Carbonized Polymer Dots: A Brand New Perspective to Recognize

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Perspective

Carbonized Polymer Dots: A Brand New Perspective to Recognize Luminescent Carbon-Based Nanomaterials Songyuan Tao, Tanglue Feng, Chengyu Zheng, Shoujun Zhu, and Bai Yang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01384 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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The Journal of Physical Chemistry Letters

Carbonized Polymer Dots: A Brand New Perspective to Recognize Luminescent Carbon-Based Nanomaterials Songyuan Tao1, Tanglue Feng1, Chengyu Zheng1, Shoujun Zhu2, Bai Yang1,* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, P. R. China. 2 Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, 35 Convent Dr, Bethesda, MD 20892 USA. 1

Abstract Carbon dots (CDs), as emerging luminescent nanomaterials, possess excellent but complex properties, bringing about extensive attention and lots of controversy. In this perspective, we put forward the concept of “carbonized polymer dots”, and emphasized the important role of polymerization and carbonization during the formation of CDs. We explored the common characters and clarified the complicated relationship of CDs, based on the reasonable classification of graphene quantum dots, carbon quantum dots and carbonized

polymer

dots.

Moreover,

different

perspectives

were

provided

for

comprehensive analysis about the essence of CDs, including quantum dots, molecules and polymers. The photoluminescence mechanism has been classified into molecule state, carbon core state, surface/edge state and crosslink enhanced emission effect for the further understanding of complicated phenomena.

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Carbon materials, such as graphite, diamond, fullerene, carbon nanotube, and graphene, have always been the focus of attention in the field of material science due to the abundant reserves and environmental friendliness.1-3 The macro-sized carbon material lacks a suitable band gap, making it difficult to serve as an ideal luminescent material. Researchers have been working on how to endow carbon materials with appropriate energy levels and luminescence. Recently, a series of luminescent nano-sized carbon has been successfully prepared by “top-down” cutting or “bottom-up” synthesis.4-7 The class of these carbon-based nanomaterials typically exhibit small size (generally below 10 nm), bright fluorescence (the highest quantum yield over 90%), low toxicity (biocompatible and metabolizable), and great stability (thermostabilization and photobleaching resistance).7 Different from traditional luminescent materials, the fluorescence of carbon-based nanomaterials is found to originate from special luminescence centers, and simultaneously display some characteristic properties. It has always been a tricky problem to reasonably classify and evaluate a new class of materials, especially for those with diverse structures and properties. Researchers adopted various definitions to describe the obtained luminescent nanodots, such as carbon dots (CDs),6 carbon nanoparticles (CNPs),4,8 carbon nanodots (CNDs)9, graphene quantum dots (GQDs),10,11 carbon quantum dots (CQDs)12, 13, polymer dots (PDs)14,15 and so on.16 Some definitions seem too general to distinguish the numerous relatives of CDs, some even have overlap and conflict. Unification and standard are urgently needed for further development of CDs. This perspective emphasizes the balance of polymerization and carbonization, and put forward the concept of “carbonized polymer dots, CPDs,” providing a comprehensive perspective to recognize and reveal the essence of luminescent carbon-based nanomaterials. As emerging luminescent nanomaterials, the study on luminescent behaviors of CDs is significant but challenging. Researchers attempted to draw inspiration from fully researched semiconductor quantum dots (SQDs), but gradually found the distinct difference either in photoluminescence (PL) performance or mechanism.17 Table 1 illustrates several representative features of most SQDs and carbon-based nanodots for concise explanation, not excluding some exceptional cases. For typical SQDs, PL generally arises from the intrinsic band-edge emission of electron-hole recombination, occasionally involved by dopant


