This episode was made in partnership with the Kavli Prize
The Kavli Prize honors scientists for breakthroughs in
astrophysics, nanoscience and neuroscience, transforming
our understanding of the big, the small and the
complex. [♪ INTRO]
The phrase “surface-level” usually means something… at
least mildly insulting. If something’s surface-level,
it’s superficial. It’s not interesting.
Well, there’s an entire field of science that begs to differ.
It’s the science of coating a thing with another thing, and
making that thing do something wildly different. These coatings are called monolayers, because each
layer of the material is just a single molecule thick, spread
out over a surface.
Coating glass with certain monolayers can make it easier,
not harder, to see through, and that’s just the start. From
TVs to pacemakers, from engineering microscopic circuits
to studying and controlling the behavior of cells, we’re here
today to show that surfaces are more than skin-deep.
Some optical, electrical and even biological processes
are easier to deal with on thin surfaces than they are in
big, bulky volumes. Think about painting
complicated designs on a canvas compared to sculpting
something in the round. You can more easily manipulate stuff on a
flat plane than in big blobs.
That’s true even for tiny stuff, and layers can be a great
way to manipulate things down to the scale of nanometers.
Scientists first got a handle on monolayers in the early 20th
century, after noticing that certain kinds of glass actually
transmitted more light when they were tarnished. It all comes down to how light waves transmit
and reflect off surfaces.
If a wave of light heads straight towards a pane of glass,
most of it will go through the material, but some small
amount will also be reflected back. That reflected light
makes up the glare that can make it harder to see through.
One way of dealing with this is with a film. As in a thin
layer of material rather than the theatrical movie kind,
although we’ll come to those in a moment too! If you put a transparent film with
precisely the right refractive index and
width on the glass, it lets incoming light enter the material as normal,
but it has another effect as well.
Light will be reflected from the surface of the glass like
before, but it will also be reflected from the surface of the
film. That sounds like it
ought to increase the total amount of reflected light, making things worse.
But this is the part where we remember that light is a wave,
with peaks and troughs. If the film is just the right width,
the peaks and troughs of the reflected light from the glass
and the film interfere and align to cancel out! In short, the film actually winds
up with getting rid of the reflections and reducing the
glare, which makes the glass super clear and transparent.
Sounds convenient, but creating these films isn’t easy.
That was the problem facing scientists around the 1930s.
Enter star physicist Katherine Blodgett. Building on the
work of her mentor Irving Langmuir, she developed a
process involving a chemical reaction between acids and
other compounds inside a container. After the reaction, a film would
form on top of the liquid solution, consisting of a thin
layer of something called cadmium arachidate.
This is a type of molecule with one end that dissolves
readily in water, and one that doesn’t. Blodgett’s process
lined them all up neatly across the water, all pointing the
same direction, since the water-loving ends would end up
facing the water’s surface. If a piece of glass was dunked
into this water beforehand, you could carefully pull it out
so that the film, just a single molecule wide, neatly transfers
itself onto the glass. Kind of like pulling
out a strawberry from a pot of melted chocolate so it's all coated evenly.
After that, voila, you’ve got yourself a layer of molecules on
a piece of glass. The resulting films are called Langmuir-Blodgett
monolayers, or LBMs, named after their inventors. The
specialized container used for creating them is called a
Langmuir-Blodgett trough. The trough, along with the
method they used for lifting monolayers out intact,
is what allowed them to apply LBMs onto solid surfaces, like glass.
By repeating this process as many times as needed to get
the right thickness, Blodgett managed to create a film of
material on glass capable of reflecting less than one
percent of white light falling on it. That made the glass amazingly clear
and was extremely useful. For example,
glass developed using her process produced crystal clear lenses used
for movie projectors in the late 1930s.
The same kind of lenses were even used in the periscopes
built for submarines and spy planes in the second world
war! But Blodgett
and Lamguir’s research on monolayers wasn’t just confined to glass, or even a particular
chemical process.
Using Langmuir-Blodgett troughs with different chemical
reactions, researchers could apply even, molecule-sized
layers onto materials for all kinds of uses. Basically, a kind
of Swiss army knife of nano-scale engineering! For instance, you might be
familiar with OLED TVs or computer monitors. They use organic light emitting
diodes to create a picture. But they’re pretty inefficient,
especially the blue kind. Because of the crystal
structure inside the OLED, the light kind of bounces around inside it
before escaping, getting absorbed by the material
and losing energy instead of coming out as visible light.
But if we could control the thickness, pattern and shape of
the OLED layers, we might be able to make them more
efficient at releasing light. A 2016 study by French researchers found that
constructing these layers with LBMs did make them super
efficient, and that by carefully tweaking the different layers,
they could even understand why. Which means LBMs could
help us save energy when using those big old screens we like looking at so much.
But as incredible as LBMs are, they’re not always the
best way of neatly arranging a layer of molecules onto a
surface. In some cases,
they’re chemically unstable on the surface
they’re applied to, meaning they won’t stick around in their
organized, layered structure for long. And what’s more,
using a Langmuir-Blodgett trough can be a slow and delicate process.
It only works on flat surfaces, and it’s one and done. No
tweaking once you’ve pulled your surface out. But, as is often the case in chemistry,
there’s more than one way to arrange an
organized layer of molecules onto a surface. You know, as they say.
By 1946, chemists had seen that in some cases they could
simply dunk a surface into a solution, and have the
molecules arrange themselves onto it in a neat layer.
In 1980 chemist Jacob Sagiv, at the Max Planck Institute in
Germany, showed that it was possible to create monolayers on certain kinds of
surfaces without all that messing around with a water trough.
