Magnetically boosted 1D photoactive micro-swarm for COVID-19 face mask disruption

The pandemic has dramatically increased the consumption and mismanagement of plastic-based personal protective equipment, especially polypropylene (PP) face masks, generating large quantities of microplastics with potential ecological and human health impacts. Conventional degradation of PP in natural environments is extremely slow, and existing treatment capacities lag behind the growing waste stream. The study aims to develop and demonstrate a magnetically actuated, photoactive 1D microswarm that can actively navigate within fibrous PP mask membranes, adhere to their surfaces, and accelerate photo-oxidative degradation under visible light. By combining Fe3O4-based magnetic nanochains with a Bi2O3/Ag photocatalytic shell and positive surface charge, the authors hypothesize that magnetic boosting will enhance photocatalyst coverage and interfacial contact, thereby increasing the efficiency of PP degradation compared to passive photocatalyst exposure.

Micro/nanorobots have shown promise in therapeutics, biofilm eradication, environmental remediation, and microplastic treatment. Light-driven micromotors enable simultaneous propulsion and photochemistry but often suffer from slow speeds, limited directional control, and reliance on toxic fuels like H2O2. Hybrid magnetic-photoactive designs can provide fuel-free, robust actuation with precise 2D/3D control and enhanced catalytic efficiency. Prior work demonstrated benefits for oral biofilm destruction, degradation of nerve agents, and removal of organic pollutants. However, magnetically powered photoactive microswarms specifically targeting solid plastic waste disruption, particularly fibrous PP membranes from commercial masks, remain underexplored. This study builds on visible-light photocatalysts such as Bi2O3 (narrow bandgap ~2.5–2.8 eV) and their heterostructures with Ag to enhance charge separation and light absorption, addressing gaps in efficient, guided photocatalytic degradation within complex 3D microfiber networks.

Synthesis and structure: Fe3O4 nanoparticles (NPs) were synthesized solvothermally from FeCl3·6H2O, trisodium citrate, and sodium acetate in ethylene glycol (200 °C, 10 h). 1D Fe3O4 nanochains (NCs) were assembled via a magnetic field-assisted sol-gel process: partial TEOS hydrolysis produced a SiO2 layer, particles were aligned under a static magnetic field, then overcoated by SiO2 to yield free-standing 1D NCs with a smooth silica shell (~50.2 ± 7.7 nm). An urchin-like Bi2O3 shell was grown solvothermally on NCs (Bi(NO3)3·5H2O in EG/EtOH, 160 °C, 5 h), followed by ~20 nm Ag nanoparticle deposition via chemical reduction (AgNO3, trisodium citrate; NaBH4) and PDDA functionalization to render a positive surface charge. Final dimensions: average chain length 6.9 ± 1.4 µm and width 0.56 ± 0.07 µm. Characterization: SEM/TEM/HRTEM and EDX for morphology and composition; XRD for phase identification (Fe3O4, β-Bi2O3, Ag); BET N2 adsorption for surface area and porosity; UV-Vis spectroscopy and Tauc plot for bandgap (~2.55 eV); zeta potential for surface charge (PP debris −8.0 ± 0.6 mV; Fe3O4 NCs −29.7 ± 0.4 mV; Fe3O4/Bi2O3/Ag −7.8 ± 0.7 mV; after PDDA +47.7 ± 1.0 mV). Magnetic actuation and motion analysis: A triaxial electromagnetic coil generated transversal rotating magnetic fields (5 mT typical) with tunable frequency and plane angle. 1D microrobots performed vertical tumbling propulsion synchronized with field frequency. Velocities were measured from microscopy videos. Manual steering and automated navigation modes (perpendicular and random trajectories) were demonstrated, including collective swarming without aggregation due to superparamagnetic Fe3O4 and colloidal stability. Adhesion quantification on PP fibers: Rectangular PP membrane pieces (3 × 4 mm) from commercial FFP-2 masks were soaked in microrobot dispersion (0.2 mg/mL). One group underwent magnetic swarming (5 mT, 3 Hz, 30 min, random mode); a static control was left undisturbed. Hyperspectral dark-field microscopy (HDFM) with spectral mapping (particle filter thresholds: microrobots peak 650 ± 50 nm, intensity >5000; PP network peak 600 ± 10 nm, intensity >1000) quantified coverage via pixel count ratio of microrobot to microfiber pixels. Photodegradation protocol and analyses: After the swarming step or control treatment, samples were irradiated under a 300 W Ultra-Vitalux lamp at 30 cm for 30 h (UV-visible solar-like). Post-irradiation analyses included SEM for morphology; BET for surface area and pore volume; FTIR-ATR for chemical changes and carbonyl index (area 1850–1650 cm−1 normalized by 1500–1420 cm−1); TGA/DTG for thermal stability; SEM-EDX for elemental composition (oxygen content). Reaction byproducts in the liquid phase were profiled by SPME-GC-MS. Cytotoxicity of reaction solutions was evaluated using resazurin assays on HeLa cells at various concentrations (2 h and 72 h exposures). Magnetic collection of treated PP fragments and microdebris was demonstrated using an external magnet.

