Ammonium sensor

Alcohol dehydrogenase, 178 Alkaline error, 149 Alkaline phosphatase, 185 Alkanethiols, 46, 123 Alkoxide precursor, 120 Amino acids, 92, 187 Ammonium sensor, 181, 182 Amperometric sensors, 172 Aniline, 35, 39... 

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

Enzyme sensors are based primarily on the immobilization of an enzyme onto an electrode, either a metallic electrode used in amperometry (e.g., detection of the enzyme-catalyzed oxidation of glucose) or an ISE employed in potentiometry (e.g., detection of the enzyme-catalyzed liberation of hydronium or ammonium ions). The first potentiometric enzyme electrode, which appeared in 1969 due to Guilbault and Montalvo [140], was a probe for urea with immobilized urease on a glass electrode. Hill and co-workers [141] described in 1986 the second-generation biosensor using ferrocene as a mediator. This device was later marketed as the glucose pen . The development of enzyme-based sensors for the detection of glucose in blood represents a major area of biosensor research. 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

Another way for covalent immobilisation is to synthesise indicator chemistry with polymerizable entities such as methacrylate groups (Figure 4). These groups can then be copolymerized with monomers such as hydrophobic methyl methacrylate or hydrophilic acryl amide to give sensor copolymers. In order to obtain self-plasticized materials, methacrylate monomers with long alkyl chains (hexyl or dodecyl methacrylate) can be used. Thus, sensor copolymers are obtained which have a Tg below room temperature. Similarly, ionophores and ionic additives (quaternary ammonium ions and borates) can be derivatised to give methacrylate derivatives. 

The participation of cations in redox reactions of metal hexacyanoferrates provides a unique opportunity for the development of chemical sensors for non-electroactive ions. The development of sensors for thallium (Tl+) [15], cesium (Cs+) [34], and potassium (K+) [35, 36] pioneered analytical applications of metal hexacyanoferrates (Table 13.1). Later, a number of cationic analytes were enlarged, including ammonium (NH4+) [37], rubidium (Rb+) [38], and even other mono- and divalent cations [39], In most cases the electrochemical techniques used were potentiometry and amperometry either under constant potential or in cyclic voltammetric regime. More recently, sensors for silver [29] and arsenite [40] on the basis of transition metal hexacyanoferrates were proposed. An apparent list of sensors for non-electroactive ions is presented in Table 13.1. 

Figure 5 Five-channel enzyme sensor for the simultaneous determination of glucose, lactate, glutamate, glutamine, and ammonium. MFM, microfiltration module WV, valves P, pumps DC, dialysis cell B, blank reactors MC, reactor D, biosensor flow cell. (Adapted with permission from Ref. 34.)...

By immobilizing Mn(III)-tetrakis(4-sulfonatophenyl)-porphyrin on dioctadecyl-dimethyl ammonium chloride bilayer membranes incorporated into a PVC film, Kuniyoshi et al. [65] developed an epinephrine CL sensor, which allowed determination of epinephrine down to 3 pM with an RSD of 1.0% for 50 pM of this biological compound. Compared with the previously reported epinephrine CL sensor [66], the present authors noted that the alkaline carrier solution, at high concentration levels, caused gradual deterioration of the immobilized catalyst, and this problem could be solved by the use of immobilization techniques other than ion exchange, e.g., solubilization of the catalyst that has octadecyl groups in the bilayer molecules. 

Zhang s group [82] recently presented a novel CL sensor combined with FIA for ammonium ion determination. It is based on reaction between luminol, immobilized electrostatically on an anion-exchange column, and chlorine, electrochem-ically generated online via a Pt electrode from hydrochloric acid in a coulometric cell. Ammonium ion reacts with the chlorine and decreases the produced CL intensity. The system responds linearly to ammonium ion concentration in a range of 1.0-100 pM, with a detection limit of 0.4 pM. A complete analysis can be performed in 1 min, being satisfactorily applied to the analysis of rainwater. 

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. 

Fig. 20a. 10. Schematic of the sensing principle of a urea optical sensor based on an ammonium-sensitive membrane employing anionic dye and neutral carrier.

E. Wang, L. Zhu, L. Ma and H. Patel, Optical sensors for sodium, potassium and ammonium ions based on lipophilic fluorescein anionic dye and neutral carriers, Anal. Chim. Acta, 357 (1997) 85-90. 

H. Chen and E. Wang, Urea optical sensors based on ammonium ion selective polymer membranes, Anal. Lett., 33(6) (2000) 997-1011. 

The sensor reported by Shirai(69) used a natural carboxylic polyether antibiotic (Aem = 481 nm) for the detection of magnesium and calcium. Detection limits of I0 5 and KT4 M, respectively, were reported but, interference from other metals was difficult to overcome. Ishibashi(69) used a bulkier hexadecyl-acridine orange dye (Xem = 525 nm) plasticized in a PVC membrane for the fluorescent detection of ammonium ions. Signal interference due to superfluous ions and poor detection limits of KT5 M restricted the use of the probe. 

Therefore, the ISE potential depends on the CO2 partial pressure with Nernstian slope. Contemporary microporous hydrophobic membranes permitted the construction of a number of gas probes, developed mainly by the Orion Research Company (for a survey see [143]. The most important among these sensors is the ammonia electrode, in which ammonia diffusing through the membrane affects the pH at a glass electrode. Other electrodes based on similar principles respond to SO2, HCN, H2S (with an internal S ISE), etc. The ammonia probe has a better detection limit than the ammonium ion ISE based on the non-actin ionophore. The response time of gas probes depends mostly on the rate of diffusion of the test gas through the microporous medium [77,143]. 

One of the pitfalls of microbial sensors, viz. their low selectivity, can be overcome by combining cells with an immobilized enzyme. Thus, creatinine deaminase (CDA, EC 3.5.4.21) hydrolyses creatinine to N-methylhydantoin and ammonium ion, the ammonia produced being successively oxidized to nitrite and nitrate ion by nitrifying bacteria. These bacteria have not yet been characterized but are known to be a mixed culture of Nitrosomonas sp. and Nitrobacter sp. The reaction sequence involved is as follows ... 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

The analytes typically determined by using this type of sensor are those usually addressed by gas-diffiision systems, viz. ammonia (or ammonium ion), carbon dioxide (or carbonates) and oxygen. The detection system used is most frequently photometric, fluorimetric or potentiometric, and can be integrated with or connected to the sensing microzone. The description below is based on the two choices shown in Fig. 5.4. 

In 1991, Simon et al. [9] reported a flow-through chemical sensor for the determination of ammonium ion in water samples using a sensing... 

In 1985, Ruzicka and Hansen established the principles behind flow injection optosensing [13-15], which has subsequently been used for making reaction-rate measurements [16], pH measurements by means of immobilized indicators [17,18], enzyme assays [19], solid-phase analyte preconcentration by sorbent extraction [20] and even anion determinations by catalysed reduction of a solid phase [21] —all these applications are discussed in Chapters 3 and 4. Incorporation of a gas-diffusion membrane in this type of sensor results in substantially improved sensitivity (through preconcentration) and selectivity (through removal of non-volatile interferents). The first model sensor of this type was developed for the determination of ammonium [13] and later refined by Hansen et al. [22,23] for successful application to clinical samples. 

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