Insights on the Molecular Characteristics of Molecularly Imprinted Polymers as Monitored by Sum Frequency Generation Spectroscopy

Abstract

Sensing in molecularly imprinted polymers (MIPs) requires specific interactions of the imprinted polymer and the approaching template molecule. These interactions are affected by the morphology of the polymer surface, the affinity of the template molecule to the polymer network, and the steric approach. In this particular study, a template molecule, metronidazole, is studied with respect to the typically used methacrylic acid-based imprinted polymer using a combination of bulk and surface techniques. The resulting infrared (IR) spectra exhibited the presence of the template molecule in the polymer matrix as well as their efficient removal after washing. Dipping of the MIP according to what is expected of facile sensing in an aqueous solution of metronidazole did not show any presence of the template molecule in the bulk of the MIP, as observed by IR spectroscopy. However, using sum frequency generation (SFG) spectroscopy, the CH aromatic stretch of the imidazole ring positioned at ∼3100 cm −1 was observed at the polymer surface, including its inner pores or cavities, and at the buried polymer-fused silica interface after dipping. SFG studies have also shown the vibrational signatures of the polymer matrix, the presence of the template molecule on the surface, and the detection of residual template molecules after washing. Increasing the washing time to 50 min has proven to be less effective than increasing the washing cycles to three. However, after the third cycle, reorganization of the polymer matrix was evident as also the complete removal of the template molecule. The observed changes from the acquired images using scanning electron microscopy and atomic force microscopy show the structural morphologies of MIPs and a good distribution of the pores across the MIP surface. The study demonstrates the importance of combining both bulk and surface characterization in providing insight into the template molecule−polymer network interactions.

Molecular Insights into Adhesion at a Buried Silica-Filled Silicone/Polyethylene Terephthalate Interface

“…Sum frequency generation (SFG) vibrational spectroscopy is a second-order nonlinear optical spectroscopic technique, which is capable of detecting the presence, coverage, and orientation of functional groups at surfaces and interfaces including buried solid/solid interfaces in situ . − Because of the selection rules of a second-order nonlinear optical process, only surfaces and interfaces without inversion symmetry can generate SFG signals, avoiding the interference of signals generated from the bulk material. − SFG has excellent sub-monolayer surface/interface specificity and has been developed into a powerful technique to investigate molecular structures at the buried polymer interfaces. ,,− Many traditional surface-sensitive analytical techniques (e.g., XPS) require debonding of composite parts to obtain structural information of the original buried interface, which may destroy information about the original interface . Practically, good adhesion is often defined by the observation of cohesive failure of the adhesive or substrate, which occurs because the interface is sufficiently strong so that it cannot be separated cleanly.…”

Effect of Surfactant Concentration and Hydrophobicity on the Ordering of Water at a Silica Surface

“…SFG vibrational spectroscopy (or SFG in short) is a second-order nonlinear optical technique which is submonolayer surface sensitive because of its special selection rule: under the electric dipole approximation, SFG signal can only be generated from a medium with no inversion symmetry. − SFG probes the macroscopic second-order nonlinear susceptibility χ (2) of a material, while χ (2) is nonzero only for a sample with no inversion symmetry. − , Most bulk media are centrosymmetric, thus do not have SFG signal, while surfaces and interfaces lack inversion symmetry and can generate SFG signals. For a charged interface like silica–water interface, it is believed and shown that there are bulk (3) contributions from the electric double layer/diffuse layer near the silica–water interface.…”

“…Here we studied nonionic surfactants and water at the silica–surfactant solution interface. SFG is a surface-specific second-order nonlinear optical spectroscopy probing second-order nonlinear susceptibility χ (2) of a medium, which has been developed into a powerful tool to investigate surfaces and interfaces. − We have applied SFG to study a variety of complex surfaces and interfaces including biological molecules, polymers, as well as surfactants. ,− The silica–water interfaces have been extensively studied by SFG in various aspects. − Silanol groups on a silica surface can be deprotonated in an aqueous solution under neutral pH (SiOH ⇄ SiO – + H + ), which results in a negatively charged silica surface in contact with water. , Different surface structures of silica can form different types of hydrogen bonding with water at the buried silica–water interface, making water molecules ordered at the immediate silica–water interface (the nearest water layer on silica), which contribute to SFG signals. − …”

Interfacial Structure and Interfacial Tension in Model Carbon Fiber-Reinforced Polymers

“…Sum frequency generation (SFG) vibrational spectroscopy is a second-order nonlinear optical technique with intrinsic interface sensitivity. − SFG has been extensively employed to study molecular structures of solid/liquid interfaces. − We have demonstrated that the valuable molecular information for polymer/solution interfaces obtained by SFG can contribute to the elucidation of interfacial behaviors of polymer, − water, − and biological molecules. ,− To gain fundamental understandings of the interfacial behaviors in CFRP, SFG was applied to characterize the molecular structures of carbon fiber/sizing solution and plastic/sizing solution interfaces in this study.…”