Effect of nanoporous structure and polymer brushes on the ionic conductivity of poly(methacrylic acid)/anode aluminum oxide hybrid membranes

Abstract

Anode aluminum oxide (AAO) porous materials have been widely used in ionic translocation for many biological and chemical studies. However, the lack of stimuli-response of this material limits its applications for the precise control of ionic transportation by the external environment. In this study, we functionalized the internal nanopores of AAO membranes to generate polyelectrolyte-filled pH-responsive membranes whose ionic conductivity could be readily controlled by changing the pH value. AAO membranes with different pore sizes (25, 75 and 100 nm) were modified with poly(methacrylic acid) (PMAA) by a “grafting-to” approach. Thermogravimetric and SEM analysis revealed that the extent of PMAA infiltration strongly depends upon the relative sizes of the nanopores and the PMAA concentration. Increasing the size of the nanopores enables the infiltration of PAA solution with a higher concentration. Electrochemical impedance spectroscopy demonstrated that the membrane conductivity decreases from 7.87 × 10⁻⁴ S cm⁻¹ at pH 1 to 5.72 × 10⁻⁵ S cm⁻¹ at pH 7. The functionalized AAO nanopores showed significant sensitivity to pH value, whereas a valve effect was observed in the pH range between 4 and 5. Our fabricated PMAA-AAO membranes show promising potential to be used as pH sensors and smart valves in micro-/nano-total analysis chips for biomedical and chemical applications.

---

Introduction

Ionic transportation through bio-inspired nano-channels, or bio-mimics, has been showing promise in revealing important information about biological and chemical processes.^1,2 Bio-mimic interfaces with adjustable chemical and/or physical properties can adapt to surrounding environments and can also recognize and manipulate specific ions and molecules on the micro/nano-scale.^3–9 Porous structures have been used to construct straight channels with stimuli-responsive functions in response to changes in pH,^10–14 temperature,^15–18 light,¹⁹ humidity,²⁰ electric and magnetic fields,^21,22 ion strength and concentration of chemical species.^23,24 To achieve smart membranes, modifying the internal pores or channels with functional molecules or macromolecules is necessary, especially for porous membranes with ordered nanoscale-channels in confined geometries.

Ordered porous materials with versatile character conferred by their individual compositions, morphologies, and functionalities have gradually become predominant in materials research due to their distinctive material features, such as well-defined pore textures and mechanical properties.²⁵ For instance, anodized aluminum oxide (AAO) membranes with cylinder pores have been widely exploited in various fields and are already commercially available,^26,27 mostly due to their advantages, including straight and ordered pore channels, homogeneous pore size distribution, uniform membrane thickness, promising mechanical strength, reusability, and good resistance to microbial and thermal impacts.^27,28

However, the lack of stimuli-response of these membranes limits their applications in the study of ionic transportation due to the need for precise control of the surface within the nanopores. In order to functionalize the inner surface of AAO membranes, several methods have been explored, including self-assembly processes,^29,30 polymer grafting,³¹ sol–gel processing,³² electrochemical methods³³ and deposition.³⁴ The attachment of a weak polyelectrolyte inside the nanopores is an alternative functionalization approach that allows fine control of the surface chemistry, functionality, density and thickness of the coating. Covalent grafting is generally carried out through either “grafting-to” or “grafting-from” approaches via atom transfer radical polymerization (ATRP),^35,36 reversible addition–fragmentation chain transfer polymerization (RAFT)^13,18 or plasma-induced graft polymerization.^31,37 The resulting polymer brushes contain multiple binding sites that give rise to high binding capacities. Using a “grafting-from” approach, previous studies have reported thermally-responsive brushes (poly(N-isopropylacrylamide) (PNIPAM)),³⁸ pH-responsive brushes (poly(dimethylamino)ethyl methacrylate (PDMAEMA),¹⁷ poly(acrylic acid) (PAA),³⁹ poly(γ-benzyl-L-glutamate) (PBLG)^40,41) and chemical species-responsive brushes (poly(2-hydroxyethyl methacrylate) (PHEMA) derivatives).⁴² Considering these, it is more challenging to obtain functional polymers using the “grafting-from” method than the “grafting-to” method. For polymer-grafted hybrid materials, an ideal pore-filled stimuli-responsive porous membrane should have a straight-pore structure, good uniformity and controlled functionality (i.e. suitable molecular weight, fewer impurities, and significant “gateable” ionic filtration). For example, the infiltration of polyelectrolytes into mesoporous silica has been extensively explored by Caruso and collaborators and subsequently exploited to build mesoporous polymer-based spheres for delivery purposes.^43–46

