Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X61: Self-Propelled Active Enzymes and Nanoscale Active MatterFocus
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Sponsoring Units: GSOFT DBIO GSNP Chair: Jennifer Ross, University of Massachusetts Amherst Room: BCEC 258B |
Friday, March 8, 2019 8:00AM - 8:12AM |
X61.00001: Design of graphene-based catalysts for Belousov Zhabotinsky reaction D Jaya Prasanna Kumar, Sachin Verma, Kabeer Jasuja, Pratyush Dayal Use of hybrid materials containing structures at different length scales has gained significant traction because of their ability to provide multifunctional characteristics to soft materials. Here, we focus on the Belousov-Zhabotinsky (BZ) reaction, which represents a nonlinear chemical oscillator, and show that its dynamics can be tuned by using hybrid 2D materials, i.e., graphene-based nanosheets decorated with different nanoparticles (NPs). Specifically, we demonstrate that through the careful choice of NP decorations in designing our catalytic mats, frequency of chemical oscillations in the BZ reaction system can be increased four times. Further, we reveal that this observed behavior is attributed to enhanced access to active catalytic sites on NPs, as well as the rapid shuttling of electrons facilitated by the highly conductive graphene platform. We also perform modelling and simulation studies and our results reveal a strong correlation between the rate of charge transfer and the frequency of chemical oscillations. In essence, we showcase the ability of a 2D material, like graphene, to influence the dynamics of an oscillatory chemical reactions and anticipate that our approach will open up new avenues to tune the dynamics of chemical oscillators. |
Friday, March 8, 2019 8:12AM - 8:24AM |
X61.00002: Self-moving reaction droplets synergized by graphene-based nanocomposites D Jaya Prasanna Kumar, Pratyush Dayal Chemo-mechanical transduction to perform locomotion is one of the characteristics of biological systems that has inspired the design of self-moving biomimetic systems. Here, we harness self-oscillating Belousov-Zhabotinsky (BZ) reaction, synergized by graphene-based catalytic mats to demonstrate spontaneous motion of the BZ reaction droplet in surrounding oil. Specifically, we synthesize graphene-based catalytic mats by decorating Ce and Ru nanoparticles (NP) on graphene nanosheets, thereby creating a 0D-2D heterostructures, and subsequently, use these mats to catalyze the BZ reaction. Our results demonstrate spontaneous locomotion of BZ droplets in oil bath due to the Marangoni flow that is brought about by the interaction between BZ reaction intermediates and surrounding oil. We further demonstrate that the velocity of the droplet motion is dependent upon the conductivity of the catalytic mats. In particular, we show two-fold increase in the droplet velocity when RuNP decorated graphene sheets are used to catalyze the BZ reaction instead of the traditional solution based BZ catalysts. Our findings can be used to design self-moving synthetic objects and opens up new avenues to control their behaviour through the use of 0D-2D hybrid nanomaterials. |
Friday, March 8, 2019 8:24AM - 8:36AM |
X61.00003: Enzyme-coated liposomes as dual-direction self-propulsive motors Ambika Somasundar, Subhadip Ghosh, Farzad Mohajerani, Paul Cremer, Darrell Velegol, Ayusman Sen Directional migration in response to specific chemical signals is critical for the survival of biological organisms. This enables living cells to move towards food, escape away from toxins, transport cargo and coordinate collective behavior. Controlling the motion of a motor either towards or away from chemical species is the first step in designing adaptive life-like synthetic motors. Model protocells derived from phospholipids and other amphiphiles have been studied and their movement through catalysis has been observed. However, control of directionality based on chemical cues (chemotaxis) has been difficult to achieve. In this talk, I will discuss both positive and negative chemotaxis of autonomous liposomal protocells based on the interplay between positive enzymatic catalysis-induced chemotaxis and solute-phospholipid interaction-based negative chemotaxis. In doing so, I will systematically rule out currently available mechanisms of colloidal transport and propose a potentially new and previously unrecognized mechanism of transport due to the Hofmeister effect. This opens up the possibility of other mechanisms for the transport of biological colloids. |
Friday, March 8, 2019 8:36AM - 9:12AM |
