Salicylate Sensors

Acetylsalicylic add is widely used as an analgesic and anti-inflammatory agent. In the body it is hydrolyzed to salicylate. The assay of salicylate by enzyme electrodes is based on the reaction of salicylate hydroxylase (EC 1.14.13.1)  

Fonong and Rechnitz (1984a) entrapped the enzyme physically with a dialysis membrane at the sensing tip of a carbon dioxide selective electrode. The sensor was suitable for salicylate concentrations in the range 0.04-2.2 mmol/l. 

Rahni et al. (1986b) combined immobilized salicylate hydroxylase with an oxygen electrode. With a linear measuring range of 0.01-0.7 mmol/l, salicylate concentrations in serum could be measured without preconcentration. The sensitivity of the enzyme electrode for aspirin and gentisate was 20 times lower than for salicylate. 

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The salicylate sensor responds to pH changes and its detection limit deteriorates as the pH increases. The selectivity coefficient for the salicylate relative to OH , log iTfai, OH = 4.2 (at pH 7.2), limits practical applications of the salicylate sensor for in situ monitoring of salicylate ions in biological systems. 

The unequivocal proof is furnished by the crystal inclusion behavior of simple 2-hydroxy-3-naphthalenecarboxylic acid 25a46 , and its 1-chloro derivative 25b37 since both allow the formation of a crystalline adduct ( clathratocomplex 19)) with dimethylformamide with the expected 1 1 stoichiometric ratio37. Thus, the salicylic acid function (2-hydroxycarboxylic acid group) is shown to be an excellent sensor, or a good complementary site for the dimethylformamide molecule in solid state inclusion. 

Below the outer membrane is a filter, usually composed of an anionic polymer, e.g. based on salicylate. Its precise composition and dimensions (thickness, pore size, amount and type of plasticizer, fillers, etc.) are optimized in order to tailor the diffusion rates of material crossing the filter from the analyte solution toward the working electrode of the sensor. Ideally, some uncharged molecules, such as H2O2, will traverse the filter so fast that, in effect, the filter is invisible to... 

The most widely used sensor for chloride ions in clinical analyzers is based on an ion-exchanger, a quaternary alkylammonium chloride, dispersed in a plastic membrane. It is not an ideal sensor due to the interference of lipophilic anions (e.g., salicylates, bromides) and lip-ophylic cations (e.g., bacteriostatic agents, anesthetics) and a relatively poor selectivity towards hydrogen carbonates (bicarbonates). However, compared to charged anion- and neutral carrier-based membranes that have been tested, it is still the best-suited for automated analyzers. 

Although the ISEs based on cobyrinates have good selectivity for nitrite over several anions, they also respond to salicylate and thiocyanate. To eliminate this interference, the nitrite-selective electrode based on ionophore 2 was placed behind a microporous gas-permeable membrane (GPM) in a nitrogen oxide gas-sensor mode (75). NOx was generated from nitrite in the sample at pH 1.7 and, after crossing the GPM, was trapped as nitrite by an internal solution that was buffered at pH 5.5 (0.100 M MES-NaOH, pH 5.5, containing 0.100 M NaCl). The internal solution was "sandwiched" between the nitrite-selective electrode and the GPM. 

Figure 6 shows the selectivity behavior of this NOx gas sensor. The sensor had a sub-Nernstian response toward nitrite, with slopes in the range of -45 to -50 mV/decade. Further, the response observed with salicylate and thiocyanate was diminished substantially, as compared to that obtained with the original nitrite-selective electrode (Figure 3). In addition, the gas sensor described here does not suffer interferences from nitrate, bicarbonate, acetate, benzoate, or chloride. These excellent selectivity properties of the sensor are a combination of the selectivity characteristics of the nitrite-selective electrode and the additional discrimination provided by the GPM.

Figure 6. Selectivity pattern of the NOx gas sensor. The sensor was exposed to 0.010 M H2SO4 containing the following anions nitrite (1), salicylate (2), thiocyanate (3), benzoate (4), nitrate (5), chloride (6), bicarbonate (7), acetate (8). (Adapted from ref. 15.)...

Chromogenic octamethyl calix[4]pyrrole-based sensors (e.g., 165-167) for antipyretic carboxylates such as naproxen, ibuprofen, and salicylate, without bias by bicarbonate or carboxy termini of blood plasma proteins, have been described <2005JA8270>. The formation of a sensor-anion complex results in partial charge transfer and a dramatic change in color. 

Keywords Lanthanide Sensor Sensitized luminescence Dipicolinate Macrocycle Ternary complex Bacterial spore Ancillary ligand Gadolinium break Catecholamine Salicylic acid Salicylurate. 

The exceptional selectivity of this sensor allowed for the direct monitoring of salicylate in blood and serum with very good precision and accuracy. 

Tetrakis(4-A, A -dimethylaminobenzene)porphyrinato-manganese(III) acetate was used as a novel carrier for a selective iodide ion electrode (Farhadi et al, 2004). The sensor exhibited not only excellent selectivity to iodide ion compared to Cl and lipophilic anions such as CIO4 and salicylate, but also a Nernstian response for iodide ion over a wide concentration range from 1.0 X 10 to 7.5 X 10 mol-l h The potentiometric response was independent of the pH of the solution in the pH range 2-8. The electrode could be used for at least 2 months without any considerable divergence in the potential. The electrode was applied to the determination of iodide in seawater samples and drug formulations. 

As described above, significant progress in the design of anion and gas selective membrane electrodes has been made. While further work is needed to understand fully the response mechanisms and to improve the performance of the new thiocyanate, salicylate, and sulfite selective membrane electrodes, each of these sensors appears to offer adequate selectivity for use in real sample measurements. In addition, by carefully... 

Most potentiometric anion-selective electrodes are based on anion exchangers such as quaternary ammonium salts. The selectivity pattern of these sensors correlates with anion lipophilicity. Highly hydrated anions such as fluoride, bicarbonate and chloride are difficult to monitor due to significant interference from more lipophilic anions like perchlorate, salicylate and nitrate which may be present in the analyzed sample. The anions can be classified according to their lipophilicity resulting in the classical Hofmeister series (ClOi > SCN > salicylate > I > NOJ > Br >... 

Figure 20. Potentiometric response of a sensor prepared with a membrane containing (TPP)Co(il) to chloride a (x), salicylate (A) and nitrite ( ) anion (a), Dynamic response of electrode toward increasing nitrite concentration and reversibility in buffer solution (b).

LLCs are promising candidates as probes for humidity sensors due to the distinct quenching effect of water molecules which can reversibly coordinate to lanthanide ions. The decrease in lifetime of the Dq transititMi of europiumflll) perchlorate was used for the determination of small amounts of water in DMF and DMSO [106]. Wang and Li have presented luminescent nanospheres for die determination of small amounts of water (0.05-3.0 vol%) in ethanol [107]. They coated silica nanoparticles with a thin layer of a salicylic acid-La /Tb " coordination compound. The green fluorescence peaking at 549 nm corresponds to the D4 transition of Tb and is strongly quenched by trace amounts of water. The nanoparticles can be excited at wavelengths around 350 nm. 

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