Practical aspects of self-organization of nanoparticles: experimental guide and future applications

For the last two decades, research efforts of large number of scientists were focused on different aspects of nanoparticle (NP) self-assembly. The reason is that NP self-organization processes are striking in simplicity as well as in complexity of the resulting structures. The self-assembled NP superstructures also allow one to obtain unique physical/chemical properties, which are different from the ones of individual nanoparticles and/or bulk materials. Since the discovery of NP self-assembly, we see a gradual and inevitable increase in complexity of the produced NP superstructures. Some of the most recent and diverse examples of such complex systems we see in this themed issue of Journal of Materials Chemistry (Fig. 1). Initially, the dimers or trimers of NPs connected with antibodies or DNA were considered to be the state-of-the-art. After that, scientists from different universities were able to make NP chains and extended supercrystals. This stage was followed by more intricate systems, for instance, rings, sheets, ribbons, tetrameters of different shapes, and other superstructures^1–5 (Fig. 1). The discrete assemblies of NP bore stronger and stronger similarity to the complexity of molecules. In respect to extended superstructures, we also see the appearance of macroscopic gels^6,7 and larger diversity of supercrystals made from NPs of different shapes. One can also notice that the complexity of the NP superstructures often competes with the sophistication of self-organized structures known in biology, which is related to fundamental similarity of anisotropic forces acting between the NP, and, for instance globular proteins.⁸ As such self-assembled superstructures with strong resemblance to viruses have been obtained.⁹ At this point the field is transitioning to dynamic NP assemblies, and many scientists are searching for NP systems that can make superstructures with pre-determined photonic and electronic properties.

Fig. 1 Representative results from the self-assembly of nanoparticles. A: Assembly of Au nanorods containing 3-4 layers of particles.² B: Long-range-self-assembled monolayer of 10 nm Co nanoparticles (L. Peña, DOI: 10.1039/c1jm11647a). C: Self-assembled Fe₃O₄ chain (M. R. Gao, DOI: 10.1039/c1jm13517a). D: Hexagonal close-packed Au nanoparticles (S. K. Eah, DOI: 10.1039/c1jm11671a). E: Self-assembly of 6.5 nm PbSe and 5.0 nm Au nanoparticles.³ F: Mesoscopic ring of magnetic Co@CoO core-shell nanoparticles (A. Wei, DOI: 10.1039/c1jm11916h). G: Ring structures from the self-assembly of polymer-tethered Au nanorods.⁴ H: Bundles of twisted ribbons from self assembly of CdTe nanoparticle.⁵

The processes of NP self-assembly of nanoparticles could be divided into two steps. The first step is the engineering of individual NPs. At this point one needs to consider what are the interactions one needs to have between them to obtain specific one dimensional (1D), two-dimensional (2D), and three-dimensional (3D) assemblies. It should be pointed out several fundamental guidelines to NP engineering enabling efficient processes of self-organization: (1) Anisotropy of NP interactions is quite critical if one wants to obtain complex superstructures. The strength of different interactions could vary from one NP pair to another. However, the localization of weak and strong “spots” in the force field around the NPs in respect to its core or geometrical features must not change appreciable between the NPs. Anisotropy of the force field can result from geometry of NPs (rod, disk, cube, tetrahedron…) or from the “patchy” distribution of electron density. The combination of both factors is not unusual as well. (2) One of the special cases of NP self-assembly is when they are coated with proteins, DNA oligomers, or other biomolecules which exhibit strong specificity in binding to each other. This approach is very useful because of excellent knowledge base about interactions of proteins, DNA, etc, as well as the availability of well-defined biological components that can be applied in a tinker-toy fashion as connectors between the NPs. It can be stated that for a NP carrying a sufficiently long biomacromolecule one can always find some conditions, assembly partners, and templates when such NP can be assembled. (3) Monodispersity of NPs is not an overwhelmingly important parameter considering the self-organization phenomena if anisotropy is strong enough. It is beneficial but not required. Monodispersity is critical for self-assembly only when the force field is rather isotropic. (4) Excluding the case of NP conjugates with DNA and proteins, one should expect the best results of NP assembly when the thickness of the stabilizer shell is relatively thin. Short stabilizers markedly increase anisotropy of the NP interactions. At the same time, long stabilizers greatly increase steric repulsion which prevents aggregation, preserve the uniform distance, between the NP, but also removes specificity in attraction patterns between NPs. Coincidentally, the electronic interactions between the NP cores for short stabilizers are the strongest. This is beneficial for numerous prospected applications.

The second step is to carefully choose the reaction conditions to allow self-assembly to take place. The guiding principles to the selection of such conditions are the following: (1) In most cases repulsion between the NP dominate. The art of self-assembly is to reduce it to the degree when the energy of attractive forces become comparable to that of repulsion. Under such conditions the NPs have the opportunity to form agglomerates whose structure is governed by the anisotropy of forces and not by the kinetics of coagulation. Strong dominance of the attractive interactions will lead to stochastic agglomerates. (2) There are multiple reports about the NP self-assembly processes in organic solvents, and some of such data are present in this issue of Journal of Materials Chemistry. They are both exciting and fundamentally important. Nevertheless, the choice of aqueous solutions to observe self-organization processes will be wiser because there are a greater variety of forces acting in aqueous media between the NPs. This is true for the NP carrying biological ligands (e.g. connectors) and those that do not. Water-soluble NPs are also expected to give greater variety of superstructures. (3) Kinetics of NPs assembly could be slow and take a few days. This is the result of fairly low concentration of NPs in solution and the slow rotation/tumbling of NPs liquid media. (4) When changing the assembly conditions trying to find a special combination of stabilizer, particle size, temperature, pH, ionic strength, solvent, etc, which results in a most desirable self-assembled structure, one should also remember that some of these conditions can alter crystallinity, geometry, and/or chemical composition of the NPs due to Ostwald ripening or oxidation. While making the matters more complex, such ongoing chemical transformation of NPs can be of great value and can lead to even more unusual superstructures competing in sophistication with those found in biology. They can also be of practical value. For instance spontaneous recrystallization greatly improves the structure of self-assembled semiconductors for charge transfer.

In perspective, I see the self-assembly of nanoparticles as a powerful tool for bottom-up techniques in addition to the simplicity for the large area preparation and the low manufacturing cost. My prediction is that within five years we shall see multiple examples of electronic, sensor, optical, and other devices utilized self-assembled superstructures in addition to some early demonstrations that have been already reported in the literature. One of the great benefit of the self-organized systems is that they can easily traverse different scales and be integrated with microscale devices.

I would like to thank all the authors who contributed to this themed issue. Their efforts give the excellent examples about the self-assembly of nanoparticles. I also want to thank the editorial and production staff of the Journal of Materials Chemistry for their superb assistance. Finally, I hope that this themed issue will provide a useful and valuable reference and will inspire many readers to widen their field of research.

Nicholas Kotov, University of Michigan, USA.

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