Antifouling Performance of Carbon-Based Coatings for Marine Applications: A Systematic Review

Although carbon materials are widely used in surface engineering, particularly graphene (GP) and carbon nanotubes (CNTs), the application of these nanocomposites for the development of antibiofilm marine surfaces is still poorly documented. The aim of this study was, thus, to gather and discuss the relevant literature concerning the antifouling performance of carbon-based coatings against marine micro- and macrofoulers. For this purpose, a PRISMA-oriented systematic review was conducted based on predefined criteria, which resulted in the selection of thirty studies for a qualitative synthesis. In addition, the retrieved publications were subjected to a quality assessment process based on an adapted Methodological Index for Non-Randomized Studies (MINORS) scale. In general, this review demonstrated the promising antifouling performance of these carbon nanomaterials in marine environments. Further, results from the revised studies suggested that functionalized GP- and CNTs-based marine coatings exhibited improved antifouling performance compared to these materials in pristine forms. Thanks to their high self-cleaning and enhanced antimicrobial properties, as well as durability, these functionalized composites showed outstanding results in protecting submerged surfaces from the settlement of fouling organisms in marine settings. Overall, these findings can pave the way for the development of new carbon-engineered surfaces capable of preventing marine biofouling.


Introduction
Carbon nanomaterials, such as graphene (GP), carbon nanotubes (CNTs), fullerenes, and diamond-like carbon, are recognized for their proven antimicrobial and antiadhesive properties [1]. Graphene consists of a single-layer sheet of sp 2 -hybridized carbon with a two-dimensional honeycomb structure [2]. GP materials show a high specific surface area, electron conductivity, and thermal stability, making them attractive for applications like photocatalysis, energy production, and storage [3,4]. These nanocomposites stand out as some of the strongest and thinnest materials available and have significantly better electrochemical properties than CNTs, which are formed by rolling up either a single graphene sheet, single-walled carbon nanotubes (SWCNTs), or a series of concentric graphene sheets, multi-walled carbon nanotubes (MWCNTs) [5]. CNTs, therefore, exhibit a concentric cylindrical structure with a diameter in the order of nanometers (depending on the number of walls) and a length of several microns (100 µm) extendable to up to a few millimeters (about 4 mm) [6]. Due to their unique properties, such as a remarkable mechanical strength, high thermal conductivity, and structural stability, CNTs are promising nanomaterials for several applications, namely in the industrial, environmental, and medical fields [7][8][9].
The antimicrobial and antifouling (AF) performance of carbon nanomaterials, as well as their outstanding mechanical properties, have led to their application in coatings for The application of these nanomaterials can increase the mechanical strength of the final composite [42] and its ability to prevent or delay biofouling [43].
Both GP and CNTs have shown good antimicrobial activity against Gram-positive and Gram-negative bacteria, as well as bacterial spores [1,5]. Regarding CNTs specifically, SWCNTs have exhibited significantly higher antibacterial activity than MWCNTs against Gram-positive bacteria [44]. However, the mechanisms of action behind these carbonbased nanomaterials are still not fully understood, due to their complexity and the wide array of factors that may influence their antibacterial activity, including their composition and geometry, as well as the type, morphology, and growth state of bacteria (planktonic or sessile) [7,45]. It is hypothesized that the antibacterial properties of these nanomaterials rely mainly on mechanical factors: their sharp structures act as ''nano-darts'' that pierce bacterial membranes, leading to cell death [44]. Nevertheless, other authors defend that CNTs produce not only mechanical damage with consequent cell disruption and release of intracellular content (a primary killing mechanism), but also generate oxidative stress [46] (Figure 2). It has also been reported that the length, diameter, surface area, concentration, and chemical modifications of CNTs play a significant role in both the AF and antimicrobial activity of these carbon materials [46][47][48].
