self-assembly of nanoscopic structures

Modern science has many capabilities that are used to achieve many goals. Sometimes they contradict each other, but this is only at first glance. Some studies are aimed at achieving maximum control over the system, while others try to achieve the goal of the system itself with minimal human intervention. Both are necessary to simplify a process while simultaneously increasing its productivity. Scientists from the University of California at San Diego (USA) have created a system of nanoscopic elements that can independently assemble into checkerboard structures when in contact with water. What principles formed the basis for this research, what were the difficulties of implementation, and what practical application does the created system have? We will find answers to these questions in the scientists' report.

Basis of the study

Long before the T-1000 showed everyone its self-assembly, scientists around the world studied this feature of some systems with the utmost meticulousness. Self-assembly is the process by which the components of a system spontaneously organize themselves into some final structure. This process can be carried out to create large-scale architectures using colloidal nanocrystalline building blocks. These colloidal nanocrystalline systems are minimally composed of solid particles, grafted ligand particles, and a carrier solvent. They have been demonstrated to self-assemble into beautifully ordered superlattices through solvent evaporation or through chemical bonding between grafted ligands. These nanocrystals often exhibit complex nascent phase behavior due to their inherently nonrigid shape and nonadditive nanoscale interactions. As a result, the valence and coordination geometries of nanocrystals within the resulting superstructure remain difficult to control and predict even within relatively simple two-dimensional (2D) lattices.

Over the years of research, the possibility of using synthetic systems of the chemical principle inherent in biological systems with the ability to self-assemble has been achieved. The result was porous two-dimensional lattices, such as checkerboard structures. Scientists have previously demonstrated the assembly of a synthetic two-dimensional protein array using fragmented protein building blocks. They modified a C4-symmetric protein with cysteine residues to allow angle-angle binding with a valence of 4. Lateral binding of the protein resulted in the formation of two-dimensional solution-grown crystals with open square pores. Similar two-dimensional arrays have also been assembled using artificial DNA nanostructures as building blocks.

The checkerboard-type structures created in all of these self-assembly systems required precise control of the valence of the building blocks, facilitated by the following two principles: complementarity of shapes (i.e., square tiling); site-selective binding (i.e., corner-to-angle).

To implement these principles, very specific chemical interactions were required, accessible only to biomolecular systems. This is the problem, because it has not yet been possible to achieve a similar result using inorganic nanocrystals.

The scientists note that biomolecular binding interactions typically result in fragile assemblies, which impose severe constraints on solvent conditions and often require specific charge conditions to avoid denaturation (in proteins) and surface binding (in DNA). The assembly of inorganic nanocrystals requires more robust interparticle interactions that can accommodate a wider range of experimental conditions. Typical self-assembly approaches to control the valence of nanocrystals use either anisotropic particle shapes (e.g., nanocubes and nanorods) or anisotropic surface chemistry (e.g., patch colloids*).

Patchwork particles* are micron- or nano-sized colloidal particles that have an anisotropic pattern either due to modification of the chemical composition of the particle surface (“enthalpy patches”), due to the shape of the particles (“entropy patches”), or both.

However, even these highly anisotropic nanocubes (NC from

nanocube

) are subject to strong driving forces for crystallization, which results in the formation of a close-packed assembly rather than optimally interconnected nanocrystals. Therefore, the ability to generate precisely interconnected nanocrystal lattices using exclusively nonspecific chemical interactions that are widespread and compatible with any inorganic nanocrystal would be a breakthrough in the field.

In the paper we are reviewing today, scientists talk about the self-assembly of inorganic checkerboard lattices by exploiting competition between several types of nonspecific intermolecular interactions associated with polymer-grafted metal nanocrystals. Interfacial forces, entropy-driven steric forces, hydrophobic forces, and particle shape are all chemically programmed and integrated to implement nanocrystal binding, valence, and orientation (1a).

