Abstract-This paper conducts a comprehensive study on 1 and 2-degree-of-freedom (DoF) piezoelectric bulk acoustic wave (BAW) disk resonators operating in the contour extensional mode as mass sensors. Laser bombardment was utilized to remove well-defined quantities of silicon to introduce mass perturbations. The weak mechanical coupling in The 2-DoF BAW resonator system showed a mode localization effect with over two to three orders of magnitude enhancement in mass sensitivity (compared to 1 and 2-DoF resonant sensors with frequency shift as the output metric), using amplitude ratio (AR) as the output signal. Moreover, the experimental results matched well with finite element (FE) simulated results and theoretical computations. Compared with previously reported aerosol mass sensor based on 2-DoF wine-glass (WG) mode BAW resonators, the presented 2-DoF device achieved a 2.14 times better quality factor (Q) of ~3800 in air and ~147.71 (AR change) and 2.97 (frequency shift) times enhancement in mass resolution. Furthermore, this work affirms better stability using AR, with a comparable level to frequency shift. This alters the commonly deemed fact that AR is inferior to the frequency shift in terms of stability. Besides, the proposed devices demonstrated Q factors of ~128 for the 1-DoF device and ~219 for the 2-DoF device in liquid. In particular, the proposed 2-DoF resonators achieved a Q factor of ~375 when only one resonator and the coupling beam were immersed in tap water. For the first time, it is shown that coupled BAW resonators are able to work in liquid with reasonable Q factors.