Microscale Motion and Light

Interfaces between fluids and photo-switchable substrates provide a unique mechanism to manipulate liquid droplets precisely by creating and adapting a heterogeneous wettability landscape. Because droplets respond to changes in the wettability, such interfaces afford a means to keep them in non-equilibrium and thereby induce new states of dynamic wetting. We present a boundary element method to determine the Stokes flow inside a droplet with a curved free surface and a flat interface at the substrate, where we apply the Navier boundary condition to permit motion of the contact line. In general, boundary element methods solve integral equations by inverting the discretized integrals of the unknown field variable such as fluid velocity on the surfaces of the domain of interest. In our approach we parametrize the free surface of the droplet by smooth functions and, using an automatic differentiation algorithm, we calculate its local metric and curvature only where necessary. This efficiently supplies high-resolution data for an iterative domain-splitting integration scheme capable of treating singular integrands, which are typical for the boundary element method. Following rigorous validation, we investigate how droplets respond to specific spatiotemporal wettability patterns that either move or deform the droplet. For example, we generate directed motion by traveling patterns and vary their properties to maximize droplet speed.

Self-propelled microrobots are seen as the next step of micro and nanotechnology. The biomedical and environmental applications of these robots in the real world needs their motion in the confined environments, such as in veins or spaces between the grains of soil. Here, self-propelled trilayer microrobots have been prepared using electrodeposition techniques, coupling unique properties of the green bismuth with the layered crystal structure, magnetic nickel (Ni), and catalytic platinum (Pt) layer. These Bi-based microrobots are investigated as active self-propelled platforms that can load, transfer and release both doxorubicin (DOX), as a widely used anticancer drug, and arsenic (As) and chromium (Cr) as hazardous heavy metals. The significantly high loading capability of such variable cargo is due to the high surface area provided by the rhombohedral layered crystal structure of bismuth, as well as the defects introduced through the oxide layer formed on the surface of bismuth. The drug release is based on an ultrafast electroreductive mechanism in which the electron injection to microrobots and consequently to the loaded objects causes an electrostatic repulsion between them and thus an ultrafast release of the loaded cargos. Remarkably, we present magnetic control of the Bi-based microrobots inside a microfluidic system equipped with an electrochemical setup as a proof-of-concept to demonstrate (i) heavy metals/DOX loading, (ii) a targeted transport system, (iii) on-demand release mechanism, as well as (iv) the recovery of the robots for further usages.

Active colloidal particles that harness chemical energy from the environment and transform it into directed motion have attracted great interest in the recent years. Several applications have been envisaged for these particles from pollutant removal to targeted drug delivery ,and anti-cancer therapies . However, to manufacture optimized active colloids for specific advanced technological applications one has to achieve fundamental understanding of the mechanism that drives their motion and how it is influenced by the properties of the surrounding medium . For instance, previous studies reported a significant influence of the ionic strength on the speed of the active colloid , which could be detrimental for their applications. To gather fundamental understanding of the mechanisms driving the motion, we develop a model for the self-propulsion of a chemically active colloid that releases a strong electrolyte at its surface. By solving the relevant governing equations using the finite element method and a perturbative expansion, we compute the velocity of the active particle as a function of the main physico-chemical parameters such as surface charge, salt concentration and surface reaction rate. Our results show that both ionic diffusophoresis and self-electrophoresis contribute to the motion of the colloid. The theoretical predictions are validated against measurements of the velocity of active particles that are propelled by the conversion of urea into ammonia and carbon dioxide reaction via a catalytic reaction. Our results could inspire novel design of catalytic micromotors that can achieve directed motion through high-salinity environments.

We study the dynamics of active Janus particles that self-propel in aqueous solution by light-activated catalytic decomposition of chemical "fuel." In experiments, the particles, initially sedimented at a bottom wall, exhibit wall-bound states of motion when illuminated from underneath the wall. Upon increasing the intensity of the light above a threshold value, the particles lift off the wall and move away from it, i.e., they exhibit a photo-gravitactic behavior. Using optical and holographic microscopy, we track the particles in both regimes of motion. Theoretically, we develop a model of a photo-active self-phoretic particle that explicitly accounts for "self-shadowing" of the light by the opaque catalytic face of the particle. We find that self-shadowing can drive "phototaxis" (rotation of the catalytic cap of the particle towards the light source) or "anti-phototaxis," depending on the properties of the particle. Incorporating the effect of thermal noise, we show that the distribution of particle orientations is captured by a Boltzmann distribution with a nonequilibrium effective potential. Within the framework of the model, we rationalize the experimental results, identifying photochemical activity and phototactic response as key mechanisms behind the observations. Overall, our findings show that photo-active particles exhibit a rich "tactic" orientational response to light, which could be harnessed to program complex three-dimensional particle trajectories.

The affects of particle chirality upon the dynamics of self-propelled microscale helices is the focus of this study. We fabricate light-activated particles that have a spherical inert head and a helical tail made from photoactive TiO2, and thus the catalytic portion of the particle, which allows for self-propulsion, has a chiral morphology. We find that left- and right-handed mirror-image enatiomorphs, which despite their handedness are otherwise identical, exhibit differences in dynamics that is strictly due to chiral affects; altering the morphology of the tail, using a dynamic fabrication process, allows us to learn how microswimmer chirality affects their movement.

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Active particles are microscopic particles, which can inject energy locally and made available by recent progress in colloidal science. They are ideal "pump-probes" to explore the emergent properties in non-equilibrium soft systems and control the behavior of soft matter and self-assembly at the microscale. In this talk, I will demonstrate how we can use dissipative building blocks activated by light to control self-assembly and show the robust formation of dynamical superstructures using active particles that consume fuel to carve non-equilibrium pathways. Using tailored particles, we will build self-spinning microgears and demonstrate that non-equilibrium phenomena can be harnessed to shape interactions and form hierarchical structures [1]. Next, I will show how we can active particles added to a material to regulates its activity internally and boost the annealing of a colloidal monolayer [2]. It opens a broad range of novel opportunities to thermal treatments, where the properties of matter are not controlled macroscopically but microscopically and in real time by active dopants. [1] Aubret et al, Nature Physics, 2018 [2] Ramananarivo et al, submitted

