KR20160149727A

KR20160149727A - Surface engineered graphene quantum dots and synthesizing method of the same

Info

Publication number: KR20160149727A
Application number: KR1020150087265A
Authority: KR
Inventors: 김범준; 조한희
Original assignee: 한국과학기술원
Priority date: 2015-06-19
Filing date: 2015-06-19
Publication date: 2016-12-28

Abstract

The present invention relates to surface-modified graphene quantum dots and a synthesizing method thereof. According to the present invention, a surface of graphene quantum dots can be modified through a single step reaction of coupling and/or ring cleavage at the relatively low temperature. In addition, according to the present invention, a hydrophilic group and a hydrophobic group of the graphene quantum dots can be appropriately regulated, and the present invention can be applied as an effective surfactant because of being stable for several months.

Description

Surface-modified graphene quantum dots and synthesizing method of the same [

The present invention relates to surface-modified graphene quantum dots and methods for their synthesis. Specifically, the present invention relates to a surface modification method for controlling the hydrophobic or hydrophilic surface activity of a graphene quantum dot, and the graphene quantum dot synthesized according to the present invention can be effectively used as a surfactant.

Nanoparticles (NPs) with modified surface properties can control the interfacial properties of two immiscible polymers or fluids. Thus, it can act as an efficient surfactant and produce new structured materials such as bicontinuous solar cells, membranes for catalyst supports, photonic bandgap materials and asymmetric-structured particles (Binks, BP, Colloid Interface Sci . 2002, 7, 21-41; Boker, A. et al ., Soft Matter 2007, 3, 1231-1248; Kwon, T. et al ., ACS Macro Lett . 404; Kim, J. et al ., J. Am. Chem. Soc . 2010, 132, 8180-8186). Unlike conventional organic surfactants, NP surfactants have interesting photonic, magnetic, electrical and catalytic properties, which can be combined with polymeric or fluid matrices to produce synergistic effects. Also, due to the quasi-irreversible adsorption at the interface between the blends by the particle surfactant, highly stable emulsions can be easily formed (Hore, MJA et al ., Macromolecules 2013, 47, 875-887; Srivastava, S. et al ., Adv. Mater . 2014, 26, 201-234; Melle, S. et al ., Langmuir 2005, 21, 2158-2162). However, the NP surfactant has difficulty in controlling the position of dispersion in the matrix due to the difficulty of surface modification, and thus shows a limit to the efficiency (Srivastava, S. et al ., Adv. Mater . 2014, 201-234; Hong, RY et al, J. Appl Polym Sci 2007, 105, 2176-2184;....... Shenhar, R. et al, Adv Mater 2005, 17, 657-669). Therefore, in order to precisely control the surface properties of NPs, an appropriate method should be developed. However, each type of NPs is difficult because it has inherent surface properties that require different methods to adjust the surface chemistry (Caruso, F., Adv. Mater . 2001, 13, 11-22).

Nanometer sized graphite derivatives, referred to as graphene quantum dots (GQDs), have received academic attention due to their excellent physical, mechanical and photoelectrical properties (Zhang, Z. et al ., Science . 2012, 5 , 8869-8890; Cheng, H. et al , ACS Nano 2012, 6, 2237-2244;. Zhuo, S. et al, ACS Nano 2012, 6, 1059-1064;.. Zheng, XT et al, ACS Nano 2013, 7, 6278-6286). It consists of a hydrophobic base plane similar to the well known graphene oxide (GO), but has a larger surface area per unit volume than a micron sized GO (Kim, J. et al ., J. Am. Chem. Soc . 2010, 132, 8180-8186; Zhu, Y. et al ., Adv. Mater . 2010, 22, 3906-3924; Dreyer, DR et al ., Chem. Soc. Rev. 2010, 39, 228-240). Thus, GQDs have the potential to be used as effective surfactants, suitable for the development of ultrafine emulsions or colloidal particles. In addition, distinct properties such as adjustable luminescent emission, biocompatibility and long term resistance to photobleaching enable the promising application of GQD-stabilized emulsions, for example in applications such as bio-imaging and fluorescence sensors (Zhang , Zhuo, S. et al ., ACS Nano 2012, 6, 1059 (1995), Zhen et al ., Science , 2012, 5, 8869-8890; Cheng, H. et al ., ACS Nano 2012, 6, 2237-2244; Zhen, XT et al ., ACS Nano 2013, 7, 6278-6286). However, GQDs are rarely used as solid surfactants because it is difficult to regulate surface properties in a systematic manner with high reproducibility (Yang, H. et al ., ACS Macro Lett . 2014, 985-990). Unlike the GO sheet, the surface of the GQDs is highly hydrophilic due to excessive oxygen functionality on the basal plane and rim, which limits solubility in organic solvents (Kim, J. et al ., J. Am. Chem. Soc ., 2010, 132, 8180-8186; Zhu, Y. et al ., Adv. Mater . 2010,22, 3906-3924; Dreyer, DR et al ., Chem. ). Recently, it has been reported that the surface of GQDs can be modified by a chemical-grafting approach (Zhu, S. et al ., Adv. Funct. Mater . 2012, 22, 4732-4740; Tetsuka, H et al ., Adv. Mater . 2012, 24, 5333-5338; Qian, Z. et al ., RSC Adv ., 2013, 3, 14571-14579). Nevertheless, chemically modified GQDs should be used as surfactants, since accurate and systematic control of the surface properties of GQDs by the chemical graft approach has not been achieved.