As for graphene-based nanomaterials, it is also assumed that both physical and chemical factors come into play. Similar to CNTs, the sharp edges of GP sheets cause membrane damage [49]. Thanks to the large surface area of graphene-based materials, they can also lead to bacterial cell entrapment [50]. Moreover, GP is assumed to be able to induce oxidative stress through the formation of reactive oxygen species (ROS), which disrupt microorganisms' DNA and proteins ( Figure 2). According to the literature, the mechanisms through which GP-based nanomaterials induce cell inhibition/death are not only dependent on their own physical and chemical properties (e.g., dimensions, number of layers, functionalization), but also on factors related to the production of the surface (e.g., graphene loading, nanoparticle dispersion, aggregation) [51][52][53].
Thanks to these appealing properties, the use of these nanomaterials for improved marine AF coatings is currently on the rise [41]. As such, assessing the effectiveness of GPand CNTs-based coatings in preventing marine biofouling, namely on pioneer bacterial attachment and biofilm formation, can contribute to optimizing and reaching a better understanding of their AF properties. However, the currently available data regarding the potential of carbon-based coatings to prevent marine biofouling need to be critically discussed to assist researchers in the design of improved marine surfaces. The application of these nanomaterials can increase the mechanical strength of the final composite [42] and its ability to prevent or delay biofouling [43].
Both GP and CNTs have shown good antimicrobial activity against Gram-positive and Gram-negative bacteria, as well as bacterial spores [1,5]. Regarding CNTs specifically, SWCNTs have exhibited significantly higher antibacterial activity than MWCNTs against Gram-positive bacteria [44]. However, the mechanisms of action behind these carbon-based nanomaterials are still not fully understood, due to their complexity and the wide array of factors that may influence their antibacterial activity, including their composition and geometry, as well as the type, morphology, and growth state of bacteria (planktonic or sessile) [7,45]. It is hypothesized that the antibacterial properties of these nanomaterials rely mainly on mechanical factors: their sharp structures act as "nano-darts" that pierce bacterial membranes, leading to cell death [44]. Nevertheless, other authors defend that CNTs produce not only mechanical damage with consequent cell disruption and release of intracellular content (a primary killing mechanism), but also generate oxidative stress [46] ( Figure 2). It has also been reported that the length, diameter, surface area, concentration, and chemical modifications of CNTs play a significant role in both the AF and antimicrobial activity of these carbon materials [46][47][48].
As for graphene-based nanomaterials, it is also assumed that both physical and chemical factors come into play. Similar to CNTs, the sharp edges of GP sheets cause membrane damage [49]. Thanks to the large surface area of graphene-based materials, they can also lead to bacterial cell entrapment [50]. Moreover, GP is assumed to be able to induce oxidative stress through the formation of reactive oxygen species (ROS), which disrupt microorganisms' DNA and proteins ( Figure 2). According to the literature, the mechanisms through which GP-based nanomaterials induce cell inhibition/death are not only dependent on their own physical and chemical properties (e.g., dimensions, number of layers, functionalization), but also on factors related to the production of the surface (e.g., graphene loading, nanoparticle dispersion, aggregation) [51][52][53].
Thanks to these appealing properties, the use of these nanomaterials for improved marine AF coatings is currently on the rise [41]. As such, assessing the effectiveness of GPand CNTs-based coatings in preventing marine biofouling, namely on pioneer bacterial attachment and biofilm formation, can contribute to optimizing and reaching a better understanding of their AF properties. However, the currently available data regarding the potential of carbon-based coatings to prevent marine biofouling need to be critically discussed to assist researchers in the design of improved marine surfaces.

Study selection and Characterization
A total of 152 articles were found using the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analysis) search methodology. This number was increased to 163 studies with the inclusion of 11 records retrieved through other sources (prior searches and references of the chosen publications). After screening out duplicates, a total of 159 articles were assessed based on title and abstract. Out of these, 127 records were disqualified for not meeting the prerequisites for inclusion. Lastly, 2 records were ruled out upon thorough analysis of the remaining 32 full-text publications since these were nonoriginal articles. Therefore, 30 studies were included in the qualitative synthesis (Figure 3).

