Metal oxide single-component light-powered micromotors for photocatalytic degradation of nitroaromatic pollutants

ARTICLE OPEN Metal oxide single-component light-powered micromotors for photocatalytic degradation of nitroaromatic pollutants Xia Peng 1 , Mario Urso 1 and Martin Pumera 1,2,3,4 ✉ Mass transfer is a key parameter in heterogeneous reactions. Micro/nanomachines, a promising technology for environmental applications, significantly enhance the performance of conventional purification treatments because of the active motion ability and thus enhanced diffusion (superdiffusion) of these photocatalysts, which in turn leads to dramatically improved mass transfer and higher degradation capability compared to stationary microparticles. However, the design of micromotors generally involves noble metals, for instance, Au and Pt, to achieve an effective autonomous motion. Considering the expensive fabrication cost and complicated steps, we present Pt-free single-component light-powered WO 3 micromotors capable of enhanced diffusion and effective degradation of nitroaromatic compounds in water. These microswimmers, synthesized by a hydrothermal method, which is highly scalable at low cost, followed by calcination, exhibit fuel-free light-driven motion due to asymmetric light irradiation. Picric acid (PA) and 4-nitrophenol (4-NP) were selected as representative nitroaromatic contaminants and photocatalytically decomposed by WO 3 micromotors thanks to the close contact with the micromotors promoted by their self-propulsion. This work provides a lowcost, sustainable, scalable method for enhancing mass transfer by creating moving catalysts with broad application potential for water cleanup. npj Clean Water (2023) 6:21 ; https://doi.org/10.1038/s41545-023-00235-z INTRODUCTION Nitroaromatic compounds are organic composites that comprise one or more nitro groups (-NO 2 ) connected to the aromatic ring 1 . Taking advantage of these groups, nitroaromatic compounds have been broadly applied to fabricate diverse industrial products, such as explosives, pesticides, and dyes 2 . Unfortunately, the nitro group also hinders the biodegradation of these compounds, resulting in continuous environmental accumulation. Indeed, their extensive employment has led to severe contamination of groundwater and soil, which is usually associated with drinking water quality, severely posing a threat to human health 3,4 . Traditional wastewater purification approaches mainly depend on biological oxidation and physical procedures (i.e., activated carbon adsorption, nano-filtration) 5–7 . The biological process is inadequate as these compounds are barely degraded due to their high constancy in the water system. Physical methods only transfer the pollutants instead of destructive removal, which indicates that further treatments are necessary. Advanced oxidation processes (AOPs) have been extensively explored in degrading organic contaminations in wastewater 8–10 . AOPs generally involve highly reactive oxygen species (ROS), for instance, hydroxyl radical (•OH) and superoxide radical (•O 2- ), which can oxidize hazardous chemical species in water 11 . Photocatalysis is a promising lightdriven AOP technology, necessitating a photocatalyst and proper light irradiation to generate ROS that break down organic pollutants. Recently, nanostructured TiO 2 , ZnO, and Fe 2 O 3 photocatalysts have received great interest 12–17 . For instance, Reddy et al. proposed Cu-doped ZnO nanoparticles for RhB dye degradation under light irradiation 18 . Similarly, Z-scheme binary heterostructured nanocomposites (i.e., MoS 2 /g-C 3 N 4 and ZnWO 4 / NiFe 2 O 4 ) were employed for the efficient photocatalytic degradation of toxic organic pollutants 19,20 . However, these approaches are restricted because of the passive diffusion of photocatalytic materials, requiring constant