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Functional Carbon Dots Derived from Biomass and Plastic wastes

January 2023
Green Chemistry 25(17)

January 2023
25(17)

DOI:10.1039/D3GC01763J

Authors:

Tong Liu

Zhejiang University of Technology

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Carbon dots (CDs) have emerged as a promising class of zero-dimensional (0D) carbon nanomaterials due to their outstanding photoluminescence and electrical properties, along with their non-toxic, non-hazardous, and biocompatible advantages. These unique properties have sparked considerable interest in their applications in various fields. The utilization of biomass and plastic wastes as carbon sources to prepare CDs has become a significant area of research interest in recent years, driven by the increasingly strict environmental regulations and the growing volume of waste generated during production processes. In this context, a comprehensive and timely review of the fabrication, structures, properties, and applications of biomass and plastic wastes-derived CDs would significantly expand this field of research. Here, we present an overview of the fabrication methods of biomass and plastic wastes-derived CDs, employing either a top-down or bottom-up strategy. Furthermore, the most recent advances in the structures and properties of these CDs are highlighted and their utilization in polymer composites derived from biomass and plastic wastes are critically reviewed and summarized. Finally, the pending challenges and prospects for future research on polymer/biomass and plastic wastes-derived CDs composites for various applications are discussed.

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Green Chemistry

TUTORIAL REVIEW

Cite this: Green Chem., 2023, 25,

6581

Received 24th May 2023,

Accepted 1st August 2023

DOI: 10.1039/d3gc01763j

rsc.li/greenchem

Functional carbon dots derived from biomass and

plastic wastes

Tairong Kuang, *

Mengyao Jin,

Xinrui Lu,

Tong Liu,

Henri Vahabi,

Zhipeng Gu

and Xiao Gong *

Carbon dots (CDs) have emerged as a promising class of zero-dimensional (0D) carbon nanomaterials

due to their outstanding photoluminescence and electrical properties, along with their non-toxic, non-

hazardous, and biocompatible advantages. These unique properties have sparked considerable interest in

their applications in various ﬁelds. The utilization of biomass and plastic wastes as carbon sources to

prepare CDs has become a signiﬁcant area of research interest in recent years, driven by the increasingly

strict environmental regulations and the growing volume of waste generated during production pro-

cesses. In this context, a comprehensive and timely review of the fabrication, structures, properties, and

applications of biomass and plastic wastes-derived CDs would signiﬁcantly expand this ﬁeld of research.

Here, we present an overview of the fabrication methods of biomass and plastic wastes-derived CDs,

employing either a top-down or bottom-up strategy. Furthermore, the most recent advances in the struc-

tures and properties of these CDs are highlighted and their utilization in polymer composites derived

from biomass and plastic wastes are critically reviewed and summarized. Finally, the pending challenges

and prospects for future research on polymer/biomass and plastic wastes-derived CDs composites for

various applications are discussed.

1. Introduction

Carbon dots (CDs), a novel carbon-based nanomaterial and by-

product of carbon nanotubes preparation and purification,

were first discovered in 2004 by Xu et al.

Later in 2006, Sun

et al.

successfully prepared carbon nanoparticles with out-

standing fluorescence properties and designated them as CDs.

CDs typically have a spherical shape with particle sizes <10 nm

and primarily are comprised of amorphous carbon and nano-

crystalline regions of sp

hybridized graphitic carbon.

These nanomaterials exhibit several desirable properties,

including stable photoluminescence,

3–6

low toxicity,

good

biocompatibility

8,9

and environmental compatibility,

ease of

surface modification

and stable chemical properties,

making them useful for various applications such as fluorescent

labeling,

13–16

bio-bacterial chemicals,

photocatalysis,

18–20

and

flame retardation.

The preparation methods of CDs can be

divided into top-down and bottom-up approaches, with the

primary diﬀerence being the sources of CDs. The top-down

approach involves the conversion of large particles into nano-

sized small particles using methods such as arc discharge,

laser

and electrochemical methods.

On the other hand, the

bottom-up approach employs small molecule precursors as

carbon sources and decomposes them into nanoscale particles

through chemical methods, hydrothermal,

microwave,

etc.

The conventional synthesis of CDs involves the use of

organic chemical reagents as reaction precursors, along with

expensive experimental instruments. However, this approach

contradicts the principles of economic viability and environ-

mental sustainability. As the global economy rapidly develops,

accompanied by an increase in living standards and industrial

advancements, the production of both municipal and indus-

trial waste has escalated. The long-term accumulation of such

waste poses irreparable damage to the environment and poten-

tial health risks. Recognizing the urgency to address this issue,

researchers have discovered that waste materials can serve as

eﬀective carbon sources for synthesizing CDs. Remarkably,

CDs derived from these waste materials exhibit comparable

structural and performance characteristics to those produced

through traditional synthesis methods. With a growing empha-

sis on environmental concerns, numerous researchers have

embraced the concept of green synthesis, leveraging waste

materials to promote resource recycling and sustainable prac-

tices. This paradigm shift towards utilizing waste as a valuable

Zhejiang Key Laboratory of Plastic Modification and Processing Technology,

College of Material Science and Engineering, Zhejiang University of Technology,

Hangzhou, P. R. China. E-mail: kuangtr@zjut.edu.cn

LMOPS, Université de Lorraine, CentraleSupélec, Metz F-57000, France

Research Institute of Sun Yat-Sen University in Shenzhen, Shenzhen, P. R. China

State Key Laboratory of Silicate Materials for Architectures,

Wuhan University of Technology, Wuhan, P. R. China. E-mail: xgong@whut.edu.cn

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resource aligns with the escalating emphasis on environmental

responsibility. Researchers are now actively exploring innovative

and eco-friendly approaches, such as green synthesis, to

produce CDs. These methods not only reduce reliance on costly

reagents and experimental instruments but also contribute to

the eﬃcient recycling of waste materials, thus fostering a sus-

tainable and environmentally conscious future.

26–30

Biomass and plastic wastes are major waste sources that are

diﬃcult to dispose of, especially non-biodegradable plastics.

The utilization of these waste materials as carbon sources for

the preparation of CDs not only solves the waste recycling

issues but also eﬃciently utilizes these valuable resources.

Biomass wastes, being nontoxic and environmentally compati-

ble, are widely used renewable and recyclable materials and

thus an ideal carbon source for synthetic CDs. Green tea,

durian shells,

coconut,

custard apple peel,

soy residues

and other biomass waste materials have been reported as

eﬀective raw materials for synthesizing CDs. In addition to

their general properties of good fluorescence and easy surface

treatment, CDs exhibit excellent biocompatibility and are

highly suitable for biomedical applications.

Furthermore,

with the increasing environmental pollution caused by plastic

waste, it has become essential to recycle and safely dispose of

plastic waste. Using plastic waste as a carbon source for the

synthesis of CDs not only broadens the synthesis route of CDs

and reduces production costs but also promotes high-value re-

cycling of plastic waste. Polymeric reverse osmosis mem-

branes,

polyurethane foams,

polyolefins,

etc., have been

eﬀectively used as raw materials to synthesize CDs.

The utilization of waste-derived CDs has become a promi-

nent research topic, particularly in the development of

polymer/CDs composites.

40–46

Compared with single polymers,

the addition of CDs can enhance the performance and utiliz-

ation value of composite materials. Previous studies have

shown that polymer/CDs composites exhibit outstanding pro-

perties such as light conversion

47,48

and chemical selection,

thereby demonstrating their great potential in the fields of

solar cells, capacitors, chemical catalysts, and beyond. In this

review, we provide a comprehensive overview of the prepa-

ration, structures, properties, and applications of biomass and

plastic wastes-derived CDs (Fig. 1). First, the conventional

methods of CDs synthesis, including top-down and bottom-up

approaches are introduced before discussing the green syn-

thesis of CDs using biomass and plastic wastes and their struc-

tural properties. Subsequently, various applications of

polymer/waste-derived CDs nanocomposites in optical, electri-

cal, chemical, and biological fields are described. Finally, exist-

ing challenges for synthesizing wastes-derived CDs and their

applications in polymer/CDs nanocomposites are analyzed,

and provide an outlook on their future research directions.

2. Synthesis methods of CDs

The synthesis of CDs can be achieved using either top-down or

bottom-up approaches, each with its unique characteristics

(Fig. 2). The top-down approach involves the conversion of

larger carbonaceous materials into CDs using techniques such

as arc discharge, laser ablation, and electrochemical synthesis.

On the other hand, the bottom-up method employs molecular

precursors that are decomposed using combustion/thermal

treatments, microwave treatments, or other similar methods to

form CDs. Subsequently, these raw products must be purified

using separation methods such as centrifugation, dialysis, and

electrophoresis.

55–58

2.1 Top-down route for synthesizing CDs

Table 1 summarizes the top-down methods for preparing CDs.

In the arc discharge method, a positive electrode is positioned

a few millimeters from the graphite rod, which is evaporated

by the arc, resulting in soot deposition on the wall.

