Surface structure and fluorescence characteristics of concentrated carbon point

Journal Pre-proof Surface structure and fluorescence characteristics of concentrated carbon point Jiuli Cao, Xueqin An, Simin Han

PII:

S0927-7757(19)31194-X

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124201

Reference:

COLSUA 124201

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

10 September 2019

Revised Date:

18 October 2019

Accepted Date:

4 November 2019

Please cite this article as: Cao J, An X, Han S, Surface structure and fluorescence characteristics of concentrated carbon point, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124201

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Surface structure and fluorescence characteristics of concentrated carbon point Jiuli Cao, Xueqin An* and Simin Han School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China; * Corresponding author, E-mail: [email protected]

-p

ro of

Graphical abstract

lP

re

The concentration-dependent fluorescence properties of carbon dots was observed and its mechanism was probed.

Abstract

na

The concentration-dependent fluorescence behaviour of carbon dots (CDs) was observed in concentrated CDs solution, and it was found that the

ur

excitation-independent fluorescence behaviour gradually transitioned to excitation-dependent fluorescence behaviour with the increase of CDs

Jo

concentration. The fluorescence characteristics of CDs solution with various concentrations were explained from the perspective of the surface states by using the fluorescence decay spectra of CDs for the first time. A close relationship between the surface microstructures and the optical properties was demonstrated. Mechanism of concentration-dependent generation fluorescence in concentrated CDs solution was presented and a possible aggregation model

of CDs was given. This work offers a novel perspective for tuning the emission wavelength of CDs and endowing them with potential various applications.

Key words: Carbon dots; Surface structure; Fluorescence characteristics; Relationship between the surface microstructures and the Fluorescence

Jo

ur

na

lP

re

-p

ro of

characteristics; Aggregation model of CDs

1 Introduction Fluorescent carbon dots (CDs) as a fascinating class of carbon nanomaterials have attracted increasing attention due to their tunable fluorescence (FL) emission properties, chemical inertness, low toxicity, and good biocompatibility[1, 2]. Therefore, these attractive merits make CDs extensively applied in drug delivery[3], fluorescence ink[4], markers for bioimaging[5, 6], sensing[7] and photocatalysising[8], etc. The most interesting feature of CDs is their tunable optical property. Generally, the tunable optical

ro of

property of CDs can be assigned to quantum size effect[9, 10] or surfacerelated defective sites[11, 12]. However, exact origins of their tunable FL

property remain debatable, and further systematic research is needed to explore these mechanisms more clearly.

Thus, a lot of efforts have been put into

-p

tuning FL emission of CDs, mainly including heteroatom doping[13], surface passivation[14] and size control[15].

re

In addition to the above methods, the concentration can also be used as a method to regulate the FL spectra. Up to now, the change of FL spectra due to

lP

concentration has been observed in fluorescent materials, such as organic dyes[16-18] and nanomaterials[19, 20]. However, only few studies have been

na

reported on concentration-dependent FL properties of CDs. For instance, the maximum emission wavelength of CDs shows red shifts as the concentration increases[21] and the CDs exhibit excitation-independent fluorescence behavior

ur

at higher concentration and excitation-dependent behavior during lower concentration[22]. Until now, the concentration-dependent FL mechanisms of

Jo

CDs remain unclear. FL lifetime is an intrinsic property and relies on microstructures of the fluorescent materials, and FL decay spectra of CDs can give some insight on their microstructures. The interpretation of this mechanism by FL lifetime relied on microstructures of the fluorescent materials is rarely reported. The study of the FL lifetime of CDs at different concentrations is helpful for understanding the relationship between surface microstructures and the concentration-dependent FL properties.

In this work, CDs with concentration-dependent FL properties were successfully synthesized. The morphologies and particle sizes of the CDs were obtained by high resolution transmission electron microscopy (HRTEM) and dynamic light scattering (DLS) technique. The optical properties of CDs at different concentrations were studied. Effect of CDs concentration on the FL properties of CDs was explored, and the relationship between the fluorescence characteristics and CDs concentration was obtained. The surface microstructures of CDs at various concentrations were explored by using FL

ro of

decay spectra to find relationship between the surface microstructures and optical properties. A concentration-dependent FL mechanism was presented and a possible aggregation model of CDs was given.

