Single entity electrochemistry of hard and soft nanoparticles using direct particle coulometry, blocking, and catalytic amplification methods.

Abstract

In the last decades, single entity electrochemistry (SEE) has emerged as a powerful technique allowing for characterization of nanoparticles, detection of molecules, enzymes, cells, viruses, and even one single atom at a time. The advantages of these techniques are many including the possibility of study electron transfer kinetics, better understanding of the nanoparticles (NPs) behavior in the medium, sizing of the NP, gives an insight of the NPs homogeneity, requires low concentration of the analyte, fast straightforward method, and highly sensitive (one particle detection limit). The herein project is a meticulous study of hard and soft nanoparticles of interest using SEE methods. The first material to be described is nano zero-valent iron (nZVI) particles which are currently in use for environmental remediations. Traits such as magnetic behavior, ion sequestering capability, and biocompatibility makes this material a focus of interest in different scientific fields such as environmental and health sciences. Direct oxidation of single nZVI particles is reported in the fourth chapter of the thesis. Anodic particle coulometry (APC) was used as the sizing method for nZVI particles characterization, and the data analysis was compared to standard sizing characterization methods such as microscopy, nanoparticle tracking analysis (NTA), and dynamic light scattering (DLS). Since nZVI particles are prompt to agglomeration due to their magnetic property, an emulsion droplet experiment was purposed to delimit the nZVI particles interaction and enhance dispersion. This time, a water to oil (w/o) emulsion was prepared where an aqueous solution of picomolar (pM) concentrations in nZVI was injected to an organic matrix and analyzed using SEE. The results are interpreted and discussed in the subsequent pages. As nZVI particles are used for environmental remediation, a modified version of them emerges as a waste product. One of the remediation processes nZVI particles have been used for is water and soil cadmium removal. Cadmium is a heavy metal of concern due to its toxicity and thread to the environment and human health. A change in the morphological structure of the nZVI particles was observed when exposed to high ppm concentrations of cadmium. NZVI particles exposed to 100 ppm Cd²⁺ were studied for the first time using SEE. Direct reduction of the particle was obtained, and the data was analyzed using cathodic particle coulometry (CPC). CPC permits the sizing of the particle and the obtained results are discussed hereinafter. Interestingly, a staircase response was also obtained for the most concentrated sample of particles suggesting a blocking mechanism. The amplitude of the steps (ΔI) was also used for sizing approximations. A third nanomaterial was characterized using SEE. In this case copper/zinc superoxide dismutase (Cu/Zn SOD) was detected using SEE for the first time. Cu/Zn SOD is a metalloprotein that belongs to a family of catalytic enzymes which functionality is to convert superoxide to oxygen and hydroxide in the body. They work as a body defense against reactive oxygen species (ROS) that escape the mitochondria during oxidative phosphorylation, therefore are a natural antioxidant. The overexpression of at least one type of SOD has been related to certain types of cancer such as gastric cancer, colorectal cancer, and lung cancer. Its detection in fM concentrations was achieved for the first time using catalytic amplification technique. This opens the door for possible applications such as the development of a biosensor for certain types of cancers, among others.