Detaching adhesive oil staining from a surface by water

Using household detergents to clean oil stains has always caused global concerns, as these detergents negatively impact the ecosystem and are toxic. Therefore, it is essential to effectively attenuate the adhesion force between oil stains and substrates to create an easy and detergent‐saving cleaning pathway. To address this challenge, we herein develop a strategy to reduce the strength of oil adhesion on common substrates by ∼20 times through a lamination layer, which contains phase‐transitioned lysozyme nanofilm (PTL) and cellulose nanocrystals (CNCs). The resultant CNC/PTL coating significantly enhances the capability of cleaning oil stains in an underwater detergent‐free manner; this strategy is applicable to edible oil packaging material and tableware, without impairing the usability and aesthetics of these materials. This coating exhibits excellent mechanical stability and regeneration characteristics through simple soaking, ensuring its robustness in real applications in an infinite life cycle. By eliminating 100% detergent in routine cleaning, the CNC/PTL coating demonstrated remarkable cost‐effectiveness, saving 57.7% of water and 83.3% of energy when washing tableware only with water. This work presents an ingenious design to create oil‐repellent packaging materials and tableware toward detergent‐free water‐cleaning pathways, thereby greatly reducing the negative environmental impact of surfactant emissions.


INTRODUCTION
In daily life, detergents used to remove oil stains contain many anionic surfactants, such as sodium alkyl sulfonate, which can cause serious environmental pollution.However, the market size for global dishwashing detergent was valued at USD 17.98 billion in 2021 and is expected to expand at a compound annual growth rate of 8.2% from 2022 to 2028. [1]his suggests that the demand for detergents is consistently high.As a result, conventional household detergents are usually discarded with sewage water or directly discharged into surface water, after which they penetrate different environmental compartments, such as water, soil, or sediment. [2] major concern is that these detergents, when discarded with wastewater, seriously inhibit the growth of aquatic organisms and inevitably pose risks to various aspects of the ecosystem. [3]In addition, bioaccumulation of surfactants through food chains and potable water arouses a reduction in the activity of various enzymes in the human body, resulting in pathological changes. [4,5]Considering the disadvantages of traditional synthetic detergents, natural, ecological, and sustainable detergents will become available on the market.The natural detergents currently available on the market are surfactants extracted from plants, including seed saponins, [6] fibers and carbohydrates, [7] or fatty alcohols based on natural fats and oils. [8]Other eco-friendly detergents, such as microbial metabolites, [9] enzymes, and lipase, are also useful for cleaning protein stains. [10]Although natural detergents are safer than synthetic detergents, scaling up the production of these detergents is limited due to their low yields and high S C H E M E 1 Schematic illustration of the preparation of the cellulose nanocrystal/phase-transitioned lysozyme (CNC/PTL) coating and subsequent surfactant-free oil-stain removal by water on daily articles.production costs; in addition, these detergents cause damage to wool or silk fabrics and skin allergies. [11,12]Moreover, these methods can only reduce the harm of surfactants to a limited extent and cannot solve the fundamental problem.Therefore, it is necessary to develop a transformative route to replace the use of detergents from the source.
The detergent cleaning process is dependent on the formation of oil stain-encapsulated stable micelles in water by surfactants and effective reduction in the interfacial adhesion of oil stains on a surface.During this process, stronger intermolecular interactions between detergents and oil stains replace the interactions between oily stains and solid surfaces.Based on the underwater superoleophobic properties of organisms, such as fish scales, nacres, and seaweed surfaces, [13][14][15][16][17][18] underwater superoleophobic surfaces are potential candidates for reducing the interaction between oil stains and substrate surfaces.Researchers have designed and developed a series of underwater superoleophobic interfacial materials. [13,14]However, few studies have reported that these surfaces were successfully used on tableware and food packaging material to reduce the use of cleaning agents.The corresponding reasons for these failures are as follows: (1) most of them are unsuitable for industrial-scale production and practical utility; (2) most of them exhibit limited transparency because of the extensive light-scattering effect from surface micro/nanostructures [13,19] and are usually vulnerable because of their poor mechanical strength, leading to a loss of underwater superoleophobicity in a short time in practical applications; [20,21] and (3) most of them are based on the use of fluoro-containing compounds that are highly toxic.Above all, applying transparent, nontoxic, and mechanically stable underwater superoleophobic materials remains a major challenge, which greatly limits their application in surfactant-reducing/free oil cleaning from a surface.
In this work, we underline a strategy to combine amyloidlike protein adhesion at surfaces with the decoration of cellulose nanocrystals (CNCs), [22,23] which exhibit great potential for practical application of superoleophobic surfaces in surfactant-free cleaning of oil stains on surfaces (Scheme 1).This concept can impart virtually arbitrary surfaces with long-lasting antifouling properties to achieve oil stain removal with only water without compromising usability and aesthetics.The central feature of this concept is that any target surface can be rapidly coated with CNC coating using amyloid-like phase-transitioned lysozyme (PTL) 2D nanofilms [24][25][26][27] as the intermediate interfacial adhesive layer.The introduction of the PTL nanofilm noticeably enhanced the interfacial adhesion of the CNC coating on virtually arbitrary surfaces, thereby significantly overcoming the interfacial stability challenge of the CNC coating on general substrates.The resultant CNC/PTL coating could be sustained well in a variety of stringent treatments of ultrasonication, cyclic friction, water flushing, and 3 M peeling without affecting the corresponding antifouling performance.In addition, the coating can withstand prolonged contact and impact of foods, as well as food corrosion and various washing approaches in practical applications, including hand and dish-washing processes.This technique can be further applied on demand to regenerate the coating in a simple water immersion or spraying step that can be repeated infinitely; thus, the long-lasting underwater oil repellence can be maintained endurably.Compared to conventional detergent-based cleaning solutions, this detergent-free water cleaning mode is more efficient for cooking utensils or food packaging materials, significantly saving water and energy by at least 57% and 83%, respectively; thus, the promise to exclude 100% detergent use for routine oil cleaning from daily tableware and packaging materials can be achieved.Notably, this coating method exhibits great commercial potential, as it can transform current mainstream protocols for cleaning tableware from detergent strategies to water rinsing methods, thus greatly promoting the sustainable development of a green society.

