Imagine a fish swimming in the ocean. On its own, the fish is an easy target for predators,
and can only find a limited amount of food. But when many fish form a school,
a highly organized structure behaving in unison, the fish gain power in numbers and organization.
As if behaving as one large organism, a school of fish has an enhanced ability to swim,
find food, and evade predators. This ability for a larger structure to retain and
enhance the functions of its smaller components is also observed in an exciting field of chemistry:
nanocrystals. A crystal is a
solid composed of atoms arranged in a repeating, symmetrical pattern in three dimensional space.
Nanocrystals are extremely small crystals where at least one dimension is less than 100 nanometers.
The largest nanocrystal is just 1/1,000th of the width of a sheet of paper!
Did you know that nanocrystal technology is already finding its way into our lives? For
example, Quantum Dots are very small nanocrystals - just 2-10nm or 10-50 atoms in size along one
direction! - used in television monitors to produce displays with higher color accuracy,
saturation, and brightness than traditional liquid crystal displays.
In the oil industry, zeolite nanocrystals are being used to refine crude oil into diesel fuel.
Nanocrystals are also already being used in drug delivery, semiconductor manufacturing, and optics.
Nanocrystals exhibit special magnetic, electronic, and optical properties
as a result of their small size. However, like a single fish, a single nanocrystal is often
too small to be useful. One goal of nanocrystal research is to create nanocrystal superstructures.
These are assemblies of nanocrystals in an ordered three-dimensional configuration,
capable of enhancing the properties of each individual nanocrystal.
There are two broad methods to synthesize superstructures from nanocrystals:
spontaneous formation and guided formation. Spontaneous formation relies on the evaporation
of a solvent - the solution that the nanocrystals are suspended in. As the solvent evaporates,
the nanocrystals are forced closer together until they form an ordered structure. On the other hand,
guided formation depends on the chemical and physical interactions (like hydrogen
bonding and electrostatic attraction) to assemble into an ordered structure based on
nanocrystal-to-nanocrystal interactions, or nanocrystal to solvent interactions.
Superstructures can be single-component, made from a single type of nanocrystal, or multi-component,
made from two or more types of nanocrystals. Scientists believe that nanocrystals and their
superstructures have great potential to create new materials with enhanced function. For example,
a key advantage of synthesizing multicomponent nanocrystal superstructures is that their
functions can be more easily predicted from the individual nanocrystals materials used to create
them. This differs from a trial-and-error approach of making new materials - where scientists cannot
reliably predict the function of a product from its starting materials. For example,
we would not predict that table salt -- which is used to season food -- is made of sodium
(a soft, highly reactive metal) and chlorine (a yellow-green gas commonly used as a disinfectant),
because table salt does not retain the properties of either sodium or chlorine.
Meanwhile, in 2014, A group of scientists synthesized a superstructure that does
retain properties of its starting material. They combined magnetic iron oxide nanoparticles
(Fe3O4) with cadmium chalcogenide (CdSe/CdS) quantum dot nanocrystals, and coated them with a
silica protection layer. The scientists discovered they were able to magnetically manipulate the
resulting superstructure in living cells due to the magnetic properties of iron oxide,
and track it using the fluorescent properties of quantum dots. The silica coating allowed
the complex to safely enter biological systems. With multicomponent nanocrystal superstructures
such as this, we can expand the ways we observe biological systems.
Nanocrystals may be about the nanoscale, but their potential impact on modern living may be enormous.
Soon we may see nanocrystal superstructures designed to combine two or more catalysts,
allowing chemical reactions to become more efficient;
smart windows that can let in custom amounts of visible light and infrared radiation;
wearable sensors; and flexible energy storage devices. Gadgets we thought existed
only in science fiction may find themselves assembling in our reality.