Antifouling coatings: recent developments in the design of surfaces that prevent fouling by proteins, bacteria, and marine organisms

The major strategies for designing surfaces that prevent fouling due to proteins, bacteria, and marine organisms are reviewed. Biofouling is of great concern in numerous applications ranging from biosensors to biomedical implants and devices, and from food packaging to industrial and marine equipment. The two major approaches to combat surface fouling are based on either preventing biofoulants from attaching or degrading them. One of the key strategies for imparting adhesion resistance involves the functionalization of surfaces with poly(ethylene glycol) (PEG) or oligo(ethylene glycol). Several alternatives to PEG-based coatings have also been designed over the past decade. While protein-resistant coatings may also resist bacterial attachment and subsequent biofilm formation, in order to overcome the fouling-mediated risk of bacterial infection it is highly desirable to design coatings that are bactericidal. Traditional techniques involve the design of coatings that release biocidal agents, including antibiotics, quaternary ammonium salts (QAS), and silver, into the surrounding aqueous environment. However, the emergence of antibiotic- and silver-resistant pathogenic strains has necessitated the development of alternative strategies. Therefore, other techniques based on the use of polycations, enzymes, nanomaterials, and photoactive agents are being investigated. With regard to marine antifouling coatings, restrictions on the use of biocide-releasing coatings have made the generation of nontoxic antifouling surfaces more important. While considerable progress has been made in the design of antifouling coatings, ongoing research in this area should result in the development of even better antifouling materials in the future.     

Resistance of galactoside-terminated alkanethiol self-assembled monolayers to marine fouling organisms

Self-assembled monolayers (SAMs) of galactoside-terminated alkanethiols have protein-resistance properties which can be tuned via the degree of methylation [Langmuir 2005, 21, 2971-2980]. Specifically, a partially methylated compound was more resistant to nonspecific protein adsorption than the hydroxylated or fully methylated counterparts. We investigate whether this also holds true for resistance to the attachment and adhesion of a range of marine species, in order to clarify to what extent resistance to protein adsorption correlates with the more complex adhesion of fouling organisms. The partially methylated galactoside-terminated SAM was further compared to a mixed monolayer of ω-substituted methyl- and hydroxyl-terminated alkanethiols with wetting properties and surface ratio of hydroxyl to methyl groups matching that of the galactoside. The settlement (initial attachment) and adhesion strength of four model marine fouling organisms were investigated, representing both micro- and macrofoulers; two bacteria (Cobetia marina and Marinobacter hydrocarbonoclasticus), barnacle cypris larvae (Balanus amphitrite), and algal zoospores (Ulva linza). The minimum in protein adsorption onto the partially methylated galactoside surface was partly reproduced in the marine fouling assays, providing some support for a relationship between protein resistance and adhesion of marine fouling organisms. The mixed alkanethiol SAM, which was matched in wettability to the partially methylated galactoside SAM, consistently showed higher settlement (initial attachment) of test organisms than the galactoside, implying that both wettability and surface chemistry are insufficient to explain differences in fouling resistance. We suggest that differences in the structure of interfacial water may explain the variation in adhesion to these SAMs.     

Microbicidal activity of TiO2 nanoparticles synthesised by sol–gel method

In this study, the authors investigated antimicrobial activity of TiO₂ nanoparticles (NPs) synthesised by sol–gel method. As synthesised TiO₂ NPs were characterised by X‐ray diffraction, scanning electron microscopy and ultraviolet‐visible absorption spectroscopy. The antimicrobial activity of calcined TiO₂ nanoparticle samples was examined in day light on Gram positive bacteria (Staphylococcus aureus, Streptococcus pneumonia and Bacillus subtilis), Gram negative bacteria (Proteus vulgaris, Pseudomonas aeruginosa and Escherichia coli) and fungal test pathogen Candida albicans. The synthesised TiO₂ NPs were found to be effective in visible light against Streptococcus pneumonia, Staphylococcus aureus, Proteus vulgaris, Pseudomonas aeruginosa and Candida albicans.Inspec keywords: titanium compounds, microorganisms, nanomedicine, biomedical materials, nanofabrication, sol‐gel processing, ultraviolet spectra, visible spectra, X‐ray diffraction, scanning electron microscopy, nanoparticles, antibacterial activityOther keywords: microbicidal activity, titanium dioxide nanoparticle, sol‐gel method, antimicrobial activity, X‐ray diffraction, scanning electron microscopy, ultraviolet‐visible absorption spectroscopy, Gram positive bacteria, Staphylococcus aureus, Streptococcus pneumonia, Bacillus subtilis, TiO₂ , Candida albicans, fungal test pathogen, Escherichia coli, Pseudomonas aeruginosa, Proteus vulgaris, Gram negative bacteria     