Unexpectedly, fluorescent nanoparticles were obtained by Xu

et al.

while preparing single-walled carbon nanotubes

(SWNTs) from the soot of arc discharge. To obtain CDs, the

soot was oxidized with nitric acid, and the extracted suspen-

sion was subjected to gel electrophoresis. The prepared CDs

exhibited blue-green, yellow, and orange fluorescence, with a

quantum yield (QY) of 0.016 when measured at an excitation

wavelength of 366 nm. The proportion of carbon, hydrogen,

nitrogen, and oxygen in the elemental analysis was 53.93%,

2.56%, 1.20%, and 40.33%, respectively.

Fig. 1 Carbon source, properties and applications of waste-derived

CDs

50–54

including polyoleﬁn plastics and polyester plastics (reproduced

source (reproduced from ref. 51 with permission from American

microbial sources (reproduced from ref. 53 with permission from

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The laser ablation method is a well-known bottom-up

approach to synthesize CDs. The technique involves the use of a

laser to irradiate a carbon target, leading to the decomposition

ofthetargetintoCDs.Sunet al.

developed a variation of this

method, whereby CDs were synthesized using laser ablation on

carbon targets in the presence of water vapor with argon as a

carrier gas. Specifically, a mixture of graphite powder and

cement was hot pressed, then stepwise baked, cured, and

annealed in argon. The Nd:YAG (1064 nm, 10 Hz) laser was used

for ablation in argon at 900 °C and a pressure of 75 kPa. The

resulting sample was then refluxed in nitric acid solution for

12 hours, and the surface was passivated to obtain the final CDs.

Notably, the QY of the synthesized CDs ranged from about 4%

to more than 10% when measured at an excitation of 400 nm.

The electrochemical method is an approach for synthesiz-

ing CDs by electrolysis using a high-purity graphite anode. In a

study by Zhou et al.,

carbon nanocrystals with strong blue

emission were synthesized through the electrochemical treat-

ment of multi-walled carbon nanotubes (MWCNTs) grown on

carbon paper by chemical vapor deposition (CVD). The electro-

chemical cell was assembled using MWCNTs as the working

electrode, a platinum wire as the counter electrode, and silver/

silver chloride (Ag/AgClO

) as the reference electrode. The

applied voltage was varied between −2.0 and 2.0 V, with a scan

rate of 0.5 V s

−1

. The electrolyte solution changed from color-

less to yellow, and then to dark brown with blue luminescence

under a UV lamp, indicating the exfoliation of CDs from

MWCNTs and their aggregation in solutions. Furthermore,

after the evaporation of acetonitrile from the solution, purified

CDs were obtained by dissolving the remaining solid in water

and subjecting it to dialysis.

2.2 Bottom-up route for synthesizing CDs

Table 2 provides a summary of the bottom-up preparation

methods. Hydrothermal treatment utilizes water as a solvent

in sealed pressure vessels to facilitate chemical reactions. This

method is a popular choice for synthesizing CDs due to its

Fig. 2 Schematic representation of the top-down and bottom-up methods for synthesizing carbon dots (CDs).

Table 1 Synthesis methods for CDs based on the top-down approach

Top-down method Process Advantages Disadvantages Ref.

Arc discharge Place the positive electrode a few

millimeters away from the

graphite rod evaporated by the arc

Good fluorescence performance Low yield, complex

composition, many impurities,

diﬃcult purification process

1 and

60–63

Laser ablation Use a laser to irradiate a carbon

target and decompose it into CDs

Fast, eﬀective, and highly tunable Expensive experimental

instruments and organic

passivators, uneconomical

2, 22, 61,

63 and 64

Electrochemical

method

Prepare CDs by electrolysis of

high purity graphite anode

Simple and reproducible, and the

obtained CDs have superior

performance without further

modification

Harsh experimental conditions

and easy agglomeration of

products

23, 59–61,

63 and 65

Table 2 Synthesis methods for CDs based on the bottom-up approach

Bottom-up

method Process Advantages Disadvantages Ref.

Hydrothermal

method

Water is used as the solvent for

chemical reactions in sealed pressure

vessels to carbonization of carbon

source

Cost eﬀectiveness, ease,

controllable, economic,

environmental protection

Long synthesis duration 66, 67

and

73–76

Microwaves

method

Use a microwave to assist in the

carbonization of carbon source to

obtain CDs

High quantum yield (QY), and

low equipment requirements

The size of the obtained CDs is

diﬃcult to control, and the large size

CDs are diﬃcult to separate by

dialysis

60,

73–75

and 77

Pyrolysis

method

The process of thermal decomposition

in an inert atmosphere at high

temperatures and controlled pressures

Simple procedure, short

synthesis duration

Diﬃcult to be scaled up, Broad size

distribution

61, 73

and 74

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ease of use, controllable reaction conditions and cost-

eﬀectiveness.

66,67

Hydrothermal methods use green biomass

materials, including cabbage,

bacteria,

prunus mume,

cornflour

and apple juice,

as precursors. The hydrothermal

method oﬀers several advantages, including simplicity, con-

venience, low cost, and environmental friendliness.

Microwave-assisted technology have been established as an

eﬃcient method for synthesizing CDs, with high reaction rate

being a notable advantage. Pajewska-Szmyt et al.

utilized a

microwave-assisted method to synthesize nitrogen (N)–sulfur

(S)-CDs from a mixture of citric acid and either glutathione or

thiourea. The resulting CDs exhibited a QY of 26% and an

average diameter of 14.557 nm, which is larger than those

reported in other studies. Consequently, these N–S-CDs hold

promise for use in mercury ion sensors. However, the method

suﬀers from uncontrollable reaction conditions,

which is a

significant disadvantage.

Pyrolysis is a process that involves the thermal decompo-

sition of a precursor materials in an inert atmosphere at high

temperatures and controlled pressures. This process results in

solid residues containing carbon, which may be obtained from

various carbon-based materials, including plant waste, plastic

waste, and other carbon-based materials. Precursors typically

undergo dehydration, polymerization, carbonization and sub-

sequent formation of CDs.

61,73,74

Hsu et al.

utilized coﬀee

grounds waste as the carbon precursor to synthesize CDs

through pyrolysis method. During heating, the coﬀee groups

waste underwent dehydration, polymerization, carbonization

and passivation, ultimately producing blue fluorescent CDs

with a diameter of 5 ± 2 nm and a quantum yield of 3.8%. The

synthesized CDs were subsequently used as probes and sur-

faces for cell imaging and matrices for surface-assisted laser

desorption/ionization-mass spectrometry (SALDI-MS) for the

detection of angiotensin I and insulin.

3. Green synthesis of waste-derived

CDs

Over the last decade, there has been a significant focus on

green synthesis of CDs through bottom-up methods using

biomass waste or plastic waste as precursors, as depicted in

Fig. 3. Traditional CDs synthesis methods typically employ a

large number of organic solvents that are not only expensive

and toxic but also cause environmental pollution. By contrast,

utilizing green biomass or recyclable plastic waste as precur-

sors for CDs synthesis oﬀers several advantages, such as ease

of operation, availability of raw materials, and alignment with

the principles of sustainability and waste reduction.

3.1 Biomass waste

Biomass is a promising feedstock for the production of a

range of chemicals, energy, and fuels due to its abundant

availability, low cost, renewability, biocompatibility, and biode-

gradability. Diverse types of biomass including wood and

woody biomass, herbaceous and agricultural biomass, aquatic

Fig. 3 Timeline of CDs derived from biomass and plastic wastes,

81–97

including orange waste peels (reproduced from ref. 81 with permission from

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biomass, animal and human biomass waste, as well as semi-

biomass mixtures, such as contaminated biomass and indus-

trial biomass waste, e.g., municipal solid waste, garbage-

derived fuel, sewage sludge, and removal of wood and other

industrial organic wastes, have been explored for biofuels and

biochemicals production.

98–100

Biomass waste represents a sustainable, non-toxic and

abundant source of carbon that can be employed in the syn-

thesis of CDs. Despite the potential of biomass waste, a con-

siderable amount of this material is discarded or burned in

the open air, resulting in environmental pollution and health

hazards. In recent years, researchers have been exploring the

feasibility of using biomass waste from various sources, such

as vegetation, animals, and microorganisms, as raw materials

for the production of CDs. These eﬀorts oﬀer a promising

avenue for the valorization of biomass waste and the develop-

ment of sustainable and eco-friendly technologies for CDs

synthesis.

3.1.1 Vegetal source. Table 3 presents a comprehensive

summary of plants that have been used to prepare CDs, includ-

ing empty fruit bunch,

101

beetroot,

102

tobacco straw,

103

sugar-

cane bagasse,

104

etc. These plants exhibit a high abundance of

glucose and cellulose, which results in an ample presence of

carbon (C) and oxygen (O) elements in the surface compo-

sition of CDs. Consequently, the surface of CDs is rich in

oxygen-containing functional groups, including hydroxyl and

carboxyl groups. These groups contribute to the hydrophilicity

of CDs. Furthermore, some vegetal sources contain hetero-

atoms such as N and S, which introduce a self-doping eﬀect to

CDs. This self-doping eﬀect enhances the fluorescence

eﬃciency of CDs. Therefore, the properties of CDs derived

from vegetal sources heavily rely on the specific choice of

carbon sources. Additionally, CDs derived from vegetal sources

generally exhibit a spherical shape with sizes typically below

10 nm. However, it is worth noting that the luminescent colors

of these CDs are predominantly in the blue and green spectra.

