214 results
(View BibTeX file of all listed publications)
2023
Multiplane Diffractive Acoustic Networks
Athanassiadis, A. G., Schlieder, L., Melde, K., Volchkov, V., Schölkopf, B., Fischer, P.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 70(5):441-448, IEEE, March 2023 (article)
Biomolecular actuators for genetically selective acoustic manipulation of cells
Wu, D., Baresch, D., Cook, C., Ma, Z., Duan, M., Malounda, D., Maresca, D., Abundo, M. P., Lee, J., Shivaei, S., Mittelstein, D. R., Qiu, T., Fischer, P., Shapiro, M. G.
Science Advances, 9(8):eadd9186, AAAS, Washington, 2023 (article)
2022
Hierarchical optofluidic microreactor for water purification using an array of TiO2 nanostructures
Kim, H., Kwon, H., Song, R., Shin, S., Ham, S., Park, H., Lee, J., Fischer, P., Bodenschatz, E.
npj Clean Water, 5, pages: 62, Macmillan Publishers Limited, Basingstoke, November 2022 (article)
Magnetic Micro-/Nanopropellers for Biomedicine
Qiu, T., Jeong, M., Goyal, R., Kadiri, V., Sachs, J., Fischer, P.
In Field-Driven Micro and Nanorobots for Biology and Medicine, pages: 389-410, 16, (Editors: Sun, Y. and Wang, X. and Yu, J.), Springer, Cham, 2022 (inbook)
In nature, many bacteria swim by rotating their helical flagella. A particularly promising class of artificial micro- and nano-robots mimic this propeller-like propulsion mechanism to move through fluids and tissues for applications in minimally-invasive medicine. Several fundamental challenges have to be overcome in order to build micro-machines that move similar to bacteria for in vivo applications. Here, we review recent advances of magnetically-powered micro-/nano-propellers. Four important aspects of the propellers – the geometrical shape, the fabrication method, the generation of magnetic fields for actuation, and the choice of biocompatible magnetic materials – are highlighted. First, the fundamental requirements are elucidated that arise due to hydrodynamics at low Reynolds (Re) number. We discuss the role that the propellers’ shape and symmetry play in realizing effective propulsion at low Re. Second, the additive nano-fabrication method Glancing Angle Deposition is discussed as a versatile technique to quickly grow large numbers of designer nano-helices. Third, systems to generate rotating magnetic fields via permanent magnets or electromagnetic coils are presented. And finally, the biocompatibility of the magnetic materials is discussed. Iron-platinum is highlighted due to its biocompatibility and its superior magnetic properties, which is promising for targeted delivery, minimally-invasive magnetic nano-devices and biomedical applications.
Comment on “Molecules, the Ultimate Nanomotor: Linking Chemical Reaction Intermediates to their Molecular Diffusivity”
Günther, J., Fillbrook, L. L., Majer, G., O’Leary, D. J., Price, W. S., Ryswyk, H. V., Fischer, P., Beves, J. E.
ACS Nano, 16(7):9973-9976, July 2022 (article)
Diffusion mechanisms of DNA in agarose gels: NMR Studies and Monte Carlo Simulations
Bochert, I., Günther, J., Fischer, P., Majer, G.
The Journal of Chemical Physics, 156(24):245103, June 2022 (article)
We report on the diffusion mechanism of short, single-stranded DNA molecules with up to 100 nucleobases in agarose gels with concentrations of up to 2.0% with the aim to characterize the DNA–agarose interaction. The diffusion coefficients were measured directly, i.e., without any model assumptions, by pulsed field gradient nuclear magnetic resonance (PFG-NMR). We find that the diffusion coefficient decreases, as expected, with an increase in both DNA strand length and gel concentration. In addition, we performed Monte Carlo simulations of particle diffusion in a model network of polymer chains, considering our experimental conditions. Together, the Monte Carlo simulations and the PFG-NMR results show that the decrease in diffusion coefficients in the presence of the agarose gel is due to a temporary adhesion of the DNA molecules to the surface of gel fibers. The average adhesion time to a given gel fiber increases with the length of the DNA strands but is independent of the number of gel fibers. The corresponding magnitude of the binding enthalpies of DNA strands to gel fibers indicates that a mixture of van der Waals interactions and hydrogen bonding contributes to the decreased diffusion of DNA in agarose gels.
Amplification of Acoustic Forces Using Microbubble Arrays Enables Manipulation of Centimeter-Scale Objects
Goyal, R., Athanassiadis, A. G., Ma, Z., Fischer, P.
Physical Review Letters, 128(25):254502, June 2022 (article)
Manipulation of macroscale objects by sound is fundamentally limited by the wavelength and object size. Resonant subwavelength scatterers such as bubbles can decouple these requirements, but typically the forces are weak. Here we show that patterning bubbles into arrays leads to geometric amplification of the scattering forces, enabling the precise assembly and manipulation of cm-scale objects. We rotate a 1 cm object continuously or position it with 15 μm accuracy, using sound with a 50 cm wavelength. The results are described well by a theoretical model. Our results lay the foundation for using secondary Bjerknes forces in the controlled organization and manipulation of macroscale structures.
Toward Maximally Electromagnetically Chiral Scatterers at Optical Frequencies
Garcia-Santiago, X., Hammerschmidt, M., Sachs, J., Burger, S., Kwon, H., Knöller, M., Arens, T., Fischer, P., Fernandez-Corbaton, I., Rockstuhl, C.
ACS Photonics, 9(6):1954-1964, May 2022 (article)
Designing objects with predefined optical properties is a task of fundamental importance for nanophotonics, and chirality is a prototypical example of such a property, with applications ranging from photochemistry to nonlinear photonics. A measure of electromagnetic chirality with a well-defined upper bound has recently been proposed. Here, we optimize the shape of silver helices at discrete frequencies ranging from the far-infrared to the optical band. Gaussian process optimization, taking into account also shape derivative information on the helices scattering response, is used to maximize the electromagnetic chirality. We show that the theoretical designs achieve more than 90% of the upper bound of em-chirality for wavelenghts 3 μm or larger, while their performance decreases toward the optical band. We fabricate and characterize helices for operation at 800 nm and identify some of the imperfections that affect the performance. Our work motivates further research both on the theoretical and fabrication sides to unlock potential applications of objects with large electromagnetic chirality at optical frequencies, such as helicity filtering glasses. We show that, at 3 μm, a thin slab of randomly oriented helices can absorb 99% of the light of one helicity while absorbing only 10% of the opposite helicity.
Plasmonic Nanostructure Engineering with Shadow Growth
Jang-Hwan, H., Kim, D., Kim, J., Kim, G., Fischer, P., Jeong, H.
Advanced Materials, 35(34):2107917, March 2022 (article)
Physical shadow growth is a vacuum deposition technique that permits a wide variety of 3D-shaped nanoparticles and structures to be fabricated from a large library of materials. Recent advances in the control of the shadow effect at the nanoscale expand the scope of nanomaterials from spherical nanoparticles to complex 3D shaped hybrid nanoparticles and structures. In particular, plasmonically active nanomaterials can be engineered in their shape and material composition so that they exhibit unique physical and chemical properties. Here, the recent progress in the development of shadow growth techniques to realize hybrid plasmonic nanomaterials is discussed. The review describes how fabrication permits the material response to be engineered and highlights novel functions. Potential fields of application with a focus on photonic devices, biomedical, and chiral spectroscopic applications are discussed.
