100 ps performance typically requires bulky, high-power electronics, limiting the scalability of these results for clinical systems. In this context, FastIC emerges as a promising candidate for achieving high timing performance with low power consumption. To evaluate FastIC capabilities, I measured the CTR of single-channel detectors using modern SiPMs and scintillators. Using a 3 mm-thick LSO crystal coupled to an FBK SiPM, CTR values of approximately 76 ps FWHM were obtained. When using LYSO crystals with dimensions comparable to those in commercial scanners, CTR values around 127 ps FWHM were achieved. Measurements with pure Cherenkov radiators demonstrated that FastIC, in combination with SiPMs, can process prompt light with CTR performance comparable to that of high-power electronics reported in the literature. In time-of-flight mass spectrometry (TOF-MS), SiPM-based detec-tors offer a promising alternative to conventional microchannel plate (MCP)-based detectors by addressing key limitations such as restricted ion rates, aging effects, the need for high vacuum conditions and low sensitivity for high mass-to-charge ratio (m/z) ions. Furthermore, due to limitations in MCP readout systems, current detectors are unable to achieve simultaneous sub-nanosecond time and sub-millimeter spa-tial resolution. This constraint prevents the measurement of full three-dimensional velocity distributions of molecular fragments in velocity-map imaging mass spectrometry (VMImMS). The proposed ion detector consists of a fast scintillator, an array of SiPMs, and FastIC-based readout electronics. Two configurations are considered: direct ion detection using the scintillator, or enhanced signal amplification by employing first a single-stage MCP to convert the ions to thousand of electrons. A small prototype was developed, comprising a 16-channel SiPM array, a fast organic scintillator, and FastIC readout electronics. This prototype was tested in a custom-built velocity-map imaging TOF-MS instrument and used to acquire TOF-MS spectra of C3H6 and CF3I molecules. The detector successfully measured ions with m/z of 196 and 18, achieving time resolutions of approximately 3.3 ns and 2.5 ns FWHM, respectively, thus demonstrating its ability to reconstruct TOF-MS spectra with high precision. A key feature of the detector is its potential to operate at ion fluxes up to 109 cm−2 s−1, exceeding the performance of conventional MCP-based detectors. The single-stage MCP configuration enhances sensitivity to ions with high m/z ratios while maintaining a high maximum ion processing rate. Two critical effects were investigated to assess the feasibility of using SiPMs for MCP readout: the impact of SiPM dark count rates (DCRs) on detector sensitivity, and the influence of time walk introduced by fluctuations in the MCP electron yield on time resolution. The first effect was mitigated by adjusting the FastIC discriminator threshold to achieve DCRs comparable to those of MCPs. An optimal threshold was identified at the 8 photoelectron (phe) level for an SiPM bias voltage of 55 V using a Hamamatsu device. To address the second effect, time walk was corrected by measuring signals of varying intensities from a picosecond laser, yielding a corrected time resolution of approximately 120 ps FWHM. To achieve the sub-millimeter spatial resolution required for full 3D velocity imaging, a novel camera design was proposed. This design employs an MCP for signal amplification and an optical window to distribute scintillation light across multiple SiPMs. Monte Carlo simulations indicated that spatial resolutions below 200 µm can be attained, demonstrating the system capability to achieve sub-millimeter precision. Additional VMImMS simulations confirmed that, with the achieved time and spatial resolution, the detector can successfully measure the three-dimensional velocity distributions of molecular fragments. The detector technologies developed in this thesis present significant opportunities across a broad range of applications. In medical imaging, improved time-of-flight resolution in PET detectors enhances image quality and sensitivity, enabling faster scans and supporting the development of more accessible and cost-effective systems. In mass spectrometry, a SiPM-based ion-to-photon detector facilitates the de-sign of compact and portable instruments suitable for high-throughput screening in fields such as drug discovery and toxicology. When combined with a MCP, the detector can achieve enhanced sensitivity to ions with high m/z ratios, expanding the potential of TOF-MS for pesonalized medicine. In velocity-map imaging, the capability to record three-dimensional molecular movies broadens its applicability to the study of more complex molecular systems.
The Multisymplectic Geometry of Classical Field Theories on Finite-Dimensional Covariant
(Universitat de Barcelona, 2025-07-24) Guerra IV, Arnoldo; Román-Roy, Narciso; Universitat de Barcelona. Facultat de Física
[eng] This thesis presents new developments in the De Donder–Weyl formulation of classical field theories, which treats space and time on an equal footing. The formal Lagrangian geometric construction of field theories takes place on manifolds of jets, giving rise to the standard Euler–Lagrange equations, while the equivalent Hamiltonian formulation of De Donder–Weyl takes place on affine dual jets, giving rise to the Hamilton–De Donder–Weyl equations. Manifolds of jets (and their duals) act as finite-dimensional phase spaces, and their multisymplectic geometry acts as a generalization of symplectic geometry in classical mechanics, underpinning variational calculus and providing a powerful set of tools for the investigation of Noether symmetries and bonds. In addition, the multisymplectic construction of relativistic field theories preserves covariance throughout the analysis of these theories and, by working on sections of jet manifolds (and their duals) over space-time, it is possible to derive the covariant phase space formalism from Zuckerman [152], Crnković and Witten [34], and Lee and Wald [108], best known in physical literature and, in general, of infinite dimension. The original contributions of this thesis include new properties of De Donder–Weyl bonds and natural symmetries, which we present as recently proven mathematical propositions and can be found in the following publications: [63, 75, 76]. This work also provides a multisymplectic construction of the Poisson parentheses introduced by Marsden et al. [116] to obtain the Hamilton–De Donder–Weyl equations from the action functional, in direct analogue with the analysis of classical mechanics. In addition, we show how our geometric interpretation of these Poisson parentheses also leads to field equations and discuss their relationship to the infinite-dimensional covariant formalism mentioned above. Finally, we offer a first step towards understanding the multisymplectic geometry associated with the BRST–BV analysis of gauge theories in the Lagrangian framework. After constructing the BRST symmetry, we find the corresponding multimoment application and show that it provides the standard BRST charge except for a total derivative. The novelties presented in this thesis offer a new selection of geometric techniques that allow us to move from the infinite-dimensional language of local functionals to the finite-dimensional language of vector fields and differential forms, treating space and time on an equal footing.
