Lightweight Gas Sensor Based on MEMS Pre-Concentration and Infrared Absorption Spectroscopy Inside a Hollow Fiber.

This paper reports on a compact and lightweight sensor for analysis of gases/vapors by means of a MEMS-based pre-concentrator coupled to a miniaturized infrared absorption spectroscopy (IRAS) module. The pre-concentrator was utilized to sample and trap vapors in a MEMS cartridge filled with sorbent material and to release them once concentrated by fast thermal desorption. It was also equipped with a photoionization detector for in-line detection and monitoring of the sampled concentration. The vapors released by the MEMS pre-concentrator are injected into a hollow fiber, which acts as the analysis cell of the IRAS module. The miniaturized internal volume of the hollow fiber of about 20 microliters keeps the vapors concentrated for analysis, thus allowing measurement of their infrared absorption spectrum with a signal to noise ratio high enough to identify the molecule, despite the short optical path, starting from sampled concentration in air down to parts per million. Results obtained for ammonia, sulfur hexafluoride, ethanol and isopropanol are reported to illustrate the sensor detection and identification capability. A limit of identification (LoI) of about 10 parts per million was validated in the lab for ammonia. The lightweight and low power consumption design of the sensor allowed operation onboard unmanned aerial vehicles (UAVs). The first prototype was developed within the EU Horizon 2020 project ROCSAFE for the remote assessment and forensic examination of a scene in the aftermath of industrial or terroristic accidents.

Cavitand Decorated Silica as a Selective Preconcentrator for BTEX Sensing in Air.

The monitoring of benzene and other carcinogenic aromatic volatile compounds at the ppb level requires boosting both the selectivity and sensitivity of the corresponding sensors. A workable solution is the introduction in the devices of preconcentrator units containing molecular receptors. In particular, quinoxaline cavitands (QxCav) resulted in very efficient preconcentrator materials for the BTEX in air to the point that they have been successfully implemented in a commercial sensor. In this work, we report a highly efficient quinoxaline-based preconcentrator material, in which the intrinsic adsorption capacity of the QxCav has been maximized. The new material consists of silica particles covalently coated with a suitable functionalized QxCav derivative (QxCav@SiO2). In this way, all the cavities are exposed to the analyte flux, boosting the performance of the resulting preconcentration cartridge well above that of the pure QxCav. It is noteworthy that the preconcentrator adsorption capacity is independent of the relative humidity of the incoming air.

Editorial for the Special Issue on MEMS and Microfluidic Devices for Analytical Chemistry and Biosensing.

The outbreak of the SARS-CoV-2 pandemic has made the general public aware of the breakthrough technologies which were developed in recent years for state-of-the-art biosensing, and terms such as clinical specificity and sensitivity are now widely understood [...].

Tuning the conformational flexibility of quinoxaline cavitands for complexation at the gas-solid interface.

The selectivity and efficiency of benzene and toluene uptake at the gas-solid interface by quinoxaline cavitands is strongly enhanced by partial rigidification of the receptor cavity and immobilization of the cavitand onto silica gel particles.

Compact GC-QEPAS for On-Site Analysis of Chemical Threats.

This paper reports on a compact, portable, and selective chemical sensor for hazardous vapors at trace levels, which is under development and validation within the EU project H2020 "RISEN". Starting from the prototype developed for a previous EU project, here, we implemented an updated two-stage purge and trap vapor pre-concentration system, a more compact MEMS- based fast gas-chromatographic separation module (Compact-GC), a new miniaturized quartz-enhanced photoacoustic spectroscopy (QEPAS) detector, and a new compact laser source. The system provides two-dimensional selectivity combining GC retention time and QEPAS spectral information and was specifically designed to be rugged, portable, suitable for on-site analysis of a crime scene, with accurate response in few minutes and in the presence of strong chemical background. The main upgrades of the sensor components and functional modules will be presented in detail, and test results with VOCs, simulants of hazardous chemical agents, and drug precursors will be reported and discussed.

A MEMS-Enabled Deployable Trace Chemical Sensor Based on Fast Gas-Chromatography and Quartz Enhanced Photoacousic Spectoscopy.

This paper reports on a portable selective chemical sensor for hazardous vapors at trace levels, which combines a two-stage purge and trap vapor pre-concentration system, a Micro-Electro-Mechanical-System (MEMS) based fast gas-chromatographic (FAST-GC) separation column and a miniaturized quartz-enhanced photoacoustic spectroscopy (QEPAS) detector. The integrated sensing system provides two-dimensional selectivity combining GC retention time and QEPAS spectral information, and was specifically designed to be rugged and suitable to be deployed on unmanned robotic ground vehicles. This is the first demonstration of a miniaturized QEPAS device used as spectroscopic detector downstream of a FAST-GC separation column, enabling real-world analyses in dirty environments with response time of a few minutes. The main modules of the GC/QEPAS sensor device will be described in detail together with the system integration, and successful test results will be reported and discussed.

