Assembly of nanoscale building blocks into hierarchical superstructure by self-assembly is one of the most pursued topics in nanoparticles chemistry. The possibilities obtainable when individual components arrange themselves into an ordered structure are limitless and of great interest. Spherical colloidal clusters have been proposed to possess remarkable collective supermodes with large local field enhancements, and a spectral response extending from the near-infrared to deep into the mid-infrared region. As such, these superstructures hold great promise as a sensing platform that is capable of addressing the whole spectral domain of vibrational molecular fingerprints, while simultaneously exploiting the advantages of their plasmonic constituents , .
By exploiting these properties of the self-assembled metamaterials supercluster, we report the experimental measurement of near and mid- IR plasmonic collective modes by monitoring the Raman scattering of 4-Mercaptobenzoic acid with a confocal microscope. The strongly enhanced Raman signal allows measurement of the plasmonic mode with a lateral resolution lower than 300 nm and a vertical one of 300 nm. As the supercluster structure possesses tunable optical modes, different plasmonic responses are mapped according to the cluster size and the excitation wavelength.
Moreover, as SERS allows sensitive detection of molecules, the remarkable and tunable modes excitable inside the spherical colloidal clusters can provide an efficient platform for ultra-sensitive molecular spectroscopy. To this end proof-of-principle implementation of the superclusters as an efficient platform for pH sensing of the surrounding medium is reported .
 V. A. Turek, L. N. Elliott, A. I. I. Tyler, A. Demetriadou, J. Paget, M. P. Cecchini, A. R. Kucernak, A. A. Kornyshev, and J. B. Edel, “Self-assembly and applications of ultraconcentrated nanoparticle solutions,” ACS Nano, vol. 7, no. 10, pp. 8753–8759, 2013.
 A. Lauri, L. Velleman, X. Xiao, E. Cortes, J. B. Edel, V. Giannini, A. Rakovich, and S. A. Maier, “3D Confocal Raman Tomography to Probe Field Enhancements inside Supercluster Metamaterials,” ACS Photonics, vol. 8, no. 4, p. pp 2070-2077, 2017.
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