Ultrasound-responsive systems as components for smart materials
Athanassiadis, A., Ma, Z., Moreno-Gomez, N., Melde, K., Choi, E., Goyal, R., Fischer, P.
Chemical Reviews, 122(5):5165-5208, March 2022 (article)
Smart materials can respond to stimuli and adapt their response based on external cues from their environments. Such behavior requires a way to transport energy efficiently and then convert it for use in applications like actuation, sensing or signaling. Ultrasound can carry energy safely and with low losses through complex and opaque media. It can be localized to small regions of space and couple to systems over a wide range of timescales. However, the same characteristics that allow ultrasound to propagate efficiently through materials make it difficult to convert acoustic energy into other useful forms. Recent work across diverse fields has begun to address this challenge, demonstrating ultrasonic effects that provide control over physical and chemical systems with surprisingly high specificity. Here, we review recent progress in ultrasound-matter interactions, focusing on effects that can be incorporated as components in smart materials. These techniques build on fundamental phenomena such as cavitation, microstreaming, scattering and acoustic radiation forces to enable capabilities such as actuation, sensing, payload transport and delivery, and the initiation of chemical or biological processes. The diversity of emerging techniques holds great promise for a wide range of smart capabilities supported by ultrasound, and poses interesting questions for further investigations.
Second harmonic Rayleigh scattering optical activity of single Ag nanohelices in a liquid
Ohnoutek, L., Olohan, B. J., Jones, R. R., Zheng, X., Jeong, H.
Nanoscale, 14(10):3888-3898, February 2022 (article)
Dynamic ultrasound projector controlled by light
Ma, Z., Joh, H., Fan, E. D., Fischer, P.
Advanced Science, 9(9):2104401, January 2022 (article)
Dynamic acoustic wavefront control is essential for many acoustic applications, including biomedical imaging and particle manipulation. Conventional methods are either static or in the case of phased transducer arrays are limited to a few elements and hence limited control. Here, a dynamic acoustic wavefront control method based on light patterns that locally trigger the generation of microbubbles is introduced. As a small gas bubble can effectively stop ultrasound transmission in a liquid, the optical images are used to drive a short electrolysis and form microbubble patterns. The generation of microbubbles is controlled by structured light projection at a low intensity of 65 mW cm–2 and only requires about 100 ms. The bubble pattern is thus able to modify the wavefront of acoustic waves from a single transducer. The method is employed to realize an acoustic projector that can generate various acoustic images and patterns, including multiple foci and acoustic phase gradients. Hydrophone scans show that the acoustic field after the modulation by the microbubble pattern forms according to the prediction. It is believed that combining a versatile optical projector to realize an ultrasound projector is a general scheme, which can benefit a multitude of applications based on dynamic acoustic fields.
Roadmap on soft robotics: multifunctionality, adaptability and growth without borders
Mazzolai, B., Carpi, F., Suzumori, K., Cianchetti, M., Speck, T., Smoukov, S. K., Burgert, I., Keplinger, T., Siqueira, G., Vanneste, F., Goury, O., Duriez, C., Nanayakkara, T., Vanderborght, B., Brancart, J., Terryn, S., Rich, S., Liu, R., Fukuda, K., Someya, T., Calisti, M., Laschi, C., Sun, W., Wang, G., Wen, L., Baines, R., Patiballa, S. K., Kramer, R., Rus, D., Fischer, P., Simmel, F., Lendlein, A.
January 2022 (article)
Soft robotics aims at creating systems with improved performance of movement and adaptability in unknown, challenging, environments and with higher level of safety during interactions with humans. This Roadmap on Soft Robotics covers selected aspects for the design of soft robots significantly linked to the area of multifunctional materials, as these are considered a fundamental component in the design of soft robots for an improvement of their peculiar abilities, such as morphing, adaptivity and growth. The roadmap includes different approaches for components and systems design, bioinspired materials, methodologies for building soft robots, strategies for the implementation and control of their functionalities and behaviour, and examples of soft-bodied systems showing abilities across different environments. For each covered topic, the author(s) describe the current status and research directions, current and future challenges, and perspective advances in science and technology to meet the challenges.
Review of data processing of functional optical microscopy for neuroscience
Benisty, H., Song, A., Mishne, G., Charles, A. S.
Neurophotonics, 9(4):041402, SPIE, Bellingham, WA, USA, 2022 (article)
Emergent dynamics of light-induced active colloids probed by XPCS
Zinn, T., Narayanan, T., Kottapalli, S. N., Sachs, J., Sottmann, T., Fischer, P.
New Journal of Physics, 24(9):093007, IOP Publishing, Bristol, 2022 (article)
2021
Rods in a lyotropic chromonic liquid crystal: emergence of chirality, symmetry-breaking alignment, and caged angular diffusion
Ettinger, S., Dietrich, C. F., Mishra, C. K., Miksch, C., Beller, D. A., Collings, P. J., Yodh, A. G.
Soft Matter, 18(3):487-495 , December 2021 (article)
Following molecular mobility during chemical reactions: no evidence for active propulsion
Fillbrook, L. L., Günther, J., Majer, G., O’Leary, D. J. P. W. S., Van Ryswyk, H., Fischer, P., Beves, J. E.
Journal of the American Chemical Society, 143(49):20884-20890, December 2021 (article)
The reported changes in self-diffusion of small molecules during reactions have been attributed to “boosted mobility”. We demonstrate the critical role of changing concentrations of paramagnetic ions on nuclear magnetic resonance (NMR) signal intensities, which lead to erroneous measurements of diffusion coefficients. We present simple methods to over-come this problem. The use of shuffled gradient amplitudes allows accurate diffusion NMR measurements, even with time-dependent relaxation rates caused by changing concentrations of paramagnetic ions. The addition of a paramagnetic relaxation agent allows accurate determination of both diffusion coefficients and reaction kinetics during a single experi-ment. We analyze a copper-catalyzed azide-alkyne cycloaddition ‘click’ reaction, for which boosted mobility has been claimed. With our methods, we accurately measure the diffusive behavior of the solvent, starting materials and product, and find no global increase in diffusion coefficients during the reaction. We overcome NMR signal overlap using an alter-native reducing agent to improve the accuracy of the diffusion measurements. The alkyne reactant diffuses slower as the reaction proceeds, due to binding to the copper catalyst during the catalytic cycle. The formation of this intermediate was confirmed by complementary NMR techniques and density functional theory calculations. Our work calls into question recent claims that molecules actively propel or swim during reactions, and establishes that time-resolved diffusion NMR measurements can provide valuable insight into reaction mechanisms.
Optical Activity in Third‐Harmonic Rayleigh Scattering: A New Route for Measuring Chirality
Ohnoutek, L., Jeong, H., Jones, R., Sachs, J., Olohan, B., Rasadean, D., Pantos, G., Andrews, D., Fischer, P., Valev, V.