Accordingly, the present inventors have completed the present invention based on experiments to develop a surface modification method for applying stable nano-sized GQDs as a surfactant.

Accordingly, it is an object of the present invention to provide a method for synthesizing a graphene quantum dot having a hydrophilic surface and a hydrophobic surface modifiable surface, which can be applied as a stable and effective surfactant, and a graphene quantum dot synthesized thereby.

In order to accomplish the above object, the present invention provides a process for preparing a mixture comprising: a) preparing a mixture of a compound containing a hydrophobic group in a solvent, DMAP (4-dimethylaminopyridine) and a graphene quantum dot dissolved therein; b) adding DCC (N, N'-dicyclohexylcarbodiimide) to the mixture of step a), heating and cooling to room temperature; c) filtering and precipitating the cooled mixture in step b) to obtain a precipitate; And d) washing the precipitate obtained in the step c) with diethyl ether to remove unreacted materials. The present invention also provides a method for synthesizing Graphene quat dots (GQDs).

In the present invention, the graphene quantum dot is a nanometer-sized graphite derivative. Specifically, the graphene quantum dot refers to a material having a dot size of 10 nm or less so that quantum phenomenon may occur in order to convert graphen, which is a conductive material, into a semiconductor form. (Zhang, Z. et al ., Science , 2012, 5 (1), 5), which has been applied to various fields such as biosensors, photosensors, and bioimaging due to its physical and mechanical photoelectric properties including its unique quantum mechanical properties due to its small size , 8869-8890). Previously, however, it was not easy to control the surface properties in a systematic manner, and it was rarely used as a solid surfactant. Therefore, in the present invention, a relatively simple and effective method for modifying the graphene quantum dot surface has been developed and applied as a surfactant.

In the present invention, the synthesis method involves a single-step opening and grafting onto the surface of GQDs. Specifically, the surface-unmodified raw GQDs used in the present invention were prepared by chemical oxidation (Dong, Y. et al ., J Mater Chem 2012, 22, 8764-8766) Containing groups such as a hydroxyl group, a hydroxyl group, a hydroxyl group, a hydroxyl group, a hydroxyl group, a carboxyl group, a hydroxyl group, a carboxyl group, a carboxyl group and a hydroxyl group. The surface modification according to the invention also takes place via a single step reaction of DCC coupling with the carboxyl group of these original GQDs and / or opening of the epoxy moiety (see example 1.2).

In one embodiment of the present invention, the compound containing the hydrophobic group in step a) may be an alkylamine having 1 to 6 carbon atoms.

In the present invention, the alkylamine means a compound in which one to three hydrogens in the alkyl group are substituted with an amine group, and specifically includes alkylamine having 1 to 6 carbon atoms. In the present invention, the surface of graphene quantum dots is modified by appropriately adding a compound containing a hydrophobic group. When the number of carbon atoms is 6 or more, the hydrophobic property becomes too strong. On the other hand, if the number of carbon atoms is too small, the electrical properties become poor. Therefore, an alkylamine having an appropriate carbon number should be added according to the purpose.

In one embodiment of the present invention, the solvent of step a) may be THF (tetrahydrofuran) or DMF (N, N-dimethylformamide).

In one embodiment of the present invention, the compound including a hydrophobic group and the graphene quantum dot in the step a) may be mixed at a weight ratio of 1:10 to 10: 1, but preferably 1: 4 to 4: 1 Can be mixed in a weight ratio.

