The Effect of the Capping Agents of Nanoparticles on Their Redox Potential

Din Zelikovich, Chemistry, Institute of Chemistry, Jerusalem, Israel (din.zelikovich@mail.huji.ac.il)

In the last years, nontoxicity has become at the forefront of research due to its relevance and importance to the environment and human health. Although the exposure risk to engineered nanomaterials depends on their physicochemical properties including their surface chemistry, the regulations are based mainly on the particle size. Since the use of nanoparticles is unavoidable, there is an urgent need for small, easy-to-use, and field-available sensors for the detection of nanoparticles based on their physicochemical properties including their surface chemistry.

Engineered metallic nanoparticles, which are found in numerous applications, are usually stabilized by organic ligands influencing their interfacial properties. We found that the ligands affect tremendously the electrochemical peak oxidation potentials of the nanoparticles. In this work, identical gold nanoparticles were ligand exchanged and carefully analyzed to enable a precise and highly reproducible comparison. The peak potential difference between gold nanoparticles stabilized by various ligands, such as 2- and 4-mercaptobenzoic acid, can be as high as 71 mV, which is substantial in energetic terms.

A detailed study supported by DFT calculations aimed to determine the source of this interesting effect. The DFT simulations of the ligand adsorption modes on Au surfaces were used to calculate the redox potentials through the thermodynamic cycle method. The DFT results of the peak potential shift were in good agreement with the experimental results for a few ligands; however, showed some discrepancy, which was attributed to kinetic effects. The kinetic rate constant of Au nanoparticles stabilized by 4‑mercaptobenzoic acid was found to be twice as large as that of the Au nanoparticles stabilized by citrate. Finally, these findings could be applied to some novel applications such as determining the distribution of nanoparticle population in a dispersion as well as monitoring the ligand exchange between nanoparticles.