Hydrothermal synthesis of biomass-derived CQDs: Advances and applications

2.1 Plant-based carbon sources

Plant-based carbon sources have garnered significant attention in biomass-based CQDs research due to their wide availability, low cost, and renewability. These carbon sources not only allow for the effective utilization of agricultural and industrial waste, reducing environmental impact, but can also be extracted from edible plants. Based on their sources and applications, plant-based carbon sources can be further divided into plant waste-based carbon sources and edible plant-based carbon sources.

2.1.1 Plant waste-based carbon sources

Plant waste offers significant advantages as a carbon source due to its wide availability, low cost, and environmental friendliness. It is typically rich in lignin, cellulose, and other rigid, hard-to-degrade components, which not only serve as abundant carbon sources but also impart unique electronic properties during the hydrothermal synthesis of CQDs. In particular, lignin and lignin-like structures, under appropriate degradation conditions, can promote electron conjugation, thereby enhancing the photostability of CQDs. The common feature of plant waste is its richness in complex organic substances (such as lignin and cellulose), which facilitate CQD synthesis but may also influence their optical properties. By optimizing reaction conditions and refining the hydrothermal synthesis process, the QY of waste-derived CQDs can be effectively improved, while leveraging their unique structural characteristics to enhance photostability and expand their application potential. Table 2 summarizes the current status of research on the hydrothermal synthesis of CQDs from plant waste-based carbon sources.

Table 2

Hydrothermal synthesis of CQDs from plant waste-based carbon sources

Carbon source	Synthetic conditions	Size (nm)	QY (%)	Ref.
Kenaf	200°C, 24 h	—	76.12	[29]
Dwarf banana peel	200°C, 24 h	2.5–5.5	23	[37]
Banana peel	200°C, 24 h	3.6	20	[38]
Citrus limetta peel	185°C, 24 h	3	—	[39]
Pomelo peel	200°C, 3 h	2–4	6.9	[40]
Orange peel	180°C, 3 h	3.5–5.5	35.37	[41]
Avocado peel	180°C, 24 h	2.22	—	[42]
Palm kernel shell	160°C, –	4.5	2.4	[43]
Phytic acid	200°C, 4 h	4.03	22	[44]
Eucalyptus leaf (Eucalyptus globulus)	180°C, 24 h	6.33	60.7	[45]
Willow leaf	200°C, 12 h	—	—	[46]
Psidium guajava L. (Guava) leaf	180°C, 5 h	2.9	9.2	[47]
Mango leaf	200°C, 35 min	5–10	16	[48]
Prosopis juliflora leaf	180°C, 5 h	8	7.88	[48]
Apple peel	200°C, 18 h	1.96	9.43	[49]
Lychee shell	180°C, 9 h	—	7.86	[50]
Rice residue	200°C, 12 h	2.7	23.48	[51]
Wheat straw	250°C, 10 h	1.7	9.2	[52]
Corn cob powder	180°C, 12 h	2.54	1.05	[53]
Wolfberry straw	200°C, 24 h	1.7–2.5	59	[54]
Starch fermentation wastewater	180°C, 10 h	4.2	24.5	[55]
Cigarette smoke	180°C, 2 h	6.3	—	[56]
Banana petiole	200°C, 5 h	3.27	8.53	[57]
Poplar wood powder	200°C, 6 h	5–10	—	[58]
Salix wood powder	180°C, 12 h	2.19	—	[59]
Pistachio shells	200°C, 1 h	4	15	[60]
Magnolia flower	200°C, 12 h	Purple flower: 3.9	Purple flower: 11.9	[61]
		White flowers: 10.1	White flowers: 10.4
Parthenium hysterophorus leaf	140°C, 1 h	4.4	10.2	[62]
Rhizome of Acorus calamus	200°C, 6 h	6	15	[63]
Laurel leaf	180°C, 5 h	3.8	49.9	[64]
Kiwi fruit (Actinidia Deliciosa) peel	200°C, 24 h	5	18	[65]
Wasted coffee ground	220°C, 20 h	2.18	—	[66]
Poplar aspen wood powder	200°C, 6 h	3.4–4.6	47.4	[67]
Cherry blossom flowers	200°C, 20 h	4.86	—	[68]
Alkali-soluble xylan	200°C, 12 h	6.92	16.18	[69]
Xylose	180°C, 4 h	3.5–4	6.2	[70]
Cellulose	300°C, 24 h	3.8	10.9	[71]
Oxidized cellulose	200°C, 10 h	1.86	30.3	[72]
Pre-hydrolyzed lignin	220°C, 11 h	5.91	13.5	[73]
Corncob lignin	240°C, 10 h	7.83	46.38	[74]
Alkali lignin	200°C, 12 h	5.05	30.5	[75]

Atchudan et al. [37,38] synthesized CQDs with high QYs of 23 and 20%, respectively, using dwarf banana peels and common banana peels as carbon sources via the hydrothermal method. Figure 3(a) illustrates the detailed synthesis process of luminescent nitrogen-doped CQDs derived from dwarf banana peels, demonstrating that during hydrothermal treatment, the organic molecules and elements in the peels decompose under high temperature and pressure, forming CQDs with luminescent properties. Figure 3(b) presents the plausible formation mechanism of CQDs synthesized from common banana peels, highlighting the decomposition of organic molecules in the peels, which contributes to the high QY of the resulting CQDs. These illustrations clearly reveal the influence of different banana peel sources on CQD synthesis and underscore the critical role of the hydrothermal method in the synthesis process. Han et al. [41] achieved a QY of 35.37% using orange peel as a carbon source, completing the synthesis in just 3 h, making it highly efficient and promising. Xing et al. [44] used phytic acid to produce CQDs with a QY of 22%. The high QY is attributed to the multifunctional groups and doping effects of phytic acid. Phytic acid molecules contain abundant phosphate and hydroxyl groups, which participate in CQDs formation during hydrothermal synthesis and create a wealth of surface functional groups. Phosphorus from phytic acid can be doped into the CQDs structure to form phosphorus-doped carbon quantum dots (P-CQDs), significantly enhancing their optical properties and QY. Johny et al. [45] synthesized CQDs from eucalyptus leaves, achieving an ultra-high QY of 60.7%. This was attributed to the high carbohydrate, cellulose, and lignin content in eucalyptus leaves, which are also rich in hydrocarbon groups and organic compounds like phenols and alcohols – functional groups that facilitated surface functionalization of the CQDs during synthesis [76]. Xu et al. [54] optimized reaction conditions, including reaction time, temperature, and wolfberry straw concentration (7.5 g/L), to achieve a QY of 59% by heating at 200°C for 24 h. Man et al. [55] prepared CQDs with a QY of 24.5% using starch fermentation wastewater, demonstrating not only the reuse of waste materials but also a reduction in environmental pollution. During the synthesis, residual amino acids and other organic compounds in the wastewater formed reactive groups like –COOH and –C═O on the CQDs surface, enhancing their fluorescence properties. Long et al. [64] synthesized green carbon quantum dots (G-CDs) with a remarkable QY of 49.9% using laurel leaves as the carbon source, o-phenylenediamine (OPD) as the nitrogen source, and N,N-dimethylacetamide (DMAC) as the solvent through a one-step hydrothermal method. Figure 3(c) illustrates the synthesis process of these green CQDs: laurel leaves, OPD, and DMAC were mixed and reacted under hydrothermal conditions. Structural characterization revealed the presence of abundant nitrogen-containing functional groups on the surface of the CQDs, which played a crucial role in enhancing their PL properties, significantly improving both QY and fluorescence intensity. This study provides a novel approach for the synthesis of green CQDs and highlights the impact of different raw materials and reaction conditions on CQD performance. Gong et al. [67] synthesized water-soluble CQDs with a QY of 47.4% from poplar wood powder, and found that these CQDs were rich in –OH, NH₃, and –COOH functional groups. Liu et al. [72] found that the chemical accessibility and reactivity of cellulose were significantly improved after oxidation treatment. Cellulose also provides additional functional groups, contributing to improved CQD QYs. Yang et al. [73] synthesized sulfur-doped carbon quantum dots (SCQDs) using hydrolyzed lignin as a carbon source. These SCQDs exhibited excellent fluorescence stability in acidic environments, with a QY of 13.5%. The sulfur doping introduced numerous sulfur atoms on the surface of the CQDs, forming new surface states that promoted radiative recombination, thereby increasing the QY. Yang et al. [74] used corn kernel lignin as a carbon source and applied a co-doping technique with magnesium and nitrogen, which significantly increased the CQD QY to 46.38%. This dual doping improved CQD optical properties and broadened their emission spectrum, making them suitable for various optical applications. Zhu et al. [75] synthesized CQDs with a QY of 30.5% using alkali lignin as the carbon source. The high yield was attributed to the aromatic ring structures in alkali lignin, which contains numerous aromatic rings and oxidized branched chains rich in unsaturated bonds (e.g., C═C and C═O), making it an ideal carbon source for CQDs synthesis.

