Carbon nanodots-catalyzed free radical polymerization of water-soluble vinyl monomers

Abstract

Carbon nanodots made from green tea (T-CNDs) catalyzing free radical polymerization of water-soluble vinyl monomers is reported. T-CNDs-catalyzed polymerization yields a higher molecular weight as well as a narrower polydispersity index than without T-CNDs, which is attributed to good electron accepting ability of T-CNDs and redox initiators formation.

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Carbon nanodots (CNDs) are a new class of carbon nanoparticles with size below 10 nm, which consist of an amorphous to nanocrystalline core with major sp²-hybridized carbon and an amorphous shell containing abundant surface functional groups.¹ As a family of low-cost, non-toxic and highly fluorescent nanoparticles, CNDs have shown tremendous potential for a wide range of chemical and biomedical applications.^1,2 In particular, their unique optical properties have attracted increasing interest in recent years. Although bare CNDs possess tunable emissions, upon surface passivation with organic or polymeric materials, the surface defects become more stable to facilitate more effective radiative recombination of trapped electrons and holes, thus achieving brighter fluorescence emissions.^1e,3 Many literature have reported deliberate surface functionalization of CNDs to be used in synthesis of multi-responsive, fluorescent biopolymers, e.g. via conjugation surface amine groups with various vinyl monomers.⁴ On the other hand, CNDs could be utilized for fabrication of light-emitting diodes (LED) or solar cells in composite of a conductive polymer matrix such as polyaniline and poly(4-styrene sulfonate).^3b,5

The more fascinating feature of CNDs revealed by Sun et al. is that the photo-excitation of carbon cores in CNDs results in efficient charge separation, i.e., photo-excited CNDs are both excellent electron donors and acceptors.⁶ This photo-induced electron transfer properties of CNDs were confirmed by Kang et al. in similar fluorescence decay studies and utilized in catalyzing a series of organic transformation reactions under visible light.⁷ Recently, band-gap engineering by combining CNDs with TiO₂,⁸ or with C₃N₄ (ref. 9) for high efficient solar water splitting has been boosted, in which the CNDs served as not only visible light harvesting media but also an electron scavenger to avoid the process of electron–hole recombination.¹⁰

The good electron accepting ability of CNDs has tempted us to use them as a catalyst for free radical polymerization of vinyl monomers. It is expected that the good electron scavenger will make the concentration of free radicals in the reaction system keep at a low level so that the polymer molecular weight can be increased. In addition, the sp²-hybridized CNDs tend to activate C=C bond in the vinyl monomer, and thus make the monomer be initiated more easily. In this communication, an important vinyl monomer, sodium 4-styrenesulfonate (NaSS) was selected as a model to perform the experiment, since its polymer (PSSNa) has broad industrial applications,¹¹ and PSSNa nanocomposites are promising in future LEDs.^3b Conventionally, the polymerization of NaSS is initiated in water with potassium persulfate (KPS), suggesting that water-soluble CNDs are preferred. In the past decade, a variety of CNDs with functional surface groups have been synthesized by using diverse natural products as carbon sources.¹² During these carbonization processes, different oxygen- or nitrogen-containing residues remain on the surface of CNDs,^12,13 which have significant impact on their quantum yield (QY), photoluminescence (PL), surface charge, and water solubility. Herein, a kind of highly water-soluble CNDs was prepared by using green tea as a carbon source via a pyrolysis approach. Green tea is naturally-abundant in China and rich of catechin. One molecule of catechin contains five phenolic hydroxyl groups (Fig. S1†). Fig. 1a illustrated a typical synthesis procedure comprising of two steps: first pyrolysis of green tea at 450 °C for 1.5 h and subsequent extractions by using water and ethanol alternately. The final product showing blue fluorescence under an UV light was named as T-CNDs. Fig. 1b shows UV-Vis absorbance and PL emission spectra of the T-CNDs dispersed in water. It can be seen a strong absorption peak at 243 nm assigned to the π–π* electron transition.^12h Meanwhile, a strong PL emission at 410 nm was shown at 330 nm excitation.