Various parts of plants such as shells, skins, rhizomes, straws

and residues have been utilized as raw materials, resulting in

CDs with diﬀerent properties. Generally, hydrothermal or

microwave treatments are employed in the preparation

process.

CDs can be produced using various types of plant residues

and wastes such as sago waste,

105

tea residues,

106

and so on.

Wang et al.

employed hydrothermal treatment of soy resi-

dues to produce CDs, which were nearly spherical and exhibi-

ted bright blue fluorescence under ultraviolet (UV) lamp

irradiation. The prepared CDs showed a diameter ranging

between 10–20 nm and were rich in hydroxy groups and N

doping. Similarly, Brachi et al.

107

utilized an innovative dry-

heating method with γ-Al

as a catalyst to prepare CDs from

agro-industrial residues (i.e., grape marc, tomato seeds and

peels, sugar beet pulp). The obtained CDs showed a uniform

spherical morphology with a diameter ranging between

2–4 nm and demonstrated a blue dispersion under UV light

irradiation. Xie et al.

108

reported a pyrolysis and hydrothermal

method to synthesize CDs from reed straw, and the obtained

Table 3 Synthesis of CDs derived from vegetal sources

Vegetal source Synthesis methods Temperature or power Time Size (nm) Quantum yield (%) Applications Ref.

Corncob Hydrothermal method 180 °C 9 h 1.5 ± 0.5 1.89 Detection of melamine 91

Corncob Hydrothermal method 220 °C 12 h ∼4 54 Fluorescence probe 110

Patera Hydrothermal method 200 °C 10 h 5 ± 0.45 83 Detection of Hg

ions 111

Eggplant Hydrothermal method 220 °C 8 h 2–5—Electrode 112

Orange peel Hydrothermal method 200 °C 12 h 2.03 —Electrocatalysts 113

Empty fruit bunches Hydrothermal method 180 °C 8 h 2.9 —Photocatalyst 114

Palm powders Hydrothermal method 200 °C 7 h 3.54 ± 0.82 0.9 Photocatalyst 115

Orange peels Hydrothermal method 180 °C 12 h 2–7 12.3 Photocatalyst 81

Green teas Hydrothermal method 200 °C 3 h 13 —Hg

sensor 31

Cauliflower leaves Hydrothermal method 220 °C 8 h 2–5—Supercapacitors 116

Tobacco leaves Hydrothermal method 220 °C 12 h 6.3 13.7 Fluorescent probe 117

Watermelon peel Pyrolysis method 220 °C 2 h 2.0 ± 0.5 —Optical imaging probe 54

Coconut coir Pyrolysis method 600 °C 3 h 10 48 Sensory probe 33

Peanut shells Pyrolysis method 400 °C 4 h 1.8–4.2 10.58 Probes for copper ion 118

Cotton Pyrolysis method 300 °C 2 h 8 —Tunable white light emission 119

Chia seeds Pyrolysis method 350 °C 6 h 2–64–25 Lectro-chemical sensor 120 and 121

Black soya beans Pyrolysis method 200 °C 4 h 5.16 ± 0.30 38.7 ± 0.64 Imaging intracellular Fe

122

Allium sativum peel Pyrolysis method 315 °C 3 h ∼2—Transparent sunlight conversion 123

Water hyacinths Pyrolysis method 180, 200, 220 °C 6, 8, 10 h 3–10 15.12 Detection sensors for ferric ions 124

Konjac flour Pyrolysis method 470 W 1.5 h 3.37 22 Cell imaging 125

Ginger and galangal herbs Microwave 450 W 5–40 min ∼10 —— 126

Edible mushrooms Microwave method 400 W 7 min ∼10 —Fluorescence imaging of human epithelial cells 127

Dragon fruit peels Microwave method 230 W 5–30 min ∼10 —Growth enhancer 128

Garlic Microwave method 700 W 2 min 20 54 Anti-oxidative eﬀects 129

Flour Microwave method 180 °C 20 min 1–4 5.4 Selective detection of mercury(II) ions 130

Guar gum Microwave method —25-30 min ∼30 7.5 —131

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CDs ranged from 5 to 10 nm. They pyrolyzed reed straws at a

low oxygen concentration and 500 °C for 5 h, followed by

heating using a hydrothermal method at 200 °C for 4 h. The

collected supernatant was dialyzed to obtain bright yellow CDs

solutions. Zhang et al.

109

synthesized CDs using agricultural

waste reed by a one-step hydrothermal method (Fig. 4a). The

waste reed was heated at 180 °C for 3 h and centrifuged and

dried the obtained mixture to obtain CDs powder. The pre-

pared CDs were nearly spherical with an average diameter of

2.7 nm. CDs–Cu

O/CuO synthesized using an ultrasonic

method was found to be an excellent electrode material. The

preparation process usually involves hydrothermal or micro-

wave treatments, which are eﬀective in producing CDs with

diﬀerent properties using diﬀerent kinds of raw materials.

The utilization of plant-based residues such as shells and

peels for the production of CDs has garnered interest in recent

years. Chen et al.

132

demonstrated the potential of grape peels

for CDs synthesis via pyrolysis, while Abdullah et al.

133

employed hydrothermal and solvothermal methods to obtain

CDs from palm shells. Zhang et al.

134

proposed a hydro-

thermal method for the synthesis of CDs from acorn cups

(Fig. 4b), where the dispersion of acorn cups powder and water

was heated at 200 °C for 8 h, filtered and centrifuged to obtain

yellow solution non-doped CDs (1-CDs). An alternative method

involved using ethylenediamine (EDA) mixed with water to syn-

thesize nitrogen-doped CDs (2-CDs) which exhibited a signifi-

cantly increased particle size and looser carbon sphere struc-

ture. 1-CDs have a particle size distribution ranging from 2 to

6 nm, with an average diameter of 4.16 nm, whereas 2-CDs

have a particle size distribution ranging from 4 to 7 nm, with

an average diameter of 5.23 nm. Furthermore, CDs were com-

bined with chitosan (CS) and polyvinyl alcohol (PVA) to obtain

PVA/CS/CDs films which showed excellent UV barrier and high

transmittance under visible light. In another study, Niu

et al.

135

reported a one-step microwave-assisted method for

synthesizing CDs from passion fruit peels (Fig. 4c). The pre-

pared B-CDs had a diameter range of 1.7–4.2 nm and were uti-

lized to develop a molecularly imprinted fluorescent probe for

the identification and detection of tetracycline (TC). These

studies demonstrate the potential of utilizing plant-based resi-

dues for the production of CDs and their potential appli-

cations in various fields.

The utilization of plant materials as carbon sources for CDs

synthesis has been explored by several researchers.

Palanichamy et al.

136

synthesized CDs through a simple hydro-

thermal method using the petals of Chrysanthemum

Morifolium flower as precursors. Srivastava et al.

137

also

employed hydrothermal treatment to synthesize CDs from

marigold flowers. Ge et al.

138

prepared nitrogen, sulfur, and

phosphorus co-doped CDs (NSP-CDs) from the seeds of green

pepper using a hydrothermal method. The synthesized

NSP-CDs possessed an average particle size of approximately

3.2 nm and could be used as a fluorescent probe for Fe

detec-

tion and cell imaging. Gomes et al.

139

prepared CDs using zuc-

chini as a precursor and utilized the obtained CDs for the fab-

rication of supercapacitor electrodes (Fig. 4d). Similarly, Şenol

et al.

140

synthesized CDs using cranberry as a green precursor

via a hydrothermal method. The obtained CDs had a particle

size of approximately 5 nm and a quantum yield of 14.5%. In

addition, the obtained CDs showed high sensitivity and

eﬃciency in detecting Fe

and ClO

−

, indicating their potential

as low-cost and high-performance fluorescent sensors.

3.1.2 Animal source. The use of animal materials as

carbon sources for CDs synthesis is gaining attention due to

their wide availability, renewable nature, and eco-friendliness,

despite being less explored than plant-based sources. The

Fig. 4 Biomass waste-derived CDs: (a) hydrothermal preparation of

CDs from reed straw (reproduced from ref. 109 with permission from

acorn cups (reproduced from ref. 134 with permission from Elsevier,

peels (reproduced from ref. 135 with permission from Springer Nature,

(reproduced from ref. 139 with permission from Elsevier, copyright

2019).

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abundance of chitin and protein in animal materials presents

promising technical routes for CDs synthesis. In contrast to

vegetal sources, animal sources oﬀer not only abundant C and

O elements for the synthesis of CDs but also a substantial

presence of N elements. This presence of N elements contrib-

utes to the formation of N-containing functional groups, such

as amino groups, on the surface of CDs derived from animal

sources. These N-containing groups confer unique properties

to the CDs, including high quantum yield (QY) and excep-

tional fluorescence characteristics. As a result, CDs derived

from animal sources exhibit strong fluorescence emission. Su

et al.

