Ultrasonic activation of inert poly(tetrafluoroethylene) enables piezocatalytic generation of reactive oxygen species

Reactive oxygen species (ROS) such as •OH, •O₂⁻ and ¹O₂ are powerful redox agents needed across biological, chemical, and environmental applications. Piezoelectric catalysis uses mechanical vibrations or ultrasound to polarize a material, creating internal electric fields that separate charge carriers and drive redox reactions to generate homogeneous ROS in water. Existing inorganic (e.g., BaTiO₃, ZnO, BiFeO₃) and polymeric (e.g., PVDF) piezoelectric materials often have low piezoelectric coefficients (d33 ≈ 3–105 pC/N), stability issues, or environmental concerns (e.g., PZT). Nonpolar polymer electrets like PTFE can exhibit high apparent piezoelectric responses (reported d33 up to ~600 pC/N) and excellent chemical stability, but conventional PTFE polarization typically requires complex, high-voltage processes. This study aims to develop a simple ultrasonic activation route to induce permanent polarization in inert PTFE particles, evaluate their piezoelectric properties, and demonstrate efficient piezocatalytic ROS generation and its potential applications.

Prior work has established piezocatalysis as an advanced oxidation process where mechanical/ultrasonic excitation enhances charge separation and redox activity, enabling ROS generation for applications including pollutant degradation and sonodynamic therapy. Conventional piezoelectrics (BaTiO₃, ZnO, BiFeO₃) and PVDF show limited piezocatalytic performance due to modest piezoelectric coefficients and stability issues. PTFE and other polymer electrets have shown large apparent piezoelectric coefficients, surpassing PVDF and approaching inorganic ceramics, with PTFE being highly inert and stable. However, PTFE electret formation typically requires high electric fields and complex processing, limiting practical deployment. The literature underscores the need for chemically stable, environmentally benign, high-performance piezocatalysts, motivating exploration of ultrasound-induced PTFE polarization and its catalytic implications.

- Materials and activation: Commercial PTFE particles (1–5 μm) were ultrasonically irradiated to induce electret formation. A PTFE membrane was also subjected to treatments (ultrasound, mechanical stretching, compaction at 25 MPa, electric field ~134 kV/m) for PFM characterization. - Ultrasonic conditions: PFM activation used 60 kHz, 10 W ultrasound. Electrical and catalytic tests employed an ultrasonic cleaner (Branson 3000-CXPH, 40 kHz, 110 W). Activation durations included up to 1 h. - PFM characterization: Piezoresponse force microscopy (conductive AFM with 10 V AC applied to tip) measured amplitude and phase to evaluate piezoelectric response of PTFE before and after treatments; PVDF served as a reference. AFM topography and PFM phase/amplitude maps were collected to identify localized electret domains. - Electrical output: A 10 mm PTFE membrane ultrasonically treated for 1 h was contacted with copper meshes and connected to a digital multimeter (DMM-6500). Mechanical loads (5 g, 100 g, 200 g corresponding to specified pressures) and ultrasound excitation (on/off) were applied and voltage outputs recorded. - ROS detection by ESR: Spin-trapping ESR at ambient temperature detected •OH and superoxide using DMPO; singlet oxygen using DMPO/DMSO; TEMP was used for ¹O₂ detection. To assess oxygen’s role, water was deoxygenated by argon bubbling for 30 min prior to irradiation. Reactions were conducted in 1.5 mL tubes with specified volumes of deionized water, catalysts (PTFE, controls), and spin traps, and sampled after 1–5 min ultrasound. - Catalytic tests: Dye degradation and organic transformation experiments used PTFE powders (e.g., 12.5 mg in 50 mL dye solution in a 4.8 cm diameter beaker; water height ~1 cm). The beaker was positioned to match liquid levels with the ultrasonic bath. Aliquots were taken over time, centrifuged to remove PTFE, and analyzed spectrophotometrically. Additional targets included nitrobenzene and 4-chlorophenol (with HPLC analysis: water:methanol 30:70 v/v, 0.5 mL/min). Comparators included ultrasound alone, ultrasonicated polyethylene (PE), TiO₂, and PVDF particles. - Disinfection tests: A PTFE membrane attached to a beaker’s inner wall was used to treat E. coli suspensions under ultrasound; surviving colonies on agar plates were enumerated before and after 15 min irradiation. Scanning electron microscopy assessed cellular morphology. Electrospun PTFE fibers were also evaluated for in situ disinfection under ultrasound. Through-tissue sonocatalysis was explored using a commercial ultrasound therapy device to compare ROS generation versus BaTiO₃ and sonolysis controls. - Mechanistic analysis: PFM results and ESR signals under aerobic and anaerobic conditions were combined with a proposed reaction scheme in which ultrasonically induced polarization and dynamic stress release surface/space charges that react with water and oxygen to produce •OH, •O₂⁻, ¹O₂, H₂O₂, and related species.