Laser & Photonics Reviews, 15(11):2100235, September 2021 (article)
In 3D isotropic liquids, optical third-harmonic generation is forbidden, with circularly polarized light (CPL). Yet the associated nonlinear susceptibility directly influences the optical properties at the fundamental frequency by intensity dependence (Kerr effect). Here, the hidden third-harmonic optical properties upon CPL illumination are revealed by demonstrating a new effect, in hyper-Rayleigh scattering. This effect is succinctly enunciated: the intensity of light scattered at the third-harmonic frequency of the CPL incident light depends on the chirality of the scatterers. It is referred to as third-harmonic (hyper) Rayleigh scattering optical activity (THRS OA) and was observed from Ag nanohelices randomly dispersed in water. The first analytical theory model for the new effect in nanohelices is also provided, highlighting the role of localized transition dipoles along the helical length. THRS OA is remarkably user-friendly. It offers access to intricate optical properties (hyperpolarizabilities) that have so far been more easily accessible by computation and that are essential for the understanding of light−matter interactions. The new effect could find applications in hyper-sensitive characterization of the chirality in molecules and in nanostructures; this chirality plays a fundamental role in the function of bio/nano-machinery, with promising applications in next generation technologies.
Chemically active micromotors
University of Stuttgart, Stuttgart, July 2021 (phdthesis)
Motion is a mark of living systems. It is realised by energy conversion to perform vital tasks and is thus of great importance for all living systems. One approach to achieve motion is by including active motion of micro/nano objects. Unlike in the fluid at the macro scale, active swimming cannot be achieved by reciprocal movements at the micro scale. Breaking symmetry at the micro scale thus becomes a critical issue. The challenge is that this often requires outside intervention to build systems that already show symmetry breaking. And another challenge is that there are few examples where active microscale motion can cause a macroscopic effect, or facilitate a useful application.
In the first part of the thesis, the first challenge is addressed and a new route of spontaneous symmetry breaking is developed. Microscale motion in artificial chemical systems has thus far been realised in chemical motion. These are microparticles that are fabricated to possess two different halves, known as Janus particles. One half is catalytically active and drives the self-phoretics. The Janus micromotors are generally fabricated using fabrication techniques such as PVD, CVD. These techniques require deposition onto a surface, which limit the number of structures that can be fabricated.
In this work, we show that two species of isotropic (symmetric) micro particles, one is a photocatalytically active particle TiO2, the other is a passive SiO2 particle can spontaneously form a dimer structure. Under UV illumination, a chemical gradient is generated around the photo active particles. The passive particle is attracted toward the highest chemical concentration of the reaction product towards the active particle. A dimer forms that starts to self-propel. The speed of the dimer can be controlled by adjusting the UV intensity.
The mechanism of the dimer formation is examined and shown to be due to a diffusiophoretic interaction between the active and the passive particle. The interaction force and the propulsion of the dimer swimmers are examined. The role of salts, particle size and concentration are studied. An additional repulsion interaction is observed between two active particles. An optimal volumetric particle density of ≤ 2% is identified for dimer formation and the dimers remain active for > 20s. This thesis thereby demonstrates a self-assembly route where the chemical activity causes dimer formation and thus spontaneous symmetry breaking which does not require any physical fabrication steps.
Most work thus far has studied the behaviour of individual chemical micromotors (Janus particles) at the micro scale. To induce a macroscopic effect and facilitate an application using individual micro/nano active particles requires cooperative effects of many "micromotors". Therefore, we developed a novel fabrication method which allows a large number of Janus structures to be assembled in an ordered manner. We fabricated an array of photoactive Janus micro structures on a surface by glancing angle deposition (GLAD) onto a photolithography patterned substrate. Illuminating the surface of Janus array structures with UV light initiates the water splitting reaction, which produces an osmotic flow around the micro structures. The osmotic flow at each structure is coupled with the flows generated by the neighbouring particles. The microscopic osmotic flow thereby results in a macroscopic fluid flow. By adjusting the spacing between single micro structures, an optimised pumping velocity is achieved with a micro pillar diameter of 2 μm and a spacing of ∼ 2 μm. We compared the pumping performance of the micro pillar array with other topological chemical structures, such as micro Janus bar arrays and 2D micro Janus disk arrays, and find that the 3D structure is essential to generate a chemical gradient on the surface. We believe that this is the first chemical micropump formed by chemically active Janus structures. The active pumping surface can provide a flow speed of up to 4 μm/s.
This active surface consisting of micropillar arrays can be easily integrated in most microchannels and serve as an on-board micropump. A theoretical model and numerical simulations are presented to describe the microchannel pumping. The theory reproduces the experimentally measured flow profiles very well. We have thus established a new type of chemical pump, which can wirelessly pump fluid in a microchannel, and the pumping volume rate and flow profile can be modified simply by changing the nature and orientation of the self-pumping walls.
Combinatorial growth of multinary nanostructured thin functional films
Barad, H., Alarcon-Correa, M., Salinas, G., Oren, E., Peter, F., Kuhn, A., Fischer, P.
Materials Today, 50, pages: 89-99, July 2021 (article)
The rapid generation of material libraries with multidimensional gradients is important for the discovery of new functional materials. Here we report an integrated fabrication scheme, based on glancing angle physical vapor deposition, to form a thin-film materials library with controlled variations in nanoshape, multinary composition, and oxidation state on a single large area substrate. We demonstrate the versatility of the method by growing an octonary materials system, which we characterize with high-throughput methods, and reveal variations in several physico-chemical properties. Among others, we examine the materials library in the frame of the oxygen evolution reaction and show that nanostructuring leads to NiO clusters that are active towards such a reaction. Our scheme can be readily extended to include more starting elements, and can be transferred to other deposition methods, making this an adaptable and versatile platform for combinatorial materials science.
Dynamic Acoustic Levitator Based On Subwavelength Aperture Control
Lu, X., Twiefel, J., Ma, Z., Yu, T., Wallaschek, J., Fischer, P.
Advanced Science, 8(15):2100888, June 2021 (article)
Acoustic levitation provides a means to achieve contactless manipulation of fragile materials and biological samples. Most acoustic levitators rely on complex electronic hardware and software to shape the acoustic field and realize their dynamic operation. Here, the authors introduce a dynamic acoustic levitator that is based on mechanically controlling the opening and (partial) closing of subwavelength apertures. This simple approach relies on the use of a single ultrasonic transducer and is shown to permit the facile and reliable manipulation of a variety targets ranging from solid particles, to fluid and ferrofluidic drops. Experimental observations agree well with numerical simulations of the Gor'kov potential. Remarkably, this system even enables the generation of time-varying potentials and induces oscillatory and rotational motion in the levitated objects via a feedback mechanism between the trapped object and the trapping potential. This is shown to result in long distance translation, in-situ rotation and self-modulated oscillation of the trapped particles. In addition, dense ferrofluidic droplets are levitated and transformed inside the levitator. Controlling subwavelength apertures opens the possibility to realize simple powerful levitators that nevertheless allow for the versatile dynamic manipulation of levitated matter.
Comment on “Using NMR to Test Molecular Mobility during a Chemical Reaction”
Fillbrook, L. L., Günther, J., Majer, G., Price, W. S., Fischer, P., Beves, J. E.
