1 Introduction

With the rapid development of industrialization, worldwide oil spill and industrial oily wastewater have discharged various toxic compounds to the water systems and thus threaten the global ecosystem [1, 2]. Superwetting materials, showing different interfacial effects on oil and water, are effective means for oil/water separation [3,4,5,6,7,8,9,10]. For the purpose of separating the contaminants and pure water with higher flexibility, smart response of superwetting materials become the most promising matter since their surface structure could alternate under external stimuli such as pH [11,12,13,14,15], light [16,17,18], temperature [19, 20], electric field [21, 22], gas [23] and pressure [24]. Their stimuli-responsive behavior can result in switchable wettability between superhydrophobicity and superhydrophilicity, offering different affinity to water and oily matters during the separation.

Among these smart switchable materials, pH-responsive superwetting materials with porous structures have great advantages on convenient operation, rapid response and uncomplicated equipment [25,26,27,28]. Generally, the pH-responsibility comes from polymeric coatings that can reversibly switch their chemical conformation or polarity to change the surface energy and surface wettability [29,30,31]. Zhang et al. [30] grafted pH-sensitive poly(2-vinylpyridine)-based block copolymer on non-woven textiles and polyurethane sponges. The resulted material realized switchable superoleophilicity and superoleophobicity and exhibited excellent oil/water separation performance. Dai [32] fabricated a three-dimensional structural collagen fiber matrix modified by poly(methacrylic acid-co-glycidyl methacrylate) copolymer. The functional material can be tuned to absorb or release oil with pH variation, which is beneficial for recycling the absorbed oil. Yi [33] developed a novel membrane based on reduced graphene oxide and polyacrylic acid (PAA) grafted carbon nanotube. Benefited from the pH-responsive property of PAA, the pore size of the membrane can be narrowed in acid solution and enlarged in basic solution, providing controllable separation efficiency (SE) over a wide pH range. However, the layer of functional polymer is intolerant toward acids, bases and organic solvents, and microscale/nanoscale rough structures on surface are inevitably weak towards mechanical abrasion [34]. The low stability of pH-responsive materials limits their application in practical oil/water separation and can be further optimized.

Mussel-inspired chemistry provides a facile and promising surface-modification approach to introduce special wettability behavior and hierarchical structures onto surfaces [35,36,37,38,39]. Polydopamine (PDA) is quite similar in molecular structure to the dihydroxyphenylalanine in mussel foot proteins. It has been proved to show strong adhesive ability on nearly all kinds of substrates, offering new opportunities in oil/water separation [40,41,42,43,44]. Huang [45] used PDA as a small molecular bioadhesive to prepared the superhydrophobic polyurethane sponge with hierarchically structured surface. The as-prepared sponge remained excellent absorption capacities towards pump oil and chloroform after five absorption cycles. Kang [46] presented a durable superhydrophobic glass wool (GW) fabricated by the combination of PDA chemistry and chemical vapor deposition of polydimethylsiloxane. The water contact angle (WCA) of the superhydrophobic GW remained above 150° after being kept in diesel and petrol for even 30 days. This was attributed to the inherent stability of the PDA nanoparticles towards oil.

Herein, we described a facile method to fabricate a random copolymer-coated stainless steel mesh with pH-switchable wettability to efficiently separate oil/water mixture, as illustrated in Scheme 1. The dopamine@polymer@SiO2 nanoparticles (DPS) stainless steel mesh was fabricated by dipping meshes into dopamine solution and then treated with the mixture solution of poly(2-(dimethylamino) ethyl methacrylate)-co-poly(2-hydroxyethyl methacrylate)-co-poly(2,2,3,4,4,4-hexafluorobutyl methacrylate) (PDMAEMA-co-PHEMA-co-PHFBMA) and SiO2 nanoparticles. Stainless steel mesh was chosen for its high compressive strength, corrosion resistance and firmness to provide efficient and durable foundation for oil/water separation. In our previous work, the functional copolymer was synthesized by simple free radical polymerization [47, 48]. PDMAEMA segments gave the pH responsiveness, HEMA segments rose the hydrophilicity and the HFBMA segments reduced surface energy. PDA enhanced the adhesion between the components and SiO2 nanoparticle provided the required surface roughness. The as-prepared meshes exhibited intelligent pH-switchable wettability, excellent mechanical and chemical stability. The microstructure and surface properties of the DPS stainless steel meshes were investigated and the oil–water SE and recyclability of the mesh were evaluated. The mussel inspired durable pH-responsive mesh is promising in high-efficient oil/water separation.