B radiative decays at LHCb: measurement of B→ K∗γ isospin asymmetry and preparation of Run 3 analyses
(2025-09-08) Lobo Salvia, Aniol; Graugés Pous, Eugeni; Marin Benito, Carla; Universitat de Barcelona. Facultat de Física
[eng] The Standard Model (ME) of particle physics is the theory that describes fundamental particles and their interactions, except gravity. Despite its success and accuracy in predicting phenomena, it is known that it is not a complete theory. It does not describe dark matter or energy, the asymmetry between matter and antimatter in the Universe, or the origin of neutrino mass. Consequently, it is vitally important to investigate possible extensions of this model and measure processes that may reveal new physics. An experiment that contributes to this task is the LHCb, one of the four main detectors located in the LHC particle accelerator operated by CERN. His study focuses on high-precision measurements of particles involving heavy quarks b and c. Rare radiative decays of B mesons are an ideal platform to test EM and study its possible extensions. These decays cannot occur through tree diagrams within the description of the ME, but take place through loops in which unknown particles could participate, thus altering the observable quantities. This thesis presents three interconnected topics within the study of rare radiative decays of B mesons: the experiment's calorimeter monitoring tools, the radiative inclusive selections in the trigger system, and the measurement of the isospium asymmetry of the B→K*γ decay. The measurement of the isospium asymmetry of the B→K*γ decay has been done using data from Run 2, which covers the years 2015 to 2018. The other two tasks have been carried out within the framework of Run 3, starting in 2021. Between these two Runs, an ambitious improvement program, called Upgrade I, has made it possible to increase the capacity of the detector. Some of the subdetectors have been completely renewed, the data reading system has been improved to facilitate 40 MHz reading and the hardware phase of the trigger has been dismantled, becoming the first large detector with a completely software-based trigger. This set of improvements has made it possible to increase the instantaneous luminosity at which the detector operates by a factor of five, thus gaining sensitivity and its precision measurements. The monitoring of the calorimeter makes it possible to detect anomalies in the data collection and provides information on the origin of the problem in order to be able to solve it quickly. It has been a critical function during the commissioning period of Run 3, until the system has reached its stable operation. Trigger selections inclusive of radiative processes make it possible to capture events of this type and rule out those that constitute background noise, thus keeping the volume of data stored within the limits of the system. The selections for the new instantaneous high luminosity environment have been optimized and the BDT multivariable method has been used in order to obtain the best possible signal efficiency. Finally, the measurement of the isospy asymmetry of the B→K*γ decay is obtained for the first time with data from the LHCb and with an accuracy comparable to the measurements of other experiments, validating the ME and applying constraints on its possible extensions, in particular extensions of supersymmetry and extra Higgs sectors.
Artificial 3D Skeletal Muscle Tissues to Model and Investigate Disease Onset and Progression Mechanisms
(Universitat de Barcelona, 2025-07-22) Mughal, Sheeza; Ramón Azcón, Javier; Fernández-Costa, Juan Manuel; Universitat de Barcelona. Facultat de Física
[eng] This thesis investigates the innovation and application of three-dimensional (3D) in vitro skeletal muscle tissue models to study and model muscle pathophysiology, particularly in idiopathic and corticosteroid-induced myopathies. Based on the historical evolution of cell culture techniques, this work highlights the limitations of conventional two-dimensional systems in reproducing native muscle structure, bioenergetics, and functional features. In the first section of this work, 3D human skeletal muscle constructs were developed using human muscle progenitor cells and exposed these tissues to sera from patients with Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and long COVID (LC-19). Structural, functional and transcriptomic analyses post-treatment revealed disease-specific, time-dependent disturbances in mitochondrial function, protein homeostasis, and inflammatory signaling. The disease pathology in these models demonstrated mitochondrial stress, IL-6–mediated catabolism, and metabolic reprogramming toward glycolysis, with ME/CFS samples showing particularly severe contractile deficits. In the second part, a steroid myopathy model was produced by exposing 3D muscle tissues to dexamethasone. Taurine supplementation effectively restored contractile function by promoting protein synthesis and suppressing proteolysis by regulating the AKT/mTOR pathway modulation, even in the continued presence of dexamethasone. Together, these models provide powerful platforms for investigating disease mechanisms and testing therapeutic interventions, offering new mechanistic insights into muscle dysfunction. These insights are particularly important in conditions with unknown etiologies. Future prospects include developing an in vitro construct together with immune and neural components, increasing patient cohorts, and improving these models for clinical translation, particularly for testing patient stratification.
