Meng Li Liua , Lin Yangb , Rong Sheng Lib , Bin Bin Chena, Hui Liub* , and Cheng Zhi Huangab*

Photoluminescent (PL) carbon dots (CDs) as a new type of carbon nanomaterial have attracted an increasing attention owing to their fascinating properties. Herein, we develop a facile, energy-efficient, large-scale route to simultaneously  prepare highly PL CDs with a quantum yield of up to 35.3% at room temperature. This PL CDs can further separate out  green-emissive amorphous carbon nanodots (CNDs) and yellow-emissive crystalline graphene quantum dots (GQDs) through silica gel column. The as-prepared both CNDs and GQDs, even if having the same particle-size distribution and  chemical groups, have different surface oxidation degree. As XPS characterized, the yellow-emissive crystalline GQDs have  much higher surface oxidation degree than that of green-emissive amorphous CNDs. Further finding is that the  characteristic emission peaks of CDs show an obvious red shift from 518 to 543 nm with the increase of the surface  oxidation degree, which can be attributed to the decrease of their band gap between the lowest unoccupied molecular  orbital (LUMO) and the highest occupied molecular orbital (HOMO). That is, the difference of band gap is closely related to  the oxidation degree of CDs, rather than the particle size or chemical groups. Moreover, the amorphous CNDs are very  easily photobleached under 140 W xenon lamp irradiation as compared to the crystalline GQDs, indicating that the  photostability is dependent on the crystalline structure of CDs, which is beneficial for the top designment and  development of suitable CDs for different application purposes.

Introduction

Photoluminescent (PL) carbon dots (CDs) have aroused wide  attention in recent years owing to their fascinating properties,  including tunable PL property, good water solubility, excellent  biocompatibility and easy preparation.1-5 Therefore, CDs have  been widely applied in the field of fluorescence imaging, drug  delivery and photodynamic therapy6 . The common CDs encompassing a series of spherical and single-sheet carbonbased nanoparticles are divided into graphene quantum dots  (GQDs), carbon quantum dots (CQDs) and carbon nanodots  (CNDs) according to their nature, quantum confinement and  crystalline structure.7 Owing to their high promise in the field  of applied technology, a wide variety of methods have been  developed following different technological routes to produce CDs, such as laser irradiation8 , hydrothermal synthesis1, 2 ,  electrochemical etching9 , ultra-sound and microwave-assisted  synthesis10, 11 . Although these approaches are facile and rapid,  there are still many limitations from complicated experimental  set-ups to the demand of high energy consumption such as  comparatively high temperature supply. 12 Room temperature  synthesis without extra energy supply is the most ideal  approach for preparing the PL CDs. Among them, chemical  oxidation is the most common method, but it is limited by the low quantum yield (< 5%) and the treatment of strong acids or  base in the most of synthesis process, which increases environmental issues owing to the difficulty to thoroughly  remove the excessive oxidizing agent. 12-14 Therefore,  developing a simple, green, energy-efficient and large-scale way for preparing highly PL CDs at room temperature remains  highly desirable.

The optical properties of CDs, which seem to greatly depend  on their preparation methods, routes and raw materials, still remain unclear. Surface-state15, 16 , free zigzag sites17 ,  conjugated π-domains18 and quantum size effect (sizedependent shape and corresponding edge variations) 19 have  been proposed to explain their PL mechanism, which relates PL origin of the as-prepared CDs with different precursors.  However, it is greatly improper because the CDs have different components and structures. 16 Therefore, it is still desirable to  study the PL mechanism through controllable synthesis of CDs with different emissions.

In this work, we report a one-pot synthesis of highly PL CDs  on a large scale by simply keeping the mixture of  triethylenetetramine and p-benzoquinone at room temperature (Scheme 1). CNDs with amorphous structure and  GQDs with crystalline structure can be obtained from the asprepared CDs solution by column separation, and their quantum yield are as high as 35.3% and 17.5% respectively, which are the highest value yet recorded for CDs prepared at  room temperature. Both of the green-emissive CNDs and the  yellow-emissive GQDs have the same size distribution and  chemical groups such as amine groups and Schiff base  structures, but their PL and photostability are significantly different. Owing to the reduced band gaps, the PL red shift  shows a direct relationship with the degree of surface oxidation of the CDs (Scheme 1), rather than quantum size  effect. Moreover, we find that the crystalline GQDs own a  good photostability, while the amorphous CNDs are easily  photobleached, indicating that the photostability is dependent  on the crystalline structure of CDs.