141

demonstrated the utility of chitin-rich crab shells as

carbon precursors for the synthesis of both fluorescent CDs

and small organic molecules through a low temperature hydro-

thermal treatment process (Fig. 5a), in which 68% of the

organic compounds were obtained. The CDs exhibited exci-

tation and emission spectra with an emission center at

460 nm when excited at 360 nm, and demonstrated excellent

selectivity for Fe

. Guo et al.

synthesized CDs from turtle

shells by ball milling and carbonizing at high temperatures

(under nitrogen protection at 500 °C for 5 h) (Fig. 5b).

Subsequently, the carbonized product was ground into powder

and transferred to deionized water for 12 h by ultrasonic treat-

ment. After centrifugation, dialysis and freeze-drying, brown

CDs powder was collected. These CDs displayed excellent

fluorescence properties, with a photoluminescence quantum

yield of 45%, and could be used as fluorescent ink for inkjet

printing patterns with single-signal anti-counterfeiting func-

tion. Furthermore, Marinovic et al.

142

successfully syn-

thesized CDs from lobster shells by hydrothermal treatment

and demonstrated their utility as sensitizers for TiO

-based

nanostructured solar cells. The performance of these solar

cells was shown to be dependent on the chemical compo-

sition and structure of the CDs used. These studies provide

promising examples of using animal materials as carbon

sources for CDs synthesis, which is of interest due to the

wide availability and environmentally friendly nature of these

materials.

Fig. 5 Biomass waste-derived CDs: (a) CDs prepared by hydrothermal method using crab shell chitin powder as carbon source (reproduced from

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Several studies have reported the preparation of CDs from

various animal-derived waste materials. In addition to chitin

and other polysaccharides present in shrimp and crab shells,

animal protein has also been utilized for synthesizing CDs.

Liu et al.

143

developed a method for preparing CDs from pork

ribs extracted from kitchen garbage. The process involved car-

bonizing the raw materials at 700 °C for 5 h, dispersing the

resulting powder in sulfuric acid solution, and subjecting the

mixture to hydrothermal treatment at 200 °C for 10 h to obtain

nitrogen-containing CDs with a QY of up to 14.9%. The CDs

could be used for detecting pesticide residues in water. Su

et al.

144

utilized expired milk and water to prepare nitrogen-

containing CDs with bright fluorescence properties (Fig. 5c).

The CDs formed a fluorescent CDs (FCDs)/Fe

complex with

, making them suitable for the sensitive detection of Fe

Moreover, due to the bright fluorescence characteristic of CDs,

it is possible to prepare ink with excellent light stability. As its

raw materials originate from non-toxic and harmless milk pro-

ducts, the ink prepared by these CDs may be used for textile

printing and flexible wearable products. Patra et al.

145

syn-

thesized CDs from spider silk using a simple and green

method. A mixture of acetone, ethanol, and deionized water

was used to clean and dry the spider silk, which was then

heated for four hours at 800 °C in a tube furnace. It was then

ground into a fine powder and the CDs were collected.

Although the CDs possessed general CDs properties, their

unique properties and applications require further investi-

gation. Ahn et al.

146

reported the synthesis of nitrogen-doped

CDs from ground cat feed powder through hydrothermal car-

bonization at various temperatures (Fig. 5d). The CDs exhibi-

ted excellent fluorescent properties with a QY of up to 28%

and could be used for highly selective Fe

fluorescence detec-

tion probes. Additionally, the CDs showed promising potential

for biological imaging, with a survival rate of cultured cells up

to 80%.

3.1.3 Microbial source. In the realm of CDs synthesis, the

exploration of microorganisms as potential biomass sources is

becoming increasingly prevalent due to their wide distri-

bution, rapid breeding cycle and low cost. In particular,

marine algae have been identified as rich sources of carbon,

such as sodium alginate, that can be harnessed for the pro-

duction of CDs. Microorganisms possess a wealth of bio-

molecules such as proteins, nucleic acids, lipids, and polysac-

charides, which are abundant in C, O, N, phosphorus (P), S,

and other elements. The inclusion of heteroatoms like N, P,

and S in microorganisms oﬀers opportunities for surface pas-

sivation, which can enhance the fluorescence eﬃciency of the

resulting CDs. Moreover, microorganisms are characterized by

their high carbon content. Consequently, CDs derived from

microorganisms exhibit stronger fluorescence emission,

higher QY values, and excellent water solubility. Chang et al.

dissolved microalgae in deionized water, stirred the mixture at

room temperature for 2 h at high speed, and then subjected

the uniform suspension to an 800 W microwave for 5 min to

synthesize CDs (Fig. 6a). The resulting solid powder was

further ground to collect CDs products, which were found to

enhance the photocatalytic performance of TiO

nanoparticles.

Under normal visible light, the TiO

/CDs composite can be

used as an eﬀective photocatalyst for degrading organic

methylene blue. Similarly, Singh et al.

147

utilized waste sea-

weeds as raw materials after agar extraction, followed by pyrol-

ysis at 400 °C for 4 hours, and sterilized in a polytetrafluoro-

ethylene (PTFE) liner autoclave at 160 °C for 12 hours to

produce CDs. The fluorescence QY of the aqueous solution

was 24%, oﬀering potential applications in stealth inks for

secret communication. Atienzar et al.

148

employed pyrolysis of

sodium alginate in brown algae in an argon atmosphere fol-

lowed by annealing and laser ablation to obtain CDs. The sus-

pension exhibited strong fluorescence with increasing laser

ablation time, suggesting the potential of such CDs for bio-

imaging applications.

In addition to marine algae, microbial sources have also

been explored for the green synthesis of CDs, which exhibit

universal fluorescence properties and unique advantages in

detecting microbial cell activity, leading to extensive research

in this field. Hua et al.

separated Staphylococcus aureus or

Fig. 6 Biomass waste-derived CDs: (a) preparation of CDs from microalgae by microwave method (reproduced from ref. 53 with permission from

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Escherichia coli from a culture medium by centrifugation and

resuspended them in pure water (Fig. 6b). The bacterial sus-

pension was then heated in a stainless-steel autoclave at

200 °C for 24 h, resulting in the formation of CDs. The pre-

pared CDs displayed good dispersibility and stability in

aqueous solutions and were capable of selectively staining

dead microbial cells without aﬀecting living ones. Similarly,

Zhang et al.

149

cultured Bacillus cereus MYB41-22 in a liquid

medium at 37 °C for 24 h, which was then transferred to a

PTFE-lined stainless steel autoclave and cultured at 200 °C for

12 h to synthesize CDs. The obtained CDs exhibited good bio-

compatibility, multicolor fluorescent properties, and quick

excretion in mice, indicating their potential application in bac-

teria, cellular, and animal imaging. Furthermore, Kousheh

et al.

150

prepared CDs from Lactobacillus suspension and

embedded them in a cellulose nanopaper to improve the flexi-

bility of the nanopaper with good UV barrier activity,

suggesting the potential of CDs in food packaging and anti-

counterfeiting packaging.

3.2 Plastic waste

Plastic materials have become essential in modern life due to

their attractive features, such as low cost, lightweight, high

transparency, good ductility and thermal stability. Despite

their benefits, plastic wastes remain a serious environmental

issue because they are non-biodegradable. Thus, the conver-

sion of waste plastics into usable products is the focus of sig-

nificant research eﬀorts. A promising approach involves

synthesizing CDs from waste plastics, which is not only an eco-

friendly but also an economically viable method. Generally,

pyrolysis carbonization and solvothermal methods are used to

synthesize CDs from plastic waste (Fig. 7). These CDs exhibit a

high abundance of oxygen-containing functional groups on

their surface. They possess a small and spherical structure,

excellent hydrophilicity, and display remarkable photo-

luminescence properties. However, one limitation is that to

enhance their fluorescence eﬀect, the addition of organic

reagents is necessary for achieving heteroatom doping.

Additionally, it is noteworthy that these CDs predominantly

emit blue light, limiting the range of luminescent colors

observed. Bhardwaj et al.

151

demonstrated that single-system

white emission CDs and white light-emitting diodes (LEDs)

could be prepared from various types of waste plastic, includ-

ing disposable surgical and N95 masks, gloves, syringes,

bottles and plastic storage containers. This method not only

addresses plastic pollution but also provides a new opportu-

nity to overcome the energy crisis. Plastic wastes can be classi-

fied into polyester, polyolefin or other categories based on

their raw materials. In this article, we describe CDs prepared

from diﬀerent plastic wastes.

3.2.1 Polyolefin plastics. Polyolefins, a class of polymer

materials, are known for their wide availability, low cost and

remarkable comprehensive properties, with polyethylene (PE)

and polypropylene (PP) being the most commonly utilized

members. Several studies have reported the preparation of CDs

from waste polyolefin plastics, such as, plastic bags made of

PE and, plastic cups made of PP in recent years.

CDs can also be synthesized from waste PE plastic as a pre-

cursor, as reported by Gautam et al.

in their recent study

(Fig. 7a). The authors utilized a simple acid-mediated heat

treatment approach, wherein PE sheets were heated in sulfuric

acid at 140 °C–150 °C for 1 h. Subsequently, H

was added

slowly and, after cooling to room temperature, the mixture was

diluted, filtered with water, and extracted by a liquid funnel.

