Cucurbit[8]uril‐Regulated Nanopatterning of Binary Polymer Brushes via Colloidal Templating

Abstract

Sub‐micrometer/nanoscale patterned polymer brushes are prepared by employing cucurbit[8]uril (CB[8]) as a supramolecular recognition motif to assemble functional silica colloids onto Au surfaces as a sacrificial nanopatterning template. By employing CB[8]‐mediated host–guest interactions at the interface, it is possible to readily generate nanopatterned materials in aqueous media under ambient conditions.

Interest in the fabrication of micro‐/nanopatterned polymer surfaces with high precision has grown substantially in recent years, on account of potential applications in surface‐based technologies including photovoltaic devices,1 semiconductors,2, 3 photonic crystals,4 nanofluidic devices,5 and platforms for tissue engineering.6 In particular, development of less stringent alternative approaches to traditional nanoimprint lithography methods, which use photons or electrons to modify the chemical and physical properties of surface‐bound resins, is crucial in many areas, for example, photonics, chip‐based sensors, and most biological applications.7 Polymer brushes are widely applied in surface engineering as alternatives to self‐assembled monolayers due to their mechanical and chemical robustness, coupled with a high degree of synthetic flexibility and facile incorporation of functional moieties.8 Patterned polymer brushes have attracted considerable interest not only in a myriad of applications, ranging from semiconductors9 to biomaterials,10 but also as useful tools to study fundamental questions of surface‐tethered polymer films, such as swelling behavior or molecular organization.11, 12 Despite numerous strategies such as microcontact printing,13 electron beam,14 or UV irradiation15 assisted nanolithography techniques for both laboratory and industrial use to prepare patterned polymer surfaces, few approaches have been reported for the preparation of laterally patterned binary brushes on the sub‐micrometer scale under ambient conditions.16, 17

The monolayer colloidal crystal (MCC) templated self‐assembly process has proven to be a promising alternative strategy to fabricate ordered arrays with well‐defined micro and nanostructures because of its flexibility and controllable morphologies.18, 19, 20, 21 Since Deckman et al. reported the formation of identical sub‐micrometer microcolumnar structures in 1982,22 the MCC templating approach has been employed to prepare various ordered micro‐/nanostructures such as nanosphere arrays,23 nanoring arrays,24 and hierarchical polymer surfaces,25 by different methods including dip coating,26 spin coating,27 electrochemical deposition,28 etc. A main challenge that remains in the area of MCC templating is the controllable and reversible immobilization of colloidal particles on surfaces.29 For example, in the fabrication of hierarchical binary structures, the surface‐attachment should be strong enough so the colloids do not fall off the surface during the assembly of the first required material, yet need to be easily removed from the surface on demand to leave void spaces available for sub­sequent material deposition.30, 31

We herein demonstrate a solely supramolecular route as a promising alternative process for the preparation of sub‐micrometer and nanopatterned brush surfaces. Cucurbit[8]uril (CB[8]) is a pumpkin‐shaped macrocyclic host that has received tremendous interest in the supramolecular community, on account of its ability to simultaneously incorporate two guest molecules (e.g., viologen (MV²⁺) as the first guest and azobenzene (Azo) or naphthol (Np) as the second guest) and their exceptional recognition properties in aqueous medium.32, 33, 34, 35 Recently, our group reported a CB[8]‐based rotaxane structure on Au surfaces, where CB[8] is threaded onto a MV²⁺ core and prevented from dissociation away from the surfaces. The reversible formation of colloidal monolayers on Au surfaces through the host–guest interaction between these surface‐bound CB[8] rotaxanes and Azo functionalized silica microspheres (Azo‐Si) was also described.36, 37 We now further explore the application of these supramolecularly assembled colloidal monolayers in the MCC templated formation of sub‐micrometer and nanopatterned polymer brushes.

The four‐step protocol for the patterning of binary brush surfaces is illustrated in Figure 1 . First, a trans‐Azo‐Si colloidal crystal monolayer was assembled on a CB[8]‐rotaxane functionalized Au surface using a dip coating technique, which allows the monolayer deposition of particles in hexagonal arrangement on the substrate (Figure 1A). The Azo‐Si colloids were prepared by postmodification of bare silica microspheres with 4‐hydroxyazobenzene‐functionalized silane with a hydro­dynamic diameter of 455 nm (polydispersity, PD = 0.046) as measured by dynamic light scattering (see Figure S1B in the Supporting Information). The ordered hexagonal arrangement of Azo‐Si on the surface was verified by both scanning electron microscopy (SEM) and atomic force microscopy (AFM) (Figure 2 ), which showed a diameter of 333 nm for the particles. This MCC serves as a template in the next step for the supramolecular assembly of the first polymer, a naphthol functionalized poly(ethylene glycol) (M _n = 2000 g mol⁻¹, Np‐PEG_2k), on the surface.

