Microwave-to-Terahertz Sensing in Microfluidic Systems

The microwave-to-terahertz frequency range represents a unique window for the label-free assessment of biological liquids and its cellular constituents. The high losses of water at microwave frequencies – in combination with purpose-designed integrated resonator and transmission line structures – enable fast measurements on small liquid volumes down to single cell size. Since the cell membranes are transparent for electromagnetic radiation in this frequency range, the response represents a global view of the intercellular water- and protein/DNA content, which is also affected by surface water and the concentration of ions. As a first practical example, the fast and accurate determination of the haemoglobin concentration in sub-microlitre blood samples at microwave frequencies was recently demonstrated. For the terahertz frequency range, where collective vibrational modes of biomolecules become noticeable against the dominant contribution of water relaxation, we recently demonstrated broadband surface wave structures for highly efficient liquid detection.