Study Selection and Characterization
A total of 152 articles were found using the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analysis) search methodology. This number was increased to 163 studies with the inclusion of 11 records retrieved through other sources (prior searches and references of the chosen publications). After screening out duplicates, a total of 159 articles were assessed based on title and abstract. Out of these, 127 records were disqualified for not meeting the prerequisites for inclusion. Lastly, 2 records were ruled out upon thorough analysis of the remaining 32 full-text publications since these were nonoriginal articles. Therefore, 30 studies were included in the qualitative synthesis ( Figure 3).

Study selection and Characterization
A total of 152 articles were found using the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analysis) search methodology. This number was increased to 163 studies with the inclusion of 11 records retrieved through other sources (prior searches and references of the chosen publications). After screening out duplicates, a total of 159 articles were assessed based on title and abstract. Out of these, 127 records were disqualified for not meeting the prerequisites for inclusion. Lastly, 2 records were ruled out upon thorough analysis of the remaining 32 full-text publications since these were nonoriginal articles. Therefore, 30 studies were included in the qualitative synthesis (Figure 3).  With the increasing search for environmentally friendly AF materials, the study of carbon nanomaterials and their unique properties has been rising during the past decade. These nanomaterials are generally incorporated into commercial AF paints based on polymeric matrices, such as polydimethylsiloxane (PDMS), to assess how they influence the physicochemical properties and AF performance of coatings.
Due to the appealing physical and chemical properties it possesses, such as high hydrophobicity, low surface energy, adhesion, and endurance, PDMS stands out as one of the most widely used polymers for AF purposes [54]. However, PDMS also presents certain limitations, namely, its mechanical weakness and propensity to become damaged when immersed in a marine environment [55]. Thus, any additive capable of improving the efficacy and durability of PDMS-based AF coatings is extremely promising.
CNTs-modified PDMS composites have gained worldwide attention due to their facile fabrication, ecological stability, and remarkable AF performance [56]. Furthermore, the incorporation of GP or CNTs in polymeric coatings has been shown to improve their physical properties, namely in terms of mechanical strength [42,57], increasing their capability to delay marine biofouling.
In this systematic review, 30 full-text articles focused on carbon materials incorporated into polymeric coatings and their AF properties in the marine biofouling context were reviewed and summarized, as shown in Tables 1 and 2. Among the selected publications, 12 focused on the incorporation of pristine or modified MWCNTs and 11 used graphene oxide (GO), either on its own or functionalized with other nanoparticles. In addition, five studies addressed the AF performance of other GP forms. PDMS stood out as the most used nanocoated polymer, with a total of 14 out of the 30 studies.
Based on the reviewed articles, Figure 4 represents the evolution that the number of studies focused on carbon-nanocoated polymeric materials for marine AF purposes has shown over the years. carbon nanomaterials and their unique properties has been rising during the past d These nanomaterials are generally incorporated into commercial AF paints based ymeric matrices, such as polydimethylsiloxane (PDMS), to assess how they influe physicochemical properties and AF performance of coatings.
Due to the appealing physical and chemical properties it possesses, such as h drophobicity, low surface energy, adhesion, and endurance, PDMS stands out as the most widely used polymers for AF purposes [54]. However, PDMS also presen tain limitations, namely, its mechanical weakness and propensity to become da when immersed in a marine environment [55]. Thus, any additive capable of imp the efficacy and durability of PDMS-based AF coatings is extremely promising.
CNTs-modified PDMS composites have gained worldwide attention due to th ile fabrication, ecological stability, and remarkable AF performance [56]. Furthermo incorporation of GP or CNTs in polymeric coatings has been shown to improve thei ical properties, namely in terms of mechanical strength [42,57], increasing their cap to delay marine biofouling.
In this systematic review, 30 full-text articles focused on carbon materials in rated into polymeric coatings and their AF properties in the marine biofouling c were reviewed and summarized, as shown in Tables 1 and 2. Among the selected cations, 12 focused on the incorporation of pristine or modified MWCNTs and 1 graphene oxide (GO), either on its own or functionalized with other nanoparticles dition, five studies addressed the AF performance of other GP forms. PDMS stood the most used nanocoated polymer, with a total of 14 out of the 30 studies.
Based on the reviewed articles, Figure 4 represents the evolution that the num studies focused on carbon-nanocoated polymeric materials for marine AF purpo shown over the years. Considering that no restrictions were applied to the search in terms of publ year, the fact that the first article identified on the subject dates to only 2008 prov this strategy is fairly recent. In addition, the increasing number of studies publishe the last few years confirms the relevance of this topic as a research matter.
In this systematic review, particular attention was given to carbon-based co with application in the marine field and their antifouling potential against micr macrofoulers. Considering that no restrictions were applied to the search in terms of publication year, the fact that the first article identified on the subject dates to only 2008 proves that this strategy is fairly recent. In addition, the increasing number of studies published over the last few years confirms the relevance of this topic as a research matter.
In this systematic review, particular attention was given to carbon-based coatings with application in the marine field and their antifouling potential against micro-and macrofoulers. Graphene coatings were effective in decreasing the adhesion and expression levels of adhesion genes of biofilm-producing bacteria Halomonas spp. [58] Silicone rubber Paracoccus pantotrophus In vitro study Artificial seawater Quasi-static assay (7 days) Dynamic assay (7 days, varying speeds within the 0.2-0.5 m/s range) Under quasi-static conditions, the graphene-silicone membranes showed similar AF performance to that of the control surface (rigid polystyrene sheet). Under dynamic conditions, the graphene-based membranes showed better AF performance than the control surface, with around 40% reduction in colony-forming units (CFUs). [59] Graphenesilver nanocomposites

