Precision engineering of nanorobots: Toward single atom decoration and defect control for enhanced microplastic capture

RESEARCH ARTICLE www.afm-journal.de Precision Engineering of Nanorobots: Toward Single Atom Decoration and Defect Control for Enhanced Microplastic Capture Anna Jancik-Prochazkova, Hana Kmentova, Xiaohui Ju, Stepan Kment, Radek Zboril,* and Martin Pumera* Nanorobots are being received with a great attention for their move-sense-and-act capabilities that often originate from catalytic decomposition of fuels. In the past decade, single-atom engineering has demonstrated exceptional efficiency in catalysis, energy-related technologies, and medicine. Here, a novel approach involving point defect engineering and the incorporation of platinum (Pt) single atoms and atomic level species onto the surface of titanium dioxide nanotubes (TiO2-NT)-based nanorobots is presented and its impact on the propulsion capabilities of the resulting nanorobots is investigated. The achievement of point defect engineering is realized through the annealing of TiO2-NT in a hydrogen atmosphere yielding to the point-defect decorated nanotube (TiO2-HNT) nanorobots. Subsequently, the atomic level Pt species decorated TiO2nanotube (TiO2-SA-NT) nanorobots are achieved through a wet-chemical deposition process. Whereas TiO2-SA-NT nanorobots showed the highest negative photogravitaxis when irradiated with ultraviolet (UV) light, TiO2-HNT nanorobots reached the highest velocity calculated in 2D. Both TiO2-HNT and TiO2-SA-NT nanorobots demonstrated a pronounced affinity for microplastics, exhibiting the capability to irreversibly capture them. This pioneering approach utilizing point-defect and atomic level Pt species nanorobotics is anticipated to pave the way for highly efficient solutions in the remediation of nanoand microplastics and related environmental technologies. A. Jancik-Prochazkova, X. Ju, M. Pumera Future Energy and Innovation Laboratory Central European Institute of Technology Brno University of Technology Purkyˇ nova 123, Brno 61200, Czech Republic E-mail: [email protected].cz The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202402567 © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/adfm.202402567 1. Introduction Single-atom and atomic-level catalysis has gone through a tremendous boom in the last decade.[1,2]Single atoms have proven to be highly efficient in synthetic organic chemistry,[3]photocatalysis,[4] electrocatalysis,[5]and other chemical conversions with the potential to become a versatile new generation of heterogeneous catalysts.[6,7]Dispersing isolated single-atom metal catalysts on a desired support allows access to every active site and enhances the catalyst turnover frequencies and turnover numbers.[8]When combined with the optimization of the host matrix, single-atom engineering provides a significant degree of flexibility, allowing customization of reaction pathways, catalyst selectivity, and the precise design of desired structures.[9] The field of nanorobotics, which entails the design of miniature self-propelled autonomous engines endowed with advanced functionality for diverse applications, is rapidly gaining prominence as an emerging area of research.[10]Nanorobots can undergo chemical propulsion using fuels such H. Kmentova, S. Kment, R. Zboril Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute (CATRIN) Palacký University Olomouc Olomouc 78371, Czech Republic E-mail: [email protected] X.Ju,M.Pumera QuantumMaterialsLaboratory CenterforNanoroboticsandMachineIntelligence DepartmentofChemistryandChemicalBiology MendelUniversityinBrno Zemedelska1,Brno61300,CzechRepublic S.Kment,R.Zboril NanotechnologyCentre CentreforEnergyandEnvironmentalTechnologies(CEET) VŠB–TechnicalUniversityofOstrava 17.listopadu2172/15,Ostrava-Poruba70800,CzechRepublic Adv. Funct. Mater. 2024,34, 2402567 2402567 (1 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.afm-journal.de as hydrogen peroxide (H2O2), glucose, or urea, among others.[11] Additionally, external propulsion methods can be used, including light,[12]ultrasonic, magnetic,[13]or electric field.[14]Furthermore, the integration of multiple fields can be combined synergistically to improve propulsion capabilities in various modes.[15] The ability to move is beneficial for improving the distribution of nanorobots in the desired medium and allows the achievement of the specific task in the “on-the-fly” regime with high effectiveness.[16,17]These nanorobots find primary application in tasks such as cargo transport,[18]capture and degradation of pollutants,[19]detection of analytes,[20]antimicrobial treatment,[21]and various mechanical procedures,[22]etc. Nanorobots, demonstrating notable efficiency in active motion, have also found extensive applications in the environmental[23,24] and biorelated context.[25–27]Currently, one of the main focuses lies on further downsizing of nanorobots to enhance their efficiency.[28]Ultimately, nanorobots endowed with swarming capabilities are preferred over individual macroscopic robots to perform more intricate tasks.[29] However, as advanced nanorobots continue to decrease in size, the need to modify the surface requires the utilization of smaller and more intricate structures, which allows integration of a multitude of functionalities.