One of the main challenges in the field of synthetic and biological microrobots is the development of robust control systems to achieve the desired guidance and/or actuation. In that regard, two main control approaches had been followed: (i) either relying on the use of external energy source (i.e. light, magnetic field, electric field)[1] or (ii) taking advantage of a tactic behavior (i.e. phototaxis, chemotaxis, pH-taxis)[2]. However, the full automation, where the robot’ path and actuation are predetermined and programmed, is still not well-stablished in the micro and nanomotors’ community. So far, such refined control had only been achieved in magnetic micromotors.[3] By implementing visual tracking systems, we were able to demonstrate the feasibility of not only tracking such robots, but also performing closed-loop path-planning and force control. Micro-force sensing mobile microrobots (µFSMM) with sharp features allowed vision-based force sensing while fully programming its path.[4] By integrating colored tracking fiducials in the micrometric robot’ body, we were able to go one step forward by developing a less computationally expensive color-based tracking algorithm that provides real-time data to the end user, allowing a force-guided transport of a micro-object that mimics a cellular construct.[5] Color integration is key to provide unique features for vision-based algorithms that can be extended to diverse microrobot’ configurations at different scales to achieve programmable and safe object manipulation. The scope of such vision-based control methods is not only of interest in tissue engineering applications and mechanobiology studies, but also to gain a better insight on full microrobots automation.

We investigate the free swimming of thermally actuated Janus particles that are irreversibly trapped at a water-oil interface. The particle's motion is triggered by thermal gradients that form across the particle body due to the selective heating of a hemispherical gold coating. In comparison to thermophoretic motion in bulk liquid (1), here the formation of surface tension gradients leads to Marangoni flows that drastically enhance the resulting motion. We confirm theoretical predictions on the single particle velocity (2) but also observe a transition from moderate speeds into a high-speed regime of yet unknown origin. References: (1) H.-R. Jiang, N. Yoshinaga, and M. Sano, Phys. Rev. Lett., 105, 2010. (2) A. Würger, Journal of Fluid Mechanics, 752, 2014.

Metamaterials or negative index materials are [4-7] novel artificial materials with negative electric permittivity and negative magnetic permeability which tend to absorb and focus a large amount of the incident light. Solar cell technology or Photovoltaics are very useful technologies for generating the electrical power from the sunlight due to the increasing need of energy. The absorption, reflections, and transmissions are very important parameters which influence photovoltaic solar cell efficiency [1-4]. The waveguides containing different materials can be modeled to be solar cells. The thin films of silicon oxide and silicon nitride as anti-reflection coatings to minimize the reflection losses of incident solar insolation on silicon photovoltaic cells was presented and discussed as one example. Other materials as amorphous, microcrystalline materials and organic semiconductors had also been used in the industry of solar cells. In this work, various multilayers waveguide structure containing metamaterials are proposed to enhance the absorption of solar light. In addition, the comparisons between the two types of structures are analyzed and discussed. The proposed waveguide structures are examined in terms of absorption and transmission of light for various physical properties of the structures. The proposed waveguide structures could be very excellent candidates for future solar energy technology. Key words: Solar energy, Metamaterials, Antireflection, and Multilayers. References [1] Antonio Luque (Editor), Steven Hegedus (Co-Editor), Handbook of Photovoltaic Science and Engineering, 2010 [2] D. Buie, M.J. McCannb, K.J. Weberb, C.J. Dey, Solar Energy Materials & Solar Cells 81 (2004) 13–24 [3] M.Y. Ghannam MY, A.A.Abouelsaood, A.S.Alomar, J. Poortmans, Solar Energy Materials and Solar Cells,l 94(2010)850-856. [4] V. G. Veselago, Sov. Phys. Usp., 10 (1968) 509. [5] R.A.Shelby, Dr.R.Smith, S.Schultz, Science 299, (2001)77-79. [6] S.Taya and M.M.Shabat, Appl.Phys A, vol.103, (2011) 611-614 [7] M.F.Ubeid, M.M.Shabat, M.O|.Sid-Ahmed, Indian Journal of Physics (Springer),86(2)(2012) 125-128.

In 1676, using candle light for illumination and a small glass sphere as the lens, Antonie van Leeuwenhoek revealed to the world the beauty and complexity of a microscopic universe densely populated by living microorganisms. Today, using lasers, spatial light modulators, digital cameras and computers, we investigate the statistical and fluid mechanics of microswimmers in ways that were unimaginable only 50 years ago. With light we can image bacteria as they swim rapidly through 3D environments, apply gentle and controllable force fields or sculpt the 3D structure of their environment. In addition to shaping the physical world outside cells we can also use light to control the internal biological state of microorganisms that are genetically engineered to respond to light signals. I will review our recent work on the interactions between light, matter and bacteria, from the fundamental problems of rectification and non-equilibrium steady states in active matter to the use of genetically modified bacteria as propellers for micro-machines or as a "living" paint that can be controlled by light.

Interfacial energetics and processes in photoelectrochemical energy conversion Jan Philipp Hofmann, Eindhoven University of Technology The storage of intermittent solar energy in chemical bonds, i.e. Solar Fuels, is a promising way to a future greener energy infrastructure. Chemical specificity in the investigation of the multiscale interfacial phenomena during semiconductor-driven photoelectrochemical (PEC) redox reactions is key to a comprehensive understanding and modelling of the processes occurring in Solar Fuel generators. In this talk, I will give an account on the relevant interfacial processes occurring during photocatalytic and photoelectrochemical water splitting for the conversion and storage of solar energy in chemical bonds. Since charge transfer at interfaces plays a major role in determining the overall efficiency of photoelectrodes, the engineering of interface energetics is a viable approach to overcome efficiency losses due to unfavorable charge carrier separation and transport. As a showcase, the design of CuBi$_2$O$_4$ photoelectrodes will be discussed. Results from lab and synchrotron based photoemission spectroscopies (XPS, resonant XPS, HAXPES) based surface photovoltage spectroscopy under both ultra-high vacuum and in-situ conditions will be presented which were used to design improved performance CuBi$_2$O$_4$ photocathodes. In CuBi$_2$O$_4$, surface photoreduction leads to Cu0 surface states, which enhance band bending at the interface in an advantageous manner to aid charge carrier separation. By tailoring the energetic alignment of the back contact hole transport layer (NiO), an overall enhancement of PEC performance can be obtained. In contrast to CuBi$_2$O$_4$, hematite Fe$_2$O$_3$ photoanodes suffer from Fe$^{2+}$ reduced surface states in that they prevent the formation of a sizeable surface photovoltage via polaron formation. Next to assessing interfacial energetics, time-resolved mid-IR experiments on oxide semiconductors will be presented to discuss the interplay between semiconductor bulk excitation and surface chemical changes as a result of above-bandgap excitation.

