Short Communication Electrochemical Formation of Prussian Blue in Natural Iron-Intercalated Clay and Cinder Matrixes Jyh-Myng Zen,* Annamalai Senthil Kumar, and Huey-Wen Chen Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan; e-mail: jmzen@dragon.nchu.edu.tw Received: October 22, 1999 Final version: December 9, 1999 Abstract Natural iron-intercalated clays and waste cinder (from steel industry) were converted into an ef®cient catalyst by electrochemical formation of Prussian blue (PB) directly inside the matrix. The basic electrochemical features for the PB-modi®ed electrodes were investigated by means of cyclic voltammetry technique, in terms of E fwhm , surface charge (q), surface excess (G), site-site interaction model, and ®lm resistivity (R f ), etc. The iron intercalated PB electrodes show superior electrochemical activity and stability over the classical GCE/PB electrodes and were analyzed by the interaction parameter `r'. The more ef®cient electrocatalytic function of the iron intercalated PB electrodes than the GCE/PB was demonstrated with guanine oxidation reaction. Keywords: Clay, Cinder, Prussian blue, Guanine We report here the ®rst electrochemical evidence for the for- mation of Prussian blue (PB) directly inside the natural iron- riched materials of nontronite and cinder. Earlier studies on clay modi®ed electrodes (CMEs) in the presence of Fe(CN) 6 3 focussed only on the interfacial surface characterization of the modi®ed clays [1±8]. Neither the clay ®lm formed by rapid drying nor the spin-coated CMEs showed the PB formation on the intercalated iron ions [1±8]. Since only a very small amount of clay can be deposited on the electrode surface, the con- centration of iron available for PB formation is even lower. Furthermore, the CME prepared by simple evaporation technique has highly porous aerogel structures and the ®lms eventually ``sloughed off'' as platelet to platelet distance increased to a minimum of energy interaction due to osmotic swelling in aqu- eous solutions [9]. Indeed, the nature of making the electrode, the amount of the material, and its operating concentration, etc., make a crucial modi®cation in the interfacial-structures that may in¯uence the PB formation. In this study, we solve the above problems by preparing the clay=carbon paste electrode (clay=CPE) and cinder=carbon paste electrode (cinder=CPE), which contain a relatively higher amount of clay or cinder. Two natural iron-riched modi®ers of nontronite, [(Si 7.25 Al 0.75 )(Fe 2.75 3 Al 0.85 Mg 0.33 Ti 0.05 )O 20 (OH) 4 X 0.98  , SWa- 1] [10], and industrial waste cinder were speci®cally used in this study. The iron content of cinder was experimentally determined as 7272.8 ppm=g by atomic absorption spectroscopy. To verify the iron-carbon intercalated nature of the cinder powders, same measurements were also done for 1 M HNO 3 treated (ca. 2 h at 60  C) cinder powders. The result of 6921.1 ppm=g is only about a 5 % decrease in iron content con®rming the expectation. Another one of the most studied smectite clays, [montmorillonite, (Si 7.84 Al 0.16 )(Fe 0.26 3 Al 3.22 Mg 0.44 Fe 0.12 )O 20 (OH) 4 X 0.68  , SWy- 1], was also chosen to prepare the SWy-1=CPE for comparison. Note that, even though the amount of iron centers in nontronite is about 7.2 times greater than that in montmorillonite, nontronite has isomorphous substitution in the tetrahedral sheet and mon- tmorillonite is a dioctahedral mineral with substitution mainly in the octahedral sheet [10, 11]. Figure 1 shows the continuous cyclic voltammetric (CV) response of the SWa-1=CPE, SWy-1=CPE, and cinder=CPE in 2 mM K 3 [Fe(CN) 6 ] and 0.1 M KCl=HCl (pH 2) solution. Scheme 1 illustrates the formation of PB directly into the iron 542 Scheme 1. # WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 1040±0397/00/0704±0542 $17.50:50=0 Electroanalysis 2000, 12, No. 7