[30,31]From this point of view, point defect engineering and single atom incorporation approaches toward surface functionalization are highly advantageous.[32,33]One of the recent achievements was the attachment of single-atom Cu catalysts to the surface of jellyfish-like nanorobots for tumor therapy.[34]In that study, single Cu atoms catalyzed the decomposition of hydrogen peroxide into toxic reactive oxygen species (ROS) that inhibited tumor growth, while the accessible main body of the nanorobots was available for self-thermophoretic propulsion induced by NIR light illumination. Here, we report the first application of nanorobots decorated with single atom and atomic level Pt species in environmental technologies. Platinum was chosen because it represents a well-studied single-atom and atomic-level species catalyst. Furthermore, platinum is a commonly used metal to form Janustype nano-/microrobots, as it catalytically decomposes H2O2that induces a chemical and electrochemical gradient leading to efficient propulsion. We fabricated titanium dioxide (TiO2)nanotubes (TiO2-NT)-based nanorobots and decorated their surface with point defects (TiO2-HNT nanorobots) and atomic level Pt species (TiO2-SA-NT nanorobots) and investigated the influence of defect engineering and atomic level Pt species engineering on the resulting propulsion. The nanorobots were propelled under M. Pumera Advanced Nanorobots & Multiscale Robotics Laboratory Faculty of Electrical Engineering and Computer Science VSB – Technical University of Ostrava 17. listopadu 2172/15, Ostrava 708 00, Czech Republic M. Pumera Department of Chemical and Biomolecular Engineering Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea M. Pumera Department of Medical Research China Medical University Hospital China Medical University No. 91 Hsueh-Shih Road, Taichung City 40402, Taiwan ultraviolet (UV) irradiation in the presence of H2O2as a fuel. It was observed that the nanorobots exhibited negative photogravitaxis, resulting in efficient 3D motion. The designed nanorobots were subjected to microplastic capture tests, revealing a synergistic effect between point defect engineering and atomic level Pt species implantation on the efficiency of microplastics capture. 2. Results and Discussions 2.1. Fabrication and Characterization of Nanorobots TiO2nanotubes (TiO2-NT) were grown on a Ti metal foil using an electrochemical anodization process that was adopted from previous reports.[32,35]Scanning electron microscopy (SEM) micrographs in Figure S1A (Supporting Information) clearly show the morphology of the separated nanotubes with a spacing between tubes of ≈100 nm. An individual nanotube has an average diameter of ≈250 nm, a wall thickness of ≈20 nm, and length of ≈4μm. The TiO2-NT were further annealed in a hydrogen atmosphere to introduce defects to the structure; these point-defect nanorobots are labeled TiO2-HNT nanorobots. Finally, the TiO2-HNT nanorobots were decorated with atomic level Pt species (TiO2-SA-NT nanorobots) using a dark deposition technique. As demonstrated in Figure S1 (Supporting Information), no changes in the morphology of the nanorobots were observed after point defect engineering, single atom, and atomic level Pt species decoration. The schematic illustration of nanorobots fabrication is shown in Scheme 1. To evaluate the morphology of the nanorobots in detail and to verify the presence of atomic level Pt species on the surface, high-resolution transmission electron microscopy (HRTEM) was used to study the TiO2-SA-NT nanorobots (Figure 1A). Clearly, the general morphology agrees with the previous SEM characterization of the original TiO2-NT (Figure S1, Supporting Information). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) micrographs and the corresponding energy-dispersive X-ray spectroscopic (EDX) elemental mapping (Figure 1B,C) revealed that platinum was distributed on the TiO2surface of the nanorobots primarily in the form of single atoms (indicated by red circles) and small clusters consisting of individual single atom assemblies (indicated by magenta squares). The original HAADF-STEM micrograph without any highlights is presented in Figure S2 (Supporting Information). Furthermore, the uniformity of the decoration of atomic level Pt species throughout the length of the nanotube was evident from the mapping images and spectra in Figure 1C and Figure S3 (Supporting Information), respectively. Considering the presence of both single atoms and their assemblies, we further generalize these formations as atomic-level Pt species. Recent developments, along with our research, have demonstrated that the partial clustering of Pt single atoms into atomic level Pt species can serve as (co-)catalysts and enhance desired catalytic, electrocatalytic, and photocatalytic reactions,[36,37]including hydrogen production via water splitting.[38,39]This enhancement is linked to the unique electronic interactions and surface attributes caused by Pt atoms clustering into atomic-level species, which effectively promote photocatalytic processes. X-ray photoelectron spectroscopy (XPS) was performed to study the chemical states of the nanorobots. XPS survey spectra Adv. Funct. Mater. 