Sandwich membrane method

The selectivity here is directly proportional to complex formation constants and can be estimated, once the latter are known. Several methods are now available for determination of the complex formation constants and stoichiometry factors in solvent polymeric membranes, and probably the most elegant one is the so-called sandwich membrane method [31], Two membrane segments of different known compositions are placed into contact, which leads to a concentration polarized sensing membrane, which is measured by means of potentiometry. The power of this method is not limited to complex formation studies, but also allows one to quantify ion pairing, diffusion, and coextraction processes as well as estimation of ionic membrane impurity concentrations. 

M.M. Shultz, O.K. Stefanova, S.S. Mokrov, and K.N. Mikhelson, Potentiometric estimation of the stability constants of ion-ionophore complexes in ion-selective membranes by the sandwich membrane method theory, advantages, and limitations. Anal. Chem. 74, 510-517 (2002). 

Conversely, the fundamentals for the UDL he on the coextraction of counterions into the membrane therefore, the membrane is no longer permselective (Donnan failure) [9]. Ideally, when the ionophores are saturated by ions, the ion-ionophore complex functions as an ion-exchanger and the membrane shows an anion Nernstian response. The UDL can be estimated from the membrane composition, formation constant and coextraction coefficients obtained from the so-called sandwich membrane method [73]. 

Figure 33 Determination of the stability of ionophore complexes with the sandwich membrane method Only the left hand membrane contains ionophore.

The design and implementation of a portable fiber-optic cholinesterase biosensor for the detection and determination of pesticides carbaryl and dichlorvos was presented by Andreou81. The sensing bioactive material was a three-layer sandwich. The enzyme cholinesterase was immobilized on the outer layer, consisting of hydrophilic modified polyvinylidenefluoride membrane. The membrane was in contact with an intermediate sol-gel layer that incorporated bromocresol purple, deposited on an inner disk. The sensor operated in a static mode at room temperature and the rate of the inhibited reaction served as an analytical signal. This method was successfully applied to the direct analysis of natural water samples (detection and determination of these pesticides), without sample pretreatment, and since the biosensor setup is fully portable (in a small case), it is suitable for in-field use. 

The actual blotting process may be accomplished by one of two methods passive (or capillary) transfer and electroblotting. In passive transfer, the membrane is placed in direct contact with the polyacrylamide gel and organized in a sandwich-like arrangement consisting of (from bottom to top) filter paper soaked with transfer buffer, gel, membrane, and more filter paper. The sandwich is compressed by a heavy weight. Buffer passes by capillary ac-... 

Despite these improvements, there are other important biosensor limitations related to stability and reproducibility that have to be addressed. In this context, enzyme immobilisation is a critical factor for optimal biosensor design. Typical immobilisation methods are direct adsorption of the catalytic protein on the electrode surface, or covalent binding. The first method leads to unstable sensors, and the second one presents the drawback of reducing enzyme activity to a great extent. A commonly used procedure, due to its simplicity and easy implementation, is the immobilisation of the enzyme on a membrane. The simplest way is to sandwich the enzyme between the membrane and the electrode. Higher activity and greater stability can be achieved if the enzyme is previously cross-linked with a bi-functional reagent. 

The microfluidic structure was manufactured in glass by classical photolithography and wet-chemical HF etching methods [164], Holes for fluid connection were drilled. A polymer membrane was prepared in-house and served as a top plate to give a glass-polymer sandwich chip. Pressure sealing was applied. 

Transfection efficacy of naked DNA can be increased by physical methods such as electroporation and sonication. Electroporation employs electric pulses to punch holes in the cell membrane, usually smaller than 10 nm but larger than oligonucleotides. With the use of electroporation, DNA was delivered into the cytosol of cells by diffusion. Since its introduction in 1982, in vivo transfection has been achieved in skeletal muscle, fiver, skin, tumors, testis, and the kidney. Tsujie et al. (2001) developed a method to target glomeruli using electroporation in vivo wherein injection of plasmid DNA via the renal artery was followed by application of electric fields. The kidney was electroporated by sandwiching the organ... 

OPLC is a method in which the mobile phase is pumped through a layer that is sandwiched between a rigid support block and a flexible plastic membrane... 

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