Metal Oxide Chemiresistors: A Structural and Functional Comparison between Nanowires and Nanoparticles.

Metal oxide nanowires have become popular materials in gas sensing, and more generally in the field of electronic and optoelectronic devices. This is thanks to their unique structural and morphological features, namely their single-crystalline structure, their nano-sized diameter and their highly anisotropic shape, i.e., a large length-to-diameter aspect ratio. About twenty years have passed since the first publication proposing their suitability for gas sensors, and a rapidly increasing number of papers addressing the understanding and the exploitation of these materials in chemosensing have been published. Considering the remarkable progress achieved so far, the present paper aims at reviewing these results, emphasizing the comparison with state-of-the-art nanoparticle-based materials. The goal is to highlight, wherever possible, how results may be related to the particular features of one or the other morphology, what is effectively unique to nanowires and what can be obtained by both. Transduction, receptor and utility-factor functions, doping, and the addition of inorganic and organic coatings will be discussed on the basis of the structural and morphological features that have stimulated this field of research since its early stage.

A Statistical Analysis of Response and Recovery Times: The Case of Ethanol Chemiresistors Based on Pure SnO2.

Response and recovery times are among the most important parameters for gas sensors. Their optimization has been pursued through several strategies, including the control over the morphology of the sensitive material. The effectiveness of these approaches is typically proven by comparing different sensors studied in the same paper under the same conditions. Additionally, tables comparing the results of the considered paper with those available in the literature are often reported. This is fundamental to frame the results of individual papers in a more general context; nonetheless, it suffers from the many differences occurring at the experimental level between different research groups. To face this issue, in the present paper, we adopt a statistical approach to analyze the response and recovery times reported in the literature for chemiresistors based on pure SnO2 for ethanol detection, which was chosen as a case study owing to its available statistic. The adopted experimental setup (of the static or dynamic type) emerges as the most important parameter. Once the statistic is split into these categories, morphological and sensor-layout effects also emerge. The observed results are discussed in terms of different diffusion phenomena whose balance depends on the testing conditions adopted in different papers.

Monitoring Fish Freshness in Real Time under Realistic Conditions through a Single Metal Oxide Gas Sensor.

The realization of an unobtrusive and effective technology able to track fish freshness in real time and inform on its edibility is highly demanded, but still unachieved. In the present paper, we address this issue through a single metal oxide gas sensor working in temperature modulation mode. The system can work without an external reference air source, which is an appealing feature for its possible integration in domestic refrigerators. Tests were carried out using fresh sea bream fillets as case study and working both inside the refrigerator and at room temperature. Parallel gas chromatography-mass spectrometry and microbiological characterization indicated the marked dependence of both the microbiological condition and the gas-phase composition from the individual sample and from the storage temperature. Despite such a large variability, which may be expected in real applications, the proposed system provided similar responses whenever the total bacterial population approached and exceeded the edibility threshold of 107 CFU/g.

Morphological Effects in SnO2 Chemiresistors for Ethanol Detection: A Review in Terms of Central Performances and Outliers.

SnO2 is one of the most studied materials in gas sensing and is often used as a benchmark for other metal oxide-based gas sensors. To optimize its structural and functional features, the fine tuning of the morphology in nanoparticles, nanowires, nanosheets and their eventual hierarchical organization has become an active field of research. In this paper, the different SnO2 morphologies reported in literature in the last five years are systematically compared in terms of response amplitude through a statistical approach. To have a dataset as homogeneous as possible, which is necessary for a reliable comparison, the analysis is carried out on sensors based on pure SnO2, focusing on ethanol detection in a dry air background as case study. Concerning the central performances of each morphology, results indicate that none clearly outperform the others, while a few individual materials emerge as remarkable outliers with respect to the whole dataset. The observed central performances and outliers may represent a suitable reference for future research activities in the field.

