Raman spectroscopy, an ideal tool for studying the physical properties and applications of metal-organic frameworks (MOFs).

Metal-organic frameworks (MOFs) are a unique family of materials constructed by coordinating metal ions or clusters to bridging organic ligands. Many of these materials are well known for their intricate structures, and exceptional gas adsorption properties, and have potential applications in the separation of alkanes, catalysis, energy storage, surface-enhanced Raman spectroscopy (SERS) based detections, and diagnostics. In situ or in operando Raman spectroscopic studies provide real-time information about the different processes and associated structural changes in MOFs. In the last few decades, there has been phenomenal growth in the publications on MOFs containing insights from Raman spectroscopy. Such studies have helped the research community in identifying the adsorption sites, defect sites, structural or spin transitions, reaction centers, intermediates, etc. In this review, we present the current research status of Raman spectroscopy in probing the structure, guest adsorption, catalytic activity, and reaction mechanisms of MOFs, and their application in energy storage and SERS detection. We highlight the advancements in the Raman spectroscopy technique that have facilitated in situ studies in atmosphere as well as various chemical environments. We briefly discuss the relevance of computational studies in understanding phonon modes and predicting the stability of MOFs. Although this review is particularly focussed on works related to Raman spectroscopy of MOFs, we do discuss infrared studies on MOFs, where such results or analyses are missing from the Raman studies. These discussions have been provided with the intent to develop similar analysis techniques or methods in Raman spectroscopy research.

Magnetic Stress-Driven Metal-Insulator Transition in Strongly Correlated Antiferromagnetic CrN.

Traditionally, the Coulomb repulsion or Peierls instability causes the metal-insulator phase transitions in strongly correlated quantum materials. In comparison, magnetic stress is predicted to drive the metal-insulator transition in materials exhibiting strong spin-lattice coupling. However, this mechanism lacks experimental validation and an in-depth understanding. Here we demonstrate the existence of the magnetic stress-driven metal-insulator transition in an archetypal material, chromium nitride. Structural, magnetic, electronic transport characterization, and first-principles modeling analysis show that the phase transition temperature in CrN is directly proportional to the strain-controlled anisotropic magnetic stress. The compressive strain increases the magnetic stress, leading to the much-coveted room-temperature transition. In contrast, tensile strain and the inclusion of nonmagnetic cations weaken the magnetic stress and reduce the transition temperature. This discovery of a new physical origin of metal-insulator phase transition that unifies spin, charge, and lattice degrees of freedom in correlated materials marks a new paradigm and could lead to novel device functionalities.

Raman Evidence of Multiple Adsorption Sites and Structural Transformation in ZIF-4.

The zeolitic imidazolate framework, ZIF-4, exhibits soft porosity and is known to show pore volume changes with temperatures, pressures, and guest adsorption. However, the mechanism and adsorption behavior of ZIF-4 are not completely understood. In this work, we report an open to narrow pore transition in ZIF-4 around T ∼ 253 K upon lowering the temperature under vacuum (10-6 Torr) conditions, facilitated by C-H···π interactions. In the gaseous environment of N2 and CO2 around the framework, characteristic Raman peaks of adsorbed gases were observed under ambient conditions of 293 K and 1 atm. A guest-induced transition at ∼153 K resulting in the opening of new adsorption sites was inferred from the Raman spectral changes in the C-H stretching modes and low-frequency modes (<200 cm-1). In contrast to a single vibrational mode generally reported for entrapped N2, we show three Raman modes of adsorbed N2 in ZIF-4. The adsorption is facilitated by dispersive and quadrupolar interactions. From our temperature-dependent Raman results and theoretical analysis based on the density functional tight-binding approach, we conclude that the C-Hs are the preferred adsorption sites on ZIF-4 in the following order: C4-H, C5-H > C2-H > center of the Im ring (interacting with C-H centers) > center of the cavity. We also show that with an increasing concentration of N2 adsorbed at low temperatures, the ZIF-4 structure undergoes shear distortion of the window formed by 4-imidazole rings and consequent volumetric expansion. Our results have immediate implications in the field of porous materials and could be vital in identifying subtle structural transformations that may favor or hinder guest adsorption.

Erythroid spectrin binding modulates peroxidase and catalase activity of heme proteins.