To create large-scale 2D arrays, the scientists performed self-assembly of nanocrystals at air-water interfaces, which bypasses substrate interactions or solvent restrictions associated with evaporation-induced assembly. Colloidal silver NCs are used as the main building block of nanocrystals due to their anisotropic shape, which allows them to adopt different orientations relative to the interface (e.g., face-up, edge-up, or tip-up) and mediate different interparticle bonds (e.g., face-to-face or rib-rib). The scientists modified the surface of Ag nanocubes using a mixture of two ligands: a short, predominantly hydrophobic graft that introduces attractive interactions between NCs, and a long, predominantly hydrophilic graft that introduces steric repulsion between NCs. Having two types of grafts allows for simultaneous control of particle orientation and NC connectivity, which is difficult to achieve with a single graft system.

Research results

Image #1

On 1b shows scanning electron microscopy (SEM) images of a checkerboard lattice scanning electron microscopy). The lattice data was obtained using a proprietary materials design approach that involved iterative feedback between coarse-grained molecular dynamics simulations (CG MD from coarse-grained molecular dynamics) and experiments on the assembly of nanocrystals. Ag nanocubes were modified using ligand feedstocks consisting of a mixture of thiolated polyethylene glycol (PEG-SH) chains and 1-hexadecanethiol (C₁₆-SH). The resulting mesoscopic assemblies (in this case deposited on a solid substrate) exhibit edge-to-edge NC junctions with a valency of 4 and a nearly ideal internal bond angle of 90°. On 1c And 1d shows the corresponding structure predicted by CG MD simulations, which further confirms that the nanocubes are oriented face-up at the air-water interface. On 1e shows the 2D Fourier transform of an SEM image of a checkerboard lattice, from which local four-fold NC symmetry was extracted using angular cross-correlation analysis (ACC from angular cross correlation) (1f).

Image #2

The scientists note that the checkerboard lattice represents only a small part of the rich structural phase space that can be accessed using the developed nanocube assembly system. On 2a shows the CG model used to construct the phase diagram of the mesostructural assembly, where the cores of Ag nanocubes were modeled as rigid lattices, hydrophobic and hydrophilic ligands as flexible chains of beads of length l_Ho = 3σ and l_Hi = 6σ, and the solvent molecules are like single beads of size σ, setting the length scale of the system.

Appropriate bead-to-bead potentials were implemented to account for van der Waals attractions between nanocube cores, attractive interactions between hydrophobic ligands, steric repulsion between hydrophilic ligands, and surface tension of the air–water interface.

On 2b the phase diagram is presented depending on two parameters of the system: the total grafting density of ligands Γ_g (in units of CG chains of beads per σ²) and the percentage of hydrophobic ligands on the surface of nanocubes. For each condition, MD simulations were performed using the CG model for 16 grafted nanocubes, and the resulting structures were qualitatively classified as dispersed, one-dimensional, or two-dimensional. They then varied in nanocube connectivity and interface orientation to yield a total of six different phases (i.e., 2D face-to-face, checkerboard, 1D face-to-face, 2D edge-to-edge, 1D edge-to-edge, and scattered).

The checkerboard lattice is formed only if a dense ligand corona with a high hydrophobicity content is grafted onto the nanocube. These conditions result in a face-up orientation of isolated nanocubes (2c), which is a key condition for the formation of a chessboard. The model shows that these upward-facing nanocubes are ≈80% immersed in the aqueous subphase (inset in 2c), which is confirmed by optical spectroscopy data.

Even though the two ligands are uniformly distributed on the nanocube surface, the longer hydrophilic ligands are pulled away from the nanocube surface to minimize their interaction with the shorter hydrophobic ligands and maximize their interaction with water. The stretching of hydrophilic ligands, together with their low grafting density, exposes the edges of the nanocube, which are mostly hydrophobic (inset in 2c). On 2d shows the orientational free energy landscape calculated for one nanocube as a function of the Euler angles (θ,φ). This confirms that the face-up orientation is indeed the most stable compared to the edge-up and top-up orientations.

