Plasmon-enhanced Raman scattering is a powerful approach to detecting and characterizing proteins in live and dynamic biological systems. However, the selective detection/enhancement of specific residues as well as spectral diffusion and fluctuations have complicated the interpretation of enhanced Raman spectra and images of biological matter. In this work, we demonstrate that the amino acid tryptophan (Trp) can capture an electron from an excited plasmon, which generates a radical anion that is resonantly enhanced: a visible excited electronic state slides into resonance upon charging. This surface enhanced resonance Raman scattering (SERRS) mechanism explains the persistence of Trp signatures in the SERS and TERS spectra of proteins. Evidence for this picture includes the observation of visible resonances in the UV–vis extinction spectrum, changes in the ground state vibrational spectrum, and plasmon-resonance dependent behavior. DFT calculations support the experimental observations. The behavior observed from the free Trp molecule is shown to explain the SERS spectrum of the Trp-cage protein. In effect, resonant Raman scattering from radicals formed through plasmonic excitation represents an under-investigated mechanism that may be exploited for chemical sensing applications.

While it is generally accepted that van der Waals (vdW) forces govern gecko adhesion, several studies indicate contributions from non-vdW forces and highlight the importance of understanding the adhesive contact interface. Previous work hypothesized that the surface of gecko setae is hydrophobic, with nonpolar lipid tails exposed on the surface. However, direct experimental evidence supporting this hypothesis and its implications on the adhesion mechanism is lacking. Here, we investigate the sapphire-setae contact interface using interface-sensitive spectroscopy and provide direct evidence of the involvement of acid-base interactions between polar lipid headgroups exposed on the setal surface and sapphire. During detachment, a layer of unbound lipids is left as a footprint due to cohesive failure within the lipid layer, which, in turn, reduces wear to setae during high stress sliding. The absence of this lipid layer enhances adhesion, despite a small setal-substrate contact area. Our results show that gecko adhesion is not exclusively a vdW-based, residue-free system.

Challenges investigating molecules on plasmonic nanostructures have limited understanding of these interactions. However, the chemically specific information in the surface-enhanced Raman scattering (SERS) spectrum can identify perturbations in the adsorbed molecules to provide insight relevant to applications in sensing, catalysis, and energy conversion. Here, we demonstrate spectrally resolved SERS imaging, to simultaneously image and collect the SERS spectra from molecules adsorbed on individual nanoparticles. We observe intensity and frequency fluctuations in the SERS signal on the time scale of tens of milliseconds from n -mercaptobenzoic acid (MBA) adsorbed to gold nanoparticles. The SERS signal fluctuations correlate with density functional theory calculations of radicals generated by the interaction between MBA and plasmon-generated hot electrons. Applying localization microscopy to the data provides a super-resolution spectrally resolved map that indicates the plasmonic-induced molecular charging occurs on the extremities of the nanoparticles, where the localized electromagnetic field is reported to be most intense.

Surface-enhanced Raman scattering (SERS) offers increases in chemical sensitivity associated with the Raman signal from molecules interacting with plasmonic nanoparticles and with use for diverse applications. This signal enhancement is the result of a combination of the enhanced electromagnetic field generated at the surface of the metal nanostructures and possible chemical enhancements specific to the molecule being detected. These chemical effects can alter the chemical identity of the analyte and manifest as differences in the SERS spectrum relative to the spontaneous Raman spectrum. In this work, we examine changes in the vibrational spectrum resulting from the previously hypothesized formation of a radical anion of the amino acid tryptophan on gold (Au) nanoparticles. Density functional theory calculations are used to model changes in the vibrational frequencies attendant to the tryptophan radical anion and correlated with the experimentally observed vibrational modes in the SERS spectrum of tryptophan (Trp) on Au nanoparticles. The calculated vibrational frequencies are in close agreement with the experimental data. N-Acetyltryptophanamide (NATA) is shown to have the same trends in the calculated and experimental vibrations, indicating that the captured electron is localized to the indole ring structure. Changes in the SERS spectrum with pH are readily explained by the calculations, further indicating that the observed signal originates from the ground-state radical anion in these experiments. This evidence for the capture of a hot electron by tryptophan and the resulting changes in the vibrational spectrum are important for molecular understanding of proteins and other molecules as detected by SERS.

The excitation of plasmon resonances on nanoparticles generates locally enhanced electric fields commonly used for sensing applications, and energetic charge carriers can drive chemical transformations as photocatalysts. The surface-enhanced Raman scattering (SERS) spectra from mercaptobenzoic acid (MBA) adsorbed to gold nanoparticles (AuNP) and silica-encapsulated gold nanoparticles (AuNP@silica) can be used to assess the impact of energetic charge carriers on the observed signal. Measurements were recorded using a traditional point-focused Raman spectroscopy and a wide-field spectral imaging approach to assess changes in the spectra of the different particles at increasing power density. The wide-field approach provides an increase in sampling statistics and shows evidence of SERS frequency fluctuations from MBA at low power densities, where it is commonly difficult to record spectra from a point-focused spot. The increased spectral resolution of the point spectroscopy measurement provides improved peak identification and the ability to correlate the frequency fluctuations to charged intermediate species. Interestingly, our work suggests that isolated nanoparticles may undergo frequency fluctuations more readily than aggregates.