Introduction to Photoluminescent Carbon Dots

Carbon dots (CDs) were first discovered during the electrophoretic purification of single-walled carbon nanotubes produced from arc-discharge soot by Scrivens et al. in 2004, and were called fluorescent carbon at the time.¹  After nearly 20 years of vigorous development, photoluminescent CDs have evolved into several subtypes, including carbon nanoclusters (CNCs), carbon nanodots (CNDs), carbonized polymer dots (CPDs), carbon quantum dots (CQDs), and graphene quantum dots (GQDs) based on differences in their structure.²  Clarification of the structure is of great significance for revealing the photophysical properties of CDs, such as their photoluminescence (PL) mechanism and photostability.

Currently, the preparation of CDs usually necessitates the polymerization and carbonization of precursors by high-energy treatments, including heat, electricity, and laser, or the use of a strong oxidizing agent (a strong alkali or acid) during synthesis, such as chemical oxidation, the hydro/solvothermal method, the electrochemical method, microwave-assisted synthesis, and the direct carbonization method.^3–5  These strategies not only consume a lot of energy, but they also negatively impact the environment, which contradicts the concept of sustainable chemistry.⁶  Given these drawbacks, sustainable synthesis approaches evolve as the need arises, including biomass synthesis without the use of harmful chemical reagents and self-exothermic synthesis without the need for an external energy supply.⁴  Together with traditional synthesis methods, these newly established strategies provide a solid platform for producing CDs with good photophysical properties.

CDs have been widely used in a variety of fields, including sensing,^{2, 3}  imaging,⁷  cancer therapy,⁸  light-emitting diodes,⁹  solar photovoltaics,¹⁰  and catalysis,^{11, 12}  due to their notable advantages such as excellent electron transfer capability, good biocompatibility, stable PL, and high chemical stability. Meanwhile, CDs have advanced rapidly in green chemistry application domains such as water treatment,¹³  air pollutant removal,¹⁴  environmental analysis,¹⁵  and crop yield improvement.¹⁶  This should be attributed to the constant refinement of the synthesis methods and functional design, which allows for the synthesis of a wide range of CDs with innovative structures and functions.

Focusing on green pathways this book focuses on the green synthesis of CDs and their applications in bioanalytical, catalytic, biomedical, and environmental sciences. In this chapter, we have covered the fundamentals of CDs, such as their structure, classification, formation, synthesis strategy, PL mechanism, and role in plant growth. Chapter 2 provides a thorough discussion of biomass synthesis strategies for CDs. Chapter 3 outlines the most recent advancements in intelligent synthesis of CDs assisted by machine learning, which provides a powerful tool for attaining precise CD synthesis. Chapter 4 discusses large-scale synthesis of CDs and their applications in pollutant adsorption and photodegradation. Chapter 5 introduces the functionalization methodologies and catalytic application of CDs. Chapter 6 summarizes the sensing strategy and the related detection mechanisms of CDs. Chapter 7 provides an overview of the use of CDs in ion sensing. Chapter 8 discusses CD applications in drugs, antibiotics, and toxin sensing. Chapter 9 focuses on the use of CDs in imaging and therapy. Chapter 10 provides a comprehensive discussion of the use of CDs in cancer therapy. Finally, we look forward to exciting prospects for the green development of photoluminescent CDs. We hope that this book will help readers gain a better knowledge of CDs, a novel type of carbon nanomaterial, and will encourage their rapid advancement in the field of sustainable chemistry.

Photoluminescent CDs are the generic term for a variety of luminescent carbon nanoparticles that are primarily composed of carbon elements. With carbon and oxygen as their only constituent elements, bare CDs are regarded as having the simplest elemental compositions. In general, carbon sources like activated carbon,¹⁷  gas soot,¹⁸  and carbon fibers¹⁹  can be chemically oxidized to produce bare CDs. Typically, the surface of bare CDs only contains hydroxyl, carbonyl, or carboxyl groups, and the lack of diversified chemical groups limits the functional properties of bare CDs. Worse still, the low quantum yield (QY <2%) of bare CDs further limits their applications. Moreover, bare CDs prepared by chemical oxidation must undergo a complicated post-processing (e.g., strong acids used for oxidation need to be neutralized). Because of this, it was difficult to make extensive use of the bare CDs that were developed early on.

Surface passivation, proposed by Sun et al.,²⁰  is a potent technique for increasing the luminescent efficiency of bare CDs because it can stabilize emission from surface energy traps. Polymers have been successfully employed as surface passivation agents for modifying the surface and improving the QY of bare CDs. Inspired by surface passivation, functionalization strategies for CDs, including surface modification and heteroatom doping, have been developed to enhance the photophysical performance of CDs.²¹  Non-metallic and metallic elements as the heteroatoms can be doped into the structure of CDs to modify their composition and electron distribution as well as optical characteristics. Meanwhile, surface modification can introduce abundant active sites and chemical groups onto the surface of CDs. Consequently, the functionalization strategies not only endow CDs with unique photophysical properties but also introduce various components, complicating the structure of the CDs.

