Photocatalytic TiO2 Micromotors for Removal of Microplastics and

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Energy, Environmental, and Catalysis Applications

Photocatalytic TiO2 micromotors for removal of microplastics and suspended matter Linlin Wang, Andrea Kaeppler, Dieter Fischer, and Juliane Simmchen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06128 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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ACS Applied Materials & Interfaces

Photocatalytic TiO2 micromotors for removal of microplastics and suspended matter LinLin Wang, † Andrea Kaeppler, ‡ Dieter Fischer ‡ and Juliane Simmchen *†

†Physical

Chemistry TU Dresden, Zellescher Weg 19, 01062 Dresden, Germany

‡Leibniz-Institut

für Polymerforschung Dresden, Hohe Straße 6, 01069 Dresden,

Germany

ABSTRACT: Environmental contamination is a major global challenge and the effects of contamination are found in most habitats. In recent times, the pollution by microplastics has come to the global attention and their removal displays an extraordinary challenge with no reasonable solutions presented so far. One of the new technologies holding many promises for environmental remediation on the microscale are self-propelled micromotors. They present several properties that are of academic and technical interest,

ACS Paragon Plus Environment

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such as the ability to overcome the diffusion limitation in catalytic processes, as well as their phoretic interaction with their environment. Here, we present two novel strategies for the elimination of microplastics using photocatalytic Au@Ni@TiO2 -based micromotors. We show that individual catalytic particles as well as assembled chains show excellent collection and removal of suspended matter and microplastics from environmental water samples.

KEYWORDS: Photocatalytic micromotors, magnetic assembled chains, microplastics, removal, active passive interaction, environmental remediation

INTRODUCTION Micromotors, being self-propelled nano/microscale devices, have attracted considerable attention in the last decade, owing to the fascinating physics as well as the envisioned functions in different fields, including biomedical1–7 and environmental6–12 applications. Particularly, micromotors hold huge potential for environmental remediation, such as organic degradation,13–17 oil removal,18-20 and heavy metal removal.21–26 Microbial contamination could be detected and remediated,27,28 and a general assessment of water


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quality via micromotors was presented.29,30 Compared to traditional chemically catalytic micromotors, photocatalytic micromotors bear the additional advantage of an immediate on/off switch and they can employ water as non-toxic fuel and light as renewable energy source.31,32,33 Light is easily controllable and does not lead to any waste products during the propulsion, thus extending the benefits of micromotors in the diverse environmental applications.10,11 Especially, for water purification, photocatalytic micromotors display exceptional contributions.16,17,34,35 Among the problems of water contamination, plastic contamination attracts global attention owing to its persistence and damage for marine organisms.36 Microplastics, as one of the main environmental pollutant, are defined as fragments less than 5 mm in size,37,38 which do not easily sediment and frequently pass by filtration systems, holding potential threats even for marine organisms and human beings.39,40,41 Fish and other marine creatures confuse these microplastics with food, and since humans are a part of the food chain, we are also influenced by the microplastics. Recently, microplastics also have been found in human stools42,43 for the first time proving that the threat is ubiquitous, not only endangering to the marine organisms. Fortunately, many countries are starting


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to tackle the problem, for instance, by banning the production of personal care products containing plastic microbeads.44–46 However, due to the persistence of microplastics, the threat will still be here for long time even if all the countries banned the microbeads products,44 and microplastics from other sources still pose the threat to the environment.41 In this manuscript, we use a photocatalytic TiO2 based micromotor (Au@mag@TiO2, mag = Ni, Fe) for microplastics removal. This catalytic micromotor can move efficiently both in peroxide and water under UV illumination owing to the photocatalytic reaction occurring on the particles. Phoretic interactions induced by photocatalytic activity of these individual micromotors show collective properties with passive particles in very low concentration of peroxide. Moreover, to tackle the challenge of peroxide free microplastics removel, we also develop assembled chains by assembling individual particles under an external magnetic field. These are motile in pure water and display shoveling effects under magnetic field both in water and peroxide. Here, both catalytic individual swimmers and multiparticle assemblies exhibit exceptional behaviors for passive particles removal. Subsequently, we show on real life samples the suitability to the collect microplastics and suspend matter based on these two strategies.