Some considerations about the climate variability and change in the Mediterranean region
(Universitat de Barcelona, 2025-09-04) Cos i Espuña, Pep; Doblas-Reyes, Francisco Javier; Marcos Matamoros, Raúl; Universitat de Barcelona. Facultat de Física
[eng] Climate change has many effects, and while global warming caused by human activity is widely recognised, regional-scale changes are particularly important yet uncertain. To effectively communicate, plan protective measures, and adapt to these changes, it is crucial to improve the regional climate information and expand the knowledge of regional climate variability. This thesis focuses on the Mediterranean region, not only because it is where we are located, but also due to its high population density, distinct climate, and the expected intensification of climate change beyond the global average. The thesis begins with an analysis of temperature and precipitation projections from the latest phase of the Coupled Model Intercomparison Project (CMIP6). It considers multiple sources of uncertainty, including different climate models and emission scenarios, and compares the results with those from the previous CMIP5 multi-model set. Findings confirm that the Mediterranean is a climate-change hotspot, showing significant warming and drying trends, especially in summer. Despite differences in global warming projections, CMIP6 projects a warmer future than CMIP5, the pat-tern of amplified regional warming remains consistent. Uncertainty increases toward the end of the 21st Century, driven by the differences in emission scenarios, under-scoring the urgent need for mitigation efforts. Without emission reductions, summer mean temperatures (JJA) could rise by up to 8◦CZ by 2100. Precipitation projections vary widely, but drying trends become clearer under higher emission scenarios, highlighting the region’s future vulnerability to drought and water shortages. The study employs weighting methodologies on the multi-model ensembles to constrain their sampling and performance issues. The thesis then examines the near-term climate in the region, focusing on methods to estimate Mediterranean summer temperatures over the next 20 years from CMIP6 projections. At this timescale, internal climate variability plays a dominant role in uncertainty. The study evaluates different methods that incorporate internal variability by selecting simulations that match the recent climate state. This comparative assessment of constraining methods for short-term climate projections is novel and provides a framework for testing them against observational data. Results show that effectiveness varies depending on the method used and the area considered within the Mediterranean region. Selection approaches based on sea surface temperatures appear promising for improving the estimates but require further refinement for reliability. The research highlights the importance of evaluating the methods to estimate future climate against past observations. Given the complexity of the constraining methodologies that attempt to capture internal variability in uninitialized simulations, the study proposes a framework to improve the understanding of the results and guide future constraining techniques. The thesis concludes addressing another process that affects the Mediterranean climate and has received little attention: the Saharan warm air intrusions, which are not necessarily linked to dust storms and transport. These intrusions can impact extreme temperatures across large parts of the Euro-Mediterranean region throughout the year. The study identifies a rising trend in these events during summer, winter, and autumn over the historical period. It also identifies large-scale atmospheric circulation patterns associated with their occurrence. Understanding these mechanisms could improve climate estimates and serve as an additional metric for evaluating climate models. Initial results indicate that CMIP6 models perform in a variety of ways when reproducing the historical frequency, seasonality, and trends of the intrusion events. This research explores open questions about the Mediterranean climate, enhances understanding of its variability, and proposes ways to improve the quality of available climate information using climate simulations and observational products.
Toroidal nematics: Point defects, disclination lines and walls
(Universitat de Barcelona, 2025-05-29) Rojo-González, Javier; Fernández-Nieves, Alberto; Universitat de Barcelona. Facultat de Física
[eng] Confining partially order fluids can lead to frustration if the locally preferred state can not be achieved everywhere simultaneously. When this happens, the configuration of the material can not be given by the local minimum, and a plethora of compatible frustrated configurations become possible. In this case, the ground state configuration results from the minimization of the total energy. This is specially exemplified by nematic liquid crystals where the anisotropic building blocks reduce their free energy by aligning parallel to each other. Perfect parallel alignment can become impossible everywhere when the nematic is confined within different shapes while fixing the orientation at the boundaries. How this frustration is resolved and which configuration the liquid crystal acquires depends on the geometry and topology of the boundary and might require the presence of regions with no defined alignment, called defects. In this thesis we mainly focus on experimentally studying the configurations acquired by a nematic liquid crystal confined to a stable toroidal droplet with different boundary conditions. We first study the case with tangential alignment at the surface. In this case, we find stable defect populated configurations when the torus is heated to the isotropic phase and cooled back to the nematic phase. They consist mainly on defect pairs whose number and location is random and that turn out to be metastable with an energetic barrier that is high enough for them to not spontaneously annihilate. We also report the appearance of a configuration not fully elucidated that we call ”X” pattern and that can have a solitonic character. Note that all these features are observed even when there is no topological requirement for the torus to have defects. We then study the effect of applying magnetic fields on planar nematic tori with and without defect pairs and find that stable inversion walls form. In the defect free case, two splay-bend walls form, while when a defect pair is present, either one of the walls does not appear or an extra wall is formed depending on the initial location of the defect pair. Interestingly, we find that the mobility of the defects allows for the defect pair wall to reorganize into different types of walls, becoming a wall of the twist type to reduce energy. We also show how, by heating and cooling our nematic droplets in an external magnetic field, the number of defect pairs generated can be controlled. We finish our work on nematic tori by studying the case with perpendicular anchoring at the surface. We recover previous experimental results and find that the anchoring strength plays a significant role in the appearance of a configuration characterized by four dark brushes forming a spiral. Moreover, for certain conditions, stable configurations filled with disclination lines are found and experimentally studied using epifluorescence microscopy. Aside from our experimental work, from a theoretical perspective, we use the decomposition of a director field distortion into four normal modes to derive the compatibility conditions for a 2D and 3D nematic. For that, we use a simple approach based on imposing that the director field variation has to be the same when going through two different paths up to second order. This approach gives an intuitive interpretation of the different equations and provides a simple way to relate them to their counterpart in curved space through the concept of holonomy. Finally, we also discuss some side projects collaterally related to the main thesis work. We first discuss our studies on water evaporation of chitosan hydrogels. We describe the constructed setup to generate the gels and our experiments that track the mass evolution of the gels in time. Our results suggest that the evaporation rate for our hydrogels is slower than pure water due to an increase in the energy barrier for the water molecules to escape the gel, thus bringing about a kinetic character to the process. We also show the design, construction and first tests on an upscaled prototype for the production of these chitosan hydrogels. We end by briefly describing our work with thermorresponsive hydrogels that present a constant volume phase transition leading to phase coexistence between a swollen and shrunken phases when the temperature is increased rapidly. We mainly show how cylindrical gels suddenly bend due to a spontaneous polarization of the region that shrinks, which is in accordance with the current theory for this process.
