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Nacre-inspired underwater superoleophobic films with high transparency and mechanical robustness - Nature Protocols
Abstract .
Underwater superoleophobic materials have attracted increasing attention because of their remarkable potential applications, especially antifouling, self-cleaning and oil–water separation. A limitation of most superoleophobic materials is that they are non-transparent and have limited mechanical stability underwater. Here, we report a protocol for preparing a transparent and robust superoleophobic film that can be used underwater. It is formed by a hydrogel layer prepared by the superspreading of chitosan solution on a superhydrophilic substrate and biomimetic mineralization of this layer. In contrast to conventional hydrogel-based materials, this film exhibits significantly improved mechanical properties because of the combination of high-energy, ordered, inorganic aragonite (one crystalline polymorph of calcium carbonate) and homogeneous external hierarchical micro/nano structures, leading to robust underwater superoleophobicity and ultralow oil adhesion. Moreover, the mineralized film is suitable for neutral and alkaline environments and for containing organic solvent underwater and can be coated on different transparent materials, which has promising applications in underwater optics, miniature reactors and microfluidic devices. In this protocol, the time for the whole biomimetic mineralization process is only ~6 h, which is significantly shorter than that of traditional methods, such as gas diffusion and the Kitano method. The protocol can be completed in ~2 weeks and is suitable for researchers with intermediate expertise in organic chemistry and inorganic chemistry.

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Introduction .
Special wettability of solid surfaces is a ubiquitous phenomenon in nature and has attracted much attention because of its potential applications in diverse fields, such as oil–water separation, anti-biofouling and drag reduction 1 , 2 . Some biological organisms, such as fish scales, columnar nacre and seaweed, have functional surfaces with underwater superoleophobic properties that have inspired researchers to design and create novel interfacial materials 2 , 3 . There is evidence to suggest that underwater superoleophobic materials can be used to address rapidly growing issues in the marine environment, such as highly frequent oil-spill problems 4 . Until now, various inorganic and organic materials have been used to fabricate underwater superoleophobic surfaces similar to those found in nature 5 , 6 , 7 , 8 , 9 , 10 . However, most of these materials are unsuitable for industrial-scale production and practical utility: most inorganic materials have limited transparency because of the extensive light-scattering effect from surface micro/nano structures 2 , 3 , 11 , and organic materials do not have sufficient mechanical strength 12 , 13 .
In managing marine oil spills, for example, the mechanical durability of underwater superoleophobic materials is not negligible 6 . These materials always suffer from mechanical damage in practical applications, such as fluid flush in seawater and inevitable scraping or rubbing, resulting in a gradual loss of superoleophobicity 13 . Thus, improving the mechanical strength of the material is often an effective strategy to keep its underwater superoleophobic properties 7 , 12 , 13 , 14 , 15 . In addition, the transparency of underwater superoleophobic materials can meet specific needs in numerous emerging fields such as droplet microfluidic systems. For instance, when using microfluidic devices to prepare various types of emulsion droplets (e.g., single, double and triple emulsions), oil droplets tend to adhere to the microchannel surface with weak underwater oleophobic ability, thus affecting the normal operation of microfluidic devices 16 . If an underwater superoleophobic material with high transparency and mechanical stability is applied to the microchannel surface, it cannot only prevent the adhesion of oil droplets on the surface, but also maintain the transparency of the microchannel surface for optical imaging. Furthermore, this integration of high transparency and mechanical stability also makes it a promising application in underwater optical equipment (e.g., underwater cameras and diving goggles) 2 , 11 , 17 , which is difficult for traditional underwater superoleophobic materials.
Therefore, the development of advanced underwater superoleophobic surfaces with high transparency and mechanical performance integrated in a facile way is urgently required to feed the demand for practical applications in numerous emerging fields such as underwater optics, miniature reactors and microfluidic devices 2 , 11 , 18 , 19 , 20 .
Development of the protocol .
Biological structures such as fish scales, columnar nacre and seaweed provide inspiration to design and create novel interfacial materials with underwater superoleophobic properties 5 , 7 , 8 , 10 . Generally, hydrogel is an ideal candidate for underwater superoleophobic materials because of the three-dimensional network structure filled with abundant water molecules, which has a biophysical similarity to mucus 7 , 10 , 21 , 22 . With this unique feature, a silicon template replication method was used to roughen chemically cross-linked polyacrylamide hydrogels, which exhibited fish scale–like underwater superoleophobicity and excellent underwater transparency 10 , 23 .
However, these hydrogels are usually vulnerable because of their poor mechanical strength, which often leads to the rapid loss of underwater superoleophobicity under harsh conditions 6 , 13 . To address the above issues, double-polymer networks and inorganic-reinforced nanomaterials have been introduced into hydrogels to improve their mechanical robustness 7 , 14 , 23 , 24 . However, the transparency and mechanical properties are still difficult to obtain simultaneously because these poorly transparent inorganic platelets are generally distributed randomly within the hydrogel.
In nature, mollusks provide us with a case for assembling an organic/inorganic composite hybrid with ordered distribution at micro/nanometer scale in mild conditions 25 . By secreting organic molecules in biomineralization, these living organisms can regulate the size, morphology and orientation of the inorganic components, thus forming organic-inorganic hybrid composites with excellent mechanical and optical properties 26 . When exfoliating a thin film from natural nacre (e.g., Anodonta woodiana ), the film was not only highly transparent underwater but also mechanically robust 17 . Inspired by this interesting phenomenon, we thought that the process of biomimetic mineralization might suggest an efficient strategy to achieve advanced underwater superoleophobic materials with integrated high transparency and mechanical robustness.
Here, we present a detailed protocol for developing a transparent and mechanically robust underwater superoleophobic film with both high underwater transparency and mechanical robustness by combining superspreading and biomimetic mineralization strategies (Fig. 1 ). The prepared nacre-inspired mineralized (NIM) films were composed of aragonite (one crystalline polymorph of calcium carbonate) platelets as the inorganic component and chitosan (CS) derivatives (CS modified by methacrylic anhydride (MA), CSMA) as the organic framework, which highly resembled the thin film of natural nacre in terms of chemical compositions and hierarchical micro/nano structures (Fig. 2 ). Because of the proper combination of high-energy, orderly, inorganic aragonite and surface hierarchical micro/nano structures (Fig. 3 ), these NIM films exhibit high underwater transparency and outstanding mechanical properties at the same time.
Fig. 1: Schematic of the fabrication procedure of the nacre-inspired mineralized (NIM) film based on superspreading and biomimetic mineralization. First, a chitosan modified by methacrylic anhydride (CSMA) solution droplet is added, and spontaneous and complete spreading occurs at the superhydrophilic surface/oil interface, forming a thin superspreading layer. Then, the CSMA hydrogel film is fabricated by photo-cross-linking. Finally, the underwater superoleophobic NIM film is obtained by biomimetic mineralization of the negatively charged amorphous calcium carbonate (ACC) particles on the CSMA film.