Preparation of the CNC/PTL coatings
The nontoxic reducing agent cysteine helps break the Cys6-Cys127 disulfide bonds in the lysozyme molecule, triggering its unfolding and subsequent amyloidlike aggregation.This process can be verified by the far-UV circular dichroism (CD) spectra of the reaction solution.
The spectra showed a partial loss of α-helix peaks at 208 and 222 nm of native lysozyme and a subsequent transition to an antiparallel β-sheet at 216 nm (Figure 1A).Then, a large number of unfolded protein monomers were assembled into nanoscale particles in a short time.Subsequently, these protein nanoparticles with a typical diameter of 50 nm, aggregated at the air/water or liquid/solid interface and resulted in a continuous PTL nanofilm (Figure 1B).According to our previous studies, [28,29] a PTL nanofilm with a thickness of approximately 9 nm can be formed on a target substrate by simply immersing it in the reaction solution for 20 min.The PTL-coated substrates were subsequently soaked in the CNC suspension solution.The CNCs were extracted from cotton linters and exhibited a rod-like morphology with a length of 188.97 ± 15.23 nm and a diameter of 16.83 ± 2.25 nm (Figure S1).With the deposition of CNCs, the resultant CNC/PTL coating exhibits a morphology of densely packed rod-like cellulose nanocrystals (Figure 1C), with a thickness of 15 nm and RMS roughness of 3.4 nm (Figure 1D and Figure S2).The CNC/PTL coating barely changed the transmittance of the substrate, as the quartz coated by the CNC/PTL coating remained highly transparent with a transmittance higher than 95% in the visible wavelength range (Figure 1E).This result is in stark contrast to classical coating techniques in the literature, [30] such as tannins with deep coloration (Figure S3).The hydrophilicity of the PTL-coating was significantly enhanced by the further deposition of hydrophilic CNCs, as the water contact angle (WCA) of the PTL-coated silicon wafer was reduced from 80.1 ± 0.5 • to 30.1 ± 0.5 • (Figure 1F and Figure S4a,b).The CNC/PTL-coating exhibited excellent underwater super-oleophobicity, as the coating shows ultra-low oil adhesion underwater (Figure 1F and Figure S4c).An ultralow oil sliding angle (OSA, 1.2 ± 0.1 • ) was obtained and oil droplets completely bounced off from the coating surface.
The immersion time of the substrate in the CNC suspension plays an essential role in controlling the morphology and wettability of the resultant coatings.When the soaking time in a 0.10 wt% CNCs suspension was prolonged from 1 to 6 min, the corresponding oil contact angle (OCA; Figure 1G) and the packing closeness of the CNCs gradually increased (Figure S5).In comparison, the underwater OSA decreased noticeably (Figure 1G).This result was further characterized by a laser scanning confocal microscopy (LSCM), where the fluorescence from the FITC-PTL nanofilm (prepared by FITC-labeled lysozyme) was gradually shielded by the CNC coating (Figure S5).An immersion time of 5 min was then determined to produce a reliable OCA and OSA at 156.3 • and 0.8 • , respectively.The results were further supported by quartz crystal microbalance (QCM) analysis (Figure S6), in which at concentrations of CNCs ≥ 0.05 wt%, the equilibrium time for the adsorption of CNCs on the PTL layer was less than 5 min (Figure S6d).The wettability of the CNC/PTL coating was also determined by the concentration of the CNCs in the suspension.To obtain a close-packed CNC/PTL coating with good underwater superoleophobicity, the effective concentration of the CNC suspension was determined to be as low as 0.06 wt% (Figure 1H and Figure S6d).In addition to the dip-coating approach, spray coating and spin coating, which are commonly used in industry at a large scale, are also suitable for preparing robust CNC/PTL coatings on desired substrates (Figure 1I).The PTL nanofilm can adhere to various material surfaces, endowing the CNC/PTL coating strategy with excellent universality. [31,32]As shown in Figure 1J, the CNC/PTL films could be readily coated on various substrates, including metals (stainless steel, etc.), inorganics (glass, silicon wafer, ceramics), and polymers (e.g., polyvinyl chloride, polypropylene, polyethylene glycol terephthalate [PET], polycarbonate [PC]).These CNC/PTL-coated surfaces showed effective underwater superoleophobicity, exhibiting good repellency toward a broad range of oils in a water medium, such as petroleum ether, liquid paraffin, n-hexane, and edible oil such as rapeseed oil (with an underwater OCA > 150 • and OSA < 3 • ) (Figure 1K).