Mathematical Ideas in Quantum Computing
(2025-05-15) Lin, Ruge; Latorre, José Ignacio; Universitat de Barcelona. Facultat de Física
[eng] This thesis can be classified into the research field of quantum algorithms. A typical paper in this field usually focuses on what kind of things we can do with a quantum computer, whether they are useful or advantageous compared to classical computers, and how we can do it more efficiently. Since the ideal fault-tolerant large-scale quantum computer will not be available soon, many papers focus on noisy intermediate-scale quantum (NISQ) devices, how to characterize them through various benchmarking protocols, and how to make them useful through error mitigation or variational algorithms. Most probably, anyone who ends up opening this thesis is already an expert in certain topics, but they can still find some “out of the box" elements in one chapter or another. The results presented in the thesis might not be directly useful, but hopefully, these elements can provide insights and inspiration. The main body of this thesis is organized into six independent chapters. Each chapter is complete and self-consistent. All these chapters share a similarity: They provide some mathematical ideas of quantum computing in one aspect or another. Since part of the research is conducted in collaboration, the pronoun “we" is used throughout the work. In the first chapter, we focus on actions that transform quantum states into quantum states. We begin with a more theoretical approach. These actions naturally process a group structure. And we ask, under what conditions is this group finite? Given an arbitrary finite group, how can we construct these actions? How can these actions be useful to quantum computing? The set of these actions is named "Quantum representation of finite group," and answers to these questions are provided in the main text. Then, in the second chapter, we switch to a more practical scenario. The inspiration came from a problem that appeared in post-quantum cryptography, the dihedral coset problem, which uses quantum algorithms to recover a secret of the group hidden in quantum states. We simplified this problem and designed an interactive protocol for verifying the computational capacity of a quantum device. Holding a small quantum computer, the verifier prepares quantum state samples that encode a secret and sends them one by one to the prover via a one-way quantum channel. The prover then uses his quantum device to recover the secret with a certain probability. The device’s capacity is then reflected in the accuracy of the solution. This interactive protocol can be used locally on a single device as a benchmarking protocol. In the third chapter, we explore quantum entanglement. We present a new method for visualizing entanglement during a quantum evolution, such as the execution of a quantum algorithm. We name this method "Entanglement trajectory" and show that it can provide a "fingerprint" for each quantum process. We determine the boundaries of these trajectories with Random Matrix Theory and use different examples and entanglement measurements to show that this trajectory has an intrinsic quantum meaning and provides more information about the system. In the fourth chapter, we elaborate on the entanglement presented in a succession of numbers. First, we give a numerical and analytical description of the average entropy of succession, which can provide a standard for studying any successions. Then, we simulate the example of k-almost primes and its unions for different system sizes and explore their particular feature. In the fifth chapter, we will show three different methods of using a quantum annealer to solve Multivariate Quadratic (MQ) problems. In the sixth chapter, we present a data encoder in the fixed Hamming weight subspace, which is parameter-optimal and can be extended to a sparse and binary data encoder. We will give a small summary for each chapter. At the end of this thesis, we will provide an overall conclusion for our readers as “take-home messages".
A multiscale study on multivalence targeting
(Universitat de Barcelona, 2025-12-01) Xie, Zhendong; Battaglia, Giuseppe; Samitier i Martí, Josep; Universitat de Barcelona. Facultat de Física
[eng] Precision drug design utilizes detailed biological information, such as genetic profiles, cellular phenotypes, and molecular expression patterns, to develop therapeutics that are highly specific, effective, and safe. In this work, we demonstrate the potential of nanomedicine in the development of precision drugs. We establish a set of design principles to guide nanoparticle selectivity, grounded in thermodynamics, kinetics, and pharmacokinetics. Nanomedicine can leverage the multivalent effect to enhance binding avidity. To better understand nanoparticle–cell interactions, we developed a thermodynamic model called the phenotypic targeting theory (PAT). Generally, a larger number of low-affinity ligands can effectively increase binding selectivity. We applied PAT to study poly(2-(methacryloyloxy)ethyl phosphorylcholine)-poly(2-(diisopropylamino)ethyl methacrylate) (PMPC-PDPA) nanoparticles with various topologies. The model predicts the binding fraction and association constant for different nanoparticle architectures. Based on PAT, we analyzed the selectivity parameter of spherical nanoparticles as a function of particle radius and the degree of polymerization of a single PMPC chain. Since PMPC targets three distinct receptors, we also explored how changes in receptor density affect the binding fraction. These results provide a quantitative framework for optimizing nanoparticle design for selective targeting. We extended the PAT to include the targeting of large receptors, such as the low-density lipoprotein receptor-related protein 1 (LRP1) and the mannose receptor (MRC1/CD206). When targeting such large receptors, both osmotic steric penalty and compression steric penalty must be taken into account. Additionally, a steric penalty arises from unbound ligands. When an excessive number of functional ligands is used, this steric penalty increases, leading to a reduction in binding strength, ultimately resulting in negligible or no binding. This steric effect is influenced by both the number of functional ligands and an interference parameter. To account for these factors, we redefined the selectivity parameter as a function of the number of ligands rather than the number of receptors. We then analyzed this revised selectivity parameter in relation to binding energy, the interference parameter, the steric penalty per unbound ligand, and the number of available binding sites. After developing the PAT, a thermodynamic model, we extended it to describe the binding kinetics of nanoparticle-cell interactions. We investigated the binding kinetics of PMPC nanoparticles with various topologies, fitting experimental data using a mathematical model. By integrating PAT with a kinetic framework, we clarified key parameters governing multivalent interactions, including the on-rate, dissociation rate, and endocytosis rate. In addition, we explored the intracellular trafficking of PMPC nanoparticles with different topologies to gain deeper insight into their cellular fate. We also applied the model to fit the binding kinetics between mUNO-decorated nanoparticles, functionalized with varying ligand densities, and THP-1-derived macrophages. To expand the applicability of our model to more complex biological scenarios, we developed an advanced framework that accounts for binding kinetics, endocytosis, exocytosis, receptor recycling, and receptor synthesis. Additionally, we introduced a chaotic dynamics model to simulate receptor oscillation behavior observed in certain experimental conditions. We anticipate that this chaos-based model can be further refined and applied to better interpret uptake experiments involving dynamic receptor regulation. To quantify therapeutic efficiency, we developed a physiologically based pharmacokinetic (PBPK) model to describe the absorption, distribution, metabolism, and excretion (ADME) of nanoparticles. We applied this model to two distinct scenarios. The first involved non-specific interactions using PMET (poly(methionine)) nanoparticles, while the second was based on the Phenotypic Targeting Theory (PAT) applied to PMPC nanoparticles. For the PMET nanoparticles, we quantified key pharmacokinetic parameters and analyzed their influence on targeting selectivity. In the case of PMPC nanoparticles, we proposed a strategy to tune nanoparticle design parameters in order to enhance targeting selectivity, ensuring that the nanoparticles preferentially interact with specific cell types. We aim to use in silico methods, including thermodynamic, kinetic, and pharmacokinetic modeling, to assist researchers in designing more selective nanomedicines and gaining deeper insights into nanoparticle-cell interactions.