Figure 3

(a) Illustration of the synthesis procedure of fluorescent HN-CDs from dwarf banana peel. Reprinted from Atchudan et al. [37], copyright (2020), with permission from Elsevier. (b) Plausible formation mechanism of CQDs from the banana peel by hydrothermal process. Reprinted from Atchudan et al. [38], copyright (2021), with permission from Elsevier. (c) Schematic diagram of CQDs synthesized using Laurel leaves as a carbon source. Reprinted from Long et al. [64], Copyright (2023), with permission from Elsevier.

2.1.2 Edible plant-based carbon sources

The chemical composition and structural characteristics of edible plants have a significant impact on the QY of CQDs. Edible plants rich in nitrogen- or oxygen-containing functional groups provide more active sites, which contribute to enhanced optical properties. Additionally, plants with abundant aromatic rings and flavonoids improve the conjugation of electrons, thereby optimizing the light absorption and emission properties of CQDs. Therefore, selecting suitable edible plant materials and optimizing their chemical composition and structural features can effectively regulate the optical performance of CQDs. Table 3 summarizes the current research on hydrothermal synthesis of CQDs from edible plant-based carbon sources.

Table 3

Hydrothermal synthesis of CQDs from edible plant-based carbon sources

Carbon source	Synthetic conditions	Size (nm)	QY (%)	Ref.
Lentinus polychrous Lèv	200°C, 12 h	6	6.4	[77]
Northern Shaanxi potatoes	200°C, 12 h	5	16.96	[78]
Traditional Chinese medicine Codonopsis pilosula	180°C, 3 h	1.6–4.6	—	[79]
Tobacco	200°C, 3 h	2.14	27.9	[80]
Honeysuckle	200°C, 9 h	1.76	—	[81]
Pakchoi (Brassica rapa subsp. chinensis)	200°C, 10 h	3	10.28	[82]
Highland barley	200°C, 24 h	5.8	14.4	[83]
Maojian (a kind of famous green teas)	200°C, 3 h	13	12.79	[84]
Rambutan seed	200°C, 20 h	3.07	16.97	[85]
Orange juice	200°C, 11 h	1.86	31.7	[86]
Phyllanthus acidus (P. acidus)	180°C, 8 h	4.5	14	[87]
Lime (Citrus aurantifolia)	180°C, 6 h	—	—	[88]
The unripe fruit of Prunus persica (peach)	180°C, 5 h	8	15	[89]
Papaya	200°C, 5 h	2–6	18.98	[90]
Pyrus pyrifolia (pear) fruit	180°C, 6 h	2	10.8	[91]
Lycium ruthenicum	180°C, 9 h	4.05	10.63	[92]
Chionanthus retusus (C. retusus) fruit	180°C, 6 h	5	9	[93]
Seville orange (Citrus aurantium)	130°C, 12 h	4.8	13.3	[94]
Ganoderma lucidum	180°C, 6 h	4.09	6.26	[95]
Tylophora indica plant	180°C, 10 h	4.3	16.1	[96]
Mopan persimmons	150°C, 4 h	3.18	8.39	[97]
Chinese herbal residues	200°C, 8 h	2.8	4	[98]
Zanthoxylum bungeanum	180°C, 5 h	B-CDs: 3.5	B-CDs: 13.1	[99]
		G-CDs: 4	G-CDs: 11.2
		R-CDs: 7.5	R-CDs: 9.8
Aloe carazo leaf	200°C, 12 h	5.64	21.4	[100]
Tapioca starch	175°C, 1 h	9.54	1.167	[101]
Pine pollen	200°C, 20 h	3.77	—	[102]
Citron fruit	180°C, 7 h	4	34.5	[103]
Chlorogenic acid	230°C, 2 h	2–5	—	[104]
Ginsenoside Rb1	200°C, 24 h	7.14	—	[105]
Quinoa saponin	200°C, 10 h	2.25	22.24	[106]
Proanthocyanidin	200°C, 10 h	9.78	—	[107]
Folic acid	200°C, 5 h	6.8	41.8	[108]
Zucchini	180°C, 6 h	2.5∼3.5	12.19	[109]