Micro/nanomotors that self-propel in different liquid media and perform complex tasks, have attracted worldwide attention in the last decade. Different studies have shown that the active motion of these micromachines leads to an enhanced micromixing, favoring mass transfer and decontamination rates of different water cleaning processes. Since a real-world application of these devices is still limited due to the need of precious metals in their structure and the high cost associated with their fabrication, we present low-cost tubular micromotors solely based on graphitic carbon nitride photocatalyst. These metal-free C3N4-based tubular micromotors are powered by visible light irradiation, which enables their remote motion control by simply turning on/off the light irradiation. Moreover, taking advantage of their adsorptive capabilities and inherent fluorescence, the removal of a heavy metal from contaminated water with the concomitant optical monitoring of its adsorption by fluorescence quenching was demonstrated. Interestingly, the speed of C3N4 micromotors was enhanced in the presence of common toxic pollutants found in wastewater, showing a promising application for environmental remediation.

The emerging field of self-propelling micro/nanorobots is teeming with a wide variety of novel micro/nanostructures, which are tested here for self-propulsion in a liquid environment. As the size of these microscopic movers diminishes into the fully nanosized region, the ballistic paths of an active micromotor become a random walk of colloidal particles. To test such colloidal samples for self-propulsion, the commonly adopted “golden rule” is to refer to the mean squared displacement (MSD) function of the measured particle tracks. The practical significance of the result strongly depends on the amount of collected particle data and the sampling rate of the particle track. Because micro/nanomotor preparation methods are mostly low-yield, the amount of used experimental data in published results is often on the edge of reproducibility. To address the situation we perform MSD analysis on an experimental as well as a simulated dataset. These data are used to explore the effects of MSD analysis on limited data and several situations where the lack of data can lead to insignificant results.

Templated electrodeposition is a highly interesting technique for the synthesis of nanomaterials in any desired shape and composition. Using electrodeposition, a wide range of materials can be synthesized in a simple and cost-effective manner, while using a template to guide the electrodeposition provides a method for perfect pattern replication. The most common nanostructures that are made by templated electrodeposition are nanowires and nanotubes that are deposited inside the pores of track-etched polycarbonate or anodized alumina membranes. By exchanging the electrolyte during templated electrodeposition, it becomes possible to deposit radially and axially segmented nanowires as well, which we for instance employed for the deposition of segmented Ag | ZnO nanowires and coaxial TiO2-Ag nanowires that were both used for photocatalytic H2 formation [1-3]. Furthermore, we’ve also deposited segmented Pt | Au nanowires that were used for autonomous movement in H2O2. And as can be seen in Figure 1, this principle could even be further extended for the formation of multifunctional nanowires comprised of six independently deposited material segments combining photocatalysis and micromotion in a single nanowire [1]. References: [1] Maijenburg et al., Small, 7 (2011) 2709 [2] Maijenburg et al., J. Mater. Chem. A, 2 (2014) 2648 [3] Maijenburg et al., J. Vis. Exp., 87 (2014) e51547

Frank E. Osterloh, Department of Chemistry, University of California, Davis, CA 95616, USA, fosterloh@ucdavis.edu, http://orcid.org/0000-0002-9288-3407 The identification of an artificial photosynthesis method to turn solar energy into globally usable amounts of fuel is considered one of the most important challenges today. Photochemical water splitting with particle-based systems has the greatest potential to achieve this goal. Because of the total integration of components for light absorption and water electrolysis, particle-based photocatalysts (see Figure below) can be over one order of magnitude cheaper than photoelectrochemical or photovoltaic cells. Currently, the development of photocatalysts for the overall water splitting reaction is limited by intrinsic materials issues and by an incomplete understanding of the photochemical charge carrier separation and recombination in small particles. This talk will discuss these obstacles and present ways to overcome them using recent examples from the literature and from the author’s own laboratory.

Microswimmers made from a photocatalytic material show strong morphology- and materials-dependent dynamics. I will discuss how active motion and self-assembly of light-activated, self-propelled particles can be fine-tuned by designing specialized hybrid structures with complex shape and composition.

Self-motile mesoporous ZnO/Pt-based Janus micromotors accelerated by bubble propulsion that provide efficient removal of explosives and dye pollutants via photodegradation under visible light are presented. Decomposition of H2O2 (the fuel) is triggered by a platinum catalytic layer asymmetrically deposited on the nanosheets of the hierarchical and mesoporous ZnO microparticles. The size-dependent motion behavior of the mesoporous micromotors is studied; the micromotors with average size ∼1.5 μm exhibit enhanced self-diffusiophoretic motion, whereas the fast bubble propulsion is detected for micromotors larger than 5 μm. The bubble-propelled mesoporous ZnO/Pt Janus micromotors show remarkable speeds of over 350 μm s–1 at H2O2 concentrations lower than 5 wt %, which is unusual for Janus micromotors based on dense materials such as ZnO. This high speed is related to efficient bubble nucleation, pinning, and growth due to the highly active and rough surface area of these micromotors, whereas the ZnO/Pt particles with a smooth surface and low surface area are motionless. We discovered new atomic interfaces of ZnO2 introduced into the ZnO/Pt micromotor system, as revealed by X-ray diffraction (XRD), which contribute to enhance their photocatalytic activity under visible light. Such coupling of the rapid movement with the high catalytic performance of ZnO/Pt Janus micromotors provides efficient removal of nitroaromatic explosives and dye pollutants from contaminated water under visible light without the need for UV irradiation. This paves the way for real-world environmental remediation efforts using microrobots.

