Mass and stiffness spectrometry of nanoparticles and bio-molecules by nanoelectromechanical systems

Abstract

Mass spectrometry (MS) is a technique used frequently in mass measurements in order to identify mass of the molecules. Nanoelectromechanical systems are highly sensitive to adhered species, thus using NEMS devices, it is possible to perform NEMS-MS where not only the inertial mass of the molecules but also the position of the adhered particle can be found out by resolving the adsorbateinduced frequency shifts in the first two modes.By using frequency shifts obtained from three mechanical modes, it is possible to obtain stiffness of the adsorbate in addition to its mass and position on the resonator when the Youngs modulus of the analyte and the resonant structure are comparable. For soft analytes, multimode information can be used to obtain shape properties of analytes and allows for image reconstruction from global image features. In order to conduct our experiments, we fabricate NEMS resonators whose transduction method is electrothermal actuation and piezoresistive detection. Fabrications of the devices are completed in National Nanotechnology Research Center (UNAM) in Bilkent University and Sabancı University Nanotechnology Research and Application Center (SUNUM).Initially, low vacuum apparatus is built to perform NEMS-MS using Electrospray Ionization(ESI) for molecule delivery. In order to direct particles to resonator, the fabrication of a doubly clamped beam is planned in a way that the orifice was etched through silicon wafer from the backside with KOH etch.This fabrication method, however, is tedious and hard to fabricate consistently.Then, Matrix Assisted Laser Desorption and Ionization (MALDI) is implemented to deliver particles towards the resonator.Different analyte types which are gold nanoparticles, centrosome organelles of HeLa cells and M13ke bacteriophages are used in the experiments.We use first four out-of-plane modes of the doublyclamped beam resonator for real-time study of the adsorbates. For biomolecule

detection, care was taken to prevent uniform coverage of matrix molecules. Phaselocked-loop(PLL) operation is simultaneously performed for the first four modes of the resonator.Using frequency shifts of the four modes due to the adsorption, we propose a method in which we assume the analytes adhered on the beam are hemispherical to obtain mass and stiffness, size and positions of the analytes. Using three mechanical modes, stiffness, mass and position vaues are calculated.