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levels.18 The exciton confinement is size-dependent, thus the PL properties are mainly dominated by quantum confinement effect when quantum dots (QDs) are smaller than their exciton Bohr radius. Researches on QDs are focused on the regulation of intrinsic energy levels related to particle size and doping, and the surface effect related to ligands and defects. Well-controlled SQDs with uniform size distribution exhibit excitation-independent emission and narrow emissive full width at half maximum (FWHM), the PL lifetime of which reaches up to 101~102 ns. For carbon-based nanodots, diverse structures determine abundant luminescent properties, bringing about more complex PL origins. The local π-domains with imperfect conjugated structure and limited size are generally accompanied by defects, doping, quasi-molecule and sub-fluorophore, that facilitates the formation of various individual energy levels rather than continuous band gap. Many factors have been proven to contribute to the luminescence of CDs. Although CDs have the same size as QDs, it is unremarkable in CDs for the intrinsic emission derived from quantum confinement effect due to the existence of multiple PL centers. The PL of most CDs always reflects size-independence, excitation-dependence, broad emissive FWHM and short lifetime (100~101 ns).7 Obviously, the developed theories of QDs or graphene are not fully applicable to explain the diverse PL behavior of CDs, calling for further exploration on luminescence essence of CDs in categories.17



Table 1. Comparison on luminescent performance of SQDs (e.g., IV, II-VI, IV-VI, III-V SQDs and perovskite QDs) and carbon-based nanodots (e.g., GQDs, CQDs and CPDs).

There is a big debate about how to reasonably recognize and divide CDs. “CDs” is a general term for a variety of nano-sized luminescent carbon materials, that always possesses at least one dimension less than 10 nm in size and fluorescence as instinct properties, consisting of sp2/sp3 carbon and oxygen/nitrogen-based groups. CDs are mainly prepared by pyrolysis of graphite-based materials or polymerization of small molecular precursors.7,8,19 The former tends to be exfoliated into few-layer (GQDs) or multi-layer (CQDs) graphite structure.10,19 The latter tends to form highly-crosslinked polymer cluster (CPDs) by random dehydration, or well-defined conjugated π structure (CQDs) by further carbonization or special planar design of precursor.6,12 It is most reasonable to classify CDs into GQDs, CQDs and CPDs at current stage, covering almost all different categories. In terms of synthesis, “top-down” method mainly cuts graphite-based materials with perfect sp2 carbon structures, such as graphite powder, carbon rods, carbon fibers, carbon


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nanotubes, carbon black, and even candle soot, the preparation process of which involves oxidation and exfoliation.19 “Bottom-up” method focuses on the crosslinking reactions of molecules or polymers rich in amino, hydroxyl, carboxyl or other active groups, in which polymerization and carbonization play a vital role for the formation of particles.6,20 In terms of structure, GQDs possess graphite structure within five layers and connected chemical groups on the edges.21,22 They are anisotropic, with lateral dimensions larger than height, that can serve as an ultra-small two-dimensional material.23,24 GQDs possess similar properties to graphene oxide, the PL of which are determined by conjugated π-domains, and greatly affected by surface or edge structure. GQDs exhibit certain crystallinity and size effect. As the class of CDs with simplest and clearest structure, GQDs are ideal models for the study on PL mechanism.25,26 Different from the planar structure of GQDs, CQDs are usually three-dimensional multiple-layer graphite structure with similar horizontal and vertical dimensions. Besides incomplete exfoliation by “top-down” route, researchers have also proved the feasibility of “bottom-up” approach to prepare CQDs by periodic reaction of coplanar molecules with high symmetry, such as phenol, aromatic amine, and other derivatives.11,12,27 Notably, “CPDs” is a updated concept, that has been proposed recently to reveal the forming essence of a class of CDs and simultaneously emphasize the importance of polymerization and carbonization, originally named as polymer carbon dots or even polymer dots.20,28-33 In general, “top-down” route leads to more regular morphology, while “bottom-up” method brings about more abundant properties. There are increasing studies concentrated on selecting and regulating molecule or polymer precursors for better performance (Figure 1). In addition to the few aforementioned molecules with high symmetry (CQDs), the vast majority of multi-functional precursors have no symmetric structure. That means non-selective random polymerization will facilitate spatial extension in all directions. As illustrated in Figure 1, the final product seems to be a tangled coil, consisting of a hydrophobic core with highly crosslinking or slightly graphitization, and hydrophilic polymer chains on external surface, properly named CPDs. CPDs are as diverse as polymers, even with more abundant compositions, changeable structures and regulated properties. The stability of CPDs is better than that of polymers due