Miao et al. [80] synthesized CQDs with a QY of 27.9% using tobacco as the carbon source, attributing the high yield to the one-pot hydrothermal synthesis. The abundant carbon content and surface oxygen-containing functional groups, such as carbonyl and hydroxyl, provided stability and favorable optical properties during CQDs formation. Pakchoi (Brassica rapa subsp. chinensis), commonly known as baby bok choy, is a leafy vegetable whose chlorophyll and organic molecules serve as rich precursors for CQDs formation. Cui et al. [82] used pakchoi to prepare CQDs with a QY of 10.28%, enhanced by combining solvent extraction and solvothermal methods, which resulted in a significant single-excitation dual-emission property. Xie et al. [83] synthesized CQDs using highland barley as a carbon source, achieving a high QY of 14.4%. Figure 4(a) illustrates the synthesis process of these green CQDs, where highland barley was pyrolyzed under hydrothermal conditions at elevated temperatures, resulting in the successful production of CQDs with high QY. The carbohydrates and proteins abundant in highland barley were transformed into efficient fluorescence emission centers during the pyrolysis process, significantly enhancing the PL properties of the CQDs. Xu et al. [84] demonstrated excellent fluorescence properties and high QY of CQDs synthesized from Maofeng tea leaves. Figure 4(b) illustrates the formation process of CQDs derived from tea leaves, where hydrothermal treatment successfully converted the natural components of tea leaves into high-yield CQDs. The polyphenols and other organic compounds abundant in tea leaves were transformed into efficient luminescent centers during the thermal process, significantly enhancing the PL properties of the CQDs. These efficient luminescent centers endow the material with broad potential for multifunctional applications, particularly in bioimaging and sensing. Li et al. [86] synthesized CQDs with a QY of 31.7% from orange juice and ethylenediamine, largely due to the abundant surface defects and functional groups. Nitrogen doping effectively disrupted the large conjugated carbon structure, enhancing fluorescence performance. Phyllanthus acidus (P. acidus), a natural material rich in organic acids and antioxidants, is an environmentally friendly and easily accessible carbon source. The polyphenolic compounds in P. acidus aid in the synthesis of CQDs with a QY of 14% [87]. Liao et al. [99] reported a method for synthesizing multicolor fluorescent CQDs using Zanthoxylum bungeanum (Sichuan pepper) as the carbon source. Figure 4(c) illustrates the synthesis process of these multicolor fluorescent CQDs. By varying the nitrogen source, the researchers utilized a hydrothermal method to produce blue, green, and red fluorescent carbon dots (B-CDs, G-CDs, and R-CDs) with stable optical properties and high QYs. The emission wavelengths of these CQDs were 492, 523, and 601 nm, with corresponding QYs of 13.1, 11.2, and 9.8%, respectively. Furthermore, the CQDs demonstrated excellent salt tolerance, photobleaching resistance, and chemical stability, highlighting their broad potential for applications, particularly in the field of anti-counterfeiting fluorescent inks. These properties make them ideal materials for anti-counterfeiting technologies, capable of maintaining stable luminescence under various environmental conditions. Serag et al. [108] synthesized CQDs with a high QY of 41.8% using folic acid as the carbon source. The high QY is due to folic acid’s ability to form CQDs with high crystallinity and uniform particle size under high temperature and pressure in hydrothermal conditions. High crystallinity leads to fewer defects and a more regular lattice structure, reducing non-radiative recombination and energy loss, thereby improving fluorescence performance and QY. Folic acid also contains abundant nitrogen, which can be doped into CQDs during synthesis, further enhancing fluorescence properties and QY.

Figure 4

(a) The preparation process of the N-CDs by the hydrothermal method from Highland barley. Reprinted from Xie et al. [83], open access under the Creative Commons license. (b) Illustration of the formation process of (AA)-enhanced CQDs from green teas by hydrothermal treatment and their multi-function application. Reprinted from Xu et al. [84], copyright (2019), with permission from Elsevier. (c) Synthesis of multicolor fluorescent CDs from Zanthoxylum bungeanum as the carbon source via the hydrothermal method. Reprinted from Liao et al. [99], copyright (2023), with permission from Elsevier.

2.2 Animal-based carbon sources

The choice of carbon source is critical to the optical properties and application potential of CQDs. Animal-based carbon sources differ significantly from plant-based carbon sources in terms of chemical composition and structure. Unlike plant-based carbon sources, animal-derived materials primarily consist of proteins, amino acids, and chitin. These components not only provide nitrogen sources but also react with other components of the carbon source, influencing the surface functional groups and optical properties of CQDs, thereby enhancing their QY and photostability. Compared to plant-based sources, animal-based carbon sources can impart strong nitrogen-doping characteristics to the surface of CQDs due to their rich nitrogen content, which contributes to improved optical performance. Additionally, animal-based carbon sources often exhibit strong surface functionalization capabilities, making them more promising for applications in sensing and catalysis.

The variations in chemical composition and structure among different animal-based carbon sources also result in differences in CQDs’ performance. Generally, animal-derived materials lack the complex cellulose and lignin structures found in plant-based sources. However, their high protein and amino acid content, along with certain specific macromolecules such as chitin and their ability to bind metal ions, allow CQDs synthesized from these sources to exhibit unique advantages in photostability, functionalization, and metal doping.

In summary, animal-based carbon sources possess distinct advantages in nitrogen doping, surface functionalization, and metal ion doping, which significantly influence the optical properties and application potential of CQDs. When selecting an appropriate carbon source, it is essential to consider the impact of its chemical composition and structural characteristics on CQDs’ performance. Table 4 summarizes the current research on hydrothermal synthesis of CQDs from animal carbon sources.

Chen et al. [112] synthesized CQDs from crayfish shells with a QY of 10.68%. Crayfish shells, rich in chitin and protein, are ideal precursors for nitrogen-rich carbon dots. The preparation process is inexpensive and simple, and it effectively utilizes large amounts of crustacean waste, reducing environmental pollution. Hastuti et al. [111] used chicken feathers as a carbon source to synthesize CQDs, achieving an ultra-high QY of 57%. They found that the hydrothermal time and temperature significantly influenced the yield, with the highest yield of 57% obtained after 7 h of reaction at 200°C. Additionally, functional groups formed during hydrothermal treatment, such as C═C, O–H, C–N, C–H, C═O, and C–O–C, and their transformations positively impacted the optical properties of the CQDs. Shen et al. [113] synthesized metal-doped CQDs using horse spleen ferritin as a carbon source, doping the CQDs with Co²⁺, Fe²⁺, Cu²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Zn²⁺, and Gd²⁺. The QYs of the nine resulting samples (including a blank control) ranged from 5.81 to 10.61%, with the Zn-doped CQDs exhibiting the highest QY, indicating that metal-doped CQDs from ferritin possess good optical properties. Narimani and Samadi [124] synthesized novel CQDs using chicken drumsticks as a carbon source via a green, low-cost hydrothermal method without the use of surface passivating agents or chemical reagents. The resulting CQDs exhibited blue fluorescence emission, a high QY of approximately 32.86%, and excellent water solubility. Liu et al. [132] synthesized CQDs from Bombyx mori silk with a QY of 61.1%. These nitrogen-doped CQDs exhibited excellent optical properties and stability, maintaining photostability under strong acids or bases, high salt concentrations, and most organic solvents.

• Interestingly, although both studies used cow milk as a carbon source to synthesize CQDs, differences in synthesis conditions and post-processing steps led to significant variations in QY and size. Kumar et al. [117] used a simple hydrothermal method to carbonize milk components under high temperature and pressure, producing CQDs with an average size of approximately 7 nm and a QY of 38%. In contrast, Han et al. [118] incorporated an additional ethyl acetate extraction step and ultraviolet (UV) irradiation to form complexes with silver nanoparticles, yielding smaller CQDs (1–5 nm). However, the more complex purification and functionalization processes led to a reduction in QY. The two studies also differed in their approach to surface functionalization. The CQDs synthesized by Kumar et al. had stabilized surface functional groups, making them suitable for sensing applications. In contrast, Han et al. introduced amphiphilic functional groups, resulting in silver nano-complexes with antimicrobial properties. Thus, although both studies used the same carbon source, the variations in synthesis and treatment significantly impacted the physicochemical properties and potential applications of the CQDs.

