Thursday, November 21, 2013

Nanoparticles Could Improve Chemical Sensors

Scanning electron microscope images of SnO2 “nano-flowers” synthesized for gas sensor researchIMAGE COURTESY OF MARK ANDIO
Patricia Morris, associate professor of materials science and engineering, leads a team of researchers who have developed new methods for making materials for gas sensors that could be used to detect toxic industrial chemicals and biological warfare agents.
The goal is to design a material with long-term stability that responds quickly and accurately to a variety of chemicals at very low concentrations. These sensor devices are similar to the human nose, which coordinates signals from hundreds of thousands of sensory neurons to identify gases. Similarly, the artificial sensor uses a combination of electrical responses from sensor arrays to identify the concentration of a specific gas.

The group’s efforts include synthesizing metal-oxide particles in the form of nanoparticles, nano-structured materials and hollow particles for use as the sensing material to increase sensor performance. NiO and SnO2nanoparticles, for example, are created with a particle size between five and 10 nanometers. Five nanometers (billionths of a meter) is approximately 50 atoms in diameter.“These are sensors that a soldier could wear on the battlefield, or a first responder could wear to an accident at a chemical plant,” Morris says.
In order to make the particles, precursors are placed in a Teflon-lined pressure vessel that is heated in an oven. The combination of temperature and pressure involving the correct precursors leads to the formation of nanoparticles in such a way that they have large surface areas to capture molecules from the air, enabling the sensor to detect very small quantities of a substance.
For instance, the SnO2nano-structured particles, sometimes referred to as “nanoflowers” due to their appearance, have many surfaces advantageous for adsorption and detection of hazardous gases. These materials have shown fast response and recovery times — meaning quicker detection — compared to other metal-oxide materials used for sensors.
Once the metal-oxide materials are synthesized, the particles are suspended in liquids designed to have the proper viscosity and surface tension in order to deposit the materials controllably on sensor platforms. Morris and her colleagues use a sophisticated inkjet printer that dispenses picoliter-volume drops onto very small microsensor substrates consisting of a silicon chip fitted with a platinum heater and gold electrodes to monitor the material’s electrical resistance. The change in the material’s resistance corresponds to a change in the surrounding atmosphere.
“Each material is sensitive to specific gases, and both oxides can be used in an array to selectively detect a gas in a complex background,” says Mark Andio, a materials science and engineering doctoral student working with Morris on the nano-structured oxides research.
Now that the researchers know the various chemical steps that take place during the synthesis of the materials, they can devise ways to add chemical dopants to the nanoparticles to change the function of the sensor — for instance, to speed up the response rate.