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The interaction between organic molecules and oxidized catalyst surfaces has frequently been used to study the fuel crossover from the anode to the cathode in direct liquid fuel cells. In such experiments, the oxidized surface is put in contact with the fuel under open circuit conditions, and the evolution of the potential is registered. The open circuit potential (OCP) vs. time features can then inform on the reactivity of the fuel with the oxidized surface and provide valuable information not only to applications in fuel cells but also to the electrochemical reform of those molecules to produce clean hydrogen. In this paper, we present an experimental investigation of the open circuit interaction between ethanol or 2-propanol with oxidized platinum surfaces. Besides the OCP time traces, we have also employed cyclic voltammetry and fast oxide reduction sweep in the presence of the alcohols. Comparable reaction currents are obtained in the cyclic voltammogram, but the electro-oxidation of 2-propanol sets in at considerably lower overpotentials than that of ethanol. At the high potential region, both the magnitude and the potential of the current peak are nearly identical in both cases. In contrast, under open circuit conditions, the interaction of ethanol with the oxidized platinum surface is more pronounced than that found for 2-propanol, and these results are corroborated by the facile reduction of the platinum oxides along the fast backward sweep for the case of the latter.

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In biological or abiotic systems, rhythms occur, owing to the coupling between positive and negative feedback loops in a reaction network. Using the Semenov-Whitesides oscillatory network for thioester hydrolysis as a prototype, we experimentally and theoretically analyzed the role of fast and slow inhibitors in oscillatory reaction networks. In the presence of positive feedback, a single fast inhibitor generates a time delay, resulting in two saddle-node bifurcations and bistability in a continuously stirred tank reactor. A slow inhibitor produces a node-focus bifurcation, resulting in damped oscillations. With both fast and slow inhibitors present, the node-focus bifurcation repeatedly modulates the saddle-node bifurcations, producing stable periodic oscillations. These fast and slow inhibitions result in a pair of time delays between steeply ascending and descending dynamics, which originate from the positive and negative feedbacks, respectively. This pattern can be identified in many chemical relaxation oscillators and oscillatory models, e.g., the bromate-sulfite pH oscillatory system, the Belousov-Zhabotinsky reaction, the trypsin oscillatory system, and the Boissonade-De Kepper model. This study provides a novel understanding of chemical and biochemical rhythms and suggests an approach to designing such behavior.

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Water and its dissociated species at the solid‒liquid interface play critical roles in catalytic science; e.g., functions of oxygen species from water dissociation are gradually being recognized. Herein, the relationship between oxide identity (PtOHads, PtOads, and PtO2) and electrocatalytic activity of platinum for ethanol electrooxidation was obtained in perchlorate acidic solution over a wide potential range with an upper potential of 1.5 V (reversible hydrogen electrode, RHE). PtOHads and α-PtO2, rather than PtOads, act as catalytic centers promoting ethanol electrooxidation. This relationship was corroborated on Pt(111), Pt(110), and Pt(100) electrodes, respectively. A reaction mechanism of ethanol electrooxidation was developed with DFT calculations, in which platinum oxides-mediated dehydrogenation and hydrated reaction intermediate, geminal diol, can perfectly explain experimental results, including pH dependence of product selectivity and more active α-PtO2 than PtOHads. This work can be generalized to the oxidation of other substances on other metal/alloy electrodes in energy conversion and electrochemical syntheses.

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Many drugs adjust and/or control the spatiotemporal dynamics of periodic processes such as heartbeat, neuronal signaling and metabolism, often by interacting with proteins or oligopeptides. Here we use a quasi-biocompatible, non-equilibrium pH oscillatory system as a biomimetic biological clock to study the effect of pH-responsive peptides on rhythm dynamics. The added peptides generate feedback that can lengthen or shorten the oscillatory period during which the peptides alternate between random coil and coiled-coil conformations. This modulation of a chemical clock supports the notion that short peptide reagents may have utility as drugs to regulate human body clocks.

