Indian Institute of Science Bangalore
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Investigation of Optical and Electro-optical Effects at Material Interfaces
No full textThe key questions addressed in this thesis are related to light-matter interactions at the material interfaces and are related to both plasmonic as well as Pockels effects. Plasmonics enables the design of compact photonic circuits with sub-micron electric field confinements and authorizes optical signal processing at the nanoscale. At the same time, the Pockels effect is necessary for designing on-chip interferometers and ultra-high-speed modulators. These two are crucial for integrated optics and photonics engineering. Integrated optics deals with miniaturizing large-scale optical signal processing circuit functionalities on a small footprint. This leads to performance enhancement as well as low power consumption. An electro-optic modulator is one of the most integral parts of an integrated photonic circuit. LiNbO3 is a well-known material with asymmetric crystal symmetry with a high Pockels coefficient of around 40 pm/V. However, integrating asymmetric crystals on a chip leads to slow and expensive fabrication processes. Instead, considering amorphous and poly-crystalline materials, their fabrication procedure is cost-effective and can be deposited rapidly using solution-processed techniques. In the first part of this thesis, we present an observation of the Pockels effect at the interface of a sol-gel spin-coated amorphous titanium dioxide and a poly-crystalline metal. We have found nonlinear two-dimensional susceptibility χ2D(2)(omega; omega, 0) at the interface of these materials of the order of 10^7pm^2/V. The order of magnitude is similar to a recent report of χsurface(2)(2ω; ω, ω)~ 10^6pm^2/V for the silicon-air interface. This work presents the interface of centrosymmetric TiO2 and metal as a new electro-optic material. In the next part, we argue the possibility of free carrier absorption as the probable physics instead of the pure field Pockels in the observations regarding the electro-optic effect described in the beginning. We study theoretical models expressing the dependence of free carrier absorption and dispersion on the mobility of the medium. We note a quench in free carrier absorption for very low mobilities. Since our TiO2 dielectric is amorphous and has low measured mobility, we reject the free carrier absorption as a possibility. However, the plasma dispersion effect can dominate at higher mobilities. We then describe a mechanism to inject and modulate carriers up to 10^19 cm^-3 in high mobility.
(μ≥ 1 cm^2V^-1s^-1) TiO2 with low intrinsic carrier concentration using ohmic contacts. Finally, we study these electro-modulated devices' optical properties using the transfer matrix method. It is possible to confine electromagnetic radiation at the interface of a metal and dielectric to nanoscale by converting it into surface plasmon polariton (SPPs). The SPPs are excellent tools for studying TiO2 and metal interface's electro-optical properties. In the last part of the thesis, we study the temporal coherence properties of these SPPs propagating on the surface of a metal and a dielectric. Despite the heavy loss, the coherence properties of the SPPs are preserved. We experimentally demonstrate this conservation of coherence up to 80 μm of propagation. Ultimately, we propose a miniaturized design of a plasmonic electro-optic modulator with TiO2 as dielectric
Linear and Nonlinear Ultrafast Time Resolved Spectroscopy of Topological Insulators, Weyl Semimetals and Semiconducting Nanowires
No full textUltrafast time resolved spectroscopy has recently gained immense significance due to its potential capability to explore the dynamics of photoexcited carriers in condensed matter systems. The technique has proved to be a very powerful tool in understanding the relaxation process of photogenerated carriers by providing insight into various scattering mechanisms of charge carriers, which, in turn, find significance for practical applications in optoelectronic and photonic devices. On the other hand, condensed matter physics have also witnessed a drastic change over the past few years with the introduction of topological band structure, leading to the discovery of many new materials with extraordinary properties. This has motivated the research community to investigate various scattering mechanisms of charge carriers in topological materials, both in bulk crystals as well as in nanomaterials, where the later further modifies the band structure due to confinement effects. In this work, we report our studies on understanding the carrier relaxation dynamics in topological insulators (Bi_2 Te_3,SnBi_4 Te_7), type -1 Weyl semimetals (TaAs, TaP, NbAs and NbP) and semiconducting nanowires (Te NWs) using ultrafast time resolved spectroscopy. The work mainly covers the linear and second order response of the photoexcited carriers in these materials, studied using time resolved terahertz (THz) spectroscopy and time resolved second harmonic generation (TR - SHG), respectively. The experimental results are then explained using theoretical models emphasizing on different analytical calculations for understanding the carrier relaxation dynamics, thereby giving more microscopic insight into various physical processes
Prompt and Displaced Signatures of Physics Beyond the Standard Model
No full textThe quest to understand our Universe’s fundamental particles and their interactions has led us to the
Standard Model (SM) of particle physics. Despite successfully explaining the weak, electromagnetic, and
strong interactions, the SM fails to explain many experimental observations and theoretical questions.
