Acetylcholine electrode

The acetylcholine electrode was first constructed (Tran-Minh et al., 1978) by immobilizing AChE on a glass electrode that was sensitive to acetic acid [101]. The sensitivity of the sensor reaches a maximum for pH values between 8.3 and 8.6, and the biosensor is still 90 % active after 2 months use. Other acetylcholine electrodes were later constructed following the same principle [25, 108]. 

Baum and Ward reported the potentiometric determination of a pesticide using an acetylchoUne-sensitive liquid membrane electrode. They achieved detection limits in the useful concentration ranges of 10-100 ng/mL for paraoxon and 50-300 ng/mL for tetram. The detection limit of their method seemed to be determined by interference from choline, a product of the enzyme reaction, and the detection limit of the acetylcholine electrode itself. 

Katz E, WiUner 1, Wang J (2004) Electroanalytical and bioelectroanalytical systems based on metal and semiconductor nanoparticles. Electroanalysis 16 19-44 Pardo-Yissar V, Katz E, Wasserman J, Willner 1 (2003) Acetylcholine esterase-labeled CdS nanoparticles on electrodes Photoelectrochemical sensing of the enzyme inhibitors. J Am Chem Soc 125 622-623... 

Figure 6.8 ACh release and cortical activity. Correlation between acetylcholine release and EEG activity after injections of leptazol (LEPmgkg intravenously) into the urethane anaesthetised rat. ACh was collected in a cortical cup incorporating EEG recording electrodes. Mean values SE, n — 6 (unpublished data, but see Gardner and Webster 1977)...

Among cations, potassium, acetylcholine, some cationic surfactants (where the ion-exchanger ion is the / -chlorotetraphenylborate or tetra-phenylborate), calcium (long-chain alkyl esters of phosphoric acid as ion-exchanger ions), among anions, nitrate, perchlorate and tetrafluoro-borate (long-chain tetraalkylammonium cations in the membrane), etc., are determined with this type of ion-selective electrodes. 

A particular interest for clinical applications was a possibility for detection of dopamine by its oxidation on nickel [19], cobalt [65], and osmium [66] hexacyanofer-ates. Except for oxidation of dopamine, cobalt and osmium hexacyanoferrates were active in oxidation of epinephrine and norepinephrine. For clinical analysis it is also important to carry out the detection of morphine on cobalt [67] and ferric [68] hexacyanoferrates, as well as the detection of oxidizable amino acids (cystein, methionine) by manganous [69] and ruthenium [70] hexacyanoferrate-modified electrodes. In general, oxidation of thiols was first shown for Prussian blue [71] and nickel hexacyanoferrate [72], This approach has been used for the detection of thiols in rat striatum microdialysate [73], Alternatively, the detection of thiocholine with Prussian blue was employed for pesticide determination in acetylcholine-esterase test [74],... 

Tor [7] developed a new method for the preparation of thin, uniform, self-mounted enzyme membrane, directly coating the surface of glass pH electrodes. The enzyme was dissolved in a solution containing synthetic prepolymers. The electrode was dipped in the solution, dried, and drained carefully. The backbone polymer was then cross-linked under controlled conditions to generate a thin enzyme membrane. The method was demonstrated and characterized by the determination of acetylcholine by an acetylcholine esterase electrode, urea by a urease electrode, and penicillin G by a penicillinase electrode. Linear response in a wide range of substrate concentrations and high storage and operational stability were recorded for all the enzymes tested. 

Kato T, Liu JK, Yamamoto K, Osborne PG, Niwa O. 1996. Detection of basal acetylcholine release in the microdialysis of rat frontal cortex by high- performance liquid chromatography using a horseradish peroxidase-osmium redox polymer electrode with pre-enzyme reactor. J Chromatogr B 682 162-166. 

Fig. 9.7. Determination of acetylcholine by differential pulse polarography with hanging electrolyte drop electrode. Acetylcholine concentrations 0-0, 1-0.5 ppm, 2-1 ppm, 3-2 ppm, 4-5 ppm.

Let me illustrate the effect of at least one of these losses by first describing the role of acetylcholine in the cortex of a normal brain (yours). Imagine that, using an electroencephalogram or EEG, I have attached some electrodes to the front half of your head to record the electrical activity occurring inside your brain. Next, I calmly inform you that as soon as I ring a bell (at the point in time shown by the number i in Fig. 2—2) a... 

