Fabrication of sodium alginate nanofibrous membranes with bead-on-string structure for effective air filtration

Airborne dust particles carrying pathogens threaten public health, driving demand for eco-friendly air filters. While sodium alginate (SA) offers sustainability advantages over petroleum-based nanofibers, its conventional smooth, densely packed membranes suffer from limited performance. Inspired by biological micro/nanostructures, we engineered bead-on-string SA nanofibers via green electrospinning. Rheological tests and SEM characterization revealed the mechanism of bead-on-string formation. The morphology transformation of SA fibers from smooth to bead-on-string structures was systematically investigated by adjusting the solute concentration of SA spinning solutions (2.4-3.0 wt%) and the volume ratio of ethanol (0-15 wt%) as a green co-solvent. Dense 3.0 wt% membranes achieved >99 % efficiency but suffered high resistance (330 Pa). Reducing concentration introduced bead-on-string structures, enlarging pores and lowering efficiency marginally while drastically reducing pressure drop. At the optimal 2.6 wt%, bead-on-string morphology minimized resistance (~100 Pa, 67 % reduction vs. 3.0 wt%) and delivered the highest quality factor (QF = 0.0284 Pa^-1), demonstrating the optimal balance of efficiency and low airflow resistance. This work demonstrates a sustainable route to high-performance air purification materials.