The fluorescence mechanism of carbon dots, and methods for tuning their emission color: a review

Abstract

Carbon dots (CDs) display tunable photoluminescence and excitation-wavelength dependent emission. The color of fluorescence is affected by electronic bandgap transitions of conjugated π-domains, surface defect states, local fluorophores and element doping. In this review (with 145 refs.), the studies performed in the past 5 years on the relationship between the fluorescence mechanism and modes for modulating the emission color of CDs are summarized. The applications of such CDs in sensors and assays are then outlined. A concluding section then gives an outlook and describes current challenges in the design of CDs with different emission colors.

Introduction

Carbon dots (CDs) have a typical size of less than 10 nm [1, 2]. CDs consist of two parts: one is a spherical-like core formed by stacking multiple graphene fragments in an ordered or disordered manner; the other is rich functional groups distributed on the surface of CDs [2, 3]. Notably, CDs exhibit tunable photoluminescence (PL), multi-color emission associated with excitation [4,5,6], low toxicity and good biocompatibility [7,8,9,10,11]. All these unique properties make CDs have potential applications in biomedicine, sensing and optoelectronics [12,13,14,15,16].

The modulating the color-emitting of CDs have always been the focus of attention [17, 18]. Once the chemical structure of CDs is well controlled during the preparation process, it is possible to clarify the factors affecting the fluorescence mechanism of CDs and ultimately regulate the color-emitting of CDs. In general, the fluorescence behavior of CDs is affected by the relationship between the carbon core and surrounding chemical groups [19]. For some CDs with large conjugated π-domains and few surface chemical groups, the band gap of conjugated π-domains is considered to be the carbon-core states fluorescence center. By adjusting the size of the conjugated π-domains, the color-emitting of CDs can be adjusted. For surface defect states mechanism, the surface chemical groups of CDs have various energy levels and result in more emissive traps. A higher degree of surface oxidation can lead to more surface defects and the red-shift of emission wavelengths. For the molecule states mechanism, the fluorescence center is composed of organic fluorophores formed by small-molecule carbonization. Some fluorescent small molecules or fluorophores are attached to the surface or interior of carbon skeleton and show emission directly. Moreover, doping is an effective method for modulating the color-emitting, QY and decay lifetime of CDs. There are many reviews papers on CDs but the relationship between the fluorescence mechanisms and color-emitting modulation of CDs has not been systematically summarized so far [2, 20,21,22,23].

The present review highlighted the fluorescence mechanism and the color-emitting modulation methods of CDs, as well as resulting fluorescence behaviors and applications. We mainly discussed the modulation of CDs with blue-, green-, yellow-, red-light emission and multicolor and mentioned their applications. This paper will provide a theoretical basis for the structure and properties control of CDs.

Fluorescence mechanism of CDs

Bandgap transitions of conjugated π-domains

A major feature of CDs is the quantum confinement effect (QCE), which appears when CDs are smaller than the exciton Bohr radius. The QCE refers to the valence band and conduction band change from the continuous energy band to discrete energy level and the band gap increases with the decrease of the three-dimensional size of the material at the nanometer level, which results in the band gap transition in the ultraviolet-visible region and greatly improves the fluorescent quantum yield (QY).

For some CDs with large conjugated π-domains and few surface functional groups, fluorescence is essentially derived from the QCE of conjugated π electrons, and the band gap of conjugated π-domains is considered to be the carbon-core states fluorescence center [24,25,26]. The larger the size of CDs conjugated π-domain, the smaller the band gap and the more redshift of the emission peak. The conjugated π-domains with confined size results in the separation of the valence band of the CDs from the conduction band, and the conduction band electrons directly transition to an empty state of the valence band, thereby enabling “direct recombination” of electrons and holes, resulting in a so-called band gap fluorescence. The sp² carbon clusters in CDs are dispersed in the sp³ carbon skeleton, so the optical properties of CDs are determined by the π-electron state of the sp² carbon cluster [27,28,29]. Since the electron energy levels of π and π* in the sp² carbon cluster are affected by the σ and σ* states in the sp³ carbon skeleton, the radiation transition of the electron-hole pairs in the sp² carbon cluster can produce fluorescence. By adjusting the size of the conjugated π-domains, the band gap of CDs can be tuned, and it is expected to modulate the color-emitting of CDs.

The size of the conjugated π-domains can be adjusted using different reaction solvents and the color-emitting of CDs can be further modulated. Tian et al. [30] prepared multicolor-emitting CDs1 using citric acid (CA), urea and three different solvents (water, glycerol, and dimethyl formamide) via a solvothermal method. The solvents control the dehydration and carbonization processes of precursors, resulting in the formation of conjugated π-domains of different sizes in CDs1, which in turn leads to different emission colors from blue to red. They further prepared CDs1 phosphors/ PDMS composites and WLEDs by covering red and green CDs1 phosphors on luminescent blue InGaN chips (Fig. 1).

Fig. 1

Schematic presentation of CDs1 fabrication

The color-emitting of CDs can be tuned by varying precursors and reaction time. Yuan et al. [31] synthesized multicolor-emitting CDs2 by controlling the fusion and carbonization of CA and diaminonaphthalene (DAN). The emission peaks of the blue, green, yellow, orange, and red CDs were centered at 430, 513, 535, 565 and 604 nm, respectively. The QYs were 75%, 73%, 58%, 53% and 12%, respectively. CDs2 showed average sizes of about 1.95, 2.41, 3.78, 4.90 and 6.68 nm, respectively. As the size of CDs2 increases(conjugated π-domain increases), the bandgap decreases gradually, resulting in the tunable fluorescence emission from blue to red. The monochrome LEDs from blue to red and WLEDs were respectively prepared using CDs2 and CDs2 blended PVK as the emission layer (Fig. 2).

Fig. 2

Schematic presentation of CDs2 fabrication

The color-emitting of CDs also can be modulated by changing the molar ratio of precursors and the reaction temperature. More recently, Miao et al. [23] prepared the multicolor-emitting CDs3 by using different molar ratios of CA to urea at different temperatures. The emission of CDs3 can be adjusted from 430 to 630 nm by controlling the degree of graphitization and the number of surface -COOH groups. The QYs of the three CDs3 of blue, green and red were 52.6%, 35.1% and 12.9%, respectively.

With the increase of the effective conjugation length and the surface -COOH groups, the band gap caused by π-electron delocalization in the sp² domain is reduced, and the emission wavelength is red-shifted. CDs3 can be evenly dispersed into epoxy resins to form transparent CDs/epoxy composites for multicolor WLED (Fig. 3).

Fig. 3

Schematic representation of CDs3 fabrication

The bandgap transitions of conjugated π-domains to describe the fluorescence of CDs is actually based on the premise of single-layer graphene fragments, which is not enough to explain the complex structure of multiple graphene fragments stacked like CDs. Therefore, the explanation of the fluorescence properties of CDs based on nanometer size and quantum size effects has certain limitations. Quantum size effects is often used in conjunction with surface defect states.

Surface defect states

Another fluorescence mechanism is that the fluorescence of CDs is dominated by its rich surface defect states. The surface defect refers to a boundary region or a spherical shell that is distinct from the carbon core region or body. The spheroidal region contains various chemical groups derived from sp² and sp³ hybrid carbons, or other surface functional groups [32, 33], or dangling bonds. Due to the diversity and complexity of surface defect states, fluorescence originating from surface defect states is characterized by pleochroism and excitation-dependent luminescence. Surface defects are mainly generated by surface oxidation and can act as a capture center for excitons, resulting in surface defect state fluorescence [34, 35].

Surface defect fluorescence is caused by radiation relaxation from the excited state to the ground state. sp³ and sp² hybrid carbon on the surface of CDs and other surface defects can lead to multicolor emissions of their local electronic states. When the light of a specific wavelength illuminates CDs, the photons whose energy satisfies the optical band gap will transition and accumulate in the adjacent surface defect traps, and return to the ground state to emit visible light of different wavelengths. The higher the degree of surface oxidation of CDs, the more surface defects and emission sites, resulting in the red-shift of emission wavelength [36,37,38].

The surface state is not composed of isolated chemical groups, but a fluorescence center formed by the synergy of carbon core with associated chemical groups. Among them, some groups are fluorescence activation state or silence state under a certain condition. Functional groups have different energy levels and may produce a series of emission traps [39,40,41]. The energy levels of functional groups may be related to their ability to supply electrons. The stronger the ability of functional groups to provide electrons, the higher the energy they generate. Obviously, the emission wavelength can be adjusted by changing the chemical groups on the surface of CDs.

Different degrees of surface oxidation will facilitate the color-emitting modulation of CDs. Ding et al. [42] synthesized multicolor-emitting CDs4 using p-phenylenediamine and urea through a hydrothermal method. After column chromatography, CDs4 exhibit excitation-independent fluorescence from blue to red. The QYs of R-CDs4 was 24%. Different degrees of oxidation of the CDs4 resulted in different surface states, and the band gap gradually decreased, which ultimately determined the fluorescence red shift. Because of their excellent fluorescence properties and low cytotoxicity, CDs4 can be used to image HeLa cells and live nude mice (Fig. 4).

