Kinetics of the interfacial curing reaction for an epoxy-amine mixture

A better understanding of the chemical reaction between epoxy and amine compounds at a solid interface is crucial for the design and fabrication of materials with appropriate adhesive strength. Here, we examined the curing reaction kinetics of epoxy phenol novolac and 4,4'-diaminodiphenyl sulfone at the outermost interface using sum-frequency generation spectroscopy, and X-ray and neutron reflectivity in conjunction with a full atomistic molecular dynamics simulation. The reaction rate constant was much larger at the quartz interface than in the bulk. While the apparent activation energy at the quartz interface obtained from an Arrhenius plot was almost identical to the bulk value, the frequency factor at the quartz interface was greater than that in the bulk. These results could be explained in terms of the densification and orientation of reactants at the interface, facilitating the encounter of the reactants present.

Probing protein aggregation at buried interfaces: distinguishing between adsorbed protein monomers, dimers, and a monomer-dimer mixture in situ

Protein adsorption on surfaces greatly impacts many applications such as biomedical materials, anti-biofouling coatings, bio-separation membranes, biosensors, antibody protein drugs etc. For example, protein drug adsorption on the widely used lubricant silicone oil surface may induce protein aggregation and thus affect the protein drug efficacy. It is therefore important to investigate the molecular behavior of proteins at the silicone oil/solution interface. Such an interfacial study is challenging because the targeted interface is buried. By using sum frequency generation vibrational spectroscopy (SFG) with Hamiltonian local mode approximation method analysis, we studied protein adsorption at the silicone oil/protein solution interface in situ in real time, using bovine serum albumin (BSA) as a model. The results showed that the interface was mainly covered by BSA dimers. The deduced BSA dimer orientation on the silicone oil surface from the SFG study can be explained by the surface distribution of certain amino acids. To confirm the BSA dimer adsorption, we treated adsorbed BSA dimer molecules with dithiothreitol (DTT) to dissociate these dimers. SFG studies on adsorbed BSA after the DTT treatment indicated that the silicone oil surface is covered by BSA dimers and BSA monomers in an approximate 6 : 4 ratio. That is to say, about 25% of the adsorbed BSA dimers were converted to monomers after the DTT treatment. Extensive research has been reported in the literature to determine adsorbed protein dimer formation using ex situ experiments, e.g., by washing off the adsorbed proteins from the surface then analyzing the washed-off proteins, which may induce substantial errors in the washing process. Dimerization is a crucial initial step for protein aggregation. This research developed a new methodology to investigate protein aggregation at a solid/liquid (or liquid/liquid) interface in situ in real time using BSA dimer as an example, which will greatly impact many research fields and applications involving interfacial biological molecules.

Sensitivity of sum frequency generation experimental conditions to thin film interference effects

Sum-frequency generation (SFG) spectroscopy has furthered our understanding of the chemical interfaces that guide key processes in biology, catalysis, environmental science, and energy conversion. However, interpreting SFG spectra of systems containing several internal interfaces, such as thin film electronics, electrochemical cells, and biofilms, is challenging as different interfaces within these structures can produce interfering SFG signals. One potential way to address this issue is to carefully select experimental conditions that amplify the SFG signal of an interface of interest over all others. In this report, we investigate a model two-interface system to assess our ability to isolate the SFG signal from each interface. For SFG experiments performed in a reflective geometry, we find that there are few experimental conditions under which the SFG signal originating from either interface can be amplified and isolated from the other. However, by performing several measurements under conditions that alter their interference, we find that we can reconstruct each signal even in cases where the SFG signal from one interface is more than an order of magnitude smaller than its counterpart. The number of spectra needed for this reconstruction varies depending on the signal-to-noise level of the SFG dataset and the degree to which different experiments in a dataset vary in their sensitivity to each interface. Taken together, our work provides general guidelines for designing experimental protocols that can isolate SFG signals stemming from a particular region of interest within complex samples.