2024,34, 2402567 2402567 (2 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 38, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202402567 by Technical University Ostrava, Wiley Online Library on [27/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Scheme 1. A) Schematic illustration of the fabrication of TiO2-HNT nanorobots toward TiO2-SA-NT nanorobots. B) Illustration of the capture of microplastics under UV irradiation. Figure 1. Structural and chemical characterization of TiO2-SA-NT nanorobots. A) HR-TEM image of a TiO2-SA-NT nanorobot. B) HAADF-STEM image of the surface of a TiO2-SA-NT nanorobot showing selected Pt single atoms in red circles and Pt small clusters in magenta squares (the micrograph not highlighted is presented as Figure S2 in Supporting Information). C) HAADF-STEM images and EDX elemental mapping of a fragment of a TiO2-SA-NT nanorobot, scale bar 200 nm. D) XPS core level spectra of Pt 4f, Ti 2p, and O 1s of TiO2-SA-NT nanorobots. E) EPR spectra and F) XRD diffractograms of TiO2-NT, TiO2-HNT, and TiO2-SA-NT nanorobots, diffractograms are compared with a noncalcined amorphous TiO2material (referred to as “TiO2 not treated”). Adv. Funct. Mater. 2024,34, 2402567 2402567 (3 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 38, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202402567 by Technical University Ostrava, Wiley Online Library on [27/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de of the original TiO2-NT and TiO2-HNT nanorobots (Figures S4 and S5, Supporting Information) demonstrate the presence of Ti and O as expected. For the TiO2-SA-NT nanorobots, Pt was clearly detected in addition to Ti and O in the survey spectra. The survey spectrum of the TiO2-SA-NT nanorobots that demonstrates the presence of platinum is shown in Figure S5 (Supporting Information). The core-level spectrum of Pt 4ffrom the TiO2-SA-NT nanorobots is presented in Figure 1D. The spectrum was deconvoluted into two peaks centered at 72.9 and 76.2 eV, corresponding to Pt𝛿+4f7/2 and Pt𝛿+4f5/2 (0<𝛿≤2), respectively. According to previous studies, Pt clusters in the nature of metallic Pt0exhibit a binding energy of 4f5/2 to the lower region (71.1–71.6 eV), while Pt─O species have been reported with a binding energy to higher values over 74 eV.[40]In our case, the platinum signal was detected at 72.9 eV, which indicates the presence of atomic Pt species (with the possibility of small contributions from Pt clusters) captured on the surface,[41]which can further interact with surface available oxygen.[42]The atomic percentage of Pt dispersed on TiO2 for the TiO2-SA-NT sample is quantified to be 2.6±0.1 at.% by XPS, further indicating its atomic level Pt species dispersion on the substrate. However, it should be noted that this is a relatively overestimated value since XPS is a surface-sensitive technique with an information depth of <10 nm. Most Pt atoms are located on the surface of the defect sites of TiO2nanotubes, whose wall thickness is ≈100 nm. The core-level spectrum of O 1swas deconvoluted into two peaks centered at 529.6 and 531.0 eV. The peak of oxygen at lower binding energy (529.6 eV) is attributed to the bulk oxygen of TiO2, while the broadened peak at 531.0 eV is highly likely due to adsorbed OH and moisture residues on the surface of the oxide, as previously reported.[43,44]Although the reduction at high temperatures is expected to result in the formation of TiO2-HNT nanorobots with an enriched amount of reduced TiO2-x oxides, it is highly likely that the number of defects and associated Ti3+species cannot be quantified by XPS. This assumption is further confirmed by the Ti 2pspectra of the three samples of nanorobots (Figure 1D; Figure S4, Supporting Information). The core-level spectra (Figure 1D; Figure S4,Supporting Information) of Ti 2p show signals at 458.4 and 464.1 eV corresponding to 2p3/2 and 2p1/2, respectively. These doublets can be attributed to Ti4+species without observation of Ti3+species. It should be noted that detection of Ti point defects using XPS is restricted by its surface sensitivity and the extremely low concentration of present defects.[45] Further confirmation of the point-defect structure of the synthesized TiO2-HNT nanorobots is provided by electron paramagnetic resonance (EPR) spectroscopy, which possesses a higher sensitivity to structural defects compared to XPS. Figure 1E shows the EPR envelopes of a series of TiO2-based nanorobots. The TiO2-NT nanorobots show a very weak and unresolved resonant line at g≈1.99. The observed signal aligns with previously reported spectra and g-values, indicating the presence of Ti3+sites within the crystalline structure of the TiO2nanotubes.[4]Thermal annealing of the TiO2nanotubes in the hydrogen atmosphere leads to a significant increase in the recorded EPR signal intensities of the resulting TiO2-HNT nanorobots. Significant enhancement of signals associated with the Ti3+species embedded in the lattice in the TiO2-HNT nanorobots is observed as a sharp resonant line at 326 mT corresponding to g=1.995. In addition to that, in the region of the larger field (B>325mT) we observe a wide line modulation at g=1.947 corresponding to the surface exposed Ti3+sites.[46]These results further confirm the point-defect structure of TiO2-HNT nanorobots. For TiO2-SA-NT nanorobots prepared by decoration of TiO2nanotubes with atomic level Pt species, we observed a decrease in signals associated with both Ti3+sites embedded in the lattice at g=1.995 and with the Ti3+ surface exposed at g=1.936, probably due to the galvanic displacement reaction (Pt4+→Pt𝛿+,Ti 3+→Ti4+,𝛿≈2).