-p

2. Materials and methods 2.1 Materials

re

Citric acid (CA) (≥ 99.5 %), Ethylene imine polymer (PEI) (>99%, MW=10000) were purchased from Aladdin (Shanghai) Co., Ltd. Potassium

lP

hydrogen phosphate (≥ 99.5 %) was supplied by LingFeng Chemical Reagent (Shanghai) CO., Ltd. Disodium hydrogen phosphate (≥ 99 %) was purchased from Titan Scientific Co., Ltd. All reagents were used directly without further

na

purification, and all aqueous solutions were prepared with ultrapure water (18.2 MΩ).

ur

2.2 Synthesis and characterization of carbon dots (CDs) Synthesis of CDs: The CDs were synthesized by hydrothermal process.

Jo

Briefly speaking, 0.2 g of citric acid (CA) and 0.36 g ethylene imine polymer (PEI) were added to 15 mL of ultrapure water to form a well-distributed solution. Then the solution was transferred into a 45 mL Teflon-lined stainless steel autoclave and heated at the temperature of 200 °C for 12 h. After the end of the reaction, the autoclave was cooled down naturally. Purification of CDs solution: took out the solution from the reaction and the aqueous solution was centrifuged at 15 000 rpm for 15 min to remove the large particles. The solution

was then dialyzed for 72 h with dialysis bag (molecular weight cutoff MWCO=200) to remove small molecular impurities. The resulting purified aqueous solution of CDs was yellowish and stable for several months when they were stored at 4°C . Characterizations: UV-visible absorption spectrum of the samples was obtained by UV2450 spectrophotometer (Hitachi, Japan). The steady-state and time-resolved FL spectra of CDs were acquired by using FLS920 fluorescence spectrometer (Edinburgh, UK). The high resolution transmission electron

ro of

microscopy (HRTEM) images of the CDs were observed by a JEM-2100 electron microscope (JEM, Japan) operating at 200 kV. The fourier transform infrared (FTIR) spectrum of CDs was measured in the range of 500 cm -1 to

4000 cm-1 by a NICOLET iS10 (Thermo Fisher, America) spectrometer. The

-p

particle size of CDs was measured by using a dynamic light scattering (DLS)

technique (Zeta-sizer ZEN 3600, Malvern instruments Ltd, UK). The powder

re

X-ray diffraction (XRD) spectrum was collected on a D/max 2550 VB/PC (Rigaku, Japan) with Cu Kα radiation. The X-Ray photoelectron spectroscopy

Fisher, America).

lP

(XPS) spectra were performed by a ESCALAB250 spectrometer (Thermo

na

2.3 Concentration-dependent fluorescence emission To investigate the effect of the CDs concentration on the optical properties of CDs, the purified CDs solution was diluted into CDs aqueous solution with

ur

concentration of 0.03 g/L, 0.06 g/L, 0.12 g/L, 0.36g/L, 0.74 g/L, 1.48 g/L, 2.47 g/L, respectively. In order to exclude pH differences in CDs solutions of

Jo

different concentrations, 0.2 mol/L phosphate buffer solution (PBS, pH=7.0) were used to prepare the CDs solution with various concentrations. Then, the optical properties of the CDs in various concentrations were probed in different ways, and all measurements were carried out at room temperature. 3. Results and discussion 3.1 Synthesis and characterization of the as-prepared carbon dots (CDs)

In this work, CDs were synthesized by hydrothermal process and the synthesis procedure was illustrated in Fig. S1A. The main factors influencing the synthesis process were the mass ratio of CA to PEI, reaction temperature and reaction time. In the synthesis process, CA and PEI were as carbon source and nitrogen source, respectively[23], the mass ratio between CA and PEI was adjusted in parallel reactions, and a ratio of 1:1.8 was found to be optimal for the strongest FL (Fig. S1B). Similarly, the optimal reaction temperature and reaction time were determined to be 200 ℃ and 12 h, respectively (Fig. S1 C

ro of

and D). The colour of as-prepared CDs solution was yellowish (Fig. 1A insert, left item), which showed bright blue fluorescence (FL) under the excitation of