Furthermore, an ongoing challenge in the synthesis of ionic transport channels is understanding the relationship between functionality and responsiveness. In this work, we describe the functionalization of a pH-responsive poly(methacrylic acid) (PMAA) within the nanopores of AAO membranes with different pore sizes (25, 75 and 100 nm), which demonstrated promising controllability in proton-dependent ionic transport. The implementation of the pH-valve nanochannels is based on the “grafted-to” approach, while PMAA with narrow molecular weight distribution was synthesized via a RAFT polymerization. The synthesized PMAA showed several advantages, including: (1) uniform distribution of molecular size, which allows for the determination of the relationship between brush thickness and degree of loading; (2) high stability under a range of solution conditions as a weak polyelectrolyte, thus enabling quantitative study of the effects of charge density and conformation of macromolecules as they correspond to pH value.

Experimental section

Materials

AAO membranes (100 μm in thickness, 25 mm in diameter, 25 nm, 75 nm and 100 nm pore size, respectively) were purchased from Nanjing XFNANO Materials Tech Co., Ltd, China. Poly(methacrylic acid) (PMAA) (the degree of polymerization is 100, M_n = 7900, PDI = 1.08) were synthesized by a RAFT method based our previous work.⁴⁷ Details of the synthesis and characterization of the PMAA brushes are presented in the ESI (Fig. S1 and S2†). γ-Glycidoxypropyltrimethoxysilane (GPTMS) and other chemicals were of analytical grade and used without any additional purification. All solutions were prepared using deionized Milli-Q water.

All pH buffer solutions were prepared according to a literature procedure. In a typical preparation procedure, specified volumes of 0.1 M HCl (pH 2 to 4) or 0.1 M NaOH (pH 5 to 6) were added to 50 mL of a solution containing 0.1 M potassium hydrogen phthalate (pH 2 to 5) or 0.1 M NaH₂PO₄ (pH 6), and the solution volume was diluted to 100 mL using deionized water. The pH was then measured using a pH meter (PHS-3C, Shanghai Leici Instrument Co., China) and adjusted by the addition of HCl, NaOH, or the appropriate salt. The hydrodynamic radius of gyration (R_g) was measured using static laser scattering (SLS) and dynamic laser scattering (DLS) (BI-200SM, Brookhaven Co., USA). The result is depicted in Fig. S3.†

Preparation of PMAA-grafted AAO membranes

The AAO membrane was immersed in a solution of 5 mL GPTMS-30 mL ethanol-2 mL sodium acetate buffer solution (50 mM, pH 5.0) for activation. The polytetrafluoroethylene device containing the above solution was placed under vacuum for 20 min. The membrane was left in this solution and was ultrasonicated for 15 min under ambient pressure before being rinsed with ethanol and then cured by heating under vacuum at 85 °C for 2 h.

PMAA was dissolved in deionized water to form a PMAA solution. In order to graft the PMAA chains, the activated membrane was ultrasonicated in 10 mL of the above PMAA solution for 15 min at room temperature and reacted for 6 h at 60 °C. The membrane was placed onto a sintered glass filter holder and the remaining PMAA solution was sucked slowly through the membrane. The hybrid membrane was carefully cleaned by ultrasound for 15 min and continuously washed with deionized water for 2 h at room temperature in order to remove excess unanchored PMAA chains. This was followed by drying the wet membrane in an oven at 115 °C for 1 h and then slowly cooling it to room temperature. Non-covalently bound PMAA was removed by washing the membrane with deionized water. The reaction schema for grafting PMAA onto the AAO membrane is shown in Fig. S4.† The amounts of PMAA grafted onto the AAO membranes were measured by thermogravimetric analysis (TGA, TA SDTQ600) in the range of 100–800 °C with a heating rate of 20 °C min⁻¹ in a nitrogen atmosphere.