Fig. 1 (a) Illustrated synthesis procedure of T-CNDs; (b) UV-Vis absorbance and PL emission (λ_ex = 330 nm) spectra of the T-CNDs dispersed in water; (c) TEM image of T-CNDs (inset: a magnified image of an individual T-CND); (d) ¹H NMR spectrum of T-CNDs.

The QY of the T-CNDs in water was estimated to be as high as 64% by selecting quinine sulfate as a standard (Table S1†). Fig. 1c is a representative TEM image of the mono-dispersed T-CNDs with an average size of 3.83 nm (Fig. S2†). A magnified TEM image of an individual T-CND (Fig. 1c, inset) reveals the lattice spacing of 0.206 nm comparable to the (100) facet of graphite, which is consistent with the previous reports of CNDs made from oxidizing candle soot¹⁴ or pyrolysis of polyethylenimine.^4c The ¹H NMR (Fig. 1d), FT-IR and ¹³C NMR (Fig. S3b†) spectra confirm the existence of abundant phenolic hydroxyl residues and carboxylic acid moieties on the surfaces of T-CNDs, which probably derived from decomposition of the catechin in green tea. The zeta potential of T-CNDs was measured to be −30.5 mV due to the presence of surface phenolic hydroxyl groups. In addition, the total residues content was estimated to be 51.1 wt% from the TGA–DTG curve of the T-CNDs (Fig. S4†), among which carboxylic accounted for 35.2% while hydroxyl accounted for 15.9%.¹⁵ Table S2† lists the elemental analysis data of T-CNDs, in which the carbon content agrees with that of the TGA result. Moreover, the PL intensity of the T-CNDs showed a pH-dependent behaviour (Fig. S5†) and a maximum value was achieved in a basic aqueous solution (pH = 10), which further attests to the “acidic” property of the surface groups on T-CNDs. Intentionally, we evaluated fluorescence quenching effects on T-CNDs by an electron acceptor (2,4-dinitrotoluene) and an electron donor (N,N-diethylaniline, DEA), respectively. As shown in Fig. 2 and S6† luminescence decay time as well as photoluminescence (PL) intensity of T-CNDs in water has decreased remarkably no matter after adding 2,4-dinitrotoluene or DEA. The decay time of T-CNDs was shortened from 4.5 ns to 2.0 ns and 2.2 ns, respectively. The corresponding Stern–Volmer quenching constants of 44.5 M⁻¹ and 37.56 M⁻¹ can be determined from linear regression of the luminescence decays. This result confirms that T-CNDs are both excellent electron donors and electron acceptors.⁶ Fig. 3a shows that PL from T-CNDs in water can be quenched by KPS or NaSS, respectively. Moreover, a simple mixing of the three chemicals results in a minimum PL intensity. PL quenching results indicate the presence of electron transfer among KPS, T-CNDs and NaSS.

Fig. 2 Luminescence decays of the T-CNDs with (a) 2,4-dinitrotoluene and (b) DEA (the insets show Stern–Volmer plots for the quenching of luminescence quantum yields (λ_ex = 485 nm) of the T-CNDs by 2,4-dinitrotoluene and DEA, respectively).

Fig. 3 (a) PL emission spectra (λ_ex = 330 nm) of T-CNDs, (T-CNDs + KPS), (T-CNDs + NaSS) and (T-CNDs + KPS + NaSS) in water; (b) EPR spectra of aqueous solutions of T-CNDs, (T-CNDs + KPS) and (T-CNDs + KPS + NaSS) under visible light irradiation for 20 min; (c) CV curves of T-CNDs and (T-CNDs + KPS); (d) ¹H NMR spectra of monomer NaSS (black line) and (NaSS + T-CNDs) (red line) in D₂O.

