Femtogram Mass Detection

Femtogram mass detection has been achieved with cantilever oscillators at Oak Ridge National Lab. Once set to vibrating at MHz frequencies with a diode laser, the tiny cantilevers (tiny slivers of silicon as small as 2 microns long and 50 nm thick) are exposed to an atmosphere of small particles or molecules. Depending on how the cantilever is coated, some of the particles will be absorbed onto the surface of the cantilever, altering its resonance frequency in a measurable way. In a recent test the vapor used was an acidic substance, which was absorbed with a mass change that was noticeable at the 5 fg mass scale. Other subject particles, such as DNA, proteins, cells, or trace amounts of various chemical contaminants, should be detectable by this process. The experiment was carried out at ambient conditions, with no vacuum or cryogenic temperatures. According to Panos Datskos of Oak Ridge (865-574-6205) the mass sensitivity of the device can be sharpened to the molecular level if the resonance frequency can be raised from about 2 MHz at present up to 50 MHz. (Lavrik and Datskos, Applied Physics Letters, 21 April; see figure; website at www.ornl.gov)