What are the main Life Sciences TERS applications?

TERS offers several benefits in studying biomolecules compared to other spectroscopic and microscopic techniques. For example, it can overcome low signal-to-noise that hampers some biological applications and thus avoid the need to use larger sample volumes. This is because TERS offers enhanced Raman signal similar to Surface Enhanced Raman Spectroscopy (SERS) with the added benefit of nanometer resolution and hence it can potentially target single molecules⁹⁸. Alternative methods to study nanoscale chemical composition such as super-resolution fluorescence based methods or methods to study morphology such as scanning electron microscopy are available; however, they almost always require fluorescence labeling or heavy metal staining. TERS has the capability to provide nanoscale chemical composition and morphology in a label-free manner. TERS has been used to study a number of biomaterials such as amino acid and nucleobase monolayers, proteins, macromolecular protein assemblies, nucleic acids, cell surfaces, and cell surface interactions.

Some of the earliest TERS studies on biomaterials focused on pure components, like nucleobases^99-101 and amino acids^101-104. It was shown that the normal nucleobases A, T, G, C adsorbed separately on a surface in picomole quantities could be differentiated based on their TER spectra⁹⁹. Later, they were also identified in nucleic acids with TERS, which makes TERS a novel method of label-free sequencing^{105,106,107,108}. TER spectra have been obtained from cystine and histidine monolayers, which revealed different ionization states adsorbed on a gold surface^109,110. A similar degree of chemical specificity has also been demonstrated in proteins. For example, TER spectra obtained from cytochrome c showed not only distinct spectral features of amino acid and heme but also variations due to different molecular orientation¹¹². This is not surprising, since TER spectra differs from normal Raman and SER spectra by avoiding an averaged signal and probing only a few molecules at a time.

More recently, TERS has been applied tor the study of more complex biological samples. The surface chemical compositions of large protein assemblies, such as amyloid fibrils and peptide nanotapes, have been studied. For example, the surface composition of amino acids such as cysteine, tyrosine, proline, histidine, and the composition of secondary structure elements such as the α-helix and β-sheet regions of insulin fibrils have been characterized¹⁰⁴. Further, different polymorphs of insulin fibrils have been differentiated based on such surface characterization. This is important because, polymorphs of the same fibrils have different cytotoxicity levels, which is influenced by surface properties such as hydrophobicity. TERS has the potential to probe only the surface composition and avoid signal corresponding to bulk fibril structure.

The biochemical composition of other multi-component systems, such as the surface of bacterial cells, viral cells and human cells has been studied^{114,115,116,117}. The contributions of surface lipid molecules, RNA and protein were identified in the resulting TER spectra. For example, the compositions of viral coat proteins and RNA were identified from tobacco mosaic virus¹¹⁴. More recently, cell surface interactions such as antigen-antibody interaction have been studied using TERS. The antibodies were conjugated to nanoparticles which can be located on a cell surface by conventional microscopy upon binding to surface antigen, following which those areas were specifically studied by TERS¹¹⁹. This allowed a targeted method of studying small areas of interest on large, complex, micrometer-sized cell surfaces. Membrane protein has also been detected on a human erythrocyte cell surface immersed in liquid¹²⁰. This promising method allows biomolecules to be studied in their native condition and reduces sample heating due to the laser and photobleaching, and it remains to be explored further.