developing area of nanochemistry. A particularly interesting topic in this fi eld is the synthesis and properties of NPs with concave nanoscale geometries. These NPs have distinct "spiky" shapes, with apexes that give rise to multiple optical, chemical, biological effects. NPs typically have Platonic geometries, which include tetrahedrons, cubes, hexahedrons, octahedrons, dodecahedrons, and icosahedrons, representing fi ve regular tessellations of the sphere. Some examples of Archimedean and Johnson solids can also be found among NPs. [ 1-3 ] From a geometrical perspective, all shapes mentioned above are convex. Concave NPs are much less common and, while known, [ 4-9 ] their formation mechanism is still enigmatic. A large surface area, plasmonic hot spots, strong near-infrared absorbance, and special interactions with cells and cell components [ 10-27 ] differentiate concave spiky particles from NPs with spherical and/or other convex shapes. All of these properties have high practical relevance and, apart from the fundamental interest, make it possible to substantially advance NP applications in catalysis, photonics, electronics, plasmonics, sensing, biological labeling, imaging, and cancer therapy. In general, the spiky particles are thermodynamically unstable and can potentially transform into spherical particles with time. [ 28 ] The synthesis conditions for them were identifi ed serendipitously, which does negate their importance or uniqueness but rather underlines the need to understand the underlying reasons for such growth pattern seeming deviating from thermodynamic control elaborated for "smooth" particles with Platonic shapes. While many papers do not describe the mechanism of formation of the spikes and focus primarily on the unusual morphology and optical properties of corresponding dispersions, [ 17,29,30 ] the formation of the spiked or bristled particles is currently attributed to different tendency of the stabilizers exemplifi ed by thioglycolic acid, [ 31 ] poly(vinylpyrrolidone) (PVP), [ 6 ] or cetyltrimethylammonium bromide (CTAB), [ 7 ] to absorb on diverse crystal faces. [ 6,8 ] The pivotal role of surface defects on NP seeds increasing in numbers under rapid growth conditions was also suggested as potential reason for the growth of spikes. [ 7 ] These explanations involving the thermodynamically controlled variability of stabilizer density and kinetically controlled density of defects as well as the combination thereof, however,
