Potassium sensors

Mercury layers plated onto the surface of analytical electrodes serve as Hquid metal coatings. These function as analytical sensors (qv) because sodium and other metals can be electroplated into the amalgam, then deplated and measured (see Electro analytical techniques). This is one of the few ways that sodium, potassium, calcium, and other active metals can be electroplated from aqueous solution. In one modification of this technique, a Hquid sample can be purified of trace metals by extended electrolysis in the presence of a mercury coating (35). 

The majoiity of the various analyte measurements made in automated clinical chemistry analyzers involve optical techniques such as absorbance, reflectance, luminescence, and turbidimetric and nephelometric detection means. Some of these ate illustrated in Figure 3. The measurement of electrolytes such as sodium and potassium have generally been accomphshed by flame photometry or ion-selective electrode sensors (qv). However, the development of chromogenic ionophores permits these measurements to be done by absorbance photometry also. 

Detection of Bromine Vapor. Bromine vapor in air can be monitored by using an oxidant monitor instmment that sounds an alarm when a certain level is reached. An oxidant monitor operates on an amperometric principle. The bromine oxidizes potassium iodide in solution, producing an electrical output by depolarizing one sensor electrode. Detector tubes, usefiil for determining the level of respiratory protection required, contain (9-toluidine that produces a yellow-orange stain when reacted with bromine. These tubes and sample pumps are available through safety supply companies (54). The usefiil concentration range is 0.2—30 ppm. 

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... 

Koskela SJM, Fyles TM, James TD (2005) A ditopic fluorescent sensor for potassium fluoride. Chem Commun 7 945-947... 

Calcium sensors are merely representative of a much wider class of ion sensors, albeit probably the best understood. Fluorescent probes have now been developed for a wide range of metal ions of biological interest, particularly sodium, potassium, magnesium, and zinc. 

Kawabata Y., Tahara R., Imasaka T., Ishibashi N., Fiber-optic potassium ion sensor using alkyl-acridine orange in plasticized poly(vinyl chloride) membrane, Anal. Chem. 1990 62 1528. 

Murkovic I., Lobnik A., Mohr G.J., Wolfbeis O.S., Fluorescent potential-sensitive dyes for use in solid-state sensors for potassium ion, Anal. Chim. Acta, 1996 334 125. 

The sensor layer consists of a selective ionophore (e.g. valinomycin for potassium), a lipophilic anionic site (borate) and the cationic PSD. Before interaction with potassium, a lipophilic ion pair between the cationic PSD and borate anion is formed in the polymer layer. When valinomycin (also contained in the layer) selectively extracts potassium into the layer, then the positively charged valinomycin-potassium complex forms an ion pair with... 

Figure 7. Schematic representation of the microenvironment of the cationic PSD diOC16(3) in a potassium sensor before (A) and after (B) extraction of potassium from the aqueous into the lipophilic membrane phase. The sensor membrane is composed of valinomycin, diOC16(3) and a lipophilic borate salt dissolved in plasticized PVC.

Clinical chemistry, particularly the determination of the biologically relevant electrolytes in physiological fluids, remains the key area of ISEs application [15], as billions of routine measurements with ISEs are performed each year all over the world [16], The concentration ranges for the most important physiological ions detectable in blood fluids with polymeric ISEs are shown in Table 4.1. Sensors for pH and for ionized calcium, potassium and sodium are approved by the International Federation of Clinical Chemistry (IFCC) and implemented into commercially available clinical analyzers [17], Moreover, magnesium, lithium, and chloride ions are also widely detected by corresponding ISEs in blood liquids, urine, hemodialysis solutions, and elsewhere. Sensors for the determination of physiologically relevant polyions (heparin and protamine), dissolved carbon dioxide, phosphates, and other blood analytes, intensively studied over the years, are on their way to replace less reliable and/or awkward analytical procedures for blood analysis (see below). 