Metal Oxide Gas Sensors, a Survey of Selectivity Issues Addressed at the SENSOR Lab, Brescia (Italy).

This work reports the recent results achieved at the SENSOR Lab, Brescia (Italy) to address the selectivity of metal oxide based gas sensors. In particular, two main strategies are being developed for this purpose: (i) investigating different sensing mechanisms featuring different response spectra that may be potentially integrated in a single device; (ii) exploiting the electronic nose (EN) approach. The former has been addressed only recently and activities are mainly focused on determining the most suitable configuration and measurements to exploit the novel mechanism. Devices suitable to exploit optical (photoluminescence), magnetic (magneto-optical Kerr effect) and surface ionization in addition to the traditional chemiresistor device are here discussed together with the sensing performance measured so far. The electronic nose is a much more consolidated technology, and results are shown concerning its suitability to respond to industrial and societal needs in the fields of food quality control and detection of microbial activity in human sweat.

Does routine completion angiogram during embolectomy for acute upper-limb ischemia improve outcomes?

BACKGROUND: Since 1963, Fogarty balloon catheter thromboembolectomy is usually adopted as the gold standard treatment for acute limb ischemia. As the success of the procedure depends on complete removal of all thromboembolic material, intraoperative arteriography can be used after arterial thromboembolectomy as a guide for extension of the procedure. It is still a matter of debate whether intraoperative angiography should be routinely performed in all cases or only in selected cases, depending on intraoperative findings, when the surgeon suspects an incomplete disobstruction. Most published evidence derives from analysis of lower-limb thromboembolectomies. The aim of our retrospective study was to elucidate the value of routine completion angiogram in acute arterial embolism of the upper limb. METHODS: Clinical and demographic data of 100 patients with acute embolic upper-limb ischemia were prospectively recorded during an 18-year period in a central hospital vascular unit setting. The relevance of intraoperative angiography was retrospectively analyzed. The procedures were divided into two groups: group A, when intraoperative angiography was performed in selected cases (selective angiography); and group B, when angiography was performed as a routine procedure in all cases (routine angiography). All factors associated with reocclusion and mortality were investigated to produce meaningful information that could assist the surgeon to predict outcomes. RESULTS: Cumulative reocclusion and mortality rates at 24 months were 14.0% and 70.0%, respectively. After upper-limb arterial embolectomy, the rate of extension of the procedure was significantly higher in group B than in group A (26.0% vs. 4.0%, P = 0.002). At 24 months after embolectomy, group B resulted in a lower incidence of reocclusion compared with group A (12.0% vs. 2.0%, P = 0.05), whereas there was no statistical difference between the two groups in terms of mortality (P > 0.05). On univariate analysis, the factor associated with increased 2-year reocclusion rate was only the avoidance of completion angiography, although it lost some of its predictive value on multivariate analysis. Factors associated with increased 2-year mortality rate on univariate analysis included age >80 years, diabetes mellitus [DM], and antiplatelet drug use. Only DM was significantly associated on multivariate analysis. CONCLUSION: Routine use of intraoperative angiography influences outcome after embolectomy for upper-limb acute arterial occlusion. Routine use of intraoperative angiography, compared with selective use, results in a higher rate of extension of the procedure for residual lesion and in a lower rate of reocclusion at 24 months. In prevention of reocclusion, completion angiogram has a hazard ratio of 5.44 on multivariate analysis. Postoperative late mortality is mainly affected by old age and DM.

Nanostructured metal oxide gas sensors, a survey of applications carried out at SENSOR lab, Brescia (Italy) in the security and food quality fields.

In this work we report on metal oxide (MOX) based gas sensors, presenting the work done at the SENSOR laboratory of the CNR-IDASC and University of Brescia, Italy since the 80s up to the latest results achieved in recent times. In particular we report the strategies followed at SENSOR during these 30 years to increase the performance of MOX sensors through the development of different preparation techniques, from Rheotaxial Growth Thermal Oxidation (RGTO) to nanowire technology to address sensitivity and stability, and the development of electronic nose systems and pattern recognition techniques to address selectivity. We will show the obtained achievement in the context of selected applications such as safety and security and food quality control.