Development of 3D printing technologies for manufacturing microfluidic devices for bioengineering applications
(Universitat de Barcelona, 2025-06-20) Subirada, Francesc; Samitier i Martí, Josep; Rodriguez Trujillo, Romen; Universitat de Barcelona. Facultat de Física
[eng] Advancements in 3D printing have revolutionized microfluidic device fabrication, offering unprecedented flexibility and precision in the design and production of complex geometries. This thesis presents a comprehensive study utilizing a custom-made DLP-SLA 3D printer and commercially available resins to develop and evaluate microfluidic devices for applications in particle separation and fluid mixing under laminar flow conditions. A detailed analysis of printing errors was conducted, highlighting the challenges posed by over-curing effects, which significantly impact the ability to fabricate closed microchannels with high dimensional accuracy. The research involved a dual approach of computational simulations and experimental validations to optimize device performance. Spiral microfluidic devices were designed and tested for particle separation, achieving efficient separation of particles larger than 75 µm under laminar flow conditions, with Reynolds numbers carefully analyzed to ensure optimal performance. Additionally, staggered herringbone mixers were developed to enhance mixing efficiency through chaotic advection. The results demonstrated effective mixing of fluids, confirming the versatility of these devices in achieving homogenous mixing even at low Reynolds numbers. This work underscores the potential of DLP-SLA 3D printing in fabricating both open molds for PDMS replication and fully enclosed microchannels, offering a scalable and customizable alternative to traditional lithographic methods. The findings contribute to advancing the field of microfluidics, providing insights into the interplay between fabrication techniques, material properties, and device functionality. Future applications of these devices extend to biomedical diagnostics, chemical synthesis, and lab-on-a-chip systems, highlighting their relevance and impact in both research and industry.
Leveraging metabolism as a cellular state reporter for liver health and disease via hyperpolarized magnetic resonance spectroscopy
(Universitat de Barcelona, 2025-04-29) Herrero Gómez, Alba; Marco Rius, Irene; Ramón Azcón, Javier; Universitat de Barcelona. Facultat de Física
[eng] Tissue-engineered cellular models represent an innovative, ethical, and controlled research tool, offering an ideal platform to study disease mechanisms and test emerging therapies in vitro. However, their application is limited by the difficulty associated with reproducing the metabolic environment of native organs. Hyperpolarized Magnetic Resonance Spectroscopic Imaging (HP-MRSI) has emerged as a technique capable of evaluating cellular metabolism non-invasively. Despite its potential, the integration of HP-MRSI with tissue engineering has not yet been achieved, mainly due to technical challenges related to their compatibility and the reproducibility of metabolic environments in vitro. This thesis explores these incompatibilities, identifying the main challenges to combine tissue engineering with HP-MRSI. A milestone of this work is the creation and characterization of 3D cellular models and culture platforms compatible with HP-MRSI. Furthermore, this study has successfully identified metabolic signatures of Metabolic-Associated Fatty Liver Disease (MAFLD) in vivo, opening new possibilities to reproduce them in vitro, using HP-MRSI to model disease progression in the laboratory. The conclusions of this thesis establish a foundation for the future use of HP-MRSI in 3D cell models, with the potential to reduce dependence on animal models in preclinical research. Through the application of HP-MRSI to in vitro studies, this work paves the way for more efficient and ethical research into metabolic diseases to develop new treatments. The integration of these technologies represents a significant advancement in the field of metabolic imaging and biomedical research.
Solution-processed Engineering Strategies for Chalcogenide Thermoelectric Nanomaterials
(2025-02-20) Nan, Bingfei; Cabot i Codina, Andreu; Universitat de Barcelona. Facultat de Física
[eng] The chapters of this PhD thesis cover the work carried out in the period 2020-2024 by the PhD candidate Bingfei Nan at the Catalonia Institute for Energy Research (IREC) in Sant Adrià de Besòs, Barcelona, funded by China Scholarship Council (No. 202004910311). The thesis is mainly devoted to the development the high-performance and precisely controllable chalcogenide thermoelectric (TE) nanomaterials via a solution-processed bottom-up engineering strategy. This thesis is divided into five chapters. Basic research background, universal TE concepts, synthetic methods and various objectives are comprehensively introduced in the Chapter 1. The core experimental research work from Chapter 2 to Chapter 5 is presented one by one, involving a series of solution synthesis and characterization of chalcogenide TE building blocks. The work in this dissertation is based on the overall purpose of bottom-up solution processing and developing high-performance TE materials, and specifically aims to eliminate some of the major challenges in this field through the following strategies: 1) lattice thermal conductivity is reduced by various defects produced by secondary phases causing phonon scattering; 2) improvement of the Seebeck coefficient is achieved by increasing band convergence; 3) electrical conductivity is improved by increasing carrier concentration in the matrix by introducing metal nanoparticles (NPs) in a modulation doping scheme. I detail a thiol-free SnTe precursor that can be thermally decomposed to produce SnTe-Cu2SnTe3 by introducing Cu1.5Te in Chapter 2. The presence of Cu in SnTe and the segregation of the semimetallic Cu2SnTe3 phase effectively optimize the electrical conductivity while reducing the lattice thermal conductivity without affecting the Seebeck coefficient. In Chapter 3, NaSbSe2 nanocrystals (NCs) alloyed with SnTe NPs can adjust carrier concentration and band convergence. The SnTe-NaSbSe2 alloys induce multiscale defects that are beneficial to the reduction of lattice thermal conductivity. A novel colloidal quaternary Ag2SbBiSe4 is presented in Chapter 4. A modulation doping strategy based on the blending of semiconductor Ag2SbBiSe4 NCs and metallic Sn NCs is demonstrated to control the charge carrier concentration and carrier mobility. In chapter 5, I present a room temperature, aqueous-phase synthesis approach to generate Ag2Se and Bi2S3 particles, and to incorporate Bi2S3 into the Ag2Se matrix to improve the Seebeck coefficient and power factor.
