Membrane sensors

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. 

Following the discovery that the fluorescence of metalloporphyrins is strongly quenched by oxygen57, optical sensor membranes were developed that are suitable for phosphorescent sensing of oxygen58. Table 1 summarizes fundamental articles on optical sensors for oxygen until the year 2000. 

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. 

Figure 5. Mechanism of ion-exchange of an analyte ion (E) and a proton (H+) between the sensor membrane and the aqueous phase.

In the case of co-extraction, a selective anion-carrier (ionophore) extracts the analyte anion into the lipophilic sensor membrane. In order to maintain electroneutrality, a proton is co-extracted into the membrane where it protonates a pH indicator dye contained in the polymer membrane. Due to protonation, the dye undergoes a change in either absorption or fluorescence. (Figure 6 and Tables 13 and 14). 

Absorbance of a nitrite selective sensor membrane Experimental (o) and theoretical ( ) (composed of ETH 2439, NI 1, and PTTFPB in calibration plots plasticized PVC) exposed to different concentrations of nitrite... 

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.

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

When the Zincon ion-pair is exposed to an aqueous sample containing the analyte, the latter diffuses into the sensor membrane to react with the indicator, and gives a colour transition from pink to blue at near neutral pH. The pKa value of Zincon for the color transition from pink to blue is above 13, therefore, the sensor membrane is virtually insensitive to pH changes. However, due to the high complexation constant of Zincon for copper and zinc, the response of sensor membrane is irreversible and must be evaluated kinetically12. 

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],... 

M. Fibbioli, W.E. Morf, M. Badertscher, N.F. de Rooij, and E. Pretsch, Potential drifts of solid-contacted ion-selective electrodes due to zero-current ion fluxes through die sensor membrane. Electroanalysis 12, 1286-1292 (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. 

T. Mayr, I. Klimant, O.S. Wolfbeis and T. Werner, Dual lifetime referenced optical sensor membrane for the determination of copper(II) ions, Anal. Chim. Acta, 462(1) (2002) 1-10. 

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. 

Ion-selective sensors for chloride are commercially available [103]. Both fundamental and practical information concerning the theory, design, and operation of chloride-selective electrodes is available from a recent textbook [57]. Sensor membranes for potentiometric chloride detection have been formed from Ag2 S (as ion... 

Ion-Selective Electrodes based on Bis-Thiourea Receptors. Bis-thiourea derivatives 14, 15, and 17, which have a good membrane solubility, sufficient lipophilicity to prevent leaching into the aqueous sample solution, and a low tendency for self-aggregation in nonpolar solvents, were incorporated into PVC matrix liquid membranes for ISEs. While membrane electrodes based on the dibutyl derivative 14 gave a phosphate response almost identical to that of a conventional anion-exchanger electrode, a membrane electrode based on the phenyl-substituted bis-thiourea 15 exhibited a slightly improved phosphate response, which seems to be the result of improved complexation of phosphate in the sensor membrane. 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

The main challenge in designing clinically useful sensors is definitely the production of the electroactive element, i.e., the sensor membrane. The membrane is the place where the chemical recognition and discrimination processes occur. The membrane dictates, overwhelmingly, the quality of signal and durability of the sensor. Only a restricted number of membranes can be and are used in routine electrolyte and blood gas measurements ... 

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