Elsevier

Carbon

Volume 142, February 2019, Pages 673-684
Carbon

Bottom-up synthesis and structural design strategy for graphene quantum dots with tunable emission to the near infrared region

https://doi.org/10.1016/j.carbon.2018.10.047Get rights and content

Abstract

Despite recent advances in the fabrication of graphene quantum dots (GQDs) with excellent fluorescence performance, it has been challenging to extend the fluorescence emission to deep red and short wave near-infrared. Herein, we present a strategy to reach the goal via hydrothermal treatment of polythiophene derivatives which mainly comprises a polythiophene conjugate skeleton, lots of benzene ring structure and alkyl chain. This structure is thermally converted into a doped crystalline GQDs at 170 °C for 20 h with the maximum fluorescence emission at 700 nm. In addition, the length of alkyl chain also has a regulatory effect on emission wavelength of final products, which enables the chemical molecular-level structural design of GQDs with specific light emission waveband.

Introduction

Many carbonaceous materials with various morphologies have been investigated, such as fullerene, carbon nanotubes, nanofibers, porous carbon, graphene and their hybrids. Among these materials, graphene sheets with several nanometer, were generally called as graphene quantum dots (GQDs). Because of the unique physical and chemical properties such as excellent fluorescence, easy surface functionalization, high resistance to photobleaching, low toxicity and good biocompatibility, the GQDs have drawn considerable attention in fields of biotechnology and medicine recently [[1], [2], [3]]. Generally, the preparation strategies of GQDs were classified as “top-down” and “bottom-up” ways. The top-down strategy is based on cleaving or breaking down of carbonaceous materials including graphite, graphene, carbon fibers, carbon nanotubes, graphene oxide, and carbon black via hydrothermal, strong acid oxidation and electrochemical exfoliation. The GQDs derived from top-down way almost all had fluorescence-emission in relatively short wavelength and lacked effective control on morphology and size distribution. As for the bottom-up way, it gives us more space to design products with desired performance. Bottom-up methods, relying on the chemical reaction from precursors or assembly of small building blocks, are convenient in controlling the morphologies and structures of nanomaterials. Citric acid [4], pyrene [5], glucose [6] and the mixture or derivatives are usually used to prepare GQDs by approaches, such as pyrolysis or hydrothermal treatment and microwave-assisted hydrothermal treatment. Xin Yan et al. used developed oxidative condensation reactions to synthesize stable colloidal graphene quantum dots with tunable size successfully [7]. Lately, carbon nanobelts with cylindrical shape have been synthesized through iterative Wittig reactions followed by a nickel-mediated aryl-aryl coupling reaction [8]. Moreno et al. have succeeded in preparing nanoporous graphene with atomic precision by bottom-up design of the molecular precursors [9]. These organic synthesis methods supported us for more controllability of the structure and morphology. And currently, many efforts on element doping have been carried out to change the electronic structure of GQDs and tune their optical and electrical properties [[10], [11], [12], [13], [14]]. But the element doping through bottom-up routes mainly focus on the selection of diversified organic precursors that contain desired heteroatoms and carbonization conditions. The thermal conversion process could bring undesired by-products and poorly controlled radical reactions. Hence, if doped atoms couldn't permeate to the carbon matrix effectively, the modulation for electronic structure and optical properties of GQDs would be limited. So turning the emission wavelength of GQDs to a longer wavelength remains a challenge. Up to now, there are few reports of carbon dots with long-wavelength emissions. For example, Mandal et al. reported efficient orange-emissive carbon dots with optimal emission at 590 nm [15]. Zhang and co-workers reported efficient red-emissive CNDs with optimal emission at 615 nm and an on–off fluorescent probe was prepared for detecting glutathione (GSH) based on aggregation-induced fluorescence quenching [16]. Yang and co-workers reported highly efficient red emissive nitrogen-doped carbonized polymer dots with optimal emission at around 630 nm [17]. Although carbon dots with long wavelength emissions have been obtained, their emission wavelengths in the deep red or NIR ranges still have space to explore. Recently, Wang et al. made pioneering use of polythiophene derivatives as carbon sources to synthesize GQDs with deep red emission and succeeded in applying GQDs to the fields of photodynamic therapy and photothermal therapy [18,19]. Tang et al. reported an approach to generate or to enhance the near-infrared luminescence from the green luminescent carbon nanodots. Experiments reveal that the near-infrared emission is significantly enhanced by thermally assisted growth in vacuum, which was attributed to the formation of the larger nano-domains from the small carbon clusters at elevated temperatures [20]. These works have made pioneering explorations in deep red and near-infrared light, but still need to explore in further realizing the tunability of long wave emission and the control of structure. The excellent performances of deep-red and near-infrared light prompt us to further control the evolution process from molecular-level and get the GQDs with controllable morphology and long wavelength emission.

