Detection of pathogenic bacteria using a homogeneous immunoassay based on shear alignment of virus particles and linear dichroism.

Biomolecular detection has for a long time depended on a relatively small number of established methodologies. Many of these depend on the detection of a ligand-antibody complex using some kind of optical technique, e.g., chemiluminescence. Before this measurement can be made, the ligand-antibody complex generally has to be separated from bulk contaminants. This process involves the implementation of a heterogeneous assay format involving immobilization of the antibody onto a solid support to enable washing of the unbound ligand. This approach has a number of inherent issues including being slow and complex and requiring the use of expensive reagents. In this paper, we demonstrate how we have harnessed a biologically inspired nanoparticle to provide the basis for a homogeneous assay which requires no immobilization. The method relies on using fluid shear flow to align a fiber-like nanoparticle formed from a filamentous virus, M13, combined with a ligand-specific antibody. This renders the particle visible to linear dichroic spectroscopy, which provides an easily interpretable signal. The addition of the target ligand (in this case Escherichia coli O157) leads to the formation of a nanoparticle-ligand particle that is unable to align, leading to the perturbation of the linear dichroism signal.

Single peptide assembly onto a 1.5 nm Au surface via a histidine tag.

Nanoparticle surfaces functionalized with proteins or other biomolecules provide a mechanism for interfacing the unique properties of nanomaterials with biological samples. In most of these studies, the biomolecule is conjugated to a gold nanoparticles (AuNP) surface through the thiol group of native or introduced cysteine residues. Here we demonstrate the direct attachment of a hexa-histidine tagged (His(6)) peptide to a 1.5 nm AuNP. Binding occurs via a specific interaction between the Ne of the His imidazole, forming a 1:1 stoichiometric complex. Given the widespread use of histidine tags in producing recombinant proteins, this approach promises to expand the applications of AuNP in biological applications.