Figure 1.

Stepwise preparation of nanopatterned dual polymer brushes via the reversible host–guest complexation of CB[8]. A) Assembly of Azo‐Si colloids on the CB[8]‐rotaxane functionalized surface through the heteroternary complex formation of (trans‐Azo·MV²⁺)⊂CB[8]. B) Preparation of Np‐PEG_2k brushes on the void spaces around Azo‐Si colloids on the surface via the incorporation of Np functionalities into the CB[8]‐rotaxane cavities. C) Removal of the Azo‐Si colloids from the surface by applying UV light (350 nm, 1 min), with the trans‐Azo photoisomerized to cis‐Azo and be expelled out of the CB[8] cavity. D) Grafting of the Azo‐PEG_5k brushes on the empty spaces vacated by Azo‐Si colloids.

Figure 2.

Microscopy images of the Azo‐Si monolayer colloidal crystal. A) SEM image, B) AFM 2D topography image (3 μm × 3 μm), and C) AFM 3D topography image (3 μm × 3 μm).

In the second step, as illustrated in Figure 1B, Np‐PEG_2k was used to fill in the interstitial vacancies between the Azo‐Si colloids leading to polymer brush structures through the formation of heteroternary complexes (Np‐PEG·MV²⁺) ⊂ CB[8] at the interface, using methodology previously demonstrated by our group.38 The SEM and AFM topography images do not exhibit much difference before (Figure 2) or after (Figure 3 ) the grafting of Np‐PEG_2k brushes, because the Azo‐Si particles are much larger compared to the height of the Np‐PEG brushes. The advantage of using the supramolecular chemistry to attach brushes on the surface in a “grafting to” fashion is its ability to carry out the brush assembly in ambient conditions without the requirement of further chemical reactions and any special instruments, e.g., cyclic voltammetry (CV) for electropolymerization.

Figure 3.

Microscopy images showing the ordered monolayer Azo‐Si particles with their gaps filled with Np‐PEG_2k brushes. A) SEM image, B) AFM 2D topography image (3 μm × 3 μm), and C) AFM 3D topography image (3 μm × 3 μm).

The high hexagonal ordering of the colloidal particles on the surface was well‐preserved after the assembly of Np‐PEG_2k brushes (Figure 3); the assembly process involves the immersion of the substrate in an aqueous solution of Np‐PEG_2k (5 × 10⁻³ m) for 30 min on a shaker (200 rpm). Note, this robust immobilization of the Azo‐Si colloids to the surface is only made possible by employing the CB[8]‐mediated host–guest interaction at the interface to “stick” the particles onto the surface. In an analogous control experiment where Azo‐Si colloids were dip coated on an unfunctionalized Au substrate, the Azo‐Si particles fell off the surface during the assembly process of the Np‐PEG_2k brushes as water washes off the particles and destroys the ordering. It is noteworthy that although Np exhibits a higher binding constant (6.1 × 10⁵ m ⁻¹) to MV ²⁺ ⊂ CB[8] than Azo derivatives (1.4 × 10⁴ m ⁻¹), the MCC structure of the Azo‐Si microspheres was well‐maintained during the assembly of Np‐PEG_2k brushes. This is likely on account of the multivalent nature of the binding between the Azo‐Si colloids and the CB[8]‐functionalized substrate.33, 34, 39

