SCIENCE CHINA Materials ARTICLES mater.scichina.com link.springer.com Published online 16 January 2017 | doi: 10.1007/s40843-016-5160-9 Sci China Mater 2017, 60(2): 141–150 Production of yellow-emitting carbon quantum dots from fullerene carbon soot Qinghong Zhang1,2, Xiaofeng Sun1,3, Hong Ruan1, Keyang Yin1 and Hongguang Li1* ABSTRACT Carbon quantum dots (CQDs) have emerged as because of their potential toxicity [4–6]. In 2004, carbon a new generation of photoluminescent nanomaterials with quantum dots (CQDs) were accidentally discovered by wide applications. Among the various synthetic routes for Xu et al. [7] during the purification of single-walled car- CQDs, the acid-refluxing method, which belongs to the group bon nanotubes (SWNTs), which quickly gained immense of “top-down” methods, offers the advantage of large-scale attention during the following years owing to their good production of CQDs and uses cheap and abundantly available starting materials. In this study, we evaluated the potential biocompatibility, high photostability, and tunable photolu- of fullerene carbon soot (FCS), a by-product obtained during minescent properties [8–11]. the synthesis of fullerene, as the starting material for CQD To fully realize the promising applications of CQDs, production. It was found that FCS can be successfully con- their large-scale and low-cost production is necessary. verted to CQDs in high production yield in mixed acids, i.e., Thus far, various methodologies have been reported for the concentrated HNO3 and H2SO4, under mild conditions. The synthesis of CQDs. These methodologies can be mainly fluorescence quantum yield (Φ) of the as-produced CQDs is classified into two categories, namely, the “top-down” and in the range of 3%–5%, which is the highest value for CQDs “bottom-up” methods. The former includes laser ablation obtained from “top-down” methods. Importantly, the CQDs [12–14], plasma treatment [15], electrochemical oxidation prepared by this method show emission in the yellow range of the visible light, which is advantageous for their various [16–19], and acid-refluxing [20–31], while the latter is potential applications. Further investigations reveal that the mainly focused on pyrolysis using simple heating and hy- CQDs are highly photostable over a wide pH range and show drothermal or microwave treatment [32–35]. Among these good resistance against ionic strength and long-term UV irra- methods, the acid-refluxing of raw carbon materials offers diation. This further expands their potential use under harsh a large-scale production of CQDs as it only needs simple conditions. instruments and can be easily scaled-up using standard Keywords: carbon quantum dots, fullerene, carbon soot, photo- industrial equipment. Theoretically, nearly all carbon-rich luminescent, top-down resources such as carbon nanotubes (CNTs) [20], carbon fibers [21], activated carbon [22], coals [23–25], petroleum INTRODUCTION coke [26–28], and carbon soot [29–31] can be used as the Fluorescence imaging is indispensable in optical and bi- starting materials. For the large-scale production of CQDs, ological studies. Quantum dots (QDs) are considered CNTs and carbon fibers can be ideal candidates if their as great substitutes of conventional organic dyes and prices can be further lowered in the future. Coal is abun- genetically engineered fluorescent proteins used in fluo- dant and cheap, and is thus a promising raw material for rescence imaging of biological samples because of their the synthesis of CQDs by acid reflux. However, the CQDs excellent photostability, sharpe emission bands, and tun- obtained by using coal as the raw material suffer from able emission wavelengths [1–3]. However, conventional both low production yield (Y) and fluorescence quantum semiconducting QDs contain heavy metal elements, which yield (Φ) because of the complicated composition of coal. are harmful to humans when used in biological imaging Carbon soot is regarded as a promising starting material 1 Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China 2 China Research Institute of Daily Chemical Industry, Taiyuan 030001, China 3 University of Chinese Academy of Sciences, Beijing 100049, China * Corresponding author (email: [email protected]) February 2017 | Vol.60 No.2 141 © Science China Press and Springer-Verlag Berlin Heidelberg 2017 ARTICLES SCIENCE CHINA Materials to obtain high-quality CQDs as it is normally obtained added to this mixture. The resulting mixture was then cup by burning and the composition as well as the size dis- sonicated for 1 h, heated to 80–120°C under stirring and tribution of the carbon granules present in it is relatively allowed to react for 12–36 h. After the reaction mixture uniform and the carbon content is usually very high. A was cooled down to room temperature, it was subjected to limitation is that currently the carbon soot used for CQD