365 nm UV irradiation (Fig. 1A insert, right item). The UV-visible absorption, excitation and emission spectra of CDs are presented in Fig. 1A. There is an

-p

absorption peak at 360 nm in the UV-visible absorption of CDs, which is

consistent with the excitation wavelength. The FL spectra display an emission

re

maximum at 444 nm at the excitation of 360 nm. Moreover, the quantum yield (QY) of CDs was determined by using quinine sulphate as FL standards, which

lP

was 69% (the detailed measurement process and diagram see Fig. S2). The QY of the CDs is much higher than that (about 15%) of CDs without N-doping[24].

na

It is likely that N doping can promote radiative recombination and lower non-

Jo

ur

radiative recombination[25].

Fig. 1 (A) Typical UV-visible absorption spectrum (the black curve), FL excitation spectrum (the blue curve) and FL emission spectrum (the red curve) of the CDs (0.12 g/L). The inset shows digital photographs of the CDs under visible light

irradiation (left) and 365 nm UV light irradiation (right). (B)

XRD diffraction pattern of the CDs. (C) FTIR spectrum of the CDs. XPS spectra of CDs: (D) C 1s, (E) N 1s and (F) O 1s.

The XRD pattern of CDs (Fig. 1B) displays a broad peak center at 23.2°, which is attributed to highly disordered carbon atoms, showing that CDs are mainly composed of amorphous carbon. Fourier transform infrared (FTIR) was

ro of

employed to provide convincing evidences for the surface state of CDs. As

displayed in Fig. 1C, the peak from 1375 cm-1 to 1462 cm-1 can be identified as COO- group[26]. A small band from 2833 cm-1 to 2947 cm-1 is ascribed to the

C-H bonds. A sharp peak associated with amide linkage (-CONH-) and bending

-p

vibration of -NH are found at 1650 cm-1, 1557 cm-1, respectively[27].

Moreover, the surface groups were also investigated by XPS spectra. The high-

re

resolution C 1s analysis reveals three peaks observed with the binding energies of about 284.6 eV, 285.6 eV and 287.3 eV, which are attributed to C-C/C=C,

lP

C-N and C=O species, respectively (Fig. 1D). The three peaks of N 1s highresolution spectrum at 398.7 eV, 399.5 eV and 400.4 eV are attributed to C-N-

na

C, N-(C)3 and N-H (Fig. 1E). The two peaks of O 1s high-resolution spectrum at 531.6 eV and 532.8 eV are attributed to C=O and C-OH/C-O-C bands (Fig.

ur

1F). The surface components of the CDs determined by the XPS are good agreement with FTIR results. It demonstrates that the surface of the CDs is

Jo

filled with hydrophilic oxygen functional groups and nitrogen functional groups. The stability of CDs was an important factor to assess their applications. We further investigated the influence of ionic strength and photo irradiation to the FL intensity of the CDs (Fig. S3). As shown in Fig. S3A, the FL intensity of the CDs was relatively constant with increasing concentrations of NaCl in aqueous solution up to 1 mol/L, suggesting high stability of the CDs even under high ionic-strength conditions. In Fig. S3B, the CDs was exposed

under a 365 nm UV lamp for various time spans to probe their photo-stability. The FL intensity of the CDs almost unchanged, indicating that CDs has better FL stability. Therefore, the as-prepared CDs exhibited good stability in this article. We argued the functional groups on the surface of the CDs could improve the hydrophilicity and stability of the CDs. 3.2 Concentration-dependent fluorescence properties of the CDs It is noteworthy to mention that the most interesting part in this work is studying the concentration-dependent optical properties of CDs. Fluorescence

ro of

spectra of CDs at various excitation wavelength are obtained when the concentration of CDs is less than 0.12 g/L, as an example, the normalized FL emission spectra of CDs at 0.03 g/L are shown in Fig. 2A. The emission

wavelength is invariable as the excitation wavelength varies, which is much

-p

different from the previous report[28]. This results illustrate that the emission peaks position of CDs maintains unchanged with the variation of excitation

re

wavelength at low concentration. Fluorescence spectra of CDs at various excitation wavelength are also obtained while the CDs concentration is in the

lP

range of 0.12 g/L-2.47 g/L, for example, Fig. 2B shows the normalized FL emission spectra of CDs at 2.47 g/L. The emission peaks clearly shift to longer

na

wavelength as the excitation wavelength gradually increases. It is found that the emission peaks of CDs depend on excitation wavelength at higher concentration. The results are similar to that of other CDs whose emission

ur

wavelength depends on excitation wavelength[29]. More information about the emission wavelength variation with excitation wavelength at various CDs

Jo

concentrations are provided in Fig. S4.