Desenvolupament i implementació de tècniques de microscòpia òptica per la mesura 3D de superfícies de manufactura additiva
(Universitat de Barcelona, 2025-07-18) Vilar Solé, Narcís; Duocastella Solà, Martí; Carles, Guillem; Universitat de Barcelona. Facultat de Física
[cat] La manufactura additiva ha revolucionat els paradigmes tradicionals de fabricació, oferint una llibertat de disseny i unes capacitats de personalització sense precedents. No obstant això, les complexitats inherents a aquests processos de manufactura requereixen tècniques de metrologia òptica precises i fiables per garantir la integritat dels components fabricats. Aquesta tesi està enfocada en la investigació de la metrologia òptica tradicional i d’avantguarda, introduint tècniques i sistemes òptics innovadors dissenyats per abordar els desafiaments de la fabricació additiva. L’estudi comença amb una anàlisi dels processos de fabricació additiva existents, avaluant-ne els punts forts i febles. Posteriorment, es valoren les tècniques de metrologia més avançades, identificant-ne les limitacions per capturar geometries intrincades i propietats dels materials de fabricació additiva. Per abordar les limitacions crítiques de la metrologia tradicional en la manufactura additiva, aquesta tesi introdueix un nou sistema òptic que millora l’eficiència i la velocitat de mesura. La metrologia tradicional afronta reptes com l’elevada rugositat superficial i les variacions en la reflectivitat, que dificulten la precisió i l’escalabilitat en els processos de manufactura additiva. El sistema proposat integra un disseny òptic personalitzat, incloent-hi una lent de tub dissenyada a mida per mitigar aquests desafiaments. Amb aquesta millora en les capacitats de microscòpia, aquesta tesi tanca la bretxa entre les tècniques de metrologia existents i els requisits creixents de la manufactura additiva. A més, es presenten diversos dissenys òptics per a un objectiu de microscopi, especialment personalitzats per abordar els reptes únics en la inspecció de peces fabricades additivament. A la segona part es presenta una tècnica innovadora de metrologia òptica que permet velocitats de captura topogràfica sense precedents, superant significativament el rendiment dels mètodes convencionals. La integració d’aquest mètode en els fluxos de treball de la manufactura additiva es facilita mitjançant tècniques del control del focus axial, com l’enfocament remot i les lents varifocals. A més, la recerca explora els principis fonamentals del mètode, aprofitant propietats òptiques com la interferència i la polarització, alhora que es demostra la seva versatilitat mitjançant la implementació en un microscopi confocal. Aquestes aportacions milloren col·lectivament l’estat de l’art en la metrologia òptica. En conclusió, aquesta tesi doctoral fa contribucions significatives als camps de la fabricació additiva i la metrologia òptica a través d’avanços innovadors en sistemes òptics, el desenvolupament de dissenys d’objectius de microscopi personalitzats i la introducció d’una nova tècnica de metrologia òptica per a la captura topogràfica.
Few Interacting Particles in Low-Dimensional Quantum Systems: From Harmonic Oscillators to Fractal Lattices
(Universitat de Barcelona, 2025-02-21) Rojo Francàs, Abel; Juliá-Díaz, Bruno; Universitat de Barcelona. Facultat de Física
[eng] Quantum mechanics is a fascinating field to explore, where its effects are usually non- trivial and counterintuitive. In this Thesis, we focus on systems with a small number of particles in low dimensions, where the quantum effects are enhanced. We consider experimentally realistic systems, considering either an external harmonic confinement or a lattice potential. These systems can be created nowadays in ultracold atom laboratories worldwide, where they can control the dimensionality, the external potential, as well as the number of particles and the interactions. Our goal is to understand the static and dynamic properties of different systems involving few particles and a wide range of interactions. In particular, we focus on the one-dimensional harmonic trap, fractal lattices, and one-dimensional three-site lattices. In each system, we consider a contact interaction potential defined by a delta function in the continuum space or an on-site interaction for the lattices. In addition, we compute different properties, such as energy spectra, densities, pair correlations, mean square displacement, and population of each site, in both static and time-dependent cases. We combine analytical and numerical techniques to study the different systems. In particular, the numerical part is mostly done using exact diagonalization techniques. With this approach, we can only examine systems with few particles due to the intrinsic limitations of the method, although it provides the capacity to obtain any property needed. The case of the harmonic oscillator trap includes an additional limitation due to the exact diagonalization method, as the basis must be truncated, resulting in upper-bound energies. We show how to correct this error and obtain a better estimate of the energy and the density by using the analytical solution of the two-particle system. We demonstrate that this correction works for a larger number of particles by computing results for up to eight particles. For the systems with harmonic oscillator confinement, we compute the energy spectrum as a function of the interaction strength, the density for different interaction values, and pair correlations. We present the results for the symmetric SU (N ) case and then study systems with broken interaction symmetry. We present different interaction configurations, and we explore the ground state structure, where the correlations play an important role. We also consider the impurity case, where one particle interacts with all the bath particles but the bath particles do not interact with each other. We show that the impurity system can be mapped to an effective one particle in a double-well model, showing two phases: the miscible and the immiscible. Afterwards, we study fractal lattice systems, where the sites and the tunneling connections are configured in a non-standard scheme creating an effective finite representation of a fractal structure. Under this situation, we explore the effect on the transport of a single-particle obtaining the diffusion exponent and relating it to spectral properties. We demonstrate that the fractal slows down the motion of the particle and that this effect is robust against a random potential. Using the slower dynamics, we show how the fractal system can preserve information about the initial phase of the wavefunction for much longer times than a regular lattice. We also explore the entanglement of a two-particle system and how the interactions affect entanglement creation. Finally, we study a three-site lattice where each site has a different energy and the couplings are time-dependent. We implement the spatial adiabatic passage protocol, that transfers a single particle from the first to the third site, and generalize it for a few particle systems with interactions. Due to the interactions, the adiabaticity of the protocol is lost, but we demonstrate that it is possible to populate the third state for certain interaction strength values. As a result, since the final state populates the most energetic site of the system, we propose this setup as a quantum battery model.
