Nitrogen and sulfur doped carbon dots coupled cellulose nanofibers: A surface functionalized nanocellulose membranes for air filtration

The current study demonstrates a process to produce a hybrid bio-organic cellulose nanofiber (CNF) membrane with high filtration performance and mechanical strength. This was accomplished by 1) tailoring the functional groups of CNFs, notably using the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical oxidation and 2) coupling them with carbon dots (CDs). The surface chemistry of CNF was modified through TEMPO oxidation. Subsequently, a coupling reaction was conducted with CDs synthesized hydrothermally, as well as synthesized N/S-CDs derived from palm bunch. The impact of these modifications on the formation of air filters was investigated. Modification and coupling of surface modified CNF were evaluated by advanced analytical techniques. The toxicity of pristine and N/S-CDs coupled cellulose nanofibers was studied by cytotoxicity assay. Vacuum filtration and sequential compression molding techniques were applied to fabricate CNF filters, and the particle filtration efficiency of different filters was determined. The coupling of N/S-CDs to CNF significantly enhanced the tensile strength of the membranes. Coupling of N/S-CDs to TEMPOCNF membrane showed the highest tensile strength (9.3 ± 1.9 MPa), while N/S-CDs coupling to CNF membrane exhibited slightly lower tensile strength (9.0 ± 1.3 MPa). The average pore diameter of the filters was slightly reduced after the CNF surface modification, which led to higher differential pressure across the filters. Both the pristine and TEMPO-modified CNF filters displayed high filtration efficiency for 0.3 μm-aerosol particles (∼70–81 %) and 0.1 μm-aerosol model particles (∼50–68 %), which was in a range of bacteria and viruses. This study provided detailed information about the fabrication of modified CNF filters with high air filtration efficiency for micro/nano-sized particles using cost-effective biodegradable raw materials.