A detailed study of the aqueous synthesis of composite 50-150 nm magnetite-gold core-shell nanoparticles with the ability to engineer the coverage of gold on the magnetite particle surface is presented. This method utilizes polyethyleneimine for the dual functions of attaching 2 nm gold nanoparticle seeds onto magnetite particles as well as preventing the formation of large aggregates. Saturation of the magnetite surface with gold seeds facilitates the subsequent overlaying of gold to form magnetically responsive core-shell particles, which exhibit surface plasmon resonance. In-depth characterization and quantification of the gold-shell formation process was performed using transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive spectroscopy, and inductively coupled plasma optical emission spectroscopy. Dynamic light scattering studies also showed that PEI coating of synthesized particles served as an excellent barrier against aggregation. The ability of the gold shell to protect the magnetite cores was tested by subjecting the particles to a magnetite-specific dissolution procedure. Elemental analysis of dissolved species revealed that the gold coating of magnetite cores imparts remarkable resistance to iron dissolution. The ability to engineer gold coverage on particle surfaces allows for controlled biofunctionalization, whereas their resistance to dissolution ensures applicability in harsh environments.

Herein, we demonstrate the use of modified gold-coated magnetic nanoparticles as 'dispersible electrodes' which act as selective capture vehicles for electrochemical detection of prostate-specific antigen (PSA). A key advantage of this system is the ability to quantify non-electrochemical active analytes such as proteins with unprecedented detection limits and fast response times.

Presented herein is a detailed study into the controlled adsorption of polyethylenimine (PEI) onto 50 nm crystalline magnetite nanoparticles (Fe(3)O(4) NPs) and how these PEI-coated Fe(3)O(4) NPs can be used for the magnetic capture and quantification of ultratrace levels of free cupric ions. We show the ability to systematically control the amount of PEI adsorbed onto the Fe(3)O(4) magnetic nanoparticle surfaces by varying the concentration of polymer during the adsorption process. This in turn allows for the tailoring of important colloidal properties such as the electrophoretic mobility and aggregation stability. Copper adsorption tests were carried out to investigate the effectiveness of PEI-coated Fe(3)O(4) NPs in copper remediation and detection. The study demonstrated that the NPs ability to bind with copper is highly dependent on the amount of PEI adsorbed on the NP surface. It was found that PEI-coated Fe(3)O(4) NPs were able to capture trace levels (approximately 2 ppb) of free cupric ions and concentrate the ions to allow for detection via ICP-OES. More importantly, it was found that due to the amine-rich structure of PEI, the PEI-coated Fe(3)O(4) NPs selectively adsorb toxic free cupric ions but not the less toxic EDTA complexed copper. This unique property makes PEI-coated Fe(3)O(4) NPs a novel solution for the challenge of separating and quantifying toxic cupric ions as opposed to the total copper concentration of a sample.

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.

Physical and electrochemical properties of gold nanoparticle-based electrodes are highlighted. Polycrystalline gold electrodes are passivated by a self-assembled monolayer, then the immobilization of gold nanoparticles "switch on" the electrochemical reactivity of ruthenium. Herein, gap-mode Raman studies show that the location of the nanoparticles is on the top of the monolayer, meaning that the "switching on" cannot be attributed to a direct electrical contact between nanoparticles and the gold support. This "switching on" feature is also not affected by the size of the gold nanoparticles with a range of diameters between 4 and 67 nm. Further, the charge of the nanoparticles is investigated by grafting chemical groups onto the nanoparticles which is observed to alter the electron-transfer kinetics. The variation in rate constant however is insufficient to attribute the "switching on" phenomenon to a possible adsorption of the redox species onto the nanoparticles.