to the carbonization, while the compatibility of CPDs is better than that of QDs due to the reserved polymer chains. As a link between polymers and QDs, CPDs not only combine the advantages of both, but also are environmentally friendly, non-toxic and low cost simultaneously.34 Firstly, compared with metal element, carbon as the main constituent element makes CPDs nontoxic and metabolizable. Secondly, the raw materials of CPDs are widely available, rich in variety and low in price, including organic small molecules, polymers and even biomass. Thirdly, several facile preparation methods have been proven to be effective, such as hydrothermal, solvothermal or microwave methods. Fourthly, CPDs possess abundant surface groups, making CPDs hydrophilic enough without further modification, that is conducive to apply in living organisms.

Figure 1. Synthesis and structure of CPDs.

The vast majority of CDs obtained by “bottom-up” route should belong to the range of CPDs in a strict sense. The synthesis of “bottom-up” method requires precursors to possess certain polymerizable elements, including dehydratable functional groups, crosslinkable acting sites or additive unsaturated bonds (Figure 1).35 The formation of CPDs need to undergo a series of complex changes, mostly involved in hydrothermal crosslinking polymerization (HTCP). Figure 2 describes the whole process of HTCP, experiencing from precursors to final complete carbonization. The first stage mainly involves the growth of precursors. Effective intermolecular collisions lead to dehydration between functional groups, forming longer polymer chains with certain crosslinking. As temperature rises, more intense thermal motion and newly initiated reaction pathway bring the reaction to the next


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stage (Figure 2). Entanglement happens in chain segments of preformed polymer chains, generating numerous random coils. Owing to the shortened spatial distance, crosslinking further proceeds in interior of polymer clusters and the structures get more compact and stable, that is the initial birth of CPDs and subsequently accompanied by slight carbonization. In the next process, polymeric structure is gradually reduced with the increase of carbonization degree, while microcrystalline regions and lattices appear in the interior of CPDs. When completely carbonized, PL intensity of the obtained product is greatly reduced, that is already beyond the study range of CPDs and even CDs as luminescence materials (Figure 2).20,36,37 Notably, some small molecules, formed during the reaction by intramolecular cyclization or fragmentation of unstable structures, can also serve as new building blocks to participate in the generation of CPDs.38 It has been confirmed these fluorophores have great contributions to PL emission, whether they are bonded to CDs or involved in the formation of carbon skeletons.39,40 Complete purification is necessary to avoid the interference from free fluorophores in solution. In general, the involved process of HTCP in water-dispersed nano-domains is similar to seed growth and soap-free emulsion polymerization in polymer synthesis, while the formed particles are restricted and stabilized by hydrophilic-hydrophobic interaction. The model of HTCP summarizes the effect of polymerization, crosslinking and carbonization on different growth stages of CPDs, in favor of separately discussing the complex PL origin.

Figure 2. A diagram to describe the reaction process of hydro-thermal crosslinking polymerization (HTCP) to prepare CPDs by “bottom-up” route.

It is reasonable to understand the relationship between GQDs, CQDs and CPDs in the following way. GQDs possess the fundamental structure of graphite layer to construct CQDs, that can be approximately regarded as fully exfoliated CQDs. The exfoliation degree during



“top-down” process determines the final product is GQDs or CQDs. Besides, some GQDs may experience redox reaction, resulting in random aggregation and structure transformation, that eventually change into amorphous CPDs. Similarly, CQDs can be regarded as a special type of CPDs in an extreme state, the Internal polymerization behavior of which can be strictly regulated. It is crucial for the proper selection of precursors with symmetrical factors to obtain CQDs by “bottom-up” method. Appropriate carbonization is conducive to promoting the conversion of CPDs to CQDs. In addition to classification discussed above, the PL origin of CDs is another recognized difficulty with many controversies, especially for CPDs with diverse structures. CDs probably exhibit distinct characteristics, such as molecule,41 polymer,20 or quantum dot,42 due to the differences in multiple classes, including GQDs, CQDs and CPDs.20 The studies on CDs can be inspired by the research method of molecule, polymer or quantum dot, and mainly focus on the following four parts. The first is to study the distribution of electronic energy levels in a limited nano-sized space as other QDs do. The gaps of CDs usually originate from heteroatom-doped carbon core rather than pure graphite structure.42,43 The second is to analyze the influence of the composition or structure of the edge or surface on luminescence.44,45 The third is to confirm luminescent organic molecule or some derived structure forming during the synthesis process.43,46 The fourth is to reveal the enhancement effect on fluorescence by the highly crosslinked polymeric structures. Such effect was firstly proposed as crosslink-enhanced emission (CEE) effect to explain strong emission in non-conjugated system (Scheme 1).47 Structures only with sub-fluorophores (C=O, C=N, N=O or amino-based groups, C-O) instead of typical conjugated fluorophore groups should not emit strong PL in the usual sense. Interestingly, PL can be enhanced by covalent bond crosslinking, supramolecular interaction or rigid aggregation. CEE effect has been verified to be widespread in CDs and significant to PL, especially for CPDs due to the existence of polymeric structures.29,33