Halomonas pacifica
In vitro study Static assay Marine broth 26 • C, 24 h The nanocomposite displayed significant bacterial biofilm inhibition (99.6% reduction) and antiproliferative effects on marine microalgae (growth inhibition greater than 80%), whereas surfaces coated with graphene alone did not display any AF properties when compared to the control surface. [60] Dunaliella tertiolecta Isochrysis sp.  Composite-based paint showed great self-polishing AF performance in natural seawater. [71] Polyaniline/pphenylenediaminefunctionalized graphene oxide

Epoxy resin
Organisms in a simulated marine environment (e.g., guppy fish, spirulina algae, and dwarf hair grass) In vitro study Simulated marine environment 25-27 • C, 3 months The anticorrosion and AF properties of commercialized epoxy coatings were improved by the addition of the functionalized graphene oxide composite. [72] Antibiotics 2022, 11, 1102 8 of 18 [73] Graphene oxide-boehmite nanorods

Graphene-Based Coatings
Several authors have demonstrated the promising AF activity of GP coatings against marine bacteria [58,59] (Table 1). According to Jin, Zhang, et al., under dynamic conditions, graphene-based membranes were able to reduce bacteria adhesion by 40% [59].
However, a records analysis reveals that the current trend is to study the potential of modified/functionalized GP. The functionalization of GP with silver nanoparticles has demonstrated remarkable antibiofilm effects, with a 99.6% inhibition rate for Halomonas pacifica and over 80% for Dunaliella tertiolecta and Isochrysis sp. [60]. Recently, guanidine functionalized GP has also shown promising antibacterial and diatom antiadhesion properties, with reduction rates of up to 95% and up to 99.2%, respectively. Moreover, the field trial revealed no fouling or surface deterioration for 2 months [61].
Furthermore, laser-induced GP coatings reduced Cobetia marina surface coverage by up to 80% after 36 h of exposure [62].
These results indicated that functionalized GP coatings can be successfully applied for the development of AF marine surfaces.

