Chromatographic selectivity of graphene capillary column pretreated with bio-inspired polydopamine polymer

Abstract

This work reports the first example of employing bio-inspired polydopamine (PDA) in capillary column fabrication of a graphene stationary phase (denoted as G-PDA@capillary) for gas chromatographic (GC) separations. The as-fabricated G-PDA@capillary column achieved improved column efficiency of 3400 plates per m and separation performance in contrast to the graphene column without PDA (G@capillary). In particular, it exhibited high selectivity and resolving ability for alkane isomers, alcohol isomers, substituted benzenes with diverse groups and many other types of analytes. Additionally, the G-PDA@capillary column showed good repeatability with RSD values less than 0.02% for run-to-run, 0.16% for day-to-day and 5.0% for column-to-column, respectively, and thermal stability up to 300 °C. This work demonstrates the feasibility of the proposed strategy by integrating graphene sheets with PDA coating, which is efficient in addressing the current problem with graphene sheets and exploring its full potential in GC separations.

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1 Introduction

Due to its high specific surface area and large delocalized π-electron system,^1–4 graphene (G) as well as its analogs such as graphene oxide (GO) has attracted growing attention in separation science. Graphene sheets have been employed as adsorbents in solid-phase microextraction^5–8 and as stationary phases in chromatography, including capillary electrochromatography (CEC),^9,10 liquid chromatography (LC)¹¹ and gas chromatography (GC).^12–14 The previous reports on GC have shown the advantageous features of the graphene stationary phase¹³ over the GO phase^12,14 in terms of thermal stability and separation performance. Though graphene sheets are of high thermal stability, enhancing their stability on a capillary column is still a challenging task. The reported methods for obtaining bonded graphene on a substrate material (fiber, capillary, planar surface, etc.) generally involve the following multiple steps: first bonding a coupling agent onto the surface of a substrate, then, grafting GO sheets with the coupling agent and reducing GO into graphene (also termed as reduced GO). The general method is applicable but tedious and time-inefficient. Undoubtedly, finding a more facile and efficient way with good repeatability would be valuable for exploring the potential of graphene in GC. As known, column pretreatment is an important step in the fabrication of a capillary GC column. For graphene stationary phase, a pretreatment method that could produce a stable and uniform layer on a bare capillary and be capable of further anchoring graphene sheets firmly onto the capillary was highly expected.

Polydopamine (PDA) is a bio-inspired novel polymer, which can be in situ formed with extremely high adhesion strength on almost any type of substrate surfaces irrespective of materials nature by spontaneous polymerization of dopamine (DA) under the mild conditions of pH 8.5 Tris buffer in the presence of oxygen.^15,16 Though its exact structure is still not clear, PDA is basically identified as the composite of dihydroxyindole and indoledione units,¹⁶ which can further adhere materials of interest onto its surface via chemical bonding or molecular interactions. Due to these fascinating features, PDA have found its wide use in surface modification of various materials^17,18 and other areas.¹⁹ But no related reports are available in GC up to now except a couple of reports in capillary electrophoresis (CE),^20–22 showing the high stability of PDA coating on fused-silica capillary columns and improved column performance. Additionally, PDA has high thermal stability that the weight loss of PDA polymer was about 50% up to 800 °C,²³ showing its great potential in capillary GC. Accordingly, integrating PDA coating with graphene sheets through their intermolecular interactions could be an ideal strategy for simultaneously addressing the aforementioned problems in column fabrication and achieving improved column performance for graphene stationary phase.

Herein, we report the chromatographic performance of as-fabricated capillary column (denoted as G-PDA@capillary) in GC separations following the above strategy. It was evaluated in terms of chromatographic parameters, separation performance, thermal stability and column repeatability. The retention behaviours and possible mechanism of G-PDA@capillary column were also explored by diverse types of analytes, including alkane isomers, alcohol isomers, substituted benzenes with diverse groups and other types of analytes. To the best of our knowledge, this is the first example of applying graphene stationary phase with PDA coating in GC separations. This proposed approach shows a promising potential for graphene and its analogs in GC and in separation science.