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Fig. 2: The NIM film is similar to natural nacre in both chemical composition and physical structure. a , b , Surface hierarchical micro/nano structures of natural nacre ( a ) and the NIM film ( b ). c , d , Enlarged micrographs of the aragonite platelet of natural nacre ( c ) and the NIM film ( d ). The insets show the diameter distribution of nanograins on the corresponding films. e , X-ray diffraction profiles indicate that the aragonite crystal structure of the NIM film is the same as that of the nacre film. Figure adapted with permission from ref. 17 , Wiley.
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Fig. 3: Morphology characterization of the NIM film. a , b , Scanning electron microscopy images and corresponding energy dispersive spectroscopy mapping images of the CSMA film ( a ) and the NIM film ( b ). c , Cross-polarized light micrograph of the NIM film in air. d , High-resolution transmission electron microscopy image reveals the aragonite nature of the NIM film. Figure adapted with permission from ref. 17 , Wiley.
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In this protocol, we use calcium and carbonate not in the form of ions but as amorphous precursor nanoparticles similar to those found in natural nacre for biomineralization 27 , 28 . Traditional mineralization approaches such as diffusion of CO 2 and the Kitano method often require the fabrication of amorphous calcium carbonate (ACC) as precursors in advance, limiting the speed of the mineralization process because of the slow rate of gas diffusion (24 h) or evolution (6 d) 29 , 30 . In contrast, our mineralization process in this protocol directly used calcium carbonate as precursors, making the mineralization process faster. Importantly, this NIM film can be coated on a series of transparent and flat supporting materials, such as glass, polystyrene (PS), polyethylene terephthalate (PET) and polypropylene (PP), by combining the superspreading technique with a biomimetic mineralization process thereby making the materials promising coatings in underwater optics, miniature reactors and microfluidic devices. This protocol describes an efficient, interdisciplinary approach for rapid, large-scale production of biomineralized materials with unique features. It will be of interest to audiences covering fundamental and applied research in biomaterials, biochemistry, inorganic chemistry and interfacial engineering.
Applications of the method .
Recently, underwater superoleophobic surfaces have been explored for many applications including bio-adhesion 31 , 32 , oil-droplet manipulation 19 , 33 , microfluidic technologies 18 , self-cleaning 22 , 34 , marine antifouling 7 , 24 and oil–water separation 35 , 36 , 37 . Importantly, the ingenious integration of transparency and mechanical robustness into underwater superoleophobic materials significantly expands their potential applications in emerging fields, such as underwater optics and microfluidic devices 2 , 11 , 17 .
Underwater optics .
In a recent report, we successfully used this film as oil-repellent windows of diving instruments and underwater cameras, taking advantage of its high transparency and underwater superoleophobicity 17 . The result demonstrates that the NIM film-coated lens can repel oil away from the coated lens surface and retains its high transparency in the oil–water environment (Fig. 4 ).
Fig. 4: Transparency of NIM films and their potential applications as underwater transparent oil-repellent coatings. a , Quantitative comparison of the transparency of NIM-coated and uncoated transparent substrates, including glass, PS, PET and PP. Data are expressed as mean?±?s.d. ( n ?≥?3). b , The mechanical properties of NIM films are much higher than those of the previously reported underwater superoleophobic materials. c , The NIM-coated substrate shows an excellent oil-repellent property and high transparency. The coated substrate is contaminated by mineral oil before immersing into water. d , Optical images of swimming goggles coated with the NIM film after immersion in a mixture of mineral oil and water. e , The photograph was taken in a mixture of mineral oil and water by using an underwater camera with a NIM-coated lens. It shows the high transparency and self-cleaning capability of the coating under water. The inset is a photo of the underwater camera lens coated with the NIM film placed in oily water. The NIM film is coated on the protective glass substrate rather than on the lens itself to reduce the risk of lens damage. All mineral oils are pre-dyed with oil red O. MPC, 2-methacryloyloxyethyl phosphorylcholine; PDA, polydopamine; PNIPAM, poly( N -isopropylacrylamide); PVA, poly(vinyl alcohol); TXE, polyethylene glycol tert -octylphenyl ether nonionic surfactant/epoxy. Figure adapted with permission from ref. 17 , Wiley.
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Oil–water separation .
Subsequently, we have successfully prepared a biomineralized mesh with underwater superoleophobicity, showing scalable and robust oil–water separation with high efficiency (Fig. 5 ) 35 . We also showed in these studies that nacre-like features impart excellent mechanical properties to the NIM films, leading to mechanical robustness. Especially the hardness and Young’s modulus of NIM films are higher than those of reported underwater superoleophobic materials including double-network hydrogels and organic-inorganic composite materials. Hence, the NIM films can achieve durable superoleophobicity, even after suffering harsh treatments such as sand-grain impingement and knife scratch, which is promising for underwater oil-repellent materials 17 , 35 .
Fig. 5: NIM coatings for oil–water separation. a , SEM image of the bio-inspired mineralized mesh (BMM). Based on the combination of dip coating and biomimetic mineralization, the BMM was obtained by depositing a mineralized layer on the surface of commercial nylon mesh. b , c , Photographs of oil–water separation. The BMM was fixed between two quartz tubes, and the oil–water mixture was put into the upper tube. The separation process was driven merely by the gravity of the liquids without other external forces. Silicone oil was dyed by oil red O, and water was dyed blue by methylene blue trihydrate. d , Photograph of the BMM with a large area. e , The oil–water separation efficiency of the BMM for different types of oil. f , The separation efficiency of silicone oil–water mixtures from different recycles. Data are expressed as mean ± s.d. ( n ≥ 3). Figure adapted with permission from ref. 35 , Wiley.
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Microfluidic devices .
Another application of this material is in microfluidic devices. Droplet microfluidic systems, for example, require microchannel surfaces with multifunctional properties (i.e., high transparency, mechanical robustness and superwettability) to avoid cross-contamination between channels and make them easier to clean after use (which improves their reusability) 18 , 38 , 39 , 40 . If the surfaces are transparent, it is possible to obtain a clear view of the microdroplets during the liquid transportation process; this real-time analysis is crucial for monitoring the reactions between microdroplets in microfluidic channels or miniature reactors 2 , 18 , 39 . Superwettability (e.g., superoleophobicity) also makes further miniaturization of channels possible, because channels with these properties are less likely to become blocked. The risk of blocking is especially relevant for high-viscosity liquids, in which it can result in high pressure conditions 40 , 41 .
