Twenty-Five Years of PM-IRRAS: Advancing the Understanding of Organized Molecular Films on Gold Electrodes

This article provides a comprehensive review of advances in polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) for studying organized molecular films at electrode surfaces over the past 25 years. After briefly reviewing the history of PM-IRRAS and the principles of data processing, we describe its application to a wide range of molecular films formed on electrodes. PM-IRRAS produces infrared spectra of molecular films adsorbed on metal surfaces, which closely resemble conventional transmission IR spectra, allowing accurate determination of band positions, widths, and intensities. The technique provides unique molecular-level information, including band assignments, molecular mobility, conformation, physical state, molecular orientation within the film, and hydrogen bonding between solvent molecules and the polar headgroups of film constituents. Furthermore, PM-IRRAS offers significant advantages for in situ investigations of electrochemical interfaces under potential control by minimizing interference from the absorption of aqueous solutions. The combination of potential control with PM-IRRAS provides a means of investigating changes in film structure and stability under applied electric fields. The quantitative power of PM-IRRAS is demonstrated by its application to diverse systems ranging from physisorbed films of insoluble surfactants, weakly and strongly chemisorbed monolayers, supported lipid bilayers, interactions of antimicrobial peptides with lipid bilayers, peripheral proteins bound to phospholipid bilayers, amyloid plaque formation, molecular recognition reactions (Watson–Crick complex formation), and electrochemical reduction of graphene oxide. These examples collectively illustrate the versatility of PM-IRRAS in addressing fundamental questions about molecular organization, interfacial dynamics, and activity. The ability of PM-IRRAS to deliver detailed molecular-level insight into a broad spectrum of thin-film systems highlights its versatility and underscores its promise as an essential tool in surface science and biophysics.