Surface grafting of fluorescent carbon nanoparticles with polystyrene via atom transfer radical polymerization

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Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Surface grafting of fluorescent carbon nanoparticles with polystyrene via atom transfer radical polymerization Bo Liao

a,* ,

Peng Long a, Benqiao He b, Shoujun Yi a, Qingquan Liu a, Rongxiang Wang

a

a

School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China b

A R T I C L E I N F O

A B S T R A C T

Article history:

Fluorescent carbon nanoparticles (f-CNPs) were grafted with polystyrene using a ‘‘grafting

Received 20 October 2013

from’’ method via atom transfer radical polymerization. The fluorescent carbon nanoparti-

Accepted 15 February 2014

cles grafted with polystyrene (f-CNP-g-PSt) were characterized using nuclear magnetic res-

Available online 20 February 2014

onance, Fourier transform infrared spectroscopy, transmission electron microscopy, gel permeation chromatography, dynamic light scattering and fluorescence spectroscopy. The synthesized f-CNP-g-PSt are fluorescent in solution or in the solid state with the appropriate excitation wavelength and exhibit better dispersibility and processability compared to f-CNPs. Ó 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Fluorescent carbon nanoparticles (f-CNPs) have attracted a great deal of scientific attentions in recent years due to their high potential for applications in biological labeling and imaging [1–3], sensors [4–6] and other optoelectronic devices [7–11]. In comparison to quantum dots and fluorescent dyes, the f-CNPs exhibit high fluorescent stability and are nontoxic [12]. To date, the synthesis and optical properties of f-CNPs have been extensively investigated [13–21]. However, f-CNPs, like other inorganic nanoparticles, such as CdS and Fe3O4, have drawbacks that can limit their applications because they cannot be easily processed into functional films or fibers. Therefore, to improve their properties and broaden their applications, f-CNPs should be modified with other functional groups. Many methods including chemical or physical adsorption of preformed polymers have been employed to functionalize

* Corresponding author. E-mail address: [email protected] (B. Liao). http://dx.doi.org/10.1016/j.carbon.2014.02.051 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved.

these inorganic nanoparticles to improve their dispersibility and processability [22–24]. Among these methods, grafting on the surface of the nanoparticles is one of the most popular approaches. In this approach, the surface of the nanoparticles is typically modified with functional groups and then these functional groups are linked to a polymerization mediator, such as an initiator, comonomer, or chain transfer agent. Then, the polymerization can proceed from the surface of the nanoparticles. Recently, the surface modification of carbon nanomaterials, such as non-fluorescent nanodiamonds [25–27] and carbon nanotubes [28–30], by employing surface initiated polymerization has been reported to improve their dispersion. Previously, these non-fluorescent nanodiamonds or carbon nanotubes required tedious surface oxidization with a mixture of concentrated sulfuric and nitric acids to change their surface properties prior to surface initiated polymerization. To the best of our knowledge, there are no reports on the surface

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modification of f-CNPs employing surface initiated polymerization. However, it is possible that surface initiated polymerization can modify f-CNPs because there are many active groups, such as carboxyl and hydroxyl groups, on the surface of the f-CNPs. Herein, we report the surface grafting of f-CNPs with polystyrene via atom transfer radical polymerization (ATRP) to broaden the applications of f-CNPs. We performed a detailed characterization of the structure and properties of the fluorescent carbon nanoparticles grafted with polystyrene (f-CNP-g-PSt). The size of the f-CNP-g-PSt can be controlled by changing the grafting lengths. The f-CNP-g-PSt, whether in solution or as a solid film, can be excited and fluoresce and have fluorescent behaviors that are similar to f-CNPs. The synthesized f-CNP-g-PSt composites show better processability and dispersibility compared to the f-CNPs, indicating that it can be readily processed into such as fluorescent plastic films and fibers, which can aid the applications of f-CNPs in many fields.

2.

Experimental

2.1.

Materials

Ethylenediaminetetraacetic acid disodium salt (EDTAÆ2Na) was obtained from Aldrich. 2-Bromo-2-methylpropionyl bromide, N,N,N 0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA) and triethylamine were purchased from Acros and used without purification. Styrene was purchased from Aldrich. The inhibitor in styrene was removed by passing the styrene through a column of alumina and distilling in a vacuum. CuBr was obtained from Aldrich and purified according to a previously published procedure [31]. Tetrahydrofuran (THF), ethanol and dichloromethane were obtained from the Beijing Chemical Reagent Company. These chemicals were previ˚ molecuously distilled and maintained in the presence of 4 A lar sieves to eliminate any traces of water prior to use. The water used was double-distilled water.

2.2.

