Carbon-dot nanoparticles break down water pollutants quickly and safely

May 22, 2021

Carbon dots could eventually be used to clean up after oil spills in the ocean. (AP Photo/Dave Martin)

When exposed to the sun, versatile particles called carbon dots degrade some water contaminants and appear to be nontoxic to sea creatures, according to a new study, which presents a budding "green chemistry" solution that significantly outperforms previous attempts.

University of Miami chemists led the research and said that the new and highly effective nanoparticles they developed could become a fast, cheap and safe method of water remediation or wastewater treatment. They published their findings April 28 in the Journal of Colloid and Interface Science.

Discovered less than two decades ago, carbon dots are a category of microscopic particles with a wide breadth of potential applications being studied for use in drug delivery, bioimaging, explosives detection and more. Carbon dots are usually under 10 nanometers wide and structured with an inner core surrounded by an outer shell.

Carbon dots have numerous properties that make them useful for scientists. They are semiconductors that are useful for driving chemical reactions without being spent. They also do not contain metals, so they primarily leave behind carbon dioxide and water when decomposed.

In the new study, chemists developed new carbon dots to be used in photocatalytic degradation, a water-purifying technique that breaks down pollutants with reactions driven by sunlight-activated catalysts.

Specifically, the nanoparticles were designed to break down pollutants by reacting with water to create reactive oxygen species. These are highly reactive molecules containing oxygen, and they include hydrogen peroxide and hydroxyl radicals, the latter being produced by removing an electron from a hydrogen-oxygen ion called hydroxide.

The target of the reactive oxygen species were pollutants with aromatic chemical structures, which contain six-carbon hexagonal rings and are susceptible to being damaged by reactive oxygen species.

"The [hydroxyl] radical, because it's already lost one electron, it has to grab another electron from the environment,"  said co-lead author Yiqun Zhou, a postdoctoral chemistry researcher at the University of Miami. "Aromatic structures contain a lot of delocalized electrons. So, once the radical grab[s] one electron from them, their structures are not going to be stable anymore and tend to be reformed or degraded."

Much like when the carbon dots themselves degrade, the degraded contaminants would be reduced into carbon dioxide, water and simple mineral acids, none of which are contaminants, according to the researchers.

Zhou, who led the application of carbon dots for the project, had previously helped develop gel-like carbon dots, which glow in ultraviolet light and effectively break down water contaminants when exposed to light, compared with other carbon dots.

The carbon dots were tested on water mixtures containing three differently colored dyes and the petroleum-derived chemical phenol, which all contain an aromatic ring in their chemical structure. Light fixtures mimicking solar radiation drove the degradation reactions.

Zhou described how one of his students accidentally discovered that the gel-like carbon dot completely degraded the dyes within 20 minutes, much more rapidly than the three hours a similar process had taken in the past. The student was using carbon dots that were embedded with graphitic carbon nitride, which also has semiconductor properties.

The embedded carbon dots were newly developed in this study and were found to speed up the degradation even faster than standard gel-like carbon dots.

"That's why we feel it's super significant," Zhou said. "In real life, 30 minutes, you degrade everything, all the pollutants. That's so crazy; that's too great."

The phenol was also completely degraded by the embedded carbon dots, accomplished in a little more than 2 1/2 hours. Zhou believes the process can be sped up with higher concentrations of carbon dots.

The results, according to the postdoc, indicated that the carbon dots could even break down many of the compounds in petroleum, some of which have aromatic chemical structures. The dots could eventually be used to clean up after oil spills in the ocean, he said.

The carbon dots also passed a test of toxicity in sea life. Eggs of sea urchins were incubated in waters containing various concentrations of carbon dots. No difference between their survival rates was found, leading the researchers to conclude that the nanoparticles are biocompatible and generally not toxic to sea-urchin embryos.

Given the apparent efficacy, speed and safety of the gel-like carbon dots, the researchers said their nanoparticles "will open up new horizons for diverse photocatalytic applications." Furthermore, they are easily and quickly produced when compared with other carbon dots, according to Zhou.

The postdoc said he and his lab are planning to test carbon dots on real-world water samples, which contain a much more complex mix of compounds than the few dyes and the phenol tested for the study. They will also investigate whether carbon dots can break down microplastics and further test whether they are toxic to life.

The study "Gel-like carbon dots: a high-performance future photocatalyst," published April 28 in Journal of Colloid and Interface Science, was authored by Yiqun Zhou, Jiuyan Chen, Emel K. Cilingir, Brian Walters, Keenan J. Mintz, Christian Martin, Braulio C.L.B. Ferreira, Wei Zhang, Sajini D. Hettiarachchi, Leonardo F. Serafim, Patricia L. Blackwelder, Athula H. Wikramanayake and Roger M. Leblanc, University of Miami; Ahmed E. ElMetwally, University of Miami and Egyptian Petroleum Research Institute; and Wenquan Shi and Zhili Peng, Yunnan University.

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