Basic tests on the stability of the CNC/PTL coating
Most reported underwater superoleophobic coatings can be easily damaged and lose their oil resistance in practical applications (especially in the marine environment) due to their weak adhesion to substrates. [20,21]In contrast, in this work, the PTL nanofilm acts as an excellent interfacial binder, enabling the CNC/PTL coating to adhere firmly to a virtually arbitrary substrate, resulting in durable oil repellency underwater.First, a 180 • peeling test was done to quantify the adhesive strength of the coating (Figure S7).The adhesion strength of the CNC/PTL coating on the glass substrate was at least five times higher than a CNC coating formed on a blank glass substrate (Figure 2A).Second, the resistance of the CNC/PTL coating toward bending was tested.The underwater OCA on the coating did not show any decrease after the CNC/PTL-coated PET substrate was bent at 180 • for 4000 bending cycles (Figure 2B).Furthermore, the coating retained its intact morphology appearance after the bending tests (Figure S7).Third, the CNC/PTL coating exhibits excellent thermal stability; after heating at 180 • C for 72 h, the coating maintained its underwater superoleophobicity, because the underwater OCA on the coated substrate remained above 150 • (Figure 2C).
Furthermore, a series of stringent tests were done and demonstrated the robustness of the CNC/PTL coating during multiple harsh treatments, including 100 cycles of rubbing tests by using sandpaper (grit no.400) under a slider weighing 200 g and moved for 5 cm (with a friction force of 0.74 N), ultrasonic treatment for 30 min, 50 cycles of 3 M adhesive tape peeling, exposed to a water jet for 60 min at a flow rate of 6.25 mL/s (with a force of 0.01 N), and chemical erosion in the pH = 2 and pH = 12 solutions for 2 h.Under these severe destabilization conditions, the underwater OCA on CNC/PTL surfaces still maintained above 150 • (Figure 2E), and the corresponding surface topography (e.g., RMS roughness) and thickness remained unchanged (Figure 2F,G).In contrast, the morphology of the CNCs directly deposited on the blank substrate was largely damaged under these conditions (Figure 2D).In addition, we also found that the coating could withstand consecutive blade scrapes for at least 50 cycles (Figure S7).The above mechanical stability tests suggest that common abrasion in daily life may not compromise the CNC coating on the PTL-primed substrate.
The strong binding between the CNCs and the PTL coating was further reflected by QCM analysis, in which CNCs at a concentration of 0.10 wt% produced rapid and signifi-cant adsorption on the PTL-primed chip surface; in contrast, nearly no CNCs absorbed on the blank chip and native lysozyme-coated (Lys-coated) chip surfaces (Figure 3A-C and Figure S8).Moreover, no CNC was observed on Lyscoated silicon wafer (Figure S8).This binding was further highlighted by immersing a substrate in which a partial area was coated by the PTL coating, and the corresponding scanning electron microscopy (SEM) image showed that the PTL-coated part exhibited a uniform layer of CNCs, while the blank area had no obvious adsorption of CNCs (Figure S9).The strong adsorption activity of CNCs on PTL coating was ascribed to the hydrogen bonding and electrostatic interactions between PTL and CNCs, as demonstrated by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectra, and Zeta potential analysis.The high-resolution C 1s spectrum of the PTL nanofilm reflected multiple signals for a variety of functional structures (Figure 3D studies, [27,28,30,31] these chemical structures are favorable for forming multiple binding modes with the underlying substrates to achieve adhesion generality to various material surfaces, including coordination bonding, electrostatic forces, hydrogen bonding, and hydrophobic forces.Furthermore, the multifunctional structures of CNCs, mainly including carboxyl and hydroxyl groups, can form hydrogen bonds with functional groups on the PTL surface, as reflected by the shift to high binding energies in the C 1s , O 1s , and N 1s spectra (Figure 3D-F and Figure S10).With the appearance of the characteristic peak at 1023 cm −1 for the CNCs (Figure 3G), the -NH bending vibration peaks in the amide II band, -COOH, and amide I bands are further redshifted from the initial 1571 cm −1 , 1310 cm −1 , and 1649 cm −1 in PTL to 1556 cm −1 , 1303 cm −1 , and 1634 cm −1 in CNCs/PTL (Figure 3H).In these tests, the occurrence of hydrogen bonding between CNCs and PTL will average the electron clouds of the functional groups of PTL and CNCs, resulting in a redshift in the position of the IR peak. [32,33]Furthermore, the binding of CNCs on PTL is also strengthened by the electrostatic interactions between them; CNCs are negatively charged with a Zeta potential of −10.64 eV.In comparison, PTL is positively charged with a Zeta potential of 19.2 eV (Figure 3I).
Overall, the above results demonstrate that the PTL nanofilm can serve as an excellent intermediate binder to adhere to the underlying substrate and bind with the CNC toppings, making the CNCs stably adhere to a surface (Figure 3J).In contrast, the CNCs deposited on the blank substrate without the PTL coating lack this interfacial adhesion; as a result, the CNC layer easily peeled off after being flushed with water for a few seconds (Figure S11).Moreover, this method is not limited to CNCs; other nanocellulose-based coatings (e.g., cellulose nanofibers [CNF] and bacterial cellulose [BC]) can also be deposited to obtain CNF/PTL and BC/PTL composite coatings (Figure S12).As already recognized in science and engineering fields, the poor adhesion between CNCs and substrates has always been a challenge toward limiting its applications.In this work, the introduction of PTL has solved this problem, allowing CNCs to adhere stably on various substrates via a universal and scale-up engineered surface coating method.

