Membrane Modification with Polydopamine to Improve Performances — A Mini Review

This review paper explores the use of polydopamine (PDA) as an additive in membrane fabrication. PDA is a versatile material with superadhesive and hydrophilic properties that make it a popular choice for enhancing membrane performance. The addition of PDA to membrane surfaces has been shown to improve permeation properties and increase fouling and biofouling resistance. The functional groups of PDA are hydrophilic, which increases membrane permeation and reduces hydrophobic-hydrophobic interactions between membranes and contaminants. Surface modification with PDA has been found to be effective in improving the fouling resistance of the membrane. Moreover, PDA's strong antibacterial properties have been attributed to its ability to kill bacteria contaminants by reducing the surface charge of the membrane, by contact killing, and by silver ion (Ag) reduction. However, the review also highlights cases where modification with PDA has led to a substantial decrease in permeation performance due to pore shrinkage and clogging by coating, which may lead to fouling. Therefore, the review emphasizes the importance of controlling the pure water permeability of coated membranes by varying modification conditions such as the concentration of the coating solution and coating time duration. Overall, the review concludes that PDA is a promising additive for improving membrane performance, but careful consideration of modification conditions is necessary to avoid potential drawback issues.


Introduction
Ever since the pilot invention of commercial membrane in the 1960s, a large number of remarkable developments in membrane technology have been achieved both scientifically and commercially. Some of them have been applied in large-scale industries, some of which include potable water production (Jarvis et al., 2022;Judd and Carra, 2021;Yu and Graham, 2015), desalination by reverse osmosis (RO) (Wu et al., 2014), wastewater treatment by membrane bio-reactors (MBR) (Deowan et al., 2016), lithium-ion batteries (Cao et al., 2014;Jiang et al., 2016;Shi et al., 2016), membrane-based fuel cells (Branco et al., 2016;Shi et al., 2023;Wang et al., 2023), and medical-related application like wounddressing material (Ao et al., 2023;Luo et al., 2017;Markov et al., 2022). Membrane technology is favored for its superiority, which meets sustainability criteria in terms of environmental impacts, ease of use, compactibility, and adaptability (Le and Nunes, 2016).
Good performances, such as high permeability and rejection, are greatly desired and important for the membrane. To earn the best performance, membranes have to be fabricated meticulously through a selection or combination of materials and a modification of manufacturing techniques. The compatibility of the polymer with the additive plays an important role in the performance of the membrane. A large and growing body of literature has been published on the use of different types of additives, both organic and inorganic, in an attempt to improve the performance of the membrane (Ahmad et al., 2013;Fahrina et al., 2021;Mulyati et al., 2022Mulyati et al., , 2020bMulyati et al., , 2020aQian et al., 2022;Wahab et al., 2019). Herein, the authors focus on the use of polydopamine (PDA) as one of the most frequently utilized additives in membrane fabrication. This paper will provide an overview of the use of PDA as a modifier in membrane fabrication and the effects it has on the morphology and performance of the resulting membrane.

Overview of Polydopamine as Modifier in Membrane Fabrication
Polydopamine (PDA) is a highly hydrophilic material. The polymerization mechanism of dopamine involves the oxidation of catechol into quinine through a process under base conditions. One important characteristic of PDA is its robustness and strong adhesion properties to almost any type of surface, regardless of the chemical properties of the substrate. PDA is a kind of biological glue and can even stick well to the most highly nonadherent material ever known, i.e., poly (tetrafluoroethylene) (PTFE) (Lee et al., 2007). The ability to attach to organic and hydrophobic surfaces is the most prominent advantage of PDA (Liu et al., 2014). By means of atomic force microscopy (AFM), Yu et al. demonstrated that the adhesion mechanism of PDA to the organic surface relies on the oxidation of catechol to quinone in the oxidation process under base conditions, causing the formation of double covalent bonding to the surface of the organic material via the aryl-aryl or Michael's addition reaction (Yu and Deming, 1998).  (Liebscher et al., 2013).
