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Surface-enhanced Raman spectroscopy probes adsorbates on a plasmonic substrate and offers high sensitivity with molecular identification capabilities. In this study, we present a refined methodology for considering the supporting substrate in the computation of the Raman spectra. The supporting substrate is taken into account by employing a periodic slab model when doing the geometry optimization and normal mode analysis, and then the Raman spectrum is calculated for the isolated molecule but with the normal modes from the surface structure. We find that the interaction with the surface induces internal distortion in the molecule, and spectral shifts in the computed Raman spectrum. By comparing a low temperature surface-enhanced Raman spectroscopy measurement of Rhodamine 6G (R6G) with the computed Raman spectra of a series of adsorption geometries, we propose that the binding state captured in the experiment tends to possess the least internal distortion. This binding state involves upward orientation of ethylamine on R6G, and our calculations indicate that this is the lowest energy adsorption structure. Following this route, it is possible to infer both a molecular orientation and an adsorption geometry of the molecule from its Raman spectrum. Importantly, we note that, if the substrate correction is established to play a role, we also discuss that this corrected approach still has several shortcomings that significantly limit its overall accuracy in comparison with experimental spectra.

Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) combines the atomic-scale imaging capability of scanning probe microscopy with the single-molecule chemical sensitivity and structural specificity of surface-enhanced Raman spectroscopy. Here, we use these techniques in combination with theory to reveal insights into the influence of intermolecular interactions on the vibrational spectra of a N-N′-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) self-assembled monolayer adsorbed on single-crystal Ag substrates at room temperature. In particular, we have revealed the lifting of a vibrational degeneracy of a mode of PDI on Ag(111) and Ag(100) surfaces, with the most strongly perturbed mode being that associated with the largest vibrational amplitude on the periphery of the molecule. This work demonstrates that UHV-TERS enables direct measurement of molecule–molecule interaction at nanoscale. We anticipate that this information will advance the fundamental understanding of the most important effect of intermolecular interactions on the vibrational modes of surface-bound molecules.

Native electron capture dissociation (NECD) is a process during which proteins undergo fragmentation similar to that from radical dissociation methods, but without the addition of exogenous electrons. However, after three initial reports of NECD from the cytochrome cdimer complex, no further evidence of the effect has been published. Here, we report NECD behavior from horse spleen ferritin, a ∼490 kDa protein complex ∼20-fold larger than the previously studied cytochrome cdimer. Application of front-end infrared excitation (FIRE) in conjunction with low- and high-m/zquadrupole isolation and collisionally activated dissociation (CAD) provides new insights into the NECD mechanism. Additionally, activation of the intact complex in either the electrospray droplet or the gas phase produced c-type fragment ions. Similar to the previously reported results on cytochrome c, these fragment ions form near residues known to interact with iron atoms in solution. By mapping the location of backbone cleavages associated with c-type ions onto the crystal structure, we are able to characterize two distinct iron binding channels that facilitate iron ion transport into the core of the complex. The resulting pathways are in good agreement with previously reported results for iron binding sites in mammalian ferritin.

Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was used for the first time to spatially resolve local heterogeneity in redox behavior on an electrode surface in situand at the nanoscale. A structurally well-defined Au(111) nanoplate located on a polycrystalline ITO substrate was studied to examine nanoscale redox contrast across the two electrode materials. By monitoring the TERS intensity of adsorbed Nile Blue (NB) molecules on the electrode surface, TERS maps were acquired with different applied potentials. The EC-TERS maps showed a spatial contrast in TERS intensity between Au and ITO. TERS line scans near the edge of a 20 nm-thick Au nanoplate demonstrated a spatial resolution of 81 nm under an applied potential of −0.1 V vs Ag/AgCl. The intensities from the TERS maps at various applied potentials followed Nernstian behavior, and a formal potential (E0′) map was constructed by fitting the TERS intensity at each pixel to the Nernst equation. Clear nanoscale spatial contrast between the Au and ITO regions was observed in the E0′map. In addition, statistical analysis of the E0′map identified a statistically significant 4 mV difference in E0′on Au vs ITO. Electrochemical heterogeneity was also evident in the E0′distribution, as a bimodal distribution was observed in E0′on polycrystalline ITO, but not on gold. A direct comparison between an AFM friction image and the E0′map resolved the electrochemical behavior of individual ITO grains with a spatial resolution of ∼40 nm. The variation in E0′was attributed to different local surface charges on the ITO grains. Such site-specific electrochemical information with nanoscale spatial and few mV voltage resolutions is not available using ensemble spectroelectrochemical methods. We expect that in situredox mapping at the nanoscale using EC-AFM-TERS will have a crucial impact on understanding the role of nanoscale surface features in applications such as electrocatalysis.

Nanoparticle encapsulation inside zirconium-based metal–organic frameworks (NP@MOF) is hard to control, and the resulting materials often have nonuniform morphologies with NPs on the external surface of MOFs and NP aggregates inside the MOFs. In this work, we report the controlled encapsulation of gold nanorods (AuNRs) by a scu-topology Zr-MOF, via a room-temperature MOF assembly. This is achieved by functionalizing the AuNRs with poly(ethylene glycol) surface ligands, allowing them to retain colloidal stability in the precursor solution and to seed the MOF growth. Using this approach, we achieve core–shell yields exceeding 99%, tuning the MOF particle size via the solution concentration of AuNRs. The functionality of AuNR@MOFs is demonstrated by using the AuNRs as embedded probes for selective surface-enhanced Raman spectroscopy (SERS). The AuNR@MOFs are able to both take-up or block molecules from the pores, thereby facilitating highly selective sensing at the AuNR ends. This proof-of-principle study serves to present both the outstanding level of control in the synthesis and the high potential for AuNR@Zr-MOF composites for SERS.

Atomic layer deposition (ALD) of TiO2was performed in tandem with in situ surface-enhanced Raman spectroscopy (SERS) to monitor changes in the transient surface species across multiple ALD cycles. A self-assembled monolayer of 3-mercaptopropionic acid was used as a capture agent to ensure that nucleation of the titanium precursor (titanium tetraisopropoxide [TTIP]) occurs. Comparisons between the Raman spectra of the neat precursor and the SER spectra of the first ALD cycle of TiO2reveal typical ligand exchange chemistry taking place, with self-limiting behavior and intact isopropoxide ligands. However, subsequent cycles show drastically different chemistry, with no isopropoxide ligands remaining at any point during the second and third cycles. Continuous exposure of either TTIP or isopropyl alcohol after the first cycle shows unlimited chemical vapor deposition (CVD)-type growth. Comparisons with alternative precursors (aluminum isopropoxide, titanium tert-butoxide, and titanium propoxide) and DFT calculations reveal that, for the TTIP precursor, isolated TiO2sites play a role in the dehydration of off-gassing isopropyl alcohol. The resulting propene then undergoes oligomerization into six-carbon olefins before polymerizing into indistinguishable carbon products that accumulate on the surface. The emergence of the dehydration chemistry is expected to be exclusively the result of these isolated TiO2sites and, as such, is expected to occur on other surfaces where TiO2ALD is feasible. This work showcases how seemingly innocuous ALD can evolve into a CVD process when the products can participate in various side reactions with newly made surface sites.