Fig.2 (A) The normalized FL emission spectra of CDs at concentration of 0.03 g/L and (B) CDs at concentration of 2.47 g/L with various excitation wavelength.

To provide insight into the effect of concentration on optical properties of CDs, the FL emission spectra of CDs at different concentrations were measured at the excitation wavelength of 360 nm (Fig. 3A). As shown in Fig. 3B (the red line), the emission peaks position of CDs hardly changes when the concentration of CDs is less than 0.12 g/L, however, the wavelength displays red shifts from 444 nm to 478 nm with the CDs concentration changing from

ro of

0.12 g/L to 2.47 g/L. The FL emission intensity of CDs increases with CDs concentration which is below 0.12 g/L. By contrast, the FL intensity of CDs

continuously decreases during higher concentration of CDs (Fig. 3B, the black line), which is resulted from collisional quenching and self-absorption

-p

quenching[30]. The FL excitation spectra of CDs at different concentrations

were also measured at the emission wavelength of 444 nm (Fig. S5). For CDs

re

concentration less than 0.12 g/L, the FL excitation spectra exhibit one excitation peak and the FL intensity of CDs increases with its concentration.

lP

However, it can be seen that at higher CDs concentration, the FL intensity of CDs decreases and the FL excitation spectra show two excitation peaks, which reveals that CDs may have multiple surface energy trap[31]. Based on the FL

na

emission spectra and FL excitation spectra of CDs at different concentrations, it can be concluded that the turning point of the concentration is about 0.12 g/L,

ur

indicating that the FL from one kind of radiative recombination is gradually

Jo

transformed into two or more kinds with the CDs concentration increasing[32].

Fig.3 (A) The normalized FL emission spectra of CDs at different concentrations with the excitation wavelength of 360 nm. (B) The emission peaks position (the red line) and the FL intensity (the black line) at different CDs concentrations at 360 nm excitation.

In order to study the change of optical properties caused by CDs concentration, the particle sizes of CDs at various concentrations were obtained by using dynamic light scattering (DLS). As shown in Fig. 4D (the DLS data see Table. S1), the average particle sizes are nearly invariable when the CDs concentration is less than 0.12 g/L, but the average particle sizes increase when concentration ranges from 0.12 g/L to 2.47 g/L. To further investigate the effect

ro of

of concentration on particle sizes, the morphologies and particle sizes of CDs

with different concentrations were probed by the high resolution transmission

electron microscopy (HRTEM) (Fig. 4A-C). When the CDs concentrations are

-p

0.03 g/L, 0.74 g/L and 2.47 g/L, the average particle sizes of CDs are 3±0.8 nm, 17±0.6 nm and 35±4.5 nm, respectively. The average particle sizes of CDs

re

obtained by DLS measurement are a somewhat larger than that of HRTEM results, which is mainly because the DLS considers the overall hydrodynamic

lP

diameter that includes particles, surface functional groups as well as absorbed molecules and ions[33]. These results reveal that the particle sizes of CDs are highly associated with their concentrations. Accordingly, It can draw a

na

conclusion that the concentration-dependent particle sizes of CDs can be attributed to their interactions which are similar to intermolecular forces[22].

ur

When the CDs concentration is less than 0.12 g/L, the increase of interparticle distance weakens the interaction among the particles so that the CDs

Jo

aggregation do not occur, which makes the particle size small and substantially unchanged. While the CDs concentration is larger than 0.12 g/L, the interparticle distance decreases and the interaction among the particles strengthens, resulting in the aggregation of CDs. And a possible aggregation model of CDs is given in Fig. 4D.

ro of

Fig. 4 HRTEM micrographs of CDs: (A) at concentration of 0.03 g/L, (B) 0.74 g/L and (C) 2.47 g/L. (D) Average particle sizes of CDs measured by DLS at different concentrations. (E) FL lifetime line: τ1 (the black line) and τ2 (the red line) at different CDs concentrations, respectively.

