The ability to assemble nanoscopic components into larger structures and materials depends crucially on the ability to understand in quantitative detail and subsequently "engineer" the interparticle interactions. This Review provides a critical examination of the various interparticle forces (van der Waals, electrostatic, magnetic, molecular, and entropic) that can be used in nanoscale self-assembly. For each type of interaction, the magnitude and the length scale are discussed, as well as the scaling with particle size and interparticle distance. In all cases, the discussion emphasizes characteristics unique to the nanoscale. These theoretical considerations are accompanied by examples of recent experimental systems, in which specific interaction types were used to drive nanoscopic self-assembly. Overall, this Review aims to provide a comprehensive yet easily accessible resource of nanoscale-specific interparticle forces that can be implemented in models or simulations of self-assembly processes at this scale.

Self-assembly of charged, equally sized metal nanoparticles of two types (gold and silver) leads to the formation of large, sphalerite (diamond-like) crystals, in which each nanoparticle has four oppositely charged neighbors. Formation of these non-close-packed structures is a consequence of electrostatic effects specific to the nanoscale, where the thickness of the screening layer is commensurate with the dimensions of the assembling objects. Because of electrostatic stabilization of larger crystallizing particles by smaller ones, better-quality crystals can be obtained from more polydisperse nanoparticle solutions.C rystalline aggregates composed of one or more types of metallic and/or semiconductor nanoparticles (NPs) are of great interest for the development of new materials with potential applications in areas such as optoelectronics (1), high-density data storage (2), catalysis (3), and biological sensing (4). To date, methods for the crystallization of two-dimensional (2D) and 3D NP superlattices have relied on the differences in the sizes of component particles and on attractive van der Waals or hard-sphere interactions between them. This strategy has been successful in preparing several types of lattices Esuch as AB (5), AB 2 (6), AB 5 (7), and AB 13 (6)^, but the all-attractive nature of the interparticle potentials limits its applicability to relatively few and usually (8) close-packed structures.To overcome this limitation, we and others (8, 9) have focused on systems of NPs interacting via electrostatic forces; such forces provide a basis for ionic, colloidal (9), or even macroscopic (10) crystals, but, despite promising attempts (8, 11), have not been successfully exploited for controllable or predictable long-range organization of matter at the nanoscale. Here, we report electrostatic self-assembly (10) (ESA) of oppositely charged, nearly equally sized metallic NPs of different types into large 3D crystals characterized by sphalerite (diamond-like) (12) internal packing, and of overall morphologies identical to those of macroscopic diamond or sphalerite crystals (Figs. 1 to 4). Formation of these nonclose-packed structures results from the change in electrostatic interactions in the nanoscopic regime, where the thickness of the screening layer becomes commensurate with the dimensions of the assembling particles, and is facilitated by the presence of smaller, charged NPs in the crystallizing solutions that stabilize larger NPs by what can be termed a nanoscopic counterpart of Debye screening.We used Ag and Au NPs coated with w-functionalized alkane thiols (13): HS(CH 2 ) 10 COOH (MUA) and HS(CH 2 ) 11 NMe 3 þ Cl j (TMA) (Fig. 1A). These NPs were prepared according to a modified procedure (14) Esee Supporting Online Material (15)^and had average diameters of 5.1 nm (with dispersity s 0 20%) for Au and 4.8 nm (s 0 30%) for Ag (Fig. 1B). We chose this pair as a model system, because the average sizes of Au NPs passivated with MUA Eself-assembled monolayer (SAM) thickness 0 1.63 nm (16)^and Ag NPs cov...