In the third step (see Figure 1C), the Azo‐Si particles were removed from the surface to expose a dotted pattern with “free” surface‐bound CB[8]‐rotaxanes, capable of further complexation with second guests. The Azo‐Si colloids serve as a sacrificial template for the preparation of ordered Np‐PEG_2k brushes and their controlled removal was achieved by the trans‐to‐cis isomerization of the Azo functionalities on the periphery of the particle surfaces upon UV light irradiation (350 nm, 1 min). While the trans‐Azo can be incorporated into the CB[8]‐rotaxane structure via heteroternary complex formation and “stick” the Azo‐Si colloids onto the surface, the cis‐Azo will dissociate from the CB[8] cavity, resulting in the release of the Azo‐Si colloids from the surface. SEM analysis (Figure 4 A) confirmed that the Np‐PEG_2k brushes were retained as a monolayer array on the surface and self‐assembled not only in‐between but also at the interstitial void spaces underneath the Azo‐Si. As shown in Figure 4B,C, AFM topography measurements of the ordered Np‐PEG_2k network depict a periodical cavity of 333 nm in diameter, which matches the size of the Azo‐Si colloidal template. The AFM line profile as shown in Figure 4D reported an average height of the cavity equivalent to 10 nm. Surfaces modified with ethylene glycol units have shown to resist nonspecific adsorption of proteins and cells, thus may find potential applications as biomedical coatings.40, 41, 42, 43

Figure 4.

Microscopy images showing the inverse pattern of Np‐PEG_2k brushes after the removal of Azo‐Si colloids. A) SEM image, B) AFM 2D topography image (3 μm × 3 μm), and C) AFM 3D topography image (3 μm × 3 μm). D) The line profile taken along the blue line as indicated by arrow in (B).

In order to prepare ordered dual polymer brushes, as schematically illustrated in Figure 1D, the cavities created by removing the Azo‐Si colloids were back‐filled with a second polymer Azo‐PEG_5k. Azo‐PEG_5k is a poly(ethylene glycol) (M _n = 5000 g mol⁻¹) end‐functionalized with an azobenzene group. A longer PEG chain was used in Azo‐PEG_5k compared to Np‐PEG_2k to provide a contrast in heights between the two polymer brushes, in order to obtain a visible pattern. After immersing the substrate in a solution of Azo‐PEG_5k (5 × 10⁻³ m) for 30 min on a shaker (200 rpm), the adsorption of the Azo‐PEG_5k brushes into the cavities via CB[8]‐directed heteroternary complex formation can be clearly seen. As shown by the SEM and AFM topography images in Figure 5 A–C, the periodical patterning is well‐maintained after the assembly of the second polymer Azo‐PEG_5k. An average increase of ≈30 nm in height of the inside cavities after grafting Azo‐PEG_5k brushes was determined in the line profile obtained from a zoomed‐in scan with AFM (see Figure 5D,F).

Figure 5.

Nanopatterned dual brushes of Np‐PEG_2k and Azo‐PEG_5k. A) SEM image, B) low‐magnification AFM 2D topography image (3 μm × 3 μm) and C) AFM 3D topography image (3 μm × 3 μm) of the dual brushes. D) High‐magnification AFM 2D topography image (1.3 μm × 1.3 μm). E) Approach force–distance curves of the Np‐PEG_2k brushes (brown line, taken on the dark areas in (D)) and the Azo‐PEG_5k brushes (yellow line, taken on the bright areas in (D)) on the dual brush pattern. F) The line profile taken along the blue line as indicated by arrow in (B).

Force‐distance curves of Np‐PEG_2k and Azo‐PEG_5k were recorded by approaching a bare Si₃N₄ AFM tip to different areas of the surface (Figure 5D), to further confirm the existence of dual polymer brushes on the surface and their difference in terms of brush height. The brown force spectroscopy curve in Figure 5E was taken on the dark area and exhibited a brush height of 9 nm for the Np‐PEG_2k brushes, while the yellow curve was obtained on the bright area and resulted in a thickness of 26 nm for the Azo‐PEG_5k brushes. Note that the slight inconsistency in brush height between AFM topography and force spectro­scopy is due to the fact that the topography images were taken in tapping mode, while the force–distance curves were measured in contact mode.

Importantly, the patterned supramolecular Np‐PEG_2k brushes (Figure 1D) remained stable on the CB[8]‐functionalized surface during the subsequent assembly of the longer Azo‐PEG_5k brushes. Previously, we have shown that Np‐PEG brushes did not exhibit any significant decrease in height after 5 h of continuous washing with an aqueous solution of a small molecule Np derivative ([Np] = 1 × 10⁻³ m).38 Here, the stability of the Np‐PEG_2k brushes in the presence of a solution of Azo‐PEG_5k is even greater as the likelihood of competitive surface displacement with end‐functionalized Azo‐PEG_5k “polymer chains” is substantially lower. The observed robust nature of the supramolecular PEG brushes is likely due to a multivalent “carpet effect” created by the presence of multiple H₂O bridges exploiting hydrogen bonds between oxygen atoms of neighboring PEG chains.44 These H₂O bridges are only possible when the distance between two adjacent PEG polymer chains is necessarily low.

