Screening for explosives is a public safety issue. 2,4,6-trinitrotoluene (TNT) is readily detectable by detecting its common and low-boiling contaminant, 2.4-dinitrotoluene (DNT).
Some other explosives are more difficult to detect. An example is the issue of detecting 2,3-dimethyl-2,3-dinitrobutane (DMNB).
Law requires this contaminant to be in all commercial plastic explosives. However, the molecular structure of DMNB makes it difficult to latch onto (detect) the molecule.
Jing Li (Rutgers, State University of New Jersey) and coworkers have used porous metal-organic frameworks for detecting explosives. Detection is fast at low concentrations.
Porous metal-organic frameworks: Molecular sponges.
The scientists' detection units, porous metal-organic frameworks, can be thought of as "molecular sponges." They are porous crystals, and can entrap small molecules within the pores.
An important feature is their extremely high surface area, enabling the sponges to soak up a large quantity of molecules within them. This feature is being explored for use in many applications, such as absorbing pollutants and hydrogen storage.
These scientists' metal-organic frameworks have a pore volume of 0.17 cubic centimeters per gram, a pore diameter of approximately 0.75 nanometers, and a surface area of 483 square meters per gram. This surface area is 9% of a football field packed within only one gram of material.
Detecting explosives with molecular sponges.
It is clear that metal-organic frameworks can be used to soak up a large quantity of molecules. How can this be detected?
At room temperature, these scientists' sponges emit fluorescent light when they are illuminated with UV light. However, when they entrap the explosives DNT and DMNB, they lose their ability to emit fluorescent light.
Thus, if they are not luminescent, explosives are present. This is the basis for their sensing ability.
When a thin (5 micrometers) film of the scientists' molecular sponges were exposed to low concentrations (in the neighborhood of 1 part per million) of vapor from the explosives, approximately 85% loss of fluorescence is observed within 10 seconds. The sensing is therefore fast, and easily detectable.
Using thicker films (30 micrometers), the response is slower (hours). However, the eventual response is similar, suggesting that while using films thinner than 5 micrometers may give an even faster response, it will probably not be more sensitive.
The loss of fluorescence is reversible by boiling the explosive vapors away for 1 minute. The molecular sponges can therefore be easily reused.
Easy access to open pores within the molecular sponges is important. When the pores are filled with solvent molecules, their response to explosive vapors is reduced by approximately 90%.
Further development.
It seems that these scientists' metal-organic frameworks do not report on the concentration of explosive vapor, but rather simply tell the user that explosives are present. Since information on the concentration is unimportant as long as low concentrations can be detected, this does not seem like a limitation for practical purposes.
The scientists envision improving on their molecular sponges by tuning their molecular composition to respond selectively to different analytes. This will enable the user to identify the specific explosive present, a practical requirement unless other instrumentation is available for further analysis once preliminary identification has been made.
The composition of metal-organic frameworks can in fact be readily tuned, in terms of their pore shapes, fluorescence response, and other characteristics. The scientists' vision therefore seems obtainable, and may render their molecular sponges useful for law enforcement personnel who need to rapidly screen for explosives.
for more information:
Lan, A.; Li, K.; Wu, H.; Olson, D. H.; Emge, T. J.;
Ki, W.; Hong, M.; Li, J.
A luminescent microporous metal-organic framework for the fast and
reversible detection of high explosives.
Angew. Chem. Int. Ed. 2009, 48, 2334-2338.