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The Belousov-Zhabotinsky (BZ) self-oscillating gel is a unique actuator suited for studying the behavior of intelligent soft robots. However, the traditional BZ self-oscillating polyacrylamide (PAAm) gel is easily broken and is slow to response to stimuli, which limits its practical application. Therefore, the preparation of BZ gels with sensitive responses to external stimuli and desirable, robust mechanical properties remains a challenge. In this work, PAAm-activated nanogels with unpolymerized double bonds are used as nanocrosslinkers to synthesize a nanogel crosslinking-based BZ (NCBZ) self-oscillating PAAm gel, whose mechanical properties, for example, antipuncture, cutting, and tensile properties, are superior to those of traditional PAAm BZ-self-oscillating gels. The oscillatory period of the traditional gel is much longer than that of the corresponding homogeneous BZ system, resulting from the slow response of the gel to changes in redox potential, whereas large, interconnected pores inside the NCBZ gel provide efficient channels for rapid species transport, supporting fast response of the gel, which results in almost the same period of chemomechanical oscillations as the homogeneous system under the same conditions. Scanning electron microscopy results show that the NCBZ gel is more stable than the traditional BZ PAAm gel after 7 h of oscillation. Our results make it possible to prepare robust gel motors and provide promising application prospects for smart soft robots, actuators, sensors, tissue engineering, and other applications.

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The electrodissolution of Au(111) in anaerobic cupric/ammonia/thiosulfate solutions, typical of a non-equilibrium dissipative system, was investigated via in situ electrochemical atomic force microscopy. At a specific initial concentration ratio of aqueous ammonia to cupric ions, the pit number and average pit area increase autocatalytically, while the pit depth increases monotonically during dissolution. A further increase in this initial concentration ratio leads to oscillatory dynamics in the pit number and average pit area while the pit depth fluctuates between one and two atoms. Mechanistic analysis indicates that alternation between formation and dissolution of a sulfur film results in periodic pitting, which produces gold dissolution layer by layer. This work presents a new dissolution mode, i.e., periodic layer dissolution generated by oscillatory pitting processes in addition to a pitting mode with a continually increasing depth, and the use of high initial concentration ratios of ammonia to cupric ion to accelerate the elimination of passivating sulfur film for Au dissolution.

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Active media that host spiral waves can display complex modes of locomotion driven by the dynamics of those waves. We use a model of a photosensitive stimulus-responsive gel that supports the propagation of spiral chemical waves to study locomotive transition and programmed locomotion. The mode transition between circular and toroidal locomotion results from the onset of spiral tip meandering that arises via a secondary Hopf bifurcation as the level of illumination is increased. This dynamic instability of the system introduces a second circular locomotion with a small diameter caused by tip meandering. The original circular locomotion with large diameter is driven by the push-pull asymmetry of the wavefront and waveback of the simple spiral waves initiated at one corner of gel. By harnessing this mode transition of the gel locomotion via coded illumination, we design programmable pathways of nature-inspired angular locomotion of the gel.

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Stem cells derived from human amniotic membrane (hAM) are promising targets in regenerative medicine. A previous study focused on human amniotic stem cells in skin wound and scar­free healing. The present study aimed to investigate whether hydrogen peroxide (H2O2)­induced senescence of human dermal fibroblasts (hDFs) was influenced by the anti­aging effect of conditioned medium (CdM) derived from human amniotic stem cells. First, the biological function of two types of amniotic stem cells, namely human amniotic epithelial cells (hAECs) and human amniotic mesenchymal stem cells (hAMSCs), on hDFs was compared. The results of cell proliferation and wound healing assays showed that CdM promoted cell proliferation and migration. In addition, CdM from hAECs and hAMSCs significantly promoted proliferation of senescent hDFs induced by H2O2. These results indicated that CdM protects cells from damage caused by H2O2. Treatment with CdM decreased senescence­associated ß­galactosidase activity and improved the entry of proliferating cells into the S phase. Simultaneously, it was found that CdM increased the activity of superoxide dismutase and catalase and decreased malondialdehyde by reducing H2O2­induced intracellular reactive oxygen species production. It was found that CdM downregulated H2O2­stimulated 8­hydroxydeoxyguanosine and γ­H2AX levels and decreased the expression of the senescence­associated proteins p21 and p16. In conclusion, the findings indicated that the paracrine effects derived from human amniotic stem cells aided delaying oxidative stress­induced premature senescence.

