Surface-Enhanced Raman Spectroscopy of Soft-Landed Polyatomic Ions and Molecules

Abstract

Surface-enhanced Raman spectroscopy (SERS) was used to detect and characterize polyatomic cations and molecules that were electrosprayed into the gas phase and soft-landed in vacuum on plasma-treated silver substrates. Organic dyes such as crystal violet and Rhodamine B, the nucleobase cytosine, and nucleosides cytidine and 2'-deoxycytidine were immobilized by soft landing on plasma-treated metal surfaces at kinetic energies ranging from near thermal to 200 eV. While enhancing Raman scattering 10(5)-10(6)-fold, the metal surface effectively quenches the fluorescence that does not interfere with the Raman spectra. SERS spectra from submonolayer amounts of soft-landed compounds were sufficiently intense and reproducible to allow identification of Raman active vibrational modes for structure assignment. Soft-landed species appear to be microsolvated on the surface and bound via ion pairing or pi-complexation to the Ag atoms and ions in the surface oxide layer. Comparison of spectra from soft-landed and solution samples indicates that the molecules survive soft landing without significant chemical damage even when they strike the surface at hyperthermal collision energies.

In situ SIMS analysis and reactions of surfaces prepared by soft landing of mass-selected cations and anions using an ion trap mass spectrometer

“…Since the introduction of the basic concept of SL [6], the method has been applied to small molecules, clusters, peptides, proteins, oligonucleotides, and even viruses, with deposition onto different substrates [10 -15]. Although the first successful surface modification was reported in 1983 [16], applications aimed at on the preparation of unique surfaces are more recent [17][18][19]. SAMs provide a particularly convenient and flexible platform for creating materials with specific chemical properties and they have been used extensively for studying ion/ surface collision phenomena, including SL experiments [10, 20 -22].…”

“…Infrared reflection absorption spectroscopy (IRRAS) confirmed the ␣-helical conformation of the soft-landed peptide. Previous surface-enhanced Raman spectroscopy (SERS) studies of soft-landed protein surfaces proved that substantial biological functionality is retained in the hyperthermal collision event during SL [18]. Soft-landed biological or polyatomic organic molecules can be probed by fluorescent imaging, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) [5,18,29].…”

“…Previous surface-enhanced Raman spectroscopy (SERS) studies of soft-landed protein surfaces proved that substantial biological functionality is retained in the hyperthermal collision event during SL [18]. Soft-landed biological or polyatomic organic molecules can be probed by fluorescent imaging, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) [5,18,29]. Clearly, controlled deposition of organic and biomolecular ions onto surfaces represents a new approach for obtaining molecular level understanding of the interactions of these molecules/ions with a variety of hydrophobic and hydrophilic substrates.…”

“…Analysis can be performed ex situ using various analytical methods after removing the substrate from the vacuum system. Ex situ methods available to investigate modified soft-landed surfaces include matrix-assisted laser desorption/ionization (MALDI) and nanoESI mass spectrometry [4,18]. In these experiments, the material is rinsed off the surface for further analysis.…”

“…In these experiments, the material is rinsed off the surface for further analysis. On-surface ex situ investigations of soft-landed species can use SERS, fluorescence, and bioassays, as well as microscopic techniques [4,18]. Other ex situ techniques also reported include IRRAS, temperature-programmed desorption (TPD), and SIMS [8,34,35].…”