Synthesis of the f-CNPs

The f-CNPs were synthesized via a hydrothermal method that we previously reported [13]. 2 g of EDTAÆ2Na was added to 20 mL of water in a hydrothermal reactor and sealed, heated to 220 °C and maintained for 24 h. After the reaction, the reactants were cooled to room temperature. A brown mixture was obtained and dried by vacuum rotary evaporation at 70 °C. The resulting brownish black solid was dissolved in 20 mL of ethanol. Then, the obtained solution was centrifuged at a high speed (16,000 r min1) for 10 min to remove the deposits. The upper yellow solution exhibited strong blue–green luminescence with irradiation by 365 nm light and contained the f-CNPs. The f-CNPs solution was dried to obtain a powder for further use.

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Fig. 1 – Schematic representation of the synthesis of f-CNPBr. (A color version of this figure can be viewed online.)

bone-dry f-CNPs (0.2000 g) was dispersed in 10.0 mL of anhydrous CH2Cl2 and 0.4046 g (4 mmol) of triethylamine was added. Then, 0.9192 g (4 mmol) of 2-bromo-2-methylpropionyl bromide dissolved in 5 mL of anhydrous CH2Cl2 was added dropwise over 30 min at 0 °C. Then, the mixtures were stirred for 2 h at 0 °C followed by stirring at room temperature for 48 h. After the reaction, the precipitated triethylamine hydrochloride was removed by filtration, and the CH2Cl2 solvent was removed by vacuum rotary evaporation. Then, the product was washed with aqueous sodium hydroxide (saturated) and water and precipitated by centrifugation at least three times. The supernatant was discarded. The resulting f-CNP-Br were obtained by collecting the precipitant and then dried overnight under vacuum at 40 °C.

2.4.

Synthesis of f-CNP-g-PSt

As shown in Fig. 2, 5 mg of f-CNP-Br, 3.6 mg (0.025 mmol) of CuBr, 8.7 mg (0.050 mmol) of PMDETA, 0.25 mL of toluene, and a certain amount of styrene were placed in a 10 mL dried flask, and then, the flask was evacuated and back-filled with nitrogen three times. Next, the flask was sealed by melting the glass. The flask was immediately placed in an oil bath at 110 °C. At the end of the reaction, the viscosity increased dramatically. The resulting mixture was diluted with THF and passed through a column with activated Al2O3 to remove the catalyst. The resulting solution was concentrated by vacuum rotary evaporation to remove the majority of the THF and precipitated in 100 mL of methanol. The f-CNP-g-PSt were obtained by filtration and dried overnight.

2.5.

Measurements

Transmission electron microscopy (TEM) was performed on a Hitachi-7650 electron microscope operating at 100 kV using a micro grid as the support membrane. Fluorescence spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer at room temperature. The fluorescence lifetime and quantum yield were measured with an Edinburgh FLS980 steady/transient fluorescence spectrometer. Fourier

2.3. Synthesis of fluorescent carbon nanoparticle initiators (f-CNP-Br) The f-CNP-Br synthesis is shown in Fig. 1 (all of the operations were performed under a nitrogen atmosphere). A sample of

Fig. 2 – Schematic representation of the synthesis of f-CNPg-PSt. (A color version of this figure can be viewed online.)

transform infrared (FTIR) spectra were obtained using a Nicolet Avatar 360 FTIR spectrophotometer. The proton nuclear magnetic resonance (1H NMR) measurements were recorded on a Bruker Advance Digital 500 MHz spectrometer. The hydrodynamic diameter distributions were measured with a Malvern Mastersizer 2000. The molecular weight was measured using Waters Series 2414 gel permeation chromatography (GPC) with polystyrene as the standard and THF as the eluent at the flow rate of 1 mL/min.

3.

Results and discussion

3.1.

Characterization

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The 1H NMR spectra of f-CNPs, f-CNP-Br and f-CNP-g-PSt are shown in Fig. 3. As shown in Fig. 3, the peaks at 2–4 ppm in the spectrum of the f-CNPs correspond to the resonance signals of AOH on the surface; after the f-CNPs were converted to f-CNP-Br, these peaks were weaker and a new peak at 1.9 ppm from the resonance signal of ACH3 of the 2-bromo2-methylpropionyl group appeared, as shown in the spectrum of f-CNP-Br. These results indicate that the f-CNPs were converted to the initiators of f-CNP-Br. In the spectrum of the fCNP-g-PSt, the proton resonance signals of the ACHA and ACH2A groups are located at 1.2–1.7 ppm (Peak b) and 1.7– 2.0 ppm (Peak c), respectively. The aromatic protons are located at 6.3–7.2 ppm (Peak d), while the resonance signals of the ACH3 in the 2-bromo-2-methylpropionyl group shifted to 0.9 ppm (Peak a) [32]. The ratio of the area of Peak c to that of Peak a is approximately 20, which indicates that the grafting length is approximately 120 repeat structure units. The 1H NMR results confirmed that the f-CNP-g-PSt composites have been successfully synthesized via ATRP. Fig. 4 shows the FTIR spectra of the f-CNPs, f-CNP-Br and fCNP-g-PSt. The Peak at 1738 cm1 was attributed to the stretching vibration of the carbonyl in the 2-bromo-2-methylpropionyl group in f-CNP-Br, which did not appear in the spectra of f-CNPs. The intensity of the peak at 3423 cm1 from

