Multimode Self-Oscillating Vesicle Transformers

Engineering synthetic materials that mimic the complex rhythmic oscillatory behavior of living cells is a fundamental challenge in science and technology. Up to now, the reported synthetic model system still cannot compete with nature in oscillatory modes and amplitudes. Presented here is a novel alternating copolymer vesicle that exhibits drastic and multimode shape oscillations in real time, which are controlled by polymer concentrations and driven by the Belousov—Zhabotinsky oscillatory reaction, including swelling/deswelling, twisting/detwisting, stretching/shrinking, fusion/fission, and multiple division. Some of them, especially the fission oscillation, have not been observed before. In addition, the oscillation magnitude with regard to diameter is much larger than that of previously reported self-oscillating vesicles. Such a self-oscillating vesicle transformer would extend the complexity and capacity of membrane deformations in synthetic systems, approaching those of natural cells.     

Evolved Colloidosomes Undergoing Cell‐like Autonomous Shape Oscillations with Buckling

In living systems, there are many autonomous and oscillatory phenomena to sustain life, such as heart contractions and breathing. At the microscopic level, oscillatory shape deformations of cells are often observed in dynamic behaviors during cell migration and morphogenesis. In many cases, oscillatory behaviors of cells are not simplistic but complex with diverse deformations. So far, we have succeeded in developing self‐oscillating polymers and gels, but complex oscillatory behaviors mimicking those of living cells have yet to be reproduced. Herein, we report a cell‐like hollow sphere composed of self‐oscillating microgels, that is, a colloidosome, that exhibits drastic shape oscillation in addition to swelling/deswelling oscillations driven by an oscillatory reaction. The resulting oscillatory profile waveform becomes markedly more complex than a conventional one. Especially for larger colloidosomes, multiple buckling and moving buckling points are observed to be analogous to cells.     

Chemically Controlled Spatiotemporal Oscillations of Colloidal Assemblies

We report an autonomous oscillatory micromotor system in which active colloidal particles form clusters, the size of which changes periodically. The system consists of an aqueous suspension of silver orthophosphate microparticles under UV illumination, in the presence of varying concentrations of hydrogen peroxide. The colloid particles first attract each other to form clusters. After a short delay, these clusters abruptly disperse and oscillation begins, alternating between clustering and dispersion of particles. After a cluster oscillation initiates, the oscillatory wave propagates to nearby clusters and eventually all the clusters oscillate in phase‐shifted synchrony. The oscillatory behavior is governed by an electrolytic self‐diffusiophoretic mechanism which involves alternating electric fields generated by the competing reduction and oxidation of silver. The oscillation frequency is tuned by changing the concentration of hydrogen peroxide. The addition of inert silica particles to the system results in hierarchical sorting and packing of clusters. Densely packed Ag₃PO₄ particles form a non‐oscillating core with an oscillating shell composed largely of silica microparticles.     

The oscillatory adsorption of self‐assembled organosilane films on aluminium oxide surfaces

The time‐dependent oscillatory growth mechanism of organosilane film self‐assembly on aluminium oxide has been investigated using X‐ray photoelectron spectroscopy. While this unusual oscillatory process has been reported for the trifunctional silane, propyltrimethoxysilane, we report here, for the first time, that this oscillatory behaviour is also present during the self‐assembly of the difunctional silane propylmethyldimethoxysilane. The presence of multiple oscillations in this growth mechanism is also first reported for propyltrimethoxysilane and propylmethyldimethoxysilane as a function of exposure time. Multiple oscillations indicate that the 3‐component model that is used to describe and fit a single coverage oscillation must be reconsidered and contain additional components to account for the multiple oscillations seen experimentally. The absence of such oscillatory behaviour in the growth of the monofunctional organosilane propyldimethylmethoxysilane, which in fact follows a Langmuir‐type growth mechanism, indicates that this measurable oscillatory behaviour is because of the ability of multifunctional silanes to oligomerise both on the substrate and in solution.     