Scanning electron micrographs (SEM, Hitachi S-4700) were taken to analyze the morphologies of the AAO membranes. The samples were previously coated with platinum. Fourier transform infrared spectroscopy (FTIR, Nicolet 6700 spectrophotometer) was used to study the grafting procedure of organic molecules to the AAO surface; the AAO membranes were tableted in KBr disks and the experiments were conducted in the range of 400 to 4000 cm⁻¹.

Electrochemical impedance spectroscopy (EIS)

EIS was used to investigate the barrier properties of the PMAA-grafted membranes for different pH values. EIS was performed with a model CHI605B potentiostat-galvanostat (Shanghai Chenhua Instrument Co., Ltd, China) interfaced to a personal computer.⁴⁸ A beaker type three-electrode cell, which consisted of a pH buffer solution prepared as described above, equipped with a sample current collector on an AAO membrane as the working electrode, an Ag/AgCl electrode as the reference electrode, and a Pt plate (1 cm × 1 cm) as a counter electrode, was used.⁴⁹ A Luggin capillary tip was kept as close as 1 to 2 mm to the working electrode, and was set to minimize error due to iR drop in the electrolyte. The measurements were made at the open circuit potential with a 5 mV AC perturbation. All data were controlled in the range from 10⁴ to 10⁻¹ Hz using 48 points and were fitted with an equivalent circuit model to determine resistance values. At each pH value, sufficient time was allowed to ensure that the membrane reached a stable state as evidenced by the accumulation of spectra that did not change with time.

Results and discussion

PMAA/AAO composite membranes via a “grafting-to” method

Compared to other methods, the “grafting-to” methods offer greater control of the amount of PMAA brushes through adjusting the reaction conditions. For instance, the concentration of PMAA (M_n = 7900, PDI = 1.08, characterized in ESI Fig. S1 and S2†) solution enables efficient adjustment of the grafted amount of polymer brushes. The R_g is 7.3 nm, determined by SLS (Fig. S3†), which is smaller than the pore diameters of the AAO membranes. This means that random coils of PMAA can easily enter the bigger holes in the AAO membrane. Moreover, the hybrid membranes were carefully cleaned to dismantle any physically adsorbed PMAA chains. To accurately determine the loading of the PMAA brushes, a TGA method was applied for quantitative analysis. Fig. 1 shows the weight losses of PMAA/AAO composite membranes with 25 nm pore size. This illustrates that the neat AAO membrane is quite thermally stable, losing less than 5% weight after 800 °C calcination. When the concentration of PMAA solution was increased from 5 to 15 wt%, the amount of grafted polymer increased from 2.8 to 5.3 wt%. It is noted that at a lower grafting amount (2.8 wt%), the weight loss differs slightly compared to higher grafting amounts. This may suggest that the PMAA brushes cover less of the porous surface of the AAO membrane. This phenomenon can be partly supported by comparing the SEM images of various grafted hybrid membranes (Fig. 3a and b; more detailed images are shown in Fig. S5a and b†). At higher grafting amounts (4.5 and 5.3 wt%), the PMAA brushes can preferably coat the porous surface of the AAO membrane, and the weight losses show obvious steps around 420 °C, which is attributed to the pyrolysis of the PMAA brushes. Fig. 2 summarizes the detailed grafted amounts of composite membranes with various pore sizes (25, 75 and 100 nm, respectively). It can be seen that as the concentration of PMAA solution increases, the amount of grafted PMAA on the AAO membrane surface increases. However, in the case of the AAO membrane with 100 nm pores, the amount of PMAA is an increment of 8.1 wt% from a 15 wt% concentration of PMAA solution. The amount of grafted polymer brushes is limited due to the steric hindering and surface density of the chemical groups. A higher concentration of PMAA solution is too viscous and cannot readily permeate the nano-scale pores.