In the present invention, as the ratio of the hydrophobic group-containing compound increases, the surface-modified GQDs exhibit hydrophobicity. Particularly stable when the hydrophobic group-containing compound and the graphene quantum dots are mixed at a ratio of 3: 4 See example 2)

In one embodiment of the present invention, heating in step b) may be performed at 30-50 ° C for 10-15 hours, but preferably at 40 ° C for 12 hours.

Also, the present invention provides a graphene quantum dot having a surface modified by the above method.

The present invention also provides a surfactant comprising a graphene quantum dot modified by the above-described method.

In the present invention, the surfactant is a substance that adsorbs to an interface, which is an interface at which two other materials are in contact with each other, and reduces the surface tension thereof. The surfactant is a substance that reduces the surface tension of liquids, liquids, liquids and gases, liquids and solids, Applicable in all phases. In the present invention, the surfactant specifically refers to a solid nanoparticle surfactant, which controls the interfacial properties of two unmodified polymers or fluids. The nanoparticle surfactant according to the present invention is useful for the production of materials such as solar cells, films for catalyst supports, and photonic bandgap materials due to its magnetic and electrocatalytic properties (Binks, BP Colloid Interface Sci . 2002, 7, 21 -41)).

According to the present invention, it was confirmed that the surface of the graphene quantum dot can be modified through a single step reaction of coupling and / or exchange at a relatively low temperature. Further, according to the present invention, the hydrophilic group and the hydrophobic group of the graphene quantum dot can be appropriately regulated, and it is possible to apply the surfactant effectively and stably for several months.

Figure 1 shows the XRD profile (Figure 1a) and Raman spectrum (Figure 1a) under a 514 nm laser of GH0 , respectively.
Figure 2 shows the preparation route of surface modified GQDs having hexylamines. The photograph shows the solubility of GQDs.
FIG. 3A is a transmission infrared spectrum of GH0 , GH25 , GH50 , GH75 and GH100 , and the illustration shows a vector-normalized transmission infrared spectrum for a specified peak of a carboxyl group of about 1720 cm ^-1 .
Figure 3b shows the vector-normalized transmission infrared spectra of GH25 , GH50 , GH75 and GH100 ranging from 3200 to 2600 cm <" ¹ >.
Figure 4 shows the carbon 1s XPS spectrum of GH0 (Figure 4a) and GH100 (Figure 4b), respectively, and Figure 4c shows the nitrogen 1s XPS spectrum of surface modified GQDs.
Figure 5 shows an atomic force microscope (AFM) image of GQDs and a corresponding histogram of the size distribution. Correspond to GH0 (Fig. 5A), GH25 (Fig. 5B), GH50 (Fig. 5C), GH75 (Fig. 5D) and GH100 (Fig.
Figure 6a is a graph showing the average height of synthesized GQDs measured by AFM and Figure 6b is a graph showing the chemical structure of the model GQD system with two grafted hexylamines on the base plane and its equilibrium geometry Fig.
Figure 7 is a histogram of transmission electron microscopy (TEM) images of GQDs and corresponding size distributions. Each GH0 (FIG. 7a) (sapdo: HRTEM image of GH0), GH25 (Fig. 7b), GH50 (Fig. 7c), GH75 (Fig. 7d): a graph of the (sapdo HRTEM image of a GH75) and GH100 (Fig. 7e) .
8 is a graph showing the average size of synthesized GQDs measured from a TEM image.
Figure 9 is a photograph showing the solubility of modified GQDs in a water / chloroform solvent system, in which 3 mg of each kind of five different GQDs were separately dissolved in a mixture of 2 mL water / chloroform (1: 1 v / v) .
10 is a photograph showing the solubilities of GH50 , GH75 and GH100 soluble in DCM, toluene, CB and DCB, respectively.
Figure 11a is a photograph of a Pickering emulsion stabilized by GH25 , GH50 and GH75 , and is a DCB-in-water pickling solution stabilized by GH25 (Figure 11b), GH50 (Figure 11c) and GH75 Is an optical microscope image of the emulsion. GH0 and GH100 could not stabilize Pickering emulsion. The illustration in Figures 11b-d shows the size distribution and average size of the emulsion droplets.
FIG. 12 shows the SEM image size distribution of PS colloidal particles prepared by mini-emulsion polymerization, using GH25 (FIG. 12A), GH50 (FIG. 12B) and GH75 (FIG. The illustration in Figures 12A-C shows the size distribution and average size of PS colloidal particles.
13 is a UV-visible absorption spectrum (FIG. 13A) and a PL emission spectrum (FIG. 13B) of GH25 , GH50 and GH75 excited at 365 nm wavelength.
14 is optical (Fig. 14A) and fluorescence microscope images (Fig. 14B) of PS colloidal particles stabilized by GH25 , and the excitation wavelength is 450 nm.
Figure 15 is a photograph of graphite powder in MeOH after several months with no dispersant (Figure 15a) and with GH0 , GH25 , GH50 , GH75 and GH100 as dispersants (Figure 15b).
Figures 15c-e also show optical microscope images and size distribution histograms of the graphite dispersion using GH25 (Figure 15c), GH50 (Figure 15d) and GH75 (Figure 15e), respectively.

