The primary objective of this study is to determine electrokinetic properties of platinum nanoparticle (PtNP) monolayers deposited on mica sheets under controlled conditions of pH, ionic strength and temperature using an electrokinetic cell applied in the streaming potential measurements. The platinum nanoparticles (PtNPs) used in the research were prepared by a chemical reduction method using sodium borohydride as a reducing agent of chloroplatinic acid and trisodium citrate as a stabilizing agent of formed nanoparticles. In the first stages of research, the PtNPs dispersed in the suspensions were characterized using dynamic light scattering and electrophoretic light scattering techniques. It was found that the PtNPs exhibit an average size equal to 22±5 nm. The PtNPs were negatively charged, and their zeta potential was equal to -60±3 mV and -48±2 mV at ionic strength 10-4 and 10-2 M, respectively. The deposition process of PtNPs and formation of monolayers were conducted under controlled conditions of pH, ionic strength, and temperature. Because PtNPs as well as mica, used as a solid substrate, are negatively charged in a broad range of pH and ionic strength, an efficient formation of the monolayers was carried out after the modification of surface with cationic polyelectrolyte poly(allylamine hydrochloride) (PAH). The control of deposition time as well as ionic strength of PtNP suspension used in the experiments enable the formation of homogeneous monolayers. The structure and coverage of PtNP monolayers were determined based on the micrographs obtained from scanning electron microscopy (SEM).
In the next stage of studies, the electrokinetic properties of PtNP monolayers of given coverage were studied. Streaming potential Es of mica sheets, bare and covered by platinum nanoparticle monolayers, was determined in the plane-parallel cell. Es, occurring when an electrolyte solution of NaCl was flowing through the cell under regulated and constant hydrostatic pressure difference, was measured using the two Ag/AgCl electrodes connected to an electrometer [1]. The classical equations derived by Maryan Smoluchowski were used to convert streaming potential results into the surface zeta potential [2].
Different pH and ionic strength conditions were created to investigate the electrokinetic properties of PtNP monolayers and their stability over time. It was established that the electrokinetic properties of PtNP monolayers are dependent on the surface coverage. Dense PtNP monolayers of coverage equal to 25% were negatively charged in broad range of pH and ionic strength. Moreover, it was found that the desorption process of PtNPs from the monolayers was rather negligible. Thereby, with the use of streaming potential measurements the electrokinetic properties of monolayers were determined. The knowledge about these parameters should be useful for platinum-based catalysts and substrates used in modern spectroscopic techniques such as e.g., surface enhanced Raman spectroscopy (SERS) and tip enhanced Raman spectroscopy (TERS).