[35] It further indicates the successful anchoring of Pt single atoms and atomic level Pt species at the defect site of the metal oxide support.[47] Finally, the crystalline structure of the nanorobots was studied by X-ray diffraction (XRD) (Figure 1F). Reduction of pristine TiO2-NT nanorobots under hydrogen flow at 500 °C created a defective nanostructure but did not reveal any crystal transformation after treatment with high-temperature reduction. Notably, all samples exhibit nearly identical XRD patterns corresponding to a pure anatase phase. Specifically, the peak at 2𝜃=29.5° corresponds to the diffraction of the anatase (101) lattice planes (JCPDS No. 01-075–2547). XRD measurement was performed directly on the samples grown on the Ti substrate foil; therefore, the peaks at 2𝜃=44.9, 47.1, 62.5, 84.4°are assigned to titanium from the Ti foil used as the substrate for anodization (JCPDS No. 04-005-7594). The peak at 2𝜃=31.8°that corresponds to the rutile (110) phase (JCPDS No. 034–0180) can be seen in a very minor proportion even in the diffractogram of the original TiO2NT nanorobots, which confirms a high structural purity of the nanorobots. For comparison, the diffractogram of the TiO2sample was detected prior to the annealing procedure to demonstrate the amorphous nature of the as-synthesized TiO2material. In general, our results demonstrate the successful point defect engineering and incorporation of a single Pt atoms alongside the atomic level Pt species on the surface of photocatalytic TiO2-based nanorobots. 2.2. Propulsion of Nanorobots The evaluation of the propulsion of nanorobots constituted a pivotal aspect in the investigation of point defect engineering and the implantation of atomic-level Pt species on the surface of TiO2based nanorobots. As expected, with respect to the photocatalytic properties of the crystalline structure of anatase,[48]the propulsion induced by UV light of the nanorobots was observed in an aqueous environment in the presence of hydrogen peroxide (H2O2) that was applied as fuel. Representative videos demonstrating the propulsion of nanorobots are shown in Videos S1– S3 (Supporting Information). It is worth noting that no buffer or surfactant was applied during the propulsion experiments. For all nanorobots tested, only Brownian motion was observed under UV irradiation without fuel addition (refer to Figure S6, Supporting Information, for typical tracks of nonfuelled nanorobots), suggesting that neither type of nanorobot is suitable for pure photocatalytically driven propulsion in the fuel-free environment.[49]At the same time, no propulsion was observed when 5 wt% H2O2was applied without UV irradiation; not even when studying the propulsion abilities of TiO2-SA-NT nanorobots. Clearly, the decomposition of H2O2over atomic level Pt species is not an efficient driving force to propel TiO2-SA-NT nanorobots through the diffusiophoresis mechanism.[50]This Adv. Funct. Mater. 2024,34, 2402567 2402567 (4 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 38, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202402567 by Technical University Ostrava, Wiley Online Library on [27/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 2. 2D-propulsion analysis of the synthesized nanorobots. A) MSD curves of TiO2-SA-NT nanorobots: fuelled under UV irradiation (a); under dark conditions (b); and not fuelled under UV irradiation (c). B) Time-lapse micrographs demonstrating the light-dependent behavior of the nanorobots in the presence of 5 wt% of H2O2under alternating UV irradiation and dark conditions. C) Average velocities of nanorobots as a function of H2O2 concentration (wt%) and UV irradiation. D) Typical trajectories of nanorobots exposed to UV irradiation in 5 wt% H2O2collected for 20 s. observation is demonstrated in Figure 2Aby presenting representative mean squared displacement (MSD) curves of TiO2-SANT nanorobots. As suggested by the MSD curves, the enhanced diffusion mode that leads to the propulsion of nanorobots occurs only in the presence of H2O2under UV irradiation.[51]This result is in agreement with a previous study by X. Peng et al.[52] who demonstrated that the amount of platinum on the surface has a crucial effect on the resulting mobility of microrobots. In general, the propulsion of nanorobots occurred when H2O2 was applied as fuel at a concentration greater than 1 wt% along UV irradiation. As Figure 2B clearly suggests, once UV irradiation is turned off, the nanorobots are no longer propelled and only Brownian motion is observed. The propulsion under UV irradiation in the presence of H2O2is fully reversible, which means that they quickly resume movement after each round of UV exposure. This suggests that H2O2is photocatalytically decomposed on the surface of TiO2-based nanorobots providing chemical and electrochemical gradients in the direct environment of an individual nanorobot that induces autophoretic motion, as discussed further in more detail.