A limited understanding of the PDA binding mechanism as well as its high adhesion capability invites the interest of scientific researchers to learn about and create functional substrates and hybrid materials from it. Another interesting feature of polydopamine is its binding ability with various multivalent metal ions such as Fe 3+ , Mn 2+ , and Cu 2+ . The bond with the metal ions is due to the large number of functional groups in the polydopamine, namely quinone, carboxy, amino, imine, and phenol groups. It is worth noting that different bonding sites of polydopamine will be activated under different conditions. In addition to being a metal ion binder, polydopamine is also capable of reducing several noble metal ions, such as Au 3+ , Ag + , and Pt 3+ , under basic conditions. The ability of polydopamine to play a role as a reducing agent is due to the redox properties of its monomer units (Liebscher et al., 2013).
Surfaces of various nanomaterials such as TiO2 (Feng et al., 2015;Shao et al., 2012), graphene (Yang et al., 2016) and clay (Yang et al., 2011) have been modified with PDA due to the special adhesive properties. The goal is to enhance the interfacial interaction between the nanomaterial and the polymer matrix. The results of the study suggest that the interfacial layer of PDA not only facilitates nanomaterial dispersion into the polymer matrix due to increased interfacial interaction (a combination of covalent and noncovalent interactions such as charge transfer, ηstacking, hydrogen bond interaction, and hydrophobic interactions), (Lee et al., 2009(Lee et al., , 2007(Lee et al., , 2006Yu et al., 2013) but also rewards the substrate with the traits possessed by PDA.
In recent years, there has been a dramatic increase in the utilization of mussel-inspired PDA as a surface-modifying agent, and it has been extensively applied in membrane technology for various removal processes such as heavy metals (Dong et al., 2014;Habibi et al., 2015;Wu et al., 2015), dye (Yang et al., 2022), antibacterial in medical field (Luo et al., 2017;X. Wang et al., 2016), separators on Li batteries (Cao et al., 2014;Shi et al., 2016) and others.
Lee et al (Lee et al., 2007) has developed a simple and rapid technique in modifying various materials with polydopamine through dopamine polymerization using tris-buffer. The presence of oxygen and Tris-HCl base solutions as a medium assisted dopamine molecules to be oxidized, intramolecularly decyclized, and self-polymerized into particles. These particles then accumulate and grow into thin layers of polydopamine, which is rich in bioactive groups like OH and NH2, which can provide an effective platform for further modification (Jiang et al., 2011;Lee et al., 2007;Muchtar et al., 2019). The thin layer of PDA attaches strongly to the membrane surface under alkaline conditions through covalent and noncovalent bonds such as ηstacking, hydrogen bonding, and electrostatic interactions (Jiang et al., 2011;Lee et al., 2007) and forms the PDA thin layer. Figure 2 illustrates the polymerization stage of dopamine to produce polydopamine. It can be seen that after polymerization, a whole bunch of functional groups, such as planar indoles (-NH-CH=CH-), amine group (-NH), carboxylic acid (CH2O-R), catechol or quinone, and indolic/catecholic π-system are formed. These functional groups provide a super-adhesion peculiarity to polydopamine on all substrate types (Liu et al., 2014). In membrane modification, polydopamine can be added by blending it into the casting solution, and then the membrane is fabricated through the NIPS or in situ method (Jiang et al., 2014), However, the coating technique is the most commonly used, in which the fabricated membrane is dipped or immersed in polydopamine solution. Polydopamine can be used directly as a solo modifier , and as a sticking medium for other additives that have low compatibility with the membrane materials. After being coated with PDA, the membrane will have a surface that is rich in reactive groups, can serve as a versatile platform for secondary reactions, and may immobilize molecules with functional groups like -NH2 or -OH (Lee et al., 2009). Chen et al. performed a coating modification with dopamine solution on the membrane, which was beforehand functionalized with N-Methylglucamine gluconate (NMG). The amine group in NMG may react with dopamine through Michael's addition reaction, which aids in adjusting the self-polymerization process of dopamine, resulting in the formation of a homogeneous PDA coating (Chen and He, 2017).
As stated previously, the PDA coating can be used to separate layers to create composite membranes with excellent adhesion between active layers and substrate surfaces (Faure et al., 2013;Pan et al., 2009;Wei et al., 2010;Yang et al., 2013;Ye et al., 2011). Therefore, a number of authors have employed PDA as a medium to graft hydrophilic materials onto the membrane surface.
