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Adsorption of polymers on single-walled carbon nanotubes (SWCNTs) has enabled developments in molecular sensing, in vivo imaging, gene and drug delivery, and chirality sorting applications. Noncovalent functionalization offers a modification route that preserves the pristine atomic structure, thus retaining the intrinsic near-infrared fluorescence of the SWCNTs for sensing or imaging functions, and offers a reversible binding mode for cargo delivery or chirality separation processes. However, noncovalent adsorption is an inherently dynamic process, where exchange occurs between molecules in the bulk solution and molecules on the surface, into what is known as the ‘corona phase’. In the case of polymers on SWCNTs, the nature, strength, and kinetics of both the polymer binding and unbinding processes are important contributors to the success of polymer-SWCNT based technologies. Understanding this binding process is especially important for intended uses of functionalized SWCNTs in biological environments, where native biomolecules compete with the original polymer to occupy the SWCNT surface. Binding of proteins and other biomolecules to the SWCNT not only disrupts the intended functionality of the nanoparticle, but further leads to potentially adverse biocompatibility outcomes. We present an assay to study the process of corona exchange dynamics between solution-phase and corona-phase polymers on SWCNTs. Real-time tracking of polymer and protein adsorption and desorption events are conducted with systematic variation of the molecular entities and solution conditions. This assay exploits the quenching property of fluorophores in close proximity to the SWCNT surface to monitor ligand binding and unbinding events. Binding profiles are thus extracted from the experimental assay and used to inform a kinetic model of the system. This study encompasses exchange of SWCNT-adsorbed ssDNA and either ssDNA, surfactant, or protein added to the bulk solution with multiplexed fluorescence tracking of all entities. Moreover, the kinetics of these binding profiles are validated against an orthogonal and more ubiquitously-used platform monitoring solvatochromic shifting of the near-infrared SWCNT spectrum as a proxy for SWCNT corona perturbations (Beyene et al., Nano Letters, 2018; Heller et al., PNAS, 2011; Jena et al., ACS Appl. Mater. Interfaces, 2017). The work presented herein develops an understanding of the fundamental corona exchange mechanism, contextualizes the nature of the ligand exchange process versus SWCNT solvatochromic shifting, and provides insight into performance of these designed SWCNT-based systems in biologically relevant, protein-rich conditions. |