Fig. 4

Schematic presentation of CDs4 fabrication and its fluorescence mechanism

By changing the functional groups on the surface can tune the color-emitting of CDs. Yuan et al. [43] synthesized green light-emitting G-CDs5 and red light-emitting R-CDs5 using perylene as the precursor. The QYs of G-CDs5 and R-CDs5 were 81% and 80%, respectively. The alkali can cause the increase in the quinone structure of R-CDs5, resulting in an electron-rich property. Rich electrons can raise HOMO to higher energy levels, causing a red shift of fluorescence. This highly efficient G-CDs5/MTES and R-CDs5/APTES can be made by doping G-CDs5 and R-CDs5 into MTES and APTES with QYs of 80% and 78%, respectively. Integrating G-CDs5/MTES and R-CDs5/APTES on a blue LED chip, a blue pumped CDs phosphor based three-color warm WLED was prepared (Fig. 5).

Fig. 5

Schematic presentation of CDs5 fabrication

Surface states mechanism was considered to be the key fluorescence mechanism of CDs, which provide a primary way to modulate the color-emitting of CDs through controlling the surface states. However, we cannot ignore the role of the carbon-core states in fluorescence emission, especially in short wavelength regions.

Effects of fluorescent molecules or fluorophores

The fluorescent molecules or fluorophores in CDs are small organic molecules or chromophores similar to dye molecules. These fluorescent small molecules or fluorophores are connected on the surface or interior of the carbon backbone and can exhibit fluorescence emission directly [44]. Such molecular fluorescence mechanisms are more common in the preparation of CDs by small molecule condensation/carbonization [45]. As the carbonization temperature or time increases, the carbon core is formed by dehydration of the initial molecule or formation of a fluorophore. In general, the QYs of fluorescent molecules in such CDs is much higher than the band gap fluorescence efficiency caused by QCE of the conjugated π-domains, but the molecular luminescence stability is lower than that of band gap fluorescence.

Based on the molecule states fluorescence mechanism, Gude et al. [46] prepared orange-red light-emitting CDs6 from easily available sources like sucrose, fructose and glucose at low temperature of 70 °C. CDs6 are composed of aggregated hydroxymethylfurfural (HMF) derivatives and exhibit excitation wavelength independent fluorescence emission at 590 nm. There is only one type of chromophore (HMF derivatives) in CDs6. Aggregated HMF derivatives are responsible for fluorescence and a large Stokes shift (~ 150 nm) of CDs6 (Fig. 6).

Fig. 6

Schematic presentation of CDs6 fabrication

The interaction between bandgap fluorescence and molecular fluorescence further improves the fluorescence mechanism and has a large effect on the color-emitting of CDs [47]. Song et al. [48] fabricated CDs7 using CA and ethylenediamine (EDA) as precursors. A bright blue fluorophore (imidazo[1,2-a]pyridine-7-carboxylic acid, 1,2,3,5 -tetrahydro-5-oxo-, IPCA) was obtained by separation of CDs7. A separate fluorophore IPCA attached to the surface or inside of carbon core is the true molecular state fluorescence center. In CDs with lower carbonization temperatures, molecular fluorescence dominates, while in CDs with higher reaction temperatures, carbon core plays a dominant role. As the temperature increases, a part of the molecular fluorophore is consumed to form a crystalline carbon core. IPCA has polar groups, rigid ring structure and low cytotoxicity, and has broad application prospects in biomedicine (Fig. 7).

Fig. 7

Schematic presentation of CDs7 fabrication

Afterwards, Ehrat et al. [49] synthesized CDs8 by pyrolysis of CA and EDA at different synthesis times. In the initial phase, the molecular fluorophore dominates the blue fluorescence of CDs8. However, over time, the aromatic domains form and increase, causing the fluorescence red shift. A molecular fluorophore may serve as a seed for the formation of aromatic domains (Fig. 8).

Fig. 8

Schematic presentation of CDs8 fabrication

Effects of doping with other elements

Pristine CDs containing only C and O elements usually emit blue fluorescence and have low QYs, and most require further surface modification or functionalization to improve luminous efficiency, which often limits the application and development of CDs. Other element doping can introduce new surface properties to CDs and alter structural defects, surface function groups, and interactions among carbon atoms with their neighboring atoms [50, 51]. These new surface states can suppress or eliminate the original O-states and facilitate a high yield of radiative recombination, resulting in doped CDs with higher QYs and excitation-independent emissions. The doped elements might also be embedded within the carbon core structure of CDs to create new energy levels or change their initial band gap. In summary, doping elements can effectively modulate the color-emitting of CDs and contribute to a deeper understanding of the fluorescence mechanism of CDs.

The QY refers to the fraction of the excited state molecule that returns to the ground state by emitting fluorescence. In other words, the QY is the ratio of emitted photons to absorbed photons. The QY of CDs is influenced by different synthetic method, precursor, surface passivation, and element doping. Element doping (with nitrogen, sulfur, other heteroatoms and metal ions) can well control the band gap and electron local density of CDs, greatly improving their QY. Different surface states derived from doping effects would have a primary influence on the QY of CDs.

The PL decay lifetime of CDs represents the time that CDs spend in the excited state before photon emission returns to the ground state. It is an intrinsic property of CDs, which is usually on the low nanosecond scale. It also can be tuned by different fabrication methods, precursors, surface passivation, or doping with elements. Element doping appears to be an important factor affecting the PL decay lifetime of CDs. Due to element doping, the surface of CDs develops passivated defects, resulting in a longer PL decay lifetime.

The level of QY and the length of decay lifetime are two important indicators for measuring the potential of CDs in bioimaging and sensing. Therefore, in order to widely apply CDs in the field of biological imaging and sensing, QY needs to be improved, and the decay life also needs to be extended.

Effects of nitrogen doped CDs and nitrogen morphology

Since the radius of the N atom is similar to that of the C atom and the N atom is easily incorporated into the carbon material skeleton, the N-doped CDs are the most studied and most potential doping system [52,53,54]. The effect of N doping on the fluorescence properties of CDs is mainly divided into two aspects [55,56,57]: the red-shift of emission wavelength and the fluorescence QY increase [58, 59]. The rearrangement of electron hole and radiation theory is mainly used to explain the fluorescence mechanism of heteroatom-doped CDs. The nitrogen atom produces a new surface state energy level resulting in a red-shift of emission wavelength. Nitrogen-doped surface states can facilitate a high yield of radiative recombination and depress non-radiative recombination. The electron-donating amino-groups on the CDs surface can enhance the conjugation degree of conjugated systems, increasing the electron transition from the ground state to the lowest excited singlet state, and thus contribute to higher QY indirectly of the CDs [60,61,62].

Differences in nitrogen content caused by doping can modulate the color-emitting, QY and decay lifetime of CDs. Jiang et al. [63] synthesized CDs9 using three different phenylenediamine isomers in ethanol through a solvothermal method. Under 365 nm UV excitation, CDs9 emitted red, green and blue fluorescence in solution and in the polymer matrix. The difference in the color-emitting, QY and decay lifetime of CDs9 was due to the increase in nitrogen content (B-CDs9, G-CDs9, and R-CDs9 were 3.69%, 7.32%, and 15.57%, respectively). The maximum emission wavelength of B-CDs9, G-CDs9 and R-CDs9 were 435, 535, and 604 nm under excitation at 365 nm, respectively. The fluorescence QY was 4.8, 17.6 and 26.1%, respectively. The decay lifetime was 0.99, 4.44 and 9.39 ns, respectively. CDs9 can prepare multicolor emission PVA films, multicolor displays and multiplexed biological imaging reagents (Fig. 9).

Fig. 9

Schematic presentation of CDs9 fabrication and its fluorescence mechanism

N-related functional groups produced by doping will have a great effect on the color-emitting of CDs. Pan et al. [64] prepared full-color emission CDs10 via microwave assisted heating CA and formamide solution at 160 °C for 1 h. CDs10 exhibited excitation-independent fluorescence from blue to red. The QY of CDs10 was 11.9%, 16.7% and 26.2% when the excitation was 360, 450, and 540 nm, respectively. The blue light-emitting of CDs10 should come from an aromatic structure, while the green and red light-emitting are due to the presence of C=N/C=O and C-N related functional groups. CDs10 can be used to detect a variety of metal ions, and as biomarker reagents for deep tissue imaging (Fig. 10).

Fig. 10

Schematic presentation of CDs10 fabrication

The color-emitting of CDs can be modulated by controlling nitrogen-doped surface states. The QY increased and fluorescence lifetime was prolonged as the nitrogen content increases. Yuan et al. [65] prepared CDs11 with the nitrogen-content changes from 5.5% to 19.9% by controlling the molar ratios of CA and guanidine hydrochloride. CDs11–1, CDs11–2 and CDs11–3 showed emission-dependent emission, while CDs11–4, CDs11–5 and CDs11–6 with high surface -NH₂ groups density showed excitation-independent emission. The surface states of CDs11–6 be completely passivated by electron-donating -NH₂ groups, leading to single energy level and showing high fluorescence emission efficiency. Nitrogen-doped surface states promoted a high yield of radiative recombination of the surface-trapped electrons and holes in CDs11. Furthermore, tunable fluorescent CD11 has been used as an effective fluorescent probe for semi-quantitative monitoring of intracellular Hg²⁺ (Fig. 11).

