Ligand Adsorption Research Articles

Disagreements Between Calorimetric and Van't Hoff Enthalpies of Adsorption II: Effect of pH and pH Buffers on Phenobarbital Adsorption to Activated Carbon

The reported inconsistencies between the van't Hoff equation and calorimetry hinder the utility of thermodynamics in biochemical and pharmaceutical research. A novel thermodynamic approach is developed herein for ligand adsorption with a focus on the interpretation of calorimetric data in the presence of concurrent proton exchange reactions. Such exchange reactions typically result in a pH-dependence of calorimetric measurements that obscures intrinsic binding enthalpies. It is shown that for the adsorption of phenobarbital to activated carbon, the measured calorimetric enthalpy is a result of three linked acid/base equilibria. A model was established to predict the intrinsic binding enthalpy using 1) the adsorbate's pKa and 2) the adsorbate's enthalpy of protonation. The observed calorimetric enthalpy of binding exhibited both pH and buffer-dependence and was between -5 and -42 kJ/mol. Meanwhile, the predicted intrinsic enthalpy (-25.1 kJ/mol) of binding was in excellent agreement with the measured intrinsic enthalpy (-25.6 kJ/mol). Corrections to the observed calorimetric enthalpies allowed comparisons with enthalpies obtained from the van't Hoff method. It is shown that the predicted intrinsic calorimetric enthalpy agrees well with the van't Hoff enthalpies in instances where observed enthalpies significantly deviated. This treatment is general and is not specific to phenobarbital or activated carbon.

Effect of Surface Functionalization on the Magnetization of Fe3O4 Nanoparticles by Hybrid Density Functional Theory Calculations.

Surface functionalization is found to prevent the reduction of saturation magnetization in magnetite nanoparticles, but the underlying mechanism is still to be clarified. Through a wide set of hybrid density functional theory (HSE06) calculations on Fe3O4 nanocubes, we explore the effects of the adsorption of various ligands (containing hydroxyl, carboxylic, phosphonic, catechol, and silanetriol groups), commonly used to anchor surfactants during synthesis or other species during chemical reactions, onto the spin and structural disorder, which contributes to the lowering of the nanoparticle magnetization. The spin-canting is simulated through a spin-flip process at octahedral Fe ions and correlated with the energy separation between O2– 2p and FeOct3+ 3d states. Only multidentate bridging ligands hamper the spin-canting process by establishing additional electronic channels between octahedral Fe ions for an enhanced ferromagnetic superexchange interaction. The presence of anchoring organic acids also interferes with structural disorder, by disfavoring surface reconstruction.

Coalescence of Au-Pd Nanoropes and their Application as Enhanced Electrocatalysts for the Oxygen Reduction Reaction.

Lattice distortions and defects can lead to a strain effect that greatly affects the electronic structure of the noble metal surface and the chemical adsorption of ligands on the surfaces. Introducing defects is an efficient strategy to improve the activity of noble metal catalysts. Herein, a fusion approach is developed to fine-tune the defects and lattice strain in Au-Pd nanowires. Specifically, braided strands in Au-Pd nanoropes gradually coalesce to form solid nanowires upon H2 O2 treatment and heating, leading to a series of Au-Pd nanowires with various amounts of defects. Owing to the 1D morphology, as well as the optimized lattice strain and surface electronic structure, the intermediate Au-Pd nanowire obtained after 60min heating (denoted as Au-Pd NW60 ) exhibits excellent catalytic activity and stability toward the oxygen reduction reaction, with the half-wave potential at 0.918V, 45mV higher than that of the commercial Pt/C; and specific activity reaches up to 1.7mA cm-2 , 7.3 times higher than that of the Pt/C.