Jiang carried out an experiment in which he and his research team made a hydrophilic membrane by immersing a polyethylene porous membrane into the dopamine solution before adding heparin to the reactive layer of the PDA coating (Jiang et al., 2010). The results indicated that the modification reduced platelet adhesion and increased the anticoagulation activity of the membrane. Zhu and co-workers immobilized BSA to a polyethylene (PE) membrane using PDA as a spacer. The results showed that hydrophilicity of the membrane increased significantly after coating with PDA and binding with BSA (Jiang et al., 2011). In addition, McCloskey et al. conducted a series of trials in which they modified the already PDA-coated membrane with superhydrophilic mPEG-NH2 to improve membrane surface functionalization (McCloskey et al., 2010). In a follow-up study, they coated the surfaces of RO, UF, and MF membranes with PDA and then grafted them with PEG (McCloskey et al., 2012). In another study, PEG was combined with polyvinylpyrrolidone (PVP) to obtain a membrane with higher hydrophilicity (T. . When graphene oxide membrane was made on Al2O3 support, PDA was used as a linker (adhesive). After modification, the stability of the membrane increased. The membrane was not easily peeled off from the support even with strong sonification treatment due to the high adhesive nature of PDA (Feng et al., 2015;Ryu et al., 2010). PDA coating on the membrane has no effect on mass transfer resistance, support, or rejection, but it did successfully augment the flux performance (Xu et al., 2016).
It can also be noted that besides functioning as a chelating agent, PDA also has another special ability: it is capable of reducing noble metal ions such as Au 3+ , Ag + , and Pt 3+ under alkaline conditions. This reduction capability is related to the character of the constituent monomer units. Polydopamine contains a large number of catechol functional groups that can release electrons when they are oxidized to quinone groups. This phenomenon triggers the reduction of metal cations (Liu et al., 2014;Sileika et al., 2011).

Effects on Membrane Morphology
From the literature study, it can be observed that the most obvious change seen in the membranes after the surface modification with PDA is the discoloration to brownish or dark brown, which is the effect of the polymerization reaction of dopamine to polydopamine (Feng et al., 2015;Karkhanechi et al., 2014;Vaselbehagh et al., 2014). Another influence of PDA on the morphology is decreasing porosity or pore size of the membrane (Arena et al., 2014;Fan et al., 2016;Li et al., 2014). In addition to pore decrease, the modification with PDA also affects the surface roughness of the membrane (related to the nodular structure generated in the self-polymerization process of dopamine) . Modification with PDA could increase the surface roughness of the membrane due to the formation of nano-aggregate PDA on membrane surfaces (Ball et al., 2012;Cheng et al., 2012;Ou et al., 2009;Shin et al., 2011;Xi et al., 2009;Zangmeister et al., 2013). . Membranes with high surface roughness are more susceptible to fouling than those with smooth surfaces (Vaselbehagh et al., 2014). In addition, the PDA aggregation on the surface may limit the action of functional groups, which results in a decreased permeability of the membrane .
The opposite tendency towards membrane roughness was also reported. There are also cases where modifications with PDA decrease surface roughness (Freeman et al., n.d.;Xi et al., 2009). This change in roughness may be due to variations in the porosity and pore size of the membrane (Xi et al., 2009). In the blending modification method, Jiang reported that PDA functions as a pore-forming agent in the membrane formation process. Nonpolymerized dopamine and some PDA nanoparticles are released from the film into the coagulation bath during the NIPS process, leading to the formation of pores in the membrane. In addition, the increased thermodynamic immisibility induced by PDA as a non-solvent additive facilitates phase separation and enlarges deep macropores. Thus, the porosity and structure of the inner macropore of the PVDF/PDA membrane increased (Jiang et al., 2014).
Researchers have attempted to solve the problem of pore occlusion caused by PDA particles. Fang et al. offered a novel notion in which the interior pores of the UF membrane can serve as a particular platform for incorporating PDA into the membrane rather than the membrane surfaces. The tissue structure of the internal pores can be prevented from aggregation by immobilizing PDA along the walls of the internal pores, and the PDA adsorptive sites can be exposed as a whole (Fang et al., 2017).

Effect on Permeation Performance
Generally, an improvement in water permeation is observed after coating with PDA. This is because the superhydrophilic characteristics of PDA increase the wetness of membrane pore hence water transport occurs more easily (Arena et al., 2011). The hydrophilicity of the membrane modified with PDA increased due to the presence of the hydroxyl groups in the PDA.