Scheme 1
scheme 1

Schematic illustration of the preparation of DPS stainless steel mesh

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2 Experimental

2.1 Materials

2-(Dimethylamino) ethyl methacrylate (DMAEMA), 2-hydroxyethyl methacrylate (HEMA), and 2,2,3,4,4,4-hexafluorobutyl methacrylate (HFBMA) were purchased from Aladdin Biochemistry Co. Ltd. and were filtered to remove the inhibitor through alkaline alumina column for further use. Dopamine hydrochloride (98.0%) was purchased from Aldrich Chemical Company. Silica nanoparticles (7–40 nm) and 2,2′-azobis-(isobutyronitrile) (AIBN) were purchased from Macklin Biochemistry Co. Ltd. The stainless steel meshes (500 mesh size) were obtained from Shanghai Jiya Industrial Wire Mesh Products Co., Ltd. Meshes were washed with acetone, ethanol, and deionized water in sequence under ultrasonication to remove impurities and dried at 60 °C for 24 h. Kapok fiber was purchased from Nanning Weiyu Home Textile Co., Ltd. It was immersed into ethanol under ultrasonication and dried up for later use. Other reagents were used without further purification.

2.2 Synthesis of PDMAEMA-co-PHEMA-co-PHFBMA

Copolymer PDMAEMA-co-PHEMA-co-PHFBMA (PDHH) was synthesized through free radical polymerization with following details. A mixture of DMAEMA (12.59 g), HEMA (2.60 g), HFBMA (5.00 g), and AIBN (0.16 g) were dissolved in purified toluene (100 mL) under ultrasonication and then poured into a dry eggplant-type flask with a magnetic stirrer. The system was removed oxygen by nitrogen gas for 0.5 h and stirred for another 4 h with a thermostat at 65 °C under nitrogen protection. The obtained copolymer was precipitated into excessive n-hexane for three times and then dried under vacuum oven at 65 °C.

2.3 Preparation of PDA modified stainless steel mesh

Dopamine hydrochloride was dissolved in deionized water to prepare a solution with a mass concentration of 2 g/L and Tris buffer was used to adjust solution pH to 8.5. The stainless steel mesh was immersed in the solution for 0.5 h under ultrasonication and then stirred for 24 h. After reaction, the PDA mesh was washed by ethanol and water for several times and was dried in oven at 60 °C.

2.4 Fabrication of the dopamine@polymer@SiO2-coated (DPS) stainless steel mesh

Silica nanoparticles (THF, 1 wt%) and copolymer PDHH were dispersed into tetrahydrofuran to form a coating solution. The DPS mesh was prepared by immersing the PDA mesh into the solution for 0.5 h. The DPS mesh was dried in a vacuum oven at 120 °C for 3 h. Finally, after washing in ethanol to remove residuals, the mesh was dried at 60 °C. In addition, a coated mesh without being modified with dopamine was prepared as polymer@SiO2-coated (PS) stainless steel mesh for comparison.

2.5 Fabrication of oil collector

1.0 g NaClO2 was ultrasonically dissolved in 100 mL deionized water, and acetic acid was added dropwise with a syringe until the solution pH was adjusted to 4.5. Then, 1.0 g kapok fiber was added and was poured into a single-neck flask equipped with a mechanical stirrer. The reaction was stirred at 650 rpm and 80 °C for 2 h. After the reaction, the fibers were washed with deionized water until the pH value of the filtrate reached 7. The DPS stainless steel meshes were cut into small rectangle blocks (2 × 4 cm) and folded into squares (2 × 2 cm). Pretreated kapok fiber was filled into the stainless steel mesh bag and the oil collector was prepared after the edges were sewed on with fishline.

