Microenvironment‑engineered biocatalytic metal–organic framework nanomotors for selective and transformative water decontamination

As the demand for efficient water remediation technologies continues to grow, the limitations of conventional catalysts in terms of energy consumption, selectivity, and carbon emissions become more pronounced. Now, researchers from The University of New South Wales, in collaboration with South China Normal University, Harbin Institute of Technology, and University of Science and Technology of China, led by Professor Kang Liang, have presented a comprehensive strategy for developing biocatalytic nanomotors with tunable microenvironments for active and selective water decontamination. This work offers valuable insights into the development of next-generation nanomotor systems that can overcome these limitations while enabling transformative pollutant removal.

Why Biocatalytic Nanomotors Matter

· Energy Efficiency: Biocatalytic nanomotors operate at low chemical fuel levels without external agitation, addressing the "energy-intensive" barrier in conventional advanced oxidation processes.

· Selective Decontamination: By engineering the microenvironment with tailored surface charge and nanoconfinement, the system achieves charge-based selective enrichment of target pollutants, overcoming the "selectivity challenge" in mixed contaminant scenarios.

· Transformative Remediation: Mimicking natural enzyme pathways, the nanomotors transform toxic phenolic pollutants into recoverable polymeric products rather than mineralizing them, enabling chemical energy recovery with lower carbon footprint.

Innovative Design and Features

· Dual-Enzyme System: The nanomotors encapsulate catalase and horseradish peroxidase within ZIF-8 metal-organic frameworks. Catalase provides jet-like bubble propulsion while maintaining safe hydrogen peroxide levels, and peroxidase catalyzes pollutant oxidation without direct competition for fuel.

· Microenvironment Engineering: A synergistic etching and surface engineering strategy using tannic acid creates tailored microenvironments with optimized surface charge and hierarchical porosity, transforming dense MOF structures into yolk-shell architectures.

· Biomimetic Inspiration: The design draws inspiration from bombardier beetles, which utilize peroxidase and catalase enzymes with hydrogen peroxide as fuel to generate propulsion while oxidizing hydroquinone for defense.

Enhanced Catalytic Performance and Selectivity

· Exceptional Propulsion: The TA-engineered nanomotors achieve a maximum velocity of 1113 ± 12 μm s⁻¹ at 0.3% hydrogen peroxide, significantly outperforming pristine enzyme@ZIF-8 nanomotors (523 ± 18 μm s⁻¹) and most previously reported micro/nanomotors.

· Charge-Selective Removal: Surface engineering with tannic acid reverses the MOF surface charge from positive to negative, enabling selective preconcentration of cationic methylene blue (reaction rate: 0.39 μM s⁻¹) while repelling anionic methyl orange, achieving a separation factor of approximately 73 within 5 minutes.

· Nanoconfinement Effects: Etching-induced voids facilitate rapid mass transfer to enzyme active sites, with finite element simulations confirming that hollow structures shorten diffusion pathways and allow target molecules to accumulate in the cavity around enzymes.

Applications in Emerging Contaminant Remediation

· Bisphenol A Removal: The biocatalytic nanomotors achieve ~98% bisphenol A removal within 2 minutes through enzyme-mediated polymerization into recoverable oligomers (dimers, trimers, tetramers), validated by UHPLC-MS/MS analysis.

· Environmental Resilience: The system retains over 90% removal efficiency across broad pH range (5–10), in the presence of background ions (Cl⁻, HCO₃⁻) and natural organic matter (up to 10 mg/L), and demonstrates robust performance in real tap water and river water samples.

· Recyclability and Stability: The oxidative nanomotors preserve over 80% initial activity after 10 catalytic cycles, with negligible enzyme leakage (<6%) and maintained structural integrity confirmed by PXRD, SEM, and FTIR analyses.

Mechanistic Insights and Future Outlook

· Polymerization Pathway: UHPLC-MS/MS reveals that peroxidase-catalyzed hydrogen abstraction transforms bisphenol A into phenoxy radicals that couple into high-degree oligomers with increased hydrophobicity, enabling facile separation by filtration.

· Scalable Fabrication: The synthesis is readily scalable with mild, enzyme-compatible conditions, and the optimal TA concentration of 6 g L⁻¹ provides a self-limiting etching environment that preserves enzyme conformation.

· Challenges and Opportunities: The study highlights the need for extending this microenvironment engineering strategy to other enzyme-MOF combinations and exploring applications in continuous flow systems for industrial water treatment. Future research will focus on optimizing the multilevel microenvironment surrounding enzymes on a rational and predictive basis.

This comprehensive study establishes a robust biocatalytic nanomotor platform for selective and transformative water decontamination. It highlights the importance of interdisciplinary research in materials science, biotechnology, and environmental engineering to drive innovation in sustainable water treatment. Stay tuned for more groundbreaking work from Professor Kang Liang at The University of New South Wales and collaborators!