Dissymmetrical Nanoparticles


Serge Ravaine Centre de Recherche Paul Pascal, CNRS

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I need hardly say that one of my first actions on reaching England was to look up my old friend, Hercule Poirot.

I found him installed in one of the newest type of service flats in London. I accused him (and he admitted the fact) of having chosen this particular building entirely on account of its strictly geometrical appearance and proportions.

‘But yes, my friend, it is of a most pleasing symmetry, do you not find it so?’(…).

Through many years of friendship in Agatha Christie's novels, Captain Hastings will finally learn that for the famous Belgian detective Hercule Poirot, beauty of things could not be complete without symmetry. Probably since the very first days of the mankind, symmetry has been considered as an evidence of aestheticism and harmony and consequently as a main criteria for beauty, not only in arts, but also in science. This could be resumed in a few words, quoted by the Greek philosopher and scientist Aristotle in Metaphysica: “The main species of beauty are orderly arrangement, proportion (σϒμμετρια, symmetria), and definiteness; and these are especially manifested by the mathematical sciences.” To give a single example of a field where symmetry has played and still plays a major role, one can consider the case of chemistry and highly recommend the reading of books from Magdolna and Istvan Hargittai who have thoroughly studied the matter.

Given all the positive influence of symmetry in so different areas of knowledge, it may be surprising that some people could be attracted by the mysteries of dissymmetry. In 1848, Louis Pasteur was probably the most famous scientist to raise the question while studying what he called the “molecular dissymmetry,” later recognized as the foundation of the stereochemistry. One of the main conclusions of his works was that only natural substances are dissymmetrical and optically active, what he summarized by “la vie est fonction de la dissymétrie de l'Univers (life is a function of the dissymmetry of the Universe).” A few years later, around 1880, Pierre Curie began to study the relationship between symmetry and properties of crystals. From his early observations, he concluded that a physicochemical phenomenon, such as piezoelectricity, could not occur without a change in the symmetry elements of the system, and he summed up the idea saying that “c'est la dissymétrie qui crée le phénomène (dissymmetry creates the phenomenon).”

Since these pioneering studies, dissymmetry at the molecular or crystalline scale has been thoroughly explored, but surprisingly, this has not been extended to supramolecular systems and particularly colloids. In this review, we aim to demonstrate that the specific geometry or design of dissymmetrical nanoparticles should be as worthy of interest as symmetrical particles in several applications of the modern physical and chemical sciences.