Hereinafter, the present invention will be described in detail with reference to examples. However, these examples are intended to further illustrate the present invention, and the scope of the present invention is not limited to these examples.

<Experimental Examples> Preparation and characterization of experimental materials

1. Experimental material

Vulcan CX-72 carbon black was purchased from Cabot Corporation. Styrene, hexylamine, DCC, 4-dimethylaminopyridine (DMAP) and toluene were purchased from Aldrich. Prior to the emulsion polymerization, styrene was purified by passing through an alumina column. Azobisisobutyronitrile (AIBN) was purchased from Junsei and purified by recrystallization from ethanol. Deionized water was used in all experiments.

2. Preparation of raw GQDs and surface modified GQDs

The raw GQDs were prepared by chemical oxidation (Dong, Y. et al ., J Mater Chem 2012, 22, 8764-8766). Briefly, CX-72 carbon black was refluxed with nitric acid (6 M) for 48 hours. Thereafter, the resulting mixture was cooled to room temperature and centrifuged at 4000 rpm for 20 minutes. The supernatant was carefully collected and the solvent was evaporated to give a reddish brown powder. GQDs were redispersed in tetrahydrofuran (THF) and filtered through an ultrafilter through a 0.22 μm microporous membrane.

Hexylamine (25 mg, 0.247 mmol), DMAP (60.3 mg, 0.494 mmol) and GQDs (100 mg) in the prepared state were dissolved in THF (20 mL) with nitrogen bubbling to produce surface modified GQDs. &Lt; / RTI > and the mixture was stirred for 30 minutes. DCC (203.3 mg, 0.988 mmol) was then added and the mixture was heated at 40 < 0 > C for 12 hours. After cooling to room temperature, the mixture was passed through Whatman # 1 filter paper. The filtrate was centrifuged and the precipitated product was washed several times with diethyl ether to remove unreacted hexylamine. The product was named GH25 . A similar procedure was used to synthesize GH50 , GH75 and GH100 with different weight ratios of hexylamines to the raw GQDs, i.e. 50, 75 and 100 wt%, respectively.

3. Preparation of pickling emulsion

A; (DCB o -dichlorobenzene) -; a solution of the modified GQDs ride GH0 and GH25 (3 mg) ion (DI deionized) can be dissolved in dichlorobenzene (2 mL) and 1 mL of the GH50, GH75, and GH100 o , Which was then sonicated for 10 minutes. Then, 1 mL of DCB was added to the GHO and GH25 solutions and 2 mL of water was added to GH50 , GH75 and GH100 to make the same total amount of solution (3 mL) for each sample. The solution was emulsified at 15,000 rpm for 5 minutes using a homogenizer.

4. Preparation of polystyrene colloidal particles

Polystyrene (PS) colloidal particles were prepared by non-uniform mini-emulsion polymerization. In a 50 mL vial, GQDs (7.5 mg), styrene (100 mg), octadecane (5 mg) and AIBN (4 mg) were dispersed in methanol (MeOH) and stirred for 20 minutes. The mixture was then sonicated for 10 minutes. The resulting mixture was transferred to an ampoule and then degassed three times before performing the radical reaction. The reaction mixture was stirred at a constant rate at 70 < 0 > C for 24 hours. After the polymerization, the PS colloidal particles were quenched by pouring the reaction mixture into MeOH at room temperature. The PS colloidal particles were separated by filtration and washed several times with a mixture of MeOH, MeOH / distilled water (50:50 v / v) and distilled water, and finally dried at room temperature.

5. Preparation of Dispersion of Graphite

For solid dispersion experiments, graphite powder (Asbury, 3763) and 3 mg of GQDs were added in 3 mL of MeOH in a weight ratio of 5: 1 (graphite / GQD). Thereafter, the dispersion was ultrasonically decomposed for 30 minutes using an ultrasonic mill. The solution was then centrifuged at 1,000 rpm for 5 minutes to remove the finely dispersed mass. The supernatant was carefully collected and stored at room temperature for several months. Thereafter, an image of the dispersed graphite was obtained by an optical microscope.