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The thiourea dioxide-periodate reaction has been investigated under acidic conditions using phosphate buffer within the pH range of 1.1-2.0 at 1.0 M ionic strength adjusted by sodium perchlorate. Absorbance-time series are monitored as a function of time at 468 nm, the isosbestic point of the I2-I3- system. The profile of these kinetic runs follows either sigmoidal-shaped or rise-and-fall traces depending on the initial concentration ratio of the reactants. The clock species iodine appears after a well-defined but reproducible time lag even in substrate excess, meaning that the system may be classified as an autocatalysis-driven clock reaction. It is also demonstrated that the age of the thiourea dioxide solution markedly shortens the Landolt time, suggesting that the original form of thiourea dioxide (TDO) rearranges into a more reactive form and reacts faster than the original one. The behavior found is consistent with that recently observed in other oxidation reactions of TDO. To characterize the system quantitatively, a 22-step kinetic model is constructed from adapting the kinetic model of the TDO-iodate reaction published recently by supplementing it with six different reactions of periodate. By the help of seven fitted rate coefficients a sound agreement between the measured and calculated absorbance-time traces is obtained.

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Reactions of hexathionate with thiosulfate and sulfite have been investigated by high-performance liquid chromatography via monitoring the concentration-time series of tetrathionate, pentathionate, hexathionate, and thiosulfate simultaneously within the pH range of 4.0-5.0. In both reactions, elementary sulfur forms; more significant sulfur precipitation may be observed in the case of the hexathionate-thiosulfate reaction, but slight turbidity in the other system means that elementary sulfur also appears in a detectable amount in the hexathionate-sulfite reaction. Initial rate studies have revealed that the formal kinetic orders of both reactants in both systems are clearly unity but pH-dependence can only be observed in the case of the hexathionate-sulfite reaction. The proposed kinetic model appears to suggest that nucleophilic attack of sulfite and thiosulfate may also occur on the ß- or γ-sulfur of the polythionate chain and breakages of the α-ß, ß-γ, and γ-γ' bonds are all conceivable possibilities to drive the reactions. Consequently, the generally accepted sulfur-chain elongating effect of thiosulfate on longer polythionates is also proven to be accompanied by sulfur-chain shortening pathways, eventually leading to the formation of elementary sulfur.

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The sulfide-chlorine dioxide reaction was found to have two distinct kinetic stages at alkaline conditions. The first stage proceeds so rapidly that it can only be measured by a stopped-flow technique at low temperature and leads to the parallel formation of polysulfide and sulfate as sulfur-containing products. At the same time, chlorite, chlorate, and chloride are produced from chlorine dioxide in detectable amounts, suggesting a complex stoichiometry. A nine-step kinetic model including short-lived intermediates like sulfide radical and •HSClO2- is proposed to describe the kinetic data in this rapid stage. In an excess of chlorine dioxide, the first stage is followed by a significantly slower one to be measured by conventional UV-vis spectroscopy at room temperature. Considering that tetrasulfide is formed during the first rapid course of the reaction, the subsequent slow kinetic stage can only be described by the direct oxidation of tetrasulfide by chlorine dioxide and, surprisingly, the tetrasulfide-catalyzed disproportionation of chlorine dioxide.

