Supramolecular Cross-Link-Regulated Emission and Related

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Supramolecular Cross-Link-Regulated Emission and Related Applications in Polymer Carbon Dots Tanglue Feng,† Shoujun Zhu,‡ Qingsen Zeng,† Siyu Lu,§ Songyuan Tao,† Junjun Liu,† and Bai Yang*,† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡ Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, 35 Convent Drive, Bethesda, Maryland 20892, United States § College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Kexue Road, Zhengzhou 450001, China ABSTRACT: Involvement of clear photoluminescence (PL) mechanism in specific chemical structure is at the forefront of carbon dots (CDs). Supramolecular interaction exists in plenty of materials, offering an inherent way to administrate the optical and photophysical properties, especially in terms of newly developed polymer carbon dots (PCDs). However, supramolecular-interaction-derived PL regulation is always ignored in the shadow of many kinds of PL factors, and we still have a limited understanding on the distinct chemical structure and mechanism of supramolecular effect in PCDs. Herein, several distinct photoluminescent phenomena of PCDs under aqueous and solid state are reviewed in terms of supramolecular cross-linking, with highly emphasizing the importance of supramolecular cross-link-enhanced emission (SCEE) effects, and the regulated function of supramolecular interaction’s intensity and types between PCDs for special PL behaviors of PCDs. In addition, we categorize the photoluminescent phenomena in PCDs into the following aspects: supramolecular cross-link-enhanced dilute-solution-state emission, concentration-controlled multicolor emission, supramolecular regulation for quenching-resistant solid-state fluorescence, as well as supramolecular cross-link-assisted room-temperature- phosphorescence (RTP) under solid states. Furthermore, the applications of PCDs in light-emitting diodes (LED), solar cells, and anticounterfeiting and data encryption, etc., are presented, based on the distinct supramolecular cross-link-regulated photoluminescent phenomena, especially the solidstate emission. Finally, a brief outlook is given, highlighting the currently existing problems and development direction of supramolecular cross-link-regulated emission in PCDs. KEYWORDS: polymer carbon dots (PCDs), cross-link-enhanced emission (CEE), supramolecular interaction, supramolecular CEE, solid-state emission, room-temperature phosphorescence (RTP) molecular cross-link-regulated emission,39,50−52 etc. However, as a special method of constructing aggregation state, supramolecular strategy, which has emerged in previous works but been not clearly proposed and well-summarized, can offer a special way to tune the optical and photophysical properties of PCDs in terms of supramolecular interaction. In terms of cross-link, similar to restriction of intramolecular motion mechanism of aggregation-induced emission (AIE) phenomenon,53−55 the supramolecular cross-linking can inhibit the nonradiation decay channel of excited state even form energy transfer, enhancing photoluminescence of PCDs, even changing PL colors. On the one hand, as our previous articles,56−59 the cross-link-enhanced emission (CEE) effect

1. INTRODUCTION As our understanding of the emerging polymer carbon dots (PCDs), one of the fluorescence carbon nanomaterials, has advanced, increasing numbers of innovative properties are being developed, including bright luminescence, high optical and chemical stability, good water solubility, photobleaching resistance, good biocompatibility, and low toxicity, as well as low cost, etc.1−4 It is particularly true that these excellent properties made PCDs promising in diverse research fields, for instance, chemical and biological sensing,5−8 bioimaging,3,9−11 theranostic, 12−16 light-emitting diodes, 17−20 photocatalysts,21−25 and solar cells,26,27 etc. Generally, there are four types of chemical structures in PCDs, including nonconjugated chains,28−30 functional groups,31−33 conjugated ringlike structure,34−37 and supramolecular interaction-derived cross-link structure.36,38,39 Therefore, researchers can regulate the photoluminescence (PL) properties of PCDs by multiple strategies, such as element doping,34,40,41 conjugation strategy,35,42−44 surface modification,45,46 composite engineering,47−49 supra© XXXX American Chemical Society

Special Issue: AIE Materials Received: September 29, 2017 Accepted: November 22, 2017 Published: November 22, 2017 A

DOI: 10.1021/acsami.7b14857 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Review

ACS Applied Materials & Interfaces

Figure 1. Supramolecular cross-link-assisted synthesis and emission of PCDs. (A) Synthesis of BPLPs. Reproduced with permission from ref 63. Copyright 2009 National Academy of Science of the United States of America. (B) Construction of multifunctional PCDs from hydrophilic PEI and hydrophobic PLA amphiphilic copolymer. Reprinted with permission from ref 64. Copyright 2013 Nature Publishing Group. (C) Schematic illustration of the relationship between different products in the PCDs prepared from citric acid (CA) and ethylenediamine (EDA). Reproduced with permissio from ref 72. Copyright 2015 Royal Society of Chemistry.

was proposed and proved to elucidate the PL mechanism of PCDs. Typically, the PL properties of some PCDs were attributed to some potential chemical groups (named as subfluorophores), such as heteroatom-containing double bonds (CO, CN, NO) and single bonds (amino-based groups, C−O). These subfluorophores show generally weak fluorescence while showing increased PL with suitable immobilization, such as covalent/supramolecular cross-linking or even physical aggregation.56,59 This experimental success is also reflected in the growing number of reported cases.39,52,60−62 That is, supramolecular cross-linking can be used to enhance the photoluminscence by inhibiting the motion of fluorescence center, and we named the supramolecular cross-link-enhanced emission effect as SCEE effect. On the other hand, the intensity and types of supramolecular cross-linking between PCDs are important, which provides us a rational design thought for regulating the PL properties of PCDs. The regulation for intensity and types of supramolecular cross-linking possible can form energy (or charge) transfer, or generate new narrow bandgap, thus enhancing the fluorescence and even changing PL colors. Given its increasing importance as the key chemical structure for modulating the PL of PCDs, we focus this review on some unique photoluminescent phenomena of PCDs in terms of supramolecular cross-linking, as well as related applications. In sections 2 and 3, we elucidate distinct fluorescence properties of PCDs under dilute solution state, concentrated solution state, individual solid state, and solid-state composites, respectively. We highly emphasize the importance of regulating the intensity and types of supramolecular interaction between PCDs, for example, different concentration-controlled multicolor emission, and the realization of quenching-resistant photoluminescent PCDs under solid state by weakening the π−π interaction cross-link. In section 4, distinct roomtemperature phosphorescence (RTP) properties of PCDs

under solid state is reviewed in terms of hydrogen bond. In section 5, based on these distinct supramolecular cross-linkregulated photoluminescent phenomena, especially the solidstate emission, we review some applications of PCDs in light emitting diodes (LED), solar cells, fingerprint detection, anticounterfeiting and data encryption, etc. Finally, a brief outlook is given, with highlighting the currently existed problem, corresponding solutions and development direction in the future. We hope that this review provides accurate definition on distinct supramolecular-regulated emission of PCDs and their applications, eventually promoting the further development of PCDs.