6. Characterization

The size of the GQDs and the shape of the polymer particles stabilized by the modified GQDs were measured by field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800), transmission electron microscopy (TEM) JEOL 2000FX) and an optical microscope (Nikon, Eclipse 80i). TEM samples were prepared by dropping an aqueous suspension of GQDs onto a Cu grid coated with a porous carbon film followed by solvent evaporation. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra were collected using Bruker ALPHA. X-ray photoelectron spectroscopy (XPS) was performed using Thermo VG Scientific ESCA 2000. An atomic force microscopy (AFM) image was obtained on a scanning probe microscope (Veeco). X-ray diffraction (XRD) was obtained using RIGAKU D / MAX-2500. Raman spectra were collected using LabRAM ARAMIS. The UV-visible absorption spectra and the photoluminescence (PL) emission spectra of the surface-modified GQDs were measured using a UV-1800 spectrophotometer (Shimadzu Scientific Instruments) and a Horiba Jovin Yvon NanoLog spectrophotometer, respectively. The emission quantum yields (QYs) of the surface-modified GQDs were calculated using the following equation (1) based on rhodamine B as a standard:

Where I is the measured integrated emission intensity,? Is the refractive index of the solvent, A is the optical density, the subscript "st" means a standard with a known quantum yield, "x "Means a sample.

&Lt; Example 1 > Measurement of properties of GQDs

According to Experimental Example 2, the original GQDs and the surface modified GQDs were synthesized, and their molecular characteristics were confirmed by the following method.

1.1 Observation of GQDs

The synthesized raw GQDs were labeled GHO . The resulting GHO showed a broad (002) XRD peak with a center of 21.1 DEG (0.419 nm interlayer spacing), similar to that from GQDs synthesized by thermal degradation (Fig. et al ., J Mater Chem 2012, 22, 8764-8766).

In addition, when GH0 was excited with a 514-nm laser, Raman spectra showed two distinct carbon-related bands, the D band at about 1356 cm ^-1 and the G band at about 1605 cm ^-1 , (Fig. 1B) (Cheng, H. et al ., ACS Nano 2012, 6, 2237-2244). GHO had various oxygen-containing groups such as epoxy, hydroxyl and carboxylic acid groups at the base plane and edges. The functional groups may provide a reactive site for interaction with organic molecules.

1.2 Observation of surface modified GQDs 1

Figure 1 shows that the surface chemistry of GQDs was systematically modified by varying the amount of hexylamine molecules. The molecule was chemically bonded onto the GQD surface by a single-step reaction of DCC coupling with the carboxylic acid moiety and / or opening of the epoxy moiety at a mild temperature of 40 ° C. As a result, four different kinds of GQDs were successfully obtained and they were designated as GH25 , GH50 , GH75 and GH100 according to the weight ratio of added hexylamine, i.e. 25, 50, 75 and 100 wt%, respectively.

ATR-FTIR measurements were performed to monitor the grafting reaction of hexylamines on the surface of GQDs (Figure 3). Several important vibration peaks of functional groups including carboxylic acid, epoxide and amide groups were monitored and compared. For GHO , strong stretching vibration of C = O was observed at 1720 cm ^-1 due to the carboxylic acid group at the edge of GQD. However, the intensity of the C = O stretching vibration gradually decreased in the order of GH25 , GH50 , GH75 and GH100 . Because the carboxylic acid groups were converted to amide groups, they exhibited new stretching vibration of C = O at 1644 cm ^-1 and bending vibration of NH at 1577 cm ^-1 . In addition, the stretching vibration of the secondary amine was observed at 3050-3150 cm ^-1 , whereas the epoxide band at 1035 cm ^-1 was reduced in strength due to the ring opening reaction with the hexylamine of the epoxide group. In addition, as the amount of grafted hexylamine increased in the order of GH25 , GH50 , GH75 and GH100 , the asymmetric stretching vibration peak at 2854 and 2927 cm ^-1 was increased. High resolution carbon 1s (C 1s) and nitrogen 1s (N 1s) XPS measurements were performed to further support the correct surface modification of GQDs (Figure 4). Table 1 below summarizes the atomic concentrations of the different GQDs measured by XPS.

The change between the GH0 and GH100 C 1s spectra showed a significant change in the ratio of carboxylic acid and amide groups. For GH100 , the C (O) -O peak at 289.4 eV disappeared, but the amide bond (C (O) -N) peak at 288.1 eV, as most of the carboxylic acid groups reacted with hexylamine. In the N 1s XPS spectrum, it was apparent that the total amount of nitrogen increased in the order of GH25 , GH50 , GH75 and GH100 , indicating that GQD surface properties were systematically controlled by successful grafting of hexylamine molecules onto GQDs.