Probing the electrode-solution interfaces in rechargeable batteries by sum-frequency generation spectroscopy

An in-depth understanding of the electrode-electrolyte interaction and electrochemical reactions at the electrode-solution interfaces in rechargeable batteries is essential to develop novel electrolytes and electrode materials with high performance. In this perspective, we highlight the advantages of the interface-specific sum-frequency generation (SFG) spectroscopy on the studies of the electrode-solution interface for the Li-ion and Li-O₂ batteries. The SFG studies in probing solvent adsorption structures and solid-electrolyte interphase formation for the Li-ion battery are briefly reviewed. Recent progress on the SFG study of the oxygen reaction mechanisms and stability of the electrolyte in the Li-O₂ battery is also discussed. Finally, we present the current perspective and future directions in the SFG studies on the electrode-electrolyte interfaces toward providing deeper insight into the mechanisms of discharging/charging and parasitic reactions in novel rechargeable battery systems.

Probing Molecular Behavior of Carbonyl Groups at Buried Nylon/Polyolefin Interfaces in Situ

Nylon and maleic anhydride (MAH)-grafted polyolefin-based thin co-extruded multilayer films are widely used in packaging applications encountered in daily life. The molecular structure of the nylon/MAH-grafted polyolefin buried interface and molecular bonding between these two chemically dissimilar layers are thought to play an important role in achieving packaging structures with good adhesion. Here, the molecular bonds present at a nylon/maleic anhydride (MAH)-grafted polyethylene buried interface were systematically examined in situ for the first time using sum frequency generation (SFG) vibrational spectroscopy. The carbonyl stretching frequency region of the SFG spectra of a nylon/MAH-grafted polyethylene buried interface showed the presence of hydrolyzed MAH groups grafted to the polyethylene chain and very low levels of unreacted MAH enriched at the buried interface. The ability of SFG to detect these molecular species at the buried interface yields important understanding of the interfacial molecular structure and provides the basis for subsequent in situ studies of the bonding reaction between the grafted MAH and nylon directly at the interface. This understanding may guide the design of multilayer films with improved properties such as enhanced adhesion between polymer layers. The approach used in this study is general and is applicable to study the molecular characteristics of other buried interfaces of significance, such as buried interfaces involving polymers in solar cells, polymer semiconductors, and batteries. Nylon impact modification is another area of interest where the interaction between the MAH-grafted elastomer and the continuous phase of nylon is important.

Hydrogen-Bond-Driven Chemical Separations: Elucidating the Interfacial Steps of Self-Assembly in Solvent Extraction

Chemical separations, particularly liquid extractions, are pervasive in academic and industrial laboratories, yet a mechanistic understanding of the events governing their function are obscured by interfacial phenomena that are notoriously difficult to measure. In this work, we investigate the fundamental steps of ligand self-assembly as driven by changes in the interfacial H-bonding network using vibrational sum frequency generation. Our results show how the bulk pH modulates the interfacial structure of extractants at the buried oil/aqueous interface via the formation of unique H-bonding networks that order and bridge ligands to produce self-assembled aggregates. These extended H-bonded structures are key to the subsequent extraction of Co²⁺ from the aqueous phase in promoting micelle formation and subsequent ejection of the said micelle into the oil phase. The combination of static and time-resolved measurements reveals the events underlying complexities of liquid extractions at high [Co²⁺]:[ligand] ratios by showing an evolution of interfacially assembled structures that are readily tuned on a chemical basis by altering the compositions of the aqueous phase. The results of this work point to new principles to design-applied separations through the manipulation of surface charge, electrostatic screening, and the associated H-bonding networks that arise at the interface to facilitate organization and subsequent extraction.

Insight into the Mechanisms Driving the Self-Assembly of Functional Interfaces: Moving from Lipids to Charged Amphiphilic Oligomers

Polymer-stabilized liquid/liquid interfaces are an important and growing class of bioinspired materials that combine the structural and functional capabilities of advanced synthetic materials with naturally evolved biophysical systems. These platforms have the potential to serve as selective membranes for chemical separations and molecular sequencers and to even mimic neuromorphic computing elements. Despite the diversity in function, basic insight into the assembly of well-defined amphiphilic polymers to form functional structures remains elusive, which hinders the continued development of these technologies. In this work, we provide new mechanistic insight into the assembly of an amphiphilic polymer-stabilized oil/aqueous interface, in which the headgroups consist of positively charged methylimidazolium ionic liquids, and the tails are short, monodisperse oligodimethylsiloxanes covalently attached to the headgroups. We demonstrate using vibrational sum frequency generation spectroscopy and pendant drop tensiometery that the composition of the bulk aqueous phase, particularly the ionic strength, dictates the kinetics and structures of the amphiphiles in the organic phase as they decorate the interface. These results show that H-bonding and electrostatic interactions taking place in the aqueous phase bias the grafted oligomer conformations that are adopted in the neighboring oil phase. The kinetics of self-assembly were ionic strength dependent and found to be surprisingly slow, being composed of distinct regimes where molecules adsorb and reorient on relatively fast time scales, but where conformational sampling and frustrated packing takes place over longer time scales. These results set the stage for understanding related chemical phenomena of bioinspired materials in diverse technological and fundamental scientific fields and provide a solid physical foundation on which to design new functional interfaces.