2. SUPRAMOLECULAR INTERACTION-REGULATED EMISSION IN SOLUTION-STATE PCDS The concentration of PCDs in solvent determines the interparticle distance, which influences the surface electron distribution of PCDs by supramolecular cross-linking. Thus, the supramolecular cross-linking-regulated PCD emission in solution is elucidated from dilute solution and concentrated solution, respectively. 2.1. SCEE Effect of PCDs in Dilute Solution. SCEE coexisting generally in covalent bond-based PCDs system, together with covalent CEE, plays important role in the fabrication and photoluminescence of PCDs. Highly crosslinked fluorescent polymers or assembled hybrids, a type of PCDs, were reported by supramolecule cross-link-assisted synthesis at facile conditions, with the PL of PCDs enhanced. Yang’s group reported the synthesis of aliphatic biodegradable photoluminescent polymers (BPLPs) and cross-linked variants (Figure 1A) from small molecules.63 In the cross-linked variants, the supramolecular interaction (multiple hydrogen bonds) CEE together with covalent bond CEE contributed to the enhanced PL. Sun’s group reported the synthesis of photoluminescent copolymer (PEI−PLA) (Figure 1B) for B


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Figure 2. Supramolecular cross-link-assisted synthesis and emission of PCDs. (A) (a) Chromatograms for the elution of PCDs monitored by PDA at 254 nm (b, d). High-resolution mass spectra and (c, e) 1H NMR spectra of (b, c) fraction I and (d, e) fraction II. Reproduced with permission from ref 39. Copyright 2017The Royal Society of Chemistry. (B) Formation of supra-CDs and water-induced PL enhancement behavior. Reproduced with permission from ref 74. Copyright 2015 Wiley−VCH.

Figure 3. Supramolecular cross-linking structures play a major role in the PL of PCDs. (A) Formation mechanism of the HMF derivative in PCDs. (B) HMF derivatives’ supramolecular cross-linking structure inside PCDs. Reprinted with permission from ref 52. Copyright 2016 PCCP Owner Societies. (C) (a) Schematic illustration about the energy states and electron transition process of PCDs. (b) Illustration of the hydrogen bond (HB) effect-dominated PCD emission mechanism between molecule states and solvents. (c) Digital photograph of different PCD−polymer composites. Reproduced with permission from ref 51. Copyright 2017 Royal Society of Chemistry.

amine (Cys).36 The author found the dehydration between CA and Cys produced a molecular fluorophore with the photoluminescent quantum yield (PLQY) of 0.88. Furthermore, they proposed that the molecular fluorophore can self-assemble into amorphous PCD core with PLQY of 0.66 through hydrophobic interaction and π−π stacking interaction. In fact, similar works67,71−73 had been reported by our (Figure 1C) and other groups before this work though the mechanism of supramolecular interaction-induced self-assembly of molecular fluorophore into PCDs were not explicitly proposed in these literatures. Recently, Fang’s group39 investigated in detail the structure of molecular fluorophore assembly and PL mechanism of highly fluorescent citric-acid derived PCDs. The author found that molecular fluorophores (Figure 2A(b,c)), a fivemembered ring fused 2-pyridone organics, were the origin of strong fluorescence, and the individual molecular units could

biomedical application by self-assembling hydrophilic polyethylenimine PEI with hydrophobic polyactide (PLA), in which supramolecular interaction-derived compact network structure was main origin of enhanced PL due to the fixation of subfluorophore.64 Other types of photoluminescent PCDs, such as triblock copolymer (PEG-PAsp(MEA)-PEI),65 biohybrid (aggregated DNA dots),66 were also synthesized through self-assembly route. Furthermore, the SCEE effect also exists in partly carbonized PCDs. In general, the formation of PCDs undergo in sequence dehydration/polycondersation, assembly/cross-linking and carbonization from small molecules or polymer precursors,36,39,67−70 and supramolecular interaction plays a crucial role in assembly and cross-linking process. Yu’s group prepared highly blue fluorescent N,S-doped PCDs through one-pot hydrothermal condensation from citric acid (CA) and cysteC


Review


Figure 4. Concentration-dependent emission in PCDs. (A) Concentration-dependent PL behaviors in PCDs: (a) 0.12, (b) 0.5, (c) 2.0, and (d) 8.0 mg/L. Reproduced with permission from ref 86. Copyright 2016 American Chemical Society. (B) Optical photographs of R-, O-, Y-, G-, and BPCDs under ambient (top) and white light irradiation (bottom). (C) TEM and HRTEM images of the (a) Blue-PCDs and (c) Red-PCDs and size histograms of the (b) Blue-PCDs and (d) Red-PCDs. Reproduced with permission from ref 87. Copyright 2016 The Royal Society of Chemistry. (D) Concentration-mediated luminescence colors ranging from blue to red. Reproduced with permission from ref 88. Copyright 2016 Royal Society of Chemistry.

link together through hydrogen bond, generating dimer, trimer, etc., and the presence of star-shaped polymer DETA@5CA (Figure 2A(d,e)), also proved the contribution of hydrogen bonds to the formation of PCD assembly hybrid. Thus, the molecular fluorophores, as-fabricated dimer, trimer, together with supramolecule (star-shaped polymer DETA@5CA), were considered as the main origin of bright PL and spherical morphology of the as-synthesized PCDs. In the aforementioned supramolecular cross-link-based PL system, the Supramolecular CEE has a minor contribution on the PL of PCDs in contrast with covalent bond CEE. However, supramolecular interaction-based cross-linking structure could also play a major role in enhancing PL for some specific PCDs. Qu’s group prepared water-induced photoluminescent supraPCDs from alkyl chains functionalized-PCDs by self-assembly as shown in Figure 2B.74 Typically, the alkyl chains functionalized PCDs can self-assemble into a supra-PCD aggregation state in toluene, which show weak fluorescence, and it is surprised that the supra-PCDs can show water-induced PL enhancement owing to the decomposition of supra-PCDs in water. Obviously, the supramolecular hydrophilic/hydrophobic interaction plays a crucial role in the formation of supra-PCDs and the generation of water-induced PL enhancement behaviors of supra-PCDs. Mandal’s group reported the synthesis of PCDs with orange-red emission employing sucrose as the carbon sources.52 The hydroxymethylfurfural (HMF) derivative (Figure 3A) in the PCDs was confirmed by multiple spectroscopic techniques. Considering the HMF derivative has no charge transfer donor−acceptor moiety and no extended conjugation, which can be proved by the medium polarityindependent emission maximum and single exponential PL lifetime, the author demonstrated that the HMF derivative can form cross-linked structures (Figure 3B) through noncovalent interactions such as dipole−dipole, π−π stacking, and van der Waals interactions, generating red-shifted orange-red emission.