2 Experimental

Materials and equipments

Tris(hydroxymethyl)aminomethane hydrochloride and dopamine hydrochloride were purchased from Aladdin Industrial Corp. (Shanghai, China). 1-Butanol, methyl hexanoate and ethyl salicylate were from Tokyo Chemical Industry Co. (Tokyo, Japan). Naphthalene and biphenyl were from Sinopharm Chemical Reagent Co. Ltd (Beijing, China). n-Hexane, n-heptane, n-octane, n-decane, n-undecane, isooctane, 2,2-dimethylbutane, 2,3-dimethylbutane, 3-methylpentane, heptanal and decanal were purchased from J&K Scientific. Ltd (Beijing, China). Benzene, n-dodecane 2-pentanone and 1-nitropropane were from Alfa Chemical Co., Ltd (Tianjin, China). The rest of the chemicals were from Beijing Chemical Reagent Co. (Beijing, China). All the chemicals were at least of analytical grade.

Untreated fused-silica capillary tubing (0.25 mm i.d.) was purchased from Yongnian Ruifeng Chromatogram Apparatus Co., Ltd (Hebei, China). A HP-5MS capillary column (10 m × 0.25 mm, i.d., 0.25 μm film thickness, 5% phenyl polysiloxane) was from Agilent Technologies. An Agilent 7890A gas chromatograph equipped with a flame ionization detector (FID) and an autosampler was employed in GC separations. All the separations were performed under the following conditions: nitrogen of high purity (99.999%) as carrier gas, injection port and FID detector at 300 °C, flow rate at 1 mL min⁻¹ unless otherwise specified. Oven temperature programs for each of the separations were individually provided in their figure captions.

Synthesis of graphene product

Graphene was prepared by a hydrothermal method,^24,25 characteristic of few-layer stacked sheets with highly three-dimensional cross-linking porous structure. Briefly, homogeneous graphene oxide aqueous dispersion (2.0 mg mL⁻¹) was sealed in a Teflon-lined autoclave (cylindrical, 30 mL) at 180 °C for 3 h. Then the autoclave was naturally cooled down to room temperature, yielding a graphene gel. After filtration, the moist graphene gel was pre-frozen for 30 min under liquid nitrogen, and subsequently vacuum dried at 40 Pa for two days to sublimate the ice crystals. At last, the freeze-dried graphene was kept at 4 °C for further use. The graphene product was characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), which agreed well with the results provided in ref. 24. In addition, we also employed thermal gravimetric analysis (TGA) to evaluate its thermal stability. As shown in Fig. S1,† the as-prepared graphene began to show a weight loss at a temperature of exceeding 400 °C under nitrogen, indicating its good thermal stability.

Fabrication of capillary columns

Three types of capillary GC columns were statically fabricated, i.e., graphene coated on a capillary pretreated with PDA layer (G-PDA@capillary), graphene on a bare capillary without PDA (G@capillary) and a capillary only coated with PDA layer (PDA@capillary). The latter two types of the columns were used for comparison. The procedures for column fabrication were performed as follows.

Prior to coating, a bare fused-silica capillary column (10 m × 0.25 mm, i.d.) was rinsed with dichloromethane for 10 min and then heated in an oven at 100 °C for 10 min. After the column was successively flushed with 1 M HCl and 1 M NaOH for 30 min, respectively, it was rinsed with ultrapure water until the eluate was neutral and then dried in an oven at 200 °C for 3 h under nitrogen. Afterwards, the column was filled with a dopamine solution (1.2 mg mL⁻¹ dopamine in 10 mM Tris–HCl buffer, pH 8.5) and stayed for 24 h at ambient temperature.²⁰ Next, the solution in the capillary was flushed out and washed with water for 10 min to remove the residual dopamine and dried under nitrogen. After repeating the coating process one more time, PDA@capillary column was obtained. The PDA coating on the inner wall of the capillary column showed orderly crumpled morphology (Fig. S2†).

Further, PDA@capillary column was statically coated with a dispersion of graphene sheets in dichloromethane (0.015%, w/v) at 40 °C. After the column was filled with the dispersion, one end of the capillary was sealed and the other end was connected to a vacuum system to gradually remove the solvent. Afterwards, the coated capillary column was then conditioned from 40 °C to 180 °C at 1 °C min⁻¹ and held at the high-end temperature for 10 h under nitrogen at 1 mL min⁻¹. As a result, G-PDA@capillary column was fabricated and the schematic diagram is also shown in Fig. 1. By the same procedure, the capillary with graphene directly coated on a bare capillary, G@capillary, was also obtained.

Fig. 1 Schematic diagram for the fabrication of G-PDA@capillary column.