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Scheme 1. Representation of the covalent-bond, supramolecular-interaction, and rigidity-aggregated CEE effect in non-conjugated polymers or CPDs. Reproduced by permission of Wiley-VCH from Ref. [47].

The PL mechanism of CDs can be correspondingly attributed to the carbon core state affected by quantum confinement effect or the structure of conjugated π-domains,26,47-49 the surface/edge state determined by the hybridization of the carbon skeleton and the connected chemical groups,24,50-53 the molecule state solely controlled by the organic fluorophore bonded to the surface or interior of core,39,40,48 and the CEE effect amplified by crosslinking of sub-fluorophore.32,54,55 Table 2 illustrates a comprehensive reference to analyze PL mechanism of CDs.

Table 2. Main research aspects on PL mechanism of CDs.

Better performance of CDs have been explored, realizing satisfactory achievements in many hot research fields including biology, energy, display and smart response.10,12,56-58 Clear standards are needed to regulate further development of CDs. There are three critical issues



to be noticed. Firstly, the obtained products must be fully purified to avoid the interference from strong PL of free fluorophores, especially for CDs synthesized from small molecule precursor. Secondly, fundamental research on the essence of CDs need sustained attention.45,59 Researchers should establish and develop reasonable classification to amplify respective advantages of CDs with different structures, such as quantum, molecular, and polymeric characteristic. Thirdly, sufficient characterization should be carried out to clarify the luminescent type of CDs for systematic comparison and propose a feasible regulation method.60 In summary, there are still many fields to be explored in the research of CDs. The perspective has analyzed relationship of various CDs and proposed a relatively rational classification based on the current research results. It stresses the importance of the balance of polymerization and carbonization, and shares our perspectives on some notable issues in further study.

Acknowledgments This work was financially supported by the National Science Foundation of China (NSFC) under Grant Nos. 21774041, 51433003, the National key research and development program of China (2016YFB0401701), the Fundamental Research Funds for the Central Universities, JLU and JLUSTIRT (2017TD-06).