Graphene Oxide-Based Coatings
Graphene oxide, the most studied form of GP within the marine AF coatings context, has shown both in vitro and in situ high AF activity [63][64][65] (Table 1). In fact, surfaces containing GO 0.36 wt% when incubated for 10 days under dynamic conditions almost completely inhibited diatom adhesion [64] (Figure 5).
However, GO is often functionalized with metal nanoparticles, such as silver or alumina, and other compounds. These nanocomposites aim to provide GO with enhanced antimicrobial properties by improving particle dispersibility and strengthening the contact between the carbon nanomaterials and surrounding microorganisms. GO-silver nanoparticles coatings developed by Liu et al. and Zhang and Mikkelsen showed improved antibacterial and antialgal properties, and more than 80% average biofilm inhibition against H. pacifica (Figure 6), respectively [66,67]. Besides silver, alumina and silica nanoparticles have also been used in conjunction with GO, having both demonstrated excellent antimicrobial properties against a wide range of organisms [68,69]. In turn, the functionalization of GO with polyaniline/p-phenylenediamine conferred anticorrosion and AF properties to commercialized epoxy coatings [72]. Likewise, the modifications of GO materials with compounds such as cuprous oxide, acrylic acid, or boehmite nanorods, produced AF coatings with high self-cleaning performance and durability in marine environments (up to 6 months) (Figure 7) [70,71,73].
In general, these results suggest that GO composites have promising AF and anticorrosion properties, which are so desirable in the marine industry.

Carbon-Nanotubes-Based Coatings
Up to date, several studies have reported the effectiveness of CNT-based coatings in the prevention and control of marine biofouling. Table 2 describes the studies demonstrating the efficacy of these coatings in marine environments, which refer essentially to MWCNTs.
Concerning the application of pristine MWCNTs (p-MWCNTs) for marine coatings, the obtained results differ. While some studies demonstrated that the incorporation of these carbon nanomaterials on PDMS improves its AF performance by reducing the abundance of pioneer eukaryotic microbes [79] and the adhesion strength of adult barnacles [75,78], other studies revealed that p-MWCNT-based coatings did not affect the settlement of micro-and macrofoulers [76,77].
In addition, recent studies have strived to assess the influence of carboxyl-and hydroxyl-modified MWCNTs on the AF performance of marine coatings. Sun and Zhang performed a thorough study of these modified surfaces by carrying out a two-month field trial focused on the addition of MWCNTs with varying hydroxyl content % (w/w), carboxyl content % (w/w), diameter, and length into PDMS coatings. Results showed that the type of MWCNTs had an impact on the coating's AF behavior, as well as on pioneer

Carbon-Nanotubes-Based Coatings
Up to date, several studies have reported the effectiveness of CNT-based coatings in the prevention and control of marine biofouling. Table 2 describes the studies demonstrating the efficacy of these coatings in marine environments, which refer essentially to MWCNTs.
Concerning the application of pristine MWCNTs (p-MWCNTs) for marine coatings, the obtained results differ. While some studies demonstrated that the incorporation of these carbon nanomaterials on PDMS improves its AF performance by reducing the abundance of pioneer eukaryotic microbes [79] and the adhesion strength of adult barnacles [75,78], other studies revealed that p-MWCNT-based coatings did not affect the settlement of micro-and macrofoulers [76,77].
In addition, recent studies have strived to assess the influence of carboxyl-and hydroxyl-modified MWCNTs on the AF performance of marine coatings. Sun and Zhang performed a thorough study of these modified surfaces by carrying out a two-month field trial focused on the addition of MWCNTs with varying hydroxyl content % (w/w), carboxyl content % (w/w), diameter, and length into PDMS coatings. Results showed that the type of MWCNTs had an impact on the coating's AF behavior, as well as on pioneer