Hemoglobin oxidation due to oxidative stress and disease conditions leads to the generation of ROS (reactive oxygen species) and membrane attachment of hemoglobin in-vivo, where its redox activity leads to peroxidative damage of membrane lipids and proteins. Spectrin, the major component of the red blood cell (RBC) membrane skeleton, is known to interact with hemoglobin and, here this interaction is shown to increase hemoglobin peroxidase activity in the presence of reducing substrate ABTS (2', 2'-Azino-Bis-3-Ethylbenzothiazoline-6-Sulfonic Acid). It is also shown that in the absence of reducing substrate, spectrin forms covalently cross-linked aggregates with hemoglobin which display no peroxidase activity. This may have implications in the clearance of ROS and limiting peroxidative damage. Spectrin is found to modulate the peroxidase activity of different hemoglobin variants like A, E, and S, and of isolated globin chains from each of these variants. This may be of importance in disease states like sickle cell disease and HbE-ß-thalassemia, where increased oxidative damage and free globin subunits are present due to the defects inherent in the hemoglobin variants associated with these diseases. This hypothesis is corroborated by lipid peroxidation experiments. The modulatory role of spectrin is shown to extend to other heme proteins, namely catalase and cytochrome-c. Experiments with free heme and Raman spectroscopy of heme proteins in the presence of spectrin show that structural alterations occur in the heme moiety of the heme proteins on spectrin binding, which may be the structural basis of increased enzyme activity.

Unusual CO2 Adsorption in ZIF-7: Insight from Raman Spectroscopy and Computational Studies.

Here, we use Raman spectroscopy to investigate temperature-dependent changes in the atomic-scale structure of the zeolitic imidazolate framework ZIF-7 in a CO2 atmosphere and uncover the mechanism of maximal CO2 adsorption at 206 K. At 301 K, the Raman spectra of ZIF-7 at various CO2 gas pressures reveal a narrow-pore (np) to large-pore (lp) phase transition commencing at 0.1 bar as a result of adsorption of CO2, as evident in the appearance of Fermi resonance bands of CO2 at 1272 and 1376 cm-1. Moreover, the Raman inactive bending mode of CO2 becomes active due to geometrical distortion of adsorbed CO2. It further splits into two peaks due to hydrogen bonding interactions between CO2 and the benzene ring of the benzimidazole linker ZIF-7, as supported by our computational studies. In addition, the interaction between CO2 molecules plays a key role. Upon reducing the temperature at 1 bar CO2 gas pressure, ZIF-7 exhibits softening of the imidazole puckering mode and the Fermi resonance CO2 band due to interactions between CO2 and the framework through hydrogen bonding. At 206 K, substantial modification in the lattice mode and disappearance of the Raman inactive CO2 bending mode confirm the changes in the size of the pore cavity through structural rearrangements of CO2.

Insights on Aggregation of Hen Egg-White Lysozyme from Raman Spectroscopy and MD Simulations.

Protein misfolding and aggregation play a significant role in several neurodegenerative diseases. In the present work, the spontaneous aggregation of hen egg-white lysozyme (HEWL) in an alkaline pH 12.2 at an ambient temperature was studied to obtain molecular insights. The time-dependent changes in spectral peaks indicated the formation of ß sheets and their effects on the backbone and amino acids during the aggregation process. Introducing iodoacetamide revealed the crucial role of intermolecular disulphide bonds amidst monomers in the aggregation process. These findings were corroborated by Molecular Dynamics (MD) simulations and protein-docking studies. MD simulations helped establish and visualize the unfolding of the proteins when exposed to an alkaline pH. Protein docking revealed a preferential dimer formation between the HEWL monomers at pH 12.2 compared with the neutral pH. The combination of Raman spectroscopy and MD simulations is a powerful tool to study protein aggregation mechanisms.

Functional Monochalcogenides: Raman Evidence Linking Properties, Structure, and Metavalent Bonding.

Pressure- and temperature-dependent Raman scattering in GeSe, SnSe, and GeTe for pressures beyond 50 GPa and for temperatures ranging from 78 to 800 K allow us to identify structural and electronic phase transitions, similarities between GeSe and SnSe, and differences with GeTe. Calculations help to deduce the propensity of GeTe for defect formation and the doping that results from it, which gives rise to strong Raman damping beyond anomalous anharmonicity. These properties are related to the underlying chemical bonding and consistent with a recent classification of bonding in several chalcogenide materials that puts GeTe in a separate class of "incipient" metals.

Minerals to Functional Materials: Characterization of Structural Phase Transitions and Raman Analysis of a Superionic Phase in Na6Co(SO4)4.