The organic phase was then subjected to rotary evaporation to

obtain solid CDs powder PE-CDs. These CDs had a size of

1.5–7 nm, with an average size of 4 nm. Chaudhary et al.

153

conducted a study where plastic waste, specifically plastic

bags, cups, and bottles primarily made from PE, PP, and poly-

ethylene terephthalate (PET), was utilized as raw materials for

the synthesis of CDs. The plastic waste was processed through

hydrothermal treatment at 200 °C for 5 hours, followed by cen-

trifugation, filtration, and lyophilization to obtain powdered

CDs. Three types of CDs were successfully produced, each with

QY of 60%, 65%, and 69%, respectively. These CDs demon-

strated excellent biocompatibility and were eﬀectively utilized

for the selective sensing of Escherichia coli.

PP plastic has been identified as a viable source for the syn-

thesis of CDs, as demonstrated by Aji et al.

and Chaudhary

et al.

154

The former utilized a heating process, with PP plastic

waste added to ethanol, to obtain CDs (Fig. 7b). The size of the

resulting CDs was found to be less than 20 nm, with the

average particle size ranging between 15 nm and 8 nm as the

temperature was increased from 200 °C to 300 °C. The CDs dis-

played desirable photoluminescence properties, making them

suitable for various applications such as bioimaging and

photocatalysis. In a similar vein, the latter employed a pyrol-

ysis method to synthesize CDs from plastic cups, which were

found to have blue emission under UV irradiation, high glass

transition temperature, and good thermal stability. Notably,

these CDs were applied in the development of sensors to

detect sulfite ion pollutants.

3.2.2 Polyester plastics. Polyester is a class of polymers

formed by polycondensation of polyols and acids. Among them,

polyethylene terephthalate (PET) is an engineering plastic with

excellent performance and versatile applications. It is extensively

utilized for the production of plastic beverage bottles, but its high

production cost and diﬃculty in recycling pose a significant chal-

lenge. Therefore, researchers have suggested a pyrolysis method

to carbonize PET and generate CDs for diverse applications.

Hu et al.

152

synthesized CDs from oxidized waste PET

plastic bottles through a pyrolysis method followed by a hydro-

thermal treatment with H

solution (Fig. 7c). The procedure

involved heating the waste PET plastic bottles in air at 350 °C

for 2 h, and then hydrothermally treating them with H

solution at 180 °C for 12 h. After purification, centrifugation,

and freeze-drying, the resulting CDs had an average size of

approximately 6 nm, displayed blue fluorescence under UV

irradiation, and exhibited a QY of 5.2%. Moreover, the CDs

contained abundant functional groups on their surfaces, and

their fluorescence could be selectively quenched by Fe

ions

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and restored by adding pyrophosphate anions (PPi). These fea-

tures enable the application of the CDs in sensing systems for

detecting Fe

Hu et al.

have developed a novel carbon-dot catalyst by uti-

lizing PET oxidation and sulfuric acid sulfonation (Fig. 7d).

The process involved placing the waste plastic bottles in a cru-

cible, heating it in air for 2 h at 300 °C, and treating it with

(98%) in a brown oxide at 120 °C for 6 h. After the treat-

ment, the CDs powder was obtained via dilution, filtration,

dialysis, and freeze-drying. The average size of the produced

CDs was approximately 3 nm, diﬀerent from those generated

by H

, as they included a small amount of sulfur. The pres-

ence of sulfur indicated a small number of acid groups on the

surface of CDs. These CDs were found to be eﬃcient in gener-

ating 5-hydroxymethylfurfural (HMF) from fructose at low

temperatures, which suggested a new method for the conver-

sion of biomass into biofuel.

In addition to PET, polylactic acid (PLA) is also a viable pre-

cursor for the synthesis of CDs. Lauria et al.

155

successfully

depolymerized PLA particles in deionized water at 95 °C, and

the resulting dispersion containing degraded PLA oligomers

was then transferred to an autoclave lined with PTFE.

Subsequently, the autoclave was naturally cooled to room

temperature from 180–240 °C at diﬀerent intervals, and CDs

were collected via 0.22 µm membrane filtration. This study

represents the first demonstration of a green and industrially

scalable synthesis method for photoluminescence CDs using

common PLA waste as a carbon source, indicating great poten-

tial for the development of sustainable approaches to the syn-

thesis of CDs.

3.2.3 Other plastics. In addition to the aforementioned

methods for producing CDs from waste polyolefins and poly-

esters, there has been considerable interest in utilizing other

high-usage plastic materials for CDs synthesis.

Fig. 7 Plastic waste-derived CDs: (a) synthesized CDs by pyrolysis from polyethylene (PE) (reproduced from ref. 95 with permission from Elsevier,

(c) synthesized CDs by solvothermal method from polyethylene terephthalate (PET) (reproduced from ref. 152 with permission from Elsevier, copy-

thesized CDs by pyrolysis from PP, polysulfone (PSU), polyamide (PA) and polyester (reproduced from ref. 37 with permission from Elsevier, copyright

2021); (f ) synthesized CDs by pyrolysis from poly (ethylene imine) (PEI) (reproduced from ref. 52 with permission from Royal Society of Chemistry,

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Polystyrene (PS) foam, a non-degradable and photodegrada-

tion-resistant materials, poses significant challenges in the re-

cycling industry, with a large portion ending up in landfills.

Converting waste PS foam into CDs oﬀers an eco-friendly solu-

tion to this pressing issue. Ramamurthy et al.

successfully

synthesized water-soluble CDs from expanded PS (EPS)

through a one-step solvothermal approach. EPS was combined

with chloroform and ethylenediamine (EDA) in an autoclave,

followed by heating at 150 °C for 8 hours. The resulting solid

CDs, after drying, centrifugation, dialysis, and freeze-drying,

had an approximate diameter of 4 nm and a QY of around

20%. These CDs exhibited remarkable photoluminescence pro-

perties and excellent water solubility, making them ideal fluo-

rescent probes for the selective and sensitive detection of Au

Another method for producing nonpolar CDs from waste PS

foam was developed by Srivastava et al.

156

This low-energy,

acid-free, alkali-free, and catalyst-free approach utilized micro-

wave pyrolysis at a temperature range of 270–390 °C to obtain

graphene quantum dots. The resulting CDs had an average

size of approximately 5.5 nm, a QY of 15%, and were highly

soluble in polar solvents. Coating cotton fabric with these CDs

imparted durable and excellent hydrophobicity and self-clean-

ing performance.

Polymer reverse osmosis membranes consist of PP, polya-

mide (PA), polysulfone (PSU) and polyester, which are non-

degradable polymer materials. It is therefore essential to

recycle reverse osmosis membranes in a proper manner. Lisak

et al.

proposed a process for converting waste polymer

reverse osmosis membranes into CDs through pyrolysis and

hydrogen peroxide carbonization (Fig. 7e). Waste reverse

osmosis membranes were heated at 600 °C for 30 minutes in a

nitrogen atmosphere, the pyrolytic carbon was ground into

small particles, and CDs were synthesized using H

. After

several days of dialysis, the CDs powder was prepared by

freeze-drying. The resulting CDs had an average particle size

ranging from 2.6 to 6.8 nm and exhibited selective binding

capabilities for Fe

Huang et al.

have explored the potential of pyrolyzing

polyimide to obtain pyrolytic carbon (PPC) as a precursor for

CDs with tunable sizes from 3 to 10 nm (Fig. 7f ). The study

established a correlation between the chemical structure of

raw materials and the properties of graphene products, includ-

ing their size and yield. By utilizing graphitizable polymers

with double continuous (sp

and sp

) structures as precursor,

the yield of CDs was enhanced.

4. The toxicity of CDs

Toxicity determines whether nanofillers can be applied in

polymers. CDs exhibit low toxicity and excellent biocompatibil-

ity, making them valuable in various applications such as food

packaging, bioimaging, drug delivery, and cell therapy. CDs

synthesized from biomass sources have been found to possess

even lower toxicity compared to those prepared using tra-

ditional methods.

157,158

Feng et al. utilized winter melon as a carbon source to

prepare nitrogen-doped CDs (N-CDs) through a one-step

hydrothermal method.

159

These N-CDs were eﬀectively

employed as bioimaging agents for hepG2 cells (human liver

hepatocellular carcinoma). To evaluate the biocompatibility of

the synthesized N-CDs, a CCK-8 assay was performed on

hepG2 cells. Results showed that even at concentrations

ranging from 0 to 0.8 mg mL

−1

, the cell viability remained

unaﬀected for 24 hours, exceeding 90%. These findings indi-

cate the absence of significant toxicity and the favorable cyto-

compatibility of N-CDs with hepG2 cells.

Ramasamy et al. employed a hydrothermal method to syn-

thesize plant-derived CDs from Prosopis juliflora leaves extract

(PJ-CDs) for biological imaging of Caenorhabditis elegans.

160

The biological imaging study using PJ-CDs revealed no signifi-

cant reduction in cell viability, even at concentrations as high

as 1 mg ml

−1

. Additionally, no morphological changes were

observed after incubation with PJ-CDs, indicating the low tox-

icity and excellent biocompatibility of PJ-CDs.