Fig. 1 TGA curves of PMAA/AAO composite membranes with 25 nm pore size, grafted with different PMAA amounts.

Fig. 2 Relation between the grafted amount of PMAA brushes and the dosage concentration of PMAA.

Fig. 3 illustrates that the morphology of the original AAO membrane and the PMAA/AAO composite membranes with different pore sizes and 15 wt% concentration of PMAA solution. The neat AAO membranes have almost perfect vertical nano-scale pores, and the diameters are clearly uniform. After the “grafting-to” process of PMAA, thick polymer layers are obvious in all the nanopores, and the diameter of the modified nanopores becomes narrower. In the 25 nm pore sample (Fig. 3b), it can be observed that the morphology of the nanopores is oblique and rough, indicating that the polymer brushes may block these smaller pores. In contrast, larger pores (75 and 100 nm) show straight channels, despite the thicker polymer layers (Fig. 3d and f). These phenomena are important factors that affect the electrochemical properties of the PMAA/AAO composite membranes. In the case of a diluted concentration of PMAA solution, the grafted polymer brushes are diminished; therefore, the polymer “jams” the nanopores and the pore sizes are reduced, particularly for the AAO membrane with 25 nm pores. More SEM results from different preparation conditions are presented in Fig. S5 and S6,† which present a top-view of the AAO membranes and different thicknesses of polymer layers.

Fig. 3 SEM images of fractures of pristine AAO membranes with various pore size (a, c and e) and composite membranes: sample (b) with 25 nm pore size was grafted with 5.3 wt% PMAA; sample (d) with 75 nm pore size was grafted with 7.5 wt% PMAA; sample (f) with 100 nm pore size was grafted with 8.1 wt% PMAA.

The functional groups contained in the PMAA/AAO composite membranes were also identified using FT-IR in order to verify the chemical groups. The respective FTIR curves are presented in Fig. 4. The peaks related to the carboxylic groups appeared in an obvious broad absorption peak between 1500 to 1750 cm⁻¹. This broad signal is very different from that of carboxylic acids and their salts. Due to strong hydrogen bonds, the carboxylic group can be associated and the stretching vibration signal of the carbonyl groups is around 1700 cm⁻¹.

Fig. 4 The FTIR spectra of the pristine AAO membrane (25 nm pore size) and different composite membranes.

Due to the strong hydrophilicity of PMAA and the porous surface of the AAO membranes, the carboxylic groups may be moist, resulting in the widening of this signal peak. Two bands at 1454 and 1240 cm⁻¹ are attributed to the in-plane bending vibrations of associated hydroxy groups (–OH) and the stretching vibration of the C–O bonds, respectively. In addition, compared with the original AAO membrane, two additional peaks for methylene groups (–CH₂–) also clearly appear at 2918 cm⁻¹ and 2856 cm⁻¹ in the FTIR spectra of the PMAA/AAO composite membranes. The broad band at 3420 cm⁻¹ is attributed to the stretching vibration of the O–H groups of PMAA. The characteristic bands of methylene and hydroxyl groups demonstrated the successful grafting of PMAA onto the AAO membrane surface.⁵⁰

pH-responsive performance

To investigate the effect of pH value on ionic conductivity, we fabricated a three-electrode cell system which was characterized using EIS measurements in various pH buffer solutions. The EIS was measured through the three-electrode cell system to obtain the bulk resistance (R_b) of the grafted AAO films, which can enable the calculation of the ionic conductivity through the nano-scale porous channels following the equation:
where γ is the ionic conductivity of the porous film, R_b is the bulk resistance of the porous film, D is the thickness of the carbon working electrode, and S is the area of the modified porous film. To design a smart flow control system, it is necessary to analyze the gating effect as a function of pore size, degree of polymerization, and grafting density. The performance of the nanoporous system at a given grafting condition is determined by the maximum change in permeability between the closed and open states.

