**4. Conclusion and outlook**

In general, the density of assemblies is a critical parameter for SERS activity. The interparticle distances determine the coupling strength. Here, the coupling threshold of the smallest, in other words, weakest coupling partner limits the interaction strength [117]. For a nanofilm, that is, a dense layer of close-packed particles, this limitation is overcome aided by collective long-range modes. For instance, a layer of nanotriangles prepared by film-casting and solvent evaporation shows nicely arranged edge-to-edge and tip-to-tip ordered arrangements in the monolayer (**Figure 9C**) [118]. The resulting extrinsic hot spots have been shown allowance for hot electron-driven catalytic reactions such as the dimerization of 4-NTP to 4,4′-dimercaptoazobenzene (DMAB), followed by SERS spectroscopy in a dry state. Radziuk and Möhwald reported close-packed nanofilms of 30-nm sized AgNPs on silica microparticles for intracellular SERS (**Figure 9D**) [119]. The primary (of individual microparticles) and secondary hot spots (between adjacent microparticles) were utilized for chemical imaging of live fibroblasts [120]. These AgNP-decorated microparticles have been compared to AgNPs with a 5 nm silica

**Figure 9.** Ordered assemblies designed for high SERS activity. (A) Nanoparticle clusters of discrete coordination numbers. Copyright 2012 Wiley, adapted with permission [109]. (B) DNA-directed 2D core/satellite structures. Copyright 2012 Wiley, adapted with permission [110]. (C) Monolayer of close-packed nanotriangles. Copyright 2017 ACS, adapted with permission [118]. (D) AgNP-decorated silica microparticles. Copyright 2015 ACS, adapted with permission [119]. (E) Patterned supercrystal array of close-packed vertically aligned nanorods. Copyright 2012 Wiley, adapted with permission [124]. (F) Supercrystals of NPs ranging from the nano- to the microscale. Copyright 2012 ACS, adapted with permission [121]. (G) Pattern of pyramidal supercrystal architectures. Copyright 2017 ACS, adapted with permission [126]. (H) Macroscale patterns of quasi-infinite linear NP arrangements. Copyright 2014 ACS, adapted with

shell and silica NPs with a 5 nm thin Ag coating.

permission [61].

168 Plasmonics

This chapter gave an overview of plasmonics in the field of sensing. Starting with nanoparticles with specific morphological and optical properties up to the complex arrangements of 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.

**Author details**

**References**

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Christian Kuttner1,2,3\*

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

Universität Dresden, Dresden, Germany

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

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

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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 closely linked to the analytical question.

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 interfaces and nanoparticle surfaces [131].

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 development.
