Role of tungsten disulfide quantum dots in specific protein-protein interactions at air-water interface.

The intriguing network of antibody-antigen (Ab-Ag) interactions is highly governed by environmental perturbations and the nature of biomolecular interaction. Protein-protein interactions (PPIs) have potential applications in developing protein-adsorption-based sensors and nano-scale materials. Therefore, characterizing PPIs in the presence of a nanomaterial at the molecular level becomes imperative. The present work involves the investigation of antiferritin-ferritin (Ab-Ag) protein interactions under the influence of tungsten disulfide quantum dots (WS2 QDs). Isothermal calorimetry and contact angle measurements validated the strong influence of WS2 QDs on Ab-Ag interactions. The interfacial signatures of nano-bio-interactions were evaluated using sum frequency generation vibration spectroscopy (SFG-VS) at the air-water interface. Our SFG results reveal a variation in the tilt angle of methyl groups by ∼12° ± 2° for the Ab-Ag system in the presence of WS2 QDs. The results illustrated an enhanced ordering of water molecules in the presence of QDs, which underpins the active role of interfacial water molecules during nano-bio-interactions. We have also witnessed a differential impact of QDs on Ab-Ag by raising the concentration of the Ab-Ag combination, which showcased an increased inter-molecular interaction among the Ab and Ag molecules and a minimal influence on the methyl tilt angle. These findings suggest the formation of stronger and ordered Ab-Ag complexes upon introducing WS2 QDs in the aqueous medium and signify the potentiality of WS2 QDs relevant to protein-based sensing assays.

Interfacial molecular structure of phosphazene-based polymer electrolyte at the air-aqueous interface using sum frequency generation vibrational spectroscopy.

The change induced in the physicochemical properties of polymer while hosting ions provides a platform for studying its potential applications in electrochemical devices, water treatment plants, and materials engineering science. The ability to host ions is limited in very few polymers, which lack a detailed molecular-level understanding for showcasing the polymer-ion linkage behavior at the interfacial region. In the present manuscript, we have employed sum frequency generation (SFG) vibrational spectroscopy to investigate the interfacial structure of a new class phosphazene-based methoxyethoxyethoxyphosphazene (MEEP) polymer in the presence of lithium chloride salt at the air-aqueous interface. The interfacial aspects of the molecular system collected through SFG spectral signatures reveal enhanced water ordering and relative hydrogen bonding strength at the air-aqueous interface. The careful observation of the study finds a synchronous contribution of van der Waals and electrostatic forces in facilitating changes in the interfacial water structure that are susceptible to MEEP concentration in the presence of ions. The observation indicates that dilute MEEP concentrations support the role of electrostatic interaction, leading to an ordered water structure in proximity to diffused ions at the interfacial region. Conversely, higher MEEP concentrations promote the dominance of van der Waals interactions at the air-aqueous interface. Our study highlights the establishment of polymer electrolyte (PE) characteristics mediated by intermolecular interactions, as observed through the spectral signatures witnessed at the air-aqueous interface. The investigation illustrates the polymer-ion linkage adsorption effects at the interfacial region, which explains the macroscopic changes observed from the cyclic voltammetry studies. The fundamental findings from our studies can be helpful in the design and fine-tuning of better PE systems that can offer improved hydrophobic membranes and interface stability for use in electrochemical-based power sources.

Ion-Specific Water-Macromolecule Interactions at the Air/Aqueous Interface: An Insight into Hofmeister Effect.

The specificity of ions in inducing conformational changes in macromolecules is introduced as the Hofmeister series; however, the detailed underlying mechanism is not comprehensible yet. We utilized surface-specific sum frequency generation (SFG) vibrational spectroscopy to explore the Hofmeister effect at the air/polyvinylpyrrolidone (PVP)/water interface. The spectral signature observed from the ssp polarization scheme reveals ion-specific ordering of water molecules following the Hofmeister series attributed to the ion-macromolecule interactions. Along with this, the presence of ions does not reflect any significant influence on the structure of the PVP macromolecule. However, the ppp-SFG spectra in the CH-stretch region reveal the impact of ions on the orientation angle of vinyl chain CH2-groups, which follows the Hofmeister series: SO42- > Cl- > NO3- > Br- > ClO4- > SCN-. The minimal orientation angle of CH2-groups indicates significant reordering in PVP vinyl chains in the presence of chaotropic anions ClO4-, and SCN-. The observation is attributed to the ion-specific water-macromolecule interactions at the air/aqueous interface. It is compelling to observe the signature of spectral blue shifts in the OH-stretch region in the ppp configuration in the presence of chaotropic anions. The origin of spectral blue shifts has been ascribed to the existence of weaker interactions between the interfacial water molecules and the backbone CH- and CH2-moieties of the PVP macromolecules. The ion-specific modulation in water-macromolecule interactions is endorsed by the relative propensity of anion's adsorption toward the air/aqueous interface. The experimental findings highlight the existence and cooperative participation of ion-specific water-macromolecule interactions in the mechanism of the Hofmeister effect, along with the illustrious ion-water and ion-macromolecule interactions.

