A Reaction–Diffusion–Reaction System for Forming Periodic Precipitation Bands of Cu-Fe-Based Prussian Blue Analogues

Hisashi Hayashi,Tomoko Suzuki

doi:10.3390/app11115000

Abstract

We propose a simple and novel system to form precipitation patterns of Cu-Fe-based Prussian blue analogues (Cu-Fe PBA) in agarose gel through coupled electrochemical reactions, reactant ion diffusion influenced by electric field, and precipitation reactions. The spatiotemporal evolution, spatial distribution, and crystallite morphologies of the precipitates were investigated by visual inspection, Fe Kα intensity distribution measurements, and optical and scanning electron microscope observations. The observed precipitation patterns and their evolution depended on the applied voltage. Multicolored periodic precipitation bands were stochastically formed under cyclic alternating voltage (4 V for 1 h and then 1 V for 4 h per cycle). The distances between adjacent bands were randomly distributed (0.30 ± 0.25 mm). The sizes and shapes of the crystallites generated in the gel were position-dependent. Cubic but fairly irregular crystallites (0.1–0.8 μm) were formed in the periodic bands, whereas definitely cube-shaped crystallites (1–3 μm) appeared close to the anode. These cube-like reddish–brown crystallites were assigned to Cu-FeII PBA. In some periodic bands, plate-like blue crystallites (assigned to Cu(OH)2) were also present. Future issues for potential applications of the observed periodic banding for selective preparation of Cu-Fe PBA crystallites were discussed.

Highlights

Liesegang bands are formed spontaneously when a precipitation reaction is coupled to the diffusion of reagents in a sample tube filled with hydrogels [1,2,3]
We propose a simple reaction–diffusion–reaction (RDR)field, setup,and where electrochemical reactions, diffusion of reactant ions influenced by electric preelectrochemical reactions, diffusion of reactant ions influenced by electric field, and precipitation reactions are coupled to form periodic precipitation patterns in agarose gel (Figcipitation reactions areyet coupled to form periodic precipitation patterns agarose gel ure 1c), which has not been investigated
It was found that the Cu-Fe Prussian blue analogues (PBA) tended to produce a continuous precipitation band in agarose gels, regardless of preparation conditions such as the initial electrolyte concentration and gel density

Summary

Introduction

Liesegang bands (periodic precipitation bands of slightly soluble inorganic compounds in gels) are formed spontaneously when a precipitation reaction is coupled to the diffusion of reagents in a sample tube filled with hydrogels [1,2,3]. In this RD setup, two electrolytes are loaded in separate columns in a single sample tube. The longer gel column is placed at the bottom, where a continuous precipitation zone (sometimes called the “turbulent zone” [4]) and Liesegang bands form. The shorter column on top (in the form of a gel or aqueous solution) has a higher electrolyte concentration. These electrolytes are denoted as the “inner” and “outer” electrolytes, respectively

Methods

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Discussion

Conclusion