Surface Enhanced Raman Scattering Selectivity in Proteins Arises from Electron Capture and Resonant Enhancement of Radical Species

Highly sensitive detection of tryptophan based on Schiff base reaction and surface-enhanced Raman spectroscopy

In this study, a simple, rapid and sensitive method combining surface-enhanced Raman spectroscopy and Schiff base reaction was developed for the detection of tryptophan. This method does not require product separation to obtain a significant Raman signal of the derivatized product, and the derivatization reaction can be controlled by experimental parameters such as reaction temperature, time, concentration of derivatization reagent and concentration of sodium nitrite. The characteristic peak of the derivative of tryptophan (1620 cm-1) was selected for quantitative analysis, and the intensity of the characteristic Raman spectrum peak showed a linear relationship with the concentration of tryptophan (10-8-10-4 mol/L) in the range of with a correlation coefficient R2 of 0.9922. This assay combines surface-enhanced Raman spectroscopy and Schiff base reaction, which is characterized by high sensitivity and easy operation, and has good application prospects in the detection of tryptophan in food.

Whispering-Gallery Mode Optoplasmonic Microcavities: From Advanced Single-Molecule Sensors and Microlasers to Applications in Synthetic Biology

Optical microcavities, specifically, whispering-gallery mode (WGM) microcavities, with their remarkable sensitivity to environmental changes, have been extensively employed as biosensors, enabling the detection of a wide range of biomolecules and nanoparticles. To push the limits of detection down to the most sensitive single-molecule level, plasmonic nanorods are strategically introduced to enhance the evanescent fields of WGM microcavities. This advancement of optoplasmonic WGM sensors allows for the detection of single molecules of a protein, conformational changes, and even atomic ions, marking significant contributions in single-molecule sensing. This Perspective discusses the exciting research prospects in optoplasmonic WGM sensing of single molecules, including the study of enzyme thermodynamics and kinetics, the emergence of thermo-optoplasmonic sensing, the ultrasensitive single-molecule sensing on WGM microlasers, and applications in synthetic biology.

SERS probing of fungal HET-s fibrils formed at neutral and acidic pH conditions

Advances in precision medical diagnostics require accurate and sensitive characterization of pathogens. In particular, health conditions associated with protein misfolding require an identification of proteinaceous amyloid fibrils or their precursors. These pathogenic entities express specific molecular structures, which require ultra-sensitive, molecular-level detection methods. A potentially transformative technique termed nanoplasmonics employs electro-optical phenomena in the vicinity of specially engineered metal nanostructures. A signature application of nanoplasmonics exploits enhancement of inelastic scattering of light in specific locations near metallic nanostructures, known as surface-enhanced Raman scattering (SERS). We applied SERS complemented with confocal microscopy imaging for ultra-sensitive, non-invasive, and label-free characterization of the fungal prion HET-s (218-289) as a model for β-sheet rich amyloid structures. This characterization employed Au-coated dielectric supports as plasmonic substrates. After confirming the formation of HET-s fibrils at both pH 7.5 and 2.8 using negative staining transmission electron microscopy, we subjected the fibril-containing solutions to multimodal analysis using confocal microscopy and SERS. The SERS spectral fingerprints from all HET-s samples expressed vibrational markers for β-structure, unstructured backbone, and aromatic side-chains. However, relative intensities of major SERS bands were pronouncedly different for the two pH levels. We have analyzed potential origins of the most pronounced SERS bands and proposed hypothetical mechanistic models that could explain the observed SERS fingerprints from HET-s fibrils grown at pH 7.5 and 2.8.

Controlling the Fluctuating Tip-Enhanced Raman Spectra of Chloramben on Silver Nanocubes

Tip-enhanced Raman (TER) scattering from molecules residing at plasmonic junctions can be used to detect, identify, and image single molecules. This is most evident for flat molecules interrogated under conditions of extreme temperatures and pressure. It is also the case for (bio)molecular systems that feature preferred orientations/conformations under ambient laboratory conditions. More complex molecules that can adopt multiple conformations and/or feature different protonation or charge states give rise to complex TER spectra. We illustrate how the latter can be controlled in the case of chloramben molecules coated onto plasmonic silver nanocubes. We show that characteristic molecular Raman spectra cannot be obtained when tunneling plasmons are operative, i.e., when the tip is in direct contact with the chemically functionalized plasmonic nanoparticles. We rationalize these observations and propose an approach to less invasive and hence more analytical TER spectral imaging.

