Phosphate sensor

The entanglement generally reduces the leaching of active components from the matrix and so extends the operational lifetime of the ISE. However, PTC can also be blended with different amounts of vinyl chloride/vinyl acetate/vinyl alcohol copolymer (VAGH) which permits grafting of alkyl phosphate sensors and phosphonate mediators and 90 further reduces leaching effects. 

Figure 16,12 Molecular structures of selected PET (photoinitiated electron transfer)-based fluorescent phosphate sensors...

One of the most important sensors needed is one that reliably monitors cell density. An IR fiber-optic cell density probe has been used for this because it can directly monitor cell growth (without dilution) in high-cell-density bacterial fermentations. The ability to do an online sample filtration through the use of hollow fibers or rotating filters has made possible continuous, online measurement of glucose, lactate, and other metabolites. However, glucose, nitrogen substrate, and phosphate sensors that can withstand repeated system sterilization are still needed. 

The result will often be the eutrophication of lakes and coastal waters [238]. The area of phosphate sensors, including both chemical and biosensors, has been reviewed in 1998 by Engblom [239]. Phosphate biosensors have been reviewed by Amine and Palleschi [240]. 

Pyruvate oxidase requires the presence of thiamine pyrophosphate (0.1 mmol/1) and Ca2+ (2.5 mmol/1) for maximum activity. It should be used in 40 mmol/1 Tris buffer, pH 6.5-7.5, containing 0.5 mmol/1 phosphate. At higher phosphate concentrations substrate inhibition occurs this effect has been utilized in a phosphate sensor based on immobilized PyOD (Tabata and Murachi, 1983). Since PyOD is relatively unstable, for biosensors the enzyme has been immobilized by physical entrapment in, e.g., collagen (Mizutani et al., 1980), poly(vinyl chloride) and acetylcellulose (Kihara et al., 1984a,b). 

Efforts to produce a phosphate-selective ISE have been hindered by its diverse spe-ciation and lability in biological samples a recent review describes various potentiometric and amperometric approaches to this problem [22]. Of the potentiometric approaches, selectivity is most often achieved using inorganic or organometal-lic extracting agents such as organotin compounds in liquid-membrane ISEs, cobalt complexes or metallic cobalt in coated-wire and metallic electrodes, respectively. Nickel phosphate, silver phosphate, and mixtures of lead precipitates have also been used in phosphate ISEs. All of these sensors suffer from limited selectivity. Enzyme-based sensors for phosphate have also been a topic of research as yet, however, no commercial phosphate sensor exists [22]. 

Table 3.2 Characteristics of PVC calcium ISEs based on calcium bis[di(4-alkylphenyl) phosphate] sensors and various mediators (16)... 

Table 3.3 Characteristics of PVC ISEs with calcium bis di-[4-(l, 1,3,3-tetramethylbutyl) phenyl] phosphate sensor and various mediators (17). Viscosity and relative permittivity at 25 °C are listed below each solvent mediator... 

The overall improvement in performance of PVC calcium ISEs based on either (3) or (4) compared with the original didecylphosphoric acid sensor (1), is evident from Tables 3.2-3.4, although the dramatic (unexpected) improvement in sodium selectivity could not have been predicted. There is little difference in the performances of these aryl type phosphate sensors (membranes 9v. 13, Table 3.2) but the isomer (4) was the easier to synthesize since the branched chain 4-(l, 1,3,3-tetramethylbutyl)phenol starting compound needed for its synthesis (22) was much more readily available... 

Unusual interferences. PVC calcium ISEs based on the same di[4-(l, 1,3,3-tetramethylbutyl)phenyl]phosphate sensor but with trioctyl phosphate solvent mediator are superior to any others examined, including DOPP, in... 

Calcium phosphate sensors are commonly based on solvents with low permittivities, e.g. DOPP (s = 6.2), whereas neutral carrier types may require highly polar solvents, typically 2-nitrophenyl octylether (2-NPOE), e = 23.6 (28). Another characteristic feature of neutral carrier systems (Table 3.12) is the presence of lipophilic anions, e.g. tetraphenylborate (KTPB) which was first incorporated to reduce the interference by lipophilic sample anions (36) and so dislodge interferents from the membrane phase. Today tetrakis(4-chlorophenyl)borate, KTpClPB, is generally preferred because its water solubility is smaller than that of KTPB by a factor of about 1000, and its... 

Measurements of nitrate in soils and waste waters using the TOAN-DBP electrode and the brucine method agreed closely. All these nitrate ISEs (66,67) are subject to serious interference from iodide, chlorate and perchlorate. This feature can in turn be exploited, e.g., the Corning nitrate exchanger may be readily converted to a viable chlorate-sensing cocktail using an extraction technique as described for the uranyl phosphate sensor (section 3.2.10). 

CALCIUM ION-SELECTIVE ELECTRODES WITH COVALENTLY-BOUND ORGANO-PHOSPHATE SENSOR GROUPS... 

Keil, L., Moody, G.J. and Thomas, J.D.R. (1978) An evaluation of PVC matrix membrane calcium-selective electrodes based on nitrated (octylphenyl)phosphate sensors and phosphonate mediators. Ana/. Chim. Acta, 96, 171-175. 

M.D. Phillips, Fluorescent phosphate sensors, M.NatSc(Chem) Dissertation, The University of Birmingham, Birmingham, 2000. 

Crystallographic studies have revealed stractural similarities between the kinesin and myosin protein families, and there are short amino acid stretches displaying sequence conservation (75). These findings have led to the proposal that kinesins and myosins have a common ancestor. This precursor protein apparently also led to the G protein superfamily, which has a phosphate sensor domain that is very similar to the ATP binding sites of kinesins and myosins. As proposed for evolution of the bacterial flagellar motor, transport and locomotion functions appear to be evolutionarily connected. 

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