3 Results and discussion

Column efficiency and McReynolds constants

Column efficiency of G-PDA@capillary column was measured by isothermal determination of n-dodecane at 120 °C at different flow rates of carrier gas and the Golay plot was obtained. As shown in Fig. 2, the height equivalent to a theoretical plate (HETP) attained the minimum over the flow rate of 0.2–0.4 mL min⁻¹ and the corresponding column efficiency was 3400 plates per m. McReynolds constants of the five probe analytes, namely benzene (X′), n-butanol (Y′), 2-pentanone (Z′), 1-nitropropane (U′) and pyridine (S′), on G-PDA@capillary column were determined at 120 °C to evaluate the column polarity. As listed in Table 1, G-PDA@capillary column exhibits close McReynolds constants to the commercial HP-5MS column, suggesting its weakly polar nature. For comparison of graphene with the conventional stationary phase in retention behaviours and separation performance, this commercial column was also adopted in this work. Notably, G-PDA@capillary column exhibits slightly higher Y′ and S′ values than the commercial column, indicating its relatively stronger H-bonding and dipole–dipole interactions with H-bonding analytes possibly due to the residual oxygen-containing groups such as carbonyl and hydroxyl groups in graphene sheets synthesized from graphene oxide.^24–26

Fig. 2 Golay plot of G-PDA@capillary column.

Table 1 McReynolds constants of G-PDA@capillary at 120 °C^a

	Stationary phase	X′	Y′	Z′	U′	S′	Sum
a X′: benzene, Y′: 1-butanol, Z′: 2-pentanone, U′: 1-nitropropane, S′: pyridine.
I	Graphene	687	687	694	751	790
	HP-5MS	686	662	693	751	766
	Squalane	653	590	627	652	699
ΔI	Graphene	34	97	67	99	91	388
	HP-5MS	33	72	66	99	67	337

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Separation performance

Separation performance of G-PDA@capillary was examined in comparison to G@capillary and PDA@capillary by a mixture consisting of n-alkanes, alcohols and aldehydes. The latter two types of analytes are prone to exhibit tailing or distorted peaks in GC. As shown in Fig. 3, G-PDA@capillary achieved the best separation result with improved resolution and narrow peak shapes. In contrast, G@capillary well separated most of the analytes except the coelution of the last peak pair (n-dodecane/n-decanal) whereas PDA@capillary failed to resolve all of them. Clearly, the improved performance of G-PDA@capillary column suggested the advantage of integration of graphene stationary phase with PDA, which may facilitate the even coating of graphene sheets on PDA layer via their intermolecular interactions. In the following, separation performance of G-PDA@capillary column was further investigated with critical analytes including nonpolar alkane isomers, polar alcohol isomers and other types of analytes with diverse structures and varying polarities.

Fig. 3 Separations of the mixture of n-alkanes, alcohols and aldehydes on PDA@capillary, G@capillary and G-PDA@capillary columns. Peaks: (1) n-heptanal, (2) 1-hexanol, (3) n-decane, (4) 1-heptanol, (5) n-undecane, (6) 1-octanol, (7) n-dodecane and (8) n-decanal. Oven temperature: 40 °C for 1 min to 160 °C at 10 °C min⁻¹.

Fig. 4 shows the separation of alkane isomers (a) and butanol isomers (b) on G-PDA@capillary column. Observably, it achieved good resolution of the hexane isomers (peaks 1–4) and of isooctane and n-heptane (peaks 5/6) (Fig. 4a). The first four analytes eluted in the order of 2,2-dimethylbutane, 2,3-dimethylbutane, 3-methylpentane and n-hexane. Clearly, a more branched analyte eluted earlier due to its weaker dispersive interaction with the stationary phase. Moreover, the G-PDA@capillary column achieved baseline resolution (R = 1.74) of isooctane and n-heptane, which are difficult to be separated due to their close boiling points, 99 °C and 98.4 °C, respectively. Interestingly, isooctane with a slightly higher boiling point retained shorter than n-heptane, possibly owing to its more branched structure as stated above. For the polar butanol isomers in Fig. 4b, G-PDA@capillary column also exhibited nearly baseline separation, suggesting its high resolving ability for polar analytes. The slight tailing of the alcohols may originate from their H-bonding interaction with the residual hydroxyl and carboxyl groups in graphene stationary phase. Additionally, aromatic positional isomers of trichlorobenzenes, dichlorobenzenes and nitroanilines were also determined with good results (Table 2). The above results demonstrated the high selectivity and resolving ability of G-PDA@capillary for alkene isomers, alcohol isomers and aromatic positional isomers.

