Nanoelectromechanical (NEM) resonators, which are nanosized slabs of material vibrating at a certain frequency, can weigh tiny particles with extraordinary sensitivity. They do this by measuring the change in frequency of the resonator when an object is placed on top of it. Indeed, these cantilevers can weigh extremely light objects, including single molecules and even single protons.
However, the problem is that these sensors only work in vacuum and so cannot be used to weigh biological samples that need to be kept in fluid. The viscosity of liquids seriously degrades the sensitivity of the devices because the mechanical vibrations of the resonator are hindered by viscous drag. Their mechanical quality factor (QM) is also dramatically lowered and the surrounding fluid, which moves with the cantilever itself, adds unwanted mass to the resonator. Indeed, atomic force microscope (AFM) cantilevers (the most common form of cantilever) in liquid have very low quality factors (of less than five) and huge added fluid mass (tens of times that of the resonator itself).
Now, a team of researchers led by Hong Tang has overcome this problem with a new optomechanical micro-wheel resonator integrated in a microfluidic system. The device, which resonates at very high frequencies of 170 MHz and has a mass of just 75 picograms, can weigh objects as light as 3.5 attograms in water.
Very-high-Q optical resonances in water
The device has very-high-Q optical resonances in water thanks to the fact that it operates at near-visible wavelengths of 780 nm, which happily fall within the biological transparency window – that is, the wavelength at which biological tissue absorbs light. This means that it could be useful in biological and medical applications. “The resonator also features a compact design that nicely integrates nanophotonic components with microfluidics, providing a versatile platform for lab-on-a-chip technology,” explains team member King Yan Fong.
The nano-optomechanical cantilever is a suspended micro-wheel structure made out of a 200 nm thick silicon nitride (SiN) slab. When the device is immersed in water, the viscous damping becomes so great that the device no longer self-oscillates. However, thanks to its very high optical Q, we can still measure its thermomechanical motion, explains Tang, and so use this motion to detect the presence of an object.
“We believe that our work is a technical breakthrough in the field of mass sensing using nanomechanical resonators, and hope that it will impact research at the forefront of biological and chemical sensing, fluidic dynamics and microfluidics,” he tells nanotechweb.org.
The device is detailed in Nano Letters DOI: 10.1021/acs.nanolett.5b02388.
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