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Wave number/cm-1 Fig. 4 – FTIR spectra of f-CNPs, f-CNP-Br and f-CNP-g-PSt. (A color version of this figure can be viewed online.) the AOH group decreased for f-CNP-Br compared to that for fCNPs. These results indicate that the 2-bromo-2-methylpropionyl group was substituted for hydrogen in the AOH group on the f-CNPs surface. After grafting polymerization on the surface of f-CNP-Br, the bands in the range of 755 and 694 cm1 and at 1600 cm1 were observed, which correspond to the aromatic rings [32,33]. These results suggest that the polystyrene was grafted onto the f-CNPs surface. Fig. 5 shows the GPC results of f-CNP-g-PSt synthesized with different weight ratios of styrene to f-CNP-Br initiators under nearly the same conversion (50%). The number average weight (Mn) and polydispersity index (PDI) of the synthesized f-CNP-g-PSt are also shown in Fig. 5. The ratios increased from 70 to 350, as shown in Fig. 5, and Mn of f-CNP-g-PSt increased from 44,976 to 264,159 g/mol. The increase in molecular weight is most likely due to the increase in the grafting length under the same grafting density. The results are confirmed by the

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Fig. 3 – 1H NMR spectra of f-CNPs, f-CNP-Br and f-CNP-g-PSt. (A color version of this figure can be viewed online.)

Fig. 5 – GPC chromatographs of f-CNP-g-PSt synthesized with different ratios of styrene to f-CNP-Br under nearly the same conversion (50%) (the weight ratios of styrene to fCNP-Br are 70, 150, 180, 210, 240, 270, and 350 from low to high). (A color version of this figure can be viewed online.)

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results of the hydrodynamic diameter distributions shown in Fig. 6. The hydrodynamic diameters of f-CNP-g-PSt dispersed in THF increased as the molecular weight of f-CNP-g-PSt increased. According to the grafting length and Mn, we can calculate the grafting density and determined that the grafting

density is approximately 14 polymer chains per carbon nanoparticle when the grafting length is approximately 120 repeat structure units (Mn of the f-CNP-g-PSt is 177,117 g/mol and ignoring the molecular weight of f-CNPs). This result indicates that there are approximately 14 polymer chains grafted from the surface of the f-CNPs in each of the synthesized f-CNP-g-PSt with different grafting lengths. The morphologies of f-CNPs and f-CNP-g-PSt with different grafting lengths were also investigated by TEM, as shown in Fig. 7. f-CNPs cannot be aggregated. All of the f-CNPs are nearly the same size with diameters of approximately 3 nm, as shown in Fig. 7. However, the structures of f-CNP-g-PSt differ from the structure of f-CNPs. Due to shielding of the polymer chains, the nanostructure of the single f-CNP cannot be identified in the f-CNP-g-PSt composites particles. The particle size of the f-CNP-g-PSt composites is larger than that of f-CNPs. As the molecular weight increases, the size of the fCNP-g-PSt composite particles increased. When Mn is 44,976 g/mol (the grafting length is approximately 31 repeat structure units), the average size is approximately 5 nm. When Mn increases to 264,159 g/mol (the grafting length is approximately 181 repeat structure units), the size increases to approximately 20 nm. The TEM results confirmed that the increase in the size of f-CNP-g-PSt is due to the increase in the grafting lengths, which is consistent with the results mentioned above. These results also confirmed that the f-CNP-g-PSt composites with different grafting lengths have been successfully synthesized.

Fig. 7 – TEM images of f-CNPs and f-CNP-g-PSt with different grafting lengths. (a) f-CNPs; (b) f-CNP-g-PSt (Mn = 44,976 g/mol, 31 repeat structure units); (c) f-CNP-g-PSt (Mn = 147,746 g/mol, 101 repeat structure units) and (d) f-CNP-g-PSt (Mn = 264,159 g/ mol, 181 repeat structure units).

