Fabrication of thin-film composite nanofiltration membrane employing polyelectrolyte and metal–organic framework (MOF) via spin-spray-assisted layer-by-layer assembly

Spin-assisted layer-by-layer (LbL) assembly is an innovative method for producing nanostructured thin films, offering enhanced efficiency and precision over traditional dip-coating techniques. This method enables significantly faster deposition and bilayer cycle times while optimizing material usage. In addition, spray LbL systems present advantages in speed and scalability for large-area substrates. By integrating these approaches, spin-spray-assisted LbL assembly allows for rapid assembly and extensive coverage of substrates. In this study, we demonstrated the efficacy of spin-spray LbL assembly in fabricating a thin-film composite nanofiltration (NF) membrane.

In this work, TFC NF consists of multiple layers of polyelectrolyte, and a metal–organic framework (MOF-303) was fabricated to enhance the membrane's biofouling resistance. Polyethyleneimine (PEI) and poly (sodium-4-styrene sulfonate) (PSS) were sprayed alternately and deposited on the top of a spinning polyethersulfone (PSF) ultrafiltration support to construct thin-film composite NF.

The resulting membrane was examined using standard nanofiltration membrane testing, and its performance is comparable to commercial NF. For instance, five bilayers of PEI/PSS NF membrane, i.e., (PEI/PSS)₅, showed a rejection rate of 42.65 ± 0.17 % and a permeability of 9.46 ± 0.14 l/h.bar.m^2, while a commercial NFX membrane from Synder Inc. showed a rejection rate of 53.66 ± 3.23 % and a permeability of 3.51 ± 0.42 l/h.bar.m². We fabricated a PEI/PSS membrane coated with MOF303 as the outermost layer. From our preliminary investigation, (PEI/PSS)5-MOF303 with an MOF concentration of 0.05 wt% exhibited a rejection rate of 18.94 ± 1.58% and a permeability of 0.91 ± 0.13 l/h.bar.m². Although this is still a preliminary result, this work shows that spin- and/or spin-spray-assisted layer-by-layer assembly is a promising method for fabricating membranes for various applications. Some further details, such as surface characteristics, are also provided in this work.