Chemically sensitive membranes deposition

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. 

The first prototype of a technologically improved IWAO was developed and tested with a membrane based on a new H+-selective ketocyanine dye and a commercial cadmium ionophore [39]. Its incorporation in an IWAO allows a highly sensitive and portable optical system to be obtained for an situ chemical analysis as well. The authors propose a flow injection analysis (FIA) system for the determination of cadmium in water samples using a cadmium-selective IWAO, as an alternative method to the ones generally used in analytical control laboratories. It permits enhanced sensitive signals in short response times by taking advantage of the very thin membranes deposited over the circuit. 

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. 

Perovskite-structured membranes, in the form of thin films supported on porous ceramic or metal substrates, have been studied extensively in the past decade. Thin films offer several advantages including reduced material cost, improved mechanical strength and possibly higher H2 flux. Chemical vapor deposition (CVD) [99], electrochemical vapor deposition (EVD) [100] and sputtering [101] represent typical methods. However, dense films have been difficult to obtain by these methods. It was found that the continuity and gas-tightness of the deposited films were very sensitive to the morphologies and pore size of substrates. 

However, the conventional methods of biomolecule immobilization (covalent binding, physical adsorption, cross-linkings, and entrapment in gels or membranes) present some disadvantages, such as low reproducibility and stability of bioselective elements or poorly controlled spatial deposition. In this sense, evaluating the surface modifications of the biomolecule plays an important role in biosensor development. Nonetheless, improvements are required in many areas, for example, prevention of nonspecific binding, sensitivity, or bioactivity. Some of these problems can be addressed by using different chemical surface modification approaches. 

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