Molecular Modeling Guided Drug Designing for the Therapeutic Treatment of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a systemic inflammatory disorder that can cause destructive joint disease, significant disability, and increased mortality. RA is the most frequent of all chronic inflammatory joint diseases, and its prevalence frequency in Pakistan is 1.6 per thousand people. Different cytokines and receptors were involved in the triggering of RA, including interleukin-6 (ILR-6), major histocompatibility complex (MHC) antigen human leukocyte (HLA-DR) receptor, and CD20. Several studies illustrated RA as an inherent immune response and triggered due to the “shared epitope.” Therefore, the involvement of all these receptors (IL-6, HLA-DR, and CD20) leads to the neurological, ocular, respiratory, cardiac, skin, and hematological manifestations that have been considered a potential therapeutic target for drug design. Various herbal, natural, and synthetic source inhibitors of interleukin-6 (IL-6), human leukocyte (HLA-DR), and CD20 were studied and reported previously. Reported inhibitors are compared to elucidate the best inhibitor for clinical trials, leading to the orally active drug. In this study, a computer-aided drug designing approach disclosed the potential inhibitors for all receptors based on their distinct binding affinity. Moreover, drug suitability was carried out using Lipinski’s rule by considering the adsorption, distribution, metabolism, and excretion (ADME) of ligands. Results elucidated “calycosin 7-O-glucoside” and “angeliferulate” as putative ligands for IL-6 and HLA-DR, respectively. However, the pharmacokinetic properties (ADMET) revealed angeliferulate as an effete ligand for the biological system compared to calycosin 7-O-glucoside. Based on docking, drug toxicity profiling or pharmacokinetics, and MD simulation stability, this study highlights orally active therapeutic inhibitors to inhibit the activity of pivotal receptors (IL6, HLA-DR, and CD20) of RA in humans. After clinical trials, the resultant inhibitors could be potential therapeutic agents in the drug development against RA.

Growth Mechanism of Spiky Nb2O5 Nanoparticles and their Electrochemical Property

Spiky nanoarchitecture composed of numerous nanorods and a spherical core has applications in various fields because of its high performance and durability. However, controlling the spiky structure is challenging because of an unclear formation mechanism. This study investigates the growth mechanism of spiky nanostructure by hydrothermally synthesizing Nb2O5 with various conditions and additives, and it is shown that adsorption and gradual decomposition of ligands promote the formation of the spiky shape. Using oxalic acid as an additive and synthesizing at 160 °C, which is below oxalic acid's decomposition temperature, spherical Nb2O5 with a rough surface that consists of a nanostructure with less than 5 nm size forms, and its surface is protected by oxalate ions. When the synthesis temperature increases to 180 °C, oxalate ions are decomposed, and nanorods grow on the spherical particles. The results show that oxalate ions suppress particle growth, promote self‐assembly, and control subsequent particle growth, resulting in complex nanostructures. When used as anode material in lithium‐ion batteries, the spiky Nb2O5 nanoparticles show a higher capacity (143 mAh g−1) than collapsed spiky particles (92 mAh g−1) and nanorods (37 mAh g−1) because of fast lithium‐ion diffusion on the surface nanostructure and high dispersibility.

Indirect chronovoltametric dosage of alkaline earth metals. The contribution of the chemist Ion Vatamanu to the development of electrochemical methods of analysis

The paper presents the results of a series of investigations on the indirect chronovoltametric dosing of alkaline earth metals, carried out in the Laboratory of the Analytical Chemistry at the Institute of Chemistry, currently called the Laboratory of Physicochemical Methods of Research and Analysis, conducted in 1971–1993 by the doctor of chemical sciences Ion Vatamanu. During the elaboration of the presented polarographic methods for determining the ions of alkaline-earth metals, a series of techniques were used to increase the sensitivity and selectivity of the determination. Based on two indirect polarographic methods for measuring alkaline earth metals, developed by Ion Vatamanu and coworkers and analyzed in this paper, two fundamental physicochemical principles were applied: (I) anion-induced adsorption of ligands adsorbed on the electrode surface and (II) thermodynamic determination of the optimal dosing conditions by calculating the conditional constants or the equilibrium chemical composition of complex reactant systems. The developed methods were used in the electrochemical analysis of standard alloy samples based on various metals, in the analysis of semiconductor systems, ferrous metals, nickel electrolytes, copper plating, cadmium plating, chromium plating, aluminum oxidation, wastewater analysis in galvanic baths, clays and limestones from Moldovan deposits, determination of pesticides, tanning extracts, dyes for the textile industry, etc.

