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.
The influence of the length of a self-assembled monolayer (SAM) linker on the electrochemical performance of electrode-linker-gold nanoparticle molecular constructs is investigated. Electrodes were first modified with amino-1-alkanethiols of four different lengths (C=2, 6, 8, and 11). The SAM showed progressively greater blocking ability to ruthenium hexamine as the length of the alkyl chain increased to the point where no significant Faradaic peak was observed for the amino-1-undecanethiol SAM. Upon the attachment of gold nanoparticles, distinct Faradaic electrochemistry of the ruthenium hexamine was observed for all four length SAMs with the electrochemistry being similar to that observed on a bare electrode. The charge transfer resistance to this Faradaic process was observed to be insensitive to the length of the intervening SAM, indicating it is electron transfer between the redox species and the nanoparticles, rather than tunneling across the SAM, which is the rate-limiting step. Some comments on the mechanism of charge transfer are provided. When forming multilayers of the linker-nanoparticle constructs, fabricated in a stepwise manner, whenever the distal species was the SAM the Faradaic process was blocked and whenever it was the nanoparticle a distinct Faradaic process was observed. With up to five layers of linker-nanoparticles, there was little increase in charge transfer resistance and again the charge transfer resistance was insensitive to the length of the linker.
Monitoring enzyme secretion in tissue culture has proved challenging because to date the activity cannot be continuously measured in situ. In this Letter, we present a solution using biopolymer loaded photonic crystals of anodized silicon. Shifts in the optical response by proteolytic degradation of the biopolymer provide label-free sensing with unprecedented low detection limits (1 pg) and calculation of kinetic parameters. The enhancement in sensitivity relative to previous photonic crystal sensors constitutes a change in the sensing paradigm because here the entire pore space is responsive to the secreted enzyme rather than just the pore walls. In situ monitoring is demonstrated by detecting secretion of matrix metalloprotease 9 from stimulated human macrophages.
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.
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.
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