agitation. Furthermore, most of them involved more than one component. Consequently, it is highly desirable to explore more facile and practical strategies to facilitate the removal of pollutants. Light-powered micromotors, micro/nano-sized materials that exploit energy from a powerful, renewable, and abundant source, such as light, and convert it into motion, have received considerable interest 21–26 . Previous research has shown that the active locomotion of light-powered micromotors based on photocatalytic materials can overcome the limitation of passive diffusions of photocatalysts by enhancing the interactions with targeted pollutants 27–31 . For instance, Ma et al. developed tubular micromotors based on TiO 2 , magnetic Fe 3 O 4 nanoparticles, and Pt for the effective degradation of rhodamine 6G 30 . Moreover, lightpowered ZnO/Pt micromotors with H 2 O 2 -free light-driven propulsion ability and hematite/Pt Janus microrobots were explored for nitroaromatic explosives decomposition 31,32 . However, a noble metal coating was required in both cases to unlock the selfpropulsion ability, increasing fabrication costs and complexity. Therefore, low-cost and simpler micromotors remain to be explored for future practical applications. Here, we demonstrate the photocatalytic degradation of nitroaromatic pollutants by noble metal-free single-component light-powered WO 3 micromotors. Compared to Au-WO 3 @C Janus micromotors proposed by Ren et al. for the photodegradation of dye pollutants 25 , single-component WO 3 micromotors without the need for additional noble-metal coating were prepared by a facile hydrothermal reaction followed by a calcination process. Once exposed to asymmetric light illumination, the micromotors 1 Future Energy and Innovation Laboratory, Central European Institute of Technology, Brno University of Technology, Purkynova 123, 61200 Brno, Czech Republic. 2 Department of Medical Research, China Medical University Hospital, China Medical University, No. 91 Hsueh-Shih Road, TW-40402 Taichung, Taiwan. 3 Faculty of Electrical Engineering and Computer Science, VSB - Technical University of Ostrava, 17. listopadu 2172/15, 70800 Ostrava, Czech Republic. 4 Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea. ✉email: [email protected] www.nature.com/npjcleanwater Published in partnership with King Fahd University of Petroleum & Minerals 1234567890():,; can exhibit fuel-free self-actuation in pure water, with remote control and rapid response over on/off switching of UV-light. Motion behavior in different conditions demonstrated that H 2 O 2 enhanced the motion and diffusion of micromotors. Finally, these WO 3 micromotors were applied for the effective photodegradation of nitroaromatic compounds, among which picric acid (PA) and 4-nitrophenol (4-NP) were selected as models because of their hazardous nature and high stability in water. Such simple and lowcost micromotors capable of degrading toxic substances hold great potential in the micro/nanomotors systems for environmental remediation. RESULTS Characterization of WO 3 micromotors WO 3 micromotors were successfully prepared via the combination of a simple hydrothermal method and a calcination process, as described in Fig. 1a. Specifically, a homogenous solution consisting of the tungsten precursor and glucose was sealed under the hydrothermal condition, followed by an annealing process in air to promote the formation of single-component WO 3 micromotors. Scanning electron microscopy (SEM) images in Fig. 1b illustrate the obtainment of WO 3 microspheres with sizes varying from 1 to 2 µm. A more detailed analysis displays a highly rough surface, ascribed to a hierarchical structure composed of assembled nanoparticles (Fig. 1c). Energy-dispersive X-ray spectroscopy (EDX) elemental mapping images validate the existence and even distribution of the W