We report on the fabrication of nanometer-scale mass sensors with subattogram sensitivity. Surface micromachined polycrystalline silicon and silicon nitride nanomechanical oscillators were used to detect the presence of well-defined mass loading. Controlled deposition of thiolate self-assembled monolayers on lithographically defined gold dots were used for calibrated mass loading. We used a dinitrophenyl poly(ethylene glycol) undecanthiol-based molecule (DNP-PEG4-C11thiol) as a model ligand for this study. Due to the fact that the gold mass is attached at the distance l0 from the end x=l of the cantilever beam, an additional moment evolves in the boundary condition of the oscillator, which was taken into consideration through the rotational inertia of the attached mass. We showed that the corresponding correction of the frequency is on the order of γ(l0/l), where γ is the attached mass normalized to the mass of the beam. The rotational inertia correction to the frequency is on the order of γ(l0/l)2. The adopted approach permits accurate determination of the eigenfrequency in the framework of the Euler–Bernoulli beam when rotational inertia of the attached mass is included. From the resonant frequency shift, the mass of the adsorbed species was determined and compared to results obtained by other techniques. Utilizing vacuum encapsulation, we demonstrate sensing capability in the attogram regime of the adsorbed self-assembled monolayer.

The ability to detect small amounts of materials, especially pathogenic bacteria, is important for medical diagnostics and for monitoring the food supply. Engineered micro- and nanomechanical systems can serve as multifunctional, highly sensitive, immunospecific biological detectors. We present a resonant frequency-based mass sensor, comprised of low-stress silicon nitride cantilever beams for the detection of Escherichia coli (E. coli)-cell-antibody binding events with detection sensitivity down to a single cell. The binding events involved the interaction between anti-E. coli O157:H7 antibodies immobilized on a cantilever beam and the O157 antigen present on the surface of pathogenic E. coli O157:H7. Additional mass loading from the specific binding of the E. coli cells was detected by measuring a resonant frequency shift of the micromechanical oscillator. In air, where considerable damping occurs, our device mass sensitivities for a 15 μm and 25 μm long beam were 1.1 Hz/fg and 7.1 Hz/fg, respectively. In both cases, utilizing thermal and ambient noise as a driving mechanism, the sensor was highly effective in detecting immobilized anti-E. coli antibody monolayer assemblies, as well as single E. coli cells. Our results suggest that tailoring of oscillator dimensions is a feasible approach for sensitivity enhancement of resonant mass sensors.

We have demonstrated high-sensitivity detection of bacteria using an array of bulk micromachined resonant cantilevers. The biological sensor is a micromechanical oscillator that consists of an array of silicon-nitride cantilevers with an immobilized antibody layer on the surface of the resonator. Measured resonant frequency shift as a function of the additional cell loading was observed and correlated to the mass of the specifically bound Escherichia coli O157:H7 cells. Deposition and subsequent detection of E. coli cells was achieved under ambient conditions.