Micro-weighing based biosensor with using an adaptive interferometry

Mass of a macroscopic object is easily measured by a suitable balance. However, this approach becomes inapplicable if it is necessary to measure the masses of micro- and nano-objects such as biomolecules, cells, viral particles. Alternative approach for mass measurement is based on using of micromechanical resonator as an inertial balance which oscillation frequency depends by added mass value. There are various ways to measure the micro-oscillators resonant frequency: optical, piezoelectric transducers, electrostatic, etc. Optical frequency measurement methods are the most versatile. Measurements by optical methods make it possible to measure vibrations in a non-contact way. In this work, an adaptive holographic interferometer with implemented for measurement of oscillator frequency. A silicon micro-cantilever with dimensions 125×30×2.5 µm³, which plays the role of the sensing element, is mounted on the positioning system. The cantilever was fixed on a piezoquartz plate to excite vibrations by sinusoidal electrical signal. Cantilever vibrations were measured by the adaptive interferometer. The interferometer object wave being reflected from the cantilever propagates through the semi-insulating photorefractive CdTe crystal, where it is mixed with the reference wave. Wave coupling provides phase-to-intensity transformation in the object wave. Changes in the intensity of the object wave were recorded using a photodetector connected to an analyzer of the spectrum of electrical signals. The recorded spectrum of the electrical signal was the amplitude-frequency spectrum of the cantilever oscillations. Cantilever-type microoscillators, as a rule, have a high QF of oscillations, therefore they always have a clearly defined resonant frequency, which for the cantilever used in this work was 253442±23 Hz. In the experiment, mass of absorbed molecules of Bovine serum albumin (BSA) was measured on different concentration of water solution. It is shown that biosensor is able to measure concentration of BSA water solution with concentration in 0.2 mg/ml.