In-situ probing of interfacial molecular structure of polymers and nanomaterials using sum frequency generation vibrational spectroscopy

Abstract

The knowledge of the interfacial structure of material under desired environmental conditions is of considerable significance due to its ubiquity in various fields of science and technology. The interfacial molecular arrangement is quite distinct and unique as compared to that of bulk phases. The confined interfacial complex structure is adequately approximated in the nanometric length scale. There is a big challenge to achieve sufficient specificity for the interfacial region to exclude the contribution from the bulk phase. The sum frequency generation (SFG) vibrational spectroscopy has shown excellent intrinsic interface-selectivity to probe the interfacial molecules. It is a second-order nonlinear optical technique used to elucidate the molecular structure of aqueous surfaces and buried interfaces under different experimental conditions. Polymer materials are widely applicable as biosensor materials, adhesives, biomedical implants, corrosion-resistant materials, antifouling, and lubricants. These exhibit excellent surface properties such as adhesion, friction, biocompatibility, and wettability. The investigation of the surface molecular structures is essential to determine the molecular activities of the polymer because the surface properties are entirely different from the bulk properties of the polymers. Also, biocompatible nanomaterials are widely used in the field of photonics, electronics, sensing, polymer nanocomposites, antifouling coatings, and drug delivery. Water plays a crucial role during the synthesis of biocompatible nanoparticles and their various activities in the aqueous medium. It is exciting to probe the molecular structure of nanoparticles and their impact on the aqueous environment at the air-aqueous interface. We have applied SFG vibrational spectroscopy to explore different molecular systems based on polymer and nanomaterials in probing the molecular conformation and dynamics of the molecules residing at the air-polymer and air-aqueous interfaces. The silica (SiO2) particles of a well-known silica precursor tetraethyl orthosilicate (TEOS) are synthesized via a promising route as a solution-gelation inorganic polymerization process. These particles have a wide range of applications in various emerging fields due to their tunable morphology. In the aqueous sol-gel process, water plays a dual role as a ligand and solvent during the preparation of silica oxides as well as silica nanoparticles from TEOS. There are reaction parameters that are involved in an aqueous sol-gel procedure, such as hydrolysis and condensation rate of TEOS, pH, temperature, the molar ratio of TEOS and water, etc. The molecular-level information is essential to control the reaction route mediating the silica particle synthesis with well-defined morphology and homogeneity. In this study, we present the systematic investigation of the in-situ interfacial molecular structure of TEOS at the air-TEOS interface under different polarization combinations and its dynamics at the air-aqueous interface using SFG vibrational spectroscopy. SFG intensity of CH-stretching modes has been found to be decreased continuously with time for each concentration of TEOS in water, which signifies the elimination of its ethoxy groups due to hydrolysis. This hydrolysis process of TEOS leads to the formation of negatively charged species (Si-O-) and silanol (Si-OH) groups in an aqueous medium which is revealed from the time-dependent overall enhancement of SFG signal in broad OH stretching region. Hydrolysis leads to the enhancement of water ordering to maximum due to the increase in electrostatic interactions between negatively charged species and water molecules at the air-aqueous interface. Hydrolysis of TEOS further follows the condensation of hydrolyzed moieties. The condensation effect (formation of Si-O-Si bridging) is visible through a continuous decrease in the enhanced SFG signal in the OH stretching region at the air-aqueous interface. After hydrolysis and condensation of TEOS in the aqueous phase, the end product appears as silica nanoparticles with particle size from 1.75 nm to 4.67 nm for the increasing concentration of precursor TEOS in water. The organic nanomaterials as multianalyte sensors are extensively investigated to design and establish better selectivity and predictability for a variety of metal ions in the aqueous environment. These nanomaterials in the aqueous solutions have drawn increasing attention as chemical sensors such as detecting selective metal ions to control water pollution, biosensors, and bio-imaging agents. In this study, we have determined the sensing activities of organic nanomaterials as organic nanoparticles (ONPs) and carbon dots (CDs) in the aqueous bulk media and at the air-aqueous interface