Nanoparticles (NPs) decorated with ligands combining photoswitchable dipoles and covalent cross-linkers can be assembled by light into organized, three-dimensional suprastructures of various types and sizes. NPs covered with only few photoactive ligands form metastable crystals that can be assembled and disassembled ''on demand'' by using light of different wavelengths. For higher surface concentrations, self-assembly is irreversible, and the NPs organize into permanently cross-linked structures including robust supracrystals and plastic spherical aggregates.azobenzene ͉ colloids ͉ crystallization ͉ dynamic ͉ photoswitchable S elf-assembly (1) induced and controlled by light ‡ is of continuing interest as a promising route to new types of structures and materials (2-4) with potential applications in optics (5), sensing (6), and delivery systems (7). Although considerable progress has been achieved in implementing light-induced self-assembly (LISA) at both colloidal (8, 9) and (macro)molecular scales (10, 11), the underlying phenomena and/or experimental methods have not proven effective at the nanoscale. For example, nanoscopic components of dimensions significantly smaller than the wavelength of light cannot be efficiently addressed and assembled by using optical confinement techniques [e.g., laser interference (8) and optical trapping (9)], on which virtually all colloidal LISA systems are based. At the same time, LISA based on light-induced interactions between nanoscale components coated with photoswitchable molecules (12, 13) has invariably led to disordered precipitates rather than crystalline assemblies. Here, we describe a system that circumvents these limitations, and in which photoisomerization of dithiol molecules bound onto the surfaces of metal nanoparticles (NPs) mediates their LISA into ordered, three-dimensional suprastructures: light-reversible or irreversible crystals (Figs. 1A and 2) and supraspheres of various sizes (Figs. 1 A and 3). The degree of structural reversibility depends on the strength of light-induced, dipole-dipole interactions between the NPs and on the extent of covalent binding between them. Remarkably, for low surface concentrations of dithiol ligands, the assemblies are fully reversible and can be toggled between crystalline and disordered states multiple times by using light of different wavelengths. For higher concentrations, the ligands can permanently cross-link the assemblies, making them either mechanically/thermally robust (crystals) or flexible (spherical aggregates, ''supraspheres''). Results and DiscussionInterparticle Interactions. Our experiments were based on gold nanoparticles (5.6 nm in diameter) prepared according to a modified literature procedure (14) (also see Materials and Methods). The AuNP solutions in toluene/methanol (0-30% v/v methanol content) were stabilized by dodecylamine (DDA) capping agent and didodecyldimethylammonium bromide (DDAB) surfactant. To such solutions, different amounts of photoactive trans-azobenzene dithiol ligands [4,4Ј-bis(11-me...

Fans of the "Mission Impossible" movies might recall the selfdestructing messages used to brief the secret agent on the details of his new mission. Even beyond the realm of fictitious espionage, materials that store textual or graphical information for a prescribed period of time are desirable for applications in secure communications. [1, 2] Furthermore, if such materials are rewritable, they can help to limit the use of traditional paper, thereby reducing the costs, both industrial and environmental, [3] associated with paper production and recycling. To date, most research on self-erasing media has relied on the use of photochromic molecules [4][5][6][7] -that is, molecules that isomerize and change color when exposed to light of appropriate wavelength-embedded in or attached to a polymeric or gel matrix. In one widely publicized example, Xerox Corporation recently announced [8] the development of photochromic paper that self-erases in 16 to 24 h. While writing with light can be both rapid [9] and accurate, [5,7] photochromic "inks" are not necessarily optimal for transforming light-intensity patterns into color variations, because they have relatively low extinction coefficients, [10] are prone to photobleaching, [11] and usually offer only two colors corresponding to the two states of photoisomerizing molecules. [10] Herein, we describe a conceptually different self-erasing material in which both the "writing" and self-erasure of color images are controlled by the dynamic non-equilibrium aggregation [12] of photoresponsive metal (here, gold and silver) nanoparticles (Au and AgNPs "inks") embedded in thin, flexible organogel films. When exposed to UV light, the trans-azobenzene groups coating the NPs isomerize to cisazobenzene with a large dipole moment. [13] As a result, the NPs aggregate into supraspherical (SS) assemblies, [13][14][15][16] whose apparent color depends on the duration of UV irradiation (Figures 1 and 2). Since the SS are metastable and fall apart spontaneously in the absence of UV irradiation, the two-color and multicolor images written into the films gradually self-erase (Figures 2 and 3). The erasure times can be controlled by the number of dipoles induced on the nanoparticles and can also be accelerated by exposure to visible light or by heating the material. Multiple images can be written into the same film either concurrently or after erasure.In a wider context, the present system demonstrates the flexibility and promise of non-equilibrium nanostructures to create "smart" materials capable of changing their properties or function on demand in response to external stimuli.Our experiments were based on AuNP (5.6 AE 0.6 nm diameter) or AgNP (5.3 AE 0.3 nm diameter) inks coated with mixed self-assembled monolayers (mSAMs) of dodecylamine (DDA) and photoswitchable azobenzene-terminated thiol (4- Figure 1. Reversible aggregation of photoactive nanoparticles. a) Structural formula of trans-4-(11-mercaptoundecanoxy)azobenzene (trans-MUA). b) UV irradiation of nanoparticles (here, gold) cove...

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