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Hydrophobic Carbon Dots from Aliphatic Compounds with One Terminal Functional Group Keyang Yin, Dandan Lu, Li-Ping Wang, Quanxin Zhang, Junying Hao, Gaozhan Li, and Hongguang Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04479 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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The Journal of Physical Chemistry

Hydrophobic Carbon Dots from Aliphatic Compounds with One Terminal Functional Group Keyang Yin,†,|| Dandan Lu,‡ Liping Wang,*,§ Quanxin Zhang,† Junying Hao,*,† Gaozhan Li,‡ Hongguang Li*,‡ †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Lanzhou

730000, China ‡

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education,

Jinan 250100, China §

College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China

||

University of Chinese Academy of Sciences, Beijing 100049, China

Corresponding authors: +86-635-8230919. E-mail: [email protected] (Liping Wang) +86-931-4968236. E-mail: [email protected] (Junying Hao) +86-531-88363597. E-mail: [email protected] (Hongguang Li)

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ABSTRACT: In the preparation of carbon dots (C dots) through pyrolysis, organic molecules with multiple functional groups are popular, and monosubstituted compounds, such as aliphatic amines, are used only as an auxiliary component. Here, we provide a strategy to transfer aliphatic amines and aliphatic aldehydes to C dots at the absence of any other reagent under mild condition. In a typical experiment, C dots were facilely obtained by refluxing dodecylamine (C12-NH2) in chlorobenzene, which show blue emission with wavelength-dependent characteristics. The C dots are soluble in a variety of organic solvents and can be further processed by either gel permeation chromatography or silica gel column chromatography, which gives highly pure C dots with absolute fluorescence quantum yield over 10%. The C dots are photoluminescent in solvent-free state and can be used as fluorescent ink. They are also compatible with polymers such as poly(methyl methacrylate) and polydimethylsiloxane. Influence of the type of functional group was checked. Blue-emitting C dots was obtained as well by refluxing dodecanal in chlorobenzene, but using compound bearing carboxylic or hydroxy group only showed little success.


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INTRODUCTION As a new generation of photoluminescent (PL) nanomaterials, carbon dots (C dots) have attracted great attention due to their superior characteristics such as ease of preparation, environmentally benign character, high chemical stability, high anti-photobleaching capability, low cytotoxicity and good PL properties.1-5 To date, various methods for the preparation of C dots have been reported. In the early stage, C dots were obtained by the top-down method by cutting carbon nanotubes6 and graphite-containing carbon target,7 which was followed by other bulk carbon materials including candle soot,8 natural gas soot,9 petroleum,10 coal,11 carbon fibre,12 activated carbon,13 fullerene14 and hair fibre.15 Later, the focus of C dots preparation gradually shifted to pyrolysis by using various organic compounds.16,17 The heating can be performed in an open system, or in a closed autoclave with the help of hydrothermal or microwave treatment. Although pyrolysis seems simple which is normally carried out in one-pot process, the transformation of the organic compound as a function of temperature and time is rather complicated. It is generally accepted that with increasing temperature, condensation will occur among the functional groups of the organic compounds followed by carbonization of the whole compound.18-21 According to this rule, organic compounds with multiple functional groups are popular to be used as starting materials. Indeed, the majority of the organic compounds tested up to now meet this criterion, and preparation of C dots by using compound bearing only one functional group succeeded commonly at harsh conditions where strong base22 or oxidative reagent23 is present. Currently, there are numerous reports on water soluble C dots with applications in bioimaging and fluorescence sensing.1-5,24 In comparison, studies on C dots preferentially soluble in nonaqueous solutions, termed hydrophobic C dots hereafter, are relatively rare. As an important branch of hydrophobic C dots, those bearing aliphatic chains have received considerable attention16,22,23,25-35 and their promising applications in composite materials34 and light-emitting diode27,28,35 have been ACS Paragon Plus Environment


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demonstrated. A typical way to obtain aliphatic C dots relies on the use of a binary mixture which contains a carbon source and a capping agent.16,25-29 An alternative strategy is by using alkylated polythiophene30,31 or gallic acid32,33 as both the carbon source and the capping agent. Aliphatic C dots can be also obtained by post-functionalization of water soluble C dots through covalent34 or noncovalent bonds.17 Besides, some surfactants were reported to be able to produce aliphatic C dots in harsh conditions.22,23,35 Aliphatic compound with only one terminal group represents a big class of organic molecules which are abundant and cheap. Up to now, some of them (typically aliphatic amines) have been used in the production of C dots merely as the capping agent.16,25-29,34 Indeed, judging from the molecular structure, compound with such a structural motif does seem to be unpopular in the preparation of C dots through pyrolysis. In this paper, we report that blue-emitting aliphatic C dots can be facilely obtained by refluxing aliphatic amine or aliphatic aldehyde in chlorobenzene (CB) at moderate temperature (132 C) at the absence of any other reagent. The so-obtained C dots show good solubility in a variety of common organic solvents and can be further purified by gel permeation chromatography (GPC) or silica gel column chromatography, which gives highly pure C dots with absolute fluorescence quantum yield () over 10%. Compared to other strategies to produce aliphatic C dots, current method owns obvious advantages including readily-available starting materials, simplified process, safer operation and low energy consumption. The success in the production of C dots using such simple organic compounds greatly widens the scope of the starting materials for C dots.

