The last (double) issue of 2023 offers eleven original research articles on: pulsed plasma deposition and polymerization of carvone and its antimicrobial properties;1 photocatalytic and antibacterial chlorine-doped indium sulfide thin films;2 mesoporous membrane permeable to ethylenediamine;3 magnetite nanoparticles precipitated from ionic solutions without stabilizers;4 sputtering of tungsten trioxide thin films and examination their optical properties;5 stability of Leidenfrost droplets;6 durable fluoride-free antifouling coating;7 liquid flow drag reduction in bioinspired textured channels;8 self-healing epoxy resin coating;9 yttrium oxide and graphene-based composite coating for magnets;10 and self-cleaning superhydrophobic coating for concrete buildings.11
Antimicrobial coatings protect against a wide spectrum of bacteria, germs and mold, and these coatings gained a broad popularity among researchers and industry in recent years,12–14 including coatings for medical devices and implants that prevent infections.15,16 In a new contribution, Masood et al.1 demonstrate the use of plasma polymerization technique in deposition and polymerization of carvone that result in antimicrobial coating. Carvone is a member of the terpenoid family of chemicals, found naturally in many essential oils (extracted from seeds of caraway, spearmint, dill and other plants) that exhibits antimicrobial, anticancer, anti-inflammatory, antidiabetic and neurological properties, and many other pharmacological effects. By using low-pressure pulsed-wave plasma polymerization with optimized deposition parameters, the authors show that the molecular structure of the deposited carvone can be less fragmented and retain moieties associated with C–O and C=O bonds, producing smooth coatings. The antimicrobial and biofilm formation testing with Escherichia coli and Staphylococcus aureus bacteria revealed a strong correlation between bactericidal effect and the presence of C–O and C=O bonds.
The second paper of this issue also relates to antimicrobial coatings used against bacterial infections that could benefit healthcare facilities and medical devices. The international team of researchers from Tunisia, Portugal and Romania reports on the surface chemistry, morphology, structure, photocatalytic and antibacterial potential of pure and chlorine-doped indium sulfide thin films produced by spray pyrolysis.2 The indium sulfide films demonstrate a photocatalytic efficiency higher than 80% and strong antibacterial functioning against the multidrug-resistant bacteria Pseudomonas aeruginosa. The obtained results also show an increase of antibacterial activity with increasing chlorine content of the doped film. This innovation should motivate other laboratories to explore the use of chlorine-doped indium sulfide coatings in biomedical fields.
The development of highly selective membranes that are permeable to a gaseous analyte and impermeable to the gas matrix is still a challenge.17 In a new contribution from Xinjiang University in China,3 the authors describe fabrication and testing of a titanium metal–organic framework membrane used for gas sensors. The membrane of a mesoporous structure, fabricated on a titanium dioxide film composite optical waveguide substrate, exhibits positive responses to ethylenediamine, nitrogen dioxide, methylamine and trimethylamine when exposed to several types of benzenes, amines and acidic gases. Further modification of the framework membrane with trimethylindolinonaphthospirooxazine significantly improves the selectivity of the membrane towards ethylenediamine, demonstrating potential applications in monitoring this analyte during the manufacture and use of industrial products such as epoxy resins, coolant oils, fungicide and many others. This is a significant contribution to the engineering field of membrane-based gas sensors.
Magnetite nanoparticles are attractive as adsorbents in water and wastewater cleaning technologies, drug-delivery carriers in biomedicine and many other applications due to their magnetic properties, non-toxicity, large specific surface area, stability, biocompatibility and simplicity in synthesis, and ease of guidance and recovery.18 In a new study, Xia et al.4 investigated the effects of the ammonium hydroxide/ferric ion and ferrous/ferric ion ratios, concentrations of both hydrochloric acid and iron salt, reaction temperature and time on precipitation synthesis without stabilizers, along with characterization of the physical and chemical properties of synthesized magnetite nanoparticles. It was found that the crystalline size of nanoparticles varied from about 12 to 20 nm and depended on pH, reaction temperature, aging time and the total concentration of iron. Without a stabilizer, the nanoparticles aggregated to a hydrodynamic diameter of 0.3–1.2 µm that was controlled by colloidal forces. This study demonstrates the application of a coprecipitation method in reproducible synthesis of magnetite nanoparticles.
