The optomechanical chip will analyze anything
MIPT physicists have developed an ultra-sensitive nanomechanical biosensor
Dmitry Fedyanin and Yuri Stebunov, young researchers from the Laboratory of Nanooptics and Plasmonics at MIPT, have developed an ultra-compact, highly sensitive nanomechanical sensor for analyzing the chemical composition of various substances. Their development is also capable of detecting biological objects: for example, markers of viral diseases that appear as a result of the immune system's response to such incurable and intractable diseases as AIDS, hepatitis, herpes and many others.
The proposed sensor will allow detecting cancer markers, the presence of which in the body signals the appearance and growth of a cancerous tumor. The sensitivity of the device is best characterized by one digit: according to the authors, the sensor is able to record in real time the appearance of particles weighing only a few kilodaltons. One dalton is, roughly speaking, the mass of one proton or neutron, and several thousand daltons correspond to the mass of single molecules of proteins or DNA. Thus, the new sensor will make it possible to diagnose diseases long before they become available for detection by any other methods, which will open the way for medical diagnostics of the future.
The device, which is described in an article published in the journal Scientific Reports, is an optical, or, more precisely, an optomechanical chip.
"We have been following the progress in the field of micro- and nanomechanical biosensors for a long time, but so far there has not been a simple and scalable technology for parallel monitoring ready to work outside of laboratory conditions, so our goal was not only high sensitivity and compactness of the sensor, but also scalability and compatibility with standard microelectronics technologies", – explain the researchers.
Unlike similar devices, the proposed sensor lacks complex components, and it is manufactured in a standard CMOS process used in microelectronics. Despite this, there is not a single electrical circuit in the sensor, and the sensor design is so simple that it can be divided into only two parts: a photonic (or plasmon) nanowave for controlling the optical signal and a cantilever hanging over this waveguide.
Scheme of a nanooptical sensor with a nanoscale cantilever. Here and below are the images of the authors of the study.
A cantilever (literally "beam") is a long and thin strip inextricably linked to a chip. It has microscopic dimensions (5 micrometers long, 1 micrometer wide and only 90 nm thick), but fundamentally the behavior of the cantilever is not much different from the ruler hanging from the edge of the table and pressed against the tabletop: the hanging end can make mechanical vibrations at a certain frequency. The differences between the considered cantilever and a ruler clamped at one end, in fact, are only in the size and frequency of mechanical vibrations, which is determined by the material and geometric parameters: if an ordinary ruler oscillates at a frequency of tens of hertz, then a microscopic cantilever is characterized by a megahertz frequency. In other words, it makes several million vibrations per second!
An example of a freely oscillating and fixed beam at one end is a tuning fork.
The frequency of vibrations depends on the size of the tuning fork and the properties of the material.
In the waveguide passing under the cantilever, two optical signals propagate at once: the first sets the cantilever in motion, and the second allows reading information about this movement. The inhomogeneous electromagnetic field of the control signal induces a dipole moment (or, more simply, causes positive and negative charges to diverge in different directions; a combination of two charges at some distance from each other is called a dipole) on the cantilever and simultaneously affects this dipole, forcing the cantilever to move.
The sinusoidal control signal swings the cantilever, and it begins to oscillate with an amplitude of up to 20 nanometers. In turn, the moving cantilever acts on the second, reading, optical signal, the output power of which depends on the position of the cantilever.
What is optical fashion? Light waves, propagating in a space limited by mirrors, overlap each other and as a result fill it unevenly, form areas with greater and lesser intensity. Such intensity distributions are called modes. For clarity, the picture shows not a nanoscale waveguide, but an optical resonator, part of a laser of quite ordinary dimensions.The key role in the effective rocking of the cantilever is played by highly localized optical modes of nanowaves, which create a large gradient in the intensity of the electric field.
Since the characteristic size of changes in the electromagnetic field in such systems is in the tens of nanometers, researchers use the term "nanophotonics", and this is the case when the prefix "nano" is not a tribute to fashion at all! Without reducing the waveguide with a cantilever to nanoscales, the chip simply would not be able to work – a large cantilever cannot be swung by freely propagating light, and the effect of chemical changes in its surface on the oscillation frequency would not be so noticeable.
The vibrations of the cantilever make it possible to determine the chemical composition of the medium in which the chip is located. It helps that the frequency of mechanical vibrations depends not only on the size and properties of the materials, but also on the mass of the entire oscillatory system, which changes in the case of a chemical reaction of the cantilever with the medium. By covering the cantilever with different reagents, it is possible to achieve its selective reaction with certain substances or even biological objects. If antibodies to certain viruses are applied to the cantilever, it will catch these viral particles from the analyzed medium. Vibrations with viruses attached to the beam or simply with a layer of reaction products will occur with a smaller (or larger) amplitude, and the electromagnetic wave propagating through the waveguide will be scattered by the cantilever somewhat differently, which is fixed at the output of the circuit by changing the intensity of the reading signal.
The calculations carried out by the researchers showed that the new device will combine high sensitivity with comparative ease of manufacture and miniature dimensions, allowing it to be used as an element of any portable devices (for example, smartphones, wearable electronics, etc.) that can work, including in the field. On a single chip with a size of several millimeters, it will be possible to assemble together a multitude (namely up to several thousand) of such sensors configured to detect various particles or molecules. At the same time, due to the simplicity of the design, it is expected that the price of the device will slightly depend on the number of sensors, which distinguishes it from competitive solutions.
The work was funded from funds received within the framework of the project part of the state assignment of the Ministry of Education and Science of the Russian Federation No. 16.19.2014/K.
P.S. You can read the original article of the researchers (Dmitry Yu. Fedyanin & Yuri V. Stebunov, All-nanophotonic NEMS biosensor on a chip) in the journal Scientific Reports.
The MIPT press service thanks Alexey Arsenin for his invaluable help in preparing the material.
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