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Physicists develop ultra-sensitive 'nanomechanical' biosensor

10 June 2015

The sensor can analyse the chemical composition of substances and detect biological objects, like viral disease markers following immune response.

The principle of the sensor (image courtesy of Dmitry Fedyanin and Yury Stebunov)

Two young researchers working at the Moscow Institute of Physics and Technology, Dmitry Fedyanin and Yury Stebunov, have developed an ultra-compact, highly sensitive nanomechanical sensor for analysing the chemical composition of substances and detecting biological objects, such as viral disease markers, which appear when the immune system responds to incurable or hard-to-cure diseases, including HIV, hepatitis, herpes, and many others. The sensor will enable doctors to identify tumour markers, whose presence in the body signals the emergence and growth of cancerous tumours.

According to its developers, the sensor can track changes of just a few kiloDaltons in the mass of a cantilever in real time. One Dalton is roughly the mass of a proton or neutron, and several thousand Daltons are the mass of individual proteins and DNA molecules. So the new optical sensor will allow for diagnosing diseases long before they can be detected by any other method, which the researchers believe will pave the way for next-generation diagnostics.

"We've been following the progress made in the development of micro- and nanomechanical biosensors for quite a while now and can say that no one has been able to introduce a simple and scalable technology for parallel monitoring that would be ready to use outside a laboratory," say the researchers. "So our goal was not only to achieve the high sensitivity of the sensor and make it compact, but also make it scalable and compatible with standard microelectronics technologies."

Unlike similar devices, the new sensor has no complex junctions and can be produced via the standard CMOS process. The sensor doesn't have a single circuit, and its design is very simple. It consists of two parts: a photonic (or plasmonic) nanowave guide to control the optical signal, and a cantilever hanging over the waveguide.

The cantilever measures 5 micrometres long, 1 micrometre wide and is 90 nanometres thick, and is connected firmly to a chip. It is set in motion - vibrating at MHz frequencies - by one of two optical signals passing through the waveguide (the second optical signal is used to read the signal containing information about the cantilever's movement).

The non-homogeneous electromagnetic field of the control signal's optical mode transmits a dipole moment to the cantilever, impacting the dipole at the same time so that the cantilever starts to oscillate.

The sinusoidally modulated control signal makes the cantilever oscillate at an amplitude of up to 20 nanometres. The oscillations determine the parameters of the second signal, the output power of which depends on the cantilever's position.

The highly localised optical modes of nanowave guides, which create a strong electric field intensity gradient, are key to inducing cantilever oscillations. Without the nanoscale waveguide and the cantilever, the chip simply wouldn't work. A large cantilever cannot be made to oscillate by freely propagating light, and the effects of chemical changes to its surface on the oscillation frequency would be less noticeable.

It is the cantilever oscillations that make it possible to determine the chemical composition of the environment in which the chip is placed. That's because the frequency of mechanical vibrations depends not only on the materials' dimensions and properties, but also on the mass of the oscillatory system, which changes during a chemical reaction between the cantilever and its environment.

By placing different reagents on the cantilever, the researchers were able to make it react with specific substances or even biological objects. If antibodies to certain viruses are placed on the cantilever, it capture the viral particles in the analysed environment. Oscillations will occur at a lower or higher amplitude depending on the virus or the layer of chemically reactive substances on the cantilever, and the electromagnetic wave passing through the waveguide will be dispersed by the cantilever differently, which can be seen in the variation of intensity of the readout signal.

The researchers believe the high-sensitivity biosensor can be manufactured relatively easily and its tiny dimensions will allow its deployment in portable devices, such as smartphones and wearable electronics. One chip, several millimetres in size, will be able to accommodate several thousand such sensors, configured to detect different particles or molecules.

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