2.3 Microbial-based carbon source

Unlike plant-based and animal-based carbon sources, microbial-based carbon sources, such as yeast powder and yeast cell walls, offer abundant carbon resources due to their complex cell wall structures, which are rich in polysaccharides, proteins, and lipids. Microbial-based carbon sources not only exhibit high biocompatibility but also allow for the optimization of the optical properties of CQDs by adjusting reaction conditions. During the synthesis of CQDs, the polysaccharides and proteins in microbial carbon sources provide ample carbon content, while the lipid components contribute to improved hydrophobicity and surface activity. Moreover, the diversity and complexity of microbial carbon sources enable multiple possibilities for surface modification and the introduction of functional groups in CQDs. Table 5 summarizes the current research on the hydrothermal synthesis of CQDs using microbial-based carbon sources.

Mirseyed et al. [137] demonstrated that yeast cell walls can be used as a carbon source to synthesize CQDs with a QY of 19%. Figure 5 illustrates the composition and structure of yeast cell walls, which consist of inner and outer layers. The inner layer is primarily composed of chitin and β-1,3- and β-1,6-glucans, while the outer layer is made up of mannoproteins and glycoproteins. Yeast cell walls are rich in carbon sources, such as glucans and mannoproteins, which are transformed into the core structure of CQDs during the hydrothermal carbonization process. Additionally, yeast cell walls contain heteroatoms such as phosphorus, nitrogen, and sulfur, which are naturally doped into the CQDs during synthesis. This doping not only modulates the electronic structure of CQDs but also significantly enhances their fluorescence intensity and QY. Furthermore, heteroatom doping optimizes the optical properties of CQDs, improving their biocompatibility and multifunctionality. The surface of yeast cell walls is also enriched with functional groups, including hydroxyl, carboxyl, and amino groups, which play a passivating role on the surface of CQDs. These functional groups reduce surface defects, thereby enhancing the optical properties and chemical stability of CQDs. Overall, yeast cell walls exhibit strong potential as an efficient carbon source for the synthesis of CQDs and show promising applications in fields such as bioimaging and sensing. Kalpana et al. [141] selected exopolysaccharides as the carbon source for synthesizing CQDs due to their unique advantages. Exopolysaccharides, naturally occurring high-molecular-weight polymers produced by bacteria, are rich in various functional groups such as hydroxyl, carboxyl, and amino groups. These functional groups provide excellent surface modification during the synthesis process, reducing surface defects and enhancing both optical performance and chemical stability of the CQDs. Additionally, exopolysaccharides contain abundant elements like carbon, nitrogen, and oxygen, which can effectively be converted into the core structure of CQDs during hydrothermal carbonization. The nitrogen content, in particular, helps modulate the electronic structure of the CQDs, further improving their fluorescence intensity and QY. More importantly, the biocompatibility and environmental friendliness of exopolysaccharides make them an ideal carbon source for synthesizing high-performance CQDs. These unique properties not only enhance the fluorescence performance of CQDs but also bolster their potential applications in fields such as biomedicine and environmental protection. Therefore, using exopolysaccharides as a carbon source for CQDs synthesis offers significant advantages, promoting the efficient synthesis of CQDs and optimizing their optical and biological properties.

Figure 5

Schematic illustration of the Saccharomyces cerevisiae’s cell wall and the components of its two layers. Reprinted from Mirseyed et al. [137], open access under the Creative Commons license.

3.1 Anti-counterfeiting

Biomass-based CQDs show significant potential for anti-counterfeiting applications, primarily due to their excellent fluorescence properties and stability [142]. CQDs synthesized via the hydrothermal method are simple and cost-effective to produce, and their use in anti-counterfeiting heavily depends on their solid-state fluorescence. However, particle aggregation often leads to a phenomenon known as aggregation-caused quenching (ACQ), where fluorescence is significantly reduced in the solid state compared to in solution [143]. One effective way to mitigate this quenching is by dispersing CQDs in a polymer matrix [144]. Poly (vinyl alcohol) (PVA) is commonly considered a suitable matrix due to its excellent surface passivation ability and strong compatibility with CQDs [145]. Additionally, other materials such as carboxymethyl cellulose (CMC), polyacrylamide (PAM), and polyethylene glycol (PEG) [114] have been used to further enhance the performance and applicability of CQDs. These CQDs can be uniformly dispersed in various media, including deionized water and ethanol, facilitating the preparation of diverse anti-counterfeiting materials. Common tools for applying CQDs in anti-counterfeiting include fountain pens, fine brushes, filter paper, and inkjet printers. Techniques used in this field include fluorescent ink, phosphorescent ink, transparent films, and latent fingerprint detection.

In practice, biomass-based CQDs can be applied through various methods, including handwriting, inkjet printing, stamping, spraying, flexographic printing, and screen printing. These techniques ensure that anti-counterfeiting marks remain invisible under daylight but become highly fluorescent under UV light. Thanks to their photostability and biocompatibility, CQD-based anti-counterfeiting materials maintain excellent performance even after long-term storage and multiple uses. Fluorescent anti-counterfeiting technology is widely adopted due to its strong security, fast recognition, and ease of use [146,147]. Traditional fluorescent materials, such as organic fluorescent dyes, suffer from poor stability and resistance to photobleaching, and some azo-based dyes pose toxicity concerns [148,149]. In contrast, the environmentally friendly nature of biomass-based CQDs minimizes their environmental impact during production and application, making them ideal for large-scale production with strong environmental protection benefits. By leveraging these advantages, the application of biomass-based CQDs in anti-counterfeiting not only enhances the reliability of the technology but also significantly reduces environmental impact, showing great potential for future development. Table 6 summarizes the applications of CQDs in anti-counterfeiting and outlines the characteristics required for effective anti-counterfeiting functionality.

Solvent-based inks release large amounts of environmentally harmful volatile organic compounds (VOCs), making this a major global concern. In contrast, water-based inks do not emit VOCs that contribute to global warming (GW) and are becoming increasingly popular due to their environmentally friendly properties. Previous studies have reported the formulation of water-based flexographic inks using organic molecules as fluorescent pigments [150]. Ullal et al. [101] examined the impact of various components (glycerol, surfactants, defoamers, and waxes) in their security inks on the absorbance and emission characteristics of CQDs. The results showed that the addition of wax caused a 17 nm redshift, shifting the emission peak from 473 to 490 nm. The fluorescence intensity of CQDs remained largely unaffected when mixed with glycerol alone, but it decreased steadily when dispersed in surfactants or defoamers, indicating possible interactions with CQD surface groups. Additionally, the dispersion of CQDs in wax significantly reduced fluorescence intensity.