What’s more, supramolecular cross-link were also found between PCDs and solvents, which can also change PL properties of PCDs. Yu’s group reported efficient red-emission PCDs from p-phenylenediamine via solvothermal method, and found that as-prepared PCDs showed solvent-dependent multicolor emission from green to red, with fluorescence peak position red-shifting with the increasing of solvent polarity.75 Song’s group investigated in detail the solvent polarity-dependent photoluminescent mechanism, attributing the unique spectral shift to hydrogen bond (HB) effect as shown in Figure 3C, which was verified by fluorescence lifetime and equation simulation.51 2.2. Concentration-Controlled Emission of PCDs. For PCDs, there are several strategies to regulate the PL colors, such as separation by column chromatography to extract fractions with specific emission,76,77 introduction of bigger conjugated sp2 system inside PCDs by employing aromaticcontaining precursors,18,78−80 solvent engineering,51,75,81,82 postfunctionalization utilizing surface modifying agent,45,46 and exploiting efficient synthesis methods,83,84 etc. However, besides aforementioned methods, concentration control strategy provides us another facile route to regulate the emission colors of PCDs. It mainly involves the supramolecular cross-link-induced emission mechanism regarding the concentration dependent PL in PCDs. Typically, the concentration of PCDs determines the interparticle distance, and influences the surface electron distribution of PCDs by supramolecular crosslinking (a type of aggregation state), thus allowing the concentration of PCDs to tune their emission states.85 Wang’s group prepared hydrophobic PCDs through hydrothermal treatment of ionic liquid of 1-ethyl-3-methylimidazolium bromide, and the as-obtained PCDs exhibited concentration-dependent PL behaviors as shown in Figure 4A, which were used for multicolor imaging in vitro and in vivo. The redshifted fluorescence could be ascribed to the energy or charge D


Review


Figure 5. Self-quenching resistant solid-state emission of PCDs prepared from small molecules. (A) Preparation of N-doped PCDs, and photoluminescent photograph of N-doped PCDs under daylight and under a 365 nm UV beam. Reproduced with permission of Royal Society of Chemistry from ref 92. (B) (a) Photoluminescent photograph of PCD powder under daylight and UV-lamp. (b) FTIR spectra of SFPCDs (red line) and SNPCDs (green line), and high-resolution C 1s, N 1s, O 1s spectra of SFPCDs and SNPCDs. Reproduced with permission from ref 93. Copyright 2016 Royal Society of Chemistry. (C) The optical properties contrast between nitrogen-doped PCDs and boron-and-nitrogen codoped PCDs. (D) Schematic structures of the corresponding PCDs. Reproduced with permission from ref 41. Copyright American Chemical Society. (E) Dilute aqueous-solution and solid-state photoluminescent photographs of PCDs under (a) daylight and (b) UV light. (F) Photographs of starch (No. 1) and photographs of PCD/starch composites of b-PCDs (No. 2), g-PCDs (No. 3), and y-PCDs (No. 4) under (a) daylight and (b) UV light. (G) PCDs/PVA film with full-color emission under UV light. Reproduced with permission from ref 89. Copyright 2017 Wiley−VCH.

structure, which easily result in luminescence self-quenching. However, the supramolecular cross-linking provide a potential way to conquer the solid-state quenching of PCDs. 3.1. Self-Quenching-Resistant Solid-State Fluorescence in PCDs. Many fluorescent small molecular dyes with typical conjugated π-domains fluorophores generally suffer from fluorescence quenching under solid state because of strong π−π interaction (energy-transfer process).90,91 Likewise, if there exists strong π−π interaction or other strong supramolecular interaction in PCDs, the solid-state PCDs generally show only weak even no fluorescence. Our group reported highly fluorescent PCDs from citric acid and ethylenediamine, with a fluorescent small molecule (IPCA)72,73 found, a conjugated ring-like molecule, which is the main origin of strong aqueous solution fluorescence. However, the as-obtained PCDs show no fluorescence under solid state, which possibly was resulted from the strong π−π interaction-caused luminescence quenching between molecule fluorophores. What’s more, most of reports about PCDs only involve aqueous solution fluorescence in general, and the solidstate fluorescence properties were rarely illuminated because of the π−π interaction-caused luminescence quenching. In fact, we claim that the solid-state luminescence can be greatly enhanced by regulating the supramolecular cross-linking of PCDs to appropriate point, especially reducing even avoiding the fabrication of strong suparmolecular interaction between fluorescence centers. 3.1.1. Supramolecular Strategy for Solid-State Emission of Small Molecule Precursor-Derived PCDs. Small molecule

transfer induced by supramolecular cross-linking between fluorphores.86 Analogously, Yu’s group also reported concentration-dependent full-color fluorescence (Figure 4B) of PCDs prepared from an N-methylpiperazine precursor in MgAPO-44 zeolite by calcination and NaOH treatment. The author claimed that such tunable PL might mainly result from the energy transfer between multiple emitters in the nanoparticles, especially in the highly concentrated PCDs.87 The energy transfer resulted from the supramolecular cross-link-derived aggregation state between adjacent nanoparticles as proved by high-resolution TEM (Figure 4C), and the cross-linking degree determined the energy transfer degree then PL color. Bai’s group also reported multicolor (blue to red) photoluminescent PCDs (Figure 4D) with stable luminescence character by changing the concentration of PCDs in toluene.88 It should be noted that for the concentration-mediated multicolor emission, the high-concentration (i.e., long-wavelength emission) PCDs generally show lower PLQY compared to the low-concentration (i.e., short-wavelength emission) PCDs as reported by our group89 because most of the PCD PL intensity decreases with the increase of excitation wavelength, and the multicolor emission of PCDs are mainly caused by energy transfer between PCD particles.