Tungsten trioxide is an attractive semiconducting material with electrochromic and gasochromic characteristics, and has been the focus of research by the team from Changzhou University in China.18 In their second report published in Surface Innovations, the effects of sputtering power, working pressure, substrate bias and substrate temperature on the morphological, structural and optical properties, and subsequently electrochromic performance of tungsten trioxide, are presented and discussed.5 The study demonstrates a prospect of manipulation of the amorphous and crystalline structure of tungsten trioxide films of varying porosity through control of operational conditions of radio-frequency magnetron sputtering. This report should benefit researchers who explore simplified, reproducible and cost-effective methods of fabrication of tungsten trioxide thin films.
The Leidenfrost effect refers to a liquid droplet that is levitated on its own vapor above a hot surface.19 The research team from the University of Texas at Arlington (TX, USA) investigated, both theoretically and experimentally, the stability of the Leidenfrost droplets in circular configuration.6 The study reveals for the first time that the Leidenfrost droplet instability can be suppressed by a thin rod inserted into the liquid droplet. The authors confirm this stability experimentally in circular aluminum and glass vessels using water and isopropyl alcohol droplets. This report is an excellent example of a strong scientific contribution that combines both theory with experimentation. Although practical applications of the Leidenfrost effect remain sketchy at this moment, this research has potential to inspire inventors in designing micro-systems for a controlled and accelerated liquid droplet transports of reduced drag and miniature chemical reactors, to name a couple of possibilities.
Historically, antifouling coatings have been commonly applied to the hulls of boats and ships and many other aquatic navigational structures and structural elements to reduce attachment and buildup of subaquatic organisms, although modern applications also include coatings on medical materials and devices as well.20 In the seventh original report of this issue, Zhang et al. 7 describe their invention of a transparent hydrophobic fluoride-free coating with robust mechanical properties. The coating, made of a cross-linked and branched polymeric matrix with hard zirconium dioxide ceramic nanoparticles, endows remarkable protection of the substrate against exposure to harsh chemical conditions.
Natural materials and living organisms continue to inspire scientists and researchers with the development of new synthetic materials, structures and devices.21,22 Inspired by the texture of leaves of indocalamus and rice, Xu et al.8 fabricated closed channels on the surface of a silicon wafer. The channels were decorated either with micropillars made of polydimethylsiloxane or covered with a polydimethylsiloxane layer having micropits. Then, the authors studied a laminar flow of water and paraffin oil in these channels and show up to 5 and 27% pressure drop reductions for paraffin oil and water, respectively. The experimental work is supported by modeling and analysis of skin–friction drag. This interesting study opens new engineering opportunities for designing low-flow drag coatings and surfaces that could attract the attention of laboratories working on microfluidic devices, pipelines and aquatic navigation vessels.
Self-healing materials, including coatings, are an emerging class of smart materials designed for self-induced repairs in response to local damage. These materials can also restore original structural characteristics and functionalities and prevent catastrophic failure. In an interesting new contribution on this topic, Wen et al.9 describes novel polythiourethane/epoxy anticorrosive coatings capable of self-healing because of incorporation of dynamic disulfide bonds into epoxy. Results from tensile strength and electrochemical measurements show that the polythiourethane/epoxy anticorrosion coating could heal scratches completely in a couple of hours at elevated temperature. This invention might motivate other laboratories to design novel epoxy resin coatings for corrosion protection with extended service lifetime and reduced maintenance due to self-healing performance.
The next paper also describes a novel coating for corrosion protection. Zou et al.10 invented nickel–yttrium oxide/nickel–graphene composite coatings with specific application for NdFeB magnets. The authors investigated the effect of graphene and yttrium oxide addition to nickel coatings, in terms of structure, chemistry and corrosion properties. Experimental results indicate a good passivation barrier of the formulated coating, with excellent stability and corrosion resistance. The NdFeB magnets are commonly used in rail transit systems and need to be protected from corrosive humidity and reactive constituents of surrounding environment. This invention has potential to improve the service lifetime of magnets and widen their application.
In the final contribution to this issue, the researchers from the Dalian University of Technology in China reveal their invented formula for painting concrete buildings and stones for self-cleaning from dust by rain droplets.11 The authors formulated their coating by blending ester/methacrylate-based emulsion with natural sand and fluoroalkylsilane. The coating applied to the exterior walls of concrete buildings exhibits superhydrophobic and self-cleaning properties with anticorrosion characteristics and outdoor durability. By adding iron oxide dyes, the researchers also demonstrate that the coating can be colorful, adding aesthetic value.
As indicated in the previous Editorial, this journal offers many original and review papers on superhydrophobic materials and coatings. For example, a couple of quality reviews published in 2022 by Wang et al.23 and Chambers et al.24 are strongly recommended for additional reading.
We would appreciate any feedback and valuable suggestions from contributing authors and readers on any developments to Surface Innovations that could make it more appealing to the readers in the years to come.