Bovine Serum Albumin protofibril-like aggregates formation: solo but not simple mechanism.

We report an experimental study on the model protein Bovine Serum Albumin (BSA), with the aim of elucidating the mechanisms by which a fully folded globular protein undergoes different aggregation pathways leading to the formation of amyloid fibrils or amorphous aggregates. We observe thermally induced formation of fibrillar structures at pH far from the protein isoelectric point. The increase of electrostatic repulsion results in protein destabilization and in modifications of inter and intra-molecular interactions leading to the growth of fibril-like aggregates stabilized by inter-molecular-ß sheets. The aggregation kinetics is studied by means of fluorescence techniques, light scattering, Circular Dichroism (CD), infrared spectroscopy (FTIR) and Atomic Force Microscopy (AFM). Changes in protein secondary structures turn out to be the driving mechanism of the observed aggregation and they progress in parallel with the growth of Thioflavin T emission intensity and scattering signal. This concurrent behavior suggests a mutual stabilization of elongated protofibril-like structures and of protein conformational and structural changes, which lead to a more rigid and ordered structures. Our results give new insights on BSA self-assembly process in alkaline conditions clearly providing new pieces of evidences of the interplay of several and interconnected mechanisms occurring on different time and length scales.

Response Times of Metal-Oxide Chemiresistors: Comparison Between the Isothermal and Temperature Modulation Modes.

The response time is one of the main functional parameter for gas sensors, including metal oxide chemiresistors. This parameter is widely investigated for devices working in isothermal mode but it is much less investigated for gas sensors working in temperature modulation mode. In this work, considering ethanol as target gas, we compare the response times of a metal oxide chemiresistor working according to these two modes. In order to compare them, we worked with nearly the same average temperature in both cases, supplying a constant voltage to the heater in the isothermal mode and a squared voltage wave in temperature modulation. Our results show that, depending on the average working temperature, one mode or the other may be faster. At high average working temperature, the response time recorded with the isothermal mode is shorter than the thermal-period of the temperature modulation mode. Lowering the average working temperature, the response time increases for both modes, but the increase is more marked for the isothermal mode, which become slower than the temperature modulation one.

High-Pressure Synthesis and Gas-Sensing Tests of 1-D Polymer/Aluminophosphate Nanocomposites.

Recently, filling zeolites with gaseous hydrocarbons at high pressures in diamond anvil cells has been carried out to synthesize novel polymer-guest/zeolite-host nanocomposites with potential, intriguing applications, although the small amount of materials, 10-7 cm3, severely limited true technological exploitation. Here, liquid phenylacetylene, a much more practical reactant, was polymerized in the 12 Å channels of the aluminophosphate Virginia Polytechnic Institute-Five (VFI) at about 0.8 GPa and 140 °C, with large volumes in the order of 0.6 cm3. The resulting polymer/VFI composite was investigated by synchrotron X-ray diffraction and optical and 1H, 13C, and 27Al nuclear magnetic resonance spectroscopy. The materials, consisting of disordered π-conjugated polyphenylacetylene chains in the pores of VFI, were deposited on quartz crystal microbalances and tested as gas sensors. We obtained promising sensing performances to water and butanol vapors, attributed to the finely tuned nanostructure of the composites. High-pressure synthesis is used here to obtain an otherwise unattainable true technological material.

Direct integration of metal oxide nanowires into an effective gas sensing device.

