Underwater Thermoresponsive Surface with Switchable Oil‐Wettability between Superoleophobicity and Superoleophilicity

An underwater thermoresponsive surface that can switch between superoleophobicity and superoleophilicity is fabricated with a combination of mixed brushes, containing thermoresponsive poly(N-isopropylacrylamide) and underwater oleophilic heptadecafluorodecyltrimethoxysilane, and nanostructured silicon nanowire arrays. Temperature-induced underwater adhesion switching between low-adhesive superoleophobicity and high-adhesive superoleophobicity is achieved on a pure poly(N-isopropylacrylamide)-modified nanostructured silicon nanowire array. Smart control of the surface wettability and adhesion has been of great significance because of its wide applications in self-cleaning,1 bioseparation,2 microfluidic devices,3 tunable optical lenses,4 and so on. Through combining surface chemical compositions and surface roughness, various stimuli-responsive surfaces with tunable wettability, and adhesion in air have been fabricated and displayed great potential in practical applications.5 Besides the controllable wetting behavior of surfaces in atmosphere, surfaces with tunable wettability and adhesion in liquid environment have recently emerged as a new focus6 due to their great value for such applications, as antifouling, water industry components, medical engineering and so forth. In 2009, inspired by the self-cleaning property of fish scales, our group reported hydrogel-based underwater superoleophobic surface with low oil-adhesion for the first time.7 At the same time, Jung and Bhushan further proposed a theoretical model to predict underwater contact angles of oil droplets.8 Subsequently, a number of underwater superoleophobic surfaces, either inorganic9 or organic,10 have been fabricated. With the cooperation of responsive molecules and surface topography, underwater smart surfaces responding to light,11 electrocity12 or temperature13 have been reported. However, these smart surfaces can only tune the underwater oil-wettability in a limited range. Recently, voltage- and pH-responsive surfaces that can switch between underwater superoleophobicity and superoleophilicity have been successfully prepared.14 These kind of smart surfaces have exhibited great potential for on-demand separation of oil/water mixtures.15 However, fabrication of underwater temperature-responsive surfaces with switchable underwater wettability between two extreme states is still challenging. In this study we report underwater temperature-responsive surface that can switch between superoleophobicity and superoleophilicity by grafting mixed brushes on silicon substrate. The mixed brushes are composed of heptadecafluorodecyltrimethoxysilane (HFMS) as the oleophilic component in aqueous environment and poly(N-isopropylacrylamide) (PNIPAAm) as the thermoresponsive component. Moreover, temperature-induced underwater low adhesive superoleophobicity and high adhesive superoleophobicity can be achieved by grafting pure PNIPAAm on silicon nanowire arrays (SiNWAs). We synthesized the mixed brush-modified surface through combining silane coupling chemistry16 and surface-initiated atom-transfer radical polymerization17 (SIATRP) (Figure S1, Supporting Information). First, mixed silane agents containing HFMS and 3-aminopropyltrimethoxysilane (ATMS) were introduced onto the silicon surface via silane coupling chemistry. Second, initiators were immobilized onto the surface through the reaction between amino groups and acyl bromide groups. Subsequently, PNIPAAm was grafted onto the surface by SIATRP. X-ray photoelectron spectroscopy (XPS) data show four peak components with binding energies at about 284, 532, 400, and 690 eV corresponding to C1s, O1s, N1s, and F1s, respectively (Figure S2a–c, Supporting Information), suggesting the successful modification of the surface with mixed brush. While for the surface modified with pure ATMS, the peak at about 690 eV disappears (Figure S2d, Supporting Information). The atomic force microscopy (AFM) images show that flat silicon wafer modified with mixed brush is heterogeneous (Figure S3a–c, Supporting Information), while that modified with pure PNIPAAm is relatively homogenous (Figure S3d, Supporting Information). It was envisaged that the grafted PNIPAAm chains could conceal or expose oleophilic component (i.e., HFMS) by virtue of a thermoresponsive conformational change of the PNIPAAm molecules.18 At room temperature, the grafted PNIPAAm is in a hydrated state due to intermolecular hydrogen bonding between PNIPAAm chains and water molecules, and extends outward to shield the HFMS, resulting in a superoleophobic surface (left in Figure 1a). Upon increasing the temperature to 60 °C, the grafted PNIPAAm chains suddenly become dehydrated and shrink because of the dominant intramolecular hydrogen bonding of PNIPAAm chains, thus exposing the HFMS and leading to a superoleophilic surface (right in Figure 1a). For example, when nanostructured SiNWAs were modified with the mixed brush, underwater contact angle of 2 μL 1,2-dichloroethane (DCE) droplet is about 157° at 20 °C (left in Figure 1b). In contrast, underwater DCE contact angle is only about 3° at 60 °C (right in Figure 1b). More importantly, this temperature-induced switching between superoleophobicity at 20 °C and superoleophilicity at 60 °C is repeatable for at least five cycles (Figure 1c). Hence, by incorporating thermoresponsive PNIPAAm along with underwater oleophilic HFMS on SiNWAs, we can achieve reversible underwater wettability between superoleophobicity and superoleophilicity. During our experiments, we found that surface compositions significantly influenced the responsiveness of the surfaces. As shown in Figure 2a, dynamics of underwater DCE contact angles is quite different for SiNWA with different chemical compositions. For bare SiNWA, the DCE contact angle is temperature-independent and always maintains at about 