The precursors can carbonize or polymerize to a complex core structure with a variety of chemical groups on the surface due to the ambiguous growth mechanism. Generally, CDs are composed of a sp²/sp³ carbon skeleton as the carbon core, and chemical groups or polymer chains are widely distributed on the surface. The carbon core mainly includes the graphite lattice and amorphous carbon form, which not only affects the photostability of CDs, but is also closely related to their optical properties.²²  In addition, surface chemical groups can influence the dispersion of CDs in aqueous solution and also adjust their optical band gap. Based on structural differences (see Figure 1.1), CDs can be divided into five subtypes: (1) highly disordered carbon nanoclusters (CNCs), which lack a carbon core and a crystal structure; (2) carbonized polymer dots (CPDs), which have a polymer/carbon nanohybrid structure consisting of a carbon core and numerous polymer chains or chemical groups on the surface; (3) highly carbonized carbon nanodots (CNDs), which have some groups on their surface but lack a clear crystal structure and polymer feature; (4) highly crystallized carbon quantum dots (CQDs), which have an obvious crystal structure and functional group on the surface; and (5) single or few-layered graphene quantum dots (GQDs), which have obvious graphene lattices and chemical groups on the edge or within the interlayer defect.² 

Currently, “top-down” and “bottom-up” techniques have been developed for the preparation of photoluminescent CDs (see Figure 1.2).²³  The former is achieved by physically and chemically splitting carbon materials such as activated carbon and graphene. The latter involves the pyrolysis or carbonization of organic small molecules, macromolecules and even biomass materials.

The “top-down” strategy can be accomplished by physically, chemically, or electrochemically exfoliating sheet-like or bulky carbon materials such as carbon soot, activated carbon, graphene, and carbon nanotubes. Cutting carbon fibers into GQDs with powerful oxidants like H₂SO₄ and HNO₃ is a typical example.¹⁹  The breakdown of the bond between carbon atoms is essential to the entire formation mechanism. The CDs prepared using the “top-down” strategy usually have perfect sp² structures, but always lack strong luminescence because epoxy and carboxylic groups produced by chemical oxidation induce non-radiative recombination of localized electron–hole pairs, suppressing the luminescence.²⁴  Additionally, CDs obtained by this approach lack diverse active sites, which restricts their further applications. The “top-down” method is rarely used in green synthesis of CDs because of the restrictions on raw materials.

The “bottom-up” strategy is realized by pyrolysis, carbonization, or step-wise chemical fusion of organic molecules. During the formation of CDs, organic molecules typically go through four stages: condensation, polymerization, carbonization, and passivation.⁴  First, by undergoing condensation reactions such as amidation, aldol condensation, Schiff base condensation, and radical reaction, organic precursors can create chain compound intermediates. Second, by covalent, non-covalent, or other interactions, the intermediates are further polymerized to produce polymer-like nanoparticles. Third, especially at high temperatures, the polymer-like nanoparticles will be carbonized to create a carbon core. Finally, a surface passivation strategy is usually used to effectively improve the luminescent efficiency of CDs. Unlike “top-down” synthesis, the “bottom-up” technique is a frequently employed mechanism in the green synthesis of CDs. This is due to the fact that the majority of biomass materials, such as grass, sweet pepper, and grape juice, are composed of organic molecules, which can be carbonized into CDs by a “bottom-up” technique at high temperatures.

The synthesis of CDs can be divided into traditional synthesis methods, such as chemical oxidation, the electrochemical method, the hydro/solvothermal method, microwave-assisted synthesis, and the direct carbonization method, and sustainable synthesis methods, such as base catalysis, self-exothermic synthesis, and the reduction method, based on the differences in the synthesis equipment and energy type.

Chemical oxidation is typically seen as a “top-down” process, which has frequently been used to break down bulky carbon materials such as carbon soot¹⁸  and activated carbon²⁵  to obtain CDs. This technique can introduce -OH and -COOH groups to the surface of CDs, making them hydrophilic and negatively charged.²⁶  But the introduction of -COOH groups can cause the non-radiative recombination of localized electron–hole pairs, resulting in low QY of CDs.²⁴  Moreover, the difficulty of completely removing an excessive amount of an oxidizing agent (concentrated HNO₃ or H₂SO₄) is a common issue.

Ajayan’s group¹⁹  produced GQDs with high crystallinity by oxidation cutting micrometer-sized carbon fibers in one step. Pitch carbon fibers with stacked graphitic submicrometer domains can convert to GQDs after 26 h in a solution of concentrated H₂SO₄ and HNO₃. It is interesting to note that GQDs of different sizes can be manufactured by simply changing the reaction temperature from 80 to 120 °C. Wu’s group²⁷  prepared GQDs with carbene-like zigzag structures by chemically oxidizing micrometer-sized graphene sheets in concentrated H₂SO₄ and HNO₃ at 200 °C for 10 h. The resulting GQDs exhibit an excitation-dependent blue fluorescence, with a QY of 6.9%, and oxygen-containing functional groups, such as C=O/COOH, OH, and C-O-C groups, can be introduced at the edge and on the basal plane during the oxidation. Meanwhile, Mao’s group²⁶  produced multicolor CDs in concentrated HNO₃ for 12 h using candle soot as the carbon source. The oxidative acid has the ability to break down the carbon materials into nanosized CDs, solubilize the CDs, and alter their luminescent characteristics. Three pure CD components with relatively low QYs of 0.8%, 1.9%, and 0.8% can be extracted from the original CDs. Because of the poor QY, surface passivation is typically required after preparation.

