Science & Technology

Carbon dots: A futuristic solution for sustainably managing aquatic environment?

The inexpensive nanomaterial is becoming more popular in applications like sensing and bioimaging

Carbon dots are expected to bring about more exciting opportunities for environmental applications. Photo: iStock

Modern technology like nanomaterials or carbon dots (CD) might be the solution to environmental issues like water pollution. 

The urban development of modern society has resulted in the introduction of harmful and toxic pollutants into waterbodies, disturbing the integrity of the aquatic environment. Novel technological developments like nanotechnology provide innovative solutions for sustainable and efficient environmental cleanup.

Nanotechnology is a field of research and innovation concerned with building ‘things’ — generally, materials and devices — on the scale of atoms and molecules. A nanometre is one-billionth of a metre: ten times the diameter of a hydrogen atom. 

The attractive properties of carbon nanomaterials have been an extensively studied area as they push the limits and capabilities of technology. 

CDs are one of the youngest members of the carbon nanomaterial family. They were discovered in 2004 and have an average diameter of less than 10 nanometres. 

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The nanomaterials have garnered the attention of researchers mainly due to their convenient availability from both organic and inorganic materials. In addition, CDs have the potential to be used instead of quantum dots, which are more toxic and less biocompatible. 

The technology is produced from various raw materials leading to their wide range of beneficial physicochemical characteristics. These are many surface functional groups, extremely small sizes, large surface areas, excellent water dispersibility and superior ability in charge transport. 

The surface functional groups in CDs contribute rich oxygen-containing moiety on their surface and are favourable for water solubility and further functionalisation for various applications. 

CDs possess remarkable optical properties, which differ peculiarly based on the precursor used for synthesis. 

The dots show continuous and broad absorption spectra, intense fluorescent activity, excellent photostability and highly tunable photoluminescence.

Since they are good electron donors and acceptors, they are becoming more popular as candidates in applications like sensing and bioimaging. Moreover, CDs are inexpensive, highly biocompatible, and environment-friendly.

Green synthesis of CDs

Generally, the synthesis of carbon dots can be categorised into “top-down” and “bottom-up” methods. The top-down approach converts large carbon structures into quantum-sized carbon dots by laser ablation, arc discharge, and chemical or electrochemical oxidation. 

In the bottom-up method, CDs are produced from carbonising small molecule precursors by pyrolysis, carbonisation, hydrothermal processes or microwave-assisted synthesis.

Numerous studies are going on for the green synthesis of carbon dots with high efficiency. 

CDs have been produced from water hyacinth waste, which showed green fluorescence under UV light. They were also proven to be a fluorescent sensor to detect pretilachlor herbicide causing trouble in aquatic bodies. 

This study paved the way for an efficient method to control the water hyacinth menace in aquatic bodies apart from pollutant detection.

In 2016, a study put forward green synthesis of fluorescent carbon dots from eutrophic algal blooms, which can be used as a biomarker for bioimaging. This was a completely green and rapid approach to CD synthesis and an alternate way to handle algal blooms in the recovery process of eutrophication.

CDs for managing aquatic environment

The fascinating properties of carbon dots have enabled them to be used in various environmental applications. 

  • Pollutant sensing
  • Contaminant adsorption
  • Water treatment 
  • Pollutant degradation
  • Antimicrobial

CDs provide an excellent possibility for fluorescence and colourimetric environmental pollutants detection. They are widely used as a fluorescent nanoprobe for pollutant detection because of their high fluorescence emission. 

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They also enable the detection of pollutants with colour change by the colourimetric method. 

The technology can provide many surface adsorption sites due to their small size and large specific surface area. Nitrogen-doped CDs were formed for the adsorption of ions like Cd2+ and Pb2+ from wastewater (Sabet et al, 2019). 

CDs can also be useful for water treatment as they are promising nano-fillers in fabricating thin-film nanocomposite membranes where they can form chemical bonds with other compounds. 

Studies on CDs in combination with thin-film nanocomposite membranes reported increased surface hydrophilicity, solute selectivity, and improved anti-fouling performance.

The technology can also be useful for pollutant degradation by providing a cutting-edge approach for next-generation photocatalysis. Photocatalysis includes reactions that take place by utilising light and a semiconductor.

Organic pollutants in polluted water can act as electron and hole transferring agents, while carbon dots act as photosensitiser. 

The efficiency of methylene blue and rhodamine B degradation is improved in the presence of pollutants with a photosensitiser (CD), a study by scientists S Bhunia and colleagues found. Recently, thio-urea and citric acid-based carbon dots were used for the degradation of harmful dyes.

Antimicrobial mechanisms of CDs mainly include physical/mechanical destruction, oxidative stress, photocatalytic effect and inhibition of bacterial metabolism. CDs in contact with the bacteria cell under visible or natural light could efficiently generate reactive oxygen species. This can damage deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), causing bacteria death.

Recent advancements 

Aquaculture farms use CDs for water quality management. Prawn-shell-derived N-doped carbon nanodots as fluorophores were used in a study for selective and sensitive determination of nitrogen dioxide in water. 

CDs obtained from grass carp successfully detected mercury ions in lake water, demonstrating its potential for diverse applications (Liua et al, 2019).

Zhang et al, 2021 demonstrated a highly selective sensing strategy for ammonia detection based on nitrogen and sulphur co-doped CDs in a wide linear range of 2.0–80.0 millimoles per litre. 

Further, metal sensing-CDs loaded with titanium dioxide-nanocomposite were applied in aquaculture for photocatalytic bacterial deactivation. This is the first report using nanocomposite to disinfect V harveyi-mediated acute-hepatopancreatic necrosis disease (Alexpandi et al, 2020).

CDs have already demonstrated their promising potential for environmental applications in preliminary studies as new carbon-based nanomaterials. However, more efforts should be focused on synthesising new CDs with high quantum yield and specific surface groups to prepare CDs-based sensors with improved sensitivity and accuracy.

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The efficiency of pollutant degradation can be increased by optimising the material design through advanced characterisation techniques. Similarly, further study on synthetic raw materials, conditions, methods, and adsorption mechanisms is necessary to produce CDs-based absorbents with superior adsorption capability. 

Future research should devote to the standardised synthesis and purification processes to obtain CDs with homogeneity, high purity, and high selectivity. 

Although many difficulties and challenges are faced on the way to development, CDs are expected to bring about more exciting opportunities for environmental applications.

CDs will be an inevitable part of the scientific toolbox for environmental science and engineering in the future eith the development and integration with other nanotechnology.

Views expressed are the authors' own and don’t necessarily reflect those of Down To Earth

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