Carbon-Nanotubes-Based Coatings
Up to date, several studies have reported the effectiveness of CNT-based coatings in the prevention and control of marine biofouling. Table 2 describes the studies demonstrating the efficacy of these coatings in marine environments, which refer essentially to MWCNTs.
Concerning the application of pristine MWCNTs (p-MWCNTs) for marine coatings, the obtained results differ. While some studies demonstrated that the incorporation of these carbon nanomaterials on PDMS improves its AF performance by reducing the abundance of pioneer eukaryotic microbes [79] and the adhesion strength of adult barnacles [75,78], other studies revealed that p-MWCNT-based coatings did not affect the settlement of microand macrofoulers [76,77].
In addition, recent studies have strived to assess the influence of carboxyl-and hydroxyl-modified MWCNTs on the AF performance of marine coatings. Sun and Zhang performed a thorough study of these modified surfaces by carrying out a two-month field trial focused on the addition of MWCNTs with varying hydroxyl content % (w/w), carboxyl content % (w/w), diameter, and length into PDMS coatings. Results showed that the type of MWCNTs had an impact on the coating's AF behavior, as well as on pioneer eukaryotic communities [55]. Conversely, Ji et al. demonstrated that most carboxyl-and hydroxylmodified coatings had weak modulating effects on pioneer biofilm communities [81], while Sun and Zhang showed that the AF behavior of these modified CNTs varied for different pioneer biofilm bacteria [55].
In turn, the production of fluorinated MWCNTs polymer-based coatings showed a promising antiadhesion effect against pseudobarnacles [84] and an impressive 98% reduction rate against E. coli [85].
Furthermore, the infusion of lubricants, such as silicone oil, into the polymeric matrices was revealed to be a promising approach to MWCNTs-based coatings [74,80,83]. This strategy aims to develop long-term superhydrophobic fouling release (FR) surfaces that leach lubricant over time, creating an isolation oil layer that protects the surface from deformation or damage caused by friction and reinforces its AF properties [83].
Altogether, these data provide important findings that should be considered in the development of new CNTs-based antifouling marine coatings.

Other Carbon-Nanomaterials-Based Coatings
Apart from GP and CNTs, the AF potential of other carbon nanomaterials has been explored. Recently, Luo et al. synthesized atomic chromium-graphitic carbon nitride coatings and demonstrated their in situ activity to control marine biofouling for approximately 2 months [86].

Qualitative Assessment
In order to assess the validity of the obtained results and their predictive value, the 30 selected articles were scored according to an adapted MINORS scale ( Table 3). Out of a maximum score of 24, the studies obtained a mean score of 21.5 ± 2.0.
All articles clearly stated the aim of the work, presented adequate methodologies, and provided enough information about the composition and fabrication method of the tested coating (criteria one, two, and four; mean score of 2.00). Additionally, 29 out of the 30 articles reported at least three replicates/independent experiments for each assay, as well as an adequate control group (criteria three and five; mean score of 1.93). Moreover, most studies provided sufficient information about the experimental setup used (criterion eight; mean score of 1.90) and implemented appropriate surface characterization methods (criterion six; mean score of 1.87).
Although these results are overall positive, some limitations were also found. It is noteworthy that 17 out of the 30 selected studies were carried out solely in vitro, most under static conditions (criterion seven; mean score of 1.73). This is quite unfortunate since experimental setups that mimic the marine environment (e.g., hydrodynamic conditions, day-to-night light variation) or in situ studies can be much more reliable for most applications. Moreover, a considerable number of studies did not report the number of organisms (e.g., cell concentration) that the surfaces were exposed to (criterion nine; mean score of 1.76), and only evaluated the performance of AF coatings for short-term adhesion (criterion 10; mean score of 1.76). Lastly, only 12 out of the 30 assessed publications clearly mentioned the implementation of statistical tests appropriate to the dataset (criterion 12; mean score of 0.87). Since a statistical analysis is a crucial part of producing trustworthy results and predictions, this is considered to be a critical flaw of most articles found.
Despite the overall high score of the selected studies, due to the wide array of methodologies used to test the AF properties of the coatings and the lack of proper statistical analysis, it is essential to highlight the importance of conducting further tests with robust, established methodologies and adequately analyzing the results, to facilitate the comparison between studies and draw more reliable conclusions about AF marine coatings. Moreover, one of the key issues in the development of new AF surfaces is the necessary screening of candidate surfaces (usually tested in vitro in a first step prior to in situ testing) before scale-up and final performance evaluation. Since it has been shown that hydrodynamics can severely affect biofilm formation [87][88][89][90] and gene expression by fouling organisms [91,92], it is recommended that these tests be performed in controlled hydrodynamic conditions that mimic the final application scenario [93].

