Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy carrier and as a chemical reactant. There are an immense number of sensors reported in the literature for hydrogen detection and in this work these sensors are classified into eight different operating principles. Characteristic performance parameters of these sensor types, such as measuring range, sensitivity, selectivity and response time are reviewed and the latest technology developments are reported. Testing and validation of sensor performance are described in relation to standardisation and use in potentially explosive atmospheres so as to identify the requirements on hydrogen sensors for practical applications.

Hydrogen Sensors - A review

This review presents an overview of 20 years of worldwide development in the field of biosensors based on special types of surface acoustic wave (SAW) devices that permit the highly sensitive detection of biorelevant molecules in liquid media (such as water or aqueous buffer solutions). 1987 saw the first approaches, which used either horizontally polarized shear waves (HPSW) in a delay line configuration on lithium tantalate (LiTaO3) substrates or SAW resonator structures on quartz or LiTaO3 with periodic mass gratings. The latter are termed “surface transverse waves” (STW), and they have comparatively low attenuation values when operated in liquids. Later Love wave devices were developed, which used a film resonance effect to significantly reduce attenuation. All of these sensor approaches were accompanied by the development of appropriate sensing films. First attempts used simple layers of adsorbed antibodies. Later approaches used various types of covalently bound layers, for example those utilizing intermediate hydrogel layers. Recent approaches involve SAW biosensor devices inserted into compact systems with integrated fluidics for sample handling. To achieve this, the SAW biosensors can be embedded into micromachined polymer housings. Combining these two features will extend the system to create versatile biosensor arrays for generic lab use or for diagnostic purposes.

Surface acoustic wave biosensors: a review

Stille polycondensation for synthesis of functional materials.

A review of miniaturised humidity sensors is presented. Recent achievements in capacitive-, hygrometric-, gravimetric-, optical- and integrated sensors are discussed. Attention is paid to a general perspective of miniaturised humidity sensors, emphasising on integration issues and technological challenges. A table summarising most of the reviewed devices is included.

Recent achievements in miniaturised humidity sensors—a review of transduction techniques

‘Electronic nose’ systems involve various types of electronic chemical gas sensors with partial specificity, as well as suitable statistical methods enabling the recognition of complex odours. As commercial instruments have become available, a substantial increase in research into the application of electronic noses in the evaluation of volatile compounds in food, cosmetic and other items of everyday life is observed. At present, the commercial gas sensor technologies comprise metal oxide semiconductors, metal oxide semiconductor field effect transistors, organic conducting polymers, and piezoelectric crystal sensors. Further sensors based on fibreoptic, electrochemical and bi-metal principles are still in the developmental stage. Statistical analysis techniques range from simple graphical evaluation to multivariate analysis such as artificial neural network and radial basis function. The introduction of electronic noses into the area of food is envisaged for quality control, process monitoring, freshness evaluation, shelf-life investigation and authenticity assessment. Considerable work has already been carried out on meat, grains, coffee, mushrooms, cheese, sugar, fish, beer and other beverages, as well as on the odour quality evaluation of food packaging material.

‘Electronic Noses’ and Their Application to Food

This paper deals with some advances in surface acoustic wave (SAW) sensor research relating to the use of both a new chemically interactive material (metal porphyrin) and new amplifiers for low-noise SAW oscillators, which are illustrated and discussed in some detail. The trend toward SAW matrixes, implemented on silicon, required for more sensitive chemical image systems, has led to the observation of coupling effects between adjacent SAW oscillators, even if the electronics were not implemented on Si. This effect must be considered at design level in order to achieve a higher resolution value in detecting either single chemical species or chemical images.

Advances in SAW-based gas sensors

The sensitivity to relative humidity (RH) variations in the range 0%–90% of a platinum polyyne, namely poly-[1,4-dihexadecyloxy-2,5-diethynylbenzene-bis(triphenylphosphine) platinum(II)] (Pt-P-HDOB) membrane was investigated by means of a surface acoustic wave (SAW) based sensor. A thin film of polymeric membrane was spin deposited on the free surface of the device and the resulting acoustic velocity and attenuation perturbations allowed the acoustic characterization of the membrane by means of the perturbation theory. The SAW sensor was able to reveal even very low (<10% RH) humidity conditions at room temperature, with high reproducibility, repeatability and stability.

Sensitivity of a platinum-polyyne-based sensor to low relative humidity and chemical vapors

Aluminum nitride thin and thick films were grown on Al2O3(0001) substrates by reactive radio frequency-sputtering technique at 180°C. The AlN films, 0.022–6.2μm thick, were stress-free, uniform, transparent, and extremely adhesive to the substrate. Their structural properties were investigated by x-ray diffraction measurements: the resulting films were highly c-axis oriented, with a full width at half maximum of the (0002) rocking curve in the range from 1.6° to 1.0° for AlN film thickness ranging between 0.022 and 6.2μm. The crystalline quality of AlN films was extremely good even at high thicknesses, as shown by the presence of the AlN(0004) reflection and by the narrow (0.12°–0.20°) diffraction peaks. Optical measurements of the transmission in the visual and infrared region demonstrated that the AlN films have low absorption and scattering. The extinction and the absorption coefficients, α and Ke, were estimated at λ⩾600nm(α=850±50cm−1,Ke=0.0040±0.0005). The piezoelectric strain constant d33 was measu...

Structural, optical, and acoustic characterization of high-quality AlN thick films sputtered on Al2O3(0001) at low temperature for GHz-band electroacoustic devices applications

Structural, morphological and acoustic properties of AlN thick films sputtered on Si(001) and Si(111) substrates at low temperature

AlN films, 1.6–6.3 μm thick, were sputtered at 200 °C on Si(100) and Si(111) substrates. The films were crack-free, uniform, and c-axis oriented. The experimental phase velocities of surface acoustic waves (SAW) propagating in the AlN/Si structures were estimated and showed only a small discrepancy (20–40 m/s) compared to the calculated theoretical values. A SAW resonator (SAWR)-based chemical sensor, operating at about 700 MHz, was implemented on AlN/Si. The SAWR surface was covered with a polymer film sensitive to relative humidity (RH) changes, already tested for RH sensing in previous works on SAW delay lines implemented on AlN/Si and ZnO/Si and operating at about 130 MHz. The RH mass sensitivity and the detection limit of the SAWR sensor improved by 38% and by one order of magnitude, respectively, compared to the delay line-based sensors previously tested.

Cinzia Caliendo

Papers

Advances in SAW-based gas sensors

Sensitivity of a platinum-polyyne-based sensor to low relative humidity and chemical vapors

Structural, optical, and acoustic characterization of high-quality AlN thick films sputtered on Al2O3(0001) at low temperature for GHz-band electroacoustic devices applications

Structural, morphological and acoustic properties of AlN thick films sputtered on Si(001) and Si(111) substrates at low temperature

High-frequency, high-sensitivity acoustic sensor implemented on ALN/Si substrate