No abstract

Droplets emitting surface-active chemicals exhibit chemotaxis toward low-pH regions. Such droplets are self-propelled and navigate through a complex maze to seek a source of acid placed at one of the maze's exits. In doing so, the droplets find the shortest path through the maze. Chemotaxis and maze solving are due to an interplay between acid/base chemistry and surface tension effects.

Self-assembly of nanoscopic components into ordered "suprastructures" is a promising route to new types of nanomaterials with applications in catalysis, [1] optoelectronics, [2] and biological sensing, [3] to name just a few. Whereas numerous strategies (mostly, evaporation-driven [4][5][6][7][8] but also based on electrostatic interactions [9][10][11][12] or DNA base-pairing [13] ) have been developed to crystallize spherical particles into large two-dimensional lattices and three-dimensional crystals, the self-assembly of components of lower symmetries (rods, plates, etc. [14] ) remains challenging. For such particles, crystallization is often hindered by strong van der Waals (vdW) attractions, [15] which lead to indiscriminate "sticking" of the particles rather than to orientation-specific self-assembly. Several groups [4,5] have recently demonstrated the assembly of nanorods (with the help of cationic or Gemini surfactants) and of columnar structures made of platelets [6,16] (where entropic forces [15] facilitate organization at high volume fractions), but crystallization of high-aspect ratio (width: thickness) polygonal nanocrystals has not been reported. Such polygons-especially, metallic nanotriangles-and their arrays are of particular interest because of the presence of sharp edges, where electromagnetic fields are concentrated into "hot-spots" which enhance SERS (surface-enhanced Raman spectroscopy) sensing abilities.[17-21] Herein, we demonstrate the assembly of metallic nanotriangles (NTs) into ordered monolayer and multilayer structures. The remarkable feature of this method is that self-assembly is facilitated by repulsive electrostatic interactions between the triangles introduced by charged surfactants.[22] These repulsions weaken the strong vdW attractions between the flat faces of the triangles and effectively serve as a "molecular lubricant" that allows the particles to fine-tune their mutual orientations. Confocal Raman imaging shows that ordering of the metallic triangles into large arrays enhances Raman scattering and significantly increases the SERS sensitivity relative to randomly aggregated NTs. In a general context, the use of repulsive forces appears a versatile strategy for "softening" strong attractive potentials in self-assembling systems at the nanoscale.Approximately equilateral nanotriangles (NTs; side a = 158 AE 14 nm; thickness h = 8.7 AE 0.8 nm; Figure 1 a,b), were synthesized by sequential seeded growth from citrate-stabilized gold seeds (see the Experimental Section and reference [25]). The as-synthesized AuNTs were then purified by two sedimentation and redispersion steps with fresh deionized water to remove excess hexadecyltrimethylammonium bromide (CTAB) surfactant as well as isotropic side-products. The purified NTs, stabilized by CTAB, were subsequently functionalized with self-assembled monolayers (SAMs) of N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride (TMA, ProChimia, Poland), which displaces the CTAB surfactant.In a typical experiment, self-assembly was achieved...

The reversible molecular template-directed self-assembly of gold nanoparticles (AuNPs), a process which relies solely on noncovalent bonding interactions, has been demonstrated by high-resolution transmission electron microscopy (HR-TEM). By employing a well-known host-guest binding motif, the AuNPs have been systemized into discrete dimers, trimers, and tetramers. These nanoparticulate twins, triplets, and quadruplets, which can be disassembled and reassembled either chemically or electrochemically, can be coalesced into larger, permanent polygonal structures by thermal treatment using a focused HR-TEM electron beam.

Properties of self-assembled monolayers (SAMs) can be tailored by the curvature of the underlying surface. This is so because on a curved support the density of SAM headgroups is always smaller than that of the surface-attachment sites. This density difference increases with increasing curvature and is most pronounced for SAMs formed on nanoscopic particles. This Perspective describes systems in which nanoscale curvature causes pronounced changes in the pK(a) of acid-presenting SAMs or in the electrochemical potential of redox-active molecules (including supramolecular "switches") attached to nanoparticles. It is suggested that in nanoparticles having regions of different curvature these geometrical differences can translate into site-selective charging; such "patchy" particles could be used as building blocks of pH-sensitive assemblies.