Chemically sensitive membranes

Figure 3. Ionic leakage paths in chemfet structures a.Schematic illustration of ionic leakage paths around the chemically sensitive membrane. Leakage through the membrane also occurs but is not illustrated b. Schematic illustration of leakage at the surface of a standard ion sensitive field effect transistor.

The integration of chemically sensitive membranes with solid-state electronics has led to the evolution of miniaturized, mass-produced potentiometric probes known as ion-selective field effect transistors (ISFETs). The development of ISFETs is considered as a logical extension of coated-wire electrodes (described in Section 5.2.4). The construction of ISFETs is based on the tech-... 

A major problem in ISFET fabrication is connected with the packaging of the chip because only a very small part of the electronic structure has to be exposed to the solution while the rest must be perfectly shielded. Any ionic leakage path from the solution around the chemically sensitive membrane and along the surface of the gate dielectric will discharge the potential generated in the sensitive membrane. 

Figure 1. Stylised chemical sensor comprising a conducting cable or track to convey the electronic signal to the outside world, a transducer to sense the chemical signal and convert it into an electronic form, and a chemically sensitive film or membrane at which the molecular binding event occurs.

The rapid expansion of reverse osmosis technology during the past two decades has resulted in the development of a variety of new membranes. Unique polymer systems and fabrication methods have led to the production of membranes with significantly improved performance and reliability. In spite of these developments little is known about chemical sensitivity or life expectancy of reverse osmosis membranes used in desalting applications. Manufacturers are consequently reluctant to guarantee their products for long runs especially in unique chemical environments. 

Osada et al. [52] have also grafted PMAA to a porous substrate in order to achieve an environmentally sensitive membrane. This sensitivity was termed a chemical valve function because mechanochemical forces caused the pores to enlarge and contract. PMAA was grafted onto poly(vinyl alcohol) (PVA) films which had a mean pore radius of 4 pm. The water permeation of the membrane was strongly affected by the conformational state of the PMAA grafts. At low pH the chains were contracted and the water permeability was... 

The chemical sensitivity or life expectancy of reverse osmosis membranes is very important for manufacturing application. Thus chlorine is the most well known reagent for water disinfection. Glaster et al. 61 inspected the influence of halogens on the performance and durability of reverse osmosis membranes. Cellulose acetate was unresponsive to halogen agents but polyamide-type membranes deteriorated rapidly when exposed to halogens. 

The ion controlled diode was an initial attempt to isolate the active electronics from the chemical solution by producing a metallic-like via that allows the isolation of the chemically sensitive region from an area where electronic components could be deposited (41,42). However, the limited precision of the non-standard microfabrication techniques made this process difficult and costly. Since this device is still essentially a capacitive membrane-insulator-semiconductor structure like the chemfet, the same problems of hermetic isolation of the gate remain. 

On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

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