Silver-based coatings or silver deposited surfaces are well-known to provide bacteriostatic/bactericidal, fungistatic/fungicidal and algistatic/algicidal properties to surfaces (Devasconcellos et al., 2012). Silver is known to have inhibitory effects against different kinds of microorganisms (Goswami et al., 2015; Kumari et al., 2016; Naskar et al., 2018). Therefore, it finds applications in medical and consumer products, and in environmental applications (Kumar et al., 2014; Rasheed et al., 2017). Nano-sized silver, i.e., silver nanoparticles, have found increasing application as an antimicrobial agent due to high surface to volume ratio and increased surface reactivity (Kumari et al., 2016). Several studies about the antimicrobial activity of silver nanoparticles and their action mechanism have been reported (Kumar et al., 2014; Durán et al., 2016). Silver formulations containing silver ions or silver nanoparticles are generally considered biologically benign and safer than any other heavy metal-based formulations (Rasheed et al., 2017). The bacteriostatic/bactericidal properties of nanosilver can also be useful in antifouling applications against microfouling (Devasconcellos et al., 2012; Li et al., 2013). Only a few studies investigated the effect of nanosilver on eukaryotes or macrofouling organisms. For example, it has been shown that a silver nanoparticle coating prevents the adhesion of marine and freshwater algae (Ren et al., 2014). In another study, nanosilver prevented mussel settlement through modification of the structure of biofilms (Yang et al., 2016b).

The mechanism of the antifouling properties of silver nanoparticles is not yet clear and debatable. One of the most accepted mechanisms is based on the ability of Ag⁺ ions to anchor to cell walls. It is shown that Ag⁺ ions penetrate into the bacterial cell and interacts with thiol groups of most of the vital enzymes, leading to deactivation of the enzyme, which ultimately stops bacterial growth leading to the death of the bacterial cell (Kim et al., 2007; Kumar et al., 2014; Pareek et al., 2018). The study with eukaryotes suggested that contact killing of the cells by Ag⁺ ions is the main antifouling mechanism (Ren et al., 2014). The formation of free radicals by AgNPs is another proposed mechanism. Electron spin resonance (ESR) spectroscopy studies suggested that free radicals generated from AgNPs damage cell membranes when they come in contact with bacteria, resulting in membrane rupture and ultimately cell death (Danilczuk et al., 2006; Kailasa et al., 2019). However, antimicrobial activity of AgNPs depends on several factors including physico-chemical parameters such as size (Ginjupalli et al., 2018; Kumar et al., 2019b), morphology (El-Zahry et al., 2015), crystallinity (Kumar and Münstedt, 2005), surface coatings of nanoparticles (Brobbey et al., 2019), and microbial species in the environment (Pareek et al., 2018).

Graphene is an allotrope of a single layer of carbon atoms arranged in a sp²-bonded hexagonal lattice (Figure 4). They are naturally found as the building blocks of graphite. Graphene materials show high specific surface area, electron conductivity and thermal stability that make it attractive for several environmental applications like photocatalysis, energy production and storage (Sun et al., 2015; Ozer et al., 2017; Xu et al., 2018). Carbon nanotubes (CNTs) represent a hollow, concentric cylindrical structure, typically with a diameter of a few nanometers and a length varying from a few nanometers to several microns (100 μm) up to a few millimeters (4 mm) (Venkataraman et al., 2019). Nanotubes are primarily classified into three categories; single-walled carbon nanotubes (SWCNTs), double-wall carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs). The environmental application of CNT’s mainly involves the fabrication of nanocomposite materials with biocidal properties and fouling release activities (Upadhyayula and Gadhamshetty, 2010; Dasgupta et al., 2017).

CNTs and graphene exhibit good antimicrobial activity towards both Gram-positive and Gram-negative bacteria as well as bacterial spores (Lukowiak et al., 2016; Al-Jumaili et al., 2017; Sun et al., 2020). CNTs, especially SWCNTs were shown to have significantly greater antibacterial activity than MWNTs, probably due to their smaller sizes, enabling membrane perturbation (Figure 5) (Kang et al., 2008). Additionally, CNTs can impact the recruitment of macro-fouling organisms (Beigbeder et al., 2008). Nano-sized carbon black more effectively prevented settlement of the barnacle Amphibalanus amphitrite larvae compare to single-layer graphene oxide (Mesarič et al., 2013). Just 0.5% weight percent of CNT affected the composition of biofilms by increasing the abundance of Proteobacteria and decreasing the abundance of Bacteroidetes, which in turn decreased the settlement of Mytilus coruscus (Yang et al., 2016a).