3. SUPRAMOLECULAR CROSS-LINK-REGULATED SOLID-STATE EMISSION IN PCDS Distinct from the solution-state PCDs, PCDs in solid state generally show nondispersive compact (aggregation state) E


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Figure 6. Quenching resistant solid-state emission of PCDs prepared from (A, B) polymers or (C, D) silane coupling agent precursors. (A) Photographs of PCDs from PVA with diethylenetriamine (1), tetraethylenepentamine (2), ethylenediamine (3), or individual PVA (4) under daylight and UV lamp. (B) Schematic diagram of the relationship between fluorescence properties and interparticles spacing distance. Reproduced with permission from ref 38. Copyright 2016 Wiley−VCH. (C) Photographs of B-, G-, and T-Si-PCD/SiO2 nanocomposite powders under irradiation of daylight (top) and UV light (bottom). Reproduced with permission from ref 40. Copyright 2015 American Chemical Society. (D) Schematic diagram for the preparation of photoluminescent PCDs and the fabrication of dual-fluorescence PCD/starch composites. Reproduced with permission from ref 102. Copyright 2017 Wiley−VCH.

solid-state PLQY value to date. The author claimed that the bright solid-state emission could be attributed to abundant graphitic structures in the core and well-distributed surface states as a result of boron-and-nitrogen codoping as shown in Figure 5D.41 In fact, well-distributed surface states possibly play leading role in solid-state fluorescence owing to proper supramolecular cross-linking degree. More recently, our group achieved the red emission of PCDs under solid state (Figure 5E) from maleic acid and EDA via microwave-assisted method, and the aqueous-solution PCDs showed blue fluorescence, with PCD/polymer composites showing multicolor emission when changing the concentration of PCDs in matrix as shown in Figure 5F,G.89 We think that it can be ascribed to the precise control of facile microwave conditions (form a moderate carbonization degree), which generated red-shifted solid state emission due to supramolecular cross-link-derived energy transfer, and different supramolecular cross-linking degree in polymer matrix can generate different PL color. Currently, advances in our knowledge of supramolecularregulation emission of PCDs, and innovation in the synthesis of PCDs with solid-state emission are improving. Nevertheless, the most reported PCDs from small molecules show weak even no fluorescence, and challenges remain in this emerging field related to the bright emission at long wavelength. We believe that the multiple supramolecular cross-linking regulation strategies (such as hydrogen bonds, van der Waals interactions, host−guest interactions, and coordination interaction, etc.) will offer probably potential ways to render PCDs generate energy transfer, even form new narrow band gap, then unique optical properties. 3.1.2. Polymer Precursor-Derived Solid-State Emission. There were several reports about polymer precursor-derived solid-state emission. We think that the using of polymer

precursor-derived PCDs generally possess a high carbonization degree, which will result in the formation of π-domains even ring-like molecules connected on the PCD surface because of sufficient dehydration and carbonization. The formed strong π−π interaction under solid state will cause fluorescence quenching. Thus, supramolecular cross-linking regulation strategy provides us a method to enhance the solid-state emission by regulating the types and/or intensity of supramolecular interaction. Typically, inhibiting and avoiding the formation of strong supramolecular interaction in PCDs probably can conquer the fluorescence quenching by some particular strategies, such as carbonization degree control, surface charge engineering, etc. Zhang’s group reported the synthesis of yellowish-green fluorescent PCDs under solid states from calcium citrate and urea by microwave-assisted hydrothermal method as shown in Figure 5A.92 We speculate that the unique solid-state fluorescence can be attributed to the increased electrostatic repulsion between adjacent particles as a result of the presence of calcium ion on the surface of PCDs. Zhou’s group prepared solid-state fluorescent PCDs (SFPCDs) and solid-state nonfluorescent PCDs (SNPCDs) (Figure 5B (a)) from identical precursors (citric acid and urea) through precise control of the microwave reaction conditions.93 The entirely different solid-state emission properties could be attributed to different carbonization degree as shown in Figure 5B (b), which determine the amounts of chemical groups and molecule fluorophores on the surface of PCDs, reflecting the different supramolecular cross-linking degree between adjacent nanoparticles. Kim’s group synthesized boron-and-nitrogen codopped PCDs (BN-PCDs) from CA and EDA and boric acid via microwave-hydrothermal treatment. The as-prepared BN-PCDs exhibited exceptionally bright solid states fluorescence (Figure 5C) with the high PLQY of 67.6%, a champion F


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Figure 7. PCD-based composites with quenching resistant solid-state emission. (A) Schematic illustration of the formation mechanism of starch/ PCD phosphors. (B) (a) Optical photographs of starch/green-PCDs (left) and starch/blue-PCDs (right) phosphors under room light (top) and UV lamp (down). (b) Luminescent blocks fabricated from starch/blue-PCD (left) and starch/green-PCD (right) phosphors in epoxy silicone resin under room light (above) and UV lamp (below). Reproduced with permission from ref 20. Copyright 2014 Royal Society of Chemistry. (C) Multicolor composites films based on PCDs with polymers (160 and 240 represent the prapration temprature). Reproduced with permission from ref 114. Copyright 2014 Nature. (D) Preparation of luminescent PCD/PDMS films by one-pot thermal treatment route. Distinct multicolor PCDs were prepared by using different precursors. Reproduced with permission from ref 115. Copyright 2016 The Royal Society of Chemistry. (E) Chemical structure of (a) TMA-POSS, (b) photoluminescent photographs, and (c) TEM images of PCD@TMA-POSS powders under daylight (top) and UV light (bottom). Reproduced with permission from ref 116. Copyright 2015 Royal Society of Chemistry. (F) Fabrication mechanism of PCD@BaSO4 hybrid phosphors. Reproduced with permission from ref 120. Copyright 2017 Wiley−VCH.

3.1.3. Silane Coupling Agent Precursor-Derived Solid-State Emission. The using of silane coupling agent precursor is another strategy to conquer the self-quenching of solid-state PCDs. We take (3-aminopropyl)-trimethoxysilane (APTMS) for example. The reactive amino group can react with other organic acids, participating in the nucleation and growth of PCDs by continuous dehydration, polymerization, carbonization. The long-chain structures contribute to the formation of highly cross-linked polymer network structure in PCDs, and the alkoxy can hydrolyze or alcoholysis with alcoholic hydroxyl group, forming silica network structure on the surface of PCDs. The highly cross-linked polymer and silica net-structure could weaken and reduce strong supramolecular interaction between PCDs under solid state, avoiding the strong π−π interactioncaused fluorescence quenching. Zhang’s group reported the highly luminescent PCDs under solid state first by using CA as carbon source, ethanediaminecontaining organosilane (AEAPMS) as a coordinating solvent.97 The fluorescent PCD films or monoliths on the selected substrate or vessel can be obtained via simply heating at low temperature. What’s more, the as-prepared Si-PCDs were also functionalized with other silane coupling agents to fabricate luminescent gel glasses.98 However, the as-synthesized Si-PCDs was water-insoluble. Chang’s group reported the preparation of strongly fluorescent Si-PCDs under solid state