The development of photochemical systems capable of mimicking the natural photosynthesis by driving useful chemical transformations has attracted significant interest motivated by the need to meet various environmental concerns and to secure the future supply of clean and sustainable energy. The activity, stability, and selectivity of such systems is determined not only by the ability of materials to absorb light and create charges, but also by efficient separation of the charges and their fast and selective reaction with substrates. In this respect, the effective coupling of well-designed co-catalysts with light absorbers plays a crucial role. The talk will focus on the problem of interfacing different light absorbers with various types of cocatalysts for water remediation[1] and solar water splitting.[2,3] One of the most important features of effective cocatalysts in photo(electro)catalytic systems are their optical properties since undesired parasitic absorption of light by the catalysts should be avoided. In this respect, properties of TiO2 powders modified with single Cu(II) and Fe(III) co-catalytic sites will be discussed. The results demonstrate that the single Cu(II) and Fe(III) ions act as effective cocatalytic sites, enhancing the charge separation, catalyzing "dark" redox reactions at the interface, and improving thus the normally very low quantum yields of UV light-activated TiO2 photocatalysts.[1] Similarly, our recent efforts in introducing efficient catalysts into porous photoanodes for light-driven water oxidation will be reviewed. The focus will be on distinct advantages of ultrasmall (1-2 nm) cobalt oxyhydroxide nanoclusters as compared to more conventional (larger) cocatalyst particles.[3] Finally, strong electrolytes effects on the photoanode performance will be discussed based on photoelectrochemical and spectroscopic investigations, combined by theoretical calculations. References: [1] S. Neubert, D. Mitoraj, S. A. Shevlin, P. Pulisova, M. Heimann, Y. Du, G. K. L. Goh, M. Pacia, K. Kruczala, S. Turner, W. Macyk, Z. X. Guo, R. K. Hocking, R. Beranek, J. Mater. Chem. A 2016, 4, 3127. [2] M. Bledowski, L. Wang, S. Neubert, D. Mitoraj, R. Beranek, J. Phys. Chem. C 2014, 118, 18951. [3] L. Wang, D. Mitoraj, S. Turner, O. V. Khavryuchenko, T. Jacob, R. Hocking, R. Beranek, ACS Catal. 2017, 7, 4759.

Nano/micromachines with autonomous motion are the frontier of nanotechnology and nanomaterial research. These self‐propelled nano/micromachines convert chemical energy obtained from their surroundings to propulsion. They have shown great potential in diagnostic and therapeutic applications. This work introduces a high‐speed tubular electrically conductive micromachine based on reduced nanographene oxide (n‐rGO) as a platform for drug delivery and platinum (Pt) as the catalytic inner layer. n‐rGO/Pt micromachines are loaded with doxorubicin (DOX) by a simple physical adsorption with a very high loading efficiency, displaying single‐ or multistrand wrapping of DOX monomers on the micromachine cylinders. More importantly, it is found that electron injection into DOX@n‐rGO/Pt micromachines via electrochemistry leads to expulsion of DOX from micromachines in motion within only a few seconds. An in vitro study confirms this efficient release mechanism in the presence of cancerous cells. The unique properties of the n‐rGO/Pt micromotor enable the effective management of DOX release at the tumor site and thus enhances the therapeutic efficiency and reduces the side toxicity toward the healthy tissue. These micromachine drug carriers combine the high loading capacity of conventional carbon‐based drug carriers with a fast and efficient electrochemical drug‐release mechanism.

Ever since the first reports on photocatalytic CO$_2$ reduction$^1,2$ research in this direction has been performed for almost 40 years. The reaction allows not only to recycle CO$_2$, but also to provide a source of renewable hydrocarbons (Figure 1). Yet, no industrial process has so far been realized, which is predominantly due to the extremely low yields and selectivities achieved so far. A thorough literature search furthermore revealed that many studies fail to prove that hydrocarbon formation truly originated from CO$_2$.$^3$ The potential of the photocatalyst may be overestimated, if impurities contribute to product formation. To evaluate the potential of photocatalytic CO$_2$ reduction, we conducted a thorough study on TiO$_2$ as model photocatalyst. Under conditions of highest purity it is demonstrated that continuous production of CH$_4$ from CO$_2$ as carbon source is truly possible.$^4$ Reaction conditions are optimized with respect to light intensity and reactant concentration,$^4,5$ and potential intermediates of the reaction are identified under batch conditions.$^6$ It is also verified that the reaction is indeed light-induced, but that it contains classical catalytic steps that are accelerated by higher temperature.$^5$ Surface modification with noble metals and structural design is shown to alter the product distribution.$^7$ However, yields remain low, and product formation ceases after some time. Apparent quantum yields and productivities, determined with TiO$_2$ powders and powder thin films, are almost negligible$^8$. The catalytic cycle is most likely not closed, since oxygen formation in the gas phase does not occur. If oxygen is added to the reaction mixture, product formation completely ceases.$^4$ It is thus concluded that commercial TiO$_2$ can never be a viable option to realize an industrially feasible process. Instead, directions for future material and process development are suggested and discussed that bear the promise to realize such a process in the future.

As compared to living systems like bacteria or more complex organisms, active particles are simple machines that just propel due to the consumption of energy. They obey physical interactions that are the result of their propulsion mechanism but essentially lack a system of sensorial inputs and feedback loops which control the activity their biological counterparts. We have developed active self-thermophoretic particles that can be controlled by light in all of their aspects of motion. Using a real time detection and feedback control we introduce artificial sensorial interactions among particles and of particles with their environment to explore which simple control mechanisms provide behavioral features in living systems. Our experiments show that sensorial information and sensorial delays drive structure formation and dynamics in ensembles of active particles. The integration of machine learning algorithms into the feedback processes to optimize strategies under environmental pressure highlights the importance of noise for the development of behavior.

After a brief overview of artificial intelligence, machine learning and deep learning, I will present a series of recent works in which we have employed deep learning for applications in photonics and active matter. In particular, I will explain how we employed deep learning to enhance digital video microscopy [1], to estimate the properties of anomalous diffusion, and to improve the calculation of optical forces. Finally, I will provide an outlook for the application of deep learning in photonics and active matter. References [1] S. Helgadottir, A. Argun and G. Volpe, Digital video microscopy enhanced by deep learning. arXiv 1812.02653 (2018).

