Zwitterionic polymers and poly(ethylene glycol) (PEG) have been reported as promising nonfouling materials, and strong surface hydration has been proposed as a significant contributor to the nonfouling mechanism. Better understanding of the similarity and difference between these two types of materials in terms of hydration and protein interaction will benefit the design of new and effective nonfouling materials. In this study, sum frequency generation (SFG) vibrational spectroscopy was applied for in situ and real-time assessment of the surface hydration of the sulfobetaine methacrylate (SBMA) and oligo(ethylene glycol) methacrylate (OEGMA) polymer brushes, denoted as pSBMA and pOEGMA, in contact with proteins. Whereas a majority of strongly hydrogen-bonded water was observed at both pSBMA and pOEGMA surfaces, upon contact with proteins, the surface hydration of pSBMA remained unaffected, but the water ordering at the pOEGMA surface was disturbed. The effects of free sulfobetaine, free PEG chains with two different molecular weights, and PEG coated gold nanoparticles on the surface hydration of proteins were investigated. The results indicated that free sulfobetaine could strengthen the protein hydration layer, but free PEG chains greatly disrupt the protein hydration layer and likely directly interact with the protein molecules. In contrast to free PEG, the PEG chains anchored on the nanoparticles behave similarly to the pOEGMA surface and could induce strong hydrogen bonding of the water molecules at the protein surfaces.

No abstract

Surface hydration has been proposed as the key nonfouling mechanism of zwitterionic materials. Because these materials have various chemical structures and will be used in complex environments, in situ probing of their surfaces in different aqueous environments is crucial to understanding their surface hydration properties. In this work, the surfaces of three zwitterionic polymer brushes in aqueous solutions with salts or varied pH were probed using sum frequency generation (SFG) vibrational spectroscopy. The SFG spectra indicate the ordering of the polymer brushes in water and the strong hydrogen bonding of the interfacial water molecules. The ordering of water at the surfaces of carboxybetaine polymers changed with pH, but at the sulfobetaine polymer surface, this ordering was not affected by pH. The interfacial ordering of water also decreased when salt ions were associated with the polymers. Ions from different salts had different interfacial binding affinities depending on the ion type and the polymer structure, as deduced from the interfacial water signals. These SFG results reveal important surface hydration properties of zwitterionic polymers that will guide their applications in complex environments.

A novel ultrasensitive multiplexed immunoassay method was developed by combining alkaline phosphatase (ALP)-labeled antibody functionalized gold nanoparticles (ALP-Ab/Au NPs) and enzyme-Au NP catalyzed deposition of silver nanoparticles at a disposable immunosensor array. The immunosensor array was prepared by covalently immobilizing capture antibodies on chitosan modified screen-printed carbon electrodes. After sandwich-type immunoreactions, the ALP-Ab/Au NPs were captured on an immunosensor surface to catalyze the hydrolysis of 3-indoxyl phosphate, which produced an indoxyl intermediate to reduce Ag(+). The silver deposition process was catalyzed by both ALP and Au NPs, which amplified the detection signal. The deposited silver was then measured by anodic stripping analysis in KCl solution. Using human and mouse IgG as model analytes, this multiplexed immunoassay method showed wide linear ranges over 4 orders of magnitude with the detection limits down to 4.8 and 6.1 pg/mL, respectively. Acceptable assay results for practical samples could be obtained. The newly designed strategy avoided cross talk and the need of deoxygenation for the electrochemical immunoassay and, thus, provided a promising potential in clinical applications.

Understanding the surface hydration of nonfouling materials such as zwitterionic polymers and poly(ethylene glycol) (PEG) aids in the design of new and effective nonfouling materials. Sum frequency generation (SFG) vibrational spectroscopy is a powerful technique used to probe water structures at solid/liquid interfaces. However, SFG signals of H 2 O consisting of symmetric and asymmetric stretches and Fermi resonance overlap heavily, which complicates the interpretation of the spectra and leads to controversy. In this work, isotopically diluted water was used instead of H 2 O to study the surface hydration of three zwitterionic polymers and a PEG coating. Because the water signal contains only an O−H stretch, strongly and weakly hydrogen-bonded water structures were easily distinguished. SFG results showed a majority of strongly hydrogen-bonded water molecules at the nonfouling polymer surfaces. For comparison, the water spectra at the surfaces of poly(methyl methacrylate) and poly(ethylene terephthalate) suggested significant amounts of weakly hydrogen-bonded water. The effects of pH on the surface hydration of the nonfouling polymers were also investigated. The materials respond to pH differently because of their different structural formulas. The flip of the water molecules at a carboxybetaine polymer surface in response to pH was also observed.