Pick up and dispose of pollutants from water via temperature-responsive micellar copolymers on magnetite nanorobots

The study addresses the need for efficient, environmentally friendly, and reusable nano/micromotors for water remediation, focusing on the pickup and disposal of toxic pollutants such as arsenic ions and the pesticide atrazine. Conventional magnetic nano/micromotors often rely on noble-metal catalysts and chemical fuels (e.g., H2O2), which limit stability and lifespan in aqueous environments. The research proposes polymer-modified, fuel-free magnetic nanorobots that leverage temperature-responsive behavior to reversibly capture and release contaminants. The purpose is to combine precise magnetic actuation with thermoresponsive micellar copolymers to enhance adsorption via dynamic motion and to enable on-demand release by temperature cues, thereby improving efficiency, reusability, and practicality for water treatment.

Prior work demonstrates diverse magnetically or light-driven micromotors for environmental remediation, including degradation of organic pollutants and removal of heavy metals, oils, and dyes. Many systems employ noble-metal catalysts (Pt, Au) and/or require chemical fuels, which can oxidize and reduce lifespan, complicate fabrication, and raise environmental concerns. Polymer-based micromotors have been explored (e.g., poly(amino acid) micromotors for heavy metals; PEDOT:PSS-based systems for selective pickup), but often involve complex fabrication and limited mechanical robustness and reusability. Magnetic micromotors have shown promise due to accurate, fuel-free actuation and long-lasting motion. However, reusable nanomotors typically require harsh desorption media (acids or organic solvents), which can damage active surfaces. Thermoresponsive block copolymers (e.g., Pluronic-type PEO–PPO–PEO) exhibit reversible micellization with temperature, offering a route to dynamic capture/release without harsh chemicals. This work builds on these insights by integrating PTBC with Fe₃O₄ to provide temperature-controlled aggregation/separation for pollutant pickup/disposal under magnetic actuation.

Synthesis: Fe₃O₄ nanoparticles were prepared via coprecipitation by adding 3 mL of 5 M NH₄OH dropwise into an equimolar solution of FeCl₃·6H₂O (15 mL) and FeCl₂·4H₂O (7.5 mL) under stirring, followed by addition of 100 mg oleic acid and heating at 80 °C for 5 h. Oleic-acid-functionalized Fe₃O₄ nanoparticles were magnetically separated, washed, and redispersed. They were then mixed with 0.1 g PTBC and stirred overnight to afford PTBC-coated Fe₃O₄ (TM nanorobots), subsequently washed by magnetic separation. Characterization: FTIR confirmed oleic acid and PTBC functionalization (e.g., –CH₂ stretches at 2933 and 2834 cm⁻¹; COO⁻ stretches at 1439 and 1531 cm⁻¹; Fe₃O₄ band at ~565 cm⁻¹; PTBC C–O stretches 1000–1200 cm⁻¹). XRD (2θ 20–70°, step 0.039°) showed Fe₃O₄ crystalline peaks ((220), (311), (222), (400), (511), (440), (533)) retained after PTBC coating with reduced intensity. TEM revealed nonregular hexagonal particles ~200 ± 25 nm with a visible PTBC shell; EDS mapping confirmed Fe, C, O. STEM/EDS at 25 °C showed aggregation; at 5 °C more dispersion. Zeta potential shifted from −7.178 mV (oleic-acid Fe₃O₄) to −4.445 mV after PTBC coating. Motion evaluation: A transversal rotating magnetic field (triaxial coils) of 3 mT amplitude and 0.5–4 Hz frequency was applied; motion recorded by inverted optical microscope (Olympus CKX53) with high-speed cameras. Velocity was quantified for 30 nanorobots across frequencies; optimal speed near 3 Hz. Pollutant removal protocol: For adsorption isotherms, varying TM nanorobot dosages (3.1–18.6 mg) were added to 2 mL solutions of arsenic (5 mg L⁻¹) or atrazine (5 mg L⁻¹) pre-cooled to 5 °C. A transversal rotating magnetic field (3 mT, 3 Hz) was applied for 10 min at ambient temperature (~25 °C) to promote intermicellar aggregation and pickup. TM nanorobots were magnetically separated; supernatants analyzed by UV–vis. Pickup capacity qe = (C0 − Ce)V/m; pickup efficiency = (C0 − Ce)/C0 × 100. Time studies varied motion duration 10–100 min at optimized dosage (18.6 mg). Disposal (desorption) protocol: Loaded TM nanorobots were placed in 2 mL ultrapure water and cooled below 5 °C to expand PTBC and release pollutants. Disposal capacity qae = C1V/m; disposal efficiency = qae/qe × 100. After disposal, nanorobots were magnetically recovered, rinsed, and reused for up to 10 cycles. Low-level arsenic (≥25 µg L⁻¹) was quantified by ICP-OES. Additional tests: High-concentration pollutant tests (100 mg L⁻¹) at 18.6 mg and 93 mg TM dosages; ionic strength effects probed with NaCl (1–100 mM); real-water matrices (tap and filtered/diluted river water) and complex samples spiked with 1 µM rhodamine B plus pollutants. Instrumentation included FTIR (Nicolet 6700), XRD (PANalytical X’Pert PRO), UV–vis (Shimadzu UV-2450), ICP-OES (Spectro Arcos), TEM (Jeol 2200 FS).