precursors can make as-synthesized PCD self-dispersion due to the formation of cross-linked polymer network structure in PCDs, which weaken strong supramolecular interaction between PCDs. What’s more, the using of polymer precursors can reduce the fabrication of π-domains and ring-like molecules due to the insufficient carbonization in contrast to small molecule precursors, which avoid the strong π−π interactioncaused fluorescence quenching. Chen’s group reported luminescent PCD films synthesized from poly(styrene-coglycidyl methacrylate) (PS-co-PGMA) photonic crystals.94 Subsequently, they reported another fluorescent PCDs under solid state via pyrolysis of poly(acrylic acid) (PAA) in the presence of glycerol.95 Other photoluminescent solid-state PCDs with the using of PVA precursor38,96 were also reported. Thereinto, Liu’s group38 ascribed the red-shifted solid-state fluorescence (Figure 6A) in contrast to aqueous-state fluorescence to proper interparticle spacing control (Figure 6B). The polymer precursors-derived PCDs may offer a proper interparticle distance, which not only protect the fluorescence from quenching but also make interparticle energy transfer then red-shifted emission under solid state. What’s more, the author also mentioned that overclose interparticle distance (the fabrication of excessive supramolecular cross-linking) will result in fluorescence quenching as shown in Figure 6B. G


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Figure 8. Room-temperature phosphorescence (RTP) properties of PCDs. (A) Photoluminescent spectra of PCDs and PCD/PVA composites: dispersed in water (blue line) and PVA matrix under UV light (cyan line), and after UV light has been turned off (olive line). (B) Time-resolved phosphorescence spectrum. (C) Proposed phosphorescence mechanism of PCDs dispersed in the PVA matrix. Reproduced with permission from ref 121. Copyright 2013 Royal Society of Chemistry (D) Schematic diagram of supramolecular cross-linking model of RTP emission. Reproduced with permission from ref 124. Copyright 2017 Royal Society of Chemistry. (E) Photoluminescent photograph of PCDs embedded into melting recrystallization urea and biuret matrices under UV light and after UV light has been turned off, and schematic illustration of possible energy structures of CN bonds and phosphorescent emission processes. Reproduced with permission from ref 50. Copyright 2016 American Chemical Society.

into the aqueous-solution red fluorescent PCD system, can also generate red fluorescent PCDs under solid state. For example, Xie’s group synthesized red fluorescent PCDs under solid state from 3-isocyanatopropyltriethoxysilane (IPTS) and p-phenylenediamine via solvothermal method.103 3.2. PCD-Based Solid-State Fluorescent Composites. The supramolecular cross-linking of PCDs under solid state is generally excessive owing to the presence of some strong supramolecular interaction (such as π−π interaction), and effective methods to conquer excessive supramolecular crosslinking for bright solid-state emission are required. In addition to the above-mentioned self-quenching-resistant strategies, compositing PCDs with polymer or inorganic matrix is another efficient strategy to conquer the excessive supramolecular crosslink-caused luminescence quenching. 3.2.1. Polymer Matrix. Polymer has been considered to be good antiaggregation agent for many luminescent materials, such as nanocrystals,104 nanoclusters,105 etc. For PCD/polymer composites, the polymers can disperse the cross-linked PCDs under solid state, and their functional groups can form hydrogen bond with PCD surface groups. Shen and co-workers developed strong luminescent PCD/starch composites (Figure 7A, B) with high PLQY of about 50%.20 The author thought that the formative hydrogen bonds between hydroxyl groups of starch particles and carboxyl and amide groups of PCD surface, suppressed the nonradiative decay processes, thus generating quenching-resistant emission. Analogous luminescent PCD/ polymer composites were also reported by employing other polymers. These polymers include poly vinyl alcohol (PVA),106−108 epoxy resin,109 poly(methyl methacrylate) (PMMA),110−112 poly(N,N′-dimethylacrylamide) (PDMAA),68 silicone,113 etc. Interestingly, Zeng’s group found that the PCDs dispersed in different polymer matrixes can presented different PL colors as shown in Figure 7C, even

from APTMS via one-pot hydrothermal method. The asprepared Si-PCDs exhibited excellent photostability and high water-solubility.99 Similar works100,101 with more detailed optical properties characterization, were also reported by Liu’s group from CA and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEATEMS or KH-792). The PCDs with long-wavelength fluorescence under solid state have been the subject interests for researchers, contributing to bioimaging and multicolor light-based applications. Chang’s group prepared multicolor luminescent solidstate Si-doped PCDs (Figure 6C) via hydrothermal method followed by calcination of powder or film. They attributed the heat-induced PL red-shifting to the combination of several factors, including the formation of SiO2 in PCDs due to the hydrolysis of the excess APTMS, change in sizes, and composition of Si-PCDs during heating. 40 These heatinduced-structure changes could make supramolecular crosslinking changed, influencing the energy transfer and PL colors. However, it should be noted that the PLQY of the as-prepared blue-, green- and tan-emitting Si-PCD powders were only 6.7, 0.6, and 0.1%, respectively. Recently, Lei’s group reported solidstate red-fluorescent Si-PCDs with upper PLQY of 9.60% and great water-solubility from CA and 3-(2-Aminoethylamino) propyl-dimethoxymethylsilane (AEAPMS or KH-602), with acetone acting as solvent as shown in Figure 6D.102 It should be noted that the as-prepared PCDs exhibited blue fluorescence under aqueous solution with PLQY of 50.7%, namely showing dual-fluorescence morphologies emission between solid state and liquid state. The large red-shift of PL wavelength was ascribed to the supramolecular cross-link-induced Förster resonance energy transfer (FRET) as proved by solventdependent PL in aqueous solution, humidity and concentration-determined PL colors for solid PCD/starch nanocomposites. What’s more, introducing silane coupling agents H


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ACS Applied Materials & Interfaces when the same PCDs were used.114 What’s more, Raz Jelinek’s group reported the in situ synthesis of color-tunable (green, yellow, and orange) and strongly luminescent PCD/PDMS composites (Figure 7D) by employing different PCD precursors through a simple one-pot thermal treatment,115 which provided a new strategy for constructing highly colortunable fluorescent light emitters. 3.2.2. Inorganic Matrix. Inorganic matrix can also be used to weaken the excessive supramolecular cross-linking (π−π interaction) of PCDs under solid state. Rogach’s group reported the fabrication of highly luminescent solid-state luminophores with strong deep blue emission and a record high PLQY of 60% by integrating water-soluble PCDs with an octa(tetramethylammonium)-functionalized polyhedral oligomeric silsesquioxane (TMA-POSS) as shown in Figure 7E.116 Other inorganic matrixes, such as SiO2,117 NaCl118,119 were also employed to form solid-state luminescent composites. Moreover, supramolecular assembly strategy was also used to fabricate solid-state luminescent hybrids. Qu’s group fabricated PCD/BaSO4 hybrid (Figure 7F) in an easy and low-cost process by sequentially assembling Ba2+ and SO42− ions onto the surface of PCDs.120 The as-fabricated PCD/BaSO4 hybrid exhibited excellent photothermal stability with PLQYs of 27%, as well as remarkable resistance to strong acid/alkali and common organic solvents.