Hyphenation of a MEMS based pre-concentrator and GC-IMS.

A micro-electro-mechanical system (MEMS) based pre-concentrator filled with a standard Tenax TA adsorbent as well as with a synthetic receptor designed to adsorb 3-hydroxy-3-methylhexanoic acid (3H3MHA), a particular metabolite only available from human beings, was adapted to a custom made ion mobility spectrometer with gas-chromatographic pre-separation (GC-IMS). This combination was compared to a traditional sample loop GC-IMS. The application of a pre-concentrator is highly beneficial for the GC-IMS as analysing technique. By variation of the adsorbed sample volume, the system can be adapted to changing sample concentration ranges easily, thus increasing sensitivity significantly. Detection limits of few hundred ppqV could be obtained in this work for eucalyptol and 3 human metabolites (benzaldehyde, 2-ethyl-1-hexanol and decanal) as exemplary analytes. Moreover, the appropriate choice of selective pre-concentration phases in the pre-concentrator enables an adaptation of sampling to the composition of the mixture. Relevant compounds in very low concentrations can be amplified by using specially designed cavitands while interfering substances could be suppressed. This was successfully demonstrated by detecting 3H3MHA, a compound exclusively available in human sweat, which can be used to locate trapped or hidden individuals.

On the potential of ion mobility spectrometry coupled to GC pre-separation - A tutorial.

In this tutorial, we want to demonstrate the significant potential of ion mobility spectrometry (IMS), an analytical technique for identification and quantification of gas-phase compounds, in particular combined to other useful analytical tools. Coupled to gas-chromatographic pre-separation (GC-IMS), selectivity can be improved significantly, thus enabling the analysis of complex, humid mixtures. In-line pre-concentration can improve sensitivity down to the ppqV range. Furthermore, the use of non-radioactive ionisation sources in the near future could gain acceptance and will avoid legal restrictions. Hence, with suitable and controlled sampling, implementation of appropriate substance and pattern databases and data evaluation software, GC-IMS as rapid, selective and sensitive analytical tool has shown its high potential for many applications in process and quality control, medicine (diagnostics, medication and therapy control), biology, safety and security. In the present tutorial, we want to demonstrate this capacity on behalf of examples from the application fields mentioned above, with particular focus on controlled sampling, pre-concentration and pre-separation as well as on data treatment and interpretation.

In Search of the Ultimate Benzene Sensor: The EtQxBox Solution.

In this work we report a comprehensive study leading to the fabrication of a prototype sensor for environmental benzene monitoring. The required high selectivity and ppb-level sensitivity are obtained by coupling a silicon-integrated concentration unit containing the specifically designed EtQxBox cavitand to a miniaturized PID detector. In the resulting stand-alone sensor, the EtQxBox receptor acts at the same time as highly sensitive preconcentrator for BTEX and GC-like separation phase, allowing for the selective desorption of benzene over TEX. The binding energies of the complexes between EtQxBox and BTX are calculated through molecular mechanics calculations. The examination of the corresponding crystal structures confirms the trend determined by computational studies, with the number of C-H···N and CH···π interactions increasing from 6 to 9 along the series from benzene to o-xylene. The analytical performances of EtQxBox are experimentally tested via SPME, using the cavitand as fiber coating for BTEX monitoring in air. The cavitand EFs are noticeably higher than those obtained by using the commercial CAR-DVB-PDMS. The LOD and LOQ are calculated in the ng/m3 range, outperforming the commercial available systems in BTEX adsorption. The desired selective desorption of benzene is achieved by applying a smart temperature program on the EtQxBox mesh, which starts releasing benzene at lower temperatures than TEX, as predicted by the calculated binding energies. The sensor performances are experimentally validated and ppbv level sensitivity toward the carcinogenic target aromatic benzene was demonstrated, as required for environmental benzene exposure monitoring in industrial applications and outdoor environment.

A supramolecular approach to sub-ppb aromatic VOC detection in air.

Micromachining technology is coupled to a selective pre-concentration material for the development of a portable sub-ppb level monitoring system for aromatic volatile organic compounds (VOC); the high sensitivity of Metal Oxide (MOX) gas sensors is combined with a supramolecular concentration unit to increase selectivity and reduce the detection limits.