We describe a robust method to assemble nanoparticles into highly ordered superlattices by inducing aqueous phase separation of neutral capping polymers. Here we demonstrate the approach with thiolated polyethylene-glycol-functionalized gold nanoparticles (PEG-AuNPs) in the presence of salts (for example, KCO) in solutions that spontaneously migrate to the liquid-vapor interface to form a Gibbs monolayer. We show that by increasing salt concentration, PEG-AuNP monolayers transform from two-dimensional (2D) gas-like to liquid-like phase and eventually, beyond a threshold concentration, to a highly ordered hexagonal structure, as characterized by surface sensitive synchrotron X-ray reflectivity and grazing incidence X-ray diffraction. Furthermore, the method allows control of the inplane packing in the crystalline phase by varying the KCO and PEG-AuNPs concentrations and the length of PEG. Using polymer-brush theory, we argue that the assembly and crystallization is driven by the need to reduce surface tension between PEG and the salt solution. Our approach of taking advantage of the phase separation of PEG in salt solutions is general (i.e., can be used with any nanoparticles) leads to high-quality macroscopic and tunable crystals. Finally, we discuss how the method can also be applied to the design of orderly 3D structures.

Taking advantage of the aqueous biphasic behavior of polyethylene glycol (PEG)/salts, recent experiments have demonstrated self-assembly and crystallization of PEG-grafted gold nanoparticles (PEG-AuNPs) into tunable two-dimensional (2D) supercrystals by adjusting salt concentration (for instance, KCO). In those studies, combined experimental evidence and theoretical analysis have pointed out the possibility that similar strategies can lead to three-dimensional (3D) formation of ordered nanoparticle precipitates. Indeed, a detailed small-angle X-ray scattering (SAXS) study reported herein reveals the spontaneous formation of PEG-AuNPs assemblies in high-concentration salt solutions that exhibit short-range 3D order compatible with fcc symmetry. We argue that the assembly into fcc crystals is driven by partnering nearest-neighbors to minimize an effective surface-tension gradient at the boundary between the polymer shell and the high-salt media. We report SAXS and other results on PEG-AuNPs of various Au core diameters in the range of 10 to 50 nm and analyze them in the framework of brush-polymer theory revealing a systematic prediction of the nearest-neighbor distance in the 3D assemblies.

Ion-specific effects on the assembly and crystallization of polyethylene-glycol-grafted Au nanoparticles (PEG-AuNPs) at the vapor–liquid interface are examined by surface sensitive synchrotron X-ray scattering methods. We show that monovalent salts, such as KCl and NaCl, that do not advance phase separation of pure PEG at room temperature induce two-dimensional (2D) self-assembly and crystallization of PEG-AuNPs with some distinctions. Whereas for KCl the 2D hexagonal coherence length of the PEG-AuNP superlattices is remarkably large compared to other salts (over micron-sized crystalline grains), NaCl induces coexistence of two hexagonal structures. Using various salts, we find that the value of the lattice constant is correlated to the ionic hydration entropy consistent with the Hofmeister series.

Nanoparticle-based clusters permit the harvesting of collective and emergent properties, with applications ranging from optics and sensing to information processing and catalysis. However, existing approaches to create such architectures are typically system-specific, which limits designability and fabrication. Our work addresses this challenge by demonstrating that cluster architectures can be rationally formed using components with programmable valence. We realize cluster assemblies by employing a three-dimensional (3D) DNA meshframe with high spatial symmetry as a site-programmable scaffold, which can be prescribed with desired valence modes and affinity types. Thus, this meshframe serves as a versatile platform for coordination of nanoparticles into desired cluster architectures. Using the same underlying frame, we show the realization of a variety of preprogrammed designed valence modes, which allows for assembling 3D clusters with complex architectures. The structures of assembled 3D clusters are verified by electron microcopy imaging, cryo-EM tomography and in-situ X-ray scattering methods. We also find a close agreement between structural and optical properties of designed chiral architectures.