Chemical oxidation, a formerly widely used synthesis method, can directly break down bulky carbon materials to produce nanosized CDs, but it also comes with numerous drawbacks, including a lengthy reaction time (often over ten or more hours), the requirement for strong oxidants (concentrated H₂SO₄ and HNO₃), and low luminescent efficiency of the CDs produced (QY is usually less than 2%). Additionally, blue emission is where most of the fluorescence of CDs created by chemical oxidation is concentrated. As a result, in recent years, researchers have focused on developing more sophisticated methods to prepare photoluminescent CDs.

The hydro/solvothermal method has emerged as an effective technique for producing CDs with novel structural and functional characteristics. Organic precursors can be polymerized or carbonized into CDs in aqueous solution or organic solvents at high temperatures. Organic precursors frequently experience condensation, polymerization, carbonization, and passivation during the production of CDs.⁴  Keep in mind that the reaction temperature needs to be lower than the melting point of the reactant because too high a temperature will cause the reactant to decompose. Reactant ratio, solvent type, reaction temperature, and reaction time all have a major impact on the structural properties of CDs, such as the degree of carbonization and surface state, which directly affect the optical characteristics of CDs. Furthermore, direct carbonization of natural biomass sources into CDs is possible when utilizing this technique.

The hydro/solvothermal method makes it possible to synthesize full-color emission CDs, which overcomes the limitation that CDs prepared through chemical oxidation typically have blue luminescence. Typically, Huang’s group²⁸  prepared full-color emission CQDs with absolute QYs higher than 70% by a one-pot hydrothermal carbonization of triethylamine and 3,4,9,10-perylenetetracarboxylic dianhydride (see Figure 1.3). Meanwhile, Xiong’s group²⁹  used the hydrothermal carbonization of urea and p-phenylenediamine to synthesize CDs that had bright and stable luminescence in gradient colors from blue to red under a single wavelength of UV light. Additionally, using the three distinct isomers of phenylenediamines, Lin’s group³⁰  developed three CDs with vivid luminescence in red, green, and blue, respectively, using a simple solvothermal method.

During the hydro/solvothermal process, the reaction solvent plays significant roles in the structural and luminescent properties of CDs by controlling the degree of dehydration and carbonization of the precursor.³¹  Due to the high polarity of water and the resulting poor dispersibility of conjugated structures, which is detrimental to the formation of the conjugated luminescent center, the CDs formed in aqueous solution typically emit blue light.³²  As a result, solvents with low polarity are preferred because CDs with large, conjugated structures and narrow optical bandgaps would be produced in organic solvents. A solvent-engineered molecular fusion technique was developed by Wu’s group³³  for the production of CQDs with multicolor bandgap fluorescence (see Figure 1.4). The lateral sizes of the CQDs can be controlled by the synthesis solvents. Red CQDs (r-CQDs) with a longest wavelength emission at 620 nm can be prepared by using DMF as the solvent because DMF not only has a good ability to dissolve organic-soluble 1,3,6-trinitropyrene precursors, but also helps to promote the epitaxial growth of the graphene core of CQDs. Additionally, the yellow CQDs (y-CQDs) produced in ethanol solvent have the highest degree of graphitization and few defects, which accounts for their high QY. The solvent can also affect the functional groups of CQDs. Therefore, the structure, size, composition, and functional groups of CQDs are significantly influenced by solvents.

Acid solution also has a sizable impact on adjusting the luminescent characteristics of CDs in addition to organic solvents. Wang’s group³⁴  demonstrated a one-step acid reagent engineering method for the synthesis of highly efficient full-color fluorescent CQDs by using mild acids such as folic acid, boric acid, and tartaric acid. The suitable acid is adopted to introduce different functional groups with various ratios into the structure of CQDs and to passivate the surface of CQDs, controlling particle sizes, reducing defects, and enhancing optical performance. In addition, strong acids such as HNO₃, H₂SO₄, and HCl can also affect the carbonization degree of CDs. Yang’s group³⁵  demonstrated that HCl can modify the luminescent characteristics of CDs by altering the pH level of the reaction medium. The redshift of emission is caused by a highly acidic environment, which increases the carbonization degree and enlarges the conjugated sp² structure of the CDs. Likewise, Xiong’s group³⁶  revealed that the dehydration and carbonization processes can be controlled by H₂SO₄, which increases the degree of graphitization of CDs, increasing the size of sp² conjugated domains and causing a redshifted excitation (620 nm) and emission (715 nm). Furthermore, Yang’s group³⁷  showed that HNO₃ not only acts as a catalyst to quicken reaction rates but also readily reacts with o-phenylenediamine through an electrophilic reaction and further influences the structure of CPDs through reduction, protonation, polymerization, and carbonization reactions occurring under conditions of high temperature and high pressure. By changing the HNO₃ dosage, it is possible to control the size of conjugated aromatic π systems of CPDs, which in turn controls their optical characteristics.

