The Five Senses of Sensors - Smell

An electrochemical nose, also called an e-nose or micro nose, is an artificial olfaction device with an array of chemical gas sensors, a sampling system, and a system with a pattern-classification algorithm used for analysis of gases, vapors, or odors. Simply put, it is an array made up of chemical gas sensors housed in a device that uses pattern recognition to recognize, identify, and compare odors.

There are many classes of e-noses, including those with conductive polymer, surface acoustic wave, calorimetric, and polymer composite. Often there are several types or classes of sensors used. Chemical data sources include infrared spectrometers, gas chromatographs, and mass spectrometers.

The e-nose evolved from the work of U.K. scientists attempting to understand the olfactory biological process. They were able to discriminate 20 pure compounds and complex essential oils using an array of electrochemical metal oxide (MOX) sensors.

In human olfaction, lungs bring odor to the epithelium layer, while the e-nose uses a pump. Hair, membranes and mucus act as filters and concentrators in the human nose. Comparatively, an inlet sampling system provides filtration in the e-nose model. The human epithelium represents millions of sensing cells that interact with odors, while the e-nose uses sensors that interact differently with odors. In humans, the chemical response to odors is converted to electronic nerve impulses much in the same way that the e-nose chemical sensors react with the odor to produce electric signals.

Early research gave way to commercial possibilities as low-cost and higher-performing chemical sensors became more readily available. As simultaneous advances in patter recognition and classification algorithms took place, these technologies were combined with low-power CMOS microprocessors that allowed for instrumentation to be portable.

Electronic noses are devices that identify volatile organic compounds (VOCs) using multiple, cross-reactive sensors. Measurements or results are based on such changes in properties in polymers or on electrochemical oxidations at heated metal oxide surfaces as mass, volume, and conductivity. Detecting compounds at low concentrations and the discrimination of similar chemical compounds is still problematic.

There are a many chemical gas sensors, systems, and modules available to assist with development. One such module is a gas sensor module series by Parallax capable of measuring methane, carbon monoxide, propane, and alcohol. In the case of this series, a 4-pin SIP header on the Gas Sensor Module makes it easy to connect to a breadboard or SIP socket. Connection to a 5-V microcontroller is straightforward, requiring two I/O pins. For a 3.3-V MCU, an additional resistor and npn switching transistor would be required. See schematic below.

Figure 1: Schematic of Parallax gas sensor module connection

A colorimetric sensor is a more recent advancement in capturing odors and volatile organic compounds (VOCs) using thin films of multiple chemically response dyes. In this scheme, the colors change depending on a range of interactions. The device is particularly effective at discriminating among smells in a simple device. Its best feature, however, is that since it does not respond to water vapor, it does not reflect changes in humidity--which is a major obstacle to e-nose applications.

Colorimetric sensor arrays depend on digital imaging and pattern recognition. Moving to sensor arrays coupled with the power of digital imaging, increases the sensitivity of detection and identification and makes possible unique fingerprinting of complex odorant combinations.

How the e-nose works

Chemical sensors in the e-nose have a sensitive layer that translates a chemical interaction into a unique signal pattern, which is interpreted by pattern recognition/classification algorithms. Sensor systems incorporate reaction, sampling, preconditioning, electronics, data pretreatment, and pattern recognition and classification. An array of sensors can be in the form of different polymer types or by using metal oxide (MOX) semiconductors.

The e-nose works when molecules of any chemical element are place on a sensor's surface. The change of sensor resistance is measured when it is exposed to odors. A resulting pattern is displayed that is unique to that element. This allows samples to be compared. Responses can relate not only to odor, but also to explosiveness, flavor, or bacterial content, making it ideal for use in several applications.

Applications

Although the most common use is for the food and drink industry, there are myriad existing and potential applications for e-nose technology, such as:

• Detecting toxins or other hazardous elements such as CO, which is odorless to humans
• Detection and identification of bacteria and infectious disease
• Alcohol sensor systems for use without active participation by the subject
• Qualitative and quantitative analysis in the petroleum industry
• Explosive detections
• Detection for environmental applications
• Quality control in the automotive industry
• Discriminating between clean and contamination in milking systems
• Cosmetic raw materials analysis
• Space applications
• Monitoring hazardous substances
• Chemical warfare detection
• Plant disease diagnosis

Interestingly, the technology is said to not yet outperform dogs used to sniff out drugs.

The future of e-nose technology

E-nose adoption experienced a somewhat rocky start. Capabilities were initially exaggerated and performance did not match early hype. Today, however, the technology can manage extremely complex tasks and new applications based on continuing research. Future systems will use higher order sensors with advanced signal processing techniques to provide the even greater sensitivity, adaptability, and selectivity. Chemical sensor systems will continue to grow in numbers, enabling portability compared with early systems.