KPS, a common peroxide initiator, is highly soluble in water and can generate free radicals of SO₄⁻˙ via thermal degradation, SO₄⁻˙ further reacts with water and then produce HO˙. In the present work, the role of T-CNDs was investigated by applying electron paramagnetic resonance (EPR) measurements to the reaction system (see ESI† for details). Fig. 3b shows a narrow EPR signal centered at g = 1.9998, which reveals the presence of unpaired electrons at the surface of T-CND.¹⁶ However, the amplitude of the EPR signal dropped greatly when KPS was added into the solution, at the same time, PL quenching occurred as shown in Fig. 3a. This coincidence confirms an electron transfer from KPS to T-CNDs and the number of unpaired electrons on T-CNDs was reduced, namely, T-CNDs can act as radical scavengers for SO₄⁻˙ as well as HO˙ in solution.

In addition, CNDs have excellent chemical reduction capability, as previously reported.¹⁷ KPS is a strong oxidant, so a pair of redox initiator (T-CND/KPS) might be formed at the first stage of the polymerization. Usually, redox initiators are used in emulsion polymerization of vinyl monomers, based on oxidants such as persulfate and peroxides combined with reducing agents such as ascorbic acid, FeSO₄ and tetramethyl ethylene diamine.¹⁸ Compared with thermal initiator solely, a pair of redox initiator can generate free radicals at lower temperatures, and more importantly, they can give only one radical whereas one molecule of KPS in aqueous solution dissociates to give two radicals, which means the concentration of free radicals in T-CND/KPS-initiated polymerization can be lower than that in KPS-initiated polymerization. Further, formation of the redox initiator (T-CND/KPS) was investigated by using cyclic voltammetric (CV) technique. CV techniques have been widely used to determine the band gap of CNDs,^3b and study their catalytic property of being a peroxidase mimic^8b or a catalyst for H₂O₂ disproportionation to O₂.⁹ Fig. 3c shows CV behaviour of T-CNDs and T-CNDs + KPS in 0.5 M NaSO₄ solution. Two obvious oxidation peaks at 0.99 V and 1.59 V arise from electrochemical oxidation of the phenolic hydroxyl groups at the surface of T-CNDs. As reported, the oxidation of phenol would first result in the formation of hydroquinones and benzoquinones, and then further oxidized to small organic acids (maleic, formic, oxalic acid), finally mineralized to carbon dioxide and water at high anode potentials.¹⁹ While adding KPS into the electrolyte, it can be seen that the oxidation peak at 0.99 V shifts negatively to 0.75 V, suggesting that the reductive T-CNDs and the oxidative KPS would combine into redox pairs (T-CNDs/KPS).

On the other hand, both evidences in Fig. 3a and b have demonstrated there is also electron transfer from T-CNDs to NaSS. Further, ¹H NMR spectrum of (T-CNDs + NaSS) in D₂O (Fig. 3d) shows the peaks of the protons on vinyl groups and the neighbouring protons on phenyl groups had obvious shifts to downfield compared to that in NaSS, whereas protons on phenyl closer to sulfonate group shows no shift, suggesting an electron transfer from T-CND to NaSS may activate C=C bond of NaSS.

Kinetic data were obtained by removing aliquots at regular intervals of 20 min. Approximately 1 mL of the reaction solution was transferred under N₂ by double-tipped needle into an ice box (−4 °C) to terminate the reaction. And then the reaction solution was freeze-dried. ¹H NMR spectra were recorded in D₂O to determine the conversion at the time of sampling (Fig. S7†). As shown in Fig. 4a, conversion rates vs. time was obtained from ¹H NMR studies for the polymerization of NaSS with or without T-CNDs. Compared with the control polymerization without T-CNDs, the monomer conversion rates in T-CNDs-catalyzed polymerization are significantly higher, almost double in the first 40 min, probably because T-CNDs/KPS redox pairs formed at the beginning can generate free radicals more easily and initiate the polymerization in a shorter time. Fig. 4b shows real-time PL emissions of the reaction solution containing T-CNDs, KPS and NaSS during the whole polymerization process. Obviously, PL of T-CNDs was much weakened in the first 50 min due to capturing free radicals of SO₄⁻˙/HO˙ in the solution and initiating NaSS to produce monomer radicals. With the polymerization proceeding, more monomers have been initiated and added to the polymer chain while less free radicals remained in the solution, as a result, more and more T-CNDs were released, and thus PL of the solution became stronger again.

