In this work, poly(ethersulfone) (PES) ultrafiltration (UF) hollow fibers (HF) were modified by introducing TiO 2 nanoparticles (TiO 2 -NPs) in the polymeric dope, to endow them with photocatalytic properties. Different dope compositions and spinning conditions for producing "blank" PES UF fibers with suitable properties were investigated. PEO-PPO-PEO (Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol, Pluronic ® (Sigma-Aldrich, Milan, Italy) was finally selected as

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... he additive and a suitable dope composition was identified. After the detection of an appropriate dope composition and the optimization of the spinning parameters, PES-TiO 2 HF was produced. The optimized composition was employed for preparing the mixed matrix HF loaded with TiO 2 NPs. The effect of different TiO 2 NP (0.3-1 wt %) concentrations and bore fluid compositions on the fiber morphology and properties were explored. The morphology of the produced fibers was analyzed by Scanning Electron Microscopy (SEM). Fibers were further characterized by measuring: pore size diameters and thickness, porosity, and pure water permeability (PWP). The photocatalytic activity of the new membranes was also tested by UV light irradiation. The model "foulant" methylene blue (MB) was used in order to prove the efficiency of the novel UF membrane for dye photo-degradation. confined within the walls of a laboratory, membrane applications have become widespread in industry, and have even started to enter into our family homes; for example, with the growing popularity of home water purification systems. Especially concerning water treatment, membrane based pressure driven operations (i.e., micro-, ultra-, nanofiltration, and reverse osmosis), together with membrane reactors, contactors, and integrated membrane systems, are generally acknowledged as suitable, reliable, sustainable, and cost-effective alternatives to traditional filtration, purification, and desalination procedures [1] [2] [3] . The heart of each process is the membrane itself, along with its performance, often expressed in terms of flux and selectivity. Other aspects that are equally important concern the chemical and mechanical resistance and, above all, the susceptibility to fouling. Fouling and, especially, biofouling, is a major drawback of any membrane process; indeed, the latter is often referred as its "Achilles' heel" [4] . It impairs membrane performance (flux decline) and forces us to stop processes to carry out membrane cleaning or module replacement, leading to discontinuity in operation, reduction of the membrane lifetime, and to the increase of costs. Although this problem cannot be completely eliminated, a set of strategies can significantly reduce its effects, moving the balance needle towards the pros, rather than cons, of membrane processes. Different approaches are currently under investigation, from research on new membrane materials and testing of innovative preparation techniques, to the optimization of module and plant configuration. Among all the possible strategies, tailoring the membrane material and properties is a fundamental basis of any process, which might be further improved. Furthermore, as recently pointed by Jhaveri and Murthy [5], irreversible fouling, which leads to permanent flux decline, can only be reduced by modification of the membrane preparation techniques. Several studies have highlighted the possibility of drastically reducing membrane susceptibility to fouling by modifying well-known conventional polymers (i.e., PES, PSf, PVDF, and PVC) using nanoparticles (NPs) or the composites of nanoparticles [5] . Both mixed matrix and thin-film nanocomposite membranes can be prepared. In the first case, NPs are dispersed into the polymer casting solution before the phase inversion step; in the second case, NPs self-assemble onto the membrane surface via dip-coating or other methods. Different nano-sized materials can be used for this purpose, such as metal and metal-oxide NPs, zeolites, carbon-based nanomaterials (nanotubes, graphene, graphene oxide), or even combined hybrid nanomaterials (GO-SiO 2 , GO-TiO 2 ) [6-12]. Even though nanoparticle composite membranes have recently shown promising results [13], titanium dioxide (TiO 2 ) still retains much attention due to its outstanding photo-catalytic and antibacterial properties that are coupled to stability, high hydrophilicity, non-toxicity, biocompatibility, and low cost [14] . Concerning polymeric membrane materials, the sulfone family, and, in particular, polyethersulfone (PES), is among the leading choices for the preparation of both flat sheets and hollow fibers. PES has the highest hydrophilicity, due to its highest percentage of sulfone groups, together with good stability and resistance, ease of processing, and compatibility with several additives. Indeed, PES was widely used in the past for the preparation of membranes for several purposes [15] [16] [17] [18] [19] [20] ; it is still under investigation for the preparation of polymer/TiO 2 nanocomposite membranes. Both the incorporation of NPs in the membrane matrix and coating on the membrane surface were investigated, and in general resulted in increased hydrophilicity, sustained photocatalytic activity, and improved resistance to fouling. The most relevant results of the recent works on PES/TiO 2 membrane preparation are shown in Table 1 ; papers are divided in two categories, depending on the strategy employed for NP immobilization (incorporation in the membrane matrix vs. deposition onto its surface).