Sensors membrane-based

Murkovic Steinberg I., Lobnik A., Wolfbeis O.S., Characterisation of an optical sensor membrane based on the metal ion indicator Pyrocatechol Violet, Sensors Actuators B. 2003 90 (1-3) 230-235. 

Ion-selective electrodes (ISEs) with sensor membranes based on sensor molecules plus suitable plasticising solvent mediators are best fabricated with such components physically entangled in a thin poly(vinyl chloride) (PVC) membrane. 

In attempts to establish a relationship between cavity sizes of crown-4 derivatives and lithium selectivities, Kitazawa et al (58) examined the potentiometric behaviour of a series of fourteen highly lipophilic crown-4 compounds with 12-, 13-, 14-, 15- and 16-member rings respectively and the fixed compositions of each of the otherwise identical PVC sensor membranes based on (24-27) make for a facile comparison of their respective potentiometric behaviour (Table 3.15). In general, the 13-membered ring compounds are much less selective than 14- and 15-membered ring compounds. Benzo-13-crown-4 derivatives also showed poor lithium selectivities, and... 

Despite the universal adoption of PVC, some investigations have centred on the prospect of alternative matrix materials. Except for poly(vinylisobutyl ether) (71), poly(methyl methyl acrylate) (72), poly(methyl acrylate) (39) and poly(vinyl chloride/alcohol) copolymer (73), practically no functional calcium ISEs were realized with poly(vinylidene chloride) (19), cellulose acetate, ethyl cellulose, collodion or pyroxylin (11). Photocured sensor membranes based on acrylates are considered in section 3.4.4. 

Possible applications of MIP membranes are in the field of sensor systems and separation technology. With respect to MIP membrane-based sensors, selective ligand binding to the membrane or selective permeation through the membrane can be used for the generation of a specific signal. Practical chiral separation by MIP membranes still faces reproducibility problems in the preparation methods, as well as mass transfer limitations inside the membrane. To overcome mass transfer limitations, MIP nanoparticles embedded in liquid membranes could be an alternative approach to develop chiral membrane separation by molecular imprinting [44]. 

GP 1] [R 10] By proper heater design, membrane-based reactors with internal heaters allowed one to reach quasi-uniform temperatures at the membrane, which determines the catalyst temperature [19]. This thermal uniformity was checked during reaction, i.e. when large heats were released in the oxidation of ammonia and needed to be transferred out of the reaction zone (Figure 3.29). Thin-film-coated temperature sensors in the center and at the edges of the membrane served to monitor the lateral temperature difference. 

The design of bioeompatible (blood compatible) potentiometric ion sensors was described in this chapter. Sensing membranes fabricated by crosslinked poly(dimethylsiloxane) (silicone rubber) and sol gel-derived materials are excellent for potentiometric ion sensors. Their sensor membrane properties are comparable to conventional plasticized-PVC membranes, and their thrombogenic properties are superior to the PVC-based membranes. Specifically, membranes modified chemically by neutral carriers and anion excluders are very promising, because the toxicity is alleviated drastically. The sensor properties are still excellent in spite of the chemical bonding of neutral carriers on membranes. 

Chemical structure of the Zincon- Response of a Zincon-based sensor membrane tetraoctylammonium ion pair to different pM concentrations of copper(II) at... 

While ionophore-free membranes based on classical ion exchangers are still in use for the determination of lipophilic ions, such sensors often suffer from insufficient selectivity, as it is governed solely by the lipophilicity pattern of ions, also known for anions as the Hofmeister sequence. This pattern for cations is Cs+ > Ag+ >K+ > NH > Na+ > Li+ > Ca2+ > Mg2+ and for anions CIOT > SCN- > I > Sal- > N03- > Br > N02- > Cl- > OAc- HC03- > SO - > HPO4. While the ion exchanger fixes the concentration of hydrophilic analyte ions in the membrane on the basis of the electroneutrality condition within the membrane, the second key membrane component is the ionophore that selectively binds to the analyte ions. The selectivity of... 

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],... 

R.P. Buck, V.V. Cosofret, E. Lindner, S. Ufer, M.B. Madaras, T.A. Johnson, R.B. Ash, and M.R. Neuman, Microfabrication technology of flexible membrane-based sensors for in-vivo applications. Electroanalysis 7, 846-851 (1995). 

B. Fu, E. Bakker, J.H. Yun, V.C. Yang, and M.E. Meyerhoff, Response mechanism of polymer membrane-based potentiometric polyion sensors. Anal. Chem. 66, 2250-2259 (1994). 

Another problem that is common for all membrane-based solid-state sensors is the ill-defined membrane-metal interface. A large exchange current density is required to produce a reversible interface for a stable potentiometric sensor response. One approach to improving this interface is to use conducting polymers. Conducting polymers are electroactive n-conjugated polymers with mixed ionic and electronic conductivity. They... 

H.J. Yoon, J.H. Shin, S.D. Ixe, H. Nam, G.S. Cha, T.D. Strong, and R.B. Brown, Solid-state ion sensors with a liquid junction-free polymer membrane-based reference electrode for blood analysis. Sens. Actuators B. 64, 8-14 (2000). 

Silicon-based pressure sensors are amongst the most common devices making use of this process. A thin low-n-doped epitaxial layer on the wafer determines an etch stop depth and thus the thickness of e.g. the pressure sensor membrane. 

Phenol red immobilized PVA membrane for an optical pH sensor is developed based on the same approach, since the molecular structure of phenol red is similar to that of phenolphthalein. Phenol red was first reacted with the formaldehyde to produce hydroxymethyl groups, and then it was attached to PVA membrane via the hydroxymethyl groups. The changes of spectra characteristics after immobilization, the ionic strength effects, response time, reproducibility and long-term stability of the sensor membrane are discussed by Z. Liu et al. [170],... 

Figure 3.38 — Integrated flow-through sensors. (A) With electrochemical generation of the luminescent reagent. The flow stream path follows the line between the analyte inlet and the outlet to waste. (B) With immobilization of a phosphor (length, 3 cm internal diameter, 2 mm) 1 immobilized phosphor 2 CFG 3 quartz wool plug 4 KEL-F caps 5 hand-tightened screw 6 stainless steel capillaries. (C) Sensor based on reflectance measurements. The sensor membrane is fixed on a Plexiglas disc. Reflectance spectra are measured from the rear side. (Reproduced from [267] and [269] with permission of the American Chemical Society and Elsevier Science Publishers, respectively).

The fact that the species transferred across the sensor membrane (the analyte or reaction product) must be a gas limits application of this type of flowthrough sensor, which, however, is still more versatile than are the sensors based on integrated separation (gas diffusion) and detection [4] described in Section 4.2 in fact, while these latter can only exploit physico-chemical properties of the analytes transferred, sensors based on triple integration allow the implementation of a (bio)chemical reaction and formation of a reaction product, so they are applicable to a much wider variety of systems with adequate sensitivity and selectivity. 

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