Fig. 4 (a) Conversion rates vs. time obtained from ¹H NMR studies for the polymerization of NaSS with or without T-CNDs reacted at 70 °C; (b) PL emission spectra (λ_ex = 330 nm) of the reaction solution containing T-CNDs + KPS + NaSS at different polymerization time.

Subsequently, we conducted a series of polymerizations of NaSS for reaction time of 6 h to ensure a complete conversion. Fig. S8† shows ¹H NMR spectrum of the T-CNDs-catalyzed polymer, showing typical signals of PSSNa,¹¹ but the T-CNDs moieties are invisible because of their low content (<0.5 wt%) in the polymer. UV-Vis absorbance spectra (Fig. S9a†) of the T-CNDs-catalyzed PSSNa and the control sample without T-CNDs are almost same, however, the stronger PL intensity (Fig. S9b†) of the T-CNDs-catalyzed PSSNa than the control sample reveals the presence of T-CNDs in the former. Besides, the UV-Vis absorption of PSSNa has an obvious blue-shift compared to that of NaSS due to less conjugation degree between vinyl bond and benzene ring in a NaSS molecule when the C=C bond of vinyl changed to C–C bond during polymerization. As a result, the PL emission wavelength of PSSNa presents a blue-shift, too.

For further exploring the advantages of T-CNDs-catalyzed polymerization, four products of PSSNa polymerized with or without T-CNDs at different temperatures of 50 °C and 70 °C were prepared (see also samples A, B, C and D in ESI†), and then their molecular weights were measured by means of gel permeation chromatography (GPC). Fig. S10† shows GPC curves of the four products. Table 1 has listed their weight-average molecular mass (M_w), number-average molecular mass (M_n) and polydispersity index (PDI, M_w/M_n) data. Obviously, T-CNDs-catalyzed PSSNa have greater molecular weights as well as narrower PDI than those polymerized without T-CNDs at both 50 and 70 °C. Although T-CNDs-catalyzed polymerization is not a real ‘living’ radical polymerization since no dormant species was introduced, a narrower molecular weight distribution of PSSNa has further proved that T-CNDs can capture free radicals in the reaction system due to their good electron accepting ability. However, there are two other competing factors, one is temperature. KPS is a typical thermal initiator, its dissociation speed can be greatly accelerated by increasing temperature of 5–10 °C, and then the concentration of free radicals will expand exponentially.²⁰ Therefore, PSSNa synthesized at 70 °C has a lower polymer molecular weight than that synthesized at 50 °C even though both catalyzed by T-CNDs. The other negative impact may come from light irradiation. As shown in Fig. S11,† EPR signals of not only DMPO⁻·OH but also many other radical species trapped by DMPO would appear when a visible light produced by a 300 W Xe-lamp with a 420 nm cut-off optical filter was used to irradiate the aqueous solution containing either KPS or KPS + T-CNDs. GPC data listed in Table S3† demonstrate that T-CNDs catalyzed PSSNa polymerized at 50 °C under the visible light irradiation still present a slightly catalytic effect compared with the control without T-CNDs, nevertheless, it has much lower molecular weights and broader PDI than T-CNDs-catalyzed PSSNa-50 that polymerized at the same temperature but without light irradiation (Table 1).

Table 1 GPC data of PSSNa and PMMA polymerized under different conditions^a

	Temperature (°C)	T-CNDs	Mn × 10^4 (g mol^−1)	Mw × 10^4 (g mol^−1)	PDI
a PSSNa-70 and PSSNa-50 refer to PSSNa polymerized at 70 and 50 °C, respectively.
PSSNa-70	70	—	15.6	32.3	2.06
	70	10 mg	23.0	34.5	1.50
PSSNa-50	50	—	17.0	31.5	1.85
	50	10 mg	25.8	35.5	1.39
PMMA	60	—	1.45	2.62	1.81
	60	10 mg	1.80	3.06	1.70