There are also some reaction conditions that result in high pressure; for these applications, researchers require materials that have high mechanical stability to reduce the deformation of microfluidic systems under high pressure 41 , 42 , 43 . Moreover, in droplet microfluidic systems, the wetting properties of microchannels are of critical importance for droplet formation and stabilization 16 . For instance, when using microfluidic devices to prepare emulsions, oil droplets tend to adhere to the surface of microchannels with weak underwater oil repellency, thus disrupting the flow tendency of water and leading to fouling in microchannels 2 , 16 , 40 , 42 . Therefore, it is necessary to combine the mechanical stability and transparency of underwater superoleophobic coatings to meet some specific demands of microfluidic device applications such as droplet microfluidics. As a proof of concept, the modification of our NIM film on the surface of the microfluidic channel not only prevents the adhesion of oil droplets on the surface of the microchannel, but also preserves the transparency of the microchannel surface for optical imaging (Fig. 6 ). When the mixture of oil and water flowed into the channel, the NIM-modified microchannel could effectively inhibit the adhesion of oil droplets, while the bare microchannel could easily adhere to the oil droplets (pre-dyed with oil red O). After rinsing the channel with water, the optical images clearly showed that no oil remained on the surfaces of the NIM-coated channel, whereas oil always adhered firmly on the surfaces of the bare channel. These experimental results confirmed that the use of NIM coatings not only helps in monitoring fluid motion (e.g., microemulsion) in microfluidic channels because of their good transparency, but also keeps a high flow rate by reducing oil droplet fouling and blocking.
Fig. 6: The NIM coating for anti-oil in microfluidic channels. a , Photograph of the microfluidic chip. b , Optical images of microchannels with and without NIM coatings. c , The NIM coating as an underwater superoleophobic surface is used for preventing oil droplets fouling and blocking in the microchannel. When a mixture of oil and water flows into the channel, the NIM-modified microchannel can effectively inhibit the adhesion of oil droplets, whereas the bare one can easily be adhered to by oil droplets (pre-dyed with oil red O). After rinsing the channel with water, no oil remained on the surface of the NIM-coated channel, whereas oil still firmly adhered on the surface of the bare one.
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Comparison with other methods .
To achieve underwater superoleophobicity, high-surface-energy materials with hierarchical micro/nano structures are often used to trap water molecules on their surfaces, forming a water cushion to repel oil droplets 5 , 6 , 10 , 37 . Similar to those found in nature, various materials, such as hydrogels 22 , metal oxide 5 , polyelectrolyte assemblies 44 , calcium alginate 7 and conducting polymer 45 , have been pursued to obtain underwater superoleophobic surfaces. Although some of these strategies have achieved excellent underwater superoleophobicity, the low mechanical stability of these coating materials greatly limits their practical applications 12 , 13 . For instance, hydrogels are usually used as underwater superoleophobic surfaces with good transparency, but they are usually vulnerable because of their poor mechanical strength, which leads to the loss of underwater superoleophobicity in a short time under harsh marine conditions 13 , 34 . Although double-network hydrogels and inorganic-reinforced composite hydrogels have been successfully used in the fabrication of mechanically robust underwater superoleophobic surfaces, their mechanical properties and transparency are generally mutually exclusive 11 , 17 . In addition, metal oxides are applied in underwater superoleophobic surfaces with high mechanical strength, but they are opaque and easily corrode in marine environments 5 , 6 . Compared with the developed technology for preparing underwater superoleophobic surfaces, the realization of both underwater superoleophobicity and transparency yet remains a significant challenge, which might require more rigorous structures at micro/nanometer scales.
In recent years, the technology of femtosecond laser microfabrication has been successfully used to fabricate underwater superoleophobic surfaces. This production process has advantages that include a negligible heat-affected zone, a no-contact process, a precise ablation threshold and high resolution 3 , 20 . In particular, this technology can be used to make a very wide range of materials, some of which have underwater superoleophobic surfaces. The types of materials include semiconductors, glasses, metals, polymers, ceramics and even biological tissues 20 . To achieve underwater superoleophobicity, the surface of solid materials is directly ablated by laser to form various hierarchical micro/nano structures 46 . However, the current machining process is relatively time consuming as well as extremely destructive to the surface of materials 20 , 46 . Generally, the mechanical stability of the femtosecond laser–induced underwater superoleophobic surfaces greatly depends on the properties of the body material. Although the femtosecond laser–ablated silica glass surface has excellent superoleophobicity and good light transmittance in water, it exhibits poor transmittance in air 11 . In addition, femtosecond laser–induced underwater superoleophobic surfaces always require expensive precision instruments and complex conditions, which further limit its widespread application 20 , 46 . It would be better to use a simpler and more effective method to develop advanced underwater superoleophobic coatings with both high transparency and mechanical robustness.
In this protocol, the underwater superoleophobic NIM film exhibits excellent integration of high transparency and mechanical robustness. Inspired by natural nacre, we present a facile strategy for fabricating underwater superoleophobic materials by integrating superspreading with a biomimetic mineralization strategy. Because of the combination of high-energy, ordered, inorganic aragonite and homogeneous external hierarchical micro/nano structures, these underwater superoleophobic NIM films exhibit outstanding mechanical properties and high transparency. The hardness and tensile strength of NIM films are 2.48 ± 0.59 GPa and 113.07 ± 12.64 MPa, respectively, as determined by the nanoindentation and tensile tests, respectively. Both the hardness and Young’s modulus of NIM films are higher than those of reported underwater superoleophobic materials including double-network hydrogels and organic-inorganic composite materials. Hence, NIM films can keep durable superoleophobicity, even after suffering harsh treatments such as sand-grain impingement. Especially, mechanical properties of NIM films are much higher than those of previously reported underwater transparent superoleophobic materials 7 , 14 , 24 , 47 . Therefore, this film offers new opportunities to develop highly transparent coatings for various underwater applications.
Advantages and limitations of the protocol .
The major advantage of the advanced underwater superoleophobic films described in this protocol is the integration of high transparency and mechanical performance in a facile way. Here, we provide calcium and carbonate not in the form of ions but as amorphous intermediate nanoparticles as found in natural mollusks 27 , 48 . Unlike traditional mineralization approaches such as gas diffusion and the Kitano method, these amorphous precursors can easily allow for high mass flux and rapid crystallization speed, independent of gas diffusion and evolution 30 , 49 , 50 , 51 . By using the ACC-mediated biomimetic mineralization process described in this protocol, it takes only a few hours to finish the preparation of the mineralized film; this is substantially less time than that required for traditional methods (e.g., 24 h for the gas-diffusion method 52 , 53 , 54 and >6 d for the Kitano method 55 , 56 ). We have also shown that this process can be used to make large-sized mineralized films (Supplementary Fig. 1 ). It is worth mentioning that the amorphous intermediate nanoparticles should be at an appropriate concentration to enable the formation of a uniform and continuous NIM film. The precursor concentration in this protocol described can serve as a basis for further optimization. As shown in Supplementary Fig. 2 , the NIM films exhibit excellent solvent-resistance ability, which is a considerable advantage over polymer-based films 37 . Moreover, the good biocompatibility and special superwettability of the mineralized film gives it potential applications in medical fields, such as functional coating for implant materials 54 , 57 , 58 and antifouling coating for optical equipments 17 . Despite these advantages, this approach also has several limitations. The mineralization of the CSMA film led to the wettability transition, which accounts for the gradual increase in inorganic CaCO 3 components with high energy and the generation of surface micro/nano structures in the ACC-mediated biomimetic mineralization process. Increasing the thickness of the CSMA film can accelerate the mineralization process and thus increase the thickness of the NIM film (Supplementary Fig. 3 ), but a rapid rate of mineralization may lead to a decrease in transparency 17 . Thus, the uniform and continuous micro/nano scale structures of NIM films are critical for achieving transparency, which largely depends on the thickness and homogeneity of the CSMA film. Although NIM films can be easily coated on various transparent, flat substrate surfaces by combining superspreading and biomimetic mineralization, this process does not work on substrates that are not flat. For surfaces that are not flat, we demonstrate that NIM films can coat substrates like nylon mesh and capillaries by using a combination of dip coating and biomimetic mineralization (Figs. 5 and 6 ).