A simple and large-area scalable methodology has been set up for direct integration of metal oxide nanowire bundles into a functional device for gas sensing. It is based on sequential application of two consolidated techniques, namely high temperature vapour transport and condensation for fabrication of metal oxide nanowires, and wet etching of a sacrificial layer. The alumina substrate patterned with a silicon dioxide sacrificial layer does not influence the growth of nanowires and remains unaltered under the high temperature process. The sacrificial layer is finally removed under hydrofluoric acid, the metal oxide nanowires do not suffer modifications and a clean substrate surface can be obtained for deposition of stable metal contacts. The methodology was proven effective for application in a gas sensor device. Electrical measurements indicate that a slightly rectifying Schottky junction is present at low temperatures (up to T = 150 degrees C) between nanowires and platinum electrodes, which vanishes as the temperature increases and under high voltage (bias voltage above approximately 3 V). The results foresee the possibility of growth and integration of nanowire bundles directly into devices, overcoming the need for expensive and time-consuming nanomanipulation techniques.

The posterior approach in the treatment of popliteal artery aneurysm: feasibility and analysis of outcome.

BACKGROUND: This review evaluates the results of our 18-year experience with surgical treatment of popliteal artery aneurysms (PAAs), examining the effects of the variables of clinical presentations, surgical technique, graft material, and runoff on operative results in the management of popliteal aneurysms. METHODS: We reviewed 49 PAAs consecutively repaired in 35 patients. We preferentially use, if possible, the posterior approach for repair of popliteal aneurysms. We repaired aneurysms passing above the Hunter canal using a medial approach to allow for adequate exposure of the proximal neck of the aneurysm. We separately analyzed the results of patients who underwent the posterior approach (group A) and those that underwent the medial approach (group B). Primary, primary assisted, and secondary patency were established using life-tables analysis. RESULTS: In our experience, the posterior approach was used in 38 repairs (77.6%), followed by graft interposition (group A). PAAs were asymptomatic in 29 (59.2%) of 49 cases. Among 20 symptomatic PAAs, nine (18.4%) caused intermittent claudication, one (2.0%) caused rest pain and trophic wound, and the remaining 10 limbs (20.4%) presented with acute ischemia and limb threat. A total of 11 popliteal aneurysms (22.4%) required repair with a medial approach (group B) because the extension of the aneurysm was proximal to the adductor hiatus. The primary patency rates at 6 and 8 years were 94.3 and 83.8%, respectively, for group A and 100% (p = .43) and 19.1% (p = .001) for group B, the respective assisted primary patency rates were 97.3 and 86.3%, in group A and 100% (p = .43) and 19.1% (p = .001) for group B. The secondary patency rates at 6 months and 8 years were 97.3 and 97.3%, respectively, in group A and 90.9% (p = .34) and 77.9% (p = .05) in group B. Amputation occurred in two (4.1%) of 49 limbs and 30-day postoperative mortality was 2.0% (1/49 patients). There was no statistical difference in amputation rate in symptomatic and asymptomatic limbs, and in group A and B. CONCLUSION: We believe that the posterior approach is the gold standard surgical therapy to treat PPAs not extending above the Hunter canal. In our experience, the posterior approach was possible in 77.6% of cases. It has excellent patency and prevents further aneurysm expansion by completely interrupting the collateral circulation to the aneurysm sac. In contrast, the posterior approach had a slightly higher tibial nerve injury (p = .43), especially during the learning curve. The preoperative symptoms and the use of venous material for reconstruction affect significantly long-term patency.

Cellulose Fibers Enable Near-Zero-Cost Electrical Sensing of Water-Soluble Gases.

We report an entirely new class of printed electrical gas sensors that are produced at near "zero cost". This technology exploits the intrinsic hygroscopic properties of cellulose fibers within paper; although it feels and looks dry, paper contains substantial amount of moisture, adsorbed from the environment, enabling the use of wet chemical methods for sensing without manually adding water to the substrate. The sensors exhibit high sensitivity to water-soluble gases (e.g., lower limit of detection for NH3 < 200 parts-per-billion) with a fast and reversible response. The sensors show comparable or better performance (especially at high relative humidity) than most commercial ammonia sensors at a fraction of their price (<$0.02 per sensor). We demonstrate that the sensors proposed can be integrated into food packaging to monitor freshness (to reduce food waste and plastic pollution) or implemented into near-field-communication tags to function as wireless, battery-less gas sensors that can be interrogated with smartphones.