Elucidating the Molecular Structure of Hydrophobically Modified Polyethylenimine Nanoparticles and Its Potential Implications for DNA Binding.

The structural properties of the polyethylenimine (PEI) polymer are generally tuned and selectively modified to reinforce its potential in a broad spectrum of applied domains of medicine, healthcare, material design, sensing, and electronic optimization. The selective modification of the polymer brings about changes in its interfacial characteristics and behavior. The current work involves the synthesis of naphthalimide conjugated polyethylenimine organic nanoparticles (NPEI-ONPs). The interfacial molecular structure of NPEI-ONPs is explored in an aqueous medium at pH 7.4 using surface tensiometry and sum-frequency generation vibrational spectroscopy (SFG-VS). The hydrophobic functionalization rendered a concentration-dependent surface coverage of NPEI-ONPs, where the SFG-VS analysis exhibited the molecular rearrangement of its hydrophobic groups at the interface. The interaction of NPEI-ONPs with double-stranded DNA (dsDNA) is carried out to observe the relevance of the synthesized nanocomposites in the biomedical domain. The bulk-specific studies (i.e., thermal denaturation, viscometry, zeta (ζ) potential, and ATR-FTIR) reveal the condensation of dsDNA in the presence of NPEI-ONPs, making its structure more compact. The interface-sensitive SFG-VS showcased the impact of the dsDNA and NPEI-ONP interaction on the interfacial molecular behavior of NPEI-ONPs at the air-aqueous interface. Our results exhibit the potential of such hydrophobically functionalized ONPs as promising candidates for developing biomedical sealants, substrate coatings, and other biomedical domains.

Investigation of Water Evaporation Process at Air/Water Interface using Hofmeister Ions.

Evaporation is an interfacial phenomenon in which a water molecule breaks the intermolecular hydrogen (H-) bonds and enters the vapor phase. However, a detailed demonstration of the role of interfacial water structure in the evaporation process is still lacking. Here, we purposefully perturb the H-bonding environment at the air/water interface by introducing kosmotropic (HPO4-2, SO4-2, and CO3-2) and chaotropic ions (NO3- and I-) to determine their influence on the evaporation process. Using time-resolved interferometry on aqueous salt droplets, we found that kosmotropes reduce evaporation, whereas chaotropes accelerate the evaporation process, following the Hofmeister series: HPO4-2 < SO4-2 < CO3-2 < Cl- < NO3- < I-. To extract deeper molecular-level insights into the observed Hofmeister trend in the evaporation rates, we investigated the air/water interface in the presence of ions using surface-specific sum frequency generation (SFG) vibrational spectroscopy. The SFG vibrational spectra reveal the significant impact of ions on the strength of the H-bonding environment and the orientation of free OH oscillators from ∼36.2 to 48.4° at the air/water interface, where both the effects follow the Hofmeister series. It is established that the slow evaporating water molecules experience a strong H-bonding environment with free OH oscillators tilted away from the surface normal in the presence of kosmotropes. In contrast, the fast evaporating water molecules experience a weak H-bonding environment with free OH oscillators tilted toward the surface normal in the presence of chaotropes at the air/water interface. Our experimental outcomes showcase the complex bonding environment of interfacial water molecules and their decisive role in the evaporation process.

Explicating the molecular level drug-polymer interactions at the interface of supersaturated solution of the model drug: Albendazole.

Supersaturation as a formulation principle relates to the aqueous solubility of poorly soluble drugs in solution . However, supersaturation state of drugs tends to crystallize because of its thermodynamic instability thereby compromising the solubility and biopharmaceutical performance of drugs. The present study aims to investigate the supersaturation potential of albendazole (ABZ) and its precipitation via nucleation and crystal growth. We hypothesized the use of polymers will avoid ABZ precipitation by interacting with drug molecules. The drug polymer interactions are characterized using conventional methods of Fourier transform infrared (FTIR), Nuclear magnetic resonance (NMR) and Polarized light microscopy (PLM). We have used a novel approach of sum frequency generation (SFG) vibrational spectroscopic in exploring the drug polymer interactions at air-water interface. Recently we have reported the SFG for e rifaximin-polymer interactions (Singh et al., 2021). The supersaturation assay, saturation solubility studies and nucleation induction time analysis revealed polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP K30) as effective precipitation inhibitors thereby enhancing the ABZ equilibrium solubility and in vitro supersaturation maintenance of ABZ. Further, modification in the solid state of ABZ has confirmed the influence of polymers on its precipitation behaviour. We conclude that PVA and PVP K30 act as nucleation and crystal growth inhibitor, respectively for the precipitation inhibition of ABZ.

