X-ray Photoelectron Spectroscopy, Time-of-Flight Secondary Ion Mass Spectrometry, and Principal Component Analysis of the Hydrolysis, Regeneration, and Reactivity of N-Hydroxysuccinimide-Containing Organic Thin Films

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Abstract
N-Hydroxysuccinimide (NHS) esters are widely used as leaving groups to activate covalent coupling of amine-containing biomolecules onto surfaces in academic and commercial surface immobilizations. Their intrinsic hydrolytic instability is well-known and remains a concern for maintaining stable, reactive surface chemistry, especially for reliable longer term storage. In this work, we use X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to investigate surface hydrolysis in NHS-bearing organic thin films. Principal component analysis (PCA) of both positive and negative ion TOF-SIMS data was used to correlate changes in the well-defined NHS ester oligo(ethylene glycol) (NHS-OEG) self-assembled monolayers to their surface treatment. From PCA results, multivariate peak intensity ratios were developed for monitoring NHS reactivity, thin-film thickness, and oxidation of the monolayers during surface hydrolysis. Aging in ambient air for up to 7 days resulted in hydrolysis of some fraction of bound NHS groups, oxidation of some resident thiol groups, and deposition of adventitious hydrocarbon contaminants onto the monolayers. Overnight film immersion under water produced complete hydrolysis and removal of the NHS chemistry, as well as removal of some of the thiolated OEG chains. NHS regeneration of the hydrolyzed surfaces was assessed using the same multivariable peak intensity ratio as well as surface coupling with amine-terminated molecules. Both aqueous and organic NHS regeneration methods produced surfaces with bound NHS concentrations ∼50% of the bound NHS concentration on freshly prepared NHS-OEG monolayers. Precise methods for quantifying NHS chemistry on surfaces are useful for quality control processes required in surface technologies that rely on reliable and reproducible reactive ester coupling. These applications include microarray, microfluidic, immunoassay, bioreactor, tissue engineer-ing, and biomedical device fabrication.

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