The solutions to these problems require new physics beyond the SM (BSM), motivating the hunt for any
indication of BSM physics. We study various probes of BSM physics, ranging from deviations of precision
measurements from SM predictions which capture indirect hints of new physics, to searching directly
for BSM particles at colliders or dark matter experiments. We explore signatures from prompt decay
of BSM particles having very small lifetimes and the exotic array of signatures arising from long-lived
BSM particles (LLPs). While the former has been the primary focus of most BSM searches, the latter
possibility has recently gained attention, demanding a more careful examination to ensure we are not
missing any part of the BSM parameter space.
The discovered Higgs boson is a leading portal connecting new physics particles to the SM particles,
and this motivates our studies in the first part of the thesis. We begin by exploring the parameter space
of the phenomenological Minimal Supersymmetric Standard Model (pMSSM), a well-motivated BSM
model, with a neutralino thermal dark matter (DM) contributing to the invisible Higgs boson decay
(M_DM ≤ M_h/2). We consider both positive and negative values of the higgsino mass parameter (µ) and
track down the region of parameter space consistent with the current collider and astrophysical constraints.
Our investigation shows that the recent experimental results put this scenario under severe tension.
Experimental searches have largely constrained the parameter space with prompt BSM particles leading
to conventional signatures in many popular BSM theories. We, therefore, shift to non-conventional
displaced scenarios in our subsequent study of long-lived mediator particles being pair produced from the
decay of the Higgs boson (M_LLP ≤ M_h/2) and their subsequent decay into standard model particles.
We compute the projected sensitivity of using the muon spectrometer of the CMS detector at the high
luminosity version of the Large Hadron Collider (HL-LHC) experiment for different production modes
of the Higgs boson and various decay modes of the mediator particle, along with dedicated detectors for
LLP searches like CODEX-b and MATHUSLA. Subsequently, we study the prospects at the hadronic
future circular collider (FCC-hh), expected to reach a centre-of-mass energy of 100 TeV. We
propose the DELIGHT detectors for dedicated LLP searches at the FCC-hh and study their sensitivity.
The second part includes studies focusing on different kinds of LLP signatures using various simplified
BSM scenarios as benchmarks. We demonstrate how the structure of collider detectors creates a distinction
between the energy deposition pattern of prompt jets and displaced jets, where the latter comes from
an LLP decay. Given the diverse applications of machine learning (ML) techniques and their promising
results in various BSM searches, we study the usefulness of a convolutional neural network (CNN) in
learning these differences and discriminating displaced jets from prompt jets, playing an essential role in
LLP searches. Finally, we end on an optimistic note, which is motivated by the question of what can be
said about the properties of the LLPs, like their mass and especially lifetime, once they are discovered.
We discuss the challenges and remedies in estimating the mass and lifetime of an LLP, for a wide range
of signatures, provided a few such LLP events are observed at HL-LHC
Role of Sirt2 in Stress-induced Muscle Atrophy
No full textSkeletal muscle is one of the essential organs in our body, responsible for various functions such as locomotion, transport, protein storage, and thermoregulation. Under certain pathological conditions, there is a reduction in muscle mass known as muscle atrophy. Skeletal muscle atrophy occurs when there is an upsurge in protein degradation and a reduction in protein anabolism. Aging-associated reduction of muscle weight is known as sarcopenia. Individuals suffering from sarcopenia have severe degeneration, particularly in fast twitching (type II) muscle fiber.
