Supporting Information
Carbon paste electrode
Graphite is widely used as basis for CPE electrode because of its electrochemical inertness.
Since graphite does not experience a reduction or oxidation reaction in a wide potential range,
the potential limits in its use are due to the electrolysis of water in aqueous solutions.
Hydrogen evolution is observed at negative potential values, and oxygen evolution is
achieved at positive potential values. In Fig. S1 are presented the cyclic voltammograms
(CVs) performed with a GPE, without the presence of the CoFe2O4/Au nanocomposite (blank,
graphite and binder only) in the same aqueous solutions used for the nanoparticles
electrochemical characterization. The CVs are flat in a wide potential interval and no
interferring peaks are visible in the regions used for the electrochemical characterization. An
anodic current due to the oxygen evolution is visible at very positive potentials: E>1.2Vvs
SCE for the 1M HClO4/1M NaCl solution. For the case of the 1 M HClO4 solution, oxygen
evolution took place at E >1.35V vs SCE.
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Fig S1 CVs performed with the blank GPE, without the presence of the CoFe 2O4/Au
nanocomposite, in the 1M HClO4, and the 1M HClO4/1M NaCl solutions. Scan rate 50 mV
cm-2, counter electrode Pt wire.
Bulk Au electrode oxidation
In order to compare the data collected in the characterization experiments of core@shell
nanoparticles. Gold oxidation and reduction reactions have been investigated on a Au bulk
electrode, in 1M HClO4 solutions. These experiments were performed with and without presence of
ionic iron and chloride.
The reason for carrying out the experiments in the presence of chloride is because chloride can form
stable complexes with Au and iron ions. Therefore, in a NaCl aqueous solution, oxidation and
reduction potential of Au are expected to be shifted to more negative potentials. In Fig. S2 are
shown the CVs for Au bulk electrode in 1M HClO4/ 1M NaCl solution with and without the
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presence iron ions. It can be observed that in the presence of chloride (Fig. S2a, inset), a higher
anodic current can be reached due to the fact that Au oxidation is followed by its dissolution and not
limited the formation of an oxide layer.
Au reduction peak (BR) and iron reduction peak (AR) are partly overlapped (Fig. S2b, inset).
Therefore, if Au concentration in solution is high (meaning that the scan reaches a potential higher
than 0.95V vs SCE, the iron reduction peak in this experimental conditions) the AR peak is
completely masked by the Au reduction peak and the presence of iron is no longer detectable in the
cathodic sweep of the voltammogram. Nevertheless the presence of the iron can be investigated
analyzing the anodic sweep, where Fe2+ oxidation peak (AO) is clearly visible.
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Fig S2 CVs for Au bulk electrode in a 1M HClO4/ 1M NaCl solution with and without the
presence iron ions, Fe(NO3)3. Scan rate 50 mV cm-2, counter electrode Pt wire.
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