Fig. 5 shows that the ionic conductivities of the grafted-AAO membranes significantly dropped when the pH value was increased from 4 to 5, indicating that the graft chains form a pH-stimulus “gate” in the nanopores of the AAO membrane. The neat AAO membrane without grafted PMAA showed higher ionic conductivity than the PMAA-grafted membranes. The response sensitivity of the hybrid membrane depends on the amount of grafted PMAA. A low surface density of PMAA did not significantly affect the ionic conductivity, while a high surface density of PMAA reduced the ionic conductivity; both were almost independent of pH value.

Fig. 5 The membrane conductivity derived from the impedance spectra upon variation of the pH value. (a) AAO membranes with 25 nm pores. (b) Nyquist plot (Z_im versus Z_re) for the electrochemical impedance measurements performed at different pH values. The pore size of the AAO membrane is 25 nm. The amount of grafted PMAA was 4.5 wt%. (c) AAO membranes with 75 nm pores. (d) AAO membranes with 100 nm pores.

The results for the membrane with 25 nm nanopores (Fig. 5a) suggest that the surface density of grafted PMAA in a moderate range (i.e. 4.5 wt%) showed the highest sensitivity to the environmental conditions. When the weight loading of grafted PMAA on the membrane was about 4.8%, the ionic conductivity through the PMAA-grafted membrane was strongly dependent on pH and changed rapidly between pH 4 and 5 (in according with a previous report (PMAA pK_a = 4.5)⁵¹), which indicates that the mesoporous membrane can be controlled by pH. Fig. 5b particularly presents the impedance spectra for various pH values in the form of Nyquist plots where the semicircle domains approximately correspond to the membrane resistance values for the electrochemical processes. The average membrane conductivity, calculated from the reciprocal resistance, decreases as the pH increases. The membrane conductivity decreases from 7.87 × 10⁻⁴ S cm⁻¹ at pH 1 to 5.72 × 10⁻⁵ S cm⁻¹ at pH 7; notably, the membrane conductivity plummets from 6.64 × 10⁻⁴ S cm⁻¹ at pH 4 to 6.47 × 10⁻⁵ S cm⁻¹ at pH 5, a 90% decline, suggesting a measurable barrier to ion transfer. The ionic conductivity of the membrane hardly changes at pH values lower than 4 or higher than 5. As illustrated in Fig. 6 (at low pH), PMAA chains can be protonated and ionized in aqueous solution with grafted polymer swelling, excellently demonstrating their strong hydrophilicity; on the other hand, in the high pH region, PMAA is deprotonated to form an extended structure that covers the pores. The PMAA response to pH is reversible, as evidenced by the similar membrane conductivities obtained upon first decreasing pH incrementally from 7 to 1 and then increasing pH incrementally from 1 back to 7.

Fig. 6 Illustration of pH-responsive ionic permselectivity through a porous membrane grafted with ionizable PMAA.

Increasing the weight loading of PMMA on the AAO membrane surface significantly decreases the membrane conductivity. For the membrane with 25 nm pores (Fig. 5a), a thick layer of PMAA brushes (5.3 wt%) will impede the transport of protons through the modified pores, which has been confirmed by the SEM results (Fig. 3b). Moreover, the approximately 90% drop of the membrane conductivity between pH 4 and 5 is greatly reduced, indicating that an excessive amount of PMMA can congest pores of a certain diameter (25 nm) and disrupt the “gate” on/off status. Fig. 5c shows that with a 7.5% weight loading of PMAA, the membrane conductivity of the 75 nm pores exhibits a noticeable roll-off from 6.11 × 10⁻⁴ S cm⁻¹ at pH 1 to 7.71 × 10⁻⁵ S cm⁻¹ at pH 7, with an 87% drop from 5.76 × 10⁻⁴ S cm⁻¹ at pH 4 to 7.47 × 10⁻⁵ S cm⁻¹ at pH 5, similar to the phenomenon observed for the modified AAO membrane with 25 nm pores. In the case of the membrane with 100 nm pores (Fig. 5d), the ionic conductivities were slightly affected by the PMAA brushes. Even with 8.2 wt% grafted PMAA, the conductivity decreased from 7.94 × 10⁻⁴ S cm⁻¹ to 3.95 × 10⁻⁴ S cm⁻¹ when the pH value increased from 1 to 7. This result corresponds with the simulation results, which show that polymer brushes of 100 DPn cannot block a concave cylinder pore of 100 nm diameter.^52,53 In summary, PMAA-functionalized AAO membranes present pH-responsive characteristics, which are precisely controlled by the polymerization conditions. The endurance of the aforementioned hybrid membranes was also preliminarily evaluated. These membranes can sustain 10 cycles of pH-reversible tests and show minimal loss of ionic permselectivity (as shown in Fig. S7†). Owing to their excellent pH-stimulus, the pore size and weight loading of the PMAA brushes are of significant influence; suitable conditions can cooperate and exert a powerful “gate” control method.