From the above experiment, we can conclude that there are two kinds of

-p

surface states in CDs solution. The uniform surface states and the complex surface states from more oxygen-related groups may contribute to the

re

concentration-dependent FL properties (Fig. 4D, aggregation model). When at low concentration, the single CDs with uniform surface states caused by less

lP

oxygen-related groups show a considerably shorter wavelength emission and excitation-independent FL behavior (Fig. 2A) [34]. While at higher

na

concentration, the aggregation of CDs can increase the oxygen-related groups on the surface,[35] and more oxygen-related groups can introduce complex

ur

surface states to trap the excitons under excitations. The radiative recombination of those surface-trapped excitons can give the FL emission with

Jo

corresponding energy[34]. Consequently, higher concentration of CDs with more complex surface states exhibit excitation-dependent FL behavior (Fig. 2B) and two excitation peaks in the FL excitation spectra (as shown in Fig. 3C) [36]. Meanwhile, the complex surface states of CDs lead to electron rapid relaxation from excited states to substates, which corresponds to longer wavelength emission (Fig. 3B, the red line) [32]. It suggests that the surface states change is driven by CDs concentration in the solution.

FL lifetime is an intrinsic characteristic of fluorescent reporter, which is usually independent of signal amplitude that is a function of factors such as reporter light attenuation, laser power, light attenuation[37], but it relies on the microstructures of CDs[38]. Thus FL lifetime can give some information about the microstructures of CDs. In order to further explain the concentrationdependent FL properties, FL decay curves of CDs at different concentrations were measured at their respective optimal excitation and emission wavelengths. All results of FL decay curves of CDs are summarized in Fig. S6. The

ro of

fluorescent decay curves of CDs at different concentrations could be fitted by a double-exponential function. It is found that there are two different emission

sites on the surface of the samples, which accords with intrinsic state emission reflected by τ1 and defect state emission reflected by τ2[39]. The concentration

-p

of CDs has little influence on τ1 which almost remains a constant in the course

of experiment (Fig. 4E, the black line). The reason of this phenomenon is that τ1

re

is ascribed to the intrinsic state emission which belongs to the internal structure of the CDs.[40, 41] Therefore, the differences of surface energy defect caused

lP

by the CDs concentration barely impact on it. Surprisingly, the value of τ2 does not change during the concentration of CDs below 0.12 g/L and the τ2 of CDs

na

increases from 9.01 ns to 11.03 ns when the CDs concentration changes from 0.12 g/L to 2.47 g/L (Fig. 4E, the red line). When CDs concentration is less than 0.12 g/L, the τ2 is basically unchanged because the CDs possess uniform

ur

surface states due to less oxygen-related groups. However, the complex surface states caused by more oxygen-related groups at higher concentration can

Jo

significantly hamper relaxation of the excited electronic states of CDs, which leads to increment in FL lifetime[42, 43]. These phenomena also support the conclusion that the FL of CDs is derived from the surface defects states[44]. 4. Conclusion In short, CDs with concentration-dependent FL properties have been successfully synthesized. The CDs exhibit excitation-independent FL behavior during the concentration less than 0.12 g/L and excitation-dependent FL

behavior with the concentration varying from 0.12 g/L to 2.47 g/L. Red-shifts of the emission wavelength from 444 nm to 478 nm are observed when the concentration of CDs increases from 0.12 g/L to 2.47 g/L. The fluorescence properties of CDs were explained from the view of morphologies and particle sizes by HRTEM and DLS analysis. It is worthwhile to illustrate the change of fluorescence characteristics of CDs caused by the surface states probed through the FL decay spectra. Relationship of the microstructures and macroscopic properties was explored, and a possible aggregation model of CDs was given.

ro of

The concentration-dependent FL properties of the CDs can provide a new way to adjust the emission wavelength and put forward many new applications in the future.

Prof. Xueqin An

re

East China University of Science and Technology

-p

Declaration of interest statement

na

lP

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgment

ur

This research was supported by the National Natural Science Foundation

Jo

of China (21473055 and 21773063).

