Interfacial properties of grafted zwitterionic polymers and interaction with proteins at the nanoscale

Ahmed, Syeda Tajin

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https://hdl.handle.net/2142/113810 Copy

Description

Title

Interfacial properties of grafted zwitterionic polymers and interaction with proteins at the nanoscale

Author(s)

Ahmed, Syeda Tajin

Issue Date

2021-09-21

Director of Research (if dissertation) or Advisor (if thesis)

Leckband, Deborah E

Doctoral Committee Chair(s)

Leckband, Deborah E

Committee Member(s)

• Gruebele, Martin
• Kong, Hyunjoon
• Rogers, Simon A

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Zwitterionic polymers
• Surface science
• Interfacial forces, Surface forces
• protein adsorption

Abstract

Zwitterionic polymers are considered to be great candidate for surface modifications of biosensors and implantable materials because of their super hydrophilicity and ability to prevent non-specific protein adsorption or ‘fouling’. They also provide steric and electrostatic stabilization for colloid or nanoparticles in electrolytes, protein/drug stabilization and prevent marine fouling. Their functional performance as biocompatible, non-fouling coatings in different environment such as salt concentration in medium, types of salt, temperature etc. affect their solubility, swelling behavior and molecular level surface properties. In this thesis, the design parameters of grafted polysulfobetaine thin films- one of the commonly known polyzwitterions, are identified to be- grafted chain densities, polymer molecular weight, and/or film thickness and demonstrated how these parameters tune protein adsorption and surface forces at varying ionic strength of the surrounding medium and at different grafting densities. The first part of this study address the research question- why do grafted zwitterionic polymers display excellent non-fouling properties but directly interact with proteins in solution? The results of the study reported that proteins do adsorb on this so-called non-fouling polysulfobetaine grafted chains. The amount of adsorbed proteins follow a bell shaped curve, with the maximum adsorption happening at low (non-overlapping mushrooms) grafting densities and a low adsorption high (dense brush) grafting densities. This adsorption profile is a signature of ternary adsorption of proteins on weakly attractive grafted polymer chains is well described by theory, and it allows us to both test our hypothesis that the polysulfobetaines form segment-protein attractive interactions by overcoming the osmotic repulsion of insertion into the grafted chain layer as well as identify design parameters to tune protein adsorption on such zwitterionic thin films. In the second part of the study, the surface forces between grafted polysulfobetaine chains and mica were measured as a function of distance in order to investigate the influence of ionic strength, polymer molecular weight and surface density on the range of interactions and the amplitude of interfacial forces. The repulsive forces generated by thin films were quantified with sub nanometer resolution in distance by using a Surface Force Apparatus (SFA). SFA, based on multiple beam interferometry, can directly inform us of the efficacy of polysulfobetaines as entropic barriers in colloid or nanoparticle stabilization in salt solutions. The results from this study highlighted the potential for using sparsely grafted chains for developing non-fouling coatings and/or particle stabilization, whereas previous reports only focused on densely grafted brushes of polysulfobetaines. Next, surface force measurements were performed between a statistical copolymer consisting of non-ionic oligoethylene glycol and zwitterionic polysulfobetaine polymers at high and low grafting densities and testing surface mica at varying zwitterionic composition. Here, we tested the hypothesis that although the monomer constituents are chemical structurally different, they are well-mixed and non-interacting and thus, their influence on steric repulsive forces depend on the zwitterionic content in the copolymer chains. The ionic strength dependence of the chain extension and repulsive forces increased proportionally with the sulfobetaine content, reflecting the increasing influence of charged monomers and their interactions with ions in solution. These results suggested that ethyene glycol and sulfobetaine behave as non-interacting, miscible monomers that contribute independently to the polymer extension and chain interactions with ions. These findings have important practical implication in stabilizing proteins/drugs by differential interactions of zwitterionic and non-ionic counterparts in the copolymer chains. Prior to beginning my research project in surface science, I had investigated protein mediated binding kinetics by using Micropipette Aspiration Assay (MPA), the results of which are included in the fifth chapter of this thesis. Finally in the last chapter of my thesis, I discussed future directions of the project. Here I elaborated on using thin films of polyzwitterions to tune surface interactions with proteins for direct, in-situ measurements of protein folding dynamics of immobilized, high value proteins or drugs at the interface.

Graduation Semester

2021-12

Type of Resource

Thesis

Permalink

http://hdl.handle.net/2142/113810

Copyright and License Information

Copyright 2021 Syeda Tajin Ahmed

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