An important consideration is that the NIM film, mainly composed of CaCO 3 crystals, easily tends to decompose in harsh chemical environments, including the presence of acids and chelating agents, which may limit some specific usage scenarios (Fig. 7 ). For instance, when mineral films are used for oil–water separation, the mixture of oil and water containing acid needs to be pre-adjusted to neutral or alkaline to increase the service life of the film, rather than separating oil and water containing a large amount of acid directly. Therefore, the development of transparent and mechanically robust underwater superoleophobic materials for application in complex environments is an issue that needs to be solved in the near future.
Fig. 7: Chemical stability of the NIM film. a , Variation of underwater oil contact angle of the NIM film after immersing in EDTA solution with different concentrations for 24 h. b , Cross-polarized light micrographs of the NIM film after immersing in EDTA solution with different concentrations for 24 h. c , Variation of underwater oil contact angle of the NIM film after immersing in the solution with different pH values for 24 h. d , Cross-polarized light micrographs of the NIM film after immersing in the solution with different pH values for 24 h. Data are expressed as mean ± s.d. ( n ≥ 3).
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Experimental design .
In this protocol, NIM films are fabricated by combining the superspreading technique and biomimetic mineralization process. The superspreading technique provides a thin liquid layer with a controllable and stable environment as a confined reactor for the preparation of the CS-based organic thin films for subsequent biomimetic mineralization (Fig. 1 ). To achieve photo-cross-linking ability, CSMA is synthesized and used in the preparation of organic thin films by the superspreading technique (Steps 1–16). Then, we detail how to achieve CaCO 3 crystal growth on the surface of the superspreading-based organic CSMA film by the ACC-mediated, short-time biomimetic mineralization process (Steps 17–20). Finally, we provide detailed characterizations of NIM films, including microscopic morphology, crystal phase, underwater superoleophobicity, mechanical property and transparency (Steps 21–36).
Synthesis of CSMA .
On the basis of previously established methods 59 , CSMA conjugates are synthesized by single-step chemo-selective N -acylation between CS and MA (Supplementary Fig. 4 ). The synthesis reaction is typically performed for 1.0 g of CS and can be further scaled to a desired size (reactions using 0.5–2.0 g of CS have been successfully performed in our laboratory by linear scaling of all reagents). Characterization of CSMA functionalization is highly recommended to be performed on every newly synthesized and purified batch, to ensure that the intended degree of methacryloyl substitution has been obtained (Steps 1–7). The degree of substitution here mainly depends on the molar ratio of MA to CS, which is a key parameter for determining the solubility of CSMA and the cross-linking density of CSMA hydrogel.
Photo-polymerization of CSMA hydrogels .
Here, CSMA hydrogel is obtained via the photopolymerization of methacryloyl carbon-carbon double bonds under UV light irradiation ( λ = 365 nm). We recommend a systematic study of the influence of CSMA concentration on polymerization, which is very important to subsequently control the thickness of the CSMA film by the superspreading technique (Steps 8 and 9).
Fabrication of CSMA films based on the superspreading method .
The superspreading technique involves coating a thin liquid layer on an immersed gel surface. The gel surface with the liquid layer acts as a confined reactor making it possible to prepare polymer films with controlled thicknesses. In this protocol, a droplet of CSMA precursor solution rapidly and entirely spreads on the superhydrophilic substrate immersed in silicone oil and forms a thin solution layer within seconds. After a few minutes of photo-polymerization by UV radiation, the thin precursor solution layer is converted into a CSMA hydrogel film with uniform thickness and a smooth surface (Steps 10–16). In particular, the thickness of the superspreading-based CSMA film can be precisely controlled from the nanoscale to the micron scale by regulating the concentration or volume of the CSMA precursor solution (Supplementary Fig. 5 ). In the process of superspreading, the thickness of the CSMA film varies linearly with the concentration or volume of CSMA precursor solution.
ACC-mediated biomimetic mineralization of the CSMA film .
Before the biomimetic mineralization process, ACC is decorated with poly(acrylic acid) (denoted as PAA) molecules to improve its long-term stability; this is done by modifying a previously reported method (Step 17) 60 . Through the electrostatic interaction between the -COO ? groups of PAA and the -NH 3+ groups of CSMA, a large number of ACC particles in the solution can be attracted to the CSMA film vicinity, resulting in high supersaturation with CaCO 3 on the surface of the CSMA film. The high supersaturation of CaCO 3 drives crystallization, causing aragonite cores to form randomly on the CSMA film surface and then grow laterally. With increasing mineralization time, the NIM film with a Voronoi pattern is eventually formed by the lateral growth of aragonite platelets on the CSMA film (Step 19). Crystal growth based on biomimetic mineralization of the negatively charged ACC on the CSMA film is the most crucial step toward obtaining the NIM film.
To ensure the effectiveness of biomimetic mineralization, the ACC solution used should be prepared fresh and should not be kept for a long time. We strongly recommend that the ACC solution used in each mineralization process be stored at room temperature (RT) for ≤3 d. In addition, the temperature in the mineralization process should not be <25 °C, because low temperature leads to a slow mineralization rate.
Materials .
Reagents .
Caution .
When handling the chemicals used in this protocol, always wear suitable personal protective equipment, including a laboratory coat, nitrile gloves, safety goggles and, where indicated, a face shield and respirator. For any chemical listed in this protocol, appropriate institutional and governmental safety guidelines must be followed. Please refer to the appropriate materials safety data sheets.
CS (viscosity >400 mPa s; Innochem, cat. no. 9012-76-4; store it at RT)
Critical .
Other molecular weights of CS can also be used to prepare CSMA by following the same protocol, if necessary. However, this may affect the viscosity during synthesis and the properties of the resulting hydrogels.
Acetic acid (Aladdin, cat. no. A116166; store it at RT)
Caution .
Acetic acid is a volatile and toxic organic compound, which may cause skin irritation, serious eye damage and respiratory irritation if inhaled. Always wear a face shield and respirator. Avoid any direct contact.
MA (Aladdin, cat. no. M102519; store it under nitrogen at 4 °C)
Caution .
Contact with MA can cause skin irritation, respiratory irritation and eye damage. When handling MA, always wear appropriate personal protective equipment, including a long-sleeved laboratory coat, a respirator, safety goggles and a face shield. Avoid any direct contact.
2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (I2959; Aladdin, cat. no. H137984; store it at RT)
Liquid nitrogen
Caution .