EXPERIMENTAL SECTION Materials. Octylamine (C8-NH2, 99%), dodecylamine (C12-NH2, 98%), cetylamine (C16-NH2, 98%), dodecanal (C11-CHO, 95%), dodecanoic acid (C11-COOH, 98%) and dodecanol (C12-OH, > 99.0%) were purchased from Aladdin Biochemical Technology Co., Ltd (Shanghai, China). ACS Paragon Plus Environment

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Poly(methyl methacrylate) (PMMA, average Mw ~350000) was from Sigma-Aldrich. CB (analytical grade) was from Tianjin Chemicals (China). Polydimethylsiloxane (PDMS, SYLGARD silicone elastomer 184) and the curing agent were purchased from Dow Corning Corp (America). Glycerol (analytical grade) was purchased from Sinopharm Chemical Reagent Co., Ltd. Other solvents were obtained from local suppliers with the quality of analytical grade. All the chemicals were used without further purification unless otherwise stated. 3,4,5-trioctadecyloxybenzaldehyde

was

synthesized

following

the

procedures

reported

previously.36 Detailed synthetic procedures and characterizations of the molecule can be found there. Synthesis of C dots. The C dots were prepared using a standard apparatus for traditional organic synthesis. In a typical experiment, desired amount of the starting material was added to a 100 mL three-neck flask. After removing air by repeated vacuum-argon cycles for three times, 50 mL CB was injected and the mixture was refluxed under stirring. For the experiments aiming to see the effects of different starting materials, the concentration of each starting material and the refluxing time was fixed at 0.2 molL-1 and 48 h, respectively. Totally six monofunctional small organic molecules and one bi-functional were selected. After the reaction stopped, the mixture was naturally cooled to room temperature and the organic solvent was removed by rotary evaporation. The crude product was then re-dissolved in 50 mL CB for qualitative comparison and PL measurements. The C dots prepared from C12-NH2, which show superior properties, were selected for further examination. The concentration was varied from 0.2 to 2.0 molL-1 and the refluxing time was changed from 6 to 48 h. Further purification was also carried out wherever necessary to get the yield of the C dots and to facilitate the quantitative analysis of the PL properties. In brief, the crude product (dissolved in toluene) was passed through a GPC column (Bio-Beads S-X1, 200-400 mesh) using toluene as an eluent to remove the unreacted C12-NH2 and other impurities with small ACS Paragon Plus Environment


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molecular weights. Alternatively, the crude product (dissolved in ethanol) was purified by silica gel column chromatography using ethanol as an eluent. Sample Preparation Method for Solubility Test. To test the solubility of the C dots in various solvents, a stock solution of 72 mg purified C dots in 2 mL ethanol was prepared. To a series of clean glass bottles, 250 μL of the stock solution was added. After ethanol was totally evaporated at 50 oC in a vacuum oven, 3 mL of each solvent was added and the mixture was homogenized thoroughly by vortex. Solubility was judged by visual inspection as well as PL measurements. Preparation of the PL Inks and Polymer-Based Composite Materials. For the preparation of the PL ink, a stock solution of the C dots in ethanol was prepared with a concentration of 36 mgmL-1. To a clean and dry glass bottle, 0.25 mL of the stock solution was added and ethanol was allowed to evaporate totally. Then, 3 mL of pre-mixed ethanol/glycerol mixture with a volume ratio of 1:1 was added. The mixture was then homogenized by vortex followed by sonication for 10 min. The as-prepared ink was easily filled in a gel ink pen for writing. For the preparation of the C dot/PDMS composite films, 15.0060 g PDMS, 1.5109 g curing agent and 21 mg C dots were mixed and homogenized by stirring. Then, 3.02 mL and 6.04 mL of the mixture were respectively added to a petri dish with a diameter of 3.5 cm. After air bubbles were threw out under vacuum, the mixture was solidified at 70 C for 27 h. For the preparation of the C dot/PMMA composite films, a stock solution of the C dots in dichloromethane (DCM) was prepared with a concentration of 29 mgmL-1. Then, a series of mixtures containing C dots and PMMA were prepared by mixing the DCM solution of C dots and solid PMMA. The total weight of C dots and PMMA was fixed at 500 mg and the weight percentage of C dots was varied. To fully mix C dots and PMMA, 3.5 mL DCM was added to each sample for total dissolution of the mixture. The homogeneous solution was then poured into a petri dish with a diameter of 5.5 cm for air-drying. ACS Paragon Plus Environment

