Citric Acid Cross-Linked Nanocellulose-Based Paper for Size-Exclusion Nanofiltration

This article explores the effect of cross-linking of nanocellulose with citric acid for the development of novel paper filters for potential application within nanofiltration, including sterile (virus) filtration. Cladophora cellulose paper sheets were cross-linked by first soaking in 16 wt % citric acid in the presence of 1 wt % sodium hypophosphate overnight and then curing at 160 °C for 10 min in a hot-press. The cross-linked paper filter samples were then characterized with FTIR, AFM, N₂ gas adsorption, and tensile strength analysis (dry and wet strength). The particle retention properties were further studied with respect to filtering of 20 nm Au nanoparticles with SEM and comparing the UV absorbance intensity of the starting solution and the filtrate. The wet strength of the paper filter was greatly improved following the cross-linking, although in the dry state, the paper becomes brittle. The improved wet strength of the paper filter enables increasing the pressure gradient applied for filtration without compromising the integrity of the filter. This is the first report in which a fully nature-derived paper filter is capable of removing tracer particles as small as 20 nm. It is concluded that citric acid cross-linking of nanocellulose is beneficial for developing paper based sterile (virus) removal industrial filters.