Design of a Tunable Self‐Oscillating Polymer with Ureido and Ru(bpy)3 Moieties

An upper critical solution temperature (UCST)‐type self‐oscillating polymer was designed that exhibited rhythmic soluble–insoluble changes induced by the Belousov–Zhabotinsky (BZ) reaction. The target polymers were prepared by conjugating Ru(bpy)₃, a catalyst for the BZ reaction, to ureido‐containing poly(allylamine‐co‐allylurea) (PAU) copolymers. The Ru(bpy)₃‐conjugated PAUs exhibited a UCST‐type phase‐transition behavior, and the solubility of the polymer changed in response to the alternation in the valency of Ru(bpy)₃. The ureido content influences the temperature range of self‐oscillation, and the oscillation occurred at higher temperatures than conventional LCST‐type self‐oscillating polymers. Furthermore, the self‐oscillating behavior of the Ru‐PAU could be regulated by addition of urea, which is a unique tuning strategy. We envision that novel self‐oscillating polymers with widely tunable soluble‐insoluble behaviors can be rationally designed based these UCST‐type polymers.     

Recent Advances in Self‐Oscillating Polymer Material Systems

In 1996, we first reported self‐oscillating polymer gels exhibiting autonomous swelling‐deswelling oscillations driven by the Belousov‐Zhabotinsky reaction. In contrast to conventional stimuli‐responsive gels, the self‐oscillating gel can autonomously and periodically change its volume in a closed solution without any external stimuli. Since the first report, the novel concept of self‐oscillating gels has been expanded into various polymer and gel systems. Herein, we summarize recent advances in self‐oscillating polymers and gels.     

New Experimental and Mechanistic Investigation on the KSCN‐H2O2‐NaOH‐Cu(II)‐Catalyzed Oscillating System (Orbàn–Epstein Reaction): Inhibitory Effects by Diphenols

The KSCN‐H₂O₂‐NaOH‐Cu(II)‐catalyzed system is one of the few reactions in which chemical oscillations can be observed in batch conditions. In the present paper, this oscillating reaction was revisited in a wide range of initial concentrations of all components in batch. A mixture with a long lasting oscillation time (1 h 34 min) and a great number of oscillations (24) was found and used to investigate the effect of temperature. An Arrhenius‐type temperature dependence was observed from which an apparent “average activation energy” E_av = 76 ± 5 kJ for the overall oscillatory reaction was observed. A mechanistic study based on a modified model analyzed by the stoichiometric network analysis approach gave a satisfactory agreement between calculated and experimental oscillating behaviors and temperature dependence. The addition of the three diphenols (catechol, resorcinol, and hydroquinone) causes perturbations similar to those observed in the Briggs‐Rauscher oscillating system, i.e., an inhibition of the oscillatory regime. These inhibitory effects were described in detail, and a reasonable qualitative interpretation is given.     

Dissipative and Autonomous Square‐Wave Self‐Oscillation of a Macroscopic Hybrid Self‐Assembly under Continuous Light Irradiation

Building a bottom‐up supramolecular system to perform continuously autonomous motions will pave the way for the next generation of biomimetic mechanical systems. In biological systems, hierarchical molecular synchronization underlies the generation of spatio‐temporal patterns with dissipative structures. However, it remains difficult to build such self‐organized working objects via artificial techniques. Herein, we show the first example of a square‐wave limit‐cycle self‐oscillatory motion of a noncovalent assembly of oleic acid and an azobenzene derivative. The assembly steadily flips under continuous blue‐light irradiation. Mechanical self‐oscillation is established by successively alternating photoisomerization processes and multi‐stable phase transitions. These results offer a fundamental strategy for creating a supramolecular motor that works progressively under the operation of molecule‐based machines.     