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Instruments and Methods. High-resolution transmission electron microscopy (HRTEM) images were recorded on a JEOL 2100 instrument operating at 200 kV. For sample preparation, a drop of sample solution (20 L) was placed on a holey-carbon coated copper grid (300 meshes) and dried by an infrared lamp. Fluorescence spectra of the solution of C dots were obtained by a spectrofluorometer (LS-55). Fluorescence spectra of the C dot/PMMA films were recorded on a fluorescence spectrometer (Thermo Scientific Lumina). Fourier transform infrared (FTIR) spectra were recorded on a VERTEX-70/70v spectrometer (Bruker Optics, Germany) using KBr pellets. UV-vis measurements were carried out on a computer-manipulated spectrometer (UV-vis 4100, Hitachi, Japan). Thermogravimetric analysis (TGA) was carried out with DSC 822e (Piscataway, NJ) under nitrogen with a scanning speed of 10 °C·min-1. The absolute fluorescence quantum yields were measured with a spectrofluorometer (FLSP920, Edinburgh Instruments LTD) equipped with an integrating sphere, which consists of a 120 mm inside diameter spherical cavity. 3 mL of sample solution was sealed in a quartz cell (1 cm × 1 cm) with a plug. The same volume of solvent was used as the blank sample. Photos were taken by a digital camera (SONY DSC-TX300). X-ray diffraction (XRD) pattern was recorded on a PANalytical X’Pert Powder diffractometer (PANalytical, Holland) that operated in the reflection mode with Cu Ka radiation (l=1.54178 a). X-Ray photoelectron spectroscopy (XPS) spectra were recorded on an X-ray photoelectron spectrometer (ESCALAB 250) with a monochromatized Al K X-ray source (1486.71 eV). Photos of the samples were taken with a digital camera (SONY DSC-TX300). Elemental analysis was carried out with Vario EL CUBE (Elementar).

RESULTS AND DISCUSSION The Initial Discovery. The motivation of using monosubstituted aliphatic compound to prepare C dots dates back to our recent work on the covalent functionalization of graphene.36 As demonstrated before,33 the commercially-available graphite was first exfoliated into graphene in CB ACS Paragon Plus Environment


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with the help of an alkylated Percec monodendron (3,4,5-trioctadecyloxybenzaldehyde). The as-obtained graphene dispersion was then heated to reflux for in situ alkylation through [3+2] cycloaddition at the presence of sarcosine. After the reaction, the alkylated graphene deposited at the bottom, leaving a yellowish supernatant which is fluorescent under 365 nm UV irradiation. Initially, we thought that this luminescent solution contains debris of graphene with solvophilic groups. However, later we found that this yellowish solution appeared as well by refluxing 3,4,5-trioctadecyloxybenzaldehyde alone (0.2 molL-1) in CB. This color change should not come from the CB used as in control experiment, no color change was observed at all if the pure solvent was refluxed (Figure S1). To get more information, CB was removed under reduced pressure and the crude product, which was re-dissolved in toluene, was subjected to GPC. The fluorescent species was successfully grouped into several sections with different PL characteristics (Figure 1a). Totally five fractions were collected during the first round of separation (denoted as 1#-5#). With increasing retention time (Rt), the wavelength of maximum emission (em) gradually shifts to shorter wavelength (Figure 1b). From PL properties recorded at different excitations (Figure S2), the emission of 1# and 2# shows limited dependence on the excited wavelength (ex) and the curves are symmetric.  was determined to be 8.18% and 6.99%, respectively. The emission of 3# and 4# is wavelength-dependent. Especially, 3# became trailing on the column and its emission curves slightly overlapped with that of 4#. Besides the main peak at 431 nm, a shoulder peak is noticed around 486 nm for 3#, indicating that it still contains heterogeneous fluorescent species with different PL properties. To fully separate this section, it was subjected to the second round of GPC, which yielded another four portions (denoted as 3#-1, 3#-2, 3#-3 and 3#-4, Figure S3). With increasing Rt, it was found that the intensity of the emission at longer wavelength decreases while that at shorter wavelength increases, which is consistent with the trend observed in the first round of ACS Paragon Plus Environment

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GPC. Measurement of  on 3#-3 gave a value of 6.93%. Like 1# and 2#, 5# also shows wavelength-independent PL properties. TEM observations were carried out on 1# which showed the presence of spherical objects with a mean diameter of 3 nm (Figure 1c, d). Combining the results from PL measurements and TEM observations, it can be concluded that the fluorescent species produced by refluxing 3,4,5-trioctadecyloxybenzaldehyde in CB are C dots. (a)

(b)

(c)

5#

5#

1#

500 nm

1#

(d) Counts / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


350

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1

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Figure 1. (a) Separation of the as-prepared C dots by GPC with toluene as the eluent. Totally 5 fractions were collected, which were denoted as 1# to 5 # based on the order of the outflow. (b) Photos under 365 nm UV irradiation (top) and emission spectra excited at 360 nm (bottom) of the separated C dots. (c,d) TEM image and corresponding size distribution of the C dots which were first eluted (i.e., 1#). C dots from Aliphatic Compounds with Different Functional Groups. The production of C dots from 3,4,5-trioctadecyloxybenzaldehyde provides a new method to produce aliphatic C dots. It should be noted that the temperature adopted during pyrolysis is only 132 C (i.e., the boiling point of CB), which is a relatively moderate value in the production of C dots. To check the universality of this strategy, the scope of the starting material was expanded and monosubstituted aliphatic compounds with different terminal functional groups were selected, as illustrated in Figure ACS Paragon Plus Environment


