Carbon dots fluoresce their way through the rainbow as liquids change

Fluorescent materials usually emit a single consistent color. They are typically made from dyes or metallic components. In contrast, researchers reporting in ACS Central Science have created carbon dots that can fluoresce in a range of colors under UV light, depending on the concentration and the liquid they're in. The team incorporated the new carbon dots into plastic decorations and used them to create a data encryption system.

LEDs, solar cells and some data protection or anticounterfeiting technologies rely on materials that absorb invisible UV light and then emit visible, colored light. But their glow is usually a single distinct color—think of the bright blue of white clothing or teeth under a blacklight. Siyu Lu, Lin Ai and colleagues wanted to make more versatile fluorescent materials that shift between different colors in response to their surroundings. They were especially interested in these materials for environmental sensing and dynamic information encryption.

Surfaces tuned for shifting colors

The researchers turned to carbon dots—tiny particles that can fluoresce brightly and whose surfaces are adjustable. By tuning those surfaces, they found that the same dot could fluoresce in different colors in varying conditions.

First, the researchers heated two carbon-based compounds in an organic solvent under pressure, forming particles with carbon-rich cores and chemically responsive surfaces. They made two versions, CD1 and CD2, using different ratios of the starting materials.

In tests, CD1 fluoresced red in acidic water and green in alkaline water. Shifting the pH caused the color change to reverse repeatedly without changing the dots' brightness. Similarly, when suspending CD1 and CD2 in different liquids—for example, water, toluene, methanol and ethanol—or varying the dots' concentration, the researchers observed that the dots changed their fluorescence, emitting colors from blue to red.

From colorful crafts to encrypted information

Next, the researchers mixed carbon dot solutions with polymers to make blue, green, yellow and red materials. They incorporated carbon dot-polymer powders into crafts, including molded Chinese characters and decorative bottles, and folded flexible films into different shapes.

They also used the dots' color changes to create a multidimensional encryption system, encoding information through unique light, liquid and acidity conditions. Together, these findings demonstrate potential uses of the new carbon dots in data security and anticounterfeiting technologies.

"This offers a new approach to designing smart optical materials that can sense their surroundings and convey information," says Lu, a corresponding author of the study.

The researchers are "actively exploring opportunities to commercialize the CDs, particularly for information encryption," says Lu. Next, they will evaluate long-term stability, large-scale manufacturing and production cost.