**Author details**

these into ordered and even hierarchical superstructures, there is *much diversity* of accessible nanostructures. Despite this diversity, it must be considered that the structure-property relations need to be well understood to design *functional materials* with tailored properties.

For colorimetric detection, this represents the fabrication of uniform systems with high spectral sensitivity. Changes in particle size, shape, composition, and arrangement can be used for this purpose. This gives access to many possible applications in the field of biosensing, ion/ temperature sensing, mechanosensing, to name a few. In each case, the plasmonic response of a system to a *specific internal or external stimulus* represents the central design criterion. For colorimetry, the task of plasmonics is very direct, meaning that the plasmonic response of a material is evaluated directly. For this reason, the plasmonic material/substrate is often

Building on this, SERS spectroscopy demonstrates the power of *surface-enhanced analytics*. Here, plasmonics serve to provide the enhancement as a means to an end. The central task is the detection of chemical information—often independent of the actual plasmonic material. In fact, the analysis is, in many cases, blind to the plasmonic material. This opens the door for optimizing the nanostructure for *higher signal yields*. In this overview, we have found various structural approaches to increase the sensitivity. The confined and localized enhancement in these structures gives access to unprecedented details about the chemistry of and at material

As a final thought, one needs to bear in mind that some questions persist regarding the enhancement mechanism in action. For multiparticle systems and complex nanostructures, the prediction of hot spot localization and intensity becomes increasingly challenging. Several examples have shown the surprising circumstance of highest SERS response at unexpected conditions. In retrospect, it is likely that a fair share of SERS studies in the literature might be affected by particle aggregation. For that reason, targeted investigations are necessary to explain such phenomena. Also, the accurate prediction of SERS activity is still challenging both on a singleparticle level as well as in multiparticle assemblies. In view of these points, besides increasing the sensitivity, the *sensing robustness* of the system must be a central design criterion in the

C.K. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 799393 (NANOBIOME). Honest gratitude is addressed to Andreas Fery for his fruitful suggestions, his critical think-

closely linked to the analytical question.

interfaces and nanoparticle surfaces [131].

development.

170 Plasmonics

**Acknowledgements**

ing, and his continued support.

The author declares no competing financial interest.

**Conflict of interest**

Christian Kuttner1,2,3\*

\*Address all correspondence to: ckuttner@cicbiomagune.es

1 BioNanoPlasmonics Laboratory, CIC biomaGUNE, Donostia–San Sebastián, Spain

2 Leibniz-Institut für Polymerforschung Dresden e.V., Dresden, Germany

3 Cluster of Excellence Centre for Advancing Electronics Dresden (cfaed), Technische Universität Dresden, Dresden, Germany