Differential dynamic microscopy (DDM) is a technique that exploits optical microscopy to obtain local, multi-scale quantitative information about dynamic samples, in most cases without user intervention. It is proving extremely useful in understanding dynamics in liquid suspensions, soft materials, cells, and tissues. In DDM, image sequences are analyzed via a combination of image differences and spatial Fourier transforms to obtain information equivalent to that obtained by means of light scattering techniques. Compared to light scattering, DDM offers obvious advantages, principally (a) simplicity of the setup; (b) possibility of removing static contributions along the optical path; (c) power of simultaneous different microscopy contrast mechanisms; and (d) flexibility of choosing an analysis region, analogous to a scattering volume. For many questions, DDM has also advantages compared to segmentation/tracking approaches and to correlation techniques like particle image velocimetry. The very straightforward DDM approach, originally demonstrated with bright field microscopy of aqueous colloids, has lately been used to probe a variety of other complex fluids and biological systems with many imaging methods, including dark-field, differential interference contrast, wide-field, light-sheet, and confocal microscopy. The number of adopting groups is rapidly increasing and so are the applications. Here, we briefly recall the working principles of DDM, we highlight its advantages and limitations, we outline recent experimental breakthroughs, and we provide a perspective on future challenges and directions. DDM can become a standard primary tool in every laboratory equipped with a microscope, at the very least as a first bias-free automated evaluation of the dynamics in a system. References [1] R. Cerbino, V. Trappe, Phys. Rev. Lett. 100, 188102 (2008) [2] F. Giavazzi, R. Cerbino, J. Opt. 16, 083001 (2014) [3] R. Cerbino, P. Cicuta, J. Chem. Phys 147, 110901 (2017)

Molecular metal oxides, or polyoxometalates, are nanoscale molecular units comprised of early, high-valent transition metals and oxo ligands. The compound class offers unique photoactivity, and the linkage of additional photoabsorbers to POMs by chemical means enables light-absorption and utilization across the UV-Vis range. This presentation will showcase how POMs can be used for light-driven productive chemistry under homogeneous conditions and when deposited on nanostructured functional substrates. Current applications in energy conversion as well as future directions of this research will be discussed, and challenges along the way will be highlighted.

Photocatalysis, Carbon Nitride, Hydrogen evolution, Organic Synthesis Porous g-C3N4 with a bandgap of 2.8 eV can be synthesized from different precursors, e.g. by simple pyrolysis of inexpensive, environmentally friendly, active oxygen-evolving urea. Photocatalytic hydrogen evolution in the presence of methanol as a sacrificial reagent and Pt as co-catalyst showed for g-C3N4 prepared from urea higher rates than for such prepared from thiourea or melamine [1]. This can on one hand be attributed to the more porous structure and higher surface area, but on the other hand transient photocurrent and electrochemical impedance measurements revealed a higher photoinduced charge carrier separation being attributed to a slightly more negative flat-band potential. This causes a higher driving force of the electrons to react with the surface-adsorbed protons. Solid-state NMR data confirmed a lower degree of polymerization in the urea-derived g-C3N4, corresponding to additional structural defects which might lead to more photocatalytic active sites. Photocatalysis-assisted synthesis of organic molecules is a wide field; however, mainly homogeneous catalysis is employed. Metal complexes of Ru or Ir and organic dyes like eosin Y are used as homogeneous photocatalysts. However, the difficult reusability of the catalysts as well as the use of precious transition metals limit their applicability. As heterogeneous photocatalysts mainly the wide band gap material TiO2 or the photocorrosive CdS have been established. Graphitic C3N4 is an attractive heterogeneous alternative, which shows visible light activity and has proven to be able to generate organic molecules, for example ketals [2]. As an example we examined the use of g-C3N4 for the shown ketalization reaction. Aim of the work is to optimize the selectivity by modification of the carbon nitride being modified via exfoliation and nanocasting. [1] M. Ismael, Y. Wu, D.H. Taffa, P. Bottke, M. Wark, New J. Chem 2019, 43, 6909-6920. [2] Y. Zhao, M. Shalom, M. Antonietti, Appl. Catal., B 2017, 207, 311–315.

BiVO4 is one of the most promising anode materials for photoelectrochemical (PEC) water splitting [1]. We will review the common strategies to improve the PEC activity of BiVO4 photoanodes, including doping, nanostructures, heterojunctions, and electrocatalysts. There is a growing awareness that sustainable PEC water splitting requires over thousands of operation hours under harsh PEC conditions. While stable in neutral-pH media, BiVO4 suffers from photocorrosion. Recent stability studies showed conflicting results of V dissolution from BiVO4 either together with Bi [2] or alone [3]. We performed in-operando dissolution study by applying our development [4] in illuminated scanning flow cell, integrated with inductively coupled plasma mass spectrometry. V dissolution prevails at the initial stage. Then, Bi and V dissolve close to stoichiometry. We systematically examined time and potential-resolved dissolution, and suggest oxidation of Bi3+ as a main mechanism of BiVO4 photocorrosion. Electron microscopy after photocorrosion reveals Bi-rich particles on the BiVO4 surface that correlate to the off-stoichiometric V dissolution and serve a role in passivation. With insights gained from the powerful combination of in-operando dissolution measurement and correlative microscopy, we will discuss future opportunities in photocatalyst design, including electrochemical surface activation and passivation. [1] Rohloff M. et al. Sustainable Energy Fuels 1: 1830 (2017). [2] Toma F.M., et al. Nat. Commun. 7: 12012 (2016). [3] Lee D.K. & Choi K.S. Nat. Energy 3: 53 (2018). [4] Knöppel J., Zhang S., et al. Electrochem. Commun. 96: 53 (2018). [5] We acknowledge financial support from DFG under SPP 1613.