and O elements, as shown in Fig. 1d, e. The X-ray diffraction (XRD) pattern of WO 3 micromotors is displayed in Fig. 1f, which agrees with the standard monoclinic phase (JCPDS 83-0951) 33 . In fact, diffraction peaks are observed at 2θ=22.9°, 23.4°, 24.1°, 26.4°, 28.1°, 32.8°, 33.1°, 41.6°, 50,43° and associated with the (002), (020), (200), (120), (112), (022), (202), (222), and (400) crystalline planes of WO 3 . The chemical states of WO 3 micromotors were additionally determined by X-ray photoelectron spectroscopy (XPS). Figure 1g shows the main peaks ascribed to W 4fand O 1s, highlighted in the XPS wide spectrum. The highresolution spectrum of W 4fdemonstrates two distinguished peaks in Fig. 1g. The binding energies of W 4f 7/2 and W 4f 5/2 , positioned at 35.3 eV and 37.3 eV, respectively, match well with reported values of the W 6+ oxidation state for WO 3 microspheres. The high-resolution spectrum of O 1sdisplays two peaks at 531.8 534 532 530 528 Intensity [a.u.] Binding energy [eV] Measured Backgroud Fitted W-O WO(OH) 40 38 36 34 Intensity [a.u.] Binding energy [eV] Measured Backgroud Fitted W4f7/2 W4f5/2 W 4f 20 25 30 35 40 45 50 55 60 (112) (400) (222) (420) (202) (120) (022) (200) (002) Intensity [a.u.] 2 Theta [degree] (020) 1200 1000 800 600 400 200 0 W 4s W 5s C 1s O KLL W 4pW 4dW 4f Intensity [a.u.] Binding energy [eV] O 1s O 1s fg hi a W precursor Mixing Hydrothermal reaction Annealing Stirring Centrifuge Drying Glucose WO bc e d Fig. 1 The preparation process and characterization of WO 3 micromotors. a Schematic illustration of the preparation of WO 3 microspheres by hydrothermal and calcination processes. b(the scale bar is 2 µm) and c(the scale bar is 1 µm) SEM images of WO 3 micromotors. Elemental mapping images for dW and eO. fXRD pattern. gXPS wide spectrum. hHigh-resolution W 4fXPS spectrum. iHigh-resolution O 1sXPS spectrum. X. Peng et al. 2 npj Clean Water (2023) 21 Published in partnership with King Fahd University of Petroleum & Minerals 1234567890():,; and 530.3 eV binding energy (Fig. 1i) 34 . The peak at 531.8 eV is ascribed to OH groups on the WO 3 micromotors’surface, and the other peak agrees with the lattice oxygen in the crystal structure of WO 3 micromotors. Motion behavior of WO 3 micromotors The single-component WO 3 micromotors display autonomous motion when exposed to UV-light illumination both in fuel-free and H 2 O 2 solutions. Previous research has elucidated the locomotion mechanism of single-constituent photocatalytic micromachines under light irradiation 35,36 . As illustrated in Fig. 2a, the incidence of photons with higher energy than the optical bandgap of WO 3 micromotors triggers the generation of electron-hole pairs. According to previous works 37–39 , the edge of the conduction band (CB) of WO 3 micromotors (E CB =0.77 V NHE , where E CB is the conduction band edge at the normal hydrogen electrode (NHE)) lies below the photocatalytic hydrogen evolution threshold. The photogenerated electrons in the CB could not react with H + to generate H 2 , whereas they can decompose H 2 Oor H 2 O 2 in protons (H + ). Meanwhile, the photogenerated holes in the valence band (VB) can contribute to breaking down H 2 OorH 2 O 2 into •OH, O 2, and H + . The non-uniform light exposure of WO 3 micromotors results in an asymmetrical generation of these chemical species, establishing a product gradient leading to their movement by self-phoresis 40,41 . An optical bandgap of 2.72 eV, representing the minimum energy to excite the electrons from the VB to the CB, was determined from the absorption spectrum of WO 3 micromotors (Fig. 2b) using the Tauc plot (inset in Fig. 2b) 42 . Figure 2c exhibits the time-lapse images of a WO 3 micromotor’s trajectory at intervals of ∼5 s in pure water and 1% H 2 O 2 (Supplementary Videos 1 and 2). WO 3 micromotors exhibit Brownian motion without light