• Thermoresponsive behavior: PTBC-coated Fe₃O₄ nanorobots disperse at ~5 °C and aggregate at ~25 °C, enabling temperature-controlled pickup (aggregation) and disposal (dispersion) of pollutants.
• Magnetic propulsion: Under 3 mT rotating fields, velocity increased with frequency up to ~3 Hz, reaching 6.21 µm s⁻¹ (1.78 µm s⁻¹ at 0.5 Hz); above ~4 Hz move-out reduced synchrony and speed.
• Arsenic removal (5 mg L⁻¹, 2 mL): After 10 min at 3 Hz, 3 mT, pickup increased with dosage from 15.2% (3.1 mg) to 59.7% (18.6 mg). Disposal efficiency after cooling reached 44.4%. Control (no magnetic field) pickup was 26.8% at high dosage. Extending motion time to 100 min increased pickup to 65.2%. Reuse over 10 cycles showed pickup decreasing from 65.2% to 38% and disposal from 48% to 31%.
• Atrazine removal (5 mg L⁻¹, 2 mL): After 10 min, pickup rose from 14.2% (3.1 mg) to 53.8% (18.6 mg); control pickup 32.2% at high dosage. Disposal after cooling reached 34.6%. Extending to 100 min yielded 61.5% pickup. Over 10 reuse cycles, pickup decreased from 61.5% to 31.8% and disposal from 38.4% to 28%.
• Low-level arsenic (25 µg L⁻¹): Post-treatment supernatant arsenic was below ICP-OES detection, indicating near-complete removal in this regime.
• Capacities (Langmuir fits, close to experimental): At 25 °C, maximum pickup capacities were 3.4 mg g⁻¹ (arsenic) and 3.3 mg g⁻¹ (atrazine). At 5 °C, disposal capacities were 2.3 mg g⁻¹ (arsenic) and 2.0 mg g⁻¹ (atrazine).
• High concentration tests (100 mg L⁻¹): With 18.6 mg TM nanorobots, pickup efficiencies were 2.68% (arsenic) and 2.10% (atrazine). At 93 mg dosage, pickup increased to 11.13% and 10.21%, respectively.
• Real-water matrices: Pickup in tap water reached ~73% (arsenic) and 72% (atrazine); in river water, 65% and 63%, respectively, compared to ultrapure water. In presence of 1 µM rhodamine B dye, pickup decreased markedly (<10% for arsenic, ~15% for atrazine).
• Ionic strength: Increased NaCl (up to 100 mM) reduced nanorobot motion and substantially decreased arsenic pickup, though aggregation behavior persisted.
• Surface properties: Zeta potential shifted from −7.178 mV (oleic-acid Fe₃O₄) to −4.445 mV after PTBC, supporting interactions with positively charged contaminants. TEM after 10 cycles showed an intact PTBC protective film, indicating corrosion resistance.

The study demonstrates that integrating a thermoresponsive Pluronic triblock copolymer with magnetite nanoparticles yields nanorobots that can both actively enhance adsorption via magnetic propulsion and enable reversible capture/release of pollutants by modest temperature changes. The magnetic actuation facilitates microconvection and improves collision frequency between contaminants and the adsorbing PTBC matrix, evidenced by higher pickup under motion versus static controls. The PTBC shell undergoes dehydration-driven intermicellar aggregation above its CMT, trapping arsenic ions and atrazine; cooling re-expands the shell to release the payload, allowing straightforward recovery and reuse without harsh desorption chemicals. These results address the practical limitations of prior nanomotor systems (fuel/catalyst dependence, poor reusability) by offering a biocompatible, fuel-free, and recyclable platform. Performance in real-water matrices indicates robustness but also highlights competition effects from co-solutes and dyes. The observed decline over cycles and sensitivity to ionic strength suggest further optimization of polymer properties (molecular weight, branching, functional groups) could improve charge density, binding affinity, and durability. Overall, the approach is relevant for targeted water treatment applications where temperature control and magnetic fields can be implemented to concentrate, remove, and regenerate sorbents efficiently.

This work introduces facilely fabricated, biocompatible, thermosensitive magnetic nanorobots (PTBC-coated Fe₃O₄) that efficiently pick up and dispose of arsenic ions and atrazine by toggling temperature between ~25 °C and ~5 °C under magnetic actuation. The system achieves significant pickup enhancements relative to static controls, allows repeatable release and recovery over multiple cycles, and shows applicability in real-water samples. The nanorobots retain Fe₃O₄ crystallinity and stability, and exhibit controllable motion with optimal speeds at ~3 Hz. The platform can be generalized by tailoring surface functional groups and PTBC molecular characteristics to target different contaminants and improve performance. Future work should optimize polymer architecture (higher molecular weight, branching), expand functionalization (e.g., OH, NH₂, COOH) for selectivity, assess long-term mechanical/chemical stability and fouling resistance, and scale up deployment strategies (e.g., magnetic capture modules) in water-treatment systems.

• Pickup efficiencies and capacities, while significant, are modest in mg g⁻¹ terms and decrease over repeated cycles (pickup and disposal decline after 10 cycles), indicating gradual performance loss.
• Performance is sensitive to ionic strength; higher salinity reduces propulsion and arsenic pickup, potentially limiting efficacy in saline or high–ionic strength waters.
• Presence of co-contaminants (e.g., rhodamine B dye) markedly suppresses pickup due to site competition and matrix effects.
• Some aggregation persists even at low temperature, and complete separation may not be achieved in all conditions.
• Disposal efficiencies are lower than pickup and do not fully recover adsorbed loads, leading to gradual capacity loss.
• Reported optimal conditions use controlled laboratory temperatures and magnetic fields; practical implementation will require system-level engineering for temperature control and magnetic actuation in flow environments.