an ice-water bath and a large amount of water (> 6 times) synthesis is mainly obtained by burning candles [29], nat- was added slowly followed by the addition of K2CO3 to ural gas [30], or tires [31]. Thus, the soot produced is quite neutralize the excess acids (until pH = 7). The mixture limited in quantity and is difficult to be gathered. Hence, was then centrifuged (Anke TGL-16G) for 30 min at a finding an alternative source of carbon soot is imperative rate of 10,000 r/min to remove large particles followed by for large-scale and low-cost production of CQDs. dialysis for 5–7 days with frequent changing of water using As one of the earliest discovered carbon nanomaterials, a dialysis bag of 500 Da to give the final product. fullerenes have received considerable attention since their In a typical experiment for stepwise dialysis, the mixing discovery owing to their unique structural, optical, and volume ratio of HNO3 and H2SO4 was fixed at 1:1 and the electrical properties. With the smooth progress in the fun- total volume was 4 mL. 50 mg of FCS was added. Refluxing damental research on fullerenes their commercialization was performed at 100°C for 36 h. After centrifugation, the has also increased. Nowadays, tons of fullerenes are pro- sample was first dialyzed with a dialysis bag with a molecu- duced every year, thus producing a large amount of carbon lar weight cut-off of 100 Da to remove the inorganic salts soot (>85 wt.%) as the by-product. It would be of great sig- and tiny fragments present in it. The sample within the nificance if this soot can be converted into more valuable bag was then successively subjected to dialysis using the products. The use of fullerene carbon soot (FCS) in capac- bags with molecular weight cut-offs of 500, 1000, and 2000 itor electrodes or as catalyst supports for the reduction of Da. During each step in the dialysis process, the dialysate NO have been reported [36,37]. In a recent work, Hu et was concentrated to ~5 mL and the final sample within the al. [38] reported that FCS can be used for producing sin- dialysis bag had a volume of ~15 mL. The solutions of the gle-walled CNTs in high Y. Herein, we presented a detailed as-produced CQDs were concentrated using rotary evapo- evaluation of the potential of FCS to be used as the start- ration or lyophilized wherever necessary. ing material for the production of CQDs. It was found that the CQDs obtained from FCS exhibited a high Y and Φ and Characterizations had yellow-emitting characteristics. Moreover, they could Transmission electron microscopy (TEM) images were endure long-term UV irradiation and were stable in solu- taken on a JEM-1011 system. High-resolution TEM tions with a wide pH range and high salinity. All of these (HRTEM) images were recorded on a HRTEM JEOL 2100 merits open up a variety of potential applications for CQDs. system operating at 200 kV. The specimens were prepared by drop-casting the sample solution onto a carbon-coated EXPERIMENTAL SECTION copper grid followed by drying at room temperature. X-ray diffraction (XRD) pattern was obtained using a Materials D8 Advance X-ray diffractometer with a wavelength The FCS used for the preparation of CQDs was purchased (λ) of 0.15418 nm. Raman spectra were recorded on a from Suzhou Dade Carbon Nanotechnology Co., Ltd. and Renishaw Raman microscope using a 514-nm laser exci- Jiangsu XFNANO Materials Tech Co., Ltd. It was obtained tation at room temperature. Fourier transform infrared as a by-product during the manufacture of fullerenes. Con- (FTIR) spectra were obtained using an FTIR spectropho- centrated HNO3 (65%–68%) and H2SO4 (95%–98%) were tometer (VERTEX-70). UV-vis spectra were obtained obtained from Laiyang Kangde Chemicals Co., Ltd. and by a UV-vis spectrophotometer (U-4100). X-ray pho- were used as received. Dialysis bags with varying molecu- toelectron spectroscopy (XPS) was carried out using an lar weight cut-offs were provided by Membrane Filtration X-ray photoelectron spectrometer (ESCALAB 250) with Products, Inc. Beijing Thai technology Co., Ltd. a monochromatized Al Kα X-ray source (1486.71 eV). Thermogravimetric analysis (TGA) was carried out using a Synthesis of CQDs DSC 822e (Piscataway, NJ) under nitrogen with a scanning For optimizing the synthetic conditions, concentrated speed of 5°С min−1. Fluorescence spectra were obtained HNO3 and H2SO4 were pre-mixed by cup sonication using a spectrofluorometer (LS-55). Elemental analysis (model number: 230T) for 40 min. 50–250 mg FCS was was carried out using an organic elemental analyzer (Vario 142 February 2017 | Vol.60 No.2 © Science China Press and Springer-Verlag Berlin Heidelberg 2017 SCIENCE CHINA Materials ARTICLES EI III). The fluorescence lifetimes and time resolved emis- sion fluorescence spectra of the CQDs were measured on an Edinburgh Instruments FLS920 with a time correlated single photon containing (TCSPC) method.