Stochastic Detection of Terrorist Agents and Biomolecules in a Biological Channel - Nanopores: Sensing and Fundamental Biological Interactions - page 320

13.3 Detection of Explosive and Chemical Warfare Agents

13.3.1 2,4,6-Trinitrotolune

2,4,6-Trinitrotolune (TNT) is an explosive used in military and industrial
applications and also a priority pollutant listed by the US Environmental Protection
Agency [ 52 - 54 ]. To detect TNT, aromatic surface function groups could be
introduced inside the

HL protein pore (Fig. 13.5 ). For example, the Met amino
acid residues at position 113 of the wild-type

a

HL pore can be replaced with Phe,
Tyr, or Trp side chains [ 13 ]. Note that position 113 is close to the constriction area
of the

a

HL protein channel and this position has been employed for the develop-
ment of numerous nanopore sensors for a variety of analytes [ 3 , 9 ]. When a TNT
molecule enters the mutant protein pore, it will bind near that position (containing a
cluster of seven aromatic residues) due to the aromatic-aromatic interaction, thus
causing current blockage (Fig. 13.5 ). Since the event frequency was linearly related
to the concentration of the added TNT, and other related nitroaromatic compounds
had different current signatures (e.g., residence times and amplitudes) from those of
TNT [ 13 ], the method should be able to find a potential application in identifying
and quantifying TNT in an aqueous environment.

a

13.3.2 Liquid Explosives

Liquid explosives are usually binary mixtures where the two individual components
alone are nonexplosives [ 55 ]. Since they can be transported easily and without being
noticed, liquid explosives may be smuggled on the airplane to be the secret terrorist
weapon. In fact, recently, there have been some reports about the failed plot to attack
airplanes with liquid explosives [ 56 ]. Nanopore sensors have been developed as an
effective platform to detect the liquid explosive component and sensitizers. For
example, hydrazine (HZ) is a component of the liquid explosive, Astrolite. When
hydrazine is mixed with ammonium nitrate, a powerful explosive is made. Diethy-
lamine (DEA), triethylamine (TEA), and morpholine (CM) are liquid explosive
sensitizers for nitromethane. The identification of amine-type liquid explosive
components and the associated sensitizers was achieved by using boromycin as a
molecular adaptor [ 14 ]. The cleft formed by the boromycin structure can accommo-
date monovalent cations such as potassium, ammonium, amine compounds, etc.
(Fig. 13.6 ). When the host-guest complex enters the pore, current blockage events
could be observed. Since different analyte-boromycin complexes showed signifi-
cantly different event mean residence times and/or amplitudes (Fig. 13.6 ), the
differentiation and even simultaneous detection of liquid explosive components in
aqueous environments could be conveniently achieved.