Thursday, April 29, 2010

Tunable Bio-Functionalized Nanoelectromechanical Systems Having Superhydrophobic Surfaces For Use In Fluids

Boston University scientists Kamil L Ekinci and Victor Yakhot have developed tunable, bio-functionalized, nanoelectromechanical systems (Bio-NEMS), micromechanical resonators (MRs), nanomechanical resonators (NRs), surface acoustic wave resonators, and bulk acoustic wave resonators having superhydrophobic surfaces for use in aqueous biochemical solutions. Suitable coating materials for providing superhydrophobicity can include nanoparticles, such as carbon nanotubes, nanobricks, or nanoturf. 

Detailed in U.S. Patent Application 20100107285, the MRs, NRs or Bio-NEMS include a system resonator that can vibrate or oscillate at a relatively high frequency and to which an analyte molecule(s) contained in the solution can attach or upon which small molecular-scale forces can act; a device for adjusting a relaxation time of the solution, to increase the quality (Q-factor) of the resonator inside the solution, to reduce energy dissipation into the solution; and a device for detecting a frequency shift in the resonator due to the analyte molecule(s) or applied molecular-scale forces.

The resonator can include roughness elements that provide superhydrophobicity and, more particularly, gaps between adjacent asperities for repelling the aqueous solution from the surface of the device.

The figure (11) is a diagram of an exemplary superhydrophobic NEMS resonator. 
NEMS-based sensing using a vibrating or oscillating NEMS resonator can be patterned on a silicon-on-insulator (SOI) wafer, e.g., using electron beam lithography, metal deposition, lift-off techniques, and various etching techniques, and are operated in flexural modes using optical and/or capacitive techniques. By actuating the NEMS sensor harmonically at or near its fundament resonant frequency, small frequency shifts can be detected with a high degree of sensitivity. Hence, as analyte molecules or a single molecule attach to the functionalized layer on the surface of the device , frequency shifts can be measured that provide indicia of the nature and concentration of the analyte molecules. 

In this size regime, NEMS provides relatively high fundamental resonant frequencies, very small active masses, relatively low intrinsic energy dissipation, a relatively high intrinsic quality (Q-) factor, very small heat capacities, and so forth. In combination, these attributes can provide unprecedented sensitivities at relatively high operating frequencies with a potential for a wide range of sensing, actuating, and signal processing applications. 

The invention  is applicable to tunable resonator systems for improved detection and imaging of atomic force microscopy (AFM) or magnetic resonance force microscopy (MRFM) inside an aqueous solution. The systems for AFM and MRFM use would include a system resonator, e.g., a cantilever, micro-cantilever, nano-cantilever, and the like, that is subject to the action of molecular-scale forces, which produce resonant frequency shifts as well as means for detecting the resonant frequency shifts due to the molecular-scale forces and means for adjusting the relaxation time of the solution to increase a quality (Q-) factor of the resonator and to reduce energy dissipation into said solution. 

The Boston University MR/NR devices for use in aqueous biochemical solutions do not suffer from the disadvantages of conventional devices including energy loss in liquids. Specifically, it provides a sensor-level solution that minimizes energy dissipation to the fluid, i.e., reduces fluidic friction, and that optimizes the resonator signal-to-noise ratio. It provides technology that increases the quality (Q-)factor of MRs/NRs when the resonator is being used in such fluids and, moreover, that is tunable as a function of the relaxation time of the fluid.  It  provides a Bio-NEMS device with such a high Q-factor that it is capable of single bio-molecular detection with a very high mass resolution.

With resonant Atomic Force Microscopy (AFM), the very same problem makes AFM imaging and force measurements in water quite challenging. For example, in AFM, a vibrating micro-cantilever can be used to detect small nanoscale forces from the surface for mapping the surface topography. However, in water, the force sensitivity, and hence, the signal-to-noise ratio decreases because the Q-factor of the cantilever decreases, e.g., to about 10.  Accordingly, a variety of advanced probe microscopy techniques, such as AFM and magnetic resonance force microscopy (MRFM), will benefit immensely from the development of micro-cantilever sensor probes with increased Q-factor for use in water. 

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