with no matrix used.124 We think that the using of polymer precursors can make as-synthesized PCDs partly maintain polymeric structure. Thus, cross-linked polymer network structure could be formed by covalent-bond cross-linking by dehydration condersation and supramolecular interaction crosslinking by H-bond between interchains, which can effectively lock the emissive species. Inorganic crystalline structure is an alternative to fabricate PCD-based RTP hybrids. Most reported works illuminated that the confine effects of inorganic crystalline structure were the main origins of PCD RTP properties by rigidifying triplet excitons (potential phosphorescence center) then suppressing nonradiative relaxation. These inorganic crystalline structures contain layered double hydroxides (LDHs),125 KAl(SO4)2· xH2O.126 However, Zhou’s group reported the supramolecular interaction-assisted RTP emission by one-pot heating treatment for the mixture of urea and as-prepared PCDs. They elucidated that the CN bond on the surface of PCDs was potential phosphorescence center, and the RTP emission was ascribed to the fabrication of composite matrices (the melting recrystallization urea and biuret fabricated from the heating urea), which played a dual role to suppress the vibrational dissipation of long-lived triplets by combining the rigidity of the melting recrystallization urea and hydrogen bond-induced cross-linking effects between biuret and PCDs as shown in Figure 8E.50 Currently, the investigation of the PCD RTP phenomenon is still in preliminary stage, more efforts need to be done to solve some crucial science issues, including origin of the PCD RTP phenomenon, and the relationship between the fluorescence and phosphorescence in PCDs. Moreover, it is highly desired to find new strategies to improve RTP performances (color, PLQY, etc.), achieve more self-confinement PCD RTP systems without matrices, even delay fluorescence by confining PCDs in other rigidity matrixes containing porous or cagelike structure. The construction and regulation of efficient supramolecular cross-linking network based on multiple types of supramolecular interaction, will provides us powerful routes to improve RTP performances.

4. SCEE EFFECT-ASSISTED ROOM-TEMPERATURE PHOSPHORESCENCE OF PCDS The concepts of PCDs, derived from the research field of CDs, are just put forward recently, and there are still many novel properties waiting to be exploited. The room-temperature phosphorescent (RTP) property of PCDs is one of the interesting properties, which was achieved by integrating asprepared PCDs in matrix materials, such as polymer, inorganic crystalline materials. We emphasize that the supramolecular interaction between PCDs and matrix materials plays an important role in the generation of RTP phenomenon. For polymer matrix, there exists many oxygen/nitrogen containing functional groups (such as C = O, −NH2, −OH, etc.), which can form large amounts of hydrogen bonds with the potential triplet state groups (such as C = N, C = O) of PCDs under the solid state. The as-fabricated supramolecular cross-linking could effectively lock the emissive species, and inhibit their intramolecular motions and the nonradiative relaxation channel, generating RTP emission. In 2013, Zhao’s group reported the first example of PCD-based RTP (Figure 8A) by embedding PCDs into poly(vinyl alcohol) (PVA) matrix, and its phosphorescent lifetime was up to the subsecond (380 ms) as shown in Figure 8B. The generation of phosphorescence was attributed to the triplet state of CO bonds, but it cannot produce RTP phenomenon when PCDs were dispersed in water, cellulose paper, PEG 20000, etc.121 That is, the PVA matrix plays a crucial role in the generation of RTP. The author thought that large amounts of hydrogen bonds could be formed between the CO of PCDs and the hydroxyl group of PVA as shown in Figure 8C. The hydrogen bond-induced cross-linking fixation effects effectively inhibit the nonradiative relaxation channel then promote the generation of RTP. Subsequently, similar phenomena were also reported by many groups by dispersing as-prepared PCDs into poly(vinyl alcohol) (PVA),49,122 polyurethane (PU).123 Furthermore, there was a report about the self-confined RTP phenomenon (Figure 8D) of PCDs prepared from PVA and ethylenediamine,

5. RELATED APPLICATIONS In the aforementioned sections, we mainly focus on the supramolecular cross-link-regulated photoluminescence properties of PCDs, emphasizing highly the solid-state emissions of PCDs. Thus, this section mainly introduce some solid-state emission-derived applications, including phosphor light emitting diodes, down-conversion solar cells, RTP-derived anticounterfeiting and data encryption applications, with few supramolecular-regulated aqueous-state applications involved. We hope this section could encourage researchers to expand the applications of these novel materials. Of course, some biological applications, such as bioimaging, drug delivery, and therapy, also belong to the applications ranges of PCDs mentioned in this review, but the biologically related applications will not be elucidated in this review considering there have been number of reviews for the biological applications of PCDs. 5.1. Phosphor Light-Emitting Diodes. PCDs present considerable potential in light emitting diodes (LEDs) owing to excellent PL properties, especially as phosphor conversion layer. The PCD-based phosphors LEDs possess definite superiority in contrast to the rare earth iron and quantum dots-based phosphors, including great optical stability, cheap raw materials and low toxicity. Generally, the most of asI


