**Author details**

a consequence of the strong hot-spot confined in the IPS region. Periodic 3 × 3 arrays of nanostructures show higher E-field enhancement compared to single nanostructures, and a lower E-field enhancement with respect to the dimer layout, which is in good agreement with the E-field enhancements observed in **Figure 14**. Nanostructures arranged in the form of chain show the highest E-field enhancement (a factor of 62) in comparison to the other geometries. **Figure 17** shows the SERS signal intensity at 1077 cm−1 as a function of the arrangements of nanostructures at 1 fM p-MA concentration. As in the previous situation, the highest SERS

**Figure 17.** Normalized SERS signal intensity variation at 1077 cm−1 with respect to different arrangements of the eight-

In summary, the engineering of 3D multi-branched (up to 10 branches) nanostructures for sensing of analyte molecules at ultra-low concentrations, down to 1fM, is demonstrated to be highly feasible. Numerical simulations were performed to understand the underlying physics of high electric field enhancement of the plasmonic nanostructures. The advancement of the 3D fabrication methods enables the realization of uniform, homogenous and reproducible SERS devices. Reflection and SERS measurements were carried out to evaluate the MB nanostructure performances. Within this context, we demonstrated the importance of the geometry, IPS and polarization on SERS signal enhancement using 3D five-branched nanostar dimers (with sub-10 nm IPS). The elevated 3D geometry shows the advantage of high E-field enhancement over 2D geometry due to decoupling from the underlying substrate of the strong optical-near fields localized at the metal/dielectric interface. In particular, the 3D geometry enables direct interaction of analytes with hot-spot spatial regions, which are severely affected by solid dielectric substrates in the 2D geometry case. This kind of SERS architectures is particularly important in miniaturized lab-on-chip Raman detection systems, thus allowing the exploitation of lower laser powers with no consequencie over the device sensitivity. Moreover, the low-cost recycling capability of the 3D geometry counterbalances the production cost and time defined by the lithographic process. The effect of metal layer composition on SERS signal enhancement of p-MA molecules, and recycling capabilities of

intensities were observed for chains, as a result of the higher hot-spot density.

**4. Conclusions and outlook**

branched 3D PM nanostructures at 1 fM p-MA concentration.

30 Raman Spectroscopy

Anisha Chirumamilla1,2, Manohar Chirumamilla<sup>2</sup> \*, Alexander S. Roberts3 , Andrea Cerea<sup>4</sup> , Esben Skovsen<sup>2</sup> , Francesco De Angelis<sup>4</sup> , Remo Proietti Zaccaria4,5, Peter Kjær Kristensen<sup>2</sup> , Roman Krahne<sup>4</sup> , Duncan S. Sutherland<sup>1</sup> , Sergey I. Bozhevolnyi3 , Kjeld Pedersen<sup>2</sup> and Andrea Toma<sup>4</sup> \*


5 Cixi Institute of Biomedical Engineering, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, China