Contact of liquid nitrogen with the skin or eyes may cause serious frostbite injury. When handling liquid nitrogen, always wear appropriate personal protective equipment, including a long-sleeved laboratory coat, thermo-insulated gloves, safety goggles and a face shield.
Silicone oil (5 centistokes (cSt); Beijing Zhongkeshangde Technology Co., Ltd.)
NaOH (Aladdin, cat. no. S111498; store it at RT)
Caution .
NaOH can cause skin corrosion and serious eye damage. Avoid any direct contact.
HCl (Sinopharm Chemical Reagent, cat. no. 10011018; store it at RT)
Caution .
HCl can cause skin corrosion and serious eye damage. Avoid any direct contact.
PAA (molecular weight: 5,100; Sigma-Aldrich, cat. no. 447013; store it at RT)
Critical .
Other molecular weights of PAA can also be used to prepare mineralized films by following the same protocol, if necessary. However, this may affect the properties of the materials and the as-prepared crystals.
CaCl 2 (Sinopharm Chemical Reagent, cat. no. 10005861; store it at RT)
Na 2 CO 3 (Sinopharm Chemical Reagent, cat. no. 10019260; store it at RT)
MgCl 2 (Sinopharm Chemical Reagent, cat. no. XW77863034; store it at RT)
MgSO 4 (Sinopharm Chemical Reagent, cat. no. 20025118; store it at RT)
Deionized water (18.2 MΩ cm; Millipore water purification system)
Ethanol (≥99.7%; Sinopharm Chemical Reagent, cat. no. 10009218; store it at RT)
Caution .
Ethanol is flammable and can cause moderate eye and skin irritation.
Equipment .
Round-bottom flask with a magnetic stir bar
Critical .
Use a stir bar of sufficient size with a powerful stirrer to ensure good dispersion of MA. Alternatively, an overhead stirrer with propeller agitator can be used instead of a magnetic stir bar.
Dialysis membrane with a 3,500-Da MWCO
Lyophilizer (Christ, model no. Alpha 2-4LD plus)
Plasma cleaner (Harrick Plasma, model no. PDC-002)
Contact-angle system (Dataphysics, model no. OCA25)
Dynamic contact-angle measuring instrument (Dataphysics, model no. DCAT21)
UV-LED curing system (NBET, XE300WUV)
Caution .
UV irradiation can seriously damage skin and eyes. Avoid any direct contact.
Oven (Shanghai Bluepard Instruments, model no. DZF-6050)
pH meter (Mettler Toledo, FE20K)
Temperature-controlled water bath (Lichen, model no. DF-101S)
Untreated microscope glass slides
Critical .
Ensure that the same brand or type of glass slides is used consistently to exclude variation of hydrogel properties due to different light permeation through different glass slides.
Stereomicroscope (Olympus, model no. LV100ND)
Critical .
Ensure that the microscope has parts for polarizing devices.
UV-visible absorption spectrophotometer (Shimadzu, model no. UV-2250)
Reagent setup .
CSMA precursor solution .
To prepare 1.0% (wt/vol) CSMA precursor solution, 1.0 g of CSMA sponge is dissolved in 100 ml of deionized water containing 1.0% (wt/vol) acetic acid and 0.5% (wt/vol) I2959.
Critical .
Other concentrations of CSMA can also be used to prepare CSMA solution by following the same protocol, if necessary. This solution can be stored for ≥1 week at 4 °C in a dark place.
ACC solution .
To prepare 100 ml of ACC solution, 50 ml of Na 2 CO 3 (10 mM) is added into 50 ml of a precursor solution containing CaCl 2 (10 mM) and PAA (0.02% (wt/vol)) at RT with stirring for 15 min. Then, the pH of the solution is adjusted to 10.5 by the stepwise addition of a 1 mM NaOH aqueous solution. Finally, ACC solution is further purified by filtration.
Critical .
ACC solution should be made up fresh.
Artificial seawater .
Add 5.3452 g of NaCl, 0.2306 g of CaCl 2 , 0.4520 g of MgCl 2 and 0.6496 g of MgSO 4 to 200 ml of deionized water. Then, the artificial seawater solution is further purified by filtration. This solution can be stored for ≥1 month at 4 °C.
Procedure .
Synthesis of CSMA .
Timing 6–8 h (setup), 5–7 d (dialysis) and 3–4 d (lyophilization)
1 Dissolve 1.0 g of CS in 100 ml of deionized water containing 1.0% (wt/vol) acetic acid in a three-neck round-bottom flask with a magnetic stir bar. Stir rigorously for 60 min at RT to facilitate complete CS dissolution.
2 While stirring moderately, heat the mixture to (and keep at) 60 °C in a water bath until the CS is fully dissolved and the solution becomes clear.
Caution .
Perform CSMA functionalization in a chemical safety fume hood, and wear appropriate personal protective equipment, including a laboratory coat, a respirator, nitrile gloves and safety goggles.
Critical step .
Ensure that all glassware and laboratory equipment are clean, to avoid chemical contamination.
Critical .
CSMA is light sensitive; maintain lyophilized CSMA and solutions containing CSMA in the dark, for example, by wrapping the dialysis setup and conical centrifuge tubes in aluminum foil.
Troubleshooting
3 While stirring vigorously, slowly add 1.0 g of MA (a very viscous liquid) per 1.0 g of dissolved CS for methacryloyl functionalization. The reaction will be opaque because of the emulsion of MA. Allow the reaction to proceed in a water bath for 6 h while maintaining the temperature at 60 °C.
Critical step .
The reaction time and temperature, as well as the mass ratio of MA to CS, can influence the degree of CSMA functionalization. Under the premise of controlling the reaction time and temperature, the degree of modification of CSMA can be varied by changing the ratio of MA to CS.
Critical step .
Ensure adequate stirring during CSMA functionalization while minimizing air uptake. Insufficient stirring will lead to visible phase separation. Use a glass pipette when handling MA, because organic solvents may dissolve plastic pipette tips.
Troubleshooting
4 After the reaction period, transfer the solution to a dialysis membrane with a 3,500-Da MWCO and dialyze at RT against a large volume of demineralized or ultrapure water for 5 d in a chemical safety fume hood. Change the water at least twice daily.
Critical step .
Methacrylic anhydride and acid byproduct are cytotoxic. It is therefore crucial to fully remove these contaminants by dialysis before proceeding to Step 4. Dialysis is completed when the CSMA solution appears clear and when the odor of residual methacrylic anhydride or methacrylic acid byproduct is no longer noticeable.
Troubleshooting
5 After dialysis, transfer the solution to 50-ml tubes and snap-freeze them in liquid nitrogen.
Caution .
Contact of liquid nitrogen with the skin or eyes may cause serious frostbite injury. When handling liquid nitrogen, always wear appropriate personal protective equipment, including a long-sleeved laboratory coat, thermo-insulated gloves, safety goggles and a face shield.
Pause point .
Samples can be stored at ?80 °C for ≥1 month.
6 Transfer all frozen sample to the freeze-dryer without allowing the solutions to thaw, and lyophilize them until the polymer is fully dry (this takes 3–4 d). To maintain a barrier, cover the 50-ml tubes with filter paper and seal them with rubber bands before lyophilization.