Fabrication of Self‐Oscillating Micelles with a Built‐In Oxidizing Agent

Various biological behaviors are fueled by “respiration”, which is an example of catabolism. So far, we have reported various self‐oscillating soft materials exhibiting bioinspired dynamic movements. These autonomous polymer systems are driven by the Belousov–Zhabotinsky (BZ) reaction, which is analogous to the tricarboxylic acid (TCA) cycle that is an integral part of respiration. However, in the BZ reaction, the external addition of an oxidizing agent is necessary to initiate the oxidation process, which is realized by intracellular moieties such as ubiquinone in living systems. Herein, we realized self‐oscillating micelles that are driven without the external addition of an oxidizing agent. This was achieved by embedding the oxidizing source into the structure of the self‐oscillating AB diblock copolymers. This strategy introduces a new function equivalent to intracellular oxidizing moieties, and is useful for the design of completely autonomous bioinspired materials.     

Cu(II)‐Metallacryptands Self‐Assembled to Vesicular Aggregates Capable of Encapsulating and Transporting an Anticancer Drug Inside Cancer Cells

Crystallographically characterized M₂L₄ type cationic Cu(II)‐metallacryptands [MC(X)] derived from a series of bis‐pyridyl‐bis‐urea ligands (L_X; X = O, S, C) are self‐assembled to single‐layered vesicular aggregates in DMSO, DMSO/water, and DMSO/DMEM (biological media). One such vesicle is MC(O)‐vesicle that is demonstrated to be able to load and release (pH responsive) an anticancer drug, namely doxorubicin hydrochloride (DOX). DOX‐loaded MC(O)‐vesicle is also successfully transported within MDA‐MB‐231 cells—a highly aggressive human breast cancer cell line. Such self‐assembling behavior to form vesicular aggregates by metallacryptands (MCs) is hitherto unknown.     

Vesicle fission of giant unilamellar vesicles of liquid-ordered-phase membranes induced by amphiphiles with a single long hydrocarbon chain

Vesicle fissions are very important processes of biomembranes in cells, but their mechanisms are not clear and are controversial. Using the single giant unilamellar vesicle (GUV) method, we recently found that low concentrations (less than the critical micelle concentration (CMC)) of lysophosphatidylcholine (lyso-PC) induced the vesicle fission of GUVs of dipalmitoylphosphatidylcholine/cholesterol(6/4) (DPPC/chol(6/4)) membranes and sphingomyelin/cholesterol membranes (6/4) in the liquid-ordered (lo) phase. In this report, to elucidate its mechanism, we have investigated the effect of low concentrations (much less than their CMC) of other amphiphiles with a single long hydrocarbon chain (i.e., single long chain amphiphiles) on DPPC/chol(6/4) GUVs as well as the effect of the membrane composition on the lyso-PC-induced vesicle fission. We found that low concentrations of single long chain amphiphiles (lyosophosphatidic acid, octylglucoside, and sodium dodecyl sulfate) induced the shape change from a prolate to two spheres connected by a very narrow neck, indicating that the single long chain amphiphiles can be partitioned into the external monolayer in the lo phase of the GUV from the aqueous solution. As the single long chain amphiphile concentrations were increased, all of them induced vesicle fission of DPPC/chol(6/4) GUVs above their threshold concentrations. To elucidate the role of cholesterol in the single long chain amphiphile-induced vesicle fission, we investigated the effect of lyso-PC on GUVs of dioleoyl-PC (DOPC)/chol(6/4) membranes in the Lalpha phase; no vesicle fission occurred, indicating that cholesterol in itself did not play an important role in the vesicle fission. Finally, to elucidate the effect of the inclusion of DOPC in the lo-phase membrane of GUVs on the lyso-PC-induced vesicle fission of the DPPC/chol(6/4) GUV, we investigated the effect of low concentrations of lyso-PC on GUVs of DPPC/DOPC/chol membranes. With an increase in DOPC concentration in the membrane, the threshold concentration of lyso-PC increased. At and above 30 mol % DOPC, no vesicle fission occurred. On the basis of these results, we have proposed a hypothesis of the mechanism of the single long chain amphiphile-induced vesicle fission of a GUV of a lo-phase membrane.     

