Potentiometric amino acid electrodes

The interaction of hydrogen peroxide with the carbon surface groups appears to be the major contributor to the potentiometric response. Several amino acid electrodes monitoring the NH4 ion have also been reported (68, 274-277). 

Potentiometric enzyme-based electrodes have found application in clinical, pharmaceutical, food and biochemical analyses to enable the selective determination of a wide range of important enzyme substrates, including amino acids, esters, amides, acylcholines, /Mactam antibiotics, sugars, enantioselective drugs and many others [74]. 

Lvova et al. [100] Caffeine, catechines, sugar, amino acid L-arginine Natural coffee, black tea and different sorts of green teas Glucose oxidase/with glutaraldehyde and BSA, then covered with an aromatic polyurethane membrane Carbon paste screen-printed electrodes/ potentiometric (different active components on each of 30 sensors to construct an array) Iron hexacyanoferrate Prussian Blue, Fein[Fen(CN)6]3... 

Kinetic measurements of H2O2 formation result in fast electrode responses (less than 12 s) using chemically bound L-amino acid oxidase (LAAO) covering a platinum electrode (170) to assay for cysteine, leucine, tyrosine, phenylalanine, tryptophane, and methionine. Potentiometric-selective electrodes for amino acids are Constructed by immobilizing LAAO on chemically modified graphite... 

It is evident from the equation that potentiometric CO2 electrodes as well as amperometric O2 or H2O2 electrodes can be used as transducers. Both potentiometric and amperometric sensors have been covered by a layer of oxalate oxidase protected by a dialysis membrane (Bradley and Rechnitz, 1986 Rahni et al.f 1986a). The sensors had a pH optimum at pH 3.5-4. Diffusion control was reached at 1 U oxalate oxidase per electrode. Oxalate determination was not affected by ascorbic acid or amino acids. The hydrogen peroxide-detecting sensor (Rahni et al., 1986a) has been used to measure oxalate in urine diluted 1 40. 

L-Amino acid oxidase [EC 1.4.3.2] Uaminoacid + Oj + H2O - a-ketoacid + HjOj + NHj l amino acid NHj,02.H202 Potentiometric electrode, FET... 

Amino acid decarboxylases [EC 4.1.1.n] Aminoacid - Amine + CO2 Amino acid CO2 Potentiometric electrode, FET... 

Another example deals with amino acid analysis using immobilized specific microorganisms in combination with selective electrodes (35). Thus, glutamine could be analyzed by an electrode consisting of a potentiometric ammonia gas sensor and a layer of the bacterium Sarcina flava (American type culture collection 147) trapped in the volume between a NHo-permeable membrane on the surface of the electrode and a dialysis membrane in contact with the surrounding solution (Fig. 10). Using this electrode, steady state potentials were reached within 5 minutes. 

Potentiometric electrodes Ammonia or carbon dioxide gas sensing probes can be employed in the same way as the oxygen electrode, with the immobli-zed microbial layer held in place on the surface of the gas-permeable membrane. The ammonia probe is commonly used to determine amino acids by detection of ammonia production by microbial deamination of amino acids. Carbon dioxide probes are alternatives to oxygen electrodes for monitoring respiratory activity. 

Many potentiometric en2yme electrodes for widely different analyte species can be prepared merely by choosing the appropriate ISE transducer and immobilized enzymatic reagent. Table 5 summarizes the enzymes and ISEs/gas sensors used to construct biosensors for a number of important biomolecules, ranging from urea and creatinine to amino acids, nucleotides, and even glucose and penicillin. In the... 

A pronounced case is in the direct potentiometric titration of amino functions on polymer, since the titrant is an about 0.5% solution of 72% aqueous perchloric acid in trichloroethane/ethanol, which is added to the whole batch of polymer in a mixture of glacial acetic acid/dichloromethane (1 1) [134]. One titration lasts about 1 hour because of the slow reaction of the glass-calomel combination of electrodes on changing potentials and is usually repeated once. To neutralize free amino groups on polymer, each equivalent of perchloric acid of the titrant transports about 2 equivalents of water into the reaction mixture. These moist conditions endanger extended Merrifield syntheses with trifunctional amino acids by proton catalyzed side reactions including detachment from polymer as well as partial acid hydrolysis of the peptide as we found. Since each titration has to be followed by another deprotonation with tertiary base while all polar groups on polymer... 

Applications.—Amino-acid anions have been shown to be transported across organic solvent membranes against their concentration gradient as counterions to metal-cation transport by macrocylic crown-type carriers. Optically active chiral crown ionophores have been used in liquid membrane electrodes for direct potentiometric determination of the enantiomeric excess in chiral ammonium... 

Changing the NH3 electrode appropriately to a different potentiometric sensor enables a wide range of species to be detected, ranging from glucose to amino-acids. 

Amino acid oxidase can thus be used to measure amino acid concentrations amperometrically, using an oxygen electrode, or potentiometrically, using a pNH4 electrode (Figure 4.16). For potentiometric measurements, a glass electrode that is sensitive to monovalent cations is more widely used. 

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