One of the first CNT-based antifouling coatings was developed in 2008 (Beigbeder et al., 2008). It was based on the incorporation of synthetic multi-wall CNTs in silicone coatings. Nowadays, CNT-based coatings are actively used as antifouling and anti-corrosion marine coatings in the construction and oil and gas industries (Dustebek et al., 2016). CNTs are generally loaded into a polymeric matrix for application in protective coatings. Epoxy resins have been mostly used in reinforcement with different types of carbon nanotubes (single-wall, double-wall and multi-wall) (Jin F.-L. et al., 2015; Deng et al., 2015; Shen et al., 2016). CNTs act as an efficient adsorbent due to their large surface area, π-bond electrons on the surface, providing more chemically active sites on the nanotubes, and hollow and layered structures. CNTs have high tensile strength and Young’s modulus, and in comparison to steel, CNTs have hundreds and tens of times the higher tensile strength and Young’s modulus (Walters et al., 1999). The reinforcement of the paint matrix with CNTs improves mechanical properties in the coatings/paints. Dustebek et al. (Dustebek et al., 2016) investigated the effect of CNTs on the mechanical strength of a self-polishing antifouling resin-based paint and found that the increase in the amount of MWCNT leads to an improvement in the mechanical strength of self-polishing antifouling paints. Antifouling paints with 0.5% (w/w) and 0.7% (w/w) of MWCNTs showed a significant improvement in impact resistance (Dustebek et al., 2016). This study was further supported by another research that evaluated the effect of reinforcement of MWCNT and graphene oxide (GO) on mechanical properties of poly-dimethyl siloxane (PDMS) marine coatings (Cavas et al., 2017). Incorporation of MWCNTs in composites increased both tensile strength and percent of elongation of PDMS marine coatings. These studies indicate that CNT and GO beside of antifouling properties can improve the mechanical properties of nanocomposite coatings.

Numerous mechanisms of antifouling action of CNTs and GO have been proposed, but the actual mechanism is still not clear. A study suggested that different antifouling mechanisms can be involved (Mesarič et al., 2013). First, CNTs and GO are toxic to microbes and larvae of macrofouling organisms. The toxic effect of CNTs is widely known (Francis and Devasena, 2018). CNTs can increase cell apoptosis, oxidative stress, and inflammation. Second, CNTs and GO can inhibit larval settlement by a non-toxic way simply interfering with the attachment and adhesion of cyprid larvae (Mesarič et al., 2013).

The physical size (in particular the length), diameter, surface area, concentration, treatment time, etc., play a role in the antifouling activity of CNTs. Yang et al. (2010) investigated the effect of length of SWCNTs’ on antimicrobial activity to Salmonella cells using three different lengths of SWCNTs (<1 μm, 1–5 μm, and ca.5 μm) (Yang et al., 2010). Longer SWCNTs were reported to exhibit stronger antimicrobial activity by aggregating bacterial cells more effectively, whereas shorter SWCNTs were reported to aggregate between themselves without involving many bacterial cells (Figure 6). The tube diameter also plays an important task in the inactivation of microorganisms. Smaller diameters CNTs (<10 nm) can damage cell membranes by interaction with the cell-surface, while large diameter CNTs (15–30 nm) mostly interact by their sidewalls with the bacteria (Upadhyayula and Gadhamshetty, 2010).

Metal oxide nanoparticles (NPs), such as ZnO, TiO₂, SnO₂, are highly photoactive and used for antimicrobial, self-cleaning, self-healing, anti-corrosion and anti-biofouling applications. Table 1 summarizes the antifouling effects of various photocatalytic metal oxide nanostructures on aquatic prokaryotes (bacteria) and eukaryotes (microalgae, barnacles, bryozoans, etc.). In general, investigators studied the antifouling effect of metal oxide nanoparticles, nanowires, nanorods, and hybrid nanostructures. ZnO, TiO₂ and hybrid metal-metal oxide, as well as hybrid carbon-metal oxides nanostructures were the most commonly used systems for the inhibition of biofouling (Table 1). ZnO nanoparticles have attracted more attention compared to other metal oxides due to their low cost and ready availability (Naskar et al., 2018; Wallenhorst et al., 2018). As per the FDA (Food and Drug Administration, United States) guidelines, ZnO is listed as a safe material for various uses. ZnO NPs are used in the food processing and packaging, as well as in the agriculture sector because of their biocompatibility, low toxicity and antimicrobial properties (Garcia et al., 2018). Photocatalysis of various toxic organic dyes and inorganic pollutants in industrial wastewater has been performed using ZnO and TiO₂ semiconductor oxides under light irradiation. TiO₂ is widely used in self-cleaning coatings and paints but their application is limited to outdoors, as they require activation by UV light (Xu et al., 2017; Adachi et al., 2018). The most common bacteria used for testing include Gram-positive (B. subtilis, Micrococcus) and Gram-negative (E. coli, P. aeruginosa) pathogens (Table 1). In opposite, the most common eukaryotes used in metal oxide nanostructures studies are environmental species of microalgae and invertebrate larvae.

