Facile, gram-scale and eco-friendly synthesis of multi-color graphene quantum dots by thermal-driven advanced oxidation process
Graphical abstract
Introduction
Graphene Quantum Dots (GQDs) are excellent zero-dimensional carbon nanomaterials with unique optical properties and good biocompatibility [1], [2], [3]. As a member of the graphene family, GQDs have superior electrical conductivity, and their bandgap can be adjusted by modulating the size due to quantum confinement effects. Their extremely stable fluorescence, tunable photoluminescence emissions and high color purity has offered many advantages for replacing toxic light-conversion inorganic quantum dots [4], [5], [6], [7], or unstable fluorescent dyes in optical devices. For the present, various researches have been focused on preparing multi-color GQDs, which have been studied emphasizedly for biological fluorescent probes and white light-emitting devices (WLEDs) [8], [9], [10]. Xiong et al. prepared PL-tunable carbon dots (CDs) of bright full-color emissions from blue to near-infrared region, which are successfully applied in the fields of cell and even in vivo imaging [8]. Fan et al. synthesized multi-color luminescent CDs, and the Lmax of the blue-light dot-based devices could reach ~136 cdm−2 which is the best reported performance of carbon quantum dots-based monochromatic electroluminescent LEDs [9]. These studies fully demonstrate the potential and benefits of GQDs as efficient fluorescent materials in high-performance optical devices and applications.
So far, versatile synthetic methods and diverse precursors have been attempted to prepare GQDs. Among these traditional preparation methods cutting sp2-carbon materials, strong oxidizing agents and acids (e.g., NaClO3, KMnO4, HNO3, and H2SO4) are frequently used for stripping precursors. In spite of its high efficiency, these cutting reagents are extremely dangerous and the accompanying environmental pollution caused by the reaction by-products including inorganic salts and acids has undoubtfully increased the difficulty of operation and also preparation cost. Besides, the cutting methods can hardly guarantee achieving GQDs with uniform size, due to the restriction of reaction conditions. To solve the hurdles, new environmental-friendly shearing agents have been exploited successively to improve the cutting efficiency and simplify the operation process [11], [12], [13], [14], [15], [16]. Zhou et al. prepared GQDs of large size with graphene oxide as the precursor and as the cutting agent under UV irradiation [11]. Fan et al. reported an electrochemical exfoliation method by immersing graphite rods in K2S2O8 solution by using as the cutting agent [12]. Furtherly, Yang et al. used W18O49 nanowire as a catalyst to shear graphene oxides into GQDs [14]. Zhou et al. have attempted UV-Fenton reaction to prepare GQDs of large-size (~40 nm) for DNA detection [15], [16]. However, these strategies generally prepared GQDs from graphene oxides which are expensive raw materials, and it still remains difficult to achieve gram-scale synthesis of GQDs with high crystalline quality, ultra-small uniform size and super fluorescent property at one time. Therefore, how to enhance the yield of GQDs with superior optical performance utilizing low-cost sustainable and industrializable methods becomes highly demanding and particularly important.
Besides, precise regulation of the luminescent properties of GQDs is greatly desired for practical importance. As common in the investigation of a fluorescent material, systematic tuning of the spectral property and energy structure of the GQDs based on chemical unit engineering has attracted a great interest. To finely control the optical properties of GQDs, synergistic strategies to systematically tailor the structure of GQDs have been developed, including tuning the bandgap (separation between the highest occupied molecular orbital and lowest unoccupied molecular orbital), and surface states [8], [9], [17], [18], [19]. Specifically, bandgap narrowing could be realized by enlarging π conjugated sp2-carbon systems or introducing an intermediate n-orbital via surface modification. In 2012, Tetsuka et al. used graphene oxide and ammonia to prepare amino-functionalized GQDs, and firstly discovered the importance of size regulation and nitrogen doping in bandgap tuning [18]. In 2018, Yan et al. prepared GQDs modified with various polyaromatic rings to expand the π-conjugate structure, successfully achieving efficient band gap modulation and application for CO2 reduction [17]. Despite the tremendous strategies, there still is a great challenge to systematically study the PL mechanism for precisely controlling the optical properties of GQDs. Actually, the aim of expanding the applying scope of GQDs determines the necessity of controllable bandgap tuning for multi-color fluorescence, which requires deeper mechanism research.
Herein, an improved thermal-drvien advanced oxidation process (AOP) has been utilized for the first time to achieve high-yield synthesis of well-crystalline GQDs. The AOP reagent could provide abundant strong oxidizer which could shear cheap graphene oxides into GQDs with uniform and small size. The AOP-driven cutting strategy is environmental-friendly by avoiding the use of strong acids, and the reaction by-products could be easily removed via simple filtrate. The formation mechanism of the GQDs via stripping the graphite oxides has been researched. Furtherly, to develop more possibilities in the application field, nitrogen-doping of the GQDs via modification with different nitrogen resources was accomplished, which successfully and precisely manipulate the band structures and fluorescent properties of GQDs. To study the PL mechanism, the band structures of different GQDs were then calculated via utilizing linear sweep voltammetry to determine the conduction band minimum (CBM) and valence band maximum (VBM). Based on the novel and excellent fluorescent property, the PL-tunable GQDs have been demonstrated as a fluorescent probe in in vitro/in vivo bioimage and used for preparing a white-light-emitting composite film.
Section snippets
Materials
All the chemicals are commercially available and used without further purification. Graphite oxide powders were purchased from The Sixth Element (Changzhou) Materials Technology Co., Ltd. M-Phenylenediamine, o-Phenylenediamine, p-Phenylenediamine were purchased from A J&K Scientific Technology Co., Ltd. Ethylenediamine (EDA) were purchased from Sigma Co., Ltd. Quinine sulfate, rhodamine 6G and rhodamine B (RhB) were purchased from Shanghai Lanji Technology Development Co., Ltd. Polyvinyl
Formation mechanism of F-GQDs
Oxidized GQDs were synthesized via the improved AOP reaction using graphite oxides as the precursors, during which the multi-layer graphite oxides were cut into 0D graphene quantum dots, shown in Scheme 1. In details, the GO flakes are rich in oxygen groups and could be dispersed in water. When the Fe3+ and H2O2 were added in the GO solution, the dark GO solution turned light brown (Fig. S1), accompanied by a large amount of bubbles which can be assigned to oxygen. After the hydrothermal
Conclusions
In our study, a thermal-driven AOP strategy was exploited to efficiently stripping large cheap graphite oxides into GQDs of uniform and ultra-small size with an average yield of 60%. The obtained GQDs were furtherly modified with different nitrogen-containing resources to tune the band structure, successfully achieving PL-tunable GQDs with blue, green, yellow, orange and red emissions. It is found that the obtained GQDs have an average size distribution of 2–5 nm and the corresponding AFM
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We greatly appreciate the financial supports from the National Natural Science Foundation of China (51702212, 51802195), Science and Technology Commission of Shanghai Municipality (18511110600, 19ZR1435200), Innovation Program of Shanghai Municipal Education Commission (2019-01-07-00-07-E00015), Program of Shanghai Academic Research Leader (19XD1422900).
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Bowen Lyu and Hui-Jun Li contributed equally to this work.