Myocardial infarction — a heart attack — occurs when the blood supply to heart muscle suddenly stops, causing cell damage. The simultaneous release of the cardiac protein, a complex of troponin I, T and C, makes it the preferred marker for the diagnosis of various heart disorders, including myocardial infarction.

Enzyme-linked immunosorbent assay (ELISA) is widely used for the detection of troponin I. In ELISA, anti-antigen antibodies are anchored onto a substrate surface. Target antigens bind to the antibodies, and subsequent binding of enzyme-linked antibodies results in an optical signal. However, the random organization of the antibodies on the two-dimensional surface limits access of target protein markers to the antibodies, thereby limiting the sensitivity of this procedure.

Fig. 1: Schematic representation of the controlled orientation of antibody probes in the three-dimensional assay.

Now, Jeewon Lee at Korea University and colleagues1 have successfully incorporated genetically engineered virus nanoparticles into three-dimensional diagnostic assays for the first time (Fig. 1). Their approach makes it possible to detect the myocardial infarction protein marker at ultra low levels.

Using Escherichia coli bacteria, the researchers synthesized DNA-free spherical nanoparticles from the hepatitis B virus and linked the nanoparticle ends to histidine amino acids, which have a strong affinity for nickel. They then attached the nanoparticles to three-dimensional nickel nanowire arrays or high-porosity polyvinylidene fluoride (PVDF) membranes.

The researchers also functionalized the surfaces of their nanoparticles with antibody binding sites. Exposure of the nanoparticles to anti-marker antibodies yielded high density, highly organized three-dimensional antibody arrays.

“Unlike conventional ELISA, the protein markers are captured by the antibodies carpeting the surface of virus nanoparticles that are already bound to another three-dimensional nanostructure, providing a large surface area for attaching the viral particles,” says Lee.

The sensitivity of the three-dimensional virus-based assay was 6–7 orders of magnitude greater than that for ELISA assays. The assay also detected troponin I in blood samples of clinical myocardial infarction patients at concentrations lower than the clinical ELISA cutoff value.

“The genetically engineered virus nanoparticles used in this study can attach many different kinds of anti-marker antibodies and can therefore be applied to the detection of a variety of protein markers,” says Lee. The researchers are currently developing biosensors for the early diagnosis of autoimmune diseases.