Advanced Transition Metal Oxide Nanomaterials for Energy Technologies
(Universitat de Barcelona, 2025-02-10) Yang, Linlin; Cabot i Codina, Andreu; Martinez Alanis, Paulina R.; Universitat de Barcelona. Facultat de Física
[eng] Transition metal oxides (TMOs) are compounds formed from transition metals and oxygen, known for their complex and versatile structures, including diverse crystal forms and multiple oxidation states. This versatility gives TMOs unique electronic, magnetic, optical, and thermal properties, making them valuable in advanced technological applications. The ability of TMOs to engage in redox reactions, coupled with their stability abundant availability, and cost-effectiveness, makes them ideal for applications such as electrochromic devices, catalysis, and batteries. Electrochromic smart windows (ESWs) offer an attractive option for regulating indoor lighting conditions. Electrochromic materials based on ion insertion/desertion mechanisms also present the possibility for energy storage, thereby increasing overall energy efficiency and adding value to the system. However, current electrochromic electrodes suffer from performance degradation, long response time, and low coloration efficiency (CE). In Chapter 2, I detail the synthesis mechanism of defect-engineered brookite titanium dioxide (TiO2) nanorods (NRs) with different lengths and investigate their electrochromic performance as potential energy storage materials. The controllable synthesis of TiO2 NRs with inherent defects, along with smaller impedance and higher carrier concentrations, significantly enhanced their electrochromic performance, including improved resistance to degradation, shorter response times, and enhanced CE. The electrochromic performance of TiO2 NRs, particularly longer ones, is characterized by fast switching speeds (20 s for coloration and 12 s for bleaching), high CE (84.96 cm2 C−1 at a 600 nm wavelength), and good stability, highlighting their potential for advanced electrochromic smart window applications based on Li+ ion intercalation. This work was published in Small in 2023. Direct urea fuel cells (DUFCs), generating electric power from the electrooxidation of urea have great potential as a cost-effective technology to simultaneously treat urea-containing wastewater and produce electricity. DUFCs release only gaseous products, not generating new waste, and they are characterized by a relatively high theoretically open circuit voltage (OCV) of 1.147 V, similar to that of hydrogen fuel cells. However, the relatively low OCVs and peak power densities realized so far have hindered their commercialization. Therefore, improving the OCVs and peak power densities of DUFCs using low-cost and abundant transition oxide nanoparticles as catalysts is required to ensure practical significance. In Chapter 3, I detail the production of self-supported electrodes consisting of NiO nanosheets vertically grown on CuO nanowires and use them to realize the urea oxidation reaction (UOR). Such electrodes show excellent UOR performance requiring 1.39 V vs. reversible hydrogen electrode (RHE) to achieve 100 mA cm-2. Besides, DUFCs provide OCV and power densities up to 0.88 V and 11.35 mW cm-2. Electrochemical characterization and Raman spectroscopy prove the formation of NiOOH to enable the UOR. Mott-Schottky analysis and ultraviolet photoelectron spectroscopy show the NiO/CuO p-p heterostructure to facilitate the charge transfer from CuO nanowires to NiO nanosheets. Besides, at a local level, density functional theory calculations show that the presence of CuO modulates the electronic states of Ni at the very NiOOH/CuO interface, which results in stretched Ni-O bonds and a uniquely elongated N-H bond of urea that favor its oxidation. This work was published in Nano Energy in 2023. Rechargeable aqueous zinc-air batteries (ZABs) have emerged as a promising candidate technology for energy storage applications owing to their high energy density, safety, and environmental friendliness. However, ZABs are limited by the sluggish kinetics of the multiple electron-proton coupling processes involved in the oxygen evolution reaction (OER) that takes place at the air cathode during ZAB charging. Therefore, the development of transition oxide nanoparticles as highly efficient, low- cost, and durable OER catalysts is crucial for the realization of high-performance ZABs, among other electrochemical technologies. Beyond optimizing electronic energy levels, the modulation of the electronic spin configuration is an effective strategy, often overlooked, to boost activity and selectivity in a range of catalytic reactions, including the OER. This electronic spin modulation is frequently accomplished using external magnetic fields, which makes it impractical for real applications. In Chapter 4, I detail the synthesis of Ni/MnFe2O4 heterojunctions and apply them in OER. I investigate the spin modulations of the reconstrued NiOOH/MnFeOOH during the OER by the heterojunctions without an external magnetic field. NiOOH/MnFeOOH shows a high spin state of Ni, which regulates the OH− and O2 adsorption energy and enables spin alignment of oxygen intermediates. As a result, NiOOH/MnFeOOH electrocatalysts provide excellent OER performance with an overpotential of 261 mV at 10 mA cm−2. Besides, rechargeable ZABs based on Ni/MnFe2O4 show a high OCV of 1.56 V and excellent stability for more than 1000 cycles. This outstanding performance is rationalized using density functional theory calculations, which show that the optimal spin state of both Ni active sites and oxygen intermediates facilitates spin-selected charge transport, optimizes the reaction kinetics, and decreases the energy barrier to the evolution of oxygen. This work was published in Adv. Mater. in 2024. The main conclusions of this thesis and some perspectives for future work are presented in the last
Enhanced Optical Response in Arrays of Multifunctional Plasmonic Nanostructures
(Universitat de Barcelona, 2025-04-11) Rodríguez Álvarez, Javier; Fraile Rodríguez, Arantxa; Labarta, Amílcar; Universitat de Barcelona. Facultat de Física
[eng] This thesis is devoted to the study of two plasmonic nanostructures that present a 3-fold symmetry, namely, inverted honeycomb lattices of bars and twisted stacks of triskelion nanostructures. These systems are particularly interesting due to the inherent geometric frustration originated by the mis- match between dipolar excitations, and the odd parity associated with the 3-fold symmetry. The combination of FDTD simulations with the fabrica- tion and subsequent far-field and near-field optical characterization of such structures allows for a multifaceted description of the system. In this thesis, a remarkable agreement between experiments and simulation is achieved, demonstrating the effectiveness of this combined approach in elucidating the response of plasmonic systems. This thesis begins with a fundamental overview of the field, laying the groundwork for the essential concepts necessary for understanding the main findings presented in subsequent sections. These results are discussed in detail through the publications derived from this research. Furthermore, simulation methodologies, nanofabrication techniques, and characterization methods are introduced, as they form the core of the research presented herein. Publications I and II are devoted to the study of inverted honeycomb plas- monic lattices. Here we prove the potential of such structures as refractive index sensors, taking advantage of the sharp SLR and the well-defined spec- tral dependence with the refractive index. The general sensing capabilities of this SLR can easily be expanded due to the out-of-plane electric field of hot spots spanning hundreds of nanometers away from the structure surface, providing a huge potential sensing volume. From a fundamental point of view, we have successfully characterized the plasmonic response of this system through state-of-the-art EELS experiments and FDTD simulations. By using EELS, both bright and dark modes can be detected. In particular, we present the first observation of resonances with an anti-ferroelectric arrangement of the dipolar excitations of the slits in the honeycomb lattice that occur with such spatial periodicity so that their unit cell has twice the area of the honeycomb lattice. The samples presented in this part have been fabricated by EBL and specially dedicated FIB milling using Au ions to avoid contamination of the sample. Publications III and IV focus on the study of two stacked triskelia nanostruc- tures, and their response as a function of the geometry of the structure, in particular, the twist angle between them. The triskelion motif is character- ized by its 3-fold symmetry and inherent two-dimensional chirality in 2D. This system holds two coupled plasmonic resonances tunable by control- ling the angle between both triskelia. We have demonstrated that a simple bonding-antibonding model is insufficient to fully elucidate the behavior of these two resonances. Instead, we have observed a continuous evolution of the excited modes as a function of the angle between the elements. Further insight into the combination of such resonances with SLR is proposed. The fabrication of this structure by successive EBL over large areas and high degree of alignment are also detailed.