Conclusions

In this work, we grafted a weak polyelectrolyte (PMAA) onto the nanoporous surface of AAO membranes, which functionalized the internal nanopores as “gates” responding to different pH values and presented a structural transformation with “recoiling-stretching” chain aggregation. Comprehensive studies were undertaken to understand the controlling factors of the pH-stimuli “gate” using three modified AAO membranes with different pore sizes. The PMAA brushes were synthesized by a RAFT polymerization and a “grafting-to” method with good control of the molecular weights and grafted amounts. The higher the concentration of the polymer, the narrower the modified pore size of the AAO membranes; particularly, the AAO membrane with 25 nm pores was blocked by a 5.3 wt% amount of PMAA brushes. To elicit an efficient pH response, the combination of the right size nanopores and a suitable loading of PMAA brushes inside the nanopores is essential. In the case of the AAO membranes containing 25 nm nanopores, a 4.8 wt% amount of PMAA is optimal for a pH-sensitive composite membrane, contributing sensitive ionic conductivity, most notably an abrupt 90% transition between pH 4 and 5. In other cases, grafting 7.8 wt% PMAA brushes onto a composite membrane with 75 nm pores causes a similar 87% roll-off of ionic conductivity when the pH value is increased from 4 to 5; the ionic conductivity of composite membranes with 100 nm pores is less affected by PMAA brushes, even with a higher loading of 8.2 wt%, indicating that the “gate” cannot be totally shut down in 100 nm nanopores with the PMAA being used here. Our presented work provides an efficient and easy functionalization strategy to manipulate ionic permselectivity in nanopores, with efficiency controllable by the concentration of the polymer and the size of the nanochannel. Furthermore, our fabricated pH-responsive nanopores could find wide use in bio-mimics of “gated” ionic transport, as well as “smart valves” in micro-/nano-fluidic systems.

Acknowledgements

This material is based upon work funded by Natural Science Foundation of China under Grant No. 21274131, 51273178, 51203139 and 51303158. This work is also funded by Natural Science Foundation of Zhejiang Province under Grant No. LY15E030005. Tairong Kuang and Dajiong Fu would like to acknowledge the support of the Chinese Scholarship Council and the financial support. Last but not least, all the laboratory members and co-workers are appreciated for their kindly discussion and assistance.