Poly(3-alkylthiophenes) (P3ATs), an unusual class of polymers, have been widely studied for their good solubility, processibility, environmental stability, electroactivity, and other interesting properties [[21], [22], [23], [24]]. P3ATs were achieved by introducing alkyl group into the β-position of the thiophene ring, whose bandgap Eg could be tuned by controlling the extent of the effective conjugation in the poly(thiophene) backbone [25]. So, the red shift of the absorption and emission spectra resulting from a decrease of Eg will reflect the enhancement of conjugate level.

Herein, we selected the P3ATs as precursor, aiming to base the conjugate thiophene skeleton while converting R moiety into an extended conjugate structure, to synthesize doped GQDs. Structure-preserving carbonization has been proved to be feasible by self-assembly of block-copolymers or surfactant templates [[26], [27], [28]]. Moreover, the conversion of organic precursor into ordered carbonaceous frameworks has also been realized [29]. So combining the two possible features of P3ATs that the structure-preserving for conjugate thiophene skeleton and the conversion into ordered carbonaceous frameworks for R moiety, it was worthy of our research on the structure evolution of P3ATs. Because its alkyl chains have very rich carbon yields in carbonization reactions, which contributes to extending the conjugated structure based on conjugate thiophene skeleton. In order to introduce nitrogen and simplify the assembly process, with the help of Suzuki reaction, the 3-substitution group of P3ATs was designed as the structure shown in the Scheme 1. Wang et al. have successfully prepared GQDs with such precursors, and have studied its potential application in the biomedical field in detail [18,19]. But the evolution mechanism and the relationship between structure and performance of such precursor are still unclear. So In this work, we provide detailed examination on the relationship between the synthesis conditions (temperature, duration) and product properties, which is highly important for obtaining a good understanding of the reaction mechanism and optimal strategies. Besides, efforts have been made to provide a detailed account on the structural evolution of P3ATs during hydrothermal process and an in-depth insight on the structure–property relationship of GQDs. Rely on this structure, high water-dispersible GQDs were prepared through hydrothermal treatment and the fluorescence emission was extended to deep red and short wave near-infrared by adjusting length of alkyl side chains. The results suggest that the present method has certain feasibility in aspect of synthesizing desired carbonaceous materials by well-designed chemical structures of precursor.

Section snippets

Sources of materials

4-Bromobenzyl bromide, N,N-dimethyldodecylamine, N,N-dimethyltetradecylamine, N,N-dimethylhexadecylamine, thiophene-3-boronic acid, tetrakis (triphenylphosphine) palladium(0) were purchased form Alfa Aesar. Other reagents were purchased from Beijing Chemical Regent Co. All reagents were of analytical grade and used as received without further purification. Noted, CHCl3 were distilled from MgSO4 to remove residual water. Milli-Qultrapurewater (18.2 MΩ) was used in experiment.

Characterization

1H and 13C spectra

Results and discussion

For the sake of convenient analyzation, the detailed study was mainly focused on the P3ATs whose R moiety consist of a phenyl ring and alkaneC16 (abbreviated as PAT16). To investigate the evolution process of PAT16, UV–vis absorption and PL spectra were firstly performed. Fig. 1a describes the UV–vis absorption spectra of PAT16 at different hydrothermal temperatures for 12 h (untreated, 120 °C, 150 °C, 170 °C, respectively). As shown in Fig. S1, the absorption of compound 1 centres at 260 nm

Conclusions

In summary, the conversion of P3ATs into GQDs was presented. The successive thermal conversion process actually comprises three steps. Firstly, the agminated P3ATs relax to loose structure to increase the contact area with hydrothermal environment. Secondly, volatile moieties like alkyl chains turn into fusion state with cyclodehydrogenation and nano-conjugate phase separates from matrixs, which caused by the structural characteristics of P3ATs mentioned above. And finally, the separated parts

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by National Science Foundation of Shandong Province (ZR2016EMM20, ZR2016EMQ09), Shenzhen Science and Technology Research and Development Funds (JCYJ20160331173823401) and the National Natural Science Foundations of China (51472150). The authors deeply acknowledge Prof. Huaijin Zhang, Haohai Yu, Xiaoqiang Yu and Qi Fang for their support in this project. The authors thank Dr. Shouzhi Wang and Yingying Jin for assistance with cyclic voltammetry characterization.

References (79)

  • J. Bukowska et al.

    In situ Raman studies of polypyrrole and polythiophene films on Pt electrodes

    Synthetic Met

    (1990)
  • E. Agosti et al.

