A review of carbon quantum dots and their applications in wastewater treatment

doi:10.1016/j.cis.2020.102124

Advances in Colloid and Interface Science

Volume 278, April 2020, 102124

https://doi.org/10.1016/j.cis.2020.102124 Get rights and content

Highlights

•
Carbon quantum dots (CQD) is a class of carbon nanomaterials with sizes 1-10 nm
•
CQD possess strong tunable photo-luminescent properties
•
Plant-based materials can be used as raw materials in the fabrication of CQD
•
CQD can be applied as an effective semiconductor photo-catalyst for photo-catalysis
•
CQD can be functionalised as an indicator for metal ions detection

Abstract

Carbon quantum dots (CQDs) are a fascinating class of carbon nanoparticles with sizes around 10 nm. The unique properties of CQDs are low toxicity, chemical inertness, excellent biocompatibility, photo-induced electron transfer and highly tunable photoluminescence behaviour. Sustainable raw materials are commonly used for the fabrication of CQDs because they are cost-effective, eco-friendly and effective to minimise waste production. CQDs can be fabricated using laser ablation, microwave irradiation, hydrothermal reaction, electrochemical oxidation, reflux method and ultrasonication. These methods undergo several chemical reactions such as oxidation, carbonisation, pyrolysis and polymerisation processes to produce CQDs. Due to small particle sizes of CQDs, they possess strong tunable fluorescent properties and highly photo-luminescent emissions. It also contains oxygen-based functional groups and highly desired properties as semiconductor nanoparticles. Therefore, CQDs are promising nanomaterials for photo-catalysis, ions sensing, biological imaging, heavy metal detection, adsorption treatment, supercapacitor, membrane fabrication and water pollution treatment. This review paper will discuss the physical and chemical properties of CQDs, raw materials and methods used in the fabrication of CQDs, the stability of CQDs as well as their potential applications in wastewater treatment and biomedical field.

Graphical abstract

Section snippets

Introduction to quantum dots

Nowadays, nanotechnology is one of the studies by researchers that is beneficial to majority fields of life sciences such as in spine surgery [1], agro-food industry [2], castable refractory [3] and dentistry [4]. In recent years, progress in the development of high-quality quantum dots (QDs) has provided opportunities [[5], [6], [7]] to evaluate the phenomena associated with the uniqueness of quantum confinement properties [8].

QDs have been recognised as a discovery in nanotechnology for

Carbon quantum dots (CQDs)

Carbon quantum dots (CQDs) are generally defined as a fascinating class of carbon nanoparticles that mainly consist of carbons with sizes around 10 nm [27]. CQDs are fluorescent nanomaterials due to their strong quantum confinement effect with highly tunable photo-luminescent and optoelectronic properties [28]. The oxygenated-functional groups on the surfaces of CQDs have modified the surface structures of CQDs and particle sizes to exhibit the quantum confinement effects. Besides that,

Raw materials used to fabricate CQDs

According to past studies, CQDs have been fabricated from various natural carbon sources, such as citric acid [30,60,[64], [65], [66]], graphitic micro-particles [67], denatured milk [46], zein biopolymer [68], sodium hydroxide [69], dried leaf [28], broccoli [70], food wastes [49], pomelo fruit [41], ammonium citrate [47], ginkgo leaf [71], grass [39], humic acid [72], ascorbic acid [65] and gelatin [73].

From previous researches, very limited studies have reported the use of waste biomass to

Methods used in previous studies to fabricate CQDs

Various methods and techniques have been used to fabricate CQDs. One of them is sonication of multi-walled carbon nanotubes in a mixture of concentrated nitric and sulfuric acids for 2 h, which was followed by refluxing at 80 °C for 8 h. After the reaction ends, sodium hydroxide was used to dilute the acidic medium before it was dialysed by using a dialysis membrane to remove the excess acidic solutions [85]. The use of concentrated acids was to break down and functionalised its precursor

Methods used to characterise CQDs

Some several instruments or analyses are commonly used to characterise the CQDs. Functional groups on the CQD surfaces can be analysed by Fourier Transform Infrared Spectroscopy (FTIR). Transmission electron microscopy (TEM) can be carried out to obtain the data on surface morphology, nanostructure and particle size of CQDs. Besides that, UV-Vis absorption spectra over a range of 200–800 nm can be used to check the optical behaviour of CQDs while photoluminescence (PL) can be used to analyse

Application of CQDs as photo-catalysts

One of the major problems in photo-catalyst materials is the recombination of electron and hole pairs, which reduces the performance of photo-catalyst. Therefore, CQDs can be considered as promising materials to produce high-efficiency photo-catalysts [129] due to their good performance in trapping and transferring of electrons, which have similar properties as conventional semiconductors [60].

