Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.

Supported lipid structures and human cells (human dermal derived keratinocyte, HaCaT) were investigated using tip-enhancedRaman spectroscopy (TERS) to use the high spatial resolution capabilities of TERS, which is assumed to be less than 10 nm, to determine specific components on the cell surface. As lipids are a main component of cellular membranes, the correlation of spectral properties of pure lipids with respect to the complex biological sample was investigated. Induced by dynamic structural changes as well as nanoscale effects, a particular spectral feature of the lipid TERS spectra is found to vary, and a similar spectral deviation appears among the TERS spectra measured on the cell. Modifications of the cell surface alone cannot cause such behaviour. In contrast to soft lipid agglomerates, the cells were fixed and therefore hampered for intrinsic structural changes. Hence, the main contribution for the cell TERS spectra variation results from nanoscale effects, determined by different spectral characteristics compared to conventional Raman spectroscopy. The present results demonstrate the capability of TERS to provide a detailed and fast insight into the composition of the cell surface, even allowing the detection of single components.

TERS (tip-enhanced Raman scattering) provides exceptional spatial resolution without any need for labelling and has become a versatile tool for biochemical analysis. Two examples will be highlighted here. On the one hand, TERS measurements on a single mitochondrion are discussed, monitoring the oxidation state of the central iron ion of cytochrome c, leading towards a single protein characterization scheme in a natural environment. On the other hand, a novel approach of single molecule analysis is discussed, again based on TERS experiments on DNA and RNA, further highlighting the resolution capabilities of this method.