Probing the Bovine Hemoglobin Adsorption Process and its Influence on Interfacial Water Structure at the Air-Water Interface.

*These authors contributed equally to this work.The molecular-level insight of protein adsorption and its kinetics at interfaces is crucial because of its multifold role in diverse fundamental biological processes and applications. In the present study, the sum frequency generation (SFG) vibrational spectroscopy has been employed to demonstrate the adsorption process of bovine hemoglobin (BHb) protein molecules at the air-water interface at interfacial isoelectric point of the protein. It has been observed that surface coverage of BHb molecules significantly influences the arrangement of the protein molecules at the interface. The time-dependent SFG studies at two different frequencies in the fingerprint region elucidate the kinetics of protein denaturation process and its influence on the hydrogen-bonding network of interfacial water molecules at the air-water interface. The initial growth kinetics suggests the synchronized behavior of protein adsorption process with the structural changes in the interfacial water molecules. Interestingly, both the events carry similar characteristic time constants. However, the conformational changes in the protein structure due to the denaturation process stay for a long time, whereas the changes in water structure reconcile quickly. It is revealed that the protein denaturation process is followed by the advent of strongly hydrogen-bonded water molecules at the interface. In addition, we have also carried out the surface tension kinetics measurements to complement the findings of our SFG spectroscopic results.

Elucidating the Molecular Mechanism of Drug-Polymer Interplay in a Polymeric Supersaturated System of Rifaximin.

Supersaturated drug delivery system (SDDS) enables the solubility and sustained membrane transport of poorly water-soluble drugs. SDDS provides higher drug concentration in the dispersed phase and equilibrium in the continuous phase, which corresponds to amorphous solubility of the drug. Rifaximin (RFX) is a nonabsorbable BCS class IV drug approved for the treatment of irritable bowel syndrome and effective against Helicobacter pylori. RFX shows slow crystallization and precipitation in an acidic pH of 1.2-2, leading to obliteration of its activity in the gastrointestinal tract. The objective of the present study is to inhibit the precipitation of RFX, involving screening of polymers at different concentrations, using an in-house developed microarray plate method and solubility studies which set forth hydroxypropyl methylcellulose (HPMC) E15, Soluplus, and polyvinyl alcohol to be effective precipitation inhibitors (PIs). Drug-polymer precipitates (PPTS) are examined for surface morphology by scanning electron microscopy, solid-phase transformation by hot stage microscopy, the nature of PPTS by polarized light microscopy, and drug-polymer interactions by Fourier transform infrared and nuclear magnetic resonance spectroscopy. Besides, the unfathomed molecular mechanism of drug-polymer interplay is discerned at the air-water interface using sum-frequency generation spectroscopy to correlate the interfacial hydrogen bonding properties in bulk water. Surprisingly, all studies disseminate HPMC E15 and Soluplus as effective PIs of RFX.

Multifunctional Receptor with Tunable Selectivity: A Comparative Recognition Profile of Organic Nanoparticles with Carbon Dots.

The exponential growth in the research field of water pollution control demands the evolution of novel sensing materials for regulation and quantification of metals ions. Within this context, the current work reports a new strategy for the synthesis of carbon dots from the hydrothermal treatment of organic nanoparticles. The organic nanoparticles are found to be selective towards Cs(I) ions with a detection limit of 5.3 nM, whereas the highly fluorescent carbon dots are found to be selective towards Ag(I) ions with a detection limit of 4.8 nM. Both sensing systems illustrate rapid sensing with a working pH range from 4-9. The interfacial molecular restructuring of the sensing systems in the aqueous phase has been investigated in the absence and presence of targeted metal ions using a sum frequency generation vibrational spectroscopic tool. The practical applicability of the sensors was checked in environmental samples. This work opens new avenues for the exploration of temperature-guided sensing modulation in nanomaterials.

IL@CQD catalyzed active ester rearrangement for the detection and removal of cyanide ions.