The Journal of Physical Chemistry Letters, 12(25):5932-5937, June 2021 (article)
A study reported in The Journal of Physical Chemistry Letters (Wang et al., 2021, 12, 2370) of “boosted mobility” measured by diffusion NMR experiments contains significant errors in data analysis and interpretation. We carefully reanalyzed the same data and find no evidence of boosted mobility, and we identify several sources of error.
Light- and magnetically actuated FePt microswimmers
Kadiri, V. M., Günther, J., Kottapalli, S. N., Goyal, R., Peter, F., Alarcon-Correa, M., Son, K., Barad, H., Börsch, M., Fischer, P.
The European Physical Journal E, 44(6):74, June 2021 (article)
Externally controlled microswimmers offer prospects for transport in biological research and medical applications. This requires biocompatibility of the swimmers and the possibility to tailor their propulsion mechanisms to the respective low Reynolds number environment. Here, we incorporate low amounts of the biocompatible alloy of iron and platinum (FePt) in its L10 phase in microstructures by a versatile one-step physical vapor deposition process. We show that the hard magnetic properties of L10 FePt are beneficial for the propulsion of helical micropropellers with rotating magnetic fields. Finally, we find that the FePt coatings are catalytically active and also make for Janus microswimmers that can be light-actuated and magnetically guided.
Panoramic imaging assessment of different bladder phantoms – an evaluation study
Hackner, R., Suarez-Ibarrola, I., Qiu, T., Lemke, N., Pohlmann, P., Wilhelm, K., Fischer, P., Miernik, A., Wittenberg, T.
Urology, 156, pages: e103-e110, June 2021 (article)
Objective: To evaluate “panoramic image stitching” for cystoscopy, a novel technique to augment an urologist's field of view transoperatively in real-time during a cystoscopic “keyhole” procedure, 3-D bladder phantoms provide a suitable setting. Thus, the objective is the evaluation of different 3-D printed bladder phantoms with respect to their ability to be used for extended experiments of panoramic cystoscopy.
Results: Panoramas of all phantom and endoscope combinations were computed. Using landmarks (south pole, north pole, equator) in the phantoms, maximum extension of the panoramas was assessed. The computed panoramas yield maximum extensions between 270o (0-degree cystoscope) and 330o (video cystoscope). Deformable phantoms yield larger panoramas than the rigid models.
Soft urinary bladder phantom for endoscopic training
Choi, E., Waldbillig, F., Jeong, M., Li, D., Goyal, R., Weber, P., Miernik, A., Grüne, B., Hein, S., Suarez-Ibarrola, R., Kriegmair, M. C., Qiu, T.
Annals of Biomedical Engineering, 49(9):2412-2420, May 2021 (article)
Bladder cancer (BC) is the main disease in the urinary tract with a high recurrence rate and it is diagnosed by cystoscopy (CY). To train the CY procedures, a realistic bladder phantom with correct anatomy and physiological properties is highly required. Here, we report a soft bladder phantom (FlexBlad) that mimics many important features of a human bladder. Under filling, it shows a large volume expansion of more than 300% with a tunable compliance in the range of 12.2 ± 2.8 – 32.7 ± 5.4 mL cmH2O−1 by engineering the thickness of the bladder wall. By 3D printing and multi-step molding, detailed anatomical structures are represented on the inner bladder wall, including sub-millimeter blood vessels and reconfigurable bladder tumors. Endoscopic inspection and tumor biopsy were successfully performed. A multi-center study was carried out, where two groups of urologists with different experience levels executed consecutive CYs in the phantom and filled in questionnaires. The learning curves reveal that the FlexBlad has a positive effect in the endourological training across different skill levels. The statistical results validate the usability of the phantom as a valuable educational tool, and the dynamic feature expands its use as a versatile endoscopic training platform.
Large Area Patterning of Nanoparticles and Nanostructures: Current Status and Future Prospects
Barad, H., Kwon, H., Alarcon-Correa, M., Fischer, P.
ACS Nano, 15(4):5861-5875 , April 2021 (article)
Nanoparticles possess exceptional optical, magnetic, electrical, and chemical properties. Several applications, ranging from surfaces for optical displays and electronic devices, to energy conversion, require large-area patterns of nanoparticles. Often, it is crucial to maintain a defined arrangement and spacing between nanoparticles to obtain a consistent and uniform surface response. In the majority of the established patterning methods, the pattern is written and formed, which is slow and not scalable. Some parallel techniques, forming all points of the pattern simultaneously, have therefore emerged. These methods can be used to quickly assemble nanoparticles and nanostructures on large-area substrates into well-ordered patterns. Here, we review these parallel methods, the materials that have been processed by them, and the types of particles that can be used with each method. We also emphasize the maximal substrate areas that each method can pattern and the distances between particles. Finally, we point out the advantages and disadvantages of each method, as well as the challenges that still need to be addressed to enable facile, on-demand large-area nanopatterning.
Progress in robotics for combating infectious diseases
Gao, A., Murphy, R., Chen, W., Dagnino, G., Fischer, P., Gutierrez, M., Kundrat, D., Nelson, B., Shamsudhin, N., Su, H., Xia, J., Zemmar, A., Zhang, D., Wang, C., Yang, G.
Science Robotics, 6(52):eabf1462, March 2021 (article)
The world was unprepared for the COVID-19 pandemic, and recovery is likely to be a long process. Robots have long been heralded to take on dangerous, dull, and dirty jobs, often in environments that are unsuitable for humans. Could robots be used to fight future pandemics? We review the fundamental requirements for robotics for infectious disease management and outline how robotic technologies can be used in different scenarios, including disease prevention and monitoring, clinical care, laboratory automation, logistics, and maintenance of socioeconomic activities. We also address some of the open challenges for developing advanced robots that are application oriented, reliable, safe, and rapidly deployable when needed. Last, we look at the ethical use of robots and call for globally sustained efforts in order for robots to be ready for future outbreaks.
Advanced Diffusion Studies of Active Enzymes and Nanosystems
Universität Stuttgart, Stuttgart (und Cuvillier Verlag, Göttingen), February 2021 (phdthesis)
Enzymes are fascinating chemical nanomachines that catalyze many reactions, which are essential for life. Studying enzymes is therefore important in a biological and medical context, but the catalytic potential of enzymes also finds use in organic synthesis. This thesis is concerned with the fundamental question whether the catalytic reaction of an enzyme or molecular catalyst can cause it to show enhanced diffusion. Diffusion measurements were performed with advanced fluorescence correlation spectroscopy (FCS) and diffusion nuclear magnetic resonance (NMR) spectroscopy techniques. The measurement results lead to the unraveling of artefacts in enzyme FCS and molecular NMR measurements, and thus seriously question several recent publications, which claim that enzymes and molecular catalysts are active matter and experience enhanced diffusion. In addition to these fundamental questions, this thesis also examines the use of enzymes as biocatalysts. A novel nanoconstruct – the enzyme-phage-colloid (E-P-C) – is presented, which utilizes filamentous viruses as immobilization templates for enzymes. E-P-Cs can be used for biocatalysis with convenient magnetic recovery of enzymes and serve as enzymatic micropumps. The latter can autonomously pump blood at physiological urea concentrations.
