M. Urbiztondo

Explosives detection using nanoporous coatings

Zeolite-coated cantilevers provided with internal heating elements have been developed and used for the selective detection of nitroderivates, in particular o-nitrotoluene as an example of an explosive-related molecule. In particular, Co exchanged commercial BEA zeolites have been deployed of rectangular Si cantilevers by microdropping technique. In particular, two different strategies have been demonstrated to increase the zeolite modified cantilevers performance: the sensing coating and the operating temperature. As a result, o-nitrotoluene LOD values below 1 ppm are attained at room temperature conditions; whereas the interference of toluene at concentrations below 1000 ppm is completely suppressed by heating the cantilever.

Gas Sensing with Silicon-Based Nanoporous Solids

Publisher Summary This chapter examines how Si-containing nanoporous solids can be used to improve the performance of gas sensors. The two most important qualities of a gas sensor are its sensitivity and its selectivity. In view of the molecular recognition properties that nanoporous solids can afford, the strategy followed by many research groups to increase sensor selectivity involves the use of nanoporous solids as added elements on already existing platforms of sufficient sensitivity. The added nanoporous materials may be in the form of isolated entities or be a fully grown film completely coating the sensor surface. Materials containing micro- and mesopores, such as silicon-based nanoporous solids, are attracting substantial interest in the gas sensing field. Considerable improvements of sensor selectivity have been achieved through the deployment of (often functionalized) Si-based films as targets for the desired gas phase analytes, or as barriers to avoid the effect of interfering molecules. Adding to this are the considerable refinements occurred during the last decade on the synthesis of porous silicon-based films. The full potential of hybrid materials comprising organic (or bioorganic) functional groups and nanoporous inorganic structures has not yet been realized in the sensor field. In the case of Si-based materials such as silica and zeolites, the porous structure of the solid combined with the rich chemistry related to the presence of hydroxyl surface groups will allow the hosting/grafting of specific molecules, opening up a wealth of sensing possibilities.

Fast microwave synthesis of Pt-MFI zeolite coatings on silicon micromonoliths: application to VOC catalytic combustion

Abstract This paper describes the preparation of a ZSM-5 zeolite layer onto conventional Si wafers and Si micromonoliths, by using microwave-assisted seeded hydrothermal synthesis. A homogeneous ZSM-5 zeolite layer up to 680 nm was obtained over the external surface of a Si micromonolith (surface to volume ratio >300,000 m-1) after only 4 min of hydrothermal synthesis at 453 K. As a proof of concept, the catalytic activity of the prepared ZSM-5 coated micromonoliths, after a conventional Pt ion-exchange process, was demonstrated in the combustion of traces of n-hexane (200 ppmV) in air.

Explosives Detection by array of Si μ-cantilevers coated with titanosilicate type nanoporous materials

An array comprising four Si μ-cantilevers coated with nanoporous functionalized ETS-10 crystals sub-micrometric in size has been developed as a multisensing platform for explosives recognition in vapor phase. The detection capabilities of the proposed device have been tested for common taggants [such as 1-methyl-2-nitro-benzene (o-MNT)] and explosives (commercial detonation cord, a plastic tube filled with pentaerythritol tetranitrate (PETN); and C-4, a mixture of cyclotrimethylenetrinitramine (RDX), binders and plastifiers). The general strategy for the detection of explosives in vapor phase is based on the characteristic fingerprint each one produces as a result of the dissimilar chemical interactions between the ETS-10 coated μ-cantilevers and the target molecules emanating from the explosives and swept by ambient air. A portable lock-in amplifier has been implemented to exploit the truly benefits of the array in terms of portability, reduced size, and energy consumption. Such low-power electronic interface is capable of creating the excitation signal as well as obtaining the response values of four resonating μ-cantilevers simultaneously. The resulting sensing platform has successfully been applied for the o-MNT, PETN, and RDX detection at trace level.

Portable lock-in amplifier for microcantilever based sensor array. Application to explosives detection using Co-BEA type zeolites as sensing materials

Recent advances in microcantilever-based sensors have led to a significant increase in sensitivity, making them a competitive solution in highly demanding applications as explosives detection. However, these sensors face severe challenges related to: reliability, sensitivity, reproducibility and throughput; that have yet to be solved for commercial applications. This paper describes our efforts in this direction, particularly on the reproducible detection of nitroaromatic type explosives by means of parallelization combined with: i) nanoporous solids as sensing materials; and, ii) a portable low-power electronic readout interface capable of both excitation and measurement of the multisensing platform. The response of the sensor array, comprising 4 microcantilevers, due to presence of 2-nitrotoluene, a common explosive taggant, has been properly monitored. The obtained results with 4 identical Co-BEA coated Si microcantilevers underline the importance of a proper sensing material degassing on the sensor performance.

Zeolite based microconcentrators for volatile organic compounds sensing at trace-level: fabrication and performance

A novel 6-step microfabrication process is proposed in this work to prepare microfluidic devices with integrated zeolite layers. In particular, microfabricated preconcentrators designed for volatile organic compounds (VOC) sensing applications are fully described. The main novelty of this work is the integration of the pure siliceous MFI type zeolite (silicalite-1) polycrystalline layer, i.e. 4.0 ± 0.5 μm thick, as active phase, within the microfabrication process just before the anodic bonding step. Following this new procedure, Si microdevices with an excellent distribution of the adsorbent material, integrated resistive heaters and Pyrex caps have been obtained. Firstly, the microconcentrator performance has been assessed by means of the normal hexane breakthrough curves as a function of sampling and desorption flowrates, temperature and micropreconcentrator design. In a step further, the best preconcentrator device has been tested in combination with downstream Si based microcantilevers deployed as VOC detectors. Thus, a preliminar evaluation of the improvement on detection sensitivity by silicalite-1 based microconcentrators is presented.

Grace: Development of Pd-Zeolite Composite Membranes for Hydrogen Production by Membrane Reactor

This chapter presents the development of Pd-zeolite composite membranes for hydrogen production by membrane reactor. Pd-zeolite composite membranes are prepared over the external surface of macroporous α-alumna tubular supports by secondary growth of zeolite layers followed by Pd modification. Pd nanoparticles filtration and/or impregnation + in situ reduction of an organic Pd precursor are explored as deposition techniques devoted to enhance the H 2 separation performance of the non-defect free A-type zeolite membranes. The Pd deposition aims toward the partial blockage of the non-selective intercrystalline pathways, which may account for a significant fraction of the total permeation flux. The Pd-zeolite composite substrates are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and EDX. The study of the permeation properties of these substrates for single (N 2 ) and binary mixtures (H 2 –CO 2 ) before and after Pd modification reveals some improvements in terms of H 2 separation performance. The impregnation + in situ reduction of palladium acetylacetonate solution (Pd(acac) 2 ) carried out over KA zeolite membranes previously seeded with Pd nanoparticles appears as the most adequate among the tested methods. Separation factors for H 2 -CO 2 binary mixtures up to 145 have been achieved, although further optimization is required to improve the H 2 permeation fluxes (around 10 −8 mol H 2 /m 2 s Pa).