Sum-Frequency Generation Vibrational Spectroscopy: A Nonlinear Optical Tool to Probe the Polymer Interfaces

Electric Fields in Polymeric Systems

Polymer-based electronic devices are limited by slow transport and recombination of newly separated charges. Built-in electric fields, which arise from compositional gradients, are known to improve charge separation, directional charge transport, and to reduce recombination. Yet, the optimization of these fields through the rational design of polymeric materials is not prevalent. Indeed, polymers are disordered and generate nonuniform electric fields that are hard to measure, and therefore, hard to optimize. Here, we review work focusing on the intentional optimization of electric fields in polymeric systems with applications to catalysis, energy conversion, and storage. This includes chemical tuning of constituent monomers, linkers, morphology, etc. that result in stronger molecular dipoles, polarizability or crystallinity. We also review techniques to characterize electric fields in polymers and emerging processing strategies based on electric fields. These studies demonstrate the benefits of optimizing electric fields in polymers. However, rational design is often restricted to the molecular scale, deriving new pendants on, or linkers between, monomers. This does not always translate in strong electric fields at the polymer level, because they strongly depend on the monomer orientation. A better control of the morphology and monomer-to-polymer scaling relationship is therefore crucial to enhance electric fields in polymeric materials.

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.

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.

Algae Adhesion onto Silicone is Sensitive to Environment-Induced Surface Restructuring

Resistance to algae contamination is an important characteristic of insulators used in overhead power distribution in coastal environments. It is therefore important to understand the parameters governing algae adhesion onto polymer insulator materials such as silicone. Flow cell-based shear experiments were conducted in order to characterize the adhesion strength of algae onto polydimethylsiloxane surfaces, comparing fresh polymer substrates with those that have been soaked in water and saline solutions for 1 month. Both freshwater algae and seawater species could withstand considerably less drag force and were therefore more easily removed when the polymer was soaked in salt water. The polymer surface was found to be unaltered in terms of its roughness, contact angle, and lack of water uptake; no macroscopic surface characterization was therefore able to account for the differences in cell adhesion strength resulting from the soaking treatment. Surface-specific nonlinear vibrational spectroscopy, however, revealed subtle differences in the orientation of surface methyl groups that resulted from the water and saline exposure.

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.