Critical step .
Ensure that samples do not thaw when transferred to the lyophilizer. Once dry, store polymers under a vacuum at ?20 °C to avoid absorption of water and hydrolysis of CS.
Critical step .
Once dry, store frozen samples under a vacuum at ?20 °C to avoid absorption of water and hydrolysis of CS.
Pause point .
Lyophilized CSMA can be stored for ≥1 year at ?20 °C.
7 Characterize the degree of modification by using 1 H NMR spectroscopy with deuterium oxide (D 2 O) as the solvent.
Dissolve 4.0 mg of CSMA in 0.8 ml of D 2 O with shaking for 30 min at RT to ensure complete dissolution.
Transfer the solution to an NMR tube by using a pipette and seal the tube with a cap.
Acquire spectra according to the manufacturer’s instructions.
As shown in Supplementary Fig. 4 , calculate the degree of substitution (DS) of CSMA by determining the ratio of integrated area of the H a′ peak at 2.8 ppm to that of the methylene (H b ) peaks at 5.5 and 5.8 ppm according to equation ( 1 ).
$${\mathrm{DS}} = \frac{{{{A}}\left( {5.5\& 5.8\,{{{\mathrm{ppm}}}}} \right)/2}}{{{{A}}\left( {2.8\,{{{\mathrm{ppm}}}}} \right)}} \times 100\%$$
(1)
where A (5.5&5.8 ppm), A (2.8 ppm) are the area of methylene protons’ peak (H b ) at 5.5 and 5.8 ppm, and the ring protons’ (H a′ ) peak of CSMA residues at 2.8 ppm, respectively.
Critical step .
We recommend that Step 7 be performed each time a new batch of CSMA is made, to ensure that the intended degree of methacryloyl substitution has been obtained. According to the experimental conditions in Step 3, we usually obtain a functionalized CS with a substitution degree of ~11% for subsequent experiments.
Troubleshooting
Preparation of CSMA hydrogel precursor solutions .
Timing ~1 h
8 Dissolve the CSMA sponge obtained in Step 6 in deionized water containing 1.0 wt% acetic acid and 0.5 wt% I2959 to obtain a CSMA solution (e.g., 1.0 wt%).
Critical step .
The concentration of CSMA can be adjusted according to the final concentration required in Step 10.
Critical step .
The dissolution of photo-initiator and CSMA in aqueous solution can be promoted by ultrasound. For example, ultrasonic treatment of 1.0 wt% CSMA solution at RT for 1 h can achieve good dissolution.
Pause point .
The precursor solution can then be stored at 4 °C in a dark place for ≤1 week, to avoid hydrolyzation of the methacryloyl groups and to allow time to complete the next step.
Photo-polymerization of CSMA hydrogels .
Timing ~10 min
9 Perform the photo-cross-linking reaction by exposing the CSMA hydrogel precursor to UV light irradiation ( λ = 365 nm) for 2–3 min.
Critical step .
The UV exposure time can be substantially reduced (e.g., ~60 s) if a higher concentration of CSMA is used.
Critical step .
The influence of CSMA concentration on polymerization should be further studied, which is very important to subsequently control the thickness of CSMA film by the superspreading technique in Step 11. According to the experimental conditions in Step 3, functionalized CS with a substitution degree of ~11% can usually be obtained. In terms of this degree modification, gel cannot be obtained when CSMA solution concentration is <0.5 wt%. Therefore, the lower concentration of CSMA solution cannot be used to prepare the following superspreading-based stable films.
Fabrication of CSMA films .
Timing 7–8 h
10 Treat the glass surface that will be used as the reaction substrate with plasma (high power for 10 min) and immediately immerse it in deionized water.
Critical step .
It is important to immerse the plasma-treated glass in water, because the treated glass surface is superhydrophilic and can form a water film in the water, which facilitates superspreading in the following steps.
11 Immerse the plasma-treated glass in silicone oil with a depth of ~1 cm.
Critical step .
Choose a silicone oil that has low viscosity to facilitate the superspreading of the CSMA solution. We typically use silicone oil with a viscosity range of 5–20 cSt.
12 Add CSMA solution to the silicone oil layer on the hydrophilic surface by using a syringe. The superspreading-induced confined water layer is formed on the hydrophilic surface within seconds.
Critical step .
The thickness of the superspreading-based solution layer can be precisely controlled by regulating the volume of the CSMA precursor solution.
Troubleshooting
13 Irradiate the CSMA precursor solution with UV light ( λ = 365 nm) for ~2 min. A CSMA hydrogel film forms inside the confined water layer.
Critical step .
Avoid introducing air bubbles, because entrapped oxygen inhibits radical polymerization of the hydrogel precursor solution and leads to incomplete cross-linking.
Critical step .
Ensure that the superspreading-based solution layer is horizontal and uniform during photo-cross-linking.
Troubleshooting
14 Gently remove the as-prepared CSMA film from the silicone oil and rinse successively with absolute ethyl alcohol to remove the residual silicone oil.
Caution .
Ethanol is flammable and can cause moderate eye and skin irritation. Avoid any direct contact with fire.
Critical step .
Ensure that the residual silicone oil on the film is completely removed by ethanol.
15 Transfer the CSMA film to the baking box at 60 °C for >3 h until the film is fully dry. Then, soak the dried film in the 2 M NaOH aqueous solution for ≥10 min to remove the residual acetic acid.
Caution .
NaOH can cause skin corrosion and serious eye damage. Avoid any direct contact.
Critical step .
Ensure that the residual acetic acid in the film is completely neutralized by NaOH.
16 Repeatedly wash the CSMA film with deionized water (a total of three times). Then, transfer the CSMA film to the baking box at 60 °C for >3 h and store at RT until you are ready to perform Step 19.
Critical step .
Ensure that the residual NaOH in the film is completely removed by deionized water.
Pause point .
The CSMA film can be stored for ≥1 month at RT.
ACC-mediated biomimetic mineralization of the CSMA film .
Timing 6–7 h
17 To prepare 100 ml of ACC solution, add 50 ml of Na 2 CO 3 (10 mM) into 50 ml of a precursor solution containing CaCl 2 (10 mM) and PAA (0.02% (wt/vol)) at RT with stirring for 15 min.
Critical step .
This solution should be prepared fresh and should not be kept for a long time (no more than 3 d before use).
Critical step .
Ensure that the mixture solution is adequately stirred when Na 2 CO 3 solution is slowly added to the precursor solution containing CaCl 2 and PAA.
Troubleshooting
18 Adjust the pH of the solution to 10.5 by the stepwise addition of a 1 mM NaOH aqueous solution.
Caution .
NaOH can cause skin corrosion and serious eye damage. Avoid any direct contact.
19 Mineralize the dried CSMA film by simply immersing it in the ACC solution for 6 h. The film should be left in a 25 °C water bath for the entire mineralization process.
Critical step .