Review


properties, endowing PCD long-wavelength photoluminescence (up to 650 nm) and narrow spectral widths. The monochrome LED devices (Figure 10D) were then achieved by using the functional PCD/PMMA composite as phosphor.46 What’s more, the single solid-state fluorescent PCDs prepared from small molecules was also used to fabricate full-color and white color LEDs by our group as shown in Figure 10E,89 which was a new strategy of constructing full-color and white color LEDs using only a kind of solid-state fluorescent PCDs. It should be noted that the PCDs possessed short-wavelength aqueous-solution emission (blue) and long-wavelength solidstate emission (red) as shown in Figure 6A. PCDs are increasingly used to fabricate color-conversion layer of LEDs, but it is far from satisfactory in aspects of luminance, color purity, etc. Thus, there is a long way to go to achieve high-performance PCD phosphor LEDs. First, some extra strategies need to be employed to conquer preferably the fluorescence quenching of PCDs under solid state, such as searching excellent matrixes beneficial to the solubility and dispersity of PCDs. What’s more, it is highly desired to design and prepare the brightly multicolor luminescent PCDs under solid state for achieving bright multicolor LEDs. Finally, the fabrication procedure of devices need to be optimized, especially the thickness control of phosphor coated on the GaN LED chips, which plays a crucial role in the luminance and color purity of phosphor-based LEDs. 5.2. Down-Conversion Solar Cells. Zhang’s group applied the solid-state luminescent PCDs prepared from ascorbic acid and N-(2-aminoethyl)-3-aminopropyltriethoxysilane (KH791) as light down-conversion layers of inverted polymer−fullerenebased bulk heterojunction (BHJ) solar cells (Figure 11A), improving the power conversion efficiency (PCE) by about 12% (Figure 11B).129 They found that the PCDs can boost certainly the light-harvesting of solar cells in near-ultraviolet and blue-violet portions of sunlight on the basis of the luminescent down-shifting behavior of PCDs as shown in Figure 11C. In the future, the solid-state luminescent PCDs will possibly achieve more breakthroughs in other types of solar cells. In terms of high-efficiency Si solar cells, although the down-conversion of graphene quantum dots (GQDs) with no solid-state fluorescence has been achieved,130 there’s still some room for improvement when employing the solid-state luminescent PCDs. 5.3. Other Applications. In addition to the photoelectric applications, the solid-state luminescent PCDs could be applied in other fields. Zhang’s group employed the as-synthesized PCDs as fingerprint detection tool based on solid-state fluorescent properties.92 In terms of the RTP properties of PCDs, it can be used as anticounterfeiting ink and dataencryption applications, such as color-encoding and timeresolved identification. In 2013, Zhao’s group observed the RTP phenomenon of PCDs and applied it in anticounterfeiting (Figure 11D).121 Similar modes (Figure 11E, F) were also reported by other groups.49,50 Qu’s group utilized waterinduced photoluminescence behaviors of supra-PCDs for water-jet fluorescence printing and sweat pore imaging by water or sweat-induced PL of supra-PCD/paper composites, which provided us a new application thoughts of PCDs in information storage, information security, and protection and medical diagnose.74

prepared PCDs can be dispersed directly in various polymer matrices to form PCD/polymer composites (section 3.2.1) phosphors, which were coated on ultraviolet or blue emissive GaN chips for LEDs applications as shown in Figure 9.68 In

Figure 9. (A) Photograph of poly(N,N′-dimethylacrylamide) (PDMAA)/PCD phosphor white LEDs. (B) Emission spectra of white LEDs. (C) Color coordinate (0.29, 0.31) of LEDs. The bulk polymer/PCD composites phosphor’s photograph (D) under daylight and (E−G) under UV lamp at different temperature. Reproduced with permission from ref 68. Copyright 2016 American Chemical Society.

addition, the distinct self-quenching-resistant fluorescent PCDs under solid state (section 3.1.2 and 3.1.3) were also reported to construct phosphors by mixing with polymer, with elevated performances presented. Inorganic matrix-dispersed PCDs (section 3.2.2) were also reported to fabricate PCD-based composite phosphors by combining with polymer. Moreover, liquid-phase-based LEDs without matrix were also achieved. Bai’s group reported liquid-state PCD-based multicolor (blue to red) LEDs with stable luminescence characters by changing the concentration of PCDs in toluene as shown in Figure 10A. The PCDs were stored in a glass box and packaged with a UVLED chip.88 Furthermore, PCDs can also be combined with other luminescent materials to fabricate composites phosphor for white LEDs, such as quantum dots,88,116 conjugated polymer dots,127 rare earth,128 etc. For example, Liu’s group constructed dual-emitting core−shell PCD/silica phosphor (PCDSP) from PCD, tetraethoxysilane (TEOS) and Sr2Si5N8:Eu2+ via one-pot sol−gel method as shown in Figure 10B, achieving white light emission. Then, PCDSP were mixed with epoxy resin to fabricate white LEDs with a high color rendering index (CRI) of 94.128 However, the above-mentioned reports are almost white color phosphor LEDs for daily illumination, only few other colors LEDs with inferior color purity were obtained owing to broad spectral widths. To achieve the monochrome phosphor LEDs devices based on PCDs for multicolor displays, Rogach’s group reported a series of monochromatic phosphor LEDs based on monodisperse PCDs prepared from CA and N-(βaminoethyl)-γ-aminopropylmethyldimethoxysilane (AEAPMS) via pyrolysis method, in which the colors were determined by the thickness of the down-conversion layers or the concentration of PCDs in the polymer matrix as shown in Figure 10C.19 Rhee’s group chemically modified the surface of PCDs using a series of para-substituted anilines to control their PL J


Review


Figure 10. LEDs applications of PCDs. (A) PCDs (a) LED PL spectra, (b) CIE color coordinates, and (c) LED photographs of blue, green, yellow, and red luminescence of UV-LED under 3 V. Reproduced with permission from ref 88. Copyright 2016 Royal Society of Chemistry. (B) Formation process of PCDSP through one-pot sol−gel method, the left inset is the photographs of PCDSP under daylight (left) and UV light of 365 nm (right), the right inset is photograph of a PCDSP-based white LEDs. Reproduced with permission from ref 128. Copyright 2015 Royal Society of Chemistry. (C) (a) Schematic diagram of the monochromatic PCD-LEDs with PCDs embedded in a PMMA matrix with a variable layer thickness on top of a UV-LED chip. (b, c) Thickness of the PCD active-layer-dependent PL colors of PCD-LEDs. Reproduced with permission from ref 19. Copyright 2015 Royal Society of Chemistry. (D) Emission spectra of multicolor LEDs. The intensity is normalized for easy comparison, the inset is the photoluminescent photographs of LEDs during operation. Reproduced with permission from ref 46. Copyright 2015 Nature. (E) (I) Photograph, (II) emission spectra, and (III) CIE color coordinate of the LEDs by mixing PCD/starch composites or PCD powder with curable resin, with (a) blue, (b) green, (c) yellow, (d) red, and (e) white light, respectively. Reproduced with permission from ref 89. Copyright 2017 Wiley−VCH.