Origin of the Overpotential for the Oxygen Evolution Reaction on a Well-Defined Graphene Electrode Probed by in Situ Sum Frequency Generation Vibrational Spectroscopy

To develop an efficient material for the cathode of the lithium-oxygen (Li-O₂) secondary battery, the oxygen reduction and evolution reactions (ORR and OER) on a well-defined graphene electrode have been investigated in a typical organic solvent, dimethyl sulfoxide (DMSO). The adsorption and desorption behaviors of the solvents on the graphene electrode surface were evaluated by an intrinsically surface-selective vibrational spectroscopy of sum frequency generation (SFG) during the ORR and OER. After the initial ORR depositing lithium peroxide (Li₂O₂) on the graphene electrode surface in a LiClO₄/DMSO solution, the SFG spectroscopy revealed that the subsequent OER oxidizing the Li₂O₂ preferentially proceeds at the interface between the Li₂O₂ and graphene rather than that between the Li₂O₂ and bulk solution. Therefore, the OER tends to reduce the electric conductivity between the Li₂O₂ and graphene by decreasing their contact area before a large part of the deposited Li₂O₂ was oxidized, which elucidates the origin of the high overpotential for the OER.

Surface-Restructuring Differences between Polyrotaxanes and Random Copolymers in Aqueous Environment

In the present study, we investigated the surface reorganization behaviors and adsorption conformations of fibrinogen on the surface of polyrotaxanes containing different amounts of α-cyclodextrin (α-CD) by using surface-sensitive vibrational spectroscopy sum frequency generation (SFG). For comparison, behaviors of the surface restructuring and fibrinogen adsorption on the random copolymers containing similar terminal groups were also investigated. It was found that larger amounts of BMA moieties of polyrotaxanes form ordered surface structures after immersion in water for 48 h. Furthermore, the polyrotaxane surfaces exhibit a much higher capability of fibrinogen adsorption than the random copolymer surfaces. The water-induced surface restructuring of the polyrotaxane films slightly affects the adsorption structure of the fibrinogen molecules.

Effect of immobilization site on the orientation and activity of surface-tethered enzymes

Tethering peptides and proteins to abiotic surfaces has the potential to create biomolecule-functionalized surfaces with useful properties.

Molecular Interactions between Graphene and Biological Molecules

Applications of graphene have extended into areas of nanobio-technology such as nanobio-medicine, nanobio-sensing, as well as nanoelectronics with biomolecules. These applications involve interactions between proteins, peptides, DNA, RNA etc. and graphene, therefore understanding such molecular interactions is essential. For example, many applications based on using graphene and peptides require peptides to interact with (e.g., noncovalently bind to) graphene at one end, while simultaneously exposing the other end to the surrounding medium (e.g., to detect analytes in solution). To control and characterize peptide behavior on a graphene surface in solution is difficult. Here we successfully probed the molecular interactions between two peptides (cecropin P1 and MSI-78(C1)) and graphene in situ and in real-time using sum frequency generation (SFG) vibrational spectroscopy and molecular dynamics (MD) simulation. We demonstrated that the distribution of various planar (including aromatic (Phe, Trp, Tyr, and His)/amide (Asn and Gln)/Guanidine (Arg)) side-chains and charged hydrophilic (such as Lys) side-chains in a peptide sequence determines the orientation of the peptide adsorbed on a graphene surface. It was found that peptide interactions with graphene depend on the competition between both planar and hydrophilic residues in the peptide. Our results indicated that part of cecropin P1 stands up on graphene due to an unbalanced distribution of planar and hydrophilic residues, whereas MSI-78(C1) lies down on graphene due to an even distribution of Phe residues and hydrophilic residues. With such knowledge, we could rationally design peptides with desired residues to manipulate peptide-graphene interactions, which allows peptides to adopt optimized structure and exhibit excellent activity for nanobio-technological applications. This research again demonstrates the power to combine SFG vibrational spectroscopy and MD simulation in studying interfacial biological molecules.