The electrochemical method is used as a quick and efficient strategy for the preparation of photoluminescent CDs. The applied potential and current density can be adjusted to control the size, compositional, and luminescent characteristics of CDs. Electrochemical methods include “top-down” electrochemical etching and “bottom-up” electrochemical synthesis.

With electrochemical etching, carbon materials like graphite, carbon nanotubes, and carbon fiber electrodes can be cut chemically to form CDs.^38–40  When compared to CDs produced by chemical oxidation, electrochemical etching can form CDs with a narrow size range and a comparatively high QY. Cyclic voltammetry was used initially by Ding’s group³⁹  to fabricate blue fluorescent CDs in an acetonitrile solution. Tetrabutylammonium perchlorate electrolyte is thought to function as an intercalator to crack the multiwalled carbon nanotube electrode and enable the production of luminescent CDs. Furthermore, using a graphite electrode in alkaline alcohols, Liu’s group⁴¹  developed an electrochemical oxidation strategy for producing highly fluorescent (QY = 11.2%) CQDs with an average diameter of 4 nm and high crystallinity. When maintained in ambient circumstances, the colorless CQDs’ dispersion progressively changes to bright yellow due to the oxygenation of surface species over time. The size of the CQDs is largely controlled by the applied potential, and a higher applied potential can lead to larger-sized CQDs. Additionally, the introduction of alcohols can slightly regulate the radical’s activity, resulting in the smooth synthesis of CQDs.

By transferring an electric current between two or more electrodes spaced apart by an electrolyte, the “bottom-up” electrochemical synthesis of CDs can be achieved. The electrochemical synthesis system is made up of electrochemically active molecules in the electrolyte and inert electrodes. The synthesis occurs within the electric double layer close to the electrode, which exhibits an extremely large potential gradient of 1 × 10⁵ V cm⁻¹. The synthesis of CDs usually involves the following processes: the decomposition of molecular precursors, the polymerization of carbon substances, the growth and oxidation of CDs.⁴²  Chang’s group⁴³  prepared blue CDs from glycine in NH₄OH solution during a two-hour period at 25 °C. Following electro-oxidation, electro-polymerization, carbonization, and passivation, CDs can be formed. The NH₄OH concentration, applied voltage, and reaction time have a significant impact on the luminescent characteristics of CDs. The NH₄OH, as a surface passivation reagent, can boost the luminescent efficiency of CDs. In addition, CDs with high QY tend to be synthesized when applied voltage and reaction time are increased appropriately.

Due to electrochemical oxidation, this approach typically allows for the introduction of numerous oxygen-containing groups into the surfaces of CDs. Recently, Long’s group⁴⁴  produced N-doped CQDs with green fluorescence using pyrocatechol and ethylenediamine as both precursors and electrolytes. The electrochemical oxidation technique can introduce many oxygenous groups to the surface of CQDs, improving their dispersibility. The Fe³⁺-mediating ON–OFF–ON–OFF fluorescence principle can be used to monitor alkaline phosphatase activity using CQDs with a high QY of 30.6%.

Due to its simplicity and time-saving simultaneous, homogenous, and efficient heating, microwave-assisted synthesis has often been used to produce luminescent CDs. Prior to carbonization into nanosized CDs, microwave treatment first causes the dehydration and pyrolysis of reaction precursors. The size of the CDs is often uniform, and it roughly correlates with the duration of the microwave treatment.⁴⁵  Under microwave radiation, CD synthesis typically takes only a few minutes and has a high QY. Guo’s group⁴⁶  developed a one-step microwave-assisted method for preparing organosilane-doped CDs within 5 minutes. CDs have a QY of 65.8% in their solid form, 2.5 times higher than in their solution. The high QY is a result of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane effectively acting as distance barrier chains to inhibit the quenching brought on by aggregation. The prepared CDs are used to fabricate a white-light emitting device with color coordinates of (0.32, 0.36) and a correlated color temperature of 6071 K, thanks to both their good film-forming ability and thermal stability.

