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Chemistry

Identification by smell

Electronic nose identifies wood species and can help combat illegal logging

Set of gas sensors used to analyze vapors from volatile substances

Eduardo CesarSet of gas sensors used to analyze vapors from volatile substancesEduardo Cesar

from Agência FAPESP

Researchers at the University of São Paulo’s Chemistry Institute (IQ-USP) in Brazil have built “electronic noses” capable of identifying and classifying different types of wood and plastic by odor as well as detecting early-stage deterioration of oranges due to fungal contamination.  Some of the devices were developed through the FAPESP-funded project “New conjugated polymers for solar cells and electronic noses.”  The technology is very simple and inexpensive and has several applications,” say Jonas Gruber, a professor at IQ-USP and principal investigator for the project.  Each “e-nose” consists of an array of gas sensors that change the electrical conductivity of some of the materials that they are made of (including conductive polymer, a type of plastic) as they interact with vapors from volatile substances such as amines, alcohols, ketones and aromatic compounds.

Variations in the array’s conductance generate a specific electrical signal that is converted into a digital signal. This signal is read by a computer program, which, in a matter of seconds, identifies the type of volatile substance in contact with the device.  “The e-nose responds differently according to the nature of the gas that comes into contact with the polymeric materials in the sensors,” Gruber explains. One feat in particular permitted the development of these e-noses. Gruber and his group at IQ-USP synthesized and characterized new conductive polymers derived from two specific classes of polymer: poly-p-phenylenevinylene (PPV) and poly-p-xylylene (PPX), which were then used these to build sensors.  “We were the first to use PPV in gas sensors,” Gruber says. “The advantages are low production cost, low power consumption, and the ease with which the characteristics of the devices can be changed by introducing structural modifications into the polymer chains.”

The sensor construction technique used by the researchers consists of depositing a conductive polymer film with a thickness of a few hundred nanometers (billionths of a meter) on a board the size of a cell phone chip with two interdigitated metal electrodes (interlocked, but with minute gaps between them) so that the film connects the two materials.  When the sensor is exposed to vapor from a volatile substance, the film’s electrical resistance changes. “Each sensor costs one Brazilian Real, and we use between four and seven sensors per e-nose on average,” Gruber says.

Equipment differentiates species of wood after logging

EDUARDO CESAREquipment differentiates species of wood after loggingEDUARDO CESAR

One of the devices was developed to identify and classify different species of wood. The researchers expect it to be used by environmental police to combat illegal logging of endangered tree species in Brazil’s tropical forests.  Protected species such as mahogany (Swietenia macrophylla) are often hard to distinguish from species that can be legally logged and marketed, such as Spanish cedar (Cedrela odorata).  Both belong to the same family and are so similar that mahogany is often marketed as Spanish cedar Gruber explains.

“You can distinguish between mahogany and Spanish cedar when you look at them in the forest, but once logged, they can only be distinguished by histological analysis [of the plant tissues] performed in the lab by a botanist,” Gruber says.  E-noses facilitate the identification of these and other species, such as Brazilian walnut (Ocotea porosa) and Ocotea catharinensis (local common name: canela-preta). All they need is a small shaving from the trunk, which releases volatile compounds. The sensor array identifies the species in under a minute.  “Because Spanish cedar and mahogany are different species and belong to different genera, e-noses can distinguish them with 100% accuracy,” Gruber says. “In the case of walnut and canela-preta it’s a little harder because they belong to the same genus. Even so, identification by e-nose is correct 95% of the time.”

Prototype of the e-nose at the USP laboratory

Eduardo CesarPrototype of the e-nose at the USP laboratoryEduardo Cesar

Aged cachaça
The e-nose for wood identification aroused the interest of researchers at the Cane Spirit Chemistry Development Laboratory (LDQA), part of the University of São Paulo’s São Carlos Chemistry Institute. They wanted a way to distinguish cachaça aged in oak casks from cachaça aged in barrels composed of other wood species that are considered inferior. According to Gruber, consumers prefer the flavor and odor of cachaça aged in oak casks, so it sells for a higher price. Because it is imported from Canada, however, import restrictions often apply. Native species increasingly used to age cachaça include cherry (Hymenaea courbaril), rosewood (Jacaranda mimosifolia), jequitibá (Cariniana estrellensis) and walnut (Ocotea porosa). According to Gruber, the producer may claim that oak has been used.