In search of promising Na+ ion conductors, we have detected a superionic phase in a Vantoffite mineral, Na6Co(SO4)4, at 570 °C, thus enhancing the use of minerals to produce futuristic solid state electrolytes. Na6Co(SO4)4 crystallizes concomitantly to produce di- and tetrahydrate forms from an aqueous solution. Both the crystal forms belong to a triclinic system, space group P1. The mineral transforms to a dehydrated phase as established by in situ single crystal X-ray diffraction at 217 °C and is shown to be isostructural with its Mn analogue. Even though thermal analysis indicates a single structural phase transition at 450 °C, the features associated with in situ powder X-ray diffraction as well as in situ Raman spectroscopy signify a second phase transition ≈540 °C and the behavior of ionic conductivity leads to a superionic phase (σ ≈ 10-2 S/cm at 570 °C). These observations are significant for the development and understanding of mineral based solid electrolytes.

Understanding the Mechanism of Ferroelectric Phase Transition in RbHSO4: A High-Pressure Raman Investigation.

Previous high-pressure dielectric and diffraction studies on rubidium hydrogen sulfate (RbHSO4) observed ferroelectric phase transition below 1 GPa pressure. We have performed high-pressure Raman spectroscopy studies on RbHSO4 up to a maximum pressure of 5.15 GPa and at ambient temperature to understand the microscopic origin and mechanism of ferroelectric transition. On the basis of the pressure dependence of Raman mode frequencies and their full-width at half-maxima, we observed a transition around a pressure of 0.3 GPa, similar to the ferroelectric transition discovered in dielectric measurements, followed by another transition around 2.4 GPa. These phase transitions are evident from the appearance/disappearance of Raman-active modes and the change in the slope of frequencies with pressures. From the pressure dependence of the S-O and S-OH frequencies, we deduce that HSO4- ion ordering results in ferroelectric phase transition around 0.3 GPa. Further, the transition around 2.4 GPa pressure is associated with significant changes in the stretching and bending vibrational frequencies and indicates a structural phase transition with possible lowering of the crystal symmetry. Interestingly, no significant changes are observed in the Raman spectrum around 1 GPa, at which a phase transition was noticed in earlier X-ray and dielectric studies.

Pressure-Induced Phase Transitions in Germanium Telluride: Raman Signatures of Anharmonicity and Oxidation.

Pressure-induced phase transitions in GeTe, a prototype phase change material, have been studied to date with diffraction which is not sensitive to anharmonicity-induced dynamical effects. GeTe is also prone to surface oxidation which may compromise surface sensitive measurements. These factors could be responsible for the lack of clarity about the phases and transitions intervening in the phase diagram of GeTe. We have used high-pressure Raman scattering and ab initio pseudopotential density functional calculations to unambiguously establish the high-pressure phase diagram and identify three phases up to 57 GPa, a low-pressure rhombohedral phase, an intermediate pressure cubic phase, and a high-pressure orthorhombic phase. We detect substantial broadening and softening of Raman modes at low pressure and identify the transition regions and possible intermediate phases.

In Situ Growth of Self-Assembled ZIF-8-Aminoclay Nanocomposites with Enhanced Surface Area and CO2 Uptake.

Self-assembly of metal-organic framework (MOF) nanoparticles (NPs) with a functional material can result in MOF nanocomposites having new and advanced properties along with the fabrication of new nanoscopic structures. However, such assembly of MOFs has not been realized to date. Here we report self-assembled nanocomposites of the zeolitic imidazolate framework (ZIF-8) and layered aminoclay (AC) for the first time, and the ZIF-8@AC composites exhibit significantly enhanced adsorption properties in comparison to those of pristine ZIF-8 nanoparticles. Four different composites denoted as ZIF-8@AC-1, ZIF-8@AC-2, ZIF-8@AC-3, and ZIF-8@AC-4 were synthesized by varying the clay content, and their AC contents were found to be 12.1, 18.3, 22.2, and 27.2 wt %, respectively. The composites were thoroughly characterized by PXRD, FTIR, Raman, and various microscopic techniques (FESEM, TEM, and STEM). The formation of the composites is driven by the specific interaction between unsaturated Zn(II) sites of ZIF-8 nanoparticles and NH2 groups of the aminoclay, which was validated from ζ potential and Raman spectroscopic measurements. The adsorption studies of the desolvated composites were also carried out in detail. The best performance is achieved with one of the composites, which exhibits a 42% increase in BET surface area while CO2 uptake at 298 K is doubled in comparison to the ZIF-8 nanoparticles.

Bi4TaO8Cl Nano-Photocatalyst: Influence of Local, Average, and Band Structure.