References

1. S. S. Patel, B. J. Belmont, J. M. Sante and M. F. Rexach, Cell, 2007, 129, 83–96.
2. C. Dekker, Nat. Nanotechnol., 2007, 2, 209–215.
3. W. Sparreboom, A. van den Berg and J. C. T. Eijkel, Nat. Nanotechnol., 2009, 4, 713–720.
4. P. E. Boukany, A. Morss, W. C. Liao, B. Henslee, H. C. Jung, X. L. Zhang, B. Yu, X. M. Wang, Y. Wu, L. Li, K. L. Gao, X. Hu, X. Zhao, O. Hemminger, W. Lu, G. P. Lafyatis and L. J. Lee, Nat. Nanotechnol., 2011, 6, 747–775.
5. X. Hou, W. Guo and L. Jiang, Chem. Soc. Rev., 2011, 40, 2385–2401.
6. X. Hou, H. Zhang and L. Jiang, Angew. Chem., Int. Ed., 2012, 51, 5296–5307.
7. W. Guo, Y. Tian and L. Jiang, Acc. Chem. Res., 2013, 46, 2834–2846.
8. S. Horike, D. Umeyama and S. Kitagawa, Acc. Chem. Res., 2013, 46, 2376–2384.
9. K. Gao, L. Li, L. He, K. Hinkle, Y. Wu, J. Ma, L. Chang, X. Zhao, D. Gallego-Perez, S. Eckardt, J. Mclaughlin, B. Liu, D. F. Farson and L. J. Lee, Small, 2014, 10, 1015–1023.
10. M. Tagliazucchi, O. Azzaroni and I. Szleifer, J. Am. Chem. Soc., 2010, 132, 12404–12411.
11. M. Tagliazucchi and I. Szleifer, Soft Matter, 2011, 7, 5669–5676.
12. G. J. A. A. Soler-Illia and O. Azzaroni, Chem. Soc. Rev., 2011, 40, 1107–1150.
13. Q. Zhao, M. Yin, A. P. Zhang, S. Prescher, M. Antonietti and J. Yuan, J. Am. Chem. Soc., 2013, 135, 5549–5552.
14. F. Chen, X. Jiang, T. Kuang, L. Chang, D. Fu, J. Yang, P. Fan and M. Zhong, Polym. Chem., 2015, 6, 3529–3536.
15. F. Schacher, M. Ulbricht and A. H. E. Müller, Adv. Funct. Mater., 2009, 19, 1040–1045.
16. H. C. Lee, H. Y. Hsueh, U. S. Jeng and R. M. Ho, Macromolecules, 2014, 47, 3041–3051.
17. S. Ma, J. Liu, Q. Ye, D. Wang, Y. Liang and F. Zhou, J. Mater. Chem. A, 2014, 2, 8804–8814.
18. Q. Li, X. He, Y. Cui, P. Shi, S. Li and W. Zhang, Polym. Chem., 2015, 6, 70–78.
19. A. Brunsen, J. Cui, M. Ceolin, A. del Campo, G. J. Soler-Illia and O. Azzaroni, Chem. Commun., 2012, 48, 1422–1424.
20. J. Wang, M. Su, J. Qi and L. Chang, Sens. Actuators, B, 2009, 139, 418–424.
21. L. Chang, M. Howdyshell, W. Liao, C. Chiang, D. Gallego-Perez, Z. Yang, J. Byrd, L. Wu, N. Muthusamy, L. J. Lee and R. Sooryakumar, Small, 2015, 11, 1818–1828.
22. B. V. V. S. P. Kumar, K. V. Rao, S. Sampath, S. J. George and M. Eswaramoorthy, Angew. Chem., Int. Ed., 2014, 53, 13073–13077.
23. J. Elbert, F. Krohm, C. Rüttiger, S. Kienle, H. Didzoleit, B. N. Balzer, T. Hugel, B. Stühn, M. Gallei and A. Brunsen, Adv. Funct. Mater., 2014, 24, 1591–1601.
24. Q. Zhao, J. W. C. Dunlop, X. Qiu, F. Huang, Z. Zhang, J. Heyda, J. Dzubiella, M. Antonietti and J. Yuan, Nat. Commun., 2014, 5, 4293.
25. W. Li, Q. Yue, Y. Deng and D. Zhao, Adv. Mater., 2013, 25, 5129–5152.
26. A. M. M. Jani, D. Losic and N. H. Voelcker, Prog. Mater. Sci., 2013, 58, 636–704.