In the biomimetic mineralization process, aragonite cores first form on the surface of the CSMA film at the nucleation sites, followed by lateral growth. With increasing mineralization time, the NIM film with a Voronoi pattern is eventually formed by the lateral growth of aragonite platelets on the CSMA film.
Critical step .
The temperature in the mineralization process should not be <25 °C; too low a temperature will lead to too slow a mineralization rate.
Critical step .
Ensure that the entire mineralization is static.
Critical step .
Ensure that the amount of mineralized solution is sufficient. Typically, 10 ml of mineralized solution is sufficient for the mineralization of a 20 mm × 20 mm CSMA film.
Troubleshooting
20 Rinse the NIM film with deionized water and dry at RT.
Critical step .
The NIM film can be easily detached from the substrate after being immersed overnight in 2 M NaOH aqueous solution.
Pause point .
The mineralized film can be stored at RT for several years.
Microstructure characterization of the NIM film .
21 There are several options to characterize the microstructure of the NIM film, including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), optical microscopy, optical polarizing microscopy and atomic force microscopy (AFM).
(A) SEM characterization of the NIM film
Timing ~1 h
(i) To obtain electrical conductivity of the sample, fix the CSMA film on the sample table with conductive double-sided tape and spray with gold by ion sputtering (10 mA, 60 s).
Critical step .
Blow the sample surface clean with nitrogen before ion sputtering.
Critical step .
Protect the morphology of the sample surface during storage and handling.
Critical step .
Ensure that the sample has good electrical conductivity.
(ii) Transfer the sample to the electron microscope observation room.
(iii) Acquire SEM images by using a Quanta FEG 250 field-emission scanning electron microscope at an acceleration voltage of 10 kV in high-vacuum mode.
(B) EDS characterization of the NIM film
Timing ~1 h
(i) Acquire EDS data by using a Hitachi SU1510 variable-pressure scanning electron microscope equipped with an Oxford Penta FET Precision detector.
Critical step .
Protect the morphology of the sample surface during storage and handling.
Critical step .
Blow the sample surface clean with nitrogen before EDS characterization.
(C) Optical microscopy characterization of the NIM film
Timing ~0.5 h
(i) Place the NIM film between two clean microscope glass slides and flatten them.
Critical step .
Protect the morphology of the sample surface during storage and handling.
Critical step .
Blow the sample surface clean with nitrogen before optical microscopy characterization.
(ii) Observe the morphological features of the CaCO 3 crystals by optical microscopy and/or optical polarizing microscopy.
(D) AFM characterization of the NIM film
Timing ~1 h
(i) Blow the NIM film surface obtained in Step 18 clean with nitrogen.
(ii) Perform AFM tests of CSMA films and NIM films by using a Bruker FastScan Bio atomic force microscope.
Critical step .
Protect the morphology of the sample surface during storage and handling.
Crystal-phase characterization of the NIM film .
Timing ~0.5 h
22 Cut the NIM film obtained in Step 18 into pieces with scissors and then grind them into powder in a mortar.
23 Take an appropriate amount of powder into the glass groove of the sample table and gently press flat with clean glass pieces.
24 Identify the crystal phases of the obtained precipitates by X-ray diffraction (XRD) using Ni-filtered Cu Kα radiation ( λ = 1.5406 ?) (scanning range: 5–80°).
Characterization of underwater superoleophobicity of the NIM film .
Timing ~0.5 h
25 Carefully draw up the oil droplets (1,2-dichloroethane, 3 μl) with a syringe and drop these onto the surface of the NIM film immersed in water.
26 Measure the oil contact angles (OCAs) by using an OCA 20 machine (Data Physics) under ambient conditions.
27 Calculate the average value from at least five measurements performed at different positions on the same sample; this is reported as the OCA.
Critical step .
Ensure that the oil droplets are the same size for each measurement of contact angle.
Critical step .
Ensure that no bubbles adhere on the surface of the CSMA film immersed in deionized water.
28 Measure the adhesion force of the oil droplet by using a high-sensitivity microelectron mechanical balance system (Data Physics DCAT 11) in an aqueous environment.
An oil droplet (1,2-dichloroethane, 10 μl) is suspended underwater by using a copper ring.
The copper ring is attached to the microbalance.
Under the water, the NIM film surface is controlled to move upward toward the suspended oil droplet at a constant speed of 0.005 mm/s until it makes contact with the oil droplet; when it makes contact, it is controlled to move downward again.
Changes in the force experienced by the NIM film and the shape of the oil droplet are simultaneously recorded during the whole measuring process.
29 Determine the influence of external pressure on underwater oil adhesion by preloading different forces on the oil droplet and then measuring differences in the force and shape changes of the droplet as described in Step 28.
The oil adhesion force is measured by using a high-sensitivity microelectromechanical balance system in a water environment.
The average value of at least five measurements performed at different positions on the same sample is adopted as the oil adhesion force.
Ensure that no bubbles adhere on the surface of the CSMA film immersed in deionized water.
Ensure that the hung oil droplets are the same size for each measurement of the oil adhesion force.
Mechanical characterization of the NIM film .
Timing ~2 h
30 For tensile mode testing, separate the NIM film from the glass (see Step 20) and cut it into 10-mm wide × 50-mm long pieces for the next step.
31 Perform tensile tests at a loading speed of 1 mm/min with a distance of 10 mm between the clamps by using a motorized test station (Mark-10 ESM301).
32 Simultaneously record the change of tensile force and displacement during the whole tensile measuring process.
Critical step .
The average value of at least five measurements performed is adopted as the tensile strength.
Critical step .
Ensure that there are no cracks in the edge of the sample before the tensile test.
33 For nanoindentation testing, select the flat NIM film described in Step 20.
34 Determine the indentation position on the NIM film by using an optical microscope at a magnification factor of 1,000.
35 Perform nanoindentation tests of the NIM film by using a Nano Indenter G200 (Keysight). The indented depth for all samples is ~500 nm.
Critical step .
Ensure that the indentations are performed on the calcium carbonate crystals of the NIM film.
Transparency characterization of the NIM film .
Timing ~0.5 h
36 Record the transmission spectra of different films in the wavelength range of 300–900 nm by using a 2600 UV-visible spectrometer in double-beam mode with air as the reference.
Critical step .
The NIM film is separated from the glass when it is used for transparency tests.
Troubleshooting .
Troubleshooting advice can be found in Table 1 .
Table 1 Troubleshooting table Full size table
Timing .
Steps 1–7, synthesis of CSMA: 9–12 d
Steps 8–16, fabrication of CSMA films: 8–9 h
Steps 17–20, ACC-mediated biomineralization of the CSMA film: 6–7 h
Steps 21–36, characterization of the NIM film: ~7 h
Anticipated results .
The composition and hierarchical structure of the NIM film .