Fig. 11

Schematic presentation of CDs11 fabrication

The complex N-doping process leads to the existence of multiple nitrogen species in the structure of CDs, and the different configurations of N atoms have unlike effects on the different properties of N-doped CDs [66]. The nitrogen forms present in CDs can be mainly classified into four types: amino type, pyrrole type, pyridine type and graphite type. The amino N is distributed on the surface of CDs, while the pyrrole N and pyridine N can be distributed at the edge and center of graphene structure, and the graphite N is regularly doped in the graphene skeleton. The fluorescence properties of CDs can be further demonstrated by the clear nitrogen form and the distribution. Most researchers believe that amino groups can improve the fluorescence of CDs by self-passivation [67, 68].

The increasing graphite nitrogen is beneficial for modulating the color-emitting of CDs. The aromatic amine or terminal aliphatic amine of CA can cause C-N cross-linking during carbonization to form graphite nitrogen structure, which is beneficial for red light-emitting of CDs. To further investigate the relationship between nitrogen form and the fluorescence properties of CDs, Hola et al. [69] prepared multicolor emission CDs12 using urea and CA in formamide via a solvothermal process. CDs12 were separated by column chromatography based on differences in surface charge. In water, the QYs of B-CDs12, G-CDs12, Y-CDs12, and R-CDs12 were 13.3%, 10.0%, 11.6%, and 4.0%, respectively. The increasing graphite nitrogen in CDs12 caused the energy gap decrease, finally resulting in the red-shift of emission wavelength. R-CDs12 had great application potential in the field of biomedicine (Fig. 12).

Fig. 12

Schematic presentation of CDs12 fabrication

The number and speciation of N-functional groups on the surface are related to modulating the color-emitting of CDs. Liu et al. [70] synthesized blue, yellow and orange fluorescence CDs13 using m-phenylenediamine, o-phenylenediamine and p-phenylenediamine as precursors by microwave method. CDs13 showed wavelength-independent excitation. The QYs of three CDs13 were 14.3, 45 and 7.5%, respectively. The non-amino N can break the conjugated structure, creating many defect states to produce more polyaromatic structures, and finally enhance fluorescence. The C=O/ -CONH- groups can give rise to local isolation that will induce deformations in the plane of CDs13, thus cause the energy gap decrease, finally resulting in the red-shift in emission. Moreover, CDs13 have been applied in multicolor cellular imaging, starch/CDs composite phosphors and LEDs illumination (Fig. 13).

Fig. 13

Schematic presentation of CDs13 fabrication and its fluorescence mechanism

Effects of nitrogen and sulfur co-doped CDs

In addition to the N element, other types of non-metallic element doped CDs have also been reported, but the scale of study is far less than that of N-doped CDs. S-doped CDs are the second type of doped CDs that occur after N-doping, where the S element may be in the form of a thiol, thioether or sulfonic acid group [71, 72]. The S element that usually alone exists has no significant effect on the fluorescence emission wavelength, which may be related to its difficulty in embedding the carbon skeleton. Since heteroatom co-doping can produce a unique electronic structure, there is a synergistic effect between the N and S doped in CDs. The atomic radius of N and S is closer to C, and the availability of five valence electrons in N and six in S helps to bind N and S with C. The N and S element can increase the probability of electronic transition from the ground state to the excited state, thereby leading to a change in bandgap energy and significantly increasing QY of CDs.

The role of the S atom may be to suppress the surface state of the O atom and stabilize and enhance the surface state of the N atom. Bao et al. [73] synthesized N, S co-doped CDs (CDs14) using l-cysteine and CA as precursors. The maximum emission wavelength of CDs14 is 415 nm under excitation of 345 nm UV light. The fluorescence QY is calculated to be 16.9%. The fluorescence emission of CDs14 is mainly the recombination of electrons and holes caused by surface-doped nitrogen and/or sulfur atoms. The S atom provides a synergistic effect on the function of the N atom, which greatly accelerates the radiation transition of the N-related surface states, resulting in fluorescence dominated by nitrogen-containing functional groups. CDs14 showed good fluorescence activity and low toxicity, and were widely used in bioimaging and other fields (Fig. 14).

Fig. 14

Schematic presentation of CDs14 fabrication and its fluorescence mechanism

Different surface states derived from doping effects would have a vital influence on the modulating the color-emitting of CDs. Guo et al. [74] fabricated multicolor CDs15 with blue to NIR tunable emission at a single excitation wavelength by varying the ratio of PT1 and PT2 in the polythiophene derivatives via hydrothermal method. CDs15 have an average size of about 3–6 nm, good water solubility and low cytotoxicity. The increasing nitrogen content and the introduction of quaternary ammonium structure are the main reasons for tunable emission colors, while the sulfur element plays a synergistic effect. Multicolor CDs15 have been used in HeLa cells imaging due to their low cytotoxicity and good biocompatibility (Fig. 15).

Fig. 15

Schematic presentation of CDs15 fabrication and its fluorescence mechanism

Moreover, some new functional groups can be introduced into the CDs by doping N and S elements, the emission wavelength of N, S co-doped CDs can be significantly red-shifted. Miao et al. [75] developed strong red emission N, S-CDs (CDs16) using CA and thiourea as sources by a one-step hydrothermal route. Some new functional groups such as thiophene, pyrrole ring, and -SCN were introduced onto CDs16 by introducing S and N elements. Due to the conjugation of sp² carbon with thiophene and pyrrole units, the emission wavelength of CDs16 can be red-shifted. CDs16 showed sensitivity for Fe³⁺. Doxorubicin was loaded onto CDs16 to form the CDs16-Dox, which was applied in fluorescence imaging of nude mice (Fig. 16).

Fig. 16

Schematic presentation of CDs16 fabrication

Effects of other heteroatoms doped CDs

The type of heteroatom in the doped CDs is not limited to nitrogen and sulfur, and includes various elements such as phosphorus, boron, fluorine and selenium. Moreover, there is not only one heteroatom-doped CDs, but also a plurality of heteroatoms co-doped with CDs [76, 77]. The different types, positions and doping patterns of elements in CDs have different effects in modulating the color-emitting of CDs. Some can adjust the emission wavelength, and some can significantly improve the QY. Doping with heteroatoms greatly enhances PL emission of CDs, and the synergistic effect of co-doping facilitates the formation of band gaps, resulting in the high QY.

Patra et al. [78] synthesized nitrogen-containing CDs (N-CDs17), Phosphorus-doped CDs (P-CDs17) and Boron-doped CDs (B-CDs17) using CA, diethylenetriamine (DETA), o-phosphoric acid (o-PA) and boric acid (BA) by a hydrothermal method. The electron-rich P atom acts as an n-type donor to enhance the fluorescence properties of the CDs and contribute to the radiation transition pathway, while the electron-deficient B atom acts as a p-type dopant, which increases the non-radiative recombination pathway, resulting in a decrease in fluorescence. Therefore, the electron and hole transfer processes of n-type and p-type doping in CDs open up novel applications for a new generation of doped semiconductor materials (Fig. 17).

Fig. 17

Schematic presentation of CDs17 fabrication

F-doping method can effectively modulate the color-emitting of CDs. Zuo et al. [79] used o-phenylenediamine or 1,2-diamino-4,5-difluorobenzene and tartaric acid through solvothermal method to obtain undoped CDs (CDs18) and F-doped CDs (F-CDs18). The former can emit fluorescence at 500 nm (480 nm excitation) or 550 nm (540 nm excitation), while the latter shows a large red-shifted fluorescence at 550 nm or 600 nm. The QY of CDs18 was 28%. After the F doped on CDs18, F-CDs18 showed higher QY of 31%. F-CDs18 can be used as a novel probe for efficient cell imaging of a variety of normal cells and cancer cells. In addition, F-CDs18 are also the Ag⁺ detection probe which have been applied in environmental monitoring (Fig. 18).

Fig. 18

Schematic presentation of CDs18 and F-CDs18 fabrication

The doped CDs show improved fluorescence and much higher QY than those of oxidized-CDs. Xie et al. [80] oxidized CNPs in Chinese ink and cut it into oxidized-CQDs, and then obtained Se-doped CDs (Se-CDs20) by reduction and in-situ doping reaction. Taking S and Se as examples, the electronegativity of the heterojunction may determine the emission wavelength of the doped CDs. Since the electronegativity of S and Se is smaller than C, it can be used as an electron acceptor, which causes the fluorescence emission peak of CDs to be red-shifted. The S element has a weak influence on the fluorescence position of CDs, while the Se-CDs20 can emit a noticeable yellow fluorescence. The yellow fluorescence of Se-CDs20 may be a special energy level from the formation of surface C-Se bonds. These doped CDs have a great potential in the development of various CDs-based functional materials and sensing devices (Fig. 19).