The direct carbonization method can produce luminescent CDs by directly carbonizing organic molecules at high temperatures without the use of any solvents. It is significant that the synthesis processes of CDs can be seen with the naked eye. Huang’s group⁴⁷  has recently developed a two-step carbonization method for the preparation of L-cysteine-modified chiral CQDs. Citric acid is first pyrolyzed to form the CQDs with carboxyl groups at 200 °C for 20 minutes. By a dehydration-condensation reaction, which can be accomplished by the evaporation of water produced in the reaction, these carboxyl groups can bond with hydroxyl groups. The amino groups of L-cysteine can then interact with the carboxyl groups on the surface of CQDs for 60 minutes at a temperature of 200 °C to form L-cysteine-modified chiral CQDs. It is important to note that the temperature should not be higher than the temperature at which incorporating molecules decompose. The prepared chiral CQDs exhibit bright blue fluorescence with a high QY of 68%. Importantly, carbonization synthesis preserves the chiral characteristics of L-cysteine, most likely resulting from chiral imprint or chiral induction.

The sustainable synthesis method for CDs includes base catalysis, self-exothermic synthesis, and the reduction method. Without using an external energy source, these strategies can produce luminescent CDs in an efficient manner at ambient temperature. Because organic molecules in the biomass are difficult to polymerize and carbonize at room temperature, it is currently challenging to prepare CDs utilizing biomass materials by these processes.

A strong base can cause the polymerization of organic molecules such as saccharide⁴⁸ , p-benzoquinone²² , ascorbic acid⁴⁹ , PEG-200⁵⁰ , albumin⁵¹ , and cetylpyridinium chloride monohydrate⁵²  to form luminescent CDs. Without specialized equipment or an external energy source, luminescent CDs can be produced using this method at ambient temperatures, although the synthetic process often takes a long time. Huang’s group²²  prepared green fluorescent CNDs with a QY of 35.3% and yellow fluorescent GQDs with a QY of 17.5% at room temperature using triethylenetetramine and p-benzoquinone as the precursors (see Figure 1.5a). Triethylenetetramine not only creates an alkaline environment, but also can produce CDs through Schiff base condensation reaction with p-benzoquinone. Furthermore, the degree of surface oxidation, not particle size or functional groups, is what gives CDs their luminescent origin. Moreover, Choi’s group⁵²  produced both water-soluble CDs and organic soluble CDs utilizing cetylpyridinium chloride monohydrate as the carbon source in the presence of NaOH at room temperature. As obtained, the CDs have a homogeneous particle distribution, superior photostability, and high fluorescence, with a QY of 7.2% for water-soluble CDs and 16.7% for organic soluble CDs.

Self-exothermic synthesis, which produces CDs with bright fluorescence at room temperature without the need for an external energy source, is a relatively simple, affordable, and effective process. The as-prepared CDs with high QY are stable and can be used for cellular imaging, visual analysis, and other applications. Instead of using heating devices like drying ovens, the heat generated by a certain exothermic reaction can quickly polymerize and carbonize the organic molecules. Huang’s group⁵³  prepared green fluorescent CQDs by combining hydroquinone with H₂O₂ at room temperature, followed by a reaction with ethylenediamine. The polymerization of p-benzoquinone, which is a product of the oxidation of hydroquinone, and ethylenediamine can be effectively promoted by the heat from the decomposition of H₂O₂ catalyzed by ethylenediamine. The p-benzoquinone can then interact with ethylenediamine via Schiff base condensation to produce CQDs. The as-prepared CQDs with an absolute QY of 24.6% can be employed for the visual detection of vitamin B12. Furthermore, Huang’s group⁵⁴  produced single-layered GQDs utilizing hydroquinone and triethylenetetramine as the precursors (see Figure 1.5b). Meanwhile, triethylenetetramine can also cause the H₂O₂ to decompose and produce a significant amount of heat, which can help carbonize the products of the Schiff base condensation between p-benzoquinone and triethylenetetramine, leading to the formation of single-layered GQDs. The as-prepared GQDs can specifically bind with Al³⁺ ions to produce an aggregation-induced emission effect. Using H₂O₂ as an oxidant and primary heat source prevents the formation of hazardous gases and environmental concerns in addition to avoiding the use of concentrated acid and the introduction of contamination by metal impurities.⁵⁵ 

At normal temperatures, some organic compounds like carbon tetrachloride (CCl₄) can polymerize into CDs in the presence of hydride reducing substances like lithium aluminium hydride (LiAlH₄)^{56, 57}  and lithium triethylborohydride (Li(C₂H₅)₃BH).⁵⁸  With this technique, narrow size distribution monodisperse CDs can be produced. By chemically reducing carbon precursors in reverse micelles, Doyle’s group⁵⁶  proposed a simple method for producing highly fluorescent CQDs at room temperature. The hydrophilic interior of tetraoctyl ammoniumbromide reverse micelles is used to dissolve the CCl₄, and the carbon source is then easily reduced with LiAlH₄ to synthesize blue emissive CQDs. A similar reduction synthesis of CQDs was later developed by their group utilizing several reducing agents, allowing control over the CDs’ size, shape, and surface state as well as their high crystallinity and monodispersity.⁵⁸  With the decrease in reducibility, the CQDs will also become much more polydisperse and irregular in shape. The QY of CQDs increases to 27% after passivation with allylamine, making them potentially useful in the domains of bioimaging and ion sensing.

