Journal of Colloid and Interface Science 452 (2015) 43–53
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Surface charge control for zwitterionic polymer brushes: Tailoring surface properties to antifouling applications Shanshan Guo a, Dominik Jan´czewski b,c,⇑, Xiaoying Zhu b,⇑, Robert Quintana b, Tao He b, Koon Gee Neoh a,d,⇑ a
NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Kent Ridge, Singapore 117576, Singapore Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore Laboratory of Technological Processes, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland d Department of Chemical & Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260, Singapore b c
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 31 January 2015 Accepted 7 April 2015 Available online 15 April 2015 Keywords: Surface charge Zeta potential Antifouling Surface-initiated ATRP Polymer brushes Mixed charge
a b s t r a c t Hypothesis: Electrostatic interactions play an important role in adhesion phenomena particularly for biomacromolecules and microorganisms. Zero charge valence of zwitterions has been claimed as the key to their antifouling properties. However, due to the differences in the relative strength of their acid and base components, zwitterionic materials may not be charge neutral in aqueous environments. Thus, their charge on surfaces should be further adjusted for a specific pH environment, e.g. physiological pH typical in biomedical applications. Experiments: Surface zeta potential for thin polymeric films composed of polysulfobetaine methacrylate (pSBMA) brushes is controlled through copolymerizing zwitterionic SBMA and cationic methacryloyloxyethyltrimethyl ammonium chloride (METAC) via surface-initiated atom transfer polymerization. Surface properties including zeta potential, roughness, free energy and thickness are measured and the antifouling performance of these surfaces is assessed.
⇑ Corresponding authors at: Laboratory of Technological Processes, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland. Fax: +48 22 234 5504 (D. Jan´czewski). Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore. Fax: +65 6872 0785 (X. Zhu). NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Kent Ridge 117576, Singapore. Fax: +65 6779 1936 (K.G. Neoh). E-mail addresses: [email protected] (D. Jan´czewski), zhux@ imre.a-star.edu.sg (X. Zhu), [email protected] (K.G. Neoh). http://dx.doi.org/10.1016/j.jcis.2015.04.013 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
44
S. Guo et al. / Journal of Colloid and Interface Science 452 (2015) 43–53
Findings: The zeta potential of pSBMA brushes is 40 mV across a broad pH range. By adding 2% METAC, the zeta potential of pSBMA can be tuned to zero at physiological pH while minimally affecting other physicochemical properties including dry brush thickness, surface free energy and surface roughness. Surfaces with zero and negative zeta potential best resist fouling by bovine serum albumin, Escherichia coli and Staphylococcus aureus. Surfaces with zero zeta potential also reduce fouling by lysozyme more effectively than surfaces with negative and positive zeta potential. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Biofouling is a serious problem in numerous applications such as biological implants [1,2], water purification systems [3] and marine equipment [4,5]. A universal protocol to eliminate biofouling on surfaces exposed to various aqueous environments has been a difficult task due to the complexity of biofoulants present in the environment [6]. Thus, development of novel surface modification protocols is a necessity [6]. Modifying surfaces using zwitterionic polymers such as carboxybetaine and sulfobetaine has proved to be a promising strategy to reduce settlement of marine organisms [7–9], protein adsorption [10,11,12] and bacterial adhesion [13]. Zwitterionic polymers bear both positively and negatively charged moieties in the repeating unit, featuring fixed charge stoichiometry and excellent hydration ability [14–16]. Mixed-charge materials have also been studied as alternatives to zwitterions. Chen et al. [17] have shown that mixed self-assembled monolayers (SAMs) containing tetraalkylammonium groups and monovalent acid thiols resist fibrinogen adsorption when the surface composition of counter charges is 1:1. Bernards et al. [18] demonstrated that mixed-charge polymer brushes grafted from surfaces via surface-initiated atom transfer polymerization (SI-ATRP) can eliminate protein adsorption when the monomer ratio of positively charged [2-(meth-acryloyloxy)ethyl]trimethylammonium chloride and negatively charged 3-sulfopropyl methacrylate potassium salt in the feed solution is 1:1. Chen et al. [19] further showed that hydrogels formed from valence-balanced polyampholyte aminoethylmethacrylate hydrochloride and 2-carboxyethyl acrylate highly resist protein adsorption. These results suggested that zero charge valence from either zwitterionic or mixed-charge groups is the key to antifouling properties of these surfaces because electrostatic interaction is presumably minimized in this way [19]. Previous investigations on the antifouling properties of zwitterionic or mixed-charge materials have mainly focused on examining the ratio of oppositely charged groups on surfaces. However, the electrostatic forces of charged surfaces in a medium depend not only on the formal charge sign of functional groups but also on the relative strength of their acid and base components. As a result, the actual surface charge in a medium depends on pH of the medium which may affect the degree of ionization of surface functional groups and ions in the medium which may cause differential ion adsorption at the solid–liquid interface [20]. Particularly, understanding how surface charge varies with the pH of media is important for designing antifouling surfaces tailored to different applications. For example, to remain charge neutral in the marine environment, surface structure should be adjusted to the seawater pH which typically has a value of 8 [21]. In biological applications, surface charge should be adjusted according to the physiological pH which is 7.4 [10]. The quantification of surface charge has been realized by zeta (f) potential measurement [22]. It effectively measures the surface charge by measuring the charge compensation by ions in solution [23]. Despite the importance of f potential measurement in providing quantitative information on the actual surface charge, the study of the relationship between f potential and antifouling performances of zwitterionic and mixed-charge surfaces has been
rather limited. In a previous study, Schroeder et al. [24] prepared antifouling hydrogels from equimolar quantities of positively charged [2-(acryloyloxy)ethyl] trimethyl ammonium and negatively charged 2-carboxyethyl acrylate. They determined the f potential of the synthesized surfaces at pH 7.4 to be between 3 mV and 9 mV. Similarly, Dobbins et al. [25] prepared antifouling hydrogels from positively charged [2-(methacryloyloxy)ethyl]trimethylammonium chloride and negatively charged 3-sulfopropyl methacrylate potassium salt and found the f potential of the surfaces to be between 3 mV and 1 mV at pH 7.4. Previous studies have examined the f potential of zwitterionic polymer powders and zwitterion-modified particles. Mary et al. [26] measured the f potential of propylsulfonate dimethylammonium ethylmethacrylate (sulfobetaine carrying a carboxylate functional group) polymers and showed that these polymers bear negative f potential if salt-free. Polzer et al. [27] showed that the f potential of colloidal particles modified with zwitterionic sulfobetaine was negative at pH 7 throughout the range of potassium chloride concentration examined. Wu et al. [28] studied colloidal particles coated with zwitterionic block copolymers consisting of dimethylammoniopropyl sulfonate (sulfobetaine) groups and found the surface to be anionic between pH 2 and 10. Notably, the f potential of zwitterionic sulfobetaine polymer chains tethered on flat surfaces in the form of polymeric brush, relevant to antifouling applications, has never been studied. Several works reported adjustment of the isoelectric point (pI) of surfaces using mixed-charge groups via SAMs. Lin et al. [29] mixed COOH (from 16-mercaptohexa-decanoic acid) and NH2 (from 8-amino-1-octanethiol) groups in the solution and achieved PIs from 3.5 to 6.5. Kuo et al. [30] mixed SH (from 3-mercaptopropyltrimethoxysilane) and NH2 (from 3-aminopropyltrimethoxysiliane) groups and obtained PIs from
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Surface charge control for zwitterionic polymer brushes: Tailoring surface properties to antifouling applications Shanshan Guo a, Dominik Jan´czewski b,c,⇑, Xiaoying Zhu b,⇑, Robert Quintana b, Tao He b, Koon Gee Neoh a,d,⇑ a
NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Kent Ridge, Singapore 117576, Singapore Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore Laboratory of Technological Processes, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland d Department of Chemical & Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260, Singapore b c
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 31 January 2015 Accepted 7 April 2015 Available online 15 April 2015 Keywords: Surface charge Zeta potential Antifouling Surface-initiated ATRP Polymer brushes Mixed charge
a b s t r a c t Hypothesis: Electrostatic interactions play an important role in adhesion phenomena particularly for biomacromolecules and microorganisms. Zero charge valence of zwitterions has been claimed as the key to their antifouling properties. However, due to the differences in the relative strength of their acid and base components, zwitterionic materials may not be charge neutral in aqueous environments. Thus, their charge on surfaces should be further adjusted for a specific pH environment, e.g. physiological pH typical in biomedical applications. Experiments: Surface zeta potential for thin polymeric films composed of polysulfobetaine methacrylate (pSBMA) brushes is controlled through copolymerizing zwitterionic SBMA and cationic methacryloyloxyethyltrimethyl ammonium chloride (METAC) via surface-initiated atom transfer polymerization. Surface properties including zeta potential, roughness, free energy and thickness are measured and the antifouling performance of these surfaces is assessed.