Silver nanoparticles is a mostly used antimicrobial agent that synthesized with environment-friendly methods and can be non-toxic for mammals. Titanium oxide nanotubes have been used to improve the functional properties of silver nanoparticles because of their superior specific surface area both on inner and outer surfaces of the tubular structure. Yee et al., 2017 proposed an innovative 2-steps hydrothermal synthesis of a silver–titania nanotube (Ag/TNT) composite that showed nanotubular TiO₂ structures with Ag nanoparticles uniformly dispersed throughout the nanomaterial (Yee et al., 2017). The inhibitory properties of the composite material against biofilm were studied and the result demonstrated the Ag/TNT reduced the biofilm formation of marine bacterium Halomonas pacifica by 98% compared to bare titania nanotubes. Additionally, Ag/TNT also showed growth inhibition of marine microalgae Dunaliella tertiolecta and Isochrysis sp (Yee et al., 2017). A ternary nanocomposite (PDMS/GO-Al₂O₃ NRs) superhydrophobic coatings were also developed from PDMS, and graphene oxide anchored with alumina nanorods (GO-Al₂O₃ NRs) hybrid sheet via solution-casting method (Selim et al., 2018; He et al., 2021). The results showed that the well-dispersed GO-γ-Al₂O₃ NRs hybrid sheets increased the contact angle (151°), decreased the surface free energy (13.25), and improve micro-nano roughness on the surface, and thus the developed nanocomposites may have promising antifouling applications in the shipping industry (Selim et al., 2018). In a recent study, silicone/graphene-based two novel superhydrophobic nanocomposite were developed, and the results shows PDMS/GO-γ-AlOOH nanorod composite had better antibacterial activity than PDMS/RGO nanocomposite against different bacterial strains might be due high surface area and stabilizing effects of the GO-γ-AlOOH hybrid nanofillers. The GO-γ-AlOOH (3 wt%) nanostructured coating had profound superhydrophobic antifouling properties due to their homogeneity, and high WCA of 151° and a rough surface (Selim et al., 2022).

Aluminum oxides are inexpensive, non-toxic, and one of the most stable inorganic materials. Boehmite is an aluminum oxide hydroxide (γ-AlOOH) particle that contain hydroxyl groups on its surface. Boehmite nanoparticle has the orthorhombic structure, in which the boehmite nanoparticles’ surface are covered with OH groups. Boehmite nanoparticles can improve hydrophilicity and surface properties of the membrane due to presence of extra hydroxyl groups on their surface, and thus reduce fouling of the resulted membranes (Vatanpour et al., 2012; Farjami et al., 2021). Among the alumina compounds, boehmite has highest hydrated surface and hydrophilicity. The effects of two different solvents, dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP) were evaluated on performance of the pristine and EPVC/nano-boehmite nanocomposite membranes, and the results showed that the membranes prepared in NMP solvent using boehmite nanoparticles had higher anti-fouling and fouling resistance properties (Farjami et al., 2021).

Antifouling properties and toxicity of metal oxide nanomaterials depend on the biological species, with some species being more sensitive than others (Dobretsov et al., 2020). Additionally, physico-chemical properties of nanomaterials such as particle size, shape, surface charges, and surface groups affect their toxicity. The smaller size of nanomaterials can easily enter through cell membranes and other barriers of the living organisms. The amount of uptake of nanomaterials decreases with the increased particle size (Hoshyar et al., 2016). The basic principle of metal oxide based photocatalytic disinfection however involves the generation of highly reactive intermediate species, and these species lead to the photocatalytic destruction of adsorbed entities (organics, micro-organisms, and macro-organisms).

Different surface charges and groups on nanostructures exhibit different surface oxidation reactivity resulting in the generation of ROS at varying levels. The use of TiO₂ or ZnO is limited to UV light irradiation due to their wide bandgap and the high recombination rate of photo-generated charges. Thus, efforts have been focused on achieving high photocatalytic efficiency with these materials especially under visible light. In order to render ZnO, TiO₂, SnO₂ and other semiconductor materials suitable for visible light photocatalysis, several techniques have been developed, such as surface modification of semiconductor materials, creation of oxygen vacancies, doping of metal/non-metal atoms, depositing noble metals, and coupling with other semiconductors or carbon materials (Rehman et al., 2009; Youssef et al., 2018). Doping with metal or nonmetal makes TiO₂ and ZnO to absorb visible light. Doping metal or non-metal atoms can generate impurity levels in the bandgap of semiconductors altering the photoelectric properties to cover its spectral absorbance in the visible region (Bora et al., 2017; Raizada et al., 2019).

