Compact Electronic noses can monitor air quality, diagnoze diseases to biohazards like covid-19, explosives and nerve gas

Government and local law enforcement agencies have employed canines for decades to sniff  explosives and such banned goods as fresh produce, exotic wildlife, undeclared currency and illicit drugs.   Now researchers are developing electronic noses imitating the sense of smell of humans and animals especially dogs.

The smells are composed of molecules, which has a specific size and shape. Each of these molecules has a corresponding sized and shaped receptor in the human nose. When a specific receptor receives a molecule it sends a signal to the brain and brain identifies the smell associated with the particular molecule. The electronic noses work in a similar manner of human. The electronic nose uses sensors as the receptor. When a specific sensor receives the molecules, it transmits the signal to a program for processing, rather than to the brain.

It has been reported that the human nose has around 400 scent receptors and can detect at least one trillion odors (Bushdid et al., 2014). Although the human nose can rate a smell, individuals’ judgments may be bias, and human nose cannot be used to sense toxic gases. In addition, human nose has detection limits for difference gases. Those limitations prevent the human nose from being a universal tool for all smell-related discrimination and classification.

The electronic nose is an array of chemical sensors, connected to a pattern-recognition system that responds to odors passing over it. Different odors cause different responses in the sensors, and these responses provide a signal pattern characteristic of a particular aroma.Typically, the volatile molecules react with the sensing materials of the gas sensor and cause irreversible changes in electrical related properties, such as conductivity.  The computer evaluates the signal pattern and can compare the aromas of different samples, using pattern recognition algorithms such as artificial neural network (ANN), to perform discrimination and classification.

Compared with traditional gas analytical equipment including, GC–MS, high-performance liquid chromatography (HPLC), and Fourier transform infrared (FT-IR) spectrometry, e-nose is a relatively inexpensive and less time-consuming approach. These electronic noses can assist security agencies to  detect explosives in venues like stadiums, schools, airports, hospitals or any other crowded place, which offer an attractive target to  terrorists.

The best-known electronic nose is the breathalyser. As drivers breathe into the device, an ethanol specific chemical sensor measures the amount of alcohol in their breath. This chemical reaction is then converted into an electronic signal, allowing the police officer to read off the result. Alcohol is easy to detect, because the chemical reaction is specific and the concentration of the measured gas is fairly high. But many other gases are complex mixtures of molecules in very low concentrations. Building electronic noses to detect them is thus quite a challenge. The electronic nose can detect gas molecules with more specificity and sensitivity than Breathalyzers, which can confuse acetone for ethanol in the breath. The distinction is important, for example, for patients with Type 1 diabetes who have high concentrations of acetone in their breath.