M. Latino

Photoreduction of mesoporous In2O3: mechanistic model and utility in gas sensing.

A model is proposed for the drop in electronic resistance of n-type semiconducting indium oxide (In(2)O(3)) upon illumination with light (350 nm, 3.5 eV) as well as for the (light-enhanced) sensitivity of In(2)O(3) to oxidizing gases. Essential features of the model are photoreduction and a rate-limiting oxygen-diffusion step. Ordered, mesoporous In(2)O(3) with a high specific surface area serves as a versatile system for experimental studies. Analytical techniques comprise conductivity measurements under a controlled atmosphere (synthetic air, pure N(2)) and temperature-resolved in-situ Fourier transform infrared (FTIR) spectroscopy. IR measurements reveal that oxygen vacancies form a donor level 0.18 eV below the conduction band.

CdO-based nanostructures as novel CO2gas sensors

Crystalline Cd(OH)(2)/CdCO(3) nanowires, having lengths in the range from 0.3 up to several microns and 5-30 nm in diameter, were synthesized by a microwave-assisted wet chemical route and used as a precursor to obtain CdO nanostructures after a suitable thermal treatment in air. The morphology and microstructure of the as-synthesized and annealed materials have been investigated by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and thermogravimetry-differential scanning calorimetry. The change in morphology and electrical properties with temperature has revealed a wire-to-rod transformation along with a decreases of electrical resistance. Annealed samples were printed on a ceramic substrate with interdigitated contacts to fabricate resistive solid state sensors. Gas sensing properties were explored by monitoring CO(2) in synthetic air in the concentration range 0.2-5 v/v% (2000-50 000 ppm). The effect of annealing temperature, working temperature and CO(2) concentration on sensing properties (sensitivity, response/recovery time and stability) were investigated. The results obtained demonstrate that CdO-based thick films have good potential as novel CO(2) sensors for practical applications.

Real-time monitoring of breath ammonia during haemodialysis: use of ion mobility spectrometry (IMS) and cavity ring-down spectroscopy (CRDS) techniques

BACKGROUND The diffusion of high-performance analytical technology has opened prospects for breath diagnosis as a non-invasive diagnostic tool. In this study, ion mobility spectrometry (IMS) and cavity ring-down spectroscopy (CRDS) techniques were used to analyse ammonia gas (NH3) in real-time in breath from patients undergoing haemodialysis (HD) treatment and any correlation with blood urea nitrogen (BUN) levels and Kt/V were investigated. METHODS We studied 20 patients on intermittent HD treatment. The first breath samples were taken before the start of dialysis and further breath samples were taken every hour during the treatment and after the end of the session. An evaluation was also made of 20 healthy volunteers, acting as controls [healthy subjects (HS)]. RESULTS Breath ammonia concentrations were higher in CRDS-HD (914.5±301.4 versus 280±120 parts per billion (p.p.b.), P<0.0001) and IMS-HD patients (964.4±402.4 versus 280±120 p.p.b., P<0.0001) than in HS. We assessed real-time variations in the levels of NH(3) and showed a continuous decrease in the levels of NH3. Expired NH3 correlated directly with BUN levels, both in the IMS-HD (P=0.002; r=0.84; P=0.009; r=0.76) and in the CRDS-HD group (P=0.005; r=0.80; P=0.008; r=0.77), respectively, both before and at the end of dialysis. A direct correlation with Kt/V was found in both groups studied (IMS-HD: P=0.003; r=0.82; CRDS-HD: P=0.006; r=0.79). CONCLUSIONS Breath monitoring of NH3 with IMS and CRDS techniques could be useful to assess the real-time clinical status of patients during HD. By using pre-dialysis ammonia values, an approximate calculation of the Kt/Vurea ratio can be established.

Pt-TiO2/MWCNTs Hybrid Composites for Monitoring Low Hydrogen Concentrations in Air

Hydrogen is a valuable fuel for the next energy scenario. Unfortunately, hydrogen is highly flammable at concentrations higher than 4% in air. This aspect makes the monitoring of H2 leaks an essential issue for safety reasons, especially in the transportation field. In this paper, nanocomposites based on Pt-doped TiO2/multiwalled carbon nanotubes (MWCNTs) have been introduced as sensitive materials for H2 at low temperatures. Pt-TiO2/MWNTs nanocomposites with different composition have been prepared by a simple wet chemical procedure and their morphological, microstructural and electrical properties were investigated. Resistive thick-film devices have been fabricated printing the hybrid nanocomposites on alumina substrates provided with Pt interdigitated electrodes. Electrical tests in air have shown that embedding MWCNTs in the TiO2 matrix modify markedly the electrical conductivity, providing a means to decrease the resistance of the sensing layer. Pt acts as a catalytic additive. Pt-TiO2/MWNTs-based sensors were found to be sensitive to hydrogen at concentrations between 0.5 and 3% in air, satisfying the requisites for practical applications in hydrogen leak detection devices.

