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Part 1
BUILDING UNITS
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Chapter 1
Self-Assembly Systems
My ten-year-old son loves building action figures using LEGO bricks (LEGO, please see References). He has many LEGO products, “which I bought for him, of course.” He first built the action figures that he was supposed to build by following the instructions. Once he built enough number of them in many different forms, he then began to build his own action figures by using the parts from the different boxes. Whenever I am watching him building new forms of action figures, in many cases with new functions, I am amazed by how an “unbiased” child's mind can do such a creative and fun thing. I love watching him doing that, and, of course, enjoy the new action figures so much. What is also amazing is the flexibility of those tiny parts. They are small and simple but at the same time so elegantly and functionally designed. It seems to me that their core structures are composed of just a couple of different basic segments. These basic segments are simple yet diverse, and easy to assemble. One segment from one part perfectly fits with the complementary segments from all other parts even from other types of action figures. By following this simple rule, my son keeps building his own action figures with a high variety and different size scales.
My approach to self-assembly begins with the segmental analysis of self-assembly building units. (The term building block is used roughly ten times more than the term building unit in the literature. But the term building block may bring an unintended implication that it is limited to sizable materials rather than encompassing a wide range of different entities. Thus, the term building unit will be used in this book with the intention that it includes any type of entity that can be assembled into any type of self-assembled system.) It does not totally come from my son's LEGO playing, but it has definitely helped me build up this concept. It is not about making self-assembly analysis more complicated. There is a very simple way to address self-assembly issues that are seemingly widely dispersed. And we can benefit from it, not just in nanotechnology but in other areas of modern technology as well. It may not look like a conventional scientific approach toward natural phenomena. But it is indeed possible to understand self-assembly with a very simple rule.
1.1 Self-Assembly
Figure 1.1 presents a schematic explanation of the self-assembly process based on the concept of force balance. A full description of this concept has been discussed elsewhere (Lee, 2008). For almost all of the self-assembly processes, major interactions between their building units, regardless of the types and sizes, occur through relatively weak intermolecular or colloidal forces. These include hydrogen bond, van der Waals interaction, hydrophobic force, π-π interaction, steric interaction, depletion force, solvation/hydration forces, and so forth. Strong bonds such as covalent bond, coordination bond, or ionic bond are rarely involved with self-assembly processes. These weak intermolecular or colloidal forces can be classified into three distinctive groups whose delicate balance determines the process and outcome of the self-assembly. They are attractive driving force, repulsive opposition force, and directional/functional forces. The attractive driving force acts to bring self-assembly building units together, thus initiating the self-assembly process. Once this attractive process takes place, the repulsive opposition force, which is originated by another segment within the self-assembly building unit, acts to balance the attractive process, which places the building units at a certain critical state. Self-assembly is established at this critical point and self-assembled aggregates begin to appear at this point as well. The third group, directional/functional forces, are the forces that can guide this balancing process between the attractive and repulsive forces. Depending on the nature of the self-assembly system, the directional/functional forces can act as either an attractive force or a repulsive force. In most cases, it is the directional/functional forces that give the self-assembly system (or self-assembled aggregate) unique structural functionalities.
Self-assembly occurs through the delicate balance between at least any two groups of the forces. For example, it can be between the attractive force and repulsive force, between the attractive force and the directional force that has the capability of the repulsive force, between the repulsive force and the directional forces that have the capability of the attractive force, or between all three groups. But, to become a self-assembly, it always has to fulfill both the “self” aspect and the “assembly” aspect, and at the same time should have the actual outcomes, that is, “self-assembled aggregates.” Therefore, there always has to be the force that gives the “self” aspect to the self-assembly building units and the balance that can ensure both the structural integrity and dynamic flexibility of the self-assembled aggregates. On the other hand, this observation leads us to the justification that, once the conditions (intrinsic ones of the building unit and environmental ones) for this force balance are met between any building units, they will come close (“self” aspect) and form the aggregates (self-assembled aggregates) at a certain point of the process (“assembly” aspect) regardless of their types and sizes.
Certainly, the conditions that can induce thermodynamic equilibrium between the self-assembly building units at the balance point will ensure the self-assembly process. Equilibration process means bringing the building units together (“self” aspect), and equilibrium state means holding the self-assembled aggregates flexible yet with structural integrity (“assembly” aspect). However, there can be kinetic conditions that can also ensure the two aspects of the self-assembly processes and self-assembled aggregates. There can be a certain point (or points) during any types of kinetic processes where the building units can be close (“self” aspect) and maintain the state until they escape from the point (“assembly” aspect). This leads to the conclusion that self-assembly processes do not always have to be driven thermodynamically. They can also be driven by kinetic processes. Self-assembled aggregates can maintain their structural integrity and ensure dynamic flexibility not just by keeping them at equilibrium state but by a kinetically stable “force well” also.
Figure 1.2 shows an arbitrary energy profile between self-assembly building units as a function of their coordination of assembly. The interaction energy (or force) between building units is varied as the distance between them (coordination) is changed. The profile can have just monotonic characteristics if it is assumed that the interaction is through only attractive or repulsive force. However, they can also go throug...
