Bringing ancient technology into the light

When Roman artisans incorporated tiny silver and gold particles into glass and created an opulent chalice for the social elite, they crafted a technology that now, some 1,600 years later, could provide low-cost water quality testing in developing communities. The chalice, known as the Lycurgus Cup, changes from green to red when light is shone from behind, and a biosensor developed by Assistant Professor Gang Logan Liu uses the same color-changing nanotechnology to identify liquid and gas molecules. 

 

The Lycurgus Cup has been the center of scientific interest since the late 1990s, when it was discovered that the color-changing phenomenon was the result of the microscopic metal particles— mostly between 50 to 70 nanometers in diameter. The particles vibrate differently depending on the direction of the light, and different wavelengths are reflected accordingly. Silver nanoparticles, it was discovered, reflect blue light when illuminated from the front, and gold nanoparticles reflect green. Aluminum and copper have similar properties, but they reflect light at wavelengths beyond, or at the margin of, the visible spectrum. 

 

Although the cup itself has never been subjected to scientific experimentation—rather, the silver and gold particles were discovered by inspecting shards of similar glass—Liu suspected that the color of the light might change depending on the substance that filled the cup. “We are very interested to see the interaction of such a device with liquids and gases,” he said. “Of course, the current, modern-day archeologists, they cannot try using the real piece because that’s too valuable to be tried by anybody in an experiment. But we can do it: we can make a replica.”

 

Their replica exhibits the color-changing properties of the cup, but they shrunk it to almost one millionth of the original size. Each cup has a diameter of about 180 nanometers, and the inside walls are studded with silver and gold particles, each about 50 nanometers in diameter. Because these cups are so tiny, however, when the light shines on it from different directions, the human eye cannot detect the cup, let alone the color change. To make the transformation visible, Liu and his students created billions of the cups in a highly ordered array, lining them up, side-by-side on a plastic substrate using a nanoimprint fabrication process.

 

The resulting device is about one square centimeter, and excepting the silver and gold, which, per device, are used in almost negligible quantities, the materials are inexpensive, as is the fabrication method. They were then able to test whether different liquids cause the device—or the Lycurgus Cup, if a transubstantiation is allowed—to change colors. They poured different liquids onto the surface—water, organic solvents, various alcohols and oils, bio-molecular solutions, and so on—and they were excited to see that each substance generated a perceptibly different color.  

 

“We think our sensitivity is at least one hundred times better than most of those nanoparticle-based solutions out there,” Liu said, referring to a common chemistry procedure that involves gold nanoparticles suspended in solution. “Let’s say, your color change for a nanoparticle solution may be from ‘green color number one’ to ‘green number two,’ you know, two green colors, but in our case, it changes from green to red. So anyone who sees color differences can see that clearly.”

 

Differentiating between the fine-grade color variation inherent to the nanoparticle solutions often requires a spectrometer, an expensive piece of lab equipment, used to quantify wavelengths of light. The human eye couldn’t detect the differences accurately enough. “But in our case, the color change is so dramatic, so apparent to your eyes, we don’t have to use that,” Liu said. “Which means we can do a very cool scientific experiment without using very expensive scientific instrumentation.”

 

The actual price point for a reusable biosensor created using Liu’s technology would likely be under $50, possibly below $10. “It’s very similar to if you transform an expensive supercomputer into something that anyone can afford, like a laptop. So you can do most of the computing on a laptop as well.” Liu said, “So here we can do most of the biosensing, like what you usually do by using a very expensive set of equipment.”

 

Villagers could test the water quality of the community well. Physicians could determine cholesterol levels in blood. Sommeliers could verify wine vintages. The applications are, in essence, inconceivably many. Unlike the original Lycurgus Cup, which was probably used on special occasions by patricians of ancient Rome, this sensor could be used by almost anyone, anywhere. “The technology makes things better, but not necessarily more expensive, actually cheaper, more portable,” Liu said. “I want to make that day come earlier; that will really help a lot of people.”