[53]Since H2O2acts as fuel during propulsion, the average velocity of nanorobots increased with increasing concentration of H2O2(Figure 2C). Interestingly, TiO2-HNT nanorobots showed a higher average velocity compared to other samples. Furthermore, their trajectories exhibited a distinct pattern different from those of the other samples (Figure 2D): Most TiO2-HNT nanorobots tend to move in circles, whereas most TiO2-NT and TiO2-SA-NT nanorobots have irregular trajectories. We also observed that nanorobots exhibited negative photogravitaxis when irradiated vertically from the bottom of the substrate (Figure 3). As suggested in Figure S7 (Supporting Information), the nanorobots at the substrate level were getting out of focus and eventually disappearing from the view field when irradiated from the bottom, indicating their upward movement. Interestingly, prior to moving upward, the nanorobots self-oriented and stood up; this behavior was previously observed for TiO2and inorganic semiconductor-based microrobots.[54–56]To elucidate this phenomenon, the mechanism of propulsion of nanorobots needs to be discussed in more detail. TiO2-based materials act as semiconductors; that is, upon absorption of UV light, electrons, and holes can be separated in the conduction and valence bands, respectively (Figure 3A). Electrons and holes accumulated on the surface of nanorobots can subsequently react with H2O2 and form chemical and electrochemical gradients that induce autophoretic propulsion (Figure 3A,B). Based on the morphology and homogeneity of the structures and the direction of UV irradiation, the chemical and electrochemical gradients are not formed symmetrically around the individual nanorobots, which can lead to their self-orientation, stand-up motion, and eventually Adv. Funct. Mater. 2024,34, 2402567 2402567 (5 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 38, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202402567 by Technical University Ostrava, Wiley Online Library on [27/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 3. Mechanism of “stand-up” motion of nanorobots toward propulsion on the z-axis. A) Generation of a chemical gradient after UV irradiation around a TiO2-NT nanorobot. B) Schematic illustration of the gradient of photogenerated chemical species and their reorganization during the “standup” motion of the TiO2-NT nanorobot during the illumination from the bottom. C) Schematic illustration showing the light-induced self-orientation leading to the 3D motion of a single TiO2-NT nanorobot. D) Micrographs demonstrating the “stand-up” motion of nanorobots. Scale bar 5 μm. 5 wt% H2O2was applied as fuel for this experiment. propulsion in 3D space.[54,55]Figures 3B,C schematically suggest the self-orientation of the nanorobots that leads to the stand-up motion and subsequent motion on the z-axis due to the formation of an asymmetric chemical gradient.[55,56] Figure 3D then shows micrographs of the self-orientation of the nanorobots under UV irradiation followed by motion on the z-axis. We further carried out an in-depth quantitative analysis of the photogravitaxis and calculated the ratio of the nanorobots that disappeared out of focus for each sample after 20 s of UV irradiation. The strongest photogravitaxis effect was observed for TiO2-SA-NT nanorobots, where only 27% of the nanorobot population stayed in the focal plane. However, in the case of platinum-free nanorobots, that is, TiO2-NT and TiO2-HNT, 53% and 56% of the nanorobots, respectively, stayed in the focal plane. The speed of photogravitaxis was exactly the same for all three types of nanorobots; in all cases, it occurred within 6–10 s after UV exposure. This phenomenon is demonstrated in the supporting videos that were recorded for 20 s (Videos S1–S3, Supporting Information). It must be noted here that the average velocity discussed in Figure 2C was calculated from the tracks that were recorded for 10 s, and it was calculated only in the 2D projection without taking into consideration the vector of direction. Therefore, it can be assumed that the actual velocity of the nanorobots will be even higher considering the motion in 3D. In fact, this assumption could explain the lower velocity observed in the case of TiO2-SA-NT nanorobots compared to TiO2-HNT nanorobots, as TiO2-SA-NT nanorobots exhibit a greater tendency to negative photogravitaxis. To conclude the propulsion of the nanorobots, no significant effect of the atomlevel Pt species implantation nor point defect engineering on the propulsion of the nanorobots compared to pristine TiO2-NT nanorobots was found. All three samples were propelled in 3D space in the presence of fuel under UV irradiation due to negative photogravitaxis. Whereas TiO2-SA-NT showed the highest negative photogravitaxis ability (z-axis), TiO2-HNT nanorobots reached the highest velocities calculated in 2D (xy plane). In addition, a schooling behavior that is typical for photocatalytic TiO2-based nano-/microrobots[53]was observed among Adv. Funct. Mater. 