## 1 Introduction  
Through billions of years of evolution, creatures in nature have developed various remarkable functions for adapting themselves to the environment. Green plants, for instance, have developed photocatalysis, in order to utilize light energy to survive and thrive. _Ruellia devosiana_ (a tropical plant) has developed a successful method of reducing fouling caused by organisms through the use of leaves with a superhydrophilic surface [1]. Inspired by these natural functions, advanced materials and processes have been developed to improve human life. Mussels can attach themselves to almost any surface type such as rocks, pilings, aquatic plants, and floating debris, by the help of strong adhesive byssal (or byssus) threads. This adhesive property is attributed to a special chemical, dopamine [2]. Inspired by this function, dopamine was utilized as a versatile coating material, and it has been proved that it can self-polymerize on most surfaces of materials in base conditions [2]. Recently, it was demonstrated that dopamine polymerizes on nanoparticles [3; 4; 5; 6; 7], which allows nanoparticles to connect with bulk materials. This versatile connection may have a great potential for environmental applications, such as the immobilization of photocatalysts in photo-reactors, or the coating of photocatalysts on substrates to reduce fouling in air or water. Among photocatalytic materials, titanium dioxide (TiO\({}_{2}\)) is of interest for solar energy conversion and environmental applications, because of its superhydrophilic and photocatalytic properties. The combination of these two properties yields a material which has a good self-cleaning function on the surfaces of TiO\({}_{2}\). Similar to its function in _Ruellia devosiana_, the superhydrophilicity of TiO\({}_{2}\) is capable of letting water spread over the surfaces, forming an ultrathin water film which can significantly prevent pollutants from attaching. When illuminated by UV (ultraviolet) light, TiO\({}_{2}\) degrades organic pollutants through photocatalytic reactions, and keeps the surface clean. This self-cleaning function enables TiO\({}_{2}\) to be used in a variety of environmental applications [8]. In membrane filtration, the incorporation of TiO\({}_{2}\) nanoparticles into membrane matrixes or surfaces has received significant attention in recent years, and has been shown to improve the membranes' anti-fouling performance [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. Several strategies for integrating TiO\({}_{2}\) into TFC membranes have been reported, including self-assembly [16; 21; 22; 23], entrapment [24], and chemical binding [25; 26]. For the entrapment method, the entirely enfolded TiO\({}_{2}\) nanoparticles do not yield the same performance of high hydrophilicity and good photochemical reactivity as the exposed bare nanoparticles, because of the shelter provided by the polymer matrix [24]. Furthermore, the higher concentration of TiO\({}_{2}\) nanoparticles inside the thin film may decrease the rejection of TFC membranes [24]. The chemical binding method can also improve the anti-fouling performance of the membranes, but this method can only be applied to certain kinds of TFC membranes, and it modifies the structure of the membrane surface [25]. When compared with these methods, the self-assembly method yields the best anti-fouling or self-cleaning performance of TiO\({}_{2}\), as the bare nanoparticles can be exposed to the environment. However, the main drawback of this method is the weak binding force between nanoparticles and membrane surfaces, which greatly limits the application of this modification method [27]. In order to overcome this limitation, mussel-inspired polydopamine has been used and has been reported to successfully bind TiO\({}_{2}\) nanoparticles on the membrane surface [28]. Additionally, this dopamine-modified TiO\({}_{2}\) is reported to enable adsorption in the visible light spectrum, and can be excited by visible light energies below 3.2 eV (\(\sim\)387.5 nm) [29].