them into hierarchal functional structures remains a challenge. [1][2][3][4][5][6][7][8][9] Naturally, the primary route to overcome this challenge has been to explore conditions that allow controlled self-assembly either by manipulating the medium in which the NPs are embedded in and/or by functionalizing them with "smart programmable molecules" (complementary single-stranded DNA, for instance). [1][2][3][4][5][6][7][8][9] Specifi cally-organized 2D and 3D NPs have been highly desirable to theoretical engineers who conceive metamaterials with novel photonic, electronic, and magnetic properties where the NPs play similar roles to those of atoms in functional materials such as, insulators, semiconductors, or metals. [ 8,[10][11][12][13][14][15][16][17][18] Major advances have been made in the last decade in laying out engineering rules for crystallization of 3D [ 1,2,4,6,[19][20][21][22][23][24][25] and 2D [ 5,[26][27][28][29][30] superlattices; however, the stability, scale of production, and specifi c organization are hurdles that need to be overcome to render these assemblies technological viability. These shortcomings can in part be affected by refocusing the exploration to fundamental understanding of molecular length scale mechanisms involved in self-assembly. The 2D self-assembly into long-or short-range order to a certain extent lessens the complexity inherent in 3D systems providing a suitable playground to unraveling the underlying interactions that can in turn be employed to the assembly process in 1D, 2D and 3D structures. Indeed, it has been demonstrated that singlestranded DNA-functionalized AuNPs (ssDNA-AuNPs) can form so-called Gibbs layers by controlling salt concentrations and even spontaneously crystallize as 2D hexagonal structures at the vapor/solution surfaces. [ 27,28 ] However, the mechanism by which these DNA-complexed AuNPs (or any other DNAcomplexed NPs) migrate to the vapor/aqueous interface, or the forces that lead to crystallization have not been fully addressed yet. We have undertaken this synchrotron X-ray study to answer these questions and to determine the interactions that lead to the spontaneous accumulation and crystallization of ssDNA-AuNPs by manipulating salt concentrations. More details on the preparation of the materials, their characterization, and the methods we use are provided in the Experimental Section below and in the Supporting Information online. Whereas Campolongo and co-workers use parallel small-angle X-ray scattering (parSAXS) from a drop-vapor interface [ 27,28 ] (1.5 µL droplet) Surface sensitive synchrotron X-ray scattering and spectroscopy are used to monitor and characterize the spontaneous formation of 2D Gibbs monolayers of thiolated single-stranded DNA-functionalized gold nanoparticles (ssDNA-AuNPs) at the vapor-solution interface by manipulating salt concentrations. Grazing incidence small-angle X-ray scattering and X-ray refl ectivity show that the noncomplementary ssDNA-AuNPs dispersed in aqueous solution spontaneously accumulate at the vapor-liquid...

Controlled self-assembly of nanoparticles into ordered structures is a major step in fabricating nanotechnology based devices. Here, we report on the self-assembly of high quality superlattices of nanoparticles in aqueous suspensions induced via interpolymer complexation. Using small angle X-ray scattering, we demonstrate that the NPs crystallize into superlattices of FCC symmetry, initially driven by hydrogen bonding and subsequently by van der Waals forces between the complexed coronas of hydrogenbonded polymers. We show that the lattice constant and crystal quality can be tuned by polymer concentration, suspension pH and the length of polymer chains. Interpolymer complexation to assemble nanoparticles is scalable, inexpensive, versatile and general.

Magnetotactic bacteria that produce magnetic nanocrystals of uniform size and well-defined morphologies have inspired the use of biomineralization protein Mms6 to promote formation of uniform magnetic nanocrystals in vitro. Small angle X-ray scattering (SAXS) studies in physiological solutions reveal that Mms6 forms compact globular three-dimensional (3D) micelles (approximately 10 nm in diameter) that are, to a large extent, independent of concentration. In the presence of iron ions in the solutions, the general micellar morphology is preserved, however, with associations among micelles that are induced by iron ions. Compared with Mms6, the m2Mms6 mutant (with the sequence of hydroxyl/carboxyl containing residues in the Cterminal domain shuffled) exhibits subtle morphological changes in the presence of iron ions in solutions. The analysis of the SAXS data is consistent with a hierarchical core-corona micellar structure similar to that found in amphiphilic polymers. The addition of ferric and ferrous iron ions to the protein solution induces morphological changes in the micellar structure by transforming the 3D micelles into objects of reduced dimensionality of 2, with fractal-like characteristics (including Gaussian-chain-like) or, alternatively, plateletlike structures. Small angle X-ray scattering (SAXS) studies in physiological solutions reveal that Mms6 forms compact globular threedimensional (3D) micelles (approximately 10 nm in diameter) that are, to a large extent, independent of concentration. In the presence of iron ions in the solutions, the general micellar morphology is preserved, however, with associations among micelles that are induced by iron ions. Compared with Mms6, the m2Mms6 mutant (with the sequence of hydroxyl/carboxyl containing residues in the C-terminal domain shuffled) exhibits subtle morphological changes in the presence of iron ions in solutions. The analysis of the SAXS data is consistent with a hierarchical core−corona micellar structure similar to that found in amphiphilic polymers. The addition of ferric and ferrous iron ions to the protein solution induces morphological changes in the micellar structure by transforming the 3D micelles into objects of reduced dimensionality of 2, with fractal-like characteristics (including Gaussian-chain-like) or, alternatively, platelet-like structures.

We have created two-dimensional (2D) binary superlattices by cocrystallizing gold nanoparticles (AuNPs) of two distinct sizes into √3 × √3 and 2 × 2 complex binary superlattices, derived from the hexagonal structures of the single components. The building blocks of these binary systems are AuNPs that are functionalized with different chain lengths of poly(ethylene glycol) (PEG). The assembly of these functionalized NPs at the air−water interface is driven by the presence of salt, causing PEG-AuNPs to migrate to the aqueous surface and assemble into a crystalline lattice. We have used liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) to examine the assembly and crystallization at the liquid interface.