Nanoscale Chemical Imaging of Supported Lipid Monolayers using Tip‐Enhanced Raman Spectroscopy

Abstract

Visualizing the molecular organization of lipid membranes is essential to comprehend their biological functions. However, current analytical techniques fail to provide a non‐destructive and label‐free characterization of lipid films under ambient conditions at nanometer length scales. In this work, we demonstrate the capability of tip‐enhanced Raman spectroscopy (TERS) to probe the molecular organization of supported DPPC monolayers on Au (111), prepared using the Langmuir–Blodgett (LB) technique. High‐quality TERS spectra were obtained, that permitted a direct correlation of the topography of the lipid monolayer with its TERS image for the first time. Furthermore, hyperspectral TERS imaging revealed the presence of nanometer‐sized holes within a continuous DPPC monolayer structure. This shows that a homogeneously transferred LB monolayer is heterogeneous at the nanoscale. Finally, the high sensitivity and spatial resolution down to 20 nm of TERS imaging enabled reproducible, hyperspectral visualization of molecular disorder in the DPPC monolayers, demonstrating that TERS is a promising nanoanalytical tool to investigate the molecular organization of lipid membranes.

Nanoscale chemical characterization of biomolecules using tip-enhanced Raman spectroscopy

“…Early TERS characterization on dry phospholipid mixtures revealed the possibility to identify most of the chemical groups of lipids, including ester and phosphate ones, 40 albeit not in all experimental conditions. 15,16,40 CH stretching and bending vibrations were however more frequently observed than the other vibrations. 15,16,40 Therefore, even though CQO ester bands could be used as reliable lipid markers, 32,39 CH stretching ones were preferentially selected for TERS mapping purposes owing to the intensity of the corresponding TERS bands and their high wavenumber in a spectral region devoid of other spectral contributions.…”

“…clean Au-coated Si/SiO 2 wafers, Au or Ag single crystals). 8,11,[14][15][16][17] Gold is often preferred due to its chemical stability and the additional electromagnetic enhancement (410 6 ) that it can generate. It has been established that the resonant excitation of a TERS tip induces a dipole at the tip apex.…”

“…This latter constraint limits the applicability of ''gap-mode'' TERS in biology although single biomolecules and nanoscale biomaterials can be characterized. 11,12,[15][16][17] The sample deposition onto metal STM substrates is typically based on direct solution drop-casting, microcontact printing of a biomolecule-impregnated stamp (followed by rinsing and drying in a flow of argon or nitrogen gas), or on transferring Langmuir-Blodgett films. 2,15,16 In AFM, an insulating material is perfectly suitable as substrate, although gold films have also been employed in ''gap-mode'' configuration.…”

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

“…The molecular organization of a dipalmitoylphosphatidylcholine (DPPC) monolayer, which is an important constituent of pulmonary surfactants, on Au (111) was investigated by STM-TERS imaging of the C-H stretching at 2800-3000 cm −1 . 141 The higher intensity ratio of the TERS peak at 2930 cm −1 , which was assigned to the Fermi resonance associated with the terminal methyl symmetric stretch and the C-H bending mode, 142,143 to that at 2850 cm −1 , which was assigned to the methylene symmetric stretch of the lipid chains, 142,143 represents higher molecular disorder. The regions were clearly distinguished from the lower molecular disorder regions with 20 nm spatial resolution.…”

“…The regions were clearly distinguished from the lower molecular disorder regions with 20 nm spatial resolution. 141 In a mixture of protein and drug, 122 moreover, the binding between them was investigated by noncontact AFM-TERS to avoid contamination. Indeed, the concentration of the drug depended on the position.…”