Originally posted May 20, 2011

Making blocks with atoms is more difficult than you may think

May 20, 2011

My oldest, who just turned four, has been “practicing” her letters lately. Of the many things that come easily to her – her early ability to speak and enunciate has always amazed me – writing letters is just not one of those things. (Now, don’t get me wrong. I’m not concerned about her “advancement” in this area. Because there are so many things that she just does so well naturally, I’m just caught off guard at the things that don’t come easily to her.) But, being able to watch her develop is fascinating and one of the great joys of parenthood. Back to her letters … My daughter has always loved to draw and color. But her transition from hesitantly-drawn, squiggly lines to lines drawn in confidence is unmistakable (as the pictures on our home refrigerator will show). And her confidence is coming through in her structured drawing as well. Her first name starts with a K, and she has been determined to get that K down pat.

Recently, watching my daughter draw, I have been struck by how are lives are mirroring one another. She is interested in drawing circles and squares and the letter “K”. I am interested in using metal atoms to make spheres and cubes and pyramids and other not-so-simple shapes out of metal atoms.

As a chemist, there are good reasons to want to do this. Metal nanoparticles have been used for many different purposes: solar energy storage, catalysis, and pigments – among other things. A particle’s ability to do any of these things is dependent upon a number of different factors. First and foremost, the kind of metal atoms that make up a specific particle define many of the particle’s properties. For example, let’s say you start off with a bunch of iron atoms that you fashion into a sphere (by arranging them atom-by-atom). The properties of that sphere are going to be directly linked to the properties of an individual iron atom. So, a sphere of iron atoms is going to be different from a sphere of nickel atoms. While the make-up is important, the size and shape of the particles also determines what kind of properties the particles will have.

The Shape of Things

Spherical nanoparticles change colors as they grow in size. The larger they are, the more “blue” they appear. The color change is due to how the individual metal atoms collectively interact with one another. As you add more and more atoms to the sphere, these collective interactions are altered more and more from the original properties of the bare atom.

Silver nanoparticles change color as they change their size. Image Source

Interestingly, many of these spherical particles also luminesce (glow). And, you guessed it, the colors that they glow are dependent upon their size. Scientists are using these particles for all sorts of projects: from watching biology to making better solar cells.

Images of a cell that use quantum dots (metal nanoparticles) of different sizes and compositions to image different cellular compartments. Image Source

Cubic and pyramidal particles have their own uses as well. It turns out that the corners of these particles are hotspots for enhancing the electronic properties of the particles. The enhancement helps scientists to be able to detect single molecules that find themselves at the corners of a nanoparticle, which is no small feat.

This image is from a really interesting paper in Nature Materials from the Alivisatos group at Berkeley. A great description of the research can be found here.

Some nanoparticles are also fantastic catalysts (i.e. they make chemical reactions easier to carry out). Like in all solid surface-based catalysts, it has been suggested that irregularities in the surface (cracks or other places where a single metal atom isn’t surrounded by other atoms) are the most likely places for catalysis to occur. This is because metal atoms “want” to be surrounded by other metal atoms. The other atoms help to stabilize the single atom that they surround. Take the stack of oranges shown below as illustrative of the point I’m trying to make.

The way oranges stack – analogous to the way some metallic atoms stack – can illustrate how oranges/atoms with a lower number of interactions have more “instability” and increased reactivity. Image Source

The orange in the middle at the very bottom of the stack is completely surrounded by other oranges. It is stable. The oranges on the outside of this structure are less stable than the one in the middle. By this analogy, the orange/atom at the top of the stack is the least stable (because it has the fewest number of interactions) and most likely to search out other atoms and molecules to interact with. So, in a cubic or a pyramidal nanoparticle, the corners and any cracks – where the atoms have the least number of interactions – are going to be the most reactive. (This description holds for larger surfaces as well. The reaction between Mentos and Diet Coke, which I’m sure you’ve all seen before, is brought about because of the large number of cracks in the surface of the mentos).

Taking Shape

But how do we go about reproducibly making nanoparticles of a particular size and shape. With her drawing, after her initial steps that amounted to scribbling, she started slowly drawing her lines and shapes and letters to get them right. Scientists have done similar things by methodically writing words and making shapes with individual atoms by lining them up one-by-one. And, like my daughter, their shapes aren’t the most quickly written, but they certainly are beautiful.