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Additionally, we applied T-CNDs-catalyzed free radical polymerizations to other vinyl monomers such as acrylamide (AM) and methyl methacrylate (MMA) (see ESI† for details). The digital photographs of polymerization process of AM in aqueous system are shown in Fig. S13,† a faster gelling rate with T-CNDs than the control without T-CNDs demonstrates that T-CNDs are effective for catalysing polymerizations of water-soluble vinyl monomers. For catalysing polymerization of oil-soluble monomer (MMA), we got the GPC traces of PMMA (Fig. S12†) and the data listed in Table 1, from which almost no catalytic effect can be observed for PMMA synthesized with T-CNDs. Different from NaSS and AM, both MMA and its initiator (azo-bis-isobutyronitrile, AIBN) are oil-soluble, so polymerization of MMA was performed in toluene. However, hydrophilic T-CNDs have a poor solubility in toluene, which prevent them from interacting with AIBN and MMA. Therefore, T-CNDs-catalyzed free radical polymerizations is not suitable for oil-soluble monomers.

Notably, an initiator, e.g., KPS is necessary to T-CNDs-catalyzed polymerization of water-soluble vinyl monomers because it has been proved that T-CNDs solely can't initiate polymerization of NaSS neither under visible light irradiation nor heating at 70 °C (see ESI† for details). This fact make it clear that T-CNDs alone cannot provide free radicals, but they will play an important role in free radical polymerization when accompanied with an oxidative initiator.

As illustrated in Scheme 1, a possible polymerization mechanism of NaSS based on experimental evidences is proposed as follows. T-CNDs are highly water-soluble with a reductive activity, they can form redox initiator pairs with oxidative initiator, KPS. A pair of T-CND/KPS redox initiator has a higher initiating rate than KPS alone, in the meantime, T-CND can activate the double bond of NaSS and make the monomer be initiated more easily. These two reasons lead to a remarkably higher conversion rate of monomers during the whole polymerization. The formation of the redox initiator (T-CND/KPS) has been confirmed by the results of fluorescence quenching experiment and EPR and CV measurements, and the activation of NaSS by T-CNDs has been demonstrated by the results of ¹H NMR. More importantly, a pair of T-CND/KPS redox initiator can give only one radical of SO₄⁻˙, whereas one molecule of KPS in aqueous solution dissociates to give two radicals. Moreover, T-CNDs can capture free radicals in the reaction system due to their good electron accepting ability. Both reasons assure that the total concentration of radical species in T-CND/KPS-initiated polymerization is lower than that in KPS-initiated polymerization. According to classical theory of free radical polymerization,²⁰ the kinetic chain length is inversely dependent on the radical concentration, as described in the following equation: v = (k_p/2k_t)[M]/[M˙]. In the present case, the monomer concentration [M] is the same for both polymerizations with T-CNDs or without T-CNDs, whereas the radical concentration [M˙] in the former become much lower than that in the latter, therefore T-CNDs-catalyzed PSSNa yields higher molecular weight as well as narrower PDI than the PSSNa polymerized without T-CNDs.

Scheme 1 The proposed mechanism of T-CNDs-catalyzed polymerization of NaSS.

Conclusions

In this work, T-CNDs prepared from green tea exhibited an obvious catalytic ability for free radical polymerization of water-soluble vinyl monomers such as NaSS and AM. The T-CNDs-catalyzed polymerization shows a remarkably higher conversion rate of monomer and then yields a higher molecular weight as well as narrower PDI than that polymerized without T-CNDs. The catalytic mechanism can be attributed to good electron accepting ability of T-CNDs and redox initiators (e.g. T-CNDs/KPS) formation at the first stage of polymerization, leading to a lower radical concentration in the reaction system. Moreover, T-CNDs-catalyzed PSSNa or PAM retains mostly the optical properties of T-CNDs that are useful for future applications in flexible optoelectronic devices. We expect to exemplify the versatility of CNDs-catalyzed radical polymerizations in more unexpected areas and practical uses.

Acknowledgements

This work was financed by the National Natural Science Foundations of China (Grant No. 51272002 and 51403002). All of co-authors would like to thank Prof. Shiyong Liu at University of Science and Technology of China for his helpful suggestions on the manuscript.