Hong and Yang [102] used ethanol as a solvent, enabling faster drying than water-based inks, with fewer wrinkles and reduced spreading on paper, which is beneficial for anti-counterfeiting applications. No differences in the thickness or surface morphology of the paper were observed, suggesting that the ink penetrated the paper and interacted with its chemical composition, resulting in excellent invisibility of the anti-counterfeiting patterns.

As shown in Figure 6, Shen et al. [140] conducted a study to evaluate the potential of CQDs in the field of information encryption, using QR codes (Quick Response codes) as an example. Certain patterns (e.g., ii and iii) were replaced with encryption patterns drawn using CQD-based ink. When the ink was still wet, the encrypted patterns were already difficult to identify (e.g., ii). Once the ink was completely dried, the patterns achieved a fully encrypted effect (e.g., iii). Moreover, under natural light (daylight conditions), the encrypted text within the QR code could not be recognized by smartphone software. However, under 365 nm UV light (e.g., iv), the hidden portions of the QR code became visible and could be decrypted using smartphone software. Furthermore, the study also utilized another type of CQD fluorescent ink (synthesized from oxidized carbon sources) to conduct similar experiments, achieving comparable results (e.g., v to viii). Specifically, only under 365 nm UV light (e.g., viii) could the encrypted patterns and the complete QR code be revealed, successfully enabling the encryption and decryption of information. The experiments demonstrated that both types of CQD-based inks exhibit excellent application potential in fields such as painting, information encryption, and anti-counterfeiting.

Figure 6

Application of CQD fluorescent ink. - QR Code anti-counterfeiting. Reprinted from Shen et al. [140], copyright (2023), with permission from Elsevier.

Zhang et al. [138] synthesized CQDs using spirulina as the carbon source, whose fluorescence color can be dynamically regulated by adjusting the pH of the system. In the experiment, two equal-volume (10 mL) solutions of water, ethanol, and glycerol (mixed in a ratio of 1:3:1) were stirred for 1 h, and their pH values were adjusted to 1.0 and 9.0, respectively. Subsequently, 30 mg of CQDs were dissolved in each solution and stirred for another hour, resulting in red and blue fluorescent inks. These CQD-based nanocomposites were used to fabricate programmable red and blue fluorescent inks capable of complex information encryption and decryption to safeguard critical data. During the encryption-decryption process, triethylamine (TEA) and hydrochloric acid (HCl) were used as encryption and decryption agents, respectively, to achieve pH-induced color switching. For example, Figure 7(a) illustrates the encryption-decryption-re-encryption process of the target protected information “CD” within the word “ENCODE.” Figure 7(b) further shows the conversion of the encoded “CD” information through ASCII binary coding. By switching the CQDs solution color between red and blue under UV light, dynamic data encryption and decryption were successfully achieved. This safe ink demonstrates significant potential in dynamic anti-counterfeiting, multi-level data encryption and decryption, and reversible data encoding under UV light.

Figure 7

(a) Encryption-decryption-re-encryption process of the protected information “CQDs” within “ENCODE” using pH-responsive CQDs ink. (b) Reversible fluorescence encoding and decoding of “CQDs” based on ASCII binary code. Reprinted from Zhang et al. [138], copyright (2023), with permission from Elsevier.

Ni et al. [114] verified the anti-counterfeiting effect of phosphorescent films using water. When sprayed with water, the blue fluorescence emission remained unchanged, while the green phosphorescence was quenched immediately. The phosphorescence intensity of the CQDs-PVA film decreased significantly under water vapor and recovered once dried. This phenomenon is due to water disrupting the hydrogen bonding between CQDs and PVA, rather than the luminescent centers of the CQDs. The weakened hydrogen bonding reduces system rigidity, enhancing molecular vibrations and rotations. The phosphorescence lifetime of the CQDs-PVA film was shortened as water was gradually added to ethanol, destabilizing the excited triplet state and increasing non-radiative transitions.

Ma et al. [97] addressed the ACQ process by adding sugar to form CQDs-Sugar complexes. The sugar did not interfere with the excitation-dependent behavior of the CQDs but significantly enhanced emission intensity (by 81% at 0.7 g/mL sugar), increased QY to 13.74%, and extended fluorescence lifetime by 1.73 ns.

Additionally, Krushna et al. [62] synthesized CQDs using leaves of Parthenium hysterophorus (silverleaf nightshade) as a carbon source via a hydrothermal method. These CQDs were applied in innovative chromogenic fingerprint analysis and anti-counterfeiting applications by integrating latent fingerprint detection, porosity analysis, and the YOLOv8 deep learning algorithm. In this process, computational techniques and deep learning algorithms played a crucial supporting role in CQDs-based experiments. Specifically, deep learning algorithms were employed for the preprocessing of fingerprint images, model training, and predictive analysis (Figure 8a–c), significantly improving the accuracy and efficiency of fingerprint recognition. The experimental results demonstrate that this approach provides robust technical support for the application of CQDs in fingerprint detection, anti-counterfeiting, and related fields.

Figure 8

The computational analysis process for fingerprint identification involves three steps: (a) Pre-processing, (b) training model, and (c) prediction process, respectively. Reprinted from Krushna et al. [62], copyright (2024), with permission from Elsevier.

Sandeep et al. [60] synthesized CQDs using pistachio shells as a carbon source through a hydrothermal method and applied them to efficient personal identification by combining latent fingerprint detection techniques and poroscopy. Unlike the approach of Krushna et al., the authors utilized Python programming to enhance fingerprint images, enabling image refinement, feature extraction, and detailed micro-level analysis. This significantly improved detection accuracy and further expanded the application potential of CQDs in the field of anti-counterfeiting. Specifically, Figure 9(a)–(c) presents the fluorescent images of fingerprints collected from various surfaces, including paper, OHP sheets, and steel locks, revealing substantial differences in fingerprint visualization based on surface types. Figure 9(d)–(f) demonstrates the extraction of level-I to -III fingerprint features, including bifurcations, short ridges, cores, and scars, covering a total of 13 micro-level features. The efficient extraction and analysis of these features highlight the effectiveness and precision of combining CQDs with Python-based algorithms for latent fingerprint detection and detailed feature analysis, providing novel insights into the application of CQDs in anti-counterfeiting and personal identification.

Figure 9

(a–c) Fluorescence images of FPs collected from different surfaces of (paper, OHP sheet, and steel lock). (d–f) Extracted features of levels I–III. Reprinted from Sandeep et al. [60], copyright (2024), with permission from Elsevier.

3.2 Treatment of dye-contaminated wastewater

In this application, biomass-derived CQDs demonstrate significant advantages in dye wastewater treatment. Biomass CQDs possess excellent environmental properties, with non-toxic basic components and renewable raw material sources, making them a green catalyst. Biomass CQD-based catalysts exhibit high catalytic efficiency and rapid reaction rates, effectively degrading dyes under light irradiation without producing additional harmful by-products. This gives them a clear advantage over traditional methods in wastewater treatment. Furthermore, compared to carbon dots and other nanostructures, biomass CQD composites generally have larger surface areas and porous volumes [151]. This not only enables more dye molecules to be adsorbed onto the catalyst surface, thereby improving catalytic efficiency, but also enhances catalytic performance by extending the electron-hole recombination period. As a result, biomass CQD composites exhibit higher efficiency and stronger performance in photocatalysis, showcasing exceptional catalytic properties and significant potential in dye wastewater degradation.