160° owing to the trapped water in the gap of the SiNWAs (Figure 2b). For SiNWA modified with pure PNIPAAm (PNIPAAm-SiNWA), although the PNIPAAm chains adopt dehydrated states at 60 °C, they cannot repel the trapped water in the gap. Thus, the DCE contact angle only slightly decreases with time at 60 °C (Figure 2c). With the introduction of oleophilic HFMS (molar ratio of HFMS/ATMS = 171.2), however, the oil droplet will gradually replace the trapped water and penetrate into the gap owing to the strong interaction between the oil droplet and the oleophilic HFMS. Thus, the DCE contact angle decreases gradually with time and superoleophilicity is achieved (Figure 2d). We also found that the chemical composition of the mixed brush modified SiNWA (HFMS/PNIPAAm-SiNWA) significantly influences the dynamic wettability. At 60 °C, the HFMS/PNIPAAm-SiNWA can reach underwater superoleophobic more rapidly with increasing content of HFMS (Figure S5, Supporting Information). These results demonstrate that we can obtain temperature-driven underwater switching between superoleophobicity and superoleophilicity with the assistance of mixed brushes containing thermoresponsive PNIPAAm molecules and underwater oleophilic HFMS. As discussed above, pure PNIPAAm modified SiNWA does not display thermo­responsive changes of wettability and the DCE contact angle on PNIPAAm-SiNWA is as high as 150° even at 60 °C (Figure 2a). However, we found that the adhesion forces between DCE droplets and the PNIPAAm-SiNWA could be reversibly tuned through controlling the surrounding temperatures. At 20 °C, the PNIPAAm chains are in fully hydrated state, which will prevent the contact between the DCE droplet and the SiNWA, leading to a low adhesive underwater superoleophobic state (left in Figure 3a). On the other hand, at 60 °C the collapsed PNIPAAm chains become hydrophobic, affording strong interaction between the hydrophobic PNIPAAm molecules and the DCE droplet. At the same time, the collapsed PNIPAAm chains provide larger contact area with the DCE droplet. Thus, a high adhesive underwater superoleophobic state generates (right in Figure 3a). When changing surrounding temperature between 20 and 60 °C, an underwater adhesion switch could be obtained (Figure 3b). It should be noted that the thickness of the modified PNIPAAm is of critical importance to the adhesion switches. It has been reported that when the thickness of the PNIPAAm layer is >30 nm, PNIPAAm modified surfaces are hydrophilic even at temperatures above the lower critical solution temperature (LCST).19 Thus, PNIPAAm-SiNWA with PNIPAAm thickness of about 32 nm exhibits underwater low adhesive superoleophobicity regardless of temperatures.20 On the other hand, PNIPAAm-SiNWA with PNIPAAm thickness of about 20 nm has been used for reversible capture and release of cancer cells.21 In the present study, the thickness of the PNIPAAm layer is about 18 nm (Figure S4d, Supporting Information), which can switch between hydrated state and restricted hydrated state. As a result, the PNIPAAm-SiNWA shows temperature-dependent adhesion switch. Besides the thickness of PNIPAAm layer, another important parameter that dominants the adhesion switch is the surface topography.22 To investigate the influence of surface topography on adhesion switch, we compared three kinds of PNIPAAm modified surfaces with different morphologies: PNIPAAm modified flat silicon wafer (PNIPAAm-Si), PNIPAAm modified SiNWA (PNIPAAm-SiNWA), and PNIPAAm modified microstructured silicon posts along with SiNWA (PNIPAAm-Micro/NanoSi). Underwater contact angle of the DCE droplet on PNIPAAm-Si is 135.9° ± 3° at 20 °C and 124.5° ± 4.5° at 60 °C, exhibiting typical Wenzel states.23 Thus, the adhesion forces between the DCE droplet and PNIPAAm-Si are both higher than 50 μN below or above the LCST of PNIPAAm (Figure 4a). During the measurements, we found that the adhesion forces between the DCE droplets and the PNIPAAm-Si were so high that small parts of the DCE droplets were left on the substrates (Figure 4b). In contrast, for PNIPAAm-Micro/NanoSi, the hierarchical structures might provide a “lotus state”24 like the lotus leaf in air, thereby very low adhesion forces at both 20 and 60 °C (Figure 4a, Table S1, Supporting Information). Especially for the PNIPAAm-Micro/NanoSi with S = 15 μm, the adhesion forces are almost undetectable (Figure 4c). While for the PNIPAAm-SiNWA, the adhesion force is only about 6 μN at 20 °C, but the adhesion force increases to about 35 μN at 60 °C with the cooperation of thermo­responsive PNIPAAm molecules and nanostructured SiNWA (Figure 4d). Therefore, a thermoresponsive underwater adhesion switch has been obtained. In conclusion, through introduction of mixed brushes containing thermo­responsive PNIPAAm and underwater oleophilic HFMS on nanostructured SiNWA, smart surface that can switch between underwater superoleophobicity and superoleophilicity has been achieved. Although the pure PNIPAAm modified SiNWA (PNIPAAm-SiNWA) maintains underwater superoleophobicity regardless of the surrounding temperature, the adhesion forces between the DCE droplets and the PNIPAAm-SiNWA are responding to temperature changes: low adhesive underwater superoleophobicity at 20 °C and high adhesive underwater superoleophobicity at 60 °C. We believe that this study will provide a clue for the design of underwater smart surfaces, which might be applied for miniature reactor, smart separation, and controllable bioadhesion. This research was supported by the National Research Fund for Fundamental Key Projects (Grant Nos. 2012CB933800, 2013CB933000, 2009CB930404, and 2012CB934100), the National Natural Science Foundation (Grant Nos. 21404109, 21121001, and 91127025), and the key Research Program of the Chinese Academy of Sciences (Grant No. KJZD-EW-M01). As a service to our authors and readers, this journal provides supporting information supplied by the authors. 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