Not only for water treatment, polydopamine has also improved the performance of membranes for other applications, for instance, gas separation. Wang et al. fabricated metal organic framework (MOF)/polyimide (PI) mixed matrix membranes coated with polydopamine. The compatibility and gas flux improved; however, other gas-separation factors decreased (T. . The increase in permeation properties occurs due to the increase in inner space of the polymer matrix, which results in good compactability between the polymer and the filler. In addition, osmotic water flux from the forward osmosis (FO) membrane increased up to six times. The increased wetness of the membrane support layer increases the rate of solute transport through the membrane support layer. The osmotic concentration and pressure of the draw solution will increase at the membrane interface. The salt flux increased due to increased wetness after modification (Arena et al., 2014).
In contrast, several studies have reported a decrease in the permeation of pure water through membranes after modification with PDA (Kasemset et al., 2017;McCloskey et al., 2010). Despite the increased membrane hydrophilicity, the PDA coating could also cause a decrease in flux due to the plugging of some channels on the surface of the membrane after coating with PDA (Fang et al., 2017). Since the polymerization reaction that takes place during the coating process can occur anywhere on the membrane surface (especially in more polar domains), some of the PDA particles may deposit into membrane pores (Kim et al., 2014). McCloskey and the research team (McCloskey et al., 2010) reported that there was a decrease in the flux in the PDA-coated porous membranes (UF, MF); The largest decrease occurred in the UF membrane (up to 40%), whereas for the NF membrane, the decline was 1%, and the reduction of total flux for the non-porous membrane (RO) was 25%. These data indicate that PDA deposition is more disruptive to water permeation in small pore-sized membranes due to clogging.

It
can be concluded that, although modification with PDA can improve membrane performance in terms of water permeation due to its hydrophilic nature, PDA also plays an opposite role because PDA particles can cause pore plugging on the surface or inside the membrane matrix. One of the solutions that can fix or reduce this problem is to design the modification conditions, for instance, concentration and coating time. Karkhanechi et al. reported a decline in membrane permeability with long immersion times in dopamine solution. because a longer immersion time increases the coating thickness. With the increasing thickness of the PDA layer on the membrane surface, the diffusional resistance will increase as well (Karkhanechi et al., 2014). The long deposition time also diminishes the mechanical properties of the membrane, although in another piece of literature it is mentioned that the longer the coating time, the more hydrophilic the obtained membrane is (Fan et al., 2016).
On the other hand, the concentration of the coating solution also affects the morphology of the membrane. The higher the concentration of PDA, the more PDA particles it contains and the larger the pore size; hence, there will be particle clumping on the membrane surface (Vaselbehagh et al., 2014). PDA concentration also affects the mechanical properties of the membrane. Elongation at the break of the membrane increases with increasing loading of PDA. This is because the nanoparticles of the PDA can absorb energy during the tensile deformation process, causing the brittleness of the membrane to diminish. However, excessive loading of the PDA can reduce elongation at break as well (Jiang et al., 2014).

Hydrophilic and Antifouling Agent
Today, the majority of commercial membranes are made of highly hydrophobic materials such as polysulfone, PVDF, PES, polyether ketone (PEK), polyacrylonitrile, etc. These materials are chosen because of their respective characteristics, which are good and suitable for a particular application, but it is undeniable that the hydrophobic nature of these materials is a major problem that should be avoided at all costs. The main reason why the hydrophobic nature of the membrane is not desirable is that it facilitates the occurrence of fouling. The hydrophobic surface of the membrane means the absence of hydrogen bond interactions on the boundary layer between the membrane surface and water (Gao et al., 2011). This phenomenon can lead to severe fouling, which can degrade the performance and shorten the life of the membrane.
The abundant hydroxyl group makes polydopamine a superhydrophilic material that is often used as an antifouling agent in membrane separation technology. The hydrophilic membrane surface may inhibit biofoulant attachment due to the steric hindrance effect of the hydrophilic layer (Elimelech and Phillip, 2011).