Smrithi et al. [109] synthesized CQDs using zucchini as the carbon source through a hydrothermal method. These CQDs exhibit excellent light absorption properties, low electron-hole recombination rates, and biocompatibility, making them ideal photocatalytic materials. Studies have shown that under visible light irradiation, CQDs can effectively degrade harmful organic dyes in water, such as crystal violet dye. When combined with hydrogen peroxide (H₂O₂), the catalytic degradation efficiency reaches 99.9%. Compared to traditional semiconductor catalysts, CQDs not only demonstrate better dispersion but also offer a simpler synthesis process without the need for additional doping or complex post-treatment steps.

Saafie et al. [29] synthesized CQDs from kenaf and demonstrated their excellent photocatalytic degradation performance under optimal synthesis conditions. As shown in Figure 10, although CQDs can remove a portion of methylene blue (MB) dye through physical adsorption in the absence of light, the removal efficiency is significantly enhanced under light irradiation. Specifically, under 120 min of light exposure, the CQDs can remove up to 90% of MB dye, highlighting their great potential for water pollution treatment. This result indicates that light irradiation significantly enhances the photocatalytic degradation ability of the CQDs, thereby greatly improving the dye removal efficiency. To further investigate the performance of CQDs in the photocatalytic process, the study also conducted a kinetic analysis of the MB degradation reaction. The results showed that the degradation process of the sample followed a pseudo-first-order kinetic model, with a degradation rate constant of 0.023/min, indicating a high decolorization efficiency. The article further proposed a photocatalytic degradation mechanism based on CQDs: when CQDs absorb photons, holes (h⁺) are generated in the valence band (VB), and electrons (e⁻) are generated in the conduction band. These excited electrons and holes participate in the generation of superoxide radicals (–O₂) and hydroxyl radicals (–OH), which react with water and oxygen, ultimately degrading the MB dye molecules.

Figure 10

Comparison of the degradation performance of optimized CQDs (C200-0.5-24: 200°C, 0.5 g precursor, 24 h) under visible light irradiation and in total darkness. Reprinted from Saafie et al. [29], open access under the Creative Commons license.

Liu et al. [152] synthesized CQDs using folic acid as the carbon source. Under different reaction conditions, CQDs exhibited strong degradation ability, achieving nearly 99.69% degradation of nicotine from wastewater within 90 min, indicating their high catalytic performance. Specifically, the reaction temperature significantly affected the catalytic efficiency, with the best catalytic activity observed at 160 and 180°C; the reaction time had less impact on the degradation effect, with a 1 h reaction time being both efficient and energy-saving; increasing nicotine concentration improved the degradation efficiency, and CQDs exhibited excellent catalytic ability in high-concentration nicotine wastewater; the presence of light notably enhanced the degradation efficiency, demonstrating the importance of light in the catalytic reaction; pH value influenced the catalytic effect, with the best degradation performance of CQDs under alkaline and neutral conditions, while acidic conditions led to poorer results; finally, cyclic usage tests showed that CQDs maintained high catalytic activity after six cycles, demonstrating excellent stability and reusability. These results suggest that by optimizing the reaction conditions, CQDs can efficiently and stably treat nicotine wastewater.

Sabet and Mahdavi [153] synthesized CQDs from grass as a carbon source and investigated their application in the treatment of dye-contaminated wastewater. As shown in Figure 11(a)–(f), these CQDs exhibit excellent photocatalytic performance, effectively degrading various dyes such as Acid Blue, Acid Red, Eosin Y, MB, Eriochrome Black T (ECBT), and Methyl Orange under both UV and visible light irradiation. Furthermore, due to their high specific surface area, the CQDs also demonstrate strong adsorption capability for heavy metal ions, with removal efficiencies of 75 and 37% for Cd²⁺ and Pb²⁺, respectively. Therefore, nitrogen-doped CQDs not only efficiently remove organic dyes but also serve as effective adsorbents for heavy metal ions, offering promising potential for the treatment of dye-contaminated wastewater.

Figure 11

Degradation of different dyes in the presence of CQDs: (a) Degradation of Acid Blue. (b) Degradation of Acid Red. (c) Degradation of Eosin Y. (d) Degradation of MB. (e) Degradation of ECBT. (f) Degradation of Methyl Orange. Reprinted from Sabet and Mahdavi [153], copyright (2019), with permission from Elsevier.

3.3 Sensors

Biomass-derived CQDs exhibit significant potential in metal ion sensing due to their excellent fluorescence properties, high stability, high dispersibility, and environmental friendliness. In sensing applications, the fluorescence properties of CQDs can be altered through interactions with metal ions, leading to fluorescence quenching or enhancement. Common mechanisms include static quenching, dynamic quenching, inner filter effect, photo-induced electron transfer, and energy transfer, such as Förster resonance energy transfer, Dexter electron transfer, and surface energy transfer [154]. These mechanisms involve effective electron or energy transfer processes that promote non-radiative electron/hole recombination annihilation, resulting in fluorescence quenching of metal ions. Moreover, the rich surface functionalization of CQDs enables interactions with specific analytes, adjusting their fluorescence properties. Through adsorption or chemical reactions, the surface structure of CQDs is altered, allowing for sensitive detection of target analytes. To date, these fluorescence modulation mechanisms (including both quenching and enhancement) have been widely applied in the detection of various molecules and metal ions, further broadening the application prospects of CQDs as sensors in various fields.