Fig. 19

Schematic presentation of Se-CDs19 fabrication

Effects of CDs associated with metal elements

Metal element doping can also modulate the color-emitting, QY and decay lifetime of CDs to some extent [81,82,83,84]. Yellow-emitting CDs (Mn-CDs20) have been synthesized using CA, urea, manganese acetate and toluene by Zhang et al. [85]. Compared with CDs20, Mn-CDs20 have an increase extent of conjugated π-domains and surface state, the emission wavelength was red-shifted. That is, Mn²⁺ can polymerize more CA molecules under dissolved heat conditions, thereby ensuring a high degree of conjugation of the sp²-domain and the formation of more surface groups. The fluorescence QY and lifetime of CDs20 was 46% and 8.11 ns, respectively. Due to the Mn²⁺ doped on CDs20, Mn-CDs20 surface developed passivated defects, which showed higher QY (68.6%) and longer lifetime (8.75 ns). An ultraviolet-pumped LED was fabricated using Mn-CDs20. The LED displays warm orange light with a color purity of 99% (Fig. 20).

Fig. 20

Schematic presentation of CDs20 and Mn-CDs20 fabrication

Wu et al. [86] synthesized Cu-CDs21 using Na₂[Cu (EDTA)] as raw material by one-step pyrolysis. Schiff-base-like structure transformed into a Cu coordination complex chelated with carbon matrixes. Cu-CDs21 showed copper content of 2.1%. Although Cu-CDs21 have no special performance in terms of fluorescence properties, their electron acceptor and donor capacities are 2.5 and 1.5 times higher than those of ordinary CDs. Cu-CDs21 have been used as photocatalysts for the photo-oxidation of 1,4- DHP in water solution (Fig. 21).

Fig. 21

Schematic presentation of Cu-CDs21 fabrication

Cheng et al. [87] used zinc ions and CA as precursors to prepare strong yellow photoluminescence Zn²⁺-doped CDs (Zn-CDs22) by solvothermal method. Compared with ordinary CDs22 synthesized without adding a zinc source, the addition of zinc causes the emission peak to shift from the blue band to the yellow band. Zinc exists in the form of Zn-O bonds, thereby promoting some kind of formation of defect states. The fluorescence QY and lifetime of CDs22 was 12% and 5.5 ns, respectively. Due to the Zn²⁺-doped on Zn-CDs22, QY (51.2%) and decay lifetime (6.8 ns) is longer than that of CDs22. A bifunctional membrane was fabricated using colloidal PS-co-PMAA microspheres and Zn-CDs22. Zn-CDs22 have a good application prospect in the fields of inkjet printing, anti-counterfeiting, sensing and LED (Fig. 22).

Fig. 22

Schematic presentation of CDs22 and Zn-CDs22 fabrication and its fluorescence mechanism

Feng et al. [88] prepared a novel Co²⁺-doped CDs (Co-CDs23) with an average size of 2.93 nm by hydrothermal method using 1-(2-pyridylazo)-2-naphthol and cobalt chloride as raw materials. The Co element introduces a new energy level for Co-CDs23, similar to charge transfer, resulting in orange fluorescence of Co-CDs23 with a QY of 6.2%. In the presence of Cr (VI), the interaction of Cr (VI) with the surface groups of Co-CDs23 produced an internal filter effect (IFE), which had potential for identifying Cr (VI) in real samples (Fig. 23).

Fig. 23

Schematic presentation of Co-CDs23 fabrication

In many CDs with metal elements, the forms of many elements are changed, which may be caused by the change of defect states caused by functional groups, or the new radiation transition caused by metal elements. In the future, only the strict, careful and thorough separation of metal element doped CDs can reveal the state of metal elements, indicating the fluorescence mechanism of metal doped CDs.

Throughout these existing reports, although people are trying to explore the mechanism of doping to improve the fluorescence characteristics of CDs, there is still no authoritative conclusion. Usually because of too many variables, it is difficult to accurately synthesize doped CDs with the desired fluorescence properties.

Different color-emitting CDs

CDs have different color-emitting and can cover blue to red range, even ultraviolet and near infrared [57, 89, 90]. In recent years, many synthetic methods have been developed to prepare and optimize the optical properties of CDs. Systematic studies of different color-emitting CDs prepared by diverse precursors, methods and reaction conditions will help to clarify the factors affecting the fluorescence mechanism of CDs.

Blue light-emitting CDs

Although a variety of CDs have been reported, the most common is the blue light-emitting CDs. Most of these CDs showed strong fluorescence emission and high QY, but the Stokes shift was small and the photobleaching resistance was relatively poor.

Glutathione (GSH) containing free thiols, carboxyl and amine groups can form a protective shell around CDs. Wang et al. [91] synthesized blue-emitting CDs24 using glucose and GSH via a hydrothermal process. The QY of CDs24 in aqueous solution is 7.2%. The surface passivation of CDs24 resulted in their blue fluorescence emission. The surface state and size effect that affect the band gap of CDs24 are the cause of complex fluorescence behavior. CDs24 have great potential as novel luminescent thermometers and pH sensors, especially their fluorescence intensity is temperature sensitive and varies greatly over a temperature range from 15 to 60 °C (Fig. 24).

Fig. 24

Schematic presentation of CDs24 fabrication

By doping N and S elements, the emission wavelength of N, S co-doped CDs can be significantly red-shifted. The heterocyclic aromatic structure of caffeine is a fusion of a pyrimidine ring and an imidazole ring, and is an effective precursor for the synthesis of CDs. Kim et al. [92] used caffeine, ammonium persulfate as raw materials to obtain o-CDs25 by a simple one-pot solid state synthesis method. The QY of o-CDs25 was 38%. When adding urea, u-CDs25 exhibited bright blue emission with a QY of up to 69%. CDs25 can establish a selective, sensitive fluorescence platform for detecting Ag⁺ in aqueous media (Fig. 25).

Fig. 25

Schematic presentation of CDs25 fabrication

The fluorescence enhancement may originate from polyaromatic structures induced by incorporated nitrogen atoms on CDs. Diamines can act as nitrogen precursors to form polyaromatic structures, resulting in stronger fluorescence emissions. Qian et al. [93] fabricated N-doped CDs (CDs26) through a solvothermal approach using CCl₄ and diamines. CDs26 emit bright blue fluorescence and the QY is greatly elevated to 36.3% due to the introduction of N atoms. The average fluorescence lifetime of CDs26 was 10.3 ns. The pyridinic and pyrrolic N atoms can be regarded as defect structures, the emergence of these defects would lead to formation of more polyaromatic structures and give rise to stronger emission. CDs26 can be used as a multi-function sensor for the detection of pH, Ag⁺, Fe³⁺and H₂O₂ in aqueous solution. The fluorescence intensity of CDs26 is inversely proportional to the pH value, ranging from 5.0 to 13.5. In addition, CDs26 can be applied to human Hela cells imaging (Fig. 26).

Fig. 26

Schematic presentation of CDs26 fabrication

Metal element doping can also modulate the color-emitting and QY of CDs by chelation enhanced fluorescence mechanism. Lin et al. [94] prepared N, P-CDs (CDs27) using o-phosphorylethanolamine and CA as precursors via hydrothermal treatment. CDs27 with blue-green emission had a maximum emission wavelength at 435 nm when the excitation was 325 nm. CDs27 with QY of 8.17% showed excitation-dependent emission. CDs27 exhibited a fluorescence enhancement effect in the presence of Cd²⁺ from 0.5–12.5 μM with a LOD of 0.16 μM. The phenomenon may be attributed to chelation enhanced fluorescence mechanism via the coordination reaction between functional groups on the surface and Cd²⁺. The method has been used as a nanoprobe for detection of Cd²⁺ in serum and urine samples (Fig. 27 and Table 1).

Fig. 27

Schematic presentation of CDs27 fabrication

Table 1 An overview on blue-emitting carbon dots

Green light-emitting CDs

Green light-emitting CDs are also more common in recent years, but have lower QYs than blue light-emitting CDs. It is still a challenge to prepare highly stable water-soluble green light-emitting CDs.

Different surface functional groups will effectively modulate the color-emitting of CDs. CDs with different optical properties can be prepared by different synthesis methods. Wang et al. [95] prepared microwave-synthesized CDs (m-CDs28) by microwave method using CA and urea as precursors. The electrochemical-synthesized CDs (e-CDs28) were prepared by using graphite rod as carbon source. Ultrafast spectroscopy reveals that a particular edge state consisting of several carbon atoms at the edge of the carbon skeleton and chemical groups with C=O is the origin of green emissions. The optical properties of CDs are mainly determined by the competition between different emission centers and traps (Fig. 28).

Fig. 28

Schematic presentation of m-CDs28 and e-CDs28 fabrication

The surface states can capture the excited state energy, resulting in a significant red shift in the emission wavelength of CDs. Yang et al. [96] synthesized green light-emitting CDs29 by hydrothermal treatment of ethylene diamine in the presence of phenol. The fluorescence QY and lifetime of CDs29 was 15% and 3.83 ns, respectively. CDs29 showed a maximal emission wavelength at 513 nm under 410 nm excitation. The green emission of CDs29 was attributed to the capture of excited state energy by surface states. A fluorescence method for the insecticide cartap quantitation was developed by using green fluorescence CDs29 and Au NPs based on IFE. The method was used for the detection of cartap in the 5–300 nM with a detection limit of 3.84 nM (Fig. 29).

Fig. 29

Schematic presentation of CDs29 fabrication

Polyethyleneimine (PEI) is an amino-rich cationic polyelectrolyte to prepare fluorescence CDs. However, high molecular weight PEI is more toxic than low molecular weight oligoethylenimine (OEI). Zheng et al. [97] synthesized green fluorescence CDs30 through carbonization of OEI and β-cyclodextrin (βCD) via a heating process in phosphoric acid. The QY of CDs30 is 30%. CDs30 containing carboxyl and carbonyl groups are doped with nitrogen and phosphorus. CDs30 have been used in biomedical fields such as cell labeling and imaging. In addition, the cytotoxicity of Dox-loaded CDs30 nanocomposites on H1299 cancer cells was enhanced compared with that of free Dox (Fig. 30).

Fig. 30

Schematic presentation of CDs30 fabrication

Active dry yeast (ADY) contains a variety of carbon compounds such as amino acids, nucleotides, sugars and vitamins, which can be used as a precursor for large-scale synthesis of CDs. Folic acid (FA) has high affinity and stability and is an ideal ligand for Folate receptor (FR), an important tumor biomarker. Zhang et al. [98] used ADY as a raw material to prepare bright green light-emitting CDs31 by simple and rapid microwave method. Folic acid- conjugated CDs (FA-CDs31) were further prepared by covalent coupling of CDs31 and FA by EDC/NHS reaction. The surface of CDs31 has abundant -OH, -COOH and -NH₂ groups and self-passivation of surface traps. FA-CDs31 can be used for targeting and imaging of FR-positive cancer cells. FA-CDs31 have broad prospects for cancer diagnosis and treatment (Fig. 31).

Fig. 31

Schematic presentation of CDs31 and FA-CDs31 fabrication

Different surface functional groups will facilitate the color-emitting modulation of CDs. Mohan et al. [99] fabricated fluorescent CDs32 using ascorbic acid via hydrothermal method in the presence of ethylene glycol. CDs32 emit wavelengths of 382 nm and 538 nm under excitation of 320 nm and 420 nm, respectively. Under 365 nm excitation, CDs32 exhibit dual-band emission, one at 435 nm and the other at 538 nm. The green fluorescence of CDs32 is attribute to the formation of molecular states including C=O and -COOH on the surface of CDs32. The blue fluorescence is attribute to all kinds of surface states of CDs32 including both mono- and double-bonded oxygen chemical groups. CDs32 have potential application prospects in cell imaging (Fig. 32).

Fig. 32

Schematic presentation of CDs32 fabrication

The doped of phosphorus as a heteroatom into CDs can produce new optical properties. Jiang et al. [100] used sucrose as carbon source to prepare green light-emitting CDs33 via a solvothermal process. CDs33 with a QY of 6% exhibited stable green fluorescence at 520 nm. CDs33 with good reproducibility have a reversible linear response to temperature from 20 to 85 °C as well as the temperature resolution better than 0.5 °C. The green fluorescence of CDs33 can be attributed to the modulation of chemical and electronic properties caused by the introduction of P atoms. Moreover, CDs33 have potential as a fluorescent probe for temperature monitoring and cell imaging (Fig. 33).

Fig. 33

Schematic presentation of CDs33 fabrication

Rare-earth elements have special properties due to their unique atomic structure. It can be used as a heteroatom for the preparation of co-doped CDs, thereby improving the optical properties of the original CDs. Yang et al. [101] fabricated La-doped CDs (La-CDs34) that the emission shifted from blue to green (445 to 510 nm) by microwave pyrolysis. The QY of CDs34 and La-CDs34 was 10.21% and 12.42%, respectively. The fluorescence lifetime of CDs34 and La-CDs34 was 10.34 ns and 9.8 ns, respectively. The fluorescence enhancement of La-CDs34 may be caused by local structural changes caused by La-O bonds. Since La³⁺ do not bind to surface oxygen atoms, but combine with oxygen atoms inside La-CDs34, which promotes electron or hole distribution, increases charge transfer and reduces the energy gap. The combined effect enhances the PL performance of La-CDs34. La-CDs33 were successfully used for sensitive imaging of Fe³⁺, and the LOD was 91 nM in live HeLa cells (Fig. 34 and Table 2).

Fig. 34

Schematic presentation of CDs34 and La-CDs34 fabrication

Table 2 An overview on green-emitting carbon dots

Yellow light-emitting CDs

The common photodamage of biological substrates by autofluorescence and short-wavelength excitation light limits the biological application of blue and green light-emitting CDs [102, 103]. Currently, yellow light-emitting CDs with large Stokes shifts have been prepared, but QYs are generally low. The preparation of yellow light-emitting CDs with high QYs typically relies on the use of specific precursors in special solvents, these CDs exhibit high ability in biological applications.

The emission colors can be adjusted by changing the surface states of CDs. The 2,3- phenylenediamine (PD) with larger conjugated structure is favorable for the preparation long-wavelength emission CDs. Based on it, our groups developed yellow light-emitting CDs35 via solvothermal method using CA and PD as the precursors [104]. CDs35 showed a fluorescence QY of 24% and a stokes shift of 188 nm. The fluorescence emission peak centered at 568 nm under the excitation of 380 nm. Due to the high oxidation degree of -COOH on CDs35 surface, the energy band gap is reduced, leading in yellow light-emitting. CDs35 has been applied to the detection of Ag⁺ and GSH in living cells, and will be further applied in environmental monitoring and biomedicine (Fig. 35).

Fig. 35

Schematic presentation of CDs35 fabrication

Dicyandiamide is an excellent N-containing precursor for the preparation of CDs. Li et al. [105] synthesized green-yellow light-emitting N-doped CDs (CDs36) using dicyandiamide and 1,2 -diaminobenzene as the precursors by one-pot hydrothermal process. The optimal excitation and emission wavelength are at 420 and 556 nm. CDs36 in aqueous solution have two typical UV-visible absorption peaks at 276 nm and 360 nm. The transition at 360 nm is due to the trapping of excited state energy by the surface states, resulting in strong emission. CDs36 can measure Ag⁺ with a LOD of 5 × 10⁻⁸ mol L⁻¹ in water and have potential applications for imaging HepG2 cells (Fig. 36).

Fig. 36

Schematic presentation of CDs36 fabrication

Bright yellow N-doped CDs (CDs37) with the emission maximum at 568 nm were prepared using 1,2,4-triaminobenzene as carbon precursors via a solvothermal reaction by Jiang et al. [106] The QYs were 32.5% in ethanol. There is only one dominant emission state in CDs37, and the -COOH groups and/or C-N related structures in CDs37 are responsible for the yellow light-emitting. CDs37 and Ag⁺ can be used to detect cysteine in human plasma samples, which has potential application prospects in the cellular imaging and bifunctional sensing (Fig. 37).

Fig. 37

Schematic presentation of CDs37 fabrication

Nitrogen atoms on CDs surface will provide lone pairs of electrons, leading to Fermi level shift, band gap and optical properties change. Ding et al. [107] prepared the N-doped CDs (CDs38) with bright orange fluorescence emission at 592 nm using p-phenylenediamine as the precursor by microwave method. The prepared CDs38 show excitation wavelength independence and sensitive pH response. The optimal excitation and emission wavelength were at 535 nm and 592 nm, respectively. The surface of CDs38 have -OH and -NH₂ groups. Furthermore, CDs38 has been successfully used as a fluorescent probe for A-549 cell imaging and has potential application prospects in the detection of pH in living cells (Fig. 38).

Fig. 38

Schematic presentation of CDs38 fabrication

Bandgap transitions of conjugated π-domains and surface defect states can jointly modulate the color-emitting of CDs. Mutuyimana et al. [108] synthesized yellow light-emitting CDs39 using 4-acetamidobenzaldehyde and 4-aminoacetenilide hydrochloride via hydrothermal process. CDs39 showed a maximum emissive wavelength at 550 nm under 470 nm excitation. The yellow fluorescence of CDs39 derived from the bandgap transitions of conjugated π-domains and/or surface defect states. The fluorescence of CDs39 was quenched by Cr⁵⁺ via static quenching effect. CDs39 were used as a fluorescent nanoprobe for the determination to Cr⁵⁺ in the range of 1–400 μM with a LOD of 0.13 μM. This method was successfully applied to the detection of Cr⁵⁺ in water and biological tissue samples (Fig. 39).

Fig. 39

Schematic presentation of CDs39 fabrication

Based on the bandgap of conjugated π-domains and surface states mechanism, Xu et al. [109] prepared yellow light-emitting CDs (CDs40) using glucose and H₃PO₄ via microwave method. The amphibious CDs40 show strong yellow fluorescence in both water and organic solvents. The surface of CDs40 has oxygen-containing and phosphorus-containing functional groups. The degree of oxygen-containing groups and the size of the particles can tune the emission wavelength of CDs40. Furthermore, CDs40 can detect chromium by fluorescence colorimetry, and the LOD of Cr (VI) ions is 0.02 × 10⁻⁶ M. It has broad prospects in practical applications such as bio-imaging, optical equipment and Cr (VI) visual field nanoprobe (Fig. 40).

Fig. 40

Schematic presentation of CDs40 fabrication

Co-doping of P and Cl can facilitate the formation of band gaps and enhanced fluorescence emission of CDs41. Wang et al. [110] prepared P, Cl-CDs (CDs41) via one-pot hydrothermal treatment using maltose, phosphoric acid and hydrochloric acid as C, P, and Cl sources, respectively. CDs41 exhibited yellow fluorescence with a QY of 14.9%. They have excitation/emission maxima at 390/590 nm. The fluorescence of CDs41 were quenched by Fe³⁺ due to their strong coordination with Fe³⁺. This fluorescence nanoprobe was successfully applied to the determination of Fe³⁺ in water and human serum samples in the range from 0.1–8.0 μM, and the detection limit is 60 nM (Fig. 41).

Fig. 41

Schematic presentation of CDs41 fabrication

By conjugated sp²-domain controlling and surface charges engineering, Qu et al. [111] developed high-efficiency orange-emitting CDs42 by solvothermal pathway with CA and urea as precursors. DMF is a good solvent for the formation of CDs42 with large conjugated sp² domains and is the basis for orange bandgap emission. The emission of CDs42–1 and CDs42–2 were centered at 580 nm and 590 nm under 540 nm excitation, with QYs of 46% and 6%. The surface metal ion-functionalization increased the carboxylate groups on the inner surface of CDs42–1, leading to an electron-rich property. The orange emission of CDs42–1 and CDs42–2 are bandgap emission of the conjugated sp² domain in the nitrogen-rich multilayer graphene-like core. A highly efficient orange emissive starch/CDs42–1 phosphor (QY of 21%) was prepared, and warm WLEDs were realized using only green and orange emissive starch/ CDs42 phosphors (Fig. 42).

Fig. 42

Schematic presentation of CDs42, CDs42–1 and CDs42–2 fabrication

Phenylenediamines having three isomers of o-phenylenediamine, m-phenylenediamine and p-phenylenediamine are excellent precursors for the preparation of CDs. Song et al. [112] prepared CDs43 with strong yellow fluorescence by microwave-assisted method using o-phenylenediamine as the carbon precursor. The emission peak of CDs43 solution was centered at 573 nm under 400 nm excitation, and the QY is 38.5%. The fluorescence behavior of CDs43 is attributed to differences in the molecular structure of the phenylenediamine isomers that might form CDs43 with different morphology, size, or chemical groups. CDs43 can be used for fluorescence imaging of SW480 cells and detection of Fe³⁺ and H₂O₂, the LOD for Fe³⁺ and H₂O₂ is 16.1 nM and 28.1 nM, respectively (Fig. 43).

Fig. 43

Schematic presentation of CDs43 fabrication

Coal tar is a cheap and abundant by-product containing polycyclic and heterocyclic aromatic molecules. Geng et al. [113] used coal tar to nitrate and then toluene solvent heat treatment to synthesize orange fluorescent CDs44. The QYs of CDs44 in the toluene solution was 29.7%. The orange-emitting CDs44 exhibit excitation wavelength dependence. As the excitation wavelength increases, larger sized CDs44 can be excited and emit fluorescence at long wavelengths, resulting in the red-shift of emission wavelength. In addition to size effects, the distribution of different emission sites on CDs44 may also cause the redshift. The PEGylated liposomes encapsulating CDs44 are useful for bioimaging. Liposomes-CDs44b have been successfully used in vitro and in vivo imaging (Fig. 44).

Fig. 44

Schematic presentation of CDs44 fabrication

Metal element doping may affect surface defect states fluorescence, making bandgap transitions of conjugated π-domains modulate the color-emitting of CDs. Feng et al. [114] successfully synthesized manganese-doped CDs (Mn-CDs45) using 1-(2-pyridylazo)-2-naphthol as both a carbon source and a metal ion chelating agent. The addition of Mn²⁺ changes CDs from the original blue fluorescence to yellow fluorescence. The blue fluorescence in CDs comes from the surface defect state, while the yellow fluorescence is caused by the bandgap transitions of conjugated π-domains. That is to say, the change of the fluorescence position is not because the Mn element induces a new energy level, but Mn²⁺ annihilates the surface defect state luminescence by binding with the N atom in the defect, so that the bandgap transitions of conjugated π-domains becomes dominant. In addition, Mn-CDs45 have been applied in bioimaging, fluorescent inks, nanoprobe and fluorescent films (Fig. 45 and Table 3).

Fig. 45

Schematic presentation of CDs45 and Mn-CDs45 fabrication

Table 3 An overview on yellow-emitting carbon dots

Red light-emitting CDs

Red light-emitting CDs shows optimal tissue penetration, minimal damage to biological samples, and low background interference [115]. In addition, red CDs can provide missing building blocks for full-color emission spectroscopy, for example, white light emitting diodes. Therefore, the development and promotion of long-wavelength emission CDs are of great significance in the field of lighting and biomedical. Currently, The QYs of red light-emitting CDs are generally low. In order to improve the fluorescence behaviors of red CDs, a large number of studies including the selection of the reaction precursors and optimization of the synthetic approach have been continuously conducted.

N-doping is a facile and effective way to improve the optical properties of CDs. Wang et al. [17] prepared red light-emitting CDs46 with maximum emission wavelength 615 nm by a microwave-assisted method using p-phenylenediamine as the precursor. The PL QY of CDs46 was 15%. CDs46 were successfully applied as the fluorescent probe for the detection of glutathione (GSH). Furthermore, CDs46 based turn-on fluorescent nanothermometry have potential applications in living cell imaging and temperature sensing (Fig. 46).

Fig. 46

Schematic presentation of CDs46 fabrication

Under the same synthesis conditions, Sun et al. [116] fabricated red light-emitting CDs47 by one-step microwave-assisted method using p-phenylenediamine as carbon and nitrogen source. The PL QY of CDs47 was 15%. CDs47 had a maximum emission wavelength at 620 nm when the excitation was 470 nm. The excitation-independent FL behavior suggested that the FL properties of CDs47 are mostly dependent on the surface states. Additionally, CDs47 were used as a nanoprobe for detection of pH and Fe³⁺ in aqueous solution and E. coli bacteria cell, simultaneously. The LOD for Fe³⁺ is 15 nM in the 0 to 30 μM range (Fig. 47).

Fig. 47

Schematic presentation of CDs47 fabrication

Co-doping of N, B and S atoms enables CDs to exhibit excellent optical properties, good biocompatibility and low cytotoxicity. Huang et al. [117] synthesized NBS-CDs (CDs48) by a one-step hydrothermal method by using 3-aminobenzeneboronic acid and 2,5-diaminobenzenesulfonic acid as precursors. The maximum excitation and emission wavelength of CDs48 are 520 nm and 605 nm, respectively. CDs48 shows an excitation wavelength-independent feature. Some related functional groups (-OH, -COOH, and -NH₂) are existed on the surface of CDs48. The doping of N, B, and S atoms can result in the relative uniform size distribution and surface state of CDs48. Furthermore, red light-emitting CDs48 were used for the detections of Ag⁺ and L-cysteine with detection limits as low as 0.35 mM and 0.045 mM, respectively (Fig. 48).

Fig. 48

Schematic presentation of CDs48 fabrication

Phenylboronic acid (PBA) might be used as a mediator to shorten the contact time between CDs and cells, which is conducive to the applications of CDs in the field of bio-probes. Li et al. [118] fabricated red light-emitting N, S co-doped CDs (CDs49) using CA and thiourea via a hydrothermal method. After coupling with PBA, CDs49-PBA was synthesized. At 550 nm excitation, the maximum emission wavelength is 593 nm. The PL QY in water was 23%. After doping the heteroatoms S and N, the excitation and emission wavelengths of CDs49 show a significant red shift. The surface of CDs49 contains -NH₂, -COOH, -SCN and -OH groups. CDs49-PBA show high sensitivity to Fe³⁺ and are expected to be used as an effective detection platform for other biological samples and environmental systems (Fig. 49).

Fig. 49

Schematic presentation of CDs49 and CDs49-PBA fabrication

Formamide containing carbonyl and amino groups plays a key role in the successful preparation of long-wavelength red emission CDs. Sun et al. [119] prepared red light-emitting CDs50 by one-step microwave assisted heating of CA formamide solution. The PL QY of CDs50 was 22.9%. CDs50 were composed of rich amino and hydroxyl functional groups. Moreover, CDs50 can selectively stain RNA-rich nucleolus and had been successfully applied to photothermal cancer therapy in vitro (Fig. 50).

Fig. 50

Schematic presentation of CDs50 fabrication

One molecule of 1,3-dihydroxynaphthalene can as the smallest sp² domain with -OH groups. Under the strong oxidation of KIO₄, this domain can form large conjugated sp² clusters with -OH groups. Wang et al. [120] have devised a DCDP(dehydrative condensation and dehydrogenative planarizationmethod) using 1,3-dihydroxynaphthalen as carbon source to synthesize CDs51. The QY of red light-emitting CDs51 was up to 53%. In the decoupled DCDP route, DC enables size tuning while DP ensures a high degree of π-conjugation. CDs51 are a conjugated sp² cluster of uniform size with -OH groups at the edges, producing a strong red emission. For the first time, an UV-pumped CDs51 phosphors-based warm WLED was prepared with a color rendering index of 97 (Fig. 51).

Fig. 51

Schematic presentation of CDs51 fabrication

Biomass CDs with superior optical properties and low cytotoxicity can reduce resource waste and realize sustainable development. Ding et al. [121] synthesized high-efficiency red light-emitting CDs52 by solvothermal method using lemon juice. The obtained CDs52 with a QY of 28% were monodisperse with an average diameter of 4.6 nm, and exhibited excitation-independent emission at 631 nm. The surface state of CDs52 and the nitrogen-derived structure in the CDs52 core synergistically caused intense red fluorescence. The low cost, ecologically friendly synthesis method and good optical properties make CDs52 have great application value in the field of biological imaging and optoelectronics (Fig. 52).

Fig. 52

Schematic presentation of CDs52 fabrication

Bagasse is a kind of natural raw material for the preparation of biomass CDs. Jiang et al. [122] prepared red light-emitting CDs53 by one-step carbonization of bagasse with H₂SO₄ and H₃PO₄ at 100 °C. CDs53 are mainly composed of sp² graphitized carbon atoms with a small amount of sp³ carbon defects. The water-dispersed CDs53 are sulfur-doped and rich in -COOH on the surface. The polyvinylidene fluoride-loaded CDs53 can selectively detect gaseous ammonia with a low detection limit, which can meet the sensitive monitoring of toxic ammonia pollution (Fig. 53).

Fig. 53

Schematic presentation of CDs53 fabrication

Polymers such as polythiophene phenylpropionic acid (PPA) have a larger conjugated structure and can be used as a precursor to prepare novel CDs. Ge et al. [123] fabricated CDs54 using PPA as the precursor. CDs54 showed red light-emitting from 500 to 800 nm with a peak at 640 nm. The red-emitting CDs54 exhibited a photothermal conversion efficiency of up to 38.5% under 671 nm laser irradiation. CDs54 were carboxylate-terminated and sulfur-doped with an average size of 10 nm. what’ more, CDs54 can be used as active agents in fluorescent, photoacoustic, and thermal theranostics in living mice (Fig. 54 and Table 4).

Fig. 54

Schematic presentation of CDs54 fabrication

Table 4 An overview on red-emitting carbon dots

Multicolor-emitting CDs

Obtaining the fluorescence emission of the entire visible spectrum is considered to be a key requirement for the successful implementation of CDs in almost all practical applications [115, 124, 125]. Although many reports have proposed fluorescence-related emission, the relative wavelength shift often does not cover the entire visible spectrum, the fluorescence emission intensity is significantly reduced. Such optical displacements cannot be truly marked as tunable. What is really needed is a method of changing the chemical and physical properties of CDs to modulate the color-emitting of CDs without affecting the peak emission intensity [31, 126,127,128]. The multicolor emission of CDs is primarily derived from different surface states.

Different surface oxidation degree will facilitate the color-emitting modulation of CDs. CDs55 with a QY of up to 35.3% were synthesized by Schiff base reaction using triethylenetetramine (TETA) and p-benzoquinone [129]. CDs55 can be further separated into green G-CDs55 and yellow Y-CDs55 through a silica gel column. The QYs are 35.3% and 17.5%, respectively. G-CDs55 show an excitation-independent feature but Y-CDs55 are excitation-dependent. G-CDs55 have a higher surface oxidation degree than Y-CDs55, resulting in red-shifted emission from 558 nm to 543 nm, which can be attributed to the reduction of band gap. Moreover, Y-CDs55 with good photostability can be widely used in cell imaging and analysis, while G-CDs55 are easier to be photobleached (Fig. 55).

Fig. 55

Schematic presentation of CDs55, G-CDs55 and Y-CDs55 fabrication and its fluorescence mechanism

The solvents can adjust the dehydration and carbonization of precursors, which then vary oxygen-containing functional groups on CDs surface, resulting in different optical properties of CDs. Hu et al. [130] mixed CA or ethylene glycol (EG) with ethylenediamine endcapped polyethylenimine (PEI-EC), and added a dehydrating agent (H₃PO₄) or a reducing agent (NaBH₄) to prepare tunable fluorescence emission CDs56 from blue (400 nm) to NIR (710 nm). CDs56 rich in -COOH on the surface mainly emit blue fluorescence. As the CO-C or C-O in CDs increases, the band gap of surface states of CDs56 gradually decreases, resulting in the red-shift of emission wavelength. CDs56 can be widely used in lighting and biosensing (Fig. 56).

Fig. 56

Schematic presentation of CDs56 fabrication

Controlling surface -NH₂ groups density by varying the reaction temperature is a facile way to modulate the color-emitting, QY and decay lifetime of CDs. CDs57 were synthesized by using CA and urea as precursors via hydrothermal method [131]. CDs57 prepared at 240 and 160 °C have excitation-dependent and excitation-independent fluorescence, respectively. CDs57–1 prepared at lower temperatures have a higher density of -NH₂ groups on their surface, which results in the band gap narrowed, further leading to the redshift of emission and higher QY (44.7%) and longer lifetime (7.13 ns). The fluorescence QY and lifetime of CDs57–2 was 20.8% and 5.21 ns, respectively. CDs57 have been used to fabricate multicolor CDs57/polymer composites, which have potential applications in the field of LED. In addition, CDs57 can also be used as probes for the detection of Be²⁺.The detection limit was 23 μM (Fig. 57).

Fig. 57

Schematic presentation of CDs57 fabrication and its fluorescence mechanism

Controlling surface states using C=N functional groups is an effective way to modulate the color-emitting of CDs. Han et al. [127] fabricated multicolor-emitting CDs58 using hydroquinone and EDA via hydrothermal method. CDs58 were separated via column chromatography and showed bright yellow, green, and blue fluorescence. The multicolor emission of CDs58 was primarily connected with the C=N functional groups on the surface. The band gap decreased with increasing content of C=N, resulting in the red shift of the fluorescence emission. Moreover, CDs58 exhibited stable fluorescence emission and excellent water solubility, these properties promoted their application in fluorescent ink and pH sensing (Fig. 58).

Fig. 58

Schematic presentation of CDs58 fabrication and its fluorescence mechanism

The -COOH and -NH₂ groups can modulate the color-emitting of CDs by electron transfer according to the electron absorption and electron donation behavior of chemical groups. Dhenadhayalan et al. [132] used CA and EDA as precursors to synthesize carboxyl and amine functionalized CDs (B-CDs59 and G-CDs59) by microwave method. The QYs of B-CDs59 and G-CDs59 were 7.2 and 21.8%, respectively. The multicolor-emitting of CDs is mainly derived from carbon core and surface states. When -COOH is present on the surface of B-CDs59, electrons are transferred from carbon core to surface states. In contrast, when -NH₂ is present on the surface of G-CDs59, the orphaned electrons of nitrogen in -NH₂ increases the electron density, reducing the energy gap of surface states, resulting in the red-shift of emission wavelength (Fig. 59).

Fig. 59

Schematic presentation of CDs59 fabrication

Furthermore, controlling surface states using -COOH and -OH is a facile way to modulate the color-emitting of CDs. Sun et al. [133] synthesized bright blue-emitting CDs (B-CDs60) by hydrothermal method using CA and urea (mass ratio 1:1) as precursors. In the microwave method, the mass ratios were 1:1 and 1:2, and blue-green CDs (GB-CDs60) and green CDs (G-CDs60) were prepared. CDs60 exhibited excitation dependent, but the strongest emission redshift from 415 nm to 525 nm. Blue and green fluorescence are related to surface hydroxyl groups and carboxyl groups, respectively. Luminescent elastomers having strong tensile were prepared using CDs60. TiO2 nanowire arrays modified with CDs60 have potential applications in photovoltaic converters (Fig. 60).

Fig. 60

Schematic presentation of CDs60 fabrication

Carboxyl groups and phosphate radical on CDs surface can introduce new energy levels into their electronic structures, resulting in CDs with tunable emission colors. B-CDs61 with a QY of about 31.59% were synthesized by using folic acid as the precursor [134]. Phosphoric acid and oxalic acid were used to modify the surface state of B-CDs61, resulting in the decrease of band gap energy and obtaining green and yellow-green CDs (G-CDs61 and YG-CDs61). The redshift of the treated CDs61 was 70 nm and 105 nm, respectively. The band gap energy before and after oxalic acid treatment was 2.35 eV and 1.75 eV, respectively. CDs61 as an efficient probe were applied in vitro and in vivo bioimaging (Fig. 61).

Fig. 61

Schematic presentation of CDs61 fabrication and its fluorescence mechanism

The function groups from different solvents can bring about different surface states and introduce new energy levels into their electronic structure, and thus contribute to the modulation of the color-emitting of CDs. Yuan et al. [135] fabricated long-wavelength red emission CDs62 using 1,2,4-triaminobenzen as carbon source and polyethylene glycol 200 (PEG200) as passivant through solvothermal method. Furthermore, multicolor-emitting CDs62 are observed when CDs62 are dissolved in ethyl acetate, ethanediamine, oleylamine and DMSO. Diverse function groups have different ability to donor electrons, S=O>-NH₂>-OCO-. Multicolor-emitting CDs62 can be used to construct warm WLEDs (Fig. 62).

Fig. 62

Schematic presentation of CDs62 fabrication and its fluorescence mechanism

The precursors with larger conjugate structures and high temperature conditions are more advantageous for obtaining long-wavelength emitting CDs. Fu et al. [136] designed a model system containing only three types of polycyclic aromatic hydrocarbons (PAHs) to mimic the optical properties of CDs63, in which anthracene, pyrene and perylene embedded in a matrix dominated by sp³ hybrid carbon. The Stokes shift is large due to exciton self-trapping in stacked PAHs molecules. The transfer of energy from PAHs with larger energy gaps to PAHs with smaller energy gaps, results in the red-shift of emission wavelength. CDs63 can be used for wavelength tunable nanolasers or organic light-emitting diodes (Fig. 63).

Fig. 63

The model system of CDs63 containing only three types of PAHs

Increasing the quinone structure on the surface by alkali induced can tune the color-emitting of CDs. Yuan et al. [137] obtained alkali induced R-CDs64 using perylene with a large conjugated structure and strong fluorescence activity. In addition, green emission amino silane functionalized CDs (G-SiCDs64) were obtained by heating CA and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEATMS). The QYs of R-CDs64 and G-SiCDs64 were 80% and 49%, respectively. Since the alkali induced surface electronic states, the quinone structure of R-CDs64 increases, which is responsible for red emission. R-CDs64 were doped into the amino silane to achieve solid red light-emitting. G-SiCDs64/R-CDs64 gel glasses were prepared by doping R-CDs64 into G-SiCDs64 sol, which can be potentially applied in three-color WLEDs (Fig. 64).

Fig. 64

Schematic presentation of R-CDs64 and G-SiCDs64 fabrication

Reasonable adjustment of surface oxidation degree or size can be beneficial to modulate the color-emitting of CDs. By carefully changing the nitric acid concentration and reaction conditions, the size and surface oxidation degree of CDs can be adjusted. The tunable color-emitting CDs65 were synthesized by oxidation and ultrafiltration of carbon fibers (CFs) [138]. The emission wavelengths of CDs65can be tuned from 430 to 610 nm. The higher the degree of surface oxidation or conjugated system, the smaller the surface energy gap, resulting in the red-shift of emission wavelength. CDs65 as an efficient probe was applied in Vero cells imaging (Fig. 65).

Fig. 65

Schematic presentation of CDs65 fabrication

Reasonable adjustment of surface functional groups or size also facilitates the modulation of the color-emitting of CDs. B-CDs66 with controllable size and surface functional groups were fabricated by using lauric gallate via heat treatment [139]. Moreover, the ester groups on the surface of B-CDs66 can be turned into carboxylate groups by basic hydrolysis to gain G-CDs66. Nucleation-related emission dominated the fluorescence characteristics of B-CDs66, while the carboxyl as the surface emission traps induced the red shift emission of the de-esterified G-CDs66 (Fig. 66).

Fig. 66

Schematic presentation of B-CDs66 and G-CDs66 fabrication and its fluorescence mechanism

By varying the composition of four different solvents during the synthesis, the size and graphite N content of CDs can be changed, and the color-emitting of CDs was finally modulated. Ding et al. [140] synthesized CDs67 using L-glutamic acid and o-phenylenediamine via solvothermal method. By changing four different solvents in synthetic process, CDs exhibited tunable emission colors from blue (443 nm) to NIR (745 nm). The QYs of B -, G -, Y -, and R -CDs67 are 54%, 41%, 51%, and 43%, respectively. CDs67 had large conjugated π-domains and rich surface functional groups. CDs67 have been successfully used to make CDs67/PVA films and as bioimaging fluorescent probes (Fig. 67).

Fig. 67

Schematic presentation of CDs67 fabrication

By changing the composition of reaction solvents, the size and bandgaps of CDs were systematically manipulated, thereby modulating the color-emitting of CDs from blue to red. Based on the molecular fusion mechanism, Zhan et al. [141] fabricated multicolor-emitting CDs68 by changing the composition of reaction solvents using 1,3,6- trinitropyrene (TNP) as the carbon and nitrogen sources. The QY of CDs68 was up to 59%. The color-emitting of CDs68 whose surface were passivated by nitrogen-containing groups can be modulated by QCE and slightly altered by surface functional groups. Fluorescence imaging study in Hela cells and the Hela tumor-bearing nude mice served to assess the bioimaging capabilities of lipo-R-CDs68 in vitro and in vivo (Fig. 68).

Fig. 68

Schematic presentation of CDs68 fabrication

The color-emitting of CDs can be modulated by varying the concentration. Chen et al. [142] fabricated the concentration dependent CDs69 using CA and ethanolamine by hydrothermal method. The color tunable fluorescent CDs69 with emission wavelengths ranging from 585 to 514 nm. Surface, edge, and core states were three fluorescent centers in CDs69. The pyrrolic-N at the surface of CDs69 was the fluorescent center of orange light. Under the action of dimethylformamide solvent, CDs69 showed stronger emission than that in water. In addition, there was an energy transfer process between three fluorescent centers. Concentration-induced CDs69 have been used to prepare warm white LED (Fig. 69).

Fig. 69

Schematic presentation of CDs69 fabrication and its fluorescence mechanism

Surface states mechanism is considered to be the key fluorescence mechanism of CDs, which provides a primary way to modulate the color-emitting of CDs. Controlling the surface states and element doping are facile and effective way to improve the optical properties of CDs. In particular, N-related functional groups produced by doping will have a great effect on the color-emitting of CDs.

It is unclear how different preparation methods or precursors affect the fluorescence mechanism of CDs. The use of natural starting materials (such as leafs, bark, coffee beans, menure, etc.) that are available in certain regions only makes method hardly reproducible by others. Users prefer methods that are based on the use of materials that are available (a) anywhere, (b) at any time (not just during certain seasons), and (c) in constant quality. If CDs are prepared with pure organic chemicals, different effects may occur. Defined organic chemicals such as CA is the most commonly used precursor at the moment, because the reproducible preparation of CDs is particularly sensitive to the kind of starting material. However, under the same synthesis conditions, precursors with larger conjugate structures and intense fluorescence activities are more advantageous for obtaining long-wavelength emissions CDs. These precursors facilitate the self-assembly of carbon core, and the different functional groups in precursors will contribute to the formation of the surface state of CDs.

Furthermore, the solvents can adjust the dehydration and carbonization of precursors. By simply varying the composition of reaction solvents, the bandgaps of CDs also can be manipulated, modulating the color-emitting of CDs (Table 5).

Table 5 An overview on multicolor-emitting carbon dots

Conclusion

In this review, we summarized the fluorescence mechanism of CDs and focused on the modulation of different color-emitting CDs. The fluorescence of CDs is usually believed to originate from the carbon-core states and surface states. Among them, surface states mechanism was considered to be the key fluorescence mechanism of CDs. Controlling the surface states is the most primary and easy method to modulate the color-emitting of CDs. Different functional groups can form various surface defect states, thus introducing many energy level band gaps to regulate the color-emitting of CDs. In addition, element doping (with nitrogen, sulfur, other heteroatoms and metal ions) is a facile and effective way to improve the optical properties of CDs. In particular, N-related functional groups produced by doping will have a great effect on the color-emitting of CDs. For some CDs with large conjugated π-domains and few surface chemical groups, the bandgap of conjugated π-domains is considered to be the carbon-core states fluorescence center. However, it is difficult to control the size of the conjugated π-domains in CDs, so the quantum size effect is not easy to use modulate the color-emitting of CDs. Some fluorescent small molecules or fluorophores are attached to the surface or interior of carbon skeleton and can show fluorescence emission directly. There are few literatures on the use of molecule states mechanisms to modulate the color-emitting of CDs, because reactions are difficult to be controlled. Once the chemical structure of CDs is well controlled during the synthesis process, it is possible to clarify the fluorescence mechanism of CDs and large-scale synthesis different color-emitting CDs, and eventually promote the use of CDs in biomedicine and optoelectronics fields in future work.