Notes and references

1. (a) S. N. Baker and G. A. Baker, Angew. Chem., Int. Ed., 2010, 49, 6726; (b) H. Li, Z. Kang, Y. Liu and S. T. Lee, J. Mater. Chem., 2012, 22, 24230; (c) P. G. Luo, S. Sahu, S. T. Yang, S. K. Sonkar, J. Wang, H. Wang, G. E. LeCroy, L. Cao and Y. P. Sun, J. Mater. Chem. B, 2013, 1, 2116; (d) Y. Song, S. Zhu and B. Yang, RSC Adv., 2014, 4, 27184; (e) S. Y. Lim, W. Shen and Z. Gao, Chem. Soc. Rev., 2015, 44, 362.
2. (a) Y. Guo, Z. Wang, H. Shao and X. Jiang, Carbon, 2013, 52, 583; (b) Y. P. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S. Fernando, P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H. Wang, P. G. Luo, H. Yang, M. E. Kose, B. Chen, L. M. Veca and S. Xie, J. Am. Chem. Soc., 2006, 128, 7756; (c) L. Cao, X. Wang, M. J. Meziani, F. Lu, H. Wang, P. G. Luo, Y. Lin, B. A. Harruff, L. M. Veca, D. Murray, S. Y. Xie and Y. P. Sun, J. Am. Chem. Soc., 2007, 129, 11318; (d) P. Huang, J. Lin, X. Wang, Z. Wang, C. Zhang, M. He, K. Wang, F. Chen, Z. Li, G. Shen, D. Cui and X. Chen, Adv. Mater., 2012, 24, 5104; (e) H. Li, R. Liu, S. Lian, Y. Liu, H. Huang and Z. Kang, Nanoscale, 2013, 5, 3289.
3. (a) L. Cao, M. J. Meziani, S. Sahu and Y. P. Sun, Acc. Chem. Res., 2013, 46, 171; (b) L. Bhattacharjee, R. Manoharan, K. Mohanta and R. R. Bhattacharjee, J. Mater. Chem. A, 2015, 3, 1580; (c) Y. Dong, R. Wang, H. Li, J. Shao, Y. Chi, X. Lin and G. Chen, Carbon, 2012, 50, 2810.
4. (a) P. Zhang, W. Li, X. Zhai, C. Liu, L. Dai and W. Liu, Chem. Commun., 2012, 48, 10431; (b) L. Cheng, Y. Li, X. Zhai, B. Xu, Z. Cao and W. Liu, ACS Appl. Mater. Interfaces, 2014, 6, 20487; (c) J. Yin, H. Liu, S. Jiang, Y. Chen and Y. Yao, ACS Macro Lett., 2013, 2, 1033.
5. (a) H. Zhang, T. Cui, Y. Wang, J. Zhao, W. W. Yu and A. L. Rogach, ACS Nano, 2013, 7, 11234; (b) J. K. Kim, M. J. Park, S. J. Kim, D. H. Wang, S. P. Cho, S. Bae, J. H. Park and B. H. Hong, ACS Nano, 2013, 7, 7207; (c) C. M. Luk, B. L. Chen, K. S. Teng, L. B. Tang and S. P. Lau, J. Mater. Chem. C, 2014, 2, 4526; (d) G. Zhang, M. Yan, X. Teng, H. Bi, Y. Han, M. Tian and M. Wang, Chem. Commun., 2014, 50, 10244.
6. X. Wang, L. Cao, F. Lu, M. Meziani, H. Li, G. Qi, B. Zhou, B. A. Harruff, F. Kermarrec and Y. P. Sun, Chem. Commun., 2009, 25, 3774.
7. (a) Y. Han, H. Huang, H. Zhang, Y. Liu, X. Han, R. Liu, H. Li and Z. Kang, ACS Catal., 2014, 4, 781; (b) H. Li, R. Liu, W. Kong, J. Liu, Y. Liu, L. Zhou, X. Zhang, S.-T. Lee and Z. Kang, Nanoscale, 2014, 6, 867; (c) R. Liu, H. Huang, H. Li, Y. Liu, J. Zhong, Y. Li, S. Zhang and Z. Kang, ACS Catal., 2014, 4, 328.