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RESULTS

Photocatalytic propulsion We initially prepared the TiO2 particles by a modification of our previously published method.47 To obtain magnetic Au@mag@TiO2 micromotors, TiO2 particles were coated with a 10 nm nickel or alternatively iron layer and then with 30 nm of gold. This photocatalytic micromotor can move efficiently in both, water and peroxide when irradiated with UV light, based on photocatalytic chemical reaction, which had been shown previously.31,48,49,50,51 In comparison to earlier motors,

31,52

our Au@TiO2

micromotors achieve very fast swimming speeds47 and therefore more pronounced phoretic phenomena, enabling the here presented application. As shown in the scheme of photocatalytic propulsion of micromotor in water (Figure 1A), under UV illumination, photo-excitation processes takes place within TiO2, creating conduction band electrons and valence band holes. We assume, that holes involve in the oxidation reaction of water at TiO2 hemisphere, which produces protons. Subsequently, the electrons must migrate to the metal cap and involve into the reduction of protons at this hemisphere. The


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consumption of protons leads to a fluid flow toward the metal cap, thus contributing the photocatalytic propulsion of Au@Ni@TiO2 micromotors. Similar to the case in water, the catalytic decomposition of H2O2 via oxidation-reaction mechanism occur on the micromotors in H2O2 solution (Figure 1B), propelling the micromotors. It can actively selfpropel with the speed of 14.72 µm s-1 in H2O and 65.52 µm s-1 in 0.1% H2O2 solution with UV light, as shown in the Figure 1A, B (Video S1, S2). The motion track lines for micromotors during 2 s are displayed. Just as noted earlier with Au@TiO2 micromotors, the micromotors achieve faster speeds in H2O2 compared to H2O, but steady motion in both fuels.47

Figure 1. (A, B) Scheme of photocatalytic propulsion of Au@Ni@TiO2 micromotor, track lines and speed of micromotors during 2 s in H2O and 0.1 % H2O2 solution with 315 mW UV light, respectively. The insets are the optical images of active-passive particles


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behaviors based on activity induced interaction in H2O and H2O2 solution. Scale bar, 5 µm.

Interactions Besides looking at active - active interactions,53,54,55,56,57,58 in earlier studies active particles have been used for cargo transport,59 partially based on surface properties,60 or on particle morphology for light activated microswimmers.61 The interaction between passive particles and active (metal) capped swimmers induced by photocatalytic activity was studied previously62 as well as in our earlier work.47 We found that the interactions between micromotors with passive particles differ among the swimming modes, which leads to different behaviors in different fuels: in water the mixture of active and passive particles exhibited weak attractive interaction between single active and passive particles thus they cannot form patterned clusters (shown in the insets of Figure 1A). In peroxide solution (Figure 1B), passive particles can be collected based on strong attractive phoretic interaction, forming certain patterns.47


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For the magnetic Au@mag@TiO2 particles, we find similar behaviors. Especially pronounced in H2O2, strong attractive phoretic interactions with passive particles enable assembling of larger motile structures as well as motion of passive structures (see the model in Figure 2). In the sections below, when we refer to ‘phoretic interactions’ we mean ‘strong attractive phoretic interactions’. However, for the phoretic interaction induced cluster formation during the microparticles removal, more complex consideration are required. Our hypothesis on the origin of the interaction is that they are dominantly caused by the activity of the microswimmers. Since a complete theoretical description of this phenomenon is still missing it is rather difficult to address and terminology has to be chosen carefully. We will therefore refer to hydrodynamic effects as caused by the fluid motion around the particle. The term phoretic will be used when effects or interactions are caused by the chemical fields or gradients produced by the (photo) chemical reactions. Both are difficult to decouple experimentally and the interactions between them can be very complex. Especially when electric charges are involved, (which we cannot exclude and for our system) or generated protons play a role in the mechanism and locally alter the pH, the differentiation between both can become impossible. We would like to point


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that out to the reader but since in purely deforming (hydrodynamic) systems generally no attractive interactions with tracer particles are observed we will use the term 'phoretic' in this manuscript. This label is needed to simplify the distinction between the two types of interactions and the simplification does not mean that we underestimate the purely hydrodynamic contribution to the interaction.