1.3 Observation of surface modified GQDs 2

The average height of the surface modified GQDs was measured from the AFM image and the average size thereof was measured from the TEM image of each sample. As shown in Fig. 5, GH0 was monodisperse with an average height of 0.97 nm, which is mostly mono or bilayer (Zhu, S. et al ., Chem. Commun . 2011, 47, 6858-6860 ). Interestingly, upon grafting of hexylamine, the average height of GHlOO was sharply increased to 2.98 nm and showed a pronounced increase tendency in the order of GH25 , GH50 , GH75 and GH100 (Fig. 6A). The increase in average height was due to the chain length of hexylamine grafted onto the basal plane of GQDs, which can be well supported by molecular simulations (Zhu, S. et al ., Adv. Funct. Mater . 2012, 22 , 4732-4740).

The simulation was performed at the B3LYP / 6-31G (d, p) level on the basis of the GQD model with two hexylamines grafted on the base plane, using density functional theory (DFT) 6b). Since the chain length of hexylamine molecules extended in equilibrium geometry was about 1 nm, the calculated height of hexylamine-grafted GQDs was about 3 nm, which was consistent with our experimental results.

Also, the graft of hexylamine affected the size of GQDs. 7, the size was increased from 12.4 nm to 13.5 nm for GH0 on GH100, which demonstrates that the hexylamine molecule has been successfully grafted onto GQD corner. In addition, the calculated average size of the modified GQDs based on the simulation was increased when hexylamine molecules were grafted to the corners of GQDs , which is consistent with a sharp increase in size between GH0 and GH25 (FIG. 8). , The average size of the modified GQDs was gradually increased from 12.4 nm to 13.5 nm for GH0 on GH100 as the amount of the grafted hexylamine increases. However, the experimentally measured size (13.5 nm) of GH100 did not exactly match the calculated value. In the present invention, this is presumed that not all edge portions are grafted to the hexylamine chain, and that the grafted chains can be directed in different directions from the base plane of the GQDs. Also, as shown in the illustration of Figures 7a and d, the high resolution TEM image of GH0 and GH75 showed a crystalline structure with a lattice constant of 0.24 nm, corresponding to the (1120) lattice of graphene, Suggesting that the crystalline structure of GQDs was retained after amine grafting (Peng, J. et al ., Nano Lett . 2012, 12, 844-849).

&Lt; Example 2 > Effect of dissolution of modified GQDs

2.1 Modified surface chemical effect investigation

To investigate the effect of the modified surface chemistry on the solubility of GQDs, 3 mg of each of the five different GQDs was dissolved separately in a 2 mL water / chloroform (1: 1 v / v) mixture 9). While GH0 was only soluble in the water phase, most GH25 , GH50 and GH75 were located at the interface between the water phase and the organic phase, reducing the interfacial tension. On the other hand, GH100 was only soluble in organic phase. Thus, the surface properties of the modified GQDs were systematically controlled in the range from the hydrophilic surface to the hydrophobic surface. In addition, GH25 , GH50 , GH75 and GH100 were dispersed in other hydrophobic solvents such as dichloromethane (DCM), toluene, chlorobenzene (CB) and DCB to demonstrate the solubility of the surface modified GQDs. As shown in Figure 10, GH50 , GH75 and GH100 were clearly dispersed in the solvents without any aggregation. However, GH25 was only partially dispersed due to the relatively strong hydrophilic surface properties.

2.2 Possibility of use as surfactant

Next, droplets of micrometer sized organic solvent in water were generated to investigate the possibility of surface modified GQDs for their use as surfactants. For each of the five different GQDs, a ¹ mg.ml ^-1 GQD solution in a DCB / water (1: 2 v / v) mixture was emulsified with a homogenizer at 15,000 rpm for 5 minutes to produce an oil-in-water emulsion . After vigorous stirring, GH0 and GH100 remained in a single phase, i.e., in water and DCB, respectively, thus failing to form a pickling emulsion. Conversely, as shown in FIG. 11 bd, GH25 , GH50 and GH75 reduced the total interfacial energy by replacing the oil-water portion at the interface and resulted in a dispersion of oil droplets in water (Binks, BP, Colloid Interface Sci 2002, 7, 21-41; Boker, A. et al ., Soft Matter 2007, 3, 1231-1248).