Glucocorticoid signaling is one of the significant components influencing type II muscle fiber atrophy. Many post-translational modifications are known to alter the function of the glucocorticoid receptor. To enhance our knowledge of Glucocorticoid receptor-mediated muscle degeneration, we explored the role of SIRT2, a class III histone deacetylase member, in glucocorticoid receptor-mediated skeletal muscle degeneration. Various organs express SIRT2, including muscles. Nevertheless, the importance of SIRT2 in skeletal muscle atrophy remains poorly understood.
PART I: Development and validation of a glucocorticoid-based model to study muscle atrophy in primary myotubes
Traditional models utilize cell lines to study skeletal muscle atrophy in vitro. However, experiments performed in vitro using cell lines suffer from poor reproducibility and are often inconsistent with the results observed in vivo. On the other hand, primary myotube culture is a superior model for studying the skeletal muscle phenotype in a cell-autonomous manner. We standardized the isolation of primary myotubes from neonatal murine pups. We also standardized and validated the treatment of dexamethasone (a glucocorticoid receptor agonist) to induce muscle atrophy in primary myotubes. The results show that dexamethasone treatment can induce muscle atrophy in primary culture. Further, we observe an increase in atrogenes expression and a reduction in myotube diameter. Overall our data show that dexamethasone treatment is a simple, reliable, and efficient means to induce muscle atrophy in vitro.
PART II: Understanding the role of SIRT2 in stress-induced muscle atrophy
In the current study, we have focused on the role of SIRT2 (majorly a cytoplasmic protein) in muscle under dexamethasone-induced stress conditions. We characterized dexamethasone-mediated muscle atrophy in primary myotubes, SIRT2-deficient mice, and muscle-specific SIRT2-transgenic mice. Dexamethasone treatment decreases SIRT2 levels in primary myotube cultures, as well as in the skeletal muscles of mice. SIRT2 depletion aggravates stress-induced myotube atrophy, and consistent with this, SIRT2 overexpression protects against stress-induced myotube atrophy in vitro. SIRT2-deficient mice are more susceptible to stress-induced muscle atrophy than the wild-type control.
Increased expression of SIRT2 in cardiac-specific SIRT2-transgenic mice protects them from developing cardiac hypertrophy. However, the role of SIRT2 in skeletal muscles is currently poorly understood. To address this problem, we developed a novel animal model, a muscle-specific SIRT2-transgenic mouse, in the current study. Our data suggest that these mice have increased levels of SIRT2, specifically in skeletal muscle, and do not have any visible structural or functional defects. Further, upon corticosteroid (dexamethasone) treatment, these mice displayed resistance against reduced muscle weight and increased performance compared to controls. Muscle-specific transgenic mice also showed reduced expression of muscle atrophy markers.
Our results indicate that SIRT2 depletion leads to a decrease in the inhibitory phosphorylation of glucocorticoid receptors. Consistent with this, muscle-specific SIRT2-transgenic mice display increased phosphorylation of glucocorticoid receptors. Mechanistically, SIRT2 directly interacts with and deacetylates glucocorticoid receptors reducing its DNA binding affinity.