2024,34, 2402567 2402567 (6 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 38, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202402567 by Technical University Ostrava, Wiley Online Library on [27/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de samples under UV light irradiation. As demonstrated in Video S4 (Supporting Information), TiO2-HNT nanorobots exhibited a tendency to agglomerate under UV irradiation in the presence of 3wt%H 2O2. Similarly, this phenomenon was also observed in the case of TiO2-SA-NT nanorobots. However, in the latter case, the effect of clustering was not that significant; it could be related to the fact that TiO2-SA-NT nanorobots showed higher negative photogravitaxis that hindered spontaneous agglomeration in the xy focal plane. The difference in the motility abilities among the samples is not surprising since defects incorporation, as well as metal insertion into the structures of catalysts, have a great influence on the resulting catalytic and electrochemical performance of materials.[57,58]Considering the fact, that the propulsion of nanorobots is an interplay between photocatalytic processes and induced electrochemical gradients, it is reasonable to expect that the incorporation of surface defects in metal oxide-based microrobots would significantly influence the resulting speed, directionality, and the stability of their continuous motion. For example, in the studies of A. M. Pourrahimi et al.[59,60]it was demonstrated that hindering surface defects in microrobots led to enhanced recombination of electrons and holes that were formed upon UV irradiation which led to more regular trajectories, higher velocities, and improved photocatalytic activity. On the other hand, the presence of surface defects supported the enhanced diffusion and phoretic motion of the microrobots fueled with H2O2.InastudybyR.Liuetal. [61]it was described that the distribution of surface defects plays a crucial role in the resulting trajectories. According to the symmetry of the surface defects among the body of nanorobots, they are preferably shaken, rotated, or translated. 2.3. Microplastics Capture TiO2-derived nano-/microrobots have previously been reported to be suitable materials for the capture of nano- /microplastics.[23,53,54]Therefore, we further explored this ability with our nanorobots while investigating the effect of implanted atomic-level Pt species and surface point defects. First, the affinity between the nanorobots and the microplastics was tested by observing the colloidal solutions containing the nanorobots alongside the microplastics. As model microplastics, PS spheres with an average diameter of 4.8–5.8 μmwereused without any surface decoration to mimic real microplastics to the best possible extent. To enable the “on-the-fly” regime of microplastic capture, the H2O2concentration was set to 3 wt%. As demonstrated in Figure 4, the TiO2-NT nanorobots were intact to microplastics in the presence of 3 wt% H2O2in dark conditions. On the contrary, TiO2-HNT and TiO2-SA-NT nanorobots tended to capture microplastics over time by aggregating around them immediately after mixing. To explain the spontaneous clustering of nanorobots, it must be taken into account that TiO2-based microrobots rich in hydroxyl groups can undergo an acid-base reaction in an aqueous environment and form H+ and OH−ions. Assuming different diffusivities of the generated H+and OH−ions, a local electric field is formed around the nanorobots, which leads to ionic diffusiophoresis which is a driving force for the clustering of nanorobots.[62]It is worth noting that diffusiophoretic phenomena simultaneously influence passive particles, such as PS spheres in our case, leading to their spontaneous capture.[49,62]The capture of microplastics is also supported by an electrostatic attraction between the nanorobots and microplastics (Figure S8, Supporting Information). The Zeta potential of the pristine PS spheres dispersed in deionized water was negative −8.2 ±0.6 mV. Similarly, a slightly negative zeta potential of −0.7 ±0.5 mV was detected for TiO2-NT nanorobots; the negative value of the zeta potential can explain the lack of interaction between microplastics and TiO2-NT nanorobots. On the other hand, the zeta potential of TiO2-HNT and TiO2-SA-NT nanorobots was positive, that is, 0.5 ±0.7 and 2.5 ±0.1 mV, respectively, suggesting a possible electrostatic interaction during microplastics capture. Investigating further the nature of the interaction of nanorobots with microplastics, we performed a control experiment to monitor the interaction of TiO2-HNT and TiO2-SA-NT nanorobots without applying H2O2as fuel. Interestingly, no microplastic capture was observed in the 5-minute time interval in the case of both nanorobot samples (Figure S9, Supporting Information), suggesting a crucial contribution of H2O2to phoretically induced clustering phenomena. Being aware that TiO2-based materials exhibit photocatalytic behavior and are capable of light-driven schooling behavior that allows efficient clustering around pollutants and eventually their transport,[63]nanorobots were exposed to UV irradiation to observe their influence on microplastic capture. Figure 5shows that UV irradiation has a strong effect on nanorobots and their ability to cluster and capture microplastics. The TiO2-NT nanorobots did not show any ability to cluster and capture microplastics, only the effect of negative photogravitaxis was observed. On the other hand, the TiO2-HNT and TiO2-SA-NT nanorobots clustered and captured microplastics. As expected, the clustering ability is higher in case of TiO2-HNT nanorobots, as they showed a lower contribution of negative photogravitaxis; therefore, they tended to stay at the bottom of the glass slide and interact with each other. Regardless of negative photogravitaxis, both