In Situ Formation of Zwitterionic Ligands: Changing the Passivation Paradigms of CsPbBr3 Nanocrystals.

CsPbBr3 nanocrystals (NCs) passivated by conventional lipophilic capping ligands suffer from colloidal and optical instability under ambient conditions, commonly due to the surface rearrangements induced by the polar solvents used for the NC purification steps. To avoid onerous postsynthetic approaches, ascertained as the only viable stability-improvement strategy, the surface passivation paradigms of as-prepared CsPbBr3 NCs should be revisited. In this work, the addition of an extra halide source (8-bromooctanoic acid) to the typical CsPbBr3 synthesis precursors and surfactants leads to the in situ formation of a zwitterionic ligand already before cesium injection. As a result, CsPbBr3 NCs become insoluble in nonpolar hexane, with which they can be washed and purified, and form stable colloidal solutions in a relatively polar medium (dichloromethane), even when longly exposed to ambient conditions. The improved NC stability stems from the effective bidentate adsorption of the zwitterionic ligand on the perovskite surfaces, as supported by theoretical investigations. Furthermore, the bidentate functionalization of the zwitterionic ligand enables the obtainment of blue-emitting perovskite NCs with high PLQYs by UV-irradiation in dichloromethane, functioning as the photoinduced chlorine source.

Disagreements Between Calorimetric and Van't Hoff Enthalpies of Adsorption: A New Langmuir-like Model to Account for the Effect of Solvent Displacement Stoichiometry

The reported inconsistencies between calorimetry and the van't Hoff equation hinder the utility of thermodynamics in pharmaceutical research. In ligand binding or adsorption assays, it is believed that the van't Hoff equation falls short because of the lack of stoichiometric treatment in the equilibrium constant. A new modified Langmuir-Like equation that accounts for the stoichiometry of solute adsorption and solvent displacement is proposed in this work. The performance of the model was evaluated by studying the adsorption of phenobarbital from aqueous solutions by commercial activated carbon. The amount of water occupying the adsorption sites was estimated by graphical analysis of the ‘knee point’ of water-vapor adsorption isotherms and was found to correlate well with the relative percentage of hydroxyl and carbonyl surface groups. It was found that one phenobarbital molecule displaces 2-6 water molecules from the adsorption site. It is shown that adsorption enthalpy was not affected by the adjustment for stoichiometry, supporting the notion that the van't Hoff enthalpy is intrinsic and is independent of the stoichiometry of solvent displacement in Langmuir-based binding. The widely reported disparities between the van't Hoff and calorimetric enthalpies are unlikely to be from a stoichiometric origin.

Improving Rare-Earth Mineral Separation with Insights from Molecular Recognition: Functionalized Hydroxamic Acid Adsorption onto Bastnäsite and Calcite.

Enhancing the separation of rare-earth elements (REEs) from gangue materials in mined ores requires an understanding of the fundamental interactions driving the adsorption of collector ligands onto mineral interfaces. In this work, we examine five functionalized hydroxamic acid ligands as potential collectors for the REE-containing bastnäsite mineral in froth flotation using density functional theory calculations and a suite of surface-sensitive analytical spectroscopies. These include vibrational sum frequency generation, attenuated total reflectance Fourier transform infrared, Raman, and X-ray photoelectron spectroscopies. Differences in the chemical makeup of these ligands on well-defined bastnäsite and calcite surfaces allow for a systematic relationship connecting the structure to adsorption activity to be framed in the context of interfacial molecular recognition. We show how the intramolecular hydrogen bonding of adsorbed ligands requires the inclusion of explicit water solvent molecules to correctly map energetic and structural trends measured by experiments. We anticipate that the results and insights from this work will motivate and inform the design of improved flotation collectors for REE ores.