Analysis of ligand density distribution (insets in 2d) shows that when considering both the cross-sectional area of the nanocube core and the stretching of hydrophilic ligands in the interfacial plane, the face-up orientation maximizes the closed interfacial area. The attraction between nanocubes through their exposed hydrophobic edges, combined with steric repulsion from hydrophilic ligands (which prevents face-to-face contact), is what promotes the formation of edge-to-edge contacts of ≈ 90° and a nanocube binding valence of 4. Free calculations energy for two approaching nanocubes (2e), along with the decomposition of the total free energy into contributions from hydrophobic and hydrophilic ligands, show that edge-to-edge contacts, where steric repulsion from hydrophilic ligands is weak, are favored. For face-to-face contacts, steric repulsion suppresses the attractive contribution of hydrophobic ligands. Assembly into a checkerboard lattice requires a delicate balance of these two interactions: moving the phase diagram to lower hydrophobic content reduces the attraction between nanocubes, resulting in dispersed or one-dimensional morphology; transition of the phase diagram to a higher hydrophobicity content and/or lower Γ_g results in face-to-face joints and more compact morphologies.

During practical testing of the system, polymer-combined Ag nanocubes were used with different nanocube core sizes and ligand lengths, chemical compositions, and association densities.

The scientists synthesized colloidal Ag nanocubes (≈80 nm length) and performed ligand exchange reactions to replace the capping agents of the nanocubes with a mixture of hydrophilic (PEG) and hydrophobic (alkyl chains or polystyrene) ligands. This made it possible to establish in practice the conditions under which the system will form a chess-type structure.

Total graft density measured by inductively coupled plasma mass spectrometry (ICP-MS from inductively coupled plasma mass spectrometry), ranges from 0.826 to 1.636 ligands/nm², which is consistent with the values expected for nanocubes functionalized with these two different ligands. The hydrodynamic radii of various PEG-grafted nanocubes measured by dynamic light scattering show that the grafted nanocubes have PEG-core aspect ratios in the range of 0.255–0.375, which closely matches the aspect ratios in the CG MD simulations. Assembly was accomplished by distributing functionalized nanocubes at the air-water interface and allowing the nanocubes to self-assemble over a period of hours to several days.

The resulting structures were transferred to a silicon substrate and analyzed using SEM (scanning electron microscope). ACC analysis was performed to identify specific mesophases with q values corresponding to 1/(2a) edge-to-edge or 1/(3a) face-to-edge connections and n values corresponding to 1D (n = 2) and 2D (n = 4) symmetry.

Image #3

On 3a–3e shows SEM images of assemblies obtained for nanocubes functionalized with PEG20K (50 μM) at a fixed concentration and increasing concentration of C₁₆ in ligand raw materials, which corresponds to the following mesophases:

dispersed (0 µM, Γ_g = 0.5 nm^-2);

edge-fin 1D (3 µm, Γ_g = 1.6 nm^-2);

checkerboard (6 µm, Γ_g = 1.4 nm^-2);

face-to-face 2D (9 µm, Γ_g = 1.1 nm^-2);

face-to-face 2D (15 µM, Γ_g = 0.8 nm^-2).

–

a histogram of the Fourier coefficients obtained for each nanocube assembly is presented. Assembly experiments using nanocubes of different sizes (60–100 nm) and different chemical compositions of hydrophobic ligands (1-octadecanethiol, polystyrenethiol, and 2-naphthalenethiol) show that the nanocubes accept similar mesophases, all else being equal. However, checkerboard-shaped nanocubes are observed only for part of the samples obtained at the same molar ratios of raw materials as the nanocubes on

–

, with 1D edge-to-edge nanocubes observed as the predominant mesophase. This confirms the narrow window of experimental parameters for staggered lattice formation, consistent with the phase diagram in

Image #4

SEM images at low magnification show that the formation of the staggered lattice is not limited to a small area of the sample, but rather represents a major assembly product (4a). However, it was observed that many of the experiments resulted in the formation of 1D edge-to-edge and 1D edge-to-edge mesophases in coexistence with checkerboard structures. This is also consistent with the phase diagram, which shows a transition zone where multiple assembly morphologies coexist. Most importantly, this coexistence is noticeable for nanocubes grafted with a ratio of hydrophobic to hydrophilic ligands of 7:3 and a normalized grafting density of ≈ 0.75 (4b). This confirms the sensitive nature of the staggered lattice formation.

