Nickel oxide electrodes sensitivity

Nickel oxide has also been used as the electrode material for NO sensors [264—274]. One of the advantages of NiO electrodes is the improved sensor response at high temperatures. Figure 13.23 shows the sensitivities of XO, sensors with WO3 [254—259, 262] or NiO [264—269] electrodes. As shown above for CO sensors, the responses of sensors vhth WO3 electrodes decrease as the temperature is increased above 600 °C. The responses of sensors with NiO electrodes, however, remain high up to operating temperatures of900 °C. The response of NiO electrodes can be increased even further with the addition of ruthenium [268]. 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

Defects in a SCR, which is present under reverse bias, can be tested in a similar way. Figure 10.6 c shows the same wafer as in Fig. 10.6 e after removal of the oxide and under cathodic polarization in the dark. Hydrogen bubbles caused by the dark current now decorate nickel silicide precipitates that short-circuit the SCR. Nickel precipitates are known to increase the dark current of a p-type Si electrode under reverse bias by orders of magnitude [Wa4]. If the bias is increased the copper silicide precipitates also become visible, as shown in Fig. 10.6 d. This method, like defect etching (Fig. 10.4f), is only sensitive to precipitated metals. Metals that stay in solution, like iron, do not show up in defect mapping and have to be determined by other methods, for example diffusion length mapping. 

Most popular schemes used to collect analytes are based on coordination reactions and electrostatic attraction. Common examples include the accumulation of nickel onto dimethylglyoxime-containing surfaces [39], the uptake and voltammetry of mercury on a diphenylcarbazide-carbon paste electrode [40], the use of surface-bound crown ethers for the collection and measurements of lead [41], or of trioctylphosphine oxide for the preconcentration of uranium [42], and the utility of polyelectrolyte-coated electrodes for the electrostatic collection of counterionic reactants [43,44], Bioaccumulation through binding to surface-bound microorganisms [45] or biocatalytic processes [46] can also offer the desired sensitivity and selectivity enhancements. 

This detection system is coupled with anion-exchange chromatography (48,49), which has the same alkaline pH requirements. The sensitivity of PAD is considerably higher than that yielded using the refractometric detector, with detection limit being of the order of 10 pmol (29). On the other hand, its response also depends heavily on the nature of the carbohydrate. Finally, other alternatives described in the literature include the use of nickel-based or copper-based electrodes and catalytic oxidation (50). 

Several metal oxides (platinum, gold," nickel, copper, ) and cobalt phtalo-cyanine have been employed as surface bound mediators for carbohydrate detection. In a dc amperometric mode of operation detectors based on these mediators exhibit a significant loss of response with time and/or exposure to analyte. Various potential pulse programs have circumvented this stability problem, but at the expense of sensitivity and complexity of the instrumentation. Silver electrodes coated with electrogenerated silver oxide exhibit electrocatalytic activity with respect to carbohydrate oxidation. This paper describes our efforts to utilize an electrode as a carbohydrate detector in a dc amperometric mode. 

Detection Methods. — A nickel-titanium alloy electrode for stable and sensitive electrochemical detection of carbohydrates has been reported." Similarly, several copper(II) oxide modified electrodes were highly sensitive for constant-potential amperometric detection of picomole levels of carbohydrates (Glc, Xyl, xylitol) in alkaline solution in flow through systems (anion-exchange h.p.l.c. and flow injection analysis), although problems with day-to lay reproducibility remained to be solved."... 

Prabakar and Narayanan (2010) demonstrated the amperometric determination of BHA using a wax composite electrode modified with nickel hexacyanoferrate (NiHCF) coupled to a FIA system. The modified electrode, in comparison to the bare electrode, gave a considerable improvement in the sensitivity (approximately sevenfold) and decreased the oxidation potential of BHA. The potential chosen for amperometric determinations was 0.4 V versus Ag/AgCl/3 mol KCl (a decrease of 300 mV in the overpotential... 

Kazemi et al. [121] reported the electrochemical fabrication of a conducting polymer of 5,10,15,20-tetra(4-sulfophenyl) porphyrin-nickel as a nanostructured electrocatalyst for hydrazine oxidation. An electrode modified with this electrocatalyst provided a detection limit of 0.11 pmol and a sensitivity of 1 pA L pmoF for the determination of hydrazine, and showed good stability and reproducibility. 

---