Flexible atomic layer deposition system for coating porous materials

Herein, we describe an atomic layer deposition (ALD) system that is optimized for the growth of thin films on high-surface-area, porous materials. The system incorporates a moveable dual-zone furnace allowing for rapid transfer of a powder substrate between heating zones whose temperatures are optimized for precursor adsorption and oxidative removal of the precursor ligands. The reactor can both be evacuated, eliminating the need for a carrier gas during precursor exposure, and rotated, to enhance contact between a powder support and the gas phase, both of which help us to minimize mass transfer limitations in the pores during film growth. The capabilities of the ALD system were demonstrated by growing La2O3, Fe2O3, and LaFeO3 films on a 120 m2 g−1 MgAl2O4 powder. Analysis of these films using scanning transmission electron microscopy and temperature-programmed desorption of 2-propanol confirmed the conformal nature of the oxide films.

Reduced‐Dimensional Engineering toward 2D R‐P (OAm)2CsPb2Br7 Perovskite by Metal Ion Enabled Ligands Confinement Effect

AbstractThe dimension is a leading parameter in customization of unique optoelectronic properties of halide perovskite, while it is still challenging to achieve dimension control beyond the well‐developed composition regulation‐based strategy considering their intrinsic feature of ionic crystal. In this paper, ligands‐directed confinement effect is emphasized in rendering the reduced‐dimensional engineering of halide perovskite, and quasi‐2D (OAm)2CsPb2Br7 (n = 2) nanosheets are obtained by introducing a large amount of metal ions. As a representation, with Mg2+ participation, the initial adsorption of organic ligands (i.e., OA and OAm) to inorganic components (i.e., [PbBr6]4−) is promoted, conferring a ligands‐directed surface reconstruction to form the lamellar structure composed of alternate organic and inorganic layers. Namely, the inorganic–organic lamellar ensemble will play as a soft template, which effectively restricts the growth of [PbBr6]4− in the c‐axis direction even after the Cs‐OA injection, thus producing the anisotropic layered perovskite of (OAm)2CsPb2Br7 from CsPbBr3 scenario. Notably, such reduced‐dimensional engineering enables a remarkable luminescence tailoring, of which the deep blue emission centered at 439 nm is achieved. In addition, benefiting from such universal strategy, the luminescence is also dependent on the species of introduced metal ions (e.g., Ca2+, Co2+, Sr2+, and Mn2+).

Controlled membrane interactions by lipid coated quantum dots

Quantum Dots (QDs) are being employed in a wide range of biological application due to their superior fluorescence characteristics. Biocompatibility of QDs is usually achieved by exchange of as-synthesized surface ligands with ligands that impart the particle with water solubility properties. An alternative approach for surface functionalization is ligand adsorption. This approach is based on weak interactions between the alkane chains of the as-synthesized surface ligands and a hydrophobic element of an adsorbed ligand with a functional head group/s.

NMR illuminates the ligand and surface adsorption of autolysin-amidase from Staphylococcus epidermidis

Bacterial biofilms on medical devices and implants pose a serious health challenge, and the interaction of bacterial surface proteins to both biotic and abiotic surfaces is an important first step towards bacterial colonization and biofilm development. The autolysin (AtlE) surface protein of S. epidermidis functions in cell wall homeostasis, cell separation and promote biofilm. The amidase catalytic domain (AmiE) of AtlE is a 26 kDa zinc-dependent peptidoglycan peptidase and is thought to play a role in bacterial surface attachment.