Similar behavior has been also reported in very different systems63,64 and origins of this behavior and the differences among the fuels are currently under research and to be considered in a separate work. Herein, we will focus on the collective behaviors induced by the phoretic interaction in very low concentration of peroxide and analyze the influence of different passive materials and irregular shapes. In earlier works, only spherical model particles had been considered. This newly found robustness of the phoretic interactions, apart from being interesting as phenomenon, hints towards huge potential for collecting and removing microplastics, which we will demonstrate in the following. We use polystyrene (PS) passive particles as a model system for microplastics. As displayed in Figure 2 (Video S3), in the mixture of individual catalytic Au@Ni@TiO2


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micromotors and PS particles, micromotors can collect PS particles very efficiently through phoretic interactions, and transport large numbers of them. Especially since our particles are much faster, this behavior more efficient than in previous systems thus providing a promising strategy to use these individual micromotors for microplastics removal based on phoretic interaction.

Figure 2. Scheme of phoretic interaction between Au@Ni@TiO2 micromotors and PS particles; time-lapse optical images illustrating Au@Ni@TiO2 micromotors collecting PS particles by phoretic interaction in 1.67% H2O2 solution with 315 mW UV light.

Microplastics In order to study the occurrence of microplastics in our daily life and test our system on these, diverse personal care products, which might contain microplastics, were extracted


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by vacuum filtration. These products include washing powders, toothpastes and face cleansing creams. SEM images of different extracts from products are shown in Figure3 A-C (more see in Figure S1). Further analytical methods including Zetapotential measurements, IR and Raman spectra are used to identify the composition of the extracts. Details of the individual samples can be found in the SI. From the selection of products we checked, it seems that the EU guidelines have been successfully banning microbeads, and in most of the European products abrasive components have been replaced by other (inorganic) materials, such as silica and zeolites. However, in an imported product, namely sample 13 (Figure 3C), we could still identify microplastic compounds. The analytical identification resulted in a mixture of polyethylene and synthetic wax (more details see in Figure S2). Moreover, to get a better estimate of the occurrence of microplastics in the environment, samples were collected in the Baltic Sea and Warnow River, elaborated and investigated by Raman microscopy (see details in the experimental section). SEM images of extracts from Baltic Sea and Warnow River are shown in Figure 3D and Figure 6F, respectively. Microplastics could be confirmed in both extracts, for instance, the sample from Warnow


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River, contained 8 microplastics out of 1542 particles, including polypropylene (PP), polyethylene (PE), polystyrene copolymer, polytetraflouroethylene (PTFE), polyethylene terephthalate (PET) and pigments (more details in table S2).

Figure 3. SEM images of extracts from (A) toothpaste (sample 5); (B) washing powder (sample 11); (C) face cleansing cream (sample 13) and (D) Baltic Sea.

Removal by phoretic interactions To test the applicability of the phoretic interaction for removing the remnants of the personal care products, individual catalytic micromotors are mixed with the extracts. As displayed in Figure 4 (Video S4), the extracts from sample 11 consisting of zeolite, are collected rapidly by micromotors and move together in 0.1% H2O2 solution with UV light


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illumination. This indicates that the phoretic interaction can be applied to collect not only regularly shaped laboratory synthetic sample but also real samples from daily life, made of different, even irregularly shaped materials.

Figure 4. Removal of personal care product (washing powder sample 11) in 0.1% H2O2 solution with 315 mW UV light.

Despite the promising results employing phoretic interactions for microparticles removal at ultralow peroxide contents, we still aim at developing a removal strategy independent of the employed fuel which means that it needs to work in water as well. Generalizing, one can say that the effect achieved by phoretic interactions strongly resembles pushing the passive particles. Here, we want to test the hypothesis, that we can achieve efficient ‘pushing’ independent from the phroetic interaction in peroxide, and that we can still


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remove passive particles by ‘shoveling’ in water, even there is no strong attractive interaction.

