An introduction to mesoscopic chemistry : from nanomaterials to mesocrystals

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The term "meso" comes from the Greek mesos, meaning median, in the middle, in between. The mesoscopic scale is intermediate between the atomic and molecular scale (1-20 Å) and the micron scale, where the macroscopic world sensitive to touch begins. For the sake of clarity, the mesoscopic scale lies between 2 nanometers (the size of a grown man divided by a billion) and 1 micron (the size of a small bacterium, i.e. one meter divided by a million). Mesoscopic chemistry covers both the synthesis and the study of the construction modes of chemical objects with sizes in this intermediate scale (2 nm-1 μm), the two- or three-dimensional assembly of well-defined objects located in this size range, and the study of the physical properties of the resulting materials. This innovative theme is attracting a great deal of interest in both the chemical and physical sciences. At a fundamental level, recent studies into the processes of solid formation are challenging the simple nucleation-growth models currently taught. This research shows that many seemingly monocrystalline materials can be the result of a non-classical crystallization phenomenon based on ordered aggregation, giving rise to mesocrystals whose habit resembles that of a monocrystal: this mechanism is called mesocrystallization. In fact, the two modes of growth - nucleation-growth or mesocrystallization - can be concomitant or sequential, although a "nucleation" stage enabling the formation of the nanobricks needed to build the mesocrystal is always present in the early stages.

The living world has been making and effectively using mesomaterials since the dawn of time. Nature is a model in this field, as it builds numerous mineral or hybrid edifices (coccoliths, diatoms, magnetic biominerals) by legochemistry, associating nano- or meso-bricks with prefabricated objects to create ordered crystalline or amorphous meso-assemblies, often with hierarchical structures. In particular, in this first lecture, we developed the example of magnetic biominerals serving as "GPS" for certain micro-organisms (magnetotactic bacteria) and animals (pigeon, lobster, lizard, turtle...) which use the inclination and/or intensity of the Earth's geomagnetic field to orient themselves.

This introductory lesson also showed the interest and some of the specific properties of meso-bricks, whose synthesis, size, shape and surface composition must be mastered by the chemist in order to control the modes of aggregation and organization that enable mesocrystals to be produced. In particular, nano-objects of controlled size, due to their high surface-to-volume ratio or via their specific mechanical (hardness) or optical properties (plasmon resonance which gives small gold particles their coloring, quantum confinement which generates the strong luminescence of certain semiconductor nanocrystals such as zinc sulfide...), are particularly interesting in the fields of probes and imaging, nanomedicine, new catalysts, new materials for energy. Ultradivatized gold is an extraordinary material, used in potions and medicinal treatments, as well as for its plasmonic colorings in Antiquity (glass coloring), then in the Middle Ages (stained glass) and at the start of the industrial era (Cranberry rose). After presenting mesoscopic gold and its properties, we discussed a few examples of materials at the interface of the mesoscopic world and new technological developments. These examples are particularly relevant to the medical field (imaging, therapy) and to new high-performance batteries. They provided a rapid illustration of this burgeoning field.

After this introduction, all the lectures and seminars that followed were devoted to describing and discussing this field, rich in fundamental questions, conceptual exchanges and applications. The following five lectures and seminars highlighted, more specifically, the new developments underway or the research needed to build "tailor-made" functional meso-assemblies for tomorrow.