�Artificial nose  detects dangerous vapors Researchers have developed a way to sniff out solvent gases by combining a common mineral zeolite with a metallic compound based on rhenium.They report that in the presence of the compound each gas has a photoluminescent �fingerprint� with a specific intensity lifetime and color.The challenge for Angel Mart­ assistant professor of chemistry and bioengineering at Rice University and his team of student researchers was to get their large metallic particles through the much smaller pores of a zeolite cage. They designed a process in which small chemical components enter the cage find each other and self-assemble into rhenium complexes. Then they re stuck like a ship in a bottle.�We sequentially load the individual parts of the complex into the zeolite� Martí says. �The parts are smaller than the pores but when they self-assemble inside the zeolite they re trapped.� Once washed to eliminate complexes that form outside the zeolites the compound is ready for use. The study is published in the journal Angewandte Chemie.The relatively simple technique which two undergraduate alumni initially developed and studied could provide a scalable inexpensive platform to monitor toxic vapors from industrial solvents.Solvents are liquid chemicals often petroleum-based that are widely used to dissolve solid materials. They are found in paints thinners aerosol sprays dyes marking pens adhesives and other products. They also evaporate quickly. Solvent vapors which are hazardous to inhale and can be highly flammable are often denser than air and gather at floor level where they can build to dangerous amounts unless detected.Mart­ says platinum gold palladium and copper salts are often used to detect vapors because they change color in the presence of solvents. The rhenium-based supramolecular complex was known to fluoresce in the presence of some solvents but dealing with vapors is a different story.�If the complexes are in a solid state they are too close to each other and gases can t interact with them� he says. �So we started thinking of ways to create space between them.�Enter zeolites. �These zeolites are cages with big cavities and small pores� Mart­ says. �The pores are big enough at about 7.4 angstrom for most gas-phase molecules to enter. The question was how to trap the bigger rhenium complexes inside.�Other groups have trapped ruthenium complexes in zeolites but these complexes were not ideal to detect solvents. Then researchers developed the method to put rhenium complexes inside zeolites. The results were outstanding Mart­ says.The caged complexes strongly signal the presence of a vapor by the color and intensity of their photoluminescent glow in ultraviolet light.Mart­ says nobody had studied the third key property the amount of time the complex remains in an excited state. That ranges from less than 1000 nanoseconds for water and ammonia to �a quite long� 4000-plus nanoseconds for pyridine. It s different for every type of vapor he says.�We concluded that every individual vapor has a set of photophysical properties that is unique for that solvent� he says. �Each one has a unique fingerprint.�With the ability to detect three distinct characteristics for each vapor a team led by graduate student Avishek Saha built a 3D plot to map the fingerprints of 17 types of solvents. They found categories of solvents nonpolar alcohols protics (which include water) and aprotics tended to gather in their own areas.�That s another interesting thing� Mart­ says. �Different solvent groups occupy different areas in the map. So even if a solvent hasn t been studied our material will help people recognize the category it falls into.�He says the group plans to test more solvents and suggested the material may also be useful for detecting the presence of other volatile species like explosives.The Welch Foundation supported the research.