Nitrate sensors

Nitrate sensor Viologen-fimctionalized polyvinylpyridinium (PV) Nitrate reductase [189]... 

Chiang, R.C. Cavicchioli, R. Gunsalus, R.P. Identification and characterization of narQ, a second nitrate sensor for nitrate-dependent gene regulation in Escherichia coh. Mol. Microbiol., 6, 1913-1923 (1992)... 

Urushi, a natural lacquer, has been used on the ISFET surface [89] to make, amongst other devices, a chloride sensor. The natural lacquer has a long curing time (10 days) but this can be shortened to 2-3 days with the use of formaldehyde as a crosslinking agent [90] and has been successfully used in the production of a nitrate sensor. 

Other siloxane polymers are of interest because their very low glass transition temperatures render plasticisers superfluous. However their low polarity and high hydrophobicity means that the simple commercial polydimethyl-siloxanes have high membrane resistances [92], probably due to the low solubility of ions in such media. Plasticisers could aid this but would defeat the object of using polysiloxanes in the first place. Incorporation of more polar moieties as co-monomers in the polymer, such as cyanopropyl [93], lowered membrane resistance by a factor of up to 20, with the incorporation of triflu-oropropyl groups leading to sensors for nitrate and cations [94]. Studies have also been made on using photopolymerisable crosslinkers, for example in the construction of a nitrate sensor [95]. 

As earlier reported for electrochemical sensing, often the active chromo-phore will be dispersed in a polymeric matrix. For example, Mohr and Wolfbeis reported a nitrate sensor [121] where the active chromophore is a rhodamine B dye which had been modified with an octadecyl side chain to render it hydrophobic and prevent leaching. The dye was dispersed in a plasticised PVC membrane containing a hydrophobic anion carrier (tridodecylmethylammo-nium chloride). On exposure to nitrate, the fluorescence of the dye increased. This membrane, however, only displayed Hofmeister-type selectivity and was also affected by pH. Replacing the quaternary ammonium anion carrier with a palladium phospine chloride carrier led to selectivity for nitrite [ 122], probably due to a preferential interaction between Pd and nitrite ion. 

Figure 21. (A) The assembly of an integrated nitrate sensor electrode by the cross-linking of a microperoxidase-11-nitrate reductase (cytochrome-dependent, EC 1.9.6.1) affinity complex on an Au electrode. (B) Cyclic voltammograms of the integrated MP-ll-NR monolayer-modified Au electrode (roughness factor ca. 15). (a) 0.1 M phosphate buffer, pH 7.0 (b) in the presence of KNO3, 20 mM. Potential scan rate, 5 mV s . Inset electrocatalytic cathodic currents [E = —0.6 V vs. SCE) transduced by the modified electrode at different concentrations of KNO3. Measurements were performed under argon.

Kamonsawas J, Sirivat A, Niamlang S, Horm-nirun P, Prissanaroon-Ouajai W. Electrical conductivity response of poly(phenylene-vinylene)/zeolite composites exposed to ammonium nitrate. Sensors 2010 10(6) 5590-603. 

After the successful fabrication of PVC-calcium ISEs, viable PVC nitrate sensors were similarly constructed with commercial nitrate cocktails (66). Their selectivity for some parameters is quite similar, despite their different compositions—a tris(substituted 2-phenanthroline)nickel(II) exchanger in the Orion 92-07-02 cocktail and tridodecylhexadecylammonium nitrate sensor in the Corning 477316 cocktail (Table 3.18). Their respective lifetimes were 3 and 11 weeks (66). 

Poly(urethane) can be substituted for PVC in nitrate sensor membranes (67). Nonetheless, such materials are unlikely to usurp the well-proven poly(vinyl chloride) matrix. It is essential to employ a proven PVC, e.g. Breon III EP, Flowell 470 or Fluka S 704, otherwise non-functional or poor-quality ISEs may be produced even with a well-tried cocktail. However, Silopren (a silicone rubber) offers a distinct advantage in that lipophilic anions do not interfere with its valinomycin electrode, and it is considered to have universal applicability for body fluids (41). 

A nitrate sensor has been also reported by Kim et al.." This sensor uses a simple electrochemical system composed of a silver sensing electrode, a silver oxide reference electrode, and a platinum counter electrode in 0.01 M NaOH concentrically distributed electrolyte. These micro-electrodes are microfabricated on a silicon substrate, providing an electrochemical microcell ion which a single microfluidic channel is used to deliver the reagents. Nitrate concentration is detected using double-potential step chronocoulometry in which the current accrued from nitrate reduction to nitrite is integrated. 

Kim D, Goldberg IB, Judy JW (2009) Microfabricated electrochemical nitrate sensor using double-potential-step chronocoulometry. Sens Actuat B 135 618-624... 

Safety has been greatly increased by use of the continuous nitration processes. The quantity of nitroglycerin in process at any one time is greatly reduced, and emulsification of nitroglycerin with water decreases the likelihood of detonation. Process sensors (qv) and automatic controls minimize the likelihood of mnaway reactions. Detonation traps may be used to decrease the likelihood of propagation of an accidental initiation eg, a tank of water into which the nitrated product flows and settles on the bottom. 

The sensor s fnnction rests on the property of lead dioxide to sorb nitrogen oxides (NOJ, forming lead nitrate. In this case the lead nitrate concentration is proportional to the nitrogen oxide concentration in the air. The sorption of the nitrogen oxides is a reversible process The Pb02 is regenerated completely within a few minutes when exposed to clean air (free of nitrogen oxides). 

The first interest in the electroreduction of N02 or NO catalyzed by metal complexes is to model the activity of nitrite reductase enzymes.327 There is also an extensive growth in studies related to the development of metal complex-based electrochemical sensors for NO determination in biological and environmental samples 328 329 Nitrate disproportionates to nitric oxide and nitrate in aqueous solution. 

The ion sensing scheme based on the use of potential-sensitive or polarity-sensitive dyes (PSDs) was extended to other anions. Both the clinically significant chloride ion124 and the environmentally important nitrate anion125 can be sensed in the desired concentration ranges. Such sensors have the unique advantage of having a virtually pH-insensitive response. 

Y. Mourzina, Y. Ermolenko, T. Yoshinobu, Y. Vlasov, H. lwasaki, and MJ. Schoning, Anion-selective light-addressable potentiometric sensors (LAPS) for the determination of nitrate and sulphate ions. Sens Actuators, B 91, 32—38 (2003). 

A submersible flow injection-based sensor has been used to determine nitrite and nitrate in seawater. Detection limits of 0.1 pm for nitrate and a linear range of 0.1-0.55 pM were achieved [173]. 

A new in situ probe [25] was presented for the continuous measurement of ammonium and nitrate in a biological wastewater treatment plant. Based on the use of electrochemical measurement, the sensor can be immersed and requires minimum maintenance. The tests carried out to compare its performance with those of other procedures (including UV for nitrate) showed that the results were rather close, with a detection limit of 0.1 mg L 1 for both analytes. 

Anions play key roles in chemical and biological processes. Many anions act as nucleophiles, bases, redox agents or phase transfer catalysts. Most enzymes bind anions as either substrates or cofactors. The chloride ion is of special interest because it is crucial in several phases of human biology and in disease regulation. Moreover, it is of great interest to detect anionic pollutants such as nitrates and phosphates in ground water. Design of selective anion molecular sensors with optical or electrochemical detection is thus of major interest, however it has received much less attention than molecular sensors for cations. 

Organism Sensor kinase Response regulator Nitrite exclusion a n 5 y Nitrate reductase large small Nitrite transporter... 

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