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The hydrogen peroxide-sulfite-ferrocyanide reaction shows excellent photosensitivity under pulse light illumination, which can be used to efficiently control the spatiotemporal dynamics of the nonlinear chemical system. Here, we numerically simulated the photoinduced pH oscillations by integrating two models that describe the oscillatory and photosensitive behaviors, respectively. A dynamic transition from the low-pH steady state to oscillations can be obtained using light illumination. In accordance with the simulation results, oscillatory dynamics was experimentally obtained under light illumination through excitation of a low-pH steady state. In the reaction-diffusion medium, corresponding multiple pulse waves were observed under suitable acid concentration and illumination conditions. Hence, light illumination can be efficiently employed to tune pattern formation in pH dynamic systems. Especially, the observation indicates that the local oscillations and pulse waves were promoted by diffusion in the gel reactor when comparing with dynamics in a homogeneous system.

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Hydrodynamic flows can exert multiple effects on an exothermal autocatalytic reaction, such as buoyancy and the Marangoni convection, which can change the structure and velocity of chemical waves. Here we report that in the chlorite-trithionate reaction, the production and consumption of chlorine dioxide can induce and inhibit Marangoni flow, respectively, leading to different chemo-hydrodynamic patterns. The horizontal propagation of a reaction-diffusion-convection front was investigated with the upper surface open to the air. The Marangoni convection, induced by gaseous chlorine dioxide on the surface, produced from chlorite disproportionation after the proton autocatalysis, has the same effect as the heat convection. When the Marangoni effect is removed by the reaction of chlorine dioxide with the Congo red (CR) indicator, an oscillatory propagation of the front tip is observed under suitable conditions. Replacing CR with bromophenol blue (BPB) distinctly enhanced the floating, resulting in multiple vortexes, owing to the coexistence between BPB and chlorine dioxide. Using the incompressible Navier-Stokes equations coupled with reaction-diffusion and heat conduction equations, we numerically obtain various experimental scenarios of front instability for the exothermic autocatalytic reaction coupled with buoyancy-driven convection and Marangoni convection.

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The trithionate-iodate reaction has been studied spectrophotometrically in an acidic medium at 25.0 ± 0.1 °C in phosphoric acid/dihydrogen phosphate buffer, monitoring the absorbance at 468 nm at the isosbestic point of the iodine-triiodide ion system and at I = 0.5 M ionic strength adjusted by sodium perchlorate. The main characteristics of the title system are very reminiscent of those found recently in the pentathionate-iodate and the pentathionate-periodate reactions, the systems paving the way for classifying clock reactions. Thorough analysis revealed that the direct trithionate-iodate reaction plays a subtle role only to produce a trace amount of iodide ion via a finite sequence of reactions, and once its concentration reaches a certain level, then the reaction is almost exclusively governed by the trithionate-iodine and iodide-iodate reactions. The title reaction, as expected, was experimentally proven to be autocatalytic with respect to iodide ion. A simple three-step Landolt-type kinetic model is proposed to describe adequately the most important kinetic features of the title system that can easily be extended to a feasible sequence of elementary and quasi-elementary reactions.

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Periodic to-and-fro migration is a sophisticated mode of locomotion found in many forms of active matter in nature. Providing a general description of periodic migration is challenging, because many details of animal migration remain a mystery. We study periodic migration in a simpler system using a mechanistic model of a photosensitive, active material in which a stimulus-responsive polymer gel is propelled by chemical waves under the regulation of an illumination gradient sensed by the gel, which plays a role analogous to the environment in periodic animal migration. The reciprocating gel migration results from autonomous transitions between retrograde and direct wave locomotion modes arising from the gradient distribution of the illumination intensity. The local dynamics of the chemical waves modulates the asymmetry between push and pull forces to achieve repeated reorientation of the direction of locomotion. Materials that display similar intelligent, self-adaptive locomotion might be tailored for such functions as drug delivery or self-cleaning systems.

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Crawling motion mediated by retrograde and direct waves, that is, in the opposite or the same direction, respectively, as the muscular wave that generates it, is a fundamental mode of biological locomotion, from which more complex and sophisticated locomotion modes involving outgrowths such as limbs and wings may have evolved. A detailed general description of muscular wave locomotion and its relationship with other modes of locomotion is a challenging task. We employ a model of a photosensitive self-oscillating gel, in which chemical pulse waves and a stimulus-responsive medium play roles analogous to nerve pulses and deformable muscles in an animal, to generate retrograde and direct waves under non-uniform illumination. Analysis reveals that the directional locomotion arises from a force asymmetry that results in unequal translation lengths in the push and pull regions associated with a pulse wave. This asymmetry can be modulated by the kinetic parameters of the photosensitive Belousov-Zhabotinsky reaction and the performance parameters of the gel, enabling a transition between retrograde and direct wave locomotion.