Three months after the preparation of the emulsion, an optical microscope image was obtained. Most of the emulsified droplets were about 2-3 μm in diameter, indicating a high interfacial activity of GH25 , GH50 and GH75 . However, each sample had a different volume fraction of the residual emulsion; Respectively, with respect to the GH25, GH50 and GH75 9, 77 and 76% by volume (Fig. 11a) (He, Y. et al ., ACS Appl. Mater. Interfaces 2013, 5, 4843-4855). In addition, the GH50 and GH75 emulsions were stable for coalescence for at least several months. This is because the hexylamines grafted onto the GQDs significantly suppressed aggregation of the GQDs by reducing the strong? -Π interaction between the GQDs.

Example 3: Confirmation of use of surface-modified GQDs as a surfactant

3.1 Confirmation of amphibia of GQDs

The amphiphilic properties of GH25 , GH50 and GH75 make them suitable for use as surfactants in heterogeneous polymerization. Mini-emulsion polymerization of styrene was carried out at 70 캜 for 24 hours using GQDs as a surfactant. When GH0 and GH100 were used, no PS colloidal particles were produced, which is consistent with the results from Pickering emulsion. Conversely, mini-emulsion polymerization using GH25 , GH50 and GH75 produced spherical PS colloidal particles of ultra-fine size with a smooth surface, similar to the surface morphology of particles stabilized by conventional organic surfactants ( 12). This surface morphology differs from the rough surface morphology observed in PS colloidal particles formed when a large GO sheet is used as the stabilizer (Thickett, SC et al ., ACS Macro Lett . 2013, 2, 630-634; Che Man, SH et al ., J. Polym. Sci., Part A: Polym Chem ., 2013, 51, 47-58). All mini-emulsion polymerization conversions were greater than 90%, demonstrating the effectiveness of the surfactant behavior of GH25 , GH50 and GH75 . The average sizes of PS colloidal particles stabilized by GH25 , GH50 and GH75 were 655, 641 and 278 nm, respectively. In addition, the particle size distribution was narrowed in the order of GH25 , GH50 and GH75 , indicating that GH75 had the highest surface activity of the GQDs used in this study due to well-balanced amphipathies.

3.2 Investigation of luminescence characteristics of GQDs

In addition, the tunable luminescent emission of GQDs can be synergistically combined with the properties of the surfactant for promising applications such as fluorescent labeling and bio-imaging. The inventors measured the UV-visible absorption spectra and the photoluminescence (PL) emission spectra of GH25 , GH50 and GH75 to investigate the luminescent properties of GQDs.

As shown in FIG. 13A, a strong absorption band for surface modified GQDs was observed at about 278 nm, which was attributed to the amide group at the corner of GQDs (Sandeep Kumar, G. et al ., Nanoscale 2014, 6, 3384 -3391).

Figure 13b shows the PL emission spectra of GH25 , GH50 and GH75 under excitation at 365 nm. All surface modified GQDs emitted green luminescence with a maximum emission peak of 540 nm, regardless of the amount of hexylamine grafted. This indicates that the PL emission of GQDs is due to the emission of excited electrons from the lowest unoccupied molecular orbital (LOMO) of the carbene at the zigzag edge to the highest occupied molecular orbital (HOMO) (Zhu, S. et al ., Adv. Funct. Mater . 2012, 22, 4732-4740; Tetsuka, H. et al ., Adv. Mater . 2012, 24, 5333-5338) .

Carbenes at the zigzag edges of the original GQDs were also expected to be well conserved after the hexylamine chains were grafted onto the corners of the GQDs, because the sites were not affected by surface modification. Thus, PL release was well observed in surface modified GQDs, GH25 , GH50 and GH75 . The QYs of surface modified GQDs were calculated based on rhodamine B as a standard. QYs of GH25 , GH50 and GH75 decreased slightly in the order of 1.91, 1.56 and 1.41%, respectively. The -CONHR and CNHR groups formed by surface modification induced non-radioactive recombination of the local electron-hole pairs, and the QY gradually decreased as the amount of grafted hexylamine increased (Zhu, S. et al . , Adv. Funct. Materials, 2012, 22, 4732-4740). To demonstrate the possibility of surface modified GQDs as fluorescent surfactants, optical and fluorescence microscopic images of PS particles stabilized by GH25 were measured. Figure 14 shows that the PS colloidal particles stabilized by GH25 have green emission, indicating that the fluorescent GQDs are strongly coated on the surface of the PS particles.