In Sagiv’s version, the molecules in the solution were
individually attracted onto the solid surface, sticking
themselves onto it in a particular direction. In a short
amount of time, they arranged themselves into a monolayer, firmly stuck onto the material.
In fact, by mixing and matching specific molecules and
organizing different surfaces, it was possible to create
patterns – something you couldn’t get with the pull-out-the-strawberry method.
Since these monolayers assemble themselves, they’re
called, no awards for guessing, self-assembled monolayers or SAMs.
Sagiv’s work on SAMs was a huge step, allowing him to
create monolayers on a variety of surfaces. Specifically,
solids with a layer of oxides on their surfaces were
especially well suited to the process. That might seem super specific,
but lots of materials exposed to air react
with oxygen and end up with oxides on their surface, so the ability
to make SAMs on them opens a lot of doors.
For example, this made it possible to make electrical
circuits at the nano-scale, as in, technology that’s only
dozens of molecules or so in size. Using SAMs, Sagiv was able to create
monolayers that could conduct
currents through them, which is a crucial step for engineering electrical circuits
that are only a few molecules thick!
Which you’d want for things like transparent displays or
smart contact lenses – things where you don’t want the
electronics getting in the way. But while this works for
things like electronics, SAMs would have even more potential if they could be put onto
materials without oxides, like bare metal surfaces.
A few years later in 1983, the chemists Ralph Nuzzo and
David Allara reported in the Journal of the American
Chemical Society that they’d created a SAM on a surface
of bare gold. These kinds of SAMs
were a game changer. Like Blodgett and Langmuir had shown forty years
earlier, covering the surface of a thing with
something else can vastly change what you can do with that thing.
The SAMs based on Nuzzo and Allara’s design could be
made from lots of different organic materials and made as
precise as a few atoms in width. Researchers have since used them to change the
electrical properties of metals, make them more resistant
to corrosion, and in particular, study biological processes.
See, long before humans got into the molecular engineering game, nature was chock-full
of microbiological processes carrying out
different functions at the nanoscale. And a lot of these happen at some
kind of boundary, like the surface of a cell.
Which makes SAMs a great model system for studying such processes.
For instance, in one 1991 study, German researchers
embedded molecules of biotin onto an SAM, which itself
was bonded onto gold. In nature, biotin has particularly
strong reactions with a protein called streptavidin, that
plays a key role in biochemical processes. The SAM basically gave the researchers a nanoscale
hotbed for studying the chemical interactions between the
two in a well-controlled way! As well as studying nature,
researchers also quickly realized they could use the
microbiological tools it provided.
One instance is what’s called a DNA microarray. These
are a sort of fancy chip with a lot of DNA stuck on them.
Since a strand of DNA will connect with its complementary
strand, a microarray can tell you how much of that complementary DNA is present in a given sample.
Like, the level of certain genes expressed in samples from
patients who do or don’t have a particular sort of cancer
can be very different, and so a DNA array can be a kind of
genetic magnifying glass to help diagnose a patient.
Early on, SAMs were a promising way of creating microarrays like these because
they could be used to hold a bunch of molecules, like DNA, in place.
The challenge was placing each strand of DNA in a particular location on the array,
since you have to know what location on the chip corresponds to what DNA
sequence. Which meant finding a way of creating SAMs
with specific and intricate patterns. By 1992, a team of Harvard
researchers led by the chemist George Whitesides had cracked
the problem using what’s called soft lithography.
The solution was basically drawing a particular pattern
onto gold with the substance that the SAM would be made
out of. That laid the
foundation for another idea. Instead of drawing it out, they created the pattern
they wanted on a kind of stamp,
covered it in the substance the monolayer would be made of and stamped it on a surface
to create an SAM.
Later, Whitesides’ team came up with another way to
achieve the same effect, only this time, the idea was to
make a kind of mold in the shape of the film you wanted to
create and put it on top of a surface, like gold, and fill it
with the substance of the film. After a short while,
remove the mold and you’ve got yourself an SAM with a particular shape!
There’s a lot going on here, so to recap, by controlling the
exact shape and structure of a SAM, we can create surfaces which hold specific
biological molecules in place. That makes it easier to study them,
but also to use them, like in a DNA array.
Better still, SAMs made with particular patterns can hold all
kinds of useful and important molecules on surfaces.
Pacemakers, for instance, can often be attacked by the
body at the point where the electrical contacts are in touch
with your organs, because the immune system identifies it
as something foreign to the body. And the resulting inflammation does a
lot more harm than good.
But, by coating the contact points with SAMs, they can be
cloaked with anti-inflammatory molecules that prevent the
body from going haywire. A little like an
invisibility cloak for medical equipment. Nothing to see here, just normal heart things.
I could keep going, but hopefully you’ve seen that the
ability to stick a thing to a surface has an incredible range
of applications from medical devices to consumer
electronics. In 2022 Sagiv, Nuzzo,
Allara and Whitsides were all awarded the Kavli Prize in
Nanoscience for the remarkable tools they worked to develop in
creating and studying SAMs.
At the time of this video, there are already ten thousand
patents involving SAMs, from chemical sensors to solar
cells. So at least on the
nanoscale, we’ve only begun to scratch the surface… or at least build on it.
Thanks to the Kavli Prize Foundation for partnering with us
on this video. The Kavli Prize honors scientists for
breakthroughs in astrophysics, nanoscience and neuroscience, transforming our
understanding of the big, the small and the complex.
Now that you’ve learned about the nanoscience award,
you can check out the research that earned a 2022 Kavli
Prize in neuroscience and astrophysics. [♪ OUTRO]