Gas sensing properties and p-type response of ALD TiO2 coated carbon nanotubes

Amorphous titanium dioxide-coated carbon nanotubes (CNTs) were prepared by atomic layer deposition (ALD) and investigated as sensing layers for resistive NO2 and O2 gas sensors. By varying ALD process conditions and CNT structure, heterostructures with different metal oxide grain size, morphology and coating thickness were synthesized. Higher responses were observed with homogeneous and continuous 5.5 nm thick films onto CNTs at an operating temperature of 150 °C, while CNTs decorated with either discontinuous film or TiO2 nanoparticles showed a weak response close to the one of device made of bare CNTs. An unexpected p-type behavior in presence of the target gas was also noticed, independently of the metal oxide morphology and thickness. Based on previous works, hypotheses were made in order to explain the p-type behavior of TiO2/CNT sensors.

Novel carbon nanotubes-based hybrid composites for sensing applications

In this chapter book is reported about the development and applications of carbon nanotube (CNT)-based hybrid composites material for gas sensing devices. Gas sensors are employed in many applications spanning from security, environmental and pollution monitoring, healthcare, indoor and outdoor fields. The great number of practical applications is due to their low cost, small dimensions, easiness of use and attitude for being organized in arrays. Metal oxides and conductive polymers are the conventional sensing elements in use today. Hybrid composites are materials offering new advantages and are promising candidates for the development of high performance sensor devices. Further, with the advent of new synthesis methods at the nanoscale, this let chemical gas sensors to improve their properties and figures of merit. Indeed, owing the small size and large surface to bulk area ratio of the grain nanoparticles, this results in an increased sensitivity. However, dealing with the fabrication of sensors with nanostructured materials some problems must be taken into account, such as the synthesis and deposition methods of the sensing material (finalized to optimize grain size, porosity, film thickness, etc.) and the optimization of the sensor parameters (operating temperature, sensitivity, response and recovery time). In the chapter book there will be reported the main transduction phenomena involved on the working conditions of resistive sensor devices based on hybrid composites. Then the authors will focus the attention on two composite typologies: inorganic/CNT and organic/CNT composite materials. The first topic, about inorganic-carbon nanotube composites, will deal about the development of sensing materials based on metal oxide/CNTs composites. It will be shown as it is possible to enhance gas sensing properties towards specific gas targets using CNTs as conductive media to help to transduce any adsorption/chemical reaction on the semiconducting layer into an electrical response, i.e. by means of resistive sensors. The case that will be reported, is about the development of resistive devices obtained by employing a Pt/TiO2/CNT composite as sensing layer for monitoring high hydrogen concentration in inert atmosphere at near room temperature.

A dirhodium(II,II) complex as a highly selective molecular material for ammonia detection: QCM studies

The well known axial coordination capabilities of dirhodium(II,II) complexes towards Lewis bases have been exploited for the development of functional molecular materials for selective ammonia monitoring. On the basis of literature data and experimental evidence, the [Rh2(form)4] (1) (form = N,N′-di-p-tolylformamidinate anion) complex has been selected, among a series of dirhodium(II,II) derivatives, as the most suitable for this aim. By exposure to gaseous ammonia, 1 readily reacts, both in solution and in the solid state, with the gas to afford the corresponding ammonia-axial adduct. The reaction is reversible and is accompanied by a significant color change from dark yellow-green to red-brown due to the adduct formation, as confirmed by RGB, UV-vis and FT-IR spectroscopic investigations. Solid state UV/vis spectra reveal that while ammonia coordination occurs steadily, as judged by the blue shift observed for the λmax (470 for 1vs. 430 nm for the adduct), its complete release, in free air, takes more than a week. The potentiality of 1 for the selective ammonia detection in the headspace of aqueous ammonia solutions has been examined by QCM investigations. The dynamic response obtained from such studies is consistent with a reversible binding process occurring at the layer surface, which is operating once a fraction of the analyte is “irreversibly” bound to the sensor layer. The QCM response shows an excellent linearity in a wide concentration range (1–30 wt%) without any saturation effect up to high ammonia levels and a high selectivity to ammonia among a wide range of interfering gases.

Novel sensing materials for breath analysis devices

The development of new transition metal complexes (TMCs) sensing materials, based on di-rhodium (II,II) complexes with a lantern structure, is investigated. Specifically, in this work is reported a study carried out with the Rh2(form)4 complex having the formamidinate (form = N.N-p-tolylformamidinate anion) bridging ligand in the equatorial positions. Thick films of the complex have been deposited on interdigitated alumina substrate and their electrical characteristics have been evaluated. Rh2(form)4 behaves as a p-type semiconductor, showing a large decrease in forward and reverse current in presence of ammonia vapors in air. The preliminary results reported promise a practical application of the developed devices in the breath analysis for clinical diagnostics.

A neural network approach for safety monitoring applications

In this paper a new approach for safety monitoring of dangerous gases in the industrial plants is proposed. A single artificial neural network is used for determination of the gas concentrations based on sensor array measurements, performing at the same time compensation of the temperature and humidity influence on the sensor outputs. The obtained results show good accuracy in gas concentration estimation, enabling efficient risk warning.

Sensing Properties of SnO2/CNFs Hetero-Junctions

The sensing properties of SnO2/CNFs (CNFs = carbon nanofibers) prepared by Atomic Layer Deposition (ALD) have been investigated. By means of a novel ALD approach, which was adapted from the non-hydrolytic sol–gel route, has been possible to achieve the coating of the inner and outer surface of carbon nanofibers with a highly conformal metal oxide film of controllable thickness. The characteristics of oxygen and nitrogen dioxide sensors based on the hybrid nanomaterials have been related to the formation of a p-n heterojunction at the interface between the CNFs and the SnO2 coating.