Overall, we observed that the SIRT2 protein level reduces upon treatment with glucocorticoid signaling activator Dexamethasone. Similarly, SIRT2-deficient mice were more susceptible to dexamethasone-mediated muscle degeneration. In comparison, Upon SIRT2 overexpression in primary myotubes, we found that the muscle fibers are protected against dexamethasone-mediated muscle degeneration. Similarly, muscle-specific SIRT2-transgenic mice were protected against stress-mediated muscle atrophy. We also determined two novel acetylation sites around the DNA binding domain of the Glucocorticoid receptor, critical for its DNA binding capability. We suspect that SIRT2 regulates these acetylation sites. Collectively, these findings suggest that SIRT2 plays a protective role against stress-induced muscle atrophy, possibly via alteration of glucocorticoid signaling
Engineering of Nanomaterials: Application in Antibacterial Activity, Bio-Analyte Detection and Environmental Remediation
No full textInfectious diseases caused by pathogenic bacteria are creating a global health problem. In the recent report of World Health Organization (WHO), it has been mentioned that around 7 lacks people are dying each year worldwide due to drug resistant microbials. After discovery of the lifesaving “wonder drug” molecule penicillin, it was extensively used for the treatment of bacterial infection diseases. However, the excessive use of antibiotics leads to the development of antimicrobial resistance in the pathogenic bacterial strains to overcome the bactericidal effect of antibiotics. The drug-resistance bacteria follow multiple pathways to show resistance towards the existing antimicrobial agents and eventually make them abortive. The prevalence of these drug resistant bacterial strains poses a serious threat to the present medical system. Therefore, there is an urgency to develop advanced antimicrobial agents which can restrict the spread of pathogenic bacteria to eradicate infectious diseases. In this context, the current advancement in the field of nanotechnology would help us to develop nanomaterial-based antimicrobial agents which could be one of the possible alternatives of conventionally used antibiotics. There are numerous reports, which established that nanomaterials such as graphene oxide, carbon nanotube, noble metal nanoparticles, metal oxides like ZnO2, MnO2 etc. have possessed antibacterial activity. In particular, the use of nanosized molybdenum disulfide (MoS2), a transition metal dichalcogenide showed a great potential to utilize for the development of potent antibacterial agents owing to its unique chemical and photophysical properties. Two-dimensional MoS2 nanosheets provide a large surface to volume ratio for the effective interaction with the bacterial cell membrane. For better biological interactions of MoS2 nanomaterials, its surface modification can be easily achieved through functionalization using thiol ligand molecule. Functionalization also enhances its aqueous dispersibility in manyfold. In this thesis work, I have utilized MoS2 nanomaterials and their nanocomposites to develop nanomaterial-based effective antimicrobial agents for the pathogenic bacterial strains using multiple strategies. To extend my work towards the development of nanomaterial-based antibacterial agents, I have explored antibacterial activity of the supramolecularly self-assembled nanosized cage molecule to eradicate drug-resistant bacteria. Apart from antibacterial activity, I have also expanded the scope of applicability of our newly developed nanomaterials in the direction of bio-analyte detection and environmental remediation such as degradation of organic pollutant and detoxification of the chemical warfare agent
Urolithin-A mitigates synaptic dysfunctions associated with epilepsy through VDAC1 inhibition: A novel therapeutic approach
No full textUrolithin-A mitigates synaptic dysfunctions associated with epilepsy through VDAC1 inhibition: A novel therapeutic approach
Epilepsies are a group of neurological disorders with a prevalence of 70 million people worldwide and are characterized by spontaneous, unprovoked, and variably synchronized seizures. Despite being presented with distinct aetiologies, epileptic seizures occur due to imbalances in excitation/inhibition (E/I) activity that disrupts neuronal ensemble activity. A large group of anti-epileptic medications (AEMs) available in the market constitutes a symptomatic approach by targeting the E/I imbalance involved in seizure generation and propagation, mediated by ion channels and neurotransmitter receptors. However, these drugs produce undesirable secondary effects and are ineffective in treating 30% of patients. Therefore, the development of new adjuvant drugs is required that act on other novel components of underlying pathogenesis: oxidation, mitochondrial dysfunctions, inflammation, apoptosis, and activity-dependent synaptic modifications. Such novel approaches might provide a homeostatic boost to the neuronal network and restore physiological imbalances. Recently, interest in compounds of natural origin for anti-epileptic potential has renewed, owing to less toxicity, safer profiles, and the probability of finding novel targets. Thus, identifying safer and tolerable natural products targeting the novel downstream mediators represents a promising treatment approach against epilepsy.
