Nanoparticles (NPs) are envisaged as both building blocks to form advanced materials [1][2][3] and appropriate models to gain insight into condensed-matter physics; [4][5][6][7] this is because of their unique dimensions and the innovative physical properties derived thereof. All these promising applications of the NPs are primarily dependent on the capability of controlling their spatial coupling. Self-assembly of NPs is driven by various forces, such as van der Waals interactions, which lead to either dense and close-packed clusters or extended crystals.[8] The assembled structures are a result of a thermodynamic balance between these interactions. By capping the NPs with molecular ligands to minimize the van der Waals attractions and induce dipolar interactions, it was recently possible to tune the anisotropic interactions that dominate the course of the self-assembly process, thus leading to anisotropic self-assembly of the NPs into one-dimensional (1D) chains, [9][10][11] two-dimensional (2D) freestanding films, [12] or quasi-2D fractal chain networks.[13] However, the control of the anisotropic self-assembly of NPs (for example, the chain length) has been scarcely studied, which makes the thermodynamic picture revealed in the self-assembly studies hard to extend to other self-assembled systems, such as crystallization or phase transition-this is particularly true in the case of aqueous media where the electrostatic interactions are not at all negligible.Electrostatic interactions are long-range interactions which ubiquitously exit in aqueous systems. Electrostatic repulsive and attractive interactions between charged particles are usually regarded as isotropic, thus driving the selfassembly of the particles in an isotropic crystallization fashion. Recently, Lilly et al. successfully regulated the electrostatic interactions between aqueous CdTe NPs by using dimethyl sulfoxide, thus controlling the transformation of the NPs into nanowires. [14] This result suggests an interesting but usually ignored effect of the electrostatic interactions on the anisotropic self-assembly of charged NPs in aqueous media. Here, we demonstrate the profound anisotropic character of electrostatic repulsions during the self-assembly of charged NPs in the presence of a short-range anisotropic dipolar interaction, thus endorsing the anisotropic selfassembly of charged NPs into chains. Of particular importance is the fact that tuning the electrostatic repulsion between gold NPs by using the ionic strength, and especially the polarity, of the colloidal suspensions led to a fine control of the length of the gold-NP chains, that is, the number of NPs in them.We chose negatively charged gold NPs (with a size of 14 nm and stabilized by citrate) as the model system because the transverse configuration of their plasmon resonance is highly sensitive to the electronic coupling between particles, [15] thus enabling us to monitor the particle-chain growth in situ by using spectroscopic methods. The colloidal stability of charged particles in an...

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Aurophilic interactions (AuI⋅⋅⋅AuI) are crucial in directing the supramolecular self‐assembly of many gold(I) compounds; however, this intriguing chemistry has been rarely explored for the self‐assembly of nanoscale building blocks. Herein, we report on studies on aurophilic interactions in the structure‐directed self‐assembly of ultrasmall gold nanoparticles or nanoclusters (NCs, <2 nm) using [Au25(SR)18]− (SR=thiolate ligand) as a model cluster. The self‐assembly of NCs is initiated by surface‐motif reconstruction of [Au25(SR)18]− from short SR‐[AuI‐SR]2 units to long SR‐[AuI‐SR]x (x>2) staples accompanied by structure modification of the intrinsic Au13 kernel. Such motif reconstruction increases the content of AuI species in the protecting shell of Au NCs, providing the structural basis for directed aurophilic interactions, which promote the self‐assembly of Au NCs into well‐defined nanoribbons in solution. More interestingly, the compact structure and effective aurophilic interactions in the nanoribbons significantly enhance the luminescence intensity of Au NCs with an absolute quantum yield of 6.2 % at room temperature.