2a. After refluxing 0.2 molL-1 of each target molecule in CB for 24 h, the as-obtained mixtures were collected and checked by visual observations. The mixture using C11-COOH or C12-OH as the starting material remained colorless. Under 365 nm UV irradiation, blue emission was noticed (Figure 2b). However, static fluorescence measurements showed that the curves are ill-defined and the PL intensities are limited (Figure S4). These observations indicate that under current experimental condition, using aliphatic compounds bearing carboxy and hydroxy as starting materials only leads to the formation of C dots with low yields and poor PL properties. The mixture using dodecanal (C11-CHO) or dodecylamine (C12-NH2) as the starting material became yellowish after pyrolysis and showed strong blue emission under UV irradiation (Figure 2b). From static fluorescence measurements (Figure 2c,d), well-defined curves were recorded in both cases which showed a strong dependence on ex. This wavelength-dependent PL property is typical for C dots.1-5,24 The maximum emission of the sample from C12-NH2 occurs at a wavelength of 483 nm, which is 20 nm longer than that of the sample from C11-CHO (463 nm). OH

(b)

(c) 400

Room light

dodecanol (C12-OH)

PL Intensity

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200

300 200 100

(f)

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C12-OH

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Room light

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dodecanoic acid (C11-COOH)

(e)

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(d) 600

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PL Intensity

(a)

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400

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Concentration of C12-NH2 / molL-1

0.0

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520

540

Figure 2. (a) Structures of molecules with different functional groups used for the preparation of C dots. (b) Photos of as-prepared C dots in CB by refluxing 0.2 molL-1 of each target molecule for 24 h. (c,d) Emission of as-prepared C dots from C11-CHO (c) and C12-NH2 (d) at different ex. The ACS Paragon Plus Environment

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dashed lines are guides for the eyes. (e) Photos of as-prepared C dots in CB obtained by refluxing 1.0 molL-1 C12-NH2 for different times. (f) PL intensity and corresponding em for C dots shown in e. The emissions were recorded with a ex range of 370-430 nm (for samples with 48 h and 24 h ) and 360-420 nm (for samples with 12 h and 6 h) with a step of 10 nm. (g) Statistics of the yield and mass obtained for C dots obtained by refluxing different amount of C12-NH2 in CB for 24 h followed by purification through silica gel chromatography (see below). (h) PL intensity and corresponding em for as-prepared C dots obtained by refluxing 1.0 molL-1 C8-NH2, C12-NH2 and C16-NH2 in CB for 24 h. Emissions were recorded with a ex range of 370-430 nm for C8-NH2, 380-440 nm for C12-NH2, and 410-470 nm for C16-NH2 with a step of 10 nm.

chlorobenzene reflux

C dots aliphatic chain

-NH2 or -CHO

Scheme 1. Illustration of the preparation of C dots from formyl- and amino-based aliphatic compounds. The observations presented above indicate that formyl- and amino-based aliphatic compounds can be used to produce C dots by using the strategy originally developed from the pyrolysis of 3,4,5-trioctadecyloxybenzaldehyde, as illustrated in Scheme 1. The monosubstituted aliphatic compounds with different terminal functional groups perform differently in the production of C dots, which could be ascribed to the different activation energy among the condensation of each functional group as well as the different decomposition temperature of each compound. To prevent the possibility that the production of the C dots is caused by the impurities from the starting material (for example C12-NH2) and/or the solvent (i.e., CB), both of them from different suppliers were tested and the results are given in Figure S5. In all the cases, yellowish solutions with blue emission were obtained after pyrolysis. We have also checked the influence of oxygen, and found it ACS Paragon Plus Environment


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is not a key factor. In control experiments, both the sample obtained under protection with argon and that prepared in air are yellowish without noticeable difference by the naked eyes. From Figure 2b, one can see that the color of the mixture containing C dots from C12-NH2 is heavier than that from C11-CHO. Consistent with this observation, the PL intensity of the sample from C12-NH2 is also higher as evidenced from Figure 2c,d. These results indicated that C12-NH2 underwent a larger extent of pyrolysis than C11-CHO under current experimental condition and is thus more suitable to be used as the starting material. For this reason, in the following we will focus our attention on the C-dots from aliphatic amines. Factors Determining the Quality of the C dots. To make clear the factors determining the quality of the C dots, a survey was made by using C12-NH2 as a typical starting material. Although factors influencing the production of C dots in aqueous solutions have been extensively investigated,19-21,37 those in nonpolar organic solutions remain largely unexplored. First, the influence of the pyrolysis time was investigated. Figure 2e gives photos of samples obtained by refluxing 1.0 molL-1 C12-NH2 in CB for different times. It can be seen that the samples obtained after refluxing for 48 h and 24 h are obviously yellowish, indicating the successful production of C dots. Samples with refluxing time below 24 h are colorless, which is indicative of an incomplete pyrolysis. From the emission curves recorded at different ex (Figure S6) and corresponding statistics (Figure 2f), a continuous bathochromic shift of em from 456 nm to 472 nm was observed with increasing refluxing time. The PL intensity first increases, reaching a maximum at 24 h and then drops at 48 h. Next, the influence of the concentration of C12-NH2 (cC12-NH2) was investigated at a fixed pyrolysis time of 24 h. Figure 2g shows the statistics of the mass obtained and yield for each sample. It was found that the mass obtained did not always increase with increasing cC12-NH2. Instead, it drops when cC12-NH2 exceeds 1.0 molL-1. The yield, which is calculated by the ratio of the weight of purified C dots (see below for the purification) to that of the starting material (i.e., ACS Paragon Plus Environment