In this protocol, a thin CSMA film forms on a surface by superspreading. Calcium and carbonate are added as amorphous intermediate nanoparticles, and aragonite cores form at the nucleation sites in the CSMA film. Lateral growth of aragonite platelets results in the formation of Voronoi patterns. The NIM film is composed of microscale platelets and nanoscale granules (~44.5 nm) with distinct Voronoi patterns, which highly resemble the nacre films in terms of chemical composition and hierarchical micro/nano structure (Fig. 2a-d ). In addition, CaCO 3 crystal in the NIM film has a typical aragonite peak of (221) but not calcite peak of (104), demonstrated by the XRD pattern analysis (Fig. 2e ). The spatial distribution of carbon and calcium elements indicates the successful biomimetic mineralization of the CSMA film (Fig. 3a and b ). The distinctive dark and bright mosaics under cross-polarized light and the clearly resolved lattice fringes of the fabricated NIM film confirm that the aragonite platelets are made up of a single type of crystal (Fig. 3c and d ). These results are similar to those for the analysis of a nacre film.
Mechanically robust underwater superoleophobicity of the NIM film .
The biomimetic mineralization of the CSMA film also induced the wettability transition of the films from the hydrophilic state to the superhydrophilic state in air and from the oleophobic state to the superoleophobic state under water (Fig. 8 ). This difference is probably caused by the generation of surface CaCO 3 micro/nano structures with high energy. The NIM film shows excellent and stable underwater oil-repellent performance even under large external pressures, high ion strength and salinity (Fig. 9a–c ). Particularly, the hardness and Young’s modulus of the NIM films are higher than those of other reported underwater superoleophobic materials including double-network hydrogels and organic-inorganic composite materials because of the high crystallinity of the aragonite (Fig. 9d ) 6 , 7 , 13 , 14 , 23 , 24 , 53 . Their superoleophobicity is durable even after harsh treatment such as sand-grain impingement (Fig. 9e ). If the surfaces are damaged by more-severe external impact (e.g., sandpaper abrasion), the underwater oil-repellent property of NIM films can be restored by remineralization (Fig. 9f ). This is achieved by immersing them in an ACC solution (Steps 17–20), a process that can be repeated multiple times.
Fig. 8: The influence of biomimetic mineralization on surface wettability and underwater oil adhesion. a , Schematic diagrams show the growth process of the NIM film. b , Optical images of the crystal growth with different mineralization times. c , The curve shows the crystal growth rate over time on the CSMA film. d , The increase in the hydrophilicity in air and oleophobicity under water of the NIM film along with the increase of mineralization time. e , The underwater oil adhesion of the NIM film significantly declines as the mineralization time increases. Data are expressed as mean ± s.d. ( n ≥ 3). Panels b – e adapted with permission from ref. 17 , Wiley.
Full size image
Fig. 9: Mechanically robust underwater superoleophobicity of NIM films. a , Ultralow adhesive force of oil droplets under a series of preload forces ranging from 10 to 100 ?N. b , Stability of the underwater superoleophobic NIM films in high-salt solutions. c , Long-term stability of NIM films in seawater. d , Both the hardness and Young’s modulus of NIM films are much higher than those of the previously reported underwater superoleophobic materials. The upper left inset shows a typical residual indent of the Berkovich diamond tip on the NIM film. e , Mechanical robustness of NIM films under different external impacts. f , NIM films damaged by severe external forces could be easily restored by re-mineralization; this process could be repeated several times. The black and blue data points correspond to the underwater oil adhesion of the restored and damaged NIM film, respectively. Data are expressed as mean ± s.d. ( n ≥ 3). DN, double network; HEC, hydroxyethyl cellulose; MMT, montmorillonite; PVDF-GN, polyvinylidene fluoride-graphene nanosheet. Figure adapted with permission from ref. 17 , Wiley.
Full size image
Transparency of the NIM film .
NIM films also exhibit high transparency in air and water because of the combination of orderly inorganic aragonite and homogeneous external hierarchical micro/nano structures. NIM films have been applied to various transparent substrates, such as glass slides and PS, PET and PP surfaces, and high transparency has been retained (Fig. 4a ). Importantly, the Young’s moduli of NIM films are much higher than those of the previously reported underwater transparent superoleophobic materials (Fig. 4b ).
We compared square glass coverslips with and without NIM coatings by immersing them into an oil-polluted water tank (Fig. 4c ). Those coated with NIM showed high transparency and excellent oil-repellent properties both inside the tank and when they were removed from it. A similar experiment using swimming goggles gave the same results: the coated area was transparent and clean, whereas the uncoated area was stained with mineral oil (Fig. 4d ). This approach also works for the production of oil-repellent windows for underwater cameras (Fig. 4e ). The NIM film-coated camera lens repels oil and retains the same level of high transparency in the oil–water environment as would be expected in pure water.
These results demonstrate that NIM films offer new opportunities for developing highly underwater transparent superoleophobic coatings for applications in emerging fields.
Reporting summary .
Further information on research design is available in the Nature Research Reporting Summary linked to this article.
Data availability .
The main data supporting the findings of this study are available within the article and its Supplementary Information files.
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Acknowledgements .
We acknowledge support from the Key Research Program of the Chinese Academy of Sciences (XDPB24), the National Natural Science Foundation of China (21875269, 22035008, 31771026 and 51403158) and the International Partnership Program of the Chinese Academy of Sciences (1A1111KYSB20200010).
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CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, P. R. China
Wei Chen,?Ruhua Zang,?Shutao Wang?&?Jingxin Meng
School of Ophthalmology & Optometry, Eye Hospital, Wenzhou Medical University, Wenzhou, P. R. China
Wei Chen,?Liming Xu?&?Bailiang Wang
University of Chinese Academy of Sciences, Beijing, P. R. China
Wei Chen,?Ruhua Zang,?Shutao Wang?&?Jingxin Meng
School of Materials Science and Engineering, Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, P. R. China
Pengchao Zhang
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinghuangdao, P. R. China
Shaokang Yu
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Contributions .
W.C., S.W., B.W. and J.M. conceived and designed the experiments. W.C., P.Z., S.Y., R.Z. and L.X. performed experiments. W.C., P.Z., S.W. and J.M. analyzed and interpreted the data, developed the methodology and wrote the manuscript. S.W., B.W. and J.M. performed data interpretation, method development and editing of the manuscript.
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Correspondence to Shutao Wang , Bailiang Wang or Jingxin Meng .
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Key references using this protocol
Chen, W. et al . Adv. Mater . 32 , 1907413 (2020): https://doi.org/10.1002/adma.201907413
Yu, S. et al . Adv. Mater. Interfaces 8 , 2100852 (2021): https://doi.org/10.1002/admi.202100852
Key data used in this protocol
Chen, W. et al . Adv. Mater . 32 , 1907413 (2020): https://doi.org/10.1002/adma.201907413
Yu, S. et al . Adv. Mater. Interfaces 8 , 2100852 (2021): https://doi.org/10.1002/admi.202100852
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Chen, W., Zhang, P., Yu, S. et al. Nacre-inspired underwater superoleophobic films with high transparency and mechanical robustness. Nat Protoc (2022). https://doi.org/10.1038/s41596-022-00725-3
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Received : 03 December 2020
Accepted : 20 May 2022
Published : 15 August 2022
DOI : https://doi.org/10.1038/s41596-022-00725-3
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