S. Costea

Surface Acoustic Wave devices and their sensing capabilities

Since mid 60s, Surface Acoustic Wave (SAW) devices have been using extensively in electronics for telecommunication. However, their potential use in the sensors field is only a matter of recent interest. This paper presents an overview of the sensing mechanisms that allow detection of temperature, strain (for pressure & torque), mass, conductivity (e.g. gas detection) and viscosity by means of SAW based devices. Design and technological challenges are presented, as well as solutions to overcome them in order to obtain reliable devices. The possibility of using such devices as passive wireless sensors is also discussed.

Physical properties of AlxIn1−xN thin film alloys sputtered at low temperature

In this paper, we report on the structural, optical, and electrical properties of a wide compositional range of AlxIn1−xN thin layers deposited on glass and polyethylene terephthalate substrates. AlxIn1−xN layers of controlled composition were obtained by a simple reactive magnetron co-sputtering protocol, using a single aluminium target with indium insets, by varying the Al/In target surface area ratio, and the composition of the deposition atmosphere. The relevant physical properties were investigated and discussed. It is shown that the texture of the thin films is dependent on the cation ratio, while the bowing parameters of lattice constants and band gap values are larger than those of epitaxial layers.

Selection of gas sensing materials using the Hard Soft Acid Base theory; application to Surface Acoustic Wave CO 2 detection

The Hard Soft Acid Base (HSAB) theory is introduced as a new tool to select or design sensitive materials for carbon dioxide detection with SAW-BAW (Surface Acoustic Waves - Bulk Acoustic Waves) devices. According to HSAB, CO2 is hard acid, thus small organic or inorganic molecules, or polymers which can act as hard bases could be suitable candidates as sensing layers for carbon dioxide detection. As a consequence of this theory, we propose the following polymers as potential candidates for CO2 sensing: simple polyallylamine, N-substituted polyallylamine, polydiallylamine and polyvinylamine, and mixtures of these polymers. The SAW device coated with one of the selected polymers, polyallyamine, shows good sensitivity for CO2 concentration (in the range 500–5000 ppm), long term stability and repeatability.

Amino groups-based polymers for CO 2 detection; A comparison between two sensing mechanism models

Two CO2 sensing mechanisms, based on the Hard Soft Acids Bases (HSAB) and Bronsted-Lowry theories, are discussed and compared. They are evaluated by selecting amino groups-based coating layers, which are deposited on Surface Acoustic Wave (SAW) devices for CO2 detection. Experimentally measured CO2 sensitivities of different coating layers, such as polyallylamine (PAA), polyethyleneimine (PEI), nanocomposite matrix based on PAA-aminocarbon nanotubes and PEI-aminocarbon nanotubes, emeraldine, 4-sulfocalix[4]arene-doped polyaniline, matrix based emeraldine and carbonic anhydrase (PACA) are compared and evaluated according to their corresponding sensing mechanism.

New ligand selection rule for quantum dot functionalization

Hard Soft Acid Base (HSAB) theory is introduced as a new tool to select ligands (molecules) for quantum dot covalent functionalization. According to HSAB, only soft acid-soft base bond is covalent. Since most of the transition metal semiconductor cations on the surface of the quantum dots are soft acids, the approach we propose is to select anchors which are soft bases. Following this strategy, the suitable anchors for surface modification of the quantum dots are selected. Moreover, a plethora of bifunctional ligands for assembling quantum dots onto the surface of titania are identified among the existing molecules. The functionalization of a semiconducting organic polymer backbone with HSAB-adequate anchors for the design of polymer-quantum dot hybrid interface is presented. In addition, our new approach coherently explains the heuristic approaches described in the literature.

Pyrene-1-butyric acid-doped polyaniline for fluorescence quenching-based oxygen sensing

The synthesis of pyrene-1-butyric acid (PBA)-doped polyaniline (PANI) and its oxygen sensing properties, through fluorescence quenching, are reported. The structures of both undoped PANI (emeraldine) and PBA-doped PANI are investigated by means of Fourier Transform InfraRed (FT-IR) spectroscopy. The O2 sensing capability of the synthesized layer is demonstrated through fluorescence spectroscopy performed at different air pressure values. PBA-doped PANI is expected to lead to a fluorophore with better stability and reliability than free PBA. This could bring real benefits to the overall functioning of industrial O2 sensors based on fluorescence quenching.

Structural health monitoring using an array of spatially distributed sensors

This paper presents the use of an array of spatially distributed sensors for the development of a complete system designed for structural health monitoring. The system includes wireless and wired sensors and has a low cost implementation. We have build a reduced size system for the proof of concept, using a Resistive Temperature Detector with Infra-red connectivity, monitored by a Local Station communicating to a Central Station, allowing the characterization of an entire structure. If implemented in a campus of buildings, the Local Stations collect data from the sensors and send it to the Central Station. When risks arise, the closest Local Station and the Central Station trigger alarms, depending on the degree of danger. The administrator of the building is sent an e-mail message, letting him know the type of the alarm that was triggered and the location of the danger.All the tasks mentioned above are automated using dedicated software applications developed in LabView and C#. A commercially available software package is provided by National Instruments [1], but it was developed with concern to wireless sensors. In addition to that one, we adapted our system so that it could also integrate wired sensors.