Networks in the nanoworld
Carlos Briones, Susanna C. Manrubia y José Ángel Martín-Gago
Centre of Astrobiology (CSIC-INTA), Madrid
(Excerpt from Atrapados en la red: nanomundo, vida sociedad)

When we observe the world around us, we perceive that inorganic as well as organic matter, inert materials as well as living beings, are characterised by an ability to self-organise forming ordered structures and networks. Atoms arrange their electrons in a rigorous manner, molecules assemble or adjust to each other in coordination to construct structures of higher complexity… Nature’s fixation with organisation has aroused the curiosity of researchers, who have attempted to make out the hidden order of multiple systems and processes to better understand the fundamental mechanisms and laws that govern such organisational networks at every level.

It is obvious that in order to form organised structures, an electron, an atom or a molecule need to recognise similar units and have them behave in a specific manner by means of some sort of force. Today we know that four types of interactions or forces govern natural processes. The influence of the first two can only be felt at minute distances, shorter than atomic nuclei (of the order of a femtometre, that is, a billionth of a millimetre): the strong nuclear force is responsible for holding the units of the atomic nucleus together (protons and neutrons); on the other hand, the weak nuclear force has to do with the interactions between the particles that form protons and neutrons (known as quarks ) and makes possible certain types of natural radioactivity.

In contrast to these, the other two fundamental forces have a long scope of action, since their effects can reach any distance, theoretically an infinite range. Besides, we are much more familiarised with these two forces, since they govern the processes that our senses can perceive. Gravitation, or gravity, is the mutual attraction experienced by two objects as a function of their mass, and it is responsible for large-scale movement in the universe, for instance the organisation of the planets around the Sun. It is also responsible for our having “our feet on the ground” and for apples falling off trees. Last of all, electromagnetism or the electromagnetic force governs the behaviour of matter as a function of its electric charge, and can be attractive (between particles with different-sign charges, such as the electron and the proton) or repulsive (between same-sign charges). The electromagnetic force is involved in the physical and chemical transformations undergone by atoms and molecules, and is responsible for the formation of structures—and networks—among them. Thus, at the scale typical of molecules (of the order of a nanometre, that is, a millionth of a millimetre) electromagnetic interactions are the only ones that have a perceivable effect. In other words, electromagnetism is the basis of chemistry, the engine of the nanoworld. There are many manifestations of this force in our daily life (in fact, our life is electromagnetism): objects have colour, there are foods that we like and foods we do not, our car and mobile phone work… and when we shake somebody’s hand, our hands do not blend with each other, and we do not permanently fuse with the person we are greeting.

Thus, rooted in the domain of molecules and dominated by electromagnetic interactions, nanoscience has emerged as the experimental framework destined to shape the relationship between mankind and matter in the 21st century. Two of its derivatives, nanotechnology and bionanotechnology, are the interdisciplinary tool used in laboratories to coordinate atoms, inorganic molecules or biomolecules in order to construct higher structures with specific functionalities, just as atoms get arranged in a lattice or the way that living beings assemble simple molecules to synthesize larger and more complex ones. Just like physics was the “star among the sciences” in the first half of the 20th century, and molecular biology the star of its second half, nanotechnology will rule over the century that has just opened. Much remains to be known about the organising capabilities of nature before it can be imitated with accuracy, but there is no doubt that we are on our way: today’s nanoscience will be tomorrow’s nanotechnology.

Among the nano-objects that have been designed already in the labs, perhaps the most promising are carbon nanotubes. These are constructed by folding planes of carbon atoms (connected to each other in a hexagonal lattice reminiscent of honeycombs) to generate three-dimensional arrangements. The 2-D lattice becomes a 3-D lattice. It is as if the net of a fisherman was folded or curled upon itself and at the same time its size shrunk to a thousand-millionth of the original. This generates a tunnel of nanometric dimensions that can be used to conduct electrical currents or store molecules, among other applications.

The technique that has contributed most to the development of nanotechnology has been electron microscopy, which includes the “atomic force microscope” and the “scanning tunnelling microscope”. These new and revolutionary technologies derive from the quantum properties of matter, that is, of the laws and behaviours that rule the world below the nanometre (inhabited by atoms, electrons, atomic nuclei…). The quantum world is ruled by laws different from the ones we take for granted in our Cartesian world: what our reason may dismiss as absurd and nonsensical (for instance, the idea of an object not being in one specific spot, but rather delocalised and with a certain probability of being in any of a number of accessible places) is a perfect description of the organisation among and within atoms. In addition to the peculiarities of the quantum world, the new scanning probe microscopy techniques have been a technological revolution, and made possible what scientists had been dreaming about for at least the whole past century: to see not only the molecules… but the atoms! They have been called, deservedly, “the eyes of nanotechnology”. But they are also its hands, for these microscopy techniques also allow us to act on the molecules or atoms, to move, manipulate, and arrange them, or to alter the structures or the networks of electromagnetic interactions in which they are embedded. The new technologies allow us to not only see the network, but to construct it too.