Optical sniffer detects poison gas

US researchers have developed an optoelectronic nose that can sniff out toxic gases. The sensor is fast and inexpensive and could be used to detect high exposure risk to hazardous industrial chemicals.

A handheld device that sniffs the air and uses a postage-stamp size array of nanoporous dots to reveal the presence of toxic industrial gases, including chlorine, hydrogen sulfide, and ammonia has been developed by Ken Suslick and his team at the University of Illinois Urbana-Champaign.

"Our device is simply a digital multidimensional extension of litmus paper," Suslick explains, "it has a six-by- six array of different nanoporous pigments whose colours change depending on their chemical environment." To make the dots, the team converted various soluble dyes into stable, nanoporous organically modified silica, ormosil, pigments. "These ormosils are ideal optical substrates with transparency well into the UV," Suslick told SpectroscopyNOW.

The pattern of colour changes in the array presents a unique chemical fingerprint for almost any toxic gas as well as providing information about the concentration of the gas in the sampled air. The device "reads" the array and software compares the digital output with a library of colour fingerprint patterns stored on-board. "We can identify and quantify toxic industrial chemicals in a matter of seconds," says Suslick.

To make their sensor arrays, the team "printed" the tiny coloured dots, each of which contains a different sensitive pigment on to an inert backing material, which can be paper, plastic or glass. In one application of the system, the array is digitally imaged with an ordinary flatbed scanner or an inexpensive electronic camera before and after exposure to an odour-producing substance. An important difference between this technology and earlier electronic "noses" is that these colorimetric sensors are not affected by changes in relative humidity.

Industrial chemists and chemical engineers could wear a badge holding the array to provide a warning of exposure to potentially harmful chemicals, much as nuclear workers have radiation badges to protect them in the workplace.

To test the application of their colour sensor array, Suslick and his colleagues, Liang Feng, Jonathan Kemling, Christopher Musto, chose nineteen representative examples of toxic industrial chemicals, including nitric acid and sulfur dioxide at concentrations known to be immediately dangerous to life or health.

The new technology is based on much stronger interactions pigment molecule and gaseous analyte than earlier systems. The approach means that the array is responsive to a diverse set of toxic chemicals. They saw no misclassifications of specific chemicals in 140 trials. Importantly, they confirmed gas-stream concentrations using in-line analysis by Fourier transform infrared spectroscopy (FTIR) in real time with a multi-gas analyser for most analytes. For homonuclear diatomic compounds, such as fluorine and chlorine gas, they used Draeger detector tubes to confirm the levels of those gases.

The initial laboratory studies used an inexpensive flatbed scanner to image, the arrays. The digital image is then fed to software for analysis. However, the team has also worked with Sung Lim of iSense, a company based in Palo Alto, California to develop a fully functional prototype handheld device. The handheld sniffer uses inexpensive white LED illumination and an ordinary electronic camera, which makes the whole process more portable, sensitive, faster, and less expensive. It will be similar to a card-scanning device, Suslick emphasises, pointing out that the system is now being commercialized. "Future work will use hyperspectral imaging, especially into the UV where many of these pigments also show chemically responsive changes," says Suslick.

As with the mammalian sense of smell, the array approach to poison gas detection is based on a composite response to the specific chemicals' reactivities. "Our optoelectronic nose largely overcomes the limitations of prior electronic nose technologies," the researchers say, and it works for the identification of a wide range of chemicals at low concentrations.

"One of the nice things about this technology is that it uses components that are readily available and relatively inexpensive," adds team member David Balshaw, "Given the broad range of chemicals that can be detected and the high sensitivity of the array to those compounds, it appears that this device will be particularly useful in occupational settings."