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C12-NH2), decreases continuously with increasing cC12-NH2. The influence of the alkyl chain length was also investigated by refluxing 1.0 molL-1 C8-NH2, C12-NH2 and C16-NH2 in CB for 24 h. The results are summarized in Figure 2h and Figure S7. It was found that a decrease in the alkyl chain length leads to an improved PL intensity and a hypochromatic shift of em. On the contrary, an increase in the alkyl chain length causes a decrease in PL intensity and a bathochromic shift of em. Thus, it can be concluded that the yield and PL properties of the C dots could be adjusted by screening the starting material and/or by optimizing the experimental condition.

(a)

1#

2#

1#

3#

2#

(c)

3#

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Figure 3. (a) Photos of three samples (1#-3#) separated by GPC on the as-prepared C dots by refluxing 1.0 molL-1 C12-NH2 in CB for 24 h. The samples were organized following the order of elution. Samples 1#-3# are obtained by drying samples 1#-3# followed by redispersion in CB with a concentration of 1.0 mgmL-1. (b-d) Properties in solution of the C dots prepared by refluxing 1.0 ACS Paragon Plus Environment


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molL-1 C12-NH2 in CB for 48 h followed by purification with silica gel column chromatography using ethanol as an eluent. (b) Photos of 3.0 mgmL-1 C dots in solvents with varying polarity. (c) Emission at varying ex of a 5.0 mgmL-1 ethanol solution. (d) PL intensity at ex = 390 nm of the ethanol solutions with different concentration. (e) Time-resolved PL decay curves at ex = 390 nm in CB and DMF (5.0 mgmL-1). Purified C dots from C12-NH2. Solution Properties. Similar with the C dots from 3,4,5-trioctadecyloxybenzaldehyde, those from C12-NH2 can be also well separated from the starting material with GPC. Figure 3a shows photos of three samples (denoted as 1#-3#) separated by GPC, which were organized following the order of elution, on the as-prepared C dots by refluxing 1.0 molL-1 C12-NH2 in CB for 24 h. From the emission curves recorded at different ex and corresponding statistics (Figure S8), a continuous hypochromatic shift of em was observed with increasing Rt, which is consistent with the phenomenon observed from the separation of C dots from 3,4,5-trioctadecyloxybenzaldehyde (see Figure 1c). The as-prepared samples from GPC were dried, from which solutions with the same concentration (1.0 mgmL-1) were prepared (Figure 3a, 1#-3#). It was found that the sample with a shorter Rt has a heavier color. which is indicative of a larger extent of pyrolysis. As GPC works mainly based on the size, the observations mentioned above indicate that the PL properties of the C dots are influenced by their sizes, which is also consistent with previous reports.38,39 Interestingly, we found that the as-prepared C dots can be also purified by silica gel column chromatography. With ethanol as an eluant, the fluorescent fraction run fast on thin layer chromatography (TLC) with a Rf value approaching 1 (Figure S9). The C dots preferentially show blue emission on TLC without tailing, indicating that the differences in the polarities are limited. This is quite different with the C dots prepared previously in water where multiple components could be separated after silica gel column chromatography.40 After dried, the purified C dots can be ACS Paragon Plus Environment

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easily dissolved in a variety of organic solvents including hexane, toluene, DCM, dioxane, and N,N-dimethylformamide (DMF), but they only show limited solubility or are insoluble in solvents with high polarities such as dimethyl sulfoxide (DMSO) and water. The solubility of the C dots in most of the solvents is quite high. For example, in ethanol the value is determined to be over 50 mgmL-1, which is over 6.4 % by weight. From photos under irradiation of 365 nm UV, all the solutions show blue emission. The emission curves at different ex for an ethanol solution of 5.0 mgmL-1 (Figure 3c) showed wavelength-dependent PL properties, and the maximum emission occurs at ex = 390 nm with a em of 456 nm. The similar PL character of the purified and as-prepared sample indicates that the starting material and the small fractions involved in the as-prepared sample have negligible influence on the PL properties. This situation also differs from the case by using mixtures of organic compounds as starting materials. Typically in systems containing citric acid and amines,18-21 the components are quite complicated, among which small molecules with strong photoluminescence have been detected.19.20 In current system, the number of fluorescent species seems much smaller, presumably due to the much simpler structure of the starting material. Measurements on samples with different concentrations at a ex of 390 nm showed that the PL intensity increases first, reaches a maximum at a concentration of 5.0 mgmL-1 and then drops (Figure 3d). Meanwhile, a slight bathochromic shift was observed with increasing concentration of the C dots. The PL properties of the C dots in different solvents were also examined by static fluorescence measurements. The results are summarized in Figure S10 and Table S1. At a concentration of 3.0 mgmL-1, the maximum emission occurs at the same excitation (390 nm) in hexane, toluene, DCM, dioxane, ethanol and DMF. em varies between 452.5 nm and 462 nm, with the sample in hexane and in DMF being the smallest and highest value, respectively. The maximum PL intensity follows the order of DMF > dioxane > DCM > ethanol > toluene > hexane. The PL property in DMSO shows larger changes where em exhibits 23.5 nm ACS Paragon Plus Environment


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hypochromatic shift compared to that recorded in ethanol. In H2O, no meaningful emission could be recorded due to the too low solubility of the C dots. Table 1. The lifetime and quantum yield () of the purified C dots in DMF and CB. The raw C dots were prepared by refluxing 1.0 molL-1 C12-NH2 in CB for 48 h followed by purification with silica gel chromatography using ethanol as an eluent.

