Colorless nanoparticles used to create lightweight, colorful paint

Do you know more than 50 percent of microplastic pollution in our oceans comes from color paints? Almost every object that people throw into the ocean, whether it be a broken toy, a small bottle cap, or a shoe, has some sort of color coating. While you might try to collect all the plastic objects that are thrown into the oceans, there is no way to gather the microplastics that have already mixed into the water.

Particles derived from paint aren’t only a problem in the ocean; they also mix into the air that you breathe. In 2010, scientists studied the effect of chemicals that are used in commercial wall paint on children’s health. They found that kids who sleep in rooms with walls coated with paint having high levels of volatile organic compounds (VOCs) are more likely to develop medical conditions like eczema and asthma.

So does that mean commercial paint materials will continue to degrade our environment and our health? Well, there is a new ray of hope. Researchers from the University of Central Florida (UCF) recently published a study that describes “plasmonic paint,” a lightweight, eco-friendly material that has the potential to replace most colored coatings. They claim that their plasmonic paint is also the lightest paint in the world because it avoids the use of pigments and all the materials needed to hold the pigments in place.

Pablo Cencillo Abad, the first author of the study and a researcher at UCF’s NanoScience Technology Center, told Ars Technica, “Our ultralight paint is the lightest in the world, and its use instead of conventional pigments can help reduce the overall weight of objects, which is critically beneficial for the aerospace industry.”

He added, “As an example of a ready application, consider the case of a large object like a Boeing 747 jumbo jet. Conventional paint requires a large amount of paint, usually over 1,000 lb (454 kg), to coat such an object. However, only about 3 lb (1.4 kg) of our plasmonic paint would be needed, which represents an astonishing 400-fold reduction in weight.”

Plasmonic geometry creates colors

Plasmonics is the branch of science that deals with how electron movement affects the transit of light in metals. The proposed plasmonic paint produces colors by employing structural coloration, the phenomenon that gives peacocks and butterflies their bright, eye-catching colors. The geometrical arrangement of the feathers, skin cells, and scales in these animals alters the travel of light rays, making them bend at different angles and produce different colors.

The structural coloration in the plasmonic paint is inspired by butterflies. It is composed of two colorless materials: aluminum nanoparticles and aluminum oxide nanostructures. Just by altering the way these particles are arranged, the UCF team can manipulate visible light and create any color, giving rise to the world's first full-color structural paint.

“Our structural paints use aluminum nanoparticles to control the spectral components of light and generate a large palette of visible colors simply by changing the parameters of the structural production. When light hits our structure, the electrons of the metal of the particles start oscillating, capturing certain colors and reflecting others (this effect is called a plasmonic resonance). Importantly, the particular colors being absorbed are determined by the specific morphology of the nanoparticles. Hence, when we change the particles' size, they absorb different colors and produce different hues,” said Abad.

Pigment-based colors, on the other hand, work by absorbing certain wavelengths of light using pigment molecules. For example, the green color in plants results from chlorophyll molecules absorbing blue and red light and reflecting green. Here, colors are simply the result of the properties of the material used. Structural coloration produces color not by virtue of the material, but by controlling how it interacts with light.

An advantage of structural paints over chemical pigmentation is that while pigment molecules can dissociate with time and thus lose color, structural coloration can be made of very stable materials that will retain the color unless the structure is physically damaged. So as compared to standard paint, plasmonic paint is more durable.

A win-win

According to the study authors, durability isn’t the only advantage plasmonic paints have to offer. For instance, they don’t have VOCs or any pollutants that are commonly found in conventional paints. Plus, they are nontoxic, as they are not chemically synthesized with pigment molecules or color dyes but are self-assembled through nanofabrication methods.

So a wall or an object coated with plasmonic paint won’t have the same detrimental effect on human health and the environment as is the case with many paints. Also, since plasmonic nanoparticles are designed to selectively remove a few wavelengths of light in the visible spectrum and reflect everything else, they absorb much less heat compared to traditional paints (which absorb infrared light along with visible light), and thus may reduce power and air conditioner usage in indoor spaces and vehicles.

Plasmonic paints are also very flexible. Currently, if a paint manufacturer needs to produce a new shade of pigment-based color, he or she will require a new pigment molecule. Plasmonic paints can produce a wide range of new colors with a single formulation. All they need to do is change the geometrical arrangement of the nanostructures in their paint.

What’s more surprising is that you only need one coating of plasmonic paint to cover an entire wall or an object. Chemical pigments rely on volumetric effects, so one needs to coat an object with several layers to make sure that the volume or thickness of the paint is enough to reflect the desired color frequencies—typically, from micrometers to millimeters of paint. Since plasmonic paint can control and manipulate the way light interacts with them, “We can make them fully reflective even with a single layer of nanometric thickness, 500 to 1,000 times thinner than human hair,” said Abad.

There are no limitations to producing plasmonic paint on a large scale. The main ingredient used here is aluminum, which is the most abundant metal found in Earth’s crust. “Our paint-making process uses techniques that are already common in the electronics, semiconductor, and coating industries. This means that the infrastructure and expertise required to produce the paint already exist, and the process can easily be scaled up to meet demand. Therefore, it is a highly practical and feasible solution for large-scale commercial applications,” Abad told Ars Technica.

Is it ready for use?

Plasmonic paint promises to be a sustainable, scalable, lightweight, and eco-friendly alternative to conventional color coating products, but it’s not yet ready to be sold in stores. The study authors argue that it is in the early stages of development and has a high production cost. They are exploring new applications and refining their production methods to make plasmonic paint more efficient and cost-effective.

Abad and his team are also working on plasmonic heat radiation shields and trying to hybridize their paint with biodegradable functional polymers. This would allow for changes in color in response to external stimuli, making the coating ideal for environmental sensing or even “smart” paintings. “We firmly believe that this technology has the potential to revolutionize the paint industry and enable new innovations in a wide range of fields,” said Abad.

Science Advances, 2023. DOI: 10.1126/sciadv.adf7207 (About DOIs)

Rupendra Brahambhatt is an experienced journalist and filmmaker. He covers science and culture news, and for the last five years, he has been actively working with some of the most innovative news agencies, magazines, and media brands operating in different parts of the globe.