Carbon Quantum Dots (CQDs) have been extensively employed in various fields of science such as sensing, catalysis, and drug delivery. In this work, ionic liquid coated CQDs (IL@CQDs) have been used as catalysts for the rearrangement of a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) amide coupling intermediate. The rearranged product was confirmed by single crystal structure analysis and it was processed into organic nanoparticles (ONPs). An optical detection method was used to scrutinize the anion sensing properties. The ONPs were found to be sensitive and selective for the recognition of highly toxic cyanide ions through enhancement in the fluorescence intensity. The cyanide ion co-ordinates with the -NH groups of the product and restricts the rotation of molecules around the single bond. The ONPs coated over silica-beads were also showing CN- recognition in the solid state. The detection limit for CN- sensing was found to be 3.8 nM, and real sample analysis depicts more than 90% accuracy in detection.

The structure of water-DMF binary mixtures probed by linear and nonlinear vibrational spectroscopy.

Attenuated total reflectance Fourier transform infrared spectroscopy and sum frequency generation (SFG) vibrational spectroscopy have been employed to probe the molecular structure of N,N-dimethylformamide (DMF) and water mixture by varying the concentration of DMF. From the bulk studies, we observed a gradual decrease in the intensity with a continuous blue shift in the OH-stretch region with the increase in the DMF concentration. In contrast, no significant blue shift in the OH-stretch region is noticed from the SFG spectra collected from the air-aqueous binary mixture interface as a function of DMF concentration. However, the impact of DMF is found to be disruptive in nature toward the existing hydrogen bonding network of the pristine water at the interfacial region. Interestingly, in the CH-stretch region, the vibrational signatures of the DMF molecule show blue shifts, as proposed in earlier studies. We have calculated the molecular tilt angle of the methyl group of the DMF molecule as a function of DMF concentration. For the case of neat DMF, the observed tilt angle is ∼17.7° with respect to the surface normal. The value of tilt angle decreases with the decrease in DMF concentration and reaches a value of ∼1.7° for a mole fraction of 0.5, and it further increases with the decrease in DMF concentration. It achieves a value of ∼20° for the dilute DMF mole fraction of 0.05 in the binary mixture. This indicates that DMF molecules at the air-binary mixture interface are placing their methyl groups more toward the normal for the intermediate DMF concentrations.

Three Dimensional Nano "Langmuir Trough" for Lipid Studies.

A three-dimensional-phospholipid monolayer with tunable molecular structure was created on the surface of oil nanodroplets from a mixture of phospholipids, oil, and water. This simple nanoemulsion preparation technique generates an in situ prepared membrane model system with controllable molecular surface properties that resembles a lipid droplet. The molecular interfacial structure of such a nanoscopic system composed of hexadecane, 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC), and water was determined using vibrational sum frequency scattering and second harmonic scattering techniques. The droplet surface structure of DPPC can be tuned from a tightly packed liquid condensed phase like monolayer to a more dilute one that resembles the liquid condensed/liquid expanded coexistence phase by varying the DPPC/oil/water ratio. The tunability of the chemical structure, the high surface-to-volume ratio, and the small sample volume make this system an ideal model membrane for biochemical research.

Sum frequency and second harmonic generation from the surface of a liquid microjet.

The use of a liquid microjet as a possible source of interest for Second Harmonic Generation (SHG) and Sum Frequency Generation (SFG) spectroscopy is examined. We measured non-resonant SHG scattering patterns from the air/water interface of a microjet of pure water and observe a strong enhancement of the SHG signal for certain scattering angles. These enhancements can be explained by the optical properties and the shape of the liquid microjet. SFG experiments at the surface of a liquid microjet of ethanol in air show that it is also possible to measure the coherent vibrational SFG spectrum of the ethanol/air interface in this way. Our findings are useful for future far-UV or X-ray based nonlinear optical surface experiments on liquid jets. In addition, combined X-ray photoelectron spectroscopy and SHG/SFG measurements are feasible, which will be very useful in improving our understanding of the molecular foundations of electrostatic and chemical surface properties and phenomena.

Charge asymmetry at aqueous hydrophobic interfaces and hydration shells.

Guilty as charged: Water is often modeled as a dielectric continuum, but the molecular structure of water is asymmetric. Two ions that have a virtually identical size, shape, and structure, but an opposite charge sign have been investigated to see whether charge makes a fundamental difference to water structuring. The spectroscopic data for the hydration and interface structures are found to be remarkably different for opposite charges.

Molecular-level surface structure from nonlinear vibrational spectroscopy combined with simulations.

