Immunoassay Based On Optical Diffraction

Purdue University scientists have developed a rapid and sensitive immunoassay method based on the spontaneous assembly of analyte molecules into an optical diffraction grating (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200703222). Ghanashyam Acharya, David H. Thompson, and coworkers demonstrated the technique by trapping S-adenosyl homocysteine (SAH), a diagnostic marker for cardiovascular disease, in a molecular-sandwich-type configuration. Using standard microcontact printing methods, the team patterned gold chips with rows of nucleic acid aptamers designed to bind SAH's adenine group. The researchers also treated SAH samples with antibodies coupled to magnetic beads in order to bind the antibodies to SAH's homocysteine moiety. Exposure of the patterned chips to solutions of SAH bound to the tagged antibodies led to formation of micrometer-sized rows of sandwich-type complexes. By shining laser light on the assembly, the group produced characteristic diffraction patterns that enabled them to confirm the presence of SAH at concentrations in the low picomolar range.