X61.00004: Enhanced Diffusion and Chemotaxis of Catalytically Active Enzymes Invited Speaker: Ramin Golestanian Enzymes have been recently proposed to have mechanical activity associated with their chemical activity. In a number of recent studies, it has been reported that enzymes undergo enhanced diffusion in the presence of their corresponding substrate, when this substrate is uniformly distributed in solution. Moreover, if the concentration of the substrate is non-uniform, enzymes and other small molecules have been reported to show chemotaxis---biased stochastic movement in the direction of the substrate gradient---typically towards higher concentrations of this substrate, with a few exceptions. The underlying physical mechanisms responsible for enhanced diffusion and chemotaxis at the nanoscale, however, are still not well understood. We will review the available experimental observations of both enhanced diffusion and chemotaxis, and discuss critically the different theories that have been proposed to explain the two. We put particular emphasis on an equilibrium model recently introduced by us, which describes how the diffusion of dumbbell-like modular enzymes can be enhanced in the presence of substrate, thanks to a binding-induced reduction of the internal fluctuations of the enzyme. We then turn to chemotaxis, beginning with an overview of the chemotaxis-like diffusiophoretic behavior of micron-sized colloids in solute gradients, followed by a discussion of why chemotaxis at the nanoscale requires special consideration. Next, we review the experimental evidence of nanoscale chemotaxis, and describe a number of shortcomings and pitfalls in the phenomenological models for chemotaxis introduced in some of those works. Finally, we discuss a microscopic model for chemotaxis including both non-specific interactions and binding between enzyme and substrate recently developed by us, which overcomes many of these shortcomings, and is consistent with the experimental observations of chemotaxis. |
Friday, March 8, 2019 9:12AM - 9:24AM |
X61.00005: Single Molecule Imaging of Nanoscale Self-Propelled Active Matter Mengqi Xu, Jennifer Ross, Lyanne Valdez, Ayusman Sen Active matter composed of ensembles of self-propelled particles display exciting emergent properties. When coupled to larger objects, enzymes can act as the propulsion of self-propelled particles, and recent studies have shown that these enzymes have exciting self-propelled properties, as well. Studies using fluorescence correlation spectroscopy (FCS) or macroscopic observation have revealed that enzymes can perform enhanced diffusion depending on the substrate concentration. The mechanism for enhanced diffusion remains unclear. We report new results using single particle tracking with total internal reflection (TIRF) microscopy to observe this emergent activity of active enzymes to test proposed mechanisms that could result in enhanced mobility of active enzymes. Using urease as a model enzyme, we observe that the diffusion of individual enzymes increases three fold as a function of substrate. This diffusion is unaffected by the background concentration of enzymes. Further, we find that the oligomerization state is unchanged by the presence of the substrate, implying that the enzyme does not significantly change size upon binding of substrate. This work effectively eliminates some previously-postulated theories of the mechanism of enzymatically enhanced diffusion. |
Friday, March 8, 2019 9:24AM - 9:36AM |
X61.00006: Engineering active matter at the nanoscale with DNA nanotechnology Ibon Santiago Self-generated movement in response to internal or environmental events is a key characteristic of living organisms that distinguishes them from non-living things. Progress in bio and nanotechnology has enabled the creation of active particles that can convert energy into motion1. Catalytic self-propelled motors are of fundamental interest in statistical biophysics and are also highly necessary in the context of artificial cellular systems.Although much research has been done on micron-sized motors, progress in catalytic nanomotors of sub-100 nm is still in its infancy2. |
Friday, March 8, 2019 9:36AM - 9:48AM |
X61.00007: Catalytic enzymes are active matter Ah-Young Jee, Tsvi Tlusty, Steve Granick Recent studies suggest that the enhanced diffusion of catalytically-active enzymes is promoted by super-diffusive “kicks” generated by the catalytic events. Pursuing this idea, we have explored methods to vary systematically the turnover frequency, kcat, which defines how many substrate molecules are consumed in a given time. Our measurements using fluorescence correlation spectroscopy in super-resolution mode (FCS-STED) enable us to illuminate this matter. The contrast between motions in the presence of inhibitor and of substrate is especially interesting. |