While the pathogenesis of epilepsy is highly complex, with multiple stage-specific levels of anatomic, transcriptomic, and circuitry changes, aberrations in the synaptic transmission represent an essential common hallmark of initiation, epileptogenesis, and chronic illness phases of epilepsy. Studies on the protein-protein interactions have identified Punica granatum (Pomegranate) as one of the several natural products targeting synaptic proteins with anti-epileptic treatment potential. Additionally, ellagic acid (the primary polyphenolic precursor of UA found in Pomegranate) has been shown to exert anti-epileptic effects in epileptic models of mice. However, parental extracts undergo reduced bioavailability in the plasma; therefore, several studies have shown that the beneficial effects of the precursors are primarily mediated by gut microbial catabolite product, Urolithin-A (UA).
UA supplementation has been shown to benefit central nervous system (CNS) disorders such as Parkinson’s and Alzheimer’s disease, attenuating oxidative stress in neuronal cells; however, its effects on epilepsy remain elusive. In this study, we show that the UA exerts anti-epileptic effects with disease-modifying potential resulting in fewer seizure-susceptible neuronal activity in different experimental models of epilepsy with diverse aetiologies. Anti-epileptic results were scored based on behavioral paradigms and studies of synaptic transmission at both structural and functional levels in the sub-acute picrotoxin-induced chemoconvulsive and chronic genetic models with a gain of function in sodium channel bang senseless (bss1). UA administration had no effects on the epileptiform firing induced in acute brain slices of mice, indicating that the anti-epileptic effects of UA are, indeed, through target mediators of the underlying pathogenesis of seizures; therefore, slow in action. Furthermore, the results were validated in the Pentylenetetrazol (PTZ)-induced chronic kindling mouse model. Extracellular and whole-cell patch clamp electrophysiology studies have shown that the effects mediated by UA were restricted to the post-synaptic level without affecting pre-synaptic functions. Using a combination of affinity purification with synthesized UA-attached beads and molecular-docking studies, we identified a complex of voltage-dependent anion channel-1 (VDAC-1) and β-Tubulin as the direct interactor of UA in the mouse brain. Transcriptomics studies in Drosophila and mice revealed disruption in the regulation of several downstream targets hinting at VDAC-1 inhibition. Finally, VDAC-1 knockdown in the sub-acute epileptic model of Drosophila mimics the effects of UA.
Thus, our study provides an exciting and novel platform for developing effective and potent anti-epileptic therapy of Urolithin-A mediated modulation of VDAC1
Secure Computation Protocol Suite for Privacy-Conscious Applications
No full textAs an alternative to performing analytics in the clear, there is an increasing demand for developing privacy-preserving solutions that aim to protect sensitive data while still allowing for its efficient analysis. Among the various privacy-enhancing technologies, secure multiparty computation (MPC) is a promising approach that enables multiple parties (n) to jointly process their private inputs while ensuring that no coalition of at most t<n parties, under the control of an adversary, learns any information other than the intended output. In this thesis, we identify various such real-world applications that demand privacy-preserving solutions and address these via MPC. We consider a broad range of applications that span across healthcare, finance and even social sectors. For each application under consideration, we identify the desirable MPC setting (e.g., number of computing parties n) and security notion to be achieved when designing the solution. Based on this, we either design new MPC frameworks that provide improved security guarantees and efficiency or enhance the existing frameworks.
Although we make application-specific design choices, the common theme while designing secure protocols for all is to design as efficient a solution as possible. In this regard, we make the following common design choices across all applications. First, we consider an honest majority among the computing parties (i.e., t<n/2), which is known to render efficient protocols in comparison to the dishonest majority (i.e., t<n). Second, we focus on designing secure protocols in the preprocessing paradigm, where expensive input-independent computations are pushed onto a preprocessing phase, thereby making way for a fast and efficient input-dependent online phase. Finally, our protocols are designed to operate on the ring algebraic structure to capitalize on the efficiency gains obtained from utilizing the CPU architecture. We next elaborate on the specific applications considered in the thesis and the contributions therein.
