Strong-axis bending mode vibrations for resonant microcantilever (bio)chemical sensors in gas or liquid phase

The frequency stability, limit of detection (LOD), and sensitivity of a coated-cantilever chemical sensor operating in a dynamic mode are mainly determined by its mechanical quality factor. While a coated-cantilever operating in the gas phase exhibits a large reduction in Q-factor, immersion in liquids results in even greater Q-factor reduction, due to viscoelastic damping and losses in the surrounding liquid. Two different bending vibration modes are studied in order to understand their influence on the sensor sensitivity and to minimize the losses induced by the surrounding medium, thus maximizing the Q-factor and the detection limit of the biochemical microsensor. The two particular vibration modes involve both "first-mode" flexural vibrations (but in different orthogonal planes), and are known as "weak-axis bending" (WAB) mode and "strong-axis bending" (SAB) mode. Using Sader\´s model, the expression for the Q-factor is analysed for the case of immersion in a viscous fluid. A comparison of sensor performance in both WAB and SAB modes is made. In particular, the effects of the different operating modes on the resonant frequency, Q-factor, sensitivity, and LOD are examined. The results indicate that the SAB mode may have certain advantages over the more conventional WAB mode in enhancing the sensor sensitivity and detection limit, even for the case in which the WAB and SAB devices have identical resonant frequencies.