Removal by shoveling To increase the area to push microparticles, we developed a motile assembly of magnetic catalytic particles, i.e. magnetic micromotor chains. Similar observations have been made on Janus colloids,65 peanut shaped particles66 and other materials.67 These assembled chains can be controlled and perform a direct pushing effect (shoveling) under magnetic field, thus can remove microplastics by shoveling independent of the fuelling. Magnetic chains are formed through the interaction between magnetic Ni layers inside the Au@Ni@TiO2 micromotors under external magnetic field. The scheme in Figure 5 A shows the formation of the chain formed by individual Au@Ni@TiO2 micromotors under magnetic field, while Figure 5B displays the SEM image. When irradiated with UV light, these assemblies can move directionally in both, H2O and H2O2 solution under magnetic field. As shown in Figure 5C,D (Video S5, S6), the assembled chain can move


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directionally at a speed of 7.35 µm s-1 in H2O, and at a speed of 26.96 µm s-1 in 0.1% H2O2 solution with UV light. Despite being slower than single micromotors, the assemblies benefit from the larger area covered by the chain and can achieve considerable shoveling effects, thus holding greater potential for microplastics removal in real environmental filed. Here, we want to point out that Ni displays a certain toxicity for environment. We coat the Ni with a thin Au layer for protection, which does not completely guarantee sealing and might still be dangerous for the environment. Therefore, equivalent experiments are carried out with a magnetic Fe layer instead of Ni. As shown in Figure S3, the magnetic Au@Fe@TiO2 micromotors show self-propelled motion both in water and 0.5% peroxide solution with UV light. Due to the magnetic properties, the assembled chains also form in presence of an external magnetic field and the phoretic effects in peroxide are equivalent. Thus the micromotors with Fe magnetic layer are an excellent alternative for microparticle removal under magnetic field. Here, to compare the effect of removal by individual particles and assembled magnetic chains, the Au@Ni@TiO2 are used in the subsequent experiments.


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Figure 5. (A) Scheme and (B) SEM image of chain formation of magnetic Au@Ni@TiO2 micromotors under magnetic field; (C, D) Track lines and speed of Au@Ni@TiO2 micromotors chains during 2 s in H2O and 0.1% H2O2 under 315 mW UV light and magnetic field, respectively.

To verify the shoveling effect on microplastics and suspended matter removal by assembled Au@Ni@TiO2 chains independent of fuel under magnetic field, different experiments have been carried out in H2O2 solution (Figure 6A,B, and 6 C,D) and H2O (Figure 6E,F). In order to observe the removal effect more intuitively, the area of the UV light is limited and the amount of matter is quantified before and after removal to obtain a ‘removal efficiency’ (Figure 6B).


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Firstly, removal of suspended matter is performed in H2O2 solution. In the first row (A,B) the suspended matter consists of zeolites from washing powder (sample 10, SEM in the inset, Figure 6B). In Figure 6A can be observed how the suspended matter (Video S7), is collected in 0.1% H2O2 and shoveled by the chains out of the irradiated area. After 120 s, most of the zeolites are removed from UV illuminated area, leading to a removal efficiency of about 77%. The removal of microplastics, extracted from face cleansing cream sample 13, from a 0.2% H2O2 solution is displayed in Figure 6C, Video S8. Here, similar to zeolites, the microplastics are also removed efficiently with a removal efficiency of about 71% (Figure 6D), demonstrating that the novel magnetic assembled chains perform excellently by shoveling different microscale materials out of illuminated areas in H2O2 solution. Differently to phoretic interactions, this effect can be also used in pure H2O, as displayed with the microplastics sample from Warnow River in Figure 6E (Video S9). In only 40 s, 12 out of 18 microplastics are shoveled from UV area by magnetic assemblies, resulting in a removal efficiency of 67% (Figure 6F). Overall, the successful removal of suspended


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matter and microplastics from both, H2O2 solution and H2O suggests that shoveling effect can work independent of the fuelling system under magnetic guiding.

Figure 6. Magnetic Au@Ni@TiO2 chains remove different extracts under magnetic field in different conditions: (A) washing powder sample 10 in 0.1% H2O2 under 63 mW UV light; (C) face cleansing cream sample 13 in 0.2% H2O2 under 63 mW UV light; and (E) microplastics sample from Warnow River in H2O under 315 mW UV light. (B, D, F) amount


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of microparticles in the initial (before removal) and final (after removal) state. The insets are the SEM images. Scale bar, 10 µm.