and fuel-free propulsion when illuminated by UV-light. A longer trajectory can be observed when 1% H 2 O 2 is introduced due to the more pronounced product gradient around the micromotor. The corresponding speeds of these micromotors are also depicted in Fig. 2d and Supplementary Fig. 1a, further indicating the light-controlled on/off movement in pure water and H 2 O 2 . The motion of WO 3 micromotors under different conditions was further studied by calculating the mean squared displacement (MSD). As reported in Fig. 2e, the MSD of WO 3 micromotors in 1% H 2 O 2 without UV-light irradiation follows a linear increase within 1 s, indicating merely Brownian motion. The same is observed for the micromotors in pure water (Supplementary Fig. 1b). When UVlight is involved, the MSD of micromotors in pure water and H 2 O 2 manifests a parabolic increase with time owing to the lightinduced micromotors’self-propulsion (Fig. 2e and Supplementary Fig. 1b) 32,43,44 . According to MSD analyses, the diffusion coefficients (D) of WO 3 micromotors were plotted and inserted in Fig. 2e. Under UV-light irradiation in pure water, D is up to 2.0 ± 0.1 µm 2 s −1 , whereas a tenfold increase is noted in 1% H 2 O 2 (20 ± 1 µm 2 s −1 ). To study the parameters that regulate the photocatalytic locomotion of WO 3 micromotors, different concentrations of H 2 O 2 were introduced, as depicted in Fig. 2f. The speed of micromotors was progressively increased from 5±1μms −1 to 26 ± 2 μms −1 as a higher concentration of H 2 O 2 (1% H 2 O 2 ) was added. Nitroaromatic pollutants degradation Water contamination caused by nitroaromatic compounds poses severe environmental hazards due to their wide usage for the manufacture of explosives, pesticides, and pharmaceuticals and their non-biodegradable nature with high persistence 2 . Recently, various environmentally friendly treatments that employ active propelled micro/nanomotors have been demonstrated for the degradation or absorption of nitroaromatic compounds 6,45–48 . Nevertheless, most micromotors involve a primary half-coating with expensive Pt or Au to achieve the asymmetrical structure for light-activated propulsion or chemically-driven motion based on H 2 O 2 decomposition. There is a strong desire to develop easily fabricated, low-cost, and recyclable micromotors as candidates to facilitate their practical applications. Therefore, we employed the single-component WO 3 micromotors to degrade nitroaromatic compounds in water. PA and 4-NP (molecular structures in the insets in Figs. 3a and 4, respectively) were both selected as representative nitroaromatic contaminants. PA was also employed to investigate the reusability and photodegradation mechanism of WO 3 micromotors. PA photocatalytic degradation capability of WO 3 micromotors was estimated under UV-light irradiation up to 2 h in 1% H 2 O 2 . Figure 3a shows the UV-Vis spectra of PA solutions at different reaction times. The intensity of the absorbance peak after the treatments with micromotors decreases with time, suggesting the degradation of PA. The degradation efficiency of micromotors was estimated according to Eq. (1) Degradation Efficiency ¼C0Ct C0 ´100%(1) where C 0 represents the original PA concentration, while C t is the PA concentration at the time t. Specifically, the decomposition efficiency of PA is 72% after 2 h treatment, as shown in Fig. 3c (red line). Additionally, several control experiments were accomplished to assess other contributions to the PA photocatalytic degradation using WO 3 micromotors: 1) PA +UV-light; 2) PA +UV-light + H 2 O 2 ;3)PA+H 2 O 2 ;4)PA+WO 3 +UV-light; 5) PA +WO 3 +H 2 O 2 . As shown in Supplementary Fig. 2a, negligible decreases in the peak intensity can be observed after 2 h photodegradation without micromotors. For fuel-free motion (PA +WO 3 +UV-light) and no motion (PA +WO 3 +H 2 O 2 ), the