© (2006) IEEE

Detecting trace amounts of substances is important for many biosensing applications, where the target can range from DNA to illegal drugs. For really precise detection, the compound under study must not be modified, but existing techniques often involve adding labelling markers that change the molecular environment. Now a team of Taiwanese researchers have developed a terahertz microchip that can detect minute concentrations of drugs and other substances in a non-invasive way1.

Biomolecules can be distinguished according to their terahertz 'fingerprints' – the unique region of the spectrum in which they absorb terahertz light. But for a truly practical terahertz sensor, the radiation source would ideally be incorporated into existing lab-on-a-chip technology. Unfortunately, such devices typically contain glass substrates, which absorb terahertz waves very easily.

Ja-Yu Lu and colleagues, reporting in Photonics Technology Letters, have overcome this hurdle with a biosensor that successfully couples a cell having a glass substrate to a terahertz transmitter. The compound under study sits in microchannels within a sample cell, where edge coupling between the glass substrate and terahertz-radiation source enables light to be directly and completely transmitted to the sample. Because the sample lies inside the near field of the source — where the radiation is up to 150 μm shorter than the terahertz wavelength – it experiences intense, localized radiation. The result is a highly sensitive probe that requires less than five milliwatts of optical power.

Absorption spectra were collected for different trace white powders: cocaine, amphetamine, potato starch, lactose and flour. The microchip was shown to distinguish each compound and could detect drug powders with weights of just ten nanograms. This sensitivity matches that of current forensic techniques used to detect drugs, yet preserves the native state of the sample. By incorporating microfluidic channels into the device, water-based molecular sensing could become possible and improved sensitivity could open up detailed studies of molecular dynamics.