一种新的振荡体系的研究 Ⅰ. 四极质谱对溶解气体振荡释放现象的考察

本文对溶解气体振荡释放体系使用了四极质谱进行追踪检测。确认了该体系为一种振荡的物理体系。对溶解气体振荡释放过程的实验考察表明该体系对溶解气体的放空流量等均有阈值要求。该体系对温度的变化是敏感的。在本文的实验条件下, 体系对压力、搅拌等扰动是稳定的。体系内的丙烯与合成气在一定的条件下还可以发生一种“协同”作用。实验考察表明, 溶解气体振荡释放过程的产生是由于体系在过饱和状态下成核机制所致。在非平衡条件下, 气泡生长与上升过程所造成的负反馈条件是形成周期性现象的根本原因。本机制应对溶解度大而且沸点相差也大的二元气液体系具有普适性。     

Spontaneous Membrane Generation and Extension in a Dipeptide Single Crystal and Phospholipid Mixed System

Self‐reproduction is one of the most important characteristics of lipid vesicles for origin of life research. Most vesicle self‐reproduction systems are based on fatty acid vesicles and spontaneous phospholipid vesicle production is difficult owing to the relatively high stability of these vesicles. Now, spontaneous phospholipid vesicle generation and extension in a dipeptide/phospholipid system is demonstrated. Dissolution of the dipeptide crystal provides both the driving force and phospholipid constituents for vesicle generation and extension. This study provides a new system to enhance the understanding of vesicle self‐reproduction mechanisms.     

光引发的甘油三乙酯反应的振荡

在(30±0.1)℃条件下,以1对5W节能灯(电子荧光灯)为光源,设计出以甘油三乙酯为底物,环已烷水混合体系的Beluosov-Zhabotinskii振荡反应,并研究了改变光源、有机溶剂、各组分浓度以及加入各类表面活性剂形成O/W型乳液后,各种因素对此类光引发振荡反应的影响。实验表明,能够引发反应产生振荡的光源范围是较大的,通过光谱实验,分解实验和元素分析说明了环已烷在此反应中基本是惰性,并通过机理模型的计算,初步讨论了光照使原单调反应产生振荡的原因。     

Control of the Dynamic Motion of a Gel Actuator Driven by the Belousov‐Zhabotinsky Reaction

A novel self‐oscillating gel actuator with gradient structure, which generates a pendulum motion by fixing one edge of the gel without external stimuli was achieved. The gel was synthesized by copolymerizing the ruthenium catalyst for the Belousov‐Zhabotinsky reaction with N‐isopropylacrylamide and 2‐acrylamido‐2‐methylpropane sulfonic acid. Furthermore, we clarified that the period and amplitude for the self‐oscillating behavior of the gel actuator are controllable by changing the composition, temperature, and size of the gel. The maximum amplitude of the novel gel actuator is about a 100 times larger than that of the conventional self‐oscillating gel system.

     

Mass transfer from an oscillating microsphere

The enhancement of mass transfer from single oscillating aerocolloidal droplets having initial diameters approximately 40 microm has been measured using electrodynamic levitation to trap and oscillate a droplet evaporating in nitrogen gas. The frequency and amplitude of the oscillation were controlled by means of ac and dc fields applied to the ring electrodes of the electrodynamic balance (EDB). Elastic light scattering was used to size the droplet. It is shown that the mass transfer process for a colloidal or aerocolloidal particle oscillating in the Stokes flow regime is governed by a Peclet number for oscillation and a dimensionless oscillation parameter that represents the ratio of the diffusion time scale to the oscillation time scale. Evaporation rates are reported for stably oscillating droplets that are as much as five times the rate for evaporation in a stagnant gas. The enhancement is substantially larger than that predicted by quasi-steady-flow mass transfer.     