Theoretical and experimental examination of SFG polarization analysis at acetonitrile–water solution surfaces

Polarization analysis of SFG spectroscopy is thoroughly examined in collaboration of SFG measurements and MD simulations.

Salt Effects on Surface Structures of Polyelectrolyte Multilayers (PEMs) Investigated by Vibrational Sum Frequency Generation (SFG) Spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy was employed to investigate the surface structures of polyelectrolyte multilayers (PEMs) constructed by sequentially alternating adsorption of poly(diallyldimethylammonium chloride) (PDDA) and poly(styrenesulfonate) (PSS). It was found that the surface structures and surface charge density of the as-deposited PEMs of PDDA/PSS significantly depend on the concentration of sodium chloride (NaCl) present in the polyelectrolyte solutions. Furthermore, it was found that the surface structure of the as-deposited PEMs is in a metastable state and will reach the equilibrium state by diffusion of the polyelectrolyte chain after an aging process, resulting in a polyelectrolyte mixture on the PEM surfaces.

Phase transition behaviors of the supported DPPC bilayer investigated by sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM)

The phase transition behaviors of a supported bilayer of dipalmitoylphosphatidyl-choline (DPPC) have been systematically evaluated by in situ sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM).

Structural Reorganization and Fibrinogen Adsorption Behaviors on the Polyrotaxane Surfaces Investigated by Sum Frequency Generation Spectroscopy

Polyrotaxanes, such as supramolecular assemblies with methylated α-cyclodextrins (α-CDs) as host molecules noncovalently threaded on the linear polymer backbone, are promising materials for biomedical applications because they allow adsorbed proteins possessing a high surface flexibility as well as control of the cellular morphology and adhesion. To provide a general design principle for biomedical materials, we examined the surface reorganization behaviors and adsorption conformations of fibrinogen on the polyrotaxane surfaces with comparison to several random copolymers by sum frequency generation (SFG) vibrational spectroscopy. We showed that the polyrotaxane (OMe-PRX-PMB) with methylated α-CDs as the host molecule exhibited unique surface structures in an aqueous environment. The hydrophobic interaction between the methoxy groups of the methylated α-CD molecules and methyl groups of the n-butyl methacrylate (BMA) side chains may dominate the surface restructuring behavior of the OMe-PRX-PMB. The orientation analysis revealed that the orientation of the fibrinogen adsorbed on the OMe-PRX-PMB surface is close to a single distribution, which is different from the adsorption behaviors of fibrinogen on other polyrotaxane or random copolymer surfaces.

Probing Site-Specific Structural Information of Peptides at Model Membrane Interface In Situ

Isotope labeling is a powerful technique to probe detailed structures of biological molecules with a variety of analytical methods such as NMR and vibrational spectroscopies. It is important to obtain molecular structural information on biological molecules at interfaces such as cell membranes, but it is challenging to use the isotope labeling method to study interfacial biomolecules. Here, by individually (13)C═(16)O labeling ten residues of a peptide, Ovispirin-1, we have demonstrated for the first time that a site-specific environment of membrane associated peptide can be probed by the submonolayer surface sensitive sum frequency generation (SFG) vibrational spectroscopy in situ. With the peptide associated with a single lipid bilayer, the sinusoidal trend of the SFG line width and peak-center frequency suggests that the peptide is located at the interface beneath the lipid headgroup region. The constructive interferences between the isotope labeled peaks and the main peptide amide I peak contributed by the unlabeled components were used to determine the membrane orientation of the peptide. From the SFG spectral peak-center frequency, line width, and polarization dependence of the isotope labeled units, we deduced structural information on individual units of the peptide associated with a model cell membrane. We also performed molecular dynamics (MD) simulations to understand peptide-membrane interactions. The physical pictures described by simulation agree well with the SFG experimental result. This research demonstrates the feasibility and power of using isotope labeling SFG to probe molecular structures of interfacial biological molecules in situ in real time.