CQDs synthesized using rose petals as the carbon source were applied for the detection of the pesticide diazinon [155], demonstrating excellent sensitivity and selectivity. Figure 12(a) illustrates the gradual decrease in the fluorescence intensity of CQDs with increasing diazinon concentrations, indicating that diazinon effectively quenches the fluorescence of CQDs. Figure 12(b) further shows that within the diazinon concentration range of 0.02–10 µM, the change in fluorescence intensity (F ₀−F) exhibits a strong linear relationship with diazinon concentration, with particularly notable linearity in the range of 0.02–1 µM. The study revealed a detection limit of 0.01 µM for this method, with a relative standard deviation (RSD) of 3.5%, indicating high detection sensitivity. Moreover, interference from other pesticides in diazinon detection was minimal, and the recovery rate of diazinon in different water samples ranged from 97.8 to 102.2%, validating the reliability and accuracy of this CQDs-based nanosensor for real sample detection. These results highlight the significant potential of rose-derived CQDs in environmental monitoring, particularly for pesticide residue detection. CQDs synthesized from tobacco as a carbon source were used to differentiate and quantify tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC) [80]. The LODs for TC, OTC, and CTC were 5.18, 6.06, and 14 nM, respectively. Cao [81] synthesized fluorescent Y-CDs through a one-step hydrothermal treatment of yeast powder. These Y-CDs acted as green fluorescent probes for the detection of dopamine (DA), with an LOD of 30 nM and a linear range of 0.05–150 µM. Additionally, the authors synthesized a different type of Y-CDs by adjusting the mass of yeast powder and NaOH concentration. These Y-CDs were used to construct “On-Off-On” sensors for the sensitive and selective detection of Hg⁺ and cysteine, with LODs of 35 and 78.5 nM, respectively. The authors also prepared pH-sensitive fluorescent H-CDs from honeysuckle as a carbon source. These H-CDs were used to detect the food additive Sunset Yellow (SY), with an LOD of 11 nM and a linear range of 0–40 µM. CQDs synthesized from pre-hydrolyzed lignin served as fluorescent sensors for detecting the illegal food additive Sudan I [73], with an LOD of 0.12 µM. A CQD sensor based on CQDs synthesized from crayfish shells was developed for the detection of 4-nitrophenol [112]. The sensor showed good performance, with recoveries ranging from 95.80 to 103.55% and low RSDs between 0.91 and 4.59%, indicating minimal loss during detection and excellent reproducibility and reliability. A fluorescent sensor for the detection of pyrophosphatase was constructed using CQDs synthesized from willow leaves as the carbon source [46], with recoveries between 98.67 and 102.7%, and RSDs below 5%, demonstrating accurate and reliable results. CQDs synthesized from rambutan seeds were used to selectively and sensitively detect Congo red dye [85] through a fluorescence quenching effect, with a limit of detection (LOD) of 0.035 mM. The sensor was also effective in detecting Congo red in real water samples (tap water and lake water). In addition, John et al. synthesized CQDs using rhizomes of Acorus calamus [63] as the carbon source for the fluorescence detection of mulberry pigments and the catalytic reduction of rhodamine B and SY. Hu et al. synthesized CQDs using Cordyceps militaris [139] as the carbon source for the detection of alcohol in alcoholic beverages. Ni et al. [114] synthesized CQDs using chitosan as the carbon source and developed a novel room-temperature phosphorescent material, which was used to efficiently, stably, and reliably detect trace amounts of water in various organic solvents. Ye et al. [119] synthesized CQDs from chicken bones, using them as fluorescent probes for the detection of laundry detergent. Wu et al. [122] synthesized CQDs from chicken cartilage and established an H₂O₂ biosensor, which was applied to glucose detection in human serum.

Figure 12

Calibration curve. (a) The change in fluorescence of the CDs with the increase in concentration of Diazinon. (b) A linear correlation of (F ₀ − F) values to the concentration of Diazinon in the range from 0.02 to 1 µM. Reprinted from Shekarbeygi et al. [155], copyright (2020), with permission from Elsevier.

Moreover, many biomatrix CQDs have been applied as sensors for the detection of metal ions. The fluorescence intensity of CQDs is highly sensitive to metal ions, with most detections based on fluorescence quenching [156]. Table 7 summarizes the applications of CQDs as sensors for metal ion detection, listing their LODs and linear ranges.

3.4 Biomedical research

Biomass-based CQDs have demonstrated significant potential in the field of medical drug delivery due to their exceptional properties. First, CQDs exhibit excellent biocompatibility and high water solubility [158], enabling their rapid dispersion in vivo and efficient binding with cells, thereby minimizing toxic side effects. Moreover, their large specific surface area and enhanced cellular uptake capability make them ideal drug carriers [159], allowing for the effective delivery of therapeutic agents to target tissues. The optical properties of CQDs, such as fluorescence and resistance to photobleaching, also enable real-time monitoring during drug delivery, providing valuable insights into drug distribution and therapeutic efficacy. Through targeted delivery, CQDs can reduce side effects on non-target tissues, thereby improving the therapeutic outcomes. Notably, in the treatment of diseases such as cancer, CQDs can function as intelligent drug delivery systems, enhancing treatment efficiency and efficacy.

CQDs synthesized using ginsenoside Rb1 as a carbon source (RBCQDs) have been proposed as a novel nanomedicine with potential applications in the treatment of intracerebral hemorrhage (ICH) [105]. Figure 13 illustrates the synthesis process of RBCQDs and their therapeutic mechanism. High-purity RBCQDs were synthesized via a hydrothermal method by mixing ginsenoside Rb1 with ethylenediamine (C₂H₈N₂). Compared to traditional iron chelators, RBCQDs exhibit superior antioxidant capacity and iron chelation properties, along with excellent biocompatibility and water solubility. In ICH models, RBCQDs significantly alleviate secondary brain injuries post-ICH by clearing excess iron ions (Fe²⁺ and Fe³⁺) and reactive oxygen species (ROS) in the meningeal system. Additionally, this nanomedicine reduces oxidative stress and iron-induced cytotoxicity, further promoting the recovery of neurological function. This study provides a new direction for the application of nanomedicine in ICH treatment, while also validating the feasibility of synthesizing CQDs from natural products. It lays a foundation for the development of biomaterials with enhanced therapeutic efficacy and safety for biomedical applications. Curcumin, known for its diverse biological activities, faces limitations in clinical applications due to poor stability, low water solubility, and limited bioavailability. To address these issues, curcumin was loaded onto CQDs synthesized using folic acid as the carbon source, forming a novel nanocomposite [108]. This composite demonstrated significant antibacterial activity against both Gram-negative and Gram-positive bacteria. Moreover, it exhibited pronounced cytotoxicity in cervical cancer cells (HeLa) with high folate receptor expression, while showing no toxic response in hepatocellular carcinoma cells (HepG2) with low folate receptor expression, indicating strong tumor-targeting properties. CQDs synthesized using chlorogenic acid as a carbon source have demonstrated potential as anticancer nanozymes [104]. Figure 14(a) presents tumor images from different experimental groups, where the tumor volume in the CQDs-treated group was significantly smaller than that in the control group and the group treated with the conventional anticancer drug sorafenib. Figure 14(b) compares the tumor weights across different groups on day 12, showing that the tumor weights in the CQDs-treated groups at doses of 5 and 20 mg kg⁻¹ were significantly reduced, exhibiting a clear dose-dependent effect. The study revealed that chlorogenic acid-derived CQDs effectively inhibit tumor growth by inducing ferroptosis in tumor cells and activating the tumor immune microenvironment. Additionally, these CQDs exhibited excellent biocompatibility and low toxicity, reducing the side effects commonly associated with conventional anticancer therapies. These findings highlight the promising potential of chlorogenic acid-derived CQDs in tumor immunotherapy and other areas of nanomedicine, providing new insights for the development of more efficient and safer anticancer treatment strategies. CQDs synthesized using cell walls from Saccharomyces cerevisiae as a carbon source exhibited strong antimicrobial activity against both Gram-positive and Gram-negative bacteria [137]. The antibacterial activity is likely attributed to their negative surface charge and ability to generate ROS, which can damage bacterial cell membranes and DNA, leading to bacterial death. Phosphorus doping enhanced ROS generation by introducing additional free electrons, further improving the antibacterial effect. Additionally, these CQDs displayed significant antioxidant activity, highlighting their potential in medical and pharmaceutical applications. CQDs synthesized from Ganoderma lucidum [95] as a carbon source generated large amounts of hydroxyl radicals (–OH) through Fenton catalysis, exhibiting super-oxidizing capability that induced oxidative stress in tumor cells, ultimately leading to tumor cell apoptosis.