Periodic Nucleation of Calcium Phosphate in a Stirred Biocatalytic Reaction

Highly ordered superstructures composed of inorganic nanoparticles appear in natural and synthetic systems, however the mechanisms of non‐equilibrium self‐organization that may be involved are still poorly understood. Herein, we performed a kinetic investigation of the precipitation of calcium phosphate using a process widely found in microorganisms: the hydrolysis of urea by enzyme urease. With high initial ratio of calcium ion to phosphate, periodic precipitation was obtained accompanied by pH oscillations in a well‐stirred, closed reactor. We propose that an internal pH‐regulated change in the concentration of phosphate ion is the driving force for periodicity. A simple model involving the biocatalytic reaction network coupled with burst nucleation of nanoparticles above a critical supersaturation reproduced key features of the experiments. These findings may provide insight to the self‐organization of nanoparticles in biomineralization and improve design strategies of biomaterials for medical applications.     

A comparative study of temperature dependence of induction time and oscillatory frequency in polymer‐immobilized and free catalyst Belousov‐Zhabotinsky reactions

Environment‐sensitive poly(N‐isopropylacrylamide) (PNIPAM) microgel particles with covalently bonded ruthenium(4‐vinyl‐4′‐methyl‐2,2′‐bipyridine) bis (2,2′‐bipyridine) [Ru(vmbipy)(bipy)₂] display periodic size changes when placed in Belousov‐Zhabotinsky (BZ) reaction substrates. The temperature dependency of the induction time and oscillatory frequency of the BZ reaction in this polymer‐immobilized catalyst system were compared to the bulk BZ reaction with the catalyst in the solution phase. Prolonged induction times are observed for the immobilized catalyst, compared with free catalyst, while little difference is observed on the oscillation frequency. The Arrhenius frequency factor calculated using the induction time for the immobilized catalyst BZ reaction is about seven times smaller than that for the free catalyst Ru(bipy) case. On the other hand, the Arrhenius frequency factors calculated using the oscillatory frequency are almost the same, showing similar reaction kinetics during the BZ oscillations. The tunability of the induction time using a polymer matrix, as we observed here, while maintaining similar oscillatory behavior, should provide a new dimension to control the self‐assembling of BZ active particles. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 847–854, 2009     

Periodic Oscillatory Motion of a Self‐Propelled Motor Driven by Decomposition of H2O2 by Catalase

A self‐propelled motor driven by the enzymatic reaction of catalase adsorbed onto a filter paper floating on an aqueous solution of H₂O₂ was used to study nonlinear behavior in the motor's motion. An increase in the concentration of H₂O₂ resulted in a change from no motion to irregular oscillatory motion, periodic oscillatory motion, and continuous motion. The mechanisms underlying oscillation and mode bifurcation are discussed based on experimental results on O₂ bubble formation and growth on the underside of the motor.     

A Potentiometric Study of the Briggs–Rauscher Oscillatory Reaction in a Solution of Nonionic Surfactants and tert‐Butanol

The concentration effect of nonionic surfactants (Triton X‐100, Brij 30, Brij 58, Tween 20, and Tween 80) and tert‐butanol was investigated on the Briggs–Rauscher oscillatory reaction in a stirred batch reactor at 25 ± 0.1°C in both the absence and presence of nonionic surfactant and tert‐butanol as well. The addition of Triton X‐100, Brij 58, and Tween 20 influenced the oscillatory parameters in a similar fashion: a decrease of the induction period until its disappearance, an increase of the oscillation period, an increase of the oscillation amplitude, an increase of the duration of the oscillation, and a gradual increase of the oscillation numbers. The addition of Brij 30 has no significant effect on the oscillation parameters of the Briggs‐Rauscher oscillatory reaction. The effect of tert‐butanol on the Briggs–Rauscher oscillatory reaction is very similar to the effect of Brij 58; however, we were unable to observe the disappearance of the induction period on the studied concentration range. The addition of Tween 80 to the reaction mixture has a similar effect as the addition of Triton X‐100, Brij 58, or Tween 20, except for the induction period, which in the case of Tween 80, it becomes larger. The observed effects are explained in terms of micellar catalysis or inhibition, that is, in a different extent of individual reactants of solubilization.