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Plasmonics: How Researchers Are Using Metals To Create Powerful Biosensors

Plasmonics: How Researchers Are Using Metals To Create Powerful Biosensors

In the battle against breast cancer, the drug Herceptin is a steady ally, keeping some kinds of tumors at bay and helping people live longer. Yet nearly all cancers eventually develop resistance.

Until recently, the underlying cause of this resistance eluded researchers. But then chemist Wei Wang developed a technique to track how individual Herceptin molecules attach to cancer cells. He found that a protein in the membranes of the resistant cells was deforming the receptor molecules that Herceptin grabs onto, giving the drug no handhold.

This medical insight owes a debt to physics — specifically, the ability of metals to steer light on nanometer scales, a field of research known as plasmonics. By introducing cancer cells to Herceptin on one side of a gold wafer and watching changes in how light bounced off the other side, the researchers could see how the two parties — cancer cell and cancer drug — interacted, thus revealing a crucial mechanism of Herceptin resistance.

People have been using metals to manipulate the passage of light for centuries, though only recently has the phenomenon been used to understand diseases like cancer. Large sheets of shiny metal — a.k.a. “mirrors” — are like Do Not Enter signs for photons, reflecting back these light particles in mostly unchanged states. But microscopic metal flakes are different: They act more like traffic cops, allowing certain colors of light to pass through and blocking others.

One of the oldest examples of this phenomenon in action is a fourth century Roman chalice known as the Lycurgus Cup. Normally, the glass cup appears green and opaque. But if you illuminate the goblet from within, the glass glows a translucent red. That’s because nanometer-sized particles of gold and silver suspended in the glass reflect green light and allow red light to pass.

In creating the cup, Roman craftsmen had stumbled upon a synergy between electrons, metals and light that no one would understand for another 16 centuries: The electrons in some metals will resonate when tickled with just the right wavelength of light, which alters the path of the light itself.

Today, the field of plasmonics is flourishing. In the last 20 years or so, researchers have taken a much more deliberate approach to exploiting this behavior, creating tailored nanostructures that compress and manipulate light into volumes roughly the size of single molecules.

The ability to focus light on the nanoscale turns out to have scores of potential applications, says Caltech plasmonics pioneer Harry Atwater. The many uses are helping to solve problems in chemical sensing, data transfer, cancer therapy and navigation for self-driving cars. “That’s why this field is so exciting and why it’s been so compelling … it’s so interdisciplinary,” Atwater says.

The ability to manipulate light and electrons with nanometer precision turns out to be useful in a wide range of applications such as super hi-res imaging, targeted therapies for tumours, guidance for self-driving cars, and perhaps for even more fanciful goals, such as making everyday objects invisible. Image: Knowable Magazine

Shedding light on biomolecules

One of the most successful applications of plasmonics is biosensing, wherein researchers try to detect the presence (or absence) of biologically relevant molecules. Normally, these are much too small to see with light, and though there are ways to tag them, these techniques are often expensive or cumbersome or alter the molecules in ways that hinder their study. Plasmonics offers an alternative by confining light to molecule-sized volumes. Under such circumstances, “what you can achieve is very strong interaction of light with matter,” says Hatice Altug, a researcher at the Federal Institute of Technology in Lausanne, Switzerland. And that makes the light very sensitive to changes in, or the presence of, individual molecules within those volumes. (For a more thorough review of how plasmonics helps with biosensing, see this 2018 paper in Chemical Reviews.)

Wang, of China’s Nanjing University, and colleagues wanted to bring this power to the field of drug development. Part of the process of designing new drugs is understanding how the molecules interact with cells in the body, but that typically requires monitoring the interaction one cell at a time, which is very labor intensive. If there were ways to see many drug molecules interacting with many cells simultaneously, Wang says, that could really speed things along.

One tried-and-true technique for tracking such interactions is electrochemistry — running an electrical current through a collection of molecules and cells. By tracking changes in the current, researchers can measure the rate at which molecules attach to and detach from the cells.