The addition of polydopamine to the surface has successfully increased the hydrophilicity of the MWNT filler before it is further modified on the polysulfone membrane. The fouling tendency was reduced by the repulsive interaction between membrane surfaces and protein molecules. This minimizes the capture of protein molecules on the membrane surface. In addition, the negatively charged polydopamine causes the membrane to be negatively charged, resulting in a respulsive effect on the bovine serum albumine (BSA) being negatively charged as well. The hydrophilic nature of the membrane comes from the presence of OH-and NH2-functional groups. However, at concentrations other than 0.05 and 0.1%, the contact angle increased, making the membrane more hydrophobic. After modification with PDA, the membrane has better mechanical strength. The high stickiness and mold-like adhesive properties of PDA help it adhere strongly to the hydrophobic surface of substrates (Sianipar et al., 2016). Cao et al. (Cao et al., 2014) fabricated a PDAcoated PVDF membrane for Li-battery application. Improved performance and stability of battery cells using PDA-PVDF separators are allegedly due to modifications with PDA that contribute hydrophilicity to PVDF, hence the increased uptake of electrolyte and ionic conductivity. In addition, it is hypothesized that the catechol adhesion to PDA polymerization leads to a stronger adhesive interaction between electrodes and PDA-PVDF membrane compared to electrodes with nonmodified PVDF membrane.
Karkhanechi modified the RO membrane with PDA, and the resulting composite membrane had enhanced antifouling properties caused by the bactericidal trait of protonated amine groups from PDA (Karkhanechi et al., 2014). In addition, its high adhesive properties enhanced the mechanical properties of the modified membrane (Lee et al., 2009;Sianipar et al., 2016). It was also reported that the adhesion of bovine serum albumin (BSA) to UF polysulfone membrane, PVDF microfiltration membrane, and polyamide RO membrane plummeted to 96% after modification with PDA (McCloskey et al., 2010).
The surface modification of the UF-MFC membrane using PDA reduced membrane resistance due to enhanced accessibility of electrolytes (from the increased hydrophilicity) and improved biofouling resistance due to characteristic changes in the membrane surface. Increased hydrophilicity on the UF membrane surface resulted in increased electrolyte retention in the interspace between the electrolyte solution and the membrane surface, for which it can be summed up that the membrane resistance problems caused by inadequate ion transport can be overcome by PDA coating (Ryou et al., 2011). Increased hydrophilicity improves ionic conductivity as well as the accessibility of electrolytes to membrane surfaces; therefore, after coating with PDA, it is possible that the resistance of the UF membrane in the MFC system declines by holding more electrolytes around the surface or within the pore support layer without raising the concentration of the electrolyte solution. It can be concluded that the more hydrophilic the membrane UF, the lower the resistance caused by ionic displacement (Kim et al., 2014).
Based on Table 1, it appears that the contact angle of the various PDA-coated substrates generally falls within the same range. These results are in accord with those mentioned in some other literatures (Liu et al., 2014). Changes in contact angle after modification with PDA are claimed to occur due to changes in surface roughness (Jiang et al., 2011;Ou et al., 2009). However, in Kasemset et al.'s study, no significant change in surface roughness occurred; hence, a decrease in contact angle allegedly happened due to increased hydrophilicity on the surface of the membrane (Kasemset et al., 2017). The improved hydrophilicity of the membrane after modification with PDA is due to a rise in the number of hydrophilic functional groups on the membrane surface since PDA is rich in the -OH and -NH groups (Vaselbehagh et al., 2014).
As with morphology and permeation, modification conditions also affect the wettability of the membrane. It was found that the contact angle decreased with the deposition time, and the duration of immersion in the coating solution suggested by Kasemset et al. was no more than 60 minutes (Kasemset et al., 2016). Vaselbegh and his team reported that the membrane contact angle decreased with increasing loading of dopamine and remained constant at concentrations above 1 kg/m3. The polydopamine layer inhibits the adsorption of SDBS on the membrane surface, so fouling will be reduced. This is due to a boost in the number of negative and hydrophilic charges on the membrane surface, so the negative effects of surface roughness can be covered (Vaselbehagh et al., 2014).