References [1] S.Y. Lim, W. Shen, Z. Gao, Carbon quantum dots and their applications, Chem Soc Rev, 44 (2015) 362-381. [2] X. Liu, J. Liu, B. Zheng, L. Yan, J. Dai, Z. Zhuang, J. Du, Y. Guo, D. Xiao, NDoped carbon dots: green and efficient synthesis on a large-scale and their application in fluorescent pH sensing, New Journal of Chemistry, 41 (2017) 10607-10612. [3] Z. Zhang, Y. Shi, Y. Pan, X. Cheng, L. Zhang, J. Chen, M.-J. Li, C. Yi, Quinoline derivative-functionalized carbon dots as a fluorescent nanosensor for sensing and

ro of

intracellular imaging of Zn2+, J. Mater. Chem. B, 2 (2014) 5020-5027.

[4] M. Xu, G. He, Z. Li, F. He, F. Gao, Y. Su, L. Zhang, Z. Yang, Y. Zhang, A green heterogeneous synthesis of N-doped carbon dots and their photoluminescence applications in solid and aqueous states, Nanoscale, 6 (2014) 10307-10315.

-p

[5] B. Chen, F. Li, S. Li, W. Weng, H. Guo, T. Guo, X. Zhang, Y. Chen, T. Huang, X.

re

Hong, S. You, Y. Lin, K. Zeng, S. Chen, Large scale synthesis of photoluminescent carbon nanodots and their application for bioimaging, Nanoscale, 5 (2013) 1967-

lP

1971.

[6] P.G. Luo, S. Sahu, S.-T. Yang, S.K. Sonkar, J. Wang, H. Wang, G.E. LeCroy, L. Cao, Y.-P. Sun, Carbon “quantum” dots for optical bioimaging, Journal of Materials

na

Chemistry B, 1 (2013) 2116.

[7] Y. Dong, R. Wang, G. Li, C. Chen, Y. Chi, G. Chen, Polyamine-functionalized

ur

carbon quantum dots as fluorescent probes for selective and sensitive detection of copper ions, Anal Chem, 84 (2012) 6220-6224.

Jo

[8] L. Cao, S. Sahu, P. Anilkumar, C.E. Bunker, J. Xu, K.A. Fernando, P. Wang, E.A. Guliants, K.N. Tackett, 2nd, Y.P. Sun, Carbon nanoparticles as visible-light photocatalysts for efficient CO2 conversion and beyond, J Am Chem Soc, 133 (2011) 4754-4757.

[9] K. Jiang, S. Sun, L. Zhang, Y. Lu, A. Wu, C. Cai, H. Lin, Red, green, and blue luminescence by carbon dots: full-color emission tuning and multicolor cellular imaging, Angew Chem Int Ed Engl, 54 (2015) 5360-5363.

[10] L. Bao, C. Liu, Z.L. Zhang, D.W. Pang, Photoluminescence-tunable carbon nanodots: surface-state energy-gap tuning, Adv Mater, 27 (2015) 1663-1667. [11] H. Ding, S.B. Yu, J.S. Wei, H.M. Xiong, Full-Color Light-Emitting Carbon Dots with a Surface-State-Controlled Luminescence Mechanism, ACS Nano, 10 (2016) 484-491. [12] L. Bao, Z.L. Zhang, Z.Q. Tian, L. Zhang, C. Liu, Y. Lin, B. Qi, D.W. Pang, Electrochemical tuning of luminescent carbon nanodots: from preparation to luminescence mechanism, Adv Mater, 23 (2011) 5801-5806.

ro of

[13] S. Yang, J. Sun, X. Li, W. Zhou, Z. Wang, P. He, G. Ding, X. Xie, Z. Kang, M. Jiang, Large-scale fabrication of heavy doped carbon quantum dots with tunable-

photoluminescence and sensitive fluorescence detection, J. Mater. Chem. A, 2 (2014) 8660-8667.

-p

[14] Y.-P. Sun, B. Zhou, Y. Lin, W. Wang, K.A.S. Fernando, P. Pathak, M.J.