Determination of lentiviral titer by surface enhanced Raman scattering

Lentiviruses are commonly used to deliver genetic code into host cells for biomedical applications, such as gene therapy, pharmaceuticals, and vaccine development. Knowing the infectious titer of these virus particles is critical for development in these areas. Current methods of determining viral titer often require cell culture, where a cell is infected and the inserted genetic code is expressed in a known number of cells, which can require days or weeks to prepare and analyze samples. To provide a more rapid method of determining viral titer, the use of surface enhanced Raman spectroscopy (SERS) was explored. SERS provides both chemical and structural information by using plasmonic metallic nanostructures to amplify the Raman signal. Two different lentiviruses, one with a vector encoding a GFP gene and the same virus without the GFP gene included, were analyzed by SERS in viral production media at various concentrations. The SERS response was demonstrated to be sensitive to the incorporation of the GFP gene into the viral vector. Chemometric analysis using multivariate curve resolution (MCR) was able to identify a component in the observed SERS spectra that correlated with the concentration of GFP containing virus particles. Using the MCR model and the SERS response, the viral titer of lentivirus encoding for GFP was determined. The viral titer determined by SERS agreed well with expression of the GFP in infected cells. The SERS response using different metals and excitation wavelengths was also explored. Overall, this work demonstrates the utility of SERS for rapid determination of lentiviral titer.

Nanoscale chemical characterization of biomolecules using tip-enhanced Raman spectroscopy

Nanoscale chemical and structural characterization of single biomolecules and assemblies is of paramount importance for applications in biology and medicine. It aims to describe the molecular structure of biomolecules and their interaction with unprecedented spatial resolution to better comprehend underlying molecular mechanisms of biological processes involved in cell activity and diseases. Tip-enhanced Raman scattering (TERS) spectroscopy appears particularly appealing to reach these objectives. This state-of-the-art TERS technique is as versatile as it is ultrasensitive. To perform a successful TERS experiment, special care and a thorough methodology for the preparation of the TERS system, the TERS probe tip, and sample are needed. Intense efforts have been deployed to characterize nucleic acids, proteins and peptides, lipid membranes, and more complex systems such as cells and viruses using TERS. Although the vast majority of studies have first been performed in dry conditions, they have allowed for several scientific breakthroughs. These include DNA and RNA sequencing, and the determination of relationships between protein structure and biological function by the use of increasingly exploitative chemometric tools for spectral data analysis. The nanoscale determination of the secondary structure of amyloid fibrils, protofibrils and oligomers implicated in neurodegenerative diseases could, for instance, be connected with the toxicity of these species, amyloid formation pathways, and their interaction with phospholipids. Single particles of different viral strains could be distinguished from one another by comparison of their protein and lipid contents. In addition, TERS has allowed for the evermore accurate description of the molecular organization of lipid membranes. Very recent advances also demonstrated the possibility to carry out TERS in aqueous medium, which opens thrilling perspectives for the TERS technique in biological, biomedical, and potential clinical applications.

Spectrally Resolved Surface-Enhanced Raman Scattering Imaging Reveals Plasmon-Mediated Chemical Transformations

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.

Unveiling the interaction of protein fibrils with gold nanoparticles by plasmon enhanced nano-spectroscopy

The development of various degenerative diseases is suggested to be triggered by the uncontrolled organisation and aggregation of proteins into amyloid fibrils. For this reason, there are ongoing efforts to develop novel agents and approaches, including metal nanoparticle-based colloids, that dissolve amyloid structures and prevent pathogenic protein aggregation. In this contribution, the role of gold nanoparticles (AuNPs) in degrading amyloid fibrils of the model protein lysozyme is investigated. The amino acid composition of fibril surfaces before and after the incubation with AuNPs is determined at the single fibril level by exploiting the high spatial resolution and sensitivity provided by tip-enhanced and surface-enhanced Raman spectroscopies. This combined spectroscopic approach allows to reveal the molecular mechanisms driving the interaction between fibrils and AuNPs. Our results provide an important input for the understanding of amyloid fibrils and could have a potential translational impact on the development of strategies for the prevention and treatment of amyloid-related diseases.

From SERS to TERS and Beyond: Molecules as Probes of Nanoscopic Optical Fields

A detailed understanding of the interaction between molecules and plasmonic nanostructures is important for several exciting developments in (bio)molecular sensing and imaging, catalysis, as well as energy conversion. While much of the focus has been on the nanostructures that generate enhanced and nano-confined optical fields, we herein highlight recent work from our groups that uses the molecular response in surface and tip enhanced Raman scattering (SERS and TERS, respectively) to investigate different aspects of the local fields. TERS provides access to ultra-confined volumes, and as a result can further explore and explain ensemble-averaged SERS measurements. Exciting and distinct molecular behaviors are observed in the quantum limit of plasmons, including molecular charging, chemical conversion, and optical rectification. Evidence of multipolar Raman scattering from molecules additionally provides insights into the inhomogeneous electric fields that drive SERS and TERS and their spatial and temporal gradients. The time scales of these processes show evidence of cooperative nanoscale phenomena that altogether contribute to SERS and TERS.