Secure computation over graphs via traditional security notion:
Operating on graph-structured data is ubiquitous due to the modelling capabilities of graphs, and this finds use in analysing various systems like social networks, biological networks, transportation networks, etc. However, privacy concerns arise when analysing graphs that model sensitive data. To address this, we design privacy-preserving solutions for two popular graph algorithms---local clustering and graph convolutional networks.
-- Secure local clustering: Identifying a cluster around a target node in a graph, termed local clustering, finds use in several applications, including fraud detection, targeted advertising, community detection, etc. We design solutions for privacy-preserving local clustering, which is done for the first time in the literature. Keeping efficiency in mind for large graphs, we build over the best-known honest-majority 3-party framework of SWIFT (USENIX'21) and enhance it with some of the necessary yet missing primitives. To further enhance efficiency, we design the protocols using the GraphSC paradigm, which provides a generic secure framework for efficiently evaluating graph algorithms. Since this paradigm relies on a secure shuffle primitive, we also design an efficient secure 3-party shuffle protocol.
We note that secure shuffle is a versatile primitive that finds widespread use in various other applications as well (which may not involve computations over a graph), such as electronic voting, oblivious RAM, and anonymous broadcast, to name a few. Hence, as a by-product of our shuffle protocol, we are also able to securely realise an anonymous broadcast system. As the name suggests, anonymous broadcast enables a set of N clients to anonymously broadcast their messages while guaranteeing that none learns about the association between a message and the identity of its sender. Hence, while anonymous broadcast may not be inherently associated with graph computations, we diverge slightly to demonstrate how our shuffle protocol can be employed to realize anonymous broadcast in the 3-party setting, as considered in prior works. In the process, not only do we design a more efficient anonymous broadcast system compared to the state-of-the-art, but our system also provides improved security guarantees and properties such as censorship resistance that were missing in the prior solution.
-- Secure graph convolutional networks: Graph convolutional networks (GCNs) are gaining popularity due to their ability to effectively model and learn from complex graph-structured data. We put forth Entrada, a framework for securely evaluating GCNs. For efficiency and accuracy reasons, Entrada builds over the 4-party framework of Tetrad (NDSS'22) and enhances the same by providing the necessary primitives. Moreover, Entrada leverages the GraphSC paradigm to further enhance efficiency and entails designing a secure and efficient shuffle protocol specifically in the 4-party setting. This, to the best of our knowledge, is done for the first time and may be of independent interest.
Stepping beyond traditional security for financially and socially relevant problems:
Most protocols in the small-party setting that are designed to attain the strongest security notion of guaranteed output delivery (GOD), rely on entrusting an honest party, identified as the trusted third party (TTP), with inputs in the clear to carry out the computation. However, this may not be desirable for certain applications that deal with highly sensitive data. Another drawback of traditional MPC protocols is the view leakage attack, where a malicious adversary may send its view to an honest party, thereby enabling the latter to obtain the underlying secret information. To address these drawbacks in the traditional MPC definition, Alon et al. (CRYPTO'20) propose the notion of MPC with Friends and Foes (FaF). Thus, departing from the traditional MPC model, we identify the need to design FaF-secure MPC protocols for applications that deal with highly sensitive information, where information leakage must be prevented even against quorums of honest parties. Specifically, we consider the applications of secure dark pools and secure allegation escrow systems. Keeping efficiency at the centre stage, we design FaF-secure 5-party computation protocols (5PC) that consider one malicious and one semi-honest corruption and constitute the optimal setting for attaining an honest majority.
-- Secure dark pools: Dark pools are private security exchanges that allow investors to trade financial instruments outside of the prying eyes of the public and ensure the trade remains unexposed until it is completed. Dark pools are traditionally operated by centralized trusted brokers, who, in the past, have been known to misuse insider information. This necessitates designing solutions that guarantee privacy even against the dark pool operator. Hence, given the sensitive nature of financial data that is involved in the computation and the drawbacks present in the traditional MPC solutions, we design FaF secure solutions for the same in the 5PC setting. We design improved solutions for the continuous double auction (CDA) and volume-based matching algorithms that are used in dark pools. We benchmark the performance of these secure matching algorithms and observe improvements in comparison to the prior works.
