Toward side-selective modification of microdialysis sampling polyethersulfone (PES) membranes

by Phillips, Sarah J.; Stenken, Julie Ann

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Microdialysis (MD) sampling is a diffusion-based sepn. method that has the ability to sample any analyte that can diffuse across the membrane. However one challenge for MD is that for sol. proteins greater than 10 kDa the relative recovery (RR) using 100 kDa MWCO PES membranes is between 1 and 5 Ì?at flow rates of 0.5 and 1.0 μL/min. There are two major barriers that lead to these low recovery values - nonspecific adsorption (NSA) and poor solute mass transport. To overcome these two barriers, the modification of polyethersulfone-based (PES) MD membranes has been initiated by Laccase, an oxidase enzyme that catalyzes the oxidn. of phenols, polyphenols and anilines by one-electron abstraction. Laccase has been used to modify flat sheet PES membranes using 4-hydroxybenzoic acid (4-HBA) to create a hydrophilic polymer brush network. By functionalizing the MD membranes with carboxylic acid functional groups one can envision adding different agents to different sides of the membrane which may increase mass transport by selectively attaching targeted affinity agents. To confirm the laccase chem. with the MD PES membrane, XPS was used to confirm covalent attachment of 3-chloro-4-hydroxybenzoic acid. To confirm that mass transport of large mol. wt. solutes was not affected by the NSA of laccase, and the addn. of the polymer brush, FITC-labeled dextrans (4 and 10 kDa) were used in microdialysis recovery expts. This study focuses on reducing the NSA on the membrane by making the PES membrane more hydrophilic using a laccase catalyzed reaction to add 4HBA polymers onto the membrane surface. By obtaining a more hydrophilic membrane surface, irreversible protein adsorption will be reduced. Here we have shown using XPS that the attachment of 3-chloro-4-hydroxybenzoic acid is confirmed by the presence of a carbon-chlorine peak, and an increase in the carboxylic acid functional group and the ether linkage. This study also shows that laccase does not adsorb onto the membrane in a way that alters the RR of FITC-4/10, and that the optimal reaction time to attach 4HBA to a CMA 20 microdialysis probe is 2 h for the outside and 1 h for the luminal side of the probe.