Enzyme-catalyzed modification of PES surfaces: Reduction in adsorption of BSA, dextrin and tannin Norhan Nady a,b,d , Karin Schroën a,⇑ , Maurice C.R. Franssen b,⇑ , Remco Fokkink c , Mohamed S. Mohy Eldin d , Han Zuilhof b , Remko M. Boom a a Laboratory of Food Process Engineering, Wageningen University, Bomenweg 2, 6703 HD Wageningen, The Netherlands b Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands c Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands d Polymeric Material Research Department, Advanced Technology and New Materials Research Institute (ATNMRI), New Boarg El-Arab City 21934, Alexandria, Egypt article info Article history: Received 16 February 2012 Accepted 11 April 2012 Available online 19 April 2012 Keywords: Enzyme-catalyzed modification Poly(ethersulfone) membrane Laccase Phenolic acid Anti-fouling surface Reflectometry Adsorption Surface structure abstract Poly(ethersulfone) (PES) can be modified in a flexible manner using mild, environmentally benign com- ponents such as 4-hydroxybenzoic acid and gallic acid, which can be attached to the surface via catalysis by the enzyme laccase. This leads to grafting of mostly linear polymeric chains (for 4-hydroxybenzoic acid, and for gallic acid at low concentration and short modification time) and of networks (for gallic acid at high concentration and long exposure time). The reaction is stopped at a specific time, and the mod- ified surfaces are tested for adsorption of BSA, dextrin and tannin using in-situ reflectometry and AFM imaging. At short modification times, the adsorption of BSA, dextrin and tannin is significantly reduced. How- ever, at longer modification times, the adsorption increases again for both substrates. As the contact angle on modified surfaces at short modification times is reduced (indicative of more hydrophilic surfaces), and keeps the same low values at longer modification times, hydrophilicity is not the only determining factor for the measured differences. At longer modification times, intra-layer reactivity will increase the amount of cross-linking (especially for gallic acid), branching (for 4-hydroxybenzoic acid) and/or collapse of the polymer chains. This leads to more compact layers, which leads to increased protein adsorption. The modifications were shown to have clear potential for reduction of fouling by proteins, polysaccha- rides, and polyphenols, which could be related to the surface morphology. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Membrane fouling is a serious problem in membrane filtration. The first step of membrane fouling is adsorption of components from the feed. This adsorption process depends on the nature of the components, the (surface) material of the membrane, and on the operating conditions [1–5]. Poly(ethersulfone) (PES) is a popu- lar material for membranes, as it lends itself well for membrane preparation through phase inversion, and yields mechanically and chemically robust membranes. However, its hydrophobicity makes it (as usually proposed) intrinsically susceptible to adsorp- tion by, e.g., proteins [4,6,7]. Besides proteins, also other foulants such as polysaccharides and polyphenols pose problems in prac- tice, and have been investigated individually [8–10] or in combina- tion [11,12]. Obtaining a (more) hydrophilic membrane surface has been the aim of many researchers in order to reduce (or even prevent) adsorption [13–15], but the precise mechanism of fouling by adsorption is complex. Hydrogen bonding, hydrophobic interac- tions, p–p stacking and changes in water structure at the mem- brane have been proposed as (parts of) adsorptive mechanisms [9,10,16]. In addition, a decrease in the Gibbs energy (G) of the sys- tem (i.e., protein, surface, and solvent) leads to adsorption of fou- lants. For that, any change in the enthalpy, entropy, and the system temperature should affect the adsorption process [17,18]. For example, the release of water from the surface or protein mol- ecules (i.e., dehydration processes) with a concomitant large entro- py gain leads to increased protein adsorption and an overall decreased Gibbs energy [17–19]. On the other hand, the surface structure is also sometimes considered as a factor to influence pro- tein adsorption [5,17,20–22]. Steric hindrance and the osmotic ef- fect of hydrated coated/grafted polymer branches on the surface contributes to the reduction of adsorption by keeping the foulant molecules at a distance behind a barrier of adsorbed water mole- cules (i.e., hydration layer). Moreover, the strength and thickness 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.04.019 ⇑ Corresponding authors. Fax: +31 317 482237 (K. Schroën), fax: +31 317 484914 (M.C.R. Franssen). E-mail addresses: karin.schroen@wur.nl (K. Schroën), maurice.franssen@wur.nl (M.C.R. Franssen). Journal of Colloid and Interface Science 378 (2012) 191–200 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis