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Nanoscience & Nanotechnology
Tuning the optical properties of carbon dots (CDs) and figuring out the mechanisms underneath the emissive phenomena have been one of the most cutting-edge topics in the development of carbon-based nanomaterials. Dual-emissive CDs possess the intrinsic dual-emission character upon single-wavelength excitation, which significantly benefits their multi-purpose applications.
- dual-emissive carbon dots
- optical mechanism
- ratiometric fluorescence
- synthesis strategies
- white-light-emitting diodes
1. Introduction
Multi-emissive materials have never been in such high demand in the areas spanning photoelectronics and biomedical photonics. Among the photoelectronic devices, white-light-emitting diodes (WLEDs) with low energy consumption, long lifetime, and environmental friendliness can be applied in various fields [1,2]. By combining fluorophores covering the entire visible light range, researchers have devised a variety of WLEDs in the past [3,4,5]. In the biomedical area, fluorescent sensors with multi-band emissions, i.e., ratiometric sensors, are appealing due to the merits of excluding interferences from environmental factors (light source power, auto-fluorescence molecules in the matrix, etc.) [6,7]. This self-calibration effect is also vital for ratiometric fluorescent probes applied in bioimaging [8]. For the sake of achieving unique multi-emissive properties, single-emissive fluorophores (molecular dyes [9], semiconductor quantum dots [10], gold nanoclusters [11], upconversion nanoparticles [12], etc.) were usually integrated by either mixing or assembling. For multi-component systems, several imperfections should be addressed in practical applications. For instance, phase separation and color aging are still challenging problems for WLEDs comprised of multiple fluorophores, while the tedious preparation/purification and batch-to-batch variation in relative emission intensity are common for ratiometric fluorescent sensors/probes. These problems prompt researchers to exploit alternatives to the traditional multi-component fluorescent systems. In 2009, Peng’s group reported a Cu-doped InP/ZnSe quantum dot with adjustable dual-emissive property (bandgap emission and Cu dopant emission), opening the prelude of research on “dual-emission in one dot” [13]. Unlike the multi-component fluorescent systems, the relative emission intensity, as well as the emission wavelength of the dual-emissive semiconductor quantum dots, was customized during their synthesis, revealing superior optical consistency in the subsequent applications. Zhang et al. [14] and Peng et al. [15] fabricated WLEDs by the use of dual-emissive Cu:CdS/ZnSe and Mn:ZnCuIn/S quantum dots, respectively. Simultaneously, the ratiometric fluorescent sensings/imagings of organophosphate [16], folic acid [17], metal ions [18,19], and pH [20,21] were achieved using the dual-emissive quantum dots.
Carbon dots (CDs) are zero-dimensional emissive particles with diameters within 10 nm. Due to their appealing properties, including high quantum yield (QY), photo-bleaching resistance, excellent hydrophilicity, satisfying biocompatibility, easy to synthesize, abundant precursor sources, and so forth, CDs have become ideal alternatives to conventional organic dyes or semiconductor quantum dots [22,23]. In 2004, Scrivens’s group first discovered fluorescent carbon nanoparticles during the purification of arc-synthesized single-walled carbon nanotubes [24]. Thereafter, a diverse range of CD preparation methods have been exploited. Overall, CDs can be prepared by “top-down” (arc discharge [25], laser ablation [26], nanometer etching [27], etc.) or “bottom-up” (pyrolyzation [28], hydrothermal/solvothermal [29], microwave assistant pyrolysis [30], etc.) methods, which have been well-documented before. In comparison, the “bottom-up” methods are the most extensively approbatory because of the merits, including (1) large-scale and low-cost CD synthesis is attainable, (2) CDs reveal superior QY and uniformity, and (3) the composition and size of CDs can be tailored by precursor selection or changing the nucleation/growth conditions.
2. Synthesis Strategies for Dual-Emissive CDs
Synthesis strategy, the precursors and reaction conditions in particular, produces the most remarkable impacts on the properties of nanomaterials. CDs are not exceptional. It is well known that CDs’ chemical and optical properties are established as prepared and can rarely be changed in subsequent treatments. Optimizing the synthesis strategy is thus the first choice to acquire CDs with desirable QYs or emission wavelength. As far as we know, dual-emissive CDs have not been obtained from the “top-down” approaches. Therefore, researchers dedicatedly discuss the “bottom-up” approaches in preparing dual-emissive CDs in this section. From the CD formation mechanism during the hydrothermal/solvothermal treatments of precursors, the products, as well as the degree of graphitization of CDs, rely heavily on the treatment condition and the precursor composition [45]. Yang’s group systematically investigated the chemical procedure of CD synthesis by pyrolyzing citric acid and ethylenediamine [46]. CDs are achievable upon the precursors that are successively condensed/polymerized and carbonized in the system. In case the intermediate molecule (precursor dimer) or polymer possesses the conjugated structure, CDs are not the sole fluorophore in the systems that have not experienced harsh carbonization conditions. Hence, controllable carbonization is an effective strategy for preparing CDs exhibiting emissions from carbon skeleton and molecular/polymeric centers.
