Microgel sensors: Nerve gas responders

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  • Published: May 1, 2014
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Microgel sensors: Nerve gas responders

Stacked microgels

Writing in the journal Angewandte Chemie, a Canadian research team has demonstrated the optical properties of stacked microgel structures and how they might change in response to variations in light levels, the pH or temperature of the surroundings or even the presence of particular substrates including nerve gases. Image: Wiley/Angew

Writing in the journal Angewandte Chemie, a Canadian research team has demonstrated the optical properties of stacked microgel structures and how they might change in response to variations in light levels, the pH or temperature of the surroundings or even the presence of particular substrates including nerve gases.

Materials scientists and technologists are keen to develop "intelligent” materials that can respond to external stimuli as they could have wide applications in sensors and diagnostic devices as well as in microelectromechanical systems (MEMS) and microrobotics. Now, Michael Serpe, Qiang Matthew Zhang and Wenwen Xu of the Department of Chemistry at the University of Alberta have used the polymer poly(N-isopropylacrylamide) functionalized with triphenylmethane leucohydroxide to construct etalons. An etalon is an interferometer consisting of two reflecting plates with another material sandwiched between the plates. The team hopes their devices in which the sandwiched polymer derivative can swell and contract depending on external stimuli and will be useful in point of care diagnostics, water quality monitoring systems, and polymer-based muscles and actuators.

The upper etalons

The researchers explain that their gels are comprised of cross-linked molecules that can effectively hold a liquid within their pores, it is the presence of these filled voids that gives them the potential to swell and contract again. In their microgel format, in which the stable gel particles exist as a colloid, the material is swollen at temperatures below 32 Celsius but at higher temperatures they collapse and shrink. The team constructed small stacked structures, their etalons where a wafer-thin layer of the microgel is held between two thin layers of gold. When the gel swells up, the gold sheets are pushed apart and they move back together when the gel shrinks. Serpe and colleagues explain that the optical properties of the stack change significantly as the distance between the gold layers changes, meaning that they can be observed to “respond” to a change in temperature.

Of course, while temperature response is important. Developing gels that react to other stimuli is vital for a wide range of applications. The incorporation of triphenylmethane leucohydroxide (TPL) into the microgels endows the gel with the ability to respond to a variety of stimuli. Indeed, the TPL can absorb red laser light, this leads to a localized temperature rise that again causes the microgel to contract reducing the gap between the gold plates. The distance between the gold layers can be made to increase by irradiating with ultraviolet light. This excites the TPL molecules causing them to dissociate into leuco cations and hydroxy anions, which leads to water absorption and so swelling of the microgel. Such behaviour points to the application of the red light/ultraviolet effects in the development of optical components that are adjustable based on incident electromagnetic energy. Such a device might be used to deliver a drug to a particular disease site in the body using remote triggering. Light transmitted through the skin following implantation of a transporter device would cause the device to open up and release its cargo.

Nervous

In yet another variation on the theme, the microgel can react to a change in pH value. Acidic conditions lead to the formation of leuco cations, it is their positive charge that leads to water take up and swelling of the gel. If the pH is raised again to alkaline levels, the microgel shrinks again. The researchers point out that pH changes are often coincident with conditions within a malignant tumour and so could be used to release selectively an anticancer drug only in the presence of the tumour itself and so reduce the risk of damage to healthy tissues.

Finally, the presence of organophosphates also cause changes in the TPL molecules, release leuco cations and causing the now-familiar gel swelling. Given that the swelling changes the optical properties of the etalon it would be a relatively straightforward matter to devise a quantitative detector for organophosphates, including well-known nerve agents such as tabun.

"The next step for these devices is to evaluate their drug delivery ability in response to light/pH triggering," Serpe told SpectroscopyNOW.com. "Although we have many other experiments planned as well."

Related Links

Angew Chem Int Edn, 2014, 53, online: "Optical Devices Constructed from Multiresponsive Microgels"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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