Decreasing the oil adhesion on surfaces by the CNC/PTL coatings
The underwater superoleophobic CNC/PTL coating is favorable for lowering the adhesion of edible oils on a surface.This work showed that a broad spectrum of edible oils with different viscosities and surface tensions exhibited ultralow adhesion force to the CNC/PTL coatings in water, in which the rolling angles are less than 10 • (Figure 4A,B and Table S1).In contrast, the underwater OCAs of these edible oils on the blank glass surfaces were low and the corresponding rolling angles were over 90 • , showing the strong adhesion force of edible oils on the blank surfaces.For instance, the adhesion force of sesame oil on a blank silicon wafer surface is 18.49 times higher than that on the CNC/PTL-coated silicon wafer (Figure 4B,C and Figure S13).
To test the self-cleaning ability of the CNC/PTL coating, two rapeseed oil droplets were dropped onto the CNC/PTLcoated and blank glass substrates, respectively.As shown in Figure 4D, no oil residue was observed on the CNC/PTLcoated glass substrate after placing it in water.In contrast, we observed rapeseed oil residue on the blank glass surface even after rinsing with running water.Therefore, the CNC/PTL-coated glass surface showed excellent underwater self-cleaning properties.The removal of oil stains underwater is attributed to the hydrophilicity of the CNC/PTL coating.When the coated substrates are immersed in water, the water molecules prefer to bind with the hydrophilic CNCs, [34][35][36] forming a further fusion into a dense water layer on the CNC/PTL coating surface, creating a low-energy state interface.On the other hand, oil molecules on the oil droplets are often incompatible with water molecules, thereby tending to aggregate and form spherical shapes rather than adhere to the coating surface, effectively removing the oil stains from the surface.This process was also recorded with a high-speed camera (Movies S1 and S2).
To more intuitively evaluate the oil residues on blank and CNC/PTL-coated surfaces, the rapeseed oil was dyed with Nile red (a hydrophobic fluorescent dye), and the oil residues on both surfaces after water rinsing were imaged by inverted LSCM.Uniform fluorescent signals were detected on the blank PET surface (Figure 4E); in contrast, no obvious fluorescence signal was caught on the CNC/PTL-coated PET surface (Figure 4F).A quantitative test of the residual oil was further conducted based on fluorescence intensity normalization.The blank glass substrate with dyed rapeseed oil residues exhibited a strong characteristic peak of Nile red at 588 nm.The normalized intensity indicated that approximately 85% of the oil remained on the substrate after water rinsing.In contrast, no emission peak of Nile Red at 588 nm was observed on the CNC/PTLcoated surface, indicating that almost 100% of oil residues were removed (Figure 4G).Therefore, the CNC/PTL coating has excellent underwater oil-repellency ability.Based on the above results, the biomass-based CNC/PTL coating has great application prospects to achieve surfactant-free oil stain cleaning by water alone, which is highly desirable for easy and economical packaging material recycling and sustainable development.
We then investigated the possibility of coating CNC/PTL on packaging material surfaces to achieve surface oil removal with only water rinsing, without adding any detergents (Figure 5A).As shown in Figure 5B, the CNC/PTL-coated and blank bowls made of ceramic, plastic, glass, and stainless steel were contaminated with rapeseed oil and then washed with water.It can be observed that the coated bowls were almost clean without any rapeseed oil residues, while there are still large oil stains on the inner wall of the blank bowls.In addition to plant oil, the CNC/PTL coating also repels animal oil in a detergent-free water-cleaning process.Beef tallow was used to contaminate the coated and blank bowls and similar results after water rinsing were observed (Figure 5C).One thing to note, we use warm water (T ∼ 40 • C) to clean beef tallow stains because they solidify in cold water and cannot be cleaned.Then, the wettability of the washed dishes after water rinsing-induced oil removal was tested.The results showed that the underwater superoleophobicity (OCA) and hydrophilicity (WCA in the air) on the CNC/PTL-coated tableware surfaces after oil staining and water washing were close to those before oil contamination (Figure 5D and Figure S14).Compared with traditional detergent rinsing approaches, this water-based cleaning approach can save about 57.7% water, 83.3% electricity, and 100% surfactants (Figure 5E).Based on these significant savings, this approach was further extended to other daily articles, such as ion pans and bottles.Even after heating (210 • C) and cooling for a few cycles, the oil on the CNC/PTL-coated pan surface can be easily rinsed off by water, while a layer of oil still adhered to the unmodified pan and commercial nonstick pan surfaces (Figure 5F and Figure S15).In addition, we assessed the adhesive stability of the CNC/PTL coating after being subjected to a 210 • C heat treatment by performing 50 tape peeling tests using 3 M tape.It was observed that the coating maintained its integrity throughout the tests (Figure S15).Widely used packaging bottles could also be coated with CNCs/PTL.As shown in Figure 5G,H, the rapeseed oil and sesame oil that paint inside of the CNC/PTL-coated bottles can be easily washed away by water, while the oil stains on blank bottles were difficult to clean.