    Electronic and dynamical effects from the unusual features of the Raman spectra of oligo and polythiophenes

    Synthetic Met

    (1999)
  • J. Zhan et al.

    A solvent-engineered molecule fusion strategy for rational synthesis of carbon quantum dots with multicolor bandgap fluorescence

    Carbon

    (2018)
  • Z.Q. Song et al.

    Multifunctional N,S co-doped carbon quantum dots with pH- and thermo-dependent switchable fluorescent properties and highly selective detection of glutathione

    Carbon

    (2016)
  • A.B. Bourlinos et al.

    Pyrolytic formation and photoluminescence properties of a new layered carbonaceous material with graphite oxide-mimicking characteristics

    Carbon

    (2009)
  • J.G. Zhou et al.

    Electronic structure and luminescence center of blue luminescent carbon nanocrystals

    Chem. Phys. Lett.

    (2009)
  • K. Hola et al.

    Photoluminescence effects of graphitic core size and surface functional groups in carbon dots: COO- induced red-shift emission

    Carbon

    (2014)
  • S. Zhu et al.

    Investigation of photoluminescence mechanism of graphene quantum dots and evaluation of their assembly into polymer dots

    Carbon

    (2014)
  • P.W. Gong et al.

    Small but strong: the influence of fluorine atoms on formation and performance of graphene quantum dots using a gradient F-sacrifice strategy

    Carbon

    (2017)
  • M.Y. Li et al.

    Organic amine-grafted carbon quantum dots with tailored surface and enhanced photoluminescence properties

    Carbon

    (2015)
  • S.Y. Lim et al.

    Carbon quantum dots and their applications

    Chem. Soc. Rev.

    (2015)
  • H. Feng et al.

    Functional carbon quantum dots: a versatile platform for chemosensing and biosensing

    Chem. Rec.

    (2018)
  • D. Qu et al.

    Three colors emission from S,N co-doped graphene quantum dots for visible light H2 production and bioimaging

    Adv. Opt. Mater.

    (2015)
  • X. Yan et al.

    Synthesis of large, stable colloidal graphene quantum dots with tunable size

    J. Am. Chem. Soc.

    (2010)
  • G. Povie et al.

    Synthesis of a carbon nanobelt

    Science

    (2017)
  • C. Moreno et al.

    Bottom-up synthesis of multifunctional nanoporous graphene

    Science

    (2018)
  • K.P. Gong et al.

    Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction

    Science

    (2009)
  • Q.Q. Li et al.

    Nitrogen-doped colloidal graphene quantum dots and their size-dependent electrocatalytic activity for the oxygen reduction reaction

    J. Am. Chem. Soc.

    (2012)
  • S.B. Yang et al.

    Efficient synthesis of heteroatom (N or S)-doped graphene based on ultrathin graphene oxide-porous silica sheets for oxygen reduction reactions

    Adv. Funct. Mater.

    (2012)
  • V. Gude et al.

    Molecular origin of photoluminescence of carbon dots: aggregation-induced orange-red emission

    Phys. Chem. Chem. Phys.

    (2016)
  • C. Wang et al.

    Green synthesis of red-emitting carbon nanodots as a novel "turn-on" nanothermometer in living cells

    Chemistry

    (2016)
  • J. Liu et al.

    One-step hydrothermal synthesis of nitrogen-doped conjugated carbonized polymer dots with 31% efficient red emission for in vivo imaging

    Small

    (2018)
  • J.C. Ge et al.

    A graphene quantum dot photodynamic therapy agent with high singlet oxygen generation

    Nat. Commun.

    (2014)
  • J. Ge et al.

    Red-emissive carbon dots for fluorescent, photoacoustic, and thermal theranostics in living mice

    Adv. Mater.

    (2015)
  • X. Wen et al.

    Near-infrared enhanced carbon nanodots by thermally assisted growth

    Appl. Phys. Lett.

    (2012)
  • T.A. Chen et al.

    Regiocontrolled synthesis of poly(3-alkylthiophenes) mediated by Rieke Zinc - their characterization and solid-state properties

    J. Am. Chem. Soc.

    (1995)
  • H.A. Ho et al.

    Optical sensors based on hybrid aptamer/conjugated polymer complexes

    J. Am. Chem. Soc.

    (2004)
  • M. Leclerc

    Optical and electrochemical transducers based on functionalized conjugated polymers

    Adv. Mater.

    (1999)
  • Y.L. Tang et al.

    Direct visualization of enzymatic cleavage and oxidative damage by hydroxyl radicals of single-stranded DNA with a cationic polythiophene derivative

    J. Am. Chem. Soc.

    (2006)
  • Cited by (63)

    View all citing articles on Scopus
    View full text