Self-assembled fatty acid crystalline coatings display superhydrophobic antimicrobial properties

Abstract

Superhydrophobicity is a well-known wetting phenomenon found in numerous plants and insects. It is achieved by the combination of the surface's chemical properties and its surface roughness. Inspired by nature, numerous synthetic superhydrophobic surfaces have been developed for various applications. Designated surface coating is one of the fabrication routes to achieve the superhydrophobicity. Yet, many of these coatings, such as fluorine-based formulations, may pose severe health and environmental risks, limiting their applicability. Herein, we present a new family of superhydrophobic coatings comprised of natural saturated fatty acids, which are not only a part of our daily diet, but can be produced from renewable feedstock, providing a safe and sustainable alternative to the existing state-of-the-art. These crystalline coatings are readily fabricated via single-step deposition routes, namely thermal deposition or spray-coating. The fatty acids self-assemble into highly hierarchical crystalline structures exhibiting a water contact angle of ∼165° and contact angle hysteresis lower than 6°, while their properties and morphology depend on the specific fatty acid used as well as on the deposition technique. Moreover, the fatty acid coatings demonstrate excellent thermal stability. Importantly, this new family of coatings displays excellent anti-biofouling and antimicrobial properties against Escherichia coli and Listeria innocua, used as relevant model Gram-negative and Gram-positive bacteria, respectively. These multifunctional coatings hold immense potential for application in numerous fields, ranging from food safety to biomedicine, offering sustainable and safe solutions.

Keywords: Anti-biofouling; Antimicrobial; Biocidal; CA, contact angle; CAH, contact angle hysteresis; SFA, saturated fatty acid; Saturated fatty acids; Spray coating; Superhydrophobic coatings; Thermal deposition.

Graphical abstract

Fig. 1

HR-SEM images of the deposited coatings before and after heating. Cross-sectional views of the coatings: a1) palmitic acid (16C), b1) stearic acid (18C), c1) arachidic acid (20C), d1) behenic acid (22C), e1) lignoceric acid (24C), f1) cerotic acid (26C). Scale bar is 2 ​μm a2-f2) Planar views of the as deposited coatings, respectively. Scale bar is 4 ​μm. Insets – planar view of the coatings treated thermally for 24 ​h at 50 ​°C, respectively. Scale bar is 4 ​μm.

Fig. 2

Characteristic properties of deposited fatty acid coatings as a function of carbons number in the molecule. Thermally treated coatings are presented by the red color. a) CA of water and ethylene glycol on the as-deposited coatings (black squares and triangles) and on the thermally treated coatings (red squares and triangles). b) Water CAH on the as-deposited coatings (black rhombi) and thermally treated coatings (red rhombi). c) Roughness of as-deposited coatings (black squares) and thermally treated coatings (red squares). d) Coating thickness of as-deposited coatings. Error bars correspond to standard deviation.

Fig. 3

X-ray diffraction patterns of powdered fatty acids (black lines), as-deposited coatings (red lines) and thermally treated coatings (blue lines). a) Palmitic acid (16C), b) Stearic acid (18C), c) Arachidic acid (20C), d) Behenic acid (22C), e) Lignoceric acid (24C), f) Cerotic acid (26C).

Fig. 4

HR-SEM and XRD characterization of stearic acid (18C) spray coatings based on various solvents: a) diethyl ether, b) acetone, c) ethanol. a1-c1) cross-sectional views, respectively. Insets: water CA and roughness values. a2-c2) planar views of spray coatings, respectively. Scale bar is 4 ​μm a3-c3) XRD of spray coatings before and after heating, respectively (blue and red lines, respectively).

Fig. 5

CLSM orthogonal projection images of the (a) E. coli and (b) L. innocua adhered onto surfaces after 48 ​h: 1) Uncoated glass (control), 2) Spray-coated palmitic acid (16C),3) Spray-coated stearic acid (18C), 4) Spray-coated arachidic acid (20C). Scale bar is 80 ​μm. Pie charts: relative bacteria cells density (RCD) per unit area, normalized per unit depth (cells μm⁻³); green and red sections represent live and dead cells, respectively. HR-SEM images of the (c) E. coli and (d) L. innocua adhered onto the surfaces: 1) Uncoated glass (control), 2) Spray-coated palmitic acid (16C), 3) Spray-coated stearic acid (18C), 4) Spray-coated arachidic acid (20C). Scale bar is 4 ​μm. Bacteria cells are false-colored to ease observation.