In practical applications, the long-term contact stability and mechanical shaking stability of the coatings are crucial for edible oil packaging materials, especially considering their use in shelf-life storage and goods transportation.The CNC/PTL-coated packaging bottle filled with oil was placed on a shaker at 120 r/min for 7 days.After that, the superoleophobicity state of the coating was not compromised, showing an underwater OCA > 150 • (Figure 6A).The anti-staining stability of the CNC/PTL coating was further evaluated by washing the coated dishes via different approaches.These included 600 rinsing washes with water at a flow rate of 6.25 mL/s (a force of 1.05 × 10 −2 N), 150 rubbing washes with the rough fibrous side of a dishwashing brush, and 15 cycles of washing in a dishwasher at a temperature of 60 • C, with each wash for 30 min.After all these tests, the CNC/PTL-coated tableware still had an underwater OCA over 150 • (Figure 6B and Figure S16).Therefore, the oil-repellent capacity of CNC/PTL-coated tableware can withstand various cleaning approaches.Moreover, after the packaging bottles were filled with oil and left to stand for a month, the oil in the bottle could still be easily washed away with water, presenting an oil repellency with an underwater OCA over 150 • (Figure S17).Furthermore, the ability to resist food erosion in practical uses was evaluated.When corrosive sour soup (contains vinegar, salt, soy sauce, and pepper, pH = 4.58) or hot pot soup (contains chili, butter, spices, sugar, etc.) was placed in the CNC/PTL-coated bowl for 72 h, after cleaning the bowl with water, the resultant underwater OCA on the bowl surface remained higher than 150 • (Figure 6C).In addition, the stability of the CNC/PTL coating at 60 • C for 10 days was further verified by using 4% acetic acid and 50% ethanol as food simulants (Figure 6D).In addition to the single-time coating, the stable CNC suspension without sedimentation (being stable for at least 10 months) allowed the coating to be easily regenerated by spraying or dipping the CNC solution onto a target substrate (Figure S18).This also indicates that the rest of the CNC suspension after the first coating can be reused to prepare the next coating film, enabling highly efficient utilization of the CNC suspension and significant cost savings.The PTL coating can also be regenerated by spraying or dipping in the lysozyme phase transition solution. [23,31]This regeneration procedure renders a coating-at-will (CAW) concept (Figure S19); as a result, a facile refresh can be performed on the surface by supplying fresh CNC/PTL coating on demand (typically after 150 brush washes or 15 dishwasher washes) to achieve infinite cycled use (Figure 6E,F).With this CAW concept, we can prepare PTL/CNC coatings on dishes in the kitchen by ourselves by simply placing the preparation solu-tions in containers such as detergent pods, tandem bottles, and dual-nozzle sprayers (Figure S20).Taking the spraying approach an example, PTL nanofilms on the cleaned dishes can be obtained by spraying it with a dual-nozzle sprayer (filled with lysozyme and cysteine solution), and then another spray with a single-nozzle sprayer (filled with CNC solution) for the CNC coatings on the PTL-coated dishes.Then the PTL/CNC-coated dishes were prepared and endowed with the ability to clean oil stains only with water.
Considering its practical applications in food packaging and tableware, the in vitro cytotoxicity of CNC/PTL coatings was then assessed using a tetrazolium bromide reduction (MTT) cell viability assay on mouse fibroblasts (L-929).According to the standard test procedure (GB/T 16886.5-2017,GB/T 16886.12-2017/ISO10993-12:2012), the corresponding viability of L-929 cells with the use of CNC/PTL extract, lysozyme, and cysteine, respectively, was not significantly different from that of L929 cells on the control DMEM solution, indicating that the CNC/PTL, lysozyme, and cysteine were not toxic to mouse fibroblasts (Figure 7A-C).Additionally, the anti-nonspecific adsorption of proteins and bacteria for the CNC/PTL coating was further evaluated by using fluorescein isothiocyanate-labeled bovine serum albumin (BSA-FITC) as a model protein, as well as Escherichia coli and Staphylococcus aureus as model microbes.It was then found that the CNC/PTL coating surface did not exhibit obvious adsorption of BSA, E. coli and S. aureus, while a large amount of them was found on the blank substrate surface (Figure 7D,E and Figure S21).The resistance to protein/microbe adhesion on the CNC/PTL coating was ascribed to the close-packed CNCs with a hydration layer acting as a barrier layer to prevent the proteins/microbes from contacting the substrate and being easily washed away by water. [33]ased on excellent oil self-cleaning performance and biological safety, the requirements for sustainable expansion of application are further explored for CNC/PTL.From the perspective of material preparation cost and environmental impact, the cost to clean 100 m 2 tableware by CNC/PTL is approximately $0.12 (Table S2).In addition, a complete life cycle assessment (LCA), including environmental derating, was also conducted, which helps judge the overall sustainability and safety of materials.According to the LCA, the carbon footprint for treating 100 m 2 of CNC/PTL-coated tableware with water and 100 m 2 of blank tableware with the traditional detergent in different washing approaches (hand and machine washing) was analyzed.When washing by hand, the carbon dioxide emission for the CNC/PTL group is 26.54% of that of the detergent group (Figure 8A).In comparison, the carbon dioxide emission of CNC/PTL is only 16.76% of that of detergent when washing by dishwasher (Figure 8B).Thus, the environmental impact of CNC/PTL in both hand and machine-washing processes is significantly lower than that of detergent.In addition, the carbon footprint analysis of the CNC/PTL group to wash 100 m 2 of tableware showed that when washing tableware by hand, the carbon dioxide generated for tap water accounts for the largest proportion (Figure 8C); when cleaning tableware by machine, the carbon dioxide caused for electricity accounts for more than 90%; and the carbon dioxide generated for preparing the CNC/PTL coating accounts for only approximately 1% (Figure 8D).This further indicates that the preparation of CNC/PTL may produce low carbon dioxide emissions and such emissions will be significantly reduced after the industrialization of CNC/PTL in the future.