Transient increase of Tc and Jc in superconducting/metallic heterostructures
Ionescu, A., Bihler, M., Simmendinger, J., Miksch, C., Fischer, P., Cristiani, G., Rabinovich, K., Schütz, G., Albrecht, J.
Materials Chemistry and Physics, 263, pages: 124390, February 2021 (article)
The presence of a metallic layer can influence the properties of high-temperature superconductors underneath. We investigate the influence of metallic structures deposited in form of nanoparticles or continuous layers by electron beam evaporation or ion beam sputtering on the properties of Y1Ba2Cu3O7-x (YBCO) thin films. To generally avoid diffusion of metal atoms an additional barrier layer is introduced. Detailed measurements of the magnetic moment of the superconductor as a function of temperature and magnetic field have been performed using SQUID magnetometry. It is found that the modification of the superconducting properties of coated YBCO strongly depends on the deposition method of the metal on top rather than the type of metal (Ni or Ag), its magnetic properties (ferromagnetic or paramagnetic) or its morphology (nanoparticles or thin film). The main result is a transient increase of the critical temperature Tc and critical current density Jc that was observed for samples prepared by electron beam evaporation.
Melt Electrospinning Writing of Magnetic Microrobots
Su, Y., Qiu, T., Song, W., Han, X., Sun, M., Wang, Z., Xie, H., Dong, M., Chen, M.
Advanced Science, 8(3):2003177, January 2021 (article)
Tailoring Binding Abilities by Incorporating Oxophilic Transition Metals on 3D Nanostructured Ni Arrays for Accelerated Alkaline Hydrogen Evolution Reaction
Kim, J., Jung, H., Jung, S., Hwang, J., Kim, D. Y., Lee, N., Kim, K., Kwon, H., Kim, Y., Han, J. W., Kim, J. K.
Journal of the American Chemical Society, 143(3):1399-1408, January 2021 (article)
Comment on “Boosted molecular mobility during common chemical reactions”
Günther, J., Fillbrook, L. L., MacDonald, T. S., Majer, G., Price, W. S., Fischer, P., Beves, J. E.
Science, 371(6526):eabe8322, January 2021 (article)
The apparent “boosted mobility” observed by Wang et al. (Reports, 31 July 2020, p. 537) is the result of a known artifact. When signal intensities are changing during a nuclear magnetic resonance (NMR) diffusion measurement for reasons other than diffusion, the use of monotonically increasing gradient amplitudes produces erroneous diffusion coefficients. We show that no boosted molecular mobility is observed when shuffled gradient amplitudes are applied.
Soft liver phantom with a hollow biliary system
Tan, X., Li, D., Jeong, M., Yu, T., Ma, Z., Afat, S., Grund, K., Qiu, T.
Annals of Biomedical Engineering, 49(9):2139-2149, 2021 (article)
Hepatobiliary interventions are regarded as difficult minimally-invasive procedures that require experience and skills of physicians. To facilitate the surgical training, we develop a soft, high-fidelity and durable liver phantom with detailed morphology. The phantom is anatomically accurate and feasible for the multi-modality medical imaging, including computer tomography (CT), ultrasound, and endoscopy. The CT results show that the phantom resembles the detailed anatomy of real livers including the biliary ducts, with a spatial root mean square error (RMSE) of 1.7 ± 0.7 mm and 0.9 ± 0.2 mm for the biliary duct and the liver outer shape, respectively. The sonographic signals and the endoscopic appearance highly mimic those of the real organ. An electric sensing system was developed for the real-time quantitative tracking of the transhepatic puncturing needle. The fabrication method herein is accurate and reproducible, and the needle tracking system offers a robust and general approach to evaluate the centesis outcome.
Slippery micropropellers penetrate the vitreous humor
(US20210170056A1), 2021 (patent)
Microparticles actively propel through the vitreous humour and reach the retina in porcine eyes. The slippery micro helical propellers are constructed by the combination of glancing angle deposition technique and the fusion of the slippery liquid layer. The magnetically propulsion in the vitreous humour relies on the matched size of the propeller to the collagen network of the vitreous, and the anti-adhesion coating of the collagen fiber bundles. Clinical optical coherence tomography observed the displacement of the slippery micropropellers through the vitreous to the macular area on the retina. The slippery micropropellers realize the controllable massive movements to the retina in 30 mins, while exerting the travelling distance of above one centimeter. The injection of the slippery micropropellers, the magnetically-powered controllable propulsion in the vitreous, and the optical coherence tomography imaging technique, constitute an intact method for rapid targeted ocular delivery, providing a promising approach towards ophthalmologic applications.
Magnetic field generator
Qiu, T., Fischer, P.
(US20210228298A1), 2021 (patent)
A magnetic field generator that comprises at least three groups of magnets, the magnetic moment of each magnet being rotatable about a rotation axis, wherein each group comprises at least two magnets, and each group has an orientation in the sense that the rotation axes of the magnetic moments of the magnets of the same group extend in the group's orientation. The orientations of the different groups are linearly independent.
2020
Chiroptical spectroscopy of a freely diffusing single nanoparticle
Sachs, J., Günther, J., Mark, A. G., Fischer, P.
Nature Communications, 11, pages: 4513, September 2020 (article)
Chiral plasmonic nanoparticles can exhibit strong chiroptical signals compared to the corresponding molecular response. Observations are, however, generally restricted to measurements on stationary single particles with a fixed orientation, which complicates the spectral analysis. Here, we report the spectroscopic observation of a freely diffusing single chiral nanoparticle in solution. By acquiring time-resolved circular differential scattering signals we show that the spectral interpretation is significantly simplified. We experimentally demonstrate the equivalence between time-averaged chiral spectra observed for an individual nanostructure and the corresponding ensemble spectra, and thereby demonstrate the ergodic principle for chiroptical spectroscopy. We also show how it is possible for an achiral particle to yield an instantaneous chiroptical response, whereas the time-averaged signals are an unequivocal measure of chirality. Time-resolved chiroptical spectroscopy on a freely moving chiral nanoparticle advances the field of single-particle spectroscopy, and is a means to obtain the true signature of the nanoparticle’s chirality.
A Hierarchical 3D TiO2/Ni Nanostructure as an Efficient Hole‐Extraction and Protection Layer for GaAs Photoanodes
Alqahtani, M., Kafizas, A., Sathasivam, S., Ebaid, M., Cui, F., Alymani, A., Jeong, H., Lee, T., Fischer, P., Parkin, I., Grätzel, M., Wu, J.
ChemSusChem, 13(22):6028-6036, September 2020 (article)
Photoelectrochemical (PEC) water splitting is a promising clean route to hydrogen fuel. The best‐performing materials (III/V semiconductors) require surface passivation, as they are liable to corrosion, and a surface co‐catalyst to facilitate water splitting. At present, optimal design combining photoelectrodes with oxygen evolution catalysts remains a significant materials challenge. Here, we demonstrate that nickel‐coated amorphous three‐dimensional (3D) TiO2 core‐shell nanorods on a TiO2 thin film function as an efficient hole‐extraction layer and serve as a protection layer for the GaAs photoanode. Transient‐absorption spectroscopy (TAS) demonstrated the role of nickel‐coated (3D) TiO2 core‐shell nanorods in prolonging photogenerated charge lifetimes in GaAs, resulting in a higher catalytic activity. This strategy may open the potential of utilizing this low‐cost (3D) nanostructured catalyst for decorating narrow‐band‐gap semiconductor photoanodes for PEC water splitting devices.