1 / ns

2 / ns

/ ns

2



in DMF

1.84 (50.71%)

8.92 (49.29%)

5.33

1.170

10.83%

in CB

2.33 (51.75%)

7.58(48.25%)

4.86

1.028

9.03%

The PL properties of the purified C dots were further investigated by time-resolved fluorescence measurements. Two solvent systems with different polarities have been chosen, i.e., DMF and CB. As shown in Figure 3e, the C dots exhibit a biexponential decay with an averaged lifetime () of 5.33 ns in DMF and 4.86 ns in CB. The  was determined to be 10.83% in DMF, which is slightly larger than that in CB (9.03%). The parameters of the purified C dots are summarized in Table 1. Structural Features. The structural features of the C dots purified by silica gel column chromatography were fully investigated, and the results are summarized in Figure 4. The size and morphology of the C dots were checked by HRTEM observations. From the typical image shown in Figure 4a, the diameters of the C dots range from 1.5 to 4.0 nm with an average value of 2.4 nm. XRD measurement indicates that the cores of the C dots are of some order characterized by the peaks around 20o, 24o and 40o (Figure S11). However, to obtain clear lattice structures of the C dots is difficult because the C dots are very sensitive to the electron beam due to the presence of peripheryl aliphatic chains. TGA curve shows that the C dots are stable up to 130 oC, after which the C dots gradually decompose and only trace amount of solid remains (below 3 wt%) at a temperature of 475 C (Figure 4b).


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FTIR spectrum of the C dots was recorded with close comparison to that of C12-NH2. The spectra in the range of 3500-2700 cm-1 are given in Figure 4c. For C12-NH2, the asymmetric and symmetric methylene stretching locates at 2920 and 2854 cm-1, respectively, which shift to 2926 and 2856 cm-1 in the C dots. This hypochromatic shift indicates that the alkyl chains are more disordered in the C dots.41,42 The stretching vibration of free N-H, which locates at 3334 cm-1 for C12-NH2, disappears in the C dots. This result indicates that chemical reaction occurs for the -NH2 group during pyrolysis, which is expected to play a key role in the formation of C dots. In addition, differences have been observed between the spectrum of C12-NH2 and that of C dots in the range of 1730-600 cm-1 (Figure S12). The medium band at 720 cm-1 in C12-NH2 can be attributed to the shear vibration of methylene in the alkyl chain whose carbon atoms are no less than four.43,44 This band still exists at the same place in C dots, indicating the reservation of the alkyl chain during pyrolysis. The peak at 1651 cm-1 can be attributed to the stretching vibration of amide I band.45 This band becomes stronger and broader in C dots, indicating that the -NH2 groups have converted, at least partially, to amide bonds during pyrolysis. (b)100

(a)

(c)

(d)

C12-NH2

60 97.6%

40 20

1

2 3 4 Diameter / nm

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100 nm

100 200 300 400 500 600 700 T / C

Solvent peak

(e)

9

5

TMS

7

(f)

5

3

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Intensity / a.u.

Transmittance / a.u.

80

Weight / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C dots

3400 3200 3000 2800 -1 Wavenumber / cm

TMS

120

 / ppm

80

40

-1

N-(C)3

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Solvent peak

160

Raman Shift / cm

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C=N

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C-O-C

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C=O

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282

538

536

534

532

530

Binding Energy / eV

528

406 404 402 400 398 396 394

Binding Energy / eV

Figure 4. Structural characteristics of the C dots purified by silica gel chromatography using ethanol as an eluent. The raw C dots were obtained by refluxing 1.0 molL-1 C12-NH2 in CB for 48 ACS Paragon Plus Environment


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h. (a) TEM image and size distribution of the C dots dispersed in ethanol. (b-d) TGA (b), FTIR (c) and Raman spectra (d). (e,f) 1H NMR (e) and 13C NMR (f) of the C dots dispersed in CDCl3. (g-i) High-resolution of the XPS spectra of C1s (g), O1s (h) and N1s (i) of the C dots. The Raman spectrum of the C dots is shown in Figure 4d, from which a variety of peaks were observed. This character is typical for carbonous materials bearing functional groups. Two bands at 1360 and 1575 cm-1 can be distinguished, which can be assigned to the D band and G band. Typically, the appearing of the G band indicates that there are graphitic carbons in the C dots.27,40,46,47 This conclusion gains further proof from NMR measurements. From 1H NMR (Figure 4e), besides the peaks from alkyl chains which locate in relatively high magnetic field, abundant peaks were also observed in downfield which are typical for protons attached to -conjugated unit. From 13C NMR (Figure 4f), while the peaks below 100 ppm are from sp3 carbons, those above 100 ppm should originate from sp2 ones. All of these evidences confirm that during pyrolysis, the aliphatic amine has been converted to more complicated, -conjugated structures. The chemical composition of the C dots were further analyzed by XPS and elemental analysis. From XPS spectrum (Figure S13), presence of carbon, nitrogen and oxygen was confirmed. The high-resolution spectra of each peak with deconvolution results are given in Figure 4g-i. For C1s (Figure 4g), four main peaks were obtained at 284.8, 286.0, 286.3 and 289.1 eV, which are attributed to C-C, C-N, C-O and C=N/C=O, respectively. For O1s (Figure 4h), two peaks were obtained at 531.2 and 533.5 eV, which are attributed to C=O and C-OH/C-O-C, respectively. For N1s (Figure 4i), two peaks were obtained at 400.3 and 401.3 eV, which can be ascribed to the N-(C)3 and N-H, respectively.48 Result from elemental analysis shows that the weight percentages of C, H and N are 79.08%, 12.14% and 3.52%, respectively, which give a content of O of 5.26% after calculation.