While CDs prepared using traditional organic solvents as

carbon sources also exhibit low toxicity, their addition concen-

tration and cell survival rate fall short in comparison to

biomass-derived CDs. Bi et al. synthesized red-emissive CDs

(R-CDs) through a solvothermal method, employing citric acid

as the carbon source, N,N-dimethylformamide as the nitrogen

source, and formamide as the solvent.

161

The cytotoxicity of

R-CDs was evaluated using the classical MTT method. The

results demonstrated that even at a high concentration of

300 μgmL

−1

, the cell survival rate remained at 80%.

Nonetheless, the maximum concentration achieved by R-CDs still

falls short compared to the CDs derived from biomass sources.

In summary, the low toxicity and excellent biocompatibility

of CDs make them highly promising for a wide range of appli-

cations. Biomass-derived CDs oﬀer even greater advantages in

terms of reduced toxicity. Further research and development

in this area hold significant potential for sustainable and safe

material design.

5. Various application of polymer/

CDs composites

The incorporation of CDs into polymer matrices has emerged

as a promising strategy to enhance the properties of polymer-

based materials. This is attributed to the availability of abun-

dant surface functional groups on the polymers, which can

interact with the functional groups on the surface of CDs to

form strong interfacial bonding. As a result, Polymer/CDs com-

posites have found widespread applications in diverse fields

including optics, electricity, chemistry and biology. Recently,

there has been growing interest in using waste-derived CDs as

fillers for composite materials. Several studies have demon-

strated that the performance of CDs obtained from biomass is

comparable to that of conventionally prepared CDs, while

oﬀering the added advantage of being more environmentally

sustainable.

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5.1 Optical applications

The optical properties of CDs have generated significant

research interest due to their potential for a wide range of appli-

cations. The fluorescence spectra of CDs have been shown to

exhibit superior performance and photoluminescence mecha-

nisms compared to both semiconductor quantum dots and

organic dyes. CDs also display excitation-dependent photo-

luminescence and have been found to possess high QYs (>80%)

in contrast to inorganic semiconductor quantum dots.

162–164

Recently, researchers have utilized CDs derived from waste

biomass materials to prepare polymer composites with excellent

optical properties. This approach not only addresses urban pol-

lution challenges but also has the potential to significantly

impact the optical field. For example, Sun et al.

165

successfully

synthesized CDs from natural lignocellulose via the hydro-

thermal method and incorporated them into PVA to prepare

PVA/CDs film. The CDs exhibited well-dispersed in the PVA

matrix, overcoming issues of aggregation-induced fluorescence

quenching, thus highlighting the potential of CDs-based com-

posites in intelligent applications.

In addition to the work by Sun et al., several studies have

investigated the application of PVA/CDs composites in the

optical field. Roza et al.

166

prepared CDs using cassava juice by

microwave-assisted carbonization and blended them with PVA

to produce luminescent films using an ultrasonic-assisted

melt blending method. The CDs retained their optical pro-

perties after incorporating to the PVA matrix, and did not

disrupt the internal structure of PVA. The PVA/CDs composite

materials were prepared into thin films and the film had the

best fluorescence intensity and lowest absorbance, but it had

decreased absorbance compared with CDs and PVA/CDs com-

posite solutions. Additionally, due to hydrogen bonds formed

between PVA and CDs, the CDs were uniformly dispersed in

the PVA matrix. It is therefore not surprising that composite

materials are widely used in the optical field. Eskalen et al.

167

used waste corn husk as a carbon source to prepare CDs by the

hydrothermal method, and compounded CDs with TiO

and

PVA to obtain a PVA/TiO

/CDs composite film. The addition of

CDs decreased the thermal stability, permeability, and band

gap of the composite film, but increased its photocatalytic

degradation ability to rhodamine B. As a result, CDs enhanced

the photocatalytic eﬃciency of polymer nanocomposites sig-

nificantly. Xu et al.

168

synthesized CDs from cyanobacteria

using the hydrothermal method and combined them with PVA

and cellulose nanofibers (CNF) to create an optical-film

materials with good photoluminescence function. The func-

tional groups of the prepared CDs/CNF/PVA films were similar

to those of CDs and did not react to produce new functional

groups. As the CDs content increased, the emission intensity

of the composite film gradually decreased. Tests of water resis-

tance demonstrated that the addition of CDs increased the

water resistance of the product. Optical resistance research has

shown that composites with a higher content of CDs have

better optical properties and could be used as soft packaging

materials, anti-counterfeiting materials, UV/IR optical barriers

etc. (Fig. 8). CDs and CNF were made from biomass sources

such as cyanobacteria and coconut petioles, and the addition

of CDs led to a remarkable improvement in various properties.

Chen et al.

169

used spoiled milk to synthesize CDs by a one-

step hydrothermal method, and mixed CDs with PVA to obtain

fluorescent polymer composite films. The composite films

exhibited good flexibility and transparency under visible light

and blue light characteristics under ultraviolet light. The pre-

pared CDs/PVA films had potential applications in diﬀerent

fields, such as solid-state fluorescence thin film and intelligent

packaging materials.

CDs can be used to prepare composites with desirable

optical properties by blending with various polymers. Das

et al.

synthesized double-doped (Mg and N co-doped) CDs

with high luminescent through microwave-assisted pyrolysis

using acid bean as a carbon source (Fig. 9). The resulting com-

posites were prepared by solution casting with polyvinyl pyrro-

lidone (PVP). The polymer composite film exhibited bright

luminescence under UV light and remained colorless and

highly transparent under visible light. Polymer composite

films are extensively employed in various light-emitting appli-

cations, such as flexible LED displays, diode light-emitting

devices.

Overall, the utilization of CDs and polymers is a promising

approach to enable the functionalization of polymers by lever-

aging the exceptional optical properties of CDs while simul-

taneously achieving favorable dispersion of CDs within the

polymer matrix. These advantages have significantly extended

the application scope of CDs in various domains.

5.2 Electrical applications

The broad range of applications for diﬀerent polymers is

limited by their electrical properties compared to traditional

conductors. However, CDs, as zero-dimensional materials,

possess unique photoelectric properties. By combining CDs,

which can be synthesized through green and environmentally

friendly methods, with a polymer matrix, the electrical appli-

cation of polymers can be greatly enhanced. In recent years,

CDs have primarily been used in composites with polyacryloni-

trile (PAN), polyaniline (PANI), or polypyrrole (PPy) to develop

batteries and supercapacitors or in composites with PVDF to

create films with high conductivity.

Oskueyan et al.

170

have demonstrated the successful syn-

thesis of high eﬃciency supercapacitor electrode materials

using polyaniline–carrot-derived CDs/polypyrrole–graphene

nanocomposites (PCPG) (Fig. 10a). The CDs were prepared

from carrots as the raw materials and compounded with poly-

mers to form the PCPG composite. The electrochemical per-

formance of the PCPG was evaluated, and the results indicated

that polyaniline–(10%) carrot-derived CDs/PCPG exhibited the

best performance with a maximum specific capacitance of 396

−1

at 1 A g

−1

. The composite also showed remarkable stabi-

lity, retaining 62% of the initial capacitance at a current

density of 5 A g

−1

and 65% of the initial capacitance after 1000

cycles. Arthisree et al.

171

have successfully extracted CDs from

rice flour and prepared composite materials with PAN/PANI

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polymer matrix (Fig. 10b). The maximum specific capacitance

of composite system (PAN/PANI@G-1.5 wt%) was 589.2 F g

−1

−2

at 0.67 A g

−1

. The optimal composite system was

designed as a prototype battery and utilized in electronic and

portable devices. The authors also prepared CDs from rice

flour by the hydrothermal method

172

and incorporated them

based composite’s light transmittance to the light at wavelength between 200 and 2500 nm; (b) UV light; (c) infrared light; (d) visible light.

Fig. 9 Schematic representation of the synthesis and applications of dually doped CDs (DC-dots) (reproduced from ref. 47 with permission from

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into a ternary composite nanomaterial {polyvinylbutyral (PVB)/

PANI/poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrene

sulfonate (PSS)

CDs} was prepared by incorporating the best

loading concentration of PVB and CDs into PANI/PEDOT:PSS.

The electrochemical impedance spectroscopy of the composite

showed that the equivalent series resistance and RCT values

were significantly lower than those of the original system. The

composite (PVB/PANI/PEDOT:PSS + CDs 1 wt%) exhibited a

higher pseudo capacitance value (4992.1 F g

−1

−2

at 0.14 A

−1

) than pure polymer and other PANI-based materials

reported. The potential-time response did not change signifi-

cantly when the composite was continuously charged and dis-

charged for 3 h. Overall, this new PVB-based ternary polymer–

CDs composite demonstrated improved pseudocapacitance

performance and functional stability, making it an excellent

choice for a wide range of electronic tunable applications.

de Oliveira et al.