Except for regulation of the synthesis conditions, precursors with specific structures can directly or indirectly render CDs with a new emission band beyond the carbon core emission. The optical and structural properties of precursors could be “semi-reserved” in CDs due to incomplete carbonization, leading to the introduction of molecular emission centers (direct manner) or cross-linking among CDs for the generation of new emission bands (indirect manner). Doping with heteroatoms is also a feasible strategy to prepare dual-emissive CDs by introducing new surface properties, creating trap states, or causing electronic interactions among carbon atoms with the neighboring dopant atoms [47,48].
2.1. Controllable Carbonization
By carefully adjusting the polymerization and carbonization procedure of precursors o-phenylenediamine (OPD) and lysine (Lys) during hydrothermal treatment, Chen’s group synthesized CDs with tunable blue and green emissions [49]. In the absence of Lys, self-polymerization of OPD molecules and carbonization of OPD polymer chains resulted in the formation of blue/green emissive CDs. Adding Lys can suppress the carbonization of the OPD polymer but enhance the self-polymerization of OPD molecules, leading to improved blue emission. Simultaneously, the green emissive OPD-Lys co-polymer endowed the CDs with a new green emission. As such, the relative green-to-blue emission intensity was adjustable by regulating the mass ratio of OPD and Lys. Unlike the precursor composition-regulated carbonization, Kainth et al. utilized different oxidation and dehydration capacities of mineral acids to regulate the carbonization degree of precursor [50]. In their work, the dual-emissive CDs were synthesized from sucrose, which was acid-oxidized by the mixture of H2SO4 and H3PO4 with the assistance of a microwave. The green-to-blue intensity ratio of the CDs was influenced by the ratio of acids because H2SO4 and H3PO4 played independent roles. H3PO4 promoted the carbonization at slower kinetics to produce green-emissive surface defects, whereas H2SO4 caused the blue emissions due to its stronger oxidizing ability to oxidize C-H into O=C-H or C-OH and its dehydrating property to generate unsaturation from C-C. The carbonization process could be enhanced if the concentration of H2SO4 was too high, leading to the elimination of surface or edge functional group-induced longer wavelength emission in the CDs. Therefore, fixation of a particular molar ratio of H2SO4 and H3PO4 (1:2) was necessary to achieve the CDs with appropriate dual emissions. As the most facile approach, adjusting the treatment temperature and time on the precursor(s) is adopted by Liu et al. [51]. They synthesized green/red dual-emissive CDs using 2,5-diaminotoluene sulfate and ethanol and found that the intensity of red emission was dependent on the solvothermal conditions (time/temperature/solvent volume). The optimizable green-to-red emission ratio indicated that external conditions can directly impact the carbonization degree, as well as the optical properties of CDs.
2.2. Semi-Reservation of Precursor Structure
It has been well-documented that the chemical properties of CDs can be tailored by precursor engineering. For instance, folic acid-derived CDs were capable of targeting and imaging cancer cells, of which the folic acid receptor was overexpressed [52,53]. CDs derived from metal chelators were useful for the detection of metal ions, which were complexed by the CDs and caused their fluorescence quenching [54,55,56]. Inspired by the property customizability of CDs, the dual-emissive character can be achieved by the use of precursors with π-conjugated structures. Porphyrins are a family of macrocycle compounds with an extended π-electron system. Researchers synthesized a series of green/red dual-emissive CDs using 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin (TSPP) and citric acid. Researchers deduced that the green emission originated from the carbon core, while the red emission was linked to the partially carbonized TSPP residues. By adjusting the precursor ratio (citric acid-to-TSPP), the relative green-to-red emission intensity was customizable [57]. Similar works were reported by Shi’s group and Guo’s group [58,59]. Dual-emissive CDs were solvothermally synthesized from leek and cabbage, respectively. As is well known, a certain amount of pigments in biomass resources are porphyrin derivatives, which are considered responsible for introducing red emission to CDs. Except for porphyrin-based precursors, other types of molecules with aromatic structures were frequently adopted for the synthesis of dual-emissive CDs. The aromatic precursors carrying N or S elements can produce CDs simultaneously emitting longer wavelength fluorescence (yellow or red emission) and shorter wavelength fluorescence (blue or green).
