A large part of analog electronics is related to real-world sensors and processing their signals. We have sensors and computational systems that are exceptionally good at capturing and decoding sound and speech, light and images, pressure, motion, and heat and temperature. However, we still have some major challenges when attempting to sense and classify for taste and smell, largely because they are based on chemistry rather than physics. Nonetheless, researchers continue to work on the electronic nose since such a general-purpose sensor for smell or taste would be quite useful.
The most successful way to assess an unknown general smell or taste at present is to use gas or liquid chromatography, which requires preparation and care and certainly is not as quick and easy as taking a whiff or taste and then saying “hmmmm….smells/tastes like such and such.” True, the chromatography is far more sensitive and resolves more components than a nose. But there is still a desire to have a simple sensor that can directly detect a taste or smell.
Thus far, however, most of the efforts have been tailored to sense and measure only a single or closely related class of chemical compounds. Such e-sensors do have one advantage over the biological nose: they are much more sensitive and can provide quantitative results. But don’t be misled: while the headlines tout the “electronic nose,” the reality is that these noses remain far more limited than biological ones in the molecular spectrum they can capture, assess, and identify.
For example, a team at the University of Massachusetts (Amherst) recently developed bioelectronic ammonia-gas sensors (Figure 1), which they maintain are among the most sensitive ever made. Ammonia is very important to agriculture, environmental science, and biomedicine and is very dangerous in high concentration. It is also produced normally during metabolic processes in the human body and in nature in water, soil and air, even in tiny bacteria molecules. So even if limited to sensing ammonia, it’s a good “nose” to have.
Figure 1 In this artistic representation, protein nanowires (light green) harvested from Geobacter (background) are sandwiched between electrodes (gold) to form bioelectronic sensor for detection of biomolecules (red). (Image source: Photo courtesy of University of Massachusetts Amherst/Yao lab)
The team’s sensor uses electric-charge-conducting protein nanowires derived from the bacterium Geobacter sulfurreducens to provide biomaterials for electrical devices. The nanowire-based sensor is extraordinarily sensitive and also responds to a broad range of ammonia concentrations (10 to 106 parts per billion) according to their academic paper “Bioelectronic protein nanowire sensors for ammonia detection” published in Nano Research.
The back story associated with this bacterium is also interesting: over 30 years ago, senior paper author and microbiologist Professor Derek Lovley discovered Geobacter in river mud. These microbes grow hair-like protein filaments that work as nanoscale “wires” to transfer charges for their nourishment and to communicate with other bacteria. The professor notes than these proteins can also be “tuned” to be sensitive to other chemicals, making them somewhat more universal. Despite this tuning capability, however, it remains a fairly “narrowband” sensor, which is both good and bad depending on the application and system objective.
There are, of course, non-biological approaches to creating an e-nose for targeted smells. One technique uses 250-GHz RF-based stimulated spectroscopy (Figure 2), while others use Raman scattering in the optical spectrum (the inelastic scattering of photons) to allow sensing at a distance.
Figure 2 The rotational spectroscopy setup (using a receiver from Virginia Diodes, Inc.) shows the complexity and advanced technologies needed for this type of 250-GHz gas-sensing arrangement (Image source: University of Texas/Dallas).
Still, the goal of a wide-ranging, universal smell sensor based on electronic, optical, or bio-electronic principles that can handle a wide range of chemical, is elusive. Perhaps there will be some sort of microchannel with lined with diverse reactants that produce tagged electrochemical reactions, or perhaps there will be a chromatograph-channel on a chip with a sample heater at the input and a flow sensor at the output. Or maybe someone will figure out how to electrochemically emulate the noses of living creatures, which have different degrees of smell sensitivity and span. The bloodhound is probably the best known (Figure 3), but humans can also smell (and taste) across a wide range of chemical as can many animals of all sizes and types. Assessing smells (and tastes) and their implications is both an inborn characteristic and a learned response.
Figure 3 When a bloodhound dog breathes in, the air separates into distinct paths, one (red) flowing into the olfactory area and the other (blue) passing through the pharynx (black) to the lungs (Image source: PBS/Brent Craven).
It’s no surprise that there is a tradeoff: electronic sensors can provide extremely high sensitivity and dynamic range but for a limited set of substances, as they are tuned to fairly specific molecules. In contrast, the living-creature biological-based sensor – whether human, bloodhound, or even insect – generally doesn’t have the high sensitivity (although it may come close, in some cases), yet often does have very wide spectral range. Although we do understand to some extent how the biological “nose” works, we don’t fully understand the mechanics or the interplay between the sensor and the brain. After all, there are animals with very small brains that are pretty good at sensing both food and danger.
Have you ever had to implement a “smell” sensor function? Was it for a specific molecule or group, or for a much-more general class? How did that work out? What issues did you encounter?