The photo-catalyst can be excited by photons with a supply of energy to generate electron-hole pairs,

The potential biomedical applications of CQDs

Apart from wastewater treatment, CQDs have attracted the attention in biomedical applications as they possess outstanding biocompatibility, good solubility in aqueous solution and desirable optical properties. Tunable fluorescence and quantum size effect of CQDs make them the promising materials in the biological imaging in observing various cellular processes. Fig. 25 shows some of the biomedical applications that can be applied using CQDs.

The biological imaging of CQDs has been discovered by

Stability of CQDs

CQDs were previously stored at 4 °C for a month to evaluate its stability [112]. The fluorescence of CQDs was analysed, and it was found that the fluorescence intensity of CQDs was maintained at 95% of its original intensity. This observation revealed that the CQDs might possess excellent stability with power storage. According to another research, CQDs showed good stability in a high ionic strength solution for at least 2 h and remained stable after being kept for two months in a 4 °C

Conclusion and prospects of CQDs

This review paper introduces the recent progress in the study of CQDs, focusing on their chemical and physical properties, raw materials used, fabrication methods and application fields. Raw materials that have been used to fabricate CQDs including citric acid, ascorbic acid, graphite, plant sources, fruits, carbohydrates, gelatin, cholesterol, glucose, bio-waste lignin and ammonium citrate. However, fabrication of CQDs from sustainable materials (plant-based sources and carbon wastes) should

Declaration of Competing Interest

None

Acknowledgements

This work was supported by the Unit Perancangan Ekonomi Negeri Selangor, Selangor State Government (Geran Kecil Alam Sekitar Tahun 2019); and UCSI University (Pioneer Scientist Incentive Fund (PSIF) Proj-In-FETBE-047).

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Historical PerspectiveA review of carbon quantum dots and their applications in wastewater treatment

Highlights

Abstract

Graphical abstract

Section snippets

Introduction to quantum dots

Carbon quantum dots (CQDs)

Raw materials used to fabricate CQDs

Methods used in previous studies to fabricate CQDs

Methods used to characterise CQDs

Application of CQDs as photo-catalysts

The potential biomedical applications of CQDs

Stability of CQDs

Conclusion and prospects of CQDs

Declaration of Competing Interest

Acknowledgements

World Neurosurg

J Food Eng

Ceram Int

Curr Opin Colloid Interface Sci

Opt Commun

Curr Opin Solid State Mater Sci

Phys B Condens Matter

J Lumin

Mater Lett

Opt Mater (Amst)

Chinese Chem Lett

J Mol Catal A Chem

Adv Drug Deliv Rev

J Non Cryst Solids

Opt Laser Technol

Appl Surf Sci

Comput Mater Sci

Int J Biol Macromol

Inorg Chem Commun

Thin Solid Films

Mater Today Chem

Int J Biol Macromol

Ceram Int

J Hazard Mater

J Colloid Interface Sci

Carbon N Y

TrAC Trends Anal Chem

Appl Surf Sci

Sensors Actuators B Chem

Sol Energy

Sensors Actuators B Chem

Mater Sci Eng C

Int J Biol Macromol

Talanta

Mater Lett

Sol Energy Mater Sol Cells

Trends Food Sci Technol

Talanta

Electrochim Acta

Mater Chem Phys

Mater Chem Phys

Colloids Surfaces B Biointerfaces

Org Electron

J Lumin

J Ind Eng Chem

Water Res

Appl Surf Sci

Mater Lett

Historical Perspective
A review of carbon quantum dots and their applications in wastewater treatment