173

have prepared flexible supercapacitor

electrodes by incorporating CDs and graphene nanosheets

into eggshell membranes (ESM) by an ultrasonic method, fol-

lowed by polymerizing polypyrrole on the ESM substrate. The

flexible electrode demonstrated excellent area-specific capaci-

tance, and the synergistic eﬀects of polypyrrole, CDs, and

graphene nanosheets were conducive to high-performance

devices under diﬀerent bending states, with good capaci-

tance retention (96.3%) at high current density (10 mA

−2

). Before the polymerization of polypyrrole, the mechan-

ical and electrochemical properties of the flexible electrodes

could be modified by adding carbon derivatives into a flex-

ible matrix.

5.3 Chemical detection application

CDs are a promising class of luminescent materials that

display high sensitivity and selectivity to chemical substances.

The luminescence properties of CDs can be greatly influenced

Fig. 10 Diagram showing the the formation of polyaniline–carrot derived CDs/polypyrrole–graphene (PCPG) nanocomposite (a) (reproduced from

the pseudocapacitance activity of the PAN/PANI@green synthesised graphene quantum dot (G-CDs) nanocomposite (b) (reproduced from ref. 171

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by various ions and compounds. Recent research has focused

on utilizing CDs in conjunction with molecularly imprinted

polymers (MIPs) to create fluorescent probes with specific

functions for chemical detection. Chen et al.

174

successfully

synthesized fluorescent CDs using mango peel as the carbon

source and mixed it with MIPs to obtain CDs@MIPs, which

could selectively capture the target mesotrione through

specific interactions between the target compound and the

recognition cavity. Similarly, Ensafi et al.

175

synthesized CDs

using rosemary leaves as the carbon source and modified

them with MIP to create fluorescent optical sensors for detect-

ing thiabendazole (TBZ) in fruit juice. The detection range was

0.03–1.73 μgmL

−1

, and the detection limit was 8 ng mL

−1

Their work provided a simple and low-cost method for detect-

ing TBZ with a detection range of 0.03–1.73 μgmL

−1

and a

detection limit of 8 ng mL

−1

. Rezaei et al.

176

used cedar as the

carbon source to prepare green source CDs (GSCDs) and

coated a layer of silica film on the surface of GSCDs to create

MIPs–GSCDs, which showed specificity to phenobarbital, an

epileptic drug. The potential application of this fluorescence

sensor was demonstrated by determining phenobarbital in

human plasma samples.

In recent studies, CDs have been explored for their poten-

tial in the preparation of composite materials for selective ion

detection. One such study, conducted by Issa et al.,

177

involved

the extraction of CDs from hydroxymethyl cellulose-rich

hollow palm fruits, which were subsequently mixed with poly-

vinyl alcohol (PVA) to prepare a composite membrane with

remarkable chemical selectivity (Fig. 11a). The resulting mem-

brane demonstrated exceptional eﬃcacy in removing chro-

mium ions from water quality, with a removal rate as high as

91%. Moreover, the feasibility of utilizing the composite mem-

brane in tap and drinking water was also evaluated, with prom-

ising removal eﬃciency.

In addition to preparing fluorescent probes with MIP and

selective composite films with PVA, Nayak et al.

178

proposed a

novel approach for ion detection involving the fabrication of

composite hydrogels with PVP (Fig. 11b). Fluorescent CDs

were synthesized from lemon juice via the hydrothermal

method and then mixed them with PVP to prepare hydrogel.

The resulting PVP–CDs hydrogel exhibited strong hydrogen

bonding and induction eﬀects, as well as π–πinteraction

between CDs and PVP main chain, enabling eﬃcient adsorp-

tion of dyes in water and eﬀective elimination of Gram-positive

and Gram-negative bacteria in polluted water. The hydrogel

was also found to be repaired by mild pickling and could be

reused up to four times.

5.4 Biological applications

CDs, particularly those extracted from biomass, are attractive

for various biomedical applications due to their non-toxic,

harmless and biocompatible nature. Therefore, it has a

number of obvious advantages over other materials when it

comes to biological probes, antibacterial sterilization, and bio-

logical detection. Maruthapandi et al.

179

synthesized nitrogen-

doped CDs (N@CDs) from bovine serum protein by hydro-

thermal method and used them as an initiator to polymerize

dopamine and form polydopamine (PDA)–N@CDs. The

obtained (PDA)–N@CDs showed good antibacterial properties,

and the composite colloid at low concentration was able to

Fig. 11 Diagram showing the preparation of CDs and PVA–CDs ﬁlms from CMC waste (a) (reproduced from ref. 177 with permission from MDPI,

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completely remove Methicillin-resistant Staphylococcus aureus

(MRSA) without additional nanoparticles. The PDA captured

the microorganisms in order to interfere with microbial

growth and mitosis. Because of the strong adhesion of (PDA)–

N@CDs to the bacterial membrane, nutrients and wastes were

limited from diﬀusing outside the cytoplasm matrix, which

resulted in cell death and antibacterial activity.

Deng et al.

180

used papaya peel as a carbon source to syn-

thesize blue high-fluorescence CDs in a one-step hydrothermal

method and combined them with silver ions to form CDs-

loaded silver nanoparticle (AgCDs) (Fig. 12). Subsequent to the

synthesis of polymer microspheres via coordination polymeriz-

ation, electrostatic adsorption was utilized to combine the

negatively charged AgCDs and positively charged PEI-modified

microspheres. The resultant AgCDs@polymer nanospheres

(PNS)–polyethylenimine (PEI) exhibited desirable biocompat-

ibility and conductivity, and were employed as a sandwich

electrochemical luminescence immunosensor for the detec-

tion of human chorionic gonadotropin (HCG). The immuno-

sensor demonstrated low detection limit, excellent stability,

high reproducibility, and superior selectivity under optimal

experimental conditions. Furthermore, the immunosensor was

successfully utilized to determine HCG levels in human

serum.

Hakkarainen et al.

181

prepared reduced graphene oxide

carbon nanoparticles from cellulose and added caﬀeic acid

(CA) to prepare reduced nanographene oxide (r-nGO-CA) for

further enhancement. Poly (ε-caprolactone) (PCL) and

r-nGO-CA composite films were also prepared and showed that

r-nGO-CA and r-nGO were eﬀective enhancers that significantly

increased storage modulus. The creep resistance of the system

was improved after CDs were added. Osteoblasts did not

exhibit toxic eﬀects and the material retained PCL’s good bio-

compatibility, suggesting that it may be useful for bone tissue

engineering.

Chen et al.

182

synthesized CDs using lactose as a carbon

source via a one-step hydrothermal method. Then, used N,N-

methylene bisacryamide (MBA) as a cross-linking agent and

CDs as a nanofiller to prepare CDs/polyacrylamide (PAM) com-

posite hydrogel by cross-linking with monomer acrylamide

(AM). The CDs/PAM hydrogel exhibited bright blue fluo-

rescence emission, which could be used as an alternative

material for the optical visualization of cartilage in biomedical

engineering.

6. Conclusions and future

prospective

Carbon dots (CDs) have emerged as a highly promising nano-

material due to their low toxicity, excellent optical properties,

and good biocompatibility. In this comprehensive review, we

have explored the synthesis, structure, and properties of CDs

derived from biomass and plastic waste, as well as their appli-

cation in polymer composites. The utilization of biomass

waste sources, including plants, animals, and microorganisms,

Fig. 12 A process for preparing polyethylenimine (PEI) modiﬁed polymer nanospheres (PNS) (a), papaya peel–synthesized CDs (b), and immobiliz-

ation of Ab2 on AgCDs@PNS-PEI and construction of the immunosensor (c) (reproduced from ref. 180 with permission from Springer Nature, copy-

right 2020).

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has been extensively investigated for the synthesis of CDs.

These carbon sources oﬀer numerous advantages such as

recyclability, ease of extraction, and environmental compatibil-

ity, making them particularly suitable for biomedical appli-

cations. Additionally, the utilization of plastic waste as a

carbon source for CD synthesis not only presents an innovative

solution to address plastic pollution but also oﬀers cost-

eﬀective production of CDs.

CDs demonstrate remarkable performance when incorpor-

ated into polymer composites, owing to their unique features

such as photoluminescence and surface modification capabili-

ties. Exciting applications in areas like nano-light-emitting

devices, supercapacitor electrodes, and ion detection have

been demonstrated. However, it is important to note that chal-

lenges and issues persist in this field (Fig. 13).

In terms of synthesis methods, it is evident that many

current approaches for CDs synthesis using biomass and

plastic waste as carbon sources yield relatively low quantities,

limiting their scalability for large-scale industrial production.

Further research and development eﬀorts are necessary to

overcome these limitations and enhance the production

eﬃciency of CDs.

Regarding the carbon sources employed, it is worth

noting that the range of plastics suitable for CD synthesis is

comparatively limited when compared to biomass waste

sources. While PP, PS, and PET have been extensively

studied, the feasibility of employing other plastic wastes as

carbon sources for CDs synthesis requires additional

investigation.