Scheme 1 The room temperature synthesis of the CNDs and GQDs by  simply keeping the mixture of TETA and p-benzoquinone and their PL  origin.

Experimental

Materials

Triethylenetetramine (TETA) is from Aladdin Reagent Co., Ltd.  (Shanghai, China). p-Benzoquinone is from Sigma-Aldrich  (Shanghai, China). Mili-Q purified water (18.2 MΩ cm) is used  to prepare solutions throughout the experiment.

Instrumentations

The UV-vis absorption spectra of the CNDs and GQDs are from  a Hitachi U-3010 spectrophotometer (Tokyo, Japan). The  elemental compositions of CNDs and GQDs are obtained from an ESCALAB 250 X-ray photoelectron spectroscopy (XPS). The  Fourier transform infrared (FT-IR) spectra of CNDs and GQDs are collected on a Hitachi FTIR-8400S Fourier Transform  Infrared spectrometer (Tokyo, Japan). The transmission  electron microscope (TEM) and high-resolution TEM (HRTEM) data of CNDs and GQDs are performed on a Tecnai G2 F20 field  emission transmission electron microscope (FEI, USA). The PL spectra of CNDs and GQDs are recorded with a Hitachi F-2500  fluorescence spectrophotometer (Tokyo, Japan). The Raman  spectra of CNDs and GQDs on the silver nanoparticles (AgNPs) solution are scanned through a LabRAM HR800 Laser confocal  Raman spectrometer (Horiba Jobin Yvon, France). The  fluorescence lifetimes of CNDs and GQDs are measured with a  FL-TCSPC fluorescence spectrophotometer (Horiba Jobin Yvon,  France).

Preparation and separation process of the CNDs and GQDs

p-Benzoquinone (30 mg) is firstly dissolved in 4.8 ml water. Dark brown solution is immediately observed with the addition  of 0.2 ml TETA at room temperature. With the standing of the  mixture at room temperature for 20 h, PL CDs can be available.  After being lyophilized, the precipitate is dissolved with  methanol, and the filtrate is further purified and separated by  silica gel column (Mobile-phase, CH3OH:CH2Cl2= 10:1 and solidphase, 300-400 mesh silica gel) to obtain clear CNDs and GQDs.  Finally, the CNDs and GQDs solution are dried by lyophilization, respectively, and dispersed in water for further use

Photostability investigation of the CNDs and GQDs

For the investigation of the photostability, 5 ml CNDs (10  µg/ml) solution and GQDs (30 µg/ml) solution are respectively  placed in the dark for illumination for times between 0 and up  to 30 min with a CEL-HXF300/ CEL-HXUV300 xenon (Xe) lamp  (Beijing, China). The power output of the lamp is adjusted from  112 W to 196 W through keeping voltage at 14 V. The CNDs exposed for 0, 5 10, 30 min of illumination under the power of  140 W are obtained respectively for further characterization.

Cellular imaging of the CNDs and GQDs

HEp-2 cells (1 105 cells per ml) in RPMI 1640 supplemented  with 2% fetal bovine serum are added to culture dish (1 ml).  The cells are first cultured for 24 h in an incubator (37 o C, 5%  CO2). After treatment by 1 % TritonX-100, the culture medium  being replaced with 900 μl RPMI 1640 which contains 100 μl CNDs (5 μg/ml) or GQDs (10 μg/ml) solution for another 8 h.  Finally, following by removing the culture medium, the culture  dish is washed with PBS buffer for three times. Then the cells are mounted on microscope slides for imaging.

