Ozone is a problematic air pollutant that causes serious health
problems. A newly developed material not only quickly and selectively
indicates the presence of ozone, but also simultaneously renders the gas
harmless. As reported by Chinese researchers in Angewandte Chemie, the porous "2-in-one systems" also function reliably in very humid air.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Ozone (O3) can cause health problems, such as difficulty
breathing, lung damage, and asthma attacks. Relevant occupational safety
regulations therefore limit the concentrations of ozone allowable in
the workplace. Previous methods for the detection of ozone, such as
those based on semiconductors, have a variety of disadvantages,
including high power consumption, low selectivity, and malfunction due
to humid air. Techniques aimed at reducing the concentration of ozone
have thus far been based mainly on activated charcoal, chemical
absorption, or catalytic degradation.
A team led by Zhenjie Zhang at Nankai University (Tianjin, China) set
themselves the goal of developing a material that can both rapidly
detect and efficiently remove ozone. Their approach uses materials known
as covalent organic frameworks (COFs). COFs are two- or
three-dimensional organic solids with extended porous crystalline
structures; their components are bound together by strong covalent
bonds. COFs can be tailored to many applications through the selection
of different components.
The researchers selected easily producible, highly crystalline COFs
made of aromatic ring systems. The individual building blocks are bound
through connective groups called imines (a nitrogen atom bound to a
carbon atom by a double bond). These are at the center of the action.
The imine COFs indicate the presence of ozone through a rapid color
change from yellow to orange–red, which can be seen with the naked eye
and registered by a spectrometer. Unlike many other detectors, the imine
COF also works very reliably, sensitively, and efficiently at high
humidity and over a wide temperature range. In the presence of water,
the water molecules will preferentially bind to the imine groups.
Consequently, the researchers assert, a hydroxide ion (OH−)
is released, which reacts with an ozone molecule. The positively charged
hydrogen atom remains bound to the imine group, causing the color
change. If more ozone than water is present (or the ozone-laden air is
fully dry), the excess ozone binds to the imine groups and splits them.
Each imine group degrades two molecules of ozone. This also causes a
color change and the crystalline structure slowly begins to collapse.
The imine COF thus doesn't just detect the ozone, but also reliably and
efficiently breaks the harmful gas down. This makes it more effective
than many of the traditional materials employed for this purpose.
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About the Author
Dr. Zhenjie Zhang is
a full Professor of inorganic chemistry at Nankai University. He is
currently focusing on developing new crystalline porous materials (e.g.,
MOFs, COFs, and cages) for value-added product separation and
purification, as well as creating smart materials for sensors,
actuators, or robots. He is a recipient of the ACS-DIC Young
Investigator Award and the CCS JINGQING Chemistry Investigator Award.
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