2.6 Characterization

The morphologies of the as-prepared meshes were observed by scanning electron microscopy (SEM, Hitachi S-3400N). The WCA of as-prepared material surfaces was measured three times by a contact angle measuring instrument (JC2000D2) with 2 μL of liquid at ambient temperature.

2.7 Oil/water separation experiments

The separation capability of the DPS stainless steel mesh was gauged through a simple filtering device. Six different kinds of oil–water mixtures (n-hexane, cyclohexane, kerosene, dichloromethane, chloroform and carbon tetrachloride) were prepared by taking same volume of oil and water, using Oil Red O to dye the oil, and mixing the oil and water. The SE and flux of the meshes were calculated according to following equations:

$$SE=\frac{m_1}{m_0}\times 100\%$$
$$Flux=\frac{V}{S\times t}$$

where m0 and m1 are the weight of water or the solvent before and after separation, respectively, V is the volume of the filtrate, S is the effective filtration area, and t is the filtration time.

2.8 Oil/solvent absorption capacity

The oil collector was dipped into various solvents or oils until reaching the absorption equilibrium, after that the excess liquid was removed for mass measurement. The measurement process needs to be fast to avoid solvent evaporation. The absorption capacity Q of the oil collector was calculated according to the following equation:

$$Q=\frac{m_1-{m}_0}{m_0}$$

2.9 Durability tests

To evaluate the mechanical stability of DPS mesh, a mesh loaded with 100 g weight was put on the abrasive paper with selected roughness (500CW, 1000CW, 1500CW). For each cycle, the mesh was pulled along the ruler for 10 cm. The surface morphology was observed and WCA was recorded after each 5 cycles.

For the acid/alkali resistance test, DPS meshes were immersed into 1 M HCl solution, 1 M NaOH solution, 1 M NaCl solution, hot water, and ice water for 1 h, respectively. After dried in oven at 80 °C, the WCAs were measured and dichloromethane/alkaline water separation performance was evaluated.

In the UV-irradiation resistance test, DPS meshes were treated under UV irradiation for a series of time periods at 25 ± 2 °C with a relative humidity of 60 ± 2%. WCAs were measured once UV irradiation finished and then heavy oil/water separation performance was evaluated as above.

3 Results and discussion

The DPS stainless steel mesh was prepared by dip-coating of stainless steel mesh into PDA solution and copolymer PDHH suspension containing SiO2, shown in Scheme 1. The copolymer PDHH with pH-responsiveness was synthesized via simple radical polymerization (Fig. S1) [47]. The FT-IR and 1H NMR spectra (Figs. S2 and S3) indicated that the copolymer has been successfully synthesized.

3.1 Surface morphology and pH-switchable wettability of DPS meshes

The surface wettability and morphology of the DPS mesh were measured to confirm the dip-coating process resulted in the pH-sensitive superwetting surface on meshes. Figure 1a revealed that the DPS mesh was hydrophobic under alkali condition, and turned hydrophilic under acid condition. However, the pristine mesh absorbed all droplets quickly (Fig. 1b) and exhibited nonselective wettability. Acidic water (pH = 1) could be fully absorbed within 10 s by DPS mesh (Fig. 1c). In contrast, the DPS mesh at alkaline water (pH = 13) showed superhydrophobicity with an WCA of 153° (Fig. 1d) due to the pH-responsive surface on the mesh (Fig. 1e). Figure S4 showed the process in which an alkaline water droplet rolled off the DPS mesh with a sliding angle less than 15°.

