A micromechanical biosensor with inherently differential readout

Abstract

The ability to detect biomolecules in real-time and without the use of labels has significant benefits for systems biology in terms of cost, time and throughput. Cantilever-based micromechanical sensors detect biomolecular adsorption by means of surface-stress-induced cantilever bending. This technique enables sensitive, scalable and label-free detection of biomolecules in real-time. However, micromachined cantilevers are extremely sensitive to nonspecific chemical effects and temperature changes. This thesis explores a micromechanical sensor that suppresses disturbances by generating an inherently differential signal with respect to a reference surface. The thesis covers the design, fabrication, characterization of the sensor, and its application to protein detection using aptamers; receptor molecules produced in vitro. The sensor is composed of two adjacent cantilevers that form a sensor-reference pair, whereby only the sensing surface is activated with receptor molecules that are specific to the ligand to be detected. The relative, or differential bending between the two cantilevers is directly measured using interferometry. Through direct differential detection, disturbances affecting both cantilevers are suppressed at the measurement level. This eliminates the need for separate detection of each cantilever's motion and off-line processing of the individual signals. At high frequencies, the resolution of the sensor is only limited by its sub-angstrom-level thermomechanical noise. At lower frequencies (frequencies of interest), the resolution is limited by 1/f-type noise which can be reduced by as much as an order of magnitude by direct differential detection, enabling clear observation of receptor-ligand binding

(cont.) reactions in real-time.