Example 4 Identification of Dispersant Uses of Modified GQDs

Another major use of surfactants is their use as dispersants for insoluble solids by reducing the interfacial energy between solids and liquids (Kim, J. et al ., J. Am. Chem. Soc . 2010, 132, 8180-8186). To investigate the surfactant behavior of surface modified GQDs for solid dispersion, a model system with graphite as insoluble solids and MeOH as solvent was selected (Fig. 15A).

The graphite powder and GQDs were dispersed in MeOH in a mass ratio of 15: 1 and the dispersion was sonicated for 30 minutes using an ultrasonic mill. The dispersion was then centrifuged at 1000 rpm for 5 minutes for separation. Since GQDs have a pi-conjugated aromatic ring in the base plane, this can be adsorbed to the graphite surface through a pi-pi interaction, which reduces the interfacial energy between graphite and MeOH. Thus, GH25 , GH50 and GH75 produced good dispersions of graphite powder. However, GH100 did not stabilize the graphite powder well. This was because a large amount of hexylamine grafted onto GH100 could prevent interaction with graphite. It was observed that 1 mg of GH25 , GH50 and GH75 were dispersed in 0.8, 1.8 and 3.0 mg of graphite, respectively, whereas GH0 and GH100 were dispersed only in 0.1 mg and 0.2 mg of graphite, respectively. It should also be noted that the graphite dispersions with GH25 , GH50 and GH75 were very stable for at least several months after production (Fig. 15B). In comparison, it has been previously reported that the dispersion of graphite by GO is stable for several days (Kim, J. et al ., J. Am. Chem. Soc . 2010, 132, 8180-8186). As shown in Fig. 15c-e, the size of the dispersed graphite was very different and decreased from 13.4 to 2.6 탆 in the order of GH25 , GH50 and GH75 . In particular, GH75 with well-balanced amphipathic properties exhibited excellent surfactant behavior in oil-water systems and was also the most effective dispersant for graphite.

That is, according to the present invention, a series of modified GQDs having tailored surface properties are synthesized and their use as an efficient surfactant in an immiscible blend can be confirmed. Extensive control of the surface properties of GQDs, from highly hydrophilic to fully hydrophobic properties, has been achieved in a systematic way by grafting a controlled amount of hexylamine onto the GQD surface. Also, since the reaction conditions were very mild at 40 ° C, the reaction did not affect any inherent properties of GQDs. Amphipathic GQDs, namely GH25 , GH50 and GH75, were placed at the interface between the two immiscible blends, and thus preferred surfactant behavior was achieved with modified GQDs (see Example 2). Specifically, the GQDs stabilized mini-emulsion polymerization of Pickering emulsion and PS colloidal particles, the size of which was less than a few hundred nanometers. GQDs were also used to control the dispersion of graphite in MeOH. The strategy of the present invention for controlling the surface activity of GQDs is a simple and versatile way to extend the range of uses from emulsifiers to dispersants. In addition, synergistic combinations of the fluorescence and surfactant properties of surface modified GQDs are applicable to techniques such as fluorescent labeling and bio-imaging (see Examples 3 and 4).

The present invention has been described with reference to the preferred embodiments. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the disclosed embodiments should be considered in an illustrative rather than a restrictive sense. The scope of the present invention is defined by the appended claims rather than by the foregoing description, and all differences within the scope of equivalents thereof should be construed as being included in the present invention.

Claims

Method of synthesizing surface-modified graphene quatum dots (GQDs) comprising the steps of:
a) preparing a mixture of a compound containing a hydrophobic group in a solvent, DMAP (4-dimethylaminopyridine) and a graphene quantum dot dissolved therein;
b) adding DCC (N, N'-dicyclohexylcarbodiimide) to the mixture of step a), heating and cooling to room temperature;
c) filtering and precipitating the cooled mixture in step b) to obtain a precipitate; And
d) washing the precipitate obtained in step c) with diethyl ether to remove unreacted material.

The method according to claim 1, wherein the compound containing a hydrophobic group in step a) is an alkylamine having 1 to 6 carbon atoms.

The method according to claim 1, wherein the solvent in step a) is THF (tetrahydrofuran) or DMF (N, N-dimethylformamide).

The method according to claim 1, wherein the compound including a hydrophobic group and the graphene quantum dot are mixed at a weight ratio of 1:10 to 10: 1 in the step a).

The method according to claim 1, wherein the heating in step b) is carried out at 30-50 ° C for 10-15 hours.

6. A surface-modified graphene quantum dot synthesized by the method of any one of claims 1 to 5.

A surface active agent comprising graphene quantum dots modified in the surface of claim 6.