Magnetic Metalloimmunoassays: Influence of Particle Shape and Size on Galvanic Exchange-Based Electrochemical Detection

Abstract

The electrooxidation of silver (Ag) offers improved signal sharpness and intensity, enabling precise measurements and enhanced resolution in electrochemical sensing. Leveraging silver nanoparticles (AgNPs) as indirect labels in electrochemical assays provides a distinct advantage for detecting various biomarkers at clinically relevant concentrations. In our recent work, we have introduced an electrochemical approach that involves a galvanic exchange (GE) reaction between electrodeposited gold and AgNPs, followed by anodic stripping voltammetry (ASV), with potential applications in immunosensing. In this assay protocol, specific monoclonal antibodies immobilize the biomarker of interest separately on AgNPs and magnetic beads. Subsequently, magnetic bead-AgNP conjugates are attracted to the electrode surface by a magnetic force. However, to achieve the expected analytical performance of the bioassay, it is imperative to characterize the composition, size, shape, and surface modifications of the AgNPs. Consequently, our research focuses on investigating the electrochemical properties of a model bioconjugate formed between magnetic microbeads (MB) and AgNPs through Streptavidin-biotin interactions under various assay conditions. Notably, our recent studies reveal that conducting the GE cycle for three repetitions significantly enhances the Ag signal, yielding an improvement of approximately two-fold or greater compared to previously reported methods. Previous studies have shown that altering the shape of AgNPs, transitioning from nanospheres to other forms like nanocubes and nanodisks enhances both the limit of detection and the range of analyte detection. However, despite these enhancements, spherical AgNPs remain the most stable and easily synthesized option, characterized by excellent homogeneity. Recently, we compared AgNPs with diameters ranging from 20 to 100 nm paired with magnetic microbeads with diameters spanning from 50 nm to 3 µm to identify the optimal combination. Preliminary results indicate that the Mb (3 µm)-AgNP (50 nm) conjugate exhibits the highest Ag charge and Ag collection efficiency. Nonetheless, critical knowledge gaps persist regarding the fundamental understanding of the electrogenerated GE process and we are continually working on refining the electrochemical and assay parameters to develop a robust universal platform for GE-based electrochemical immunosensing.