No abstract

Tailoring the interaction between surfaces and nanoparticles (NPs) affords great opportunities for a range of applications, including sensors, information storage, medical diagnostics, and filtration membranes. In addition to controlling local ordering and microscale patterning of the NPs, manipulating the temporal factors determining the strength of the interaction between NP and surface enables dynamic modulation of these structural characteristics. In this contribution we demonstrate robust polymer brush-NP hybrids that exhibit both reversible swelling and reversible NP adsorption/desorption. Polymer brush functionality is tailored through post-functionalization of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes on flat solid substrates with alpha-amine conjugates ranging from perfluoro alkanes to poly(ethylene glycol) of varying molecular weights. The type of functionality controls NP affinity for the surfaces. In the case of poly(ethylene glycol) (PEG), the molecular weight (MW) of the PEG dictates adsorption and desorption phenomena. Higher MW PEG chains possess increased binding affinity toward NPs, which leads to higher relative Au-NP densities on the PHEMA-g-PEG brushes and concurrent sluggish desorption of NPs by thermal stimulus. Adsorption and desorption phenomena are further modulated by NP size yielding a system where adsorption and desorption are controlled by a delicate balance between the competitive energetics of polymer brush chelation versus solvation.

Amphiphilic polymer coatings were prepared by first generating surface-anchored polymer layers of poly(2-hydroxyethyl methacrylate) (PHEMA) on top of flat solid substrates followed by postpolymerization reaction on the hydroxyl terminus of HEMA’s pendent group using three classes of fluorinating agents, including organosilanes, acylchlorides, and trifluoroacetic anhydride (TFAA). The distribution of the fluorinated groups inside the polymer brushes was assessed by means of a suite of analytical probes, including contact angle, ellipsometry, infrared spectroscopy, atomic force microscopy, and near-edge x-ray absorption fine structure spectroscopy. While organosilane modifiers were found to reside primarily close to the tip of the brush, acylchlorides penetrated deep inside PHEMA thus forming random copolymers P(HEMA-co-fHEMA). The reaction of TFAA with the PHEMA brush led to the formation of amphiphilic diblocks, PHEMA-b-P(HEMA-co-fHEMA), whose bottom block comprised unmodified PHEMA and the top block was made of P(HEMA-co-fHEMA) rich in the fluorinated segments. This distribution of the fluorinated groups endowed PHEMA-b-P(HEMA-co-fHEMA) with responsive properties; while in hydrophobic environment P(HEMA-co-fHEMA) segregated to the surface, when in contact with a hydrophilic medium, PHEMA partitioned at the brush surface. The surface activity of the amphiphilic coatings was tested by studying the adsorption of fibrinogen (FIB). While some FIB adsorption occurred on most coatings, the ones made by TFAA modification of PHEMA remained relatively free of FIB.

On the basis of their versatile structure and chemistry as well as tunable mechanical properties, polymer brushes are well-suited as supports for enzyme immobilization. However, a robust surface design is hindered by an inadequate understanding of the impact on activity from the coupling motif and enzyme distribution within the brush. Herein, horseradish peroxidase C (HRP C, 44 kDa), chosen as a model enzyme, was immobilized covalently through its lysine residues on a N-hydroxysuccinimidyl carbonate-activated poly(2-hydroxyethyl methacrylate) (PHEMA) brush grafted chemically onto a flat impenetrable surface. Up to a monolayer coverage of HRP C is achieved, where most of the HRP C resides at or near the brush-air interface. Molecular modeling shows that lysines 232 and 241 are the most probable binding sites, leading to an orientation of the immobilized HRP C that does not block the active pocket of the enzyme. Michaelis-Menten kinetics of the immobilized HRP C indicated little change in the K(m) (Michaelis constant) but a large decrease in the V(max) (maximum substrate conversion rate) and a correspondingly large decrease in the k(cat) (overall catalytic rate). This indicates a loss in the percentage of active enzymes. Given the relatively ideal geometry of the HRPC-PHEMA brush, the loss of activity is most likely due to structural changes in the enzyme arising from either secondary constraints imposed by the connectivity of the N-hydroxysuccinimidyl carbonate linking moiety or nonspecific interactions between HRP C and DSC-PHEMA. Therefore, a general enzyme-brush coupling motif must optimize reactive group density to balance binding with neutrality of surroundings.

We describe the formation of polyampholytic block copolymer brushes and their assembly in solution. Specifically, we employ "surface-initiated" activators regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP) sequentially to form diblock copolymer grafts comprising blocks of poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) and poly(sodium methacrylate) (PNaMA) on flat impenetrable silica surfaces, i.e., SiO(x)/PNaMA-b-PDMAEMA and SiO(x)/PDMAEMA-b-PNaMA. Protonation of the PNaMA block results in formation of poly(methacrylic acid) (PMAA). We demonstrate that ARGET-ATRP of NaMA provides a convenient route to preparation of PMAA, which is an alternative method to the more traditional approach based on preparing PMAA by polymerizing tert-butyl methacrylate (tBMA) followed by cleavage of the tert-butyl group. We also discuss conformational changes of the individual polyelectrolyte blocks in solution as a function of solution pH by monitoring adsorption behavior of functionalized polystyrene spheres.