Sensitive, label-free DNA diagnostics based on near-field microwave imaging.

Biological assays often rely on "reporter labels" to enhance measurement sensitivity, for example, by incorporation of a fluorescent dye or a nanoparticle into a nucleic acid or a protein. Use of labels, however, complicates sample preparation, increases assay costs, and can cause experimental artifacts by interfering with assay thermodynamics or limiting label stability. We evaluate near-field microwave imaging (NFMI) as an alternative, label-free technique for molecular diagnostics. Using DNA monolayers as an experimental model, NFMI is demonstrated to achieve sensitivities comparable to conventional fluorescence bioassays. Moreover, NFMI is shown to be compatible with imaging at resolutions required by microarray applications, as demonstrated by monitoring DNA hybridization in an array format.