Luminescent CDs have significant UV–optical absorption with a lengthy tail that extends into the visible and even the NIR region.^59,60  The relationship between the electron transition and absorption spectra of CDs is depicted in Figure 1.6.⁶¹  The carbon core represents the sp² conjugated domains inside the CDs, and the carbon shell refers to the functional groups on the CDs’ surface. The short wavelength band below 300 nm (see Figure 1.6, Band I) corresponds to a π → π* transition involving aromatic sp² carbons (C=C bond), whereas the tail from 300 to 400 nm (see Figure 1.6, Band II) can be attributed to the inherent absorption of the n → π* transition of the C=O bond in the carbon core.^32,62–67  The absorption band above 400 nm is due to the surface state transition with electron lone pairs (see Figure 1.6, Band III-V).^63–65  It should be noted that the broad surface-state absorption band and the n → π* transition typically overlap rather than being isolated or separated. As a result, the emission tuning is not significantly disrupted, and the color gradient is quite smooth as the excitation wavelength changes.⁶⁴  Meanwhile, the low-energy absorption band at approximately 300 nm may be caused by n → π* and π → π* charge transfer transitions or interlayer charge transfer with a strong π → π* component.⁶²  Deprotonation, medium environment, structural/energetic disorder, and excitonic coupling also have some influence on the optical spectra of CDs.⁶² 

For the purposes of directing the synthesis of CDs with tuned luminescence, an understanding of the luminescence mechanism is extremely important. Despite the fact that numerous studies have been published, the luminescence mechanism of CDs is still unknown and up for debate. CDs prepared using diverse precursors and methods typically have a variety of intricate parts and architectures. In other words, CDs produced utilizing various synthetic techniques, precursors, and post-treatments have dissimilar optical properties, proving that they are a more complex system than first thought. Therefore, proposing a universal concept to explain the luminescence mechanism of CDs is of great significance.

Currently, the reported optical mechanisms can be divided into two categories: core-related emission and surface-related emission. Core-related emission includes the quantum confinement effect, conjugated structures, and free zigzag sites, and surface-related emission includes surface or edge defects, crosslink-enhanced emission, and multi-emissive centers.

When a semiconductor crystal is nanometer-sized, the crystal boundary has a significant impact on electron distribution, causing some size-related properties like the bandgap and energy relaxation dynamics.⁶⁸  Numerous semiconductor materials have been investigated for this phenomenon, also referred to as the confinement effect. Typically, bulk graphene composed of atomic layered sheets is a two-dimensional crystal with a zero bandgap and a zero effective mass of charge carriers.^{68, 69}  As a result, unlike other semiconductors, the energy of carriers in graphene follows scaling laws with respect to size.

Due to the light constituent elements and the dimensionality of graphene, the GQDs as a new zero-dimensional quantum confined system possess some special properties.⁷⁰  Choi’s group⁷¹  explored size–dependent absorption and fluorescence spectra of GQDs with different sizes (5–35 nm) and morphologies (see Figure 1.7). The quantum confinement effect is used to explain how the absorption peak energy of GQDs reduces as their size increases. The red-shifted emission behavior is shown by the authors to be caused by the growth in GQD size.

In addition to GQDs, the sp² conjugated system in the carbon core is also crucial to the optical properties of CQDs. The increased sp² conjugated domain causes the decreased band gap. As a result, the gradually red-shifted fluorescence of CQDs is attributed to the decreased band-gap energy because of their increasing conjugated structure.³⁸  Fan’s group⁷²  prepared bright multicolor bandgap fluorescent CQDs from blue to red with a QY up to 75%. The gradually increased size of the CQDs from 1.95–6.68 nm is responsible for the red-shifted fluorescence wavelength. Meanwhile, the first excitonic absorption peak wavelength further demonstrates the bandgap transitions in CQDs, which is due to the quantum confinement effect.

The electrical structure of GQDs can be considerably influenced by their edge structure.⁷³  In light of this, the free zigzag sites on the edge of GQDs may have an impact on their fluorescence.⁷⁴  Wu’s group²⁷  demonstrated that the fluorescence of GQDs results from free zigzag sites with a triplet ground state that resembles a carbene. A new UV-vis absorption band at 320 nm is produced by the significant edge effect of GQDs. Additionally, the 320 nm (3.86 eV) and 257 nm (4.82 eV) electronic transitions observed in the excitation spectra of GQDs can be interpreted as transitions from the σ and π orbitals (highest occupied molecular orbitals, HOMOs) to the lowest unoccupied molecular orbital (LUMO). The blue emission is the result of the irradiation decay of activated electrons from the LUMO to the HOMO since the two transitions directly contribute to the blue fluorescence.