Figure 13

Schematic representation of RBCQDs synthesis and their application in cerebral hemorrhage treatment. Reprinted from Tang et al. [105], open access under the Creative Commons license.

Figure 14

(a) Tumor photos. (b) Tumor weight from different groups at the 12th day after ChA CQDs treatment. Reprinted with permission from Yao et al. [104], copyright (2022) American Chemical Society.

Biomass-based CQDs, with their outstanding properties for medical drug delivery, also exhibit broad application potential in the field of bioimaging. Due to their excellent biocompatibility and high water solubility [160], CQDs can serve as non-toxic imaging probes for cell labeling and disease diagnosis. Their multicolor PL and broad absorption spectrum enable multimodal imaging, providing clear images across various imaging modalities. Moreover, the high photostability and resistance to photobleaching of CQDs allow them to maintain stable brightness during prolonged imaging, further enhancing imaging accuracy and reliability. The low cytotoxicity and biodegradability of CQDs also ensure their safety for long-term applications. Therefore, CQDs not only play a critical role in drug delivery but also serve as ideal probes in bioimaging, offering precise support for disease monitoring and diagnosis. Table 8 summarizes the applications of biomass-based CQDs in bioimaging.

3.5 Environmental sustainability

As biomass-based CQDs, they inherently exhibit low environmental impact and excellent eco-friendly characteristics, making them highly promising for applications in environmental protection and sustainable development. The surface of CQDs is rich in functional groups such as hydroxyl, carboxyl, and amino groups, which facilitate hydrogen transfer pathways, endowing CQDs with remarkable antioxidant properties that mitigate the negative effects of environmental stress on plants [107]. Moreover, the unique optical properties of CQDs, particularly their UV fluorescence, enable them to regulate light capture and energy conversion in plant photosynthesis, thereby improving plants’ efficiency in utilizing light energy. The potential of CQDs in environmental sustainability has been extensively explored, especially in the context of green energy conversion, where they demonstrate exceptional catalytic performance and minimal environmental impact. Therefore, biomass-based CQDs not only promote plant growth and enhance stress resistance but also offer innovative solutions for environmental protection and sustainable energy production.

As shown in Figure 15, Johny et al. [45] conducted an environmental impact analysis of the CQDs synthesis process using the life cycle assessment approach, specifically employing the ReCiPe2016 midpoint (H) method. The results revealed that citric acid, commonly used in traditional CQDs synthesis, is the primary contributor to many environmental impact categories. However, by utilizing biomass sources, such as eucalyptus leaves, as the carbon precursor, the reliance on citric acid can be significantly reduced, thereby mitigating the environmental impacts of the synthesis process. The figure illustrates that compared to CQDs synthesized without biomass, the use of eucalyptus leaves as the carbon source leads to substantial improvements across multiple environmental impact categories, such as GW stratospheric ozone depletion (SOD), and LU. Notably, the impacts on LU and ozone layer depletion are reduced by nearly half, highlighting the significant sustainability advantages of biomass-based CQDs synthesis strategies. Furthermore, the study indicated that using biomass not only decreases the overall environmental impact but also enhances resource efficiency in terms of WC and mineral resource scarcity. These findings suggest that integrating biomass as a raw material and optimizing CQDs synthesis processes can simultaneously achieve high-performance material production and significant reductions in environmental impact. This provides valuable guidance for advancing green chemistry synthesis technologies and promoting sustainable development.

Figure 15

Comparison of environmental impact analysis of hydrothermally synthesized CQDs with and without biomass using the ReCiPe2016 midpoint (H) method. Impact categories include GW, SOD, ionizing radiation, ozone formation-human health (OF-HH), fine particulate matter formation (FPM), ozone formation-terrestrial ecosystem (OF-TE), terrestrial acidification, freshwater eutrophication, marine eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, human carcinogenic toxicity, human non-carcinogenic toxicity, LU, mineral resource scarcity, fossil resource scarcity, and water consumption (WC). Reprinted from Johny et al. [45], open access under the Creative Commons license.

CQDs synthesized from willow wood powder as a carbon source have demonstrated significant potential in enhancing photosynthesis [59]. CQDs effectively capture UV light and convert it into blue light, thereby improving the efficiency of chloroplasts in utilizing light energy. Figure 16 illustrates the mechanism by which CQDs facilitate light energy capture and enhance chloroplast photosynthesis. By interacting with Photosystem II, CQDs convert light energy into chemical energy, driving the generation of NADPH and the synthesis of ATP, thus enhancing the overall efficiency of photosynthesis. Experimental results showed that chlorophyll content in legume leaves treated with CQDs significantly increased, particularly at low concentrations, where nitrogen-doped CQDs exhibited superior enhancement effects. This effect is attributed to the critical role of nitrogen-doped CQDs in light harvesting and energy conversion. This study provides a theoretical foundation for the development of CQD-based agricultural yield enhancement technologies while highlighting the potential application of biomass-derived functional CQDs in promoting photosynthesis. CQDs synthesized from proanthocyanidins as a carbon source demonstrated significant efficacy in alleviating salt stress in rice plants [107]. Studies revealed that rice seedlings treated with these CQDs exhibited a substantial increase in fresh weight and chlorophyll content under salt stress conditions, indicating their ability to mitigate the adverse effects of salt stress. Moreover, these CQDs enhanced the antioxidant capacity of the plants by increasing antioxidant enzyme activity in the rice seedlings, reducing oxidative damage. Waste Cooking Oil (WCO) refers to the oil discarded during cooking processes, and once released into water bodies, it can cause significant environmental pollution. For instance, just 1 L of WCO can contaminate up to 500,000 L of natural water. Therefore, the recycling and treatment of WCO are of utmost importance. Kalpana et al. [141] utilized exopolysaccharides as a carbon source to synthesize CQDs and applied them in biodiesel production. Experimental results demonstrated that CQDs derived from exopolysaccharides exhibit excellent catalytic performance, effectively enhancing the conversion efficiency of WCO into biodiesel. Compared to traditional catalysts, CQD catalysts show superior stability and lower environmental impact. Moreover, when combined with sodium hydroxide and other base catalysts, the biodiesel yield can reach 96.97%. Thus, the application of CQDs derived from exopolysaccharides in biodiesel production not only improves conversion efficiency but also provides a more environmentally friendly alternative.

Figure 16

Schematic diagram of CQDs promoting light energy capture by chloroplasts to enhance photosynthesis. Reprinted from Fu et al. [59], open access under the Creative Commons license.