The use of visible light in organic synthesis has received a lot of attention over the last decade, although the idea was promoted already more than 100 years ago.1 Visible light is easily generated, safe, leaves no trace as a reagent in the reaction mixture and can provide energy to initiate kinetically hindered or even endothermic reactions. This improves the efficiency of known reactions in organic synthesis, but also enables hitherto unknown transformations.2 However, the use of visible light has also a severe drawback: The energy of visible light photons is small compared to chemical bond energies. The energetic barrier for photoreduction and radical generation can be overcome using more than one visible light photon. By consecutive photoinduced electron transfer (conPET)3 a colored and persistent radical anion of the photocatalysts is generated, which is subsequently again excited gaining additional redox energy. Transfer of the electron to a substrate closes the catalytic cycle. Our first generation of conPET catalysts, perylene diimides (PDI), suffered from low solubility and strong aggregation. Rhodamine 6G, a second-generation conPET catalyst, is commercially available, highly soluble and provides upon visible light excitation a reduction power of up to -2.4 V vs SCE.4 A different strategy, combining the energy of two photons by energy and electron transfer reaches even quantum yields of up to 12 %.5 We discuss synthetic applications of photocatalysis including the utilization of carbon dioxide as a C1 building block in synthesis,6 the selective C-F bond activation7 and dual photo-metal catalytic cross-coupling strategies.8 References 1. G. Ciamician, Science 1912 September 27. 2. L. Marzo, S. K. Pagire, O. Reiser, B. König, Angew. Chem. Int. Ed. 2018, 57, 10034. 3 I. Ghosh, T. Ghosh, J. I. Bardagi, B. König, Science 2014, 346, 725. 4. I. Ghosh, B. König, Angew. Chem. Int. Ed. 2016 55, 7676. 5. I. Ghosh, R. S. Shaikh, B. König, Angew. Chem. Int. Ed. 2017, 56, 8544. 6. Q.-Y. Meng, S. Wang, G. S. Huff, B. König, J. Am. Chem. Soc. 2018, 140, 3198; Q.-Y. Meng, S. Wang, B. König, Angew. Chem. Int. Ed. 2017, 56, 13426. 7. K. Chen, N. Berg, R. Gschwind, B. König, J. Am. Chem. Soc. 2017, 139, 18444. 8. A. Savateev, I. Ghosh, B. König, M. Antonietti, Angew. Chem. Int. Ed. 2018, 57, 2.

Synthetic nano- and micromotors interact with their surrounding and realize various scenarios of collective behaviour. For example, being exposed to an interactive environment, such micromotors can be used for biomedical applications, like drug delivery or toxin removal. Here, we report on the collective behaviour of phototochemical blue light driven Ag/AgCl-based Janus swimmers surrounded by a dense matrix of passive silica beads in pure water. Once illuminated by light, swimmers are set into motion due to the generation of a local chemical gradient around each particle. This gradient contributes to the modulation of the electric potential around Janus swimmers and results in the repulsion of surrounding passive beads to a certain distance away from a Janus swimmer (Figure 1). The analysis of this radius of exclusion allows to conclude about the asymmetric interactions of a swimmer with passive beads, in front of the AgCl cap and behind it. We provide an insight into this phenomenon by performing the angular analysis of radii of exclusion and their time evolution at the level of a single particle. We are convinced that such complex mixture of active and passive colloidal particles provides not only a novel insight to the interactive effects between active-passive particles, but also can offer better understanding of the application potential of the system in e.g. particles transport at micro- and nanoscale, and chemical sensing.

Recently, visible light-driven Ag/AgCl-based spherical Janus micromotors have been demonstrated, which couple plasmonic light absorption with the photochemical decomposition of AgCl [1]. These new Janus micromotors reveal high motility in pure water, with mean squared displacements (MSD) 100× higher than previously studied visible light-driven Janus micromotors, which was achieved by their design and suppression of the rotational diffusion. In presence of passive beads, the visible light-actuated exclusion effect between clusters of Janus motors and passive beads is demonstrated [2]. This mixed system with complex interactions offers promise for implications in light-controlled propulsion transport and chemical sensing. References: [1] Xu Wang, L. Baraban, Anh Nguyen, Jin Ge, V. R. Misko, J. Tempere, F. Nori, G. Cuniberti, J. Fassbender, and D. Makarov, High-motility visible light-driven Ag/AgCl Janus micromotors, Small 14, 1803613 (2018). [2] Xu Wang, L. Baraban, V. R. Misko, F. Nori, G. Cuniberti, Tao Huang, J. Fassbender, and D. Makarov, Visible light actuated efficient exclusion between Ag/AgCl micromotors and passive beads, Small 14, 1802537 (2018).

Nano- and microswimmers are objects that are propelled in fluids by catalysis of chemical reactions, or by input of light, heat, electrical, magnetic or acoustic energy. Together with Ayusman Sen and other colleagues at Penn State we have explored the use of catalysis and photocatalysis to power Janus rods that contain catalytic or semiconducting segments. In both cases, the movement of the particles is powered by locally generated electric fields. Despite the difference in propulsion mechanisms, (photo)catalytic swimmers are subject to the same external forces as natural microswimmers such as bacteria. Their biomimetic collective behaviors, including momentum transfer, fluid pumping, swarming and predator-prey interactions, arise from locally generated concentration gradients and are interesting as models of living active matter. In collaboration with Mauricio Hoyos at ESPCI (Paris) we discovered that asymmetric microparticles also undergo autonomous motion in fluids when they are excited by low power ultrasound. Acoustic propulsion, particularly when combined with magnetic, chemical, or photochemical propulsion for steering and assembly, is potentially useful for diagnostic and biomedical applications because it is salt-tolerant and does not involve toxic chemical fuels. When they are confined to an acoustic pressure node, bimetallic Janus microrods are propelled along their long axis by locally generated streaming of the fluid. By bringing the resonant acoustic cavity “on board” in the form of a trapped air bubble, one can make powerful microswimmers that move at speeds of millimeters per second. These bubble-based swimmers have greater flexibility than bimetallic acoustic swimmers in that they do not require an acoustic standing wave for propulsion. Therefore they can move autonomously in three dimensions and can be propelled at relatively long distances from the source of acoustic power.