8. (a) H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C. H. A. Tsang, X. Yang and S.-T. Lee, Angew. Chem., Int. Ed., 2010, 49, 4430; (b) H. Ming, Z. Ma, Y. Liu, K. Pan, H. Yu, F. Wang and Z. Kang, Dalton Trans., 2012, 41, 9526.
9. J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S.-T. Lee, J. Zhong and Z. Kang, Science, 2015, 347, 970.
10. L. Cao, S. Sahu, P. Anilkumar, C. E. Bunker, J. Xu, K. A. S. Fernando, P. Wang, E. A. Guliants, K. N. Tackettll and Y. P. Sun, J. Am. Chem. Soc., 2011, 133, 4754.
11. P. D. Iddon, K. L. Robinson and S. P. Armes, Polymer, 2004, 45, 759.
12. (a) B. De and N. Karak, RSC Adv., 2013, 3, 8286; (b) J. Xu, Y. Zhou, S. Liu, M. Dong and C. Huang, Anal. Methods, 2014, 6, 2086; (c) Z. Wu, P. Zhang, M. Gao, C. Liu, W. Wang, F. Leng and C. Huang, J. Mater. Chem. B, 2013, 1, 2868; (d) J. Yu, N. Song, Y. Zhang, S. Zhong, A. Wang and J. Chen, Sens. Actuators, B, 2015, 214, 29; (e) V. N. Mehta, S. Jha, H. Basu, R. K. Singhal and S. K. Kailasa, Sens. Actuators, B, 2015, 213, 434; (f) J. Gong, X. Lu and X. An, RSC Adv., 2015, 5, 8533; (g) D. Sun, R. Ban, P. Zhang, G. Wu, J. Zhang and J. Zhu, Carbon, 2013, 64, 424; (h) X. Teng, C. Ma, C. Ge, M. Yan, J. Yang, Y. Zhang, P. C. Morais and H. Bi, J. Mater. Chem. B, 2014, 2, 4631; (i) L. Zhu, Y. Yin, C. Wang and S. Chen, J. Mater. Chem. C, 2013, 1, 4925.
13. (a) Y. Hu, J. Yang, J. Tian, L. Jia and J.-S. Yu, RSC Adv., 2015, 5, 15366; (b) Q. Wang, X. Liu, L. Zhang and Y. Lv, Analyst, 2012, 137, 5392; (c) B. Chen, F. Li, S. Li, W. Weng, H. Guo, T. Guo, X. Zhang, Y. Chen, T. Huang, X. Hong, S. You, Y. Lin, K. Zeng and S. Chen, Nanoscale, 2013, 5, 1967.
14. L. Tang, R. Ji, X. Cao, J. Lin, H. Jiang, X. Li, K. S. Teng, C. M. Luk, S. Zeng, J. Hao and S. P. Lau, ACS Nano, 2012, 6, 5102.
15. (a) U. Zielke, K. J. Huttinger and W. P. Hoffman, Carbon, 1996, 34, 983; (b) J. L. Figueiredo, M. F. R. Pereira, M. M. A. Freitas and J. J. M. Orfao, Carbon, 1999, 37, 1379.
16. Z. M. Markovic, B. Z. Ristic, K. M. Arsikin, D. G. Klisic, L. M. Harhaji-Trajkovic, B. M. Todorovic-Markovic, D. P. Kepic, T. K. Kravic-Stevovic, S. P. Jovanovic, M. M. Milenkovic, D. D. Milivojevic, V. Z. Bumbasirevic, M. D. Dramicanin and V. S. Trajkovic, Biomaterials, 2012, 33, 7084.
17. L. Shen, M. Chen, L. Hu, X. Chen and J. Wang, Langmuir, 2013, 29, 16135.
18. N. K. Svelko, R. Pirri, J. M. Asua and J. R. Leiza, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 2917.
19. G. Gao, M. Pana and C. D. Vecitis, J. Mater. Chem. A, 2015, 3, 7575.
20. P. J. Flory. Principles of Polymer Chemistry, Cornell University Press, 1953, ch. 4, p. 133.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06273c

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