(31d) Electrospun Copper-Based Electrostatic-Catalytic (EC) Nanofiber for Ultrahigh Flow Catalytic Air Sterilizer and Purifier

The Global Burden of Disease study reports that air pollution is the 4th leading cause of death after high blood pressure, smoking and poor diet. According to the World Health Organization, about 95% of the world’s population, more than 7 billion people, live in areas where the air is unhealthy as defined by the WHO. Around 49% of the people in the world, live with indoor air pollution from the use of non-clean energy such as wood, coal, crop waste or animal dung for cooking and heating. This air pollution is prevalent in low- and middle-income areas of Asia and Africa.

Poor indoor air quality affects people’s health, and the young and the elderly are most susceptible to this effect. Air pollutants mainly include nitrogen oxides (NOx), volatile organic compounds (VOCs), suspended particles and bioaerosols. Pollutant concentrations tend to be higher indoors than outdoors. Small particles (i.e., PM2.5) can extend deep into the lungs, causing irritation and damage that can lead to long-term respiratory disease. Bioaerosols can transmit diseases and pathogens, including Covid-19, SARS-CoV-2, and seasonal influenza viruses.

In response, Governments are actively pursuing initiatives to improve air quality and prevent disease. In recent years, there have been a number of innovative air treatment methods developed, such as X-ray enhanced electrostatic fields, cold plasma treatments, microwaves, UV irradiations, and ion emissions technology.

However, filtration technology remains the most effective and economical means for removing particulates and bioaerosols with adsorbents and photocatalytic processes for treating gaseous pollutants.^[9] The high-efficiency particulate air (HEPA) filter has a removal efficiency of 99.97 % for fine particulates (i.e., 0.3 microns) but suffers from high flow resistance owing to its low porosity (< 30 %).^[10,11] In order to maintain a 1-year service life, the airflow across disposable HEPA filters must be kept low to prevent a rapid pressure build-up from particulate deposits. Also, filters are susceptible to microbial colonization, especially in warm and humid weather, such as in Hong Kong. Microbes trapped on the filter can become a source of air recontamination, posing a significant risk of diseases and allergies.

Thus, low-cost and scalable metallized electrostatic-catalytic (EC) nanofiber fabrication method is developed in this work. To create the EC-nanofiber filter, the team developed manufacturing processes for growing oriented 1D structures on surfaces such as the CuO nanorods on a copper wire mesh and copper-metalized nanofibers. These 1D nanostructures increase the fiber’s surface area by ten to a hundred folds, and the sharp tips accumulate up to 30 kV/m of electrostatic charge, thus significantly increasing its efficiency for particulate filtration and capture. Furthermore, the sizeable electrostatic charge can disrupt microbial membrane and viral envelope, resulting in decreased viability. Catalysts on the surface catalyze the oxidation of gaseous pollutants and generate reactive oxygen that damages microbes. Selecting materials with intrinsic antimicrobial properties such as CuO render the surface virucidal, bactericidal, and sporicidal.

The research aim is to deliver a new integrated air treatment technology based on an EC-nanofiber filter. Nano-scale polymer fibers were obtained as the substrate by electrostatic spinning. Then electroless plating was carried out to obtain the metallized fibers. CuO nanorods were successfully obtained on the surface of metal fibers by surface oxidation regrowth method. Finally, a layer of PDDA coating was generated on the CuO nanorods via immersion, making the filter positively charged.

Serial characterization showed changes of fibers in the preparation process and the successful growth of needle-like monoclinic structured CuO nanorods on the final PDDA-CuO NRs/PAN surface. By adding PDDA layer, the negatively charged fiber becomes positively charged. In terms of stability, the PDDA samples were able to withstand temperatures of 280°C without significant mass loss, and had a tensile strength of 459.38 MPa.

The process can combine the metal’s damage to cell membranes and the charge carried by itself with the nanofibers, which can be applied to air filtration to improve the filtration efficiency, bacterial killing rate and VOCs removal rate. PDDA-CuO NRs/PAN has a filter efficiency of more than 95% with particles from 0.3 to 10μm and kills 99% of bacteria and viruses with 10 minutes of contact. However, the ability of fibers to sterilize spores could be improved. For VOCs removal, PDDA-CuO NRs/PAN had longer VOCs adsorption time and higher adsorption capacity with relatively stable adsorption profiles.