Enhancing the Efficiency and Stability of CsPbI3 Nanocrystal-Based Light-Emitting Diodes through Ligand Engineering with Octylamine

Bright perovskite nanocrystals with high color purity are promising for high-definition displays; however, their complex postprocessing process limits their large-scale production. Using a trace of octylamine to regulate the dynamic balance between surface ligand adsorption and desorption, we develop a facile method to obtain stable CsPbI3 nanocrystal films that combine excellent optical and electrical properties. The octylamine additive maintains the integrity of nanocrystals and inhibits the formation of defects, greatly enhances the phase- and the photoluminescence-stability, and thus the operational stability of the corresponding light-emitting devices (LEDs); enables in situ substitution of oleylamine, which reduces the spacing between adjacent nanocrystals and thus leads to closely packed nanocrystal films with promoted charge carrier transport capability; and also increases the activation energy for ion migration, indicating that the number of ion vacancies─namely, the ion migration paths─has been decreased. As a result, the octylamine additive leads to a 4- and 6-fold increase in LED external quantum efficiency (EQE) and operational lifetime, respectively. LEDs based on octylamine-assisted Zn-doped CsPbI3 nanocrystals show a high EQE of 15.3%.

N-Heterocyclic Carbene Ligand Stability on Gold Nanoparticles in Biological Media.

The ability to functionalize gold nanoparticle surfaces with target ligands is integral to developing effective nanosystems for biomedical applications, ranging from point-of-care diagnostic devices to site-specific cancer therapies. By forming strong covalent bonds with gold, thiol functionalities can easily link molecules of interest to nanoparticle surfaces. Unfortunately, thiols are inherently prone to oxidative degradation in many biologically relevant conditions, which limits their broader use as surface ligands in commercial assays. Recently, N-heterocyclic carbene (NHC) ligands emerged as a promising alternative to thiols since initial reports demonstrated their remarkable stability against ligand displacement and stronger metal–ligand bonds. This work explores the long-term stability of NHC-functionalized gold nanoparticles suspended in five common biological media: phosphate-buffered saline, tris-glycine potassium buffer, tris-glycine potassium magnesium buffer, cell culture media, and human serum. The NHCs on gold nanoparticles were probed with surface-enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS). SERS is useful for monitoring the degradation of surface-bound species because the resulting vibrational modes are highly sensitive to changes in ligand adsorption. Our measurements indicate that imidazole-based NHCs remain stable on gold nanoparticles over the 21 days of examination in all tested environments, with no observed change in the molecule’s SERS signature, XPS response, or UV–vis plasmon band.

Predicting ligand-dependent nanocrystal shapes of InP quantum dots and their electronic structures

InP quantum dots serve as solid candidates for the next-generation displays, yet their limited external quantum efficiencies have been the primary concern towards establishing self-luminous QD displays. At the heart of the problem lies our lack of understanding of how surface ligands affect the InP quantum dot properties. Here, we use density functional theory calculations to study the effect of ligand chemistry (amines, carboxylate ions, and halide ions) and coverage on the InP surface energies, equilibrium crystal shapes, and density of states. In terms of ligand chemistry, amine adsorption leads to (111)In facet-dominant octahedral Wulff shapes, while high coverage of halide results in (100)In facet-dominant cubic shapes. The computed density of states shows that the n-type defects in bare (111)In surfaces disappear upon anion adsorption, while the trap states in bare (100)In surfaces persist either with n-type or p-type upon ligand adsorption. The divergence between thermodynamically stable InP Wulff shapes and trap-suppressed InP facets call for mixed ligation strategies.

Just Scratching the Surface: In Situ and Surface-Specific Characterization of Perovskite Nanocrystal Growth

The model of nucleation and growth proposed by LaMer has guided our understanding of nanocrystal (NC) formation for over 70 years. However, LaMer’s model does not account for the effects of surface–ligand interactions on the availability of precursor, the resulting burst nucleation, or the growth of the NC. These interactions play a critical role in determining the electronic properties and physical structure of perovskite NCs. The surfaces of perovskite NCs are dynamic, with multiple processes occurring simultaneously, such as the adsorption and desorption of ligands and growth via incorporation of precursor or oriented attachment of small crystallites. These unstable surfaces are difficult to characterize during growth. A model of NC growth that includes the behavior and impact of ligands at the NC surface would enhance rational syntheses of perovskite nanomaterials with targeted properties. Here we discuss techniques and strategies that have provided insight into the nature of NC surfaces, emphasizing the need for new, multimodal, in situ measurements to characterize the role of surface ligands during perovskite NC growth.