Fig. 4 Separations of alkane isomers (a) and butanol isomers (b) on G-PDA@capillary column. Peaks for A: (1) 2,2-dimethylbutane, (2) 2,3-dimethylbutane, (3) 3-methylpentane, (4) n-hexane, (5) isooctane and (6) n-heptane; for B: (1) isobutanol, (2) 2-butanol, (3) 2-methyl-1-propanol and (4) 1-butanol. Oven temperature at 33 °C for (a) and 40 °C for (b). Flow rate at 0.4 mL min⁻¹ for (a) and 1.0 mL min⁻¹ for (b), respectively.

Table 2 Separation of aromatic positional isomers on G-PDA@capillary column^a

Analytes	Elution order	tR	k	α
a Oven temperature programs: (a) 80 °C for 1 min to 120 °C at 10 °C min^−1, (b) 40 °C for 1 min to 90 °C at 10 °C min^−1 and (c) 100 °C to 160 °C at 20 °C min^−1 and held at 160 °C for 7 min. Flow rate at 1.0 mL min^−1 for (a) and (c) and 0.5 mL min^−1 for (b).
Trichlorobenzene^a	1,3,5-	2.56	4.24
	1,2,4-	2.85	4.82	1.14
	1,2,3-	3.07	5.28	1.09
Dichlorobenzene^b	m-	7.72	6.67
	p-	7.88	6.83	1.02
	o-	8.43	7.37	1.08
Nitroaniline^c	o-	3.14	5.70
	m-	3.96	7.44	1.31
	p-	5.98	11.75	1.58

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Graphene stationary phase was found to exhibit specific retention for aromatic analytes via π–π stacking interactions in our previous work.¹³ In the present work, a more extensive investigation was made for a wide range of substituted benzenes. As shown in Fig. 5, these analytes were well resolved on G-PDA@capillary column. However, it was found that the three analytes of nitrobenzene, 1,3,5-trichlorobenzene and 2,6-dimethylphenol (peaks 8–10) eluted in the order against their boiling points, which was also against the order expected by π–π stacking interaction. This finding suggested the possible contribution of π–π electron-donor and acceptor (EDA) interaction to the retention of graphene for the aromatics. Since graphene surface is of varied electron density, i.e., relatively electron-rich near edges and electron-deficient close to the center,²⁶ graphene stationary phase may act as an amphoteric adsorbate for π-acceptors and π-donors, respectively. Accordingly, the three analytes eluted in the above order as a result of their different strength of π–π EDA interaction with graphene phase. Briefly, the retention of graphene stationary phase for specific aromatics also involves π–π EDA interaction in addition to π–π stacking interaction.

Fig. 5 Separation of the mixture of 12 aromatics on G-PDA@capillary column. Peaks: (1) toluene, (2) chlorobenzene, (3) m-xylene, (4) bromobenzene, (5) m-chlorotoluene, (6) benzyl chloride, (7) n-butylbenzene, (8) nitrobenzene, (9) 1,3,5-trichlorobenzene, (10) 2,6-dimethylphenol, (11) m-nitrotoluene and (12) ethyl salicylate. Oven temperature: 40 °C for 1 min to 160 °C at 20 °C min⁻¹.

On the basis of above results, the separation performance of G-PDA@capillary column was further investigated by a more complex mixture of different types of analytes while a commercial HP-5MS column was used for comparison. As shown in Fig. 6, G-PDA@capillary well resolved the analytes whereas the commercial column overlapped or coeluted some of them, namely phenetole/2-chlorophenol (peak 9/11), 1,3,5-trichlorobenzene/o-chloroaniline (peak 13/14) and ethyl salicylate/1-decanol (peak 17/18). Noticeably, 2-chlorophenol retained longer on G-PDA@capillary, leading to its later elution than phenetole on the column possibly due to its stronger π–π and H-bonding interactions with the graphene phase. In summary, the above results demonstrated the high selectivity and resolving ability of G-PDA@capillary column for diverse types of analytes varying from nonpolar to polar and from aliphatic to aromatic compounds and showed its potential in GC separations.

Fig. 6 GC separations of the mixture of 20 analytes on G-PDA@capillary column and commercial HP-5MS column. Peaks: (1) n-heptane, (2) toluene, (3) n-hexanal, (4) 1-bromopentane, (5) m-xylene, (6) cyclohexanone, (7) methyl hexanoate, (8) o-chlorotoluene, (9) phenetole, (10) 1,2-dichlorobenzene, (11) o-chlorophenol, (12) 1-octanol, (13) 1,3,5-trichlorobenzene, (14) o-chloroaniline, (15) naphthalene, (16) 1-bromononane, (17) ethyl salicylate, (18) 1-decanol, (19) 1-bromodecane and (20) biphenyl. Oven temperature: 40 °C for 1 min to 160 °C at 10 °C min⁻¹.