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Inspired by the biological growth that takes place in time-varying external fields such as light or temperature, we design an open reaction-diffusion system in order to investigate growth dynamics. The system is composed of the Belousov-Zhabotinsky (BZ) oscillatory reaction coupled with a copolymer gel consisting of NIPAAm and a photosensitive ruthenium catalyst. When subject to a unidirectional flow of the BZ reactants, the system displays groups of chemical waves whose structure depends upon the period and amplitude of illumination. Simulations of a modified six-variable Oregonator model exhibit all the complex wave groups found in our experiments. Studying this growth structure may aid in understanding the influence of periodic environmental variation on complex growth processes in living systems.

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Based upon a former study, the chlorite-trithionate reaction can avoid the side reactions arising from the well-known alkaline decomposition of polythionates, making it a suitable candidate for investigating spatial front instabilities in a reaction-diffusion-convection system. In this work, the chlorite-trithionate reaction was investigated in a Hele-Shaw cell, in which fingering patterns were observed over a wide range of reactant concentrations. A significant density increment crossing the propagating front indicates that the fingering pattern is generated as a consequence of the buoyancy-driven instability due to the density changes of solute when the gap thickness is less than 4 mm. The velocity of the steepest descent in the propagating front depends almost linearly on the gap thickness but displays a saturation-like profile on the trithionate concentration as well as a maximum on the chlorite concentration. Numerical simulation using the Stokes-Brinkman Equation coupled to the reaction-diffusion processes, including hydrogen ion autocatalysis and consumption, reproduces the observed fingering fronts.

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There has been steady interest in the aqueous reaction of ClO2• with sulfur(IV) since the 1950s, and a wide variety of rate laws and mechanisms have been proposed. In neutral-to-alkaline media, the reaction is challenging to study because of its great rate. Here it is shown that benzaldehyde can be used as an additive to slow the reaction and make its rates more amenable to study. The rates can be quantitatively modeled by a mechanism that includes reversible binding of sulfur(IV) by benzaldehyde and a rate-limiting mixed second-order reaction of ClO2• with SO3(2-). The latter reaction occurs through parallel electron transfer from SO3(2-) to ClO2• and oxygen-atom transfer from ClO2• to SO3(2-).

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The kinetics of the reactions of tetrathionate with S(IV) species and with thiosulfate in slightly acidic and neutral media were studied concurrently at 25.0 ± 0.1 °C by simultaneous high-performance liquid chromatography monitoring of the concentrations of polythionates (including trithionate, tetrathionate, and pentathionate), thiosulfate, and sulfite. The tetrathionate-sulfite and tetrathionate-thiosulfate reactions were found to be first-order with respect to both reactants. The tetrathionate-sulfite reaction was found to be pH-dependent under the conditions studied. In contrast, the tetrathionate-thiosulfate reaction was experimentally demonstrated to be pH-independent at neutral medium, where the pKa2 value of sulfurous acid plays a key role, whereas under slightly acidic conditions, between pH 4 and 5 the consumption of tetrathionate during the course of reaction was found to become pH-dependent. We show that the pH dependencies in both systems can be readily explained by the reactivity difference between sulfite and bisulfite toward the ß-sulfur of the tetrathionate. A simple two-step kinetic model incorporating the protonation equilibrium of sulfite is proposed on the basis of the simultaneous evaluation of the kinetic curves of the two systems, which allowed us to determine reliable rate coefficients for both the forward and backward reactions. Furthermore, the powerful ability of simultaneously evaluating the two chemical systems to yield reliable rate coefficients of the kinetic model is demonstrated.

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