Raul Calavia

Multifrequency interrogation of nanostructured gas sensor arrays : a tool for analyzing response kinetics

This paper presents a unique perspective on enhancing the physicochemical mechanisms of two distinct highly sensitive nanostructured metal oxide micro hot plate gas sensors by utilizing an innovative multifrequency interrogation method. The two types of sensors evaluated here employ an identical silicon transducer geometry but with a different morphological structure of the sensitive film. While the first sensing film consists of self-ordered tungsten oxide nanodots, limiting the response kinetics of the sensor-chemical species pair only to the reaction phenomena occurring at the sensitive film surface, the second modality is a three-dimensional array of tungsten oxide nanotubes, which in turn involves both the diffusion and adsorption of the gas during its reaction kinetics with the sensitive film itself. By utilizing the proposed multifrequency interrogation methodology, we demonstrate that the optimal temperature modulation frequencies employed for the nanotubes-based sensors to selectively detect hydrogen, carbon monoxide, ethanol, and dimethyl methyl phosphonate (DMMP) are significantly higher than those utilized for the nanodot-based sensors. This finding helps understand better the amelioration in selectivity that temperature modulation of metal oxides brings about, and, most importantly, it sets the grounds for the nanoengineering of gas-sensitive films to better exploit their practical usage.

Nanostructure Initiator Mass Spectrometry for tissue imaging in metabolomics: Future prospects and perspectives ☆

Mass spectrometry-based metabolomics provides a new approach to interrogate mechanistic biochemistry related to natural processes such as health and disease. Physiological and pathological conditions, however, are characterized not only by the identities and concentrations of metabolites present, but also by the location of metabolites within a tissue. Unfortunately, most relevant MS platforms in metabolomics can only measure samples in solution, therefore metabolites are typically extracted by tissue homogenization. Recent developments of imaging-MS technologies, however, have allowed particular metabolites to be spatially localized within biological tissues. In this context, Nanostructure-Initiator Mass Spectrometry (NIMS), a matrix-free technique for surface-based analysis, has proven an alternative approach for tissue imaging of metabolites. Here we review the basic principles of NIMS for tissue imaging and show applications that can complement LC/MS and GC/MS-based metabolomic studies investigating the mechanisms of fundamental biological processes. In addition, the new surface modifications and nanostructured materials herein presented demonstrate the versatility of NIMS surface to expand the range of detectable metabolites.

Fluctuation-enhanced sensing with organically functionalized gold nanoparticle gas sensors targeting biomedical applications

Detection of volatile organic compounds is a useful approach to non-invasive diagnosis of diseases through breath analysis. Our experimental study presents a newly developed prototype gas sensor, based on organically-functionalized gold nanoparticles, and results on formaldehyde detection using fluctuation-enhanced gas sensing. Formaldehyde was easily detected via intense fluctuations of the gas sensors resistance, while the cross-influence of ethanol vapor (a confounding factor in exhaled breath, related to alcohol consumption) was negligible.

A multisensor system for monitoring the quality of carbon dioxide in the beverage industry

In this work, we report for the first time that a system based on metal oxide resistive gas sensors can successfully determine the presence of trace-level pollutants in a carbon dioxide stream. The successful use of metal oxide sensors in the absence of oxygen (oxygen concentration below 15 ppm) has enabled a new type of analyzer to assess the quality of carbon dioxide, offering continuous monitoring capabilities, ruggedness, simplicity, wireless communications and low cost.

Active Control of the Surface Potential of Nanostructured Layers

The objective of this letter is to show the first results obtained with a control designed to keep the average surface potential of a nanostructured layerconstant. This condition is equivalent to keeping constant the resistivity of the layer measured at a constant reference temperature. The proposed closed-loop control achieves this objective by adequately changing the average temperature of the temperature waveforms applied to the nanostructured layer. Experiments are shown on which the control is applied to a layer made of Au-functionalized WO3 nanoneedles.

Active surface potential control in nanostructured MOX layers

The objective of the paper is to present the implementation of the first sigma-delta control of Surface Potential (SP) for nanostructured layers made of metal oxides in gas sensors. The objective of this type of controls is to improve the time response and reliability of these sensors. This is done by generating adequate temperature waveforms aiming at generating a constant surface potential in the nanostructures. By enforcing the condition constant SP the dynamics of all state variables is altered.

Identification of Tequila with an Array of ZnO Thin Films: A Simple and Cost-Effective Method

An array of ZnO thin film sensors was obtained by thermal oxidation of physical vapor deposited thin Zn films. Different conditions of the thermal treatment (duration and temperature) were applied in view of obtaining ZnO sensors with different gas sensing properties. Films having undergone a long thermal treatment exhibited high responses to low ethanol concentrations, while short thermal treatments generally led to sensors with high ethanol sensitivity. The sensor array was used to distinguish among Tequilas and Agave liquor. Linear discriminant analysis and the multilayer perceptron neural network reached 100% and 86.3% success rates in the discrimination between real Tequila and Agave liquor and in the identification of Tequila brands, respectively. These results are promising for the development of an inexpensive tool offering low complexity and cost of analysis for detecting fraud in spirits.

Compact device for CO 2 optical sensing using macroporous silicon photonic crystals

A photonic crystal based on a macroporous silicon structure has been fabricated and successfully used for the detection of carbon dioxide. In this paper, the device and the measurement results are presented. The sensing device here described uses an optical approach to the detection of the gas. The use of a photonic crystal allows creating a compact device working in the medium infrared spectral range. A 700 nm square lattice macroporous structure was fabricated by electrochemical etching, creating a photonic gap centered at the 4.2 μm, a CO2 absorption line. The obtained results rely only on the absorption spectra measurement.

Benzene gas sensor based on screen-printed PZT cantilevers

Benzene gas detections at ppm level are performed at room temperature with a piezoelectric resonant type sensor. For this purpose, self-actuated PZT cantilevers made of multi-layered Au/PZT/Au beam (8×2×0.1 mm3) partially released from an alumina substrate, are fabricated thanks to the association of screen-printing and sacrificial layer technique. Then, PZT cantilevers are functionalized with different organic (polypyrrole and polyaniline) and inorganic (active carbon and tin dioxide) coatings. The high sensitivity (tens of hertz/ppm) is achieved by using the unusual 31s longitudinal vibration mode of the PZT cantilever, unlike classical modes like transversal bending resonating mode. With the second “in-plane” 31-longitudinal mode of the PZT beam, benzene detection at sub-ppm level is expected with the organic and inorganic sensitive coatings. These very interesting results already demonstrate the potentialities of coated PZT cantilevers for gas detection and can be extended to species detection in liq...

Multi-frequency interrogation of nanostructured gas sensor arrays

In this paper, for the first time two types of highly sensitive nanostructured metal oxides are specifically designed to reveal the physicochemical mechanisms enhanced by multi-frequency interrogation. Both types of films employ identical silicon transducers but while the first one is designed for its response kinetics to be limited by gas diffusion, the second one is designed for its response kinetics to be limited by reaction. Indeed, our results employing a multi-frequency approach show that the optimal temperature modulation frequencies needed to selectively detect hydrogen are significantly higher for the second type of sensors.