⇑ Corresponding authors at: Laboratory of Technological Processes, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland. Fax: +48 22 234 5504 (D. Jan´czewski). Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore. Fax: +65 6872 0785 (X. Zhu). NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Kent Ridge 117576, Singapore. Fax: +65 6779 1936 (K.G. Neoh). E-mail addresses: [email protected] (D. Jan´czewski), zhux@ imre.a-star.edu.sg (X. Zhu), [email protected] (K.G. Neoh). http://dx.doi.org/10.1016/j.jcis.2015.04.013 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
44
S. Guo et al. / Journal of Colloid and Interface Science 452 (2015) 43–53
Findings: The zeta potential of pSBMA brushes is 40 mV across a broad pH range. By adding 2% METAC, the zeta potential of pSBMA can be tuned to zero at physiological pH while minimally affecting other physicochemical properties including dry brush thickness, surface free energy and surface roughness. Surfaces with zero and negative zeta potential best resist fouling by bovine serum albumin, Escherichia coli and Staphylococcus aureus. Surfaces with zero zeta potential also reduce fouling by lysozyme more effectively than surfaces with negative and positive zeta potential. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Biofouling is a serious problem in numerous applications such as biological implants [1,2], water purification systems [3] and marine equipment [4,5]. A universal protocol to eliminate biofouling on surfaces exposed to various aqueous environments has been a difficult task due to the complexity of biofoulants present in the environment [6]. Thus, development of novel surface modification protocols is a necessity [6]. Modifying surfaces using zwitterionic polymers such as carboxybetaine and sulfobetaine has proved to be a promising strategy to reduce settlement of marine organisms [7–9], protein adsorption [10,11,12] and bacterial adhesion [13]. Zwitterionic polymers bear both positively and negatively charged moieties in the repeating unit, featuring fixed charge stoichiometry and excellent hydration ability [14–16]. Mixed-charge materials have also been studied as alternatives to zwitterions. Chen et al. [17] have shown that mixed self-assembled monolayers (SAMs) containing tetraalkylammonium groups and monovalent acid thiols resist fibrinogen adsorption when the surface composition of counter charges is 1:1. Bernards et al. [18] demonstrated that mixed-charge polymer brushes grafted from surfaces via surface-initiated atom transfer polymerization (SI-ATRP) can eliminate protein adsorption when the monomer ratio of positively charged [2-(meth-acryloyloxy)ethyl]trimethylammonium chloride and negatively charged 3-sulfopropyl methacrylate potassium salt in the feed solution is 1:1. Chen et al. [19] further showed that hydrogels formed from valence-balanced polyampholyte aminoethylmethacrylate hydrochloride and 2-carboxyethyl acrylate highly resist protein adsorption. These results suggested that zero charge valence from either zwitterionic or mixed-charge groups is the key to antifouling properties of these surfaces because electrostatic interaction is presumably minimized in this way [19]. Previous investigations on the antifouling properties of zwitterionic or mixed-charge materials have mainly focused on examining the ratio of oppositely charged groups on surfaces. However, the electrostatic forces of charged surfaces in a medium depend not only on the formal charge sign of functional groups but also on the relative strength of their acid and base components. As a result, the actual surface charge in a medium depends on pH of the medium which may affect the degree of ionization of surface functional groups and ions in the medium which may cause differential ion adsorption at the solid–liquid interface [20]. Particularly, understanding how surface charge varies with the pH of media is important for designing antifouling surfaces tailored to different applications. For example, to remain charge neutral in the marine environment, surface structure should be adjusted to the seawater pH which typically has a value of 8 [21]. In biological applications, surface charge should be adjusted according to the physiological pH which is 7.4 [10]. The quantification of surface charge has been realized by zeta (f) potential measurement [22]. It effectively measures the surface charge by measuring the charge compensation by ions in solution [23]. Despite the importance of f potential measurement in providing quantitative information on the actual surface charge, the study of the relationship between f potential and antifouling performances of zwitterionic and mixed-charge surfaces has been
rather limited. In a previous study, Schroeder et al. [24] prepared antifouling hydrogels from equimolar quantities of positively charged [2-(acryloyloxy)ethyl] trimethyl ammonium and negatively charged 2-carboxyethyl acrylate. They determined the f potential of the synthesized surfaces at pH 7.4 to be between 3 mV and 9 mV. Similarly, Dobbins et al. [25] prepared antifouling hydrogels from positively charged [2-(methacryloyloxy)ethyl]trimethylammonium chloride and negatively charged 3-sulfopropyl methacrylate potassium salt and found the f potential of the surfaces to be between 3 mV and 1 mV at pH 7.4. Previous studies have examined the f potential of zwitterionic polymer powders and zwitterion-modified particles. Mary et al. [26] measured the f potential of propylsulfonate dimethylammonium ethylmethacrylate (sulfobetaine carrying a carboxylate functional group) polymers and showed that these polymers bear negative f potential if salt-free. Polzer et al. [27] showed that the f potential of colloidal particles modified with zwitterionic sulfobetaine was negative at pH 7 throughout the range of potassium chloride concentration examined. Wu et al. [28] studied colloidal particles coated with zwitterionic block copolymers consisting of dimethylammoniopropyl sulfonate (sulfobetaine) groups and found the surface to be anionic between pH 2 and 10. Notably, the f potential of zwitterionic sulfobetaine polymer chains tethered on flat surfaces in the form of polymeric brush, relevant to antifouling applications, has never been studied. Several works reported adjustment of the isoelectric point (pI) of surfaces using mixed-charge groups via SAMs. Lin et al. [29] mixed COOH (from 16-mercaptohexa-decanoic acid) and NH2 (from 8-amino-1-octanethiol) groups in the solution and achieved PIs from 3.5 to 6.5. Kuo et al. [30] mixed SH (from 3-mercaptopropyltrimethoxysilane) and NH2 (from 3-aminopropyltrimethoxysiliane) groups and obtained PIs from