Friday, March 8, 2019 9:48AM - 10:24AM |
X61.00008: Directing Self-Propelled Enzymes and Enzyme-Coated Vesicles Using Chemical Signals Invited Speaker: Ayusman Sen The ability to move both towards and away from specific chemical signals is a critical survival mechanism in living systems. The motion itself arises from the harnessing of free energy from enzymatic catalysis. A variety of enzymes has been shown to undergo positive chemotaxis, moving up their substrate concentration gradient. We propose a general expression for the active movement of an enzyme in a concentration gradient of its substrate. The proposed model takes into account both the substrate-binding and catalytic turnover step, as well as the enhanced diffusion effect of the enzyme. The model is general, has no adjustable parameters, and only requires three experimentally defined constants to quantify chemotaxis. Model enzyme-functionalized synthetic protocells have also been studied and they exhibit both positive and negative chemotaxis based on the interplay between positive enzymatic catalysis-induced chemotaxis and solute-lipid interaction-based negative chemotaxis. Controlling the extent and direction of chemotaxis holds considerable potential for designing cell mimics and delivery vehicles that can reconfigure their motion in response to environmental conditions. |
Friday, March 8, 2019 10:24AM - 10:36AM |
X61.00009: Exploring, exploiting, and ignoring history dependence in active droplet simulations Joseph Albert, Vincent Henry Crespi Active droplets are an appealing model system for studying self-propelled particles because of their high symmetry and simple composition. Nonetheless, their mechanism of action is complex - they are driven by a self-maintained concentration gradient that results in history-dependent motion. |
Friday, March 8, 2019 10:36AM - 10:48AM |
X61.00010: Sideways self-propulsion of Janus rods Naveen Reddy, Dugyala Venkateshwar Rao, Jan Fransaer, Christian Clasen The sideways self-propulsion behaviour of Janus rods will be presented [1-2]. Janus rods are prepared by consecutively sputter-coating platinum and gold on different sides of aligned polystyrene nano/micro-fibers produced via electrospinning. Self-propulsion is induced via the reaction of hydrogen peroxide at the Janus particle interface, and the effect of the particles shape on their self-propulsion trajectories is studied. We show that the self-propulsion trajectories change from straight to circular when the particle shape is changed from a straight to an ‘L’ shaped rod. In order to understand and quantitatively describe the particle shape effects, we have adopted a mathematical model developed by Hagen et al [3] to predict their trajectories. We show that the trajectory of irregularly shaped rods depends only on the particle shape. The predicted trajectories for various particle shapes are in good agreement with the experimental observations. We show that these sideways self-propulsion rods are effective in cargo transportation. |
Friday, March 8, 2019 10:48AM - 11:00AM |
X61.00011: Designing Self-propelled, Chemically Active Sheets: Wrappers, Flappers and Creepers Abhrajit Laskar, Oleg Shklyaev, Anna Christina Balazs Catalyst-coated, hard particles can spontaneously generate fluid flows, which in turn propel the particles through the fluid. If the catalyst-coated object were a deformable sheet, the self-generated flows could affect not only the sheet’s motion, but also its shape. By developing models that capture the interrelated chemical, hydrodynamic and mechanical interactions, we uncover novel behavior emerging from the previously-unstudied coupling between active, soft sheets and the surrounding fluid. The chemically-generated flows “sculpt” the sheet into various forms that yield different functionalities, which can be tailored by modifying the sheet’s geometry, patterning the sheet’s surface with different catalysts and employing cascades of chemical reactions. These studies reveal how to achieve both spatial and temporal control over the position and shape of active sheets and thus, utilize the layers to autonomously and controllably trap soft objects, perform logic operations and execute multi-stage processes in fluid-filled microchambers. |
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