- Ultrasonic activation converts inert PTFE particles/membranes into permanent piezoelectric electrets with markedly enhanced PFM amplitude and phase contrast, indicating formation of electret domains and strong triboelectric properties. - Cavitation-driven high transient pressures (up to ~1,000 kPa) and electric fields (~10 kV/m) during ultrasound are proposed to induce charge injection/trapping and localized noncentrosymmetric regions, producing the electret state. - PFM quantification shows PTFE exhibits much higher piezoresponse than PVDF (reported PTFE amplitude on the order of ~155 pm vs PVDF ~6.5 pm; text also notes large fold increases), consistent with superior piezocatalytic activity. - ESR confirms generation of multiple ROS under ultrasound with PTFE: hydroxyl radicals (DMPO–OH), superoxide (DMPO–OOH/DMPO–O₂−), and singlet oxygen (TEMP–¹O₂). Generation of •OH was also observed under argon, indicating oxygen is not strictly required for all ROS pathways. - Proposed reactions include electron generation at polarized PTFE–water interface, reduction of O₂ to O₂−, water reduction to H and OH−, and secondary formation of H₂O₂ and additional •OH. - Catalytic performance: For methyl orange (representative dye), PTFE achieved 89.7 ± 2.9% removal in 60 min with a pseudo-first-order rate constant k = 2.81 h⁻¹, exceeding ultrasound alone (0.053 h⁻¹), ultrasonicated PE (0.057 h⁻¹), and TiO₂ (0.059 h⁻¹). PVDF particles showed k = 0.175 h⁻¹, significantly lower than PTFE. - Broad reactivity: Near-complete (>90%) transformation was achieved for multiple compounds; ~50% dechlorination observed for 4-CP, reflecting non-selective oxidation by strong ROS. - Disinfection: A PTFE-coated beaker under ultrasound inactivated 99.7% of E. coli in 15 min, while controls and ultrasound-only showed negligible antibacterial activity. SEM indicated substantial cellular damage only in the PTFE + ultrasound condition. - Comparative advantage: Under similar conditions, PTFE piezocatalysis outperformed previously reported piezoelectric catalysts (as summarized in the paper’s Supplementary Table 1).

The study addresses the need for stable, high-performance piezocatalysts by demonstrating that ultrasound can activate inert PTFE into an efficient electret-based piezoelectric material. The induced polarization and charge trapping enable dynamic charge release under ultrasonic stress, producing electrons and holes that react with water and dissolved oxygen to yield •OH, •O₂⁻, ¹O₂, and H₂O₂. The enhanced PFM response of PTFE relative to PVDF correlates with substantially higher catalytic rates in pollutant degradation and strong disinfection efficacy, supporting the hypothesis that larger piezoelectric response drives higher ROS production. ESR evidence under both aerobic and anaerobic conditions clarifies that multiple ROS pathways contribute, some independent of dissolved oxygen. The chemical inertness and stability of PTFE, combined with facile ultrasonic activation and high ROS yields, make PTFE electrets compelling for environmental remediation and biomedical sonodynamic applications. The disinfection demonstration suggests a strategy for in situ ROS generation (e.g., pipe coatings) to reduce reliance on residual chemical disinfectants and mitigate disinfection by-products. Through-tissue tests indicate potential as a biocompatible sonosensitizer for therapy, provided acoustic parameters and catalyst loading are optimized to avoid tissue damage.

Ultrasonic irradiation provides a simple, mild route to induce permanent polarization in inert PTFE particles and membranes, creating piezoelectric electrets that generate multiple reactive oxygen species with efficiencies surpassing conventional piezocatalysts. The work establishes the mechanistic basis for ROS formation via dynamically released charges under acoustic stress and demonstrates broad applicability in pollutant degradation and microbial inactivation, with promise for sonodynamic therapy. Future research should optimize acoustic microenvironments (power, frequency, duty cycle), PTFE morphology (particles, membranes, electrospun fibers), and surface area-to-volume ratios, as well as explore device integration (e.g., pipeline coatings, biomedical implants), long-term stability/cyclability, and safety in biological contexts.

The text indicates that local ultrasonic power must be carefully controlled to avoid damage to biological tissue during through-tissue applications, implying a need to optimize acoustic intensity and exposure. Effective performance also depends on the ratio of PTFE surface area to liquid volume within acoustic microenvironments. Detailed long-term durability, recyclability, and by-product analyses under varied water chemistries were not fully elaborated and warrant further study.