In addition to being carbonized and embedded into the carbon skeleton to directly generate one of the emission bands in dual-emissive CDs, the aromatic residues were also evidenced to be modified on the surface of CDs, leading to inter-particle interaction. Additional emission peaks thus arose due to the electronic coupling. 1,3,6,8-Pyrenetetrasulfonic acid contains a large aromatic plane that can be used for the synthesis of CDs with appealing properties. Being served as the precursor, Jainth et al., synthesized blue/green CDs via the atmospheric pressure air plasma treatment [62]. They found that the emission character of CDs was concentration-dependent, exhibiting purple emission at low CD concentrations but green emission at high CD concentrations. The dual-emission was achievable at a CD concentration of at least 1 g/L. The uncarbonized PTSA molecule on the surface of CDs was considered pivotal for green emission. The green emission was caused by the formation of PTSA dimers driven by π-π interactions. As a consequence, the green emission was switchable by tuning the concentration of CDs. The inter-particle interaction-induced additional emission bands are more commonly seen in those CDs with solid-state fluorescence or phosphorescence. For instance, Yang’s group synthesized dual-emissive CDs with p-aminosalicylic acid and citric acid [63]. The CDs were featured with blue and red emissions in the solid state. The red fluorescence was ascribed to the supramolecular cross-linking between adjacent particles. The surface groups, determined by the precursor type, were crucial for providing the driving forces (π-π interaction, H-bond) and intermediating the supramolecular cross-linking.
2.3. Heteroatom Doping Effect
Heteroatom doping has been vastly adopted to modulate fluorescent properties and is considered the most promising engineering approach to produce highly fluorescent CDs. The doping methods, advantages compared to the undoped CDs, as well as applications of doped CDs have been systematically reviewed recently [64,65,66]. Herein, the preparations of unconventional dual-emissive CDs via heteroatom doping strategy will be focused. Esranur et al. synthesized dual-emissive CDs from 3-aminophenylboronic acid (APBA) and boric acid (BA) [67]. The dual-emissive feature of CDs was available only in the presence of high BA amounts. They evidenced that boron was capable of penetrating the CD lattice. Therefore, as the amount of BA increased, the structure of CDs became different, and boron-based energy transitions emerged, giving rise to the dual-emissive property of CDs. Duan’s group prepared orange/red dual-emissive CDs using OPD as the precursor and Al(NO3)3·9H2O as an assistant [68]. Graphitic N appeared in the CD structure (confirmed by the XPS measurements), contributing to the emergence of an additional emission band beyond the intrinsic CD emission.
This section discusses the strategies for producing dual-emissive CDs from different perspectives demonstrated in the published articles. However, in most cases, the dual-emission of CDs is a combinational effect of the mentioned factors. It is noticed that nearly all the precursors used for the synthesis of dual-emissive CDs contain at least one kind of heteroatom, such as S, N, or B. Either being embedded into the graphitic core or forming dangling chemical groups, the heteroatoms were inclined to generate new emission centers in CDs. In addition, precursors with π-domains were more likely to be used in the synthesis of dual-emissive CDs. In comparison to the chain compounds, aromatic structures were, in fact, more stable during the carbonization process due to π-π stacking, allowing the formation of molecular state emissions. Finally, the degree of carbonization produced the most significant impact, which was dominant for bandgap emission or other types of luminescence. Therefore, the unique optical property was achievable only by carefully controlling the carbonization process for all of the dual-emissive CDs.
Another critical issue to be emphasized is that adequate confirmation of the dual-emissive property of CDs is necessary based on the thorough isolation of CDs in the synthesis products. As a common phenomenon for CDs prepared via the bottom-up approaches, the emissive molecular intermediates can significantly impact CDs’ fluorescence. Using citric acid and urea as the typical precursors, Kasprzyk et al. presented molecular insights into the fluorescence of CDs synthesized under different conditions [69]. By sufficient dialysis and analyzing the chemical structures in and out of the dialysis bags, they found that citrazinic acid and 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT) were responsible for the emission colors, as well as the high quantum yields of blue-emissive CDs (synthesized in sealed reactors) and green-emissive CDs (synthesized without solvent), respectively. As for dual-emissive CDs, researchers should pay more attention to the discrimination of the actual “two-emission in one” CDs and the physical mixtures of CDs/molecular emitter. By the use of glutathione dissolved in formamide, Macairan et al., synthesized the “CDs” with blue and red emissions and investigated their optical mechanism [70]. However, in an updated research, Ganjkhanlou et al., revealed that the emissions at different wavelengths originated from a mixture of physically separate compounds but not the sole CDs [71]. The compounds were identified as blue-emissive CDs and red-emissive porphyrin derivatives, which were separable by adding kaolinite and HCl. Therefore, robust verification of fluorescence behavior is an essential task for the synthesis of dual-emissive CDs.
This entry is adapted from the peer-reviewed paper 10.3390/nano13212869
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