6. CONCLUSIONS AND OUTLOOKS In summary, PCDs possess outstanding physicochemical properties, especially bright PL properties. In contrast with other light-emitting materials, PCDs exhibit some exact advantages including low toxicity, abundant raw material, simple preparation and post-treatment procedure, etc. On the basis of both experimental and theoretical results, the supramolecular cross-linking and SCEE are crucial in PCDs, with highly emphasizing great function in assisting efficient synthesis, mediating the PL behaviors and exploiting novel applications. However, we suppose that the supramolecular cross-linking-regulated emission in PCDs is still in its infant stage, and there are several crucial science problems to be solved, such as controllability of supramolecular cross-linking degree for further improving the optical properties, solid-state applications exploitation and carrier conductivity improvement for optoelectronic applications. Currently, the supramolecular cross-linking degree (intensity and types) is generally uncontrollable because of the hard characterization for supramolecular interaction. Thus, researchers could not purposely obtain PCDs with certain optical and electrical properties by regulating the cross-linking degree (such as the size and orientation of supramolecular interaction, existed spatial position, etc.). Thus, the controllability of

supramolecualr cross-linking degree may be a significant research direction in order to purposely gain the distinct PCDs with certain structures and properties. Especially, abundant supramolecular interaction types (such as hydrogen bonds, van der Waals interactions, host−guest interactions, and coordination interaction, etc.) could be employed for designing unique supramolecular cross-linking structure, which present an opportunity for the further development of PCDs. Taking the investigation of PCD RTP for example, if the controllability of supramolecualr cross-linking degree is achieved, the RTP performances can be promoted and aqueous-state RTP can be obtained. The unique solid-state emission gives PCDs potential use in light-emitting applications. Currently, the investigation for PCD solid-state emission is still in the preliminary stage. Increasing efforts are required to improve luminescent performances for high-performance applications, such as the study of related mechanisms, PL color regulation, the achievement of high PLQY, etc. More importantly, it is highly desired to broaden the application ranges based on unique solid-state emission of PCDs in addition to the mentioned applications in this review. Finally, solid-state luminescent PCDs can probably make a breakthrough in electroluminescent light-emitting diodes fields as a result of more easy recombination between electron and K


Review


Figure 11. Applications of quenching-resistant PCDs under solid state in solar cells, anticounterfeiting and color-encoding. (A) Schematic architecture of the BHJ solar cells with luminescent down-shifting (LDS) layer. (B) J−V curves of the P3HT:PCBM-based BHJ solar cell with and without the LDS layer. (C) Wavelength dependences of EQE of the P3HT:PCBM-based solar cell with and without the LDS layer. Reproduced with permission from ref 129. Copyright 2014 Elsevier. (D) Letters “‘ciomp’” written on a common printing paper are invisible under daylight and even UV light, but appear when the UV light has just been turned off in a darkroom. Reproduced with permissio from ref 121. Copyright 2013 Royal Society of Chemistry. (E) Anticounterfeiting applications of PCD/PVA composites ink’s RTP in paper money. Reproduced with permission from ref 49. Copyright 2016 Wiley−VCH. (F) Security protection applications of RTP property of PCDs. Reproduced with permission from ref 50. Copyright 2016 American Chemical Society. (3) Lim, S. Y.; Shen, W.; Gao, Z. Carbon Quantum Dots and Their Applications. Chem. Soc. Rev. 2015, 44, 362−381. (4) Yuan, F.; Li, S.; Fan, Z.; Meng, X.; Fan, L.; Yang, S. Shining Carbon Dots: Synthesis and Biomedical and Optoelectronic Applications. Nano Today 2016, 11, 565−586. (5) Song, Y.; Zhu, C.; Song, J.; Li, H.; Du, D.; Lin, Y. Drug-Derived Bright and Color-Tunable N-Doped Carbon Dots for Cell Imaging and Sensitive Detection of Fe3+ in Living Cells. ACS Appl. Mater. Interfaces 2017, 9, 7399−7405. (6) Zhu, A.; Qu, Q.; Shao, X.; Kong, B.; Tian, Y. Carbon-Dot-Based Dual-Emission Nanohybrid Produces a Ratiometric Fluorescent Sensor for in Vivo Imaging of Cellular Copper Ions. Angew. Chem., Int. Ed. 2012, 51, 7185−7189. (7) Song, Y.; Zhu, S.; Xiang, S.; Zhao, X.; Zhang, J.; Zhang, H.; Fu, Y.; Yang, B. Investigation into the Fluorescence Quenching Behaviors and Applications of Carbon Dots. Nanoscale 2014, 6, 4676−4682. (8) Ding, C.; Zhu, A.; Tian, Y. Functional Surface Engineering of CDots for Fluorescent Biosensing and in Vivo Bioimaging. Acc. Chem. Res. 2014, 47, 20−30. (9) Pan, L.; Sun, S.; Zhang, A.; Jiang, K.; Zhang, L.; Dong, C.; Huang, Q.; Wu, A.; Lin, H. Truly Fluorescent Excitation-Dependent Carbon Dots and Their Applications in Multicolor Cellular Imaging and Multidimensional Sensing. Adv. Mater. 2015, 27, 7782−7787. (10) Qu, D.; Sun, Z.; Zheng, M.; Li, J.; Zhang, Y.; Zhang, G.; Zhao, H.; Liu, X.; Xie, Z. Three Colors Emission from S, N Co-Doped Graphene Quantum Dots for Visible Light H2 Production and Bioimaging. Adv. Opt. Mater. 2015, 3, 360−367. (11) Liu, J.; Lu, S.; Tang, Q.; Zhang, K.; Yu, W.; Sun, H.; Yang, B. One-Step Hydrothermal Synthesis of Photoluminescent Carbon Nanodots with Selective Antibacterial Activity against Porphyromonas Gingivalis. Nanoscale 2017, 9, 7135−7142. (12) Zheng, M.; Ruan, S.; Liu, S.; Sun, T.; Qu, D.; Zhao, H.; Xie, Z.; Gao, H.; Jing, X.; Sun, Z. Self-Targeting Fluorescent Carbon Dots for Diagnosis of Brain Cancer Cells. ACS Nano 2015, 9, 11455−11461. (13) Feng, T.; Ai, X.; An, G.; Yang, P.; Zhao, Y. Charge-Convertible Carbon Dots for Imaging-Guided Drug Delivery with Enhanced in Vivo Cancer Therapeutic Efficiency. ACS Nano 2016, 10, 4410−4420. (14) Jian, H. J.; Wu, R. S.; Lin, T. Y.; Li, Y. J.; Lin, H. J.; Harroun, S. G.; Lai, J. Y.; Huang, C. C. Super-Cationic Carbon Quantum Dots

hole, especially narrow-bandgap emission. However, the low carrier conductivity of PCDs may become a great obstacle, especially for long-chain alkyl containing PCDs and PCD-based composites, which are detrimental to the transportation of carrier. Thus, to achieve high-performance electroluminescent LEDs applications based on solid-state luminescent PCDs, it is highly urgent to develop strongly luminescent and selfquenching-resistant PCDs by rational synthesis design, with no long-chain alkyl-containing precursors or matrix employed to synthesize PCDs or combined with PCDs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shoujun Zhu: 0000-0002-2412-6121 Bai Yang: 0000-0002-3873-075X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (2016YFB0401701); the National Science Foundation of China (NSFC) under Grants 51433003, 51373065, and 21504029; and JLU Science and Technology Innovative Research Team 2017TD-06.

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Review

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