Repeatability, reproducibility and thermal stability

G-PDA@capillary column were evaluated for repeatability, reproducibility and thermal stability by relative standard deviation (RSD%) on retention times of the analytes in the mixture of 12 aromatics. Table 3 lists the obtained data for the run-to-run and day-to-day repeatability on one column and the column-to-column reproducibility on four different columns. As shown, the RSD% values were less than 0.02 for run-to-run, 0.16 for day-to-day and 5.0 for column-to-column, respectively, demonstrating the good repeatability and reproducibility of G-PDA@capillary column. For thermal stability, the obtained results on retention times of the analytes over the temperature range of 180–300 °C are shown in Table 4. The RSD% values were below 3.3, suggesting that the column retention remained almost unchanged after it was subjected up to 300 °C, demonstrating the excellent thermal stability of G-PDA@capillary column.

Table 3 Repeatability and reproducibility of G-PDA@capillary column in retention time (t_R, min) for separation of the mixture of 12 analytes

Analytes	Run-to-run (n = 4)		Day-to-day (n = 4)		Column-to-column (n = 4)
	Mean	RSD (%)	Mean	RSD (%)	Mean	RSD (%)
(1) Toluene	1.56	0.01	1.56	0.10	1.56	5.0
(2) Chlorobenzene	2.10	0.01	2.10	0.11	2.10	3.1
(3) m-Xylene	2.27	0.02	2.27	0.09	2.27	3.0
(4) Bromobenzene	2.80	0.01	2.80	0.10	2.81	1.6
(5) m-Chlorotoluene	2.98	0.01	2.99	0.09	2.99	1.4
(6) Benzyl chloride	3.47	0.02	3.48	0.13	3.49	0.83
(7) n-Butylbenzene	3.74	0.01	3.75	0.13	3.75	1.3
(8) Nitrobenzene	4.16	0.01	4.17	0.15	4.20	1.1
(9) 1,3,5-Trichlorobenzene	4.49	0.01	4.50	0.14	4.54	1.3
(10) 2,6-Dimethylphenol	4.72	0.01	4.73	0.16	4.77	1.6
(11) m-Nitrotoluene	4.98	0.01	5.00	0.16	5.03	1.0
(12) Ethyl salicylate	5.41	0.01	5.42	0.16	5.45	0.97

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Table 4 Thermal stability evaluated by retention times (t_R, min) for separation of the mixture of 12 analytes on G-PDA@capillary column after the column was conditioned up to each of the indicated temperatures for 2 h

Analytes	180 °C	200 °C	220 °C	240 °C	260 °C	280 °C	300 °C	RSD%
(1) Toluene	1.57	1.57	1.56	1.55	1.53	1.50	1.45	2.9
(2) Chlorobenzene	2.11	2.11	2.10	2.08	2.06	2.01	1.93	3.2
(3) m-Xylene	2.28	2.28	2.27	2.25	2.22	2.17	2.08	3.3
(4) Bromobenzene	2.81	2.81	2.80	2.78	2.75	2.69	2.58	3.1
(5) m-Chlorotoluene	2.99	2.99	2.99	2.96	2.93	2.86	2.75	3.1
(6) Benzyl chloride	3.49	3.48	3.48	3.46	3.42	3.36	3.23	2.8
(7) n-Butylbenzene	3.75	3.75	3.74	3.72	3.68	3.62	3.51	2.4
(8) Nitrobenzene	4.18	4.17	4.18	4.16	4.13	4.11	4.04	1.2
(9) 1,3,5-Trichlorobenzene	4.51	4.51	4.50	4.47	4.43	4.34	4.19	2.7
(10) 2,6-Dimethylphenol	4.73	4.72	4.73	4.69	4.65	4.57	4.40	2.6
(11) m-Nitrotoluene	5.01	5.01	5.00	4.99	4.95	4.87	4.74	2.0
(12) Ethyl salicylate	5.44	5.43	5.43	5.41	5.36	5.28	5.15	2.0

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4 Conclusions

This work introduced the bio-inspired PDA coating into GC column fabrication, which has not been reported up to now to the best of our knowledge. In this work, integration of graphene stationary phase with PDA on a capillary achieved improved column efficiency and separation performance over G@capillary column. G-PDA@capillary column exhibited high selectivity and resolving ability for analytes of different types owing to its multiple types of interactions including π–π stacking, π–π EDA, H-bonding, dipole–dipole and dispersion interactions. Also, it showed good repeatability, reproducibility and thermal stability up to 300 °C. The proposed facile and efficient strategy also provides a new alternative to GC column fabrication that may also be applicable for other types of stationary phases.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21075010, 21174019) and the 111 Project B07012 in China for this work.

