Dechang Yi

Detection of trinitrotoluene via deflagration on a microcantilever

We describe in detail the detection of deflagration of trinitrotoluene (TNT) deposited on a piezoresistive microcantilever and point out its possible use for explosive-vapor detection. The deflagration of TNT causes the cantilever to bend (due to released heat) and its resonance frequency to shift (due to mass unloading). Explosive vapors provide unique responses that are absent for “interferences” such as water or alcohol vapors. The proposed sensor makes possible a sensitive, miniature explosives detection device that may be deployed in large numbers. The minimum amount of TNT detected on the cantilever depends on the cantilever dimensions and was ≈50 pg for the batch of cantilevers used.

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.

Detection of adsorbed explosive molecules using thermal response of suspended microfabricated bridges

Here we present a thermophysical technique that is capable of differentiating vapor phase adsorbed explosives from nonexplosives and is additionally capable of differentiating individual species of common explosive vapors. This technique utilizes pairs of suspended microfabricated silicon bridges that can be heated in a controlled fashion. The differential thermal response of the bridges with and without adsorbed explosive vapor shows unique and reproducible characteristics depending on the nature of the adsorbed explosives. The tunable heating rate method described here is capable of providing unique signals for subnanogram quantities of adsorbed explosives within 50 ms.

Observation of an anomalous mass effect in microcantilever-based biosensing caused by adsorbed DNA

Quantifying adsorbed mass using resonance frequency variation in a microcantilever is an established technique. However, when applied to adsorbed mass determination in liquids, the resonance frequency variations represent several contributions. While the discrepancy between the apparent and real adsorbed mass is negligible for measurements in air, it can be significant in liquids. Here we present an anomalous effect of adsorbed DNA on the resonance frequency of microcantilevers which cannot be explained using current models. Our findings suggest that the measured frequency shifts may be explained on the basis of a hydrodynamic interaction caused by the adsorbed molecules on the cantilever.

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.

Receptor-free nanomechanical sensors

Nanomechanical response of molecular adsorption has been demonstrated as the basis for a number of extremely sensitive sensors. Molecular adsorption on microcantilevers results in nanomechanical motion due to adsorption-induced surface stress variation. Chemical selectivity in nanomechanical sensors is achieved by immobilizing receptors on the cantilever surface. Although receptor-based detection has high selectivity for biomolecular detection, it fails when applied to small molecule detection. Nanomechanics, however, offer new possibilities for achieving chemical selectivity that do not use any receptors. For example, small thermal mass or high temperature sensitivity of a cantilever beam could be used for detecting molecular adsorption using photothermal effects and physical property variation due to temperature. Here we describe two such techniques for achieving chemical selectivity without using any receptor molecules.