Silica@zirconia Core@shell Nanoparticles for Nucleic Acid Building Block Sorption.

The development of delivery systems for the immobilization of nucleic acid cargo molecules is of prime importance due to the need for safe administration of DNA or RNA type of antigens and adjuvants in vaccines. Nanoparticles (NP) in the size range of 20–200 nm have attractive properties as vaccine carriers because they achieve passive targeting of immune cells and can enhance the immune response of a weakly immunogenic antigen via their size. We prepared high capacity 50 nm diameter silica@zirconia NPs with monoclinic/cubic zirconia shell by a green, cheap and up-scalable sol–gel method. We studied the behavior of the particles upon water dialysis and found that the ageing of the zirconia shell is a major determinant of the colloidal stability after transfer into the water due to physisorption of the zirconia starting material on the surface. We determined the optimum conditions for adsorption of DNA building blocks, deoxynucleoside monophosphates (dNMP), the colloidal stability of the resulting NPs and its time dependence. The ligand adsorption was favored by acidic pH, while colloidal stability required neutral-alkaline pH; thus, the optimal pH for the preparation of nucleic acid-modified particles is between 7.0–7.5. The developed silica@zirconia NPs bind as high as 207 mg dNMPs on 1 g of nanocarrier at neutral-physiological pH while maintaining good colloidal stability. We studied the influence of biological buffers and found that while phosphate buffers decrease the loading dramatically, other commonly used buffers, such as HEPES, are compatible with the nanoplatform. We propose the prepared silica@zirconia NPs as promising carriers for nucleic acid-type drug cargos.

Observation of Surface Ligands-Controlled Etching of Palladium Nanocrystals.

Selective adsorption of ligands on nanocrystal surfaces can affect oxidative etching. Here, we report the etching of palladium nanocrystals imaged using liquid cell transmission electron microscopy. The adsorption of surface ligands (i.e., iron acetylacetonate and its derivatives) and their role as inhibitor molecules on the etching process were investigated. Our observations revealed that the etching was dominated by the interplay between palladium facets and ligands and that the etching exhibited different pathways at different concentrations of ligands. At a low concentration of iron acetylacetonate (0.1 mM), rapid etching primarily at {100} facets led to a concave structure. At a high concentration (1.0 mM), the etch rate was decreased owing to a protective film of iron acetylacetonate on the {100} facets and a round nanoparticle was achieved. Ab initio calculations showed that the differences in adsorption energy of inhibitor molecules on palladium facets were responsible for the etching behavior.

Nanoscale cooperative adsorption for materials control

Adsorption plays vital roles in many processes including catalysis, sensing, and nanomaterials design. However, quantifying molecular adsorption, especially at the nanoscale, is challenging, hindering the exploration of its utilization on nanomaterials that possess heterogeneity across different length scales. Here we map the adsorption of nonfluorescent small molecule/ion and polymer ligands on gold nanoparticles of various morphologies in situ under ambient solution conditions, in which these ligands are critical for the particles’ physiochemical properties. We differentiate at nanometer resolution their adsorption affinities among different sites on the same nanoparticle and uncover positive/negative adsorption cooperativity, both essential for understanding adsorbate-surface interactions. Considering the surface density of adsorbed ligands, we further discover crossover behaviors of ligand adsorption between different particle facets, leading to a strategy and its implementation in facet-controlled synthesis of colloidal metal nanoparticles by merely tuning the concentration of a single ligand.