Roberto Palombari

Protonic conductivity of layered zirconium phosphonates containing -SO3H groups. II. Ac conductivity of zirconium alkyl-sulphophenyl phosphonates in the range 100–200°C, in the presence or absence of water vapour

Abstract Two zirconium alkyl-sulphophenyl phosphonates with α-layered structure were prepared and characterized by TG, XRD and impedance measurements. Their formulas are: Zr(O 3 PC 6 H 4 SO 3 H) 0.85 (O 3 PC 2 H 5 ) 1.15 ·3.7H 2 O, with interlayer distance 18.5 A, and Zr(O 3 PC 6 H 4 SO 3 H) 0.97 (O 3 PCH 2 OH) 1.03 ·4.9H 2 O, with interlayer distance 19.6 A. In both cases the crystallization water is completely lost at temperatures lower than 140°C and the decomposition of the organic moieties starts above 200°C. Reproducible conductivity values were found for anhydrous zirconium ethyl-sulphophenyl phosphonate in the range 100–180°C (σ≈10 −5 S cm −1 at 180°C in dry N 2 with an activation energy of 15.2 kcal/mol). A continuous conductivity decrease was instead observed in this temperature range for zirconium hydroxymethyl-sulphophenyl phosphonate. The conductivity of both compounds is greatly enhanced by the presence of water vapour. Measurements were carried out at 100 and 140°C; the highest conductivity (σ≈10 −2 S cm −1 ) was found for Zr hydroxymethyl phosphonate at 100°C and 60% relative humidity.

All solid state hydrogen sensors based on pellicular α-zirconium phosphate as a protonic conductor

Abstract All solid state hydrogen sensors have been obtained by covering suitable solid reference electrodes by a very thin (0.01–0.1 mm) and compact pellicle of α-zirconium phosphate and then by sputtering a thin film of platinum on such a pellicle. If thin plates of metallic thallium or of interstitial hydrides (such as TiH x and ZrH x ) are employed as reference electrodes, the sensors exhibit a nernstian behaviour for H 2 in inert gas mixtures. The response is rather fast (90% in 10 s at H 2 concentration of 1000 ppm). A mixed potential is measured for H 2 in air. The sensors have been tested from room temperature until 200°C but it is likely that they can work at higher temperatures. All solid state hydrogen sensors based on pellicular α-zirconium phosphate are very easy to be realized and are very promising for obtaining very small devices.

NO Synthesis in Human Saliva

Human saliva contains nitrate that is converted into nitrite by the activity of facultative, anaerobic bacteria of the oral cavity. Nitrite can be reduced to NO in the acidic gastric milieu; some NO may also form in the mouth at acidic pH values. In this paper, we show that bacteria ( S. salivarius, S. mitis and S. bovis ) isolated from saliva, may contribute to NO production in human saliva. NO formation by bacteria occurs at neutral pH values and may contribute to the antibacterial activity of saliva.

Amperometric sensor for carbon monoxide based on solid state protonic conduction

Abstract An amperometric three electrode sensor for CO operating at room temperature and using a solid state proton conductor as an electrolyte has been developed. The sensing electrode is made of platinum black, while the reference and counter electrodes consist of TiHx sheets. A thin film of zirconium phosphate is used as a proton conductor. The minimum CO concentration which can be detected is about 1 vpm, with a response time of about 20 s for 90% of the whole response, at 100% relative humidity.The response becomes lower with decreasing relative humidity and disappears at a relative humidity as low as about 50%. Ethanol interfaces to a great extent, while methane is not approciably detected.

Use of NiO, anodically doped with Ni(III), as reference electrode for gas sensors based on proton conductors

Abstract NiO electrodes, prepared by pressing NiO and α-zirconium phosphate powders (1:1 or 1:2 in weight) were anodically doped with Ni(III). The use of these electrodes as solid references of gas sensors based on solid state protonic conductors has been investigated. A very low drift of potential (≤0.1 mV per day) and an exchange current density of the order of about 2 μA/cm 2 were found. Because of the good stability of its potential and the relatively high exchange current density, this kind of electrode can be suitably used as both reference and as counter-reference electrode in potentiometric and amperometric sensors, respectively.