using fluorescence spectroscopy and SFG vibrational spectroscopy, respectively. Fluorescence spectroscopic results have shown that ONPs can preferably sense the Cs (I) ions, whereas CDs have selective sensing towards Ag (I) ions in the bulk aqueous media. Further, SFG is employed to determine the interfacial structure of water molecules in the presence of ONP and CDs in the aqueous media. Furthermore, we have probed spectral features of ONPs and CDs on interaction with Cs (I) and Ag (I) ions, respectively, at the air-aqueous interface through SFG spectroscopy. The SFG spectroscopic results are provided molecular-level insights about the hydrophobic interaction between CDs and interfacial water molecules and which become more dominant in the presence of Ag (I) ions. SFG studies on concentration variation of Ag (I) ions in an aqueous solution of CDs make evident their binding and ordering of CDs at the air-aqueous interface. But ONPs find to be SFG inactive either in the presence or absence of Cs (I) ions, and most probably, ONPs may be preferred to stay in bulk phase. Also, these spectral features provide comparative information for the impact on interfacial water structure during its interaction with either ONPs or CDs. The finding of this research work demonstrates the molecular level alterations at the air-aqueous interface due to hydrothermal treatment of ONPs to develop CDs and during their binding activities with particular analytes. Interfacial molecular structural information of a polymer material plays a significant role in physics, chemistry, and biology to understand the microscopic properties of interfaces. We have investigated the impact of molecular weight on the surface structure of the polydimethylsiloxane (PDMS) using SFG vibrational spectroscopy, contact angle measurements, and Atomic force microscopy (AFM). The contact angle measurements have shown that the hydrophobicity of PDMS surface is increased with its molecular weight. AFM results have also revealed the RMS surface roughness values are enhanced with the variation in the molecular weight. The SFG spectra at the air-polymer interface are collected from 2800 cm-1 to 3000 cm-1 for various PDMS films under different polarization schemes. Fresnel factor corrections are applied to these SFG spectra of polymer films and obtain the molecular conformation and orientation of the functional groups at the air-polymer interface. The tilt angle of methyl functional groups of PDMS is obtained for its different molecular weights varied from 43.4° to 55.6°. The tilt angle of the methyl groups are found to decrease with a molecular weight of PDMS polymer with the same polydispersity index (PDI). The methyl groups become more oriented towards normal and formed ordered conformation at the surface with the increase in the molecular weight of PDMS polymers with similar PDI values. The increase in hydrophobicity and surface roughness of PDMS polymers with molecular weight can be correlated to enhancement in the ordered conformation of methyl groups at polymer surface with molecular weight. More molecular-level insights are obtained from the SFG spectral results for the conformation of methyl groups of PDMS polymer with variation in the molecular weight. The polymers in pharmaceutical science research have attracted remarkable attention due to their application to improve the drug solubility in the aqueous phase to retain the solution- and solid-state supersaturation. The polymers are extensively utilized for improvement and optimization of the solubility of the drug in the aqueous media and have numerous applications in obtaining a supersaturated state with better drug solubility. We have performed a systematic study to analyze the drug-polymer interaction by examining the individual response of two different drug systems and various polymers in the relevant media (RM) and then the respective combination of drug and polymers in the RM. We have applied SFG spectroscopy to explore the molecular-level interactions between the drug and polymer in the RM at the air-aqueous interface. The interfacial interactions of different polymers as hydroxypropyl methylcellulose (HPMC) E15, Soluplus, and Poly(vinyl alcohol) (PVA) with Rifaximin (RFX) in the RM are studied using SFG vibrational spectroscopy. These studies have provided evidence about the strong interfacial molecular interactions between RFX and HPMC E15 in the aqueous media in comparison to that of the other polymers. SFG vibrational spectroscopy is also employed to extract the molecular level alterations during the interactions of ABZ with polymers as PVA and polyvinyl pyrrolidone K30 (PVP K30). These studies have revealed the better interfacial molecular interactions between ABZ drug and PVA in the RM. Thus, SFG spectroscopy is found to be suitable in probing the interfacial molecular-level behavior during drug and polymer interactions in aqueous media.