A Study of Biointerfacing polymeric membrane barriers for sensors
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The major problems associated with sensors are interference from electroactive chemicals and biofouling in vivo. The problem of interference can be solved using various polymer membrane barriers that can be used as inner as well as outer membrane. The polymer membrane barriers can be modified with various macrolmolecules to enhance selectivity and improve biocompatibility of a sensor. Needle electrodes were constructed from gold and platinum as working electrodes and stainless steel as a reference electrode. Flat tip disc type platinum needle electrodes proved to be a better choice for use as amperometric sensors as compared with an extruded tip needle electrode design. The flat tip needle type of electrode is a step toward microelectrodes from macro electrodes in the needle type of electrodes assembly. The flat tip needle type of electrode is essentially a flat disc type of electrode. Hence, in this study the flat disc type of electrode (The Rank Cell) was used for investigations of polymer membranes for potential use in the needle type of electrodes. Various polymer membranes were tested using a flat disc platinum electrode. Membranes tested include commercially available pre-cast membranes as well as electropolymerised films. Commercially available poly(carbonate) membranes of 0.05, 0.2 and 5.0 micron pore diameter, 0.22 micron pore diameter poly(vinylidene fluoride) membranes and PVP free poly(carbonate) membranes were tested. Electropolymerised films included the polyphenol and polyphenol red films. Selectivity tests to various electroactive compounds such as hydrogen peroxide, catechol, ascorbic acid, uric acid and acetaminophen with all the above mentioned membranes and films were carried out and were repeated after macromolecules were adsorbed onto membrane surfaces to modify the membranes. The macromolecules used for membrane modification included albumin, fibrinogen, lysozyme and hyaluronan. Results show that polycarbonate membranes are a better choice as a membrane material in precast polymer membranes because these select for hydrogen peroxide. Moreover, the smaller pore size polycarbonate membranes provide linearity of electrode response. Results with poly(vinylidene fluoride) A study of biointerfacing polymeric membrane barriers for sensors membranes were not always reproducible when testing with hydrogen peroxide. In the case of electropolymerised films, phenol red is a better membrane material than phenol film as it selects for hydrogen peroxide and excludes interfering electroactive agents. Fouling studies with proteins (albumin, fibrinogen and lysozyme) and foetal calf serum show that a sensor suffers from loss in sensitivity on exposure to proteins and serum solution. However, when coated with hyaluronan, especially the polycarbonate membrane of 0.05 micron pore diameter, showed enhanced selectivity for hydrogen peroxide. Similar results were obtained with albumin but not with fibrinogen. Increase in concentration of protein, exposure time and pore size results in a greater reduction in sensitivity. The surface characteristics of all the polymer membranes modified with macromolecules were investigated by scanning electron microscopy and atomic force microscopy. The results showed that macromolecules adsorb at the surface of membranes as well as inside of the pores in the membranes. Aggregates of proteins as well as deposits of varying thickness were observed depending on concentration and exposure times. Macromolecule adsorption in real time on a bare gold electrode surface as well as on poly(carbonate) was investigated using a Quartz Crystal Microbalance. These studies showed that the macromolecules adsorb on the surface of the polymer membranes within seconds. The macromolecules modify the surface and pore of a polymer membrane and affect the selectivity of the polymer membranes.
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