Scalable Fabrication of Molybdenum Disulfide Nanostructures and their Assembly
Huang, Y., Yu, K., Li, H., Liang, Z., Walker, D., Ferreira, P., Fischer, P., Fan, D.
Advanced Materials, 32(43):2003439, September 2020 (article)
Molybdenum disulfide (MoS2) is a multifunctional material that can be used for various applications. In the single‐crystalline form, MoS2 shows superior electronic properties. It is also an exceptionally useful nanomaterial in its polycrystalline form with applications in catalysis, energy storage, water treatment, and gas sensing. Here, the scalable fabrication of longitudinal MoS2 nanostructures, i.e., nanoribbons, and their oxide hybrids with tunable dimensions in a rational and well‐reproducible fashion, is reported. The nanoribbons, obtained at different reaction stages, that is, MoO3, MoS2/MoO2 hybrid, and MoS2, are fully characterized. The growth method presented herein has a high yield and is particularly robust. The MoS2 nanoribbons can readily be removed from its substrate and dispersed in solution. It is shown that functionalized MoS2 nanoribbons can be manipulated in solution and assembled in controlled patterns and directly on microelectrodes with UV‐click‐chemistry. Owing to the high chemical purity and polycrystalline nature, the MoS2 nanostructures demonstrate rapid optoelectronic response to wavelengths from 450 to 750 nm, and successfully remove mercury contaminants from water. The scalable fabrication and manipulation followed by light‐directed assembly of MoS2 nanoribbons, and their unique properties, will be inspiring for device fabrication and applications of the transition metal dichalcogenides.
Spatial ultrasound modulation by digitally controlling microbubble arrays
Ma, Z., Melde, K., Athanassiadis, A. G., Schau, M., Richter, H., Qiu, T., Fischer, P.
Nature Communications, 11, pages: 4537, September 2020 (article)
Acoustic waves, capable of transmitting through optically opaque objects, have been widely used in biomedical imaging, industrial sensing and particle manipulation. High-fidelity wavefront shaping is essential to further improve performance in these applications. An acoustic analog to the successful spatial light modulator (SLM) in optics would be highly desirable. To date there have been no techniques shown that provide effective and dynamic modulation of a sound wave and which also support scale-up to a high number of individually addressable pixels. In the present study, we introduce a dynamic spatial ultrasound modulator (SUM),which dynamically reshapes incident plane waves into complex acoustic images. Its trans-mission function is set with a digitally generated pattern of microbubbles controlled by a complementary metal–oxide–semiconductor (CMOS) chip, which results in a binary amplitude acoustic hologram. We employ this device to project sequentially changing acoustic images and demonstrate the first dynamic parallel assembly of microparticles using a SUM.
Microchannels with Self-Pumping Walls
Yu, T., Athanassiadis, A. G., Popescu, M. N., Chikkadi, V., Güth, A., Singh, D. P., Qiu, T., Fischer, P.
ACS Nano, 14(10):13673-13680, 2020 (article)
When asymmetric Janus micromotors are immobilized on a surface, they act as chemically powered micropumps, turning chemical energy from the fluid into a bulk flow. However, such pumps have previously produced only localized recirculating flows, which cannot be used to pump fluid in one direction. Here, we demonstrate that an array of three-dimensional, photochemically active Au/TiO2 Janus pillars can pump water. Upon UV illumination, a water-splitting reaction rapidly creates a directional bulk flow above the active surface. By lining a 2D microchannel with such active surfaces, various flow profiles are created within the channels. Analytical and numerical models of a channel with active surfaces predict flow profiles that agree very well with the experimental results. The light-driven active surfaces provide a way to wirelessly pump fluids at small scales and could be used for real-time, localized flow control in complex microfluidic networks.
Characterization of active matter in dense suspensions with heterodyne laser Doppler velocimetry
Sachs, J., Kottapalli, S. N., Fischer, P., Botin, D., Palberg, T.
Colloid and Polymer Science, 299(2):269-280, August 2020 (article)
We present a novel approach for characterizing the properties and performance of active matter in dilute suspension as well as in crowded environments. We use Super-Heterodyne Laser-Doppler-Velocimetry (SH-LDV) to study large ensembles of catalytically active Janus particles moving under UV illumination. SH-LDV facilitates a model-free determination of the swimming speed and direction, with excellent ensemble averaging. In addition, we obtain information on the distribution of the catalytic activity. Moreover, SH-LDV operates away from walls and permits a facile correction for multiple scattering contributions. It thus allows for studies of concentrated suspensions of swimmers or of systems where swimmers propel actively in an environment crowded by passive particles. We demonstrate the versatility and the scope of the method with a few selected examples. We anticipate that SH-LDV complements established methods and paves the way for systematic measurements at previously inaccessible boundary conditions.
Motion, Symmetry & Spectroscopy of Chiral Nanostructures
Universität Stuttgart, Stuttgart (und Springer, Cham), July 2020 (phdthesis)
Nanostructures are of interest for a broad spectrum of potential applications. For biomedical tasks, small nanoswimmers could help realize targeted drug delivery or minimally invasive surgery, which necessitates a comprehensive understanding and control of their motion. As an optical sensor, single plasmonic nanostructures can enhance the weak optical signals associated with biologically-important handed (chiral) molecules, and thus potentially lead to much higher detection sensitivities and improved selectivities. Both, the motion and spectroscopic behavior of nanostructures is closely related to the symmetry they possess. For propulsion at small scales it is well known that symmetry is important. For instance, the bacterial flagellum has a chiral corkscrew shape to allow non-reciprocal motion. One can therefore wonder if chirality is essential, and if indeed this is the simplest shape for propulsion, or if other structures – possibly ones that are simpler to fabricate – can also show propulsion when they are rotated. Another aspect that relates to the symmetry is the spectroscopic observation of nanostructures. Especially in chiral structures both the shape itself as well as the orientation in space determines optical signals. It is therefore important to be able to dis-entangle these effects.
To shed light on this, this thesis presents experiments that capture the active motion and chiroptical spectroscopy of artificial nanostructures at low Reynolds number (Re), and examines the role chirality plays in these phenomena. Against previous expectations and reports, it is shown that a propeller does not need to be chiral to locomote and the first truly achiral propeller at low Re is reported. Similarly, a careful examination of the spectroscopy of plasmonic nanostructures shows that structures do not need to be chiral to give rise to chiroptical signals. A novel spectrometer was constructed to observe individual nanostructures. Thereby it has been possible to identify an observable, which has not been measured previously – that permits the true chiroptical spectrum to be obtained from a single nanoparticle suspended in solution. The basis for the presented experiments is the fabrication of micro- and nanostructures with simple and complex, achiral and chiral shapes via Glancing Angle Deposition (GLAD).