DISCUSSION Since the international attention has been focusing on the plastic problem, in central Europe many individuals are aware of the hazard and avoid products containing added microplastics. An easy way to control of the ingredients of individual products can be achieved by different apps, pinpointing that European care products rarely contain microplastics, which we could confirm in our analysis. However, the international market still has several products in use containing large proportions of added microplastics. Additionally, degrading plastic based clothing, storage containers and waste products remain a massive source of microplastics and plastic based microfibers. For this it is only natural, that in open waters, such as Warnow River and Baltic Sea, the suspended matter regularly contains microplastics. We extracted and analyzed these microscale matters and showed that in the laboratory scale, we can reliably remove them from selected areas.


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To do so, we developed light driven micromotors, and employed either individual micromotors in peroxide or larger assembled chains. The first removal strategy works through non-selective phoretic interactions of the catalytic swimmers that lead to large patterned passive structures around individual photocatalytic microswimmers. The fact that we can use a wide variety of materials with random properties shows the robustness of these phoretic attraction. However, for the here used system the phoretic interactions have the condition that the presence of low concentration of peroxide is required, which might not ideal for some extreme water cleaning applications. A way around the addition of peroxide is the use of shoveling in water, which would be rather inefficient using single microswimmers. For this, we introduced catalytic chains of magnetic assemblies that form spontaneously when a magnetic field is applied to these magnetic particles. These chains move as a collective and form kind of a barrier that displaces all microscale obstacles that come into their way. Employing this strategy, we were able to remove the extracted microplastics and random suspended matter from Warnow River, achieving about 67% of clearage efficiency in pure water. Also this effect is very robust and material independent. A faster elimination can be achieved by adding


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a low percentage of peroxide, but the same sample can also be used in pure water and still achieves a very good output. The versatility and robustness of the observed effects are clearly a plus for both systems, but also directly cause their biggest drawback: a lack of selectivity. Despite the fact, that especially the phoretic interaction is currently under intense mechanistic investigation, an in depth understanding is yet to be achieved. However, it might pave the way to ‘smarter’ collector particles.

CONCLUSIONS We have developed a novel passive particle removal system formed by photocatalytic Au@Ni@TiO2 micromotors and characterized both, the motion of individual particles and assembled chains in different fuels, namely H2O and H2O2. Two strategies for removal of microplastics and suspended matter have been proposed and applied to both, suspended matter and microplastics from personal care products and extracted from open waters. For individual micromotors the removal was caused by phoretic interaction, while the


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chained assemblies were relying on shoveling or pushing interactions. The latter system works efficiently independent of the fuel: in dilute peroxide solution and water, which is a plus for microplastics removal in practical environment. We could also show for the first time that the removal of matter by light driven micromotors is not limited to the certain materials and shapes. The greatest challenge for the future is to increase the selectivity and enable the micromotors to recognize microplastics, in order to improve the microplastics removal efficiency.

METHODS Materials and reagents Titanium (IV) iso-propoxide (TTIP) was purchased from Alfa Aesar Co. Ltd. Dodecylamine (DDA) was purchased from Fluka. Polyvinylpyrrolidone (PVP, K-30), 2,2′azo-bis(isobutyronitrile) (AIBN), acrylic acid, and styrene were obtained from Sigma Co. Ltd. Nickel and Iron powder were bought from Sigma Co. Ltd. Personal care products including toothpaste, washing powder and face cleansing cream were bought from


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supermarket. All other reagents including ethanol, methanol and acetonitrile were of analytical grade and used as received without further treatment. Synthesis of magnetic Au@Ni@TiO2 and Au@Fe@TiO2 micromotors 700 nm TiO2 particles were synthesized via hydrolysis and condensation reaction of TTIP.47 In brief, 0.18 mL of water was added to mixture solution of 105 mL methanol and 45 mL acetonitrile. Then 0.28 g of DDA was dissolved in the mixture under stirring. After stirring for 10 min, 1 mL of TTIP was added dropwisely and stirred for 12 h. Then the particles were washed with methanol three times and were calcined in tubular furnace under nitrogen flow for 2 h at 600 °C, and then the black TiO2 particles were obtained. For the magnetic particles, the monolayers of TiO2 particles were prepared first by dropcasting 200 μL of suspension onto glass slides. Then 10 nm of Ni (or Fe) layer was first deposited on the top of TiO2 particles via thermal deposition. Then 30 nm of Au layer was deposited. Finally, the magnetic Au@Ni@TiO2 (or Au@Fe@TiO2) particles were released by ultra-sonication. Extractions of suspended matter and microplastics from personal care products, river and sea