Development of a Neurovascular Microfluidic Model with an Endothelial Barrier-Integrity Sensor for Pharmaceutical Innovation in Neurodegenerative Diseases
(Universitat de Barcelona, 2025-03-14) Palma Florez, Sujey; Mir Llorente, Mònica; Lagunas Targarona, Anna; Universitat de Barcelona. Facultat de Física
[eng] Neurodegenerative diseases represent a significant health challenge, as no cure is currently available. The unique anatomy of the blood-brain barrier, which limits drug delivery to the brain, as well as the lack of predictive pre-clinical models, contribute to delays in the drug discovery process. This research presents the development of two distinct 3D models aimed at advancing pharmaceutical innovation for neurodegenerative diseases. We designed a blood-brain barrier-on-a-chip device that incorporates trans-endothelial electrical resistance electrodes for real-time assessment of barrier integrity. The 3D microfluidic design involves the co-culture of brain endothelial cells, pericytes, and astrocytes, before being fully characterized to verify the proper development of the blood-brain barrier. The developed platform was then utilized to evaluate the permeability and toxicity of novel nanotherapeutic agents for neurodegenerative diseases. Furthermore, a more advanced in vitro model for drug discovery of neurodegenerative diseases was assembled. This model incorporated human induced-neurons along with other cell types to create a neurovascular-unit in vitro. Specifically, we co-cultured human induced neurons with oligodendrocytes, astrocytes, pericytes, and endothelial cells within a microfluidic device. Finally, we monitored the release of neurofilament light, a potential biomarker of neuronal degeneration, in response to exposure to two different neurotoxic agents. Quantifying neurofilament light release enables assessment of neurodegenerative progression and evaluation of potential therapeutic interventions to mitigate disease advancement. The blood-brain barrier-on-a-chip and neurovascular-unit-on-a-chip models represent a versatile and scalable platform that offers a cost-effective, human-relevant alternative to traditional animal models. These platforms facilitate drug screening and accelerate the discovery of treatments for neurodegenerative diseases.
Directed cell migration: forces, shapes, and fluctuations in tissues. From active hydrodynamics to experiments
(Universitat de Barcelona, 2025-03-21) Pi Jaumà, Irina; Casademunt i Viader, Jaume; Universitat de Barcelona. Facultat de Física
[eng] This thesis investigates collective cell migration in epithelial tissues, under the framework of active soft matter physics. We model epithelial tissues as active polar fluids, since their components, the cells, have an internal source of energy and align, polarizing, in order to generate movement. Despite the myriad of extremely complex chemical interactions and signaling cascades within cells, ultimately motion must be governed by the most basic laws of physics. The focus of this thesis, therefore, is to use a simple and phenomenological model, encoding all these complex interactions in terms of the physical forces and mechanical parameters of the tissue. This allows us, in a very simplified but effective way, to have a generic model to model several relevant scenarios in collective cell migration. One of the main focuses of the thesis is the collective durotaxi, a phenomenon by which the migration of cellular aggregates is directed by the rigidity of the extracellular environment. Comparing the model with in vitro experiments, we identify the existence of an optimal stiffness of the substrate that maximizes the migration speed, and we observe that the wetting properties of the aggregates are closely linked to it. In addition, in the same way that the tensile forces made by cells are influenced by the stiffness of the substrate or the environment, we also study the inverse feedback mechanism, that is, how these forces can modify the rigidity of the environment. We find that their stiffening can be a mechanism to induce a jump to a much higher state of traction and trigger the spreading of the tissue. Durotaxi could have implications in fundamental biological processes where there is directed movement of cells, such as in embryonic development or cancerous metastasis. Apart from durotaxis, other factors that contribute to targeted migration are examined, such as the influence of tissue shape and fluctuations. We experimentally demonstrate that an asymmetry in shape can induce spontaneous tissue movement, without requiring global polarization, and we classify different modes of migration according to the parameters of the model. By carrying out epithelial tissue migration experiments, where we control their initial shape, we corroborate some of these predictions, especially the distinction between an anisotropic and an isotropic expansion. Finally, to incorporate the inherent variability of biological systems, we introduce stochastic noise into the model and study the consequences of different noise sources. The noise can be internal in the dynamics of polarity, or external in a coupling parameter between the cells and the substrate, related to the adhesion kinetics of the ligands. Although the model captures well some aspects of the fluctuations of the tensile forces in the experiments, such as their magnitude with the distance from the center of the fabric or the temporal correlations, other aspects remain to be explored in more depth, such as the spatial correlations and the diffusion coefficients of the center of mass of the fabric. In conclusion, this work provides a framework to decipher the physical mechanisms underlying collective cell migration, thus contributing to the knowledge of various biological processes, such as tissue regeneration, wound closure or tumor progression. The development of simple but effective theoretical models can also inspire new experimental designs, allowing a fruitful integration between theory and experiments in our quest to explain nature.

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