If a chemically active, spherical particle is close to a liquid-fluid interface, the distribution of reactant and product molecules is spatially inhomogeneous both at and along the interface, as well as along the direction normal to the interface, including the region around the particle. The inhomogeneous distribution of chemical species along the interface can induce local variations of the surface tension. This gives rise to interfacial stresses and hence leads to the onset of the so-called Marangoni hydrodynamic flow. Such flows propagate into the bulk and drive the particle close to or far away from the interface. This effective interaction is long-ranged and may provide an alternative mechanism to control particle accumulation at liquid-fluid interfaces [1]. Proceeding towards the issue of collective dynamics, we have developed a mean-field model for the dynamics of the large-scale spatial distribution of a monolayer of spherically symmetric, active particles at a liquid-fluid interface [2,3]. The model accounts for direct pair interactions as well as for hydrodynamic interactions, including the Marangoni flow induced by the response of the interface to the chemical activity of the particles. It has been shown that, in spite of the intrinsic non-equilibrium character of the system, the monolayer evolves to a ``pseudo-equilibrium'' state, in which the Marangoni flows impose the coexistence of the thermodynamic phases (e.g., gas, liquid, solid) which are associated with the direct interaction. In particular, in the case of a ``soft'' repulsion $\propto r^{-3}$, which models electrostatic or magnetic interparticle forces, it was shown that, for a sufficiently large mean number density, two-dimensional phase transitions (freezing from liquid to hexatic, and melting from solid to hexatic) should be observable in a radially stratified, ``onion-like'' structure within the monolayer. Finally, the relevance of these results for potential experimental realizations is critically assessed and discussed [1,3]. References 1. A. Dominguez, P. Malgaretti, M.N. Popescu, and S. Dietrich, Effective interaction between active colloids and fluid interfaces induced by Marangoni flows, Phys. Rev. Lett. 116, 078301 (2016). 2. A. Domínguez, P. Malgaretti, M.N. Popescu, and S. Dietrich, Collective dynamics of chemically active particles trapped at a fluid interface, Soft Matter 12, 8398 (2016). 3. A. Dominguez and M.N. Popescu, Phase coexistence in a monolayer of active particles induced by Marangoni flows, Soft Matter 14, 8017 (2018).

In the past decade, liquid-crystal network (LCN) technology has been used for the development of soft actuators based on the controlled reversible changes of the order parameter of the oriented polymer network. Various deformations ranging from simply bending or curling to complex origami type of morphing are demonstrated to lift weights, mimic nature in shape and color, or transport materials. Herein, we propose the use of a liquid crystal network for soft robotics where the various molecular accessories are assembled in the two dimensions of a coating. For instance, the LCN surface deforms dynamically into a variety of pre-designed topographic patterns by means of various triggers, such as temperature, light and the input of electric field. These microscopic deformations exhibit macroscopic impact on, for instance, tribology, haptics, laminar mixing of fluids in microchannels and directed cell growth. Another robotic-relevant function we brought into the LCN coating is its capability to secret liquids under UV irradiation or by an AC field. This controlled release is associated with many potential applications, including lubrication, controlled adhesion, drug delivery, agriculture, antifouling in marine and biomedical devices, personal care and cosmetics. With this we bring together a tool box to form two-dimensional soft robots designed to operate in area where man and machine come together.

Soft microrobots consist of soft stimuli-responsive materials that endow the microrobots with actual robotic functionality, such as on-board sensing or actuation. For instance, photoresponsive polymeric materials, i.e. materials that respond to light with mechanical deformation, can be used in microrobots as actuators that are wirelessly powered and controlled by light. One example of such materials is represented by photoresponsive liquid-crystal elastomers (LCEs), in which light is absorbed by embedded dye molecules and triggers a liquid-crystal phase transition resulting in a macroscopic shape change of the material. We adopted structured light for powering and controlling the actuation of microrobots consisting of monolithic photoresponsive LCEs, resulting in complex motion patterns such as biomimetic peristaltic deformations. We thus realized the first microrobots that swim like biological microorganisms, that is, by internally-generated periodic body-shape changes. Here the effects of the structured light control parameters on the LCE actuation are analyzed by modelling the response dynamics of the material in different environments, and their complex role in controlling the microrobots movement and locomotion is elucidated. This fundamental understanding provides the basis for optimizing the locomotion performance of the microrobots in different environments, as well as for achieving an even wider variety of complex movements, thus further enhancing the versatility of light-controlled soft microrobots.

The combination of biological components and artificial ones emerges into what we called hybrid machines/bots. Alike bacteria or small swimmers found in nature, artificial nanobots convert bio-available fuels to generate propulsion force to swim at the nanoscale. One of the dreams in nanotechnology is to engineer small vehicles which can eventually be applied in vivo for medical purposes. Major advances have been demonstrated towards that end, however, questions like -how to swim at the nanoscale, how to achieve motion control and how to image these nanobots- need to be properly addressed. Here, I will present our recent developments in the field of nanomotors that can autonomously swim and perform complex tasks in vitro [1]. Our hybrid “bots” combine the best from the two worlds: biology (enzymes, single cells and tissues) and (nano)technology (nanoparticles, 3D Bioprinting) providing remote control, guidance and actuation. We demonstrated the efficient transport and the enhanced release of drugs into cancer cells [2] and spheroids [3], sensing [4] capabilities and their use in water remediation [5]. [1] S. Sánchez, et al. Angew.Chem.Int.Edit. 2015, 54,1414-1444; J. Katuri,et al. Acc. Chem. Res. 2017, 50, 2−11; Patino et al. Acc. Chem. Res 2018, 51 (11) 2662-2671. [2] X. Ma, A. C. Hortelao, T. Patiño, S. Sánchez. ACS Nano. 2016, 10 (10), 9111-9122. [3] A. C. Hortelao et al. ACS Nano 2018, 13, (1) 429-439. [4] T. Patino et al. NanoLett. 2019. DOI: 10.1021/acs.nanolett.8b04794 [5] J. Parmar et al. J. Am.Chem.Soc. 2018 140 (30), 9317-9331

Herein, we present the concept of TiO2/Pt1 Janus micromotors laying “breadcrumbs”; that is, these micromachines can move/return to a home position without external guidance after their external energy input is stopped. Such autonomy of motion opens the door for truly independent applications of micromotors in the “deliver-and-return” fashion.2 On the other hand, we have demonstrated that these light powered TiO2/Pt Janus micromotors have high efficiency for the “on-the-fly” photocatalytic degradation of 2,4-DNT and 2,4,6-TNT in pure water under UV irradiation through the redox reactions that induced the photogenerated of holes and electrons on the micromotor surfaces. The produced local electric field propels the micromotors as well as oxidative species that are able to photodegrade 2,4-DNT and 2,4,6-TNT more efficiently compared with static TiO2/Pt Janus particles that is thanks to the enhanced mixing and mass transfer in the solution by movement of these micromotors.3 Such fuel free light-powered micromotors used for explosive degradation represent an important contribution since their leakage leads to serious environmental pollution and threatens human health for that reason, it is imperative to remove or decompose them rapidly and efficiently.4 References 1.H. Wang, M. Pumera. Emerging materials for the fabrication of micro/nanomotors. Nanoscale 2017, 9, 2109. 2.L. Kong, C. C. Mayorga‐Martinez, J. Guan, M. Pumera. Smart Microdevices Laying “Breadcrumbs” to Find the Way Home: Chemotactic Homing TiO2/Pt Janus Microrobots. CHEM-ASIAN J. 2019, https://doi.org/10.1002/asia.201900004. 3.L. Kong, C. C. Mayorga-Martinez, J. Guan, M. Pumera. Fuel-Free Light-Powered TiO2/Pt Janus Micromotors for Enhanced Nitroaromatic Explosives Degradation. ACS Appl. Mater. Interfaces 2018, 10, 22427. 4.H. Wang, M. Pumera. Micro/Nanomachines and Living Biosystems: From Simple Interactions to Microcyborgs. Adv. Funct. Mater. 2018, 28, 1705421.