The formation of a checkerboard lattice is also hampered by two types of assembly defects, shown in 4c: vacancies (when there is no nanocube in the lattice) and triangles (when three nanocubes are assembled through edge-edge bonds). CG MD simulations show that checkerboard assembly occurs through a cluster-cluster aggregation mechanism and that vacancies occur when clusters that are not complementary in shape are assembled.

On 4d–4m presents a catalog of clusters (<4 nanocubes) observed both in experiments and in CG MD simulations. Calculations confirm that the chessboard cluster (4j) exhibits the lowest free energy (−108 ± 11 kBT) among similarly sized clusters that represent metastable states.

With the exception of triangular clusters (4f, 4g), all clusters are potential structures for the formation of a checkerboard lattice, which can form if the clusters are connected to each other by complementary forms or if vacant spaces appear in the clusters filled with freely diffusing individual nanocubes.

Triangular defects have a similarly large but slightly less favorable free energy (−97 ± 7 kBT) than a checkerboard cluster. Thus, they cannot be restored during the assembly process. After formation, they disrupt the structural arrangement of nearby clusters (4c). Closer examination of this defect reveals that three nanocubes are connected through an “edge-edge” that has a compressed internal angle and a wrinkled orientation at the air-water interface (4n). Although this requires the nanocubes to be rotated slightly away from their most preferred face-up orientation (2d), this rotation mitigates some of the steric repulsion between the connected nanocubes.

The analysis showed that increasing the length of the 6σ hydrophilic graft to 8σ resulted in all nanocubes now participating in the formation of a checkerboard lattice without any triangular defects. On 4o–4t shows snapshots of the assembly process of 16 nanocubes grafted with these longer hydrophilic ligands. The nanocubes assemble into small clusters of various shapes, which eventually grow and merge into a checkerboard lattice. At l_Hi = 9σ the large steric repulsion between the nanocubes becomes too great, causing them to assemble into a 1D edge-to-edge mesophase. This is defect suppression by adjusting l_Hi supported by assembly experiments using nanocubes with PEG grafts of different molecular weights.

For a more detailed understanding of the nuances of the study, I recommend taking a look at scientists' report And Additional materials to him.

Epilogue

In the work we reviewed today, scientists presented a new method for achieving self-assembly of nanoscopic elements into “chessboard”-type structures.

The elements that participated in self-assembly consisted of a silver crystal with a mixture of hydrophobic and hydrophilic molecules attached to the surface. When a suspension of these nanocubes hits the surface of the water, they are positioned so that their angular edges touch. As a result, a structure of nanocubes and vacant spaces, reminiscent of a chessboard, is formed on the surface of the water.

The self-assembly process is not controlled by humans, but is the result of a certain surface chemistry of the nanocubes. The high density of hydrophobic molecules on the surface brings the cubes closer together, minimizing their interaction with water. Meanwhile, long chains of hydrophilic molecules cause enough repulsion to create voids between the cubes.

Scientists say such structures could have many interesting practical applications. For example, the structure of a nanocube chessboard interacts with light in an interesting way. As the scientists explain, the spaces between the cubes, especially near the corner edges where the cubes meet, can act as tiny hot spots that focus or trap light. This could be useful for creating new types of optical elements, such as nanofilters or waveguides. But this remains to be tested in practice, which is what scientists intend to do in future studies.

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