![]() The proposed system is illustrated in Figure 1. Recently, we proposed a new class of systems to stochastically form Liesegang-band-like, periodic precipitation bands of Cu–Fe-based Prussian blue analogs (Cu–Fe PBA) in agarose gel through the coupled processes of 1) electrochemical reactions to generate reactant ions, 2) diffusion of the reactant ions influenced by the electric field in the gel, and 3) reactions of the reactant ions to form precipitates, that is, reaction–diffusion–reaction (RDR) processes. ![]() Because only limited systems have been reported to form Liesegang bands, it is scientifically interesting and technologically important to widely explore systems in which Liesegang-band-like precipitation patterns could form. The shorter column on top (in the form of a gel or aqueous solution) has a higher electrolyte concentration, and the longer gel column is placed at the bottom, in which a continuous precipitation zone and/or Liesegang bands form. In conventional experiments to observe Liesegang bands, two electrolytes are loaded into separate columns in a single sample tube. Liesegang banding continues to attract considerable scientific interest as a self-organization phenomenon that is potentially applicable to micro- and nanofabrication. Liesegang bands, which are the periodic precipitation bands of slightly soluble compounds via reaction–diffusion (RD) processes in hydrogels, have been continuously investigated since their discovery by Liesegang in 1896. The application of cyclic alternating voltages (particularly, 4 V for 1 h and 1 V for 4 h) was effective in generating Liesegang-band-like periodic bands, particularly for the Cu–Fe PBA system. Experiments using a Ti cathode suggested that the formation and subsequent decomposition of PB or Cu–Fe PBA at the cathode surface are important for forming precipitation band(s) in the gel near the cathode. Higher voltage applications suppressed the propagation of the Cu–Fe PBA band to the anode side and caused the PB band to disappear. Higher initial 3‒ concentrations deepened the color of the generated patterns. Longer voltage applications promoted propagation of the Cu–Fe PBA band to the anode side and caused the discrete PB band to disappear. ![]() In the Cu–Fe PBA system, a relatively long precipitation band of Cu(OH) 2 was also generated on the anode side by OH − ions produced on the cathode as a byproduct. Under the application of 2 V for 20/50 h, the PB/Cu–Fe PBA formed a discrete precipitation band on the anode/cathode side in an agarose gel containing 0.050 M 3‒ ions. These patterns strongly depended on the type of metal electrode, applied voltage, initial 3‒ concentration, and elapsed time after voltage application. In agarose gel containing 3‒ ions and sandwiched between two metal rods (Ti, Fe, or Cu) with a voltage of 1‒5 V applied for 20–100 h, reaction–diffusion–reaction (RDR) processes (that is, electrochemical reactions at metal rods to generate reactant ions, diffusion of the reactant ions influenced by the electric field in agarose gel, and reactions of the reactant ions to form/decompose precipitates) were coupled to generate diverse precipitation patterns of Prussian blues (PB) or Cu–Fe-based Prussian blue analogs (Cu–Fe PBA). Department of Chemical and Biological Sciences, Faculty of Science, Japan Women’s University, Tokyo, Japan. ![]()
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