Cost‐efficient conversion coatings for corrosion protection prepared by the sol‐gel process

For several years, chromate compounds were used as effective and inexpensive corrosion inhibitors. Studies showed that these materials were toxic and carcinogenic. A subsequent ban on these materials has led to extensive research to develop effective alternative inhibitors. The sol‐gel method appeared as an effective technique forming protective silane layers from hydrolyzed tetraethylorthosilicate (TEOS) and glycidoxypropyltrimethoxy silane (GPTMS) in an aqueous sol–gel process. The final cross‐linking of the layer is realized by heat treatment to form thin, fully dense, protective films of silane on diverse metal surfaces. Most sol‐gel layers use a considerable amount of solvent VOC (v olatile o rganic c ompound) in order to achieve a dispersion of silane with water. However, in a wide range of industries, the high amount of VOC is the major hindrance for the application of silane layers. This article shows a coating concept without VOC, which is cost‐efficient, effective for corrosion protection and applicable for metal surfaces. The corrosion protection effect is given by means of comprehensive impedance spectroscopy measurements.     

Development and characterization of sol‐gel composite coatings on aluminum alloys for corrosion protection

For several years, Cr^IV compounds were used as effective and inexpensive corrosion inhibitors. Studies showed that these materials were toxic and carcinogenic. This has led to extensive research to develop alternative inhibitors. Organo‐silicate hybrid coatings appeared as an effective technique for forming protective layers on different metal alloys. A silane film was obtained by dip coating of the sample in sol solution prepared from the hydrolysis of 3‐glycidoxypropyltrimethoxysilane (GPTM), tetraethylorthosilicate (TEOS) with acid as catalyst and water as solvent instead of VOC (volatile organic compound). The sol solution was aged at ambient temperature to to enhance the effectiveness of the solution for the hydrolysis process. The sample was cured at 150 °C to ensure cross‐linking of the film. The experiments have shown that heat treatment leads to increased density and corrosion resistance of the films. Organic and inorganic inhibitors were added in different concentrations to improve the protection and self‐healing properties of the coating even after long‐time immersion in corrosive solution. The protection performance of the film was evaluated by electrochemical impedance spectroscopy (EIS) in 0.5 M NaCl solution (pH value 7). The surface morphologies of the treated samples were investigated using SEM.     

Dielectric and piezoelectric properties of sol–gel derived Ca doped PbTiO3

Synthesis of Ca doped PbTiO₃ powder by a chemically derived sol–gel process is described. Crystallization characteristics of different compositions Pb_1−xCa_xTiO₃ (PCT) with varying calcium (Ca) content in the range x = 0–0.45 has been investigated by DTA/TGA, X-ray diffraction and scanning electron microscopy. The crystallization temperature is found to decrease with increasing calcium content. X-ray diffraction reveals a tetragonal structure for PCT compositions with x ≤ 0.35, and a cubic structure for x = 0.45. Dielectric properties on sintered ceramics prepared with fine sol–gel derived powders have been measured. The dielectric constant is found to increase with increasing Ca content, and the dielectric loss decreases continuously. Sol–gel derived Pb_1−xCa_xTiO₃ ceramics with x = 0.45 after poling exhibit infinite electromechanical anisotropy (k_t/k_p) with a high d₃₃ = 80 pC/N, ′ = 298 and low dielectric loss (tan δ = 0.0041).     

Photoluminescence properties of an oligo-phenylkene vinylene derivative doped in polymer/silica hybrid sol–gel matrix

An oligo-phenykene vinylene derivative 4,4′-(1,4-phenylene dithenylene)-bis-(N-methyl pyridinium iodide) (OPVD) was introduced into poly(hydroxyethyl methacrylate) (PHEMA)-impregnated silica composite film by the sol–gel process. The X-ray diffraction and UV–VIS reveal that this method can restrict the crystallization of OPVD and OPVD molecules tend to form J-aggregates in the composite film. The results of photoluminescence of OPVD in the composite film indicate that as compared with OPVD itself, the emission peak of OPVD in the composite film undergoes a blue shift about 80 nm, fluorescence intensity enhances and the photostability is much better, which make such composite film of more practical significance.     