TiO2-HNT and TiO2SA-NT nanorobots showed high efficiency in microplastic capture. As demonstrated in Figure S10 (Supporting Information), all microplastics observed were captured by nanorobots when exposed to 3 wt% H2O2and UV irradiation. It should be mentioned that the microplastic capture process was irreversible in our case, which means that once the UV light source was switched off, the nanorobots did not release the microplastics but stayed in close proximity. Figure S9 (Supporting Information) shows the results of the control experiments when the interaction of microrobots with microplastics was monitored under UV irradiation in the absence of H2O2. As demonstrated, neither clustering nor microplastic capture occurred when fuel was not applied, suggesting that phoretic forces generation is essential not only for propulsion abilities (as discussed previously in Figure 2A,C), but also for microplastic capture experiments. The overall results suggest that the precision engineering of nanorobots including atomic-level species decoration and defect control is crucial for an efficient microplastic capture. To discuss the future approaches toward water remediation, two general approaches in nano-/microrobotic technologies can be considered. First, the combination of our concept with the concept of magnetic nano-/microrobots would enable the removal of micro/nanoplastics using an external magnetic field.[64] Adv. Funct. Mater. 2024,34, 2402567 2402567 (7 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 38, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202402567 by Technical University Ostrava, Wiley Online Library on [27/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 4. Interaction of nanorobots with microplastics. Interaction of A) TiO2-NT, B) TiO2-HNT, and C) TiO2-SA-NT nanorobots with microplastics in 5 minutes under dark conditions. The micrographs in the left panel were taken at t=0 min, and the images on the right were taken at t=5min.3wt% H2O2was applied as fuel. The scale bar of 50 μm is the same for all images. And second, the photocatalytic abilities of the TiO2material are expected to enable in situ degradation of micro-/nanoplastics.[65] Depending on the requirements, nanorobot engineering, such as controlling single atomic species and surface defects, can lead to precise adjustment of microplastic capture efficiency based on specific demands. While surface defects supported schooling behavior, the presence of Pt atomic level species improved the propulsion of nanorobots in 3D. In addition, using Pt single atoms and/or atomic level species can provide the resulting nanorobots with additional abilities that are typical for Pt atomic level species, i.e., catalytic and photocatalytic abilities[35,4]that are commonly required in water remediation technologies.[23] 3. Conclusion In conclusion, TiO2-based nanorobots were fabricated and modified by point defect engineering and enhanced with atomic level Pt species through single-atom engineering. The effect of the modifications on the propulsion of the nanorobots was investigated. It was observed that the nanorobots showed a strong effect of negative photogravitaxis. Especially, atomic level Pt species decorated nanorobots exhibited a powerful propulsion in 3D. On the other hand, point defect formation on the surface of nanorobots leads to higher velocities in the x-y plane up to 3.5 μms −1in the presence of fuel and under UV irradiation. The surface modifications, i.e., point defect engineering and atomic level Pt species implantation, enabled efficient and irreversible microplastic capture under UV irradiation that caused nanorobots clustering. The proof-of-concept results pave the way toward efficient microplastic remediation from aqueous environments and related environmental technologies with the use of single-atom nanorobotics. 4. Experimental Section Nanorobots Preparation:Amorphous TiO2nanotubes were grown on Ti foil (0.125 mm, Advent, 99.6+%) by electrochemical anodization using a typical organic electrolyte (diethylene glycol, 40 mL of deionized water and 6 g of ammonium bifluoride (NH4HF)). The degreased Ti foil was anodized at 60 V for 3 hours at 40 °C. The TiO2nanotubes were then annealed in air at 450 °C for 2 h (heating rate at 2 °C·min−1) resulting in the TiO2NT sample. To create point defects, the reduction process was carried out Adv. Funct. Mater. 2024,34, 2402567 2402567 (8 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 38, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202402567 by Technical University Ostrava, Wiley Online Library on [27/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 5. Interaction of nanorobots with microplastics under UV irradiation. UV-induced clustering and negative photogravitaxis of A) TiO2-NT, B) TiO2-HNT, and C) TiO2-SA-NT nanorobots in the presence of captured microplastics. Micrographs were taken within 1 minute while irradiating the sample with UV light, i.e., micrographs in the left panel were taken at t =0 min, and the images on the right were taken at t=1min.3wt%H 2O2 was applied as fuel. in a ceramic crucible placed in a tube furnace using a hydrogen stream (100 mL ·min−1) at 450 °C for 1 h (heating rate at 10 °C·min−1). The resulting product with point defects was termed TiO2-HNT. To produce TiO2-SA-NT nanorobots, the