CONCLUSION
In conclusion, we achieved a detergent-free water method of cleaning various kinds of oil stains on a variety of surfaces by constructing a biocompatible underwater superoleopho-bic cellulose/protein composite layer on a surface in a green, energy-saving, and low-cost process.The first PTL priming layer is simply obtained from the rapid phase transition of lysozyme stimulated by a nontoxic reducing agent (cysteine) through a thiol-disulfide exchange reaction.Due to the abundant, versatile functional groups within PTL, it can attach to almost any material surface through multiple binding modes.The CNCs are then applied to the PTL layer by spin coating, spray coating, or dip coating for easy production of the antifouling CNC/PTL composite coating.The functional groups on the surface of PTL can further noticeably enhance the stability of the CNC coating through hydrogen bonding and electrostatic interactions with CNCs.As a result, the CNC/PTL coating exhibits excellent mechanical stability to maintain its underwater superoleophobic properties after mechanical friction, ultrasound, 3 M tape peel bending, or hydrodynamic rinsing.The resulting stain-resistant CNC/PTL coating can impart a surface that can easily remove oil stains from packaging materials and tableware with mere water, with additional resistance to protein and bacterial adhesion.Furthermore, the CNC/PTL coating exhibits nontoxicity and can easily adhere to the interior space of substrates that are not flat (such as bottles and bowls) by withstanding shaking, long-term oil contact, food corrosion, and various cleaning approaches in practical application scenarios.In addition, coating regeneration can be easily performed to refresh the CNC/PTL layer at any time by simply immersing the layer in the CNC/PTL suspension to achieve infinite oil-stain resistance.Compared with the conventional detergent-based cleaning mode, the CNC/PTL-induced water cleaning on oil stains can save 57.7% of water and 83.3% of energy consumption, and eliminate 100% of detergents during routine oil cleaning from a daily article surface.Due to the simple recycling and low cost of this material, the coating exhibits significant prospects for large-scale application and shows great potential to replace synthetic detergents in cleaning oil stains on packaging materials and tableware.

