Interfacial water are often referred to as water molecules in close proximity (&lt; 2 nm) or bound to the surface. Identifying its roles in the interactions of nanomaterials (NMs) with cell membranes is the key for enhancing the performance of NMs in various applications (e.g., as carriers for targeted drug delivery) and minimizing unintended consequences (e.g., cytotoxicity). Model membrane systems (e.g., lipid monolayer, bilayer and vesicles) consist of one or more categories of phospholipids are commonly used as platforms to study NM-membrane interactions. This presentation will highlight the latest developments of surface-sensitive analytical tools that are used to probe the interfacial water involved in such interactions. Surface-enhanced IR absorption spectroscopy (SEIRAS) and sum-frequency generation spectroscopy (SFGS) are discussed below as examples of how these state-of-the-art techniques are being used in this context. SEIRAS exploited the enhancement (as high as 103) of the electromagnetic field occurring near (~10 nm) plasmon-resonant NMs to characterize the different composition of membrane lipids [e.g., hydrophilic head (CH3 and CH2-enriched) and hydrophobic tail (carbonyl-enriched)] and the associated water distribution by monitoring the primary dipole-active vibrational modes of lipids that typically absorb in the mid-IR. In the presence of ionic and polar groups on the surface, the adsorbed water molecules can form strongly hydrogen-bonded or ordered structure, in contrast to the randomly orientated ones in bulk water. The former is usually centered at lower wavelength (~3150 cm-1) of the OH stretching band (~3000 to 3800 cm-1), while the latter is featured at higher wavelengths. The position of the OH band is highly dependent on the extent of hydrogen bonding in water molecules. SFGS is a second-order nonlinear technique that provides molecule orientation and structural information (e.g., lipid flip-flop). With signals strictly localized to the surface/interface, SFGS generally has greater sensitivity to interfacial water than SEIRAS. While SEIRAS and SFGS provide robust information spectroscopically, ultra-high resolution atomic force microscopy is recently emerging as a complementary tool that offers powerful visualization of interfacial water. In sum, this presentation focused on illustrating how characterization of hydration dynamics contributes to a more unified picture for important phenomena at the nano-bio interface.

Probing interfacial water at the nanomaterial-membrane interfaces: A conspectus of state-of-the-art analytical techniques

ACS Spring Meeting 2021

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