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This paper demonstrates the development of a novel nanoparticle-enhanced surface plasmon resonance (SPR) sensing platform for the selective and sensitive in situ detection of nickel(II) ions at concentrations as low as 50 parts per trillion (211 pM). An enhancement in selectivity was achieved by designing a surface sandwich assay involving two different ligands each selective toward nickel(II) ions, namely, N-[5-(3'-maleimidopropylamido)-1-carboxypentyl]iminodiacetic acid (NTA) and polyhistidine. Maleimido-modified NTA was first immobilized on an alkanedithiol-modified gold thin film, followed by the sequential adsorption of Ni(II) ions. Next, polyhistidine-functionalized quasispherical gold nanoparticles, designed to enhance the SPR sensitivity, were specifically adsorbed onto surface Ni(II)-NTA complexes. This process was monitored by real-time SPR. The ability to detect Ni(II) ions as low as 50 parts per trillion (ppt) is a remarkable improvement compared to other optical and colorimetric techniques utilizing nanoparticles and is comparable to what can be achieved by state-of-the-art inductively coupled plasma mass spectrometry (ICP-MS). The improved selectivity for Ni(II) ions by the sandwich assay approach was confirmed by comparing measurements involving other divalent cations such as Zn(II), Pb(II), and Cu(II), some of which individually possess binding affinities toward either the NTA or histidine moieties.

A novel electrochemical detection methodology is described for the femtomolar detection of proteins which utilizes both DNA aptamer-functionalized nanoparticles and a surface enzymatic reaction. Immunoglobulin E (IgE) was used as a model protein biomarker, which possesses two distinct epitopes for antibody (anti-IgE) and DNA aptamer binding. A surface sandwich assay format was utilized involving the specific adsorption of IgE onto a gold electrode surface that was pre-modified with a monolayer of aptamer-nanoparticle conjugates followed by the specific interaction of alkaline phosphatase (ALP) conjugated anti-IgE. To clearly demonstrate the signal enhancement associated with nanoparticle use, anodic current measurements of the ALP catalyzed oxidation of the enzyme substrate 4-aminophenylphosphate (APP) were also compared with electrode surfaces upon which the aptamer was directly attached. The detection of an unlabelled protein at concentrations as low as 5 fM is a significant improvement compared to conventional electrochemical-based immunoassay approaches and provides a foundation for the practical use and incorporation of nanoparticle-enhanced detection into electrochemical biosensing technologies.