Dioctyl sebacate (DOS) with relative permittivity e of 3.9 and 2-nitrophenyl octyl ether (NPOE) with e = 23.9 are the traditionally used sensor membrane plasticizers. The choice of a plasticizer always depends on a sensor application. Thus, NPOE appears to be more beneficial for divalent ions due to its higher polarity, but for some cases its lipophilicity is insufficient. Furthermore, measurements with NPOE-plasticized sensors in undiluted blood are complicated by precipitation of charged species (mainly proteins) on the sensor surface, which leads to significant potential drifts. Although calcium selectivity against sodium and potassium for NPOE-based membranes is better by two orders of magnitude compared to DOS membranes, the latter are recommended for blood measurements as their lower polarity prevents protein deposition [92],... 

Simultaneous and continuous measurements of extracellular pH, potassium K+, and lactate in an ischemic heart were carried out to study lactic acid production, intracellular acidification, and cellular K+ loss and their quantitative relationships [6, 7], The pH sensor was fabricated on a flexible kapton substrate and the pH sensitive iridium oxide layer was electrodeposited on a planar platinum electrode. Antimony-based pH electrodes have also been used for the measurement of myocardial pH in addition to their application in esophageal acid reflux detection. 

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. 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

Until recently, it was accepted that the fundamental limit of detection of these sensors was at micromolar levels of the target ion in an aqueous sample, and the main application has been the determination of ions like sodium, potassium and calcium in blood samples, where the... 

Amperometric cells, sensors using, 22 271 Amperometric measurements, 14 612 Amphetamine, 3 89-90 Amphibole asbestos, 1 803 3 288 crystal structure, 3 297-298 exposure limits, 3 316 fiber morphology, 3 294-295 silicate backbone, 3 296 Amphibole potassium fluorrichterite, glass- ceramics based on, 12 637 Amphiphile-oil-water-electrolyte phase diagram, 16 427-428 Amphiphile-oil-water phase diagrams,... 

The fourth type of mediator-based cation optical sensing is using potential sensitive dye and a cation selective ionophore doped in polymer membrane. Strong fluorophores, e.g. Rhodamine-B C-18 ester exhibits differences in fluorescence intensity because of the concentration redistribution in membranes. PVC membranes doped with a potassium ionophore, can selectively extract potassium into the membrane, and therefore produce a potential at the membrane/solu-tion interface. This potential will cause the fluorescent dye to redistribute within the membrane and therefore changes its fluorescence intensity. Here, the ionophore and the fluorescence have no interaction, therefore it can be applied to develop other cation sensors with a selective neutral ionophore. 

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. 

Figure 6.20 Action of a fluorescent PET potassium cation sensor as a molecular switch using a macrocyclic electron donor and anthracene fluorophore...

Bychkova and Shvarev [16] recently prepared nanosensors (0.2-20 pm) for sodium, potassium and calcium using the precipitation method. Similarly to the previous works, the plasticized poly(vinyl chloride) included a phenoxazine chro-moionophore, a lipophilic ion exchanger and a cation-selective ionophore. The dynamic range of the very selective sensors was 5 x 10 4-0.5 M for sodium, 1 x 10 5-0.1 M for potassium and 2 x 10 4 - 0.05 M for calcium. As was demonstrated by Bakker and co-workers [45] a particle caster can be used can be used for preparation of much larger beads (011 pm). 

Brasuel M, Kopelman R, Miller TJ, Tjalkens R, Philbert MA (2001) Fluorescent nanosensors for intracellular chemical analysis decyl methacrylate liquid polymer matrix and ion-exchange-based potassium PEBBLE sensors with real-time application to viable rat C6 glioma cells. Anal Chem 73 2221-2228... 

The probe was selective for potassium determination no significant fluorescence signal was observed with other metals. However, Na+ andLi+ induced a hypsochromic shift of the absorbance maximum in the NIR dye in free solution. Although the NIR dye offered some selectivity for potassium in analysis, the selectivity was enhanced further by the polymer matrix. Furthermore, the sensitivity of the probe to KOH and KCI was almost identical. This may indicate that the sensor response was not influenced by a pH change or the presence of anions. The fluorescence response of the immobilized NIR dye with KOH and KCI is shown in Figure 7.11. 

K. Suzuki, H. Ohzora, K. Tdida, K. Miyazaki, K. Watanabe, H. Inoue, and T. Shirai, Fibre-optic potassium ion sensors based on aneuttal ionophore and a novel lipophilic anionic dye, Anal Chim Acta 237, 155-164(1990). 

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