Pulses of electricity increase a cell membrane’s conductivity (red areas), which creates an opening in the barrier that allows delivery of molecules like drugs or DNA. Visualizing the process is possible thanks to a plasmonics-based electrochemical imager, which reveals when the membrane opens. Image: Knowable Magazine

But electrochemistry doesn’t allow researchers to pinpoint these interactions with any spatial precision. A researcher might measure electrical current flowing through some cells and know that it’s hindered by molecules attaching, but be blind to exactly where. In the case of Herceptin resistance, that would mean knowing that there’s been an overall change in how frequently Herceptin is sticking to cells and then breaking off, but not knowing where the change is happening.

Guided by the light

To get the precision they needed, Wang and colleagues created a plasmonics-based electrochemical imager. As he and others describe in the 2017 Annual Review of Analytical Chemistry, instead of just applying a voltage to some cells and tracking how the ensuing electrical current changes as molecules arrive and depart, they also measured how light was reflected and absorbed by a gold wafer to see just where the current got held up, and by how much.

It’s the same principle as the Lycurgus Cup. In this case, however, the light is aimed at one gold wafer, 50 nanometers thick. On one side of the wafer is a small box filled with an electrolyte solution in which scientists can place molecules and cells. A red light illuminates the other side, and a camera measures how much of this red light bounces off the wafer.

The researchers apply an oscillating voltage to the whole setup, and this sends current coursing through the solution, drawing electrons into and out of the gold. The shifting density of electrons in the gold changes how much light gets reflected to the camera. At some densities, the light bounces off, while at others the light energy triggers waves of electrons in the gold known as plasmons that skim its surface, much like ripples moving across a pond. As the voltage goes up and down, electrons scurry in and out of the gold, and the amount of reflected light pulses in sync.

Cell-molecule interactions are detectable because the setup is affected by the behaviour of those molecules and cells in the fluid on the other side of the plate. Cells sitting close to the backside of the gold will hold back some of the current — casting an electrical “shadow” on the gold plate, pinpointing the cell’s location. Now if a molecule attaches to the cell’s membrane, the cell’s electrical properties will change, which will slightly brighten or dim its electrical shadow. This changes how light reflects off the metal at that spot, and the camera records the changes.

Light normally bounces off a thin metal film. But at just the right angle, the light triggers a plasmon wave — ripples of the metal’s electrons — and little light gets reflected. The exact angle depends partly on what molecules are present on the other side of the metal film. When molecules in solution, such as a cancer drug, attach to other molecules on the metal film, this pairing up alters the reflected light, which researchers can monitor for insight into how the molecules are interacting. Image: Knowable Magazine

Wang’s team used such an approach to figure out how Herceptin behaved differently around tumor cells that had developed resistance to the drug. They took resistant and nonresistant tumor cells and stuck them on the gold film. Then they added Herceptin to the electrolyte solution. Normally, Herceptin attaches to a protein in the cell membrane and thereby inhibits growth. This binding alters the plasmon wave of electrons and thus the light reflected off the metal.

But the researchers found that the reflections looked different near places on the gold that had been layered with drug-resistant tumor cells. Some of the proteins in those cells let go of the Herceptin dozens of times faster than normally, which altered the electrical current through those cells. This, in turn, changed whether plasmons formed in the gold and modified the pattern of light reflection.

With a view to drug development, Wang says that he has now begun studies of neuron and heart cells, in particular the protein channels used to shuttle sodium and potassium ions in and out of the cells. These ion channels are “the windows of communication between the cell and its environment,” he says. “If you’re developing a new drug, you need to evaluate the potential side effects to the heart.”

It is slow, laborious work, Wang adds, but a plasmonics-based electrochemical imager could speed things up.

Plasmonics is also being applied to other kinds of imaging, delivering resolutions that traditional microscopes cannot achieve and acting as a highly sensitive biosensor for a range of applications. Researchers hope that these tabletop devices can be shrunk into handheld ones; Altug, for one, envisions a future with portable, low-cost, easy-to-use plasmonics-based drug detectors and other biosensors that don’t require trained researchers. They might even be attached to cellphones.

“You could use it in the field to monitor water, security in an airport or in resource-limited settings such as a third-world country,” she says. “We’re not quite there yet, but there is a big push.”

Christopher Crockett is an astronomer-turned-science-journalist living in Arlington, Virginia. This article originally appeared in Knowable Magazine, an independent journalistic endeavour from Annual Reviews.

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