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Fig. 8 – Fluorescence spectra of f-CNPs, f-CNP-Br and f-CNP-g-PSt with different grafting lengths dispersed in THF at different excitation wavelengths. (a) f-CNPs, (b) f-CNP-Br, (c–f) f-CNP-g-PSt (the grafting lengths are 31, 101, 143 and 181 repeat structure units in (c–f), respectively). (A color version of this figure can be viewed online.)

3.2.

Fluorescent properties of f-CNP-g-PSt

The fluorescence spectra of f-CNPs, f-CNP-Br and f-CNP-g-PSt with different molecular weights in THF are shown in Fig. 8. The f-CNPs and f-CNP-Br in THF exhibit similar fluorescent properties. In the excitation wavelength range of 320–500 nm,

the fluorescence of f-CNPs is dependent on the excitation wavelengths. At the same excitation wavelength, the fluorescence emission wavelengths are nearly the same. All of the synthesized f-CNP-g-PSt exhibit a fluorescence emission that depends on the excitation wavelength in the range of 320–500 nm. The emission wavelengths of the f-CNP-g-PSt

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Time (ns) Fig. 9 – Time resolved fluorescence spectra of f-CNPs, f-CNPBr, and f-CNP-g-PSt with different grafting lengths (the grafting lengths are 31, 101 and 181 repeat structure units in f-CNP-g-PSt (a–c), respectively) in THF (kex = 405 nm, kex = 475 nm). (A color version of this figure can be viewed online.)

are almost same as those of f-CNPs at the same excitation wavelength. This result indicates that the fluorescence emission properties of f-CNPs do hardly change even though there was polymeric grafting of the surface and that the fluorescence emission properties are barely dependent of the grafting length. In addition, we investigated the fluorescence lifetime and quantum yield of f-CNPs, f-CNP-Br and f-CNP-g-PSt in THF. Fig. 9 shows the time-resolved fluorescence spectra of f-CNPs, f-CNP-Br and f-CNP-g-PSt with different grafting lengths dispersed in THF. At an excitation wavelength of 405 nm, the fluorescence of f-CNP-Br decays the fastest (i.e., the mean lifetime is 3.13 ns), and the fluorescence of f-CNPs decays the slowest (i.e., 4.80 ns). For f-CNP-g-PSt, the decay in the fluorescence decreased as the grafting length increased. This result suggests that the fluorescence lifetime of the f-CNPs can be decreased by the 2-bromo-2-methylpropionyl groups. The distance between the 2-bromo-2-methylpropionyl groups and the f-CNPs increased as the grafting length increased and caused the fluorescence lifetime of the f-CNPs to recover. At an excitation wavelength of 365 nm, the quantum yield of f-CNP-Br is the highest (9.5%). However, the quantum yields

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Fig. 10 – Fluorescence spectra of the films prepared from the f-CNP-g-PSt with different grafting lengths at different excitation wavelengths. The insets in (a–d) are the photographs of the films when exposed to the 365 nm UV lamp (the grafting lengths are 31, 101, 143 and 181 repeat structure units in (a–d), respectively). (A color version of this figure can be viewed online.)

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of f-CNP-g-PSt can be hardly related to the grafting length. fCNP-g-PSt with different grafting lengths exhibit nearly the same quantum yield (2%), which are nearly identical to that of f-CNPs dispersed in THF. This result indicates that the grafting length does not influence the quantum yield but does influence the fluorescence lifetime. Fig. 10 shows the fluorescence spectra at different excitation wavelengths as well as the images of the f-CNP-g-PSt composite films when exposed to the 365 nm UV lamp. All of the f-CNP-gPSt films with different grafting lengths exhibit similar fluorescence properties and display bright blue–green fluorescence. All of the films can be excited and fluoresce under the appropriate excitation wavelengths. This result implies that the fluorescence of the f-CNP-g-PSt composites cannot be quenched after aggregation into films. This property of the f-CNP-g-PSt composites would aid in future applications of f-CNPs.

4.

Conclusions

The f-CNP-g-PSt composites have been successfully synthesized using ‘‘grafting from’’ via ATRP. Our results have demonstrated that the ‘‘grafting from’’ method immobilized polystyrene macromolecules on the surface of f-CNPs. The size of the f-CNP-g-PSt composites can be controlled by changing the grafting length as a function of the ratio of styrene monomers to f-CNP-Br initiators during the synthesis. The f-CNP-g-PSt composites that are in solution or in the solid state can be excited and fluoresce. The f-CNP-g-PSt composites have the potential to be used in a more diverse set of applications because they can be readily processed into fluorescent films or fibers.

Acknowledgements The authors gratefully acknowledge financial support from the Natural Science Foundation of China (Project Nos. 21174104, 51343004 and 51373051).

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