Meziani, B.A. Harruff, X. Wang, H. Wang, P.G. Luo, H. Yang, M.E. Kose, B. Chen,

re

L.M. Veca, S.-Y. Xie, Quantum-Sized Carbon Dots for Bright and Colorful Photoluminescence, J. Am. Chem. Soc., 128 (2006) 7756-7757.

lP

[15] B. Zhu, S. Sun, Y. Wang, S. Deng, G. Qian, M. Wang, A. Hu, Preparation of carbon nanodots from single chain polymeric nanoparticles and theoretical

na

investigation of the photoluminescence mechanism, J. Mater. Chem. C, 1 (2013) 580586.

[16] L. Zang, Y. Che, J.S. Moore, One-Dimensional Self-Assembly of Planar π-

ur

Conjugated Molecules: Adaptable Building Blocks for Organic Nanodevices, Acc. Chem. Res., 41 (2008) 1596-1608.

Jo

[17] F. Wuerthner, T.E. Kaiser, C.R. Saha-Moeller, J-Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials, Angew. Chem., Int. Ed., 50 (2011) 3376-3410. [18] I.T. Sugiarto, Isnaeni, K.Y. Putri, Analysis of dual peak emission from Rhodamine 6G organic dyes using photoluminescence, Journal of Physics: Conference Series, 817 (2017) 012047.

[19] S. Chen, J.W. Liu, M.L. Chen, X.W. Chen, J.H. Wang, Unusual emission transformation of graphene quantum dots induced by self-assembled aggregation, Chem Commun (Camb), 48 (2012) 7637-7639. [20] X.-F. Zhang, S. Liu, X. Shao, Fluorescence of chemically derived graphene: Effect of self-rolling up and aggregation, Journal of Luminescence, 136 (2013) 32-37. [21] C. Wang, T. Hu, Z. Wen, J. Zhou, X. Wang, Q. Wu, C. Wang, Concentrationdependent color tunability of nitrogen-doped carbon dots and their application for iron(III) detection and multicolor bioimaging, J Colloid Interface Sci, 521 (2018) 33-

ro of

41. [22] X. Meng, Q. Chang, C. Xue, J. Yang, S. Hu, Full-colour carbon dots: from

energy-efficient synthesis to concentration-dependent photoluminescence properties, Chem Commun (Camb), 53 (2017) 3074-3077.

-p

[23] W.S. Zou, Q.C. Zhao, J. Zhang, X.M. Chen, X.F. Wang, L. Zhao, S.H. Chen,

Y.Q. Wang, Enhanced photoresponsive polyethyleneimine/citric acid co-carbonized

re

dots for facile and selective sensing and intracellular imaging of cobalt ions at physiologic pH, Anal Chim Acta, 970 (2017) 64-72.

lP

[24] J. Gong, X. An, X. Yan, A novel rapid and green synthesis of highly luminescent carbon dots with good biocompatibility for cell imaging, New J. Chem., 38 (2014)

na

1376-1379.

[25] D. Qu, M. Zheng, L. Zhang, H. Zhao, Z. Xie, X. Jing, R.E. Haddad, H. Fan, Z. Sun, Formation mechanism and optimization of highly luminescent N-doped

ur

graphene quantum dots, Sci. Rep., 4 (2014) 5294. [26] S. Jahan, F. Mansoor, S. Naz, J. Lei, S. Kanwal, Oxidative Synthesis of Highly

Jo

Fluorescent Boron/Nitrogen Co-Doped Carbon Nanodots Enabling Detection of Photosensitizer and Carcinogenic Dye, Anal. Chem. (Washington, DC, U. S.), 85 (2013) 10232-10239. [27] Y. Dong, R. Wang, H. Li, J. Shao, Y. Chi, X. Lin, G. Chen, Polyaminefunctionalized carbon quantum dots for chemical sensing, Carbon, 50 (2012) 28102815.

[28] Y. Deng, D. Zhao, X. Chen, F. Wang, H. Song, D. Shen, Long lifetime pure organic phosphorescence based on water soluble carbon dots, Chem. Commun. (Cambridge, U. K.), 49 (2013) 5751-5753. [29] S. Sahu, B. Behera, T.K. Maiti, S. Mohapatra, Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bioimaging agents, Chem. Commun. (Cambridge, U. K.), 48 (2012) 8835-8837. [30] Y. Song, S. Zhu, S. Xiang, X. Zhao, J. Zhang, H. Zhang, Y. Fu, B. Yang, Investigation into the fluorescence quenching behaviors and applications of carbon

ro of

dots, Nanoscale, 6 (2014) 4676-4682. [31] M. Xu, S. Xu, Z. Yang, M. Shu, G. He, D. Huang, L. Zhang, L. Li, D. Cui, Y.

