Single microcantilevers may bend due to thermal drift or interaction with their environment, in particular if operated in a liquid. Furthermore, nonspecific physisorption of molecules on the cantilever surface or nonspecific binding to receptor molecules during measurements may contribute to the drift.

To exclude such influences, simultaneous measurement of reference microcantilevers aligned in the same array as the sensing microcantilevers is crucial [1]. The difference in signals from the reference and sensor microcantilevers yields the net bending response, and even small sensor signals can be extracted from large microcantilever deflections without being dominated by undesired effects. When only single microcantilevers are used, no thermal-drift compensation is possible. To obtain useful data under these circumstances, both microcantilever surfaces have to be chemically well-defined. One of the surfaces, typically the lower one, has to be passivated; otherwise the microcantilever response will be convoluted with undesired effects originating from uncontrolled reactions taking place on the lower surface. With a pair of microcantilevers, reliable measurements are obtained. One of them is used as the sensor microcantilever (coated typically on the upper side with a molecule layer that shows affinity to the molecules to be detected), whereas the other microcantilever serves as the reference. It should be coated with a passivation layer on the upper surface so as not to exhibit affinity to the molecules to be detected. Thermal drifts are cancelled out if difference responses, i.e. difference in deflections of sensor and reference microcantilevers, are taken (differential measurements). Alternatively, both microcantilevers are used as sensors (sensor layer on the upper surfaces), and the lower surface has to be passivated. It is best to use an array of microcantilevers, in which some are used either as sensor or as reference microcantilevers so that multiple difference signals can be evaluated simultaneously. Thermal drift is cancelled out as one surface of all microcantilevers, typically the lower one, is left uncoated or coated with the same passivation layer.

1.Lang HP, Berger R, Andreoli C, Brugger J, Despont M, Vettiger P, Gerber C, Gimzewski J, Ramseyer JP, Meyer E and Güntherodt HJ (1998) Appl Phys Lett 72:383.