Growth of the PTL 2D nanofilm
[25][26] Substrates (including metals, glass, silicon wafers, and plastics) were then immersed in the phase transition solution and incubated at room temperature for 20 min.Then, a robust PTL nanofilm coating was prepared on the target substrate.Atomic force microscopy (AFM) measurements were done to study the morphologies and thicknesses of the nanofilms.

Preparation of the CNC/PTL antifouling coating
The PTL-coated substrates were immersed in the CNC suspension (0.10 wt%) for 5 min.After thoroughly washing with water and drying with nitrogen, the biomass-based surfaces were obtained and showed antifouling abilities.The corresponding morphology and thickness of the coating are characterized by AFM.
The CNC/PTL coating also can be prepared by spray coating (spraying CNC solution with a distance of 0.15 m from the target surface) and spin-coating approaches (with a spinning speed of 3000 rpm/min).

Characterization
SEM was conducted on an FEI Quanta 200 electron microscope (SU8020, Hitachi) with a primary electron energy of 5 kV.AFM was performed by a Dimension ICON (Bruck).Far-UV circular dichroism spectrum was recorded by using a Chirascan spectrophotometer (Applied Photophysics Ltd, England).The WCA, underwater OCA, and underwater oil slide angle (OSA) measurements were performed by an interface/tensiometer (DCAT 21 and OCA 20, Dataphysics).The FTIR spectra were recorded on a Tensor 27 (Bruck) spectrometer over the range of 4000-600 cm −1 to analyze the functional groups of the materials.The transmittance of the CNC/PTL coating adhered to a quartz plate was collected by a U-3900/3900H (Hitachi).The XPS spectra were obtained using an X-ray photoelectron spectrometer (AXIS ULTRA, Kratos Analytical Ltd.).The binding energies were calibrated by setting the C 1s peak at 284.6 eV.The fluorescence spectra were collected by an F-7000 fluorescence spectrophotometer (Hitachi).LSCM observation was conducted on FV1200 (Olympus).The adsorption of molecules on a surface was evaluated by a QCM with dissipation monitoring (QCM-D) of Q-Sense Explorer (Biolin Scientific).

Chemical stability
The chemical corrosion durability tests of the coatings were done by immersing the CNC/PTL-coated surfaces in the acid (pH = 2) and base solution (pH = 12) for 2 h.Then, the corrosion extent of the coating was checked with contact angle and AFM measurements.

Mechanical stability
A CNC/PTL-coated glass substrate was placed face-down on sandpaper (grit no.400) under a slider weighing 200 g and was then dragged on the sandpaper 5 cm.The corresponding static friction coefficient () of the coating was thus obtained.The maximum static sliding friction (F f ) then was calculated by the following equation: F f =  × F N (F N is the gravity of the slider).
For the dynamic water flushing test, the CNC/PTL-coated silicon wafer was placed under water at a flow rate (Q) of 6.25 mL/s for 60 min.The impact force (F) could be calculated by the theorem of momentum as follows: [27] F = Q(v 2 − v 1 ) m is the mass of the water, V is the volume of water impacted on the surface in a short time (t), S is the cross-sectional area of the water flow, v 1 and v 2 are the initial and final velocities of the water flow, respectively, and  is the density of water.
For the ultrasonic durability test, the CNC/PTL-coated substrates were subjected to ultrasonic (40 kHz) treatment for 30 min; for the tape stripping durability test, the CNC/PTL-based antifouling coating was peeled 100 times.The corresponding morphologies and thicknesses of the coating before and after treatments are characterized by AFM, and the wettabilities of these coatings were characterized by contact angles.
The sample was fixed on a stainless steel plate, and the other side with the CNC/PTL coating was tightly attached by double-sided adhesive tape.Then, loading with a weight of 10 kg was placed on the sample for 24 h.After removing the loading, the 180 • peeling test was applied to the sample.

Thermal stability test
The CNC/PTL-coated silicon wafer was placed at 180 • C for 72 h.

Testing the oil resistance of the CNC/PTL-coated tableware
Four materials (ceramic, plastic, glass, and stainless steel) were soiled with the same amount of rapeseed oil and beef tallow to simulate the oil stains on the tableware.The tableware contaminated with rapeseed oil was washed with water at room temperature, and the tableware contaminated with beef tallow was washed with warm water (40 • C) until the oil stain was cleaned.Then, the contact angle on the washed tableware surface was measured.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interests.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.

F I G U R E 1
Characterization of the cellulose nanocrystal/phase-transitioned lysozyme (CNC/PTL) coating.(A) Circular dichroism (CD) spectra of native lysozyme and PTL.(B,C) Atomic force microscopy (AFM) images of the PTL and CNC/PTL coatings on silicon wafer surfaces.(D) The thickness of the PTL and CNC/PTL coatings.(E) UV/vis spectrum of the CNC/PTL coating with the inset showing a transparent CNC/PTL coating prepared on the quartz surface.(F) CNC/PTL films showing hydrophilicity and underwater superoleophobicity.(G,H) Underwater oil contact angle (OCA) and sliding angle (OSA) of chloroform droplets (10 μL) on the close-packed CNC/PTL coatings fabricated by immersing the PTL-coated substrate in the CNC suspension at (G) different times and (H) CNC concentrations.(I) Underwater OCA and OSA of the CNC/PTL surface prepared by different approaches.(J) Morphology of CNC/PTL nanofilms fabricated on various substrates.The insets are the underwater OCAs of the CNC/PTL-coated substrates.(K) Underwater OCA and OSA of different oil droplets on the CNC/PTL coatings.

