Micro-differential thermal analysis detection of adsorbed explosive molecules using microfabricated bridges.

Although micromechanical sensors enable chemical vapor sensing with unprecedented sensitivity using variations in mass and stress, obtaining chemical selectivity using the micromechanical response still remains as a crucial challenge. Chemoselectivity in vapor detection using immobilized selective layers that rely on weak chemical interactions provides only partial selectivity. Here we show that the very low thermal mass of micromechanical sensors can be used to produce unique responses that can be used for achieving chemical selectivity without losing sensitivity or reversibility. We demonstrate that this method is capable of differentiating explosive vapors from nonexplosives and is additionally capable of differentiating individual explosive vapors such as trinitrotoluene, pentaerythritol tetranitrate, and cyclotrimethylenetrinitromine. This method, based on a microfabricated bridge with a programmable heating rate, produces unique and reproducible thermal response patterns within 50 ms that are characteristic to classes of adsorbed explosive molecules. We demonstrate that this micro-differential thermal analysis technique can selectively detect explosives, providing a method for fast direct detection with a limit of detection of 600x10(-12) g.

Speciation of energetic materials on a microcantilever using surface reduction.

Although microcantilevers have been used to detect explosives with extremely high sensitivity using variations in adsorption-induced bending and resonance frequency, obtaining selectivity remains a challenge. Reversible chemoselectivity at ambient temperatures based on receptor-based detection provides only limited selectivity due to the generality of chemical interactions. The oxygen imbalance in secondary explosives presents a means to achieve receptor-free speciation of explosives using surface reduction of adsorbed molecules. We demonstrate highly selective and real-time detection of Trinitrotoluene (TNT) using a copper oxide-coated cantilever with a surface reduction approach. Not only can this technique exclusively differentiate explosives from nonexplosives, but also it has the potential to specify individual explosives such as TNT, pentaerythritol tetranitrate (PETN), and RDX. This technique together with receptor-based detection techniques provides a multimodal approach for achieving very high selectivity.

Vibration response of microcantilevers bounded by a confined fluid.

Hydrodynamic predictions of fluid velocity and pressure distribution are made for fluid in a confined space bounded by a vibrating microcantilever and a fixed surface. The results are used to quantify damping factors and predict frequency response amplitudes of a microcantilever vibrating near a fixed surface. The theoretical predictions of vibration response compare favorably with experimental data.