-- Secure allegation escrow system: The rising issues of malpractices have led victims to seek comfort by acting in unison against common perpetrators (e.g., the #MeToo movement). To increase trust in the system, cryptographic solutions are being designed to realize secure allegation escrow (SAE) systems. In this regard, we identify privacy issues present in prior works and put forth an SAE system to arrest all these breaches. Given the highly sensitive nature of allegation data, we choose to realise the system under FaF security as opposed to traditional security notions. We also provide additional features which were absent in the state of the art. We benchmark the proposed system with the FaF secure 5PC protocols to showcase the practicality of our solution.
Secure computation with a constant number of parties:
Unlike the applications considered above that demanded operating with a specific number of parties, the latter may vary depending on the application. Hence, we provide a generalization which allows instantiating the MPC protocol with an arbitrary (constant) number of parties (n). Our generalized protocols continue to operate in the honest majority setting to capitalize on the efficiency benefits that this setting provides over the dishonest majority, which thereby facilitates attaining an efficient solution for the end application. We design two different protocols that are secure against a semi-honest and a malicious adversary, respectively. We also design a wide range of building blocks that facilitate the secure realization of various applications, including but not limited to genome sequence matching, biometric matching, and even deep neural networks, and showcase the practicality of the designed protocols by benchmarking these applications.
In this way, we design a range of building blocks in various MPC settings that can facilitate secure realizations for the above-mentioned privacy-conscious applications
Binding, activation, and oxidative addition of dihydrogen using Ir-pincer systems and implications in hydrogenation catalysis
No full textBinding of unreactive small molecules, such as H2, N2, CO2, CH4, etc. on a transition metal center and their subsequent activation are fundamental problems that have been extensively studied in organometallic chemistry for the past four decades. Towards this direction, activation of the H–H bond in H2, has been a topical area of research interest. The ground breaking discovery of Kubas’ complex, [W(η2–H2)(CO)3(PR3)2] in 1984,2 laid the foundation in treating other electronically saturated molecules with strong sigma bonds such as the C–H bond in alkanes in a similar manner for their activation and functionalization. From the discovery of the first dihydrogen complex to a few recently reported sigma methane complexes, the advancement in the binding and activation of either H2 or CH4 using metal complexes is not only restricted to academic research pursuits but also to pave the way for futuristic concepts such as “hydrogen economy” or “methane economy”.
In the pursuit of developing a single organometallic system (catalyst) showcasing versatile reactivity towards H2, both in terms of binding and activation, an iridium pincer complex with PPh3 as a co-ligand, [Ir(H)(Cl)(iPr)4(POCOP)(PPh3)] has been synthesized. Two different complexes derived from the same iridium pincer backbone: a weakly coordinated iridium dinitrogen cationic complex, trans-[Ir(H)(N2)(iPr)4(POCOP)(PPh3)]+,3 and a four coordinate, neutral iridium complex, [Ir(iPr)4(POCOP)(PPh3)] ,4 with two vacant sites favoring controlled binding of H2 and the subsequent scission of the H–H bond on the iridium center, has been elucidated. An unusual intramolecular H-atom site exchange occurring between H2 and the hydride ligands trans-disposed to one another, in a trans-[Ir(H)(η2–H2)(iPr)4(POCOP)(DMAP)]+ has been investigated in detail.5
In addition to the binding of H2 to give a σ-H2 complex and its cleavage to form a classical dihydride complex, attempts were made to study the systematic elongation of the H–H bond along the continuum for the oxidative addition of H2 to a metal center. A study of an equilibrium between cis-[Ir(H)(η2–H2)(iPr)4(POCOP)(PPh3)]+ to its tautomeric form [Ir(H)3(iPr)4(POCOP)(PPh3)]+ as a function of pressure of H2, has also been investigated.6 These studies were carried out to design and develop better performance catalysts for hydrogenation. The results of these studies will be presented.UGC, SERB Government of Indi
Comprehensive Investigation of the Mechanical Behavior of Nial-x (X=pt, Pd) Alloys: Effect of Composition, Orientation and Temperature