F I G U R E 2
Basic adhesion stability tests of the cellulose nanocrystal/phase-transitioned lysozyme (CNC/PTL) coating on the substrate.(A) The peeling strength of CNC/PTL coating and CNC coating during the 180 • peeling test.(B) Underwater oil contact angle (OCA) on the CNC/PTL surface after different bending cycles.The inset shows the bending angle of a CNC/PTL-coated polyethylene glycol terephthalate (PET) sample at 180 • .(C) OCA on the CNC/PTL surface after incubation at 180 • C for different times.(D) Morphology of the CNC/PTL coatings and CNC coating (dispensing CNCs on the blank substrate) after different treatments.(E) Roughness, (F) thickness, and (G) underwater OCA on the CNC/PTL coatings before and after various stringent treatments.
), including thiols (C-S), aliphatic carbon (C-H/C-C), amines (C-N), hydroxyls (C-O), amides (O=C-N), and carboxyl groups (O=C-O).According to our previous F I G U R E 3 Analysis of the interaction between cellulose nanocrystals (CNCs) and the phase-transitioned lysozyme (PTL) coating.(A) Quartz crystal microbalance with dissipation monitoring (QCM-D) measurement of the adsorption of CNCs (with a concentration of 0.10 wt%) on the PTL-coated chip.(B) The adsorption of CNCs on the PTL-coated chip at different times.(C) Comparison of the adsorption of CNCs (0.10 wt%) on coated and blank chip surfaces.The chemical components of the as-prepared PTL nanofilm and CNC/PTL coatings are characterized by (D-F) X-ray photoelectron spectroscopy (XPS) survey scan and (G,H) Fourier transform infrared (FTIR) spectra.(I) Zeta potential of PTL nanofilm and CNC suspension.(J) Schematic illustration of the interaction between the CNCs and the PTL.

F I G U R E 4
Evaluation of the wettability and adhesion of edible oils on the cellulose nanocrystal/phase-transitioned lysozyme (CNC/PTL)-coated surfaces.(A) Underwater oil contact angle (OCA) and oil sliding angle (OSA) of various edible oils on the CNC/PTL-coated surfaces.(B) Underwater oil adhesion force measurements on CNC/PTL-coated and blank silicon wafer surfaces.(C) The optical images for the shapes of oil droplets taken during the adhesive force measurements (the process of the oil drops contacting and detaching the substrate surface).(D) Self-cleaning of glass surfaces contaminated with rapeseed oil in water.Laser scanning confocal microscopy (LSCM) images showing Nile red-stained oil adhered to the (E) blank and (F) CNC/PTL-coated polyethylene glycol terephthalate (PET) surfaces, and the corresponding fluorescence spectra are shown in (G).

F I G U R E 5
Surfactant-free oil removal on dishes and packaging materials by water.(A) Schematic diagram of the water cleaning process on the oilstaining cellulose nanocrystal/phase-transitioned lysozyme (CNC/PTL)-coated surface.Photographs of the CNC/PTL-coated and blank bowls contaminated with (B) rapeseed oil and (C) beef tallow before and after water rinsing.(D) Comparison of underwater oil contact angle (OCA) on uncontaminated and oilcontaminated CNC/PTL-coated bowls surfaces after water washing.(E) Energy and surfactant savings for cleaning oil stains on the CNC/PTL-coated surface (compared to the unmodified surface).(F-H) Photographs of blank and CNC/PTL-coated (F) iron pans and (G,H) packaging bottles contaminated with (G) rapeseed oil or (H) sesame oil after water rinsing.

F I G U R E 6
Evaluation of the long-term stability of the cellulose nanocrystal/phase-transitioned lysozyme (CNC/PTL) coating toward practical applications.(A-D) The endurance for single-time coating by harsh treatments for different times.Underwater oil contact angle (OCA) after the CNC/PTL-coated bowls were subjected to (A) shaking and contact tests for 7 days, (B) washing with flushing water in 600 cycles, (C) soaking in sour soup and spicy hot pot soup for 72 h, and (D) treatment with 4% acetic acid and 50% ethanol solution for 10 days at 60 • C. (E,F) Underwater OCA of coating-at-will (CAW)-based regenerated CNC/PTL coating on bowls after washing by (E) dishwashing brush and (F) dishwasher for different cycles.In each cycle (150 brush washes or 15 dishwasher washes), the CNC/PTL coating was regenerated by the CAW-based coating.

F I G U R E 7
Cytotoxicity and antibacterial adhesion of the cellulose nanocrystal/phase-transitioned lysozyme (CNC/PTL) coating surfaces.Cytotoxicity of the (A) CNC/PTL coating, (B) lysozyme, and (C) cysteine.Fluorescence (left) and scanning electron microscopy (SEM, right) images showing that Escherichia coli and Staphylococcus aureus adhered to the (D) CNC/PTL-coated substrate surface and (E) blank substrate surface.S. aureus and E. coli were dyed with SYTO 9. F I G U R E 8 Life cycle assessment (LCA) of the cellulose nanocrystal/phase-transitioned lysozyme (CNC/PTL) coating.The carbon footprint to wash 100 m 2 tableware by (A) hand washing and (B) machine washing (carbon footprint data of detergent coming from China Products Carbon Footprint Factors Database [http://lca.cityghg.com/pages/item-view/9491]).(C,D) The proportion of each part of the carbon footprint to wash 100 m 2 tableware with the coating of CNC/PTL.

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C K N O W L E D G M E N T S P.Y. is grateful for funding from the National Science Fund for Distinguished Young Scholars (No. 52225301), the National Key R&D Program of China (Nos.2020YFA0710400 and 2020YFA0710402), the 111 Project (No. B14041), the Innovation Capability Support Program of Shaanxi (No. 2020TD-024), and the International Science and Technology Cooperation Program of Shaanxi Province (No. 2022KWZ-24)