Potentiometric sensor for oxygen based on O2H2 mixed potential of a composite Pt-metal hydride electrode

Abstract A very potentiometric sensor for oxygen operating at room temperature is described. This sensor is based on the variation of the oxygen-hydrogen mixed potential established at the sensing electrode when a constant activity of the hydrogen adsorbed on the catalytic metal (usually platinum) is maintained. This constant activity of the adsorbed hydrogen was obtained simply by sputtering a thin layer of platinum directly over a suitable metal hydride (ZrHx, ZrH2, NbH). Sheets of titanium covered by a layer of TiHx were used as reference electrodes, while pellicular α-zirconium phosphate was preferred as the solid protonic conductor. Preliminary experiments showed that these sensors are sensitive and very fast. However, a gradual worsening of the properties was noted, essentially due to the formation of an oxide layer at the hydride-proton conductor interface. This problem has been solved by protecting the surface of the metal hydride with a layer of palladium.

Use of solid state protonic conductors for oxygen potentiometric sensor at room temperature

Abstract The use of oxygen sensors based on oxygen conductors such as stabilized zirconia is restricted at temperatures 300°C. Oxygen sensors operating at room temperature are however required for many purposes, especially in environmental and biological analyses. We have therefore studied the realization of a simple solid state sensor for oxygen at room temperature, which utilizes a thin film of pellicular zirconium phosphate or NAFION as protonic conductor, a titanium hydride plate as solid state reference electrode and a sensing catalytic electrode of Pt. Such a sensor is based on the H 2 -O 2 mixed potential which is produced at the sensing electrode when short pulses of current are applied. Potential measurements are periodically performed after a suitable delay time forms the end of each pulse. The potential is a linear function over a large range of log P o 2 , while, in the most favourable conditions, the sensitivity limit is of a few parts per millions.

Nitric oxide in ischemic and reperfused human muscle.

BACKGROUND Biochemical events explaining the pathology of ischemia-reperfusion in the muscle are still debated. Nitric oxide (NO) has been postulated to be implicated in these phenomena, but the short half-life of this compound makes it difficult to measure. METHODS In this paper, we used an amperometric solid-sate sensor to measure NO concentrations in frozen human muscles before, during and after a period of ischemia. We also measured cytochrome oxidase activity and malondialdehyde (MDA). RESULTS NO increased during ischemia but it soon returned to normal values upon reperfusion. On the other hand, cytochrome oxidase that also decreased in ischemic muscle did not increase during the reperfusion and malondialdehyde only increased during reperfusion, indicating the occurrence of peroxidative reactions in this situation. CONCLUSIONS NO is implicated in the ischemia/reperfusion pathology, but it is difficult to relate whether this is connected to cytochrome oxidase activity and malondialdehyde formation, also modified in this ischemia-reperfusion model.

Preparation, proton transport properties and use in gas sensors of thin films of zirconium phosphate with γ-layered structure

Pellicular γ-zirconium phosphate (γ-ZP(p)), i.e. sheets made up entirely of oriented lamellae of Zr(PO4)(H2PO4)·2H2O (γ-ZP), have been obtained by filtering colloidal dispersions of exfoliated γ-ZP in water/acetone. The ac-conductivity of γ-ZP(p) and γ-ZP was measured in the temperature range 20/–20°C on samples previously conditioned at relative humidities between 90 and 5%. In both cases, the conductivity dependence on material hydration indicates the presence of a non-negligible bulk transport at low relative humidities. For each relative humidity the conductivity data have been parameterised on the basis of the Arrhenius equation. Activation energy and pre-exponential factor values suggest the presence of the same conduction mechanism in both materials. The conductivity of γ-ZP(p) measured by applying the electric field parallel to the sheets ranges from 3·10−4 to 1·10−5 S cm−1 for relative humidity decreasing from 90 to 11%, being an order of magnitude higher than that of γ-ZP. Since the pellicular and microcrystalline material have very similar surface areas (11–12 m2/g), the higher conductivity of γ-ZP(p) is mainly due to the preferred particle orientation parallel to the electric field.

Electrochemical determination of nitric oxide and of its derivatives

Nitric oxide (NO) is one of the simplest odd electron species. Furthermore, it is relatively hydrophobic, which is consistent with its role as a diffusible intracellular messenger or as an immune effector. NO is generated in biological systems and plays important roles as a regulatory molecule. The main problem in NO analysis is its extreme reactivity; in aerated water solution it is transformed into nitrite and nitrate, inactive biological forms. Moreover, it may lose an electron forming the NO(+) ion, involved in the synthesis of nitrosothiols (RSNOs). The main problems encountered in the analytical determination of free NO and of RSNOs in biological systems are the low stability and the very low concentration of these compounds. The determination of nitrates and nitrites may also be difficult when their concentration is in the nmolar range. We describe an electrochemical assay for the determination in the same sample of free NO and of its derivatives in nmolar range. Owing to its high sensitivity, the procedure could also be applied to environmental analyses.