Search Strategy, Study Eligibility, and Data Extraction
Previously published studies evaluating the effectiveness of carbon nanomaterials, namely GP and/or CNTs used to produce AF or FR coatings for the prevention or control of the attachment of micro-or macrofoulers in marine settings were systematically reviewed based on the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) Statement [94].
The search was conducted until June 6th, 2022, using two electronic databases, Sci-enceDirect and PubMed, through selective combinations of relevant keywords: "marine biofilm", "biofouling", "antifouling", "coatings", "graphene", and "carbon nanotubes". Published full-text articles in English were assessed for eligibility. Studies were screened based on title, abstract, and, ultimately, full content.
The reference sections of all included articles were carefully examined for additional articles that were not identified through the database search. The main inclusion criteria were: (1) in situ studies concerning the application of GP and/or CNTs composites as AF/FR coatings in the marine environment; (2) in vitro studies focused on assessing the properties of GP and/or CNTs composites, including their antiadhesion, antimicrobial, anticorrosion activities, and AF/FR performance against marine micro-and macrofoulers. As for the main exclusion criterion, any nonoriginal articles, such as reviews or reports, were discarded.
For each selected article, information concerning the materials and composition of the coatings used, identification of the tested fouling organisms, experimental setup, and relevant conclusions were extracted and analyzed by two independent reviewers.

Quality Assessment
Selected studies were subjected to a quality assessment procedure based on an adapted Methodological Index for Non-Randomized Studies (MINORS) scale [95]. MINORS is a validated instrument designed to evaluate methodological quality, as well as to detect potential biases in nonrandomized surgical studies. Although there are no methodological indices to measure the risk or the quality of nonclinical studies, the MINORS scale can be adjusted to other scientific contexts and still serve as a valuable quality assessment tool [96]. As such, the MINORS scale was adapted to the specific context of this systematic review and used to evaluate the overall quality and predictive value of each publication.

Conclusions
Among the solutions found to control marine biofouling, the application of coatings with antifouling properties on submerged surfaces has been the most promising approach. Due to their outstanding properties, carbon nanomaterials have proven to be promising in the production of antimicrobial and antifouling surface coatings.
This systematic review demonstrated that, over the last few years, there has been an increasing interest in the synthesis and evaluation of carbon-based coatings, in particular those containing GP or CNTs, to prevent and control marine biofouling. Most of the reviewed studies investigated the efficacy of modified/functionalized GP or CNTs, which demonstrated improved AF performance compared to their pristine forms.
Although these studies provided promising results, most AF coatings were only evaluated in vitro and, therefore, in situ or case studies are missing to validate their realworld application. It is also noteworthy that 26.7% of studies only qualitatively evaluated the AF performance of developed coatings or did not disclose the extent of attachment reduction and/or foulers inactivation. Moreover, the degradation or bioaccumulation rates of carbon-based coatings in the marine environment, as well as their toxicity for nontarget organisms were not addressed in the reviewed studies.
The development of more accurate and reliable in vitro test methods that are able to mimic the hydrodynamic conditions observed for each target application is of paramount importance in obtaining reliable data. This is crucial because in situ tests should not be performed with surfaces that release compounds for which the leaching and toxicity profiles have not been determined. With an increasing environmental conscience from the public and regulatory organizations, novel coatings must be nontoxic to nontarget organisms, durable, and amenable to large-scale production in a sustainable way. This is a multidisciplinary endeavor requiring the involvement of different stakeholders who have to find a