degradation efficiencies are merely 28% and 5%, respectively, which are lower than the efficient motion with H 2 O 2 (PA +WO 3 +UV-light +H 2 O 2 ). These results suggest that UV-light and the addition of H 2 O 2 are not sufficient to degrade PA. The reusability of materials has been considered one of the most critical factors for their practical applications. Figure 3b shows that the degradation efficiency still exhibits up to 55% after several consecutive degradation cycles (Supplementary Fig. 2b), which indicates that these micromotors preserved their high photocatalytic capability. Photogenerated radicals (i.e., h + ,•OH, O 2- ) are crucial for degrading organic pollutants. Owing to the low CB position of WO 3 micromotors (E CB =0.77 V NHE ), the reduction of O 2 (O 2 +e - →•O 2- (aq), −0.33 V NHE ) cannot happen under light irradiation 37 . Consequently, photogenerated h + and •OH are considered the main radicals that can break down PA. Upon UVlight irradiation, •OH radicals can be formed in two paths: the oxidation of water and reduction of H 2 O 2 adsorbed on the surface of WO 3 micromotors as follows. H2Oþhþ!OH þHþ(2) H2O2þe!OH þOH(3) A suitable potential value for the reduction of H 2 O 2 is E CB = 0.87 V NHE49 .WO 3 has a higher E CB (0.77 V NHE ), so this reaction can happen due to being energetically favorable. Therefore, the enhanced photodegradation efficiency achieved by PA +WO 3 +UV-light +H 2 O 2 relies on the photocatalytic capability of WO 3 micromotors and also the enhanced radical production on the surface of WO 3 micromotors because of the presence of H 2 O 2 .In order to further understand the degradation mechanism, radical trapping experiments were performed 50 . In this regard, EDTA (10 mg L −1 ) and isopropanol (0.25 μLmL −1 ) were chosen as typical scavengers to capture h + and •OH, respectively 51 . As shown in Fig. 3c, the involvement of isopropanol sharply decreased the photo-induced degradation efficiency of the micromotors, X. Peng et al. 3 Published in partnership with King Fahd University of Petroleum & Minerals npj Clean Water (2023) 21 resulting in 37% (Supplementary Fig. 3b). Therefore, the addition of isopropanol decreased the amount of free •OH species, resulting in a markable decrease in the PA photodegradation efficiency. On the contrary, the addition of EDTA accelerated the photodegradation process, enhancing the photoactivity of the micromotors after 2 h light exposure with 1% H 2 O 2 (Supplementary Fig. 3a). On these bases, it can be concluded that the radicals •OH are the key photogenerated chemical species responsible for PA oxidation (Fig. 3d). 4-Nitrophenol (4-NP) is a poisonous and bio-rebellious nitroaromatic contaminant that can harm human health considerably 8 .In this work, WO 3 micromotors were also used to degrade 4-NP. Initially, control experiments were run to exclude the interference of other factors (UV-light, H 2 O 2 , and their combination). As depicted in Supplementary Fig. 4a, no noticeable decline in the absorbance intensity can be observed when micromotors are not involved in control experiments. Figure 4demonstrates the 4-NP degradation after the treatment with WO 3 micromotors under light exposure in water (free-fuel motion) and in the presence of H 2 O 2 (fuel-driven motion) for different durations. Obviously, micromotors powered by the fuel presented an enhanced degradation activity, resulting in an efficiency of 40%, higher than fuel-free motion (11%). This phenomenon can be explained by the lower propulsion speed of micromotors in pure water without any agitation. DISCUSSION Here, we reported the large-scale synthesis of precious metal-free single-component WO 3 micromotors via a facile hydrothermal method, which is easily scalable, combined with calcination. We demonstrated that such WO 3 micromachines exhibit self-propulsion upon light exposure, even