Fig. 1
figure 1

Photographs of the acidic water (pH = 1) and alkaline water (pH = 13) on the (a) DPS mesh and (b) pristine mesh. WCA of the DPS mesh with (c) acidic (pH = 1), (d) alkaline (pH = 13) water droplets. (e) FESEM image of the DPS mesh: magnified 500 times

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The surface chemical compositions of three different meshes (pristine mesh, PDA mesh and DPS mesh) were characterized by XPS spectra in Fig. 2. For the pristine mesh, it mainly has the characteristic peaks of C, O and Fe elements. After modified by PDA, the characteristic peak of the Fe element basically disappeared and was replaced by the appearance of N element at 399 eV from PDA. The N/C and O/C atomic ratios of pristine mesh were 0.069 and 0.835, respectively, whereas the N/C ratio increased to 0.123 and the O/C ratio decreased to 0.252 in PDA mesh (Table 1). The ratios were in good agreement with the theoretical values of PDA which confirms that PDA was uniformly distributed on the mesh.

Fig. 2
figure 2

XPS spectra of three different stainless steel mesh: pristine mesh, PDA mesh and DPS mesh

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Table 1 Surface chemical composition of the pristine mesh and PDA mesh from XPS spectra
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Moreover, the approximate thickness of dopamine@polymer@SiO2 layer was estimated by Automatic Mercury Porosimeter. The pore diameter before and after the dip coating experiments were 36.12 and 35.48 μm, respectively. The DPS layer was about 0.32 μm.

$$d=\frac{d_1-{d}_0}{2}$$

where d1 is the pore diameter of pristine mesh and d0 is the pore diameter of the DPS mesh.

To further demonstrate the effect of copolymers on pH-response, PDHH solutions with different concentrations (0.25, 0.5, 1, 2, 3 wt%) were prepared and used to modify the meshes respectively. WCA (Table 2) and surface morphologies (Fig. 3) were characterized for comparison. After modification by PDHH solutions (0.25, 0.5, 1 wt%), a rougher state of surface could be clearly observed due to the uniform distribution of polymer and the surface roughness increased significantly with the rise of concentration. When pH ≥ 7, the WCA increased to 145.6° with the increase of concentration, which was in agreement with the results of FESEM. However, as the concentration of PD4H1H1 solution reached 2 and 3 wt%, solution would flow during the volatilization of THF, eventually formed continuous smooth surface and led to smaller WCA in neutral/alkalic condition. Thus, the concentration of PD4H1H1 was optimized as 1 wt%.

Table 2 The WCA of stainless steel mesh modified by polymer solution of different concentration
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Fig. 3
figure 3

FESEM images of the (a) non-treated stainless steel mesh and different concentration polymer modified stainless steel mesh, (b) 0.25 wt%, (c) 0.5 wt%, (d) 1 wt%, (e) 2 wt%, (f) 3 wt%

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The stability of functional coating surfaces is very important for their practical applications. However, the interaction between the functional coating and substrate is usually not strong enough for general special wetting materials [49]. The PDA layer was introduced to enhance the adhesion strength via the covalent bonds between hydroxyl- and amine-functionalized groups [50,51,52]. And a coated mesh without dopamine modification was prepared as PS stainless steel mesh for comparison. As displayed in Fig. S5a, the PDHH copolymer and SiO2 nanoparticles clumped together and piled up at the intersection of stainless steel wire of PS mesh, nearly blocking the mesh. After preprocessed by PDA, the surface of DPS mesh was covered by uniformly distributed copolymer and SiO2 nanoparticles (Fig. S5b). Moreover, the DPS mesh showed stable pH responsiveness under reversible cycles between pH = 1 and pH = 13 while the responsive performance of PS mesh decreased after two cycles (Fig. 4).