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PL Properties in Solvent-Free State. For most water-soluble C dots, fluorescence quenching will occur when forming aggregates or in solvent-free state. However, in special designs where the cores of the C dots are effectively separated, fluorescence quenching could be avoided. A typical example is the C dots which are functionalized with imidazolium cations and bear NTf2- as the counter-anions.49,50 In current case, the cores of the C dots are surrounded by alkyl chains, which are expected to separate the cores of the C dots and suppress aggregation. This assumption gains support from visual observations. As seen from the insets of Figure 5a, both the as-obtained C dots in the round-bottom flask and the purified C dots prepared on quartz can emit strong fluorescence. To get more details, the optical properties of the C dots in solvent-free state have been investigated with close comparison to those obtained in ethanol. UV-vis measurements showed that the n-π* transition at 280 nm,51-53 which is sharp in a good solvent such as ethanol, becomes unconspicuous in solvent-free state (Figure 5a). In addition, the absorption within the visible and near infrared regions enhances greatly. PL measurements indicate that the ex at which the maximum emission was observed shifts from 390 nm in ethanol (see Figure 3c) to 370 nm in solvent-free state (Figure 5b). On the contrary, an opposite trend of em was noticed, i.e., it changes from 456 nm in ethanol to 462 nm in solvent-free state. Besides, the curves become unsmooth in solvent-free state, which are different from the smooth ones observed in ethanol. It is clear that although the C dots retain the PL property in solvent-free state, subtle changes occur both for the absorption and emission. We suppose that this is partially caused by the concentration effect. As the C dots show wavelength-dependent emission, an increase in concentration will cause a bathochromic shift of the emission spectra, which has been well-documented in the literatures and also been confirmed by the results obtained in current work (see Figure 3d). Based on the concentration-dependent emission, multicolor luminescent ionogels



can be developed by simply adjusting the thickness of the hybrid material.54 Of course, the possible interactions among the cores of the C dots can not be excluded at the moment. (a)

0.8

ex / nm

(b)

330 340 350 370 390 400 410

0.4

PL Intensity

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Absorbance

I: on quartz (solvent-free) -1 II: in ethanol (0.2 mgmL ) I

0.2 II

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400

500

600

Wavelength / nm

700

800

(c)

350

(e)

400

450

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433 nm

I

II

III

IV

(f)

I

600

cC-dots / wt%

write

(d)

550

Wavelength / nm

0 0.05 0.1 0.5 1.0 3.0

PL Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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II

350

400

450 500 em / nm

550

600

Figure 5. Properties of C dots in solvent-free state and in composite films. The C dots used are the same as in Figure 4. (a) UV-vis absorption of C dots on quartz and in ethanol. Insets are photos of as-purified C dots in a round-bottom flask (left) and on quartz (right). (b) Emission at different ex of the C dots on quartz. (c) Photos of the letters written by a gel ink pen on silica plate taken under 365 nm UV irradiation. (d) Photos of PMMA films doped with 0.05 wt% (I), 0.1 wt% (II), 0.5 wt% (III) and 1.0 wt% (IV) C dots. The thickness of the film is 0.230.03 mm. (e) Emission at different ex of the C dot-doped PMMA films. (f) Photos of PDMS films doped with 0.13 wt% C dots with a thickness of 1.0 mm (I) and 2.0 mm (II). Applications as PL Inks and in Polymer-Based Composite Materials. The emission of the C dots in solvent-free state makes them an ideal candidate for the preparation of PL inks. To obtain an ink with a suitable viscosity, a mixture of solvents was selected which contains ethanol and glycerol ACS Paragon Plus Environment