The first part of this thesis considers active motion exhibited by highly symmetric micro- and nanoswimmers. Owing to the scallop theorem, reciprocal motion does not lead to a net translation at low Re, and other swimming strategies must be exploited, e.g. rotation-translation coupling. Previously, chirality was assumed to be required to efficiently couple rotation to translation at low Re, as is demonstrated by the corkscrew-shaped bacterial flagellum. However, a symmetry analysis suggested that much simpler shapes are potentially also propulsive if they are rotated in a non-trivial way. This would have important implications as then a novel class of micro- and nanoswimmers with highly symmetric, and easier to fabricate shapes, become feasible. Here, the propulsion characteristics of V-shaped objects that are driven by means of an external torque are investigated. The torque was either exerted by an external magnetic or an electric field and the ”V”-shaped particle had a corresponding dipole moment. Experiments with macroscopic as well as microscopic rigid bodies revealed that the orientation of the dipole with respect to the body plays a crucial role for their ability to convert a rotation into a translation. Symmetry arguments are developed, which accurately predict whether or not the object is propulsive at all, and additionally if it moves uni- or bi-directionally. It is thereby unequivocally shown by theory and experimental evidence that chirality is not a prerequisite for efficient rotation-translation coupling at low Re but that propulsive objects are necessarily chiral if they are driven magnetically. Surprisingly, rotation of a truly achiral object will also lead to a propulsion, which is experimentally demonstrated for the first time utilizing an electrically driven ”V”-shaped particle. Because chirality, i.e. Pb-odd symmetry, is not a requirement, an extended analysis that also includes the object’s symmetry under charge conjugation (CÒ) is employed to explain and predict the correct propulsion characteristics of arbitrary shaped objects by means of rotation-translation coupling.
In contrast, spherical objects are unable to utilize rotation-translation coupling. However, beads with unequal chemical reactivities on the two ”faces” of their spherical body, so called Janus particles, can show enhanced diffusion if a suitable chemical substrate is provided. A catalytic reaction converts the intrinsic asymmetry of the particles into a pressure gradient, which leads to a self-generated motion. Most reports of self-propulsion to date concern larger microparticles. In this thesis, it is examined if this means of propulsion can also be realized in sub-micron sized Janus particles. The exact propul-sion characteristics are not fully clear in most chemically-driven particles and how their enhanced diffusion scales with size. Because of their small size the observation of individual particles is not possible and thus high density particle ensembles are observed by light scattering techniques, i.e. Dynamic Light Scattering (DLS), Differential Dynamic Microscopy (DDM) and Super-Heterodyne Laser Doppler Velocimetry (SH-LDV). In contrast to DLS and DDM, SH-LDV is identified as a versatile and suitable technique for the characterisation of active motion exhibited by Janus nanoparticles. First results on their size-dependent enhanced diffusion are presented, which show that this form of symmetry-breaking is effective at the smallest of scales.
The second part of this thesis considers the spectroscopic response exhibited by chiral and achiral shaped nanoparticles measured in a novel single-particle spectrometer. It is based on dark-field microscopy and contains a balanced detection setup, which was built to record the Circular Differential Scattering Intensity (CDSI) of a single nanoparticle, i.e. the difference in scattering intensities for left- and right-circularly polarized light. This is known as chiroptical spectroscopy and typical setups are limited to the examination of stationary single particles. However, problems arise as their spectra appear distorted owing to a fixed light-object symmetry as well as interactions with the surface to which the particles are adhere. In contrast, the approach employed in this thesis opens up the possibility to record chiroptical spectra in ”one shot” and hence observe single mobile nanoparticles away from a surface. Crucially, a freely suspended particle in a liquid randomly reorients due to Brownian motion, which leads to an isotropic sampling of all spatial orientations. Based on this principle, this work presents time-resolved spectroscopic observations that yield snapshots for a particular alignment during the re-orientation of the particle, and the average of the time-series of snapshots provides a true chiroptical spectrum of a single nanoparticle in bulk solution away from a surface. Remarkably, this is the first time that the chiroptical spectrum of a freely diffusing nanoparticle has been observed. Experiments confirm that the novel approach detects intrinsic chirality of a single nanoparticle and additionally it is shown how even achiral particles can exhibit apparent chirality for stationary orientations. A magnetic and plasmonic nanohelix, whose alignment is controlled in an external magnetic field validates the crucial dependence of the chiroptical spectra on the light-object symmetry. Finally, the ergodicity of this chiroptical spectroscopy is demonstrated by showing the equality between time-averaged single-particle and traditional ensemble-averaged spectra. This has important consequences as now the same information de-duced by typical measurements conducted on many particles in a cuvette is recovered by utilizing only one nanoparticle. The results presented herein show that the single-particle spectrometer is a promising platform for novel sensing applications.
Evaluation of nanorobots for targeted delivery into the retina
Schnichels, S., Goyal, R., Hurst, F., Ziemsen, F., Qiu, T., Fischer, P.
61, pages: 1355, Investigative Ophthalmology & Visual Science,The Association for Research in Vision and Ophthalmology, June 2020 (conference)
A major challenge in the treatment of eye diseases in general and retinal diseases in particular, is to deliver drugs to their target sites. Traditional intravitreal injection is based on random, passive diffusion of molecules. In order to specifically address certain structures of the retina, the use of novel particles from biomaterial research promises a more targeted application. The major challenge for such particles is the narrow macromolecular matrix of ocular tissue (including the vitreous body), which acts as a barrier and prevents the penetration of particles. Novel nanorobots from material research - more precisely: nanopropellers - that can be actively controlled through the vitreous body to reach the retina present a chance to reach desired targets in the retina.
Biocompatible magnetic micro‐ and nanodevices: Fabrication of FePt nanopropellers and cell transfection
Kadiri, V. M., Bussi, C., Holle, A. W., Son, K., Kwon, H., Schütz, G., Gutierrez, M. G., Fischer, P.
Advanced Materials, 32(25):2001114, Wiley-VCH, Weinheim, May 2020 (article)
The application of nanoparticles for drug or gene delivery promises benefits in the form of single‐cell‐specific therapeutic and diagnostic capabilities. Many methods of cell transfection rely on unspecific means to increase the transport of genetic material into cells. Targeted transport is in principle possible with magnetically propelled micromotors, which allow responsive nanoscale actuation and delivery. However, many commonly used magnetic materials (e.g., Ni and Co) are not biocompatible, possess weak magnetic remanence (Fe3O4), or cannot be implemented in nanofabrication schemes (NdFeB). Here, it is demonstrated that co‐depositing iron (Fe) and platinum (Pt) followed by one single annealing step, without the need for solution processing, yields ferromagnetic FePt nanomotors that are noncytotoxic, biocompatible, and possess a remanence and magnetization that rival those of permanent NdFeB micromagnets. Active cell targeting and magnetic transfection of lung carcinoma cells are demonstrated using gradient‐free rotating millitesla fields to drive the FePt nanopropellers. The carcinoma cells express enhanced green fluorescent protein after internalization and cell viability is unaffected by the presence of the FePt nanopropellers. The results establish FePt, prepared in the L10 phase, as a promising magnetic material for biomedical applications with superior magnetic performance, especially for micro‐ and nanodevices.
