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A novel method for chemically preparing densely gathered palladium nanoparticles (PdNPs) on a gallium-doped zinc oxide (GZO) electrode is reported. We found that PdCl42– could be captured directly on a bare GZO electrode surface simply by immersing a piece of GZO in an aqueous solution of K2PdCl4, resulting in a GZO electrode color change from transparent to yellow. The subsequent reduction of the composite electrode in an aqueous solution of NaBH4 produced a further color change to blackish-violet even after the PdCl42–-captured GZO had been washed and dried. X-ray photoelectron spectroscopy analysis confirmed the presence of Pd as PdCl42– and Pd0 on the GZO before and after reduction with NaBH4, respectively. The surface images collected using field emission scanning electronic microscopy revealed that the PdNPs were densely gathered and connected to one another. Although bare GZO electrodes have not been used extensively in voltammetric analysis, the present modification using densely gathered PdNPs shows promise as a new approach to constructing Pd-functionalized electrodes as confirmed by measuring voltammetric responses. The simple capture of metal complex precursors and nanostructuring followed by a simple reduction reaction on GZO is promising as a new and efficient fabrication method that conserves precious starting materials.

We previously showed that pamoic acid (PA) could be utilized as a reducing and capping reagent for preparing monodispersed gold nanoparticles (AuNPs) with diameters of 10.8 nm. Here, we show that the size of these PA-capped AuNPs can be varied in a controlled manner by changing the pH used in the preparation process. By changing the added amount of NaOH in the PA solution before mixing with HAuCl 4 , several types of AuNPs were prepared at different pH values. The absorption spectra showed a red shift of the characteristic peak of the AuNPs with decreasing pH, indicating the formation of larger AuNPs. The TEM results confirmed this relationship between AuNP size and pH, and the zeta potentials indicated that the AuNPs capped with PA were more stable than the AuNPs prepared by citrate reduction method. Furthermore, the PA-capped AuNPs showed much higher fluorescence intensities than those of citrate-capped AuNPs, which is due to the fluorescence of PA. The sizes of PA-capped spherical AuNPs could also be controlled by a seed-mediated growth approach. However, the original PAcapped AuNPs, with diameters of 10.8 nm, exhibited the highest fluorescence intensities among the various types of PA-capped AuNPs that were grown. In addition to conferring fluorescence properties to the AuNPs, the capping by PA also provided the nanoparticles with carboxylate groups.

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