27. W. Lee and S. J. Park, Chem. Rev., 2014, 114, 7487–7556.
28. M. Aramesh and J. Cervenka, Nanomedicine, One Central Press, Manchester, UK, 2014.
29. X. Gong, L. L. Han, Y. A. Yue, J. R. Gao and C. Y. Gao, J. Colloid Interface Sci., 2011, 355, 368–373.
30. Z. Wang, Y. W. Li, X. H. Dong, X. F. Yu, K. Guo, H. Su, K. Yue, C. Wesdemiotis, S. Z. D. Cheng and W. B. Zhang, Chem. Sci., 2013, 4, 1345–1352.
31. R. Barbey, L. Lavanant, D. Paripovic, N. Schüwer, C. Sugnaux, S. Tugulu and H. A. Klok, Chem. Rev., 2009, 109, 5437–5527.
32. S. B. Lee, D. T. Mitchell, L. Trofin, T. K. Nevanen, H. Sörderlund and C. R. Martin, Science, 2002, 296, 2198–2200.
33. Y. Qin, L. Liu, R. Yang, U. Gösele and M. Knez, Nano Lett., 2008, 8, 3221–3225.
34. C. Mu, J. Zhang and D. Xu, Nanotechnology, 2010, 21, 015604.
35. A. Brunsen, C. Díaz, L. I. Pietrasanta, B. Yameen, M. Ceolín, G. J. A. A. Soler-Illia and O. Azzaroni, Langmuir, 2012, 28, 3583–3592.
36. A. Andrieu-Brunsen, Se. Micoureau, M. Tagliazucchi, I. Szleifer, O. Azzaroni and G. J. A. A. Soler-Illia, Chem. Mater., 2015, 27, 808–821.
37. P. Jain, G. L. Baker and M. L. Bruening, Annu. Rev. Anal. Chem., 2009, 2, 387–408.
38. Q. Fu, G. V. R. Rao, S. B. Basame, D. J. Keller, K. Artyushkova, J. E. Fulghum and G. P. López, J. Am. Chem. Soc., 2004, 126, 8904–8905.
39. C. Song, W. Shi, H. Jiang, J. Tu and D. Ge, J. Membr. Sci., 2011, 372, 340–345.
40. K. H. Lau, H. Duran and W. Knoll, J. Phys. Chem., 2009, 113, 3179–3189.
41. L. Sun, J. Dai, G. L. Baker and M. L. Bruening, Chem. Mater., 2006, 18, 4033–4039.
42. P. Jain, L. Sun, J. Dai, G. L. Baker and M. L. Bruening, Biomacromolecules, 2007, 8, 3102–3107.
43. A. S. Angelatos, Y. Wang and F. Caruso, Langmuir, 2008, 24, 4224–4230.
44. Y. Wang, A. S. Angelatos, D. E. Dunstan and F. Caruso, Macromolecules, 2007, 40, 7594–7600.
45. Y. Wang and F. Caruso, Chem. Mater., 2006, 18, 4089–4100.
46. X. Gong, L. L. Han, J. R. Gao and C. Y. Gao, Colloids Surf., A, 2012, 396, 299–304.
47. F. Chen, D. Dai, J. Yang, Z. Fei and M. Zhong, J. Macromol. Sci., Part A: Pure Appl.Chem., 2013, 50, 1002–1006.
48. F. Chen, W. Zhou, H. Yao, P. Fan, J. Yang, Z. Fei and M. Zhong, Green Chem., 2013, 15, 3057–3063.
49. L. Chang, C. Liu, Y. He, H. Xiao and X. Cai, Sci. China: Chem., 2011, 54, 223–230.
50. T. R. Kuang, H. Y. Mi, D. J. Fu, X. Jing, B. Y. Chen, W. J. Mou and X. F. Peng, Ind. Eng. Chem. Res., 2015, 54, 758–768.
51. N. Schüwer and H.-A. Klok, Langmuir, 2011, 27, 4789–4796.
52. S. A. Egorov, A. Milchev, L. Klushin and K. Binder, Soft Matter, 2011, 7, 5669–5676.
53. S. P. Adiga and D. W. Brenner, J. Funct. Biomater., 2012, 3, 239–256.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08990e

‡ These authors contributed equally to this work.

---