Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for nondestructive and label-free surface chemical characterization at nanometer length scales. However, despite being considered nondestructive, the interaction of the TERS probe used in the analysis can alter the molecular organization of the sample. In this study, we investigate the role of the atomic force microscopy (AFM) feedback (contact mode and tapping mode) on molecular perturbation in TERS analysis of soft samples using a self-assembled monolayer (SAM) of 2-chloro-4-nitrobenzene-1-thiol (Cl-NBT) as a test sample. Surprisingly, the tapping mode shows a consistently higher TERS signal resulting from a minimal perturbation of the Cl-NBT SAM compared to the contact mode. This study provides novel insights into the choice of the correct AFM-TERS operation mode for nanoscale chemical analysis of soft and delicate samples and is expected to expedite the growing application of TERS in this area.

Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful tool for correlative topographical and chemical imaging at the nanoscale. Herein, we examine the recent progress in the application of TERS to study two-dimensional molecular materials.

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We tested a paper-based platform ("Aptapaper") for the upconcentration and analysis of small molecules from complex matrices for two well-characterized aptamers, quinine and serotonin binding aptamers (QBA and SBA, respectively). After incubating the aptapaper under conditions that ensure correct aptamer folding, the aptapaper was used to upconcentrate target analytes from complex matrices. Aptapaper was rinsed, dried, and the target analyte was detected immediately or up to 4 days later by paper spray ionization coupled to high-resolution mass spectrometry (PS-MS). The minimum concentrations detectable were 81 pg/mL and 1.8 ng/mL for quinine and serotonin, respectively, from 100 mM AmAc or water. Complementary characterization of the QBA aptapaper system was performed using an orthogonal fluorescence microscopy method. Random adsorption was analyte-specific and observed for quinine, but not serotonin. This aptapaper approach is a semiquantitative (10−20% RSD) platform for upconcentration of small metabolites by mass spectrometry.

In the analysis of polymers by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), a commonly observed ionization pathway is cation adduct formation, as polymers often lack easily ionizable (basic/acidic) functional groups. The mechanism of this process has been hypothesized to involve gas-phase cation attachment. In previous experiments, a split sample plate set up has been introduced, enabling separate deposition of the components on individual MALDI plates. The plates are divided by a small gap of a few micrometers, allowing simultaneous laser irradiation from both plates, while precluding the possibility of any other interactions prior to ablation. Here, we extend on these studies by using different polymer-salt combinations to test the generalizability of a gas phase ionization process. Clear evidence for in-plume ionization is presented for the model polymers poly(methyl methacrylate) and polystyrene. Furthermore, the contribution of in-plume processes to the overall ion formation by cationization is considered, providing a first estimate for the importance of this pathway.