The average structure, local structure, and band structure of nanoparticles of photocatalyst Bi4TaO8Cl, an Aurivillius-Sillen layered material, has been studied by powder neutron Rietveld refinement, neutron pair distribution function technique, Raman scattering, and density functional theory calculations. A significant local structural deviation of nano-Bi4TaO8Cl was established in contrast to the local structure of bulk-Bi4TaO8Cl. Local structure was further supported by Raman scattering measurements. Through DFT calculations, we identify specific features in the electronic band structure that correlate lower secondary structural distortions in nano-Bi4TaO8Cl. Increased distortion of TaO6, decreased Ta-O-Ta bond angle, and increased octahedral tilt in the local structure of nano-Bi4TaO8Cl influence the band structure and the electron hole pair migration. Therefore, in addition to morphology and size, the local structure of a nanomaterial contributes to the photocatalytic performance. Trapping experiments confirm the role of superoxide radical in the photocatalysis mechanism of this material. Such studies help in developing new functional materials with better photocatalytic efficiency to address energy and environmental issues.

SERS and MD simulation studies of a kinase inhibitor demonstrate the emergence of a potential drug discovery tool.

We demonstrate the use of surface-enhanced Raman spectroscopy (SERS) as an excellent tool for identifying the binding site of small molecules on a therapeutically important protein. As an example, we show the specific binding of the common antihypertension drug felodipine to the oncogenic Aurora A kinase protein via hydrogen bonding interactions with Tyr-212 residue to specifically inhibit its activity. Based on SERS studies, molecular docking, molecular dynamics simulation, biochemical assays, and point mutation-based validation, we demonstrate the surface-binding mode of this molecule in two similar hydrophobic pockets in the Aurora A kinase. These binding pockets comprise the same unique hydrophobic patches that may aid in distinguishing human Aurora A versus human Aurora B kinase in vivo. The application of SERS to identify the specific interactions between small molecules and therapeutically important proteins by differentiating competitive and noncompetitive inhibition demonstrates its ability as a complementary technique. We also present felodipine as a specific inhibitor for oncogenic Aurora A kinase. Felodipine retards the rate of tumor progression in a xenografted nude mice model. This study reveals a potential surface pocket that may be useful for developing small molecules by selectively targeting the Aurora family kinases.

Crystal Structure and Band Gap Engineering in Polyoxometalate-Based Inorganic-Organic Hybrids.

We have demonstrated engineering of the electronic band gap of the hybrid materials based on POMs (polyoxometalates), by controlling its structural complexity through variation in the conditions of synthesis. The pH- and temperature-dependent studies give a clear insight into how these experimental factors affect the overall hybrid structure and its properties. Our structural manipulations have been successful in effectively tuning the optical band gap and electronic band structure of this kind of hybrids, which can find many applications in the field of photovoltaic and semiconducting devices. We have also addressed a common crystallographic disorder observed in Keggin-ion (one type of heteropolyoxometalate [POMs])-based hybrid materials. Through a combination of crystallographic, spectroscopic, and theoretical analysis of four new POM-based hybrids synthesized with tactically varied reaction conditions, we trace the origin and nature of the disorder associated with it and the subtle local structural coordination involved in its core picture. While the crystallography yields a centrosymmetric structure with planar coordination of Si, our analysis with XPS, IR, and Raman spectroscopy reveals a tetrahedral coordination with broken inversion symmetry, corroborated by first-principles calculations.

Guest dependent Brillouin and Raman scattering studies of zeolitic imidazolate framework-8 (ZIF-8) under external pressure.

We have investigated the pressure dependence of the acoustic modes of zeolitic imidazolate framework (ZIF-8) in different pressure transmitting mediums and also under non-hydrostatic conditions using high pressure Brillouin spectroscopy. Our study shows the pressure induced flexibility and dynamics of ZIF-8 framework as well as a huge increase in the acoustic velocities on applying external pressure, illustrating the role of guest in enhancing the elastic properties of the framework. In fact, the elastic constant C11 of the guest incorporated ZIF-8 increases by ∼183% on applying a pressure of only 1.47 GPa. The pressure transmitting medium also plays an important role in controlling the gate opening behaviour of ZIF-8. Pressure dependent Raman study shows significant changes in the modes of ZIF-8 as well as that of that of the pressure transmitting medium which is entrapped within the framework, indicating that the interaction between the framework and guest is responsible for the medium dependent changes observed in the Brillouin spectra.

Revealing the trehalose mediated inhibition of protein aggregation through lysozyme-silver nanoparticle interaction.