Interface-mediated spontaneous symmetry breaking and mutual communication between drops containing chemically active particles
Singh, D. P., Domínguez, A., Choudhury, U., Kottapalli, S. N., Popescu, M. N., Dietrich, S., Fischer, P.
Nature Communications, 11, pages: 2210, May 2020 (article)
Symmetry breaking and the emergence of self-organized patterns is the hallmark of com-
plexity. Here, we demonstrate that a sessile drop, containing titania powder particles with
negligible self-propulsion, exhibits a transition to collective motion leading to self-organized
flow patterns. This phenomenology emerges through a novel mechanism involving the
interplay between the chemical activity of the photocatalytic particles, which induces Mar-
angoni stresses at the liquid–liquid interface, and the geometrical confinement provided by
the drop. The response of the interface to the chemical activity of the particles is the source
of a significantly amplified hydrodynamic flow within the drop, which moves the particles.
Furthermore, in ensembles of such active drops long-ranged ordering of the flow patterns
within the drops is observed. We show that the ordering is dictated by a chemical com-
munication between drops, i.e., an alignment of the flow patterns is induced by the gradients
of the chemicals emanating from the active particles, rather than by hydrodynamic
interactions.
Spectrally selective and highly-sensitive UV photodetection with UV-A, C band specific polarity switching in silver plasmonic nanoparticle enhanced gallium oxide thin-film
Arora, K., Singh, D., Fischer, P., Kumar, M.
Advanced Optical Materials, 8(16):2000212, May 2020 (article)
Traditional photodetectors generally show a unipolar photocurrent response when illuminated with light of wavelength equal or shorter than the optical bandgap. Here, we report that a thin film of gallium oxide (GO) decorated with plasmonic nanoparticles, surprisingly, exhibits a change in the polarity of the photocurrent for different UV bands. Silver (Ag) nanoparticles are vacuum-deposited onto β-Ga2O3 and the AgNP@GO thin films show a record responsivity of 250 A/W, which significantly outperforms bare GO planar photodetectors. The photoresponsivity reverses sign from +157 µA/W in the UV-C band under unbiased operation to -353 µA/W in the UV-A band. The current reversal is rationalized by considering the charge dynamics stemming from hot electrons generated when the incident light excites a local surface plasmon resonance (LSPR) in the Ag nanoparticles. The Ag nanoparticles improve the external quantum efficiency and detectivity by nearly one order of magnitude with high values of 1.2×105 and 3.4×1014 Jones, respectively. This plasmon-enhanced solar blind GO detector allows UV regions to be spectrally distinguished, which is useful for the development of sensitive dynamic imaging photodetectors.
Acoustofluidic Tweezers for the 3D Manipulation of Microparticles
Guo, X., Ma, Z., Goyal, R., Jeong, M., Pang, W., Fischer, P., Dian, X., Qiu, T.
2020 IEEE International Conference on Robotics and Automation (ICRA 2020), pages: 11392-11397, IEEE, Piscataway, NJ, IEEE International Conference on Robotics and Automation (ICRA 2020), 2020 (conference)
Non-contact manipulation is of great importance in the actuation of micro-robotics. It is challenging to contactless manipulate micro-scale objects over large spatial distance in fluid. Here, we describe a novel approach for the dynamic position control of microparticles in three-dimensional (3D) space, based on high-speed acoustic streaming generated by a micro-fabricated gigahertz transducer. Due to the vertical lifting force and the horizontal centripetal force generated by the streaming, microparticles are able to be stably trapped at a position far away from the transducer surface, and to be manipulated over centimeter distance in all three directions. Only the hydrodynamic force is utilized in the system for particle manipulation, making it a versatile tool regardless the material properties of the trapped particle. The system shows high reliability and manipulation velocity, revealing its potentials for the applications in robotics and automation at small scales.
Investigating photoresponsivity of graphene-silver hybrid nanomaterials in the ultraviolet
Deshpande, P., Suri, P., Jeong, H., Fischer, P., Ghosh, A., Ghosh, G.
Journal of Chemical Physics, 152(4):044709, January 2020 (article)
There have been several reports of plasmonically enhanced graphene photodetectors in the visible and the near infrared regime but rarely in the ultraviolet. In a previous work, we have reported that a graphene-silver hybrid structure shows a high photoresponsivity of 13 A/W at 270 nm. Here, we consider the likely mechanisms that underlie this strong photoresponse. We investigate the role of the plasmonic layer and examine the response using silver and gold nanoparticles of similar dimensions and spatial arrangement. The effect on local doping, strain, and absorption properties of the hybrid is also probed by photocurrent measurements and Raman and UV-visible spectroscopy. We find that the local doping from the silver nanoparticles is stronger than that from gold and correlates with a measured photosensitivity that is larger in devices with a higher contact area between the plasmonic nanomaterials and the graphene layer.
A High-Fidelity Phantom for the Simulation and Quantitative Evaluation of Transurethral Resection of the Prostate
Choi, E., Adams, F., Gengenbacher, A., Schlager, D., Palagi, S., Müller, P., Wetterauer, U., Miernik, A., Fischer, P., Qiu, T.
Annals of Biomedical Engineering, 48(1):437-446, January 2020 (article)
Transurethral resection of the prostate (TURP) is a minimally invasive endoscopic procedure that requires experience and skill of the surgeon. To permit surgical training under realistic conditions we report a novel phantom of the human prostate that can be resected with TURP. The phantom mirrors the anatomy and haptic properties of the gland and permits quantitative evaluation of important surgical performance indicators. Mixtures of soft materials are engineered to mimic the physical properties of the human tissue, including the mechanical strength, the electrical and thermal conductivity, and the appearance under an endoscope. Electrocautery resection of the phantom closely resembles the procedure on human tissue. Ultrasound contrast agent was applied to the central zone, which was not detectable by the surgeon during the surgery but showed high contrast when imaged after the surgery, to serve as a label for the quantitative evaluation of the surgery. Quantitative criteria for performance assessment are established and evaluated by automated image analysis. We present the workflow of a surgical simulation on a prostate phantom followed by quantitative evaluation of the surgical performance. Surgery on the phantom is useful for medical training, and enables the development and testing of endoscopic and minimally invasive surgical instruments.
Vision Statement: Interactive Materials–Drivers of Future Robotic Systems
Advanced Materials, 32(20):1905953, January 2020, Invited Vision Statement (article)
A robot senses its environment, processes the sensory information, acts in response to these inputs, and possibly communicates with the outside world. Robots generally achieve these tasks with electronics-based hardware or by receiving inputs from some external hardware. In contrast, simple microorganisms can autonomously perceive, act, and communicate via purely physicochemical processes in soft material systems. A key property of biological systems is that they are built from energy-consuming ‘active’ units. Exciting developments in material science show that even very simple artificial active building blocks can show surprisingly rich emergent behaviors. Active non-equilibrium systems are therefore predicted to play an essential role to realize interactive materials. A major challenge is to find robust ways to couple and integrate the energy-consuming building blocks to the mechanical structure of the material. However, success in this endeavor will lead to a new generation of sophisticated micro- and soft-robotic systems that can operate autonomously.