Fig. 24.4. Amperometric enzyme microelectrode array responses to acetylcholine between the concentration range 0-5 mM (n — 3, S.D. < + 10%). Inset shows a typical current transient response for an AChE electrode when exposed to 2.5 mM acetylthiocholine chloride.

Enzyme micro-electrode arrays, on exposure to differing concentrations of the substrate acetylthiocholine chloride (Fig. 24.4), demonstrate that above concentrations of 1 mM, responses tend towards a plateau. For this reason, all sensory inhibitory responses to pesticides were recorded in the presence of 2 mM acetylcholine. It should be noted that since sensor responses are recorded in the order of hundreds of nA, it is clear that some current amplification must be operating to achieve currents of this order of magnitude. This is particularly obvious when working electrodes of 0.5 cm2 were used, which only present a combined microelectrode array area of approximately 1 x 10 5 cm 2 per screen-printed electrode (if the total number of micro-electrodes that can be produced by this technique is 2 x 105 cm 2 [2-4]). 

Acetylcholine Sensors. The general scheme for determination of the neurotransmitter acetylcholine is outlined in Figure 11. In this scheme, acetylcholine is first converted catalytically to choline by the enzyme acetylcholinesterase. The choline produced reduces the FAD redox centers of choline oxidase, and electron transfer from these centers to the electrode is facilitated by the polymeric relay system. 

Figure 12. (left) Steady-state current response of acetylcholine sensors based on polymer C, in pH 7.0 phosphate buffer under N2-saturated conditions. Each point is the mean result for five electrodes. 

Kakutani et al. described an ion-transfer voltammetry and potentiometry method for the determination of acetylcholine with the interface between polymer-nitrobenzene gel and water [13]. The PVC-nitrobenzene gel electrode was prepared as described by Osakai et al. [14]. The transfer of acetylcholine ions across the interface between the gel electrode and water was studied by cyclic voltammetry, potential-step chronoamperometry, and potentiometry. The interface between the two immiscible electrolyte solutions acted as the ion-selective electrode surface for the determination of acetylcholine ions. 

Baum reported a potentiometric determination of acetylcholine activity using an organic-cation-selective electrode [15], The performance of a liquid membrane electrode selective for acetylcholine (Corning No. 476.200) was investigated. Measurements of the potential difference at various concentration of acetylcholine were made against a calomel reference-electrode. 

A quantitative method was reported by Maslova for the determination of acetylcholine in biological tissues by polarographic analysis utilizing a rotating platinum electrode [18]. The principle of the method is based upon a polarographic analysis of the iron ions which remain after the formation of a specific Fe-acetylhydroxamic acid complex. Using this method, it was shown that 1 mL of peripheral blood of a healthy adult man contained 6.6 ng of acetylcholine. 

Matsue et al. [19] recommended an electrochemical determination method using Nation coated electrodes for the determination of electroinactive medicines. Acetylcholine was determined voltammetri-cally with the use of a Nation 117-coated vitreous carbon working electrode and a saturated calomel electrode. The method is based on competition between (ferrocenylmethyl) trimethylammonium ion and the drug cations for entry into the Nation layer. The presence of acetylcholine caused a decrease in the peak current for (ferrocenylmethyl) trimethylammonium ion, from which the acetylcholine concentration is determined. The method was described to have permitted the determination of 1 pM-1 mM of acetylcholine. 

Osakai et al. used a microcomputer-controlled system for the ion transfer voltammetry procedure [20]. The system used is based on a NEC PC-9801 microcomputer, which was designed by using a polarizable oil-water interface as an ion-selective electrode surface. The system was applied to the determination of acetylcholine ion by cyclic, differential pulse, and normal pulse voltammetry at the PVC-nitrobenzene gel electrode. The amperometric measurement was carried out with voltage pulses of short durations and constant amplitude. 

Hale et al. reported the use of an enzyme-modified carbon paste for the determination of acetylcholine [21], The sensor was constructed from a carbon paste electrode containing acetylcholineesterase and choline oxidase, and the electron transfer mediator tetrathiafulvalene. The electrode was used for the cyclic voltammetric determination of acetylcholine in 0.1 M phosphate buffer at +200 mV versus saturated calomel electrode. Tetrathiafulvalene efficiently re-oxidized the reduced flavin adenine dinucleotide centers of choline oxidase. The calibration graph was linear up to 400 pM acetylcholine, and the detection limit was 0.5 pM. 

---