Antibacterial (Antibiofouling)
Biofouling is a fouling caused by the build-up or accumulation of microorganisms on the membrane surface, followed by cell growth and multiplication, which will eventually form a biopolymer matrix with a complex structure or biofilm on the membrane surface (Abushaban et al., 2022;Hilal et al., 2004;Jadhav et al., 2021;RAZI et al., 2012). Biofouling is a more serious membrane issue in comparison to fouling by organic matter, because microorganisms can produce secondary pollutants in the form of metabolism that will stick to and grow on the The enhancement of antibacterial properties in a material (in this case, a membrane) can be done through several strategies. The first is through adhesion resistance. Adding materials that have functional groups with similar electrostatic charge polarity as bacteria can increase adhesion properties on the membrane surface due to the formation of electrostatic repulsion between bacteria and the membrane surfaces (Liu et al., 2010). Almost all bacteria are negatively charged in aquatic suspension due to functional groups such as carboxyl and hydroxyl in the cell wall components (Katsikogianni et al., 2004). The electrostatic repulsion force between the bacteria and the membrane surface can be explained by calculating the interaction energy using DLVO (Derjaguin-Landau-Verwey-Overbeek) and XDLVO (extended DLVO) theory. Park and research team (Park et al., 2005) proved that the more negativelycharged membranes have greater electrostatic repulsion with bacteria. PDA plays a major role in the prevention of biofouling by lowering the surface charge of the membrane due to deprotonation of phenolic hydroxyl groups (Kim et al., 2014) and deprotonation of amine groups (Karkhanechi et al., 2014). By lowering the surface charge of the membrane, the bonding with a negative charge on the bacterial cell wall will be difficult to occur; hence, food transport will be blocked and bacterial growth can be inhibited (Luo et al., 2017).
The next strategy, contact killing, is the method in which the membrane surface is modified with antibacterial or chemicals containing the ammonium kwartener group (Herzberg et al., 2011;RAZI et al., 2012), such as polydopamine. Ding et al. (Ding et al., 2012) developed a two-stage polycarbonate coating technique for antibacterial and antifouling by reacting a diblock copolymer with a polydopamine-coated layer via a thiol group owned by PEG.  The last method is the biocide leaching (release killing) approach, which is done by introducing the biocidal chemicals to the surface of the material to kill the bacteria contained in the feed and reduce the accumulation on the membrane surface (Razi et al., 2012;Sileika et al., 2011). Utilizing the reductive properties of PDA, Sileika has sparked a novel and simple immersion strategy for substrate coatings that can inhibit bacterial attachment and actively kill bacteria through the release of silver ions. The strategy is done by coating the membrane with PDA, followed by the grafting of antifouling polymers and silver nanoparticles sourced from a silver salt solution (Sileika et al., 2011). The nanosilver solution will be reduced by the PDA to produce silver ions (Wang et al., 2013) that are biocidal and capable of lysing bacteria (Cao et al., 2010;Karkhanechi et al., 2013).
Based on review of the literature on the properties of PDA-modified membranes in terms of morphology, permeability, and antiorganic and bio-fouling performances, it can be concluded that the modification technique employed to introduce polydopamine into the membrane system has a major impact. It is difficult to decide and select only one best strategy that offers the best merits because each technique of modification offers unique advantages, as illustrated in Fig. 8. However, despite the constraints of their modification procedure, we can strive to acquire the highest quality of the modified membrane by engineering the modification parameters including PDA concentration, coating time, and so on.

Conclusion
The superadhesive and hydrophilic properties of polydopamine make it one of the most favored additives in membrane fabrication. In most cases, the addition of polydopamine to the membrane surface successfully enhanced the membrane's performance in terms of permeation properties as well as fouling and biofouling resistance. The increasing permeation is due to the hydrophilic nature of the functional groups owned by PDA. Surface modification with PDA reduces hydrophobichydrophobic interactions between membranes and contaminants, which improves the fouling resistance of the membrane. In addition, polydopamine has an outstanding antibacterial trait. PDA kills the bacteria contaminants by reducing the surface charge of the membrane, by contact killing, and by silver ion (Ag) reduction. However, there were cases where a substantial decrease in permeation performance happened after modification with PDA due to pore shrinkage and clogging by coating. This should be avoided as much as possible because the phenomenon may lead to fouling. In general, the pure water permeability of the coated membranes can be controlled by varying modification conditions such as concentration of coating solution and coating time duration.  Fang, X., Li, J., Li, X., Pan, S., Zhang, X., Sun, X., Shen, J., Han, W., Wang, L., 2017.