Zhang, Hydrophilic and blue fluorescent N-doped carbon dots from tartaric acid and various alkylol amines under microwave irradiation, Nanoscale, 7 (2015) 15915-

-p

15923.

[32] S. Khan, A. Gupta, N.C. Verma, C.K. Nandi, Time-Resolved Emission Reveals

Dots, Nano Lett, 15 (2015) 8300-8305.

re

Ensemble of Emissive States as the Origin of Multicolor Fluorescence in Carbon

lP

[33] S.C. Ray, A. Saha, N.R. Jana, R. Sarkar, Fluorescent Carbon Nanoparticles: Synthesis, Characterization, and Bioimaging Application, J. Phys. Chem. C, 113

na

(2009) 18546-18551.

[34] Y.-M. Long, C.-H. Zhou, Z.-L. Zhang, Z.-Q. Tian, L. Bao, Y. Lin, D.-W. Pang, Shifting and non-shifting fluorescence emitted by carbon nanodots, Journal of

ur

Materials Chemistry, 22 (2012) 5917. [35] Z.Q. Xu, J.Y. Lan, J.C. Jin, T. Gao, L.L. Pan, F.L. Jiang, Y. Liu, Mechanistic

Jo

studies on the reversible photophysical properties of carbon nanodots at different pH, Colloids Surf B Biointerfaces, 130 (2015) 207-214. [36] Z. Yang, M. Xu, Y. Liu, F. He, F. Gao, Y. Su, H. Wei, Y. Zhang, Nitrogendoped, carbon-rich, highly photoluminescent carbon dots from ammonium citrate, Nanoscale, 6 (2014) 1890-1895. [37] G. Ma, Background-Free In vivo Time Domain Optical Molecular Imaging Using Colloidal Quantum Dots, ACS Appl. Mater. Interfaces, 5 (2013) 2835-2844.

[38] S. Khan, A. Gupta, N.C. Verma, C.K. Nandi, Time-Resolved Emission Reveals Ensemble of Emissive States as the Origin of Multicolor Fluorescence in Carbon Dots, Nano Lett., 15 (2015) 8300-8305. [39] R.-J. Fan, Q. Sun, L. Zhang, Y. Zhang, A.-H. Lu, Photoluminescent carbon dots directly derived from polyethylene glycol and their application for cellular imaging, Carbon, 71 (2014) 87-93. [40] F. Liu, M.H. Jang, H.D. Ha, J.H. Kim, Y.H. Cho, T.S. Seo, Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin

ro of

of blue and green luminescence, Adv Mater, 25 (2013) 3657-3662. [41] X. Zhang, Y. Zhang, Y. Wang, S. Kalytchuk, S.V. Kershaw, Y. Wang, P. Wang, T. Zhang, Y. Zhao, H. Zhang, T. Cui, Y. Wang, J. Zhao, W.W. Yu, A.L. Rogach,

Color-Switchable Electroluminescence of Carbon Dot Light-Emitting Diodes, ACS

-p

Nano, 7 (2013) 11234-11241.

[42] C.J. Jeong, G. Lee, I. In, S.Y. Park, Concentration-mediated multicolor

re

fluorescence polymer carbon dots, Luminescence, 31 (2016) 897-904. [43] S. Hu, A. Trinchi, P. Atkin, I. Cole, Tunable photoluminescence across the entire

54 (2015) 2970-2974.

lP

visible spectrum from carbon dots excited by white light, Angew Chem Int Ed Engl,

na

[44] L. Wang, S.-J. Zhu, H.-Y. Wang, S.-N. Qu, Y.-L. Zhang, J.-H. Zhang, Q.-D. Chen, H.-L. Xu, W. Han, B. Yang, H.-B. Sun, Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots, ACS Nano, 8

Jo

ur

(2014) 2541-2547.