Investigation of Reduced Graphene Oxide and a Nb-Doped TiO2 Nanotube Hybrid Structure To Improve the Gas-Sensing Response and Selectivity.

The precise detection of flammable and explosive gases and vapors remains an important issue because of the increasing demand for renewable energy sources and safety requirements in industrial processes. Metal oxides (TiO2, SnO2, ZnO, etc.) are very attractive materials for the manufacturing of chemical gas sensors. However, their gas selectivity issues and further improvement in the sensing response remain a significant challenge. The incorporation of metal oxides with two-dimensional (2D) graphene oxide (GO) is considered to be a promising approach to obtaining hybrid structures with improved gas-sensing performance. Herein, we report the development of GO and niobium-doped titanium dioxide nanotube (NT) hybrid structures with a tunable selectivity and sensing response against hydrogen gas, achieved by properly controlling the degree of reduction and concentration of GO. The effects of these parameters are systematically studied in terms of the response amplitude and selectivity. It was found that, compared to undoped titanium dioxide nanotubes, the hybrid material with an optimal concentration of reduced-GO and the introduction of niobium shows an increase in hydrogen response of about an order of magnitude and a simultaneous reduction of the response to possible interfering compounds such as carbon monoxide and acetone, thus providing enhanced selectivity. This research may provide an efficient way to enhance the chemical sensing performance of metal oxide nanomaterials.

Sensitivity-Selectivity Trade-Offs in Surface Ionization Gas Detection.

Surface ionization (SI) provides a simple, sensitive, and selective method for the detection of high-proton affinity substances, such as organic decay products, medical and illicit drugs as well as a range of other hazardous materials. Tests on different kinds of SI sensors showed that the sensitivity and selectivity of such devices is not only dependent on the stoichiometry and nanomorphology of the emitter materials, but also on the shape of the electrode configurations that are used to read out the SI signals. Whereas, in parallel-plate capacitor devices, different kinds of emitter materials exhibit a high level of amine-selectivity, MEMS (micro-electro-mechanical-systems) and NEMS (nanowire) versions of SI sensors employing the same kinds of emitter materials provide significantly higher sensitivity, however, at the expense of a reduced chemical selectivity. In this paper, it is argued that such sensitivity-selectivity trade-offs arise from unselective physical ionization phenomena that occur in the high-field regions immediately adjacent to the surfaces of sharply curved MEMS (NEMS) emitter and collector electrodes.

Finely Tuned SnO2 Nanoparticles for Efficient Detection of Reducing and Oxidizing Gases: The Influence of Alkali Metal Cation on Gas-Sensing Properties.

Tin dioxide (SnO2) nanoparticles were straightforwardly synthesized using an easily scaled-up liquid route that involves the hydrothermal treatment, either under acidic or basic conditions, of a commercial tin dioxide particle suspension including potassium counterions. After further thermal post-treatment, the nanomaterials have been thoroughly characterized by Fourier transform infrared and Raman spectroscopy, powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and nitrogen sorption porosimetry. Varying pH conditions and temperature of the thermal treatment provided cassiterite SnO2 nanoparticles with crystallite sizes ranging from 7.3 to 9.7 nm and Brunauer-Emmett-Teller surface areas ranging from 61 to 106 m2·g-1, acidic conditions favoring potassium cation removal. Upon exposure to a reducing gas (H2, CO, and volatile organic compounds such as ethanol and acetone) or oxidizing gas (NO2), layers of these SnO2 nanoparticles led to highly sensitive, reversible, and reproducible responses. The sensing results were discussed in regard to the crystallite size, specific area, valence band energy, Debye length, and chemical composition. Results highlight the impact of the counterion residuals, which affect the gas-sensing performance to an extent much higher than that of size and surface area effects. Tin dioxide nanoparticles prepared under acidic conditions and calcined in air showed the best sensing performances because of lower amount of potassium cations and higher crystallinity, despite the lower surface area.