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39. Song, Y.; Zhu, S.; Zhang, S.; Fu, Y.; Wang, L.; Zhao, X.; Yang, B. Investigation from chemical structure to photoluminescent mechanism: a type of carbon dots from the pyrolysis of citric acid and an amine. J. Mater. Chem. C 2015, 3, 5976-5984. 40. Zhu, S.; Zhao, X.; Song, Y.; Lu, S.; Yang, B. Beyond bottom-up carbon nanodots: Citric-acid derived organic molecules. Nano Today 2016, 11, 128-132. 41. Schneider, J.; Reckmeier, C. J.; Xiong, Y.; von Seckendorff, M.; Susha, A. S.; Kasák, P.; Rogach, A. L. Molecular Fluorescence in Citric Acid-Based Carbon Dots. J. Phys. Chem. C 2017, 121, 2014-2022. 42. Sun, Y.-P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K. A. S.; Pathak, P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H.; et al. Quantum-Sized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756-7757. 43. Sharma, A.; Gadly, T.; Neogy, S.; Ghosh, S. K.; Kumbhakar, M. Molecular Origin and Self-Assembly of Fluorescent Carbon Nanodots in Polar Solvents. J. Phys. Chem. Lett. 2017, 8, 1044-1052. 44. Swift, T. A.; Duchi, M.; Hill, S. A.; Benito-Alifonso, D.; Harniman, R. L.; Sheikh, S.; Davis, S. A.; Seddon, A. M.; Whitney, H. M.; Galan, M. C.; et al. Surface functionalisation significantly changes the physical and electronic properties of carbon nano-dots. Nanoscale 2018, 10, 13908-13912. 45. Margraf, J. T.; Strauss, V.; Guldi, D. M.; Clark, T. The Electronic Structure of Amorphous Carbon Nanodots. J. Phys. Chem. B 2015, 119, 7258-7265. 46. Wang, W.; Wang, B.; Embrechts, H.; Damm, C.; Cadranel, A.; Strauss, V.; Distaso, M.; Hinterberger, V.; Guldi, D. M.; Peukert, W. Shedding light on the effective fluorophore structure of high fluorescence quantum yield carbon nanodots. RSC Adv. 2017, 7, 24771-24780. 47. Zhu, S.; Song, Y.; Shao, J.; Zhao, X.; Yang, B. Non-Conjugated Polymer Dots with Crosslink-Enhanced Emission in the Absence of Fluorophore Units. Angew. Chem., Int. Ed. 2015, 54, 14626-14637. 48. Krysmann, M. J.; Kelarakis, A.; Dallas, P.; Giannelis, E. P. Formation Mechanism of Carbogenic Nanoparticles with Dual Photoluminescence Emission. J. Am. Chem. Soc. 2012, 134, 747-750. 49. Qu, S. N.; Zhou, D.; Li, D.; Ji, W. Y.; Jing, P. T.; Han, D.; Liu, L.; Zeng, H. B.; Shen, D. Z. Toward Efficient Orange Emissive Carbon Nanodots through Conjugated sp(2)-Domain Controlling and Surface Charges Engineering. Adv. Mater. 2016, 28, 3516-3521. 50. Miao, X.; Qu, D.; Yang, D.; Nie, B.; Zhao, Y.; Fan, H.; Sun, Z. Synthesis of Carbon Dots with Multiple Color Emission by Controlled Graphitization and Surface Functionalization. Adv. Mater. 2018, 30, 1704740. 51. Ritter, K. A.; Lyding, J. W. The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons. Nat. Mater. 2009, 8, 235-242. 52. Bao, L.; Liu, C.; Zhang, Z.-L.; Pang, D.-W. Photoluminescence-Tunable Carbon Nanodots: Surface-State Energy-Gap Tuning. Adv. Mater. 2015, 27, 1663-1667. 53. Li, D.; Jing, P.; Sun, L.; An, Y.; Shan, X.; Lu, X.; Zhou, D.; Han, D.; Shen, D.; Zhai, Y.; et al. Near-Infrared Excitation/Emission and Multiphoton-Induced Fluorescence of Carbon Dots. Adv. Mater. 2018, 30, 1705913. 54. Tong, D.; Li, W.; Zhao, Y.; Zhang, L.; Zheng, J.; Cai, T.; Liu, S. Non-conjugated polyurethane polymer dots based on crosslink enhanced emission (CEE) and application in Fe3+ sensing. RSC Adv. 2016, 6, 97137-97141.



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Comparison on luminescent performance of SQDs (eg. IV, II-VI, IV-VI, III-V SQDs and perovskite QDs) and carbon-based nanodots (eg. GQDs, CQDs and CPDs). 258x231mm (150 x 150 DPI)



Synthesis and structure of CPDs. 279x100mm (150 x 150 DPI)


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A diagram to describe the reaction process of hydro-thermal crosslinking polymerization (HTCP) to prepare CPDs by “bottom-up” route. 290x87mm (96 x 96 DPI)



Representation of the covalent-bond, supramolecular-interaction, and rigidity-aggregated CEE effect innonconjugated polymers or CPDs. 237x157mm (96 x 96 DPI)


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Main research aspects on PL mechanism of CDs. 354x159mm (150 x 150 DPI)



Balance of Polymerization and Carbonization 120x120mm (150 x 150 DPI)


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Carbonized Polymer Dots: A Brand New Perspective to Recognize

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