Fig. 4
figure 4

WCA of DPS mesh and PS mesh for pH conversion cycles between pH = 1 and pH = 13

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3.2 Oil/water separation performance of DPS meshes

The separation property of DPS mesh was investigated with sand core filter for different oil/water mixtures, as shown in Fig. 5a, n-hexane/acidic water mixture (1:1, v/v) was slowly poured into the funnel and was driven only by gravity. Acidic water immediately permeated through the DPS mesh and flowed into the flask underneath owing to the superhydrophilicity whereas n-hexane was blocked above the mesh (Video S1). In contrast, when poured the dichloromethane/alkaline water mixture (1:1, v/v) into (Fig. 5b), dichloromethane rapidly passed through the funnel (Video S2). However, the alkaline water was obstructed because the PDMAEMA blocks collapsed in deprotonation process and exhibited high repellency to water in alkali environment. After filtration, oil and water phases were completely separated, and other phases were not presented in the filtrate.

Fig. 5
figure 5

Oil/water separation process of (a) n-hexane/acidic water mixture and (b) dichloromethane/alkaline water mixture. The oil used were dyed with Oil Red O. SE of different oil/solvents mixed with (c) acidic water or (d) alkaline water. SE of (e) n-hexane/acidic water mixture and (f) carbon tetrachloride/alkaline water mixture at different cycles

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During the separation, the filtering time and liquid weight (before and after separation) were recorded, respectively. Therefore, the SE and flux of the DPS mesh for different organic oil pollutants were calculated. As shown in Fig. 5c, d, the SE of the meshes were all above 98% for various immiscible oil/water mixtures with only slight differences due to the density of oils. After each separation, the meshes were washed with ethanol and then dried for reuse. To evaluate the recyclability of the as-prepared meshes, the separation with the n-hexane/acidic water and carbon tetrachloride/alkaline water mixture were taken for examples (Fig. 5e, f). After ten cycles of separation, the SE of the DPS meshes were above 97% and fluxes were more than 1 × 104 L m−2 h−1.

PS mesh was also tested with n-hexane/acidic water mixture for comparison. In the second cycle, the SE decreased from 97% to 92%. The oil phase and the water phase could not be separated completely and the flux increased apparently in the next cycle due to the weak adhesion between the coating and the mesh. As displayed in Fig. 6a, b, the micro-nano rough structures on the surface of the DPS mesh still existed after ten cycles and showed little difference with the original DPS mesh. On the contrary, few SiO2 nanoparticles remained on the surface of PS mesh after three cycles (see Fig. 6c, d). The layer of copolymer and SiO2 nanoparticles was obviously peeled off, which reduced the surface roughness and changed pore size, causing the poor SE and increase of flux. These results well proved that the DPS mesh possesses favorable recyclable property.

Fig. 6
figure 6

FESEM imagines of DPS mesh after separating n-hexane/acidic water mixture for ten cycles: (a) magnified 500 times, (b) magnified 5000 times. PS mesh after separating n-hexane/acidic water mixture for three cycles: (c) magnified 500 times, (d) magnified 5000 times

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3.3 Durability of DPS meshes in harsh environments

The durability of oil/water separating meshes in harsh environments, such as mechanical abrasion, acid/alkali immersion and UV irradiation, is required in practical applications. A simple abrasion (Fig. S6) test was performed with abrasive papers (500CW, 1000CW, 1500CW) to evaluate the mechanical stability of DPS mesh. The relationship between pH-switchable behavior and sliding distance was displayed in Fig. 7.

Fig. 7
figure 7

WCA of DPS mesh after each 50 cm sliding distance on different sandpapers

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The surface structures on the mesh were destroyed to various degree after repeated abrasion. The three dimensional braided structure of the stainless steel mesh and the PDA layer protected hierarchical structures almost kept inside the DPS mesh (Fig. 8a) whereas the surface of PS mesh underwent a serious damage (Fig. 8b). The WCA under alkaline conditions gradually declined with the increase of the sandpaper roughness and the sliding distance, nevertheless, the DPS mesh was still pH-switchable and superhydrophobic with a WCA of 150 ± 0.5° even after 40 cycles of abrasion test.

