Sensitive flower: Explosive glow

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  • Published: Sep 1, 2012
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Sensitive flower: Explosive glow

Go with the glow

 Selective Visual Detection of TNT at the Sub-Zeptomole Level Credit: Wiley/Pradeep et al 

Researchers in India have developed a shapely approach to nanoparticle enhancement that allows them to detect TNT at sub-zeptomole concentrations of TNT.

Thalappil Pradeep, Ammu Mathew and P. R. Sajanlal of the Indian Institute of Technology Madras have used a clever combination of micro- and nano-structures as sensors to detect tiny quantities of the explosive material TNT, trinitrotoluene. Their gold mesoflowers, are flower-shaped gold particles about 4 micrometres in diameter, which act as supports for clusters that contain precisely fifteen silver atoms and are embedded in the protein bovine serum albumin. Irradiation at an appropriate wavelength leads to red luminescence of the silver clusters and the gold intensifies this process. The distinctive shape of the mesoflowers means that they are readily identifiable under an optical microscope by visual inspection. The shape might also be exploited in image recognition of micrographs, something that is far more difficult if the particles being observed are spherical.

Security at airports relies on highly sensitive detection of explosive materials, but highly capable and easy to use sensor technologies are equally applicable in the early detection of a wide range of diseases based on different chemical and biochemical markers and the detection of hazardous and harmful environmental agents. Higher sensitivity means a reduced rate of false-positives, which is important in security, medicine and the environment where the consequences of inappropriate remediation can be problematic, to say the least.

Writing in the international edition of Angewandte Chemie this month, the Madras team has introduced a specific detection method for the explosive TNT that can reveal the presence of even a single molecule of the compound.

The test hinges on the fact that if a drop of suspect material in solution is applied to the sensor, the amino groups of the bovine serum albumin react to form a Meisenheimer complex; this reaction only occurs with TNT in this context. The formation of the complex extinguishes the red luminescence of the silver clusters. To make the change in luminescence even more apparent under the microscope, the team embedded a green fluorescent dye adsorbed on a silicon dioxide layer covering the gold mesoflowers. All the while the silver clusters glow red, the green fluorescence is suppressed, but the presence of TNT molecules quenches the red glow and the green dye can shine through, this colour change is easily seen through a fluorescence microscope.

The team explains that at a TNT concentration of 1 ppb (part per billion) the red glow is extinguished but even 1 ppt (part per trillion) is enough to give rise to a marked change in colour. The team has also augmented the luminescence method with SERS (surface-enhanced Raman scattering). This, the researchers explain, works well with one class of their flower-shaped sensors, allowing them to achieve a detection limit bellow the sub-zeptomolar range (10-21 moles). A single mesoflower is enough to kick the sensor into action allowing the detection of a mere nine molecules of TNT in a sample solution. The team is now working on scaling the technology to make a viable sensor device.

Sensitive device

Additionally, the Madras team was able to use a similar sensor strategy to detect the toxic heavy metal mercury at close to the same sensitivity level. "Our concept could also be used for the ultra-trace analysis of other substances through the incorporation of specific ligands on the sensors," Pradeep explains.

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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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