pH-Regulated Chemical Oscillators

Abstract

The hydrogen ion is arguably the most ubiquitous and important species in chemistry. It also plays a key role in nearly every biological process. In this Account, we discuss systems whose behavior is governed by oscillations in the concentration of hydrogen ion. The first chemical oscillators driven by changes in pH were developed a quarter century ago. Since then, about two dozen new pH oscillators, systems in which the periodic variation in pH is not just an indicator but an essential prerequisite of the oscillatory behavior, have been discovered. Mechanistic understanding of their behavior has grown, and new ideas for their practical application have been proposed and, in some cases, tested. Here we present a catalog of the known pH oscillators, divide them into mechanistically based categories based on whether they involve a single oxidant and reductant or an oxidant and a pair of reductants, and describe general mechanisms for these two major classes of systems. We also describe in detail the chemistry of one example from each class, hydrogen peroxide-sulfide and ferricyanide-iodate-sulfite. Finally, we consider actual and potential applications. These include using pH oscillators to induce oscillation in species that would otherwise be nonoscillatory, creating novel spatial patterns, generating periodic transitions between vesicle and micelle states, stimulating switching between folded and random coil states of DNA, building molecular motors, and designing pulsating drug delivery systems. We point out the importance for future applications of finding a batch pH oscillator, one that oscillates in a closed system for an extended period of time, and comment on the progress that has been made toward that goal.

Viewpoint: Homeostasis as Inspiration—Toward Interactive Materials

“…An equivalent system was implemented with thiol chemistry that employed thioester hydrolysis in water as an initiator, thiol‐disulfide exchange as an auto amplification step and conjugate addition reactions of the thiols to maleimide and acrylamide for inhibition . There are also inorganic reaction networks based on similar principles . In absence of a flow reactor, however, there are only very few autonomously oscillating chemical systems: the inorganic Belousov–Zhabotinsky (BZ), the Briggs–Rauscher, and the Bray–Liebhafsky reactions.…”

Chemically coded time-programmed self-assembly

“…Nonlinear chemical dynamics operating under conditions outside equilibrium can offer a possible solution to overcome this issue. [14][15][16][17] At the material level, diverse modes of periodic actuation with no external stimuli have been elaborated via coupling nonlinear (oscillatory or even simple clock-type) chemical reactions with physicochemical processes in elastic materials. 2,[18][19][20][21][22][23][24] Autonomous periodic viscosity changes have been brought about through subsequently building and breaking chemical or physical links between the building blocks.…”

Front dynamics of pH oscillators with initially separated reactants

“…Contrary to the BZ reaction, the pH oscillators do not show periodic behaviour under batch conditions. These reactions are capable to generate large amplitude sustained pH-oscillations only in a continuously-fed stirred tank reactor (CSTR) and transient periodic behaviour in a semi-batch or a gel-fed reactor [9]. The reaction-diffusion dynamics of pH-oscillators have been explored in open one-sidefed reactors, where the feed of the initial reagents is made from one surface of the reaction medium, which is often a hydrogel.…”