Fig. 8
figure 8

FESEM imagines of (a) DPS mesh and (b) PS mesh after 500 cm sliding distance on 500CW sandpaper

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To verify the durability of DPS meshes towards acidic and alkaline environments, we soaked the meshes into five various solvents (1 M HCl solution, 1 M NaOH solution, 1 M NaCl solution, hot water, ice water) for 12 h. After immersion, the WCAs were about 150 ± 1° and the SE were about 97 ± 0.5% (Fig. 9). These results proved that the DPS mesh showed good stability in harsh environments, the copolymer and SiO2 coating didn’t peel off significantly due to the adhesion of the PDA.

Fig. 9
figure 9

The WCA and SE of DPS mesh after immersion into different solutions

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Oil/water separating materials are inevitably exposed to sunlight in outdoor use. As a part of sunlight, ultraviolet (UV) light may cause decrease in super-wettability and further worsen the oil/water separation performance. Thus, WCAs and SE changes of DPS meshes were studied after treatment of UV irradiation for various time from 6 to 24 h. As shown in Fig. 10, only a slight decrease was observed in both WCA and SE after exposure to UV-irradiation. The WCAs were about 151 ± 1° and the SE were above 97% even after 24 h exposure. Hence, DPS mesh proved to be durable under UV irradiation, making it potentially conducive to long-term outdoor use.

Fig. 10
figure 10

The WCA and SE of DPS mesh after treatment of UV irradiation for various time from 6 to 24 h

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Kapok fiber is a natural plant fiber with low density, good buoyancy, huge hollowness and good hydrophobicity. These characteristics endow kapok fiber higher oil sorption performance than common natural materials and commercial oil sorbent [53, 54]. Pristine kapok fiber assembly is difficult to retain oils effectively due to its smooth wax-covered surface and pectin coatings (Fig. S4). To remove the coatings, the kapok fiber was pretreated with NaClO2 solution. In this work, we designed an oil collector with kapok fibers and DPS meshes. The as-prepared oil collector was placed on neutral water/Red O dyed n-hexane (Fig. 11a). n-Hexane passed through the mesh and was absorbed by the oil collector within a few seconds, leaving the clear water phase without any visible oil droplets. The superhydrophobicity of the DPS mesh and the oleophilicity of kapok fibers in neutral environment played the vital role. Besides, the oil collector can also absorb heavy oil in water (Fig. 11b). At first, the oil collector floated on water in a hydrophobic situation. Once the oil collector was pressed into water and contacted the oil, the oil was quickly absorbed. The collector showed excellent selective absorption performance for the oil phase in water.

Fig. 11
figure 11

Optical images of the process of DPS mesh selectively absorb (a) light oil (n-hexane) and (b) heavy oil (dichloromethane)

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Moreover, the collector filled with 0.05 g of fiber assembly was used to evaluate the absorption capacities for twelve types of common oil pollutants in daily life and industry. As shown in Fig. 12, the absorption capacities of the oil collector for these oils were in the range of 39.67–79.56 g/g, much higher than some other oil-absorbing materials such as a biomimetic polypropylene foam (9.4 g/g) [55], electrospinning carbon nanofibrous membrane (44.9 g/g) [56] and [ADT-(CF3)2] modified polyurethane sponge (35.0–77.2 g/g) [57].

Fig. 12
figure 12

Absorption capacity of oil collector to oil/solvent

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4 Conclusion

In summary, a durable DPS stainless steel mesh has been successfully fabricated by dip-coating of mesh into PDA solution and random copolymer suspension containing SiO2 nanoparticles. The DPS mesh is pH-switchable wettability between superhydrophilicity-superoleophobicity and superhydrophobicity-superoleophilicity. The resulted mesh exhibited remarkable oil/water SE ≥ 98%) and desirable recyclability. The WCA of DPS mesh was about 150 ± 1° and the SE remained above 96% after abrasion test, acidic/alkali solution immersion and UV irradiation, exhibiting good durability in harsh environments compared with PS mesh due to the strong adhesion of PDA. Furthermore, an oil collector with excellent oil absorption capacity (39.67–79.56 g/g) was developed by as-prepared mesh and kapok fiber. The facile fabrication, switchable wettability, together with excellent durable stability, made the DPS mesh promising in large-scale oil/water separation.