Growth of mung bean in the presence of CNDs and GQDs

Clean mung bean seeds are selected with nearly equal sizes  and plumpness and then are arranged in 6-well plates (12  seeds for each well and 3 wells for each group). Running water  and CNDs/GQDs solution (prepared with running water) with  different concentration (20, 50, 100 g/ml) are added into the  above groups, respectively, and then cultured at 37 °C without  the light. After 3 d, the bean sprouts are harvested.

Results and discussion

Synthesis and structure of the CNDs and GQDs

The highly PL CDs can be easily prepared on a large scale  through simply keeping the mixture of TETA and pbenzoquinone for 20 h at room temperature (Fig. S1, ESI). The  p-benzoquinone and TETA can cause Schiff base condensation between the amine group (-NH2) of TETA and carbonyl group  (C=O) of p-benzoquinone20, and then forming CDs under the  action of strong alkali (TETA)12, 21 . The FT-IR spectra (Fig. S2, ESI)  confirm that Schiff base structure (C=N band) exist in CNDs and  GQDs, further revealing that the formation of CDs is due to  Schiff base condensation between p-benzoquinone and TETA.  Interestingly, after column separation, the as-prepared CDs  mixture is found to be divided into two kinds of pure CDs with  different PL (green emissive CNDs and yellow emissive GQDs),  which can be attributed to the difference of surface oxidation  and structure. The mass ratio of CNDs and GQDs is about  1:14.2, with the quantum yield of up to 35.3% and 17.5%,  respectively, which are the highest value yet recorded for CDs prepared at room temperature (Table 1). Therefore, the asprepared CDs are potent to be applied in biosensing,

Fig. 1 The TEM and HRTEM images of the CNDs and GQDs. (a) TEM and  (b) HRTEM image of CNDs; (c) TEM and (d) HRTEM image of GQDs.

Table 1 Comparison of the quantum yield (QY) of the CNDs and  GQDs prepared in our work with CDs obtained through other room  temperature synthesis approaches. (CNPs represent carbon  nanoparticles) CDs Oxidizing  agent Absolute QY  (%) Relative QY  (quinine sulfate as  reference, %) Reference CNPs HNO3 2 / 14 CDs HNO3 12.6 / 22 CNPs HNO3 0.43 / 23 CQDs NaOH / 2.2 12 CDs H2SO4 13 / 24 CDs HNO3 1.6 / 25 CQDs H2SO4 / 1.95 13 CNDs / 35.3 / This work GQDs / 17.3 / This work cellular imaging and other analytical field. Moreover, the  synthesis process does not need the treatment of strong acid,  external heating or additional energy input. Thus the proposed  room temperature synthesis can be used for large-scale  industrial production of CDs owing to its simplicity, low-cost  and environment-friendly potential.

The TEM and HRTEM images indicate that the CNDs and  GQDs have completely different structural features. The CNDs  with a narrow diameter distribution range (20  8 nm) are  quasi-spherical without a crystal lattice (Fig. 1a and b), which  are consisted of aggregated or cross-linked structures from monomers or linear polymers. 26, 27 The GQDs also have a  similar size distribution to the CNDs (Fig. 1c), but they have  obvious lattice spaces at 0.25 nm and 0.35 nm (Fig. 1d), which  correspond to the (020) and (002) planes of graphitic carbon,  respectively. 28, 29 The atomic force microscope (AFM) image of  the CNDs (Fig. S3a-b, ESI) further reveals that the CNDs are  quasi-spherical carbon nanoparticles. However, a typical topographic height of the GQDs is about 1-2 nm (Fig. S3c-d,  ESI), indicating that most of the GQDs consist of 3-6 graphene  layers. 30

The D band and G band in the Raman spectrum correspond  to disordered structure and graphitic structure of carbon  materials, respectively.31 The weak G band in the CNDs and strong G band in the GQDs (Fig. S4, ESI) further confirm that  the CNDs are noncrystalline structures, while the GQDs are  mainly composed of nano-crystalline graphite. 16, 32 Moreover, the average fluorescence lifetimes of the CNDs and GQDs are  4.49 ns and 3.84 ns, respectively (Fig. S5, ESI). All above results  indicate that the highly PL CNDs and GQDs have been  prepared by a facile, energy-efficient and large-scale synthesis  approach at room temperature