Nano Sensors for Bacteria

Nanoelectromechanical systems (NEMS), as the name implies, are electrical and mechanical devices whose sizes are of the order of one nanometre (a billionth of a metre). They are the successors of the micro-scaledevicescalledMEMS, which various groups at IIT-M have worked on. NEMS find applications in the detection of adulterants in food, of bacterial antigens and thus the bacteria themselves, of hazardous gases, and in communication using frequencies in the gigahertz-range. One of the most promising applications to come out of IIT-M is a “diving board” sensor for detecting bacteria.

Before a diver jumps off a diving board, he or she usually makes a few small jumps, causing the springboard to move up and down. As soon as the diver jumps off it, it begins vibrating at a much higher frequency. To an engineer, a diving board is a type of structure called a cantilever. Cantilevers are beams that are attached to a support only at one end. They have been around for a long time and are quite ubiquitous today. They are very common in construction, and can be seen in towers, bridges, and roofs. The wings of an aircraft, being anchored to the body of the plane at just one end, are also cantilevers.

When shrunk to the nano-scale, cantilevers can perform functions which are, initially, quite baffling. For over a decade now, nano-scale silicon cantilevers have been used as sensors. These sensors are so small, that ten of them stacked side-by-side would appear about as big as a grain of dust.

To understand how cantilevers can be used as sensors, we must go back to the example of the diving board. A diving board, or any cantilever for that matter, vibrates with a particular frequency called its resonant frequency. This depends on the mass on the cantilever. When the diver is standing on the board, there is a constant downward force on the cantilever (the diver’s weight) “damping” its vibrations. When the diver jumps off it, the damping force is removed, and the cantilever begins vibrating at a higher frequency.

 

A polysilicon cantilever.
A polysilicon cantilever
IMAGE CREDIT: Prof.Enakshi Bhattacharya.

 

To exploit this property of cantilevers for use in sensors, the following apparatus is used: A laser source is set up over the cantilever and the light reflected by the free end of the cantilever is sensed by a detector. A piezoelectric vibrator, similar to quartz crystals used in wristwatches, sits on the other end of the cantilever. The cantilever is vibrated at different frequencies and the reflected beam gives the highest intensity when the cantilever vibrates at its resonant frequency.

By a process called functionalization, several layers of chemicals are deposited on the cantilever to make it sensitive to a specific chemical, antigen, compound, or even a whole virus. Now, only that specific particle can bind to it. The resonant frequency of a cantilever is first found without any weight on it. Then, the cantilever array – a single cantilever is never used – is placed in a buffer solution with the sample (blood serum or urine) dissolved in it. Iftheparticletowhichthecantilever is functionalized is present in the solution, it binds to the cantilever. Measuring the difference in frequencies of the mass-free versus the mass-loaded vibrations allows researchers to tell whether the specific particle is present in the solution or not.

This method helps in detecting the presence of a bacteria or a virus in a sample in much lesser time than conventional methods.

To detect bacteria, the cantilevers are coated with antibodies which target specific proteins on the surfaces of the bacterial cells, called antigens. Since each bacterium produces many antigens, this method is more sensitive than looking for the bacteria themselves, and can show the presence of amounts of antigens or chemicals that cannot be detected by conventional methods.

The MEMS Centre of IIT Madras has been successful in obtaining urine profiles using this method. Now, their focus of research in the area of biomolecular detection is to detect antigens present in the blood serum or urine samples to detect disease-causing agents.

Gaurav Kathel, an MS student there, says, “By conventional methods, testing the blood of a person suspected to be infected with malaria takes 4–5 hours. However, by this method, the process takes only 20 minutes and the presence of even a single malaria-causing protozoan can be detected.”

Even though cantilever sensor technology is a hot topic of research in the field of NEMS, there is still huge scope for further research. Arrays of these sensors could be used to analyse blood for the presence of viruses, or water for the presence of pollutants. The extremely high resolution of the devices would allow for more accurate, and earlier, warnings than were ever possible before.