TOKYO—Toshiba Corporation (TOKYO: 6502) developed a compact, high-sensitivity odor sensor that detects specific odor, for on-site maintenance and inspection of social infrastructure facilities. Using a quartz crystal resonator (QCR) *1 based compact and inexpensive sensor, and proprietary technology that evenly coats a thin film of a metal organic framework (MOF)*2, which well absorbs specific odors, Toshiba has realized high-sensitivity detection performance for the sensor, although it is less expensive and smaller than conventional analyzers with a volume ratio of less than few tenths in comparison.
Recently, Toshiba has applied this technology to musty odor that arise in drinking water sources and tap water, and succeeded to detect 2-methylisoborneol (“2-MIB”), the main cause of such odors, with a concentration of 0.2ppbv (2 parts per 10 billion) in air. This level of performance satisfies the standards for an aqueous concentration of 10ng/L and under (1g per 100 million within 1L of water), which represents the water quality standards for 2-MIB based on the Waterworks Act of Japan*3.
The detection of musty odor typically requires the installation of a large analyzer. Additionally, inspections based on sensory evaluations that depend on the experience and olfactory sense of the inspector are also conducted in great number. The utilization of this technology in odor anomaly inspections conducted at purification plants is anticipated to result in greater automation and efficiency of musty odor inspections at those plants. Additionally, coating the sensor with different types of thin film makes it possible to apply this technology in sensors for detecting non-musty off-odors, such as oily, scorched, or metallic odors. Going forward, this technology holds the promise of being applied to anomaly inspections in the areas of quality and production control at food and beverage manufacturers in addition to the maintenance and inspection of social infrastructure facilities.
Toshiba will announce the details of this technology at the 103rd CSJ Annual Meeting, which will be held on the Noda Campus of the Tokyo University of Science starting on March 22.
Background of the Development
In recent years, there have been cases in which musty odors emitted from drinking water sources and tap water leading to complaints from residents who use that water. The main cause of this has been identified as 2-MIB, a secondary metabolite produced by algae that propagates in lakes, swamps and at dams. Under the Waterworks Act of Japan, water quality standards for 2-MIB are set forth as an aqueous concentration of 10ng/L and under. This is the most stringent standard among the standards set for numerous substances that exist. Currently, at many purification plants, initial determinations are made through sensory evaluations based on the experience and olfactory sense of operators. However, there are growing needs for automated determinations of odor for more efficient inspections. At the same time, automated determinations require large, dedicated analyzers such as those installed at chemical analysis centers. For this reason, the automation of odor inspections has not yet progressed. A compact, high-sensitivity sensor was required to popularize that automation.
Features of the Technology
In order to solve this situation, Toshiba started to develop a high-sensitivity odor detection technology that uses a QCR, which constitutes a compact, inexpensive sensor. While QCR typically vibrates at a fixed resonance frequency, when objects are attached or adhered to the sensitive film on those resonators, thereby increasing its mass, the resonance frequency decreases in proportion with the incremental mass. Therefore, QCR are used as high-sensitivity mass sensors. As this sensor will be used in the air, if the object is a gaseous substance, it functions as a gas or odor sensor. However, to detect trace amount of 2-MIB dissolved in water, a pretreatment to vaporize the water is necessary.
Having verified that vaporizing pretreated water whose aqueous concentration of 2-MIB is 10ng/L results in a concentration of 0.2ppbv in air, Toshiba aimed to develop technology that enables sensors to detect 2-MIB with that air concentration.
Toshiba identified the property of a specifically-structured MOF of effectively absorbing 2-MIB molecules and applied that MOF as the sensitive film for the QCR. MOF is known as a superior material that demonstrates considerable adhesion performance when its structure is altered to match the object substance. For some time, attempts were made to directly trigger the growth of a thin crystal film of MOF on a QCR. However, as this was accompanied by a chemical reaction in the solvent, conditions such as those for the mixture of raw materials, the selection of solvent types, and reaction temperatures and times were stringent. As such, this approach was ill-suited to mass-production. There were also possibilities of residual impurities in the film.
Toshiba established a technology that enables the uniform coating and formation of a thin film with the ideal thickness on the ideal location on a QCR by pre-synthesizing MOF nanoparticles with a diameter of roughly 10nm to 20nm, dispersing the particles in solvent after refining them, and turning them into ink. Furthermore, the company identified coating conditions for the formation of multiple mesopores*4 between MOF nanoparticles. The presence of mesopores facilitates the diffusion of 2-MIB molecules to the inside of the film, causing the MOF nanoparticles to effectively adhere to the resonator. Because the adhered 2-MIB molecules desorb with the application of heat, etc. and are discharged to outside the film once more through the mesopores, repeated use is possible (See Figure 2). It is also possible to take advantage of the degree of freedom afforded by the structural design of a MOF to apply it to substances other than 2-MIB.
By combining the MOF sensitive film that it recently developed with a QCR, Toshiba developed a compact odor sensor with a volume ratio of less than few tenths compared to that of conventional dedicated analyzers, and used it to successfully detect 2-MIB with a concentration of 0.2ppbv in air. According to research by Toshiba, the development of a compact, low-cost odor sensor for mass-production with a sensitivity not exceeding 1ppbv for 2-MIB is an industry first*5.
The MOF sensitive film recently developed by Toshiba is dedicated to enhancing sensitivity to 2-MIB. Going forward, the company aims to address dependability and other issues and complete a musty odor detection system at an early stage. Furthermore, Toshiba will continue its development efforts with a view to applying the technology to other areas such as maintenance and inspections of social infrastructure facilities other than purification plants and quality and production control at food and beverage manufacturers by coating the sensor with different types of thin film to match the object of inspection.
Figure 1: Cross-section photograph of recently developed MOF nanoparticle thin film
Figure 2: Principle of odor sensor on which recently developed MOF nanoparticle thin film was formed by coating
Figure 3: Outer appearance of odor sensor developed by Toshiba Corporation (left) and enlarged photograph of chamber area (right)
*1: Quartz Crystal resonators vibrates at a fixed resonance frequency when an AC electric field is applied to them. Because that frequency decreases in proportion to an increase in mass, such as that caused by the adherence of substances, quartz crystal resonators are known as high-precision mass sensors.
*2: Metal organic frameworks are a new porous substance with nanometer-sized pores. They are made up of metal ions and organic linker molecules.
Because the size of the pores can be set flexibly, examinations are being made to use the substance for gas storage and separation and in catalysis as well as for sensors.
*3: From the drinking water quality standards (DWQS) that includes 51 Items (described on the Ministry of Health, Labour and Welfare of Japan website)
*4: Pores with a size between 2nm and 50nm.
*5: According to research by Toshiba Corporation dated March 2023.