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For the extraction from personal care products, 1 g of products was dissolved in 200 mL water. Then, a vacuum filtration device was used to filter the products solution. During the process of filtration, a membrane with 0.2 µm holes was used. After the filtration, the extracts were washed with water three times by vacuum filtrated device. The final extracts were obtained after drying in a vacuum oven for 4 h. Environmental microplastics samples from Warnow River and Baltic Sea were extracted using an enzymatic and chemical purification protocol.68 Parts of the extracts were filtered using Si filters (10 µm holes).69 The residues onto the Si filter were analyzed by Raman microscopy. Subsequently they were dispensed with MilliQ-water for SEM investigations and removal experiments. Measurement details (Zeta Potential, SEM, FTIR and Raman) are described in the SI. Motion of individual micromotors and assembled chains Motion of individual Au@Ni@TiO2 micromotors and assembled chains in water and H2O2 solution were investigated by an optical microscope (Carl Zeiss Microscopy GmbH, Germany) equipped with a Zeiss Colibri lamp. The UV light (385 nm, 315mW) was used


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in these experiments. For the directional motion of magnetic assembled chains, a permanent magnet was set up to control of the formation and motion. Removal of carboxylated polystyrene bead (PS-COOH) by individual Au@Ni@TiO2 micromotors 3 µm carboxylic polystyrene particles were synthesized by dispersion polymerization.70 In brief, 1.35 g of PVP was dissolved in 102 mL of ethanol and 10 mL of water in a 250 mL round-bottom flask under N2 gas flux with stirring at 280 rpm. Then 0.3 g of acrylic acid was added. The mixed solution containing 0.3 g of AIBN and 15 g of styrene was added to the previous solution. After N2 gas flux for 30 min, the solution was heated to 70 °C for 24 h in oil bath and stirring lasted for 24 h. The obtained particles were washed with ethanol and water three times. For the removal of PS-COOH particles, 5 µL of active particles, 5 µL of PS-COOH (1 mg mL-1, 3 µm) particles, and 5 µL of 5 % H2O2 were mixed on the cleaning slide. Removal of microplastics and suspended matter from water by Au@Ni@TiO2 micromotors.


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For the removal of suspended matter by individual micromotors, 7 µL of active particles, 2 µL of extract solution (2.5 mg mL-1) and 1 µL of 1% H2O2 solution were mixed on the cleaning slide. For the removal of suspended matter and microplastics by assembled chains, the light area was limited and magnet was set up to form assembled chains and to control the motion direction. No H2O2 solution was added when carried out in water. The ratio of removed microparticles was determined by quantification of microparticles in the irradiated area before and after removal by counting all particles with light microscopy.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications websitr at DOI: Additional figures and tables of extracts from personal care products and Warnow River (PDF)


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Video S1, Video S2, Video S3, Video S4, Video S5, Video S6, Video S7, Video S8, Video S9

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT J.S. and LL.W. thank the Volkswagen foundation for the Freigeist fellowship (grant number 91619), as well as the Kärcher foundation. LL.W. would like to acknowledge the China Scholarship Council (CSC) for the financial support. The help with the extractions performed by S. Görke are greatly acknowledged. A.K. thanks for financial support by the BONUS MICROPOLL project funded jointly by the EU and BMBF (03F0775A). The environmental microplastics samples (river Warnow, Baltic Sea) were taken within the Leibniz Competition project MirkOMIK, funded by the German Leibniz Association (grant


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number

SAW-2014-IOW-2).

Sampling

and

purification

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were

done

by

Sonja

Oberbeckmann and Matthias Labrenz (both Leibniz Institute for Baltic Sea Research Warnemünde), what is greatly acknowledged.

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Table of Contents: from left to right: contaminated water, a micromotor collecting microplastic spheres and clean water 338x118mm (104 x 104 DPI)


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Photocatalytic TiO2 Micromotors for Removal of Microplastics and

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