We propose a facile and robust carbohydrate-mediated method for the prevention of nanoparticle induced denaturation and aggregation of proteins. Using label-free plasmon-enhanced Raman spectroscopy measurements, the mechanistic principles of trehalose stabilization in a model protein-nanoparticle system are elucidated for the first time, facilitating its further application in diagnostic and therapeutic nanoplex development.

Nano-morphology induced additional surface plasmon resonance enhancement of SERS sensitivity in Ag/GaN nanowall network.

The GaN nanowall network, formed by opening the screw dislocations by kinetically controlled MBE growth, possesses a large surface and high conductivity. Sharp apexed nanowalls show higher surface electron concentration in the band-tail states, in comparison to blunt apexed nanowalls. Uncapped silver nanoparticles are vapor deposited on the blunt and sharp GaN nanowall networks to study the morphological dependence of band-edge plasmon-coupling. Surface enhanced Raman spectroscopy studies performed with a rhodamine 6G analyte on these two configurations clearly show that the sharp nanowall morphology with smaller Ag nanoparticles shows higher enhancement of the Raman signal. A very large enhancement factor of 2.8 × 10(7) and a very low limit of detection of 10(-10) M is observed, which is attributed to the surface plasmon resonance owing to the high surface electron concentration on the GaN nanowall in addition to that of the Ag nanoparticles. The significantly higher sensitivity with same-sized Ag nanoparticles confirms the unconventional role of morphology-dependent surface charge carrier concentration of GaN nanowalls in the enhancement of Raman signals.

Dielectric and Raman investigations of structural phase transitions in (C2H5NH3)2CdCl4.

Temperature-dependent Raman and dielectric measurements have been carried out on (C2H5NH3)2CdCl4 single crystals. Raman studies reveal the presence of two structural phase transitions below room temperature at 216 K and 114 K. The phase transitions are marked by anomalies in temperature dependence of wave-number and full width half maximum (FWHM) of several vibrational modes. The transitions are also accompanied by anomalies in dielectric measurements. Raman and dielectric data indicate that the transition at 216 K is order-disorder in nature and is driven by re-orientation of organic ions, while the transition at 114 K is due to coupling between the CdCl6 octahedron and the organic chain. Further high temperature dielectric measurements reveal the presence of one more structural phase transition around 473 K across which dispersion in dielectric parameters is observed. The activation energies and relaxation time obtained for high temperature dielectric phases are characteristic of combined reorientation motions of alkyl ammonium cations.

Effect of pore occupancy on the acoustic properties of zeolitic imidazolate framework (ZIF)-8: A Brillouin spectroscopic study at ambient and low temperatures.

Brillouin spectroscopy is used to study the effect of pore occupancy on the elastic constants by incorporating various guest molecules into zeolitic imidazolate framework (ZIF)-8. A systematic study on the effect of mass and polarizability of the guest has been carried out by incorporating alcohols of varying chain lengths at room temperature. The interaction between the guest and host affects the elastic properties, lifetimes and guest dynamics inside the pores. The elastic anisotropy was seen to reduce upon incorporation of the guests. We have also studied the temperature dependence of the acoustic modes on gas adsorption to understand the framework flexibility. The Brillouin shift of the acoustic modes increases upon temperature dependent gas adsorption with transverse acoustic modes exhibiting a larger shift. This suggests a hardening of otherwise low shear modulus of ZIF-8. Our findings give insight into the role of guest molecules and temperature in tuning the elastic properties of ZIF-8 which is important for practical applications.

Photoluminescence tuning of Na1-xKxNdW2O8 (0.0 ≤ x ≤ 0.7) nanoparticles: synthesis, crystal structure and Raman study.

A series of Na1-xKxNdW2O8 (0.0 ≤ x ≤ 0.7) nanoparticles have been synthesized by an efficient glycothermal technique for the first time. SEM measurements confirmed the particle size ranges from 30-200 nm with ellipsoidal shaped morphology. Combined X-ray and neutron diffraction and Raman spectroscopy techniques were utilized in order to investigate the influence of K(+) ion substitution in NaNdW2O8. K(+) ion substitution in the crystal lattice introduced a change in the Nd-O bond length and the Nd-O-W bond angle of NaNdW2O8. The photoluminescence intensity increased up to the threshold composition x = 0.4. K(+) ion substitution resulted in blue shifted emission of NaNdW2O8. Size mismatch, the Nd-O-W angle and local disorder contributed to the observed difference in luminescence properties. Also, the chromaticity diagram for this blue emitting phosphor showed the possibility of tuning the emission by incorporation of K.