“Some distillers sell cachaça labeled as aged in cherry for less than oak-aged spirit, but the opposite is also true: you find cachaça aged in barrels made from native wood but with oak on the label and selling for R$200 a bottle,” Gruber says.  To try to protect consumers from this kind of false advertising, the researchers adapted IQ-USP’s e-nose for the analysis of cachaça samples. “The device is able to ‘sniff’ a cachaça and identify the wood species used,” Gruber says.

This particular e-nose was developed during the post-doctoral project of Alexandre Ataide da Silva, entitled “Distinguishing hydroalcoholic wood extracts and monitoring their stages of aging using gas sensors, gas chromatography (GC-MS) and multivariate analysis.”

Oranges contaminated by fungi have a different odor

Léo RamosOranges contaminated by fungi have a different odorLéo Ramos

Plastic identification
Researchers at IQ-USP have also developed a device to identify plastics for recycling.  According to Gruber, different types of plastic, such as PVC, polyethylene and polypropylene, cannot be mixed when recycled because they contain incompatible resins.  One of the techniques used to identify and classify plastics is infrared spectroscopic analysis of samples dissolved in appropriate solvents. However, infrared spectroscopy requires a laboratory staffed by professionals qualified to operate an infrared spectrometer.

The e-nose developed by Gruber’s research group identifies plastics from the gas emitted in combustion.  The researchers built a small combustion chamber to hold a sample of approximately 300 milligrams for incineration.  The e-nose “sniffs” the fumes emitted during combustion and identifies the type of plastic from the volatile compounds that the plastic releases.  “Polyethylene produces carbon dioxide and water during combustion, whereas a polyamide like nylon, for example, produces nitrogen oxides as well as carbon dioxide and water. The e-nose perceives those differences” Gruber explains.

The researchers also developed an e-nose for early detection of contamination of oranges (after being harvested) by Penicillium digitatum.  This fungus, together with Elsinoe australis and Guignardia citricarpa, causes severe economic losses in countries that are major citrus producers, such as Brazil, Gruber says. The e-nose can detect contamination by the fungus while oranges are in silo storage.  “The device detects contamination as soon as day two and, in a matter of seconds, identifies infection of the oranges by the fungus based on the volatile metabolites that it releases.”

According to Gruber, some of the e-noses developed by his group are protected by patents. The idea is for interested firms to license the technology to produce and sell the devices.  “We aim to produce low-cost e-noses. Some of the commercially available devices cost as much as $20,000,” Gruber says. One of the reasons for the high price, he explains, is that these devices have 20-30 sensors and are not designed for specific applications.  “We develop e-noses for specific applications so we can reduce the number of sensors that they contain and greatly reduce production costs.”

Projects
1. New conjugated polymers for solar cells and electronic noses (No 2011/51249-3); Grant Mechanism: Regular Research Award: Principal Investigator: Jonas Gruber (USP); Investment: R$80,782.65 US$27,507.50 (FAPESP).
2. Distinguishing hydroalcoholic wood extracts and monitoring their stages of aging using gas sensors, gas chromatography (GC-MS) and multivariate analysis (No 2012/15539-0); Grant Mechanism: Scholarships in Brazil – Doctoral degree (Alexandre Ataíde da Silva); Principal Investigator: Douglas Wagner Franco (USP); Investment: R$160,441.00 (FAPESP).

Scientific articles
ESTEVES, C. H. A. et al. New composite porphyrin-conductive polymer gas sensors for application in electronic noses. Sensors and Actuators B: Chemical. V. 193, p. 136-41. March 2014.
GRUBER, J. et al. A conductive polymer based electronic nose for early detection of penicillium digitatum in post-harvest oranges. Materials Science and Engineering:C. V. 33, No. 5, p. 2766–69. July 2013.

 

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