• Microrobot properties: Hierarchical Fe3O4/Bi2O3/Ag 1D NCs with high surface area (BET: Fe3O4 6.5 m2/g; Fe3O4/Bi2O3 40.7 m2/g; Fe3O4/Bi2O3/Ag 58.8 m2/g) and mesoporosity; visible-light responsive bandgap ~2.55 eV; strong positive zeta potential after PDDA (+47.7 ± 1.0 mV) opposing PP’s negative charge (−8.0 ± 0.6 mV).
• Motion performance: Tumbling propulsion synchronized with field frequency; linear velocity increase up to step-out at 5 Hz; maximum average velocity 13.2 µm/s at 5 Hz (5 mT).
• Adhesion enhancement: After 30 min swarming (5 mT, 3 Hz), HDFM spectral mapping showed pixel count ratio of adhered microrobots to microfiber area 0.207 ± 0.058 versus 0.008 ± 0.003 for static controls, a ~26-fold increase, indicating substantially improved coverage via active swarming.
• Morphological degradation: Following 30 h light irradiation, swarmed samples exhibited widespread cracks and cavities (~100–500 nm) across PP fibers, unlike smooth surfaces in controls.
• Surface area and porosity of PP membranes: BET surface area increased 1.9× from 6.7 ± 0.4 to 12.8 ± 1.6 m2/g; total pore volume increased 1.8× from 0.01110 to 0.02025 cc/g after swarming + light, consistent with extensive surface damage.
• Chemical oxidation: FTIR-ATR revealed new carbonyl (~1712 cm−1) and hydroxyl/hydroperoxyl (~3350 cm−1) bands in swarmed samples. Carbonyl index increased ~26-fold versus non-swarms under the same illumination, indicating strong photo-oxidation.
• Thermal stability: TGA showed 10% weight loss temperature decreased by ~20 °C (from ~440 °C control to ~421 °C swarmed), with DTG shoulder at lower temperature, evidencing reduced polymer stability due to chain scission.
• Elemental composition: SEM-EDX oxygen content increased from 6.6 ± 0.6 wt% (control) to 13.4 ± 1.0 wt% (swarmed), ~2.04× higher, aligning with oxidation indicators.
• Byproducts and toxicity: SPME-GC-MS detected volatile/semivolatile organic byproducts (e.g., m/z 41, 43, 44, 55 species such as acetaldehyde). In vitro resazurin assays in HeLa cells indicated negligible cytotoxicity across tested conditions.
• Practicality: Demonstrated magnetic collection of treated PP membranes and microdebris for potential integrated removal and degradation workflows.

The study demonstrates that magnetically actuated 1D microrobot swarms can overcome mass transport and contact limitations in fibrous PP mask membranes by actively navigating through 3D networks and maintaining prolonged interfacial contact. The positive surface charge of PDDA-functionalized microrobots enhances electrostatic adhesion to negatively charged PP fibers. The Bi2O3/Ag photocatalytic shell provides visible-light activity and increased surface area, supporting efficient generation of oxidative species at the interface. Quantitative metrics—26-fold higher adhesion, 1.9× surface area and 1.8× pore volume increases of PP after irradiation, ~26-fold higher carbonyl index, lower TGA decomposition onset, and doubled oxygen content—collectively corroborate accelerated photo-oxidative degradation compared to passive exposure. These results address the research goal of enhancing degradation of pandemic-related PP waste and highlight advantages of magnetic-photoactive hybrids: fuel-free actuation, precise 3D control in occluded microstructures, improved photocatalyst coverage, and downstream magnetic retrieval. The negligible observed cytotoxicity of reaction solutions and feasibility of magnetic collection further support potential environmental remediation applications.

This work introduces a hierarchically structured Fe3O4/Bi2O3/Ag 1D microrobot capable of magnetically guided swarming and visible-light photocatalysis to disrupt PP microfibers from commercial COVID-19 masks. The programmed rotating magnetic fields create fish-schooling-like collective behavior that markedly increases adhesion to PP fibers, enabling efficient interfacial photo-oxidation. After 30 h illumination, significant morphological, chemical, and thermal indicators of degradation are achieved relative to non-swarming controls. The approach integrates remote manipulation, enhanced photocatalysis, and magnetic collection, suggesting a promising pathway for treating fibrous plastic wastes. Future research should focus on improving catalytic efficiency toward deeper depolymerization, scaling magnetic actuation systems, and reducing materials cost to enable practical, high-throughput applications.

Complete degradation of solid plastics remains challenging; photocatalytic depolymerization is limited and typically requires tens to hundreds of hours. The need for specialized magnetic actuation hardware may hinder scalability, and the materials and synthesis costs for microrobots require reduction for large-volume treatment. Further optimization of photocatalytic materials, swarm dynamics, reactor design, and process integration is needed to enhance efficiency and scalability.