Urchins-like nanoparticles

For urchins-like nanoparticles the effects of branches dimensions and of their mutual interactions on the optical response of Ag 

The first Ag multi-branches structures presented in Fig. 3.13 are 40 nm Ag spherical cores with multi hemispheroidal Ag spines protruding from it, each with a semimajor (/lo) and a semiminor (ro) axis, respectively, of 20 and 6 nm. The angular spacing between the spines is taken responding to the formula A = with c = 0, 

These structures are excited with a plane-wave propagating in the X direction and polarized along they direction. DDA tests done on the NUO structure revealed no significant modifications of the spectra for a 3D rotation of the scatterer and this in agreement with the fact that multi-tips objects like these have a major probability of favorable alignment with the incident radiation, as compared to simpler shapes such as nanorods [65]. 

Concerning the spines radius effects, the spectra of structures with a A = 7t/4 coverage (e.g., like the structure NUl, see Fig. 3.13b), a spine height of 20 nm, and a radius changing in the range 6 12 nm are reported in Fig. 3.14. 

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Bakr, 0. M., Wunsch, B.-H., and Stellacci, F. (2006) High-yieid synthesis of multi-branched urchin-like gold nanoparticles, Chem. Mater., 18, 3297-3301. 

In order to collect SERS spectra, nanoscaled structures are usually either fixed onto large surfaces or free in suspension in the form of nanoparticles. Suitable particles are available in a variety of forms simple spherical particles, rods, [13] rice-like shapes, [14] sea urchins [15], or even cages [16]. Their size can vary from 1 to 500 nm, and this factor has an important impact on their surface plasmon resonance properties. 

While amorphous precursors were already found and discussed for biomineral systems as a sophisticated way towards minerals as they avoid high salt concentrations with their associated high osmotic pressures, the other mechanisms emerged from purely synthetic systems. Only very recently, the first evidence was reported that, for example, mesocrystals can be found in biominerals like aragonite platelets in nacre [114] or cal-cite nanocrystals in sea urchin spines [116]. Much has still to be done to understand the forces that control the perfect nanoparticle alignment in mesocrystals, as well as the exact building mechanism and the possibilities for manipulating these structiues. However, the toolbox of crystalUzation is clearly extended now. 

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