Furthermore, the application of waste as carbon sources in

CDs synthesis for polymer composites is currently constrained,

and a comprehensive understanding of the mechanisms gov-

erning the behavior of CDs in polymer matrix is yet to be fully

elucidated. Notably, the challenge of nanoparticle agglomera-

tion when introduced into the polymer matrix can lead to a

decrease in material performance. To address this issue, we

propose leveraging the rich functional groups present on the

surface of CDs. Strengthening the chemical interaction

between CDs and polymers enhances their integration within

the polymer chains. Additionally, CDs can directly participate

in the polymerization process by combining with monomers,

enabling precise control over the molecular structure and

ensuring the accurate incorporation of CDs into the polymer,

ultimately leading to the development of composite materials

with exceptional performance.

In conclusion, this review underscores the significant

potential of CDs derived from biomass and plastic waste.

While challenges exist, the continuous exploration of synthesis

methods, diversification of carbon sources, and deeper under-

standing of the behavior of CDs in polymer composites will

pave the way for their widespread application and realization

of their full potential.

Fig. 13 SWOT analysis of biomass and plastic wastes-derived CDs and their applications.

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Abbreviations

CDs Carbon dots

SWNTs Single-walled carbon nanotubes

CVD Chemical vapor deposition

MWCNTs Multi-walled carbon nanotubes

SALDI-MS Surface-assisted laser desorption/ionization-mass

spectrometry

EDA Ethylenediamine

CS Chitosan

PVA Polyvinyl alcohol

TC Tetracycline

NSP-CDs Nitrogen, sulfur, and phosphorus co-doped CDs

LEDs Light-emitting diodes

EPS Expanded polystyrene

PP Polypropylene

PET Polyethylene terephthalate

PEI Poly (ethylene imine)

PE Polyethylene

PA Polyamide

PSU Polysulfone

PPi Pyrophosphate anions

HMF 5-Hydroxymethylfurfural

PLA Polylactic acid

PPC Pyrolytic carbon

CNF Cellulose nanofibers

PVP Polyvinyl pyrrolidone

N-CDs Nitrogen-doped CDs

C Carbon

N Nitrogen

QY Quantum yield

UV Ultraviolet

PAN Polyacrylonitrile

PANI Polyaniline

PPy Polypyrrole

PCPG Polypyrrole–graphene nanocomposites

PVB Polyvinylbutyral

PEDOT Poly(3,4-ethylenedioxythiophene)

PSS Polystyrene sulfonate

ESM Eggshell membranes

MIPs Molecularly imprinted polymers

TBZ Thiabendazole

GSCDs Green source CDs

N@CDs Nitrogen-doped CDs

PDA Polydopamine

MRSA Methicillin-resistant Staphylococcus aureus

AgCDs CDs-loaded silver nanoparticle

PNS Polymer nanospheres

HCG Human chorionic gonadotropin

CA Caﬀeic acid

r-nGO-CA Reduced nanographene oxide

PCL Poly (ε-caprolactone)

MBA N,N-Methylene bisacryamide

PAM Polyacrylamide

AM Acrylamide

PJ-CDs Plant-derived CDs from Prosopis juliflora leaves

extract

R-CDs Red-emissive carbon dots

O Oxygen

P Phosphorus

FCDs Fluorescent carbon dots

Ag Silver

AgClO

Silver chloride

Conﬂicts of interest

The authors declare no competing interests.

Acknowledgements

This work was financially supported by the National Natural

Science Foundation of China (No. 52173046), Natural Science

Foundation of Zhejiang Province (No. LZ21E030002), Science

and Technology Planning Project of Shenzhen (No.

JCYJ20190813153409172), and the Ningbo Scientific and

Technological Innovation 2025 Major Project (No. 2020Z097).

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Synthesis, Characterization, and Infrared Blocking Efficiency of Polyvinyl Alcohol Composites Filled with Cadmium Sulfide and Zinc Sulfide NPs

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This research used electroblown nanofibers made from a blend of polyvinyl alcohol (PVA) and sodium alginate (SA), activated by carbon dots obtained from bovine blood (BCDs), to create a new solution for active food packaging. The nanoscale size of BCDs revealed low toxicity on chondrocyte and lung cell types, ensuring safety in biomedical applications. The insertion of BCDs into the nanofiber matrix lowered surface hydrophobicity but strengthened water solubility and mechanical strength. These nanofibers demonstrated exceptional antioxidant activities and exhibited inhibitory effects against both E. coli and S. aureus . Conducting real food trials on chicken fillets disclosed the large reduction in viable bacteria count and lipid oxidation, consequently extending the durability of the product. Noteworthy is the eco-conscious element of these nanofibers; in soil burial experiments, the packing material disintegrated within a mere 57 days, underlining its negligible environmental impact. These environmentally friendly electroblown nanofibers, supplemented with BCDs, represent a viable future for sustainable food packaging. In seamlessly coupling food safety with eco-friendly methods, this methodology offers a comprehensive and practical answer for the packaging industry.

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ENVIRON RES

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Carbon dots (CDs) or carbon quantum dots (CQDs) have emerged as rising stars in the carbon family due to their diverse applications in various fields. CDs are spherical particles with a well-distributed size of less than 10 nm. Functional CDs are promising nanomaterials with low toxicity, low cost, and enormous applications in the field of bioimaging, optoelectronics, photocatalysis, and sensing. Plastic is non-biodegradable and hazardous to the environment, however extremely durable and used in abundance. During the COVID-19 pandemic, the use of plastic waste, particularly masks, goggles, face shields, and shoe cover, has increased tremendously. It needs to be recycled in a productive way as plastic wastes take hundreds or thousands of years to degrade naturally. The conversion of plastic waste into magnificent CDs has been reported as one of the key alternatives for environmental sustainability and socio-economic benefits. In this review, synthetic routes for the conversion of plastic wastes into CDs utilizing hydrothermal, solvothermal, pyrolysis, flash joule heating, and characterization of these CDs using different techniques, such as Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and transmission electron microscope, have been discussed. Furthermore, potential applications of these plastic-derived CDs in sensing, catalysis, agronomics, and LED lights are summarized herein.

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Disposable medical masks are widely used to prevent respiratory infections due to their ability to block virus particles from entering the human body. The coronavirus disease 2019 (COVID-19) pandemic highlighted the importance of medical masks, leading to their widespread use around the world. However, a large number of disposable medical masks have been discarded, some carrying viruses, which have posed a grave threat to the environment and people's health, as well as wasting resources. In this study, a simple hydrothermal method was used for the disinfection of waste medical masks under high-temperature conditions as well as for their transformation into high-value-added carbon dots (CDs, a new type of carbon nanomaterial) with blue-emissive fluorescence, without high energy consumption or environmental pollution. Moreover, the mask-derived CDs (m-CDs) could not only be used as fluorescent probes for sensing sodium hydrosulfite (Na2S2O4), which is widely used in the food and textile industries but is seriously harmful to human health, but also be used for detecting Fe3+ which is harmful to the environment and human health due to its wide use in industries.

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Article

Nov 2022

PET-based nitrogen and phosphorus-doped Carbon Dots (NP-CDs) was prepared by solvothermal method with PET waste as raw materials. In terms of structure, NP-CDs were spherical structures with an average particle size of 2.8 nm, and its surface contains amino groups. The as-prepared NP-CDs exhibit excitation-independent emission properties from 300 nm to 430 nm, and the optimal excitation and emission wavelength was 340 nm and 460 nm. In terms of application, the as-prepared NP-CDs could be used to a pH sensor in the range of 2.3 to 12.3. When NP-CDs were dispersed in polyvinyl alcohol (PVA) matrix, excellent light blocking films(LBFs) for UV and blue light were obtained. When the ratio of NP-CDs is 14%, it still does not affect the line of sight, and the barrier rate to UV(380 nm) and blue(430 nm) light is as high as 90% and 80%. In addition, the WLED device with a colour coordinate of (0.29,0.32) and a colour temperature of 8022 K can be obtained with the combination of NP-CDs dispersed in PVA matrix and 395 nm LED chips.

Glutathione assisting the waste tobacco leaf to synthesize versatile biomass-based carbon dots for simultaneous detection and efficient removal of mercury ions

Article

Dec 2022

High-efficient conversion of the waste biomass into eco-friendly novel bifunctional materials for selective monitoring and effective removal of mercury ions (Hg²⁺) is of great significance to achieve the environment sustainable development, but has not attracted enough attention. Herein, we exploited glutathione (GSH) assisting waste tobacco leaf as the biomass-related precursors to successfully yield a novel red fluorescence emission biomass-based carbon nanodots (CDs) with a remarkably large Stokes shift of ∼ 232 nm through one-pot hydrothermal method. Owing to the synergistic Hg-S bond and the coordination effects of the Hg²⁺ to the abundant –COOH, –NH2 and –OH groups on the surface of CDs, this proposed biomass-based CDs showed a red fluorescence decrease response for Hg²⁺ with a high selectivity and a lower detection limit of 0.45 μM in environmental water samples. Furthermore, this binding endowed this proposed biomass-based CDs possessed a high removal efficiency of above 99.4% for Hg²⁺ as well. This work showed a novel insight into recycling the waste tobacco leaf into novel, economic, efficient, reliable, and eco-friendly biomass-based CDs for simultaneously precise traceability and high-efficient removal of Hg²⁺ pollution in environmental matrices.

Functional Carbon Dots Derived from Biomass and Plastic wastes

Abstract

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