No full textGas turbine engines in the turbine section operate at temperatures exceeding 1300°C, which can cause high-temperature oxidation and reduce the lifespan of turbine blades. To counter this, thermal barrier coating (TBC) systems are employed, which not only provide thermal insulation but should also possess mechanical durability, resist strain, and prevent phase transitions. The TBC system comprises a bond coat, thermally grown oxide, and ceramic topcoat. NiAl, an intermetallic compound, is the main constituent of the bond coat and is crucial in the aerospace industry. The study aims to investigate the mechanical behaviour of NiAl by studying the effect of ternary and quaternary additions of noble elements like Pt and Pd. The study involved preparing alloys with varying Pt and Pd concentrations. Site occupancy studies were carried out using XRD and DFT calculations. The results showed that Pt and Pd strongly prefer to occupy the Ni sublattice in both stoichiometric and Ni-rich NiAl due to their good bonding interaction with Al, as revealed by density of states curves. The effects of addition of Pt and Pd along with the stoichiometry and orientation on the mechanical properties of NiAl-X alloys were also investigated using high throughput nanoindentation techniques. The addition of Pt and Pd was found to decrease the modulus and increase the metallic character of bonding in NiAl, which was demonstrated through density states calculations. Elastic constants were obtained using the indentation modulus with the help of the theoretical model proposed by Vlassak and Nix and compared with those obtained from DFT calculations and the values were in reasonable agreement with each other. It was found that the hardness of the alloys was dependent on their orientation and stoichiometry, and the addition of Pt and Pd resulted in an increase in hardness due to solid solution strengthening. The effect of temperature on the nanoindentation hardness of the alloys was investigated up to 600°C. The study also investigates the effect of Pt and Pd on the brittle to ductile transition temperature (BDTT) of stoichiometric NiAl. To achieve this, binary, ternary (Ni-10Pt-50Al), and quaternary (Ni-5Pt-5Pd-50Al) alloys were tested using uniaxial compression tests performed up to 1000 °C. The results indicated that the addition of Pt and Pt-Pd significantly increased the BDTT of NiAl
Delineating the dynamics of neutrophil response in multiple inflammatory stimuli and its consequences
No full textIn this thesis, we investigate the regulation of neutrophils, an important component of the immune system, and its implications on inflammation and pathological conditions. We use biomaterial implants to induce chronic sterile inflammation in mice and examine the effects on neutrophil production rates, maturation times, and lifespan in various tissue compartments. We demonstrate that in case where inflammation leads to emergency granulopoiesis (EG), we observe increased neutrophils progenitors, reduced maturation time in the bone marrow, reduced lifespan in circulation, and prolonged residence time at the inflammatory site. These changes aim to maintain high neutrophil numbers at the site of inflammation while keeping the circulating neutrophil count constant. However, this also results in an increase in immature neutrophils in circulation.
Next, we explore the response of neutrophils to a second inflammation in mice undergoing EG. In a PVA sponge model of second inflammation, we observe increased presence of immature neutrophils and CD38+ M1-type macrophages at the second inflammatory site, indicating heightened inflammatory reactions. Next, a lung inflammation model is employed to mimic a clinically relevant scenario, again showing increased infiltration of immature neutrophils, and CD38-expressing M1-type macrophages in the lungs of mice with EG, which correlates with increased lung damage over time. Although the exact mechanism remains to be determined, the initial data suggests that inflammatory responses are intensified at the second inflammation site in mice with EG.
Lastly, our study demonstrates that while the phenotype and numbers of neutrophils are altered in mice with EG, they still possess the ability to eliminate bacteria in the presence of bacterial infections at the second inflammation site. In conclusion, the research highlights the alterations in neutrophil phenotype, numbers, and function following high-grade inflammatory insults, which subsequently impact the innate immune response to a second inflammation, resulting in similar bacterial control but increased immunopathology