Towards mode shape independent nanoelectromechanical mass spectrometry under atmospheric conditions

Nanoelectromechanical systems are being utilized in mass sensing applications owing to their small footprints and high mass sensitivities, targeting masses that are unreachable by most other techniques (>10 MDa) while also being able to measure masses in the kDa range. Recently, these sensors were deployed under atmospheric conditions by integrating a polymeric focusing lens, which increased capture efficiencies and decreased the system cost, both of which have been significant challenges in the NEMS-MS field. However, when deployed under atmospheric conditions, the displacement profiles of these sensors become dependent on the environment due to dissipative effects, unlike symmetric displacement profiles that follow the mode shapes in vacuum. Since the frequency shift by particle landing depends on the landing position, the corrections to the governing equations make the analysis more accurate, especially when the quality factor of the sensor is low. Therefore, devices that enable single mode mass sensing under atmospheric conditions remove the need for corrections and enable mode shape independent mass sensing. Here, we propose devices with uniform regions in their mode shapes, which removes the position dependency when calculating the mass of a landing particle. The uniform mode shapes are analyzed and validated by simulations and experiments using 200 nm fluorescent polystyrene nanoparticles whilst considering the landing positions of the particles and frequency shifts. After the uniform mode shape validation, we conducted 40 nm gold nanoparticle mass sensing experiments. Therefore, the proposed devices allowed us to perform mode shape independent mass sensing under atmospheric conditions.