without H 2 O 2 fuel. Specifically, they displayed perceptive on/off motion capability with light exposure. From MSD analyses, it can be proved that micromotors showed pure Brownian motion without light, whereas self-propulsion was achieved under asymmetric UV illumination. The active motion and photocatalytic activity of WO 3 micromotors were applied to degrade toxic nitroaromatic pollutants, such as PA and 4-NP. The micromotors degraded 70% of PA and 40% of 4-NP in water without any external agitation. The photodegradation mechanism was also investigated through radical trapping experiments, which confirmed that •OH is the key ROS responsible for pollutant degradation. The obtained results indicate the possibility of using single-component 300 400 500 600 700 800 2.0 2.5 3.0 3.5 4.0 0 1 2 3 4 [hv ] 2 Enengy [eV] Absorbance [a.u.] Wavelength [nm] 0.0 0.2 0.4 0.6 0.8 1.0 0 10 20 30 40 50 R 2 =0.979 UV-off UV-on Linear fit Parabola fit MSD [ µm2 ] t [s] R 2 =0.999 0 5 10 15 20 H 2 O 2 D [µm 2 S -1 ] Light off Light on Water 0 5 10 15 20 0 2 4 6 8 10 12 14 16 18 20 Speed [µmS -1] Time [s] 00.11 0 5 10 15 20 25 30 Speed [μm S -1 ] H2O2 [%] ab cd ef H 2 OH 2 O 2 0 s6 s11 s16 s 0 s6 s11 s16 s Asymmetrical illumination WO3micromotor Self-propulsion H2O H2O2 O2+ 2H+ ·OH + H+ H2O2 + 2H+ 2H2O Fig. 2 Motion behavior of WO 3 micromotors. a Schematic illustration of the propulsion mechanism of light-powered WO 3 micromotors. bUV-Vis spectrum of WO 3 micromotors. The inset shows the bandgap estimation from the corresponding Tauc plot. cTime-lapse images of a WO 3 micromotor in 1% H 2 O 2 at intervals of ∼5 s. Scale bars are 5 µm. dInstantaneous speed values and eMSD plots of WO 3 micromotors in 1% H 2 O 2 without/with UV-light irradiation. The inset shows the corresponding diffusion coefficients according to MSD plots fitting. fComparison of micromotors’speed in different concentrations of H 2 O 2 . Error bars represent the standard deviation, n=5 independent replicates. X. Peng et al. 4 npj Clean Water (2023) 21 Published in partnership with King Fahd University of Petroleum & Minerals photocatalytic micromotors to eliminate non-biodegradable and hazardous pollutants from industrial sewages. METHODS Synthesis of WO 3 micromotors Firstly, 50 mL of deionized (DI) water was placed in an 80 mL beaker. Then, 1 mmol Na 2 WO 4 was dissolved in the prepared DI water with constant magnetic stirring. Afterward, 25 mmol of glucose was added to the prepared suspension. The final mixture was magnetically mixed until obtaining a homogeneous solution. Then, the solution was transferred into an autoclave. The autoclave was sealed and placed in a preheated oven at 200 °C and lasted for 20 h. When the reaction ended, the autoclave was taken out from the oven and naturally cooled down to room temperature. The precipitated product was cleaned using DI water and ethanol and dried in an oven overnight. Then, the dried products were sealed with aluminum foil and calcined at 550 °C in air. The obtained green-colored product was collected for further experiments. Characterization of micromotors SEM images of WO 3 micromotors were acquired by a Tescan MIRA 3 XMU instrument. EDX mapping analysis was conducted by an EDX detector (Oxford Instruments) coupled to the SEM. The chemical states of WO 3 micromotors were characterized by a Kratos Analytical Axis Supra instrument. Motion experiments The micromotors’motion was recorded by an inverted microscope (Nikon ECLIPSE Ts2R) and a camera (BASLER acA1920155uc). Specifically, a 5 μL aqueous suspension containing WO 3 micromotors was used for motion experiments. Three concentrations of H 2 O 2 (Merck, 30%) were mixed with the previous suspension, achieving final concentrations of 0, 0.1, and 1%, to observe and record the videos of micromotors’motion with a frame rate of 25 fps. No surfactants were involved