Vibrational sum-frequency generation spectroscopy is valued for its ability to selectively probe molecules at a variety of interfaces without the use of extrinsic chromophores. The spectra contain valuable information regarding the molecular structure and the interfacial environment through the observation of vibrational resonances associated with specific moieties. Chemical information is obtained by close inspection of the frequencies of these bands and the amplitude of the response under conditions of different beam polarizations. Such sensitivity motivates the development of techniques that can provide structural details. We illustrate several approaches by which various types of calculations and molecular simulations may be used to enhance the sought structural interpretation of experimental data. By applying these techniques to the adsorbate molecules, interfacial water, and the substrate surfaces themselves, we are able to achieve a holistic picture of the adsorption environment.

Throwing Salt into the Mix: Altering Interfacial Water Structure by Electrolyte Addition.

Interfacial water commonly distinguishes itself from the bulk phase by adopting a polar, ordered structure. It is well-established that electrolytes can act to perturb this structure; however, the nature of this perturbation remains a topic of interest. In this study, surface- and structure-sensitive nonlinear vibrational spectroscopy is used to monitor electrolyte-induced changes in interfacial water structure. Solution ionic strength was varied over 5 orders of magnitude, and spectra were collected from two mineral surfaces (fused silica and calcium fluoride) and two polymer surfaces (polystyrene and poly(methyl methacrylate)). Analysis of the spectra reveals striking similarities and differences between these four aqueous interfaces; trends in overall intensity do not always follow changes in the spectral shape. Our results reveal the complex interplay between surface charge, ion adsorption, and hydrophobicity in determining interfacial water structure in the presence of dissolved ions.

Mixed ß-pyrrole substituted meso-tetraphenylporphyrins and their metal complexes: optical nonlinearity using degenerate four wave mixing technique.

We have investigated the roles of structural modification and polar effects in the optical nonlinearities of a series of selectively mixed ß-pyrrole functionalized tetraphenylporphyrin, MTPP(CHO)(R)2 (R = H, Br, 2-thienyl, phenyl (Ph), phenylethynyl (PE) compounds and their metal (Cu(II), Zn(II)) complexes in toluene. In the present study, we have used phase conjugation geometry of the four wave mixing process to measure the third order nonlinear susceptibility (χ(3)) and the second order hyperpolarizabilty ((γ)) with picosecond laser pulse excitation at 532 nm. An increase in the values of χ(3) and (γ) for electron-withdrawing groups was observed whereas an opposite trend was noticed for the electron-donating groups at the ß-pyrrole positions. In the Cu(II) and Zn(II) complexes of substituted free base porphyrins, the distortion of the macrocyclic ring may be responsible for the reduction of the values of χ(3) and (γ). From fluorescence measurements, it has been found that the electron-donating and electron-withdrawing substituted groups at ß-pyrrole positions and also the macrocyclic ring distortion of the porphyrin lead to increased radiationless transitions.

Surface impurities are not responsible for the charge on the oil/water interface: a comment.

The origin of the charge on oil/water interfaces that can be found from electrokinetic mobility measurements is a long-standing issue that has invoked different explanations. Sum frequency scattering (SFS) shows that impurities are likely not a general cause for the charge.

Comparison of scattering and reflection SFG: a question of phase-matching.

We present a comparison between sum frequency scattering (SFS) and reflection mode sum frequency generation (R-SFG). We have used scattering theory to describe both scattering experiments as well as reflection mode experiments. The interfacial vibrational spectrum of nanoscopic oil droplets dispersed in water was probed with SFS as well as with R-SFG. Spectra recorded in phase-matched R-SFG mode and spectra recorded with SFS from the same sample are different, which shows that different interfaces are measured. Scattering spectra at different scattering angles agree with nonlinear light scattering theory. We further present experiments with polymer films aimed at quantifying the comparative strength of R-SFG and SFS experiments.

Methyl groups at dielectric and metal surfaces studied by sum-frequency generation in co- and counter-propagating configurations.

The optimum experimental geometry for visible-infrared sum-frequency generation experiments depends rather sensitively on the molecules adsorbed at the surface, their orientation, and the nature of the adjacent bulk media. We consider the commonly encountered case of methyl groups situated at air-water, air-gold, and polymer-water interfaces. We provide expressions that may be used to determine the optimal visible and IR beam incident angles, considering the symmetric and antisymmetric modes separately and then together. The analysis is carried out for co-propagating (collinear and non-collinear geometries) and counter-propagating configurations. We first consider that one or more vibrational modes are of interest, and the goal is to study them quantitatively under a single polarization scheme; our results enable the user to set the beam angles for such an experiment. In the second case, molecular orientation information is desired, and so the calibrated response is required in all accessible polarization schemes for full characterization of the nonlinear susceptibility tensor.