Indium-doped ZnO thin films deposited by the sol–gel technique

Conducting and transparent indium-doped ZnO thin films were deposited on sodocalcic glass substrates by the sol–gel technique. Zinc acetate and indium chloride were used as precursor materials. The electrical resistivity, structure, morphology and optical transmittance of the films were analyzed as a function of the film thickness and the post-deposition annealing treatments in vacuum, oxygen or argon. The obtained films exhibited a (002) preferential growth in all the cases. Surface morphology studies showed that an increase in the films' thickness causes an increase in the grain size. Films with 0.18 μm thickness, prepared under optimal deposition conditions followed by an annealing treatment in vacuum showed electrical resistivity of 1.3 × 10^− 2 Ωcm and optical transmittance higher than 85%. These results make ZnO:In thin films an attractive material for transparent electrodes in thin film solar cells.     

Preparation and optical properties of sol–gel derived organic–inorganic hybrid waveguide films doped with disperse red 1 azoaromatic chromophores

We report here on TiO₂/organically modified silane (ormosil) organic–inorganic hybrid waveguide films doped with disperse red 1 (DR1) azoaromatic chromophores and derived by a low-temperature sol–gel process for photonic applications. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane and methyltrimethoxysilane mixed with tetrabutyl titanate are used as matrix precursor for the hybrid films. Third-order nonlinear and photo-responsive properties of the hybrid films are studied by using a z-scan technique and a UV–vis absorption spectroscopy. Results indicate that the hybrid films have a large third-order nonlinear susceptibility and an obvious trans-to-cis photoisomerization under UV light irradiation. The planar waveguide and structural properties of the hybrid films are also characterized by a prism coupling technique, thermal gravimetric analysis, and Fourier-transform infrared spectroscopy. These results indicate that the as-prepared hybrid films are promising candidates for integrated optics and photonic applications, which allow directly integrating on the same chip waveguide devices with the functionalized devices.     

Inhomogeneous distribution of organic molecules adsorbed in sol–gel glasses

The effects of the porous matrix upon the radiative characteristics of quinine sulphate doped sol–gel glasses are investigated. The broadenings of the absorption and fluorescence spectra are explained by the attachment of the molecules on distorted sites or in a non-planar fashion, creating an inhomogeneous distribution of adsorbed molecules. For this reason, each emitting center relaxes with its own characteristics. This inhomogeneous distribution is also supported by the non-exponential and the wavelength dependence of the fluorescence decay.     

Structural and optical properties of vanadium pentoxide sol–gel films

The V₂O₅ films were obtained using sol–gel procedure. The composition and mesostructure of the layers were investigated with the UV and Raman spectroscopy, as well as with electron microscopy. We showed that the changes in the properties of thin layers accompanying the variation of film thickness are connected with the changes in the microstructure of the film rather than with changes in its composition. The thin V₂O₅ layers obtained in the present study are composed of disordered clusters; their mean size is 4–13 nm.     

Behavior of thymol blue analytical pH-indicator entrapped into sol–gel matrix

The preparation of transparent monolithic silica doping with thymol blue has been carried out by the acid catalyzed sol–gel reaction of tetraethylorthosilicate in the presence of thymol blue. The immobilized thymol blue pH-indicator shows similar behavior as its solution counterpart. The UV/VIS spectra indicate that the thymol blue retains its structure during the sol–gel reactions in terms of response to pH. Thymol blue can be regarded as uniformly distributed in the sol–gel matrix, and the use of SDS surfactant has positive effects on the immobilized thymol blue monolithic disk by homogenizing the polymerizing system. This research shows that thymol blue indicator can be used as solid pH sensor.     

Preparation and characterization of ultrafine alumina via sol–gel polymeric route

Ultrafine alumina powder was prepared through resin formation between urea and formaldehyde. Aluminium stearate soap was introduced during resin preparation. Ethylene glycol was used to terminate the thermosetting reaction. Calcination of the product was carried out at 700, 1000, 1100, 1300 and 1400 °C to obtain aluminium oxide.IR and Raman spectroscopic analysis indicated the occupation of Al³⁺ at different sites in the polymer network (CO, NH₂, CO, NH, and CH₂OH).X-ray diffraction of powder calcined at 1000 °C revealed the presence of a mixture of α- and θ-alumina together, while a mixture of α- and β-alumina phases were obtained on calcination at 1400 °C. Transmission electron microscope (TEM) examination of the powder fired at 700 °C showed uniform grains in the form of clusters with average size between 22.02 and 30.5 nm. Clusters are multi-particles as evident from the electron diffraction pattern. Crystallite size of alumina powder calcined at 1000 °C was found to be ≈25.67 nm, while that of powder calcined at1400 °C was ≈30.52 nm. The calculated specific surface area of alumina powder calcined at 1000 °C was 59.17 m² g⁻¹, while that calcined at 1400 °C was 49.77 m² g⁻¹.     