References

1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669.
2. J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, T. J. Booth and S. Roth, Nature, 2007, 446, 60–63.
3. A. K. Geim, Science, 2009, 324, 1530–1534.
4. M. J. Allen, V. C. Tung and R. B. Kaner, Chem. Rev., 2010, 110, 132–145.
5. S. L. Zhang, Z. Du and G. K. Li, Anal. Chem., 2011, 83, 7531–7541.
6. J. Fan, Z. L. Dong, M. L. Qi, R. N. Fu and L. T. Qu, J. Chromatogr. A, 2013, 1320, 27–32.
7. W. P. Zhang, J. Zhang, T. Bao, W. Zhou, J. W. Meng and Z. L. Chen, Anal. Chem., 2013, 85, 6846–6854.
8. B. T. Zhang, H. F. Li, X. X. Zheng, Y. G. Teng, Y. Liu and J. M. Lin, J. Chromatogr. A, 2014, 1370, 9–16.
9. Q. S. Qu, C. H. Gu and X. Y. Hu, Anal. Chem., 2012, 84, 8880–8890.
10. X. Liu, X. L. Liu, M. Li, L. P. Guo and L. Yang, J. Chromatogr. A, 2013, 1277, 93–97.
11. X. Q. Zhang, S. Chen, Q. Han and M. Y. Ding, J. Chromatogr. A, 2013, 1307, 135–143.
12. Q. S. Qu, Y. Q. Shen, C. H. Gu, Z. L. Gu, Q. Gu, C. Y. Wang and X. Y. Hu, Anal. Chim. Acta, 2012, 757, 83–87.
13. J. Fan, M. L. Qi, R. N. Fu and L. T. Qu, J. Chromatogr. A, 2015, 1399, 74–79.
14. Y. Feng, C. G. Hu, M. L. Qi, R. N. Fu and L. T. Qu, Chin. Chem. Lett., 2015, 26, 47–49.
15. H. Lee, S. M. Dellatore, W. M. Miller and P. B. Messersmith, Science, 2007, 318, 426–430.
16. J. Liebscher, R. Mrówczyński, H. A. Scheidt, C. Filip, N. D. Hădade, R. Turcu, A. Bende and S. Beck, Langmuir, 2013, 29, 10539–10548.
17. Q. Ye, F. Zhou and W. M. Liu, Chem. Soc. Rev., 2011, 40, 4244–4258.
18. S. M. Kang, N. S. Hwang, J. Yeom, S. Y. Park, P. B. Messersmith, I. S. Choi, R. Langer, D. G. Anderson and H. Lee, Adv. Funct. Mater., 2012, 22, 2949–2955.
19. Y. L. Liu, K. L. Ai and L. H. Lu, Chem. Rev., 2014, 114, 5057–5115.
20. X. B. Yin and D. Y. Liu, J. Chromatogr. A, 2008, 1212, 130–136.
21. R. J. Zeng, Z. F. Luo, D. Zhou, F. Cao and Y. Wang, Electrophoresis, 2010, 31, 3334–3341.
22. R. P. Liang, X. N. Wang, C. M. Liu, X. Y. Meng and J. D. Qiu, J. Chromatogr. A, 2014, 1323, 135–142.
23. L. J. Zhu, Y. L. Lu, Y. Q. Wang, L. Q. Zhang and W. C. Wang, Appl. Surf. Sci., 2012, 258, 5387–5393.
24. Y. X. Xu, K. X. Sheng, C. Li and G. Q. Shi, ACS Nano, 2010, 4, 4324–4330.
25. Z. L. Dong, C. C. Jiang, H. H. Cheng, Y. Zhao, G. Q. Shi, L. Jiang and L. T. Qu, Adv. Mater., 2012, 24, 1856–1861.
26. X. Wang, S. Huang, L. Zhu, X. Tian, S. Li and H. Tang, Carbon, 2014, 69, 101–112.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14111g

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