Recently, Terahertz time domain spectroscopy (THz-TDS) has emerged as a successful method to measure the refractive index of thin solid films without sample perturbation. However, a need remains for a new non-invasive biosensor technology having application, for example, in healthcare, food monitoring, and weapon detection. This invention is directed to a method for detecting specific associations between a tethered molecule and an untethered target molecule, thus providing a method of probing bimolecular interactions using THz radiation.