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Abstract: The gold-coated magnetic nanoparticles (Au@MNPs) have been proposed to act as “dispersible electrodes” in sensing applications with the dual advantages of reduced detection time and increased sensitivity. As a sensor the Au@MNPs would be modified with ligands to capture the analyte, with exposure of a magnetic field bringing the nanoparticles back to a macroelectrode for electrochemical detection of the amount of analyte. In this paper, the potential of Au@MNPs as dispersible electrodes is assessed via (1) characterization of how they assemble onto a macroelectrode upon application of a magnetic field, (2) their electron transfer behaviour to redox species in solution and redox species attached to the Au@MNPs and (3) how low a detection limit is possible. Visualization with atomic force (AFM), scanning electron (SEM) and light microscopy showed the particles formed a coral-like structure on the electrode. Electron transfer kinetics of the Au@MNPs-modified electrode was shown to be similar to a bare electrode by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemistry of the Os(bpy)2(AMP) modified Au@MNPs enabled an estimation of the smallest number of redox molecules per Au@MNP that could be detected. It was estimated that as little as 0.144±0.007 redox molecules per nanoparticle could be detected with the lowest actual detected number being 0.79±0.11 molecule per nanoparticle. [Copyright &y& Elsevier]

Herein, we introduce the concept of utilizing conductive gold-coated magnetic nanoparticles as ‘dispersible electrodes’, which serve as the active element in the selective capture and direct electro-analytical quantification of analytes. This concept reduces response times and decreases detection limits by bringing the sensor to the analyte rather than the conventional paradigm of the analyte finding the sensor. [ABSTRACT FROM AUTHOR]

Abstract: The effect of gold attachment on the physical characteristics, cellular uptake, gene expression efficiency, and biocompatibility of magnetic iron oxide (MNP) vector was investigated in vitro in BHK21 cells. The surface modification of magnetite with gold was shown to alter the morphology and surface charge of the vector. Nonetheless, despite the differences in the surface charge with and without gold attachment, the surface charge of all vectors were positive when conjugated with PEI/DNA complex, and switched from positive to negative when suspended in cell media containing serum, indicating the adsorption of serum components onto the composite. The cellular uptake of all MNP vectors under the influence of a magnetic field increased when the composite loadings increased, and was higher for the MNP vector that was modified with gold. Both bare magnetite and gold-coated magnetite vectors gave similar optimal gene expression efficiency, however, the gold-coated magnetite vector required a 25-fold higher overall loading to achieve a comparable efficiency as the attachment of gold increased the particle size, thus reducing the surface area for PEI/DNA complex conjugation. The MNP vector without gold showed optimal gene expression efficiency at a specific magnetite loading, however further increases beyond the optimum loading decreased the efficiency of gene expression. The drop in efficiency at high magnetite loadings was attributed to the significant reduction in cellular viability, indicating the bare magnetite became toxic at high intracellular levels. The gene expression efficiency of the gold-modified vector, on the other hand, did not diminish with increasing magnetite loadings. Intracellular examination of both bare magnetite and gold-coated magnetite vectors at 48h post-magnetofection using transmission electron microscopy provided evidence of the localization of both vectors in the cell nucleus for gene expression and elucidated the nuclear uptake mechanism of both vectors. The results of this work demonstrate the efficacy of gold-modified vectors to be used in cellular therapy research that can function both as a magnetically-driven gene delivery vehicle and an intracellular imaging agent with negligible impact on cell viability. [Copyright &y& Elsevier]