Defects can adjust the luminescence of CDs. The defects are produced from two overlapping spectral bands with intrinsic and extrinsic states, or from O-related functional groups at the surface or edge of CDs. The oxygen level of the CDs is directly related to their red-shift emission. The degree of surface defects increases with surface oxidation level.⁷⁵  These defects can then capture excitons, and the radiation produced by the recombination of these trapped excitons is what produces the red-shift emission.⁷⁵ 

The defects will be greater and the red shift of the luminescence will be more pronounced the higher the oxygen content on the surface or edge. Seo’s group⁷⁶  demonstrated that the formation of the intrinsic state in GQDs is what causes the blue emission and the strong absorption peak on the higher energy side. However, the abundant O-related groups, or defect states, in graphene oxide quantum dots, which have more oxygen than GQDs, are what cause their green fluorescence. Seo’s group⁷⁷  further revealed that the optically excited carriers in GQDs transfer from intrinsic to extrinsic states as the GQDs fluorescence. The carrier transfer, which lengthens the relaxation period, is the cause of the substantially slower recombination times for short excitation wavelengths compared to long excitation wavelengths. Additionally, the basal plane’s and/or the edge’s varied oxygen functional groups are associated with the red-shift emission behavior of GQDs. Meanwhile, Xiong’s group²⁹  produced a number of purified CDs with excitation-independent luminescence ranging in color from blue to red. As the red shift of fluorescent emission progresses, so does the degree of surface oxidation of CDs. The surface-state-related luminescence can be caused by defects that surface oxidation can create and which act as capture centers for excitons.⁷⁸  On the other hand, the band gap is closely linked to the oxygen species that are present.⁷⁹  As the amount of oxygen on the surface of CDs increases, the band gap narrows, causing red-shifted emission to come from an increased level of surface oxidation (see Figure 1.8a).

The surface functional groups of CDs, such as C=O and C=N groups which act as multi-emissive centers, have been shown to be closely related to the luminescence of the CDs. Diverse fluorophores or energy levels can be introduced into CDs by various surface functional groups. Liang’s group⁸⁰  reported multicolor nitrogen-doped CDs with tunable fluorescence from dark blue to red and even white emission. Due to their similar oxygen contents, the surface functional groups on these full-color CDs, rather than the degree of surface oxidation, are what determines their fluorescence. In CDs with different surface states, functional groups like C=O and C=N groups can create two new energy levels (HOMO − 1 and HOMO), as well as new electron transitions from HOMO − 1 and HOMO energy levels to the LUMO (π*), with the exception of the HOMO − 2(π) energy level. As a result, when the electrons in the N-related defect states return to the HOMO, the emission can be shifted to the long wavelength region.

Furthermore, Zhang’s group⁸¹  showed the multi-emissive behavior of CDs at the single particle level using a single particle fluorescence imaging technique, which provided conclusive evidence that full-color emissions are a single particle behavior. Results show that the functional groups (C=O and C=N) may effectively introduce new energy levels for electron transitions and produce continuously variable full color emissions. Meanwhile, Wang’s group⁸²  further investigated the emission behavior of CDs at the single particle level using total internal reflection fluorescence microscopy. The authors show that a single CD has several emission centers and the ability to emit different colors in an excitation-dependent manner (see Figure 1.8b). As a result, several small fluorescent molecules on the surface and emissive graphitic fragments inside the carbon core hybridized to produce the full-color emission of CDs. The complex full-color emission behavior in the CDs is the result of both direct electron transitions of the multiple emission centers separately and energy transfer between various emission centers.

Crosslinking has the effect of enhancing luminescence, which is known as crosslink-enhanced emission (CEE). Crosslinking can be broadly classified as chemical or physical, and the related interactions involved into covalent or noncovalent bonding. From the perspective of interaction modes, Yang’s group⁸³  divided CEE into covalent-bond CEE and noncovalent-bond CEE (supramolecular-interaction CEE, ionic-bonding CEE, and confined-domain CEE) (see Figure 1.8c). Nonconjugated systems that exhibit CEE typically contain electron-rich sub-luminophores such as hydroxyl, carboxylic, imine, amine, and imine moieties. The distance between these functional groups is shortened by crosslinking, which leads to an overlap and coupling of the electron clouds. Such through-space interactions can create sublevels by splitting inherent energy levels. Chemical bonds are formed during covalent-bond crosslinking, which occasionally results in the creation of new energy levels and luminescence. So, by altering energy levels, CEE can affect the emission wavelength. By narrowing the energy gap between the singlet and triplet states (ΔE_S−T), radiative transitions can even reverse their course.

CPDs with a polymer/carbon nanohybrid structure are a typical crosslinking system, having a carbon core and many polymer chains or chemical groups on the surface. The surface of CPDs should have numerous hydrophilic flexible polymer chains, while the interior should be a densely crosslinked network. Chemical crosslinking is successful at enhancing the luminescence of CPDs because it produces new luminescent structures by improving immobilization and through-space interactions.