in all experiments. The micromotors were exposed to a UV-light source (Cool LED pE-100, 1.6 W cm −2 ) with a 365 nm wavelength. In order to observe the on/off behavior of micromotors, the light source was switched on/off at defined time intervals (~5 s). The WO 3 micromotors’velocities and trajectories were calculated from the recorded videos and tracked by the NIS Elements Advanced Research software. The diffusion coefficient of micromotors was 0 20 40 60 80 100 120 0.0 0.2 0.4 0.6 0.8 1.0 noitartnecnoC[C t /C 0 ] Time [min] Micromotors EDTA Isopropanol 300 350 400 450 500 550 0.0 0.2 0.4 0.6 0.8 1.0 1.2 ].u.a[ecnabrosbA Wavelength [nm] 0 min 10 min 30 min 60 min 90 min 120 min 12345 0 10 20 30 40 50 60 70 ]%[n o ita d arge D AP Recycling Times ab cd OH NO2 NO2 O2N PA hv H 2 O ·OH PA Degradation products CB VB 2.72 eV H 2 O 2 Fig. 3 Photocatalytic degradation of PA by WO 3 micromotors. a UV-Vis spectra of PA after photocatalytic degradation by WO 3 micromotors exposed to UV-light irradiation in 1% H 2 O 2 ;bReusability assessment of WO 3 micromotors under 5 successive cycles of PA degradation; cRadical scavenger experiments conducted by employing EDTA and isopropanol; dProposed WO 3 photodegradation mechanism. Error bars represent the standard deviation, n=3 independent replicates. OH NO 2 4-NP 0 20 40 60 80 100 120 0.5 0.6 0.7 0.8 0.9 1.0 Concentration [C t /C 0 ] Time [min] 4-NP + WO3 + H2O2 4-NP + WO3 + UV-light + H2O2 Fig. 4 Photocatalytic degradation of 4-NP using WO 3 micromotors under UV-light irradiation in 1% H 2 O 2 .Error bars represent the standard deviation, n=3 independent replicates. X. Peng et al. 5 Published in partnership with King Fahd University of Petroleum & Minerals npj Clean Water (2023) 21 calculated according to the following equations based on the results of MSD plot fitting: MSD ¼4DΔt(4) MSD ¼4DΔtþv2Δt2(5) Pollutants degradation experiments In all experiments, 2 mg mL −1 of micromotors, 50 μMaqueous solution of picric acid (PA, Merck, 99%), and 1% H 2 O 2 were consecutively placed in UV-transparent cuvettes. Once UV-light (356 nm, 9 W) was turned on, the cuvettes were put in a closed box for in total of 120 min. Control experiments without UV-light irradiation or H 2 O 2 were performed to elucidate the contribution of micromotors. After a different light exposure time, the mixtures were centrifuged at a certain speed for 3 min to obtain the final solution without the micromotors. A UV-Vis spectrophotometer (Jasco V-750) was used to measure the absorbance spectra of the solutions. The absorbance peak at 354 nm was considered to calculate the degradation efficiency. The degradation of 150 μM 4-nitrophenol (4-NP, Sigma Aldrich, 99%) followed the same procedures as PA. DATA AVAILABILITY The data that support the findings of this study are available from the corresponding author upon reasonable request. Received: 17 August 2022; Accepted: 20 February 2023; REFERENCES 1. Kovacic, P. & Somanathan, R. Nitroaromatic compounds: environmental toxicity, carcinogenicity, mutagenicity, therapy and mechanism. J. Appl. Toxicol. 34, 810–824 (2014). 2. Ju, K. S. & Parales, R. E. Nitroaromatic compounds, from synthesis to biodegradation. Microbiol. Mol. Biol. Rev. 74, 250–272 (2010). 3. 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M.U. designed the experiments and contributed to data interpretation. X.P. and M.P. originated the idea. M.U. and M.P. supervised the research. All authors have given approval to the final version of the manuscript. COMPETING INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41545-023-00235-z. Correspondence and requests for materials should be addressed to Martin Pumera. Reprints and permission information is available at http://www.nature.com/ reprints Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. 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