Development of photo-patternable organo-mineral hybrid films from the sol–gel condensation of alkoxysilanes

We describe the synthesis and refractive index characterisation of four organo-siloxanes materials, applicable to the fabrication of photo-patternable channel waveguides. By combining aliphatic, aromatic and vinylic alkoxysilane precursors, organically modified silicates materials have been synthesised using the sol–gel process. Control of the refractive index was achieved through the variation of aliphatic and aromatic alkoxide content within the materials. It is clearly demonstrated that the presence of aromatic groups significantly increases the refractive index to 1.562. Conversely, the addition of vinylic groups decreases the value to 1.490. These findings are attributed to the atypical polarisability exhibited by aromatic and aliphatic precursors, which strongly affect the overall molar refraction.     

Synthesis and microstructure of boron-doped alumina membranes prepared by the sol–gel method

Pure and boron-doped γ-Al₂O₃ membranes have been synthesized by the sol–gel method. The thermal stability of the unsupported alumina membrane was studied by determining the pore structure (including average pore size, pore volume and BET surface area). The average pore size of the pure alumina membrane increased sharply after sintering at temperatures higher than 1000°C. Addition of 16% boron can considerably stabilize the pore structure of the unsupported alumina membrane. The pore diameter for the B-doped membrane was stabilized within 13 nm after sintering at 1200°C for 5 h. The substantial increase in the pore size for the pure alumina membrane at the sintering temperature of 1000–1200°C was accompanied by the phase transformation from γ-Al₂O₃ to -Al₂O₃. The addition of boron can raise the temperature of this phase transformation significantly and, thus, improves the thermal stability of the membranes.     

Investigation on sol–gel boehmite-AlOOH films on Kapton and their erosion resistance to atomic oxygen

To improve the atomic oxygen (AO) erosion resistance of Kapton, boehmite-AlOOH films were deposited on it by sol–gel method and AO exposure experiments were performed in a ground-based AO simulator. The results indicate that the AlOOH-coated samples show an improved AO resistance and their erosion yield is one order of magnitude less than that of pristine Kapton. Furthermore, the AlOOH-coated Kapton remained optically stable under AO exposure. The AlOOH film structure before and after AO exposure was analyzed by scanning electronic microscope, atomic force microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. It was found that after AO exposure, the film structure tends to transfer from an octahedral coordination for AlOOH to an octahedral and tetrahedral mixed-coordination for γ-Al₂O₃. This implies that a more stable γ-Al₂O₃ structure could be formed in AlOOH film during AO exposure. The AO erosion mechanism of the coated Kapton was discussed.     

Structural investigations of sol–gel derived silicate gels using Eu ion-probe luminescence

Undoped and Eu³⁺-doped CaF₂–SiO₂ gels were prepared by the sol–gel method and their optical properties have been studied. The UV–VIS–NIR absorption and photoluminescence spectra have shown the bands typical for the Eu³⁺ ions transitions. When the Eu-doped gel is annealed at temperatures up to 800 °C (i.e. above the CaF₂ crystallisation peak at 460 °C) the photoluminescence spectra intensity increase, the 590 nm (⁵D₀→⁷F₁) and 620 nm (⁵D₀→⁷F₂) luminescence bands become comparable and a structuring of the 620 nm band is observed. The phonon sidebands peaks associated with the ⁵F₀→⁷D₂ transition of the Eu³⁺ ion were observed at around 1000 and 620 cm⁻¹ and have been assigned to the Si–O and Ca–O bonds, respectively. A phonon sideband signal in the range of 300–400 cm⁻¹ was attributed to Ca–F bonds in the precipitated CaF₂ phase. From the optical absorption, photoluminescence and phonon sidebands spectra we have concluded that in the gels annealed at 800 °C, the Eu³⁺ ions are incorporated into the silica network and in the precipitated CaF₂ phase.     

AFM characterisation of sol‐gel TiO2 films deposited on stainless steel

One‐layer, two‐layer and three‐layer titania films on AISI 304 stainless steel were deposited by sol‐gel process and dip‐coating method. Two sols were prepared by using titanium isopropoxide as a precursor, propanol as a solvent, nitric acid as a catalyst and acetylacetone for peptization. Both of the prepared sols contained the same amount of mentioned components, the only difference was in the addition of polyethylene glycol (PEG) as templating reagent to one of the sols. After calcination at 550°C, deposited films on the stainless steel substrate were characterized by atomic force microscope (AFM). Influence of layer number as well as addition of polyethylene glycol on morphology of titania films was analysed and discussed. Generally, by increasing the number of layers and by addition of polyethylene glycol, roughness parameters increase by changing surface topography. The surface topography analysis is very important when choosing the adequate industrial application of prepared layers.