TiO2-HNT sample was dipped in a 10 mL solution of methanol (50 vol%) containing a solution of H2PtCl6·6H2O (5 ·10−3m). The samples were kept in a closed container for 24 h in an Argon atmosphere under dark conditions for 30 minutes while being magnetically stirred. The TiO2-SA-NT sample was then sequentially soaked in ethanol and deionized (DI) water for 15 min each. Subsequently, the samples were dried in an N2stream. The resulting nanorobots were removed from the substrate using a scalpel and transferred to DI water to form colloidal solutions for further experiments. Characterization:The samples on the Ti foil substrate were examined in top and cross–sectional view, under a Jeol-7900F scanning electron microscope (SEM) with an acceleration voltage of 5 kV. The morphology of the separated nanotubes was investigated by field emission SEM (S-4800, Hitachi, Japan). High-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopic (EDX) mapping were obtained by a high-resolution transmission electron microscope (HR-TEM, FEI Titan G2 60–300). Electron paramagnetic resonance (EPR) spectra were acquired on a JEOL JES-X320 spectrometer operating at X-band (∼9.14–9.17 GHz) with a variable He temperature set-up ES-CT470 apparatus. The EPR envelopes were collected at 80 K. The quality factor (Q) was maintained above 6000 for all measurements. High-purity quartz tubes (Suprasil, Wilmad, ≤0.5 OD) were used as a sample holder and the accuracy of the g-values was determined by comparison with a Mn2+/MgO standard (JEOL standard). The microwave power was set to 1.0 mW to avoid power saturation effects. A modulation width of 0.7 mT and a modulation frequency of 100 kHz were used. The chemical composition and chemical states of the sample surfaces were studied by XPS (KRATOS Axis Supra) with a monochromatic Al K𝛼(1486.7 eV) excitation source. All spectra were calibrated to the adventitious C 1s peak at 285 eV. Peak deconvolution was carried out using Casa XPS software. The crystalline structure of the samples was characterized by X-ray diffraction (XRD, X’pert Philips PMD diffractometer) operating with graphite monochromatized Cu irradiation (wavelength: 0.154056 nm). For a typical experiment, 5 μL of colloidal solution containing a suitable amount of microrobots was mixed with 5 μL of hydrogen peroxide solution on a glass slide and the propulsion of nanorobots was observed using an inverted microscope (Nikon ECLIPSE Ti2) equipped with a digital camera (Hamatsu, C13440). Filter cubes were used to select the UV range of 361– 389 nm (463 mW⋅cm–2) from the white-light LED source (CoolLed, pE-300 lite). Videos were recorded for 20 s at 25 fps at 40×magnification and typical trajectories were identified using the NIS software. To study the capture of microplastics, polystyrene microspheres with an average diameter of 4.8–5.8 μm (Cospherics) were used as model microplastics. The colloidal solution of microplastics was mixed with the nanorobots and fuel at the desired concentration. Zeta potential was measured using Zetasizer Ultra (Malvern Panalytical Ltd.). Statistical Analysis:The tracking experiments were performed by monitoring the motion of nanorobots using NIS software to record the videos at the speed of 25 fps. Average velocities were calculated by processing recorded videos that captured the propulsion of at least 60 nanorobots using ImageJ software with the TrackMate[66]plugin. The resulting tracks were processed using a Python code to calculate the average velocities and the mean square displacement (MSD); the pre-set drift corrections were performed automatically.[67]The presented velocities and the corresponding standard deviations were calculated as an average of at least three processed videos. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements The work was supported by the ERDF/ESF project TECHSCALE (No. CZ.02.01.01/00/22_008/0004587). CzechNanoLab project LM2023051 funded by MEYS CR was gratefully acknowledged for the financial support of the measurements at the CEITEC Nano Research Infrastructure. S.K. thanks the Czech Science Foundation for financial support (project no. 19–27454X) and the support from the European Union’s Horizon 2020 project SAN4Fuel (HORIZON-WIDERA-2021-ACCESS-03-01: 101079384). This work was produced with the financial support of the European Union under the REFRESH – Research Excellence For Region Sustainability and High-tech Industries project number CZ.10.03.01/00/22_003/0000048 via the Operational Programme Just Transition. The authors would like to thank Dr. Eirini Ioannou for performing the SEM characterization. The authors acknowledge Dr. Zdenek Badura for conducting EPR measurements and analyzing the data. Open access publishing facilitated by Vysoke uceni technicke v Brne, as part of the Wiley - CzechELib agreement. Conflict of Interest The authors declare no conflict of interest. Author Contributions A.J.P. and H.K. contributed equally to this work. A.J.P. performed propulsion experiments, nanorobots-in-motion experiments, capture of Adv. Funct. Mater. 2024,34, 2402567 2402567 (9 of 11) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 38, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202402567 by Technical University Ostrava, Wiley Online Library on [27/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License