An image of individually placed cobalt atoms on a copper surface as described in this paper.

Unfortunately, making new particles atmo-by-atom, while precise, is a very time consuming and inefficient. And, while it is interesting to look at individual nanoparticles, loads/scads/heaps/insert-ambiguous-imprecise-term-here are needed to be useful. As with many other useful chemicals, biological systems have evolved with the ability to create complex nanoparticles very reproducibly. Through the brilliance of Nature, a single molecule can be transformed into multiple different structures (as evidenced by several of the structures calcite – CaCO₃ – shown in the figure below).

The many different structures that calcite – CaCO₃ can take. Top left: Large image -Calcite grown in a lab. Image Source. Inset – a minuscule crystal of calcite magnified. Image Source. (Its amazing how microscopic structures have their shapes conserved as they grow larger)
Top right: Coccolithophores are a type of algae that grow calcite in really amazing shapes. Image Source. Bottom left: Mollusks use calcite of a different shape to make their shells. Image source. Inset – the structural form of calcite found in these shells. Image Source. Bottom right: Mammals have calcite crystals in their inner ear. Image source. Inset – Structural form of calcite from the inner ear. Image Source.

The figure above shows calcite in different forms. Of particular note is the shape of calcite grown in a laboratory. In a lab, calcite is a cube. When left to its own devices, calcite will always form a cube. The cube is the thermodynamic minimum energy structure for calcite. That means, if calcite grows slowly, it will always form a cube. But, and this is what Nature has figured out, if calcite is forced to grow quickly, it can take any number of shapes including the ones shown above.

Forcing the particles to grow quickly is only part of Nature’s solution. These particles must also be “told” what shapes they must take. So, proteins are often used to be templates that control the specific type of structures that are formed. Bone, for example, contains calcium phosphate-based molecules that grow larger structures templated around the protein collagen.

In an attempt to mimic Nature, research from Yu Huang’s group at UCLA set out to look for specific protein sequences that can force nanoparticles to form in different shapes. Using platinum as their metal of choice and a technique called phage display (which is usually used to look at protein-protein interactions), they found two distinct peptides that would bind to different shapes of nanoparticles. One peptide was more attracted to tetrahedral particles. The other peptide was more attracted to cubic particles. Further, the work shows that these peptides can help platinum atoms form into cubes or tetrahedrons dependent on which peptide is in solution while the nanoparticle is forming.

Image from the Huang paper in Nature Chemistry on peptide sequences that can direct the formation of specific nanoparticle shape. Image source.

Aside from proteins and peptides, small molecules are also used to force and stabilize nanoparticle shape. These molecules are often called surfactants because they stabilize the interface between the particle and the solution surrounding it. Arguably, Paul Alivisatos’s group is the best in the world at making novel shapes by growing nanoparticles of various composition in the presence of specific surfactant molecules. They have developed methods for creating dots, rods, terapods, hollow spheres, and other exotic shapes.

A composite of the types of nanoparticle structures that the Alivisatos group is developing. Image source.

Unfortunately, the state of this research seems to be that it is as much art as it is science. We are still figuring out the reasons why surfactant molecules can stabilize one shape over another. We are still trying to understand how to precisely control shape and size. Fortunately there are a couple of excellent scientists working to put a little more knowledge into the science of nanoparticle synthesis. Included among these is Jonathan Owen’s group at Columbia. They using advanced robotics to control nanoparticle synthesis in order to figure out what factors (mixing time, temperature, surfactant concentration, etc) are most important when trying to make particles of a specific size and shape. The work being done in this area will help a new generation of scientists make more intricately designed and end-use targeted nanoparticles.

Growing Pains

My daughter is continuing her drawing at a prolific rate. Her water colors and pictures are covering our refrigerator and walls at home. In the same way, the early stages of nanoparticle research gave the world some pretty stunning pictures, which certainly have captivated my imagination. As with my daughter and her drawing, many scientists still want their research to move a little faster, have a little more meaning. Like all of us, our first attempts at writing our name and drawing shapes were blurry stabs at something we wanted to be a little more meaningful, a little more impressive. But these first steps were crucial in how we would develop our skills in art and penmanship later in life, whether or not our art can stand up to Vincent van Gogh. I will continue to be amazed at nanoparticle research and am in a state of constant wonderment over how the field will grow and develop.

-mrh