Currently, many studies have shown that core-related emission and surface-related emission together determine the luminescence of CDs.^78,82,84  In general, the graphitic core is the primary source of the blue-green emission, but the long-wavelength emission is closely related to the C=O and C=N groups found in the molecular residues. Lin’s group⁸⁵  demonstrated the fluorescence behaviors of CDs with multi-emissive states. The authors show that the long wavelength fluorescence (above 400 nm) results from the radiative relaxation from the excited state to ground state due to the presence of various oxygen-containing groups on the CD’s surface, as opposed to the short wavelength fluorescence (below 400 nm), which results from the radiative recombination of electron–hole pairs from the carbon core. Particularly, the amine group surface modification is what starts the red-shifted emission. Lone pair electrons in amine nitrogen groups increase electron density by reducing the energy gap, which causes the emission to shift to a longer wavelength. Meanwhile, Huang’s group²⁸  proposed that the quantum confinement effect and the surface state work in concert to explain the fluorescence of CQDs. By using a one-pot hydrothermal synthesis method and further separation by a silica gel column, a series of full-color emitted CQDs are prepared. Time-correlated single photon counting experiments reveal that the fluorescence progress of CQDs is affected by transition of the electrons. Due to quantum confinement effects, it is possible to coarsely modify the fluorescence properties of these CQDs by varying the size of the crystalline carbon core, and to finely modify them by altering the surface functional groups.

Due to their low toxicity, small size, and high QY, photoluminescent CDs have potential uses in nano-enabled agriculture. According to reports, CDs are advantageous for plant growth, resistance, and photosynthesis. Particularly in photosynthesis, CDs can convert light or transfer energy to chloroplasts by absorbing a wide range of UV light and increasing the electron transport rate, which enhances the photosynthetic activity of the plant. The impact of CDs on plant photosynthesis was investigated by Liu’s group⁸⁶  utilizing chloroplasts and rice plants (see Figure 1.9a). The internalized CDs (CD1:0.2) with a moderate QY of 46.42% convert UV radiation into photosynthetic active radiation when light reaches the chloroplast, which is then absorbed by the chlorophyll in rice leaves, increasing the electron transport rates and actual photosynthetic efficiency in the light reaction of photosynthesis. More assimilatory powers (NADPH and ATP) are then formed for the subsequent carbon assimilation. In the meantime, the application of CD1:0.2 to rice seedlings boosts the carboxylase activity of RuBisCO, which can aid in the assimilation of carbon. Thus, in CD1:0.2-treated rice seedlings, more photosynthetically active radiation can be turned into chemical energy, thereby increasing rice seedling growth. As a result, there are noticeable improvements in the growth of CD1:0.2-treated rice, including increases in shoot length (18.99%), dry weights of shoot (64.31%) and root (64.31%). In a similar vein, Kang’s team⁸⁷  showed that CDs can significantly boost ribulose bisphosphate carboxylase oxygenase activity and encourage the growth of dicotyledons (soybean, tomato, and eggplant), ultimately resulting in a 20% increase in yields.

By boosting photosynthesis and the transfer of carbohydrates, CDs can control root exudates and rhizosphere microorganisms, ultimately improving the drought tolerance of the plant. By introducing CDs into the soil, Wang’s group⁸⁸  developed a successful method for improving the N-fixing ability of nodules, controlling rhizosphere activities, and eventually enhancing N and water uptake in soybeans under drought stress (see Figure 1.9b). The application of CDs under drought stress boosts soybean nitrogenase activity by 8.6% compared to a control (drought stress) and raises N content in soybean shoots and roots by 18.5% and 14.8%, respectively. Additionally, due to increased N bioavailability, the proteins, fatty acids, and amino acids in soybean grains are improved by 3.4%, 6.9%, and 17.3%, respectively. Similarly to this, Wang’s group⁸⁹  demonstrated that foliar-sprayed CDs can influence carbohydrate metabolism, rhizosphere characteristics, and drought tolerance in maize seedlings.

Additionally, CDs have good disease resistance. The effects of CDs on rice plant growth and disease resistance were investigated by Kang’s group.⁹⁰  Because CDs may penetrate cells, get to the nucleus, relax DNA structure, and raise the expression of thionin (Os06g32600), CDs exhibit a strong and lasting effect on disease resistance in different rice plant seedling ages (see Figure 1.9c). The CDs are also degraded to form CO₂ and plant hormone analogues, which subsequently stimulate the growth of the rice plant while the CO₂ is transformed into carbohydrates by the Calvin cycle of photosynthesis. These procedures result in a 14.8% increase in the overall rice output and an expansion of the rice plant. Similar findings were reported by Wang’s group⁹¹ , who discovered that foliar application of CDs (10 mg L^-1) reduces the severity of illness by 71.19% while also increasing the fresh biomass of tomato shoots by 55.86%. The disease resistance mechanism includes the following: (1) N-doping enhanced the reactive oxygen species scavenging capability of CDs, resulting in higher oxidative stress alleviation in tomatoes; (2) CDs activated the salicylic acid and jasmonic acid dependent systemic acquired resistance in tomatoes, causing the inhibition of in vivo pathogen growth; (3) CDs activated the antioxidative enzyme activity in tomatoes, resulting in lower pathogen induced oxidative stress.

The authors appreciate the financial support from the National Natural Science Foundation of China (22134005 and 21874109) and Chongqing Talents Program for Outstanding Scientists (cstc2021ycjh-bgzxm0178).