A composite structure based on reduced graphene oxide and metal oxide nanomaterials for chemical sensors.

A hybrid nanostructure based on reduced graphene oxide and ZnO has been obtained for the detection of volatile organic compounds. The sensing properties of the hybrid structure have been studied for different concentrations of ethanol and acetone. The response of the hybrid material is significantly higher compared to pristine ZnO nanostructures. The obtained results have shown that the nanohybrid is a promising structure for the monitoring of environmental pollutants and for the application of breath tests in assessment of exposure to volatile organic compounds.

A novel electronic nose as adaptable device to judge microbiological quality and safety in foodstuff.

This paper presents different applications, in various foodstuffs, by a novel electronic nose (EN) based on a mixed metal oxide sensors array composed of thin films as well as nanowires. The electronic nose used for this work has been done, starting from the commercial model EOS835 produced by SACMI Scarl. The SENSOR Lab (CNR-INO, Brescia) has produced both typologies of sensors, classical MOX and the new technologies with nanowire. The aim of this work was to test and to illustrate the broad spectrum of potential uses of the EN technique in food quality control and microbial contamination diagnosis. The EN technique was coupled with classical microbiological and chemical techniques, like gas chromatography with mass spectroscopy (GC-MS) with SPME technique. Three different scenarios are presented: (a) detection of indigenous mould in green coffee beans, (b) selection of microbiological spoilage of Lactic Acid Bacteria (LAB), and (c) monitoring of potable water. In each case, the novel EN was able to identify the spoiled product by means of the alterations in the pattern of volatile organic compounds (VOCs), reconstructed by principal component analysis (PCA) of the sensor responses. The achieved results strongly encourage the use of EN in industrial laboratories. Finally, recent trends and future directions are illustrated.

Functionalised zinc oxide nanowire gas sensors: Enhanced NO(2) gas sensor response by chemical modification of nanowire surfaces.

Surface coating with an organic self-assembled monolayer (SAM) can enhance surface reactions or the absorption of specific gases and hence improve the response of a metal oxide (MOx) sensor toward particular target gases in the environment. In this study the effect of an adsorbed organic layer on the dynamic response of zinc oxide nanowire gas sensors was investigated. The effect of ZnO surface functionalisation by two different organic molecules, tris(hydroxymethyl)aminomethane (THMA) and dodecanethiol (DT), was studied. The response towards ammonia, nitrous oxide and nitrogen dioxide was investigated for three sensor configurations, namely pure ZnO nanowires, organic-coated ZnO nanowires and ZnO nanowires covered with a sparse layer of organic-coated ZnO nanoparticles. Exposure of the nanowire sensors to the oxidising gas NO(2) produced a significant and reproducible response. ZnO and THMA-coated ZnO nanowire sensors both readily detected NO(2) down to a concentration in the very low ppm range. Notably, the THMA-coated nanowires consistently displayed a small, enhanced response to NO(2) compared to uncoated ZnO nanowire sensors. At the lower concentration levels tested, ZnO nanowire sensors that were coated with THMA-capped ZnO nanoparticles were found to exhibit the greatest enhanced response. ΔR/R was two times greater than that for the as-prepared ZnO nanowire sensors. It is proposed that the ΔR/R enhancement in this case originates from the changes induced in the depletion-layer width of the ZnO nanoparticles that bridge ZnO nanowires resulting from THMA ligand binding to the surface of the particle coating. The heightened response and selectivity to the NO(2) target are positive results arising from the coating of these ZnO nanowire sensors with organic-SAM-functionalised ZnO nanoparticles.