Origin of the response of nanomechanical resonators to bacteria adsorption

Resonant microcantilevers are being actively investigated as sensitive mass sensors for biological detection. By performing experiments of adsorption of the bacteria Escherichia coli on singly clamped microcantilevers, we demonstrate that the effect of the added mass is not the only and may not be the main origin of the response of these sensors. The experiments show that the magnitude and sign of resonance frequency shift both depend critically on the distribution of the adsorbed bacterial cells on the cantilever. We relate this behavior to the added mass that shifts the resonance to lower frequencies and the higher effective flexural rigidity of the cantilever due to the bacteria stiffness that shifts the resonance to higher frequencies. Both effects can be uncoupled by positioning the cells where each effect dominates, near the free cantilever end for measuring the added mass or near the clamping for measuring the increase of flexural rigidity.One of the authors (D.R.) acknowledges the fellowship funded by the Autonomic Community of Madrid. This work was supported by the Spanish National Research Council (CSIC), Project No. 200550M056.Peer reviewe