The antifouling mechanisms of nanostructured metal oxides are based on the release of metal ions, such as Ag⁺ and Zn²⁺ ions from respective nanomaterials as well as the formation of ROS (Figure 7). Exposure of metal oxides with UV/visible light irradiation leads to the transfer of electrons from the valence band to the conduction band generating electron-hole pairs. The photo-generated exciton if separated can efficiently reduce and/or oxidize a compound adsorbed on their surface by generating •OH radicals, and/or by the generation of O₂ ⁻ anions (Figure 7). Both the radicals and anions can react with pollutants transform them to lesser harmful byproducts or degrade them (Allahverdiyev et al., 2011; Carré et al., 2014; Khan et al., 2015). Similarly, ROS and metal anions prevent fouling by micro- and macro-organisms (Al-Fori et al., 2014). Production of ROS at low dosage levels resulting in harsh oxidative stress that causes the damage of genetic material such as DNA and RNA, oxidation of lipid, alteration of protein, inhibition of enzymes, etc. (Figure 7), while prokaryotic and eukaryotic cell death occurs at higher concentrations of ROS (Pan et al., 2010; Allahverdiyev et al., 2011; Carré et al., 2014). The generated ROS are short-lived and only affect organisms that come in direct contact with them (Bora et al., 2017).

Metal oxide nanorods and nanowires based photocatalytic coatings have attracted recent attention owing to its ease of controlled fabrication processes (Wei et al., 2005; Inderan et al., 2015; Jin et al., 2015a; Jin et al., 2015b; Wang et al., 2018). ZnO nanorod-based photocatalytic coatings have been found effective against micro- and macro-fouling on a glass substrate and fishing net substrates in the laboratory (Al-Fori et al., 2014; Spirescu et al., 2021) and field experiments, respectively (Sathe et al., 2017) (Figure 8). ZnO nanorods have increased stability, reduced loss of nanoparticles compared to ZnO nanoparticles and can be fabricated into various types of support materials (Al-Fori et al., 2014). Additionally, ZnO nanorods have lower toxicity compared to ZnO nanoparticles, which was confirmed by experiments with bacteria, microalgae, larvae of invertebrates and adult mussels (Falfushynska et al., 2019; Dobretsov et al., 2020).

Apart from purely polymer and organic molecule mixed coatings, organic/inorganic hybrid composites prepared by blending nano-sized inorganic fillers in polymer matrix have been investigated on the fouling-release and fouling-degrading performance. And the nanomaterial-based antifouling coatings have been proved as one of the most promising coatings for biofouling prevention, amongst others (Tobaldi et al., 2017). While metal oxide nanostructures are capable of preventing of micro- and macro-fouling, their effectiveness is low due to their solubility. Additionally, nanoparticles can be lost and cause harmful effects on the environment. In order to enhance stability, reduce environmental impact, and enhance the antifouling effect, metal oxide nanostructures are usually attached to support substrata or incorporated into a stable matrix of coating. For example, ZnO nanorods attached to glass substrata was effective against the marine bacterium Acinetobacter sp., the marine microalga Tetraselmis sp. and the bryozoan Bugula neritina under the artificial sunlight (Al-Fori et al., 2014).

Various nanostructure-based nanocomposite antifouling coatings reported in the literature are summarized in Table 2. ZnO and TiO₂ nanoparticles are most commonly used, while Cu₂O, CuO and Al₂O₃ are also incorporated into nanocomposite coatings (Table 2). PDMS has certain advantages as a coating. It has a hydrophobic surface and, thus, used for fouling release antifouling applications (Zhang and Chiao, 2015). Biopolymers, such as chitosan, have been used to develop environmentally friendly nanocomposite antifouling coatings (Carteau et al., 2014; Krishnan et al., 2015; Pounraj et al., 2018). Chitosan and ZnO or TiO₂ based nanocomposite coatings have been reported as effective against several marine microorganisms including biofilm-forming bacterium Vibrio fischeri (Baniamerian et al., 2018), Pseudoalteromonas nigrifaciens, and diatoms Navicula sp. (Al-Naamani et al., 2017). While metal oxides have photocatalytic properties that prevent biofouling at the light, the addition of chitosan enhances antifouling properties in dark conditions. Most of the studies with ZnO nanoparticles or nanorods nanocomposite antifouling coatings were conducted in the laboratory conditions (Table 2). Since in the field there are many different species of biofouling organisms, as well as coatings are exposed to different physical and biological factors, it is important to test coatings the field conditions in order to prove their antifouling activity.