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with a volume ratio of 1:1. The concentration of the C dots is 3.0 mgmL-1. The ink can be facilely filled in a gel ink pen which is commercially available. When letters were written on a silica gel plate, they exhibit a bright emission under 365 nm UV irradiation (Figure 5c). The C dots are rich in aliphatic chains, which makes them highly compatible with a variety of polymers. In current work, two typical C dot/polymer composite films were prepared. The first series are C dot/PMMA composite films with a fixed thickness (0.230.03 mm) and varying weight percentage of C dots. Figure 5d gives photos of four typical films under 365 nm UV irradiation which contains 0.05, 0.1, 0.5 and 1.0 wt% C dots, respectively. One can see that the film with the lowest content of C dots (i.e., 0.05 wt%) is already luminescent. With increasing content of C dots, the color of the film becomes heavier. This phenomenon can be also confirmed by PL measurements, as shown in Figure 5e, from which one can see that the PL intensity continuously increases with increasing content of C dots (up to 3.0 wt%). It should be noted that at the moment, the content of C dots in PMMA has not reached the upper limit due to their high compatibility with the polymer matrix, which is advantageous over most of other C dots which are hardly or only slightly compatible with polymers. From Figure 5e, one can also see that the emission of the C dots in PMMA becomes wavelength-independent, which is different from that of C dots in ethanol and solvent-free state. Besides, peak splitting was observed where a main peak at 433 nm was detected together with two shoulder peaks at 410 and 460 nm, respectively. Compared to the cases in ethanol and solvent-free state, a hypochromatic shift of the main emission peak was confirmed. These interesting spectral changes should be ascribed to the interactions between C dots and the polymer matrix. Preparation of the C dot/PDMS composite films was also tried. In this case, the content of C dots was fixed at 0.13 wt% and the thickness of the film was varied. Due to the different properties between PMMA and PDMS, the method adopted in film preparation is different (for details, see ACS Paragon Plus Environment


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experimental section). Figure 5f gives photos of two typical films under 365 nm UV irradiation with a thickness of 1.0 and 2.0 mm. One can see both films appear deep blue emission, which indicates that the C dots are also compatible with PDMS matrix.

CONCLUSIONS In summary, we have shown that organic molecules with only one functional group, such as aliphatic amine and aliphatic aldehyde, can be also used as carbon sources for the preparation of C dots. This finding greatly expands the scope of the starting materials adopted in the research of C dots, which are limited to compounds with multiple and heterogeneous functional groups. The C dots were obtained through pyrolysis at moderate temperature without any other reagent, providing a new method to facilely obtain hydrophobic C dots. The as-prepared C dots can be further purified by either GPC or silica gel column chromatography. They exhibit blue emission with wavelength-dependent PL characteristics and are highly soluble in a variety of organic solvents. Due to the segregation of the PL center by the alkyl chains, fluorescence quenching is avoided and the C dots are photoluminescent in solvent-free state, making them good candidates for applications as fluorescent inks. They are also compatible with polymers including PMMA and PDMS, opening the door for the applications in fluorescent sensing and optoelectronic devices. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Photos of CB before and after reflux, additional PL spectra of C dots from different starting materials and in different solvents, photo of the TLC plate with the C dots and C12-NH2 eluted by ethanol, XRD, FTIR and XPS spectra of the C dots from C12-NH2. Acknowledgements


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The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21875129, No. 51835012, No. 51602317, No. 51805515) and the Shandong provincehigher education science and technology program (J16LA01). Appendix A. Supplementary data Supplementary data to this article can be found online at https://XXXX References (1) Baker, S.; Baker, G. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem. Int. Ed. 2010, 49, 6726-6744. (2) Zheng, X.; Ananthanarayanan, A.; Luo, K.; Chen, P. Glowing Graphene Quantum Dots and Carbon Dots: Properties, Syntheses, and Biological Applications. Small 2015, 11, 1620-1636. (3) Qi, B.; Bao, L.; Zhang, Z.; Pang, D. Electrochemical Methods to Study Photoluminescent Carbon Nanodots: Preparation, Photoluminescence Mechanism and Sensing. ACS Appl. Mater. interfaces 2016, 8, 28372-28382. (4) Lim, S.; Shen, W.; Gao, Z. Carbon Quantum Dots and Their Applications. Chem. Soc. Rev. 2015, 44, 362-381. (5) Wang, R.; Lu, K.; Tang, Z.; Xu, Y. Recent Progress in Carbon Quantum Dots: Synthesis, Properties and Applications in Photocatalysis. J. Mater. Chem. A 2017, 5, 3717-3734. (6) Xu, X.; Ray, R.; Gu, Y.; Ploehn, H.; Gearheart, L.; Raker, K.; Scrivens, W. Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126, 12736-12737. (7) Sun, Y.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K.; Pathak, P.; Meziani, M.; Harruff, B.; Wang, X.; Wang, H.; et al. Quantum-Sized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756-7757. (8) Liu, H.; Ye, T.; Mao, C. Fluorescent Carbon Nanoparticles Derived from Candle Soot. Angew. Chem. Int. Ed. 2007, 46, 6473-6475. (9) Tian, L.; Ghosh, D.; Chen, W.; Pradhan, S.; Chang, X.; Chen, S. Nanosized Carbon Particles from Natural Gas Soot. Chem. Mater. 2009, 21, 2803-2809. (10) Wu, M.; Wang, Y.; Wu, W.; Hu, C.; Wang, X.; Zheng, J.; Li, Z.; Jiang, B.; Qiu. J. Preparation of Functionalized Water-Soluble Photoluminescent Carbon Quantum Dots from Petroleum Coke. Carbon, 2014, 78, 480-489. ACS Paragon Plus Environment


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Hydrophobic Carbon Dots from Aliphatic ... - ACS Publications

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