CV was employed to evaluate the accessible area (A _sur) of the modified gold substrates at the different stages of the self‐assembly process outlined in Figure 1 of the binary polymer brushes. Figure 6 shows typical voltammograms for the Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ oxidation at the modified gold electrode. The CB[8]‐functionalized gold electrode is redox‐responsive as indicated by the black curve in Figure 6, the current (i _p) decreased from 90 to 31 μA (a reduction of 66%) after assembly of the Azo‐Si colloids on the electrode (blue curve).45 As i _p is directly proportional to A _sur under constant scan rate, this significant decrease in i _p can be attributed to the fact that the well‐ordered Azo‐Si colloidal crystal blocks the electron‐transfer event thereby reducing A _sur.46 A further decrease in i _p to only 3 μA (90% overall decrease in current) was observed (red curve) after the assembly of Np‐PEG_2k brushes; the electrochemically inactive PEG polymer brushes cover the remaining, accessible gold surface, further reducing A _sur. The removal of Azo‐Si colloids by UV light irradiation exposed spherically patterned CB[8]‐functionalized surfaces causing an increase in A _sur and resulting in a boost in i _p to 39 μA (a 12‐fold increase in current, brown curve). Finally, a gold electrode modified with patterned binary polymer brushes containing both Np‐PEG_2k and Azo‐PEG_5k exhibited a negligible i _p (green curve), as the dense layer of polymer brushes rendered the gold electrode completely insulating and stopped the flow of electrons to the surface.

Figure 6.

Cyclic voltammograms of Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ (0.5 × 10⁻³ m) in 0.5 m KCl solution obtained using the modified gold substrate as the working electrode, a silver chloride electrode as the reference electrode. Scans start from −0.2 to 0.6 V, with a scan rate of 50 mV s⁻¹.

In this stepwise approach to prepare nanopatterned dual polymer brushes, the changes in the composition of surface‐bound materials can also be followed by water contact angle measurements. As shown in Figure 7 , a contact angle (CA) of 98° was measured on the substrate with hexagonally ordered Azo‐Si colloids. After filling the gaps with Np‐PEG_2k on the surface, the CA decreased slightly to 94°. The CA was further reduced to 66° after removal of Azo‐Si colloids with UV irradiation. Note that this value is higher than the CA for both the CB[8]‐rotaxane functionalized surface (43°) and the homogeneous Np‐PEG_2k brush‐modified surface (61°).38 The CA increased to 74° after back‐filling the void spaces on the surface vacated by the Azo‐Si spheres, and again this value is higher than the CA for either the Np‐PEG_2k brushes (61°) or Azo‐PEG_5k (61°) brushes grafted to the surface. This slight increase in hydrophobicity (61° to 74°) is likely because the nanopatterned structures on the surface make it more hydrophobic.47 The observed phenomena that the nanopatterned dual‐composite structures made the surface more hydrophobic than surfaces modified with either of the individual components opens up opportunities to fabricate sup­er­hydrophobic surfaces using our supramolecular assembly strategy.

Figure 7.

Water contact angle measurements for the four stages in the stepwise preparation of nanopatterned dual polymer brushes.

In conclusion, we have developed a facile and litho­graphy‐free approach to creating topologically and chemically defined polymer surfaces under mild and ambient conditions by combining the techniques of MCC‐templated patterning, host–guest complexation and orthogonally controlled supramolecular self‐assembly. Dual composition nanopatterned brushes have been prepared by employing CB[8]‐rotaxanes as supramolecular linking agents on gold surfaces to controllably “stick” trans‐Azo functionalized silica colloids onto the surface in a hexagonal arrangement, which serve as a template for the assembly of short polymer brushes. The formed MCC is photoresponsive and can be reversibly disassembled upon UV light irradiation to create void spaces for the grafting of a second, longer polymer brush to form a dual pattern on the nanoscale. To the best of our knowledge, this is the first example of host–guest recognition directed sub‐micrometer nanopatterning of binary brushes using MCC as a template. This facile supramolecular approach provides a platform to prepare nanopatterned composite brushes with sophisticated structures under mild conditions, and may prove useful in a number of potential applications including dual responsive sensor films and biomedical coatings.

Supporting information

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Supplementary