FUNDAMENTAL UNDERSTANDING AND APPLICATIONS OF NANOEMULSIONS FOR ULTRASENSITIVE ELECTROCHEMICAL ANALYSIS

Abstract

We demonstrate a new electroanalytical approach for the ultrasensitive electrochemical analysis using nanoemulsions (NEs) uniquely combined with single entity electrochemistry (SEE). First, we investigate the relationship between the structure and relevant electrochemical activity of NEs. We employ SEE to elucidate the interfacial structure of NEs and their corresponding electrochemical activities, which cannot be unequivocally defined by general microscopic techniques. Throughout this work on nanostructural effect of NEs, we could optimize the most suitable composition of NEs, showing facile electron transfer kinetics across the NE interface as well as a high monodispersity. Particularly, the application of SEE for optimized NEs allowed us to in-situ measure the partition coefficient at intact NEs. Although partition coefficient for NEs is a critical physicochemical property determining the uptake of delivery compounds, it has never been explicitly measured by existing ex-situ analytical techniques with intact NEs. Herein, we employ SEE to directly study the partitioned 2-aminobiphenyl (2-ABP) from aqueous bulk media into NEs. The direct electrolysis of 2-ABP in each NE enables us to in situ quantitatively measure the partition coefficient at intact NEs. Our study revealed an unprecedentedly large partition coefficient of 1.9 (± 1.4) × 1010 implying intermolecular interaction as well as the thermodynamic distribution, which was validated by molecular simulations. Based on the fundamental understanding of NEs, we finally demonstrated a new electrochemical method for ultra-trace level analysis by combining SEE and NEs. Innovatively, this approach enables to in situ separate, preconcentrate, and even detect analytes with a simple instrumentation. Using ferrocenemethanol (FcMeOH) as model analytes, we successfully established our method, where FcMeOH partitioned from water, and preconcentrated in each NE was quantitatively analyzed by SEE. Notably, the extraction is efficient to reach about 8 orders of magnitude of preconcentration factor under the true equilibrium, thus leading to a detection limit of 0.2 ppb. Our approach is readily applicable to investigate other aromatic toxicants dissolved in the water, thereby showing a broad applicability for ubiquitous aromatic contaminants and offering great prospects as a sensor for environmental applications.