Current treatment of cancer is usually based on the administration of potent cytotoxic drugs able to destroy dividing cells as tumoral ones. Chemotherapy has constituted the main therapeutic approach for the treatment of aggressive tumors for over 100 years. However, one of its main drawbacks is the lack of selectivity which leads to numerous side effects and compromise the patient life. In the recent years, special attention has been devoted to the design of nanocarriers able to selectively transport these cytotoxic drugs to tumoral cells and once there, to release their cargo in a controlled manner in the presence of certain stimuli. Thus, stimuli originated by the own disease process (pH, redox changes, etc.) or that can be externally generated (magnetic fields, light, ultrasounds, etc.) have been widely explored in order to design activatable nanocarriers. Among them, light constitutes an ideal stimulus for drug delivery purposes due to it allows spatially and temporally control by finely selecting the area and the exposure time. Moreover, light can be locally applied to internal tumoral areas using optic probes. Herein, the development of light-sensitive nanocarriers for delivering cytotoxic agents to tumoral cells will be presented. These nanocarriers have been specially engineered to respond to different wavelengths such as UV, visible and near infrared in order to provide a wide arsenal of possibilities for the treatment of solid tumors located in different localizations.

PH-switchable enzymatic nanoreactors by light-driven proton transfer Silvia Morenoa, Priyanka Sharana,b, Johanna Engelkea, Brigitte Voita,c,d, Albena Lederera,b, Dietmar Appelhans*,a a Leibniz Institute of Polymer Research Dresden, Hohe Straße 6, 01069 Dresden, Germany. b Organic Chemistry of Polymers, Technische Universität Dresden, 01062 Dresden, Germany. E-mail: applhans@ipfdd.de; moreno@ipfdd.de bDepartment of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany c Organic Chemistry of Polymers, Technische Universität Dresden, 01062 Dresden, Germany d Center for Advancing Electronics Dresden, 01062 Dresden, Germany Engineering of multifunctional vesicular (multi)compartments for mimicking specific cellular functions1 is one promising approach in areas such as medicine or systems biology. These vesicular compartments have to fulfil various key properties (e.g. tuneable by external stimuli, controlling membrane functions for exchanging biomolecules, controlled release of biomolecules, retaining cargo inside of vesicular cavity), while multicompartments should also possess orthogonal-responsive membrane properties to control spatiotemporal and spatially separated biological pathways for establishing protocells.2 Herein, a self-adaptive enzymatic nanoreactor that drives a catalytic reaction by a temporal control of the membrane permeability through a reversibly switchable light-driven proton transfer was established using pH-responsive and crosslinked polymersomes.3-6 Merocyanine (MEH) was used as photo-active proton donator to modulate the permeability of Glucose Oxidase loaded polymersomes through cyclic, temporally controlled protonation and deprotonation of its membrane. Several key parameters were studied obtaining an excellent cyclic, temporal biocatalytic “ON−OFF state” of the enzymatic nanoreactors through light-driven proton transfer. The nanoreactor membranes can undergo several times light-driven proton transfers without losing their functional effectiveness and resulting a cumulative effect. This light-driven approach opens new possibilities in the design and fabrication of cell mimics and protocells.1,2 References 1 Schwille, P. Science 2011, 33, 1252. 2 Mann, S. Acc. Chem. Res. 2012, 45, 2131. 3 Gumz, H. et al. Adv. Sci. 2018, accepted. 4 Gaitzsch, J. et al. Angew. Chem. Int. Ed. 2012, 51, 4448. 5 Gräfe, D. et al. Nanoscale 2014, 6, 10752. 6 Liu, X. et al. Angew. Chem. Int. Ed. 2017, 56, 16233.

I will give short overview of our Center for Advanced Functional Nanorobots activities.

The development of synthetic nanomotors for technological applications for life science or nanomedicine is a key focus of current basic research. Enzyme-powered nanomotors offer more advantages, due to their biocompatible and biomimetic properties, capable of actuating in biological systems without side effects. Herein, we not only explored another enzyme to the library of biocatalysts with the ability to power motor motion, but also shows an enzymatic motor that retains activity in organic solutions, degrading the triglyceride. The biofriendly propulsion of lipase powered MSNs was realized through the biocatalytic reaction of lipase and its representative water-soluble substrate (triacetin as fuel), exhibiting the enhanced motion (i.e., ~50% increase of diffusion coefficient, Figure 1) at low triacetin concentration (e.g. < 10 mM) compared to Brownian motion, with the lifetime of ~40 min. Particularly, lipase here not only served as the power engine, but also functionalized as a cleaner for the lipid globules (e.g. tributyrin) in PBS solution, which offered a potential tool for dealing with the lipid storage diseases in the biomedical fields.

Self-organization is observed in many systems throughout nature, from the microscopic level of bacteria colonies to the macroscopic level of bird flocks, fish schools, and even human traffic patterns. The recent discovery of artificial self-propelled particles has led to the study the interactions of these synthetic microswimmers that mimic the collective behavior seen in biological systems. We will describe an autonomous photochemical oscillatory micromotor system in which active colloidal particles form clusters whose size changes periodically. After a cluster oscillation initiates, the oscillatory waves propagate to nearby clusters and eventually all the clusters oscillate in phase-shifted synchrony. The oscillatory behavior is governed by an electrolytic self-diffusiophoretic mechanism which involves alternating electric fields generated by competing reduction and oxidation reactions. Furthermore, the photochemically active system can communicate with larger inactive tracer particles which drives the reversible assembly of the latter into colloidal crystals.